introduction drilling characteristics · 2006. 11. 21. · introduction the standard procedures for...

Taylor, B., Fujioka, K., et al., 1990Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 126

2. EXPLANATORY NOTES1

Shipboard Scientific Party2

INTRODUCTIONThe standard procedures for drilling operations and prelimi-

nary shipboard analyses of the material recovered during DeepSea Drilling Project (DSDP) and Ocean Drilling Program (ODP)drilling have been regularly amended and upgraded since drill-ing began in 1968. In this chapter, we have assembled informa-tion that will help the reader understand the basis for our pre-liminary conclusions and also help the interested investigator se-lect samples for further analysis. This information concerns onlyshipboard operations and analyses described in the site reportsin the Initial Reports volume of the Leg 126 Proceedings of theOcean Drilling Program. Methods used by various investigatorsfor shore-based analysis of Leg 126 data will be detailed in theindividual scientific contributions published in the ScientificResults volume.

Authorship of Site ChaptersThe separate sections of the site chapters were written by the

following shipboard scientists (authors are listed in alphabeticalorder in parentheses, and no seniority is necessarily implied):

Site Summary: Fujioka, B. TaylorBackground and Objectives: B. TaylorSeismic Stratigraphy: Cooper, Klaus, B. TaylorOperations: Grout, Huey, JanecekLithostratigraphy and Accumulation Rates: Colella, Hiscott, Ja-

necek, Marsaglia, Nishimura, Rodolfo, TazakiBiostratigraphy: Aitchison, Firth, Herman, Isiminger-Kelso,

KaihoIgneous Petrology: Gill, LaPierre, R. Taylor, TorssanderIgneous Geochemistry: Gill, LaPierre, R. Taylor, TorssanderSediment/Fluid Geochemistry: EgebergPaleomagnetics: Cisowski, KoyamaPhysical Properties: Dadey, KlausDownhole Measurements: Lovell, Pezard

Following the text of each site chapter are summary core de-scriptions ("barrel sheets" and igneous rock visual core descrip-tions) and photographs of each core.

Survey DataSurvey data collected prior to Leg 126 and used in the site se-

lection process are discussed in a separate chapter (see Taylor etal., this volume). The underway geophysics data collected aboardthe JOIDES Resolution during Leg 126 are discussed in a sepa-rate chapter entitled "Underway Geophysics" (this volume). Theunderway data include bathymetry, magnetics, and seismic re-flection profiles.

1 Taylor, B., Fujioka, K., et al., 1990. Proc. ODP, Init. Repts., 126: CollegeStation, TX (Ocean Drilling Program).

2 Shipboard Scientific Party is as given in the list of participants preceding the

Drilling CharacteristicsWater circulation down the hole is open; hence, cuttings were

lost onto the seafloor and could not be examined. The onlyavailable information about sedimentary stratification in un-cored or unrecovered intervals, other than from seismic data orwireline-logging results, is from an examination of the behaviorof the drill string as observed and recorded on the drilling plat-form. Typically, the harder a layer, the slower and more difficultit is to penetrate. A number of other factors, however, deter-mine the rate of penetration, so it is not always possible to relatedrilling time directly to the hardness of the layers. Bit weightand revolutions per minute, recorded on the drilling recorder,influence the penetration rate.

Drilling DeformationWhen cores were split, many showed signs of significant sed-

iment disturbance, including the concave-downward appearanceof originally horizontal bands, haphazard mixing of lumps ofdifferent rock types (mainly at the tops of cores), and the near-fluid state of some sediments recovered from tens to hundredsof meters below the seafloor. Core deformation probably oc-curred during any of several steps in which the core sufferedstresses sufficient to alter its physical characteristics: cutting, re-trieval (with accompanying changes in pressure and tempera-ture), and core handling on deck.

Shipboard Scientific Procedures

Numbering of Sites, Holes, Cores, and Samples

ODP drill sites are numbered consecutively from the first sitedrilled by the Glomar Challenger in 1968. A site number refersto one or more holes drilled while the ship was positioned overone acoustic beacon. Multiple holes were drilled at a single siteby pulling the drill pipe above the seafloor (out of hole), movingthe ship some distance from the previous hole, and then drillinganother hole.

For all ODP drill sites, a letter suffix distinguishes each holedrilled at the same site. For example, the first hole drilled wasassigned the site number modified by the suffix A, the secondhole took the site number and the suffix B, and so forth. Notethat this procedure differs slightly from that used by DSDP(Sites 1 through 624), but it prevents ambiguity between site-and hole-number designations. It is important for sampling pur-poses to distinguish among the holes drilled at a site, becauserecovered sediments or rocks from different holes usually do notcome from equivalent positions in the stratigraphic column.

The cored interval was measured in meters below seafloor(mbsf) The depth interval assigned to an individual core rangesfrom the depth below the seafloor that the coring operation be-gan to the depth that the coring operation ended (Fig. 1). Forexample, each coring interval is generally up to 9.7 m long (thenominal capacity of a core barrel). Coring intervals may beshorter and may not necessarily be adjacent if separated bydrilled intervals. In soft sediments, the drill string can be "washedahead" with the core barrel in place without recovering sedi-

13

SHIPBOARD SCIENTIFIC PARTY

JOIDESResolution

Drilled(but not cored) area

Sub-bottom bottom YJ?Δ

Y//i Represents recovered material

BOTTOM FELT: distance from rig floor to seafloor

TOTAL DEPTH: distance from rig floor to bottom of hole(sub-bottom bottom)

PENETRATION: distance from seafloor to bottom ofhole (sub-bottom bottom)

NUMBER OF CORES: total of all cores recorded,including cores with norecovery

TOTAL LENGTHOF CORED SECTION: distance from sub-bottom top to

sub-bottom bottom minus drilled(but not cored) areas inbetween

TOTAL CORE RECOVERED: total from adding a, b, c,and d in diagram

CORE RECOVERY (%): equals TOTAL CORERECOVEREDdivided byTOTAL LENGTH OFCORED SECTIONtimes 100

Figure 1. Diagram illustrating terms used in discussing coring opera-tions and core recovery.

ments. This is achieved by pumping water down the pipe at highpressure to wash the sediment out of the way of the bit and upthe space between the drill pipe and the wall of the hole. If thin,hard, rock layers are present, then it is possible to get "spotty"sampling of these resistant layers within the washed interval andthus to have a cored interval greater than 9.7 m. In drilling hardrock, a center bit may replace the core barrel if it is necessary todrill without core recovery.

Cores taken from a hole are numbered serially from the topof the hole downward. Core numbers and their associated coredintervals in meters below seafloor usually are unique in a givenhole; however, this may not be true if an interval must be coredtwice because of caving of cuttings or other hole problems.Maximum full recovery for a single core is 9.5 m of rock or sedi-ment contained in a plastic liner (6.6-cm internal diameter) plusabout 0.2 m (without a plastic liner) in the core catcher (Fig. 2).The core catcher is a device at the bottom of the core barrel thatprevents the core from sliding out when the barrel is being re-trieved from the hole. In certain situations (e.g., when coringgas-charged sediments that expand while being brought on deck),recovery may exceed the 9.5-m maximum.

A recovered core was divided into 1.5-m sections that werenumbered serially from the top (Fig. 2). When full recovery wasobtained, the sections were numbered from 1 through 7, withthe last section possibly being shorter than 1.5 m (rarely, an un-usually long core may require more than 7 sections). When lessthan full recovery was obtained, there will be as many sectionsas needed to accommodate the length of the core recovered; forexample, 4 m of core would be divided into two 1.5-m sectionsand a 1-m section. If cores are fragmented (recovery less than100%), sections are numbered serially and intervening sectionsare noted as void, whether shipboard scientists believe that thefragments were contiguous in situ or not. In rare cases, a sectionless than 1.5 m may be cut so that features of interest can bepreserved (e.g., lithologic contacts).

By convention, material recovered from the core catcher wasplaced below the last section when the core was described andlabeled core catcher (CC). In sedimentary cores, it was treatedas a separate section. The core catcher was placed at the top ofthe cored interval in cases where material was only recovered inthe core catcher. However, information supplied by the drillersor by other sources may allow for more precise interpretation asto the correct position of core-catcher material within an incom-pletely recovered cored interval.

A recovered rock (basalt, gabbro, peridotite, or serpentinite)core was also cut into 1.5-m sections and numbered serially;however, each piece of rock then was assigned a number. Frag-ments of a single piece were assigned a single number, and indi-vidual fragments were identified alphabetically. The core-catchersample was placed at the bottom of the last section and treatedas part of the last section rather than separately. The scientistswho completed the visual core descriptions described each litho-logic unit, noting core and section boundaries only as physicalreference points.

When the recovered core was shorter than the cored interval,as was usually the case, the top of the core was equated with thetop of the cored interval by convention so that consistency inhandling the analytical data derived from the cores could bemaintained. Samples removed from the cores were designatedby the distance measured in centimeters from the top of the sec-tion to the top and bottom of each sample removed from thatsection. In curated hard-rock sections, sturdy plastic spacerswere placed between pieces that did not fit together to protectthem from damage in transit and in storage. Therefore, the cen-timeter interval noted for a hard-rock sample has no direct rela-tionship to that sample's depth within the cored interval, but isonly a physical reference to the location of the sample withinthe curated core.

14

EXPLANATORY NOTES

Fullrecovery

Sectionnumber

1

Top

Partialrecovery

Partialrecoverywith void

_çç_jpCore catcher

sample

Sectionnumber

Emptyliner

Top

ELOσ>

II

1CD

Oü

\< \i

Core catchersample

Sectionnumber

1

2

3

4

~cc~

Voi

dV

oid

. -••.i" J u w

ILU

7T7 - - = Top

oü

Core catchersample

Figure 2. Diagram showing procedure for cutting and labeling core sections.

A full identification number for a sample consists of the fol-lowing information: leg, site, hole, core number, core type, sec-tion number, piece number (for hard rock), and interval in centi-meters measured from the top of section. For example, a sampleidentification of "126-787A-10R-1, 10-12 cm" would be inter-preted as representing a sample removed from the interval be-tween 10 and 12 cm below the top of Section 1, Core 10 (R des-ignates that this core was taken with the rotary core barrel) ofHole 787A during Leg 126.

All ODP core and sample identifiers indicate core type. Thefollowing abbreviations are used: R = rotary core barrel (RCB);H = hydraulic piston core (HPC; also referred to as APC, oradvanced hydraulic piston core); P = pressure core barrel; X =extended core barrel (XCB); B = drill-bit recovery; C = center-bit recovery; I = in-situ water sample; S = sidewall sample; W= wash-core recovery; and M = miscellaneous material. APC,XCB, RCB, and wash cores were cut on Leg 126.

Core Handling

Sediments

As soon as a core was retrieved on deck during Leg 126, asample was taken from the core catcher and given to the paleon-tological laboratory for an initial age assessment. The core wasthen placed on the long horizontal rack, and gas samples weretaken by piercing the core liner and withdrawing gas into a vac-uum tube. Voids within the core were sought as sites for gassampling. Some of the gas samples were stored for shore-basedstudy, but others were analyzed immediately as part of the ship-board safety and pollution prevention program. Next, the corewas marked into section lengths, each section was labeled, and

the core was cut into sections. Interstitial water (IW) and or-ganic geochemistry (OG) samples were taken. In addition, someheadspace gas samples were scraped from the ends of cut sec-tions on the catwalk and were sealed in glass vials for light-hy-drocarbon analysis. Each section was then sealed at the top andbottom by gluing on color-coded plastic caps: blue to identifythe top of a section and clear for the bottom. A yellow cap wasplaced on section ends from which a whole-round sample hadbeen removed. The caps were usually attached to the liner bycoating the end liner and the inside rim of the cap with acetoneand then taping the caps to the liners.

From the deck, the cores were carried into the laboratory,where the sections were again labeled with an engraver to markthe full designation of the section more permanently. The lengthof the core in each section and the core-catcher sample weremeasured to the nearest centimeter; this information was loggedinto the shipboard CORELOG database program.

Next, the whole-round sections from the APC and XCB coreswere run through the multisensor track (MST). This includedthe gamma-ray attenuation porosity evaluator (GRAPE) and P-wave logger (PWL) devices, which measured the bulk density,porosity, and sonic velocity, as well as an additional meter thatdetermined the volume magnetic susceptibility. After the corewas equilibrated to room temperature (approximately 3 hr), ther-mal conductivity measurements were performed before the coreswere split.

Cores composed of rather soft material were split lengthwiseinto working and archive halves. The softer cores were split witha wire or saw, depending on the degree of induration. Hardercores were split with a band or diamond saw. Since cores on Leg126 were split with wire from the bottom to top, older material

15


could have been transported up the core on the split face of eachsection. One should, therefore, be aware that the very near-sur-face part of the split core could be contaminated.

The working half was sampled for shipboard and shore-basedlaboratory studies. Each extracted sample was logged into thesampling computer database program by the location and thename of the investigator receiving the sample. Records of allsamples that were removed are kept by the curator at ODP. Theextracted samples were sealed in plastic vials or bags and la-beled. Samples were routinely taken for shipboard physical prop-erty analyses. These samples were subsequently used for cal-cium carbonate determinations (coulometric analyses); these dataare reported in the site chapters.

The archive half was described visually. Smear slides weremade from samples taken from the archive half and were sup-plemented by thin sections taken from the working half. Sec-tions from the archive half that did not show evidence of drill-ing disturbance were run through the cryogenic magnetometer.The archive half was then photographed with black-and-whiteand color film, a whole core at a time. Close-up photographs(black-and-white) were taken of particular features for illustra-tions in the summary of each site.

Each half of the core was then put into a labeled plastic tube,sealed, and transferred to cold-storage space aboard the drillingvessel. At the end of the leg, the cores were transferred from theship in refrigerated airfreight containers to cold storage at theGulf Coast Repository at the Ocean Drilling Program, TexasA&M University, College Station.

Igneous and Metamorphic Rocks

Igneous and metamorphic rock cores were handled differ-ently from sedimentary cores. Once on deck, the core catcherwas placed at the bottom of the core liner, and the total core re-covery was calculated by shunting the rock pieces together andtaking measurements to the nearest centimeter. This informa-tion was logged into the shipboard CORELOG database pro-gram. The core was then cut into 1.5-m-long sections and trans-ferred to the lab.

The contents of each section were transferred into 1.5-m-long sections of split core liner, where the bottom of the ori-ented pieces (i.e., pieces that clearly could not have rotated topto bottom about a horizontal axis in the liner) were marked witha red wax pencil. This ensured that orientation would not belost during the splitting and labeling process. The core was splitinto archive and working halves, and a plastic spacer was usedto separate individual pieces, and/or reconstructed groups ofpieces, in the core liner. These spacers may represent a substan-tial interval of no recovery. Each piece was numbered sequen-tially from the top of each section, beginning with number 1; re-constructed groups of pieces were assigned the same number,but they were lettered consecutively. Pieces were labeled on therounded surface, rather than on the sawn surface. If the piecewas oriented, an arrow that pointed to the top of the section wasadded to the label.

The working half was sampled for shipboard laboratory stud-ies. Records of all samples are kept by the curator at ODP. Mini-core samples were routinely taken for physical properties andmagnetic studies. Some of these samples were later subdividedfor X-ray fluorescence (XRF) analysis and thin sectioning, sothat as many measurements as possible were made on the samepieces of rock. At least one minicore was taken per lithologicunit when recovery permitted, generally from the freshest areasof core. Additional thin sections, X-ray diffraction (XRD) sam-ples, and XRF samples were selected from areas of particularinterest. Samples for shore-based studies were selected at a sam-pling party held after the drilling ended.

The archive half was described visually, then photographedwith black-and-white and color film, one core at a time. Bothhalves of the core were shrink-wrapped in plastic to prevent rockpieces from vibrating out of sequence during transit, put into la-beled plastic tubes, sealed, and transferred to cold-storage spaceaboard the drilling vessel. At the end of the leg, the cores weretransferred from the ship in refrigerated air-freight containers tocold storage at the Gulf Coast Respository at the Ocean DrillingProgram, Texas A&M University, College Station.

SEDIMENT CORE DESCRIPTION FORMS

Sediments and Sedimentary RocksGraphic and verbal core descriptions, smear-slide and thin-

section petrographic data, sample locations, CaCO3 concentra-tions, physical property measurements, and biostratigraphic in-formation constitute the shipboard data summarized on the coredescription forms (barrel sheets) in this volume (Fig. 3). Thesesheets represent time-constrained field notes taken on boardship. Some ambiguities or discrepancies may be present.

The core description forms (Fig. 3), or "barrel sheets," sum-marize the data obtained during shipboard analysis of each sed-iment core, which have been recorded in detail on a section-by-section basis on visual core description forms, or VCDs. Infor-mation recorded on the VCDs is available as a searchabledatabase through the ODP Data Librarian. The following dis-cussion explains ODP conventions used in compiling the coredescriptions and the exceptions to these procedures adopted byLeg 126 scientists.

Core DesignationCores are designated as to leg, site, hole, core number, and

core type, as previously discussed (see "Numbering of Sites,Holes, Cores, and Samples" section, this chapter). In addition,the cored interval is specified in terms of meters below sea level(mbsl) and meters below seafloor (mbsf). On the basis of drill-pipe measurements (dpm), as reported by the SEDCO coringtechnician and the ODP operations superintendent, depths arecorrected for the height of the rig-floor, dual-elevator stool abovesea level to give true water depth and the correct mbsl.

Age DataMicrofossil abundance, preservation, and zone assignment,

as determined by shipboard paleontologists, appear on the coredescription form under the heading "Biostratigraphic Zone/Fos-sil Character." The geologic age determined from paleontologicand/or paleomagnetic results is shown in the "Time-Rock Unit"column. Calcareous nannofossils and planktonic foraminifersprovided most age determinations. Detailed information on zo-nations and terms used to report abundance and preservation ispresented in the "Biostratigraphy" section (this chapter).

Paleomagnetic, Physical Properties, and Chemical DataColumns are provided on the core description form to record

paleomagnetic results, physical properties values (wet-bulk den-sity, porosity, and compressional velocity), and chemical data(% CaCO3 determined by coulometric analysis). Additional in-formation on the shipboard procedures used in collecting thesetypes of data is found in the "Paleomagnetism," "Physical Prop-erties," and "Inorganic Geochemistry" sections of this chapter).

Graphic Lithology ColumnThe lithology of the recovered material is represented on the

core description forms by a single symbol or by a group of twoor more symbols (see Fig. 8, this chapter) in the column titled"Graphic Lithology."

16

EXPLANATORY NOTES

SITE

z

IME

-RO

CK

U

I -

HOLE

BIOSTRAT. ZONE/

FOSSIL CHARACTER

COocLUU-

2

1OLj_

CO

AN

NO

FOS

SI

z

CO

RA

DIO

LAR

IAN

IATO

MS

Q

PRESERVATION:G=GoodM=ModerateP=Poor

ABUNDANCEVA=Very abundantA=AbundantC=CommonF=FewR=RareT=TraceB=Bε rre 1

mc

sA

LEO

MA

GN

E

Q_

COLU

SE

HYS

. P

RO

PE

α.

de

nsi

ty

T 3

CO

rosi

ty

oQ.

üoCD

>

HE

MIS

TRY

O

• — •

CD

bona

t

CO

o

EC

TIO

N

CO

1

2

4

5

6

7

CC

CORE

ETE

RS

2

0.5—

1.0—

-I

1 I

I I

I I

I I

I I

I I

I

-

-

-

-

—

_

_

-

—

-_

—

-

-

-

i i

i M

i

ii

-

=

MM

-

-

GRAPHICLITHOLOGY

srΦ

σ>

CO

sym

bo

l

>.σ>oo

lie li

th

CO

:og

i

>>Φ

Φ

fUR

B.

RIL

LIN

G D

IS"

o

COLUDC

tD.S

TR

UC

T

CO

co

cCO

i n

Igu

res

!

u.cCO

ymb

o

CO

o>,ΦΦΦ

CO

CORED INTERVAL

AM

PLE

S

CO

XRF

XRD

OG

IW

*

#

PP

LITHOLOGIC DESCRIPTION

- < — X-ray fluorescence sample

-**— X-ray diffraction sample

Organic•*— geochemistry

sample

Interstitial-watersample

«•*— Smear slide

- < — Thin section

Whole-roundphysical properties sample

Figure 3. Core description form (barrel sheet) used for sediments and sedimentary rocks.

17


Figure 4 illustrates the following discussion of how mixedrock types and very thin intercalated beds are drafted in thegraphic columns as well as the special symbols used to describeLeg 126 cores. Where an interval is split by a solid vertical line,the sediment or sedimentary rock is an essentially homogeneousmixture of the two constituents, although each is represented byits own symbol. Examples are mixtures of biogenic and inor-ganic components, or mixtures of gravel and finer siliciclasticsediment. Carbonate content is not easily estimated from smearslides, and so, where chemical analyses are absent, carbonatevalues in the graphic column and the smear slide summary mustbe used only as first approximations. Where chemical analyseshave been run, carbonate contents have been adjusted to con-form to shipboard chemical analyses, as shown in Section 1,30 cm, and Section 2, 10 cm, in Figure 4.

In a mixed biogenic-siliciclastic lithotype such as nannofos-sil-rich clay, the terrigenous constituent is noted on the right,the biogenic constituent(s) on the left side of the column, inproportions approximately equal to their respective abundancesin the sediment. For example, in the uppermost interval of thecore illustrated in Figure 4, the left 20% of the column has anannofossil-ooze symbol, whereas the right 80% has a clay sym-bol, indicating nannofossil-rich clay or claystone composed of20% nannofossils and 80% clay. The symbols do not indicatethe quantity of primary volcanic material in the nonbiogenic

o

Carb.20.2

Carb.49.7

α>co

- 2 "

Graphiclithology

a.CO

CO

Figure 4. The graphic column and adjacent portions of the core descrip-tion form (barrel sheet), illustrating Leg 126 modifications of standardODP usage. See text for explanation.

fraction; this information is provided instead in the "LithologicDescription" column.

Intervals composed of two or more sediment types that aretoo finely interbedded for each layer to be depicted individuallyshow the relative abundances of the lithotypes graphically bydashed lines that vertically divide the interval into appropriatefractions. Thus, the interval in Section 1, 50-130 cm, consists ofthin intercalations of clay (30%), clayey silt and silty clay (col-lectively 40%), and silt (30%). Some sense of the thickness ofthe layers is provided by the bedding lines in the "Sed. Struc-tures" column. The numerous thin ash layers in the Pliocene-Quaternary sediments and the sedimentary rocks of Leg 126cores are significant and required special treatment. Rather thanbeing lumped into a vertically subdivided fraction of a core in-terval, the presence of these layers is noted in the "Sed. Struc-tures" column by the letter "A."

In cases where sediment types with significant biogenic com-ponents are finely interbedded with biogenic-free lithotypes, onlythe lithotype with biogenic content is divided into its compo-nents by a vertical solid line. Thus, the interval from Section 1,130 cm, to Section 2, 20 cm, consists equally of thin sand andnannofossil clay intercalations; the nannofossil clay is composedof 50% nannofossils and 50% clay, even though only 25% ofthe full column width is occupied by the symbol for nannofossilooze.

Three minor, but genetically significant, mineral componentsencountered in Leg 126 cores were given special symbols, whichappear within circles in the "Graphic Lithology" column. Thesewere pyrite (Py), manganese oxides (Mn), and gypsum (Gy).Some of the sedimentary rocks in Leg 126 cores are hydrother-mally altered. This is represented in the graphic column, as shownin the lower portions of Figure 4, by a special pattern that runsvertically along the right side of the column if the protolith isobvious, or by a pattern that occupies the full column if theoriginal lithology could not be recognized.

Some cores recovered on Leg 126 have intercalations of sedi-mentary material and igneous rocks. Intervals described by ig-neous petrologists are indicated by the symbol "IM" (see "Hard-Rock Core Description Forms" section, this chapter).

Sediment Disturbance

The coring technique, which uses a 25-cm-diameter bit witha 6-cm-diameter core opening, results in varying degrees of me-chanical disturbance of recovered core material. This is illus-trated in the "Drilling Disturbance" column on the core descrip-tion form (using the symbols in Fig. 5). Blank regions indicate alack of drilling disturbance. Drilling disturbance is recognizedin soft and firm sediments according to the following catego-ries:

1. slightly deformed: bedding contacts are slightly bent;2. moderately deformed: bedding contacts have undergone

extreme bowing;3. highly deformed: bedding is completely disturbed, some-

times showing symmetrical diapirlike or flow structures; and4. soupy: intervals are water saturated and have lost all as-

pects of their original bedding.

The degree of fracturing in indurated sediments and igneousrocks is described by means of the following categories:

1. slightly fractured: core pieces are in place and contain lit-tle drilling slurry or breccia;

2. moderately fragmented: core pieces are in place or partlydisplaced, but original orientation is preserved or recognizable(drilling slurry may surround fragments);

18

EXPLANATORY NOTES

Drilling deformation symbolsSoft sediments

Slightly disturbed

Moderately disturbed

Highly disturbed

Sedimentary structure symbols

_L

Soupy

Hard sediments

Slightly fractured

Moderately fractured

Highly fragmented

Drilling breccia

Figure 5. Drilling disturbance symbols used on Leg 126 core descriptionforms.

3. highly fragmented: pieces are from the interval cored andare probably in the correct stratigraphic sequence (although theymay not represent the entire section), but the original orienta-tion is completely lost; and

4. drilling breccia: core pieces have lost their original orien-tation and stratigraphic position and may be mixed with drillingslurry.

Sedimentary StructuresThe symbols used to describe the primary biogenic and phys-

ical sedimentary structures, as well as secondary structures suchas microfaults, dewatering veinlets, and mineral-filled fractures,are given in Figure 6. The symbols appear in the "Sed. Struc-ture" column of the core description form.

w

/AX

Interval over whichprimary sedimentarystructures occur

Graded interval

Graded bedding (normal)

Graded bedding (reversed)

Planar laminae

Wavy laminae/beds

Wedge-planarlaminae/beds

Lenticular laminae/beds

Cross-laminated

Cross-bedded

Flaser bedding

Convoluted/contorted

Slump blocks orslump folds

Water-escape andveinlet structures

Micro-fault

Fracture

Injection

Mineral-filled fracture

w6

43

O

syyyy

F

Slight bioturbation

Heavy bioturbation

Shells (complete)

Shell fragments

Wood fragments

Isolated pebbles andcobbles

Isolated mud clasts

Sharp contact

Gradational contact

Scoured contact

Load casts

Desiccation cracks

Current ripples

Fining-upward

Coarsening-upward

Figure 6. Sedimentary structure symbols for sediments and sedimentaryrocks used on Leg 126 core description forms.

A common structure in the sediments from Leg 126 is a de-creasing concentration upward or downward of approximatelyequivalent-sized particles; it occurs above and below beds of vit-ric ash, where the ash concentration decreases into the overlyingand underlying background sediment. Termed "fining upward"or "coarsening upward" (in deference to existing definitions),or reduction of particle abundance, this structure is interpretedto be the result of animal activity redistributing ash into the sed-iment (Bramlette and Bradley, 1942). Gradual fining and coars-ening upward are also indicated within very thick sand/sand-stone or gravel/conglomerate beds that may span several coresor sections.

19


Color

Colors were determined by comparison with the Munsell andGeological Society of America soil-color charts (Munsell SoilColor Charts, 1975). Colors were determined immediately afterthe cores were split because chemical changes may occur whendeep-sea sediments are exposed to the atmosphere (Moberly andKlein, 1976). Information on core colors is given in the text ofthe "Lithologic Description" on the core description forms and,where appropriate, in the site chapters.

SamplesThe locations of samples taken from each core for shipboard

analysis is indicated in the "Samples" column on the core de-scription form. The symbol "*" indicates the location of smearslide samples. The symbols "#," "XRD," and "XRF" indicatethe location of samples for shipboard thin sections, X-ray dif-fraction analysis, and X-ray fluorescence analysis, respectively.The symbols "IW," "OG," and "PP" designate the location ofsamples for whole-round interstitial water geochemistry, frozenorganic geochemistry, and physical properties analyses, respec-tively.

Although not indicated in the "Samples" column, the posi-tions of samples for routine physical property (porosity [%],wet-bulk density [g/cm3], and geochemical [% CaCO3]) analy-ses are indicated by a dot, and the analytical results are pre-sented in the "Physical Properties" and "Chemistry" columns.Paleomagnetic results (normal and reversed polarity intervals)are indicated in the "Paleomagnetics" column.

Shipboard paleontologists generally base their age determi-nations on core-catcher samples, although additional samplesfrom other parts of the core may be examined when required.An examination of such samples may lead to the recognition ofzonal boundaries in the core; these are indicated in the appro-priate column. All paleontological sample locations are indi-cated, even if they are barren.

Lithologic Description—TextThe lithologic description that appears on each core descrip-

tion form (barrel sheet) consists of two parts: (1) a heading thatlists all the major sediment types (see "Sediment Classification"section, this chapter) observed in the core; and (2) a more de-tailed description of these sediments, including data on color,location in the core, significant features, etc. In cases wherethere are thin beds of minor lithology, a description includinglocation information is included in the text, but the beds may betoo thin (< 10 cm) to appear in the graphic lithology column.

Smear Slide SummaryA table summarizing data from smear slides and thin sec-

tions (where available) appears on each core description form.The table includes the section and interval from which the sam-ple was taken, whether the sample represents a dominant ("D")or a minor ("M") lithology in the core, and the percentage ofsand, silt, and clay, together with all identified components (to-tal 100%). As explained in the following section, these data areused to classify and name the recovered material.

SEDIMENTARY AND PYROCLASTIC ROCKCLASSIFICATION

The sediment and pyroclastic classification scheme used dur-ing Leg 126 is descriptive in that it uses composition and textureas the only criteria to define rock types. Such genetic terms aspelagic, hemipelagic, turbidite, debris flow, ash, lapilli tephra,etc., do not appear in this classification. A primary name isbased upon, and named after, a component or components thatconstitute 60% or more of the material. This classification

scheme differs from some prior DSDP and ODP classificationsin the use of compositional modifiers.

Data for classifying sedimentary and pyroclastic materialscome principally from shipboard visual analyses of smear slidesthat use a petrographic microscope. These are qualitative opti-cal estimates of particle characteristics (mineralogy, biogenictype, grain size, relative particle abundance, etc.) and differfrom quantitative analyses of grain size, carbonate content, andmineralogy. Discrepancies between operators as well as operatorinconsistencies are to be expected with these procedures.

Composition and TextureThe term "clay" is used for clay minerals and terrigenous or

pyroclastic material <4 µm in size without regard to origin. Bi-ogenic components of silt or clay size are not described in tex-tural terms. Thus, a sediment with 50% silt-size nannofossilsand 50% terrigenous clay is called a nannofossil clay, not a nan-nofossil silt. The shipboard sedimentologists have chosen to usethis classification (1) to maintain an internal consistency in thenaming of intermediate mixtures of nonbiogenic and/or bio-genic components, and (2) to provide simple descriptive modi-fiers that indicate the important components and their abun-dance in the deposits.

We have made a special effort to avoid genetic terminologyin the naming of volcaniclastic sediments and pyroclastic mate-rials. Therefore, we do not use terms otherwise employed byvolcanologists, such as ash/tuff, lapilli tephra/tuff, and breccia.Instead, we use the size terms of Wentworth (1922; see Fig. 7) andthen indicate composition with a modifier. The sediments mayconsist entirely of volcanic ejecta, but their genesis does notplay a part in description and is instead discussed only in the in-terpretative sections of this volume.

IndurationThe determination of induration is subjective. For calcare-

ous sediments and calcareous sedimentary rocks, three classesof induration or lithification are recognized (Gealy et al., 1971):(1) soft = ooze, which has little strength and is readily de-formed under the pressure of a finger or the broad blade of aspatula; (2) firm = chalk, which is partially lithified, readilyscratched with a fingernail or the edge of a spatula; and (3) hard= well-lithified and cemented limestone or dolomite that is dif-ficult or impossible to scratch with a fingernail.

For terrigenous sediments and pyroclastic rocks, lithified va-rieties are generally indicated by the addition of the suffix"-stone" to the name for soft materials. For gravels, however,the precise origin is generally unambiguous, so we use differentterms for their lithified epiclastic and pyroclastic equivalents:"conglomerate" for epiclastic rocks, and "lapilli tuff" or "ag-glomerate" for pyroclastic rocks, depending on their grain size(lapilli = 4-64 mm diameter).

Rules for Classification1. The principal name is determined by the component or

group of components (e.g., total biogenic silica or biogenic car-bonate) that constitute(s) at least 60% of the sediment or rock,except for subequal mixtures of biogenic and nonbiogenic mate-rial. The main components are as follows:

a. Nonbiogenic: If the total of a nonbiogenic component is>60%, the main name is determined by the relative proportionsof sand, silt, and clay sizes when plotted on a modified Shepard(1954) classification diagram (Fig. 8). Examples of nonbiogenicprincipal names are clay, silt, silty clay, or sand. For nonbio-genic sediments that are composed mainly of primary volcanicparticles, the modifiers "vitric" (for glassy fragments), "vol-canic-lithic" (for polycrystalline volcanic grains), and "crystal"

20

EXPLANATORY NOTES

MILLIMETERS

1/2 —

1/4 —

1/8 —

1/32 -

1/64

1/128

1/256

4096

1024

61 —

16

3.36

2.83

2.38

1.68

1.41

1.19

i oo

0.84

0.71

0.59n Rn

0.42

0.35

0.30

' 0 π 5

0.210

0.177

0.149

0.105

0.088

0.074

0.053

0.044

0.037

0 031

0.0156

0.0078

.. 0 0039

0.0020

0.00098

0.00049

0.00024

0.00012

0.00006

µm

c n n

420

350

300

°50

210

177

149

1 05

105

88

74

53

44

37

31

15.6

7.8

3 9

2.0

0.98

0.49

0.24

0.12

0.06

PHI(0)

-20

-12

-10

6

-40 „,

-1.75

-1.5

-1.25

-0.75

-0.5

-0.25,,_ go

0.25

0.5

0.751 /->

1.25

1.5

1.750 n

2.25

2.5

2.75

3 0

3.25

3.5

3.75

4.25

4.5

4.75

5 0

6.0

7.0

- 8 0

9.0

10.0

11.0

12.0

13.0

14.0

WENTWORTH SIZE CLASS

Boulder (-8 to -12 0)

Cobble (-6 to-8 0)

Pebble (-2 to-6 0)

Granule

Very coarse sand

Coarse sand

Medium sand

Fine sand

Very fine sand

Coarse silt

Medium siltFine siltVery fine silt

Clay

VE

L

<

QZ<GO

0

Figure 7. Grain-size categories used for classification of terrigenous sed-

iments (after Wentworth, 1922).

(for unit crystals) may be used for clarity. These modifiers areplaced immediately before the textural name; both the texturaland compositional terms form the principal name.

b. Biogenic: If the total of the biogenic components is>60%, the principal name is ooze, or an appropriate term thatdenotes the sediment induration (see below).

c. Mixed sediments: In mixtures of biogenic and nonbio-genic material where neither component exceeds 60%, we haveestablished a convention that the principal name will consist oftwo parts: (1) the name of the major fossil component(s), hy-phenated if necessary with the least common fossil listed first,followed by (2) the textural name appropriate for the terrige-nous/volcanogenic components (Fig. 8). For example, if nanno-fossils form 40%-60% of a sediment that contains nothing elsebut clay, then the name is nannofossil clay, even if nannofossilsare somewhat more abundant than clay.

2. A component with an abundance of 30%-60% qualifiesfor major modifier status. Major modifiers are:

a. Nonbiogenic: silty, clayey, sandy, vitric-sandy, etc.b. Biogenic carbonate: nannofossil, foraminifer, or calcare-

ous (should be as specific as possible).c. Biogenic silica: diatom, radiolarian, or siliceous biogenic

(should be as specific as possible).

Note that for sediments containing from 40%-60% biogenicmaterial, this rule is partly redundant with Rule lc.

3. A component with an abundance of 10%-30% qualifiesfor minor modifier status and is hyphenated with the word rich.Example: nannofossil-rich clay; vitric-rich silty clay.

4. An unusual, important component that constitutes 5%-10% (e.g., bioclasts, zeolite, granules, large foraminifers) canbe indicated with a minor modifier that consists of the compo-nent name hyphenated with the word bearing. Example: zeolite-bearing clay. This is an optional modifier.

5. The most abundant accessory component appears closestto the principal name. Major and minor modifiers are listed inorder of decreasing abundance to the left of the principal name.

Example: foraminifer-rich nannofossil clay(10%) (30%) (60%)

BIOSTRATIGRAPHY

General Remarks

The general correlation between the biostratigraphic zonesand the magnetic polarity reversal record, the seafloor magneticanomalies, and an absolute time scale is based on the scheme ofBerggren et al. (1985). Some minor changes are discussed in theindividual sections for each fossil group.

Age assignments are based mainly on core-catcher samples.Additional samples were studied when the core-catcher sampleswere found to be barren or restricted to narrow time intervals,or where boundaries or unconformities occur. Sample locations,preservation, and abundance for each fossil group are indicatedon the barrel sheets.

Calcareous Nannofossils

The calcareous nannofossil zonations of Okada and Bukry(1980) and Sato et al. (1988) were used to identify nannofossilzones. The Pliocene/Pleistocene boundary is approximated bythe first occurrence (FO) of Gephyrocapsa oceanica (Backman,Duncan, et al., 1988). The Oligocene/Miocene boundary is de-fined by the last occurrence (LO) of Sphenolithus ciperoensis orReticulofenestra bisecta (at the top of Subzone CP 19b). A list ofnannofossil datums and ages, based on Berggren et al. (1985)and Backman, Duncan, et al. (1988), is given in Table 1.

Abundance and Preservation

Smear slides were prepared from all samples for the examina-tion of calcareous nannofossils. The abundance of nannofossilswas estimated as follows:

A = abundant (> 10 specimens/field of view),C = common (1-10 specimens/field of view),F = few (1 specimen/2-10 fields of view), andR = rare (1 specimen/> 10 fields of view).

The state of preservation was estimated as:

G = good (minor or no signs of dissolution and/or over-growth),

21


Sand

50

0 5 10 60

O

> ,cσü

ring

bea

*

ring

bea

*

Nannofossilrich clay

Diatom-rich clay

Nannofossilclay

Diatomclay

Nannofossilooze

Diatomooze

100% biogenic

Biogeniccarbonate

100 90

Nannofossil ooze70 40

Biogenicsilica

0% Clay

Foraminiferal chalk

CB1

Nannofossil chalk

CB6

Basic igneous rocks

CB5

Siliceous ooze

SR4

Breccia

SB3

Diatom ooze

SrSR3

Foraminifer ooze

SB1Nannofossil-foraminifer andforaminifer-nannofossil chalk

CB7

Carbonate ooze

CB2Nannofossil-foraminifer andforaminifer-nannofossil ooze

CB3

3 O<

CB4

Sand

T6

Silt

T5

Clay

T1

Silty sand,sandy silt; V / : .:'.; >; .•.v

T7

Muddy sand

AS12

Sandy mud

T4

Clayey sand,sandy clay

T9Clayey silt,silty clay

T8

Gravel

SR1

Conglomerateand breccia

SR2Agglomerate,and breccia

V3

Hydrothermally alteredsedimentary rock

AS8

22

Figure 8. Textural classification scheme for clastic sediments and rocks; procedure for naming mixtures of biogenicand terrigenous/volcanogenic materials; and graphic symbols for lithologies, used on core description forms (barrelsheets; see Fig. 4, this chapter). The textural classification scheme is modified from Shepard (1954) by subdivision ofthe central triangular field into muddy sand and sandy mud. The sand-, silt-, and clay-sized fractions are defined ac-cording to the Wentworth (1922) grade scale. Asterisks indicate an unusual component, such as zeolite, that is presentin amounts of 5°7o-lO°Io, which give a sediment or rock the initial modifier "*-bearing," as in "zeolite-bearing silt-stone."

EXPLANATORY NOTES

Table 1. List of calcarous nannofossil datumsand associated ages for the Eocene to Pleisto-

Event

FOLOLOLOFOLOLOLOLOLOLOFOLOFOLOF 0LOFOLOLOFOFOFOFOLOFOFOFOFOLOLOF0LOFOFOLOLOLOFOLOLOLOLOFOF 0LOLOFOLOLOFOFOLOFOLOFOLOFO

Species

Emiliania huxleyiPseudoemiliania lacunosaHelicosphaera selliiCalcidiscus macintyreiGephyrocapsa oceanicaDiscoaster brouweriDiscoaster pentaradiatusDiscoaster surculusDiscoaster tamalisSphenolithus spp.Reticulofenestra pseudoumbilicaPseudoemiliania lacunosaAmaurolithus tricorniculatusDiscoaster asymmetricusAmaurolithus primusCeratolithus rugosusCeratolithus acutusCeratolithus acutusTriquetrorhabdulus rugosusDiscoaster quinqueramusAmaurolithus primusDiscoaster berggreniiDiscoaster neorectusDiscoaster loeblichiiDiscoaster hamatusCatinaster calyculusDiscoaster hamatusCatinaster coalitusDiscoaster kugleriSphenolithus heteromorphusHelicosphaera ampliapertaSphenolithus heteromorphusSphenolithus belemnosSphenolithus belemnosDiscoaster druggiiReticulofenestra bisectaSphenolithus ciperoensisSphenolithus distentusSphenolithus ciperoensisReticulofenestra umbilicaCoccolithus formosusDiscoaster saipanensisDiscoaster barbadiensisIsthmolithus recurvusChiasmolithus oamaruensisChiasmolithus grandisChiasmolithus solitusReticulofenestra umbilicaNannotetrina fulgensChiasmolithus gigasChiasmolithus gigasNannotetrina fulgensDiscoaster lodoensisDiscoaster sublodoensisTribrachiatus orthostylusDiscoaster lodoensisTribrachiatus contortusDiscoaster diastypus

Age(Ma)

0.2750.4601.37*1.45*1.6*1.89*2.35*2.41*2.65*3.45*3.56*3.73.74.14.44.6*4.6*4.94.95.56.58.28.58.58.9

10.010.010.811.5*13.2*16.218.6*18.6*20.5*23.223.725.228.230.233.8*34.9*36.7*37.0*37.839.840.042.344.445.447.048.849.850.4*52.653.755.356.356.5

Note: FO = first occurrence and LO = last occur-rence. The list is based on Berggren et al. (1985)and Backman, Duncan, et al. (1988), with thelatter marked by an asterisk. The FO of Pseudo-emiliania lacunosa is based on Knüttel (1986).

M = moderate (slight to moderate dissolution and/or over-growth), and

P = poor (severe dissolution and/or heavy overgrowth).

Planktonic ForaminifersFor correlations among the biostratigraphic zones, the mag-

netic reversal record, and the absolute time scale, Blow's (1969)zonal scheme was used. The correlations of planktonic foramin-

ifer datums to magnetic polarity stratigraphy summarized byBerggren et al. (1985) were adopted in this study (Table 2).

Abundance and PreservationFor abundance estimates of planktonic foraminifer species,

the following scale was used:

B = barren (no foraminifers),R = rare (1-5 tests in the residue),F = few (6-11 tests in the residue),C = common (12-25 tests in the residue), and

A-VA = abundant to very abundant (> 26 tests in the resi-due).

Preservational characteristics were divided into three catego-ries:

Good: >90% of specimens unbroken and minor signs ofdissolution or fragmentation,

Moderate: 30%-90% of specimens show dissolution effects(sugary white texture, broken chambers, preservation of onlythe most resistant structures), and

Poor: samples dominated by fragments and partially dissolvedchambers.

Sample PreparationSamples were disaggregated in a 30% hydrogen peroxide so-

lution and then washed through a 63-µm sieve. For consolidatedsediments, the following procedure was used: the sample was (1)covered with 30% H2O2 for 10-14 hr in an oven at around40°C; (2) decanted; (3) covered with kerosene for 4-6 hr; (4) de-canted; (5) covered with H2O for several hours in an oven at50°-100°C; (6) decanted; and (7) wet sieved through a 250-meshscreen (63 µm).

Benthic ForaminifersThe biostratigraphic determination of deep-water benthic for-

aminifer assemblages follows Kaiho (in press). Several large turn-overs in deep-water faunas occurred during the Cenozoic: nearthe Paleocene/Eocene boundary (e.g., Tjalsma and Lohmann,1983), near the middle/late Eocene boundary (e.g., Tjalsmaand Lohmann, 1983), and near the middle middle Miocene (e.g.,Woodruff and Douglas, 1981; Woodruff, 1985). Kaiho (in press)studied benthic foraminifers from DSDP samples of Cenozoicage from the world oceans and Paleogene samples from NewZealand and divided the Cenozoic into the following four zonesin accordance with these turnovers.

Event

FO

LO

LO

Species

Cibicidoideswellerstorfi

Nuttallidestruempyi

Stensioinabeccariiformis

Benthicforaminifer

zone

CD4

CD3

CD2

CD1

Age

middle middleMiocene

middle/lateEocene

Paleocene/Eocene

PaleobathymetryPaleobathymetric determinations were estimated by benthic

foraminifers following Ingle (1980), Woodruff (1985), Kaiho andHasegawa (1986), Van Morkhoven et al. (1986), Akimoto (1989),and Yasuda (1989).

23


Table 2. List of planktonic foraminifer species datums, zonalboundaries, and associated ages.

Event

LOFO

Species

Globigerinoides fistulosusGlobigerina calida calida

Pliocene/Pleistocene boundary

FOLOFOLOFOFOLOFOFO

Globorotalia truncatulinoidesGloboquadrina altispiraGlobigerinoides fistulosusSphaeroidinellopsis spp.Globorotalia inflataGloborotalia tosaensisPulleniatina primalisSphaeroidinella dehiscensGloborotalia tumida

Miocene/Pliocene boundary

FOLOFOFOFOLOFOFOFOFOFOFOFOLOFOLOFO

Globigerinoides conglobatusGloboquadrina dehiscensPulleniatina primalisGloborotalia plesiotumidaNeogloboquadrina acostaensisGloborotalia siakensisGlobigerina nepenthesSphaeroidinellopsis subdehiscensGloborotalia fohsiGloborotalia praefohsiGloborotalia peripheroacutaOrbulina suturalisPraeorbulina sicanusCatapsydrax dissimilisGlobigerinatella insuetaGloborotalia kugleriGloborotalia kugleri

Oligocene/Miocene boundary

LOFOLOLOLOLO

Paragloborotalia mendacisGlobigerinoides primordiusParagloborotalia opimaStreptochilus cubensisGlobigerina ampliaperturaPseudohastigerina

Eocene/Oligocene boundary

LOFOLOLOLOLOFOLOLOLOFOFOFOLOFOFOFO

hantkeninids, GlobigerinathekaGlobigerina tapuriensisTurborotalia cerroazulensis groupGlobigerinatheka semiinvolutaAcarininaMorozovella spinulosaGlobigerinatheka semiinvolutaSubbotina frontosaGlobigerinatheka beckmanniAcarinina bullbrookiTurborotalia pomeroliGlobigerinatheka indexMorozovella lehneriMorozovella aragonensisHantkeninaMorozovella aragonensisMorozovella formosa

Paleocene/Eocene boundary

LOLOFOFOFOFOLO

Morozovella velascoensisPlanorotalites pseudomenardiiPlanorotalites psuedomenardiiMorozovella velascoensisMorozovella pusillaMorozovella conicotruncataMorozovella angulata

Zonalboundary

N23/N22

N22/N21

N21/N19

N19/N18N18/N17

N17b/N17aN17/N16N16/N15N15/N14N14/N13N13/N12N12/N11N11/N10N10/N9N9/N8N8/N7N7/N6N6/N5N5/N4N4/P22

P21/P22P21a/P21bP20/P21P19/P20

P17/P18

P14/P15

P13/P14

P11/P12P11/P12P9/P10P7/P8P6/P7

P4/P5P3/P4

P3a/P3bP3a/P3bP2/P3

Age(Ma)

1.6?

1.92.92.93.03.03.13.55.15.2

5.35.35.88.08.6

10.411.311.813.514.014.615.216.317.617.920.123.7

23.724.528.230.032.834.0

36.636.636.737.640.641.141.342.043.043.044.745.046.046.052.055.256.1

57.858.861.061.762.062.062.3

Note: The list is based on Berggren et al. (1985).

Dissolved Oxygen in Deep WaterPredominant morphologies of benthic foraminifers found in

poorly oxygenated deposits differ from those present in highlyoxygenated deposits. Therefore, the morphologies of benthicforaminifer tests were used to infer relative amounts of dissolvedoxygen in Cenozoic deep-sea bottom water. All the calcareousbenthic foraminifers were classified into three categories: aero-bic, anaerobic, and intermediate forms (Kaiho, in press). Ratiosof aerobic vs. aerobic plus anaerobic forms were calculated asestimates of changes in the oxygen content of deep oceanicwaters.

Abundance and PreservationFor abundance estimates of benthic foraminifers, the follow-

ing scale was used:

B = barren (no foraminifers),R = rare (< l specimen/10 cm3),F = few (1-10 specimens/10 cm3),C = common (10-100 specimens/10 cm3), andA = abundant (>IOO specimens/10 cm3).

Preservational characteristics were divided into three catego-ries:

Good: >90% of specimens unbroken,Moderate: 30%-90% of specimens showing dissolution ef-

fects, andPoor: samples dominated by fragments and partially dissolved

chambers.

Sample PreparationSamples approximately 20 cm3 in volume were washed

through a 250-mesh screen (63-µm opening) and dried. Uncon-solidated sediments were soaked in water, and, when necessary,disaggregation was aided by the addition of small amounts ofhydrogen peroxide. Hydrogen peroxide was also routinely addedto more consolidated sediments. Kerosene was used to disaggre-gate highly consolidated sediments.

RadiolariansThe Cenozoic radiolarian zonation of Sanfilippo et al. (1985),

derived for the tropical equatorial Pacific, was used at all sites.Sanfilippo et al. (1985) summarized the taxonomy and evolu-tionary lineages of all stratigraphically important radiolariantaxa commonly found in low-latitude regions of this zonation.In suggesting tentative "absolute" ages for radiolarian datumlevels and zonal boundaries, the schemes of Nigrini (1985) andBarron et al. (1985), established on the basis of DSDP Leg 85sites in the equatorial Pacific, were followed (Table 3).

The Quaternary biostratigraphic datums listed by Foreman(1981) were also used. Although much of the material obtainedon Leg 85 could not be directly dated paleomagnetically, therewere sufficient duplicate sites in which all major micro fossilevents could be identified, some of which had been correlated tothe polarity time scale in nearby piston cores. Thus, the ages ofPacific radiolarian events estimated by Foreman (1981), Nigrini(1985), and Barron et al. (1985) provided a satisfactory workingmodel.

AbundanceQualitative assessments of the abundance (abundant, com-

mon, few, rare, trace, absent) and preservation (good, moder-ate, poor) of radiolarians in each slide were recorded. In assess-ing the relative abundances of individual taxa, the followingsemiquantitative criteria were used:

24

EXPLANATORY NOTES

Abundant: > 20% (of the total assemblage),Common: 5%-20% (of the total assemblage),Few: l%-5% (of the total assemblage),Rare: O.l<Vò-l Vo (of the total assemblage),Trace: <0.1% (of the total assemblage), andAbsent: not found in an examination of the slide.

Sample Preparation

To obtain clean radiolarian concentrates for microscopic ex-amination, the sediments were disaggregated, sieved to removethe clay-silt fraction, and acidified to eliminate the calcareouscomponents. A 5-cm3 sample was placed in a 400-cm3 beakercontaining 150 ml of lO Vo hydrogen peroxide and a small amountof Calgon (to aid in disaggregating the sediment). If calcareouscomponents were evident, they were dissolved by adding hydro-chloric acid. The residue was sieved through a 63-µm sieve, andthe remaining siliceous microfossils were pipetted evenly ontolabeled glass slides. The accompanying water was then evapo-rated under a heat lamp, after which the remaining residue wasmounted in a suitable medium (in this instance, Norland Opti-cal Adhesive) and covered with a 22 × 50-mm cover slip. Oneslide was prepared and examined for each sample. The wet resi-due was retained in the event that reexamination of the materialproved to be necessary.

PALEOMAGNETISMPaleomagnetic studies performed on board JOIDES Resolu-

tion included measurements of remanent magnetization andmagnetic susceptibility of sedimentary, igneous, and metamor-phic material. The measurement of remanence was generally ac-companied by alternating field (AF) or thermal demagnetiza-tion to remove secondary magnetizations.

InstrumentsTwo magnetometers, a Molspin spinner magnetometer and a

2-G Enterprises (Model 760R) pass-through, cryogenic, super-conducting rock magnetometer, were routinely available for themeasurement of remanence on board JOIDES Resolution. AnAF demagnetizer (Model 2G600) capable of alternating fieldsup to 25 mT was on line with the cryogenic magnetometer. Bothwere controlled by a FASTCOM4 multiserial communicationboard in an IBM PC-AT compatible computer. To measure thearchive half of the core, Leg 126 paleomagnetists used a BASICprogram (126CORE) modified from the SUPERMAG programwritten by M. Koyama and C. Helsley during Legs 124E and126.

The spinner magnetometer was controlled by a DigitalPRO350 computer and was used only to measure individual dis-crete samples. In addition to these two standard sample magne-tometers, a prototypic, portable, fully automatic spinner (FAS)magnetometer with a built-in AF demagnetizer (maximum field= 50 mT) and magnetic susceptibility anisotropy meter, wasavailable for discrete sample measurement on Leg 126. This in-strument was designed and built by N. Niitsuma and M. Ko-yama of Shizuoka University.

The superconducting quantum interference device (SQUID)sensors in the cryogenic magnetometer measure magnetizationover an interval approximately 20 cm long. Each axis has aslightly different response curve. The widths of the sensor re-gions imply that as much as 150 cm3 of core contributes to thesensor signals, thereby blurring sharp magnetic transitions. Thelarge volume of core material within the sensor region permitsan accurate determination of remanence for weakly magnetizedsamples despite the rather high background noise related to themotion of the ship. The SQUID electronics operate at the 1 ×scale and use the flux-counting mode for most measurements.

Remanent Magnetization Measurements

The maximum AF demagnetizing field allowed for sectionsfrom the archive half is either 15 mT or the median destructivefield, whichever is lower. Discrete samples from the workinghalf of the core were demagnetized with the Schonstedt GSD-1AF demagnetizer and the TSD-1 thermal demagnetizer as wellas the AF demagnetizer that is built into the automatic spinnermagnetometer.

SedimentsRemanence measurements of sediments were performed by

passing continuous archive-half core sections through the cryo-genic magnetometer. Measurements of NRM before and afterAF demagnetization were generally taken at 3- and 5-cm inter-vals. At reversal boundaries, high-density measurements at 0.5-cm intervals, at varying AF demagnetization levels, were per-formed. Because measurements are averaged over the broad re-sponse regions of the three sensors, deconvolution is required torecover highly detailed changes of magnetization. This workwill be done during shore-based studies.

Discrete samples were taken from APC, XCB, and RCB sed-iment cores by pressing a standard plastic sampling box (7 cm3)into the soft core material. The uphole direction was marked onthe box. For more consolidated sediments, cubes were cut outwith a spatula or minicores were cut with a drill press. Up tothree discrete samples per section were taken from APC, XCB,and RCB working cores when the archive half gave a consistentpaleomagnetic signal, except at certain reversal boundaries, wherean even greater sampling density was employed. Azimuthal ori-entation of discrete samples from selected XCB and RCB coreintervals was attempted by noting bedding and structural featuredip directions, when detectable, in sample coordinates. These di-rections were referenced to the dip directions indicated by seis-mic profiles or to compass-oriented bedding and fracture-dipdirections, as determined by the downhole formational micro-scanner, so as to derive field-coordinate directions.

Igneous and Metamorphic RocksDrilled minicores (1-in. diameter) of the igneous portions of

the cores were obtained to determine the polarity of the various"hard rock" units encountered during drilling. Archive-half sec-tions that contained igneous rock pieces > 20 cm in length weremeasured in the cryogenic magnetometer at 5-cm intervals.

Magnetic StratigraphyThe magnetostratigraphic time scale of Berggren et al. (1985)

was adopted for use during Leg 126.

Magnetic Susceptibility MeasurementsMagnetic susceptibility measurements were routinely made

on all cores employing a Bartington Instrument magnetic sus-ceptibility meter (model M.S.I) with a M.S.l/CX 80 mm whole-core sensor loop set at 0.47 kHz. The susceptibility meter wason line with the GRAPE, and the PWL with the MST. Becausethe susceptibility of most core material on Leg 126 was high, allmeasurements were made in the low-sensitivity mode. In addi-tion to characterizing the cored sediments, susceptibility datawere used for correlation among holes and sites. The correlationof susceptibility data was performed by observing peak-to-peakspacings and general trends in the susceptibility curve.

PETROLOGY

Core Curation and Shipboard SamplingAs standard procedure, basement rocks recovered during cor-

ing were examined by petrologists to determine where the core

25


Table 3. List of radiolarian species datums and zones, and associated ages.

Zone

B. invaginata

C. tuberosa

A. ypsilon

A. angulare

P. prismatium

S pen tas

S. peregrina

L). penultima

D. antepenultima

D. petterssoni

D. alata

C. costata

O. WOlJJll

Approximateage

3

1 17

0.420 450.700.740.770.85

• .00.15.15.50.55.60.85

2.352.472.5

- 2.63.273.353.533.783.803.803.884.3

- 4.454.65.57

?f. A

6.86.97.0

7.6

8 6

11.411.4

11.5

— 1 3 . J

16.2

16.517 3

— i / . J

17.3

Top orbottom

DD

Tr>

TTBBTBTTTBTTBBTTTTBTBTT

BTTT

B

TBTBT

TTTTTBTTBB

BBTTTTTTBT

TTBTBTTt>

oBB

Species

Stylatractus universus

Pterocorys campanulaAnthocyrtidium nosicaaePterocorys hertwigiiAntocyrtidium euryclathrum

Lamprocyrtis nigriniaeLamprocyrtis neoheteropontsAnthocyrtidium michelinae

j lct UL•idfliUffl Uf lòfrlti I lliffl — ^ —

Anthocyrtidium angulareTheocorythium vetulumAnthocyrtidium jenghisiPterocorys sabaeTheocorythium trachelium

tjucnocorys peregrinaPhormostichoartus fistulaLychnodictyum audaxPhormostichoartus doliolumAmphirhopalum ypsilonSpongaster pentasSpongaster tetrasAnthocyrtidium prolatumSolenosphaera omnitubus

Pterocanium prismatiumAcrobotrys tritubusCalocycletta caepaSiphostichartus corona

Solenosphaera omnitubusD. antepenultima D. penultima

Spongaster berminghamiDictyocoyne ontongensisAcrobotrys tritubusBotryostrobus miralestensis

D. laticonus D. antepenultimaL. neotera L. baccaCyrtocapsella japonicaLithopera thornburgiCyrtocapsella tetraperaCyrtocapsella cornutaCarpocanopsis cristataPhormostichoartus doliolumDorcadospyris alataStichocorys wolffiiCyrtocapsella japonica

L. renzae L. neoteraLithopera thornburgiPhormostichoartus corbulaDorcadospyris alataCalocycletta virginisCarpocanopsis bramletteiCalocycletta costataDidymocyrtis violinaDidymocyrtis tubariaLithopera renzaeDocadospyris forcipata

Eucyrtidium diaphanesCarpocanopsis cingulataSiphostichartus coronaDidymocyrtis prismaticaCarpocanopsis cristataCarpocanopsis favosaLychnocanoma elongatai~*nlr\rvnl0ttn r nvtπin

Dorcadospyris dentataCalocycletta caepa

Note: The list is based on Barron et al. (1985) and Nigrini (1985).

26

EXPLANATORY NOTES

Table 3 (continued).

ZoneApproximate

ageTop orbottom Species

S. delmontensis

C. tetrapera

L. elongata

D. ateuchus

18.2

20.3

21.121.3

21.521.9

22.1

22.1

22.622.624.1

T. tuberosa

33.0

36.0

T. bromia

P. goetheana

P. chalara

41.0

42.0

43.0

P. mitra

P. ampla

45.0

46.5

T. triacantha

Stichocorys wolffiiDorcadospyris ateuchusDidymocyrtis tubariaDidymocyrtis violinaStichocorys delmontensisTheocyrtis annosa

48.0

Carpocanopsis bramletteiCalocycletta serrataCalocycletta virginisC. pegetrum C. leptetrumBotryostrobus miralestensisCarpocanopsis cingulataCyrtocapsella tetraperaCyrtocapsella cornutaCarpocanopsis favosaCalocycletta serrataArtophormis gracilisDorcadospyris papilioLychnocanoma elongataCarpocanopsis cingulataDorcadospyris forcipataLithocyclia angustaDorcadospyris papilioTheocyrtis annosaLithocyclia cruxTheocyrtis tuberosaT. triceros D. ateuchusDidymocyrtis prismaticaLychnodictyum audaxA. barbadensis A. gracilisDictyoprora mongolfieriLithocyclia cruxL. aristotelis L. angusta —Calocylas turrisLychnocanoma bandycaThyrsocyrtis bromiaThyrsocyrtis lochitesThyrsocyrtis tetracanthaThyrsocyrtis triacanthaCarpocanistrum azyxEusyringium fistuligerumPodocyrtis goetheanaLychnocanoma bandycaPodocyrtis chalaraCalocyclas turrisCarpocanistrum azyxThyrsocyrtis bromiaThyrsocyrtis tetracanthaTheocotylissa ficusSethochytris triconiscusL. ocellus L. aristotelisPodocyrtis goetheanaPodocyrtis trachodesPhormocyrtis striata striataTristylospyris tricerosP. mitra P. chalaraCryptoprora ornataPodocyrtis amplaEusyringium lagenaArtophormis barbadensisThyrsocyrtis lochitesSethochytris triconiscusPodocyrtis fasciolataP. sinuosa P. mitraPodocyrtis trachodesE. lagena E. fistuligerumPodocyrtis dorusPodocyrtis fasciolataTheocotyle venezuelensisP. phyxis P. amplaTheocotyle nigriniaeTheocotyle conicaP. diamesa P. phyxisTheocorys anaclastaThyrsocyrtis hirsutaThyrsocyrtis robustaT. tensa T. triacanthaEusyringium lagena

27



Zone

D. mongolfieri

T. cryptocephala

P. striata striata

B. clinata

B. bidartensis

Approximateage

AQ n

- 50.0

- 51.0

- 55.5

Top orbottom

B

TRD

BBT

TTBBBB

BTTTTTBB

BBB

B

B

B

Species

Podocyrtis dorusT. cryptocephala T. conicaCalocycloma castumj-» ^ if

uiayoprora mongoijienP. aphorma P. sinuosaThyrsocyrtis robustaTheocotyle venezuelensisBuryella clinatarp „ . ' _ _ • • - T" rtr1ltl/^/^^nfc^/^

Podocyrtis platypusLamptonium sanfilippoaeThyrsocyrtis rhizodonPodocyrtis platypusPodocyrtis diamesaPodocyrtis aphormaPhormocyrtis striata exquisita P. striata striata

Theocotylissa fimbriaPterocodon amplaBekoma bidartensisBuryella tetradicaThyrsocyrtis tarsipesLithocyclia ocellusThyrsocyrtis tensaT. alpha T. ficusLamptonium sanfilippoaeTheocotyle nigriniaeThyrsocyrtis hirsuta

Theocotylissa fimbriaT. auctor T. alphaCalocycloma castumLamptonium pennatum L. fabaeforme fabaeforme

should be split that would best preserve unique features and/orto expose important structures. The core was split into archiveand working halves using a rock saw with a diamond blade.Care was always taken to ensure that orientation was preservedduring splitting and prior to labeling, usually by marking theoriginal base of each piece with red crayon. Each piece wasnumbered sequentially from the top of each section on the top,uncut surface of the rock. Pieces that could be fitted together(reassembled like a jigsaw puzzle) were assigned the same num-ber, but lettered consecutively (e.g., 1A, IB, 1C, etc.).

Plastic spacers were placed between pieces with different num-bers, but not between those with different letters and the samenumber. The presence of a spacer may or may not represent asubstantial interval of no recovery. When the original unsplitpiece was sufficiently long, such that the top and bottom couldbe distinguished before removal from the core liner (i.e., thepiece could not have rotated top to bottom about a horizontalaxis in the liner during drilling), an arrow was added to the labelpointing to the top of the section. Because pieces were free toturn about a vertical axis during drilling, azimuthal orientationwas not possible.

After the core was split, the working half was sampled forshipboard physical properties, magnetics, XRF, XRD, and thin-section studies. These samples were often of minicores and,when appropriate, were stored in seawater prior to measure-ment. When recovery permitted, samples were taken from eachlithologic unit. The archive half was described on the visualcore description (VCD) form, which is used for such nonde-structive physical properties measurements as magnetic suscep-tibility, and was photographed before storage.

Visual Core Descriptions

Igneous VCD forms were used in the description of the igne-ous rock cores (Fig. 9). In Figure 9, the left column is a graphicrepresentation of the archive half. A horizontal line across theentire width of the column denotes a plastic spacer glued be-tween rock pieces inside the liner. Oriented pieces are indicatedon the form by an upward-pointing arrow to the right of thepiece. Shipboard samples and studies are indicated on the VCDin the column headed "shipboard studies," using the followingnotations: XRD = X-ray diffraction analysis, XRF = X-rayfluorescence analysis, TSB = petrographic thin section, and PP= physical properties analysis.

Because igneous and metamorphic rocks were classifiedmainly on the basis of macroscopic mineralogy and texture,shipboard petrologists followed a checklist of features that en-sured consistent and complete descriptions. To this end, VCDswere entered directly into a computerized database (HARVI).The describer is prompted by the HARVI program with a set ofquestions pertaining to petrologic features that must all be diag-nosed and entered in the database. The descriptions on the VCDforms will, to a certain extent, reflect the rigidity of the HARVIprogram. It is relevant, therefore, to discuss the organization ofthe database briefly. Fine-grained rocks were entered into a dataset called HARVI-F, and coarse-grained rocks were entered intothe data set HARVI-C. Each record was checked by the data-base program for consistency, and a hard copy was printed in aformat that resembles the final barrel sheets. The "clean-copy"print-out was directly pasted onto a copy of the barrel sheet forsubsequent curatorial record keeping.

28

EXPLANATORY NOTES

126-793B-1R-1

UNIT II: DIABASE

cm

0-

5 0 •

100

4D

XRD

t XRF

\

Pieces 1-7

CONTACTS: Upper baked; lower chilled.PHENOCRYSTS: In glomeroporphyritic clots and at lower chilled margin.

Olivine - 5-7%; 1-3 mm; subhedral; altered to smectite.Clinopyroxene - 2-15%; 2-6 mm; euhedral; twinned, with exsolution lamellae.Orthopyroxene - 1 % ; 1-2 mm; sometimes rounded.

GROUNDMASS: Plagioclase (41%, 0.1-0.5 mm); clinopyroxene (18%, 0.1-0.5 mm);orthopyroxene (<1%, 0.1 mm); opaque (4%, 0.05-0.5 mm); glass (26%); vesicles (8%).

VESICLES: 8%; 0.1 mm; equant; random.Miaroles: In bands and inclusions.

COLOR: Gray.STRUCTURE: Sill.ALTERATION: Smectite.VEINS/FRACTURES: 1%; 5-10 mm; subhorizontal; roughly at 10 cm spacing.

TSB

150-

CORE/SECTION

Figure 9. Sample visual core description (VCD) form used for metamorphic and igneous rocks.

29


When describing sequences of fine- to coarse-grained rocks,the core was subdivided into lithologic units according to thecriteria of changes in petrographic types, mineral abundances,and rock-clast types. For each lithologic unit, section, and rocktype, the following information was recorded as appropriate us-ing the HARVI system:

A.I. Leg, site, hole, core number and type, and section.A.2. Unit number (consecutive downhole), position in the

section, number of pieces of the same lithologic type being de-scribed, the name of the describer, and rock name.

A.3. Color of the rock according to the Munsell chart (re-corded when the rock piece was dry) as well as the presence oflayering and deformation and the character of each.

A.4. The number of mineral phases and their distributionwithin the unit. For each mineral phase, abundance, averagesize in millimeters, shape, the degree of alteration, and any ad-ditional comments were given.

A.5. Groundmass texture: glassy, microcrystalline, fine-grained (< 1 mm), medium-grained (1-5 mm), or coarse-grained(>5 mm). Relative grain size changes within the unit (e.g., coars-ening from Pieces 1 to 5) were also noted.

A.6. Presence of alteration and characteristics, includingstructures imposed during the formation of secondary minerals,type of alteration and abundance, and abundance of secondaryminerals.

A.7. Vesicles: percentage abundance, distribution, size,shape, and fillings and their relationships (including proportionof vesicles that were filled by alteration minerals).

A.8. Structure: massive flow, pillow lava, thin flow, breccia,hyaloclastite, etc., and comments.

A.9. Alteration: fresh (<2%), slight (2%-10%), moderate(10%-40%), high (40%-80%), very high (80%-95%), or com-plete (95%-100%). The type, form, and distribution of the al-teration were also given.

A. 10. Veins/Fractures: percent present, width, orientation,fillings, and relationships. The relationship of the veins andfractures to the alteration was also noted.

A. 11. Comments: appropriate notes on the continuity of theunit within the core and the interrelationship of units.

Basalts are termed aphyric, sparsely phyric, moderatelyphyric, or highly phyric, depending upon the proportion of phe-nocrysts visible with the hand lens or binocular microscope (ap-proximately 10X). Basalts were called aphyric if phenocrystsclearly amounted to < 1 % of the rock, sparsely phyric if theyranged from 1% to 2%, moderately phyric if 2%-10%, andhighly phyric if phenocrysts amounted to > 10% of the rock.Basalts were further classified by phenocryst type (e.g., a mod-erately plagioclase-olivine phyric basalt contains 2*70-10% phe-nocrysts, mostly Plagioclase, with subordinate olivine).

During Leg 126, the following volcanic rock names were usedfor the initial core descriptions. Basalts have any combinationof olivine, augite, and Plagioclase phenocrysts. Andesites differin that they have essential orthopyroxene and Plagioclase phe-nocrysts plus or minus augite and olivine. Dacites and rhyoliteswere not distinguished from each other petrographically. Theydiffer from andesites either by containing essential quartz or bylacking orthopyroxene. Boninites lack Plagioclase and have moreorthopyroxene than clinopyroxene or olivine.

More specific nomenclature was determined from chemicalanalyses. For consistency, we used the MgO-SiO2 fields adoptedduring Leg 125. However, we did name some fields differently(Fig. 10) to avoid using "picrite" for pyroxene-rich basalts and"boninite" for plagioclase-rich rocks. Broadly speaking, the"high-Mg series" corresponds to the "low-Ca boninitic series"of Crawford et al. (1989), and our "low-Mg series" corresponds

20

10

IBasalt

Andesite

High-Mg series

Low-Mg series

Dacite

40 50 60 70SiO2(wt%)

80

Figure 10. Nomenclature used to describe volcanic rocks recovered dur-ing Leg 126.

to the island-arc tholeiitic series of Jakes and Gill (1970). (How-ever, the low-Mg series can be calcalkaline, low- to high-K, andlow- or high-Al in detail.) Because of the delay that occurred be-tween the initial core descriptions and the later chemical analy-ses, there are discrepancies between the lithostratigraphic andgeochemical sections of this report. The geochemical classifica-tion is preferred.

Serpentinized rocks were classified on the basis of the origi-nal phase proportions present in the protolith, using the IUGSclassification system for ultramafic rocks. For example, "ser-pentinized dunite" is a rock with >90% modal olivine origi-nally present that has some specified proportion of serpentini-zation.

Synoptic versions of the Leg 126 igneous/metamorphic VCDforms are published with this volume and are available on mi-crofilm at all three ODP repositories.

Thin-Section DescriptionsThin-section billets of basement rocks were examined to (1)

confirm the identity of petrographic groups in the cores; (2) un-derstand the textures and interrelationships of the mineral phasesbetter; (3) help define unit boundaries indicated by hand-speci-men core descriptions; and (4) define the secondary alterationmineralogy. Percentages of individual mineral phases were esti-mated and were reported in the detailed thin-section descriptionsheets (available on microform at the repositories). The terminol-ogy for thin-section description was used in the same manner asin the macroscopic descriptions. In cases where discrepanciesarose between hand-specimen and thin-section analyses over thecomposition and abundance of mineral phases, the thin-sectiondescriptions were used in preference.

X-Ray Diffraction AnalysesA Philips ADP 3520 X-ray diffractometer was used for the

XRD analyses of the mineral phases. Instrument conditions innormal use were as follows: CuK-alpha radiation with a Ni fil-ter, 40 kV, 35 mA, Goniometer scan from 2° to 70° 20, step size= 0.02°, with a count time = 1 s per step.

Samples normally were ground with the Spex 8000 MixerMill or, when the sample was very small, an agate mortar andpestle. The material was then pressed into the sample holders,or smeared on glass plates and placed in the sample holders, foranalysis.

Diffractograms were interpreted with the help of a computer-ized search and match routine using Joint Committee on Pow-der Diffraction Standards (JCPDS) powder files.

30

EXPLANATORY NOTES

X-Ray Fluorescence AnalysisPrior to analysis, samples were normally crushed in the Spex

8510 Shatterbox with a tungsten carbide barrel. This producedsome tantalum and massive tungsten contamination of the sam-ple. If post-cruise studies for these elements, or instrumentalneutron activation analyses were envisaged with the shipboardXRF powders (as on Leg 125), samples were crushed with an ag-ate-lined barrel.

A fully automated wavelength-dispersive ARL8420 XRF (3kW) system equipped with a Rh target X-ray tube was used todetermine the major oxide and trace element abundances ofwhole-rock samples. Analyses of the major oxides were carriedout on lithium borate glass disks doped with lanthanum as a"heavy absorber" (Norrish and Hutton, 1969). The disks wereprepared from 500 mg of rock powder, ignited for 2 hr at about1030°C, mixed with 6.0 g of dry flux consisting of 80% lithiumtetraborate and 20% La2O3. This mixture was then melted at1150°C in a Pt-Au crucible for about 10 min and poured into aPt-Au mold using a Claisse Fluxer. The 12:1 flux-to-sample ra-tio, and the use of the lanthanum absorber, made matrix effectsinsignificant over the normal range of igneous rock composi-tions. Hence, the following linear relationship between X-rayintensity and concentration holds:

Ci = (Ii × mi) - bi,

where

Ci - the concentration of oxide / (wt%);Ii = the net peak X-ray intensity of oxide i;

mi = the slope of the calibration curve for oxide / (wt%/cps); and

bi = apparent background concentration for oxide / (wt%).

The slope, mi, was calculated from a calibration curve de-rived from the measurement of well-analyzed reference rocks(BHVO-1, G-2, AGV-1, PCC-1, JGB-1, JP-1, BR, DR-N, UB-N). The background, bi, was determined on blanks or was de-rived by regression analysis from the calibration curves.

Systematic errors resulting from short- or long-term fluctua-tions in X-ray tube intensity were corrected by normalizing themeasured intensities of the samples to those of a standard thatwas always run together with a set of six samples. To reduceshipboard weighing errors, two glass disks were prepared foreach sample. Accurate weighing was difficult on board the mov-ing platform of the JOIDES Resolution and was performedwith particular care, therefore, since weighing errors can be amajor source of imprecision in the final analysis. Loss-on-igni-tion (LOI) values were determined by drying the sample at 110°Cfor 8 hr and then weighing them before and after ignition at1030°C.

Trace element determinations used pressed-powder pellets pre-pared by pressing (with 7 tons of pressure) a mixture of 5.0 g ofdry rock powder (dried at 110°C for >2 hr) and 30 drops ofpolyvinylalcohol binder into an aluminum cap. A modifiedCompton scattering technique based on the intensity of the RhCompton peak was used for matrix absorption corrections (Rey-nolds, 1967).

An outline of the computation methods is given below.

1. Input X-ray intensities: Dead-time corrected X-ray inten-sities were read into the program from an ARL result file.

2. Make drift corrections: All peak and background intensi-ties were corrected for machine drift by using a one-point cor-rection of the form:

Di = Si/Mi, andIdi = Ii × Di,

where

Di = the drift factor for element /, generally 1.00 ± 0.01;Si = the peak intensity for element /, measured on a syn-

thetic standard ("POOP") at the time of calibration;Mi - the measured peak intensity for element i, measured

on "POOP" at any time after the calibration;// = the uncorrected peak or background intensity, ele-

ment /; andIdi = the drift corrected peak or background intensity, ele-

ment i.

3. Subtract backgrounds. Peak intensities were corrected fornonlinear backgrounds by measuring a peak to average back-ground ratio (BFi). This was determined for each element, onsynthetic and natural blank standards, at the time of calibra-tion.

BFi = PKi/AVBgi.

Thus, when measuring unknowns, the true or modified back-ground (MBgi) was calculated by multiplying the average mea-sured background for element / (AVBgi) by the BFi. This newmodified background value was then subtracted from the peakintensity (PKi) to arrive at the net peak intensity (NETPKi) forelement i.

MBgi = AVBgi × BFi, andNETPKi = PKi - MBgi.

4. Remove spectral interferences. During calibration, interfer-ences were measured on synthetic pellets containing pure quartzand the interfering element. A ratio of the interference intensityto the net peak of the interfering element was calculated and as-sumed constant with respect to concentration. When measuringan unknown, the net interference (INTFERiJ) was calculatedand removed by

INTFERiJ = NETPKi × IRiJ andCNETPKj = NETPKj - INTFERiJ,

where

INTFERiJ — the net interference intensity of element / onelement j and

CNETPKj - the net intensity of element j with the inter-ference element / removed.

In the case of mutually interfering elements, an iterative ap-proach to this same calculation was used until all the elementsinvolved converge on their respective corrected values.

5. Measure mass absorption coefficients. To correct for ma-trix differences between samples, three separate mass absorp-tion coefficients were determined following a modification ofthe Compton scattering technique of Reynolds (1967). Measuredintensities from the rhodium K-series Compton, FeKa, and TiKalines were compared with the calculated absorption coefficientsof Rb (ARb), Cr (ACr), and V (AV), respectively. From thiscomparison, three equations can be written to describe the rela-tionship between each coefficient and its respective line. Thethree equations derived from the Leg 126 calibration were asfollows:

ARb = \04/[(Rhcps × 0.0698) + 57.79],ACr = ARb/[(Fecps × 1.044 × 10~6) + 0.081], and

AV = ACr/[(Ticps × 6.140 × 10~6) + 0.778).

By using this method, unknowns were measured and correctedfor matrix differences without calculating the absorption coeffi-cients for each sample.

31


6. Calculate concentrations. Once all the spectral and ma-trix corrections have been calculated, the equation to calculateelemental concentrations reduces to:

Ci = (CNETPKi × Ai)/Ki,

where

Ci — the concentration of element / in parts per mil-lion;

CNETPKi = corrected net peak intensity, element /;Ai = mass absorption coefficient for element /; andKi = calibration factor (ppm/cps) for element /.

The value Ki is analogous to the calibration curve slope (mi) formajor elements; it was determined in the same manner on natu-ral rock (AGV-1, UB-N, G-2, BHVO-1, RGM-1, DRN) and syn-thetic (compressed, spiked quartz powders) standards.

Details of the analytical conditions for each element deter-mined are given in Table 4. Tables 5-7 summarize informationabout the precision and accuracy of XRF analyses during Leg126. Given the results presented in these tables, the major ele-ment data are precise to within 0.5%-2.5% and are consideredaccurate to ±1% for Si, Ti, Fe, Ca, and K, and to ±3%-5%for Al, Mn, Na, and P. Periodic summations to 101-102 wt%occurred but were irreproducible and attributed to weighing er-rors or gremlins. The trace element data are considered accurateto ±2%-3% or 1 ppm, whichever is greater, for Rb, Sr, Y, andZr, and to ±5%-10% or 1 ppm for the others, except for Baand Ce. Precision also was within 3% for Ni, Cr, and V at con-centrations > 100 ppm.

The range of trace element concentrations over which theseclaims hold true encompasses those encountered during Leg126—from basalts with 10% MgO to rhyolites with 75% SiO2—with the following exceptions: Zr may be systematically 5 ppm

too low, although agreement for BIR was excellent; Rb may be0.5 ppm too low; Cr may be up to 5 ppm too low below 25 ppm;Ce contents are thought to be accurate to ± 15 ppm so that con-centrations between 0 and 25 ppm are indistinguishable, whichapplies to most Leg 126 results; and Ba is accurate to within10% at concentrations >250 ppm, but most Leg 126 resultswere < 100 ppm where Ba was essentially uncalibrated. More-over, BIR, with a nominal 6 ppm Ba concentration, gave resultsranging from -10 to + 35 ppm. Nonetheless, average concen-trations for seafloor Sumisu Rift basalts agree with those mea-sured at UCSC and L-DGO to within 5 ppm at ranges from 10to 15 ppm. Consequently, Ce and Ba contents less than 20 and30 ppm, respectively, indicate that these concentrations are in-deed below those levels, but these values are too imprecise for usto be confident of more than that.

SEDIMENT/FLUID GEOCHEMISTRY ANDELECTRICAL RESISTIVITY

Fluid Geochemistry

Hydrocarbon GasesAs required by safety considerations, the concentrations of

the hydrocarbons methane (Cj), ethane (C2), and propane (C3)were monitored in the sediment cores at intervals of about 10 m.The hydrocarbon gases were extracted from the bulk sedimentby means of a headspace-sampling technique. A 5-cm3 plug ofsediment was taken as soon as the core arrived on deck, using aNo. 4 cork borer. This sample was placed immediately in a glassvial sealed with a septum and metal crimp and heated to 70°C.The gas driven off was drawn into a syringe and injected intoa Hach-Carle AGC Series 100 model 211 gas chromatographequipped with a thermal conductivity detector. Where high con-centrations of propane were suspected, a second sample wastaken by the same method and analyzed for C, through C6 hy-

Table 4. X-ray fluorescence analytical conditions, Leg 126.

Element

SiO2

TiOs

A12O3aFe2O3

MnOMgOCaONa2PK2OP2O5

RhNbZrYSrRtZnCuNiCrFeV

JiO2bCebBa

Line

KαKαKαKαKαKαKαKαKαKαK-CKαKαKαKαKαKαKαKαKαKαKαKαLα

Crystal

PET(002)LIF(200)PET(002)LiF(200)LiF(200)TLAPLiF(200)TLAPLiF(200)Ge(lll)LiF(200)LiF(200)LiF(200)LiF(200)Lif(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(200)LiF(220)LiF(220)LiF(200)LiF(220)LiF(220)

Detector

FPCFPCFPCFPCKrSCFPCFPCFPCFPCFPCScintScintScintScintScintScintScintScintScintFPCFPCFPCFPCFPCFPC

Collimator

CoarseFineCoarseFineFineCoarseCoarseCoarseFineCoarseFineFineFineFineFineFineFineFineCoarseFineFineFineFineCoarseCoarse

Peakangle

(degrees)

109.2586.14

145.2757.5262.9844.87

113.1654.71

136.65140.94

18.5921.3722.5323.7825.1326.6041.7945.0248.6769.3585.37

122.8486.14

137.92128.53

Backgroundoffset

(degrees)

00000

±0.800

-1.20000

±0.35±0.35±0.40±0.40±0.60±0.40±0.40±0.60±0.50

-0.40 + 0.70-0.50±0.50±1.50±1.50

Total count time(s)

peak background

4040

1004040

20040

20040

10010020010010010010060606060406040

100100

00000

2000

200000

20010010010010060606060406040

100100

Note: All elements were analyzed under vacuum on goniometer 1 at generator settings of 60 kV and 50mA. FPC = flow proportional counter using P 1 0 gas; KrSC = sealed krypton gas counter; and Scint= Nal scintillation counter.

a Total Fe as Fe2O3.Calibrated but not analyzed on basalts.

32

EXPLANATORY NOTES

drocarbons using a Hewlett-Packard 5980A Natural Gas Ana-lyzer, a gas chromatograph equipped with Poropak-Q, molecu-lar-sieve, and silicone-oil-coated columns, and a flame ioniza-tion detector. After the headspace gas had been analyzed, theactual volume of the sediment sample was measured to the near-est 0.5 cm3 by the displacement method using water. Details ofthis method and the complete configuration of both gas chro-matographs are given in the Explanatory Notes for Leg 112(Suess, von Huene, et al., 1988).

Interstitial Water

On Leg 126, interstitial water was obtained by squeezing andby in-situ extraction using the Barnes pore-water sampler. Wesqueezed 10-cm-long, whole-round sections of cores that werecut from the core as soon as it arrived on deck by slicing theplastic core liner and capping both ends. Following the electricalresistivity measurements, the core was removed from the coreliner, scraped with a stainless-steel spatula to remove the outer,contaminated layer, and placed in a stainless-steel squeezer (Man-heim and Sayles, 1974). The squeezer was placed in a Carver hy-draulic press and squeezed at pressures of up to 40,000 psi. In-terstitial water was collected directly into a 50-mL plastic sy-ringe, from which the various aliquots for analysis were ejectedthrough an on-line 0.45-µm polysulfone filter mounted in a Gel-man "acrodisc" disposable filter holder. Squeezed interstitialwaters are designated here as "IW" samples.

The in-situ pore-water sampler is the new version designedby Dr. R. Barnes and first used on Leg 110 (Barnes, 1988). Thistool collects interstitial water while making simultaneous mea-surements of sediment temperature and pore pressure. It is low-ered on the sand line to the end of the drill string, where it islocked into an assembly just above the bit. To keep the hole freeof fill during the descent of the sampler, the hole is flushed withdrilling fluid with the bit just off bottom. After the sampler islatched into place, the bit is lowered into the bottom with thesampling probe projecting about 20 cm through the bit. A timer-operated valve opens and interstitial water is drawn under nega-tive pressure through two 40-µm stainless-steel filters and one1-µm polyester filter into the sampler. There it passes through astainless-steel entry tube of 12 mL volume into two sample coilsarranged in series and separated by an open ball valve, and theninto a steel overflow cylinder of 1200 mL volume that also con-tains the sample coils and valving. The length of time requiredto fill the Barnes sampler varies with the permeability of thesediment, but typically is about 15 min. Interstitial water sam-ples collected with the Barnes tool are designated here as "BW".On removal from the stainless-steel coil, they are filtered at0.45 µm as for the squeezed samples.

Drilling Mud Filtrate

To evaluate the extent of contamination by drilling mud fil-trate, samples of drilling mud were taken when new batcheswere prepared. The drilling mud filtrate was separated by cen-trifugation, filtered and treated similar to interstitial water sam-ples. Drilling mud filtrate is designated here as "MW".

Sample Analysis

The waters were analyzed immediately on recovery for pH,alkalinity by potentiometric titration, salinity by refractive in-dex, and, when an odor was detected, hydrogen sulfide by col-orimetry using methylene blue. Aliquots were acidified to a pHof about 2 by adding concentrated HC1, refrigerated, and ana-lyzed within a few days for sodium, strontium, lithium, potas-sium, iron, and manganese by atomic absorption spectropho-tometry. When required, ionization suppressors were added tosamples, blanks, and standards. All samples were diluted towithin the linear range of the instrument. Calcium and magne-

sium were determined by titration as described by Gieskes andPeretsman (1986). A separate aliquot was taken for the determi-nation of chloride, silicon, sulfate, and ammonium. The con-centration of chloride was determined by means of argentiomet-ric titration; sulfate by ion chromatography; and silicon andammonium by colorimetry. All shipboard anion analyses werecarried out using standard ODP techniques as detailed by Gies-kes and Peretsman (1986) and described briefly in the Explana-tory Notes for Leg 112 (Suess, von Huene, et al., 1988). IAPSOstandard seawater was used as the primary standard for the de-termination of chlorinity, calcium, magnesium, and sulfate andas a secondary standard for lithium, potassium, strontium, andsodium.

Electrical ResistivityThe electrical resistivity and formation factor of the sedi-

ments was measured by means of a four-electrode configuration(Wenner configuration) similar to the one used by Manheim andWaterman (1974). The electrodes were made of 2-mm-diameterstainless-steel rods. Specially designed equipment allows the elec-trodes to be inserted into the short end of the whole-round sec-tions in exactly (fraction of a millimeter) the same position rela-tive to the core liner. A 20-kHz square-wave current was appliedto the outer electrodes and the difference in potential betweenthe two inner electrodes was measured. The size of the current(typically 50 mAmps) was measured over a resistor in the outercircuit. The temperature of the sediment was measured with athermocouple. For an infinite and homogeneous medium theresistivity was given by the equation,

AV/I,

where

R = the specific resistivity,a = the distance between the electrodes,

AV = the difference in potential between the inner elec-trodes, and

/ = the current (Jakosky, 1940).

However, because of edge effects and the large diameter of theelectrodes relative to the distance between them (13 mm), the "2x i × a" term in the equation had to be replaced by a cell con-stant. The cell constant was determined by measuring the resis-tivity of a 35%o NaCl solution held in a container made from acore liner and of the same length as the whole rounds on whichthe electrical resistivity was measured. Determination of the for-mation factor involved the following steps:

1. Correction of the measured apparent resistivity of the ref-erence fluid to 25 °C by means of the temperature calibrationcurve (Fig. 11).

2. Determination of the cell constant as the ratio betweenthe theoretical resistivity of the reference fluid at 25°C (0.195ohm-m) to the temperature-corrected theoretical resistivity ofthe reference fluid from Step 1.

3. Determination of the resistivity of the sediment at themeasuring temperature by means of the cell constant and themeasured current and voltage.

4. Correction of the sediment resistivity to 25 °C by meansof the temperature calibration curve (Fig. 11).

5. Making the assumption that the resistivity of the pore wa-ter of a given salinity (estimated from the chloride concentra-tion) has the same resistivity as pure sodium chloride solution,and using the calibration curve (Fig. 12) to estimate the pore-water resistivity at 25°C.

6. The formation factor is the ratio between the output fromStep 4 to 5.

33


Table 5. Consensus values and Leg 126 values (in wt%) obtained for major and trace element standards during initial calibration of shipboardX-ray fluorescence.

Element

SiO2 [1][2]

TiO2

A12O3

Fe2O3

MnO

MgO

CaO

NaO2

K2O

P2O5

H2O +H 2 O -CO2

LOI

Total [1][2]

Element

NB[1][2]

Zr

Y

Sr

Rb

Zn

Cu

Ni

Cr

V

Ce

Ba

BIR-1 JN-2anhydrous anhydrous

48.04*47.96

0.96*0.97

15.5315.5711.34*11.440.18*0.189.71*9.4

13.34*13.52

1.82*1.810.030.0230.0210.02660.090.08

0.17

100.8100.73

BIR-1

2.30.4

181616*16.7

107*107

2.20.5

7054.7

125*120.4166*157.3373*397312*321.9

1.6—6

—

JB-2

0.8'0.8

52*46.426*25.1

178*172.2

6.25.6

110*108.5230*191

14.213.327.4-26.3

578*623

6.55.1

208251

53.4*53.44

1.191.16

14.73*14.9414.39*14.350.20.234.68*4.89.93*9.952.04*2.030.43*0.4240.1*0.1030.310.07

0.38

100.71101.05

JB-3

" 2.31.7

100*96.128*28.1

395*410.9

1314.2

106*90.5

197*19138.838.8

* 60.4*55.34

383*37320.5—

251*242.6

JB-3anhydrous

51.16*51.45

1.45*1.43

16.93*17.4711.91*11.920.160.185.21*5.259.88*9.922.83*2.720.8*0.7860.29*0.2910.20.03

0.23

100.39101.19

BHVO

19*17.5

180*17026.5*27.9

420*386

9.1*8.4

113*100140*133.4120*115.3300*292.5320*328.3

38*39.2

135*129.8

All 92-29-1anhydrous

49.39*49.48

1.75*1.74

15.49*15.5110.85*10.960.18*0.187.52*6.94

11.16*11.13.07*2.870.17*0.1610.16*0.171

0.230.23

99.5198.88

BEN BR

173 100*105 103.8265* 250*271.2 269.6

30* 30*29.7 2

1370* 1320*1355 1330.7

47* 47*46 47

120* 85*100 [156.3]72* 72*77.6 75.3

267* 260*274 265360* 380*385 345.3235* 240*272 242.6152* 140145.2 144.5

1025* 1050*1090 1161

BHVOanhydrous

49.69*50.152.7*2.74

13.6*13.6512.3*12.490.17*0.187.26*6.82

11.39*11.522.29*2.140.52*0.520.28*0.27

-0 .69

-0 .69

100.89101.17

JA-1

1.71.8

87*85.431.4*31.8

266*262

11.810.590.6*85.641.7*43.2

1.82.77.3

105*110.4

136

307*295

BEN BRanhydrous anhydrous

39.58*39.872.7*2.71

10.43*10.2413.3*13.230.21*0.21

13.62*14.0814.37*14.563.3*3.211.44*1.481.09*1.052.240.50.743.48

96.5697.16

DRN

6*7.5

125*12529*28.3

400*395.7

70*73.1

145[264]

50*4716*15.84228.7

225*2164428

385398

GH

83*95

150*15370*88.8109

390*411.6

8546.8142.85*5.36

—5

—7053.2

850—

39.43*39.872.72.7

10.49*10.2313.31*13.170.21*0.21

13.71*14.1514.23*14.293.11*3.151.45*1.441.08*1.052.48

2.48

97.2497.78

STM-1

270*268.5130*0

133746*51.5

700*726.9120*120.4230

[295.3]4*42.14.72.6

25.2

260*271.5560*596

UBNanhydrous

44.93*45.060.130.123.33.179.59.50.140.15

40.1239.86

1.371.390.110.190.020.020.05

10.841.260.39

12.49

87.1886.97

MicaMg

120*128.42011.7_—

2.527.2

1300*1326

——

46.6

110123.6100*99.6

—_—_—

JA-1anhydrous

64.75*64.40.880.96

15.14*15.377.02*6.920.154*0.1581.63*1.935.78*5.73.9*3.950.83*0.7770.160.160.80.26

1.06

99.1899.27

RGM-1

9*9.8

245*243

22.425.7

106*108.7152*159.836

[17.5]11*11.8674

10.4135.7

4952

800*818.4

DRN STM-1anhydrous anhydrous

54.03*54.13

1.081

1.117.84'18.099.82'9.890.21'0.234.45'4.637.16'7.173.02'3.141.73'1.760.220.22

0

99.56100.36

MicaFe

270*281.4

80*76.52534.9

55.6

2200*2057

——

410.73540.890*87.7

135*152.8————

*

»

n

r

»

»

G2

12*12.2

300*319.5

10*9.6

480*500170*178.990*83.8i1111.65*5.383.8

35*30.6

160*188.6

1880*1877

60.63*59.940.1530.14

18.82*18.745.3*5.310.22*0.230.0920.171.12*1.139.07*9.214.38*4.350.160.14

0

99.9599.36

AGV-1

15*13.6

231*229.7

19*19.5

656*645.9

67*65.689

3 127.25957.516*15.310

1125*115.87150.1

1208*1102

Note: The consensus values are largely from Govindaraju (1984), revised in some cases to conform to experience in other XRF labs (Stork et al., 1987). The majorelement results from these sources were recalculated to an anhydrous basis by multiplying published values by 1.0 plus the sum of (H2O + + H2O ~ + CO2) givenat the base of each column. The Leg 126 entries are those obtained during calibration for ignited powders. The trace and major element calibration curves werebased on the standards that have an asterisk for that element. [1] = consensus values and [2] = Leg 126 values. Entries in brackets are analytically uncertain.

The uncertainty introduced by making the assumption that thepore-water resistivity was the same as for a NaCl solution of thesame salinity was small when the disturbance of the sedimentscaused by inserting the electrodes and by drilling and retrieval isconsidered.

Sediment GeochemistrySediments were analyzed on board ship for inorganic carbon

and for total nitrogen, carbon, and sulfur. The organic carboncontent of the sediments was calculated by difference. Inorganiccarbon and total nitrogen, carbon and sulfur were determinedon all sediment samples taken for physical properties measure-

ments and on the squeezed sections. All samples were freezedried prior to analysis.

Total inorganic carbon was determined using a Coulometrics5011 Coulometer equipped with a System 140 carbonate carbonanalyzer. Depending on the carbonate content, 15-70 mg ofground and weighed sediment was reacted in a 2N HC1 solution.The liberated CO2 was titrated in a monoethanolamine solutionwith a colorimetric indicator, while monitoring the change inlight transmittance with a photo-detection cell.

Total nitrogen, carbon, and sulfur were determined with aN/C/S analyzer (Model NA 1500) from Carlo Erba Instruments.Bulk samples were combusted at 1000°C in an oxygen atmo-

34

EXPLANATORY NOTES


NBS 688anhydrous

48.01*48.43

1.16*1.16

17.22*17.5310.27*10.340.17*0.178.33*8.34

12.07*12.622.13*2.10.19*0.190.130.1

0.050.05

99.63100.93

GHanhydrous

76.42*76.280.080.08

12.612.831.35*1.150.050.050.030.130.7*0.693.88*4.044.8*4.810.01*0.010.460.140.30.9

99.0299.17

Mica Mganhydrous

39.32*39.08

1.67*1.68

15.61*15.389.71*9.70.27*0.27

20.9419.020.080.010.1230.18

10.27*10.40.01

2.090.310.152.55

95.4593.17

JDF2anhydrous

50.33*49.9

1.951.88

13.68*13.4813.41*13.370.22*0.226.56*6.48

10.68*10.752.742.860.190.230.24*0.21

0

10099.38

RGM-1anhydrous

73.42*73.410.280.28

13.8*13.961.891.760.040.030.290.371.2*1.24.16*4.244.34*4.350.05*0.050.38

0.38

99.0999.27

K1919anhydrous

49.73*49.25

2.832.83

13.75*13.62

12.21*12.10.180.176.83*6.33

11.32*11.252.35*2.280.53*0.530.28*0.26

0

100.0198.62

JGB-1anhydrous

44*43.76

1.64*1.6

17.89*17.6915.35*15.210.170.177.93*7.56

12.13*12.091.251.360.2tf0.230.05*0.051.230.04

1.27

99.498.45

Mica Feanhydrous

35.65*35.132.6*2.56

20.21*19.6326.58*26.330.36*0.364.71*4.830.450.380.310.199.07*9.210.470.392.910.430.193.53

96.8895.48

G2anhydrous

69.51*69.060.48*0.5

15.47*15.512.68*2.410.030.030.750.91.97*1.924.08*4.094.48*4.480.13*0.130.5

0.080.58

9998.45

AGV-1anhydrous

59.9659.84

1.07*1.06

17.2917.376.86*6.80.1*0.11.531.94.97*4.924.35*4.392.94*2.960.51*0.480.78

0.020.8

99.7899.02

JP-1anhydrous

43.7443.310.010.020.64*0.6498.61*8.680.120.12

46.1446.5

0.58*0.570.0220.030.0030.005

2.680.4

3.08

96.7996.80

sphere with the addition of vanadium pentoxide, converting or-ganic and inorganic carbon to CO2 and sulfur to SO2. Thesegases, along with nitrogen, were then separated by gas chroma-tography and measured with a thermal conductivity detector.

PHYSICAL PROPERTIESPhysical property measurements made on cores recovered dur-

ing Leg 126 included MST logging (GRAPE, magnetic suscepti-bility, and compressional sonic velocity measurements), thermalconductivity, undrained vane shear strength, Hamilton Framecompressional velocity, electrical resistivity, index properties, andcarbonate analyses. Downhole temperatures were measured atone site. Representative samples of the core or section were takenin areas of least disturbance. Sample selection and spacing de-pended on the rate of core recovery, the type of measurement(see below), as well as the thickness and homogeneity of the re-covered sequences.

The testing methods employed during Leg 126 are discussedin the order in which they were performed on each core. TheMST logging and soft-sediment thermal conductivity determi-nations were performed on whole cores. Other measurementswere conducted on discrete samples taken from the split cores.

Gamma-Ray Attenuation Porosity EvaluatorThe gamma-ray attenuation porosity evaluator (GRAPE)

makes continuous measurements of wet-bulk density on wholecores by comparing the attenuation of gamma rays through thecores with attenuation through an aluminum standard (Boyce,1976). Individual core sections were placed horizontally on theMST and moved on a conveyor belt through the GRAPE sen-sors. The attenuation of the gamma rays passing through theliner and core was measured every 2.5 cm, and bulk density wascalculated from the attenuation values. The GRAPE data werefiltered to remove values that resulted from gas, gaps, or end-cap effects before averaging over 0.2-m intervals. All bulk den-sity data are reported in units of g/cm3.

P-Wave Logger

The P-wave logger (PWL), which also operates on the MST,produces a 500-kHz compressional wave pulse at a repetitionrate of 1 kHz. Both signal and receiving transducers are alignedperpendicular to the core axis. A pair of displacement transduc-ers monitor the separation between the compressional wave trans-ducers, allowing variations in the outside liner diameter withoutdegrading the accuracy of the velocities. Measurements weretaken at 2.5-cm intervals.

Water was applied to the core liner to improve acoustic con-tact between the transducers and the liner. As with the GRAPE,generally only APC and the first few XCB cores were measured.The deeper XCB cores and all RCB cores have annular voids be-tween the core and the liner which caused loss of transmissionof compressional waves. The PWL data were filtered to removedata that resulted from gas, gaps, or end-cap effects. Weak re-turns with signal strengths below a threshold value of 200 alsowere removed. All velocity data are reported in units of km/s.

Thermal ConductivityWhole-round cores were allowed to equilibrate to room tem-

perature for at least 3 hr, until drift rates were reduced to lessthan 0.1 mV/min prior to running thermal conductivity mea-surements. The thermal conductivity techniques used are de-scribed by Von Herzen and Maxwell (1959) and Vacquier (1985).All thermal conductivity data are reported in units of W/m K.

Soft-Sediment Thermal Conductivity

Needle probes connected to a Thermcon-85 unit were in-serted into the sediment through holes drilled into the core linerand the thermal drift was monitored. An additional probe wasinserted into a reference material. Once the temperature (esti-mated from voltage) stabilized, the probes were heated and thecoefficient of thermal conductivity was calculated as a functionof the change in probe resistance. When the sediment became

35


Table 6. Elemental wt% obtained during multiple analyses of major element standard AII-92-19-1 run throughout Leg 126.

Result file

SiO2

TiO2

A12O3

Fe2O3

MnOMgOCaONaO2

K2OP2O5

Total

Run 1

49.731.74

15.4610.890.157.14

11.063.020.170.16

99.52

Run 2

49.411.73

15.4710.910.156.96

11.063.040.170.16

99.06

Run 3

49.71.74

15.4710.860.157.5

11.13.070.170.16

99.92

Run 4

49.411.73

15.4310.760.157.08

11.043.030.170.16

98.96

Run 5

49.511.73

15.510.870.157.12

11.092.980.170.17

99.29

Run 6

49.651.74

15.5910.90.157.08

11.112.990.170.16

99.54

Run 7

49.621.73

15.4110.870.157.08

11.112.880.160.17

99.18

Run 8

49.61.74

15.510.90.156.97

11.062.770.160.16

99.01

Run 9

49.781.73

15.6810.910.156.93

11.12.650.160.15

99.24

Run 10

49.361.74

15.3810.850.147.17

11.062.780.160.15

98.79

Run 11

49.371.73

15.4110.770.156.6

11.092.750.160.15

98.18

Run 12

49.51.74

15.4610.920.156.95

11.082.810.160.15

98.92

Run 13

49.151.72

15.4210.770.156.83

10.972.780.170.15

98.11

Run 14

49.521.74

15.5610.840.156.91

11.12.880.170.16

99.03

Run 15

49.371.74

15.510.830.147.22

11.112.940.160.15

99.18

Note: SD = standard deviation and Real dev. = real deviation.

too stiff to allow easy insertion of the probe, holes were drilledinto the core material prior to insertion. An attempt was madeto insert the probes at locations along each core section that ap-peared to be the least disturbed. However, an annulus of dis-turbed sediment and drill fluid was often present along the in-side of the liner, preventing visual identification of disturbanceand resulting in possible erroneous values.

Hard-Rock Thermal Conductivity

Thermal conductivity measurements on well-lithified sedi-ments and rocks were conducted on split cores using the half-space technique with a needle probe partially embedded in aslab of insulating material. The surface of a sample of split corewas polished with 240-, then 600-grit sandpaper, and placed ontop of the slab. Dow Corning 3 silicone was used to improve thethermal contact between the slab and the sample. The sampleand the slab were immersed in a saltwater bath. After reachingthermal equilibrium, the probe was heated and measurementsof resistance changes in the probe were made over a 6-min inter-val. Thermal conductivity was determined from the most linearportion of the resulting temperature vs. log-time plot.

Undrained Shear StrengthUndrained shear strength was determined by means of a

Wykeham-Farrance motorized vane shear apparatus equippedwith a four-bladed vane that has a diameter of 1.28 cm and alength of 1.28 cm. The vane was inserted into the split-core sec-tion perpendicular to the core axis, to a point where the top ofthe blade was covered by between 2 and 4 mm of sediment. Thevane was then rotated at a rate of 89°/min until the sedimentfailed. The undrained shear strength was calculated from thepeak torque (stress) obtained at failure. Shear strength testingwas suspended at the point where significant disturbance oc-curred or where the material became too stiff for the vane topenetrate without severe cracking. No shear strength measure-ments were done on rotary-drilled cores. All shear strengths arereported in units of kPa.

Hamilton Frame Compressional Wave VelocityCompressional-wave velocity measurements were taken on

discrete samples that were sufficiently competent to provide ad-equate signal strength. Velocities were calculated from the deter-mination of the travel time of a 500-kHz compressional wavethrough a measured thickness of sample, using a HamiltonFrame Velocimeter and Tektronix DC 5010 counter/timer sys-tem. Samples of soft sediment were taken with a special paral-lel-sided sampling tool. A double-bladed diamond saw was usedin more lithified sediments. Basement rock samples were ob-

tained using either a double-bladed diamond saw or a 2.5-cm-diameter rock corer. Sample thicknesses (distance) were mea-sured directly with calipers.

Zero traveltimes for the velocity transducers were estimatedby linear regressions of the traveltime vs. distance for a series ofaluminum and lucite standards. Filtered seawater was used toimprove the acoustic contact between the sample and the trans-ducers. Velocities were not recorded when insufficient or ex-tremely variable signals were obtained.

Electrical Resistivity /Formation FactorThe electrical resistivity of soft sediments was measured per-

pendicular and parallel to the core axis with a Wenner electrodearray composed of two current and two potential electrodes. Al-ternating current with a voltage of 200 kHz was applied to thesediment and the resulting resistances were measured with aWayne-Kerr Precision Component Analyzer. Formation factorswere calculated as:

formation factor = sediment resistivity/pore-fluid resistivity.

Pore-water resistivity was measured with the same electrode ar-ray in a seawater-filled core liner; care was taken to simulate thesame configuration as used for the sediment measurements.

Index PropertiesIndex properties, wet- and dry-bulk density, porosity, water

content, and grain density were determined on samples of sedi-ment and rock (Lambe and Whitman, 1969). Where possible,samples for index property analyses were taken adjacent to com-pressional wave velocity and/or vane shear strength measure-ments. Samples were weighed using two calibrated Scientech202 electronic balances. An average of 200 sample weighingscompensated for ship motion. Samples were freeze dried for atleast 18 hr (Site 787) or oven dried at 105°C for 24 hr (Sites 788-793). Wet and dry volumes were measured with a Quantochromehelium penta-pycnometer.

Porosities and water contents are reported as percentages;water contents are on a dry-weight basis (Lambe and Whitman,1969). Wet- and dry-bulk densities and grain density are re-ported as g/cm3. All values were corrected for the salt contentof the pore fluid, assuming seawater salinity of 35‰.

Calcium CarbonateCalcium carbonate was calculated from weight percent inor-

ganic carbon, assuming that all inorganic carbon is in the formof CaCO3. Inorganic carbon was determined on index propertysamples with a Coulometrics Carbon Dioxide Coulometer.

36

EXPLANATORY NOTES


Run 16

49.381.74

15.4510.790.156.84

11.022.820.170.15

98.5

Run 17

49.251.73

15.4810.810.157.01

10.962.860.160.15

98.56

Element

SiO2

TiO 2

A12O3

Fe 2 O 3

MnOMgOCaONaO 2

K 2 O 5

Avg. total

Average

49.491.73

15.4810.850.157.02

11.072.89

0.170.16

99.00

SD

0.180.010.090.0600.180.050.0800

0.40

Real dev.(%)

0.360.410.560.522.832.490.492.752.922.69

EĖ

rrD5O

_ l

-0.50

-0.54

-0.58

-0.62

-0.66

-0.70

-0.74

Freeze-dried samples were dissolved in 2.0 N HO, and inor-ganic carbon was estimated from the amount of CO2 evolved(see Backman, Duncan, et al., 1988).

Heat Flow

Downhole-temperature measurements were successfully com-pleted at Site 792. The Uyeda probe, a Vi-in.-diameter thermis-tor probe, was attached to the end of the coring shoe and ex-tended beyond the shoe into undisturbed sediment. The electri-cal resistance in the sediment was recorded for approximately20 min during each deployment at 1-min intervals. During thistime, the temperature probe was effectively decoupled from shipmotion by means of a heave compensator. Electrical resistancedata was then converted into temperatures when the probe ar-rived back on deck. Reported temperatures are extrapolated toin-situ equilibrium values.

DOWNHOLE MEASUREMENTS

Measurement Descriptions

Downhole measurements determine the physical and chemi-cal properties of formations adjacent to the borehole. Interpre-tation of these continuous, in-situ measurements can yield astratigraphic, lithologic, structural, geophysical, and geochemi-cal characterization of the site. After coring is completed, acombination of sensors are lowered downhole on a 7-conductorcable, and each of several measuring devices continuously mon-itored properties of the adjacent formation. Of the dozens ofdifferent combinations commonly used in the petroleum indus-try, four combinations of Schlumberger sensors were used onLeg 126: (1) the geophysical combination, (2) the geochemicalcombination, (3) the formation microscanner (FMS), and (4) avertical seismic profile (VSP). Each combination was not neces-

8 12 16 20Temperature (°C)

24

Figure 11. Temperature calibration curve used for the correction of mea-

sured apparent electrical resistivity of a reference fluid and of sediment

resistivity.

. • I ' I • I • I • I • I • I ' I ' I ' .

. 1 . 1 . 1 . 1 . 1 . 1 . . .

-0.5 —

-1.00 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Log (NaCI)

Figure 12. Calibration curve of pure sodium chloride solution used for

the estimation of pore-water resistivity.

sarily deployed in each hole; reference should be made to indi-vidual site chapters for details.

The geophysical combination used on Leg 126 consisted ofthe dual induction tool (DIT), the sonic tool (LSS), the litho-density tool (HLDT), and the L-DGO temperature tool (TLT).This tool combination measures electrical resistivity, compres-

Table 7. Elemental wt% obtained during multiple analyses of trace element standard BIR-1 runthroughout Leg 126.

Result file

NbZrYSrRbZnCuNiCrVTiO 2

CeBa

Run 1

0.815.916.5

106.50

55.9125.4154.7391.7336.4

0.94—

22.1

Run 2

0.716.417

107.50

88.496.6

156.4—__——

Run 3

0.915.316.6

105.90

92.596.7

156.1388.9310.1

0.94_—

Run 4

0.516.116.5

106.10.6

90.596.6

156.4402.2326.2

0.95_

11

Run 5

115.216.2

106.60.1

91.499.1

156.9395.5328.7

0.94

22.4

Run 6

0.715.316.8

106.9-0.069195.8

155.6398.6327.1

0.93- 534.8

Run 7

0.81517.3

108.10

89.398.4

157.3391315.8

0.955.90

Run 8

0.415.816.4

106.30.1

88.999.3

156.4403.6336

0.9400

Average

0.715.616.7

106.70.1

86.0101.0156.2395.9325.8

0.90.3

15.1

SD

0.200.490.350.740.21

12.239.950.805.739.770.015.46

13.88

Note: SD = standard deviation.

37


sional wave velocity, formation density, and the spectral contentof naturally occurring radiation.

The geochemical combination consists of a natural gamma-ray spectrometry tool (NGT), an aluminum clay tool (ACT), aninduced gamma-ray spectrometry tool (GST), and the L-DGOtemperature tool (TLT). This tool combination measures the rel-ative concentrations of 11 elements, including silicon, calcium,iron, sulfur, manganese, hydrogen, chlorine, potassium, tho-rium, and uranium.

The FMS is a microresistivity imaging device that was de-ployed for the first time in the Ocean Drilling Program duringLeg 126. It allows for the visual characterization of the boreholewall from measurements related to the formation^ electrical re-sistivity in the vicinity of the borehole wall.

Vertical seismic profiles were conducted in two of the holesdrilled. The VSPs can provide much information on the seismicnature of reflective interfaces penetrated by the borehole, as wellas provide predictive information on the nature of, and depthto, interfaces below the total depth of the borehole.

Measurement DevicesA brief description of in-situ sensors used during Leg 126 is

given below. A more detailed description of the physical princi-ples of these sensors and their applications is provided in Schlum-berger (1972), Hearst and Nelson (1984), and Ellis (1987).

Electrical Resistivity

The dual induction instrument (DIT) provides three differentmeasurements of electrical resistivity, each one with a differentradial depth of investigation. Two induction devices ("deep"and "medium" resistivity) send high-frequency alternating cur-rents through transmitter coils, creating magnetic fields that in-duce secondary (Foucault) currents in the formation. Theseground-loop currents produce new inductive signals that areproportional to the conductivity of the formation. The signalsare recorded by a series of receiving coils, and the measuredconductivities are converted to resistivities. A third device, the"spherically" focused resistivity (SFR), measures the currentnecessary to maintain a constant voltage drop across a fixed in-terval. The vertical resolution is on the order of 2 m for the twoinduction devices and about 1 m for the spherically focused re-sistivity.

To a first-order approximation, resistivity responds to the in-verse square root of porosity (Archie, 1942). In addition, watersalinity, clay content, and temperature are important factors con-trolling the electrical resistivity of rocks. Other factors that mayinfluence the resistivity of a rock include the concentration ofhydrous and metallic minerals, the presence of vesicles, and thegeometry of interconnected pore spaces.

Sonic Velocity

The digital sonic measurement (LSS) uses two acoustic trans-mitters and two receivers to measure the time required for soundwaves to travel along the borehole wall over source-receiver dis-tances of 2.4, 3.0, and 3.6 m. The raw data are expressed as thetime required for a sound wave to travel through 0.31 m of for-mation; these traveltimes are then converted to sonic velocities.First arrivals for the individual source-receiver paths are used tocalculate the velocities of the different waves traveling in the for-mation (compressional, shear, etc.). Only compressional wavevelocity is determined on board the ship, but the full sonic wave-forms are recorded for postcruise processing to determine shearwave and Stoneley wave velocities. The vertical resolution of thetool is 0.61 m. Compressional wave velocity is primarily con-trolled by porosity and lithification; decreases in porosity andincreases in lithification generally cause velocity to increase.

Temperature

The L-DGO temperature tool is a self-contained tool thatcan be attached to any of the sensor combinations. Data fromtwo thermistors and a pressure transducer are collected every0.5-5.0 s and stored within the tool. Once the in-situ measure-ment is completed the data are transferred to a shipboard com-puter for analysis. The fast-response thermistor, though low inaccuracy, is able to detect sudden, very small temperature excur-sions caused by fluid flow from the formation. The slow-re-sponse thermistor has a high accuracy and can be used to esti-mate the temperature gradient. Data are recorded as a functionof time, with conversion to depth being based on the pressuretransducer (or preferably, on simultaneous recording by Schlum-berger of both depth and time).

Lithodensity Measurement

The lithodensity tool (HLDT) uses a Ce137 gamma-ray sourceto measure the resulting flux at fixed distances from the source.Under normal operating conditions, attenuation of gamma raysis chiefly caused by Compton scattering (Ellis, 1987). Forma-tion density is extrapolated from this energy flux by assumingthat the atomic weight of most rock-forming elements is ap-proximately twice the atomic number. A photoelectric effect fac-tor (PEF) is also provided. Photoelectric absorption occurs inthe energy window below 150 Kev and depends on the energy ofthe incident gamma ray, the atomic cross section, and the natureof the atom. This measurement is almost independent of poros-ity and can therefore be used directly as a matrix lithology indi-cator. The radioactive source and detector array are placed in atool that is pressed against the borehole wall by a spring-loadedcaliper arm. Excessive roughness of the borehole wall allowsdrilling fluid between the skid and the formation. Consequently,the density measurement will give rise to false low values. Ap-proximate corrections can be applied using caliper data. Thevertical resolution of the measurement is about 0.30 m.

Gamma-Ray Spectrometry Measurement

This induced gamma-ray device (GST) consists of a pulsedsource of 14 Mev neutrons and a gamma-ray scintillation detec-tor. A surface computer performs spectral analysis of gamma-ray counts resulting from the interactions of neutrons emittedby the source with atomic nuclei in the formation. Characteris-tic sets of gamma rays from six elements dominate the spec-trum: Ca, Si, Fe, Cl, H, and S. As their sum is always unity,they do not reflect the actual elemental composition. Conse-quently, ratios of these elements are used in interpreting the li-thology and porosity of the formation and the salinity of theformation fluid.

Aluminum Clay MeasurementAluminum abundance as measured by the aluminum clay

tool (ACT) is determined by neutron-induced (Californium chem-ical source), gamma-ray spectrometry. The contribution to thegamma-ray spectrum by natural radiation is removed by placingNal gamma-ray detectors above and below the neutron source;the one above measures the natural radiation before activation,and the one below, the induced radiation after activation. It isthen possible to subtract the naturally occurring component fromthe total measured after activation. Calibration to elementalweight per cent (wt%) is performed by taking irradiated coresamples of known volume and density and measuring theirgamma-ray output while placed in a jig attached to the loggingtool.

38

EXPLANATORY NOTES

Natural Gamma-Ray MeasurementThe natural gamma-ray measurement (NGT) measures the

natural radioactivity of the formation. Most gamma rays areemitted by the radioactive isotope K40 and by the radioactive ele-ments of the U and Th series. The gamma radiation originatingin the formation close to the borehole wall is measured by ascintillation detector mounted inside the sonde. The analysis isachieved by subdividing the entire incident gamma-ray spectruminto five discrete energy windows. The total counts recorded ineach window, for a specified depth in the well, are processed atthe surface to give elemental abundances of K, U, and Th.

Radioactive elements tend to be most abundant in clay min-erals, and consequently the gamma-ray curve is commonly usedto estimate the clay or shale content. There are rock matrixes,however, for which the radioactivity ranges from moderate toextremely high values, as a result of the presence of volcanicash, potassic feldspar, or other radioactive minerals.

Borehole Inclination Measurement

The general purpose inclination tool (GPIT) contains a three-component accelerometer and a three-component magnetome-ter. The device is used to measure the orientation of the down-hole sensors within the borehole, and the orientation (or de-viation) of the borehole itself, as well as to compensate forinstrument accelerations in the axis of the borehole during dataacquisition. It is included as a part of the FMS.

Formation Microscanner

The formation microscanner (Fig. 13) produces high-resolu-tion borehole images from electrical conductivity measurements(Ekstrom et al., 1986; Pezard and Luthi, 1988). Its applicationin the context of the oil industry since 1986 has been precludedin the Ocean Drilling Program because of diameter constraintsimposed by the internal diameter (4.125 in.) of the drill pipe. Amodified sensor was consequently developed by Schlumbergerfor ODP and first deployed during Leg 126. The FMS has a ver-tical resolution of approximately 1 cm, but a detection thresh-old for conductive features on the order of microns. With thisfine-scale detection ability (raw data points are recorded every2.5 mm), it allows for the detailed study of subsurface struc-tures. This compares with typical conventional downhole mea-surements that are averaged over 150 mm; the sampling rate ofthe FMS is consequently 60 times larger than most other log-ging devices. The FMS has four orthogonal pads that are pressedagainst the borehole wall. In the original version developed bySchlumberger, two adjacent pads each carry an array of 27closely spaced electrodes, from which two electrical images arederived. The electrode currents probe the conductivity of therock to a depth of a few centimeters into the borehole wall, thusresponding to such variations in physical and chemical proper-ties of the rock as porosity or surface conduction when clayminerals are present. The series of conductivity traces are dis-played side by side, then coded into an image where black repre-sents the most conductive values and white, the most resistiveones. The new ODP sensor was designed in such a way that fourimages (of 16 traces each), instead of 2, are recorded simultane-ously (Fig. 14). In addition, the resolution of the images is im-proved because of the smaller size of the electrode array. Oncethe data have been acquired in the borehole, the images are pro-cessed on a devoted workstation to allow for on-site comparisonwith the cores.

Possible applications of the FMS-derived images include theorientation of cores, the mapping of fractures, foliations, sedi-mentary features, and the direct analysis of depositional envi-ronments (with information on transport direction, structure ofthe pore space, nature of contacts, and depositional sequences).

Electronics

Inclinometry

Hydraulics

^ Current return

Pad array

Figure 13. Sketch of the formation microscanner (FMS). Four imagesare recorded from four pad arrays of 16 electrodes each. The 4 antipo-dal arrays are located on separate pads that open up in the borehole justas an umbrella would.

Vertical Seismic Profiles

The technical specifications and experimental procedure forthe vertical seismic profiles (VSP) are described in great detailby Phillips (Shipboard Scientific Party, 1988). During Leg 126the sound source used was the JOIDES Resolution^ large-vol-ume (400 in.), high-pressure (2000 psi/106 bar) water gun (SSIP400 Model 02). This choice was made on the basis of providingthe best possible results from a single experiment within thetime constraints of the program.

39


A

0 6.5

Figure 14. A. Sketch of an FMS pad with a 16-electrode array. B. Geometry of the FMS array. Dimensions arein millimeters.

Data Quality

Downhole data quality may be seriously degraded in exces-sively large sections of the borehole or by rapid changes in thehole diameter. Electrical resistivity and velocity measurementsare least sensitive to such borehole effects. The nuclear measure-ments (of density, neutron porosity, and both natural and in-duced spectral gamma rays) are most seriously impaired becauseof the large attenuation by the borehole fluid. Corrections canbe applied to the original data to reduce the effects of these con-ditions.

Different logs may have small depth mismatches, caused byeither cable stretch or ship heave during recording. Small errorsin depth matching can impair the results in zones of rapidlyvarying lithology. To minimize such errors, a hydraulic heave

compensator adjusts for ship motion in real time. Downholedata cannot be precisely matched with core data in zones wherecore recovery is low because of the inherently ambiguous place-ment of the recovered section within the interval cored.

Data AnalysisThroughout the acquisition of downhole measurements, in-

coming data are observed in real time on a monitor oscilloscopeand simultaneously recorded on digital tape. After the comple-tion of a set of downhole measurements, tapes are reformattedto allow interpretation using shipboard software; an interactivelog-interpretation software package with a versatile array of ma-nipulative and plotting options is available for this purpose.The precise nature of the interpretation procedure varies foreach site. Although initial appraisal of the downhole data is car-

40

EXPLANATORY NOTES

ried out on board ship, further analyses and interpretation areundertaken after the end of the leg.

Reprocessing of Downhole DataRaw count rates for six elements (Ca, Si, Fe, S, Cl, and H)

are obtained in real time by the Schlumberger data acquisitionsoftware. In the past, these count rates have commonly exhib-ited some inversion interference from chlorine; this is manifestas a strong but spurious correlation between chlorine and cal-cium, and weaker correlations with other elements. This inter-ference, which is attributable to the dominance of the inducedgamma-ray spectrum by chlorine, has until now been unavoid-able in ODP holes because the minimum pipe diameter is toosmall to permit the use of a boron sleeve for chlorine count sup-pression (see Harvey and Lovell, in press). On Leg 126, however,a modification to the tool allowed the inclusion of a slimmerboron sleeve.

The data is reprocessed after the cruise using proprietarySchlumberger software. The gamma-ray spectrum at each depthis inverted for titanium, gadolinium, and potassium in additionto the six elements listed above. Though gadolinium is presentin concentrations of only a few parts per million, its neutron-capture cross section is so large that gadolinium can account forlO o-SOI/o of the total gamma-ray spectrum. Inclusion of theseadditional elements improves the quality of the overall inver-sion, particularly the accuracy of calculated calcium abundance,by converting sources of unaccounted variance to signals. Thepotassium concentrations determined, however, are less accuratethan those from the NGT, and the hydrogen concentrations sim-ilarly are less accurate than those from the neutron tool (CNT).

Aluminum concentrations from the ACT require correctionfor variations in cable speed. Changes in speed affect the timelag between neutron irradiation of the formation and recordingof the induced gamma-ray spectrum because the number of in-duced gamma rays decreases rapidly with time. Post-cruise cor-rection for this effect is possible with techniques used in landholes; this correction may be less reliable in ODP holes whereship heave affects the measurement in spite of mechanical com-pensation devices.

After geophysical and geochemical downhole measurementcombinations have been completed, it is possible to reprocessthe geochemical downhole data further. The relative abundancesof Ca, Si, Fe, Ti, Al, K, S, Th, U, and Gd are used to calculate alog of predicted photoelectric effect. The difference betweenthis calculated curve and the actual photoelectric curve may beattributed to the only two major elements not directly mea-sured, Mg and Na. Major elements are converted from volumepercent to weight percent using downhole data curves of totalporosity and density. Major elements are expressed in terms ofoxide weight percent, based on the assumption that oxygen is50% of the total dry weight.

If GST data are available, but not enough log types are runto permit complete solution for oxide weight percentage, onefurther processing step is made. Omitting chlorine and hydro-gen, the yields of the other GST-derived elements (Ca, Si, Fe,Ti, S, K, and Gd) are summed, and each is expressed as a frac-tion of this total yield. This procedure corrects for porosity andcount-rate variations. Although the absolute abundance of eachelement is not determined, downhole variations in relative abun-dance are indicated.

Downhole sonic data obtained in real time are not based onfull-waveform analyses but on a threshold-measuring techniquethat attempts to detect the compressional wave arrival by first-break criteria. Occasionally, this technique fails, and either thethreshold is exceeded by noise, or the amplitude of the firstcompressional arrival is smaller than the threshold. The latterof these effects is known as cycle skipping and creates spurious

spikes on the sonic log. Such problems may often be eliminatedby reprocessing the data.

In-Situ Stress MeasurementsThe FMS was used during Leg 126 to determine principal

horizontal stress directions from elongation caused by stress-in-duced borehole breakouts. In an isotropic, linearly elastic rocksubject to differential stresses, breakouts form along the bore-hole wall as a result of compressive stress concentrations exceed-ing the strength of the rock. Under these conditions, the break-out orientation will develop in the direction of the least princi-pal horizontal stress. It has been demonstrated in different areasthat stress orientations deduced from breakouts are consistentwith other independent indicators (Bell and Gough, 1979; Zo-back et al., 1988). See individual site chapters for detailed ex-planations of FMS operations.

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