13. summary and conclusions - joides resolution · ing from submarine erosion or non-deposition are...
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13. SUMMARY AND CONCLUSIONS
The Shipboard Scientific Party1
SIERRA LEONE RISE(K. J. H.)
Sedimentary History
It was not possible to penetrate the earliest sedimentaryrecord of the Sierra Leone Rise; therefore, no coreswere collected which would shed light on the pre-Upper Cretaceous history. Unfortunately, it is notpossible to answer the simple questions: Were thefirst sediments deposited on an oceanic crust? Werethey shallow- or deep-water sediments? However,whatever the previous history was, the region aboutSite 13 was already a part of a deep ocean during theearly Late Cretaceous time.
The sedimentary history of the section which waspenetrated by drill can be divided into four majorepochs:
1. Pelagic sedimentation with much terrigenous influxin the Late Cretaceous.
2. Deposition of siliceous planktons in the Eocene.
3. Red clay sedimentation in the Middle Tertiary.
4. Deposition of calcareous planktons in the Pliocene.
The lowest Cretaceous unit (3-13A-7-1) of red shaleswith chert intercalations represents deposition underbathyal or abyssal conditions below the compensationdepth for calcium carbonate. Only rare specimens ofdeep-water arenaceous foraminifera were preserved.The red color renders this unit superficially similar tothe Upper Cretaceous red pelagic formations of theTethyan Geosyncline in Mediterranean countries (forexample, Capas rojas of the Betic Cordilleras, couchesrouges of the Prealps, and scaglia rossa of the Apen-nines). The Tethyan formations are, however, mainlycalcareous-including abundant planktonic foramini-fera—and were obviously deposited at a somewhatshallower depth.
1A. E. Maxwell and R. P. von Herzen, Woods Hole Oceano-graphic Institution, Woods Hole, Massachusetts; J. E.Andrews,University of Hawaii, Honolulu, Hawaii; R. E. Boyce andE. D. Milow, Scripps Institution of Oceanography, La Jolla,California; K. J. Hsu, Geologisches Institut, Zurich, Switzer-land; S. F. Percival, Princeton University, Princeton, NewJersey; and, T. Saito, Lamont-Doherty Geological Observatoryof Columbia University, Palisades, New York.
During Senonian time the bottom depth at this sitebecame more or less equal to the compensation depthfor calcium carbonate. Radiolarian and calcareousoozes were laid down alternately to form the mainbody of Unit 3-13A-6. Red shales are also present asintercalations, suggesting a gradual depth decreasesince the earlier time. The oozes have been lithified inpart to form cherty limestone and dolomitic chertinterbeds. Gradations from pure chert to pure carbon-ate were represented. These rocks bear a lithologicalsimilarity to some of the Cretaceous deep-water chertylimestones of Mexico or of Venezuela. Such similaritymay not be entirely coincidental, as a paleogeographi-cal reconstruction on the model of continental driftwould place Site 13 fairly close to the Venezuela Coastduring the Late Cretaceous.
The deposition of calcareous nannoplankton clays ofthe Unit 3-13A-2-1 may represent a further decrease inoceanic depth at the drill site during the latest Creta-ceous times. The calcium carbonate content increase,from about 6 per cent in lower Senonian clays (3-13A-5) to about 40 per cent in Maestrichtian marl oozes,reflects progressively more conducive conditions forthe preservation of calcareous nannofossils. Radiolari-ans are present, but calcareous planktonic foraminiferaare absent or very rare in these clays. The absence ofthe latter has been a puzzle. The problem will bediscussed more thoroughly in a later section of thereport by expounding the idea that selective solution,combined with other factors, may explain the prefer-ential preservation of calcareous nannofossils to theexclusion of calcareous foraminifera.
The Late Cretaceous sediments at Site 13, compared toall other cores of Leg 3, are characterized by a highterrigenous content. The average rate for Unit 3-13A-2-1, computed on the basis of 200 to 230 meters (656to 754 feet) in 15 to 20 million years, is about 1.2 ±0.2 cm/t.y. Since the sediments include, on the whole,about 20 to 30 per cent calcium carbonate, the rate ofterrigenous deposition was approximately 1. Thisrepresents a high influx of terrigenous sediments intothis area during the Late Cretaceous as insolubleterrigenous residues. Red clay sediments were com-monly accumulated at an order of magnitude slowerrate.
There was a reduction in the terrigenous influx whenthe uppermost Maestrichtian foraminiferal chalk ooze
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was deposited. A major change occurred in the Eocenewhen the sediments laid down consisted almost exclu-sively of siliceous planktons, radiolarians and diatoms(Unit 3-13-3-1). The rarity of calcareous planktons(commonly 1 per cent or less) suggests accumulationat, or beneath the compensation depth for calciumcarbonate; those that remain are calcareous nanno-fossils, with no planktonic foraminifera being preserved.Intercalated in the oozes are several thin, but hardchert layers of uncertain origin. At least 40 meters(131 feet) of the siliceous oozes were deposited duringthe 4-million year Middle Eocene interval, representinga 1 cm/t.y. rate. On the other hand, the whole earlyTertiary section was probably not more than 100meters (328 feet) thick (for some 40 million years),giving an average sedimentation rate of 0.25 cm/t.y.Apparently, there were times (during the Paleoceneand, probably, also the Oligocene) when little or nosediments were deposited. The occurrence of siliceousoozes at Site 13 supports the hypothesis of the JOIDESAtlantic Advisory Panel that a strong equatorial cur-rent system flowed between the Atlantic and thePacific during the Eocene. Also noteworthy is the factthat such an Eocene siliceous ooze formation is absentat the sites along the 30°S traverse outside of theequatorial current system.
Deposition of brown zeolitic clay sedimentation tookplace at the Sierra Leone Rise area during the middleTertiary. This condition apparently persisted from theOligocene to the Pliocene here, as well as at severalsites in the North Atlantic drilled by the Leg 2 cruise.In contrast, regional red clay sedimentation at 30°S(represented by the Discovery Formation) was restrictedto a shorter time span, for example, to the MiddleMiocene at Site 15. The average rate of brown claysedimentation was 0.3 cm/t.y.; the actual rate might besomewhat higher (0.5 cm/t.y.) if the Oligocene epochwas represented by a hiatus here, as may be impliedby the study of drill bit samples.
Geological Problems
Site 13 was selected to study the Cretaceous andTertiary equatorial floras and faunas of the Atlantic,and their relationship to the ancient current system,and to identify the reflector similar to Horizon A ofthe western North Atlantic Basin. Several problems ofgeneral interest arose during the course of shipboardinvestigation:
1) Oceanic sedimentation. Leg 3 findings confirm theresults by previous drilling that disconformities result-ing from submarine erosion or non-deposition are notuncommon in deep marine sediments. The agentswhich prevented deposition, or caused erosion may bemechanical (e.g., strong bottom currents), or chemical(e.g., dissolution of calcareous and siliceous planktons).
Interesting, too, is the fact that the planktonic organ-isms in a pelagic sediment may consist almost exclu-sively of one type of organism: for example, siliceousoozes, calcareous nannofossil clays. There seems tohave been an effective sorting mechanism which re-flects the production and/or dissolution rates of pelagicorganisms. Furthermore, the conditions must haveremained sufficiently constant so that sediments ofuniform lithology could have accumulated to tens orhundreds of meters in thickness.
2) Oceanic diagenesis. The occurrences of multiplelayers of hard sedimentary rocks bear testimony tolithification under oceanic conditions. Cherts are presentas thin layers in units containing abundant Radiolaria.Calcareous nannofossil clay stones (Unit 3-13A-2-1) aresilicified in part, also. Some cherts are definitely dolo-mitic (Unit 3-13A-6). There was no association ofcherts with turbidites, since there were no turbiditesidentified at Site 13. Because there appears to be nofirm evidence of a transition between the cryptocrystal-lized chert and the well-preserved radiolarian, it ispossible that chert might have been formed from thelithification of chemical precipitated silica gels.
The occurrence of red shale indicates that pelitic sedi-ments can also be lithified under submarine conditionswith moderate overburden (some 450 meters). Thechange from red shale to green gray nannofossil clay-stone during the Late Cretaceous might be related toa decrease in bottom depth and corresponding changein PQ of bottom waters.
3) Nature of reflectors. Drilling at Site 13 confirmsdiscoveries by earlier Deep-Sea Project drilling thatthe Horizon "A" reflector consists of one or moreEocene chert layers at some locations. However, whathad been interpreted as a "rough basement reflector"may actually be an Upper Cretaceous chert layer, at432 meters (1417 feet). It is possible that the "base-ment" lies at no greater depth beneath this chert, forthere is some evidence from the CSP record (Chapter 3,Figures 1 and 2) that the last reflector consists of tworeflections only slightly separated in reflection time.Alternatively, the basement may actually be consider-ably deeper and not detected on the acoustic record.In any case, the results introduce a caution to keep anopen mind on the nature of the "Second Layer" in theoceans, and leave the possibility open that the oceanicsediments in this part of the Atlantic may be thickerthan previously supposed.
RIO GRANDE RISE(K. J. H. & J. E. A.)
Sedimentary History
The earliest cored sediment here is a coquina of uncer-tain age. This megafossil debris was probably depositedon a relatively shallow bank and sorted by winnowing
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currents. The water depth was not likely to haveexceeded a few hundred meters, so as to permit thegrowth of red algae by photosynthesis.
The bank must have subsided to a depth which renderedthe growth of benthonic fauna and flora unfavorableduring the Campanian, the time when pelagic sedimen-tation began—and has continued intermittently tilltoday.
The pelagic sediments of the Rio Grande Rise includeabundant foraminifera, and those of the Cretaceouscontain Inoceramus and other megafossils. Commonly,less than 50 per cent of the constituents are clay-sizedparticles (mainly nannofossils). This relatively coarsecomposition may have resulted from (1) the commonpreservation of the calcareous planktonic foraminiferaat depths above the carbonate compensation depth, or(2) the winnowing of the fines from the Rio GrandeRise by bottom currents, or from a combination ofthese factors. Foraminifera are the dominant constitu-ent of the Quaternary sediments, in contrast to thedominance of nannofossils in the Tertiary. This changemay reflect an increase in the rate of production ofthe pelagic foraminifera during the late Cenozoic asBramlette (1958) has suggested.
The sedimentary history of the Rio Grande Rise wasinterrupted by a major unconformity, which covereda span over much of the Tertiary. In contrast to siteson the flanks of the Mid-Atlantic Ridge, where solutionunconformities are not uncommon, there is no indica-tion that the Rio Grande Rise ever subsided below thecarbonate compensation depth during the Tertiary.Therefore, it is thought that the stratigraphic hiatuson the Rio Grande Rise may have been related tosubmarine bottom current actions.
The unconformity at Site 21 represents a hiatus whichprobably spanned from the Middle Eocene to MiddlePleistocene, some 45 million years. Instead of assum-ing that currents were always strong enough to preventdeposition during this interval, the alternative of sub-marine erosion seems more probable. When the strati-graphy at the two Rio Grande Rise sites is compared,it is noted that the well-developed Upper Oligoceneand Lower Miocene section at Site 22, more than 130meters (426 feet) thick, and deposited at a 1.3 cm/t.y.rate or faster, is absent at Site 21. The possibility,however, of an incomplete equivalent being within the17-meter (56-foot) uncored interval cannot be ex-cluded. As Site 21 is located on the side of a deepsubmarine valley (Chapter 11, Figure 1), it is probablethat an Oligocene-Miocene section had also been depos-ited there before it was removed by bottom currenterosion.
Bottom currents are probably active today on the RioGrande Rise. Less than 10 meters (33 feet) of
Quaternary foraminiferal chalk oozes are present atSite 22; and, this thin veneer may represent a transientdeposit which could be removed in the future byerosion.
Geological Problems
The original aims of drilling on the Rio Grande Risewere to sample the Cenozoic sediments at Site 22 andthe Mesozoic sediments at Site 21, in order to obtaininformation on the distribution of planktonic organ-isms in the Southern Hemisphere. Except for thecoquina, sediments older than Campanian were notpenetrated by drill, and the Cenozoic section sampledis incomplete because of a major depositional hiatus.Nevertheless, valuable information concerning sedi-mentary processes on an oceanic rise has been obtained,and the cores provide the materials for research tofurther our understanding of oceanic sedimentationand diagenesis.
At Site 21, it was not possible to penetrate the lithifiedcoquina to reach basement. If the Late Cretaceous isconsidered the continuation of an earlier trend, itseems probable that the earliest sediments on the Risecould have been an even shallower water deposit.However, as in the Sierra Leone Rise region, theattempt to reach the crystalline basement was frus-trated. Therefore, several questions must remain un-answered: Is the Rise a fragment of a continentalcrust left behind as South America drifted westwards?Or is it an oceanic basalt rising above the surroundingregion as a group of seamounts or guyots?
The high porosity of the coquina and its lithificationpresent a puzzle. Equally puzzling is the deposition ofthe Oligocene Chalk Marker, consisting almost exclu-sively of one species of the nannoplankton Braarudo-sphaera. This chalk is absent at Site 21, probablybecause of post-depositional removal. It is 80 centi-meters thick at Site 22, some 3000 kilometers west ofits easternmost occurrence at Site 17.
Partial lithification of the Rio Grande Rise sedimentsis not uncommon. All the reflectors penetrated bydrill turn out to be sedimentary rocks. The Braarudo-sphaera Chalk is sufficiently lithified to be a weakacoustical reflector. Silicification may have enhancedthe hardening of the Eocene chalk oozes, so that thesemore or less friable cherty carbonate rocks constitutea strong reflector on the Rio Grande Rise, which couldbe considered a correlative of the Eocene Horizon Ain the North and Equatorial Atlantic. The Cretaceouspelagic oozes show signs of progressive lithificationwith burial. The deeper cores contain relatively feweridentifiable nannofossils and more crypto-crystallinecalcite, which apparently formed as a recrystallizationproduct at the expense of nannofossils. The deepest
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Campanian oozes have been consolidated into friablerocks (3-21-8, Sections 4-6), and the coquina belowmay be hard enough to serve as the basement reflector.
The occurrence of Inoceramus in Rio Grande Risesediments is not surprising. Inoceramus typically occursin Cretaceous pelagic deposits of ancient geosynclines,and the Inoceramus-beanng pelagic ooze at Site 22 islithologically very similar to the Seewen Limestone ofthe Alps (R. Trümpy, personal communication). Thequestion whether Inoceramus was a deep-water ben-thonic genus or acquired a pelagic habitat whileattached to floating sea plants could be resolved aftera stable isotope analysis of the recovered materials.
MID-ATLANTIC RIDGE SEQUENCE
Paleontology (T. S. & S. P.)
An almost continuous composite stratigraphic section,ranging in age from Maestrichtian (late Cretaceous) tolate Pleistocene was recovered in seven core holes(Sites 14 through 20) drilled on the Mid-AtlanticRidge, along an east-west traverse at about 30°SLatitude. The stratigraphic correlation of all sites isbased on the calcareous microfossil zonation shown inChapter 2, Figure 1 and the stratigraphic interval re-covered from each site is shown in Figure 1.
All the sites yielded calcareous sediments characterizedby an abundance of calcareous nannoplankton andplanktonic foraminifera. Only minor amounts of sili-ceous microfossils, radiolarian and diatoms were en-countered in the late Tertiary and Quaternary on theeastern flank of the Mid-Atlantic Ridge (Sites 17 and18), in association with a large diatom species, Ethmo-discus rex (Ratt.) Hendy. This paucity or completelack of siliceous microfossil element in planktonicfossil assemblages is the most remarkable feature whichmakes the Mid-Atlantic Ridge sequences distinct fromother sequences recovered from the sites (Sites 13 and22) relatively close to the continents of Africa andSouth America. At these two sites, near the marginalpart of the oceans, siliceous sediments consisting pre-dominantly of Radiolaria characterize the pre-Oligocenesequence, and these sediments were associated withchert layers. The fact that the pre-Oligocene sedimentswere repeatedly penetrated at two sites on the Mid-Atlantic Ridge without encountering any chert layersseems to indicate that the occurrence of chert layersin the South Atlantic is closely associated with theabundance of siliceous microfossils in sediments.
The calcareous floral and faunal assemblages from theMid-Atlantic Ridge holes include those which thrivedin various climatic conditions, ranging from tropicalto cold-temperate. Most of the floras and faunas priorto the mid-Tertiary are diverse and bear a close resem-blance to those from the tropical regions of the world.However, the planktonic foraminifera encountered inthe Upper Miocene to Upper Pleistocene sections at
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two southernmost sites (Sites 15 and 16) are temper-ate ones. These faunas are characterized by relativelyfew species and differ markedly from the diversetropical counterpart of the same age. Age determina-tions of these temperate assemblages were difficult attimes because of the absence of the diagnostic speciesof the late Tertiary planktonic microfossil zones cur-rently in use for stratigraphic correlation. The calcar-eous nannoplankton from the upper Miocene to upperPleistocene sediments at this latitude are apparentlymore ubiquitous, and they include many species whichhave been reported from tropical areas.
The abundance of well-preserved calcareous nanno-plankton and planktonic foraminifera in sedimentsfrom the Mid-Atlantic Ridge sections makes these sitesideal for use in establishing standard reference sectionsfor parts of the Quaternary, Tertiary and Upper Creta-ceous. Tentative zonal assignments have been made forthe time interval from the Maestrichtian to the MiddleMiocene for the planktonic foraminifera and from theDanian to Middle Miocene for the calcareous nanno-plankton.
One of the more important findings is the presence ofseveral layers of Braarudosphaera rosa chalk in theOligocene section of this region. They occur mostfrequently in upper Oligocene, but are also present inlower Oligocene. The chalk consists almost exclusivelyof calcite shields of a golden-brown algae, Braarudo-sphaera rosa, both as complete specimens and isolatedfragments, with an admixture of a few other species ofcalcareous nannoplankton. In the Upper Oligocenesection, these chalk layers usually occur near theboundary of the Globorotalia opima opima and Globor-otalia ampliapertura planktonic foraminiferal Zones.In the Lower Oligocene, they are present in the Globi-gerina sellii/Pseudohastingerina barbadiensis Zone. Al-though a modern representative of the pentalith(golden-brown algae), Braarudosphaera bigelowi, isknown to be abundant in sediments of very shallowwater origin-approximately 20 fathoms—all the otherflora and fauna associated with the Oligocene chalkindicate bathyal depths. Since this type of chalk isfound widely distributed in the South Atlantic, occur-ring in holes at Sites 14, 17, 19 and 20 on the Mid-Atlantic Ridge and in Hole 22 on the Rio Grande Rise,its occurrence may indicate that unusual Oceanographicconditions prevailed in this region for a short geologictime interval. These conditions might either have causedthe "bloom" of Braarudosphaera rosa, or inducedcurrents which transported shallow-water sediments todeep water over, geographically, a very wide area. Amodern analogy to the bloom hypothesis would be theso-called "Red Tide," which is the bloom of certaindinoflagellates. Whereas the Red Tide bloom is ofshort duration—several weeks, these Oligocene golden-brown algae blooms might have lasted several hundredor several thousand years to produce these chalks.
The most interesting finding from the paleontologicstudies is the correlation of paleontologic ages of sedi-ments immediately overlying the basalt basement withages of the basement predicted by the sea-floor spread-ing hypothesis. The planktonic foraminifera and cal-careous nannoplankton were extremely useful fordating these sediments in all seven sites. By using thechronostratigraphic chart of W. A. Berggren (Chapter 2,Figure 1), which shows the correlation of known radio-metric age dates with the planktonic foraminiferal zon-ations, equivalent radiometric age dates for these sedi-ments were determined. At times, calcareous nanno-plankton were found in the baked sediments associatedwith the basalt, thus giving a closer approximation tothe age of the basalt. The result indicates that the agesof sediments overlying the basement exhibit somesymmetry around the Ridge axes and flanks of theMid-Atlantic Ridge, and show striking agreement withthe ages of the oceanic crust derived from geomagneticanomaly patterns.
A comparison of the results of the studies of the cal-careous nannoplankton found in the shipboard reportsand the reports by the shore laboratories—one by Bukryand Bramlette (Chapter 18) and one by Gartner (Chap-ter 19)—show basic agreement in the age and zonaldeterminations. Slight variations are found near thePliocene-Pleistocene boundary and in the Oligocene,in both cases the age determinations used were basedon the planktonic foraminifera. Slight variations inzonal determinations occur near the boundaries of thezones; however, most zonal boundaries are in agreement.
Lithology (K. J. H. & J. E. A.)
The sediments of the Mid-Atlantic Ridge province arealmost exclusively pelagic. No turbidity-current de-posits or mass-slide debris were found, except for theCretaceous slump block in the Paleocene at Site 20.Yet, the lithological changes in time and in spacepermitted the establishment of nine formations. Theyare in descending order:
Albatross OozeBlake OozeChallenger OozeDiscovery ClayEndeavor OozeFram OozeGazelle OozeGrampus OozeHirondelle Ooze.
The differences here are sufficiently obvious that eightof these formations were recognized on-board ship;only the Gazelle Ooze was added as a new formationduring the course of synthesizing shipboard data forthe Leg 3 Report.
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A composite stratigraphic section of the Mid-AtlanticRidge formations has been constructed and is shownby Figure 2. The formations have been recognizedmainly on the basis of lithological changes with time.Initially, only two criteria were selected, namely:(1) variations in foraminifer a content, and (2) differ-ences in non-carbonate content (with correspondingcolor changes). Using these criteria, the topmost foram-inifera-rich nannofossil chalk ooze has been designatedAlbatross Ooze. The Blake Ooze is distinguished fromthe Albatross by a decrease in foraminifera content.The Challenger Ooze (marly chalk oozes) is darker incolor and has a higher average terrigenous content.The Discovery Clay is a red clay formation. TheEndeavor Ooze (marl oozes) includes more calcareousplanktons. The Fram Ooze is a nannofossil chalk oozeformation almost devoid of foraminifera. The GrampusOoze lies directly above a basalt basement, and it ischaracterized by an increased foraminifera-content anda darker color toward the base. The cycle of change isthus complete. The only disharmonious note is thepresence of a marly chalk ooze unit beneath the FramOoze at Sites 19 and 20, which has led to theestablishment of the Gazelle Formation.
The correlation of the Hirondelle Ooze required somesubjective judgment. The Unit 3-20C-5-1 at Site 20shows the same increase of foraminifera toward thebase as is characteristic of the Grampus Formation.Yet, its overall lithology is similar to the sediments ofabout the same age at Site 21, and it is distinctly dif-ferent from the Grampus Ooze by the presence ofintercalated pink dolomitic chalk oozes. At this pointa third criterion, namely, the presence of pink oozes,was chosen to establish the Hirondelle as a formation.
The type section for each of the formations was chosenwith the following considerations: preferably, the topand bottom contacts of the unit fell within coredintervals, and the lithology of the type section wasfairly representative of the formation. In general, thetype-section is the most completely developed (thickest)section, but not necessarily so.
The distribution of the various Mid-Atlantic Ridgeformations in time and depth is summarized by Figure3. Shown graphically are: total depth of holes, coredintervals, and the formations and their ages. Also shownfor comparison are the stratigraphic columns of Sites 21and 22 on the Rio Grande Rise. Several obvious con-clusions can be deduced from a quick glance:
(1) Basalt basement has been reached at all the sevensites on the Mid-Atlantic Ridge.
(2) The age of the sediments above the basalt bears adirect relation to the distance from the Ridge axis:older sediments are found farther away from the axis.
(3) The thickness of the sediments bears no simplerelation to the age of the sediments. Paradoxically, theholes with the youngest basement ages (Miocene inHoles 16 and 18) have the greatest sediment thicknesses(176 and 179 meters, respectively); whereas, the holewith the oldest basement age (Upper Cretaceous inHole 20C) has the thinnest sedimentary sequence(72 meters). More than likely this relationship resultsfrom the non-random selection of sites.
(4) The nature of the topmost formation is related tothe distance from the Mid-Atlantic Ridgé axis, and tothe present depth of the drill sites. The Albatross Oozeis present at Sites 15, 16, 17 and 18, at distances rang-ing from 221 to 718 kilometers from the Ridge axisat depths ranging from 3527 meters (11,568 feet) to4265 meters (13,989 feet). The Endeavor Ooze ispresent at Site 14, at 745 kilometers from the Ridgeaxis and 4343 meters (14,245 feet) depth. The Dis-covery Clay (covered by a thin veneer of local units ofsimilar lithology) is present at Sites 19 and 20, and isfound 1010 and 1303 kilometers from the Ridge axisand4677meters (15,340feet) and4506 meters (14,780feet) deep, respectively.
(5) The nature of the sediments below the topmostformation bears no simple relation to the present depthof the drill site, indicating changes in the depositionalenvironment at each site with time. Paradoxically, thesedimentary sequence (Eocene to Quaternary) at thedrill site where the present oceanic depth is the greatesthas the highest average calcium carbonate content (seeChapter 15).
(6) The sediments at sites less than 500 kilometersfrom the Mid-Atlantic Ridge axis (Sites 15,16 and 18)are mainly Neogene, whereas the sediments farther outare mainly Paleogene (Sites 14, 17, 19 and 20).
(7) The formations on the whole are not isochronous.The base of the formations older than Middle Miocene(Hirondelle, Grampus, Gazelle, Fram, Endeavor andDiscovery) tends to become older at drill sites fartheraway from the Ridge axis. On the other hand, the trendis reversed for younger formations (Challenger, Blakeand Albatross), the base of which is either nearly syn-chronous or younger at drill sites farther away from theMid-Atlantic Ridge axis.
(8) The Oligocene Braarudosphaera Chalk is a remark-able time-stratigraphic marker; and, it has been recog-nized at drill sites some 2800 kilometers apart (Sites 17to 22). This chalk is absent in the Ridge crest sites(Sites 15, 16 and 18) because the age of the basalt base-ment is not older than Miocene.
(9) The stratigraphy has a symmetry about the Ridgeaxis, so that the sedimentary sequence at a drill site ismore similar to that at a site nearly equidistant from
446
FORMATION NAMEö TYPE SECTION
FORMATION SYMBOLLITHOLOGICAL DESCRIPTION
ö TYPE SECTION THICKNESSTIME RANGE OF FORMATIONS EPOCH m.y.
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AlbαtrossOoze
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DiscoveryClay
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EnaeavorOoze
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FramOoze
DI-17A/2/4
GazelleOoze
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GrampusOoze
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HirondelleOoze
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FORAMINIFER AL NANNO-FOSSIL CHALK OOZES, VERYPALE BROWN TO WHITE.
61 m
NANNOFOSSIL CHALKOOZES, WHITE
73.5 m
NANNOFOSSIL CHALK OOZES.MARLY IN VARIOUS SHADESOF BROWN.
41 m
RED CLAYS, ZEOLITIC,MARLY OOZES INTERBEDSPRESENT LOCALLY.
26 mNANNOFOSSIL CHALK & MARLOOZES, BROWN WITH REDCLAYS LOCALLY. 13 m
NANNOFOSSIL CHALK OOZES,VERY PALE BROWN,UNIFORM.
54 m
NANNOFOSSIL MARL OOZESAND CLAYS, BROWN.
22m
FORAMINIFERAL NANNOFOSSILCHALK OOZES, DARKER SRICHER IN FORAMINIFERANEAR BASE.
36mNANNOFOSSIL CHALK OOZES,PARTLY RECRYSTALLIZEDPINK. 1 6 m
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Figure 2. A composite stratigraphic section of Mid-Atlantic Ridge formations showing formation type and relativethickness. The numbers identify core and location within it where lithologic unit was encountered.
447
Figure 3. Summary of formations, ages and cores recovered from Mid-Atlantic Ridge and Rio Grande Rise sites.
the axis on the other flank of the Ridge than it is tothat of its immediate neighbor (comparing Sites 14 and17, and Sites 15 and 18).
The graphical summary, using formations as strati-graphic units, cannot express the lateral variationswithin each of those units. These can best be expressedby the statistical summaries of the shore-based analysesof the calcium carbonate- and foraminifera-contents.The sites in Tables 1 and 2 have been arranged in theorder of their distance to the Ridge-axis, namely, 16,15, 18, 17, 14, 19 and 20. The variation trends, witha few exceptions, are as follows:
(1) The calcium carbonate-content of each formation,in general, decreases with the distance from the Ridge-axis; the same formation is least calcareous at the sitesfarthest away.
(2) The foraminifera-content also decreases with thedistance from the Ridge-axis; the same formation con-tains, in general, less foraminifera at the sites fartheraway from the Ridge-axis.
These trends are particularly well manifested by theEndeavor Ooze, so that this unit at the lower flank sites(19 and 20) is lithologically similar to the DiscoveryClay at the site close to the Ridge crest (15). Likewise,the Fram Ooze at the lower flank sites is similar to theEndeavor Ooze at the upper flank sites; and, the farthestout Grampus Ooze is similar to the Fram Ooze. Suchlateral lithological variations explain the fact that theformations had to be defined on the basis of verticallithological variations at each site, but not on the basisof lithotypes alone.
TABLE 1Summary of Calcium Carbonate Content a
Formation
Albatross
Blake
Challenger
Discovery
Endeavor
Fram
Gazelle
Grampus
Hirondelle
Site16
90.7(16)
90.7(25)
89.8(17)
-
-
-
-
-
-
Site15
76.8(10)
91.1(10)
80.1(6)
46.9(3)
80.0(8)
-
-
83.8
-
Site18
?
(i)?
7
7
78.9(5)
83.8(9)
-
82.6
-
Site17
86.2(5)
87.6(6)
-
-
76.9(10)
84.9(25)
-
(2)
-
Site14
-
-
-
-
57.6(6)
76.8(15)
-
7
(0)
-
Site19
-
-
-
0.0
42.8(6)
74.5(8)
67.9(14)
78.6(23)
-
Site20
-
-
-
7
37.4(7)
72.8(6)
34.1(10)
-
74.3
Averageof Sites
84.6
89.9
85.0
23.5
62.3
78.6
50.1
81.1
74.3
aNumber of samples analyzed is given in parenthesis.-Denotes formation absent at site.?Denotes value unknown, not sampled, or not sufficiently sampled. (See Chapter 15c);
449
TABLE 2Summary of Sand Fraction in Sediments a
Formation
Albatross
Blake
Challenger
Discovery
Endeavor
Fram
Gazelle
Grampus
Hirondelle
Site16
21A(21)
3.4(25)
7.3(17)
-
-
-
-
-
—
Site15
11.9(13)
3.4(8)
3.3(6)
1.0(4)
1.8(8)
-
-
9.7(6)
—
Site18
?
(i)
?
7
2.2(5)
4.2(9)
-
11.7(11)
—
Site17
25.0(5)
4.4(6)
-
-
1.5(10)
3.5(25)
-
?
(2)
—
Site14
-
-
-
-
0.4(6)
2.1(17)
-
2.0(20)
—
Site19
-
-
-
0.3(11)
0.2(6)
1.1(8)
0.4(12)
1.5(23)
—
Site20
-
-
-
9
(2)
0.5(9)
0.4(6)
0.4(10)
-
2.1(11)
Averageof Sites
19.4
3.7
5.3
0.7
1.1
2.3
0.4
6.2
2.1
Number of samples analyzed is given in parenthesis.—Denotes formation absent at site.?Denotes value unknown, not sampled, or not sufficiently sampled. (See Chapter 14c).
The sedimentation rates of the different formationsvary with lithology. The rates range from about 1.8 cm/t.y. for a foraminiferal chalk ooze unit to 0.02 cm/t.y.for a red clay unit—some two orders of magnitudedifference. As a whole, the chalk ooze formations havebeen accumulated at a net rate of about 1 cm/t.y.,whereas the red clays accumulated at about 0.1 cm/t.y.Since each formation is more argillaceous toward theouter flank, a trend of a corresponding decreasing rateof sedimentation has been recognized.
In order to eliminate the uncertainties resulting fromproduction and dissolution of calcareous planktons onnet accumulation rate, the depositional rate has beencomputed for the non-carbonate component of eachformation. As Table 3 shows, the values still vary from0.07 to 0.33 cm/t.y. (Hirondelle Ooze excepted). Thisvariation is probably real, reflecting varying terrigenous
influx, although the errors resulting from assigningabsolute ages to paleontologically determined stagesmay have exaggerated the difference. The grand averagerate of non-carbonate deposition is 0.16 cm/t.y.,approximately one-tenth that of a chalk ooze, whichcontains some 10 per cent non-carbonate impurities(for example, the Albatross Ooze at Site 16).
A synthesis of the facts so far presented here led to theadoption of a hypothesis that dissolution played themost important role in determining the lithology of theMid-Atlantic Ridge sediments and their net rates ofaccumulation.
That the red clays represent the insoluble residues ofchalk oozes seems obvious, especially when the deposi-tional rate of a sediment is compared to its calciumcarbonate content. On the other hand, whether the
450
TABLE 3Summary of Sedimentation Rates a
Formation
Albatross
Blake
Challenger
Discovery
Endeavor
Fram
Gazelle
Grampus
Hirondelle
Site16
1.8(0.17)
1.2(0.11)
1.0(0.10)
-
-
-
-
-
Site15
1.2(0.28)
0.6(0.05)
0.6(0.12)
0.3(0.16)
?
-
-
?
-
Site18
?
?
?
?
?
2(0.4)
-
2iP 4)
-
Site17
0.5(0.07)
0.45(0.06)
-
-
0.3(0.07)
0.75(0.18)
-
7
-
Site14
-
-
-
-
1.0(0-4)
-
0.45(?)
-
Site19
-
-
-
7
0.35(0.20)
0.6(0.15)
0.3(0.10)
1.2(0.26)
-
Site20
-
—
-
0.02(0.02)
0.15(0.09)
0.4
(o.n)0.2
(0.13)
-
0.1(0.03)
Average
1.2(0.17)
0.8(0.07)
0.8(0.11)
0.16(0.09)
0.27(0.12)
0.96(0.25)
0.25(0.12)
1.2(0.33)
0.1(0.03)
aAll figures are rates in centimeters per thousand years. The figures in parenthesis are rates of sedimentation of non-carbonate component.
—Denotes formation absent.?Denotes rate uncertain.
rarity or absence of foraminifera in a nannofossil chalkooze can be attributed to dissolution is a debatablequestion.
Bramlette (1958) emphasized the role of production indetermining the composition of calcareous planktons ina pelagic ooze. Production rates, no doubt, need to beconsidered. The high foraminiferal content of the Alba-tross Ooze is probably related, at least in part, to thegreater-than-average production rates of foraminiferaduring the Quaternary. Table 1 indicates that thechalk ooze formations on the east flank sites have aslightly higher foraminifera-content than those at acorresponding distance on the west flank—a second-order variation which may reflect the relatively highproduction rates east of the Ridge axis (see Berger,1968). However, several lines of evidence suggest that
calcareous foraminifera are more readily soluble inocean water than calcareous nannofossils, so that thepaucity of foraminifera in the nannofossil sediments ofthe South Atlantic could be attributed largely to differ-ential dissolution. The indications are:
(1) The parallel trend in variations of the foraminiferaand the calcium carbonate content of the Mid-AtlanticRidge formations suggest that dissolution which in-creases the non-carbonate impurities of a sediment alsotends to decrease its foraminifera-content.
(2) The calcareous planktons present in siliceous oozesare commonly exclusively nannofossils, with little orno planktonic foraminifera. This fact suggests that thelast calcareous planktons to go into solution are nanno-fossils, but not foraminifera.
451
(3) Organic coating of calcareous plankton skeletonshas a protective function (Suess, 1969). Smaller nanno-fossil particles may tend to be better protected by.sucha coating than larger, more porous foraminifera tests.
(4) The more soluble foraminifera species are nowbeing readily dissolved at 3000-meter oceanic depth,considerably above the carbonate-compensation depth,so that deeper pelagic oozes include more resistantforms (Berger, 1967). The fact that the only foram-inifera found in some more marly nannofossil oozes ofthe Ridge province are species resistant to dissolution(for example, Globorotalia index, G. suteri in theGazelle Ooze, Site 19) suggests that all but a trace of theoriginal foraminiferal fauna has been removed by dis-solution from such nannofossil sediments.
These arguments may not be definitive. Nevertheless,they are sufficient to justify the adoption of a workinghypothesis that the formations of the Mid-AtlanticRidge province represent originally calcareous sedimentsthat have undergone different degrees of dissolution.Accordingly, five different dissolution facies have beenpostulated, and their physical properties described asexemplified by the South Atlantic sediments:
(1) Alytic facies (Gr. 'aλvToS, not dissolved): Chalkoozes with no evidence of dissolution. The Quaternaryforaminifera oozes of the Rio Grande Rise may belongto this facies.
(2) Eolytic facies (Gr. * ecoç, dawn; λt>Toq, dissolved):Chalk oozes with signs of initial dissolution. The Ridgesediments with 10 per cent, or less, terrigenous matter,but more than 10 per cent foraminifera may belong tothis facies.
(3) Oligolytic facies (Gr. 'okigoç, slight; XVTOÇ, dis-solved): Chalk oozes with signs of slight dissolution,particularly the dissolution of foraminifera. The Ridgesediments with 10 to 30 per cent terrigeneous matter,and less than 10 per cent foraminifera may belong tothis facies.
(4) Mesolytic facies (Gr^Zαoç, moderate; XOTOÇ, dis-solved): Marl oozes with signs of considerable dissolu-tion, only nannofossils and the more resistant forami-nifera species are preserved. The Ridge sediments with30 to 70 per cent terrigenous matter, and less than 3per cent foraminifera may belong to this facies.
(5) Hololytic facies (Gr. '-óλoç, complete; λµroç, dis-solved): Red clays belong to this facies; all calcareousplanktons, except perhaps some nannofossils, have beendissolved. Using these terms, the genesis of the forma-tions can be interpreted as follows:
Albatross Ooze, eolytic.
Blake Ooze, oligolytic.
Challenger Ooze, oligolytic.
Discovery Clay, mesolytic at a more crestal site (15),otherwise hololytic.
Endeavor Ooze, mesolytic.
Fram Ooze, oligolytic.
Gazelle Ooze, mesolytic.
Grampus Ooze, oligolytic, almost eolyticat more crestal sites.
Hirondelle Ooze, mainly mesolytic, but oligolyticat the base ofHole20C.
These interpretations permit the derivation of two sim-ple generalizations on the sedimentary history of theMid-Atlantic Ridge province:
(1) Each time marker is represented by a more nearlyalytic formation toward the Ridge crest, and by a morenearly hololytic formation farther out.
(2) The Tertiary succession undergoes a cyclic change:first from an almost eolytic facies to a hololytic facies,then a return to the eolytic facies, although the secondhalf of the cycle was developed at the upper flank sitesonly.
The different dissolution facies appear to be an expres-sion of the varying depths of the accumulation-sites asrelated to the carbonate-compensation depth. Thepresent compensation-depth for calcite is approximately4500 meters (14,760 feet) in the Pacific (Bramlette,1958; 1961) and is about the same in the South Atlanticat 30°S latitude, as indicated by the distribution of themodern hololytic sediments there. This depth is relatedto an increase in the rate of calcite dissolution, and notto a boundary of equilibrium solubility (Peterson,1966; Hudson, 1966); the latter may be several hun-dred meters deep only (Berner, 1965; Hudson, 1966;Berger, 1967). The level of compensation-depth duringthe past has been a matter of much speculation. Thereis evidence that ocean waters were warmer during theTertiary and Cretaceous than at present (Emiliani andEdwards, 1953; Lowenstam, 1964). However, whetherthe compensation level should be deeper or shallowerfor those warmer oceans is a debatable point.
Hudson (1966) emphasized the effect of temperatureon kinetics and postulated a shallower compensationdepth for the warmer Cretaceous Chalk Sea. Thisunorthodox approach did not consider the effect of thedegrees of undersaturation on kinetics. That the calcitewas being rapidly dissolved in the cold waters below4500 meters (14,760 feet), but not at a more nearlysaturated warmer level is an evidence that departurefrom equilibrium exerts the predominant control onthe calcite-dissolution by ocean waters.
452
Arrhenius (1952), Bramlette (1958), and Riedel andFunnell (1964) all postulated that the calcite-compen-sation-depth may have been considerably greater duringthe Tertiary. Bramlette (1958) suggested a 6700-meter(21,976-foot) compensation-depth for a Tertiary bot-tom-water temperature of 12°C; this figure is probablytoo high. Assuming a non-subsiding ocean floor, Heath(1969) found an apparent maximum compensationdepth of 5200 meters (17,056 feet) some 35 millionyears ago (Oligocene). He recognized, however, thatthis depth may reflect the post-depositional subsidenceof sea floor; there may have been very little changes ofthe actual compensation-depth.
A further complicating factor is the effect of cold,carbon dioxide-rich bottom currents. Berger (1967)suggested the term lysocline as the level at which thesolution rate increases drastically, which is 500 metersor more above the compensation-depth. He cited evi-dence to show that this surface is not horizontal butinclined in the South Atlantic, because the AntarcticBottom Water is responsible for the pronounced abys-sal calcium carbonate dissolution. As this water at 30°Slatitude is confined to a path west of the Mid-AtlanticRidge on its way northward, the lysocline is probablyhundreds of meters shallower on the west side than onthe east side of the Ridge. Also, since depth differencebetween successive dissolution facies is probably of theorder of some hundreds of meters, it would be fool-hardy to make interpretations in terms of absolutedepth, A relative bathymetric scale for the dissolutionfacies with reference to the calcite-compensation-depthis suggested, taking into consideration the distributionof Holocene sediments at 30°S. This scale is:
Alytic facies: some 1500 meters above CCD(calcite-compensation-depth).
Eolytic facies: 500 to 1500 meters above CCD.
Oligolytic facies: 200 to 500 meters above CCD(just below lysocline).
Mesolytic facies: 200 meters or less above CCD.
Hololytic facies: below CCD.
The rapid facies changes hundreds of meters above andbelow the lysocline surface express the fact that dis-solution rate changes rapidly at those depths.
As the Tertiary facies changes of the Mid-AtlanticRidge sediments at 30°S involve changes in relativedepth of 1000 meters (3280 feet) or more, it does notappear warranted to attribute such changes to pastfluctuations in the absolute depth of calcite-compensa-tion. Furthermore, even if Heath's interpretation wereaccepted, a depression of the actual compensation-depth during the early Tertiary should result in a suc-cession of increasingly more alytic facies, if there hadbeen no crustal subsidence. Instead, a progression from
eolytic to hololytic sediments has been found. Conse-quently, these facies changes are interpreted in termsof the chronically changing depth at each depositionalsite.
The elevation difference between the crest and flank ofthe Mid-Atlantic Ridge is 3000 meters (9840 feet). Acrustal segment may have subsided some 3000 metersas it was moved from the crest to the lower flank. Theearly Tertiary eolytic-hololytic succession providesevidence for the subsiding conveyer-belt model ofsedimentation. Eolytic or oligolytic sediments werebeing deposited on a newly-created basalt basement inthe crestal areas of an ancestral Mid-Atlantic Ridge,which stood hundreds or thousands of meters above thecalcite-compensation-depth. Sediments of more holo-lytic facies were accumulated as the sea floor wasconveyed to deeper outer flanks by spreading. The seafloor at Site 20 reached the compensation-depth duringLate Oligocene, but the sea floor at Site 15 reached itonly during the Middle Miocene. The more nearly alyticNeogene sediments were deposited in the crestal areas,which remained high above the calcite-compensationdepth. This interpretative model best explains theobserved facies pattern. The fact that each time markeris represented by a more nearly alytic formation towardthe Ridge crest and by a more nearly hololytic forma-tion farther out can be considered an expression of thetopography of the ancestral Mid-Atlantic Ridge.
A final argument against the hypothesis of major dif-ferences in the calcite-compensation-depth as the causefor the various dissolution facies is the fact that suchfacies are heterochronous. Changes in dissolution faciesresulting from temporal changes in compensation-depthsmay be related to world-wide temperature changes, andshould be more nearly isochronous.
In conclusion, it is stated that the Tertiary sediments ofthe Mid-Atlantic Ridge province have a complex faciespattern, and that this pattern can be best explainedthrough the assumption of changing bottom-depth ateach site during the course of sea-floor spreading.
Interstitial Water Salinity (R. E. B.)
During the Leg 3 of the Deep Sea Drilling cruise, Mio-cene to Cretaceous interstitial water samples were col-lected having salinities ranging from 34.4 to 36.0 ppt(see Table 4). In general, salinity variations were smalland did not seem to relate systematically to depth, ageor formation. Interstitial water salinities at the SierraLeone Rise ranged from 35.2 to 35.8 ppt; Mid-AtlanticRidge flank sediments had interstitial salinities of 34.7to 35.5 ppt; and, the Rio Grande Rise salinities rangedfrom 34.4 ppt to 36.0 ppt. The largest salinity variationwas in samples that were collected from Cretaceous
453
TABLE 4Summary of Leg 3 Interstitial Salinities
Hole/Core/SectionInterval
(cm)
DepthIn Hole
"(m)
Salinityppt
Sediment Age Formation
13-2-413-2-3
15-3-515-9-5
19-1-5
19-5-319-8419-11-3
2OC-1-520C-3-1
21-3-121-5-2
21-8-3
147-150147-150
147-150147-150
147-150
147-150147-150148-150
147-150147-150
147-150147-150
147-150
27140
44139
7
80110137
738
7797
128
35.235.8
35.235.2
35.2
34.935.234.7
35.534.7
34.436.0
35.5
Pliocene < Eocene?Eocene
Lower Pliocene
Pleistocene <Upper Oligocene
Upper EoceneMiddle EoceneMiddle Eocene
Lower OligoceneEocene
Upper CretaceousLower MaestrichtianUpper CampanianCampanian
Blake OozeDrilling Artifact
Discovery Clay
Gazelle OozeGrampus OozeGrampus Ooze
Endeavor OozeGazelle Ooze
Hirondelle OozeNear Inoceramus zone
sediments of the Rio Grande Rise. Parts of the Creta-ceous section were lithified, but the samples werefrom the unlithified material.
In general, the salinity of the ocean water can vary withdepth, thermal conditions, currents, etc. Interstitialwaters from sediments can be changed by diffusion,filtering through clay-like membranes, diageneticchanges, lateral and vertical migrations, and alterationby organisms. In order to draw any conclusions fromthe salinity variation in respect to the geologic historyof the basins (for instance, low salinity corresponds toshallow depth, etc.), it would be necessary to assumeother variables were constant. These variables combinedwith the very small statistical population make it almostimpossible to draw conclusions with any certainty.
Physical Properties of the Sediments (R. E. B.)
The lithologic formations appear to be supported bymeasurements of natural gamma radiation and, to alesser extent, the mass physical properties of the sedi-ments, such as: wet-bulk density, porosity, sediment-sound velocity and a thermal conductivity". All of thefollowing measurements are from disturbed sedimentsamples; thus, they do not accurately represent in situconditions. They probably reflect nominal packings,and porosities of the particular grain size of the sedi-ment. However, they are still of interest as they maybe approximations to the corresponding properties in
unconsolidated sediments in situ. Consistent variationsin the physical properties can usually be seen to corre-late with certain formational sequences, when theseproperties are plotted against depth, and the forma-tional boundaries are drawn in.
Natural gamma radiation relates to the lithology, as itgenerally distinguishes argillaceous formations. In fine-grained sediments, clay and zeolitic minerals have ion-exchange capacities which may hold gamma-ray emit-ting isotopes; in addition, these minerals have highpotassium contents, thus producing relatively higherradiation than the pure siliceous or calcareous oozes.Examples are the argillaceous Discovery Clay, En-deavor and Gazelle Oozes compared with the non-argillaceous Blake and Challenger Oozes.
Natural gamma radiation ranges from zero to 2600counts per 1.25 minutes scan over a 7.62-centimetercore segment, with an average of about 200 to 400counts. The higher natural gamma radiation can berelated to the presence of clay minerals, zeolites,phosphates and, possibly hematite, dolomite, and someopaque minerals. When two or more of these mineralswere combined, high gamma counts were usually re-corded. A relatively high natural gamma radiation atSite 16 decreases exponentially from the sedimentsurface. The time lapse of this strata suggests that thiscould be caused by gamma-ray emitting Th2 3 0 orU 2 3 4 isotopes or possibly other radionuclides withsimilar half lives.
454
The lithologic formations appear to have distinct nat-ural gamma signatures within the Mid-Atlantic Ridgesedimentary province (Figure 4). The Albatross Oozeemits a slightly higher natural gamma-radiation countat the surface which immediately and exponentiallydecreases with depth. The remainder of the Albatrossand the underlying Blake Oozes have hole averagedcounts of 300 to 100 per 7.62-centimeter core segmentper 1.25 minutes, respectively. The Challenger Ooze,beneath the Blake, normally has equivalent or slightlyhigher relative average hole counts of 200 to 400,which is also distinctively less than the Discovery Clay.
The Discovery Clay, Endeavor Ooze, and Fram Oozesequence emits higher hole averaged gamma counts,which decrease sequentially from 1100 to 1700 (Dis-covery Clay), down to 350 to 1600 (Endeavor Ooze),and 250 to 500 (Fram Ooze).
Less radiation is emitted from the Fram Ooze, however,than from the underlying Gazelle Ooze, which hasaverage-hole counts of 500 to 1200. Beneath the GazelleOoze, the Grampus and Hirondelle Oozes emit relativelyless radiation with average hole counts of about 200 to400, and 400, respectively.
Other mass physical properties, such as, porosity,sound velocity and heat conductivity, show distinctsignatures within formations, but their validity isquestionable. Their validity depends upon the sedimentcores being undisturbed samples; thus they are alwayssuspect of being artifacts resulting from the drillingand coring operations.
Porosity and wet-bulk density appear to be primaryproperties that correlate to the lithologic formations(Figure 5), with the sediment sound velocity andthermal conductivity relationships being generally re-lated to wet-bulk density and porosity. In general,porosity decreases with increasing depth, and was typi-cally lowest in sediments that were immediately over-lying the basalt basement. Porosity consistently variesinversely with wet-bulk density, sound velocity andthermal conductivity. The porosity decrease appears tobe a combined function of compaction, sediment andfossil-type and grain-size distribution. The lithologicformations usually have distinct porosity and wet-bulkdensity differences.
The Albatross, Blake and Challenger Oozes have pro-gressively decreasing moderate average porosities of72 to 48 per cent (density: 1.52 to 1.88 g/cc), but thesequence lacks distinguishing signatures. The DiscoveryClay, being of finer grain-size, has moderate but higherporosities of 57 to 72 per cent, averaging 66 per cent(density: 1.48 to 1.70 g/cc); the Endeavor Ooze alsohas moderate average hole porosities ranging from 52
to 65 per cent (density: 1.56 to 1.63 g/cc) with aslightly lower overall average of 60 per cent, but stillgreater than the low average porosities of 50 to 59 percent (density: 1.65 to 1.85 g/cc) in the underlyingFram Ooze. The Braarudosphaera Chalk in the FramOoze has densities up to 2.00 g/cc with a correspondingporosity of 43 per cent. The Gazelle and GrampusOozes have low to moderate average porosities of 48to 62 per cent (density: 1.62 to 1.82 g/cc) and 57 to59 per cent (density: 1.71 to 1.84 g/cc), respectively.Water content, obviously, has interrelationships similarto those of porosity.
Uncorrected shipboard sediment sound velocities rangefrom 1.47 to 1.72 km/sec, averaging about 1.54 km/sec.In general, these velocities are lower than had beenexpected, but they appear to be in reasonable agree-ment with the velocities determined from a comparisonof the depths of cored and observed seismic reflectors.Although these velocities generally increased withincreasing depth (below sediment surface), they appearto be primarily a function of the sediment type. Forexample, the Albatross and Blake OozeS have moderateaverage-hole sound velocities of 1.50 to 1.53 km/sec,slightly increasing with depth, but without any distinctsignatures; but, the Challenger Ooze tends to haveslightly higher velocities with hole averages of 1.53 to1.55 km/sec (Figure 6). While the Discovery Clay haslow average hole velocities of 1.49 to 1.51 km/sec, theEndeavor, Fram and Gazelle Oozes have moderate tohigh average-hole velocities of 1.50 to 1.54, 1.53 to1.56, and 1.51 to 1.58 km/sec, respectively. Theunderlying Grampus Ooze has slightly lower averagehole velocities of 1.53 to 1.56 km/sec, and the bound-ary between the Gazelle and Grampus Oozes appearsto be characterized by the lowest part of a broadvelocity decrease and increase. The Hirondelle Oozetends to have low to high velocities, averaging 1.55km/sec.
Penetrometer measurements were too sparse to defi-nitively correlate penetration to other mass physicalproperties of the sediments or the lithologic forma-tions with any certainty. This is the result of coredisturbance and the complete penetration of the pene-trometer needle. Yet, in general, penetration appearsto irregularly decrease with depth, and the minimumvalues correlate directly to sound velocity. In a fewcases, needle penetration inversely correlates withporosity (directly with wet-bulk density) and naturalgamma radiation. This high gamma radiation-low needlepenetration relationship may result from the plasticityof the clay- and zeolite-type minerals, and grain-sizedistribution. The porosity relationship may be second-ary, the primary relationship being with the plasticityof clay-type minerals. Clay minerals usually comprisethe finer-grained fraction of sediments, which, inrespect to unconsolidated fine-grained sediments,
455
GAMMA RADIATION VERSUS LITHOLOGIC FORMATIONS
DEPTHM
o
I0 -
20 -
30 -
40
50
60
70 -
80 -
90 -
100
110
120
130
140
150
160
170
180
SITE 20 (c)COUNTS / 3* CORE / l'/4 MIN.0 1000 2000 3000
t
i ,.
r* ALMTITO3
iN.
CHALLENSEROOZE
r
X ----
---
30
40
50
60
70
80
90
100
U0
120
130
140
150
160
170
180
Figure 4. Summary of Natural Gamma Radiation Measurements.
DEPTH M
Or
SITE 209/CC
14 1.6 1.8 2.0
SITE 19
1.4 1.6 I.B 2.0
WET- BULK DENSITY VERSUS LITHOLOGIC FORMATIONSSITE 14 SITE 15 SITE 16
1.4 1.6 IB 2.0 1.4 1.6 1.8 2.0 1.4 1.6 I.B 2.0
SITE IB
1.4 1.6 1.8 2.0
SITE 179/CC
1.4 1.6 1.8 2.0 DEPTH M
"|0
-
-
-
70
80
90
100
110
120
130
140
150
160
170
180
Figure 5. Summary of Porosity and Wet-Bulk Density Measurements.
SOUND VELOCITY VERSUS LITHOLOGIC FORMATIONS
DEPTHM
0 -
I0 -
20 -
30 -
40 -
50 -
60 -
70 -
80 -
90 -
100
110
120
130
140
150
160
170
180 -
SITE 20
1.5 1.6 1.7 km/sec.
SITE 19
1.5 1.6 1.7 km/sec.
SITE 14
1.5 1.6 1.7 km/sec.
SITE 15
1.5 1.6 1.7 km/sec.
SITE 16
1.5 1.6 1.7 km/sec
ΠANTICRIDGEAXIS
I DISCOVERT CLAYC -.*| ENDEAVOR OOZE ^
X 5 OISCOVERY\ C L * T /
SITE 18
15 1.6 1.7 km/sec.
SITE 17
15 1.6 1.7 km/sec.
y ALBATROSS
*~ E
/ I ENDEAVOR AND' / GRAMPUS OOZES
/ CORED AT I7A AND I7B
DEPTHM0
I0
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
Figure 6. Summary of Sound Velocity Measurements.
normally have greater porosities. The pure nannofossilchalk oozes tended to be penetrated easily, as theyacted like an aggregate of marble with very littleplasticity.
Thermal conductivities range from 2.0 to 3.5 × I0"3
cal °C"1 cm"1 sec"1, and average about 2.9 X I0"3
These data have an apparent inverse correlation withporosity and, to a greater degree, with the water contentby weight. In general, thermal conductivity appears toincrease with increasing depth, which was, also, afunction of porosity, and which is in turn controlledby sediment type and compaction.
In the Mid-Atlantic Ridge sedimentary province, theAlbatross Ooze, Blake Ooze and Challenger Ooze haverelatively increasing thermal conductivities, respectively,although absolute conductivities vary at each site. Theseformations have the following range of averages: TheAlbatross Ooze ranges from 2.5 to 2.9 × I0"3; theBlake Ooze ranges from 2.8 to 3.0 × I0"3 ; and, theChallenger Ooze averages about 3.0 × I0"3 . The under-lying Discovery Clay has a markedly lower conductivitythan the Challenger Ooze. However, the DiscoveryClay, Endeavor Ooze and Fram Ooze also have se-quential relative increases. Thermal conductivities inthe Discovery Clay range from 2.0 to 2.9 × I0"3 ;Endeavor Ooze averages range from 2.1 to 3.0X I0"3 ;and, Fram Ooze averages range from 2.9 to 3.2 X I0"3 .Meanwhile, the underlying Gazelle and Grampus Oozesgenerally have lower relative conductivities (2.3 to3.2 X I0"3) than either the Fram Ooze or HirondelleOoze (2.9 to 3.1 X I0"3).
Sea Floor Spreading (R. P. V. H. & A. E. M.)
From an examination of the paleontological ages ofthe fossils contained therein, the age of the sedimentsrecovered from immediately above the basalt basementat each Mid-Atlantic Ridge site has been shown toincrease with distance from the Ridge-axis. In order tocalculate the rate of sea-floor spreading from thesedata, it is necessary to know the distance each site hasmoved from its point of origin on the Ridge-axis. Theprocedure of calculating this distance is not alwaysstraightforward, because the Mid-Atlantic Ridge hasnumerous offsets and changes in strike along its length(Heezen and Tharp, 1965).
Although the Ridge has not been surveyed in detailnear latitude 30°S, there are sufficient ship tracksalong which geographical data have been recorded toenable a rough determination of the location of theRidge-axis. Figure 7 shows the location of the axisbased upon geographical information, such as, thecharacteristic crestal topography (Vacquier and VonHerzen, 1964), magnetic anomalies associated with itand the locations of earthquake epicenters (Stover,1968). From Figure 7, a simple way to determine the
distance a site has moved away from the Ridge-axis isto measure the distance from the site to the nearestpoint on the Ridge-axis. If this procedure is followed,the displacement near 29.5°S suggests that Sites 15and 16 are associated with Ridge Segment C, Sites 17and 18 with Segment B, and all other sites are nearlyequidistant from either B or C. Linear distances of thesites from the Ridge-axis are summarized in Table 5,with errors in the distances representing a subjectiveuncertainty in the axis location. The paleontologicalages and their estimated uncertainties have been deter-mined utilizing the chronostratigraphic chart of Berg-gren (1969a); see Chapter 2, Figure 1. A more recentcompilation (Berggren, 1969b), based upon radiometricdates within continental sequences, gives somewhatdifferent ages for some paleontological stages, particu-larly during mid-Tertiary.
From a hypothesized fitting of the continents ofAfrica and South America, Bullard et al. (1965) showedthat the present positions of these continents can beexplained as a rotation about an axis at 44°N, 30.6°W.Other authors (Morgan, 1968; LePichon, 1968) haveproposed rotation axes farther north, based upon thetrends of linear magnetic anomalies and fracture zones,and they have inferred differential rates of spreadingalong the Ridge-axis. The rotation axes farther northmay be representative of recent spreading, whereasthat at 44°N may represent the rotation axis for theaverage motion since the beginning of continentalrifting.
For a rigid rotation on a sphere, the trace of the motionof any part of the surface would lie along a smallcircle with the rotation axis at the center. The distancetraversed by any site on the spreading ocean floorwhich is rotating away from the Mid-Atlantic Ridgeaxis would be slightly different from that obtainedsimply by measuring the linear distance to the nearestRidge-axis. More importantly, the original location ofany site along the Ridge-axis will be different in thetwo cases.
The last two columns of Table 5 show the distances ofdrilling sites from the ridge axis for both linear androtational spreading hypotheses, using the rotationalaxis at 62°N, 36°W proposed by Morgan (1968), forthe latter. The major difference in distances of sitesfrom the Ridge-axis is for Site 17, which for the caseof rotation is associated with Segment C (Figure 7)of the Ridge-axis. Site 18, however, remains associatedwith Segment B, so that the difference in distances ofSites 17 and 18 from the Ridge-axis is greater thantheir actual geographic separation. Similar results areobtained with the other proposed rotation axes for theSouth Atlantic.
A fracture zone that displaces the Ridge crest by,perhaps, extending eastward between Sites 17 and 18
459
30° W 20*
30° -
1
—
Site21
-
i
20
i i
19 14
• *
15
• ARGO (Vacquier andVon Herzen, 1964)
A Lamont vessels
I^DSDP Leg 3
! 1
1
16 ;
/•
j
A
/A
\/AI
/c
1
—
18 ^17
-
139{
Figure 7. Location of Leg 3 sites relative to the axis of the Mid-Atlantic Ridge.
TABLE 5 a
Magnetic Anomaly Number and Ages, Paleontological Ages, andDistances of Mid-Atlantic Ridge Sites from the Axis
Site No.
16
15
18
17
14
19
20
21C
Magnetic AnomalyNo.
5
6
—
—
13-14
21
30
—
MagneticAge of
Basement(m.y.)
9
21
_ a
34-38a
38-39
53
70-72
PaleontologicalAge Sediment
Above Basement(m.y.)
11 ± 1
24 ± 1
26 ± 1
33 ±2
40+1.5
49 ±1
67 ±1
>76
Distance
Linear
191 ±5
380 ±10
506 ± 20
643 ± 20
727 ± 10
990 ±10
1270 ±20
1617 ±20
from ridge axis (km)
Rotation at62°N, 36°W
221 ±20
422 ± 20
506 ± 20
718 + 20
745 ± 10
1010+10
1303 ±10
1686 ±10
fThe number of the magnetic anomaly and its age has been taken from Heirtzler, et al. (1968).Location of these sites within the characteristic magnetic anomaly pattern is uncertain.
cBasement rock not reached at Site 21.
may explain this apparent uncertainty. Some inde-pendent support for this interpretation comes from therecording of unusually large depths between Sites 17and 18, which are sometimes characteristic of fracturezones (Menard, 1964; Heezen and Tharp, 1965). If thefracture zones of the South Atlantic trend somewhatnorth of due E, a direction consistent with the pro-posed axes of rotation, then this fracture zone may bethe same as that displacing the Ridge crest at 291/á°Slatitude between Segments B and C (Figure 7).
When the values in Table 5 are plotted (see Figures 8and 9), they show an overall linear fit of ages versusdistance from the Ridge-axis. When the nearest Ridge-axis is selected for measurement of distances, theaverage spreading rate, back to 66 million years B. P.,for these drilling sites appears to be slightly less than2 cm/yr (Figure 8). For a rotation axis in the NorthAtlantic, the best-fitting spreading rate appears to benearly equal to 2 cm/yr (Figure 9). The small differencebetween these two interpretations probably is repre-sentative of the uncertainty in such data. Likewise, ifthe more recent paleontological ages of Berggren(1969b) are accepted, the average rates may be slightlyincreased in both cases, but will still be near 2 cm/yr.
Sites 17 and 18 were selected on the east side of theMid-Atlantic Ridge to test symmetry of the spreadingpattern. The sediment ages at these sites seem to fitwell with the pattern for the western flank Mid-AtlanticRidge sites in Figure 8, but they may be significantlyyounger for the rotation axis interpretation (Figure 9).The lack of fit for the eastern flank sites in the lattercase may result from some E-W asymmetry in thespreading pattern or from fracture zone displacements,which may occur between the Ridge crest and Sites 17and 18. Unfortunately, the magnetic anomaly patternon the east flank of the ridge is not well enough deter-mined to be of much help in resolving this minorproblem.
The scatter of points in Figures 8 and 9 may be inter-preted to indicate past changes in the sea-floor spread-ing rate (Langseth, et al, 1966; Ewing and Ewing,1967). For shorter term linear spreading, the range ofspreading rates corresponding to variations in slopebetween points of Figure 8 is from 1.2 to 6.1 cm/yr.For rotational spreading (Figure 9), the range is 0.4 to4.2 cm/yr. Similar ranges result if the alternate pale-ontological dates of Berggren (1969b) are used. Becauseof the uncertainty of the data, it does not appearwarranted to conclude that sea-floor spreading hasactually varied that much over periods of 5 to 10million years. Further, there seems to be little supportin the data for cessation of sea-floor spreading forperiods of more than 10 million years, as has beenproposed for the Miocene-5.5 to 22.5 million years-and the Paleocene—53 to 65 million years (LePichon,
1968). This would necessitate greater ranges in spread-ing rates than have been observed if periods of low orno spreading are postulated.
Another possible explanation for some of the scatterin the data may be the normal distribution about theRidge-axis in which new rocks are injected in the seafloor (Matthews and Bath, 1967; Harrison, 1968).For some clearly-defined magnetic profiles acrossridges, the standard deviation of the normal distributionappears to be as small as 3 kilometers; on others, itappears to be about 5 kilometers. For the SouthAtlantic, where the pattern appears relatively regular,the standard deviation is probably not more than 5kilometers. In this case, 95 per cent of the sea floorhas an origin within 10 kilometers of its positionpredicted from ideal sea-floor spreading (±2 standarddeviations, or ±10 kilometers, to either side of theRidge axis). In other words, there is only a 5 per centprobability that any part of the sea floor originatedmore than 10 kilometers away from its most probableposition within the sea-floor spreading scheme, assum-ing that the process has remained constant throughoutthe time required to form the sea floor. This additional10-kilometer uncertainty might explain some of thedata scatter in Figures 8 and 9.
Additional relative displacements of the ocean floorbetween drilling sites due to fracture zones (Figure 7)may contribute to the scatter of the data in Figures 8and 9. The relatively small scatter implies that any suchdisplacements have been less than 50 to 100 kilometers.It is most unlikely that displacements between sitescould be balanced out by changes in spreading rates.Therefore, such data scatter seems well within theuncertainties of the methods used in this study; and,the simpler interpretation of constant spreading rateback to at least 66 million years B. P. seems as likelyfrom these data.
A South Atlantic spreading half-rate of 2 cm/yr hasbeen deduced earlier from studies of the marine mag-netic field anomalies (Dickson, et al, 1968). Themagnetic anomaly ages predicted for the drilling sitesare listed in Table 5. Reasonably good agreementexists between predicted magnetic ages and thosedetermined from paleontology, especially consideringthe uncertainty in locating the site in relation to thecenter of the magnetic anomaly. Perhaps the onlysignificant departures from the predicted ages are atSites 19 and 20. Both sites had ages somewhat youngerthan predicted. This suggests that the proposed geo-magnetic time scale (Heirtzler, et al, 1968) may requiresome minor revision for ages of 50 million years, orgreater, in the same direction but probably not aslarge as that suggested by LePichon (1968).
Overall, it is concluded that the sea floor of the SouthAtlantic has been spreading at an essentially constant
462
4 0 0 1600800 1200
DISTANCE, km
Figure 8. Plot of age of sediment immediately above basementas a function of nearest distance to Ridge axis.
4 0 0 1600800 1200
DISTANCE, kmFigure 9. Plot of age of sediment immediately above basement as
a function of distance from Ridge axis as determined from anassumption that the Ridge is spreading about an axis of rota-tion at 62° N, 36° W.
463
rate for the past 67 million years. Accuracy of the datadoes not permit a valid interpretation of changingspreading rates. Further, the spreading half-rate of2 cm/yr, as determined from the drilling, is in agree-ment with and has provided a critical test for bothsea-floor spreading and magnetic stratigraphy.
HISTORY OF SOUTH ATLANTIC BASIN(K. J. H. &J. E. A.)
To interpret the geologic history of the South Atlantic,two major premises have been adopted. Namely:
(1) The sea floor of the South Atlantic has beenspreading apart since the Cretaceous at an averagehalf-rate of 2 cm/yr with new basalt crust being con-tinually created at the crestal region; and
(2) The South Atlantic formations represent dissolu-tion fades, the depths of which are related to thecalcite-compensation-depth.
The validity of these premises has been discussedpreviously. A graphic presentation of the interpretationis shown in Figure 10. In this figure a series of strati-graphical sections is reconstructed for five referencedates: (a) end of Eocene (37 million years), (b) end ofOligocene (26 million years), (c) end of Lower MioceneAquitanian (23 million years), (d) end of Miocene(6 million years), and (e) present. The following pro-cedures were used:
(1) The average spreading rate from the reference dateto present is computed. This rate differs somewhatfrom the average determined from the age of thesediments above basement because stratigraphy mustbe taken into consideration. For example, Site 15,which is now about 400' kilometers from the axis,includes Aquitanian sediments and, therefore, shouldappear in the stratigraphic-section for the referencedate 23 million years before present (Figure 10c). Thisis only possible if a spreading rate of 1.5 cm/yr, orless, is assumed. An alternative is, of course, to changethe absolute age of the Aquitanian to accommodatethe 2.0 cm/yr average rate. However, in order not toprejudice interpretation by changing the time-scalemidstream, a slightly variable spreading rate has beenassumed for the construction of the interpretativediagrams.
(2) The width of the new crust, created since thereference date, has been computed, and it is appro-priately shown in the diagrams. Accordingly, the loca-tion of each drill site is moved closer to the Ridge-axisin the section for that date. Those sites which are closerto the Ridge-axis than the half-width of the new crustare eliminated from the section. The spreading-rate hasbeen so computed that no sediments older than thereference date are present at the eliminated sites.
(3) The bathymetry of the drill site, during the past isdetermined by relating the depositional depth of theuppermost sediment at that time to the calcite-compensation-depth, using the quantitative relationspostulated in an earlier section on the lithology. Thecompensation-depth is now about 4500 meters (14,760feet) deep, and it was either about the same or severalhundred meters deeper during the Tertiary.
(4) The elevation of the Ridge-axis at each referencedate is extrapolated, based mainly on the interpretedbathymetry of the nearby sites.
(5) The lithostratigraphy is shown by a columnarsection for each site. The time lines are drawn betweensuch columns to indicate the age of the formations andthe relation between lithostratigraphy and chronology.
The distances between sites, the bathymetry and thestratigraphical thickness have been shown by differentscales for effective illustration; the vertical exaggera-tion of water-depth is 100-fold, and the vertical exag-geration of sediment-thickness is 5000-fold. Thus con-structed, the top line of each section illustrates the pasttopography, and the bottom illustrates the sediment-basalt contact.
Figure 10 shows that the sedimentary regime in theSouth Atlantic basin has not changed radically duringthe last 80 million years. A relatively thin blanket ofchalk oozes, with some marl oozes and red clays hasbeen covering the Ridge. No turbidity-current depositswere found here; such sediments probably were trappedin basins near the continents (such as, the Brazilianand Angola Basins). The formations of the Ridgeprovince have been distinguished mainly by theirdifferent degrees of dissolution. In contrast, however,production rate and current erosion may play a moreimportant role in the Rio Grande Rise province.Whereas unconformities between the Ridge sedimentsresulted largely from dissolution, the hiatus of theRise sediments has been almost certainly the work ofmechanical removal. Fragmentary data, especially thosefrom Site 22, suggest that the Rise remained high,while the Ridge widened and, at times, deepened duringthe Tertiary.
Assuming an average spreading rate of 2 cm/yr, it ispossible to estimate that the separation of SouthAmerica and Africa began some 130 million yearsago, or during the Early Cretaceous. The discovery ofuppermost Jurassic deep-sea sediments in the NorthAtlantic east of the Bahama Islands (Initial Reports ofthe Deep Sea Drilling Project, Volume I) is consistentwith such a postulate. Geological comparison ofBrazil and Gabon show a striking similarity of theAptian and older rocks in these countries; and, thus,their separation has been dated as middle Cretaceous,
464
Albatross OozeBlake OozeChallenger OozeDiscovery Clay
RIDGE AXISCCD
+ 2 5 0 0 m
Endeavor OozeFram Ooze
Ga Gazelle OozeG r Grampus Ooze[Rj Hirondelle Ooze
§HH§| Pleistocene
^jgj>jl Pliocene & younger
pI-I-I-j Pliocene
j Miocene & younger
Miocene
UJJJJJJUJU Lower Miocene
H> jj Oligocene
^jiftj Paleocene
K•iff&S•l Maestrichtian
y))'^i! Campanian
1 UK. | Upper Cretaceous
Vetical exaggeration -of water depth: 100 Xof sediment thickness;
5000 X
water depthabove compen-sation depth
-500J
(e)Present stratigraphy
Elevation of ridge axis500 m above calcitecompensation depth.
(d)End Miocene(6 my ago)Spreading rote
2.0 cm/year
(c)Lower Miocene
End Aquitanian(23 my ago)Spreading rate
1.5 cm/year
(b)End Oligocene(26 my ago)Spreading rate
1.8 cm/year
(a)End Eocene(37 my ago)Spreading rate
19 cm/year
1000 500 0DISTANCE FROM AXIS-KM
500.
Figure 10. Reconstruction of history of the South Atlantic Basin, taking into consideration sea floor spreading,paleontological ages and lithological formations.
465
or some 120 million years ago (Allard and Hurst, 1969).Unfortunately, the results of the Leg 3 drilling cannotgive a definitive answer on the age of separation of thecontinents since the oldest dated sediments in theEquatorial Atlantic are Senonian (< 90 million years),and in the South Atlantic they are Campanian (< 80million years). Yet, the evidence does indicate that theSouth Atlantic came into being either during LateJurassic or Early Cretaceous, and the ocean was alreadysome 3000 kilometers wide as the Campanian (oldestcored sediment) at Site 21 was deposited.
The oldest Cretaceous sediments of the Rio GrandeRise province were deposited in relatively shallowwaters, either on a fragment of a foundered continent,or on a group of subsiding guyots. The oldest coredsediments of the Ridge province are Maestrichtian, orsome 70 million years old. By the end of the Eocene(Figure 10a), the Ridge became a distinct topographicfeature. Still, the Atlantic was not sufficiently deep forhololytic red clays to have been deposited. The veryslow rate of deposition of the Maestrichtian, Paleoceneand Lower Eocene sediments in the Ridge provinceand the existence of disconformities indicate consid-erable dissolution of calcium carbonate, particularlyof planktonic foraminifera, so that the HirondelleFormation at Site 20 is considerably thinner and con-tains much less foraminifera than its counterpart on theRio Grande Rise.
The Grampus Ooze first appeared during the MiddleEocene at Site 19. From then on until Early Miocenethis oligolytic and locally eolytic unit invariably in-cludes the first sediments deposited upon the newlyformed basalt crust (see Figure 10a, b & c). TheGrampus Ooze can thus be considered the crestal depos-its on the ancestral Mid-Atlantic Ridge. The Middle andUpper Eocene flank deposits constitute the mesolyticGazelle Formation. Site 20 must have deepened to thecompensation depth during the Late Eocene time, sothat the Upper Eocene deposits are virtually missingthere, probably having been removed by dissolution.
There may have been a slight uplift of the oceanbottom, or a slight depression of the absolute depthof calcite-compensation during the Early Oligocene.In any case, an oligolytic Fram Ooze was superposedon the flank above the mesolytic Gazelle. The ratesof the Fram Ooze sedimentation are also significantlyhigher. By the end of the Oligocene, however, theflank sites (Sites 19 and 20) dropped below the calcite-compensation depth, while the crestal area stood some1000 meters (3280 feet) higher to permit eolytic sedi-mentation (the Grampus Ooze of Site 18).
A noteworthy sedimentation event took place duringthe middle Oligocene, when the Braarudosphaera Chalkwas deposited. This chalk unit extended across the
entire basin at this time, and it is thickest on the RioGrande Rise. The reason for this impressive bloom ofBraarudosphaera rosa and the exclusion of other cal-careous planktons is unknown. The condition wasapparently recurrent, because more than one suchchalk layer was found at Site 20.
A general subsidence must have commenced duringthe Early Miocene. The Lower Miocene sediments atSite 15, which then lay close to the Ridge axis, are anoligolytic Endeavor Ooze, suggestive of a deeper Ridge-axis. Accordingly a height for the crest of calcite-compensation depth plus 400 meters has been postu-lated, namely: 4100 meters (13,448 feet) assumingthe present calcite-compensation depth, or 4600 meters(15,088 feet) assuming Heath's (1969, Figure 5) calcite-compensation depth. The subsidence led also to anencroachment of the compensation-depth laterallytoward the Ridge-axis. The subsidence continued dur-ing the Middle Miocene, when the oligolytic EndeavorOoze was superposed by a mesolytic Discovery Clayunit at Site 15. Solution rates on this deep Mid-AtlanticRidge of low relief must have been rapid. The MiddleMiocene is either represented by a red clay barren ofcalcareous planktons, or by a solution unconformityat the Ridge sites farther out than Site 15.
The condition of Middle Miocene hololytic sedimen-tation was not restricted to the South Atlantic. TheMiocene at Site 10 on the flank of the northern Mid-Atlantic Ridge shows largely zeolitic red clays (InitialReports of the Deep Sea Drilling Project, Volume II).Mid-Tertiary red clays barren of fossils in the CapeVerde Basin (Site 12, Initial Reports of Deep SeaDrilling Project, Volume II), and on the Sierra LeoneRise (Site 13) were also deposited during a time-spanincluding the Middle Miocene. It is possible that thegreat depth of the Middle Miocene Atlantic may havecoincided with a period of slower sea-floor spreading.This is supported for the period from the end ofAquitanian to the end of Miocene where the sedimen-tation pattern suggests a slower than 2.0 cm/yr rate.
The Mid-Atlantic Ridge was rejuvenated during theLate Miocene. By the end of the Miocene, the newcrestal area stood at about 1500 meters (4920 feet)above the calcite-compensation depth (at 300-meter—9840-foot—depth, assuming the present calcite-compen-sation depth). At Site 16, located near the Ridge crest,the Challenger Ooze is almost eolytic. This unit is defi-nitely oligolytic at Site 15, some 200 kilometers to thewest. Farther out at Sites 14 and 17, the sea floorremained below the compensation depth where theUpper Miocene was represented by a red clay and byunconformity, respectively.
The uplift of the Ridge may have continued duringthe Plio-Pleistocene. The present crestal elevation of2-kilometer depth is probably as high as ever. The
466
eolytic and oligolytic Albatross Ooze is now beingdeposited to a distance some 650 kilometers awayfrom the Ridge axis (Site 17). Farther out, at Sites 14,19 and 20, the ocean floor has remained largely belowthe compensation depth since Late Oligocene or EarlyMiocene. Younger calcareous plankton deposits areeither absent, or are only present as a very thin veneer.
Throughout the Tertiary the Rio Grande Rise mayhave stood relatively high, probably not much differentfrom the present 2000-meter (6560-foot) depth. Alyticsediments were deposited, only to be removed locallyby active bottom-current erosion.
From this interpretation of the geologic history of theSouth Atlantic, there evolves a model of nearly constantsea-floor spreading with the ocean floor having signifi-cant vertical movements in the crestal region. Changesin the Ridge elevation may be related to small, butsignificant, changes in the spreading rate. The idea ofa lapse in sea-floor spreading has been suggested byEwing and Ewing (1967) based upon interpretations ofthe seismically determined sediment-thickness. In addi-tion, the narrowness of the high heat flow at the axialregion of the Mid-Atlantic Ridge led Langseth andothers (1966) to suggest an increase in the rate ofspreading during the last 10 million years. Schneiderand Vogt (1968) also postulated non-steady Atlanticspreading, on the basis of additional evidence furnishedby discontinuities in Ridge topography and in ampli-tudes of magnetic anomalies. The idea that sea-floorspreading virtually stopped for 30 or 40 million yearsduring the Tertiary does not appear tenable in the lightof the drilling results, which show an approximatelyconstant spreading rate of 2 cm/yr since the Cretaceous.On the other hand, the possibility of variations inspreading rate has not been excluded. In fact, acceptingthe time-scale used on board ship, the spreading ratefor different Cenozoic Stages may be computed to varyby a factor of 2 or more. Such variations are subject tointerpretation and they have been attributed in part tothe imperfect knowledge of the absolute ages of pale-ontologically determined stages and zones. On theother hand, unmistakable evidence of a non-steadysea-floor depth and, perhaps, spreading is furnished by
the sedimentary record. If there only had been steadyspreading and flank subsidence, the geologic columnat each site should be a regular succession of graduallydeepening sediments. The superposition of oligolyticand eolytic Upper Miocene and younger sedimentsover sediments that had been accumulated in muchdeeper waters (at Sites 15 and 17) indicates a lateCenozoic rejuvenation of the Mid-Atlantic Ridge.
The cause of crestal relief is related, no doubt, to theunusually light mantle, which now underlies the Ridge(LePichon, et al, 1965; Talwani, et al, 1965). Thislight mantle density is in turn related to the abnormallyhigh flows measured on the axial portion of the Ridge(Vacquier and Von Herzen, 1964; Langseth, et al,1966). As a newly created crust was conveyed aside toflank regions, underlain by a mantle of normal density,or the mantle was altered (thermal, chemical) to amore normal structure, subsidence must have occurred.The crestal uplift and the flank subsidence are probablynot simply the result of thermal expansion alone.Mantle-density variations caused by phase changes orother processes, as well as, isostatic subsidence must betaken into consideration to explain the relief of oceanbottoms (Hsu, 1965; Hsu and Schlanger, 1968; Schnei-der and Vogt, 1968).
In conclusion, it is emphasized that deviations from theaverage linear spreading rate of 2 cm/yr are probablysmall as shown by Figures 8 and 9). On the other hand,small variations may be related to significant topo-graphical evolutions, which in turn leave a remarkableimprint on the sedimentary record. There are stillsome uncertainties, about the possible variations of thespreading rate and the Ridge height during the earlyTertiary. However, several lines of evidence supportthe concept that the spreading was relatively slowerduring the Middle Miocene and has been rejuvenatedsince Late Miocene (some 15 or 10 million years ago).This rejuvenation may have raised the crestal axis bysome 2000 meters (6560 feet) at a rate of about 15cm/t.y. This rate of vertical movement is about thesame order of magnitude as that observed in thePacific (Hsu and Schlanger, 1968), and it is practicallyidentical to the uplifting rate of the Colorado Plateau(Holmes, 1964, p. 1096).
467
REFERENCES
Abernethy, S. H., 1965. Improved equipment for apulse method of sound velocity measurement inwater, rock, and sediment. U. S. Navy ElectronicsLaboratory, San Diego, California, Technical Memo-randum-824.
Allard, G. 0. and Hurst, V. J., 1969. Brazil-Gabon geo-logic link supports continental drift. Science. 163,528.
American Commission on Stratigraphic Nomenclature,1961. Code of stratigraphic nomenclature. AAPGBull. 45, 645.
Arrhenius, G. O. S., 1952. Sediment cores from theeast Pacific. Rept. Swedish Deep-Sea Exped. 5,fasc. 1,227 p.
ASTM, 1965. Standard method of test for penetrationof bituminous material. American Society for Test-ing of Materials, Designation D-5-65. 1007.
Banner, F. T. and Blow, W. H., 1965. Progress in theplanktonic foraminiferal biostratigraphy of the Neo-gene. Nature. 208(5016), 1164.
Berger, W. H., 1967. Foraminiferal ooze: solution atdepths. Science. 156, 383.
, 1968. Planktonic foraminifera: selectivesolution and paleoclimatic interpretation. Deep-SeaRes. 15,31.
Berggren, W. A., 1965. Some problems of Paleocenelower Eocene planktonic foraminiferal correlations.Micropaleontology. 11,278.
, 1969. Rates of evolution in some Cenozoicplanktonic foraminifera. Micropaleontology. 15 (3),351.
, 1969b. Cenozoic chronostratigraphy, plank-tonic foraminiferal zonation and the radiometrictime scale. Manuscript in preparation.
Berner, R. A., 1965. Activity coefficients of bicarbon-ate, carbonate and calcium ions in sea water. Geo-chim. Cosmochim. Acta. 29, 947.
Blow, W. H., 1959. Age, correlation and biostratigraphyof the Upper Tocuyo (San Lorenzo) and Pozonformations, Eastern Falcon, Venezuela. Bull. Am.Paleontol. 39 (178), 67.
, 1969. Late Middle Miocene to Recent plank-tonic foraminiferal biostratigraphy. In Proceedingsof the First International Conference of PlanktonicMicro fossils; Geneva 1967. P. Brönnimann andH. H. Renz (Eds.). Leiden (E. J. Brill) 199.
Bolli, H. M., 1957a. The genera Globigerina and Globo-rotalia in the Paleocene—Lower Eocene LizardSprings formation of Trinidad, B. W. I. U. S. NatMuseum Bull. 215, 61.
, 1957b. Planktonic foraminifera from theEocene Navet and San Fernando formations ofTrinidad, B. W. I. U. S. Nat. Museum Bull. 215,155.
, 1957c. Planktonic foraminifera from theOligocene-Miocene Cipero and Lengua formations ofTrinidad, B. W. I. U. S. Nat. Museum Bull. 215, 97.
, 1966. Zonation of Cretaceous to Pliocenemarine sediments based on planktonic foraminifera.Boll. Asoc. Venezolana Geol. Mineria Petrol. 9 (1), 3.
Bolli, H. M. and Bermudez, P. J., 1965. Zonation basedon planktonic foraminifera of middle Miocene toPliocene warm-water sediments. Boll. Asoc. Venezo-lana Geol. Mineria Petrol. 8, 121.
Boyce, R. E., 1968. Electrical resistivity of modernmarine sediments from the Bering Sea. /. Geophys.Res. 73, 4759.
Bramlette, M. N., 1958. Significance of coccolithopho-rids in calcium carbonate deposition. Bull. Geol.Soc. Am. 69, 121.
, 1961. Pelagic sediments. In Oceanography.M. Sears (Ed.). Washington, D. C. (AAAS Publ.) 345.
, 1965. Massive extinctions in biota at the endof Mesozoic time. Science. 148(3678), 1696.
Bramlette, M. N. and Martini, E., 1964. The greatchange in calcareous nannoplankton fossils betweenthe Maestrichtian and Danian. Micropaleontology.10(3), 291.
Bramlette, M. N. and Sullivan, F. R., 1961. Coccolitho-phorids and related nannoplankton of the earlyTertiary in California. Micropaleontology. 7(2), 129.
Bramlette, M. N. and Wilcoxon, J. A., 1967. MiddleTertiary calcareous nannoplankton of the Cipero sec-tion, Trinidad, W. I. Tulane Studies Geol. 5 (3), 93.
Brier, C, Bennin, R. and Rona, P. A., 1969. Preliminaryevaluation of a core scintillation counter for bulk-density measurement in marine Sediment cores./. Sediment. Petrol. 39 (4), 1509.
Brujewicz, S. W. and Zaitseva, E. D., 1958. On thechemistry of Bering Sea sediment. Inst. Okeanol.Akad. Nauk, CCCP. 26, 8.
, 1959. Chemical features of marine interstitialsolutions (Abstract). Preprints of abstracts of papersto be presented at the First International Oceano-graphic Congress, Am. Assoc. Adv. Sci., New York,958.
Brujewicz, S. W. and Zaitseva, E. D., 1960. On thechemistry of the sediments of the northwestern partof the Pacific. Inst. Okeanol. Akad. Nauk, CCCP.42,3.
Bukry, D., 1969. Upper Cretaceous coccoliths fromTexas and Europe. Univ. Kansas Paleontol. Contr.,Protista, art. 2, 1.
Bukry, D. and Bramlette, M. N., 1968. Stratigraphicsignificance of two genera in Tertiary calcareousnannoplankton. Tulane Studies Geol. 6 (4), 149.
, 1969. Personal communications.
Bullard, E. C. and Day, A. A.,. 1961. The flow of heatthrough the floor of the Atlantic Ocean. Geophys.J. 4, 282.
Bullard, E., Everett, J. E. and Smith, A. G., 1965. Thefit of the continents around the Atlantic, a sympos-ium on continental drift. Phil. Trans. Roy. Soc.London, A. 258,41.
468
Bullard, E. C, Maxwell, A. E. and Revelle, R., 1956.Heat flow through the deep sea floor. Advan. Geo-phys. 3, 153.
Deep Sea Drilling Project 1968-69. Preliminary Reports:Leg 2, Leg 2 and Leg 4. (all unpublished reports).
Dickson, G. O., Pitman, III, W. C. and Heirtzler, J. R.,1968. Magnetic anomalies in the South Atlantic andocean floor spreading. /. Geophys. Res. 73, 2087.
Dietz, R. S., 1961. Continent and ocean basin evolutionby spreading of the sea floor. Nature. 190 (4779),854.
Doig, Ronald, 1968. The natural Gamma-ray flux: insitu analysis. Geophysics. 32, 2, 311.
Eames, F. E., Banner, F. T., Blow, W. H. and Clarke,W. J., 1962. Fundamentals of Mid-Tertiary Strati-graphical Correlation. Cambridge (University Press).
Emiliani, C. and Edwards, G., 1953. Tertiary oceanbottom temperatures. Nature. 171, 887.
Evans, H. B. and Cotterel, C. H., 1968. Gamma-rayattenuation density scanner. In DSDP, 1968, CoreDescription Manual. Part V: Shipboard Treatmentof Cores. Unpublished technical manual.
Evans, H. B., Lucia, J. A. and others, 1968. NaturalGamma-ray core scanner. In DSDP, Core Descrip-tion Manual. Part V: Shipboard Treatment of Cores.Unpublished technical manual.
Ewing, M. and Ewing, J., 1967. Sediment distributionon the Mid-Ocean Ridges with respect to spreadingof the sea floor. Science. 156, 1590.
Foreman, H. P., 1968. Upper Maestrichtian Radiolariaof California. Paleontol. Assoc, London, Spec.Paper. 3, 82 p.
Gartner, S., Jr., 1968. Coccoliths and related calcar-eous nannofossils from Upper Cretaceous depositsof Texas and Arkansas. Univ. Kansas Paleontol.Contr. 48, 56 p.
Hamilton, E. L., 1956. Low sound velocities in highporosity sediments. /. Acoust. Soc. Am. 28, 16.
., 1959. Thickness and consolidation of deepsea sediments. Bull. Geol. Soc. Am. 70, 1399.
, 1960. Ocean basin ages and amounts oforiginal sediments. /. Sediment. Petrol. 30, 370.
, 1963. Sediment sound velocity measurementsmade in situ from bathyscaph TRIESTE. /. Geophys.Res. 68, 5991.
, 1964. Consolidation characteristics and re-lated properties of sediments from experimentalMohole (Guadalupe Site). /. Geophys. Res. 69 (20),4257.
, 1965. Sound speed and related physicalproperties of sediments from experimental Mohole(Guadalupe Site). Geophysics. 30 (2), 257.
Hamilton, E. L., Shumway, G., Menard, H. W. andShipek, C. J., 1956. Acoustic and other physicalproperties of shallow-water sediments off San Diego./. Acoust. Soc. Am. 28, 1.
Harrison, C. G. A., 1968. Formation of magneticanomaly patterns by dyke injection. /. Geophys.Res. 73,2137.
Hay, W. W., 1964. Utilisation stratigraphique des Dis-coastéridés pour la zonation du Paleocene et de1'Eocene inferieur. Bur. Réch. Gėol. Mineral. Mém.28,885.
, 1967. Nomina conservanda proposita. Taxon.16, 240.
Hay, W. W. and Mohler, H. P., 1967. Calcareous nanno-plankton from early Tertiary rocks at Pont Lavau,France, and Paleocene-early Eocene correlations./. Paleontol. 41 (6), 1505.
Hay, W. W., Mohler, H. P., Rota, P. H., Schmidt, R. R.and Boudreaux, J. E., 1967. Calcareous nannoplank-ton zonation of the Cenozoic of the Gulf Coast andCaribbean-Antillean area and transoceanic correla-tion. Trans. Gulf Coast Assoc. Geol. Soc. 17, 428.
Hays, J. D., Saito, T., Opdyke, N. D. and Burckle, L. H.,1969. Pliocene-Pleistocene sediments of the Equa-torial Pacific: their paleomagnetic, biostratigraphicand climatic record. Bull. Geol. Soc. Am. 80, 1481.
Heath, G. R., 1969. Carbonate sedimentation in theabyssal Equatorial Pacific during the past 50 millionyears. Bull Geol. Soc. Am. 80, 689.
Heezen, B. C. and Tharp, M., 1965. Tectonic fabric ofthe Atlantic and Indian Oceans and continentaldrift, a symposium on continental drift. Phil. Trans.Roy. Soc. London. 258, 70.
Heirtzler, J. R., Dickson, G. O., Herron, E. M., Pitman,III, W. C. and LePichon, X., 1968. Marine magneticanomalies, geomagnetic field reversals, and motionsof the ocean floor and continents. /. Geophys. Res.73(6), 2119.
Hess, H.H., 1962. History of ocean basins. In PetrologicStudies. (A volume to honor A. F. Buddington)Boulder (Geol. Soc. Am.) 599.
Holmes, A., 1964. Principles of Geology. Great Britain(Ronold Press).
Horn, D. R., Horn, B. M. and Delach, M. N., 1968. Sonicproperties of deep-sea cores from the north Pacificbasin and their bearing on the acoustic provinces ofthe north Pacific. Technical Rept #10, CU-10-68NAVSHIPSN00024-67-C-1186, December 1968.
Hsu, K. J., 1965. Isostasy, crustal thinning, mantlechanges, and the disappearance of ancient landmasses. Am. J. Sci. 263, 97.
Hsu, K. J. and Schlanger, S. O., 1968. Thermal historyof the upper mantle and its relation to crustal historyin the Pacific Basin. Proc. XXVII Intern. Geol.Congr. Prague. 1,91.
Hudson, J. D., 1966. Speculations on the depth rela-tions of calcium carbonate solution in Recent andancient seas. Marine Geol. 5, 473.
Ingelman, K. R. and Hamilton, E. L., 1963. Bulk densi-ties of mineral grains from Mohole samples (Guada-lupe Site). /. Sediment. Petrol. 33, 474.
469
JOIDES, 1965. Ocean drilling on continental margin.Science. 150, 709.
Kermabon, A., Gehin, C, Blavier, P. and Tonarelli, B.,1969. Acoustic and other physical properties of deep-sea sediments in the Tyrrhenian Abyssal Plain.Marine Geol. 7 (2) 129.
Koczy, F. F., 1963. Age determination in sediments bynatural radioactivity. In The Sea. M. N. Hill (Ed.).New York (Interscience) 3, 816.
Koczy, F. F. and Rosholt, J. N., 1962. Radioactivity, inoceanography. In Nuclear Radiation in Geophysics.H. Israel and A. Krebs (Eds.). Berlin (Springer-Verlag) 18.
Lai, D. and Peters, B., 1967. Cosmic ray produced radio-activity on the earth. In "Cosmic Rays II": Encyc-lopedia of Physics. S. Fulgge and K. Sitte (Eds.).New York (Springer-Verlag) 46, 551.
, 1966. Crustal structure of the mid-oceanridges 5. Heat flow through the Atlantic Oceanfloor and convection currents. /. Geophys. Res.71, 5321.
Laughton, A. S., 1954. Laboratory measurements ofseismic velocities in ocean sediments. Proc. Roy.Soc. London, A. 222, 336.
, 1957. Sound propagation in compactedocean sediments. Geophysics. 22, 233.
LePichon,X., 1968. Sea-floor spreading and continentaldrift. /. Geophys. Res. 73, 3661.
LePichon, X., Houtz, R. E., Drake, C. L. and Nafe,J. E., 1965. Crustal structure of the mid-oceanridges, 1, Seismic refraction measurements. /. Geo-phys. Res. 70, 319.
Lowenstam, H. A., 1964. Paleotemperatures of thePermian and Cretaceous periods. In Problems .inPaleoclimatology. A. E. M. Nairn (Ed.). London(Interscience) 227.
Luterbacher, H. P. and Silva, I. P., 1964. Biostratigrafiadel limite Cretaceo—Terziario nelFAppennino Cen-trale. Riv. Ital. Paleontol. 70 (1), 67.
Lynch, E. J., 1962. Formation Evaluation. New York(Harper and Row) 422.
Manheim, F. T., 1967. Evidence for submarine dis-charge of water on the Atlantic continental slope ofthe southern United States, and suggestions forfurther search. Trans. TV. Y. Acad. of Sci. Series IL29 (7), 839.
, 1966. A hydraulic squeezer for obtaininginterstitial water from consolidated and unconsoli-dated sediments. U. S. Geol. Surv. Profess. Papers.550-C, 256.'
Matthews, D. H. and Bath, J., 1967. Formation of mag-netic anomaly pattern of the Mid-Atlantic Ridge.Geophy. J. 13, 349.
Mauchline, J. and Templeton, W. L., 1964. Artificialand natural radioisotopes in the marine environment.In Oceanography and Marine Biology, (an AnnualReview). H. Barns (Ed.). London (George Allen &Unwin Ltd.) 2, 229.
Maxwell, A. E., 1958. "The outflow of heat under thePacific Ocean" (Ph.D. dissertation, Scripps Institu-tion of Oceanography, La Jolla, California).
Menard, H. W., 1964. Marine Geology of the Pacific.New York (McGraw-Hill).
Moore, D. G. , 1964. Shear strength and related proper-ties of sediments from experimental Mohole (Guada-lupe Site). /. Geophys. Res. 69, 4271.
Moore, D. G. and Shumway, G., 1959. Sediment thick-ness and physical properties: Pigeon Point Shelf,California. /. Geophys. Res. 64 (3), 367.
Morgan, W. J., 1968. Rises, trenches, great faults, andcrustal blocks./. Geophys. Res. 73, 1959.
Nafe, J. E. and Drake, C. L., 1957. Variation withdepth in shallow and deep water marine sedimentsof porosity, density and the velocities of compres-sional and shear waves. Geophysics. 22, 523.
, 1963. Physical properties of marine sedi-ments. In The Sea. M. N. Hill (Ed.). 3, 794. NewYork (Interscience).
Parker, F.L., 1967. Late Tertiary biostratigraphy (plank-tonic foraminifera) of tropical Indo-Pacific deep-seacores. Bull. Am. Paleontol. 52, 115.
Paterson, N. R., 1956. Seismic wave propagation inporous granular media. Geophysics. 21, 691.
Pessagno, E. A., 1967. Upper Cretaceous planktonicforaminifera from the western Gulf Coastal Plain.Palaeontographica Americana. 5, 245.
Peterson, M. N. A., 1966. Calcite rates of dissolutionin a vertical profile in the central Pacific. Science.154, 1542.
Philips, J. D., Berggren, W. A., Bertels, A. and Wall, D.,1968. Paleomagnetic stratigraphy and micropaleon-tology of three deep-sea cores from the centralNorth Atlantic Ocean. Earth & Planet. Sci. Letters.4, 118.
Prospero, J. M. and Koczy, F. F., 1966. Radionuclidesin oceans and sediments. In Encyclopedia of Ocean-ography. R. W. Fairbridge (Ed.). New York (Rein-hold Publishing Corp.) 730.
Ratcüffe, E. H., 1960. The thermal conductivity ofocean sediments./. Geophys. Res. 65, 1535.
Rhodes, D. F. and Matt, W. E., 1966. Quantitativeinterpretation of gamma ray spectral logs. Geophy-sics. 31, 410.
Richards, A. F., 1962. Investigations of Deep-SeaSediment Cores, Part II, Mass Physical Properties.Technical Rept. 106, U. S. Navy HydrographicOffice. Washington, D. C, 146.
Riedel, W. R., 1959. Oligocene and Lower MioceneRadiolaria in tropical Pacific sediments. Micropaleon-tology. 5(3), 285.
Riedel, W. R. and Funnell, B. M., 1964. Tertiary sedi-ment cores and microfossils from the Pacific Oceanfloor. Quart. J. Geol. Soc. London. 120, 305.
Schlumberger, 1966. The formation density log. InLog Interpretation Principles Schlumberger (Ed.).Paris (Keeran-Servant).
470
Schneider, E. D. and Vogt, P. R., 1968. Discontinuitiesin the history of sea-floor spreading. Nature. 217,1212.
Schreiber, B. C, 1968. Sound velocity in deep sea sedi-ments./. Geophys. Res. 73, 1259.
Shishkina, O. V., 1966. The geochemistry of halogenesin marine and oceanic sediments and interstitialwaters. (Abstract). Second International Oceano-graphic Congress, Moscow. Russia (Publishing HouseNaukam) 333.
Shishkina, O. V. and Bykova, 1961. Chemical compo-sition of silt interstitial water in the Atlantic Ocean.Soviet Oceanography. 25, 151.
Shumway, G., 1958. Sound velocity vs. temperature inwater-saturated sediments. Geophysics. 23, 494.
, 1960. Sound speed and absorption studiesof marine sediments by a resonance method, Part I,Part II. Geophysics. 25, 451, 659.
Siever, R., Garrels, R. M., Kanwisher, J. and Berner,R. A., 1961. Interstitial waters of recent marinemuds off Cape Cod. Science. 134, 1071.
Siever, R., Beck, K. C. and Berner, R. A., 1965. Com-position of interstitial waters of modern sediments./. Geol. 73(1),39.
Stover, C. W., 1968. Seismicity of the South Atlanticocean./. Geophys. Res. 73, 3807.
Stradner, H., 1963. New contributions to Mesozoicstratigraphy by means of nannofossils. Sixth WorldPetrol. Congr. Frankfurt. Sec. 1, Paper 4, 167.
Suess, Erwin. 1968. Interaction of Organic CompoundsWith Calcium Carbonate - I: AssociationPhenomena and Geochemical Implications.Unpublished Ph.D. dissertation. Lehigh University,Bethlehem, Penn.
Sutton, G. H., Berckheimer, H. and Nafe, J. E., 1957.Physical analysis of deep sea sediments. Geophysics.22,779.
Talwani, M., LePichon, X. and Ewing, M., 1965.Crustal structure of the mid-ocean ridges, 2, Com-puted model from gravity and seismic refractiondata./. Geophys. Res. 70, 341.
Vacquier, V. and von Herzen, R. P., 1964. Evidencefor connection between heat flow and the Mid-Atlantic Ridge magnetic anomaly. /. Geophys. Res.69, 1093.
von Herzen, R. P. and Maxwell, A. E., 1959. Themeasurement of thermal conductivity of deep-seasediments by a needle-probe method. /. Geophys.Res. 64, 1557.
Winokur, R. S. and Chanesman, S., 1966. A pulsemethod for sound speed measurements in coredocean bottom sediments. U. S. Naval OceanographicOffice,!. M. 66-5. 10.
471