deep sea drilling project initial reports volume 84debris flow deposits where the strength of the...

14. MASS MOVEMENT ALONG THE INNER WALL OF THE MIDDLE AMERICA TRENCH,COSTA RICA1

Miriam Baltuck, Department of Geology, Tulane UniversityElliott Taylor, Department of Oceanography, Texas A&M University

andKristin McDougall, Branch of Paleontology and Stratigraphy, U.S. Geological Survey, Menlo Park, California2

ABSTRACT

The sediments of Deep Sea Drilling Project Site 565 and University of Texas Marine Science Institute Cores IG-24-7-38to -42 taken on the landward slope of the Middle America Trench exhibit characteristics of material subject to reworkingduring downslope mass flow. These characteristics include a generally homogeneous texture, lack of sedimentary struc-tures, pervasive presence of a penetrative scaly fabric, and presence of transported benthic foraminifers. Although thesefeatures occur throughout the sediments examined, trends in bulk density, porosity, and water content, and abruptshifts in these index physical properties and in sediment magnetic properties at Site 565 indicate that downslope sedi-ment creep is presently most active in the upper 45 to 50 m of sediment. It cannot be determined whether progressivedewatering of sediment has brought the material at this depth to a plastic limit at which sediment can no longer flow(thus resulting in its accretion to the underlying sediments) or whether this depth represents a surface along whichslumping has occurred. We suspect both are true in part, that is, that mass movements and downslope reworking accu-mulate sediments in a mobile layer of material that is self-limiting in thickness.

INTRODUCTION

The movement of sediment on the continental slopeis usually attributed to turbidity currents and slump-ing—single events to periodic emplacements of sedi-ments as a result of mass gravity flows (e.g., Doyle andPilkey, 1979). These mechanisms and other types of massgravity flows (fluidized-liquidized flow, grain flow, anddebris flow) classified by Middleton and Hampton (1976)can be recognized in trie sedimentary record by charac-teristic sedimentary textures and structures (Fig. 1). Com-monly these deposits have abrupt or scoured basal con-tacts with the underlying sediment. Other features suchas grading, laminations, dish structures, grain orienta-tion, and grain sorting are variable depending on themechanism of emplacement. Sediments from the conti-nental slope offshore Costa Rica (DSDP Site 565 andUTMSI cores IG-24-7-38 to -42; Fig. 2) have featuressimilar to a grain flow deposit, but the differences sug-gest a more continuous mechanism of sedimentation.

Sediments recovered at Site 565 on the the Costa Ri-can slope are massive and generally lack grading. Al-though these features are common to a grain flow de-posit, the textural homogeneity and paucity of sedi-mentary structures of the Site 565 sediments suggest acontinuously operating sedimentary mechanism: down-slope creep. Such a process would disrupt depositionalsedimentary features and could produce fabric and tex-tures diagnostic of large-scale sedimentary reworking.

von Huene, R., Aubouin, J., et al., Init. Repts. DSDP, 84: Washington (U.S. Govt.Printing Office).

2 Addresses: (Baltuck) Department of Geology, TUlane University, New Orleans, Louisi-ana 70118; (Taylor) Department of Oceanography, Texas A&M University, College Station,Texas; (McDougall) Branch of Paleontology and Stratigraphy, U.S. Geological Survey, MenloPark, California.

We examined Costa Rican slope sediments from DSDPSite 565 and from five piston cores collected in 1978during a University of Texas Marine Science Institute(UTMSI) cruise to evaluate continuous downslope massmovement as a significant mode of slope sediment trans-port and to identify a means of documenting this trans-port (Table 1). We examined seismic profiles of the slope,sediment texture and structure, physical properties, pa-leomagnetics, in situ and transported benthic foramini-fers, and sediment fabric.

Several lines of evidence emerged from our study thatindicate the effects of plastic grain flow, or creep: (1) thegeneral lack of sedimentary structures, (2) generally ho-mogeneous texture, (3) overconsolidation of sediment inupper part of the sediment column at Site 565, (4) scat-tered paleomagnetic orientations indicating a disturbedsection, (5) mixed assemblages of benthic foraminiferscontaining both in situ fauna and species transportedfrom upslope, (6) presence of a penetrative fabric heredescribed as a "scaliness," and (7) high sediment accu-mulation rates such as might be anticipated from amass-ing sediment from upslope. Thus we envision a simplemodel of material in continuous creeping movement inthe upper sediment of the slope. As transport reworksthe sediment it dewaters and approaches its "plastic lim-it"; as sediment flow is impeded at the base of the moremobile upper meters, the sediment is plastered along thefirmer underlying material. Small-scale creep could leadto slope failure and larger-scale slumping events, espe-cially in a tectonically active area such as the MiddleAmerica Trench, where earthquake activity will contrib-ute another mechanism for triggering these events.

GEOLOGIC SETTINGThe Middle America slope is an extension of the Cen-

tral American continental framework that is overlain by

551

M. BALTUCK, E. TAYLOR, K. McDOUGALL

Turbidity current Fluidized/liquefied flow Grain flow

J

Rippled or flat top

Ripple drift micro-x-laminationLaminated

Good grading("distributiongrading")

Flutes, tool markson base

Sand volcanoes or flat topconvolute laminationFluid escape "pipes"

Dish structure?

Poor grading("coarse tail grading")

?Grooves, flame andload striationsstructures on base

Debris flow

Flat top

No grading?

Massive,grain orientationparallel to flow

Reverse gradingnear base?Scours, injectionstructures

Irregular top(large grains projecting)Massive,poor sorting,random fabricPoor grading, if any("coarse tail")Basal zone of"shearing"Broad "scours"?Striations at base

Figure 1. Structure and textures of deposits from single-mechanism mass gravity flows. No vertical scale is implied (from Middleton and Hampton,1976).

11

10c Costa Rica

88° 87° 86° 85° 84°

Figure 2. Location map of Site 565 and of track lines and the five piston cores collected during UTMSI Cruises IG 24 and 29.

83C

Table 1. Site and piston core locations.

Piston coreor site

IG-24-7-38

IG-24-7-39

IG-24-7-40

IG-24-7-41

IG-24-7-42

DSDP Site 565

Latitude, longitude

09°22.4'N84°58.1'W09°38.0'N85°19.8'W09°31.0'N85°27.0'W09°15.5'N85°31.2'W09°9.4'N85°35.O'W09°43.69'N86°05.44'W

Waterdepth(m)

1179.58

104.24

1243.58

2467.05

3427.17

3111

Recoveredsediment

(m)

10.09

2.70

6.63

9.00

4.93

287.28

Cenozoic marine sediments. A thick layer of Quaternaryand Neogene fine-grained sediments derived from theweathering of onshore Tertiary and Quaternary volcanicsdrapes over most of the slope. Site 565 from DSDP Leg84, and the UTMSI piston cores (IG-24-7-39 to -42) aresituated on the Costa Rican continental margin betweentwo submarine canyons. UTMSI core IG 24-7-38 wastaken in the southernmost of these two canyons.

Site 565 was drilled at a depth of 3111 m and recov-ered a thick section of Pleistocene through Miocene sed-iment that comprises a single sedimentary unit of mas-sive mud and mudstone. The Pleistocene section of Site565 includes the upper 90 to 100 m of sediment (Sec-tions 565-1 to -10) and portions of planktonic forami-niferal Zones N23 and N22 (Stone and Keller, this vol-

552

MASS MOVEMENT

ume) and calcareous nannoplankton zones Emiliana hux-leyi and Geophyrocapsa oceanica (Filewicz, this volume).Similar massive Pleistocene and Miocene muds and mud-stones were also encountered in the UTMSI cores. ThePleistocene assemblages correlate with planktonic fora-miniferal Zone N23. The Miocene assemblage (sampleIG-24-7-42, 493 cm) is from a white chalk deposited atabyssal depths under oceanic conditions away from thecontinental margin and will not be considered further inthis study. The UTMSI cores recovered 2.7 to 10.0 m ofsediment and were drilled at depths ranging from 104 to3427 m (Fig. 2; Table 1).

SEISMIC PROFILES

Seismic reflection profiles of the continental slope atSite 565 (UTMSI Profile CR7) appear in figures 10 and11 of the Site 565 report (this volume). Drilling penetrat-ed only the upper 328 m of slope sediment. This upperunit shows few bedding reflectors and these are lateral-ly discontinuous and of irregular trend. Other UTMSIprofiles of the Costa Rican slope show similarly disrupt-ed bedding in the upper sediments. Thus on a largescale, seismic profiles show a lack of bedding consistentwith the massiveness of the mud and mudstones recov-ered in cores from this site.

SEDIMENT TEXTURE AND STRUCTURE

Grain size analysis for sediments at Site 565 (Taylorand Bryant, this volume) show a silty clay texture with agradual trend toward increased clay content with depth.There are a few distinct ash beds, and mottling and bur-rowing occur throughout the hole. Graded bedding (clay-ey silt to silty clay) and faint current laminations also oc-cur sporadically. The sand is generally dispersed through-

out the sediment rather than concentrated in specificbeds. Small pebbles (under 15 mm in diameter) of semi-consolidated mudstone are also dispersed throughoutthe massive mud, a characteristic of continental margindebris flow deposits where the strength of the matrixmud prevents internal sorting or grading (Stanley andUnrug, 1972). The Unsorted dispersal of sand- and silt-sized grains throughout the sediment sections may be afurther expression of matrix strength.

PHYSICAL PROPERTIES

A rapid change in physical properties occurs in theupper 20 to 30 m, as indicated by a sharp decrease inwater content and porosity and increase in bulk density(Fig. 3). Water content decreases from 60 to 40% fromthe sediment surface to 20 m sub-bottom. Below 20 mthe water content decreases to about 30% at a depth ofabout 45 to 50 m. Contact marked by physical proper-ties occurs at this last depth and is indicated by an abruptincrease in water content that fluctuates around 35% toa depth of 130 m.

The compaction of sediments under gravitational load-ing results in a decrease of water content and porositywith consequent increases of bulk density and shearstrength. Skempton (1970) studied the rate of increasingshear strength as related to burial depth for marine sedi-ments. He defines a range of shear strength versus effec-tive overburden pressure that varies between 0.2 and 0.5for sediments having undergone normal consolidation.Results of shear strength measurements of Sites 565 whenplotted against effective overburden pressure yield a ra-tio of 0.7. This value is slightly higher than the rangedefined by Skempton and possibly reflects the effect ofanother process, such as downslope transport and parti-

40

80

120

160

•? 200

240

280

320

Water content

n 30 40 50

Porosity (%)

40 50 60 70 80 90 1.1

Bulk density (Mg/m3)

1.3 1.5 1.7 1.9

Shear strength (kPa) Grain size (%)

2.1 20 40 60 80 ? 1,° 2,° 3,°

CaCO

2

8o

A Air-comparison pycnometer—consolidation sampleo Gravimetric• 2-min. GRAPE

Figure 3. Downhole trend of index geotechnical properties at Site 565 (from Taylor and Bryant, this volume). Profiles of physical properties for sam-ples from DSDP Site 565 reflect the abrupt changes occurring in sediments above an apparent contact at 40 m. The grain size analysis profileexhibits the fairly homogenous texture of sediments recovered from this site.

553


cle rearrangement, aiding in the consolidation of sedi-ments.

Despite the lithologic homogeneity and massive ap-pearance of the sediment, there is a distinct break inphysical properties at a sub-bottom depth of 45 to 50 m.We suspected this was related to plastic reworking ofsediment during downslope creep and sought tangibleevidence of sediment transport in benthic foraminiferalassemblages.

PALEOMAGNETICS

Paleomagnetic properties of the sediments at Site 565are described in the Site 565 report (this volume). Thenatural remanent magnetism intensities are initially high(10~5 emu) but decrease to I0" 6 emu at about 20 m sub-bottom. Between 20 and 50 m, the inclinations are scat-tered and the magnetic intensity decreases; from 50 to100 m, the intensity decreases rapidly to about 2 × I0" 7

emu, and below 100 m intensity is so low that initial de-magnification steps destroyed any measurable remanentmagnetism. The absence of measurable magnetic incli-nation is attributed to the rearrangement of magneticparticles because of bioturbation, slumping, or plasticmass flow of sediments. Such mixing would be expectedto destroy the original magnetic orientation, but if thesediment is sufficiently fluid to permit reorientation ofmagnetic particles, the remagnetization would occur inalignment with the ambient field (Verosub et al., 1979).

Experiments by Verosub et al. (1979) with sedimentremagnetization indicate that the critical water contentnecessary to permit remagnetization is greater than theinitial water content of the uncompacted sediment, thatis, disturbed sediment will not remagnetize to the ambi-ent magnetic field even if it has a high water content.The reworked sediment of the Costa Rican slope thusprobably retains its original magnetic intensity and incli-nation in the smaller fragments of sediment created dur-ing reworking. The addition of magnetic vectors occur-ring in the smaller sediment fragments could result incancellation of magnetic vectors and produce the mag-netic incoherence observed in these sediments. Whetheror not this occurs in the sediment at Site 565, it is inter-esting that trends are particularly definable in the upper40 to 50 m of the section.

BENTHIC FORAMINIFERS

Transported foraminifers are common throughout Site565 and the UTMSI cores, frequently averaging greaterthan 50% of the total assemblage. These assemblagesindicate transport is the result of both continuous down-slope movement and rapid displacement associated withslides, debris flows, or turbidity currents. This conclu-sion is based on biofacies analysis, which relates watermasses, physical properties, water depths, and benthicforaminiferal species or assemblages with specific waterdepth ranges (Bandy, 1961; Ingle, 1980; Douglas, 1979,1981; also see McDougall, this volume, for a more com-plete discussion of the techniques and methods used).Several biofacies may be represented in a single sample,thus, the paleobathymetry is determined by the deepestbiofacies present and reflects the minimum depth of de-

position. Paleoecologic interpretations are affected byselective preservation and predation, and by transport,mixing, and environmental changes. Selective preserva-tion and predation alters the species abundances, andcan be observed directly from the condition of the testsor by comparisons of species present with dissolutionindexes for benthic foraminifers (Corliss and Honjo,1981). Transport and mixing alters the composition ofthe assemblages. Although the effects of transport andmixing are suggested by the condition of the tests, theyare more easily detected along with environmental shiftsby changes in diversity, concentrations of small or simi-lar-sized specimens, or changes in the biofacies pat-terns. Changes in the biofacies patterns include offsetsor lobes of a particular biofacies (Bandy, 1961; Douglas,1979, 1981; Ingle, 1980). Peak abundances expressedgraphically as lobes of shallower water species (Fig. 5) inone or two samples indicate slides or debris flows, where-as offsets or large-scale changes in biofacies abundancestend to reflect environmental shifts or more continuousdownslope movement. The Pleistocene section of DSDPSite 565 and the UTMSI cores contain examples of bothmodes of transport, but a slow gradual downslope move-ment appears to have been the dominant mechanism.

Biofacies analysis of the UTMSI cores indicates inmost cases that there has been little change between thelate Pleistocene and the present site of deposition (Figs.4, 5; Table 2). An environmental change is noted in Core41 between samples at 300 and 500 cm. Foraminiferssuggest that deposition occurred in the abyssal biofaciesfor samples 500 to 900 cm, and in the lower bathyal bio-facies for samples 100 to 300 cm. Although decreasingwater depths cannot be discounted, the faunal change inCore 41 between 300 and 500 cm is believed to resultfrom an adjustment of the water masses, following max-imum glaciation.

Overall, the biofacies in the UTMSI cores indicate agradual and continuous downslope movement of ben-thic foraminiferal faunas (Fig. 5). Shallower water bio-facies are represented in each of the cores, but the abun-dance generally decreases with water depth. There is anincrease in abundance of shelf species in core IG-24-7-42,which may be the result of ponding of sediment at thebase of the slope and in the Trench. The only inner shelfspecies that reach the slope are several specimens of Quin-queloculina lamarkiana. These are broken and worn, in-dicating transport. Outer shelf, upper bathyal, and oxy-gen minimum zone species that reach the lower slope in-clude platelike forms such as Bolivina bicostata andEpistominella bradyana or fragile thin-walled specimensof Chilostomella oolina and Globobulimina pacifica.All these species have probably been transported in thewater column rather than within the sediment, becausethe tests are easily suspended and are too fragile to with-stand much transport. The larger, heavier tested formssuch as Cassidulina, Hoeglundina, and Gyroidina pres-ent in the lower slope assemblages often have frag-mented or worn tests. The living range of these species(Uchio, 1960; Smith, 1964) indicates some transportwas necessary to move these species downslope, but maynot have been as great as indicated by the biofacies anal-

554

MASS MOVEMENT

500

1000

~ 1500

-2! 2000

-2 2500

3000

3500

4000

4500

Upper bathyal 400 m —

IG-24-7-39

<+uu m 1Oxygen minimum zone

Upper middle bathyal

Lower middle bathyal

500 m —

1500 m•

— 2000 m-

IG-24-7-40

IG-24-7-38

Lower bathyalS565

IG-24-7-41

•3500 m•

Abyssal

IG-24-7-42

Figure 4. Location of UTMSI piston cores with respect to present benthic foraminiferal biofacies. Depth ranges for the biofacies are taken from stud-ies of recent foraminiferal faunas and physical-chemical properties off Central America (see McDougall, this volume, for discussion and refer-ences).

ysis, which considers primarily the upper depth limit ofa species. The condition of most of the tests suggeststransport occurred as part of the sediment load.

Benthic foraminiferal assemblages from Site 565 (Ta-ble 3) indicate deposition occurred within the lower ba-thyal and abyssal biofacies with paleodepths close to4000 m and under the influence of both the Pacific DeepWater (PDW) and Antarctic Bottom Water (AABW).Major changes in the benthic foraminiferal faunas cor-respond to the glacial-interglacial cycles of the late Qua-ternary and to changes in the water masses. Glacial con-ditions (cold events, Fig. 6) are correlated with eventsdated at 0.7 (Core 565-2), 0.9 (Core 565-6), and 1.2 to2.5 (Cores 565-7 to 565-10) Ma (Ingle, 1967, 1973; Kent,Opdyke, and Ewing, 1971; Bandy, 1972; Olsson, 1974;Shackleton and Opdyke, 1977; Keller and Ingle, 1981).Dissolution associated with the cold events (e.g., Plio-cene/Pleistocene boundary, 1.2 to 2.5 Ma cold event) isbelieved to be the result of expansion of the cold AABWduring the glacial maxima. In Cores 565-1, the onset ofthe warm event and associated transgression is accom-panied by a decrease in water depths or more probablyan adjustment of the water masses as the production ofthe AABW declines. Taxonomic notes and paleoenviro-nmental considerations for Site 565 species are given inMcDougall (this volume), and age and paleoceanograph-ic interpretations are discussed by Filewicz, by Stone

and Keller, and by McDougall (this volume) and aresummarized in Figure 6.

Transported fauna are common throughout the Pleis-tocene and average at least 50% of the fauna. Biofa-cies patterns indicate transported faunas are present as aresult of two kinds of downslope movement: a rapid sin-gle event (mass gravity flow) usually affecting one bio-facies; and a continuous downslope movement, whicheffects all biofacies. Specimens transported during in-tervals of rapid downslope movement include platelikebolivinids from the outer shelf-upper slope, cylindricalforms such as Buliminella and Chilostomella from theupper slope. Abundances of these species are anoma-lously high in some samples, suggesting rapid down-slope movement such as in Sections 565-2-2, 565-2-5,and 565-2-6; 565-3-5; and 565-4-1 (Fig. 6). The larger,more robust transported specimens in these samples tendto be from the adjacent shallower water biofacies and/or have broken or altered tests (molds, fillings, or over-growths). Continuous downslope transport is recognizedby the broader more consistent abundances of biofaciesfaunas common throughout the Pleistocene (Fig. 6).Transported foraminifers in these intervals include a va-riety of shapes, sizes, and states of preservation. Shal-lower water species are represented by continuous lowabundances rather than by sporadic peak abundances.Similar patterns were noted in the UTMSI cores.

555

1000

1

2

3

E 4

Biofacies5 |-

IS = Inner shelfOS = Outer shelf

° 6 l~ UB = Upper bathyalOp = Shallow oxygen minimumUMB = Upper middle bathyalLMB Lower middle bathyalLB = Lower bathyalA = AbyssalMISC = Miscellaneous species

(broad ranges and/or range unknown)

10 L

Figure 5. Biofacies trends for the UTMSI cores based on benthic foraminifers. Cores are arranged from shallowest (IG-24-7-39) to deepest (IG-24-7-42). Biofacies groups are the percent of speci-mens with upper depth limits in that biofacies (Table 2). The distribution and abundance of each biofacies in the IG cores indicate that deposition occurred in the outer shelf (Core IG-24-7-39),upper middle bathyal (Cores IG-24-7-38 and IG-24-7-40), lower bathyal (Core IG-24-7-41, samples at 100 and 300 cm), and abyssal (Cores IG-24-7-41, samples at 500, 700, and 900 cm, andIG-24-7-42).

Figure 6. Biofacies and diversity trends for the Pleistocene benthic foraminiferal assemblages of Site 565. Solid triangles (sample column) and hachured lines (diversity column) indi-cate poorly preserved or barren samples. Diversity and biofacies are based on counts of 300 ± specimens where possible. Biofacies groups are the percent of specimens with upperdepth limits in that biofacies (McDougall, this volume). Biofacies present in this core range from the inner shelf to abyssal. Depth ranges and symbols for the biofacies are given inthe previous figures (Figs. 4 and 5) and in Table 2. Age and ecologic interpretations are from Filewicz; McDougall; and Stone and Keller (this volume).


Table 2. Faunal list, UTMSI cores.

UTMSIsamples

core/cm levelor interval)

39/49-53

39/25038/10038/300

38/50040/5040/25040/45040/650

41/10041/30041/50041/70041/900

42/300

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Note: Benthic foraminiferal occurrences for [he UTMSI cores are given as percent total fauna, and an x indicates a percent of less than 1. Biofacies abundances are also given in percent of total fauna. Dot indicates species is notpresent.

SEDIMENT FABRIC

In contrast to the general uniformity of sediment tex-ture at Site 565, a penetrative fabric best described as a"scaliness" occurs in several intervals of mud and mud-stone. The fabric is a primary sedimentary feature rath-er than a product of drilling disturbance, for it is ob-served over a range of consolidation from firm mud toundisturbed mudstone blocks in a matrix of drilling-dis-turbed mud. This fabric was observed in the near-sur-face sediments recovered in the UTMSI cores as well asat subsurface depths well below 150 m at Site 565.

The scaliness is horizontal to subhorizontal; mudbreaks into smooth "polished" concoidal flakes of lessthan 2 mm in length. Figure 7 illustrates scaly splittingof firm mud. The scaly fabric is striking under very highmagnification. Small (under 2-mm) pieces of scaly mudand mudstones were selected from the Site 565 sedimentsfor examination under the scanning electron microscope(SEM). Figure 8A-D shows SEM photographs from thisstudy. Flakes of a montmorillonite clay are aligned inlarger stacks incorporating small blocky fragments ofother materials. Such alignment is an expected featureof clays because of their phyllosilicate crystalline struc-ture (Burst, 1976), but the size of aligned flakes indi-cates an aglomeration of clay crystals. In a continentalslope setting, current activity could be an agent of earli-er horizontal orientation of the clayey sediment, but theincorporation of blocky fragments in the clayey stacksbelies sorting and aligning by current activity.

Downslope creep and scaly fabric are described insome soil science texts (e.g., Jackson, 1969), and wesuggest that this process occurs here on the Costa Ricanslope. Plastic mass flow of sediments downslope wouldcontinuously rework materials into a closer-knit fabric,smearing clayey sediment into alignment and grindingother fine materials into the clayey base. Dewateringduring this movement would result in the overconsolida-

ted sediment near the top of the sediment column ob-served at Site 565. On a broader scale, such flow de-stroys sedimentary bedding, resulting in the massivityevidenced in the seismic profile of the upper unit at Site565 as well as in the cores recovered at the site.

DISCUSSION OF SEDIMENTARY PROCESSES

The upper 45 to 50 m of sediment at Site 565 are lith-ologically indistinguishable from sediment recovered atgreater depths downhole: the entire 328 m is placed in asingle lithologic unit. Geotechnical and paleomagneticproperties, however, suggest that different processes arepresently occurring in the upper part of the section, andsedimentologic and faunal characteristics suggest thatthese are related to downslope transport and mixing ofsediment.

The unique structure of the upper sedimentary sec-tion at Site 565 provides us with the opportunity to tryseveral approaches of characterizing or recognizing a grad-ual, downslope transport mechanism. The fact that sed-iments posses different rheological behaviors and shearstrength characteristics and rest on various degrees ofslopes on continental margins and slopes leads to theconclusion that downslope transport mechanisms canvary from a slow, downslope creep to slumps, debrisflows, or turbidity currents. Any one of these transportmechanisms can in turn be the precursor of subsequentcatastrophic failures.

The nature of a deposit undergoing mass movementis dependent on the strength characteristics of the sedi-ment resisting the gravitational impulse acting on thepile along a given inclined plane. The possibility of fre-quent seismic disturbance in the region of the MiddleAmerica Trench suggests the gravitational impulse mayoccasionally be bolstered by stresses resulting from seis-mic activity. The presence of excess pore pressure gener-ated downhole at Site 565 (see Site 565 report, this vol-ume) may result from the subduction process burying

558

MASS MOVEMENT

Table 2. (Continued).

O (J ü ü (J

4 I ,^ T3 ~

III1 I 1 Ic i: Ö ea S a .s

sediment of both the oceanic plate and slope deposits.Excess pore pressure produces a decreased effective stresswithin the sedimentary column. Upon reduction of theeffective stress, the sediment loses its shear strength, whichin turn may lead to failure even on slopes as gentle as 1 °(Prior and Coleman, 1981).

Movement within the sediment mass would tend torearrange particles (fabric) and aid in compacting a givenunit. This rearrangement would be reflected in quicklychanging physical properties in the affected zone. A rap-id decrease of index properties with a normally consoli-dating sediment is not unusual. Porosity-depth relation-ships presented by Hamilton (1976) reflect a 5% decreasein porosity in the upper 40 m for calcareous sediments,2% for diatomaceous ooze, and approximately 3% forterrigenous sediments. Bryant et al. (1981) present otherporosity-depth profiles showing a wide variety of valuesfor the reduction of porosity in the upper 20 m, rangingfrom a decrease in porosity of 2% for a 75%-CaCO3 sed-iment with 35 to 55% sand-size grains to a change in po-rosity of 14% for a 10 to 15%-CaCO3 sediment with80% clay-size grains. These same profiles show an ex-pected 6% decrease in porosity for a 5 to 20%-CaCO3

sediment with 60% clay-size grains in a 20-m-interval sur-face. Site 565 has a composition similar to this latter sedi-ment, and porosity values here drop from a surface highof 80 to 66% at 19 m, a change of 14% in less than 20 m.This rapid decrease of porosity and increase in bulk den-sity at Site 565 indicates a forced dewatering process.

Atterberg limits for Site 565 sediments yield two dif-ferent classifications on the Casagrande Plasticity Chart(fig. 4, in Taylor and Bryant, this volume). The upper-most sample from Section 565-1-4 is classified as an inor-ganic clay of medium to high plasticity, whereas the sam-ple from Section 565-1-4 is classified as an inorganic clayof medium to high plasticity, whereas the sample fromSection 565-10-6 is defined as a micaceous or diatoma-ceous fine sand and silt of high plasticity. Sediment clas-

sification using Atterberg limits reflects rheological be-havior more than sediment composition. The similarityof composition of these two samples has already beennoted in this chapter, and their rheological character canbe appreciated in figure 4 of Taylor and Bryant (this vol-ume). The shift of the natural water content with depthtakes the sediment from a state near its liquid limit (120-140%) for surface and shallow sediment to one ap-proaching the plastic limit (60-70%). The rheologicalcharacter of the sediment is one in which the sedimentbehaves less plastically with depth until the natural watercontent becomes less than the plasticity index. The Atter-berg limits at Site 565 reflect the near fluid nature of sec-tion 565-1-4 and the practical plastic limit of sediments insection 565-10-6.

Moreover, because of the textural and lithologic ho-mogeneity of sediment at Site 565 we can compare theliquid and plastic limits determined for these sedimentswith the decreasing water content downcore (Fig. 9).The water content steadily decreases from above the liq-uid limit to below the plastic limit at about 45 to 50 m,then increases below this depth to a fairly constant valuejust above the plastic limit that was determined for thesesediments.

The truncation of the layer of increasing shear strengthand bulk density at approximately 50 m defines a zoneof presently more active sediment movement. A state ofoverconsolidation for this unit reflects the dewateringprocess associated with downslope movement. Booth(1979) recognized flow deposits in the Gulf of Mexicofrom the appearance of an overconsolidated sedimentwith low water contents overlying a normal to under-consolidated section below. It would appear that Site565 fulfills these observations. The injection of pore wa-ter from the zone of movement into the uppermost lay-ers will decrease the apparent state of consolidation ofthese latter sediments and produce higher water con-tents, thus decreasing the effective normal stress. These

559



UTMSlsamples

(core/cm level

or interval)

39/49-53

39/150

39/250

38/100

38/300

38/500

40/50

40/250

40/450

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higher water contents, in turn, shift the rheological na-ture of the sediment toward a more fluid state such asmeasured in Section 565-1-4.

Lowe (1979) describes sediment gravity flows in termsof the rheological nature of the deposit. The range ofwater contents within the liquid-plastic behavior fielddefined by Atterberg limits for sediments at Site 565supports the idea of a spectrum of downslope transportprocesses, going from a viscous creep movement nearthe base of the 45-m layer to mud flows or cohesive de-bris flows near the surface.

An illustration of the velocity profile resulting fromthe envisioned downslope movement mechanism is shownin Figure 10. Conditions defining this velocity profileset the depth of minimum velocity at 47 m, but thepoint of the figure does not depend on halting the flowentirely. The sediment at the base of this mobile 45 to50 m is moving much more slowly than the more fluidoverlying materials: essentially it is left behind by thefaster-moving upper layer. At this level in the sedimentsection, sediment water content has decreased to belowits plastic limit (Fig. 9), providing a second indicationthat this material will no longer flow.

The nature of the boundary at about 47 m is of inter-est. The boundary could represent a slowly shifting ho-rizon of sediment accumulation, where overlying clayeysediment is continuously plastered onto it as the sedi-ment reaches its plastic limit. Smearing of clayey sedi-ment could result in a relatively impermeable layer, ef-fectively sealing the upper sediments from the sectionbelow. Thus buildup of pore pressure resulting from de-watering of the subducting sediments of the Cocos Plateor from formations of gas hydrates ubiquitous to thesediments on this continental slope (see Organic Geo-chemistry section, Site 565 report, this volume) couldoccur in the sediments below the zone of downslopecreep. Alternatively, this layer of abrupt change in phys-ical properties could indicate the basal surface of a slump.

Buildup of a pile of sediment too consolidated for plas-tic flow at its base could result in slope instability andlocal failure through slumping. This can be enhanced bythe buildup of pore pressure at the base of the layer act-ing to lubricate the slump surface. A shift in the benthicforaminiferal biofacies and diversity at about 47 m sub-bottom indicates a slight unconformity or hiatus (Fig. 6),which supports the possibility of a slump surface at thisdepth.

We consider the answer to lie in a combination ofthese alternatives where mass movement and downslopereworking accumulates sediment in a mobile layer ofmaterial that is self-limiting in its thickness. At the baseof the layer shearing and reworking accretes the clayeymaterial to underlying slope sediments, creating a lowpermeability boundary of nonflowing clayey sedimentthat seals the mobile layer from the underlying material.Buildup of pore pressure below and accumulation of athick pile of sediment above leads to local instabilityand slumping along the surface. The few laterally dis-continuous reflectors in seismic profiles of the Costa Ri-can slope may represent ancient slump surfaces.

Canyon sedimentation on continental slopes is gener-ally treated in terms of depositional events such as tur-bidity flows, slumps, or debris flows, but continuoussedimentation processes that could lead to these eventsare somewhat neglected. The muds and mudstones ofthe Costa Rican slope are typical hemipelagic depositsof an active margin inner slope cut many canyons. Thetexture and fabric of sediment in this geologic settingleads us to suggest that downslope creep is a process ofsediment transport of greater importance than has beenheretofore recognized.

Analysis of benthic foraminiferal assemblages and de-tection of scaly fabric can be useful documentation ofdownslope creep, and shifts in physical properties andthe paleomagnetic record may be used to define the lay-er of sediment movement.

560

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2

ACKNOWLEDGMENTS

We thank Dick Buffler and William Behrens for permission to ex-amine and sample UTMSI Cores IG-24-7-38 to -42, Bill Sliter and Ho-ma Lee for their thoughtful and thorough reviews of this paper, andNeil Lundberg for thought-provoking conversation on the subject ofplastic mass flow as a mechanism of downslope sediment transport.

REFERENCES

Bandy, O. L., 1961. Distribution of foraminifera, radiolarian, anddiatoms in sediments of the Gulf of California. Micropaleontolo-gy, 17:1-26.

, 1972. Neogene planktonic foraminiferal zones, Californiaand some geologic implications. In Lipps, J. H. (Ed.) Eastern Pa-cific Plankton Biostratigraphy and Paleoecology, Paleogeogr. Pa-leoclim. Paleoecol., 12:131.

Booth, J. S., 1979. Recent history of mass-wasting on the upper conti-nental slope, northern Gulf of Mexico, as interpreted from theconsolidation states of the sediment. Soc. Econ. Paleontol. Miner-al. Spec. Publ., 27: 153-164.

Bryant, W. R., Bennett, R. H., and Katherman, C. E., 1981. Shearstrength, consolidation, porosity and permeability of oceanic sedi-ments. In Emiliani, C. (Ed.), The Sea (Vol. 7): New York (Intersci-ence), 1555-1615.

Burst, J. F., 1976. Argillaceous sediment dewatering. Ann. Rev. EarthSci., 4:293-318.

Corliss, B. H., and Honjo, S., 1981. Dissolution of deep-sea ben-thonic foraminifera. Micropaleontology, 27:356-378.

Douglas, R. G., 1979. Benthic foraminiferal ecology and paleoecolo-gy: a review of concepts and methods. In Lipps, J. H., et al.(Eds.), Foraminiferal Ecology and Paleoecology, Short Course No.6: Tulsa (Soc. Econ. and Paleontol Mineral.), pp. 21-53.

, 1981. Paleoecology of continental margin basins: a moderncase history from the borderland of southern California. In Lipps,J. H., et al. (Eds.), Depositional Systems of Active ContinentalMargin Basins: Short Course Notes: Los Angles (Pacific Section,Soc. Econ. Paleontol. Mineral.), pp. 121-156.

Doyle, L. J., and Pilkey, O. H., 1979. Geology of Continental Slopes:Tulsa (Soc. Econ. Paleontol. Mineral.).

Hamilton, E. L., 1976. Variations of density and porosity with depthin deep sea sediments. J. Sed. Petrol., 46:280-300.

561


Ingle, J. C. Jr., 1967. Foraminiferal biofacies variation and the Mio-cene/Pliocene boundary in southern California. Bull. Am. Pa-leontol., 52:217-394.

, 1973. Neogene foraminifera from the northeastern PacificOcean, Leg 18 DSDP. In Kulm, L. D., von Huene, R., et al., In-it. Repts. DSDP, 18: Washington (U.S. Govt. Printing Office),517-567.

„, 1980. Cenozoic paleobathymetry and depositional historyof selected sequences within the southern California continentalborderland. In Sliter, W. V., (Ed.), Studies in Marine Micropaleon-tology and Paleoecology: A Memorial Volume to Orville L. Ban-dy, Cushman Found. Foram. Res. Spec. Publ., 19:163-195.

Jackson, M. L., 1969. Soil Chemical Analysis—Advanced Course (2nded., 8th Printing, 1973): Madison, Wisconsin (Dept, of Soil Sci-ence, Univ. of Wisconsin).

Keller, G. 1978. Late Neogene planktonic foraminiferal biostratigra-phy and paleoecology of the northeastern Pacific: evidence fromDSDP Sites 173 and 310 at the North Pacific Front. J. Foraminif.Res., 8:332-349.

Keller, G., and Ingle J. C , 1981. Planktonic foraminiferal biostratig-raphy, paleoceanographic implications, and deep sea correlationsof the Pliocene-Pleistocene Centerville Beach Section, NorthernCalifornia. Geol. Soc. Am. Spec. Pap., 184:127-136.

Kent, D., Opdyke, N. D., and Ewing, M., 1971. Climatic change inthe North Pacific using ice rafted detritus as a climatic indicator.Geol. Soc. Am. Bull., 82:2741-2754.

Lowe, D. R., 1979. Sediment gravity flows: their classification andsome problems of application to natural flows and deposits. Soc.Econ. Paleontol. Mineral. Spec. Publ., 27:75-82.

Middleton, G. V., and Hampton, M. Z., 1976. Sediment gravity flows:mechanism of flow and deposition. In Turbidites and Deep WaterSedimentation, Short Course: Los Angeles (Pacific Section Soc.Econ. Paleontol. Mineral.), pp. 1-38.

Olsson, R. K., 1974. Pleistocene paleoceanography and Globigerinapachyderma (Ehrenberg) in Site 36, DSDP, Northeastern Pacific.J. Foraminif. Res., 4:47-60.

Prior, D. B., and Coleman, J. M., 1981. Resurvey of active mud slides,Mississippi Delta. Geo-Marine Lett., 1:17-21.

Shackleton, N. J., and Opdyke, N. D., 1977. Oxygen isotope and pa-leomagnetic evidence for early Northern Hemisphere glaciation.Nature, 270:216-219.

Skempton, A. W., 1964. Long-term stability of clay slopes. Geotech-nique, 14:77.

, 1970. The consolidation of clays by gravitational compac-tion. /. Geol. Soc. London, 125:373-411.

Smith, P. B., 1964. Ecology of benthonic species. U.S. Geol. Surv.Prof. Paper, 429B:B1-B55.

Stanley, D. J., and Unrug, R., 1972. Submarine channel deposits, fluxo-turbidities and other indicators of slope and base of slope environ-ments in modern and ancient basins. In Rigby, J. K., and Hamblin,W. K. (Eds.), Recognition of Ancient Sedimentary Environments:Soc. Econ. Paleontol. Mineral. Spec. Publ., 16:287-340.

Uchio, T , 1960. Ecology of living benthonic foraminifera from theSan Diego, California area. Cushman Found. Foram. Res. Spec.Publ., 5:1-71.

Verosub, K. L., Ensley, R. A., and Ulrick, J. S., 1979. The role of wa-ter content in the magnetization of sediments. Geophys. Res. Lett.,6:226-228.

Yen, B. C , 1969. Stability of slopes undergoing creep deformation. J.Soil. Mech. Found. Div. Am. Soc. Civ. Eng., 95:1075-1096.

Date of Initial Receipt: 10 February 1984Date of Acceptance: 4 May 1984

Figure 7. Typical scaly sediment fabric observed at Site 565, Sample565-16-4, 115-120 cm.

562

MASS MOVEMENT

50 µm 12.5µm

25 µm 25 µm

Figure 8. Scanning electron microscope photomicrographs of scaly mud from Site 565. A. Sample 565-11-2, 125 cm. Stacks of clay-ey sediment incorporating blocky fragments of other sediment constituents including shell fragments. B. Sample 565-11-2,125 cm. Closer view of the aligned stacks of Figure 8A, illustrating the agglomerate nature of the aligned stacks of sediment.Large crystalline plugs are incorporated into layers of smaller flakes. C. Sample 565-11-2, 52 cm. Tabular and cubic (pyrite?)crystals in clayey stacks. Arrows indicate incipient splitting of larger crystals possibly in response to shearing. D. Sample 565-11-2,52 cm. Multiple layers with large-scale stacks.

563


Table 3. Faunal list, Site 565.

Sample(cm interval)

I I

1-1, 70-741-2, 77-81l.CC2-1, 70-742-2, 77-81

2-3, 77-792-4, 76-802-5, 52-562-6, 70-762-7, 30-34

2,CC3-1, 64-703-2, 66-703-3, 66-703-4, 66-70

3-5, 66-703-6, 64-683,CC4-1, 10-144-2, 10-14

4-3, 10-144-4, 10-144-5, 10-145-1, 84-885-2, 84-88

5-6, 84-885-7, 44-485,CC5 (below)6-1, 39-43

6-2, 39-436-3, 39-436-4, 39-436-5, 39-436-6, 39-43

6-7, 39-436.CC7-1, 58-627-3, 58-627-4, 56-62

7-5, 58-627-6, 58-627,CC8-1, 54-588-2, 54-58

8-3, 54-588-4. 54-588-6, 54-588,CC9-1, 34-38

9-4, 54-589-6, 46-509-7, 11-159.CC10-1, 70-74

10-2, 73-7410-3, 76-8010-5, 86-9010-6, 86-90

Note: Benthic foraminiferal occurrences are giv= oxygen minimum zone; UMB = upper

ercent total fauna; an x indicates less than 1%. Biofacies abundances are also given in percent total fauna; IS = inner shelf; OS = outer shelf; UB = upper bathyal; O2bathyal; LMB = lower middle bathyal; LB = lower bathyal; A = abyssal; dot or blank space indicates species is not present.

564


MASS MOVEMENT

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565



(cπSample I 3-

IJS

I I| J

d i t0 . 0 . 3

1-1, 70-741-2, 77-811,CC2-1, 70-742-2, 77-81

2-3, 77-792-4, 76-802-5, 52-562-6, 70-762-7, 30-34

2,CC3-1, 64-703-2, 66-703-3, 66-703-4, 66-70

3-5, 66-703-6, 64-683,CC4-1, 10-144-2, 10-14

4-3, 10-144-4, 10-144-5, 10-145-1, 84-885-2, 84-88

5-6, 84-885-7, 44-485,CC5 (below)6-1, 39-43

6-2, 39-436-3, 39-436-4, 39-436-5, 39-436-6, 39-43

6-7, 39-436,CC7-1, 58-627-3, 58-627-4, 56-62

7-5, 58-627-6, 58-627,CC8-1, 54-588-2, 54-58

8-3, 54-588-4, 54-588-6, 54-588,CC9-1, 34-38

9-4, 54-589-6, 46-509-7, 11-159,CC10-1, 70-74

10-2, 73-7410-3, 76-8010-5, 86-9010-6, 86-90

X X X

566

MASS MOVEMENT

Tkble 3. (Continued).

567



Sample(cm interval)

1-1, 70-741-2, 77-81I,CC2-1, 70-742-2, 77-81

2-3, 77-792-4, 76-802-5, 52-562-6, 70-762-7, 30-34

2,CC3-1, 64-703-2, 66-703-3, 66-703-4, 66-70

3-5, 66-703-6, 64-683,CC4-1, 10-144-2, 10-14

4-3, 10-144-4, 10-144-5, 10-145-1, 84-885-2, 84-88

5-6, 84-885_7, 44-485,CC5 (below)6-1, 39-43

6-2, 39-436-3, 39-436-4, 39-436-5, 39-436-6, 39-43

6-7, 39-436,CC7-1, 58-627-3, 58-627-4, 56-62

7-5, 58-627-6, 58-627,CC8-1, 54-588-2, 54-58

8-3, 54-588-4, 54-588-6, 54-588,CC9-1, 34-38

9-4, 54-589-6, 46-509-7, 11-159,CC10-1, 70-74

10-2, 73-7410-3, 76-8010-5, 86-9010-6, 86-90

I j*I I"δ «

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MASS MOVEMENT


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Taxa

Sample(cm interval)

1-1,70-741-2, 77-811,CC2-1, 70-742-2, 77-81

2-3, 77-792-4, 76-802-5, 52-562-6, 70-762-7, 30-34

2,CC3-1, 64-703-2, 66-703-3, 66-703-4, 66-70

3-5, 66-703-6 64-683,CC4-1, 10-144-2, 10-14

4-3, 10-144-4, 10-144-5, 10-145-1, 84-885-2, 84-88

5-6, 84-885-7, 44-485,CC5 (below)6-1, 39-43

6-2, 39-436-3, 39-436-4, 39-436-5, 39-436-6, 39-43

6-7, 39-436,CC7-1, 58-627-3, 58-627-4, 56-62

7-5, 58-627-6, 58-627,CC8-1, 54-588-2, 54-58

8-3, 54-588-4, 54-588-6, 54-588,CC9_1 34-38

9-4, 54-589-6, 46-509-7, 11-159,CC10-1, 70-74

10-2, 73-7410-3, 76-8010-5, 86-9010-6! 86-90

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2

1414205047

2822

81411

3142220

5••

4125

8215

2 j

A

•

221

,

1••

•

323

•

•

•

3••

••2

1•3

••

10

5•2

••

112

•

•

2••

5•••5

Dry-water content (%)

80 100 120 140

60 -

Figure 9. Comparison of natural water content of sediments of Site565 to liquid and plastic limits of Section 565-1-4.

Increasing velocity —

0

10

1 2 0

dept

torn

bot

nOT 3 0

40

<

I

-

—

—

1

1

/

/

1

1

j

/

/

1

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//

—

-

1 1

Figure 10. Calculated downslope velocity profile for potential creepfor sediments in the upper 47 m at Site 565.

570

deep sea drilling project initial reports volume 84debris flow deposits where the strength of the...

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