31. terrigenous silt and clay facies: deposits … · terrigenous silt and clay facies: deposits of...

31. TERRIGENOUS SILT AND CLAY FACIES:DEPOSITS OF THE EARLY PHASE OF OCEAN BASIN EVOLUTION

Peter B. Andrews, Sedimentation Laboratory, New Zealand Geological Survey, Christchurch, New Zealandand

A. Thomas Ovenshine, United States Geological Survey, Menlo Park, California

ABSTRACTA terrigenous silt and clay facies has been shown to be the first

sediment to accumulate on basement (both oceanic and continental)at many sites drilled during Leg 29, and at many DSDP sites in theIndian Ocean and Tasman Sea drilled during earlier legs. It is dividedinto two subfacies: a burrow-mottled subfacies which predominatesand a bedded subfacies which is rare.

The burrow mottled subfacies is dark olive-gray to dark brown,burrow mottled, organic-rich, pyrite, glauconite, and micronodule-bearing. It is a poorly sorted, clayey silt to clay, composed of land-derived detritus, and is characterized by a very restricted flora andfauna, the latter being dominated by siliceous, and agglutinatingbenthonic foraminifera. The upper third of the subfacies is silicifiedwherever the overlying sediments are rich in siliceous microfossils. Itis inferred that the subfacies accumulated where bottom waters wereoxygenated, but circulation was sluggish. It is further inferred thatthe subfacies accumulated during the early sea-floor-spreading phasein the evolution of the respective ocean basins.

The bedded subfacies forms units that alternate with the burrowmottled subfacies at one site. It consists of slightly graded bedsclayey silt. The microfossils are shallow-water benthonic foram-inifera and neritic nannofossils. Graded beds are interpreted as tur-bidites.

INTRODUCTION

Units of the terrigenous silt and clay facies occur atfour of the Leg 29 sites (Figure 1), and comprise the bulkof the sequence cored at three of the sites. At all foursites the silt and clay facies forms the basal part of thesequence (Figure 2). It is Late Cretaceous in age at Site275, Eocene at Site 280, late Eocene and Oligocene atSite 282, and Paleocene and Eocene at Site 283. Ter-rigenous silt and clay also formed the basal part of thesequence at many sites drilled in the Tasman Sea andIndian Ocean during DSDP Legs 21, 22, 25, 26, and 27(Table 1). At most of these localities the terrigenous siltand clay is Cretaceous to Eocene in age. However, atone locality (Site 261) it is Late Jurassic, and at twolocalities (Sites 250 and 257 in the Indian Ocean) itranges up into the Pliocene or Quaternary. In mid-latitude and subtropical locations in the Indian Ocean,Tasman Sea, and southwest Pacific Ocean the ter-rigenous silt and clay facies is the first sediment to accu-mulate on newly formed basaltic oceanic crust. Inaddition, at places it is the first sediment to accumulatethe outer edge of deep plateaus underlain by continentalcrust (Sites 207 and 275). Its widespread occurrence insuch positions must have paleogeographic and paleo-oceanographic significance.

FACIES CHARACTERISTICSAs its descriptive name implies, the facies consists

predominantly of silt and clay eroded from exposedlandmasses. Microfossils are either largely or entirelyabsent, and macrofossils are extremely rare. The facies isdivided into two subfacies on the basis of visiblesedimentary structures. The most common subfacies ischaracterized by burrow-mottling. The second subfaciesis less common and distinguished by thin bedding that atplaces is subtly graded (more accurately described ascolor banding). These subfacies occur at both shallowand deep marine sites. At Site 282 units of the burrow-mottled subfacies alternate with units of the beddedsubfacies.

Burrow-mottled SubfaciesThis subfacies occurs over the southern Lord Howe

Rise (Site 207, Leg 21), at the edge of the CampbellPlateau (Site 275), at the foot of the South Tasman Rise(Sites 280 and 282), and in the central Tasman Sea (Site283. At Site 207 the subfacies rests on continental crustof subaerial rhyolite flows and tuffs (Burns et al., 1973).At Site 275, although the base of the sequence was notreached, it is known to rest on continental crust(Summerhayes, 1969). Basement rock at other sites islisted in Table 1.

1049

P. B. ANDREWS, A. T. OVENSHINE

ANIA

#283

I5O°E

Figure 1. Location of drill sites discussed in detail; Site 207- Leg 21; Sites 275, 280, 282, and 283 - Leg 29.

The subfacies has color variations of green-gray,olive, dark olive-gray, gray-brown, dark brown, orolive-black. The dark brown colors are characteristic ofthe lower part of the sequence at Sites 280,282, and 283,and the entire sequence at Site 207. The sediments aregenerally burrow-mottled, however, they range frommassive to sparsely mottled, to intensely mottled, or tohomogeneous. The intensity and distribution of mottl-ing varies stratigraphically, however, moderate tointense mottling predominates (Figure 3). It mostcommonly takes the form of subhorizontal, dis-continuous, or wispy irregular lamination, that gives thelower parts of a sequence a pronounced subhorizontalfabric (Figure 4). The irregular lamination is interpretedto represent compressed elliptical burrows, becauseindividual laminations commonly transect both "older"and "younger" layers (see also Luyendyk et al., 1972).However, some subhorizontal discontinuous lamination(Site 275) cannot be shown to be of burrow origin, andthus a current origin cannot be discounted.

At Site 280 lenticular lamination occurs at onehorizon low in the sequence and is regarded as current

lamination. Silt and clay lamination occurs in a thin(<l.O m) zone of penecontemporaneously deformedsediment just above the intrusive basalt at the site. Thisis probably depositional in origin, rather than theproduct of soft sediment flowage (Figure 5). Rare 1-4-cm beds at Site 282 probably reflect short-termvariations in sediment supply. With these possibleexceptions, the subfacies is characterized by burrow-mottling.

Where individual burrows are outlined, they take oneof five forms. The most common are elongate (5 × 20mm) horizontal to vertical tapering, elliptical burrows,lacking internal structures. Subequant blotches (20-100mm diameter), with diffuse and irregular margins, arealso common. Distinctive vertical or oblique ellipticalburrows up to 100 mm diameter, and with stubby pairedlateral galleries, are found sparsely distributed at twosites (280, 283). Slender (1-2 mm wide) vertical tubesalso occur in limited numbers at both sites. Large,angular, subhorizontal, and stratiform mottles, thatoccur in small numbers at Site 280, appear to bemodified thin beds, rather than mottles.

1050

SILT AND CLAY FACIES

WEST01

EAST

200H

3 0 0 -

4 0 0 -

5 0 0 -

6001-

SITE 282

4202m(water depth)

SITE 280

4176m{water depth)

SITE 283

4 7 2 9 m(water depth)

SITE 207

1389 mwater depth)

SITE 275

2 8 0 0 m(water depth)

glaucomte

chert

zeolites

basalt

rhyolitic tuff

rhyolite flows

unconformablθ boundary

conformable boundary

nanno ooze

foram ooze

diatom ooze

siliceous ooze

silt

silty clay/clayey silt

clay

Figure 2. Stratigraphy at the sites discussed in detail; Site 207 - Leg 21; Sites 275, 280, 282, and 283 - Leg 29.

1051


TABLE 1DSDP Sites Referred to in This Report

Leg

21222525

25252525252526262626262626272727

27292929

29292929

Site

*207212239

240A241242245246248249250

251A253254

256257258259260261263

*275277278

*280A281

*282*283

Ocean or Sea

TasmanEast IndianWest IndianWest IndianWest IndianWest IndianWest IndianWest IndianWest IndianWest IndianWest IndianWest IndianEast IndianEast IndianEast IndianEast IndianEast IndianEast IndianEast IndianEast IndianEast IndianSouthwest PacificSouthwest PacificSouthwest PacificSoutheast IndianSoutheast IndianSoutheast IndianTasman

Latitude

36°57.75'S19°11.34'S21°17.67'SO3°29.24'SO2°22.24'S15°5O.65'S31°32.O2'S33°37.2l's29°31.78'S29°56.99'S33°27.74'S36°3O.26'S24°52.65'S3O°58.15'S23°27.35'S3O°59.16'S33°47.69'S29°37'S16°09;S12°57'S23°2θ'S5O°26.34'S52°13.43'S56°33.42'S48°57.44'S47°59.84'S42°14.76'S43°54.60;S

Longitude

165°26.06'E99°17.84'E51°40.73'E50°03.22;E44o40.77'E41°49 .23 'E52°18.1l'E45°O9.6θ'E37 28.48'E36°04.62'E39°22.15'E49°29.08'E87°21.97'E87°53.72'E

100°46.46;E108°20.99'E112°28.42'E112°42'E110°18'E117°54'E110°58'E176°18.99 'E166°04.29'E160°11.48;E147°14.08'E147°45.85 'E

143°29.18'E154°16.96 'E

WaterDepth (m)

1389624349715082405422754857103049942088511934891962125353615278279347125709568750652800121436754176159142024729

TotalPenetration

(m)

513521

326202

1174676397203434412739499559344270327525

346331579746

62

473439524169311592

Oldest Sediments

MaestrichtianLate CretaceousMaestrichtianLate CretaceousTuronian-CenomanianLate EoceneEarly PaleoceneEarly EoceneMiddle PaleoceneEarly-Late CretaceousSantonian-ConiacianEarly MioceneMiddle EoceneProbably MioceneEarly CretaceousAlbianAlbianNeocomianBarremianKimmeridgianAp tian-B arrem ianMaestrichtianMiddle PaleoceneMiddle OligoceneMiddle EoceneLate EoceneLate EoceneMaestrichtian

Basement Rock

Subaerial rhyoliteBasalt flowsBasalt flowsBasaltNot reachedNot reachedBasaltNot reachedBasaltBasalt flowsBasalt flowsBasaltAsh and basalt flowsBasalt flowsBasalt flowsBasalt flowsNot reachedBasalt flowsBasalt sill?Basalt flowsNot reachedNot reachedNot reachedBasalt flowsBasalt sillSchistBasalt flowsBasalt flows

Note: Asterisked samples provide most of information discussed.

Individual burrow mottles (Figure 4) stand outbecause their color contrasts with that of thesurrounding sediment. In some cases the color contrastreflects concentration in the mottles of dark mineralssuch as glauconite, pyritized material, micronodules, ormineralized diatoms. Mineralized diatoms occur at thetop of the subfacies at several sites. They are the sourceof the dark color of tiny spot-like, and wispy mottles.

The common occurrence of burrow mottling indicatesthe presence of an abundant and active infauna duringsediment deposition. Skeletal remains are not found, butthe abundant presence of organic carbon throughout thesubfacies lends support to this conclusion. Examples ofthis organic carbon content are: Sites 275 (one measure-ment, 0.9%), 280 (values average 0.5% throughout, andmeasure 0.6%-2.2% in the lower 180 m), 282 (valuesrange from 2.4% to 5.7%), and 283 (values range from0.1% to 0.7%, and average 0.4%).

SEDIMENT TEXTURE

The subfacies consists predominantly of silty clay,with clay, and clayey silt subordinate (Figure 6). Sand-silt-clay sediments are confined to a thin transition zonebetween the terrigenous facies and the overlyingsiliceous ooze at Site 275.

The sediments are very poorly sorted (Figure 7), withthe bulk of a sample subequally distributed over the silt

and clay size range (Table 2). Some samples arebimodal, with a minor secondary mode in the fine sandto coarse silt range.

Sediments that are the most remote from land,both at present, and at the time of deposition, are finest,and those closest to land are coarsest. Specific examplesinclude: Site 283, central Tasman Sea, clay and siltyclay; Site 280, at the foot of the South Tasman Rise, siltyclay and clayey silt facies; and at both Site 275 on theouter edge of the Campbell Plateau, and Site 282 at thefoot of the continental slope west of Tasmania, thesediments consist of silty clay, in which the percentageof sand is greater than at most other sites (Figure 6).This gradual decrease in size with distance from existingcontinental landmasses suggests that the sedimentsoriginated as land-derived detritus. The sediment wasvery gradually transported in suspension, and byvarious bottom currents, toward the deep-ocean basins.

Mineralogy

The sediment mineralogy is largely consistent withderivation from continental rocks. X-ray diffractionanalysis results are presented in Table 3. Examination ofsieved fractions show that sand-size particles generallyinclude the same range of minerals. Sieve fractions alsoshow that the mica recognized by X-ray diffraction con-sists of biotite, muscovite, and schistose rock fragments.Sedimentary rock fragments (siltstone and chert) are a

1052


Figure 3. Slabbed core sections showing the intensity ofburrowing characteristic of the burrow-mottled subfacies.The bar is 1 cm long, (a) Sample 29-280A-9-1, 47-50 cm;(b)Sample 29-275-4-1,117-123cm; (c) Sample 29-280A-10-1, 102-107 cm; (d) Sample 29-280A-10-1, 75-80 cm.

notable component in the vicinity of the South TasmanRise and Tasmania.

The authigenic minerals, glauconite, manganese oriron micronodules, and pyrite, occur throughout thesubfacies. Volcanic glass, palagonite, zeolites, dolomite,gypsum, barite, and basaltic rock fragments are sparseor rare and irregularly distributed.

Glauconite, occurring in amounts ranging from atrace to 15%, is bright green, generally round and botry-

oidal, and sand to coarse silt size. Some grains arerectangular to irregular in shape. Rare grains displayedin a thin section from Core 280A-12-1, probably formedby the alteration of mica (see also Dudley and Margolis,this volume). Scattered grains of glauconite, includingsome botryoidal forms have developed by the infillingand mineralization of siliceous microfossils, especiallydiatoms (Sites 275 and 280). However, a study of thinsections suggests that most glauconite grains haveoriginated by mineralization of clay minerals in the siltyclay host sediment.

At one horizon in the upper part of the subfacies atSite 280, glauconite is so abundant that the sediment is agreensand. Glauconite forms 45%-69% of this greensandwhich is a layer 34 cm thick at 235 meters subbottom.The bright green botryoidal glauconite occurs in a siltyclay matrix. At 292 meters, a second distinctiveglauconitic silty sandstone forms a bed approximately20 cm thick. This bed is unusual in that it is graded. Thelower 10 cm includes many 3-10-mm diameter roundedclasts of pale to dark green (glauconitized?) claystone(Figure 8). Sand and coarse silt-size quartz, micas,Plagioclase, rock fragments, and fragments of probablevolcanic glass occur throughout and comprise 35% ofthe sediment. Glauconite grains make up about 15% ofthe sediment. Many grains are various shades of brownand are regarded as detrital. The bed is interpreted to bea turbidite.

The diagenetic minerals pyrite and micronodules arealso typical of the subfacies. Shipboard calculationsfrom smear slides suggest that the dark brown to black,spherical micronodules occur in amounts up to 20%.Thin-section study suggests that some proportion of thisamount is in fact spherical framboidal pyrite. The pyriteis widespread in the subfacies. Pyrite also occurs asirregular aggregates, as pseudomorphs after fecalpellets, and in the walls of diatom skeletons. The latteroccurrence was found in the upper part of the sequenceat Site 275.

A study of the clay mineral distribution shows thatmontmorillonite is most abundant at the deep-oceansites and least abundant at the submerged plateau sites(especially Site 275). This distribution is probably moreconsistent with a volcanic origin than a tropicalweathering origin, the two origins of montmorillonite inoceanic sediments cited by Lisitzin (1972). The distribu-tion and relative abundance of chlorite and kaolin islargely controlled by latitude in modern oceanicsediments. At present, chlorite is most abundant in highlatitudes and kaolin is most abundant at tropicallatitudes (Lisitzin, 1972). Diffraction analysis of bulksamples of the terrigenous facies shows that chlorite isabsent at the northernmost sites ( 35°-40°S, Sites 207and 282) and relatively abundant at the two mostsouthern sites ( 50°S, Sites 280 and 275). Conversely,with the exception of Site 207, kaolin is most abundantat the northernmost site and decreases in relative abun-dance until rare at Site 280. Kaolin is absent at the mostsoutherly site (275). In light of these trends, the absenceof kaolin at Site 207 is puzzling.

1053


cmI—0

— 2

— 4

— 6

— 10

— 12

— 14

A B C D

Figure 4. Core sections of the burrow-mottled subfacies. (a) Subhorizontal, discontinuous, and irregular lamination; inter-preted to represent compressed elliptical burrows. Sample 29-280A-17-5, 25.5-40 cm; (b) Burrows (circular cross-section,tubular long-section) with lateral galleries; bleached zone around each burrow. Sample 29-280A-19-4, 80-95 cm; (c) Strati-form mottle; possibly a deformed bed of clay (lower); flat-topped burrow mottles (top and lower third). Sample 29-280A-19-2, 56-72 cm; (d) Subhorizontal elliptical burrows; marked by concentrations of glauconite. Sample 29-280A-13-2,17.5-32.5 cm.

SilicificationThe upper part of the subfacies is characteristicallysilicified wherever the overlying sediments are rich inscliceous microfossils. The silicified zone is rich in thesecondary silica minerals, cristobalite and tridymite(Figure 9 and Table 3). In some sequences the silicifiedzone is stiff to semilithified, but where silicification ismost advanced, for example, Site 280, the sediment isthoroughly cemented and cannot be disaggregated, evenby sonic treatment. The topmost part of the subfacies ischertified.

At Site 280, and to a lesser extent at Site 283, there is aprogressive change upward from semilithified silt andclay, to silicified silt and clay rich in cristobalite andtridymite, to interbedded chert and silicified silt andclay, and finally to siliceous ooze. This progressionprovides stratigraphic documentation that chertificationof marine muds rich in siliceous microfossils involvesdissolution of the microfossils, especially diatoms. The

dissolution is succeeded by precipitation of the silica toform cristobalite and tridymite and a gradual inversionof the cristobalite and tridymite to microcrystallinequartz (chert). An identical paragenesis has previouslybeen proposed based on general observation (Bergerand von Rad, 1972) and experimental work (Green-wood, 1973).

Silicification is absent and the secondary mineralscristobalite and tridymite are absent from terrigenoussilt and clay that contains calcareous fossils rather thansiliceous microfossils, and which is overlain by cal-careous ooze, rather than siliceous ooze (for example,Site 282).

Fauna and FloraThe presence of a relatively diverse soft-bodied

burrowing infauna has been inferred. In light of this, it issurprising that at all sites (except 282) the epibenthic andplanktonic fauna and flora are so restricted, at least

1054


cm— 0

— 2

— 4

— 6

-― 8

Figure 5. Radiograph of cross-laminatedclayey silt; from zone of penecontem-poraneously deformed sediment thatlies just above the contact with the in-trusive basalt; natural scale. Sample29-280A-21-2, 54-63 cm.

until the later stages of terrigenous sediment accumula-tion. The subfacies at Site 282 is exceptional in that itcontains sparse amounts of sponge spicules (6% onaverage), and nannoplankton (3%-13%, according tocarbonate analyses). The nannoplankton are poorlypreserved, diverse at some horizons, yet very restrictedat many others. At Site 282 the subfacies is also notablefor the scattered occurrence of rare bivalve fragments, asolitary coral, shallow-water benthonic foraminiferaand low diversity planktonic foraminifera.

Elsewhere the fauna and flora are very restricted.Agglutinating foraminifera are the only benthicorganisms that occur at all sites (207, 275, 280, 283). Thelarge tubiform foram Bathysiphon occurs in scatterednumbers at Site 283 and is abundant throughout thelower 250 meters of the sequence at Site 280, where theonly other foraminifera recorded is Cyclammina.Bolivinopsis spectabilis, several specimens of which occurat one horizon, is the only foraminifera known at Site275. Small numbers of siliceous and agglutinating foram-inifera occur throughout the sequence at Site 283, with

their preservation improving and diversity increasingdownward (Webb, this volume). The only other benthicfossil recovered was a small colonial worm-like to cone-shaped chitinous microfossil of unknown affinity. Thismicrofossil occurs at the base of the sequence at Site 280and at several horizons at Site 283. The smallerspecimens tended to be attached to sand grains (Perch-Nielsen, Chapter 25, this volume).

Pelagic microfossils including dinoflagellates, silico-flagellates, archeomonads, spores, and pollen occur inmodest numbers and varying diversity at severalhorizons at most sites. Rare well-preserved Radiolariaoccur at Site 275. Planktonic foraminifera are absent.Nannofossils also are absent, except at Site 280, wheresmall nannofossils representing a very limited number oftaxa occur sporadically.

The bulk of the subfacies is therefore characterized bya very restricted epibenthic and planktonic fauna andflora. At Sites 275, 280, and 283 the characterizationchanges in the upper part of the terrigenous-richsequence. Siliceous microfossils, particularly diatoms,but also radiolarians and sponge spicules, appear inincreasing amounts. At the top of the sequence at Site275, the microfauna is diverse, but the specimens arealmost all broken. At Site 280, diatoms are rare,commonly dissolved or pyritized in the chertified upperpart of the subfacies, but dinoflagellates, pollens, andspores are rich at some horizons. At Site 283 diatoms,radiolarians, silicoflagellates, and sponge spicules arefew and poorly preserved over the upper 100 meters ofthe subfacies. However, they become increasinglydiverse and the assemblage rich and moderately to wellpreserved over the uppermost 20-30 meters. At all threesites, this transition zone is succeeded by terrigenoussiliceous oozes, in which well-preserved diatoms aredominant. At Site 207, the terrigenous facies gradesupward into clay-rich nannofossil chalk, then chertifiedchalk, then siliceous fossil-bearing foraminiferal andnannofossil ooze.

Thus, at the four sites (275, 280, 283, and 297) there isa rapid deterioration in skeleton condition and numbersdown-section until the sequence becomes barren ofsiliceous microfossils. This is interpreted to be directlylinked to their dissolution and the precipitation of thesecondary silica minerals, cristobalite and tridymite. It isapparent that the upper and even the middle parts of thedeposited sequence were either siliceous-rich silty clay orsilty clay-rich siliceous ooze. It is not clear whether thenonsilicified lower part of the sequence accumulated inthe form of siliceous-rich silty clay, or as just a relativelynonfossiliferous silty clay. Two sites have provided adetailed sediment record down to basement (280, 283),and most of the siliceous and agglutinating benthic for-aminifera that occur in the sequences, especially thestriking and large Bathysiphon, come from below thesilicified zone of terrigenous sediments. It could beargued that the initial deposits were largely barren ofsiliceous microfossils, and the lower limit of silicificationrepresents the point in the sequence at which bio-genically productive cool ("Antarctic") water rich insiliceous micro-organisms reached each site. This in-terpretation is preferred.

1055


SAND BURROW - MOTTLED SUBFACIES(41 samples)

2 7 5

280

282

283

BEDDED SUBFΔCIES(12 samples)

282graded beds

CLAY

gradational zone

at top of unit

25.

SILT

Figure 6. Ternary sand-silt-clay diagram showing the textural characteristics of the burrow-mottled and the bedded subfacies.The three coarse samples come from a transitional zone where terrigenous silt and clay grades up into radiolarian-rich diatomooze at Site 275.

Depositional Conditions

The burrow-mottled terrigenous silt and claysubfacies is widespread in the Southern Oceans in avariety of bathymetric positions. At many locations it isthe first sediment deposited upon basement (largelynewly formed oceanic basalt, but at a few locationscontinental crust). The main feature that must beexplained is the almost total absence of the planktonicmicrofossils that elsewhere dominate oceanic sediments.

Burrow-mottles of various forms suggest that adiverse and relatively abundant infauna existed, aconclusion supported by the high concentration oforganic carbon and the widespread occurrence of pyrite,some pseudomorphous after fecal pellets. These featuressuggest that the environment of deposition was mod-erately oxygenated, and certainly not euxinic. The

overlying waters were oxygenated, and possibly mod-erately agitated. This is indicated by the characteristicoccurrence of manganese micronodules throughout thesubfacies; micronodules do not form in stagnant waters.However, the bottom currents responsible for wateragitation must have been weak, as primary stratificationis rare. Sparse stratification may have been destroyed bythe burrowing infauna, but many sections are onlyslightly burrowed, so that any original primarystratification would still be visible.

The very restricted nature of the epibenthic micro-fauna further suggests that bottom water conditionswere ecologically limiting. The almost total absence ofplanktonic calcareous microflora and microfauna (ex-cept at Site 282) probably means that the depositionalsites lay below the carbonate compensation depth. High

1056


99-99

99-90

99-00

EN

T

oerLUGL

Lü>

_ l

Z)o

95-00

90-00

80-00

70-00

60-00

5000

4000

30-00

20-00

lO OO-

500-

100-

0 1 0 -

001

bedded subfαcies

mottled subfαcies

2 3 4 5 6 7 8 9

DIAMETER

I0 II

Figure 7. Cumulative frequency curves showing the grain size distribution in a characteristic range of samples from the twosub fades.

1057


TABLE 2Sand-Silt-Clay Proportions and Median Size Values for Samples from

the Burrow-Mottled Subfacies

Sample(Interval in cm)

*275-2-6, 60275-2-6, 101

*275-2-6, 136275-4-2, 112

*275~4-2, 118*280A-8-2, 116*280A-10-6, 42

280A-10-6, 57*280A-13-3, 57280A-14-1, 78

*280A-15-2, 20280A-17-1, 88

*280A-17-4, 100280A-18-1, 105280A-19-3, 80

*280A-19-3, 90*280A-20-3, 130280A-21-2, 116

*280A-22-l, 100282-114,51

*282-ll-4, 100282-11, CC282-15, CC282-16-1,58

*282-16-l,78282-17-3,75

*282-17-3,94283-8-2, 56

*283-8-2, 75283-9-2, 68

*283-9-2, 88283-11-2,84

•283-11-2, 120283-12-2,77

*283-12,CC283-13-2, 75283-14-3, 55

*283-14-3,69283-15-2, 78

*283-16-2, 89283-16-2, 106283-17-4, 83

*283-17-4, 109

Sand(%)

41.533.146.5

5.111.30.00.30.15.6

27.88.50.71.81.34.17.12.21.11.9

6.74.86.6

12.14.95.94.34.8

0.51.90.20.40.00.80.00.40.10.54.60.30.60.11.00.5

Süt(%)

31.036.829.942.943.532.547.640.961.319.579.133.246.932.635.150.335.429.849.423.244.232.943.846.551.749.348.8

29.637.444.536.125.016.450.231.620.225.229.025.714.613.523.525.7

Clay(%)

27.530.123.652.045.267.552.159.033.152.712.466.251.366.060.842.662.469.148.770.051.060.544.148.642.446.446.4

69.960.755.363.574.982.849.868.079.674.366.473.984.886.375.673.8

Median(05O>

4.26—

4.09-

7.4010.058.20

_6.42

-4.80

—8.10

--

7.108.96

—7.92

—

8.16———

7.20—

7.66_

8.90-

9.13—

11.20

9.41__

9.76—

10.28——

9.54

Classification

Sand-silt-claya

Sand-silt-claya

Sand-silt-clay a

Silty claySilty claySilty claySilty claySilty clayClayey siltGlauconitic sandy claySütSilty claySilty claySilty claySilty clayClayey siltSilty claySilty clayClayey siltSilty claySilty claySilty claySilty claySilty clayClayey siltClayey siltClayey siltSilty claySilty claySilty claySilty claySilty clayClayClayey siltSilty clayClaySilty claySilty claySilty clayClayClayClaySilty clay

Note: Samples marked with an asterisk were analyzed in theSedimentation Laboratory, New Zealand Geological Survey;the remainder were analyzed by the Deep Sea Drilling Project.

transition zone.

1058


TABLE 3Mean Composition of the Terrigenous Silt and Clay Facies

Based on X-Ray Diffraction Analysis of Bulk Samples

Mineral

QuartzK-feldsparPlagioclaseMicaMontmorilloniteClinoptilolitePyriteChloriteKaolinGypsumCalciteAragoniteCristobaliteTridymite

Site207

(2)

7.11.43.01.23.31.11.8——-—-

69.211.4

Mottled SubfaciesSite275

(2)

33.614.1

8.627.0

2.61.61.01-4———-

9.01.3

Site280

(7)

35.94.37.2

26.213.40.80.93.60.1-—-

7.30.2

Site283

(9)

32.83.42.0

21.827.8

-0.91.71.50.80.2-

6.50.5

Site282

(6)

37.21.72.5

13.811.510.34.0—

9.3-

9.20.4--

BeddedSubfacies

Site282

(10)

28.52.64.1

10.116.70.80.4—

5.30.4

30.21.0--

Note: Numbers in parentheses indicate number of samples.

detrital sedimentation rates cannot account for theabsence of calcareous nannofossils in the New Zealand-Tasman Sea area. However, it may account for theiroccurrence in only limited numbers in the terrigenoussilt and clay facies at many Indian Ocean sites. Becauseof the paucity of fossils, sedimentation rates cannot becalculated at many sites. At two sites where calculationis possible, the rates are relatively high (8 cm/1000 yr atSite 263 and 4 cm/1000 yr at Site 280).

In summary, these features suggest that the very fine-grained terrigenous sediments accumulated at variousoceanic sites where bottom waters were oxygenated, butcirculation was sluggish. In the New Zealand-TasmanSea area, the subfacies is succeeded upward by mixeddetritus and siliceous ooze and then siliceous ooze. Inmuch of the Indian Ocean, the subfacies is succeededupward by nannofossil and mixed foraminiferal andnannofossil ooze. These sequences suggest that circula-tion in each ocean basin was restricted for a long time,but that gradually an integrated circulation patternevolved for each basin. Cool (Antarctic) waters con-sequently were introduced into the southwest PacificOcean and the Tasman Sea, and warm (subtropical)waters were introduced into the Indian Ocean.

Bedded Subfacies

This subfacies is characterized by color-graded andslightly size-graded bedding and by varying amounts(5%-15%) of nannofossils. The beds characterize unitsthat alternate with units of the burrow-mottled sub-facies. At Site 282 bedded subfacies occurs throughouttwo zones: one, 55-105 meters, and the other, 114-192meters subbottom (Figure 2).

The bedded subfacies is a green-gray, light olive-gray,and olive-gray silty clay and clayey silt. Color-gradedbeds 20-90 cm thick are common, grading upward fromolive-gray to green-gray, or conversely from light at the

cm-0

— 2

— 4

10

—12

— 14

Figure 8. Part of a graded bed of glauconiticsilty sandstone. Rounded clasts (3-10 mmdiameter) of glauconitized claystone areabundant in the lower part of the bed.Sample 29-280A-14-1, 69-84 cm.

base to dark at the top. The interbed intervals aregenerally green-gray. Most color-graded beds are siltierthan the interbed intervals, but sand content is the same(Figure 6). Detailed analysis shows that some beds aredetectably size-graded (Figure 10), the lower intervalbeing reverse graded, a common feature in graded beds.Some beds appear to be amalgamated, consisting of suc-cessive graded intervals. Nannofossil content andcharacter appear to be markedly different between thegraded beds and the interbed intervals. One detailed

1059


Site 280

Hole 2 8 0 A

RELATIVE ABUNDANCE

UNITS I a 2

UNIT

UNIT

UNIT

3

4

5Δ

silty diatom ooze

and diatom detritial

silt.

UNITsilty clay

silt with

5Bor clayey

chert.

UNIT 5C

clayey siltstone or

silty claystone

basalt intrusion

Figure 9. Sequence at Site 280. (Hole 280A), showing therelative frequency and relationship between the disap-pearance of siliceous microfossüs and the developmentof silicification (expressed by the presence of chert,cristobalite, and tridymite). The relationships here pro-vide stratigraphic evidence that siliceous microfossilsprovide the silica that crystallizes to form cristobalite,tridymite, and, through inversion, chert.

analysis showed that a graded bed contained abundant,moderately preserved nannofossils, whereas the interbedcontained small numbers of poorly preserved nanno-fossils. No variation in nannofossil content or characterthroughout the bed could be detected.

Thin beds (1-2 cm thick) that are sparsely scatteredthroughout the subfacies are rich in glauconite. Somehorizons are intensely mottled, the mottling all butmasking thin beds, but generally the subfacies is sparselymottled.

Carbonate analyses show that the bedded subfaciescontains 15%-40% calcareous microfossils, most ofwhich are nannofossils. Shallow-water microfossils forma significant part of the fossil assemblage; benthonicforaminifera, neritic nannofossils, and ascidian scleritesoccur throughout. No bivalve shell fragments or solitarycorals were seen.

Compositionally, the subfacies is very similar to theburrow-mottled subfacies with which it alternates.Calcium carbonate content is higher (average of 24%versus 9%), and the sponge spicule, nannofossil, andglauconite content is markedly higher than in theburrow-mottled subfacies. Organic carbon content ismuch lower (average of 0.3% versus 3.3%), pyrite signifi-cantly lower, and micronodule content very similar orslightly lower than in the mottled subfacies. The detritalmineralogy is very similar in content and proportions inboth subfacies, although sand-size rock fragments andround quartz grains are notable in the bedded subfacies.

Depositional ConditionsSite 282 in 4202 meters of water is near the foot of the

continental slope off western Tasmania. Its position isvery similar to that of Site 280.

The terrigenous facies at Site 282 is very similar tothat at Sites 207, 275, 280, and 283, with the exceptionthat shallow marine benthonic organisms (foram-inifera, solitary corals, bivalve fragments, and ascidiansclerites) and neritic nannofossils occur in both sub-facies. The color-grading, subtle size-grading, andrelative abundance of nannofossils in the graded beds ofthe bedded subfacies suggest that they represent theperiodic incursion of turbidity currents into thedepositional area. The beds are regarded as distalturbidites and appear to consist largely of sedimentreworked from the continental shelf and continentalslope. The occurrence of shallow benthonic and neriticfossils throughout both subfacies suggests that shelfmaterial was continually reworked into deep water atthis locality. However, it was only periodically that suf-ficient sediment was entrained for density currents toform and distinct beds to be deposited at the foot of thecontinental slope. The frequency of turbidite depositionand the volume of sediment deposited appear to havebeen low, as the rate of sedimentation is the same forboth subfacies—2.4 cm/1000 yr, a moderate rate.

CONCLUSIONSThe terrigenous silt and clay facies appears to be the

normal sediment accumulation in moderate- to deep-ocean positions wherever planktonic organisms do notrichly populate the overlying water column. The faciesalso accumulates where the depositional sites are sodeep that microfossils, especially calcareous micro-fossils, are dissolved before they can become in-corporated in the sediment. At some deep sites, cal-careous sediments interbedded with terrigenous silt and

1060


99-99

99-90

9900

9500

9000

8000O

er 7θ ooLu°- 6000LU 5000j= 40-00

=J 3000

O2000

1000

500

100

010

001

29-282-13-4-23-5 top

29-282-13-4- 26-5

29-282-13-4- 29

29-282-13-4-30-5

29-282-13-4-32 base

4 5 6 7

DIAMETER

8 I0 II

Figure 10. Cumulative size frequency curves for a series of samples showing grading in a bed from the bedded subfacies atSite 282.

1061


clay are interpreted to represent turbidites (Sites 212,260, and 282 of the Indian Ocean). Elsewhere, it isusually terrigenous sediment that accumulates uponbasement. Apparently terrigenous sediment is only re-placed by organic sediments, either siliceous or calcar-eous, when continental landmasses have drifted suf-ficiently far apart for an integrated circulation pattern tohave developed, covering most or all of the evolvingocean basin. This sequence of events is especially clear inthe areas covered by DSDP Legs 21 and 29 (Sites 207,275, probably 277, 280, 282, and 283) and in large partalso is applicable on both the west and east sides of theIndian Ocean (Sites 241, 248, 256, 258, 259, and263).Wherever the sequence is dominantly organicsediment in these areas, the sequence has either formedafter spreading has developed and an integrated circula-tion pattern evolved (Sites 240[?], 245[?], 251, 278) or ithas formed on shallow topographic features whose veryelevation above the surrounding topography generates apersistent local circulation that promotes biogenicallyrich overlying waters (Sites 242, 246, 253, 254, 277, 281,and 284). These conclusions are based largely on thosesites in the western Pacific, Tasman, and Indian oceanswhere basement was reached, or very closelyapproached.

In the areas covered by Legs 21 and 29 (southwestPacific, Tasman Sea, southeast Indian Ocean), thechange from terrigenous sediments to mixed biogenicand terrigenous sediments or purely biogenic sedimentsappears to be closely related to the sea-floor-spreadinghistory determined for each ocean segment. Magneticlineation data show that the New Zealand subcontinent(including the Campbell Plateau and Lord Howe Rise)began to spread away from Antarctica about 80-83million years ago. The New Zealand subcontinent beganto spread away from Australia about 78 million yearsago, and Australia began to spread away from Antarc-tica about 56 million years ago. The latter is about thesame time that spreading in the Tasman Sea concluded(Weissel and Hayes, 1972; Christoffel and Falconer,1972; Hayes and Ringis, 1973).

The change from terrigenous sedimentation to largelysiliceous biogenic sedimentation in the vicinity of theCampbell Plateau (Site 275) began in the late Campa-nian 70-75 million years ago). On the southern LordHowe Rise (Site 207) the change from terrigenous tomixed calcareous and siliceous sedimentation began atthe end of the Maestrichtian, or the beginning of thePaleocene (65 million years ago). The change to siliceoussedimentation in the central Tasman Sea (Site 283)began early in the Eocene (about 54 million years ago),and the change to mixed siliceous and terrigenoussedimentation in the vicinity of the South Tasman Rise(Site 280, southeast Indian Ocean) began about the endof the Eocene (38 million years ago). Even if one infersthat siliceous microfossils originally occurred (that is,presilicification and chertification) much lower in thesequence at each of these four sites, the depositionalorder is still the same, and is still consistent with thespreading history appropriate to each location.

This order of events lends support to the thesis thatthe terrigenous facies accumulated in ocean basins

under limited circulation conditions. Only when thecontinental landmasses had drifted far apart did anintegrated oceanic circulation pattern develop and anabundant and diverse plankton appear. With itsappearance, terrigenous bottom sediments becamediluted with siliceous microorganisms that populatedthe overlying oceanic waters.

Probable On-Land Equivalents of the Terrigenous FaciesSeveral Maestrichtian sedimentary rock units

exposed in eastern New Zealand closely compare withthe terrigenous facies as described and may have asimilar origin. In Marlborough, South Island, theWoolshed Formation (Webb, 1971) consists of up to 750meters of dark brown and dark gray, siliceous, pyriticcarbonaceous mudstone. It is characterized by a markedparting parallel to the bedding and splintery fracture.The upper part of the unit is increasingly siliceous and atthe top contains several 0.31.7-meter-thick bands ofdark brown chertified shale (MacPherson, 1952). It issucceeded conformably by bedded chert with greensandpartings (Mead Hill Flint of Webb, 1971), which in turnis conformably succeeded by a white, partly chertifiedfine-grained limestone (Amuri Limestone). To thenortheast the Woolshed Formation grades into theMirza Formation, Webb (1971), which is up to 300meters thick. It is a fine-banded clayey siltstone, withscattered lenticular nodules of chert and is conformablyoverlain by sandstone and siltstone with chert lenses(Butt Formation). The Butt Formation is in turn suc-ceeded by the Amuri Limestone. The Woolshed andMirza formations are characterized by a limited faunaconsisting of siliceous and agglutinated foraminiferaand appear very similar to the mottled subfacies.

A third Maestrichtian unit, the Whangai Formation,which also compares closely with the terrigenous facies,is widespread in southern Hawkes Bay, North Island(Lillie, 1953; Kingma, 1971). It is a uniformly pale gray(weathers white), hard, micaceous, siliceous argillite, upto 900 meters thick. Conchoidal fracture is charac-teristic at some places. It is thoroughly burrowed, andfine discontinuous banding is faintly discernible at manylocations. The upper part is commonly dark gray andchertified. Glauconitic mudstone and greensand arecommon near the top of the unit. The lower part of theunit is characterized by a restricted fauna of siliceousand agglutinating foraminifera, but the upper partcontains a much more diverse fauna which includessome calcareous benthonic and planktonic fora-minifera and nannofossils. The Whangai Formation issucceeded by variously colored, soft mudstone, green-sand, and hard blue-gray siltstone.

The silicified character of these units may result fromdissolution of siliceous microfossils, and redeposition ofthe silica as cristobalite and tridymite, as noted in theburrow mottled subfacies. Remnant siliceous micro-fossils have not been reported from the units, but Radio-laria occur in sediments overlying the Whangai Forma-tion (Finlay, in Lillie, 1953), and large sponges areknown from Amuri Limestone where it overliesWoolshed and Mead Hill formations at WoodsideCreek, Marlborough. With the knowledge provided by

1062


Leg 29 results, careful collecting may well show thatsiliceous microfossils, especially diatoms, were original-ly characteristic of the Woolshed, Mirza, and Whangaiformations, and that the sediments were depositedunder conditions very similar to those proposed for theburrow mottled subfacies.

ACKNOWLEDGMENTS

The following New Zealand Geological Survey staff gaveinvaluable assistance in the preparation of thispaper: G. Richards (size analysis, sectioning, and photog-raphy), L. Leonard (drafting), and C. Johnstone (typing). Weextend to them our grateful thanks.

REFERENCES

Berger, W. H. and von Rad, U., 1972. Cretaceous andCenozoic sediments from the Atlantic Ocean: In Hayes, D.E., Pimm, H. C, et al., Initial Reports of the Deep SeaDrilling Project, Volume 14: Washington (U.S. Govern-ment Printing Office), p. 787-954.

Burns, R. E., Andrews, J. E., et al., 1973. Initial Reports of theDeep Sea Drilling Project, Volume 21: Washington (U.S.Government Printing Office).

Christoffel, D. A. and Falconer, R. K. H., 1972. Marinemagnetic measurements in the southwest Pacific Ocean andthe identification of new tectonic features. In Hayes, D. E.

(Ed.) Antarctic oceanology II: The Australian-NewZealand Sector, Antarctic Res. Ser., 19: Washington (Am.Geophys. Union), p. 197-209.

Greenwood, R., 1973. Cristobalite: Its relationship to chertformation in selected samples from the Deep Sea DrillingProject: J. Sediment. Petrol., v. 43, p. 700-708.

Hayes, D. E. and Ringis, J., 1973. Sea floor spreading in theTasman Sea: Nature, v. 243, p. 454-458.

Kingma, J. T., 1971. Geology of Te Aute Subdivision: NewZealand Geol. Surv. Bull. 70.

Lillie, A. R., 1953. The geology of the DannevirkeSubdivision: New Zealand Geol. Surv. Bull. 46.

Lisitzin, A. P., 1972. Sedimentation in the world ocean: Soc.Econ. Paleontol. Mineral. Spec. Publ. 17, Tulsa,Oklahoma.

Luyendyk, B. P., Davies, T. A., et al., 1972. Site 258,Unpublished Hole Summaries Leg 26, Deep Sea DrillingProject.

MacPherson, E. O., 1952. The stratigraphy and bentoniticshale deposits of Kekerangu and Blue slip, Marlborough:New Zealand J. Sci. Technol. Ser. B., v. 33, p. 258-286.

Summerhayes, C. P., 1969. Marine geology of the NewZealand Subantarctic sea floor: New Zealand Oceanog.Inst. Mem. 50.

Webb, P. N., 1971. New Zealand Lare Cretaceous(Haumurian) foraminifera and stratigraphy: A summary:New Zealand J. Geol. Geophys., v. 14, p. 795-828.

Weissel, J. K. and Hayes, D. E., 1972. Magnetic anomalies inthe southeast Indian Ocean. In Hayes, D. E. (Ed.),Antarctic Oceanology II: The Australian-New ZealandSector, Antarctic Res. Ser., 19: Washington (Am. Geophys.Union), p. 165-196.

1063

31. terrigenous silt and clay facies: deposits … · terrigenous silt and clay facies: deposits of...

Documents