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Lithological and texturalcomposition of Recent sediments

from the Maurice Ewing BankBJÔRN A. MALMGREN

Department of GeologyStockholm University

S-113 86 Stockholm, Sweden

The Maurice Ewing Bank is located at the eastern endof the Falkland Plateau, in the southwestern AtlanticOcean (figure 1). The bank has been subjected to inten -sive deep-sea drilling during recent years, as reflectedin Deep Sea Drilling Project (DSDP) leg 36 and ARA IslasOrcadas cruises 7 and 11, because it is the only part ofthe Atlantic sector of the Southern Ocean that has beensufficiently shallow to receive calcium carbonate sedi-mentation during most of its history (Ciesielski andWise, 1977).

The youngest sediments of the Maurice Ewing Bankconsist of a 1-2-meter thick layer of Plio-Pleistocene gla-cial-marine silts, sands, gravels, and oozes blanketing amuch thicker sequence of older (Jurassic through Mio-cene) deposits (Ciesielski and Wise, 1977). A zone rep-resenting the polar front (antarctic convergence) ex-tends across the bank. The northern (subantarctic)

boundary of this zone extends across the northern partsof the bank; the southern boundary (antarctic) extendsacross the southern part (Gordon and others, 1977).Thus the sediment samples examined provide good cov-erage across the polar front zone.

The lithologic composition of core-top sediments hasbeen determined by counts of more than 500 grains ineach of 25 core-top samples (topmost 1-2 centimeters)from Eltanin cruises 9 and 22, and Islas Orcada$ cruises7 and 11 (figure 1). Water depths ranged between 1,300and 3,600 meters. Counts were made of particles fromthe larger-than-90-micrometer size fraction. Mediangrain sizes were determined by standard sieming meth-ods and calcium carbonate contents by the Hülsemann(1966) gasometric method.

The following categories of particles were distin-guished: Whole (= identifiable) planktonic foramini-fera, fragments of planktonic foraminifera, benthonicforaminifera, radiolarians, diatoms, quartz (and feldsparin subordinate frequencies), and "other particles." The"other particles" group is dominated by glauconitegrains, but also contains manganese micronodules andvolcanic rock fragments. The glauconite grains are oftwo types—one a yellowish light green with a rough sur-face, the other a dark-green type with a smooth surface.The light-green variety appears to be mainly formedwithin the tests of the radiolarian suborder S/umellaria.

A latest Quaternary age of core tops containing plank-tonic foraminifera was confirmed by the presence of theGloborotalia truncatulinoides zone (less than about 200,000

480 W 460 41+0 L7° ,r0

Figure 1. Locations of Was Orcadas and Eltanln core tops from Maurice Ewing Bank (modified after Ciesleiski and Wise, 1977).The position of the polar front is after Gordon, Georgi, and Taylor (1977).

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years in subantarctic waters; Kennett, 1970). The occur-rence of typical cool subantarctic planktonic foramini-feral faunas in the core tops to the north of the polarfront indicates deposition under a subantarctic regime;this confirms a Holocene age for these samples. Absenceof the radiolarian Stylatractus universus supports a latestQuaternary age of core tops lacking calcium carbonate(less than about 410,000 years; Hays and Shackleton,1976).

In the southern ocean, the sediment type on the oceanfloor is largely controlled by the position of the polarfront (Goodell, 1973). Generally, calcareous oozes—oozes having a calcium carbonate (CaCO3) contentgreater than 30 percent—predominate to the north ofthe polar front at depths shallower than about 4,500meters. Siliceous oozes (mainly diatoms and radiolari-ans) predominate to the south of this boundary.

Q-mode factor analysis (with varimax rotation) ena-bles three different types of sediments to be distin-guished on the Maurice Ewing Bank: (1) calcium car-bonate-rich sediments (20-65-percent CaCO3) as shownby high loadings for the calcium carbonate content andby the frequencies of planktonic foraminifera and frag-ments of planktonic foraminifera; (2) sediments com-posed of quartz grains (40-75 percent) and "other par-ticles" (20-40 percent); and (3) siliceous oozes (about50 percent radiolarians and 0-30 percent diatoms).

The distribution of calcium carbonate-rich sedimentsgenerally reflects the position of the polar front (figure2), but there are some local deviations attributablemainly to topography. Thus, at the apex of the bank,the calcium carbonate-rich sediments are distributed tothe south of the northern boundary of the polar front,whereas, at the western flank, the boundary of carbonatesedimentation is located to the north of the polar front.The frequencies of quartz and "other particles" arebetween 15 and 65 percent, and the frequencies of ra-diolarians and diatoms are between 2 and 40 percent inthe zone of carbonate sedimentation.

Quartz and "other particles" are deposited to thesouth of the area dominated by carbonate sedimenta-tion. Radiolarians are relatively rare within this zone(less than 20 percent) compared with frequencies ofmore than 50 percent within the zone of siliceous ooze(figure 2). If a substantial fraction of quartz grains onthe Maurice Ewing Bank were ice-rafted (Plafker andothers, 1977), this indicates extensive deposition of gla-cial marine sediments in the area immediately to thenorth of the polar front on the western flank and withinthe polar front zone in other parts of the bank. Thisdistribution, therefore, would suggest relatively rapidmelting of sediment-bearing ice bergs close to the polarfront but rapidly tapering to the north in warmer su-bantarctic waters.

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53°Figure 2. Distribution of surface sediments on Maurice Ewing Bank. Sediments are calcium carbonate-rich sediments (20-65percent calcium carbonate); glacial marine sediments, mainly quartz (40-75 percent) and glauconite (20-40 percent); siliceousoozes (more than 50 percent radlolarians and diatoms). Distribution determined by 0- mode factor analysis of relative fre-quencies of different sediment particles in the greater-than-90-micrometers size fraction. Figures at sample points indicate

factor loadings.

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Grain size distributions do not show any clear relationto sediment type. Mean grain sizes are relatively largein areas where quartz and "other particles" dominate(2-2.5 4)) but vary more (2-4.5 4)) in the calcium car-bonate-rich sediments. Small mean sizes in the siliceousoozes (about 4.5 4)) reflect the abundance of radiolariansand diatoms.

I thank Dennis Cassidy (Florida State University), whogenerously helped in sampling; Hans G. Cronbiad(Stockholm University), who analyzed calcium carbonatecontents; James P. Kennett (University of Rhode Island),who offered comments on the manuscript; and AndersGranlund (Stockholm University), who performed ra-diolarian identifications.

References-Ciesielski, P. F., and S. W. Wise, Jr. 1977. Geologic history of

the Maurice Ewing Bank of the Falkland Plateau (southwestAtlantic sector of the Southern Ocean) based on piston anddrill cores. Marine Geology, 25: 175-207.

Goodell, H. G. 1973. The sediments. In Marine Sediments of theSouthern Oceans. Antarctic Map Folio Series, folio 17. NewYork: American Geographical Society.

Gordon, A. L., D. T. Georgi, and H. W. Taylor. 1977. Antarcticpolar front zone in the western Scotia Sea—summer 1975.Journal of Physical Oceanography, 7: 309-28.

Hays, J . D., and N. J . Shackleton. 1976. Globally synchronousextinction of the rabiolarian Stylatractus universus. Geology, 4:649-52.

Hülsemann, J . 1966. On the routine analysis of carbonates inunconsolidated sediments. Journal of Sedimentary Petrology,36: 622-25.

Kennett, J. P. 1970. Pleistocene paleoclimates and foraminiferalbiostratigraphy in subantarctic deep-sea cores. Deep-Sea Re-search, 17: 125-40.

Plafker, G., S. Bartsch-Winkler, and A. T. Ovenshine. 1977.Paleoglacial implications of coarse detritus in DSDP Leg 36cores. In Initial Reports of the Deep-Sea Drilling Project, vol. 36,ed. P. F. Barker, I. W. D. Dalziel, et al., pp. 857-64. Wash-ington, D.C.: U.S. Government Printing Office.

Cenozoic sedimentary hiatusessouth of Australia

DOUGLAS F. WILLIAMS

Department of GeologyUniversity of South Carolina

Columbia, South Carolina 29208

MICHAEL T. LEDBETTER

Department of GeologyUniversity of Georgia

Athens, Georgia 30602

Deep-sea drilling with the Glomar Challenger and theextensive coring program aboard USNS Eltanin and ARA

Islas Orcadas in the Southern Ocean showed the existenceof extensive regions of bottom scour and erosional dis-conformities in the deep basins surrounding Antarctica(Kennett et al., 1972; Watkins and Kennett, 1972; Ken-nett and Watkins, 1976; Kennett, 1977; Kennett et al.,1974). In the region between Australia and Antarctica,antarctic bottom water from the South Indian Basintraverses the Southeast Indian Ridge and enters theSouth Australian Basin (Kennett and Watkins, 1976).The bottom water flow then separates into major east-ward and northward directions, which supply waters tothe South Tasman and Wharton basins, respectively.

To better understand the effect of this bottom waterflow on the sedimentological conditions along the south-ern continental rise and Naturaliste Plateau of Australia,we carried out paleomagnetic and biostratigraphic stud-

ies on 27 piston cores taken during cruise 55 of the USNS

Eltanin (figure 1). Of these cores, 15 have been includedin a lithologic and biostratigraphic study of Mallet andHeezen (1977).

We used the foraminiferal zonations of Kennett (1973)and Kennett and Vella (1975) to correlate the magneticpolarity records with the established geomagnetic polar-ity scale (Cox, 1969). We took the lowest occurrence ofGloborotalia truncatulinoides to delineate the Plio-Pleisto-cene boundary and the base of the Gilsa event in theMatuyama reversed epoch, and used the presence ofGloborotalia puncticulata and the absence of C. truncatu-linoides to date Pliocene sediments. The Globorotalia con-omiozea-miozea conoidea bioseries differentiated Early Gil-bert and Late Miocene deposits.

Of the 14 cores with sufficient biogenic material fora biostrati graphic survey, 10 cores have evidence of dis-conformities. The tops of 6 of the cores (E-55-01, 55-17,55-19, 55-32, and 55-35) have Late Brunhes sections

AUSTRALIA

X,3435.37,39033-:---_------ --- -

30+ 32-.•'26 22j\24S \ - S

CRUISE 55 /2 \\/CORE LOCATIONS

• PISTON CORE 8TRIGGER CORE-0 PISTONCORE ONLY

Figure 1. Locations of EItanIn 55 piston cores used in pa-leomagnetic and biostratigraphic study of sedimentary hia-tuses along continental rise of southern Australia and Nat-

uraliste Plateau.

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