Bottom-current erosion in the SouthAtlantic sector of the southern ocean

MICHAEL T. LEDBETrER and PAUL F. CIES!ELSK!

Department of GeologyUniversity of Georgia

Athens, Georgia 30602

Disconformities in the sedimentary record of the southernocean have been attributed to erosion by two major bottom-water masses in the antarctic and subantarctic regions. Erosionof the seafloor in areas more than 4,000 meters deep has beenattributed to scour by Antarctic Bottom Water (AABW) in theIndian Ocean (Kennett and Watkins 1976; Watkins and Ken-nett 1971, 1972). Erosion of shallow rises in the south Atlanticsector of the southern ocean (Ciesielski, Ledbetter, and Eli-wood in press; Ciesielski and Wise 1977) has been attributedto Circumpolar Deep Water (cDw). We have chosen a traverseOf ARA Islas Orcadas piston cores from Antarctica to Africa(figure 1) to examine the pathways and periods of increasedbottom-water production (shallow and deep) in the WeddellSea region.

The magnetic polarity of each of the 25 piston cores wasdetermined at approximately 10-centimeter intervals. Thebiostratigraphic age was determined on the basis of a diatom

Figure 1. The location of piston cores from Islas Orcadas shown onbathymetry from Goodell (1973).

zonation (McCollum 1975; Weaver and Gombos in press) and,where required, a silicoflagellate zonation (Busen and Wise1977; Ciesielski 1975; Weaver 1976). Biostratigraphic sampleswere taken above and below magnetic reversals and at moreclosely spaced intervals where disconformities were indicated.Piston core segments were assigned an age (figure 2) by cor-relation to the magnetostratigraphic time scale (LaBrecque,Kent, and Cande 1977; Mankinen and Dalrymple 1979). Themagnetos tratigraphy and biostratigraphy of the 25 piston coresreveal disconformities during the last 5.5 million years of sed-imentation in this area (figure 2). Periods of erosion or non-deposition of sediment have been identified by hiatuses inthe sedimentary record. These hiatuses are a function of thephysiographic province where the core is located.

The age of sediment at or very near the seafloorTeveals thatthe present velocity of bottom currents in the South Atlanticsector of the souther ocean is high enough to erode or inhibitdeposition of sediment in three areas. The most recent sedi-ment from the deep Weddell Basin is lower Pliocene to Mio-cene (figure 2) and forms the Weddell Basin scour zone. Ero-sion of sediments near the crest of the Atlantic-Indian Ridgehas exposed lower Pleistocene sediments in the uppermostsediment column. One core near the continental rise adjacentto the Cape Basin recovered Pliocene sediment near the sea-floor.

Scour by AABW within the deep, northern Weddell Basin hasexposed upper Pliocene sediment with a northern limit ofscour at the base of the ridge (figure 2). We have designatedthe Weddell Basin, including the area south of the Maud Rise,as a scour zone since erosion or winnowing by AABW hasremoved sediment deposited since the Miocene to early Pli-ocene.

A second large scour zone is found on the crest of the Atlan-tic-Indian Ridge (figure 2). This shallow disconformity is sep-arated from the Weddell Basin scour zone by a zone of sedi-mentation along the south flank of the ridge. The disconform-ity is maintained by high-velocity CDW which has eroded orinhibited deposition at the crest of the ridge, leaving lower tomiddle Pleistocene sediments at the seafloor.

The third disconformity at or near the seafloor is found inone core (figure 2) from near the connection of the West Agul-has Basin and the Cape Basin (figure 1). Lower Pleistocenesediment is found at the seafloor in this deep site where ero-sion is caused by high-velocity AABW which flows in a clock-wise gyre in the Cape Basin (Embley and Tucholke 1976).

In addition to the disconformities at the present seafloor,three major hiatuses were identified within the sedimentaryrecord (figure 2). The youngest hiatus is restricted to the Atlan-tic-Indian Ridge, while the oldest hiatus is apparently morewidespread and the third hiatus is found only on the southernflank of the Atlantic-Indian Ridge.

Four cores near the crest and south flank of the Atlantic-Indian Ridge have a hiatus in the sedimentary record from themiddle Pliocene to early-middle Pleistocene (figure 2). Thesecores are from water depths of less than 3,850 meters and arepresently in the axis of high-velocity CDW which has left adisconformity at the seafloor. The scour zone on the Atlantic-Indian Ridge during the middle Pliocene to early Pleistocenerepresents an increase in the velocity of CDW within an areasimilar to the present core of high-velocity CDW. This hiatusis very similar to one identified on the Maurice Ewing Bankat a similar water depth (Ciesielski, Ledbetter, and Ellwood inpress).

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Figure 2. Ages of cores in figure 1 determined on the basis of the assignment of core segments to the magnetostratigraphic time scaleusing a diatom zonation. Three areas (Weddell Basin, Atlantic-Indian Ridge, and Cape Basin) have a disconformity at the present seafloor.

A buried disconformity on the south flank of the Atlantic-Indian Ridge at the northern boundary of the Weddell Sea(figure 1) is delineated in two cores (io 11-67 and 70). Thehiatus extends from the middle Pliocene (3.5 million years) toearly-middle Pleistocene (1.8-0.8 million years). Since bothcores are much deeper (>4,500 meters) than the disconformityin cores on the Atlantic-Indian Ridge (<3,850 meters), thebottom-current responsible for the scour was the deeper AABW.

The cessation of scour in the early-middle Pleistocene to thenorth of the present Weddell Basin scour zone represents amajor reduction in the thickness or lateral extent of the AABW.

A third buried disconformity was delineated on the profileof cores. Middle Pliocene sediment unconformably overliesupper Miocene sediment in three cores (figure 2). This discon-formity is more widespread than others since it extends fromthe southern flank of the Atlantic-Indian Ridge to the southernWeddell Basin. The water mass responsible for erosion is prob-ably the AABW; however, no conclusion can be drawn concern-ing the areal extent or watermass responsible without a greatercore coverage.

The intensification of bottom circulation that results in dis-conformities may be caused by climatic deterioration (Cie-sielski, Ledbetter, and Ellwood in press; Kennett and Watkins1976; Moore et al. 1978; Watkins and Kennett 1972). The exactage of the increase in bottom-water velocity cannot be deter-mined, however, since it falls within the hiatus. The age spanof the disconformities in this region encompasses some majorclimatic coolings that could be responsible for intensificationof benthic circulation. The cause-and-effect relationship of thepaleoclimatic fluctuations and bottom-water paleocirculationmust await more precise dating of the circulation events bytracing scour zones into marginal winnowing areas wheredating of high-velocity pulses may be delineated (Allison andLedbetter in press; Huang and Watkins 1977; Ledbetter, Wil-liams, and Ellwood 1978).

This work was supported by National Science Foundationgrant DPP 79-05111. We thank D. Cassidy for his help in sam-pling Islas Orcadas piston cores stored at the Antarctic CoreStorage Facility.

ReferencesAllison, E. T., and Ledbetter, M. T. In press. Timing of bottom-water

scour recorded by sedimentological parameters in the South Aus-tralian Basin. Marine Geology.

Busen, K. E., and Wise, S. W., Jr. 1977. Silicoflagellate stratigraphy.In P. R. Barker et al., Initial reports of the Deep Sea Drilling Project,Vol. 36. Washington, D.C.: U.S. Government Printing Office.

Ciesielski, P. F. 1975. Neogene and Oligocene silicoflagellates fromcores recovered during antarctic leg 28, Deep Sea Drilling Project:Biostratigraphy and paleoecology. In D. F. Hayes et al., Initial reportsof the Deep Sea Drilling Project, Vol. 28. Washington, D.C.: U.S.Government Printing Office.

Ciesielski, P. F., Ledbetter, M. T., and Ellwood, B. B. In press. Thedevelopment of antarctic glaciation and the Neogene paleoenviron-ment of the Maurice Ewing Bank. Marine Geology.

Ciesielski, P. F., and Wise, S. W., Jr. 1977. Geologic history of theMaurice Ewing Bank of the Falkland Plateau (southwest Atlanticsector of the southern ocean) based on piston and drill cores. MarineGeology, 25, 175-207.

Embley, R. W., and Tucholke, B. E. 1976. A continuous erosional zonein the Cape, Agulhas and Mozambique Basins. Geological Society ofAmerica, Abstracts with Programs, 8, 854.

Goodell, H. G. 1973. Marine sediments of the southern ocean. In V. C.Bushnell (Ed.), Antarctic map folio series (Vol. 17). New York: Amer-ican Geophysical Union.

Huang, T. C., and Watkins, M. D. 1977. Antarctic bottom water veloc-ity: Contrasts in the associated sediment record between theBrunhes and Matuyama epochs in the South Pacific. Marine Geology,23, 113-132.

Kennett, J . T., and Watkins, N. D. 1976. Regional deep-sea dynamicprocesses recorded by late Cenozoic sediments of the southwesternIndian Ocean. Geological Society of America Bulletin, 87, 321-329.

LaBrecque, J . L., Kent, D. V., and Cande, S. C. 1977. Revised mag-

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netic polarity time scale for Late Cretaceous and Cenozoic time.Geology, 5, 330-335.

Ledbetter, M. T., Williams, D. F., and Ellwood, B. B. 1978. Late Pli-ocene climate and south-west Atlantic abyssal circulation. Nature,272, 237-239.

Mankinen, E. A., and Dalrymple, C. B. 1979. Revised geomagneticpolarity time scale for the interval 0-5 m.y.B.l'. Journal of Geophys-ical Research, 84, 615-626.

McCollum, D. W. 1975. Antarctic Cenozoic diatoms: Leg 28, Deep SeaDrilling Project. In D. E. Hayes et al., Initial reports of the Deep SeaDrilling Project, Vol. 28. Washington, D.C.: U.S. Government Print-ing Office.

Moore, T. C., Jr., van Andel, Tj. H., Sancetta, C., and Pisas, N. 1978.Cenozoic hiatuses in pelagic sediments. Micropaleontology, 24,113-138.

Watkins, N. D., and Kennett, J. P. 1971. Antarctic bottom water: Majorchange in velocity during the late Cenozoic between Australia andAntarctica. Science, 173, 813-818.

Watkins, N. D., and Kennett, J . P. 1972. Regional sedimentary dis-conformities and upper Cenozoic changes in bottom water velocitiesbetween Australia and Antarctica. American Geophysical Union, Ant-arctic Research Series, 19, 273-294.

Weaver, R. M. 1976. Late Miocene and Pliocene radiolarian paleobiogeog-raphy and biostratigraphy of the southern ocean. Unpublishe4 doctoraldissertation, Florida State University

Weaver, F. M., and Combos, A. M., Jr. 1981. Southern high-latitudediatom biostratigraphy. In J . E. Warme, R. C. Douglas, and E. L.Winterer, The Deep Sea Drilling Project: A decade of progress (Specialpublication 32). Tulsa, Okla.: Society of Economic Paleontologistsand Mineralogists.

Seafloor spreading model for theWeddell Basin

JOHN LABRECQUE

Lamont-Doherty Geological ObservatoryPalisades, New York 10964

This article describes current work in two areas: analysis ofmarine magnetic anomaly data to develop a model of seafloorspreading in the Weddell Basin and preparation of a bathy-metric map of the Indo-Atlantic Basin.

Weddell Basin model. Analysis of marine magnetic anomalydata has been extremely useful in describing the developmentof oceanic basins and the evolution of continental margins. Apaucity of appropriate data within the Antarctic has delayedapplication of these same techniques to the antarctic marginsand oceanic basins.

The Weddell Basin recently has been the focus of severalgeophysical cruises, including those of the British AntarcticSurvey vessels and the ARA Islas Orcadas. Analysis of themarine anomaly data gathered during these cruises has yieldeda distinct magnetic anomaly pattern which can be correlatedto the magnetic anomaly pattern M29 to 18, or ages of LateJurassic to late Eocene, respectively (figure 1) (LaBrecque andBarker 1981). The present data set covers only portions of theeastern Weddell Basin; however, coverage is sufficient toextrapolate the seafloor spreading model for the Weddell Basinas a whole. It is hoped the model will serve as a guide inplanning future expeditions to the region.

The marine magnetic data suggest that the Weddell Basinwas formed at the southern flank of a north-south spreadingsystem that was initiated during the Early Jurassic breakup ofGondwana. The anomaly lineation pattern indicates that thepoles of rotation that describe the Mesozoic separation ofGondwana with respect to West Antarctica remained very nearthe Antarctic Peninsula during most of the Mesozoic. There-fore, a Gondwana reconstruction similar to that of DeWit (1977)is supported by the recent data. During the Cenozoic, theWeddell spreading center has been progressively subductedbeneath the Scotia Plate, with only the America AntarcticRidge surviving to the present.

Indo-Atlantic Basin map. Preparation of a new bathymetricmap of the Indo-Atlantic Basin was also completed in 1981(figure 2). The map, prepared as part of the fifth edition of theGeneral Bathymetric Chart of the Oceans (GEBCO) map series,shows all recent track lines, including the complete Islas Orca-das data set. Bathymetry is contoured at a 250-meter intervalon a Mercator projection. Several new features have beendefined on the map, including the Meteor and Islas OrcadasRises, which manifest a Paleocene truncation of the Falkland/Aguihas fracture zone (LaBrecque and Hayes 1979). EnduranceRidge, located southeast of the South Orkney Platform, hasbeen named in honor of the ship Endurance, which broke up

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Figure 1. Magnetic anomaly profiles In the Weddell Sea from IslasOrcadas cruises and Project Magnet flight 720. Model parameters:layer depth, 6 kilometers; layer thickness, 500 meters. Skewnessparameter 0 = 0; transition zone width, w = 12 kilometers; mag-netization, 0.01 electromagnetic units per cubic centimeter.Spreading rates: 35-80 million years ago = 0.9 centimeters peryear; 80-108 million years ago = 0.45 centimeters per year;108-165 million years ago = 0.6 centimeter per year. Flow linesfor South American motion with respect to Antarctica in a fixedposition are shown as stippled lines. Ages along flow lines areexpressed as magnetic anomalies and are underlined.

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