Journal of Conference Abstracts

Cool-Water Carbonates, ODP Leg 182, Great Australian Bight

D. A. Feary¹, A. C. Hine², M. J. Malone³ & J. A. Simo (Simo@geology.wisc.edu)⁴, M. Andres, C. Betzler, G. Brooks, C. A. Brunner, A. E. Holbourn, N.P. James, Q. Y. Li, B. C. Ladner, H. Machiyama, H. Matsuda, R. .M Mitterer, S. Shafik, F. Surlyk, P. Swart, U. G. Wortmann & Members of the Scientific Party

¹ Australial Geological Survey, Australia

² Dept. Marine Science, Univ. South Florida, St Petersburg, FL, USA

³ ODP, Texas A&M University, College Station, TX, USA

⁴ Dept Geology and Geophysics, University Wisconsin, Madison, WI, USA

Leg 182 recovered Cenozoic carbonate deposited in a middle and high latitude setting. Data collected are critical for understanding cool-water carbonates, their temporal evolution and response to changes in ocean temperature, sea level, and diagenesis. One of the most interesting aspects of Leg 182 was the drilling of a thick (> 550 m thick) Pleistocene shelf-edge wedge. Sediment of this wedge is bioturbated, and composed of mud and fine to very fine sand-size skeletal grains (wackestone is the dominant carbonate texture). Preliminary observations suggest that most grains (dominantly bioclasts and foraminifers) were produced on the shelf and subsequently transported to the slope environment. Overall accumulation rates were high, exceeding 40 cm/ky, which is similar to many modern shallow-water, tropical carbonates, and twice the rate of Pleistocene Bahamian slopes. Pleistocene sediment in the cores with the highest sedimentation rates show recrystallization, cementation, dissolution, and dolomitization at shallow depths and at higher rates that previously thought. Partial lithification starts at 120-235 mbsf, dolomite becomes substantial at 100-200 mbsf, and foraminifer preservation decreases considerable because of cementation overgrowth at 60-70 mbsf. Another relevant aspect of the Pleistocene shelf-wedge is the presence of numerous bryozoan-rich mounds, now at water depths of ~200-350 m. These mounds are dominantly muddy, have tongues of bioclastic sands, and are characterized by diverse bryozoan growth morphologies. The mounds are oriented parallel to the slope, have synoptic relief of up to 20 m and can be hundreds of meters in lateral extent. These are among the first modern analogs to similar mounds that were an important part of the carbonate depositional systems in earlier Cenozoic successions in the Great Australian Bight and in Phanerozoic strata elsewhere. A third important discovery of Leg 182 was the presence of a brine with salinities between 80 and 105, and a horizontal top cutting across seaward dipping strata and sequence boundaries. In the Pleistocene section, the presence of the brine combined with high supply of organic matter and high rates of sedimentation are probably responsible for the extent of diagenesis observed including dolomitization. The initial results of Leg 182 provide a new perspective and models of cool-water carbonates and their diagenesis.

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