Cool-Water Carbonates and Bryozoan Mounds in the Great Australian Bight
Miriam S. Andres1, Stefano M. Bernasconi2, Judith A. McKenzie3, and OPD Leg 182 Scientific Party4
1Earth Sciences, Chevron, San Ramon, CA
2Dept. of Earth Sciences, ETH-Zurich, Zurich, Switzerland
3Dept. of Earth Sciences, ETH-Zurich, Zurich, Switzerland
4Ocean Drilling Program, College Station, TX
Cool-water carbonates, in contrast to their warm-water counterparts, are neither temperature nor light dependent nevertheless deposit thick and extensive sedimentary and architectures in geologic time. The Great Australian Bight (GAB) today comprises the largest cool-water carbonate environment; a 200 km wide, ramp-type shelf facing the Southern Ocean. Wind-and wave-swept year round the high-energy environment sweeps biogenic material seaward where it accumulates on the shelf edge and slope. Prominent build-ups identified from seismic straddle the upper slope for several kilometers. Thus, one of the primary objectives of Ocean Drilling Program Leg 182 was to investigate the nature of this depositional system and evaluate its sedimentologic response to fluctuating sea-level and changing oceanographic conditions.
Continuous cores from an impressive 450-m thick Pleistocene wedge allow for high-resolution stratigraphic studies. Furthermore, ODP Leg 182 for the first time successfully cored and confirmed the presence of modern Bryozoan mounds. In currently 200-350 m of water depth they form up to 65 m of relief. Their internal structure is characterized by a floatstone texture and lack of synsedimentary cementation.
High sedimentation rates (50/kyr) and well-preserved foraminifera allow for a high-resolution isotope stratigraphy and in conjunction with magnetostratigraphy provide for an accurate age model. Rising and falling sea-level is thus reconstructed for the past 780 kyrs. Variations in the isotope stratigraphy correlate with simultaneous fluctuations in the carbonate mineralogy (low vs. high MgCa), petrophysical properties and down-hole Natural Gamma Ray measurements. Unlike a tropical reef platform, carbonate production on a cool-water carbonate ramp does not shut down completely during sea-level lowstands, but moves seaward along the ramp as sea-level falls.
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