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Tracking Eustatic Sea-Level Change in the Southern Hemisphere: Three-Dimensional Seismic Images of the Development of Stacked, Prograding late Paleogene-early Neogene clinoforms, Northern Carnarvon Basin, North West Shelf (NWS), Australia Donna L. Cathro 1,2 (512-232 3234; dcathro@mail.utexas.edu) James A. Austin Jr. 2 (512-471 3250; jamie@utig.ig.utexas.edu) 1Dept. of Geological Sciences C1110, University of Texas at Austin, Austin, TX 78712-1101, United States 2University of Texas, Inst. for Geophysics, 4412 Spicewood Springs Rd, Bldg 600, Austin, TX 78759, United States Late Paleogene-early Neogene mixed carbonate clastic-siliciclastic sediments beneath the NWS are characterized by stacked clinoforms prograding to the west; the complex development of this stratal configuration can only be studied properly using nested 2D and 3D multichannel seismic (MCS) data tied to exploration wells. The stratigraphic complexity is enhanced by the long-term structural evolution of the NWS, from an intracratonic basin (Paleozoic), through four phases of extension (Permian, Late Triassic-Early Jurassic, Callovian-Oxfordian and Tithonian-Valanginian) and associated passive margin development, inversion (late Cenomanian-Turonian), and finally to collision (latest Oligocene-Present). Convergence at the northern Australia plate boundary 700 km to the north, and continuing thermal subsidence, still influence Tertiary stratal configurations in the Northern Carnarvon Basin.\Previous studies, restricted to closely spaced (1-3 km), but only 2D (MCS) profiles, have not been able to describe the lateral development of seismic discontinuities within clinoformal sequences due to insufficient data coverage. In contrast, we use laterally extensive (70 km, NE-SW along strike), high-density (line spacing 12.5-25 m), moderate resolution (dominant frequencies 35-50 Hz) 3D MCS data provided by Woodside Energy, Ltd. (Australia), directly tied to 16 exploration wells. These data are completely enclosed within a regional (600 km NE-SW), widely spaced (10-100 km line spacing), lower resolution (dominant frequency $\sim$30 Hz) 2D seismic grid acquired by the Australian Geological Survey Organisation (AGSO) between 1991 and 1994.\Initial interpretation of the 3D volume has revealed previously unmapped drainage networks associated with the clinoforms, both along strike and down dip. Drainage patterns incise clinoform (inclined) surfaces dipping between 2.5 and 6 degrees. For example, down-dip drainage on an upper Oligocene surface consists of multiple, bifurcate and singular, u-shaped, straight-trunk channels up to 4 km long and spaced at 300-1100 m intervals, but restricted to near the clinoform toe. These incisions clearly truncate underlying strata, are 250 m wide and 5-10 m deep up-dip, 150 m wide and 10-15 m deep mid-length, and 600 m wide and 5-10 m deep down-dip. In contrast, a mid-Miocene surface shows a single, predominantly straight channel, 6 km up-dip from and parallel to the clinoform breakpoint. This feature is u-shaped in cross-section, $>$300 m wide, up to 30 m deep, and at least 3.5 km long before any evidence of meandering is observed.\Paleobathymetry based on analyses of benthic foraminifera from well-samples being prepared by AGSO will also be assessed to decipher depositional environments of the interpreted 3D stratal architecture. Ultimately, both sets of analyses will be combined with a regional 2D MCS interpretation and associated backstripping to determine the relationship between the observed clinoform wedge and the evolving tectonic framework. The long-term objective is to compare and contrast these results with studies on similar clinoformal geometries underway in the Northern Hemisphere (e.g., offshore New Jersey and in the Bahamas), with the ultimate goal being to unravel a eustatic signal from this globally important seismic architecture. |
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