Basin 4 of the Brazos-Trinity Slope System, Western Gulf of Mexico: The Terminal Portion of a Late Pleistocene Lowstand Systems Tract
R.T. Beaubouef, V. Abreu, J.C. Van Wagoner, 2013. "Basin 4 of the Brazos-Trinity Slope System, Western Gulf of Mexico: The Terminal Portion of a Late Pleistocene Lowstand Systems Tract", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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An ultra-high resolution, short-offset 3D seismic survey (EBHR3D) has been used to study the sedimentary fill of an intra-slope basin in the East Breaks area of the Gulf of Mexico. The site chosen for the seismic program is the fourth and southernmost basin (Basin 4) in the Brazos-Trinity Slope System. The Brazos-Trinity Slope System is a set of latest Pleistocene salt-withdrawal basins that are connected by channels in the upper to middle portion of the Texas continental slope. They are filled with sediment delivered to the slope by the ancestral Brazos and Trinity rivers and associated shelf edge deltas. Together, the linked shelf and slope depositional systems form a late Pleistocene lowstand systems tract. The seismic survey has been designed to target a large submarine fan at the top of the basin-filling succession (the Upper Fan), but imaging of the entire 250 m (maximum) of basin fill is excellent. The results are providing detailed information regarding deep water deposition far surpassing what is possible from outcrop or conventional subsurface studies. The data provide unprecedented images of the three-dimensional geometry and internal architecture of these deepwater deposits. The fill of Basin 4 records a stratigraphic evolution that includes a “ponded” fill stage followed by a “perched” fill stage. Contrasting deposit geometry and stacking patterns occur during these two stages of evolution. The perched fill of the basin contains the Upper Fan, which is located in the shallowest portion of the subsurface beneath an extensive Holocene drape. The Upper Fan represents the terminal, distributive complex of the lowstand system tract. It is a basinward-tapering wedge of sediment that contains both channel-form and sheet-like depositional elements. The prominent stratigraphic features interpreted from the Upper Fan are: (1) off-lapping, clinoform reflection patterns; (2) distributary channel systems linked to channel mouth lobes; (3) down-fan progression in architecture from channel-form elements to more sheet-like elements; and (4) down- and across-fan decrease in sand percent and/or grain size inferred from seismic attributes. In these and other ways, the stratigraphy of the Upper Fan is similar to that commonly observed for modern and ancient river deltas.