Feeder Channel-Belts and Coeval Shelf-Edge Evolution in the Lower Wilcox Clastic Wedge, South Texas: Implications for Sand Selivery Into the Deepwater of the Gulf of Mexico

Abstract

The shelf edge is a critical ‘connector’ between shelf deltas and deep-water sediment gravity flows. The shelf edge progrades basinwards mainly by the incremental addition of regressive deltaic sediment as well as muddy slope deposits but is itself supplied by feeder fluvial systems. The discovery of deep-water Wilcox turbidites in the Gulf of Mexico trigger the discussion on how these sands were delivered, as far away as 400 km from the coeval shelf edge. Light is shed on this question by the analysis of the dimensions and geometry of feeder channel belts and of the shelf-edge trajectory, based on 300+ well logs, ∼200m cores, supplemented by seismic data in South Texas. Examination of 50km long, dip-oriented transects through the Lower Wilcox clastic wedge from coastal-plain to shelf and upper slope shows that there were 18 stratigraphic sequences of regressive-transgressive character, spanning some 5 million years, with an average thickness of 35m. The shelf edge was constructed by very rapid progradation (highest rate at ∼80km/Ma) and the dip-oriented transects suggest that 1) ∼60% of regressive shelf-edge deltas did not arrive at previous shelf edges; 2) ∼30% of the sequences (Lower Wilcox) created ∼80% of the entire shelf-edge progradation distance; 3) much of the sediment that prograded over the Cretaceous carbonate platform was captured by growth-faulted subbasins. Geometry and dimensions of Wilcox channel belts along strike-oriented transects are documented. Paleohydraulic calculations for Wilcox paleochannels suggest that the water discharge through the largest of Wilcox channel is at least Mississippi river size (>15,000m³/s). The paleogeography maps based on log pattern maps and net sandstone maps show that the large and deep channel belts seen in proximal areas do not persist to the shelf edge, but disperse into numerous smaller and shallower channels, in the manner of many modern river-dominated deltas. For this reason, the large channel belts seen within the lower Wilcox are not interpreted as incised valleys. This work so far implies 1) the success of deepwater sand delivery was limited to a small number of sequences of the entire Lower Wilcox interval (less than 40%); 2) the necessary of canyons to route sediment to deepwater areas because of the high capture efficiency of the growth-faulted subbasins; 3) the importance of high sediment supply to push Lower Wilcox shorelines close to the shelf edge, in the absence of cross-shelf valleys.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016