Depositional Processes at the Lower Wilcox Shelf–Slope Transition Zone

Abstract

Recognition of sand bypass at the shelf margin is key to deep-water exploration. This study examines the shelf margin architecture of the Lower Wilcox Group in Texas by combining wireline log with 3D seismic data. During the early Paleocene a relatively extensive (50 km wide), shallow-shelf platform extended across Central Texas making it difficult for the deltas to reach the shelf edge. The seaward pinchout of Lower Wilcox sandstone-rich shoreface deposits is about 20 km updip from the contemporaneous shelf edge indicating that sand remained on the inner and middle shelf and that the shelf margin grew through mud accretion. Highstand sea-level conditions favored the generation of hyperpycnal flows that incised into shelf deposits, with sand bypassed onto upper and middle slopes. In areas of shale withdrawal, extensional features such as growth faults produced long, linear to arcuate strike-elongate depocenters within hanging-walls of faults and dictated sediment delivery pathways. Our work suggests that significant volumes of deep-water sands were deposited from hyperpycnal flows initiated by direct river effluents that accumulated on the upper slope. High-density hyperpycnal flows created sand-filled slope-channel complexes 10–20 m thick and 200 m to more than 1 km wide that served as conduits for bypass to the basin floor. Unconfined, low-density hyperpycnal flows deposited lobes on the upper slope. Lobes spread 10–20 km laterally and 2–4 km downdip, with a maximum total sand thickness of 100 m. A high net-to-gross ratio (0.6) suggests the sand-rich component of the flow was deposited on the upper slope, while finer-grained sediment continued downslope. The shelf-margin architecture exhibited by this sequence serves as an example of hyperpycnal flows being the main initiator of turbidity currents for sand accumulation on the slope.