Depositional Framework and Genesis of Wilcox Submarine Canyon Systems, Northwest Gulf Coast

William F. Galloway, William F. Dingus, Richard E. Paige

Wilcox (late Paleocene-early Eocene) slope systems of the Texas coastal plain contain two families of paleosubmarine canyons that exhibit distinctly different characteristics and stratigraphic settings: Yoakum and Lavaca type canyons occur as widely separated features within the generally retrogradational middle Wilcox interval. Four such canyons exhibit high length to width ratios, extend far updip of the contemporaneous shelf edge, were excavated deeply into paralic and coastal-plain deposits, and were filled primarily by mud. Fills consist of a lower onlapping unit and capping progradational deposits that are genetically related to deposition of the upper Wilcox fluvial-deltaic sequence. Significantly, the canyon fills correlate with widespread transgressive marine mud tones (the Yoakum shale-Sabinetown Formation and "Big Shale"). In contrast, Lavaca-type canyons form a system of erosional features created along the rapidly prograding, unstable lower Wilcox continental margin. Five distinct canyons, each at a slightly different stratigraphic and geographic position, are all characterized by low length to width ratios and broad, flat floors. They extended at most a few tens of kilometers across the contemporaneous shelf platform, but excavated large volumes of prodelta and slope facies. Their fills consist of both mudstone and sandstone, the latter of which locally exhibits discrete channel-form or mounded-lobe geometries; cores reveal chaotic-slump and debris-flow sedimentary packages. Canyon fills correlate with local shale units that separate sandy d ltaic and shore-zone facies sequences of the contemporaneous lower Wilcox deltaic system. Spatially, the lavaca-type canyons are closely associated with large slumps and locally mapped intraformational listric faults that sole out within the Paleocene or Upper Cretaceous sections.

Comparative analysis of the two canyon systems suggests a general process model for submarine canyon formation on prograding basin margins. Key elements are (1) depositional loading of the continental margin creating instability, (2) initiation of a large-scale slump, family of slumps, or listric bedding-plane fault creating a depression or indentation in the margin, and (3) headward and lateral expansion of the depression by slumping and density-underflow erosion. Extent of canyon evolution varies according to time and submerged space available for maturation; short, broad canyons form on narrow shelves of actively prograding margins, and elongate mature canyons form in retrogradational or transgressive settings.

Recognition and systematic exploration of these and similar retrogradational slope systems may offer one of the few remaining opportunities for significantly increasing oil reserves of the northwestern Gulf of Mexico.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.