Abstract

Recent discoveries of thick, oil-bearing Paleocene to Eocene-age Wilcox Group sands in the deep waters of the northern Gulf of Mexico have heralded an important new exploration play. Wilcox deltaic reservoirs have been exploited onshore since the 1920s, but the thickness and high sand-to-shale ratio of the deepwater section was a surprise to many geologists working the Gulf of Mexico. Several authors propose Paleocene-Eocene isolation of the Gulf of Mexico from a Messinian-like drawdown to explain characteristics of the deepwater Wilcox play. As evidence, they cite (a) large slope canyons that could only have been cut by a large-scale drawdown, and (b) thick, Wilcox sands in basin-floor settings, which require a large drawdown to have been deposited >400 km from the contemporary shelf.

We propose a simpler model for Wilcox canyons and the extensive sand-rich basin-floor systems. First, modern continental margins have canyons of similar scale, but were not subject to a Messinian-like sea-level drawdown. Second, basin-floor fan lengths are related to drainage-basin area, feeder river gradients, and lengths of the longest fluvial channels. From studies of detrital zircons, Wilcox drainage basins must have been >1,000,000 km², with river channels > 2000 km in length: fan lengths for such systems can exceed 300–500 km. Hence, neither the canyon systems nor the basinward extent of Wilcox fan sediments requires an extraordinary explanation–it was simply a large system. Third, we suggest the sand-rich nature of the Wilcox trend reflects a Greenhouse world, with little high amplitude glacio-eustacy to drive the back and forth cross-shelf transit of river mouths, such that there was a long-lived and persistent connection between river mouths and slope canyons.