Paleocene-Eocene Wilcox Group Submarine Canyons and Thick Deepwater Sediments of the Gulf of Mexico: Very Large Systems in a Greenhouse World, Not a Messinian-Like Crisis
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 1920’s, 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 draw-down, 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 suggest such a model is not necessary to explain the occurrence of Wilcox deepwater sand in such distal locations. We use source-to-sink scaling relationships from published literature to support the alternative, that these are simply large systems in a Greenhouse world, with little high-frequency glacio-eustasy. 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. Recent studies of Wilcox provenance from detrital zircons indicate that drainage basins must have been >1,000,000 km2, with the longest river channels >2000 km in length: in the modern world, fan lengths for such systems can easily exceed 3-500 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 the well-documented Paleocene-Eocene Greenhouse climate, with negligible global ice volume to drive high frequency and high amplitude glacio-eustasy and the back and forth cross-shelf transit of river mouths. Instead, we envision river systems that remained docked at the shelf margin, and a long-lived, persistent connection between river mouth sand sources and slope canyons.
The alternative views we outline here do not refute the isolation and draw-down model. However, such a model is not necessary to explain characteristics of Wilcox sediment dispersal systems in the deepwater Gulf of Mexico.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California