Stratigraphic Framework and Sedimentary Facies of a Clastic Shelf-Margin: Wilcox Group (Paleocene-Eocene), Central Louisiana.

Abstract

A strike-oriented trend of Wilcox oil and gas production in central Louisiana marks the location of an early Tertiary clastic shelf-margin. The shelf-margin contained a central unstable region flanked by two stable regions. The stable regions occurred where there was no significant progradation beyond the location of the underlying shelf-margins. Conversely, the unstable region occurred where progradation extended basinward of the underlying shelf-margins. Seven depositional sequences can be recognized within the shelf-margin. The vertical arrangement of these sequences shows that migration of the margin was negligible throughout Wilcox deposition, thereby suggesting a balance between subsidence and deposition. Through numerical simulation, it was concluded that published values for global cyclic sea-level fluctuations cannot be used to account for the development of these sequences. Rather, a sea-level which experiences variable rates of fall over a 0.5 $\times$ 10$\sp6$ year interval could account for the origin of such sequences. A shale-filled submarine canyon system occurs within the unstable region of the margin. Morphologically, the canyon cross-sectional profile resembles that of an entrenched fluvial system. Conventional cores through sandstone bodies from stable and unstable regions of the margin exhibit similarities in vertical sequence and interval thickness. Both sandstone bodies represent truncated progradational shoreface sequences which were associated with shelf-margin deltas. A computer program (DEPSIM) was developed in order to account for a significant difference observed between transition zone facies of the two shoreface sequences. The shoreface sequence from the unstable region of the margin contains a well-developed transition zone facies in contrast to that from the stable margin. Results from the program suggest that the difference between these two sequences may be explained by the fact that the sequence from the unstable region of the margin formed during a relative sea-level rise. The shoreface sequence from the stable region formed in response to a falling sea-level which resulted in extensive lateral translation of the shoreline. A vertical profile through the simulated shoreface sequence which formed during a sea-level fall therefore exhibits a poorly developed transition zone facies.