Abstract: The Stratigraphy, Sedimentology and Chronostratigraphy of the Mobile Bay Fan, Northeastern Gulf of Mexico

Donald B. Macurda, Jr., Gary Jones, H. Roice Nelson, Jr.

The stratal architecture of submarine fans and distribution of sandy lithofacies remains one of the most vexatious problems confronting explorationists today. The pre-Wisconsin Mobile Bay Fan in the northeastern Gulf of Mexico provides a case history which offers insights in the distribution of sand-prone facies from a proximal to distal position.

The channel axis of the Mobile Bay Fan is well imaged in the GLORIA imagery acquired as part of the study of the Exclusive Economic Zone by the U.S. Geological Survey. The channel axis is extremely sinuous; downdip it is buried by the younger Wisconsin lobe of the Mississippi Fan. An extensive seismic grid with a 6.5 km (4 mi) spacing was used to study the fan over a distance of 250 km (160 mi).

The earliest portion of the fan is a debris flow, followed by the development of a prominent sinuous channel-levee complex. The evolution of the channel can be mapped as the seismic facies of the channel deposits are very distinct from those of the levee and overbanks. Initially the channel was more linear; it became more sinuous with time. Downdip, a younger channel developed by expanding from the older one. The seismic facies of the channel axis have a low amplitude updip; in a medial position they have a high amplitude, discontinuous pattern. An amplitude map of the reflections of the levee and overbank shows an increase perpendicular to the channel axis as one moves downdip. Drilling of the Mississippi and Amazon fans has shown the net/gross of sand/shale rises downdip; both along and perpendicular to the axis of the fan. The more limited seismic data from these fans calibrates the data from the Mobile Bay Fan. Seismic facies and sequence analysis shows that the Mobile Bay Fan is a sand-efficient system that transported sand well downdip into medial and distal portions of the fan. Levees and overbank regions become more prospective downdip. Variations in the sinuosity of the channel system reflects change in slope on the basin floor. As explorationists, we must be able to differentiate sand-rich from sand-efficient fans, as there are major differences in where sands are to be found.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994