Abstract: Significance of Erosion Surfaces in Lower Miocene Deltas, Southwest Louisiana, Gulf of Mexico Basin

Marc B. Edwards

Three kinds of erosion surfaces were recognized in lower Miocene (Siph. davisi-Planulina) deltaic depocenters near the paleoshelf margin. Understanding their characteristics and origin is important to both field development and exploration in the area. Each surface shares some characteristics of "sequence boundaries."

The first type of erosion surface is recognized by inference only, as it displays no detectable downcutting or truncation of underlying strata over most of its area. These surfaces coincide with a flooding surface, except locally where they extend downdip to isolated sandstone bodies. Thus, detailed mapping identified subtle surfaces of sediment bypass.

The second type of erosion surface has a channel form with relief usually of 50-200 ft. These surfaces truncate flooding surfaces and older erosion surfaces, and exhibit a variety of patterns in map view, including radial and rectilinear. Channelized deposits are interpreted, on the basis of their relationship to subjacent depositional facies, to range from distributary channels on prograding deltas to incised valleys on subaerially exposed marine shelves. There is no clear distinction between the two end members.

Whether multiple erosional features occurring between two flooding surfaces existed contemporaneously or sequentially could not be resolved.

The third type of erosion surface is regionally extensive and broadly strike oriented, truncates rotated blocks of section containing older flooding and erosion surfaces, and is overlain by a regionally extensive wedge of bathyal shale (Abbeville). These surfaces formed subaqueously from shelf-margin failure. They occur much less frequently than channel erosion surfaces to which they appear to have no genetic relationship.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994