ABSTRACT

Three kinds of erosion surfaces were recognized in lower Miocene (Siphonina davisi-Planulina) deltaic depocenters near the paleoshelf margin in the southwest Louisiana Gulf Coast Basin. 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 prominent updip and is associated with nonmarine to inner neritic microfaunas. The channellike forms have a relief typically of 50 to 200 ft, may truncate older flooding and erosion surfaces, and exhibit a variety of patterns in map view, including radial and rectilinear. Sandstone-rich channelized deposits range from distributary channels on prograding deltas to incised valleys on subaerially exposed marine shelves. The two end members have contrasting implications for reservoir prediction and fill styles, but many examples are intermediate types that do not fall in one end member type or the other. Where laterally adjacent erosional features occur between two flooding surfaces, timing relationships were ambiguous.

The second type of erosion surface is associated with outer neritic to bathyal microfaunas. These surfaces are broadly strike oriented, are regionally extensive, and have much higher relief than the channel features. They truncate rotated blocks of section containing older flooding and erosion surfaces and are overlain by local sediment gravity flow sandstones and a wedge of bathyal shale (Abbeville facies). These erosion surfaces formed subaqueously by shelf-margin collapse. They are much less common than channelized erosion surfaces, and no genetic relationship between the two is evident.

The third type of erosion surface occurs at the base of strike-oriented, isolated sandstone bodies in an otherwise shaly, outer shelf setting. Away from these sandstones, however, there is no evidence of erosion. Thus, this erosion surface formed in association with lowstand, stranded shoreline deposits.