Origin and Timing of Sea Floor Erosion on Louisiana Continental Slope

Michiel C. Van Den Bold, Thomas F. Moslow, James M. Coleman

Erosional stripping of sediments on the Louisiana upper continental slope (between 210 and 640 m water depth in the Garden Banks area) is commonly observed on high-resolution seismic profiles in association with significant (> 10 m) topographic relief, such as fault scarps and salt or shale diapirs. Many of the erosional features are located adjacent to diapiric structures and faults which, owing to their excellent potential as hydrocarbon traps, serve as likely sites for engineering and geotechnical projects (i.e., drilling platforms, cable routes, guy wires, pipelines, and salvage operations). Therefore, it is of paramount importance for geologists and engineers to understand the timing, nature, and origin of the submarine processes that produced this erosion.

Density distributions of Globorotalia menardii were determined from samples in gravity cores that penetrate erosional surfaces to distinguish a Holocene versus Pleistocene age (biozones, X, Y, and Z) of sedimentary deposits and depositional events. In 10 of the cores, the G. menardii curves show pelagic, Holocene (Z zone) deposits averaging 70 cm thick, indicating that the processes producing the erosion are no longer active. The uppermost 1 m of the six remaining cores is different in both sedimentology and micropaleontology from the pelagic Holocene deposits in the other 10 cores. Random distribution of G. menardii, differing grain size and calcium carbonate distributions, and an irregular basal contact with underlying deposits, suggest a nonpelagic, possibly mass-movement-related o igin. Four of these sediment gravity flows or "slumps" were deposited on pelagic sediment of Holocene age (Z zone), whereas two "slumps" were deposited on hemipelagic sediment of Pleistocene age (Y zone). Although the timing of emplacement of these "slump" deposits is, at best, varied, Pleistocene (Y zone) deposits were not found at the top of any cores.

The erosion that is observed on high-resolution seismic profiles took place during the end of the late Wisconsinan sea level lowstand or early during the Holocene sea level rise. Because of the rapid melting of the ice sheet, large amounts of sediment were deposited on the outer shelf and upper slope in a short period. Owing to rapid deposition and burial, these overpressured sediments on steep gradients yielded to sediment instability processes, such as mass movements and sediment gravity flows. As a result, sea-floor scarps, scars, and bed truncations are commonly observed on high-resolution seismic profiles in the study area and on much of the continental slope of the northern Gulf of Mexico.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.