FIDUK, J. CARL, Department of Geological Sciences, UTIG, Univ. of Texas at Austin, Austin, TX

ABSTRACT: Plio-Pleistocene Sediment Accumulation Rates in the Garden Banks and East Breaks Areas

Mapping of six paleontologic extinction surfaces on the upper continental slope in the Garden Banks/East Breaks OCS areas, using a 12,000 km grid of seismic data, has been used to calculate sediment accumulation rates. Variation in sediment accumulation rates along the slope and through time reflect changes in relative proximity to the sediment source and/or changes in available accommodation space. The accommodation space available at any location on the slope can be greatly increased by salt withdrawal.

Isopach mapping of six Plio-Pleistocene intervals, covering 2.8 Ma to thepresent, reveal a major period of sediment loading/salt withdrawal between approximately 1.0-0.6 Ma. This interval correlates to sediments between the extinctions of Hyalinea balthica and Trimosina denticulata. During this time, sediment accumulation rates averaged 4000-5000 m/my with a maximum rate of 9000 m/my. For 1.9 million years prior to this time, sediment accumulation rates averaged only 750-1500 m/my with a maximum rate near 2200 m/my. Following this time to the present, sediment accumulation rates fell to an average of 2000-2667 m/my with a maximum rate measured at 7300 m/my.

Increased sediment influx during 1.0-0.6 Ma appears to be associated with a major third order sea level lowstand. Sediments are particularly concentrated in the center of the Garden Banks area. Accumulation rates during this influx approached two orders of magnitude higher than normal hemipelagic rates (100-150 m/my average for the Gulf slope). The restriction of such dramatically increased accumulation rates to this area suggests that sediment influx was accompanied by large scale salt withdrawal. Salt evacuated at this time appears to be immediately downslope in the form of a large salt sheet.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.