NEOGENE BIOSTRATIGRAPHY AND SEDIMENTATION RATE ANALYSIS OF DEEP OIL WELL SAMPLES, BLOCK 555 MISSISSIPPI CANYON, GULF OF MEXICO

The nannofossil biostratigraphy was determined for a deep oil well in the northeastern Gulf of Mexico, Block 555 Mississippi Canyon. Biostratigraphic analysis of the Late Miocene through Early Pleistocene section allowed sedimentation rates to be calculated using absolute ages assigned to last appearance datums of marker species, and their respective depths in the well. Sedimentation rates in the present study compare favorably with changes in deposition proposed by others for this area of the Gulf, during the Neogene.

The study area received lower rates of sediment supply between 7.53 and 6.32 Ma, than during later time periods. The main depocenter of the Mississippi River was probably to the west, and high sea level stand at the close of this interval, resulted in open basin conditions. Between 6.32 and 5.71 Ma, sedimentation rates rose to 1967 ft/my, a trend which peaked at rates of 14,750 ft/my between 5.71 and 5.59 Ma. This increase in sediment delivery is correlative with a worldwide decline in sea level which culminated in the latter part of the Messinian. Sedimentation rates of 449 ft/my, between 5.59 to 3.92 Ma, suggest a rise in sea level which trapped sediment shoreward.

Rising sedimentation rates between 3.92 and 3.7 Ma may be related to sediment influx from northeastern Gulf slope retreat and the migration of turbidity currents into the area. Unconformities in the data set provide evidence for the migration of mass transport systems which eroded the base sequences between 3.54 and 1.93 Ma and 1.6 and 1.26 Ma.