OLIGOCENE BIOSTRATIGRAPHY AND SEDIMENTATION RATE ANALYSIS OF THREE GULF OF MEXICO BOREHOLES: BLOCK 526 PORT ISABEL, BLOCK 555 MISSISSIPPI CANYON, BLOCK 627 ALAMINOS CANYON

Late Oligocene sediments from three deep boreholes; Mississippi Canyon MC555, Alaminos Canyon AL627, and Port Isabel PI526 were biostratigraphically correlated in a NE-SW transect across the Gulf of Mexico. Changes in sedimentation rate were calculated using the last occurrence datums of nannofossil species between the ages of 26.93 and 23.13 Ma with biostratigraphic focus on the interval between Spenolithus predistentus, 26.93 Ma and S. distentus, 26.84 Ma.

In the west central Gulf of Mexico, a sedimentation rate of 24,000ft/my was recorded at PI526, between 26.93 Ma and 26.84 Ma. This unusually high rate of deposition may be attributed to the Hackberry collapse and retrograde retreat of the continental margin along the Texas/Louisiana shoreline (Galloway, 2000). During the same interval, missing section is recorded in the north central Gulf of Mexico (MC555), evidenced by the simultaneous downhole occurrence of Dictyococcites bisectus, Sphenolithus distentus and S. predistentus. The absence of these sediments may have been due to faulting, or erosion by mass transport systems. Coeval sediments were not retrieved at AL627, which was located north and east of PI526.

During the latest Oligocene (26.84 to 23.13 Ma) Sedimentation rates declined in the western Gulf. PI526 experienced a drop in sedimentation rate to 921ft/my between 26.84 and 24.43 Ma, followed by a rise to 1827ft/my between 24.43 and 23.13 Ma. These values reflect normal fluctuations in deposition at the edge of the Rio Grande depo-center. AL627, also, experienced low sedimentation rates which declined from 49ft/my between 26.84 Ma and 24.43 Ma, to 23ft/my between 24.43 Ma to 23.13 Ma. AL627 was located slightly north and east of the Rio Grande depo-center, and experienced rates in agreement with open basin conditions.