Abstract:
Calcareous nannofossils and stable isotopes were analyzed to determine the timing and causes of extensive Miocene debris flows and Oligocene slumps recovered at five sites drilled by Ocean Drilling Program (ODP) Cruise 207 on the Demerara Rise in the western Equatorial Atlantic. The drill sites lie at water depths ranging from 1900 to 3200 m along the northernmost "nose" of the Rise, which extends some 220 km seaward of the shelf break along the Suriname and French Guyana coasts. Highly colorful upper Miocene mass transport deposits (MTDs) at Site 1261 span 59 m in thickness and contain coherent displaced blocks up to 60 cm thick ranging from early (nannofossil Zone NN4) to late (Zone NN11b) Miocene in age. Displaced Oligocene sequences range up to 25 m in thickness. Five possible modes of emplacement have been considered for the Miocene and Oligocene sediment failures. These include canyon incision and infilling, clathrate dissociation, sediment loading, current activity, and seismic triggering. Calcareous-nannofossil age dates on the Miocene debris flows at Hole 1261A indicate that these deposits were emplaced during Subzone NN11b (7.2 - 5.5 Ma). Compared to debris flows drilled previously along the much-studied New Jersey Transect, those at Site 1261 are fundamentally different, i.e., not the result of canyon incision and infilling due to seal-level changes. Instead, they are reminiscent of less extensive Miocene debris flows drilled by ODP Leg 154 on Ceara Rise some 700 km to the SE. We attribute the emplacement of the Miocene MTDs at Site 1261 to high mass accumulation rates combined with a seismic trigger. Current reorganization and/or sea-level changes best account for the Oligocene slumps.