Chevron Energy Technology, United States

The Demerara Rise is a deep-water plateau situated off the coast of South America in the equatorial Atlantic. Its geographic position makes its sedimentary record a desirable target to investigate tectonic and paleoceanographic signals in the Atlantic Ocean. Numerous mass-transport deposits (MTDs) were interpreted from seismic reflection profiles on the outer reaches of the plateau. One such MTD was drilled and sampled during Ocean Drilling Program Leg 207, Site 1261. This MTD overlies a regionally correlated middle to late Miocene unconformity that resulted from extensive erosion that caused removal of up to 220 m of sediment down to Eocene strata. Calcareous nannofossils were analyzed to determine the age of the 59-m-thick MTD lithologically described as slope conglomerate. Its age and corresponding stratigraphic sequence are used to infer possible causative factors in its emplacement. The slope conglomerate contains highly colored sandy carbonate clasts and silty-sandy turbidite rocks supported in a nannofossil-rich hemipelagic claystone matrix. The carbonate clasts are early Miocene (nannofossil Zone NN4), and the claystone matrix is late Miocene (Subzone NN11b) in age; stratigraphic relationships place the slope conglomerate sequence within nannofossil Subzone NN11b (7.2-5.5 Ma). A normal listric growth fault is located 5 km upslope of the drill site. These stratigraphic and structural relationships suggest causative mechanisms for sediment mass failure and generation of the MTDs. The age of the studied slope conglomerate provides an upper age limit (7.2 Ma) for the regional Miocene unconformity. The unconformity correlates in time with deepening of Fram Strait off Greenland, shoaling of the Isthmus of Panama, and establishment of polar glaciations. These changes resulted in intensification of bottom water currents and enhanced upwelling along continental margins throughout the Atlantic. This current intensification possibly led to increased erosion and generation of locally steep slopes along the flanks of Demerara Rise. Probable seismic activity, as indicated by the proximal shallow fault, initiated sediment mass failure and emplacement of the studied conglomerate. Similar erosion and mass-transport activity is reported throughout the Atlantic during this period of time, indicating the importance of these paleoceanographic changes to continental-margin development.