Abstract 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.

A biostratigraphically complete but intensely bioturbated Cretaceous/Tertiary (K/T) boundary section was taken during drilling at Ocean Drilling Program Leg 113 Site 690 on the Maud Rise (65°S) in the Weddell Sea off East Antarctica. The boundary, which is contained in a relatively undisturbed core, has been delineated by lithostratigraphic, paleontological, and geochemical methods. The first occurrence of the calcareous nannofossil Biantholithus sparsus is used to biostratigraphically estimate the boundary horizon, and a distinct color change between dark brown, clay-rich Tertiary sediments and light-colored Cretaceous chalks is used to more precisely delimit the boundary between 41.5 and 41.8 cm in section 4 of core 15. An iridium peak of 1,566 ± 222 parts per trillion (ppt) was detected in the same section at 39 to 40 cm. Multiple but less intense Ir peaks were also detected below the boundary. Dark-colored burrows sampled below the boundary contain up to 17 percent Tertiary nannofossils estimated to have been displaced at least 1.3 m by large-bodied bioturbators. In addition, the multiple peaks in Ir abundance below the boundary are attributed to the redeposition of Ir by this intense bioturbation. Such processes may account for some multiple Ir peaks reported at other K/T boundary sections. We suggest that serious consideration should be given to the problems of bioturbation when attempting to biostratigraphically determine any marine boundary horizon and, in reference to the K/T boundary, when interpreting multiple Ir peaks as being the result of multiple extraterrestrial impacts. Similar caution should be exercised during micropaleontological studies to determine the succession of biological extinctions and evolutionary first appearances across any boundary interval.

Abstract Exploration of the Southern Ocean by Glomar Challenger began in the Pacific sector with a boldly executed set of holes along a cruise track extending into ice-laden waters adjacent to the Antarctic continent. These holes, along with additional sections drilled in subantarctic waters off New Zealand and Australia, established the basis for regional biostratigraphic zonations of the smaller-sized microfossil groups (coccoliths, diatoms, and siiicoflagellates). Because these microfossils could be examined readily in smear-slide preparations of raw sediments, a rapid age-dating technique evolved called smear-slide biostratigraphy. In addition, a preliminary history of Antarctic refrigeration leading to severe Late Tertiary continental glaciations was documented. Exploration of the Atlantic sector, limited primarily by unfortunate weather conditions, provided additional evidence of severe Late Miocene glaciation and its role in shaping the circum-Antarctic current into a powerful geologic force. The successful drilling of critical reference sections on the Falkland Plateau also opened the area to efficient exploration by conventional piston cores sited using techniques of shipboard smear-slide biostratigraphy.