ORRIN H. PILKEY, THE “BLACK SHELL TURBIDITE” AND SEDIMENTOLOGY OF BASIN PLAINS

Orrin H. Pilkey, through studies of basin plains in the Western North Atlantic, has made numerous contributions to our understanding of sedimentation in these “lakes of sediment.” This work, in collaboration with colleagues and students, demonstrates that basin plains are dominated by turbidity currents, most of which enter from canyon mouths. Orrin has pointed out that basin plains are not just the distal parts of deep sea fans, and that basin plain deposits are overlooked and probably misidentified in the rock record. He has also shown that many basin plain deposits are very sandy and would make good reservoirs. Orrin has been particularly fascinated with the “instantaneous” deposition of enormous volumes of sediment over large areas and has emphasized that such big events are necessary for the formation/maintenance of basin plains. He predicted the presence of large deposits such as the Black Shell Turbidite in the Hatteras Abyssal Plain. This large turbidite has a volume of at least 100 km3, covered an area of over 50,000 km2, and traveled at least 500 km. It originated about 16,000 years ago as a massive slump, probably triggered by an earthquake, on the upper slope and outer shelf off Cape Hatteras. The presence of shallow water/estuarine mollusks in the deposit suggests that the slump may have taken a “bite” out of the shoreline. The slump presumably evolved into a debris flow and then a turbidity current as it moved downslope through the Hatteras Canyon and fan systems. Once it entered Hatteras Abyssal Plain deposition of coarse material did not begin until it had moved some distance onto the plain. Coarse material was deposited in the central and western part of the plain whereas fine material was ponded along the eastern margin of the plain. Through studies of individual turbidites such as the Black Shell Turbidite, as well as more general studies of basin plains, Orrin and his coworkers have provided valuable information on the nature of large sedimentation events and on how basin plains are filled.