Black Shell turbidite, Hatteras Abyssal Plain, western Atlantic Ocean

Abstract

An upper Pleistocene turbidite with a volume of at least 1011 m3 (100 km3) has been traced in 35 piston cores over a 44,000 km2 area of the Hatteras Abyssal Plain. Entering the plain from the Hatteras Canyon System, the flow traveled uninterrupted for at least 500 km in a southerly direction and resulted in a tongue-shaped turbidite with a width between 100 and 140 km and a thickness of as much as 400 cm. This turbidite appears to be one of the largest, if not the largest, single turbidite yet to be correlated between deep-sea cores and to be mapped on an abyssal-plain floor. Correlation of the unit is based on grain size, mineralogy, relative thickness, and similarity of sequences in the cores. Because correlation between cores is based largely on sand-layer characteristics, the turbidite cannot be traced beyond the last occurrence of a distinct sand layer in the distal (southerly) direction. The turbidite is characterized by its high percentage (2% to 50%) of blackened mollusk shell fragments, which led to the informal name Black Shell turbidite, and by a coarser grain size than other turbidites in the same cores. The maximum thickness of the turbidite's sand part is in the center of the abyssal-plain basin, whereas maximum thicknesses of lutite and of the total turbidite are displaced east of the center line of the depositional basin. Depending on lateral position in the flow, the sands texturally comprise a wide range of graywackes, and mineralogically they constitute a suite ranging from lithic arkoses to quartzarenites. Sand petrology indicates that the fluvially derived terrigenous fraction came from United States mid-Atlantic coast rivers, whereas molluscan and foraminiferal bioclastic components indicate an initial shallow-shelf origin from the vicinity of Cape Hatteras, North Carolina. It appears that the turbidity current began as a massive shelf-edge slump. Sand came from the shelf edge; mud was picked up on the upper continental slope. The flow apparently evolved from a slump into a high-concentration flow as it moved down-slope. Characteristics in the axial center zone, such as poor sand sorting, high mud content, Bouma AE sequences, and discontinuous vertical grading, suggest deposition from a high-concentration flow. The nature of the characteristics changed (better sorting, more continuous vertical grading, more complete Bouma sequences, and lower mud content) as the flow spread, reflecting deposition from a low-concentration turbidity current.