Sedimentology and stratigraphy of box cores from the HEBBLE site on the Nova Scotian continental rise

Abstract

Fifty box cores were obtained from the 2 × 4 km site of the High Energy Benthic Boundary Layer Experiment (HEBBLE) on the lower Nova Scotian continental rise at 4820 m. The oldest unit (occurring in two gravity cores) is a pre-Holocene dark grey gravelly mud, probably of debris flow origin. Above this is a soft grey mud overlain by brown sandy mud whose sand is largely foraminiferal with some terrigenous material from interbedded turbidites. In some places this unit has a stiff brown gravelly mud at its base which is always overlain by sandy mud containing dispersed gravel. It is suggested that the dispersed gravel originated from the underlying gravelly mud and has been dispersed upwards by the action of burrowing organisms. Foraminifera indicate a Holocene age down to about 65 cm and thus a net accumulation rate of about 6 cm ka −1. The surface unit of brown mud, named the HEBBLE Mud, is spatially variable in both thickness and sand content which average about 4 cm and 6%, respectively. Sand percentage is not related to silt size or percentage but varies inversely with deposition rate and thickness, suggesting a relation to local winnowing. The size and peak height of the main silt mode (10–17 μm) are closely correlated and the peak height is inversely related to the percentage of clay. This is attributed to fractionation during deposition of the mud wherein fine silt and clay deposition is suppressed at flow speeds from 10 to 18 cm s −1 leaving coarser silt. There is more clay on the upstream and downstream sides of a low spur suggesting spatial variation of shear stress that may be due to a standing internal wave added to the mean flow. Silt-mode size is variable downcore although clay percentages are similar to those at the surface. The overall silt size is probably controlled by turbidite input, thus silt size is likely to be an unreliable indicator of current speeds down core, though peak height of the silt mode may be satisfactory.