Holocene contourite sand sheet on the Barra Fan slope, NW Hebridean margin
Published:January 01, 2002
Dorrik A. V. Stow, Julie E. Armishaw, Richard Holmes, 2002. "Holocene contourite sand sheet on the Barra Fan slope, NW Hebridean margin", Deep-Water Contourite Systems: Modern Drifts and Ancient Series, Seismic and Sedimentary Characteristics, D. A. V. Stow, C. J. Pudsey, J. A. Howe, J.-C. Faugères, A. R. Viana
Download citation file:
Surficial sediment analyses, bottom photographs and current-meter data, coupled with studies of sidescan and 3.5 kHz echodata clearly show the imprint of a strong, seasonally-affected, slope current on the Hebrides shelf and slope. The present day shelf sediments are in fact relict, coarse-grained material (gravel with boulders) of Pleistocene glacial derivation, which have been modified during the Holocene by winnowing, sea-floor polishing and transport of the finer fraction across the shelf to the northwest. On the outer shelf and upper slope the sharp change from gravel to sand is marked by a physiographic-textural boundary, herein termed the sandline, which occurs at a depth of between 170 m and 300 m. Further down the continental margin on the lower slope, the lower limit of the sand-rich facies is marked by the mudline. This typically occurs at a depth of around 1200 m, and marks the depth of substantially increased clay-sized material. Above the mudline the long-term (Holocene to present), time-integrated signature of bottom current flow has resulted in the relatively slow accumulation of a mid-slope sandy contourite deposit, the Barra contourite sand sheet. This covers an area of 1000–1500 km2 with an estimated sand volume of 30 000 m3. Below the mudline the clay-rich deposits represent a hemipelagic drape with only minor bottom current influence and intense reworking by benthic organisms.
Figures & Tables
Deep-Water Contourite Systems: Modern Drifts and Ancient Series, Seismic and Sedimentary Characteristics
Countourites are a widespread but poorly known group of sediments linked to the action of powerful bottom currents in deep water. Although we know they are especially common along continental margins and through oceanic gateways, they have been surrounded by contoversy since they were first recognized in the early 1960s. Where correctly recognized and decoded they can provide one of the keys to our better understanding of bottom water circulation and of the ocean–climate link. They are part of the spectrum of deposits that confronts the oil industry as exploration moves into progressively greater water depths.
This memoir is an important outcome of the International Geological Correlation Project 432 on Bottom Currents, Contourites and Palaeocirculation. It includes 30 papers involving over 75 key scientists from around the world. Following an introductory state–of–the–art paper by the editors, there are 25 separate case studies on modern drifts and four on ancient contourite series. Each contribution highlights the specific geological and oceanographic setting, bathymetry, physiographic and stratigraphic context, seismic attributes and sedimentary characteristics of that drift. Case studies range from some of the well-documented North Atlantic drifts to those much less known from the Mediterrenean, from important syntheses of the Gulf of Cadiz and Vema Channel Gateway, to completely new data on South Atlantic, Pacific and Antartic margin systems. The four papers on ancient series from Japan, China and Cyprus serve to emphasise the complex nature and subtle characteristics of contourites, which make their identification a scientific challenge.
This volume is dedicated to the memory of Charlie Hollister (1936–1999), one of the founding fathers and pioneers of countourite research.