The Barra Fan: A bottom-current reworked, glacially-fed submarine fan system

Abstract

On the basis of sedimentary structures, textures and ichofauna, seven depositional facies have been recognised in cores from the Barra Fan region of the Hebrides slope. Sedimentary facies are combined into three genetic groups A–C that represent their primary mode of deposition: (A) sandy to muddy contourite facies A, A1, and A2, which represent an overall coarsening upward sequence caused by an increase in bottom-current velocity over time; (B) disorganised glaciomarine deposits, including the glaciomarine dumpstone of facies B and facies B1 deposited as a result of ice-rafting and glaciomarine sedimentation; and (C) hemipelagite facies C and C1 which are the result of normal marine hemipelagic settling. It is most likely that all contourite facies (A–A2) identified in this study are Holocene (<10,000 y) in age and that the underlying glaciomarine and hemipelagite facies (groups B and C) were deposited during the last glacial period.In terms of its diverse morphological elements and varied facies, this high latitude submarine fan is truly composite in nature. However, a broad three-stage glacial to post-glacial evolutionary system is recognised, during which different processes and process combinations were dominant. During the last glacial period (low stand system tract), downslope processes, including debris flows and minor turbidity currents, sculpted an irregular slope topography. Little in the way of bottom current activity was evident. Staged deglaciation was accompanied by rising sea level and high sedimentation rates across the region. This created an unstable margin prone to extensive sliding and slumping, that masked any effects of an incipient alongslope current. The present high stand system tract is dominated by a strong slope current that became fully active during the early part of the Holocene. Low sedimentation rates, diminished sediment supply and strong bottom current activity led to development of the sandy contourite sheeted drift system across the mid-slope region. Little downslope movement is evident during this period, so that the slope topography is being slowly smoothed and remoulded by active bottom currents.