Sedimentary evolution of the Lower Clair Group, Devonian, west of Shetland: climate and sediment supply controls on fluvial, aeolian and lacustrine deposition

Sandstone units in the Devonian Lower Clair Group vary from (a) thick, wellsorted, medium sands deposited by aeolian processes, to (b) amalgamated fluvial channel deposits of coarser sand, to (c) thin sheets of fine sand deposited in floodplain or shallow lake settings. The six lithostratigraphic subdivisions (units I to VI) of the group are differentiated by changes in the predominance of fluvial, aeolian and lacustrine facies which are in turn controlled by sediment supply and climate. During periods of high sediment supply and relatively humid climate (Units II, IV and V), fluvial conditions dominated in the form of sandy to pebbly fluvial distributary systems on the alluvial plain. The sand body characteristics vary from stacked, coarse channel fills deposited by high energy braided rivers (Unit II) to decimetre sand sheets interpreted as the deposits of poorly channelised flow at the margins of the terminal fan (Unit V). At times of relative aridity, the fluvial system retreated and aeolian reworking resulted in extensive sheets of well-sorted sands deposited as dunes or more commonly on sand-flats (Unit III). Periods of wetter climate and reduced clastic input resulted in lacustrine facies fed by rivers which formed lake deltas which were coarse, fan deltas (Unit I) of fine-grained deltas (Unit VI). The Devonian Clair Basin is an example of deposition in a basin of internal drainage which was predominantly controlled by climatic and sediment supply variations: a predictive model for sand body character and distribution can be developed using an understanding of these controls on the depositional systems.