Facies analysis of a back-tilted lacustrine basin in a strike-slip zone, Lower Devonian, Scotland

Lower Devonian (Lower Old Red Sandstone) basins developed along active faults in northern Scotland. The Dingwall–Strathpeffer Basin is a half-graben which thickens into the Helmsdale–Polinturk Fault, a splay off the major strike-slip Great Glen Fault Zone. Facies analysis allows the distinction of facies associations which represent alluvial fan, perennial lake and playa mudflat–ephemeral stream environments. Syn-sedimentary faulting at the margin was the major control on alluvial fan sedimentation. The lacustrine facies were fringed by alluvial facies, but at times of high water level extended to the active basin margin. Sedimentary structures and pseudomorphs after gypsum suggest an arid to semiarid environment in which the alluvial fan, stream and floodplain sedimentation was largely ephemeral. During arid periods, the basin-centre lake environment was constricted and the surrounding mudflats experienced intense evaporation, while after flooding episodes the lake increased in size. The thickest lacustrine sequences occur close to the basin-margin alluvial fans due to back-tilting of the fault blocks. Fault motion and/or climatic fluctuations caused significant interdigitation of the facies. The mixture of alluvial fan and organic-rich lacustrine facies close to the bounding fault offers some potential for trapping of hydrocarbons.