Walls Formation, Western Shetland: distal alluvial plain deposits within a tectonically active Devonian basin

Synopsis The Walls Formation of western Shetland is a severely deformed, structurally isolated sequence of sandstones and siltstones of probable middle Devonian age. The sequence is probably extremely thick, and can be subdivided into a number of distinct sedimentary lithofacies. These include: trough and planar cross-stratified sandstones; very low angle to horizontally stratified sandstones; small scale trough cross-laminated sandstones; ripple cross-laminated silty sandstones; alternating siltstones and sandstones; and siltstones. The lithofacies occur in three distinct lithofacies associations, which represent deposition within high sinuosity (meandering) streams, sheltered floodplains and low sinuosity (braided) streams respectively. The braided stream sequences are the most common, and include sheet flood deposits which may indicate that the streams were ephemeral in nature. The sandstones are generally finegrained, and arkosic to subarkosic in composition; in places they contain high proportions of a varied suite of heavy minerals. Syndepositional seismicity is reflected by overturned and convolute lamination which is abundant throughout the succession. The Walls Formation represents first-cycle erosion of the emerging Caledonian mountains, and deposition upon a distal alluvial braidplain within a rapidly subsiding basin. Basin development may have been controlled by strike-slip tectonics, related to movement on the transcurrent Melby and Walls Boundary Faults.