CYCLICITY IN THE SARCLET GROUP, A CONGLOMERATE/ARKOSE FACIES IN THE LOWER DEVONIAN [OLD RED SANDSTONE FACIES] IN NORTHERN SCOTLAND

The Sarclet Group [late Lower Devonian - Emsian - Old Red Sandstone facies] forms the core of a much faulted dome on the east cost of Caithness, northern Scotland. Exposures are limited to steep sea cliffs, most of which are inaccessible. The base of the sequence is not exposed, but is assumed to be unconformable on the high grade metamorphics and associated granitic intrusives of the Caledonian orogene. The oldest unit exposed is the Sarclet Conglomerate Formation [70m+] which passes up into the Sarclet Sandstone Formation [85m]. These red bed units are interpreted as an alluvial fan/braided river complex. Higher beds in the Group have been interpreted as a playa/ distal fluvial complex. The basal sediments are compositionally immature; extraformational pebbles clasts are a polymict assemblage of granites and high grade metamorphics. Pebbles of this type are likely sourced by the Caledonian orogene. A small percentage of basalt clasts cannot be tied to a known source. Associated sandstones are arkoses. Four fining-upwards units [conglomerate - arkose] can be traced for approximately two kilometers along strike. Pebbles in the conglomerates of these units show both vertical and lateral variations in composition. These variations suggest that the cyclicity may have a tectonic component of control; the tectonic component may be provided by a pre Middle Devonian phase of left lateral strike slip movement on the Great Glen fault.