Devonian correlations, environments and tectonics across the Great Glen Fault
New mapping, facies analyses, aerial photo interpretation, palaeocurrent, conglomerate provenance and clast size data are combined with a compilation of previously recorded, published and unpublished data to revise the stratigraphy of the Old Red Sandstone (ORS) around the SW Moray Firth, at the south end of the Orcadian Basin, astride the controversial Great Glen Fault (GGF). There was post-U.ORS compression, but previous notions of regional Lower-Middle and M.-Upper ORS unconformities, and of associated M. Devonian compression, are disproved. The ORS basin's configuration prior to later strike-slip is reconstructed. There is compelling evidence for net post-ORS dextral motion on the GGF of only 25-30 km, 8 km in the Mesozoic, the rest in the latest Devonian to early Permian. On parallel faults in the area, the data admit small dextral offsets, certainly less than 40 km total. Contrary to many syntheses, there is no compelling evidence for post-earliest Devonian sinistral motions along the Caledonides, only for dextral. The Lower ORS represents small, closed basins, with playas fed by ephemeral alluvial systems from adjacent small drainage basins. The Moray Firth M.ORS records the interaction, at the edge of a much larger basin, of ephemeral alluvial systems, supplied from much further afield, with the lakes which dominated the centre of the basin in Caithness. Aeolian dune and beach deposits are recognised for the first time in this succession, which is interpreted using a revised understanding of the relative importance of ephemeral and permanent lakes in Caithness. Arid and semi-arid climates alternated. Southern hemisphere trade winds prevailed. The Upper ORS contains braided river and aeolian dune deposits, plus sabkha sandstones which have proved to be particularly sensitive palaeoenvironmentally, indicating damp, dry, wet and, most abundantly, salt-cemented surfaces. Evaporitic features include those previously misinterpreted as adhesion ripples. True adhesion structures occur, but are rare. Contrary to previous suggestions, the climate was arid. Whereas the Middle ORS basin had been connected to the sea, by rivers, only during permanent lake episodes, the Upper ORS (latest M. to earliest U. Devonian) represents a coastal sabkha in a huge, semi-enclosed, Rann of Kutch-like gulf. The U.ORS ichnofauna is one of the oldest described from subaerial environments and includes possibly the earliest known land vertebrate trackway. Strike-slip faults of the GGF set did not control the ORS basin. In all three ORS subdivisions, subsidence and palaeogeography reflected NW-SE crustal extension. Half graben were separated by transfer faults: lines of differential subsidence and of access for drainage to the basin. Syn- and antithetic faulting caused uplift of previously subsiding areas to supply ORS clasts to later ORS alluvial fans. Within the ORS, no unconformity has to be attributed to compression. The (Emsian-Frasnian) extensional episode also affected E Greenland and W Norway. The Orcadian Lower, Middle and Upper ORS should be recognised as facies representing partly diachronous-stages in the interaction of drainage, sea level, and extensional tectonics.