Old Red Sandstone rocks of the Midland Valley of Scotland record the amalgamation history of blocks widely separated from each other in Ordovician times. As a response to the lateral juxtaposition of the Midland Valley against the Highland block, the Midland Valley, weakened during its long history of igneous activity, was subjected to transtension and transpression, which opened and closed basins resulting in much recycling of sediment and the renewed development of intermittent and sometimes prolific volcanic activity. The Old Red Sandstone comprises two cycles of basin fill: the older (the Lower Old Red Sandstone) is separated by a major unconformity from the younger (the Upper Old Red Sandstone). Each cycle begins with conglomeratic sedimentation in pull-apart basins then fines and petrographically matures upward. The cycles, initiated by faulting, record the decline in tectonic influence and the concomitant reduction in source relief. The sediment source to the Old Red Sandstone is enigmatic. The Dalradian basement to the north and Southern Uplands to the south were, in many places, extensively eroded by Late Silurian–Early Devonian times. The thickest sequence of coarse sediment in the UK, if not in Europe, therefore has no obvious nearby uplift to provide both the sediment and the persistent, substantial slopes required to deliver sediment of that calibre to the basin. To the northeast, the Greenland-Baltica collision had created a major Silurian–Carboniferous (Scandian) uplift. Major river systems draining this mountain belt entered the Lower and Upper Old Red Sandstone basins at a late stage in their development when relief was lowered sufficiently to allow access to them.
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New Perspectives on the Old Red Sandstone
From the 1960s onwards, the Old Red Sandstone of both borders of the Atlantic Ocean has acted as a test-bed for the development of new ideas on the interpretation of fluvial, lacustrine and aeolian sedimentary rocks, and the investigation of tectonically-active basins. Much of the earlier reconnaissance work is now being reviewed in the light of further detailed field study, along with new developments in the understanding of the biostratigraphy, palaeobiology, geochronology, pedogenesis and tectonics.
Three general papers review recent work on the stratigraphical and chronological analysis of the Late Silurian, Devonian and Early Carboniferous strata, and summarize present understanding of the tectonics of the basins. These are then followed by twenty-seven contributions covering new work in Eastern USA, Canada, Ireland, Britain, Norway, Greenland and Spitsbergen.