The sedimentation and provenance of the Lower Old Red Sandstone Greywacke Conglomerate, southern Midland Valley, Scotland.
PhD thesis, University of Glasgow.
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The origin of the southern Midland Valley Old Red Sandstone Conglomerates is reviewed with the specific aim of determining the nature and location of the source for the Old Red Sandstone Greywacke Conglomerate. The Silurian Midland Valley has been viewed as either i) a fore-arc basin with a rising trench-slope basin to the south (Leggett, 1980, Legget et al. 1983) or ii) an inter-arc basin with an arc in the Midland Valley and south in the location of the present day Southern Uplands (Bluck, 1983). The deposition of the Greywacke Conglomerate would thus record either the: i) culmination of a rising fore-arc and accretionary prism (Leeder et al. 1983), ii) time of emplacement between the northern section of the Southern Uplands and the Midland Valley (Thirlwall, 1988), or iii) age of the emplacement of the Southern Uplands over a missing fore-arc along a northward thrust, (Bluck, 1983). All of these models are dependent on the drainage basin for the Midland Valley within the present day Southern Uplands during Old Red Sandstone times. A re-appraisal of the sedimentology of the Greywacke Conglomerate has shown the sequence is characterised by vertically stacked small (2km) proximal alluvial fan sediments deposited in a series of separate N-S trending basins, each with one dominant fault controlled margin towards the east. The geometry and sedimentation pattern of these basins suggest the Southern Uplands Fault could not have controlled deposition of the Greywacke Conglomerate. The petrography and geochemistry of the greywacke clasts cannot be easily matched with a source in the Southern Uplands. The Southern Uplands sediments are characterised by petrographic consistency along strike. In contrast, there is a change in clast composition from the SW to the NE in the Midland Valley clasts, i.e. in the same general trend as the strike of the Southern Uplands. The evidence indicates that the source for the Greywacke Conglomerate was a basement cover of greywacke within the Midland Valley. This source may have been accreted to the Midland Valley during or prior to the obduction phase of the Ballantrae ophiolite. The source probably formed a series of highlands within the Midland Valley block since the Wenlock with major uplift of this source occuring in the uppermost Silurian-(?)Early Devonian.
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