Composition and provenance of sandstones and conglomerates Hugh F Barron, Emrys R Phillips and Richard A Smith British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA Introduction The Silurian Inliers The tectonic evolution of the Midland Valley of Scotland during the The Wenlock strata are subdivided into a number of formations The early to middle Wenlock 'quartzite' conglomerate is Lower Palaeozoic to Devonian, and its setting within Caledonian (Figure 2 ). March Wood, Eastgate, Henshaw and Cock Rig characterised by the presence of subangular to occasionally orogenic models has been the focus of much recent debate formations, which have been sampled during this study, comprise a rounded quartzite rock fragments within a medium- to coarse- (Williams & Harper 1988; McKerrowet al. 1991; Smith 1995; sequence of fine- to coarse-grained, poorly to locally moderately grained sandstone matrix. However, this heterolithic Phillipset al. 1998). Central to this debate is the sedimentology, sorted sandstones (Figures 3a to 3 c). The sandstones are conglomerate also contains fine-grained rhyolite and dacite rock structure and provenance of a sequence of Silurian sedimentary lithologically similar and are composed of a mixed assemblage of fragments as well as minor andesite, cleaved mudstone, rocks and the overlying Lower Old Red Sandstone (Lanark angular to subangular basaltic to andesitic volcanic rock fragments microgranite, psammite, quartz-diorite, siltstone and mudstone. Group). The Silurian sedimentary sequences are exposed in a with subordinate monocrystalline quartz and plagioclase (Figures 3b The quartzite conglomerate is best developed within the number of inliers which occur along the southern margin of the and 3c). Metamorphic (including metabasalt, phyllitic rock, mylonite, Lesmahagow inlier where it thins to the west. In contrast, in the psammite and quartzite) and sedimentary rock fragments are a Midland Valley of Scotland (Figure 1 ). Hagshaw Hills inlier the conglomerate thins and fines towards the minor, but relatively common component within these sandstones. Minor to accessory detrital components include muscovite, biotite, north (Bluck 1983 and references therein). An isolated lens of In general, the inliers record a transition from an older Llandovery garnet, chlorite, tourmaline, epidote, K-feldspar and amphibole. 'quartzite' conglomerate occurs at a similar stratigraphic level in transgressive marine sequence to a younger regressive terrestrial the Pentlands N Esk inlier. The variations in composition and Lower Wenlock succession. However, there are significant Two distinctive conglomerates, the'igneous' and 'quartzite' thickness of this conglomerate suggest that it was deposited by a differences in the detailed stratigraphy and thickness of the conglomerates, are present in some, but not all the inliers. The SE complex system of easterly and northerly prograding alluvial fans. sequences present within the individual inliers (Figure 2 ). This derived 'igneous' conglomerate (Henshaw Formation) is best variation probably reflects their deposition within in a number of developed in the inliers close to the Southern Upland Fault. It Lower Old Red Sandstone (Lanark Group) small sub-basins which were developed oblique to the Southern comprises subangular to subrounded, locally broken, granule to Figure 4. Tinto Hill from immediately SW of the Southern Upland Fault . The Tinto Rhyolite (Devonian) is intruded into the Greywacke Conglomerate Formation; the Upland Fault during regional sinistral strike-slip (Williams & pebble sized clasts of fine-grained andesitic to dacitic igneous rock Overlying the variably deformed LlandoveryWenlock inliers are March Wood Formation of the Eastfield Inlier forms the sloping ground at the foot Harper 1988; Smith 1995; Phillipset al . 1998). fragments and subordinate quartz within a medium- to coarse- the ?Pridoli-Lower Devonian strata of the Lanark Group which of Tinto Hill. grained sandstone matrix. Basalt, microgranite, rhyolite, trachyte, forms part of the Lower Old Red Sandstone sequence of the tonalite and felsite rock fragments are also present. Minor Midland Valley (Figures 1 and 2 ). The clastic sequence was sedimentary and metamorphic rock fragments (meta-andesite, deposited in the NE-SW-trending Lanark Basin by both axial and cleaved mudstone, psammite, quartzite, schist/phyllite and biotite- transverse drainage systems which supplied sediment from The Greywacke Conglomerate Formation thins and fines towards hornfels) have also been recognised. essentially the same source area.At the base of the Lanark Group the NW and was deposited in alluvial fans interbedded with is the laterally extensive Greywacke Conglomerate Formation sandstones which preserve palaeocurrent indicators of a source (Figures 2 and 4 ). region to the SE (Bluck 1983). In the Hagshaw Hills area mean palaeoflow indicate that the formation was mainly derived from the E, with only a minor component being derived from the S 6 (Syba 1989). The vertical stacking of the alluvial fans and 80 predominance of proximal fans indicate that subsidence kept pace or exceeded uplift of the source area during deposition of Midland Valley EDINBURGH this conglomeratic formation. Terrane meta-quartz- The Greywacke Conglomerate Formation passes upwards into GLASGOW CGF arenite predominantly red bed, high energy fluviatile sandstones of the volcanic rock overlyingSwanshaw Formation ( Figure 2 ). The Swanshaw 3 fragments 660 G Formation records an overall upward-fining sequence with a PF renewed input of coarse conglomerate towards the top of the formation; possibly the result of local uplift prior to the subsequent volcanic episode. The sandstones contain a mixed detrital 2 assemblage of andesitic to felsic rock fragments, quartz and feldspar (Figure 3d ). Other rock fragments present include LANARK 'greywacke' metasandstone, silicified red sandstone, chert, and 640 silty mudstone, as well as rare higher grade, foliated quartz metasedimentary rock fragments (psammite and quartzite). IGF E C BIGGAR F volcanic rock CMF fragments a b The stratigraphically overlying Duneaton, Biggar and Pentland Hills volcanic formations( Figure 2 ) consist of a suite of calc- D Figure 3a. Very coarse sandstone, Figure 3b. Coarse-grained sandstone, alkaline, basaltic to andesitic lavas (subaerial) with minor BF Cock Rig Fm, North Esk Inlier March Wood Fm, Eastfield Inlier intercalated tuffs and volcaniclastic sandstones and breccias Southern Uplands (Smith 1995; Phillipset al . 1998). These Lower Devonian (410- 6 AYR 20 Terrane quartz 415 Ma, Rb-Sr whole rock-mineral ages; Thirlwall 1988) lavas CCF have geochemical characteristics comparable to basalts erupted NEW quartz CUMNOCK 1 in a volcanic island-arc setting (Thirlwall 1981, 1983) and possess a strong 'within plate' continental component (Phillips et al. 1998). The interbedded sedimentary rocks are poorly sorted and poorly A KLF bedded. They contain a restricted, clast assemblage dominated B by angular, locally derived basaltic and andesitic volcanic rock 600 Midland Valley Silurian inliers fragments (with minor feldspar, pyroxene and hornblende crystal volcanic rock GIRVAN Southern Upland Fault fragments fragments). Lanark Group sedimentary rocks MOFFAT volcanic rock fragments Lanark Group igneous rocks GAF c d 580 SVF 2 2 2 2 3 3 3 Figure 3c. Coarse-grained sandstone, Figure 3d. Medium-grained sandstone, 20 40 60 80 00 20 40 Eastgate Fm, Carmichaell Inlier Swanshaw Fm, Lanark Group Figure 1. Simplified geological map of the Lanark Basin of the south Midland Valley showing the distribution of the Silurian inliers and the Siluro-Devonian Lanark Group. Study Areas: 1 - New Cumnock; 2 - Lanark; 3 - Pentland Hills. Silurian inliers : A, Craighead; B, Girvan Main; C, Lesmahagow; D, Hagshaw Hills; E, Carmichael; F, Eastfield; G, Pentland Hills. Faults: SVF, Stinchar Valley Fault; GAF, Glen App Fault; KLF, Kerse Loch Fault; BF, Bankend Fault; CCF, Carmacoup Fault; IGF, Inchgotrick Fault, CMF, Carmichael Fault; PF,Pentland Fault; CGF,Crossgatehall Fault. volcanic rock fragments New Cumnock Pentland Hills district e Figure 3e. Very coarse sandstone, Auchtitench Fm, Lanark Group Figure 5. Large-scale fluvial cross bedding in channel fill deposits, Auchtitench Auchtitench Formaton. The lower set is approximately 5m thick. Little Law Hill Quarry, Formation Dungavel Hill Conglomerate Member Roberton, Lanark district Auchtitench Formation AUC Figure 3. Thin section photomicrographs Wiston Grey Volcaniclastic Member Pentland Hills The succeedingAuchtitench Formation ( Figure 2 ) is Duneaton Lanark Biggar Volcanic Volcanic dominated by a sequence of fine- to coarse-grained, lithic-rich Volcanic Formation Formation Formation sandstones. At the base of the formation, highly weathered district LANARK GROUP Swanshaw sandstones and siltstones contain carbonate nodules/clasts and Swanshaw Formation In the Girvan and Pentland Hills areas there is a marked angular are interpreted as having formed as patchily developed soils or Upper SILURIAN - Formation Swanshaw Lower DEVONIAN LANARK GROUP Formation unconformity between this conglomeratic formation and salinas on the irregular surface of the volcanic terrain. The Greywacke Swanshaw Greywacke Upper SILURIAN - Lower DEVONIAN Conglomerate underlying Silurian strata. However, in the Hagshaw Hills, Formation Conglomerate Greywacke overlying fluviatile sandstone succession is typically less Formation Formation Conglomerate Lesmahagow, Carmichael and