Metabentonite and tuff occurrence in Silurian greywackes at Grieston Quarry, Innerleithen, Southern Uplands

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Grieston Quarry lies 1.6 km WSW of Innerleithen, Peeblesshire \[NT 3130 3618\] (Fig. 1). The rocks in the quarry comprise greywackes with intercalated pelitic beds which dip 60–65° NW and belong to the Gala Group, Telychian Stage. This succession has experienced prehnite-pumpellyite facies metamorphism ([Oliver et al. 1984][1]) which has transformed the pelitic beds into fissile flagstones, once used as roofing material. The quarry is important for yielding type specimens of the graptolite Monoclimacis griestoniensis Nicol, 1850 ([Toghill and Strachan 1970][2]). Approximately 7.5 m NW from the extreme southerly exposure of the main quarry face, a composite band 65 mm thick, comprising alternating coarse and fine layers, occurs within the greywacke sequence. The lowermost 5 mm of the band is made up of reddish grit, followed by 15 mm of friable fine-grained grey claystone, which shows no evidence of grading within it and has sharp upper and lower contacts. The next 45 mm are made up of alternating fine dark mudstone and coarse tuff laminae. Inspection of the tuff layers in thin section shows fragments of partly corroded euhedral feldspar crystals, fragments of quartz (some showing pyramid terminations) and rare zircon grains, all set in a mica and carbonate matrix. The grey claystone was sampled and prepared for XRD study. The clay fraction (<2 μm) was separated and analysed dry and after glyeolation. Chlorite and mixed-layer illite/smectite with 15% of expandable layers were found ([Reynolds 1980][3]). The non-clay fraction contains quartz, albitic plagioclase, chlorite, K-micas and traces of calcite. The claystone . . . [1]: #ref-7 [2]: #ref-10 [3]: #ref-8