Biostratigraphy of late Llandovery (Telychian) and Wenlock turbiditic sequences in the SW Southern Uplands, Scotland

Abstract

ABSTRACTIn the SW part of the Southern Uplands of Scotland the relatively thin Moffat Shale Group (late Ordovician–early Silurian) is succeeded by a thick development of Silurian greywackes, of variable turbiditic facies. This includes late Llandovery (Telychian) quartzose greywackes with interbedded thin graptolitic shales of the turriculatus and crispus biozones, in the upper part of the Gala Group, a sequencewhich is laterally equivalent to the basal part of the Hawick Group. The age of the finer-grained calcareous Hawick Group, which here includes 1. the Ross Formation, ranges from late Llandovery (turriculatus Biozone) to early Wenlock (riccartonensis Biozone). The Riccarton Group, which contains thick units of thinly-bedded siltstones and mudstones, is of Wenlock age (riccartonensis to lundgreni biozones). Within this sequence, all the biozones of the standard graptolite zonal scheme have been recognised in the area, with the exception of the crenulata Biozone of the late Llandovery (Telychian Stage) and the murchisoni and ellesae biozones of the Wenlock (Sheinwoodian Stage). Details of the graptolite biostratigraphy areclosely comparable with those of the markedly thinner sequences of northern England. Acritarchs occur throughout the sequence but are most numerous and best preserved in the Gala Group. Poorly preserved chitinozoa and spores are also present, the former occurring sporadically throughout the succession but the latter become common only in the Riccarton Group.