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

Abstract

In 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.

The Wenlock Cyrtograptus species of the Builth Wells District, central Wales

“…It has not been reported from the Wenlock of Scotland (e.g. White et al 1992). In the Czech Republic the species occurs in the perneri-ramosus Biozone (herein).…”

“…These biozones were subsequently recognised throughout Britain (e.g. Rickards 1965Rickards , 1967Rickards , 1969Bassett et al 1975;White et al 1992) and the Builth district became the British 'standard' for the Wenlock biozonation Zalasiewicz 1990). Zalasiewicz and Williams (1999, p. 265) noted that the Cyrtograptus species used for defining the various graptolite biozones in the Builth district are often rare, and their morphology incompletely understood.…”

“…flexilis) Biozone, which is recognised in northern England and southern Scotland. However, a re-examination of material from all of its recorded occurrences may be timely (see Rickards 1967Rickards , 1969White et al 1992). During our investigation of museum collections we have encountered a number of Cyrtograptus species which have been misidentified as C. linnarssoni.…”

“…During our investigation of museum collections we have encountered a number of Cyrtograptus species which have been misidentified as C. linnarssoni. These include a specimen collected by Elles from Coed Mawr which we refer to Cyrtograptus rigidus (SM A59196), one from Balmae Haven in south-west Scotland which we also refer to C. rigidus (BGS GSE14061; see White et al 1992, Fig. 7I), and a possible C. rigidus (SM A22463) from Moel Ferna in the Dee valley of Wales (see .…”

“…In northern England and Scotland C. rigidus has been used as the index fossil for the rigidus Biozone, and C. rigidus cautleyensis as an index for the younger flexilis Biozone (sensu Rickards 1967Rickards , 1969White et al 1992). No such distinction is recognised in the Czech sequence where C. rigidus is the index fossil of its eponymous middle Wenlock Biozone (e.g.…”