A chitinozoan biostratigraphy of the Upper Ordovician and lower Silurian strata of the Girvan area, Midland Valley, Scotland

ABSTRACT Forty-six samples, taken from the Upper Ordovician and lower Silurian strata of the Girvan district, Midland Valley of Scotland, yield moderately well-preserved and diverse assemblages of chitinozoans. The area was chosen to study the composition of the chitinozoan assemblages, rarely described before in Scotland, which lay at the south-eastern margin of the Laurentia palaeocontinent. These assemblages are compared with those of the same age on other palaeocontinents, representing a critical time when both the Iapetus and Tornquist oceans were closing, thus forming the Caledonide and associated orogens. It is possible to correlate the chitinozoan occurrences at Girvan with those of other parts of Laurentia (Quebec area), other palaeocontinents such as Baltoscandia (Sweden, Estonia), Avalonia (Wales, Brabant Massif) and some parts of Northern Gondwana (Saudi Arabia). For the lower Silurian, it is possible to correlate with the global chitinozoan biozonation, and a calibration between the chitinozoan occurrences and the graptolite biozonation of Girvan is established. Several levels within the Balclatchie, South Shore, Three Mile, Shalloch, Wood Burn, Lauchlan, Drumyork Flags and other formations are accurately dated.