The biostratigraphy of the Silurian rocks of the Girvan District, Scotland

Abstract

The Silurian biostratigraphy of the Girvan area is reviewed. Of the 23 Silurian formations, 7 have abundant shells and 14 have common graptolites. The Silurian is nearly 3000 m thick and was chiefly deposited in a turbidite environment. All the Llandovery zones from cyphus to crenulata inclusive are represented by graptolite faunas. In the Rhuddanian (prior to the cyphus Zone) the Lady Burn Conglomerate, Mulloch Hill Formation and Woodland Formation are shelly facies laid down by a marine transgression. There are two stratigraphical breaks, the unconformity at the base of the Silurian sequence and in the Fronian, where there is a paraconformity underneath the Lower Camregan Grits, both breaks with a transgressive sequence of animal communities above them. At the top of the succession Lower Wenlock turbidites, bearing shells at their bases, are followed by a sparse ostracodbivalve assemblage representing brackish or even fresh water conditions, culminating in ‘Old Red Sandstone’ facies beds of presumably Wenlock age, all representing the marine regression seen in other inliers in the Midland Valley of Scotland. Remarkable fossil finds from the Llandovery include Stomatograptus grandis girvanensis subsp, nov., Monodimacis crenulata itself, and the only chonetid brachiopod known anywhere from the Rhuddanian or Idwlan.