THE TRILOBITES AND BRACHIOPODS OF THE WRAE LIMESTONE, AN ORDOVICIAN LIMESTONE CONGLOMERATE IN THE SOUTHERN UPLANDS

The Wrae Limestone is a conglomerate within late Caradoc or Ashgill (l ate Ordovician) siltstones near the southern edge of the Northern Belt of the Southern Uplands. Its interpretation exemplifies problems comm on to many orogens: separation of fossiliferous lithoclast ages from t iming of final emplacement, separation of biofacies from temporal faun al correlation and the use of biofacies in determining provenance. The trilobites are closest to those of the lower Caradoc rocks at Girvan, north of the supposed Southern Uplands-Midland Valley terrane boundar y. The edge of the Midland Valley platform may thus have been the sour ce of the carbonate clasts. Some of the trilobites range through the s pectrum of biofacies present in the Lower Caradoc of Girvan. However, species of Phorocephala and Dubhglasina suggest an origin at the margi n of the Nileus association. The brachiopod assemblage is not directly comparable with any faunas in the Girvan-Appalachian belt. It is domi nated by coarse ribbed orthoids, smooth triplesiids and several plecta mbonitoids, having more in common with later Ordovician brachiopod bio facies associated with carbonate mudmounds, but overall the Wrae brach iopod assemblage is not particularly typical of any one benthic assemb lage zone or precise substrate type.