Time of deformation in the Caledonide Orogen of Britain and Ireland

Abstract

Considerable problems attach to any attempt to define precisely the timing of such geological events as deformation. The biostratigraphical evidence may be imperfect or lacking, the isotopic evidence may be imprecise or conflicting, and where both lines of evidence are available, disagreement over definition of the Geological time scale creates problems of wider correlation. The last situation is exempslified by the English Lake District wherein the biostratigraphical and isotopic data appear to be reliable and compatible but the isotopic data conflict with some recently published time scales. Of particular importance to the present study are disagreements over the ages of the Lower Palaeozoic period boundaries. Estimates for the Devonian-Silurian boundary vary from 400 to 412 Ma (Odin 1982, cf McKerrowetal. in press); the Silurian-Ordovician boundary 418 to 438 Ma (Odin 1982, cf Harland et al. 1982); the Ordovician-Cambrian boundary 495 to 513 Ma (Odin 1982, cf McKerrow et al. in press); and the Camsbrian-Vendian boundary 530 to 590 Ma (Odin 1982, cf Harland et al. 1982). Such differences evidently become critical where comparisons of timing based only on biostratigraphic evidence are made with those based solely on isotopic data. Particular difficulties also arise from the problems involved in determining the biostratigraphic base of the Devonian within rocks of Old Red Sandstone facies.