A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan

Abstract

The rocks of the Blue Lias in Dorset and Glamorgan can be divided into off-shore and near-shore facies. The off-shore facies has a characteristic pattern of regular, small-scale alternations of argillaceous calcilutites, marls and sometimes bituminous shales, the main variable being CaCO$_3$ content. Two distinct types of limestone are recognized and termed laminated and normal limestone, respectively. The marls are generally comparable with the limestones in all respects except their much lower CaCO$_3$ content. The bituminous shales are rich in bituminous matter which is arranged in fine laminae parallel to the bedding. Determinations of percentages of insoluble residues reveal a consistently large difference between the limestones and marls. Fuller chemical analyses of major constituents indicate, among other things, that nearly all the carbonate is present as CaCO$_3$. The clay mineral content consists almost entirely of illite, with subsidiary kaolinite. The strontium content of the carbonate fraction of the marls appears to be markedly higher than that of the limestones. Vertical variation in Dorset, Glamorgan and Somerset is studied and compared by a graphical method based on the limestone-shale ratio. It has been established that the Blue Lias rhythm is primary in origin, but that there has also been a limited amount of early diagenetic segregation of CaCO$_3$ to produce nodular structures. The limestone textures are accounted for by recrystallization from an original lime mud and the respective importance of several processes including drusy and grain growth and granular and rim cementation assessed. Pyrite is considered to have been formed early in diagenesis under anaerobic conditions within the sediments. Its association in some drusy cavities with calcite is explained as due to the local fall in pH of interstitial fluids. The difference between the normal and laminated limestones and marls is considered to be the result of aerobic and anaerobic bottom conditions, respectively. The microlaminae in the bituminous shales are interpreted as varves due to the annual fall of plankton into anaerobic bottom waters. Evidence is put forward that the Blue Lias rhythm may be the result of repeated epeirogenic oscillations. Rocks of the near-shore facies are confined to Glamorgan. They include (besides calcilutites and subsidiary marls) skeletal limestones, oolites, conglomerates and cherty beds; locally the rocks lie unconformably on Carboniferous Limestone. Silica is found in the form of bands of nodules and silicified limestone pebbles and shells. The facies relationships of the different rock types can be satisfactorily related to the approach of an old shoreline. The silica was almost certainly derived from detrital chert weathered from the Carboniferous Limestone. Although there is a broad similarity in the fauna between Dorset and Glamorgan, a number of important differences are recognizable. Differences between the off-shore and near-shore facies are also described; whereas the former has pelecypods and ammonites as its most conspicuous elements the latter is notable for the abundance of corals and gastropods and, locally, of ribbed pectinids. A relationship between the fauna and the sediments is recognized in three cases: (1) shell enrichment in condensed beds (with glauconite and/or collophane); (2) dwarfing and general faunal impoverishment in the laminated rocks, related to poor aeration of the sea bottom; and (3) variations in sedimentary rate and depth of sea probably account for the faunal differences between Dorset and Glamorgan in the off-shore facies. On the other hand, no relationship can be perceived in three other cases: (1) the increase in size up the succession in a number of forms, which is evolutionary; (2) the succession of different organisms due to ecological replacement and extinction; and (3) certain shell enrichments which may be due to population fluctuations. In a summary of the Blue Lias environment deductions are made about temperature, salinity, rate of sedimentation, depth of sea and current strengths.