The classification of Avonian limestones

Abstract

In systematised stratigraphical classification 'Avonian' is a term junior to 'Dinantian'; but in present ignorance of the precise correlative equivalents of Dinantian assises and Avonian zones the term is conveniently retained for the Lower Carboniferous rocks, most of them typical 'Carboniferous Limestone', of the British South-Western Province, where there is also ignorance of the precise horizons of the Fammennian-Dinantian junction and the Dinantian-Namurian junction. At the same time, the Avonian assemblage zones have significance only in the carbonate rocks of the northern part of the Province: they are not at all applicable to the Lower Culm of the southern part, and they are applicable only with uncertainty to other Lower Carboniferous provinces in Britain. 'Avonian' is of only temporary and local validity: it is not the name of a standard series. Avonian rocks are mainly shelf limestones of a variety of kinds. They display marked lateral and sequential changes that reflect the influence of many factors, of which contemporary earth-movement, with the consequent migration of facies belts and the occurrence of sharp lithological breaks and nonsequences, was of major importance. The fossiliferous members of the series reflect subtly variant biotopes that throw light on a multiplicity of shelf environments whose characteristics and distribution have strong analogies, in associations of organisms and in community balance, with near-shore shelf sediments of many of the warmer present-day seas; and recurrent inorganic limestones (mainly calcite muds and oolites) demonstrate the general shallowness of the shelf seas over very wide areas. Detailed palaeogeographical reconstruction is possible for many of the rock groups over much of the ground. A sedimentological classification of the rocks is not readily accommodated by a usual system of parameters inherited from the classification of detrital terrigenes. Nearly all the limestones are endogenic, their calcareous constituents almost never the product of hinterland erosion or of prolonged transport even when the 'clasts' in them are abraded fragments. Grain size in such sediments is commonly an accident of origin, not a reliable sign of the restlessness of the environment of deposition: a spirifer shell is much larger than the individual crinoid plates of the bed in which it lies; a stromatolitic algal sheet is the welded product of growing algal nodules, and conversely an algal nodule may show partial disintegration into 'amorphous' algal mud; a coral bush is the containing frame of the shells it houses and of the organic debris caught in its mesh, and at the same time is itself embedded in a bank of the same kinds of shells or in wrappings of the same kinds of debris; drewite mudstone, a faecal pellet, an oolith, reflect related factors in carbonate formation. 'Matrix' also has many ambiguities: it may be an accumulate virtually in situ; it may be detritus transported some distance, or fragmented and redistributed locally; it may be mainly of monogenetic provenance, in its shelly or its algal or its evaporitic constituents, and then be coarse-grained or fine-grained, apparently well-sorted, through immediately environmental 'accident,' or it may be, equally accidentally,' unsorted or ill-sorted in grain size as an 'internal' product of a mixed biotope; may be diagenetic, 'contemporaneous' penecontemporaneous' or 'subsequent.' In a naive petrographical system mechanically applied, classification can result in the imposition of mutually incommensurable incompatible criteria in attempts to distinguish between petrological kinds resistant to the system: rocks similar on the arbitrary criteria for instance of grain size) may have little mutual affinity; rocks very different on the criteria may be closely allied. Principles the classification of limestones, as they are illustrated in the Avonian rocks, need to self-consistent and comprehensive: they cannot be adequately of use in description unless they are also genetic.