The Petrology of Middle Devonian Limestones in South Devon, England

Abstract

ABSTRACT The petrographic examination of the diagenetic and metamorphic history of these Devonian limestones, in the light of work on Recent carbonates, shows them to be a predominantly bioclastic series, ranging in composition from calcilutites to coarse biomicrites and biosparites with variable allochems and pelsparites. Six main microfacies are recognized. Despite low grade metamorphism, recognizable diagenetic events include initial cementation, subsequent solution and the introduction of secondary calcite, and the formation of authigenic minerals, quartz and felspar. The rocks are locally dolomitized, and there is evidence of limited de-dolomitization. Metamorphism has resulted primarily in the production of new fabrics, the degree of recrystallization depending upon the stress environment of the rock. In extreme examples all previous textures are destroyed. Some quartz may have been introduced concurrently with the deformation but in general there is no development of metamorphic minerals. Lack of stratigraphic correlation prevents detailed environmental reconstruction, but present evidence suggests that current interpretations of these limestones as a reef formation are erroneous. The limestones are shown to be clastic accumulations of skeletal material, probably from and within areas of prolific growth of carbonate secreting organisms. These organisms at no time formed a growth lattice having topographic expression comparable to modern reefs. Separate ecological habitats are recognized, but little explanation can be offered of the limiting factors.