Deposition of Upper Jurassic fine-grained limestones in the western Subalpine Basin, France

Abstract

The fine-grained carbonate sequence on the western margin of the French Subalpine Basin provides information about the depositional conditions of the Late Jurassic “nodular” lithofacies which closely resemble the age-equivalent “Ammonitico Rosso” of the Mediterranean and Central Atlantic domains. A variety of rock textures can be distinguished including “lumpy”, nodular (restricted sense) and conglomeratic. The formation of these textures is referred to different sedimentological processes: cyclic marl-limestone deposition, bioturbation, winnowing, early lithification, gravity-displacement, etc. In addition, the development of deep-water cryptalgal carbonate constructions which include stromatolites and small mud mounds is noted. Outer-shelf deep-marine palaeoenvironments (100's m) are inferred from the sedimentary structures, biotic associations and facies distributions. The Callovian-Oxfordian discontinuity is believed to have resulted from a sediment starved environment. Part of the overlying fine-grained carbonate material was then presumably derived from an adjacent platform. Oscillations in nature and quantity of carbonate and clay input onto the margin probably reflect cyclic climatic and sea-level changes.