Distribution of coral-microbialite reefs along the French jura platform during the bimammatum zone (Oxfordian, late jurassic)

Abstract

During the Late Jurassic, the Jura platform was the site of major carbonate production involving various reef building components, particularly corals and microbialites. The existence of a detailed stratigraphic framework for the French Jura sedimentary succession during the Late Oxfordian–Early Kimmeridgian interval makes it possible to analyse the nature and distribution of these reefs within a high-resolution time frame, the Bimammatum Zone. Even though these reefs developed in a wide range of settings, their development was never optimal, indicating the presence of limiting factors. When the supply of terrigenous material was lower, the inner platform was characterized by strong carbonate production, with the development of a large ooidal shoal complex including ten-meter scale coral-microbialite patch reefs. The large size and high-density skeletal structure of some colonies indicates a favourable environment for coral and reef development (shallow, agitated, well-oxygenated waters with high light levels), despite the coral assemblage being poorly diversified and characterized by a mixed photo-heterotrophic association. The autocyclic character of the sedimentary system (i.e. major migrations of the ooidal dunes after big storm events) recurrently interrupted reef development. When the French Jura platform was subject to significant terrigenous input, the coral assemblages became well-diversified and characterized by mixed or heterotrophic-dominated associations. In the shallow and proximal areas of the inner ramp, limited available space imposed strong constraints on the reef growth, which only developed biostromal morphologies. Along the mid ramp, the reefs were composed firstly of plurimetric bioherms, then metric patches and finally coral meadows, highlighting the inversely proportional relationship of reef size to water depth. Once a certain depth was reached on the mid ramp, the presence of corals with heterotrophic feeding patterns could explain their reduced capacity to grow and build up a reef framework. Periods of greater terrigenous input, causing high turbidity and eutrophic conditions, led to frequent interruptions in reef growth both in the inner and more proximal mid ramp positions. This terrigenous influence is less pronounced in the central part of the mid ramp, where storms could alternatively stop reef development by causing migration of oncoidal sediments that then covered the small coral patches. In the distal part of the mid ramp, reef growth was reduced and relatively stable. Here, periods of reef development were controlled by long-term allocyclic processes that triggered the migration of oncoidal facial belts resulting from variations in relative sea level.