Sedimentary evolution of a coral-, microbialites- and debris-rich Upper Jurassic reef (upper Tithonian, eastern Sardinia, Italy)

This work investigates a portion of a poorly documented upper Tithonian reef (Lower Mt. Bardia Reef, LBR) developed along the northern platform margin of the eastern Sardinia Platform (ESP). The LBR was probably one of the most southerly reefs of the northern Tethys margin, and this study provides the first comprehensive description of facies and internal structure, giving insights for the depositional setting. Detailed compositional and facies analyses of LBR have been carried out in three quarries located in the Orosei Gulf (eastern Sardinia). The main reef components are represented by carbonate (bioclastic) debris, microbialites and microencrusters, and reef-building organisms (mainly corals and stromatoporoids). These components combine in various proportions, forming several facies and facies associations. Ten reef facies (RF), grouped into three main Facies Associations have been identified within the studied reef portion. The distribution and arrangement of facies reveal a sedimentary evolution, evolving from a microframework-dominated reef to a carbonate debris-dominated one, with the internal reef architecture evolving from massive, to patchy, to coarsely stratified and roughly bedded. The reef components and facies indicate that the investigated portion of the LBR corresponds to a shallow back-reef zone, characterized by well-lit and generally low-energy hydrodynamic conditions, possibly developed in back position with respect to a higher-energy zone that was closer to the edge of the platform. In this depositional context, the arrival of large amounts of debris and the evolving internal architecture are interpreted as the result of the infilling process of the back-reef zone due to the progradation of a sand apron, during a long-term regional regressive phase.