Abstract

- ABSTRACT – The Miocene of the Rhone-Provence Molassic Basin (BMRP), at the western side of the perialpine molassic bassin, is a major site of study of the foramol facies, typical of temperate water carbonate systems. It consists in a network of incised valleys formed during 3rd order eustatic falls and infilled by marine coastal to offshore deposits during subsequent sea-level rises. The field work was focused on four sequences in the north (Saumane-Vénasque : Aquatanian S0, Lower and Upper Burdigalian S1 and Lowermost Langhian S2) and in the south (Aquitanian S0 – Tortonian S7) of the BMRP. It aimed to study the relationships between the 3D architecture of the bioclastic bodies, their depositional setting and their diagenesis. The transgressive tracts are composed of bioclastic bodies that are characterized by a strong inner 3D heterogeneity resulting from compound crossbeddings formed by flooddominated subtidal dunes of various shapes and sizes. The transgresive tracts are separated vertically by bioturbated marly intervals that correspond to highstand tracts emplaced during the flooding of the valley interfluves. The geometric parameters of the reconstructed tidal dunes in the TSTs suggest a water depth of about 20 to 35 m within the transgressed valleys and the size of the siliciclastic grains a current speed ranging from 30 cm/s (ebb) to over 100 cm/s (flood). The bioclastic bodies are composed of packstones, grainstones and calcarenites typical of the Bryonoderm, Molechfor and Rhodalgal facies, that suggest an offshore setting for the related carbonate factory. The facies changes in space are graded and do not match the stratal boundaries, except for the contrast between bottomsets and foresets of large dunes. However, there is an overall good match between the paleoecologic attributes and the crossbed types, suggesting that the areas of production and deposition could have been close to each other. The facies petrography is controlled by biological, physical and diagenetical processes. The porosity ranges from 1 to 20%, resulting from early dissolution and cementation enhancing the original texture contrasts. The coarse grain-size, the high initial porosity and the early sparitic cementation of the grainstones , which form the core of the largest dunes, indicate that these bioclastic bodies might have a good reservoir potential. Analyzing the diagenesis by the mean of cathodoluminescence (CL) allowed identifying four major cementation (early phreatic, meteoric phreatic, phreatic) and dissolution stages. Namely, CL helped to infer emersion stages driven by the highest order sea-level variations that control the valley infilling. The early marine cementation prevented from too much reworking of the deposit by tidal currents and permitted a strong aggradation of the series. These relationships bring about a petrographic and geometric sketch of the valley fills in which the confinement of the currents in the submarine valleys of the BMRP and their infilling by clastic carbonates determines the location, genesis and preservation of an exceptionally good geological reservoir.