Giant subaqueous carbonate dunes: a revised interpretation of large-scale oo-bioclastic clinoforms in the middle Jurassic of the Paris Basin and its implications

Abstract

Recent developments of carbon capture and storage and geothermal subsurface prospects in the Middle Jurassic limestone of the central part of the Paris Basin have rejuvenated interest in the world class outcrops of the Bathonian Oolithe Blanche Formation on the southeastern edge of the Basin. Here, ooid-bioclast grainstones occur as large-scale (10–15 m high) clinoforms in the upper part of the Oolithe Blanche, which remain enigmatic and variously interpreted. High-resolution sequence stratigraphic correlations, constrained by biostratigraphic markers and integrated with regional palaeogeography, indicate that the clinoforms are giant dunes developed during the early part of the regressive phase of a third-order transgressive–regressive sequence (Bt3), in the northern part of the Burgundy Bathonian carbonate ramp. Carbonate sand bodies up to 15 m high with intervening troughs, formed subaqueous giant dune field(s) with calculated wavelengths of about 500–800 m. Using dune height as a proxy for calculating the palaeobathymetry of deposition, it is concluded that the dunes formed under a minimum water depth of about 60 m. A detailed facies analysis of the sediments underlying the dunes, which include beds rich in brachiopods in living position corresponding to the MFS of the third-order transgressive–regressive sequence (Bt3), supports such a water depth. The constitutive facies of the dunes themselves also reflect a significant water depth, despite the presence of ooids that are definitely transported away from their main production zone. This new interpretation of the large-scale clinoforms in the upper Oolithe Blanche and the associated revised palaeobathymetry have implications for the interpretations of Bathonian carbonates across the Paris Basin, which are relevant to ongoing projects of reservoir characterization of subsurface prospects.