Middle Triassic shallow-water limestones from the Upper Muschelkalk of eastern France: the origin and depositional environment of some early Mesozoic fine-grained limestones

Abstract

We have analyzed the sedimentary structures, depositional geometries, and petrography of a Middle Triassic fine-grained limestone succession from the Upper Muschelkalk in the intracratonic Germanic Basin. The limestones occur in a unit, several metres thick, that extends over an area of at least 2500 km 2 in eastern France. The geometry of specific wave ripples and small channels filled by lateral-accretion bedded limestones, and the cyclic sedimentation of the Upper Muschelkalk in eastern France indicate deposition on a very shallow subtidal shelf, commonly under the influence of relatively strong currents. The limestones mostly consist of microspar with a few skeletal grains. The formation of current ripples in the fine-grained limestones appear to be incompatible with an originally muddy grain size. The major part of the microsparite may thus have originated from the recrystallization of silt-size carbonate grains, e.g., calcispheres or peloids.