Pelagic Carbonate Platforms in the Geologic Record: Their Classification, and Sedimentary and Paleotectonic Evolution

Abstract: Drastic differences in thickness and abrupt lateral facies changes in synrift and postrift pelagic sequences usually indicate structural control over deposition. Although the ''highs'' and the ''basins'' in pelagic carbonate systems have long been recognized in ancient rifted continental margins, relatively little detailed information has been available on facies transitions between the two and on the geometries of pelagic sediment bodies. Outcrops in the Umbria-Marche Apennines (central Italy) provide new constraints for Jurassic paleogeographic reconstructions, allowing for the development of models for these depositional systems. Pelagic carbonate platforms (PCPs) are submarine highs that are drowned fragments of an ancestor regional peritidal carbonate platform. PCPs can be horsts or crests of rotated blocks. The extensional regime and drowning history exert a major control over the PCP type and over the composition of sediment which fills inter-PCP basins. Given the very low rates of pelagic sedimentation (a few mm/1000 yr), the effects of faulting on submarine topography are drastic. Block tilting is of great importance, creating pelagic carbonate ramps that are the source area of most of the gravity flow deposits in half grabens. Excessively rapid tilts cause nondeposition. In horst-and-graben settings, on the other hand, basins are sediment starved, and there are comparatively only minor gravity flows; mass transport there is mostly in the form of collapse of platform margins. The margins of PCPs are erosional during most of PCP history, becoming depositional when adjacent basins are filled. Stepped margins develop through fault-induced reduction of the PCP area (back stepping).