Biosedimentology of the Early Jurassic post-extinction carbonate depositional system, central High Atlas rift basin, Morocco



This study documents a Liassic example of the long-ranging effects of mass extinction on carbonate systems. Biohistoric constraints inherent in the Liassic carbonate depositional system are deciphered from normal-marine, sub-tidal deposits of the central High Atlas rift basin (Morocco) through ?Hettangian/Sinemurian to Early Toarcian times. The integration of results from the analysis of lithofacies, depositional geometries, microfacies, macrobenthos, carbonate build-ups, carbon and oxygen stable isotopes, and rare earth element + yttrium distribution patterns allows the intrinsic (or biohistoric) control on the central High Atlas deposits to be separated from extrinsic factors, such as basin development and palaeoclimate. The survival interval in the aftermath of the end-Triassic mass extinction persisted until the Early Sinemurian indicated by a severely depleted carbonate system impoverished in skeletal organisms. A tectonic pulse at the Early to Late Sinemurian boundary interval caused a basin widening with immigration of a marine fauna. However, until the latest Sinemurian (macdonelli Subzone of the raricostatum Zone) the deposits were dominated by filter-feeding benthic heterotrophs (sponges, brachiopods, polychaetes and crinoids). During this stage, primary production within the enlarged basin must have been largely planktonic and there was a net-flux of organic matter to the sea floor (oxygen minimum zone). A regional radiation of organic-walled phytoplankton is inferred to explain the selective success of the filter-feeding community and the occurrence of sponge mounds. Thus, significant effects of the end-Triassic mass extinction are still present during the Late Sinemurian. Through almost the entire Pliensbachian a highly productive, shoal-rimmed carbonate platform existed; it developed subsequent to tectonic reorganization and a marine recirculation event (radiolarian facies, Δδ13C ≈ −1·1, strongly negative Ce-anomaly). Photosymbiotic sediment producers (mainly large bivalves) now state the success of specialists and environmental equilibrium conditions. In the latest Pliensbachian the climax stage was reached with the development of a coralgal reef-rimmed carbonate platform. The Liassic carbonate depositional system experienced a terminal, multicausal Early Toarcian drowning event during which most of the large bivalves became extinct.