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

Abstract

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, Dd 13 C )1AE1, 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.

Subtidal stromatolites from the Sinemurian of the Lusitanian Basin (Portugal)

“…The fact that this Lower Jurassic phase of the basin's evolution is associated with the occurrence in the Early Sinemurian of the speciWc bioevents represented by the stromatolitic mounds bears some similarity to a situation documented in the Sinemurian of Morocco (ChaWki et al 2004;Wilmsen and Neuweiler 2008), though in the latter case the mounds are sponge-dominated. In fact, in the Moroccan High Atlas, the Early to Late Sinemurian transition is marked by the birth and growth of carbonate mudmounds (mainly sponge and thrombolite buildups, very rare stromatolites), which increase in size through time, with the Wrst and smaller structures corresponding to the latest Early Sinemurian (ChaWki et al 2004).…”

“…Inter-supratidal microbial mats, planar to low-relief stromatolites and intertidal-shallow subtidal microbial-algal boundstones have commonly been recognized in these platforms (e.g., RüVer and Zamparelli 1997;Bosence et al 2000;Azerêdo et al 2003b;Mastandrea et al 2006;Wilmsen and Neuweiler 2008; and references therein). Subtidal thrombolites and stromatolites are known in the Triassic of this palaeogeographic domain (e.g., RüVer and Zamparelli 1997; Mastandrea et al 2006) and elsewhere (e.g., Riding 2006;Ezaki et al 2008;Mary and Woods 2008).…”

“…In fact, in the Moroccan High Atlas, the Early to Late Sinemurian transition is marked by the birth and growth of carbonate mudmounds (mainly sponge and thrombolite buildups, very rare stromatolites), which increase in size through time, with the Wrst and smaller structures corresponding to the latest Early Sinemurian (ChaWki et al 2004). The onset of this moundrich interval is considered by ChaWki et al (2004) to be closely linked to sea Xoor morphologic changes related to breakup of the extensive Moroccan Sinemurian shallow platform, with the mounds disappearing towards the Sinemurian-Pliensbaquian boundary when the establishment of a larger basin resulted in persistent hemipelagic silting conditions; Wilmsen and Neuweiler (2008) also relate these sedimentary and biotic changes at the Early-Late Sinemurian boundary of Morocco to a subsidence pulse and corresponding deepening trend, integrating them in a wider biosedimentological post-extinction context.…”

Depositional environments and facies development of the Cenomanian–Turonian Galala and Maghra el Hadida formations of the Southern Galala Plateau (Upper Cretaceous, Eastern Desert, Egypt)

“…8b). Their intercalation into, and interfingering with, open-marine facies rules out a restricted, peritidal setting for these peloidal sediments as often reported in the literature (e.g., Gaździcki et al 2000;Fürsich et al 2003;Wilmsen and Neuweiler 2008;Wilmsen et al 2010a). It is suggested that the concentration of peloidal sediment occurred in response to weak bottom currents and/ or strong infaunal activity (the sediments are pervasively bioturbated).…”

REE patterns of microbial carbonate and cements from Sinemurian (Lower Jurassic) siliceous sponge mounds (Djebel Bou Dahar, High Atlas, Morocco)

“…The distinction between microbialite, which is a precipitated major component of the microbial mounds, and allomicrite (carbonate mud that represents transported particulate sediment from a variety of sources) is important because the former provides information about seawater chemistry and different processes of REE uptake at the time of precipitation, whereas the latter may or may not. Previous REE + Y data from Lower Jurassic successions in the central High Atlas rift basin (Wilmsen and Neuweiler, 2008) mostly represent whole rock solution ICP-MS analyses that diverge from seawater-like patterns with significant middle REE (MREE) enrichment. Despite the non-seawater-like nature of the patterns, Wilmsen and Neuweiler (2008) used, in part, Ce anomalies in those rocks to interpret evolving local seawater oxygenation levels through the Lower Jurassic succession.…”