Peritidal to intrashelf basin, facies transition of the Adgham Formation (Late Triassic) Al Qasim Province, Saudi Arabia

The Upper Triassic Adgham Formation outcropping on the western margin of the Arabian Basin comprises shallow mixed siliciclastic-carbonate rocks deposited on the Arabian Plate passive margin during the Neo-Tethys. It presents an example of three coeval environments: peritidal environment in the southeast, shoal environment in the middle and intrashelf basin towards the northeast and northwest. In the peritidal environment, the facies consists of shallowing-upward cycles, each of which commences with claystone, overlain by ferroan dolomitic quartz arenite, skeletal dedolomitic quartz arenite, microbial laminite, and sometimes caliche at the top. These cycles were deposited in intertidal, supratidal and terrestrial with intermittent subaerial exposure by climatic and allocyclic mechanisms. The shoal environment represents the barrier between the peritidal environment in the southeast and the intrashelf basin in the northeast. In the basal part, it consists of shallowing-upward cycles, each starts with thin claystone, capped with burrowed dolomitic quartz arenite, dedolomitic quartz arenite, while the upper part comprises shallowing-upward cycles, each of which begins with claystone, followed by intraclastic oolitic wackestone and dolomitic oolitic packstone. It was deposited on the uplifted block that was synchronous with the deposition of the Adgham Formation. The intrashelf basin interfingers with the shoal facies and deepening northeastwards and is restricted from the open marine in the northeast by reefal facies at the shelf edge from the east. It comprises shallowing-upward subtidal cycles; each consists of a lower claystone that gradationally passes upward into pelletal-peloidal dolomicrites, sandy oncolitic-peloidal dolomicrite, evaporitic dolomicrite and/or spheroidal dolomite. These facies were deposited in restricted lagoonal and intrashelf basin, in which the subtidal cycles were formed by climatic changes and autocyclic mechanism.