Depositional environments of the Triassic system in central Saudi Arabia

Facies analysis of Triassic rocks in central Saudi Arabia indicates a wide expanse of interfingering siliciclastic and carbonate rocks with some evaporites. Eight distinctive sedimentary facies have been recognized. The distribution of these facies show a systematic gradual change in their presumed depositional environments, laterally as well as vertically. The Lower Triassic Sudair facies represents a widespread regressive condition where the Upper Permian marine conditions gave way to the Lower Triassic with predominantly fine clastic deposits representing a restricted shallow marine shelf. This fine‐grained clastic facies consists mainly of unfossiliferous, laminated or massive, varicoloured shale with some silty shale, siltstone and very fine‐to fine‐grained sandstone. The facies is highly calcareous and gypsiferous in the northern area. A belt of Middle Triassic rocks of mostly non‐marine sandstone with some shale is present in the southern area passing into mixed siliciclastic‐carbonate facies of continental aspect with some intermittent emergence and nearshore conditions in the central area. This facies grades in the northern area into carbonate‐evaporite facies of restricted to more open marine shelf conditions. Thick siliciclastic deposits characterize the Upper Triassic Minjur facies, where a uniform repeated fining‐upward sequence of mainly sandstone and some shale developed in a non‐marine environment with some intermittent emergence in the northern area.