Sequence stratigraphy and facies analysis of the NE Gondwana Middle Jurassic inner-platform mixed carbonate-siliciclastic succession, Saudi Arabia

The sequence stratigraphy of the Dhruma Formation and Tuwaiq Mountain Limestone (Bajocian to middle Callovian) is based on shallow cores and outcrop measured sections along a 600-km–long N-S transect west and south of Riyadh. Correlations were extended westward in the subsurface using gamma-ray wireline logs (500 km). For the first time, the study integrates sequence stratigraphic framework with detailed semiquantitative micropaleontological analysis that allows reappraisal of stratigraphic interpretation of main biofacies associations. The outcrops provide a continuous stratigraphic record of the Middle Jurassic transgression of a large (>1000 km) epeiric tropical platform from continental braided fluvial deposits to tidal or wave-dominated mixed carbonate-siliciclastic lagoonal deposits. These formed aggraded flat-topped platform wedges and thicken northward due to evident syndepositional differential subsidence. The carbonate platforms are mud-dominated and evolved from restricted carbonate platforms that contained microbes and low-faunal diversity (early Bajocian to early Bathonian) to normal-marine carbonate platforms hosting coral/stromatoporoid-rich strata and high-faunal diversity adjacent to a deeper intrashelf basin (middle Callovian). The succession consists of two 2nd-order sequences, Dhruma and Tuwaiq sequences, separated by an unconformity (middle Bathonian hiatus). Several composite and high-frequency sequences that have a subordinate maximum flooding surface (MFS) in the lower Bathonian (zigzag Zone) and a main MFS in the middle Callovian coronatum Zone. The depositional sequences are considered to be of eustatic origin as they match well with Tethyan sea-level cycles. The carbonates developed well during late transgressive and highstand systems tract (TST and HST) of the composite sequences controlled by back stepping of the siliciclastic sources.