Microfacies, biofacies, and depositional environments of the Bajocian–Bathonian Middle Dhruma carbonates, central Saudi Arabia

Abstract

Three cliff-forming outcrops of the Dhruma Formation representing the carbonate platform in the Hafirat Nisha district west of Riyadh, Saudi Arabia, were investigated to identify and understand the microfacies variability in a carbonate ramp setting. An integrated approach including detailed field investigations, petrographic, biofacies, and micropaleontological analyses was used to construct a model of the depositional environments of the investigated outcrop sections. The depositional model indicates that the identified lithofacies were predominantly deposited in outer ramp, middle ramp, inner ramp, and lagoonal settings. The presence of the benthic foraminifera Redmondoides lugeoni and Nautiloculina oolithica and the trace fossil Thalassinoides indicates deposition in shallow-water environments and the prevalence of warm climatic conditions during the Middle Jurassic. The occurrences of calcareous and agglutinated benthic foraminiferal species Pseudomarssonella maxima, Siphovalvulina variabilis, Timidonella sarda, N. oolithica, R. lugeoni, Praekurnubia crusei, Paleopfenderina salernitana, Haurania deserta, Siphovalvulina spp., Lenticulina sp., Textulariopsis sp., and Amijella amiji imply that our studied sections are situated within the Dm-2 to Dm-5 biozones (Bajocian to early Bathonian). This is also supported by the calcareous nannofossil assemblages that show a Bajocian to early Bathonian age. The depositional sequences indicate a shift in the depositional environment from low-energy lagoonal with intermittent shoal complexes in the D2 and D3 Units to predominantly shoal complex and open-marine settings in the D4 Unit, suggesting a seaward shift towards the top of the D4 Unit. The interpreted depositional settings are similar to those previously documented in the subsurface equivalent, Faridah reservoir. This indicates that the investigated D2–D4 Units outcrops are excellent analogues of the subsurface reservoir.