MICROFACIES AND DEPOSITIONAL ENVIRONMENTS OF THE BATHONIAN-BAJOCIAN MIDDLE DHRUMA CARBONATES, CENTRAL SAUDI ARABIA

During the Middle Jurassic, an ancient carbonate ramp was extensively developed on the western margin of the Tethys Sea, in Central Saudi Arabia. Three cliff-forming outcrops of the Dhruma Formation, representing the platform, near Khashm Ad-Dhibi, the Riyadh region, were investigated to identify and understand their microfacies variability. Detailed field investigations, petrographic (thin section, XRD and SEM) and biofacies analyses were utilized to identify and quantify bulk mineralogy, and construct and interpret the depositional model of the identified facies of the Units 2 to Unit 4 of the Dhruma Formation in the study locality. Eight microfacies comprising of peloidal-skeletal wackestone, skeletal-peloidal packstone, peloidal grainstone, oolitic grainstone and skeletal oolitic packstone, skeletal floatstone, burrowed wackestone, and mudstone were identified in the investigated outcrop sections. These facies are interpreted to have been deposited in a lagoonal (i.e., peloidal skeletal wackestone, skeletal peloidal packstone), shoal complex (i.e., peloidal grainstone, oolitic grainstone, and skeletal oolitic packstone) to open marine environment (i.e., skeletal floatstone, burrowed wackestone, and mudstone). The presence of benthic foraminiferal species Redmondoides lugeoni and Nautiloculina oolithica throughout the intervals of the studied sections, indicates deposition in shallow warm water environments and the prevalence of warm climatic conditions in the Middle Jurassic. A shift in the depositional environments from mainly lagoonal and shoal complex as indicated by the microfacies belonging to the D2 and D3 Units to dominantly shoal complex and open marine settings in the D4 Unit, suggests a seaward shift towards the top of the D4 Unit. Another line of evidence to support the shift in deposition setting is the decrease in microfossil contents towards the top of stratigraphic sections.