Integrated micropaleontology, sedimentology and sequence stratigraphy of the Middle Jurassic D5-D6 Members of the Dhruma Formation, Central Saudi Arabia

The Middle Jurassic Dhruma Formation of Saudi Arabia is of economic importance because it contains three hydrocarbon reservoirs. We report the first record of physically extracted benthic foraminiferal assemblages from the Middle Jurassic D5 and D6 member outcrops near Riyadh, Central Saudi Arabia, using the acetic acid method. This study of Middle Jurassic limestone units is intended to improve our understanding about the depositional environments of the area and related bio-events. Microfossils such as foraminifera are important in this aspect in that they can be recovered from core samples where entire macrofossils such as ammonites are rare. This study focuses on the taxonomy and biostratigraphy of the foraminifera and examines their contribution to the previously established Jurassic biozonation. A total of 35 species belonging to 19 genera are identified. Ten sedimentological facies showing depositional energy variations from deep marine muds to shallow high-energy oolitic grainstones are identified. The extracted foraminifera are tied to the stratigraphic and sedimentological columns of the D5-D6 member outcrop. The depositional environment model of this portion of the formation is interpreted to be an open marine ramp setting.