Biostratigraphy and Palaeoenvironments of the Middle Jurassic Dhruma and Sargelu Formations, Onshore Kuwait

The study of fifteen cored wells shows calcareous shales, with occasional limestone interbeds, changing into alternations of wackestones, mudstones and packstones in the upper part, characteristic of the Dhruma and Sargelu formations respectively. Biostratigraphic analysis recognise moderate-abundant calcareous nannofossils and palynomorphs in the Dhruma and lower part of Sargelu, whereas benthonic and planktonic foraminifera are more frequently found in the Sargelu. The Dhruma Formation is Early-Late Bajocian, Zone NJT10 on the basis of the FO of W. manivitae and D. sellwoodii and the LO of C. superbus. The overlying Sargelu is Late Bajocian to Callovian since the LO of C. magharensis and FO of G. calloviensis occur in the lower half of this Formation. Micropaleontological assemblages, together with paleoenvironmental marker species enable to interpret variations from inner-middle neritic during the Bajocian, attaining outer neritic in the Bathonian-Early Callovian, with the deepest conditions observed towards the west, returning to shallower inner neritic conditions in the Callovian. These infer two major flooding surfaces during the deposition of these formations. Combined to a greater chronostratigraphic constraint, palaeobathymetry could provide significant insights to Jurassic history of the Arabian Plate, where local subsidence and/or tectono-eustatic controls influenced the depositional cycles observed within these units.