ABSTRACT: The Middle and Upper Jurassic section of Kuwait is more complete than the type Jurassic outcrop sections in Saudi Arabia. The lithostratigraphic nomenclature, biostratigraphy, and sequence stratigraphy have recently been revised and clarified. Based on biostratigraphic data, Middle Jurassic deposition began with the Aalenian-age to early Bajocian Upper Member of the Marrat Formation, which has no time-equivalent unit in the outcrop sections. Three sequences (J16 to J18) are proposed in this lowstand wedge, and the overlying Dhruma, Sargelu, Najmah, Jubaila, Gotnia, and Hith formations were deposited during the remainder of the Middle and nearly all of the Late Jurassic. During that time interval, the Gotnia Basin formed and filled and uplift of the Rimthan Arch first influenced sedimentation patterns in the Gotnia Basin and in the Arabian Basin to the south. Organic-rich rocks were deposited in both basins and were the source for hydrocarbons accumulations in the Arab Formation. Dhruma Formation deposition began with a rapid relative rise in sea level and differential subsidence of the underlying Marrat carbonate platform and ramp deposits in response to the inception of seafloor spreading as the Southern Neotethys began to open along the Zagros margin. Dhruma deposition began with laminated and heavily bioturbated calcareous shales in a basinal setting and aggraded upward, becoming interbedded with storm beds and wackestones of the outer and inner shelf in the lower member (Sargelu-Dhruma Transition) of the Sargelu Formation. Renewed, rapid sea level rise in the base of the upper Sargelu Limestone Member was marked by the disappearance of argillaceous content and aggradation of mudstones and skeletal wackestones and with upward shoaling into peloid-cortoid-skeletal packstones of shelf origin. Rapid deepening began during deposition of the Najmah-Sargelu Transition and continued though the lower Najmah Shale Member of the Najmah Formation. This marks the beginning of deposition of organic-rich laminated calcareous shales and argillaceous lime mudstones. The upper Limestone Member of the Najmah Formation shoals upward from a condensed interval at its base, through upper slope and outer and inner shelf settings. A hiatus separates the Najmah Formation from the overlying laminated and burrowed argillaceous mudstones of the Jubaila Formation. A second unconformity and hiatus separates the Jubaila from the overlying Gotnia Formation. This hiatus varies in duration, but the Gotnia as a whole is equivalent to the Arab Formation of Saudi Arabia. The Gotnia cycles in Kuwait contain great thicknesses of salts (primarily halite) and peritidal microbial mats, nodular and palmate anhydrite and peloidal lime mudstones to packstones deposited under peritidal conditions. Generalized isochore maps of the units of the Middle and Upper Jurassic in Kuwait show the Gotnia Basin forming during Dhruma time as a broad gentle sag of the underlying Lower and early Middle Jurassic Marrat Formation. The first clear evidence of segmentation into the Gotnia and Arabian basins, separated by the uplifted Rimthan Arch, occurs in the late Middle and early Upper Jurassic Najmah Shale. The Gotnia Basin appears to have been nearly infilled by the end of Najmah Limestone deposition. Renewed tectonic activity began during deposition of the newly- recognized Jubaila Formation and continued during deposition of the Gotnia Formation, with rapid thickening across the Rimthan Arch in southern Kuwait and with onlap and thinning northward into north Kuwait and eastward onto the Burgan Arch. Rapid salt precipitation during Gotnia Formation deposition kept up with subsidence so that it must be classified as a tectonic basin but never as a bathymetric depression. Extensive sampling of the strata from lower Dhruma through Jubaila Formations has been conducted. Several fossil groups have been studied and the results integrated with the existing nannofossil zonation to validate this sedimentologic and depotectonic interpretation and to provide a separate model for basin geometry and paleobathymetry. Those results are reported in part two of this article (in press).


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