A comparative study between the depositional environment and provenance of the Hussainiyat (Lower Jurassic) and the Rutbah (Upper Cretaceous) Formations, Western Desert of Iraq

Abstract

This work deals with the sedimentology of the Hussainiyat Formation (Lower Jurassic) and Rutbah Formation (Upper Cretaceous) in the Western Desert of Iraq with special emphasis on their depositional environments and nature of their source rocks. These formations are particularly chosen since they belong to different episodes, which are before (Hussainiyat Formation) and after (Rutbah Formation) the late Kimmerian deformation phase. Both units comprise supermature sandstone and contain a fairly similar suite of heavy detrital accessories (almost ultrastable minerals only). Such nature of the clastics impairs the identification in the usual methods, commonly carried out by sedimentary petrologists, although petrographic textures of quartz sand grains and the polycrystalline quartzitic pebbles are found useful. Five sections were described from both formations, and a total of 58 sandstone samples were collected, in addition to 153 gravel samples. The Hussainiyat Formation represents the classic model for high-sinuosity (meandering) river system, and the Rutbah Formation represents deposition in low-sinuosity (braided) river system resembling Platt-type model. Paleocurrent analysis of the Hussainiyat sandstones showed a southeasterly flow, and the Rutbah sandstone showed a northeasterly flow. Provenance analysis of the Hussainiyat and Rutbah Formations showed that they have different sources. Petrographic analysis of the quartz sand grains and the heavy minerals indicates derivation from sedimentary rock and plutonic or medium- to high-rank metamorphic rocks, but there is a marked difference between the percentages of the constituents in each formation. The textural features encountered in the majority of the studied pebbles characterize low-grade metamorphic rocks, and the statistical procedure (ANOVA) used shows that they have been derived from two different sources. Petrofacies studies show that the tectonic setting of the provenance area of both sandstones is craton interior of the continental block.