An Outstanding Record of Paleozoic Siliciclastic Sedimentation and Unconformities in the Subsurface of Saudi Arabia

Abstract This study is focused on seven different wells drilled in the Rub’ al-Khali Basin (southeastern Arabian Plate) and shows one of the few available integrations of palynological and sedimentological data across its Paleozoic stratigraphy, with implications for a better knowledge of both the geological history and the petroleum geology of this basin. Middle Cambrian strata, partially assigned to fluvial and coastal environments from the Siq and Burj formations respectively, represent the oldest intervals observed in our wells. Above, the upper Cambrian to Middle Ordovician Saq Formation is identified in all the wells except in those from the northern sector of the studied area. While the northern sector is interpreted as a local paleo-high that experienced little or no sedimentation during large parts of the Paleozoic, a core cut in the upper section of the Saq Formation is interpreted as partly deposited in a fluvial setting. A ~15 Ma unconformity is inferred between the Saq Formation and the upper members of the Qasim Formation (Middle to Upper Ordovician). The latter unit includes cores assigned to marine offshore settings. Also, parts of the uppermost interval of the Qasim Formation are likely eroded by the latest Ordovician glaciation. Uppermost Ordovician glacial strata from the Sarah Formation are present in the southern and central sectors of the studied area. Palynological assemblages indicate that at least part of the lowermost Sarah Formation belonged to relatively distal lacustrine settings. However, a core cut in the upper part of this unit exhibits intensely deformed strata probably related to glacial ice dynamics impacting fluvial or marine deposits. Above the Sarah Formation, a relatively thin section of the lower to middle Silurian Qalibah Formation is only found in three of the studied wells, a situation that is interpreted as due to major erosion during the formation of the Hercynian Unconformity (late Carboniferous). An increased organic richness, often documented elsewhere in the lowermost Qalibah Formation across the Arabian Plate, may still be present in these three wells. No Devonian strata are found in the studied area; instead, the glaciogenic Juwayl Formation (upper Carboniferous to Lower Permian) directly overlies the Hercynian Unconformity and shows large thickness differences across our wells. The latter is likely due to differences in the available accommodation space that was subsequently filled by sediments resulting from the shaping of the topography by glacial ice advances and retreats. The overlying Nuayyim Formation (Lower Permian) exhibits comparatively persistent thickness across the studied area, to include the northern sector, and suggests a period of structural quiescence, in terms of constant subsidence and fill of the available accommodation space, during its deposition. Finally, 2 cores from the upper section of the Nuayyim Formation include mixed clastic-carbonate sediments deposited in coastal to shallow-marine environments that evolved into subaerial paleosols probably due to an increased climatic aridity by the end of the Lower Permian. A corollary is that, unlike the continental aeolian systems commonly interpreted across sectors of the eastern Arabian Plate, our inferences point to the absence of aeolian dunes in our wells but the possibility that a dynamic interaction of coastal and marine systems existed in parts of the Rub’ al-Khali Basin by the early Permian.