Sedimentology and diagenetic evolution of the Upper Permian Khuff Formation in Saudi Arabia. New perspectives from biostratigraphy, U/Pb geochronology and fluid inclusions

Abstract

The Upper Permian section of the Khuff Formation is characterized by mud-dominated facies interpreted as lagoonal to peritidal sediments, interbedded with coated grain, grain-rich deposits suggesting lagoonal to high-energy shoal environments. The diagenetic overprint encompasses early dolomitization, dissolution, and burial diagenetic products such as dolomite cements and at least one phase of anhydrite. For the first time in the Khuff Formation of the Arabian Platform, in-situ LA-ICP-MS chronometry using U/Pb age dating has been performed in one well of the Rub al Khali Basin in order to refine the absolute timing of the various cement phases. The early dolomitization is interpreted as a very early replacive event resulting from hypersaline fluids, but is affected by recrystallization during the Middle Triassic (average 244.3 ± 7 Ma). Dolomite cements include a Guadalupian to base Lopingian phase (D1 – 265.5 ± 8 Ma) and a saddle dolomite (D2) dated from the Lower Jurassic (193.5 ± 15.7Ma). Fluid inclusion measurements in the latter indicate minimal precipitation temperatures of around 150 °C from rather medium salinity fluids, probably related to Lower Jurassic faulting. This study also shows the absence of calcite cements and of thermochemical sulfate reduction (TSR), usually present in the Khuff Formation at the platform-scale. The combined biostratigraphy assessment and U/Pb chronometry performed in this study suggest the section directly overlying the Middle Anhydrite to be slightly older than initially thought, and the transgression on top of the Middle Anhydrite marker extending into the Capitanian. Results also suggest that faults might have been active during the burial history allowing hydrothermal brines to percolate through studied interval.