The petrofacies of the Cambrian Thumaylah (Amin) formation outcrops in central Oman, and its potential for silica sand production

Siliciclastic successions of Early Cambrian age across arid continental margins provide globally significant archives for reconstructing palaeoenvironmental evolution, sediment routing systems, and assessing high-purity silica resources of industrial importance. In central Oman, the Thumaylah Formation (correlative with the Amin Formation) forms part of the Haima Supergroup and represents a regionally extensive succession unconformably overlying the Huqf Supergroup across the Angudan Unconformity. Despite its importance, a clear integration of outcrop-scale sedimentology, shallow subsurface data, and mineralogical controls on reservoir quality and industrial potential remains limited, constituting a key research gap in both regional stratigraphic frameworks and comparable Cambrian systems worldwide. This study addresses this gap by integrating surface outcrop observations with shallow borehole data to characterise the sedimentology, petrofacies, and mineralogical composition of the formation. The Thumaylah Formation is subdivided into three informal members: a lower claystone-dominated unit deposited in low-energy playa lake environments, a middle medium- to coarse-grained sandstone unit formed within high-energy braided fluvial systems, and an upper fine- to medium-grained quartz-rich sandstone unit associated with aeolian dune and sand-sheet deposition, with minor shallow marine influence. Petrographic and geochemical analyses indicate compositionally mature sandstones dominated by quartz, with minor chert, negligible feldspar, and kaolinite cementation exerting primary control on porosity and permeability. Provenance is interpreted to be derived predominantly from Pan-African felsic basement rocks with contributions from recycled metamorphic sources. Notably, the upper sandstone member hosts high-purity silica sand (approximately 98% SiO₂) with favourable grain morphology, suitable for hydraulic fracturing and other industrial applications. Correlation with the Amin Formation further establishes the Thumaylah Formation as a valuable local analogue for regional fluvial–aeolian systems. Overall, this study refines the stratigraphic architecture, depositional evolution, and reservoir and industrial mineral potential of the formation, offering insights applicable to analogous Cambrian siliciclastic systems globally.