Glacial and post-glacial deposits of the Unayzah Formation (Carboniferous–Permian), Saudi Arabia: facies analysis and sequence stratigraphy

The Unayzah Formation exposed on the southwestern margin of the Arabian Plate presents an example of the glacial and post-glacial facies deposited on the passive continental margin above the Hercynian unconformity. The glacial facies (lower lithologic unit) includes one depositional sequence that occurs in narrow incised valleys within the subjacent Hercynian unconformity. It consists of stratified diamictite (glacial outwash deposits), mottled mudstone–diamictite (glaciofluvial) and rooted sandstones (orthoquartzite) (subaerial weathering). These deposits show an unconformbale relation with the overjacent post-glacial facies. The contact between them coincides with the end of glaciation at the end of Carboniferous and the subsequent initial sea level rise at the Early Permian. The upper lithologic unit represents the post-glacial facies. It constitutes the main thickness of the Unayzah Formation and comprises two depositional sequences separated by an erosional surface. The main facies in the upper unit are laminated claystone–marl, burrowed dolomicrite, pebbly sandstone (orthoquartzite), sandstone–marl, heterolithic facies and evaporite. These sediments contain numerous escaped trace fossils and were deposited during the processes of meltwater release during glacial recession.