Advances in Arabian stratigraphy: Origin and stratigraphic architecture of glaciogenic sediments in Permian-Carboniferous lower Unayzah sandstones, eastern central Saudi Arabia

Abstract

In the subsurface of eastern central Saudi Arabia, deposition of lower Unayzah sediments commenced in late Carboniferous (late Moscovian = late Westphalian-Stephanian) times during the earliest stages of the late Paleozoic Gondwanan glaciation. The Unayzah C member is the lowermost unit, and comprises quartzose sandstones laid down on an extensive glacio-fluvial outwash braidplain upon the pre-Unayzah unconformity (so-called "Hercynian unconformity"). They were overridden during (probably several) glacial re-advances, when the sediments were thrust over each other during the construction of push moraines ahead of the advancing ice sheets. The upper surface of the Unayzah C member represents the final sub-glacial contact of this long-lived series of glacial advances and retreats, and demonstrably represents a significant post-Unayzah C unconformity. Subsequent (early Permian) deposits represent the final glacial retreat phase. They include the Unayzah B member and a new, un-named middle Unayzah member that underlies the Unayzah A member. The Unayzah B member is dominated by glaciogenic sediments, including highly deformed material attributed to seasonal push moraines (and hence representing minor glacial re-advances); ice-proximal, subaqueous outwash fans (high-energy lacustrine turbidites with associated dropstones, and massive diamictites); and ice-distal, glaciolacustrine deposits including laminites, stratified diamictites, ripple cross-laminated sandstones and mudrocks with associated dropstones. In contrast, the un-named middle Unayzah member is characterized by widespread, fine-grained red-beds, representing low-lying alluvial floodbasin deposits. Embedded within these occur aeolian and fluvial sandstones. A significant drainage event is thus implied between the Unayzah B member and the un-named middle Unayzah member. It is marked by a sharp contact between the two members that represents a regionally correlatable disconformity. The end of lower Unayzah deposition is represented in places by paleosol development, indicating a further disconformable contact, separating the un-named middle Unayzah member from the overlying Unayzah A member. This study significantly extends the record of the advance and retreat of the Gondwanan ice sheets in southern Saudi Arabia, and provides sufficient confidence to propose a degree of stratigraphic correlation with coeval deposits in Oman.