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The late Paleozoic (Carboniferous–Permian) Gondwanan glaciation is represented in the subsurface of eastern and central Saudi Arabia by the Hercynian (or pre-Unayzah) unconformity and the lower part of the overlying Unayzah Formation. The subsequent postglacial transgression is manifest in the upper members of the Unayzah Formation as well as the lowermost clastic deposits of the overlying Khuff Formation. Its component sediments result from ongoing climatic amelioration following the demise of the ice age, as well as tectonic influences related to the creation of the Neotethys Ocean.

The Unayzah Formation is subdivided into four stratigraphic members. Thus, directly overlying the Hercynian unconformity in many places are sandstones and minor conglomerates of the Unayzah C member. These were laid down within a widespread, braided glaciofluvial depositional system. They represent glacial outwash produced during times of glacial retreat throughout the duration of the late Paleozoic glaciation. An unknown number of glacial readvances occurred that significantly deformed these retreat-phase outwash sands and gravels, creating major glacially tectonized push moraine nappes. Those are interpreted from a number of distinctive and discrete shear zones that are uniquely associated with the Unayzah C member. The upper surface of the Unayzah C member is an unconformity that marks the final subglacial surface at the time of maximum advance of the ice.

The terminal melt-out phase of the Gondwanan glaciation is represented by the Unayzah B member. Paleomagnetic evidence suggests that this member was deposited at high latitudes, ~75° S. This member comprises a large number of depositional facies that are essentially glaciolacustrine in character. Those facies include (1) small-scale ice-contact push moraines indicative of minor glacial readvance, (2) ice-proximal sublacustrine debris flows (massive diamictites) and associated gravity flow deposits, and (3) ice-distal, sublacustrine stratified diamictites, ripple cross-laminated sandstones, and laminated mudrocks. Facies associations within the Unayzah B member consistently show evidence of sustained glacial retreat and flooding of the landscape by filling and spilling over of numerous glacial lakes. This flooding sequence probably represents the maximum climatically related postglacial transgressive event in Saudi Arabia. In the western part of the study area there is evidence that the ice remained longer, and it is tentatively interpreted as a local center for high altitude (“alpine”) glaciation.

Deposition of the Unayzah B member was terminated abruptly by a drainage event that is marked by a widespread sharp contact with the overlying unnamed middle Unayzah member. The latter member displays no unequivocally glacially related depositional facies, and paleomagnetic data suggest that it was deposited at ~55° S. It is dominated by red floodplain siltstones and very fine–grained sandstones that contain relatively isolated bodies of coarser fluvial and eolian sandstones. These eolianites display possible cold-climate characteristics. This is particularly true in the western part of the study area, which is consistent with a relatively sustained, high altitude ice cap in that area. The unnamed middle Unayzah member is capped in many places by a paleosol horizon. This represents a hiatus of unknown but probably prolonged duration and thus suggests a disconformable contact between the unnamed middle Unayzah member and the overlying Unayzah A member.

The Unayzah A member is dominated by sediments that are strongly characteristic of terrestrial deposition in a semiarid to arid environment (including ephemeral lakes and streams as well as eolian deposits). Paleomagnetic data suggest paleolatitudes ~28° S. The continental eolian clastic deposits of this member in places display a cyclicity in their stratal architecture that is related to fluctuations in the paleo–water table. These fluctuations are possibly related to distant marine transgression, which is supported by the occurrence of a distinctive bioturbated sandstone very close to the top of the Unayzah A member. That marine-influenced sandstone is observed in widely separated localities at either end of the study area and may represent the final breakthrough of transgressive marine waters close to the end of Unayzah A time.

In several places the uppermost deposits of the Unayzah A member are characterized by thick paleosols. These represent a prolonged period of nondeposition, interpreted to be directly related to thermal doming of the Arabian plate prior to rifting and opening of the Neotethys Ocean, and the consequent formation of the pre-Khuff unconformity, which terminated Unayzah deposition. Overlying the pre-Khuff unconformity are various siliciclastic facies of the eponymous Basal Khuff Clastics member of the Khuff Formation. The depositional facies of the lowermost Basal Khuff Clastics range from shallow marine in the southeastern part of the study area to predominantly fluvial in the west. This reflects the westward-directed transgression of the Khuff Formation following thermal collapse in the wake of the rifting that created the Neotethys Ocean. That tectonically related transgression reached its fullest expression with deposition of the carbonates and evaporites that dominate the upper members of the Khuff Formation. This stratigraphic evolution of the late Paleozoic in Saudi Arabia can confidently be correlated in sequence stratigraphic terms with coeval sediments laid down across the Arabian Peninsula.

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