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ABSTRACT

In well ST-8, Nafud Basin in Saudi Arabia, the pre-Unayzah unconformity (so called Hercynian unconformity) separates the Lower Carboniferous (upper Visean) Berwath Formation, from the Upper Carboniferous (upper Westphalian? and Stephanian) Unayzah C Member. The unconformity (erosion and non-deposition) corresponds to a hiatus in mid-Carboniferous time (Namurian-early Westphalian = Serpukovian-Bashkirian stages; about 327-311 Ma). The Berwath Formation is absent in outcrop, and rarely preserved in the subsurface. It consists of sandstones, carbonaceous shales and limestones deposited in an apparently regional lower coastal plain to marginal marine setting, and has a comparable thickness in two wells located nearly 1,000 km apart: ST-8 (361 ft, 110 m), and Abu Safah-29 (440 ft, 134 m) in the Arabian Gulf. The Berwath Formation is separated from the underlying Jubah Formation by a latest Tournaisian to mid-Visean hiatus, an event that is also recognised in North Africa and Brazil.

The subsurface Unayzah C Member (thickness range: 165-740 ft, 50-310 m) and overlying Unayzah B Member (55 ft, 16.8 m thick in reference well Hawtah-1) consist of clastics deposited in alluvial fan systems that are coeval with the glaciogenic: (1) Al Khlata Formation of Oman (upto 2,625 ft, 800 m), (2) Juwayl Member of the Wajid Formation in southwest Saudi Arabia (426 ft, 130 m); and (3) Akbra Shale in north Yemen (426 ft, 130 m). These rock units are assigned to the third glaciation of the Arabian Plate (AP G3), and their palaeodepocentre is located in the Rub’ Al-Khali Basin. The Unayzah C Member was deposited in continental lowlands and channels between uplifted fault blocks and broad arches, and unconformably overlies Proterozoic to Lower Carboniferous formations. The age of the younger and regionally widespread Unayzah B Member is interpreted as Early Permian (Asselian-Sakmarian). The Unayzah A Member in Hawtah-1 consists of the Lower ‘Siltstone’ Submember (135 ft, 41.2 m thick), and Upper ‘Sandstone’ Submember (91 ft, 27.7 m thick). The upper boundary of the Unayzah Formation, the pre-Khuff unconformity, is overlain by the Basal Khuff Clastics. The Unayzah A Member is generally unpalyniferous; recovered palynomorphs in the Lower Submember indicate an Early Permian age.

A tectonic compressional event uplifted the extensive NS-trending Arabian fault blocks (e.g. Hawtah and En Nala-Ghawar structures) in mid-Carboniferous time. The duration of the event coincides with the break between the Berwath Formation and Unayzah C Member. The same event caused deformation in the Oman Mountains, where open folds (about 100 m in amplitude, trending N50°E-N70°E) have radiometric ages of 329-321 Ma (i.e. late Visean and Namurian stages). The broad upper Palaeozoic swells (e.g. Central Arabian Arch and Haushi-Huqf Uplift) consist of Proterozoic to Lower Silurian successions that were uplifted without apparent regional faulting, either during the mid-Carboniferous tectonic event or earlier. The third glaciation AP G3 started in southern Arabia at about 311 Ma (or earlier), following the mid-Carboniferous tectonic event.

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