Tectonostratigraphy of the exposed Silurian deposits in Arabia

Exposed Silurian deposits in Arabia are represented by the Qalibah Group, the Qusaiba and Sharawra formations. The Qusaiba Formation is composed of dark-gray claystones and siltstones. It is disconformably underlain by the Late Ordovician–Early Silurian? Uqlah Formation or unconformably underlain by the Late Ordovician Zarqa or Sarah formations. It is disconformably overlain by the Sharawra Formation. The Early Rhuddanian basal “hot shale” of the Qusaiba Formation represents the early stage of the early Silurian marine transgression over the Gondwana broad shelf. It is a regional marker used to outline the structural configuration of the area prior to the Silurian time. The Sharawra Formation is composed of siltstone and sandstone. It is unconformably overlain by the Late Silurian?–Early Devonian Tawil Formation. Silurian deposits show a pronounced thinning from 992 m in the Tabuk area in the west and are completely missing in the northern part of the Qusayba depression in the east. The thinning of the Qusaiba shale and Sharawra sandstone is interpreted as due to depositional and erosional features, respectively. Thinning and distribution of the Early Rhuddanian shale “hot shale” is depositional which is affected by preexisting Late Ordovician paleo-highs in central Arabia. Thinning of the Sharawra sandstones is erosional which is attributed to Late Silurian tectonic movements synchronous with the Acadian uplift phase of the Caledonian tectonic movements. The main structural elements in central Arabia are represented by the north–south trending and northerly plunging Hail arch and to a much lesser extent the northwest–southeast trending and southerly plunging Qusayba high.