ABSTRACT The sub-surface Lower Palaeozoic succession in central Saudi Arabia includes a sandstone-dominated unit, up to 2,000 feet thick, which is overlain by either Silurian or Permian strata. Correlation of the sandstones with the exposed Lower Palaeozoic succession in northwest Saudi Arabia is problematical, partly because graptolite-bearing shales of the latter, notably the Hanadir Shale Member of the Qasim Formation, have not been identified in central Saudi Arabian wells. Based on Formation Microscanner (FMS) images, the succession in central Saudi Arabia was considered to correlate with the Quwarah Member (upper Caradoc to Ashgill) of the Qasim Formation, cutting down through the underlying members of the Qasim Formation (Ordovician) and the Saq Formation (Middle Cambrian to Lower Ordovician) to rest unconformably on Precambrian igneous basement. In contrast, palynological evidence presented here shows that the sandstone-dominated unit ranges in age from Early or Middle Cambrian, through Early, Middle and Late Ordovician, to Early Silurian. The palynological evidence suggests correlation with a substantial part of the Cambro-Ordovician succession at outcrop in northwest Saudi Arabia, rather than with just the Upper Ordovician. The palynological study also provides evidence for the depositional environments of the sandstone unit. Assemblages are generally of low diversity, and may be indicative of nearshore, marginal-marine conditions. Infrequent occurrences of more diverse assemblages suggest open-marine shelf sea environments for strata at some levels. Recognition of the latter has implications for stratigraphic modelling of the central Arabian succession, and might relate to episodes of coastal onlap recognised in the Llanvirn and lower Caradoc of northwest Saudi Arabia.

ABSTRACT Biostratigraphic investigations have been carried out on Ordovician chitinozoans mostly from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation in central Saudi Arabia. Among the 96 core samples processed from seven wells, about half of them (from wells Berri-84, Shedgum-239, Ain Dar-196 and Ain Dar-277) provided workable specimens, whereas the other wells, namely Ain Dar-281, Haradh-51 and Abu Jifan-25 were barren. Some chitinozoan species of Baltic affinities (e.g. Laufeldochitina striata) or of Laurentian affinities (e.g. Lagenochitina cf. pirum) are present, but most of the recovered chitinozoan species are of northern Gondwana affinities. These chitinozoans allow accurate correlation with the local chitinozoan biozones already established for northern Saudi Arabia. Precise correlation with some of the Ordovician chitinozoan biozones for the northern Gondwana regions can also be proposed (e.g. lower part of the pissotensis biozone). The available samples are not, however, sufficiently closely spaced for characterizing all the Ordovician chitinozoan biozones. It is not yet possible, therefore, to document eventual hiatuses in the Ordovician sedimentary succession of central Saudi Arabia. The investigated samples from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation are respectively dated as Ashgill, late Caradoc/earliest Ashgill, Caradoc and Llanvirn. Strata referred to the Sarah Formation are probably of topmost Ashgill age but may range into earliest Llandovery. Several new species have been observed. They are presently kept in open nomenclature until better preserved material is available for proposing well-documented diagnosis.

ABSTRACT A Kingdom-wide palynological analysis was carried out on the Palaeozoic section within the framework of the Saudi Aramco-Commission Internationale de Microflore du Paléozoïque (CIMP) Project. Thirty-three biostratigraphic assemblages and zones have been documented as a result of this study. A palynostratigraphic and lithostratigraphic review of the Palaeozoic of Saudi Arabia is provided herein.

Abstract Late Ordovician glacial deposits are of great importance in North Africa and the Middle East as a result of their significance as reservoirs for hydrocarbons and groundwater. The sedimentary record of this glaciation in NW Saudi Arabia (the Sarah Formation) is generally preserved in meridionally oriented palaeovalleys cut beneath northward-flowing ice sheets. In the Tabuk region of NW Saudi Arabia, an apparently intersecting complex of north–south- and east–west-oriented palaeovalleys occurs in the Alwizam area. Field relationships show two generations of palaeovalley incision, suggesting that the north–south-oriented palaeovalley was cut subglacially, filled, subsequently deformed and then cross-cut by the east–west-oriented palaeovalley. Abundant facetted and striated quartzite clasts occur at the base of each palaeovalley, testifying to a subglacial origin. Detailed examination of the north–south-oriented palaeovalley shows it to be well-defined with symmetrical sides. Its fill is composed of nine lithofacies grouped into four facies associations. About 80% of the fill consists of three sandstone facies: a parallel-bedded massive sandstone, a stacked scoured sandstone and a massive sandstone. Centimetre-scale extensional faults developed in soft sediments are commonly found throughout the stratigraphy, along with a glacially striated surface seen mid-way through the succession. These features provide evidence for direct ice contact, synglacial fill, and consequent reworking, cannibalization and deformation by the fluctuating ice margin.