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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.

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