Abstract

The Dibsiyah Member forms the lower part of the Wajid Sandstone in southwest Saudi Arabia. In the type locality the section, as described on the geological map, is about 150 m thick and rests unconformably on the basement. The lowermost unit consists of a thin fluvial conglomerate passing upwards into fluvially-dominated coarse to gravely cross-bedded sandstones. The main part of the member is fully marine and includes tidal dunes and sand-wave complexes, intensively bioturbated sandstones (skolithos ichnofacies) and a subordinate amount of shoreface and beach sandstones. In detail, this marine succession shows superimposed cycles with tidal dunes at the base, bioturbated tidal deposits and finally thick, entirely bioturbated intervals. These cycles are interpreted as third- to fifth-order sequences, which are in turn organised in a larger transgressive secondorder cycle as shown by the increase of the size of tidal bedforms and upward increase of bioturbation. The lack of chronological data for this formation does not easily allow a comparison with other Cambrian-Ordovician formations of Saudi Arabia. However, because both the Sajir and Risha members of the Saq Sandstone display similar transgressive trends including tidal features, we propose to correlate the Dibsiyah Member to one of the Saq Formation members as defined in northwest Saudi Arabia. If this working hypothesis is valid, the base Sanamah Member glacial erosion could have removed the entire Qasim Formation in this area. This is also supported by the total lack of storm- and wave-dominated facies, which are very common in the Qasim Formation and absent in the Dibsiyah Member.

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