Sedimentological and stratigraphic studies of the cambro-ordovician succession in northwest Saudi Arabia

摘要

In the first field study since 1963, the Cambro-Ordovician clastic succession studied in two areas in northwest Saudi Arabia is interpreted to represent two major lithostratigraphic units based on the integration of sedimentological descriptions, facies associations, facies distributions, petrographic analysis and paleocurrent direction analysis. Including the Siq Sandstone, Quweira Sandstone and parts of the Saq Sandstone, the depositional setting in the Al Ula area is dominated by high-energy, braided-fluvial, in-channel deposition with minor tidal influence. The depositional setting of these sandstone units in the Tabuk area is very similar, but significantly shows an increased tidal dominance with respect to fluvial facies when compared with the succession in the Al Ula area. Variations in facies associations and distributions between the two localities demonstrate evidence of paleogeographic and proximality implications within the overall fluvio-estuarine depositional setting. These facies associations and distributions indicate a major stratigraphic break at the base of the Upper Siq Sandstone, marked by a rejuvenation of fluvial facies. Thin section analysis indicates a petrographic break at the same boundary, marked by the complete loss of feldspars above that level. Paleocurrent directions show a major shift in flow direction, generally from NW to NE, at the same boundary. These observations suggest the need for revisiting the stratigraphic nomenclature and the definition for these sandstone units. The overall recognized characteristics of these rocks give a strong suggestion of the possible significant role of tectonic fluctuations during the Cambro-Ordovician time, but this needs further work.