Abstract: The Upper Ordovician Channel and Basin System in Arabia

CONNALLY, THOMAS C., JR., MOUJAHED I. HUSSEINI

During the Late Ordovician a continental ice sheet covered nearly 60 degrees of arc over Gondwana. This glaciation was nearly two to three times greater in extent than the Pleistocene American glaciers. The resulting sea level drop placed Arabia in a glacio-fluviatile continental environment. The Gondwana ice sheet extended into south Jordan, northern and central Arabia as evident from extensive field work by many geologists. A peripheral swell, formed by the crustal loading of the ice sheet, separated the glaciers from the paleo Tethys. The height of the glacial forebulge was several hundred meters and it extended several 100 kilometers in front of the glaciers. Glacial floods cut an extensive system of broad channels across the swell which discharged meltwater into the paleo Tethys. The channels were cut into the older Cambrian-Ordovician clastics (Saq and Qassim formations). The channel systems can be mapped by seismic. Individual channels are up to 10 kilometers wide, 500 meters deep and extend many hundreds of kilometers. The depth of the channels relative to their banks is at least equal to the sea level drop plus the height of the peripheral swell. In central Arabia, south of Riyadh, the older rocks were completely eroded leaving the Precambrian basement exposed. At the close of the Ordovician when the glaciers retreated into Africa, the region where the peripheral swell had been eroded rebounded downward. This region now consisted of deep channels and, south of Riyadh, a basin formed where the swell had been completely eroded. The channels and basin were rapidly filled by Late Ordovician rocks which superficially resemble the older Saq and Qassim formations. Biostratigraphic studies and borehole imagery, however, prove that these rocks are younger. The infill rocks restored the Cambrian-Ordovician isopach to its preglacial configuration and the post-glacial Silurian transgressive shale was then deposited on a uniform surface.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria