GEOLOGY OF WADI QIDAYD, WESTERN SAUDI ARABIA

Wadi Qidayd forms a drainage basin with an area about 1,750 km² in Western Saudi Arabia located north of the City of Jeddah. The primary wadi channel lies between the western margin of Harrat Rabat which is a Tertiary basalt flow and the Precambrian Shield. Commonly, the Tertiary basalt lies on top of soft, pelitic Tertiary sediments, protecting them from erosion. The widest reaches of the wadi channel formed in areas where the basaltic deposit is thin or absent near the southern margin of the flow. Subaerial erosion lowered the land surface substantially below the base level of the basalt plain with incising into the soft, underlying sediments. In areas where there is remaining basalt, armored hills have formed above the Tertiary outwash deposits. In the upper reaches of the wadi, the primary eroded channel connects with canyons and pre-existing topographic features eroded into the Precambrian Shield. Many of the canyon locations and orientations correspond to northwest/southwest trending Cenozoic faults and parallel trending intrusive dikes. Therefore, the geologic development of the wadi and the feeding dendritic channels were based on the preexisting Tertiary lave flow position and thickness and the structural and erosional features within the Precambrian Shield.

Alluvial deposits within the wadi system range from less than 1 m in thickness in the upper basin to nearly 35 m in certain parts of the primary wadi channel in the lower middle part of the basin. Wadi deposits within the coastal plain near the point of discharge into tidal water are at about 25 m in thickness based on a recent test core at Wadi Wisami. The sediments are poorly sorted cobbles and sands with increasing clay content from the upper to the lower basin. The lower basin deposits are interbedded with continental outwash clays and marine sediments