Quaternary Loessic Alluvial Silts of the Arabian Shield
Peter Vincent and Fayek H. Kattan with an appendix on heavy-mineral analyses by Alim A. Siddiqi
Although many wadi basins on the shield contain extensive deposits of alluvial silt little attention has been paid to these deposits. They were noted both by USGS geologists and members of an Austrian team working on the Quaternary geology of western Saudi Arabia and identified as loess, but their origin was not discussed. Loessic alluvial silts are also known as river end deposits denoting environments where low discharges do not flush sediments from the catchment. The silts, which are up to 4m thick in some basins, are light brown and contain few sedimentary structures other than thin laminae. Where eroded the silts form vertical cliffs in places with columnar structures typical of airborne loess. They often overlie fluvial gravel, the two units usually being separated by a sharp junction. As compared with northern hemisphere, windblown, glacially derived, loess, the silts on the shield are fine, and have a mean particle size ranging from 20 – 30µ. The particle size is controlled not mainly by process but by the nature of the material from which the silts were derived. In almost all cases, the silt clay mineralogy is dominated by kaolinite formed in association with tropical environments and deep weathering. Heavy mineral analyses also indicate the present of rock fragments that are presumably locally derived. These various lines of evidence indicate that the silt accumulations are the result of fluvial erosion and stripping of Tertiary, and possibly Neoproterozoic, saprolites and laterites, particularly as a result of the dissection of protective harrats. Optimally Stimulated Luminescence (OSL) dating of the silts generally ranges from 2000 BP to 98,000 BP indicating several phases of catchment desiccation. It is concluded that the majority of the silts are secondary and have been derived from deep weathering deposits. Their accumulation in wadi basins is a result of climatic changes and particularly the abrupt transition from fluvial conditions in wet phases of the Quaternary to low-energy conditions as arid environments took hold.
Vincent, P., and Kattan, F.H., 2007, Quaternary loessic alluvial silts of the Aabian Shield: Saudi Geological Survey Technical Report SGS-TR-2007-3, 28 p., 10 tables, 16 figs, 1 apps