THE USE OF THE TEXTURAL PARAMETERS OF SAND IN STUDYING THE CHARACTERISTICS AND DEPOSITIONAL PROCESSES OF COASTAL SEDIMENTS SOUTH OF JEDDAH, SAUDI-ARABIA

The subtidal sediments of four coastal environments (beach, swash zone, lagoon and nearshore bar) south of Jeddah have been studied in terms of their textural characteristics and depositional process. The skeletal grains are composed mainly of fragments of corals, benthic foraminifera, and different types of shells together with sand-sized rock fragments and quartz and feldspar grains. The skeletal material of the sediments has been derived from the mechanical breakdown of the adjacent reefs, while the small clastic portion was provided from the hinterland by rivers. Bimodal and polymodal grain-size distributions are common and can be attributed to heterogeneity in the types of skeletal debris. The size distribution of such sediments suggests that the various processes that are responsible for their transportation and deposition are wave, current and wind action. The different energy conditions and the morphology of each coastal environment seem to be reflected in the characteristics of the sediments in each environment.