Late Pleistocene gastropods from the raised reefal limestone of Jeddah,Saudi Arabia: taxonomic and palaeoenvironmental implications

A temporarily exposed 6-m succession of raised reefal limestone terrace south of Sharm Obhur (Jeddah) was investigated with regard to macrofossil gastropods and their palaeoenvironmental implications. The studied succession consists of two distinct layers. The lower layer consists of white limestone (4 m thick) of Late Pleistocene age, while the upper layer consists of shell-rich calcareous clay (2 m thick) and belongs to the last interglacial marine isotope stage 5e (MIS 5e). Its lithology was correlated with a nearby temporarily exposed 3-m succession, indicating that the deposition of the MIS 5e was not continuous, as indicated by the occurrence of a 0.4-m-thick weathered layer between the lower white limestone and upper calcareous clay. The studied 6-m succession yielded a very rich gastropod fauna with 90 species belonging to 4 subclasses, 5 orders, 16 superfamilies, 33 families and 64 genera. The clayey layer showed higher species richness (65 species) than the limestone layer (18 species), indicating a rapid change in the coral reef environment during the deposition of the interval of MIS 5e. It is assumed that, after the weathering interval, the sea level could not rebound to the same level as it was during the deposition of the white limestone, leading to deposition of calcareous clay in a shallow platform or embayment, allowing proliferation of gastropod species. Littorinimorpha and Neogastropoda species represent about 75% of the total recorded gastropod taxa, whereas Vetigastropoda and Caenogastropoda represent only 25% of the total recorded taxa. Superfamilies Cerithioidea and Stromboidea dominated the clay layer, whereas no particular species dominated the limestone layer.