Lithology, fauna and environmental conditions of the Late Pleistocene raised reefal limestone of the Jeddah coastal plain, Saudi Arabia

Abstract

The upper successions of the raised reefal limestone of Jeddah coastal plain, especially at Obhur area, were investigated through three temporary exposed geologic sections such as south and northeast Obhur and Al Salam Street. Uranium-series age datings of these successions indicated that their topmost parts were mostly deposited during the last interglacial marine isotope stage 5e (MIS 5e). Lithological–faunal analyses and field observations allowed the division of these successions into two distinct layers: white limestone (lower) and brown shell-rich calcareous clay (upper). The lower layer is enriched with in situ coral boulders of Porites sp. and faviids, as well as with the foraminifers Amphistegina sp., indicating deposition near fore reef and reef crest environments. The upper layer is enriched with molluscs, echinoids, branched (and dwarfed faviids) corals, and the foraminifers Elphidium striatopunctatum and Challengerella bradyi, indicating deposition in back reef, lagoonal environments. The boundary between these two layers is very sharp and irregular, suggesting that the transition from the fore reef to lagoonal conditions was very rapid and catastrophe on coral reef ecosystem due likely to sea-level changes accompanied with warming and increased water runoff. The upper surface of the shell-rich calcareous layer seems to be truncated, but its microfossils could indicate that it was deposited when the sea level was higher than the present mean sea level (MSL) by at least 7 m.