Facies, fossil record, and age of pleistocene reefs from the Red Sea (Saudi Arabia)

Abstract

Facies patterns within the Pleistocene reef terraces along the Red Sea coast exhibit lateral changes over short distances. These changes reflect either transitions within the depositional environment or they are related to minor or major sea level fluctuations. On the basis of quantitative distributions of biota in the field as well as in thin section it is possible to establish and map these lateral patterns. Important biota are framebuilders and secondary reef encrusters (foraminifers, coralline algae). Frequency distributions of sessile foraminifera and scleractinians are strikingly similar to those of the recent environment within diagenetically unaltered terraces. The marine reef terraces occur in different elevated levels above the present sea level. Morphological steps are caused by onlap during different sea levels, by tectonics, or by erosion during transgression. Although several morphological steps exist which obscure the terrace stratigraphy, only three reef units can be distinguished. Each unit exhibits a lateral facies development, which begins at the shore, covering the whole lagoonal facies and ends at the upper reef slope. Besides this lateral facies pattern vertical patterns occur as well, showing a transgressive sequence in the youngest (lower) and oldest (upper) unit and a regressive one in the middle unit. In top quality outcrops, like wadi sections, it is possible to differentiate within the youngest reef unit between three onlaping reef cycles. Such cycles, however, can not be seen in the middle and oldest formations. The three reef cycles within the youngest unit and the three units as well, exhibit different degrees of diagenetic alterations, which are strongly reflected by a gradual reduction in the number of biota. This reduction may be best described as a process of “sieving”. Where these differences in diagenesis are recorded, they correspond to the age of the reef units. U/Th datations of the investigated terraces reveal an age for the youngest unit between 86,000 and 118,000 years B.P.. During this time three major sea level high stands have occurred, which explain the existence of the three reef cycles. The age of the middle formation is around 205,000 years, while the age for the oldest formation can only be assumed to fit in the time span between 290,000 and 340,000 years B.P.. All these data correspond to other published datations along the Red Sea coast.