Pleistocene coral reefs are well exposed over a width of about 3 km parallel to the Red Sea coast in Sudan. Patterns of fossil corals, sedimentary facies, diagenetic features and age of the reefs were studied in order to construct a sedimentation model. A total of 29 scleractinian coral species that belong to 8 families and 16 genera were identified. Three coral communities were distinguished: distal communities predominated by pipe-like packed Galaxea facicularis, middle communities predominated by branched Porites sp. and proximal communities predominated by massive and columnar Porites columnaris. Four major reef zones and ten sub-facies were recognized. The major reef zones recognized are (1) rock-reef rim, (2) reef-front zone, (3) reef-flat zone, and (4) back-reef zone. Coralgal-foraminifera bindstone dominates the rock-reef rim, while Porites/stony coral framestone and coral framestone are typical for the reef-front zone. The reef-flat succession primarily consists of whole fossil wackestone/floatstone, Porites bafflestone, large bivalve floatstone and Porites framestone. The back-reef zone facies includes lower lag rudstone, arborescent coral framestone and massive domal microreef. δ18O analysis was conducted to estimate the age of the Pleistocene coral reefs. Results obtained were correlated to previous age dating of correlative reefs in Sudan and other parts of the Red Sea. Estimation of reef ages was mainly based on δ18O values of the reef-front zone and the observed sedimentary succession of the reefs. Most ages obtained were considered not reliable due to the limited number of analysed samples and their high content of calcite. However, δ18O values of two Porites coral samples from the reef-front zone strongly suggest equivalent ages of 120 and 122 ky that correspond to marine isotope stage MIS 5.5. Based on δ18O values and field relationship to the reef-front zone, ages of the reef-flat and back-reef zones could be assigned to MIS 9 and MIS 7 respectively. MIS 5.1 is suggested for the reef-rock rim. The relationship of the reef zones to individual MIS might relate to the predominance of a specific zone during a certain stage, while other facies were less well developed and/or later eroded by wave action. The vertical architecture of the reef-flat facies (MIS 9) suggests a change from an initial quiet water lagoon to a shallow reef environment with moderate to high water energy. The back-reef sediments (MIS 7) are dominated by Galaxea facicularis and Caulastrea connate that grew in an environment largely protected from wave action. The reef-front zone (MIS 5.5) is formed by large reef knolls dominated by massive, columnar Porites corals. Its coral association and morphology suggests a moderate to high energy environment. The reef-rock rim (MIS 5.1) onlaps the bottom slope of the reef-front zone and formed during a minor sea-level rise. Its components and fabric indicate a high-energy environment. The sub-recent beach rock is formed by the reworking of reef debris from nearby reef-cliffs (reef-front zone) and reef-rock rim in the foreslope environment and subjected to cementation by marine aragonite cement. The reef-flat succession (MIS 9) was exposed to intense freshwater dissolution, replacement and cementation. It shows evidence of stagnant freshwater phreatic diagenesis in the lower and active freshwater phreatic diagenesis in the middle and upper part. This pattern likely resulted to a change from a semi-arid to a more humid palaeoclimate. Back-reef zone (MIS 7), reef-front zone (MIS 5.5), reef-rock rim (5.1) and sub-recent beach rock exhibit features typical of early diagenesis within the marine diagenetic environment. Features of freshwater vadose diagenesis characterize the upper part of most reef zones. The proposed sedimentation model differs, when compared to previous models in the Red Sea region, in the formation of the back-reef zone during MIS 7, while previous studies assigned it to MIS 9. High diversity and excellent preservation of corals in MIS 7 reefs reflect their growth in protected structural troughs landward of the reef-flat.
|Advisor:||Dominik, WilhelmBussert , Robert|
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Subjects:||Paleoecology, Sedimentary Geology|
|Keywords:||Fossil coral reefs|
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