Master of Science - Research
School of Earth and Environmental Sciences
Manaa, Ammar, Late Pleistocene raised coral reefs in the eastern red sea – Rabigh, Saudi Arabia, Master of Science - Research thesis, School of Earth and Environmental Sciences, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3501
The Rabigh coast (Saudi Arabia) in the study area stretches for about 12 km between Al Kharrar Lagoon in the north and Sharm Rabigh in the south. Seven prominent Pleistocene coral reef sites were investigated with terrace heights ranging from 1 to 5 m above present sea level. In addition to field descriptions, 86 samples were collected from these seven sites to provide the data for this research. Of these seven sites, 4 of the sites were front reef, and 3 were back reef. In each of the front reef sites, there was a beach rock, upper and lower reef. The elevation of the upper and lower reef in the front reef sites ranges from 0.5 m to 3.20 m above present sea level. The two layers of beach rock and a back reef were identified in the study area. In the upper and lower reefs, corals were observed in almost all of the samples, with higher proportions for the upper than lower reef. Silicate minerals were rare in both lower and upper reef. The back-reef features much less coral compared to the lower and upper reef and algae was the dominant element in the beach rock. The upper reef can be part of the reef crest or the algal ridge in the reef system, such that erosion can occur at the front reef. The lower reef indicates an outer reef flat where this zone is a combination of the fore reef and lagoon environment with wave-breaking algal structures. The coral framework in the upper reef indicates a low energy environment during the formation of this reef. Within the back-reef calcareous mud was dominant, which indicates a low energy environment behind the reef crest, or a lagoon environment. Such an interpretation for the upper and lower reefs connects with transgression phases of the sea and represents slightly higher sea levels. The XRD results for the upper and lower reefs, and beach rock revealed variable percentages of aragonite followed by high-Mg calcite, and calcite, with a small increase in calcite and high-Mg calcite comparative to the lower reef. Calcite was the dominant mineral in the back reef area, with variable percentages of high-Mg calcite. The dominant diagenetic process in the Rabigh reefs was cementation. Fibrous calcite occurred in many upper and lower reef samples, and blocky calcite spar was the most common cement type in the back-reef area. Lower and upper reef were exposed to freshwater dissolution and cementation. There was also more cementation and diagenesis in the lower reef compared to the upper reef, and an equal distribution of calcite cement around most of the grains, with an average porosity of 14.8%, consistent with fresh water phreatic environment. The beach rock was suggestive of marine phreatic diagenesis. Amino Acid Racemisation (AAR) and 14C dating of bivalve shells from upper and lower reef were unsuccessful for deducing the age of these reefs. U/Th dating produced the most reliable results for the age of the reefs. The reefs were probably formed during the major highstand of isotope stage 5 where the age of the upper reef is more likely to be 122.8 ka (MIS 5e) whereas the lower reef could be MIS 7 with no evidence of major tectonics in Rabigh area during the last 125 ka. The contribution of this study is that it has produced a new coral reef model relevant to a low energy system in a dry and hot environment.