Year

2016

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

Late Pleistocene raised coral reef terraces form extensive outcrops up to 5 km wide along the Saudi coast. Six major coral reef sites, with reef heights ranging from 1 to 25 m above present sea level, were investigated covering about 1180 km from Haql in north to Al- Qunfidha in south. In addition to field descriptions, 281 samples were collected from these sites to provide the data for this research. Of these six sites, 5 of the sites show coastal successions, 3 show inland successions and 2 contain coastal beach rock. In the coastal successions, the terrace can be divided into upper and lower part in some locations whereas in other areas it appears as a single unit. The subdivision into lower and upper parts is really a diagenetic overprint on a single reef succession in exposed coastal locations. This lower part shows strong cementation up to 1 m above present sea level which occurred during the Holocene high stand sea level and extends into these reefs but does not appear 100-160 m from the exposed coastal portion of the reef. The height of the aragonite cementation gives an indication that the Holocene high stand sea level in this area was about 1-1.4 m above present sea level. The coral coverage and variety are lower in some locations compared to other locations within the same exposed coral terraces which is related to the ancient valleys in these areas. Inland successions only contain a few corals that are often transported. Most of the coral genera that occur in the exposed coral terraces along the Saudi Red Sea coast are currently living in shallow water (0-25 m) environments in the modern Red Sea coral reef system or extend down to shallow slope depths. The other fauna within these reefs relates to a very shallow marine embayment or lagoonal environment with sea level not very far above a current elevation of 5.5 m. The coastal reef succession represents part of a central to back reef facies rather than a forereef succession where the reefs grew up in a quieter back-barrier area behind the reef front. The reef could represent a very broad platform where the back reef area signifies the inner part of it. The reef front must have been farther seaward and the sequence could represent the stabilisation phase before the reef really became established. The inland succession exposed at Rabigh and Al-Ruwais is about 3-4 km inland from the present shoreline and mainly consists of poorly cemented coral rubble filled with sediment, shell fragments and some intact coral reef. The composition and the height of these back reef terraces indicate it represents a rubble bank on the landward side of the reef platform area where corals may have been reworked from more seaward parts of the upper reef. Porites coral and Tridacna shells were dated using U/Th while clastic sediment from Jeddah was dated using thermoluminescence. The pooled mean age for the coral samples is 121.5±0.2 ka suggesting MIS 5e, even for the uplifted 16-20 m high terrace in the north at Haql. In Jeddah the MIS 5e back-reef succession is overlain by fluvial sediment that gave a TL age of 66±13 ka. The structure and faunal composition of the terraces suggests that they accumulated in broad shallow embayments during the last interglacial maximum transgression. The consistent elevation of these terraces suggest that the central and southern Saudi coast has been tectonically stable for at least the past 125,000 years and the coral reef terraces (at 3-5.5 m elevation) are related to a MIS 5e sea level high stand at least 6 m above present sea level. The Saudi coast north of Duba shows progressive uplift to 16-25 m near Haql since 108 ka as a result of ongoing transform faulting in the Gulf of Aqaba.

This thesis is unavailable until Thursday, October 18, 2018

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