Paragenesis of a Pleistocene Carbonate Island in the African-Arabian Desert Belt (Al-Wajh Carbonate Platform Lagoon, NE Red Sea, Saudi-Arabia)

Quaternary carbonate islands have contributed significantly to the fundamental understanding of the interplay between climate and early diagenetic processes in carbonates. However, most of the studied islands, such as the carbonate islands on the Great Bahamas Bank, are situated in humid climate zones. Contrary to this, the Al-Wajh carbonate platform, situated within the arid African-Arabian desert belt on the NE Red Sea shelf (Saudi-Arabia), hosts a plethora of poorly studied carbonate islands. These islands were likely formed during the Last Interglacial (LIG) sea level highstand, commonly defined by Marine Isotope Stage 5e (MIS 5e: 124 – 119 ka). As such, these islands provide an excellent opportunity to give new insights into the paragenesis of carbonate islands within an arid climatic setting and an overall regressive/transgressive sequence.

This study investigates Shurayrah Island, located in the southern part of the Al-Wajh platform lagoon. Shaped by the prevailing NW wind direction, Shurayrah Island has an elongated shape, while a reef belt is established on the upwind NW side and carbonate sand spits accumulate on the leeward SE side. The main data base consisted of five drill cores with a total recovered length of 61 m and 150 thin sections. Eight lithofacies (LFT) and 17 microfacies types (MFT) were differentiated, including, amongst others, coral framestones, coral float- & rudstones and ooid-bioclast grainstones. Diagenetic analysis was based on a detailed petrographic investigation, while porosities (Φ) were measured from thin sections with digital image analysis (n = 150) and core plugs using a helium porosimeter (n = 102). 

Results reveal generally high porosities (mean Φ from thin sections = 29 %; mean Φ from core plugs = 45 %). Pore types are dominated by primary pores in the growth framework of coral framestones and secondary moldic & vuggy pores. Dissolution features are most pronounced in coral framestones, which show almost complete dissolution of original aragonite microstructures. Cement types include dog tooth, pore-filling and bladed cements, with a dominance of dog tooth cements in terms of frequency. Aragonite fibrous cements only occur scarcely and can be overgrown by dog tooth cements. Additionally, dog tooth and bladed cements are frequently observed to grow inside moldic pores.

The diagenetic analysis clearly reveals a dominance of porosity creating processes (dissolution) vs. porosity reducing processes (cementation) during paragenesis. In addition, results emphasize the importance of facies-controlled diagenesis: high primary porosities combined with metastable mineral composition of aragonite in coral framestones, result in a high meteoric diagenetic potential. Cement stratigraphy indicates a shift in the diagenetic realm, transitioning from marine (MIS 5e) to meteoric (MIS 5d – MIS 2) conditions, followed by a return to a marine setting with the Holocene Transgression (MIS 1). The overall strong meteoric diagenetic overprint suggests the influence of temporary humid phases (MIS 5c & a), during the overall >100 ka long subaerial exposure period. The observations highlight the significance of short-term climate fluctuations introducing meteoric waters for the diagenesis of carbonate islands in arid climate belts.