Diagenesis of Recent Carbonate Deposits of Abu Dhabi: A Sedimentological and Geochemical Study

Abstract

The coastal environment in Abu Dhabi is a “natural laboratory” to study the diagenetic processes of modern carbonate deposits in a ramp setting. These deposits contain various allochems and diverse early-diagenetic processes, including mechanical compaction, cementation, dissolution, and micritization. This thesis focuses on the poorly understood micritization, which is widely spread in recent carbonates and in ancient carbonates. Using an integrated approach, the study aims to find the relationship between micritization and other early diagenetic processes. This approach is based on petrography (thin sections, cathodoluminescence, scanning electron microscopy), geochemistry (stable carbon and oxygen isotopes of pore water and carbonates), and radiometric dating (radiocarbon and uranium series dating). The findings of this research work also provide new insights into diagenetic evolution of ancient carbonate deposits.

Specifically, petrography, geochemistry, radiocarbon dating, and porewater chemistry of modern carbonate sediments in the mangrove marshes and tidal channels along the Abu Dhabi coast, United Arab Emirates, were conducted to compare sediment composition, texture, and diagenesis between these two adjacent but distinct depositional environments. Tidal currents and extensive micritization of the allochems in the tidal channels have led to the enrichment by skeletal fragments (average 25%) and peloids (average 70%). Ooids, however, are relatively scarce (average 1%), which is attributed to strong tidal currents flushing them out of channels and depositing them on shoals and deltas or they simply being completely micritized. The severe environmental conditions in the tidal channels lead to the enrichment of specific communities of microorganisms to bore into allochems, promoting micritization via carbonate dissolution and reprecipitation of spheroidal microbial micrite. Spheroidal micrite with the same mineral composition as the host skeletal fragments fills the microbial borings, indicating that micritization does not involve mineralogical alteration. Radiocarbon dating of micrite and allochems suggests that microbial boring is an important source of micrite, which was transferred into the marshes from channels by tidal currents. The oxidizing environment in marshes, due to the presence of mangrove pneumatophores and crab burrows, reduces the likelihood of anaerobic respiration. In contrast, microbial sulfate reduction and carbonate dissolution induced by microbial boring in tidal channels caused an increase in the concentration of dissolved inorganic carbon (DIC) and alkalinity of porewater, resulting in more abundant aragonite and high-Mg calcite cements. Stable carbon (+2.9‰ to +4.6‰) and oxygen (+0.8‰ to +1.5‰) isotopes of the allochems and micrite corroborate derivation of DIC from seawater. The formation of rare scattered rhombic dolomite as cement only in tidal channels is attributed to microbial metabolic processes. This study provides important insights into the characteristics and controlling factors of diagenesis in modern carbonate sediments, which can have wide implications for understanding the early diagenesis of ancient limestones. This understanding will have a tangible impact on predicting the distribution of micrite, degree of micritization and help in reservoir characterization studies to quantify both macro and microporosity distribution.

Date of Award	16 May 2025
Original language	American English
Supervisor	Mohammad Alsuwaidi (Supervisor)