Depositional control on composition, texture and diagenesis of modern carbonate sediments: A comparative study of tidal channels and marshes, Abu Dhabi, United Arab Emirates
Abstract
Petrography, geochemistry, radiocarbon dating, and porewater chemistry of modern carbonate sediments in the\r\nmangrove marshes and tidal channels along the Abu Dhabi coast, United Arab Emirates, were conducted to compare\r\nsediment composition, texture and diagenesis between these two adjacent but distinct depositional environments.\r\nTidal currents and extensive micritization of the allochems in the tidal channels have led to the\r\nenrichment of skeletal fragments (average 25 %) and peloids (average 70 %). Ooids, however, are relatively scarce\r\n(average 1 %), which is attributed to strong tidal currents flushing them out of channels and depositing them on\r\nshoals and deltas. The severe environmental conditions in the tidal channels forced the microorganisms to bore\r\ninto allochems, promoting micritization via carbonate dissolution and reprecipitation of spheroidal microbial\r\nmicrite. Spheroidal micrite with the same mineral composition as the host skeletal fragments fills the microbial\r\nborings, indicating that micritization does not involve mineralogical alteration. Radiocarbon dating suggests that\r\nmicrobial boring is an important source of micrite, which is transferred into the marshes from channels by tidal\r\ncurrents. The oxidizing environment in marshes, due to the presence of mangrove pneumatophores and crab\r\nburrows, 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 porewater alkalinity and dissolved inorganic carbon (DIC) concentration, resulting in more abundant aragonite and high-Mg calcite cements. Stable carbon (+2.3‰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.
Author
Sadoon Morad
DOI
https://doi.org/10.1016/j.sedgeo.2024.106744
Publisher
Sedimentary Geology
ISSN
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