Coastal Holocene carbonates of Abu Dhabi, UAE: depositional setting, sediment distribution, and role of cyanobacteria in micritization

Abstract

The low-latitude coastal sediments of the United Arab Emirates accumulate adjacent to the gently sloping ramp-like bathymetry of the southern Arabian Gulf. Here, an extensive arid coastline system is expressed by a variety of different shallow carbonate and evaporite depositional settings. Off the western Abu Dhabi mainland coast, flanking the north and seaward side of the Khor Al Bazam lagoon, is an offshore bank. This bank is progressively accumulating carbonate sand and mud, extending an area of shoals and channels, coral banks, tidal deltas, and nearshore coastal terraces that prograde outwards from the offshore bank. South of the bank, where wave energy is minimal, cyanobacterial mats colonize protected intertidal sediments, building these seaward, and binding sediment washed onto them.Ooids, grapestones, pellets, lime mud and bioclastic grains accumulate along the western Abu Dhabi coast. The distribution and characteristics of individual grains types is controlled largely by their hydrodynamic setting, with interactions between wave and current action, sediment mobility, burial rate and cementation. Aragonite mud is confined mainly to the sheltered areas; the source of lime mud is the abundant blooms or clouds of "whiting" formed by carbonate precipitation directly from seawater. Soft and cemented aragonitic pellets accumulate in sheltered lagoons and shallow shelves with moderate exposure to wind and waves. High salinities in evaporative water bodies likely promote cementation in lagoons and ooid formation on tidal deltas. The surfaces of many carbonate grains are altered by cyanobacteria. This micritization forms envelopes enclosing grains that matches those found in ancient carbonates, and represents a useful tool that can be used as evidence of lower rates of accumulation and longer periods of exposure on the shallow seafloor.

Upper Triassic Carbonate Records: Insights from the Most Complete Panthalassan Platform (Lime Peak, Yukon, Canada)

“…Despite the lack of deposited mud, micritization was an active process in MCF 5. Micritization in marine environments is a well-known early diagenetic phenomenon that is caused either by abiotic [114] or biotic [115][116][117] processes. Part of these processes (e.g., biotically driven micritization by microboring by cyanobacteria) are particularly effective in case of low sedimentation rates (longer times at the sediment-water interface and thus longer exposure to light).…”

Acta Geologica Polonica

“…Recent whitings are observed mostly in shallow-littoral top-bank water masses (Florida, Bahama), where the resuspension mechanism may be important or dominant. However, in the case of the Persian Gulf, whitings occur in the deeper ramp setting (Wells and Illing 1964) and are related to combined enhanced carbonate saturation (due to evaporation) and massive carbon dioxide consumption by phytoplankton during algal (diatom) blooms (Wells and Illing 1964), which in turn may be induced by iron-bearing eolian dust (see Kendall and Alsharhan 2011;Nezlin et al 2010). Such a case shows that the tropical, eolian fertilized, mediterranean (foreland basin) seas, with strong evaporation and high carbonate saturation are favourable for bio-induced carbonate suspensoid formation in the water column.…”