Carbonate sediment dynamics in the Abu Dhabi lagoon - implications for low-angle inner-to-middle ramp models

Abstract

Carbonate ramps are prominent features of continental margins and intrashelf basins throughout much of Earth's history. Among the limited number of recent analogues of ancient carbonate ramps, the Persian/Arabian Gulf stands out due to its morphologically complex array of open coastal to sabkha environments. This study presents detailed field and laboratory data on spatial facies variability linked to different hydrodynamic regimes spanning from open coastal to inner lagoon settings in Abu Dhabi. Sediment transport and deposition are determined by waves and tidal currents, with spatially and temporally complex variations in the hydrodynamic regime, resulting in a complex facies mosaic. The open coastal domain is subject to persistent north-westerly trade (Shamal) winds and episodic winter storms, generating waves that affect the deposition of ooids, peloids, lithic clasts and bioclasts. In the outer lagoon, these waves interact with tidal currents and thus become less dominant towards the middle lagoon. Tidal currents are more important in the restricted inner lagoon. The outer lagoonal sediments primarily consist of ooids, bioclasts and lithic clasts and are peloid- and micrite-lean. Bimodal tidal currents, locally with velocities of two or more metres per second, are present in the relatively open regime of the middle lagoon and accumulate bar complexes dominated by skeletal carbonates. Seagrass and biofilms locally stabilise muddy carbonates and limit sediment entrainment even where significant tidal currents exist. In a landward direction, and with decreasing hydrodynamic levels, carbonate mud (micrite) is increasingly abundant, reflecting the lower hydrodynamic conditions in the restricted regime of the mid-and inner lagoon. Data shown here provide a facies distribution model in the shallow coastal portions of what might be one of the few modern analogues of a ‘marginal epeiric’ sea, shedding light on the complex interplay between coastal morphology, wave and tide energy and biological processes. This study discusses how hydrodynamics and seafloor bio-stabilisation influence the spatial carbonate facies of this highly dynamic ramp system by comparing data shown here with data from the Kuwait and Doha coasts, which have different orientations to the prevailing winds. The quantitative and refined composite inner ramp model shown here has wider significance for coastal dynamics and carbonate stratigraphy.