Controls on morphology and sedimentology of carbonate tidal deltas, Abacos, Bahamas

Although the physical processes controlling siliciclastic shoal bodies are reasonably well-understood, the sedimentary processes that shape carbonate tidal shoal systems are poorly constrained. The tidal deltas of the northwestern Abaco Islands, Bahamas, provide a unique opportunity to explore the relationships between physical and sedimentary processes in a variety of sizes and shapes of tidal deltas. The length of tidal deltas range in size from ~ 350 m to 2 km (from the inlet to the termination of the delta lobe) and shoal crests consist primarily of well-sorted ooid sands (mean grain size between 400 and 500 μm; McManus sorting factor ≤ 0.70). Coated grains are rare off the shoals, and sorting decreases with distance from the shoal crests. Velocities in the rocky inlets can exceed 2 m/s in the daily tidal cycles, and the systematic distribution of grain types reflects tidal velocity variations. Areas with the highest velocities (≥ 0.80 m/s) correspond to hard, rocky bottoms with some skeletal grains. These regions of elevated velocities are focused within the inlets, where jets of high velocity currents form during each tidal cycle. These current jets extend outward from the inlet throat, and velocities decrease outwards as the flow disperses. Velocities between 40 and 80 cm/s occur in the regions with abundant ooids, and peloids are most abundant in the lowest velocity regions (< 0.40 m/s) in the tidal delta system. At a larger scale, inlet tidal prism and delta size are closely correlated in these tidal deltas, as in their siliciclastic counterparts.The results of this study illustrate a clear relationship between quotidian physical processes and their geomorphic and sedimentologic products in a carbonate system. In this context, carbonates appear to share many geomorphic similarities with siliciclastic analogs. The results represent an important step beyond qualitative facies interpretations and towards quantitative, specified environmental and sedimentologic characterizations for carbonate systems.