Modern Marine Ooids Generated by Unidirectional Currents

Ooids represent important paleoclimate, paleoceanographic, and paleoenvironmental indicators. Although most modern marine ooids occur in tidal shoals or less frequently in wave-dominated shorefaces, a distinct oolitic system occurs on the eastern side of the Yucatan Peninsula, where the strong northward Yucatan Current impinges on the N–S-oriented Mujeres Shelf. This shelf extends for 40 km alongshore, is up to 10 km across, and is separated from the Caribbean Sea to the east by an island and a discontinuous ridge of Pleistocene strata. Rocky bottom (bedrock) floors the deepest areas at ~8–9 m water depth. Currents are uniformly northward and strong, up to 60 cm/s, and transport Halimeda, mollusks, peloids, and fine-medium sand-sized ooids. Although seagrass-covered bottom occurs near the mainland shore and island, the central part of the shelf is bare sand bottom with a hierarchy of bedforms. Current ripples are superimposed on subaqueous dunes of 0.2–1.0 m relief, and some interact to form composite dunes with 5 m slipfaces. Bedforms are generally normal to the current, although longitudinal forms also occur. These dynamics contrast with the classic sites of Holocene ooids and may represent an analog useful to understand the origin and heterogeneity of ancient oolitic systems, especially those characterized by large-scale unidirectional cross-bedding, such as the Bathonian oolites in the Paris Basin.