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Controls on Sedimentology and Organic Matter of a Coquina Beach Ridge: Fine-Grained Carbonate Lagoon System on a Holocene Carbonate Ramp: Yucatan Peninsula, Mexico


Many carbonate reservoirs represent deposition on ramp systems; yet modern carbonate ramps are poorly understood. To better understand the depositional patterns and processes of carbonate ramp systems, this project examines how sedimentology and organic matter (OM) composition varies with geomorphic position on the Holocene carbonate ramp system of the northwestern Yucatan Peninsula in Mexico. Integrating remote sensing data with field observations, grain size analysis, X-ray diffraction, and SEM analyses reveals variability across the range of nearshore geomorphic environments. In the shallow (< 2 m), protected, subtidal to intertidal, lagoon east of Isla Arena, with exception of the grainy channel that runs down the lagoon, the sediment texture is ‘mudstone’ to ‘wackestone.’ Dominant grains are bivalves and gastropods, but foraminifera, barnacles, and oysters are abundant as well. Preliminary gas chromatography and mass spectrometry results indicate a mix of terrestrial and marine OM in the OM-rich lagoon. The barrier island of Isla Arena consists of coquina beach ridges with ∼3.5 km longshore transport, but are less than 300 m across. Beach ridges are whole and fragmented bivalve and gastropod ‘grainstone.’ Just offshore (west, >300 m) of Isla Arena, the proportion of mud increases markedly in intertidal to shallow subtidal (< 1 m) seagrass flats, where sediment texture is ‘mudstone’ to ‘wackestone.’ Further offshore, as water depth gradually increases (up to ∼4 meters deep, 3 km offshore), the proportion of mud decreases, and the dominant textures are Halimeda mud-lean ‘packstone’ to ‘grainstone.’ Thus, the lagoon, beach ridges, and nearshore (< 300 m) are dominated by a heterozoan association. Sediment more than 3 km offshore is dominated almost exclusively by Halimeda bushes and grains, although gastropods, oysters, and bivalves are found in scattered seagrass beds. These patterns are interpreted to reflect effects of upwelling and energy on the sedimentology and the organic matter on a Holocene carbonate ramp. Components of this system may be a modern analogue for aspects of beach ridge coquina reservoirs in the “presalt” of offshore Brazil and Africa and of fine-grained, organic-rich, unconventional carbonate plays deposited in nearshore settings such as the Mississippi Lime. Similarly, understanding of nearshore sedimentology may be applicable to carbonate ramp reservoir analogs in general.