SEDIMENTOLOGIC VARIABILITY OF A HOLOCENE CARBONATE RAMP: YUCATAN, MEXICO

Although shallow, tropical, warm water, Holocene carbonate isolated platforms have been well studied (e.g., as in the Bahamas), modern carbonate ramps are poorly understood. Yet, many ancient stratigraphic successions represent deposition on ramp settings. To better understand the sedimentary dynamics and accumulations of ramp systems, this project tests the hypothesis that sediment size, sorting and type and organic matter type and abundance varies with geomorphic position on the Holocene ramp system on the northwestern Yucatan shelf in Mexico. Understanding the variability in sedimentologic and organic matter of ramp carbonates is important for constructing actualistic depositional models for heterogeneity within these systems and ancient analogs, including aspects of beach-ridge coquina reservoirs, and of nearshore fine-grained, organic-rich carbonate sediment.

Integrating remote sensing data with field observations of sediment size, type, texture, and sedimentary structures reveals variability across the range of nearshore geomorphic environments. The barrier island spit of Isla Arena consists of progradational coquina beach ridges that have prograded almost 3.5 km southward, but are less than 300 m across. The beach ridges are rich in 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 mud-lean packstone to grainstone. In the protected lagoon east of Isla Arena, water depths are less than 2 m across the lagoon. In contrast to the grainy sediment of the channels, most of the shallow subtidal-intertidal lagoon is mud-dominated, more muddy to the north, with abundant bivalves and gastropods, although foraminifera, barnacles, and oysters are locally abundant – the heterozoan association. Landward of the mangrove fringe, a palustrine coastal plain extends several km landward.

These patterns are interpreted to reflect effects of upwelling, energy, and geomorphology on the sizes and type of sediment (e.g., the heterozoan association) on a Holocene ramp.