ABSTRACT

Distribution of recent sedimentary facies is significantly different in four closely spaced, isolated carbonate platforms off the coasts of Belize and Yucatan (SE Mexico). These differences are largely controlled by variations in submarine topography, which are likely the result of differential subsidence and karstification from platform to platform. A variety of depositional styles and patterns is found in platform interiors that developed during the same Holocene sea-level rise. Composition and texture of surface sediment samples were used to define four major sediment types and to distinguish nine depositional environments within the platforms. Facies maps are based on analysis of 454 sediment samples and high-resolution satellite imagery.

Glovers Reef is characterized by a circular facies distribution. The nearly continuous reef margin is composed of a coral-red coralline algae-Halimeda grainstone. Mixed nonskeletal (peloidal)/skeletal wackestones and packstones are present in shallow lagoon parts (< 5 m water depth) behind the margin. The deep interior, from 5 to 18 m water depth, is characterized by mollusk-foram-Halimeda wackestones with over 860 patch reefs of coral-red algae-Halimeda-mollusk packstones. Lighthouse Reef also has a coral grainstone and coral-red algae-Halimeda grainstone rim, but the facies distribution in the interior is strongly asymmetrical. The 8-m-deep eastern lagoon is floored by mollusk-foram-Halimeda wackestones and packstones; the 3-m-deep western lagoon has the mixed peloidal/skeletal wackestone to grainstone facies. A linear trend of coalescing patch reefs and skeletal grainstone facies separates the two lagoons. Banco Chinchorro is similar to Lighthouse Reef, inasmuch as facies distribution patterns of the interior are strongly asymmetrical. Mollusk-foram-Halimeda wackestones-grainstones are present in eastern lagoon areas, whereas shallow (< 5 m) western parts of the lagoon are composed of mixed peloidal/skeletal grainstones, packstones, and wackestones. Most of Turneffe Islands is protected on the east by Lighthouse Reef. Accordingly, Turneffe has a narrower reef and skeletal grainstone rim than the other two platforms. Restricted interior lagoons are up to 8 m deep, and are enclosed by wide mangrove rims and surface sediments dominated by Halimeda-rich wackestone rich in organic matter. Coral patch reefs are absent. In contrast, the unprotected northernmost part of Turneffe has a wide reef and grainstone rim and the open lagoon area consists of mollusk-foram-Halimeda wackestones-packstones with abundant patch reefs.