Abstract

Many ancient carbonate strata represent deposition on gently dipping ramp systems, yet processes and patterns of carbonate accumulations of modern carbonate ramps are not as extensively studied as those on modern isolated platforms and rimmed shelves. To better understand controls on sediment accumulations of the shallowest parts of tropical carbonate ramp systems, this study investigates geomorphology, sediment, and organic-matter abundance on the nearshore areas of the Holocene ramp system on the northwestern Yucatán Shelf, Mexico.

Currents generated by the day-to-day easterly trade winds, and larger waves caused by winter cold fronts, have the net effect of transporting sediment and upwelled nutrient-rich (occasionally in excess of 5 mg/m³, eutrophic to hypertrophic) and cooler (∼ 16–20°C) water southward. The upper shoreface includes shore-parallel belts of mollusk- and Halimeda-rich muddy sand to gravel. Longshore sediment transport along the coast forms barrier-island spits of molluscan coquina beach ridges which have prograded southward and westward. The islands in turn act as barriers to form protected lagoons that accumulate ubiquitous fine sediment. A broad supratidal to freshwater marsh landward of the lagoons includes dominantly fine-grained carbonate sediment. Across the area, mollusks are the primary sediment contributors, although Halimeda is locally ubiquitous offshore. Barnacles, benthic foraminifera, sponges, bryozoans, and serpulid worm tubes dominate the remainder of carbonate sediment producers, although echinoderms and planktonic forams are present as well. Corals are notably absent, as are peloids and ooids. Biosiliceous organisms (diatoms, sponge spicules, dinoflagellates) contribute up to 20% of sediment throughout the shoreface and lagoon, and the greater part of carbonate mud is calcitic. This assemblage, which includes many biota of the heterozoan association, is interpreted to be related to upwelled cool nutrient-rich water, although local freshwater springs may provide nutrients in the lagoons.

The results of this study illustrate the roles of energy level, sea-surface temperature, upwelled nutrient-rich waters, and possibly freshwater mixing on the geomorphology, biota, and sedimentology of the nearshore parts of this Holocene ramp system. The insights provide an actualistic model for sedimentological character and depositional heterogeneity of shoreface regions of ancient carbonate ramps influenced by upwelling.