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Abstract

Recognition of features resulting from subaerial diagenesis of young carbonate rocks is important in defining times of subaerial exposure during the diagenetic history of ancient limestones. Meteoric- water cementation and other products of subaerial diagenesis of limestones can be characterized most unequivocally in carbonate units which were deposited in the subaerial environment and subsequently remained above marine waters. Such limestones were deposited as dune ridges along the northeast coast of the Yucatán Peninsula, Mexico. Late Pleistocene calcareous dunes accumulated along the seaward edge of a platform now 5 fm below sea level off the northeast coast of Quintana Roo, and Holocene eolianites are being deposited and lithified in the same area. Consolidated late Holocene dune rock on this coast provides a missing link in the study of progressive diagenesis from unconsolidated dune sand to lithified Pleistocene eolianite.

On the basis of grain composition and geomorphic relations, the Pleistocene eolianites can be divided into three different limestones, and the Holocene eolianites can be divided into two limestones. Each eolianite represents a different diagenetic stage. Youngest Holocene rocks have about the same composition as dune and beach sands: 75-85 percent aragonite, 15-20 percent Mg calcite, and less than 5 percent low-Mg calcite. Older ridges of the younger Holocene eolianite contain as much as 22 percent low-Mg calcite. The older Holocene eolianite contains 69-84 percent aragonite and less than 5 percent Mg calcite (this eolianite is low in Mg-calcite bioclasts).

The youngest Pleistocene eolianite originally contained as much as 45 percent Mg calcite, and there is high retention of Mg calcite in some beds. Several samples contain 20-32 percent Mg calcite (12-14 mole percent MgCO3); some samples have no Mg calcite. Aragonite ranges from 45 to 65 percent. The second youngest Pleistocene eolianite has 48-75 percent aragonite and less than 5 percent Mg calcite. The oldest eolianite contains 40-60 percent aragonite and less than 5 percent Mg calcite. Although there is a general trend toward calcitization of these dune rocks, each eolianite has a different route and rate of progressive diagenesis.

Cement types are important clues to the environment of early lithification of limestones. The Holocene eolianites contain grain-contact cement, microstalactitic druse, and large syntaxial overgrowths on echinoderm fragments. Finer grained layers are preferentially cemented. “Micrite envelopes” may form around grains in the vadose zone, and micro crystalline inclusions are common in sparry cement of the eolianites. Cement in the Holocene rocks is derived from a local source.

Much of the Pleistocene eolianite contains grain-skin cement in pores which contain “root-hair sheaths” and blocky spar in pores where the sheaths are absent. This fact suggests that early cementation was influenced by transpiration of dune plants. “Needle-fiber” cement is present in Pleistocene eolianites near ancient weathered surfaces. Pleistocene eolianites now immersed in the intertidal-subtidal environment are enriched in Mg calcite as a result of precipitation of Mg-calcite cement in the pores.

Laminated crusts and conglomerates containing “oolitic” and “pisolitic” bodies are developed on the weathered surfaces of the Pleistocene eolianites. Blackened “caliche-algal micrite” crusts were laid down around hypersaline lakes on Isla Mujeres.

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