Diagenesis of Calcareous Dune Rocks, Northeastern Yucatan Peninsula, Mexico: ABSTRACT

Abstract

Calcareous eolianites, of late Pleistocene and Holocene age, have accumulated along the northeastern coast of Yucatan. Consolidated Holocene dune rock on this coast provides a missing link in the study of progressive diagenesis from modern dune sand to Pleistocene eolianite. The Pleistocene dune rocks can be divided into 3 different limestones, and the Holocene eolianites, into 2 limestones. Each eolianite represents a separate diagenetic stage. Youngest Holocene rocks have the same composition as dune and beach sands: 75-85% aragonite, 15-20% Mg calcite, and less than 5% low-Mg calcite. Older ridges of the younger Holocene eolianite contain up to 22% low-Mg calcite. The older Holocene eolianite has 69-84% aragonite and less than 5% Mg calcite (composition is low in Mg-calcite bioclasts). The youngest Pleistocene eolianite originally contained as much as 45% Mg calcite, and there is high retention of Mg calcite in some beds. Several samples have 20-32% Mg calcite (12-14 mol % MgCO3); some samples have no Mg calcite. Aragonite ranges from 45-65%. The second youngest Pleistocene eolianite has 48-75% aragonite and less than 5% Mg calcite. The oldest eolianite contains 40-60% aragonite and less than 5% Mg calcite. Each eolianite has a different sequence and rate of progressive diagenesis toward calcitization. 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 microcrystalline inclusions are common in sparry cement of the eolianites. Much of the Pleistocene eolianite has grain-skin cement in pores which contain root-hair sheaths and blocky spar in pores where they are absent. This suggests that early cementation was influenced by transpiration of dune plants. Needle-fiber cement is present in Pleistocene eolianites near ancient weathered surfaces. The Pleistocene eolianites contain rhizocretions and root-hair sheaths, which are absent in the Holocene eolianites. 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. End_of_Article - Last_Page 368------------