Quaternary Mixing-Zone Dolomite, Eastern Yucatan Peninsula: ABSTRACT

Abstract

Dolomite occurs in Pleistocene limestones a few meters below the water table on the eastern margin of the Yucatan Peninsula. It is present in five of seven cores (surface to depth of 11.5 m) which penetrate three stratigraphic units, the youngest of which is dated at 122,000 years B.P. Each unit consists of platform-margin reefs and back-reef facies, and there are no indications of evaporites or restricted conditions during deposition. The most extensive dolomitization (up to 48 wt. %) is in the only reef-facies core from the middle unit. This core is calcite and dolomite in contrast to three cores in the back-reef facies which are calcite and aragonite. Dolomite occurs in a variety of forms including: (1) coarsely and finely crystalline pore-lining cement, (2) finely crystalline replacement of matrix and bioclasts, and (3) internal sediment in dissolution cavities. Most of the dolomite cement is precipitated in molds of aragonitic fossils. There are three types of dolospar cement: (1) limpid euhedral to subhedral crystals (Ca57 Mg43 CO3100), (2) zoned crystals of dolomite and calcian dolomite (Ca63 Mg37 CO3100), and (3) corrugat d layers of alternating calcian dolomite and calcite. The calcian-dolomite layers in the zoned crystals and in the corrugated layers are partly dissolved. Typically, adjacent pores within the same sample contain different types of dolomite and dolomite-calcite intergrowths. The complex mineralogy of the middle unit is evidence that this limestone was subjected to several changes in phreatic-water geochemistry. Concurrent work by Hanshaw and Back demonstrated the existence of a geochemically active and fluctuating phreatic environment in the zone of freshwater and seawater mixing immediately inland from the Yucatan coastline today. Similar mixing zones must have passed through the Pleistocene limestones during past sea-level changes. The geologic setting, textures, and mineralogy of these young limestones suggest that their complex variety and occurrence of dolomite is best explained by mixing-zone diagenesis. End_of_Article - Last_Page 640------------