Carbonate rocks of Eocene(?) to Recent age crop out in northern Yucatan. Subsurface data from a few widely scattered wells indicate these Tertiary and Recent rocks range in thickness from 3,000 to 5,000 feet. Cretaceous (Comanchean and Gulfian) carbonates, evaporites, and volcanics underlie the Tertiary carbonate rocks.

Samples from all outcropping stratigraphic units in northern Yucatan have been collected. These samples have been studied in the following ways: (1) in hand specimen and by etching, (2) by preparation of acetate peels, (3) by staining (silver nitrate-potassium chromate) for calcite-dolomite content and by preparation of stained peels, (4) in thin section, and (5) by "wet" chemical analysis. Samples are presently being analyzed for CaCO₃/MgCO₃ content by EDTA (ethylenediaminetetraacetic acid) titration.

Virtually all carbonate rock types are present, but deposits of foraminiferal microcrystalline carbonates predominate. Reefoid carbonates are present in only minor quantities. Dolomitization and silicification are encountered in many samples. Silicification is most intense in reefoid deposits and negligible in all other rock types. Preliminary results of analyses for calcite-dolomite content in the rocks disclose no apparent correlation of dolomitization with individual rock types. There is, however, an apparent increase in dolomite content with the age of the rocks; this aspect is being investigated more thoroughly.

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