Geochemical Investigation of Palygorskite-Rich Units in the Central Yucatan

Palygorskite-rich units in the Tertiary carbonate rocks of the Ticul region of the central Yucatan represent unusual and complex sedimentary environments. Although basic chemical precipitation models can explain the existence of these units, a detailed understanding of the geologic processes involved in generating these deposits is lacking. A geochemical evaluation of whole rock analyses of 33 samples previously investigated for archeological sourcing studies was conducted. To our knowledge these are the only bulk chemical data (including rare earths and trace metals) on these palygorskite-rich clays. Our analysis indicates that there is a significant volcanic component in many but not all units.

For 15 samples there is a well defined enrichment of LREE, a depletion of HREE and moderate to pronounced Eu anomaly consistent with a felsic to intermediate igneous source. Similar REE patterns are commonly interpreted as indicative of explosive volcanism in many altered volcanic ash layers throughout the Phanerozoic. The relatively low concentration of V, Ti, Sc, Cr, Co and Ni and the high Th/U and the high Th/Ta ratio also suggests that the tectonic source of this material is subduction-related. We suggest that the origin of the ash in these units is tephra fallout from the arc volcanism that dominated the Caribbean region during the Tertiary. Trace amounts of euhedral zircon, apatite and biotite in selected samples corroborate this interpretation.

Some of these palygorskite-rich units have low and irregular concentrations of REE, evident as chaotic distribution patterns. These units are interpreted as end member sedimentary chemical precipitation and may be discriminated from other units containing volcanic material.

Chemical fingerprinting using bulk chemical compositions and chemical compositions of phenocrysts may be possible. The potential exists for using palygorskite units for high resolution correlation in the region.