GEOCHEMESTRY OF CALICHES OF NORTHEAST YUCATAN AS EVIDENCES OF SOIL DEVELOPMENT IN KARSTIC GEOSYSTEMS

Calcretes or caliches are studied typically as evidence of periods of subaerial exposition during the emersion of a carbonate platform. This approach however, needs deeper understanding of soil formation processes where the sedimentological proxies have limits. The interpretation of geochemical data (including major, trace and rare earth elements), together with C and O stable isotope composition are useful to explain the complex history of the caliche formation.

Caliches developed on Pleistocene-Holocene eolianites of northeast coast of Yucatan Peninsula were studied in quarries along the roadway of the Maya Riviera in Quintana Roo, Mexico. Both primary (eolianites and marine limestones) and secondary carbonates were analyzed in detail. The secondary carbonates include different lamina found in the caliche strata, as well as carbonates localized in the transition zone between soil and eolinites.

Stable isotope composition clearly reveal the origin of the carbonates. The sedimentary layers (eolinite and limestones) present positive values of d¹³C meanwhile the caliches are negative. The lowest value (-10‰) was found in the caliche lamina in contact to the soil. Major elements as Al2O3 and SiO2 were similar in all study materials except for the carbonates in the bottom part of the soil, which have more elevated percentages of alumina.

Rare Earth Elements (REE) pattern is very interesting. The highest values were found in the soils and the lowest in the eolianite. Intermediate values correspond to the caliches and recrystallization zone. This pattern evidences dissolution and reprecipitation processes.

The dominant processes proposed in the caliche formation is the dissolution-recrystallization of carbonates, meanwhile some stable elements like REE are lost in the neoformed carbonate phases, and later the same components are incorporated into the loose leached soil horizons above caliche. Thus caliches are related to soil development in the area. The micromorphology of the study materials demonstrate the loss of structure and recrystallization of the original eolianite in the calcretes that agrees with the geochemical and isotopic data.