SR ISOTOPES IN GROUNDWATER AS A STRATIGRAPHIC TRACER, YUCATAN, MEXICO

Despite nearly complete bedrock exposure beneath a thin-to-nonexistent soil cover, the stratigraphy of the northern Yucatan peninsula is poorly known because tropical weathering has caused extensive mineral dissolution and recrystallization, accompanied by loss of primary textures. Low relief and near-horizontal bedding result in selective and limited stratigraphic exposure. Knowledge of regional stratigraphy comes from oil exploration drilling and from exploration of the K/T impact crater in NW Yucatan. Peninsular drainage is dominated by subterranean flow through faults and fractures, karst permeability is high, and over much of the peninsula the water table is controlled by sea level. Several factors make it viable to use groundwater geochemistry to assist in determination of Yucatan stratigraphy; these include: a lithology dominated by marine chemical sediments, lack of aquitards, flat-lying beds continuous for long distances in the horizontal plane, and scarcity of terrigenous clastic rocks. Sedimentary rocks of the peninsula all contain significant quantities of strontium derived from strontium of the seawater or groundwater in which they formed. The isotopic signature of this Sr can be used as a stratigraphic tracer. A factor contributing to the quality of the Sr isotopic signal is the scarcity in Cenozoic rocks of the peninsula of terrigenous minerals that might contain ⁸⁷Rb. In SE Yucatan and S Quintana Roo, lake water from Chichancanab and Cenote Azul are at or near saturation with celestite and have ⁸⁷Sr/⁸⁶Sr ratios of 0.7077-0.7078 comparable to Albion island K/T breccia and outlining a broad region of groundwater with a Cretaceous-Eocene Sr isotope ratio, which by analogy with similar values in Albion Island K/T boundary breccia values, suggests a region underlain by Cretaceous evaporite or K/T boundary breccia. Deep cenotes Ucil and Xcolak in north central Yucatan tap Sr-SO4-rich water with an ⁸⁷Sr/⁸⁶Sr of 0.7083-0.7084 suggesting that the region of deep karst depressions in which they lie is underlain by an Oligocene-Miocene evaporite. Intense weathering may result in significant underexposure of evaporite in the Yucatan stratigraphic column.