GEOCHEMICAL CHARACTERISTICS OF THE KARST AQUIFER SYSTEM, YUCATáN PENINSULA, MEXICO (Invited Presentation)

The Yucatán Peninsula, comprising the Mexican states of Yucatán, Campeche, and Quintana Roo (QR), is capped over most of its extent by Upper Cretaceous to Holocene karstic limestone and dolomite. In southeast Campeche and southernmost QR, where poor exposure prevents detailed geologic mapping, an important surface lithology consists of bedded gypsum/argillite mantled by residual clay and/or by poorly consolidated ejecta from the terminal Cretaceous Chicxulub bolide impact. Information obtained from a few drill core samples and from ground and surface water geochemistry suggests that, throughout the Peninsula, development of landforms, including coastal features and large poljes, is influenced by a widespread Paleogene evaporite. Proposed geomorphic effects vary from 1) formation of poljes and ephemeral streams where evaporite beds occur above the water table and dissolve readily or 2) mimicking and/or enhancing carbonate karst landforms where evaporite beds that are present below the water table dissolve slowly in groundwater approaching saturation. In addition, gypsum-bearing karst may affect coastal processes. Whereas mixing with seawater of groundwater that drains carbonate rock typically produces a water that aggressively dissolves limestone (as in northern QR), groundwater draining gypsum-bearing evaporite contains a high concentration of Ca⁺². Its mixture with seawater is less likely to dissolve limestone and may even result in precipitation. Therefore, coastal erosion/deposition produced by modern groundwater draining the gypsum terrain of southern QR is likely to be different from erosion of the carbonate-drained coast of the north. This different geochemical effect was not present before the Paleocene.

Much of southern Campeche, characterized by extensive near-surface beds of gypsum, contains shallow lakes and ponds of low-salinity surface water separated from high-sulfate groundwater by the clay blanket that mantles the area. Local groundwater is essentially undrinkable, and present population density is low. In contrast, the area around the archeological site of Calakmul once supported a large Maya population that expanded and exploited surface bodies of water of good quality for their needs. [Population of the area declined sharply about 800 CE, perhaps because of drought.]