The Icaiche Formation: Major contributor to the stratigraphy, hydrogeochemistry and geomorphology of the northern Yucatan Peninsula, Mexico

The Paleogene-Eocene Icaiche Formation, which contains bedded gypsum deposits that cover an estimated minimum area of 10000 km(2), is located in the southern parts of the Mexican states Yucatan. Campeche and Quintana Roo. The formation has been little studied because it crops out in an area with limited access, few people, and little economic activity Low population density is a consequence of the sulfate-contaminated water that is produced during weathering and dissolution of the gypsum deposits ill the formation. The Icaiche. Formation occupies a considerable part of the physiographic area known as the Elevated Interior Region (EIR) of the Yucatan Peninsula, which encompasses a terrain that the Maya occupied with varying success for almost two millennia and finally almost completely abandoned by about 950 CE. During their tenure, the. Maya survived and at times prospered in this region of unpalatable groundwater by using and adapting natural depressions (called bayous) for water supply and, somewhat later, by using abundant impermeable local days in construction of dams and ingenious systems for water storage and transport. Because of the relatively high solubility of gypsum, the. Icaiche Formation has had all outsize influence on hydrologic and geomorphic features and groundwater geochemistry' beyond where it crops out and into parts of the Yucatan Peninsula where it is present in the subsurface, particularly those places where it encounters flowing groundwater: In this study we propose that gypsum dissolution, followed by collapse of overlying rock, was important in: 1) the formation of paljes (large, partly or completely enclosed karst basins with flat floors and interior drainage) along the east and west margins of the EIR: 2) the development of deep karst in north central Yucatan state; and 3) the formation of irregular terrains, extensive breeciation of rock, and highly varying water quality in the area at the eastern margin of the EIR (between Lake Chidnancanab and Felipe Carrillo Puerto. Quintana Roo). Argillaceous sediment released during weathering of the Icaiche Formation, perhaps mixed with siliceous components of impact deposits from the Chicxulub Impact may have affected the permeability of post-Eocene marine sedimentary macs, perhaps including local fades of the Carrillo Puerto Formation, formed on the southeast margin of the Peninsula. The resulting rocks have markedly lower permeability than rocks of the more northerly lowlands of the Mexican portion of the Yucatan Peninsula that host a pervasive fresh water lens overlying a saline intrusion.