THE ICAICHE FORMATION: UNACKNOWLEDGED CONTRIBUTOR TO YUCATÁN HYDROGEOLOGY AND GEOMORPHOLOGY

Massive gypsum beds (GB) of the P_ε-E_o Icaiche Formation are estimated to be 35m thick and to crop out over an area of >10,000km² in the southern Mexican Yucatán Peninsula (MYP). The GB influence geomorphology and groundwater geochemistry well beyond their area of outcrop, reacting with groundwater as follows: 1) East and west of the outcrop area, GB dissolve in groundwater that flows off a regional high, readily dissolving gypsum and causing collapse of overlying rock and formation of large, enclosed depressions (poljes). 2) In the north coast Pockmarked Terrain, underlain by fractures from the Chicxulub meteorite impact, deeply buried GB dissolve to produce a dense pattern of sinkholes (both aguadas and deep cenotes). 3) Where exposed to slow groundwater movement near the water table, dissolving GB produce irregular terrain with extensive brecciation and highly variable water quality.

We use SO₄, Cl, Sr and ⁸⁷Sr/⁸⁶Sr to evaluate the contribution of gypsum dissolution and seawater mixing to groundwater chemistry.

Whereas rocks of the northern MYP aquifer are highly permeable and support a regional fresh water lens above a saline intrusion, Paleogene rocks of the southern MYP are impermeable and lack a regional aquifer. We propose that argillaceous sediment released during weathering of the Icaiche Formation plus siliceous components of impact deposits of the southern MYP produced the low-permeability post-Eocene marine sedimentary rocks on the southeast margin of the Peninsula. However, we suggest that in the southeastern MYP a deep, previously unrecognized artesian aquifer is present in the Barton Creek Formation (which was karstified before the K-Pg impact). It is recharged in the GB outcrop area and discharges high-SO₄ groundwater to the NNE-SSW Rio Hondo Fault zone of southeastern MYP, delivering gypsum-saturated water to the Rio Hondo and to Lake Bacalar. It probably also discharges directly to the Caribbean.

Between 1000 BCE and 950 CE a large Maya population occupied the southern MYP, using ingenious systems to capture and store precipitation. They ultimately left, probably because of drought, leaving the region almost unoccupied for a millennium. Stimulated by tourism, a population wave is moving into this area of poor and uncertain water quality. A thorough regional hydrologic study is lacking but essential.