Karst geomorphology and hydrogeology of the northeastern Yucatan Peninsula, Mexico
Tulaczyk, Slawomir M.
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A regional tectonic feature extending in a 100 km long and 30 to 40 km wide belt, the Holbox Fracture Zone, crosses the flat, karstified terrain of the NE Yucatan Peninsula controlling development of large, elongated poljes. The corridor poljes evolve from solution corridors whose floors reached the local water table. That inhibits further deepening of these forms and controls their lateral expansion to produce the typical shape of the polje with flat floors and steep walls. Polje floors reached the level of the water table by a process that combined solution deepening with Holocene rise of sea level and consequent rise of the water table. A few of the deeper solution corridors were submerged providing elongated lake basins. Continuous, pseudo-valley poljes that intersect the northern coast support periodic and/or episodic water flow. It is unlikely that the Holbox Fracture Zone, originating probably from the tectonic movements that took place in the Eocene, influenced directly the structure of surficial carbonate layers of probable Mio-Pliocene age. Therefore, I propose a simple hydrogeological model that attempts to explain how buried structures, that were previously karstified, can replicate themselves into overlying, younger layers and induce preferential development of karst forms within these rocks. The hydrological system of the area is affected not only by surficial flows but also by net loss of groundwater through evapotranspiration from the poljes and enhanced discharge of groundwater on the coast where a few permanent streams have developed. Hydraulic gradients measured on the coast west from the fracture zone reached values unusual for the northern Yucatan Peninsula, 1 to 2 m/km. This can be attributed to groundwater discharge in the seepage zone that borders the coastal aquitard layer. Despite groundwater losses, the freshwater lens thickness near the center of the Holbox Fracture Zone, estimated from the Ghyben-Herzberg principle, equals about 120 m. However, vertical electrical soundings performed in the study area did not detect the freshwater-saltwater interface, so that the evidence for the presence of the saltwater intrusion underneath this part of northeastern Yucatan remains largely circumstantial.