Geology of Yucatan

The geologic history of the Yucatan Platform is difficult to establish due to the paucity of deep well data, exposed anatomic strata, and relative inaccessibility of the inland terrain. In a quest for potential oil reserves in the early 1970's, PEMEX (Petroleos Mexicanos) drilled 10 exploratory wells into this platform. Most of these wells were drilled in the state of Yucatan, often in excess of –2000 m. Data from the test wells summarize the stratigraphy of the Mexico Isthmian portion of the Yucatan Peninsula as that of a great limestone platform (Ward and Weidie, 1978; Weidie, 1985). Paleozoic metasediments form the basement strata at varying depths exceeding -2400 m. Early Mesozoic redbed sediments (the Todos Santos Formation) overlie these Paleozoic strata unconformably, maintaining an average thickness of over 100 m.

The Cretaceous period launched the onset of numerous marine transgressions that would submerge much of this isthmus under warm shallow seas until the Pleistocene. During the Cretaceous over 1300 m of limestone strata were formed, while during the Tertiary period another 1000 m of essentially pure carbonate sediments was deposited. This includes the uppermost (Miocene-Pliocene) Carrillo Puerto Formation, a 15 m thick deposit that surrounds the shallow portions of many cave systems.

Uplift in the south-central area of the isthmus began in the Oligocene, encouraging the Carrillo Puerto Formation to be deposited in stages over the ever-expanding margins of the peninsula. This slow uplift continued until the Pleistocene. Most Quaternary deposits on the isthmus are restricted to the present margins of the peninsula. These deposits are typically thin, extending 1-3 km inland from the Caribbean. Most Pleistocene deposits are the result of marine transgressions over Eastern Quintana Roo during the Illinois and Wisconsin Ice ages. Unconsolidated Holocene deposits are restricted to the present shoreline. Presently, the study area is a low relief karst plain (elevation of 3-16 m) which sustains a shallow water table.

Although the geologic history of the study area is incomplete, it has parented a relatively stable karst platform that is predisposed to the dynamics of speleogenesis. The study area receives over 150 cm of rainfall per year (Ward and Weidie, 1978; Ward, 1985). Atmospheric carbon dioxide lends a slight acidity to this precipitation. With ground contact the precipitation absorbs tannic acids from the upper calichified zone of the Tertiary strata. Both the caliche and parent strata are very porous, favoring rapid percolation towards a shallow freshwater table. Steering a course to the Caribbean through bedding fractures, this acidified water has dissolved conduits (caves) in the parent limestone since its initial uplift from the ocean.

Sea level fluctuations during the Sangamon Interglacial through the Wisconsin post-glacial periods expedited numerous developments within the cave systems. Receding sea levels during active glaciation resulted in a considerable drop in the water table and thus, water in the drainage channels. Unsupported cave ceilings immediate to the surface collapsed creating new karst windows at odd intervals along the course of the conduits. Streambed erosion gradually broadened the existing caves while eroding new passages into deeper limestone beds (currently explored to -100 m). About 18,000 years ago sea level began a slow rise from 125 m below its present level as the Wisconsin glaciation period declined (Ward and Weidie, 1978; Coke and Perry, 1991). As the freshwater table was elevated by a rising saltwater intrusion, deeper conduits were rapidly immersed by this intrusion. Mid-level passages allowed both layers of rising water to interact freely within the fresh vacuities. Upper level passages were eventually flooded by the freshwater aquifer. Ultimately submerged, most of the cave systems came to possess both fresh and saltwater layers that sanctioned new speleological attributes for the future of these anchialine caves.

Holthuis (1973) coined the term "anchialine" to define specific attributes of inland, seemingly isolated aqueous pools. Many cave systems in eastern Quintana Roo contain anchialine pools. They contain salt or brackish waters that fluctuate with oceanic tides, although they lack a surface connection to the ocean. Anchialine caves in the study area contain an upper freshwater layer that flows over a near-static saltwater intrusion. A mixing zone occurs (enlivened by water current and passage configuration) at a distinct density interface between the fresh and salt layers (the halocline). The depth of the halocline and thickness of the freshwater layer increases with distance from the ocean, typically resulting in decreased freshwater chlorides. Periods of sustained precipitation and the pervasiveness of exposed mixing zones in cave system passages may cause random fluctuations in the presence of freshwater chlorides. As a general rule, freshwater chlorides remain nearly constant (~ 2 ppt) until cave passages reach the depth of the mixing zone. Salinity levels below the mixing zone often rise abruptly to 14 to 35 ppt (Iliffe, 1992) depending on sampled depth and distance inland.

Investigations have concluded that preferential dissolution of limestone at the mixing zone (Back et al., 1986) allied with native fracture zone speleogenesis has produced an arena for vast growth and passage complexity within these cave systems. With the addition of cenotes along the systems’ course, chemical and organic nutrients are able to enter the caves at various points to further enrich this environment.

Modern cave diving techniques have allowed explorers to study these underwater conduits many kilometers distant from their entrances. These explorations have discovered a number of the longest underwater cave systems in the world. The use of cave diving as a research tool has been applied to many scientific disciplines. Although many hydrological and archaeological discoveries have been made, perhaps the most numerous advancements have been made in the field of Biospeleology.

References

Back, W., B. B. Henshaw, J. S. Herman, and J. N. Van Driel. 1986. Differential dissolution of a Pleistocene reef in the ground-water mixing zone of coastal Yucatan, Mexico. Geology 14:137-140.

Coke, J. G. IV, E.C. Perry, and A. Long. 1991. Sea level curve. Nature 353:25.

Holthuis, L. B. 1973. Caridean shrimps found in land-locked saltwater pools at four Indo-West Pacific localities (Sinai Peninsula, Funafuti Atoll, Maui and Hawaii Islands), with the description of one new genus and four new species. Zoologische Verhandelingen 128:1-48.

Iliffe, T. M. 1992. An annotated list of the troglobitic anchialine and freshwater fauna of Quintana Roo. In D. Navarro and E. Suarez, eds. Diversidad Biologica en la Reserva de la Biosfera de Sian Ka’an Quintana Roo, Mexico. Commision Nacional para la Biodiversidad y CIGRO, Mexico. 2:197-215.

Ward, W. C., and A. E. Weidie, eds. 1978. Geology and hydrogeology of northeastern Yucatan. New Orleans Geological Society. New Orleans, LA.

Ward, W. C. 1985. Quaternary geology of northeastern Yucatan Peninsula, Part 2. Pages 23-53 in Ward, W. C., A. E. Weidie, and W. Back, eds. Geology and hydrogeology of the Yucatan and Quaternary Geology of northeastern Yucatan Peninsula. New Orleans Geological Society. New Orleans, LA.

Weidie, A. E. 1985. Geology of the Yucatan Platform, Part 1. Pages 1-19 in Ward, W. C., A.E. Weidie, and W. Back, eds. Geology and hydrogeology of the Yucatan and Quaternary Geology of northeastern Yucatan Peninsula. New Orleans Geological Society. New Orleans, LA.

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