Petrography and geochemistry of the near-shore carbonate aquiter system of the northwestern coast of the Yucatan Peninsula, Yucatan, Mexico

Includes bibliographical references (pages [367]-385)The northwest coast of the Yucatan Peninsula, Mexico, consists of an almost continuous dune ridge backed by a swamp/estuary system. Inland quarry sites as well as dune ridges represent transgressive sequences consisting of shallow subtidal deposits overlain by beach and storm deposits. Pleistocene to Recent progradation results in the observed depositional fabrics in quarry outcrops and modern beachridge complexes. Large quantities of fresh to brackish groundwater saturated with respect to calcium carbonate reside within the carbonate platform. This groundwater, confined by a continuous unit of impermeable wackestone, passes northward beneath the swamp/estuary system and arrives beneath the dune ridge charged with CO2 and with a head higher than mean sea level. In addition, a zone of unconfined brackish water lies above the zone of fresh groundwater. The brackish water table is hydrologically connected to both the Gulf of Mexico on one side and swamp/estuary systems on the other, and there is an inland component of flow within the brackish lens. For the northwestern 200 km of coast, evaporation causes precipitation of calcite cements and subsequent lithification of a portion of the permeable dune ridge. A unit commonly referred to as the Upper Laja is presently forming within this beach-dune ridge, within the unconfined brackish water lens. Metastable skeletal fragments comprising this sedimentary unit are simultaneously being altered to low-magnesium calcite. An unconsolidated zone of carbonate sediments below the Upper Laja is a result of the zone of dispersion between seawater and meteoric water. The confining wackestone beneath the dune ridge and swamp/estuary systems is a caliche crust which developed on the underlying carbonate rocks and/or sediments less than 23,000 years ago. Prior to calichification, subaerial exposure resulted in extensive meteoric diagenetic processes altering the precursor carbonate sediments and rocks of marine origin. While the unit was still sub- aerially exposed, evaporation and C02-degassing contributed to carbonate precipitation and stabilization of the caliche profile. Usually referred to as the Lower Laja, studies show it to be forming at present beneath and adjacent to the swamp/estuary systems, as calcite precipitates in remaining pore spaces.M.S. (Master of Science