Eugene C. Perry

Ring of Cenotes (sinkholes), northwest Yucatan, Mexico: Its hydrogeologic characteristics and possible association with the Chicxulub impact crater

A 180-km-diameter semicircular band of abundant karst sinkholes (Ring of Cenotes) in northwest Yucatan, Mexico, coincides approximately with a concentric ring of the buried Chicxulub structure, a circular feature manifested in Cretaceous and older rocks, that has been identified as the product of the impact of a bolide. The ring, expressed in Tertiary rocks, marks a zone of high permeability as shown by (1) the sinkholes themselves, (2) breaks in the coastal dune system and high density of springs where the ring intersects the coast, and (3) water-level transects characterized by a decline in water level toward the ring. Any direct relation that exists between the Ring of Cenotes and the Chicxulub structure bears on regional hydrogeology. If the layer or zone responsible for the ring is deeply buried, it may act as a barrier to the movement of ground water across the main flow direction. Shallower zones of horizontal permeability could result in less complete diversion of ground water. Through its influence on Yucatan aquifer characteristics, the ring may provide a link between modern environmental problems and astrogeology. Possible origins for the Ring of Cenotes are (1) faulting, perhaps reactivated by post-Eocene–mid-Miocene basin loading, (2) permeability in a buried reef complex developed in the shallow Paleocene sea around the crater rim, or (3) breccia collapse occasioned by consolidation or by solution of evaporite components. If the ring developed on ancient faults, it may outline hydrothermal systems and mineral deposits produced during Paleocene cooling of the Chicxulub melt sheet.

The Hydrogeochemistry of the Karst Aquifer System of the Northern Yucatan Peninsula, Mexico

Based on groundwater geochemistry, stratigraphy, and surficial and tectonic characteristics, the northern Yucatan Peninsula, Mexico, a possible analog for ancient carbonate platforms, is divided into six hydrogeochemical/physiographic regions: (1) Chicxulub Sedimentary Basin, a Tertiary basin within the Chicxulub impact crater; (2) Cenote Ring, a semicircular region of sinkholes; (3) Pockmarked Terrain, a region of mature karst; (4) Ticul fault zone; (5) Holbox Fracture Zone-Xel-Ha Zone; and (6) Evaporite Region. Regional characteristics result from tectonics, rock type, and patterns of sedimentation, erosion, and rainfall. The Cenote Ring, characterized by high groundwater flow, outlines the Chicxulub Basin. Most groundwater approaches saturation in calcite and dolomite but is undersaturated in gypsum. Important groundwater parameters are: SO4/Cl ratios related to seawater mixing and sulfate dissolution; Sr correlation with SO4 and saturation of Lake Chichancanab water with celestite, indicating celestite as a major source of Sr; high Sr in deep water of cenotes, indicating deep circulation and contact of groundwater with evaporite; and correlation of Ca, Mg, and SO4, probably related to gypsum dissolution and dedolomitization. Based on geochemistry we propose: (1) a fault between Lake Chichancanab and Cenote Azul; (2) deep seaward movement of groundwater near Cenote Azul; and (3) contribution of evaporite dissolution to karst development in the Pockmarked Terrain. Chemical erosion by mixing-zone dissolution is important in formation of Estuario Celestun and other estuaries, but is perhaps inhibited at Lake Bacalar where groundwater dissolves gypsum, is high in Ca, low in CO3, and does not become undersaturated in calcite when mixed with seawater.

Evolution of fluid compartmentalization in a detachment fold complex

Oxygen, carbon, and strontium isotope variations in vein-filling calcite and quartz cements and their host rocks are used to elucidate the origin, spatial and temporal evolution, and migration pathways of fluids in the detachment Nuncios fold complex, northeastern Mexico. The folded Mesozoic sedimentary sequence contains two regional paleohydrostratigraphic units separated by a unit of low permeability. Two main generations of cements are present in both paleohydrostratigraphic units. Distinct differences exist between δ 18 O, δ 13 C, 87 Sr/ 86 Sr, and fluid-inclusion temperatures of early vein-filling cements in the lower and the upper units. These differences, together with a strong correspondence between early cement and host-rock δ 18 O and δ 13 C values, suggest that early diagenetic fluids were compartmentalized between the two units. Late vein-filling cements have isotopic compositions and fluid-inclusion temperatures that converge to similar values, indicating a change to open fluid flow between the lower and upper units. We hypothesize that the fluid history of the Nuncios fold complex evolved in two main stages: (1) burial diagenesis and early folding, during which fluids were confined within individual units, and (2) late-stage folding, during which increased deformation associated with fold tightening caused the expulsion of fluid from the lower unit into the upper unit.

Carbonate geochemistry and the concentrations of aqueous Mg2+, Sr2+ and Ca2+: Western north coast of the Yucatan, Mexico

Abstract We have measured concentrations of major cations, Sr 2+ and alkalinity in groundwater, pore-water and surface-water samples collected from the western north coast of the Yucatan Peninsula, Mexico. Thermodynamic calculations indicate that almost all water samples from the study area are supersaturated with respect to calcite, aragonite, high-Mg calcite (HMC) and dolomite. Analysis of the data suggests that the geochemical processes controlling Mg 2+ , Sr 2+ and Ca 2+ concentrations differ in different environments. Mass-balance modeling has been used to calculate the amount of dissolution and/or precipitation of aragonite, low-Mg calcite (LMC) and HMC (and/or dolomite) required to produce waters with the observed Mg 2+ , Sr 2+ and Ca 2+ concentrations. Mass-balance calculations indicate that HMC and aragonite are dissolving, whereas LMC is precipitating from most groundwater and inland surface waters. The chemistry of surface waters from an extensive saline coastal swamp suggests that LMC and aragonite in surficial swamp sediments are converted to HMC or dolomite while swamp pore-water chemistry suggests that HMC is dissolving and LMC and aragonite are precipitating in deeper swamp sediments. The chemistry of pore waters suggests that they are derived from saline surface waters and that fresh to brackish groundwater from the shallow confined aquifer does not flow into swamp sediments.