ABSTRACT Sandstone petrography, detrital zircon geochronology, and sedimentology of Lower Cretaceous to Paleocene strata in the Cuicateco terrane of southern Mexico indicate an evolution from extensional basin formation to foreland basin development. The Early Cretaceous extensional basin is characterized by deposition of deep-marine fans and channels, which were mainly sourced from Mesoproterozoic and Permian crystalline rocks of the western shoulder of the rift basin. Some submarine fans, especially in the northern Cuicateco terrane, record an additional source in the Early Cretaceous (ca. 130 Ma) continental arc. The fans were fed by fluvial systems in updip parts of the extensional basin system. The transition from middle Cretaceous tectonic quiescence to Late Cretaceous shortening is recorded by the Turonian–Coniacian Tecamalucan Formation. The Tecamalucan Formation is interpreted as pre-orogenic deposits that represent submarine-fan deposits sourced from Aptian–Albian carbonate platform and pre-Mesozoic basement. The foreland basin in the Cuicateco terrane was established by the Maastrichtian, when foredeep strata of the Méndez Formation were deposited in the Cuicateco terrane, Veracruz basin, and across the western Gulf of Mexico, from Tampico to Tabasco. In the Zongolica region, these strata were derived from a contemporaneous volcanic arc (100–65 Ma) located to the west of the basin, the accreted Guerrero terrane (145–120 Ma), and the fold belt itself. By the Paleocene, sediments were transported to the foreland basin by drainages sourced in southwestern Mexico, such as the Late Cretaceous magmatic rocks of the Sierra Madre del Sur, and the Chortis block.

ABSTRACT A combined petrographic and chemical study of ejecta particles from the Cretaceous-Paleogene boundary sequence of El Guayal, Tabasco, Mexico (520 km SW of Chicxulub crater), was carried out to assess their formation conditions and genetic relation during the impact process. The reaction of silicate ejecta particles with hot volatiles during atmospheric transport may have induced alteration processes, e.g., silicification and cementation, observed in the ejecta deposits. The various microstructures of calcite ejecta particles are interpreted to reflect different thermal histories at postshock conditions. Spherulitic calcite particles may represent carbonate melts that were quenched during ejection. A recrystallized microstructure may indicate short, intense thermal stress. Various aggregates document particle-particle interactions and intermixing of components from lower silicate and upper sedimentary target lithologies. Aggregates of recrystallized calcite with silicate melt indicate the consolidation of a hot suevitic component with sediments at ≳750 °C. Accretionary lapilli formed in a turbulent, steam-condensing environment at ~100 °C by aggregation of solid, ash-sized particles. Concentric zones with smaller grain sizes of accreted particles indicate a recurring exchange with a hotter environment. Our results suggest that during partial ejecta plume collapse, hot silicate components were mixed with the fine fraction of local surface-derived sediments, the latter of which were displaced by the preceding ejecta curtain. These processes sustained a hot, gas-driven, lateral basal transport that was accompanied by a turbulent plume at a higher level. The exothermic back-reaction of CaO from decomposed carbonates and sulfates with CO 2 to form CaCO 3 may have been responsible for a prolonged release of thermal energy at a late stage of plume evolution.

A handful of investigative teams in several parts of the world are studying abundant biological communities in caves formed by sulfuric-acid speleogenesis. These caves are atypical in terms of origin, chemistry, and ecosystem properties. They prominently display sulfur minerals, characteristic cavity topologies, and notable biological diversity and biological productivity resulting directly from the conditions that produce the caves. Even long-inactive systems still harbor some of these indicators. The microbial and macroscopic ecosystems within sulfuric-acid speleogenetic caves are geologically mediated and maintained. This geological mediation is a theme connecting them with other sulfur-driven ecosystems on Earth, including deep-sea hydrothermal vents, sulfurous near-surface hydrothermal systems, and solfataras. Evidence exists for potentially significant microbial participation in the process of speleogenesis itself. Recent results confirming the high relative abundance of sulfur on Mars, an apparent sedimentary basin with high sulfate concentration, near-surface indicators of ice and water, and trace detection of reduced gases (especially methane) in the Martian atmosphere, possibly deriving from subsurface microbial sources, set the stage for suggesting that sulfuric-acid speleogenetic systems may be useful as astrobiological analogs for hypothetical Mars ecosystems. Unique speleogenetic mechanisms may occur on Mars and could provide subsurface void space suitable for habitation by such hypothetical microbial systems.