ABSTRACT Discoveries in the 1980s greatly expanded speleologists’ understanding of the role that hypogenic groundwater flow can play in developing caves at depth. Ascending groundwater charged with carbon dioxide and, especially, hydrogen sulfide can readily dissolve carbonate bedrock just below and above the water table. Sulfuric acid speleogenesis, in which anoxic, rising, sulfidic groundwater mixes with oxygenated cave atmosphere to form aggressive sulfuric acid (H 2 SO 4 ) formed spectacular caves in Carlsbad Caverns National Park, USA. Cueva de Villa Luz in Mexico provides an aggressively active example of sulfuric acid speleogenesis processes, and the Frasassi Caves in Italy preserve the results of sulfuric acid speleogenesis in its upper levels while sulfidic groundwater currently enlarges cave passages in the lower levels. Many caves in east-central Nevada and western Utah (USA) are products of hypogenic speleogenesis and formed before the current topography fully developed. Wet climate during the late Neogene and Pleistocene brought extensive meteoric infiltration into the caves, and calcite speleothems (e.g., stalactites, stalagmites, shields) coat the walls and floors of the caves, concealing evidence of the earlier hypogenic stage. However, by studying the speleogenetic features in well-established sulfuric acid speleogenesis caves, evidence of hypogenic, probably sulfidic, speleogenesis in many Great Basin caves can be teased out. Compelling evidence of hypogenic speleogenesis in these caves include folia, mammillaries, bubble trails, cupolas, and metatyuyamunite. Sulfuric acid speleogenesis signs include hollow coralloid stalagmites, trays, gypsum crust, pseudoscallops, rills, and acid pool notches. Lehman Caves in Great Basin National Park is particularly informative because a low-permeability capstone protected about half of the cave from significant meteoric infiltration, preserving early speleogenetic features.

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.

Abstract Guanajuato has a long history (450 years) of mineral exploitation and remarkable silver and gold production from a complex system of fault-veins. Despite this, it is only in the past 40 years that the systematic study of its geology has been conducted. Mid-Tertiary epithermal veins occur in all the Mesozoic and Paleogene rock units exposed in the mining district, and mineralization seems to be the result of the combination of several geologic factors, such as the occurrence of greenschists in the basal complex, a thick sequence of Early Paleogene red beds overlain by a thick succession of Oligocene volcanic rocks with the existence of one or more paleolakes when the volcanoes were active. The systematic study of the greenschists and associated plutonic and sedimentary rocks in the basal complex of Sierra de Guanajuato has contributed significant information to the concept of accretion of the Guerrero terrane to the SW end of the North American craton in the Early Cretaceous. Research on the Eocene red bed sequence suggests that early extension occurred creating fault patterns that later were reactivated during Neogene Basin and Range pulses. Immediately east of the city of Guanajuato, a thick volcanic sequence is exposed, with two pyroclastic units formed by felsic ignimbrites that almost certainly are related to a nearby caldera, which was active immediately prior to Ag-Au mineralization. The first activity pulse of the caldera produced the Bufa ignimbrite, a massive unit that displays very large thickness variations (300 to <10 m) in short distances, which we interpret as a signal that it may be an intracaldera deposit. The second explosive pulse originated the Calderones formation, a unit formed by an undetermined but large number of ignimbrites, surge deposits, layers with accretionary lapilli, and epiclastic-volcanic deposits. The Calderones formation is characterized by pervasive chloritization, which points out toward the presence of external water in the system, probably related to one or more shallow lakes within the caldera previously formed by the Bufa eruption. Lithofacies variations and stratigraphic arguments suggest that the Guanajuato caldera was probably located near the Cerro Alto de Villalpando and La Peregrina lava dome complex. Morphological and structural evidence of the caldera are masked by several pulses of younger normal faulting which affected the southern portion of the Mexican Basin and Range Province (i.e., Mesa Central).

The Guerrero Composite Terrane of western Mexico is the second largest terrane in North America. Mostly characterized by submarine volcanism and formed by five terranes, the Guerrero records vast and complex subduction-related processes influenced by major translation and rifting. It is composed of the Teloloapan, Guanajuato, Arcelia, Tahue, and Zihuatanejo Terranes. The Teloloapan Terrane is made up of Lower Cretaceous island-arc (IA) andesitic to basaltic submarine lava flows, interbedded with limestone and shallow-marine volcaniclastic rocks. The Guanajuato and Arcelia Terranes are characterized by Lower Cretaceous supra-subduction ophiolite successions formed by deep-marine volcanic and sedimentary rocks with mid-oceanic-ridge basalt (MORB), oceanic-island basalt (OIB), and island-arc basalt (IAB) signatures. These two terranes are placed between the continent and the more evolved arc assemblages of the Zihuatanejo Terrane. The Tahue Terrane is composed of Paleozoic accreted arc and eugeoclinal sedimentary rocks, Triassic rift-related metaigneous rocks, and overlain unconformably by pillow basalts, limestone, and volcaniclastic rocks. The Zihuatanejo Terrane was formed by Triassic ocean-flank to ocean-floor assemblages accreted in Early Jurassic time (subduction complexes). The subduction complexes are overlain by Middle Jurassic–evolved volcanic arc rocks, which are in turn unconformably overlain by Early and Late Cretaceous subaerial and marine arc-related volcano-sedimentary assemblages. Mesozoic stratigraphy at the paleocontinental margin of Mexico (Oaxaquia and Mixteca Terranes) is formed by Triassic submarine fan turbidites accreted during Early Jurassic time; Middle Jurassic–evolved volcanic arc rocks are unconformably covered by a Late Jurassic to Cretaceous calcareous platform. Six stages in the tectonic evolution are proposed on the basis of the stratigraphic and deformational events recorded in western Mexico: (1) A passive or rifting margin developed along the western margin of continental Mexico throughout the Triassic. A thick siliciclastic turbiditic succession of the Potosi Submarine Fan was accumulated on the paleo-continental shelf-slope and extended to the west in a marginal oceanic basin. (2) Subduction began in the Early Jurassic, and the turbidites of the Potosi Fan with slivers of the oceanic crust were accreted, forming a wide subduction prism. (3) Exhumation of the accretionary prism and development of a Middle Jurassic continental arc onto the paleo-continental margin (Oaxaquia and Mixteca Terrane) took place, and also in the Zihuatanejo Terrane. (4) Intra-arc strike-slip faulting and rifting of the Middle Jurassic continental arc took place along with migration of the subduction toward the west and development of a calcareous platform in Oaxaquia and the Mixteca Terrane (continental Mexico). (5) Drifting of the previously accreted Tahue and Zihuatanejo Terranes formed a series of marginal arc-backarc systems, or one continuously drifting arc with intra-arc and backarc basins during Early to middle Cretaceous time. (6) Deformation of the arc assemblages, and development of Santonian to Maastrichtian foreland and other basins, date the final amalgamation of the Guerrero Composite Terrane with the continental margin.