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74 NCKRI Special Paper No. 1 Figure 45. Regional structural setting of the Guadalupe Mountains (left; adapted from Koša and Hunt, 2006) and stratigraphic nomenclature of the Permian strata exposed in the Guadalupe Mountains (right; from Scholle et al., 2004). Figure 46. Plans and profiles of some Guadalupe caves: 1 = Carlsbad Cavern, plan view and profile (from Palmer and Palmer, 2000a); 2 = Spider Cave, plan view outline (by S. Allison, Carlsbad Caverns National Park); 3 = Dry Cave, profile outline (courtesy of Carlsbad Caverns National Park); 4 = Endless Cave, profiles and geology. It is generally agreed, as a broad speleogenetic concept that water rich in H2S rose from depth and reacted with oxygen at shallower levels within the reef/backreef formations to produce sulfuric acid (Jagnow et al., 2000). Evidence for sulfuric acid dissolution is abundant, coming mainly from geochemical and mineralogical findings in various caves: massive gypsum deposits in many caves (Davis, 1980), isotopically-light sulfur in massive gypsum (Kirkland, 1982; Hill, 1987) and massive sulfur (Cunningham et al., 1994), light-chain aliphatic hydrocarbons in sulfur and a number of sulfuric-acid related minerals, such as endellite, alunite, natroalunite, dickite, tyuyamunite, metatyuyamunite, aluminite and hydrobasaluminite (Hill, 1987; Palmer and Palmer, 1992; Polyak and Mosch, 1995; Polyak and Provencio, 1998). Sulfuric acid as the main dissolutional agent in the Guadalupian speleogenesis seems to be almost universally accepted, although some researchers still cast doubt on whether it was the main cave-forming mechanism (e.g. Brown, 2006), and others point out that it may be difficult to separate the effects of sulfuric and carbonic acid dissolution in a mixing zone setting where CO2 generated by carbonate dissolution is not allowed to escape from the cave-forming zone (Palmer and Palmer, 2000a). Although H2S is firmly established to be the result of sulfate reduction processes involving hydrocarbons, its exact source is still
HYPOGENIC CAVE FEATURES debated. Hill's model (1987, 1996) implies that the gas migrated updip from the adjacent Delaware Basin from the Bell Canyon Formation, although this view is not well tied with paleohydrogeology. DuChene and Cunningham (2006) suggested the Artesia Group of the Northwest Shelf as an alternative source of H2S. Another option is H2S derived from deep source rocks below the Capitan reef (DuChene, 1986). The resolution of this issue will depend on a revised paleohydrogeological model, as most of the gas reached the cave-forming zones in aqueous, not in gaseous, form (Palmer and Palmer, 2000a). In addition, results of possible hydrothermal speleogenesis during the Miocene (Phase 3 caves of Hill, 2000b; 1996) would be apparently utilized and modified by later processes invoking sulfuric acid; the effects of these processes are again difficult to separate. Available radiometric dates for sulfuric acid footprints (alunite from various caves, 12.3- 3.9 myr; Polyak et al., 1998) certainly post-date the main phase of cave formation for respective caves and do not necessarily indicate that it was related to sulfuric acid. However, the primary problem is that the paleohydrogeologic environment for speleogenesis of the Guadalupean caves is not well-discerned. Most works on speleogenesis in the Guadalupe Mountains have been focused on geochemistry, mineralogy, and speleothems but have largely left out in-depth studies of the cave-forming hydrogeologic environment. The notable exceptions include the paper by Palmer and Palmer (2000a), which provides, in addition to a sound hydrochemical background, a hydrogeological discussion based on regional morphogenetic analysis of cave patterns and morphology. DuChene and Cunningham (2006) discussed paleohydrogeological conditions based on analysis of tectonic/geomorphologic history. The principal controversy is about whether the main cave development occurred under phreatic (bathyphreatic) conditions or was caused by dissolution at the water table and/or due to subaerial processes such as condensation corrosion, involving H2S oxidation in water films and limestone/gypsum replacement. The only possibile resolution of this controversy lies in a systematic genetic analysis of cave patterns and meso-morphology, coupled with paleohydrogeological and paleogeomorphological analysis, and proper comparison with the broader context of hypogenic caves. Most researchers view caves in the Guadalupe Mountains as a result of combined bathyphreatic and water table development. According to the original model (Davis, 1980; Hill, 1987, 2000a, 2000b), bathyphreatic (deep-water phreatic, rising flow) conditions were responsible for the strong vertical development of these caves, and for the formation of vertical tubes, fissures and pits; and water table (shallow-water phreatic) conditions were responsible for the horizontal development of caves along certain levels (corresponding to past regional base levels). Palmer and Palmer (2000a) assigned most cave origins in the Guadalupe Mountains to bathyphreatic conditions, due to convergence of oxygenated water with deep-seated rising flow at depths up to 200 m below the water table or deeper. They acknowledged that the morphology of complex 3-D caves, such as Lechuguilla and Carlsbad Cavern, demonstrate rising flow patterns in both meso- and mega scales, from major feeders at the lowermost parts of the systems (such as the Rift and Sulfur Shores areas in Lechuguilla, and the Nicholson Pit and Lake of Clouds in Carlsbad Cavern) to highest outlet passages in the uppermost parts, including present entrances that served as outlets for rising groundwater (Figure 47). Different levels of the caves are connected by ascending passages, which show strong evidence for having been formed by rising aggressive water. Palmer and Palmer (2000a) further suggested that ascending complexes were formed in one stage, although they reserved the view that some chambers may post-date the systems, being enlarged at discrete episodes of water table development. An ascending flow pattern for the Guadalupe caves was also discerned, based on morphological observations, by Davis (1980). Observations by the author of this book in various caves of the region strongly support the views about their ascending transverse origin. In the meso-scale, the continuous succession of feeder-outlet and transitional features (MSRF, see Section 4.4) can be clearly traced throughout different levels and between them within the whole vertical range of caves. The sets of photographs illustrating MSRF components from various hypogenic caves (Plates 1-9, 11) include many examples from the Guadalupian caves. Overall, large and complex 3-D caves of the Guadalupe Mountains give compelling morphological demonstrations of an ever-ascending flow pattern. Arguments toward the major role of water table dissolution and condensation corrosion (e.g. Hose and Macalady, 2006) are chiefly based on 1) comparison with active H2S caves elsewhere in the world, 2) references to horizontal levels in caves, 3) references to gypsum and sulfuric acid-related minerals, and 4) references to specific morphologies. In addition, Palmer (2006) shows that the requirements for very low pH to form alunite found in the Guadalupe caves can be met only in subaerial conditions. In apparent contrast to his previously cited view, Palmer (2006) concluded that much, if not most, of the caves' volume, including the passages that ascend to entrances, has been produced subaerially due to sulfuric acid dissolution through absorption of H2S by water films condensed on walls and ceilings. The above-mentioned arguments in favor of the major speleogenetic role of the water table and vadose development are briefly addressed below. 75
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Hypogene Speleogenesis: Hydrogeolog
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Foreword In 1998, the National Cave
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