78 NCKRI Special Paper No. 1 Figure 49. A proposed regional speleogenetic model for the Guadalupe Mountains. Diagrams of tectonic situation on the left are extracted from the broader tectonic profiles in DuChene and Cunningham (2006).
HYPOGENIC CAVE FEATURES DuChene and Cunningham (2006) noted that previous work on the origins of Guadalupe caves were focused on the Guadalupe Mountains as a discrete block, rather than examining them in regional structural and tectonic context. They pointed out that the history of cave development in the Guadalupes is fundamentally tied to a regional paleohydrologic system, which developed in response to Laramide uplift of the Alvarado Ridge in New Mexico and west Texas. Their work provides an important background of the regional tectonic and geomorphic evolution, needed to decipher the paleohydrogeologic context of cave development. The speleogenetic model for Guadalupe caves suggested here is based on this evolutionary outline (Figure 49, diagrams in the left column, extracts from a broader picture of DuChene and Cunningham), and on the acknowledgement of vertical heterogeneity in initial permeability across the Capitan platform, a prerequisite for a confined aquifer system to develop (diagrams in the right column). The Alvarado Ridge began to rise in early Tertiary time, and by 38-35 myr, an elevated regional erosion surface extending across Colorado and New Mexico had developed. Prior to opening of the Rio Grande Rift, the ridge was an immense upland recharge area for aquifers that drained eastward into basins in eastern New Mexico and western Texas. The east flank of the Alvarado Ridge provided the potential for confined hydrodynamic flow through laterally-transmissive beds and horizons in the backreef and Capitan aquifers. Initial erosional entrenchment over the platform, in response to either the uplift phase and/or climatic changes, established conditions for restricted discharge and hence for rising and convergence of shallower oxygenated flow from the westward recharge areas and the intermediate/regional deep flow systems (Figure 49, stage 2). As volcanism (Oligocene) and subsequent regional heating (early Miocene) imposed a substantial thermal gradient across the sedimentary sequence, speleogenesis could proceed through the hydrothermal mechanism (“Stage 3 thermal caves” of Hill, 1996, 2000a, 2000b). As discussed above, it is still largely an open question when H2S began to enter the system, and from which source. The deep flow system could have originated from further upslope portions of the Alvarado Ridge, rising to the base of the Capitan platform from the Victorio Peak Formation or still deeper sediments, or from the Delaware Basin as suggested by Hill (1987). Palmer and Palmer (2000a) mentioned a possibility for a compaction/compressiondriven flow system to rise periodically from the basin to deliver H2S. It is plausible to assume that both hydrothermal and sulfuric acid dissolutional mechanisms operated, either simultaneously or sequentially, with alternating relative importance at different times through the main stage of rising transverse speleogenesis (Oligocene-Miocene). With the declining thermal gradient, sulfuric acid dissolution became the predominating mechanism, possibly overprinting much of the mineralogical evidence for the thermal contribution. The morphological suites of rising flow, extensively developed in the Guadalupe caves, contain strong imprints of a buoyant convection component in the morphology of already mature cave systems. As solute density variations are unlikely to be strong enough to drive buoyancy dissolution in the particular situation of the Guadalupe Mountains, the thermal density variations were probably the main cause for the free convection component, operative until the culminating phase of the confined development (see below). Vertical, inclined and quasihorizontal elements of caves including multi-story maze clusters developed within a single although geologically quite prolonged, stage of rising transverse speleogenesis, being guided by distribution of respective initial porosity systems. The culmination of this process occurred when erosion opened and locally truncated the Capitan platform, which caused vigorous discharge from the confined aquifer system (Figure 49, stage 3). The main distinction from the previous speleogenetic period was that rather pervasive cave development along all available paths changed to preferential development along select paths or zones connecting major feeders and ultimate outlets (rising springs). This phase was geologically short in each particular sector of the emerging Guadalupe Mountains, but probably added much of the volume to particularly large passages and rooms (e.g. Main Corridors in Cottonwood Cave and Carlsbad Cavern, the Big Room in Carlsbad Cavern, the Rift–entrance and the Sulfur Shores– Underground Atlanta series in Lechuguilla Cave; see Figure 47). It quickly changed to water table conditions (high initial position) in each emerging sector. The transition from a confined to unconfined situation began in the presently highest southwestern sector of the mountains (Guadalupe Peak), which was first to expose the reef from beneath the backreef confinement, and shifted in three main episodes northeastwards, as evidenced by the three distinct topography levels over the length of the ridge (DuChene and Martinez, 2000) and by progressively younger absolute dates from cave alunite (Polyak et al., 1998; Polyak and Provencio, 2000; Figure 50). The alunite dates, ranging from 12 to about 4 My between the highest and lowest caves, record the water table episodes rather than the main speleogenetic development, so that they provide upper constraints on the transition (confined to unconfined) phases in respective sectors. The water table situation (Figure 49, stage 4) migrated from the southwestern sector to the northeastern sector of the ridge through late Miocene-Pliocene. This speleogenetic stage resulted in the formation of sulfuric acid-related minerals and replacement gypsum, and increased the cave's volume due to water table and condensation dissolution. If stable water positions coincided with structurally-controlled stories, the latter 79
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