Download PDF - Speleogenesis
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HYPOGENIC CAVE FEATURES<br />
DuChene and Cunningham (2006) noted that previous<br />
work on the origins of Guadalupe caves were focused on<br />
the Guadalupe Mountains as a discrete block, rather than<br />
examining them in regional structural and tectonic context.<br />
They pointed out that the history of cave development in<br />
the Guadalupes is fundamentally tied to a regional<br />
paleohydrologic system, which developed in response to<br />
Laramide uplift of the Alvarado Ridge in New Mexico and<br />
west Texas. Their work provides an important background<br />
of the regional tectonic and geomorphic evolution, needed<br />
to decipher the paleohydrogeologic context of cave<br />
development. The speleogenetic model for Guadalupe<br />
caves suggested here is based on this evolutionary outline<br />
(Figure 49, diagrams in the left column, extracts from a<br />
broader picture of DuChene and Cunningham), and on the<br />
acknowledgement of vertical heterogeneity in initial<br />
permeability across the Capitan platform, a prerequisite for<br />
a confined aquifer system to develop (diagrams in the right<br />
column). The Alvarado Ridge began to rise in early<br />
Tertiary time, and by 38-35 myr, an elevated regional<br />
erosion surface extending across Colorado and New<br />
Mexico had developed. Prior to opening of the Rio Grande<br />
Rift, the ridge was an immense upland recharge area for<br />
aquifers that drained eastward into basins in eastern New<br />
Mexico and western Texas. The east flank of the Alvarado<br />
Ridge provided the potential for confined hydrodynamic<br />
flow through laterally-transmissive beds and horizons in<br />
the backreef and Capitan aquifers.<br />
Initial erosional entrenchment over the platform, in<br />
response to either the uplift phase and/or climatic changes,<br />
established conditions for restricted discharge and hence<br />
for rising and convergence of shallower oxygenated flow<br />
from the westward recharge areas and the<br />
intermediate/regional deep flow systems (Figure 49, stage<br />
2). As volcanism (Oligocene) and subsequent regional<br />
heating (early Miocene) imposed a substantial thermal<br />
gradient across the sedimentary sequence, speleogenesis<br />
could proceed through the hydrothermal mechanism<br />
(“Stage 3 thermal caves” of Hill, 1996, 2000a, 2000b). As<br />
discussed above, it is still largely an open question when<br />
H2S began to enter the system, and from which source. The<br />
deep flow system could have originated from further<br />
upslope portions of the Alvarado Ridge, rising to the base<br />
of the Capitan platform from the Victorio Peak Formation<br />
or still deeper sediments, or from the Delaware Basin as<br />
suggested by Hill (1987). Palmer and Palmer (2000a)<br />
mentioned a possibility for a compaction/compressiondriven<br />
flow system to rise periodically from the basin to<br />
deliver H2S. It is plausible to assume that both<br />
hydrothermal and sulfuric acid dissolutional mechanisms<br />
operated, either simultaneously or sequentially, with<br />
alternating relative importance at different times through<br />
the main stage of rising transverse speleogenesis<br />
(Oligocene-Miocene). With the declining thermal gradient,<br />
sulfuric acid dissolution became the predominating<br />
mechanism, possibly overprinting much of the<br />
mineralogical evidence for the thermal contribution. The<br />
morphological suites of rising flow, extensively developed<br />
in the Guadalupe caves, contain strong imprints of a<br />
buoyant convection component in the morphology of<br />
already mature cave systems. As solute density variations<br />
are unlikely to be strong enough to drive buoyancy<br />
dissolution in the particular situation of the Guadalupe<br />
Mountains, the thermal density variations were probably<br />
the main cause for the free convection component,<br />
operative until the culminating phase of the confined<br />
development (see below). Vertical, inclined and quasihorizontal<br />
elements of caves including multi-story maze<br />
clusters developed within a single although geologically<br />
quite prolonged, stage of rising transverse speleogenesis,<br />
being guided by distribution of respective initial porosity<br />
systems.<br />
The culmination of this process occurred when erosion<br />
opened and locally truncated the Capitan platform, which<br />
caused vigorous discharge from the confined aquifer<br />
system (Figure 49, stage 3). The main distinction from the<br />
previous speleogenetic period was that rather pervasive<br />
cave development along all available paths changed to<br />
preferential development along select paths or zones<br />
connecting major feeders and ultimate outlets (rising<br />
springs). This phase was geologically short in each<br />
particular sector of the emerging Guadalupe Mountains,<br />
but probably added much of the volume to particularly<br />
large passages and rooms (e.g. Main Corridors in<br />
Cottonwood Cave and Carlsbad Cavern, the Big Room in<br />
Carlsbad Cavern, the Rift–entrance and the Sulfur Shores–<br />
Underground Atlanta series in Lechuguilla Cave; see<br />
Figure 47). It quickly changed to water table conditions<br />
(high initial position) in each emerging sector. The<br />
transition from a confined to unconfined situation began in<br />
the presently highest southwestern sector of the mountains<br />
(Guadalupe Peak), which was first to expose the reef from<br />
beneath the backreef confinement, and shifted in three<br />
main episodes northeastwards, as evidenced by the three<br />
distinct topography levels over the length of the ridge<br />
(DuChene and Martinez, 2000) and by progressively<br />
younger absolute dates from cave alunite (Polyak et al.,<br />
1998; Polyak and Provencio, 2000; Figure 50). The alunite<br />
dates, ranging from 12 to about 4 My between the highest<br />
and lowest caves, record the water table episodes rather<br />
than the main speleogenetic development, so that they<br />
provide upper constraints on the transition (confined to<br />
unconfined) phases in respective sectors.<br />
The water table situation (Figure 49, stage 4) migrated<br />
from the southwestern sector to the northeastern sector of<br />
the ridge through late Miocene-Pliocene. This<br />
speleogenetic stage resulted in the formation of sulfuric<br />
acid-related minerals and replacement gypsum, and<br />
increased the cave's volume due to water table and<br />
condensation dissolution. If stable water positions<br />
coincided with structurally-controlled stories, the latter<br />
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