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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
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