Download PDF - Speleogenesis

76
NCKRI Special Paper No. 1
Figure 47. Projected vertical profiles through part of Lechuguilla
Cave, showing the nearly independent flow systems through the
entrance series and through the Sulfur Shores – Underground
Atlanta systems (from Palmer and Palmer, 2000a).
1) The situation with reference to active H2S caves that
are currently at the water table stage is similar to the
interpretation of maze caves of non-sulfuric acid origin
(see Section 4.3); it is tempting to extrapolate present
locally observed processes to more general interpretations
of cave genesis. Commonly-cited examples include Kane
Caves (Wyoming, USA), caves of Frasassi Gorge (central
Italy) and Cueva de Villa Luz (Tabasco, Mexico). They do
demonstrate quite aggressive subaerial dissolution by
sulfuric acid (through H2S oxidation by atmospheric
oxygen), both in flowing water and in condensed water
films, which certainly contributes to the morphology of
these caves. This is insufficient, however, to generalize
this as a major speleogenetic mechanism, responsible for
the origin of a wide category of hypogenic caves. Mass
balance considerations and the requirement for removal of
dissolved matter (not met in many occasions, especially in
extensive maze caves with diffuse outflow) make such
generalizations unfeasible. Other caves in the same regions
(or inactive parts of the same caves) include situations that
are a poor fit to the water table/subaerial speleogenesis
model (e.g. see the above description of hypogenic caves
in central Italy).
2) References to horizontal levels in caves often
include stories that are only somewhat horizontal (see
Figure 23 and description above for Frasassi Gorge caves).
Such stories do not correlate throughout adjacent caves and
even between different areas of the same caves, which
would be expected for true water table levels. They are
commonly conterminous either vertically or laterally with
clearly inclined stories within the same cave or in other
caves of the same areas. In most cases, such stories are
controlled by the distribution of initial porosity structures,
either stratigraphically concordant (in most cases) or
discordant to bedding. More discussion of “horizontality”
is given below with regard to the Guadalupe Mountains
caves.
3) Buck et al. (1994) described five types of gypsum
in Guadalupe caves, two of which are basic and most
relevant to the issue under discussion: (a) massive
subaqueous gypsum sediment that forms large bodies in
passages and rooms, and (b) subaerial gypsum crusts that
replace bedrock by sulfuric acid reaction. Thin
replacement gypsum crusts definitely form above the water
table, but these are volumetrically insufficient to account
for the caves' development. Massive gypsum sediments are
found in many caves that show no signs or possibility of
water table development, such as Monte Cucco caves in
central Italy, Amazing Maze Cave in Texas, and Yellow
Jacket, Spider, Dry, and Endless caves in the Guadalupe
Mountains. This gypsum is apparently formed in waterfilled
passages. As to sulfuric-acid related minerals, these
obviously reflect water table conditions due to the low pH
requirement (Palmer, 2006), but this does not support a
high relative significance of this environment in overall
speleogenesis.
4) Some morphologies of the active H2S caves are
commonly referred to as being specific to sulfuric acid
caves of water table/subaerial origin. Such references
include maze patterns, abrupt changes in morphology,
numerous dead-ends, ceiling cupolas, etc. However, these
morphologies are specific neither to the water table
situation nor to sulfuric acid speleogenesis. Instead, these
features are characteristic of a wide class of ascending
transverse caves, as argued throughout this book using
both theoretical reasoning and references to caves, for
which water table development and sulfuric acid
dissolution are definitely ruled out.
The author's contention is that caves in the Guadalupe
Mountains were mainly formed by rising transverse
speleogenesis, possibly by both hydrothermal dissolution
and sulfuric acid dissolution, with water table development
playing secondary, modifying roles. Previous
bathyphreatic hypotheses (Hill, 1987, 2000a, 2000b;
Palmer and Palmer, 2000a) operated with the notion of an
unconfined aquifer in the Capitan platform when
discussing rising flow, i.e. an aquifer in which the water
table under atmospheric pressure forms the upper
boundary. However, speleogenesis in the Capitan platform
prior to its exhumation and erosional lowering of the
adjacent basin surface occurred under confined conditions.
It is assumed by many workers that Salado evaporites and
younger sediments spread across the shelf regions
(Crysdale, 1987; Garber et al., 1989; Ulmer-Scholle et al.,
1993; Scholle et al., 2004; DuChene and Cunningham,
2006), so that they provided a confining cover over the
carbonate platform before they were removed during