Download PDF - Speleogenesis

20
NCKRI Special Paper No. 1
provide ideal local vertical hydraulic connections between
lateral stratiform passageways for groundwater flow, and
hence significantly affect basinal flow pattern.
Figure 9. A cupola at the ceiling of the uppermost story of
Caverns of Sonora, Texas (view from below; breadth of the photo
is about 2 m). Numerous cupolas at this story open up into a
distinct horizon of touching-vugs type porosity (“burrowed bed”),
which served as a receiving aquifer during the formation of the
cave. Developed at four stories within a vertical range of about 35
m in the layered carbonate Edwards Group, the cave passages are
mainly controlled by fractures encased in distinct beds of compact
limestone, although some more prominent fractures, and hence
passages, cross through several beds.
3.5 Recharge, cave-forming flow and
discharge in hypogene settings
The mode of recharge and discharge, and relationships
between the respective boundaries and zones, are among
the major factors that determine the style of speleogenesis
and resultant cave patterns. In hypogenic speleogenesis,
recharge of water to a given soluble formation occurs from
the adjacent formation below, the main criterion for
distinguishing this class of speleogenesis. Another
important difference is that discharge occurs also through
non-soluble formations. Hydraulic properties of adjacent
formations, and of a major upper confining formation,
impose important controls on speleogenesis in confined
settings (Klimchouk, 2000a; 2003a).
To prevent confusion that arises when referring to
stratigraphic units in terms of their solubility, the
formation that receives recharge from below and hosts
hypogenic caves will be referred to as a cave formation, or
a cave unit, the underlying source formation is a feeding
formation, and the overlying formation into which
discharge occurs is the receiving formation. All of the
formations can be generally soluble, but still with
drastically different capacities to support cave
development under given physical, chemical and
hydrokinetic conditions.
The mode of recharge, in terms of its lateral
distribution, depends on the types, distribution and
connectivity of the original porosity systems in both the
feeding formation and the cave formation as well as the
overall hydrostratigraphy of the system. In the feeding
formation, effective porosity at the contact can be diffuse
and homogenous (A1-A3 in Figure 11), diffuse and
inhomogeneous (zones of enhanced conductivity in
otherwise permeable media; B1-B3), or localized
(tectonically disrupted zones, e.g. fault zones, etc.) In the
latter case, high conductivity zones in the feeding and
receiving formations are commonly co-planar with the
respective major permeability paths across the whole cave
formation (A4). More commonly, there is a disparity of
permeability structures between the feeding and receiving
formations.
Such a disparity is almost always the case at the lower
contact of the cave formation, at its recharge boundary
(Figure 10). Permeability in the feeding formation can be
represented by various combinations of matrix, touchingvug,
fracture or conduit (prominent cross-bedding
fractures) porosity systems, but it never matches the
original permeability structure in the cave formation. The
extreme case of such a disparity is where there are virtually
no hydraulically efficient original flowpaths available in
the cave unit at its lower contact, and hence no perceptible
forced flow through it (Figure 10, F-G). Still, hypogenic
speleogenesis can operate in this situation through the free
convection mechanism.
Vertically across the cave formation, original flow
paths are almost always composed of segments of different
porosity styles and types, as discussed and exemplified in
the previous section. In fact, the relations discussed above
and shown in Figure 10 may be found between adjacent
horizons/units within the inhomogeneous cave formation
itself (see Figure 9). In addition to distinct characteristics
of different porosity segments, the connectivity constraints
between them impose strong effects on speleogenesis.
Complex 3-D structural organization of the most widely
acknowledged ascending hypogenic caves, such as gypsum
mazes in the western Ukraine, limestone mazes of the
Black Hills, South Dakota, USA, composite pattern caves
of the Buda Hills in Hungary and the Capitan reef complex
of the Guadalupe Mountains, New Mexico, USA, illustrate
the effect of this heterogeneity. It is important to
underscore that because hypogenic speleogenesis is often
the product of mixed (topography-driven and densitydriven)
flow systems, buoyancy effects are commonly
involved in establishing hydraulic connections between
different porosity segments (Section 3.8).