Download PDF - Speleogenesis

ASCENDING HYPOGENIC SPELEOGENESIS
The lithostratigraphic cyclicity of various scales is
common within thick carbonate sedimentary successions
of different ages. A number of studies demonstrate that the
distribution of porosity and permeability relates directly to
lithofacies, so that cyclostratigraphy is increasingly used
for characterization of vertical heterogeneity of porosity
and permeability (e.g. Hovorka et al., 1998; Budd and
Vacher, 2004).
Budd and Vacher (2004) show that matrix
permeability of young carbonates in the Upper Floridan
Aquifer ranges over three orders of magnitude between
different lithofacies. Cunningham et al. (2006) developed a
high-resolution cyclostratigraphic model for the carbonate
Biscayne Aquifer, Florida, and demonstrated pronounced
regular variations in porosity structure and permeability
between lithofacies, arranged in cyclic successions of three
types. Permeability of the aquifer is heterogeneous, with
values differing up to two orders of magnitude between the
lithofacies. Three types of flow zones, interbedded with
low-permeability zones, are recognized. High permeability
zones of “touching-vug” type (Vacher and Mylroie, 2002)
provide stratiform passageways for groundwater flow. In
the context of transverse speleogenesis, such diffusely
permeable beds play the role of “aquifers” in
heterogeneous sequences, with vertical flow and conduit
development occurring in the intervening less permeable
beds. This case of young eogenic carbonates is referred to
in order to illustrate the importance of detailed
hydrostratigraphic consideration for karst aquifers, which
is equally important for older successions of wellindurated
carbonate rocks. Unfortunately, such detailed
hydrostratigraphic characterizations are not commonly
available for multiple-aquifer systems containing soluble
units.
In the Ordovician Knox carbonates in Tennessee,
USA, the pattern of transgressive and regressive cycles
creates pronounced hydrostratigraphic heterogeneity
(Montanez, 1997). The facies within regressive cycles are
almost completely replaced by tight, fine-crystalline
dolomite that formed syndepositionally from evaporating
water. They became aquitards afterwards. In contrast,
transgressive cycles, which behaved like aquifers during
burial diagenesis, have considerable porosity and
permeability as they were affected by extensive dissolution
in intermediate to deep-burial settings, according to
petrographic data.
Machel (1999) refers to reef complex carbonates as
another example for depositional control on diagenesis that
leads to a pronounced differentiation into “proto-aquifers”
and “proto-aquitards” at the time of deposition. In addition
to the effect on diagenetic changes of porosity in different
facies, subsequent fracturing develops in distinct styles and
with different frequency in the facies varieties to further
accentuate permeability differences. As a result, ascending
transverse speleogenesis across such vertically
heterogeneous sequences will utilize various kinds of
original (pre-speleogenetic) porosity at different intervals
(cross-bedding and stratiform fracturing of different styles,
touching-vug porosity, etc.) and will be affected by
constraints of their poor connectivity with porosity at other
intervals. Complex 3-D structural organization of the
ascending hypogenic caves in the Permian reef complex of
the Guadalupe Mountains in the southwestern USA
perfectly illustrates the effect of this heterogeneity (see
Section 4.5). A small-scale illustration is presented by the
photo of Figure 9 taken in Caverns of Sonora, Texas, USA.
Numerous cupolas at the ceiling of the uppermost story of
the multi-story cave (view from below) open up into a
distinct horizon of touching-vugs type porosity (“burrowed
bed”), which served as an outlet boundary for the uprising
cave-forming cross-formational flow.
In many evaporitic sequences, gypsum beds of
moderate thickness alternate with densely fractured
limestone or dolomite beds that originally played a role of
aquifers and lateral “carrier” beds. They supply water,
undersaturated with respect to gypsum, to sub-gypsum
beds, from where the water enters the gypsum, with
upward flow driven by the hydraulic head gradient and/or
density gradient (Stafford et al., 2008). With the onset of
transverse speleogenesis, various units in the sequence
receive better hydraulic connection and the whole
sequence then behaves largely as a single multiple-aquifer
system. This is the situation common for many mixed
sulfate-carbonate sequences, e.g. the Permian sequences of
the fore-Ural, Pinega, and North Dvina regions in Russia;
and the Rustler and Seven Rivers Formations in the
Delaware and Roswell basins in New Mexico, USA.
Recognition of the nature of transverse speleogenesis
and aspects discussed above will help to develop more
adequate approaches to flow models in stratified
heterogeneous karst aquifers. Lateral transmission of
groundwater occurs mainly through non-karstic aquifers,
or through specific horizons of the laterally connected
fracture networks or “touching-vug” type porosity within
the soluble sequence. Ascending transverse cave
development provides for vertical communication between
such laterally conductive beds. It is important to recognize
that because speleogenesis in layered confined systems
evolves in response to generally transverse flow across
soluble dividing (originally less-permeable) beds, the
resultant conduit systems, even when mature, do not
provide for significant lateral hydraulic connection at the
basin scale. Even the largest maze systems in soluble beds
have continuous lateral extent through areas of only a few
square kilometers at a maximum, and for a few hundred
meters in any single direction. In the lateral aspect they
remain isolated clusters rather than laterally extensive
systems. However, mature transverse conduit systems
19