Download PDF - Speleogenesis

18
NCKRI Special Paper No. 1
3.4 Vertical heterogeneity in porosity and
permeability
Non-soluble rocks, as well as soluble rocks before
speleogenesis commences, are commonly characterized by
matrix and fracture porosity. In different lithologies and
lithofacies, the relative significance of the porosity types,
as well as permeabilities of respective media and
interaction of respective flow systems, varies broadly
depending on sediment type and diagenetic, tectonic and
geomorphic history. Evaporites and mature carbonates
normally have low matrix permeability and most flow is
transmitted through fractures, with still greater
concentration of flow in conduits when they evolve.
Young carbonates have more diverse and generally greater
matrix porosity, variously combined with fracture and
conduit systems. The touching-vug porosity of Vacher and
Mylroie (2002) is a specific sub-type of matrix porosity,
with greatly enlarged and interconnected pores-vugs, or it
can be considered as a sub-type of conduit porosity.
In a stratified system, vertical (layered) heterogeneity
in the original porosity and permeability structure is
commonly much greater than lateral. In addition to large
contrasts in bulk permeabilities of adjacent beds and
horizons, there are effects of limited connections between
different juxtaposed stratiform porosity systems (Figure 8
and 10). All these vertical heterogeneities play an
important role in configuring groundwater flow in general,
and particularly in ascending transverse speleogenesis.
In the simplest case, there is a thin homogeneous
fractured bed of limestone or gypsum, sandwiched
between diffuse permeability aquifers, in which each
fracture directly connects the bottom and top boundaries
(Figure 8-A). This type of “sandwich aquifer,” where a
thin carbonate unit is overlain and underlain by insoluble
strata, has been described by White (1969), who noted that
network caves are characteristic for this situation. In fact,
actual patterns of resultant caves are strongly dependent on
fracture distribution and arrangement. Network caves are
formed where there is a continuous fracture network
encased in the bed. If the soluble bed is only occasionally
fractured, single, laterally-isolated fissure-like passages
may form with both ends blind-terminated, or small
clusters of several intersecting passages. Illustrative
examples are caves encountered by mines in a thin
Miocene limestone bed in the Prichernomorsky artesian
basin, south Ukraine (see Figure 31).
Apart from major sedimentological heterogeneities in
the vertical section, such as alternating prominent beds of
contrasting lithologies that determine the principal
hydrostratigraphy in a basin, depositional environments
and facies changes within an otherwise “homogeneous”
soluble formation also play an important role in
determining secondary porosity and permeability
distribution and their subsequent evolution through burial
diagenesis and tectonism. Individual beds or formations
commonly differ in nature, patterns, and frequency of
fracture networks. Hence, these conditions will impose
strong control on the structure of subsequent hypogenic
speleogenesis.
In the Miocene gypsum formation in the western
Ukraine, which hosts the giant artesian maze caves, the
section is typically composed of two or three varieties of
gypsum differing in texture and structure. Each bed
encases laterally continuous extensive stratiform fracture
networks, largely independent of the network encased by
adjacent beds (Figure 8-B). Fracture orientation and
frequency differ between the beds (Klimchouk et al.,
1995), so fractures in one bed are rarely co-planar with
fractures in an adjacent bed, but they may have occasional
vertical connections at discrete points. Such discordance in
permeability structure between adjacent beds creates the
flow constraint effect and causes some lateral component
in the generally transverse flow. The same effect is caused
by discordance in permeability structure and overall values
between the lower and upper aquifers and respective
adjacent beds in the gypsum bed. Because of the lateral
component and good fracture connectivity at certain levels,
integrated systems of passages develop on such master
levels, which gives a misleading impression of generally
lateral cave-forming flow through a soluble unit or its
particular bed. Multi-story (three-dimensional) maze caves
with stratiform levels formed in this way may have a few
kilometers to a few hundreds of kilometers of laterally
integrated passages, which further favors the misleading
interpretation of the cave-forming flow to be lateral. In
cases where laterally connected fracture networks are subhorizontal,
the resultant cave levels are commonly
misinterpreted as levels in the evolutionary sense within
the epigenic paradigm, i.e. abandoned tiers of phreatic
development or cave levels at the water table. Another
common misinterpretation of such levels is that the
downward cave development was perched on the
underlying non-soluble bed (which is now a lowpermeability
bed as compared to the already karstified
soluble unit, but that used to be a feeding aquifer at the
time of early speleogenesis – see notes on the conversion
of the hydrostratigraphy above).
The above-described arrangement of the original (prespeleogenetic)
porosity is shown to be one of the main
controls for transverse ascending speleogenesis and the
structure of two to three story cave systems in the western
Ukraine (Klimchouk and Rogozhnikov, 1982; Klimchouk,
1990 and 1992; Klimchouk et al., 1995; see Figure 12).
The structure of the multi-story mazes of Wind and Jewel
caves in the Black Hills, South Dakota, USA is controlled
largely in the same way (Ford, 1989), although bedding,
superimposed stratiform fracture networks, and the
resultant cave “levels” here are dipping.