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14
NCKRI Special Paper No. 1
of potentiometric highs and lows are the areas of
downward and upward percolation, or zones of recharge
and discharge. Potentiometric highs correspond to
topographic highs, whereas potentiometric lows coincide
with topographic lows, most commonly river valleys.
Detailed studies of regional hydraulics in many
artesian basins around the world strongly support the
above general views. A brief summary of basinal
hydraulics that follows is derived from thorough regional
studies of the major Ukrainian (Shestopalov, 1981, 1988,
1989) and Canadian (Tóth, 1963; Hitchon, 1969a, 1969b)
basins to illustrate some important implications to
hypogenic speleogenesis.
At the basin scale, the overall surface topography,
along with lithologic and structural factors, controls
distribution of potentiometric highs and lows and
determines directions of cross-formational communication
across confining beds. Downward percolation prevails
along major divides, whereas notable upward flow is
characteristic of areas below river valleys and other
topographic lows. Regional confining formations of low
permeability reduce cross-formation communication
within multiple-aquifer systems, but do not isolate aquifers
completely. The effects of topography can be traced to
considerable depths in artesian basins. In particular, the
draining influence of major rivers commonly extends to
depths of 1000-1500 m and more (Shestopalov, 1989;
Hitchon, 1969a; Figure 4).
Permeabilities of confining beds generally decrease
with increasing depth. However, beneath river valleys the
permeabilities in confining beds are considerably (up to 10
times) greater compared to adjacent areas, due to the fact
that valleys often develop in tectonically weakened and
disrupted zones, and/or there is relaxation of rocks from
the load of formations removed by valley erosion.
Ascending transverse speleogenesis in soluble formations,
focused in the valley areas, and subsequent disruptions in
formations that lie above soluble ones, may further
contribute to this effect.
The rate of lateral flow through aquifers decreases
with depth, from the margins toward the interiors of
basins. However, beneath valleys a notable increase in
flow rates is characteristic even for the central parts of the
basins. Despite the fact that permeabilities of confining
beds diminish with depth, the rate of vertical water
exchange remains more stable than the rate of lateral flow,
so that the relative significance of cross-formation flow
increases with depth. The rate of upward percolation
across confining beds under valleys is generally higher
than the rate of downward percolation in the vicinity of
topographic divides.
The influence of adjacent structural uplifts and massifs
on basinal hydraulics is not only that the marginal recharge
areas are located there, but that within the edges of basins
there are particularly favorable conditions for enhanced
cross-formational communication between aquifers. This is
due to: 1) decreasing depths of aquifers and general
reduction in thickness of sedimentary cover; 2) gradual
transition of clay-rich facies to sandy-carbonate ones in
confining beds; and 3) increase of fracturing due to
weathering and unloading toward the basin edges
(Shestopalov, 1989). Hitchon (1969b) discussed the effects
of rapid recent uplifts in terms of upsetting the previously
attained steady-state equilibrium of fluid flow in a basin.
Figure 4. Hydraulic head distribution in cross sections, the Western Canada Sedimentary Basin showing the influence of topography,
particularly river valleys (redrawn from Hitchon, 1969a).