Download PDF - Speleogenesis

12
NCKRI Special Paper No.1
3. Ascending hypogenic speleogenesis
In karst and cave science, three major problems can be
traced that hindered proper understanding of hypogenic
speleogenesis. First, caves formed in hypogenic and
confined domains are accessible for exploration and study
largely when they are brought into the unconfined realm
due to uplift and denudation, hence when they become
relict, decoupled from their formational environment and
often partly overprinted by unconfined speleogenesis.
They were commonly interpreted, despite now apparent
contradictions, in the context of contemporary epigenic
conditions. Classical examples are the works of Davis
(1930) and Bretz (1942) who, in their theorizing of
unconfined speleogenetic conditions, included many caves
now known to have an ascending water origin (Ford,
2006). Although some of the most remarkable “atypical”
caves are now recognized to be of hypogenic origin (e.g.
Wind and Jewel caves of the Black Hills, South Dakota,
USA; Carlsbad Cavern, Lechuguilla Cave, and other caves
of the Guadalupe Mountains, USA; giant gypsum mazes of
western Ukraine), many other caves await reevaluation.
Another example is the former interpretation of the
western Ukrainian giant mazes as being formed by lateral
flow through the gypsum bed between entrenching subparallel
river valleys (Dublyansky and Smol'nikov, 1969;
Dublyansky and Lomaev, 1980). Second, even where a
hypogenic origin was assumed, speleogenetic regularities
and models devised for unconfined conditions were often
simply taken to be equally applicable to the largely
confined realm. And third, in earlier attempts to interpret
some caves (particularly network mazes) in terms of
artesian origin, old simplistic views of artesian flow were
commonly implied, which again led to apparent unresolved
contradictions.
Most stratified sedimentary basins are characterized by
considerable heterogeneity and large contrasts in vertical
permeability, which is, along with basin geometry
conditions, the main cause of the wide occurrence of
multiple-aquifer confined systems. The terms “confined”
and “artesian” refer to hydrodynamic conditions and imply
that groundwater is under pressure in a bed or stratum
confined above and below by units of distinctly lower
permeability. The potentiometric surface in such aquifers
lies above the bottom of the upper confining bed. These
terms are commonly used as synonymous and this usage is
adopted here, although “artesian” was originally applied to
aquifers in which the potentiometric surface lies above
ground level.
Confusion often arises when the terms “artesian”,
(“confined”) and “phreatic” are misleadingly understood as
being equivalent, especially where bathyphreatic
conditions are concerned. The term “phreatic” refers to
conditions where water saturates all voids in a rock or
sediment, in contrast to vadose conditions, above the water
table, where voids are water-filled only temporarily, if
ever. In this sense, phreatic unconfined and confined
conditions are alike. Moreover, water in phreatic conduits
is always confined by the host rock and possesses some
hydraulic head above the conduit ceiling. For example,
Glennie (1954) termed water rising from such deep
phreatic paths “artesian.” Jennings (1971, p.97) noted that
such usage is in a strict sense incorrect, but it serves as a
reminder that consolidated rock can act virtually as its own
aquiclude.
Klimchouk (2000a; 2003a) suggested limiting use of
the term “artesian” (“confined”) to prevailing flow
conditions in an aquifer or a multiple-aquifer system,
rather then to flow conditions within a single conduit. Use
of the term “phreatic” should be restricted to the lower
zone in unconfined aquifers, limited above by a water table
that is free to rise and fall. The distinction between
phreatic and confined conditions is important in the
context of speleogenesis (see Section 3.7).