Download PDF - Speleogenesis

EPILOGUE
Epilogue
The major points and conclusions of this book are
summarized in the abstract. This book does not pretend,
and hence does not succeed, in covering all aspects of
hypogenic karst and speleogenesis or in providing
comprehensive regional overviews. Such an attempt would
probably be premature because the conceptual framework
of hypogenic speleogenesis is still newly established and
poorly integrated into the main body of karst science.
Instead, the main goal of this work was to consolidate the
notion of hypogenic karst as one of the two major types of
karst systems and to outline an approach that would help to
see the forest behind the trees. This approach implies that
speleogenesis should be viewed in the context of regional
groundwater flow systems (not only of local systems that
evolve through a soluble rock after its exposure), and their
evolution in response to basinal processes, uplift,
denudation and geomorphic development.
Various styles of hypogenic caves that were previously
considered unrelated, specific either to certain lithologies
(e.g. western Ukrainian giant gypsum mazes) or chemical
mechanisms (e.g. sulfuric acid caves or hydrothermal
caves) appear to share common hydrogeologic genetic
backgrounds. They were formed by ascending transverse
speleogenesis, which is responsible for the remarkable
similarity of their most characteristic morphologic
features. It is suggested that confined and semi-confined
settings are the principal hydrogeologic environment for
hypogenic speleogenesis, and that vertical heterogeneity in
permeability is the principal control over hypogenic cave
development. Evidence for this is overwhelming.
However, there is a general evolutionary trend for
hypogenic karst systems to lose their confinement due to
uplift and denudation and due to their own development.
Confined hypogenic systems may experience substantial
modification or be partially or largely overprinted under
subsequent unconfined (vadose) stages, either by epigenic
processes or continuing unconfined hypogenic processes,
especially when H2S dissolution mechanisms are involved.
This means that in dealing with unconfined karst settings
and epigenic caves, a possibility of inheritance from
hypogenic cave development should not be overlooked or
underestimated. It is likely that many caves, previously
explained from the perspective of established epigenetic
models, will be re-interpreted to more adequately account
for such inheritance.
Hypogenic confined systems evolve to facilitate crossformational
hydraulic communication between common
aquifers, or between laterally transmissive beds in
heterogeneous soluble formations, across cave-forming
zones. The notion of cross-formational hydraulic
communication, quite well established in mainstream
hydrogeology, was not properly realized in karst science.
Hypogenic speleogenesis is essentially a cross-formational,
transverse, phenomenon.
One of the main characteristics of hypogenic
speleogenesis is the lack of genetic relationship with
groundwater recharge from the overlying or immediately
adjacent surface. It may not be manifest at the surface at
all, receiving some expression only during later stages of
uplift and denudation. But long before this expression
occurred, and long before we got physical access to
explore them, hypogenic caves were already there! And
they are there 3 , at some depth beneath a non-soluble
confining cover, through vast areas normally not
considered as karst, based on the traditional, largely
geomorphological karst paradigm. There is abundant
evidence of hypogenic caves, including those more than
ten times greater than the largest cave chamber directly
explored by humans, encountered by wells and mines at
depths up to many hundreds of meters. Industry geologists
and hydrogeologists deal with them routinely, but rarely
karst scientists. However, those who deal with deep-seated
karst features often fail to adequately understand them
because the mainly epigenic models for karst and caves are
readily available from geosciences texts. So, the common
approach to deep-seated karst features is to put epigenic
karst models into paleokarst wrapping. But this does not
always help to effectively deal with karst-related issues,
simply because they are often related to hypogenic karst,
not to true paleo (epigenic) karst.
The refined conceptual framework of hypogenic
speleogenesis has broad implications in applied fields and
promises to make karst and cave expertise more highlyvalued
by practicing hydrogeology, geological
engineering, economic geology, and mineral resource
industries. Any generalization of the hydrogeology of karst
aquifers, as well as approaches to practical issues and
resource prospecting in karst regions, should take into
account the different nature and characteristics of
hypogenic and epigenic karst systems.
An appreciation for the wide occurrence of hypogenic
karst systems, specific characteristics of their origin and
development, and their scientific and practical importance,
calls for revisiting and expanding the current
predominantly epigenic paradigm of karst and cave
science.
3 This statement, however, implies not an anthropocentric
definition of caves but the notion of “a karst cave as an opening
enlarged by dissolution to a diameter sufficient for
‘breakthrough’ kinetic rates to apply if the hydrodynamic setting
will permit them. Normally, this means a conduit greater than 5-
15 mm in diameter or width,” after Ford and Williams (2007).
97