Download PDF - Speleogenesis

Abstract
This book provides an overview of the principal
environments, main processes and manifestations of
hypogenic speleogenesis, and refines the relevant
conceptual framework. It consolidates the notion of
hypogenic karst as one of the two major types of karst
systems (the other being epigenetic karst). Karst is viewed
in the context of regional groundwater flow systems,
which provide the systematic transport and distribution
mechanisms needed to produce and maintain the
disequilibrium conditions necessary for speleogenesis.
Hypogenic and epigenic karst systems are regularly
associated with different types, patterns and segments of
flow systems, characterized by distinct hydrokinetic,
chemical and thermal conditions. Epigenic karst systems
are predominantly local systems, and/or parts of recharge
segments of intermediate and regional systems.
Hypogenic karst is associated with discharge regimes of
regional or intermediate flow systems.
Various styles of hypogenic caves that were
previously considered unrelated, specific either to certain
lithologies or chemical mechanisms are shown to share
common hydrogeologic genetic backgrounds. In contrast
to the currently predominant view of hypogenic
speleogenesis as a specific geochemical phenomenon, the
broad hydrogeological approach is adopted in this book.
Hypogenic speleogenesis is defined with reference to the
source of fluid recharge to the cave-forming zone, and
type of flow system. It is shown that confined settings are
the principal hydrogeologic environment for hypogenic
speleogenesis. 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 expansion. Confined hypogenic caves may
experience substantial modification or be partially or
largely overprinted under subsequent unconfined (vadose)
stages, either by epigenic processes or continuing
ABSTRACT
unconfined hypogenic processes, especially when H2S
dissolution mechanisms are involved.
Hypogenic confined systems evolve to facilitate
cross-formational hydraulic communication between
common aquifers, or between laterally transmissive beds
in heterogeneous soluble formations, across cave-forming
zones. The latter originally represented low-permeability,
separating units supporting vertical rather than lateral
flow. Layered heterogeneity in permeability and breaches
in connectivity between different fracture porosity
structures across soluble formations are important
controls over the spatial organization of evolving
ascending hypogenic cave systems. Transverse hydraulic
communication across lithological and porosity system
boundaries, which commonly coincide with major
contrasts in water chemistry, gas composition and
temperature, is potent enough to drive various
disequilibrium and reaction dissolution mechanisms.
Hypogenic speleogenesis may operate in both carbonates
and evaporites, but also in some clastic rocks with soluble
cement. Its main characteristic 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. In many
instances, hypogenic speleogenesis is largely climateindependent.
There is a specific hydrogeologic mechanism inherent
in hypogenic transverse speleogenesis (restricted
input/output) that suppresses the positive flow-dissolution
feedback and speleogenetic competition in an initial
flowpath network. This accounts for the development of
more pervasive channeling and maze patterns in confined
settings where appropriate structural prerequisites exist.
As forced-flow regimes in confined settings are
commonly sluggish, buoyancy dissolution driven by
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