Download PDF - Speleogenesis

8
NCKRI Special Paper No.1
2. Karst in the context of the systematized and hierarchical
nature of regional groundwater flow
Artesian basins are principal hydrogeologic structures
at regional scales in predominantly layered sedimentary
rocks (sedimentary basins) that contain stratiform
groundwater bodies (layered aquifers); and
hydrogeological massifs are tectonic block-faulted
groundwater bodies with an overwhelming dominance of
crosscutting fissure-conduit permeability (Zaitzev and
Tolstikhin, 1971; Pinneker, 1977). Transitional types
include disrupted basins and layered massifs. In cratonic
regions and their passive margins, large artesian basins
predominate, with subordinated hydrogeological massifs.
Folded orogenic regions are characterized by the
dominance of hydrogeological massifs, although small
artesian basins are also common. Basins and massifs are
commonly hydraulically connected, with massifs playing
the role of marginal recharge areas.
Broad understanding of karst processes as a
geological agent, one of the most powerful and universal
illustrations of groundwater as a geological agent, is
based on the growing recognition in mainstream
hydrogeology of hydraulic continuity, the systematized
nature and hierarchical organization of regional flow, and
the great role of cross-formational communication in
multiple-aquifer (multi-story) confined systems (e.g.
Pinneker, 1982; Sharp and Kyle, 1988; Shestopalov,
1981, 1989; Tóth, 1995, 1999). Principal categories of
karst-forming environments and resultant karst/
speleogenetic styles can be adequately understood and
classified only within the context of regional groundwater
flow systems, as they are regularly associated with
distinct segments and evolutionary states of these
systems. The works of Tóth (1995, 1999) provide a
particularly useful and inspiring synopsis of the nature of
the system, hierarchical organization, and the geologic
role of regional groundwater flow systems.
Speleogenesis, like other natural effects produced by
groundwater flow systems, is a result of interaction
between groundwater and its environment, driven by the
various components and attributes of the two respective
systems seeking equilibration (Tóth, 1999). To cause
speleogenetic development, dissolution effects of
disequilibria have to accumulate over sufficiently long
periods of time and/or to concentrate within relatively
small rock volumes or areas. The systematic transport and
distribution mechanism capable of producing and
maintaining the required disequilibrium conditions is the
groundwater flow system (Tóth, 1999). This is the single
fundamental reason why the principal categories of karst
and speleogenetic environments should be distinguished
primarily on the basis of hydrogeologic considerations,
rather than by the particular dissolutional mechanisms
involved.
The development of groundwater circulation is
broadly cyclic. The hydrogeologic cycle begins with
marine sedimentation that is succeeded by tectonic
subsidence and the formation of connate waters. It then
encompasses uplift, with denudation and progressive
invasion of meteoric waters into the reservoir. It may
include the intrusion of magma with release of juvenile
waters. It closes with a new marine transgression.
Groundwater circulation in a basin adjusts to the
pattern of maximum and minimum fluid potentials.
Large-scale groundwater flow in sedimentary basins can
be driven by several forces, such as sediment compaction
due to burial or tectonic compression, dehydration of
minerals, continental landscape topography gradients, and
density gradients due to temperature or solute variations.
Following uplift and establishment of the continental
regime and topography, gravity-driven flow systems of
meteoric groundwater increasingly flush out connate and
resurgent waters from a basin, although compaction-