Download PDF - Speleogenesis

HYPOGENIC CAVE FEATURES
brought into confined systems from overlying formations
during the late artesian stages, mainly via breakdown
structures.
12. Fluvial sediments are often absent but they can be
present locally where invasion streams have superimposed
onto a hypogenic system during exposure of the host
formation and erosional entrenchment. Widespread
deposition of backflooded silt sediments can occur during
transitional stages.
13. Hypogenic caves are often barren of common
infiltration speleothems unless the protective caprock is
largely or entirely stripped. If the latter is the case, they
may have abundant speleothems. “Exotic” minerals are
often present, indicative of particular geochemical
processes involved either in the cave formation or
(particularly) in later transformations of geochemical
environments during transition from confined to
unconfined conditions. Hydrothermal minerals and
minerals deposited as the products of redox reactions
characteristic of transitional zones are common.
Although briefly summarized above, morphological
features (cave patterns and cave mesomorphology) deserve
more detailed consideration because they are most
important for identifying cave-forming hydrogeologic
environments, and thus the origin of their caves.
4.2 Cave patterns
Hypogenic caves display variable, often complex
patterns. Branchwork caves, with passages converging as
tributaries in the downstream direction, the most common
pattern for epigenic speleogenesis, never form in
hypogenic settings. Elementary patterns typical for
hypogenic caves are network mazes, spongework mazes,
irregular chambers, isolated passages or crude clusters of
passages, and rising shafts. They often combine to form
composite patterns, including complex 3-D structures. A
variant of such complex patterns is distinguished as a
ramiform (ramifying) pattern, which Palmer (1991, 2000a)
described as “caves composed by irregular rooms and
galleries in a three-dimensional array with branches, that
extend outward from the central portions”“. This
description, however, is simply morphological and does
not necessarily reflect organization (outward) of the caveforming
flow.
Network maze patterns are most common for
hypogenic caves. Passages are strongly controlled by
fractures and form more or less uniform networks, which
may display either systematic or polygonal patterns,
depending on the nature of the fracture networks.
Systematic, often rectilinear, patterns are most common,
reflecting tectonic influence on the formation of fracture
networks. Polygonal patterns are guided by discontinuities
of syndepositional or diagenetic origin. Examples include
predominantly polygonal networks in the upper story of
some western Ukrainian gypsum mazes, guided by early
diagenetic structures (Figure 15) and a polygonal network
of Yellow Jacket Cave in the Guadalupe Mountains, New
Mexico, USA, guided by tepee-type syndepositional
structures (Figure 16; see also Plate 16). Fracture and cave
networks displaying different patterns may be present
within a single area or at various stories/parts of a single
cave, especially when confined to different rock units.
Figure 15. Variations in joint patterns and inherited maze patterns
between different horizons of the Miocene gypsum bed in the
western Ukraine, example from Optymistychna Cave. From
Klimchouk et. al. (1995).
Spongework maze patterns are less typical than
networks. Highly irregular passages develop through
enlargement and coalescing of vuggy-type initial porosity
in those horizons of the cave formation that have no major
fractures but interconnected pores and vugs. Clusters or
levels of spongework-type cavities are commonly
combined with other patterns in adjacent horizons to form
complex cave structures. An enlarged version of
spongework, locally called boneyard, is represented in
parts of some caves of the Guadalupe Mountains. In many
hypogenic caves, it seems that rising buoyancy currents
play a significant role in late stages of spongework
development.
31