of Dissolutional Cave Development ... lithology and geologic structure impose on most cave genesis: rock purity ...... signifi cant "hidden" discontinuities into a.
Lithologic and Structural Controls of Dissolutional Cave Development 32 .
.
Alexander Klimchouk and Derek Ford Abstract
This chapter summarizes the important general controls that lithology and geologic structure impose on most cave genesis: rock purity, the presence of interbedded clastic rocks and adjacent or interbedded sequences of sulfates and carbonates, and various kinds of initial porosity, fissures in particular. Lithologic and structural conditions for speleogenesis evolve throughout sedimentation, eogenesis, mesogenesis, and telogenesis, and change drastically between these stages. Inheritance in the evolution of different kinds of prespeleogenetic porosity causes increasing heterogeneity in their distribution and parameters, which reaches the highest degree after burial at the stage of rock emergence to the shallow subsurface and the surface. The importance of fabricselective porosity and stratigraphic elements diminishes with time in favor of fissure-network porosity. Fissures evolve at different stages of the rock evolution. Networks are composed of complex planar and curvilinear surfaces interconnecting in three dimensions, constructed from fissures of
various origins, generations, and ages. The initial structural conditions for speleogenesis thus can be highly varied depending at which particular stage speleogenesis commences. Conditions in deep-seated settings favor uniform speleogenetic development, whereas in shallow settings increased heterogeneity in fissure parameters can favor selective development. Modeling of conduit initiation and early development needs
to take into account a great variability of initial permeability structures between common geologic environments and evolutionary stages, especially rather dynamic nondissolutional changes of these structures in shallow settings. Introduction
The dissolutional cave systems that we explore will develop only in certain types
of rock under particular hydraulic conditions.
The purpose of this chapter is to summarize the important general controls that lithology and geologic structure impose on most cave genesis. In Chapter 3 3 another author presents perspectives on the significance of bedding or other .
parting planes and contacts in cave development. Chapter 3.4 then switches the focus to the hydraulic setting. Chapter 5 includes many individual case studies, grouped according to major speleogenetic environments, in which lithology and structure are discussed. Structural
seacoast or in a river gorge, we often notice that caves appear to be developing preferentially in particular beds or other patches of the rock. This may be due to favorable variations in the composition of the rock itself. They are considered first in this chapter, under the heading Factors of Lithology." More often, when in a large passage "
inside a cave, we note that it straddles
several beds whose slightly different resistances to dissolution are marked by some projection or recession. But much more apparent is the fact that the passage originated in some linear structural feature such as a bedding plane, joint, or fault which guides it. The location and orientation of most
segments of most caves are controlled by
controls of the shape of individual conduits
such fissures or the intersection of two or
and dissolutional microforms are also
more of them. This is discussed at greater length in the second part of the chapter Fissures." Evolution of effective porosity
discussed in Chapter 6.1. Looking at a great limestone cliff on the
,
"
55
5 Geologic and Hydrogeologic Controls-3.2 Lithologic and Structural Controls
3 the rock and their fissures during diagen-
near the sedimentation surface during eogenetic stages. But deeper seated mesogenetic dolomite and even incipitently-ended telogenetic dolomite forms in places where
eas is considered in the final section of the
chapter.
thermal water passes through limestone. Both kinds of rock may also contain nodules,
Factors of Lithology
stringers, or thin sheets of chert (precipitated Carbonate Rocks-
silica), which are much less soluble.
Limestone and Dolostone
The composition, depositional settings and diagenesis of the carbonate rocks are very varied. As a consequence it is claimed that there are more distinct types of limestone and
A popular classification of types of limestone :
i shown in 3.2 Table 1. Allochthonous
limestones are made of calcium carbonate
CaCCy debris such as shells and skeletons whole or fragmented) and fecal pellets :einented by calcium carbonate mud precipitated from warm sea water ( micrite ) :r larger crystals that result from early issolution and reprecipitation ("sparite"). They accumulate as detritus in environments ranging from the deep sea (many
dolostone than there are of all other sedimen-
mudstones) to intertidal flats or even
also give more details, with emphasis on their significance in karstification.
tary rocks combined (James and Choquette,
,
1988). For more details, interested readers
"
"
should consult some of the many books that are dedicated to them, such as Bathurst
] Rock Purity \ This is a very important factor in cave and \ karst development. A good working picture of the carbonate and calcareous [ (carbonate-rich) rocks is given in 3.2 Figure 1, p. 56. Calcite is much more I soluble than dolomite (see Chapter 4.1). In many karst areas, beds or zones of dolostone are seen to reduce or prohibit i cave development. However, in some I extensive caves in dolostone the fissuring | and hydraulic setting are suitable; generally, this implies comparatively large j fissure apertures, high hydraulic gradients, | or long periods of dissolution. Clay minerals and silica are the most | common insoluble impurities in the carbonate rocks. Limestone with more
(1975), Scholle et al. (1983), Moore (1989), Budd et al. (1995). The karst textbooks by
than 20 to 30% clay (argillaceous limestones, 3.2 Figure 1) forms little karst, probably because the abundant clay particles clog any tiny precave or protocave voids. Sand grains, being larger, create pore openings between them and so may not obstruct water flow as effectively.
White (1988) and Ford and Williams (1989)
rreshwater lake bottoms. Shallow lagoonal res tend to predominate in many well "
Evaporite Rocks Gypsum, anhydrite, and salt are much
ieveloped karsts, as noted. Here they usually display distinctive layering termed "beds" or bedding." Autochthonous limestones are buildups of coral, alga, and other immobile marine organisms that secrete and trap
simpler deposits, accumulating on lake and
"
sea floors, in intertidal zones, and coastal
calcium carbonate. Dunes and sandbanks
form similar mounds, but of inorganic origin. Some dolostones form as primary precipitates of the mineral dolomite
CaMg(C03)2), usually in intertidal settings where they may be interbedded with gypsum. More are secondary formed when limestone is first dissolved and then
caves are associated with bulk purities of greater than 90% CaCOj or CaMg(C03)2.
as a consequence.
Most individual beds or continuous
The diagenetic alteration may be profound (for example, gypsum to anhydrite,
formations of the sulfate and salt rocks do
structural heterogeneity at varying scales.
reprecipitated from water that is enriched ia Mg ions, as seawater usually is. The extent of the conversion can range from very little to complete. Most is believed to occur
However, most limestone and dolostone
marshlands, as a consequence of evaporative concentration of ions. Most evaporites display a strong layering (bedding) structure
or recrystallization) and may seal or destroy layering structure in some cases, overprinting other kinds of textural and
,
Some well-formed caves are known to
I develop in calcareous sandstone.
Interested readers may consult some recent reviews for details (Klimchouk, 1996;
not have enough sand or clay to prohibit caves.
! Interbedded Clastic Rocks The beds of .
[ carbonate, sulfate, or salt in many formations [ are without any clastic interbeds (layers of i clay, shale, sandstone, coal) of similar
Klimchouk and Andrejchouk, 1996a).
thickness for sequences of 10s to 100s of '
meters, even 1000 s of meters of beds in
Allochthonous limestones
Autochthonous limestones
original components not organically bound during deposition
Less than 10% >2 mm components
original components organically bound during deposition
ยป
Greater than 10%
By organisms
By organisms
By organisms
which act as baffles
which
which build
encrust
a rigid framework
No lime mud
Contains lime mud (.03 mm 2 mm
supported
component
Grain supported
Greater than 10%
grains Wackestone
Packstone
Grainstone
3
Floatstone
Rudstone
Bafflestone
2 Table 1. The R.J. Dunham (1962) classification of carbonate rocks, as modified by Embry and Klovan (1971). .
Bindstone
Framestone
Speleogenesis
56
impurities
dissolved over large areas, producing prominent flow paths that may influence later speleogenesis. Dissolution of
100%
thicker gypsum or salt can remove the structural support from overlying
in place of (impure) use -
quartzose (sandy),
carbonates, causing disturbance that ranges from enhanced fissuring to complete brecciation, depending on the rate and pattern of leaching and the thickness of the leached rock. It may greatly affect subsequent speleogenesis.
mixed rocks withsome carbonate
argillaceous (marly), glauconitic, cherty, etc.
(e.g. calcareous Such deformation and brecciation are
sandstone dolomitic shale
widely reported (see for example the discussions in Klimchouk and
(impure
Andrejchouk, 1996b; Ford, 1997; Friedman, 1997; James and Choquette, 1988; Palmer, 1995), although it is sometimes misinterpreted as results of
(impure)
/calcareous
dolomitic
dolomite
limestonf
o 03
cavern collapse in carbonate rocks alone.
10% calcareous
dolomitic limestone
dolomite
90%
dolomite
50%
Porosity in the Karst Rocks 10%
calcite
dolomite as % of carbonates 2 Figure 1. A bulk compositional classification of carbonate rocks by Leighton and Pendexter (1962) (by permission of American Association of Petroleum Geologists). 3
.
shale and sandstone sequences with limestone, dolostone, or gypsum interbeds, and the hydrogeologic systems grade from wholly karstic to nonkarstic.
Valid generalizations concerning cave development are difficult for intermediate conditions. As the frequency of shale beds
contained within a few meters of marble.
Another outstanding example is Botovskaya Cave in Siberia, described in Chapter 5.2.3.
in exposed settings generally do not favor conduit development. An obvious
Adjacent or Interbedded Sequences of Sulfates and Carbonates. These may influence fluid low and speleogenesis in several ways:
exception is fresh water/sea water mixing speleogenesis, which relies on location at the contact between contrasting hydrogeochemical zones within a porous aquifer rather than on any specific structural prerequisites.
(1967) described caves passing through mountains of schist and quartzite in Canada,
.
increases in a formation, so does the
compact sulfates. Bedding planes (especially unconformities) between carbonates and sulfates are particularly important.
but this is not always true. Groundwater penetration to initiate caves is often easier at a contact between .
Dissolutional conduit systems may develop in adjoining sandwiched limestones or
"
"
.
a kind of feedback mechanism between
diagenesis and the flow pattern. Heterogeneity of porosity and permeability due to diagenetic processes particularly increases during the periods of transition from
fluids to adjacent carbonates. Collapse of
basinal circulation of connate and
discrete cavities causes local deformation
resurgent fluids to meteoric groundwater circulation, with corresponding changes in hydrogeochemistry. Under favorable geochemical and
termed breccia pipes or geologic organs that can become important elements in improving cross-formational fluid flow.
to form and attain substantial length or depth
Initial differences in matrix porosity and its subsequent diagenetic redistribution and modification can play an important role in the guidance and focusing of fluid flow along certain horizons or zones. This further increases heterogeneity because of
Sulfates are much more readily dissolved in deep settings than carbonates, so dissolution and speleogenesis in sulfate beds becomes a factor in conveying
and fracturing in overlying carbonates, and under certain conditions, will trigger the upward stoping of vertical structures
sulfates. Just a few meters of pure karst rock beds in a clastic formation may permit caves if the hydraulic setting is favorable. This stripe karst is the dominant style of cave development in Norway, for example. Ford
Because carbonates have higher initial porosity, they commonly provide an access for fluids to the more soluble but
likelihood that the intervening limestone will be argillaceous and thus without caves,
limestone and shale-because of diagenetic effects or differential slippage there-than it is at bedding planes within the limestone. Sandstone interbeds often provide even more effective initial flow paths within an otherwise carbonate sequence.
total volume. Effective porosity is that which is sufficiently interconnected that it can be drained under given physical conditions such as by gravity.
Matrix Porosity. It is common to view the fabric selective bulk porosity as being of negligible importance for speleogenesis. This is supported by observations that very porous but little fissured young limestones
f
some carbonate groups. These strata generally yield the greatest cave development. However, many geologic formations are mixtures of beds or packages of beds of the clastic and soluble rocks. These grade to
The porosity of a given rock is the aggregate amount of void space in it, normally expressed as a percentage of the
Thin sulfate beds within carbonates or
intercalated sequences can be entirely
hydrodynamic conditions-particularly where cross-formational hydraulic
communication permits waters with contrasting chemistry to mix-this may lead to conduit inception and development,
3
.
57
Geologic and Hydrologic Controls-3.2 Lithologic and Structural Controls
either along the mixing zones themselves or in adjacent more compact and fissured zones. Thus, enhanced heterogeneity in
A
matrix porosity, either primary or due to diagenesis, can form types of structural guidance for subsequent conduit inception and development.
The development of macroscopic porosity of any kind can influence diagenetic processes in the rock matrix. For instance, Cander (1995) demonstrated
B
b cm ,
that in the quite young carbonates of the Floridan aquifer, the development of conduit flow reduces diagenetic alterations in the matrix because water-rock interaction becomes focused in the conduits and
decreased elsewhere. This changing relationship between matrix porosity and focused diagenesis around conduits (the latter falling into the realm of speleogenesis) is an example of a widespread characteristic of the hydrogeologic evolution of karst aquifers over time-that is, a decrease of the proportion of diffused
0 20 .
/
0 15
y
/
.
7 .
0 10 .
t
f
low in favor of conduit flow.
Porosity in diagenetically immature carbonates ranges between 25 and 80%. In well indurated carbonates it is greatly reduced and tends to be positively correlated with grain size and textural heterogeneity, although there is much variation. Porosity of micrite is generally
*
t
0 05 .
t
0
,
99
30
less than 2%, that of sparite between 5 and 10%. Dolomitization increases porosity to 5 to 15% in most instances. The porosity of metamorphosed carbonates is low, being
50
70
99.9 99.99
P.o/o
3 2 Figure 2. Distribution offissure width: A is a sketch of a fissure profile. B is a probability plot (from Chernyshev, 1983). .
less than 1 % in marbles.
Evaporites anneal readily and so tend to have negligible bulk porosity. The thermodynamic stability and solubility of gypsum and anhydrite are greatly affected by changes in the physical and chemical parameters that occur within common geologic environments. This is why conversions of gypsum to anhydrite and back to gypsum are common processes during diagenesis, as well as their recrystallization. Gypsum may acquire a considerable intercrystalline porosity as a result of these processes. The more important consequence is that substantial heterogeneity in rock texture, structure, and hence mechanical strength may develop across an individual bed or unit, which can cause networks of fissures to
form within it that are largely independent of those in the adjoining beds (Klimchouk etal., 1995).
Fissures
Bedding planes, joints, and faults are planar breaks that serve as the principal structural guides for groundwater flow in almost all karstified rocks. In hydro-
geology it is customary to categorize all of them together as fractures forming fracture aquifers. This is somewhat misleading, however, because most bedding planes originate as primary depositional features in '
the rock rather than as later fracture s
through
it. Here we use fissures as a general term. In common use it refers to breaks that are
visibly open, or gaping. Here we use it: ,
to encompass all planar breaks that can be significantly penetrated and modified (by
dissolution or precipitation) by circulating groundwaters, past or present.
closed and so prohibiting groundwater flow. For speleogenetic purposes, it is best to think ; in terms of the water in a fissure passing from more widely open areas to others down the hydraulic gradient via constrictions or throats highly variable in form and dimen1 sion. Minimum throat apertures for cave development will usually exceed 10 micrometers, and are probably greater than 100 micrometers in most instances. In 3.2
Figure 2 is an example of apertures measured in sample joints given in Chernyshev (1983). The area of a fissure surface that is completely closed (i.e., there
Structural engineers concerned with slope stability often apply the term penetrative discontinuities to such features (e.g. Cruden
is bedrock-to-bedrock contact with no
1976).
meters.
Except where they are truly gaping, the physical nature of these breaks is complex and difficult to measure. In quantitative modeling it is standard practice to treat the hypothetical fissure as a fracture with strictly parallel walls spaced a fixed distance apart. In reality this will never apply. Separation in the fissure is spatially most variable, with some areas of it usually being completely
The genetic importance of the different types of fissures summarized below can also vary a great deal between (and even within) individual caves. In some, bedding planes or contacts are quite predominant, and joints or faults play a very subordinate role.
aperture) is estimated to be no more than 1%, at least to a depth of several hundred
Mammoth Cave, Kentucky, (Chapter 8) is a prime example. Joint control is paramount in rectilinear maze caves such as 53 .
.
58
Speleogenesis
seem to be comparatively rare (Grotte di Castellana, Italy, is an example) but fault planes are important in many. Perhaps a '
majority of the world s larger meteoric-water caves (Chapter 5) follow combinations of these different types in differing proportions, plus their intersections (e.g. joints with bedding planes). Bedding Planes and Contacts Bedding or parting planes in sedimentary rocks are produced by some change in sedimentation or an interruption of it. The
change may be minor, e.g. from one size of carbonate grain to another a little bigger. Major changes are represented by big differences in grain size or homogeneity and, more often, by the introduction of clay by a storm or flood that leaves a paper-thin or thicker parting between the successive layers of karstic rock. Interruption is usually a brief marine emergence with some erosion and the start of meteoric diagenesis or beachrock formation, before there is submergence again. Many of the smaller subaerial unconformities that occur in young rocks tend to be diminished during diagenesis because they are preferred sites for mineral infilling or for obliteration by stylolite formation (pressure solution). The more prominent interruptions
persist as minor geologic disconformities and are widely used to divide layered sequences into individual formations, or members within formations. These may be referred to as contacts. The junctions between layered sediments and mound structures such as reefs
in carbonate deposits are another type of contact that is often preferentially penetrated by groundwaters. Only a minority of sedimentologic bedding planes are penetrable by groundwater under reasonable pressure gradients so as to function as fissures to guide cave initiation and development. The rock between any pair
of such planes thus constitutes an individual bed for speleogenetic purposes (although some impenetrable planes within it may later rupture under mechanical stress in a
Bed thickness (cm) 1001000
30-100 10-30 3-10
very thick bedded or
breakdown). A standard sedimentologic classification of bed thickness (i.e. of bedding plane separation) and of joint spacing in beds or sequences of beds is given in 3.2 Table 2. The areal extent of penetrable bedding planes or contacts varies considerably. Where bedding is thin to very thin, they may cover only a few square meters. Where it is medium or much more. Truly major planes can be followed throughout a formation, sometimes
massive. In thinly bedded rocks the dissolutional attack is too dispersed amongst the closely spaced planes and densely packed joints that interlink them. They also lack the mechanical strength to support large void spaces without breakdown. However, many cyclic depositional sequences in which bedding thickness varies in a systematic manner yield narrow packages of thin beds between greater units of massive ones.
for hundreds of kilometers. As a conse-
Penetration and cave initiation then often
quence, major bedding planes can be
is in the thin package (3.2 Plate 1, p. 61).
to thick, the extent is normally 103 to 106 m2
considered to be continuous entities when
solution caves are propagating through them, whereas joints and most faults are discrete (they terminate in comparatively short distances). This enhances the significance of bedding planes in cave genesis (Ford, 1971). Bedding planes and contacts most readily exploited by groundwater include those with substantial depositional disconformities, plus
planes with shale laminae or thicker partings (often with disseminated pyrite) and planes with nodules or sheets of chert. Perhaps the most important are those that have served as surfaces of differential slippage during tectonic events. Even if the slippage is just a few centimeters there is some slickenside
striation and brecciation that enhances
openings. Most steeply tilted and all folded strata will display some measure of differential slip. Where an overburden load is removed
abruptly, previously impenetrable bedding planes may open as a consequence of pressure release. The retreat of a thick glacier is the principal unloading mechanism. The opened planes have been termed bedding joints or sheeting fractures by
| hydrogeologists. However, the latter term should more properly be limited to the less equivocal sheeting in plutonic rocks or in limestone around quarries. Pressure release opening appears to be limited to the top one meter of strata or less in the carbonate rocks
j of Ontario, which were buried by 1500-2000 j m of ice during the last glaciation.
For caves of enterable size to develop, the bedding must normally be medium to
Joint Spacing (cm) >300 very wide
massive
thick medium thin
100-300 30-100 5-30
1-3
very thin
< 1.0
laminated
