Geology Reference
In-Depth Information
the host material (soil, sediment or rock) via a combi-
nation of cementation, replacement and displacement
(Gile et al. 1966; Arakel 1982; Machette 1985; West et
al. 1988; Rabenhorst et al. 1990).
Diagnostic criteria of caliche and paleocaliche:
see
Chap. 13 and Pl. 14/6, Pl. 128, and Pl 141/9-10.
and James 1988). Karst develops (a) at the air-lime-
stone or soil-limestone interface ('surface karst') and
(b) within carbonate bodies ('subsurface karst').
Diagnostic features:
Surface karst
or exokarst is
characterized by surface solution or corrosion features
comprising small-scale and large-scale criteria (micro-
karst, residual paleosols, karren, solution pans or
kamenitzas, and dolines or sinkholes). Large-scale geo-
morphological features (e.g. karst valley and karst tow-
ers) are responsible for characteristic landforms.
Sub-
surface karst
(see Pl. 129) is represented by caves and
vugs formed above, at and below the water table.
Caves contain (a) secondary mineral deposits (po-
rous calcareous tufa: moonmilk, and hard crystalline
sinter or speleothems) precipitated by vadose and shal-
low phreatic waters and (b) internal cave sediments and
karst breccia caused by the collapse of caves. Karst
breccias exhibit a wide variety of internal composition
depending on the site and the mode of origin (Sect.
5.3.3.3 and Sect. 15.2).
Speleothems
are precipitated from thin water films
flowing over the rock and forming flowstones and drip-
stones, or in pools of water, forming 'cave pearls'
(globoids, pisoids; vadoids: Chafetz and Butler 1980;
Peryt 1983. See Pl. 14/8). Dripstones include stalag-
mites and stalactites as well as finely curved twiglike
structures (helictites).
Carbonate sinters are made up of calcite with vari-
ous amounts of Mg, and/or aragonite, dolomite and rarer
minerals (Gonzalez and Lohmann 1988; Tietz 1988).
The mineralogical composition is often seasonally con-
trolled; it depends on the Mg/Ca ratio of the percolat-
ing pore-fluids, pCO
2
, and evaporative conditions
within the cave.
Speleothems exhibit diagnostic cave cements (Folk
and Assereto 1976; Kendall and Broughton 1978;
Assereto and Folk 1980) which can be used in order to
recognize ancient cave carbonates (Sect. 15.2.1). Com-
mon cement types in speleothems are length-slow and
length-fast calcites and dendritic calcite crystals.
Changes in mineralogical composition and cement
types of vadose-zone speleothems may reflect short and
long-ranging cyclic oscillations of climatic factors
(Railsback et al. 1994; Shopov et al. 1994). There is
increasing evidence for microbial contributions to the
growth and fabric of speleothems (Danielli and
Edington 1983; Jones and MacDonald 1989).
Controls:
Prerequisites for the formation of extant
karst and cave systems are fluids that are undersatu-
rated with respect to the country rock, and fluid flow
transporting dissolution products (Lohmann 1988;
Dreybrodt 1990). The most important intrinsic factors
influencing karst development are the general lithol-
2.4.1.2 Palustrine Carbonates
Palustrine limestones are deposits of shallow freshwa-
ter environments exhibiting extensive evidence of pe-
dogenic modification (Pl. 48). These carbonates are
common in many ancient continental and marginal-
marine sequences. The term (derived from 'paludal',
meaning marshy or swampy) was first proposed while
describing late Cretaceous-Early Tertiary limestones in
southern France (Freytet 1973, 1984; Freytet and Plaziat
1982).
Palustrine carbonates were originally considered as
near-shore deposits of extremely shallow lakes with os-
cillating lake levels and densely vegetated shorelines.
Because of a very low relief on the lake bottom, fluc-
tuation in the water table should result in an interbed-
ding of lacustrine facies (characterized by the occur-
rence of specific fossils), and pedogenic facies (char-
acterized by textures which indicate a pedogenic over-
print of dried-out parts of shores).
This interpretation presents problems since many
palustrine limestones are not associated with lake de-
posits. A suitable modern analogue for a palustrine en-
vironment may be the freshwater carbonate marshes of
the interior Everglades in Florida (seasonal wetland
model: Platt and Wright 1992). Seasonal variations in
water depth and minor topographic variations across
the area could explain the association of soil and cal-
crete criteria with dissolution features (
pseudo
-
microkarst
), and the presence of sublittoral carbonates
which were formed in perennial shallow lakes within
the wetland.
2.4.1.3 Cave Carbonates, Speleothems and Karst
Speleothems (carbonates formed in caves) are impor-
tant paleoclimatic recorders documenting short- and
long-term climatic fluctuations.
Terminology:
The term karst refers to physical struc-
tures formed by dissolution from meteoric waters. It
includes 'all of the diagenetic features - macroscopic
and microscopic, surface and subterranean - that are
produced during the chemical dissolution and associ-
ated modification of a carbonate sequence' (Choquette
