Geology Reference
In-Depth Information
rocks from the Upper Proterozoic onward (Bosak et al.
1989). Large-scale products of subaerial exposure have
remained rather similar since the Mid-Proterozoic.
However, the amount of karst formation per time unit
should have been lower during the Cambro-Devonian,
owing to the absence of terrestrial vegetation cover prior
to the Carboniferous.
formed laminae; Pl. 129/2, 3, 5), (b) microbreccias (con-
sisting of angular rock clasts and broken crystals em-
bedded within a microgranular matrix; Pl. 129/1),
(c) coarse redeposited calcite crystals and fine crystal
silt, and (d) internal vadose sediments as partial fill-
ings on floors of cavities or as matrix sediment in col-
lapse breccias; often grayish-yellow, silty and lami-
nated.
•
Speleothems
are among the more useful features for
recognizing karstification (Dreybrodt 1988; Frisia et
al. 2000). The surface of cave bedrocks may be coated
with white layers consisting of coarse sparry calcite
and explained as precipitation from phreatic conduits.
Distinctive criteria of paleospeleothems are (a) lami-
nar flowstones consisting of several cement layers,
growing directly on host rocks and associated with geo-
petal vadose internal sediment, (b) dendroid calcite sin-
ter crusts forming stalagmite and stalactite structures
(Fig. 15.4), (c) fibrous calcite cement crystals devel-
oped as optically length-slow 'coconut-meat calcite'
(Folk and Assereto 1976) or palisade calcite, often as
radiaxial crystals, (d) helictites (excentriques) in fis-
sures (Reinhold 1998), (e) alterations of large and small
crystal layers, and (f) coarse blocky calcite, occluding
voids.
• Carbonate cements
associated with local paleokarst
were studied by Dunham (1969), Steinen and Matthews
(1973) and Schönlaub et al. (1991), those of interre-
gional paleokarst by Meyers (1974, 1978, 1988). Most
15.2.1 Diagnostic Criteria of Paleokarst and
Paleospeleothem Structures
Common features of paleokarst include geomorphic-
stratigraphic characteristics, field-scale observations
and microscopic criteria (Choquette and James 1988;
Esteban and Klappa 1983; Wright 1991).
The study of paleokarst requires observations at out-
crop, macroscopic and microscopic scales as well as
geochemical studies. Valuable criteria include breccias,
dikes and fissures, cave infill sediments, speleothems,
carbonate cements, and silcretes and calcretes.
•
Breccia:
Collapse breccia (Pl. 129/1; Fig. 15.3)
formed in ancient caves are clast-supported mega- and
micro-breccias consisting of angular, sometimes also
subrounded millimeter- to tens of centimeter-sized
limestone clasts. Sorting is generally poor. Fitting may
be good or poor. The matrix is calcareous or siliciclas-
tic depending on the lithology of the host rock and that
of the overlying beds. Karst collapse breccia should
not be confused with breccia produced by interstratal
dissolution of interbedded sulfates (see Sect. 5.3.3.3).
•
Dikes and fissures
:
Filled and cemented fissures (Pl.
129/1) with widths ranging from millimeters to deci-
meters are common features of paleokarst. Fissures may
be cemented by several generations of isopachous and
granular coarse calcite cement and filled with internal
sediment, consisting of crystal silt, and clasts derived
from cave walls as well as from overlying beds. Cor-
rugated marine radial fibrous cement crystals may in-
dicate karst phases (Aubrecht 1997). Fissure filling oc-
curs subaerially by continental sediments or, subsequent
to marine transgressions, by marine sediments yield-
ing characteristic cavity-dwelling microfossils (e.g. os-
tracods). The fossils within the marine infill of fissures
and solution cavities are of enormous importance in
recognizing the age and timing of emergence and karst-
ification.
•
Cave infill sediments:
Caves can function as giant
sediment traps accumulating various sediments and de-
bris. Common structures are (a) lamination (sub-milli-
meter densely and widely spaced, light and dark paral-
lel laminae; fining-upward; laminations of locally de-
Fig. 15.4.
Speleothem carbonate.
Fingerlike, radiating, cm-
thick sinter crusts lining tens of cm-sized cavities within a
highly porous limestone characterized by 'sickle cell' struc-
tures (see Fig. 15.6). The speleothem structures are directed
upward here, but downward and sideward growth occurs as
well. The crusts consist of layers of fibrous calcite. Inter-
framework pores are filled by stalactitic cement followed by
drusy calcite mosaic. The sinter crusts formed during sub-
aerial exposure of lacustrine carbonates related to a lake level
lowstand phase. Late Tertiary (Miocene): Wallerstein,
Nördlingen Ries, Germany. From Pache et al. (2001).
