Geology Reference
In-Depth Information
Plate 129 Paleokarst and Cave Carbonates: An Early Carboniferous Case Study
The plate exhibits samples documenting microfacies criteria of a paleokarst breccia (-> 1), cave sediment
infillings (-> 2, 3, 5) and relicts of calcite speleothems developed within caves (-> 4).
Extensive subsurface karstification of Late Devonian limestones took place during the Early Carboniferous in
the central Carnic Alps, a part of the Southern Alps (Schönlaub et al. 1991) as a result of drops in sea level during
the Late Tournaisian and synsedimentary block faulting. The paleokarst is associated with an unconformity
separating Famennian to Early Carboniferous limestones from overlying siliciclastic sediments (Hochwipfel
Formation). The Famennian to Early Carboniferous sections comprise pelagic limestones and shelf limestones,
separated from the Hochwipfel Formation by a disorganized collapse breccia (-> 1). The breccia consists of
angular, mm to 0.5 m-sized clast-supported and matrix-supported clasts exhibiting different microfacies types.
The succession of events documented by field data, microfacies and geochemical signals is shown in Fig. 15.5B.
Polymetallic strata-bound sulfide ore deposits are thought to be genetically related to the Late Devonian paleorelief
and connected with the process of karstification.
1
Paleokarst collapse breccia
formed by subaerial dissolution leading to the development of a cave system. The breccia
consists of angular and subrounded limestone and chert clasts (C). The limestone clasts are Famennian fenestral mud-
stones (FM) and nodular pelagic wackestones (NW). The Early Carboniferous fine-grained matrix is a microbreccia
(MB) composed of detrital quartz, sandstone, shale clasts, and cherts. The large limestone clast at the right bears a fissure
infilled with radiaxial cements (arrow), pointing to a pre-breccia formation of fissures associated with synsedimentary
block faulting processes. Cava di Marmo southeast of the Plöcken Pass, near the Italian/Austrian boundary.
2
Densely laminated internal sediment within karst cavities
. The parallel siltstone laminae consist of quartz, calcite and
illite/muscovite. Arrows point to rock debris. Each couplet of dark and light laminae originate from one flooding of the
cavities (Ford 1985). Early Carboniferous (Middle Dinantian): Malpasso quarry, central Carnic Alps, Italy.
3
Laminated internal sediment
composed of calcite, authigenic quartz and mica deposited in karst cavities. Note parallel
planar bedding and flame and load structures (arrows). Cava Val di Collina west of the Plöcken Pass, Carnic Alps, Italy.
4
Euhedral calcite crystals lining the cave walls (speleothems).
Cathodoluminescence criteria indicate that these non-
luminescent cement crystals were formed in a freshwater phreatic environment. Same locality as -> 3.
5
Cave sediment infilling in paleokarst cavities.
Laminated filling between fractured scalenohedral calcite crystals formed
on the walls of the karst cavities. The spar calcite coating predates the densely laminated infilling. Same locality as -> 3.
-> 2-5: Schönlaub et al. (1991)
Sedimentary Event Diagenetic environment
T
ransg
re
s
s
i
o
n
o
f
t
h
e
H
o
c
hw
ip
f
e
l
Fm.
m
a
ri
n
e
p
hr
e
a
t
ic
Bedded sandstone
Formation of
collapse breccia - Pl. 129/1
Shale
freshwater phreatic
C l a s t s
Cephalopodtrilobite wackestone
Fenestral limestone
Black chert
Brachiopodcrinoid limestone
E
r
o
s
i
o
n
o
f
c
ry
s
t
a
l
s
il
t
First generation of isopachous
radial cements - Pl. 129/1
marine phreatic
Formation of
cav
e
s
e
d
im
e
nt
-
P
l.
1
2
9
/
2
,
3
,
5
Formation of caves with
calcite coating - Pl. 129/4, 5
M a t r i x
Siliciclastic, derived from over
lying Hochwipfel Formation
freshwater phreatic
Formation of fissures
Ore
c
o
n
c
e
n
t
r
at
i
o
n
Local silcretes and cherts
Fenestral limestone
surface karst/vadose
Extensive karst erosion and
l
i
m
est
o
n
e
s
o
l
ut
i
o
n
Limestone sedimentation up to
marine phreatic
A
B
TournaisianViséan boundary
Fig. 15.5.
Case study 'paleokarst'. Timing of sedimentary events and changes in the diagenetic environments in the Early
Carboniferous succession of the central Carnic Alps, Austria/Italy. Numbers refer to figures in Pl. 129. Modified from Schönlaub
(1991).
