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cinnabar solutions and cinnabar gels discharged
directly into the marshland, forming
exceptionally rich (up to 78% Hg), unusual
syngenetic sedimentary cinnabar ores
(Fig. 3). Due to the small inflow of sulphur,
part of the Hg remained in its elemental
form, creating ore-bearing deposits with
native Hg primarily in the Carboniferous
shales (impregnated with native mercury
droplets, see Figure 4), Skonca beds
(Mlakar, 1969; Mlakar and Drovenik, 1971;
Čar, 2010), and partly also in some other
rocks. The final period of Ladinian tectonic
events was accompanied by extensive volcanic
activity with outpourings of diabase
and keratophyre.
At the end of the Ladinian, tectonic
activity ended. The following Carnian
deposits are thus composed of different
quartz conglomerates and sandstones, and
shallow-water limestones and white dolomite.
In the Norian and Rhaetian stages,
fenestral and oncolitic dolomites, known
as the “Hauptdolomite”, frequently appear,
with rare limestone intercalations. In the
Jurassic, the shallow-water sedimentation
continued with mainly oolitic limestones
and dolomites. These rocks crop out only
in the Trnovski Gozd area located south of
the Idrija region. The Jurassic rocks were
followed by more or less stratified organic
shallow-water Lower and Upper Cretaceous
limestones.
The shallow-water carbonate sedimentation
was interrupted in the Upper Cretacious
by extensive and complex tectonic
events that led to the disintegration of the
Dinaric Carbonate Platform and the onset
of the sedimentation of flysch. An erosional
unconformity is present between
the Upper Cretaceous limestones and
Paleocene-Eocene flysch, which concluded
the sedimentation process in the
Idrija region (Čar, 2010).
The area later experienced intensive
Tertiary thrusting and Neogene strikeslip
tectonic activity. According to existing
data (Placer, 1999; Čar, 2010), folding and
thrusting occurred in the territory of Idrija
in the Late Eocene, and continued into the
Early and Middle Oligocene. At first a large
recumbent fold was formed which, under
ongoing compression, was internally
thrusted. Thrusting was directed towards
the south-west, forming a thrust structure
typical of western Slovenia. The lowermost
structural unit, representing the basement
of the Idrija region is the Hrušica nappe,
which is overthrusted by the Koševnik,
Čekovnik and Kanomlja thrust slices. The
Figure 5: The levelling of the Zadlog was formed along the nappe plane between the ‘Hauptdolomite’
of the Čekovnik thrust slice in the basement and the Norian-Rhaetian dolomite of the Trnovo nappe.
The Idrijca and Belca River valleys are in the background.
uppermost structural unit of the area is
represented by the Trnovo nappe (Placer,
1973, 1999). This complex folding and
extensive thrusting completely obliterated
the primary spatial relations of different
strata and created several interesting tectonic
phenomena: various tectonic windows
and tectonic half-windows, as well as
tectonic klippes and half-klippes.
The Neogene to recent geological
structure of Idrija’s terrain was formed
by numerous strong normal and then by
dextral strike-slip faults which cut and displaced
older thrust units (Mlakar, 1969;
Čar, 2010). Of the numerous faults, let
us mention only the two largest ones, the
Idrija and Zala faults.
Geomorphology
The main characteristic of the Idrija
region is a well-defined NW-SE trending
straight and narrow morphological
depression that actually represents the
wide fault zone of the Idrija fault, which
divides the Idrija hills into two morphologically
highly diversified geotectonic
blocks. Another characteristic is the valley
of the Idrijca River and its unusual course,
which is primarily due to the baserock
lithology. However, the river in some parts
also follows distinct tectonic lines. The variegated
world of the Upper Idrijca River
and the Belca stream represents a special
geomorphological unit. The south-eastern
part of the Idrija (Fig. 5) hills comprises a
vast, karstified plateau.
The Karst
The richly developed and often somewhat
specific karst phenomena in the Idrija
region are a reflection of the complex rock
composition, as well as tectonic and morphological
stratification, of this highly variegated
region. Broadly, the karst may be
divided into the high karst and the solitary
karst, which includes the other “non-karst”
territory of the region (Čar, 2010).
Typical of the high karst are deeply
karstified carbonate rocks, particularly
limestones, with all the characteristics of
classical karst. On the surface, numerous
sinkholes can be found in various geological
structures (Čar, 2010), whereas
the underground karst is characterized by
deep shafts, occasional subhorizontal short
caves, as well as short sinking streams and
swallow holes (‘covered karst’) along the
thrust edges. All the remaining areas where
high karst has not developed are formally
classified as the “non-karst” world, but certain
individual or group karst phenomena
found here are classified as solitary karst.
This type of karst is usually developed in
limestones and dolomites of varying age
found in the middle of impermeable rocks
(shales, calcareous sandstones, etc.).
Most valuable geosites in the Idrija Geopark
The Idrija ore deposit is important and
famous because of the complex Middle Triassic
tectonic activity that led to its forma-
European Geologist 34 | November 2012
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