RMZ - Materials and Geoenvironment, Vol. 49, No.1. pp. 61-84, 2002
Where does Underground Ljubljanica Flow?
Kje teèe podzemska Ljubljanica?
FRANCE ŠUŠTERŠIÈ1
Department of Geology, University of Ljubljana, Aškerèeva 12, 1000 Ljubljana, Slovenia;
E-mail: france.sustersic@ntfgeo.uni-lj.si
1
Received: January 29, 2002
Accepted: February 02, 2002
Abstract: The paper presents a field example of determination of the spatial position of the flow corridors within the krast, based on the data collected during
when mapping denuded cave features (“surface caving”). The sediments within
denuded caves provide a means of recognizing individual cave system sectors
that display different histories of sedimentation, outwash and resedimentation,
as well as possibly different sediment source areas. Such observations provide
information about the spatial relationships of karst channels within a flow corridor. Reconstructed flow corridors north-east from Planinsko polje fit well to
the results of extensive water tracing in the past.
Izvleèek: Predstavljeni so terenski rezultati ugotavljanja prostorskega položaja
glavnih pretoènih koridorjev v krasu s pomoèjo podatkov, zbranih pri kartiranju
denudiranih jam (“surface caving”). Sedimenti v denudiranih (“brezstropih”)
jamah omogoèajo, da razloèimo posamezne dele pretoènega koridorja, ki so
preživeli razlièno zgodovino sedimentacije, spiranja in ponovnega zatrpavanja,
lahko tudi iz drugaènega izvornega obmoèja. Tako dobimo informacijo o
prostorskih odnosih med kraškimi kanali znotraj pretoènega koridorja.
Rekonstruirani pretoèni koridorji severovzhodno od Planinskega polja se dobro
ujemajo s povezavami, ugotovljenimi z neoposrednim slednjem podzemskih
vod.
Key words: karst surface, karst hydrology, speleogenesis, karst of Slovenia, cave
sediments, “surface carving”
Kljuène besede: kraško površje, hidrologija krasa, speleogeneza, kras Slovenije,
jamski sedimenti, brezstropne jame
Scientific Paper – Izvirni znanstveni èlanek
61
62
INTRODUCTION
The Ljubljanica river (sensu lato) is one of
the best known karst sinking rivers, as it disappears/reappears seven times (and, thus,
carries seven names), before it becomes a
“normal” river, crossing Ljubljana, the capital city of Slovenia. Only two (out of four)
of the main groups of karst springs that contribute to the river at Vrhnika are popularly
named the Ljubljanica (sensu stricto). The
previous extensive underground water tracing experiment (GOSPODARIÈ AND H ABIÈ ,
1976) revealed that, though all of them share
the same general catchment area, functional
similarities would more reasonably suggest
three groups. However, in more general discussions such as this one, they may all be
considered together as the “Ljubljanica
springs”.
Before reaching the final springs, most of the
water feeding the Ljubljanica springs crosses
at least some of the karst poljes. ŠUŠTERŠIÈ
(1996) added support to GAMS’ (1959) idea
that the underground connections between
poljes with partly non-karstic inflow and the
downstream poljes are, to a great extent,
epiphreatic caves that are at least theoretically accessible to Man. On the other hand,
the present outflows from the poljes with
completely karstic inflow are deep phreatic,
and have remained mostly unexplored by
cavers. This is best expressed in the area between Planinsko polje (the lowest in the
string) and the Ljubljanica (sensu stricto)
springs at Vrhnika.
ŠUŠTERŠIÈ, F.
Though complicated and varying due to regional oscillations in precipitation (input),
water connections have been studied extensively, and have been well-established in the
past (GOSPODARIÈ AND HABIÈ, 1976). Since
the work of PUTICK (1887), several patterns1
of possible underground connections have
been proposed. Among these, those of
MICHLER (1954/55), and KRIVIC, VERBOVŠEK
AND D ROBNE (1976) 2 are the best known
(Figure 1).
Application of “surface caving” as a method
of systematic study of denuded, unroofed
caves (MIHEVC, 1996; see also ŠUŠTERŠIÈ,
1998, 1999) introduced a new set of data.
Together with other minor indications, these
observations permit recognition, at least to
some extent, of the main flow corridor(s) in
the area northeast of the Planinsko polje. This
paper sets out to compile and to reconsider
the existing data, and to reveal the relationships between the main stream corridor and
well-marked tectonic zones.
STUDY AREA
AND GEOLOGICAL SETTING
In this paper the main points of interest are
the outflow part of Planinsko polje and
Ravnik, a low-relief corridor about 18km
long and 1 to 3km wide, lying about 3km
northeast of the polje.
1 In his map Gradišnica (Teufelshöhle = Devil’s Hole) is shown in the wrong position.
2 The map was intended to be Annex 2 of the book (GOSPODARIÈ & HABIÈ, 1976, ref. p. 213). After it was
printed, the Yugoslavian military authorities declared it to be a military secret and did not permit its release to
public. Nor has it been published since, so Fig.1 of this paper, though simplified and mutilated, is the first
published version of this map.
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
63
Figure 1. Hydrogeological map of the treated area. Extracted from Hidrogeološka karta
1: 50 000, appendix to GOSPODARIÈ & HABIÈ, 1976). Green - predominantly dolomite; blue –
predominantly limestone; yellow - alluvium, predominantly flooded bottoms of karst poljes;
brown - nonkartsic rocks. Capital letters, added by the present author, mark main anticipated
flow corridors (see text!)
RMZ-M&G 2002, 49
64
Planinsko polje is one of the best-studied
parts of the Slovenian Classical Karst and,
thus, little background information need be
repeated here. The reader is referred to works
by ÈAR (1982), GAMS (1963), GOSPODARIÈ
(1982-a, 1982-b), G OSPODARIÈ & H ABIÈ
(1976), Š UŠTERŠIÈ and P UC (1970), and
ŠUŠTERŠIÈ (1982, 1996, 2000). However, to
aid understanding of the remaining discussion, it is necessary to be aware that the sinking river is called the Unica, that the elevation of the polje flat is about 445m a.s.l., and
the elevation of the main springs at Vrhnika
is somewhat less than 300m a.s.l.
Ravnik has been studied more recently
(ŠUŠTERŠIÈ, 1994, 1996, 1997, 1998, 2000).
Earlier geomorphological research viewed
Ravnik as a dry valley, presumably formed
by the Cerknišèica river (see ŠUŠTERŠIÈ, 1996,
Fig.2). This assumption was based only upon
morphological similarities, without any material proof. It was not until the mid 1970s
(ŠUŠTERŠIÈ, 1976; MIHEVC, 1979) that conglomerates similar to the present Cerknišèica
river transport material were first found. In
most outcrops the conglomerate appears together with flowstone, so that previous explanations were shaken rather than proved.
Structurally, the northeastern part of
Planinsko polje and Ravnik is a monocline,
with beds dipping at 25o to 30o towards the
westsouthwest. All of the rocks form part of
the c.6,850m-thick Dinaric carbonate sequence. The oldest beds, in the southeast,
comprise Late Triassic (Norian and Rhaetian)
dolomites, whereas the youngest beds, in the
northwest, are early Cretaceous limestones
and dolomites, topped by nearly 100%-pure
Late Cretaceous limestones. ŠUŠTERŠIÈ (1996,
1998) showed that the contacts between lime-
ŠUŠTERŠIÈ, F.
stone and dolomite packages within the Jurassic and early Cretaceous sequences are
especially prone to cavern formation (PEZDIÈ,
MIŠIÈ, ŠUŠTERŠIÈ, 1998).
The most important tectonic line in the area
is the Idria Fault, running along the eastern
margin of the Planinsko polje. In Ravnik two
lineaments, respectively about 3km and 5km
eastward from the polje (Figure 2, M, N),
were until recently only partially recognised
as major crush zones, and these run parallel
to both sides of the Ravnik. Some displacement may have occurred, but it is insignificant if compared to the width of the zone(s).
As demonstrated in the discussion below,
these tectonic lines act as delimiters between
zones of different cave sedimentation.
Between the lineaments is a swarm of sinistral strike-slip faults, running northortheast,
evidently having formed as a reaction to dextral strike-slip along the Idria fault. All of
the large collapse dolines that have been studied in detail lie on such faults (see ŠUŠTERŠIÈ,
1998).
At first glance the string of currently inactive collapse dolines north of the village of
Laze (Slaven dol string) (Figure 2, L) appears to be somewhat similar. ŠUŠTERŠIÈ ET.
AL. (2001) demonstrated its important role
in the speleogenesis of the underlying cave
system and argued that it deflected the main
underground streams northwards, parellel to
it. Current detaliled mapping of the wider
area revealed that this structure is more complicated than previously expected and further research is in progress.
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
65
Figure 2. Main structural elements: red - Idria fault zone; yelow – lineaments mentioned in the
text (see explanation of letter symbols in the text).
RMZ-M&G 2002, 49
66
HYDROGEOLOGY AND SPELEOLOGY
OF THE OUTFLOW PART OF
PLANINSKO POLJE
General situation
As is evident from Figure 1, an important
collector of karst water (B) should run approximately in the direction of Ravnik. However, the map is essentially a topological
view, merely indicating the mutual connections, whereas the actual spatial position of
the individual links is purely conjectural.
Nevertheless, the map (KRIVIC, VERBOVŠEK
& D ROBNE , o.c.) clearly illustrates the
author’s idea that the three main north-directed flow corridors (Figure 1, A, B, C) must
somehow be guided by the strike of the dolomite packs within the limestone.
In addition to local precipitation water, input to the Ravnik system comes from two3
directions - from the southeast (i.e. from
Cerkniško polje4 ) and directly from the west
(i.e. from Planinsko polje). Ponors at
Planinsko polje appear in distinct clusters,
which carry their own names. For current
purposes, a simple division into “southern”
and “northern” groups will suffice
(Figure 2, S, N / blue).
Southern ponor group
Most of the “southern” ponors lie along the
eastern border of the polje. The southernmost
ŠUŠTERŠIÈ, F.
in particular are fed permanently by sinking
river water. According to the latest published
data (BREZNIK, 1998, p.22) “The average inflow … into the polje is 26 m3s-1… The swallow holes can swallow a total of 60 m3s-1,
those in Babni dol5 alone being capable of
swallowing as much as 40 m3s-1”. Subtraction yields 20 m3s-1 for the “southern ponors”
when both groups are fully active. Their capacity is sufficient to keep the polje flat dry
during most of the year.
Vertical ponors dominate, whereas ponor
caves are few and of short length. Even Jama
na Lokah (cad. no.6 172), which for more
than a century was estimated to be a few hundred metres long, turned out to be no longer
than a few dozen metres (LAJOVIC, 2001). The
eastern polje margin is quite straight, implying that it is guided tectonically. Detailed
geological mapping at 1:5000 scale (ÈAR,
1982) revealed that the easternmost fracture
of the Idria fault zone runs just a few dozen
metres from the polje border, beneath the
polje’s alluvial flat. ŠUŠTERŠIÈ & JAKOPIN
(1979) demonstrated that most of the ponors
lie at the foot of the steep polje wall, rather
than in areas of gentler slope. This indicates
(a) that there is strong tectonic guidance of
the polje margin; (b) that the polje is relatively young, so that differential slope weathering has not yet been able to make it more
lobate; and (c) that sinking water prefers less
tectonised zones to more crushed ones.
3 Input from the northwest, in the area of Logatec, 6km north of Planinsko polje, is ignored in the general
discusssion and mentioned only where needed.
4 Some quantity even directly from the canyon before the river reaches the polje basin.
5 In terms of the present paper approximating to the “northern ponors”.
6 According to the central cave register of Slovenia, maintained by the Karst Research Institute, Postojna, and
Speleological Asociation of Slovenia, Ljubljana.
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
Water tracing experiments in 1975
(GOSPODARIÈ AND HABIÈ, 1976) revealed that
the highest velocities (c.6 cm s-1), within the
karst underground between the polje and the
springs, are between the “southern” ponors
and the main springs at Vrhnika. As mentioned above, there is no ponor cave that
would permit direct access into the karst underground farther from the polje. During the
highest floods water appears in Maèkovica
(cad. no. 52) and Logarèek (cad. no. 28),
caves that have their own entrances in the
slope above the polje. The latter is the larger
of the two and, as both are part of the same
system, the former, which lies closer to the
polje, is generally omitted from discussions.
The entrance shaft of Logarèek leads to an
epiphreatic channel of moderate dimensions,
at approximately 470m a.s.l., i.e. about 25m
above the present polje floor. Another drop
of about 25m makes more than 1km of passage accessible at the approximate level of
the polje. This passage is somewhat larger
than the higher one and, despite being modified by drip water, is basically tunnel-like,
i.e. epiphreatic. In cavers’ jargon it is termed
a “main drain”, and during the highest floods
in the polje it becomes partly flooded. The
bottom of the main drain is perforated by
(presumably) vadose shafts, which become
dry only during extreme draught. Recently,
cavers discovered a continuation of the upper passage, which led them to a maze, as
yet only partly explored, at the approximate
level of the “main” passage.
The main drain winds in a generally northward direction. Scuba diving revealed a
7
67
drowned passage only a few metres below
the water table, continuing in the same general direction. The end part of the cave is
accessible only in dry summers, and it was
explored for just a few hundred metres before the divers turned back without reaching
a physical end (PALÈIÈ, 2001).
Presently, Logarèek is partly flooded (to elevations of 444 to 447m a.s.l.) only at the
time of high water in the polje. GAMS (1963),
who presented a comprehensive study of
Logarèek, demonstrated that there is no
“third” completely drowned discrete channel lower than the “main” channel, as originally postulated PUTICK (1887), but that several only partly interrelated streams must pass
below the cave. Water tracing experiments
in 1975 proved a relatively direct connection between the Dolenje Loke ponor (northern part of the southern group) and Logarèek
(GOSPODARIÈ and HABIÈ, 1976, 1987 ). An
early observation (August, 30, 1928 / cit.
GAMS, o.c. p.55) that warm summer water
somehow reaches the most distant parts of
the cave, whereas colder, winter, water
remains stored in water bodies close to
the polje, was recently proved by divers
(PALÈIÈ, o.c.).
Loamy sediments (coarser-grained sediments
are not known) and various flowstone formations in Logarèek, as well as other caves
in the system, have not yet been studied. The
existence of clearly epiphreatic, partly canyon-like passages, and the lack of coarser
sediments, appear somewhat enigmatic.
Fragments of higher, older, abandoned cave
passages in the area in direct contact with
Calculated velocity of 11.4 cm s-1 or 1,500m per 4 hrs.
RMZ-M&G 2002, 49
68
the southern ponors are scarce. Unroofed
caves have not been searched for systematically, but the general impression is that they
are few.
Northern ponor group
Physically, the “northern” group of ponors
begins only a few hundred metres farther to
the northwest, along the polje margin. Effectively, water must flow a few kilometres
on the surface around Jakovica hill (which
takes the form of a peninsula during normal
floods) before reaching them. At first glance
the “northern” group is similar to the “southern” – there are as vertical, as cave ponors.
Again, the caves are short, though somewhat
larger. They appear to narrow down, rather
than being choked by collapse, and the general impression is that they are relatively
young. The situation in two artificial ponorwells that penetrated natural caverns (PUTICK,
1889; GAMS, 1963) is little different.
The northern ponors are active only at middle
and high water levels; during the summer
they generally remain dry. Nevertheless,
since PUTICK (1887) they have been regarded
as “the main ponors of Planinsko polje”.
JENKO (1954, cit. GAMS, 1963, p.9) estimates
that “… the southern ponors have a capacity
of 17 m3s-1 during the low and middle water
levels in the bed of the Unica, and they can
swallow the whole river. During flood the
swallow holes in the extreme northern corner of the polje, at the foot of Lanski vrh
(hill), absorb 43 m3s-1, or in exceptional cases
ŠUŠTERŠIÈ, F.
as much as 60 m 3s -1”.(Translated by F.
Šušteršiè)
The caves further from the polje margin differ significantly from those related to the
southern sector. Beside the main one,
Najdena jama (cad. no. 259), presently a
6,5 km-long phreatic maze, there are several
dozen fragments with accessible lengths
ranging from ten8 to a few hundred metres.
Except for Vranja jama (cad. no. 88), which
can be considered together with the nearby
Najdena jama, all of them are dry, high above
the karst water level undulations.
Najdena jama is accessible through a presumably vadose shaft formed in massive flowstone, indicating that the surface has lowered
significantly since its formation. An attempt
to interpret the cave in terms of “standard”
fluvial levels (GOSPODARIÈ, 1982-a), was not
convincing (ŠUŠTERŠIÈ, 1994). The so-called
“horizontal passages” are in fact segments
of a 3-D phreatic maze, parts of which appear horizontal due to abundant loam sedimentation. When the polje is dry, water levels in the siphon lakes range between 410m,
a few hundred metres from the polje margin,
and 396 m in the most distant parts. During
floods, when the cave can be more than twothirds water-filled, the piezometric level in
most of the cave stabilises at 435 m.
Najdena jama is well known for its enormous
quantities of mud, which testify the discharge
of high water through it to be small, even
8 According to the rules of the Speleological Association of Slovenia “caves” shorter than 10m are not registered. In this context, the term “cave” is used in the strict cavers’ meaning: accessible natural void of prescribed
dimensions. So, a short fragment of a large system is treated as a cave, in just the same way as a maze several
kilometres long, if they have independent entrances and are not connected physically. Very recently, cave
fragments, shorter then 10m, have been attributed to “unroofed caves” which have not yet been mapped as
systematically as “true” caves in this area. One may expect that this group will soon outnumber the “registered”
caves.
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
though the passages are nearly completely
water-filled during floods. At times of middle
flow, when the Unica just reaches the two
artificial ponor wells, a stream of about
1 m3s-1 appears in some passages, and these
are the only really washed ones.
Detailed monitoring of floods in Najdena
jama and on the polje revealed that, when the
water level within the cave starts to increase,
the difference between the polje and cave water levels remains nearly constant (15m), regardless of the absolute level of the water on
the polje. This was interpreted as indicating
the presence of a strong filter between the
polje and the cave (ŠUŠTERŠIÈ, 1982, p.23). In
such a situation, at least within Najdena
jama’s area of “influence”, which earlier researchers considered has the most effective
swallow holes, a significant increase in swallow hole capacity due to increases of flood
level on the polje would not be expected.
Clastic sediments in the cave were studied
extensively by GOSPODARIÈ (1982-b). He
found that the oldest are early Rissian quartz
sands and loams of unknown origin. More
interesting is the gravel, including various
types of chert. In Planinska jama, the main
inflow cave of the Planinsko polje, distinct
cherts were deposited separately during the
Rissian (GOSPODARIÈ, 1976), whereas in the
system of Najdena jama they were deposited, mixed together, at the beginning of
Würmian. Presumably, rapid erosion in
Planinska jama led to their retransportation
to caevs on the northern side of the polje.
Younger clastic sediments include various
limestone gravels, and pebbles of local rock,
69
mostly derived from cryoclastic rock spalling
during cold phases of the Würm. Input of this
material resulted in canyonisation of the
southern part of the Piparski rov 1 (ŠUŠTERŠIÈ,
1991, p.81), which extended about 500m into
the system, away from the polje, until the
pebbles (the abrasive) were worn out.
GOSPODARIÈ (o.c., pp.186-197) presumes that
the oldest flowstone within the cave is of
Riss/Würm age. An attempt to use the U-Th
series method to date the massive flowstone
around the present cave entrance failed, because the flowstone is too old (MIHEVC ,
2000).
Fragments of higher, older, abandoned cave
passages in the area in direct contact with
the northern ponors are quite abundant. Other
than Skednena jama and the previously mentioned Piparski rov 1 in Najdena jama (which
are the only locally conditioned exceptions),
all of them are clearly of phreatic origin. It
appears that they all formed within the same
tier, with a vertical extent of at least 150m
(ŠUŠTERŠIÈ, 1994). Lithological guidance is
evident, in the sense that the main inception
horizons are the upper and, especially, the
lower contacts of the dolomite packages9 .
Relatively common unroofed caves also appear to be phreatic. The sediment fill ranges
from coarse conglomerates of local rock, to
various sands, loams, diverse cave originated
septarian concretions, and abundant flowstone. Unfortunately, only pilot studies of the
sediments have yet been carried out, confirming that an admixture of flysch sand, presumably originating from the Postojna area, is
always present.
9 Interesting enough, first observed in Najdena jama by GOSPODARIÈ, 1982-a, 167, without an idea for
further discussion.
RMZ-M&G 2002, 49
70
About 3km northwards (i.e. downstream)
from Najdena jama is the Gradišnica cave.
At the bottom of a quite large, 189 m-deep
shaft is a chamber of 0.375 Mm3. Its floor
lies at an elevation of 379m, which is 65 m
below the level of Planinsko polje, and 30 m
below the deepest, “subaerial”, part of
Najdena jama. During floods, the rise of
water level in Gradišnica would not affect
the water level in Najdena jama until an elevation of somewhat more than 400 m is
achieved. Beyond this point both levels increase more or less in parallel.
Sediments in Gradišnica have not yet been
studied. The only analyzed sample of fine
sand 10 contained no characteristic flysch
components that would suggest an input from
Planinsko polje. On the other hand, quartz
grains with abundant included organic matter prove an origin to the northwest, i.e. the
Hotedršica region.
THE SITUATION IN RAVNIK
If only the accessible caves and general hydrology are considered, not much more can
be said than is shown in Figure 1. With the
exception of some insignificant, phreatic,
fragments, there are no accessible non-vadose caves. The only indications of karst
water streaming beneath Ravnik are highly
active11 collapse dolines in various states of
ŠUŠTERŠIÈ, F.
development. In some of them, such as Laška
kukava and Dolga doline, there are strong
blowing holes that at least testify to the existence of large caverns below.
With one exception, the floors of all the collapse dolines are higher than the flood water
level at Planinsko polje. Only Laška kukava
reaches down to the level of 420 m12 a.s.l.
On November 26, 2000, at the peak of one
of the highest recorded floods at Planinsko
polje, the lowest position in the doline was
completely dry, without any trace of water
between/below the boulders. So, it can be
concluded that the water table was at an elevation of about 418m or lower. Logarèek
was not visited on the same day, but according to the flood marks on the walls, the highest floods reached an elevation of about 447
to 449 m. Consequently, an estimation that
there must have been a vertical water level
difference of about a 30m is fair.
Except for some “isolated” collapse dolines,
most of them are arranged in lines. In
1954/55 MICHLER published a map, from
which it is clear that the “main stream line”
below Ravnik should simply be a joining of
the shortest connections between the main
collapse dolines. It must be noted that, decades before, many authors remarked that it
is self evident that “… large collapse dolines
in Ravnik indicate the underground stream
of Ljubljanica”. Michler’s map was merely
10 Courtesy of Miran Nagode, Jamarski klub Logatec.
11 In the sense that the scree material is evidently creeping toward one or two foci at the bottom, and disappearing downwards.
12 The doline was surveyed by compass and metre tape in 1972 (ŠUŠTERŠIÈ, 1973). The vertical difference of
106m between the highest elevation on the lip, and the lowest position at the bottom was obtained. This time,
the maps at the scale 1:5000 were not available for public use, and the polygon was not related to the national
grid system. According to the presently available map of 1:5000 the highest elevation is about 526m, and the
bottom thus becomes 420m.
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
a rather fanciful essay to elaborate “common
knowledge”.
Recently, ŠUŠTERŠIÈ (1998; ŠUŠTERŠIÈ ET. AL.,
2001) demonstrated that the positions of the
large collapse dolines indicate tectonic, rather
than purely hydrogeological control. At first
glance the Laška kukava string (Figure 2 / J)
appears to be simply a continuation of the
Slaven dol string, though rather disrupted at the
approximate position that it crosses lineament13 M. However, closer inspection revealed
that the two strings have little in common.
The more than 100 unroofed caves in part of
Ravnik, are the best studied within the
Ljubljanica catchment area (Š UŠTERŠIÈ ,
1998), and their sedimentary fill is considered later in this paper. They all formed
phreatically within the saturated zone and,
as a rule, they lie along the contacts between
dolomite and limestone beds within a sequence of thin interbedded layers (PEZDIÈ ET
AL., o.c., Fig. 1).
DISCUSSION OF ACCESSIBLE CAVE
SYSTEMS
Along the margins of Planinsko polje, the
ponor caves are small, evidently young, and
appear to be only a rudimentary adaptation
to a very recent situation (ŠUŠTERŠIÈ & PUC,
1970).
The tectonically related Slaven dol string is
the delimiter between two speleogically different areas (Figure 3). In the southern area
(fed by the “southern” ponors), there are only
71
a few non-vadose caves. Among these only
one short fragment of the top of a phreatic
jump (Brezno nad Slavendolom, cad. no,
491), is purely phreatic, whereas others appear to be at least partly epiphreatic.
Logarèek is the only fragment large enough
to present some information about the overall system. Its organization clearly displays
that it has passed through a phase of
epiphreatic, perhaps even paragenetic, shaping. This would be possible only if the sinking river brought in a mechanical load,
abraded the walls (and the ceiling), and filled
the lower passages within the tier with gravel,
before eventually forming a tunnel cave. If
compared to Najdena, the lack of abandoned,
dry, phreatic passages, as well as a near absence of fragments of such caves in the neighborhood, raises the question of whether any
extensive phreatic maze had developed before the epiphreatic phase. The present state
of knowledge implies no large-scale development of this type.
Ignoring the previously mentioned exceptions, the situation is literally reversed in the
northern area (fed by the “northern ponors”).
Najdena jama is not the only phreatic development. All of the fragmentary caves that
are not too deformed by slab spalling or collapse, as well as the many unroofed caves,
display a clear picture of an unconstrained
phreas. The question of how this contrast
is possible within such a short distance is
obvious.
If such a pattern had developed only at the
approximate level of the polje, there is a pos-
13 At the location, where the fault zone and the lineament come into contact, a broken zone about 100m wide
was mapped along the latter, and evidently continuing in both directions along it.
RMZ-M&G 2002, 49
72
ŠUŠTERŠIÈ, F.
Figure 3. Cave patterns on the northern (outflow) side of Planinsko polje
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
sible answer. During the cold phases of the
Pleistocene, when gravel production was
much greater than today, the river was capable of transporting its load into the southern ponors, but not into the northern ones.
So, epiphreatic channels might have been
formed downstream from the southern
ponors, while the channels more to the North
remained basically phreatic. There are, however, some strong objections to this explanation:
•
The existence of huge flowstone mass
at the present entrance to Najdena jama
testifies that there was very little change
during the last fifth of the Pleistocene,
and formation of the system cannot be
attributed to the remainder;
•
The vertical range of the phreatic system (at least 150m) pushes the time of
formation of the northern system so far
into the past that the available time
within the Pleistocene becomes too
short14 ;
•
Logarèek has developed in two distinct
levels, even disregarding what might
have been denuded away, or buried below the sediments. Thus, its fundamental organization cannot be the result of
just one episode;
These considerations invoke the need for a
more considered explanation.
The clear-cut difference between the types
of caves on opposite sides of the Slaven dol
string, is an indicator that, during the formation of the system(s), the string worked as a
delimiter between two completely distinct
73
hydrological zones. It appears that this at least
reflects different input areas, and possibly
also that different areas were active at different times. The ground plan of Najdena
jama (Figure 3) shows that the system is just
being dragged into contact with Planinsko
polje, whereas it must have formed in completely different, but currently unknown,
conditions. On the other hand, there are indications that Planinsko polje is quite young
(ŠUŠTERŠIÈ, 1996), certainly younger than the
“northern” system. Unfortunately, in both
systems, older cave wall scallops have been
destroyed by condensation water or corrosion beneath a loamy coating, to such an extent that even speculation on the basis of their
direction is impossible.
For current purpose a simple statement that
there are two basically different cave systems
must suffice. Further research is in progress.
UNROOFED CHANNELS IN RAVNIK
AND THEIR SEDIMENTS
Only the sedimentological features of denuded caves are considered in this paper, as
they seem to be better indicators of cave system history than do morphological features
(ŠUŠTERŠIÈ, in press). All of the denuded caverns on the surface of Laški Ravnik were
originally deep phreatic, oblique (reflecting
the dip of the enclosing strata), and relatively
small. They are filled with a variety of clastic sediments, and localized flowstone deposits also occur. For more detail, refer to
ŠUŠTERŠIÈ, 1998, 2000).
14 ZÖTL (1989, p.499) wrote “The springs of the Ljubljanica River … are absolute local base level of corrosion
and erosion. The connection and cave systems are paleokarst, and this is true to entire polje landscape.” Unfortunately, this revolutionary statement was not further supported and has remained more or less ignored by other
students of the area.
RMZ-M&G 2002, 49
74
“Basal fill”
The most widespread fill material is reddish
brown (2.5 YR 4/4, 5 YR 4/4) loam with a
minor admixture of relatively large oolitic
bauxite pebbles (derived from the Late Triassic - Carnian - beds) and coarse clasts of
black chert. Preliminary X-ray diffraction
analysis revealed the sediment to be mostly
muscovite/illite, plus mixed-layer clay minerals of illite/montmorillonite type, chlorite
plus mixed-layer clay minerals of chlorite/
montmorillonite type, calcium montmorillonite, and diaspore plus gibbsite, or just traces
of bauxite minerals (MIŠIÈ, 2000).
At first glance a potential sediment source
area in the present Cerknišèica river basin
appears obvious, but similar outcrops of
bauxite and chert do also appear at other sites
that are not significantly more remote. Recently, bauxite pebbles (but not chert) have
been found at elevations higher than the presently known primary outcrop in the
Cerknišèica river basin.
Because no flowstone is found on the underlying cave walls, such sediments appear to
have been deposited in cave systems that
were completely water-filled. These deposits are nearly ubiquitous, and lie beneath most
of the other preserved cave sediments, so the
sediment is referred to as basal fill
(ŠUŠTERŠIÈ, 1998).
Basal fill appears to be most common in
Ravnik, but it is not completely absent in
adjacent areas.
To the southeast, such sediment can be traced
back to the present Cerknišèica stream.
Northwestwards, it becomes nearly absent
north of Logatec town.
ŠUŠTERŠIÈ, F.
Laminated sandstone
The sediment in question is composed of
fine-grained quartz sand, cemented by calcite, that was later partly disintegrated again
by ongoing surface chemical weathering.
Some lamination is evident in unaffected
“sandstone”. Visual inspection reveals no
significant differences from a number of
similar occurrences distributed all over the
Julian Alps and Dinaric Alps of Slovenia
(HABIÈ, 1992).
Recently, about a dozen occurrences of similar laminated sandstone were found about
10km to the northwest, in a building site on
the northern outskirts of Logatec. There it
become clear that the earlier idea of a greater
age for the laminated sandstone (than that
of the basal fill) was correct. At some locations, structures that can best be attributed to
thixotropic soft sediment creep within the
caves were found, while on some others (neo)
tectonic deformation is more or less obvious. Unfortunately, the exposures were obliterated by heavy building machinery before
all the available information was collected.
Laminated sandstone appears to be ubiquitous, as it can be observed all across the general area of Planinsko polje, regardless of the
“zones” discussed above.
Coarse-grained cave sediments
Outcrops of conglomerates that were deposited in now-denuded caves are much less
common. Clast size varies greatly, from
coarse sand to large pebbles a few
centimetres in diameter. The clasts are predominantly of Upper Triassic dolomite
(originating in the Cerknišèica river basin),
with an admixture of local rock clasts, which
makes the flow direction unambiguous.
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
75
Figure 4. Sediment associations in the unroofed caves in Ravnik.
Many of the dolomite pebbles were hollowed
out during vadose diagenesis. Small quantities of bauxite and chert also occur, but it is
not yet possible to say whether they were
introduced as part of the stream load or simply admixed on the spot. Coarse rubble, originating from the channel roof or walls, is also
present locally.
Generally the conglomerate displays typically alluvial features, such as graded bedding and layered sedimentation. At some localities flowstone occurs with the conglomerate, and it appears that there were several alternating phases of conglomerate sedimentaRMZ-M&G 2002, 49
tion and flowstone deposition. As flowstone
is an indicator of subaerial conditions, its
mere existence indicates that conditions in
the cave system were no longer deep phreatic
at the time of conglomerate sedimentation.
In Ravnik, conglomerate-filled caverns are
restricted to the area between lineaments M
and N. Half a dozen kilometres northwest,
at Logatec, on the northwestern side of lineament K, the “conglomerate area” widens.
In parallel, pebbles of Permian sandstone,
originating from the northwest, become more
and more commonly admixed. As the prevailing component, arriving from the same
76
direction, comprise late Triassic dolomite
pebbles, appearing exactly the same as those
originating to the southeast, in the
Cerknišèica river basin, the location of mixing and the relative proportions involved are
difficult to establish.
Flowstone
In Ravnik, conglomerate and flowstone were
deposited more or less simultaneously, into
channels previously washed partly clean of
the basal fill. Relatively dry periods of flowstone deposition alternated with possibly
catastrophic flood events, during which
gravel-laden water rushed into dry caves.
The length of the time gap between basal fill
emplacement and the last phase of partial
outwash and deposition is uncertain. Determination of the age of the flowstone deposits in the denuded caverns is clearly beyond
the reach of the U-Th series method. Following a single failed attempt, at Bergen University (MIHEVC, 2000), no further direct attempts have been made to measure their
date(s) of deposition.
The dolomite and bauxite pebbles, as well
as the chert gravel within the conglomerate,
do not differ from those in late Pleistocene
and Recent Cerknišèica river alluvial fan
gravels in the Cerkniško polje. Thus, the cessation of “conglomerate” sedimentation in
Ravnik is clearly a result of the inrush of the
present Cerknišèica into the Cerkniško polje
basin, as it abandoned its old underground
routes towards the northwest15 . The oldest
ŠUŠTERŠIÈ, F.
directly dated sediment within the present
Cerknišèica alluvial fan is 55ka old
(GOSPODARIÈ AND HABIÈ, 1979, p.43), whereas
indirect observations push the beginning of
the sedimentation back to about 80ka b.p. On
the other hand, conglomerate-filled caves are
exposed in the slopes of most of the larger
collapse dolines, which suffered their essential transformation during the last glacial
episode (ŠUŠTERŠIÈ, 1998). So, it may be concluded that conglomerate (and flowstone)
sedimentation took place during the Pleistocene, but not at its very end.
ORGANIZATION OF
THE DRAINAGE SYSTEM
From consideration of the previous text one
conclusion can very clearly be reached. Some
tectonic structures (including lineaments) act
as strong delimiters between areas (blocks)
of completely different speleological and
hydrological properties.
a16 . Area northeast from lineament N and
southeast from lineament L
The basal fill is preserved at high elevations,
whereas laminated sandstone appears at
lower ones, and no conglomerate has yet been
found in this area. In areas where laminated
sandstone has not been found, there are some
larger caverns, which are accessible to humans. Close to the caverns are larger, unroofed, caves containing basal fill (GERŠL ET.
AL., 1999). Absence of conglomerate and the
presence of accessible caverns are charac-
15 One of the consequences of this catastrophic event is retransportation of chert from Planinska jama to
Najdena jama (ŠUŠTERŠIÈ ET. AL.., in press).
16 See Figure 2
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
teristic. If any collapse dolines exist in this
area, they were excluded from the drainage
system so long ago that they have completely
lost their characteristic geometry. Thus, remnants of old underground karstification exist, but there are no underground indications
of active karst streams.
b. Area between lineaments M and N, and
southeast from lineament K
This area is characterized by a number of
unroofed, originally phreatic caves, predominantly associated with alternating limestone/
dolomite beds of the Malmian (ŠUŠTERŠIÈ,
1998). Though laminated sandstone is not
completely absent, it appears to have had
been mostly washed out and replaced with
basal fill. Much later a minor part of the basal
fill was removed and conglomerate plus flowstone took its place. The direction of input
of the basal fill is quite likely from the southeast, but direct proof has not yet been found.
The conglomerate, however, contains a minor admixture of local rocks, and it is easy
to trace a source to the southeast, in the
present Cerknišèica river basin.
Highly active collapse dolines, like Vodiška
dolina, Rakovska Kukava and Laška Kukava,
testify that active underground streams (or
perhaps a single stream) exist beneath
Ravnik. Water tracing experiments do not
permit any other interpretation than the
present southeast-northwest flow direction.
The great potential difference (Dh ≥ 28 m)
across lineament M, between Logarèek and
Laška kukava, indicates that the system is
highly transmissive. So, it may be presumed
that the present flow corridor makes use of
older passages, most likely the ones that were
previously filled with basal fill, and later
washed out.
RMZ-M&G 2002, 49
77
As they are bound intimately to the Malmian,
it is possible simply to extrapolate this sequence below the elevation of 420m a.s.l.,
and east of lineament M. Since the bedding
is oblique to the general direction of Ravnik,
such a procedure is possible only in the area
where the Malmian is “trapped” between the
two lineaments, which fortunately is in the
main area of interest. Before positioning the
flow corridor positively, two constraints must
be considered. Firstly, the block between the
two lineaments has been broken down into
slices, one or two hundred metres wide, and
affected by sinistral displacement of several
dozen metres (ŠUŠTERŠIÈ , 1997). Consequently, the overall, effective dip of the rock
does not equal the one measured by compass. Secondly, there is an optimum depth
for cave channel formation (WORTHINGTON,
2001). So, very detailed mapping in the critical areas might provide data, sufficient for
relatively exact determination of the main
channels location/depth, below the present
water table.
The role of the apparent continuation of the
Slaven dol string (L) remains rather unclear.
Closer to Planinsko polje, southwest of lineament M, it is a delimiter between two completely different cave systems. North-east
from it, however, such a difference has not
been observed, at least not as regards unroofed caves. Another question is its influence on the water table. The previously mentioned absence of flood water at the base of
the doline could mean either that the general
level of the water table on both sides of the
doline is this low, or that it is this low only
on the downstream side. The volume of the
doline (4.17Mm3, ŠUŠTERŠIÈ, 2000-b, 224)
suggests that water has been penetrating
through the broken zone for a long time.
78
Thus, collapsing possibly has no significant
influence on the actual transmissivity, and the
water table might be low on both sides of the
doline.
cs. Area southwest of lineament M, and
southeast of lineament Slavendol string (L)
The area in question covers the “southern
ponors” of Planinsko polje and adjacent areas downstream. Hard information about the
water body is available only from the
Logarèek cave. During flood periods the
lower parts of the cave are water filled to a
level close to that of the water table on the
polje, and high above the still dry floor of
Laška kukava.
During summer drought the tested ponors
feeding Logarèek (see footnote no.7!) become
dry. As described above, at this time colder,
winter, water remains trapped in the areas
closer to the polje, whereas the more northern parts are influenced by warm water arriving more from the south. In general, passages of Logarèek run parallel to the Slaven
dol string on its eastern side, only a few hundred metres away (ŠUŠTERŠIÈ ET. AL, 2001,
15, Figure 3). So it may be concluded that
the main flow corridor funnels into the angle
between lineament M and the Slaven dol
string (L). In this area there are some active
collapse dolines, one of them exactly at the
intersection of the two tectonic lines. This
makes the option that the stream corridor
crosses lineament M seem more plausible
than the alternative possibility that it runs
northwest, parallel to the lineament.
cn. Area southwest of lineament M and northwest of the Slavendol string (L)
This sector is generally believed to transmit
the greatest quantities of floodwater (see
ŠUŠTERŠIÈ, F.
above). However, Breznik’s (o.c.) estimation
is not the only one, and figures from other
authors vary by a factor larger than two
(PUTICK, 1888; JENKO, 1954 [cit. GAMS, 1963];
HABIÈ, 1976). It must be noted that none of
the swallow hole capacities has been obtained
by direct measurement at their openings, but
only by calculating the riverbed water budgets. As the water level must increase significantly before the “northern ponors” receive water at all, BREZNIK’S (o.c.) statement
would be better as: “When the water level is
so high that all the northern ponors become
really active, the polje margins can absorb
40m3s-1 more than they do when the water
level is lower”.
Estimations made by various authors thus
imply that, as might be expected, the capacities of the ponors change according to the
hydrological conditions in the polje. It must
also be noted that most of the accessible cave
passages (ŠUŠTERŠIÈ, 1994), as well as unroofed caves, lie along the upper or lower
margins of two massive dolomite packages
(Figure 3). So, it is perhaps better to suggest
two transmission systems within the “northern zone”.
Some information about the water body can
be obtained within the western area, where
most of Najdena jama, Vranja jama and
Gradišnica are situated. As demonstrated
above, it is not likely that great quantities of
water would flow through there. The origin
of the sandy sediment in Gradišnica indicates
that the cave is displaced from, i.e. west of,
the main flow corridor of the waters sinking
in Planinsko polje. Consequently, the floodwater observed there could simply be
dammed by a main stream farther to the east.
A northward-trending string of large, active
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
collapse dolines (= Smreènica string) lies to
the northwest of Najdena jama, implying that
the flow corridor must cross a tectonic line.
The northerly orientation of the string would
permit both flow directions, either west to
east, or the opposite. Weak indications in the
northernmost part of Najdena jama imply that
the latter option is more likely. On the other
hand, if this were true, the stream should turn
north very soon after crossing the string. If
not, it would unavoidably influence sedimentation in Gradišnica. Without more data there
is no point hypothesizing further.
Relative rapid reactions of the water level in
Gradišnica to the flooding/emptying of the
polje indicate that an obstacle, not important
for small discharges, but with limited transmissivity for larger discharges, must exist
close by. So, as in block ‘cs’, it might be expected that the main stream runs quite close
to lineament M, parallel to it on its southwestern side, until it finds a weak point, and
the water pours into area “d”.
So, it would be expected that most of the
water leaves the polje through channels related to the eastern dolomite pack. Unfortunately, the water table is not accessible there,
and active collapse dolines appear to be
absent.
d. Area northeast from lineament M, and
northwest from lineament K
This area has not been researched in great
detail, nor have the unroofed caves been studied systematically. Being situated so far to
the northwest, it is the most probable location for the main flow corridors to turn
northeastwards, towards the Ljubljanica
springs at Vrhnika. Two aspects must be underlined.
RMZ-M&G 2002, 49
79
In this area sedimentation within the unroofed
caves becomes influenced more and more by
input from nonkarstic area farther to the north
and northwest. Only at first glance is the transition gradual. In fact, channels filled with
various materials of very different origin,
interfinger in 3-D. This implies an intensive
interchange of sedimentation, washing and
resedimentation from both sides, but with
intensive oscillations in the water regime too.
In an area of less than 2km2, in the northern
angle of the crossing of lineaments M and
K, there are about two dozen mostly very
active collapse dolines with the general name
Logaške Koliševke. They differ from those
previously mentioned in that they are not
arranged only along strings, but also into a
planar, clustered pattern. Such organization
is not very common. Undoubtedly it indicates
intensive fracturing of the parent rock
(PEÈERKIN & KATAEV, 1985) as well as strong
water streaming below. The conclusion that
they impede the drainage of block “cn” appears to be obvious. Block “b” (and indirectly, block “cs”) appears to be better
drained, and it is possible that their main
outlet turns northeastwards before crossing
lineament K.
CONCLUSIONS
The former discussion can be summed up in
the following statements:
•
A relatively direct and only slightly
impeded flow corridor runs between lineaments M and N, approximately along
the axis of Ravnik, perhaps corresponding with “links” A1 and B on Figure 1.
Though the Rakovska kukava and
Laška kukava collapse dolines indicate
80
•
•
•
•
•
ŠUŠTERŠIÈ, F.
strong, concentrated flow, observations
of the unroofed caves give an impression that a well-developed maze exists,
so that collapses cannot influence the
bulk transmissivity of the flow corridor.
The southern ponors group drains the
polje quite directly, but after encountering lineament M, water turns
northwestwards along it. At the intersection of lineament M and the
Slavendol dol string at least a part of
the flow breaks through lineament M’s
broken zone and pours into the formerly
mentioned flow corridor.
The northern ponors group seems not
to be very uniform, and nor can it take
as great a proportion of the flood water
as was formerly believed. The system
was formed in hydrological conditions
that might differ from the present ones,
to such an extent that it hasn’t yet
adapted completely. The main flow corridor runs parallel to lineament M until
close to Gradišnica, and then turns
northeast, having crossed the continuation of lineament K.
Gradišnica lies in a dead corner of the
system, and it does not transmit large if any at all - quantities of water originating from Planinsko polje.
Regional lineaments, with 100m-wide,
or wider, fracture zones generally serve
as delimiters between blocks with distinctive, local, karst hydrology.
Sinistral strike-slip faults, with fracture
zones not wider than 20m do not influence underground streaming to any
great extent. On the other hand, they
are the most prone for the location of
extremely large dolines, opened up by
ongoing undercutting, rather than by a
sudden collapse.
•
•
In this configuration, the Slaven dol
string appears to be an exception, as it
displays properties of both of the previously listed structural lines. Further
research is in progress.
The locations of large collapse dolines
are primarily important as indicators of
weakened rock, and only secondarily as
indicators of the flow corridor(s) below.
POVZETEK
Kje teèe podzemska Ljubljanica?
Kraška Ljubljanica je že vsaj 150 let v
središèu zanimanja krasoslovcev, najbolj pa
pozornost privlaèijo delno še neznane poti,
po katerih se ponikalnica pretaka med
posameznimi kraškimi polji. Še posebej
izstopa odsek med Planinskim poljem in
Vrhniškimi izviri, kjer je znanih sorazmerno
malo vodnih jam. Kljub temu, da v zadnjnih
letih ni prišlo do veèjih posmeznih odkritij,
lahko na osnovi izboljšanega splošnega
poznavanja ustroja in delovanja kraških
pretoènih sistemov ter posameznih na videz
nepovezanih terenskih opazovanj pridemo
do uporabnih zakljuèkov. Pri tem izhajamo
iz spoznanja, da se podzemske vode
pretakajo, oz. oblikujejo kanale (jame) vzdolž
jasno definiranih pretoènih koridorjev, ki jih
doloèajo veèji prelomi (oz. lineamenti, èe
jih ugotavljamo samo posredno) in litologija.
Pri slednjem izhajamo iz spoznanja, da je na
obravnavanem obmoèju v znanih
podzemskih in brezstropih (denudiranih)
jamah veèina kanalov vezanih na stik
apnenec-dolomit. Sedimenti v brezstropih
jamah nudijo informacijo o smereh
pretakanja oz. izvornih obmoèjih v
preteklost, s tem pa posredno pomagajo
RMZ-M&G 2002, 49
Where does Underground Ljubljanica Flow?
ovrednotiti hidrogeološko vlogo posameznih
lineamentov.
Na ozemlju med Planinskim poljem in
Ljubljanskim Barjem najbolj izstopata
lineamenta M in N, ki v dinarski smeri,
vzporedno z Idrijskim prelomom, potekata
vzdolž Begunjsko-Logaškega Ravnika. Tako
so zaèrtani trije pretoèni koridorji, ki
topološko ustrezajo rezultatom sledilnih
poskusov v preteklosti. Povprek na te
strukture ležita lineamenta K in L, ki v
preènodinarski smeri razmejujeta posmezne
hidrogeološko individualne bloke. Lineament L loèi enoti, ki izkazujeta popolnoma
razlièno spleogenezo v preteklosti, danes pa
– kjer sta v neposrednem stiku s Planinskim
poljem - precej razliène požiralne lastnosti.
Jugovzhodnejši blok, ki je sposoben odvesti
nizke in veèino srednjih voda Unice, je v
preteklosti doživel fazo epifreatiènega
oblikovanja, kar z drugimi besedami pomeni,
da je ponornica vnašala mehansko plavje s
površja. Obratno pa je kraško podzemlje v
severozahodnem bloku popolnoma freatièno.
Dosedanja literura meni, da so najpomembnejši ponori v tem bloku, medtem ko avtor
ugotavlja, da je to verjetno le videz. Zdi se,
da se kraško podzemlje v tem bloku ni
razvijalo v povezavi s Planinskim poljem in
da se v njegovo hidrologijo vkljuèuje šele v
najmlajšem èasu. Ugotovitve povzamemo v
sledeèe:
•
Sorazmerno neposreden in le nekoliko
oviran pretoèni koridor poteka med
lineamentoma M in N. Èeprav
posmezne udornice kažejo na krajevno
koncentriran tok, je tu bolj verjeten
labirinten splet kraških kanalov, kjer
pretoka vode posamezni udori stropov
bistveno ne moteijo.
RMZ-M&G 2002, 49
81
•
•
•
•
•
•
•
Južna skupina ponorov odvodnjava
Planinsko polje precej neposredno. Ko
pa tok trèi na lineament M, zavije
vzdolž njega proti sevrozahodu. Na
seèišèu z lineamentom L se vsaj del
vode prebije skozi lineament M in
vkljuèi v prej omenjeni pretoèni koridor.
Severna skupina ponorov ni niti enotna,
niti tako uèinkovita, kot se zdi. Jamski
splet je nastal v razmerah, ki so od
današnjih verjetno precej razlikovale in
se slednjim še ni popolnoma prilagodil.
Glavni pretoèni koridor poteka
vzporedno z lineamentom M do bližine
jame Gradišnica, kjer zavije proti
severovzhodu, èim preèka lineament K.
Gradišnica leži v mrtvem kotu sistema
in ne prevaja veèjih kolièin voda,
poniklih na Planinskem polju.
Regionalni lineamenti, s po100m in veè
širokimi zdrobljenimi conami, so
veèinoma izrazite loènice med bloki, ki
kažejo samosvojo kraško hidrologijo.
Levozmièni prelomi, katerih zdrobljene
cone niso širše od 20m, bistveno ne
vplivajo na podzemsko pretakanje. Po
drugi plati so ugodni za nastanek veèjih
udornic, ki pa ne nastajajo s hipnim
zrušenjem stropa, temveè poèasnim
spodkopavanjem.
V tem kontekstu je lineament L izjema.
Podrobno preuèevanje je v teku.
Mesta nastanka velikih udornic v prvi
vrsti odražajo oslabljeno kamnino, manj
neposredno pa so udornice pomembne
kot kazalci pretoènih koridorjev v
globini.
82
ŠUŠTERŠIÈ, F.
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