ACTA CARSOLOGICA	29/2	4	65-81	LJUBLJANA 2000
COBISS: 1.08
COLLAPSE STRUCTURES AS A CONNECTION BETWEEN THE KARST SURFACE AND UNDERGROUND (EXAMPLES FROM CROATIA)
UDORNE STRUKTURE KOT POVEZAVA MED KRAŠKIM POVRŠJEM IN PODZEMLJEM (PRIMERI S HRVAŠKE)
NENAD BUZJAK1
1 Speleological Society Dinaridi, Marulicev trg 19/II, HR-10000 ZAGREB, CROATIA
Prejeto / received: 1. 9. 2000
Izvleček	UDK: 551.435.83(497.5)
Nenad Buzjak: Udorae strukture kot povezava med kraškim površjem in podzemljem (primeri s Hrvaške)
Na hrvaškem krasu je veliko udornic. So tudi take, nastale zaradi posedanja (subsidence). Analiziranih je nekaj geomorfolo{kih elementov in oblik, nastalih z udiranjem in posedanjem (vpliv geolo{kih okoli{~in in stopnje jamskega razvoja na procese udiranja in posedanja, morfologija udornih in subsiden~nih vrta~ ter jamskih rovov pod njimi), iz dveh jam z razli~nih delov hrva{kega krasa (Dola~ina mama iz @umberka in Jama na Sredi s Cresa). Opazovanja potrjujejo, da je podiranje pomembna sestavina v razvoju jam in da je nastajanje udornih in subsiden~nih vrta~ pomemben pokazatelj razvojne stopnje krasa. Ključne besede: geomorfologija krasa, udor, udornica, posedanje, subsiden~na vrta~a, Hrva{ka, Cres, @um-berak, Dola~ina mama (jama), Jama na Sredi.
Abstract	UDC: 551.435.83(497.5)
Nenad Buzjak: Collapse structures as a connection between the Karst surface and underground (Examples from Croatia)
In the Croatian Karst area there are a lot of collapse dolines. Here we can also find structures that are the result of subsidence (subsidence dolines). In the cases of two selected caves in different parts of the Croatian Karst area (Dola~ina mama cave - @umberak Mt. and Jama na Sredi cave - Cres island) some geomorphological elements and features of collapse and subsidence processes were analysed (the influence of geological conditions and the cave's stage of development to wards collapse/subsidence the morphology of collapse and subsidence dolines and the cave passages beneath them). Observations confirm that collapse is a significant component of cave development and that formation of collapse and subsidence dolines is an important indi-cator of karst evolution.
Key words: karst gemorphology, collapse, collapse doline, subsidence, subsidence doline, Dola~ina mama cave, Jama na Sredi, @umberak, Cres island, karst, Croatia.
INTRODUCTION
The dolines are the most common surface karst forms in Croatian karst area. Besides corrosional dolines, which predominate, here one can find collapse and subsidence dolines also. Their origin and evolution is, as in the case of corrosion dolines, in the general sense conditioned by interaction of geological situation (constitution and structure), hydrogeological conditions, corrosional and erosional water activity, characteristics and processes in underground cavities (such as breakdown), and climatic conditions/changes.
The process of collapse or subsidence has the biggest influence on their appearance leading to their separation as special types of dolines.
About processes and the above-mentioned factors in the occurrence and evolution of collapse dolines and similar features in Dinaric karst area, examples can be ound in many papers (Bahun 1969; Božičevic 1985-86; Habič 1963; Roglic 1965; Sebela 1998; Sušteršič 1998 etc.).
In this paper collapse and subsidence process and their impact on doline formation will be discussed. The occurrence and evolution of both features are connected with processes in the caves below them. The characteristics of one collapse doline are observed above Jama na Sredi cave (Cres island, Northern Adriatic). The example of a subsidence-collapse doline was studied out above Dolačina mama cave (Zumberak Mt., Central Croatia).
Recently in Croatia there is a lack of studies about such karst forms. But, authors mentioned earlier showed the great importance of collapse forms in karst as reflection of karst underground evolution. Or, as F. Sušteršič once noted, these forms can be observed as "surface projections of karstification in the underground" (Sušteršič 1984).
Collapse dolines are the most markedly surface karst features. Their appearance - very steeply to overhanging rocky slopes that emphasize depth, by rock debris and boulder-covered bottom -distinguish them from other types of dolines. Those with vertical rocky walls in Croatia are called "well dolines". Their development is caused by roof breakdown in caves and the opening of underground cavities due to the lowering of karst surface. That is mostly result of fluctuations and lowering of water level (Ford&Williams 1989; Sweeting 1973; Sušteršič 1998).
The forming of these depressions is often a combination of the two or all three mechanisms with factors mentioned earlier. For example, breakdown in caves is often a result of neotectonic movements or changes in water level that expose bedrock to big oscillations of hydraulic pressure. That has great influence on their stability over large passages or chambers in caves.
JAMA NA SREDI CAVE
Position and history of research
Jama na Sredi cave is located in the southern part of Cres island, 4,3 km from town of Osor in the area of Sredi. The entrances are at an elevation of 73 m (Fig. 1).
The cave was visited and explored from various aspects since 18th century. In earlier Italian literature (published before 2nd World War) it was known as La Caverna dei Fossili or La Grotta Fortis after its first explorer, Italian abbot A. Fortis. He described it in the first scientific book about Cres-Lošinj archipelago (Fortis 1771). More than one century later it was explored by G. Pucalovich (1899). In 1922. it was visited by G. Windspach (Morton 1932) and three years later
Fig. 1: Position map.
one paper about its fauna was published (Ravasini1925). The first plan and some basic data about its morphology could be found in the book "Duemilla Grotte" (Bertarelli & Boegan 1926). In 1932. F. Morton published his speleobotanical observations from this cave (Morton 1932). Archaeologi-cal research after 2nd World War showed that the cave was inhabited from Mesolithic age (Mirosavljevic 1959; 1973). During 1996 the cave was surveyed and systematically researched in speleobotanic, meteorological and speleomorphologic sense (Buzjak et al. 1997; Buzjak 1997; Fiedler & Buzjak 1998).
The cave is very interesting due to its morphology. It is a simple cave but in its formation breakdown has been important. Therefore it has three accessible entrances (two of them can be used only by vertical caving technique). That is the reason why local people call it "jama" (pit, shaft) not "spilja" (cave). Cave's total length is 54 m and depth 19 m.
Geological situation
The area of Sredi, where Jama na Sredi is located, is built of well bedded Upper Cretaceous limestone. In the research area the bed's outcrops are rare and stratification is mostly hardly observable. The limestone is of mostly gray and partially white color. It is pure (with high percentage of CaCO3) and of micrograin structure. The presumed thickness of these beds is 500-800 m. The general extent of the beds is NNW-SSE with local aberration caused by smaller transversal fractures. The whole Cres island area is characterized by well developed isoclinal folds and typical nappe structure (Maga{ 1968; Maga{ 1973). According to some structural elements observed (dip and stretching of strata and fissure network), it also had influence on Jama na Sredi development. Owing to such geological situation (but respecting other relevant factors in relief development too) in this area deep karst occurs. The dominantly bare karst surface is covered in places by thin soil residuum (terra rossa). The predominant karst forms are shallow dolines. Other karst forms are
Speleomorphology
Jama na Sredi is divided into three morphological parts.
Entrance Chamber (Fig. 2, profile section 2) is a part of an almost completely collapsed passage in the middle of the cave. Its bottom is steeply inclined and covered by rock debris and soil. The eastern wall is steeply inclined towards the bottom and probably modified by ancient man for easier entery into the cave (Mirosavljevic 1959). The western wall is partially overhanging above the bottom (Fig. 2, cross-section B-B'). The entrance chamber is 18 m long and up to 10 m wide. Due to a sufficient amount of light and climatic characteristics influenced from the surface, this part is rich in plants (Fiedler & Buzjak 1998). At its southern end there is a preserved cave passage (Fig. 2, 1). It is 15 m long, up to 5 m wide and up to 4,5 m high. The walls and ceiling are partially covered by mostly thin flowstone, especially along the fissure with chimneys, which ex-tends parallel to the passage direction. Along that fissure, rock is crushed and partially filled by rock fragments with a reddish matrix. The bottom is covered by loam and rock debris. Near its end there was a pit about 18 m deep which is completely filled and known only thanks to earlier descriptions (Bertarelli & Boegan 1926; Morton 1932; Mirosavljevic 1959).
Both Entrance Chamber and cave passage stretch in a NW-SE direction because they were formed along major fissures of similar direction. In the wall that separates the Entrance Chamber from Pantheon Chamber and in the NE slope of Entrance Chamber a crush zone could be seen.
At the opposite side of Entrance Chamber there is a low and partially filled entrance to Pantheon Chamber (Fig. 2, 3). It is 24 m long, 10-15 m wide and up to 15 m high. The entrance to the chamber is reduced by rock debris and soil (probably deposited not only by natural processes but during archeological excavations also). To preserve it from further filling from the Entrance Chamber side it is protected by artificial supporting wall (Fig 2., cross-section B-B'). The chamber is elon-gated in a N-S direction, provoked by a fault plane that had a great part in its formation. Along this line the chamber's ceiling is partially collapsed so there is a second, vertical entrance to the cave. Its dimensions are 7x5 m. Below this opening formed by collapse, rock blocks and debris cover the bottom. The rest of the bottom is covered by mostly dry soil. In the preserved part of the ceiling there are few high chimneys. One of them is connected with the surface, but it is very narrow (average perimeter 40 cm) and used only by cave pigeons, which live in the holes in the Pantheon Chamber's ceiling.
Fig. 2: Jama na Sredi cave (Legend: 1-fault, 2-fissure, 3-crushed zone, 4-doline edge, 5-doline bottom edge, 6-doline bottom, 7-doline slopes).
Besides the previously mentioned fault plane of N-W direction, across the SW wall there is a clearly observable fault plane (60/44) which also had great importance for chamber formation (Fig. 2, cross-section C-C'). From the narrower part in rock filled with fault breccia, it becomes wider towards the contact with the chamber void and partially filled with old speleothems. Due to the block movements along it, the rock is intersected with many fissures forming a crushed zone. The water that penetrates along it, in combination with tectonic movements, had an important role in chamber modeling.
Most of the cave is dry, with small amounts of dripping water occurring along fissures and some condensed water (Pantheon Chamber). Dripping water deposits rare and small speleothems, mostly in Pantheon Chamber. In the whole cave one cannot find clear evidence of a water stream that could have had a part in its formation, except maybe a terrace-like step along the bottom of SE wall of cave passage.
The mentioned collapse in Pantheon Chamber occurred bellow a doline, of which preserved parts are clearly observable at the surface. So, if we look from the surface we see a good example of a collapse doline. Here we can see the remaining part of its bottom and most of its slopes (Photo 1). Dolines of similar shape and dimensions, but not collapsed (yet?) can be seen in vicinity of this one.
The bottom is about 3 m wide and it is gently inclined (approx. 20°) towards the entrance to Pantheon Chamber. It is covered by thin soil with many rock fragments incorporated. The slopes are steeper and rocky, without soil but overgrown by trees and bushes. The vegetation makes the doline and entrance hardly visible.
Limestone beds seen in the slopes are very crushed and even folded, especially along the fault plane on the northern and southern slope parts. At the southern slope the rate of the beds, move-ment can be seen.
It is presumed that breakdown of a chamber roof caused the present state of Entrance Chamber too. That is visible from former chamber cross-sections and by rock blocks and debris covered bottom (Fig. 2, A-A' and B-B'). The breakdown occurs along the crushed zone as observable on the walls.
The speleogenesis of Jama na Sredi cave, its collapsed entrances and collapse doline above Pantheon Chamber is very interesting from a speleomorphologic point of view. Its speleogenesis is associated with tectonic movements and the occurrence of well developed secondary rock porosity and water activity in well bedded Upper Cretaceous limestone, which resulted in a high level of karstification. The formation of the cave is a result of circulation of water between surface and underground along paths provided by fissures of tectonic origin. The main role in fissures widen-ing is corrosion by percolating water and possibly also erosion by a stream formed by precipitation water percolating from the surface (but due to the lack of evidence it cannot be firmly established). If we presume the existence of a water stream, a few phases of speleogenesis can be established: 1) widening of fissures by the stream with washing away of dissolved and eventually collapsed material, 2) after the lowering of the water level the main role in cave formation is corrosion by percolating water and breakdown followed by deposition of cave sediments, especially rock debris. The lack of larger speleothems along fissures, formation of corrosional chimneys and micro-grooves and large amounts of breakdown deposits speaks on behalf of such scenario.
The cave's roof breakdown and collapse in Entrance and Pantheon Chambers was the result of several processes. Water activity along fissures and later passage walls widened them gradually and created more and more width. It resulted in occasional breakdowns and deposition of rock
debris partially dissolved or maybe washed through now inaccessible passages. Fault lines and crushed zones were favorable for faster dissolution and widening of cave voids. Occurrence of numerous high chimneys shows that inverse karstification had a great part in their evolution also. These processes of widening of voids underground (laterally and especially upwards) in combina-tion with lowering of karst surface resulted in a more and more thin cave roof. Since the limestone beds in the roof were unstable due to tectonic movements which crushed them, all important conditions for collapse occurrence were present. At the places where the roof was the thinnest and the rock was very disturbed and unstable, collapse occurred. That was the case in Entrance Passage and the western wall of Pantheon Chamber. In the case of the collapse doline, the thinning of cave roof was accelerated by corrosion in the doline bottom. Maybe neotectonic movements along the main fault planes accelerated the collapse also. At the places with a thicker and more stable bed in the roof (cave passage and NE part of Pantheon Chamber), collapse is absent (Photo 2). Therefore the NE part of doline above Pantheon Chamber is preserved.
The collapse doline above Pantheon Chamber is therefore a result of widening of an underground chamber by water activity along fissures and cave walls. It was not completely collapsed due to the differences in thickness of beds above the cave (as a result of different conditions for dissolution) and their stability.
DOLAČINA MAMA CAVE
Position and history of research
Dolacina mama cave is located on Zumberak Mt. in Central Croatia (Fig. 1). From a speleological point of view the cave is found in a quite well researched area, but it was explored recently (February 2000.) because its entrance opened in a relatively fresh subsidence doline. The doline was noted during one of the field trips in 1998. but then no entrance was visible. Due to its position in flysch-covered carbonate beds (covered karst area) without any surface karst forms and mark-edly funnel-like steep cross-section, it was estimated as a potential entrance to speleological feature. Since its is located close to the recently often visited Rogovac cave, I had an excellent oppor-tunity to observe the subsidence and whole process that opened the entrance to the cave.
Geological situation
The part of Zumberak Mt. where the cave is situated is of Upper Jurassic (Malmian) and Upper Cretaceous limestone and breccia in the base, that were transgressively covered by Upper Cretaceous (Maastricht) flysch (Devidé-Nedela et al. 1982; Herak 1968; Herak & Bukovac 1988). In Rogovac cave, which is situated 125 m NE from Dolacina mama cave, O. Lukic established Upper Cretaceous limestone breccia in the base of the above-mentioned flysch (Lukic 1989). Dolacina mama cave is developed in similar conditions. These carbonate beds contain high levels of CaCO3; they are intersected by many fissures and therefore well karstified. But, due to the impermeable flysch cover, covered karst with lack of surface forms is developed. The only visible surface traces of karstification in carbonate beds below flysch cover are rare subsidence dolines. Another proofs for karstification are speleological features. Up to now in this area two caves (Rogovac and Dolacina mama) and one shaft were explored. They were formed by accumulation and circulation of underground water that infiltrates from the surface. The water flows mainly towards NE and rises in a
Photo 1: Southern sì ope (back) and remains of collapse doline bottom (N. Buzjak).
Photo 2: The bedrock between remains of collapse doline bottom and ceiling of Pantheon Chamber (N. Buzjak).
Photo 3: The doline above Dolačina mama cave at the beggining of a bottom collapse (March 22nd 1998. ). Doline slopes and most of the bottom are stili preserved (N. Buzjak).
few small springs of Polamanica creek basin. The amount of water is highly dependent on the amount and annual distribution of precipitation, as observed in Rogovac cave (which is a spring cave).
Speleomorphology
Dolacina mama cave is a simple cave with an entrance shaft (Fig. 3), 15 m long and 12 m deep. Its entrance was opened in a small doline. The doline was formed by subsidence of flysch beds. Its slopes are mostly covered by wet soil produced by disintegration of flysch. The flysch outcrops at the SW slope and it is very disintegrated, especially along fissures 170-350 that intersect it. In this profile it is deposited in thin layers (2-5 cm thick) and inclined towards NE (65/27). The soil from the slope slides slightly towards bottom, especially after rain and snow melting.
The vertical entrance shaft is narrow and 4,5 m deep. In the upper part it is filled by muddy soil sliding from the doline bottom. There it contains more water due to the weak water vein that rises from a fissure intersecting the wall. This water circulation, in combination with breakdown from above, formed it to its present dimensions. In this part, 8 m deep from the doline's border, there is a contact between flysch and carbonate beds in which entrance shaft (and whole cave) was formed. From the entrance, that is 45 cm wide, the shaft is bell-shaped so mud can easily slide into the cave. During the exploration of the cave, this spot showed as very dangerous for cavers. Due to the choke of mud accumulated after descending in the narrower entrance part of the shaft, we were trapped inside and, together with a team member who was left outside, dug our way out for four hours.
The main part of the cave is a chamber up to 10 m wide and 15 m long. Its cross-section is determined by the dip and stretching of strata (Fig. 3, cross-section C-C'). The chamber is widest and highest at its northern part. That is mostly result of breakdown that occurs along fault plane 114-294. The bottom there is covered by boulders and rock debris fallen from the walls and ceiling. Only a small part of the bottom below entrance shaft is covered by mud slid from the doline. Since this deposit is flysch residuum it is mixed with flysch particles. There is a weak water stream observable. It forms from water that runs (or drips) from shaft. After a few meters flowing in a shallow and narrow bed, it disappears in rock debris. The rest of the chamber bottom is covered by accumulated mud. Its thickness in places is 1,5 m (Fig. 3, C). Such quantities of sediment could be accumulated only by the larger amount of underground water that flowed through the cave in the past. The mud completely fills a possible passage at the southern end of chamber (Fig. 3, B'). The mostly flat mud surface is gently inclined towards point C' (on the ground-plan, Fig. 3) and inter-sected by a few channels centimeters deep and wide. They were formed by dripping water coming out of fissures in the rocky ceiling. This weak water streams flow towards a Mud lake (pound). The lake is 6 m long, 2,8 m wide and (in time of exploration, 6 February 2000) about 10 cm deep. Small terraces in a step that encloses the lake show that water can rise about 20 cm high. Such a rise is conditioned by the amount of precipitation because the water in the lake accumulates from drip-ping water.
In the opposite side of the chamber, there is a "shaft" formed by dripping water. The water leaks along the wall and washes out the mud, together with water disappearing in the rocky and fractured rocky bottom. It is possible that this water occurs in Rogovac cave.
The development of Dolacina mama cave is result of activity and variation in underground water level. The present cave could be just a part of a large cavity filled by sediment after lowering of water level.
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The change in water level also forced the formation of a doline where the cave entrance was opened. Its development can be explained by mechanism of subsidence doline development in combination with collapse. Such forms occur in areas of covered karst where karstifiable beds are covered by superficial deposits or unconsolidated residual soils (Donaldson 1963; Jennings 1971; Newton 1984). They develop through spasmodic subsidence and more continuous piping of these materials into widened fissures and solution pipes in the bedrock beneath. Many authors reported development of such dolines in glacial moraines, residual soils overlying various types of carbonate rocks and granite (Jennings 1971; Newton 1984). They can also be formed by solution at the top of subjacent karst beds and by gradual subsidence or recurrent spasms of subsidence of overly-ing weak bedrock through progressive removal of their support (Jennings 1971).
According to G. W. Donaldson (1963) who analyzed subsidence dolines in residuum overlying dolomite, there are four main factors that influence their development:
1)	voids into which the eroded material can be washed - fissures or cave passages,
2)	a flow of water sufficient to erode the material that must be of such permeability as to allow enough flow to cause erosion,
3)	the soil must be capable of being eroded by the flow of water passing through it,
4)	the soil should have enough inherent strength to arch over the eroded area, forming a roof.
In the doline above Dolacina mama cave all these conditions were present. There is karst bed-rock in which water circulation enlarged a fissure to a shaft leading to cave. Above the bedrock there is a flysch that is crushed and very liable to erosion producing unconsolidated residuum.
Newton listed several mechanisms that cause formation of subsidence dolines (Newton 1984):
1)	loss of buoyant support to roofs of cavities or caverns or to residual clay or other unconsolidated deposits overlying openings in the top of the bedrock,
2)	increase in the velocity of movement of ground water,
3)	increase in the amplitude of water-level fluctuations and
4)	movement of the water from the land surface to openings in the bedrock.
According to the same author main mechanism is fluctuation in water level. Its decline results in higher recharge of surface water that accelerates creation of cavities in deposits overlying karst bedrock.
Subsidence dolines result from roof failures of cavities in unconsolidated deposits overlying karstifiable beds. These cavities are created when the overlying material migrate or is eroded down-ward into fissures or larger cavities in bedrock. The typical cavity in unconsolidated deposits is circular with the configuration of the top resembling a dome or arch. The roof will collapse and fallen material will be washed down the fissure leaving enlarged cavity. When the arching cavity roof is unable to sustain its load it will collapse to form a doline (Donaldson 1963).
The doline above Dolacina mama cave was formed by a similar process (Fig. 4). Water infil-trating through flysch overlying carbonate bedrock enlarged a fissure in bedrock (1). The water flow eroded the flysch and its soft residuum washed downwards through the entrance shaft to the cave (2). In this way a cavity in the flysch deposit formed. Due to the sufficient amount of water, the cavity enlarged until its roof became unable to hold the material above it and therefore col-lapsed (Photo 3). Thus it is a combination of subsidence and collapse of material overlying karstifiable bedrock.
Since the amount of water is small and circulation is slow, it seems that there were at least two phases of subsidence. An important factor was the perimeter of the fissure and later cavity (entrance shaft as we saw it). Since it was obviously quite small, it could be choked by residuum in the past. That could temporarily interrupt movement of material which resulted in the filling of a cav-ity in flysch by collapse of material from its roof (3). That stabilized the doline bottom and inter-rupted further collapse. After water circulation moved the filling material downwards, the process was reactivated (4). The confirmation of such process could be seen from the remains of a former bottom located 2 m above present bottom (entrance to shaft) and 2,3 m below the lower edge of the doline on the northern slope (Fig. 3). During the period of repose, tree roots consolidated the residuum also (as it can be observed even today). Even then water circulated through material that choked it could enlarge a fissure or vertical passage in combination with breakdown from below. The water circulation washed away material that choked it and renewed the process below the temporary bottom. The result was forming a cavity which is still visible bellow the remains of bottom that arched it earlier (Photo 4). Enlargement of this cavity finally resulted in its roof col-lapse and the form of doline as it looks today (5). Since water washed away most of the material, shaft (or pipe) below it is semi-empty until it will probably be filled again by further movement of material from doline slopes.
CONCLUSION
The development of collapse structures above Jama na Sredi (in Entrance chamber and Pantheon chamber) was result of several processes. Water activity widened fissures and created a large cavity. In later phases of development widening resulted in occasional breakdowns and deposition of rock debris. Some of the deposits were partially dissolved or washed away. These processes of widening of voids (laterally and especially upwards) in combination with lowering of karst surface resulted in thinning of the cave roof. Due to its unstability (probably caused by neotectonic move-ments) collapse occurred. The thinning of the cave roof below the collapse doline was accelerated by corrosion in the doline bottom. At the places of thicker and more stable beds in the roof (cave passage and NE part of Pantheon Chamber), collapse is absent. We can conclude that the collapse doline above Pantheon Chamber is a result of widening of the underground chamber by water activity along fissures and cave walls. The collapse was not completed due to the differences in thickness of beds above cave (as a result of different conditions for dissolution) and their stability.
The development of Dola~ina mama cave is result of activity and variation in underground water level. The present cave could be just a part of a large cavity filled by sediment after lowering of water level. The change in water level also forced the formation of a doline where the cave entrance was opened. Its development can be explained by a mechanism of subsidence doline development in combination with collapse. It developed through spasmodic subsidence and more continuous piping of residuum into widened fissures and solution pipes in the bedrock beneath. Due to the sufficient amount of water, the cavity enlarged until its roof became unable to hold material above it and therefore collapsed. Therefore, it is a combination of subsidence and collapse of material overlying karstifiable bedrock and processes in the cave.
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UDORNE STRUKTURE KOT POVEZAVA MED KRA©KIM POVR©JEM IN PODZEMLJEM (PRIMERI S HRVA©KE)
Povzetek
Jama na Sredi, na južnem delu otoka Cresa, je enostavna jama, 54 m dolga in 19 m globoka. Zelo je zanimiva, ker je velik del njenega razvoja vezan na procese podiranja. Nastanek udornih struktur nad jamo (v Vhodni dvorani in dvorani Pantheon) je posledica različnih procesov. Voda je razširila razpoke in ustvarila veliko jamo. V kasnejših fazah razvoja je zaradi vecanja jamskih prostorov prihajalo do obcasnih udiranj in nabiranja podornega kamenja. Nekaj teh odkladnin je bilo raztopljenih ali odplavljenih proc. Zaradi vecanja jamskega prostora (bocnega in predvsem navzgor) in socasnega zniževanja kraškega površja, se je tanjšal jamski strop. Zaradi nestabilnosti (najbrž pospešene z neotektonskimi premiki) je prišlo do udora. Tanjšanje jamskega stropa pod dnom udornice je še pospešila korozija v njenem dnu. Kjer tvorijo strop bolj debele in stabilnejše plasti (jamski rov in severovzhodni del dvorane Pantheon), ni prišlo do udora.
Dolacina mama je majhna enostavna jama z navpicnim vhodnim rovom. Njen razvoj sta narekovali dejavnost in nihanje podzemeljske vodne gladine. Sedanja jama je morda le del velike jame, zapolnjene s sedimenti zaradi znižanja vodne gladine. Sprememba vodne gladine je bila tudi vzrok nastanka vrtace, v kateri se je odprl današnji jamski vhod. Nastanek te vrtace je mogoce razložiti z mehanizmom posedanja v kombinaciji z udiranjem. Nastala je zaradi obcasnih posedanj in bolj stalnega drsenja podornega gradiva skozi razširjene razpoke in korozijske cevi v kamninski podlagi. Zaradi zadostne kolicine vode se je jama vecala toliko casa, dokler strop ni postal nestabilen, ni vzdržal teže kamnine in se je zrušil.