ACTA CARSOLOGICA	33/1	15	237-244	LJUBLJANA 2004
COBISS:1.03
THE ORIGIN OF SEDIMENTS INSIDE THE COLLAPSE DOLINES OF POSTOJNA KARST (SLOVENIA)
IZVOR SEDIMENTOV V UDORNICAH POSTOJNSKEGA KRASA
(SLOVENIJA)
UROŠ STEPIŠNIK1
1 Porentova 3, 1000 Ljubljana, Slovenia. E-mail: urosstepisnik@hotmail.com
Abstract:	UDK: 551.435.83:552.52(497.4 Postojna)
Uroš Stepišnik: The origin of sediments inside collapse dolines of Postojna karst (Slovenia)
Several hundred collapse dolines are recorded on the Slovenian karst surface. Their floors are covered with boulders, scree or a soil layer. In ponor karst areas, where water transports significant amount of allochtonous material, many collapse doline floors are made level by deposits of loamy sediment.
This discussion relates to a detailed study of 15 collapse dolines near the Postojna cave system. Loamy sediment appears within several neighboring collapse dolines and covers their floors at approximately the same altitude. The sediment level preserved in the collapse dolines is commonly at the same elevation as flood loam deposits within nearby caves. It transpires that the flattening of the collapse doline floors is a result of flooding inside the karst that extended above the original floors of the collapse dolines. It is possible to predict some of the sedimentation dynamics inside the karst on the basis of the elevations of the loamy sediments within the collapse dolines. On the other hand, it is also possible to find out about collapse doline development by studying the processes inside cave systems.
Keywords: karst, collapse doline, cave sediment, sediment levels, Postojna karst, Slovenia.
Izvleček:	UDK: 551.435.83:552.52(497.4 Postojna)
Uroš Stepišnik: Izvor sedimentov v udornicah Postojnskega krasa (Slovenija)
Na kraškem površju Slovenije najdemo več sto večjih udornic. Njihova dna lahko prekrivajo podorni bloki, melišča ali prst. Na ponornem obrobju krasa, kjer voda prenaša večje količine alohtonega materiala, so dna mnogih udornic zapolnjena z ilovnatimi sedimentnimi uravnavami. Podrobneje sem preučil 15 udornic v bližini Postojnskega jamskega sistema. Pokazalo se je, da so ilovnate sedimentne uravnave na istih nadmorskih višinah v večih sosednjih udornicah hkrati. Nivoji sedimentnih uravnav so primerljivi z višinami zapolnitev poplavne ilovice v bližnjih jamskih rovih. Proces sedimentacije in uravnavanja dnov udornic v nivoje je posledica poplav, ki so segale nad dna udornic in so povzročile sedimentacijo ilovnatega materiala in uravnavanje v njihovem dnu. Zato lahko na podlagi nadmorskih višin sedimenta v dneh udornic predvidevamo dinamiko sedimentacije v kraškem podzemlju, hkrati pa lahko na podlagi razvoja jamskega sistema ugotavljamo razvoj udornic. Ključne besede: kras, udornice, jamski sedimenti, sedimentna nivoji, Postojnski kras, Slovenija.
INTRODUCTION
The Postojna karst lies in the northern part of the Pivka basin, where the Pivka and Lekinka rivers sink. A major cave system has developed here, including Postojna and many nearby caves.
There are many collapse dolines on the surface of the Postojna karst. Most of them lie above inactive cave passages or their expected continuations. They are the result of cave system disintegration. Some of them are positioned above active cave passages, and we can observe recent collapse doline development at the surface and inside the caves.
Collapse dolines in this area are dissimilar in shape and dimensions. The only characteristic that is common to several collapse dolines is the elevation of their floors. The floors of neighboring collapse dolines are commonly at the same elevation.
Habič (1963) noticed a similar phenomenon in collapse dolines between Logatec and Vrhnika in central Slovenia. He supposed that the floor levels of collapse dolines were related to a few distinct elevation zones. He thought that this was a result of cave formation at just a few levels and cessation of the undermining of collapse dolines at the moment when they emerge at the surface.
This research was intended to investigate the appearance of the floors of collapse dolines lying at the same elevation. Though there are many collapse dolines on the Slovenian karst, comparable phenomena were noticed only in some of the collapse dolines in the regions of Matarsko podolje, Divača karst, Postojna karst and near the Ljubljanica springs. The Postojna karst was chosen for the
Collapse doline	Elevation of floor
VODNI DOL (central part)	496
VODNI DOL (eastern part)	520
VODNI DOL (western part)	521
DOLGA DOLINA (central part)	522
DOLGA DOLINA (eastern part)	529
DRČA DOLINA	529
BLIND VALLEY SV. ANDREJ	529
MAGDALENA JAMA	530
BLIND VALLEY RISOVEC	530
M. JERŠANOVA DOLINA	539
V. JERŠANOVA DOLINA	539
COLLAPSE DOLINE WEST OF STARA APNENCA	547
COLLAPSE DOLINE NORTH OF V. JERŠANOVA DOLINA	549
KOLIŠEVKA	553
STARA APNENCA	553
Tab. 1: Collapse dolines with elevation of their floors. Tab. 1: Udornice z nadmorskimi višinami dnov.
study because there have been extensive past studies of its surface and underground features (Gams, 1966; Gospodaric, 1976; Šebela, 1998).
COLLAPSE DOLINES IN THE POSTOJNA KARST
The Postojna karst extends across the area of the Postojna cave system, in the northern ponor part of the Pivka flysch basin. Underground water has formed an extensive epiphreatic cave system predominantly comprising tunnel-like cave passages (Šušteršič, Knez, 1995).
On the Postojna karst 15 of the larger collapse dolines (Tab. 1) lie above the Postojna cave system and its expected continuations. The collapse dolines are of various forms and dimensions. Kafrna dolina, the smallest collapse doline in this group, is 35 m deep and has a volume of 0.14 Mm3. Vodni dol is the biggest among them, at 60 m deep with a volume of 2.9 Mm3.
Several neighboring collapse dolines have floors at the same elevation, and the floors are flat due to the presence of loam deposits. In the Postojna karst most of the collapse doline floors fall into groupings at five general levels. Those in the eastern and western parts of Vodni dol, together with the floor of Dolga dolina, lie at elevations between 520 and 522 m. The floors in the eastern part of Dolga dolina, Drča dolina and the Magdalena jama collapse doline are at elevations between 529 and 530 m. Velika Jeršanova dolina and Mala Jeršanova dolina have floors at 539 m. The floors of Stara apnenca, Koliševka, an unnamed doline north of Velika Jeršanova dolina, and a collapse doline west of Stara apnenca are at elevations between 547 and 553 m (Fig.1).
The floors of the collapse dolines at Pivka jama, Črna jama, Kafrna dolina, Kozja jama and north of Jama Koliševka are not flattened by loam deposits and do not fit into any of the groupings described above. The process of undermining is still active beneath the floors of these dolines, or their floors are not yet deep enough to reach any of the levels described.
THE ORIGIN OF THE LOAMY SEDIMENT FILL
The loamy sediment deposits that form the flat floors of the collapse dolines were examined in detail. Petrographic analysis of the sandy fraction in the sediment revealed that the loam contains particles of flysch origin. Grain-size analysis of the sediment revealed that the loam comprises mainly clay and silt size particles. Flysch-derived sandstone fragments about 1.5 cm in diameter were found in the Velika Jeršanova dolina collapse doline. These fragments were washed from an unroofed cave that is exposed higher on the slope of the collapse doline.
Loamy sediment deposits that form the level floors of investigated collapse dolines probably originate from the Pivka flysch basin, from where the River Pivka and other streams transport al-lochtonous flysch-derived sediment into this part of the karst.
Grain-size analysis confirms that the loam comprises clay and silt particles with a diameter lesser than 0.1 mm. For suspended material of this size to be deposited the water flow velocity must be less than 0.01 m/s, or the water must be stagnant (Ford, Williams, 1989).
Similar floor level elevations inside many collapse dolines suggest that the process that led to sedimentation of the fine allochtonous particles from stagnant water was active across a wide area, not just locally inside isolated collapse dolines.
It is likely that deposition of the loamy sediment fills inside the collapse dolines resulted from
Fig. 1: Loamy sediment levels in Postojna karst. 1. cave, 2. sediment level at 494 - 495 meters, 3. sediment level at 520 - 522 meters, 4. sediment level at 527 - 530 meters, 5. sediment level at 539 meters, 6. sediment level at 547 - 553 meters, 7. collapse doline.
Sl. 1: Ilovnate sedimentne uravnave na Postojnskem krasu. 1. jama, 2. sedimentni nivo na 494 - 495 metrov, 3. sedimentni nivo na 520 - 522 metrov, 4. sedimentni nivo na 527 - 530 metrov, 5. sedimentni nivo na 539 metrov, 6. sedimentni nivo na 547 - 553 metrov, 7. udornica.
flooding inside the Postojna karst. The original floors of the affected collapse dolines extended below the upper limits reached by the floodwaters that supplied the loamy sediment that formed the new flat floors.
Accepting the assumption that the loamy sediment in the collapse doline floors is of flood origin, the question arises of whether similar sediment levels also formed inside nearby cave passages and in neighboring areas whose elevation was below flood level.
Results of detailed study of the Postojna cave system by Gospodarič (1979) have been re-examined, and new field examinations carried out, in a search for flood loam deposits inside cave passages and on parts of the surface that are lower than the known sediment levels. In parts of Črna jama there are flood loam deposits at 495 m altitude. At the 520 to 522 m level there are flood loam deposits in sections of Postojnska jama, Črna jama and Jama Koliševka. Flood loams are found in sections of Otoška jama and Postojnska jama at the 529 to 530 m level. Loamy sediment is also found at this elevation within the Risovec and Sv. Andrej blind valleys. No flood sediments were found inside caves at the highest level recorded in the collapse dolines, at elevations between 535 and 547 m. This level has a large vertical range (12 m) and any associated deposits might have formed in more than one flood phase or been partly washed away after deposition.
The flood loam levels might be result of several phases of flooding. They can be found inside cave passages, collapse dolines and parts of neighboring surface which extend below flood levels. With further investigation of flood loam levels would be posible to establish dynamic of some processes inside cave system and collapse dolines.
CONCLUSION
Detailed investigation of collapse doline floors at similar elevations on the Postojna karst revealed that:
-	the floors have been leveled by deposits of loam sediment;
-	the loamy material probably originated from Pivka flysch basin;
-	the flat loamy floors of collapse dolines are grouped at or within five distinct levels or level ranges;
-	the occurrence of collapse doline floors at similar elevations might be a result of sedimentation of suspended material from floodwaters that inundated the lower parts of several neighboring collapse dolines at the same time;
-	similar flood loam deposit levels are also found inside cave passages and on nearby parts of the surface that extend below the depositional levels;
-	it is also possible to establish that development of collapse dolines is related to processes inside the cave system.
REFERENCES
Čar, J., Šebela, S., 2000: Velika Jeršanova doline - a former collapse doline. Acta carsologica, 29, 2, 201-212.
Ford, D. C., Williams, P., 1989: Karst Geomorphology and Hydrology. Wellington, Uniwin Hyman, 601.
Gams, I., 1966: On the Hydrology of the Territory among the poljes of postojna, Planina and Cerknica, Acta carsologica 4, 1, 5-54. Gospodarič, R., 1976: The Quaternary Caves Development Between the Pivka Basin and Polje of
Planina. Acta carsologica 7, 1, 5-135. Habič, P, 1963: Dolines en forme de puits, dites koliševke, et le cours d'eau soutterain (Resume).
Treci jugoslovenski speleološki kongres. Speleološki savez Jugoslavije, Sarajevo, 1-272. Knez, M., Šušteršič, F., 1995: Prispevek k slovenskemu speleološkemu pojmovniku. Naše jame, 37,1, 153-171.
Šebela, S., 1998: Tectonic Structure of Postojnska jama Cave System. Ljubljana, ZRC SAZU, 112.
IZVOR SEDIMENTOV V UDORNICAH POSTOJNSKEGA KRASA
(SLOVENIJA)
Povzetek
Na površju Postojnskega krasa se nahaja več udornic. Največ udornic leži nad neaktivnimi jamskimi rovi ali njihovimi nadaljevanji in so produkt razpadanja jamskega sistema. Udornice si med seboj niso podobne ne po oblikah ne po dimenzijah. Edina značilnost, ki je udornicam skupna, so nadmorske višine dnov nekaterih udornic. Pokazalo se je, da so dna večih sosednjih udornic pogosto na isti nadmorski višini.
Udornice, ki imajo dna na istih nadmorskih višinah, imajo v dneh večje ilovnate uravnave. Tako lahko na Postojnskem krasu najdemo pet nivojev v katerih so razporejena dna. Nadmorske višine dnov v V in Z delu Vodnega dola ter v dnu Dolge doline so razporejene v enem nivoju, ki je na nadmorski višini od 520 do 522 m. Dna v vzhodnem delu Dolge doline, v Drči dolini in udornici Magdalena jama, se nahajajo na nadmorski višini od 529 do 530 m. Dna Velike in Male Jeršanove doline sta na nadmorski višini 539 m. Nadmorska višina dnov v udornici Stara Apnenca, Jama Koliševka, udor-nici Z od Stare Apnence in v udornici S od Velike Jeršanove doline je med 547 in 553 m. Ilovnata uravnava dna na nadmorski višini 495 metrov je le v udornici Vodni dol (Tab. 1).
Ilovnati sediment, ki ga najdemo v uravnanih dneh udornic, po vsej verjetnosti izvira iz flišne Pivške kotline, od koder reka Pivka in ostali vodonosniki vnašajo alohton flišni sediment v podzemlje. Granulacijska analiza je pokazala, da prevladujejo meljasti in glinasti delci s premerom do 1 mm. Za odlaganje suspendiranih delcev te velikosti iz vode, je morala biti hitrost vode manjša od 0,01 m/s, oziroma oziroma je voda stala (Ford, Williams, 1989).
Uravnanost večih udornic na isti nadmorski višini nam kaže, da je bil proces, ki je povzročal sedimentacijo finih alohtonih delcev iz stoječe ali počasi tekoče vode vezan na več udornic hkrati. Po vsej verjetnosti je oblikovanje nivojev ilovnatih uravnav v dneh udornic vezano na poplavljanje v podzemlju Postojnskega krasa. Dna udornic so segala pod nivo poplavne vode in so bila zasedi-
mentirana do višin poplav.
S pomočjo podrobnih opisov sedimentov Rada Gospodariča (1976) v Postojnski jami in terenskim opazovanjem sem ugotavljal nivoje poplavnih ilovic v jamskih rovih in delih površja, ki segajo pod nivoje ilovnatih sedimentnih uravnav. Na nivoju 495 m. n. v. so poplavne ilovice v delih Črne jame. Na nivoju 520 do 522 m. n. v. so poplavne ilovice v delih Črne jame, v Jami Koliševki in v zgornjem Tartarju v Postojnski jami. Poplavne ilovice na nivoju od 529 do 530 m. n. v. so v delih Otoške jame in v delih Postojnske jame. Ta nivo poplavnih ilovic lahko najdemo tudi na površju v slepih dolinah Sv. Andrej in Risovec. Nivo na nadmorski višini 539 m. n. v. najdemo v Jami na poti in v Pisanem rovu Postojnske jame. Najvišji nivo, med 547 in 535, je višje od rovov Postojnske jame, zato poplavnih ilovic v jamskih rovih v tem nivoju ni. Ta najvišji nivo je na višinskem razponu 12 metrov in je deformiran zaradi površinskega spiranja ali pa je nastal v večih različnih poplavnih obdobjih (Sl. 1).
Različni poplavni nivoji so lahko posledica večih poplavnih obdobij. Poplavne ilovice v nivojih najdemo v jamskih rovih, udornicah in okoliškom površju, ki je segalo pod nivo poplav. S proučevanjem poplavnih ilovic, ki so bile odložene v istem poplavnem obdobju na površju in v podzemlju, bi bilo mogoče ugotavljati dinamiko nekaterih procesov v jamskem sistemu in v udornicah.