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Caves formed in karst terrains are often trapping big amounts of different kinds of sediments. Cave sediments can be very useful for the determination of conditions and processes that were present at the time of their deposition. They are usually well preserved so they can give us a lot of i nformation about changing of the conditions over a long period (sometimes even over a few million years). One of the key methods that can provide us with a deeper insight into the complex karst systems is magnetic susceptibility, which shows the magnetism of measured material. Magnetic susceptibility measuring allows recognition of iron-rich minerals (together with their concentration and volume), classification of different types of materials and a better understanding of tectonic events. This is also particularly important in the context of exploration of karst waters that are important from the aspect of vulnerable karst ecosystems and strategic reserves of drinking water. In this research, two caves were chosen for sediment analysis. Both caves are located in the northern part of Velebit mountain, named Nedam and Ledena jama. Nedam is Croatia's fourth deepest pit with a depth of 1021 m. Ledena jama, is 536 m deep pit that is significant by the perennial ice between depths of 50 and 160 m, and is located in a paleoglacier valley. Nedam is located on steeper and more inaccessible karst terrain. The entrances are at different altitudes – at 1420 m for Nedam, and at 1235 m for Ledena jama. That altitude difference and different terrain morphology (paleoglacier valley vs. steep karst), suggest that interesting differences in the sediments studied may be present. Sediment samples were taken at 50 m depth in Ledena jama, and at 100 m depth in Nedam. Preliminary magnetic measurements were performed using the SM30 instrument (ZH instruments). The additional analyzes made on the sediment samples were the determination of mineralogical composition – identification of clay minerals by X-ray diffraction analysis, separation of light and heavy mineral fraction, and observing under the microscope. Magnetic research provides complementary data to these methods. From this analyzes, we can deduce the province of the sediments, and some conditions that were present during their deposition and terrain formation. The mineral composition of the samples from both pits was compared. This work was done in collaboration with Northern Velebit National Park, Krasno (project Exploration of the deep pits of the Northern Velebit National Park), Department of Geology, Faculty of Science, University of Zagreb and Speleological Society Velebit, Zagreb.

Research of Northern Velebit started in the early 90s by Slovak cavers. From then on, each year at least one caving expedition is carried out in the Northern Velebit. In 22 years in the Northern Velebit area a total of 348 caves were explored of which three caves are deeper than 1, 000 m, five caves are deeper than 500 m, nine caves deeper than 200 m, and 27 caves deeper than 100 meters. Other explored caves do not reach 100 m of depth. Most of cave entrances are located at an altitude between 1, 400 to 1, 600 m. Basic morphological features of Velebit caves are verticality and the incidence of major verticals. The biggest discovered verticals are located in Patkov gušt (P553), Cave systemVelebita (P513), Meduza (P333) and in Cave system Lukina jama (P329). Five verticals are deeper than 200 m and 100 m verticals are quite common. The majority of Croatian caving associations participated in the cave research but the most of those expeditions were organized by the Speleological Committee of Croatian Mountaineering Association. During all these years of research an excellent international cooperation was formed with cavers from Slovakia, Hungary, Belgium, Polish, Lithuania, France, Italy, Switzerland, Great Britain, Slovenia, Spain, Bulgaria, USA and Serbia. In the last four years, there were four expeditions. Results of these expeditions are presented in this article.

Ronjenje u speleološkim objektima najzahtjevnija je djelatnost u speleologiji. U Hrvatskoj je ronjenje u speleološkim objektima započelo 1959. u špilji Veternici kraj Zagreba. Roniti u dubokim jamama znači najprije se spustiti duboko u jamu i tek onda roniti. U Hrvatskoj je ronjeno u svega nekoliko dubokih jama, kojima je ronjenjem povećana njihova dubina. Ronjeno je na dah i s ronilačkim aparatima, u mokrim i suhim ronilačkim odijelima. Duboke jame, dublje od 200 m, u kojima je ronjenjem povećana Klepinoj dulibi 2 ili PKD2 (-254 m), obje na srednjem Velebitu, jama Punar u Luci (-350 m) istočno od Gračaca, jamski sustav Lukina jama – Trojama (-1421 m) na sjevernom Velebitu i novootkrivena jama na Crnopcu (-512 m)., Cave diving is the most challenging of speleological activities. In Croatia cave diving began in 1959 in the Veternica cave near Zagreb. To dive in deep caves firstly means descending into caverns and then diving. In Croatia diving was carried out only in few deep caves which were also made deeper by diving. Diving was carried out as free diving and with dive apparatus, in wet and dry suits. Deep caverns, deeper than 200m, in which diving has increased their depths are: Ponor u Klepinoj dulibi 1 (-238 m), Ponor u Klepinoj Dulibi 2 or PKD2 (-254 m), both in central Velebit, sinkhole of Punar in Luka (-350 m) east of Gračac, the cave system of Lukina Hole – Trojama (-1421 m) in north Velebit and the newly discovered cave on Crnopac (-512 m).

Istraživanja speleoloških objekata središnjeg dijela sjevernog Velebita traju sedamdesetak godina (od 1930.), tijekom kojih je istraženo više od 300 špilja i jama. Značajna otkrića ostvarena su tek prije deset godina, kada je otkrivena Lukina jama. Od 1992. pa do danas otkriveno je 18 jama dubljih od 120 ni, od čega dvije dublje od 1300 m sustav Lukina jama - Trojama dubok 1392 m i Slovačka jama duboka 1301 m). U ovim istraživanjima ostvareno je nekoliko svjetski značajnih rezultata, npr. Lukina jama najokomitija je jama na svijetu, ronjenje u Lukinoj jami na dubini od 1355 m (50 m daleko i 6 m duboko) najdublje je na svijetu, jama Meduza (-707 m) ima najveću unutarnju okomicu na svijetu (duboku 450 m), a jama Patkov gust (553 m) ima drugu po veličini vanjsku okomicu na svijetu (duboku 553 m). U ovim istraživanjima uz hrvatske speleloge sudjelovali su slovački i poljski speleolozi, a Lukinu jamu posjetili su već mađarski, belgijski, nizozemski i litvanski speleolozi. Istraživanja se nastavljaju., The speleological objects on the central part of north Velebit have been under exploration for seventy years now (from 1930). During that period more than 300 caves and pits were found and investigated. The important discoveries, however, started to be realized some ten years ago after Luka's cave was found. Meaning, from the 1992 were discovered 18 caves deeper than 120m and the two deeper than 1300 m (Luka's cave-Trojma system 1392 m deep, the Slovakiam cave 1201 in deep). In these explorations were realized some results having the world-wide importance i.e. Luka's cave is the most vertical in the world, diving in it on the depth of 1335 m (length 50 m, depth 6 in) is the deepest in the world. Then there is Meduza's cave (- 707m) with its internal vertical altitude the first in the world, while Patkov gust cave (- 553 m) with its outside vertical altitude is the second in the world. Beside Croatians, in these explorations cooperated also the speleologists from Slovakia and Poland. Luka's cave was visited until now by the Hungarian, Belgian. Dutch and Lithuanian speleologists. The explorations will continue., Die Untersuchungen der speläologischen Objekte des mittleren Teils des nördlichen Velebit dauern schon ungefahr siebzig Jahre (seit 1930). Im Laufe dieser Jahre wurden mehr als 300 Höhlen und Gruben untersucht. Bedeutende Entdeckungen wurden erst vor 10 Jahren verwircklicht, als die Lukina Jama entdeckt wurde. Vorn 1992 bis zu heute wurden 18 Gruben entdeckt, die tiefer als 120 m sind (System Lukina Jama - Trojama - Tiefe 1392 m, und Slovačka /Slowakische/Jama - Tiefe 1301 m). In diesen Untersuchungen kam man zu einigen weltlich bedeutenden Resultaten, z.B. die Lukina Jama ist die senkrechteste Grube in der Welt, das Taufen in dieser Grube in der Tiefe von 1 355 m (50 in weit. und 6 m tief) ist am tiefsten in der Welt, die Grube Meduza (-707 m) hat die größte innere Senkrechte in der Welt (Tiefe 450 m), und die Grube Patkov Gust (553 m) hat nach der Größe die zweite äußere Senkrechte in der Welt (Tiefe 553 m). Neben der kroatischen Speläologen nahmen auch die slowakischen und polnischen Speläologen teil. und die Lukina Jama besuchten schon die ungarischen, belgischen und Iitauischen Speläologen.