Research of the Miljacka IV cave (cave by the mill at the Miljacka slap waterfall)

The Freatik society for karst exploration and the Zagreb Speleological Federation conducted speleodiving research of the channel in the Miljacka IV cave in the period from 17–25 August 2022 The cave research in the area of the Miljacka slap waterfall first began in the 1970s. The last research conducted in the main channel gave great impetus for the further research of the cave, which stopped in the wide channel at a depth of 15 metres. During the research, special equipment was used for diving in overhead environments where it is not possible to surface. For additional safety, separate first and second level cold-water adapted regulators were used. To measure depth, canal azimuth and water temperature, the Suunto EON Steel dive computer containing a digital compass and measuring tape was used. To film the speleological structure, two devices were used, Mnemo and ENC 2. All data obtained were compared with data obtained using standard manual measurements, and a detailed topographic map of the structure was created. The research discovered a new 240 metres of submerged channels. The length of the Miljacka IV cave is now 500 m and depth 32 m, with an elevation difference of 34 m. With the exception of the entrance section, the Miljacka IV cave currently has no known dry fossil parts of the canal, and in terms of its hydrological activity is the most active of all the caves in the Miljacka slap area. This suggests that it may be the youngest in terms of its speleogenesis, particularly since it is situated at a lower elevation than the remaining caves, and is nearer the erosion basis. However, stalactites and curtains were discovered in submerged sections of the cave, clearly indicating that it was dry at some period. The presences of speleothems that form in dry conditions indicate that in some period, the entire structure was dry. There may be different reasons for this, either that the ancient watercourse changed direction over its geological history, or did not exist at a certain period, or the position of the canal in the Miljacka IV cave changed positions due to tectonic activity (folding and splitting) and at some point of time was above the erosion base of the river. From the dry part of the cave, it is possible to enter the submerged main channel in two places. One place is immediately behind the entrance hall, and the second is from the lake at its northwestern tip. The lake access to the canal is a space that was formed by erosion and corrosion processes of strong water flow. These processes opened a low and wide passage between the horizontal layers, and several dives were needed to find the right way to the main channel. The main channel generally extends in a northwest to southeast direction for the first 120 metres, when its dimensions are 4 x 1 metre, with a side and short cross-section. In the flanks of the channel, eroded layers of lesser thickness are visible. The bottom of the channel shows eroded remnants of layers seen as sharp rock fragments of larger and smaller size, where in the parts of the channel where there are connections to secondary channels or channels that are promising for future research, there are numerous potholes. The shapes of the cross-sections are irregular lens shapes indicating that the erosion and corrosion processes were from the side (likely along the layer surface or along a more soluble layer of a different type of sedimentary rock, as layers of promina deposits and limestone alternate throughout the cave). In addition to expanding the channel sideways by creating fissure niches several metres wide, in certain parts of the channel it expanded in the ceiling along the tectonic cracks, which are likely an indicator of series of parallel fault lines and all as the result of intensive folding and splitting along the entire area (in which the caves Miljacka II and the Miljacka I-V system were formed). An analysis of the direction of the channel and its frequency indicates that the dominant direction of the channel in the cave Miljacka IV is northwest to southeast, and the azimuth of the channel is between 280°-290°/100°-110°. At several locations, the main channel has secondary channels that re-join back onto the main channel. They are of smaller dimensions and only a few metres long. After the first 70 metres, the channel suddenly changes direction sharply, in the direction 10°-190°, and after 120 metres the channel again suddenly changes directly sharply back to the direction NW-SE. The speleogenesis of this part of the channel can be connected with a series of transverse cracks in the fault zone, so that the entrance part of the cave that was indented is a part of these lateral (relaxation) cracks connect the series of parallel fault lines such as the main channel of the Miljacka II cave and the main channel of the Miljacka I-V system. In the first 20 metres of the submerged part of the main channel, an adult specimen of the olm, Proteus anguinus, was always observed. Other fauna was observed in large numbers along the entire researched part of the main channel in the parts of the channel with high water flow. At the end of the researched section (near the fifth siphon in the section called the Bioportal), large colonies of bacteria were observed. Here the water flow is strongest, and the Seacraft flow meter measured a flow of 70 m/min. The water temperature was 12.9°C. In addition to the bacterial colonies, every dive recorded members of cave fauna, particularly crustaceans of the following genera: Niphargus, Monolistra and Troglocaris. The research was stopped at a depth of 18 metres in the main channel, as divers could not continue due to the exceptionally strong flow. There are prospects for continuing this research, but in that case should be conducted in conditions of lower water levels. Further research is required to conduct topographic imaging of all the secondary channels that are connected to the main channel, and at the assumed connection of the siphon and lake on the main channel. It is interesting that in 2019, research of the Miljacka II cave near the fourth siphon reached a part where there was an exceptionally strong flow of water that sank into lower levels. It can be assumed that this is the same flow that appears in the cave Miljacka IV, and future research of that channel could confirm the connections between these two caves. The total length of the explored cave in the area of Miljacka slap waterfall (Miljacka II, Miljacka I-V system, Miljacka III and Miljacka IV) has now come to 6736 metres. Photo: Florian Launette, Ivan Miloš