Kurilo Threshold – a Key Geosite in Geopark Iskar Gorge Курилският Праг – Ключов Геотоп В Геопарк „Искърски Пролом“ Dimitar Sinnyovsky Димитър Синьовски
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СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 81, кн. 3, 2020, с. 271–273 REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 81, part 3, 2020, p. 271–273 Национална конференция с международно участие „ГЕОНАУКИ 2020“ National Conference with international participation “GEOSCIENCES 2020” Kurilo threshold – a key geosite in geopark Iskar Gorge Курилският праг – ключов геотоп в Геопарк „Искърски пролом“ Dimitar Sinnyovsky Димитър Синьовски Минно-геоложки университет “Св. Иван Рилски”, 1700 София; E-mail: [email protected] Keywords: Kurilo threshold, Geopark Iskar Gorge. Introduction the Balkan Mountain through the picturesque Iskar Gorge, then passes through the Moesian platform The first Bulgarian geopark “Iskar Defile” estab- and flows into the Danube. lished in the frame of the project for elaboration of During the early Miocene, 15–20 Ma ago, the So- the “Register and Cadastre of the Bulgarian geo- fia field was a hilly plain with small rivers flowing to logical phenomena” funded by the Ministry of En- the north, inhabited by tropical flora and fauna which vironment and Waters, was developed according to may be illustrated by restoration of Jay Matternes the concept based on the remarkable geodiversity (Fig. 1a). In the middle Miocene 14–15 Ma ago, a of the Iskar Gorge providing picturesque landscapes regional tectonic activity was manifested and the and wonderful outcrops of various rock types rang- early Miocene peneplain was cut by numerous faults, ing from Precambrian to Quaternary (Jelev et al., forming grabens and horsts. A new mountain began 2002). Despite of its high scientific and scenic value to grow from the south – Rila, and in the Sofia Plain geopark “Iskar Defile” remained in a draft version а large graben started to form, the initial subsidence due to lack of funding and disagreement between of which was compensated by fluvial sedimentation. the stakeholders and the decision makers. This re- According to Kanev (1988) the Sofia graben, which markable geomorphological formation is located started to form in the Pontian age (7 Ma ago), inter- between Kurilo threshold near Sofia and the verti- rupted the river flow through the Miocene hilly plain cal beds of the Rocks near the village of Lyutibrod. and started the formation of the Sofia Lake (Fig. 1b). It In 2018 the bottom-up initiative for establishment existed during the Pontian and Dacian stages between of geopark under the name “Iskar Gorge” was re- 7 and 4.5 Ma ago. At the beginning, the subsidence inforced by the Iskar municipalities Svoge, Mezdra was compensated by fluvial sedimentation forming and Novi Iskar, the last of which is constituent of a variegated terrigenous unit. Then typical lake sedi- the big metropolitan municipality of Sofia. This ini- mentation is established and more than 700 m thick tiative was supported by the Ministry of Environ- sediments were accumulated at its bottom forming ment and Waters and the University of Mining and three formal lithostratigraphic units: Gniljane, Novi Geology “St. Ivan Rilski” (Sinnyovsky et al., 2019). Iskar and Lozenets Formations united in Sofia Group Reinforcement of the efforts for geopark establish- (Kamenov, Kojumdgieva, 1983). ment provoked new investigations of geosites of Initially the new Sofia Lake began to flow west- scientific value among which is the Kurilo thresh- ward through a narrow channel underlined by the old which is directly responsible for the formation Upper Jurassic limestones of the Slivnitsa Forma- of the Iskar Gorge. tion (Barlozhnitsa threshold – Kanev, 1988). The restoration of the shoreline along the contour with Formation of the Iskar Gorge altitude 690 m which is the modern altitude of the Iskar Gorge is closely connected with the so called Barlozhnitsa threshold (Fig. 1b), shows that this is Sofia Basin or Sofia Lake which existed for about the only possible way for a sea connection through 2.5 Ma during the late Miocene and early Pliocene. the small Gaber Basin, Nesla Gorge and Nishava- The outflow of the Sofia Lake to the north in the Moravian Basin which was connected to the Panno- middle of the Dacian Age is the main reason for the nian Sea located on the present territory of Hungary formation of the picturesque gorge crossing the Bal- and parts of Serbia, Bosnia and Herzegovina, Ro- kan Mountain. mania, Austria, Slovenia and Slovakia. It is proved Now Iskar River takes its source from Rila by several layers in the Novi Iskar Formation con- Mountain, the second highest mountain in Eastern taining bivalves and gastropods, characteristic of Europe after Caucasus, passes through the Sofia the Pannonian Basin – part of the large Paratethys Valley between Pancharevo and Novi Iskar, crosses Basin (Kamenov, Kojumdgieva, 1983). Neverthe- 271 less, Sofia Lake flowed to the north through the so southern “lake river” flowed to the lake and the called Kurilo threshold. northern “mountain river” (Pre-Iskar) – to the Low- Kanev (1988) considered several theories for the er Danube Basin in the Moesian platform. There Iskar Gorge formation. According to him the upper was a saddle between the upper courses of the riv- course of the modern Iskar River coincides with a ers, but the erosional basis of the northern river small river flowing from Rila Mountain that fed the was obviously lower than the southern lake river, Sofia Lake from the south. Тhe formation of the so it incised faster. In this way a deeper streambed gorge started with the overflow of the Sofia Lake was formed against the streambed of the lake river, across the Kurilо threshold. There are at least three which ”attacked” the threshold from the north and suggestions about the gorge formation considering “stole” some of the water of the lake river. This pro- the way of breaking of the threshold: (1) lake-over- cess called “piratery” was carried out under the in- flow, (2) erosional and (3) antecedent origin. fluence of the lower erosional basis of the Pre-Iskar, The first theory is based on the assumption that whose valley cut faster and “ate away” the threshold the lake began to overflow, forming a stream across from the north until the lake began to flow along the the Kurilo threshold. This could be done only in the Pre-Iskar valley and formed the gorge. case of abundant water inflow from the Rila Hills. The antecedent origin suggests that the Iskar Initially, along the river bed, which began to cut into River existed before the mountain rise and it is su- the rocks, water flowed only occasionally during in- perimposed over the newly uplifted Balkan Moun- tensive rainfall and high lake level. Then the stream tain range. However, the formation of the lake started to form a channel, which gradually turned presupposes existence of a fence mountain from into a gorge, north of which, through the Danube the north. Otherwise, it could not have existed for Plain, the river flowed along the Pre-Iskar riverbed 2.5 million years, as evidenced by the lacustrine to the Lower Danube (Dacian) Basin, which was sediments of the Novi Iskar Formation. part of the Paratethys, fragmented during the Plio- The question arises as to why the lake did not cene into separate small basins. drain through the Barlozhnitsa threshold. Accord- According to the second theory the breaking of ing to Kanev (1988) the erosional basis of the the Kurilo threshold was carried out between two Djerdap threshold in Serbia (Fig. 1c) was at higher river valleys flowing in opposite directions. The altitude and the Pannonian Basin was with higher a b c d Fig. 1. Neogene history of the Sofia Valley: a, late Miocene faunal association (restoration by Jay Matternes) with which the late Miocene landscape of the Sofia field can be illustrated, before the formation of the Sofia graben; b, restoration of the shoreline along the contour with altitude 690 m of the Sofia Lake, which existed for 2.5 million years during late Miocene–early Pliocene in the present Sofia Valley; c, the Djerdap threshold between Serbia and Romania, the channel through which the Danube now crosses the Carpathian-Balkan chain; d, the Kurilo threshold through which Sofia Lake flowed to the north and formed the Iskar Gorge 272 lake basis than the Lower Danube Basin. Thus, the through Sofia Valley, Stara Planina and Danube Balkan rivers flowing into the Lower Danube Basin Plain to the Danube. incised faster than Pannonian rivers and Pre-Iskar River reached the Kurilo threshold earlier. The buffer role of the Gaber Basin has also contributed Conclusion to this. Its connection with the Nishava-Moravian Regardless of which theory is correct, there is no basin, in contrast to that with the Sofia Lake through doubt that Sofia Lake was connected with the Pan- the Barlozhnitsa threshold, took place through a nonian Basin and flowed out through the Kurilo well-formed gorge called the Nesla Gorge (Sinnyo- threshold. The first theory raises some doubts about vsky, 2020, this volume). Due to these obstacles the the reasons for the lake overflow. Apparently, pen- Barlozhnitsa threshold composed of limestones pre- vented the narrow channel between Sofia Lake and etration of lake water through the Kurilo threshold Gaber Basin to incise a deeper gorge and Sofia Lake was quite long, as evidenced by the river terraces flowed through the Kurilo threshold (Fig. 1d). preserved on both slopes up to 100 m above the cur- There is no doubt that the marine molluscs in rent riverbed. The transfusion may have occurred the sediments of the Novi Iskar Formation, depos- at certain intervals, as Kanev (1988) suggested, but ited on the bottom of Sofia Lake, indicate two-way the reason was not the intense rainfall. Sea level movement of water across the Barlozhnitsa thresh- fluctuations, due to Milankovitch climatic cycles, old.