Bashkir Shikhans: Toratau, Yuraktau and Kushtau

Description

Toratau Shikhan: N53°33′15.57 E56°05′56.78

Yuraktau Shikhan: N53°44′29.72 E56°05′49.86

Kushtau Shikhan: N53°41′55.69 E56°04′57.13

Bashkir Shikhans: Toratau, Kushtau and Yuraktau are three separate mountains (or shikhans); thanks to the unique landscapes they are deservedly the hallmark of the Republic of Bashkortostan (Southern Urals). The mountains rise majestically on the right bank of the Belaya River valley to the east of the city of Sterlitamak. Shikhans are well studied from the point of view of geological structure and palaeontological remains.

The complex of three shikhans is a perfectly preserved part of one of the largest reef systems on the planet, which existed during the Early Permian on the eastern edge of the Laurussia (298.9–290.1 million years ago). The geological sections of Toratau, Kushtau and Yuraktau show the final stages of the existence of the Palaeouralian Ocean and the formation of Pangaea, traces of catastrophic earthquakes, as well as evidence of climate change and biota of the past associated with global palaeogeographic and biospheric changes. Each of the three shikhans represents a carbonate reef massif raised to the surface by tectonic movements and transformed by weathering processes.

The uniqueness of the nominated object lies in the combination of the peculiar structure of the Earth's crust and the various rare species of plants and animals associated with it, which form a single natural geological and landscape complex.

All these features make the nominated object an ideal information platform and a centre of attraction.

Toratau, Kushtau and Yuraktau are complex natural monuments of Bashkortostan-republican significance.

Geology

Each reef massif – Toratau, Kushtau and Yuraktau – is unique and peculiar, despite the proximity of external similarity and geological structure. The shikhans are part of a single reef system and differ from each other in the specifics of palaeontological communities and features of palaeogeographic environments.

The structure of the massifs is similar to the anticline fold; landslide structures are observed in the marginal parts of the reefs. Numerous tectonic dislocations are present in reef massifs: tension fractures, as well as microfractures, slickensides and crushed zones, which are genetically related to compression. Neptunian dikes filled with carbonate rocks with fauna of the Artinskian Age are the most striking examples of tension fractures. The sub-vertical slickensides can be traced for tens of meters. The crushed zones have a width from 1 to 20 m. The amplitudes of the displacement of the blocks are up to several tens of meters. Tectonic dislocations form two large systems of fractures; zones of dolomitization and seepage of oil fluids are confined to them.

Until the moment of tectonic uplift, each of the shikhans was composed of limestone of the Asselian and Sakmarian Stages and was unconformable overlain by rocks of the Artinskian Stage.

Rocks of the Asselian Stage are subdivided from bottom to top into Kholodnolozhsky and Shikhansky Horizons. The stratotype of the Shikhansky Horizon (regional straton) is determined on the Toratau mount. The Shikhansky Horizon is divided into three fusulinid zones. The lower zone usually doesn’t come out to the surface, but is exposed by drilling under the massifs. It consists mainly of fusulinid, bryozoan and crinoid limestones. Bioherm formations are confined to the middle and upper zones. These formations are very diverse in size, confined to different hydrodynamic environments and are characterized by specific sets of frame builders. Five facies are distinguished in the biohermic Asselian limestones:1) tubiphytes, 2) tubiphytes-bryozoan, 3) tubiphytes-coral-fusulinid, 4) polyphitic, 5) polybiohermic with massive brachiopods. The last three facies are confined to the zone of maximum hydrodynamic activity – the reef ridge.

The rocks of the Sakmarian Stage are divided into two horizons: the Tastuba and Sterlitamak Horizons. There was a gradual change of communities of reef builders during the Tastuba time. Polyphitic formations were gradually replaced by bryozoan bioherms in the zone of the reef ridge. The number of tubiphytes structures is sharply reduced on reef plateaus; palaeoaplysinas and colonial rugose bioherms are widely spread. The role of colonial rugoses is decreasing in the Sterlitamak time, bryozoan and palaeoaplysina bioherms are becoming widespread.

The Toratau reef massif is composed mainly of Asselian (Shikhansky Horizon), to a lesser extent Sakmarian (Tastuba Horizon) deposits. The thickness of reefogenic limestones reaches 500 m. Tubiphytes facies and facies of polyphitic bioherms are distinguished, as well as thick bryozoan bioherms with massive and diverse brachiopods of the Tastuba Horizon. Nautiloid shells are sporadically found on the slopes of the mountain. Coral colonies are rare, palaeoaplysina and fusulinids are practically absent. There is a limestone layer with scalloped folds in the middle part of the western slope. Thin lenses of the Lower Artinskian rocks and bryozoan bioherms occur unconformably and have been preserved in several places on the massif. Small remnants of the Upper Artinskian terrigenous-carbonate strata with poor benthic fauna and ammonite shells are found in hollows on the slopes of the massif.

The Maly Shikhan massif is located 1.5 km north of Toratau (in relief it is represented by a small hill). The roof of the Asselian Stage on the Maly Shikhan is lowered by 1000 m, and the thickness of the Lower Artinskian limestones is 600 m. Crinoid limestones with massive cladoconus corals, as well as clay slabby limestones with Late Artinskian ammonites are exposed in natural outcrops on the hills top.

Outcrops of reefogenic limestone are observed on the southern and western slopes on а the Kushtau massif. The western part of the massif is composed of fusulinid, bryozoan and palaeoaplysina limestones of the Sakmarian Stage. The limestones of the southern slope contain abundant remains of brachiopods, bryozoan and various molluscs. Remains of trilobites are relatively common. The outcrops of gypsum-bearing strata of the Kungurian Stage near Urnyak village is one of the features of Kushtau.

The Yuraktau massif is composed mainly of carbonates of the Asselian (Shikhansky Horizon) and to a lesser extent of the Sakmarian (Tastuba Horizon) Stage. A thick-layered strata of a reef shoal is at the southeastern foot of the mountain, it is composed of crinoid, tubiphytes, bryozoan, patterned limestones with lenses of brachiopod banks and bioherms. Alternation of tubiphytes limestone and bryozoan bioherms is observed when climbing to the top of the mountain, they are replaced by palaeoaplysina bioherms up the slope. A variety of brachiopods, bryozoan, coral colonies, foraminifera, crinoids, molluscs and trilobites are found in limestones. The presence of horizontally overlying layers of micrite (non-reef) limestone with radiolaria and embryonic shells of ammonoids has been found. The presence of micrite interlayers indicates frequent stops of reef formation processes in the final stage of the life of the massif and the turbulent tectonic regime of the site (frequent alternation of uplift and immersion). The “coral trail” is one of the attractions of Yuraktau, it runs between the eastern and western peaks of the mountain. The trail goes along a layer of the Tastuba Horizon of the Sakmarian Stage with massive colonies of rugose. The southwestern part of the massif is disrupted by a tectonic fault. Zones of crushing and secondary dolomitization of limestone are observed along the fault. Relics of black bitumen are visible in the walls of karst voids and in leaching caverns – these are traces of the former presence of oil.

The block of the Earth's crust, representing the shallow shelf of Laurussia with biohermal structures, was raised to the surface in the Neogene as a result of tectonic movements. For about 5 million years, it was transformed by weathering processes and acquired a modern appearance in the form of isolated solitary mountains.

Relief

Toratau Shikhan rises spectacularly in the valley of the Belaya River. Absolute altitude is 402 m, relative altitude is 270 m. The mountain has the shape of a truncated cone with a base length of about 1 km in diameter, with a steep south-western and gentle north-eastern slope. The north-eastern slope is covered with deciduous forest.

Kushtau Shikhan is the central of the shikhans, it is a double-humped ridge, elongated in a submeridional direction. Its average altitude is 357 m above sea level, the relative altitude is 251 m from the level of the Belaya River; the heights of the peaks are 274.5 and 243.8 m; length is 3.8 km, width is 1–1.4 km. The southern and northwestern slopes of Kushtau are rocky covered by steppe vegetation, the rest of the slopes are forested; the Belaya River flows at the western and southern foot of the ridge.

Yuraktau Shikhan is the northernmost of the three shikhans, it has a cross-sectional diameter of 1 km; its height over the Belaya River valley is 118 m, the true altitude is 336 m. It is a beautiful single cone-shaped mountain. There are several oxbow lakes of the Belaya River near the mountain. The western slope of the shikhan is cut off by a rocky cliff rising 100 m from the foot, the northern slope is covered with forest, and the southern and southeastern slopes are steppificated. A two-vertex ridge stretches along the top of the mountain.

The solitary mountains are composed of reefogenic carbonate rocks, the isolation of the shikhans in the relief caused the uniqueness of the flora and fauna formed on them. A single natural landscape complex was formed, confirming the uniqueness of the nominated object.

Justification of Outstanding Universal Value

The nominated object is a unique geological phenomenon. The object is part of one of the largest ancient reef systems on the planet. This system of reefs can be traced according to drilling data and in geological outcrops from the Caspian Sea to the Novaya Zemlya tundra. However, only in the Southern Urals, this reef system is available for observation in well exposed natural outcrops, because a fragment of the Early Permian buried shelf with reef massifs was brought to the surface by tectonic movements in the Neogene. The massifs of Toratau, Kushtau and Yuraktau reflect the shape of the relief of the bottom of the ancient sea of the end of the Sakmarian age of the Early Permian. The reef system existed during the Asselian and Sakmarian ages (about 9 million years). It developed on the border of the shallow epicontinental sea that covered the entire Eastern European Platform and the narrow relatively deep-water Ural Strait. The conditions of palaeoecological environments provided good bioproductivity of reef systems. This was reflected in the high rates of sedimentation of biogenic carbonates. Diverse reef ecosystems coexisted and replaced each other, forming unusually diverse and specific reef facies.

The location of the nominated object on the border of the epicontinental sea and the deep-water strait, as well as the connection between Tethys and Pantalassa, provided the highest biodiversity among all known Permian reef complexes. Perhaps this situation contributed to the formation of refugia, where relict forms were preserved, and Mesozoic organisms appeared. That is why the fossils of Bashkir Shikhans are unique. Numerous species of fossil algae, foraminifera, sponges, corals, cephalopods, gastropods, bivalves, and bryozoan are found in the rocks of shikhan. Palaeoaplysinas are widely distributed here – organisms of unclear systematic position, preserved in the form of wide spongy plates. The reef-formers of huge biogerms were tubiphytes – small calcareous tubules – a problematic and widespread group in the Early Permian. Palaeontology of Bashkir Shikhans also includes single taxons of other Palaeozoic groups.

Global palaeogeographic and climatic changes of the planet are imprinted in the change of sedimentation conditions and the change of complexes of ancient animals. The Asselian and Sakmarian Ages in the outcrops of the Bashkir Shikhans correspond to intensive reef formation in the conditions of the open strait between the Tethys and Pantalassa. In the Artinskian Age, sediments accumulated under different conditions: the Ural Strait closed in the southern part and turned into a large bay. The closing of the strait coincides with the beginning of the uplift of the Urals and fixes the final reunification of all continents into the supercontinent Pangaea. Catastrophic events are reflected not only in the abrupt change of sedimentation regimes and complexes of ancient animals, but also in the Neptunian dikes, which record the strongest earthquakes. Evaporites of the Kungurian Stage, common in the peripheral parts of the shikhans, indicate a complete closure of the bay and a change of climate to a sharply arid one.

Accordingly, the uniqueness of the Nominated object is as follows:

1. Palaeontological value. High taxonomic diversity, including more than 400 species of fossil plants and animals.
2. Palaeoecological value. High diversity of types of palaeoecological communities of the shallow shelf of the Early Permian (bryozoan-palaeoaplysina, fusulinid-bryozoan, coral-brachiopod, etc.)
3. Historical development. Reflection of the dynamics of the development of the fossil reef complex from the beginning of occurrence to extinction for almost nine million years.
4. The availability of observation in natural outcrops on relatively large open areas of the mountains.

Thus, Bashkir Shikhans indicate palaeogeographic, climatic and biospheric changes on a planetary scale. The geological record of the Bashkir Shikhans stores information about important events of our planet. The nominated object is an important geological, scientific and educational value not only of the Republic of Bashkortostan, but also of the whole world.

Participants of several International congresses visited shikhans. For the first time, shikhans were presented at the XVII International Geological Congress (XVII IGC) on the southern route of the Permian excursion (1937). Kushtau and Yuraktau, which stood out with a sharp contrast against the background of the smoothed relief of the surroundings and were visible at a distance of 40–50 km. All international geological excursions in the Southern Urals are conducted with a visit to shikhans. The Shikhans were presented on excursions of the VIII International Congress on Carboniferous (1975), the 27th International Geological Congress (1984), the International Congress “Permian System of the Globe” (1991), the Xth International Congress on Corals and Sponges (2007), the XVIIIth International Congress on Carboniferous and Permian (18 ICCP) (2015). In 1992–1994, special work on the study of reef massifs was carried out by a group of geologists from the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences together with French colleagues on the topic “Paleozoic carbonates of the Urals” under the leadership of B.I. Chuvashov (Yekaterinburg, Russia). The results of the study of shikhans have been reported at international congresses, for example, in 2010, a report by coral specialist O.L. Kossova (VSEGEI, St. Petersburg, Russia) together with French scientists D. Vachard (University of Lille) and A. Izart (University of Lorraine) was presented at the International Palaeontological Congress.

Criterion (viii): The shikhans of Toratau, Yuraktau and Kushtau are part of the largest reef system that existed in the Early Permian on the outskirts of the continent of Laurussia. Reef structures reflect the facies features of the shelf of Laurussia in the Sakmarian Age of the Early Permian. These are the only Early Permian reefs that perfectly demonstrate the nature and history of the formation of reef bodies. Palaeontological remains of Bashkir shikhans are one of the most important characteristics of the state of marine biota at the junction of two ancient oceans – Tethys and Pantalassa in the northern hemisphere.

The geological sections of the shikhans reflect the most important stages of the planet's development: a) the final stages of the existence of the Palaeouralian Ocean, b) the closure of the Ural Strait in its southern part and the final stage of the formation of the Pangaea supercontinent, c) the beginning of the uplift of the Ural Mountains, d) the complete closure of the Ural Gulf, and e) reflect the global climate changes during the Early Permian.

Statements of authenticity and/or integrity

The territory of the monument “Bashkir Shikhans” is proposed for inclusion in the World Heritage List. The territory is an integral natural system, within which natural objects have been preserved in a fairly stable state for a long time; they are closely connected by a common origin and history of geological development. The nominated object is a complex natural monument of the Republic of Bashkortostan – a specially protected natural territory. Shikhan Toratau was declared as a natural monument in 1965, Shikhan Yuraktau – in 1985, and Shikhan Kushtau – in 2020. Thus, the nominated objects are protected by the administration of the region on the basis of laws and regulations of the Government of the Republic of Bashkortostan.

Despite the fact that the shikhans are located in the populated territory of the Ishimbaysky district of the Republic of Bashkortostan, the objects are not damaged, retain their geological and palaeontological integrity and form a unique colourful landscape that also represents aesthetic appeal. In addition, the population of the republic considers shikhans as objects of national pride, shikhans are sung in folk songs and legends.

The UNESCO World Heritage status will further enhance the guarantees of the preservation of Bashkir shikhans, it will protect against possible threats to integrity from human economic and recreational activities.

Comparison with other similar properties

Bashkir shikhans are unique Permian reef formations. They reflect the most important stages of the planet's development in the final stages of the existence of the Palaeouralian Ocean and the formation of Pangaea supercontinent, show traces of catastrophic earthquakes, as well as evidence of climate change and biota of the past, which was associated with global palaeogeographic and biospheric changes.

Comparison with the closest objects from the World Heritage List: Great Barrier Reef (Australia), Belize Barrier Reef Reserve System (Belize), Lagoons of New Caledonia: Reef Diversity and Associated Ecosystems (France), Tubbataha Reefs Natural Park (Philippines), Ningaloo Coast (Australia), Lena Pillars (Russia), Chengjiang Fossil Site (China), Carlsbad Caverns National Park (USA) shows, that Bashkir shikhans have some similar structural elements, but differ in geological features, time of formation. Shikhans represent a well-preserved part of one of the largest reef systems of the planet that existed in the Early Permian on the eastern outskirts of Laurussia (298.5–290.1 million years), it is the only place where the Early Permian reef system is available for observation in well preserved natural outcrops on relatively large open areas.

The Great Barrier Reef (GBR), the Belize Barrier Reef (BBR) and the Bashkir Shikhans are brought together by the fact that they are all objects of a planetary scale, but GBR and BBR are the modern largest reefs, and the Bashkir Shikhans represent a fragment of the largest Early Permian barrier reef. The objects differ in the composition of the main reef-forming communities – if GBR, BBR, Lagoons of New Caledonia, Tubbataha and Ningaloo Reefs form coral communities, the Bashkir shikhan’s reef-forming organisms were bryozoan, tubiphytes, palaeoaplysinas and other organisms. The main distinguishing feature of fossil reefs from modern ones is that the fossil reef reflects the entire history of its existence from inception to extinction over millions of years, whereas the modern reef captures the current stage of its development.

The Carlsbad Caves are formed in the Permian fossil reef, which brings them closer to the Bashkir shikhans. But despite the fact that in the Carlsbad Caves researchers can study the fossil Capitan reef from the inside, the main attention is paid to underground karst processes and unique dripstones in the caves.

Rocks of the Cambrian System, the first system of the Paleozoic Erathem, take part in the geological structure of the Chengjiang Palaeontological Reserve and Lena Pillars. Bashkir Shikhans are formed by rocks of the Lower Series of the Permian System, which completes the Paleozoic. The remains of the oldest organisms – archaeocyates, trilobites, Epiphyton algae, indicating the very beginning of the development of biodiversity, were found in the rocks of the Chengjiang Reserve and Lena Pillars. But in the Bashkir Shikhans, we are already observing a variety of marine organisms forming powerful reef structures at the final stage of Paleozoic history.

There are several World Heritage sites where palaeontological remains are protected, differing from the fossils of Bashkir Shikhans in taxonomic composition and age, for example: Australian Fossil Mammal Sites (Riversleigh / Naracoorte) (Marsupialia mammals, Oligocene–Miocene, Paleogene–Neogene Periods, 10–30 million years ago), Joggins Fossil Cliffs (Canada) (terrestrial life – traces of ancient animals walking and the remains of the forest, Pennsylvanian Series, Carboniferous Period, 323–298 million years ago); Dinosaur Provincial Park (Canada) (vertebrates – dinosaurs, Late Cretaceous period 77–75 million years ago); Dorset and East Devon Coast (Great Britain) (vertebrates and invertebrates of the Mesozoic).

According to literature data, reef massifs are known in China, Central Asia, Canada, Great Britain and the USA. Each of the shikhans of the nominated object is not a part or remnant of a large platform reef body, but a whole, separate reef structure. Each of these formations is transformed by weathering processes. Shikhans traced along the same line on a segment of about 20 km. Taking into account the size of the exposed massifs and their spectacular arrangement along one line, the nominated object has no equal. The diversity and change of facies reflect the conditions of existence and the formation sequence of the reef system. The Bashkir Shikhans in the combination with the Artinskian and Kungurian Stages’ sections represent a rocky record of the important period of the planet's development in the Late Paleozoic: the disappearance of the Palaeouralian Ocean, the uplift of the Urals, the final stages of the Pangaea formation. Palaeontological remains of the Bashkir Shikhans reflect the peculiarities of the marine community’s development at the junction of the two ancient oceans of Tethys and Pantalassa, both before and after the closure of the Ural Strait, which makes this area especially unique.