Madygen_Formation

Madygen Formation

Madygen Formation

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The Madygen Formation (Russian: Madygen Svita) is a Late Triassic (Carnian) geologic formation and Lagerstätte in the Batken and Osh Regions of western Kyrgyzstan, with minor outcrops in neighboring Tajikistan and Uzbekistan. The conglomerates, sandstones and mudstones of the 560 m (1,840 ft) thick formation were deposited in terrestrial lacustrine, alluvial, fluvial and deltaic environments.

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The formation, extending across the Fergana Valley and Fergana Range, is unique for Central Asia, as it represents one of the few known continental deposits and the Madygen Formation is renowned for the preservation of more than 20,000 fossil insects, making it one of the richest Triassic Lagerstätten in the world. Other vertebrate fossils as fish, amphibians, reptiles and synapsids have been recovered from the formation too, as well as minor fossil flora.

The lake sediments of the Lagerstätte provided fossil cartilaginous fishes and their egg capsules and unusual Triassic reptiles like Sharovipteryx and Longisquama.[1][2] The wide diversity of insect fossils was first discovered in the 1960s and first described by Russian paleontologist Aleksandr Sharov, with a notable example being Gigatitan.[3]

Description

Paleogeography of the Late Triassic, around 230 Ma. The Madygen Formation was deposited north of the Paleo-Tethys ocean.

The Madygen Formation is a 560 metres (1,840 ft)[4] thick succession of predominantly siliciclastic rocks accumulated in a tectonically induced basin, covering parts of the Fergana Range and Fergana Valley of Kyrgyzstan with minor outcrops in Tajikistan and Uzbekistan.[5] The Late Triassic layers rest on top of Paleozoic basement with local Permo-Triassic molasse sediments. The section consists of mudstones, sandstones, conglomerates and fanglomerates. This wide variety of siliciclastic rocks reflects the complex spatial and temporal pattern of depositional sub-environments including alluvial fans, sandflats, swamps, back-swamp areas, and littoral to profundal lake zones. The fluvio-lacustrine deposits of the Madygen Formation belong to one of only a few occurrences of continental Triassic beds in Central Asia.[6]

The formation was deposited during the Carnian Pluvial Event (CPE), a global humid event leading to high extinction levels of various groups globally. The CPE led to anoxic conditions, most notably in the South China Block. The area where the Madygen Formation was deposited formed part of the Cimmerian microcontinent, a slab of crust that collided with Laurasia during the Cimmerian orogeny in later Mesozoic times. This orogeny led to the disappearance of the Paleo-Tethys Ocean.

Petroleum geology

The formation grades from bottom to top from alluvial to fluvial into a thick succession of lacustrine mudstones, followed by an alluvial package, on top of which lacustrine, fluvial, deltaic and alluvial layers were deposited.[5]

The hydrocarbon potential of samples of the Madygen Formation ranges from poor to excellent. The sediments containing more than 0.5% Total Organic Carbon (TOC) may be regarded as sources of gaseous hydrocarbons rather than of oil.[7] The Hydrogen Index (HI) of outcrop samples reaches 100 and the maximum recorded maturity (Ro) is 0.8.[8]

Paleontological significance

During the 1960s, Russian paleontologists recovered an unusually rich fossil content in the type strata of the Madygen Formation, including abundant macrophytes, more than 20,000 insect remains[9] and unique small reptiles with well preserved soft tissue.[6] Spirorbis-like polychaete worm tubes, crustaceans (ostracods, kazacharthrans, malacostraca), freshwater Bivalves and gastropods are known from shallow to deeper lake environments. Non-aquatic insects are among the most common fossil remains of the Madygen Formation. These include representatives of the orders Ephemeroptera, Odonata, Notoptera, Blattodea, Titanoptera, Ensifera, Caelifera, Rhynchota, Auchenorrhyncha, Stenorrhyncha, Coleoptera, Hymenoptera, Trichoptera and Diptera. Traces of insect larvae are preserved in near-shore lake deposits.[10]

Fish remains mostly represent endemic genera assigned to the actinopterygian families Evenkiidae (Oshia), Palaeoniscidae (Ferganiscus, Sixtelia) and Megaperleidus and Alvinia. The actinopterygian Saurichthys and the dipnoan Asiatoceratodus are cosmopolitan taxa also recorded in the Madygen Formation. Two distinctive elasmobranch egg capsule types, i.e. Palaeoxyris, indicating a small Lissodus- or Lonchidion-like hybodont shark and an indeterminate capsule type, imply the presence of two different elasmobranch species which used the freshwater environments of the Madygen Formation as spawning grounds. Tetrapods are known from a probably larval urodelan (Triassurus), a small procynosuchid cynodont (Madysaurus), a gliding reptile (Sharovipteryx) and the enigmatic diapsid Longisquama.[10]

Fossil content

Amphibians

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Reptiles

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Synapsids

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Cartilaginous fishes

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Fishes

The following fish fossils were found in the formation:[10][17][18]

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Arthropods

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Flora

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Color key
Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon
 Notes
Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

Insect fauna correlations

Progonocimicidae found in the formation are also recorded in the Carnian Los Rastros Formation of Argentina, the Norian Blackstone and Mount Crosby Formations of Australia, and the Norian to Rhaetian Tologoi Formation of Kazakhstan.[37] Permochoristidae are also known from the Carnian Potrerillos and Cacheuta Formations of Argentina, Huangshanjie Formation of China, the Norian Blackstone and Mount Crosby Formations of Australia; the Norian to Rhaetian Tologoi Formation of Kazakhstan, the Sinemurian Dzhil Formation of Kyrgyzstan and the Toarcian Posidonia Shale of Germany.[38]

Orthophlebia had a relatively broad distribution in the Late Triassic as it is also found in the Sinemurian Badaowan Formation of China and Dzhil Formation of Kyrgyzstan, the Pliensbachian Makarova Formation of Russia and Sulyukta Formation of Tajikistan; the Toarcian Whitby Mudstone Formation of England, Posidonia Shale of Germany, and Cheremkhovo Formation of Russia, and the Early Jurassic Kushmurun Formation of Kazakhstan.[39]

Haglidae were also recorded in the Koldzat and Tologoi Formations of Kazakhstan, in the Carnian Cacheutá Formation of Argentina, the Carnian to Norian Molteno Formation of South Africa and Lesotho, and the Norian Mount Crosby Formation of Australia.[40]

See also

Other Central Asian Lagerstätten

References

  1. [1]
    Fischer et al., 2011
  2. [2]
    Unwin et al., 2003, pp.177-186
  3. [3]
    Shcherbakov, 2008
  4. [4]
    Fischer et al., 2018, p.25
  5. [5]
    Berner et al., 2009, p.2
  6. [6]
    Fischer et al., 2007, p.42
  7. [7]
    Berner et al., 2009, p.5
  8. [8]
    Berner et al., 2009, p.6
  9. [9]
    Kelly, 2018, p.62
  10. [10]
    Voigt et al., 2007, p.162
  11. [11]
    Schoch et al., 2010
  12. [12]
    Schoch et al., 2020
  13. [13]
    Alifanov & Kurochkin, 2011
  14. [14]
    Buchwitz & Voigt, 2012
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    Sharov, 1971
  16. [16]
    Tatarinov, 2005
  17. [17]
    Kogan et al., 2009, p.142
  18. [18]
    Franeck et al., 2013, p.54
  19. [19]
    Sharov, 1968
  20. [20]
    Aristov, 2018
  21. [21]
    Aristov, 2015
  22. [22]
    Kopylov, 2014
  23. [23]
    Aristov, 2017
  24. [24]
    Shcherbakov, 2015
  25. [25]
    Béthoux et al., 2010, p.9
  26. [26]
    Aristov & Storozhenko, 2013
  27. [27]
    Aristov & Sukatcheva, 2018
  28. [28]
    Shcherbakov, 2011
  29. [29]
    Aristov et al., 2009
  30. [30]
    Becker-Migdisova, 1953
  31. [31]
    Ponomarenko, 1969
  32. [32]
    Arnoldi et al., 1977
  33. [33]
    Storozhenko, 1993
  34. [34]
    Shcherbakov et al., 1995
  35. [35]
    Novokshonov, 1997
  36. [36]
    Gorochov, 2005
  37. [37]
    Kelly, 2018, p.190
  38. [38]
    Kelly, 2018, p.186
  39. [39]
    Kelly, 2018, p.185
  40. [40]
    Kelly, 2018, p.182

Bibliography
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