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 edit

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]

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]

Amphibians edit

Reptiles edit

Synapsids edit

Cartilaginous fishes edit

Fishes edit

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

Arthropods edit

Flora edit

Insect fauna correlations edit

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]

Bibliography edit

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