The early Jurassic fauna from the Gurumdy and Mynhajir zones of the East Pamirs

Abstract

The Southeast Pamirs is a unique region with a wide development of Jurassic marine sediments that are characterized by complete different-facies sections and diverse fossils. Of particular interest are Early Jurassic corals, which represent the first finds in mountainous Central Asia and the entire eastern Mediterranean belt. Finds of such fossils are extremely rare in other regions of the world so far. According to [1], Jurassic sediments in the Southeast Pamirs can be divided into three (Gurumdy, Mynhajir, and Istyk) structural‐facies zones. Upper Jurassic fossils are confined to the Gurumdy and Mynhajir zones, which occupy marginal parts of the Southeast Pamirs and make up a horseshoe-shaped framing around the Inner Istyk zone (Istyk Uplift), where sedimentation commenced in the terminal-Early-initialMiddle Jurassic (Toarcian‐Aalenian). The Gurumdy and Mynhajir zones constitute the southwestern and northeastern segments of this horseshoe-shaped zone, respectively (figure). In the Jurassic, both zones were elements of a single trough located along the periphery of the Istyk Uplift inside a sea basin. The Gurumdy segment of the trough experienced intense subsidence and accumulated thick sediments, while the Mynhajir segment subsided less significantly to accumulate thinner sediments of variable composition. The present communication is dedicated to the characteristic of distribution, lithology, age, and fossils of the Lower Jurassic sediments developed in the Gurumdy and Mynhajir zones of the Southeast Pamirs. Different fossil groups from Lower Jurassic sediments have been determined and described by many paleontologists, including G.K. Melnikova (corals); V.P. Makridin, S.A. Melnikova, and A.S. Dagys (brachiopods); L.D. Kiparisova, L.V. Sibiryakova, and T.F. Andreeva (bivalves); and G.Ya. Krymgol’ts, V.I. Dronov, and Yu.S. Repin (ammonites). In the present-day structure of the Gurumdy zone, Jurassic sediments are developed in the river basins draining the Vahan Ridge beginning from upper reaches of the Beik River to the Zorkul Depression. Farther northwestward, they are widespread in the basins of the Mashale, Gurumdy, Irikyak, Alichur, Karademur, Kattamarjanai, and Vatasaif rivers that drain the eastern part of the South Alichur and western part of the North Alichur ridges. In the zone under consideration, the Jurassic rocks are unconformably sandwiched between upon Permian‐Triassic strata and Tertiary sediments. The zone is characterized by the continuous Jurassic section that includes sediments ranging from the basal part of the system to upper Callovian layers. Its lower series is very thick because of the development of reeflike sequences in some areas. The total thickness of the Lower Jurassic rocks amounts to 700 m. The Jurassic sediments of the Mynhajir zone are developed in the East and West Mynhajir mountains separated by the Aksu River and along the northern foothill of the Zougan Ridge, in the Karasu River basin, and at the estuary of the North Boztere, Kaindy, and other rivers. They are unconformably sandwiched between Permian‐Triassic and Tertiary layers. The Mynhajir zone also includes a complete section extending from the basal Jurassic to the Bathonian‐Lower Callovian strata, although the Mynhajir zone differs from the previous zone in more diverse lithology and two times lower thickness of the Lower Jurassic sequence (220‐285 m). Following below is a description of the Jurassic type sections of the Gurumdy and Mynhajir zones with Early Jurassic faunal assemblages. The integral Lower Jurassic section of the Gurumdy zone is compiled on the basis of outcrops on the watershed between the Sedek and Irikyak rivers. The author of the present communication studied it first together with V.P. Novikov (in 1969), E.V. Boiko (1970), and V.I. Dronov (1970s‐1980s).