the mechetlino section (south ural): a potential limitotype of the artinskian-kungurian stage...

ISSN 1028�334X, Doklady Earth Sciences, 2011, Vol. 441, Part 2, pp. 1629–1632. © Pleiades Publishing, Ltd., 2011.Original Russian Text © B.I. Chuvashov, V.V. Chernykh, 2011, published in Doklady Akademii Nauk, 2011, Vol. 441, No. 5, pp. 657–660.

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The Artinskian–Kungurian section composed ofsandy–clayey rocks with rare limestone beds outcropson the right side of the Yuryuzan River downstream ofthe Mechetlino Settlement (Fig. 1). This sectionattracted the attention of stratigraphers as a potentiallimitotype for establishing the lower boundary of theKungurian Stage in the International StratigraphicScale (ISS). The section has been under study for sev�eral years [1–5]. Until recently, the rare occurrence ofconodonts and scarcity of reference levels character�ized by these fossils in sediments of the Artinskian–Kungurian interval were its main disadvantage. In thisregard, the section recovered by a small quarry in thewestern outskirts of the Mechetlino Settlementappeared to be a fortunate exception. This section isdominated by limestones and limy sandstones, i.e.,rocks, which are suitable for extracting conodontsfrom them by the traditional technique. All the sam�pled carbonate beds of the section yielded conodonts;moreover, in half of the rock samples weighting 10 kg,the conodont abundance exceeds 10 specimens. Theconodont remains are accompanied by ammonites. Inaddition, the section includes several tuff layers, whichgive hope to find zircon in them and date them by theisotopic method. Figure 1 illustrates the schematicstructure of the section, which is described below.

Bed 1. A shallow excavation in the basal part of thequarry recovered sandy–clayey sediments, whichenclose two 5�cm�thick tuff layers and lenticular bedsof detrital limestones up to 10 cm thick with remainsof ammonoids, bryozoans, brachiopod needles, algae,and small foraminifers. The conodont remains in thispart of the section are rare being represented by Neo�streptognathodus pequopensis Behnken and N. ruzhen�cevi Kozur. These sediments are overlain by a bed ofdark gray fine�detrital limestone (bioclastic grain�stone) up to 15 cm thick. The limestone enclosesirregularly distributed bleached isometric concretion�

like swells up to 25 cm thick. They contain accumula�tions of foraminifers, bryozoans, ammonoid remains,crinoid columnals, fragments of their stems, and algaeTubiphytes sp. The conodont assemblage consists ofN. pequopensis Behnken, N. ruzhencevi Kozur, Sweet�ognathus somniculosus n. sp., and forms transitional toN. labialis n. sp. The total thickness of the bed is 1.0 m.

Bed 2. In the basal part of the bed, the area coveredby talus is followed by a 25�cm�thick layer of greenishgray fine�grained sandstone with a sandy limestoneintercalation 5–10 cm thick located 15 cm below its top.

Bed 3. Greenish gray sandstone with a laminated,although massive structure. The thickness of the bed is0.43 m.

Bed 4. Thin�bedded platy medium�grained sand�stone divided into plates from 2–3 mm to 1 cm thick.The bed thickness is 0.5 m.

Bed 5. A characteristic sustained sandstone bed,which is divided into two parts. In its lower part up to10 cm thick, the sandstone is incoherent, slightlycemented, and oversaturated with coalified plant seedsand fragments of Calamites stems. The upper part ofthe bed is composed of hard cemented sandstone withrare similar plant remains. The bed thickness is 0.2 m.

Bed 6. The lower part of the bed is represented by athick member of gray to light gray bedded platy lime�stones consisting of plates from 10 to 40–50 cm thick.The upper part of the bed is composed of massivelimestones. Some limestone layers are separated byintercalations of gray or yellowish marlstone up to5 cm thick. The middle part of the member encloses a10�cm�thick layer of bedded bright yellow tuff. Fivelevels of this bed are characterized by conodonts (Fig. 1);four of them yielded N. pequopensis, N. ruzhencevi,N. pnevi, and N. aff. pnevi, N. labialis n. sp. The totalthickness of the bed is 3.5 m.

Bed 7. A member of alternating highly calcareoussandstones with a rough platy jointing is composed oflayers up to 40–60 cm thick. They are separated bysimilarly thick layers represented by finely alternatingplaty sandstones and mudstones 1–5 cm thick. Thethickness of the member is 4.0 m.

The Mechetlino Section (South Ural): A Potential Limitotype of the Artinskian–Kungurian Stage Boundary

Corresponding Member of the RAS B. I. Chuvashov and V. V. ChernykhReceived July 5, 2011

DOI: 10.1134/S1028334X11120117

Zavaritskii Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620219 Russiae�mail: chernykh@igg.uran.ru

GEOLOGY

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CHUVASHOV, CHERNYKH

Judging from taluses, the overlying layers are repre�sented exclusively by sandstones barren of carbonatevarieties.

The conodonts represent the main fossil group,which serves a basis for the Lower Permian zonal scale[1] and for global correlation of boundaries betweenstages in the International Stratigraphic Scale. In thisconnection, we analyze below the morphological fea�tures of conodonts in the Artinskian–Kungurianboundary interval of the above section with indicationof events in the evolutionary trend of this group, which

may be used for recognizing the lower boundary of theKungurian Stage.

The diversity of late Artinskian (Sarginskian) con�odonts established now in the Mechetlino section(Fig. 1) in Bed 1 is relatively low. Their assemblageincludes Neostreptognathodus pequopensis Behnken,N. ruzhencevi Kozur, and Sw. somniculosus n. sp. Eachof these taxa demonstrates peculiar changes in theirmorphology in the Artinskian–Kungurian interval,which are discussed below.

Fig. 1. The geographic location and structure of the stratotype of the Kungurian Stage lower boundary at the Yuryuzan River nearthe Mechetlino Settlement. (1) Limestones; (2) thin�bedded sandstones; (3) thick�bedded sandstones; (4) mudstones; (5) talus;(6) tuff; (7, 8) fossil remains: (7) ammonoids, (8) conodonts (numbers of informative samples with conodonts are indicated inbrackets).

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DOKLADY EARTH SCIENCES Vol. 441 Part 2 2011

THE MECHETLINO SECTION (SOUTH URAL) 1631

The Sarginskian was marked by the appearance ofpeculiar morphotypes with a narrow ridge�shapedplatform and papillary structure of its upper surface inthe conodont assemblage. We attributed these forms toSweetognathus somniculosus n. sp. They include speci�mens with flattened ridges, which are complicated bya median furrow in the middle part of the flattenedsurface (Fig. 2, 2). The Kungurian representatives of

this lineage distinctly demonstrate further transforma�tion of such morphotypes (Fig. 2, 1–5). The medianfurrow deepens to turn into a narrow V�shaped trench,which divides the ridgelike platform in two parapets.The further evolution of these morphotypes is reflectedin the appearance of three–five narrowings of the plat�form, which divide parapets into several wide�roundedcarinal denticles. These narrowings first appear (or are

1 2 3 4 5

Fig. 2. The evolutionary lineage of Sweetognathus somniculosus n. sp.—Neostreptognathodus labilais n. sp. (1, 2) Sw. somniculosusfrom Bed 1 (Sample 5779a); (3–5) N. labialis: (3) from the basal part of Bed 6 (Sample 5779b), (4) from the middle part of Bed 6(Sample 5902), (5) from Bed 6 (Sample 5779d); the sketch in the lower row demonstrates the magnified fragment of a carina foreach specimen.

1 2 3 4

Fig. 3. Reduction of anterior carinal denticles in the evolutionary lineage Neostreptognathodus ruzhencevi–N. aff. pnevi.(1) N. ruzhencevi from Bed 1 (Sample 5901); (2–4) N. aff. pnevi: (2, 3) from the basal part of Bed 6 (Sample 5779b), (4) from themiddle part of Bed 6 (Sample 5779d).

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CHUVASHOV, CHERNYKH

better expressed) in the posterior part of the platform,although they may be developing along the entire plat�form as well. The eventual formation of denticlesopposite to each other is crowned by the appearance ofa morphotype very similar in the structure of denticlesto the species Neostreptognathodus kamajensis Chern.,although differing from the latter by a narrower plat�form and less developed carinal denticles. This mor�photype likely represents a new species, which is pre�liminarily called N. labialis n. sp. The latter is charac�terized by peculiar morphological features and itsappearance level may be used as indicating the lowerboundary of the Kungurian Stage.

Neostreptognathodus pequopensis Behnken andN. ruzhencevi Kozur are species transient from theSarginskian Horizon to the Saraninskian Horizon.These morphotypes differ from each other mainly bycarinal denticles: mostly tubercular in N. pequopensisand laterally elongated in N. ruzhencevi. In addition,the platform in the last species is notably widened in itsmiddle part, which is not the case in the former one.Both species demonstrate a notable reduction of ante�rior carinal denticles in the Saraninskian. The Neo�streptognathodus representatives with reduced anteriordenticles are attributed to the species Neostreptogna�thodus pnevi Kozur, which originated, in the opinionof Kozur himself, from N. pequopensis Behnken. SomeNeostreptognathodus forms with smooth anterior partsof parapets and almost vertical tubercular posteriordenticles, which occur in the Saraninskian Horizon,may undoubtedly be referred to N. pnevi in Kozur’s

understanding. The appearance level of N. pnevi is tra�ditionally used as marking the lower boundary of theKungurian Stage. At the same time, similar reductionof carinal denticles is also observed in Kungurian Neo�streptognathodus ruzhencevi (Fig. 3). We preliminarilydesignate the morphotype with smoothed anteriorbranches of parapets as N. aff. pnevi. Judging fromavailable data, N. pnevi and N. aff. pnevi first appear atthe same stratigraphic level and both of them may beused for recognizing the lower boundary of the Kun�gurian Stage.

Taking into consideration all the above�mentioneddata, we believe that the Mechetlino section may serveas a stratotype for recognizing the lower boundary ofthe Kungurian Stage in the International StratigraphicScale.

REFERENCES

1. V. V. Chernykh, The Zonal Method in Biostratigraphy.The Lower Permian Conodont Zonal Scale, (IGG UrORAN, Yekaterinburg, 2005) [in Russian].

2. V. V. Chernykh and B. I. Chuvashov, Ezhegodnik�2005In�ta geologii i geokhimii UrO RAN, 36–38 (2006).

3. B. I. Chuvashov and V. V. Chernykh, Dokl. Akad. Nauk375 (3), 370–374 (2000) [Doklady Earth Sciences375A, 1345–1349 (2000)].

4. B. I. Chuvashov and V. V. Chernykh, in Geology of theUrals and Adjacent Regions IGG UrO RAN, Yekaterin�burg, 2007), pp. 201–218 [in Russian].

5. B. I. Chuvashov and V. V. Chernykh, Permophiles 44,12 (2004).