Volumina Jurassica, 2010, Viii: 5–48 The Middle Oxfordian to lowermost Kimmeridgian ammonite succession at Mikhalenino (Kostroma District) of the Russian Platform, and its stratigraphical and palaeobiogeographical importance Ewa GŁOWNIAK1, Dmitry N. KISELEV2, Mikhail ROGOV3, Andrzej WIERZBOWSKI1, John K. WRIGHT4 Key words: ammonites, biostratigraphy, Boreal zonation, Subboreal zonation, Submediterranean zonation, correlation, Oxfordian/ Kimmeridgian boundary. Abstract. The Mikhalenino section on the Russian Platform has yielded numerous ammonites from the Middle and Upper Oxfordian and lowermost Kimmeridgian, collected bed by bed. The ammonites belong mostly to the Boreal family Cardioceratidae, but also to the Sub- boreal family Aulacostephanidae; additionally at some levels there were collected various Submediterranean ammonites (Perisphinctidae, Oppeliidae and Aspidoceratidae). The co-occurrence of ammonites representative of different faunal provinces makes possible recognition of the Boreal, Subboreal, and partly also Submediterranean standard zonations. In consequence, it is possible to make a close correlation between these zonal schemes. The Oxfordian/Kimmeridgian boundary at the Pseudocordata/Baylei zonal boundary of the Subboreal zonal scheme corresponds precisely to the Rosenkrantzi/Bauhini zonal boundary. This boundary of the stages defined well faunistically in the Flodigarry section (Isle of Skye, Scotland) and proposed as a candidate for the uniform Oxfordian/Kimmeridgian boundary, can be also recognized in the Russian sec- tion studied. The boundary can be traced in the Mikhalenino section using the same criteria as used at Staffin: the appearance of the first representatives of Pictonia [M]–Prorasenia [m] (Subboreal), and the first appearance of Amoeboceras (Plasmatites) (Boreal). This indi- cates the large correlation potential of the boundary defined in this way. The research on the Mikhalenino section has provided the new palaeontological findings described in this study. These include the first discovery in the territory of the Russian Platform of ammonites of the genera/subgenera Decipia, Vineta and Pomerania (Pachypictonia). Two new species: Decipia (?) kostromensis sp. nov., and Pictonia mesezhnikovi n. sp. are described. INTRODUCTION 1929; Spijarski, 1932; Krom, 1933; Gerasimov, 1972), and the results of these investigations were of major im- The Oxfordian and lowermost Kimmeridgian deposits portance for the general foundation of the Oxfordian and which crop out along the Unzha River of the Kostroma Dis- Kimmeridgian stratigraphy of the whole Russian Platform. trict, about 500 km north-east of Moscow, although general- The detailed succession of ammonites of the Boreal family ly of small thickness, are stratigraphically among the most Cardioceratidae in the Middle and Upper Oxfordian (as well complete of the Russian Platform, and thus are especially as the lowermost Kimmeridgian according to the stratigra-​​ important for wider correlations and palaeobiogeography. phical interpretation in the present study) has been recog- These deposits have been studied for more than 120 years nized only recently by Mesezhnikov et al. (1986, 1989), and (Nikitin, 1884; Ivanov, 1909; Zhirmunskyj, 1914; Sokolov, Mesezhnikov and Kalacheva (1989). These studies resulted 1 Institute of Geology, University of Warsaw, ul. Żwirki i Wigury 93, 02-089 Warszawa, Poland; e-mail: [email protected], [email protected] 2 Geographical Department, Yaroslavl State Pedagogical University, Kotoroslnaya nab. 46, Yaroslavl 150000, Russia; e-mail: [email protected] 3 Geological Institute of Russian Academy of Sciences, Pyshevskii Lane 7, Moscow 109017, Russia; e-mail: [email protected] 4 Department of Geology, Royal Holloway, Egham, Surrey, TW20 OEX, UK; e-mail: [email protected] 6 Ewa Głowniak et al. in the recognition of the succession as a key one for elabora- paper are based on the field-work carried at Mikhalenino tion of the Upper Oxfordian–Lower Kimmeridgian Boreal during this field-meeting, as well as on the previous studies ammonite zonal scheme and the relevant foraminiferal zonal of Rogov and Kiselev (2007) on the same section. The sec- scheme for the Russian Platform (Mesezhnikov et al., 1989; tion has been revisited by the authors in June 2010 when Grigelis, 1982). The other ammonite groups (Perisphincti- supplementary field work was carried aut. The stratigraphi- dae and Aulacostephanidae), although reported previously cal distribution of ostracods and foraminifers in the Mikha- in the interval studied, have not been described in detail so lenino section using the ammonite stratigraphy of the current far. The co-occurrence of ammonites belonging to families study has been recently presented by Tesakova and Guzhov of Boreal (Cardioceratidae), Subboreal (Aulacostephanidae) (2009) and by Ustinova (2009a, b). and Submediterranean (mostly Perisphinctidae) affinity in the succession studied is of great importance for correlation of the different zonal schemes recognized in particular am- THE DETAILS OF THE SECTION STUDIED: monite provinces of Europe. LITHOLOGY AND AMMONITES The best sections of the Oxfordian to lowermost Kim- meridgian deposits are afforded by the banks of the Unzha River, the left tributary of the Volga (Fig. 1). These sections, The deposits are exposed in the steep bank of the Unzha described by Mesezhnikov et al. (1986, 1989) and subse- river valley, and are flat lying. Slumping often obscures quently discussed by Hantzpergue et al. (1998a, b), are situ- the succession, and the state of the section can vary from ated close to the town of Makariev. The section studied by year to year. The full succession exposed in 2006 from the present authors is located on the right bank of the Unzha Middle Oxfordian to lowermost Kimmeridgian consisted of River, about 15 kilometers to the north of Makariev, near the 10.25 m of dark silty and shaly clays (beds 1 to 29 accord- village of Mikhalenino. The section was recognized for the ing to Rogov, Kiselev, 2007). The section studied in 2007 first time by Rogov and Kiselev (2007; but note also the ear- by the authors showed well-exposed deposits from bed 1 lier description of some gastropods from the section by upwards to bed 21 covering the interval from the Middle Guzhov, 2004), and demonstrated during a field-trip for the Oxfordian to the lowermost Kimmeridgian (about 5 meters participants of the International Kimmeridgian W.G. Meet- in thickness) with the interval corresponding to beds 22–27 ing in Russia in June 2007. The results presented in this of Rogov and Kiselev (2007) obscured, and the topmost part Fig. 1. Locality map of the Mikhalenino section as well as other localities discussed in the text (B–C), and their position in northern Europe (A) The Middle Oxfordian to lowermost Kimmeridgian ammonite succession at Mikhalenino (Kostroma District)... 7 of the section (beds 28–29 of the Lower Kimmeridgian) ex- Ammonites found in a loose block in the rubble coming posed to about 1.9 m in thickness. The ammonites were from beds 9–10 (but without precise location) include: Decipia collected carefully bed by bed. The following lithologies cf. decipiens (Sowerby) (Pl. 3: 7) and Amoeboceras (A.) ilo­ and ammonites have been recognized in the section studied vaiskii (Sokolov) (Pl. 3: 6). (Figs 2–4). Beds 11–12: light grey-greenish silty clays, highly bioturbat- Bed 1: grey-greenish silty clays, bioturbated, containing am- ed (Chondrites) in the lowest part (0.07 m), followed monites: Cardioceras (Subvertebriceras) densiplica­ by a non-bioturbated interval (0.06 m) – together rep- tum Boden, C. (Plasmatoceras) ex gr. popilaniense resenting bed 11, whereas more highly bioturbated Boden, C. (Scoticardioceras) laevigatum Boden; interval (Chondrites) in the upper part (0.17 m) is 0.2–0.3 m (base not visible). recognized as bed 12; the ammonites from bed 11 Bed 2: grey-greenish silty clays, bioturbated, containing am- include Perisphinctes (Perisphinctes) sp. (Pl. 8: 1); monites: Cardioceras (Subvertebriceras) densipli­ the ammonites from bed 12 include: Perisphinctes catum Boden, C. (Plasmatoceras) cf. tenuicostatum (Dichotomoceras) cf. bifurcatoides Enay (Pl. 4: 12, (Nikitin), C. (Scoticardioceras) laevigatum Boden, 13), P. (D.) cf. wartae Bukowski, P. (D.) sp. juv., C. (Vertebriceras) cf. rachis Buckman; 0.28–0.30 m. Amoeboceras nunningtonense Wright (Pl. 4: 2), A. cf. Bed 3: grey silty clays, bioturbated near the top, containing nunningtonense Wright; total thickness of beds 11–12 ammonites: Cardioceras (Cawtoniceras) cawto nense equals 0.30 m. (Blake et Huddleston) (Pl. 2: 4–4a), Perisphinctes Bed 13: light grey-greenish highly bioturbated silty clays (Otosphinctes) cf. arkelli Głowniak (Pl. 2: 1, 2), with common phosphorites at the top; ammonites: P. (Kranaosphinctes) sp., Euaspidoceras sp. and Ta­ Perisphinctes (Dichotomoceras) sp. juv., P. (Dicho­ ramelliceras cf. dentostriatum (Quenstedt) (Pl. 2: 5), tomoceras) cf. bifurcatoides, Amoeboceras glosense Taramelliceras sp. and Glochiceras sp.; 0.28–0.30 m. (Bigot et Brasil), found within the bed, and Amoe­ Bed 4: grey silty clays, bioturbated in the upper part of boceras koldeweyense Sykes et Callomon (Pl. 4: 3), the bed, with glauconite, containing ammonites: Car­ found at its top and directly below the base of bed 14; dioceras (Miticardioceras) tenuiserratum (Oppel), 0.07–0.09 m. C. (Subvertebriceras) sp.; 0.25 m. Bed 14: grey, silty, shaly clays that become green 0.03 m Bed 5: grey, locally greenish