ISSN 08695938, Stratigraphy and Geological Correlation, 2014, Vol. 22, No. 1, pp. 28–43. © Pleiades Publishing, Ltd., 2014. Original Russian Text © V.V. Mitta, V.V. Kostyleva, L.A. Glinskikh, B.N. Shurygin, I.A. Starodubtseva, 2014, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2014, Vol. 22, No. 1, pp. 31–46. Middle Jurassic Stratigraphy in the Southwestern Part of the Republic of Tatarstan V. V. Mittaa, V. V. Kostylevab, L. A. Glinskikhc, B. N. Shuryginc, and I. A. Starodubtsevad aBorisyak Paleontological Institute, Russian Academy of Sciences, Moscow, Russia email: [email protected] bGeological Institute, Russian Academy of Sciences, Moscow, Russia cTrofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia dVernadsky State Geological Museum, Russian Academy of Sciences, Moscow, Russia Received June 18, 2012; in final form, April 17, 2013 Abstract—Data on the structure of the Middle Jurassic marine deposits in the vicinity of Ulyanovsk (sections of the Tarkhanovskaya Pristan–Dolinovka profile) are generalized with due regard for ammonites, bivalves, and microfossils occurring in sediments. Outcrops of the Tarkhanovskaya Pristan site represent the northern most Bajocian section of the Russian Platform, where ammonites of Tethyan origin are identified. As is estab lished, the preCallovian sandclay sequence formerly attributed to the Bathonian includes the Garantiana beds of the upper Bajocian in its middle part. The multidisciplinary biostratigraphicsedimentological research showed that downwarping of the Ulyanovsk–Saratov basin and origin of the Simbirsk Bay of the Tethyan marginal sea commenced in the Bajocian Age. The identified fossils of the upper Bajocian and lower and upper Callovian are cited in paleontological plates of this work. Keywords: Ulyanovsk area of the Volga region, ammonites, bivalves, foraminifers, ostracodes, Bajocian, Call ovian, stratigraphy, sedimentology DOI: 10.1134/S0869593814010043 INTRODUCTION Trofimuk Institute of Petroleum Geology and Geo physics (IPGG). The investigative history of Mesozoic deposits exposed along the right bank of the Volga River upstream of Ulyanovsk (Simbirsk) is long. The Upper INVESTIGATIVE HISTORY Jurassic and Lower Cretaceous sediments exposed 25 km upstream of Ulyanovsk in Ulyanovsk oblast First data on Jurassic deposits that rest on the (lectostratotype of the Volgian Stage) were of primary “variegated marl stage” (the Permian in current understanding) near the village of Tetyushi, on the interest to researchers. The Middle Jurassic rocks are right bank of the Volga River, appeared in publications exposed further upstream at the same riverbank, flank by Golovkinskii (1869), Laguzen (1874), Sintsov ing at present the Kuybyshev Reservoir (Republic of (1872), and Shtukenberg (1882). These researchers Tatarstan). Their outcrops extending from the Tarkh described deposits southward of the village of Doli anovskaya Pristan site to the abandoned settlement of novka in very general terms, however. Actually, we Dolinovka (Fig. 1) and being a continuation of the should honor A.P.Pavlov for the discovery and first above set of outcrops have been studied to a lesser comprehensive description of Middle Jurassic deposits extent, although the respective area appears to be in the Ulyanovsk area of the Volga region (Pavlov, unique in the Middle Volga region. Natural outcrops 1883, 1884). Later, Rosanov (1919), who also studied of this area exhibit the succession of Middle Jurassic outcrops near that village, confirmed the observations marine sediments overlying the erosion surface of the of Pavlov. The subsequent work by Zonov (1939) Permian variegated clays and limestones and yielding described outcrops of the Middle Jurassic rocks diverse fossils of the upper Bajocian and lower–upper exposed along the Kilna River and its tributaries. Callovian. After a long break in investigations, the upper Call Original paleontological remains listed and ovian marls containing ferruginous ooliths and ammo depicted in this work are stored in two depositories: nites Quenstedtoceras, Euaspidoceras, Kosmoceras, ammonites and bivalves at the Borisyak Paleontologi and others were described for the first time at the cal Institute (PIN); foraminifers and ostracodes at the Tarkhanovskaya Pristan site (Mitta, 2001, 2003). In 28 MIDDLE JURASSIC STRATIGRAPHY IN THE SOUTHWESTERN PART 29 48°15′ 48°30′ 48°45′ E Tetyushi N 55° 55° N Kilna R. Dolinovka R ga . en R d a e Tarkhanovskaya g B a iy Pristan site v 54° 54° S 40′ 40′ Undory Kuybyshev Reservoir 54° 54° 30′ 30′ Isheevka Cherdakly 54° 54° 20′ 20′ Oktyabr’skii Ulyanovsk 01020 km 48°15′ 48°30′ 48°45′ Fig. 1. Geographic localities of the studied sections. southwestern Tatarstan, the preCallovian part of the results obtained and additional data on found bivalves Jurassic sedimentary succession (Laishevo Sequence) (determinations by B.N. Shurygin), foraminifers, and was attributed to the Bathonian on the basis of exami ostracodes (determinations by L.A. Glinskikh) are nation of palynological assemblages macerated from summarized and interpreted. the rocks (Didenko and Zorina, 2003; Zorina, 2007). In 2010 and 2011, Mitta, Kostyleva, and Staro dubtseva resumed their field examinations of Middle DESCRIPTION OF SECTIONS Jurassic deposits within the Dolinovka–Tarkha The Middle Jurassic deposits are exposed on the novskaya Pristan area and presented preliminary coastal slope almost everywhere beginning from a results at several scientific conferences (Mitta, 2010; ravine mouth near the abandoned village of Dolinovka Kostyleva and Mitta, 2011; Kostyleva, 2011; Mitta et al., to the Tarkhanovskaya Pristan site. Their beds dip 2011; Mitta and Ditl’, 2012). In this work, all the southwestward (toward the village of Memei) under STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 22 No. 1 2014 30 MITTA et al. the Upper Jurassic clays. In places, the rocks are con Upper Bajocian cealed under taluses and this is an obstacle for corre Member II (Beds 4, 4a, 4b) is recognizable in all lating the beds and measuring their thickness. The var parts of the coastal slope, being though of variable iegated sequence of the Upper Permian is composed completeness. Its apparent thickness ranges from 9 to of alternating clay beds pale green to crimson red in 14 m. Contact with Member I was not observed. coloration, intercalated with interlayers of pale yellow marl and interbeds of calcareous conglomerates and 4. Chloritesmectitekaolinitehydromica clays, graywacke sandstones. The erosion surface of these gray, silty, noncalcareous, with fine crosslamination rocks is overlain with a stratigraphic hiatus by a sand and horizontal and lenticular undulating bedding that siltclay sequence of the Middle Jurassic that is is underlined by accumulations of yellowish gray silty divided into four members. The three lower members material and diagenetic marcasite segregations. of siliciclastic sediments correspond to the preCall Higher in the bed, clays comprise intercalations of ovian part of the sequence. The fourth member of the pale gray, slightly clayey quartz silt. Silt intercalations upper Callovian marls is separated from underlying include admixture of goethite, jarosite, and rare mar casite. The intercalations range in thickness from sediments by a hiatus of eluvial erosion type. Along the 0.1 to 2.0 m. Large (up to 2 m across) loafshaped con sloped bank, the succession of outcrops is divided into cretions of welllithified calcareous feldsparquartz the eastern (sections 1–6), central (sections 7–9), and siltstone observable in Section 4 as associated in places western (sections 10, 11) parts, differing from each with the above intercalations slide downslope, when other in structure and completeness of the sections. weathered, to the bank edge. Ripple marks, predomi Description of the Middle Jurassic beds is presented nantly cellular ones, which were produced by waves of below (Figs. 2, 3). variably oriented water flows, are observable on the upper surface of these concretions. Apparent thick ness of the bed is 3 to 10 m. ? Lower Bajocian 4a–b. Smectitekaolinitehydromica clays, dark Member I (beds 1–3): rocks of the member are brownish gray, silty, noncalcareous, with minor admix exposed in eastern sections of the Tarkhanovskaya ture of fine coaly detritus and chlorite sometimes; in Pristan site and near Dolinovka (sections 1, 3). Its pre places, the rocks reveal fine horizontal or horizontally sumable age is determined on the basis of the position undulating lamination that is detectable owing to yel of the rocks in the succession. The apparent thickness lowish gray coloration of silt laminae and occurrence of of the member is not greater than 6.5 m. The beds gypsum crystals on bedding planes. In the lower third of described below are numbered from the base upward: the unit, there are thin (not thicker than 0.04 m) inter layers of yellowish gray silt. Clays seemingly form the 1. Quartz graywacke sands, coarse to medium lensshaped bodies 2 to 6 m thick inside the claysilt grained and mediumgrained, grayish olive green matrix of Bed 4, which wedge out in all directions (“snuffcolored”), variably compact, slightly argilla (Fig. 2, sections 2, 7–9, and beds 4a, 4b). ceous, displaying cross or wedgeshaped gentle bed In central section 8, an interlayer interruptive along ding that is variably oriented and grades upward into the strike and confined to the top of Bed 4b comprises vague troughshaped bedding. Sediments comprise segregations