Reference section of Neogene-Quarternary deposits in the Uimon Basin (Gorny Altai)

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Reference section of Neogene–Quarternary deposits in the Uimon Basin (Gorny Altai) G.G. Rusanov a,b,*, E.V. Deev c,d, I.D. Zolnikov d,e,f, L.B. Khazin c, I.V. Khazina c, O.B. Kuz’mina c a

Gorno-Altaisk Expedition JSC, ul. Sovetskaya 15, Maloeniseiskoe Village, Altai Territory, 659370, Russia b Shukshin Altai State Humanities Pedagogical University, ul. Korolenko 53, Biysk, 659333, Russia c A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia d Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia e V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia f Institute of Archeology and Ethnography, Siberian Branch of the Russian Academy of Sciences, pr. Lavrentieva 17, Novosibirsk, 630090, Russia Received 28 March 2016; accepted 20 October 2016

Abstract An extraordinary-thick (400 m) section of the Neogene–Quaternary deposits is for the first time exposed by well No. 1 in the central Uimon Basin. The Miocene–Pliocene lacustrine Tueryk Formation is recognized at the base of the continuous section, verified by new paleontological data (ostracods, spores, and pollen). As assumed, overlaying deposits are represented by the Lower Pleistocene lacustrine-alluvial Beken Formation, Middle Pleistocene alluvial-proluvial Bashkaus Formation, undifferentiated Middle Pleistocene glacial, fluvioglacial, and alluvial deposits, and Upper Pleistocene lacustrine-glacial deposits. The data obtained from the core of well No. 1 undisputably demonstrate that the Uimon Basin had been developed prior the beginning of the Miocene Epoch, when it was characterized by accumulation of the lacustrine Tueryk Formation, incompletely exposed within the studied section. The presence of thick unexposed lower-Ohm interval of sedimentary filling of the basin suggests that the Uimon Basin was developed as early as the Paleogene. Therefore, the tectonic evolution and sedimentation history of the basin are assumed to have features similar to those of the Chuya and Kurai Basins of Gorny Altai. © 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Neogene; Pleistocene; ostracods; palynology; Uimon Basin; Gorny Altai

Introduction Aspects of stratigraphic subdivision, correlation and dating of certain stratigraphic units within the Neogene–Quaternary sedimentary complex of Gorny Altai are still at great controversy. A vast majority of stratotypes and parastratotypes for the Neogene and Quaternary deposits are localized in southeastern Gorny Altai, within the Chuya and Kurai intramontane basins, filled with 1 km-thick sedimentary complex (Deev et al., 2011, 2012a; Devyatkin, 1965; Nevedrova et al., 2001, 2014; Rusanov and Vazhov, 2014; Svitoch et al., 1978). In contrast, there have been no representative continuous Neogene–Quaternary sections documented in the other regions of Gorny Altai thus far.

* Corresponding author. E-mail address: [email protected] (G.G. Rusanov)

The Uimon intramontane basin, being third in size (300 km2) and least-studied among the analogous structures of Gorny Altai, is one of the most perspective objects for search of representative Neogene–Quaternary sections (Fig. 1A, B). In 1960–90s, several low-depth (up to 110 mdeep) exploration and hydrogeological wells with no core recovery were drilled in certain areas within the Basin. These wells exposed the Quaternary gravels and gravely sands, which have not been further genetically-interpreted and ageconstrained. For the first time, 53-m thick succession of supposedly Neogene-age deposits, comprised by reddish-brown clays with rounded clasts of quartz and crystalline schists, was exposed in 1951 by two wells (depths 62 and 82.5 m, respectively) in footwall of a thrust at the foot of the Terekta Range, near the Margala Village (Neshumaeva and Studenikin, 1952). Small (up to 10 m-thick) natural outcrops of reddish-brown clay with

1068-7971/$ - see front matter D 201 7, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 7.07.008

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G.G. Rusanov et al. / Russian Geology and Geophysics 58 (2017) 973–983 Fig. 1. A, Location map of the Uimon Basin (black arrow) within the Central Asia; B, shaded relief model of the Uimon intramontane basin; white dot marks location of well No. 1; C, tentative geoelectric model of the Uimon Basin, after Deev et al. (2012b), and location of well No. 1. The numbers determine electric resistivity (Ohm⋅m).

G.G. Rusanov et al. / Russian Geology and Geophysics 58 (2017) 973–983

angular pebbles, assigned to the Late Pliocene, were documented near the Kastakhta and Bashtala Villages (Shmidt, 1964). As there were no paleontological material found to constrain the age of the clays, the assignment of these strata to the Neogene was based only on its red color (Bogachkin, 1981; Rakovets, 1966; Shmidt, 1972; Zybin et al., 1988). L.A. Ragozin (1945) for the first time assumed the development of the Uimon Basin, as well as the basins in southeastern Gorny Altai, begun in the Paleogene. Presence of the Oligocene variegated clays, corresponding to the Paleogene stage of the evolution of Gorny Altai, was supposed by S.N. Bazhenova (Levitskii et al., 1964). As can be seen from the above, neither thickness, nor age, genesis, or composition of the Cenozoic strata, filling the Uimon Basin, have been comprehensively investigated thus far. On the other hand, high practical importance of such a researches is determined by a buried multilayered large-volume gold-bearing placer predicted within the Uimon Basin (Butvilovskii et al., 2011; Zybin et al., 1988). Recent electromagnetic studies yielded the Cenozoic deposits of the Uimon Basin are as thick as 870 m (Fig. 1C), which is consistent with those within the basins of southeastern Gorny Altai (Deev et al., 2012b). In 2013, well No. 1 was drilled by specialists of JSC “Gorno-Altaisk Expedition” in the central Uimon Basin (50º16′04.1″ N, 85º48′06.2″ E, 970 m asl) for investigation and subdivision of the Cenozoic strata (Fig. 1). At the depth 400 m, this well went through a thick (>300 m) member of the Quaternary strata and intruded the Upper Miocene deposits (Khazina et al., 2015; Rusanov and Tsaer, 2014). Our research is focused on composition and stratigraphic subdivision of the Neogene–Quaternary strata, exposed by well No. 1. As drilling of gravel intervals and interbeds leads to vigorous core disintegration, direct contacts between the stratigraphic subdivision identified are not recognized. These contacts are herein determined by changes in the lithology and based on correlation with similar successions, well-studied in the Chuya Basin. Clays and silts of the Tueryk Formation are exclusively display conformable and gradual layer alternation. Position of the layers within the members is precised by the gamma-log data.

Stratigraphy of the Neogene–Quaternary strata, exposed by well No. 1 Further we log the stratigraphic-genetic subdivisions recognized in well No. 1. Lacustrine-glacial (?) deposits (lgQIII) are exposed within the interval 0.0–23.0 m (Fig. 2), and comprised as follows (top to bottom): Thickness, m 1. Alternating gray fine-grained clayey sand with sporadic inclusions of granules and fine pebbles (3–5 mm) and pale yellowish-gray compacted silt. Interbeds are 10–20 cm-thick. . . . . . . . . . . . . . 1.0 2. Gray middle-fine grained polymictic sand, with abundant subangular to rounded granules and fine pebbles, composed of effusive rocks, metamorphic schists and quartz. Interbeds (10–12 cm-thick)

of pale yellowish-gray compacted silt with inclusions of granules and fine pebbles (3–5 mm) are also documented. . . . . . . . . . . . . . . 3. Subangular gravel (up to 8 cm) in gray sandy-silt matrix. Clasts are comprised by quartz-chlorite schist and microgranite porphyry. . . 4. Pale greenish-gray compacted clay, thin-bedded (layers are up to 2 mm thick), with inclusions of pebbles. . . . . . . . . . . . . . . . 5. Subangular gravel (up to 8 cm) in gray sandy-silt matrix. Clasts are comprised by rhyolite, metamorphic schist, microgranite porphyry, quartzite, andesite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Pale greenish-gray compacted clay with thin interlayers (2–3 mm) of dark gray clay. . . . . . . . . . . . . . . . . . . . . . . . 7. Subangular gravel (up to 8 cm) in gray sandy-silt matrix. Clasts are comprised by effusive rocks, chlorite–albite–quartz schist, sporadic quartzite, dolerite, medium-grained leucocratic granite. . . . . . . . . . 8. Subangular to rounded pebbles and cobbles in pale gray sandysilt matrix. Some cobbles exceed the core diameter. Pebbles and cobbles are represented by rhyolite, leucocratic granite porphyry, gabbro. 9. Pale-gray compacted clay with occasional inclusions of granules, thin-bedded (interlayers are 1–3 mm-thick). . . . . . . . . . . . . 10. Subangular to rounded pebbles in sandy-silt matrix. Intraclasts are comprised by rhyolite, quartz, granite, granite porphyry, chlorite-albite-quartz schist. An interbed of yellowish-gray compacted silt with occasional inclusions of very fine to medium pebbles is documented at the interval 20.4–20.7. . . . . . . . . . . . . . . . . . . . . . . . . .

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1.7 2.7 0.7

3.7 1.2

4.5

2.2 1.1

4.2

The core recovery varies between 59 and 70% in the sands, silts and clays, and between 20 and 35% in gravels. There are no taxonomically identifiable organic remains found in the lacustrine-glacial deposits. All specialists studied the Cenozoic deposits of the Uimon Basin have recognized the lacustrineglacial deposits, which drape the bottom of the basin, and are composed of silt, sand, loam, and gravel. However, an age interpretation of these strata has been debatable—the interval was constrained as the Middle Pleistocene (Levitskii et al., 1964), Middle–Late Pleistocene (Butenko, 2001), Late Pleistocene (MIS 2) (Butvilovskii and Prekhtel’, 2000), or even Holocene (Bailagasov et al., 2012). The OSL-age 101 ± 9 ka was obtained for the lacustrine silts (RIS0-132536), collected near the eastern margin of the Uimon Basin (5.9 km southeast of Nizhnii Uimon Village) (Zolnikov et al., 2016). This age proposedly corresponds the cool substages of MIS 5. Within the Uimon Basin, the lacustrine-glacial deposits are erosionally cut by the Upper Pleistocene alluvial sediments of the first and second terraces of the Katun’ River and alluvial fans of the Bashtalinka, Kastakhta, Bol’shaya Terekta, and Chendek Rivers. The floodplain alluvial facies of the second terrace embed shells of freshwater mollusks Planorbis planorbis L. and Bithynia sp., with the second taxon being dominate. AMS-dating of Bithynia sp. shells yielded 14C age >45.7 ka (AA95968, dated in the Arizona Laboratory, USA) (Deev et al., 2013). These Gastropods are tolerant to the habitat drying out, though being thermophilic. Associated silts are OSL-dated as 77 ± 5 ka (RIS0-132537) (Zolnikov et al., 2016). A redeposited (partially rounded) fragment of Bison priscus Boj. pelvis, dated as MIS 3 of the Late Pleistocene (concluded by A.V. Shpanskii), is documented from deposits of the Chendek River alluvial fan. Associated fan sands have the OSL-age of 89 ± 5 ka (RIS0-132534) and 79 ± 5 ka (RIS0-132533) (Zolnikov et al., 2016). Consequently, we preliminarily correspond the lacustrine-glacial deposits of well No. 1 to MIS 5. Undifferentiated glacial, fluvioglacial and alluvial (?) deposits (g,f,aQII) 184m-thick are documented within the interval 23–207 m, laying immediately below the lacustrine-

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Fig. 2. Log of well No. 1. 1, cobbles and boulders; 2, pebbles; 3, granules and fine pebbles; 4, angular granules and fine pebbles; 5, angular pebbles; 6, sand; 7, silt; 8, clay; 9, mollusk and ostracod fossils; 10, well diameter (mm).

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glacial deposits (Fig. 2). The section logged as follows (top to bottom):

rhyolite, rhyodacite, pink granite porphyry, gray subporphyric granite, and quartzite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.4

Thickness, m

The core recovery within the logged strata considerably varies at 50–70% for sands, 35-45% for pebbles, and at 10–25%—for cobbles and boulders. Nevertheless the observed lithological composition may be variously interpreted as alluvium, moraine or fluvio-glacial deposits, it is considered to leave these strata undifferentiated, since there is the only one section of this interval (the core herein studied) is known thus far. The whole logged section of the interval 23–207 m demonstrates the matrix being dominated by granules (15.4– 78%) and (7.9–67.3%) sands. Petrographic investigation of the clasts yields its transport from the Terekta Ridge by the Kastakhta, Kurunda, and Bol’shaya Terekta Rivers, and supposedly by the Koksa River. These deposits have majorly identical light- and heavy-mineral composition to the strata underlaying the Bashkaus Formation. The only difference is documented in authigenic limonite concentration, which does not exceed 3% through the section. The sandy clay matrix of the lower interval 15 contains increased concentration of gold (up to 4 mg/ton). There is no fossils identified within the interval. In contrast to the Bashkaus Formation, the logged interval is majorly gray-colored and dominated by unweathered pebbles, cobbles, and boulders. We proposedly place these strata between the underlying Bashkaus Formation, and Late Pleistocene lacustrine-glacial deposits (lgQIII), hence this interval roughly corresponds to the Middle Pleistocene (MIS 17–MIS 6). Alluvial (?) Bashkaus Formation (a,QIIbs) is a 90 mthick interval exposed in depths 207–297 m by well No. 1 in the Uimon Basin. The Formation overlaps the Beken Formation, and comprises following lithological units (top to bottom):

1. Cobbles and boulders with coarse sand matrix. The boulders exceed the core diameter. The gravels are rounded and well-rounded, unweathered, comprised by rhyolite, dacite, quartz, granite porphyry, diorite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 2. Angular to subangular granules with abundant variously-rounded pebbles (1–5 cm), and sporadic rounded and well-rounded cobbles (up to 12 cm along the core axis). The matrix is comprised by pale-gray sandy silt. The clasts are composed of rhyolite, rhyodacite. Granite and granite porphyry pebbles are also documented. . . . . . . . . . . 14.3 3. Boulders and cobbles. The clasts are rounded and well-rounded, unweathered. More than 80% of the clasts are represented by rhyolite, rhyodacite, pink granite porphyry. The other 20% are composed of sericite-quartz-chlorite schists. . . . . . . . . . . . . . . . . . . . . . . 4.2 4. Angular granules with gray silty sand matrix, and abundant variously-rounded pebbles (1–4 cm). The pebbles are composed of rhyolite and metamorphic schists. . . . . . . . . . . . . . . . . . . . . . 1.4 5. Boulders and cobbles. The clasts are rounded and well-rounded, unweathered. More than 70% of the clasts are comprised by gray rhyolite, whereas pink granite porphyry and hematitized brownish-gray quartzite and sandstones hold the other 30%. . . . . . . . . . . . . . 1.4 6. Angular granules with variously-rounded pebbles (1–5 cm), and sporadic rounded and well-rounded cobbles (up to 12 cm along the core axis). The matrix is comprised by pale-gray sandy silt. The clasts are represented by porphyry, dacite, metamorphic schists, rarely siltstones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 7. Gray very-coarse and coarse polymictic sand with subangular granules and inclusions of rounded to well-rounded pebbles and cobbles (1–7 cm). The gravels are composed of porphyry, dacite, metamorphic schists, quartz, and scarce siltstones and volcanic tuff. . . . 6.5 8. Rounded and well-rounded boulders and cobbles. The boulders exceed the core diameter and reach 14 cm along the well axis. The matrix is represented by polymictic gravely sand. All the clasts are unweathered, and composed of rhyolite, dacite, metaschists, quartzite, gabbro-dolerite, and pink granite porphyry. . . . . . . . . . . . . . . 15.3 9. Gray coarse polymictic sand with sporadic pebbles (up to 5 cm) of quartzite, granite porphyry, and dolerite. . . . . . . . . . . . . . . 1.5 10. Rounded and well-rounded boulders and cobbles with gravely sand matrix. Some clasts exceed the core diameter. The sand is coarseand very coarse-grained, polymictic. The granules and fine pebbles are subangular. The gravels are represented by metaschists, quartz- and feldspar-rhyolite, dacite, granodiorite, dolerite, diorite porphyry, and tuff. Individual clasts are comprised by marmorized limestone. . . . 2.9 11. Rounded and well-rounded pebbles and cobbles (5–10 cm) with matrix of variously rounded (from angular to well-rounded) granules and gray coarse sand. The cobbles exceeding the core diameter are documented in the lower part of the interval. The clast composition varies greatly, with dominating (~80%) banded quartz- and feldspar-rhyolite, dacite, metaschists, granodiorite, dolerite, gabbro-dolerite, gabbro-diorite, dacite porphyry, quartz, diorite porphyry, tuff, tuff-breccia, marmorised limestone and middle-grained granite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27.2 12. Rounded boulders and cobbles with gravely sand matrix. Some of clasts are larger than the core diameter. The gravels are comprised by dacite porphyry, quartz, tuff breccia, gabbro-dolerite, gabbro-diorite, effusive rocks, quartzite, middle-grained granite, metamorphic schists. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23.0 13. Pebbles (1–4 cm) with gray sandy clay matrix. The clasts are rounded to well-rounded, and display similar composition as those in the overlaying interval (see above). . . . . . . . . . . . . . . . . . . . 3.9 14. Rounded boulders and cobbles. The clasts are comprised by effusive rocks, granite porphyry, quartzite, metaschists. . . . . . . . . . 1.3 15. Pebbles and cobbles (1–9 cm) with gray sandy clay matrix. The clasts are rounded and well-rounded, unweathered, composed of the same rocks listed above (interval 12). . . . . . . . . . . . . . . . 47.5 16. Pebbles and cobbles (up to 9 cm) with yellowish-gray clayey sand in matrix. The clasts are angular to rounded and comprised by

v

Thickness, m 1. Subangular very fine to fine pebbles with yellowish-gray clayey sand matrix and scarce subangular to rounded medium to very coarse pebbles (1–5 cm) of effusive and metamorphic rocks. The sand is fineto coarse-grained. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 2. Boulders and cobbles. The clasts are variously-rounded (subangular to well-rounded) though majorly being rounded. The clasts are comprised by rhyolite, rhyodacite, pink granite porphyry, subporphyric granite, and quartzite. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 3. Gray polymictic sand with granules and fine pebbles, sporadic admixture of gray silt and individual pebbles and cobbles (2–8 cm). The granules and fine pebbles are subangular and angular. . . . . . . 2.6 4. Subangular to rounded cobbles and boulders. The clasts represented by rhyolite, rhyodacite, granite exceed the core diameter. . . . 7.3 5. Pebbles and cobbles (2–9 cm) in matrix of gravely sand with admixture of light gray clay. Clasts are variously rounded (subangular to well-rounded). The clasts are commonly flattened and elongated. Petrographic composition of the clasts majorly display effusive rocks, however pink and gray granite porphyry, quartz and metamorphic schists are also documented. . . . . . . . . . . . . . . . . . . . . . . . 3.6 6. Pebbles, cobbles, and boulders with yellowish-gray clayey sand matrix. The larger clasts exceed the core diameter and reach 15 cm along the vertical axis. The clasts are subangular to well-rounded, and composed of effusive rocks, quartzite, gabbro-diorite, granitoids and metamorphic schists. . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.8 7. Pebbles (up to 5 cm in diameter) in yellowish-gray coarse polymictic clayey sand. The pebbles of effusive rocks and metamorphic schists are subangular to well rounded. . . . . . . . . . . . . . . . . . 3.1

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8. Variously rounded pebbles, cobbles, and boulders with gravely sand matrix. The clasts of effusive rocks, granitoids and metamorphic schists are bigger than 3 cm, the largest clasts exceed the core diameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Yellowish-gray coarse polymictic clayey sand with inclusions of variously-rounded very fine and fine pebbles. . . . . . . . . . . . . . 10. Pebbles (up to 5 cm) with matrix of yellowish-gray unevengrained clayey sand (grain size varies between silt and coarse sand). Pebbles are rounded and well-rounded, elongated, and flattened. Among the clasts, effusive rocks are dominant, with subordinate amount of granitoids, metamorphic schists and quartzites. . . . . . . 11. Uneven-grained polymictic sand, cemented with yellowish-gray clay. An admixture of granules and fine pebbles (up to 40%) and subangular to rounded very coarse pebbles (3–4 cm) of effusive rocks is documented. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Subangular and rounded pebbles, cobbles, and boulders with granule-sand clay matrix. The clasts of effusive rocks and granitoids are bigger than 3 cm, the largest clasts exceed the core diameter. . . 13. Subangular to rounded pebbles and cobbles (up to 7 cm) with compacted yellowish-gray uneven-grained clayey sand matrix. There are scarce weathered reddish-brown pebbles, easily powdering in sandy clay mass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14. Subangular to well-rounded pebbles, cobbles, and boulders with granule-sand-clay matrix. Some of clasts are bigger than the core diameter. The clasts are majorly composed of effusive rocks. . . . .

4.1 2.4

3.5

8.6

8.3

4.1

5.5

The core recovery within the interval is of same ratio as for the overlying one (logged above). The deposits reveal high recovery of light minerals (65–95%), represented by quartz (50–60%) and calcite (3–10%). The heavy minerals differ considerably from those of the Tueryk and Beken Formations. Within the interval, the heavy minerals are majorly comprised by apatite (70–97%), corundum (3–30%) and authigenic limonite (3–15%). Sporadic grains of zircon, rutile, leucoxene, ilmenite, garnet, malachite, galenite, scheelite, hematite, pyromorphite, epidote and anatase are also documented. Silt-sandy matrix in the individual beds contain up to 4 mg/ton of gold. No identifiable fossils are found within the interval. Summarizing the localization within the section, lithology, presence of intensely weathered pebbles and cobbles, and mainly yellowish color of the rocks, we correlate this interval with the Bashkaus Formation logged in the Kubadru stratotype section, as well as in natural outcrops and wells in the Kurai and Chuya Basins of Gorny Altai (Devyatkin, 1965; Liskun, 1975; Svitoch et al., 1978; Ponomarev et al., 2010; Rusanov and Vazhov, 2014). Accordingly the Resolutions of Interdepartmental Stratigraphic Committee (Borisov, 2008), the Bashkaus Formation has the early Middle Pleistocene age (MIS 19–MIS 18). Lacustrine-alluvial (?) Beken Formation (laQIbk) is for the first time recognized within the Uimon Basin. In well No. 1, these strata are 42 m-thick and exposed in depths 297–339 m (Fig. 2). The Beken Formation, overlapping the Tueryk Formation, is herein comprised by (top to bottom): Thickness, m 1. Gray compacted damp clay, with 40% of the whole sediment mass represented by subangular pebbles (up to 2 cm). There are also scarce intensely weathered brownish clasts (up to 4 cm), easily powdering in clayey sand mass. . . . . . . . . . . . . . . . . . . . . . . . 2. Pebbles and cobbles (4–9 cm) with compacted yellowish-gray sandy clay matrix and admixture of very coarse sand. Subangular to rounded pebbles and cobbles are composed of effusive rocks, granite porphyry and quartzite. . . . . . . . . . . . . . . . . . . . . . . . . .

1.8

7.2

3. Compacted plastic reddish-brown calcareous clay. Downwards the bed, the clay is gradually replaced by yellowish-gray solid polymictic clayey sand (middle-fine grained). . . . . . . . . . . . . . . . 4. Pebbles (up to 6 cm) with admixture of uneven-grained sand, cemented with yellowish-brown clay. The clasts are variously rounded (subangular to well-rounded), and comprised by effusive rocks, granitoids, metamorphic schists and quartzite. . . . . . . . . . . . . . . . . 5. Horizontally-bedded thin laminated gray, brown, and brownishgray clays and yellowish-gray medium sand. Thickness of interlayers varies between 2–3 mm and 2 cm. . . . . . . . . . . . . . . . . . . . 6. Gravely sand cemented with brownish-yellow clay. . . . . . . . 7. Horizontally-bedded thin laminated (up to 2 mm-thick) calcareous clay with sporadic inclusions of granules. The lamination is composed of alternating reddish-brown, yellow and brown clay. Towards the base of the bed, the clay becomes mainly yellowish-gray. Small flattened thin laminated carbonate concretions (up to 3 cm) are registered in the clay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Dark gray compacted clay with scarce variously-rounded granules and fine pebbles (up to 0.5 cm). . . . . . . . . . . . . . . . . . . 9. Pebbles (up to 3 cm in diameter) and uneven-grained sand, consolidated with yellow clay. The pebbles are variously rounded and composed of effusive rocks, granitoids and quartzite. . . . . . . . . . 10. Thin laminated yellowish-gray compacted calcareous clay, with thin (up to 0.5 cm) interlayers of gray clay. . . . . . . . . . . . . . . 11. Gravely sand with matrix of yellowish-gray clay. The clasts are variously rounded, the pebbles are up to 3 cm in diameter; the sand is uneven-grained. The clasts are represented by effusive rocks and metamorphic schists. . . . . . . . . . . . . . . . . . . . . . . . . . 12. Variously rounded pebbles and cobbles with sand matrix. The pebbles (up to 7 cm) are comprised by effusive rocks, granitoids and quartzite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. Brown compacted, plastic clay, embedding up to 30% of very fine to coarse (up to 2.5 cm) well-rounded pebbles. . . . . . . . . . . 14. Yellowish-brown compacted silty clay with well-rounded granules and fine pebbles. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15. Brown compacted, plastic clay, embedding up to 30% of very fine to coarse pebbles. Near the base of the bed the clay becomes yellowish-gray. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16. Variously rounded coarse to very coarse pebbles with yellowish-gray clayey sand matrix. The pebbles (3–6 cm) are composed of effusive rocks, granitoids and quartzite. . . . . . . . . . . . . . . .

3.8

9.7

1.8 0.5

1.5 0.5

1.2 2.8

3.7

1.9 0.6 1.0

1.4

2.6

The core recovery varies broadly within the interval 32–45% in the coarse-grained beds, 57–58% in clays. A particle-size analysis of the clays yields faint admixture of sand (2.3–14%) and silt (0.9–19.3%). X-ray diffraction study of the clay samples reveals kaolinite, chlorite, illite, mixed layer silicates. Nevertheless calcite is localized within certain beds, its overall concentration (up to 5%) is considerably lower than in the Tueryk Formation. The clays of the Beken Formation herein logged displays nearly identical recovery rate, composition and concentrations of light and heavy minerals to those from the Tueryk Formation. The beds 1 and 8 embrace 4 mg/ton concentrations of gold. The geoelectric survey indicates a bed with resistivity of 255–543 Ohm⋅m, localized within the stratigraphic interval of the Beken Formation (Fig. 1C). This bed may correspond to silty sands and sands, corresponding to different sedimentary conditions—supposedly, lacustrine (Deev et al., 2012b). The bed 8 (depths 323.3–323.8 m) contains sporadic fossil pollen grains, identified as Betulaceae—2, Pinus s/g Diploxylon—3, Pinus s/g Haploxylon—3, Tsuga—3. There are only two pollen grains Pinus s/g Diploxylon identified in the bed 10 (depths 325–327.8 m) (Khazina et al., 2015). In the Early Pleistocene, flora of Altai Mountains, South Siberia and Mongolia was characterized by final disappearance of tsuga,

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which are not documented through all the Pleistocene strata there (Devyatkin, 1965; Svitoch et al., 1978). Hence, we interpret the strata embedding Tsuga pollen grains were accumulated no later than the Early Pleistocene. Considering the localization within the section and overall lithology, we correlate this interval with the Beken Formation, exposed with wells within the Kurai and Chuya Basins of southeastern Altai. Accordingly an explanatory note for the geological map (Ponomarev et al., 2010; Shokal’skii, 1999) and the lower Quaternary boundary, recently verified as 2.588 Ma, the Beken Formation is of the Pleistocene age. Lacustrine Tueryk Formation (lN1-2tr) is for the first time established within the Uimon Basin. The well No. 1 exposed only the uppermost 61 m of the formation, where the drilling was shut in depth 400 m (Figs. 1C, 2). The section exposed is logged as follows (top to bottom): Thickness, m 1. Light gray compacted calcareous silt with abundant interlayers of brown coarse sand, and yellowish-gray and light gray fine and medium sand. The sand layers are 3–25 cm-thick. Small (up to 2.5 cm) calcareous concretions are documented in the silt. Depth-interval 345.5–345.6 m is comprised by yellowish-gray robust sandy marl, whereas the interval 346.6–346.75 m is represented by light gray robust sandy marl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 2. Yellowish-gray clay with scarce subangular pebbles (up to 3 cm). The sand is uneven-grained. Interlayers of yellowish-gray clay (up to 2 cm) and pebbles in sandy clay matrix (up to 5–7 cm) are documented. The pebbles (up to 4 cm in diameter) are subangular to rounded. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.0 3. Yellowish-gray compacted clay with numerous thin (1–2 mm) bands of ferruginization, parallel to lamination. . . . . . . . . . . . . 2.2 4. Reddish-brown compacted massive sandy calcareous clay. Reaching the depth 364 m, angular pebbles and rounded to angular granules and fine pebbles of weathered rocks occur in the core of the well. A large cobble (up to 20 cm) of quartz-chlorite schist lays at the base of the bed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 5. Compacted massive clay. Downwards the bed, color of the clay gradually changes from brown to gray. . . . . . . . . . . . . . . . . . 1.5 6. Yellowish-gray compacted clay with inclusions of granules and fine pebbles. The core interval 367.7–368.2 m comprises a bed, filled with subangular granules and fine pebbles. . . . . . . . . . . . . . . . 5.0 7. Brownish-gray clay with thin ferruginized bands, parallel to lamination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 8. Uneven-grained clayey sand with scarce granules. The clay is yellowish-brown. The core interval 375.8–375.9 m is composed of yellowish-gray calcareous gravely sandstone with numerous small fragments of mollusk shells. . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 9. Gray clay with brown and reddish-brown ferruginized bands. . 0.5 10. Bluish-gray compacted clay with brown and reddish-brown ferruginized bands in the uppermost part of the bed. . . . . . . . . . . . 0.5 11. Compacted thin laminated (several millimeters) clay. Downwards the interval, color of the clay gradually changes from light brown to greenish-gray, grayish-green, and even black (basal layers). 0.7 12. Black compacted clay with abundant small black carbonaceous plant debris, and fragments of mollusk shells. . . . . . . . . . . . . . 1.9 13. Brown thin laminated compacted clay with an interlayer (10 cm) of uneven-grained sand at the base. . . . . . . . . . . . . . . 1.7 14. Gray compacted clay with individual granules and fine pebbles and scarce thin interlayers (up to 1 cm) of uneven-grained sand. . . 1.1 15. Yellowish-gray massive compacted clayey sand with inclusions of pebbles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 16. Reddish-brown massive compacted calcareous clay with rounded granules and angular pebbles of metamorphic schists. An interlayer of grayish-green clay is documented in the core interval 391.7– 392.0 m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 17. Yellowish-gray clay with inclusions of granules. . . . . . . . 0.5 18. Reddish-brown calcareous clay with inclusions of granules. . 0.8

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19. Compacted massive sand-silt clay. The uppermost 40 cm of the bed are gray-colored, whereas the rest of the interval is black. The clay includes sporadic black carbonaceous plant debris, subangular granules of lithologically-various rocks, numerous angular pebbles of weathered metamorphic schists and carbonates. When desiccating, the clay changes its color dramatically—within the black, gray, dark gray, grayish-green and dark brown interlayers (0.3–0.5 m) appear, displaying gradual transitions. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4

The gravely sand-dominated intervals demonstrate the core recovery of 40–60%, whereas the intervals dominated with silt and clay—of 67–97%. In major, the logged interval 339– 400 m is comprised by clays. The sands amount varies here between 2.7 and 22.2%, silt—2.1–43.1%, clays—31–95.2%. Carbonate compound gradually decreases upwards the section from 12% at the base, to 5% at the top. Mineral composition of the clays includes kaolinite-montmorillonite, with mixed layer silicates, calcite, and goethite. The logged strata are characterized by high concentrations of light minerals at 96.21–99.38% (quartz, feldspar, calcite), and low concentrations of heavy minerals (3.78–0.62%), which include major compounds (>10%, each)—magnetite, ilmenite, hematite, leucoxene, epidote, amphibole, authigenic limonite; minor compounds (<10%)—pyroxene, apatite, rutile, sphene; and individual grains of zircon and tourmaline. In beds 8, 9, 10, and 16, the clays reveal increased gold concentration up to 4–5 mg/ton. Thin marl interlayers are one of the diagnostic features of the Tueryk Formation in sections of the Chuya Basin of southeastern Gorny Altai (Devyatkin, 1965). Considering the lithology, the lower part of the logged section (depths 375.9–400 m; beds 9–19) was originally correlated with the Oligocene–Lower Miocene Kosh-Agach Formation (Rusanov and Tsaer, 2014). However, recent investigations for the first time for the region revealed spore–pollen assemblages (“spore–pollen spectra”) and ostracod fossils, which confronts the assignment of the section to the Kosh-Agach Formation (Khazina et al., 2015). The ostracods were found only in one sample from the basal black clays of the bed 11 (depth 377.5 m) (Figs. 2, 3). The complex includes Cyclocypris laevis (Müller) (5 specimens), Cytherissa hyalina Schweyer (4 specimens), Eucypris foveatus Popova (9 specimens), Ilyocypris bradyi Sars (12 specimens), Limnocythere seducta Mandelstam (7 specimens), Limnocythere inderica Scharapova (9 specimens), Limnocythere luculenta Livental (4 specimens). Five of seven mentioned species show no robust stratigraphic constraint. For instance, Lymnocythere inderica is known both from the Novaya Stanitsa and Bitekei ostracod complexes of southern West Siberia (Upper Miocene–Middle Pliocene). Ostracods Limnocythere seducta and Eucypris foveatus are abundant in the Upper Miocene–Lower Pleistocene strata (Gelasian–Calabrian/‘Eoplestocene’) (Kochki complex), however being unknown from the younger deposits. Species Eucypris foveatus and Ilyocypris bradyi are widely known from the Upper Miocene–modern sections of West Siberia. Ostracods Cytherissa hyaline and Limnocythere luculenta are exclusively known from the Late Miocene–Early Pliocene Novaya Stanitsa complex (Kaz’mina, 1989), which corresponds to the Tueryk Formation within the studied

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Fig. 3. Ostracod association from the sample C-1/8 (depth 377.5 m).

section. In Gorny Altai, species Limnocythere inderica is known only from the Early Pliocene Upper Tueryk complex (Teterina, 2012). Consequently, the ostracod assemblage from the logged section corresponds to the Lower Pliocene, and characterizes the Tueryk Formation. The upper beds of the interval (beds 1–6) contain only extremely sporadic pollen Pinus spp., P. s/g Diploxylon. The most taxonomically-diverse spore–pollen assemblages are recorded from the depths 377.7 m (bed 12), 378.5 m (bed 12), 396 m (bed 19), and 398 m (bed 19) (Fig. 4). Other strata of the Tueryk Formation within the section contain only scarce and taxonomically-poor palynomorphs. Taxonomical composition of the samples 10 (depth 377.7 m) and 11 (depth 378.5 m) is virtually the same: tree-shrub plants comprise 32.96% and 38.78%, respectively; herbal-suffruticose plants—65.93% and 58.33%, respectively. Spores are extremely scarce. The tree-shrub plants are represented by pollen of Betula spp., Betulaceae, Tsuga, Pinaceae, Pinus spp., Tilia, Ulmus, Salix, Picea. The herbaceous plants within the complex include pollen of Chenopodiaceae, Asteraceae, Artemisia, Ranunculaceae, Poaceae, Fabaceae, and mixed herbs. The identified spores include Polypodiaceae and sporadic Sphagnum. The spore–pollen associations obtained from depths 396 m, 396.5 m, and 398 m are dominated by pollen of herbs (majorly Chenopodiaceae, Asteraceae, Artemisia, Ranunculaceae), and contain extremely rare tree pollen.

To estimate and verify the age of the studied interval, further we compare the obtained palynological data with spore–pollen characteristic of the Kosh-Agach and Tueryk Formations (Bessonenko, 1963; Devyatkin, 1965). These data display wide distribution of mildly impoverished Turgai-type mixed coniferous-broad leaved flora here in the Kosh-Agach time, dominated with coniferous plants. During the first half of the Kosh-Agach time, the floral complexes were characterized by mixed flora, with dominating conifers: tsuga (20–30%), spruce (10–15%), pine (5%). Taxodiaceae are numerically-rare, but occurs constantly. The birch pollen holds averagely 5–10%, broad-leaved flora—5–8%. The middle Kosh-Agach Formation (Chuya Basin) is characterized by a substantial increase of birch pollen (up to 40–70%) and associated expansion of broad-leaved plants up to 10–20%. This maximum of birch abundance is coherent with oscillatory expansions of Ericaceae (40–80%). The uppermost KoshAgach Formation (Samakha Basin) is marked with a dramatic decrease of birch and Ericales, further lowering of the broad-leaved flora, and dominance of dark conifers (spruce— 25–40%, tsuga—15–20%, pine—20–50%. Being compared, the palynocomplex of well No. 1 considerably differs from the mentioned above. After the taxonomical identifications made by E.A. Bessonenko in (Devyatkin, (1965), the characteristic spore–pollen complex of the Tueryk Formation near the Chagan-Uzun River includes: Betula verrucosa (23,2%), Pinus sect. Haploxylon (11,5%), Pinus sibirica (9,2%), Tsuga cf. diversifolia (8, 2%),

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Fig. 4. Distribution of spores and pollen within the spore–pollen assemblages (SPA) in the studied samples. 1, pollen of herbaceous plants; 2, pollen of tree-shrub plants; 3, spores.

Pinus silvestris (5,9%), Abies sibirica (4,7%), Picea cf. obovata (4,9%), Picea sect. Excelsa (3,5%), Tilia cf. cordata (1,1%), Taxodiaceae (1,1%), Ulmus (0,9%), Juglans (0,9%), Carya (0,7%), Carpinus (0,7%), Pterocarya (0,4%). The woody plants comprise 49.9% of the complex. The herbal associations are dominated by meadow and steppe plants. The spore–pollen complex of clays from well No. 1 (depths 377.5–398 m) are typical for forest and forest-steppe vegetation. Extremely rare pollen of broad-leaved plants and considerable amount of herbaceous pollen assign this complex rather to Late Miocene–Early Pliocene ones (Tueryk), than the Oligocene–Early Miocene (Kos-Agach). These palynological data positively correlate with spore–pollen complexes of West Siberia, where the Upper Miocene Pavlodar Horizon embeds steppe and semiarid associations with xerophytes (Babushkin, 2001). Nevertheless the palynocomplexes with dominating pollen of Chenopodiaceae are typical both for the Pavlodar Horizon and the overlaying Pliocene–Pleistocene strata (Volkova, 2000), the data obtained on ostracod distribution robustly constrain the age of the embedding strata as the Late Miocene–Early Pliocene. As a conclusion, the ostracod and palynological data precisely date the middle and lower parts of the section exposed by well No. 1 as the Late Miocene–Early Pliocene, and substantiate a correlation of this interval with the Tueryk Formation of southeastern Gorny Altai, where the age of the

whole sequence of the Tueryk Formation is constrained as the Middle Miocene–Middle Pliocene (Shokal’skii, 1999). Paleogene–Neogene deposits undifferentiated (P − –N?) are not exposed by well No. 1. We nominally establish them based on results of geophysical electromagnetic prospecting (transient electromagnetic method/TEM, vertical electric sounding/VES), performed in the Uimon Basin in 2011 (Deev et al., 2012b). The prospecting revealed the low-resistivity (57 Ohm⋅m) interval 210 m-thick, overlaying the high-resistivity (1670 Ohm⋅m) basement of the basin, in the lower part of the geoelectric section (Fig. 1C). The upper part of the low-Ohm interval is exposed with well No. 1 and correlated with the Tueryk Formation. The rest volume of the interval (at least 150 m) is not exposed. On the analogy with the Chuya and Kurai Basins of Gorny Altai, we interpret the geophysical data obtained on the undiscovered low-resistivity interval with lacustrine, slope-wash, and alluvial fan low-Ohm clays, silts, and clayey sands, stratigraphically corresponding to the Tueryk, Kosh-Agach, and Karachum Formations (Deev et al., 2011, 2012a; Nevedrova et al., 2014, 2017).

Conclusions Well No. 1, drilled in the center of the Uimon intramontane basin, exposes the Neogene–Quaternary section of unique thickness. From the top to bottom, the section comprises the

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Upper Pleistocene lacustrine-glacial deposits; Middle Pleistocene glacial, fluvioglacial, and alluvial deposits; Middle Pleistocene alluvial-proluvial Bashkaus Formation, and lacustrine-alluvial Beken Formation. For the first time, the paleontological data on ostracods and spore–pollen complexes veraciously establish the Miocene–Pliocene lacustrine Tueryk Formation in the basin. The detailed multifaceted analysis of the section exposed with well No. 1 clearly demonstrates the Uimon Basin to be already developed as early as the Miocene, when acted as the accommodation space for the lacustrine Tueryk Formation. Presence of thick low-resistivity interval below indicates the Uimon Basin might develop even in the Paleogene. This assumption determines the tectonic and depositional history of the Uimon Basin to be similar with the Chuya and Kurai Basins of Gorny Altai. Sedimentary successions of all these basins demonstrate clear two-member lithological composition, characterized by considerably different electric resistivity (Deev et al., 2011, 2012a,b; Nevedrova et al., 2014, 2017). The lower complex, characterized by low-resistivity, corresponds to the Paleogene–Neogene mainly-clayey lacustrine deposits, accumulated in relatively low-rugged relief. The overlying high-Ohm complex answers the coarse-grained facies of the Quaternary strata. The second complex reaches 339 m in thickness in well No. 1. Such a dramatic change in facies composition of the Uimon, Kurai and Chuya Basins in the Early Quaternatey mirrors activation of neotectonic orogenesis in Gorny Altai, shaped its nowadays face. During that period, the Basins were developing as ramp and half-ramp structures with the thrust zones of montane margins being more strikingly developed along the northern margins (Deev et al., 2012a, 2017; Delvaux et al., 1995, 2013; Nevedrova et al., 2014). Acknowledgments. The study was carried out with financial support of the Russian Academy of Sciences Presidium Program No. 30.

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Editorial responsibility: I.S. Novikov