Geomorphological evidence of at least two Pleistocene glacial epochsis noted within the Chagan-Uzun river basin, SE Altai. A review and analysis of all available absolute dates for reference Chagan section is presented. The highest correlation amongst all TL dates is observed for the lens of glacio-lacustrine sediments – the most suitable among glacial deposits for luminescence dating, and indicates its possible Middle Pleistocene age. IRSL dates obtained from feldspar indicate a Middle Pleistocene age of moraines already in the upper part of the section. The small number of obtained IRSL dates does not allow making geochronological reconstructions of the Pleistocene glaciations, but gives the possibility for further experiments with different variation of OSL (IRSL) techniques. Strong low temperature peak in TL signal and strong response to IR stimulation are specific regional quartz features, which could be explained by combination of short transportation distance and low number of depositional cycles for mineral grains. Available radiocarbon dates of carbonate concretions from this section are not related to the age of moraine sedimentation and most likely indicate the period of the Chagan river incision into the ancient glacial deposits.
This study has shown that TL method is not valid for dating glacial sediments and TL dates cannot be utilized as chronostratigraphic markers. Generally, the Chagan section could hardly serve as a reference section for the Altai stratigraphy; available depositional correlation schemes for the Russian Altai and Siberia which are based on several TL dates obtained in the last century needs to be improved.
- glacial sedimentation
- luminescent dating
- radiocarbon dating
- Chagan section
- Russian Altai
Reconstruction of the patterns of Pleistocene glaciations in the mountains of Southern Siberia, as well as correlating the sediments within mountains of Central Asia mountain belt and platform areas of Siberia, is not possible without absolute dating of glacial deposits, but these types of continental sediments are very complex to date absolutely. By now there is a wide arsenal of different numeric techniques for age determination of Quaternary sediments (Wagner, 1998). The application, precision and accuracy of each of them vary considerably (Fuchs and Owen, 2008).
However, within the Russian Altai utilizing most of techniques are highly problematic. Organic material generally is not presented in ancient glacial sediments. Moreover, glacial sequences are beyond the radiocarbon timescale and geological materials suitable for other radiogenic dating methods are also absent. Cenozoic formation of the Russian Altai was not accompanied by volcanic activity unlike to the mountain systems of the neighboring Tuva region where at least 5 glaciations were revealed for the last 1.75 million years (hyaloclastites, the products of subglacial volcuno eruptions were dated using the K-Ar method (Yarmolyuk and Kuzmin, 2006)). In contrast to Tuva, the effusive and tephra layers are absent in the sections of the Russian Altai. Repeated developing giant ice-dammed lakes in the region (Butvilovsky, 1993; Carling
Now thermoluminescence (TL) dates of glacial deposits in several areas of the Russian Altai form the basis for the formal Altai Pleistocene stratigraphic scale and depositional correlation schemes for the Russian Altai, Altai foothill plains, Western and Eastern Siberia (Volkova and Babushkin, 2000; Quaternary System, 2008 and Volkova
Subsequently, a new set of absolute dates describing glaciations in this part of Central Asia was presented. For sediments of the Chagan section first TL dates utilizing naturally saturated etalons were published (Sheinkman, 1990) and several radiocarbon dates of carbonate concretions were obtained (Butvilovsky
In this paper, we analyze tectonic and climatic factors that controlled the number and extension of the Pleistocene glaciations in different parts of the Russian Altai; present the results of our geomorphological investigations within the SE Altai with the main focus on the Chagan-Uzun river basin, where traces of different glaciations are preserved in topography; and finally compare these results with the results of applying different absolute dating techniques (previously published and obtained in this study) for dating glacial sediments of the reference Chagan section located within this basin. We also present the first experience of applying the IRSL method for feldspar from this section; discuss applicability of different luminescence techniques for dating the Pleistocene glacial sediments in the region and correlating these deposits with Siberian stratigraphic scales. The relevance of this work is evidenced by the interest of some international teams in studying and dating glacial features in Central Asia including Russian Altai applying various numerical techniques (Gribenski
The Altai intracontinental uplift is a part of global watershed between the Arctic Ocean basin and the inland drainage basin of Central Asia (Fig. 1). The Altai Mountains are the northern part of the Central Asia collision belt. This mobile zone is clamped between Eurasia plate on the north and Djungar together with Tuva-Mongolian microplates on the west and on the east (Molnar and Tapponnie, 1975 and Novikov, 2004). As a result, the Altai intracontinental uplift stretches northwest more than 1500 km and forms a wedge shape narrowest in the southeast (about 50 km) and widest in the northwest (up to 500 km). The elevation increases in the opposite direction from 400 m a.s.l. to 4000 m a.s.l. In its northern part, the socle surface Socle surface is the surface contacted minimum marks of relief. A surface contacted top of mountains is summit surface.
Socle surface is the surface contacted minimum marks of relief. A surface contacted top of mountains is summit surface.
Within the Russian Altai the following areas, which are different in orographic and climatic features and therefore in manifestations of ancient glaciations, could be defined (Fig. 2). They are presented in a southward direction – in the direction of decreasing humidity and increasing altitudes (Fig. 3):
NE Altai, Lake Teletskoe area. There is Earth crust extension in this part of the Altai Cenozoic uplift (Novikov Chulyshman plateau, Sorulukol and Dzhulukul intermountain depressions (southern border of this area partly goes along the crest of the Kurai range). This area belongs to dextral strike-slip fault zone (Novikov SE Altai, the Kurai-Chuya system of intermountain depressions and framing ridges except the Sailugem range (southern border of this area goes along the crests of the South-Chuya and Katun ranges). This is a transpressional zone formed due to oblique thrusting. It is located on the brow of the socle surface with an altitude of about 1500 on the west (Kurai depression) to 2000 m a.s.l. on the east (Chuya depression). The tectonically driven topography roughness is about 2600–3000 m with the maximal value about 3500 m within the Katun range. This is the largest value for the whole Altai Mountains. In spite of arid climate, due to the high altitudes of the ridges (up to 4506 m a.s.l.) the SE Altai is the centre of the modern glaciation. Geomorphologically distinct moraines of the maximal Middle- and two Late Pleistocene (Early Zyryanka (MIS 4) and Sartan (MIS 2)) glaciations are marked out here, last of which (Sartan) was the smallest one (Devyatkin, 1965; Okishev, 1982; Agatova, 2005b; Zol’nikov The presence of early Pleistocene glaciation within the SE Altai was suggested by (Svitoch South Altai, the South Altai and Sailugem ranges, the Ukok plateau (southern border of this area goes along the Tabyn-Bogdo-Ula range which is affected by modern glaciation). Socle surface of the main weakly dissected tectonic blocks and intermountain depressions lies at an altitude of about 1800–2300 m a.s.l., and summit surface — at about 3000 m a.s.l. Altitudes of the Tabyn-Bogdo-Ula range are 3200–3400 m a.s.l. with the highest peak at 4374 m a.s.l. That is an area of the Late Pleistocene low mobile valley glaciers and ice caps (Sailugem range) (Rudoy and Kirjanova, 1996; Galakhov and Samoylova, 2007) and ice reservoirs (high mountain depressions within Ukok plateau and Dzhazator tectonic valley, which took the glaciers from Tabyn-Bogdo-Ula and South Chuya ranges) (Okishev, 1982; Butvilovsky, 1993; Rudoy
NE Altai, Lake Teletskoe area. There is Earth crust extension in this part of the Altai Cenozoic uplift (Novikov
Chulyshman plateau, Sorulukol and Dzhulukul intermountain depressions (southern border of this area partly goes along the crest of the Kurai range). This area belongs to dextral strike-slip fault zone (Novikov
SE Altai, the Kurai-Chuya system of intermountain depressions and framing ridges except the Sailugem range (southern border of this area goes along the crests of the South-Chuya and Katun ranges). This is a transpressional zone formed due to oblique thrusting. It is located on the brow of the socle surface with an altitude of about 1500 on the west (Kurai depression) to 2000 m a.s.l. on the east (Chuya depression). The tectonically driven topography roughness is about 2600–3000 m with the maximal value about 3500 m within the Katun range. This is the largest value for the whole Altai Mountains. In spite of arid climate, due to the high altitudes of the ridges (up to 4506 m a.s.l.) the SE Altai is the centre of the modern glaciation. Geomorphologically distinct moraines of the maximal Middle- and two Late Pleistocene (Early Zyryanka (MIS 4) and Sartan (MIS 2)) glaciations are marked out here, last of which (Sartan) was the smallest one (Devyatkin, 1965; Okishev, 1982; Agatova, 2005b; Zol’nikov
The presence of early Pleistocene glaciation within the SE Altai was suggested by (Svitoch
South Altai, the South Altai and Sailugem ranges, the Ukok plateau (southern border of this area goes along the Tabyn-Bogdo-Ula range which is affected by modern glaciation). Socle surface of the main weakly dissected tectonic blocks and intermountain depressions lies at an altitude of about 1800–2300 m a.s.l., and summit surface — at about 3000 m a.s.l. Altitudes of the Tabyn-Bogdo-Ula range are 3200–3400 m a.s.l. with the highest peak at 4374 m a.s.l. That is an area of the Late Pleistocene low mobile valley glaciers and ice caps (Sailugem range) (Rudoy and Kirjanova, 1996; Galakhov and Samoylova, 2007) and ice reservoirs (high mountain depressions within Ukok plateau and Dzhazator tectonic valley, which took the glaciers from Tabyn-Bogdo-Ula and South Chuya ranges) (Okishev, 1982; Butvilovsky, 1993; Rudoy
This analysis indicates that SE Altai is the most perspective aria for studying the chronology of the Pleistocene glaciations.
The Chagan-Uzun river basin (Fig. 3) is located in the SE Altai and includes southwestern part of the Chuya intermountain depression and framing ridges. Tributaries of the Chagan-Uzun river are originated in the highest parts of the North and South Chuya ranges and in the flattened top of the Chagan-Uzun massif, which separates the Kurai and Chuya intermountain depressions.
Flower structure (Cunningham
Mapping accumulation surfaces of glacial origin within study area (Agatova, 2005b) revealed
We suppose that washed and eroded remnants of the moraine covering outside the well-expressed terminal moraines on tectonic steps of the South Chuya range and in the Chuya depression are the traces of one (or possibly several) of the ancient piedmont glaciations. Complex of well-preserved moraines argue for valley type of the next glaciation. Stadial ramparts of the proximal moraine complex are younger because they are imbedded into moraines of the peripheral complex or cover them. Among all glacial landforms, moraines in the valleys are the youngest ones. Once again, the question of using them for reconstructing independent glaciation is open. Lack of geochronological data makes it difficult to determine the age of moraines except the nearest to the modern glacier LIA (13th–19th centuries) moraines (Agatova
Therefore, different geomorphological features of various moraine complexes and their different state of preservation argue for at least of two ancient glaciations within the SE Altai. Most likely they were separated by prolonged interglacial period when due to tectonic activity the valleys of the South Chuya range were deepened.
Following the established traditional concept of the Russian Altai Quaternary geology (Devyatkin, 1965; Ivanovsky, 1967; Popov, 1972 and Okishev, 1982) we refer well preserved ramparts of proximal and peripheral moraine complexes to the Late Pleistocene glaciation, and washed and eroded remnants of moraine cover beyond these complexes — to the Middle Pleistocene glaciation. Thus, assuming these glacial sediments in the Chuya depression and at the watersheds on the lower tectonic steps of the South Chuya range belong to a single glaciation, in transitional zones where younger sediments cover the older ones, we can expect moraine complexes at least of two glaciations to be exposed in proximal outcrops including the presented here Chagan section.
The Chagan section, located in the transition zone between the Chuya intermountain depression and South Chuya range (Fig. 4), is one of the reference sections of the Pleistocene glacial, glacio-fluvial, and glacio-lacustrine deposits in the region. This section is about 4 km long and ~200 m high containing several units (Fig. 5). Interaction of the Chagan and Taldura glaciers in the area of their conjunction determines the structural complexity of glacial thickness here. In the Chagan river valley lateral ramparts of the proximal moraine complex within the upstream part of the section are at about 2250 m a.s.l. Downstream the altitude of the section increases due to overlaying lateral moraines of the Taldura glacier.
Preglacial deposits at the bottom of the section have Low Neogene — Low Pleistocene ages (Devyatkin, 1965 and Rusanov, 2011). They include light gray and yellowish-gray lacustrine sediments of the Tueryk suite, yellowish-brown lacustrine-alluvial sediments of the Beken suite, and brown-reddish alluvial sediments of the Bashkaus suite. We have also explored sediments of the Tueryk suite (middle Miocene — middle Pliocene) in structure of the Chagan-Taldura watershed when studying the giant landslide triggered by the 2003 Chuya earthquake
Overlying glacial deposits have a basal thin (1.5–7.0 m) moraine layer with brownish redeposited sediments of the Bashkaus suite.
Above it there is a lens (up to 45–50 m) of light gray fine-grained glacio-lacustrine sediments with drop-stones and layers with carbonate concretions (up to 10–15 cm in diameter) at the unit boundaries. Svitoch
Above the lens there is a thick (about 60–80 m) gravel-dominated section of gray moraine and glacio-fluvial sediments with distinct sandy layers of fluvial origin. The glacial deposits are characterized by a high degree of facies variability, which cause difficulties in distinguishing the different glacial episodes. Before the 2003 Chuya earthquake two clearly distinguished steps were formed as a result of long-term erosion. Devyatkin (1965) interpreted them as evidence of two different glaciations but it could be also explained by different resistance of strata to erosion. Borisov (1984) and Butvilovsky
The top of the exposure has a height of 2250 m a.s.l., which coincides with the possible maximum filling of the Pleistocene ice-dammed palaeolakes in the Chuya depression (Rusanov, 2008). The timing of formation and the outburst events of these giant lakes is still debated, however, the preservation of high lake terraces here argues for moraines formation before the maximum occupancy of the lake. Thus, the age of the largest ice-dammed palaeolake could be assumed as an upper possible age of deposition processes. It could be correlated with the late Pleistocene (25–12 ka BP (Rudoy, 2005 and Rusanov, 2008)) or, in contrast, with the middle Pleistocene (Zol’nikov and Mistrukov, 2008). We assume the formation of the Chagan section as a result of lake abrasion and seismically induced landsliding. The entire Chagan section was exposed by repeated earthquake-triggered landslides. Seismic origin of this multi-kilometers outcrop in permafrost rocks was confirmed during the 2003 Chuya earthquake. A number of new seismo-gravitational cracks were developed along the brow of the valley slope. High seismicity during the accumulation of sediments in this section is evidenced by numerous seismically induced disturbances: seismic convolutions, cracks and offsets (Agatova
Paleomagnetic characteristics of the Chagan section argue for two short-term episodes of magnetic polarity reversals for glacial part of this section (Faustov
First palynology studies of the Chagan section (Svitoch
There are also only single paleontological finds in this section (Mikhailova
The first absolute dates for the Chagan section were obtained in the late 1970s. The Early and Middle Pleistocene ages of moraine deposits from Chagan section were based on three TL dates – 145 ± 13, 266 ± 30 and 476 ± 51 ka obtained with artificially saturated samples (Fig. 6)(Svitoch
At the end of the 20th century there was an attempt to revise the initial TL data using a TL technique based on applying naturally saturated etalons (Sheinkman, 1990; 2002) Sheinkman (1990) presented four dates from the upper part of the section and boundary date (≥300 ka, complete saturation) for the underlying reddish alluvial sediments at the bottom of the glacial part of the section. Later (Sheinkman, 2002) two of these ages were recalculated and location for these samples were specified as well as the TL age ≥ 100 ka for glacial-lacustrine lens was presented. Our comments to these data are given as a footnote in Fig. 6.
Sheinkman (1990) presented four dates from the upper part of the section and boundary date (≥300 ka, complete saturation) for the underlying reddish alluvial sediments at the bottom of the glacial part of the section. Later (Sheinkman, 2002) two of these ages were recalculated and location for these samples were specified as well as the TL age ≥ 100 ka for glacial-lacustrine lens was presented. Our comments to these data are given as a footnote in Fig. 6.
There are also three radiocarbon dates of carbonate concretions from the glacio-lacustrine lens at the bottom of the glacial part of the section and from thin glacio-lacustrine layers in its middle part (Fig. 6). They were obtained by Butvilovsky (Rusanov and Orlova, 2013). Radiocarbon samples have an inverse stratigraphic position in the section (the older samples are located above the younger ones) and their ages have another order of magnitude in comparison with the TL data. These dates fall into the Sartan glaciation (the end of the Late Pleistocene).
Rusanov (2011) marks out two units in the glacial part of the Chagan section. The low unit includes the basal moraine and glacio-lacustrine lens when the upper one is the single gray-colored unit of about 100 m thick. With the azimuthal unconformity the upper unit overlays the eroded surface of the lens and alluvial gravels of the Bashkaus suite. At the bottom of this upper gray-colored unit a single paleontological find (heavily damaged section of the upper tooth -
The significant discrepancy between TL dates based on different techniques as well as their disagreement with radiocarbon dates prompted us to carry out our own investigation of glacial and associated sediments of the Chagan section including applying different absolute dating techniques (radiocarbon, TL and OSL methods).
A single sample for radiocarbon analysis was collected from carbonate concretions in glacio-lacustrine lens located at the bottom of the glacial part of the section above the brownish sediments of Bashkaus suite (Fig. 7).
Sample preparation and radiocarbon age measurement was made at the Institute of Geology and Mineralogy SB RAS, Novosibirsk. The determination of carbon residual activity was done with the QUANTULUS-1220. The conventional radiocarbon age was calibrated (2-sigma standard deviation) applying the CALIB Rev 5.0 program (Stuiver and Reimer, 1993), with the IntCal09 calibration data set (Reimer
The calculated age (30189 ± 548 cal. BP; SOAN 8548 (Table 1)) correlates with the other available radiocarbon data previously obtained at the same laboratory by Butvilovsky (Rusanov and Orlova, 2013).
Obtained absolute dates. Applied techniques: RTL – radiotermoluminescence; IRSL – Infra Red stimulated luminescence; 14C – scintillation radiocarbon analysis (both calibrated (2σ) and radiocarbon (in brackets) ages are given).
N Sample Lab. code Technique Age (ka) 1 2001/04 BuGIN 504 RTL 60±11 MSU NS-4 RTL 106 ± 27 2 2001/05 BuGIN 498 RTL 25 ± 4 MSU NS-5 RTL 127 ± 32 3 2001/06 MSU NS-6 RTL 260 ± 65 4 2001/07 BuGIN 503 RTL 140 ± 17 MSU NS-7 RTL 336 ± 84 5 2001/10 BuGIN 501 RTL 124 ± 15 6 2001/13 MSU NS-13 RTL 251 ± 63 7 2001/18 MSU NS-18 RTL 151 ± 38 BuGIN 506 RTL 190 ± 23 8 2001/19 BuGIN 513 RTL 41 ± 6 MSU NS-19 RTL 290 ± 73 9 2003/02 BuGIN 664 RTL 380 ± 51 10 2003/03 BuGIN 665 RTL 310 ± 50 11 2003/05 MSU NS-305 RTL 480 ± 20 12 2003/08 BuGIN 669 RTL 253 ± 26 13 2003/10 BuGIN 670 RTL 340 ± 46 MSU NS-310 RTL 505 ± 20 14 2003/11 BuGIN 671 RTL 330 ± 57 15 CHM1 IRSL 236 ± 52 16 CHM5 IRSL ≥360 17 CHM8 IRSL ≥220 18 SOAN 8548 14C 30.189 ± 0.548 (25.470 ± 0.195)
Obtained absolute dates.
Applied techniques: RTL – radiotermoluminescence; IRSL – Infra Red stimulated luminescence; 14C – scintillation radiocarbon analysis (both calibrated (2σ) and radiocarbon (in brackets) ages are given).
Insufficient bleaching of glacial sediments is a common problem for making adequate luminescence dating (Fuchs and Owen, 2008). Deposits associated with proglacial, glacio-fluvial and glacio-lacustrine sediments have a higher potential to be bleached than subglacial and englacial ones, but the risk of insufficient bleaching is still considerable. For example, glacio-fluvial sediments may not be fully exposed to daylight due to the turbidity in the meltwater or attenuation of the daylight intensity with depth. Modern glacio-fluvial samples have been shown to exhibit residual luminescence signals which lead to overestimation of the calculated ages (Gemmell, 1997; 1999). Essentially, the effectiveness of luminescence signal zeroing is directly controlled by the geomorphic processes responsible for the sediment production and reworking. Thus, a geomorphological under standing of the glacial environment is required to identifysediments and the association with the particular geomorphological process in the context of sufficient intensity and duration of daylight exposure.
Prior to sample collecting, detailed geomorphological investigations of the Chaga-Uzun river basin were carried out in order to determine the number, patterns of and evolution of the Pleistocene glaciations in the region.
The identification of appropriate sediments for luminescence dating, as well as samples collection, was done following recommendations and suggestions of Richards (2000) and later of Benn and Owen (2002). The material for luminescence dating was derived from glacio-fluvial, glacio-lacustrine and englacial meltwater sediments to estimate the age of moraine formation equal to the age of glaciations. Several samples were collected from alluvial deposits of the Bashkaus suite, which underlies the Pleistocene complex of glacial deposits in the Chagan section, to estimate the lowest possible age of glaciations. Preference was given to deposits containing finely grained fractions. During the later field researches
TL method was used prior to the development of OSL and IRSL methods. Previously, the applicability of TL dating has been demonstrated for various sedimentary environments (Wagner, 1998 and references therein), mainly focused on those that have experienced sufficient exposure to daylight enabling a complete resetting of the luminescence signal. Luminescence dating of glacial and associated sediments is difficult, mainly due to insufficient bleaching during transportation and deposition of the sediments (Fuchs and Owen, 2008 and references therein).
In addition to the problem of zeroing of the luminescence signal, there is also a great regional diversity in physical properties of quartz grains. Quartz properties, with regards to their luminescence signal, are mainly determined by the content and composition of microimpurities which form electron traps in the mineral crystal structure. This leads to variations in the suitability of a quartz or alternative silicate minerals (such as feldspar) as a geochronometer for luminescence dating.
Generally 38 samples were collected from deposits containing sands and sandy loams within the Chagan section. Luminescence sample preparation was carried out under subdued red light applying standard chemical treatment including clay fraction washing and wet sieving (0.10–0.25 mm), hydrochloric and hydrofluoric acid etching, ultrasonic grinding with further sieving (0.1 mm), and separation of the light fraction in bromoform. Thermoluminescence dating was carried out in two laboratories which applied different radiothermoluminescence (RTL) methods. The RTL dating technique with the artificially saturated standard (Vlasov and Kulikov, 1989) was applied in the Laboratory of dosimetry, radioactivity and RTL dating, Moscow State University (MSU), while RTL age determination using naturally saturated standard (Shlukov
20 new RTL dates are presented in table (Table 1, Fig. 7). Among all studied samples there were three naturally saturated ones, which is crucial for age measurement when applying the RTL technique on the basis of naturally saturated etalons (Shlukov
11 BuGIN RTL dates (obtained with the naturally saturated etalon) indicate accumulation of glacial deposits in the Chagan section between 25 ± 4 – 380 ± 51 ka while nine MSU RTL dates (obtained with the artificially saturated standard) define this time interval as 106 ± 27 – 505 ± 20 ka. At the same time four among five samples dated in both laboratories return different ages. Moreover, there is a discrepancy with the stratigraphic position for samples dated in both laboratories. The highest correlation amongst all dates is observed for the lens of glacio-lacustrine sediments. Five dates are in the range of 260–340 ka (the Middle Pleistocene) and only one (obtained in MSU as 505 ± 20 ka, which was also dated in BuGIN as 330 ± 57 ka) stands out.
It is noteworthy that in the samples analyzed at BuGIN, from the 24 successfully prepared samples a TL signal was measured only from 14 samples, including the 3 saturated ones. Quartz extracted from the remaining ten samples gave powerful low temperature peak and therefore was unsuitable for measurements. A similar proportion of undated samples came from MSU laboratory. This fact, once, emphasizes the importance of the discussion about choosing the mineral-paleodosimeter (quartz versus feldspar) for regional luminescence dating.
The optically stimulated luminescence (OSL) method is expected to be more suitable for dating glacial sediments, mainly due to the significantly shorter time (several seconds) of daylight exposure needed for complete bleaching the sample. In 2008 we made a first attempt to apply this technique for dating deposits of glacial origin from the Chagan section. A number of samples were taken from the glacio-fluvial and glacio-lacustrine layers. It was discovered that quartz in these samples showed a strong response to IR stimulation, thought to be resulting from feldspar present as inclusions, and therefore feldspar had to be used for analysis although it is only present in very low quantities.
Initial tests on the IRSL signal obtained at 50°C stimulation temperature demonstrated relatively high De values (>300 Gy) and fading rates of up to 7%. This combination of a large natural De and high fading rates led to problems when applying a fading correction, as the corrected De value falls on the “flatter” part of the dose-response curve near/in saturation. For age estimation of the samples, the post-IR IRSL dating procedure (Thomsen
Applying this technique 3 absolute dates were obtained in Institute for Geology and Palaeontology, University of Innsbruck, Austria (Fig. 7). The Middle Pleistocene age (236 ± 52 ka (CHM1)) of the glacio-fluvial sandy layer in the upper part of the section at 2250 m a.s.l. was obtained. It is older age than all previously published dates for the upper part of the section. Geochemical analysis reveals a significant difference in content of petrogenic oxides between this layer and other glacial stratums, suggesting a possible change the source area and pattern of sedimentation in the upper part of the section. A glacio-fluvial sandy layer at a depth of about 60 m has a Middle Pleistocene age or older (>360 ka (CHM3), >220 ka (CHM8)).
The numerical age estimation for deposits of the Chagan section is important for the regional stratigraphy and region correlation schemes for the Russian Altai and Siberia. At the moment, there are 4 radiocarbon ages (three previously published), 28 TL (8 previously published) ages, and 3 IRSL ages for the upper part of the section, which includes glacial sediments of various types; and 2 previously published TL dates for underlying alluvial deposits Sheinkman (2002) reported applying nanocycle (varvocycle) technique (Afanas’ev, 1990) for testing TL dates from glacial deposits of the Chagan section. At the same time this technique was suggested for lacustrine sediments of platform areas (Afanas’ev, 1990). Correlation of the lacustrine sedimentation patterns with the Earth’s gravitational field (defined by position of the planets in the Solar system) forms the basis of this method. It is clear that in high mountain areas the glacio-lacustrine sedimentation patterns are mainly controlled by glacier runoff, proximity of the slopes and glacier itself to the lake, lake size, bottom inclination and so on. Thus we suppose that utilizing this technique for mountain areas is highly debatable, it could not be used for independent age control, and we do not analyse the nanocycle dates in this paper
Sheinkman (2002) reported applying nanocycle (varvocycle) technique (Afanas’ev, 1990) for testing TL dates from glacial deposits of the Chagan section. At the same time this technique was suggested for lacustrine sediments of platform areas (Afanas’ev, 1990). Correlation of the lacustrine sedimentation patterns with the Earth’s gravitational field (defined by position of the planets in the Solar system) forms the basis of this method. It is clear that in high mountain areas the glacio-lacustrine sedimentation patterns are mainly controlled by glacier runoff, proximity of the slopes and glacier itself to the lake, lake size, bottom inclination and so on. Thus we suppose that utilizing this technique for mountain areas is highly debatable, it could not be used for independent age control, and we do not analyse the nanocycle dates in this paper
Taking into account the physical basis of the applied luminescence techniques, OSL method is expected to be more reliable in comparison with TL. It has the advantage shorter times needed for complete bleaching of the luminescence signal, which is particularly important when dating glacial and associated sediments; it takes just a few seconds of direct sunlight exposure for the more light sensitive OSL and IRSL signals against several tens of hours for less light sensitive TL signal.
Analysis of all the available luminescence dates revealed the best correlation of results in the lens of glacio-lacustrine deposits and allows to assume its Middle Pleistocene age, that, however, requires further verification.
We also argue that it is inappropriate to correlate aleuropelite layer from glacio-lacustrine deposits in the Tedesh section (TL dates 121 ± 14 and 135 ± 15 ka were reported for these sediments in (Sheinkman, 2002)) with glacio-lacustrine lens in the Chagan section, and therefore, to utilize these TL dates for absolute age estimating glacio-lacustrine deposits in the Chagan section as it was done in (Sheinkman, 2002; Zol’nikov and Mistrjukov, 2008 and Rusanov, 2011). Such geochronological correlations are not valid, regardless of the applied dating technique. Tedesh and Chagan sections are located in 3 km from each other and are separated by the transverse tectonic riegel crossed the Chagan valley. Slope in this part of the valley is covered by talus fans with no clear transition between these sections and glacio-lacustrine sediments of the lens wedge out at the surface of the tectonic riegel.
Good correlation of TL dates for the glacio-lacustrine lens supports the thesis that glacio-lacustrine sediments are one of the most perspective ones for luminescence dating among all glacial and associated deposits. It is likely that the sedimentation patterns in the glacial lake increased the extent of bleaching when compared to the sedimentation during subglacial and englacial processes. The possible Middle Pleistocene age of this lens also does not contradict to the results of our geomorphological investigations, which argue for presence of deposits of several glacial epochs in the Chagan section. We suppose that following researches should pay special attention to this lens in the Chagan section while making further study and testing various numerical techniques for dating glacial and associated sediments.
Generally, the large scattering of obtained TL ages and their low correlation with the stratigraphic position in the section allow us to conclude that TL method does not fit for dating glacial sediments and landforms, and cannot be used for stratigraphic breakdown, as well as for making any geochronological reconstructions and correlations.
The OSL technique is expected to be more suitable for dating glacial sediments. Narama
At the same time glacial sediments within the Russian Altai are very problematic for luminescence dating. Applying OSL dating technique for fluvial and aeolian deposits Lehmkuhl
The small number of IRSL dates obtained does not allow us to assess the appropriateness of using this method for age estimation of glacial and associated sediments, which remain one of the most difficult types of continental deposits for dating. It also does not allow clearly determine the ages of glacial deposits in the Chagan section. We can only assume a relatively low number of depositional cycles for the grains in this section and a short transport which is also problematic for luminescence dating techniques.
Another important problem is the selection of the mineral-dosimeter and associated task of choosing the best dating technique. Among two the most commonly used minerals, quartz and feldspar, the luminescence signal from feldspar is an order of magnitude greater than for quartz (Aitken, 1998). Feldspar, however, is less rapidly bleached than quartz, sometimes by a factor of about 10 (Godfrey-Smith
Our study revealed that strong low temperature peak in the TL signal is a regional feature of quartz from the Chagan section. This led to failure in determining a number of TL ages. It was also discovered that regional quartz showed a strong response to IR stimulation, thought to be resulting from feldspar inclusions. In general, the mineral material from the Chagan section is not ideal for luminescence dating. This is probably due to a combination of a short transportation distance and a low number of depositional cycles the grains have undergone since they were eroded from bedrock.
Further studies of regional minerals-dosimeters should be undertaken in order to sort out the most effective luminescence dating techniques of glacial and associated sediments in the SE Altai. The small number of IRSL dates obtained does not allow us to fully evaluate appropriateness of this technique for absolute age estimation of glacial sediments in the Chagan section.
Radiocarbon dates of carbonate concretions from the bottom and the middle part of the glacial complex in the Chagan section generally correlate to each other: 22946 ± 482 cal. BP (SOAN 3115), 22264 ± 657 cal. BP (SOAN 3116), 24857 ± 470 cal. BP (SOAN 3117) (Rusanov and Orlova, 2013), and 30189 ± 548 cal. BP (SOAN 8548) (this study). It should be mentioned that this type of material is not easy and comfortable for absolute dating. Some difficulties of applying radiocarbon method for carbonate concretions dating have been previously discussed (Aitken, 1990 and Bowman, 1995). Problems of radiocarbon age measurements for lacustrine carbonate samples from the Pleistocene Lake Lahontan, Nevada, USA are presented by Lin and coauthors (Lin
Radiocarbon dates of carbonate concretions argue for the relatively young age of their formation. An inverse stratigraphic sample position in the section (the older samples are located above the younger ones) could be explained by intensive Chagan river incision into the previously accumulated glacial deposits at the end of the Late Pleistocene during the interglacial time (Butvilovsky, 1993).
Heavily damaged section of the upper tooth (
Thus, it could be stated that number of glaciations associated with the glacial sediments of the Chagan section (Borisov, 1984 and Quaternary System, 2008) is overestimated and their chronology is not currently proved. A similar conclusion was also made by Zol’nikov and Mistryukov (2008) and Rusanov (2011). Deposits, previously interpreted as Akkem moraines (the end of the Late Pleistocene) (Borisov, 1984), are actually landslide which is composed of alluvium sediments of the Bashkaus suit. Only on top it is clothed by a thin moraine cover. Generally by now the Chagan section could hardly serve as a reference section for the Altai stratigraphy. Further geochronological researches based on modern absolute dating techniques are needed to make stratigraphic breakdown and correlations.
Our detailed geomorphological investigations argue for presence at least two major Pleistocene glaciations in a region. These results support a scheme of glaciation history within the SE Altai suggested by Devyatkin (1965) and Okishev (1982). Analysis of all available numerical age estimations of glacial and associated deposits for the reference Chagan section, SE Altai, indicates that glacial environment is one of the most complex for absolute dating.
The presented luminescence ages, obtained with both naturally and artificially saturating TL techniques showed the best data correlation in the lens of glacio-lacustrine sediments at the bottom of glacial part of the Chagan section and suggested its possible Middle Pleistocene age. The age of basal moraine as well as the time of glacial sedimentation above the lens is not reliably established. Numerous TL dates revealed a large spread in the obtained ages measured in different laboratories on the basis of various techniques. The dates sometimes poorly agree with the stratigraphic position of the sample in the section. The obtained results demonstrate that TL method is not suitable for dating glacial sediments and available TL ages cannot be treated as geochronological markers. It should only cautiously be used for making any reconstructions and correlations.
Three IRSL dates indicate Middle Pleistocene age in the upper part of the section. The small number of obtained IRSL dates cannot form the basis for geochronological reconstructions of the Pleistocene glaciations in the SE Altai, but gives the possibility for further experiments with different variation of OSL (IRSL) techniques.
The available radiocarbon dates of carbonate concretions from this section are significantly younger and indicate the age inversion of their formation. Generally, they fall into interglacial period before Sartan glaciation and indicate the probable Chagan river incision into the ancient glacial deposits which were accumulated long before.
Analysis of the physical properties of minerals-dosimeters reveals specific regional quartz features such as strong low temperature peak in TL signal which often makes it impossible to measure luminescence at chronometrical peak near 300°C. Strong response to IR stimulation is provided by feldspar inclusions. Relatively low number of depositional cycles for the mineral grains in this section and a short transport distance was established. Thus, further experiments on selecting the appropriate mineral and luminescence dating technique are still relevant.
It could be stated that by now the age of Pleistocene glacial landforms and sediments within the Russian Altai remains a mainly relative determination and such age markers as “Late” or “Middle Pleistocene” mean mainly relative terms “older” — “younger”. The large spread in ages of deposits in reference sections within the Russian Altai and the weak provision of some of them with absolute dates do not allow the correlation of glaciations between different oroclimatical zones of Russian Altai, or further comparison with the other mountain systems of Central Asia and Siberia. The adopted scheme of correlation between Altai and Siberia glaciations, based on the first TL dates of the Chagan section, needs to be improved.
Obtained absolute dates.Applied techniques: RTL – radiotermoluminescence; IRSL – Infra Red stimulated luminescence; 14C – scintillation radiocarbon analysis (both calibrated (2σ) and radiocarbon (in brackets) ages are given).
|N||Sample||Lab. code||Technique||Age (ka)|
|MSU NS-4||RTL||106 ± 27|
|2||2001/05||BuGIN 498||RTL||25 ± 4|
|MSU NS-5||RTL||127 ± 32|
|3||2001/06||MSU NS-6||RTL||260 ± 65|
|4||2001/07||BuGIN 503||RTL||140 ± 17|
|MSU NS-7||RTL||336 ± 84|
|5||2001/10||BuGIN 501||RTL||124 ± 15|
|6||2001/13||MSU NS-13||RTL||251 ± 63|
|7||2001/18||MSU NS-18||RTL||151 ± 38|
|BuGIN 506||RTL||190 ± 23|
|8||2001/19||BuGIN 513||RTL||41 ± 6|
|MSU NS-19||RTL||290 ± 73|
|9||2003/02||BuGIN 664||RTL||380 ± 51|
|10||2003/03||BuGIN 665||RTL||310 ± 50|
|11||2003/05||MSU NS-305||RTL||480 ± 20|
|12||2003/08||BuGIN 669||RTL||253 ± 26|
|13||2003/10||BuGIN 670||RTL||340 ± 46|
|MSU NS-310||RTL||505 ± 20|
|14||2003/11||BuGIN 671||RTL||330 ± 57|
|15||CHM1||IRSL||236 ± 52|
|18||SOAN 8548||14C||30.189 ± 0.548|
|(25.470 ± 0.195)|