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Seismites in Quaternary sediments of southeastern Altai
Russian Geology and Geophysics 50 (2009) 546–561 www.elsevier.com/locate/rgg
Seismites in Quaternary sediments of southeastern Altai E.V. Deev a,*, I.D. Zolnikov b, S.A. Gus’kov a a
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia b Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia Received 28 March 2008; accepted 24 October 2008
Abstract We study earthquake-induced soft-sediment deformation (seismites) in reference Quaternary sections of southeastern Altai. Sediments in the sections bear signature of liquefaction and fluidization and deformation is localized in thin (few centimeters to 0.5–1.0 m) continuously striking and frequently repeated layers sandwiched between undeformed sediments. The soft-sediment deformation records coseismic motion of different slip geometries. Seismic origin is also inferred for layers and lenses of coarse colluvium slid into the lake bottom from the slopes, which intrude plane-bedded silt and sand and vary in thickness from a few centimeters to one meter. The occurrence of seismic soft-sediment deformation at different stratigraphic levels of the Quaternary and in the Upper Pliocene Beken Formation confirms the high seismicity of southeastern Altai in Quaternary time. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: seismite; Quaternary deposits; seismicity; southeastern Altai
Introduction Large earthquakes in active seismic areas leave their traces in landforms as various gravity slides, fault scarps, or long ditches, which may be indicators of slip geometry, magnitude, and location of the causative earthquakes (Keefer, 2002; Khilko et al., 1985; Malamud et al., 2004). However, this macro-scale signature commonly becomes erased by denudation in a few thousand years and is thus of no much use in paleoseismology. Evidence of older seismic events preserves as deformation structures in soft sediments called seismites (Seilacher, 1969). Seismites are known in continental, tidal, and marine facies (Enzel et al., 2000; Hempton and Dewey, 1983; González et al., 2005; Obermeier, 1998; Plaziat et al., 1990; Povolotskaya et al., 2006). Deformation structures have been best documented in Cenozoic sediments, especially in Pleistocene– Holocene sections, but structures of this kind were reported also from older strata (Jewell and Ettensohn, 2004; Mazumder et al., 2006; Netoff, 2002). Investigation into surface rupture of various types caused by the Chuya earthquake of 2003 (southeastern Altai), including structures associated with ground liquefaction and fluidi-
* Corresponding author. E-mail address: [email protected] (E.V. Deev)
zation, proved valid the existence of landforms produced by prehistoric earthquakes and, on the other hand, prompted the search for seismites in different Cenozoic sedimentary facies in the area. After the first report of seismites we discovered (Deev et al., 2005), we found many other deformation structures (Fig. 1) at different stratigraphic levels in Quaternary sediments. Therefore, the primary objective of this study was to constrain the stratigraphy of the seismites as a prerequisite for timing the paleoseismicity events. Other objectives concerned with the morphology of seismites and their formation mechanisms, as well as criteria to distinguish them from nonseismic deformation structures.
Cenozoic tectonics and seismicity of southeastern Altai The modern geodynamic models since the study by Molnar and Tapponnier (1975) commonly attribute the onset of Cenozoic tectonic deformation in Central Asia to the India– Eurasia collision which gave rise to several intracontinental orogens, such as the Altai orogen that initiated in the latest Paleogene–earliest Neogene (Dobretsov et al., 1995). Mountain growth increased especially in the Late Pliocene and produced the neotectonic framework of southeastern Altai with elevations over 4000 m and the uplift reaching 1000– 2000 m at basin-range junctions and a few tens to hundreds of meters at boundaries of smaller blocks (Novikov, 2004;
1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2008.10.004
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Fig. 1. Location map of studied Quaternary sections (1–9). 1, Kyzyl-Chin (50°03′19.2′′ N, 88°17′03.9′′ E, depth (H) = 1845 m); 2, Chagan-Uzun (50°03′39.0′′ N, 88°20′55.5′′ E, H = 1767 m); 3, Chagan (49°56′48.1′′ N, 88°06′32.5′′ E, H = 1992 m); 4, Menk (50°16′47.4′′ N, 87°40′13.5′′ E, H = 1428 m); 5, Ust’-Chuya (50°23′58.6′′ N, 86°40′18.7′′ E, H = 768 m); 6, Malya Inya (50°26′53.6′′ N, 86°40′23.3′′ E, H = 778 m); 7, Yaloman (50°29′51.1′′ N, 86°35′28.3′′ E, H = 755 m); 8, Malyi Yaloman (50°28′55.0′′ N, 86°34′35.8′′ E, H = 745 m); 9, Katun’ (50°30′23.6′′ N, 86°34′08.2′′ E, H = 735 m).
Rakovets, 1967). The activity shows up mainly as strike-slip faults in environments of compression, extension, and rotation (Buslov et al., 1999; Lukina, 1996; Thomas et al., 2002) and, besides newly formed faults, involves rejuvenated segments of Late Paleozoic and Mesozoic large faults (Sarasa–Kurai, Shapshal, Kuznetsk–Altai, Chulyshman, etc.). Quaternary fault motions are recorded in tilted and offset river and lake terraces and fans, in moraine and glaciofluvial complexes, in deformed features of glacial erosion, as well as in facies change and deformation of sediments in intermontane basins (Chuya, Kurai, etc.) (Buslov et al., 1999; Devyatkin, 1965). Current deformation is recognized by GPS measurements and repeated leveling surveys (Kolmogorova and Kolmogorov, 2002; Timofeev et al., 2006). Ongoing seismic activity in the area is evident in tens of M ≤ 4.5 (K ≥ 12) instrumental events yearly (Emanov et al., 2005; Zhalkovskii et al., 1995), while signature of high paleoseismicity is found in numerous fault scarps possibly caused by earthquakes of magnitudes up to M = 7.5 and shaking intensities up to IX (Butvilovskii, 1993; Devyatkin, 1965; Rogozhin et al., 1999). Altai may experience seismicity as high as in the nearby areas of Mongolian Altai (Khilko et al., 1985) because the Mongolian seismogenic faults extend into southeastern Altai. The expected high seismic potential was confirmed by the Altai (Chuya) event of September 2003 (M = 7.3 and intensity VIII–IX) followed by a long aftershock process (Emanov and Leskova, 2005).
Quaternary sedimentary complexes The historic background of the stratigraphy and paleogeography of southeastern Altai was discussed in detail in (Novikov, 2004). We reconstruct the chronology of main Quaternary events in the area proceeding from stratigraphic, facies, and geochronological published evidence (Devyatkin, 1965; Efimtsev, 1964; Parnachev, 1999; Sheinkman, 2002),
and our own data (Zolnikov and Mistryukov, 2008). Quaternary strata lie upon lacustrine, fluvial, and overbank deposits of the Beken Formation, which has been commonly assigned a Late Pliocene age. The Quaternary section proper begins with brown boulders, pebbles, and sands of the Pliocene (?)– Lower Quaternary Bashkaus Formation. This is generally orogenic molasse deposited in large river valleys (fluvial facies) and in intermontane basins (fluvial-colluvial-overbank facies). First Quaternary glaciations in the Altai area were hypothesized to date back to the Early Quaternary mountain growth but, if any, they left no continuous traces on the surface (Svitoch et al., 1978). The largest extent of moraines (maximum glaciation after Devyatkin (1965)) and damlake silt has been attributed to the Riss Glacial epoch equated with the Taz Glacial of the West Siberian regional stratigraphy (WSRS). The correlation with WSRS and δ18O stratigraphy or MIS (Imbrie et al., 1984) is according to (Arkhipov et al., 1997). The glacial unit corresponding to the maximum glaciation (unit I in Fig. 2) correlates with MIS 6. At that time glaciers dammed river valleys and produced damlakes that occupied the territory of the Chuya, Kurai, Uimon, and other basins. Failures of the dams caused great floods that reached hundreds of meters in depth and locally spilled over the mountain divides between the Chuya and Katun’ valleys (Butvilovskii, 1993; Rudoi, 2005). The moraine and limnic facies of the unit make up the 300–350 m thick Inya Formation (Efimtsev, 1964) of rhythmic overbank deposits, up to seven cycles in the most complete sections. In the Chagan reference section, they are alternating till and intermediate beds, with controversial TL age constraints (Borisov and Chernysheva, 1987; Sheinkman, 2002; Svitoch et al., 1978). Data by (Sheinkman, 2002) indicating that the oldest age of the Chagan moraines (Fig. 1) falls within MIS 6 appear most consistent. No less realistic is the commonly assumed range of the Bakhta superstage which encompasses the Taz (MIS 6) and Samara (MIS 8) stages of WSRS. According to (Sheinkman, 2002),
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Fig. 2. Correlation of earthquake-induced deformation structures in Quaternary sediments of southeastern Altai. 1–6, lithofacies that host seismites: silt and sand of glacial damlakes and secondary damlakes (1), mostly alluvial silt (2), sand (3), gravel (4), pebble (5), and boulders (6); 7–10, seismites: various shapes produced by liquefaction and fluidization of sediments (7), coarse colluvium layers and lenses (8), flexures and their systems and series of small joints, often with displacement of relatively plastic sediments (9), joints of different scales (10). Repetitive structures 7–10 indicate presence of several deformation units within sections. Section numbers are according to Fig. 1.
glacial deformation and floods might also partly represent cold substages of MIS 5. Therefore, the stratigraphic constraints of maximum glaciation in sections of Gorny Altai, and the related Inya sequence, should be approached with caution until better representative and geographically larger data become available. The Inya Formation is cut by high erosive first terraces of the Chuya and Katun’, in which fan alluvium rarely reaches thicknesses of 3–5 m or is often absent; the age of the alluvium corresponds to the Kazantsevo stage of WSRS (MIS 5e). The complex of high terraces encloses the 60–100 m thick Saldzhar Formation (Efimtsev, 1964) which we correlate with glacial and limnic-glacial deposits of the first post-maximum glaciation of Gorny Altai (glacial unit II). The unit may correspond to the Early Zyryanian or Ermakovian stage of WSRS (MIS 4, or possibly MIS 5b, d) and contains at least three cycles of overbank deposits. The Saldzhar Formation, in turn, makes up the base of middle terraces. Some terraces are covered with 3–5 m thick alluvium deposited during the incision of the rivers. Its age corresponding to the Karginian stage of WSRS (MIS 3) is consistent with data on Paleolithic sites (Derevy-
anko and Markin, 1987; Postnov et al., 2007). The small thickness of the alluvium is due to the fact that major rivers and their tributaries incised into deposits of glacial floods in post-Saldzhar time. The process was rather long and incision often reached the bedrock base of sediments. The second post-maximum glaciation (glacial unit III) corresponds to the Sartan glacial stage of WSRS in the glacial zone and to the subaerial unit on terrace surfaces in the periglacial zone. The subaerial sediments are loess, eolian sand derived from Karginian alluvium, and overbank debris that were deposited in a dry cold climate. The Holocene section includes the upper part of subaerial sediments, alluvium of lower terraces, and floodplain deposits.
Deformation structures in reference Quaternary sections of southeastern Altai Below we discuss the stratigraphy of deformation structures in reference Quaternary sections. The sections have been largely documented and described in detail, with facies
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analysis and age constraints, in a separate book (Zolnikov and Mistryukov, 2008). Therefore, in this study we confine ourselves to a brief summary in the text and in the chart of Fig. 2. The sections belong to two Quaternary paleogeographic zones: (i) glacial zone which was covered with glaciers (up the Chuya valley, from the Belgibash River mouth to the Chuya basin) and (ii) periglacial zone which was out of the reach of the glaciers but experienced a strong impact from related floods (from Belgibash River mouth and downstream along the Chuya and Katun’ valleys, as far as the plainland). Kyzyl-Chin section (Upper Pliocene–Quaternary), left side of the Kyzyl-Chin River, 1.5 km upstream of its exit from a gorge. The section consists of sand and silt of the Beken Formation (to 15 m thick) at the base, pebbles of the Bashkaus Formation (8–10 m) in the middle, and a gray diamictite bed (10–11 m) on the top (Devyatkin, 1965). We examined a 9.5 m high and 12 m long exposed fragment of the Beken Formation (Fig. 3, A) composed of light brown to orange plane-bedded silt and sand with five layers and lenses (0.3 to 1.5 m thick) of angular coarse clastic material. Silt is thinly laminated and interlayered with 2 to 5 cm thick sand. Silt and sand are obviously lake sediments while the coarse material is petrographically uniform colluvium (clasts of brown Devonian tuff) shed into the lake from the valley sides. The mottled-
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banded structure of the coarse layers and lenses, and their rugged boundaries, result from intrusion of colluvium into lake sediments (Fig. 3, B, C). The colluvium lenses grade on strike into beads of “floating” angular clasts, and the silt-sand layers are undulate and contorted as small disharmonic folds. Thin (a few centimeters) contorted silt-sand beds were found also outside the colluvium occurrence (Fig. 3, D). Chagan-Uzun section, 5 km upstream of the Chagan-Uzun inlet into the Chuya, consists of pale yellow thin-bedded silt and sand deposits of a glacial lake of the maximum glaciation. We studied an outcrop fragment, about 7 m high and 8 m long (Fig. 4, A), where 3–4 m thick lake sediments are overlain by diamictites (1.5–2 m of visible exposed thickness) with striated and faceted boulders as well as rounded erratic cobbles and pebbles. The diamictite bed has its base undulated in this outcrop and deformed in laterally proximal sites. It appears to belong to a moraine of unit I of post-maximum glaciation. Plane-bedded silt within the limnic sequence includes four large (up to 1 m thick) and several smaller layers of coarse angular material of local provenance (see one such layer in Fig. 4, B), which strongly vary in thickness and often pinch out off the valley sides. The coarse layers apparently formed in a submerged environment, judging by their structures of colluvium-silt mingling (Fig. 4, C). Furthermore, there are
Fig. 3. Fragment of Kyzyl-Chin section (A). 1, plane-bedded damlake silt and sand; 2, coarse colluvium, 3, bedding (here and below). B–D are explained in text.
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Fig. 4. Fragment of Chagan-Uzun section (A). 1, plane-bedded damlake silty sand; 2, coarse colluvium, 3, till. B–H are explained in text.
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traces of mudflow injection into laminated sediments, which upset and crumpled the sediment layers (Fig. 4, D). Like in the Kyzyl-Chin section, there are small-scale deformation features on the extension of the coarse layers. We discovered several cases of a peculiar relationship between deformation structures and coarse material, when a clastic layer, or isolated floating clasts, lie on a deformed layer that has a well-defined base (Fig. 4, E). The same section contains systems of meeting cracks in silt and sand (Fig. 4, F) and simple, disharmonic (Fig. 4, G), fan-shaped, or isoclinal (Fig. 4, E) folds that show no relation to coarse material. Note that the deformed layers, from a few centimeters to tens of centimeters thick, are sandwiched between undeformed lake sediments. Deformation in limnic varves of the first maximum glaciation was found also in the Chagan stratotype section (Svitoch et al., 1978) as very small volcanic craters (a few centimeters) clearly seen in jointing planes (Fig. 4, H). Ust’-Chuya section, right side of the Katun’ River, 600 m downstream of its confluence with the Chuya. This is the first section of the periglacial zone, where no glaciers existed in Quaternary time but repeated glacial floods exerted major topographic and lithofacies control. The section begins with the Inya exposed cyclic overbank deposits discordantly overlain by the Saldzhar Formation consisting of three members (Fig. 5, A). Proceeding from our previous experience, we turned special attention to thin-bedded silt and sand deposits of secondary damlakes on the top of the glacial flood cycle. According to (Parnachev, 1999), secondary damlakes arise after great floods in tributaries dammed by overbank deposits or in subsided valley fragments where denudation and previous flood deposition have made the floor rugged. The Ust’-Chuya section includes two secondary damlake beds: one on the top of the Inya Formation and the other in the middle of the Saldzhar Formation. These beds, as well as a sand-gravel layer under the Inya silt and sand, host flame- (Fig. 5, B–E) and cloud-shaped (Fig. 5, F) deformation structures. The former are intrusions of coarse material into finer-grained sediments and the latter are, vice versa, finer sediments in a coarser matrix. The deformed sediments are no more than 10 cm thick. The upper silt-sand layer is cut by cracks with their dip and rake angles of 90°/45° and 300°/55°, respectively (see Fig. 5, G for one such crack). The cracks go through about 15 cm of rocks, undeformed sediments between the cracks are 3 to 15 cm thick, and the offset reaches 5–7 cm. Katun’ section, left-side meander of the Katun’ opposite Malyi Yaloman Village. The section exposes a long lens of plane-bedded silt and sand of a secondary damlake produced by the final flood cycle of Saldzhar time (Fig. 6, A). Limnic sediments are 6.6 m thick and contain 10–15 cm thick layers with deformation structures at different levels. The structures are pillow-shaped (Fig. 6, B), disharmonic (Fig. 6, D), cyclic (Fig. 6, E), or fan-shaped folds, often with necked cores (Fig. 6, F), or look like mushrooms (Fig. 6, G) or small volcanic craters (Fig. 6, H).
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Malaya Inya section, left side of the Inya River, 3.4 km far from its inlet into the Katun’. This is a parastratotype section of the Saldzhar Formation exposed in the sides of a gully running across the Inya valley. The formation comprises three members with a complete set of lithofacies typical of cyclic overbank deposits. Each member has interbedded silt and sand of a secondary damlake at its top. The Saldzhar deposits are incised with Karginian alluvium (2 m of visible thickness), which is overlain, in turn, with a 14 m thick apron of Sartan flood deposits cut by the Holocene channel and flood plain of the Inya. See Fig. 7, A for a fragment of an outcrop at the intersection of the Inya and the gully. The exposed section consists of cross-bedded channel pebbles (up to 1.5 m of visible thickness), overbank silt and sand with a hydromorphic fossil soil (up to 0.5 m thick), and layered overbank debris reaching 2.4 m of visible thickness produced by redeposition of the Saldzhar debris. The deformed interval, about 1 m thick, encompasses the upper part of fluvial pebble and overbank silt. Deformation is especially prominent near the interface between sediments of different grain sizes and appears as sharp cyclic convolutions (Fig. 7, B) and isolated lenses (Fig. 7, C) on both sides of the interface. The deformation structures are traceable discontinuously for several tens of meters along the Inya. As for the Saldzhar Formation itself, we found numerous features of offset bedding in an outcrop in the right side of the Inya (50°27′07.1′′ N, 86°39′13.9′′ E, H = 775 m) in lacustrine silt and sand at the top of the lower overbank bed. The deformation structures are of flame (Fig. 7, D) or pillow (Fig. 7, E) shapes and of various sizes. Different section parts include from two to four deformed layers (Fig. 7, D, F, G), all at the transition between coarse sediment above and fine sediment below. Malyi Yaloman section, left side of the Malyi Yaloman River, 3.5 km upstream of its mouth. The section consists of plane-bedded Saldzhar debris, up to 3.5 m of visible thickness, plane-bedded silt and sand of the Saldzhar last secondary damlake (up to 1.5 m), Karginian cross-bedded boulder and pebble beds of alluvium that give way upsection to plane-bedded overbank sand and gravel (up to 2 m), and 10–12 m of Sartanian (MIS 2) plane-bedded overbank debris with sand interlayers. The post-Saldzhar section at the Malyi Yaloman site is obviously similar to that at the Malaya Inya site but deformation structures in it are different and appear in a more or less prominent way over a distance about 100 m. Figure 8, A shows a fragment of the outcrop with jointed debris, silt, sand, and the lower bed of cobble and pebble. The deformation structures are sealed from above by 0.2–1.0 m thick channel deposits. In addition to brittle failure of different magnitudes and slip geometries (Fig. 8, A, B, D), silt and sand bear small flexures (Fig. 8, E) and flame-shaped structures older than the joints (Fig. 8, C). The same section contains a lens of mudflow intruded into silt and sand (Fig. 8, F). The relatively small thickness (within 40 cm) of silt and sand below the lens indicates that the mudflow event occurred at the initial stage of the secondary damlake. The mudflow lens is overlain by lake sediments with
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Fig. 5. Ust’-Chuya section (A). 1, boulders; 2, pebbles; 3, debris; 4, gravel; 5, sand; 6, silt; 7, clay; 8, secondary damlake silt and sand. B–G are explained in text.
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Fig. 6. Katun’ section (A). Legend same as in Fig. 5. B–G are explained in text.
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Fig. 7. Fragment of Malaya Inya section (A). Legend same as in Fig. 5. B–G are explained in text.
wavy structures at the depth of the lens top produced by roiling of bottom sediments. Together with the overlying silt and sand, it is partly truncated by a bed of cross-bedded cobbles and pebbles. Thus, the mudflow lens may have formed by gravity sliding of colluvium into the lake.
Yaloman section, a quarry in the left side of the Katun’, 1 km southeast of the Malyi Yaloman mouth. The quarry opens the surface of middle terraces not far from their back suture and exposes lens-shaped cross-bedded pebbles with gravel and sand (Fig. 9, A), most likely of a post-Saldzhar
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Fig. 8. Fragment of Malyi Yaloman section (A). 1, joints. Other symbols as in Fig. 5. B–F are explained in text.
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Fig. 9. Yaloman section (A). Legend same as in Fig. 5. B–D are explained in text.
(i.e., Karginian) age. According to their stacking patterns, the deposits appear to be lateral accretion complexes. A sandgravel lens in a terrace surface bears signature of rupture and displacement (Fig. 9, B), and involves a system of minor flexures (Fig. 9, C). As deformation increases, the primary bedding of sediments becomes destroyed (Fig. 9, D). Note that the deformed lens is sandwiched between undeformed sediments like in the previous limnic sections. Menk section, an abandoned quarry of the Menk hydro power plant. The quarry exposes post-Saldzhar alluvium consisting of cross-bedded pebbles (1.5 m visible thickness) below 15.5 m of lenticular-cross-bedded sand with gravel and pebble overlain by solifluction loamy sand with pebble and gravel (1.5 m). This may be one of best representative post-Saldzhar sections in which there are seven deformation units within the 3.6–6.6 m interval above the sand-gravel base (Fig. 10, A). The structures are of flame (Fig. 10, C) or spherical (Fig. 10, E) shapes that result from intrusion of coarser sediments into the underlying finer sediments, or structures of more intense sediment mingling (Fig. 10, F). The deformation structures were apparently produced by several discrete events, as one may infer from their location in different lenses (Fig. 10, B) and from the position of deformed layers between undeformed ones. Furthermore, the upper deformed units in pairs of two closely spaced layers often have their base well-defined and weakly deformed (Fig. 10, C, E) and the top truncated by overlying sediments (Fig. 10, D).
Discussion Origin of deformation structures. Identifying the origin of various soft-sediment deformation structures remains problematic and is thus worth special attention. Deformation structures (including morphologically similar ones) in Quaternary sediments of southeastern Altai may be due to glacial, permafrost, solifluction, gravity, depositional, or seismic causes. Deformation associated with static or dynamic loading from glaciers is logically impossible in the periglacial zone of the area. Glacier-induced deformation of the Beken pre-Quaternary deposits in the Kyzyl-Chin section is inconsistent with available paleoclimate data. Deformation structures in the Chagan-Uzun section would appear to result from glacier loading upon the underlying wet sediments (more so that there are varves under a moraine) were there no colluvium slid into an ice-free lake. Deformation structures in the lacustrine varves that bear no traces of subaerial gaps cannot arise by cryogenic or solifluction processes which normally act in subaerial environments. Ripple structures produced by currents have quite a different morphology. Cryogenic origin may be hypothesized for convolutions reaching 0.5 m in fluvial pebbles, gravel, and sand, for instance, in the Chuya basin, in areas of karst topography formed by heave degradation. Contorted structures may be due to solifluction associated with thawed injection lenses of
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Fig. 10. Fragment of Menk section (A). 1, silty sand; 2, solifluction loam sand with pebble and gravel. Other symbols as in Fig. 5. B–F are explained in text.
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ground ice as well as with segregation ice schlieren, secondary ground ice, or ice cement. The only possible ice-related formation mechanism for the folds and joints in alluvial pebbles, gravel, and sand found in different outcrops but at the same stratigraphic level (base of overbank sediments deposited during a cold dry spell) in the Malaya Inya and Malyi Yaloman sections is thawing of secondary ground ice. However, we never saw traces of this ground ice, which must look like wedges with particular upturned margins, in the sections we studied. Thus, neither cryogenesis nor aseismic gravity sliding appear to be likely candidates responsible for the deformation. Structures similar to those we observed may arise also by a submerged gravity flow, namely, debris or mud flows into the lake bottom at the final stage of ice damlakes in glacial zones or at the waning stage of large glacial floods in periglacial zones (Zolnikov and Mistryukov, 2008). However, unlike the submerged mudflow, turbidity, and debris flow structures grading one into another, in which deformation and sedimentation are syngenetic (Zolnikov and Mistryukov, 2008), deformation in the Gorny Altai Quaternary sections is obviously independent of sedimentation. Furthermore, submerged debris-flow deposits normally display a broad variety of structures (drag folds, microdiapirs, bands with blob-shaped contours), often stretching along the flow. They are always mixed layers, often up to several meters thick, rather than thin clusters of small-scale deformation in lacustrine sediments. Thus, having analyzed the morphology and settings of deformation structures we observed in Quaternary sections, both in separate layers and in layer successions, as well as in outcrops within the same stratigraphic level, we infer that they are of seismic origin. This inference is consistent with their morphological similarity at different stratigraphic levels in different facies and paleogeographic zones, and with spontaneous occurrence of deformed intervals in similar stacking patterns. The restriction of the soft-sediment deformation structures to discrete stratigraphic levels and the presence of several deformed layers between undeformed strata indicate that the events were discrete and recurrent, as are normally the events in active seismic areas where seismic activity alternates with periods of quiescence. The seismic origin of deformation in southeastern Altai is quite reasonable because it was an area of active mountain growth associated with high seismicity in the Quaternary and remains active at present (see above). The described patterns are similar in geometry and scale to historic and fossil seismites reported from other seismic areas (see Introduction). Note that structures like those we report from the Quaternary sections were observed in mud springs produced by soil liquefaction during the Chuya earthquake of 2003 (Fig. 6, B, C). The presence of seismites in Quaternary sediments of southeastern Altai has implications for the origin of the lenses and layers of coarse colluvium in thinly laminated silt and sand. Sliding of colluvium into a lake not necessarily results from an earthquake. However, the coexistence of colluvium enclosed in lacustrine sediments with seismites in the Chagan-
Uzun and Kyzyl-Chin sections provides reliable evidence for the seismic triggers. Especially informative in this respect are the seismites in the Chagan-Uzun section which slightly predate the deposition of colluvium. This means that the formation of seismites and landsliding were triggered simultaneously, most likely by a seismic shock. Seismites may have formed at the point of seismic wave arrival, whereas the landslide reached the lake bottom in a short while (a few minutes later) and fell upon the already deformed sediment. The fact that colluvium exists in lake sediments as lenses and layers indicates its recurrent influx thus providing additional evidence for its linkage with earthquakes. We infer a seismic trigger also for the mudflow we discovered in the Malyi Yaloman section, which appears reasonable especially because silty sand in the section bears signature of liquefaction and rupture. Note that earthquake-induced lenses and layers of coarse clastic material in lakes share much similarity with submerged mudflow mixtites that arise on recession of great floods or on lake level fall, but there are essential points of difference. A seismically triggered landslide is a single event after which normal deposition resumes rapidly. Moreover, the rock textures and structures we report show that clasts mix with bottom sediments only after having fallen into the lake and thus form clearly localized and stratigraphically confined continuous layers of mingling and sediment deformation. Unlike the earthquake-induced landslide deposits, mudflow deposition associated with water level fall is a longer and multiple-event process. The flows intersect, become diluted, and overlap one another thus making more numerous mixed layers with sharp lithological transitions. Another typical feature is that sediments of mudflows from rapidly emerging lake sides are of different grain sizes. Thus, it appears quite reasonable to suggest a seismic origin of the deformation structures and attendant layers and lenses of coarse clastic material in the Quaternary sections we examined. Types, geometry, and formation mechanisms of seismites. The seismites we discovered in Quaternary sections of southeastern Altai are of different types. Most of deformation is associated with liquefaction and fluidization of wet sediments and is confined in thin (a few centimeters to 0.5–1 m), often continuous, layers that lie concordantly upon and below undeformed sediments. Most sections bear several deformed layers, which thus appear to correspond to recurrent events. Deformation structures of same geometries are found in sediments of different grain sizes. For the lack of any universal classification, these structures are usually termed in a way to give the most appropriate idea of their geometry. We distinguish isoclinal (Fig. 4, E), disharmonic (Figs. 3, D, 4, G, and 6, D), fan-shaped (Fig. 6, F), and cyclic folds, spherical, flame-, cloud-, or mushroom-shaped intrusions of one sediment type into another, or micro-volcanoes (Figs. 4, H and 6, H). Some names require additional explanation. Cyclic structures are actually sine-shaped simple folds (Figs. 6, E and 7, B). Spherical (Fig. 10, E) and pillow (Figs. 6, B and 7, E) structures are round or flattened intrusions of coarse material
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separated by columns of fine sediment. Flame-shaped structures correspond to interfingering sediments of different types (Figs. 5, B–E, 7, D, F, 8, C, 10, C); they look like spires and are often attendant with isolated lenses. Cloud-shaped structures result from upward intrusion of finer sediment into coarser material above (Fig. 5, F). Mushroom-shaped structures looking like champignons are cross sections of plastic contorted beds, with coarser sediment from below squeezed into their cores (Fig. 6, G). Typical brittle deformation structures found in the Malyi Yaloman and Chagan-Uzun sections are joints of different slip geometries (Figs. 4, F, 5, B, D). Intermediate between brittle failure and liquefaction-fluidization structures are solitary flexures or their systems, as well as systems of joints in which the displaced sediment is still relatively ductile (Figs. 5, G and 9, C). In studies of parameters and mechanisms of seismically induced strain behavior in granular porous materials (e.g., Allen, 1982; Lowe, 1975; Owen, 1987), passage of waves was hypothesized to cause change in sediment packing and fabric thus decreasing porosity and increasing pore fluid pressure. The lithostatic/hydrostatic pressure difference drives fluidization and liquefaction. The onset of the latter process is associated with plastic deformation (folding and bending). The progress of liquefaction increasingly destroys the primary sediment textures which become fluidal and massive. Fluidization produces flows of silty and sandy water along fractures of which some are feeder channels of sand volcanoes appearing on the surface. Liquefaction and fluidization are favored by fine gain sizes of sediment but impeded by high percentages of cohesive clay. According to our study (Sibiryakov and Deev, 2008), deformation in porous materials subject to seismic shaking depends on the pore fluid phase, being brittle or associated with liquefaction and fluidization if the pores are filled with gas or liquid, respectively. In the latter case, the specific pore surface area, porosity, and pore pressure are the major controls. For instance, the lower the porosity and the higher the specific pore surface area of sediment (i.e., the finer the grain size), the greater the degree of its possible liquefaction and fluidization; the presence of an impervious seam (closed porosity) is another favorable factor. This explains why liquefaction and fluidization structures occur mainly in sediments of silt to gravel grain sizes. Deformation in the pebbles of the Malaya Inya section is most likely due to liquefaction of their sand and silt cement. Fluidization of the cement may be responsible for the craters in the Taldura channel alluvium (Fig. 11, A) that arose during the Chuya earthquake of 2003. Pebble beds may be more competent in deformation relative to finer sediments between them and thus make up small horsts (Fig. 11, B). Pebbles, as well as finer sediments, may be crosscut with sedimentary dikes (Fig. 11, C) which mark seismic surface rupture. Sedimentary systems become still less stable against seismic shaking if the contact sediment layers have different specific weights (reverse density gradient systems): e.g., unstable are the systems in which the sediment above is denser
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than that below. In the Quaternary sections we examined this instability is due mainly to larger grain sizes of higher sediment layers, though package, porosity, and water content may be also important. Additional instability may arise from local slope gradients in cross-bedded sediments. Natural seismites were reproduced in many loading tests. See Fig. 11, D for models of liquefaction and fluidization structures (Moretti et al., 1999) obtained using a digital shaking table which was excited with a shock equivalent to a natural M = 7 earthquake. The model provides an excellent illustration of the mechanisms that produce seismites in systems with an impervious barrier and with a reverse density gradient. Some model structures (Fig. 11, E) are similar in geometry and scale to the natural structures we discovered (Fig. 11, F). We suggest to distinguish the coarse lenses and layers, from a few centimeters to one meter thick, in laminated sand and silt as a special type of seismites produced by gravity sliding of colluvium into lakes. These seismites arose under the water, and the slid material may have reached the lake bottom in submerged mudflows, as one may infer from lake sedimentcolluvium mingling structures, with signature of crumpling in the lake sediments and plastic deformation on extension of the coarse lenses. Age of seismites. The oldest seismites we discovered are from the Late Pliocene–Early Pleistocene Beken Formation (Kyzyl-Chin section). The presence of seismites in damlake silt of the glacial zone (Chagan and Kyzyl-Chin sections) and in sands of secondary damlakes of the Inya Formation (Ust’-Chuya section) confirms high seismic activity of the southeastern Altai during the Last Glacial maximum. The deformation structures in silt and sand at the top of the lower (Malaya Inya section), middle (Ust’-Chuya), and upper (Katun’, Yaloman) members of the Saldzhar Formation record several seismic events during the first post-maximum glaciation (Ermakovian time). Some seismites in the Malyi Yaloman section likewise document seismic activity of the area at the end of the Saldzhar deposition. Seismicity increased also in Karginian time, judging by numerous earthquake-induced deformation structures in alluvium of middle terraces (Menk, Malaya Inya, Malyi Yaloman, and Yaloman sections).
Conclusions There are several reasons to infer the seismic origin of the discussed deformation structures in the Quaternary sections from southeastern Altai. 1. All sections are located in the strongly faulted area of rapid Late Cenozoic mountain growth, where high seismicity is confirmed by instrumental data, historic accounts, and fault scarps produced by large prehistoric earthquakes. 2. Deformed layers in Quaternary sediments have a broad geographic extent and are traceable in outcrops for tens or less often hundreds of meters. 3. Deformation structures are restricted to discrete stratigraphic levels, and the deformed layers lie between unde-
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Fig. 11. A. Craters in channel alluvium of Taldura River produced by Chuya earthquake of 2003. B. Small horst from upper overbank member of Saldzhar Formation in Malaya Inya section. C. Sediment dike from Saldzhar Formation, area of Yaloman section; D. Model of liquefaction and fluidization textures obtained in laboratory tests with a digital shaking table, after (Moretti et al., 1999): 1, 2, fine (1) and silty (2) sand; 3, silt; 4, clay; 5, water table. E. Example loading texture obtained in experiment, after (Moretti et al., 1999). F. Natural equivalent from lower member of Saldzhar Formation in Malaya Inya section; darker color shows relatively coarser sediments.
formed sediments. This position indicates that deformation must result from recurrent events which is typical of seismic areas with alternating periods of activity and quiescence. 4. Deformation structures from different stratigraphic levels, facies, and paleogeographic zones share many features of similarity. 5. Seismites are similar in geometry and scale to structures observed in historic large earthquakes and to fossil seismites, as well as to structures obtained in laboratory tests that simulate seismic loading of wet soft sediments. Formation of abundant seismites in Quaternary strata of southeastern Altai was facilitated by a potentially liquefiable composition of silt-sand and gravel deposits, closed porosity and reverse density gradient systems, and slope gradient in cross-bedded sequences. The stratigraphy of seismites in Late Pliocene (Beken Formation) and Quaternary sediments (Fig. 2) confirms high seismicity of southeastern Altai in the Late Cenozoic.
We greatly appreciate constructive criticism by E.V. Artyushkov, S.I. Sherman, and Yu.K. Sovetov which made for improving the manuscript. The study was carried out as part of Integration Projects 2, 18, 56, and 69 of the Siberian Branch of the Russian Academy of Sciences.
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Editorial responsibility: V.A. Vernikovsky
Seismites in Quaternary sediments of southeastern Altai E.V. Deev a,*, I.D. Zolnikov b, S.A. Gus’kov a a
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia b Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia Received 28 March 2008; accepted 24 October 2008
Abstract We study earthquake-induced soft-sediment deformation (seismites) in reference Quaternary sections of southeastern Altai. Sediments in the sections bear signature of liquefaction and fluidization and deformation is localized in thin (few centimeters to 0.5–1.0 m) continuously striking and frequently repeated layers sandwiched between undeformed sediments. The soft-sediment deformation records coseismic motion of different slip geometries. Seismic origin is also inferred for layers and lenses of coarse colluvium slid into the lake bottom from the slopes, which intrude plane-bedded silt and sand and vary in thickness from a few centimeters to one meter. The occurrence of seismic soft-sediment deformation at different stratigraphic levels of the Quaternary and in the Upper Pliocene Beken Formation confirms the high seismicity of southeastern Altai in Quaternary time. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: seismite; Quaternary deposits; seismicity; southeastern Altai
Introduction Large earthquakes in active seismic areas leave their traces in landforms as various gravity slides, fault scarps, or long ditches, which may be indicators of slip geometry, magnitude, and location of the causative earthquakes (Keefer, 2002; Khilko et al., 1985; Malamud et al., 2004). However, this macro-scale signature commonly becomes erased by denudation in a few thousand years and is thus of no much use in paleoseismology. Evidence of older seismic events preserves as deformation structures in soft sediments called seismites (Seilacher, 1969). Seismites are known in continental, tidal, and marine facies (Enzel et al., 2000; Hempton and Dewey, 1983; González et al., 2005; Obermeier, 1998; Plaziat et al., 1990; Povolotskaya et al., 2006). Deformation structures have been best documented in Cenozoic sediments, especially in Pleistocene– Holocene sections, but structures of this kind were reported also from older strata (Jewell and Ettensohn, 2004; Mazumder et al., 2006; Netoff, 2002). Investigation into surface rupture of various types caused by the Chuya earthquake of 2003 (southeastern Altai), including structures associated with ground liquefaction and fluidi-
* Corresponding author. E-mail address: [email protected] (E.V. Deev)
zation, proved valid the existence of landforms produced by prehistoric earthquakes and, on the other hand, prompted the search for seismites in different Cenozoic sedimentary facies in the area. After the first report of seismites we discovered (Deev et al., 2005), we found many other deformation structures (Fig. 1) at different stratigraphic levels in Quaternary sediments. Therefore, the primary objective of this study was to constrain the stratigraphy of the seismites as a prerequisite for timing the paleoseismicity events. Other objectives concerned with the morphology of seismites and their formation mechanisms, as well as criteria to distinguish them from nonseismic deformation structures.
Cenozoic tectonics and seismicity of southeastern Altai The modern geodynamic models since the study by Molnar and Tapponnier (1975) commonly attribute the onset of Cenozoic tectonic deformation in Central Asia to the India– Eurasia collision which gave rise to several intracontinental orogens, such as the Altai orogen that initiated in the latest Paleogene–earliest Neogene (Dobretsov et al., 1995). Mountain growth increased especially in the Late Pliocene and produced the neotectonic framework of southeastern Altai with elevations over 4000 m and the uplift reaching 1000– 2000 m at basin-range junctions and a few tens to hundreds of meters at boundaries of smaller blocks (Novikov, 2004;
1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2008.10.004
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Fig. 1. Location map of studied Quaternary sections (1–9). 1, Kyzyl-Chin (50°03′19.2′′ N, 88°17′03.9′′ E, depth (H) = 1845 m); 2, Chagan-Uzun (50°03′39.0′′ N, 88°20′55.5′′ E, H = 1767 m); 3, Chagan (49°56′48.1′′ N, 88°06′32.5′′ E, H = 1992 m); 4, Menk (50°16′47.4′′ N, 87°40′13.5′′ E, H = 1428 m); 5, Ust’-Chuya (50°23′58.6′′ N, 86°40′18.7′′ E, H = 768 m); 6, Malya Inya (50°26′53.6′′ N, 86°40′23.3′′ E, H = 778 m); 7, Yaloman (50°29′51.1′′ N, 86°35′28.3′′ E, H = 755 m); 8, Malyi Yaloman (50°28′55.0′′ N, 86°34′35.8′′ E, H = 745 m); 9, Katun’ (50°30′23.6′′ N, 86°34′08.2′′ E, H = 735 m).
Rakovets, 1967). The activity shows up mainly as strike-slip faults in environments of compression, extension, and rotation (Buslov et al., 1999; Lukina, 1996; Thomas et al., 2002) and, besides newly formed faults, involves rejuvenated segments of Late Paleozoic and Mesozoic large faults (Sarasa–Kurai, Shapshal, Kuznetsk–Altai, Chulyshman, etc.). Quaternary fault motions are recorded in tilted and offset river and lake terraces and fans, in moraine and glaciofluvial complexes, in deformed features of glacial erosion, as well as in facies change and deformation of sediments in intermontane basins (Chuya, Kurai, etc.) (Buslov et al., 1999; Devyatkin, 1965). Current deformation is recognized by GPS measurements and repeated leveling surveys (Kolmogorova and Kolmogorov, 2002; Timofeev et al., 2006). Ongoing seismic activity in the area is evident in tens of M ≤ 4.5 (K ≥ 12) instrumental events yearly (Emanov et al., 2005; Zhalkovskii et al., 1995), while signature of high paleoseismicity is found in numerous fault scarps possibly caused by earthquakes of magnitudes up to M = 7.5 and shaking intensities up to IX (Butvilovskii, 1993; Devyatkin, 1965; Rogozhin et al., 1999). Altai may experience seismicity as high as in the nearby areas of Mongolian Altai (Khilko et al., 1985) because the Mongolian seismogenic faults extend into southeastern Altai. The expected high seismic potential was confirmed by the Altai (Chuya) event of September 2003 (M = 7.3 and intensity VIII–IX) followed by a long aftershock process (Emanov and Leskova, 2005).
Quaternary sedimentary complexes The historic background of the stratigraphy and paleogeography of southeastern Altai was discussed in detail in (Novikov, 2004). We reconstruct the chronology of main Quaternary events in the area proceeding from stratigraphic, facies, and geochronological published evidence (Devyatkin, 1965; Efimtsev, 1964; Parnachev, 1999; Sheinkman, 2002),
and our own data (Zolnikov and Mistryukov, 2008). Quaternary strata lie upon lacustrine, fluvial, and overbank deposits of the Beken Formation, which has been commonly assigned a Late Pliocene age. The Quaternary section proper begins with brown boulders, pebbles, and sands of the Pliocene (?)– Lower Quaternary Bashkaus Formation. This is generally orogenic molasse deposited in large river valleys (fluvial facies) and in intermontane basins (fluvial-colluvial-overbank facies). First Quaternary glaciations in the Altai area were hypothesized to date back to the Early Quaternary mountain growth but, if any, they left no continuous traces on the surface (Svitoch et al., 1978). The largest extent of moraines (maximum glaciation after Devyatkin (1965)) and damlake silt has been attributed to the Riss Glacial epoch equated with the Taz Glacial of the West Siberian regional stratigraphy (WSRS). The correlation with WSRS and δ18O stratigraphy or MIS (Imbrie et al., 1984) is according to (Arkhipov et al., 1997). The glacial unit corresponding to the maximum glaciation (unit I in Fig. 2) correlates with MIS 6. At that time glaciers dammed river valleys and produced damlakes that occupied the territory of the Chuya, Kurai, Uimon, and other basins. Failures of the dams caused great floods that reached hundreds of meters in depth and locally spilled over the mountain divides between the Chuya and Katun’ valleys (Butvilovskii, 1993; Rudoi, 2005). The moraine and limnic facies of the unit make up the 300–350 m thick Inya Formation (Efimtsev, 1964) of rhythmic overbank deposits, up to seven cycles in the most complete sections. In the Chagan reference section, they are alternating till and intermediate beds, with controversial TL age constraints (Borisov and Chernysheva, 1987; Sheinkman, 2002; Svitoch et al., 1978). Data by (Sheinkman, 2002) indicating that the oldest age of the Chagan moraines (Fig. 1) falls within MIS 6 appear most consistent. No less realistic is the commonly assumed range of the Bakhta superstage which encompasses the Taz (MIS 6) and Samara (MIS 8) stages of WSRS. According to (Sheinkman, 2002),
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Fig. 2. Correlation of earthquake-induced deformation structures in Quaternary sediments of southeastern Altai. 1–6, lithofacies that host seismites: silt and sand of glacial damlakes and secondary damlakes (1), mostly alluvial silt (2), sand (3), gravel (4), pebble (5), and boulders (6); 7–10, seismites: various shapes produced by liquefaction and fluidization of sediments (7), coarse colluvium layers and lenses (8), flexures and their systems and series of small joints, often with displacement of relatively plastic sediments (9), joints of different scales (10). Repetitive structures 7–10 indicate presence of several deformation units within sections. Section numbers are according to Fig. 1.
glacial deformation and floods might also partly represent cold substages of MIS 5. Therefore, the stratigraphic constraints of maximum glaciation in sections of Gorny Altai, and the related Inya sequence, should be approached with caution until better representative and geographically larger data become available. The Inya Formation is cut by high erosive first terraces of the Chuya and Katun’, in which fan alluvium rarely reaches thicknesses of 3–5 m or is often absent; the age of the alluvium corresponds to the Kazantsevo stage of WSRS (MIS 5e). The complex of high terraces encloses the 60–100 m thick Saldzhar Formation (Efimtsev, 1964) which we correlate with glacial and limnic-glacial deposits of the first post-maximum glaciation of Gorny Altai (glacial unit II). The unit may correspond to the Early Zyryanian or Ermakovian stage of WSRS (MIS 4, or possibly MIS 5b, d) and contains at least three cycles of overbank deposits. The Saldzhar Formation, in turn, makes up the base of middle terraces. Some terraces are covered with 3–5 m thick alluvium deposited during the incision of the rivers. Its age corresponding to the Karginian stage of WSRS (MIS 3) is consistent with data on Paleolithic sites (Derevy-
anko and Markin, 1987; Postnov et al., 2007). The small thickness of the alluvium is due to the fact that major rivers and their tributaries incised into deposits of glacial floods in post-Saldzhar time. The process was rather long and incision often reached the bedrock base of sediments. The second post-maximum glaciation (glacial unit III) corresponds to the Sartan glacial stage of WSRS in the glacial zone and to the subaerial unit on terrace surfaces in the periglacial zone. The subaerial sediments are loess, eolian sand derived from Karginian alluvium, and overbank debris that were deposited in a dry cold climate. The Holocene section includes the upper part of subaerial sediments, alluvium of lower terraces, and floodplain deposits.
Deformation structures in reference Quaternary sections of southeastern Altai Below we discuss the stratigraphy of deformation structures in reference Quaternary sections. The sections have been largely documented and described in detail, with facies
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analysis and age constraints, in a separate book (Zolnikov and Mistryukov, 2008). Therefore, in this study we confine ourselves to a brief summary in the text and in the chart of Fig. 2. The sections belong to two Quaternary paleogeographic zones: (i) glacial zone which was covered with glaciers (up the Chuya valley, from the Belgibash River mouth to the Chuya basin) and (ii) periglacial zone which was out of the reach of the glaciers but experienced a strong impact from related floods (from Belgibash River mouth and downstream along the Chuya and Katun’ valleys, as far as the plainland). Kyzyl-Chin section (Upper Pliocene–Quaternary), left side of the Kyzyl-Chin River, 1.5 km upstream of its exit from a gorge. The section consists of sand and silt of the Beken Formation (to 15 m thick) at the base, pebbles of the Bashkaus Formation (8–10 m) in the middle, and a gray diamictite bed (10–11 m) on the top (Devyatkin, 1965). We examined a 9.5 m high and 12 m long exposed fragment of the Beken Formation (Fig. 3, A) composed of light brown to orange plane-bedded silt and sand with five layers and lenses (0.3 to 1.5 m thick) of angular coarse clastic material. Silt is thinly laminated and interlayered with 2 to 5 cm thick sand. Silt and sand are obviously lake sediments while the coarse material is petrographically uniform colluvium (clasts of brown Devonian tuff) shed into the lake from the valley sides. The mottled-
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banded structure of the coarse layers and lenses, and their rugged boundaries, result from intrusion of colluvium into lake sediments (Fig. 3, B, C). The colluvium lenses grade on strike into beads of “floating” angular clasts, and the silt-sand layers are undulate and contorted as small disharmonic folds. Thin (a few centimeters) contorted silt-sand beds were found also outside the colluvium occurrence (Fig. 3, D). Chagan-Uzun section, 5 km upstream of the Chagan-Uzun inlet into the Chuya, consists of pale yellow thin-bedded silt and sand deposits of a glacial lake of the maximum glaciation. We studied an outcrop fragment, about 7 m high and 8 m long (Fig. 4, A), where 3–4 m thick lake sediments are overlain by diamictites (1.5–2 m of visible exposed thickness) with striated and faceted boulders as well as rounded erratic cobbles and pebbles. The diamictite bed has its base undulated in this outcrop and deformed in laterally proximal sites. It appears to belong to a moraine of unit I of post-maximum glaciation. Plane-bedded silt within the limnic sequence includes four large (up to 1 m thick) and several smaller layers of coarse angular material of local provenance (see one such layer in Fig. 4, B), which strongly vary in thickness and often pinch out off the valley sides. The coarse layers apparently formed in a submerged environment, judging by their structures of colluvium-silt mingling (Fig. 4, C). Furthermore, there are
Fig. 3. Fragment of Kyzyl-Chin section (A). 1, plane-bedded damlake silt and sand; 2, coarse colluvium, 3, bedding (here and below). B–D are explained in text.
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Fig. 4. Fragment of Chagan-Uzun section (A). 1, plane-bedded damlake silty sand; 2, coarse colluvium, 3, till. B–H are explained in text.
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traces of mudflow injection into laminated sediments, which upset and crumpled the sediment layers (Fig. 4, D). Like in the Kyzyl-Chin section, there are small-scale deformation features on the extension of the coarse layers. We discovered several cases of a peculiar relationship between deformation structures and coarse material, when a clastic layer, or isolated floating clasts, lie on a deformed layer that has a well-defined base (Fig. 4, E). The same section contains systems of meeting cracks in silt and sand (Fig. 4, F) and simple, disharmonic (Fig. 4, G), fan-shaped, or isoclinal (Fig. 4, E) folds that show no relation to coarse material. Note that the deformed layers, from a few centimeters to tens of centimeters thick, are sandwiched between undeformed lake sediments. Deformation in limnic varves of the first maximum glaciation was found also in the Chagan stratotype section (Svitoch et al., 1978) as very small volcanic craters (a few centimeters) clearly seen in jointing planes (Fig. 4, H). Ust’-Chuya section, right side of the Katun’ River, 600 m downstream of its confluence with the Chuya. This is the first section of the periglacial zone, where no glaciers existed in Quaternary time but repeated glacial floods exerted major topographic and lithofacies control. The section begins with the Inya exposed cyclic overbank deposits discordantly overlain by the Saldzhar Formation consisting of three members (Fig. 5, A). Proceeding from our previous experience, we turned special attention to thin-bedded silt and sand deposits of secondary damlakes on the top of the glacial flood cycle. According to (Parnachev, 1999), secondary damlakes arise after great floods in tributaries dammed by overbank deposits or in subsided valley fragments where denudation and previous flood deposition have made the floor rugged. The Ust’-Chuya section includes two secondary damlake beds: one on the top of the Inya Formation and the other in the middle of the Saldzhar Formation. These beds, as well as a sand-gravel layer under the Inya silt and sand, host flame- (Fig. 5, B–E) and cloud-shaped (Fig. 5, F) deformation structures. The former are intrusions of coarse material into finer-grained sediments and the latter are, vice versa, finer sediments in a coarser matrix. The deformed sediments are no more than 10 cm thick. The upper silt-sand layer is cut by cracks with their dip and rake angles of 90°/45° and 300°/55°, respectively (see Fig. 5, G for one such crack). The cracks go through about 15 cm of rocks, undeformed sediments between the cracks are 3 to 15 cm thick, and the offset reaches 5–7 cm. Katun’ section, left-side meander of the Katun’ opposite Malyi Yaloman Village. The section exposes a long lens of plane-bedded silt and sand of a secondary damlake produced by the final flood cycle of Saldzhar time (Fig. 6, A). Limnic sediments are 6.6 m thick and contain 10–15 cm thick layers with deformation structures at different levels. The structures are pillow-shaped (Fig. 6, B), disharmonic (Fig. 6, D), cyclic (Fig. 6, E), or fan-shaped folds, often with necked cores (Fig. 6, F), or look like mushrooms (Fig. 6, G) or small volcanic craters (Fig. 6, H).
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Malaya Inya section, left side of the Inya River, 3.4 km far from its inlet into the Katun’. This is a parastratotype section of the Saldzhar Formation exposed in the sides of a gully running across the Inya valley. The formation comprises three members with a complete set of lithofacies typical of cyclic overbank deposits. Each member has interbedded silt and sand of a secondary damlake at its top. The Saldzhar deposits are incised with Karginian alluvium (2 m of visible thickness), which is overlain, in turn, with a 14 m thick apron of Sartan flood deposits cut by the Holocene channel and flood plain of the Inya. See Fig. 7, A for a fragment of an outcrop at the intersection of the Inya and the gully. The exposed section consists of cross-bedded channel pebbles (up to 1.5 m of visible thickness), overbank silt and sand with a hydromorphic fossil soil (up to 0.5 m thick), and layered overbank debris reaching 2.4 m of visible thickness produced by redeposition of the Saldzhar debris. The deformed interval, about 1 m thick, encompasses the upper part of fluvial pebble and overbank silt. Deformation is especially prominent near the interface between sediments of different grain sizes and appears as sharp cyclic convolutions (Fig. 7, B) and isolated lenses (Fig. 7, C) on both sides of the interface. The deformation structures are traceable discontinuously for several tens of meters along the Inya. As for the Saldzhar Formation itself, we found numerous features of offset bedding in an outcrop in the right side of the Inya (50°27′07.1′′ N, 86°39′13.9′′ E, H = 775 m) in lacustrine silt and sand at the top of the lower overbank bed. The deformation structures are of flame (Fig. 7, D) or pillow (Fig. 7, E) shapes and of various sizes. Different section parts include from two to four deformed layers (Fig. 7, D, F, G), all at the transition between coarse sediment above and fine sediment below. Malyi Yaloman section, left side of the Malyi Yaloman River, 3.5 km upstream of its mouth. The section consists of plane-bedded Saldzhar debris, up to 3.5 m of visible thickness, plane-bedded silt and sand of the Saldzhar last secondary damlake (up to 1.5 m), Karginian cross-bedded boulder and pebble beds of alluvium that give way upsection to plane-bedded overbank sand and gravel (up to 2 m), and 10–12 m of Sartanian (MIS 2) plane-bedded overbank debris with sand interlayers. The post-Saldzhar section at the Malyi Yaloman site is obviously similar to that at the Malaya Inya site but deformation structures in it are different and appear in a more or less prominent way over a distance about 100 m. Figure 8, A shows a fragment of the outcrop with jointed debris, silt, sand, and the lower bed of cobble and pebble. The deformation structures are sealed from above by 0.2–1.0 m thick channel deposits. In addition to brittle failure of different magnitudes and slip geometries (Fig. 8, A, B, D), silt and sand bear small flexures (Fig. 8, E) and flame-shaped structures older than the joints (Fig. 8, C). The same section contains a lens of mudflow intruded into silt and sand (Fig. 8, F). The relatively small thickness (within 40 cm) of silt and sand below the lens indicates that the mudflow event occurred at the initial stage of the secondary damlake. The mudflow lens is overlain by lake sediments with
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Fig. 5. Ust’-Chuya section (A). 1, boulders; 2, pebbles; 3, debris; 4, gravel; 5, sand; 6, silt; 7, clay; 8, secondary damlake silt and sand. B–G are explained in text.
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Fig. 6. Katun’ section (A). Legend same as in Fig. 5. B–G are explained in text.
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Fig. 7. Fragment of Malaya Inya section (A). Legend same as in Fig. 5. B–G are explained in text.
wavy structures at the depth of the lens top produced by roiling of bottom sediments. Together with the overlying silt and sand, it is partly truncated by a bed of cross-bedded cobbles and pebbles. Thus, the mudflow lens may have formed by gravity sliding of colluvium into the lake.
Yaloman section, a quarry in the left side of the Katun’, 1 km southeast of the Malyi Yaloman mouth. The quarry opens the surface of middle terraces not far from their back suture and exposes lens-shaped cross-bedded pebbles with gravel and sand (Fig. 9, A), most likely of a post-Saldzhar
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Fig. 8. Fragment of Malyi Yaloman section (A). 1, joints. Other symbols as in Fig. 5. B–F are explained in text.
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Fig. 9. Yaloman section (A). Legend same as in Fig. 5. B–D are explained in text.
(i.e., Karginian) age. According to their stacking patterns, the deposits appear to be lateral accretion complexes. A sandgravel lens in a terrace surface bears signature of rupture and displacement (Fig. 9, B), and involves a system of minor flexures (Fig. 9, C). As deformation increases, the primary bedding of sediments becomes destroyed (Fig. 9, D). Note that the deformed lens is sandwiched between undeformed sediments like in the previous limnic sections. Menk section, an abandoned quarry of the Menk hydro power plant. The quarry exposes post-Saldzhar alluvium consisting of cross-bedded pebbles (1.5 m visible thickness) below 15.5 m of lenticular-cross-bedded sand with gravel and pebble overlain by solifluction loamy sand with pebble and gravel (1.5 m). This may be one of best representative post-Saldzhar sections in which there are seven deformation units within the 3.6–6.6 m interval above the sand-gravel base (Fig. 10, A). The structures are of flame (Fig. 10, C) or spherical (Fig. 10, E) shapes that result from intrusion of coarser sediments into the underlying finer sediments, or structures of more intense sediment mingling (Fig. 10, F). The deformation structures were apparently produced by several discrete events, as one may infer from their location in different lenses (Fig. 10, B) and from the position of deformed layers between undeformed ones. Furthermore, the upper deformed units in pairs of two closely spaced layers often have their base well-defined and weakly deformed (Fig. 10, C, E) and the top truncated by overlying sediments (Fig. 10, D).
Discussion Origin of deformation structures. Identifying the origin of various soft-sediment deformation structures remains problematic and is thus worth special attention. Deformation structures (including morphologically similar ones) in Quaternary sediments of southeastern Altai may be due to glacial, permafrost, solifluction, gravity, depositional, or seismic causes. Deformation associated with static or dynamic loading from glaciers is logically impossible in the periglacial zone of the area. Glacier-induced deformation of the Beken pre-Quaternary deposits in the Kyzyl-Chin section is inconsistent with available paleoclimate data. Deformation structures in the Chagan-Uzun section would appear to result from glacier loading upon the underlying wet sediments (more so that there are varves under a moraine) were there no colluvium slid into an ice-free lake. Deformation structures in the lacustrine varves that bear no traces of subaerial gaps cannot arise by cryogenic or solifluction processes which normally act in subaerial environments. Ripple structures produced by currents have quite a different morphology. Cryogenic origin may be hypothesized for convolutions reaching 0.5 m in fluvial pebbles, gravel, and sand, for instance, in the Chuya basin, in areas of karst topography formed by heave degradation. Contorted structures may be due to solifluction associated with thawed injection lenses of
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Fig. 10. Fragment of Menk section (A). 1, silty sand; 2, solifluction loam sand with pebble and gravel. Other symbols as in Fig. 5. B–F are explained in text.
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ground ice as well as with segregation ice schlieren, secondary ground ice, or ice cement. The only possible ice-related formation mechanism for the folds and joints in alluvial pebbles, gravel, and sand found in different outcrops but at the same stratigraphic level (base of overbank sediments deposited during a cold dry spell) in the Malaya Inya and Malyi Yaloman sections is thawing of secondary ground ice. However, we never saw traces of this ground ice, which must look like wedges with particular upturned margins, in the sections we studied. Thus, neither cryogenesis nor aseismic gravity sliding appear to be likely candidates responsible for the deformation. Structures similar to those we observed may arise also by a submerged gravity flow, namely, debris or mud flows into the lake bottom at the final stage of ice damlakes in glacial zones or at the waning stage of large glacial floods in periglacial zones (Zolnikov and Mistryukov, 2008). However, unlike the submerged mudflow, turbidity, and debris flow structures grading one into another, in which deformation and sedimentation are syngenetic (Zolnikov and Mistryukov, 2008), deformation in the Gorny Altai Quaternary sections is obviously independent of sedimentation. Furthermore, submerged debris-flow deposits normally display a broad variety of structures (drag folds, microdiapirs, bands with blob-shaped contours), often stretching along the flow. They are always mixed layers, often up to several meters thick, rather than thin clusters of small-scale deformation in lacustrine sediments. Thus, having analyzed the morphology and settings of deformation structures we observed in Quaternary sections, both in separate layers and in layer successions, as well as in outcrops within the same stratigraphic level, we infer that they are of seismic origin. This inference is consistent with their morphological similarity at different stratigraphic levels in different facies and paleogeographic zones, and with spontaneous occurrence of deformed intervals in similar stacking patterns. The restriction of the soft-sediment deformation structures to discrete stratigraphic levels and the presence of several deformed layers between undeformed strata indicate that the events were discrete and recurrent, as are normally the events in active seismic areas where seismic activity alternates with periods of quiescence. The seismic origin of deformation in southeastern Altai is quite reasonable because it was an area of active mountain growth associated with high seismicity in the Quaternary and remains active at present (see above). The described patterns are similar in geometry and scale to historic and fossil seismites reported from other seismic areas (see Introduction). Note that structures like those we report from the Quaternary sections were observed in mud springs produced by soil liquefaction during the Chuya earthquake of 2003 (Fig. 6, B, C). The presence of seismites in Quaternary sediments of southeastern Altai has implications for the origin of the lenses and layers of coarse colluvium in thinly laminated silt and sand. Sliding of colluvium into a lake not necessarily results from an earthquake. However, the coexistence of colluvium enclosed in lacustrine sediments with seismites in the Chagan-
Uzun and Kyzyl-Chin sections provides reliable evidence for the seismic triggers. Especially informative in this respect are the seismites in the Chagan-Uzun section which slightly predate the deposition of colluvium. This means that the formation of seismites and landsliding were triggered simultaneously, most likely by a seismic shock. Seismites may have formed at the point of seismic wave arrival, whereas the landslide reached the lake bottom in a short while (a few minutes later) and fell upon the already deformed sediment. The fact that colluvium exists in lake sediments as lenses and layers indicates its recurrent influx thus providing additional evidence for its linkage with earthquakes. We infer a seismic trigger also for the mudflow we discovered in the Malyi Yaloman section, which appears reasonable especially because silty sand in the section bears signature of liquefaction and rupture. Note that earthquake-induced lenses and layers of coarse clastic material in lakes share much similarity with submerged mudflow mixtites that arise on recession of great floods or on lake level fall, but there are essential points of difference. A seismically triggered landslide is a single event after which normal deposition resumes rapidly. Moreover, the rock textures and structures we report show that clasts mix with bottom sediments only after having fallen into the lake and thus form clearly localized and stratigraphically confined continuous layers of mingling and sediment deformation. Unlike the earthquake-induced landslide deposits, mudflow deposition associated with water level fall is a longer and multiple-event process. The flows intersect, become diluted, and overlap one another thus making more numerous mixed layers with sharp lithological transitions. Another typical feature is that sediments of mudflows from rapidly emerging lake sides are of different grain sizes. Thus, it appears quite reasonable to suggest a seismic origin of the deformation structures and attendant layers and lenses of coarse clastic material in the Quaternary sections we examined. Types, geometry, and formation mechanisms of seismites. The seismites we discovered in Quaternary sections of southeastern Altai are of different types. Most of deformation is associated with liquefaction and fluidization of wet sediments and is confined in thin (a few centimeters to 0.5–1 m), often continuous, layers that lie concordantly upon and below undeformed sediments. Most sections bear several deformed layers, which thus appear to correspond to recurrent events. Deformation structures of same geometries are found in sediments of different grain sizes. For the lack of any universal classification, these structures are usually termed in a way to give the most appropriate idea of their geometry. We distinguish isoclinal (Fig. 4, E), disharmonic (Figs. 3, D, 4, G, and 6, D), fan-shaped (Fig. 6, F), and cyclic folds, spherical, flame-, cloud-, or mushroom-shaped intrusions of one sediment type into another, or micro-volcanoes (Figs. 4, H and 6, H). Some names require additional explanation. Cyclic structures are actually sine-shaped simple folds (Figs. 6, E and 7, B). Spherical (Fig. 10, E) and pillow (Figs. 6, B and 7, E) structures are round or flattened intrusions of coarse material
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separated by columns of fine sediment. Flame-shaped structures correspond to interfingering sediments of different types (Figs. 5, B–E, 7, D, F, 8, C, 10, C); they look like spires and are often attendant with isolated lenses. Cloud-shaped structures result from upward intrusion of finer sediment into coarser material above (Fig. 5, F). Mushroom-shaped structures looking like champignons are cross sections of plastic contorted beds, with coarser sediment from below squeezed into their cores (Fig. 6, G). Typical brittle deformation structures found in the Malyi Yaloman and Chagan-Uzun sections are joints of different slip geometries (Figs. 4, F, 5, B, D). Intermediate between brittle failure and liquefaction-fluidization structures are solitary flexures or their systems, as well as systems of joints in which the displaced sediment is still relatively ductile (Figs. 5, G and 9, C). In studies of parameters and mechanisms of seismically induced strain behavior in granular porous materials (e.g., Allen, 1982; Lowe, 1975; Owen, 1987), passage of waves was hypothesized to cause change in sediment packing and fabric thus decreasing porosity and increasing pore fluid pressure. The lithostatic/hydrostatic pressure difference drives fluidization and liquefaction. The onset of the latter process is associated with plastic deformation (folding and bending). The progress of liquefaction increasingly destroys the primary sediment textures which become fluidal and massive. Fluidization produces flows of silty and sandy water along fractures of which some are feeder channels of sand volcanoes appearing on the surface. Liquefaction and fluidization are favored by fine gain sizes of sediment but impeded by high percentages of cohesive clay. According to our study (Sibiryakov and Deev, 2008), deformation in porous materials subject to seismic shaking depends on the pore fluid phase, being brittle or associated with liquefaction and fluidization if the pores are filled with gas or liquid, respectively. In the latter case, the specific pore surface area, porosity, and pore pressure are the major controls. For instance, the lower the porosity and the higher the specific pore surface area of sediment (i.e., the finer the grain size), the greater the degree of its possible liquefaction and fluidization; the presence of an impervious seam (closed porosity) is another favorable factor. This explains why liquefaction and fluidization structures occur mainly in sediments of silt to gravel grain sizes. Deformation in the pebbles of the Malaya Inya section is most likely due to liquefaction of their sand and silt cement. Fluidization of the cement may be responsible for the craters in the Taldura channel alluvium (Fig. 11, A) that arose during the Chuya earthquake of 2003. Pebble beds may be more competent in deformation relative to finer sediments between them and thus make up small horsts (Fig. 11, B). Pebbles, as well as finer sediments, may be crosscut with sedimentary dikes (Fig. 11, C) which mark seismic surface rupture. Sedimentary systems become still less stable against seismic shaking if the contact sediment layers have different specific weights (reverse density gradient systems): e.g., unstable are the systems in which the sediment above is denser
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than that below. In the Quaternary sections we examined this instability is due mainly to larger grain sizes of higher sediment layers, though package, porosity, and water content may be also important. Additional instability may arise from local slope gradients in cross-bedded sediments. Natural seismites were reproduced in many loading tests. See Fig. 11, D for models of liquefaction and fluidization structures (Moretti et al., 1999) obtained using a digital shaking table which was excited with a shock equivalent to a natural M = 7 earthquake. The model provides an excellent illustration of the mechanisms that produce seismites in systems with an impervious barrier and with a reverse density gradient. Some model structures (Fig. 11, E) are similar in geometry and scale to the natural structures we discovered (Fig. 11, F). We suggest to distinguish the coarse lenses and layers, from a few centimeters to one meter thick, in laminated sand and silt as a special type of seismites produced by gravity sliding of colluvium into lakes. These seismites arose under the water, and the slid material may have reached the lake bottom in submerged mudflows, as one may infer from lake sedimentcolluvium mingling structures, with signature of crumpling in the lake sediments and plastic deformation on extension of the coarse lenses. Age of seismites. The oldest seismites we discovered are from the Late Pliocene–Early Pleistocene Beken Formation (Kyzyl-Chin section). The presence of seismites in damlake silt of the glacial zone (Chagan and Kyzyl-Chin sections) and in sands of secondary damlakes of the Inya Formation (Ust’-Chuya section) confirms high seismic activity of the southeastern Altai during the Last Glacial maximum. The deformation structures in silt and sand at the top of the lower (Malaya Inya section), middle (Ust’-Chuya), and upper (Katun’, Yaloman) members of the Saldzhar Formation record several seismic events during the first post-maximum glaciation (Ermakovian time). Some seismites in the Malyi Yaloman section likewise document seismic activity of the area at the end of the Saldzhar deposition. Seismicity increased also in Karginian time, judging by numerous earthquake-induced deformation structures in alluvium of middle terraces (Menk, Malaya Inya, Malyi Yaloman, and Yaloman sections).
Conclusions There are several reasons to infer the seismic origin of the discussed deformation structures in the Quaternary sections from southeastern Altai. 1. All sections are located in the strongly faulted area of rapid Late Cenozoic mountain growth, where high seismicity is confirmed by instrumental data, historic accounts, and fault scarps produced by large prehistoric earthquakes. 2. Deformed layers in Quaternary sediments have a broad geographic extent and are traceable in outcrops for tens or less often hundreds of meters. 3. Deformation structures are restricted to discrete stratigraphic levels, and the deformed layers lie between unde-
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Fig. 11. A. Craters in channel alluvium of Taldura River produced by Chuya earthquake of 2003. B. Small horst from upper overbank member of Saldzhar Formation in Malaya Inya section. C. Sediment dike from Saldzhar Formation, area of Yaloman section; D. Model of liquefaction and fluidization textures obtained in laboratory tests with a digital shaking table, after (Moretti et al., 1999): 1, 2, fine (1) and silty (2) sand; 3, silt; 4, clay; 5, water table. E. Example loading texture obtained in experiment, after (Moretti et al., 1999). F. Natural equivalent from lower member of Saldzhar Formation in Malaya Inya section; darker color shows relatively coarser sediments.
formed sediments. This position indicates that deformation must result from recurrent events which is typical of seismic areas with alternating periods of activity and quiescence. 4. Deformation structures from different stratigraphic levels, facies, and paleogeographic zones share many features of similarity. 5. Seismites are similar in geometry and scale to structures observed in historic large earthquakes and to fossil seismites, as well as to structures obtained in laboratory tests that simulate seismic loading of wet soft sediments. Formation of abundant seismites in Quaternary strata of southeastern Altai was facilitated by a potentially liquefiable composition of silt-sand and gravel deposits, closed porosity and reverse density gradient systems, and slope gradient in cross-bedded sequences. The stratigraphy of seismites in Late Pliocene (Beken Formation) and Quaternary sediments (Fig. 2) confirms high seismicity of southeastern Altai in the Late Cenozoic.
We greatly appreciate constructive criticism by E.V. Artyushkov, S.I. Sherman, and Yu.K. Sovetov which made for improving the manuscript. The study was carried out as part of Integration Projects 2, 18, 56, and 69 of the Siberian Branch of the Russian Academy of Sciences.
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