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Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India
SEDGEO-05078; No of Pages 14 Sedimentary Geology xxx (2016) xxx–xxx
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Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India Naresh Rana a,⁎, Saraswati Prakash Sati a, Yaspal Sundriyal a, Navin Juyal b a b
Department of Geology, H.N.B. Garhwal University, Srinagar Garhwal, Uttarakhand 246174, India Earth Science Division, Physical Research Laboratory, Ahmedabad 380009, India
a r t i c l e
i n f o
Article history: Received 25 November 2015 Received in revised form 16 June 2016 Accepted 20 June 2016 Available online xxxx Keywords: Soft-sediment deformation structures (SSDS) Fluvial sediment Alaknanda Valley Flash floods
a b s t r a c t Valley-fill terraces and fluvio-lacustrine sediment successions were investigated for the nature and type of softsediment deformation structures (SSDS) in the Alaknanda Valley of the Garhwal Himalaya. Based on their morphologies, sediment characteristics and comparison with existing data on SSDS, these features are classified into seismic and aseismic categories. The study indicates that, despite the terrain being in the seismically active domain of the Central Himalaya, the majority of the deformation structures seem to have been generated aseismically. We attribute their genesis to uneven loading, slope failure and, most importantly, turbulent flow and sudden loading by flash floods. The study suggests that a cautious approach is needed before assigning a seismic origin to deformation structures in sediments deposited in high-energy fluvial systems. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Soft-sediment deformation structures (SSDS) include a wide range of structures that form in unlithified/unconsolidated clastic sediments (Mills, 1983; Maltman, 1984; Owen, 1996a; Hibsch et al., 1997; Van Loon, 2009; Owen et al., 2011; Ghosh et al., 2012) under the scenario of adequate driving force(s), deformational mechanism and trigger (Allen, 1982; Owen et al., 2011; Rana et al., 2013a). Temporary reduction in sediment strength due to liquefaction, fluidization or thixotropy is the main processes implicated for deformation in clastic sediments (Lowe, 1975; Owen, 1996a, 2003). These processes can be triggered by rapid sedimentation, artesian groundwater movement, earthquake shaking, storm currents and gravity flows (Lowe, 1975; Obermeier, 1996; Owen and Moretti, 2011). Despite the variety of potential triggers, these structures are widely exploited in paleoseismic studies (e.g. by Obermeier et al., 1987; Mohindra and Bagati, 1996; Moretti, 2000; Neuwerth et al., 2006; Pandey et al., 2009; Ghosh et al., 2012; Rana et al., 2013a). Seismically induced SSDS are crucial for reconstructing the paleoseismicity of an area where the historical records of earthquakes are either insufficient or not well documented. Garhwal Himalaya is a seismically active area within the central seismic gap which is considered as a potential zone for an impending large earthquake (Fig. 1B) (Khattri and Tyagi, 1983). The terrain experienced moderate to high⁎ Corresponding author. Tel.: +91 1370 267391. E-mail addresses: [email protected] (N. Rana), [email protected] (S.P. Sati), [email protected] (Y. Sundriyal), [email protected] (N. Juyal).
magnitude earthquakes in the historical as well as in the recent past. For example, the great earthquake of 1803 that devastated a large area in the Garhwal had its epicenter around Srinagar (Rajendran and Rajendran, 2005). Similarly, the 1991 Uttarakashi and 1999 Chamoli earthquakes had epicenters in the south of the Main Central Thrust (Kayal, 2010) (Fig. 1A). As mentioned above, the terrain lies in the central seismic gap, which implies that it is due for a large-magnitude earthquake in the near future. Therefore, it is important to understand the frequencies and magnitudes of past earthquakes (beyond the historical and instrumental records) in order to ascertain the recurrence interval of large-magnitude earthquakes in the region. The sedimentary records of past earthquakes in the form of SSDS are considered as potential archives that can contribute towards achieving this aim. In this study we analyze SSDS and some brittle deformation features in fluvial and fluvio-lacustrine deposits in the Alaknanda Valley with the following objectives: (i) to document the morphologies of the deformation features; (ii) to determine the driving forces, deformation mechanism and possible triggers; and (iii) to consider the paleoseismic/ environmental implications for the area. The present study is a contribution towards improving our ability to distinguish aseismic from seismic SSDS, particularly in a high-energy fluvial environment. 2. Geological and tectonic setting of the study area The geological and tectonic evolution of the Himalaya is a result of the convergence of the Indian and Eurasian plates. This convergence is active at the rate of ~ 50 mm/year (DeMets et al., 1994) and is
http://dx.doi.org/10.1016/j.sedgeo.2016.06.012 0037-0738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 1. Study area. (A) Location and seismotectonics of the Alaknanda catchment of the Garhwal Himalaya. (B) Darker polygons are the rupture zones of great earthquakes. STDS—South Tibet Detachment System, MCT—Main Central Thrust, MBT—Main Boundary Thrust, MFT—Main Frontal Thrust, SH—Sub-Himalaya, LH—Lesser Himalaya, HH—Higher Himalaya, TH—Tethyan Himalaya. 1—Badrinath, 2—Birehi, 3–—Gauchar, 4—Rudraprayag, 5—Tilwara, 6—Swit, 7—Srinagar, 8—Chauras, 9—Kirtinagar and 10—Khandukhal. Tectonic structures after Valdiya (1980). Seismic data plotted according to the USGS (http://earthquake.usgs.gov/earthquakes/eqarchives/epic/).
episodically manifested in the form of moderate to large-magnitude earthquakes. However, a zone of ~700 km length located between the rupture zones of the 1905 Kangra and 1934 Bihar–Nepal earthquakes has not experienced any great earthquake for at least the last two centuries (Fig. 1B). This zone is called the Central Seismic Gap and is considered to be the potential zone for an impending large magnitude earthquake (Khattri and Tyagi, 1983). The Garhwal Himalaya, which lies in this zone, has witnessed three earthquakes of magnitude N6, namely the Srinagar (1803), Uttarkashi (1991) and Chamoli (1999) earthquakes (Rajendran and Rajendran, 2005). Geologically from
north to south, the Alaknanda Valley can be divided into three distinct lithological zones. These are the Tethyan Sedimentary Sequence, the Higher Himalayan Crystalline and the Lesser Himalayan Metasedimentaries (Fig. 1A). The Tethyan Sedimentary Sequence is dominated by fossiliferous shales, limestone and sandstone of Precambrian to Eocene age, whereas Proterozoic–Ordovician schists, gneisses and migmatites dominate the Higher Himalayan Crystalline. The Lesser Himalayan Meta-sedimentaries are dominated by quartzite, slate, carbonate and phyllite of Precambrian age (Heim and Gansser, 1939, Srivastava and Ahmad, 1979, Kumar and Agarwal, 1975, Valdiya, 1980).
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 2. Fluvial aggradation in the Alaknanda Valley around Srinagar. A. Well sorted gravely facies with alternating sand lenses resting on bedrock. B. Gravel sequences underlain by thick sandy facies. C. Older gravel facies with few sand lenses.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Structurally, the Alaknanda Valley is traversed by the South Tibet Detachment System, and the Main Central Thrust (MCT). The former is a tectonic boundary between the Tethyan Sedimentary Sequence and the Higher Himalayan Crystalline whereas the latter defines the boundary between the Higher Himalayan Crystalline and the Lesser Himalayan meta-sedimentary rocks. In addition to this, in the south of the MCT, two subsidiary structures cut across the Alaknanda River — the Alaknanda Fault and the Tons Thrust. A widely accepted view suggests that currently the youngest Himalayan Frontal Thrust, which separates the Siwalik ranges from the Indo-Gangetic plain, is the most active structure in the Himalaya (Lave and Avouac, 2000; Bilham et al., 2001, etc.). However, there is an equally compelling argument (e.g. by Seeber and Armbruster, 1981) that the MCT which separates the Higher Himalayan Crystalline from the Lesser Himalayan Meta-sedimentaries is an equally active structure. In addition to this, recent studies have shown that it is not the MCT but the terrain to the south of it that is seismically active (Kayal et al., 2003; Tyagi et al., 2009). 2.1. Seismicity The earthquake records of the recent past indicate that seismicity is broadly concentrated in an ~60 km wide zone coinciding with the MCT. This zone is also called the Himalayan Seismic Belt (Fig. 1A). Fault plane solutions of earthquakes indicate a shallow thrusting depth in the direction of plate convergence (Pande, 2003). Based on the pattern of
damage, the 1803 high-magnitude (N7) earthquake (Oldham, 1869) is speculated to have had its epicenter in the Alaknanda Valley around Srinagar town (Rajendran and Rajendran, 2005) (Fig. 1B). Following this, based on the earthquake catalog, an earthquake of unknown magnitude occurred in 1816 around Gangotri (Bhagirathi Valley). In recent times, the 1991 Uttarkashi and 1999 Chamoli earthquakes of magnitude N 6 are attributed to slip along the Main Himalayan Thrust (Basement thrust) which reactivated local faults in the region (Kayal et al., 2003; Kayal, 2010). 3. Facies analysis of the Quaternary deposits Late Pleistocene to Holocene river-borne and debris-flow sediment constitutes the major landforms that are preserved in the form of flights of valley-fill terraces in the Alaknanda Valley (Pal, 1986; Sati et al., 2007; Ray and Srivastava, 2010; Juyal et al., 2010). The oldest valley-fill in the study area is dated to ~40 ka. However, major valley-fill aggradation occurred after the Last Glacial Maximum (Ray and Srivastava, 2010; Juyal et al., 2010), corresponding to the gradual strengthening of the Indian Summer Monsoon (Juyal et al., 2010). In addition to this, fluviolacustrine and debris-flow deposits are also observed and are dated to ~ 14 ka and ~ 45 ka respectively (Sundriyal et al., 2007; Juyal et al., 2010). Multiple fluvial sequences investigated in the study area show dominance of gravely and sandy facies (Ray and Srivastava, 2010; Juyal et al.,
Fig. 3. SSDS in lake section at Chauras near Srinagar. (A) Position of the relict lake deposits in the fluvial section. (B) The section showing the positions of different SSDS. (C) Clay beds with load structures (l—load cast; f—flame structure). (D) Closer view of load cast. (E) Photograph and sketch of pendulous structure (p).
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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2010; Chaudhary et al., 2015). Textural analysis of terrace sediments in the study area suggests dominance of gravely facies, followed by coarse sandy facies with minor contribution of silty-clay and locally-derived slope wash debris (Figs. 2 and 3). Terrace gravel occurs as valley-fill (~40 m thick) overlying the undulatory and beveled bedrock platforms. In places the deposits plug relict channels. The gravely facies represents channel bed-forms, whereas the coarser sandy facies indicate pointbar deposits, thus indicating laterally fluctuating hydrological conditions (Juyal et al., 2010). The gravel size varies between 5 and 30 cm. However, in places boulders N1 m diameter are embedded in the terrace gravels (Juyal et al., 2010). Texturally, the gravel facies are dominantly well rounded, clast supported, moderately to well sorted and imbricated. However, assorted assemblages with no imbrication and coarse sand matrix are also found, particularly towards the upper part of the valley-fill sequences; they are often mixed with angular locally-derived lithoclasts (slope wash/debris). Sandy facies are relatively thinner than gravel facies and are more prominent in the younger valley-fill sediments (Ray and Srivastava, 2010; Juyal et al., 2010). Compared to this, the fine sand and silty-clayey facies are observed in areas having high valley width-to-depth ratio (Wasson et al., 2013). The subordinate silty-clay facies are associated with fluvial and debris-flow terraces and represent temporary ponding conditions (Sundriyal et al., 2007). The sediments are dominated by planar, relatively thin beds (decimeter scale) of fine sand to clay (Fig. 3) and often contain plant remains. The ponding is ascribed to the obstruction caused by landslides/slope-debris (landslide-dammed lake, sensu Sundriyal et al., 2007). At higher elevations in the study area, lacustrine facies dominated by varves and rhythmites (Juyal et al., 2009) are associated with obstruction caused by moraines in the proglacial zone (Fig. 9). Fine sandy to clayey-silt facies form the youngest terrace at wider segments of the valley and at some sheltered locations. These deposits are usually associated with high-magnitude past floods in the study area (Wasson et al., 2013). They are fining-upward sequences with coarse to medium sand at the bottom and often clayey-silt at the top. Such sequences often exist in multiple stories, each representing a flood event. Debris-flow facies containing fragmented angular gravels with slope materials are well developed on valley slopes at higher elevations than the fluvial terraces. They form the oldest (preserved) sediment, often in the form of moderately developed terraces, and stratigraphically the oldest preserved sequence in the Alaknanda Valley. Topographically, the lithofacies discussed above overlie uneven bedrock (sloping towards the valley), unconfined and lying well above the present water table. The characteristics of the major portion of the fluvial sediments (lithofacies analyses) suggest that they traveled a long distance before deposition. Deposition occurred in a high-energy environment (under high flow regime) in the form of channel-fill which often waned and resulted in matrix-dominated gravels followed by sandy facies (Chaudhary et al., 2015). These conditions prevailed in the valley after the Last Glacial Maximum, corresponding to the intensified Indian Summer Monsoon period (Juyal et al., 2010). In addition to this, in a few places the fine sandy to clayey-silt facies can be attributed to extreme hydrological conditions; the age of these deposits is seldom more than 1 ka (Wasson et al., 2013).
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4.1. Load structures 4.1.1. Simple load structures These are the most common structure, formed when an upper layer sags or penetrates crudely into an underlying layer. Also called load cast structures, they are observed in both the fluvial and fluvio-lacustrine deposits at sand–silt, sand–mud and sand–sand interfaces. Their sizes and amplitudes vary from 2 to 10 cm. They are best observed in a fluvio-lacustrine deposit near Chauras, at a silt–mud interface (Fig. 3D–E at location 8 in Fig. 1A).
4.1.2. Pendulous load structures Pendulous load structures occur in lake deposits and are mainly associated with simple load structures. They appear as a bulbous body as described by Owen (2003) and Neuwerth et al. (2006). Their sizes vary from a few centimeters to 10 cm in length and 5 cm in diameter. Fig. 3E shows a typical pendulous load structure preserved in a relict Holocene fluvio-lacustrine deposit near Srinagar (location 8; Fig. 1A) as a bulbous structure of ~8 cm diameter of sand penetrating into underlying carbonaceous clay–silt.
4.1.3. Ball structure Small-scale ball structure occurs at sand–silt interfaces and, occasionally, at sand–sand interfaces. A typical ball structure associated with a sand–sand interface occurs in the terrace deposit at Swit near Srinagar (Fig. 4; location 6 in Fig. 1A) and can be identified by the color contrast in homogeneous sandy deposits.
4. Description and interpretation of soft-sediment deformation structures (SSDS) Various types of SSDS occur in the terrace sequences, particularly those proximal to the river channel, i.e. terrace T1 and sub-recent deposits. Morphologically, these structures can be grouped under the major categories recognized by Owen (2003) and Neuwerth et al. (2006).
Fig. 4. Field photograph and sketch of ball structure in the fluvio-colluvial deposit near Srinagar.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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4.1.4. Flame and diapir structures Flame and diapir structures are the flame-like penetration of underlying sediments into overlying layers. They develop at sand–silt and silt–clay interfaces in the lacustrine deposits (Fig. 3C), where they are often associated with other load structures. However, they occasionally occur at sand–sand interfaces in fluvial deposits around Gauchar (location 3; Fig. 1A) and Srinagar (Fig. 5A–C). Their sizes (amplitudes) vary from a few centimeters to ~15 cm. 4.1.5. Interpretation Simple load casts, one of the most common SSDS, occur in unstable discontinuous layered systems (Allen, 1982; Owen, 1996a) at the interface of denser and underlying less dense sediments, often when the sediment loses its strength (Owen, 2003; Moretti et al., 1999 and references therein). In the study area these structures are observed in fluvio-lacustrine and fluvial–flood deposits where denser sediment overlies less dense layers. Partial liquefaction and thixotropy might have reduced the strength of sediment and caused gravitational instability. However, in cases where the upper layer is finer or of nearly the same grain size, uneven loading seems the most reasonable cause for the development
of load structures. Pendulous load casts represent a more advanced stage of deformation in a reverse density gradient system and can thus develop from simple load casts by a continuous deformation process (Owen, 2003). In the study area these structures are found in fluvio-lacustrine sediments within the overall fluvial environment. Ball (like) structures of silt or silty-sand are indicative of a reverse density gradient experiencing a longer-lasting deformation mechanism or simply detachment of the sediment from overlying layer. Flame and diapir structures usually form due to a reverse density gradient, difference in porosity and shear stress (Mills, 1983; Mohindra and Bagati, 1996). 4.2. Folded laminations 4.2.1. Deformed laminae A few small-scale sand pseudo-dikes are associated with the highly folded silty-sand beds on the left bank of the Alaknanda River at Kirtinagar (Fig. 6; location 9 in Fig. 1A). In places the sand layer penetrates the overlying silty-sand layer. The laminated sand of the lower layer is disorganized (disrupted), whereas the upper layers show undisturbed parallel stratification followed by cross-bedding (Fig. 6). The
Fig. 5. Flame and diapir structures. (A) & (B) Flame and diapir structures at sand–sand interface. (C) Flame and diapir structures at sand–silt interface.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 6. Folded laminae in the lowest terrace at Kirtinagar. The deformed laminae are overlain by cross-stratified fine sand beds. The layer shown in darker gray lost all primary structures and seems to have liquefied during deformation. Note the indications (arrows) of forceful upward movement of fluidized sand. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
disturbed and folded laminae overlie medium-grained sand with multiple load structures. 4.2.2. Complex structures Complexly folded laminae of alternating sand and silt occur at a depth of around 2 m from the youngest terrace top, and are overlain by silty-sand layers on the right bank of the Alaknanda River near Gauchar (Fig. 7; location 3 in Fig. 1A). They occur in finely laminated sand and silt with clay intercalations and differ from the ball structures due to the complex folding. 4.2.3. Interpretation Folded laminae are formed by current drag or shear stress (Mills, 1983; Owen, 1996a). They occur mostly in the silty-sand layers of floodplain deposits formed during floods in the Alaknanda Valley and include structures varying from simple folds to complex flames (Figs. 12 and 13). The folded laminae indicate that a sequence of sandy silt layers was liquefied, broken and dragged by a shear stress provided by sediment-laden flood-water (for discussion see Owen, 1996a). Deformed layers restricted within parallel, undeformed beds indicate sudden surges during floods (flash floods). These surges may have occurred during the breaching of landslide-dammed lakes during high rainfall events in the upper reaches of the valley. The complex morphology of the structures is attributed to the interaction of several shear-stress mechanisms interact (Lowe, 1975). However, turbulent river flow at this confined location may provide multi-directional shear stresses which may combine with gravitational effects to deform the laminae in a complex form (Lowe, 1975). 4.3. Water-escape structures Water-escape structures occur in sand beds at the bottom of a fluvial terrace composed of sand, silt and gravel ~8 m thick near Rudraprayag
(location 5; Fig. 1A). A sand layer underlain by boulders shows flamelike features with ~15 cm amplitude formed by the alignment of sand grains of darker shades (Fig. 8).
4.3.1. Interpretation Water-escape structures include a variety of structures formed by fluid escape in unconsolidated sediments due to many reasons (Lowe, 1975). Owen (1996b) described water-escape structures driven by fluidization in high-energy fluvial deposits. In Fig. 8 the orientation of sand grains and laminae clearly indicates the forceful upward movement of fluid. Water-escape was probably driven by uneven loading exerted by boulders when the sand layer was temporarily fluidized.
4.3.2. Slump folds A sequence of clay–silts and sand exhibits close, small-scale folding in otherwise uninterrupted layers (Fig. 9) of a relict lake succession developed behind a recessional moraine in the upper catchment of the Alaknanda River near Bardrinath (location 1; Fig. 1A). The folds exhibit unidirectional orientation (isoclinal tight folds) with inclination up to 10°. The relict lake succession generally shows alternating layers of coarse and fine sediments (rhythmites) varying from clay to medium sand, with a sequence of distinct glacial varves at the base.
4.3.2.1. Interpretation. The unidirectional orientation of the folded beds of sand and silt in the fluvio-lacustrine deposit (Fig. 9) may be the outcome of a sheet slump. Such folded sheets of sand and silt have been reported from subaqueous glacially-influenced settings, even on lowangle slopes. The folded layer is interbedded with undisturbed layers, which is typical of slumping (Allen, 1982).
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 7. Field photograph and sketch of complex deformation structures in the youngest silt-dominated sand deposit near Gauchar.
4.4. Brittle deformation in sediments 4.4.1. Ground fissures Ground fissures with no appreciable vertical offset occur in massive silty-sand in the lower reaches of the Alaknanda River around Khandukhal (location 10: Fig. 1A). The fissures developed on relict overbank facies resting on gravels. Morphologically, they are vertical to near vertical and extend up to 5 m below the surface (Fig. 10). Fissures are filled either with clean gray-colored medium-grained sand (Fig. 10B) or with mixed muddy sand and local debris including fragments of the local phyllite rocks (Fig. 10A). 4.4.2. Small-scale (sedimentary) faults As well as the well aggraded terraces, fluvial aggradation along the Alaknanda Valley also occurs in discrete patches resting on the valley slopes. Normal listric faults (Fig. 11) with offsets up to ~7 cm occur in sand–silt beds of fluvial origin in the upper and middle Alaknanda Valley at Birehi and Rudraprayag (locations 2 and 4 respectively; Fig. 1A). They occur on valley slopes which are susceptible to failure. 4.4.3. Interpretation Earthquakes of magnitude Mw N6 have the ability to produce ground deformation (Caputo, 2005). Ground cracks or fissures are long, tensile
deformation structures that occur in the ground with or without vertical offset (Carpenter, 1993; Ayalew et al., 2004). They were recorded from the 1989 Loma Prieta (by Sims and Garvin, 1995) and 1999 Chamoli (by Rajendran et al., 2000; Sarkar, 2004) earthquakes. They may develop as a consequence of local ambient stress fields developed during an earthquake (Sarkar, 2004), lateral spreading due to liquefaction (Sims and Garvin, 1995), or withdrawal of pore water and subsequent shrinkage of silty-sand (desiccation cracks). Normal faults are common in the gravitationally unstable system. In the study area these faults are very small and occur on the discrete sand–silt patches which have experienced unequal slip along the slopes.
4.5. Deformation structures in flood sediments 4.5.1. Gauchar A 3 m thick and ~300 m long succession of three alternating sand– silt layers along the bank of the Alaknanda River in Gauchar shows laterally extensive folded laminae (Fig. 12). Three distinct intervals show crude fining upwards to silt layers, implying that deposition occurred in a sheltered location during low flow conditions. The otherwise parallel laminated planar beds of silty-sand are deformed into simple anticline, syncline and isocline folds and complex recumbent folds and flame-like structures. Laminae are often broken and appear massive in
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 8. Water-escape structure in the sand beds of a fluvial terrace near Tilwara. Arrows indicate paths of fluid movement.
Fig. 9. Small-scale folds (slump folds) in the relict glacial lake near Badrinath. Note the unidirectional orientation of the fold and small fault. F—fault; Fo—Fold.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 10. Field photograph and sketches of ground fissures in Quaternary sediments at Khandukhal. (A) A fissure ~5 m deep in a thick silty-sand layer, filled with sand and angular clasts. (B) A wide fissure filled with medium gray sand. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the upper part whereas they gradually form simpler structures and finally undisturbed lamination in lower parts (Fig. 12). 4.5.2. Srinagar During a June 2013 flash flood, a sand layer ~5 m thick was deposited around Srinagar town in the floodplain of the Alaknanda Valley
(Rana et al., 2013b). At one location (Bhainswara village, location 10: Fig. 1A), a fine silty-sand horizon ~ 0.5 m thick shows folded layers of varying geometry. This deformed horizon is 1 m below the top surface of the deposit and is underlain and overlain by undeformed, parallel sandy layers (Fig. 13). The magnitude of deformation decreases downwards. The morphology of the deformed laminae varies from simple
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 11. Field photograph and sketches of brittle deformation structures (fault) in Quaternary sand deposits. (A) Small-scale fault in sand beds near Rudraprayag. (B) Similar fault in sand beds near Birehi.
folds to sharp flame-like notches and complex fold structures similar to the deformation around Gauchar (Fig. 12). The amplitude of the structures varies from a few to 30 cm. 5. Trigger mechanism Seismicity has been identified as a principal trigger for soft-sediment deformation (Rosetti, 1999), although storm waves, rapid loading, flood surges and gravitational density flows are also potential triggers (Jones and Omoto, 2000; Owen et al., 2011; Owen and Moretti, 2011). Although criteria for distinguishing seismic from aseismic triggers are not unequivocal, some studies have reduced the uncertainties by providing reasonable criteria for the identification of seismically triggered SSDS (e.g. Sims, 1975; Rosetti, 1999; Moretti and Sabato, 2007; Pandey et al., 2009; Owen and Moretti, 2011 and references therein). Most of the SSDS reported here (excluding brittle deformation) seem to have experienced partial liquefaction and fluidization, particularly the load structures and water-escape structures, which suggest partial liquefaction and fluidization of underlying layers. The presence of massive gravel and sand layers suggests that overloading may be an important trigger for deformation in the underlying, relatively finer grained sediment. Deformation was further facilitated by elevated groundwater conditions during high-discharge periods. Thus, the high discharge associated with high sediment flux was responsible for overloading and subsequent deformation in the saturated sediments (Obermeier, 1996; Moretti et al., 2001; Owen, 2003). Moretti et al. (2001) established a relationship between the thickness of overloading and the depth of liquefaction, and demonstrated that even a small thickness of overload can cause liquefaction in substrate layers at a depth comparable to the overload thickness. It has been suggested that during earthquake shaking, liquefaction may fail if sediments are not saturated (Obermeier, 1996). In mountainous terrain like the Alaknanda Valley, the Quaternary deposits are
devoid of groundwater due to their unconfined nature, open riverward side and higher elevation from the river bed. It is therefore very unlikely that the terrace sediment would experience liquefaction from earthquake-induced shaking. Ground fissures similar to those reported here often occur in the mesoseismal zones of large earthquakes (Sims and Garvin, 1995; Rajendran et al., 2000; Sarkar, 2004). At Khandukhal, they are deep (~ 5 m), occur in relatively stable ground, and are filled with locallyderived secondary deposits, indicating that they post-date deposition of the silty-sand and hence cannot be interpreted as desiccation cracks. Their size, morphology and, most importantly, their occurrence in a mesoseismal zone of past earthquakes (1803 Srinagar earthquake sensu Rajendran and Rajendran, 2005) may qualify them as seismites. However, the small-scale (sedimentary) faults appear to have been generated by local factors such as slope instability caused by active riverbank erosion (Obermeier, 1996). Relict lake deposits are an ideal repository for seismites (Hibsch et al., 1997) because alternative, internal trigger mechanisms (sensu Owen and Moretti, 2011) are generally absent. The slump folds observed in the relict proglacial lake deposit at Badrinath (Fig. 9) occur in rhythmites dominated by silty-clay and overlain by coarse- to medium-grained fluvial sand. We suggest that the deformation was generated by turbulence caused by the change in the environment from the calm proglacial lake to the high-energy fluvial environment. The (complex) folded laminations were undoubtedly formed during recent floods. They occur in places where the valley width-to-depth ratio is high and the river slope is gentle. At such locations the suspended load settles to form typical fining-upward sequences, generally capped by clay. Flooding in mountain areas is generally episodic and wanes away in a day or two. Furthermore, floods typically result from the pooling and breaching of river flow, forming surges further downstream. These episodic surges act as a shear wave over the freshly deposited sediment and drag it in different directions. This complex
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 12. SSDS in the older flood deposits at Gauchar. The deformed beds exhibit a variety of morphology from simple folds to complex convolutions.
drag, along with liquefaction of the underlying layer, produces complex deformation (complex structures). As mentioned earlier, the study area is traversed by many faults and thrusts of local to regional extent (Fig. 1). These and the occurrence of earthquakes of Magnitude 6.8 (1991, Uttarkashi), 6.6 (1999, Chamoli) and the historically documented 1803 Srinagar earthquake indicate that the valley has been seismically active (Cotton et al., 1996; Rajendran et al., 2000; Rajendran and Rajendran, 2005; Kayal, 2010). In spite of this, the morphological observations and facies analysis of the SSDS do not show any genetic bias towards their seismogenic origin; in contrast, turbulent stream flow, slope instability and uneven or overloading seem to be the factors responsible for their development. 6. Discussion and conclusion We have concentrated on the recognition of SSDS in Late Quaternary sediments dominated by fluvial sequences preserved in the Alaknanda Valley, mainly focusing on a context-based approach (sensu Owen and Moretti, 2011). Our study indicates that, due to textural and hydrological constraints that prevent the development of high pore water pressure during seismic shaking, most of the late Quaternary–Holocene sediments are devoid of SSDS. However, the sporadically distributed SSDS documented here were formed by liquefaction (or partial
liquefaction), thixotropy and to some extent fluidization under the influence of various driving forces including differential loading, gravity slides and current shear. Although the terrain lies in a seismically active zone, the SSDS are mainly aseismic in origin, triggered by hillslope movements, overloading and high shear stress during flash floods. Brittle deformation mostly occurred on unstable slopes (normal faults) but ground fissures (Fig. 10) were probably seismically formed during some large earthquake like the 1803 Srinagar earthquake. Since the sediments are neither confined nor saturated, we discard the possibility of liquefaction during earthquake shaking, indicating that the absence of SSDS in such a seismically active environment cannot be interpreted as a negative indication of paleoseismicity. The limited seismogenic SSDS may not be due to activity on local structures such as the Tons Thrust which traverses the study area around Srinagar, but may be manifestations of far-field effects of high-magnitude late Quaternary earthquakes. However, the majority of the SSDS described here are aseismic in origin, forming in response to sudden loading, saturation and shear stress induced by flood surges related to episodic high-magnitude floods in the Alaknanda Valley, which are known to affect the study area on a decadal scale (Wasson et al., 2013). This study cautions that care must be taken in interpreting soft-sediment deformation structures as sedimentological expressions of paleoseismicity, even in seismically active regions.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 13. SSDS observed immediately after the 2013 flood in the Alaknanda Valley. Note the similarity of the structure (s) to Fig. 7.
Acknowledgments We thank DST and Ministry of Earth Sciences, India for financial support by research grant (DST/23(570)/SU/2005). The constructive
comments and suggestions by the guest editor Dr. Pedro Alfaro and reviewers: Dr. M Moretti and Dr. Juan Pedro Rodríguez-López helped immensely in the improvement of the manuscript. We are thankful to Dr. Owen for improving the readability of the text.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India Naresh Rana a,⁎, Saraswati Prakash Sati a, Yaspal Sundriyal a, Navin Juyal b a b
Department of Geology, H.N.B. Garhwal University, Srinagar Garhwal, Uttarakhand 246174, India Earth Science Division, Physical Research Laboratory, Ahmedabad 380009, India
a r t i c l e
i n f o
Article history: Received 25 November 2015 Received in revised form 16 June 2016 Accepted 20 June 2016 Available online xxxx Keywords: Soft-sediment deformation structures (SSDS) Fluvial sediment Alaknanda Valley Flash floods
a b s t r a c t Valley-fill terraces and fluvio-lacustrine sediment successions were investigated for the nature and type of softsediment deformation structures (SSDS) in the Alaknanda Valley of the Garhwal Himalaya. Based on their morphologies, sediment characteristics and comparison with existing data on SSDS, these features are classified into seismic and aseismic categories. The study indicates that, despite the terrain being in the seismically active domain of the Central Himalaya, the majority of the deformation structures seem to have been generated aseismically. We attribute their genesis to uneven loading, slope failure and, most importantly, turbulent flow and sudden loading by flash floods. The study suggests that a cautious approach is needed before assigning a seismic origin to deformation structures in sediments deposited in high-energy fluvial systems. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Soft-sediment deformation structures (SSDS) include a wide range of structures that form in unlithified/unconsolidated clastic sediments (Mills, 1983; Maltman, 1984; Owen, 1996a; Hibsch et al., 1997; Van Loon, 2009; Owen et al., 2011; Ghosh et al., 2012) under the scenario of adequate driving force(s), deformational mechanism and trigger (Allen, 1982; Owen et al., 2011; Rana et al., 2013a). Temporary reduction in sediment strength due to liquefaction, fluidization or thixotropy is the main processes implicated for deformation in clastic sediments (Lowe, 1975; Owen, 1996a, 2003). These processes can be triggered by rapid sedimentation, artesian groundwater movement, earthquake shaking, storm currents and gravity flows (Lowe, 1975; Obermeier, 1996; Owen and Moretti, 2011). Despite the variety of potential triggers, these structures are widely exploited in paleoseismic studies (e.g. by Obermeier et al., 1987; Mohindra and Bagati, 1996; Moretti, 2000; Neuwerth et al., 2006; Pandey et al., 2009; Ghosh et al., 2012; Rana et al., 2013a). Seismically induced SSDS are crucial for reconstructing the paleoseismicity of an area where the historical records of earthquakes are either insufficient or not well documented. Garhwal Himalaya is a seismically active area within the central seismic gap which is considered as a potential zone for an impending large earthquake (Fig. 1B) (Khattri and Tyagi, 1983). The terrain experienced moderate to high⁎ Corresponding author. Tel.: +91 1370 267391. E-mail addresses: [email protected] (N. Rana), [email protected] (S.P. Sati), [email protected] (Y. Sundriyal), [email protected] (N. Juyal).
magnitude earthquakes in the historical as well as in the recent past. For example, the great earthquake of 1803 that devastated a large area in the Garhwal had its epicenter around Srinagar (Rajendran and Rajendran, 2005). Similarly, the 1991 Uttarakashi and 1999 Chamoli earthquakes had epicenters in the south of the Main Central Thrust (Kayal, 2010) (Fig. 1A). As mentioned above, the terrain lies in the central seismic gap, which implies that it is due for a large-magnitude earthquake in the near future. Therefore, it is important to understand the frequencies and magnitudes of past earthquakes (beyond the historical and instrumental records) in order to ascertain the recurrence interval of large-magnitude earthquakes in the region. The sedimentary records of past earthquakes in the form of SSDS are considered as potential archives that can contribute towards achieving this aim. In this study we analyze SSDS and some brittle deformation features in fluvial and fluvio-lacustrine deposits in the Alaknanda Valley with the following objectives: (i) to document the morphologies of the deformation features; (ii) to determine the driving forces, deformation mechanism and possible triggers; and (iii) to consider the paleoseismic/ environmental implications for the area. The present study is a contribution towards improving our ability to distinguish aseismic from seismic SSDS, particularly in a high-energy fluvial environment. 2. Geological and tectonic setting of the study area The geological and tectonic evolution of the Himalaya is a result of the convergence of the Indian and Eurasian plates. This convergence is active at the rate of ~ 50 mm/year (DeMets et al., 1994) and is
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Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 1. Study area. (A) Location and seismotectonics of the Alaknanda catchment of the Garhwal Himalaya. (B) Darker polygons are the rupture zones of great earthquakes. STDS—South Tibet Detachment System, MCT—Main Central Thrust, MBT—Main Boundary Thrust, MFT—Main Frontal Thrust, SH—Sub-Himalaya, LH—Lesser Himalaya, HH—Higher Himalaya, TH—Tethyan Himalaya. 1—Badrinath, 2—Birehi, 3–—Gauchar, 4—Rudraprayag, 5—Tilwara, 6—Swit, 7—Srinagar, 8—Chauras, 9—Kirtinagar and 10—Khandukhal. Tectonic structures after Valdiya (1980). Seismic data plotted according to the USGS (http://earthquake.usgs.gov/earthquakes/eqarchives/epic/).
episodically manifested in the form of moderate to large-magnitude earthquakes. However, a zone of ~700 km length located between the rupture zones of the 1905 Kangra and 1934 Bihar–Nepal earthquakes has not experienced any great earthquake for at least the last two centuries (Fig. 1B). This zone is called the Central Seismic Gap and is considered to be the potential zone for an impending large magnitude earthquake (Khattri and Tyagi, 1983). The Garhwal Himalaya, which lies in this zone, has witnessed three earthquakes of magnitude N6, namely the Srinagar (1803), Uttarkashi (1991) and Chamoli (1999) earthquakes (Rajendran and Rajendran, 2005). Geologically from
north to south, the Alaknanda Valley can be divided into three distinct lithological zones. These are the Tethyan Sedimentary Sequence, the Higher Himalayan Crystalline and the Lesser Himalayan Metasedimentaries (Fig. 1A). The Tethyan Sedimentary Sequence is dominated by fossiliferous shales, limestone and sandstone of Precambrian to Eocene age, whereas Proterozoic–Ordovician schists, gneisses and migmatites dominate the Higher Himalayan Crystalline. The Lesser Himalayan Meta-sedimentaries are dominated by quartzite, slate, carbonate and phyllite of Precambrian age (Heim and Gansser, 1939, Srivastava and Ahmad, 1979, Kumar and Agarwal, 1975, Valdiya, 1980).
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 2. Fluvial aggradation in the Alaknanda Valley around Srinagar. A. Well sorted gravely facies with alternating sand lenses resting on bedrock. B. Gravel sequences underlain by thick sandy facies. C. Older gravel facies with few sand lenses.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Structurally, the Alaknanda Valley is traversed by the South Tibet Detachment System, and the Main Central Thrust (MCT). The former is a tectonic boundary between the Tethyan Sedimentary Sequence and the Higher Himalayan Crystalline whereas the latter defines the boundary between the Higher Himalayan Crystalline and the Lesser Himalayan meta-sedimentary rocks. In addition to this, in the south of the MCT, two subsidiary structures cut across the Alaknanda River — the Alaknanda Fault and the Tons Thrust. A widely accepted view suggests that currently the youngest Himalayan Frontal Thrust, which separates the Siwalik ranges from the Indo-Gangetic plain, is the most active structure in the Himalaya (Lave and Avouac, 2000; Bilham et al., 2001, etc.). However, there is an equally compelling argument (e.g. by Seeber and Armbruster, 1981) that the MCT which separates the Higher Himalayan Crystalline from the Lesser Himalayan Meta-sedimentaries is an equally active structure. In addition to this, recent studies have shown that it is not the MCT but the terrain to the south of it that is seismically active (Kayal et al., 2003; Tyagi et al., 2009). 2.1. Seismicity The earthquake records of the recent past indicate that seismicity is broadly concentrated in an ~60 km wide zone coinciding with the MCT. This zone is also called the Himalayan Seismic Belt (Fig. 1A). Fault plane solutions of earthquakes indicate a shallow thrusting depth in the direction of plate convergence (Pande, 2003). Based on the pattern of
damage, the 1803 high-magnitude (N7) earthquake (Oldham, 1869) is speculated to have had its epicenter in the Alaknanda Valley around Srinagar town (Rajendran and Rajendran, 2005) (Fig. 1B). Following this, based on the earthquake catalog, an earthquake of unknown magnitude occurred in 1816 around Gangotri (Bhagirathi Valley). In recent times, the 1991 Uttarkashi and 1999 Chamoli earthquakes of magnitude N 6 are attributed to slip along the Main Himalayan Thrust (Basement thrust) which reactivated local faults in the region (Kayal et al., 2003; Kayal, 2010). 3. Facies analysis of the Quaternary deposits Late Pleistocene to Holocene river-borne and debris-flow sediment constitutes the major landforms that are preserved in the form of flights of valley-fill terraces in the Alaknanda Valley (Pal, 1986; Sati et al., 2007; Ray and Srivastava, 2010; Juyal et al., 2010). The oldest valley-fill in the study area is dated to ~40 ka. However, major valley-fill aggradation occurred after the Last Glacial Maximum (Ray and Srivastava, 2010; Juyal et al., 2010), corresponding to the gradual strengthening of the Indian Summer Monsoon (Juyal et al., 2010). In addition to this, fluviolacustrine and debris-flow deposits are also observed and are dated to ~ 14 ka and ~ 45 ka respectively (Sundriyal et al., 2007; Juyal et al., 2010). Multiple fluvial sequences investigated in the study area show dominance of gravely and sandy facies (Ray and Srivastava, 2010; Juyal et al.,
Fig. 3. SSDS in lake section at Chauras near Srinagar. (A) Position of the relict lake deposits in the fluvial section. (B) The section showing the positions of different SSDS. (C) Clay beds with load structures (l—load cast; f—flame structure). (D) Closer view of load cast. (E) Photograph and sketch of pendulous structure (p).
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2010; Chaudhary et al., 2015). Textural analysis of terrace sediments in the study area suggests dominance of gravely facies, followed by coarse sandy facies with minor contribution of silty-clay and locally-derived slope wash debris (Figs. 2 and 3). Terrace gravel occurs as valley-fill (~40 m thick) overlying the undulatory and beveled bedrock platforms. In places the deposits plug relict channels. The gravely facies represents channel bed-forms, whereas the coarser sandy facies indicate pointbar deposits, thus indicating laterally fluctuating hydrological conditions (Juyal et al., 2010). The gravel size varies between 5 and 30 cm. However, in places boulders N1 m diameter are embedded in the terrace gravels (Juyal et al., 2010). Texturally, the gravel facies are dominantly well rounded, clast supported, moderately to well sorted and imbricated. However, assorted assemblages with no imbrication and coarse sand matrix are also found, particularly towards the upper part of the valley-fill sequences; they are often mixed with angular locally-derived lithoclasts (slope wash/debris). Sandy facies are relatively thinner than gravel facies and are more prominent in the younger valley-fill sediments (Ray and Srivastava, 2010; Juyal et al., 2010). Compared to this, the fine sand and silty-clayey facies are observed in areas having high valley width-to-depth ratio (Wasson et al., 2013). The subordinate silty-clay facies are associated with fluvial and debris-flow terraces and represent temporary ponding conditions (Sundriyal et al., 2007). The sediments are dominated by planar, relatively thin beds (decimeter scale) of fine sand to clay (Fig. 3) and often contain plant remains. The ponding is ascribed to the obstruction caused by landslides/slope-debris (landslide-dammed lake, sensu Sundriyal et al., 2007). At higher elevations in the study area, lacustrine facies dominated by varves and rhythmites (Juyal et al., 2009) are associated with obstruction caused by moraines in the proglacial zone (Fig. 9). Fine sandy to clayey-silt facies form the youngest terrace at wider segments of the valley and at some sheltered locations. These deposits are usually associated with high-magnitude past floods in the study area (Wasson et al., 2013). They are fining-upward sequences with coarse to medium sand at the bottom and often clayey-silt at the top. Such sequences often exist in multiple stories, each representing a flood event. Debris-flow facies containing fragmented angular gravels with slope materials are well developed on valley slopes at higher elevations than the fluvial terraces. They form the oldest (preserved) sediment, often in the form of moderately developed terraces, and stratigraphically the oldest preserved sequence in the Alaknanda Valley. Topographically, the lithofacies discussed above overlie uneven bedrock (sloping towards the valley), unconfined and lying well above the present water table. The characteristics of the major portion of the fluvial sediments (lithofacies analyses) suggest that they traveled a long distance before deposition. Deposition occurred in a high-energy environment (under high flow regime) in the form of channel-fill which often waned and resulted in matrix-dominated gravels followed by sandy facies (Chaudhary et al., 2015). These conditions prevailed in the valley after the Last Glacial Maximum, corresponding to the intensified Indian Summer Monsoon period (Juyal et al., 2010). In addition to this, in a few places the fine sandy to clayey-silt facies can be attributed to extreme hydrological conditions; the age of these deposits is seldom more than 1 ka (Wasson et al., 2013).
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4.1. Load structures 4.1.1. Simple load structures These are the most common structure, formed when an upper layer sags or penetrates crudely into an underlying layer. Also called load cast structures, they are observed in both the fluvial and fluvio-lacustrine deposits at sand–silt, sand–mud and sand–sand interfaces. Their sizes and amplitudes vary from 2 to 10 cm. They are best observed in a fluvio-lacustrine deposit near Chauras, at a silt–mud interface (Fig. 3D–E at location 8 in Fig. 1A).
4.1.2. Pendulous load structures Pendulous load structures occur in lake deposits and are mainly associated with simple load structures. They appear as a bulbous body as described by Owen (2003) and Neuwerth et al. (2006). Their sizes vary from a few centimeters to 10 cm in length and 5 cm in diameter. Fig. 3E shows a typical pendulous load structure preserved in a relict Holocene fluvio-lacustrine deposit near Srinagar (location 8; Fig. 1A) as a bulbous structure of ~8 cm diameter of sand penetrating into underlying carbonaceous clay–silt.
4.1.3. Ball structure Small-scale ball structure occurs at sand–silt interfaces and, occasionally, at sand–sand interfaces. A typical ball structure associated with a sand–sand interface occurs in the terrace deposit at Swit near Srinagar (Fig. 4; location 6 in Fig. 1A) and can be identified by the color contrast in homogeneous sandy deposits.
4. Description and interpretation of soft-sediment deformation structures (SSDS) Various types of SSDS occur in the terrace sequences, particularly those proximal to the river channel, i.e. terrace T1 and sub-recent deposits. Morphologically, these structures can be grouped under the major categories recognized by Owen (2003) and Neuwerth et al. (2006).
Fig. 4. Field photograph and sketch of ball structure in the fluvio-colluvial deposit near Srinagar.
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4.1.4. Flame and diapir structures Flame and diapir structures are the flame-like penetration of underlying sediments into overlying layers. They develop at sand–silt and silt–clay interfaces in the lacustrine deposits (Fig. 3C), where they are often associated with other load structures. However, they occasionally occur at sand–sand interfaces in fluvial deposits around Gauchar (location 3; Fig. 1A) and Srinagar (Fig. 5A–C). Their sizes (amplitudes) vary from a few centimeters to ~15 cm. 4.1.5. Interpretation Simple load casts, one of the most common SSDS, occur in unstable discontinuous layered systems (Allen, 1982; Owen, 1996a) at the interface of denser and underlying less dense sediments, often when the sediment loses its strength (Owen, 2003; Moretti et al., 1999 and references therein). In the study area these structures are observed in fluvio-lacustrine and fluvial–flood deposits where denser sediment overlies less dense layers. Partial liquefaction and thixotropy might have reduced the strength of sediment and caused gravitational instability. However, in cases where the upper layer is finer or of nearly the same grain size, uneven loading seems the most reasonable cause for the development
of load structures. Pendulous load casts represent a more advanced stage of deformation in a reverse density gradient system and can thus develop from simple load casts by a continuous deformation process (Owen, 2003). In the study area these structures are found in fluvio-lacustrine sediments within the overall fluvial environment. Ball (like) structures of silt or silty-sand are indicative of a reverse density gradient experiencing a longer-lasting deformation mechanism or simply detachment of the sediment from overlying layer. Flame and diapir structures usually form due to a reverse density gradient, difference in porosity and shear stress (Mills, 1983; Mohindra and Bagati, 1996). 4.2. Folded laminations 4.2.1. Deformed laminae A few small-scale sand pseudo-dikes are associated with the highly folded silty-sand beds on the left bank of the Alaknanda River at Kirtinagar (Fig. 6; location 9 in Fig. 1A). In places the sand layer penetrates the overlying silty-sand layer. The laminated sand of the lower layer is disorganized (disrupted), whereas the upper layers show undisturbed parallel stratification followed by cross-bedding (Fig. 6). The
Fig. 5. Flame and diapir structures. (A) & (B) Flame and diapir structures at sand–sand interface. (C) Flame and diapir structures at sand–silt interface.
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Fig. 6. Folded laminae in the lowest terrace at Kirtinagar. The deformed laminae are overlain by cross-stratified fine sand beds. The layer shown in darker gray lost all primary structures and seems to have liquefied during deformation. Note the indications (arrows) of forceful upward movement of fluidized sand. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
disturbed and folded laminae overlie medium-grained sand with multiple load structures. 4.2.2. Complex structures Complexly folded laminae of alternating sand and silt occur at a depth of around 2 m from the youngest terrace top, and are overlain by silty-sand layers on the right bank of the Alaknanda River near Gauchar (Fig. 7; location 3 in Fig. 1A). They occur in finely laminated sand and silt with clay intercalations and differ from the ball structures due to the complex folding. 4.2.3. Interpretation Folded laminae are formed by current drag or shear stress (Mills, 1983; Owen, 1996a). They occur mostly in the silty-sand layers of floodplain deposits formed during floods in the Alaknanda Valley and include structures varying from simple folds to complex flames (Figs. 12 and 13). The folded laminae indicate that a sequence of sandy silt layers was liquefied, broken and dragged by a shear stress provided by sediment-laden flood-water (for discussion see Owen, 1996a). Deformed layers restricted within parallel, undeformed beds indicate sudden surges during floods (flash floods). These surges may have occurred during the breaching of landslide-dammed lakes during high rainfall events in the upper reaches of the valley. The complex morphology of the structures is attributed to the interaction of several shear-stress mechanisms interact (Lowe, 1975). However, turbulent river flow at this confined location may provide multi-directional shear stresses which may combine with gravitational effects to deform the laminae in a complex form (Lowe, 1975). 4.3. Water-escape structures Water-escape structures occur in sand beds at the bottom of a fluvial terrace composed of sand, silt and gravel ~8 m thick near Rudraprayag
(location 5; Fig. 1A). A sand layer underlain by boulders shows flamelike features with ~15 cm amplitude formed by the alignment of sand grains of darker shades (Fig. 8).
4.3.1. Interpretation Water-escape structures include a variety of structures formed by fluid escape in unconsolidated sediments due to many reasons (Lowe, 1975). Owen (1996b) described water-escape structures driven by fluidization in high-energy fluvial deposits. In Fig. 8 the orientation of sand grains and laminae clearly indicates the forceful upward movement of fluid. Water-escape was probably driven by uneven loading exerted by boulders when the sand layer was temporarily fluidized.
4.3.2. Slump folds A sequence of clay–silts and sand exhibits close, small-scale folding in otherwise uninterrupted layers (Fig. 9) of a relict lake succession developed behind a recessional moraine in the upper catchment of the Alaknanda River near Bardrinath (location 1; Fig. 1A). The folds exhibit unidirectional orientation (isoclinal tight folds) with inclination up to 10°. The relict lake succession generally shows alternating layers of coarse and fine sediments (rhythmites) varying from clay to medium sand, with a sequence of distinct glacial varves at the base.
4.3.2.1. Interpretation. The unidirectional orientation of the folded beds of sand and silt in the fluvio-lacustrine deposit (Fig. 9) may be the outcome of a sheet slump. Such folded sheets of sand and silt have been reported from subaqueous glacially-influenced settings, even on lowangle slopes. The folded layer is interbedded with undisturbed layers, which is typical of slumping (Allen, 1982).
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Fig. 7. Field photograph and sketch of complex deformation structures in the youngest silt-dominated sand deposit near Gauchar.
4.4. Brittle deformation in sediments 4.4.1. Ground fissures Ground fissures with no appreciable vertical offset occur in massive silty-sand in the lower reaches of the Alaknanda River around Khandukhal (location 10: Fig. 1A). The fissures developed on relict overbank facies resting on gravels. Morphologically, they are vertical to near vertical and extend up to 5 m below the surface (Fig. 10). Fissures are filled either with clean gray-colored medium-grained sand (Fig. 10B) or with mixed muddy sand and local debris including fragments of the local phyllite rocks (Fig. 10A). 4.4.2. Small-scale (sedimentary) faults As well as the well aggraded terraces, fluvial aggradation along the Alaknanda Valley also occurs in discrete patches resting on the valley slopes. Normal listric faults (Fig. 11) with offsets up to ~7 cm occur in sand–silt beds of fluvial origin in the upper and middle Alaknanda Valley at Birehi and Rudraprayag (locations 2 and 4 respectively; Fig. 1A). They occur on valley slopes which are susceptible to failure. 4.4.3. Interpretation Earthquakes of magnitude Mw N6 have the ability to produce ground deformation (Caputo, 2005). Ground cracks or fissures are long, tensile
deformation structures that occur in the ground with or without vertical offset (Carpenter, 1993; Ayalew et al., 2004). They were recorded from the 1989 Loma Prieta (by Sims and Garvin, 1995) and 1999 Chamoli (by Rajendran et al., 2000; Sarkar, 2004) earthquakes. They may develop as a consequence of local ambient stress fields developed during an earthquake (Sarkar, 2004), lateral spreading due to liquefaction (Sims and Garvin, 1995), or withdrawal of pore water and subsequent shrinkage of silty-sand (desiccation cracks). Normal faults are common in the gravitationally unstable system. In the study area these faults are very small and occur on the discrete sand–silt patches which have experienced unequal slip along the slopes.
4.5. Deformation structures in flood sediments 4.5.1. Gauchar A 3 m thick and ~300 m long succession of three alternating sand– silt layers along the bank of the Alaknanda River in Gauchar shows laterally extensive folded laminae (Fig. 12). Three distinct intervals show crude fining upwards to silt layers, implying that deposition occurred in a sheltered location during low flow conditions. The otherwise parallel laminated planar beds of silty-sand are deformed into simple anticline, syncline and isocline folds and complex recumbent folds and flame-like structures. Laminae are often broken and appear massive in
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 8. Water-escape structure in the sand beds of a fluvial terrace near Tilwara. Arrows indicate paths of fluid movement.
Fig. 9. Small-scale folds (slump folds) in the relict glacial lake near Badrinath. Note the unidirectional orientation of the fold and small fault. F—fault; Fo—Fold.
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Fig. 10. Field photograph and sketches of ground fissures in Quaternary sediments at Khandukhal. (A) A fissure ~5 m deep in a thick silty-sand layer, filled with sand and angular clasts. (B) A wide fissure filled with medium gray sand. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the upper part whereas they gradually form simpler structures and finally undisturbed lamination in lower parts (Fig. 12). 4.5.2. Srinagar During a June 2013 flash flood, a sand layer ~5 m thick was deposited around Srinagar town in the floodplain of the Alaknanda Valley
(Rana et al., 2013b). At one location (Bhainswara village, location 10: Fig. 1A), a fine silty-sand horizon ~ 0.5 m thick shows folded layers of varying geometry. This deformed horizon is 1 m below the top surface of the deposit and is underlain and overlain by undeformed, parallel sandy layers (Fig. 13). The magnitude of deformation decreases downwards. The morphology of the deformed laminae varies from simple
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 11. Field photograph and sketches of brittle deformation structures (fault) in Quaternary sand deposits. (A) Small-scale fault in sand beds near Rudraprayag. (B) Similar fault in sand beds near Birehi.
folds to sharp flame-like notches and complex fold structures similar to the deformation around Gauchar (Fig. 12). The amplitude of the structures varies from a few to 30 cm. 5. Trigger mechanism Seismicity has been identified as a principal trigger for soft-sediment deformation (Rosetti, 1999), although storm waves, rapid loading, flood surges and gravitational density flows are also potential triggers (Jones and Omoto, 2000; Owen et al., 2011; Owen and Moretti, 2011). Although criteria for distinguishing seismic from aseismic triggers are not unequivocal, some studies have reduced the uncertainties by providing reasonable criteria for the identification of seismically triggered SSDS (e.g. Sims, 1975; Rosetti, 1999; Moretti and Sabato, 2007; Pandey et al., 2009; Owen and Moretti, 2011 and references therein). Most of the SSDS reported here (excluding brittle deformation) seem to have experienced partial liquefaction and fluidization, particularly the load structures and water-escape structures, which suggest partial liquefaction and fluidization of underlying layers. The presence of massive gravel and sand layers suggests that overloading may be an important trigger for deformation in the underlying, relatively finer grained sediment. Deformation was further facilitated by elevated groundwater conditions during high-discharge periods. Thus, the high discharge associated with high sediment flux was responsible for overloading and subsequent deformation in the saturated sediments (Obermeier, 1996; Moretti et al., 2001; Owen, 2003). Moretti et al. (2001) established a relationship between the thickness of overloading and the depth of liquefaction, and demonstrated that even a small thickness of overload can cause liquefaction in substrate layers at a depth comparable to the overload thickness. It has been suggested that during earthquake shaking, liquefaction may fail if sediments are not saturated (Obermeier, 1996). In mountainous terrain like the Alaknanda Valley, the Quaternary deposits are
devoid of groundwater due to their unconfined nature, open riverward side and higher elevation from the river bed. It is therefore very unlikely that the terrace sediment would experience liquefaction from earthquake-induced shaking. Ground fissures similar to those reported here often occur in the mesoseismal zones of large earthquakes (Sims and Garvin, 1995; Rajendran et al., 2000; Sarkar, 2004). At Khandukhal, they are deep (~ 5 m), occur in relatively stable ground, and are filled with locallyderived secondary deposits, indicating that they post-date deposition of the silty-sand and hence cannot be interpreted as desiccation cracks. Their size, morphology and, most importantly, their occurrence in a mesoseismal zone of past earthquakes (1803 Srinagar earthquake sensu Rajendran and Rajendran, 2005) may qualify them as seismites. However, the small-scale (sedimentary) faults appear to have been generated by local factors such as slope instability caused by active riverbank erosion (Obermeier, 1996). Relict lake deposits are an ideal repository for seismites (Hibsch et al., 1997) because alternative, internal trigger mechanisms (sensu Owen and Moretti, 2011) are generally absent. The slump folds observed in the relict proglacial lake deposit at Badrinath (Fig. 9) occur in rhythmites dominated by silty-clay and overlain by coarse- to medium-grained fluvial sand. We suggest that the deformation was generated by turbulence caused by the change in the environment from the calm proglacial lake to the high-energy fluvial environment. The (complex) folded laminations were undoubtedly formed during recent floods. They occur in places where the valley width-to-depth ratio is high and the river slope is gentle. At such locations the suspended load settles to form typical fining-upward sequences, generally capped by clay. Flooding in mountain areas is generally episodic and wanes away in a day or two. Furthermore, floods typically result from the pooling and breaching of river flow, forming surges further downstream. These episodic surges act as a shear wave over the freshly deposited sediment and drag it in different directions. This complex
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 12. SSDS in the older flood deposits at Gauchar. The deformed beds exhibit a variety of morphology from simple folds to complex convolutions.
drag, along with liquefaction of the underlying layer, produces complex deformation (complex structures). As mentioned earlier, the study area is traversed by many faults and thrusts of local to regional extent (Fig. 1). These and the occurrence of earthquakes of Magnitude 6.8 (1991, Uttarkashi), 6.6 (1999, Chamoli) and the historically documented 1803 Srinagar earthquake indicate that the valley has been seismically active (Cotton et al., 1996; Rajendran et al., 2000; Rajendran and Rajendran, 2005; Kayal, 2010). In spite of this, the morphological observations and facies analysis of the SSDS do not show any genetic bias towards their seismogenic origin; in contrast, turbulent stream flow, slope instability and uneven or overloading seem to be the factors responsible for their development. 6. Discussion and conclusion We have concentrated on the recognition of SSDS in Late Quaternary sediments dominated by fluvial sequences preserved in the Alaknanda Valley, mainly focusing on a context-based approach (sensu Owen and Moretti, 2011). Our study indicates that, due to textural and hydrological constraints that prevent the development of high pore water pressure during seismic shaking, most of the late Quaternary–Holocene sediments are devoid of SSDS. However, the sporadically distributed SSDS documented here were formed by liquefaction (or partial
liquefaction), thixotropy and to some extent fluidization under the influence of various driving forces including differential loading, gravity slides and current shear. Although the terrain lies in a seismically active zone, the SSDS are mainly aseismic in origin, triggered by hillslope movements, overloading and high shear stress during flash floods. Brittle deformation mostly occurred on unstable slopes (normal faults) but ground fissures (Fig. 10) were probably seismically formed during some large earthquake like the 1803 Srinagar earthquake. Since the sediments are neither confined nor saturated, we discard the possibility of liquefaction during earthquake shaking, indicating that the absence of SSDS in such a seismically active environment cannot be interpreted as a negative indication of paleoseismicity. The limited seismogenic SSDS may not be due to activity on local structures such as the Tons Thrust which traverses the study area around Srinagar, but may be manifestations of far-field effects of high-magnitude late Quaternary earthquakes. However, the majority of the SSDS described here are aseismic in origin, forming in response to sudden loading, saturation and shear stress induced by flood surges related to episodic high-magnitude floods in the Alaknanda Valley, which are known to affect the study area on a decadal scale (Wasson et al., 2013). This study cautions that care must be taken in interpreting soft-sediment deformation structures as sedimentological expressions of paleoseismicity, even in seismically active regions.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Fig. 13. SSDS observed immediately after the 2013 flood in the Alaknanda Valley. Note the similarity of the structure (s) to Fig. 7.
Acknowledgments We thank DST and Ministry of Earth Sciences, India for financial support by research grant (DST/23(570)/SU/2005). The constructive
comments and suggestions by the guest editor Dr. Pedro Alfaro and reviewers: Dr. M Moretti and Dr. Juan Pedro Rodríguez-López helped immensely in the improvement of the manuscript. We are thankful to Dr. Owen for improving the readability of the text.
Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012
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Please cite this article as: Rana, N., et al., Genesis and implication of soft-sediment deformation structures in high-energy fluvial deposits of the Alaknanda Valley, Garhwal Himalaya, India, Sedimentary Geology (2016), http://dx.doi.org/10.1016/j.sedgeo.2016.06.012