Recognizing deformation origins: a review of deformation structures and hypothesis on the perspective of sediment consolidation

ABSTRACT Key information on sedimentary or tectonic events is recorded in deformation structures formed in unlithified and lithified sediments. Disputes about the classification and identification of the two types of deformation have become increasingly relevant. The present study systematically summarizes, based on consolidation states, the genetic mechanisms of deformation. Consolidation conditions may affect deformation patterns and morphology; this can be a clue to distinguish soft-sediment deformation from tectonic deformation. Liquefaction is a typical state of unconsolidated sediment and can create clastic dikes, liquefied breccia, convolute laminae, load cast, and water-escape structures. Synsedimentary faults may be formed in weakly consolidated sediments. Most deformation structures of lithified sediments are large-scale folds and faults, but small-scale structures – especially microfolds – are difficult to distinguish from slump folds. Tectonic folds can be formed in different strata and induced by tectonic events; they differ from slump folds in morphology, distribution, and related structures. We demonstrated that consideration of liquefaction, folds in different strata – matched with the regional geological regime, related deformation structures, and micro-deformation structures – can be clues to the identification of deformation origins.


Introduction
Water-saturated sediments covered by overlying deposits gradually consolidate as the water content decreases (Yang and Van Loon 2016;Qiao et al. 2017;Liu et al. 2020). Transition from the synsedimentary period to the diagenetic stage is gradual Liang et al. 2018a). Deformation structures are formed during the processes of deposition, consolidation, and diagenesis by external forces and tectonic events (Elliott and Williams 1988;Qiao et al. 2016). The deformation structures in unlithified and lithified sediments result from soft-sediment deformation (SSD) and hard-rock deformation (HRD, also called tectonic deformation), respectively (Alsop et al. 2019a). SSD structures can be regarded as a record of the sedimentary transformation process from sedimentation to diagenesis (Van Loon 2009;Waldron and Gagnon 2011;Feng 2017a).
SSD structures are widespread phenomena in a variety of tectonic settings and sedimentary environments, including deltas, lacustrine deposits, shallow coastal deposits, and glaciers (Moretti et al. 2001;Hilbert et al. 2016;Rana et al. 2016;Shanmugam 2017aShanmugam , 2017bFeng 2017b;Zhong et al. 2020). As common sedimentary structures, they have intrigued geologists for over 100 years (Seilacher 1969;Van Loon 2009;Williams et al. 2012;Alsop et al. 2013;Rodríguez-Pascua et al. 2016). The 27th International Congress on Sedimentology and the 4th International Conference on Palaeogeography both discussed SSD structures and their significance as a special topic. In addition, a special issue of Sedimentary Geology (Volume 344) on 'The environmental significance of softsediment deformation' with 26 contributions has been published. SSD structures are induced by complex genetic mechanisms and vary in morphology and scale (Van Loon 2009;. Many processes can trigger SSD including earthquakes, slop instability, volcanic activity, tsunami, gas escape, sediment loading, and other origins (Matsumoto et al. 2008;Oliveira et al. 2009;Sarkar et al. 2011;Alsop et al. 2016;Rana et al. 2016;Sherrod et al. 2016;Liang et al. 2018b). The same deformation structures or similar morphologies can be induced by different triggering geneses. Therefore, different viewpoints on the origins of deformation structures affect the understanding of the sedimentary environment (Elliott and Williams 1988;Waldron and Gagnon 2011;Qiao et al. 2016;Alsop et al. 2019a).
SSD structures preserved in the strata may also be overprinted by later tectonic deformation. Simultaneously, small-scale tectonic deformation structures, especially contractional folds, are morphologically similar to slump folds. This makes identification of contractional folds and fabrics problematic (Qiao et al. 2017;Alsop et al. 2019a). Misidentification of deformation structures in unlithified sediments and lithified rocks may lead to a confused interpretation of the deformation origin and regional geological setting (Elliott and Williams 1988;Yang and Van Loon 2016). Therefore, the identification of soft-sediment origin versus tectonic origin is important for correct interpretation of tectonic processes, and it is also one of the key scientific problems to be solved in the present study of SSD. The crux of misidentification and debate is that similar deformation structures can be formed in both unlithified sediments and lithified rocks. Establishing criteria to identify the deformation mechanisms has therefore become key to resolving this debate. Unfortunately, clear criteria and indicators for recognizing SSDs and subsequent tectonic deformation are lacking.
Water-saturated sediments gradually consolidate with a decrease in water content during sedimentation, burial, and diagenetic processes (Van Loon 2009;Qiao et al. 2017). Deformation structures vary in the gradual process depending on sediment competence (resistance to flow) and on the physical properties resulting from the differing amounts of water contained (Yang and Van Loon 2016). Therefore, the consolidation state or water content may be clues for differentiating the deformation structures of unlithified sediments from those of lithified rocks. Most researches on deformation structures focus on their identification, origin, and classification, ignoring the influence of their consolidation states (Owen 1996;Frey et al. 2009). The aims of this paper are therefore: (1) to briefly summarize SSD structures of different origins; (2) to advance the hypothesis that water content may affect deformation structures and may therefore be a clue to recognizing SSD structures; and (3) to propose some indicators for distinguishing SSD from HRD.

Definitions and current research status of deformation structures
SSD occurs at the surface or sub-surface sediments during or after deposition, but before significant diagenesis (Maltman 1984;Waldron and Gagnon 2011;Alsop et al. 2022). Tectonic deformation structures are formed by subsequent tectonism in lithified successions or more deeply buried rocks. These two types of deformation can continue throughout a long geological history. Common structures preserved in deformed sedimentary rocks include load casts, slump folds, liquefaction-induced deformation structures, large-scale folds, faults, and other structures (Lowe 1975;Owen 1996;Onorato et al. 2016;Van Loon et al. 2016). It has been described in the literature that when SSD occurs, the detrital grains in the unconsolidated sediment are not cemented Waldron and Gagnon 2011). Conversely, the grains are cemented during tectonic deformation, some deformed rocks that have undergone metamorphism may even occur with the appearance of metamorphic minerals.
Deformation structures characterized with varied morphology and complex driving forces have concerned geologists for many decades . Significant progress has been made in classification of deformation structures, genetic mechanisms, triggering factors, and experimental simulations (Ross et al. 2011;Zeng et al. 2019;Rodríguez-López and Wu 2020). In general, classification of SSD has been variously based on the morphological characteristics of deformation structures (Lu et al. 2003), deformation mechanisms (Van Loon 2009), and driving forces (Sun et al. 2018). These approaches have resulted in different division schemes (Table 1). However, while similar deformation structures may be attributed to different mechanisms, they may be in the same consolidation state.
Certain triggers are required for SSD.  noted that there must be a driving force to change the primary sedimentary features, a deformation mechanism to change the sediments to a deformable state, and a trigger to initiate the deformation. Earthquakes are common events in geological history. Energy transferred by earthquakes can induce instability in unconsolidated sediments and trigger deformation; this leads most SSD structures to seismic interpretation in early researches ( Figure 1). In recent years, sedimentary geologists realized that some SSD structures are non-seismic origin and multi-origin (Moretti and Ronchi 2011;Feng 2017a).

Morphology
Fold, fault, fabrics, boudinage, etc. Ramsay 1967;Delitsin et al. 1968;Bons et al. 2004;Goscombe et al. 2004;Festa et al. 2010;Dabrowski and Grasemann 2014;Qiao et al. 2017;Hagke et al. 2018;Liu et al. 2020. owing to seismic disturbances. The most common ones are liquefied structures (Figure 2(a)). These deformation structures are generally preserved in thin strata (Qiao et al. 2017). However, frequent repetition of deformed layers may exist in the vertical distribution, indicating the deposition characteristics of seismic events (Lu et al. 2011). When the water-saturated sediments liquefied by earthquakes, the homogeneous mixtures can intrude into the unconsolidated sediment, puncture the laminated sediments, and form sand volcanos, clastic dikes, diapirs, convolute laminae, and water-escape structures. In the sediment of the reversed density inversion, earthquakes may create ball-and-pillow structures and load casts (Figure 2(a)). Deformation structures of seismic origin can be affected by the seismic magnitude and distance from the depocentre (Rodríguez-Pascua et al. 2000). Many deformation structures in the deformed strata are good indicators of seismic origin, but the occurrence of deformed layers between undeformed layers is one of the strongest arguments, especially when the deformed and undeformed layers have similar grain sizes (Yang and Van Loon 2016).

Freeze and thaw cycles
Deformation structures induced by freeze and thaw cycles are mainly non-interlaminar, low-velocity, and low-pressure intra-laminar displacements of clastic grains (Wang et al. 2018a;Zhong et al. 2020). Typical deformation structures include clastic dikes, diapirs, and folds ( Figure 2(c)). The deformation structures are mainly formed by periodic changes in warm and cold climates. During the warm period, the liquefied clastic layer and increased pore-water pressure induced by melted permafrost make it possible for clastic particles to move upward or laterally (Wang et al. 2018a). Under the above condition, they may form water-escape structure and clastic dikes (Zhong et al. 2020). During cold periods, permafrost expands and makes the surrounding sediments broken, forming folds, faults, veins, and other structures in the strata (Wang et al. 2018a). In contrast to those of seismic origin, deformation structures induced by freeze-thaw cycles are characterized by low- . Scale is arbitrary. a. Liquefaction induced by earthquake can form convolute laminae, clastic dikes, and sand volcanos. b. Unlithified sediments slide along the slope and form varied deformation structures. c. Clastic dikes, diapirs, and folds can be induced by freeze and thaw cycles in cold climate conditions or periglacial tundra. d. Rapid deposition of soft-sediments is another typical cause of liquefaction. e. Gas moving and accumulation in deltaic plain can form discharge pit and dome. temperature wedges, frozen fractures, and large deformation scales, and they are not bound in the single layer (Zhong et al. 2020). The deformation structures mentioned above are mostly preserved in periglacial tundra or places with cold climate.

Rapid deposition of soft-sediments
The rapid deposition of soft-sediments mainly occurs in delta and turbiditic deposits (Moretti et al. 2001;Zhong et al. 2004). Overloading of underlying layers induced by rapid accumulation of clastic particles results in temporarily transferring of grain weight to pore fluid, this induces the overpressure of pore-fluid and drastic decrease in intergranular shear strength, finally liquefaction and fluidization are induced Liang et al. 2018a). Once the water-saturated sediments are liquefied, deformation is driven by the tangential shear stress, forming structures with peculiar morphologies, notably including the water-escape cusps, convolute laminae, pillar structures, and clastic dikes (Lowe 1975;. Also, a special deformation involved in fine-grained turbidite induced by high sedimentation rate is interpreted as dome-shaped structure by Liang et al. (2018a). The dome-shaped structures show thickened laminae from their sides towards the central upper part of the structure (Figure 2(d)). A remarkable feature is that the overlaying sandstone sunk into the sag of the underlying laminae. The small energy of water-saturated mixture cannot penetrate overlying layers and form domeshaped accumulation. Preferred orientation of dome axial planes may indicate the palaeocurrent direction in the case of turbidity influence (Liang et al. 2018a).

Gas flow
Soft-sediments can also be deformed by gas escapes. Gas moving along loose parts or fissures can create discharge pits on the surface (Boudreau 2012). Some gas may be trapped by surrounding sediments that confine the increasing upward velocity that can occur close to the surface. The trapped gas gradually expands, and when the pressure is sufficiently large, it can form oval-and balloon-shaped inflation structures. Hilbert et al. (2016) demonstrated that when balloon-shaped bubbles near the surface burst or ejected, ground cracks and small explosion pits can be preserved (Figure 2(e)). These balloon-shaped inflation structures and surface fractures are different from water-escape structures in morphology and spatial configurations. Vertical pipes and concentric layered structures on the surface of the Yellow River delta plain are also generally considered to be formed by gas escape (Zhong et al. 2004). The mentioned gas escape structures can be triggered by volcanic activity, seismic activity, and intermittent flow interruptions (Zhong et al. 2004;Hilbert-Wolf et al. 2016;Sherrod et al. 2016).
In addition to the above origins, storm-induced wave movements, volcanic activity, tidal range, and even human activity can induce SSD (Bryn et al. 2005;Wang et al. 2018b). Determination of the deformation origin requires a comprehensive consideration of depositional environment, geological setting, and special deformation structures. However, different deformation structures may be formed in specific consolidation states or diagenetic stages.

Deformation structures of different consolidation states
A continuous process exists between the watersaturated sediment to lithified rock (Elliott and Williams 1988;Van Loon et al. 2013). The competence of the sediment in this complex process may be changed by the water content, which may affect the morphology and pattern of deformation structures. Previous research already demonstrated that different state of compaction or lithification can result in certain deformation structure based on the characteristics of deformation structures Yang and Van Loon 2016;Zeng et al. 2019; Rodríguez-López and Wu 2020) and experimental simulation (Owen 1996;Dasgupta 2008;Frey et al. 2009;Ross et al. 2011). However, neither consolidation nor lithification represents a clearly defined state (Davies 2005;Yang and Van Loon 2016). This would be more complex in a succession with different lithology because of the varied consolidated rate -some layers may be consolidated, while their overlying or underlying deposits are still in unlithified state (Alsop et al. 2022). Special state exists between water-saturated sediments and lithified rocks. Therefore, we interpret the consolidated state as water-saturated, weakly consolidated, and completely consolidated when deformation occurs. The hypothesis that consolidation state of sediment will affect the morphology of deformation structures is proposed, and the various deformation structures may be formed in certain consolidation state.

Deformation structures of water-saturated sediment
Water-saturated sediments are in unstable state which is primarily required to the formation of SSD structures. Water in sediment pores -influenced by overlying low-permeability sediments -can generate pore fluid pressure, resulting in the loss of internal cohesion and support strength Qiao et al. 2017;Liang et al. 2018a). Relationship between pore fluid pressure and cohesive strength due to grain weight was described by Lowe (1975) and reviewed by Alsop et al. (2022). When the fluid pressure is less than cohesive strength, the hydroplastic deformation such as fold may be formed in primary bedding. These hydroplastic deformation structures share a close morphological similarity with structures in tectonic deformation and metamorphic rocks (Alsop et al. 2019b(Alsop et al. , 2022. Liquefaction occurs when grain weight is equal to fluid pressure and is temporarily transferred to the pore fluid, resulting in consequent destruction of bedding (Obermeier et al. 2005;. The liquefied sediments can deform the existing stratification and produce pervasive structures (Owen 1996). Fluidization occurs when fluid pressure exceeds grain weight in which the primary bedding is destroyed by turbulent flow and overlying sequences injected by fluidized sediments, producing water-escape cusps, pillar structures, and clastic dikes (Lowe 1975;Li et al. 2008;Van Loon 2009;Qiao et al. 2017;Alsop et al. 2022).

Folded laminae
Seismic vibration can form autochthonous folds in laminated sediments (Qiao et al. 2011). These folds are generally small in scale, with thicknesses of 10-40 cm and wave length of 1-100 cm; the overlying and underlying sediments of folded laminae are relatively flat, and erosion or bending of the overlying laminae is absent (Figure 3(a)). Some laminae may be crushed by the seismic stress (Figure 3(b)). However, distinct from the tectonic deformation, there may be a sediment-reworking phenomenon of the overlying laminae. The axial planes of folded laminae are disordered, and no dominant direction exists. This type of fold is interpreted as convolute lamina (Allen 1977;Jiang et al. 2016) or hydroplastic folds (Qiao et al. 2017) in some literature and textbooks. The folded laminae are possibly not in ideal liquefied state but in hydroplastic state when deformation occurs, because a homogeneous mixture of sand and mud can be easily formed if the sediments are liquefied. Under the mentioned conditions, the pore water may lead to laminar flow and destruction of bedding, even create water-escape structure, which can release the high pressure of the liquefied mixture and make it difficult to form folded laminae.

Water-escape structure
Water-escape structure is common in sands and sandstone (Lowe 1975;Maltman 1984;Moretti and Ronchi 2011). The rapid drainage of pore water in unlithified sediments changes the original grain-supported condition, displacing and rearranging the particles. Water-escape structure is mostly formed in a range of sediments with high porosity and rapid accumulation including turbidity-current deposits, delta fronts, and fluvial deposits . When the pore water escapes, the sandy laminae can be washed away by the flowing liquefied homogeneous mixtures. The laminae on both sides of the drainage vein become drawn and curved along the drainage  (Figure 3(c)). The laminae are therefore discontinuous in the transverse direction and stacked vertically with others. The disc-shaped concavity points upward.

Clastic dikes and liquefied breccia
Fluidization can create the injection of sediments into overlying sequences, creating long, narrow, and veinshaped intrusions (Alfaro et al. 2002;Qiao et al. 2017). These injected clastic dikes may cross-cut deformation structures such as folds and faults, indicating that the structure was formed in unlithified sediment. The size of the clastic dikes varies from millimetres to metres. The source sediment associated with the dikes is generally visible. Clastic dikes gradually narrow and disappear with increasing distance from the source sediment (Figure 3(d)). In addition, clastic dikes intersect the bedding at high angles (sometimes even vertically) (Qiao et al. 2017). These dikes exhibit obvious homogeneity of liquefaction induced by seismic-shaking in softsediments, which can form sand blasting, water jetting, and sand volcanoes (Figure 3(e)). Appearance of a clastic dike is a typical indicator of seismically induced liquefaction (Levi et al. 2008;He et al. 2012). Particle size of the sediment during liquefaction has a certain influence on deformation; particles below 0.005 mm exhibit liquefied flow deformation, while thixotropic flow deformation occurs above the boundary size (Su and Qiao 2018).
Original mudstone may be broken into irregular breccia when the liquefied layer intensively punctures the overlying or underlying mudstone. Irregular breccia is usually filled into the liquefied sand, which is mostly seismically induced (Figure 3(f)). When the liquefied sediment cannot penetrate the overlying sediment, liquefied diapirs can form in the shapes of cones, mushrooms, and domes. The shape of the liquefied diapirs may be controlled by the energy injected during liquefaction and the competence of the soft-sediments (Qiao et al. 2011).

Load casts
In the case of inverted sediment density, the coarsegrained sediments with high relative density subside into the underlying fine-grained sediments with low relative density, forming load casts in the underlying sediments (Figure 4(a-f)) (Moretti et al. 2001). Load casts are usually in axiolitic shape and trailing shape. Argillaceous sediments squeezed into the overlying sediments can form flame structures (Figure 4(e)). The load casts separated from the host rock tend to fall through the underlying sediment to a somewhat uniform depth, forming ball-and-pillow structures (Figure 4(f)). The width and height of the load casts may indicate subsidence depth and origin (He et al. 2012). With increasing subsidence depth, the long axis of the load cast becomes larger, the short axis becomes smaller, and the load cast becomes flatter . These changes may be the result of increased buoyancy obstructing soft-sediment subsidence. The aspect ratio of load casts induced by seismicity is greater than 1, whereas width of a load cast below 0.01 m may be induced by simple overloading (Moretti and Sabato 2007).

Slump folds
Slump folds are usually formed in a range of lithologies including clastic rocks and carbonates that are not consolidated completely (Jablonska et al. 2018;Li et al. 2019). Different from folded laminae, slump folds are often induced by slumping of unconsolidated sediments along the slope, resulting in allochthonous accumulations of a certain displacement (Figure 5(a)). Morphology of fold varies from cylindrical fold to curve fold in different parts of the sliding body. The limbs of the slump fold are mostly irregular and asymmetrical ( Figure 5(b,)). Axis of the slump fold is inclined away from the depocentre of the basin. Stereoplots of fold hinges and axial surfaces are mostly distributed along a large circle or arc Marco 2011, 2013;Qiao et al. 2017).
In some folded layers, folds and fabrics can be injected by clastic dikes (Alsop and Marco 2013;Alsop et al. 2016Alsop et al. , 2019a. Folded mudstone or aragonite layers may also be eroded and truncated with irregular surfaces by sedimentary cap (Figure 5(c,)). The folded layers are usually not thick, strata between them may be undeformed. However, there may be multiple slump superpositions. The upper sediments of the earlier slump are refolded by later slump, even forming multiple and undistinguished deformation structures, but the folds and fabrics can be preserved in the lower sediment (Lu et al. 2011).
While imbricating thrust ramps are previously recognized in orogenic systems, Alsop et al. (2021) describe similar fold duplexes from a gravity-driven fold and thrust system (FATS). In the system, different layers are characterized with various shortening amount. Fanning crowns of folds can be formed above the upper detachment ( Figure 5(d)). It was speculated that expelled fluids may pond directly beneath overlying detrital-rich units that act as baffles and locally increase fluid pressure, thereby facilitating further movement along the upper detachment (Alsop et al. 2021).
The injected clastic dikes are directly linked to the underlying source layers, which may also show upward movement of the particles and internal flow banding parallel to the dike margins. However, intrusive dikes may also inject fissures and cut folds in lithified rocks induced by tectonic deformation (Palladino et al. 2016). Fold at the toe of the slump may be morphologically  similar to the sheath fold, which is usually formed in the deep ductile shear zone Qiao et al. 2017). Therefore, the recognition of slump fold and tectonic fold is difficult and controversial.
In addition to the above deformation structures formed in water-saturated sediment, more complex and atypical structures can be found in outcrop. In general, fluidization and liquefaction are common condition in which soft-sediments are deformed. However, they are not essential condition and not involved in some deformation structures such as synsedimentary faults, boudinage, and others (Liang et al. 1991;Yang and Van Loon 2016;Sun et al. 2018).

Deformation structures in the weakly consolidated state
Water-saturated sediments gradually tend towards a consolidated state after deposition (Alsop et al. 2022). Special state exists between saturated sediment and lithified rocks, but this is not a clearly defined state (Yang and Van Loon 2016). The sediments consolidated to such a state that the weakly consolidated sediment may behave in a special way and form synsedimentary faults.
Synsedimentary faults are minor faults confined within thin strata, and they may create graben structure (Figure 4(g)). Most fault planes are regular fractures parallel to others, but the overlying and underlying strata of the faulted layer are not fractured (Figure 4(h-j)). Although these faults can occur in unlithified sediments, the regular and smooth fracture surfaces indicate a more consolidated state than liquefaction and fluidization. Appearance of synsedimentary faults in the slumps indicates different internal stress fields at head and toe of slumping masses. Unlithified sediments or internal particles are re-compacted and induce a smaller volume and differential subsidence, this may be a cause of synsedimentary faults (Qiao et al. 2011(Qiao et al. , 2017. The stretched fault block can move laterally in the unconsolidated state (Yang and Van Loon 2016). Argillaceous sediments fill the space thus produced a morphology similar to that of boudinage induced by compressional stress and transverse tensile stress in lithified rocks.

Deformation structures of lithified sediment: tectonic deformation
SSD structures are mostly formed on the surface and sub-surface (Cathles et al. 2010;Shanmugam 2017a;Liang et al. 2018a;Zhong et al. 2020;Alsop et al. 2022). Alsop et al. (2022) established a synchronous failure model that a single failure may concurrently create surficial and sub-surface deformed intrastratal horizons at different stratigraphic levels. Surficial deformations are characterized with irregular erosive surfaces and overlying sedimentary caps deposited out of suspension, while sub-surface deformation cuts through entire stratigraphic sequences and affects the interbedded undeformed beds (Alsop et al. 2019a(Alsop et al. , 2022. The original clastic particles moved and rearranged without internal deformation or metamorphism during the SSD process, resulting in the relatively small-scale structures in thin strata (Qiao et al. 2017). The deformed layers are usually between undeformed layers, and their underlying and overlying layers are usually undeformed. Conversely, tectonic deformation structures of lithified sediments induced by regional tectonic events are usually large scale. Most tectonic deformation structures are formed deep underground, accompanied by cleavage, microscopic deformation, and metamorphic phenomena (Ramsay 1967;Lu et al. 2011;Wakabayashi 2017). Lithified rocks are often folded and faulted by regional contraction. Deformation mechanisms include cataclasis, diffused material migration, recrystallization, and plastic flow (Liu et al. 2020). Different tectonic settings that lithified rock deformation mechanisms occurred may be the key factors that control temperature, pressure, fluids, and rock strain mechanism (Lu et al. 2011;Qiao et al. 2017;Liu et al. 2020). However, some small-scale tectonic deformation structures are similar to the deformation of unconsolidated sediments.

Deformation structures in different consolidation states and their correlation with seismic or tectonic events
Geological records can be preserved in sediments, but the deformation structures differ between watersaturated sediments, weakly consolidated sediments, and lithified rocks. SSD records the influence of earthquakes, rapid deposition, and other triggers on sediments with different water contents. Earthquakes in particular, being a common phenomenon throughout long geological history, have induced complex deformation structures in sedimentary rocks. Thus, the correlation between deformation structures and seismic or tectonic events is discussed as follows: As mentioned in 4.1, fluidization, liquefaction, and hydroplastic deformation are typical phenomenon during SSD. Fluidization occurs when the pore-fluid pressure exceeds the overburden pressure. The seismic magnitude is usually more than 5 when homogeneous mixtures flow along fractures and form sand volcanoes and clastic dikes (Rodrıǵuez-Pascua et al. 2000). When liquefaction occurs, load structures and concordant deformation structures are created in sediments ( Figure 6). Hydroplastic deformation occurs in grain-supported unlithified sediments in which the pore-fluid pressure cannot induce liquefaction, and the water-saturated sediments tend to form hydroplastic folds (Guiraud and Plaziat 1993). The seismic magnitude is usually less than 4 in this condition (Calvo et al. 1998;Rodrıǵuez-Pascua et al. 2000).
Unconsolidated sediments that do not reach the ideal liquefaction state may also form ductile and brittle deformation structures, which are similar to tectonic deformation (Yang and Van Loon 2016). Synsedimentary faults induced by interlaminar shear stress are typical brittle deformations in these sediments. Simultaneously, slump folds are usually interpreted as hydroplastic deformation in early studies, but the fabrics, boudinage, and faults preserved in the folded layers indicated a special palaeotopography (slope) or nearly consolidated state (Qiao et al. 2017). Boudinage are common structures in lithified rocks due to a tensional tectonic background, but in unconsolidated sediments they are also mentioned in literatures (Zulauf et al. 2011;Marques et al. 2012;Yang and Van Loon 2016). The layers must consolidate to a special state that they can deform in a brittle way. Compaction and early diagenetic cementation may be the main reasons to induce such a special consolidated state. Therefore, we deduce these folds, boudinage, and faults may not be formed in watersaturated sediments but in weakly consolidated sediment.
In addition to seismic energy and consolidation state, other factors including lithology, sediment competence, and sedimentary environment may also affect the deformation structures in unlithified sediment. He et al. (2012) suggested that sediments with small differences in competence can more easily form liquefied lamination, but a larger difference in sediment density is conducive to the formation of load casts.
Lithified rocks broken by earthquakes can be transported to new places, forming exotic and cataclysmic sedimentary records. In contrast to seismic and other cataclysmic events recorded in soft-sediments on shallow surfaces, tectonic deformation of lithified rocks mostly forms faults in the shallow crust and folds in deep-seated tectonic levels. They are related to later tectonic events and preserve informative records of the regional tectonic evolution, which can provide evidence for the reconstruction of plate movements and crustal evolution (Lu et al. 2011). The deep burial environment Figure 6. Deformed structures in different consolidation stages (modified from Lowe 1975;Owen 1987;Guiraud and Plaziat 1993). P = pore-fluid pressure, Sz = overburden pressure. makes thermal action an important factor in tectonic deformation. This may induce metamorphism. Metamorphism not always occurs during regional deformation, but once metamorphism occurs, it can be a good indicator of tectonic deformation (Liu et al. 2020).

Case deformation in Lingshan Island
Deformation structures in interbedded sediments with different consolidation rates may be complex, especially the SSD structures superimposed tectonic deformation. This is a controversial issue when geologists want to interpret the genetic mechanism. The deformation structures in Lingshan Island may be good examples. Load structures, water-escape structures, synsedimentary faults, and folds are preserved in the Lower Cretaceous Fajiaying Formation in Lingshan Island, North China (Figure 4, 7). Previous studies interpreted these structures as SSD induced by slump, rapid sedimentation, or palaeoseismic shake (Lu et al. 2011;Liang et al. 2018a;Li et al. 2019). Particularly, the interpretation of folds is ambiguous.
Black mudstone and greyish yellow sandstone are the main lithologies of Fajiaying Formation. Thickness of layers differs a lot, from millimetres to metres. At the interface between unlithified sand and mud, load casts are induced by reversed density gradient. These load casts are characterized with elliptical and tailing morphology (Figure 4(a-d)). When the load casts drop to a relatively deep level, ball-and-pillow structures in the thick black mudstone are formed (Figure 4(f)). Flame structures are also formed by sand stacking into the underlying mud (Figure 4(e)). The varied morphology of load structure indicates a water-saturated state.
When the weakly consolidated sediments in tensional tectonic regime are triggered by earthquake or slump, synsedimentary faults cut the thin strata, inducing deformation with morphology similar to half-graben-like structure and graded faults (Figure 4(g-j)). These faults are usually preserved in thin layers, while the overlying and underlying thick mudstone layers are not faulted (Figure 4(j)). Sandstone layers behave in a brittle way, indicating more consolidated sandstone than mudstone. However, a special combination of multiple deformation structures is found in outcrop (Figure 4(a,)). Ball-andpillow structures are in the lower mudstone, waterescape structure in the middle part, and synsedimentary faults in the upper sandstone (Figure 4(a,)). We deduced when the deformation occurs, the upper relatively thick sandstone and lower mudstone are in different consolidated state due to the different consolidation rates. The upper sandstone layer is weakly consolidated, while the lower layers are still water-saturated and undergo typical SSD.
In addition to the mentioned SSD, enigmatic deformation structures including folds are preserved in Fajiaying Formation. Recumbent folds and inclined folds of interbedded sandstone and mudstone are typical folds in outcrop (Figure 7(a, b)). Refolding of fold limb creates a nearly vertical fold and a complex fold (Figure 7  (a,)). Morphology of complex fold is similar to chaotic structure. Broken folded sandstone and fault are preserved in the thick mudstone, and some broken limbs are in boudinage shaped (Figure 7(e)). In the typical recumbent folds and inclined folds, mudstone is thickened at fold hinge, while sandstone form parasitic fold (Figure 7(f,)), indicating more competent sandstone than mudstone (Waldron and Gagnon 2011). Simultaneously, broken fold and sandstone block with hook-shaped morphology are also preserved in black mudstone (Figure 7(h,)). In some sandstone block, broken clastic lamina is still preserved (Figure 7(j)), some of which are even folded. These broken clastic laminae are interpreted as mud clast by Li et al. (2019), but the origin bedding may indicate a nearly lithified state. Most early studies interpret these folds as slump folds (Lu et al. 2011;Yang and Van Loon 2016;Liang et al. 2018a;Li et al. 2019). Subsequent work by Qiao et al. (2017) indicated that large-scale recumbent folds and inclined folds are regional tectonic deformation. In their works, they indicated that the folds are big-scale; in the dip-isogon classification of Ramsay (1967), sandstone layers display class 3 geometry interbedded with mud layers show thicker hinges and class 1 geometry; regular and rotated surfaces between some domino boudins indicates the brittle fracture which show characteristics of consolidated deformation.
Overall, it is difficult to rationalize these complex deformation structures with a single genetic mechanism, particularly the co-occurrence of SSD and enigmatic deformation. Considering these deformation structures may be formed in different consolidated states, we proposed a possible interpretation: Sediments of varied lithology are in different consolidated rates. When triggered by earthquake and slump, load structures and water-escape structures are created in water-saturated sediments, while synsedimentary faults and boudinage are created in weakly consolidated sediments (Figure 8(a,b)). When these sediments are consolidated to a nearly lithified state or ductile state after deep burial and compaction, interbedded sandstone and mudstone are folded and broken by compression (Figure 8(c,)). The less competent mudstone thickens at fold hinge, and the more competent thick sandstone form class 3 geometry, the thin sandstone form parasitic fold. When the sandstone is broken, residual fold hinges and rootless hook-shaped folds are preserved in the thick mudstone (Figure 8(d)). Some thin and long broken sandstone are similar to clastic dikes, but they are deformed by the compression. After these deformed strata uplift to the ground, they are difficult to interpret in the outcrop. Although the fold in Qiancengya cliff is undoubtedly a tectonic fold (Figure 7(c)), other recumbent folds and inclined fold are still ambiguous. Many structures are enigmatic, like the refolded fold limbs, complex folds, and isolated boudins. More detailed work is needed to rationalize these deformation structures in Lingshan Island.

Identification of deformation structures in different sediments
Criteria for the recognition of deformation structures in unlithified sediments and lithified rocks are still unclear. Close relationship between sedimentation and tectonics assures widespread unlithified deformation in mobile belts (Helwig 1970). Slump folds and tectonic folds are typical deformation structures with similar morphologies. The collapse of unlithified sediments along the slope is similar to thickly layered jelly or to ductile rock deep underground in the condition of high temperature and pressure (Qiao et al. 2017). Therefore, it is difficult to identify deformation structures in unlithified sediments and lithified rocks, which necessitates a cautious approach to differentiating SSD and lithified rock deformation. Different criteria have been established in publications, and some deformation features previously used as indicators of HRD may be invalid or questionable, such as the axial-planar cleavage, parasitic fold, and sheath fold. These deformation structures can also be found in SSD (Elliott and Williams 1988; Waldron and Gagnon 2011; Alsop et al. 2019a). Take it another way, when we observe the deformation structures in outcrop, we can qualitatively infer whether the deformation is liquefied or completely consolidated, which may be a way to distinguish structures between SSD and HRD. The following features may be clues to distinguish SSD and HRD.

Liquefaction and fluidization in water-saturated sediments
Original characteristics of soft-sediments can be changed by liquefaction and fluidization. When water-saturated sediments liquefy or fluidize, clastic dikes and waterescape structures are typical deformation structures. In addition, liquefaction and fluidization can reverse the competence contrast between interbedded mudstone and sandstone. In normal sediment and diagenetic processes, the sandstone is more competent than the adjacent mudstone, but the competence difference is reversed when the sand is liquefied (Waldron and Gagnon 2011). In this case, mudstone is more competent than interbedded sandstone. When deformation occurs, the less competent sandstone fills the space left by the deformation. The less competent layer is thickened at the fold hinge, forming a class 3 geometry, but the more competent mudstone displays a class 1 geometry. In some cases, one can see sandstone deformed as a ptygmatic fold, while the mudstone around is undeformed. Simultaneously, structures induced by fluidization are undisputed in water-saturated sediment. Therefore, the appearance of liquefaction and fluidization in the deformation morphology is a typical indicator of SSD.

Folds and fabrics in different strata matched with regional geological regime
Previous studies have demonstrated that useful indicators of pre-lithification deformation include overprinting of sediment reworking or remobilizing structures, truncated folds, cross-cutting clastic dikes, and sedimentary fill of irregular erosive surfaces that truncate underlying structures (Figure 9(b)) (Waldron and Gagnon 2011; Alsop et al. 2019a). Ductile deformation structures, brittle deformation structures, and large-scale deformation structures in different strata, on the other hand, mostly  (Alsop et al. 2019a). More competent mudstone displays a class 1 geometry while sandstone thickened at fold hinge. (c) Fold hinging in different strata towards the same orientation indicate the same stress field and geological setting. Tectonic boudinage, sandstone block, folded fossils induced by tectonic stress are preserved in deformed bed. Basal conglomerates are on the regional unconformity, which differ from irregular erosive surface.
indicate tectonic deformation after lithification (Elliott and Williams 1988;Qiao et al. 2017). However, special deformation phenomena including parasitic fold and sheath fold mentioned in 7.1 are found both in HRD and SSD Marco 2011, 2013;Alsop et al. 2019a). These features cannot, therefore, be key indicators of tectonic folds. Especially in sedimentary rocks that have undergone low-grade metamorphic or superimposed tectonic deformation, the complex deformation structures make it more difficult to identify their origins. Subsequent studies demonstrate that it is unscientific and ineffective to identify the deformation origin solely by the deformation morphology and patterns (Feng 2017a;Alsop et al. 2019a).
Slump folds are generally discordant, but the axial plane of slump folds usually dips upslope. Previous studies have recognized a large-scale radial slump system that fold axial planes generally inclined away from the depocentre of the basin (Alsop and Marco 2012b;Alsop et al. 2019a). Thus, in a single slump cell, the fold hinges towards different orientations in different places, but the axial plane dip gradually decreases from the head to the toe of the slump in the same slump orientation (Figure 5 (a)). Conversely, tectonic events usually influence different strata. Preferred dimensional orientations are involved in stereo plots of fold hinges and axial planes. Thus, fold hinge in different strata towards the same orientation indicates the same stress field and geological setting. Simultaneously, the folds, faults, and superimposed deformation structures in different strata can be matched with the regional geological regime. If the strata have been deformed by multi-stage tectonic activity, different deformational stages can be divided according to folds and fabrics. Although the superposition of multi-stage folds may easily form complex deformation structures and chaotic deformation phenomena -which can be easily attributed to SSDthe variation in the axial plane dip angle of slump folds is different from that of tectonic folds. Therefore, the preferred dimensional orientation of folds and fabrics in different strata may be an indicator of HRD.

Related deformation structures and micro-deformation structures
Related deformation structures associated with slumps can be found in folded strata (Qiao et al. 2011). Clastic dikes in soft-sediments are similar to intrusive dikes and broken thin sandstone layer in hard rocks. However, clastic dikes are connected to underlying source layers and show internal flow banding parallel to dike margins. Liquefaction homogeneity is a typical characteristic of these clastic dikes, but the broken thin sandstone layer of tectonic origin preserves original and continuous bedding. These dikes in soft-sediments are often reappear in vertical direction and exhibit good continuity in lateral direction, but to intrusive dikes, they may be big-scale and not reappear in vertical direction.
Sedimentary infill of irregular erosive surfaces truncating underlying structures indicated an unlithified state; this can be a key diagnostic feature in establishing SSD (Alsop et al. 2019a). Different from these irregular erosive surfaces, uplift of HRD structures to the ground can create regular and straight unconformity. Basal conglomerates may be seen on the eroded deformation structures overlaid by undeformed layers (Figure 9(c)). The upper and lower sedimentary facies of unconformity are generally different and will not reappear in vertical direction if they experienced a continuous evolution. Occurrence of these unconformities is also a key diagnostic feature in establishing deformation origin.
Special sedimentary components including fossils, concretions, and igneous or metamorphic lithic clasts are typical signs of consolidated state. They can be good indicators of lithification before deformation if they are deformed by folds (Elliott and Williams 1988). Appearance of cleavage is common in HRD, but it may also be found in mudstones during SSD and is not unique to HRD (Sobiesiak et al. 2017). However, cleavage refraction can be a special feature in lithified sandstone blocks induced by the competent difference of the overlying and underlying strata (Qiao et al. 2016;Liu et al. 2020).
Another related deformation structure is boudinage, a disruption of the rock in response to bulk extension along the enveloping surface (Goscombe et al. 2004;Qiao et al. 2017). Although some rare domino boudins have been described in gravity-induced slump fold limbs (Altermann 1986;Yang and Van Loon 2016), most of the boudins, especially the torn boudins, gash boudins, and shear band boudins present a tensional tectonic regime. When boudins are formed in adjacent layers with different competences, secondary gypsum or calcite veins can fill the parallel and smooth fissures. The calcite vein injected into the smooth fissure indicates a brittle break in the lithified rock, which may be an indicator of tectonic deformation.
Liquefied mixture may be visible in the thin section of SSD structures (Qiao et al. 2017). Clastic particles are uniformly distributed, and preferred dimensional orientation of grains and minerals is absent in most fabrics. Migration of clastic particles differs between different fold limbs. These micro-deformation structures may be indicators of SSD. Conversely, presence of metamorphic minerals, quartz pressure shadows, undulatory extinction, and other microdeformation structures are features of tectonic deformation.
In conclusion, folds and fabrics in different stratamatched with the regional geological regime, liquefaction, related deformation structures, and microdeformation structures -can be clues to identify the origin of deformation. The emergence of a single point of evidence is insufficient to ascertain whether deformation structures formed in soft-sediment or in lithified rocks. As described by Elliott and Williams (1988), few indicators can stand on their own; however, taken together, they may provide the most reasonable explanation for the deformation.
Deformation structures observed in the outcrop have undergone a long evolution history. Even the softsediment structures are completely preserved after diagenesis, they may be affected by later tectonic activity. The identification of the deformation origins discussed above is an idealized interpretation, and many complex deformation structures need to be explained. In particular, it is still unclear how the diagenetic process, compaction, and tectonism affect early SSD, so caution must be applied to the interpretation of complex deformation.

Conclusion
Deformation structures vary in sediments of different consolidation states. Clastic dikes, liquefied breccia, load casts, and water-escape structures are usually formed in water-saturated sediments, syndepositional faults, and syndepositional boudinage may be formed in weakly consolidated sediments. Disputes still exist in recognizing deformation structures in unlithified sediments and lithified rocks. Some indicators are proposed for distinguishing SSD and HRD.
Liquefaction is a typical indicator of pre-lithification deformation. Folds and fabrics in different strata, matched with the regional geological regime, may be good indicators of tectonic deformation. The related deformation structures and micro-deformation structures can be clues to identify the deformation origin. Diagenetic processes, compaction, and later tectonic deformation may influence the SSD, necessitating more care in the interpretation of complex deformation structures.

Highlights:
• Soft-sediment deformation (SSD) structures of different origins are summarized. • Hypothesis that deformation structures are created in certain consolidation state is proposed. • Liquefaction is a typical indicator of SSD.
• Folds and fabrics in different strata, matched with details of the regional geological regime, may be a good indicator of tectonic deformation. • Related deformation and micro-deformation structures can be clues to identify deformation origins.