Shallow-marine massive sandstone sheets as indicators of palaeoseismic liquefaction — An example from the Ordovician shelf of Central Bolivia

The Middle to Upper Ordovician siliciclastic succession in Central Bolivia provides excellent exposures of up to 1 m thick massive sandstone beds produced by liquefaction-induced sediment remobilisation. These fine-grained massive sandstones occur in shallow-marine nearshore facies that were deposited above storm wave base. Vertical to steeply inclined clastic dykes, which penetrate up to 1.5 m of the overlying sediment pile, feed into the basal parts of massive sandstone sheets. These dykes are interpreted as pathways used by liquefied sediment during upward-directed escape from a subsurface horizon. The relatively short lateral spacing of 5 to 25 m between the individual dykes initiated lateral coalescence of ejected individual sediment bodies. As a result, massive sandstone sheets formed and can be traced laterally over several kilometres. While undeformed sandy deposits contain between 5 to 10% dispersed clay the average clay content in massive sandstone sheets is ∼ 15%. The elongated, sometimes S-shaped form of the sandy, laminated fragments reflects squeezing during movement showing that they have been deformed under shear stress during flow. The axis of maximum elongation of the fragments indicates local flow direction. As the massive sandstone sheets are likely the products of seismic shocks they provide the first evidence of tectonic activity in this part of the basin during the Middle to Late Ordovician.     

Evaluation of soft sediment deformation structures along the Fethiye–Burdur Fault Zone,SW Turkey

Burdur city is located on lacustrine sedimentary deposits at the northeastern end of the Fethiye–Burdur Fault Zone (FBFZ) in SW Turkey. Fault steps were formed in response to vertical displacement along normal fault zones in these deposits. Soft sediment deformation structures were identified at five sites in lacustrine sediments located on both sides of the FBFZ. The deformed sediments are composed of unconsolidated alternations of sands, silts and clay layers and show different morphological types. The soft sediment deformation structures include load structures, flame structures, slumps, dykes, neptunian dykes, drops and pseudonodules, intercalated layers, ball and pillow structures, minor faults and water escape structures of varying geometry and dimension. These structures are a direct response to fluid escape during liquefaction and fluidization mechanism. The driving forces inferred include gravitational instabilities and hydraulic processes. Geological, tectonic, mineralogical investigations and age analysis were carried out to identify the cause for these soft sediment deformations. OSL dating indicated an age ranging from 15161±744 to 17434±896 years for the soft sediment deformation structures. Geological investigations of the soft sediment deformation structures and tectonic history of the basin indicate that the main factor for deformation is past seismic activity.     

湖相沉积岩中的同生变形构造及其地质意义

本文以渤海湾周缘地区的东濮凹陷下第三系沙三沙四段湖相沉积岩中出现的同生变形构造为例,讨论其命名、分类、主要类型、形成机理及其在分析古沉积环境、古地理和古构造等方面的意义。这项讨论为研究陆相含油气盆地的同生变形构造提供了对比依据。     

Soft-sediment deformation structures in late Albian to Cenomanian deposits, São Luís Basin, northern Brazil: evidence for palaeoseismicity

Soft-sediment deformation structures from the Alcântara Formation (late Albian to Cenomanian), São Luís Basin, northern Brazil, consist of (1) contorted structures, which include convolute folds, ball-and-pillow structures, concave-up paths with consolidation lamination, recumbently folded cross-stratification and irregular convolute stratification that grades into massive beds; (2) intruded structures, which include pillars, dykes, cusps and subsidence lobes; and (3) brittle structures, represented by fractures and faults displaying planes with a delicate, ragged morphology and sharp peaks. These structures result from a complex combination of processes, mostly including reverse density gradients, fluidization and liquefaction. Reverse density gradients, promoted by differential liquefaction associated with different degrees of sediment compaction, led to the genesis of convolute folds. More intense deformation promoted the development of ball-and-pillow structures, subsidence lobes and sand rolls, which are attributed to denser, and thus more compacted (less liquefied), portions that sank down into less dense, more liquefied sediments. Irregular convolute stratification that grades into massive beds would have formed at periods of maximum deformation. The subsidence of beds was accompanied by lateral current drag and fluid escape from water-saturated sands. In addition, the fractures and faults record brittle deformation penecontemporaneous with sediment deposition. All these mechanisms were triggered by a seismic agent, as suggested by a combination of criteria, including (1) the position of the study area at the edge of a major strike-slip fault zone that was reactivated several times from the Albian to the Holocene; (2) a relative increase in the degree of deformation in sites located closer to the fault zone; (3) continuity of the deformed beds over large distances (several kilometres); (4) restriction of soft-sediment deformation structures to single stratigraphic intervals bounded by entirely undeformed strata; (5) recurrence through time; and (6) similarities to many other earthquake-induced deformational structures.     

Towards establishing criteria for identifying trigger mechanisms for soft-sediment deformation: a case study of Late Pleistocene lacustrine sands and clays, Onikobe and Nakayamadaira Basins, northeastern Japan

Five main deformation units, discrete sheets of deformed sediments that lie between a significant thickness of undeformed sediment, were selected for study within Late Pleistocene lacustrine sands and clays in the Onikobe and Nakayamadaira Basins, northeastern Japan. The deformed units show evidence of deformation by a variety of mechanisms including fluidization, liquefaction, brittle failure and cohesive flow. Driving forces are thought to be primarily reverse density gradient systems, but also include gravitational body force, shear stress and unequal loading. The main trigger mechanisms are firstly earthquakes, secondly overloading from volcanic sands and thirdly, to a lesser extent, subaqueous currents. Consideration is given to criteria that allow the trigger mechanism to be identified. This study shows that the following criteria can be used to identify a seismic triggering agent: (i) setting; (ii) the extent of the deformation units; (iii) absence of evidence relating to other potential trigger mechanisms; and (iv) evidence relating to other potential trigger mechanisms is present but can be seen elsewhere in the stratigraphic section associated with undeformed sediment. Conversely, the following criteria, while they are important in interpreting the driving force and deformation mechanism, have no relevance to the trigger mechanism: (i) sediment composition; (ii) deformation structures being restricted to a single stratigraphic interval (<1 m thick) (not necessarily correlatable over large areas); and (iii) similarity to structures in the literature.     

Evidence for Late Messinian seismites,Nijar Basin,south‐east Spain

The Feos Formation of the Nijar Basin comprises sediments deposited during the final stage of the Messinian salinity crisis when the Mediterranean was almost totally isolated. Levels of soft‐sediment deformation structures occur in both conglomeratic alluvial sediments deposited close to faults and the hyposaline Lago Mare facies, a laminated and thin‐bedded succession of whitish chalky marls and intercalated sands alternating with non‐marine coastal plain deposits. Deformation structures in the coarse clastics include funnel‐shaped depressions filled with conglomerate, liquefaction dykes terminating downwards in gravel pockets, soft‐sediment mixing bodies, chaotic intervals and flame structures. Evidence for soft‐sediment deformation in the fine‐grained Lago Mare facies comprises syndepositional faulting and fault‐grading, sandstone dykes, mixed layers, slumping and sliding of sandstone beds, convolute bedding, and pillar and flame structures. The soft‐sediment deformed intervals resemble those ascribed elsewhere to seismic shaking. Moreover, the study area provides the appropriate conditions for the preservation of deformation structures induced by seismicity; such as location in a tectonically active area, variable sediment input to produce heterolithic deposits and an absence of bioturbation. The vertical distribution of soft‐sediment deformation implies frequent seismic shocks, underlining the importance of seismicity in the Betic region during the Late Messinian when the Nijar Basin became separated from the Sorbas Basin to the north. The presence of liquefied gravel injections in the marginal facies indicates strong earthquakes (M ≥ 7). The identification of at least four separate fissured levels within a single Lago Mare interval suggests a recurrence interval for large magnitude earthquakes of the order of millennia, assuming that the cyclicity of the alternating Lago Mare and continental intervals was precession‐controlled. This suggestion is consistent with the present‐day seismic activity in SE Spain.     

Evidence of Late Quaternary seismicity from Yunam Tso,Lahaul and Spiti,NW Himalaya,India

A relict fluvio-lacustrine sediment of an 8 m thick section exposed at Kilang Sarai along Yunam river, near Baralacha La shows presence of cycloids or pseudonodules, ball and pillow structures, flame-like and pocket structures, sand dyke injections, bed dislocation/faulting and flow folds. Within this section four deformed levels of soft sediment structures have been identified which were dated ca. 25 ka BP at level 1 (～0.4 m from the modern river level (mrl), 20.1 ka BP at level 2 (～1.8 m mrl), 17.7 ka BP at level 3 (～2.56 m mrl) and 12.2 ka BP at level 4 (～4.25 m mrl)). Detailed study of these soft sediment structures allow us to demonstrate that deformation level 3 is not related to seismic trigger, but remaining three deformation levels (1, 2 and 4) are ascribed to seismic origin. From compilation of earlier palaeoseismological studies using soft sediment deformational structures (SSDS) in the palaeolake deposits in the adjoining area, suggest that the deformational events identified in the present study are regional in nature and thus tectonic process plays an important role in the evolution of landform in the Spiti region.     

滇中中元古界大龙口组地震灾变事件及地质意义

软沉积物变形构造是确定古地震存在的关键证据之一.笔者在云南易门地区进行野外露头剖面凋查时,在滇中新元古代大龙口组中识别出了3个地震事件层,其中发现大量的水塑性褶皱、液化构造等变形构造,主要类型包括微褶皱纹层、与液化脉有关的褶皱、与液化层有关的褶皱和受侧向挤压而成的顺层滑动、碟状泄水构造、液化脉、液化沙侵、水压破裂等构造.此外,臼齿构造与水塑性褶皱、液化构造等地震成因变形构造伴生发育,并且其脉体形态、大小、优势方位等与后者的分布、变形样式、强度等有一定的对应关系.软沉积物变形构造及臼齿构造的形态、位态及发育层位特征表明,它们的驱动机制是地震活动.迄今为止,已发现的滇中地区大龙口组震积岩均分布于罗茨-甘庄断裂的东侧,指示软沉积物变形、臼齿构造与西缘控盆断裂间存在密切的成因联系.     

Last Deglacial Soft−Sediment Deformation at Shawan on the Eastern Tibetan Plateau and Implications for Deformation Processes and Seismic Magnitudes

The eastern margin of the Tibetan Plateau is characterized by frequent earthquakes; however, research of paleo?earthquakes in the area has been limited, owing to the alpine topography and strong erosion. Detailed investigations of soft?sediment deformation (SSD) structures are valuable for understanding the trigger mechanisms, deformation processes, and the magnitudes of earthquakes that generate such structures, and help us to understand tectonic activity in the region. To assess tectonic activity during the late Quaternary, we studied a well?exposed sequence of Shawan lacustrine sediments, 7.0 m thick, near Lake Diexi in the upper reaches of the Minjiang River. Deformation is recorded by both ductile structures (load casts, flame structures, pseudonodules, ball?and?pillow structures, and liquefied convolute structures) and brittle structures (liquefied breccia, and microfaults). Taking into account the geodynamic setting of the area and its known tectonic activity, these SSD structures can be interpreted in terms of seismic shocks. The types and forms of the structures, the maximum liquefaction distances, and the thicknesses of the horizons with SSD structures in the Shawan section indicate that they record six strong earthquakes of magnitude 6–7 and one with magnitude >7. A recent study showed that the Songpinggou fault is the seismogenic structure of the 1933 Ms7.5 Diexi earthquake. The Shawan section is located close to the junction of the Songpinggou and Minjiang faults, and records seven earthquakes with magnitudes of ~7. We infer, therefore, that the SSD structures in the Shawan section document deglacial activity along the Songpinggou fault.     

河南南召盆地上三叠统太山庙组软沉积物变形构造及其古地理意义*

河南南召盆地上三叠统太山庙组中发现的软沉积物变形构造包括同沉积断层、液化均一层与泄水脉、底劈构造、塑性变形层、碎裂岩及大型负载构造。它们集中保存在太山庙组中段深湖环境中,以该层段为界,其下水体渐深,其上水体渐浅。多数软沉积物变形构造与浊流沉积砂体相伴生,也可保存在泥岩层中,其形成可能与浊流沉积过程相关,但古地震活动是主要的触发机制。软沉积物变形的类型包括液化变形、塑性变形和脆性变形,指示了高强度的古地震活动,记录了秦岭造山带印支期一次强烈的造山活动。造山带逆冲推覆作用造成南召盆地的抬升,代表了前陆盆地系统中的楔顶沉积。     

Microstructures in subglacial and proglacial sediments: understanding faults,folds and fabrics,and the influence of water on the style of deformation

Macroscopic field and micromorphological studies have been carried out on subglacially and proglacially deformed glacigenic sequences at a number of sites throughout Scotland, UK. Examination of microstructures (folds, faults, hydrofractures, plasmic fabrics) aided understanding of the deformation histories preserved in the sediments, but a similar range of structures were developed in both Subglacial and Proglacial settings. Discrimination between Subglacial and Proglacial deformation was only possible when micromorphological data was used in conjunction with larger-scale field observations. Variations in lithology and water content were controlling factors influencing the style and apparent intensity of deformation recorded. Changes in pore-water content and pressure during deformation can lead to liquefaction and hydrofracturing, with early-formed structures locally controlling the pattern of water escape. Liquefaction can also lead to homogenisation of the sediments and the destruction of earlier deformation structures, even at relatively low strains. Beds or zones of liquefied sand and silt may form highly ‘lubricated’ detachments within the sediment pile, resulting in a marked reduction in the amount of shear transmitted to underlying units. A multidisciplinary approach, involving sedimentological, geomorphological, stratigraphical and structural field observations, combined with micromorphological analysis, is recommended to confidently unravel the glacitectonic history and depositional environment of most deformed glacigenic sedimentary sequences.     

Water escape structures in coarse-grained sediments

Three processes of water escape characterize the consolidation of silt-, sand-and gravel-sized sediments. Seepage involves the slow upward movement of pore fluids within existing voids or rapid flow within compact and confined sediments. Liquefaction is marked by the sudden breakdown of a metastable, loosely packed grain framework, the grains becoming temporarily suspended in the pore fluid and settling rapidly through the fluid until a grain-supported structure is re-established. Fluidization occurs when the drag exerted by moving pore fluids exceeds the effective weight of the grains; the particles are lifted, the grain framework destroyed, and the sediment strength reduced to nearly zero. Diagenetic sedimentary structures formed in direct response to processes of fluid escape are here termed water escape structures. Four main types of water escape structures form during the fluidization and liquefaction of sands: (1) soft-sediment mixing bodies, (2) soft-sedimsnt intrusions, (3) consolidation laminations, and (4) soft-sediment folds. These structures represent both the direct rearrangement of sediment grains by escaping fluids and the deformation of hydroplastic, liquefied, or fluidized sediment in response to external stresses. Fundamental controls on sediment consolidation are exerted by the bulk sediment properties of grain size, packing, permeability, and strength, which together determine whether consolidation will occur and, if so the course it follows, and by external disturbances which act to trigger liquefaction and fluidization. The liquefaction and fluidization of natural sands usually accompanies the collapse of loosely packed cross-bedded deposits. This collapse is commonly initiated by water forced into the units as underlying beds, especially muds and clays, consolidate. The consolidation of subjacent units is often triggered by the rapid deposition of the sand itself, although earthquakes or other disturbances are probably influential in some instances. Water escape structures most commonly form in fine- to medium-grained sands deposited at high instantaneous and mean sedimentation rates; they are particularly abundant in cross-laminated deposits but rare in units deposited under upper flow regime plane bed conditions. Their development is favoured by upward decreasing permeability within sedimentation units such as normally graded turbidites. They are especially common in sequences made up of alternating fine-(clay and mud) and coarse-grained (sand) units such as deep-sea flysch prodelta, and, to a lesser extent, fluvial point bar, levee, and proximal overbank deposits.     

Experimental soft-sediment deformation: structures formed by the liquefaction of unconsolidated sands and some ancient examples

The effects of liquefaction in saturated sand bodies under a variety of driving forces are described from shaking table experiments, and structures from the geological record are presented which are analogous to the experimental structures. The collapse of sloping heaps of cross-bedded sand under a gravitational body force generates low-angle, essentially uncontorted stratification. A basal zone of shearing may be present, with steepened and folded foresets. Stretching of foresets may be accommodated on normal faults, and bottomsets may be contorted into inclined folds. In natural systems the substrate may also liquefy, causing deformation driven by an unevenly distributed confining load. Stratification in the surface bedform is flattened, and stratification in the substratum contorted. Experiments failed to produce relative displacement at the interface between stacked sand bodies. Liquefaction of gravitationally unstable systems in sands generates load structures comparable to those from sand-mud systems. Recumbent-folded deformed cross-bedding is formed by current shear over a liquefied bed, as has been inferred from field and theoretical analyses. Shear of nonliquefied sand forms angular folds. Other deformation mechanisms, such as fluidization or seepage, may generate structures similar to all of these. Local water-escape structures driven by fluidization occur in the upper parts of some liquefied sand bodies. They include cusps, sand volcanoes and clastic dykes. Transient cavities formed in some experiments and seemed to be preserved as breached cusps. Although the experiments tried to isolate individual driving forces, driving forces may operate together, and there may be a continuum between deformation driven by water escape and deformation driven by loading. Different structures from those described here may form where liquefaction develops in a buried layer as opposed to at the sediment surface.     

Wave-induced liquefaction: a modern example from the Bay of Fundy

Soft-sediment deformation features occur commonly on parts of intertidal sand bodies in Cobequid Bay, Bay of Fundy. These features are small- to intermediate-sized, slump-like bodies, 1-3 m² in area and located on the crest and upper stoss side of ebb megaripples. External modification of these slumps indicates that they formed before complete emergence. The deformed cross-bedding within these bodies extends to a depth of 0.15-0.35 m and shows that deformation occurred during slumping and flowage of liquefied sand down the megaripple stoss side. Field evidence and calculations strongly indicate that this liquefaction results from the impact of 0.1-0.3 m high waves breaking against the megaripple lee faces. Neither rapid drawdown of the water level nor earthquake shocks are reasonable alternative explanations. Indigenous wave activity provides an attractive substitute to tectonism as an explanation of soft-sediment deformation in ancient shallow-water sediments. Slow wave-induced compaction may also account for the relative scarcity of deformation structures in shallow marine sandstones.     

ROTATIONAL SLUMPS AND SLUMP SCARS IN SILURIAN ROCKS, WESTERN IRELAND

Curved or planar discordant surfaces occurring within a limited stratigraphic range immediately beneath shallow marine deposits represent penecontemporaneous shear surfaces along which slumps have moved. Rotated packets of strata frequently retained within the curved discordant surfaces show both internal and external evidence of having moved laterally. Movement is considered to have been triggered off by a sudden shock or shocks which may have been either of sedimentary or tectonic origin. The slumping occurred at the change in gradient between slope and shallow marine shelf deposits.     

鄂尔多斯盆地南部上三叠统延长组震积岩的发现及地质意义

鄂尔多斯盆地南部中生界延长组长6—长8中发育与地震有关的震积岩，通过岩心观测识别出的震积岩标志主要有微同沉积断裂、震裂缝、液化砂岩脉、振动液化卷曲变形构造、地震角砾岩、负荷构造及枕状层等。同时在塔17井中发现完整的震积岩垂向序列，序列自下而上分为下伏未震层、微断裂层、微褶皱层、碎块层及液化均一层，上覆未震层。该震积岩的发现为盆地构造演化提供了动力学解释，表明晚三叠世随着秦岭、南祁连海槽的封闭，南北向逆冲带发生强烈活动，是本区延长组震积作用的直接诱发因素。同时该震积岩的发现，为盆地西南部延长组长6—长8发育的大规模浊积岩的外界触发机制是由地震活动引起的提供了有力证据。     

Deformation styles as a key for interpreting glacial depositional environments

Lithostratigraphical and lithofacies approaches used to interpret glacial sediments often ignore deformation structures that can provide the key to environment of formation. We propose a classification of deformation styles based on the geometry of structures rather than inferred environment of formation. Five styles are recognised: pure shear (P), simple shear (S), compressional (C), vertical (V) and undeformed (U). These dictate the first letter of the codes; the remaining letters conveying the evidence. This information can be used to reconstruct palaeostress fields and to infer physical properties of sediments when they deformed. Individual structures are not diagnostic of particular environments but the suite of structures, their relative scale, stratigraphical relationships, and orientation relative to palaeoslopes and to palaeoice‐flow directions can be used to infer the environment in which they formed. This scheme is applied at five sites in west Wales. The typical succession is interpreted as subglacial sediments overlain by meltout tills, flow tills and sediment flows. Paraglacial redistribution of glacial sediments is widespread. Large‐scale compressional deformation is restricted to sites where glaciers readvanced. Large‐scale vertical deformation occurs where water was locally ponded near the ice margin. There is no evidence for glaciomarine conditions. Copyright © 2003 John Wiley & Sons, Ltd.     

塔里木盆地顺托果勒地区早中志留世震积岩——高分辨构造事件的记录

在塔里木盆地塔中隆起与满加尔坳陷结合部——顺托果勒地区的深钻井岩心中,发现了大量早中志留世软沉积变形构造。其中主要包括液化砂岩脉、液化角砾岩、触变底劈构造、触变楔、负载构造、球-枕构造和复合混插构造等。通过系统地观察软沉积变形构造的岩石组成、构造形貌及样式、垂向分布的循环性、横向分布的延展性、沉积环境及与古活动断裂的关系,确定其为震积岩。结合该区断裂早中志留世的发育特征,推测发震断裂主要可能是塔中隆起与满加尔坳陷结合部的北东向走滑逆冲断裂以及北西向剪切拉张断裂。在早志留世柯坪塔格组沉积时约4 Ma中最少发生了26次古地震事件(震级M5)。这些古地震记录不仅反映了研究区志留纪构造的活动性,也是弥补主构造运动中高频次构造事件脉动性、循环性的重要证据,为重建中古生代的古构造提供新的线索。     

Soft‐sediment deformation structures in a Pleistocene glaciolacustrine delta and their implications for the recognition of subenvironments in delta deposits

The development of soft‐sediment deformation structures in clastic sediments is now reasonably well‐understood but their development in various deltaic subenvironments is not. A sedimentological analysis of a Pleistocene (ca 13·1 to 15 ¹⁰Be ka) Gilbert‐type glaciolacustine delta with gravity‐induced slides and slumps in the Mosty‐Danowo tunnel valley (north‐western Poland) provides more insight, because the various soft‐sediment deformation structures in these deposits were considered in the context of their specific deltaic subenvironment. The sediments show three main groups of soft‐sediment deformation structures in layers between undeformed sediments. The first group consists of deformed cross‐bedding (inclined, overturned, recumbent, complex and sheath folds), large‐scale folds (recumbent and sheath folds) and pillows forming plastic deformations. The second group comprises pillar structures (isolated and stress), clastic dykes with sand volcanoes and clastic megadykes as examples of water‐escape structures. The third group consists of faults (normal and reverse) and extensional fissures (small fissures and neptunian dykes). Some of the deformations developed shortly after deposition of the deformed sediment, other structures developed later. This development must be ascribed to hydroplastic movement in a quasi‐solid state, and due to fluidization and liquefaction of the rapidly deposited, water‐saturated deltaic sediments. The various types of deformations were triggered by: (i) a high sedimentation rate; (ii) erosion (by wave action or meltwater currents); and (iii) ice‐sheet loading and seasonal changes in the ablation rate. Analysis of these triggers, in combination with the deformational mechanisms, have resulted – on the basis of the spatial distribution of the various types of soft‐sediment deformation structures in the delta under study – in a model for the development of soft‐sediment deformation structures in the topsets, foresets and bottomsets of deltas. This analysis not only increases the understanding of the deformation processes in both modern and ancient deltaic settings but also helps to distinguish between the various subenvironments in ancient deltaic deposits.     

Earthquake-related soft-sediment deformation structures in Palaeogene on the continental shelf of the East China Sea

Earthquake, as disastrous events in geological history, can be recorded as soft-sediment deformation. In the Palaeogene of the East China Sea shelf, the soft-sediment deformation related to earthquake event is recognized as seismic micro-fractures, micro-corrugated laminations, liquefied veins, ‘vibrated liquefied layers’, deformed cross laminations and convolute laminations, load structures, flame structures, brecciation, slump structures and seismodisconformity. There exists a lateral continuum, the wide spatial distribution and the local vertical continuous sequences of seismites including slump, liquefaction and brecciation. In the Palaeogene of East China Sea shelf, where typical soft-sediment deformation structures were developed, clastic deposits of tidal-flat, delta and river facies are the main background deposits of Middle-Upper Eocene Pinghu Formation and Oligocene Huagang Formation. This succession also records diagnostic marks of event deposits and basinal tectonic activities in the form of seismites.