Soft Sediment Deformation

The NW-trending Denizli basin of the SW Turkey is one of the neotectonic grabens in the Aegean extensional province. It is bounded by normal faults on both southern and northern margins. The basin is filled by Neogene and Quaternary terrestrial deposits. Late Miocene-Late Pliocene aged Kolankaya formation crops out along the NW trending Karakova uplift in the Denizli basin. It is a typical fluviolacustrine succession that thickens and coarsens upward, comprising poorly consolidated sand, gravelly sand, siltstone and marl. Various soft-sediment deformation structures occur in the formation, especially in fine-to medium grained sands, silts and marls: load structures, flame structures, clastic dikes (sand and gravely-sand dike), disturbed layers, laminated convolute beds, slumps and synsedimentary faulting. The deformation mechanism and driving force for the soft-sediment deformation are related essentially to gravitational instability, dewatering, liquefaction-liquidization, and brittle deformation. Field data and the wide lateral extent of the structures as well as regional geological data show that most of the deformation is related to seismicity and the structures are interpreted as seismites. The existence of seismites in the Kolankaya Formation is evidence for continuing tectonic activity in the study area during the Neogene and is consistent with the occurrence of the paleoearthquakes of magnitude >5.

Three Pleistocene tills can be distinguished in a coastal cli! section near Heiligenhafen, northern Germany, on the basis of structural and petrographic characteristics. The Lower and Middle Tills had previously been ascribed to the Saalian, and the Upper Till to the Late Weichselian. The former two tills are folded, and unconformably overlain by the Upper Till. In this paper, structural and sedimentological observations are used to investigate whether the Lower and Middle Tills belong to one glacial advance, or two separate (Saalian) advances, as was suggested in earlier studies based on "ne gravel stratigraphy. From the contact with local rocks to the top of the MT there is a steady increase in allochtonous components (Scandinavian rocks) and decrease in parautochtonous (chalk and #int) and autochtonous components (local Eocene siltstone and meltwater sediments). This is paralleled by a trend towards increasing deformation ("nite strain) from the bedrock to the top of the section. The most obvious aspect of this latter trend is the massive appearance of the MT which can be interpreted as the result of homogenization by repeated folding and attenuation of sediment lenses which have been incorporated into the till. This interpretation is supported by macroscopic and microscopic observations of structures in both tills. The structural analysis of the tills is based on the marked contrast in symmetry between sections parallel and perpendicular to the shear direction. Structures on all scales in the LT as well as in the MT indicate E}W (dextral) shearing, except in the western part of the section, where this is overprinted by W}E (sinistral) shearing. The sediment inclusions in the chalk-rich LT are mainly fragments of one or more strongly extended glacio#uvial delta bodies with a depositional direction towards WSW. Locally these delta sediments rest on Eocene siltstone and contain numerous angular fragments of this local bedrock. Boudins and lenses of sorted sediments are incorporated into the till and occur as &&islands of low strain'' in a high strain homogeneous matrix. It is concluded that the LT and MT do not belong to two stratigraphically separate Saalian advances. The section is alternatively interpreted as one subglacial shear zone (deformation till) with upward increasing strain and allochtonous component content. It probably formed during the Younger Saalian (Warthe) westward advance from the Baltic region. Folding of the two diamicts occurred due to lateral compression near the Late Saalian ice margin. The section was "nally overridden by the Late Weichselian Young Baltic advance, eroding the folded LT and MT and depositing the UT.

In the back-arc basin of southern Peru, the bulk of the mid-Cretaceous carbonate platform collapsed near the Turonian–Coniacian boundary (~90–89 Ma), due to slope creation and resulting oversteepening. The resulting mass-wasting deposits, namely the Ayabacas Formation, consist of a megabreccia which is organised from NE to SW in relation with two major fault systems. Facies of sediment reworking (such as brecciation, liquification, sedi-mentary dykes and soft-sediment deformation) are described and four types of resedimentation facies are define. In the northeastern part of the study area, deposits mainly consist of a mixture of very heterometric clasts and blocks (millimetric to kilometric in size), mainly carbonate but also sandy–marly in nature, floating in sandy– marly matrix that exhibits features of liquification (sedimentary dykes and flows) and plastic deformation. Here, resedimentation facies are characterized by deformations and a brecciated facies at each observation scale (from aerial photographs to thin sections) and are therefore defined as fractal or multi-scale breccias. Some clasts and large amounts of the matrix were derived from the underlying clay-rich sandstones of the Murco Formation. These materials were prone to liquification and plastic deformation, allowing them to act as a sliding sole that facilitated the slides and the downslope movement of large limestone rafts. In the southwestern part of the study area, only limestone breccias are observed, in alternation with well-stratified levels. The sliding sole of plastically deformable siliciclastic sediments that previously acted as a lubricating layer was not present here, as materials were more deeply buried. Variations in the degree of sediment lithification from northeast to southwest are inferred to have existed before the collapse and also within the sedimentary succession in the northeastern part. In particular, limestones were well-cemented at the base of the carbonate succession and formed a cap that prevented water to escape from the underlying siliciclastic materials. Such a succession allowed the formation of limestone clasts and of a slide sole constituted by water-saturated siliciclastic materials. In the southern part of the study area, the slide surface was located within the Murco Formation in the upper part of the collapse and just above the Murco Formation downslope. The collapse was frontally confined as it was blocked downslope by a topographic high that folded the whole limestone succession. In the northern part of the study area, the slide surface was also within the Murco Formation in the upper part, but occurs within the limestone succession downslope, due to higher subsidence that buried the sediments more deeply. The compressional structures affecting the limestone succession in the south are not observed there, suggesting that the toe of the collapse was not blocked here.

In the Northern Owyhee Mountains (SW Idaho), a >200-m-thick flow of the Miocene Jump Creek Rhyolite was erupted on to a sequence of tuffs, lapilli tuffs, breccias and lacustrine siltstones of the Sucker Creek Formation. The rhy-olite lava flowed over steep palaeotopography, resulting in the forceful emplacement of lava into poorly consolidated sediments. The lava invaded this sequence, liquefying and mobilising the sediment, propagating sediment subvertically in large metre-scale fluidal diapirs and sediment injectites. The heat and the overlying pressure of the thick Jump Creek Rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the Sucker Creek Formation units, and the formation of metre-scale loading structures (simple and pendulous load casts, detached pseudonodules). Density contrasts between the semi-molten rhyolite and liquefied sediment produced highly fluidal Rayleigh-Taylor structures. Local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment. The result is a 30–40-m zone beneath the rhyolite lava of extremely deformed stratigraphy. Brittle failure and folding is recorded in more consolidated sediments , indicating a differential response to loading due to the consolidation state of the sediments. The lava-sediment interaction is interpreted as being a function of (1) the poorly consolidated nature of the sediments, (2) the thickness and heat retention of the rhyolite lava, (3) the density contrast between the lava and the sediment and (4) the forceful emplacement of the lava. This study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture.

Soft-sediment deformation structures can provide valuable information about the conditions of parent flows, the sediment state and the surrounding environment. Here, examples of soft-sediment deformation in deposits of dilute pyroclastic density currents are documented and possible syn-eruptive triggers suggested.

Los sedimentos como arenas finas y limos sin cohesión poseen una alta resistencia al corte, siendo capaces de soportar grandes cargas sin producirse alteraciones en su estructura interna. Sin embargo, ya sea por causas naturales o artificiales, estos materiales pueden perder resistencia cambiando su estado al de líquidos viscosos. Este mecanismo es denominado licuefacción o licuación de suelos (Allen1982).
El presente trabajo forma parte de un estudio geomorfológico y sedimentológico que comprende la extensión del tramo continental del sistema de fallas Magallanes-Fagnano (SFMF) en Tierra del Fuego. Corresponde a una región tectónicamente activa, influenciada por el borde de placas Sudamérica al norte y de Scotia hacia el sur. Las estructuras sedimentarias que aquí se presentan fueron reconocidas en dos sitios a lo largo del área afectada por el sistema de fallas, cuyo rumbo promedio es de 290°N. En territorio argentino, el SFMF afecta al lago Fagnano a lo largo de casi 87 km en sentido oeste-este, y puede reconocerse, a lo largo de aproximadamente 68 km desde la costa oriental del lago hasta la costa atlántica, por elementos morfotectónicos como escarpes de falla, cauces desplazados y/o controlados tectónicamente y lagunas.
Con el objetivo de identificar evidencias de paleosismicidad en la región afectada por el SFMF, se realizó el relevamiento de los acantilados del margen sur del lago Fagnano y varios sitios a lo largo de la traza de la falla seleccionados a partir del análisis de imágenes satelitales.
Se reconocieron estructuras de licuefacción en 2 sitios, el sitio 1 con sedimentos naturalmente expuestos en los acantilados sur del lago Fagnano y el sitio 2 en afloramientos sin cobertura vegetal situados en la costa sur de la Laguna Udaeta, afectados ambos por el SFMF. El perfil sedimentario del sitio 1 está compuesto por till basal, con presencia de niveles glaciofluviales intercalados entre niveles de till, y pequeñas cuencas lacustres compuestas por niveles de arcilla y arena muy fina laminadas y masivas, también intercalados en till. Estas secuencias corresponden a depósitos glacigénicos, resultado de los diferentes avances glaciarios que modelaron la región a lo largo del último ciclo glacial (Coronato et al. 2009; Ponce et al. 2014). El sitio 2, está ubicado 30 km al este del lago Fagnano, la secuencia sedimentaria posee en su base niveles alternantes de arcilla de color gris oscuro y claro, corresponden a un ambiente lacustre, hacia el techo es seguida por estratos alternantes de arena de color gris oscuro y arcilla gris claro, finalmente la secuencia posee estratos antropizados de grava fina de color gris claro con abundantes raíces de árboles, que en diferentes sectores aparecen quemados. La edad de estas unidades fue asignada al Holoceno (Onorato et al. 2015).
Las estructuras de licuefacción observadas fueron agrupadas en dos tipos principales; inyecciones de arenas (Figura 1a) y cuerpos irregulares predominantemente arenosos (Figura 1b). El primer grupo muestra estructuras cuyas dimensiones varían de 8 a 50 cm de largo y de 0,5 a 6 cm de ancho. No poseen a escala visible estructuras internas tales como laminación ni clastos orientados. En general, son subverticales a verticales y están constituidas por arenas de tamaño medio a grueso. La procedencia inferida del material que rellena las estructuras es desde los niveles inferiores hacia la parte superior del perfil. El segundo grupo de estructuras tiene dimensiones variables, con medidas promedio de 35 cm de alto y 40 cm de ancho. Están rellenas por arenas de tamaño fino, medio y grueso, en algunos casos poseen gravas finas de hasta 1 cm (Figura 1b). Para este grupo de estructuras, sin estructura interna definida, la procedencia del material de relleno no es clara, pudiendo ser tanto desde los estratos superiores como desde los inferiores.
Los agentes desencadenantes que dan origen a las estructuras de licuefacción pueden estar vinculados a procesos durante la depositación o a procesos externos, como por ejemplo terremotos (Owen 1987, 1996; Audemard y De Santis 1991). Las estructuras que aquí se presentan son consideradas de origen sísmico, están situadas en un área sísmicamente activa, donde histórica e instrumentalmente se ha registrado actividad sísmica. La evidencia morfológica de actividad cuaternaria activa posee rasgos muy recientes, por ej. la escarpa creada por el terremoto de 1949, descripta por Schwartz et al. (2002) en el Lago Fagnano como un escarpe muy degradado, de entre 0,50 y 1 m. El bloque hundido de la falla posee árboles secos aún de pie, resultado de inundaciones y bloqueo de drenaje causados por el depósito de seiche asociado con el sismo de 1949. En el Río San Pablo, a unos 15 km de la cabecera del Lago Fagnano en sentido este, la escarpa mide de 5 a 11 m, con un bloque elevado hacia el norte que expone depósitos cuaternarios fluvioglaciares y la formación de niveles de terrazas sucesivas. Schwartz et al. (2002) identificaron grietas tensionales en échelon, grábenes coaxiales y valles. La evidencia estratigráfica permite la interpretación de tres eventos sísmicos durante los últimos 8 ka (Schwartz et al. 2002), con un intervalo de recurrencia de 2-2.7 ka. Recientemente, Costa et al. (2006) indicaron que las rupturas debieron ocurrir al menos en dos eventos anteriores a 1949, durante los últimos 9 ka.
Considerando que los sedimentos involucrados son susceptibles a la licuefacción y que la región de estudio es afectada intensamente por la actividad tectónica del SFMF, se propone como origen probable de estas estructuras sedimentarias de licuefacción a los terremotos de magnitud moderada-alta que afectaron Tierra del Fuego, durante los últimos 9 ka. Sin embargo el estudio en marcha sobre de los rasgos neotectónicos y paleosísmicos presentes en la región, así como el análisis micromorfológico de los rellenos de las estructuras permitirá obtener una más acabada definición de los ambientes de sedimentación, la diagénesis y la deformación postdepositacional.

Las estructuras de deformación en sedimentos blandos comprenden las alteraciones que se producen casi simultáneamente con la sedimentación; sin embargo, cuando se reconocen resulta difícil diferenciar entre su origen sedimentario y diagenético o su carácter de auténticas sismitas, pues los procesos que las generan son similares, diferenciándose tan sólo en el fenómeno desencadenante. Las estructuras de deformación dependen de las características de los sedimentos y los procesos que las afectan, como efectos por carga, debido a la presión de la columna litológica, o por presión dirigida causada por sismos.
La Laguna Udaeta (54°33'32.70" S y 66°44'52.36" O), se localiza en el centro-este de la Isla Grande de Tierra del Fuego, aproximadamente a 30 km al E de la localidad de Tolhuin y de la cabecera del Lago Fagnano (Fig. 1.a). La región abarca parte del borde de placas del sistema transformante Magallanes-Fagnano (STMF), el rumbo promedio de la estructura es de 290°N. El sistema de fallas tectónicas comprende distintos tramos dispuestos en una geometría en escalón, con sentido dominante del movimiento a lo largo de la falla de tipo sinestral, desplazándose a una velocidad de 6,6 mm/año (Smalley et al. 2003). La secuencia sedimentaria estudiada tiene aproximadamente 2,50 m de espesor (Fig. 1.b); se localiza en el margen S del cuerpo de agua y está excavada sobre la ladera del escarpe. Está constituida por tres unidades. La unidad I (desde su base y hasta 1,30 m) posee niveles alternantes de arcilla de color gris oscuro y claro, correspondiendo a un ambiente lacustre. En su parte inferior, posee cuerpos de formas irregulares arenosos muy oxidados, sin estructura interna. Por encima, la unidad II, de 0,35 m de espesor, está compuesta por estratos alternantes de arena de color gris oscuro y arcilla gris claro. En general, ambas unidades se encuentran muy deformadas (Fig. 1.c), con estructuras de escape de fluidos, plegamientos, estructuras en flama, inyecciones de arcillas, arenas finas y otras constituidas por gravas finas a medias. En su parte media se reconoce un lente con núcleo arenoso bien seleccionado, con grava fina hacia los bordes, éste es atravesado casi verticalmente por un dique clástico de aproximadamente 1 m de largo y 9 cm de ancho, constituido por arena y grava. La secuencia esta coronada por la unidad III, que está compuesta por grava fina de color gris claro de ambiente fluvial, y por encima material antropizado y muy disturbado por raíces de árboles. La edad de estas unidades es incierta, sin embargo la datación realizada en una tefra reconocida en la base arcillosa de una turbera ubicada a 350 m en sentido NO del perfil estudiado, permitiría asignarles una edad holocena.
Los antecedentes sísmicos registrados indican un significativo número de terremotos de baja a mediana magnitud que afectaron la totalidad de Tierra del Fuego (Costa et al. 2006; Sabbione et al. 2007; entre otros) desde el año 1879, incluyendo un sismo fuerte ocurrido el 2 de febrero de ese mismo año, de magnitud 7,0-7,5 (Sabbione et al. 2007), similar al ocurrido el 17 de diciembre de 1949, de magnitud 7,8. Localmente, en la periferia de la laguna, se han realizado estudios dendrocronológicos (Pedrera et al. 2014) que identificaron estructuras de deformación en los árboles como consecuencia de la ruptura del escarpe del STMF ocurrida en los años 1883+5 y 1941+10, lo cual es coincidente con los eventos sísmicos de 1879 y 1949 catalogados para Tierra del Fuego.
Las estructuras sedimentarias que son generadas por efectos de carga están relacionadas con la gravedad y actúan en las fases iniciales de la depositación. Una de las condiciones necesarias para que se produzcan las deformaciones de este modo es que los niveles suprayacentes sean de granulometría más gruesa. Por otro lado, algunos autores (Obermeir et al. 1990, 1993; Tinsley et al. 1985 y Amick et al. 1990, en Rodríguez Pascua 1997) sugieren además, una potencia mínima de 3 m como condición para que la carga sedimentaria cause deformaciones en los sedimentos subyacentes. Esta condición no se cumple para el nivel superior del perfil descripto (Unidad III), ya que si bien está constituido por gravas fluviales, su espesor es sólo de 0,80 m.
En este estudio se propone que las estructuras de deformación identificadas en el borde S de la Laguna Udaeta tendrían un posible origen sísmico. Si bien las características litológicas del perfil en el que fueron identificadas favorecen la formación de estas estructuras, sin implicar necesariamente la ocurrencia de uno o varios eventos sísmicos, es posible considerar a la sismicidad como proceso desencadenante; fundamentalmente debido a la situación tectónica en la que se encuentra respecto al sistema transformante Magallanes-Fagnano y considerando los antecedentes sísmicos registrados en el área de estudio.

The Torridonian of NW Scotland is a thick, sandstone-dominated succession that unconformably overlies Archaean to Palaeoproterozoic metamorphosed Lewisian basement of the Hebridean terrane and is unconformably overlain by the Cambrian of the Caledonian foreland. The Applecross Formation comprises over 3000 m of homogeneous, cross-bedded, coarse to pebbly red sandstone with little mudstone, and few reliable stratigraphical markers. Its accumulation pre-dates the advent of land vegetation that took place from Ordovician times. Depositional architecture is characterised by laterally extensive sandstone bodies representing a braided fluvial system with high width-to-depth ratio channels. Soft-sediment deformation structures are abundant, complex in geometry and large in scale. Using criteria relating to the driving forces, deformation mechanism and timing of deformation, distinct styles of soft-sediment deformation are recognised. They relate to liquefaction or fluidization triggered by: (1) flood-induced turbulence or rapid sedimentation in active channels; (2) seismically induced liquefaction of near-surface waterlogged sediment in active or recently active terminal splays; and (3) fluidization by upwelling groundwater in inactive parts of the system, possibly near the toes of inter-channel areas of distributive fluvial systems and triggered by earthquakes or groundwater movements. The soft-sediment deformation demonstrates that groundwater levels remained high in inactive parts of the fluvial system and this is likely a contributing factor to the unusual abundance of soft-sediment deformation in the Applecross Formation.

Trembling assisted soft-sediment deformation structures of Karewa Group exposed in Kashmir valley
have been studied to understand the depositional environment and deformational stability of the Karewa Basin in
space and time. The study reveals the dominance of lacustrine-fluvial depositional environment during the PliocenePleistocene time with interference of number of syn- and post- depositional trembling quakes of different intensities
in the basin. The association of such soft-sediment deformational structures along with major sedimentary features
distributed laterally and vertically supports the pronounce past-trembling quakesin Karewa Basin during PliocenePleistocene time resulted from southwest upliftment of Pir-Panjal range (4 Myr) during the post-Tertiary Himalayan
Orogeny. The present study of past-seismic characteristics and trends may be significant for prediction of future
earthquakes and also deduce the past-seismic intensities of great earthquakes.

Seismogenic sedimentary structures—seismites—provide an opportunity to understand the effects of syndepositional tectonics on lacustrine sedimentation. The lowermost Wilkins Peak Member (Eocene Green River Formation, Wyoming, USA) contains laterally extensive intervals displaying folds, load structures, mixed brittle–ductile deformation, microfaults, breccias and sedimentary dykes, which formed intrastratally in a low-energy, sublittoral lacustrine environment. They developed in situ by liquefaction and plastic or brittle behaviour of semi-lithified sediment, or by remobilization and injection of granular material. Their morphological attributes and lateral extent, zonation in deformation intensity, and the depositional setting all imply deformation triggered by earthquake-induced stresses in shallow-buried sediment. Their stratigraphic distribution coincides with a shift in hydrological conditions from a balanced-filled to an underfilled lake type, independent of climate change. This study is the first to ascribe seismites to a pulse of tectonic activity that caused a synchronous change in lacustrine evolution.

In 2012 and 2013 extensive sedimentological field work was conducted to study the depositional setting, the regional geology and facies relationships, and measure sections of the Green River Formation in all the subbasins, in Wyoming, Colorado and Utah (USA). Several pervasive horizons of seismically induced deformation structures were identified mainly in profundal lacustrine sediments. Deformation is represented by brittle and plastic behaviour, as well as sediment injection and mass transport. The deformed layers are confined by undeformed beds which implies sporadic short-lived events that affected only near-surface sediments with susceptible rheological state at the time.
Based on: (1) the tectonic setting of the lacustrine basins; (2) the sedimentary environment and sedimentological characteristics of the successions in which the deformed layers occur; (3) their lateral extent and proximity to known active fault systems during the time of the deposition of host sediments; (4) their recurrence at different stratigraphic levels, intervals showing large-scale sedimentary deformation structures are interpreted as result of in-situ loss of shear strength, formed by increased pore pressure and vertical or horizontal stress induced by seismic activity