Slump Folds Research Articles

An exceptional record of soft-sediment deformation within Pliocene deposits of Faro Drift (SW Iberia margin) - IODP Expedition 339 Sites U1386 and U1387

The occurrence of soft-sediment deformation structures (SSDS) have long been recognized in several types of sedimentary environments and deposits. However, their presence in contourite drift deposits is still unreported in the literature. In this work, we present the first detailed description of SSDS found within the Pliocene sedimentary record of the Faro Drift, recovered during the Integrated Ocean Drilling Program (IODP) Expedition 339. The Faro Drift is the largest contourite drift of the Contourite Drift Depositional System developed in the Gulf of Cadiz since the Late Miocene by the circulation of the Mediterranean Outflow Water. The SSDS were identified in archive-halves of core sections located between ∼458 and ∼ 510 m below seafloor (mbsf) (hole U1386C), and between ∼599 and ∼ 670 mbsf (hole U1387C). Their identification and characterization was made by visual core description, structural geometrical analysis in core-scan high-resolution images, and scanning electron microcopy (SEM) analysis in selected intervals. The SSDS were classified based on the exhibited geometry, structural configuration and respective kinematics. The main deformation process and potential trigger were inferred from the geometrical and kinematics analysis. We identified five categories of SSDS: i) microfaults (normal and thrust faults), ii) slump sheet (formed by several types of folds, such as eye-folds, fish-hook folds, spiral folds), iii) convolute bedding, iv) folds within debrite mudclasts', and v) sigmoidal-like structures. Although the first three are well known types of SSDS, the folds within debrite mudclasts' and sigmoid-like structures have been scarcely recognized and described at core-scale. The inferred deformation processes responsible for the formation of these SSDS were i) brittle deformation by hydrofracturing and compaction faulting (microfaults), ii) hydroplastic (ductile) deformation (slump folds, folds within debrite mudclasts'), iii) liquefaction (convolute bedding), iv) shearing by flow movement (sigmoid-like structures). The most probable triggering agents seem to have been overloading, downslope movement of slump sheet and debris flow, and shearing by currents.

The lower Barra Velha formation (Aptian) in the Atapu field, Santos basin: Geological model for a pre-salt succession

The characterization of facies, microfacies and facies successions allowed to develop a conceptual geological model for the lower succession of the Barra Velha Formation in the Atapu field, Aptian of the Santos Basin. The obtained data and interpretations were based on the detailed description (scale 1:10) of ca. 90 m of two almost continuous well-cores and petrographic analysis. The facies were defined based on their primary depositional attributes, which are well recognized despite diagenetic modifications. Lutites, microbialites, tufa-rocks, calcirudites constitute the main recognized facies. The clastic lutites recorded transport and deposition of terrigenous and intraformational clasts by hydrodynamic and gravitational processes in low energy and shallow lacustrine environment. Microbialites and tufa-rocks record in situ carbonate deposition in alkaline saturated waters. Injectites, diastasis cracks, loop bedding, slump folds, many of the normal and listric faults and fractures are syn-depositional structures interpreted as primarily induced by tectonics. Facies changes allow to divide the succession in three stratigraphic intervals. Lutites predominates in intervals 1 and 3 whereas tufa-rocks constitute the interval 2. In the lower part of interval 1 labile fresh siliciclastic components and tractive and gravitational structures are abundant, indicating dry climate but with sporadic rainfall during a period of relative high water level of the lake. The upward decreasing of siliciclastic fragments, the appearance of coarse microbialite and a fish mortality level in the upper part of interval 1, mark the transition to the tufa-rocks of interval 2. These latter features denote a relative dryer climate and higher alkalinity waters, and a low water level of the lake. The interval 3 lutites cover abruptly the tufa-rocks recording the drowning of the succession. This may be related to renewed rifting or to the beginning of the basin thermal subsidence.

Synsedimentary slump folding: Examples and consequences of an under-recognized process in epicratonic basins

Small-scale synsedimentary folds in the Eagle Ford Formation in southwest Texas formed during deposition in a shallow epicratonic basin on the North American craton. These folds formed by slumping (i.e., gravity-driven deformation) and are characterized by (i) plastic deformation of limestone beds, (ii) discontinuous bedding and thickness changes of limestone beds, (iii) horizontal compactional fabric at high angle to folded bedding, (iv) low-angle or recumbent axial surfaces, and (v) erosion of slumped intervals and/or onlapping by resumed sedimentation. Exposures of the Eagle Ford near the northwest margin of the Maverick Basin reveal slump folds in 1–2 m thick zones with observed lateral extents of 10's to 100's of meters to >1 km – actual extents are likely much larger. Curvilinear fold hinges changing trend by >90° over short distances, indicating markedly non-cylindrical fold geometries. Bed thickness changes reflect flowage of unlithified sediment during slumping, and subsequent compaction. Previous descriptions of slump folds in the Eagle Ford Formation have focused on examples from the hydrodynamic facies of the basal portion, but slump folds are also present in pelagic facies of the upper Eagle Ford. Slump folding locally represents significant shortening (e.g., 50%) and thickening (e.g., 100%) of slumped intervals, with up-dip absence of the same interval represented by missing section, bed terminations, boudinage, and/or extensional faulting. Repeated slumping can result in significant cumulative updip thinning and downdip thickening along depositional slopes. Our experience working unconventional reservoirs in a variety of North American epicratonic basins indicates that slump folding is common and underrepresented in the literature. Criteria for recognizing slumping in the subsurface include a combination of discontinuous and contorted beds, thickening into fold hinges, compactional fabric postdating folds, and inverted or repeated section. Natural fracture networks may be less regular than in similar but flat-lying beds. Occurrence of slump folded intervals adds complexity and unpredictability to unconventional reservoirs, with implications for reservoir quality, drilling, and optimal stimulation during completion.

Record of Late Neogene seismites in turbidite deposits of the Tafna Basin (NW Algeria)

The wide variety of soft-sediment deformation structures (SSDS) developed within deposits of the same age may hinder the interpretation of their origin. Some types of SSDS may appear similar though have different trigger mechanisms, while others may result from a specific mechanism. Furthermore, the development of particular SSDS may be influenced by several synchronous or semi-synchronous factors. This study deals with the recognition of SSDS trigger mechanisms with respect to lithological and deformational features of the deposits concerned. Turbidite deposits of late Neogene age in the Hadjret El Gat area (Tafna Basin) contain different types of SSDS associated with (1) slope processes (e.g., slump folds) and induced overburden pressure, coupled with broken beds and overloading structures, and (2) liquefaction and fluidisation phenomena, leading to the development of load structures, ball-and-pillow structures, water-escape structures and syndepositional faults. These two mechanisms of SSDS formation in the study area are thought to result from seismically-induced triggers. Recognition of a vertically-repeated, sandwich-like arrangement of deformed and undeformed layers along with the SSDS features ("trapped" within beds) suggests that these internally-deformed beds are seismites, the first recognized in the Tafna Basin of NW Algeria. Large earthquakes may trigger seismic waves energetic enough to deform strata and induce the development of SSDS. This hypothesis is supported here by tectonic evidence, given deposition of the Tafna Basin strata in the convergence zone between Africa and Eurasia, active since the late Neogene.

Early Late Triassic retro-foreland basin in response to flat subduction of the Paleo-Tethyan oceanic plate, SE Tibet

Syn-subduction basins bear significant implications to understand tectonic evolution of any fossil subduction zone. The late Paleozoic to early Mesozoic (Paleo-Tethyan) tectonics of the eastern and southeastern Tibetan Plateau (i.e., the Sanjiang Orogenic Belt) is featured by ocean-continent subduction systems. A huge pile of volcanic-absent sedimentary succession developed in the middle segment of the Sanjiang orogenic belt, its age and tectonic nature remain unclear. Detailed geological mapping and zircon U-Pb dating results demonstrate that the early Late Triassic volcanic-absent succession comprises the nonmarine Maichuqing Formation in the lower part and the shallow marine Sanhedong Formation in the upper part. The Maichuqing Formation consists of coarse to fine-grained sandstone, siltstone and mudstone with abundant basal erosional surfaces, trough and planar cross-beddings, ripples, mudcracks, and plant fragments. The Sanhedong Formation comprises predominantly bioclastic limestones interlayered with marl, calcareous-muddy siltstone, and calcareous sandstone with abundant bivalve fossils. Syn-sedimentation deformation structures, such as slump folds and associated normal faults are common, suggesting intense tectonism during deposition. Synthesizing sedimentary data, paleocurrent and provenance results, combined with other available data, demonstrate that the volcanic-absent succession deposited within a retro-foreland basin along the rear part of the Permian-Triassic Jomda-Weixi-Yunxian arc in response to flat-subduction of the Paleo-Tethyan Ocean during the early Late Triassic time.

The Broken Hill Block, a new structure and mineral exploration target areas

The Broken Hill Block contains the Paleoproterozoic Willyama Supergroup, host to the world-renowned lead–zinc–silver deposits known as the Line of Lode. A new large-scale geological structural interpretation has been developed based on the construction of 11 regional-scale cross-sections and one longitudinal section. Over an extended period of major folding during the Olarian Orogeny, two distinct sets of regional folds were produced. The longitudinal folds are of similar-type and mostly trend northeast. The cross-folds are subtle, mostly trend at right angles to the longitudinal folds and are open-style. Longitudinal folds in or near the Line of Lode have been reinterpreted. A newly interpreted major slump fold, central to the Line of Lode, allows correlation of rock units from east to west. The well-known Western Anticline and Eastern Syncline, which host the main Line of Lode, are normal drag folds, formed during the initial folding with their axes folded by the later slump folding. The lead lodes are older than the zinc lodes. We interpret previously mapped longitudinal folds, as formed by successive pulses within the one tectonic event. This resulted in increased folding complexity on a local scale, but not a regional scale, and with no substantive impact on the regional structure. Pitch and plunge data were used to identify 23 cross-folds, some more than 50 km long. Away from the Line of Lode, the new structure is simpler than previous interpretations and with no tectonic inversion of the stratigraphy, other than gravity slumping, and no regional-scale nappe structures. The combination of cross-folds and longitudinal folds has produced regional-scale domes and troughs throughout the block. The intersection of longitudinal fold axes with cross-fold crests are proposed to be particularly prospective for metallic sulfides with 25 named and prioritised mineral exploration target areas identified. The cross-folding has caused the ore bodies to plunge steeply to great depths north of the Line of Lode. Further to the northeast, the plunge is reversed bringing strata, containing potential northern extensions of the Line of Lode, closer to the surface. The retrograde schist zones, which are regional-scale faults or shears that have undergone retrograde metamorphism, postdate the longitudinal and cross-folding and are not considered to be related to thrust structures. Displacement of strata and structures is evident across the zones and is generally well understood. KEY POINTS A new set of regional folds, termed cross-folds, has been identified. In the Line of Lode, large-sale longitudinal folds include a major slump fold. The stratigraphic sequence is not inverted; nappes are for the most part not present. Broken Hill-type lead–zinc–silver deposits are preferentially found close to the intersection of longitudinal and cross-fold axes.

Spiral-shaped fabrics in metamorphic rocks: A new example of rotation during progressive deformation

This paper deals with a new example of spiral geometry in medium-grade metamorphic rocks. In this example, multi-microlayers are wrapped into millimeter-to-centimeter scale spirals and form pod-shaped structures 3 cm to >10 cm long and 1.5–2 cm across which are outlined by layers up to 3 mm thick and are named spiral pods. Their long axes are oriented at angles of 40–90° to mineral lineations, and are sub-parallel to the average orientation of curvilinear hinge lines of shear-related folds. The wrapping sense coincides with the sense determined by shear sense indicators and shear-related fold vergence. We conclude that the spiral pods developed during a single progressive non-coaxial deformation. Their development began with the formation of buckle microfolds and small shear-related tight to isoclinal folds with curvilinear hinge lines. Shear-related folds experienced refolding and subsequent disarticulating of their refolded hinges. The sense of rotation of hinge lines and rolling of hinges is consistent with the shear sense and these processes led to the formation of spiral pods at all stages of progressive non-coaxial deformation. Thus the spiral pods evolved from refolded folds with curvilinear hinge lines whereas spiral slump folds with a similar structure from cylindrical refolded folds.

Shallow marine glauconitization during the Proterozoic in response to intrabasinal tectonics: A study from the Proterozoic Lower Bhander Sandstone, Central India

Glauconite forms authigenically by the replacement of quartz and feldspar substrates within the Precambrian Lower Bhander Sandstone (LBS) of the Vindhyan Basin. The glauconite exhibits consistently high K2O (7.08–8.62 wt%) and low to moderate TFe2O3 (6.20–18.5 wt%). The X-Ray Diffraction study reveals an evolved to highly evolved glauconitic mica structure. The deposition of mudstone-dominated LBS took place in a shallow marginal marine environment within a low gradient epeiric sea. The overall fining upward succession of the LBS comprises multi-storied, meter scale progradational cycles defined by red mudstone and sandstone–siltstone alternation in ascending order. The repetitive cycles indicate recurrent low-magnitude basin subsidence of the epeiric seafloor. Slump folds and slide planes, associated with the wedge-shaped mud pebble conglomerate, mark each episodes of basin subsidence. The glauconite occurs within sandstone and the sandy matrix of a wedge-shaped conglomerate at the base of a shallowing upward cycle which starts with a grey colored mudstone. The enrichment factors of redox-sensitive elements elucidate sub-oxic depositional condition during the glauconitization. The redox-sensitive element ratios (V/Cr, V/Sc) corroborates the sub-oxic depositional condition for the glauconitic interval and oxic environment for the encasing sediments. The formation of glauconite in the LBS corresponds to the deepening of the basin in response to the tectonic subsidence. The meter scale progradational cycles, recording the transition from suboxic to oxic conditions, corresponds to tectonically-induced deepening and subsequent filling. This study recognizes the significance of intrabasinal tectonics in facilitating suitable redox conditions for the glauconitization process within Proterozoic shallow epeiric sea deposit.

Resedimented limestones in fault-controlled basins (Zorzino Limestone, Southern Alps, Norian, Italy): Facies types and depositional model

The Norian succession of the Southern Alps of Italy stores thick successions of resedimented basinal carbonates (Zorzino Limestone) that were deposited in fault-controlled, anoxic, intraplatform basins developed within a vast inner platform domain (Dolomia Principale). To characterise the basinal facies, to define their distribution within these basins and to identify depositional processes, four stratigraphic logs covering about 2000 m of total stratigraphic thickness from proximal to distal basinal settings were described bed by bed. Eight different lithofacies organised in 11 bed types were identified according to sedimentary structures, texture, and composition: each bed (about 4500 beds in total) was measured individually at cm scale and classified in one of the 11 bed types.Distribution of bed types in terms of events (i.e. number of beds) and cumulative thickness (i.e. volume of sediments produced by the different depositional processes) in the four selected logs change from the proximal to the distal parts of the basin, reflecting the existence of diverse dominant depositional processes at different places within the intraplatform basins. Bed by bed analyses of thick stratigraphic logs provide important hints about the distribution of different types of depositional processes, from proximal to distal basin. Frequent slump folds and mass flow deposits, with clasts mostly deriving from the slope facies itself, indicate instability of the slope. Sediment composition and facies types document a major role played by the reworking of primary deposits formed along the slope: basinal facies mostly consist of resedimented slope deposits, produced by sliding of sediments, rich in lime mud, accumulated along the slope and low-angle proximal basin, whose instability (likely enhanced by seismic activity related to the Norian extensional tectonics) produced most of the recognised facies types. The two-step depositional processes as well as the facies associations and bed types clearly indicate a different behaviour (in terms of prevailing depositional processes and facies associations) of the studied resedimented limestone with respect to siliciclastic deposits.The distribution of facies and the detailed analyses of bed types, recording the distribution of depositional processes at basinal scale, provide a tool for the interpretation of basin morphology, from the study of stratigraphic logs, that can be applied both to outcrop and to subsurface successions.

Soft‐sediment deformation structures in the Late Messinian Abu Madi Formation, onshore Nile Delta, Egypt: Triggers and tectonostratigraphic implications

The Upper Messinian Abu Madi Formation of the Nile Delta constitutes sediments deposited during the final stage of the Messinian salinity crisis (MSC). Several levels of soft‐sediment deformation structures (SSDS) were observed in the transgressive heterolithic fluvial facies deposited adjacent to the deep‐seated faults. The most common deformation features include diapiric structures, intraformational lithoclastic breccia, small‐scale normal faults, slump folds, and liquefied beds. Such association of SSDS typically resembles those described elsewhere as generated by a complex interplay of gravity‐driven and seismically induced liquefaction processes. The pore pressure measurements revealed high pore fluid density ~ 9.1 ppg equivalent density values which reflect a mild pressure disequilibrium in Abu Madi sediments. Pressure disequilibration is observed all over the studied section of the Abu Madi Formation and is not only restricted to the fluvial channel sandstones deposited during rapid loading. Therefore, allogenic seismic activity has been proposed as the main trigger for pressure disequilibrium and the development of SSDS in the Abu Madi Formation. The heterolithic nature of the Abu Madi sediments as well as the scarcity of bioturbation provides the favourable conditions for the preservation of the seismically induced deformation. The lateral variation in thickness suggests deposition of the Abu Madi Formation during periods of active subsidence which promotes the generation of the SSDS. Late MSC tectonism likely controlled the evolution and sedimentary facies variability of the Abu Madi canyon‐infill system, and therefore the distribution of Abu Madi facies varies significantly over very short distances. Abu Madi SSDS follows the spatial distribution observed in other “lago‐mare” deposits (e.g., Foes Formation and SE Spain), characterized by spatial variability and vertical rhythmic alternation between deformed and non‐deformed layers. Accordingly, regional tectonic instability in the circum‐Mediterranean margins during the late stage of the MSC is proposed.

Evidence for Upper Cretaceous seismites in the Abu Tartur area, Western Desert, Egypt

Upper Cretaceous tidal-flat sediments of the Abu Tartur region, Western Desert of Egypt, contain a variety of soft-sediment deformation structures (SSDSs) in the Mut Member of the Quseir Formation and the overlying Duwi Formation. Here, we document recognition of a set of SSDSs characterized by complex morphology and large size. The SSDS include: 1) water-escape structures, 2) minor recumbent, chevron, and slump folds, 3) syn-sedimentary minor faults, 4) neptunian dykes, 5) regular ball-and-pillow structures, flattened pseudo-nodules, and load casts, 6) syn-sedimentary undulations, 7) fractures and joints, 8) sand injectites, and 9) autoclastic breccia and brecciated siderite clasts. The dimensions of the SSDSs range from a few centimeters to several decimeters. Both extensional and contractional forces appear to have been involved in development of these structures. A detailed analysis of facies relationships, combined with consistent orientations of SSDSs and the regional tectonics of the target area suggests earthquakes as the triggering mechanism for sediment deformation and large-scale mass failure. The deformed strata are consistent with most of the well-known criteria of typical seismites. Thus, syn-sedimentary liquefaction was identified as the primary driving force in most of the observed deformation horizons for these Upper Cretaceous SSDSs in the Abu Tartur area.

South Tethyan passive margin paleoslope orientation inferred from soft-sediment deformation and fault kinematic analysis: a case study from the Cretaceous of Borj Cedria area, Tunisia

The Cretaceous outcrops of Borj Cedria-Bou Kournine area belongs to the NE-trending Atlas system of northern Tunisia. This area exposes sub-meridian folds associated with numerous N- to NW-trending major fault systems. This study together with previous surveys reveals that this N-trending folding is believed to be related to the inversion of the Jurassic and Early Cretaceous pre-existing fault zones and generated in response to the late compressive deformations. In addition, the study area provides evidence of soft-sediment deformations by good exposures of Cretaceous-aged slump sheets. Slump folds are usually associated with several meso-scale syn-sedimentary normal faulting together with frequent reworked blocs and occasional conglomeratic horizons. All these features indicate sedimentation on irregular seafloor topography. The aim of the present study is to investigate slump folds by applying techniques to reconstruct the contemporaneous slope gradient which has triggered soft-sediment deformations. Moreover, the brittle deformation is quantified using fault kinematic analysis together with the analysis of lithostratigraphic correlation and syn-sedimentary structures. Considerable thickness variations of Cretaceous deposits are interpreted as controlled by normal faulting activities. Likewise, fault kinematic analysis typifies a regional pure extension that trends NNW during Barremain, NNE during Albian, and probably NW during Cenomanian time. Based on slump folds analysis, the inferred sub-marine paleoslope is believed to have a northward dipping during Barremian and a NNE-dipping during Albian time. On the light of the over-mentioned interpretations, it is believed that Cretaceous sedimentation of the study area is highly controlled by major syn-depositional normal faults associated with intra basin growth faulting. These fault systems seem to be related to the Southern Tethyan expansion of the rifted continental passive margin.

Slumping as a record of regional tectonics and palaeoslope changes in the Satpura Basin, central India

Abstract Soft-sediment deformation structures play an important role in interpreting regional tectonics and basin evolution during slumping events. The Satpura Basin is interpreted as pull-apart with a monoclinal northerly palaeoslope throughout its evolution. The basin formed as a result of sinistral strike-slip faulting, induced by the ENE–WSW-trending Son-Narmada South fault in the north and the Tapti North fault in the south. We have analysed the slump folds within the basalmost Talchir Formation and related these to regional tectonics and palaeoslope changes in the Satpura Basin. The glaciofluvial strata of the Talchir Formation, exposed in the southern part of the Satpura Basin, record intricacies of folds created during slumping. Several fold styles can be distinguished, within alternations of competent sandstone and incompetent shale layers, some of which indicate buckling. Upright folds, resulting from pure shear, underwent rotation of their axial planes and fold axes during simple shear-dominated progressive deformation when the slump moved downslope. The soft-sediment deformation structures that we have studied show refolding patterns that closely resemble comparable folds known from lithified rocks. These layers with refolded structures are overlain by unde-formed sediments, which proves that they are the product of a single ongoing slumping process, rather than of successive deformation events. Our analysis of their fold axes and axial planes, together with fold vergences and thrust directions within the slumps, suggests a mean slumping direction towards the southwest. Analyses of slump folds and their relationship with regional tectonics have allowed us to reinterpret basin evolution history. The southwesterly trending palaeoslope of the basin suggest that the slope of the basin was not uniform throughout its evolution. At the opening, the oblique slip fault, which trended NE–SW, generated due to movement along the ENE–WSW basin bounding faults, was more active and triggered slumping event within the Talchir deposits in the basin. With progressive overlapping of the basin-bounding faults, the Satpura Basin gradually tilted towards the north.

Lacustrine Slope-Related Soft-Sediment Deformation Structures in the Cretaceous Gyeokpori Formation, Buan Area, SW Korea, and Volcanism-Induced Seismic Shocks as Their Possible Trigger

The Gyeokpori Formation in the Buan volcanic area primarily contains siliciclastic rocks interbedded with volcanoclastics. These sediments are characterized by a variety of soft-sediment deformation structures (SSDS). The SSDS in the Gyeokpori Formation are embedded in poorly sorted conglomerates; slump folds are also present in the formation. The deformation mechanisms and triggers causing the deformation are not yet clear. In the present study, the trigger of the SSDS in the Gyeokpori Formation was investigated using facies analysis. This included evaluation of the reworking process of both cohesive and non-cohesive sediments. The analysis indicates that the SSDS are directly or indirectly associated with the alternation of conglomerates and mud layers with clasts. These layers underwent non-cohesive and cohesive deformation, respectively, which promoted SSDS formation. The slump folds were controlled by the extent of cohesive and non-cohesive deformation experienced by the sediment layers in the slope environment. The SSDS deformation style and morphology differ, particularly in the case of reworking by slump activity. This study contributes to the understanding of lacustrine slope-related soft-sediment deformation structures.

Deformation Structures in a Large Slump Horizon, Paleoproterozoic Vempalle Formation, Cuddapah Basin, Southern India

Occurrence of slump folds and associated faults generated by soft-sediment deformation from the Paleoproterozoic Vempalle Formation, southwestern Cuddapah basin, India, is being reported here for the first time. The slump horizon is preserved within a more or less undeformed shallow to deep-water carbonate unit in the Cuddapah basin exposed near Parnapalle village, Andhra Pradesh, India. The stratigraphic framework includes the Gulcheru Quartzite, lowermost unit of the Cuddapah basin succession, deposited in an alluvial to shallow marine shelf environment, and the overlying Vempalle Formation, representing a ramp-type stromatolitic carbonate platform. The synsedimentary sliding along a steepened ramp is evidenced by northeast-verging kink-like folds with wavelength up to 400 m and an overprinting set of thrusts with ramp-flat geometry, fault-cored folds or small break-thrusts showing top-to-southwest displacement, and smaller congruent folds. From the isolated occurrence within a generally undeformed succession, association of structures, and the stratigraphic context, we suggest soft-sediment deformation at the toe of a large (kilometer-scale) slump, with the prevalent bedding-parallel anisotropy exploited for common flexural slip and ramp-flat geometry. In addition, the bedding-parallel slickensides in the Gulcheru Quartzite immediately below the Vempalle Formation indicate a top-to-east-northeast, normal sense of slip representing extensional slip at the slump head. Northeast-verging large folds in the Parnapalle slump horizon possibly represent structures formed during the translation phase of slumping. As the large northeast propagating slump was halted, the backthrust-like structures and associated folds developed at the slump toe.

Slump folds within mid-Miocene crevasse-splay deposits: a unique example from the Tomisławice lignite opencast mine in central Poland

Non-tectonic, soft-sediment deformation structures occur in mid-Miocene crevasse-splay deposits exposed in the Tomislawice lignite opencast mine in central Poland. The crevasse-splay cross-stratified siliciclastic deposits are underlain by continuously folded and relatively thick lignite beds, and overlain by a thin undisturbed layer of lignite. Only the middle part of the crevasse-splay succession is deformed plastically in the form of folds, while the lower and uppermost beds are undeformed. Most of the intraformational deformation structures are recumbent folds, while only a few can be classified as upright folds in the initial stage of their evolution. The origin of these folds is associated here with a penecontemporaneous slumping process caused by liquefaction of sandy-muddy sediments. The slumping was triggered by an increase in the inclination of heterolithic layers caused by the compactional subsidence of an organic-rich substrate – peat. This type of subsidence occurred following a sudden siliciclastic load on top of the underlying and poorly-compacted peat/lignite seam. The initiation and development of the slump folds can be explained by differentiated loading, compaction and liquefaction processes, and the introduction of a tectonic agent is unnecessary.

Are slump folds reliable indicators of downslope flow in recent mass transport deposits?

Despite the widespread use of slump folds as indicators of palaeoslope orientation, there is a lack of detailed analysis of variations in fold geometries and orientations down the length of individual slump profiles within mass transport deposits (MTDs). To address this gap in knowledge, we have systematically recorded more than 500 structural measurements of fold hinges and axial planes along a 25 m section through a mesoscopic slump profile. Our case study is performed in wet unconsolidated (late Holocene) sediments, which are only recently exposed due to falling water levels in the Dead Sea. In this situation, the modern slope is exposed and directly visible, slumping having occurred in the past few centuries. Fold hinges define broad arcs at high angles to flow in the downslope toe of the slump and progressively swing to become sub-parallel to flow in the upslope region. Greatest amounts of shortening (~35%) are recorded at the toe, suggesting that the swing in trends of fold hinges and axial planes is a consequence of differential layer-normal shear rather than downslope strain gradients. Significant variations of >90° occur in the orientation and vergence of slump folds on either side of a 10 m wide gully, which cuts the slump sheet. In some instances, folds have nucleated around longer (>10 cm) wooden sticks that were incorporated into the slump, whereas shorter wooden fragments align parallel to the flow direction. The differences in orientations of wooden sticks and wooden fragments are consistent with differential layer-normal shear on each side of a flow cell. Evaporite concretions grew within the sediments during slumping and influenced the geometry and kinematics of slump folds, suggesting that slope failure may have been a slow ‘creep’ event generated by slope instability, rather than a result of catastrophic failure associated with large earthquakes. Our work illustrates the problems associated with using partial datasets, where classical structural analysis of transects <10 m apart would incorrectly suggest slump directions opposed to one another by 90°. This study thereby highlights the extreme variability within a downslope profile of a single slump. It may therefore help explain discrepancies in regional datasets where slumps, sporadically sampled at different stratigraphic levels, may provide apparently diverse flow directions.

Distinguishing coeval patterns of contraction and collapse around flow lobes in mass transport deposits

Gravity-driven mass transport deposits (MTDs) form by the downslope-directed movement of sediment associated with slope failure. Simple models suggest that extension forms at the upslope (head) area, contraction is focussed in the downslope toe of the slump, while differential shear associated with strike-slip is restricted to the lateral margins of the slump. Although the head and toe are considered to be dominated by layer-parallel shear (LPS), differential layer-normal shear (LNS) may be generated around the lateral margins of slumps and potentially also within MTDs where flow has been separated into different ‘lobes’. Despite this realisation that LNS must form, there has been little work into the geometries and spatial relationships of resulting structures. Using the late Pleistocene Lisan Formation exposed around the Dead Sea Basin as our case study, we examine detailed (<10 m) relationships of folds and thrusts created during LNS and LPS, as well as investigating the role of broadly coeval extension that may reactivate these structures. We also undertake analysis of anisotropy of magnetic susceptibility (AMS) fabrics to determine flow and shear relationships around folds and detachments created during LNS and LPS. Our study shows that LPS results in gently-curvilinear fold hinges that arc around the transport direction while LNS results in cylindrical fold hinges developed oblique or sub-parallel to transport. Such folds may be recumbent or upright, and associated with lateral ramps marking areas of differential LNS within the MTD. These structures are interpreted to accommodate variations in the amount and direction of downslope-directed movement resulting in LNS around the margins of individual flow ‘lobes’ that are developed over tens of metres. These ‘lobes’ display broadly down-slope transport with locally radial flow that results in along-strike shortening between lobes. Our analysis of AMS fabrics shows that they are controlled by slump folds, but magnetic fabrics do not differentiate how these folds were created in zones of LPS or differential LNS. AMS taken from gouge formed along detachments marked by differential LNS provide a first-order indicator for the transport direction. In addition, AMS fabrics in gouge or fluidised layers directly beneath thrust ramps, reveals prolate fabrics marking a component of strike-parallel flow along the branching intersections of thrust ramps and flats. Extensional faults directly reactivate existing thrusts, or create new extensional faults that are sub-parallel to thrusts or cut across them at steeper angles. Extension is part of the same MTD event as a sedimentary cap that is deposited out of suspension following slope failure, overlies and locally thickens into the hangingwall of extensional faults to create ‘growth’ sequences. Extensional reactivation and ‘collapse’ of original thrusts may help explain why contraction is apparently ‘missing’ from many seismic sections across MTDs.

Soft-sediment deformation: deep-water slope deposits of a back-arc basin (middle Eocene-Oligocene Kırkgeçit Formation, Elazığ Basin), Eastern Turkey

The Kirkgecit Formation (middle Eocene-Oligocene) is exposed in an approximately E-W direction around the city of Elazig, eastern Turkey. This unit represents marly and sandy deep-marine sediments deposited in a slope setting with channels and contain soft-sediment deformation structures (SSDSs). These SSDSs include slump folds, chaotic strata, load casts, flame structures, convolute laminations, clastic dykes, water-escape structures and syn-sedimentary faults. The main underlying deformation processes are liquefaction, fluidization and gravity-induced loading. Some of the SSDS could be interpreted as seismites, which originated due to earthquakes that also triggered high-density mass flows. This interpretation is consistent with the active tectonics in the Elazig back-arc basin. The SSDS in the study area are compared with SSDS in other regions, as well as with SSDS in other marine deposits. In this way, findings obtained from the study area are compared with SSDS at different locations in the marine Kirkgecit Formation around Elazig and with similar structures developed in marine settings worldwide, such as SE Crete and NW Africa.

Folding during soft-sediment deformation

Abstract The detailed analysis of folding in rocks was in part pioneered by John Ramsay, and resulted in a range of techniques and criteria to define folds. Although folding of unlithified or ‘soft’ sediments is typically assumed to produce similar geometries to those in ‘hard rocks’, there has to date been little detailed analysis of such folds. The aim of this paper is therefore to investigate folds developed during soft-sediment deformation (SSD) by applying techniques established for the analysis of tectonic folds during hard-rock deformation (HRD). We use the Late Pleistocene Lisan Formation exposed around the Dead Sea as our case study, as the laminated lake sediments record intricacies of fold detail generated during seismically triggered slumping of mass transport deposits (MTDs) towards the depocentre of the basin. While it is frequently assumed that folds created during SSD are chaotic and form disharmonic structures, we provide analyses that show harmonic fold trains may form during slumping, although larger upright folds cannot be traced for significant distances and are more typically disharmonic. Our analysis also reveals a range of fold styles, with more competent detrital-rich layers displaying buckles (Class 1B), as well as upright Class 1A folds marked by thickened limbs. Class 1A buckle folds are generally considered to be created by flattening that overprints folds with an original Class 1B geometry. As thickened fold limbs are truncated by overlying erosive surfaces, the vertical flattening is considered to have occurred during the slump event. Different fold shapes may partially reflect variable flattening, depending on the original orientation of upright or recumbent folds, together with continued downslope-directed simple-shear deformation that modifies the fold geometry. Analysis of fold wavelength, amplitude and bed thickness allows us to plot strain contour maps, and indicates that beds defining slump folds display viscosity contrasts in the range of 50–250, which are similar to values estimated from folds created during HRD in metamorphic rocks. A range of refold patterns, similar to those established by John Ramsay in metamorphic rocks, are observed within slumps, and are truncated by the overlying sediments, indicating that they formed during a single progressive slump event rather than distinct ‘episodes’ of superimposed deformation. This study confirms that techniques developed for the analysis of folds created during HRD are equally applicable to those formed during SSD, and that resulting folds are generally indistinguishable from one another. Extreme caution should therefore be exercised when interpreting the origin of folds in the rock record where the palaeogeographical and tectonic contexts become increasingly uncertain, thereby leading to potential misidentification of folds created during SSD.