Slump Sheets 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.

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.

The ‘Base Cretaceous Unconformity’ in a basin-centre setting, Danish Central Graben, North Sea: A cored record of resedimentation and condensation accompanying transgression and basinal overturn

A cored section (North Jens-1) in the Danish Central Graben spans the ‘Base Cretaceous Unconformity’ (BCU) in a seismically conformable situation in a basin-centre setting. Multidisciplinary analysis of the core revealed that the apparent simplicity of the boundary in seismic profiles at this location disguises a complex, dynamic geological history. Dark, organic-rich, laminated hemipelagic/turbiditic Farsund Formation mudstones of late Ryazanian (Surites stenomphalus Zone) age are overlain by a thick slump sheet of upper Ryazanian mudstones, probably derived from relict rift-fault scarps on the basin floor. An erosionally-based gravity-flow bed (a hybrid density-flow deposit) of latest Ryazanian (Peregrinoceras albidum Zone) age forms the lowermost unit of the Valhall Formation. The lower boundary of this bed may record a minor hiatus across the stenomphalus–albidum zonal boundary and represents the BCU, compatible with seismic and log data. The hybrid bed is capped by an intensely burrowed omission surface that represents the first clear indication in this basin-centre location of watermass overturn and the demise of the stratified, poorly oxygenated basin. The biostratigraphy, ichnofabric and fauna/flora of the succeeding bioturbated marlstones and marly chalks of the lower Valhall Formation (Platylenticeras/Paratollia Zone) testify to increasingly oxygenated bottom waters and low sedimentation rates in a well-mixed, open marine, oligotrophic system; palynological data indicate that the onset of these conditions was linked to transgression. The North Jens-1 core illustrates the risk of assuming stratigraphic simplicity at the BCU surface in basin-centre, seismically conformable settings. Sedimentary reworking, already characteristic of the Farsund Formation setting, was probably exacerbated by transgressive erosion and accompanying sediment de-stabilization along the graben flanks.

Middle Ordovician mass‐transport deposits from western Inner Mongolia, China: Mechanisms and implications for basin evolution

AbstractSubaqueous mass‐transport processes are one of the mechanisms for transport of sediment into the deep sea. Internal structures and depositional processes of carbonate mass‐transport deposits are relatively poorly understood relative to siliciclastic facies due to their comparative paucity in the rock record. A variety of carbonate mass‐transport deposits, including slumps, debrites and deep‐channel‐confined density flow deposits, occur in Middle–Upper Ordovician slope deposits in western Inner Mongolia (Wuhai), China. These provide a rare opportunity to illustrate the emplacement history of carbonate mass‐transport deposits at the outcrop scale. The slumps and debrites host remarkable folds, chaotic beds and imbricated beds that reflect differences in both rheology and position on the slope. Individual slump sheets show gradations between undulating laminae, inclined and recumbent folds, highly deformed folds, and chaotic textures upslope from the toe region. Debrites are commonly interbedded with slump deposits, whereas imbricated beds are present in the middle and lower parts of the toes of slump sheets near the terminal wall. In the study area, thin‐bedded limestone with slump deposits of the Kelimoli Formation are overlain by fine‐grained, siliciclastic‐dominated, slope deposits of the Wulalike Formation. A thick breccia of the Wulalike Formation was deposited in a main feeder channel in south‐east Wuhai, but to the west‐north‐west the breccia was deposited in distributary channels possibly represented as a unique lower‐slope pattern of gullies. At the latter locality, the breccia was deposited solely within the channels on a steep west‐north‐west dipping slope under density‐driven flows. The mass‐transport deposits documented herein records passive to foreland basin tectonic transitions, and associated platform foundering and steepening of the slope. A slope facies model was constructed to demonstrate the spatial and temporal variations of mass‐transport deposits during basin evolution, and as such it provides a template for the interpretation of the deposits of ancient slopes that underwent passive to active tectonic transitions.

Syn-eruptive soft-sediment deformation structures in a deep submarine caldera: Havre, 2012

The 2012 rhyolitic eruption of Havre submarine caldera in the Kermadec arc generated an assemblage of new, soft-sediment deformation structures on the caldera floor. These structures are the first documented examples of syn-eruptive, soft-sediment deformation structures within a submarine caldera. They are exquisitely portrayed on a high-resolution (~1 m) bathymetric map made in 2015. Their timing has been resolved by means of stratigraphic relationships with an overlying giant pumice deposit that correlates with the pumice raft first observed on 18th July 2012 UTC. Weeks to months earlier in 2012, the largest lava was erupted from a vent on the southwestern caldera wall, flowed down to the caldera floor and advanced almost 1 km farther. A surficial layer of the caldera-floor sediment surrounding the leading edge of the lava failed, most likely in response to load and heat transfer from the lava. Failure produced a slump sheet comprising folds with arcuate crests at the toe, an undeformed domain in the middle and a basin-and-ridge domain at the head, adjacent to the lava. Subsequently, (1) the lava ruptured on two main faults that propagated across the caldera floor, and (2) strike slip on the faults moved the lava that was in between forward, simultaneously offsetting segments of the slump sheet. These two events were probably triggered by the earthquake swarm recorded on the 17th July 2012 UTC, a day before the pumice raft appeared. The leading edge of the lava then sank slowly into the underlying sediment. Sinking was accommodated by sideways expulsion of sediment from beneath the lava, modifying the topography around the edge of the lava. The caldera-floor sediment was also disturbed when part of the southwestern caldera wall collapsed, generating a small debris avalanche which is represented by a cluster of hummocks on the caldera floor. Collapse was a response to shallow intrusion of a rhyolite dyke into the southwestern caldera wall.

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.

Slump sheets as a record of regional tectonics and paleogeographic changes in South China

Slumps of various scales are recognized in the late Katian (late Ordovician) mixed carbonate and siliciclastic slope successions in the Changshan area, western Zhejiang Province, South China. In order to understand their deformational processes, triggering mechanisms, paleo-slope orientation and geological implications, a detailed sedimentological study was carried out on four outcrop sections. In the southern Changshan (the Putangkou, Zhoutang and Shikeng sections), we recognized three slumping episodes (ca. 3.5 m, 11 m, and 73 m in thickness in stratigraphically ascending order) and co-occurred debris flows. At the Sanqushan section (~11.5 km north of Changshan), thirteen slump units, each being tens of cm to 6 m in thickness, are recognized. Facies analyses and diagenetic characteristics of these slumps and their internal deformation structures suggest that they formed as a result of plastic and brittle deformation, and liquefaction and fluidization during slumping. The rip-up blocks, cracks and ruptured beds in the slumps of southern Changshan indicate extensional features, whereas recumbent and disharmonic folds in the slumps of northern Changshan represent compressional features. These spatially different features suggest that the slumps may have been generated on a paleo-slope dipping generally to the north. The case study of slumps and debris flows from the Putangkou section indicates a significant mass flow transition from a slumped mass into debris flow deposits. Autogenic triggering mechanisms of these slumps, e.g., rapid deposition, storm-wave loading, and tidal shear, are ruled out by the results of facies analysis. Instead, the slumps might have been triggered by relative sea-level changes or tectonically-induced, progressively steepening of the slope, which is likely related to the contemporaneous Kwangsian Orogeny.

Soft-sediment deformation structures related to tectonomagmatic activity: A case study from the borate-bearing lacustrine deposits of early Miocene Bigadiç Basin, NW Turkey

This study documents soft-sediment deformation structures in the lacustrine Bigadiç Basin, one of the early Miocene terrestrial basins in the extensional tectonomagmatic province of northwestern Anatolia. The focus is on the Upper Borate Unit of the basin-fill succession, which consists of sedimentary facies including massive conglomerates, stratified sandstones, siltstones, organic-rich claystones, laminated limestones, organic-rich limestones, marlstones, borates and volcaniclastics. Their deposition, including borates, occurred in a deep lacustrine environment. Possible phases of lake shallowing are represented by stratified sandstones facies. Soft-sediment deformation features include small-scale folds formed by convolution with occasional basal décollement (minor slumps); load casts and related flame structures; fragmented and mixed beds, brecciated by pore-water explusion; shallow- and deep-rooted synsedimentary extensional faults with related neptunian dykes; and large slump sheets of thick bed packages with recumbent folds. These deformation features are interpreted to be of seismo-tectonic and seismo-volcanic origin, related to syndepositional strike-slip faulting and volcanic eruptions. Seismic tremors combined with the basin floor updoming by magma intrusions caused deep-rooted normal faults and intra-basinal slumps tens of metres thick. The study may serve as a guide to similar deformation structures in the underlying part of the Bigadiç lake-fill succession and in the other Neogene basins of northwestern Anatolia – which shared the region's seismo-tectonic, magmatic, palaeogeographic and sedimentation history.

Sedimentary response to tectonic uplift of the Dukla basin margin recorded at Skrzydlna – the Menilite Beds (Oligocene), Outer Carpathians, S Poland

The Menilite Beds (Oligocene of Polish Flysch Carpathians) at Skrzydlna crops out in a structurally complex zone of the Fore-Magura Unit, which is tectonically overridden by Magura Nappe thrust form the S. The exposed sedimentary suite, representing the Dukla Basin, consists of fine-grained, well organised strata deposited in a low-energy, deep marine basin, which are abruptly overlain by poorly organised, coarse sandy conglomerate that forms a Mass Transport Deposit (MTD) complex. The MTD contains large boulders of extrabasinal rocks, massive sandstone beds with intrabasinal mudstone clasts, and slump sheets of sandstones. Above rests a fining- upwards sequence of sandstone beds interlayered with mudstones. The fine-grained facies reappear above to terminate the exposed succession. Erosional contacts and rapid facies changes, both vertical and lateral, are characteristic for the MTD unit. The sandstone-mudstone unit above contains laterally migrating erosional channels filled with massive sandy conglomerate in the lower part. Turbidites of varying density and completeness of internal structures that occur above are accompanied by an association of mixed facies including large-scale dune cross-bedding. Mineralogically, the sandstones are quartz arenites, sub-lithic arenites and wackes. Calcarenite grains – bioclasts, micrite and marl occur in substantial proportions only in the uppermost part of the succession. The point-counting data plotted on Qm-F-Lt diagram are clustered within the recycled fields: quartzose and transitional. Mineralogical maturity of the sandstones has the tendency to decrease from the sub-MTD strata upwards via the MTD unit to the lower part of the sandstone-mudstone complex; then it increases to the youngest sandstone beds with carbonate grains. These tendencies, associated with sedimentary features of the succession, reflect rapid uplift, emergence and progressive erosion of the terrigenous detritus source area, followed by tectonic stabilisation reflected by the appearance of the “carbonate factory”.

Seismogenic rotational slumps and translational glides in pelagic deep-water carbonates. Upper Tithonian-Berriasian of Southern Tethyan margin (W Sicily, Italy)

Soft-sediment deformation structures (SSDSs), which reflect sediment mobilization processes, are helpful to identify punctual events of paleoenvironmental stresses. In the upper Tithonian-Berriasian calpionellid pelagic limestone of the Lattimusa Fm. outcropping in the Barracù section (W Sicily), paleoenvironmental restoration reveals the occurrence of a deep-water flat basin, characterised by undeformed planar bedding, laterally passing to a gentle slope where the deformed horizons alternate with undeformed beds. Here, two types of gravity slides have been differentiated on the basis of different kinds of SSDSs, brittle deformation, involved lithofacies, geometry and morphology. Type 1 slump sheets, involving marl-limestone couplets, are characterised by chaotic stratification, truncated beds, recumbent folds, tension gashes, clastic dikes and small-scale listric normal faults and thrusts. They moved downslope as a rotational slump, displaying the typical wedge-shaped morphology, becoming thicker toe-wards. Type 2 slump sheets, involving bed packages of lime mudstone, are characterised by arcuate slump scars developing downwards as listric normal faults, associated with rotational beds and bulging. Displaying tabular geometries, they moved coherently along an overall bedding parallel detachment surface as a translational glide characterised by sediment creep.Regional geology indicates that the basin (Sicanian), bordered by a stepped carbonate platform, was affected by synsedimentary tectonics throughout the Mesozoic. The synsedimentary extensional tectonics that affect the Upper Triassic-Jurassic deposits with steeped normal faults, causing tilt-block and instability of the seafloor through fluidization processes, triggered the rotational slumps of the lower portion of the section. Seismic shocks, induced by outside sector tectonics, like those recorded in the adjacent Busambra stepped carbonate platform margin, triggered the rotational slumps that are not related to local-scale faults. To explain the translational glides of the upper portion of the section, the aforementioned outside sector earthquakes produced instability of the sea-floor through thixotropy of marls. In this case, the driving forces (i.e., gravitational) were favoured by the uplifting trend that is well recorded in the Southern Tethyan continental margin.We suggest that the various early deformations represent rotational slump and sediment creep that may be characteristic for slump structures in pelagic carbonates, depending on the involved lithologies (i.e., paleoenvironmental setting) and the intensity/distance of the trigger mechanism (i.e., earthquake epicentre).

Sedimentary and structural controls on seismogenic slumping within mass transport deposits from the Dead Sea Basin

Comparatively little work has been undertaken on how sedimentary environments and facies changes can influence detailed structural development in slump sheets associated with mass transport deposits (MTDs). The nature of downslope deformation at the leading edge of MTDs is currently debated in terms of frontally emergent, frontally confined and open-toed models. An opportunity to study and address these issues occurs within the Dead Sea Basin, where six individual slump sheets (S1–S6) form MTDs in the Late Pleistocene Lisan Formation. All six slumps, which are separated from one another by undeformed beds, are traced towards the NE for up to 1km, and each shows a change in sedimentary facies from detrital-rich in the SW, to more aragonite-rich in the NE. The detrital-rich facies is sourced predominantly from the rift margin 1.5km further SW, while the aragonite-rich facies represents evaporitic precipitation in the hyper saline Lake Lisan. The stacked system of MTDs translates downslope towards the NE and follows a pre-determined sequence controlled by the sedimentary facies. Each individual slump roots downwards into underlying detrital-rich layers and displays a greater detrital content towards the SW, which is marked by increasing folding, while increasing aragonite content towards the NE is associated with more discrete thrusts. The MTDs thin downslope towards the NE, until they pass laterally into undeformed beds at the toe. The amount of contraction also reduces downslope from a maximum of ~50% to <10% at the toe, where upright folds form diffuse ‘open-toed’ systems. We suggest that MTDs are triggered by seismic events, facilitated by detrital-rich horizons, and controlled by palaeoslope orientation. The frequency of individual failures is partially controlled by local environmental influences linked to detrital input and should therefore be used with some caution in more general palaeoseismic studies. We demonstrate that MTDs display ‘open toes’ where distributed contraction results in upright folding and shortening rather than distinct thrusts. Such geometries may account for some of the contraction that is apparently missing when balancing seismic sections across large off shore MTDs.

A revised sedimentary and biostratigraphical architecture for the Type Llandovery area, Central Wales

AbstractThe global standard for the Llandovery Series (early Silurian) in central Wales is re-assessed in the light of detailed geological surveying, biostratigraphical sampling and a rigorous examination of published datasets. A new sedimentary and biostratigraphical architecture is presented. Key graptolite, brachiopod, acritarch and, for the first time, chitinozoan assemblages are critically assessed. Upper Hirnantian to Aeronian strata record events that followed the Late Ordovician glacial maximum and comprise a series of progradational sequences bounded by flooding surfaces, but inferred still to be glacioeustatic in origin. Significant faunal renewals associated with many of the flooding levels underpin their potential for international recognition. Compound non-sequences are a feature of proximal parts of the system where erosion associated with fault footwall uplift was an important process. Extensive slump sheets contribute to further stratal loss and displacement in distal facies. A re-assessment of the Aeronian Stage GSSP reveals shortcomings with the biostratigraphical criteria used in its selection. Telychian portions of the succession display the disrupting effects of intra-Wenlock synsedimentary sliding; hence the relevance of key published fossil assemblages and the criteria used to erect the stage GSSP are undermined. However, the Llandovery area remains one of the best studied early Silurian successions in the world. This, together with regional considerations, supports the retention of the series standard in mid Wales where the contiguous deep-water basinal succession affords internationally cited exposure of richly graptolitic facies for the whole series and, significantly, for the post-sedgwickiiBiozone interval.

Tectonic triggering of slump sheets in the Upper Cretaceous carbonate succession of the Porto Selvaggio area (Salento peninsula, southern Italy): Synsedimentary tectonics in the Apulian Carbonate Platform

Soft-sediment deformation structures crop out in the Upper Cretaceous carbonate succession in Porto Selvaggio cove in the western Salento peninsula, Apulian foreland, southern Italy. The deformed interval is about 13m thick and occurs between shallow-water limestones and dolostones formed in peritidal and shallow subtidal environments. It comprises well-bedded grey mudstones interlayered with dark grey laminated microbioclastic wackestones characterized by couplets of closely spaced dark and bright laminae marked by the parallel orientation of calcareous microbioclasts and thin-shelled bivalves. The low biological diversity, scarcity of burrowing biota, and presence of a well preserved fish fauna provide evidence of anoxic conditions occurring in morphological depressions within the platform, and a stagnant, stratified water body affected by weak bottom currents, indicating the sudden development of a localised and short-lived intraplatform basin. Two soft-sediment deformation horizons (slump sheets) separated by undeformed limestones with similar facies occur in this part of the succession. The lower, thicker slump sheet (1.0–1.3m thick) contains asymmetric and box folds. Well-developed décollement surfaces (locally containing thick brecciated zones) cut the folds, forming small-scale thrust-sheets and indicating mixed plastic to brittle behaviour. The upper, thinner slump sheet (0.25–0.35m thick) contains only asymmetric folds, indicating plastic behaviour only. The differences in deformation style are attributed to differences in facies. Measurements of fold-axis orientations in the slump sheets show that they moved in similar directions, recording the development of a local, gently dipping palaeoslope. Autogenic (internal) trigger mechanisms are ruled out by a detailed consideration of facies. The slump sheets were triggered by allogenic, tectonic effects, either the weakening of sediment by seismic activity or the tectonically induced steepening of slopes, or a combination of both. Tectonically induced steepening is consistent with localised and sudden vertical facies changes related to the creation of an intraplatform basin. The occurrence of slump sheets in carbonate platform successions is unusual since carbonate platforms are normally associated with shelves or low-angle ramps.

Slump sheets in Upper Cretaceous limestones of the Porto Selvaggio area (Salento peninsula, southern Italy): a record of tectonics in the Apulian Platform

Slump sheets in Upper Cretaceous limestones of the Porto Selvaggio area (Salento peninsula, southern Italy): a record of tectonics in the Apulian Platform

Interpreting fold and fault geometries from within the lateral to oblique parts of slumps: A case study from the Anglo-Brabant Deformation Belt (Belgium)

Using a rigorous comparison of as many slump fold-based and slump-related fault-based methods as possible, also small complex data-sets can be used to deduce a reliable slump transport direction and sense. This is exemplified using late Early Ordovician to Middle Ordovician slump features of the Anglo-Brabant Deformation Belt. The suggested procedure involves (A) an analysis of fold data by means of (1) the mean axis method, (2) the separation arc method, (3) the axial–planar intersection method, (4) the fold hinge azimuth and interlimb angle method, (5) the fold axial surface strike and interlimb angle method and (6) the axial surface dip and dip direction method, (B) an analysis of slump-related fault data by means of (1) the mean fault orientation method, (2) the fault separation method, (3) the fault intersection method and (4) the best-fit girdle to fault poles method, and finally (C) a rigorous comparison of the results of the different methods. Several possible diagnostic criteria are put forward for distinguishing slump deformation features trending at low angles to transport direction (oblique to lateral parts of slump sheets or internal lobes or tongues) and slump deformation features trending at high angles to transport direction (frontal to central parts of slump sheets). These criteria include the relationship between fold interlimb angle and axial surface dip, the plunge of the fanning axis of the reverse slump-related faults and the plunge of the intersection of the reverse and normal slump-related faults. Such criteria greatly facilitate the interpretation of the often confusing results of many classical methods. Moreover, the plunge azimuth of the intersection of the normal and reverse slump-related faults and the mismatch between slump transport direction inferred from fold-based and fault-based methods, may both be diagnostic for dextrally deformed or sinistrally deformed slump sheet parts.

Lithostratigraphy of the late Early Permian (Kungurian) Wandrawandian Siltstone, New South Wales: record of glaciation?

The late Early Permian (273 – 271 Ma) Wandrawandian Siltstone in the southern Sydney Basin of New South Wales represents a marine highstand that can be correlated over 2000 km. A mainly fine-grained terrigenous clastic succession, the Wandrawandian Siltstone contains evidence for cold, possibly glacial conditions based on the presence of outsized clasts and glendonites, mineral pseudomorphs after ikaite, a mineral that forms in cold (0 – 7°C) marine sediments. A lithostratigraphic and facies analysis of the unit was conducted, based on extensive coastal outcrops and continuous drillcores. Eight facies associations were identified: (i) siltstone; (ii) siltstone with minor interbedded sandstone; (iii) interbedded tabular sandstone and siltstone; (iv) admixed sandstone and siltstone to medium-grained sandstone; (v) discrete, discontinuous sandstone intervals; (vi) chaotic conglomerate and sandstone in large channel forms; (vii) chaotically bedded and pervasively soft-sediment-deformed intervals; and (viii) tuffaceous siltstone and claystone. Using lithology and ichnology, relative water depths were ascribed to each facies association. Based on these associations, the unit was divided into five informal members that reveal a history of significant relative sea-level fluctuations throughout the formation: member I, interbedded/admixed sandstone and siltstone; member II, siltstone; member III, slumped masses of members I and II; member IV, siltstone and erosionally based lensoid sandstone beds and channel bodies; and member V, interbedded/admixed sandstone and siltstone with abundant tuffs. Member I marks an initial marine transgression from shoreface to offshore depths. Member II records the maximum water depth of the shelf. Member III is interpreted to be a slump sheet; plausible mechanisms for its emplacement include seismicity produced by tectonism or glacio-isostatic rebound, changes in pore-water pressures due to sea-level fluctuations, or an increase in sedimentation rates. Members IV and V record minor fluctuations in depositional environments from offshore to shoreface water depths. Member IV includes regionally extensive, large channel bodies, with composite fills that are interpreted as storm-influenced mass-flow deposits. Member V includes a greater abundance of volcanic ash. Glacial controls (isostasy, eustasy) and tectonic affects may have worked in concert to produce the changes in depositional environments observed in the Wandrawandian Siltstone.

Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gürsökü Formation (Kavak–Samsun, N. Turkey)

The Late Cretaceous Gürsökü Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gürsökü Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous.

Seismically-induced slumps in Lower-Maastrichtian peritidal carbonates of the Apulian Platform (southern Italy)

In the southeastern Murge (Apulian Foreland, southern Italy), a Lower-Maastrichtian carbonate section showing soft-sediment deformation structures is composed of peritidal lithofacies associations developed in a wide low-energy inner-platform system. Shear-induced soft-sediment deformation structures correspond to multilayer deformed horizons, 1.5 to 2 m in thickness, sandwiched between undeformed layers. Overturned and recumbent folds lying on décollement surfaces and asymmetrical fault-inception folds allow us to interpret the observed deformed horizons as slump sheets. The detailed description of the main morphological features of the deformed beds and the recognition of unconformable surfaces and undeformed beds between slumped beds allow us to distinguish three different events of deformation. Preferential orientation of slump folds has been measured and every slumped bed shows a different direction of shortening. Randomly oriented direction of shortening suggests that each deformation event was driven by the presence of local very gentle gradients developed in peritidal settings. By excluding other potential trigger mechanisms of deformation, the observed slumps are interpreted as seismically-induced slides. Even if no data about the distance of the seismic source may be obtained, near-field earthquake tectonics affected the Apulian Platform during Late Cretaceous times.

Styles of soft-sediment deformation on top of a growing fold system in the Gosau Group at Muttekopf, Northern Calcareous Alps, Austria: Slumping versus tectonic deformation

Styles of soft-sediment deformation in a syngrowth coarse-grained sediment gravity flow system are described from outcrops of the Upper Gosau Subgroup of the Northern Calcareous Alps. The deepwater sediments were deposited along the flanks and top of growing folds. Beside load and fluid escape structures, which are commonly found in sediment gravity flow systems, soft-sediment deformation is mainly related to fluidization of coarse-grained beds and downslope gliding of meter-thick sediment packages. Common structures affecting single or several beds are: hydroplastic folding of fine-grained beds in a coarse-grained fluidized matrix, symmetric or asymmetric mullions at the base of coarse-grained beds, and folding of heterolithic units into coarse-grained beds. Folds have varying geometries and styles, and fold axes are scattered. Soft-sediment folds also affect thick sediment packages. These folds are uniformly verging asymmetric metric to dekametric folds, formed at different degrees of lithification. Rheology of sandstones in such folds ranges from ductile to semi-brittle and brittle. Fold axes tightly cluster about one direction. The first group of deformational structures is attributed to slump-related deformation, whereas the second group represents folds related to tectonic deformation. A dislocation model and a shear zone model are used to interpret orientation of folds related to slump deformation. This considers processes taking place on the scale of the whole slump sheet and within the shear zone at the base of gliding units. Both models predict rotation of fold axes toward the transport direction of the slump. Field data support that fold axes scatter either about the downslope transport direction or about the strike of the slope.

Large-scale slumping deduced from structural and sedimentary features in the Lower Palaeozoic Anglo-Brabant fold belt, Belgium

Distinguishing slump folds from tectonic folds in poorly exposed areas can be difficult, especially when the scale of the slump folds exceeds outcrop scale. In the southeastern part of the single-phase deformed, Lower Palaeozoic Anglo-Brabant fold belt a comparison of cleavage/fold relationships and stratigraphic polarity shows that a 200 m thick interval of middle Caradoc fine-grained turbidites in the core of a large synform was overturned prior to tectonic deformation. This overturning is attributed to large-scale slumping, which was most likely a result of middle Caradoc seismic activity. The exposed portion of the large slump sheet contains only a few small slump folds and intraformational breccias, making up less than 5% of the exposed thickness. If the beds were not overturned, large-scale slumping would never be suspected and the small slump folds would probably be interpreted as localized features in an overall ‘stable’ sedimentary pile. This may explain why so few ancient large-scale slides and slumps have been reported: the small amount of internal deformation makes them very difficult to recognize, especially when dealing with poorly exposed areas. As such, large ancient slides and slumps may be more common than suggested by the geological literature.