Clastic Dykes Research Articles

Soft sediment deformation structures in the Maastrichtian Ajali Formation Western Flank of Anambra Basin, Southern Nigeria

Soft sediment deformation structures were recognized in the Maastrichtian shallow marine wave to tide influenced regressive sediments of Ajali Formation in the western flank of Anambra basin, southern Nigerian. The soft sediment deformation structures were in association with cross bedded sands, clay and silt and show different morphological types. Two main types recognised are plastic deformations represented by different types of recumbent folds and injection structure represented by clastic dykes. Other structures in association with the plastic deformation structures include distorted convolute lamination, subsidence lobes, pillars, cusps and sand balls. These structures are interpreted to have been formed by liquefaction and fluidization mechanisms. The driving forces inferred include gravitational instabilities and hydraulic processes. Facies analysis, detailed morphologic study of the soft sediment deformation structures and previous tectonic history of the basin indicate that the main trigger agent for deformation is earthquake shock. The soft sediment deformation structures recognised in the western part of Anambra basin provide a continuous record of the tectonic processes that acted on the regressive Ajali Formation during the Maastrichtian.

Large earthquake-triggered liquefaction mounds and a carbonate sand volcano in the Mesoproterozoic Wumishan Formation, Beijing, North China

Ten well-preserved, earthquake-triggered liquefaction mounds and a carbonate sand volcano have been found in the Mesoproterozoic Wumishan Formation (1550–1400 Ma) in the Beijing area, North China. These features crop out in a roadcut near Zhuanghuwa Village. All ten mounds occur in the same sedimentary layer and have rounded shapes with some concentric and radial fissures arising from the centre. They range from 1.5 to 4 m in diameter and from 10 cm to 30 cm in height. The carbonate sand volcano has a diameter of 110 cm and the ‘crater’ at the top has a depth of about 30 cm. Associated with these mounds and the sand volcano are many ‘normal’ sedimentary structures and numerous soft-sediment deformation structures. The former include ripple marks, cross-bedding, stromatolites and desiccation cracks, indicating deposition in a stable shallow-water peritidal platform environment. The latter include intrastratal faults and folds, seismically formed breccias and carbonate clastic dykes. The morphological features and the genesis of these liquefaction mounds are very similar to mounds formed recently by the great Wenchuan Earthquake of China (2008). Detailed thin-section study of the mounds found no signs of any kind of biological constructional process; instead it reveals some obvious fluidification and liquefaction characteristics. Comparative studies have shown that these features are probably the products of Mesoproterozoic earthquake activity. Copyright © 2013 John Wiley & Sons, Ltd.

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

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

Origin and terminal architecture of a submarine slide: a case study from the Permian Vischkuil Formation, Karoo Basin, South Africa

AbstractSubmarine mass movement deposits exposed in the Vischkuil Formation, Laingsburg Karoo Basin, South Africa, provide a rare opportunity to analyse and interpret their emplacement history and deformation processes at a scale comparable to seismic examples. An up to 80 m thick slide deposit, continuously exposed in two 2 km long sub‐parallel sections, passes from extensionally deformed material (clastic dykes and down‐dip facing low‐angle shear surfaces) down‐dip into a compressional toe zone with large (tens of metres amplitude) folds dissected by steep, up‐dip facing thrust planes. The compressional shear planes sole out onto a highly sheared décollement and cross‐cutting relationships indicate an up‐depositional dip younging in the timing of fold dissection. Lithofacies characteristics and detailed correlation of volcanic ash and other marker beds over more than 500 km2 in the bounding undeformed stratigraphy indicate a low‐gradient (<0·1°) basin floor setting. The slide is abruptly overlain by an up to 50 m thick debrite with sandy clasts supported by an argillaceous matrix. Shear loading of the debris flow is interpreted to have driven large‐scale deformation of the substrate through the generation of high shear stresses at a rheological interface due to: (i) the abrupt contact between the slide and the debrite; (ii) the coincident thickness distributions of the debrite and slide; (iii) the distribution of the most intense folding and thrusting under the thickest parts of the debrite; (iv) the preservation of fold crests with only minor erosion along fold limbs; (v) the presence of the debrite under overturned folds; (vi) the presence of laterally extensive marker beds directly above deformation units indicating minimal depositional topography; and (vii) the demonstrably local derivation of the slide as individual folded beds are mapped into undeformed strata outside the areas of deformation. The debrite is directly overlain by fine‐grained turbidite sandstone beds that show widespread vertical foundering into the debrite. This case study demonstrates that intensely deformed strata can be generated by negligible amounts of down‐dip movement in a low‐gradient, fine‐grained basin floor setting with the driver for movement and deformation being the mass imbalance resulting from emplacement of episodic debris flows. Simple interpretation of an unstable slope setting based on the presence of such deformed strata should be treated with caution.

An extensive erosion surface of a strongly deformed limestone bed in the Gushan and Chaomidian formations (late Middle Cambrian to Furongian), Shandong Province, China: Sequence–stratigraphic implications

In order to understand sequence development and sea-level fluctuations during the late Middle Cambrian to early Furongian on the North China epeiric platform, the present study focuses on a unique, subtle erosion surface of an extensive (approx. 100 km), strongly deformed limestone bed in the uppermost part of the Gushan Formation, Shandong Province, China. The Gushan Formation and the overlying Chaomidian Formation consist mainly of shales and a variety of carbonates that were deposited in subtidal environments (e.g., deep subtidal, shallow subtidal, shoreface/shoal, subtidal microbial flat, and restricted platform interior). Three third-order depositional sequences (S1–3) are identified, each of which comprises a thin transgressive systems tract (TST) and a relatively thick highstand systems tract (HST). Each sequence is bounded by a drowning unconformity (SB1), a subaerial unconformity (SB2), or a surface of submarine erosion (SB3). The upper sequence boundary (SB2) of sequence 1 (S1) is represented by a subtle erosion surface of an extensive, deformed limestone bed with a wide variety of soft-sediment deformation structures (e.g., lime mudstone breccias, chaotic wacke-packstone laminae and fragments, homogenized oolites, and clastic dykes), and is overlain by small sporadic microbial buildups and an extensive bioclastic grainstone bed. The deformed limestone was formed during early diagenesis by differential deformation processes (brecciation, liquefaction/fluidization, and injection) which were most likely induced by pore-water overpressure during the period of rapid sea-level fall. Despite the lack of subaerial exposure features (e.g., paleokarst, paleosol, etc.), the characteristics of the erosion surface (cutting well-lithified sediment below), the missing of a significant geological record (the Prochuangia biozone), and the worldwide correlatable positive carbon isotope excursion collectively indicate that the erosion surface developed under conditions of subaerial exposure after contemporaneous marine cementation of the deformed sediment. The missing of the Prochuangia biozone is most likely due to non-deposition at a subaerial hiatal surface. The erosion surface was submerged as a result of subsequent rise in sea level, where sporadic microbial buildups formed under suitable conditions. Freshly deposited, winnowed, shell-dominated transgressive lag deposits (containing Chuangia trilobite fragments, brachiopod shells, and abundant glauconite grains) formed with continued rise in sea level, which became, in turn, overlain by shale-dominated facies. The unique combination of the subtle erosion surface ( sensu stricto a subaerial unconformity) and the underlying deformed limestone bed provides an important criterion for recognizing the subtle changes in relative sea level on shallow epeiric platforms.

Soft‐sediment deformation structures in Cretaceous non‐marine deposits of southeastern Gyeongsang Basin, Korea: Occurrences and origin

AbstractA variety of soft‐sediment deformation structures formed during or shortly after deposition occurs in the Cretaceous Seongpori and Dadaepo Formations of the southeastern Gyeongsang Basin exposed along coastal areas of southeastern Korean Peninsula for 0.5–2 km. These are mostly present in a fluvial plain facies, with interbedded lacustrine deposits. In this study, the features of different kinds of soft‐sediment deformation structures have been interpreted on the basis of sedimentology of structure‐bearing deposits, comparison with normal sedimentary structures, timing and mechanism of deformation, and triggering mechanisms. The soft‐sediment deformation structures can be classified into four morphological groups: (i) load structures (load casts, ball‐and‐pillow structures); (ii) soft‐sediment intrusive structures (dish‐and‐pillars, clastic dykes, sills); (iii) ductile disturbed structures (convolute folds, slump structures); and (iv) brittle deformation structures (syndepositional faulting, dislocated breccia). The most probable triggering mechanisms resulting in these structures were seismic shocks. These interpretations are based on the following field observations: (i) location of the study area within tectonically active fault zone reactivated several times during the Cretaceous; (ii) deformation structures confined to single stratigraphic levels; (iii) lateral continuity and occurrences of various soft‐sediment deformation structures in the deformed level over large areas; (iv) absence of depositional slope to indicate gravity sliding or slumping; and (v) similarity to the structures produced experimentally. The soft‐sediment deformation structures in the study areas are thus interpreted to have been generated by seismic shocks with an estimated magnitude of M > 5, representing an intermittent record of the active tectonic and sedimentary processes during the development and evolution of two formations from the late Early Cretaceous to the Late Cretaceous.

Sub-till glaciofluvial sediments at Hultsfred, South Swedish Upland

In a gravel pit near Hultsfred, at the eastern margin of the South Swedish Upland, gravelly, sandy and silty-sandy beds covered by a thick, stratified gravelly diamicton are exposed. Large folds and shear-planes indicate post-depositional deformation of the sorted sediments, which according to OSL dating have a maximum Middle Weichselian (∼60 ka) age. The sand and gravel are interpreted as glaciofluvial sediments that were deposited laterally during a Middle Weichselian deglaciation, or possibly subglacially during the Late Weichselian. The large-scale deformation was due to glaciotectonics, caused by an overriding wet-based ice sheet during the last deglaciation. The subglacial hydrostatical pressure was high and led to the formation of downward-injected clastic dykes. A basal till, which forms the cover moraine in the area, caps the succession. The fairly frequent occurrence of sub-till sediments on the South Swedish Upland demonstrates the limited effect of glacial erosion in parts of this area. During the Last Glacial Maximum, the South Swedish Upland was an ‘island’ of slow-moving, polythermal and at least partly non-erosive ice, surrounded by faster ice-streams in the Baltic and in the Skagerrak. The ice sheet may have left its main impact on the landscape during short periods during advance and deglaciation only.

Issues associated with the distinction between climatic and tectonic controls on Permian alluvial fan deposits from the Kotzen and Barnim Basins (North German Basin)

Abstract Facies analysis focussing on coarse-grained sediments has been carried out on more than 2500 m of drill cores from seven wells from southern margins of the North German Basin (NGB). The NGB forms a central element of the Southern Permian Basin (SPB). The wells exposed conglomerates and sandstones of the Rotliegend Gruneberg and Parchim Formations deposited in the Kotzen Basin and the Barnim Basin. 17 lithofacies types have been grouped into six lithofacies associations. The studied successions are dominated by fluid gravity flow deposits (hyperconcentrated flows and stream flows) of alluvial fan and alluvial plain systems. Maximum particle size/bed thickness plots (MPS/BTh) support the interpretation as fluid gravity flow deposits. The MPS and BTh data have also been used to differentiate coarsening–thickening and fining–thinning trends of the fan systems. The dominance of water-rich mass flow processes together with sedimentary structures such as dewatering structures and outwashed tops suggests the presence of wet-type fans and plains under semi-humid to semi-arid seasonal climates in the central SPB. The investigated sediments show variation in clast composition subsequent to deep erosion processes on basin margins and changes of source areas. Synsedimentary normal faults and clastic dykes have been interpreted as indicators of tectonic activity of grabens itself and its frames. On a larger scale, then evolution from a half-graben to a graben is apparent for the Tuchen Sub-basin at least. The progradational/retrogradational cycles of the studied alluvial fan systems document combined local tectonic movements and influences of climatic changes. However, our data did not allow for a clear distinction between climatic and tectonic signals. Furthermore, a one-to-one correlation of fan cycles with depositional trends in the NGB basin centre would appear to be oversimplistic.

Funnel-shaped, breccia-filled clastic dykes in the Late Cambrian Chaomidian Formation (Shandong Province, China)

Some examples of a funnel-shaped structure filled with both angular and rounded clasts occur in a peloidal grainstone bed of the Chaomidian Formation (Late Cambrian) in the Jiulongshan area (Shandong Province, China). The clasts consist of peloidal grainstone which is the same as that of the walls of the funnels. The structures are interpreted as dykes formed by upward escape of pore water and/or fluidized sediment through a narrow conduit which was gradually widened by the force of the intruding fluid/sediment. The erosional capacity of the pressurized fluid/sediment mixture consequently diminished upwards, which explains the gradual decrease in intensity of fragmentation from the adjoining walls in this direction.

The Cenozoic CH 4-derived carbonates of Monferrato (NW Italy): A solid evidence of fluid circulation in the sedimentary column

Two types of methane-derived carbonates, each with different lithological and paleontological signatures, have been distinguished in the Oligocene–Miocene sediments of Monferrato (NW Italy): 1) Irregularly-shaped, metre-sized rock bodies characterized by the widespread occurrence of remains of large bivalves ( Lucina sp.). These bodies include two subtypes, differing in the lithological characteristics of the cemented sediments: a) Lucina-rich marly limestones, encased in Langhian outer shelf marls. They document a slow seepage of CH 4-rich fluids at the paleo-sea floor; b) Lucina-bearing cemented mud breccias, found either as cemented masses in the original stratigraphic position or as redeposited blocks within Upper Messinian chaotic (block-in-matrix) sediments. These carbonate bodies document the activity of submarine mud volcanoes during the late Messinian. In particular, they document intermittent fluid emission ranging from violent eruptive phases, responsible for the extrusion of soft mud breccias at the sea floor, to quiet degassing phases, that promoted colonization by chemosymbiotic communities and localized cementation. 2) Very irregularly shaped strongly cemented, mainly coarse-grained, clastic sediments, reaching the lateral extent of few hundred meters and the thickness of few tens of meters. They occur within Oligocene to Messinian sediments, lack remains of chemosymbiotic taxa and are highly fractured, with fractures filled with sediments and cements. These carbonate-cemented masses, here referred to as macroconcretions, result from the precipitation of carbonate cements within buried sediments flushed by ascending CH 4-rich fluids. They document a complex fluid circulation pattern in the sedimentary column, in which SO 4-rich waters were drawn from surrounding sediments by the fast and prolonged flow of CH 4-rich fluids and promoted anaerobic oxidation of methane and the precipitation of carbonate cements. The polyphase network of clastic dykes and veins that characterize macroconcretions is related to an intermittent fluid flow regime: dykes formed during fast upward flows of sediment-rich fluids whereas carbonate-filled veins formed during slower, degassing phases. The local lithostratigraphic and structural framework allow to correlate genetically and chronologically the macroconcretions to Lucina-bearing mud breccias. Consequently, macroconcretions are here regarded as the result of cementation along fluid conduits feeding Messinian mud volcanoes at the sea floor. The massive release of fluids could have played a role in triggering large-scale mass wasting events responsible for the emplacement of sedimentary mélanges including CH 4-derived rocks in the Messinian.

Paleoseismicity in cis-Baikal rift basins: Evidence and age constraints

Recent years have witnessed considerable advances in the study of deformation structures caused by soil liquefaction during seismic shocks [1‐5]. These structures are clear paleoearthquake indicators [2] that can be used for determining various parameters of past events [3‐5]. This paper deals with identification, structural and geological study, and dating of the sections that contain seismites, clastic dykes, dislocation cracks, and paleosols buried as a result of rapid tectonic downthrows in the Tunka and Barguzin valleys of the cis-Baikal Area (Fig. 1). The sections with structures indicative of past seismic events were identified along the fault zones that had been previously mapped [6, 7] and ranked as hazardous based on several features [8]. In six stations the deformed sediments were carbon dated (table). The residual carbon activity was measured at the Institute of Geology and Mineralogy (Novosibirsk). Based on a half-life of 5570 years, the age was calculated started at 1950. Below is a short description of the sections and a discussion of the results. Section T0201 in descending order consists of (1) a recent soil and vegetation horizon, 0‐0.08 m; (2) sand loam, 0.08‐0.32 m; (3) laterally persistent paleosol with a disturbed lower part, 0.32‐0.5 m; (4) mediumgrained, limonitized, and strongly deformed sand, with fragments of clay loam and paleosol that previously formed a continuous layer, 0.5‐1.15 m; (5) limonitized clay loam with signatures of seismic shocks in the form of waves, 1.15‐1.57 m; and (6) medium-grained sand with seismites in the form of mushroom- or dropshaped injections into the overlying layer, 1.57‐2.0 m. The lack of inclinations and swellings on the daylight surface of the section suggested that gravity sliding or cryogenic processes were not involved in the origin of the deformations. There are normal faults in the section with dip and dip direction 70/290 that together with the patterns in deformation (Fig. 2a) prove the seismic origin of the observed structures. The dates obtained (table) suggest that the soil fragments in layer 4 represent a once continuous paleosol distinct from layer 3.

Funnel structures and clastic dykes in Cambrian sandstones of southern Sweden indications for tensional tectonics and seismic events in a shallow marine environment

Funnel structures and clastic dykes in Cambrian sandstones of southern Sweden indications for tensional tectonics and seismic events in a shallow marine environment

Under pressure: clastic dykes in glacial settings

Clastic dykes are widespread in glacial settings, and we provide examples from Switzerland, Patagonia, Iceland and the Antarctic, ranging in age from Tertiary to recent. On the basis of these examples we establish the general characteristics of clastic dykes and proceed to establish the direction of propagation on sedimentological grounds. Micromorphological analysis reveals that sedimentation in the dykes is ruled by pressure gradients and that ordinary sedimentological rules do not apply. Clastic dykes act like safety valves in the subglacial hydraulic system. Their development depends on specific subglacial conditions like water volume and pressure, the nature of the bed, sediments or bedrock, and the hydraulic properties of the bed. A number of scenarios for clastic dyke development are presented as there is not just one set of conditions under which they form. Development of clastic dykes affects glaciodynamics, like velocity and surging and landform formation, like till thickness and bedrock disruption.

Velocities and driving pressures of clay-rich sediments injected into clastic dykes during earthquakes

SUMMARY We studied the velocities and driving pressures associated with clastic-dyke formation in the Ami’az plain, where hundreds of clastic dykes cross-cut the soft rock of the late Pleistocene lacustrine Lisan Formation, within the seismically active Dead Sea basin. Flow of clastic material into fractures and opening of the fractures are two mechanisms that occur during earthquake-induced clastic dyke emplacement. Two analytic models were established, based on field observations and experimental viscosity tests, to estimate the velocities and driving pressures that were associated with dyke emplacement: (a) a channel flow for upward injection of a clay–water mixture and (b) a profile of fracture dilation based on the elastic theory analysis. The two models predict that pressures between 1 and 10 MPa are generated in the source layer and dykes in the last stage of the injection process. In addition, the channel flow model predicts that the injection velocity reaches metres to tens of metres per second and the emplacement time of the clastic dykes is on a scale of seconds. It is suggested that the high pressure values represent the static stress drop during earthquake events or represent dynamic stresses resulting from the seismic waves which passed through the soft lacustrine rocks. In both cases, the predicted high pressure values indicate that the clastic dyke was emplaced in close proximity of an active segment of the Dead Sea Fault during the late Pleistocene-Holocene.

Hirnantian glacial and deglacial record in SW Djado Basin (NE Niger).

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid- pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.

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

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

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

Abstract This paper summarizes the results of completed and ongoing research in three areas of the Basin and Range Province of the western United States that casts doubt on the interpretation of specific regional detachment faults and the large extensional strains with which such faults are commonly associated. Given that these examples were influential in the development of ideas about low-angle normal faults, and particularly in making the case for frictional slip at dips of appreciably less than the 30° lock-up angle for μ ≈ 0.6 (where μ is the coefficient of friction), we advocate a critical re-examination of interpreted detachments elsewhere in the Basin and Range Province and in other extensional and passive margin settings. The Sevier Desert ‘detachment’ of west-central Utah is reinterpreted as a Palaeogene unconformity that has been traced to depth west of the northern Sevier Desert basin along an unrelated seismic reflection (most probably a splay of the Cretaceous-age Pavant thrust). The absence of evidence in well cuttings and cores for either brittle deformation (above) or ductile deformation (below) is inconsistent with the existence of a fault with as much as 40 km of displacement. The Pavant thrust and the structurally higher Canyon Range thrust are erosionally truncated at the western margin of the southern Sevier Desert basin, and are not offset by the ‘detachment’ in the manner assumed by those inferring large extension across the basin. The Mormon Peak detachment of SE Nevada is reinterpreted as a series of slide blocks on the basis of detachment characteristics and spatially variable kinematic indicators that are more closely aligned with the modern dip direction than the inferred regional extension direction. A particularly distinctive feature of the detachment is a basal layer of up to several tens of centimetres of polymictic conglomerate that was demonstrably involved in the deformation, with clastic dykes of the same material extending for several metres into overlying rocks in a manner remarkably similar to that observed at rapidly emplaced slide blocks. The Castle Cliff detachment in the nearby Beaver Dam Mountains of SW Utah is similarly regarded as a surficial feature, as originally interpreted, and consistent with its conspicuous absence in seismic reflection profiles from the adjacent sedimentary basin. The middle Miocene Eagle Mountain Formation of eastern California, interpreted on the basis of facies evidence and distinctive clast provenance to have been moved tectonically more than 80 km ESE from a location close to the Jurassic-age Hunter Mountain batholith of the Cottonwood Mountains, is reinterpreted as having accumulated in a fluvial–lacustrine rather than alluvial fan–lacustrine setting, with no bearing on either the amount or direction of tectonic transport. The conglomeratic rocks upon which the provenance argument was based are pervasively channelized, with erosional relief of less than 1 m to as much as 15 m, fining-upwards successions at the same scale and abundant trough cross-stratification – all characteristic features of fluvial sedimentation and not of alluvial fans. The interpretation of the Eagle Mountain Formation as having been deposited within a few kilometres of the Hunter Mountain batholith, which depends strongly on assumptions about the dimensions of alluvial fans, is therefore not required. The result is important because the Eagle Mountain offset has been viewed as representing the strongest evidence for extreme extension in this part of California, and for the existence of detachment faults of regional dimensions.

The origin of a giant downward directed clastic dyke in a kame (Bełchatów mine, central Poland)

A kame near Czubata Góra in the Pleistocene overburden of the Bełchatów open-cast browncoal mine in central Poland shows several types of deformations. A number of diapirs occur, but also a structure that shows all characteristics of a diapir but consists of sediments that apparently intruded the underlying rather than the overlying sediments. Analysis of this structure shows that it consists of material that was originally deposited as a fine-grained (mainly silty) glacilacustrine sediment. Seismic activity triggered the material to fluidise, resulting in a structureless mass. Part of the fluidised material flowed down into a fissure that had formed in the underlying sediments (probably due to tectonics in the active graben), thus forming a clastic dyke; more of the overlying silty material was later pressed down into a dyke with the shape of a downward directed diapir.

Volcanic-sedimentary features in the Serra Geral Fm., Paraná Basin, southern Brazil: Examples of dynamic lava-sediment interactions in an arid setting

The formation of volcanic–sedimentary interaction features in extreme arid environments is not a commonly described process. Specifically the occurrence of dynamically mixed sediments and juvenile igneous clasts as peperites, for water has been considered one major important factor in the processes of magma dismantling and mingling with unconsolidated sediment to form such deposits. The study area, located in south Brazil, shows a sequence of lava flows and intertrapic sandstone layers from the Paraná Basin, associated with the formation of clastic dykes, flow striations, peperite and ‘peperite-like’ breccias. Four processes are suggested for the genesis of the peperites: (a) fragmentation of the flow front and base; (b) sand injection; (c) dune collapse; (d) magma cascade downhill. The continued flow of a lava, while its outer crust is already cooling, causes it to break, especially in the front and base, fragments falling in the sand and getting mixed with it, generating the flow front ‘peperite-like’ breccia. The weight of the lava flow associated to shear stress at the base cause sand to be injected inwards the flow, forming injection clastic dykes in the cooled parts and injection peperite in the more plastic portions. The lava flow may partially erode the dune, causing the dune to collapse and forming the collapse ‘peperite-like’ breccia. The shear stress at the base of a flowing lava striates the unconsolidated sand, forming the flow striations. The sand that migrates over a cooled, jointed lava flow may get caught in the cavities and joints, forming the filling clastic dykes. These deposits are analogous to those found in the Etendeka, NW Namibia, and show that sediment–lava interactions in arid settings are widespread throughout the Paraná-Etendeka province during the onset of flood volcanism.

Formation, deformation and chertification of systematic clastic dykes in a differentially lithified carbonate multilayer. SW Iberia, Algarve Basin, Lower Jurassic

This work presents original field evidence for tectonically controlled calciclastic dyke injection and subsequent chertification in shelf carbonates during rifting in the south-westernmost part of the Eurasian continent in the Early Pliensbachian. It is shown by the detailed description of tectonic structures (faults and joints), stratigraphic discontinuities and by the distribution, orientation and morphology of the injected dykes of calciclastic sands into fine-grained carbonates, that these soft-sediment deformation structures were tectonically controlled. Extensional tectonics developed vertical tensile joints in semi-lithified, fine-grained limestones permitting upward and downward injection of loose calciclastic sands, forming clastic dykes. The frequency of injection structures along strata was constrained by the thickness of the layer into which the injection was occurring, which implies an elastic behaviour; however the curvi-planar shapes of the dykes and drop-like nodules attests the ductile behaviour of the fractured limestones, indicating transition from elastic to ductile response to deformation with time. The multi-layered system consists of just two lithotypes. The two display different mechanical behaviours, which evolved in time to more brittle conditions as lithification progressed, as shown by the cataclastic faulting and jointing of the previously formed soft-sediment deformation structures and strata under an analogous stress regime. The injection dykes were disrupted by a short-lived episode of compression, after which tectonic extension resumed, still in the Lower Pliensbachian. Sometime before the deposition of the Upper Pliensbachian a pervasive selective event of chertification occurred: only the calciclastic sandy layers, dykes and nodules were substituted by silica, thus enhancing the mechanical contrast between the primary sedimentary structures and the soft-sediment deformation structures. All the described events occurred during a time interval of approximately 2 Myr as constrained by ammonoid stratigraphy.