Clast Fabric Research Articles

Pebble morphogenesis of the Cretaceous Conglomerates from the Abeokuta Formation near the Oluwa River, Eastern Dahomey Basin, Southwestern Nigeria

Pebbles taken from the bed of the Cretaceous Abeokuta Formation paraconglomerate near the Oluwa River were used to infer the depositional setting and the nature of the source area, through the integration of bivariate and ternary (sphericity-form and Zingg diagrams) analyses. Deposition in a river environment is indicated by the high sphericity values, ranging from 0.59 to 0.88 (average 0.70). Also, bivariate plots of the maximum projection sphericity (ψp) vs. oblate-prolate index (OPI) and flatness index (FI) vs. maximum projection sphericity (ψp) point to the domination of fluvial processes. Dominantly elongated, compact elongated, compact and compact-bladed pebbles are typical for sedimentary regime with prevalence of fluvial over beach processes. Co-existence of various pebbles shapes (mainly disc, rod-, and sphere-shaped), despite of the similar, predominantly quartz composition, may occur due to the different clast fabrics. This heterogeneity also indicates various transport distances and water energies, pointing to the multiple source areas.

Gravity flows: Types, definitions, origins, identification markers, and problems

Abstract This review covers 135 years of research on gravity flows since the first reporting of density plumes in the Lake Geneva, Switzerland, by Forel (1885). Six basic types of gravity flows have been identified in subaerial and suaqueous environments. They are: (1) hyperpycnal flows, (2) turbidity currents, (3) debris flows, (4) liquefied/fluidized flows, (5) grain flows, and (6) thermohaline contour currents. The first five types are flows in which the density is caused by sediment in the flow, whereas in the sixth type, the density is caused by variations in temperature and salinity. Although all six types originate initially as downslope gravity flows, only the first five types are truly downslope processes, whereas the sixth type eventually becomes an alongslope process. (1) Hyperpycnal flows are triggered by river floods in which density of incoming river water is greater than the basin water. These flows are confined to proximity of the shoreline. They transport mud, and they do not transport sand into the deep sea. There are no sedimentological criteria yet to identify hyperpycnites in the ancient sedimentary record. (2) A turbidity current is a sediment-gravity flow with Newtonian rheology and turbulent state in which sediment is supported by flow turbulence and from which deposition occurs through suspension settling. Typical turbidity currents can function as truly turbulent suspensions only when their sediment concentration by volume is below 9% or C < 9%. This requirement firmly excludes the existence of 'high-density turbidity currents'. Turbidites are recognized by their distinct normal grading in deep-water deposits. (3) A debris flow (C: 25-100%) is a sediment-gravity flow with plastic rheology and laminar state from which deposition occurs through freezing en masse. The terms debris flow and mass flow are used interchangeably. General characteristics of muddy and sandy debrites are floating clasts, planar clast fabric, inverse grading, etc. Most sandy deep-water deposits are sandy debrites and they comprise important petroleum reservoirs worldwide. (4) A liquefied/fluidized low (>25%) is a sediment-gravity flow in which sediment is supported by upward-moving intergranular fluid. They are commonly triggered by seismicity. Water-escape structures, dish and pillar structures, and SSDS are common. (5) A grain flow (C: 50-100%) is a sediment-gravity flow in which grains are supported by dispersive pressure caused by grain collision. These flows are common on the slip face of aeolian dunes. Massive sand and inverse grading are potential identification markers. (6) Thermohaline contour currents originate in the Antarctic region due to shelf freezing and the related increase in the density of cold saline (i.e., thermohaline) water. Although they begin their journey as downslope gravity flows, they eventually flow alongslope as contour currents. Hybridites are deposits that result from intersection of downslope gravity flows and alongslope contour currents. Hybridites mimic the "Bouma Sequence" with traction structures (Tb and Tc). Facies models of hyperpycnites, turbidites, and contourites are obsolete. Of the six types of density flows, hyperpycnal flows and their deposits are the least understood.

3D outcrop modelling of large discordant breccia bodies in basinal carbonates of the Apulian margin, Italy

Large discordant breccia bodies (LDBBs) are important record keepers of the tectonic and gravitational evolution in platform-to-basinal settings, and have important implications for fluid-flow migration and compartmentalization of tight carbonate reservoirs. In the Gargano Promontory of southern Italy, LDBBs occur within a Cretaceous slope and basinal carbonate succession. We use field mapping and Unmanned Aerial Vehicle (UAV) -based Structure from Motion (SfM) Photogrammetry to document otherwise inaccessible cliff-side exposures of seismic-to subseismic-scale vertical discordant breccias. LDBBs are up to 50 m in width, more than 80 m in height and display internal chaotic or aligned clast fabrics. The formation which generally contains the LDBBs is characterized by beds of cherty pelagic limestone intercalated with calcarenites, calciturbidites and horizons of mass transport deposits. The mass-transport deposits can be correlated across the chaotic breccia bodies, indicating only slight or no vertical displacement across the adjacent walls. The bases of the breccia bodies are always hidden below current sea level, while the rarely exposed tops are capped by bedded intervals of the host rock formation. Timing and origin of the studied breccias were determined using several lines of evidence, such as stratigraphic provenance of clasts in breccias, mutual relationships of structural and sedimentologic features, and previous studies which establish that the age of dolomitizing cements in the LDBBs formed at different times and by different processes (fault shearing and solution collapse). This work investigates the size, shape and geometry of these breccia bodies whilst also providing cm-scale detail of the textural features in otherwise inaccessible outcrops. We suggest that breccias formed as a result of solution exploiting a pre-existing fracture network characterized by large-scale vertical strike-slip or oblique-slip faults. Initial displacement along these faults created a wide fault damage zone, where fluid migration was later focused to create a zone highly susceptible to solution and subsequent periodic sidewall collapse.

The Aptian Gura Râului conglomerates (Southern Carpathians): remains of an extended subaqueous gravity flow deposit from the eastern flank of the Getic Nappe

The Getic Nappe belongs to the Central Dacides or Dacia Mega Unit. Gravity flow deposits were generated by the Lower Cretaceous overthrust and uplift of this unit over the External Dacides. In the Piatra Craiului Syncline (part of the Getic Nappe), such detrital deposits are represented by the Aptian conglomerates and the uppermost Albian-Cenomanian conglomerates. Part of the infill of the syncline consists of a thick pile of NW–SE oriented conglomerates. Sedimentological study documents the presence of an association which consists of three distinct facies types, represented by: massive conglomerates; alternating conglomerates, sandstones and microconglomerates; and fining- and coarsening-upwards conglomerates. These deposits were accumulated through a series of debris flows, and hyperconcentrated or concentrated flows. The entire assemblage represents a complex routing system within submarine channels, an interpretation made by interpreting facies associations, palaeoflow directions and clast fabrics. A large part of this succession was removed by post-Albian erosion. Thus, the interpretation of the depositional system could be as a series of feeder channels or the submarine sector of a fan-delta system representing a foreland basin fill.

Electromagnetic resistivity anisotropy in till from the Kiskatinaw streamlined landform field, northeastern British Columbia, Canada

Applications of the azimuthal electromagnetic (EM) method that establish the internal structure of landforms consisting of sediment are sparse. In the Kiskatinaw streamlined landform field of northeastern British Columbia, ice-flow parallel till ridges formed beneath the Cordilleran Ice Sheet during the late Wisconsinan. EM anisotropy data from the Kiskatinaw field are systematically related to ridge elongation and orientation and, in part, to clast fabrics. However, in contrast to previous studies, we find that resistivity measurements do not necessarily record the orientation of ice flow, and electrical anisotropy in till may not be a simple indicator of ice-flow direction. For highly elongate ridges (l/w > 3), the long axes of resistivity ellipses are at high angles to the long axes of the landforms. Tills in these elongate ridges are matrix rich (>85%) and display strong unimodal ice-flow parallel clast fabrics. For more equant ridges (l/w < 3), the long axes of resistivity ellipses are at low angles to the long axes of the landforms. Tills in these ridges are clast rich (up to 60%) and display polymodal clast fabrics with both ice-flow parallel and ice-flow transverse modes. Because clast fabrics in the more equant ridges are so variable, despite unimodal resistivity patterns, and because groundwater well data indicate relatively fresh pore waters and the till matrix contains clay (average 12%), we speculate that the resistivity/conductivity anisotropy we observed may record aligned matrix clays rather than clast fabrics alone. We interpret that the highly elongate ridges indicate subglacial shear along a non-cohesive easily deformed bed, whereas the more equant ridges mark ‘sticky spots’ where high clast concentrations produced local areas that resisted subglacial shear and induced local ice-flow parallel shortening, which limited landform elongation and clast alignment.

Soft-sediment deformation structures in gravelly fluvial deposits: A record of Cretaceous seismic activity in Patagonia?

Cretaceous sedimentary rocks corresponding to the Las Plumas Member of the Cerro Barcino Formation were deposited in the eastern margin of the Somuncurá - Cañadón Asfalto Basin. The analyzed succession records alluvial settings in a fault-proximal scenario and contains a 7 m thick stratigraphic interval bearing soft-sediment deformation structures which include: pebble load casts, pseudonodules, flame structures and contorted bedding. Deformed beds are bounded by undisturbed strata and these features can be traced along the 800 m of exposures. The sedimentological features such as size and shape of SSDS, clast fabric of gravels and depositional paleoenvironment added to the proximity to a fault zone support the interpretation of the described deformation features as formed by liquefaction and/or fluidization triggered by seismic shocks during syn-tectonic deposition.

The Palaeoproterozoic Grythyttan Field in the Svecofennian Orogen, West Bergslagen, Central Sweden: structure, stratigraphy and age

A number of discrete slate belts of limited size occur in the Paleoproterozoic volcano-sedimentary Bergslagen Group of western Bergslagen in the Fennoscandian shield of south central Sweden. The Grythyttan Slate Belt, studied for more than a century, forms a single basin with the nearby Saxan Slate Belt. We use the lithostratigraphy of the Grythyttan Slate Formation of the Grythyttan belt, based on basin scale sedimentary facies associations of the volcaniclastic sediments in time and space, as a method to interpret overall tectonic structure. Contrary to traditional views, we reconstruct the GSB as a single overturned limb of a km-scale anticline with horizontal axis in the hanging wall of an east vergent thrust fault, reactivating a listric extensional fault. The fold connects the GSB to the neighboring SSB. Folding and thrusting were related to tectonic closure of a volcanic back- or intra-arc basin. The early folds were subsequently affected by strike-slip shearing and folding around vertical fold axes, which partitioned preferentially into the least competent lithologies (slates and marbles), significantly modifying the map appearance of the slate belt. The late shearing and folding resulted from accretion of Bergslagen onto the Fennoscandian continental margin during the late, Svecobaltic phase of the Svecofennian orogeny. The GSB forms a thin, intraformational wedge in the Bergslagen Group and represents a relatively short interlude with a conglomeratic alluvial fan and turbiditic volcaniclastics followed by more felsic volcanic rocks resembling those of the Bergslagen Group. We suggest that the GSB, and by inference the other slate belts, stem from calderas, as either terrestrial volcanic lakes or shallow submarine eruption centers. The conglomerates are pre-orogenic, not a post-orogenic molasse, as traditionally conceived, and their clast fabric has resulted from sedimentary processes only. Newly determined SIMS U-Pb zircon ages constrain the age of the GSB at ca. 1895 Ma.

Geometry of indicator mineral and till geochemistry dispersal fans from the Pine Point Mississippi Valley-type Pb-Zn district, Northwest Territories, Canada

An indicator mineral and till geochemical dispersal case study was conducted at the Pine Point Mississippi Valley-type (MVT) Pb-Zn district in northern Canada to document glacial dispersal down ice from a known orebody. Indicator minerals in the non-ferromagnetic heavy mineral (specific gravity >3.2) concentrate of till down ice of the mined-out O-28 Pb-Zn deposit in the east end of the district include sphalerite, galena, and pyrite, as well as secondary Pb-Zn minerals anglesite (as coatings on galena), smithsonite, and cerussite. These minerals are sufficiently physically and chemically robust to survive glacial transport as well as post-glacial weathering in the carbonate-rich till of the region. Pathfinder elements in the <0.063 mm fraction of till, determined using a modified aqua regia digestion, include Zn, Pb, Cd, Tl, and S.Evidence for multiple ice flow phases, each with an erosional and depositional record across the district, includes cross-striated bedrock surfaces, streamlined landforms, and till clast fabrics. At the O-28 pit, two main phases of ice flow eroded and dispersed Pb- and Zn-rich debris first to the southwest and then to northwest, producing a fan-shaped palimpsest dispersal train defined best by indicator minerals, but also by till geochemistry. The last ice-flow phase during deglaciation had a minimal effect on the dispersal train geometry. This case study presents another example of complex glacial transport and dispersal by multiple ice flows that have been defined by both glacial ice flow indicators and glacial transport data. Complex ice flow and transport are to be expected over large parts of the glaciated terrain of Canada.The key implications of this study to mineral exploration are that: 1) till sampling is a viable exploration method in the district; 2) in the eastern part of the district where the till is generally thin (<5 m), surface till sampling will be cost effective; further west where till cover exceeds 30 m overburden drilling methods will be needed to collect till samples at depth; 3) indicator minerals and till geochemistry (transport data) combined with the reconstructed phases of ice flow define a palimpsest pattern of glacial transport in the region.

Classification, formation, and transport mechanisms of mud clasts

ABSTRACTMud clasts are common in non-marine to marine sedimentary records, however, why lack a widely accepted classification scheme? We propose that it is the relative balance of volumetric abundance, sorting, roundness, and grain size that controls the texture and fabric of mud clasts. Nine distinct types of mud clasts are identified in the study based on quantitatified properties, and fall into two groups coarse-grained and fine-grained. The generation of mud clasts can be assigned to failure, erosion, and/or bioturbation of muddy sediment. These clasts are transported within fluid flows including Newtonian fluids, non-Newtonian fluids, and Bingham plastics (gravity flow and turbidity flow), showing various physical characteristics depended upon the density and viscosity of flows. Newtonian flows with less density and viscosity commonly form mud clasts with mature textures. In non-Newtonian (gravity-driven) flows, mud clasts are normally transported in laminar flows with high density and viscosity, developing matrix-supported mud clasts with immature textures. The study of classification, formation, and transport mechanisms of mud clasts has implications for identifying and interpreting sedimentary environments.

Middle to Late Devensian glaciation of north‐east Scotland: implications for the north‐eastern quadrant of the last British–Irish ice sheet

ABSTRACTWe present a review of over 175 years of research into the Middle to Late Devensian (Weichselian) glaciation of NE Scotland based crucially on both its lithostratigraphic and geomorphic records. The location of the region, and surrounding seabed, makes this unusually detailed record significant for deciphering the former interactions and dynamics of Scottish and Scandinavian ice within the North Sea Basin (NSB), which continue to be controversial. A 12‐stage event stratigraphy is proposed based on a parsimonious interpretation of stratigraphic relationships, till lithology and regional clast fabrics, striae, subglacial bedforms, ice‐marginal features and published geochronometry at critical sites. The record of regional glaciation supports converging evidence that the north‐eastern quadrant of the last British and Irish Ice Sheet reached its maximum spatial extent in the late Middle to early Late Devensian and later re‐expanded following widespread internal glacial reorganizations, marine transgression and partial retreat from the central NSB. Retreat was interrupted by several glacial readvances of limited extent. Field‐captured data help to identify important events that are not clearly resolved from remote sensing alone, particularly regarding growth phases of the last glaciation.

Emplacement style of pebbly mudstone melange suggested by the clast fabric

Emplacement style of pebbly mudstone melange suggested by the clast fabric

Paleoflow directions of a subaqueous lahar deposit around the Miocene Keserűs Hill lava dome complex (North Hungary) as constrained by photo-statistics and anisotropy of magnetic susceptibility (AMS)

A twofold fabric analysis by using photo-statistics on rock surfaces and low-field anisotropy of magnetic susceptibility (AMS) is presented, making it possible to infer paleoflow directions, which in turn helps to constrain the primary volcanic geomorphology of a deeply eroded mid-Miocene field (Keserűs Hill lava dome group, Visegrád Mountains, North Hungary). The analyses were carried out on the Rám Hill Pumiceous Sandstone (RHPS) that, based on its petrographic features, is considered as a subaqueously emplaced laharic deposit.The observed anisotropic frequencies of clast a-axis azimuths, the good agreement of fabric directions obtained for all and most elongated samples (elongation>2.5), the good clustering of the declinations of the K1 susceptibilities largely corresponding with the result of image analysis, allow to infer reliable flow directions. The obtained large-scale paleoflow paths show a quasi-radial pattern around the central part of the Visegrád Mountains, which quantitatively confirms the previous hypothesis on the volcanic structure, namely a central edifice-topped syn-eruptive topography.The RHPS is characterized by a clast fabric (revealed by photostatistics on cutted rock surfaces considering the lapilli-sized fraction) as strong as in subaerial PDC deposits, but by a significantly weaker magnetic fabric. The weak magnetic fabric is supposed to be the result of rock heterogeneity, thus the weakening effect of abundant lithic clasts with strong internal magnetic susceptibility anisotropy, different from the shape anisotropy of the clasts and the flow-related magnetic fabric of the fine-grained matrix.

Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

Chaotic breccias and megabreccias – locally called gash breccias – hosted within the Pembroke Limestone Group (Visean, Mississippian, lower Carboniferous) of southwest Wales are re-mapped along with spatially-related crackle and mosaic breccias. Of thirteen studied megabreccia bodies, seven lie along steep, NNW- or NNE-striking strike-slip faults originating during north–south Variscan (late Carboniferous) shortening, though reactivated during later extension. Four bodies are conformable with E–W striking, steeply-dipping bedding, and two have irregular or indeterminate margins. The bedding-parallel zones are interpreted as the dilational tips of listric normal faults, and the cross-strike faults as transtensional transfer zones. Sub-horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults. Most fissures have geometrically matched margins produced by this dilational faulting, and only locally have the indented margins indicating solutional processes. The most likely age for the main fissure extension and fill is late Triassic, based on analogous dated fills at the eastern end of the Bristol Channel Basin. The Pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones.

Deep-lacustrine transformation of sandy debrites into turbidites, Upper Triassic, Central China

Deep-lacustrine transformation of sandy debrites into turbidites in the downslope direction is evident in the Upper Triassic Yanchang Formation of the Ordos Basin, central China. This facies trend is used as a template for predicting the distribution of reservoir facies of the Huaqing oilfield, which contains 100-million-tonnes of oil reserves. Based on examination of conventional cores from 30 wells, four major types of lithofacies have been recognized: (1) fine-grained massive sandstone with floating mudstone clasts and planar clast fabric (sandy debrite); (2) fine-grained sandstone and siltstone showing contorted bedding, sand injection, and ptygmatic folding (sandy slump), (3) fine-grained sandstone with thin layers of normal grading and flute casts (turbidite), and (4) mudstone with faint laminae (suspension fallout). Thick sandy debrite units occur beneath the delta-front facies, implying progradation. The lake margin is dominated by sandy debrites, whereas the lake center is dominated by turbidites. In our study area, sandy debrites constitute the producing petroleum reservoirs, but turbidites are non reservoirs. The proposed model is characterized by (1) delta-fed multiple source (line source), (2) absence of channels, and (3) downslope transformation of proximal sandy debrites into distal turbidites. This downslope transformation of sandy debrites into turbidites may be applicable to other deep-lacustrine basins worldwide for predicting reservoir distribution.

Hydrothermal Breccia Textures and Processes: Lisca Bianca Islet, Panarea Volcano, Aeolian Islands, Italy

Panarea is a largely submarine to partly subaerial Quaternary lava dome complex-stratovolcano with a long-lived, active, shallow hydrothermal system, located in the Aeolian Islands volcanic arc of southern Italy. The emergent top of the volcano forms a small archipelago, made up of calc-alkaline basaltic andesite to rhyolite lava domes (ca. 150–20 ka). We document the facies outcropping on Lisca Bianca islet, Panarea archipelago, based on grain size, clast fabric, and degree of hydrothermal alteration, identifying coherent facies, boulder breccia facies, cobble breccia facies, pebble breccia facies, and pervasively altered andesite facies (alunite-marcasite-sulfur). The breccias all have ubiquitous jigsaw-fit clast textures, and are variably hydrothermally altered. The breccias are interpreted as hydrothermal breccias and are distinguished from primary volcanic facies based on their distinguishing characteristics. The breccias formed through a cyclical process, involving the following: stage 1: progressive build-up of fluid pressure toward the level of the tensile strength of the host andesite; stage 2: incipient fracturing of the andesite when fluid pressure approaches and then exceeds the tensile strength of the andesite under critical fracturing conditions; stage 3: pervasive fracturing of the host andesite, leading to an increase in permeability as a network of fractures develops; stage 4: declining pressure, with fluid flow rates that lead to infilling and sealing of fractures by natroalunite, thereby reducing permeability, leading to progressive build-up of fluid pressure again, and the beginning of a new cycle.

Quaternary slope processes and morphologies in the upper Sehonghong Valley, eastern Lesotho

Several high south-facing slopes in the southern Drakensberg were glaciated during the Late Quaternary, yet little is known about the slope geomorphic history on equally high slopes that appear not to have been glaciated. To this end, we examine evidence for Late Quaternary slope dynamics in a high alpine valley of eastern Lesotho, with particular attention given to palaeo-environmental signatures offered by deep colluvial mantles along the flanks of the Sehonghong River and pronival ramparts along the upper southerly-facing slopes of the adjoining Thabana-Ntlenyana. Sedimentary exposures were mapped and sampled for clast fabric, clast shape, organic matter content, granulometry and 14C age determination. Similarly, a pronival rampart was mapped and clast size, shape and fabric determined for various micro-topographic settings. A palaeosol beneath the rampart was also Radiocarbon dated. Findings suggest that the deep and extensive debris mantles on lower south-facing slopes are a product of prolonged colluviation, whilst the more stratified sequences on north-facing slopes indicate greater slope geomorphic-process variability over time. Radiocarbon ages suggest sediment accumulation along the north-facing exposure from at least ∼43 ka. It is proposed that snowcreep and snowpush on high (>3400 m a.s.l.) southeast-facing slopes during a relatively cold, yet moist period at ∼AD 300–1000 initiated boulder movement to form the pronival ramparts.

Clast fabric and mass wasting on Asteroid 25143-Itokawa: Correlation with talus and other periglacial features on Earth

Regolith on Itokawa, a 300-meter diameter near-Earth asteroid, first imaged by the Hayabusa Spacecraft in 2005, provides a new database of information on surface features and different space-form (i.e. landform) assemblages. The satellite imagery was taken from distances of near-surface and 2 km to 63 m, the latter giving image resolutions of 6 mm/pixel. Small regolith-covered asteroids produce rubble-covered surface mantles, resident on slopes ranging from 27° relative to an artificial horizon, and distributed non-uniformly across the surface. Low gravity acceleration on Itokawa means that clast movement is largely the result of global vibrations caused by impacts producing seismic shaking. Gravel-network orientations, previously correlated with imbricated stream gravels on Earth by the Hayabusa Scientific Team (Miyamoto, H., Yano, H., Scheeres, D.J., Abe, S., Barnaouin-Jha, O., Cheng, A.F., Demura, H., Gaskell, R.W., Hirata, N., Ishiguro, M., Michikami, T., Nakamura, A.M., Hakamura, R., Saito, J., Sasaki, S., 2007. Regolith migration and sorting on asteroid Itokawa. Science. 316, 1011–1014, Fig. 2F), are here partly correlated with talus and stone-banked lobe accumulations on Earth. We present ‘sliderock’ accumulations on Itokawa correlated with talus accumulations on Earth from several high, middle and low latitude Cryic localities. The presence of debris-flow accumulated clastic material indicates the presence of permafrost and issuance of meltwater at some time in the past, presumably linked to an orbital or solar radiation perturbation. The available data indicate the presence of water, at least on a punctuated time scale; along with the presence of Fe, the combination of the two suggesting weathering processes and a dynamic surface geology on what was previously considered to be a dry, barren, lifeless world.

Reinterpreting Rotherslade, Gower Peninsula: implications for Last Glacial ice limits and Quaternary stratigraphy of the British Isles

AbstractRotherslade on the Gower Peninsula in south Wales has been viewed as a key site for the reconstruction of Quaternary depositional environments in the British Isles. Since the early 20th century, and certainly since the 1980s, the accepted view has been that Rotherslade is the most westerly location on the south Gower coast where there is in situ basal till exposed and that, logically, this location marks the position of the LGM ice limit. However, reinvestigation of the sediments and their architecture, and analysis of clast fabrics and thin sections of critical sedimentary units, show that none of the exposed sediments has properties diagnostic of subglacial deposition or deformation. We postulate here that LGM ice terminated at the western side of Swansea Bay, a few kilometres to the north‐east of Rotherslade, and propose that the sedimentary sequence comprises Early to Middle Devensian periglacial sediments, overlain by a complex of Late Devensian, ice‐proximal outwash fan deposits, an assemblage of paraglacial debris and, finally, periglacial mass movement deposits. The proposed repositioning of the Late Devensian ice limit and the associated new subaerial interpretation of the sediments suggest that a reassessment of sedimentary sequences (Hunts Bay, Western Slade) and landforms (Paviland Moraine) farther west on Gower, which have attained similar stratigraphical status, is now warranted. Copyright © 2008 John Wiley &amp; Sons, Ltd.

On the Welding of Pyroclasts from Very Low-Viscosity Magmas: Examples from Kimberlite Volcanoes

Lithofacies in kimberlite pipes in southern Africa exhibit features consistent with welded pyroclastic rocks. These include flared conduit-filling geometries, abundant lithic clasts, lithic clast layers, subhorizontal clast fabrics, gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash, and lapilli and melt-coated particles. The welding dynamics of kimberlite pyroclasts differ from those of glassy, vesiculated pyroclasts in silicic volcanic systems. Low melt viscosity (∼0.1 Pa s) results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets. The glassy state is difficult to form in silica-deficient magmas because the crystallization kinetics are fast in low-viscosity melts. Three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition: (1) densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure melt, coalescing almost immediately to form a degassed homogeneous melt that subsequently crystallizes and may be texturally indistinguishable from igneous kimberlite; (2) incipiently welded kimberlite forms where the melt phase in pyroclasts is mostly crystalline with some residual melt (pyroclasts are resistant to deformation but are sticky and can sinter together); and (3) nonwelded kimberlite forms where the melt phase in pyroclasts is fully crystalline on deposition. Transitions between these pyroclast conditions may be abrupt. Gradations to nonwelded deposits and overlaps in their textural features in several kimberlite pipes suggest that welded rocks may be deposited from large-scale fluidized systems. The presence of crystals and lithic clasts may inhibit compaction deformation-welding textures by the formation of hard-particle percolation networks. Similar rocks in other kimberlite pipes may turn out to be welded rocks. The processes outlined here may be generally applicable to other low-viscosity magmas.

Possible origin of the block stream in Tom Mays Canyon, Franklin Mountains, TX

Sometime during the Pleistocene, a relatively small mass movement event occurred in which material detached from the canyon wall just north of Franklin Mountain, traveled down the canyon, and was super-elevated along the opposite side of the canyon. Determining the actual mode of transportation and the proper classification of this landform has been challenging. Near the end of the 20th century, two researchers debated, in print, the mode of emplacement for the deposit. One researcher suggested transport similar to a flash flood. The second researcher suggested several alternative possibilities: landslip accompanied by washing out of fines, frost-induced deposition or rubble flow and sieve deposition. We reexamined the deposit and based on the fabric of clasts, topographic profiles, cross-sections, estimated volume, mean gradient (H/L), and local geomorphology and geologic setting, suggest that the Tom Mays block stream was most likely formed as a rock avalanche. Freeze-thaw weathering is suggested as the trigger mechanism for the avalanche of material, which originated from preferentially weakened source rock within a large fault line that cuts the rhyolite cliffs of the west canyon wall just north of Franklin Mountain. Blocky debris was transported by means of grainflow or fragmental flow along a diverted runout within the confines of the canyon.