Clast Shape Research Articles

Subaqueous strombolian eruptions, and eruption-fed pyroclastic deposits in a Paleoproterozoic rifted-arc: Hidden formation, Flin Flon, Canada

The Hidden formation of the Paleoproterozoic Flin Flon arc assemblage comprises the stratigraphic hanging wall to the volcanogenic massive sulfide (VMS) deposits at Flin Flon, Manitoba, Canada. It represents a basaltic volcanic edifice erupted within a large subsidence structure developed in a rifted-arc environment. It is predominately pillow lavas with subordinated amounts volcaniclastic rocks.Rocks that stratigraphically overly the Schist and Mandy VMS deposits ∼5 km southeast of the main Flin Flon deposits have been correlated through litho- and chemo-stratigraphy with the Hidden formation. They display evidence of pyroclastic volcanism and concomitant subsidence. Pillow lavas, and hyaloclastite are unequivocal evidence of subaqueous eruption.The Hidden formation in the Schist – Mandy area comprises mafic tuff, mafic megabreccia, basalt lava, fluidal-clast breccia, and plagioclase crystal-rich volcaniclastic lithofacies, intruded by synvolcanic dikes and sills. The megabreccia lithofacies is characterized by angular basalt clasts (1–20 m) in a matrix of finer basalt clasts and is interpreted to have been deposited by mass flows due to slumping of the caldera wall. A within flow vertical transition in the lavas from massive to pillowed flows suggests a slowing of emplacement velocity, decrease in temperature, and an increase in the viscosity during eruption. Based on the monolithic nature of the fluidal-clast breccias, and the fluidal shapes of clasts, with delicate tails and intact glassy margins, they are interpreted to be the result of strombolian eruptions. An increase in the ingestion of water in the vent(s) lead to a change from magmatic to phreatomagmatic fragmentation. Planar and cross laminations, graded beds and the abundance of broken plagioclase crystals, in combination with scoria and lithic clasts suggest the mafic tuff and plagioclase crystal-rich volcaniclastic lithofacies were deposited by subaqueous, eruption-fed density currents derived from a phreatomagmatic eruption.Detailed mapping of the volcanic lithofacies in the Schist – Mandy area has facilitated the identification of synvolcanic faults defining inner caldera margins and volcanic vents. These structures when traced downwards through stratigraphy define structural corridors corresponding with the Schist and Mandy VMS deposits, indicating they represent long-lived structures that also acted as magmas and fluid pathways.

The glacial landsystem of a tropical glacier: Charquini Sur, Bolivian Andes

AbstractThe geomorphological signature of tropical glaciers has the potential to provide important information on the response of ice masses in high‐mountain environments to climate warming. This study investigates the glacial geomorphology of Charquini Sur, Bolivia. Detailed geomorphological mapping was conducted both in the field and from satellite imagery in order to produce a 1:4000 scale geomorphological map of the glacier foreland. Sedimentological analyses (description of physical characteristics, clast shape and roundness, particle‐size distribution) provided additional insight into the landform–sediment assemblage. Glacial landforms are well preserved and include up to 11 moraine ridge suites, seven of which are cross‐valley frontal moraine arcs. These can be linked to an existing lichenometric chronology from previous work and record glacier recession since the local Little Ice Age (LIA) maximum in the late‐1600s. Lateral moraine ridges also record continuous thinning of the glacier over this time period. Smaller groups of parallel ridges are interpreted as annual moraines formed during recession. Intermorainic areas consist of flutings and a typically thin sediment cover of subglacial, supraglacial and glaciofluvial origin, with prominent ice‐moulded bedrock protuberances in places. Analysis of the landform–sediment assemblage provides an insight into the main controls on landform genesis in the basin and implies there have been temporal changes in ice‐marginal dynamics since the LIA. We present the first landsystem model for a tropical cirque glacier, documenting its behaviour since the LIA and providing an indication of glacier response in rapidly‐warming high‐mountain environments. Copyright © 2018 John Wiley & Sons, Ltd.

Fractal Analysis of In Situ Host Rock Nepheline Syenite Xenoliths in a Micro-Shonkinite Dyke (The Elchuru Alkaline Complex, SE India)

Formation of the fragments of the wall-rock during dyking is one of the important manifestations of instantaneous magmatic events. This process is well documented at shallower depths of Earth’s crust but not at deeper levels. In this paper the in situ xenoliths of host rock nepheline syenite within a micro-shonkinite dyke emplaced at mid-crustal depths is described and the fractal theory applied to evaluate origin of the xenoliths. The nepheline syenite xenoliths are angular to oval shaped and sub-millimetre to ~50 cm long. The xenoliths are matrix supported with clasts and matrix being in equal proportions. Partly detached wall-rock fragments indicate incipient xenolith formation, which suggested that the model fragmentation processes is solely due to dyke emplacement. Fractal analytical techniques including clast size distribution, boundary roughness fractal dimension and clast circularity was carried out. The fractal data suggests that hydraulic (tensile) fracturing is the main process of host rock brecciation. However, the clast size and shape are further affected by postfragmentation processes including shear and thermal fracturing, and chemical erosion. The study demonstrates that dyking in an isotropic medium produces fractal size distributions of host rock xenoliths; however, post-fragmentation processes modify original fractal size distributions.

The Ophiolitic Mélanges in Strike‐slip Fault Zones in West Junggar, Xinjiang, NW China

The Ophiolitic Mélanges in Strike‐slip Fault Zones in West Junggar, Xinjiang, NW China

VÝVOJ LEDOVCOVÝCH SEDIMENTŮ NA KONTAKTU S ŽULOVSKÝM MASIVEM VE ŠTACHLOVICÍCH U VIDNAVY

The Žulová Upland is composed of granitoids of the Žulová batholith with relicts of Pleistocene (Elsterian) continental glaciation sediments. The investigated outcrop represents development of glacigenic sediments on rugged topography of the Žulová Upland. Investigated locality is situated on a hill located on the northern margin of the Žulová Upland. It is located near Štachlovice, local part of the Vidnava town. The exposed part of the hilltop reveals a preglacial basement covered by glacigenic sediments. The facies analysis and gravel petrology analysis of clasts with fraction 16–64 mm in b-axis were undertaken on the walls of the outcrop. The Georadar (GPR, Ground penetrating radar) was used to investigate the sedimentary landform and its relation to the basement. The granitoid basement is in places formed by elevations covered by glacigenic sediments. The height of elevation reaches 350 cmin outcrop, or ca ~400 cm according to the GPR survey. The glacitectonite, formed on the gentle side of elevation, is composed of angular blocks of granitoids of the Žulová batholith, diamicton, sand, gravel and deformed glacifluvial sand. The glacitectonite was deposited during the advancement of the continental glacier. The original subglacial cavity is enclosed by a steep side of the elevation. This cavity is filled by foreset body composed of stratified sand and gravel and nonstratified gravelly sand, gravel and diamicton. The cavity was filled by high-density turbidity currents and debris flow in subaqueous-subaerial environment. The infill of the cavity reaches ~400 cm in thickness according to the GPR survey. The cavity was filled during deglaciation in subglacial environment. The glacitectonite underlies the diamicton (supraglacial till) that was deposited as a debris flow during the retreat of the continental glacier. Unsorted gravel overlaps with erosional base the infill of the cavity, this gravel has a huge extent according to the GPR survey. This sediment represents the environment of terminoglacial stream. Gravel material of all types of glacigenic sediments is mainly composed of rocks from the Rychleby Mts. (amphibolites, Gierałtow orthogneiss, other diverse gneisses, quartzites, mica schist),quartz, and Nordic and Polish rocks. Subglacial sediments contain clasts of amphibolites (~40 %), on the other hand supraglacial and terminoglacial sediments are more polymict. Dominant subrounded shape (~60–70 %) of clasts and composition of this material indicates its origin in preglacial fluvial sediments. These fluvial sediments were deposited by river flowing from the Rychleby Mts. towards their northern foreland. The locality represents preglacial elevation of bedrock, which was glacitectonically deformed during the glaciation. Lots of different types of sediments (sub-, supra-, and terminoglacial) were deposited around the elevation during deglaciation period. The elevation was completely buried by these sediments. Deposition of these sediments was related with morphology of the elevation of bedrock. Formation of these sediments took place in environment analogous to environment of part bedrock/part till drumlin (Stokes et al. 2011).

Experimental constraints on the relationship between clay abundance, clay fabric, and frictional behavior for the Central Deforming Zone of the San Andreas Fault

AbstractThe presence of smectite (saponite) in fault gouge from the Central Deforming Zone of the San Andreas Fault at Parkfield, CA has been linked to low mechanical strength and aseismic slip. However, the precise relationship between clay mineral structure, fabric development, fault strength, and the stability of frictional sliding is not well understood. We address these questions through the integration of laboratory friction tests and FIB‐SEM analysis of fault rock recovered from the San Andreas Fault Observatory at Depth (SAFOD) borehole. Intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or SAFOD clasts extracted from the sample. Nano‐textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges; such slip planes have multiples of random distribution (MRD) values of 3.0–4.9. The MRD values measured are higher than previous estimates (MRD 1.5) that show lower degrees of shear localization and clay alignment averaged over larger volumes. The intact fault rock exhibits less well‐developed nano‐clay fabrics than the experimentally sheared materials, and MRD values decrease with smectite content. We show that the abundance, strength, and shape of clasts all influence fabric evolution via strain localization: quartz clasts yield more strongly developed clay fabrics than serpentine‐dominated SAFOD clasts. Our results suggest that (1) both clay abundance and the development of nano‐scale fabrics play a role in fault zone weakening and (2) aseismic creep is promoted by slip along clay shears with >20 wt % smectite content and MRD values ≥2.7.

A study on the granulometric and clast shape characteristic of gravel terrace deposit at Jeongdongjin area

A study on the granulometric and clast shape characteristic of gravel terrace deposit at Jeongdongjin area

Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

This paper describes the Neogene lava–sediment mingling from the Abu Treifiya Basin, Cairo–Suez district, Egypt. The lava–sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators. The identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the Abu Treifiya Basin. Characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops. The peperites and sedimentary rocks are up to 350m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area.Lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive/intrusive basaltic rocks forming peperitic breccias to lava–sediment contacts at a large to small scales, respectively. Peperites encompass five main facies types ascribed to: (i) carbonate sediments-hosted fluidal and blocky peperites, (ii) lava flow-hosted blocky peperites, (iii) volcaniclastics-hosted fluidal and blocky peperites, (iv) sandstone/siltstone rocks-hosted blocky peperites, and (iv) debris-flows-hosted blocky peperites. Soft sediment deformation structures, vesiculated sediments, sediments filled-vesicles, and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments. All the peperites in this study could be described as blocky or fluidal, but mixtures of different clast shapes occur regardless of the host sediment. The presence of fluidal and blocky juvenile clasts elucidates different eruptive styles, reflecting a ductile and brittle fragmentation. The gradual variation from fluidal to blocky peperite texture, producing the vertical grading is affected by influencing factors, e.g., the viscosity, magma temperature, confining pressure, sediment fluidization, and vapor film at the magma–sediment interactions.Peperites in the study area record deposition within a shallow marine and fluvio-lacustrine environment accumulated in a rift-related basin developed during pre- to syn-rift phase, respectively. The facies transitions (peperites) in this area resulted from the explosive and sediment depositional processes, which were mingled separately by volcanism under contrast geological conditions. The development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the Abu Treifiya Basin. Hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows.

The ground layer of the Campanian Ignimbrite: an example of deposition from a dilute pyroclastic density current

A thin, fines-poor, and lithic- and crystal-rich layer locally present, from proximal to distal areas, at the base of the pyroclastic density current deposits of the Campanian Ignimbrite eruption (39 ka), sourced from Campi Flegrei (Italy), is interpreted as a ground layer (GL). It has an average lithic and crystal content of 57 and 25 wt%, respectively. The GL rests on a paleosol or an early fall deposit and is capped by a stratified ash deposit. It is bounded by erosion surfaces and its thickness does not change systematically from the source. The occurrences of GL in places separated from the source by a stretch of sea and the angular to sub-angular shape of the lithic clasts are consistent with deposition from a dilute current. The presence of the GL up to 971 m above sea level and beyond 1400–1500-m-high mountain ridges, allows estimation of the thickness of the PDC as having been at least 1.5 km. To investigate the parameters influencing the transport and emplacement mechanisms that produced the GL, we use field and sedimentological data (grain size and componentry), from samples collected between 30 and 70 km from the source. The progressive decrease in maximum lithic clast and median grain-size with distance from the vent and increasing altitude corresponds to lateral and vertical grading of lithic clasts within the current. The coarser clasts moved toward the basal part of the current, settling in more proximal areas and at low altitude. Farther from the vent (45–50 km), the lower part of the pyroclastic current impacted against the reliefs surrounding the Campanian Plain, causing the emplacement of coarser and more poorly sorted deposits on slopes oriented toward the source.

Experimental stone tool replication at the Early Stone Age site of Sterkfontein, Gauteng, South Africa

Between 1999 and 2000 a large experimental knapping programme was carried out at the early hominin site of Sterkfontein, South Africa. Its purpose was to assess aspects of the technology of lithic production found in the Oldowan and Acheulean deposits at the site. The data was used to inform technological analysis of the Sterkfontein stone tool assemblages. In the light of more recent research on raw material selection and transport at East African Oldowan sites, we report on the conclusions drawn from the analysis of the experimental cores to assess their contribution to understanding the influence of raw materials on knapping in the Oldowan and Acheulean at Sterkfontein. Hominins practised raw material selectivity and adapted their knapping strategies to fit clast shape and lithology. The experimental programme offered the opportunity to analyse some of the variables commonly involved in reconstructing lithic behaviour from artefacts and found, as with other studies reported elsewhere, that linear relationships based on one, or a few supposedly diagnostic variables do not stand up to scrutiny.

Subsurface structure of a large basaltic maar volcano examined using geologically constrained potential field modelling, Lake Purrumbete Maar, Newer Volcanics Province, southeastern Australia

Lake Purrumbete Maar (LPM) is situated in the Late Cenozoic intraplate, basaltic Newer Volcanics Province, southeastern Australia. It is one of the largest maar volcanoes in the world with a near circular crater that is up to 2800m in diameter containing a 45m deep lake. Gellibrand Marl accidental lithics, which occurs to a maximum depth of 250m below LPM, represent the deepest excavated host rock unit present in the volcanic succession. Irregular clast shapes and peperitic textures observed in marl lithics suggest the host rock was poorly consolidated during the eruption. High-resolution lake- and land-based gravity and magnetic data were collected to conduct forward and inverse modelling of the subsurface architecture of the maar. This is done to test the assumption, based on lithics, that the diatreme is limited to 250m depth and identify the reasons behind LPM's large size. The collection of gravity data presented a unique challenge due to the nature of measuring small changes in gravitational forces (<1mGal) associated with the maar, on an inherently unstable water body. The magnetic anomaly over LPM shows several irregularly shaped high magnetic anomalies. Four 2.5-D forward models transecting LPM were constructed based on the observed potential field data. Five coalesced vents forming an undulating shallow bowl-shaped diatreme contained within the Gellibrand Marl (240m depth) were modelled to satisfy the observed magnetic response, while a large sill body at 350m depth is modelled to satisfy the observed gravity response. A second forward model, completed to satisfy the observed magnetic response, also includes coalesced shallow bowl-shaped diatremes within the Gellibrand Maar, which then taper down to become thin and steep sided and extend down to the basement (1,050m depth). Three-dimensional property and geometry inversions suggest the diatreme most likely extends to a greater depth than the initial maximum depth of 240m. This suggests the lithic-based maximum diatreme depth inferred by accidental lithics present in the volcanic successions is an underestimate.

Shape fabric development in rigid clast populations under pure shear: The influence of no-slip versus slip boundary conditions

Shape fabrics of elliptical objects in rocks are usually assumed to develop by passive behavior of inclusions with respect to the surrounding material leading to shape-based strain analysis methods belonging to the Rf/ϕ family. A probability density function is derived for the orientational characteristics of populations of rigid ellipses deforming in a pure shear 2D deformation with both no-slip and slip boundary conditions. Using maximum likelihood a numerical method is developed for estimating finite strain in natural populations deforming for both mechanisms. Application to a natural example indicates the importance of the slip mechanism in explaining clast shape fabrics in deformed sediments.

Structure and sedimentology of George VI Ice Shelf, Antarctic Peninsula: implications for ice-sheet dynamics and landform development

Collapse of Antarctic ice shelves in response to a warming climate is well documented, but its legacy in terms of depositional landforms is little known. This paper uses remote-sensing, structural glaciological and sedimentological data to evaluate the evolution of the c . 25000 km 2 George VI Ice Shelf, SW Antarctic Peninsula. The ice shelf occupies a north–south-trending tectonic rift between Alexander Island and Palmer Land, and is nourished mainly by ice streams from the latter region. The structure of the ice shelf is dominated by inherited foliation and fractures, and with velocity data indicates a largely compressive flow regime. The formation of a moraine complex at the margin of the ice shelf is controlled by debris entrained within foliation and folds. This englacial debris is of basal origin, and includes both local Mesozoic sedimentary and volcanic lithologies, and exotic crystalline rocks from Palmer Land. Folding of basal ice to a high level in the source glaciers on Palmer Land is required to bring the debris to the surface. These results have implications for understanding flow dynamics of ice shelves under compressive flow, and debris entrainment and moraine formation associated with palaeo-ice shelves. Supplementary material: Photographs of ice-shelf morphology, ice facies and ice structure; detailed descriptions of ice facies, including foliation logs, supporting evidence for interpreting sedimentary facies; complete dataset of sedimentological data, including triangular plots of clast shape, clast roundness histograms, particle-size distribution of sand-size and lower, and pie chart of local clasts versus exotic clasts from Palmer Land; and a table summarizing the characteristics of representative clasts in the ice-shelf moraine, based on thin-section analysis, with indication of their provenance are available at http://www.geolsoc.org.uk/SUP18831

WITHDRAWN: Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

Abstract This paper describes the Neogene lava–sediment mingling from the Abu Treifiya Basin, Cairo–Suez district, Egypt. The lava–sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators. The identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the Abu Treifiya Basin. Characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops. The peperites and sedimentary rocks are up to 350 m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area. Lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive/intrusive basaltic rocks forming peperitic breccias to lava–sediment contacts at a large to small scales, respectively. Peperites encompass five main facies types ascribed to: (i) carbonate sediments-hosted fluidal and blocky peperites, (ii) lava flow-hosted blocky peperites, (iii) volcaniclastics-hosted fluidal and blocky peperites, (iv) sandstone/siltstone rocks-hosted blocky peperites, and (iv) debris-flows-hosted blocky peperites. Soft sediment deformation structures, vesiculated sediments, sediments filled-vesicles, and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments. All the peperites in this study could be described as blocky or fluidal, but mixtures of different clast shapes occur regardless of the host sediment. The presence of fluidal and blocky juvenile clasts elucidates different eruptive styles, reflecting a ductile and brittle fragmentation. The gradual variation from fluidal to blocky peperite texture, producing the vertical grading is affected by influencing factors, e.g., the viscosity, magma temperature, confining pressure, sediment fluidization, and vapor film at the magma–sediment interactions. Peperites in the study area record deposition within a shallow marine and fluvio-lacustrine environment accumulated in a rift-related basin developed during pre- to syn-rift phase, respectively. The facies transitions (peperites) in this area resulted from the explosive and sediment depositional processes, which were mingled separately by volcanism under contrast geological conditions. The development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the Abu Treifiya Basin. Hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows.

A comparison of the effectiveness of clast based finite strain analysis techniques to AMS in sandstones from the Sevier Thrust Belt, Wyoming

Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques, which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres. To evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility (AMS) in sandstone samples from the Wyoming salient. The Cordilleran Mountain Belt of North America is one of the world's classic foreland fold and thrust belts. The Sevier Belt represents the thin-skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of Devonian to Cretaceous strata eastward. Deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility (AMS) to clast-based strain analyses. Studies attempting to define the relationship between AMS and finite strain have been in vogue since the link between layer parallel shortening and AMS was first established. This relationship, despite proven strong correlations between the AMS tensors and tectonic directions, is generally complicated by competing sub-fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the AMS fabric. Even with these complications, we conclude that AMS is much more sensitive to incipient tectonic fabrics than clast based methods.

Fall velocity of multi-shaped clasts

Accurate settling velocity predictions of differently shaped micro- or macroclasts are required in many branches of science and engineering. Here, a single, dimensionally correct equation is presented that yields a significant improvement on previous settling formulas for a wide range of clast shapes. For smooth or irregular clasts with known axial dimensions, a partially polynomial equation based on the logarithmic values of dimensionless sizes and settling velocities is presented, in which the values of only one coefficient and one exponent need to be adapted for different shapes, irrespective of the Reynolds number. For irregular, natural clasts with unknown axial dimensions, a polynomial equation of the same form is applied, but with different coefficients. Comparison of the predicted and measured settling velocities of 8 different shape classes as well as natural grains with unknown axial dimensions in liquids, representing a total of 390 experimental data points, shows a mean percentage error of −0.83% and a combined R2 value of 0.998. The settling data of 169 differently shaped particles of pumice, glass and feldspar falling in air were also analyzed, which demonstrates that the proposed equation is also valid for these conditions. Two additional shape classes were identified in the latter data set, although the resultant equations are less accurate than for liquids. An Excel spreadsheet is provided to facilitate the calculation of fall velocities for grains settling individually and in groups, or alternatively to determine the equivalent sieve size from the settling velocity, which can be used to calibrate settling tubes.

Provenance and sediment characteristics of contemporary gravel deposits at Sellicks Beach, eastern shore of Gulf St Vincent, South Australia

Sellicks Beach, located on the eastern shore of Gulf St Vincent, South Australia, is subject to wave-dominated processes and northward longshore transport. During winter, when wave energy is typically vigorous, gravel deposits are exposed across most of the beach, and three step-like berms are well developed. Sand is restricted to a narrow strip that is exposed only at low tide. In contrast, during summer, when wave energy is generally moderate to low, much of the gravel is covered by a thin veneer of sand and only the high berm, on the landward edge of the beach, remains as an obvious feature. Steeply dipping Neoproterozoic to Cambrian strata that outcrop strongly across Sellicks Hill are the original source rocks for the beach gravel; distinctive sedimentary textures, structures and fossils in the cobble-size clasts can be confidently matched with those of the provenance rocks. Much of the sediment entered the modern beach environment as a consequence of coastal erosion of transitional alluvial fan sediments. The oldest alluvial fan sediments are of late Pliocene to earliest Pleistocene age. Mount Terrible Gully provides a conduit for the input of fluvial sediment at the mouth of Cactus Canyon, where clasts as large as boulders accumulate across the beach. Sellicks Beach gravels are subject to longshore transport northwards. Relatively softer clasts, such as those derived from the Heatherdale Shale, are rare beyond Cactus Canyon. In contrast, quartzite clasts are more abundant towards the north. This lithological differentiation is attributed to preferential survivorship of clasts that are physically harder and chemically less reactive. The change in the shapes of clasts northwards, from predominately shingle-like ‘very platy’ and ‘very bladed’ at Cactus Canyon, to more ‘compact’ towards the boat ramp, is in accord with the more massive fabric of the surviving quartzite clasts. At Sellicks Beach, preservation of uplifted, coarse gravels, with entire and comminuted marine molluscan shells, of last interglacial age, provides evidence of neotectonism. At the landward margin of the beach, imbricated gravels in which pore spaces have been infilled with mud, and which show no evidence of modern coastal erosion, may provide evidence of continuing uplift during the recent Holocene. The geological setting, geomorphic framework and modern sedimentary regime at Sellicks Beach combine to provide an exceptionally useful outdoor laboratory for education in field geology.

An inverse approach to constraining strain and vorticity using rigid clast shape preferred orientation data

We describe a three-part approach for modeling shape preferred orientation (SPO) data of spheroidal clasts. The first part consists of criteria to determine whether a given SPO and clast shape are compatible. The second part is an algorithm for randomly generating spheroid populations that match a prescribed SPO and clast shape. In the third part, numerical optimization software is used to infer deformation from spheroid populations, by finding the deformation that returns a set of post-deformation spheroids to a minimally anisotropic initial configuration. Two numerical experiments explore the strengths and weaknesses of this approach, while giving information about the sensitivity of the model to noise in data. In monoclinic transpression of oblate rigid spheroids, the model is found to constrain the shortening component but not the simple shear component. This modeling approach is applied to previously published SPO data from the western Idaho shear zone, a monoclinic transpressional zone that deformed a feldspar megacrystic gneiss. Results suggest at most 5 km of shortening, as well as pre-deformation SPO fabric. The shortening estimate is corroborated by a second model that assumes no pre-deformation fabric.

Primary welding and crystallisation textures preserved in the intra-caldera ignimbrites of the Permian Ora Formation, northern Italy: implications for deposit thermal state and cooling history

Exceptional exposure through a Permian intra-caldera ignimbrite fill within the 42 × 40 km Ora caldera (>1,290 km3 erupted volume) provides an opportunity to study welding textures in a thick intra-caldera ignimbrite succession. The ignimbrite succession records primary dense welding, a simple cooling unit structure, common crystallisation zones, and remarkably preserves fresh to slightly hydrated glass in local vitrophyre zones. Evidence for primary syn- and post-emplacement welding consists of (a) viscously deformed and sintered juvenile glass and relict shard textures; (b) complete deposit welding; (c) subtle internal welding intensity variations; (d) vitrophyre preserved locally at the base of the ignimbrite succession; (e) persistent fiamme juvenile clast shapes throughout the succession at the macroscopic and microscopic scales, defining a moderate to well-developed eutaxitic texture; (f) common undulating juvenile clast (pumice) margins and feathery terminations; (g) a general loss of deposit porosity; and (h) perlitic fracturing. A low collapsing or fountaining explosive eruption column model is proposed to have facilitated the ubiquitous welding of the deposit, which in turn helped preserve original textures. The ignimbrite succession preserves no evidence of a time break through the sequence and columnar joints cross-gradational ignimbrite lithofacies boundaries, so the ignimbrite is interpreted to represent a simple cooling unit. Aspect ratio and anisotropy of magnetic susceptibility (AMS) analyses through stratigraphic sections within the thick intra-caldera succession and at the caldera margin reveal variable welding compaction and strain profiles. Significantly, these data show that welding degree/intensity may vary in an apparently simple cooling unit because of variations in eruption process recorded in differing lithofacies. These data imply complex eruption, emplacement, and cooling processes. Three main crystallisation textural zones are identified in the ignimbrite succession: localised vitrophyre zones, widespread microcrystalline to sparsely spherulitic pseudomorphed vitriclastic textural zones, and thick granophyric zones. These zones record a typical spectrum from rapid to prolonged cooling. The non-uniform crystallisation patterns reflect variations in deposit thickness, the relative position of deposits within the larger ignimbrite succession, the type of substrate material, and the degree of confinement in the intra-caldera setting. We support previous work suggesting that traditional welding classifications (e.g. Smith, US Geological Survey Professional Paper 354-F 1960b) are most appropriate for use within altered and/or ancient ignimbrite successions.

New methods to reconstruct clast transport history in different glacial sedimentary environments: Case study for Old Red sandstone clasts from polythermal Hrbyebreen and Bertilbreen valley glaciers, Central Svalbard

The objective of this study were Little Ice Age (LIA) to recent subglacial tills, glaciofluvial sediments of proglacial sandur, esker sediments and sediments of moraine-mound complexes of Hrbyebreen polythermal valley glacier and LIA to recent sediments of lateral moraine, frontal moraine, ice-cored moraine and glaciofluvial sediments of proglacial sandur of Bertilbreen polythermal valley glacier. Fossil (probably early Holocene) subglacial and supraglacial tills and sediments of coarse-grained glaciomarginal delta of Bertilbreen have also been studied. The research focused on Old Red sandstone clasts, for which roundness, shape and striation presence have been investigated. The results from this research led to the proposal of new methodological approaches. It is mainly the covariant plot of striations and RA (the share of striated clasts versus the share of very angular and angular clasts), which effectively differentiate subglacial tills from glaciofluvial sediments and allows for identification of the source material of moraine-mound complexes, especially the oldest, more degraded parts near the maximum LIA glacier extent. The second is the covariant plot of distance and RS index (the distance from the starting point versus the share of subangular and subrounded clasts) to present downstream roundness trends in proglacial glaciofluvial sediments. The research confirmed the crucial role of lithological properties of thinly bedded rocks on the shape of these rock clasts in sediment and a minimum impact of passive and active transport on the clast shape modification.