Clast Shape Research Articles

A Relict Pronival (Protalus) Rampart in the Tararua Range, North Island, New Zealand

ABSTRACTDebris ridges in New Zealand are routinely assumed to be ‘moraines’ and used as key Southern Hemisphere paleoclimatic sites without detailed evaluation of ridge origin. Here we assess the origin of a debris ridge adjacent to Dundas Ridge in the Tararua Range, North Island, New Zealand, through measurements of ridge morphology and sedimentary properties. The ridge has a steep c. 35° distal slope (height 18 m), compared with the c. 19° proximal slope (height 6 m), and on all transects the distal slopes contain the coarsest material (median b‐axis clast widths of 0.18‐0.25 m), compared to distal samples (0.34‐0.37 m). Clast shape (C40 range 40‐60%) and angularity (RA>65%) indicate typically angular and ‘slabby’ clasts, and along with the lack of fines, and the ca. 40‐m‐distance between the ridge crest and the foot of the backwall, lead us to reject a glacial (moraine) origin for the ridge. The single ridge morphology precludes a protalus rock glacier origin, while the lack of a broad hillslope scar and debris apron beyond the ridge excludes a landslide origin. Instead, we interpret the ridge as a pronival (protalus) rampart formed by supranival debris supply–from the ca. 200 m‐high southeastern slopes of Dundas Ridge–across a snowbed. Re‐distribution of snow by prevailing westerlies from Mt Dundas Ridge into the basin would have nourished the snowbed, which is likely to have formed during the interval 24‐18 ka BP, when a minor alpine‐style glaciation affected sectors of the Tararua Range. This is the first pronival rampart detailed in New Zealand, raising the possibility that debris ridges of pronival origin may also be present elsewhere in New Zealand's mountains. Copyright © 2012 John Wiley & Sons, Ltd.

Provenance of sand on the Poverty Bay shelf, the link between source and sink sectors of the Waipaoa River sedimentary system

The Poverty Shelf, North Island, New Zealand, is a segment of the Waipaoa Sedimentary System (WSS), a MARGINS Source-to-Sink focus site. Our petrographic analysis of sand from shelf core samples indicates that the sand fraction is mainly derived from intra- and potentially extrabasinal sources, but surprisingly, the major fluvial system that drains into Poverty Bay, the Waipaoa River (mean %QFL=xQyFzL), is not the dominant source. Only one vibracore at the mouth of Poverty Bay contained sand (%QFL=xQyFzL) potentially derived from the Waipaoa River. The shelf sand (mean %QFL=xQyFzL) more strongly resembles beach sand (mean %QFL=xQyFzL) collected along the coast, which is likely sourced from pervasive local cliff erosion of Miocene–Pliocene sedimentary units that exhibit similar sandstone detrital modes (mean %QFL=xQyFzL). Texturally, coarser, more poorly sorted and more angular sand is located along the outer shelf, while finer, well-sorted sand characterizes the mid-shelf. These findings suggest a shorter transport history for the material near the outer-shelf bathymetric high areas, and this observation along with the composition data suggests that they were sourced by erosion of locally exposed Miocene–Pliocene units.A potential extrabasinal source of shelf sediment is indicated by anomalous prolate and equant-shaped greywacke (Torlesse) and minor red chert pebbles collected in two outer-shelf box cores on the Lachlan anticline; these are not lithologies found within the terrestrial segment of the WSS or strata comprising the outer-shelf highs. The clast shapes are also distinctly different from the oblate-shaped, pebble-sized greywacke gravel clasts on beaches in Hawke Bay. Rather, these sediments are more similar to Torlesse stream gravel. Seismic and multibeam data support the possibility that during the most recent sea-level lowstand, the Hawke Bay fluvial system flowed into Poverty Canyon, bringing these unique gravels onto what is now the Poverty outer shelf. The presence of the Torlesse gravel is significant in that it highlights how the WSS was not a closed sedimentary system in the past, just as recent sedimentary research has shown in the case today.

A revised approach to discriminating sediment transport histories in glacigenic sediments in a temperate alpine environment: a case study from Fox Glacier, New Zealand

ABSTRACTClast shape measurements have developed into a standard method for reconstructing the transport histories of sediments in glacial environments. The majority of studies use the ‘RA‐C40’ covariance approach, with some researchers routinely including clasts of varying lithologies within their samples. The corollary is that variable lithological properties may control clast form and roundness, rather than debris‐transport mechanisms. Despite this, the role of lithology on clast shape in glacial environments has rarely been analysed. Furthermore, some studies have reported difficulties in using the RA‐C40 co‐variance plot in discriminating clasts that have undergone subglacial transport, and clasts that have been modified by fluvial activity. Results from a glacierized valley in a temperate alpine setting indicate that detailed analysis of clast shape where samples are of uniform lithology, although time consuming, is a useful tool in the investigation of deposits in glaciated environments. Copyright © 2012 John Wiley & Sons, Ltd.

Particle interaction inside debris flows: Evidence through experimental data and quantitative clast shape analysis

Results of analytical experiments related to the dynamic behavior of debris flows are presented. Particle interaction and the amount and type of fines content present in the matrix, aspects that govern debris flow behavior, are studied via the morphological evolution of constituent particles. Four different mixtures of water and sediment with compositions resembling debris flows were prepared and put into a rotating drum, known as a Los Angeles standard abrasion machine, and studied at different time intervals. For each sampling time, quantitative shape analysis was performed using three morphological coefficients, MC2, MC3–8, and MC30–34. Coefficient MC2 gives information about particle ellipticity. MC3–8 is related to basic irregularities at a macro scale, such as roundness. MC30–34 is related to microscale irregularities like surface texture. Coefficient MC3–8 was shown to be very sensitive to silt content in the matrix, and coefficient MC30–34 to the presence of clay. All three coefficients point to the fact that changes in shape between granulometric classes are gradual and follow a power law. Coarse granulometric classes acquire more circular and smooth profiles while small particles maintain their irregular profiles. This phenomenon is the result of clast interaction processes within mixtures. Clast collisions between particles produce comminution of the coarsest fractions, while finer particles fracture along their entire surface. The rate at which these changes occur is related to the type of fine sediment present in the matrix. Changes in particle shape are an important tool for revealing internal dynamics because they are related to clast interaction and affect sedimentation processes inside these flows. This type of experiment, although not on a real scale, provides important physical information for understanding the phenomena that occur inside debris flows, enabling the study of a variety of textural parameters and their changes at regular time intervals. The experimental patterns observed offer new insights into clast–clast interaction, rounding and comminuting theory, with important implications for understanding rheological behavior and kinematics of flows and related hazards.

Genesis, stability and preservation potential of large lateral moraines of Alpine valley glaciers – towards a unifying theory based on Findelengletscher, Switzerland

Lateral moraines are prominent features of glaciated landscapes in high-mountain environments and key landforms in glacier and palaeoclimatic reconstructions, yet, compared to smaller moraines, they have been little studied and several aspects are not well understood. We here present detailed sedimentological results from the lateral moraines of Findelengletscher in SW Switzerland to gain new insights into the formation of these landforms. The lateral moraines studied here stand up to 140 m above the valley floor, are over 3 km long and strongly asymmetrical in cross-profile, with distal slopes between 29 and 36° and proximal slopes that are commonly 41–64°, but locally reach angles of up to 80°. Recorded lithofacies comprise loose clast and matrix-supported, stratified diamicts and intercalated sorted sediments in the distal slopes and near the crestline; overconsolidated matrix-supported, massive and weakly stratified diamicts and streaked-out sorted sediment lenses in the core and proximal slopes; and partly intercalated dark-brown layers overlain by loose and consolidated diamicts exposed in near-vertical walls in the proximal flank. These are interpreted as supraglacial debris flow units with intercalated fluvial ‘wash’ horizons; glaciotectonised and subglacial traction till with boudinaged and streaked-out sediment lenses; and palaeosoils overlain by sediment produced by overtopping of the former moraine surface during a subsequent advance of the glacier. Clast shape analysis and process observations reveal that the dominant mode of transport is subglacial and glaciofluvial, and that the main mode of sediment delivery to the moraines is by debris flows after the material has been transferred from the bed via englacial debris bands and meltout at the surface. This differs from previous studies that found that a supraglacial source was dominant. Sedimentary structures, clast fabric and process observations during the 1979/1980 readvance of Findelengletscher strongly suggest that proximal layers of reworked pre-existing sediments and/or basal traction zone till have been plastered onto the moraine core in several locations, causing a high degree of overconsolidation and strongly-clustered fabric eigenvalues (S1 ≤ 0.94) with clustering parallel to the moraine crestline. This suggests that a combination of basal-lateral drag and lateral plastering produces the observed proximal stability and ensures a high preservation potential. We develop a conceptual model that describes Alpine lateral moraines as structurally complex landforms that do not just record a single event as often surmised, and we discuss implications for palaeo-glacier reconstruction and the application of numerical dating methods.

Stabilization Role of Crustose Coralline Algae During Late Pleistocene Reef Development on Isla Cerralvo, Baja California Sur (Mexico)

Abstract TIERNEY, P.W. and JOHNSON, M.E., 2012. Stabilization role of crustose coralline algae during Late Pleistocene reef development on Isla Cerralvo, Baja California Sur (Mexico). Crustose coralline algae played a fundamental role in reef establishment during the Late Pleistocene (122,143 ± 175 YBP) on Isla Cerralvo in the southern Gulf of California. Transported cobbles with a generally elongated clast shape (mean sphericity: 0.6) were encrusted by coralline red algae before locking in a N–S alignment (mean N2°W) and providing a fixed substrate for colonization by Porites and Pocillopora corals. A fringing reef grew on this pavement of clast-encrusting rhodoliths and was succeeded by additional cobble-coral cycles. Out of five stratigraphically repetitive cycles, only the second and third offer sufficient exposure to be quantified with any confidence. Census data, cobble orientations, and measurements of algal rinds were collected to characterize the transition from rhodoliths to corals. Clasts in th...

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas1This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology. Canadian Journal of Earth Sciences, 48: this issue.2MRNF Contribution BEGQ 8439-2010/2011-1. Natural

The Archean Blake River Group (BRG) of Ontario and Quebec is dominated by submarine mafic to intermediate lavas, with more restricted felsic volcanic rocks. Given the good quality of outcrop, and high level of preservation of some BRG rocks, the mafic to intermediate lavas were used in the 1970s and 1980s to better understand the evolution of massive and pillowed submarine flows, and their associated fragmental facies (pillow breccias, hyaloclastite). Potentially, the BRG could also represent a useful volcanic succession for the study of explosive submarine eruption products in the ancient record. Before this is possible, however, a regional inventory of the mafic to intermediate volcaniclastic units is needed to clarify their characteristics and origins. In this paper, we compare and contrast volcaniclastic rocks from three areas within the same formation of the northern BRG in Quebec: the Monsabrais area, the Lac Duparquet area, and the D’Alembert tuff area. Close examination reveals pronounced differences in terms of lateral continuity, thickness, grading, bedding, clast shapes, textures, etc. in the volcaniclastic rocks. These differences are interpreted to reflect vastly different emplacement processes, ranging from hyaloclastite generation as a result of self-fragmentation and lava contact with water (dominant in the Monsabrais and Lac Duparquet areas) to aqueous density currents likely fed directly by explosive submarine eruptions (dominant in the D’Alembert tuff).

Contrasted textural and taphonomic properties of high-energy wave deposits cemented in beachrocks (St. Bartholomew Island, French West Indies)

This study investigates the preservation potential of high-energy wave deposits by beachrocks located along shores contrastingly exposed to storms. In order to discriminate between clast populations originating in different formational and depositional conditions, namely fair-weather vs. extreme wave conditions, the applicability of quantitative textural and taphonomic analyses of sand- to boulder-sized sediments was evaluated using samples from beachrocks and non-cemented coarse-grained coastal deposits. The size, shape and roundness of coarse clasts were characterized by measuring axial dimensions (L, I and S), from which shape descriptors were derived. Taphonomic scores were established at macroscopic and microscopic (thin-section) scales by rating the degree of physical and biologically-mediated alteration of skeletal material. Combined with the examination of depositional morphologies and facies architectures, the procedure enables characterization of fair-weather and extreme wave-generated deposits, and provides criteria to distinguish between single-event deposits and reworked material resulting from the coalescence of sediments produced during multiple events. The distinction of single short-lived event deposits from those reflecting long-term reworking is crucial for correctly interpreting similar deposits in the geological record and evaluating the historical hazard along coastlines. Site-dependent controls such as the nature of available sediments, the geomorphology of coastal depositional settings, and the tectono-climatic context proved to be important for interpreting the origin of the high-energy wave deposits studied here. In the ongoing debate on the signatures of tsunamigenic and storm sediments in fringing coastal settings, the present study demonstrates that one promising research direction is the use of multiple sedimentological criteria to characterize event materials, including their morphology and taphonomy, combined with the precise characterization of site-specific controls.

Sedimentary petrological characteristics of lateral and frontal moraine and proglacial glaciofluvial sediments of Bertilbreen, Central Svalbard

Bertilbreen is a valley glacier located in the central part of the Spitsbergen Island, Svalbard. Glacier bedrock is composed of Devonian Old Red facies sedimentary rocks, Carboniferous clastic sedimentary rocks and Carboniferous to Permian limestones. Cobble clasts from the right-hand lateral moraine, frontal moraine and proglacial glaciofluvial sediments were studied. The upper part of the lateral moraine is composed mostly of passively transported supraglacial debris (originally unmodified scree, snow and scree/rock avalanche deposits) with a small proportion of actively transported clasts or reworked glaciofluvial sediments. Clasts in the middle part of the lateral moraine originate predominantly from the frontal moraine of a small glacier in the lateral valley. The lower part of the lateral moraine and frontal moraine of Bertilbreen are rich in subglacially transported material, which is supported by isometric clast shapes, roundness degree and common clast surface striations. Coarse gravel forms longitudinal bars in the glaciofluvial stream flowing from the glacier front. In the southern mouth of the valley, the proglacial stream grades into a braided outwash fan. Clast nature is affected by the source from the surrounding glacial deposits and bedrock outcrops, the impact of glaciofluvial transport on the clast nature increases in the braided outwash fan. Clast shapes are primarily influenced by bedding and fractures of source rocks, but are also significantly influenced by the type and proximity of material sources. Striation is cleared away the clast surface during the glaciofluvial transport. A morphostrati-graphically older glaciofluvial terrace formed by glaciofluvial sediments deposited during the glacier advance culminating during the Little Ice Age (LIA) is located at the southern end of the valley. The comparison of active proglacial stream sediments and those from older glaciofluvial terrace was done using the coarse pebble fraction.

Rounding of pumice clasts during transport: field measurements and laboratory studies

Volcanic clasts in many pyroclastic density current deposits are notably more round than their counterparts in corresponding fall deposits. This increase in roundness and sphericity reflects different degrees of comminution, abrasion and breakup during transport. We performed experimental measurements to determine an empirical relationship between particle shape and mass loss caused by particle–particle interactions. We consider, as examples, pumice from four volcanoes: Medicine Lake, California; Lassen, California; Taupo, New Zealand; Mount St Helens, Washington. We find that average sample roundness reaches a maximum value once particles lose between 15% and 60% of their mass. The most texturally homogeneous clasts (Taupo) become the most round. Crystal-rich pumice abrades more slowly than crystal-free pumice of similar density. Abrasion rates also decrease with time as particles become less angular. We compare our experimental measurements with the shapes of clasts in one of the May 18, 1980 pyroclastic density current units at Mount St Helens, deposited 4–8 km from the vent. The measured roundness of these clasts is close to the experimentally determined maximum value. For a much smaller deposit from the 1915 Lassen eruption, clast roundness is closer to the value for pumice in fall deposits and suggests that only a few volume percent of material was removed from large clasts. In neither field deposit do we see a significant change in roundness with increasing distance from the vent. We suggest that this trend is recorded because much of the rounding and ash production occur in proximal regions where the density currents are the most energetic. As a result, all clasts that are deposited have experienced similar amounts of comminution in the proximal region, and similar amounts of abrasion as they settle through the dense, near-bed region prior to final deposition.

Structure of gravel beaches and their relationship to tidal range

AbstractThe height of accretionary gravel beaches, determined by the vertical distance between the large sphere zone at the seaward margin and the large disc zone at the landward margin, is determined mainly by tidal range. As the beach builds seaward, this height translates into thickness and so the complete thickness of gravel beach accumulations is a good indicator, in the geological record, of tidal range. Beaches with a small tidal range have the zone between the large discs and the large spheres compressed and sometimes steepened. These beaches are characterized mainly by cusps and berms, both of which yield steeply dipping, seaward building, cross‐strata. Beaches built by large, persistent waves generally have a steep profile, whereas those with small wave height appear to have a shallow profile. Cusps generally have extremely well‐sorted gravels, often in an openwork fabric and with a landward crown of coarse gravel that may grade seaward into shape‐sorted, gently dipping gravels. Berms, which may extend for hundreds of metres along the coast, comprise, in section, very well‐sorted cross‐stratified gravels that may intermittently build seaward for a distance >15 m. Extensive selection pavements typify beaches with a high tidal range and comprise sheets of gravel which are well‐sorted and dip at comparatively low angles towards the sea. These selection pavements form beds that, in section, can be >50 m in seaward length; they grow from clasts, supplied to them from swash or backwash, and which are selected by the fabric of the existing sheet. In some cases, the selection pavements can be very mature having only a specific clast shape and size remaining. Total preservation of the beach sequence in the geological record is more likely to take place in areas of rapid deposition of sediments, such as the marine fringes to a fan delta.

Stratigraphy and Chronology of Late Quaternary Floodplain Sediments in a Historic Mining Area, Vils River Valley, East Bavaria, Germany

Analysis of 288 percussion drillings from depths of up to seven meters and a 120 m long excavated section provide new and detailed information on the Late Quaternary to Late Holocene floodplain evolution from the most important Central European iron-mining region of the 15th century. The generalized sequence of the Vils River floodplain is built of five units representing facies of different genesis (rock/saprolite, gravel, sand, loam, peat) that are identified by physical, chemical, and mineralogical parameters (grain size, clast shape, and content, TOC, bulk mineralogy). Along the 87 km length of the valley, different units/facies are found at the Upper Vils River (UVR 1-5) and at the Lower Vils River (LVR 1-4b). In both river sections, fine-grained material (flood loam) forms the major part (up to 4 m) of the alluvial sequence. 29 age determinations (24 14C, 4 IRSL, 1 tree-ring analysis) show increasing accumulation of flood loam in the last millennium. The data correlate with the beginning of mining at the Vils River and hence suggest a causal connection with mining activities in the catchment—both on the slopes and in the floodplain—with intense deposition of flood loams.

Rethinking conditions necessary for pseudotachylyte formation: Observations from the Otago schists, South Island, New Zealand

Pseudotachylytes and two distinct types of cataclasite in the Otago Schist at Tucker Hill, South Island, New Zealand, provide evidence for both seismic slip and aseismic creep on a normal fault zone during regional crustal extension in late Cretaceous time. Regional geologic evidence suggests that the fault had its present low-angle dip (ca. 10°) at the time it was active. ‘Type A’ cataclasites, presumably aseismic, can be recognized by bi-fractal grain size distributions, monomict composition, angular clasts of uniform textural maturity, and a crude fabric defined by oriented grains and transgranular fractures. ‘Type B’ cataclasites, possibly cosesimic, have characteristics consistent with fluidized grain flow, including heterogeneous clast shapes and types, a bimodal grain size distribution, intrusive relationships with other rocks, and the absence of any fabric or transecting fractures. Pseudotachylyte, which occurs as fault veins, injection veins and more complex types of intrusive structures, consistently cuts across and invades Type A cataclasites but is both intrusive into and included as clasts in Type B cataclasites. These relationships are consistent with a fault evolution model in which the development of a damage zone through aseismic cataclasis (Type A) facilitates the formation of pseudotachylyte in a subsequent seismic event by providing a permeable matrix through which fluids can drain in the early stages of slip, thereby maintaining frictional contact between rock surfaces. The formation of pseudotachylyte, in turn, may seal the fault zone and lead to thermal pressurization in a later seismic cycle, forming fluidized (Type B) cataclasites. Seismic slip on the low-angle normal fault zone at Tucker Hill may have occurred by two distinct modes of dynamic weakening — melt lubrication and thermal pressurization — in successive seismic events. Although there is a perception among geologists that pseudotachylyte is most likely to form in intact, crystalline rocks, geophysical models of fault zones clearly demonstrate that pseudotachylyte formation is actually suppressed in low-permeability rock because any fluids present would be unable to escape the fault zone and thermal pressurization would rapidly reduce frictional resistance. The paradigmatic occurrences of pseudotachylyte in otherwise pristine crystalline rocks probably represent somewhat exceptional circumstances (single rupture events at very high effective stress in dry rock). Coseismic frictional melts may actually be more common in hydrated rocks like the schist at Tucker Hill, but harder to recognize and also vulnerable to overprinting as a fault zone matures. In such rocks, pseudotachylyte may represent an intermediate stage in the evolution of a fault zone, the period between the formation of a high-permeability damage zone and the development of a low-permeability fault core.

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.

Kinematic vorticity analysis and evolving strength of mylonitic shear zones: New data and numerical results

The kinematic vorticity number is an important quantity in structural geology and tecton- ics, giving a nonlinear ratio of simple shear to pure shear deformation. We use natural obser- vations and numerical models to show how rigid clast methods for determining the kinematic vorticity number (W k ) are compromised where strain localization occurs at the matrix-clast interface. Our numerical results show that the critical shape factor cutoff between perma- nently rotating and stable clasts, used to determine W k , is highly sensitive to coupling between the clast and the matrix. This fi nding provides an elegant explanation for the fact that rigid clast methods tend to underestimate W k relative to other methods. We present numerically determined envelopes for clast behavior across a range of kinematic vorticity numbers, clast shape factors, and matrix-clast coupling. Our numerical models show that the shape-pre- ferred orientations of feldspar clasts trend toward the positions of mica fi sh with increasing localization at the clast boundary, suggesting that mica fi sh behave as highly lubricated clasts. Our data and numerical results show that the clast-matrix interface may be several orders of magnitude weaker than the surrounding matrix and that weak interfaces can lead to a marked drop in the bulk shear strength of faults and shear zones.

Fingerprinting ash deposits of small scale eruptions by their physical and textural features

Correlation of distal ash deposits with their proximal counterparts mainly relies on chemical and mineralogical characterization of bulk rock and matrix glasses. However, the study of juvenile fragments often reveals the heterogeneity in terms of clast shape, external surface, groundmass texture and composition. This is particularly evident in small scale eruptions, characterized by a strong variability in texture and relative abundance of juvenile fragments. This heterogeneity introduces an inherent uncertainty, that makes the compositional data alone inadequate to unequivocally characterize the tephra bed. Pyroclast characteristics, if described and quantified, can represent an additional clue for the correct identification of the tephra. The paper presents morphological, textural and compositional data on the products of an ash eruption from Middle Age activity of Vesuvius, to demonstrate the information that can be extracted from the proposed type of analysis. Juvenile fragments from five ash layers throughout the studied products were randomly hand-picked and fully characterized in terms of external morphology, particle outline parameterization, groundmass texture and glass composition. Statistical analysis of shape parameters characterized groups of fragments that can be compared with the other textural and physical parameters. The main result is that the data do not show important cross-correlation so suggesting that all of these parameters, together with accurate field data are needed for the complete fingerprinting of a tephra bed. We suggest that this approach is especially important for characterizing the products of small scale, compositionally undistinguishable, eruptions and represents the necessary step to deal with before going into more detailed compositional analyses.

Quantifying sample bias in clast fabric measurements

Sampling elongate clasts that protrude from a planar face for clast macro-fabric analysis introduces a bias into reconstructed fabrics because clasts aligned perpendicular to the sampling face are over-represented relative to those aligned parallel to the face. This study develops a probability-based mathematical analysis to quantify sample bias for a variety of clast shapes and population fabrics, including isotropic, clustered and girdled fabric styles. Bias is expressed in terms of sample eigenvalues and eigenvectors relative to those of the parent population. Results indicate that sampling always has the effect of artificially drawing fabrics towards perpendicularity to the sampling face relative to the populations from which they are drawn. This rotation generally has the effect of artificially strengthening population fabrics, by up to 30% in the case of very weakly clustered or girdled populations. However, fabric strengthening is by no means universal and sampling alters different parent populations in different, sometimes complex, ways: the analysis in this paper identifies situations where sampling can strengthen or weaken parent fabrics, where it can rotate parent fabrics (by up to 90° in the case of a very weak population fabric), and where it can even change the style of a population fabric. For example, near-isotropic population fabrics can appear clustered; weakly clustered and weakly girdled population fabrics can appear isotropic; weakly clustered population fabrics can appear weakly girdled; and girdled population fabrics can appear clustered. Overall, this analysis indicates that weakly orientated population fabrics are most susceptible to bias. Thus, a weakly clustered population fabric aligned parallel to a sample face is particularly susceptible to major sample bias in both fabric strength (artificially overestimated) and orientation (artificially rotated perpendicular to the face). Bias can be reduced either by sampling all the clasts from a cube-shaped block of sediment excavated from the face, or by sampling equal numbers of clasts from the three orthogonal faces of the same sedimentary unit.

Early-Holocene glacier fluctuations in Krundalen, south central Norway: palaeoglacier dynamics and palaeoclimate

Krundalen, a valley occupied by three eastern outlet glaciers of the Jostedalsbre ice cap, contains three groups of moraines, two of which are outside the dated neoglacial (`Little Ice Age', LIA) limits. Exposures in the outermost moraine ridge display three sedimentary units. Detailed sedimentological analyses reveal that the lower two units formed during a temporary stillstand of the ice margin inferred to have occurred because of a topographically controlled stabilization of the ice margin during glacier retreat following the Younger Dryas. A climatically controlled readvance during which the upper unit was deposited, partly eroding pre-existing sediments, followed. Continuation of this readvance led to the whole sequence being pushed up to form the present moraine. The sedimentological results including clast shape measurements imply a temperate glacier thermal regime at the time of moraine formation. The glacial palaeoenvironment was characterized by frequent oscillations and an abundant supply of glaciogenic sediment. The age of this landform can at present not be determined by numerical dating. However, the moraine is likely to have been formed in the early Holocene, during Jondal Event 2 (JE) from c. 10 550—10 450 cal. yr BP. The average equilibrium-line altitude lowering calculated for this event is c. 440 m, 340 m for the Erdalen Event 1 (EE, 10 100—10 050 cal. yr BP) moraines and c. 190 m for the LIA moraines farther upvalley. Precipitation calculated for these stages compared with present values is 160—175% (JE), 165—175% (EE) and 110% (LIA). Summer air temperatures were slightly higher than today's during the JE, much higher during the EE and are indistinguishable from present values during the LIA. The existence of a potential JE-equivalent and the extent of the EE are reported and used for glacier reconstruction for the first time in Krundalen. The sedimentological data implies that the JE did not simply consist of a single readvance but was more complex. It is argued that glacial sedimentology linked with methods of glacier reconstruction provides a powerful tool to gain insights into the dynamics and functioning of formerly glaciated areas.

Clast fabric examination of impact‐generated breccias, borehole LB‐07A, Bosumtwi, Ghana

Abstract— An impact event always creates a cloud of ejecta generated through excavation of the target. Subsequent in‐filling of the void by crater‐fill deposits provides a record of post‐impact processes. Full‐core digital photographic scans of core segments from borehole LB‐07 in the Bosumtwi impact crater provide a complete record of the in‐fill process. The shape, orientation, and size of clasts within the impact breccia were measured using a best‐fit ellipsoid approach. Clast size and variance, together with clast orientation data, suggest the impact breccias at Bosumtwi can be divided into a simple two‐fold subdivision that loosely agrees with the lithological zonation of a lower monomict breccia overlain by a polymict breccia. The lower unit is characterized by a uniform and finer‐grained clast size together with a uniform flat‐lying clast orientation. The boundary between the two zones is defined by a sharp increase in clast size. The upper zone shows an average increase in clast size with decreasing depth, but full grain size spectrum together with increased grain size deviation suggest that this is a result of mixing between two populations with different grain size distribution. The main population of clasts shows an incremental decrease of clast size with decreasing depth. The upper zone also contains weakly defined shallowly dipping clast fabrics, which may be suggestive of horizontal transport or deposition onto an inclined surface.

Generation of fine hydromagmatic ash by growth and disintegration of glassy rinds

The deposits of mafic hydromagmatic eruptions are more fine grained and variable in vesicularity than dry magmatic deposits. Blocky, equant shapes of many hydromagmatic clasts also contrast with droplet, thread, and bubble wall morphology of dry magmatic fragments. Small (<∼180 μm), blocky hydromagmatic pyroclasts have traditionally been interpreted to result from discrete vapor explosions, although such explosions tend to occur only under certain conditions. This paper considers a process of hydromagmatic ash formation that involves repeated growth and disintegration of glassy rinds on pyroclast surfaces as they deform within turbulent flows. This process, termed “turbulent shedding”, may occur during the expansion phase of vapor explosions or during turbulent but nonexplosive mixing of magma with water, steam, or water sprays. The occurrence of turbulent shedding and the resulting fragment sizes depend on the timescale for rind growth and the timescale between disturbances that remove or disintegrate glassy rinds. Turbulent shedding is directly observable in some small littoral jets at Kilauea. Calculations suggest that, in the presence of liquid water or water sprays, glassy rinds having a thickness of microns to millimeters should form in milliseconds to seconds. This is similar to the timescale between turbulent velocity fluctuations that can shred lava globules and remove such rinds. The fraction of a deposit consisting of fine ash should increase with the duration of this process: Large‐scale Surtseyan jets generate hundreds or thousands of shedding events; bubble bursts or tephra jets at Kilauea's coast may produce only a few.