Clast Size Distribution Research Articles

Transport and deposition of the 39.8 ka Campanian Ignimbrite large-scale pyroclastic density currents (Italy)

Abstract Processes of transport and deposition of large-scale pyroclastic density currents, which transport 102–103 km3 of material, are still poorly understood. The volcanic explosivity index (VEI) 7 Campanian Ignimbrite pyroclastic density current is one such poorly understood case that is subject to lively scientific debates about its origin and processes. The Campanian Ignimbrite is related to the 39.8 ka eruption that formed the partly submarine and active Campi Flegrei caldera in Italy. The related pyroclastic density current extended over an area of more than 6000 km2 on land around the caldera, with a final runout of ~75 km, and surmounted mountain ridges higher than 1000 m above sea level, even at distal reaches. We used physical characteristics of the ignimbrite to understand the transport and emplacement dynamics of the Campanian Ignimbrite pyroclastic density current. Thickness, mass, and clast-size distributions across paleotopography were used to infer that the main transport system was a dilute, density-stratified pyroclastic density current, which was at least 1.5 km thick, which was short-lived (possibly as short as ~20 min), and which had a mass flow rate of ~1011 kg/s. At its base, high-concentration undercurrents formed and interacted with the rough paleotopography, laying down a low-aspect-ratio ignimbrite sheet when on flat plains, but generating back-flows from ridges and channeling in paleovalleys. The deposit is predominantly a massive valley-pond ignimbrite facies, even across rough mountain terrains. The absence of veneer facies on steep slopes reflects the near-Newtonian rheology of the undercurrents, interpreted as fluidized dense granular flows. This work advances the interpretations of pyroclastic density current deposits, which do not necessarily directly reflect conditions in the transport system.

Origin of a polymict, mega‐clast‐bearing impact breccia in the crater floor of Wetumpka Impact Structure, Elmore County, Alabama: Insights from surface outcrops and a drill core

ABSTRACTWetumpka impact structure is a Late Cretaceous, marine‐target impact crater of about 5 km diameter. The apparent crater rim is mostly made of crystalline local basement, and the apparent crater floor consists of a mixed sediments of target lithology. These sediments are the provenance of the crater‐filling impactite sands, overlying trans‐crater slide unit, and the capping polymict impact breccia deposit, often referred to by previous workers as “central polymict breccia.” The unit has been known to contain elongated mega‐clasts of up to tens of meters in size. This study attempted to understand the mode of emplacement of this polymict breccia, which occurs in some places on the apparent crater floor and resembles a polymict proximal ejecta deposit. This work also reports the first documentation of rare, potential impact spherules in the polymict impact breccia, interpreted to be a part of distal ejecta. The presence of large, decimeter‐sized clasts in the breccia can be best explained by the movement of overturned rim flap forming part of proximal ejecta from the crater rim to the apparent crater floor during early modification stage of impact cratering. Our work highlights the bimodal clast size distribution of the polymict breccia, and so we propose that the term “mega‐clast‐bearing impact breccia” be used for this unit. We attribute a generally steep orientation of the decameter sized clasts to primary imbrication during emplacement. The emplacement of this breccia is interpreted as associated with the ejecta emplacement process that occurred before the return of marine resurge.

Radar Perspective of the Aristarchus Pyroclastic Deposit and Implications for Future Missions

The Aristarchus plateau represents one of the most complex volcanic provinces on the lunar surface and is host to the largest pyroclastic deposit on the Moon. Lunar pyroclastic deposits offer a window into the Moon’s interior and represent a valuable resource to support a sustained human presence. We present a new analysis of the Aristarchus pyroclastic deposit using Mini-RF bistatic radar data at wavelengths of 4.2 and 12.6 cm. Building on previous Earth-based Arecibo Observatory radar studies at 12.6 and 70 cm, we place further constraints on the spatial extent of the pyroclastic deposit and investigate the clast size distribution and provenance of foreign material distributed within the formation. Concentrations of blocky material >0.5 cm in diameter and suspended within the upper decimeters of the pyroclastic deposit are associated with potential buried mare flows along the rim of Vallis Schröteri and discrete pockets of primary material ejected by the Aristarchus impact. Unraveling the deposit from nonpyroclastic materials and the surrounding landscape creates new constraints with which to reconstruct the volcanic history of the region. From a future mission perspective, the identification of primary Aristarchus material distributed across the plateau offers an opportunity to sample diverse volcanic lithologies within an area that could be sampled by a single Commercial Lunar Payload Services mission. In terms of lunar resource in situ utilization, such ejected material also represents a contaminant; thus, radar data provide an invaluable tool to identify pristine pyroclastic material for mission planners.

A 3D Petrofabric Examination of Martian Breccia NWA 11220 via X‐Ray Computed Microtomography: Evidence for an Impact Lithology

AbstractThe Martian regolith breccia Northwest Africa (NWA) 11220 and paired stones represent the only known meteorites that sample a clastic sub‐surface lithology from Mars. By applying X‐ray computed microtomography to monomineralic clasts, we identify three phases that can be automatically segmented by thresholding X‐ray attenuation greyscale values: (A) feldspars, (B) pyroxene and apatite, and (C) iron‐rich oxides and sulfides, confirmed via scanning electron microscopy and Raman spectroscopy. For these three phases, we demonstrate scale invariance in size and shape for sand‐sized clasts and smaller, a characteristic commonly observed for clast populations generated by fragmentation without further sorting from sedimentary transport (e.g., Aeolian or fluvial processes). Additionally, by assessing the preferred orientation of fitted ellipses and ellipsoids to manually segmented proto‐breccia clasts in two and three dimensions, we identified a weak planar fabric that likely resulted from compaction rather than impact transport. Combining clast size distribution with evidence for nested textures inside proto‐breccia clasts, we propose that NWA 11220 has experienced a minimum of two hypervelocity impact events and should be considered a lithified impact ejecta lithology with little to no reworking via surface regolith processes.

Grain-size-reducing- and mass-gaining processes in different hydrothermal fault rocks

AbstractInteractions of hydrothermal and deformation processes in active fault zones are important in light of seismic and geothermal activities. This study investigates different tectonites of the Grimselpass breccia fault, a hydrothermally active strike-slip structure in the Central Alps (Switzerland). We combine microstructural and geochemical investigations to decipher the interaction of grain-size reduction and associated dilation (volume gain) during frictional deformation with geochemical processes such as mass gain via precipitation, grain growth and healing. Three types of tectonites were investigated in the fault zone: (1) cockade-bearing tectonites, (2) cataclasites and (3) fault gouges. They differ in terms of microstructure of clasts, mass and volume gain/loss, as well as type and degree of cementation. Clast size distributions show decreasing median values from low-strain cataclasites to high-strain cataclasites. Mass/volume gains are up to 20 %, with samples of smallest clast sizes often showing the highest mass gain. In the case of cockade-bearing tectonites large volume and mass gain occur. In particular, the precipitation of SiO2-rich cements is inferred to (i) reduce permeability and (ii) induce a gain in strength of the respective fault rocks. Location, timing and degree of healing directly control the fault architecture by deactivating fault strands from the circulation of hydrothermal fluids. In this case, new rupturing events are necessary to reactivate locked fault strands. Such feedback cycles of episodic deformation, mass/volume and clogging preserved hydrothermal circulation and deformation over the last 3.4 Ma, representing a process chain for the long-term preservation of orogenic hydrothermal systems.

Crush-origin pseudotachylyte in granitic mylonites of continental exhumed Ailaoshan-Red River shear zone southeastern Asia

Tectonic pseudotachylytes have provided valuable insights into the evolution of seismic activity during crustal deformation. However, the origin and formation processes of pseudotachylytes are still debated. Thin pseudotachylyte veins were first recognized within the granitic mylonites of the continental exhumed Ailaoshan-Red River strike-slip shear zone, southeastern Asia, which record the deformation characteristics from ductile (mylonite) to brittle deformation (ultracataclasite). Mylonitization resulted in the recrystallization of quartz aggregates/ribbons, fine-grained plagioclase, and the concentration of biotite into biotite-rich foliation planes. The pseudotachylyte veins have various angles oblique to the host rock foliation, which record superimposition of the latter brittle deformation onto former ductile fabrics. Observations of the microstructure and quartz clast size distribution (obey power-law distribution) suggest that crushing was the dominant process in the formation of pseudotachylyte in the samples. Plagioclase within the pseudotachylyte vein presents a complex texture of high-strength nanocrystalline to partly amorphous material. Detailed microstructure observation revealed that amorphous materials form by a solid-state amorphization process and is different from friction melts. The changes in composition between plagioclase clasts and amorphous materials (enriched in Ca, but depleted in Na) provide evidence of mechanochemical effect that accompanies fault motion and comminution process. When a sufficient amount of amorphous material is formed, it has great significance in fault weakening through the coseismic slip of the strain partition.

Tephrostratigraphy and morphometry of Wum Maar Volcano (Oku Volcanic Group- Cameroon Volcanic line): Implications for complex monogenetic volcanoes

Field investigations of tephra deposits and morphometric characterisation were used to constrain the eruptive history and the complexity of the Wum Maar Volcano (WMV) on the Northwestern foot of the Oku Volcanic Group (Cameroon Volcanic Line). The WMV is a compound volcanic edifice made up of an irregular horse-shoe-shaped maar-crater bordered by overlapping scoria cones and a small lava flow field. The bulk ejecta volume computed using ArcGIS.10.3 and field topographic measurements is ∼0.133 km3. The deposits were subdivided into older volcanics, a pre-maar member composed of four Strombolian scoria cones and a maar-member made up of interbedded layers of lapilli-tephra, lapilli-ash, and ash-breccia deposits. The bedding characteristics (structure, bedding dip, direction, attitude, and contact relations) coupled with the componentry analysis of maar deposits indicate phreatomagmatic-derived fallouts and pyroclastic density currents emplaced under both dry and wet conditions due to variations in magma-water ratio during the different phreatomagmatic explosions. The thick paleosol between older volcanics and the pre-maar and the maar deposits suggests that before the formation of the WMV, an eruptive activity took place and comprised an effusive outpouring of a basaltic magma batch that exploited the network of fractures within the basement rocks to reach the surface. Long after this stage, other batches of basanitic and trachybasaltic magmas richer in volatiles erupted explosively (Strombolian eruptive dynamism) around the northern and southern rims of the maar crater, leading to the emplacement of several overlapping scoria cones. The last stage was characterised by the formation of the Wum maar crater through a phreatomagmatic explosion when batches of basanitic magma came in contact with groundwater. The volcanics are sub-aphyric and host mantle xenoliths. Although evenly distributed across the different maar units, these mantle xenoliths are frequent and large enough to suggest a rapid magma ascent. The center of the maar crater compared to the location of the scoria cones suggest that the explosion locus migrated eastwards. The relative proportion of lithic granite within the pyroclastic succession increases upward (highest in MU4), and the depth of the maar varies from <10 m at the western border to about 124 m at the eastern one, suggesting a vertical and a lateral migration of the explosion locus. The intensity of the phreatomagmatic explosion reduced with time as indicated by the clast-size distribution, with a progressive reduction in water availability wherein the eruption became more internally controlled, changing in style from phreatomagmatic to Strombolian at the end. We conclude that even with very small magma volumes, monogenetic eruptions can have very complex histories, as shown by the WMV, because of the taping system, multiple vents, change in eruptive style, vertical and lateral migration of eruption foci, and multiple magma batches.

Formation and dispersal of pyroclasts on the Moon: Indicators of lunar magma volatile contents

We use new estimates of the total content and speciation of volatiles released during the ascent and eruption of lunar mare basalt magma to model the generation and behavior of gas bubbles, the disruption of magma at shallow depth by bubble expansion, and the acceleration and dispersal of the resulting pyroclasts. Lunar eruptions in near-vacuum differ significantly from those on bodies with an atmosphere: 1) exposure to near-zero external pressure maximizes volatile release to form gas bubbles; 2) the infinite potential expansion of the gas bubbles both ensures and maximizes magma fragmentation into pyroclastic liquid droplets with sizes linked to the bubble size distribution; 3) the speeds to which gas and entrained pyroclasts can be accelerated by gas expansion are also maximized. Generation of CO gas bubbles at much greater depths and pressures than bubbles of other volatiles produces bimodal (~120 and 650 μm) total pyroclast size distributions. In the near-vacuum, gas expands to pressures so low that gas-particle interactions enter the Knudsen regime, resulting counter-intuitively in the median grainsize in pyroclastic deposits first increasing, then decreasing, and finally increasing again with increasing distance from the vent, instead of decreasing monotonically as when an atmosphere is present. These complex gas-particle interactions cause clast size distributions to vary in a complex way with distance from the vent and the maximum thickness of the deposit to occur at about 75% of the maximum pyroclast range. Lunar eruptions typically evolve through four stages, which significantly influence gas release patterns. Most volatiles are released during the second, hawaiian-style eruption stage. However, elevated gas concentration can occur both in the short first stage (due to gas accumulation in the dike tip during ascent from the mantle) and in the third and fourth stages (due to reduced volume flux, increased time for gas bubble formation, growth, rise and coalescence, and strombolian activity replacing the hawaiian eruption style). Such gas concentration mechanisms can increase pyroclast ranges by a factor of ~5, but result in very much thinner deposits than if no concentration occurs. Maximum pyroclast range scales essentially linearly with total mass fraction of released volatiles; thus determination of the deposit radius around specific vents can provide data on lunar magma volatile contents. If the volatile inventory of the Apollo 17 orange glass bead picritic magma (~3400 ppm maximum) is typical, maximum ranges of the majority of pyroclasts would have been ~20 km. Such eruptions could explain 79% of the currently recognized pyroclastic deposits on the Moon. A few larger deposits and vents, such as the Aristarchus Plateau Dark Mantle and Cobra Head, suggest higher magma volatile contents. Numerous lunar vents show little evidence of associated pyroclastic deposits. Together, these observations suggest a wide range of volatile contents in lunar basaltic magma mantle source regions.

Size distribution of survivor clasts in pseudotachylyte and cataclasite: Implications for crushing and melting processes in seismic fault zones

Quartz/feldspar fragment (‘clast’) sizes were measured in thin sections of three types of fault zone rocks, e.g., melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and cataclasite (Ct), from two well-studied Precambrian shear/fault zones in the Indian craton (e.g., the Gavilgarh–Tan Shear zone in central India and the Sarwar–Junia Fault zone in western India). Logarithmic plots of clast area vs. cumulative frequency in the pseudotachylytes demonstrate a fractal clast-size distribution (c.s.d.) for the intermediate size range, whereas the finer and coarser clast size fractions clearly deviate from the fractal trend. Under-representation of the finer size clasts in the pseudotachylyte samples may be attributed to their preferential melting and removal from the clast population. The relative paucity of coarse clasts, on the other hand, is possibly due to a sampling bias against coarse clasts. The c.s.d of the cataclastic rock shows a multi-fractal character with two different slopes (i.e., lower D-value for finer clast sizes) and absence of the left-hand (finer size) fall off. This suggests less efficient crushing in the finer clast size fraction. The proportion of clasts, compared to the matrix, is very small in M-Pt, increases in C-Pt and is highest in Ct, suggesting that melting of rock/mineral fragments is a dominant process in forming M-Pt, whereas it is less significant in C-Pt, and is absent in Ct, which corroborates the microscopic observations.

Frictional Strengths of Subduction Thrust Rocks in the Region of Shallow Slow Earthquakes

AbstractEarthquakes are expected to nucleate within velocity‐weakening materials; however, at the updip limit of the subduction seismogenic zone, the principal lithologies exhibit velocity‐strengthening behavior. At an exhumed analogue for present‐day subduction at Nankai (the Mugi Mélange, Japan) two examples of paleoseismic features occur within altered basalts, suggesting that they may be velocity‐weakening. We shear altered basalt and shale matrix from the mélange in the triaxial saw cut configuration at in situ conditions of deformation (Pc = 120 MPa, T = 150 °C) for two pore fluid factors (λ = 0.36, 0.7). The shale matrix exhibits a coefficient of friction between 0.4 and 0.5, and velocity‐strengthening behavior. Altered basalt exhibits Byerlee friction and velocity‐weakening behavior. In most experiments deformation was partitioned into Riedel shears, which have a lower percentage of clasts, clast size distributions with higher fractal dimensions, and shape factors indicating rounder clasts compared to the matrix between Riedel shears. We hypothesize that earthquakes preferentially nucleate within altered basalt at the updip limit of the seismogenic zone. More complex forms of deformation (e.g., shallow very low frequency earthquakes) may occur by mixing velocity‐weakening altered basalt into the velocity‐strengthening shale matrix. In subduction zones where the matrix is composed of shale, this complex behavior is limited to shallow depths (T &lt; ~250 °C), above the updip transition to velocity‐weakening behavior for the matrix. Altered basalt is a ubiquitous subducting material, and future studies on its behavior through a range of subduction zone conditions are required for a full understanding of the mechanical behavior of subduction zones.

Conglomerate recycling in the Himalayan foreland basin: Implications for grain size and provenance

Abstract The nature of coarse sediment in rivers emerging from mountain ranges determines rates of downstream fining, the position of the gravel-sand transition, sediment entrainment thresholds, and channel morphologies. Additionally, in the stratigraphic record, clast size distributions and lithologies are used to reconstruct paleo-hydraulic conditions and source area provenance. Using Himalayan rivers, we demonstrate that the signal of first-generation clasts derived from the hinterland of a mountain range can be significantly altered by recycling older, structurally exhumed foreland deposits. The Siwalik foothills of the Himalaya comprise Neogene fluvial sandstones and quartzite-rich conglomerates with well-rounded clasts that were deposited in the Indo-Gangetic foreland basin and later exhumed by erosion, following uplift along the Himalayan mountain front. Mass balance calculations reveal that the Upper Siwalik conglomerate may contribute a significant proportion of the total gravel flux exported from the main Himalayan catchments (up to 100%) despite forming &amp;lt;1% of the catchment geology. Three end-member catchments with variable proportions of gravel flux from Siwalik conglomerates are analyzed to test for the effects of conglomerate recycling. Catchments that recycle the most Upper Siwalik conglomerate form quartzite-rich gravel bars comprising well-rounded pebbles and a narrow grain size distribution, mimicking the characteristics of the Upper Siwalik conglomerate. Conversely, catchments that recycle the least Upper Siwalik conglomerate form gravel bars with a range of Himalayan lithologies, angular quartzite pebbles and a wider grain size distribution. This study highlights that recycling of quartzite-rich conglomerate can dramatically modify the flux, lithology, grain size, and shape of gravel entering the Indo-Gangetic Plain.

Semiautomatic Digital Clast Sizing of a Cobble Beach, Nantian, Taiwan

ABSTRACT Bujan, N.; Cox, R.; Lin, L.-C.; Ducrocq, C., and Hwung, H.-H., 2018. Semiautomatic digital clast sizing of a cobble beach, Nantian, Taiwan. Surface sediment data is scarce for beaches that are made of material close to the cobble size range, partly because of the difficulty of direct sampling for sediment size analysis. An object-detection tool for semiautomatic analysis of clast geometry, originally developed for riverbeds, was applied to the coarse-clastic beach of Nantian, Taiwan. Comparison of the software with digital point counts for the common size percentiles D5, D16, D30, D50, D70, D84, and D95 indicated an average coefficient of determination of 0.990 over 10 test pictures. The surface sediment was digitally sampled at three cross-shore locations along the beach: near the shoreline, on the steep part of the beach face, and on the beach crest. Clast-size distributions, including clast area and elongation ratio, were computed from the geometry of 1000 clasts for each location and the cros...

Investigating mechanics of conglomeratic rocks: influence of clast size distribution, scale and properties of clast and interparticle cement

Conglomeratic rocks are composed of macro-particles (varying in size from gravels to cobbles and boulders with diameter greater than 2 mm) bonded by fine-grained cement particles with diameters of < 2 mm. Determination of strength and deformability of such materials in the laboratory is challenging due to sampling and testing constraints induced by the strength and stiffness contrast of the particles and the cement matrix. Alternatively, the mechanics of such materials has been studied by laboratory testing on a synthetic conglomerate composed of steel spheres and Portland cement. The physical modeling and testing was supplemented by numerical simulations in PFC3D by preparation and testing of equivalent numerical models. This paper presents the results of numerical investigations conducted on conglomerates with the objective of finding the effect of particle size distribution, scale effects and the sensitivity of variable mechanical parameters of clasts and cement. Two different clast particles were considered, namely sandstone and granite, both with Hertzian contacts in addition to steel particles. For each clast type, the following bed matrices were modeled to glue the clasts by deriving necessary micro-mechanical parameters from published literature; Portland cement, argillaceous cement and arrenaceous (quartzitic) cement. The mechanical responses of the numerical conglomerates were examined in uniaxial, triaxial and Brazilian tensile testing. Macroscopic behavior of each combination of conglomerate was then analyzed. The results show that particle size distribution affects both the mechanical response and damage pattern in the specimen, while scale, i.e. the effect of specimen size, keeping the micro-structure the same, was found to influence the strength and elastic response of the conglomerates similar to other natural rocks. The properties of clast and cement were also found to influence mechanical response of conglomerates in numerical simulation. The peak strength and the stiffness of the conglomerate increases with the increase of the stiffness and the strength of the cement matrix in uniaxial or close to uniaxial stress states, while negligible dependence on the stiffness and strength of the clast material. In contrast, the mechanical properties of clast materials were found to increase the strength and stiffness of the conglomerate in triaxial stress states, while there was negligible dependence on the strength and stiffness of the cement matrix.

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.

Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance

Gangavalli (Brittle) Shear Zone (Fault) near Attur, Tamil Nadu exposes nearly 50 km long and 1–3 km wide NNE–SSW trending linear belt of cataclasites and pseudotachylyte produced on charnockites of the Southern Granulite Terrane. Pseudotachylytes, as well as the country rock, bear the evidence of conjugate strike slip shearing along NNE–SSW and NW–SE directions, suggesting an N–S compression. The Gangavalli Shear Zone represents the NNE–SSW fault of the conjugate system along which a right lateral shear has produced seismic slip motion giving rise to cataclasites and pseudotachylytes. Pseudotachylytes occur as veins of varying width extending from hairline fracture fills to tens of meters in length. They carry quartz as well as feldspar clasts with sizes of few mm in diameter; the clast sizes show a modified Power law distribution with finer ones (<1000 \({\upmu }\)m\(^{2})\) deviating from linearity. The shape of the clasts shows a high degree of roundness (>0.4) due to thermal decrepitation. In a large instance, devitrification has occurred producing albitic microlites that suggest the temperature of the pseudotachylyte melt was >1000\(^{\circ }\hbox {C}\). Thus, pseudotachylyte veins act as a proxy to understand the genetic process involved in the evolution of the shear zone and its tectonic settings.

Geomorphological and sedimentary evidence of probable glaciation in the Jizerské hory Mountains, Central Europe

The Jizerské hory Mountains in the Czech Republic have traditionally been considered to be a highland that lay beyond the limits of Quaternary glaciations. Recent work on cirque-like valley heads in the central part of the range has shown that niche glaciers could form during the Quaternary. Here we report geomorphological and sedimentary evidence for a small glacier in the Pytlácká jáma Hollow that represents one of the most-enclosed valley heads within the range. Shape and size characteristics of this landform indicate that the hollow is a glacial cirque at a degraded stage of development. Boulder accumulations at the downslope side of the hollow probably represent a relic of terminal moraines, and the grain size distribution of clasts together with micromorphology of quartz grains from the hollow indicate the glacial environment of a small glacier. This glacier represents the lowermost located such system in central Europe and provides evidence for the presence of niche or small cirque glaciers probably during pre-Weichselian glacial periods. The glaciation limit (1000m asl) and paleo-ELA (900m asl) proposed for the Jizerské hory Mountains implies that central European ranges lower than 1100m asl were probably glaciated during the Quaternary.

Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

AbstractThe timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali‐rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K‐Ar, U‐Pb in apatite, and U‐Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre‐Amazonian high‐temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

Fractionation of grain size in terrestrial sediment routing systems

AbstractSediment is fractionated by size during its cascade from source to sink in sediment routing systems. It is anticipated, therefore, that the grain size distribution of sediment will undergo down‐system changes as a result of fluvial sorting processes and selective deposition. We assess this hypothesis by comparing grain size statistical properties of samples from within the erosional source region with those that have undergone different amounts of transport. A truncated Pareto distribution describes well the coarser half of the clast size distribution of regolith, coarse channel bed sediment and proximal debris flows (particularly their levees), as well as the coarser half of the clast size distribution of gravels that have undergone considerable amounts of transport in rivers. The Pareto shape parameter a evolves in response to mobilization, sediment transport and, importantly, the selective extraction of particles from the surface flow to build underlying stratigraphy. A goodness of fit statistic, the Kolmogorov–Smirnov vertical difference, illustrates the closeness of the observed clast size distributions to the Pareto, Weibull and log‐normal models as a function of distance from the depositional apex. The goodness of fit of the particle size distribution of regolith varies with bedrock geology. Bedload sediment at catchment outlets is fitted well by the log‐normal and truncated Pareto models, whereas the exponential Weibull model provides a less good fit. In the Eocene Escanilla palaeo‐sediment routing system of the south‐central Pyrenees, the log‐normal and truncated Pareto models provide excellent fits for distances of up to 80 km from the depositional apex, whereas the Weibull fit progressively worsens with increasing transport distance. A similar trend is found in the Miocene–Pliocene gravels of the Nebraskan Great Plains over a distance of &gt;300 km. Despite the large fractionation in mean grain size and gravel percentage from source region to depositional sink, particle size distributions therefore appear to maintain log‐normality over a wide range of transport distance. Use of statistical models enables down‐system fractionation of sediment released from source regions to be better understood and predicted and is a potentially valuable tool in source‐to‐sink approaches to basin analysis.

Numerical identification of tsunami boulders and estimation of local tsunami size at Ibaruma reef of Ishigaki Island, Japan

AbstractTsunami boulders deposited along the coast constitute important geological evidence for paleotsunami activity. However, boulders can also be deposited by large storm waves. Although several sedimentological and theoretical methods have been proposed to differentiate tsunami and storm wave affected boulders, no appropriate numerical method exists for their differentiation. Therefore, we developed a new numerical scheme to differentiate tsunami and storm wave boulders for coastal boulders on Ishigaki Island, Japan. In this area, tsunami and storm waves have emplaced numerous boulders on the reef and the coast. By conducting numerical calculations of storm waves in this region, we estimated the size of a storm wave that can explain the maximum clast size distribution of boulders on the reef. Consequently, we showed that a wave with a combination of 8 m in initial wave height and 10 s period can satisfy the above conditions when we assume mean sea level. In contrast to the boulders on the reef, all boulders deposited along the shore are heavier than the calculated possible maximum clast size distribution by the storm wave. Therefore, we confirmed these boulders as being of tsunami origin. Results of previous studies showed that they were most likely deposited or reworked by the 1771 Meiwa tsunami. Then, using the tsunami boulders, we numerically estimated the wave period and amplitude of the 1771 Meiwa tsunami, which should have had a 4–5 min period and 5.6–5.9, 6.3–7.0 m amplitude, respectively. Using the proposed scheme, it is possible to differentiate tsunami and storm wave boulders and estimate the size of past storm waves and tsunami waves, although it is noteworthy that there are exceptions for which the scheme cannot be applied.

DEPOSITIONAL ENVIRONMENT AND BONE DIAGENESIS OF THE MIO/PLIOCENE LANGEBAANWEG BONEBED, SOUTH AFRICA

Langebaanweg is a world renowned, richly fossiliferous Miocene/Pliocene bone assemblage on the West Coast of South Africa. Despite considerable palaeontological investigation of the biodiversity of the site, few studies have been conducted on the bone diagenesis. This study examines the petrography of fossil bones and teeth in relation to the host sediment matrix. The Langebaanweg bonebed is variably matrix- and clast-supported, containing abundant gravel-size bone and phosphorite clasts in a quartzose sand matrix with minor amounts of peloidal phosphate grains and kaolinitic clay. The bimodal size distribution of clasts and sand matrix indicates a mixture of beach, fluvial and aeolian sources, consistent with a riverine or aquifer-fed waterhole palaeoenvironment proximal to the sea. Both macro- and microfaunal bones exhibit a range of surface colours (cream/brown through dark orange/brown to black) indicative of secondary mineral precipitation (including manganese) and some show physical damage from trampling. Three phases of bone diagenesis are recognised; an early, rapid transformation of bone (bioapatite) to mineral carbonate hydroxylapatite, an apatite cementation phase and a single, late growth of infilling Mn-rich alteration material. Except for rare burnt bones, bones and teeth mostly preserve fine histological detail. Despite differences in secondary mineral burden and physical deformation (trampling), diagenesis is fairly uniform within the deposit. The excellent bone preservation, and the fact that the deposit has not been reworked is important for depositional models. The classic Palaeo-Berg River fluvial depositional model is challenged in this paper by an alternative possibility that the deposit was an aquifer-fed, near coastal waterhole.