Angular Clasts Research Articles

Indications of volcanic style of the Silurian Nová Ves Volcano (Hemrovy Rocks)

Studied locality of Hemrovy Rocks belongs to Nová Ves Volcanic Center, one of the Silurian volcanic centers of Prague Basin of Teplá-Barrandien area (lower Wenlock – lower Ludlow age). The aim of this work is to describe details of the volcanic rock structures and to contribute to the interpretation of the Silurian volcanic style. The macroscopic and microscopic structures of basaltic rocks in and close to the abandoned Kační Quarry in the southern part of Prague and at the adjacent locality – ridge of Hemrovy Rocks were studied. At these sites, volcaniclastics dominate over solid volcanic rocks and sediments. The volcanic rocks are represented by basalts, which form thin lava flows overlying both the volcaniclastics and sedimentary rocks. Massive fine-grained basalts pass to pillow lava facies, autobreccia or hyaloclastite breccia in-situ. These brecciated structures evolve into peperites or to unconsolidated volcaniclastic rocks of a previous eruption. Coarse-grained volcaniclastics with oversized subrounded to angular clasts of vesiculated lava or sediments within glassy matrix, sporadically with sediment admixture, are unsorted and thick-bedded. They were probably formed by gravity-driven mass flow. Basalt in the Kační Quarry probably represents a synvolcanic intrusion, as documented by its hyaloclastite rim and presence of clusters of resedimented volcaniclastics in the adjacent fine-grained sediments. Matrix of the volcaniclastics is mostly unsorted, formed mostly by quenched fragments of altered palagonitized and chloritized basaltic glass, locally scoriaceous, and fragments of chilled basaltic lava. Originally highly porous volcaniclastics, resp. hyaloclastites were secondarily cemented by calcite and silica. Secondary chlorite, calcite and silica also fill vesicles and cavities in fragments of glass and lava clasts. Dense vesicularity of lava fragments in volcaniclastics indicates an effective explosive interaction of lava with water. It is probable that much of the wet volcaniclastic material repeatedly slumped back down into the volcano’s crater to be re-ejected by subsequent phreato-magmatic explosions. Hydroclastic fragmentation was iniciated by repeated subaquatic eruptions in shallow subphotic marine zone. Unsorted crinoidal packstones mostly with trilobites, trepostomate bryozoan and volcaniclastic matrix were deposited in the vicinity of the Nová Ves Volcanic Center.

Lithofacies and Depositional Environments of the Garubathan Alluvial Fan, North Bengal, India

Abstract The sedimentology of the Garubathan alluvial fan of north Bengal foothills is a product of Himalayan neotectonics and monsoon climate. The active neo-tectonics of the Main Central Thrust (MCT), Ramgarh Thrust (RT) and Ghis Transverse Fault (GTF) produce a huge amount of angular clasts in the upper reaches of the river Chel and Mal. During the wet season, either sediment flow by gravity or turbulent stream flow or both, move the clasts to the downhill. The tectonic zone of Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT) are relative passive thrusts than the MCT, RT and GTF in the study area. A very low angle slope is observed in the area of MBT and MFT of the study area. So, the downhill deposition occurred in MBT and MFT zones of the study area. The river Chel and Mal are perinial rivers. During dry periods (pre-monsoon and post-monsoon), in the absence of sediment flow by gravity or turbulent stream flow, the low stream competency deposit the fine sediments. To identify the depositional environments three, seven and six lithofacies varieties were identified in the upper, middle and lower fan surfaces, respectively. The non-cohesive sediment flows by gravity deposited large angular boulders, cobbles and pebbles in the upper fan area. These deposits are either chaotic, unsorted, un-imbricated or graded and imbricated. Deposition of sand is scanty. Both, sediment flow by gravity and fluvial processes deposited angular to round clasts with matrix in the middle fan area. In this area the deposits are reverse graded, reverse to normally graded, laminated and cross-bedded. The high facies diversity of this area indicates highly variable nature of the depositional environments. In the lower fan area, presence of both clast-supported chaotic and matrix-supported laminated or cross-bedded layers suggests a complex nature of the depositional environment of sediment flow by gravity and fluvial processes.

Provenance of the Middle Miocene Barreiras Fan Delta in the Onshore Pará-Maranhão Basin, Northern Brazil

The demise of the carbonate platform in the Brazilian Equatorial Margin has been related to the progradation of massive siliciclastic inflow from the Cratonic Amazon River during the middle Miocene. The sediment plume did not influence the eastern portion of this drainage, including the Bragantina Platform andthe Marajó Basin, the onshore Pará-Maranhão Basin. The provenance signatures of these tectonic segments have been demonstrated in siliciclastic coastal deposits of the Barreiras Formation, whose progradation caused the suppression of the Oligocene-Miocene carbonate platform. Our provenance study is based on clast morphology, quartz cathodoluminescence, and heavy minerals of mainly Barreiras alluvial-fluvial deposits, using new and published facies data. This study opens a window of opportunity to investigate, for the first time, the proximal facies combined with paleocurrents exposed only in this area of Brazil. The restricted occurrence of proximal facies indicates an exceptional potential for preservation related to subsiding regions generated by reactivated faults in the Gurupi Belt. Provenance data indicate the strong influence of metamorphic source rocks with minor volcanic and plutonic contributions from the southeastern and southern of the Bragantina Platform and eastern Marajo Basin. Respectively, two heavy minerals assemblages were found: 1) staurolite, metamorphic quartz, and angular clasts; and 2) abundant zircon, kyanite, tourmaline, mixed quartz (metamorphic, plutonic, and volcanic), and rounded clasts. The assemblages are associated with São Luís Craton, mainly crystalline rocks from Gurupi Belt. In addition, the proximal facies of the Bragantina Platform provide rich staurolite concentration and angular clasts indicating near source area; the uplifted Marajó Basin during the middle Miocene contributed with reworked sediments containing rounded clasts and ultrastable minerals related to Silurian-Cretaceous rocks. This work complements the previous paleoenvironmental reconstruction for the Barreiras deposits, including the most robust proximal facies interpretation contributing to understanding sediment dispersion during the middle Miocene, reinforcing previously defined source areas.

Origins of clay-rich strata in Cenozoic paleochannel deposits: an example from Suttor Formation, Queensland

Cenozoic sediments cover much of Queensland’s landscape and record changes in climate and flora that evolved in tandem with the tectonic evolution in eastern Australia, associated with uplift and erosion events arising from Late Cretaceous sea-floor spreading in the Coral Sea, leading to the passive margin we see today. In eastern Australia, incised and potentially fault-bounded valleys developed on the pre-Cenozoic landscape and were variably infilled at different times by siliciclastic sediments, carbonaceous muds and peat, and basaltic lava flows. This study examines a 100 m-deep drill core through one such siliciclastic-dominated paleochannel incised into upper Permian coal measures on the western limb of the Bowen Basin. Palynological analysis, core logging, hyperspectral logging of the core, X-ray diffraction (XRD), SEM–EDS and U–Pb zircon geochronology were used to investigate the age and origins of these clay-rich deposits. Palynological analysis confirmed that the strata belong to the upper Paleogene and Neogene Suttor Formation, a time when Australia’s climate has been interpreted to shift from wet and warm to a more arid setting. Core logging revealed that the sequence in the studied channel began as a breccia/alluvial fan with chaotic angular clasts of Permian laminated siltstone, tuff and coal floating in disturbed poorly lithified sand with evidence of soft sediment deformation, possibly initiated by localised faulting. These clasts were continually reworked and broken down into a clast supported pebble conglomerate capped by a series of claystone with intermittent carbonaceous claystone and lignite, interpreted to have formed in a lacustrine environment that could be localised on the floodplain (oxbow) or more regional in extent. Above this, and interpreted from palynology as corresponding to a climatic shift to more arid conditions, are well sorted but subrounded sugar sands and intermittent clays with abundant iron staining (?paleosols) interpreted to represent sandy ephemeral channels. These are overlain by extremely weathered undated basalts. The hyperspectral logging of the core and XRD studies of the white claystone samples indicate that they are kaolinite-rich, but observations under SEM–EDS noted that their honeycomb structure mimicked that of smectite. The presence of abundant zircon suggested the possibility of syndepositional volcaniclastics, but U–Pb zircon geochronology recorded an age of 272–254 Ma (middle to late Permian). Hence local recycling of the tuffaceous smectite-rich coal measures within the incised alluvial system, assisted by intense weathering under a warm and wet climate, and potentially acid leaching from the organic-rich layers was the origin of the kaolinite-rich clay strata. KEY POINTS Clay-rich Cenozoic paleochannels in the northern Bowen Basin belong to the upper Paleogene and Neogene Suttor Formation, based on palynological study. The clay-rich strata are developed by local recycling of middle to late Permian tuffaceous smectite-rich coal measures. The interpretation based on palynology and clay mineralogy confirm that Australia experienced a shift from wet and warm to a more arid climate during the upper Paleogene and Neogene.

Hydrovolcanic activity on a continental shelf inferred from the subsurface diatreme- and crater-filling deposits of Jeju Island, Korea

Jeju Island comprises numerous tuff rings and tuff cones and their reworked deposits in the subsurface, which formed on the ca. 120-m-deep Yellow Sea continental shelf under the fluctuating Quaternary sea levels. Tens of meter-thick and massive deposits were found by chance during groundwater drilling at three sites. These deposits are interpreted as either syn-eruptive diatreme-filling deposits or post-eruptive crater-filling deposits, both of hydromagmatic volcanoes. The diatremes were cut into shelf sediment, 70 to 250 m thick, and developed generally within it below the Quaternary sea levels. Abundant external water was therefore available for explosive magma-water interactions at shallow levels. The diatreme deposit in one core shows some features attributable to extreme wetness or water saturation of the diatreme fill, such as the matrix support of larger clasts, meager vertical changes in matrix content, and an absence of features related to particle adhesion. Fluidally shaped clasts with delicate reentrant margins in the core suggest minimal particle abrasion and breakage in a water-saturated and highly fluid slurry of tephra and water that was probably filling a shallow bowl-like diatreme, which is distinguished from both phreatomagmatic and kimberlite diatremes. The diatreme deposits in other cores comprise blocky and angular clasts in a sideromelane ash matrix, suggesting phreatomagmatic explosions at a deeper level. One of the cores contains collapsed deposits of thinly stratified tuff emplaced by pyroclastic surges, indicating that the diatreme is associated with an emergent tuff ring. Both Surtseyan and phreatomagmatic eruptions are therefore interpreted to have occurred on the shelf under the controls of fluctuating Quaternary sea levels. The subsurface diatremes suggest that there can be a variety of diatremes with different sizes, shapes, and material characteristics beneath the craters of hydromagmatic volcanoes, including not only maars but also tuff rings and tuff cones.

Back-arc basin closure at the East Asian margin during Permo-Triassic boundary: Evidence from geochemistry and U-Pb zircon data of sedimentary breccia from Maizuru Terrane, Southwest Japan

The paleo Maizuru back-arc basin, which was located at the East Asian continental margin opened in Early Permian times and closed during the Permo-Triassic (P-T) boundary. The back-arc basin sediments are exposed in the present-day Maizuru Terrane located in Southwest Japan and hold the key to understand the tectonic evolution of both this back-arc basin and that of the East Asian continental margin. The opening of the Maizuru back-arc basin was accompanied by the deposition of a massive black mudstone-dominated formation, presently recognized as the Maizuru Group. The closure of the back-arc basin which was accompanied by the deposition of coarse-grained materials such as the Tonoshiki breccia made up of differently-sized extremely angular clasts implies that the P-T boundary was a tectonically active period in the Maizuru Terrane. Petrographical and geochemical studies revealed that Tonoshiki breccia can be divided into two main types, a first type mainly made up of mafic to intermediate rock clast fragments (Type I) and a second type mostly consisting of felsic rock clast materials (Type II). Both types commonly contain massive black mudstone clasts that were derived from the underlying Maizuru Group. The U-Pb detrital zircon geochronology of those two types of breccia suggests that they were deposited during two phases of the tectonic activity-induced debris flow that occurred at ca. 259 Ma and ca. 251 Ma for the first and second type, respectively. Type I was sourced from the Southern zone, whereas Type II was sourced mostly from the Northern zone of the Maizuru Terrane. This switch of provenance testifies to the intense tectonic activity that reigned during the P-T boundary. The closure of the Maizuru back-arc basin was induced by the subduction of the basement crust of the basin under the East-Asian continent during the P-T boundary.

An evolutionary model and classification scheme for nephrite jade based on veining, fabric development, and the role of dissolution–precipitation

Although nephrite jade has been collected and treasured since the Stone Age, we lack a clear understanding of how it forms during deformation and metasomatism in shear zones. Using microstructural analysis of samples from Taiwan, California, and New Zealand, we propose a conceptual model for the evolution of nephrite jade that distinguishes four nephrite types based on mode of formation and textural characteristics: (1) primary (type 1a) or folded (type 1b) vein nephrite, (2) crenulated nephrite (type 2), (3) foliated semi-nephrite (type 3), and (4) nodular or domainal nephrite (type 4). We interpret the texture of our analysed samples to represent snapshots of a progressive textural evolution similar to that experienced by other deformed and fine-grained metamorphic rocks that develop under fluid-present, greenschist-facies conditions. Our observations suggest that types 2 and 3 nephrite can evolve from vein nephrite (type 1) by the development of crenulated and foliated metamorphic fabrics, during which the most important deformation process is dissolution–precipitation. However, development of metamorphic fabrics can be interrupted by transient brittle deformation, leading to the formation of type 4 nephrite that is characterised by nodular or angular clasts of nephrite in a nephritic matrix.

Diagenesis of clastic ores of the Ishkininо Co-bearing massive sulfide deposit (Southern Urals): Mineralogical-geochemical data and thermodynamic modeling

Research subject. The transformed clastic ores (ore diagenites) of the Ishkinino Co-bearing massive sulfide deposit hosted by serpentinites of the Main Uralian Fault Zone.Materials and methods. The structures and textures of the ores were stu died. The trace element contents of sulfides and oxides were determined using LA ICP MS. The physical and chemical modeling of the diagenetic formation of accessory As minerals was conducted using the Selektor program package. Results. The clastic ores are transformed gravelites with angular and rounded clasts of serpentinites, sulfides and chromite in the psammitic matrix of the same mineral composition. No hydrothermal minerals remain in gravelites; they are replaced by crystalline pyrite-2, porous pyrite-3, anhedral pyrite-4, pyrrhotite, chalcopyrite and magnetite. Chalcopyrite and magnetite replace all sulfides, sulfarsenides, chromite and gangue minerals. Chromite occurs as fragmented crystals or inclusions into serpentinite clasts. The matrix hosts euhedral cobaltite crystals with nickeline, gersdorffite and native gold inclusions. Crystalline pyrite-2 is characterized by higher Mn, Co, Ni, Cu and Zn contents. Porous pyrite-3 exhibits higher Co, Cu and Se contents. Anhedral pyrite-4 is enriched in most trace element contents in comparison with other sulfides and pyrite generations. Chalcopyrite is characterized by higher contents of Zn and Se. Pyrrhotite contains the highest Ni and hi gher Co contents.Conclusions. The main trace elements in the ores of the deposit (Co and Ni), as well as Cu, Zn and Mn, are hosted not only in sulfides, but also in oxides. Thus, chromite contains Zn and Ni, while magnetite contains Mn and Cu. Selenium occurs in all sulfides in similar quantities. Tellurium is mostly concentrated in pyrite-4. A comparative analysis of our results with those reported on other massive sulfide deposits showed that the serpentinite-sulfide gravelites of the Ishkinino deposit had been intensely transformed during diagenesis, which resulted in low trace element contents in diagenetic sulfides. The diagenetic alteration of clastic ores led to the formation of authigenic cobaltite, gersdorffite, nickeline and native gold as a result of trace element release from primary hydrothermal minerals. Thermodynamic mode ling showed the possibility of formation of As-bearing minerals (in particular, nickeline) at temperatures of 200°C and below.

Identification of a Quaternary rock avalanche deposit (Central Apennines, Italy): Significance for recognition of fossil catastrophic mass-wasting.

Whereas deposits of extremely‐rapid, ‘catastrophic’ mass wastings >105 m3 in volume (for example, the Marocche di Dro rock avalanche in the Southern Alps and the Flims rockslide in the Western Alps) are easily recognized by their sheer mass and blocky surface, the identification of fossil catastrophic mass wastings partly removed by erosion must be based on deposit characteristics. Herein, a ‘fossil’ (pre‐last glacial) rock avalanche, previously interpreted as either a till or debris flow, is described. The deposit, informally called ‘Rubble Breccia’, is located in the intramontane Campo Imperatore halfgraben that is bounded by a master fault with up to ca 900 m topographic throw. Based on documentation from field to thin section, and by comparative analysis with post‐glacial rock avalanches, tills and debris flows, the Rubble Breccia is reinterpreted as a rock avalanche. The Rubble Breccia consists of an extremely‐poorly sorted, disordered mixture of angular clasts from sand to block size. Many clasts show fitted subclast boundaries in crackle, jigsaw and mosaic fabrics, as diagnostic of catastrophic mass wasting deposits. Intercalated layers of angular to well‐rounded clasts of coarse sand to fine pebble size, and deformed into open to recumbent folds, may represent shear belts folded during terminal avalanche propagation. The clast spectrum of the Rubble Breccia – mainly shallow‐water bioclastic limestones, Saccocoma wackestones and other deep‐water limestones and dolostones – is derived from the front range along the northern margin of the basin. Calcite cement found within the Rubble Breccia was dated with the U/Th disequilibrium method to 124.25 ± 2.76 ka bp, providing an ante‐quam age constraint to the rock avalanche event. Because catastrophic mass wasting is a common erosional process, fossil deposits thereof should be more widespread than have been identified to date, although this may be a consequence of misidentification. The criteria outlined here provide a template to identify fossil catastrophic mass wasting deposits of any age.

Evaluating Petrophysical Properties and Volumetrics Uncertainties of Sand Injectite Reservoirs – Norwegian North Sea

Sand injectites on the Norwegian Continental Shelf have proven their commercial significance. Some are already producing, e.g., Volund, Viper, Balder, Ringhorne, and Kobra Fields, while others such as in production licenses (PL) 340 and 869 have recently been discovered and appraised. Extensive literature on the geology of sand injectites has been published (e.g., Jenssen et al., 1993; Jolly and Lonergan, 2002; Huuse et al., 2003; Hurst et al., 2005). However, few references are available on the petrophysical and geophysical aspects of sand injectite reservoirs. This paper discusses the petrophysical properties of sand injectite facies, dykes, sills, and brecciated sands, along with their identification from seismic data. A perception that volumetrics of sand injectite reservoirs cannot be reliably evaluated is assessed. Sand injectites in PL 340 and 869 were interpreted as remobilized sands from the Hermod and Heimdal Formations of Paleocene age injected into the overlying Balder Formation and Hordaland Group mudstones of Eocene age. The mudstones acted as a seal, forming an intrusive stratigraphic trap. The trap geometry varied locally depending upon the dyke and sill geometries of the sandstone. Dykes had large vertical reach with the corresponding high-hydrocarbon column, while sills had low-vertical relief with large lateral extent. Intervals of brecciated sands were also observed within the injectite complex, especially where sands were thin. These brecciated sands contained large amounts of angular mudstone clasts of different dimensions suspended in an overall sandy matrix. Close examination of cored dykes made it possible to observe this, while it might not be as obvious when looking at bulk well logs. Petrophysical-log responses for clean sills and dykes behaved the same way as they would in a clean sandstone reservoir. If sills and dykes were very thin, they would also risk not being counted as net or pay (Suau et al., 1984; Dromgoole et al., 2000; Flølo et al., 2000;). Such errors can impact in-place volumes in a significant way. Sills appeared as blocky clean sand on logs, but it was difficult to differentiate a dyke from a sill or thin sands using logs. Dykes are high-angle features and are identified either by core studies or borehole images when intersected by a well or, if large enough, observable on seismic. Brecciated sand intervals appeared with cm-to-dm-scale mudstone clasts suspended in sand with approximately 40 to 60% net to gross. Log responses over these intervals indicated shaly sand or thin sands. Resistivity and thermal neutron porosity logs were highly affected by the shale clasts. For this reason, a fractional net/gross interpretation technique was used to evaluate the sand content and hydrocarbon pore volume. To further verify these results, they were compared to observations directly on the core. To qualify to what extent petrophysical logs and interpreted products thereof can be relied on to evaluate hydrocarbon volumes of sand injectite reservoirs, a high-resolution petrophysical interpretation was generated using a computerized tomography (CT) scanned core image. Core image sand counting and image-derived high-resolution bulk density logs with shale-corrected resistivity were used. Results of this high-resolution interpretation featured an excellent match with routine core analysis data and manual core observations in the core laboratory. The fractional net/gross method used is the modified Thomas-Stieber method (Johansen et al., 2018). Its results compared well to the high-resolution CT-scan image results and better evaluated the hydrocarbon pore volume of sand facie compared to the conventional bulk formation evaluation approach. This result confirms that the Thomas-Stieber method can be used for brecciated rocks, which leads to some useful recommendations on how to best log and perform a petrophysical evaluation in such reservoirs.

A method and software solution for classifying clast roundness based on the radon transform

In this paper, an algorithm for clast roundness classification based on the Radon transform is presented. The degree of roundness is determined by processing the sinogram of the clast image. The algorithm consists in applying two low-pass filters to the sinogram, obtaining the inverse Radon transform and comparing the filtered images with the original image. For rounded particles, the difference between the original image and either of the filtered images will be small. For angular clasts, the difference will be greater than for rounded clasts, due to the presence of high-frequency components. In the comparison process, each of the two filtered images are subtracted from the original image to yield two difference images. Since the data are binary, these two images present topologically unconnected regions that correspond to the particle's edges. The percentage of non-overlapping area between the original and the difference images, and the number of regions are used to classify the morphology of the clast. The results have been validated using a comparison chart designed for visual roundness estimation. The comparison chart, consisting of five roundness classes, was proposed by Russell, Taylor and, Pettijohn (Müller, 1967). Two cutoff frequencies, one to classify well-rounded, rounded and sub-rounded clasts and another for angular and sub-angular classes, were used. The proposed algorithm correctly classifies the roundness classes of the visual graph. The results provided by the algorithm were compared with the classification performed by a group of experts. The algorithm assigned 92% of the clasts to the same classes as the human experts. We also propose Gaussian models, which are useful to classify the particles into the five classes. We have developed a user-friendly software to carry out the roundness classification algorithm. This software was developed on the MATLAB platform and can be freely downloaded from the public repository.

The First Samples from Almahata Sitta Showing Contacts Between Ureilitic and Chondritic Lithologies: Implications for the Structure and Composition of Asteroid 2008 TC3.

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70-80% are ureilites (achondrites) and 20-30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal-sulfide, as well as chondrules (~130-600 μm) and chondrule fragments. The C1 material consists of fine-grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28-42), an unidentified Ca-rich silicate phase, Fe,Ni sulfides, and minor Ca-phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC-like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75-88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal-sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal-sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04-0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH- in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F-type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A-like materials, with as much as 40-70% of the latter, and <10% of OC, EC and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g/cm3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7-2.2 g/cm3). Its porosity (36%) is near the low end of estimates for the asteroid (33-50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC-like body impacted into already well-gardened ureilitic + impactor-derived debris. AhS 91A bulk samples do not show a solar wind (SW) component, so they represent sub-surface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5-9 Ma) than previously studied AhS stones (11-22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.

Fault-controlled magnetite-rich breccias in Jurassic limestones: Evidence for concealed epigenetic ore systems in the Lusitanian Basin (Portugal)?

The present work reports the first study on fault-controlled breccias cemented by low-Ti iron-oxides cropping out in Valverde (Maciço Calcário Estremenho, Lusitanian Basin, Portugal). The mineralized bodies: (i) occur in jogs of a WNW-ESE fault zone that comprises four segments, forming an irregular en échelon array of arched geometry; and (ii) are hosted in Jurassic limestones affected by a prominent metasomatic alteration halo macroscopically characterized by darker coloring, higher hardness and disseminations of pyrite, pyrrhotite, hematite and magnetite. Breccias are mainly composed of heterometric and variably modified angular clasts of limestone rocks along with minor dolerite clasts recording intense hydrothermal carbonate alteration. The breccia cements include magnetite and calcite, besides variable amounts of hematite, mixed-layered chlorite-smectite-mica, Ba-bearing phlogopite, fluorapatite, pyrite and chalcopyrite. These features are comparable with some types of Ti-poor iron oxide ores included in the “copper-deficient end-members” of the IOCG clan generated during the Phanerozoic. However, as far as can be observed, the hosting limestones at Valverde do not show calcic or magnesium skarn assemblages and massive Fe-oxide stratabound replacements are also absent.The evolution of the Valverde system can be synthetically described as a succession of two main ore-forming stages during which all of the aforementioned authigenic mineral phases grew in the following sequence: phyllosilicates, iron-oxides and redox reactions, and sulfides. Compared to the hosting limestones, the mineralized fault breccias display strong enrichments in Fe and Co (up to ≈200 times), Ba, Cu, P, Cr, Mn, Si and V (up to ≈33 times) and Rb, K, Zn, Mo, Al, Na, Pb, Mg, Sc, Zr and S (up to ≈8 times). Quantitatively, the iron grades are quite significant (rising up to ≈85 wt% Fe2O3) and the abundances in other base metals are anomalous (≈61 ≤ (Cu + Co + Zn) ≤ 280 ppm). The recognition of this exotic ore-system opens new mineral exploration perspectives for the Lusitanian Basin and other basins with similar characteristics.

A Cretaceous carbonate delta drift in the Montagna della Maiella, Italy

AbstractThe Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is compared to newly discovered contourite drifts in the Maldives. Like the drift deposits in the Maldives, the Orfento Formation fills a channel and builds a Miocene delta‐shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km2 large coarse‐grained bioclastic Miocene delta drifts in the Maldives. The composition of the bioclastic wedge of the Orfento Formation is also exclusively bioclastic debris sourced from the shallow‐water areas and reworked clasts of the Orfento Formation itself. In the near mud‐free succession, age‐diagnostic fossils are sparse. The depositional textures vary from wackestone to float‐rudstone and breccia/conglomerates, but rocks with grainstone and rudstone textures are the most common facies. In the channel, lensoid convex‐upward breccias, cross‐cutting channelized beds and thick grainstone lobes with abundant scours indicate alternating erosion and deposition from a high‐energy current. In the basin, the mounded sedimentary body contains lobes with a divergent progradational geometry. The lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channelized topset, a grainy foreset and a fine‐grained bottomset with abundant irregular angular clasts. Up to 30 m thick channels filled with intraformational breccias and coarse grainstones pinch out downslope between the megabeds. In the distal portion of the wedge, stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration. The bioclastic wedge of the Orfento Formation has been variously interpreted as a succession of sea‐level controlled slope deposits, a shoaling shoreface complex, or a carbonate tidal delta. Current‐controlled delta drifts in the Maldives, however, offer a new interpretation because of their similarity in architecture and composition. These similarities include: (i) a feeder channel opening into the basin; (ii) an excavation moat at the exit of the channel; (iii) an overall mounded geometry with an apex that is in shallower water depth than the source channel; (iv) progradation of stacked lobes; (v) channels that pinch out in a basinward direction; and (vi) smaller channelized intervals that are arranged in a radial pattern. As a result, the Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is here interpreted as a carbonate delta drift.

Roundness of survivor clasts as a discriminator for melting and crushing origin of fault rocks: A reappraisal

Roundness of survivor clasts (mineral/rock fragments) in fault rocks (e.g., pseudotachylyte and cataclasite/gouge) has been used by some workers to distinguish melt-origin from crush-origin of such rocks. Keeping in view the large overlap in the published data on the roundness of fault rocks, the reliability of such a discriminator appears somewhat uncertain. The present study attempts to reappraise the aforesaid criterion through roundness analysis of quartz and feldspar clasts in melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and fault-related cataclasite (F-Ct) collected from Sarwar–Junia Fault Zone in Rajasthan and from Gavilgarh–Tan Shear Zone in central India. Our analysis shows that roundness of clasts cannot reliably distinguish between fault rocks of melt-origin and crush-origin (especially M-Pt and F-Ct) as the roundness values overlap and a distinct limit of roundness value for each rock type cannot be established. While the roundness of clasts in M-Pt and C-Pt may be enhanced due to melt-induced rounding off of the initially angular clasts, rounding of clasts can also occur by abrasion during rolling of crushed material in F-Ct. Furthermore, anomalous thermal expansion of clasts in melt-origin pseudotachylyte may cause disintegration of larger clasts into smaller angular fragments, thereby increasing the percentage of angular clasts in melt-origin fault rocks. Therefore, roundness of survivor clasts cannot be solely used as a discriminator between melt-origin and crush-origin fault rocks.

The common origin and alteration history of the hypabyssal and volcanic phases of the Wadi Tarr albitite complex, southern Sinai, Egypt

New data and interpretations are presented for the igneous albitites of the Wadi Tarr area, southern Sinai, Egypt. The albitite masses are isolated in outcrop from any granitic intrusions and have intrusive contacts against the country rocks without any structural control. They have marginal zones of breccias with jigsaw-fit angular clasts suggesting explosive, in-situ formation. The albitites are of two types: the western, medium-grained, hypabyssal albitite and the eastern, fine-grained porphyritic albitite. The field relations suggest emplacement at different levels in a magmatic cupola: the hypabyssal texture and steeply dipping slope of the upper contact of the western albitite imply deeper emplacement whereas the gently dipping contacts and porphyritic texture of the eastern albitite masses indicate that they define the probable location of the cupola apex. Both types of albitites consist of albite (92–97%) with minor amounts of quartz, K-feldspar and biotite. The accessory minerals include Fe-oxides, augite, sulphides, zircon, rutile, xenotime, titanite, allanite and monazite. The whole-rock compositions of the hypabyssal and porphyritic albitites are closely related, but the porphyritic type has lower abundances of Sr, Ba, Y, Nb, Th and Zr. We show that the hypabyssal and porphyritic albitites have a common petrogenetic origin, most likely as late-stage cumulates from a fractionating, strongly alkaline A-type magma, consistent with the compositions of the mafic minerals. The source magma was probably a tephritic liquid; we use MELTS models to show that only a sufficiently alkaline magma follows a differentiation path that both avoids quartz saturation and encounters the alkali feldspar solvus, reaching a residual liquid in equilibrium with highly sodic feldspar. Although the MELTS results show a chemically consistent means of forming igneous albitite, they are incomplete in that physical segregation mechanisms are still required to isolate the albite from mafic minerals and or a low-temperature aqueous alteration stage is needed to leach K from the feldspar. Alteration surrounding the Wadi Tarr albitites is extensive and dominated by alkali metasomatism similar to fenitization. Alteration in the marginal breccia zone of the albitite is dominated by precipitation of amphibole and carbonate in veins and in the breccia matrix, whereas the volcanic country rocks show replacement of feldspars by sericite, carbonate and epidote as well as vein carbonate. The altered volcanic country rocks show lower concentrations of Fe2O3, Sr, Cu, Pb, Ba and Ce, accompanied by higher concentrations of Na2O and MgO compared to unaltered equivalent samples.

The Triassic of southwestern Switzerland – Marine or non-marine, that is the question!

It is often difficult to distinguish nearshore marine from non-marine non-fossiliferous quartz-rich sandstones, and in many cases the depositional environment is disputed. Detailed facies analysis is necessary to determine the environment and, although there are no ‘smoking gun’ features to delineate them, a group of specific attributes in combination can be used to differentiate fluvial from marine deposits. Primary characteristics of fluvial deposits are: fining-upward channel-based sequences lacking marine indicators, well-developed palaeosols, and unimodal palaeocurrent distributions.The Vieux Emosson Formation, a Triassic autochthonous unit of the western Swiss Alps, is one such controversial example. It lies unconformably above crystalline basement and consists of a basal sandstone and conglomerate-dominated unit, and an upper heterolithic sequence of thin sandstones and mudstones with thin dolostone beds near the top. The formation is overlain by allochthonous younger carbonates of the Tethys Ocean. The light color of the basal unit gives the appearance of quartz-rich sandstone. In part because of this and also the overlying marine carbonates, previous workers interpreted the Vieux Emosson Formation as having been deposited in a marginal marine environment, along the southern edge of the continent and bordering Tethys. However, petrographic analysis determined that the Vieux Emosson sandstones are poorly sorted subarkoses and arkoses, and facies analysis indicated deposition in a continental fluvial-lacustrine system with a north-trending palaeoslope off the Vindelician High, towards the Germanic Basin. Deposition of the lower unit was in braided streams, supported by: high-relief basal erosional surface, immature sediment, large angular clasts, amalgamated fining-upward sequences, mottled palaeosol horizons with carbonate nodules, absence of bioturbation, and a unimodal palaeocurrent pattern. The interbedded sandstone and mudstone (proximal) and the upper mudstone and dolostone (distal) units represent deposition in a terminal splay-playa environment. Desiccation cracks, dolostone, absence of body fossils and very rare trace fossils indicate deposition in a playa lake. Non-marine deposition in the region during the latest Early Triassic and early Middle Triassic helps constrain the location and timing of the initial marine transgression in the southern Germanic Basin.

Clastic polygonal networks around Lyot crater, Mars: Possible formation mechanisms from morphometric analysis

Polygonal networks of patterned ground are a common feature in cold-climate environments. They can form through the thermal contraction of ice-cemented sediment (i.e. formed from fractures), or the freezing and thawing of ground ice (i.e. formed by patterns of clasts, or ground deformation). The characteristics of these landforms provide information about environmental conditions. Analogous polygonal forms have been observed on Mars leading to inferences about environmental conditions. We have identified clastic polygonal features located around Lyot crater, Mars (50°N, 30°E). These polygons are unusually large (>100 m diameter) compared to terrestrial clastic polygons, and contain very large clasts, some of which are up to 15 metres in diameter. The polygons are distributed in a wide arc around the eastern side of Lyot crater, at a consistent distance from the crater rim. Using high-resolution imaging data, we digitised these features to extract morphological information. These data are compared to existing terrestrial and Martian polygon data to look for similarities and differences and to inform hypotheses concerning possible formation mechanisms. Our results show the clastic polygons do not have any morphometric features that indicate they are similar to terrestrial sorted, clastic polygons formed by freeze-thaw processes. They are too large, do not show the expected variation in form with slope, and have clasts that do not scale in size with polygon diameter. However, the clastic networks are similar in network morphology to thermal contraction cracks, and there is a potential direct Martian analogue in a sub-type of thermal contraction polygons located in Utopia Planitia. Based upon our observations, we reject the hypothesis that polygons located around Lyot formed as freeze-thaw polygons and instead an alternative mechanism is put forward: they result from the infilling of earlier thermal contraction cracks by wind-blown material, which then became compressed and/or cemented resulting in a resistant fill. Erosion then leads to preservation of these polygons in positive relief, while later weathering results in the fracturing of the fill material to form angular clasts. These results suggest that there was an extensive area of ice-rich terrain, the extent of which is linked to ejecta from Lyot crater.

First finding of impact melt in the IIE Netschaëvo meteorite

AbstractAbout half of the IIE nonmagmatic iron meteorites contain silicate inclusions with a primitive to differentiated nature. The presence of preserved chondrules has been reported for two IIE meteorites so far, Netschaëvo and Mont Dieu, which represent the most primitive silicate material within this group. In this study, silicate inclusions from two samples of Netschaëvo were examined. Both silicate inclusions are characterized by a porphyritic texture dominated by clusters of coarse‐grained olivine and pyroxene, set in a fine‐grained groundmass that consists of new crystals of olivine and a glassy appearing matrix. This texture does not correspond to the description of the previously examined pieces of Netschaëvo, which consist of primitive chondrule‐bearing angular clasts. Detailed petrographic observations and geochemical analyses suggest that the investigated samples of Netschaëvo consist of quenched impact melt. This implies that Netschaëvo is a breccia containing metamorphosed and impact‐melt rock (IMR) clasts and that collisions played a major role in the formation of the IIE group.

Reconstructing the transport history of pebbles on Mars.

The discovery of remarkably rounded pebbles by the rover Curiosity, within an exhumed alluvial fan complex in Gale Crater, presents some of the most compelling evidence yet for sustained fluvial activity on Mars. While rounding is known to result from abrasion by inter-particle collisions, geologic interpretations of sediment shape have been qualitative. Here we show how quantitative information on the transport distance of river pebbles can be extracted from their shape alone, using a combination of theory, laboratory experiments and terrestrial field data. We determine that the Martian basalt pebbles have been carried tens of kilometres from their source, by bed-load transport on an alluvial fan. In contrast, angular clasts strewn about the surface of the Curiosity traverse are indicative of later emplacement by rock fragmentation processes. The proposed method for decoding transport history from particle shape provides a new tool for terrestrial and planetary sedimentology.