Erosion Of Rocks Research Articles

Field testing study on the rainfall thresholds and prone areas of sandstone slope erosion at Mogao Grottoes, Dunhuang.

The Dunhuang Mogao Grotto is a famous Buddhist monument and was inscribed in the list of world cultural heritage sites by UNESCO in 1987. Water poses a major threat to the preservation of this heritage even though it is located in an arid region. This study was conducted to investigate the effect of rainfall on rock erosion. Specifically, the formation mechanism of slope runoff and the erosion threshold of rainfall were analyzed, and erosion-prone areas of the site were identified. This was carried out using field artificial rainfall simulation testing, and the results inform methods of preventing rainfall-induced cliff erosion. In addition, the rainfall threshold and erosion-prone areas obtained from the experiment were further validated and optimized using monitoring data for natural rainfall and historic documentation. The threshold value of erosive rainfall obtained by empirical statistical analysis method was found to be similar to that obtained by the runoff generation mechanism. The areas identified as prone to erosion using field tests coincided with areas of historic erosion as recorded in site documentation. Furthermore, the forecast grade of cliff slope erosion and its erosion-prone areas are determined after comprehensive analysis of the results obtained by these two methods. The research results are critical for the monitoring, early warning, and prevention of cliff slope erosion. The research methods can also be used as reference in areas for which rainfall data are missing.

Sources, Distribution, and Fluxes of Major and Trace Elements in the Yangtze River

Surface water samples were collected from 20 sampling sites in the main stream and its major tributaries of the Yangtze River from April to May 2017. The concentrations of dissolved trace and major elements were analyzed to determine the spatial variation, source identification, and riverine fluxes using various multivariate statistical techniques, including correlation analysis, principal component analysis (PCA), and cluster analysis (CA) with the goal of determining the influence of natural factors and human activities, including the operation of the Three Gorges Dam on the distribution and loading of major and trace elements in the Yangtze River water environment. Spatial distribution results showed that Cu, Zn, Pb, Cd, and As were the major elements affected by human activities in the Yangtze River, and their concentrations downstream were significantly higher than those in the middle and upper reaches (P<0.05). All elements had fairly high concentration values in both channels of the Yangtze River mainstream in Chongqing city and Hanjiang River in Wuhan city, which were mainly related to the enhanced human activities. However, the low concentrations of multi-elements in the reach of the Yangtze River in Yichang were largely caused by the retention effect of Three Gorges Project on element transport, which decreased the riverine loadings of multi-elements. Principal component analysis (PCA) indicated that Na, Mg, K, Ca, Fe, Mn, Co, Ni, Mo, Cr, and V were mainly associated with the weathering and erosion of various rocks and minerals in the river basin. And Cu, Zn, and Pb were mainly affected by enhanced human activities, such as industrial wastewater, metal smelting, and mineral mining, whereas Cd and As were mainly related to agricultural activities. The spatial distribution of trace and major elements showed that concentrations of some elements in the Yangtze River channels were enhanced by human activities. Generally, the heavy metal pollution in the Yangtze River Basin was lower than that in other rivers of the world. However, the annual fluxes of Cu, Zn, Pb, Cd, and As could have far-reaching ecological effects on the Yangtze River estuary and offshore ecological environment.

Observation and Simulation of Solid Sedimentary Flux: Examples From Northwest Africa

AbstractThe sedimentary archive preserved at passive margins provides important clues about the evolution of continental topography. For example, histories of African uplift, erosion, and deposition of clastic sedimentary rock provide information about mantle convection. Furthermore, relating histories of uplift and erosion from regions where sediment is generated to measurements of efflux is important for understanding basin evolution and the distribution of natural resources. We focus on constraining Mesozoic to Recent solid sedimentary flux to northwest Africa's passive margin, which today is fed by rivers draining dynamically supported topography. Histories of sedimentary flux are calculated by mapping stratigraphy using seismic reflection and well data courtesy of Tullow Oil Plc and TGS. Stratigraphic ages, conversion from two‐way time to depth and compaction, are parameterized using biostratigraphic and check‐shot records from exploration, International Ocean Discovery Program and Deep Sea Drilling Project wells. Results indicate that Late Cretaceous to Oligocene (∼100–23 Ma) sedimentary flux decreased gradually. A slight increase in Neogene sedimentary flux is observed, which is concomitant with a change from carbonate to clastic sedimentation. Pliocene to Recent (∼5–0 Ma) flux increased by an order of magnitude. This history of sedimentary flux and facies change is similar to histories observed at other African deltas. To constrain sources of sedimentary flux, 14,700 longitudinal river profiles were inverted to calculate a history of continental uplift. These results were used to parameterize a simple “source‐to‐sink” model of fluvial erosion and sedimentary efflux. Results suggest that increased clastic flux to Africa's deltas from ∼30 Ma was driven by denudation induced by dynamic support.

On the origin of both a recent and submerged tombolo on Prvić Island in the Kvarner area (Adriatic Sea, Croatia)

This paper analyzes the origins of two tombolos (one recent and another fossil/submerged) on the southwestern coast of Prvic Island, which is located in the Kvarner area in the northeastern part of the Adriatic Sea. A recent tombolo on Cape Pipa was formed by the erosion of Quaternary deposits and Palaeogene siliciclastic rocks. The submerged tombolo is much bigger, clearly visible on the sea bed toward the southwest. The conditions for forming a submerged and recent tombolo have occurred during a slow rise and then stagnation of sea level of the Adriatic Sea in the Holocene. The sea flooded the fossil tombolo probably in the final part of a period of rapid sea-level rise at the beginning of the Holocene when a large proportion of the Quaternary sediments were eroded. Waves from the northwest (tramuntana) and southeast (jugo) refract and diffract around the tombolo. The nourishment of the beach body happens permanently on both sides of the cape. Accumulated sediments are protected by resistant rocky blocks of breccia on the peak of Cape Pipa, acting as a natural tombolo. Due to the fact that wave directions are perpendicular to the beach coastline, they do not generate longshore currents that would erode sediment in beach bodies around Cape Pipa. Therefore, the recent tombolo is probably stable in the present climatic and oceanographic conditions in the Kvarner area.

Early Cretaceous sedimentary provenance and structural evolution of the central Sanandaj–Sirjan Zone, Iran: implications for palaeogeographic reconstructions of the northern Neo-Tethyan margin

ABSTRACT The Early Cretaceous was an important epoch in the evolution of the Earth system in which major tectonic episodes occurred, especially along the Alpine–Himalayan belt. The paucity of reliable palaeogeographic data from the central segment of this geological puzzle, however, hampers the reconstruction of a panoramic view of its Early Cretaceous palaeogeography and geodynamic setting. Here we present multidisciplinary provenance data from Lower Cretaceous strata of the overriding plate of the Neo-Tethyan subduction zone (the Sanandaj–Sirjan Zone; SSZ, of central Iran), including structural, basin-fill evolution, petrographic and geochemical analyses. Sandstone provenance analysis of Lower Cretaceous red beds suggests the occurrence of sub-mature litho-quartzose sandstones attributed to an active continental arc margin in convergent setting predominantly derived from plutonic, quartzose sedimentary and metamorphic rocks exposed in the central SSZ. Weathering indices indicate moderate chemical weathering in the source area which may be related to close source-to-sink relationships or arid climate. Our palaeogeographic reconstructions and original geological mapping indicate that the erosion of uplifted basement rocks exposed in horst blocks provided the sediment sources for the syn-extensional deposition of uppermost Jurassic–lowermost Cretaceous conglomerates and Lower Cretaceous siliciclastic red beds within a continental retro-arc basin during initiation of the ‘Neo-Tethys 2ʹ. The polyphase tectonic reactivation along the principal fault of the study area controlled the syn- and post-extensional tectonostratigraphic evolution that reflect the corresponding mechanical decoupling/coupling along the northern Neo-Tethyan plate margin.

Influence of frequency change during sandstone erosion by pulsed waterjet

ABSTRACTPulsating water jet technology for rock erosion application has been emphasized in this study. The erosion phenomena are explored as an eroded depth, eroded width and volume eroded of the sandstone at frequency f = 20,000–40,000 Hz by using pulsed water jet. The surface was eroded at pressure 20–40 MPa; standoff distance 20 mm; nozzle diameter 1.6 mm; and feed rate of the jet 125–200 mm/s. The eroded depth, eroded width, and volume of the eroded sections were evaluated through non-contact-type optical profilometer. The result shows the relationship between feed rate of the jet, pressure, and standoff distance, which affects the erosion depth and width and volume removal. At f = 40,000 Hz, the maximum value of erosion depth, width, and volume removal was obtained. The microstructural analysis Field Emission Scanning Electron Microscopy (FESEM) examined the erosion topography, plastic deformation, and failure analysis at a frequency f = 20,000–40,000 Hz. The results reveal that the frequency change affects the erosion, and failure was observed at 40,000 Hz. This is the result of the deeper and narrow eroded surface caused by the enhanced number of impacts (200–320 impact per mm) and concentrated stream of a jet at 40,000 Hz.

Assessment of rock mass erosion in unlined spillways using developed vulnerability and fragility functions

ABSTRACT Hydraulic power can lead to the erosion of rock and cause dams to be at risk of failure. Methods exist to predict the degree of erosion for rock masses in spillways; however, these deterministic approaches are unable to consider the uncertainties of rock mass parameters. We develop a methodology that determines the vulnerability of a rock mass to hydrological erosion, and this approach takes into consideration the uncertainties related to the parameters of the rock mass at the study site. Monte Carlo simulation is used to create a dataset for each class of rock for then developing fragility and vulnerability curves. The effects of each geomechanical parameter on erosion level can be determined by applying this methodology to individual spillway sites. As a result, sensitivity analysis shows that the discontinuity orientation factor is a critical parameter for explaining the erosion of a rock mass; increasing this parameter decreases the vulnerability of the rock mass to erosion. Our approach has an advantage over deterministic methods as the uncertainties of rock mass parameters have been considered. The error associated with the predicted erosion level via our probability-based approach is significantly less than the error obtained via deterministic methods.

A reconnaissance provenance study of Triassic–Jurassic clastic rocks of the Russian Barents Sea

ABSTRACTCombined U–Pb detrital zircon dating of 21 samples, along with whole-rock chemical composition and Sm–Nd isotopic studies of 39 samples of Triassic and Jurassic rocks from Franz Josef Land and wells in the southern part of the Russian (eastern) Barents Sea, were analyzed for a reconnaissance provenance study. The similarity of detrital zircon age distributions was statistically assessed using the Kolmogorov–Smirnov (K–S) test and points to a common source area for the clastic material of Triassic to Middle Jurassic age.Uralian-age detrital zircons predominate in all samples, with a comparably smaller portion of Caledonian- and Timanian-age detrital zircons. The number of Palaeoproterozoic and Archean grains is very small and becomes significant only in a few Jurassic samples. εNd(t) values gradually decrease from −1.5 to +2.5 in Lower Triassic rocks, to −2.0 to −8.2 in Jurassic rocks, suggesting an increasing influence of ancient metamorphic basement erosion in the younger Jurassic rocks. High Co/Th ratios, suggesting the erosion of mafic rocks, were mainly recorded in Lower Triassic rocks, whereas increasing Th/Sc ratios, suggesting the erosion of felsic rocks, were recorded only in some uppermost Triassic and Jurassic rocks.We identify the Urals and, in addition during the Triassic, the basement of the West Siberian Basin as the main provenance for the studied clastic rocks. By contrast, only a small volume of fine-grained clastic detritus was derived from basement erosion of the East European Craton, which was characterized by a subdued relief during this time.

Role of inherent geological and climatic characteristics on landscape variability in the tectonically passive Western Ghat, India

How does lithological variation affect the landscape and its evolution is a fundamental issue in modern and post-orogenic terrains. In the absence of active tectonic uplift, spatial variation of rock resistance and rainfall distribution are thought to primarily control the fluvial and hillslope processes. However, these lithologic and climatic controls are difficult to decouple owing to the fact that these post-orogenic terrains endure long and complex geomorphic history. To examine the relative importance of lithology and rainfall in a post-orogenic setting, we analyzed west-flowing drainage basins in the Western Ghat (WG) with a multi-proxy analysis. We relied on digital topography, sediment yield and hydrological data. We evaluated topographic relief, hypsometry, river-long profiles and sediment yield to elaborate the importance of spatial variability of rock resistance in the long term evolution of these drainage basins. The dominant increase of relief (∼200–1500m) and decrease of sediment yield (3.32–1.91ton/month/km2) from north to south of the escarpment is directly attributed to the rock erodibility and structural characteristics. Field verification suggests that despite high rainfall condition, the channels are mostly detachment limited and therefore lack the abrasive tools. We further showed that the harder lithologies sustain higher steepness. On the contrary, the spatial distribution of satellite-derived rainfall showed a negligible effect on the long profile evolution. Our analysis is also supported by the existing knowledge of chemical denudation rate and implicates that rock hardness and weatheribility defines the topographic characteristics with little or no effect of rainfall in this passive margin setting.

Sources of elevated heavy metal concentrations in sediments and benthic marine invertebrates of the western Antarctic Peninsula

Antarctica is one of the least anthropogenically-impacted areas of the world. Metal sources to the marine environment include localised activities of research stations and glacial meltwater containing metals of lithogenic origin. In this study, concentrations of nine metals (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) were examined in three species of benthic invertebrates collected from four locations near Rothera Research Station on the western Antarctic Peninsula: Laternula elliptica (mudclam, filter feeder), Nacella concinna (limpet, grazer) and Odontaster validus (seastar, predator and scavenger). In addition, metals were evaluated in sediments at the same locations. Metal concentrations in different body tissues of invertebrates were equivalent to values recorded in industrialized non-polar sites and were attributed to natural sources including sediment input resulting from glacial erosion of local granodioritic rocks. Anthropogenic activities at Rothera Research Station appeared to have some impact on metal concentrations in the sampled invertebrates, with concentrations of several metals higher in L. elliptica near the runway and aircraft activities, but this was not a trend that was detected in the other species. Sediment analysis from two sites near the station showed lower metal concentrations than the control site 5 km distant and was attributed to differences in bedrock metal content. Differences in metal concentrations between organisms were attributed to feeding mechanisms and habitat, as well as depuration routes. L. elliptica kidneys showed significantly higher concentrations of eight metals, with some an order of magnitude greater than other organs, and the internal structure of O. validus had significantly higher Ni. This study supports previous assessments of N. concinna and L. elliptica as good biomonitors of metal concentrations and suggests O. validus as an additional biomonitor for use in future Antarctic metal monitoring programs.

Dip, layer spacing, and incision rate controls on the formation of strike valleys, cuestas, and cliffbands in heterogeneous stratigraphy

AbstractLandscapes developed over heterogeneous stratigraphy exhibit a spectrum of landforms from dramatic cliffbands to hogbacks, depending on the dip and spacing of the layers. In deeply incised landscapes, a single cliffband may consist of multiple resistant layers, whereas similar stratigraphy elsewhere is separated by strike valleys into individual cuesta benches or hogbacks. This paper presents a geometric analysis, informed by a numerical landscape model, to explain the conditions for development of a strike valley floored by erodible rocks. The results define a threshold incision rate below which strike valleys are more likely to form; this threshold incision rate is proportional to the stratigraphic spacing of cliff-forming layers and a trigonometric function of dip angle. The analysis also yields a time scale for the adjustment of structural landforms to changes in regional incision rate, which is a function of dip angle and the coupling between cliff retreat rate and escarpment height. In example landscapes of the Colorado Plateau, this time scale is likely much longer than that of documented variations of incision rates due to late Quaternary climate and land-use changes. The transitional state of escarpments in layered rock may therefore contain information about regional downcutting rates over time scales different from those recorded by the fluvial network. The utility of such features will require better understanding of the coupling between incision of a foot slope and the retreat rate of the cliff above in different kinds of rocks.

Structural control on karst water circulation and speleogenesis in a lithological contact zone: The Bossea cave system (Western Alps, Italy)

Geological surveys, structural mapping, and thin section analysis helped to unravel the speleogenesis and water circulation in the Bossea cave (Northern Italy), an unconventional karst system at the contact between carbonates and underlying insoluble rocks. The surveys within the cave and in the surrounding surface outcrops revealed a complex structural setting caused by the coupling of lithological-mechanical heterogeneities with two major regional left-lateral ESE-WNW strike-slip faults. Transpression associated with the faults led to a disharmonic deformation, accommodated by a detachment surface, between the buckled basement meta-volcanics and the disharmonically-folded-marble sequence laying on top. Low-angle shear zones, recumbent or drag folds, and small stacks of duplexes are localized next to this detachment. The preferential pathways for water circulation inside the vadose zone are the folded bedding interfaces in the meta-carbonate sequence. Fracture clusters at slipping bed terminations, on the other hand, provide through-sequence connectivity. Once underground stream water flow reached the cataclastic and highly deformed meta-volcanics of the detachment core and damage zones, erosion started, removing large volumes of rocks. The downward erosion of the Permian basement rocks caused gravitational instability and subsequent roof collapses, aided by structural control in the damage zone above the detachment surface (slip surfaces in between duplexes or drag folds). This gravitational instability ultimately led to the formation of the giant halls that are characteristic of the downstream part of the Bossea cave.

Effects of arbuscular mycorrhizal fungi on inoculated seedling growth and rhizosphere soil aggregates

Arbuscular mycorrhizal fungi (AMF) inoculation have been shown to enhance host plant growth and rhizosphere soil aggregates largely due to the combined effect of extraradical hyphae and glomalin-related soil proteins but their effects on rocky desertified soils remain unknown. Extensive karst area of southwest China has undergone rocky desertification because of soil erosion and vegetation degradation. Karst soils in this area are mainly composed of silty clay loam developed from carbonate rock erosion under subtropical humid monsoon climate. We tested the hypothesis that AMF colonization of tree seedlings would improve water stable aggregates of degraded karst soil and enhance seedling growth. Seedlings of two tree species, Toona sinensis and Delavaya toxocarpa suitable for afforestation in karst area, were inoculated separately with three AMF, Funneliformis mosseae, Glomus versiforme and Rhizophagus intraradices in pots containing rocky desertified soil. Non-inoculated seedlings were used as control. After 12 months we found that the percentage of water stable aggregates (diameter >0.25 mm; WSA>0.25) in the rhizosphere soil of the inoculated seedlings was increased by 15.6–20.5% compared to control. AMF colonization significantly increased the quantity of aggregates with diameter > 2 mm compared to control but soil aggregates of size class 0.25 – 0.5 mm decreased significantly in inoculated seedlings, regardless of tree species. Shoot biomass of inoculated T. sinensis and D. toxocarpa seedling was increased by 15.7–21.2% and 8.5–20.4%, respectively compared to non-inoculated seedlings. In contrast, AMF inoculation minimally influenced the root biomass of the two tree species. The easily extractable glomalin (EEG) content and mean weight diameter (MWD) of rhizospheric soil of the inoculated seedlings increased by 28.9–75.1% and 31.7–48.1%, respectively compared to control. The AMF colonization was highly correlated with WSA>0.25, EEG and MWD. Overall results support our hypothesis that AMF colonization enhance the quantity of rhizospheric soil aggregates and improve the growth of host seedlings. We conclude that planting tree species inoculated with AMF has a potential for restoration of degraded karst ecosystem. In future research other locally adapted tree and shrub species should be tested following AMF inoculation to prepare a wider planting stocks to be used in restoration of degraded karsts.

Особливості охорони ґрунтів від ерозійної деградації та забруднення в промислових регіонах

The purpose. To find complex solution of a problem of water erosion and pollution of soils, caused by industrial wastes. Methods. Analysis of maps and sharts, monitoring of erosion processes on waste dumps of coal mines, field observation and measurement, probe of water permeability of rock refuse by method of tubes, physical simulation of blowing erosion of rock using laboratory aerodynamic device. Analytical-synthetic methods in view of fund, statistical materials and normative-legal baseline. Results. In natural conditions it is determined that from waste dumps of Public joint-stock company «Lisichanskugol» in Lugansk oblast at absence of protective plant cover as a result of water erosion about 3500 t/hectare of rock during their existence or up to their afforestation are carried out. Potential removal of rock at blowing erosion exceeds 150 t/hectare. It is shown that afforestation improves water permeability of rock in 5 times. That is the reliable factor of termination of water-erosion process and prevention of removal of pollutants in visual environment. It is proved that there is a legal opportunity and ecological expediency of including terraces in circuits of local ecological nets with their subsequent afforestation. Necessity of use in eco-net as ecological alleyways, besides fluvial net, of primary hydrographic network in the form of hollows is justified. The generality of use of terraces in local eco-net and implementation of contour-meliorative architecture of agro-landscapes on separate operations and as the complex factor of protection of soils from erosion and pollution is shown. Conclusions. On an instance of Lugansk oblast it is shown that inclusion of terraces of coal mines and mining factories in local ecological nets with the status of rebuilt terrains will stimulate their afforestation, promoting thorough lowering of removal from them of pollutants. For complex solution of problem of erosive degradation of soils and spread of pollutants in agro-landscapes it is necessary to implement contour-meliorative architecture.

Geochemical evidence of anthropogenic activity in western Mesoamerica since the Classic Period

Sediments of the La Colorada Lake provided evidence of anthropogenic perturbations in western part of the Mesoamerica over the last 2000 years. Comparison of chemical compositions of these sediments and sediments from a nearby reference site, containing both natural and perturbed deposits, suggested that erosion of the felsic rocks occurred under humid conditions during the terminal Formative Period (100 BCE–100 CE) and it caused natural sedimentation in the lacustrine basin. Deposition of sediments sourced from mixed provenances (felsic and mafic rocks) increased since the Classic Period (>100 CE). Enhancement in anthropogenic activities related to agricultural practices of the Classic and Epiclassic Periods triggered transportation of sediments from the nearby as well as long-distance sources. Farming activities and organic waste discharge of the Industrial Period (1600–2017 CE) caused the enrichment of P and Ca.

Modeling of Dynamic Rock–Fluid Interaction Using Coupled 3-D Discrete Element and Lattice Boltzmann Methods

Scour of rock is a challenging and interesting problem that combines rock mechanics and hydraulics of turbulent flow. On a practical level, rock erosion is a critical issue facing many of the world’s dams at which excessive scour of the dam foundation or spillway can compromise the stability of the dam resulting in significant remediation costs, if not direct personal property damage or even loss of life. This interaction between the blocky rock mass and water is analyzed by directly modeling the solid and fluid phases—the individual polyhedral blocks are modeled using the discrete element method (DEM) while water is modeled using the lattice Boltzmann method (LBM). The LBM mesh is entirely independent of the DEM discretization, making it possible to refine the LBM mesh such that transient and varied fluid pressures acting on the rock surfaces are directly modeled. This provides the capability to investigate the effect of water pressure inside the fractured rock mass, along potential sliding planes, and can be extended to rock falls and slides into standing bodies of water such as lakes and reservoirs. Results show that the coupled DEM–LBM implementation is able to accurately capture the interaction between polyhedral rock blocks and fluid by analytically solving for the solid volume fraction in the coupling computations using convex optimization and simplex integration; however, further performance improvements are necessary to simulate realistic, field-scale problems. Particularly, adaptive mesh refinement and multigrid methods implemented in a parallel computing environment will be essential for capturing the highly computationally intensive and multiscale nature of rock–fluid interaction.

Dust provenance in Pan-third pole modern glacierized regions: What is the regional source?

To differentiate the source of aeolian dust between the desert sources from Pan-third pole and high mountain glaciers, therefore, we investigated the spatial variability of aeolian dust sources in the Pan-third polar region. The question of whether such changes reflect variable transport pathways from a unique source in the western China area was addressed. That is, the SrNd radiogenic isotope composition of modern desert samples do not support the hypothesis of a single dust provenance at higher elevation mountain glaciers by long-distance transport; regional sources also play a significant role. Based on previous studies and the data from this study, the five isotopic regions were divided, which are controlled by the geological characteristics in western China. The results suggest that mineral dust deposited into the high-mountain glaciers originated from the free ice region because of glacier melting and the physical and chemical erosion of rocks from the surrounding mountains by local wind systems. The Pb isotopic data further demonstrated that natural dust is the source of Pb for the high-mountain glaciers of Pan-third pole. These results provide an exhaustive documentation of the isotopic signature of the regional dust reaching the glacier regions.

Mineralogy of Zirconium in Iron-Oxides: A Micron- to Nanoscale Study of Hematite Ore from Peculiar Knob, South Australia

Zirconium is an element of considerable petrogenetic significance but is rarely found in hematite at concentrations higher than a few parts-per-million (ppm). Coarse-grained hematite ore from the metamorphosed Peculiar Knob iron deposit, South Australia, contains anomalous concentrations of Zr and has been investigated using microanalytical techniques that can bridge the micron- to nanoscales to understand the distribution of Zr in the ore. Hematite displays textures attributable to annealing under conditions of high-grade metamorphism, deformation twins (r~85˚ to hematite elongation), relict magnetite and fields of sub-micron-wide inclusions of baddeleyite as conjugate needles with orientation at ~110˚/70˚. Skeletal and granoblastic zircon, containing only a few ppm U, are both present interstitial to hematite. Using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analysis and mapping, the concentration of Zr in hematite is determined to be ~260 ppm on average (up to 680 ppm). The Zr content is, however, directly attributable to nm-scale inclusions of baddeleyite pervasively distributed throughout the hematite rather than Zr in solid solution. Distinction between nm-scale inclusions and lattice-bound trace element substitutions cannot be made from LA-ICP-MS data alone and requires nanoscale characterization. Scandium-rich (up to 0.18 wt. % Sc2O3) cores in zircon are documented by microprobe analysis and mapping. Using high-angle annular dark field scanning transmission electron microscopy imaging (HAADF-STEM) and energy-dispersive spectrometry STEM mapping of foils prepared in-situ by focused ion beam methods, we identify [011]baddeleyite epitaxially intergrown with [22.1]hematite. Lattice vectors at 84–86˚ underpinning the epitaxial intergrowth orientation correspond to directions of r-twins but not to the orientation of the needles, which display a ~15˚ misfit. This is attributable to directions of trellis exsolutions in a precursor titanomagnetite. U–Pb dating of zircon gives a 206Pb/238U weighted mean age of 1741 ± 49 Ma (sensitive high-resolution ion microprobe U–Pb method). Based on the findings presented here, detrital titanomagnetite from erosion of mafic rocks is considered the most likely source for Zr, Ti, Cr and Sc. Whether such detrital horizons accumulated in a basin with chemical precipitation of Fe-minerals (banded iron formation) is debatable, but such Fe-rich sediments clearly included detrital horizons. Martitization during the diagenesis-supergene enrichment cycle was followed by high-grade metamorphism during the ~1.73–1.69 Ga Kimban Orogeny during which martite recrystallized as granoblastic hematite. Later interaction with hydrothermal fluids associated with ~1.6 Ga Hiltaba-granitoids led to W, Sn and Sb enrichment in the hematite. By reconstructing the evolution of the massive orebody at Peculiar Knob, we show how application of complimentary advanced microanalytical techniques, in-situ and on the same material but at different scales, provides critical constraints on ore-forming processes.

The Macururé Complex (Sergipano Belt, NE Brazil) in southern Alagoas state: Geology and geochronology

The Sergipano Belt is part of the Southern Subprovince of the Borborema Province (NE Brazil) and separates the Pernambuco-Alagoas Domain from the Sao Francisco Craton. The Macurure Domain is the largest unit of the Sergipano Belt and comprises a basement of Paleoproterozoic age (Jirau do Ponciano Dome) and a supracrustal succession (Macurure Complex). Here we present the results of field, petrographic and structural work and of geochronological dating conducted in an area in the southern part of Alagoas state, northeast of the city of Sao Bras. The lithological associations of the Macurure Complex include quartzite, schist and banded metasedimentary rocks (metarhythmites). The main foliation shows a gentle dip to the southwest and is associated with a SW-plunging mineral lineation. The mineral assemblage muscovite+biotite+garnet±kyanite±staurolite in schist and metarhythmite indicates medium pressure and temperature conditions (amphibolite facies) typical of Barrovian metamorphism. U-Pb dating of detrital zircon grains from a quartzite sample yielded dominantly Tonian ages, showing a large age peak at c. 985 Ma. These zircon grains are thus derived from sources related to the Cariris Velhos event. The youngest grain has a 206Pb/238U age of 939±36 Ma (2s), which is considered as the maximum age of deposition. Two smaller age populations of c. 2.0 Ga and 1.1 Ga are also present. The most probable provenance of c. 2.0 Ga zircon grains is the nearby Jirau do Ponciano Dome. The other age group is more enigmatic and could be related to erosion of A-type rocks formed during local extensional events preceding the Cariris Velhos event.

Watersheds Morphometry and Structural Interpretation of Lineaments Extracted from SRTM Data in the Mayo-Kani Division Far-Nord Region, (Cameroon)

In the Far-North Region of Cameroon, the high rate of weathering and erosion of rocks do not allow to map the hydrological and geological structures in the field. This paper aims to test the combined use of field topographic data and Shuttler Radar Topography Mission (SRTM) data to define watershed morphometry, highlight the different erosion factors, and characterize the structural lineaments of the Mayo-Kani Division, within the Far-North Region of Cameroon. All these are done using; watersheds, hydrographic network, slopes, manual lineaments and automatic lineaments maps. These treatment techniques used (filtering, enhancement and slope analyses) allows: to morphologically characterize the ten identified Mayo-Kani watersheds, to measure the erodibility rate of the watersheds using the Universal Soil Loss Equation (USLE) and to compute the fracture maps. This last operation was validated using ground truth field observations and the literature review. Fracture networks were analyzed using statistical analysis techniques by studying fracture length distribution laws.