Bedrock Lithology Research Articles

Upwelling-induced organic matter enrichment of the Upper Permian Dalong Formation in the Sichuan Basin, SW China and its paleoenvironmental implications

The organic-rich siliceous rock was developed worldwide during the Late Permian which contains plenty of information about paleoenvironment. This study reveals the vertical variation in organic matter enrichment of the Upper Permian Dalong Formation in the northern Sichuan Basin, which was related to the expansion of the Paleo-Tethys Ocean induced transgression and upwelling in the northern continental margin of the Upper Yangtze. The Dalong Formation was divided into three units from bottom to top on the basis of lithology and organic matter abundance: the lower calcareous mudstone (Unit A), the middle organic-rich siliceous shales (Unit B), and the upper marlstones (Unit C), with average TOC contents of 4.09%, 6.72%, and 1.20%, respectively. The Unit A was deposited at the early stage of the transgression under seasonal upwelling settings with moderate paleoproductivity evidenced by the moderate-high Cd/Mo ratios and concentration of P, Cu and Zn. The Unit B shows extremely high Cd/Mo ratios and Ni concentration indicating the perennial upwelling and high paleoproductivity induced by the extensive transgression. The Unit C was deposited under similar hydrographic conditions with the Unit A but lower paleoproductivity due to the effect of the latest Permian mass extinction. In addition, the redox conditions varied with the paleoproductivity and positive correlations between P and redox proxies (e.g. VEF, NiEF, ZnEF and CuEF) were observed (the Unit A: moderate paleoproductivity and suboxic-anoxic conditions, the Unit B: high productivity and anoxic-euxinic conditions, the Unit C: low-moderate productivity and oxic-suboxic conditions), which indicate that nutrients brought to primary producers by the upwelling during the transgression stimulate blooms and accelerate the consumption of oxygen leading to the anoxic-euxinic bottom water conditions. During the transgression, differences in types and strength of the upwelling result in the variation of paleoproductivity and redox conditions, and thus the organic matter abundance.

Cenozoic Tectonic Deformation Along the Pontarlier Strike‐Slip Fault Zone (Swiss and French Jura Fold‐and‐Thrust Belt): Insights From Paleostress and Geomorphic Analyses

AbstractWe present insights into the Cenozoic tectonic evolution of the region around the Pontarlier strike‐slip fault zone within the Jura fold‐and‐thrust belt by combined study of paleostress and geomorphic analyses. A preliminary separation of heterogeneous fault‐slip data and bedding‐tilt correction was performed before determining the paleostress axes orientations. The paleostress results provide evidence of multiphase deformation history consisting of four successive events. These events include a strike‐slip stress regime with ∼N‐S directed compression, a NW‐SE directed extensional stress regime, a NW‐SE trending compressional stress regime, and a strike‐slip stress regime with a mean NW‐SE directed compression. The directions of extension and compression for the second and third events are consistent with the general direction of maximum horizontal far‐field stress near the northern segment of the Pontarlier fault and generally deviate from those found near the southern segment.Geomorphic analysis of selected rivers along the southern segment of the Pontarlier fault, together with sub/surface geological observations provides new clues on Late Cenozoic strike‐slip reactivation of inherited extensional structure of Mesozoic age. These clues include (i) anomalous zones of high gradient within uniform bedrock lithology that spatially coincide with an obvious change in the depth to top Mesozoic layers; and (ii) right‐lateral offset of river channels, which is consistent with NW‐SE compression related to the youngest paleostress and current stress field. A combination of paleostress and geomorphic analyses provides a useful complementary approach for unraveling the Cenozoic tectonic evolution of the region characterized by a general lack of outcrop of Cenozoic rocks.

Testing Late Bronze Age mobility in southern Sweden in the light of a new multi-proxy strontium isotope baseline of Scania.

The Bronze Age of Sweden’s southernmost region, Scania, is complex and intriguing. One could say that Scania represented in many ways a gateway for people, ideas and material culture connecting continental Europe with Sweden. Shedding light on the dynamics of human mobility in this region requires an in depth understanding of the local archaeological contexts across time. In this study, we present new archaeological human data from the Late Bronze Age Simris II site, located in an area of Scania showing a dynamic environment throughout the Late Bronze Age, thus likely involving various forms of mobility. Because the characterization of solid strontium isotope baselines is vital for delineating human mobility in prehistory using the strontium isotope methodology, we introduce the first environmentally based multi-proxy (surface water-, plant- and soil leachates) strontium isotope baselines for sub-regions of Scania. Our results show, that the highly complex and spatially scattered lithologies characterising Scania does not allow for a spatially meaningful, geology-based grouping of multi-proxy data that could be beneficial for provenance studies. Instead, we propose sub-regional baselines for areas that don’t necessarily fully correspond and reflect the immediate distribution of bedrock lithologies. Rather than working with a Scania-wide multi-proxy baseline, which we define as 87Sr/86Sr = 0.7133 ± 0.0059 (n = 102, 2σ), we propose sub-regional, multi-proxy baselines as follows: Area 1, farthest to the north, by 87Sr/86Sr = 0.7184 ± 0.0061 (n = 16, 2σ); Area 2, comprising the mid and western part of Scania, with 87Sr/86Sr = 0.7140 ± 0.0043 (n = 48, 2σ); Area 3–4, roughly corresponding to a NW-SE trending zone dominated by horst-graben tectonics across Scania, plus the carbonate dominated south western part of Scania with 87Sr/86Sr = 0.7110 ± 0.0030 (n = 39, 2σ). Our results also reflect that the complexity of the geology of Scania requires systematic, high density, statistically sound sampling of multiple proxies to adequately constrain the baseline ranges, particularly of those areas dominated by Precambrian lithologies. The averaging effect of biosphere Sr in surface water might be beneficial for the characterization of baselines in such terranes. Our sub-regional, area-specific baselines allow for a first comparison of different baseline construction strategies (single-proxy versus multi-proxy; Scania-wide versus sub-regional). From the Late Bronze Age Simris II site, we identified six individuals that could be analysed for Sr isotopes, to allow for an interpretation of their provenance using the newly established, environmental strontium isotope baselines. All but one signature agrees with the local baselines, including the 87Sr/86Sr value we measured for a young individual buried in a house urn, typically interpreted as evidence for long distance contacts. The results are somewhat unexpected and provides new aspects into the complexity of Scandinavian Bronze Age societies.

Quantification of Gravitational Mass Wasting and Controls on Submarine Scarp Morphology Along the Roseau Fault, Lesser Antilles

AbstractDue to challenges involved in mapping the seafloor at high‐resolution (e.g., <2 m), data are lacking to understand processes that control the evolution of submarine normal fault scarps, which cover large parts of the global seafloor. Here, we use data from autonomous deep‐sea vehicles to quantify local erosion and deposition associated with a pronounced tectonic surface scarp formed by slip on the submarine Roseau normal fault (Lesser Antilles). We use high‐resolution video imagery, photomosaics, and high‐resolution bathymetric data (0.1–10 m/pixel) to identify active erosional features on the scarp including channels, steep gullies, small scarps, and debris cones. We compare volumes of erosion and deposition and find that under certain depositional conditions, debris cones effectively record the erosion signal of mass wasting from the footwall with a ratio of hanging wall deposition to footwall erosion of 0.80. We use eroded volumes to estimate earthquake‐induced landslide erosion rates for the Roseau fault of 14–46 m Myr−1. Assuming mass wasting of the Roseau fault scarp is mostly coseismic, the erosion rates for the Roseau fault imply that submarine earthquake induced mass‐wasting can occur at similar rates to various terrestrial lithological and tectonic settings. We present a process‐based model of submarine scarp degradation via retrogressive erosion in basement lithology where scarps have a gravitational stability threshold height of 20–40 m and a long‐term average slope of 30°–40°. More generally, the results presented here may be applicable to develop models of submarine landscape evolution based on degradation of normal fault scarps on the seafloor.

Lithological characterization of Ijebu-Ife, Southwest, Nigeria using aero-radioactivity data

In this work, aeroradioactivity (airborne radiometric) data was used to characterize the lithology of Ijebu Ife, Southwest Nigeria. Measurements were made using Sodium Iodide [NaI (Tl)] detector mounted on a fixed-wing aircraft by Nigerian Geological Survey. Geochemical information on potassium (40K), thorium (232Th) and uranium (238U) concentrations were extracted from the raw data after correction and normalization due to contributions from soil, atmosphere, aircraft and cosmic radiation. The data was further subjected to photogeological analysis and image-enhancing techniques using minimum curvature gridding with cell size of 25 m after which single band, ratio and ternary images were generated and used to characterize the lithology of the area. Six lithologies including granite lithology, shale lithology, sandstone lithology, Lekki lagoon lithology, schist and clay lithologies were characterized after combining the single band, ratio and ternary maps. Results of radiometric analysis showed the relative abundances of each radioelement concentration within each lithologic unit. 232Th had 16.33 ± 0.62 ppm as the highest mean value of radioelement abundance within the granite lithology with the range of 2.93–62.42 ppm. 40K had the lowest mean concentration of 0.04 ± 0.00% within the Lekki lagoon lithology. These values reflected the overall average concentration and the nature of bedrock lithologies within their respective location of occurrence. This work showed the usefulness of aeroradioactivity data in the characterization of lithologies, environmental monitoring and assessment in terms of radiological distributions and radionuclide concentrations.

An Environmental Perspective of Dracunculiasis and the Guinea Worm Eradication Programme on Human Behaviour in the Northern Region of Ghana

This article examines the role dracunculiasis (Guinea Worm Disease – GWD) and the Guinea Worm Eradication Programme (GWEP) played in changing community members’ behaviour towards the disease and their bio-physical environment. The research design that was used to collect data was the mixed method approach, in which questionnaire was administered, Focus Group Discussion (FGD) was held, and key informant interviews conducted. Krejcie and Morgan’s formula for determining sample population for research was used to determine the sample population of the study area, which was 860. It was found that bedrock lithology made community members to resort to unsafe sources of drinking water which exposed them to GWD. It was also realized that the educational programmes which the GWEP brought along with it changed community members’ behaviour towards their bio-physical environment, which ultimately resulted in eradicating the disease in the study area. It was therefore recommended that in carrying out disease eradication or control programmes, the environmental protection agency should be involved in the process of educating community members on methods of improving on the integrity of their natural and built environment to enhance the overall health status of the community.

Estimation of recharge in mountain hard-rock aquifers based on discrete spring discharge monitoring during base-flow recession

Estimation of aquifer recharge is key to effective groundwater management and protection. In mountain hard-rock aquifers, the average annual discharge of a spring generally reflects the vertical aquifer recharge over the spring catchment. However, the determination of average annual spring discharge requires expensive and challenging field monitoring. A power-law correlation was previously reported in the literature that would allow quantification of the average annual spring discharge starting from only a few discharge measurements in the low-flow season, in a dry summer climate. The correlation is based upon the Maillet model and was previously derived by a 10-year monitoring program of discharge from springs and streams in hard-rock aquifers composed of siliciclastic and calcareous turbidites that did not have well defined hydrogeologic boundaries. In this research, the same correlation was applied to two ophiolitic (peridotitic) hard-rock aquifers in the Northern Apennines (Northern Italy) with well-defined hydrogeologic boundaries and base-outflow springs. The correlation provided a reliable estimate of the average annual spring discharge thus confirming its effectiveness regardless of bedrock lithology. In the two aquifers studied, the measurable annual outputs (i.e. sum of average annual spring discharges) could be assumed equal to the annual inputs (i.e. vertical recharge) based on the clear-cut aquifer boundaries and a quick groundwater circulation inferable from spring water parameters. Thus, in such setting, the aforementioned correlation also provided an estimate of the annual aquifer recharge allowing the assessment of coefficients of infiltration (i.e. ratio between aquifer recharge and total precipitation) ranging between 10 and 20%.

Uraniferous Leucogranites in the Rössing Area, Namibia: New Insights from Geologic Mapping and Airborne Hyperspectral Imagery

Abstract This study combines historical exploration data with new mapping, underpinned by airborne hyperspectral imagery, to provide a detailed camp-scale geologic view of the Rössing uranium mine area in the Damara orogen, Namibia. The Neoproterozoic Damaran metasedimentary host rocks to uranium deposits of the Rössing area structurally overlie Paleoproterozoic basement rock. Both units were subjected to polyphase deformation and upper amphibolite to lower granulite facies metamorphism during Pan-African orogenesis. The sequence was voluminously intruded by leucogranites, where younger phases may contain ore-grade uranium as magmatic uraninite and traces of betafite, together with secondary uranium minerals. Early, postdepositional modifications to the Damaran sequence included partial dolomitization of marble units and development of evaporite dissolution and diapiric breccias. Major pre-D3 extensional structures developed in conjunction with recumbent, isoclinal folding and acted to focus the intrusion of early, mostly barren leucogranites generated primarily through anatexis of Damaran metasediments. Syn-D4 leucogranites overprint complex interference fold geometries that resulted from D3 deformation. D4 leucogranites were emplaced under predominantly ductile, transtensional conditions, into NNE-trending zones oriented highly oblique to all preexisting structures. These steeply dipping zones provided the prerequisite conditions for partial melt material to be derived from uraniferous basement lithologies. The concentration of magmatic uranium was promoted where leucogranite melt material interacted with carbonates and sulfide-bearing Damaran metasedimentary units. In the Rössing area these horizons occur at the Khan-Rössing Formation contact zone for the SJ, SK, SH, Z20, and Husab deposits and within and above the Arandis Formation for the Z19 deposit leucogranites.

Electrical resistivity tomography and digital aerial photogrammetry in the research of the “Bachledzki Hill” active landslide – in Podhale (Poland)

Podhale is a region in Poland at high risk of landslide. Sudden activation of landslides is the greatest threat to the local infrastructure. This work describes the joint use of electrical resistivity tomography (ERT) and aerial photogrammetry in the study of the active “Bachledzki Hill” landslide. In order to determine landslide dynamics, a digital version of stereo-pairs of aerial images from 1972 and 2004, as well as the airborne laser scanning (ALS) results from 2014 were used. Based on the data, first, three photogrammetric digital elevation models were generated. Then, in the ArcGIS software two differential models were generated (2004–1970 and 2014–1970) ground surface displacements in the landslide area and two other models illustrating the volume of the displaced material. Our research has shown that the landslide has been active to varying degrees since 1970. The landslide geometry, as well as the structure and lithology of the flysch bedrock were characterized using the ERT method. The resistivity images allowed the interpretation of the faults and lithology of the bedrock, as well as the determination of the depth and geometry of the landslide slip surface. Areas with water-logged rocks or rocks containing clay in the structure of the landslide are located. In this areas further movements may be generated. In addition, a voxel 3D ERT model of landslide and a model of rupture surface were generated. Both models allowed the spatial interpretation and visualization of the landslide structure and the geological structure of flysch rocks, as well as the spatial imaging of the surface rupture. The results of geophysical surveying made it possible to understand the cause of the activity of the landslide. The proposed comprehensive application of geophysical researches, geological modeling and digital aerial photogrammetry methods may become a common research methodology in the detailed study of landslides. • The aim of the photogrammetric research was to describe and understand the kinematics of landslides. • Geoelectrical methods, such as Electrical Resistivity Tomography, can be successfully used in landslide studies. • The 3D ERT voxel model facilitated geological interpretation. • Based on geophysical data and archival wells the surface of rupture model was developed using geostatistical methods.

Quantifying the competing influences of lithology and throw rate on bedrock river incision

AbstractRiver incision in upland areas is controlled by prevailing climatic and tectonic regimes, which are increasingly well described, and the nature of the bedrock lithology, which is still poorly constrained. Here, we calculated downstream variations in stream power and bedrock strength for six rivers crossing a normal fault in western Turkey, to derive new constraints on bedrock erodibility as function of rock type. These rivers were selected because they exhibit knick zones representing a transient response to an increase in throw rate, driven by fault linkage. Field measures of rock mass strength showed that the metamorphic units (gneisses and schists) in the catchments are ∼2 times harder than the sedimentary lithologies. Stream power increases downstream in all rivers, reaching a maxima upstream of the fault within the metamorphic bedrock but declining markedly where softer sedimentary rocks are encountered. We demonstrate a positive correlation between throw rate and stream power in the metamorphic rocks, characteristic of rivers obeying a detachment-limited model of erosion. We estimated bedrock erodibility in the metamorphic rocks as kb = 2.2–6.3 × 10−14 m s2 kg−1; in contrast, bedrock erodibility values were 5–30 times larger in the sedimentary units, with kb = 1.2–15 × 10−13 m s2 kg−1. However, in the sedimentary units, stream power does not scale predictably with fault throw rate, and we evaluated the extent to which the friable nature of the outcropping clastic bedrock alters the long-term erosional dynamics of the rivers. This study places new constraints on bedrock erodibilities upstream of active faults and demonstrates that the strength and characteristics of underlying bedrock exert a fundamental influence on river behavior.

Volcanic spherules condensed from supercritical fluids in the Payenia volcanic province, Argentina

Spherules can be formed by high-temperature processes during volcanic eruptions, lightning strikes and meteorite impacts. Here we report four different types of spherules and spheroidal particles associated with tephra deposits from two separate volcanic fields in the southern Payenia province of Argentina. These silicate and carbonate spherules represent <0.01% of the sampled material, with individual spherules <200 µm in size. Thirty particles were imaged. Only the transparent spherules are smooth, perfect spheres; other morphologies include ellipsoids and aggregated dumbbells. The spheroids are either hollow or solid. Major element analyses show that the spherules and spheroids have silica-rich, Fe-rich, carbonate and basaltic compositions. Chemical analysis of the carbonate spheroids shows some variability in the trace element content between the cores and rims, suggesting element mobility and loss towards the margins. All the analysed carbonate spheroids have elevated Sr/Y, La/Y and La/Ce ratios outside the range for sedimentary carbonates. All four spherule types are considered to be volcanic in origin, with the excess CO 2 required for the formation of the carbonate spherules potentially sourced from the basement lithologies. Based on the major and trace element analyses, we conclude that the silica-rich and carbonate spherules formed by instantaneous condensation from supercritical CO 2 -rich hydrous fluids saturated with dissolved silicates. Supplementary material : Appendix A, containing the full analytical dataset collected by electron probe microanalysis and secondary ion mass spectrometry, is available at https://doi.org/10.6084/m9.figshare.c.5108689

Characterization of the weathered basement rocks in the Dongping field from the Qaidam Basin, Western China: significance as gas reservoirs

Reservoir quality is a critical risk factor in basement reservoirs. Researches into basement reservoirs by petrographic analysis combined with X-ray diffraction, log identification, electron microscopy, field emission scanning electron microscopy, porosity and pulse-decay permeability and core analysis have provided insights into the characterization of the commonality, diversity and difference of the weathered basement rocks as gas reservoirs in the Dongping field. Geological structures, lithology and near-surface processes control the reservoir physical property together. From Wellblock Dp 3 to Wellblock Dp 17, the high uplift gradually transforms into the low slope area towards the center of basin, with the lithology changing as well, which results in different degrees of fracture development in the bedrock in different wellblocks. The basement lithologies are granite, granitic gneiss, and limestone with slate in Wellblock Dp3, Dp1 and Dp17, respectively. Though they all provide effective reservoir space for gas accumulation, the productivity of nature gas shows significant differences. Fractures are the main store space in the three wellblocks. The development of fractures gives rise to secondary porosity around them because of physical weathering and chemical dissolution, but they generate many inhomogeneous fractures and secondary solution pores, whether on the planar distribution or in vertical. In Wellblock Dp3, high angle fractures were generated under the action of structural stress mechanism, with a large number of secondary pores. The porosity is between 0.1 and 23.2%. In Wellblock Dp 1, low angle fractures were the main storage space, with plenty of solution pores mainly in melanocratic minerals. The porosity is between 0.1 and 18.8%. In Wellblock Dp 17, where short and dense fractures developed, the porosity is between 0.1 and 10.3%. The data indicate that the granite in the uplift in Wellblock Dp3 has better reservoir properties due to the stronger physical weathering and chemical dissolution. As the porosity gradually decreases towards the slope and low-lying area, the more favorable exploration area should be the uplift and slope area in the depression area. However, the effective caprock developed locally in Wellblock Dp3, which affected the gas accumulation. Meanwhile, the reservoirs’ petrophysical properties showed distintive variation with different depths in different wellblocks. High productivity layers are under the 200 m, 100 m and 200 m depths from the top of the basement rocks in Wellblock Dp 3, Wellblock Dp 1 and Wellblock Dp 17, respectively. This suggestion in this study will be of significance for guiding oil and gas exploration in front of the Altun Mountains.

Sediment-routing controls on sandstone bulk petrographic composition and texture across an ancient shelf: Example from Cretaceous Western Interior Basin, Utah and Colorado, U.S.A.

ABSTRACTSediment-routing controls on sandstone texture and bulk petrography have been evaluated in linked alluvial–coastal–shelfal deposits of the Upper Cretaceous Castlegate Sandstone, Blackhawk Formation, Star Point Sandstone, and Mancos Shale (Western Interior Basin, Utah and Colorado, USA) using thin-section analysis of representative outcrop samples in the context of a high-resolution sequence stratigraphic and paleogeographic framework. The studied strata record deposition from two styles of sediment-routing system within an overfilled foredeep and contiguous intra-continental seaway. First, multiple transverse drainages supplied sand to fluvial, shoreline, and shelf segments of sediment-routing systems characterized by down-dip transport distances of 150–450 km and significant strike-oriented sediment transport along the shoreline. Second, the distal shoreline–shelf segment of an axially supplied sediment-routing system was characterized by sand transport for a distance of c. 300 km.Bulk petrographic composition indicates that transverse sediment-routing systems were sourced from catchments that supplied quartz-rich sand with a subordinate lithic component, while the large axial sediment-routing system was sourced from a catchment(s) supplying slightly more feldspathic sand. Thin-section measurements of mean grain size, sorting, skewness, and ratio of minimum-to-maximum diameter (a proxy for sphericity) are similar for sandstones deposited in fluvial, shoreline, and shelf segments of the transverse sediment-routing systems and in the shoreline–shelf segment of the axial sediment-routing systems, although hydrodynamic sorting is important in locally segregating grain-size populations within each segment. Further, textural analysis of detrital quartz, feldspar, and lithic sand-grain populations shows little evidence of relative change in mean grain size or apparent grain sphericity with downsystem distance, implying that sand-grain populations of different petrographic composition did not undergo significant differential mechanical breakdown during transport. Instead, the textural characteristics of these sand-grain populations are inferred to have been controlled mainly by bedrock lithology and recycling in source catchments. The textural signal of sediment-source areas then propagated downsystem in the sand fraction of detrital sediment supply. This inference is supported by the fine- to medium-grained, well- to very well-sorted character of all sandstone samples, consistent with recycling of sandstones and quartzites from the Sevier fold-and-thrust belt.

Carbonates at the supergiant Olypmic Dam Cu-U-Au-Ag deposit, South Australia part 2: Sm-Nd, Lu-Hf and Sr-Pb isotope constraints on the chronology of carbonate deposition

Mineralization at the supergiant Cu-U-Au-Ag Olympic Dam deposit (South Australia), the ‘uranium endmember’ of the iron-oxide copper–gold (IOCG) spectrum of ore deposits, is hosted in a breccia complex developed entirely within granite of the 1.59 Ga Hiltaba Suite (Gawler Craton). Earlier studies suggested brecciation and mineralization occurred within a magmatically-driven hydrothermal system at 1.59 Ga, with a critical role for mafic–ultramafic intrusions. In contrast, recent radiometric dating of the breccia complex indicates a prolonged, multi-stage history of brecciation and mineralization from 1.59 to 0.5–0.4 Ga. Ca-Fe-Mg-Mn-carbonate gangue minerals are associated with ore minerals at virtually every stage of mineralization. In a companion study (Apukhtina et al., 2020), this mineralogically, texturally and compositionally diverse carbonate mineral suite was assigned to seven associations defined on the basis of host lithology and texture. Here we report Sm-Nd, Pb-Pb and Lu-Hf isotope ages for these carbonates, which are used to examine the chronology of carbonate deposition. Initial Sr-Nd isotopic compositions are used to place constraints on fluid sources. Sm-Nd and Pb-Pb isotope systematics of calcite veins in ~1.59 Ga IOCG ore indicate 1.59–1.55 Ga deposition ages. Likewise, locally abundant laminated siderites have Sm-Nd ages in this age interval. A world-first attempt to apply Lu-Hf dating to carbonate gangue in an ore deposit yields ages that are 70–100 Ma younger than corresponding Sm-Nd ages, presumably reflecting isotopic exchange of carbonate Lu-Hf isotope systems with host rocks. Sm-Nd ages for carbonates assigned to other carbonate associations (hosted in highly altered inferred 1.59 Ga basalt and picrite; diverse settings within granite-dominated breccia; locally abundant megaclasts of green and red bedded sandstone/mudstone sequences; ~0.82 Ga doleritic dykes) are more diverse and range from ~1.59 to 0.5 Ga. The structurally youngest carbonates (unbrecciated fluorite-barite veins; carbonate matrix in polymict conglomerate above the breccia complex) yield ~0.50 Ga Sm-Nd ages. Inferred carbonate ages are broadly consistent with radiometric dates for other hydrothermal minerals (e.g., hematite, uraninite, apatite, fluorite). They suggest that mineralization initiated at 1.59 Ga was reworked and possibly increased in size in response to large-scale tectonic, magmatic, sedimentary and hydrothermal events. Initial 87Sr/86Sr in the carbonates is higher and more variable (0.710–0.752, average ~0.721) than could be explained by ore and gangue mineral formation from magmatic-hydrothermal fluids during a single event at 1.59 Ga, a model favored in several earlier studies. By contrast, carbonate formation over a long period, as inferred from the Sm-Nd chronology presented here, would allow ingrowth of 87Sr in the granitic host rocks to develop the heterogeneous initial 87Sr/86Sr recorded in the carbonates. Carbonate-bearing fluids appear to have sourced Nd (and most likely also Sr) locally, within the host granite and breccia, with contributions from mafic rocks. The emerging evidence for protracted, multi-stage mineralization implies that single-stage models for Olympic Dam need to be revisited and that all studies of sulfide and gangue minerals in this deposit require careful radiometric dating. We speculate that the polymetallic nature and unusually large metal reserves of the Olympic Dam mineralization are related to its multi-stage formation history.

Weathering Crusts in the Glukharinyi Ore-Placer Cluster (Kolyma Terrane, Northeastern Russia) and Their Gold Potential

The Kolyma Terrane is one of the long-developed regional structures in Northeast Asia. Specifics of its neotectonic evolution promoted the wide distribution of Cenozoic overburden rocks, weathering crusts included. We studied the spatial distribution, mineralogy, chemistry, grain size composition, and morphology of native gold in this region. Only relicts of lower horizons of the weathering crust are preserved at present. Lithology of bedrocks was unfavorable for the formation of the well-developed weathering crust and its enrichment with Au. Very weak supergene alteration of native gold in placer specimens indicates their direct delivery from the bedrock source. The studied weathering crusts in this region lack economic significance for gold exploration.

Assessment of groundwater processes using censored data analysis incorporating non-detect chemical, physical, and biological data

In Europe most environmental based water quality research has focused on both nutrient and microbial contamination which can arise from agricultural processes and inadequate wastewater treatment. Recent work in Ireland has linked the presence of arsenic in groundwater at elevated concentrations at national and subnational scales with bedrock lithology serving as a strong predictor variable. Groundwater data was collected as part of an environmental impact assessment for a road construction project and this resulting groundwater geochemistry dataset was used in this present study to assess the geochemical controls of arsenic in natural waters in addition to biological and nutrient contamination. Physiochemical parameters, trace elements, nutrients, organics, and microbiological parameters were collected for every quarter for four years (2004–2008) in 67 wells. Due to differing sampling procedures and limitations in the data, only one quarter (November 2005) was used to understand groundwater geochemistry in greater detail. Multivariate statistical techniques were used to overcome the presence of non-detect data. This is an important consideration as while methods exist for chemical data, methods incorporating biological data are limited. Elevated levels of nitrate in groundwater may arise from the runoff of septic tanks and/or agricultural practices in the area. Both pesticides and polycyclic aromatic hydrocarbons were not detected in any wells signifying no anthropogenic contamination inputs. However, fuel products such as methyl tert-butyl ether were detected and potentially illustrate point source contamination, these were detected in only one well. Geochemical data indicate that elevated arsenic concentrations are present within alkali-oxic groundwaters through the desorption from Fe and Mn oxyhydroxides, i.e. alkali desorption. This study examines of the geochemistry of arsenic in groundwater in Ireland at a local scale. In addition, the multivariate methods used in this study were able to fully integrate both chemical and biological censored data, which may be applied in other regions with similar data.

Boulders as a lithologic control on river and landscape response to tectonic forcing at the Mendocino triple junction

AbstractConstraining Earth’s sediment mass balance over geologic time requires a quantitative understanding of how landscapes respond to transient tectonic perturbations. However, the mechanisms by which bedrock lithology governs landscape response remain poorly understood. Rock type influences the size of sediment delivered to river channels, which controls how efficiently rivers respond to tectonic forcing. The Mendocino triple junction region of northern California, USA, is one landscape in which large boulders, delivered by hillslope failures to channels, may alter the pace of landscape response to a pulse of rock uplift. Boulders frequently delivered by earthflows in one lithology, the Franciscan mélange, have been hypothesized to steepen channels and slow river response to rock uplift, helping to preserve high-elevation, low-relief topography. Channels in other units (the Coastal Belt and the Franciscan schist) may experience little or no erosion inhibition due to boulder delivery. Here we investigate spatial patterns in channel steepness, an indicator of erosion resistance, and how it varies between mélange and non-mélange channels. We then ask whether lithologically controlled boulder delivery to rivers is a possible cause of steepness variations. We find that mélange channels are steeper than Coastal Belt channels but not steeper than schist channels. Though channels in all units steepen with increasing proximity to mapped hillslope failures, absolute steepness values near failures are much higher (∼2×) in the mélange and schist than in Coastal Belt units. This could reflect reduced rock erodibility or increased erosion rates in the mélange and schist, or disproportionate steepening due to enhanced boulder delivery by hillslope failures in those units. To investigate the possible influence of lithology-dependent boulder delivery, we map boulders at failure toes in the three units. We find that boulder size, frequency, and concentration are greatest in mélange channels and that Coastal Belt channels have the lowest concentrations. Using our field data to parameterize a mathematical model for channel slope response to boulder delivery, we find that the modeled influence of boulders in the mélange could be strong enough to account for some observed differences in channel steepness between lithologies. At the landscape scale, we lack the data to fully disentangle boulder-induced steepening from that due to spatially varying erosion rates and in situ rock erodibility. However, our boulder mapping and modeling results suggest that lithology-dependent boulder delivery to channels could retard landscape adjustment to tectonic forcing in the mélange and potentially also in the schist. Boulder delivery may modulate landscape response to tectonics and help preserve high-elevation, low-relief topography at the Mendocino triple junction and elsewhere.

Application of combined electrical resistivity tomography and seismic reflection method to explore hidden active faults in Pingwu, Sichuan, China

Abstract Pingwu County, which is located at the northern end of the Longmenshan fault structural belt, has an active regional geological structure. For a long time, the Longmenshan fault tectonic belt has become intensely active with frequent earthquakes. According to the existing geological data, the Pingwu–Qingchuan fault passes through the urban area of Pingwu. However, because of the great changes in the original landform of Pingwu caused by the construction activities in this urban area, a precise judgment of the location of the Pingwu–Qingchuan fault according to the new landform characteristics is difficult. Here, the seismic reflection method, electrical resistivity tomography (ERT), and drilling method were used to determine the accurate location of the buried active faults in Pingwu County. The seismic reflection method and ERT are used to determine the location of faults, the thickness of overlying strata of the fault, and the basic characteristics of faults. The drilling data can be used to divide the bedrock lithology and confirm the geophysical results. The geological model of the faults can be constructed by 3D inversion of ERT, and the structural characteristics of the faults can be viewed intuitively. The results of this study can provide a basis for earthquake prevention and construction work in Pingwu. The finding also shows that seismic reflection method and ERT can effectively explore buried active faults in urban areas, where many sources of interferences may exist.

Triassic–Jurassic evolution of the eastern North China Craton: Insights from the Lushun-Dalian area, South Liaodong Peninsula, NE China

AbstractThe Lushun-Dalian area of the South Liaodong Peninsula, in NE China, located in the SE margin of the North China Craton (NCC) exposes a suite of Middle-Late Proterozoic low-grade metamorphic sedimentary rocks which can be divided into a lower competent layer, a middle incompetent layer, and an upper competent layer on the basis of lithology and deformation style. Two stages of deformation recorded both in the metasedimentary rocks and a magmatic complex intruded in them indicate that the Lushun-Dalian area is a key region to decipher the Triassic–Jurassic tectonic evolution of the eastern NCC. The earliest D1 deformation mylonitized the magmatic complex and thrusted it northeastward over the low-grade metasedimentary rocks, in which a series of NE-verging folds and NE-directed brittle thrust faults developed. The D2 deformation erased the D1 fabrics in the incompetent layer by a top-to-the-NW ductile shearing and refolded the D1 fabrics in the lower and upper competent units, producing a series of km-scale SW-plunging folds. New zircon secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb ages from the magmatic complex and the granite porphyry dikes intruded in it, combined with the unconformity between the low-grade metasedimentary rocks and the Early Cretaceous volcanic rocks, indicate that D1 and D2 occurred after 211 Ma and before the Early Cretaceous. The decrease of the deformation intensity of D1 and D2 from the Lushun-Dalian area toward the interior of the NCC in the NE and NW directions suggests that D1 was the structural response in the overriding plate to the NCC-South China Block convergence during the Late Triassic to Early Jurassic, and D2 was the structural response to the northwestward subduction of the Paleo–Pacific plate beneath the NCC in the Middle-Late Jurassic. The superimposition of D2 on D1 recorded a significant tectonic transformation from the nearly E-W–trending Tethysian domain to the NE-SW–trending Pacific domain.

U–Pb geochronology of apatite and zircon from the Brent impact structure, Canada: a Late Ordovician Sandbian–Katian boundary event associated with L-Chondrite parent body disruption

In situ LA-ICP-MS U–Pb geochronology has been performed on apatite and zircon within thermally recrystallized clast-laden and clast-poor impact melt rocks from the Brent impact structure. A total of 377 laser analyses on 120 impact melt-grown (n = 9) and impact-recrystallized zircon grains (n = 111) were obtained, from which a concordia age of 452.8 ± 2.7 Ma (MSWD 0.57, n = 11), and a weighted average mean 206Pb/238U age of 453.2 ± 2.9 Ma (MSWD 0.60) (n = 11) are calculated. A total of 300 laser analyses from 100 relict apatite grains were obtained, with an unanchored regression through all data yielding a lower intercept age of 453.2 ± 6.0 Ma (MSWD 5.8, n = 300), that overlaps within error of zircon. 207Pb/206Pb ratios obtained from feldspar clasts within clast-laden impact melt retain the same initial Pb composition as the target rocks from which they are derived, while feldspars that crystallized from impact melt have 207Pb/206Pb ratios indicative of isotopic re-equilibration between basement lithologies of two different ages. A similar variability in 207Pb/206Pb is recorded by apatite. This provides evidence for the involvement of Neoproterozoic Lake Nipissing alkaline suite, as well as Mesoproterozoic Grenville gneisses in the production of impact melt at Brent. Recrystallized apatite grains exhibit enrichments in light rare earth elements (LREEs) along neoblast grain boundaries, indicative of trace element substitution and phase precipitation during impact-induced recrystallization. An age of 452.8 ± 2.7 Ma from zircon and 453.2 ± 6.0 Ma from apatite places the impact event in the Late Ordovician, at or near the Sandbian–Katian boundary, confirming Brent’s involvement in the Middle to Late Ordovician crater cluster event—a period of enhanced impactor flux to Earth related to the L-Chondrite parent body disruption.