Lithology Of Sediments Research Articles

The ~1.85 Ga carbonatite in north China and its implications on the evolution of the Columbia supercontinent

Mantle-derived carbonatites provide a unique window in the understanding of mantle characteristics and dynamics, as well as insight into the assembly and breakup of supercontinents. As a petrological indicator of extensional tectonic regimes, Archean/Proterozoic carbonatites provide important constraints on the timing of the breakup of ancient supercontinents. The majority of the carbonatites reported worldwide are Phanerozoic, in part because of the difficulty in recognizing Archean/Proterozoic carbonatites, which are characterized by strong foliation and recrystallization, and share broad petrologic similarities with metamorphosed sedimentary lithologies. Here, we report the recognition of a ~1.85 Ga carbonatite in Chaihulanzi area of Chifeng in north China based on systematic geological, petrological, geochemical, and baddeleyite U-Pb geochronological results. The carbonatite occurs as dikes or sills emplaced in Archean metasedimentary rocks and underwent intense deformation. Petrological and SEM/EDS results show that calcite and dolomite are the dominant carbonate minerals along with minor and varied amounts of Mg-rich mafic minerals, including forsterite (with Fo > 98), phlogopite, diopside, and an accessory amount of apatite, baddeleyite, spinel, monazite, and ilmenite. The relatively high silica content together with the non-arc and OIB-like trace element signatures of the carbonatite indicates a hot mantle plume as the likely magma source. The depleted Nd isotopic signatures suggest that plume upwelling might be triggered by the accumulation of recycled crust in the deep mantle. As a part of the global-scale Columbia supercontinent, the Proterozoic tectonic evolution of the North China Craton (NCC) provides important insights into the geodynamics governing amalgamation and fragmentation of the supercontinent. The Paleo-Mesoproterozoic boundary is the key point of tectonic transition from compressional to extensional settings in the NCC. The newly identified ~1.85 Ga carbonatite provides a direct link between the long-lasting supercontinental breakup and plume activity, which might be sourced from the “slab graveyard,” continental crustal slabs subducted into asthenosphere, beneath the supercontinent. The carbonatite provides a precise constraint of the initiation of the continental breakup at ~1.85 Ga.

Changes in the geochemistry of fluvial sediments after dam construction (the Chrudimka River, the Czech Republic)

The Seč Dam Reservoir on the Chrudimka River (the Czech Republic) was studied as a possible barrier for the transport of solid particles related to fluvial pollution. To understand the damming-triggered changes in the sediment lithology and geochemistry and to establish the pollution chemostratigraphy of the reservoir deposits, we first characterized the grain-size control on the sediment composition, including concentrations of risk elements Cu, Ni, Pb, and Zn. To that end, we first examined the chemical composition of the sieved size fractions of sediments, and then we compared the grain-size parameters obtained by granulometry with the geochemical composition using either scatter plots or a compositional regression with functional covariates in framework of Bayes spaces. The onset of deposition in the dam reservoir outside of the inflow delta is marked by three major changes: (1) a decrease in the grain size and a corresponding increase in Al/Si and Ti and a decrease in K and Ca, (2) enhanced retention of the finest particulates, especially Fe and Mn, and (3) enhanced concentration of risk elements originating from local industry in the city of Hlinsko that started nearly coevally with the dam construction. In the inflow delta, these changes are not so straightforward: the lithology of the deltaic deposits resembles that of the pre-dam fluvial sediments, except for the fact they are polluted, although the pollution stratigraphy is poorly preserved. The peak of polycyclic aromatic hydrocarbons (PAHs) in the dam reservoir was found near the onset of the dam deposition. We found that Fe is the best-performing reference (normalizing) element for the geochemical background functions (GBFs), although Ti would provide better grain-size correction; clearly, post-depositional geochemical reactions also play a role. GBFs were used to calculate local enrichment factors (LEF) and show that the dam construction suppressed the downstream transport of Cu, Ni, and Zn pollution by more efficient retention and trapping of fine sediment components including Fe oxides, which are otherwise transported through the floodplain.

Mechanisms leading to the 2016 giant twin glacier collapses, Aru Range, Tibet

Abstract. In north-western Tibet (34.0∘ N, 82.2∘ E) near lake Aru Co, the entire ablation areas of two glaciers (Aru-1 and Aru-2) suddenly collapsed on 17 July and 21 September 2016. The masses transformed into ice avalanches with volumes of 68 and 83×106 m3 and ran out up to 7 km in horizontal distance, killing nine people. The only similar event currently documented is the 130×106 m3 Kolka Glacier rock and ice avalanche of 2002 (Caucasus Mountains). Using climatic reanalysis, remote sensing, and three-dimensional thermo-mechanical modelling, we reconstructed the Aru glaciers' thermal regimes, thicknesses, velocities, basal shear stresses, and ice damage prior to the collapse in detail. Thereby, we highlight the potential of using emergence velocities to constrain basal friction in mountain glacier models. We show that the frictional change leading to the Aru collapses occurred in the temperate areas of the polythermal glaciers and is not related to a rapid thawing of cold-based ice. The two glaciers experienced a similar stress transfer from predominant basal drag towards predominant lateral shearing in the detachment areas and during the 5–6 years before the collapses. A high-friction patch is found under the Aru-2 glacier tongue, but not under the Aru-1 glacier. This difference led to disparate behaviour of both glaciers, making the development of the instability more visible for the Aru-1 glacier through enhanced crevassing and terminus advance over a longer period. In comparison, these signs were observable only over a few days to weeks (crevasses) or were absent (advance) for the Aru-2 glacier. Field investigations reveal that those two glaciers were underlain by soft, highly erodible, and fine-grained sedimentary lithologies. We propose that the specific bedrock lithology played a key role in the two Tibet and the Caucasus Mountains giant glacier collapses documented to date by producing low bed roughness and large amounts of till, rich in clay and silt with a low friction angle. The twin 2016 Aru collapses would thus have been driven by a failing basal substrate linked to increasing pore water pressure in the subglacial drainage system in response to increases in surface melting and rain during the 5–6 years preceding the collapse dates.

Seismic and lithofacies characterization of a gravity core transect down the submarine Tuaheni Landslide Complex, NE New Zealand

Abstract The southern Tuaheni Landslide Complex (TLC) at the Hikurangi subduction margin displays distinctive morphological features along its distribution over the Tuaheni slope offshore Gisborne, New Zealand. We here present first analyses of a gravity core transect that systematically samples surficial sediments from the source area to the toe of this landslide complex, thus providing important new insight into shallow lithological variation in the slide complex. Geophysical and geochemical core logs and core descriptions form the basis for a characterization of representative sediment successions that are indicative of the respective slope segment of recovery. Our results show that the lithology of surficial sediments varies significantly along the length of the landslide complex. Depending on the slope segment observed, this variation includes post-Last Glacial Maximum (LGM) outer-shelf sediments, and hemipelagic drape and near-surface reworked debris avalanche deposits, as well as multiple intercalated thinner turbidites and tephra layers at the distal end of the profile. Lithological downslope variability suggests ongoing mass transport events through the late Holocene that were likely to have been limited to small mud-turbidite flows. Integration with acoustic sub-bottom imagery reveals the presence of multiple stacked mass-transport deposits at depth, contrasting with previous interpretations of a single parent failure.

Diagenetic overprint on authigenic Nd isotope records: A case study of the Bering Slope

Neodymium isotope ratios (143Nd/144Nd, εNd) have been used as a quasi-conservative water mass tracer in the Atlantic and Southern oceans. However, boundary exchange and diagenesis can limit their use in areas where extensive interactions between seawater, pore water and bottom sediments can overwrite the original water mass signature. To avoid the misapplication of this proxy and to provide a new perspective on the application of Nd isotopes to authigenic phases, we present εNd records covering the past 2.4 Myr for the authigenic (εNd, auth) and detrital (εNd, det) phases of sediments from Integrated Ocean Drilling Program site U1343 (∼1950 m water depth) on the Bering Slope. The εNd, det values range from −9.0 to −2.3 (n=70), showing binary mixing between Alaskan and Aleutian sediments. Glacial–interglacial variations are muted in the εNd, det record because sediment delivery pathways from both source areas exist regardless of climatic conditions. The εNd, auth values range from −5.9 to −0.2 (n=153) and show a strong correlation (r=0.58, n=60) with εNd, det in the deeper part of the core (>160 ka), where authigenic carbonate layers are frequently observed. We attribute this covariation to diagenetic overprinting by pore water Nd during late diagenesis. The pore water Nd that is incorporated into authigenic carbonates was initially released by marine silicate weathering, therefore indicating that sediment lithology exerts a first-order control on the εNd values of the pore water in deeply buried sediments. In the shallower part of the core (<160 ka), there is no correlation between εNd, auth and εNd, det (r=−0.23, n=10). Here, εNd, auth is typically radiogenic during interglacial periods, which may be due to the diagenetic flux from the relatively stagnant N. Pacific or the incongruent chemical weathering of freshly ground sediments on the Bering Shelf. The effect of dense water formation on εNd, auth seems to be minimal when we compare the εNd, auth records at three different depths of the Bering Sea. Our εNd dataset provides a unique perspective on the behavior of εNd, auth during late diagenesis. To apply εNd, auth as a water mass tracer to long cores containing volcanogenic sediments or in productive regions, we recommend testing for deep authigenesis using pore water profiles.

Reservoir Studies: A Case Study of Well in Offshore Depobelt, Niger Delta, Nigeria

Bioturbation the reworking activity by organisms in and around sediment surfaces is a natural day to day and important process in the marine environment. This process brings about disruption and alteration to primary sedimentary lithologies though commonly overlooked in the areas of reservoir quality assessment. In other to assess the effect of these reworking activities on reservoir quality (porosity and permeability), two core samples of approximately 32 ft, recovered from the Miocene interval of the parallic Agbada Formation, located in the South-eastern offshore Niger Delta were subjected to conventional core/thin section analysis. The resulting datasets containing digital core images. Petrophysical plug data and thin section reports were employed in this study/research work. The samples showed dominance of bioturbated lithofacies with the spread of the ruziana and Skolithos ichnofacie associations. It was revealed also that the cleaner sandstone fades have the best reservoir quality while the more bioturbated intervals of the entire cored sections had porosity and permeability values slightly higher than their rarely-lowly bioturbated counterparts.

Macrostrat: A Platform for Geological Data Integration and Deep‐Time Earth Crust Research

AbstractCharacterizing the lithology, age, and physical‐chemical properties of rocks and sediments in the Earth's upper crust is necessary to fully assess energy, water, and mineral resources and to address many fundamental questions. Although a large number of geological maps, regional geological syntheses, and sample‐based measurements have been produced, there is no openly available database that integrates rock record‐derived data, while also facilitating large‐scale, quantitative characterization of the volume, age, and material properties of the upper crust. Here we describe Macrostrat, a relational geospatial database and supporting cyberinfrastructure that is designed to enable quantitative spatial and geochronological analyses of the entire assemblage of surface and subsurface sedimentary, igneous, and metamorphic rocks. Macrostrat contains general, comprehensive summaries of the age and properties of 33,903 lithologically and chronologically defined geological units distributed across 1,474 regions in North and South America, the Caribbean, New Zealand, and the deep sea. Sample‐derived data, including fossil occurrences in the Paleobiology Database, more than 180,000 geochemical and outcrop‐derived measurements, and more than 2.3 million bedrock geologic map units from over 200 map sources, are linked to specific Macrostrat units and/or lithologies. Macrostrat has generated numerous quantitative results and its infrastructure is used as a data platform in several independently developed mobile applications. It is necessary to expand geographic coverage and to refine age models and material properties to arrive at a more precise characterization of the upper crust globally and test fundamental hypotheses about the long‐term evolution of Earth systems.

Detecting Kerogen as a Biosignature Using Colocated UV Time-Gated Raman and Fluorescence Spectroscopy.

The Mars 2020 mission will analyze samples in situ and identify any that could have preserved biosignatures in ancient habitable environments for later return to Earth. Highest priority targeted samples include aqueously formed sedimentary lithologies. On Earth, such lithologies can contain fossil biosignatures as aromatic carbon (kerogen). In this study, we analyzed nonextracted kerogen in a diverse suite of natural, complex samples using colocated UV excitation (266 nm) time-gated (UV-TG) Raman and laser-induced fluorescence spectroscopies. We interrogated kerogen and its host matrix in samples to (1) explore the capabilities of UV-TG Raman and fluorescence spectroscopies for detecting kerogen in high-priority targets in the search for possible biosignatures on Mars; (2) assess the effectiveness of time gating and UV laser wavelength in reducing fluorescence in Raman spectra; and (3) identify sample-specific issues that could challenge rover-based identifications of kerogen using UV-TG Raman spectroscopy. We found that ungated UV Raman spectroscopy is suited to identify diagnostic kerogen Raman bands without interfering fluorescence and that UV fluorescence spectroscopy is suited to identify kerogen. These results highlight the value of combining colocated Raman and fluorescence spectroscopies, similar to those obtainable by SHERLOC on Mars 2020, to strengthen the confidence of kerogen detection as a potential biosignature in complex natural samples. Key Words: Raman spectroscopy-Laser-induced fluorescence spectroscopy-Mars Sample Return-Mars 2020 mission-Kerogen-Biosignatures. Astrobiology 18, 431-453.

Three-dimensional modeling of nitrate-N transport in vadose zone: Roles of soil heterogeneity and groundwater flux

Contamination of groundwater from nitrogen fertilizers in agricultural lands is an important environmental and water quality management issue. It is well recognized that in agriculturally intensive areas, fertilizers and pesticides may leach through the vadose zone and eventually reach groundwater. While numerical models are commonly used to simulate fate and transport of agricultural contaminants, few models have considered a controlled field work to investigate the influence of soil heterogeneity and groundwater flow on nitrate-N distribution in both root zone and deep vadose zone.In this work, a numerical model was developed to simulate nitrate-N transport and transformation beneath a center pivot-irrigated corn field on Nebraska Management System Evaluation area over a three-year period. The model was based on a realistic three-dimensional sediment lithology, as well as carefully controlled irrigation and fertilizer application plans. In parallel, a homogeneous soil domain, containing the major sediment type of the site (i.e. sandy loam), was developed to conduct the same water flow and nitrate-N leaching simulations.Simulated nitrate-N concentrations were compared with the monitored nitrate-N concentrations in 10 multi-level sampling wells over a three-year period. Although soil heterogeneity was mainly observed from top soil to 3 m below the surface, heterogeneity controlled the spatial distribution of nitrate-N concentration. Soil heterogeneity, however, has minimal impact on the total mass of nitrate-N in the domain. In the deeper saturated zone, short-term variations of nitrate-N concentration correlated with the groundwater level fluctuations.

Geochemistry of sediments of the Peninsular rivers of the Ganga basin and its implication to weathering, sedimentary processes and provenance

Abstract Geochemistry of the Himalayan rivers has been a major point of contention in sedimentary geochemistry because of its global impact on ocean chemistry and climate. Among the Himalayan rivers, the Ganga and its Himalayan catchment have been studied extensively. However, the Peninsular rivers constituting a significant part of the Ganga basin have not received much attention, heretofore. In the present geochemical study of the Peninsular river's sediments, we have found that the initial chemical weathering of rocks in the catchment produces texturally, mineralogically and geochemically diverse sediments and fluvial processes further segregate them into the geochemically contrasting coarse channel and fine-grained suspended sediments. The overbank sediments possess an intermediate texture and geochemistry, but more towards the suspended sediments. The sediment geochemistry data indicates that the Peninsular river's sediments show an increase in the weathering intensity from the west (Banas river) to the east (Son river) along the climate gradient, i.e., from the arid-semiarid to the sub-tropical condition. The higher chemical index of alteration (CIA) values of the Peninsular river's sediments than the Yamuna and Ganga river sediments reflect the control of weatherable lithology in the tectonically stable, low relief and subtropical climate condition. The Yamuna and Ganga river sediments with lower CIA values show more physical weathering in the tectonically active Himalaya. The suspended and overbank sediments of the Yamuna and Ganga rivers show an increase in their CIA values after the confluence of the Peninsular rivers, indicating the contribution of more weathered materials by the Peninsular rivers. It has been found that there is a geochemical split between the channel and suspended sediments of the Peninsular rivers for sharing different sources, i.e., the dominant contribution of felsic sources to the channel and mafic sources to the suspended sediments. The weathering geochemistry, trace element systematics, and REE patterns suggest that the dominant sources of the channel sediments of the Peninsular rivers are the felsic crystallines and sedimentary lithologies of the Aravalli range, Bundelkhand and Chhotanagpur granite and gneisses, and Vindhyan sandstones. Whereas, the mafic lithologies such as the Deccan traps and mafic components of the felsic lithologies appear to be the major sources of the suspended sediments. Within the range of intermediate composition, the overbank sediment chemistry reflects more contribution of the mafic sources except for the Ken which shows a higher contribution of the felsic sources. The geochemical split between the channel and suspended sediments of the Peninsular river's sediments for the different provenances indicates differential weathering of the Deccan basalts, Bundelkhand crystalline, and Vindhyan sedimentary rocks; and also the hydrodynamic control of the Peninsular rivers during erosion, transport, and deposition. However, the Himalayan rivers do not show such contrast between the channel and suspended sediments because of the higher physical weathering and sediment mixing in the high gradient topography of the Himalayan catchment.

Single crystal elasticity of natural topaz at high-temperatures

Topaz is an aluminosilicate mineral phase stable in the hydrothermally altered pegmatitic rocks and also in subducted sedimentary lithologies. In nature, topaz often exhibits solid solution between fluorine and hydrous end members. We investigated elasticity of naturally occurring single crystal topaz (Al2SiO4F1.42(OH)0.58) using Resonant Ultrasound Spectroscopy. We also explored the temperature dependence of the full elastic constant tensor. We find that among the various minerals stable in the Al2O3-SiO2-H2O ternary system, topaz exhibits moderate elastic anisotropy. As a function of temperature, the sound velocity of topaz decreases with frac{{boldsymbol{d}}{{boldsymbol{V}}}_{{boldsymbol{P}}}}{{boldsymbol{d}}{boldsymbol{T}}} and frac{{boldsymbol{d}}{{boldsymbol{V}}}_{{boldsymbol{S}}}}{{boldsymbol{d}}{boldsymbol{T}}} being −3.10 and −2.30 × 10−4 km/s/K. The elasticity and sound velocity of topaz also vary as a function of OH and F content. The effect of composition ({{boldsymbol{x}}}_{{boldsymbol{O}}{boldsymbol{H}}}) on the velocity is equally important as that of the effect of temperature. We also note that the Debye temperature ({{boldsymbol{Theta }}}_{{boldsymbol{D}}}) of topaz at room temperature condition is 910 K and decreases at higher temperature. The Debye temperature shows positive correlation with density of the mineral phases in the Al2O3-SiO2-H2O ternary system.

О СОВРЕМЕННОЙ ДИНАМИКЕ ПРОДОЛЬНОГО ПРОФИЛЯ СРЕДНЕГО ТЕЧЕНИЯ РЕКИ ЛЕНЫ

The history of fluvial sedimentation in the middle Lena River valley in Central Yakutia in the Late Pleistocene and Holocene is revisited. It is shown that the alluvial cover of the Lena-Taatta interfluve was accumulating almost uninterruptedly throughout the Pleistocene, with incision events corresponding to major glacial epochs – Samarovo and Zyryan glaciations. The stages of incision and aggradation with the predominance of the latter alternated in the middle Lena River during the Holocene. Directed river incision is not typical for the middle Lena River neither in present nor in most periods of the Neopleistocene. The presupposed features of the modern incision are considered, and it is shown that such features are not manifested in the dynamics of the longitudinal profile and the structure of the bottom of the middle Lena River valley. The downward shift of the stage-discharge curve on the hydrological station of Tabaga, 40 km upstream from Yakutsk, is due to its enhancement by the Yakut Department of Hydrometeorology and Environmental Monitoring, but not because of hydro-morphological processes. Certain features of the structure of the riverbed and the valley of the middle Lena River, typical for the incising rivers, may be associated with modern cryogenic processes. In the process of syngenetic freezing of floodplain sediments, the certain floodplain levels can increase their relative elevation, which contributes to the floodplain consolidation. Floodplain levels can be formed due to differences in the lithology of sediments with different heaving potential. Riverside thermo-erosion associated with the thermal influence of river flow and the destructive action of the ice, erodes the frozen island front faster, than outside the permafrost area. As a result, the islands are displaced downstream, but in their upper part, there are sandbanks indicating the aggradation of sediments. Thus, there are no signs of the modern incision trend in the relief of the middle Lena River territory. Riverbed sections with predominance of local erosion and aggradation are alternated, which is primarily due to the hydraulic structure of the flow, as well as the influence of neotectonic movements.

Литология и микропалеонтология донных отложений малого озера острова Олений в Кандалакшском заливе Белого моря

Литология и микропалеонтология донных отложений малого озера острова Олений в Кандалакшском заливе Белого моря

Molybdenum, vanadium, and uranium weathering in small mountainous rivers and rivers draining high-standing islands

Rivers draining high standing islands (HSIs) and small mountainous rivers (SMRs) are known to have extremely high sediment fluxes, and can also have high chemical weathering yields, which makes them potentially important contributors to the global riverine elemental flux to the ocean. This work reports on the riverine concentrations, ocean flux, and weathering yields of Molybdenum (Mo), Vanadium (V), and Uranium (U) in a large number of small but geochemically important rivers using 338 river samples from ten lithologically-diverse regions. These redox-sensitive elements are used extensively to infer paleo-redox conditions in the ocean, and Mo and V are also important rock-derived micronutrients used by microorganisms in nitrogen fixation. Unlike in large river systems, in which dissolved Mo has been attributed predominately to pyrite dissolution, Mo concentrations in these rivers did not correlate with sulfate concentrations. V was found to correlate strongly with Si in terrains dominated by silicate rocks, but this trend was not observed in primarily sedimentary regions. Many rivers exhibited much higher V/Si ratios than larger rivers, and rivers draining young Quaternary volcanic rocks in Nicaragua had much higher dissolved V concentrations (mean=1306nM) than previously-studied rivers. U concentrations were generally well below the global average with the exception of rivers draining primarily sedimentary lithologies containing carbonates and shales. Fluxes of U and Mo from igneous terrains of intermediate composition are lower than the global average, while fluxes of V from these regions are higher, and up to two orders of magnitude higher in the Nicaragua rivers. Weathering yields of Mo and V in most regions are above the global mean, despite lower than average concentrations measured in some of those systems, indicating that the chemical weathering of these elements are higher in these SMR watersheds than larger drainages. In regions of active boundaries with andesite/dacite lithologies, rivers draining young Pleistocene rocks had higher concentrations than did older Miocene-Pliocene rocks of a similar composition. This work shows that weathering yields of Mo, V, and U from SMRs are slightly higher than from large rivers, and the age of igneous lithologies in these regions exhibits a measurable control on riverine concentrations of these elements.

The San Marcos mélange, Cordillera Septentrional of the Dominican Republic. Nature, origin and age

Understanding the nature, origin, and age of a mélange is crucial to deciphering the evolution of certain orogenic belts. Located in the Cordillera Septentrional of the Dominican Republic, the San Marcos Mélange is characterized by a blocks-in-matrix internal structure. Apparently arranged chaotically, the blocks have centimeter to kilometer sizes, include exotic ophiolitic, volcaniclastic, and intrabasinal sedimentary lithologies, and are of Cretaceous to Miocene in age, all immersed in a scaly and variably sheared argillaceous matrix. In less-deformed coherent sections, the Mélange consists of alternating sandstones, siltstones, and clays, indicative of a distal turbiditic depositional environment. Syn-sedimentary deformation is pervasive and produces systems of conjugated extensional shear fractures, veins filled with calcite, cataclastic bands, and boudinage and pinch-and-swell, symmetrical and asymmetrical structures. They are consistent with a layer parallel extension, produced in a low-energy slope setting, and indicate a low-ductility contrast between few or no lithified lithologies. Paleostress analysis of fractures and veins establishes a NE-SW trend of sub-horizontal extension, suggesting an orientation of the basin along a WNW-ESE to NW-SE axis (current coordinates). The biochronological data of nannofossils present in the matrix gives a sedimentary age for the Mélange from, at least, the top of the lower Miocene to the base of the upper Miocene. The San Marcos Mélange has therefore an essentially gravitational origin. Its deformation was temporally syn-sedimentary and was produced by processes of gravitational sliding, probably triggered by both instability processes within the basin and tectonic (earthquake shaking) mechanisms. Blocks and megablocks of ophiolitic rocks derived from the underlying Puerto Plata complex and of well-lithified, volcaniclastic and sedimentary rocks of the Imbert Fm, represent "exotic" blocks caused by the gravitational collapse of the high topographically margins of the depositional basin, slide down, and emplaced into the unconsolidated slope and basin turbiditic deposits, at least in the lower and middle Miocene. The existence of a large-scale, syn-sedimentary shearing surface directed to-the-ONO at the basal contact suggests that the whole Mélange was displaced once or several times in that direction, contributing to the chaotic nature of the unit. The San Marcos Mélange is not a subduction-zone mélange, but was caused by the submarine landslide of turbiditic sediments in a syn-collisional, piggy-back basin, transported on top of a supra-subduction zone ophiolite of the Caribbean plate.

Mineralogy of the Clay Fraction and Chemical Properties of Soils Developed from Sedimentary Lithologies of Pirambóia, Sanga-the-Cabral and Guará Geological Formations in Southern Brazil

ABSTRACT Quali-quantitative and crystallographic characterizations of the different minerals of a soil are essential for the best evaluation of its genesis, degree of weathering, active pedogenetic processes, physical and chemical properties, and its potential reserve of mineral nutrients. This study presents the results of mineralogical investigations carried out to verify the influence of the different lithologies of three geological formations of a sedimentary nature on the mineralogical composition of the soils and the relation of this mineralogical composition to soil [...]

Hydrogen isotope and molecular alteration of n-alkanes during heating in open and closed systems

We conducted a series of heating experiments using pure n-alkane mixtures and natural soil organic extracts to evaluate isotopic and molecular changes during cracking and exchange associated with thermal alteration. Experiments included controlled heating of pure n-alkane mixtures (C7–C40) and a total lipid extract from a modern wetland sediment under both ambient air and anhydrous, closed-system conditions. In an argon-purged, closed-system (at temperatures of 60–300°C), the distribution of pure n-alkane mixtures predictably shifted towards a higher proportion of shorter chain lengths (< C20), with no odd-over-even preference. During the heating process, the δ2H values of long-chain n-alkanes (> C20) became 2H-enriched by up to 12‰, while the δ2H of short chain n-alkanes were 2H-depleted by up to 28‰. When repeating these experiments with the total lipid extract from soils, we observed 2H-enrichment of individual n-alkanes and mean-weighted δ2HnC27–31 of soil n-alkanes under both ambient air (up to 15‰ at 300°C over 72h) and Ar-purged, anhydrous, closed-system (up to 10‰ at 300°C over 720h) conditions. Carbon preference indices (CPI) decreased during the thermal alteration process in tandem with isotopic changes. The average chain length (ACL) of the n-alkanes decreased by more than 1 unit during heating, even at temperatures as low as 60°C, when in the ambient air. This highlights the potential for differences in sediment lithology and gas permeability on an outcrop scale to alter primary organic signatures during thermal alteration. In the presence or absence of oxygen, the hydrogen isotopic composition of pure n-alkane mixtures and natural extracts change from the original composition, though for most compounds this was less than 10–15‰ at temperatures below 150°C. The magnitude of change is greatest for shorter chain n-alkanes and at temperatures above 150°C. We conclude that degradation and/or production during heating, such as may be expected during sediment burial or contact heating, could drive shifts in the measured hydrogen isotope composition of sedimentary leaf wax biomarkers. This is especially rapid at temperatures in excess of 200°C, but the magnitude of isotopic change is generally in the range 10–15‰ before the loss of compounds reaches levels that are below analytical relevance for isotope work. Collectively, our results highlight the need for careful consideration of the depositional environment (particularly for information pertaining to oxygen availability) when interpreting molecular and/or isotopic information from sedimentary alkanes. In addition, they also demonstrate the general fidelity of hydrogen isotopes of n-alkanes under shallow burial temperatures with only modest isotopic enrichment prior to complete loss of long carbon chain compounds.

Revisiting the dissolution of biogenic Si in marine sediments: a key term in the ocean Si budget

Of the ~240 × 1012 mol year−1 of biogenic silica (bSi) produced by diatoms and other silicifying organisms, only roughly 3%–4% escapes dissolution to be permanently buried. At the global scale, how, where and why bSi is preserved in sediment is not well understood. To help address this, I compile 6245 porewater dissolved Si concentrations from 453 sediment cores, to derive the concentration gradient at the sediment–water interface and thus diffusive fluxes out of the sediment. These range from <0.002 to 3.4 mol m−2 year−1, and are independent of temperature, depth and latitude. When classified by sediment lithology, predominantly siliceous sediments unsurprisingly have higher mean diffusive fluxes than predominantly calcareous or clay-rich sediment. Combined with the areal extent of these lithologies, the ‘best-guess’ global sedimentary bSi recycling flux is 69 × 1012 mol year−1.

Full waveform tomography to resolve desert alluvium

This paper presents shear wave velocity distribution developed from full waveform tomography on a seismic dataset recorded in the northwest Las Vegas Valley, Nevada. The setting is a deep and variable alluvial sequence, encompassing gravel, sand, silt and clay, with some carbonate cementation. Seismic data were collected along a ~144-m long array using 72 equally spaced geophones, with an accelerated drop-weight source applied at 6-m intervals. The dataset was interpreted using an advanced 2-D full waveform tomography method. The same dataset was also analyzed using Rayleigh wave dispersion, following the multichannel analysis of surface waves (MASW) method, to develop a single, 1-D velocity profile. Results from the MASW method indicated a mild velocity inversion. The waveform tomography method was able to characterize vertical and lateral velocity variations along the array length to 25m depth. Results showed significantly contrasting high- and low-velocity layers. While depth-averaged shear wave velocities from the two methods were similar, the degree of variability indicated using FWI is supported by strong variation in Standard Penetration Test (SPT) blow counts from a nearby drillhole and by expectations from sediment lithology.

Capturing digital data with handheld devices to determine the redox regime, lithology, and provenance of siliciclastic sediments and residual deposits—a review and field manual

Geomagnetic (magnetic susceptibility), geoelectrical (resistivity and conductivity, self-potential/SP, induced polarization/IP), and radiometric measurements (gamma radiation of K, U, and Th) are well adapted to the needs and wants of geoscientists and exploration geologists, in particular, who widely use them in wireline tools and for ground surveys. Miniaturization of technical components resulted in the production of handheld devices which enable field geologists to an improving of the mineralogical and chemical database and the efficiency of the routine work in the field or at a drill site. The tools and devices used by a field geologist are categorized and presented in a tripartite set. The tools for routine field work with “hammer and laptop” belong to the A-level kit. The handheld devices under consideration constitute an intermediate level (B-level) to enhance the mineralogical and chemical database using physical methods. More advanced level applications make use of short-wave infra-red mineral analyzers or portable X-ray fluorescence devices (C-level). Handheld tools are designed for one-dimensional (cross-sectioning) and two-dimensional (mapping) surveys as well as drill core and cuttings examinations during terrain analysis. They can be operated in foot-borne surveys by one field geologist and the obtained data interpreted without an exuberant computing capacity. In the present overview, siliciclastic rocks and residual deposits have been singled out for their good response to the afore-mentioned methods. Their practical use is demonstrated by means of some case histories, each standing for a particular sedimentary lithology and discussed in combination with similar applications from literature: mixed-type (calcareous) siliciclastic rocks from Mesozoic–Cenozoic basins, SE Germany; residual argillaceous/kaolin deposits on top of granites of the Variscan basement, SE Germany; Neogene red-bed sediments from the promontory of the Tien Shan, East Uzbekistan; and Cretaceous gray-bed sediments with coal seams from the Baganuur basin, Central Mongolia. Cobweb diagrams, histograms, ternary diagrams, simple x-y plots and x-y plots in combination with spider diagrams have proved to be the most suitable ways when it comes to combine the data obtained from various methods and to illustrate these results for further interpretation on screen. The geophysical methods are discussed as to their strong and weak points to cater for a solution in three important subject matters of applied and genetic sedimentology: (1) constraining the redox regime, (2) determination of the lithology and mineralogy, (3) and provenance analysis and lithostratigraphy. Magnetic methods have proved to be useful for all objectives (1, 2, and 3), gamma spectrometry can successfully be applied for objectives 1 and 2, and micro-resistivity contributes significantly to solutions of objective 2. Magnetic and gamma spectrometric methods do not need any direct contact with the sedimentary rocks and therefore can be correlated with equivalent airborne surveys and are less depending on the wetability of the substrate and climate than the geoelectrical methods under study. The final goal of this review is to create a matrix of applicability of the methods and enable the field geologist to select the most suitable type of geophysical measurement or combination of tools for a solution to one of the three issues as a function of the sediment types under study.