Changes In Clay Mineralogy Research Articles

Clay mineralogical evidence of near-equatorial Palaeocene–Eocene Thermal Maximum in Barmer Basin, India

Abstract The Palaeocene–Eocene Thermal Maximum (PETM) was a global extreme climatic event, but it is relatively unknown from lower latitudes or equatorial regions in comparison to mid- and high latitudes. The present study provides the first clay mineralogical evidence of the PETM and subsequent hyperthermal events in a near-equatorial region represented by the Akli Formation in the Barmer Basin, India. The 32 m-thick succession of the Akli Formation shows abrupt changes in smectite and kaolin abundances preceding, during and succeeding the PETM event. Within the studied section, the kaolin content increases from 5–8% pre-PETM to 30–35% during the PETM, and then again decreases to 5–6% during the post-PETM period. The smectite, however, is marked by a corresponding decrease and its transformation into kaolin in acid weathering conditions. The transformation of the smectite is first marked by hydroxy interlayering and then transformation into kaolin during the PETM. The transformation of smectite into kaolin also resulted in extensive precipitation of iron oxide in sediments. The clay mineralogical changes in the Palaeocene–Eocene transition sediments of the Akli Formation were caused by 3–5°C warming and a 25–50% increase in rainfall during the hyperthermal events. Unusually high charcoal (~20%) fragments during the Palaeocene–Eocene transition also suggest warming and widespread biomass burning during the PETM in the lower latitudes.

Elevated physical weathering exceeds chemical weathering of clays during the Paleocene-Eocene Thermal Maximum in the continental Bighorn Basin (Wyoming, USA)

The Paleocene-Eocene Thermal Maximum (PETM) global warming event at ∼56 million years before present changed catchment weathering and erosion. Increased chemical weathering of silicate minerals is thought to be an important process removing CO2 from the atmosphere. However, changes in clay mineralogy can often be explained by enhanced erosion of catchment laterites during the event. Here, we investigate chemical and physical weathering and erosive flux changes through the PETM interval in the Bighorn Basin, Wyoming, a Laramide foreland basin, in a proximal continental-interior alluvial setting. These show an increase of detrital smectite with a lag time of 20-kyr after the main onset the PETM. The smectite increase continued for at least 50-kyr after the event. In-situ, post-depositional pedogenic clay mineral formation is similar between pre-PETM and PETM soil profiles, despite large macroscopic differences between soils that formed before and during the event. Drier, hotter summers during the PETM probably caused decreased vegetation cover that, in concert with more frequent and heavier rainstorms, intensified the erosion of smectite-rich Cretaceous bentonites on the margins of the catchment, which exceeded changes in chemical weathering within the catchment. The lagged response in reaching full PETM clay mineral values can be explained by the time required for upstream sediment to reach the catchment basin floodplain. The prolonged nature of smectite enhancement after the PETM event may again relate to signal propagation times that are now even longer due to lower fluvial recycling rates. Our results indicate that chemical weathering changes were probably superceded by enhanced physical weathering and clay-mineral transport from basin margins at this continental-interior study site.

Clay diagenesis and overpressure development in Upper Cretaceous and Tertiary shales of South Texas

We studied the origins of overpressure, primarily using cuttings and wireline logs, in a nearly 4 km-thick succession of Tertiary and Upper Cretaceous shales and sandstones in the northwestern Onshore Gulf of Mexico. Our goals were: a) to specifically look for the microtextural changes in shale that are often inferred, by pore-pressure specialists, to develop in overpressured shale, and b) to test the utility of cuttings for that purpose. Overpressures begin approximately 2.5 km below the present ground surface and are moderate at the base of our study interval. Much of that section has velocity and density properties that are diagnostic of unloading, excess pore pressures generated after compaction has occurred. Our cuttings-based analyses included XRD and Rock-Eval measurements. Those datasets allowed us to define the changes in mineralogy and organic content (maturity and amount) with depth. We rule out organic maturation as a significant source of overpressure and instead identify clay diagenesis (smectite-to-illite transition) as the major source of excess pore pressure. Finally, we use SEM imaging of ion-milled samples to examine and document the nano-to micron-scale textural and mineralogic changes that occurred during diagenesis. Diagenetic quartz, present as overgrowths on quartz silt grains and microquartz intergrown with illite, is associated with the changes in clay mineralogy. Some nano-to micro-porosity can be preserved locally when detrital silt or rigid diagenetic minerals (mainly quartz and pyrite in our samples) prevent physical compaction. The stratigraphic offset between the onset of overpressure and the onset of clay diagenesis is probably due to the combined effects of the broad-scale stratigraphy (the section becomes sandier at shallower depths), and the temporal changes associated with overpressure generation and dissipation. Our experiment with cuttings demonstrated the utility and limitations of their use. Finally, our results challenge some of the assumptions about shale diagenesis that can be used during pore-pressure studies.

Tectonic and climatic controls on sediment transport to the Southeast Indian Ocean during the Eocene: New insights from IODP Site U1514

The Eocene was a critical period of global plate reorganization and it also saw the Earth's climate transition from the warmhouse state to the coolhouse state. Reconstructing the Eocene sedimentary history in the climate-sensitive Southern Ocean is important for understanding paleoenvironmental changes in response to the accelerated Australia/Antarctica separation and global cooling throughout the middle and late Eocene. Here, we present the first detailed multiproxy record of a continuous sequence from International Ocean Discovery Program (IODP) Site U1514 in the Mentelle Basin off southwestern Australia. Our aim is to reconstruct the sediment provenances and paleoenvironmental evolution in response to the abovementioned climatic and tectonic changes in the mid-high southern latitudes during the Eocene. Provenance analyses based on Sr-Nd isotopes, trace elements, and clay mineral assemblages suggest that Eocene sediments at Site U1514 predominantly originated from the southwestern Australian continent and the Naturaliste Plateau. Sediment provenance variations during the middle Eocene indicate that the onset of fast separation between Australia and Antarctica at 43 Ma caused an increased supply of volcanic materials from the Naturaliste Plateau between 43 and 38 Ma. Terrigenous inputs to the Mentelle Basin during the middle Eocene were primarily controlled by paleoclimate changes rather than tectonic processes because coeval clay mineralogical changes (higher kaolinite/smectite ratio and MARkaolinite) indicate a period of stronger physical erosion and chemical weathering on the western Australian continent that resulted in increased terrigenous materials delivered to the Mentelle Basin. Our results reveal a 5 Myr-long (43–38 Ma) warming reversal in the southern mid-high latitudes, providing an exception to the generally short-lived (10–100 kyr-long) hyperthermals that interrupted the long-term global cooling throughout the middle to late Eocene. As for the late Eocene (38–37 Ma), tectonic processes related to the sudden acceleration in seafloor spreading in the Tasman Sea led to the exposure of shallower areas, resulting in rapid detritus accumulation at the study site. During the late Eocene (37–34 Ma), major sediment provenance shifted from distal source areas (e.g., the Yilgarn Craton) to relatively proximal sources (e.g., the Leeuwin Block and Perth Basin). We interpret that the regional uplift in southwestern Australia and coeval climate cooling resulted in the diversion and inactivation of large drainage systems, thus blocking the transportation of sediment from distant regions.

Clay mineralogy of soils developed from Miocene marls of Medvednica Mt., NW Croatia: Origin and transformation in temperate climate

Five young soil profiles, formed from Miocene marls and argillaceous limestone, were investigated to determine changes in their clay mineralogy. Soil profiles were described, sampled, and their basic properties analyzed (particle size distribution, pH, cation exchange capacity (CEC), CaCO3 and organic C contents). The soils were classified according to the WRB classification as: 1) Calcaric Mollic Leptosol (Humic); 2) Calcaric Lithic Leptosol (Humic); 3) Calcaric Regosol (Loamic, Relocatic); 4) Calcaric Regosol (Siltic); and 5) Calcaric Cambisol (Densic, Endoloamic, Episiltic). Mineralogy was determined via X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) on bulk samples and clay fractions, and chemical composition was determined by inductively coupled plasma emission spectroscopy (ICP-ES). Calcite, quartz and some plagioclase alongside phyllosilicates were identified in parent material. Dolomite was found in the parent material of younger age. Clay fraction from parent material consists of expandable clays (smectite-rich illite-smectite and vermiculite), illite and kaolinite, with chlorite present in younger parent materials. In most soil horizons dissolution and leaching of less stable phases (calcite, dolomite, chlorite) occurred. Initial stage soil formation is consistent with the dominant influence of parent rock mineralogy on soil clay mineral assemblage. Analyses showed substantial homogeneity between and within profiles; nevertheless, slight changes in the clay mineralogy were noted: decrease of illite layers in illite-smectite in topsoil samples and occurrence of mixed-layered chlorite-vermiculite and/or hydroxyl or organic matter interlayering in soil horizons. Chemical composition of bulk samples indicated mobility only of alkali elements, confirming weak chemical weathering. The results show slight changes in clay mineralogy, which can be detected in young soils developed from calcareous parent material under high pH conditions. Those weak alterations of parent clay minerals assemblage, often through unstable phases, can modify soil properties, but are also indicators of subtle clay transformation in calcareous soils in temperate climate.

Comprehensive thermal maturity assessment in shales: a case study on the upper cretaceous Qingshankou formation (Songliao Basin, NE China)

The Songliao Basin is the largest Early Cretaceous lacustrine rift basin of northeast China and hosts tremendous conventional and unconventional petroleum resources. However, the distribution of shale oil is not well constrained, as production from source rock reservoirs requires certain thermal maturity cut-offs (vitrinite reflectance of > 0.8%Rr). In this study, the thermal maturity of the Qiangshankou Formation along the Southern Uplift and in the Central Depression of the Songliao Basin was evaluated by combining traditional and advanced petrographic and geochemical maturity proxies with clay mineralogical investigations. The Qingshankou Formation is one of the major source rocks of the region, but previous studies encountered difficulties in obtaining reliable maturity data, mainly due to strongly shifted Tmax values. Equally, Tmax determined during this study showed apparent shifting likely caused either by retained solid bitumen or by variations in kerogen type. In contrast, the measured vitrinite reflectance shows a reliable depth trend, meeting the shale oil cut-off maturity of 0.8%Ro at a drilling depth of ~ 2300 m, which is only reached in the Central Depression of the basin. The vitrinite reflectance trend is supported by changes in clay mineralogy, such as the progressing ordering of I/S mixed layers (“Reichweite”), increasing illitization, decreasing detrital kaolinite and correspondingly increasing authigenic chlorite contents. Maturity-related biomarker proxies are only partly valid; while the methylphenanthrene index (MPI-1) and the trimethylnaphthalene ratio (TNR-1) show reliable maturity trends even over short depth intervals, other established proxies (e.g. based on alkylbiphenyls) proved insensitive to changes from immature to peak oil mature and are, therefore, not recommended for further use in studies on Qingshankou Fm. shales. In summary, the combination of various maturity parameters improves the understanding of shale oil distribution in the central Songliao Basin, providing valuable insights for studies on similar unconventional targets in the region.

Biological weathering of phlogopite during enriched vermicomposting

As the dominant macrofauna in many soils, earthworms play a significant role in mineral weathering and nutrient release from mineral structures into available forms for root uptake. Phlogopite is a trioctahedral micaceous mineral that can be weathered to other minerals through release of its interlayer potassium (K). In this study, we investigated the effect of earthworm activities and associated microbial communities on phlogopite weathering during the production of phlogopite-enriched vermicompost with different quantities of phlogopite added. An experiment was run under laboratory conditions using three levels of phlogopite (0, 20% and 40% by weight) in triplicates. Each Petri dish contained 20 g of a mixture of cow manure and phlogopite, and vermicomposting process was performed by inoculation of eight Eisenia foetida earthworms. Changes in clay mineralogy and C/N ratio in each dish were assessed at days 0, 45, 90, 135, and 180 during vermicomposting. The results indicated that phlogopite was partially weathered to vermiculite and interstratified mica-vermiculite during the vermicomposting process. The rate of weathering was higher during the early stages of vermicomposting (the first 90 d). The weathering of phlogopite was influenced considerably by the activity of earthworms and associated microbial communities during the organic matter transformation process. At the early stages of the process, the weathering rate for different levels of phlogopite was not significantly different, but over time it accelerated steadily, and the difference increased.

Change of lithofacies in marine sediment core from Quaternary to Pre-Quaternary: A case study from the Central Indian Ocean Basin

A distinct change in lithofacies was observed from red clay to siliceous ooze in a core from Indian Ocean. Radiolarian index species were used to ascertain the Quaternary datum levels and an age of 2.0 Ma is determined up to 125 cm depth from surface. Below 185 cm depth, the sediment core lacked radiolarian test completely. At about 50 cm from core top, the sedimentation rate decreased drastically from 0.18 to 0.05 cm/kyr with a corresponding age of 500 kyr, marking Marine Isotope Stage 13. The changes in lithofacies and sedimentation rate are further accompanied by the change in clay mineralogy from smectite rich older sediments to smectite depleted younger sediments and presence of higher amount of volcanogenic materials in the older sediments. Higher values of chemical index of alteration (CIA) and lower K2O/Al2O3 in older sediments indicate dominance of chemical weathering in the older sediments which decreased gradually towards core top. Ti normalized concentrations of elements like Co, V, Cu, Ni, Ce, Y and ∑REEs exhibit lowest values at the transition zone above which the ratios increased steadily towards the core top. All these observations indicate a marked change in the climatic regime from late Quaternary onwards, prior to which the environment was less bio-productive.

Can geophysics and geochemistry combine to detect mineralisation under transported cover?

The oxidation of Fe-containing sulfide mineralization can lead to the development of natural galvanic cells that are characterized by a reduced acid chimney above the mineralization itself and pH-redox controlled chemical haloes surrounding the chimney. Such chimneys may extend through overlying transported regolith cover of varying thicknesses and composition, altering regolith mineralogy (changes to clays, loss of carbonates), element distribution patterns and geophysical characteristics such as electrical conductivity (EC). This paper compares EM and geochemical profiles in transported regolith over porphyry style Au-Mo-Cu mineralization at Mandamah (NSW) and MVT style Pb-Zn mineralization at Pitchi (Iran). At Mandamah, mineralisation is buried under 30 m of residual regolith and a further 50 m of re-weathered alluvium. Mineralised zones that intersect the base of weathering are characterized by low EM and EC in the upper 6 m of the transported regolith profile. This can be related to changes in clay mineralogy and the destruction of carbonates caused by an acid chimney that formed during a “prograde” phase of regolith development. These patterns are also variably reflected in soil pH and the distribution of selectively-extracted Ca, Mg, S, Ba and REE in the upper part of the transported regolith profile, but generally not in the elements of economic interest (Au, Cu or Mo). At Pitch carbonate-hosted Pb-Zn mineralisation is covered by 80 m of barren dolomitic sedimentary rocks and a thin layer of alluvium and colluvium. There is an increase in the EM and EC as well as Na and S in the transported regolith above mineralisation and adjacent low EC values, but again no detectable patterns in the main ore-related metals. The combined geochemical and (EM) geophysical approach provides a new approach to exploration in areas with economically prospective geology but where mineralization is buried by various forms of cover including transported regolith.

Evidence of cyclic climatic changes recorded in clay mineral assemblages from a continental Paleocene-Eocene sequence, northwestern Argentina

The continental Paleocene-Eocene sequence investigated in this study belongs to the Salta Group, deposited in an intracontinental rift, the Salta Basin (NW Argentina), that evolved from the lower Cretaceous to the middle Paleogene, and is subdivided into the Pirgua, the Balbuena and the Santa Barbara Subgroups. The Maíz Gordo Formation (200 m thick) is the middle unit of the Santa Bárbara Subgroup, deposited during late post-rift sedimentation. We studied the mineralogy of fine-grained horizons of this formation by X-ray diffraction and Scanning Electron Microscopy (SEM) in order to examine the connection between vertical changes in clay mineralogy in alluvial sediments and paleosols, and global paleoclimatic changes registered during the Paleogene. Paleosols vary from calcic vertisols in the lowermost levels, to inseptisols and gleysols in intermediate positions, to gleyed oxisols in the upper section, indicating increased chemical weathering through time.Clay mineral relative abundances vary with a general increase in kaolinite content from bottom to top. However, at one site there are significant variations in kaolinite/muscovite (Kln/Ms) that define five cycles of kaolinite abundance and Kln/Ms. that indicate cyclic patterns of paleoprecipitation and paleotemperature. These are interpreted as several short-lived hyperthermals during the Paleocene-early Eocene in the Southern Hemisphere, which correlate with well-established episodes of warmth documented from the Northern Hemisphere.

Mechanism analysis of soil amelioration and phosphorus recovery by using a mineral soil conditioner in southern China

This study aims to identify the processes and mechanisms affecting physicochemical properties of soil following application of mineral soil conditioner and to investigate the behavior of PO4 sorption among 1:1 and 2:1 clay minerals and Fe/Al oxides in a ferralsol. Three replicates of five treatments, i.e., (1) a control without any application of inorganic P (CK), (2) inorganic P fertilization (CKP), and (3) combinations of inorganic P fertilization with mineral soil conditioner applied at 750 (M1), 975 (M2), and 1200 (M3) kg ha−1, were used to explore the influence of phosphate (PO4) sorption capacity and dynamic changes in clay mineralogy in the ferralsol, as examined using geochemical modeling (Factsage 6.1) and physical measurements (zeta potential, XPS, FTIR, and XRD analysis). We have found that the new formation of illite (a 2:1 phyllosilicate) was generated from the kaolinite (a 1:1 phyllosilicate) in the ferralsol after mineral soil conditioner (MSC) application. The iron in the ferralsol as well as the K+, Mg2+, and Si4+ in the MSC, played important roles in the clay mineral transformation. Meanwhile, the observed trends of phosphorus sorption competition suggested that the PO4 sorption capacity of the 2:1 clay minerals may be close to or even higher than that of the 1:1 clay minerals and Al oxides. The effects of Fe oxides on PO4 sorption could be ignored when Al oxides were present in the ferralsol. In comparison to the control and P-fertilized treatments, the PO4-binding capacity of the clay minerals in the MSC treatments could exceed that of the Al oxides because of the new formation of 2:1 clay minerals (illite). We demonstrate that the combination of zeta potential, XRD, FTIR, and XPS analysis is a promising strategy for distinguishing relationships between clay minerals and Al oxides in terms of phosphorus sorption. The MSC application was highly effective for soil amelioration and phosphorus sorption.

Specific polarizability of sand-clay mixtures with varying ethanol concentration.

We utilise a concept of specific polarizability (c s ), represented as the ratio of mineral-fluid interface polarization per pore-normalised surface area S p , to demonstrate the influence of clay-organic interaction on complex conductivity measurements. Complex conductivity measurements were performed on kaolinite- and illite-sand mixtures as a function of varying ethanol (EtOH) concentration (10% and 20% v/v). The specific surface area of each clay type and Ottawa sand was determined by nitrogen-gas-adsorption Brunauer-Emmett-Teller method. We also calculated the porosity and saturation of each mixture based on weight loss of dried samples. Debye decomposition, a phenomenological model, was applied to the complex conductivity data to determine normalised chargeability (m n ). Specific polarizability estimates from previous complex conductivity measurements for bentonite-sand mixtures were compared with our dataset. The c s for all sand-clay mixtures decreased as the EtOH concentration increased from 0% to 10% to 20% v/v. We observe similar c s responses to EtOH concentration for all sand-clay mixtures. Analysis of variance with a level of significance α = 0.05 suggests that the suppression in c s responses with increasing EtOH concentration was statistically significant for all sand-clay mixtures. On the other hand, real conductivity showed only 10% to 20% v/v changes with increasing EtOH concentration. The c s estimates reflect the sensitivity of complex conductivity measurements to alteration in surface chemistry at available surface adsorption sites for different clay types, likely resulting from ion exchange at the clay surface and associated with kinetic reactions in the electrical double layer of the clay-water-EtOH media. Our results indicate a much larger influence of specific surface area and ethanol concentration on clay-driven polarization relative to changes in clay mineralogy.

Effects of lime treatments on marls

Marly clay was treated with calcitic and Mg-rich lime in order to determine the influence of the clay's high carbonate content on the stabilization effectiveness. The evolution of mineralogical and physical properties over the course of the treatment were studied using XRD, TEM, SEM, elemental analysis, TG, granulometry, and nitrogen sorption and correlated with the marly clay's improved geotechnical behaviour. Only a small portion of smectites and other clay minerals dissolved upon lime treatment. Changes in clay mineralogy had, thus, only very limited influence in the improvement of the material's plasticity and swelling behaviour, which was rather modified by an increase in particle size. This increase was primarily caused by aggregation induced by calcium silicate aluminium hydrate (C-(A)-S-H) formation, whereas flocculation had an only minor effect. After the initial improvement, disaggregation of clay particles occurred which resulted in a particle size decrease, most likely, caused by carbonation of C-(A)-S-H phases. These findings question the effectiveness of lime stabilization for marl using currently applied standard treatment protocols.

Impact of potassium fertilization and potassium uptake by plants on soil clay mineral assemblage in South Brazil

Potassium (K) fertilization increases crop productivity, but in some cases, crop productivity is maintained even with inadequate or low K input. In both situations, the soil mineralogy plays a role that should be investigated. The aims of this study were to quantify the potassium concentrations in several soil compartments, determine the amount of K taken up by plants, and consider soil clay mineralogical changes and K dynamics, as response to K fertilization historic and successive crops. A soil field experiment was conducted in south Brazil over a period of 15 years with different K doses, and a second experiment was conducted in a greenhouse with 11 successive plant cycles under two conditions: no K fertilization (K-poor context) and K fertilization of 30 and 90 mg kg−1 of soil (K-rich context). In the K-poor context, illite was not found in clay fraction and the non-exchangeable K and available K forms were reduced, compromising both the K uptake by plants and the crop yield. In the K-rich context, the low amount of illite was found compared to K-poor context, while relative hydroxy-aluminum interlayered vermiculite (HIV) abundance decreased in detriment of kaolinite. Furthermore, all K compartments (K in plants, available K, structural K, and non-exchangeable K) were restored when high fertilization was applied to the soil. For correct fertilization, the soil mineralogy and fertilization background should be taken into account to obtain high crop production and low K loss in agricultural lands. This will help to maintain soil K reserve, but the successive crops induce ion exhaustion, including K, affecting whole clay mineral assemblage.

Magnesium isotope fractionation during shale weathering in the Shale Hills Critical Zone Observatory: Accumulation of light Mg isotopes in soils by clay mineral transformation

Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: δ26Mg values (−0.6‰ to −0.1‰) of stream and soil pore waters are about ~0.5‰ to 1‰ lighter than the shale bedrock δ(26Mg values of +0.4‰), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg2+ concentrations but lower δ26Mg values (−1.1‰) in groundwater, ~1.5‰ lighter than the bedrock. δ26Mg values (+0.2‰ to +0.4‰) of soil samples on the planar hillslope are either similar or up to ~0.2‰ lighter than the bedrock. Hence a heavy Mg isotope reservoir – complementary to the lighter Mg isotopes in soil pore water and stream water – is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in δ26Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution–precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments (δ26Mg values of +0.3‰ to +0.5‰) accumulated on the valley floor are ~0.2‰ heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir.Our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter Mg isotopes in residual bulk soils. This example also demonstrates that transport of isotopically distinct fine particles from clay-rich systems could be a new and important mechanism to drive the Mg isotope compositions of silicate weathering residuals. This mechanism drives fractionation in an opposite direction as might be expected from previous studies, i.e. residual soils are driven to lighter Mg values and sediments become isotopically heavier.

Pedogenetic evolution of clay minerals and agricultural implications in three paddy soil chronosequences of south China derived from different parent materials

This study aims to understand how clay minerals change sequentially with paddy cultivation age and how parent materials (or original soils) affect the clay mineral behavior of paddy soils. Three paddy soil chronosequences in the hilly regions of South China, derived from purple sandy shale (PS), Quaternary red clays (RC), and red sandstone (RS), were selected to explore the dynamic changes in clay mineralogy, by comparing physical, chemical, and mineralogical properties of soil sequences. For RC and RS soils, both of which have a low K content, there was little change in the clay minerals. Long-term paddy cultivation can promote formation of illite-like minerals; however, this form of K storage was limited under present farming conditions. In PS soils, which are abundant in K-bearing minerals, the depotassication was strong, accompanied by marked transformation of clay minerals. Kaolinite-like minerals gradually decreased with paddy cultivation age; by contrast, derivative clay minerals such as secondary chlorite and halloysite gradually increased. Strong depotassication mainly occurred in the nonclay fractions. The rate of depotassication and the generation of clay fractions were much faster than in natural soils. The clay minerals of paddy soils mainly followed the feature of their original soils. Their evolutions could be distinguished based on their constituents, which are greatly affected by their parent materials. Moreover, paddy cultivation is able to modify clay mineralogy, according to the original mineralogy and paddy soil management.

Recognition criteria for distinguishing between hemipelagic and pelagic mudrocks in the characterization of deep-water reservoir heterogeneity

Determination of turbidite event magnitude and frequency remains subjective and difficult to define. This is because turbidite sedimentation events commonly include both sand and mud, with the mud component commonly excluded from bed thickness studies because of the inability to establish a genetic link to the turbidity current. Pelagic mudrock is defined as fine-grained marine sediment derived primarily from biogenic particles, whereas hemipelagic mudrock includes both biogenic and terrigenous particles. Unfortunately, these compositional definitions do not account for differences in depositional process. Scanning electron microscopy, field emission scanning electron microscopy, and x-ray diffraction analyses of 70 samples from El Rosario Formation outcrops (Baja California, Mexico) and core from the Woodford Shale (Oklahoma) illustrate this distinction. Furthermore, these laboratory measurements are calibrated to 192 outcrop samples to provide a robust method for field identification of clay fabric and mineralogy to define turbidite sedimentation units. Pelagites show organized layering of clay platelets, few flocculates, and a lower proportion of high-density minerals. Hemipelagites have disorganized and chaotic clay fabrics characterized by visible flocculates and contain a higher proportion of denser particles. There may also be a corresponding change in clay mineralogy, for example, smectite in pelagites versus kaolinite in hemipelagites. These results indicate a settling velocity greater than shear velocity in pelagites, whereas hemipelagites record the opposite condition. Turbidity currents support and suspend denser grains, generate disorganized and chaotic clay fabrics, and provide more time for flocculation. Discrimination between pelagites and hemipelagites has important implications in the determination of turbidite event frequency and magnitude, which affects vertical connectivity and continuity of sand, deposited from multipartite turbidity currents. In addition, distinction between pelagites and hemipelagites provides a better understanding of mudrock reservoir architecture.

Reflections on the residual strength of clay soils, with special reference to bedding-controlled landslides

This Glossop Lecture is about landslides and their slip surfaces at residual strength in clays. Particularly in southern England, but also elsewhere, landslides in infrastructure cuttings and many natural slopes are commonly found to be slowly moving compound landslides with a component of their basal shear surfaces following a particular bed (or ‘slide-prone horizon’). A selection of both historical and modern case records of this type of landslide are presented briefly. The geotechnical conditions that give rise to this occurrence are discussed, and the dominant factor relates to the dip of the strata, which must be of low inclination for the landslide mass to remain in place over the critical clay bed in the geological sequence after sliding has been initiated. Observations of the slip surfaces in the field lead to the conclusion that the bedding-controlled elements of this type of landslide develop along thin, slide-prone or slide-susceptible , horizons in the bedding. The question of what caused the formation of those horizons in the first place is answered by putting forward two hypotheses to explain why bedding-controlled slip surfaces form where they do, and considering the evidence for or against each of them. The conclusion is reached that despite the attractiveness of the concept that these slip surfaces form by a progressive failure mechanism at the junction of two materials with dissimilar properties, the alternative concept that they occur where there is a bed of slightly enhanced smectite content better fits the observations. The mechanisms for such local changes in clay mineralogy are linked to inputs of volcanic ash at the time of deposition. Definitive proof of concept is, however, lacking, but taking into account how clay sediments are deposited in sedimentary basins, this paper makes suggestions for future lines of enquiry. Even now, nearly a half-century after Skempton’s seminal Rankine Lecture that introduced the concept of residual strength of clays to the wider geotechnical profession, the corpus of data is rather limited. Some of the datasets are shown to exhibit remarkable similarities, and the implications of this tend to support the preferred explanation of the origin of slide-prone horizons.

Distribution and origin of minerals in high-rank coals of the South Walker Creek area, Bowen Basin, Australia

The coals of the South Walker Creek area in the northern Bowen Basin, with a vitrinite reflectance (Rvmax) of 1.7 to 1.95%, are among the highest rank coals currently mined in Australia. X-ray diffraction studies have identified an unusual mineral variation through the vertical sequence, with kaolinite-rich assemblages typical of other Australian coals at the top and bottom of the seam section and an illite–chlorite assemblage resembling a metamorphic association in the middle. Calcite and ankerite are also abundant in parts of the vertical sequence. Diaspore is abundant in the middle section of the seam in some areas, and paragonite, dickite or nacrite occurs in some locations as well.The distribution of the non-carbonate mineral assemblages cross-cuts the seam stratigraphy, and appears to be primarily controlled by faults and other structural features. Integration of the mineral distribution with optical and electron microscope data on the coal and non-coal bands, chemical data from coal ash analysis, and information from X-ray micro-tomography, suggests that the illite–chlorite and diaspore-bearing assemblages resulted from hydrothermal effects associated with fluid migration through the middle part of the coal seam. This is further confirmed by the development of metamorphic textures in a tonstein band, also altered to illite and chlorite, in the illite-rich part of the seam section, in contrast to a tonstein with more normal sedimentary textures and a kaolinite composition in the kaolinite-rich upper part of the seam.Detailed studies of cleat mineralisation suggest that fluids rich in Ca, Mg and Al were introduced relatively early in the seam's post-depositional history, associated with heat flow sufficient to alter the minerals in the coal and tonstein in the affected part of the seam section. The relatively uniform vitrinite reflectance profile observed through the seam section may indicate that the coal was already of high rank before the fluids were introduced, or that any localised heat effects on the organic matter associated with the changes in clay mineralogy were overprinted by subsequent wider-ranging rank advance processes.

Evidence of short-term clay evolution in soils under human impact

Clay minerals are still often considered as stable in soils over a century to millennium time scale despite a few examples of rapid evolution. Indeed, recent advances in X-ray diffraction pattern treatment combined with studies of soil sequences, demonstrated that changes in clay mineralogy were faster than commonly thought. In this article we present the evidence for rapid clay evolution based on X-ray diffraction pattern decomposition of soil samples collected along land-use sequences chosen to represent the main route of clay evolution in temperate soils (interlayer ion exchanges, weathering and transport).