Clay Mineral Assemblages Research Articles

Early Cretaceous black shale in the Fajiaying Formation (Lingshan Island, East China): Terrestrial record of hothouse climate

The significance of Cretaceous hothouse climate in terrestrial systems is currently unclear, and its records in terrestrial strata are rarely reported. This study investigated the sedimentary environment of the Fajiaying Formation, characterised by abundant black shale and soft-sediment deformation structures. Five lithofacies have been identified for the Fajiaying Formation, e.g. Facies A: black shale with siltstone inter-bedding, Facies B: black and gray-black shale and siltstone alternations with minor sandstone, Facies C: black and gray-black shale interbedded with gray-black sandstone, Facies D: pebbly sandstone and Facies E: gray-black shale and siltstone. The average TOC content of the shales decreases from facies A (av. 1.32%) to facies E (av. 0.46%) through facies B (av. 0.57%) and facies C (av. 0.40%). The paleosalinity gradually decreases from lithofacies A to lithofacies E, corresponding to the change of paleoclimate. In addition, Bivariate Scatter Plot of SiO2 and Al2O3 + K2O + Na2O and the clay mineral assemblages suggest an arid climate. Combined with distribution characteristics and paleontology of saliferous strata in eastern China, we determine that hothouse climate influenced the salification of lakes located in eastern China during the Early Cretaceous. Furthermore, the Fajiaying Formation in Lingshan Island was deposited in a terrestrial salified lake. Salinization of the sedimentary water bodies of the Fajiaying Formation might be related to an oceanic anoxic event. The black shale developed extensively in the Fajiaying Formation and represents the terrestrial record of hothouse climate.

Cretaceous-Paleogene (K/T) boundary in the North-Eastern Caucasus (Dagestan): sedimentology, geochemistry and biota

The Cretaceous/Paleogene (K/T) boundary interval was studied in the Okhli and Gubden sections, northeastern Caucasus. Two sediment successions represented in these sections were accumulated in different zones of the paleobasin and distinguished by their lithological and geochemical characteristics. The high-resolution study of nannofossils revealed very gradual changes in nannoplankton community at the Cretaceous/Paleogene transition. The nannofossil assemblage becomes poor from the level ca. 4 m below K/T boundary and it is very scarce in the last 2.5 m of uppermost Maastrichtian. The significant sea-level fall is evidenced in the beginning of the K/T event. It led to erosion and redeposition of Maastrichtian calcareous sediments within the thin layer of clay, covering erosional surface in the Okhli section. The clay mineral assemblage, starting from this surface contains palygorskite. The concentrations of coprolites (Decapoda?) and microspherulas (diagenetic microconcretions) with higher content of some trace elements were found within the small red lenses at the base of the clay layer overlying the Maastrichtian erosional surface. The reliable evidence of impact event are not recognized. The evolutional trends which are recorded within the Cretaceous/Paleogene transitional interval in the Okhli and Gubden sections are similar to trends recognized in studied sections of the K/T event on the adjacent territories of the North-Eastern Peri-Tethys (e.g., southern Turkmenistan and Mangyshlak Peninsula).

Cretaceous–Paleogene Boundary in the Sequences of the Northeastern Caucasus, Dagestan: Sedimentology, Geochemistry, and Biota

The results of integrated study of the Cretaceous/Paleogene transition in the northeastern Caucasus are reported. The lithological, geochemical and micropaleontological characteristics of the sediments from two sections (Okhli and Gubden, Mountainous Dagestan) accumulated in different parts of paleobasin are displayed. An analysis of nannofossil distribution at the Cretaceous/Paleogene transition allowed to define the stratigraphic position of the stage boundary and reveal the interval of sediments (1–2 m thick transitional zone) accumulated under stress conditions. The large-scale sea-level fall occurred at the onset of the Cretaceous/Paleogene boundary event is evidenced. It led to the erosion and redeposition of the Maastrichtian calcareous sediments. In Okhli section, the occurrence of palygorskite is detected in the clay mineral assemblage of the transitional zone. The local concentrations of coprolites (Decapoda?) and spherical nodules of different composition (more likely, concretions) are found in the clay layer onlapping the erosional surface at the top of Maastrichtian. There is no reliable evidence of the impact nature of these nodules. The lithological record at the Cretaceous/Paleogene transition of the studied sections is similar to those exposed in other areas of the NE Peri-Tethys (e.g., southern Turkmenistan and Mangyshlak Peninsula).

Provenance of Bengal Shelf Sediments: 2. Petrology and Geochemistry of Sand

The Bangladesh lowlands are traversed by the largest sediment flux on the planet. Detritus generated mostly in Himalayan highlands and conveyed through the Ganga–Brahmaputra rivers and Meghna estuary reaches the Bay of Bengal, where it forms a composite deltaic system. This study integrates the vast existing database on Ganga–Brahmaputra sediments of all grain sizes from clay to sand with new petrographic, mineralogical, and geochemical data on estuarine and shallow-marine sands. A large spectrum of compositional signatures was used to: (i) assess the relative supply of the Ganga and Brahmaputra rivers to estuarine and shelfal sediments; (ii) define the compositional variability of estuarine sediments and the impact exerted by hydraulic sorting and climate-related chemical weathering on provenance signals; (iii) define the compositional variability of shelf sediments and the potential hydrodynamic segregation of fast-settling heavy minerals in coastal environments and of slow-settling platy micas on low-energy outer-shelf floors; (iv) consider the potential additional mud supply from the western subaerial part of the delta formerly built by the Ganga River; and (v) draw a preliminary mineralogical comparison between fluvio-deltaic sediments and turbidites of the Bengal–Nicobar deep-sea fan, thus tracing sediment dispersal across the huge sedimentary system extending from Tibet to the equatorial Indian Ocean. All investigated mineralogical and geochemical parameters, as well as Sr and Nd isotope ratios and clay–mineral assemblages, showed a clear prevalence in sediment supply from the Brahmaputra (60–70%) over the Ganga (30–40%). Heavy-mineral suites and Sr and Nd isotope fingerprints of Bengal shelf sediments are nearly identical to those of the Brahmaputra River and Meghna estuary, also because the Brahmaputra carries almost twice as many Ca-plagioclase grains and heavy minerals including epidote than the Ganga, and these minerals control the large majority of the Sr and Nd budgets. The experience gained in modern settings can be directly extrapolated only to the recent past, because sediments older than the late Pleistocene and buried more than a few hundred meters begin to lose less durable ferromagnesian minerals by selective chemical dissolution, which makes quantitative estimates progressively less robust in more deeply buried older strata.

Asian summer monsoon influence on chemical weathering and sediment provenance determined by clay mineral analysis from the Indus Submarine Canyon

AbstractClay minerals from the Indus Canyon and eastern clinoform since ~12 ka are uniformly rich in smectite and illite, similar to those from the Holocene Indus flood plains. A systematic enrichment of smectite in the proximal delta compared to the canyon and eastern clinoform argues for preferential capture of smectite close to the river mouth since ~12 ka. There is a rapid shift to a more smectite-rich assemblage in the canyon and eastern clinoform after ~5 ka. This change is probably caused by a change in sediment source, with less direct flux from the Himalaya and more erosion of older, weathered, smectite-rich sediment from the Indus River flood plains, driven by incision of the Indus and its tributaries into the floodplain as summer monsoon rains weakened. This influx of smectite is consistent with lower kaolinite/smectite values since ~5 ka. The onset of large-scale agricultural activities since ~5 ka, especially starting with the Harappan Civilization, may also have enhanced incision and erosion of floodplain sediments over the same time period. This study reports for the first time how monsoon strength variations since ~12 ka affected the clay mineral assemblages and sediment provenance in a major submarine canyon.

Paleoclimatic record from the clay mineralogy of Quaternary sediments of drill core DWJ from Jiudong subbasin (western Hexi Corridor), NW China

AbstractThe thick sedimentary deposits of the Jiudong subbasin in the western Hexi Corridor of NW China can potentially provide important records of climate change in arid and semiarid areas subject to the interaction of the East Asian summer monsoon (EASM) and the westerlies. Here we present the results of a study of the clay mineralogy of the sediments of drill core DWJ from the Jiudong subbasin. The clay minerals consist mainly of illite, followed by chlorite, kaolinite, and smectite. The clay mineral assemblages are used to define four paleoclimatic stages since ~1.8 Ma: (1) During ~1.8–1.2 Ma, the climate was relatively cold and arid; (2) during ~1.2–0.9 Ma, it was relatively warm and wet, with strongly seasonal precipitation; (3) during ~0.9–0.3 Ma, there was a long-term trend of cooling and drying; and (4) during ~0.3–0.1 Ma, the climate was warmer and wetter than during the previous period, with strongly seasonal precipitation. The paleoclimatic record of the Jiudong subbasin since ~1.8 Ma is consistent with records from the Tengger Desert and typical monsoonal areas. This suggests that the EASM penetrated into the central regions of the North Qilian Shan and Zhangye–Jiudong subbasin during ~1.2–0.9 and ~0.3–0.1 Ma.

Decoding the origins and sources of clay minerals in the Upper Cretaceous Tununk Shale of south‐central Utah: Implications for the pursuit of climate and burial histories

AbstractClay minerals in fine‐grained marine sedimentary successions are most commonly considered to be detrital in origin and have been used extensively by geologists as indicators of palaeoclimate conditions in the hinterland. Most of these previous studies, however, were not designed to address in depth the potential effects of mixing clay minerals from multiple sources and the formation of authigenic clay minerals during early diagenesis on the ultimately observed clay mineral assemblages of fine‐grained marine sedimentary successions. Herein, clay minerals in shales and bentonites of the Tununk Shale Member in south‐central Utah were examined through integrated X‐ray diffraction and petrographic (scanning electron microscopy) analysis, to evaluate the various origins of clay minerals in this offshore mudstone succession. Clays in Tununk bentonites contain dominantly smectite (>80%) and a minor amount of kaolinite. Clays in Tununk shale samples consist dominantly of mixed‐layer illite/smectite with up to 45% illite‐like layers, small amounts of kaolinite and mica, and in places trace amounts of chlorite. Clay minerals in Tununk shale samples occur in the following three forms: (a) in clay‐dominated aggregates (i.e. smectite‐dominated altered volcanic rock fragments and illite/smectite‐dominated shale lithics); (b) in the fine‐grained matrix (mostly illite/smectite and minor amounts of mica and kaolinite); and (c) in intergranular and intragranular pore spaces (authigenic smectite, kaolinite and chlorite). Possible sources for the mixed‐layer illite/smectite in shales include (a) erosion of older smectite‐bearing mudstone successions and (b) weathering of volcanic rocks or volcanic debris that had been deposited on land. Most of the kaolinite and chlorite in the Tununk Shale were precipitated as pore‐filling cements, rather than having a terrigenous source (as weathering products). A comprehensive understanding of the multiple origins of clay minerals (e.g. terrigenous input, volcanic input, recycled sediments and diagenesis) in marine mudstone successions is critical when attempting to use clay mineral data for reconstructions of palaeoclimate and burial and thermal histories.

Authigenic, volcanogenic, and detrital influences on the Cenomanian–Turonian clay sedimentation in the Western Interior Basin: Implications for palaeoclimatic reconstructions

High-resolution changes in the mineralogical assemblages at Pueblo and Hot Springs of the Western Interior Basin (WIB, USA) were investigated for the late Cenomanian–early Turonian interval, including the Oceanic Anoxic Event 2 (OAE2). The aim of this study is to investigate the regional significance of clay mineral assemblages in the WIB by distinguishing authigenic, volcanogenic, and detrital influences. At Pueblo, the last few centimetres of the Hartland Shale and the Bridge Creek Limestone formations present an abrupt increase in R0 I–S content (up to 90% of the clay signal), which is coeval to an important incursion of subtropical Pacific seawater entered in the seaway from the Gulf of Mexico. We suggest that sea-level changes and volcanic activity expressed by bentonite horizons constitute the principal source of smectitic minerals found in the south central region of the basin. At equivalent stratigraphic interval in the Hot Springs section, situated 5° north of Pueblo (∼600 km north), the smectitic sedimentation was mixed by local detrital input in kaolinite (from 6 to 45%). The great abundance of detrital kaolinite at the northern Hot Springs site is probably due to a very different clay provenance in that part of the seaway. It appears that two local supplies of kaolinite are recorded in the S. gracile ammonite zone (up to 45%) and in the latest Cenomanian–early Turonian interval (from 6 to 31%). The particular geographic configuration of the Western Interior Basin (arrangement of landmasses and the seaway) in the tropical-subtropical environment initiate and maintain seasonally reversing movements of air mass, high evaporation flux, atmospheric heat transport, and precipitation. In this regards, we suggest that local kaolinite input reflect a more humid climate can be attributed to the local intensity of monsoonal-driven precipitation.

Late Jurassic–Early Cretaceous climate change record in clay minerals of the Norwegian-Greenland Seaway

The humid climate of northwestern Europe during the Late Jurassic–Early Cretaceous was punctuated by a widespread arid phase. There are few well constrained and complete sections that document this climate shift from semi-humid to arid, and back to more humid conditions. Detrital clay minerals formed in soils and derived through weathering may be used as palaeoclimatic proxies, and here, clay mineral assemblages from several well-constrained sections from the Norwegian Continental Shelf are presented as a N-S transect of the Norwegian-Greenland Seaway. Palaeoclimate signals in most of the clay mineral sections are complicated by diagenetic overprinting. Yet, towards the Boreal realm of the Norwegian-Greenland Seaway there is a discernible palaeoclimatic signal in the clay mineralogy of Well 16/3–4. At ~30°N, kaolinite is known to have reappeared in the clay mineralogical record in the upper part of Calpionellid Zone B (early-mid Berriasian). However, we find an absence of kaolinite at ~40°N in Well 16/3–4 from the North Sea until the late Berriasian, suggesting that initiation of the first humid phase in the Early Cretaceous Norwegian-Greenland Seaway was diachronous.

Provenance, tectonic setting, and weathering of sediments in Tumaco-1 ST-P well, Tumaco forearc basin, Colombia: Insights from petrography, heavy minerals, X-ray diffraction, and whole-rock chemostratigraphy

A chemostratigraphic evaluation based on major and trace elements (including the REE group), heavy minerals, and whole rock petrographic analysis is applied on 300 samples from the Tumaco-1-ST-P well located in the Tumaco forearc basin, Colombia. The aim of the study is to gain a better understanding of the provenance, tectonic, and paleoweathering conditions of the succession, which contains three formations (Tumaco, Tangareal del Mira, and Cascajal) ranging in age from the late Oligocene to late Pliocene.SiO2 has the highest concentration of all major elements (mean at 52%), followed by Al2O3 (mean at 14.6%), Fe2O3, MgO, and CaO are in the range of 3.2–5.3%, Na2O and K2O oscillate between 1 and 2%, and TiO2, P2O5, MnO, and Cr2O3 are <1%. Out of trace elements, Ba, Sr, V, Cr, Cu, Zn, Ni and Zr are the most abundant; their enrichment is probably due to the marine/deltaic depositional environment conditions. Th, Cs, and Nb have the highest concentrations of all trace elements. In general, concentrations of light rare earth elements are greater than high rare earth elements (LREE > HREE). Both positive and negative Eu anomalies are present in Tumaco and Tangareal del Mira formations, suggesting that rocks have two sources, mafic and felsic; the presence of both is consistent with petrographic and heavy mineral analyses.Paleoweathering indices, such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), and index of compositional variability (ICV) in the Tumaco-1-ST-P well suggest that the source of these rocks experienced initial stages of weathering compared with Post-Archean Average Australian Shale (PAAS) and Upper Continental Crust (UCC) values, indicating that the Tumaco formation samples have undergone major alteration. The clay mineral assemblage is dominated by the occurrence of illite and illite/smectite mixed layers; when coupled with the CIA index and the Al2O3-CaO-Na2O-K2O (ACNK) plot this indicates post-depositional K-enrichment. The distribution of Al2O3, TiO2, and Zr suggests a limited recycling effect.The presence of first cycle mafic-to-intermediate igneous detritus, together with felsic to intermediate igneous and metamorphic minerals, suggest that the Tumaco forearc basin detrital material was supplied by two locations. One location corresponds to the orogenic island arc system of the Western Cordillera, which is dominated by mafic and intermediate extrusive rocks (basalt and andesite). The other source, which is volumetrically less important, is the Central Cordillera, which comprises intrusive felsic rocks and metamorphic rocks (skarn, marble, and quartz-micaceous schist). This subsidiary source represents a continental arc setting and is observed mainly in the Tumaco Formation.

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin, Llanbedr (Mochras Farm) borehole, Wales

AbstractEarly Jurassic climate is characterized by alternating cold and warm periods highlighted by studies based notably on oxygen isotopes measured on belemnite guards and other marine invertebrate shells. These climatic changes include changes in the hydrological cycle, and consequently weathering and runoff conditions. In order to clarify the erosion and weathering conditions during the Pliensbachian, this study determined the mineralogical composition of the clay fraction of 132 samples taken from the entire stage drilled in the Llanbedr (Mochras Farm) borehole (Cardigan Bay Basin). The clay mineral assemblages are composed of various proportions of chlorite, illite, illite/smectite mixed‐layers (R1 I–S), smectite and kaolinite, with possibly occasional traces of berthierine. The occurrence of abundant smectite indicates that the maximum burial temperature never exceeded 70°C. Consequently, clay minerals are considered mainly detrital, and their fluctuations likely reflect environmental changes. The variations in the proportions of smectite and kaolinite are opposite to each other. Kaolinite is particularly abundant at the base of the jamesoni Zone, in part coinciding with the δ13C negative excursion corresponding to the Sinemurian/Pliensbachian Boundary Event, and through the davoei Zone, whilst smectite is abundant in the upper part of jamesoni and base of ibex zones and through the subnodosus/gibbosus subzones of the margaritatus Zone. The kaolinite‐rich intervals reflect an intensification of hydrolysis and an acceleration of the hydrological cycle, while the smectite‐rich intervals indicate a more arid climate. The spinatum Zone is characterized by a distinct clay assemblage with abundant primary minerals, R1 I–S, kaolinite reworked from previously deposited sediments or from Palaeozoic rocks, and probably berthierine originating from contemporaneous ironstone‐generating environments of shallower waters. This mineralogical change by the end of the Pliensbachian likely reflects a transition from a dominant chemical weathering to a deeper physical erosion of the continent, probably related to a significant sea‐level fall consistent with a glacio‐eustatic origin.

Elemental Compositions of Smectites Reveal Detailed Sediment Provenance Changes during Glacial and Interglacial Periods: The Southern Drake Passage and Bellingshausen Sea, Antarctica

Variations in clay mineral assemblages have been widely used to understand changes in sediment provenance during glacial and interglacial periods. Smectite clay minerals, however, have a range of various elemental compositions that possibly originated from multiple different sources. Therefore, it might be crucial to distinguish the various types of smectites by analyzing their elemental composition in order to verify the sediment provenances with certainty. This hypothesis was tested for the clay mineral characteristics in a marine sediment core from the southern Drake Passage (GC05-DP02). Rare earth elements and ε N d data had previously indicated that fine grained detritus was supplied from the Weddell Sea to the core site during interglacial periods, when the sediments contained more Al-rich smectite (montmorillonite). Indeed, marine sediments collected close to the Larsen Ice Shelf on the eastern Antarctic Peninsula continental shelf, western Weddell Sea embayment, show more Al-rich smectite components as compared with other possible West Antarctic sources, such as the Ross Sea embayment or King George Island, South Shetland Islands. Furthermore, two types of smectite (Al-rich and Al-poor) were identified in core GC360 from the Bellingshausen Sea shelf, suggesting that during glacial periods some sediment is derived from subglacial erosion of underlying pre-Oligocene sedimentary strata containing predominantly Al-rich montmorillonite. This finding reveals different sources for smectites in sediments deposited at site GC360 during the last glacial period and during the present interglacial that show only minor differences in smectite contents. For the interglacial period, two groups of smectite with a wide range of Al-rich and Mg–Fe-rich were identified, which indicate delivery from two different sources: (1) the detritus with high contents of Mg–Fe-rich smectite supplied from Beethoven Peninsula, southwestern Alexander island and (2) the detritus with higher contents of Al-rich smectite (montmorillonite) possibly derived from the subglacial reworking of pre-Oligocene sedimentary strata. These results demonstrate that the elemental compositions of smectites can be used to differentiate the sources of smectites in marine sediments, which is an important tool to define sediment provenance in detail, when down-core changes observed in clay mineral assemblages are interpreted.

Implications for Provenance since the Last Glaciations in Southeastern Andaman Sea Sediments by Clay Mineralogy

This study presents high-resolution clay mineralogy of Core MASS-III-10, located in the southeastern Andaman Sea. The study aimed to investigate terrigenous sedimentary input from various potential provenances throughout different periods of geological time. The clay mineral assemblages of MASS-III-10 reveal a high amount of smectite at 73% (65-79%), a moderate kaolinite at 12% (10-17%), and a low illite and chlorite content(&lt;15%) over the last 45 ka. Provenance analysis suggests the most fine-grained terrigenous sediments originated from the Irrawaddy Delta Shelf (IDS), with minor sediments deriving from the Andaman Islands, the East Continental Shelf (ECS), and Sumatra provenance. The results show that IDS were predominantly smectite (~44%) while the ECS largely produced kaolinite (47%). The provenance interpretation based on the smectite content revealed that over time there has been no change in the main source of sediment in the Andaman Sea, despite changes in the volume of sedimentary input. Since the last glaciation, the Myanmar provenance (including IDS) has always contributed clay minerals, while other sources made up only minor contributions to the Andaman Sea. The clay minerals from the Myanmar provenance increased when the sea level was at a low stand (MIS 2), potentially due to shoreline retreat which enabled easier transportation of sediment from other sources. During the interglacial period (MIS 1 and 3), the study found a decrease in Myanmar-sourced clay minerals, which may be because the higher sea level made clay mineral deposition more difficult.

Clay mineral compositions in surface sediments of the Ganges-Brahmaputra-Meghna river system of Bengal Basin, Bangladesh

Clay minerals are significant indicators that can be used to identify sources and transport patterns of both fluvial and marine sediments. The Ganges, Brahmaputra, and Surma-Meghna (SM) rivers are the most important rivers of the Bengal Basin (BB), loading a large amount of sediments from the Himalayas into the Bay of Bengal. Clay minerals of 132 surface sediment samples from the Ganges-Brahmaputra-Meghna (GBM) river system have been analyzed with X-ray diffraction (XRD) to show the variation of clay mineral assemblages in the sediments from upstream to downstream for the major rivers of the BB and to verify the validity of clay minerals as an important index for sediment provenance in the Bay of Bengal after these sediments are discharged into the neighboring seas. The results show that the clay mineral assemblage in the GBM river system consists dominantly of illite (average 66%), with smaller amounts of smectite, kaolinite, and chlorite (8%, 8%, and 17%, respectively). Smectite is diagnostic in the Ganges River sediments, with a higher content (23%) than that in the Brahmaputra (2%) and the SM River (1%); however, chlorite (21%) is high in the SM River sediments and the Brahmaputra River sediments have a high illite percentage (77%). Downstream, the smectite percentages in the Ganges River increase significantly, but the kaolinite percentages in sediments of the SM River increase slightly. Due to the continuous mixture of first the Brahmaputra River and then the SM River discharge, the clay mineral assemblages of this river system's sediments change greatly, e.g., smectite percentages decrease from 23% in the Ganges River to 8% in the GBM River via the stretch with 12% in the Ganges-Brahmaputra confluence river.

Paleoclimatic evolution of the SW and NE South China Sea and its relationship with spectral reflectance data over various age scales

Spectral analysis of sediment from International Ocean Discovery Program (IODP) Site U1433 in the South China Sea can be used to constrain humidity and temperature through time by constructing hematite/goethite records spanning the last 10 million years. Records in the southwest and northern parts of this basin show long-term contrasting trends, despite the fact that geochemical evidence for alteration is similar in both parts of the basin, i.e. towards less alteration since 10 Ma. However, contrasting clay mineral assemblages in the northern and southwestern areas do suggest long-term changes in the relative climate of Indochina and southern China. We interpret this to reflect the northward migration of the ITCZ since the Miocene resulting in a progressive drying of Indochina and increasingly wet conditions in South China. Sediment believed to be supplied from the Mekong River (IODP Site U1433) shows increasing chemical weathering at a time of long-term drying of the climate. This probably reflects increasing chemical weathering driven by slower transport and despite reduced rates of chemical alteration in the context of a colder, drier climate since the Miocene. In general hematite/goethite data are consistent with pollen constraints from the same drill site that show colder conditions with increasing hematite/goethite since 3 Ma and with a transition to apparently wetter conditions since the mid-Pleistocene transition (MPT). This implies that most of the sediment in the southwestern basin is weathered and eroded during warmer, wetter interglacial phases, at least since the MPT.

Reconstruction of palaeoenvironmental conditions of the Vaca Muerta formation in the southern part of the Neuquén Basin (Tithonian-Valanginian): Evidences of initial short-lived development of anoxia

Abstract The Vaca Muerta Formation is a famous source rock being largely present in Argentina. The formation was examined in the southern part of the Neuquen Basin through clay-mineral assemblage composition, as well as organic and inorganic geochemical characteristics. In total, 259 samples were analyzed on both sides of the Huincul Arch (major E-W structure that divided the southern part of the basin into two depocentres). South to this structure (Picun Leufu area), the total organic carbon content (TOC) can reach 20% but is generally ranging from 0.1 to 6%. To the north (Covunco area), average TOC does not exceed 0.5%. Clay-mineral assemblages indicate an evolution of the sources of clastic supply through time in response to evolving weathering/erosion at the regional scale. Redox-sensitive trace-element distribution shows that the particulate iron shuttle process operated, with an initially-euxinic depositional environment that evolved rapidly to oxygenated seawater and suboxic sediments. Our results suggest an episodically restricted circulation pattern that occurred at the beginning of the deposition of the Vaca Muerta Formation (highest TOC content).

Natural Antibacterial Clays: Historical Uses and Modern Advances

AbstractAntibacterial clays in nature include a variety of clay mineral assemblages that are capable of killing certain human pathogens. Although clays have been used for medicinal applications historically, only in the last decade have analytical methods and instrumentation been developed that allow researchers to evaluate the antibacterial mechanisms of various clays applied medicinally. Comparisons of the mineralogical and chemical compositions of natural clays that kill bacteria have promoted a better understanding of the mineral properties that are toxic to a broad-spectrum of human pathogens, including bacteria that have developed resistance to antibiotics. Popular literature is filled with reports of ‘healing’ clays, that, when tested against pathogens in vitro and compared to controls, do not appear to have bactericidal properties. It is important, however, to differentiate what properties make a clay ‘healing,’ versus what makes a clay ‘antibacterial.’ Most antibacterial clays identified to date buffer pH conditions of a hydrated clay outside the range of conditions in which human pathogens thrive (circum-neutral pH) and require oxidation reactions to occur. It is the change in oxidation state and pH imposed by the hydrated clay, applied topically, that leads to a chemical attack of the bacteria. Healing clays, on the other hand, may not kill bacteria but have soothing effects that are palliative. This article reviews some of the historical uses of clays in medicine but focuses primarily on the common characteristics of natural antibacterial clays and early studies of their antibacterial mechanisms. In this era of bacterial resistance to antibiotics, mimicking the antibacterial mechanisms exhibited by natural clays could be advantageous in the development of new antimicrobial agents.

Clay‐coat diversity in marginal marine sediments

AbstractThe specific mineralogy of clay grain coats controls the ability of the coat to inhibit quartz cementation in sandstones during prolonged burial and heating. How and why clay‐coat mineralogy varies across marginal marine systems is poorly understood, even though these eogenetic phenomena strongly influence subsequent mesodiagenesis and reservoir quality. The novel development of the ability to predict the distribution of clay‐coat mineralogy would represent an important development for sandstone reservoir quality prediction. In marginal marine sediments, clay minerals occur as grain‐coats, floccules, mud intraclasts, clay‐rich rock fragments or as dispersed material. However, the relationships between clay mineralogy, the amount of clay, and its distribution is poorly understood. This study focused on the Ravenglass Estuary, UK. The key aim was to develop and apply a novel methodology utilising scanning electron microscope – energy dispersive spectrometry, for the first time, on grain coats in modern sediments, to differentiate the clay‐coat mineral signature from that of the bulk sediment, and reveal the distribution of clay minerals across marginal marine sediments. The study showed that marginal marine sediments principally have their clay mineral assemblage present as clay‐coats on sand grains. These clay‐coats have a mixed clay mineralogy and are spatially heterogeneous across the range of marginal‐marine depositional environments. The study further showed that clay‐coat mineralogy is governed initially by the hydrologically‐controlled segregation of the clay minerals within inner estuarine depositional environments, and subsequently by the selective abrasive removal of specific clay mineral types during reworking and transport into the outer estuary and the marine environment. The highest relative abundance of grain‐coating chlorite was in sand‐flat and tidal‐bar depositional environments. The availability of an analogue data set, and an understanding of the controlling processes of clay‐coat mineralogy, offer crucial steps in building a predictive capability for clay‐coat derived elevated reservoir quality in deeply buried sandstones.

Clay mineralogy of an Eocene fluvial-lacustrine sequence in Xining Basin, Northwest China, and its paleoclimatic implications

The Eocene was marked by significant cooling during which the global climate was transformed from greenhouse to icehouse conditions. Notable coeval events were the India-Asia collision and the retreat of the Paratethys Sea in Asia. The Eocene section of the long and continuous sedimentary succession of the Xining Basin in Northwest China is characterized by red mudstones with intercalated gypsum and muddy-gypsiferous layers. In this study, we conducted a semi-quantitative analysis of the mineralogy of bulk samples and the clay fraction using X-ray diffraction, with the aim of characterizing the Eocene climatic evolution of the northeastern margin of the Tibetan Plateau and inland Asia. We used a new pretreatment method to address the problem of extracting sufficient clay particles from the gypsum and gypsiferous layers. The bulk mineralogy is dominated by quartz, feldspar, calcite, gypsum and dolomite; and the clay mineralogy is dominated by illite, chlorite, and smectite (including irregular mixed-layer illite-smectite (I/S)). The variations of the clay mineral assemblages indicate the occurrence of alternations between warm humid conditions and hot dry conditions, with relatively high humidity during ~52–50, ~41.5–39 and ~35–34 Ma. Comparison of the results with the timing of Tibetan Plateau uplift, transgressions and regressions of the Paratethys Sea, and the marine oxygen isotope record suggest that the Eocene climatic evolution of the study region was driven fundamentally by global climate change.

Clay Mineral and Geochemical Proxies for Intense Climate Change in the Permian Gondwana Rock Record from Eastern India.

The clay mineral assemblages and geochemical compositions of the Permian Talchir and Barakar mudstones of the Raniganj basin, India, have been used to interpret terrestrial paleoclimate. The Talchir Formation presents unequivocal evidences of the Permian global glacial climate, and the overlying Barakar Formation with braided fluvial deposits immediately follows the glacial amelioration stage to a humid warm climate. Sediments unaffected by burial diagenesis and originated from a similar source under contrasting climates are ideal for developing proxies for substantial climate shift. Illite (28.4-63.8%), illite/smectite (0-58.6%, 40-80% illite), chlorite (0-53.9%), and chlorite/smectite (5.6-29.8%) constitute the clay mineral assemblage in the Talchir Formation whereas illite (5.3-78.2%), illite/smectite (trace-34.1%, mostly 60-90% illite), and kaolinite (36.1-86.8%) dominate the clay mineral assemblage in the Barakar Formation. The Talchir mudrocks are enriched in mobile elements and depleted in alumina w.r.t. PAAS, have relatively higher K2O/Al2O3 ratios (~0.3), high ICV (1.12-1.28), and lower CIA values (52.6-65.1) compared to those of the younger Barakar mudstones. The Barakar mudstones are depleted in mobile elements w.r.t. PAAS, have relatively low ICV (0.33-0.62) and K2O/Al2O3 values (0.11-0.16), and higher CIA values (72.9-88.2). Textural, mineralogical immaturity, and rock fragments of different components of the basement seen in the Talchir sandstones show these sediments being a first-cycle sedimentary deposit. The distinctive clay mineral assemblage and major oxide composition of the Talchir mudrocks attest to a unique low intensity chemical weathering in cold arid climate. Significant presence of kaolinite as well as distinctive geochemical characters of the Barakar mudrocks marks a shift in the paleoclimate from cold arid to humid. This climatic shift is further supported by the proportion and composition of illite/smectite across the formations. The relative proportion of chlorite and kaolinite and composition of illite/smectite therefore closely corroborate the significant climate shift, and such proxies, therefore, are useful indicators of climate extremes in the geological record.