Moderate Degree Of Chemical Weathering Research Articles

Geochemical Characterization and Constraints on Provenance, Tectonic setting and Paleoweathering of the Miocene Bhuban Sandstone Exposed in Sakhan Anticline, Tripura Fold Belt, Surma Basin, India

ABSTRACT The petrography of sandstone, major oxides, trace and REE compositions of the Bhuban formation, exposed in the Sakhan Anticline of Tripura Frontal Fold Belt have been studied to determine the source rock, depositional tectonic setup and paleoweathering conditions. The average modal composition of the Bhuban Sandstone (Q89.27F4.40L6.33) classified them as quartz arenite and sub litharenite types. The modal composition data (Q-F-L) indicates a craton interior and recycled source for these sediments and deposited in an active continental basin margin as suggested by the bivariate plot of SiO2 vs. K2O/Na2O. The binary discrimination plot of Hf vs. La/Th suggests acidic and older sedimentary component as source for the studied sandstone. The ternary plot of V-Ni-Th×10 pointed towards felsic igneous provenance. Trace and rare earth elemental ratios, chondrite-normalize REE plot suggests the source of the Bhuban sediments are derived from felsic terrain. The CIA values (66.48-75.41), A-CN-K plot (A=Al2O3, CN=CaO+ Na2O, K=K2O) and distinct depletion of Na2O, CaO, U, Sr and Ba suggests moderate degree of chemical weathering of the source rocks which may indicate humid climatic conditions in the source area.

Influence of hydrothermal and upwelling events on organic matter accumulation in the gas-bearing lower Cambrian shales of the middle Yangtze Block, South China

With onoing studies on the diversification of animals during the early Cambrian in the Yangtze Block, resource effects associated with environmental changes have started to be elucidated, in which regional sedimentary events might have played an important role. The gas-rich Yichang area in the southern Huangling anticline is a potential area for shale gas development and provides a more suitable target for unravelling the debates on organic matter (OM) accumulation in the Low Cambrian marine black shales. This study integrated elemental geochemical and mineralogy time-series analyses based on core samples drilled in Yichang area on the shallow-water platform to constrain the evolution of the depositional environments and consider the forcing function behind OM enrichment in the Shuijinguo Formation. Distinctive TOC and Cu/Al peaks within the black shale deposited in the Shuijingtuo Member I suggest intensification of bioproductivity level. The chemical index of alteration (CIA), regarded as an indicator of paleoclimatic variation, increased in the upsection, and together with increasing Sr/Cu and quartz/feldspar ratio values, suggest a trend toward warming conditions and a moderate degree of chemical weathering. The geochemical proxies for the degree of water mass restriction (Mo/TOC and Co × Mn) and paleoredox (MoEF, UEF, U/Th and Corg/P) indicate that the shallow-water shelf experienced weakly-to moderately-restricted water conditions under an anoxic/euxinic environment in the Shuijingtuo Member I and transited to a moderately-restricted water mass with suboxic conditions during the late Shuijingtuo and Shipai periods. A positive Eu anomaly within the Shuijingtuo Member I provided robust evidence of hydrothermal activity. We propose here for an opening of deposition conditions on the shallow-water platform which results in enhanced upwelling and hydrothermal activity (Si, Ba, et.), thereby ultimately disrupting the original nutrient cycling for the metabolism and prompting the OM accumulation in the epicontinental sea during the early Cambrian.

Petrography and geochemistry of the Miocene Upper Red Formation sandstones in NW Iran; with an application to the origin and tectonic setting

Middle to Upper Miocene sedimentary successions in the Alborz sedimentary basin (northern Iran) form the caprock of the Lower Miocene gas reservoirs. Their stratigraphic and sedimentological studies have been of interest to geological studies. Due to their importance, in this research, two suitable stratigraphic sections in the Zanjan and Hashtroud regions (northwest of Iran) have been subjected to detailed sedimentological and stratigraphic study to determine the origin and tectonic setting. Clastic-evaporite sediments of the middle to upper Miocene (Upper Red Formation) in the northwest of Iran are about 950 m thick, which is placed on the hydrocarbon reservoirs of the Qom Formation (Lower Miocene) with a discontinuous and abrupt boundary, and are covered by conglomerate and unhardened sediments belong to Pliocene. These sediments in the northwest of Iran are divided into four lithostratigraphic units, including (1) green marls with gypsum, (2) red marls along with gray and red sandstones, (3) red and green marls, and (4) green marls. Based on microscopic studies, the sandstones of this formation are composed of sedimentary and metamorphic rock fragments, quartz, and to a lesser extent, feldspar, pyrite and chlorite, in order of importance. The matrix (carbonate mud) and, to a lesser extent, calcite and evaporite cements form the groundmass of studied sandstones. Based on petrographic studies, these fine-to medium-grained sandstones are litharenite to feldspathic litharenite with an average composition of Q38F13RF49. Also, based on the main constituent components of sandstones and geochemical analysis of major and minor elements, and by drawing different types of separating diagrams, the tectonic setting of these sandstones are considered as an active continental margins and foreland basins (collision belts between Arabia-Eurasia continental plates), which originate from fine-to medium-grained siliceous-clastic rocks, metamorphic, carbonate, and intermediate to felsic igneous rocks outcropped in the region. The data show that these sediments are influenced by arid to semi-arid climate and moderate degree of chemical weathering, mainly from the outcrops of Takab Complex and Qom Formation (Lower Miocene), which have significant outcrops and, to a lesser extent, from Eocene volcanics. The research results are consistent with the paleogeography of the region and previous studies and can be of interest to sedimentologists and other researchers related to petroleum geology.

Geological Characteristics and Paleoenvironmental Evolution of Fine-Grained Sediments in the Third Member of the Xujiahe Formation in the Western Sichuan Depression, SW China

This study investigated, in detail, the characteristics of the Late Triassic fine-grained sediments in the third member of the Xujiahe Formation (Xu-3 Member), in the Western Sichuan Depression, and the paleoenvironmental evolution during their deposition through petrological interpretation, mineralogical composition characterization, and element geochemical analysis. According to the mineralogical composition, the Xu-3 Member can be divided into two petrological types, namely clayey fine-grained felsic sedimentary rocks and lime fine-grained felsic sedimentary rocks. The main mineral components are siliceous, clay, and carbonate minerals. Through the cluster analysis of major elements, all samples could be divided into two types with different major elemental characteristics. Trace elements exhibited distinct Sr depletion, relative enrichment of large ion lithophile elements, and high field strength elements. Two REE enrichment patterns were observed, which could be attributed to differences in the provenance area and tectonic background. The paleoclimate of the sedimentary area was warm and humid, but it was hotter and drier in the southern and central parts of the depression. The change trend of paleo-productivity was consistent with the paleoclimate. The waters in the sedimentary environment were mainly brackish water to saline water, with fresh water in the southern part of the depression. The paleo-redox conditions of the waters were mainly sub-oxidation to sub-reduction, but the southern part of the depression was more oxidative. The provenance area experienced a moderate degree of chemical weathering under a warm and humid paleoclimate, same as the depositional area. However, the depositional environments differed between the northern and south-central parts of the depression.

Tracing the sedimentary provenance of the Mesoproterozoic rocks from Taoudeni Basin (∼ 1.1 Ga) Mauritania: Evidence from Sm/Nd and elemental geochemistry

The Mesoproterozoic era is a crucial period in Earth history characterized by muted oxygen levels in the atmosphere–ocean system. This period is well represented in the non-metamorphosed sedimentary rocks from the Taoudeni Basin, Mauritania. Few studies have been carried out to trace the sources of these sedimentary archives. Here, we examined the sedimentary rocks from the Atar (S4 drill core) and El Mreiti groups (S2 and S3 drill cores). This study focuses on whole-rock elemental geochemistry and Sm/Nd isotope compositions to provide insights into the provenance of the fine-grained siliciclastic sediments from the basal part of the Mesoproterozoic Atar and El Mreiti groups. Paleoweathering indices reveal a moderate degree of chemical weathering in the source areas, consistent with the low compositional maturity of the studied samples. The concentrations of the relatively immobile trace elements suggest mixing sources of sediment provenance, which is dominated by felsic source rocks for the Atar sediments. However, distinctively positive ƐNd(t) values coupled with high 143Nd/144Nd and Sm/Nd ratios and high ƒSm/Nd values of the sediments from the El Mreiti Group indicate derivation from juvenile and less evolved sources rocks. Further, the depleted mantle model ages (TDM) of the provenance protolith (1.95–2.31 Ga) support the argument that the El Mreiti sediments are dominantly sourced from the Paleoproterozoic terrane exposed in the eastern Reguibat Shield while the Atar Group were predominantly from the Archean basement rocks in the western Reguibat Shield. Moreover, Sm/Nd isotope and mixing model results support the major contributions of the felsic Archean rocks of the Reguibat Shield for the Atar Group samples. Although these strata, separated by ∼1000 km, have been described as stratigraphic equivalent, our study demonstrates distinct sediment sources between the Atar Group strata and those of El Mreiti, therefore, reinforcing the importance of sediment provenance in understanding the evolution and reconstruction of modern and ancient sedimentary systems.

The genetic relationship between paleoenvironment, mineral compositions and lithofacies in the Ordovician–Silurian Wufeng–Longmaxi sedimentary succession in the Sichuan Basin, SW China

• Five intervals (I–V) showing changes in minerals, lithofacies and paleoenvironment were identified. • High productivity and low detrital influx lead to the enrichment of nondetrital quartz in siliceous shale in Intervals I-II. • The enrichment of carbonate in calcareous shale in Interval III was due to high detrital influx. • High detrital influx and strong chemical weathering lead to an abundance of silt and clay minerals in silty–argillaceous shale and argillaceous shale in Intervals IV-V. Changes of sedimentary environments recorded in the Late Ordovician–Early Silurian sedimentary succession can be reflected in mineral composition and lithofacies of shales. Outcrops and cores from this set of shales in the Sichuan Basin, China, are analyzed. The results show that quartz, clay minerals, and carbonate are the main minerals. Five major lithofacies are developed. Based on the provenance proxies (Al 2 O 3 /TiO 2 , Th/Sc, and Zr/Sc ratios), chemical index of alteration (CIA), paleoproductivity proxies (Cu/Al and Ni/Al ratios), redox condition proxies (U/Th, V/Cr, and Ni/Co ratios), proxies of detrital influx (Ti and Al contents), and sedimentation rate (SR), the paleoenvironment is analyzed. The source rocks are dominated by felsic igneous rock. Comparison between five lithofacies revealed that the relatively low degree of chemical weathering, anoxic conditions, maximum paleoproductivity probably associated with the high frequency of volcanic activities, and minimum detrital influx and SR (0.60–5 m/Ma) led to the enrichment of nondetrital quartz and organic matter, which eventually formed siliceous shale. The moderate degree of chemical weathering, dysoxic–oxic conditions, decreasing paleoproductivity, and increasing detrital influx and SR (10–16.67 m/Ma) resulted in the increase in detrital quartz, clay minerals, and carbonate content, and the development of mixed shale and calcareous shale. Relatively strong chemical weathering, oxic conditions, minimum paleoproductivity, and maximum detrital influx and SR (100.74 m/Ma) favored the enrichment of silt and clay minerals, forming silty–argillaceous shale and argillaceous shale. This study provides an insight into the relationship between the environmental changes and shale mineral composition and lithofacies.

Petrography and geochemistry of late Devonian sandstones (Shishtu Formation): Middle East central Iran

The petrography of sandstones integrated with geochemistry is important to infer the provenance, paleoclimate, and tectonic setting. An integrated petrographic and geochemical study of the late Devonian sandstones, Shishtu Formation in central Iran was performed to determine the provenance, tectonic setting, and weathering history. These sandstones are classified into quartz arenite, subarkose, and litharenite types. Sandstones consist of quartz, feldspars, and sedimentary rock fragments, and also provenance shows a recycled orogen and craton interior. Moreover, petrographic data indicate quartz-rich sedimentary (recycled), metamorphic, and plutonic parent rocks. Major and trace elements suggest a quartzose sedimentary provenance in a passive continental margin. As indicated by the chemical index of weathering (CIA), the sandstones underwent a moderate degree of chemical weathering. Diagenetic processes consist of cementation, compaction, and dolomitization. Dolomitization in the carbonate particles of sandstones in the upper part of organic Frasnian-Famennian shale indicates the potential for producing Mg fluid and shows that this unit differs from other shales in these strata.

Provenance, source weathering, and tectonic setting of the lower Cambrian Shuijingtuo Formation in the Middle Yangtze area, China

The organic-rich shale of the Lower Cambrian Shuijingtuo Formation is an important hydrocarbon source rock in South China. The geochemical characteristics of this shale were investigated to determine the provenance, weathering conditions, sedimentary recycling, paleoclimate, and tectonic setting of the Shuijingtuo Formation in Yichang in the Middle Yangtze area. The shale is relatively enriched in CaO, P2O5, Mo, U, V, Cd, and Cu. The rare earth element (REE) patterns of the shale exhibit light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion. It has weak negative Eu and Ce anomalies, which are similar to the characteristics of passive margin deposition. The analysis of the carbonate dilution effect conducted in this study reveals that carbonate dilution has a greater effect on the MgO, Ba, Sc, and Zr contents. The provenance discrimination diagrams suggest that the shale is primarily derived from a mixture of intermediate and felsic igneous source rocks, which were similar to Tonalite-trondhjemite-granodiorites (TTGs) in composition. The chemical index of alteration (CIA) values and the A–CN–K ternary diagram indicate that the source area of the shale experienced a low degree of chemical weathering under a cold and arid climate in the early period and a moderate degree of chemical weathering under a warm and humid climate in the later period. The relationship between Zr/Sc and Th/Sc indicates that the shale was deposited by the first cycle of sedimentation rather than via reworking. The discriminant function diagrams, major element ratios, immobile trace element ratios, and ternary relationships between La, Th, Sc, Zr, and Co indicate that the source rocks were deposited in a passive margin tectonic setting. It is suggested that the Ezhong old land, which is predominantly composed of the Huangling granite and the Kongling complex (composed of intermediate and felsic igneous rocks) was the source area.

Aquatic geochemistry of a major freshwater lake in the Kashmir Himalaya: solute acquisition and denudation process in the lacustrine system.

Lakes, the main entities of lacustrine environments, are a rich archive of environmental and geogenic changes in terms of compositional variation of water and sediment. Water and sediment samples (N = 173) were collected during 2013-2014 from the Wular Lake, one oftheimportant fresh lakeswithin the Indian landmass. The study provides insights on the solutes acquisition mechanism and provenance of ionic constituents within the lake water and the sediments. Besides, the impact of catchment attributes on the lake system was in addition assessed. The hydrochemical results suggest that the chemical weathering of silicate and carbonates within the catchment shapes the lake water chemistry and characterizes the facies pattern into a hybrid type. The geochemical results of the lake sediments demonstrate that the improved abrasion rates and ensuant settling of detritus into the lake are closely linked with the prominent physical weathering over chemical weathering. The new finding of the present study is that sediments represent an unweathered basalt compositional trend, plausible provenance from mafic rocks, experiencing low to moderate degree of chemical weathering. The study found that increased encroachment within the lake catchment due to continued anthropogenic forcing is the primary source contributing the organic matter (OM) as well as the higher levels of Cl, NO3, SO4, and P to the lake. These findings corroborate with the land use-land cover changes (from the last 50years) within the lake catchment in significantly deteriorating the lake system. The study recommends that the ongoing conversion of lake peripheral areas into urban settlement and agro-horticulture land by filling activities should be restricted.

Geochemistry of the mudrocks and sandstones from the Bredasdorp Basin, offshore South Africa: Implications for tectonic provenance and paleoweathering

Abstract An inorganic geochemical investigation of mudrocks and sandstone from the southern Bredasdorp Basin, off the south coast of South Africa was carried out to unravel the provenance, paleoweathering, and tectonic setting of the basin. Seventy-seven representative samples from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3 underwent geochemical analysis involving major and trace elements. The major oxide compositions show that the sandstones could be classified as sub-arkose and sub-lithic arenite. The provenance discrimination diagrams based on major oxide geochemistry revealed that the sandstones are mainly of quartzose sedimentary provenance, while the mudrocks are of quartzose sedimentary and intermediate igneous provenances. The discrimination diagrams indicate that the Bredasdorp sediments were mostly derived from a cratonic interior or recycled orogen. The bivariate plots of TiO2 versus Ni, TiO2 against Zr, and La/Th versus Hf as well as the ternary diagrams of V–Ni–Th∗10 suggest that the mudrocks and sandstones were derived from felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance. Also, the closely similar compositions of the analysed samples and recent sedimentary rocks of the East African Rift System perhaps suggest a rifted basin tectonic setting for the Bredasdorp Basin. Chemical index of alteration (CIA) indices observed in the sandstones suggest that their source area underwent low to moderate degree of chemical weathering. However, the mudrocks have high CIA indices suggesting that the source area underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations.

Provenance and tectonic setting of the Neoproterozoic meta-sedimentary rocks at southeastern Tibetan Plateau: Implications for the tectonic affinity of Yidun terrane

The Yidun terrane is located between the Qiangtang terrane and Songpan-Ganzi Fold Belt and its tectonic affinity with other adjacent terrane is of significance to understand the tectonic evolution of southeastern Tibetan Plateau. In this study, we present integrated bulk geochemical data, detrital zircon U-Pb ages and Hf isotopes of meta-sedimentary rocks from the Qiasi Group, which represents the oldest Precambrian basement of the Yidun terrane, to constrain the depositional age and provenance of the meta-sedimentary rocks as well as the tectonic affinity of the terrane. Zircon U-Pb dating results show that detrital zircons from three samples have almost identical age spectra with two major age clusters of 800–780 Ma and 850–820 Ma at age peaks of ca. 790 Ma and ca. 827 Ma, and one minor age cluster of 910–860 Ma. The youngest group of detrital zircons reveals that the maximum depositional age of the Qiasi Group was 779–777 Ma. Low SiO2/Al2O3 (2.9–4.1), CIA (59–47), and PIA (63–47) values indicate that the sediments could be derived mainly from the low mature sources that had undergone weak to mildly moderate degree of chemical weathering. Bulk geochemical and zircon U-Pb-Hf isotopic data suggest that the provenance was composed mainly of Neoproterozoic intermediate to felsic igneous rocks. In combination with the published geological data and detrital zircon U-Pb age spectra on the western Yangtze Block, and the Kunlun-Qilian-Qaidam Block, we propose that the Neoproterozoic basement of the Yidun terrane has a close tectonic affinity with the former one.

Provenance, palaeoweathering and tectonic setting of the Ediacaran Bayandor Formation in NW Iran: Implications for the northern Gondwana continental margin during the late Neoproterozoic

The tectonic evolution of Iran during the late Neoproterozoic and early Paleozoic is usually expressed in two models including passive (conventional) and active (alternative) margin tectonic models. The passive margin model proposes rifting of peri-Gondwanan terranes and strike-slip tectonics in a non-magmatic setting while the active margin model suggests the existence of a subduction zone and the formation of a continental arc (known as “Cadomian” arc) along the northern Gondwana margin. The Ediacaran siliciclastic Bayandor Formation from the Soltanieh Mountains in western Alborz records the geological history of Iran during the late Neoproterozoic through its provenance, as aims of our studies, which allows palaeoweathering and tectonic setting reconstructions. Thin-section petrographic analysis of the Bayandor Formation sandstones reveal that the feldspar-rich sandstones including minor values of chert rock fragments have characteristics of both active continental margin settings and recycled orogen provenances. The chemical index of alteration (CIA) indicates a moderate degree of chemical weathering in the source area. Most of the Bayandor Formation sandstones were derived from a dissected arc and a minor number of the samples seem to be derived from a basement uplift and recycled orogen provenance. Geochemical discrimination diagrams using major and trace elements indicate both active continental margin and island arc tectonic settings for the Bayandor Formation sandstones. The results indicate that it is more likely that deposition of this formation took place along an active continental margin and the Cadomian basement of NW Iran comprises a series of metagranites and metamorphic rocks and may have contributed some sediment to the localized Neoproterozoic–Cambrian basins. Other sources were likely located in the basement provinces particularly the Arabian–Nubian Shield that is consistent with the results of this study.

Geochemical constraints on provenance and source area weathering of metasedimentary rocks from the Paleoproterozoic (~2.1 Ga) Wa-Lawra Belt, southeastern margin of the West African Craton

ABSTRACTThe Wa-Lawra Belt which is situated in the northern part of Ghana consists of Paleoproterozoic Birimian fine metasedimentary rocks metamorphosed to greenschist facies, particularly, in the western part. A whole-rock geochemical study of these metasedimentary rocks was undertaken to unravel their source area weathering, provenance and tectonic setting. Geochemical characteristics of the studied shales show that they are immature in nature and first cycle in origin, with little or no recycled component. Compared to Post-Archaean Australian Shales (PAAS), the studied shales indicate reduction in Zr, Hf, La, Nb, Th and Ta being the high field strength elements and evidences of transition metal enrichments in V, Ni, Sc, Co, and Cr. Major element geochemistry indicates that the shales were subjected to slight potassium metasomatism after deposition. Pre-metasomatized Chemical Index of Alteration calculations indicates that weak to moderate degree of chemical weathering took place at the sediment source area. Co-Th-La-Sc systematics reveals a combination of mafic and felsic provenances for the shales. Eu/Eu* together with values of Th/U and some abundances of trace elements show that the shales were mainly derived from juvenile rocks. Average REE model calculations suggest that the source materials are composed of about 49% basalt, 16% TTG and 35% granite.

Geochemistry and weathering history of the Balfour sandstone formation, Karoo basin, South Africa: Insight to provenance and tectonic setting

Geochemical analysis on sandstones from clastic sediments was carried out to understand the tectonic setting and subsequent post-depositional change in the Karoo basinal fill of the fluvial deposits of the Balfour Formation during the Late Permian to Early Triassic period. The major and trace element analysis reveal a relatively homogeneous provenance for the sandstones. The geochemical analysis shows that these rocks are first order mature sediments, derived from igneous and/or meta-igneous rocks of a felsic composition. The results show that the sandstone consists of SiO2 (71.58 wt. %), followed by Al2O3 (14.48 wt. %), but with low contents of Fe2O3+MgO (4.09 wt. %) and TiO2 (0.47 wt. %). These sandstones are classified as litharenites and arkoses based on the elements constituent ratio of various schemed adopted. The sandstone in the provenance discrimination diagram plots in the dissected and transitional arc fields suggesting an active margin and continental island arc provenance, preserving the signature of a recycled provenance. The Chemical Index of Alteration (CIA) ranging from 63.56 to 67.10% suggests recycling processes, and that the source area has undergone a moderate degree of chemical weathering. The geochemical characteristics of the sediment suggest the source area of uplifted terrane of folded and faulted strata with detritus of sedimentary and metasedimentary origin.

The response of transitional pedogenic characteristics of loess in the Yunxian Basin to abrupt climatic events in the northern subtropics since the Last Glacial Maximum

The transitional pedogenic characteristics of loess deposits in the Yunxian Basin have provided solid evidence for understanding the mechanisms of dust transportation, deposition, and detailed documentation of climate instability since the Last Glacial Maximum (LGM) in the northern subtropics of China. The Heinrich Stadial 1b (HS 1b, 16.9–14.7 ka B.P.) event, Bϕlling/Allerϕd (B/A, 13.9–13 ka B.P.) period, Younger Dryas (YD, 12.9–11.7 ka B.P.) event, and an environmental event (5–4 ka B.P.) during the Holocene Optimum were imprinted by the high-resolution multi-proxy of loess-paleosol in the Yunxian Basin, China. Based on granularity and chemical analyses, the loess in the Yunxian Basin can be classified as a fine, clayey loess zone that experienced a moderate degree of chemical weathering. The mechanism forming the transitional pedogenic characteristics was explained by the dust storm frequency data (1957–2007), particle-size data, and multi-proxy records of chemical weathering extent, which was mainly caused by the moderate transport distance from the common provenance and the transitional climatic characteristics of the northern subtropical zone.

Fluctuations in chemical weathering on the Yangtze Block during the Ediacaran–Cambrian transition: Implications for paleoclimatic conditions and the marine carbon cycle

The Ediacaran–Cambrian transition was characterized by profound climatic, tectonic, and oceanographic fluctuations, followed by the remarkable radiation of metazoans. To further understand the paleoclimatic and paleoenvironmental conditions during this interval, comprehensive mineralogical and geochemical analyses were performed on Ediacaran–Cambrian shales (in the Doushantuo through Jiumenchong to Bianmachong formations) at Daotuo, South China. Their major and trace element compositions and ratios indicate that all of the studied shales were derived from basically uniform source rocks that were similar to granodiorite. The chemical index of alteration (CIA) values in the Doushantuo and Bianmachong formations mostly range from 65 to 80, indicating that they experienced a moderate degree of chemical weathering and were thus deposited in a warm and humid climate. In contrast, a sharp drop in CIA values (~60–70) in the lower Jiumenchong Formation, together with decreased Al/Ti ratios that are indicative of increased eolian inputs, are suggestive of a relatively cold and arid climate for the Early Cambrian Yangtze Block. In this scenario, eolian fluxes could have supplied nutrients to the marine environment; increased upwelling, along with a rising thermocline and nutricline during the extensive marine transgression, could have returned abundant nutrients to the surface seawater, thus greatly stimulating primary productivity and intensifying anoxic conditions, which would ultimately have resulted in a high degree of organic burial in the sediments. Therefore, the negative correlation between the corrected CIA values and the total organic carbon (TOC) content may imply that climate-related changes played a role in the enrichment of organic matter during the Early Cambrian.

Depositional environments, provenance and paleoclimatic implications of Ordovician siliciclastic rocks of the Thango Formation, Spiti Valley, Tethys Himalaya, northern India

Recently published findings indicate that the Ordovician period has been much more dynamic than previously anticipated thus making this period significant in geological time. The Ordovician of India can best be studied in the Spiti region because the Spiti basin records the complete uninterrupted history of excellent marine sedimentary rocks starting from Cambrian to Paleogene which were deposited along the northern margin of India. Due to these reasons the geochemical data on the Ordovician rocks from the Spiti region is uncommon. The present geochemical study on the Ordovician Thango Formation (Sanugba Group) is mainly aimed to understand the provenance and the paleoclimatic conditions. The sandstones are the dominant lithology of the Thango Formation with intercalations of minor amount of shales. Detailed petrographic and sedimentological analysis of these rocks suggest that three major depositional environments, viz., fluvial, transitional and marine prevailed in the basin representing transgressive and regressive phases. The major and trace element ratios such as SiO2/Al2O3, K2O/Na2O and La-Th- Sc discrimination diagram suggest that these rocks were deposited in passive margin tectonic settings. Various geochemical discriminants and elemental ratios such as K2O/Na2O, Al2O3/TiO2, La/Sc, Th/Sc, Cr/Th, Zr/Sc, (Gd/Yb)N and pronounced negative Eu anomalies indicate the rocks to be the product of weathering of post-Archean granites. The striking similarities of the multi-elemental spider diagrams of the studied sediments and the Himalayan granitoids indicate that sediments are sourced from the Proterozoic orogenic belts of the Himalayan region. Chemical index of alteration (CIA) values of the studied sediments (55–72) suggest that the source rocks underwent low to moderate degree of chemical weathering. The span of the CIA values (55–72) recorded in the sediments from the Spiti region may have resulted from varying degrees of weathering conditions in the source area. This, in turn, reflects variable climatic conditions prevailing during the Ordovician period, which is consistent with the global studies. The high CIA values observed by numerous studies on siliciclastic sediments may imply warm paleoclimate attributed to an increase in atmospheric carbon dioxide and/or methane levels inducing an acidic weathering environment and enhanced silicate weathering. While the low CIA values indicates prevalence of arid-glacial conditions where negligible chemical alteration is possible. This study documents the noteworthy events like fluctuating (rise and fall) atmospheric CO2 and sea levels during the Ordovician period, a conclusion which is consistent with the other such studies on the Phanerozoic glaciation (or mass extinction) events on Gondwana supercontinent.

Geochemical and Nd-isotopic compositions of juvenile-type Paleoproterozoic Birimian sedimentary rocks from southeastern West African Craton (Ghana): Constraints on provenance and tectonic setting

The Paleoproterozic Birimian Supergroup consists of sedimentary basins composed predominantly of wackes and argillites, and volcanic belts composed predominantly of basaltic rocks. Whole rock geochemical and Nd-isotopic studies were carried out on fine-grained sedimentary rocks (shales and fine-grained sandstones) from one of the Birimian sedimentary basins, the Kumasi basin. The sedimentary rocks are immature in nature, with very little recycled materials. Relative to average early Proterozoic upper continental crust, the Birimian sedimentary rocks are depleted in the alkalis and alkaline earth elements and the high field strength elements, and enriched in transition metals. Chondrite-normalized rare earth element (REE) plots for both the shales and sandstones show fractionated LREE patterns (average LaN/SmN=3.45), with slight negative Eu-anomalies (average Eu/Eu∗=0.79) and a slightly fractionated HREE patterns (average GdN/YbN=1.84). Low to moderate degree of chemical weathering in the sediment source area is indicated by Chemical Index of Alteration values of between 52 and 80. Elemental ratios such as La/Co, Th/Co, Th/Cr, and Eu/Eu∗ that are critical to provenance suggests that the sediments were derived from rocks of mixed felsic and mafic compositions. Mixing calculations based on the REEs indicate that, the shale is best modeled with a mixture having 62% TTG, 16% granite and 22% basalt whereas the sandstone is almost 100% TTG. The εNd values for the analyzed samples, calculated at the age of deposition (i.e., 2.1Ga), range from −1.69 to 1.07, suggesting derivation from an undifferentiated source. The Nd-isotopic compositions, together with Eu/Eu∗ and Th/U values and other trace element characteristics suggest Young Undifferentiated Arc provenance, and this is in agreement with previous studies that show that the Birmian crust is juvenile and little amount of Archean crust is present in the sedimentary rocks. The geochemical and Nd-isotopic systematics further suggest that the bulk of detritus was supplied by adjacent Paleoproterozoic Birimian greenstone belts with only 10% or less contribution from the Archean crust. Trace element characteristics further suggest that deposition took place in an island arc setting.

Geochemistry of metasedimentary rocks from the Puncoviscana Complex in the Mojotoro Range, NW Argentina: Implications for provenance and tectonic setting

Low-grade metasedimentary rocks from the Lower Paleozoic Puncoviscana Complex of the Mojotoro Range in NW Argentina have been analyzed for their major and trace elements - including rare earth - in order to constrain their provenance and depositional tectonic setting. They show a moderate degree of chemical weathering with intermediate CIA (<75) values and can be classified as metawackes and metapelites. Provenance discrimination diagrams based on major element geochemistry and trace element ratios (i.e., Cr/V, Y/Ni, Th/Co, La/Sc, Th/Sc, Eu/Eu*) suggest that the sediments were derived from upper crustal, felsic source rocks. Tectonic setting discrimination diagrams based on major elements, immobile trace elements (i.e., La- Th- Sc and Th-Sc-Zr/10) and La/Yb vs. Eu/Eu* plots, support an active margin setting associated with a magmatic arc located to the east of the Neoproterozoic–Cambrian Pampean Brazilian arc that formed along the margin of a Mesoproterozoic craton.

Provenance, tectonic setting and source-area weathering of the lower Cambrian sediments of the Parahio valley in the Spiti basin, India

The geochemical study of siliciclastic rocks from the Lower Cambrian of Parahio Valley has been studied to describe the provenance, chemical weathering and tectonic setting. The K2O/Al2O3 ratio and positive correlation of Co (r=0.85), Ni (r=0.86), Zn (r=0.82), Rb (r=0.98) with K2O reflects that the presence of clay minerals control the abundances of these elements and suggests a warm and humid climate for this region. The chondrite normalized REE pattern of the samples is equivalent to upper continental crust, which reflects enriched LREE and flat HREE with negative Eu anomaly. The tectonic setting discriminant diagram log[K2O/Na2O] vs. SiO2; [SiO2/Al2O3] vs. log[K2O/Na2O]; [SiO2/20] – [K2O+Na2O] – [TiO2+Fe2O3+MgO] indicates transitional tectonic setting from an active continental margin to a passive margin. The discriminant function plot indicates quartzose sedimentary provenance, and to some extent, the felsic igneous provenance, derived from weathered granite, gneissic terrain and/or from pre-existing sedimentary terrain. The CIA value indicates low to moderate degree of chemical weathering and the average ICV values suggests immature sediments deposited in tectonically active settings. The A–CN–K diagram indicates that these sediments were generated from source rocks of the upper continental crust.