Average Chemical Index Of Alteration Research Articles

Elemental geochemistry of Cretaceous deposits in the Dahomey Basin (Nigeria): Implications for paleoclimatic and paleoenvironmental reconstructions

The Dahomey Basin, located in southwestern Nigeria, is an energy-rich frontier basin with sedimentary successions consisting of both (onshore) terrestrial and (offshore) marine deposits. Since the discovery of several bitumen seeps, tar sands, and oil and gas shows offshore the basin and the successful production of the Aje Field, the basin has gained immense interest from academia and industry. Nevertheless, surface and subsurface research focusing on paleoclimatic conditions, primary productivity, anoxic conditions, paleowater depth, and hydrodynamic influence in the Dahomey Basin is less documented. To reconstruct paleoenvironmental conditions, subsurface data from the Araromi-Obu (A1) and Idiobiolayo (A15) cores were used. The average chemical index of alteration (CIA) values ranging from 63.01 to 94.88 reflect varying degrees of weathering intensity in the sediment source area of the Cretaceous Araromi Formation. These values show a transition from relatively low to considerably high degrees of weathering in the source region of the Araromi Formation. The analysis of CIA values and Ga/Rb, Sr/Cu, Sr/Ba, Ca/Al, and 100*Mg/Al ratios suggests that the Araromi Formation was deposited in a brackish to marine environment.Furthermore, chemical alteration-sensitive geochemical indicators revealed prevailing warm, humid tropical climates during the late Cretaceous period, with likely minor arid intervals. In addition, the geochemical markers used (Ba/Al, U/Th, Ni/Co, authigenic uranium (Uau), P/Ti, and [(Fe2O3 + CaO + MgO)/(SiO2 + Al2O3)] indicate poor paleoproductivity for the Araromi Formation deposited in an oxygen-rich marginal to shallow marine environment. The upper Cretaceous period was characterised by fluctuating hydrodynamic circulation, with a weak hydrodynamic regime in core A1 and transitioning to strong hydrodynamic conditions in core A15. The inferred paleowater depth corresponds to the salinity measurements, with the A1 core displaying elevated salinity, indicating a greater depth compared to the A15 core, which was deposited in a shallower environment and displays lower salinity.

Geochemistry of the Lancang River (Upper Mekong River) overbank sediments: Implications for provenance, weathering and sedimentary characteristics

Large rivers originating from the Tibetan Plateau play a crucial role in sediment production and transportation and thus influence the ocean composition and global climate. This study focuses on the geochemical characteristics, including major, trace and rare earth elements of overbank sediments from the Lancang River (Upper Mekong), which drains southwest China and is one of the largest international rivers in Asia. We characterize sediment maturity and recycling, weathering and provenance in a typical, tectonically active cold-humid climate region. Lancang River (LR) sediments with an average chemical index of alteration (CIA) of 73 ± 6 and an average index of compositional variability (ICV) of 1.1 ± 0.6 suggest an intermediate mineralogy and chemical maturity corresponding to moderate weathering in a slightly warmer climate. The provenance of LR sediments is characterized by felsic source rocks, similar to the upper continental crust (UCC), followed by recycled sedimentary rocks, which were possibly derived from the Yidun, North Qiangtang (NQ) and Songpan-Ganzi (SGZ) terranes at upstream locations when the Lingcang basalt (LCB) provided mafic rocks for LR sediments. However, recycled materials have limited impacts on sediment maturity, as reflected by the degree of weathering. Highly physical weathering (erosion) has created a massive source for LR sediments due to active tectonics, while the impacts of climate on weathering processes have increased. Meanwhile, the overbank sediment in large rivers could be a good sampling medium for evaluating terrestrial weathering, provenance and sediment characteristics using geochemical evidence.

Quantitative characterization of vertical zonation of Mesozoic granite weathering reservoirs in the coastal area of eastern Fujian Province, China

Weathering crust reservoirs have obvious vertical zonation, which is the focus of weathering crust reservoir research, but there is a lack of quantitative characterization indexes. To achieve the quantitative characterization of granite weathering crust reservoir and provide the basis for oil exploration of granite weathering crust buried hill reservoir, in this paper, the vertical zonation of granite weathering crust reservoir is quantitatively divided by testing and analyzing the uniaxial compressive strength (UCS), magnetic susceptibility (MS), permeability, and chemical index of alteration (CIA) of the Mesozoic granite weathering crust in the coastal area of eastern Fujian. The results show that the granite weathering crust reservoir can be divided into four zones vertically: a soil zone (SZ), weathered and dissolved zone (WDZ), fracture zone (FZ), and bedrock zone (BZ). A cataclastic area is developed in the FZ and BZ, in which structural fractures are well-developed, the fracture surface density is usually greater than 200 m/m2, and the contribution to the fractures in the rock mass is up to about 50%, making this the sweet spot of the reservoir. In the SZ, the rocks are loose, and the pores are well-developed. The UCS is less than 10 MPa, and the average rate of change of the UCS (Δσ) is 0.90. The average permeability is 2823.00 mD, and the average rate of change of the permeability (Δκ) is 5.13. The average CIA is 74.9%. The average clay mineral content is 7%. The rocks in the WDZ have been significantly weathered by physical and chemical processes, and the weathering fractures and dissolution pores are well-developed. The average UCS is 18.2 MPa, and the average Δσ is 0.70. The average permeability is 143.80 mD, and average Δκ is 4.17. The average CIA is 65.3%. The average clay mineral content is 4%. Under the influence of tectonic movement and physical weathering, the rocks in the FZ have developed structural fractures and a few weathered fractures. The average UCS is 57.9 MPa, and the average Δσ is 0.18. The average permeability is 5.50 mD, and the average Δκ is 2.55. The average CIA is 61.6%. The average clay mineral content is 2%. In the BZ, the rocks are intact and hard. The average UCS is 69.9 MPa, and the average Δσ is 0.13. The average permeability is 1.46 mD, and the average Δκ is 1.43. The average CIA is 57.8%. The average clay mineral content is less than 1%. The multi-parameter combination of the UCS, Δσ, permeability, Δκ, CIA, and clay mineral content achieved good results in the division of the zones of the weathering crust. The UCS increases gradually from top to bottom, while Δσ, permeability, Δκ, CIA, and clay mineral content all decrease gradually. In addition, based on the petrophysical parameters of the rocks, including the density, resistivity, and acoustic velocity, a good division effect was also achieved, which can provide a basis for the vertical zonation of the granite buried-hill weathering crust reservoir.

Late Campanian Climatic-Continental Weathering Assessment and Its Influence on Source Rocks Deposition in Southern Tethys, Egypt

Climatic variability and silicate weathering are remarkable features throughout the Late Cretaceous period. Late Campanian black shale is considered the most significant silicate source rock in the southern Tethys. Here, we used mineralogical and geochemical data to evaluate the continental weathering intensity and climatic changes as well as their impact on the deposition of the Late Campanian black shale in the Western Desert of Egypt. The studied black shale has a relatively high concentration of Al, Fe, Mg, Ca, Sr, Ga, Co, Cr, and V when compared to the average Post-Archean Australian Shales (PAAS). The studied samples have elevated values of Ga/Rb, and low values of Rb/Sr, Sr/Cu, and K2O/Al2O3, supporting the deposition of Late Campanian shale under warm/humid conditions. Furthermore, the average chemical index of alteration (CIA, 78.6%), chemical index of weathering (CIW; 83.8%), C-value (1.26), Fe/Mn (408), and Mg/Ca (1.54) reveal the predominance of warm/humid climate. The chemical weathering proxies (CIA, CIW, PIA, LnAl2O3/Na2O) and ACNK diagram imply that the Late Campanian samples were exposed to a moderate grade of chemical alteration. The deposition of black shale occurred under high seawater salinity conditions based on Sr/Ba (Avg = 3.6). Additionally, the weathering indices are well correlated with paleoclimatic proxies, suggesting that weathering intensity is strongly affected by paleoclimate. However, chemical weathering during the Late Campanian has a weak influence on oceanic nutrient fluxes. No substantial impact of the paleoclimate during the deposition of Late Campanian black shale on water salinity was reported.

Geochemistry of the Permian–Triassic boundary strata in the southwest margin of the North China Craton: Provenance and tectonic implications

The Late Permian Sunjiagou Formation and Early Triassic Liujiagou Formation were studied to understand the palaeoclimatic and palaeoenvironmental changes in the southwest margin of the North China Craton across the Permian–Triassic boundary, as well as the tectonic evolution of their provenance sediment. Major, trace, and rare earth elements in 20 clastic rock samples from the strata above and below the boundary were analysed. Overall, the Sunjiagou Formation samples showed an average index of compositional variability (ICV) and chemical index of alteration (CIA) of 1.53 and 73.59, respectively, indicating high‐maturity sediments in a warm and humid environment with moderate chemical weathering, whereas Liujiagou Formation samples had an average ICV and CIA of 2.40 and 70.1, respectively, indicating low‐maturity sediments in a cold and dry environment with mild chemical weathering. These strata were composed of first‐cycle sediments in a tectonically active area. The provenance of the Sunjiagou Formation was dominated by quartz‐rich sedimentary rocks, whereas that of Liujiagou Formation had both quartz‐rich sedimentary rocks and intermediate volcanic rocks. The provenance tectonic settings were mainly the continental island arc and active continental margin. Thus, the study area was in a terrestrial, freshwater sedimentary environment from the Late Permian to Early Triassic, and the climate gradually changed from warm and humid to dry and hot, with multiple small‐scale periodic evolutions. Taken together, these findings further confirm that the entire study area was in an oxidizing environment and the oxidation degree in the Early Triassic was higher than that in the Late Permian.

Controlling the Iron Migration Mechanism for the Cretaceous Sediment Color Variations in Sichuan Basin, China.

Cretaceous continental sediments in Sichuan Basin, China, have different colors, and the reasons for their formation are not determined. Based on mineralogical and geochemical characteristics, red beds and nonred beds in the Upper and Lower Cretaceous sedimentary strata in the western Sichuan Basin are described and tested in this study. The test and analysis of the mineral composition, element content, and iron speciation of mudstone samples with gray-green, gray, and red colors in Cangxi, Bailong, and Guankou formations found that the change in hematite content directly causes the color difference of samples. For red mudstone, the average chemical index of alteration, chemical index of weathering, weather eluviation index (Ba), Ca/(Mg*Al), and Al2O3/SiO2 index are 67.75, 79.94, 2.07, 0.26, and 0.26, respectively, indicating that chemical weathering is the most intense. The geochemical indexes corresponding to gray samples are 64.41, 74.91, 2.08, 0.19, and 0.24, respectively. Those corresponding to the gray-green samples are 62.30, 70.68, 2.17, 0.21, and 0.24, with the weakest chemical weathering. The ratio of Cu/Zn and the enrichment factor of V show that red and nonred bed samples are formed in weak oxidation and weak reduction environments, respectively. The red sample contains the highest content of hematite iron. The gray-green sample mainly represents paramagnetic ferrous in clay minerals. The geochemical contents of the gray sample’s three iron elements are slightly different, mainly trivalent iron. The change in iron speciation content in different color samples shows that the Fe element forming hematite in red bed samples may come from the weathering of source rock and clay minerals subjected to secondary weathering. At present, it is confirmed that different colors of samples are related to different weathering degrees of source rocks, which can be related to hot, dry/humid climates. It is necessary to distinguish the climate type in combination with other indicators.

Distribution of rare earth elements in sediments of the North China Plain: A probe of sedimentation process

Rare earth elements (REE) are powerful tracers for our understanding of sedimentary provenance and depositional processes. This study investigated geochemical characteristics of REE in 226 sediment samples collected from the piedmont, central and littoral plains of the North China Plain. Results showed that total REE concentrations exhibited a decreasing trend as piedmont > central > littoral, and 82%, 70% and 67% samples had total REE concentrations higher than the value of upper continental crust. The REE concentrations were controlled by sedimentary provenance and sediment physicochemical properties. Ternary diagrams of sandstone classification and Al2O3–CaO + Na2O–K2O reflected that sediments originated from greywackes and they experienced an incipient to intermediate chemical weathering. Average chemical index of alteration (CIA) value was 68, 51 and 55 for sediments from piedmont, central and littoral plains, respectively. Sediment North American Shale Composite (NASC)-normalized patterns were characterized by light REE (LREE) enrichment over heavy REE (HREE) and coherent negative Eu anomaly (Eu/Eu* ranging between 0.57 and 1.00). Value of (La/Yb)NASC ranged from 0.86 to 1.15 (average 1.23), and from 1.03 to 1.49, and from 0.91 to 1.49 in three zones, respectively, indicating REE fractionation occurred from piedmont to littoral during sedimentation processes. A positive correlation between (La/Yb)NASC and total REE concentrations was observed in piedmont and central sediment samples, implying that LREE were preferentially enriched over HREE with an accumulation of REE. This positive correlation was not found in littoral sediment samples, where a general decreasing trend along depth for REE concentrations existed. The findings of this study have an implication for using REE as a tracer in provenance studies from a basin scale.

Mineralogical and geochemical characterisation of kaolin deposit from Debre Tabor area northwestern, Ethiopia

ABSTRACT Debre Tabor kaolin deposit is located around Debre Tabor town in Amhara region of northwestern Ethiopia. The kaolin deposit in the study area needs a detailed study to evaluate the geological, mineralogical, physical, and geochemical conditions. For this purpose, detailed geological, physical, mineralogical, and geochemical laboratory tests were performed. XRD and petrographic analysis were used to study the mineralogical composition. Geochemical analysis was determined using ICP-MS and ICP-AES. The Debre Tabor kaolin deposit is exposed along riverbanks, road cuts, hillside, and quarry sites. The laboratory results reveal that the deposit is formed from the weathering of felsic rocks mainly trachyte and tuff units. From the laboratory analysis, we found that quartz is the dominant impurities. The geological, mineralogical, and geochemical studies indicate that in situ weathering followed by leaching played a major role in the formation of the Debre Tabor kaolin deposit. The average Chemical Index of Alteration (77.98%) shows that the host rock has experienced moderate weathering and alteration under the hot and humid climatic conditions of the subtropical zone. The low bulk density (1.54 g cm−3) and Ce + Y + La vs. Ba + Sr plot correspond to the supergene type of formation. Based on the mineralogical, geochemical, and physical properties, Debre Tabor kaolin deposit can be used for refractory, ceramics, paper coating, and filler (paint) industries, but treatment is important to remove the existing impurities such as iron, quartz, and feldspars.

Provenance and Tectonic Implications of Paleozoic Strata in the South Yellow Sea Basin, China–Revealed from the Borehole CSDP-2

Well CSDP-2 is a fully coring deep drilling hole, penetrating the thick Paleozoic marine strata in the South Yellow Sea Basin (SYSB) in the lower Yangtze Block (LYB). Based on the petrological and geochemical analysis of 40 sandstone samples from the core CSDP-2, the provenance and tectonic features of Paleozoic detrital rocks from SYSB are analyzed and systematically delineated in this article. The results show that the Silurian – Carboniferous sandstones are mature sublitharenite, while the Permian sandstones are unstable feldspathic litharenite. The average CIA (chemical index of alteration) is 74.61, which reflects these sediments were derived from source rocks with moderate chemical weathering. The REE (rare earth element) patterns are characterized by LREE enrichment, flat HREE and obviously negative Eu anomaly, which are similar to that of the upper continental crust. Dickinson QFL discrimination results indicate the recycled orogeny provenance. Various diagrams for the discrimination of sedimentary provenance based on major and trace element data show all the sediments were derived predominantly from quartz sedimentary rocks, of which the Permian strata contain more felsic sedimentary rocks. Geochemical data for these detrital rocks suggest they occur at the passive continental margin and island arc settings, and the Permian sandstone presents active continental margin setting.

Geochemistry of the Middle Jurassic sediments in Gümüşhane, north‐eastern Turkey: Implications for weathering and provenance

The whole‐rock geochemistry and Sr‐Nd isotope data of Middle Jurassic sedimentary rocks in the Gümüşhane area (north‐eastern Turkey) were analysed to reconstruct the provenance, chemical weathering, sediment recycling history, and tectonic setting of the source area. Based on trace element ratios and rare earth element (REE) patterns, it was interpreted that the sandstones were derived from a source area with felsic‐mafic rocks as dominant contributors. The average chemical index of alteration (CIA) and weathering index (WIP) values are low with respect to Upper Continental Crust (UCC) in the studied sandstones and mudstones, but the index of compositional variability (ICV) values are high, thus revealing weak–moderate weathering conditions. The average Th/U ratios of the sandstones and mudstones indicate a relatively simple cycling history. The positive and low negative εNd values support the supposition that the source area did not undergo much sedimentary change. A model mixture composed of 60% granite, 5% granodiorite, and 35% gabbro could be deduced to be the potential source composition of Middle Jurassic sediments; this model was reinforced by the 87Sr/86Sr ratios and εNd(t) values of the sediments. The Middle Jurassic siliciclastic sedimentary rocks of the Gümüşhane region were likely deposited in a tectonic environment with a passive margin.

Carboniferous glaciotectonized sediments in the southernmost Paraná Basin, Brazil: Ice marginal dynamics and paleoclimate indicators

Carboniferous glaciogenic strata (Itararé Gp.) in the southernmost Paraná Basin, Brazil exhibit soft-sediment deformation features previously interpreted as glaciotectonism. These sediments were studied in detail to confirm that they were deformed by ice and to assess the nature of the glaciation, depositional environments, and paleoclimate in this region during the Carboniferous. Five outcrops were described along a railroad transect that contains a conglomerate and diamictite facies with striated and faceted clasts, a medium sandstone facies, a fine grained silt/clay rhythmite and mudstone facies with dropstones and diamictite pellets, a sandy clinoform facies, and a folded sandstone with interbedded mudstone facies. The depositional environment for these sediments is interpreted as an outwash fan and fan delta from an ice-proximal, transitional terrestrial-to-estuarine setting. Rb/K values from the rhythmites reflect a transition from a freshwater to brackish environment and the Chemical Index of Alteration (CIA) of the rhythmites ranges from 65 to 73, reflecting a relative increase in the degree of chemical weathering through time. Deformation features include widespread folding, thrust faults, hydrofractures, décollement surfaces, and piggyback fold-thrust complexes. The deformation is interpreted as evidence of a push-moraine complex formed by at least two complete ice advance/retreat cycles. The occurrence of décollement surfaces, plastically deformed proglacial sediments, and hydrofractures indicate a dynamic, warm-based or polythermal glacier. Abundant outwash sediments, rhythmites with dropstones, and the shift to average CIA values all support a temperate paleoclimate. Deformation structures indicate a NW direction of ice shove that is in agreement with the regional-scale hypothesis that a NNW flowing lobe extended out of Uruguay during the Carboniferous and terminated in the southernmost Paraná Basin. This study demonstrates that late Paleozoic glaciation in this region was more dynamic than previously understood, with high frequency fluctuations in ice marginal positions.

Application of geochemical techniques for prospecting unconventional phosphate sources: A case study of the lake sediments in Eppawala area Sri Lanka

Jaya-Ganga is a water canal constructed bisecting the phosphate deposit in Eppawala, Sri Lanka. It nourishes three lakes around the vicinity of Eppawala Phosphate Deposit (EPD). In this study, the lake sediment samples were analyzed for major oxides and phosphate solubility for prospecting unconventional phosphate sources. Analyses of the geochemical content of the surface sediments in both upstream and downstream lakes revealed higher contents of P2O5, TiO2, Al2O3, Fe2O3 and MnO and, low levels of SiO2, MgO, Na2O, K2O and CaO compared with the norms of Upper Continental Crust (UCC) values. Log plots of (Na2O/K2O) vs log (SiO2/Al2O3) and (Fe2O3/K2O) vs (SiO2/Al2O3) showed chemical immaturity and mineralogical instability of the sampled sediments. The average Chemical Index of Alteration (CIA) values of 93 for upstream and 92 for downstream portrayed extreme chemical weathering conditions in the sediment source area, facilitating the breakdown of the EPD under prolonged exposure. Besides the transportation of detrital sediments, the above could convert residual phosphate into a more soluble form enabling transportation through Jaya-Ganga and re-deposition in downstream sediments. Thus, positive anomalies of P2O5 and high phosphate solubility on surface and core sediments in the downstream lake indicated the contribution of the EPD. The results of the study reflect the potential of secondary phosphate enrichment in downstream sediments and its applicability as a low-grade phosphate fertilizer. Continuous nourishment of phosphate-bearing materials, into lake sediments, increase the quantity of the phosphate content in downstream sediments and the minable quality of the lake sediments as an economically viable phosphate source.

Provenance of the Cretaceous Debre Libanos Sandstone in the Blue Nile Basin, Ethiopia: Evidence from petrography and geochemistry

This work aims to unravel the provenance, depositional tectonic setting and source area weathering of the cretaceous Debre Libanos Sandstone in the Blue Nile Basin, Ethiopia from its petrographic and geochemical signatures. Both the average chemical index of alteration (CIA) and chemical index of weathering (CIW) of sandstones (CIA = 91.08, CIW = 96.75), shales (CIA = 81.89, CIW = 98.17) and siltstones (CIA = 98.10, CIW = 99.35) of the Debre Libanos Sandstone indicate very strong chemical weathering. The low Th/Sc ratio of sandstones (1.46), shales (0.85) and siltstones (0.81), and Th/Sc versus Zr/Sc relationship indicate sediment recycling that leads to a progressive enrichment of heavy minerals such as zircon which is the major host for Zr and Th. The increase of quartz content in sandstones and dense minerals in siltstones and shales can be related to hydraulic sorting, grain-size controlled fractionation and source composition. Trace element concentration and their ratios (La/Th, Cr/Th, Th/Sc, Cr/V, Y/Ni) with rare earth element proxies (Eu/Eu*, LaN/YbN) of the studied samples revealed felsic to mixed source rocks of upper continental crust with some input from recycled old sediments. The source area of the Debre Libanos Sandstone is from the northern and northwestern Ethiopia during the time of marine regression, retreat of the Indian Ocean from NW Ethiopia back to SE Somalia. Tectonic discrimination diagrams revealed continental rift setting for the sandstones and collision setting for shales and most of the siltstones. Furthermore, the studied sandstones were partly deposited in a passive margin setting which had undergone post-depositional K-metasomatic alteration. The shales and siltstones were deposited in active margin setting.

Provenance of sandstones in Ethiopia during Late Ordovician and Carboniferous–Permian Gondwana glaciations: Petrography and geochemistry of the Enticho Sandstone and the Edaga Arbi Glacials

We compare Ethiopian glaciogenic sandstone of the Late Ordovician and Carboniferous–Permian Gondwana glaciations petrographically and geochemically to provide insight into provenance, transport, and weathering characteristics. Although several studies deal with the glacial deposits in northern Africa and Arabia, the distribution of ice sheets and continent-wide glacier dynamics during the two glaciations remain unclear. Provenance data on Ethiopian Palaeozoic sedimentary rocks are scarce. The sandstones of the Late Ordovician glaciation are highly mature with an average quartz content of 95% and an average chemical index of alteration of 85, pointing to intense weathering and reworking prior to deposition. No evidence for sediment recycling was found. In contrast, the Carboniferous–Permian glaciogenic sandstones are less mature with an average quartz content of 75%, higher amounts of feldspar and rock fragments and a chemical index of alteration of 62. Trace and rare earth element concentrations indicate a higher input of juvenile material, most probably from proximal sources. Comparison with stratigraphically corresponding formations in Saudi Arabia shows similar geochemical patterns for the Upper Ordovician, but major differences in the Carboniferous–Permian. This supports previous assumptions of a large, uniform sediment dispersal system during the Late Ordovician glaciation, in which a combination of long transport paths and exceptionally strong weathering prior to the glaciation produced mature sandstone. During the Carboniferous–Permian, the glacial systems seem to have been more localised and glacial abrasion exposed fresh basement material.

Geochemical and isotopic characteristics of stream and terrace sediments of the Harsit Stream, NE Turkey

The current study compares and contrasts the elemental concentrations and the Nd, Sr and O isotopic compositions of sediments collected from the Harsit Stream bed sediments (HSBS) and terrace sediments (HSTS). There are positive correlations between Al 2 O 3 and SiO 2 , TiO 2 , Na 2 O and Fe 2 O 3 , K 2 O in HSBS and between Al 2 O 3 and Fe 2 O 3, MnO in the HSTS samples. Major element components of HSBS and HSTS are comparable to average upper continental crust. Both types of sediments are generally classified as greywacke, based on chemical composition and O–Sr isotopic compositions. The ɛ Nd (0) and ɛ Sr (0) values range from −0.6 to −5.7 and from 7.1 to 45, respectively, in HSBS; and from −0.7 to −4.4 and from 20 to 54, respectively, in HSTS. The δ 18 O values range from 8.8 to 11.8 (average 10.8) in HSBS and from 11.3 to 13.3 (average 12.2) in HSTS, which are generally controlled by detrital or authigenic minerals in the studied samples. The average chemical index of alteration (CIA) and, plagioclase index of alteration (PIA) values are high in HSTS, whereas the index of compositional variability (ICV) and weathering index (WIP) values are high in HSBS. Several minor differences were observed between HSBS and HSTS in terms of geochemistry and isotopic compositions that reflect different processes involved in their sedimentation.

Geochemistry of lower Silurian shale of Longmaxi Formation, southeastern Sichuan Basin, China: Implications for provenance and source weathering

Lower Silurian Longmaxi Shale (SLS) in southeastern Sichuan Basin, China, was analyzed for major and selected trace elements, and their provenance, intensity of palaeoweathering of the source rocks were analyzed based on these elements. The results show that SiO2, Al2O3 and Fe2O3, are dominant major elements with average contents of 60.59%, 15.91% and 5.87% in Upper Silurian Longmaxi Shale (USLS), and 65.14%, 13.24% and 4.68% in Lower Silurian Longmaxi Shale (LSLS). The TiO2−Zr plot, Hf (ppm) versus La/Th discriminant diagram, and abundance of Cr and Ni suggest a dominantly felsic source for the Longmaxi sediments. Average chemical index of alteration (CIA), plagioclase index of alteration (PIA) values (64.05% and 72.86%, respectively) imply low-degree chemical weathering of the source material in early Longmaxi time, and average CIA, PIA values (68.44% and 80.35%, respectively) imply moderate chemical weathering of the source material in late Longmaxi time.

Paleoclimate changes in the late precambrian: Evidence from the upper precambrian section of the South Urals

The climatic impact on the formation of fine-grained rocks from the Riphean stratotype and Vendian Asha Group on the western slope of the South Urals during the time interval lasting approximately 1200 Ma is considered. It is shown that these rocks are largely represented by “tectonosilicate-dominated” shales. This feature combined with changes in the average K2O/Al2O3 values disavows the hypothesis in (Kennedy et al., 2006), according to which the growth of free oxygen concentration in the Late Riphean and Vendian atmosphere was determined by gradual intensification of the organic carbon extraction from the biosphere by clays. The average values of the hydrolyzate module, chemical index of alteration (CIA), and several lithogeochemical parameters calculated for the Riphean and Vendian clayey rocks provide grounds for the conclusion that intensity of weathering in paleodrainage areas during the accumulation of the Upper Precambrian sedimentary successions was low. The curve reflecting changes of the average CIA values in the Upper Precambrian fine-grained siliciclastic rocks of the South Urals is similar to some extent with the “standard” CIAcorrect. curve (GonzalezAlvarez and Kerrich, 2012). It is assumed that changes in microand macrobiotic communities during the Late Precambrian were controlled to a variable extent by climate fluctuations as well. At the same time, these fluctuations most likely left the chemical composition of water in the ocean virtually unchanged, which is evident from analysis of the redox conditions in the ocean and the distribution of primary producers with the average CIAcorrect. and CIA values.

Geochemical and Nd isotopic constraints on provenance and depositional setting of the Shihuiding Formation in the Shilu Fe–Co–Cu ore district, Hainan Province, South China

The Shihuiding Formation, a subordinate succession hosting the Fe–Co–Cu ores, is a suite of Neoproterozoic terrigenous clastic rocks occurring in the Shilu Fe–Co–Cu ore district of the Hainan Island, South China. Integrated petrographical, geochemical, and Nd isotopic analyses have been carried out on 23 sandstone specimens of the Shihuiding Formation in order to understand their provenance and the tectonic setting of their deposition. The samples can be divided into two groups, quartzose sandstones (13 samples) and ferruginous sandstones (10 samples). The ferruginous sandstones have average SiO2 and Fetotal contents of 77.23wt.% and 18.09wt.%, respectively, and this contrasts with the higher average SiO2 (94.04wt.%) and lower Fetotal (2.67wt.%) contents of the quartzose sandstones. The bivariant Th/Sc and Zr/Sc ratios indicate a predominantly recycled sedimentary provenance, and the low to medium degrees of weathering are commonly indicated by an average chemical index of maturity (CIM) of 81 and an average chemical index of alteration (CIA) of 68. The Shihuiding Formation sandstones have REE contents of 21–249ppm, with LREE/HREE=9.18 and δEu=0.67. The εNd (970Ma) values of −5.7 to −3.4, and model (TDM) ages of 2099–1773Ma are compatible with a source mainly from the Paleo- to Mesoproterozoic Baoban Group, a suite of metamorphosed sedimentary rocks intruded by ca. 1450Ma granites. Quantitative provenance modeling indicates that the Shihuiding Formation sandstones are best modeled with a mixture of 29% plagioclase–amphibole gneiss (29 P), 38% quartz–muscovite schist (38 Q), and 33% granite (33 G) detritus. Mixing the εNd values of the sandstones, calculated at 970Ma, indicates that the sediment received 22–47% (average 34%) of its detritus from the Baoban Group quartz–muscovite schists. Components from hydrothermal fluids may also have been involved during deposition of the Shihuiding Formation sandstones, as revealed by a bivariant Al/(Al+Fe+Mn) vs. Fe/Ti plot, positive Eu anomalies, and negative Y anomalies relative to PAAS. Tectonic discrimination diagrams, such as La–Th–Sc and Th–Co–Zr/10, indicate a passive continental margin setting for the deposition of the Shihuiding Formation. In terms of its geological setting, we propose that the Shihuiding Formation was deposited in a peripheral foreland basin, supplied by detritus shed from the Grenville Orogenic Belt during the assemblage of the Rodinia Supercontinent.

Integrated petrographic, mineralogical, and geochemical study of the Late Cretaceous–Early Tertiary Dakhla Shales, Quseir–Nile Valley Province, central Egypt: implications for source area weathering, provenance, and tectonic setting

Late Cretaceous–Early Tertiary shales of the Dakhla Formation of the Quseir–Qena Province, central Egypt, were analyzed for major and selected trace elements to infer their provenance, source rock paleoweathering intensity, and tectonic setting. The studied formation consists of a series of marls and shales and is subdivided into two members, namely Beida Shale Member at the top and Hamama Marl Member at the base. The Dakhla Shales are texturally classified as mudstones. Mineralogically, these shales consist mainly of smectite and kaolinite. Chemical analysis of the major and trace elements generally exhibits a uniform distribution throughout the Upper Maastrichtian–Lower Paleocene sediments. SiO2, Al2O3, and Fe2O3 contents are higher than the values reported for the post-Archaean Australian shale (PAAS), while TiO2 and Na2O contents are found to be lower. Bivariate plot of Zr versus TiO2 diagram indicates that intermediate to felsic igneous rocks constitute the main supplying source rock. Average chemical index of alteration (CIA), plagioclase index of alteration (PIA), and chemical index of weathering (CIW) values (81, 92, and 93 %, respectively) imply intermediate to intense weathering of the source material in a semiarid climate. The bivariate discriminant function diagram reveals an active continental to passive margin setting for the Dakhla Shales. The developed soils were transported by rivers to the depositional basin. The inferred tectonic setting for the Late Cretaceous–Early Tertiary Dakhla Shales in Quseir–Qena Province is in agreement with the tectonic evolutionary history of central Egypt during the Late Cretaceous–Early Tertiary.

Petrography and geochemistry of the Kamlial Formation, southwestern Kohat plateau, Pakistan: implications for paleoclimate of the Western Himalayas

The Middle to Late Miocene Kamlial Formation that largely consists of sandstone and interbedded clay/mudstone sequences is exposed in the southwestern part of the Kohat plateau, which constitutes the westernmost extension of the Himalayan Foreland Basin. Whereas the sandstone is gray to brownish gray, fine- to medium-grained, and mostly thick-bedded, the interbedded clay/mudstone sequence is brownish gray to maroon red and occurs as continuous beds as well as lenses. Some mudstone/clay beds are bioturbated and seem to be pedogenic surfaces. Results of geochemical investigation of fresh (unaltered) representative sandstone and mudstone samples from three different sections of the formation in the southwestern Kohat plateau are presented and discussed. The average chemical index of alteration (CIA) values of both the sandstone (70-86) and mudstone (71-85) suggest moderate to slightly intense weathering in the source area. However, the high CIA values may also be due to the presence of abundant sedimentary rock fragments, which occur in the studied sandstone, rather than a result of severe weathering. Furthermore, the possibility of intensive chemical weathering in the Himalayas orogenic belt is highly unlikely, as it requires tectonic quiescence for a long period, higher temperature, and humidity. Accordingly, the range of the index of chemical variability values (0.6-2.1) of mudstone and low contents of Rb and Cs in both the mudstone and sandstone indicate somewhat moderate weathering. Furthermore, the Th/U and Rb/Sr ratios of the Kamlial Formation are lower than the corresponding average values for the upper continental crust and post-Archean average Australian shale, which shows that these sediments are first-cycled in origin. However, the Zr/Sc ratio indicates minor contributions from recycled sedimentary sources. The values of authigenic U and the U/Th, V/Cr, Cu/Zn, and Ni/Co ratios all suggest that the Kamlial sediments were deposited under oxidizing conditions.