High CIA Values Research Articles

Source Area Weathering and Tectonic History Inferred from the Geochemistry of the Maastrichtian Sandstone from Patti Formation, Southern Bida Basin, North Central Nigeria

Sandstones sampled from Patti Formation, Southern Bida Basin, were studied geochemically using Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES) and an Inductively Coupled Plasma Mass Spectrometry (ICP-MS) technique to evaluate their weathering and tectonic setting as well as to deduce the paleo-climatic conditions that existed during their deposition. Geochemically, SiO2 range from 73.9% to 86.2%, Al2O3 (6.7%~17.1%), Fe2O3 (1.1%~1.9%), K2O (0.1%~0.7%) while MgO, CaO, Na2O, P2O5, MnO and TiO2 were <1%. Enriched in Ba (Av. 622.94), Sr (Av. 153.63), Rb (Av. 55.08) and Zr (Av. 51.86) relatively similar in composition to UCC. High SiO2 but low other major oxides signify high mobility during processes of weathering. This was confirmed by high value (>80%) for indices like chemical index of alteration, chemical index of weathering, plagioclase index of alteration, mineralogical index of alteration and relatively lower values for weathering index of parker, recently used alpha indices (αAl E) of sodium (326.17αΑl Na<344.40), magnesium (100.54αΑl mg<398.55), calcium (12.07αΑl Ca<198.99), potassium (4.43αΑl K<64.33), strontium (0.84αΑl E<21.40), barium (0.45 αΑl Ba<10.52) and rubidium (0.0008αΑl Rb<0.06), supported by AI2O3-(CaO*+Na2O)-K2O and CIA vs. SiO2 plots that imply intense weathering in the source area. The obtained high CIA values (>80) indicates a steady-state of weathering under a warm/humid climate as confirmed by the SiO2 vs. Al2O3+ K2O + Na2O plot. High average SiO2 (75.41wt%) with K2O/Na2O ratio >1 (15.63), low Fe2O3 (1.27wt %), Al2O3 (15.82wt%) and TiO2 (0.46) suggest passive margin tectonic setting. This is supported by enriched ΣREE (209.64 ppm), ΣLREE (195.78), LREE/HREE (27.78) and negative Eu/Eu* (0.68), plots of log (K2O/Na2O) vs. SiO2 and SiO2/Al2O3 vs. K2O/Na2O. Major elements discriminant-function multi-dimensional diagram, DF1 (arcrift-col) vs. DF2 (arc-rift-col), for high-silica sediments revealed a continental rift tectonic setting. Thus, the Patti Formation sandstone underwent a high degree of weathering under a humid climatic condition within a continental rift tectonic setting.

Geochemistry of Permian carbonaceous shales from Raniganj sub-basin, Damodar Valley, India: Implications for provenance, weathering, tectonics and source of organic matter

Raniganj sub-basin, the easternmost depository of Damodar Valley encompasses the entire succession of Gondwana Supergroup, the lower and upper Gondwana sequences. The present investigation of Permian carbonaceous shales belonging to Barakar, Barren Measure and Raniganj formations of lower Gondwana sequence from this basin is an integrated study of inorganic and organic geochemical characteristics viz. major/trace elements, stable isotopic composition and kerogen type to ascertain source rock attributes such as provenance, tectonic setting, prevailing weathering/climatic conditions as well as source and type of organic matters. Trace element indices, such as La/Sc, Th/Co, Th/Sc, (Gd/Yb)CN, Eu/Eu*, Cr/V, Y/Ni, reveal that the shales are originated from felsic/granitic source rock. Prominent Eu-anomaly, enrichment of chondrite normalised light rare earth element (LREE) values and a flat heavy rare earth elements (HREE) pattern reflect post-Archean felsic source for the shales. The major element composition, especially relative enrichment of Al over readily soluble elements, Ca, Na and K yielding high CIA (74.9–84.6%) and CIW (87.1–97.5%) as well as low ICV (0.64–1.32) values, establishes significantly weathered and matured nature of the shales. The distribution of redox-sensitive elements (U/Th, V/Cr, Ni/Co, Ce/Ce*) as well as low ICV, depleted K2O/Al2O3 ratios and high CIA values point towards an oxic environment and warm-humid climatic condition during deposition of these shales. The conventional method for determining tectonic setting using major and trace element composition fails to clearly delineate the tectonic setting for the current shales. The samples show wide scattering across active and passive continental margin in the K2O/Na2O–SiO2 diagram, whereas the samples are concentrated near the active in the continental margin in the Th–Sc–Zr/10 diagram contradicting the geological framework of Raniganj sub-basin. Discriminant functions-based approach which categorises the tectonic setting into continental/oceanic arc, continental rift and continental collision settings has been proved to be suitable in the present case and reveals continental-rift setting for the shales from Raniganj sub-basin. The carbon isotopic signature (δ13Corg) of the shales reveals a range (−24.5 − −22.9‰) intermediate between that of marine and terrestrial (C3 plants) organic matters indicating possible mixed type of organic matters. The fluvio-marginal marine depositional condition obtained from K2O/Al2O3 and MgO/Al2O3 ratios further implies the possibility of mixed nature of organic matter. The geochemical parameters of the organic matter (Rock-Eval pyrolysis parameters S1, S2, TOC, HI, OI etc.) indicate that the studied shales contain thermally matured mixed Type III/II gas-prone kerogen and are derived predominantly from terrestrial sources with significant marine input.

Tonian and Cryogenian – Early Cambrian sedimentation in NW India: Implications on the transition from Rodinia to Gondwana

The Tonian Punagarh-Sindreth and Cryogenian – early Cambrian Marwar sequences in NW India encompass a geological record that could provide vital information for working out the tectonic evolution of this region and a better understanding of the Rodinia to Gondwana transition. The Tonian Punagarh and Sindreth sub-basins received a dominantly felsic detritus with negligible sorting of heavy minerals and mild to weak chemical weathering conditions. The clastic compositions and geochemical characteristics of the Punagarh and Sindreth groups suggest a back-arc basin setting and the detritus sourced from a volcanic arc and cratonic interior region. The Punagarh basin was proximal to the craton while the Sindreth basin was close to an arc system. In contrast, the Cryogenian – early Cambrian Marwar Supergroup clastic sequence contains mature quartz arenite whose high CIA (77.8–94.8) and low ICV (0.18–1.00) values suggest a strong chemical weathering in the source region and noteworthy physical sorting during transportation and sedimentation. An abundance of quartz grains (87–94%), enrichment of Zr, and detritus predominantly derived from the cratonic basement indicate deposition of Marwar sediments in a tectonically stable basin, temporally coinciding with the Gondwana assembly. Collectively, the ∼ 760 Ma active continental margin in NW India, documented by the provenance and tectonic setting of the Punagarh and Sindreth basins, is in agreement with the subduction of the peripherical Rodinia supercontinent during its breakup. On the other hand, the passive continental margin sedimentation in Marwar Supergroup points toward an open sea between NW India and western Gondwana during the Gondwana assembly.

Organo-inorganic facets of shale gas reservoir of Jharia Basin of Damodar Valley, India

The Mahuda-Kapuria block of the Jharia Basin, Damodar Valley comprises thick shale sequences of Shibabudih (>180 m) in Barakar and Hariharpur shale (>40 m) in Barren Measures Formations. The ternary organic facies of maceral shown the trend of anoxification, resulting in higher maceration as a function of terrestrial-marshy depositional conditions. Subsequently, it supported the tissue preservation and carbon enrichment contributing to the formation of shale matrix. H/C vs O/C plot shown Type III/IV kerogen transformed from fluvio-humid to dry terrestrial depositional conditions followed by diagenesis, cata- and metagenesis processes. Also, the range values of Tmax (442–558 °C) and HI (25–345 mg HC/g TOC) confirms the significantly matured source rock having the input of dominantly type III/IV kerogen. The S1 and S2 specify (0.07–2.54 mg HC/g rock, 0.58–20.14 mg HC/g rock) the presence of substantial free/adsorbed hydrocarbon gases and uncracked hydrocarbon compounds. The large content of SiO2, Al2O3, and Fe2O3 account for 85–95 % of the samples point towards granitic source rock of sediments. The high CIA and low ICV values denote the compositional maturity of the shale due to conversion of feldspars to Al-bearing clays through intense geochemical weathering and alteration. The positive trend of organic weight with Langmuir volume (VL) and TOC specify the organic carbon present in shale mainly contributes to the formation of pore containing matrix system and adsorption of methane in shale pores. The mineralogy of shale directly controls the tensile strength, uniaxial compressive strength, Young's modulus, Poisson's ratio and brittleness of shale beds. Further, it is observed that the brittleness of shale beds exhibits higher axial strain with a small region of elastic behaviour by the influence of the hydrous property of kaolinite and illite. The proposed empirical EMBI for estimation of brittleness indicated moderate brittle characteristics of shale beds.

Palynology, mineralogy and geochemistry of sediments in Tondè locality, northern part of Douala sub-basin, Cameroon, Central Africa: implication on paleoenvironment

A multi-proxy study, including mineralogy, whole rock geochemistry and palynology analyses, was conducted on 79 samples (64 bulk sediment and 10 pyrite samples) from two sites (ES and DA) located in the Tonde area, Douala sub-basin (Cameroon) to unravel the paleoenvironmental and paleoclimatic conditions prevailing in recent continental deposits. Lithologies of this locality mainly consist of pyriteous claystones (grey and red) and unconsolided sandstones. Encountered pyrite is euhedral (isolated and clusters crystals) and massive with diverse morphologies and sizes. In both sections, major and trace elements show that the clayey materials result from weathering of an intermediate source rock, probably from the surrounding basement (gneiss and micaschist), whereas the sandy materials appear to be recycled. Palynological data consists of a few index species such as Malvaceae and graminaceae which indicate Pleistocene-Holocene age deposits of continental origin (swampy bays of seaside and hinterland). This area was subjected to intense chemical alteration (high CIA values: 97.35–99.43%) as testified by the mineralogical phases, mainly consist of kaolinite, quartz and goethite. Mean annual precipitation (MAP = 221.1e0.0197(CIA-K); ±181 mm.y−1) and mean annual temperature (MAT = 46.9C + 4; ±0.6 °C) are 1575 ±181 mm.y−1 and 25.2 ± 0.6 °C for ES site and then 1566 ± 181 mm.y−1 and 20.3 ± 0.6 °C for DA, thus reflecting a subequatorial to equatorial climate. The pyrite geochemistry is characterized by a weak Al vs. ΣREE positive correlation (r2 = 0.02), a positive Eu anomaly relative to PAAS and a significant negative correlation between ΣREE and the Eu anomaly (r2 = −0.83). These results indicate that pyrite is a late diagenesis product formed by a secondary enrichment and coupled with precipitation under more reducing conditions, linked to the water table fluctuation.

Late Holocene paleoenvironmental changes inferred from Manasbal Lake sediments, Kashmir Valley, India

Paleoclimate investigations and reconstruction using the lake sediment cores of Kashmir Valley have been rarely attempted. The basin is influenced by Indian Summer Monsoon, Westerlies and the local katabatic winds. In the present study, an 80 cm undisturbed sediment core was retrieved from the Manasbal Lake situated in North western Himalaya. Sediments were dated using 14C method, and the age-depth model indicated a depositional history extending from ∼3345 yrs BP to the present. Grain size, major and trace elements, TOC, OM contents C/N ratio and diatom assemblages were applied to reconstruct the paleoenvironmental changes around the area. The A-CN-K diagram displays that the weathering intensity of the Manasbal lake sediments are moderate to highly weathered and the sediments have been derived from nearby and mixed source origin. Several wet and cold phases were identified during ∼3300 yrs BP to ∼2500 yrs BP and ∼1800 yrs BP to ∼1300 yrs BP due to intense precipitation reflect high lake level. These wet phases are marked by higher sand and silt content, C/N ratio and high allochthonous input with less abundance of diatom species, clay, OM, TOC contents and high CIA values. Consequently, low lake level and dry and cold period from ∼3345 yrs BP to ∼3300 yrs BP and ∼2500 yrs BP to ∼1800 yrs BP is observed. These inferences are corroborated with the trace elemental ratios CIA and CWI data. Our data indicate that the Westerlies and localised katabtic winds rather than the Indian summer monsoons played a major role in controlling the past climate around the Manasbal lake, Kashmir Valley.

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.

Giant bauxite deposits of South China: Multistage formation linked to Late Paleozoic Ice Age (LPIA) eustatic fluctuations

The Lower Carboniferous Jiujialu Formation bauxite deposits at Zunyi, northern Guizhou are distinguished by their great thickness (to 110 m) and cyclic alternation of bauxite ore and bauxitic claystone layers. These features are typical of the Kazakhstan subtype of karst bauxite deposits, although the genesis of this type of deposit is poorly understood. Here, we undertook a petrological, mineralogical, and geochemical study of the Zunyi deposit in drillcore ZK5600 to gain insights into its formation history. The Zunyi profile contains seven bauxite cycles, as recorded by petrological, mineralogical and geochemical features, indicating a multistage formation history. Each cycle is composed of a lower bauxitic claystone layer and an upper bauxite ore layer. The bauxite ore is characterized by high Al2O3, high Al-mineral (diaspore), and low SiO2 content. Positive Ce anomalies, high CIA values (>90), and strong depletion of major elements indicate that the bauxite ore formed through intense chemical weathering and leaching in a well-drained soil environment. The bauxitic claystone is characterized by relatively lower Al2O3 and higher SiO2 and clay mineral (mostly illite) content. This lithology exhibits both negative and positive Ce anomalies, moderate CIA values (70–80), and enrichment of some elements (e.g., Fe, K), indicating formation through moderate chemical weathering in a poorly drained soil environment. Secular variations in chemical weathering intensity and soil drainage conditions were linked to regional hydrologic changes. During the early Carboniferous, the Zunyi area was located in a karstified coastal upland setting, and its hydrologic system was affected by glacio-eustatic fluctuations. High groundwater table elevations during eustatic highstands led to formation of bauxitic claystone layers, whereas low groundwater table elevations during eustatic lowstands led to formation of bauxite ore layers.

Volcanic sources and diagenetic alteration of Permian–Triassic boundary K-bentonites in Guizhou Province, South China

Terrestrial Permian–Triassic boundary (PTB) sequences in South China usually contain one to three ash beds (K-bentonites), which provide evidence of a possible trigger for the terrestrial mass extinction and are useful as a distinct marker for correlation of the PTB boundary interval from terrestrial to marine areas. Here, we investigate the clay mineralogy and geochemistry of ash beds in the Zhejue and Jiucaichong sections, representing paludal and lacustrine environments, respectively, in order to better understand the source(s) and preservation of these ash beds. The two Zhejue K-bentonites exhibit similar compositions that are distinctly different from that of the Jiucaichong K-bentonite. The former contain mainly clay minerals with minor quartz, anatase, and lepidocrocite, and the clay minerals in these layers are mainly R1 and R3 mixed-layer illite/smectite (I/S) and well-crystallized to poorly-crystallized kaolinite, whereas the latter consists mainly of R3 I/S with minor quartz, feldspars, and anatase. The former have higher CIA values (92.2 and 93.8) than the latter (79.4), indicating more intense chemical weathering that may have been related to deposition in a paludal setting with high organic acid concentrations in pore fluids. These K-bentonites also exhibit differences in major-element chemistry (MgO and K2O contents are higher at Jiucaichong due to lesser weathering intensity and, possibly, early diagenetic uptake from pore fluids), REE chemistry (Jiucaichong exhibits a larger Eu anomaly), and Nd isotopic compositions (Zhejue K-bentonites have 143Nd/144Nd ratios of 0.512436 and 0.512400, higher than the 0.512190 ratio for Jiucaichong). Although some of these compositional differences may be related to the dissimilar chemical environments in which diagenesis occurred (i.e., paludal vs lacustrine), the differences in Eu anomalies and Nd isotopic compositions imply heterogeneous volcanic ash sources. The source of these K-bentonites was probably a continental volcanic arc located on the paleo-southern margin of the South China Craton, within which multiple volcanic vents may have been active concurrently.

Geochemistry of black shales from the Mesoproterozoic Srisailam Formation, Cuddapah basin, India: Implications for provenance, palaeoweathering, tectonics, and timing of Columbia breakup

The Mesoproterozoic Srisailam Formation, exposed along the northern part of the Cuddapah basin, India, comprises mainly medium- to fine-grained siliciclastics, and is devoid of any carbonate sediment. Preliminary sedimentological studies helped in recognizing fifteen distinct facies (five facies associations) in Chitrial outlier of the Srisailam Formation deposited in continental half-graben basin(s). Black shales (sensu lato) are minor components of the Srisailam Formation, and inferred to have deposited in deep lacustrine and prodelta facies of the half-graben(s). The black shales show restricted thickness (up to 29.0m), and are characterized by overall high ‘black shale' to ‘total shale' ratio (>0.51). Their geochemical characteristics were studied to constrain provenance, palaeoclimate, and tectonic setting of deposition of the Srisailam Formation. Further, an attempt has been made to use the Srisailam black shales as proxy for constraining the timing of breakup of the supercontinent Columbia.The Srisailam black shales are geochemically quite distinct. At similar SiO2 contents they are considerably different from PAAS. They are characterized by considerably lower ΣREE (Av. 136.0±50.4ppm) but a more conspicuous negative Eu-anomaly (Av. 0.34±0.09) than PAAS. Al2O3/TiO2 and TiO2/Zr ratios coupled with Eu/Eu*, GdCN/YbCN, La/Sc, Th/Sc, and Th/Cr ratios suggest their derivation from granite and granodiorite. The CIA values (65–90, Av. 72±9) as a whole indicate moderate chemical weathering under semiarid climate. Discriminating geochemical parameters indicate passive margin depositional setting. The combined sedimentological and geochemical characteristics reveal deposition of the Srisailam sediments in continental rift basin(s).Thick succession of black shales (with high CIA values) that deposited with shelf carbonates proxy for mantle superplume and supercontinent breakup events. The sedimentological characteristics and geochemical data of the Srisailam black shales plausibly exclude any large-scale breakup of Columbia during the interval (1400–1327Ma) of deposition of the Srisailam Formation.

Geochemistry of upland lacustrine sediments from Serra dos Carajás, Southeastern Amazon, Brazil: Implications for catchment weathering, provenance, and sedimentary processes

A multi-proxy geochemical study of surficial sediments of an upland lake (Amendoim Lake), located in the Serra dos Carajás region, Brazil, was carried out to understand catchment weathering and provenance of sediments in the basin, as well as sedimentary processes. The carbon and nitrogen isotopic compositions of organic matter in the sediment were quite homogeneous, and suggest that the organic sources for this lake are mainly composed of palms and submerged macrophytes. The R-mode factor analysis indicates that most of the trace elements, including rare earth elements (REEs), are clustered with Al (Group 1), Si is grouped with Zr and Hf (Group 4), total organic carbon (TOC) with TS and Hg (Group 2), and Fe with Mn and As (Group 3). The elements of the Group 1 show strong positive correlations with Al, suggesting that these elements are hosted in detrital minerals during laterization in the catchment basin and not significantly affected by diagenesis. The high CIA values (96–99) of sediments together with their position in A-CN-K and log (Fe2O3/K2O) vs log (SiO2/Al2O3) plots indicate intense chemical weathering in source area. However, similar geochemical signatures between lake sediments and catchment lateritic crust indicates that mechanical erosion was the dominant sediment formation process. REE patterns normalized to chondrite along with geochemical indices (Al/K, Al/Ti, La/Th, La/Al, Ti/Zr, Zr/Hf, Th/Sc, Co/Th, Ba/Sr, and Eu/Eu*) also indicate that the sediments are mainly derived from laterite crust. This study provides reliable background information to reconstruct weathering processes and lake evolution in the Serra dos Carajás area.

Late Miocene to early Pliocene climate variability off NW Africa (ODP Site 659)

A high resolution δ18O record obtained on benthic foraminifera (Cibicides wuellerstorfi) combined with clay mineralogy, major element concentrations and Sr–Nd isotopes have been investigated for the ODP Site 659 located off North Africa in order to reconstruct African monsoon intensity between 6.2 and 4.9Myr. Mineralogical and geochemical results indicate that sedimentation on the Cape Verde plateau results from a mixture of sediments deriving from the neighbouring Saharan arid region and the wet African equatorial zone (Sahelian Band), characterised by a low and a high state of weathering, respectively. Past variations in terrigenous inputs from these areas allow us to track past extension and displacement of the West African Monsoon–ITCZ system. Maximal summer insolation induced a northward migration of the inter-tropical convergence zone (ITCZ), wetter conditions in the Sahel, and a diminishing of wind erosion over the South Sahara and Sahel. In addition, there was an increase in river input to the Cape Verde plateau of sediments derived from southern sedimentary sources, which are characterised by high kaolinite contents and CIA values and low K/Al and Ti/Al ratios. On the other hand, minimal summer insolation induced a southward movement of the ITCZ, drier climate in the Sahel and stronger easterly winds resulting in an increase in dust transport from the Sahara to the Cape Verde Plateau. Dust particles are characterised by high illite and smectite content and by a low chemical state of weathering (low CIA values and high K/Al and Ti/Al ratios). Finally, our results provide new clues regarding the re-flooding of the Mediterranean Sea at the end of the MSC (5.33Myr). This event was associated with enhanced aridity in the Sahara, implying a reorganisation of the atmospheric circulation and a southward migration of the ITCZ.

Geochemistry of Mesozoic and Cenozoic sediments in the northern Qaidam basin, northeastern Tibetan Plateau: Implications for provenance and weathering

Whole-rock geochemical data of the Mesozoic and Cenozoic sediments in the northern Qaidam basin were used to reconstruct the provenance and chemical weathering history. Based on the fairly uniform REE patterns and trace element ratios, both the Mesozoic (LaCN/YbCN=10.37±1.75; Eu/Eu*=0.68±0.07; Th/Sc=1.11±0.38) and Cenozoic (LaCN/YbCN=9.77±0.62; Eu/Eu*=0.69±0.03; Th/Sc=0.97±0.14) mudstones were derived from a similar source area with acidic-intermediate rocks as dominant contributors, and two modeled mixtures composed of 60% granite, 35% quartzdiorite and 5% mafic rocks, and 55% granite, 40% quartzdiorite and 5% mafic rocks can be deduced as the potential source compositions for the Mesozoic and Cenozoic sediments, respectively. This conclusion is reinforced by the mudstone major element composition and sandstone petrography. The Early–Middle Jurassic mudstones have relatively high K-corrected CIA (84–93) and PIA (88–99) values, indicating intense chemical weathering conditions; while the Late Jurassic to Cenozoic sediments have variable corrected CIA (51–85) and PIA (50–92) values, implying predominantly mild to moderate weathering conditions. The chemical weathering history suggests that a warm and humid climate prevailed during the Early–Middle Jurassic, whereas a cool and semiarid to arid climate prevailed from the Late Jurassic up to now. This dramatic transition of climate may be attributed to multiple likely factors, including the restructuring of global atmospheric circulation patterns in response to the breakup of Pangea since Late Jurassic, and the orogenic rejuvenation and high elevation of the Qilian Mountains, northeastern Tibetan Plateau. Furthermore, the decreasing chemical weathering intensity over time indicates the increase of aridification and cooling of the climate culminating in the Late Cenozoic.

Geochemistry of Permo-Triassic mudstone of the Satpura Gondwana basin, central India: Clues for provenance

Major and trace element data of the Permo-Triassic mudstones from the intracratonic Satpura Gondwana basin, central India have been used to investigate provenance. The Satpura succession (~ 5 km thick) unconformably overlies the Precambrian basement. Sediment dispersal pattern suggests that the Precambrian rocks straddling the southern margin of the Satpura basin were the most probable source for the sediments. The lowermost unit of the Satpura succession comprises glacio-marine and glacio-fluvial deposits. The rest of the succession largely represents a variety of fluvial depositional systems with some records of fluvio-deltaic and fluvio-lacustrine sedimentation under a climatic spectrum of temperate, humid to warm, semi-arid. The present study is confined to the lower six formations namely Talchir, Barakar, Motur, Bijori, Pachmarhi and Denwa, arranged in that order from bottom to top. The oldest rocks of this sequence, the Talchir Formation, are enriched in mobile elements such as Na, Ca and Mg, depleted in alumina and have high ICV (0.93–1.29) and lower CIA values (61.82–74.89) compared to those of the younger mudstones. K 2O/Al 2O 3 ratios of the Talchir mudstones (0.24–0.28) are higher than the younger mudstones in this sequence. These results suggest that the Talchir mudstones were weathered less intensely and are thus more immature than the younger mudstones. This is consistent with the prevailing cold climate and relatively higher rate of basin subsidence. In contrast, the majority of the mudstones from the younger formations are depleted in mobile elements, have relatively low ICV (0.18–0.87) and K 2O/Al 2O 3 values (0.07–0.26), higher CIA values (76.99–92.86), and appear compositionally more mature reflecting moderate to high intensity source rock weathering. Among them, the Barakar mudstones likely have been more intensely weathered than mudstones from any of the other formations. The trace element compositions and ratios are consistent with most mudstones being derived from a felsic source with some mudstones being derived from a more mafic source. For example, La/Sc (0.805 and 0.114), La/Th (6.245, 5.072, 7.418, 6.245, and 5.072) and Eu/Eu* (1.431, 0.868, and 0.805) of some mudstones indicate mafic source. Bivariate plot of Th/Sc and Zr/Sc indicates considerable enrichment of zircon that suggests recycling from meta-sedimentary rocks. Prominent negative Eu anomaly, high LREE/HREE, and flat HREE pattern reflect sediment derivation from predominantly felsic rocks of the old upper continental crust. SiO 2/Al 2O 3 versus K 2O/Na 2O plot for the mudstones suggests that they formed in a passive margin. Also, total REE content of most of the mudstones is conformable with passive margin tectonic setting and craton interior type of provenance. Plotting in La–Th–Sc, Th–Sc–Zr/10 and La/Sc versus Ti/Zr diagrams largely indicates continental margin settings for majority of the mudstones but fails to differentiate between active and passive continental margins.

Combined statistical and petrological analysis of provenance and diagenetic history of mudrocks: Application to Alpine Tethydes shales (Sicily, Italy)

Mineralogical (X-ray diffraction) and geochemical (X-ray fluorescence) compositional data of Alpine Tethyde mudrocks from Sicily were used to unravel the interplay of provenance and diagenesis by means of multivariate statistical methods (PCA, discriminant function analysis) designed for non-negative constant-sum data. The Sicilian Alpine Tethydes comprise Cretaceous–Eocene scaly shales (Mt. Soro and Upper Scagliose Shales Units), Eocene–Oligocene variegated shales (Troina and Nicosia Units), and Miocene Numidian shales (Nicosia Unit). The use of biplots of clr-transformed variables allows discrimination of two different groups of shales. High CIA values reveal a strongly weathered source area especially for the Numidian shales. The analyses of mixed-layered clay minerals indicate that the behaviour of K2O and geochemical affine elements is controlled by the diagenetic grade, which increases from south-west to north-east in response to tectonic loading. Ratios of selected elements (Zr, Y, La, Nb, Ti, Co, Cr, Ni), considered essentially unchanged with respect to the source rocks, are similar in the scaly and variegated shales, and resemble the Variscan Peloritani metamorphic basement. In contrast, the distinct geochemical signature of the Numidian shales reveals a source similar to the average cratonic sandstone, suggesting derivation from external sources located in the foreland. Multivariate discriminant analysis performed on selected trace elements confirms the previous suggestions.

Heavy Minerals and Rare Earth Elements in Coastal and Inland Dune Sands of El Vizcaino Desert, Baja California Peninsula, Mexico

A heavy mineral (HM) study and light and heavy rare earth elements (LREE and HREE) analysis were performed in coastal and inland dune sands, El Vizcaino Desert, Central Baja California Peninsula, Mexico. Our study shows high abundances of hornblende and apatite in the El Vizcaino dune sands, suggesting a dominance of granodiorites/intermediate plutonic rocks and marine authigenic phosphorite in the dune sands. There is a relationship between unstable heavy minerals like hornblende, pyroxenes, and sphene, and heavy rare earth elements (HREE) that suggests that unstable heavy minerals are potential carriers of HREE in the dune fields. However, there is a slight depletion of HREE in relation to LREE, especially in one locality of the inland dunes probably associated with the wind regime and weathering of unstable heavy minerals in the sands. Inland, transitional, and coastal dune fields can be observed as different dune provinces by means of grouping HM and REE data in two separate dendograms. It seems that HREE are correlated with fine-grained sand sizes and correlated with high CIA values linked to slightly weathered sands.

Recycling and chemical weathering in tectonically controlled Mesozoic–Cenozoic basins of New Zealand

AbstractSedimentation in molasse basins is controlled by tectonics, however, recycling and chemical weathering play a critical role in the compositional evolution of a sedimentary succession. The Cretaceous to Pliocene molasse deposits of Central Otago, New Zealand are excellent examples of tectonically related deposits that were governed by the effects of chemical weathering and recycling. Preserved in fault‐controlled basins floored by flysch deposits of the Otago Schist, the clastic successions contain ubiquitous unconformities and lithofacies consistent with alluvial, fluvial and lacustrine depositional settings. Textural analysis of Central Otago sandstones establishes a general quartz enrichment and increased mixing of angular and well‐rounded quartz varieties up‐section, consistent with a history of sediment recycling. Rare earth element (REE) patterns, which reflect upper crustal compositions, are similar for the flysch‐type Otago Schist (Permian–Early Cretaceous), a palaeo‐weathering profile, and the overlying molasse deposits. The development of quartz arenites is also consistent with high degrees of chemical weathering, and erosion of the schist basement, which contains numerous quartz veins. Although recycling has occurred, SiO2 and TiO2 do not consistently show a negative correlation over time. This reflects erosion of previously deposited quartz‐rich sediment and the Otago weathering profile, which produced an inverse stratigraphy. CIA values range from 52 for lithic‐rich, coarse‐grained sandstones and polymictic conglomerate matrices, to 93 for coarse‐grained to pebble‐rich quartz arenites. Individual samples were split into finer‐ and coarser‐grained pairs (<2·5φ and 2·5 to −1φ) and were analysed separately. The results show that finer‐grained samples contain higher REE abundances and less SiO2, but the coarser‐grained Miocene–Pliocene samples have higher CIA values than their finer‐grained counterparts. These coarse‐grained deposits are quartz‐rich and plot erratically on tectonic discrimination diagrams, implying that using SiO2‐poor samples is more reliable for geochemical analysis. Overall, the petrographic and geochemical results indicate that the main factors controlling the composition of the Central Otago molasse deposits were source composition, chemical weathering and recycling. Studies of this nature can be conducted in Archaean tectonically controlled molasse basins that are affected by similar allocyclic factors.

Sedimentation in the Venezuelan Basin, Circulation in the Caribbean Sea, and Onset of Northern Hemisphere Glaciation

Currents of the equatorial Atlantic and Tethys Sea migrated westward through the Caribbean Sea and Panama seaway from Cretaceous to Oligocene time. At about 25 Ma currents reversed direction, allowing cool Pacific waters (Peruvian current) to enter the Caribbean. Almost coincident with the reversal was a dramatic increase in Central American explosive volcanism, which resulted in development of large volcanic centers, and an abundance of suspended volcanic detritus in Pacific and southern Caribbean waters. The detritus was transported by Pacific currents across the Caribbean to be deposited as pelagic mud in the Venezuelan Basin during 25 to 15 Ma. These cool, turbid Pacific waters led to a dramatic reduction in the diversity of hermatypic corals. At about 15 Ma Venezuelan Basin pelagic sediments record a dramatic change in provenance, with derivation of highly weathered detritus from the South American continent, probably the result of renewed uplift of the northern Andes. Exceptionally abundant kaolinite, reflected in high CIA values of the pelagic mud, indicates that materials of the Andean hinterlands suffered intense chemical weathering from at least 15 Ma to the present, with the most intense weathering occurring during Pliocene time. Weathering was somewhat less intense during the cool Pleistocene Period. These findings bear on Northern Hemisphere glaciation. With the Panama Seaway open, cool Pacific waters entered the Caribbean between 25 and 12 Ma, and probably mixed with warm equatorial Atlantic waters to form the Gulf Stream. Closure of the seaway commenced about 12 Ma and finally closed about 3.5 Ma. This resulted in a warmer Gulf Stream, derived entirely from equatorial Atlantic waters. Dramatically increased pelagic sedimentation rates in the Venezuelan Basin since 4 Ma suggest intensification of the Atlantic equatorial current and strengthening of equatorial Atlantic summer storms (hurricanes), both of which strengthened the Gulf Stream current. Resultant northward heat transport by a warm, intensified Gulf Stream led to increased precipitation and promoted glaciation in the Northern Hemisphere.