Chaowen Wang

Neogene Source-to-Sink Relations between the Pamir and Tarim Basin: Insights from Stratigraphy, Detrital Zircon Geochronology, and Whole-Rock Geochemistry

The Tarim Basin, as the largest inland basin on the planet, provides a valuable opportunity to understand the mountain building of the northern Tibetan Plateau and its effects on basin development. Here we present a synthesis of sedimentology, zircon U-Pb geochronology, and bulk-rock geochemistry of Neogene sediments in the Qimugan section, southwest Tarim Basin. Spatial variation in zircon U-Pb age distributions from early Miocene clasts at Qimugan and Oytag suggest significant dextral strike-slip on the Kashgar-Yecheng transfer system likely commenced during the Oligocene–early Miocene. Over time, age peaks of ∼20 and ∼107 Ma in a middle Miocene sample at Qimugan suggest significant headwater erosion of the ancient Yarkand River reached the southeast Pamir–Karakoram hinterland as it does today. This is coincident with a relatively steady decrease in chemical weathering of source terranes during the middle-late Miocene, suggesting a climate transition from warm and/or humid to cool and/or dry in the Pamir-Karakoram. Under global cooling, middle Miocene changes in both provenance and geochemistry at Qimugan require topographic growth of the Pamir-Karakoram interior at that time, coeval with initial formation of the fold-thrust system and doming of the Muztaghata massif in the eastern Pamir, in addition to a prominent depocenter shift and sediment load in the southwest Tarim Basin. Subsequently, stable sediment provenance and depocenters suggest the current tectonic-sedimentary configuration in the eastern Pamir–southwest Tarim Basin has been established since the middle-late Miocene. These observations can be explained by a model of crustal contraction below the southeast Pamir–Karakoram and strain propagation to the Tarim Basin, possibly related to resumed Indian crust subduction. Our results thus support compressional deformation extended to all margins of the northern Tibetan Plateau by the middle-late Miocene.

The early-Eocene climate optimum (EECO) event in the Qaidam basin, northwest China: clay evidence

Abstract Clay mineralogy and its palaeoclimatic interpretation of the early-Eocene (~53.3-49.70 Ma) sediments at Lulehe, Qaidam basin, northwest China, were investigated using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The interval of ~53.3–49.70 Ma, including the early-Eocene climate optimum (EECO) with isotopic events, was the transition period of ‘‘greenhouse’’ to ‘‘icehouse’’. Climate changes during the episode were documented in the sediments and were expressed by the proportion of clay species and clay indices, as well as by the proportion of non-clay minerals, gypsum, halite and calcite. Our results suggest that a warm and humid climate prevailed over the period ~53.3–52.90 Ma, followed by a warm and seasonally dry and humid climate in the period ~52.90–51.0 Ma and a subsequently warm and humid climate in the period ~51.0–49.70 Ma. Three warmer and more humid intervals were observed at 52.7, 51.0 and 50.5 Ma based on clay indices. The climate evolution in the Qaidam Basin during the period derived from the clay mineralogical study is in good agreement with the early Eocene global climate change, and the warm and seasonally dry and humid episode in the early Eocene in Qaidam basin is a regional response to the global early-Eocene climate optimum.

Climatic and tectonic evolution in the North Qaidam since the Cenozoic: Evidence from sedimentology and mineralogy

Clay mineralogy and bulk mineral composition of Tertiary sediments in Qaidam were investigated using X-ray diffraction (XRD) and scanning electron microscopy in order to better understand regional climate change resulting from uplift of the Northeast Tibetan Plateau. Climate change in Qaidam since ∼53.5 Ma could be divided into four stages: a warm and seasonally arid climate between ∼53.5 and 40 Ma, a cold and arid climate from ∼40 to 26 Ma, a warm and humid climate between ∼26 and 13.5 Ma, and a much colder and arid climate from ∼13.5 to 2.5 Ma, respectively. The illite crystallinity and sedimentary facies suggested that uplift events took place around >52–50, ∼40-38, ∼26-15, ∼10-8, and <5 Ma in the Qaidam region, respectively. The climate in Qaidam Basin could have been controlled by global climate prior to 13.5 Ma. As the Tibetan Plateau reached a significant elevation by ∼13.5 Ma, and the climate cycles of the East Asian monsoon might add additional influence.

CLAY MINERALOGY OF THE ZHADA SEDIMENTS: EVIDENCE FOR CLIMATIC AND TECTONIC EVOLUTION SINCE ~9 Ma IN ZHADA, SOUTHWESTERN TIBET

The clay mineralogy of the Zhada sediments was investigated, using X-ray diffraction and scanning electron microscopy, to obtain a better understanding of climatic change and uplift of the Himalayas in the Zhada region of Tibet. The sediments of Zhada basin in the late Miocene to Pliocene consist of lacustrine and fluvial deposits >800 m thick and can be subdivided into five clay assemblage zones based on their clay-mineral composition. The upward zonation is as follows: (1) smectite-kaolinite; (2) illite-chlorite; (3) chlorite-illite-kaolinite; (4) illite-chlorite; and (5) smectite, illite, and kaolinite. The ratio of chlorite + illite to kaolinite + smectite (Ch+I/K+S) and the Kübler index indicate a warm and humid climate from 9.5 to 8.4 Ma, a cold and dry climate from 8.4 to 7.2 Ma, a warm and seasonal arid climate from 7.2 to 4.5 Ma, a cool and humid climate from 4.5 to 3.6 Ma, and a warm and seasonally humid climate from 3.6 to 3.0 Ma. Intense fluctuations in the Kübler index and in the quantities of evaporite minerals dolomite, aragonite, and gypsum, during the period 7.2–4.5 Ma suggest strong climatic fluctuations between humid and seasonally humid conditions in the Zhada basin. Rapid uplift around the Zhada basin occurred at 8.4 and 3.6 Ma, with sharp subsidence at 7.2 and 4.5 Ma. Evolution of the climate at Zhada showed a different model from that of global climate change, and tectonics-led climate change was the major contributor to climate evolution in the area.

Three-component mixed-layer illite/smectite/kaolinite (I/S/K) minerals in hydromorphic soils, south China

Abstract To understand clay mineral transformations in hydromorphic conditions in the red earth sediments in Xuancheng, south China, clay mineralogy was investigated using X‑ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM). The XRD results indicated that clay minerals in the hydromorphic soils were illite, kaolinite, smectite, vermiculite, and mixed-layer illite/smectite and illite/smectite/kaolinite. Changes of the kaolinitic reflections under the various conditions suggested that the kaolinitic phase is a mixed-layer structure having kaolinite layers randomly interstratified with illite and smectite layers. HRTEM observation showed that 10 Å illite layers interstratified with both 15 Å smectite layers and 7 Å kaolinite layers in clay particles, confirming the occurrence of illite/smectite/kaolinite (I/S/K) three-component mixed-layer clays. The lattice fringes of the I/S/K clays appeared corrugated and vanishing, and also exhibited variable thickness along a lattice fringe, which were consistent with changes from illite to smectite, from smectite to kaolinite, and from illite to kaolinite, respectively. Hydromorphic conditions in the Xuancheng soils led simultaneously to the direct transformation of illite to kaolinite and the transformation of illite to smectite to kaolinite in the pedogenic processes, and the formation of I/S/K three-component mixed-layer clays as intermediate products of these processes.

Characterisation of the hydroxy-interlayered vermiculite from the weathering of illite in Jiujiang red earth sediments

The clay mineralogy and formation of hydroxy-interlayered vermiculite (HIV) in the Jiujiang red earth sediments were investigated using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and inductively coupled plasma-atomic emission spectrometer (ICP-AES) analyses. The 1.4-nm peak of HIV did not change after Mg 2+ saturation and glycol solvation, but it exhibited partial collapse to 1.0nm after K + saturation followed by heat treatment at successively higher temperatures. HIV was also characterised by FTIR adsorption bands at ~3485cm -1 and ~3415cm -1 , which did not change with increasing temperature. DSC analysis revealed that the dehydroxylation of hydroxides in the interlayer of HIV began at ~4008C, and a further dehydroxylation was confirmed by the XRD of the sample heated to ~6008C. The ICP-AES analysis of sodium citrate extracts showed that the Al concentration was higher than that of Fe, indicating that the Al was probably present as hydroxy-Al in the interlayer of HIV. The presence of hydroxy-Al polymers in the interlayer influenced both expandability and thermal properties of HIV clays from Jiujiang red earth sediments.

Clay mineralogical constraints on weathering in response to early Eocene hyperthermal events in the Bighorn Basin, Wyoming (Western Interior, USA)

Series of transient greenhouse warming intervals in the early Eocene provide an opportunity to study the response of rock weathering and erosion to changes in temperature and precipitation. During greenhouse warming, chemical weathering is thought to increase the uptake of carbon from the atmosphere, while physical weathering and erosion control sediment supply. A large ancient greenhouse warming event is the Paleocene-Eocene Thermal Maximum at 56 Ma. In many coastal sites, an increase in the abundance of kaolinite clay during the Paleocene-Eocene Thermal Maximum is interpreted as the result of reworking from terrestrial strata due to enhanced runoff caused by increased seasonal precipitation and storminess during a time of decreased vegetation cover. In the continental interior of North America, Paleocene-Eocene Thermal Maximum paleosols show more intense pedogenesis and drying, which are indicated by deeply weathered and strongly oxidized soil profiles. The weathering and oxidation could be related to temperature and precipitation changes, but also to increased time available for weathering and increased soil permeability in coarser sediment. Here, we provide evidence for enhanced climate seasonality, increased erosion of proximal laterites and intrabasinal floodplain soils, and a potential slight increase in chemical weathering during the smaller early Eocene hyperthermals (Eocene Thermal Maximum 2, including H1 and H2) postdating the Paleocene-Eocene Thermal Maximum, for which no previous clay mineral data were available. Hyperthermal soil formation at the site of floodplain deposition causes a similar, insignificant clay mineralogical change as occurred during the background climates of the early Eocene by showing small increases in smectite and decreases in illite-smectite and illite. Remarkably, the detrital sediments during the hyperthermals show a similar pedogenic-like increase of smectite and decreases of mixed-layer illite-smectite and illite, while the kaolinite and chlorite proportions remained low and unchanged. Since sedimentation rates and provenance were similar during the events, enhanced smectite neoformation during soil formation in more proximal settings, and associated reworking, is the likely process causing this clay mineralogical change. The hundreds to thousands of year time scales at which individual paleosols were formed were probably too short for significant alteration of the rocks by in situ chemical weathering despite changing climates during the two post−Paleocene-Eocene Thermal Maximum greenhouse warming episodes. The relatively small signal, however, raises the question of whether increased chemical weathering can indeed be a strong negative feedback mechanism to enhanced greenhouse gas warming over the time scales at which these processes act.

Illite–Smectite Mixed-Layer Minerals in the Alteration Volcanic Ashes Under Submarine Environment

The clay mineralogy of the clay intervals interbedded with siliceous mudstones across the Permian–Triassic boundary (PTB) in Pengda, Guiyang, Guizhou province, was investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The clay mineral assemblages of the sediments are mainly I/S clays and minor smectite, kaolinite, and illite as revealed by XRD analyses. The peak-shaped parameters BB1 and BB2 of I/S clays of the representative clay bed PL-01 are 4.7° and 4.4°, respectively, and the peak position of the low-angle diffraction is at 6.5° 2θ (13.6 Ǻ), suggesting that the I/S clays have a IS type of ordering. However, multi-order diffractions and their intensities are different from those of completely ordered 1:1 mixed-layer I/S clay rectorite, indicating that I/S clays of the Pengda section have partially ordered IS structures. HRTEM observations show that most of the I/S clays exhibit a IS stacking ordering. However, in some areas within a IS particle, smectite layer is observed in doublets, triplets, and quartets, which are interstratified by various amounts of illite layers, suggesting the presence of other irregular stacking in addition to the major 1:1 IS-ordered stacking. Transformation of smectite layer into illite layers is also observed in the I/S clays, suggesting that the Pengda I/S clays are derived from smectite illitization, in good agreement with the clay mineral assemblage. The I/S clays of the Pengda section contain up to 45–95 % smectite layer, the notably higher contents of smectite layer relative to those of other PTB stratigraphic sets in south China can be attributed to the difference in alteration and smectite illitization processes due to different sedimentary environments.

Clay mineralogy and its palaeoclimatic significance in the Luochuan loess-palaeosols over ~1.3 Ma, Shaanxi, northwestern China

To understand climate changes recorded in the Luochuan loess-palaeosols, Shaanxi province, northwestern China, clay mineralogy was studied using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM) methods. XRD results show that clay mineral compositions in the Luochuan loess-palaeosols are dominantly illite, with minor chlorite, kaolinite, smectite, and illite-smectite mixed-layer clays (I/S). Illite is the most abundant species in the sediments, with a content of 61%–83%. The content of chlorite ranges from 5%–22%, and the content of kaolinite ranges from 5%–19%. Smectite (or I/S) occurs discontinuously along the loess profile, with a content of 0–8%. The Kübler index of illite (IC) ranges from 0.255°–0.491°, and the illite chemical index (ICI) ranges from 0.294–0.394. The CIA values of the loesspalaeosols are 61.9–69.02, and the R3+/(R3+ + R2+ + M+) values are 0.508–0.589. HRTEM observations show that transformation of illite to illite-smectite has occurred in both the loess and palaeosol, suggesting that the Luochuan loess-palaeosols have experienced a certain degree of chemical weathering. The Luochuan loess-palaeosols have the same clay mineral assemblage along the profile. However, the relative contents of clay mineral species, CIA, ICI, and IC values fluctuate frequently along the profile, and all these parameters display a similar trend. Moreover, climate changes suggested by the clay index are consistent with variations in the deep-sea δ18O records and the magnetic susceptibility value, and thus, climate changes in the Luochuan region have been controlled by global climate change.

Clay Mineralogy and its Paleoclimatic Significance of the Oligocene- Miocene Sediments in the Gerze Basin, Tibet

This study collected the early Oligocene to middle Miocene sediments from the Gerze Basin of Tibet, and used X-Ray diffraction (XRD) and Scanning Electron Microscope (SEM) to discuss their clay mineralolgy, clay indices, better understand the clay mineralogy and its paleoclimatic significance. The results show that clay minerals of the Gerze Basin sediments are mainly composed of illite and chlorite, with minor amounts of smectite and kaolinite, and their relative content varies along the section. Variations of relative contents and clay indices suggest that the Gerze Basin has experienced three-stage evolution of paleoclimate: I) high illite and chlorite contents, with fluctuant smectite and low (I+Ch)/(K+S) ratio, indicative of a dominant seasonal arid climate from the early Oligocene to late Oligocene; II) higher illite and chlorite contents and larger (I+Ch)/(K+S) ratio but absence of kaolinite, indicating a colder and drier climate from the late Oligocene to early Miocene; III) high illite and chlorite contents with fluctuant (I+Ch)/(K+S) ratios and occasional occurrence of kaolinite, suggesting that the climate became warmer and more humid compared with that of stage II in the mid-Miocene. These conclusions were also reinforced by the clay morphology, which suggests that physical weathering dominated in stage II, while relatively strong chemical weathering was dominant in stages I and III. Clay minerals of the sediments mainly consist of illite and chlorite, indicating that the source rock played a significant role in clay origin. It is inferred that global cooling and the enhancement of denudation and obstruction of northward moisture due to the uplift of the Qinghai-Tibet Plateau were responsible for the provenance of illite and chlorite under weak chemical weathering. Though the Qinghai-Tibet Plateau reached a certain elevation by the mid-Miocene, yet the mid-Miocene widespread warming might have largely impacted the Gerze climate.