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1.
This study uses clay mineral assemblages, illite ??crystallinity?? (IC), chlorite ??crystallinity?? (CC), illite polytypes, the b cell-dimension of K-white mica, mineral geothermo-geobarometers and homogenization temperatures of fluid inclusions to investigate the transition from diagenesis to metamorphism in a 7?km thick Triassic flysch sequence in the well Hongcan 1, eastern Tibetan plateau. The 7,012.8?m deep borehole penetrated flysch of Upper to the Middle Triassic age and represents a unique chance to characterize low temperature metamorphic processes in an unusually thick sedimentary sequence developed on thickened continental crust. Mineral assemblage analysis reveals a burial metamorphic pattern with kaolinite and chlorite/smectite mix-layer phases present in the upper 1,500?m, and illite/smectite mixed-layer phases extending to a depth of 3,000?m. The metamorphic index mineral, graphite, was detected in sedimentary rock below 5,000?m using Raman spectroscopy. There exists a good correlation between IC and CC within the prograde burial sequence; with CC anchizonal boundaries of 0.242 and 0.314°2?? (upper and lower boundaries, respectively) corresponding to Kübler??s IC limits at 0.25 and 0.42°2??. Illite polytypism also shows an increase in the 2M 1 polytype with increasing depth, with ca. 60?% 2M 1 abundance compared to the 1M type at the surface, to 100?% 2M 1 at the bottom of the borehole. Fluid inclusion analysis show HHC-rich bearing fluids correspond to the diagenetic zone, CH4-rich bearing fluids appear at transitional zone from diagenetic to low anchizone and H2O-rich bearing fluids mark the high anchizone to epizone. Based on chlorite chemical geothermometer, calcite?Cdolomite geothermo-barometers as well as homogenization temperature of fluid inclusions, a paleotemperature range of 118?C348?°C is estimated for the well with a pressure facies of low to intermediate type.  相似文献   

2.
The Upper Red Formation (URF) comprises over 1–5 km of late Miocene siliciclastic sediments in the Central Iran Basin. The formation is dominated by volcaniclastic conglomerates and arenites. The prevailing arid conditions during most of the basin's history resulted in deposition of predominantly organic‐poor, red sediments with gypsum and zeolites. This investigation concentrates on the mineralogy and geochemistry of the URF in the southern and northern margins of the basin where the formation was buried to depths of 2.4 and 6.6 km, respectively. Fine fraction mineral separates from the southern margin consist of nearly pure smectite and zeolites at a depth of 400 m and smectite with minor quartz and calcite at 1800 m. Shallow samples (1350 m) from the northern section are rich in smectite, illite/smectite with some discrete illite and chlorite. This assemblage is progressively replaced by discrete illite and chlorite with increasing burial depth so that only these two minerals are found at depths greater than 4300 m. The initial alteration process involved replacement of glass and volcanic lithics by smectite and zeolites in both margins of the basin. Increased depth of burial in the northern margin resulted in the progressive isochemical alteration of smectite to discrete illite and chlorite. Diagenesis of clay assemblages occurred essentially in a closed system. Solute products of glass hydrolysis reactions were retained in highly alkaline, saline ground waters from which zeolites, carbonates and oxides precipitated as cements. It is unlikely that these sediments were ever significantly leached by meteoric waters or by organic acids generated during burial diagenesis. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

3.
Core U1359 collected from the continental rise off Wilkes Land, east Antarctica, is analyzed for the clay mineralogy and carbon content. The temporal variation of the clay mineralogical data shows a dominance of illite with chlorite, smectite and kaolinite in decreasing concentration. Clay mineral illite is negatively correlated with smectite which shows enrichment during 6.2–6.8, 5.5–5.8, 4.5 and 2.5 Ma. The mineralogical analyses on the silt size fraction (2–53 μm) of some selected samples were also carried out. The combined result of both the size fractions shows the presence of chlorite and illite in both size fractions, smectite and kaolinite only in clay size fraction (<2 μm) and similarity in the crystallinity and chemistry of illite in both fractions. Similar nature of illite in both fractions suggests negligible role of sorting probably due to the deposition from the waxing ice sheet. During times of ice growth, nearby cratonic east Antarctica shield provided biotite-rich sediments to the depositional site. On the other hand, the presence of smectite, only in the clay size fraction, suggests the effective role of sorting probably due to the deposition from distal source in ice retreat condition. During times of ice retreat, smectite-rich sediment derived from Ross Orogen is transported to the core site through surface or bottom water currents. Poor crystallinity of illite due to degradation further corroborates the ice retreat condition. The ice sheet proximal sediments of U1359 show that in the eastern part of Wilkes Land, the ‘warming’ was initiated during late Miocene.  相似文献   

4.
The metamorphic conditions of the Upper Permian Yangjiagou Formation in eastern Changchun, China, were evaluated based on the mineral assemblage, illite crystallinity, illite polytypism,the b dimension of illite, and the chemical composition of chlorite. The pelitic rocks in the Yangjiagou Formation are characterized by illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite in the formation has a crystallinity of 0.38-0.55 and comprises mixed 2 M_1 and1 M_d polytypes, indicating a metamorphic temperature of 200℃. Based on the chemical composition of chlorite and the chlorite geothermometer, we estimated diagenetic to very low-grade metamorphic conditions with temperatures of 185℃～204℃. The b dimension of illite varies from 8.992 A to 9.005 A.We used a mathematical algorithm to extend Guidotti and Sassi's(1986) diagram relating illite b dimension with temperature and pressure, and used this diagram, together with illite crystallinity and chlorite chemical composition, to semi-quantitatively estimate the formation pressure at1.2 kbar. These reveal that the Yangjiagou Formation has experienced very low-grade metamorphism.  相似文献   

5.
李娟  郑常青  金巍  崔芳华  施璐  王虎 《地学前缘》2011,18(2):223-230
内蒙古中部晚古生代地层由于中生代岩浆的侵入,部分发生接触变质,变质程度多处于沸石相或葡萄石绿纤石相。通过对典型剖面的泥质岩石中的粘土矿物、伊利石结晶度以及镜质体反射率的分析,得出以下结论：（1）粘土矿物以伊利石和绿泥石为主,只在极少数样品中有伊蒙混层和蒙脱石,多数缺乏伊绿混层;（2）伊利石结晶度为022~040;（3）镜质体反射率Ro值为15~40。其结果表明：在岩浆侵入地区有小规模的接触变质,为极低级变质带,并未发生大规模的低绿片岩相的区域变质,而且这种变质只影响到晚二叠世以前沉积的地层和油气,分布范围有限。  相似文献   

6.
Clay minerals from the three principal kinds of zeolitic sediments from the type area for zeolite facies alteration, the Triassic Murihiku Supergroup, Southland, New Zealand, have been studied by TEM. Bentonitic tuff consists largely of smectite and heulandite with minor illite; they occur as replacements of glass shards and are inferred to be direct alteration products of tuff alteration. Both analcime- and laumontite-rich tuffs contain chlorite, illite and mixed-layer illite-chlorite, including 11 mixed-layer sequences. Subhedral to euhedral phyllosilicate crystal shapes and other textural features imply that phyllosilicates crystallized from solution derived in part by dissolution of precursor smectite. Intralayer transitions involving illite and chlorite are inferred to be products of crystallization rather than direct alteration and replacement. Petrographically similar bentonitic and analcimized tuffs overlap each other in the stratigraphic section, supporting earlier observations that there is no systematic change in smectite relative to the illite plus chlorite derived from smectite in sections up to 8.5 km thick. The data imply that smectite may be metastable relative to illite plus chlorite. Permeability and fluid chemistry are inferred to be as significant as temperature in promoting reactions in clay minerals as well as zeolites during burial metamorphism.Contribution No. 436, from The Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109  相似文献   

7.
The clay mineralogy of the Newark Supergroup (Upper Triassic/Lower Jurassic) in the Connecticut Valley was studied by X-ray diffraction analysis. Clay minerals identified in 126 samples are illite, chlorite, smectite, kaolinite, vermiculite, expandable chlorite, mixed-layer illite/smectite, mixed-layer chlorite/smectite, and mixed-layer chlorite/vermiculite. In general, the rocks are illitic with subordinate amounts of chlorite. However, the various lithofacies in the Newark Supergroup are characterized by distinct clay-mineral assemblages. Red beds of floodplain origin contain clays mainly of detrital nature with 2M illite most abundant. Subordinate amounts of chlorite, smectite, vermiculite, kaolinite and mixed-layer illite/smectite are also present. An interstratified chlorite/vermiculite occurs in red mudstone underlying basalt flows. Lacustrine gray beds are generally characterized by the clay-mineral assemblage 1Md illite + chlorite with minor amounts of smectite ane expandable chlorite. An interstratified chlorite/smectite predominates in gray mudstone associated with perennial lake cycles in the East Berlin Formation. Black shales of deeper lacustrine origin contain the assemblage 1Md ifillite + trioctahedral smectite and traces of chlorite. Illite and smectite also occur as mixed-layer phases.In many respects, the distribution of clay minerals in the Connecticut Valley can be likened to the general scheme proposed for the Permo-Triassic basins of Europe and Africa. These display both vertical and horizontal variations in clay-mineral assemblages that reflect the chemical and spatiotemporal evolution of intrabasin depositional and diagenetic environments. Chemical data indicate that magnesium, especially, was concentrated in the black muds of large perennial lakes that intermittently occupied the Connecticut rift valley. Pore waters derived from these sediments played an important role in the development of Mg-rich 2 : 1 and interstratified clay minerals during early diagenesis.  相似文献   

8.
Thermal and hydrothermal effects of Triassic–Liassic basalt flow deposition on sedimentary series of the Argana Basin are responsible for major modifications in detrital clays, until 20 m in depth. It expressed by transformation of detrital smectite to corrensite and moreover to chlorite, and by increasing illite crystallinity. On the 2 m of sediments located immediately under the flow, magnesium-rich hydrothermal fluids have caused precipitation of new mineral phases. To cite this article: L. Daoudi, J.-L. Pot de Vin, C. R. Geoscience 334 (2002) 463–468.  相似文献   

9.
The maximum palaeotemperature of oil-bearing sandstones in the UpperTriassic in the eastern Ordos basin has been determined by using many methods including thevitrinite reflectance, fluid inclusion, apatite fission track, illite crystallinity, chlorite polytypeand diagenetic change of authigenic minerals. The thermal gradient in the Late Mesozoic wasabout 2.9-3.0℃/100m. The Upper Triassic was in a mature stage of organic matter andhydrocarbon began to be generated and migrated during this period. The palaeotemperatures ofoil-bearing sandstones were in the range of 88-110℃; those for the generation and migrationof oil ranged from 112 to 122℃. The thickness of the denuded strata overlying the UpperTriassic was 2465-2750m. The present burial depth of oil-bearing sandstones is generally from400 to 1200m. At a depth of ca. 1900m, the temperature may reach 140℃. Below this depth,organic matter was supermature and mainly generated gas.  相似文献   

10.
We analyzed the clay mineral assemblages, content and mineralogical characteristics of Hole U1438A sediment recovered from Amami Sankaku Basin during International Ocean Discovery Program (IODP) expedition 351. The results show that the clay minerals are mainly composed of illite (average 57%), smectite (average 26%), chlorite (average 14%) and minor kaolinite(average 3%). The crystallinity of illite in all samples are good (<0.4 Δ° 2θ), and the chemical indexes of illite in all samples are low (<0.4). Both indicate that illite in Hole U1438A formed in cold and dry climate. By comparing clay mineral assemblages of hole U1438A and the potential sediment sources, we suggest that smectite be mainly derived from the volcanic materials around Amami Sankaku Basin. Illite, chlorite and kaolinite are mainly derived from the Asian dust. The ratios of (illite+chlorite)/smectite show a phased increase over the last 350 ka, which is consistent with the cold and drying trend of the Asian continent since late Pleistocene. The high ratios of (illite+chlorite)/smectite and (illite+chlorite)/kaolinite during glacial period indicate that much more Asian dust was input into the Amami Sankaku Basin, which are responded to the aridity of Asian continent and strengthened east Asian Monsoon during glacial period.  相似文献   

11.
The results of the study of clay mineral alterations in Upper Pleistocene sediments of the southern trough in the Guaymas Basin (Gulf of California) due to the influence of hydrothermal solutions and heat produced by sill intrusions are discussed. Core samples from DSDP Holes 477 and 477A were taken for the analysis of clay minerals. Application of the method of modeling X-ray diffraction patterns of oriented specimens of the finely dispersed particles made it possible to establish the phase composition of clay minerals, determine their structural parameters, and obtain reliable quantitative estimates of their contents in natural mixtures. The modeling data allowed us to characterize reliably the transformation of clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin. In Upper Pleistocene sandy–clayey sediments of the southern trough, changes in the composition of clay minerals occurred under the influence of a long-living hydrothermal system. Its lower part (interval 170.0–257.5 m) with maximum temperatures (~300°C) was marked by the formation of chlorite. Terrigenous clay minerals are not preserved here. Saponite appears at a depth of 248 m in the chlorite formation zone. Higher in the sedimentary section, the interval 146–170 m is also barren of terrigenous clay minerals. Sediments of this interval yielded two newly formed clay minerals (chlorite and illite), which were formed at lower temperatures (above 180°C and below 300°C, approximately up to ~250°C), while the relatively low-temperature upper part (110–146 m) of the hydrothermal system (from ~140°C to ~180°C) includes the mixture of terrigenous and newly formed clay minerals. Terrigenous illite is preserved here. Illitization of the mixed-layer illite–smectite was subjected to illitization. The terrigenous montmorillonite disappeared, and chlorite–smectite with 5–10% of smectite layers were formed. In the upper interval (down to approximately 110 mbsf), the composition of terrigenous clay minerals remains unchanged. They are composed of the predominant mixed-layer illite–smectite and montmorillonite, the subordinate illite, mixed-layer chlorite–smectite with 5% of smectite layers, mixed-layer kaolinite–smectite with 30% of smectite layers, and kaolinite. This composition of clay minerals changed under the influence of sill intrusions into the sedimentary cover at 58–105 m in the section of Hole 477. The most significant changes are noted in the 8-m-thick member above the sill at 50–58 m. The upper part of this interval is barren of the terrigenous mixed-layer illite–smectite, which is replaced by the newly formed trioctahedral smectite (saponite). At the same time, the terrigenous dioctahedral smectite (montmorillonite) is preserved. The composition of terrigenous clay minerals remains unchanged at the top of the unit underlying the sill base.  相似文献   

12.
Abstract Fairly strong (r= 0.75–0.85) positive linear correlations were found between crystallinity indices (peak widths) measured on the first two basal reflections of chlorite and those of illite–muscovite in <2-μm fractions of a representative shale–slate–phyllite series from Palaeozoic and Mesozoic formations of northeast Hungary. The metamorphic grade ranges from late or deep diagenesis through anchizone to epizone conditions. Chlorite crystallinity values measured on air-dried and ethylene-glycol-solvated samples suggest that the effects of expandable interlayers are negligable, especially in the higher grade (～temperature) part of the series. However, the greater scattering of crystallinity values for the chlorite 001 reflection compared to those of the 002 reflection may be related to the effects of minor amounts of interlayered and/or discrete smectite and/or vermiculite. With increasing metamorphic grade and advancing equilibrium recrystallization, the chlorite compositions in different samples become more homogenous. No correlation exists between crystallinity and changes in chlorite composition as estimated from the intensity ratios of basal reflections. Hence an increase of domain size and a decrease of lattice distortion with increasing grade (～temperature) may be decisive factors affecting chlorite crystallinity. Chlorite crystallinity can be applied as a reliable regional, statistical technique complementary with, or instead of, the illite crystallinity method. The illite and chlorite crystallinity scales used here are related to Kübler's epi-, anchi- and diagenetic zones and correlated with coal rank, conodont colour alteration and mineral facies data. As the effects of the detrital white mica can be observed even in the <2-μm fractions of anchizonal metapelites, the anchizone boundaries determined solely on the base of ‘fixed’illite crystallinity values may vary with amounts of detrital and newly formed muscovite–illite. Hence a complex approach utilizing more than one method for determination of grade is preferred for petrogenetic purposes, even if relationships between crystallinity scales, coal rank and mineral facies also vary strongly in different tectonic settings and lithologies.  相似文献   

13.
The Shurijeh Reservoir Formation of Neocomian age is represented by a sandstone sequence, occasionally interbedded with shale, in the Gonbadli gas field, Kopet-Dagh Basin, Northeastern Iran. In this study X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques were used together to characterize the Shuirjeh clay minerals in 76 core samples collected from two deep Gonbadli wells. The results of XRF analysis showed high percentages of silicon and moderate to low percentages of aluminum, sulfur, calcium, potassium, sodium, magnesium, and iron in both wells. The XRD analysis indicated that the above elements were concentrated in the form of quartz, anhydrite, dolomite, calcite, plagioclase, K-feldspar, hematite, and clay minerals. Further XRD examination of the clay fraction revealed that illite, chlorite, and kaolinite were the major types of clay minerals. Unlike, glauconite, smectite, and a mixed layer clays of both the illite–smectite and chlorite–smectite types were observed only in very few samples. The percentages of individual clay minerals were determined using external standard calibration curves and successfully validated by a system of simultaneous linear equations acquired from detailed elemental information based on the XRF analysis. The error reached ± 5% for the main mineral constituent and ± 15% for minor minerals. A local regression relationship was also derived, based on the XRF elemental information, which can be used to estimate the clay contents of other Shurijeh drilled wells with data of pulsed-neutron spectroscopy tools. According to the proposed quantitative approach, the amount of illite varied considerably, reaching 18.3%. In contrast, the amounts of kaolinite and chlorite were generally small, i.e., less than 8.4%. The amount of total clay minerals changed greatly from a minimum of 5% to a maximum of 32.5%. An increase in illite with increasing burial depth and temperature was an obvious indication of deep burial diagenesis in this formation.  相似文献   

14.
X-ray diffraction methods for estimating the metamorphic grade of diagenetic, anchizone and epizone in metapelites are reviewed and applied to samples from a 7000?m+ borehole in western China and surface samples from the surrounding Zoigê area. Kübler’s illite crystallinity (IC) measurements provide more consistent results than calculated values of percentage of illite in the I/S mixed layers and percentage of I/S mixed layers. Down-borehole IC values display a typical burial metamorphic relationship between stratigraphic level and IC. A method for preparing very low grade metamorphic maps is described, and isograds plotted on a regional geological map at selected values of IC, delineating a high temperature diagenetic zone, an anchizone, and an epizone. The map shows that IC values are controlled by stratigraphic level in the north of the study area (i.e. burial metamorphism), and proximity to an igneous intrusive body in the south (i.e. contact metamorphism).  相似文献   

15.
Abstract

The characteristics and distribution of clay minerals and their effects on reservoir quality in the Huagang sandstones in the Xihu Sag, East China Sea Basin were studied by using X-ray diffraction, casting thin-sections, scanning electron microscopy, electron microprobe analysis, fluid inclusion analysis, constant-rate mercury injection and nuclear magnetic resonance. Clay minerals consist of kaolinite, chlorite, illite and illite–smectite mixed layer (I/S); kaolinite forms from dissolved feldspars, chlorite occurs as clay coatings that are transformed from clay precursors owing to the flocculation of suspended detrital clays or the crystallisation of pore fluids, and illite forms from the illitisation of detrital smectite, authigenic kaolinite and K-feldspars. Clay distribution is controlled by sedimentary environments, burial history and lithologies. Typical reservoirs in the western sub-sag are thin and developed in braided river facies at relatively shallow burial depths with clays dominated by kaolinite. However, typical reservoirs in the central inversion tectonic zone are thicker and developed in a braided delta front facies at deeper burial depths with clays mainly consisting of chlorite, illite and I/S. High-quality reservoirs are characterised by coarse granularity, high quartz content and low clay content with widespread development of chlorite coatings that inhibit quartz cements at low temperatures. At higher temperatures, the high-quality reservoirs develop more pores providing growth space for quartz cements and result in the coexistence of chlorite coatings and quartz cements. The high-quality reservoirs are controlled by their lithological characteristics rather than chlorite coatings. Illite and I/S clays create severe damage to reservoirs by reducing the size and connectivity of pore-throats.  相似文献   

16.
可可西里盐湖表层沉积物中粘土矿物的环境信息   总被引：1，自引：0，他引：1  
通过对可可西里盐湖表层沉积物中粘土矿物的研究,以期获得近年来湖区的某些环境信息.研究结果表明:沉积物中的粘土矿物主要为伊利石和绿泥石,非粘土矿物主要为石英、方解石和长石以及少量白云石、文石、闪石和石膏等.样品中＜2μm的粘土矿物以伊利石和绿泥石为主,局部样品含少量有序伊/蒙混层.根据样品分布特点和粘土矿物的组合类型,将样品划分为3个区域:1区和3区的矿物组合为伊利石、绿泥石和有序伊/蒙混层;2区为伊利石和绿泥石,无伊像混层矿物.其中1区和3区样品的结晶度(IC)范围为0.41～0.59,均值为0.51;2区样品的结晶度范围为0.25～0.40,均值为0.34,明显低于1区和3区,说明2区样品中的伊利石具有更高的结晶度.样品的伊利石化学指数(CI)均小于0.5,表明该伊利石为富Fe-Mg伊利石,主要是物理风化作用的结果.伊利石和绿泥石作为主要粘土矿物反映了近年来整个湖区的环境以干冷为主,局部区域(1区和3区)出现的少量有序I/S混层矿物则指示在短期内曾经历过短暂的干湿交替环境.此外,在酸性介质条件下,少量伊利石发生弱水解作用导致晶格中的K+淋失,并转化为伊/蒙混层矿物,同时也降低了伊利石的结晶度.  相似文献   

17.
Burial Metamorphism of the Ordos Basin in Northern Shaanxi   总被引：1，自引：0，他引：1  
Burial metamorphism has been found in the Ordos basin of northern Shaanxi. On the basis of a rather intensive study of burial metamorphism of sandstone, it has been shown that the evolution from diagenesis to metamorphism involves four stages: cementation of clay minerals, regrowth of pressolved quartz and feldspar, cementation of carbonates and formation of laumontite. On that basis it has been put forward that the laumontite is formed by burial metamorphism of clay and carbonate minerals. According to the thermodynamic data of minerals, the conditions under which laumontite is formed are T<250℃ and X_(CO_2)<0.17. High-resolution SEM and TEM studies of clay minerals in mudstone show that there occur a mixed layer assemblage of bertherine and illite/chlorite and transformation from bertherine to chlorite. On that basis coupled by the X-ray diffraction analysis the author suggests the following transformation of clay minerals during burial metamorphism: the earliest smectite-kaolinite assemblage changes into the bertherine-illite mixture with increasing depth, then into the illite/chlorite mixed layer assemblage and finally into dispersed individual illite and chlorite. The reaction of the transformation is:smectite+kaolinite+K~+=illite+chlorite+quartz According to the study of the oxygen isotope thermometry of the coexisting illitequartz pair, the temperature of the above transformation is lower than 180℃.  相似文献   

18.
周健  王河锦 《地质论评》2002,48(4):361-364
运用X射线衍射分析技术，计算了成岩带、近变质带粘土矿物C^*方向的粒度。根据阿尔卑斯造山带前陆碎屑岩粘土矿物粒度与伊利石结晶指数的关系，推导出了伊利石、绿泥石、蒙脱石、伊蒙混层、高岭石和叶蜡石的c^*方向粒度变化趋势曲线。结果表明，大多数产于成岩带和近变质带的粘矿物其c6*方向粒工为几至几十纳米。成岩带和近为质带是一维天然纳为级土帮物的产地。  相似文献   

19.
采用X射线衍射、扫描电子显微镜分析方法,对青海循化盆地晚渐新世-早中新世沉积物中黏土矿物的微观形貌、体积分数、结晶度及其古气候指示作用进行了深入的研究。分析结果表明,晚渐新世盆地沉积物中的黏土矿物主要为蒙脱石、伊利石、坡缕石和绿泥石,以蒙脱石为主,指示循化地区总体为相对温暖潮湿的气候条件,蒙脱石、伊利石和绿泥石相对体积分数及伊利石、蒙脱石结晶度值均出现明显的周期性波动,表明循化地区气候经历了干旱-温暖潮湿交替的变化;早中新世盆地沉积物中的黏土矿物主要为伊利石、蒙脱石和绿泥石,以伊利石为主,各黏土矿物体积分数及伊利石、蒙脱石结晶度值的变化范围不大,表明循化地区气候以相对持续冷干为特征。从晚渐新世到早中新世,气候条件由相对温暖潮湿到相对冷干的转变,揭示其间(约21.3Ma)出现了一次极端的降温事件,可能与青藏高原隆升及亚洲沙漠化密切相关。  相似文献   

20.
Early Jurassic climate is characterized by alternating cold and warm periods highlighted by studies based notably on oxygen isotopes measured on belemnite guards and other marine invertebrate shells. These climatic changes include changes in the hydrological cycle, and consequently weathering and runoff conditions. In order to clarify the erosion and weathering conditions during the Pliensbachian, this study determined the mineralogical composition of the clay fraction of 132 samples taken from the entire stage drilled in the Llanbedr (Mochras Farm) borehole (Cardigan Bay Basin). The clay mineral assemblages are composed of various proportions of chlorite, illite, illite/smectite mixed‐layers (R1 I–S), smectite and kaolinite, with possibly occasional traces of berthierine. The occurrence of abundant smectite indicates that the maximum burial temperature never exceeded 70°C. Consequently, clay minerals are considered mainly detrital, and their fluctuations likely reflect environmental changes. The variations in the proportions of smectite and kaolinite are opposite to each other. Kaolinite is particularly abundant at the base of the jamesoni Zone, in part coinciding with the δ13C negative excursion corresponding to the Sinemurian/Pliensbachian Boundary Event, and through the davoei Zone, whilst smectite is abundant in the upper part of jamesoni and base of ibex zones and through the subnodosus/gibbosus subzones of the margaritatus Zone. The kaolinite‐rich intervals reflect an intensification of hydrolysis and an acceleration of the hydrological cycle, while the smectite‐rich intervals indicate a more arid climate. The spinatum Zone is characterized by a distinct clay assemblage with abundant primary minerals, R1 I–S, kaolinite reworked from previously deposited sediments or from Palaeozoic rocks, and probably berthierine originating from contemporaneous ironstone‐generating environments of shallower waters. This mineralogical change by the end of the Pliensbachian likely reflects a transition from a dominant chemical weathering to a deeper physical erosion of the continent, probably related to a significant sea‐level fall consistent with a glacio‐eustatic origin.  相似文献   

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