Contact metamorphism adjacent to a teschenite intrusion

Late Permian (early Tatarian, 258 Ma) palaeosols exposed near Kiama and Dapto on the south coast of New South Wales can be recognised from their red colour, clayey and massive texture, and clay‐filled root traces. Harvey pedotype palaeosols are within the upper Jamberoo Sandstone Member of the Broughton Formation. Loveleigh pedotype palaeosols are within the basal Kiama Sandstone overlying columnar jointed flows of the Blow Hole Latite Member of the Broughton Formation. Both kinds of palaeosols are strongly ferruginised with little relict bedding, yet they are little weathered and have surprisingly high amounts of feldspar and pyroxene. Both also show deformation of subsurface layers comparable to the active layer of permafrosted soils. Root traces in these palaeosols are sparse and comparable to those of woody gymnosperms, not chambered like the known roots of Glossopteris. Plausible components of the taiga woodland represented by the palaeosols include Gangamopteris, cordaites, seed ferns, and equisetaleans. Frigid palaeoclimatic indications from the palaeosols are compatible with a palaeomagnetically determined palaeolatitude of 57–85°S, and thus indicate an equator‐pole climatic gradient in the Late Permian generally similar to that of today.     

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin,Llanbedr (Mochras Farm) borehole,Wales

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.     

The influence of salt withdrawal subsidence on palaeosol maturity and cyclic fluvial deposition in the Upper Triassic Chinle Formation: Castle Valley, Utah

Integrated fluvial sequence stratigraphic and palaeosol analysis can be used to better reconstruct depositional systems, but these approaches have not been combined to examine halokinetic minibasins. This study characterizes the temporal and spatial patterns of lithofacies and palaeosols in a sequence stratigraphic framework to reconstruct a model of minibasin evolution and identify halokinetic influences on fluvial deposition. This research documents fluvial cycles and stratigraphic hierarchy, palaeosol maturity and apparent sediment accumulation rates in the Chinle Formation within the Big Bend minibasin. This study also uses palaeosols to help identify fluvial aggradational cycle (FAC) sets. The Chinle is divided into two hectometre‐scale (10² m) fluvial sequences, six decametre‐scale (10¹ m) FAC sets, and variable numbers of metre‐scale FACs depending on proximity to the minibasin. Ten pedotypes representing 225 palaeosol profiles are recognized. The pedotypes include palaeosols similar to modern Entisols, Inceptisols, Aridisols, Vertisols and Alfisols. A maturity index (1–5) is assigned to each pedotype to assess its variability in palaeosol development. Estimated palaeosol development time is used to approximate apparent sediment accumulation rates. Increased subsidence resulted in a greater number and thicker FACs, thicker FAC sets and fluvial sequence sections, and lithofacies associations reflecting more rapid sedimentation along the minibasin axis. Palaeocurrent indicators converge towards the minibasin axis and indicate that it formed and drifted through time. Relative palaeosol maturity is inversely related to stratal thickness, and decreases towards the minibasin where episodic burial by fluvial sediment was more frequent. Metre‐scale FACs are most abundant towards the minibasin axis, and locally have Entisols and Inceptisols developed upon their upper boundaries reflecting increased sediment accumulation rates. Areas outside the minibasin are characterized by fewer FACs that are associated with more mature palaeosols. Palaeosol‐derived apparent sediment accumulation rates are as much as two orders of magnitude greater within the minibasin than in marginal areas. The combined stratigraphic, palaeocurrent and palaeosol evidence is used to develop a model for the evolution of the Big Bend minibasin that illustrates the halokinetic affect on fluvial and landscape processes.     

钱塘江下切河谷SE2孔全新世沉积物黏土矿物特征及其意义*

通过X射线衍射系统分析了杭州湾地区SE2孔全新世沉积物的黏土矿物组成,结果显示研究层段黏土矿物主要由伊利石、绿泥石、高岭石和蒙脱石组成;伊利石结晶度较好,化学风化指数普遍大于0.5,表明以化学风化为主,且风化趋势自下而上呈递减趋势。通过对比中国东南部主要河流沉积物的黏土矿物组成,认为钱塘江下切河谷全新世沉积物的黏土矿物组成具有较好的物源指示意义: 全新世 Ⅰ 段(即古河口湾和河漫滩)沉积物主要来自钱塘江上游,特征黏土矿物为高岭石,河口外物质贡献不大;Ⅱ 段(即现代河口湾和近岸浅海)沉积物包含较多蒙脱石,表明不仅包括钱塘江上游物质,河口外长江物质也开始进入钱塘江河口。黏土矿物中,高岭石对气候有较好的指示作用:中全新世高岭石含量达到最高值,反映气候最为湿热,化学风化程度最高;晚全新世含量逐渐降低,反映气候逐渐回冷,化学风化强度降低。     

Clay record of climate change since the mid‐Pleistocene in Jiujiang,south China

To obtain a better understanding of climate change in south China in the Quaternary, a clay mineralogical study was undertaken on the red earth profile at Jiujiang, using X‐ray diffraction (XRD) and particle‐size distribution analysis methods. The XRD results showed that the clay minerals of the Jiujiang red earth were mainly mixed‐layer illite–smectite (I/S), illite, kaolinite and vermiculite, with trace amounts of mixed‐layer kaolinite–smectite (K/S). Changes in clay mineral composition displayed a trend of three‐stage evolution. The higher mixed‐layer I/S clays and kaolinite contents in the lower portion suggest extremely warm and humid climates over the period c. 700 to c. 350 ka ago. A gradual decrease in I/S clays and kaolinite reveals a gradual climate change from warm/humid to cool/dry during the period c. 350 to c. 130 ka ago. The higher illite and vermiculite contents indicate a relatively cool and dry climate during the period since c. 130 ka ago. The particle‐size distribution pattern of the upper section was similar to that of the Xiashu loess, while that of the middle to lower section was similar to those of fluvially reworked red earth. A rapid increase in the abundance of large grain‐size components at 2.6 m depth indicates an intensification of the winter monsoon and a cool and dry climate during the period, in good agreement with results from the clay mineral composition and homogeneous structure. The red earth sequences in south China could probably be used to test the response of tropical to subtropical regions to global climate changes.     

Landscape ecological shift at the Permian‐Triassic boundary in Antarctica

Palaeosols across the Permian‐Triassic boundary in Antarctica provide evidence of a marked change in ecosystems at this greatest of all extinctions in the history of life on Earth. The boundary can now be recognised from evidence of carbon isotopic (δ¹³C) stratigraphy, reptiles of the earliest Triassic Lystrosaurus zone, and Late Permian glossopterid fructifications and pollen. The boundary is a profound change in palaeosols, with very different suites of pedotypes in Permian compared with Triassic sequences. Permian palaeosols include coals, rooted lithic sandstones and rooted tuffaceous silt‐stones. Triassic palaeosols in contrast are largely rooted, green‐red‐mottled claystones. These palaeosols represent a shift from Late Permian cold temperate broadleaf deciduous swamp woodlands to Early Triassic cool temperate conifer forests. Indications of more intense weathering during the earliest Triassic confirm a significantly warmer palaeoclimate in the earliest Triassic than in the latest Permian. Palaeoclimate remained humid with low evapotranspiration in both Permian and Triassic, but Triassic ecosystems were more oligotrophic, humus‐poor and more oxidised than Permian ones. Yet both Permian and Triassic palaeosols were unpodzolised, unlike soils today under such climates and vegetation. Palaeosols in Antarctica confirm several peculiarities of the earliest Triassic: (i) a global coal gap; (ii) a high‐latitude greenhouse; and (iii) a Gondwanan tuff gap. Palaeosols support evidence from fossil plants and reptiles and from carbon isotopic studies for a shift toward oligotrophic, low‐productivity ecosystems, dominated by opportunistic and stress‐tolerant organisms in the earliest Triassic. Life was difficult on land as well as in the sea following the terminal Permian mass extinction.     

Jurassic smectite and kaolinite trends of the East European Platform: implications for palaeobathymetry and palaeoclimate

Clay minerals cannot only give information on sedimentary depositional environment but also the climate regime under which sediment was weathered. The successions of non‐marine to marine Middle and Late Jurassic strata of the East European Platform show upwards increasing contents of smectite and decreasing contents of kaolinite. There is also an increasing smectite vs. kaolinite ratio laterally, in a basinward direction within individual formations. Vertical smectite and kaolinite content curves coincide broadly with the Sahagian and Jones eustatic sea‐level curve. All this implies differential settling of smaller smectite particles offshore and larger kaolinite particles onshore. Smectite formation, preservation and supply were further enhanced as the eastwards expanding Late Jurassic arid zone of Laurasia influenced weathering conditions in the sediment source areas, to the north and east of the East European Platform.     

Facies analysis and palaeoclimatic significance of ironstones formed during the Eocene greenhouse

Lower and middle Eocene ironstone sequences of the Naqb and Qazzun formations from the north‐east Bahariya Depression, Western Desert, Egypt, represent a proxy for early Palaeogene climate and sea‐level changes. These sequences represent the only Palaeogene economic ooidal ironstone record of the Southern Tethys. These ironstone sequences rest unconformably on three structurally controlled Cenomanian palaeohighs (for example, the Gedida, Harra and Ghorabi mines) and formed on the inner ramp of a carbonate platform. These palaeohighs were exposed and subjected to subaerial lateritic weathering from the Cenomanian to early Eocene. The lower and middle Eocene ironstone sequences consist of quiet water ironstone facies overlain by higher energy ironstone facies. The distribution of low‐energy ironstone facies is controlled by depositional relief. These deposits consist of lagoonal, burrow‐mottled mud‐ironstone and laterally equivalent tidal flat, stromatolitic ironstones. The agitated water ironstone facies consist of shallow subtidal–intertidal nummulitic–ooidal–oncoidal and back‐barrier storm‐generated fossiliferous ironstones. The formation of these marginal marine sequences was associated with major marine transgressive–regressive megacycles that separated by subaerial exposure and lateritic weathering. The formation of lateritic palaeosols with their characteristic dissolution and reprecipitation features, such as colloform texture and alveolar voids, implies periods of humid and warm climate followed major marine regressions. The formation of the lower to middle Eocene ironstone succession and the associated lateritic palaeosols can be linked to the early Palaeogene global warming and eustatic sea‐level changes. The reworking of the middle Eocene palaeosol and the deposition of the upper Eocene phosphate‐rich glauconitic sandstones of the overlying Hamra Formation may record the initial stages of the palaeoclimatic transition from greenhouse to icehouse conditions.     

Climate-induced sediment-palaeosol cycles in a Late Cretaceous dry aeolian sand sheet: Marília Formation (North-West Bauru Basin, Brazil)

Aeolian sand sheets, which are characterized by low relief surfaces that lack dunes, are common in arid and semi‐arid climatic settings. The surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects, or unstable such that it is affected by deflation or sedimentation. The Marília Formation (Late Cretaceous) may be interpreted as an ancient aeolian sand sheet area, where alternating phases of stability and instability of the accumulation surface have been recorded. Detailed field studies were carried out in several sections of the Marília Formation, where cyclic alternations of palaeosols and aeolian deposits were evident, using palaeopedological and facies analysis methods, supported in the laboratory by the analysis of rock samples, cut and polished in slabs, thin sections, scanning electron microscope images and X‐ray diffraction data from the clay minerals. The deposits comprise three lithofacies that, in order of abundance, are characterized by: (i) translatent wind‐ripple strata; (ii) flood deposits; and (iii) ephemeral river channel deposits. Palaeosols constitute, on average, 65% of the vertical succession. Three types of palaeosols (pedotypes) are recognized: (i) Aridisols; (ii) Entisols; and (iii) Vertisols. Erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits. The palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the Marília Formation to be defined as a flat area, dominated by aeolian sedimentation, with subordinate ephemeral river sedimentation, and characterized by a dry climatic setting with occasional rainfall. The climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development. A climate‐controlled model is proposed in which: (i) the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases; and (ii) the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation, thereby increasing the availability of sediment during the drier climatic phases. The ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment.     

Palaeosols in early Himalayan foreland basin sequences demonstrate latitudinal shift-related long-term climatic change

The Palaeogene sedimentary record in the Himalayan foreland basin contains palaeosols that are interpreted as reflecting changes in climate through time with the passage of the Indian Plate from the equator to 30° N latitude. To understand spatial and temporal variation in the occurrence of diagnostic palaeosol types, 12 exposed stratigraphic sections were investigated and studied in detail using petrographic and geochemical techniques, including scanning electron microscopy and X‐ray mineralogy. Oxisol (bauxite) is a karst bauxite that occurs in crudely bedded and laminated forms. It contains gibbsite, goethite and kaolinite and shows chemical index of alteration values close to 100, demonstrating intense weathering. No gradual change in the concentrations of trace and rare earth elements is recorded. However, their behaviour suggests that the Oxisol was produced by weathering of basalt under a warm and humid climate. Histosol (coal), found stratigraphically higher than Oxisols, contains woody structures dominated by vitrinite maceral. The Histosol is interpreted as having originated from an undisturbed peat developed in a swamp of high tree density under wet tropical climatic conditions. Up‐section, calcrete profiles of stages 3 and 4, containing rhizoliths, pellets, filamentous calcite and Microcodium, have developed by pedogenic processes. Large negative stable carbon (?8·5‰ to ?11·2‰) and oxygen isotope values (?8·5‰ to ?12·6‰) suggest that the studied calcretes formed under the influence of meteoric water and soil organic matter in dry sub‐tropical climatic conditions. The formation of Oxisol in a warm and humid climate was possible in the equatorial region when part of the Indian subcontinent was close to the equator. The coal developed under a humid climate when the northern tip of the Indian subcontinent reached the intertropical convergence zone, while the pedogenic calcrete formed once the sub‐tropical climatic zone was reached. Hence, it is suggested that these palaeosols formed in different climatic zones during northward drift of the Indian plate and that the drift brought them to their present positions between 30° N and 35° N within the sub‐tropical climatic zone.     

The Tayiba Red Beds: Transitional marine-continental deposits in the precursor Suez Rift,Sinai, Egypt

The Tayiba Red Beds, exposed in the Abu Zenima area, west-central Sinai, have been intensively studied for their clay mineralogy and charophytes assemblages. Three surface sections exposed at Wadi El-Tayiba and Wadi Nukhul were studied. The Tayiba Formation uncomformably overlies the Middle Eocene Khaboba Formation at Wadi Nukhul and the Late Eocene Tanka Formation at Wadi El-Tayiba and commonly underlies the Early Miocene Nukhul Formation with unconformable relationships. The Tayiba Formation at Wadi Nukhul consists predominantly of continental coarse clastic sediments represented by polymictic conglomerates, alternating with red to pinkish mudstone, ferruginous sandstone and varicoloured mottled siltstone with plant remains. At Wadi El-Tayiba, the Tayiba Formation is represented by marine, yellow mudstone and red siltstone, alternating with greyish and reddish-yellow argillaceous to sandy limestone, highly fossiliferous with reworked Nummulites spp. and molluscan shell fragments.The mineralogical composition of the studied clay size fraction showed that most samples are dominated by illite, together with smectite, kaolinite and illite/smectite mixed layers. The relative proportion of these constituents shows wide variation. Smectite is more abundant than other constituents at Wadi El-Tayiba. The high content of smectite is usually accompanied by a terrigenous influx in the form of kaolinite and illite, reflecting deposition in an inner neritic shallow marine environment. The sediments of Wadi Nukhul are characterised by an appreciable proportion of illite, together with an illite/smectite mixed layer and minor amounts of kaolinite, suggesting deposition in fluviatile environments.The detailed investigation of charophytes (green algae) in the investigated sections showed that Wadi El-Tayiba is nearly barren of these microflorae, except for some benthic foraminifera from a shallow marine environment. In contrast, Wadi Nukhul yielded a high frequency and great diversity of charophytes, where 15 species have been identified, described and illustrated for the first time. The utilisation of the ranges of these species allowed the subdivision of the section into three charophyte zones, which are correlated with other zones recorded in Europe, as well as the standard mammal levels in the world. These biozones strongly assign the Tayiba Formation to the Late Eocene to Late Oligocene (Late Priabonian to Chattian). A depositional model was suggested for the Tayiba Formation in west-central Sinai.     

Palaeoclimate indicators (clay minerals,calcareous nannofossils,stable isotopes) compared from two successions in the late Jurassic of the Volga Basin (SE Russia)

A study of clay mineral and calcareous nannofossil abundances in late Jurassic–early Cretaceous sediments from the Volga Basin, SE Russia, is presented. From these results, we are able to compare some general patterns of mineralogical and palaeontological change for the Volga Basin to the palaeoclimate models developed for northern Europe and beyond. The two successions examined comprise calcareous mudstones with black organic‐rich shale horizons, overlain by a series of phosphatic silty sands. Clay mineralogical results show a progressive decrease in kaolinite and the concomitant increase of smectite and illite through the middle Volgian, followed by an abrupt increase in kaolinite in the late Volgian. The clay mineral evidence suggests increasing aridity at the end of the Jurassic, similar, in part, to many western European successions. Because of differential settling of clay minerals, superimposed upon this possible climatic signature is likely to be the effect of relative sea‐level change. Calcareous nannofossil analysis from a single section reveals a shift through the middle Volgian from low nutrient, warm water assemblages dominated by Watznaueria to cooler surface water and high nutrient assemblages dominated by Biscutum constans. These observations suggest that increased aridity is also associated with climatic cooling. Black shales are associated with increased productivity, higher sea levels and increases in smectite content. Hence, periods of low (chemical) hinterland weathering during semi‐arid conditions are paradoxically associated with relatively nutrient‐rich waters, and organic‐rich shales. Comparison of published carbon and oxygen stable isotope results from this and other sections to the clay mineral and nannofossil data confirms the palaeoclimatic interpretation. This study significantly improves the published biostratigraphically constrained clay mineral database for this time period, because other European and North American successions are either non‐marine (and thus poorly dated), absent (through penecontemporaneous erosion) or condensed. Copyright © 2002 John Wiley & Sons, Ltd.     

Occurrence of palygorskite and sepiolite in upper Paleocene–middle Eocene marine deep sediments of the Lagonegro Basin (Southern Apennines—Italy): Paleoenvironmental and provenance inferences

A mineralogical and geochemical study of clay lithologies and a biostratigraphic analysis of the carbonates from the deep-sea Lagonegro Basin (Southern Apennines—Italy) have been carried out to deduce in general the provenance of clay sediments and their paleoenvironmental conditions and particular to recognize the signature of the Paleocene–Eocene climatic global warming. The analysed succession comprising a wide stratigraphic interval of the Sannio Unit, spanning between Albian to the upper Oligocene–lower Miocene, is exposed near Accettura and Stigliano villages. Eighteen clay samples were analysed by XRD, XRF, SEM, TG-DTA. Their age was framed by biostratigraphic analyses carried out on carbonate sediments. Mineral assemblage of the clay sediments includes quartz, carbonates (calcite and dolomite), feldspars (plagioclase and k-feldspars), hematite, randomly illite/smectite mixed layers with a low illite percentage, kaolinite, discrete illite–muscovite, chlorite, palygorskite and sepiolite. The low illite percentage in randomly illite/smectite mixed layers indicates low diagenetic conditions for the studied successions. These features are unique for the Cretaceous–Tertiary successions of the Lagonegro domain and are particularly significant for the preservation of the native mineralogical assemblage useful to determine the provenance and paleoenvironmental conditions of the clayey sediments. Palygorskite and sepiolite are concentrated in the upper Paleocene–middle Eocene stratigraphic interval and particularly in the upper part of the early Eocene—lower part of the middle Eocene (biozone of Blow P 9–12). Clay sediments rich in palygorskite and sepiolite show a higher P₂O₅ amount and a lower kaolinite percentage, compatible with warm and arid climatic conditions typical of the global warming event well recorded in the southern tethyan margin. Likely palygorskite and sepiolite formed in lagoonal environment in nearby carbonate platform margins and then they were transported into the Lagonegro Basin as indicated by the well developed habitus of palygorskite. During the Paleogene the Lagonegro Basin and the nearby carbonate platforms represented a key sector the southern paleodomains of the Tethys. The discovery of these minerals gives a contribution to the reconstruction paleoenvironmental conditions of the Tethian paleo-margin during the early–middle Eocene.     

An early–middle Eocene Antarctic summer monsoon: Evidence of ‘fossil climates’

A warmer and mostly ice-free South polar region prevailed during the early–middle Eocene, indicative of a low latitudinal temperature gradient. Climatic models mostly fail to reconstruct such a low gradient, demonstrating our poor understanding of the mechanisms involved in heat transfer. Here we describe a new phenomenon that shaped the southern high latitude climate during the early–middle Eocene: the Antarctic summer monsoon. Our palaeoclimatic reconstruction is based on 25 morphotypes of fossil dicotyledonous leaves from the early–middle Eocene fossil leaf assemblage of Fossil Hill from King George Island, the Antarctic Peninsula. We use a novel CLAMP (Climate Leaf Analysis Multivariate Program) calibration which includes new climatic parameters that allow us to characterise better the seasonality in precipitation. Our reconstruction indicates a warm humid temperate climate with strong seasonality in temperature and precipitation. Seasonality in precipitation indicates a rainfall rate of 6.4 ± 1.30 mm/day during summer (summer daily rate of precipitation; SDR) and a summer precipitation representing more than 60.3 ± 8.28% of annual rainfall (ratio of summer precipitation; RSP), which fulfils the definition of a summer monsoon in the modern world. This implies a seasonal alternation of high- and low-pressure systems over Antarctica during the early–middle Eocene. Such a climate regime would have impacted upon global atmospheric circulation and heat transfer. This climatic regime presents a challenge for climatic models and their ability to reconstruct accurately palaeoclimates at high southern latitudes and thereby understand latitudinal heat transfer in a ‘greenhouse Earth’ regime.     

江汉盆地始新世中、晚期古气候定量重建初探

依据江汉盆地明钾1孔中、晚始新统系统的孢粉资料, 应用化石孢子花粉亲缘植物的生态位估算孢粉组合所反映的古气候特征.气候参数曲线的变化趋势显示江汉盆地始新世中期气候参数与现今北纬2 2°~ 2 6°的数值相接近, 为中-南亚热带湿润半湿润气候.晚期气温较前期低1~ 4℃, 相当于现今北纬2 3°~ 2 8°处的数值, 为北-中亚热带气候.但是, 气候构型与现今截然不同, 具有气温高、年较差小、降水波动大的特点.始新世中期1月均温比今高5~ 9℃, 表明冬季风作用尚不明显, 晚期略显冬季风的影响.气候的降温发生在孔深2 10 0m处, 主要表现为1月均温的下降和年较差增长, 因而孢粉组合中落叶阔叶树种增多.年降水量波动强烈, 一般于30 0~ 170 0mm之间, 这有利于盐类矿物的迁移和堆积.降水量低于10 0 0mm时, 麻黄灌木花粉增多, 且与膏盐堆积深度相一致.高山深盆的干热环境是本区盐类矿物沉积的主要原因, 年降水量波动强烈且偏低, 有利于盐类的持续堆积, 从而有别于现今西北地区的盐类堆积.      

Clay Mineralogy and its Paleoclimatic Significance of the OligoceneMiocene 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 iilite 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 ilUte and chlorite contents, with fluctuant smectite and low （I＋Ch）/（K＋S） ratio, indicative of a dominant seasonal arid climate from the early Oligoeene 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 iilite 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 iUite 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.     

Palaeoweathering of Lower Palaeozoic rocks of the Brabant Massif,Belgium: a mineralogical and petrographical analysis

The Lower Palaeozoic low-grade metamorphic rocks of the Brabant Massif are largely buried below a thick cover of post-Palaeozoic strata. Along the top of the subcrop, they comprise remnants of Cretaceous to Tertiary weathering profiles that represent the lower part of thick saprolites. The alteration of the chlorite- and muscovite-dominated Palaeozoic rocks was characterized by the destruction of chlorite, accompanied by the formation of kaolinite and iron oxides and/or iron hydroxides. The first product of chlorite weathering seems to have been regularly interstratified chlorite-vermiculite or chlorite-smectite, which is now represented by interstratified chlorite-muscovite with regular ordering. Outside the thin transitional zones in which this mineral occurs, the rubefied intervals show only little variation in composition, which is due to the replacement of chlorite by kaolinite over short vertical distances and the stability of muscovite throughout the preserved parts of the saprolite. The rubefied rocks do have a somewhat different composition along the top of some profiles, which is related to an interaction with groundwater after burial, resulting in smectite formation, feldspar weathering and iron dissolution. Groundwater interaction is also responsible for the occurrence of weathering without rubefaction, outside the areas with saprolite remnants, which resulted in vermiculite, smectite and kaolinite formation. Copyright © 1999 John Wiley & Sons, Ltd.     

Sub‐Cambrian pedogenesis recorded in weathering profiles of the Arabian‐Nubian Shield

Altered crystalline rocks occur at the peneplain exposed in southern Israel and in other localities across North Africa and Arabia where they underlie an extensive blanket of Cambro–Ordovician sandstones. This study focuses on the petrography, mineralogy and geochemistry of top basement rocks of the northern Arabian‐Nubian Shield. The altered rocks are shown to be weathering profiles that can be subdivided into three horizons interpreted as apparently unweathered granite, or saprock, which grades upwards to a saprolite, topped by a thin clayey plasmic zone. The plasmic zone is enriched in iron and aluminium and is depleted in silicon, calcium, magnesium and potassium relative to the underlying saprolite. The chemical index of alteration increases upward, but does not exceed 90 and, therefore, lags behind values observed in strongly leached present‐day tropical soils. Petrographic examinations reveal iron mobility under local fluctuating redox conditions, similar to modern and Proterozoic soils. A variety of birefringence fabrics induced by shrinkage and expansion of clays during wetting and drying cycles and clay illuviation strongly indicate pedogenic processes rather than a post‐depositional alteration. Illite and ordered illite‐smectite phases coexist with smectitic illite‐smectite in the lower part of the saprolite and with kaolinite in the plasmic zone, in line with increasing chemical index of alteration. Observations are in accordance with the current profile being a remnant of a thick weathering profile whose top was truncated by fluvial incision just prior to deposition of the overlying Early Cambrian sequence. A previously documented Devonian thermal event reaching temperatures of at least 200°C overprinted the studied rocks. During burial diagenesis, illitization affected original smectite rather than kaolinite. However, in spite of the elevated temperatures, illitization was incomplete implying restricted potassium addition. The sub‐Cambrian weathering reflects warm and humid conditions in a tropical or sub‐tropical climate, in line with several plate reconstructions placing Israel at low latitudes during Cambrian time.     

Diagenesis and Very Low‐grade Metamorphism of the Upper Permian Yangjiagou Formation in Eastern Changchun,China: Evidence from Clay Mineral Geothermobarometers

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.     

Alteration geochemistry of the Nopal I uranium deposit (Sierra Peña Blanca,Mexico), a natural analogue for a radioactive waste repository in volcanic tuffs

Natural analogues provide an approach to characterize and test the long‐term modelling of a repository performance. This article presents geochemical information about the alteration conditions of the Nopal I uranium deposit, Mexico, an analogue for the proposed Yucca Mountain radioactive waste repository. Mineralization and hydrothermal alteration of volcanic tuffs are contemporaneous, according to petrographic observations. Trace element geochemistry (U, Th, REE) provides evidence for local mobilization of uranium under oxidizing conditions and further precipitation under reducing conditions. O‐ and H‐isotope geochemistry of kaolinite, smectite, opal and calcite suggests that argillic alteration proceeded at shallow depth with meteoric water at 25–75 °C, a low‐temperature context, unusual for volcanic‐hosted uranium deposits. This temperature range is compatible with some post‐closure evolution models of the proposed Yucca Mountain repository.