The effects of diagenesis on the reservoir characters in sandstones of the Late Cretaceous Pab Formation,Kirthar Fold Belt,southern Pakistan

The Maastrichtian Pab Formation in the southern part of Pakistan is composed of fine- to very coarse-grained texturally mature quartz arenite and subordinate sublitharenite varieties. The sandstones have undergone intense and complex diagenetic episodes due to burial and uplift. Diagenetic modifications were dependent mainly on the clastic composition of sandstone, burial depth and thrust tectonics. Diagenetic events identified include compaction, precipitation of calcite, quartz, clay minerals and iron oxide/hydroxide, dissolution and alteration of unstable clastic grains as feldspar and volcaniclithic fragments as well as tectonically induced grain fracturing. The unstable clastic grains like feldspar and lithic volcanic fragments suffered considerable alteration to kaolinite and chlorite. Dissolution and alteration of feldspar and volcanic lithic fragments and pressure solution were the main sources of quartz cements. Mechanical compaction and authigenic cements like calcite, quartz and iron oxide/hydroxide reduced the primary porosity, whereas dissolution of clastic grains and cements has produced secondary porosity. Chlorite coatings on clastic grains have prevented quartz cementation. Coarse-grained, thick bedded packages of fluviodeltaic, shelf delta lobe and submarine channels facies have higher average porosity than fine-grained, thin bedded and bioturbated sandstone of deeper shelf and abyssal plain environments and these facies are concluded to be possible future hydrocarbon prospects.     

鄂尔多斯盆地东缘C-P煤系致密砂岩储层敏感性分析

鄂尔多斯盆地临兴地区上古生界致密砂岩储集层中矿物组成多样、孔隙结构复杂且黏土矿物含量高,直接影响储层改造和开发效果。文章基于X 衍射、铸体薄片、气测孔渗、压汞和敏感性实验,系统研究了储层敏感性及其影响因素。结果表明研究区砂岩中石英和岩屑含量高,长石含量较低,以岩屑砂岩、岩屑石英砂岩、长石岩屑砂岩和石英砂岩为主。黏土矿物主要为伊利石、高岭石、绿泥石以及伊/蒙混层;储层普遍低孔低渗,孔隙结构较差。速敏以太原组最强,山西组最弱,与伊利石+绿泥石含量正相关,高岭石含量负相关。水敏下石盒子组最强,太原组最弱,与伊/蒙混层含量正相关。盐敏与水敏有类似特点,与伊/蒙混层含量表现出正相关。酸敏山西组最强,下石盒子组最弱,与绿泥石和铁白云石矿物含量正相关。碱敏性山西组最强,太原组最弱,受长石、石英和高岭石含量影响。相关认识有助于指导研究区钻井、压裂等施工工艺选择和排采控制。     

Diagenetic processes in cretaceous sandstones from occidental Brazilian Equatorial Margin

Despite a great interest in Brazilian Equatorial Margin exploration, very little was published on the diagenesis of sandstones from that area. A wide recognition petrographic study was performed to identify the major diagenetic processes that impacted the porosity of Lower Cretaceous sandstones of the Pará-Maranhão, São Luís, Bragança-Viseu and Barreirinhas basins. Arkoses from the Pará-Maranhão Basin show neoformed or infiltrated clay coatings, mica replacement and expansion by kaolinite and vermiculite, and precipitation of grain-replacive and pore-filling quartz, kaolinite, albite, chlorite, calcite, dolomite, siderite, pyrite and titanium oxides. Compaction, quartz and calcite cementation were the main porosity-reducing processes. Barreirinhas Basin lithic arkoses and subarkoses display clay coatings, compaction of metamorphic fragments into pseudomatrix, and precipitation of grain-replacive and pore-filling kaolinite, quartz, albite, chlorite, calcite, dolomite, TiO₂ and pyrite. The main porosity-reducing processes were calcite cementation in the subarkoses, and compaction and quartz cementation in lithic arkoses. Quartzarenites from this basin were early- and pervasively cemented by dolomite. Arkoses and lithic arkoses of the São Luís and Bragança-Viseu basins show clay coatings, pseudomatrix from mud intraclasts compaction, and precipitation of pore-filling and grain-replacive kaolinite, vermiculite, smectite, quartz, albite, chlorite, illite, calcite, dolomite, hematite, TiO₂ and pyrite. Compaction of mud intraclasts and dissolution of feldspars and heavy minerals were the main porosity-modification processes. These preliminary results may contribute to the understanding of the spatial and temporal distribution of the diagenetic processes and their impacts on the porosity of the sandstones from these basins.     

Depositional environment, sand provenance, and diagenesis of the Basal Salina Formation (lower Eocene), northwestern Peru

The Basal Salina Formation is a lower Eocene transgressive sequence consisting of interbedded shales, siltstones, and conglomeratic sandstones. This formation occurs in the Talara basin of northwestern Peru and is one of a series of complexly faulted hydrocarbon-producing formations within this extensional forearc basin. These sediments were probably deposited in a fan-delta complex that developed along the ancestral Amotape Mountains during the early Eocene. Most of the sediment was derived from the low-grade metamorphic and plutonic rocks that comprise the Amotape Mountains, and their sedimentary cover. Detrital modes of these sandstones reflect the complex tectonic history of the area, rather than the overall forearc setting. Unlike most forearc sediments, these are highly quartzose, with only minor percentages of volcanic detritus. This sand is variably indurated and cemented by chlorite, quartz, calcite, and kaolinite. Clay-mineral matrix assemblages show gradational changes with depth, from primarily detrital kaolinite to diagenetic chlorite and mixed-layered illite/smectite. Basal Salina sandstones exhibit a paragenetic sequence that may be tied to early meteoric influx or late-stage influx of thermally driven brines associated with hydrocarbon migration. Much of the porosity is secondary, resulting from a first-stage dissolution of silicic constituents (volcanic lithic fragments, feldspar, and fibrous quartz) and a later dissolution of surrounding carbonate cement. Types of pores include skeletal grains, grain molds, elongate pores, and fracture porosity. Measured porosity values range up to 24% and coarser samples tend to be more porous. Permeability is enhanced by fractures and deterred by clay-mineral cements and alteration residues.     

Characteristics and distribution of clay minerals and their effects on reservoir quality: Huagang Formation in the Xihu Sag,East China Sea Basin

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.     

Mineralogy of Holocene Sediments from the Southwestern Black Sea Shelf (Turkey) in Relation to Provenance, Sea-level and Current Regimes

Grain size and mineral composition of core sediments were used to investigate influences of various terrestrial and marine conditions, which have prevailed on the southwestern Black Sea shelf during the Holocene. Siliciclastic mud with small amounts of sand and gravel from nearby coastal hinterland is the principal sediment type, whereas sediments deposited near the shelf edge and the Istanbul Strait and off the Duru Lake （a paleo-river mouth） constitued large quantities of sand and gravel of both biogenic and terrigenic origin. Variable amounts of aragonite, 1 nmmicas, quartz, feldspars, calcite and dolomite constitute the dominant non-clay minerals in bulk sediments. The clay mineral assemblage in the 〈 2 μm fraction is made up of smectite, illite, kaolinite and chlorite. Aragonite and calcite are mainly derived from benthic accumulations, whereas feldspars （mainly plagioclase） and smectite reflect magmaticvolcanic provenance and the distribution of 1 nm-micas and chlorite correlate with nearby metamorphic sources onland. Nevertheless, grain size and mineral distribution generally indicate a combination of effects of wind and wave climate, longshore and offshore cyclonic currents, changing sea-level stands and nearby source rock and morphological conditions. It is also suggested that at least part of clay minerals could be derived from the northwesterly Danube River input.     

Diagenetic evolution of the volcaniclastic deposits: an example from Neoproterozoic Dokhan Volcanics in Wadi Queih basin,central Eastern Desert,Egypt

Wadi Queih basin hosts a ～2,500-m thick Neoproterozoic volcanoclastic successions that unconformably lie over the oldest Precambrian basement. These successions were deposited in alluvial fan, fluviatile, lacustrine, and aeolian depositional environments. Diagenetic minerals from these volcaniclastic successions were studied by X-ray diffractometry, scanning electron microscopy, and analytical electron microscopy. The diagenetic processes recognized include mechanical compaction, cementation, and dissolution. Based on the framework grain–cement relationships, precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore-lining and pore-filling clay cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cement and feldspar grains. In addition to calcite, several different clay minerals including kaolinite, illite, and chlorite with minor smectite occur as pore-filling and pore-lining cements. Chlorite coating grains helps to retain primary porosity by retarding the envelopment of quartz overgrowths. Clay minerals and their diagenetic assemblages has been distinguished between primary volcaniclastics directly produced by pyroclastic eruptions and epiclastic volcaniclastics derived from erosion of the pre-existing volcanic rocks. Phyllosilicates of the epiclastic rocks display wider compositional variations owing to wide variations in the mineralogical and chemical compositions of the parent material. Most of the phyllosilicates (kaolinite, illite, chlorite, mica, and smectite) are inherited minerals derived from the erosion of the volcanic basement complex, which had undergone hydrothermal alteration. Smectites of the epiclastic rocks are beidellite–montmorillonite derived from the altered volcanic materials of the sedimentary environment. Conversely, phyllosilicate minerals of the pyroclastic rocks are dominated by kaolinite, illite, and mica, which were formed by pedogenetic processes through the hydrothermal influence.     

Clay Minerals in Sediments of the Arctic Seas

The distribution of clay minerals in recent sediments on the Arctic shelf off the Eurasian and North American continents is considered. Prominence is given to the East Siberian and Laptev seas. The illite belt established on the basis of the composition of clay minerals in seven Arctic seas stretches from the Beaufort Sea to the White Sea and reveals a mineralogical zonality. The belt can be devided into smectite and chlorite provinces. Factors governing the formation of the Arctic illite belt and features of the distribution of individual clay minerals are discussed. The identification of the illite belt in sediments on the Arctic shelf complements the system of planetary latitudinal zones of clay minerals formulated by previous researchers.     

The Upper Miocene Injana (Upper Fars) Formation of Iraq: insights on provenance history

Petrographic, geochemical, and scanning electron microscope analyses of the sandstone and mudstone units of the Upper Miocene Injana Formation are presented. Furthermore, microprobe analysis for amphiboles, pyroxenes, garnet, and chromian spinels as common heavy mineral species present is done to support other results for better understanding of the provenance history of the Injana Formation. The sandstones of the Injana Formation consist of terrigenous carbonate lithic fragments as common type of sedimentary rock fragments in addition to chert, argillaceous, and rare sandstone fragments. They also include metamorphic and igneous lithic fragments, quartz, feldspars, and mica and generally, the sandstones are lithic arenites and immature. Scanning electron microscopic analysis for the heavy minerals shows that they have been affected by dissolution due to chemical etching and mechanical abrasion through several surface texture generated either in arid and semihumid environment or in diagenetic environment. Clay mineralogy of the mudstone units indicates the presence of illite, chlorite, kaolinite, palygorskite, and illite–smectite mixed layers. Bulk-rock and mineral phase geochemistry in addition to petrographic data suggest the derivation of the Injana Formation from a nearby sources with contribution from igneous, metamorphic, and sedimentary provenance mainly from the high lands in the northeastern parts of Iraq which comprise mainly the Zagros mountains and the older sedimentary formations.     

Alterations of cretaceous siltstones and sandstones near basalt contacts (Nûgssuaq,Greenland)

The influence of the intrusion of basaltic dykes and sills was investigated on sandstones and siltstones of the Atane Formation (Turonian-Coniacian) from the Sarqaq area (Nûgssuaq peninsula, central West Greenland).In the unaltered rock sequence, the siltstones are dominated by kaolinite, quartz and feldspar. No cementation was observed. The sandstones which are prevalent in this formation are arkoses and lithic arkoses with quartz/feldspar ratios of about 1, with variable contents of rock fragments and with minor amounts of matrix. The matrix consists mainly of kaolinite, less frequently of illite-muscovite and smectite or interstratified illite-smectite. Cement minerals include calcite, Fe-calcite, both mainly in concretions, and subordinate Fe-oxide hydrates.The porosity of the sandstones and siltstones indicates a former maximum depth of burial of 1000 m in the Sarqaq region.Adjacent to thin dykes and sills (1–2 m) the above-described rocks are altered as follows. In sandstones albite (fibrous), quartzine, smectite, goethite were all formed at the expense of kaolinitic matrix, and aragonite cement. In siltstones, interstratified illite-smectite, illite, smectite and low-cristobalite replaced kaolinite. These minerals, especially smectite, exclude temperatures above 200°C; they were probably formed during a cooling period.Adjacent to thicker basalt intrusions—only sills, over 5 m thick, and no dykes are known from this area—the following alterations are observed. In sandstones two textural types of albite were formed in addition to: (1) fibrous albite mentioned above; (2) undulose lath-shaped albite; (3) coarse-twinned non-undulose replacement albite. Moreover, brownish luminescing undulose quartz, muscovite, chlorite, epidote, nontronite and anatase occur. In siltstones the mineral association muscovite-chlorite-pyrophyllite-albite occurs, suggesting temperatures in the range of 250–500°C. Additionally a 22–26 Å mixed-layer mineral (chlorite-muscovite?) formed. At the contacts of thick as well as thin basalt intrusions the detrital grains directly adjoining the basalt are partly fractured; potassium feldspar grains show also a chemical disintegration.The temperatures calculated by application of a cooling model are higher than those indicated by the mineral alterations observed. Convective heat transfer by pore water is suggested as an explanation.     

兰州市取暖期可吸入颗粒物中单颗粒矿物组成特征

为研究兰州市2005年冬季大气可吸入颗粒物(PM10)中单颗粒的矿物组成,用能谱扫描电镜识别和统计了兰州市区(东方红广场)和郊区(榆中县)两个采样点的单矿物颗粒。结果在市区样品中识别出方解石、伊/蒙混层、石英、斜长石、伊利石、石膏、绿泥石、高岭石、浊沸石和钾长石等21种矿物,前7种占统计总量的75%以上;郊区样品中识别出20种矿物,以方解石、石英、伊利石、绿泥石、斜长石和伊/蒙混层为主(占70%以上),与市区相比缺少钾石膏、金红石和水铝酸钙而增加了硫酸镁和磷灰石。总体来看,大气PM10中的矿物颗粒可分为粘土类、长石类、碳酸盐类、硫酸盐类、氧化物类和其他六类,以粘土类和碳酸盐类矿物为主(约占60%);冬季市区颗粒物表面的“硫化”现象较郊区严重;这些矿物颗粒主要来自地表土,人为排放和大气中二次化学反应生成的矿物的贡献较小。     

Characterization and origin of spongillite-hosting sediment from João Pinheiro,Minas Gerais,Brazil

Spongillite from João Pinheiro, Minas Gerais, Brazil is mainly known for its use in brick production and in the refractory industry. Very few studies have focused on its geological context. Spongillite-rich deposits occur in shallow ponds on a karstic planation surface developed on rocks of the Neoproterozoic São Francisco Supergroup. Cenozoic siliciclastic sediments are related to this surface. A field study of these deposits and analysis of multispectral images showed a SE–NW preferential drainage system at SE, suggesting that Mesozoic Areado Group sandstones were the source area of the spongillite-hosting sediments. Mineralogical and textural characterization by optical microscopic analysis, X-ray diffraction (XRD), differential and gravimetric thermal analysis (DTA-GTA), infrared spectroscopy (IR) and scanning electron microscopy (SEM) of seven open-pit spongillite-rich deposits (Avião, Carvoeiro, Vânio, Preguiça, Divisa, Severino, Feijão) showed a sedimentological similarity between the deposits. They are lens-shaped and are characterized at the bottom by sand facies, in the middle by spicules-rich muddy-sand facies and at the top by organic matter-rich muddy-sand facies.Petrographically, the spongillite-hosting sediments and the siliclastic sediments of the Areado Group show detrital phases with similar mineralogical and textural features, such as the presence of well-sorted quartz grains and surface features of abrasion typical of aeolian reworking that occurred in the depositional environment in which the sandstones of the Areado Group were formed. Detrital heavy minerals, such as staurolite, zircon, tourmaline, and clay minerals, such as kaolinite, low amounts of illite, scarce chlorite and mixed-layer chlorite/smectite and illite/smectite occur in the spongillite-hosting sediments and in sandstones from the Areado Group. In both formations, staurolite has similar chemical composition. These mineralogical and textural features show that the sediments of the Areado Group constitute the main source of the pond sediments that host spongillite.     

Provenance of bar-top sediments at Tons river near Allahabad

The bar-top sediments at the Tons river deposited mainly from the suspension current during waning stage condition of river are collected to study their provenance on the basis of clay mineralogy, heavy minerals and magnetic properties. The clay mineral assemblages in samples predominantly consist of illite, with minor amounts of kaolinite, smectite and chlorite. The clay minerals are contributed due to (i) weathering and decomposition of shales, argillaceous limestones and pyroclastic deposits of upper Vindhyan Groups and (ii) weathering and erosion of Banda plain of Gangetic alluvium. The low ZTR index for the studied samples indicates poor sediment maturity, rapid erosion in the source region and short transportation of detritus. The transparent heavy mineral assemblages in the sediment samples predominantly consist of garnet, with minor amounts of tourmaline, zircon, hornblende, enstatite, hypersthene, rutile, tremolite, kyanite, sillimanite, andalusite, chlorite, epidote, wollastonite, and staurolite. The heavy minerals are dominantly angular to sub-angular with some rounded to sub-rounded grains. The rounded grains indicate multicyclicity and derivation fromVindhyan sandstones. The angular grains are either contributed due to erosion of primary rocks of Bundelkhand gneissic complex and or various Gangetic alluviums. The magnetic properties from sediment samples indicate that the antiferromagnetic minerals (illite, chlorite and smectite) are more concentrated in clay sized particles and it also indicated mixed source rocks for the bar-top sediment of Tons river.     

上新世以来伊豆-小笠原海脊黏土矿物的来源与古气候意义

对伊豆-小笠原海脊(ODP 782A孔)上新世以来沉积物中黏土矿物的组成、含量及矿物学特征进行了分析,结果表明:黏土矿物以伊利石(42%)和蒙皂石(42%)为主,绿泥石的平均含量为14%,高岭石的含量最低,平均仅为2%。伊利石的结晶度较好,平均为0.25°Δ2θ;化学指数较低,平均为0.31;表明伊利石主要形成于干冷的气候环境。通过将ODP 782A孔黏土矿物组合特征和含量与周边可能源区对比,并结合伊利石和蒙皂石的矿物学参数特征,我们认为蒙皂石主要来源于伊豆-小笠原海脊周边岛弧火山物质;伊利石、绿泥石和高岭石主要来自亚洲大陆风尘。上新世以来(伊利石+绿泥石)/蒙皂石比值总体上呈增加的趋势,并且在5.3~3.6、3.6~1.6、1.6~0 Ma的三个阶段表现出不同的变化特征,该比值与全球深海δ¹⁸O值所记录的全球变冷、北太平洋ODP 885/886孔风尘通量和灵台黄土沉积速率,以及日本海U1430站伊利石/蒙皂石比值所指示的亚洲内陆干旱变化的总体变化趋势和阶段性变化的时间点大致同步,表明该比值敏感地响应了全球变冷和亚洲内陆的干旱。上新世以来(伊利石+绿泥石)/高岭石比值表现为高/低交替变化,分别与中国灵台黄土磁化率高/低变化相对应,由于黄土磁化率记录了亚洲内陆干/湿变化,因此该黏土矿物比值敏感地响应了亚洲内陆的古气候变化,因而可以作为可靠的亚洲大陆干/湿变化示踪指标。     

龙虎泡地区高台子油层成岩作用及其对储集岩孔隙演化的影响

龙虚泡地区高台子油层储集岩基本上由岩屑长石粉砂岩组成。压实作用、胶结作用、溶蚀作用、粘土矿物的转化作用和交代作用对沉积岩 (物 )进行了改造。根据岩石的结构可以看出压实作用发育于早成岩阶段,方解石胶结物的结构和碳、氧稳定同位素组成说明方解石胶结作用发育于早成岩阶段。压实作用和方解石结作用对孔隙的减少起着决定性影响。长石和中基性火山岩岩屑的溶蚀作用使得孔隙有一定程度的增加。压实作用和胶结作用也阻止了自生石英的生长和次生孔隙带的形成。     

Diagenesis and reservoir quality analysis in the Lower Cretaceous Qishn sandstones from Masila oilfields in the Sayun–Masila Basin,eastern Yemen

Lower Cretaceous sandstones of the Qishn Formation have been studied by integrating sedimentological, petrological and petrophysical analyses from wells in the Masila oilfields of eastern Yemen. These analyses were used to define the origin, type of diagenesis and its relation to reservoir quality. The sandstones of the Qishn Formation are predominately quartz arenite to subarkose arenite with sublitharenite and quartz wackes displaying a range of porosities, averaging 22.33%. Permeability is likewise variable with an average of 2844.2 mD. Cementation coupled with compaction had an important effect on porosity destruction after sedimentation and burial. The widespread occurrence of early calcite cement suggests that the sandstones of the Qishn Formation lost significant primary porosity at an early stage of its diagenetic history. In addition to poikilotopic calcite, several different cements including kaolinite, illite, chlorite and minor illite–smectite occur as pore‐filling and pore‐lining cements, which were either accompanied by or followed the development of the early calcite cement. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar grains. The new data presented in this paper suggest the reservoir quality of Qishn sandstones is strongly linked to their diagenetic history; hence, the reservoir quality is reduced by clay minerals, calcite and silica cements but is enhanced by the dissolution of the unstable grains, in addition to partial or complete dissolution of calcite cements and unstable grains. Copyright © 2015 John Wiley & Sons, Ltd.     

南海北部陆架海域表层沉积物黏土矿物分布特征及物源分析

应用X射线衍射(XRD)对南海北部陆架海域225个站位表层沉积物黏土组分进行分析,结果表明,研究区黏土矿物总体以伊利石和绿泥石为主,高岭石和蒙脱石质量分数少,绿泥石、高岭石与蒙脱石质量分数呈明显的负相关关系。根据南海北部陆架海域表层沉积物中黏土矿物空间分布特征,结合邻近河流的黏土矿物组分以及洋流搬运作用,雷州半岛东部海域伊利石主要来源于广东沿海河流和珠江,绿泥石来自台湾岛,蒙脱石主要由吕宋河流提供,高岭石则由广东沿海河流和海南岛入海河流提供;雷州半岛西部海域伊利石来源于珠江,绿泥石和高岭石由红河提供,蒙脱石可能受广西入海河流携带的沉积物影响。     

Metamorphic and Thermal History of a Fore-Arc Basin: the Fossil Bluff Group, Alexander Island, Antarctica

The Himalia Ridge Formation (Fossil Bluff Group), AlexanderIsland is a 2·2-km-thick sequence of Upper Jurassic–LowerCretaceous conglomerates, sandstones and mudstones, derivedfrom an andesitic volcanic arc and deposited in a fore-arc basin.The metamorphic and thermal history of the formation has beendetermined using authigenic mineral assemblages and vitrinitereflectance measurements. Metamorphic effects include compaction,pore-space reduction, cementation and dissolution and replacementof detrital grains by clay minerals (smectite, illite/smectite,corrensite and kaolinite), calcite, chlorite, laumontite, prehnite,pumpellyite, albite and mica, with less common quartz, haematite,pyrite and epidote. The authigenic mineral assemblages exhibita depth-dependence, and laumontite and calcite exhibit a strongantipathetic relationship. Detrital organic matter in the argillaceouslayers has vitrinite reflectance values (R_o) ranging from 2·3to 3·7%. This indicates considerable thermal maturation,with a systematic increase in reflectivity with increasing depth.There is good correlation of metamorphic mineral assemblageswith chlorite crystallinity and vitrinite reflectance values—allindicating temperatures in the range of 140 ± 20°Cat the top of the sequence to 250 ± 10°C at the baseof the sequence. The temperatures suggest a geothermal gradientof 36–64°C/km and a most likely gradient of 50°C/km.It is suggested that this higher-than-average gradient for afore-arc basin resulted either from rifting during basin formationor from a late-stage arc migration event. KEY WORDS: Antarctica; diagenesis; fore-arc basin; low-temperature metamorphism; vitrinite reflectance     

Clay Minerals in an Active Hydrothermal Field at Iheya‐North‐Knoll,Okinawa Trough

Iheya‐North‐Knoll is one of the small knolls covered with thick sediments in the Okinawa Trough back‐arc basin. At the east slope of Iheya‐North‐Knoll, nine hydrothermal vents with sulfide mounds are present. The Integrated Ocean Drilling Program (IODP) Expedition 331 studied Iheya‐North‐Knoll in September 2010. The expedition provided us with the opportunity to study clay minerals in deep sediments in Iheya‐North‐Knoll. To reveal characteristics of clay minerals in the deep sediments, samples from the drilling cores at three sites close to the most active hydrothermal vent were analyzed by X‐ray diffraction, scanning electron microscope and transmission electron microscope. The sediments are classified into Layer 0 (shallow), Layer 1 (deep), Layer 2 (deeper) and Layer 3 (deepest) on the basis of the assemblage of clay minerals. Layer 0 contains no clay minerals. Layer 1 contains smectite, kaolinite and illite/smectite mixed‐layer mineral. Layer 2 contains chlorite, corrensite and chlorite/smectite mixed‐layer mineral. Layer 3 is grouped into three sub‐layers, 3A, 3B and 3C; Sub‐layer 3A contains chlorite and illite/smectite mixed‐layer mineral, sub‐layer 3B contains chlorite/smectite and illite/smectite mixed‐layer minerals, and sub‐layer 3C contains chlorite and illite. Large amounts of di‐octahedral clay minerals such as smectite, kaolinite, illite and illite/smectite mixed‐layer mineral are found in Iheya‐North‐Knoll, which is rarely observed in hydrothermal fields in mid‐ocean ridges. Tri‐octahedral clay minerals such as chlorite, corrensite and chlorite/smectite mixed‐layer mineral in Iheya‐North‐Knoll have low Fe/(Fe + Mg) ratios compared with those in mid‐ocean ridges. In conclusion, the characteristics of clay minerals in Iheya‐North‐Knoll differ from those in mid‐ocean ridges; di‐octahedral clay minerals and Fe‐poor tri‐octahedral clay minerals occur in Iheya‐North‐Knoll but not in mid‐ocean ridges.     

Provenance of Clay Minerals in the Amami Sankaku Basin and Their Paleoclimate Implications Since Late Pleistocene

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.