Mineralogical and geochemical implications of weathering rates in coastal dunes and beach sands close to a volcanic rock source in the western Gulf of Mexico,Mexico

Coastal dune (CD) and beach sand samples were mineralogically and chemically studied to assess the weathering rates in a coastal area surrounded by the Trans-Mexican Volcanic Belt (TMVB). The study area is a narrow coastal plain with sub-humid warm weather and vigorous coastal dynamics located in the Western Gulf of Mexico (WGM). Our results show that the grain size parameters remain homogeneous along nine CD and beach sites, probably due to the hydrodynamic mechanisms prevailing in the area, i.e. wind deflation, longshore currents, and waves. The minerals found in the CD and beach sands are enriched in recycled, highly corroded monocrystalline quartz (Qm), with loss of plagioclase (P) and olivine (ol), with ilmenite fractions (op) especially in the northern sites. The geochemical data show that the sands are controlled by the exhumation of the TMVB rocks composed of a high volcanic lithic content with lathwork, microlithic, and negligible vitric textures. These volcanic fractions are in agreement with the presence of lavas of calc alkaline, andesite, Na-alkaline lavas, and rhyolite tuffs derived from the TMVB. Significant Spearman Rank Correlations (SRCs) resulted from high silica, titanium, iron, magnesium, calcium, vanadium, chromium, cobalt, associated with the presence of Qm, ilmenite, clinopyroxene, shell fragments, and volcanic lithics. Monocrystalline quartz enrichments, compared to a quartz dilution effect (<Qm; < Qm) at the northern and southern beach sites, reflect the vigorous coastal dynamics. The presence of ilmenite lag deposits in the northern sites indicate that high energy hydrodynamic conditions prevail at the site. The rare earth element (REE) patterns support the mineralogical and compositional framework of the CD and beach sands as part of the TMVB exhumation. We used various chemical indices such as the Chemical Index of Alteration (CIA), the Chemical Index of Weathering (CIW), the Plagioclase Index of Alteration (PIA), and the Weathering Index of Plagioclase (WIP). The WIP values reflect the depletion of mobile elements such as Ca, Na, K, and enrichment of Qm at the northern beach sites, e.g. the Istirinchiá site. The CIA/WIP ratio of 2.13 indicates intermediate weathering under warm climatic conditions and recycling of CD and beach sands.     

闽南-台湾浅滩陆架沉积砂中石英颗粒表面的微结构

石英颗粒表面微结构的研究是随着电子显微镜应用到地质学上而发展起来的。近年来利用扫描电镜观察石英颗粒表面形态特征及差异来研究沉积物物质来源、搬运方式和沉积环境已在我国开展起来。本文是在对闽南-台湾浅滩陆架的地形、沉积物粒度研究基础上,用JSM-50A扫描电镜观察、研究石英颗粒表面微结构时,发现调查海区的(中砂)沉积物中的石英颗粒具有各种微结构,反映特有的沉积环境。     

四川盆地白垩纪沙漠石英沙颗粒表面特征

石英具有较大的硬度和较高的化学稳定性，因而其颗粒表面特征能很好地反映沉积环境。而通过扫描电镜研究石英颗粒表面微细特征是分析沉积环境行之有效的方法。虽然多数人认为四川盆地白垩纪地层存在沙漠沉积(打儿凼组和夹关组)，但仍有人对沙漠沉积的存在持怀疑态度，并认为是河流成三角洲沉积。过去关于其沉积环境的判别主要是根据沉积结构和构造，并未对其石英沙颗粒表面特征进行过系统分析。笔者对采自四川盆地白垩系不同层位地层的样品进行了石英沙颗粒表面特征系统分析。结果表明，石英沙颗粒表面特征分析可以成功地将石英沙区分为风成和水成沉积。因此，本文从石英沙颗粒表面特征方面进一步肯定了四川盆地白垩纪古沙漠的存在。     

珠峰地区晚石炭世冰海相杂砾岩石英颗粒表面结构

石英颗粒表面结构类型以及它们和形成环境间的对应关系,国外已有初步总结^[1,2,3]。对石英颗粒表面结构应用于历史环境分析,有着不同的意见^[4-6]。多数研究者认为,对于松散的或未固结的沉积物(主要为第四纪的)中的石英颗粒表面结构,具有显著的环境分析意义,而对于地质年代久远、早已半固结或固结成岩的则看法不一,有的则予以否定。因为,地质年代愈老,成岩作用(包括浅变质作用)影响也就愈大。但也有些情况表明^[7,8],虽已固结成岩,其物源区的搬运、沉积过程中产生的机械结构,仍然部分或大部分被保存下来,仍可以指示其环境。     

Microtextures of detrital sand grains from the Tecolutla,Nautla, and Veracruz beaches,western Gulf of Mexico,Mexico: implications for depositional environment and paleoclimate

Detrital sand grains from three beaches (Tecolutla, Nautla, and Veracruz) along the western Gulf of Mexico were studied by a scanning electron microscope, to investigate the depositional environment and paleoclimate. Totally, 24 microtextures are identified; among them, 13 are grouped as mechanical origin, 5 as mechanical and/or chemical origin, and 6 as chemical origin. These microtextures are nonuniformly distributed among the three beach areas. Concoidal fractures, straight and arcuate steps at Tecolutla and Veracruz beaches indicate that the sand grains were derived from the crystalline rocks. The abundance of angular outline grains at the Nautla beach supports for short transportation probably close to the source area. The domination of rounded sand grains in the Veracruz beach reveals that the sediments were derived by the aeolian mechanism. Chattermark trials at the Veracruz beach sands are indicating a wet tropical climate. Chemical features like silica globules, silica pellicle, and trapped diatoms in the Tecolutla and Veracruz beach sands suggest a silica saturated environment. Similarly, chemical etching and solution pits are common in the Veracruz beach sands, which are probably linked to the contaminated sea water. Desiccation crack at Veracruz beach sands is an indicator of temperature changes in the beach environment. Broken benthonic foraminifera Elphidium discoidale sp. present in the Veracruz beach indicates a high-energy littoral environment.     

西宁黄土石英颗粒表面结构与黄土物质来源探讨

利用电子扫描电镜分析了西宁黄土地层中的石英颗粒表面形态和机械结构特征,发现近1.2Ma以来西宁黄土石英颗粒形态以尖棱角-棱角为主,颗粒表面具有典型的剪切断口或大贝壳状断口或平行擦痕,且比例较高;有些颗粒表面还具有V形坑、水下磨光面,这类颗粒以次圆状为主,这些信息指示出,西宁黄土物质不是主要来源于沙漠环境,而是来源于青藏高原第四纪冰川和冰缘作用区.     

SOME METHODS TO STUDY SURFACE TEXTURES OF SAND GRAINS

For the study of the surface textures of quartz sand grains the normal microscope was used, besides the sofar generally applied electron microscope, and a new method was developed for using incident illumination. Grains from a desert sand and a river sand were studied, the former grains displayed textures not described yet by previous authors, the latter grains possessed a variety of textures, some of which have been found with beach sand grains, others with glacial sand grains by previous investigators.     

Pleistocene evolution of the Nile Valley in northern Upper Egypt

In the El Minia district of northern Upper Egypt, the Pleistocene deposits of the Nile Valley include (?)Early Pleistocene conglomerates of the Armant Formation and (?)Middle Pleistocene sands of the Qena Formation. These sediments are exposed along both sides of the valley in terraces at different heights, unconformably overlying Eocene limestones. Field observations suggest that the conglomerate facies of the Armant Formation was deposited in proximal and mid-alluvial fans. The sands of the Qena Formation are differentiated into two facies: a cross-bedded sandstone, representative of alluvial fan—braided stream environments, and a facies of sand interbedded with mud that may have been deposited by a meandering river. The heavy minerals of the Armant Formation are markedly different from those of the Qena Formation, suggesting derivation from different sources. Surface textures of quartz grains from the Qena sands, observed by SEM, exhibit chemical features, attributed to a fluviatile origin. Sand from the Armant Formation is characterized by mechanical and chemical surface textural features that suggest original aeolian derivation followed by later fluvial sedimentation.     

Provenance of coastal dune sands along Red Sea,Egypt

Texture, mineralogy, and major and trace element geochemistry of 26 coastal dune sand samples were studied to determine the provenance and tectonic environment of two dune fields close to the beaches of Safaga (SF) and Quseir (QS) at the Egyptian Red Sea coast. Onshore winds generate fine, moderate, moderately-well to well-sorted, coarse-skewed to near-symmetrical dune sands with mesokurtic distributions. Winds pick up and transport grains from nearby beach sands and alluvial deposits into a wide Red Sea coastal plain at the border of the beach. The mineralogical (Qt–Ft–Lt) and geochemical composition of the sands, indicate that SF and QS coastal dune sands are mature and influenced by quartz-rich sands. The average CIA values in SF and QS coastal dune sands are low relative to the range of the PAAS, suggesting an arid climate and a low intensity of chemical weathering. The SF and QS coastal dune sand samples are plotted in the recycled orogen and partly in craton interior fields suggesting recycled older sedimentary and partly metamorphic-plutonic sources. The high content of quartz with shell debris and carbonates in coastal dune sands support the recycled sedimentary beach and alluvial sand sources. The dominance of heavy minerals like amphiboles (hornblende) and biotite in the coastal dune sands also supports the effect of metamorphic-plutonic source rocks. The new tectonic discriminant-function diagrams suggest that the coastal dune sands were deposited in a passive margin of a synrift basin. The results provide a good evidence for the extension in the Red Sea rift system during Oligocene-post Pliocene, which is consistent with the general geology of Egypt.     

Kinetics of decompressional reactions in eclogitic rocks – formation of plagioclase coronas around kyanite

This paper describes a kinetic study on reaction textures in eclogitic rocks from the Sulu region, eastern China. Some of the eclogitic rocks display a decompressional reaction texture, whereby kyanite grains are surrounded by plagioclase coronas and are never in contact with quartz. The change in mineral parageneses with progress of the reaction was predicted by constructing chemical potential diagrams in a model system. The chemical potential diagrams indicated that the chemical potential of 2Na₂O + CaO (2µ_Na2O + µ_CaO) in intergranular regions between kyanite and quartz should decrease with decreasing pressure, whereas 2µ_Na2O + µ_CaO in intergranular regions between garnet and omphacite should increase with decreasing pressure. Thus, upon decompression, an inequality in chemical potential arises in the rock. To reduce this inequality, garnet and omphacite react to produce amphibole and plagioclase and release Na₂O and CaO. Then, the released Na₂O and CaO components diffuse into the regions between kyanite and quartz grains and react to produce plagioclase between them. This model also indicates that the chemical potential of SiO₂ should decrease around kyanite grains during the progress of the decompressional reaction, and Si‐undersaturated conditions should have formed around kyanite grains in spite of the presence of quartz in these eclogitic rocks. Thus, spinel or corundum that are not stable in the system with excess quartz can form as a metastable phase, as observed in eclogitic rocks from the study areas. Phase diagrams in the system with excess quartz should be carefully applied for analysis of such reaction textures.     

Fracturing mode analysis and relative age dating of faults by surface textures of quartz grains from fault gouges

Surface textures of quartz grains taken from about 250 samples of fault gouges in faults ranging in length from several metres to several tens of metres with the faults extending several tens to several hundreds of kilometres in length, are examined by means of the scanning electron microscope and are categorized into four groups from I to IV, based on the smoothness of the surface, degree of undulation and development of cavities, as mentioned in a previous paper. Based on surface features of quartz grains from faults the geological age of movement of which is known, the groups I to IV are tentatively related to the age of formation of quartz grains and the period of formation of surface textures. This correlation makes it clear that quartz-grain surface textures are an important clue to dating a relative age of faulting. River patterns, striations, granular fracture surfaces, and dimple-like textures are observed to appear on less corroded surfaces of quartz grains from fault gouges. Fractographic analysis of these surfaces of quartz grains should be a clue to revealing the mode of fracture of the fault movement which released the quartz grains into the fault gouge.     

Archaean quartz arenites in the Canadian Shield: examples from the Superior and Churchill Provinces

Units of remarkably pure Archaean quartz arenite occur in the northwestern part of the Superior Province and in the northern terrane of the Western Churchill Province (Rae Province) of the Canadian Shield. In the Superior Province, silica-cemented quartz arenites of Archaean age are well preserved in several greenstone belts. The example from the Keeyask Lake sedimentary assemblage displays tabular–planar and trough cross-beds, ripple marks, reactivation surfaces and pebble lag deposits. In spite of penetrative deformation and greenschist-grade metamorphism, primary textures are extremely well preserved, showing framework grains to be very well rounded and sorted. The succession of Keeyask Lake quartz-arenite beds is overlain by siltstones containing small-scale stratiform, domal and columnar stromatolites. A shallow-marine environment of deposition is inferred. Detrital heavy minerals include pyrite, magnetite, zircon, tourmaline, apatite, sphene and topaz. In the northern part of the Western Churchill Province (Rae Province), Archaean quartz arenites occur in northeasterly trending belts where intense structural deformation has in most places obscured or obliterated primary textures and structures. This has led to speculation that some of these units are metachert or recrystallized vein quartz, but local preservation of primary textures and structures provides clear evidence of epiclastic origin. In the example described herein, quartz arenites of the Woodburn Lake Group display sparse occurrences of trough and tabular–planar cross-beds, channels, ripple marks and pebble lag deposits. Probable environments of deposition for these quartz arenites include fluvial systems and shallow-marine shelf settings. The occurrence of unequivocal quartz-arenite clasts in beds of intercalated conglomerate provides direct evidence of at least two episodes of accumulation of almost pure quartz sand. Thin sections and polished slabs reveal frameworks of clastic quartz grains with little to no matrix (now mainly muscovite), and rare detrital grains of accessory heavy minerals, predominantly zircon and opaque iron oxides. Pyrite and other sulphides have been introduced along fractures, but some intergranular sulphide grains may be of detrital origin. The principal source for the quartz arenites in both areas must have been quartz-rich granitoid rocks. Conditions of intense chemical weathering are indicated. The widespread occurrence of extremely mature quartz arenites throughout Archaean terranes of the Canadian Shield, and in other shields of the world, are suggestive of crustal stability during early Earth history. The association of quartz arenites and ultramafic rocks, uncharacteristic of younger terranes, is now recognized in many Archaean greenstone belts of the Canadian Shield.     

Dating fault activity by surface textures of quartz grains from fault gouges

Surface textures of quartz grains from fault gouges are examined by means of the scanning electron microscope. It is disclosed that the surface morphology of the grains is variable, but quartz grains extracted from one fault gouge sample commonly show a type of texture different from those observed on the grains from the other samples. The surface textures observed under the microscope are tentatively classified into eight types; subconchoidal, orange peel-like, fish scale-like, moss-like, moth-eaten, stalactitic, pot-hole, and coral-like textures. These textures can be classified into four groups. Arranged in the order of their apparent features, it is interpreted that the progressive corrosion of quartz grains by ground water has taken place after faulting. The change of this surface feature can assist in estimating the time elapsed since the last fault activity.     

Surface textures of turbidite sand grains, Laurentian Fan and Sohm Abyssal Plain

Surface textures of quartz grains have been examined from five samples from the Laurentian Fan and Sohm Abyssal Plain, representing varied transport distances and power of the depositing turbidity current. The grains retain their primary irregular shape derived from glacial erosion, and glacial surface textures are preserved in dish-shaped depressions. These features have been superimposed by a slight rounding of edges and an abundance of collision-induced markings, particularly mechanical V-forms. The most intense current modification of this sort occurs in mid-Wisconsinan or earlier sands that have been transported over 1000 km to the distal Sohm Abyssal Plain by turbidity currents. Collision textures probably develop during grain flow on the steep continental slope: delicate resedimented shelf foraminifera are preserved in the same turbidites and most have been transported exclusively in suspension.     

浙江沿海全新世海滩岩基本特征及其古地理意义

浙江沿海全新世海滩岩是正处于中亚热带的海滩岩,迄今已发现有13处,主要出露于两侧为基岩岬角的小海湾潮间带中部的沙滩上,14C年龄主要为2 705 a±160 a BP～6 485 a±210 a BP。通过多学科手段的综合研究,初步探讨了其基本特征及古地理意义。研究结果表明,受海水作用影响产生的颗粒(如生活于潮间带-潮下带的贝类壳体及其碎片、近岸底栖的有孔虫和介形虫等生物碎屑、花岗岩岩屑等)、颗粒支撑结构及文石针胶结物是其最重要特征。在其形成过程中,适宜的地貌如处于有砂砾滩的小海湾中,充足的经海蚀搬运来的陆源岩屑、石英、长石,大量的海洋生物碎屑作为物源,合适的颗粒粒径,适中的水动力,基本稳定或缓慢下降的海面及较高的气温等,都是有利的条件。在海滩岩形成时期,浙江中、北部地区沿岸线基本稳定,或略微缓慢向海推进。东海沿岸流不是如今这种低温水团,其水温应比现今高5～6^oC,最少应高2^oC,而其低盐度性质则古今基本一致。同时,热带扩展到浙江中、北部地区,或者当时的气候分带性不如现今这么明显。     

中国东南部海岸砂与大陆架砂沉积特征比较及其环境意义

针对目前对大陆架砂成因问题的争议,本文从海陆过渡带动力关联的统一的环境体系,选取环境相关的海岸与大陆架砂质沉积做对比分析,获知东海大陆架砂的沉积特征和形成环境不具备高度的均一性,因而不是统一的大规模风成堆积,它们基本上均属于河口海岸-浅海环境下的产物。据大陆架砂的沉积特征以及当代关于沙漠化概念的内涵,质疑东海大陆架"沙漠化"的观点。     

广西北海现代海岸沉积作用

本文讨论了广西北海现代海岸各种环境的自然地理分带、地质营力、沉积物以及生物生态等特征,指出环境能量直接决定了环境类型、沉积物类型以及生物组合类型。海岸环境据其能量划分为高能海滩、低能海湾、中能岩质潮坪和砂质潮坪、低能泥质红树林潮坪以及高能潮上沙丘等6种沉积类型;混合沉积作用主要发育于高能海滩和中能潮坪沉积环境内。涠洲岛现代地壳的上升活动控制着该区海蚀地貌的形成。     

图门江下游沙丘粒度分布与石英表面结构研究

图门江下游沙丘粒度频率累积曲线形态全部为双峰,负偏态,粒度变化范围大,平均粒度数值小,标准偏差平均值为0.61;粒度分布特征方法判断沙丘沉积环境以河流成因为主。通过扫描电镜研究其石英颗粒表面结构,机械作用以贝壳状断口、V形坑和次棱角状形态组合为主,化学作用为中等-强烈溶蚀特征;石英颗粒表面结构特征表明沙丘为河口沉积;热释光测年结果表明沙丘受全新世海平面变化的影响,经历海水及风力改造。     

石英砂岩的硅质胶结作用及其对储集性的影响

作者应用阴极发光显微镜、扫描电镜、电子探针及包体测温等手段,对华北太原组石英砂岩的硅质胶结现象、形成温度、物质来源及其对储集性的影响作了研究。结果表明:太原组石英砂岩有两期硅质胶结,第二期规模大;其形成温度为130°-140℃;砂岩处于中成岩阶段晚期;硅质胶结作用是使该砂岩丧失良好储集性能的主要因素。     

Thin section and S.E.M. textural criteria for the recognition of cement-dissolution porosity in sandstones

Cement-porosity relationships are described from the Lower Triassic Sherwood Sandstone Group and the Middle Jurassic Ravenscar Group in the United Kingdom. Calcite cemented sandstones display a variety of replacement textures, with preferential replacement of grains and of overgrowth faces with high free-surface energy. Dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates. Examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains. These range from small ‘v’-shaped notches and pits, through regular and irregular shaped embayments, into large depressions. These textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones, and are interpreted to have been formed by the dissolution of pore-filling and grain replacive authigenic carbonates. This is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones. These textures indicate that part of the intergranular porosity in these sediments is secondary in origin, and has been generated by the dissolution of carbonate cements. The identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface.