Analysis Of Quartz Grains Research Articles

Beach and dune deposits on the Mediterranean coast of Andalusia (Spain) in the light of morphoscopic analysis of quartz grains

Beach and dune deposits on the Mediterranean coast of Andalusia (Spain) in the light of morphoscopic analysis of quartz grains

Geology of Kerabari - Rajarani Area with Special Emphasis on Geological Structures and Strain Analysis of the Main Central Thrust, the Lesser Himalayan Sequence, and Higher Himalayan Crystalline, Eastern Nepal

The geological study was carried out in the Kerabari - Rajarani area that covers south-eastern part of Dhankuta district and northern part of Morang district of eastern Nepal. The study was mainly focused on the lithostratigraphy, structural analysis, and strain analysis of the Main Central Thrust (MCT) and its hanging and foot walls by integrating field study and laboratory investigation. Geologically, the study area can be divided into the Sub-Himalaya, the Lesser Himalayan Sequence, and the Higher Himalayan Crystalline, respectively. The Lesser Himalayan Sequence is divided into the Bhedetar Group and the Dada Bajar Group, separated by the Chimra Thrust. The Bhedetar Group consists of the Chiuribas Formation, while the Dada Bajar Group comprises of the Ukhudanda Formation, the Mulghat Formation, the Okhre Formation, and the Patigau Formation, respectively. The key geological structures include the Main Central Thrust, the Chimra Thrust, the Main Boundary Thrust, and the Main Frontal Thrust, from north to south, respectively. The microstructural analysis of the quartz grains from the Lesser Himalayan Sequence and the Higher Himalayan Crystalline on either side of the MCT provide evidence of multiple phases of deformation. Likewise, the strain analysis of quartz grains from the MCT characterizes the MCT as the stretching fault thrust.

Single grain pIRIR dating of glacigenic deposits in the Yuzhu Peak area of Kunlun Mountains of Tibetan Plateau revealed the glaciations during Holocene period

The variations of glaciers in the Kunlun Mountains of the northern Tibetan Plateau are influenced by the complex interaction of the East Asia summer monsoon (EASM), Indian summer monsoon (ISM) and Westerlies, as well as human activity during the Holocene period. Consequently there is ongoing debates regarding the evolution of glaciers and the factors driving their changes. The reconstruction of glacial evolutions in these regions has been hindered by insufficient chronological data for moraines and other glacial landforms. In this study, glacial sediment samples were collected from five moraine sequences located on the southern and northern slopes of the Yuzhu Peak in the Kunlun Mountains. These poorly bleached moraine samples were dated using the single-grain quartz optically stimulated luminescence (OSL) and K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating methods. The luminescence characteristics analysis of quartz grains shows that single grain quartz OSL dating cannot be used to date Holocene moraine samples due to the low brightness of quartz OSL signals. Various tests were conducted to assess the reliability of K-feldspar pIR50IR170 dating, including anomalous fading tests, dose recovery, and residual dose tests. The K-feldspar pIR50IR170 ages, determined using the Minimum Age Model (MAM), indicate that during the early Holocene, glaciations have expanded by ∼800 m in length at ∼ 8 ka, and during the end of the Neoglacial period, they extended by ∼200 m in length at ∼ 1 ka. The strengthened ISM precipitation and northward movement of ISM boundary likely contributed to the glacial expansion in the early Holocene. During the end of the Neoglacial period, the increase in westerlies precipitation and cold events may have played a role in the advancement of glaciers.

Evolution of Weichselian aeolian strata on a coarse-grained substrate in a rugged piedmont topography: A case study from the foothills of the eastern Sudetes Mts., Czechia

The southern margins of the northern European loess belt on the foothills of Eastern Sudetes Mountains are less explored sedimentation zones. This study provides new data about the development of aeolian silty-sandy sediments overlying the glaciofluvial succession on the rugged topography near the village of Kolnovice. The Kolnovice sand quarry (360 × 200 m), which lies at the margin of the upland plateau, is the only active-mined outcrop on the foothills of the Eastern Sudetes and is large enough to study Pleistocene (peri-)glacial sediments. To examine the origin of these sediments, we applied lithofacies analysis (both macro-description of outcrop walls and micromorphological study of thin sections) and surface analysis of quartz grains. Periglacial structures have been identified within the sediments, allowing us to further interpret the post-sedimentary evolution of the sedimentary succession. The studied sediments resulted from colluvial redeposition of aeolian sediments, which was controlled particularly by the topography, glaciofluvial substrate, and climatic conditions. The underlying glaciofluvial sediments are the most crucial source of the studied sediments, although the fine-grained material could have been transported from more distant areas.

RECOGNITION OF PORPHYRY QUARTZ IN STREAM SEDIMENTS BY FLUID INCLUSION PETROGRAPHY AND CATHODOLUMINESCENCE MICROSCOPY: RESULTS OF SYSTEMATIC DISPERSION STUDIES AND POTENTIAL APPLICATIONS IN PORPHYRY EXPLORATION

Abstract Regional stream sediment surveys are an important exploration tool used in the search for concealed or partially concealed porphyry deposits. It is shown here that quartz contained in the coarse fraction of stream sediments can be used as an indicator mineral to supplement geochemical analyses conducted on the fine fraction, such as the measurement of the bulk cyanide leach extractable gold content. A method is proposed that allows separation of quartz grains from the coarse rejects of stream sediment samples to prepare grain mounts for petrographic analysis. Based on optical cathodoluminescence microscopy and fluid inclusion petrography, the number of porphyry quartz grains in each grain mount is then identified. Case studies conducted at Vert de Gris in Haiti and Hides Creek in Papua New Guinea show that porphyry quartz grains could be confidently identified in sediments in the catchment areas of both porphyries. Because the cost of microscopic analysis of quartz is small compared to the expense of sampling and geochemical analysis, the developed technique could be routinely used in large greenfield exploration programs. It is envisaged here that petrographic analysis of quartz grains can contribute valuable information for prioritization of targets defined based on their geochemical signatures.

New insights into sedimentological and geochemical conditions of marine Pleistocene sediments in the Tangier region (Morocco): First implications for paleoenvironmental and paleoclimatic reconstructions

The Pleistocene period is characterized by important periodic climatic variations, marked by alternating arid and humid phases. This work aims to reconstruct the prevailing Pleistocene paleoenvironment and paleoclimate in the Tangier area based on a multidisciplinary study of a marine formation dating from 125 ka. Field investigations and sedimentological, micropaleontological, mineralogical, and geochemical analyses were carried out on samples from the Pleistocene Achakkar formation. The granulometric study, combined with a morphoscopic, exoscopic analysis of quartz grains and evidence for diverse benthic foraminifera, suggests a low-energy marine depositional environment, most likely foreshore to shoreface. Moreover, analysis of clay minerals and major elements suggests mineralogical evolution, indicating an arid climate. The sedimentary successions of the Pleistocene Achakkar littoral are progressively more and more coarse; emplacement was subject to several interactive factors, such as climatic and eustatic responsible for reconstituting the geological history of the Tangier Peninsula.

Microtextural signatures in quartz grains and foraminifera from tsunami deposits of the Portuguese shelf

This study presents results from two sediment cores collected on the southern Portuguese shelf attempting to, partially, fill the knowledge gap of the offshore record of high-energy events. The results were obtained based on description of cores, microtextural analysis of quartz grains, and foraminiferal taphonomy. The lithostratigraphy corresponds to Late Holocene sedimentation, with interesting intercalations of medium sand rich with bioclastic fragments and erosive basal contact. In terms of microtextures, a high degree of mechanical marks on the grains associated with tsunami deposition was observed and reflects the high-energy hydrodynamic processes. In compositional terms, the higher presence of quartz grains in these units favors the increase of mechanical marks because grain-to-grain contact is more intense. Additionally, the geomorphological setting of the coring sites controlled the degree and type of mechanical microtextures observed. Furthermore, post-depositional changes and characteristics of the original sediment source contribute to explain the occurrence of dissolution in units of GeoB23513-02. The foraminiferal taphonomy displayed a predominance of dissolution alteration in the test surfaces that was more evident in the silty layers. On the other hand, similar to quartz grain microtextural signature, the sandy high-energy units exhibit a slight predominance of physical processes despite the still strong presence of dissolution. The sole presence of foraminifera species from the middle to the outer shelf in some units is an indication that there was little reworking of specimens. This work aims to increase the understanding of dynamics during Holocene high-energy events and to characterize their backwash phases through the different imprints left in the sedimentary offshore record of the Portuguese Algarve shelf.

Sedimentological, Lithostratigraphic, Clayey and Exoscopic Study of the Senonian Series of District Imini, Asfalou and Anmitar, Central High Atlas, Morocco

The present work deals with the sedimentological, lithostratigraphic and paleoenvironmental study of the red (Senonian) series of the two sections Asfalou and Anmitar located in the district of Imini, Asfalou. These areas are semi-arid zone in the south of the High central Atlas-Morocco. Different analysis was performed such as a sedimentological, lithostratigraphic analysis of the facies to recognise sedimentary structures and textures encountered. Also, the collection of samples for exoscopic analysis of quartz grains was conducted, and the analysis of clay procession was done. In general, all the sedimentation was spent in a continental environment (fluvial or flooded) with some rare marine upwellings that are marked by the installation of dolomite bars.

Chemically Active Elements of Reservoir Quartz Cement Trace Hydrocarbon Migration in the Mahu Sag, Junggar Basin, NW China

Element exchange and enrichment during fluid-rock interactions are common, providing potentially novel proxies to trace hydrocarbon migration in addition to the traditional organic geochemistry tracers. However, the processes, mechanisms, and geological and geochemical fingerprints of these interactions are complex, hampering the applications of hydrocarbon migration tracers. To investigate such interactions, we conducted a petrological, mineralogical, and in situ and bulk geochemical study of authigenic quartz and whole-rock samples from the Mahu Sag, northwestern Junggar Basin, northwest China. We found that dissolution, clay and chlorite formation, and overgrowth occurred on quartz grains in hydrocarbon fluid migration pathways, suggestive of strong fluid-rock interactions. In situ quantitative elemental analysis of quartz grains revealed elemental enrichment (e.g., Mn, Fe, Al, Sr, and W) in quartz overgrowth rims compared with their cores, indicating that migration of hydrocarbon-bearing fluids in reservoirs may promote elemental exchange between fluids and minerals. Whole-rock geochemical analysis showed that decreasing contents of some elements may reflect the direction of hydrocarbon-bearing fluid migration and can be monitored with three geochemical proxies, which are the MnO contents and MnO/Zr and Y/Ho ratios. Our data provide new constraints on fluid-rock interactions in petroleum reservoirs and have implications for using inorganic geochemical methods to trace hydrocarbon migration.

Sedimentary State of the Middle Sebou Valley, Between Oued Inaouene and Oued Bouchabel (Pri- Rif) - Morocco

In this paper, we will study the sedimentary state of the river Sebou, which extends between the Mesopotamia, Inaouene Valley and Bouchabel Valley, which is an area of about 36 km (Figure 1). We will analyze the sedimentary components at the bottom of the oued and the level of the floodplain through the banks of the latter. This area comes directly after the confluence of Oued Sebou with Oued Inaouene, in the area of Hamria in douar Ben Hadan, as it extends to the western borders of the center of Sidi Daoud, downstream of the bridge of Sebou in douar Oulad Ben Hamou, where the Oued Sebou takes a general direction from southeast to northwest. This course draws free river confluences in the floodplain extending to the bottom of the Sebou Valley, which is surrounded by land dominated by clay hills of different heights, and overlooking the valley with long to medium slopes. In our study, we relied on fieldwork, including field observation, identification of sections to be studied and analyzed, determination of the properties of river sediments, including coarse elements and morphometric analysis of the gravel, and we relied on choreographic work to draw crosses. Sections and we worked on the identification of the types of banks and the nature of its sedimentary components, we studied the volume of coarse sedimentary elements, in addition to the study of fine sedimentary materials, including laboratory analysis of micro-sediment grains, and we worked on the morphoscopic analysis of quartz grains.

Crystallographically controlled void space at grain boundaries in the Harkless quartzite

Electron backscatter diffraction analysis of quartz grains and imaging of void spaces at grain boundaries in two orthogonal sections of a sample from the plastically deformed and dynamically recrystallized Harkless quartzite reveals the presence of void spaces at quartz-quartz grain boundaries. The distribution of these voids correlates with the crystallographic orientation of neighboring quartz grains relative to the grain boundary. Void spaces are more prevalent along grain boundaries where the a-axis of quartz is normal to the grain boundary. These void spaces likely formed during the exhumation (decrease in temperature and pressure) of the Harkless quartzite as a consequence of the volume change associated with an anisotropic elastic change in quartz crystal lattice dimensions, in which the length of the a-axis decreases by more than that of the c-axis. This volume change results in void space preferentially forming at grain boundaries neighbored by quartz with the a-axis normal to the grain boundary. The preferred orientation of the void spaces thus matches the preferred crystal orientation of quartz in the samples, suggesting that the preexisting rock microstructure and crystal fabric plays an important role in controlling the development and distribution of porosity during exhumation, which more generally may have important implications for fluid migration and the strength of exhumed rocks.

Late Cretaceous-Cenozoic tectonic-sedimentary evolution and U-enrichment in the southern Songliao Basin

Late Cretaceous-Cenozoic tectonic-sedimentary evolution is fundamental to U (Uranium) migration and U-enrichment in the southern Songliao Basin, northeast China. To establish the genetic connection between tectonic-sedimentary evolution and U-enrichment, this paper presents a mineralogical study, apatite fission-track (AFT) analysis, and SEM-CL analysis of quartz grains. Two stages of mineralization were proposed, including the U pre-enrichment stage during the Late Cretaceous, and the ore-forming stage during the Cenozoic. Based on the density plots of the AFT ages, paleoclimate studies, and SEM-CL analysis of quartz grains, it is inferred that the U pre-enrichment was closely related to the rapid uplift of the Great Xing'an Range and the Zhangguangcai Range, and the O2-rich atmosphere during 97–90 Ma in the southern Songliao Basin. The O2-rich atmosphere increased U mobility and led to the formation of red beds during the Late Cretaceous, which widely developed U pre-enrichment in the basins of Northeast China. The mineralogical study reveals that the Qianjiadian uranium deposit developed in two stages. The diagenetic process experienced the transition from early alkaline to late acidic conditions, to promote the Stage I mineralization (U pre-enrichment). The dominant occurrence spaces for U were the dissolution pores of quartz, tightly packed with calcite cement. U-enrichment during alkaline ambient conditions was mainly via adsorption. With the increase of the buried depth, the early alkaline diagenesis returned to acidic conditions due to the abundant organic material. U(VI) was reduced to pitchblende due to the reducing capacity of the organic matter (OM). Stage II (ore-forming stage) mineralization was strictly related to the hydrocarbon re-concentration. According to the thermal history study of the CC1 and Q2 boreholes, the southern Songliao Basin experienced four episodes of rapid uplift during the Cenozoic at 83–80 Ma, 68–52 Ma, 33–23 Ma, and −8 Ma, respectively. The Cenozoic uplifts led to hydrocarbon migration at 65 Ma, 23 Ma, and present. The re-concentration of hydrocarbon prompted the U mineralization in the Qianjiadian uranium deposit in the southern Songliao Basin during the stable period of tectonic movements. The reducing conditions caused by hydrocarbons promoted U mineralization. The dominant spaces of coffinite were intergranular dissolved pores, formed during the transformation from the feldspar to the kaolinite under acidic conditions. The post-rift phase and structural inversion phase directly controlled the U pre-enrichment stage and the ore-forming stage, respectively. Overall, the tectonic-sedimentary evolution played an important role in controlling the U leaching from granites, the dominant occurrence spaces, and U recycling.

The response of a dune succession from Lençóis Maranhenses, NE Brazil, to climate changes between MIS 3 and MIS 2

In tropical Southern Hemisphere South America, late Quaternary marine core and speleothem records provide different proxies and accurate chronologies correlating millennial-scale intervals of increased precipitation with southward shifts of the Intertropical Convergence Zone (ITCZ). However, the climatic effect of these intervals on coastal landforms is poorly understood. Here we investigated the response of abrupt and long-lasting climate events during the Marine Isotope Stages 3 and 2 in a 13.8-m thick sand succession located in a large-scale coastal eolian system, the Lençóis Maranhenses Dunefield, NE Brazil, where winds and precipitation are anti-phased and controlled by the single action of the ITCZ. A chronology of the sediment overburden determined by optically stimulated luminescence (OSL) dating of 14 sediment samples obtained at 1-m intervals showed ages between 132.2 ± 7.0 and 12.9 ± 0.6 thousand years ago in stratigraphic order. A multi-proxy approach based on grain size, surface grain texture, heavy minerals, thermoluminescence (TL) sensitivity, inorganic geochemistry, reflectance, and magnetic parameters from 268 sediment samples collected at 5-cm intervals indicated periods of dune building and stabilization. Significant stabilization periods are synchronous with Heinrich Stadials 6, 4, 3, 2, and 1, and Greenland Stadial 4, within age model uncertainties. Heavy mineral analysis indicated a steady sand source to the dune succession, while TL sensitivity analysis of quartz grains showed the input allochthonous sediments during stadials. The preservation of peak dune activity during the Last Glacial Maximum is attributed to the enduring precipitation brought by the subsequent Heinrich Stadial 1 event.

Distribution of ductile deformation around the Main Central Thrust zone at the frontal part of nappe in southeastern Nepal Himalaya

AbstractA geological survey and morphological analysis of quartz grains were performed to investigate the distribution of ductile deformation caused by the Himalayan Main Central Thrust (MCT) around Dhankuta, southeastern Nepal. The MCT was mapped as the lithological boundary between the gneiss of the Higher Himalayan Crystalline (HHC) as the hanging wall and the inverted metamorphic sequence of the Lesser Himalayan Sediments (LHS) as the footwall. The MCT was found to truncate various stratigraphic levels of LHS and cuts a map‐scale gentle fold developed in the LHS. Ductile deformation was quantified by fractal dimension between size and perimeter of dynamically recrystallized quartz grains in bedded metaquartzite intercalated in both HHC and LHS. Serrate and polygonal shapes of quartz indicate large and small strain rates, respectively, when the temperature during ductile deformation was assumed to be uniform. A peak of strain rate was found at the lithological boundary with the peak width of ca. 500 m. Such a thin shear zone is favorable for producing frictional heat to promote the inverted metamorphism in LHS.

Analysis of Quaternary Sediments of Quartz Grains Applied to the Identification of the Environment of Some Ivory Beaches East of Abidjan (Côte d'Ivoire)

Morphoscopic and exoscopic analysis of quartz grains collected on the Ivorian beaches of the gulf of Guinea between Abidjan (Côte d'Ivoire) and Aforenou (Ghana) during topographic surveys between March 2007 and March 2009 on the examination of quartz at the binocular loupe, reveals blunting grains shining on all beaches (50% to 70%).
 In Abidjan area, the majority of the grains are reddish, indicates a ferrous and inherited environment. The exoscopy for the fine and detailed study of the morphology and the surface of the quartz grains was made with the Scanning Electron Microscope (SEM). This approach makes it possible to interpret traces related to events (energy level) or environments (transport, transition and storage environments).
 These observations indicate that the quartz after a long transport in a fluvial environment have been reworked in a marine environment. They were finally deposited in a low-energy aquatic continental medium marked by the polishing of the crystalline points and the siliceous corpuscles dotting the surface of the grains.
 The energy of the transport environments crossed is high in a turbulent environment through traces of shock, as large and numerous as they are. SEM examination of the samples reveals that beach quartz has generally been transported by river and recovered in an intertidal and/or subtidal environment. They have a continental and marine origin.

Luminescence chronology of late-Holocene palaeofloods in the upper Kaveri basin, India: An insight into the climate–flood relationship

Instrumental/historic records have helped to understand the extreme flood–climate relationship in the modern environment; however, few studies are available to understand their long-term relation (102–103 years) due to the poor preservation and lack of dating techniques. It remains uncertain whether extreme flooding is linked with long-term wet phases of climate or a random event caused by an unusual downpour irrespective of climate. Luminescence analysis of quartz grains from river/floodplain sediments in the Kaveri basin, southern India, showed heterogeneous bleaching. We demonstrated the successful application of various statistical age models in estimating ages of heterogeneously bleached young sediments. This study shows distinct flood clusters occurred during the times of major shifts in the monsoon climate, from fluvial dormancy to sudden outburst of monsoons (~2 ka), from warmer to colder (onset of ‘Little Ice Age’ (LIA) ~ 14th century), from colder to warmer (end of LIA ~ 19th century) and ~20th century, indicating that climatic pattern can be associated with more frequent occurrences of extreme flood events. The study also shows that the two major flood events of the 20th century reported from the upper Kaveri were produced by high intensity short duration storms, suggesting that not all wet phases are associated with major floods and all dry phases with low floods/droughts. The excellent match of the chronology with the historical data, instrumental data and published literature based on proxy data on the Indian summer monsoon validates the chronology and the potential of sedimentary archives for future palaeoenvironmental reconstruction of the study area.

Surface Textural Analysis of Quartz Grains from Modern Point Bar Deposits in Lower Reaches of the Yellow River

The surfaces of quartz grains contain characteristic textures formed during the process of transport, due to their stable physical and chemical properties. The surface textures include the information about source area, transporting force, sedimentary environment and evolution history of sediment. Surface textures of quartz grains from modern point bar deposits in the lower reaches of the Yellow River are observed and studied by scanning electron microscopy (SEM). Results indicate that there are 22 kinds of surface textures. The overall surface morphology of quartz grains shows short transporting time and distance and weak abrasive action of the river water. The combined surface textures caused by mechanical action indicate that quartz grains are transporting in a high-energy hydrodynamic condition and suffer a strong mechanical impact and abrasion. The common solution pits prove that the chemical property of transportation medium is very active and quartz grains receive an obvious chemical action. The combination of these surface textures can be an identification mark of fluvial environment, and that is: quartz grains are main subangular outline, whose roundness is higher with the farther motion distance; Surface fluctuation degree of quartz grains is relatively high, and gives priority to high and medium relief; V-shaped percussion marks are very abundant caused by mechanical action; The conchoidal of different sizes and steps are common-developed with paragenesis relationship; Solution pits are common-developed as well. The study makes up for the blank of surface textures analysis of quartz grains from modern fluvial deposits in China. It provides new ideas and evidence for studies of the sedimentary process and environmental significance, although the deep meanings of these micro textures remain to be further researched.

Tracing the Mahabharat Thrust (MT) on the basis of lithology and microstructures around Bhainse-Manahari area, central Nepal

Lithological and microstructural study carried out in Bhainse –Manahari area, central Nepal reveals that the rock sequences of the Bhainse–Manahari area can be divided into two successions: the Nawakot Complex and the Kathmandu Complex. These two Complexes are separated by a distinct thrust boundary, the Mahabharat Thrust (MT). The Nawakot Complex consists of low-grade metamorphic rocks like slate, phyllite, quartzite and limestone while the Kathmandu Complex comprises medium grade (up to garnet grade) metamorphic rocks like garnet-schist, marble and mica-schist. The Mahabharat Thrust (MT) and the Manahari Thrust (MnT) are the two major thrusts in the study area. The MT separates the rocks of the Nawakot Complex (foot wall) in the south from the rocks of the Kathmandu Complex (hanging wall) in the north. The Manahari Thrust in the western part of the study area separates the Dunga Quartzite and the older Benighat Slates lying above it. The microstructure analysis reveals that the rocks in the thrust zone show higher deformation than in the neighboring rocks, and this gradually decreases away from the MT zone. The strain analysis of quartz grains reveals that the rock sequences of the hanging wall of the MT showed pure, simple and complex shear senses and the rocks of the footwall also showed the same pattern indicating MT as a stretching fault.

Microtextural analysis of quartz grains of tsunami and non-tsunami deposits – A case study from Tirúa (Chile)

In order to estimate the tsunami hazard it is essential to reliably identify and differentiate tsunami deposits from other high-energy events like storms. Recently, the microtextural analysis of quartz grain surfaces was introduced as a method to differentiate between tsunami and other deposits. Using tsunami deposits sampled from a bank profile of the Tirúa river (central Chile), an area that was significantly affected by the 2010 and 1960 Chile tsunamis, we tested the microtextural analysis method for its capability to identify tsunami deposits. A total of 815 quartz grain surfaces of two tsunami layers, two non-tsunami marsh sediment samples, and three reference samples from nearby beach, dune and river were analyzed using scanning electron microscopy (SEM). We grouped the detected microtexture features into five microtextural families: angularity, fresh surfaces, percussion marks, adhering particles and dissolution features. Both the tsunami deposits and reference samples reveal high numbers of fresh surfaces and percussion marks. Thus, there are no statistically significant differences between tsunami, beach, dune and river deposits in characteristics and abundances in all microtextural families. Our study indicates that the microtextural analysis of quartz grains may not be a suitable method to identify tsunami deposits in Tirúa (Chile), due to local factors such as high numbers of inherited microtextures and the possible effects of the high amount of heavy minerals.

Surface textural analysis of quartz grains from varved sediments of Lake Kusai in the Hoh Xil area, Tibetan Plateau

湖泊是风成沉积的理想载体，利用扫描电镜分析石英砂的表面微结构特征，可以用于判断石英颗粒被风或水流搬运的相对距离和湖泊的水动力条件，从而判断其沉积环境与演化历史.库赛湖位于青藏高原北部可可西里地区，是青海省沙尘天气频发的地区之一，为进一步证实库赛湖年纹层粗颗粒层的风成成因，以库赛湖沉积年纹层为研究对象，结合青藏高原北部典型沙丘沉积、库赛湖冰面风成沉积、典型河流沉积和库赛湖湖滨相水成粗颗粒沉积样品，利用扫描电镜对上述沉积物石英颗粒表面特征进行观察研究，并统计35种特征的出现频率.结果表明：库赛湖地区典型风成物石英颗粒表面具有圆形轮廓、中低浮雕、翻板、新月型撞击坑、球根状边缘、直形阶梯、弓形阶梯、碟形坑等微形貌特征；典型水成物石英颗粒表面具有V型撞击坑、小型和中型贝壳状断口、少量溶蚀坑等微形貌特征；库赛湖年纹层粗颗粒层石英颗粒表面具有圆形轮廓、低浮雕、翻板、新月形撞击坑、球根状边缘、碟形坑等明显风成特征，与研究区风成物的石英颗粒表面特征相似，而与研究区典型水成物的石英颗粒表面特征差别很大.因此，库赛湖年纹层中的粗颗粒层是由风成作用形成的.;Lake sediments containing aeolian deposits in the arid areas are one of the important archives for sedimentary studies. Surface textural analysis of quartz grains by scanning electron microscopy can provide an insight into the sedimentary history of clastic sediments, depositional environment and successive sedimentary cycles. Lake Kusai is located in almost uninhabited Hoh Xil region of the Northern Tibetan Plateau, and is one of the frequently dust storm-occurred areas occurring in Qinghai Province. In order to confirm the coarse grained layers formed by aeolian processes in varved sediments of Lake Kusai, 35 types of surface features of quartz grains are analyzed in different sediments, such as coarse grained layers in varved sediments of Lake Kusai, aeolian dune sediments, fluvial sediments, aeolian dust samples in the ice of Lake Kusai, lacustrine deposits in the lakeside. The results show that quartz grains in aeolian sediments have surface microtextures of rounded outline, low or medium relief, acuate steps, straight steps, crescent percussion marks, upturned plates, bulbous edges and dish-shaped depression, etc. The quartz grains in fluvial sediments have microtextures of angular or subangular outline, medium or high relief, V-shaped percussion cracks, small or medium conchoidal fractures, and solution pits, etc. The microtextures of quartz grains in the coarse-grained layers of varved sediments in Lake Kusai are rounded outline, low relief, acuate steps, straight steps, crescent percussion marks, upturned plates, bulbous edges, and dish-shaped depression, etc, which are similar to these of the aeolian sediments, but quite different from these of fluvial sediments. Thus, it is indicated that the coarse grained layers in varved sediments of Lake Kusai are formed by aeolian processes.