A note on the characteristics and possible origins of desert varnishes from southeast Morocco

Two substrate-controlled varnish types were identified from arid southeast Morocco: (1) patchy varnish interspersed with ‘silica glaze’ on silicified limestone and (2) a complete varnish cover on partially-silicified limestone. Electron probe microanalysis was used in conjunction with scanning electron microscopy to investigate their chemical composition. In both instances, varnishes are predominantly silica and iron-rich with localized concentrations of manganese. Inorganic fixation of iron and manganese is the preferred explanation, with manganese and iron derived predominantly from external sources—possibly wind-blown dust. Some iron and silica may also be derived by outward migration from within the substrate, particularly on partially silicified limestone.     

Distribution and formation of rock varnish in southern Tunisia

Field observations, chemical analysis, electron and light microscopy and radiolabelling experiments provide information on the characteristics, formation and distribution of Fe and Mn rock varnishes in southern Tunisia. Unstable rocks are not surface-coated, but when broken they commonly reveal internal microbial solution fronts. Stable rocks are coated in Fe, and more rarely Mn, rock varnishes. Radiolabelling indicates the presence of many respiring microbes on both Fe and Mn varnishes. Microscopy shows fungi and bacteria, both on the Fe varnishes and within the rock. Only rare, microcolonial fungi occur on Mn varnishes, suggesting that the microbes are within or beneath the varnish. The abundance of microorganisms supports a biogenic origin for rock varnishes. Different microbial communities appear to be involved in Fe and Mn varnish formation. Microbes appear to live beneath the surface of unstable rocks but coat stable rocks which cannot be penetrated, apparently to avoid u.v. light and desiccation. This explains the concentration of elements that form u.v. opaque minerals and clays. The origin of these elements is external to the substrate. These results suggest that rock varnish dating and paleoenvironmental techniques can be applied in southern Tunisia.     

Ferromanganese rock varnish in north Norway: A subglacial origin

A thin, dark brown rock varnish is described from ice-smoothed bedrock on the forefield of a glacier in North Norway. It occurs only in narrow strips (ca 100 mm or less wide) which run roughly parallel to the ice front and along the top edges of small treads of a series of bedrock steps. The varnish is hard, thin (< 10 μm) and consists of an iron/manganese deposit, greatly enriched in these elements compared with the composition of the underlying gabbroic bedrock. It is suggested that the varnish formation is due to localized changes in Eh/pH conditions in subglacial regelation ice and meltwater at the top of the bedrock steps. These changes may be due to CO₂ and/or O₂ degassing from water held at higher pressures under the ice than in cavities downstream where the ice is decoupled from the bedrock. Glacier retreat rates indicate that the varnish has remained exposed subaerially at the surface for about twenty years, and its restricted occurrence suggests that it is not of biological origin but rather is essentially authigenic. Possible subglacial origins for associated iron and silica precipitates are also suggested.     

Insights into the formation of rock varnish in prevailing dusty regions

The characteristics of rock varnish from the Campo de Piedra Pomez (CPP, Andes Argentina) provides new insights into the development of rock varnish under severe dusty conditions. The CPP varnish has been analysed using SEM‐EDAX and Raman techniques. The rock coating is tens of microns in thickness and under the microscope shows a micro to cryptocrystalline appearance and a general granular texture. Silica (quartz, cristobalite and amorphous silica) is the main phase forming these coverings followed by Al, K and Na compounds. Minor Fe oxides and Mn oxides are the source for the orange (Fe) and brown (Fe and Mn) hues of the coating. The results obtained for the CPP coating show that desert varnish developed under dusty circumstances does not have the appearance of typical rock varnish. Textural characteristics include high crystallinity and granular arrangement of the components. Moreover the absence of typical microlaminations is related to the presence of aeolian mineral grains which inhibit their development. The main mechanism of formation of rock varnish under such environmental circumstances is the direct incorporation of aeolian mineral grains into the varnish. However, other physicochemical processes are also required to explain the formation of varnish components such as amorphous silica or iron oxides phases. Although the development of the desert varnish may act as a protector of the underlying pyroclastic rock, the extreme and persistent windy conditions in the CPP field are high enough to weather and erode not only the rock coating but also the original ignimbrite. As other warm desert sites on Earth, the CPP area can also be considered as possible terrestrial analogue to Mars. Some environmental attributes might be similar to those expected on the Martian surface and thus, textural similarities between the CPP varnish and the rock varnish‐like coating of Mars are likely. Copyright © 2014 John Wiley & Sons, Ltd.     

High resolution transmission electron microscopy evaluation of silica glaze reveals new textures

We present the first nanoscale investigation of silica glaze. High resolution transmission electron microscopy of a rock coating from the Ashikule Basin, Tibetan Plateau, reveals the presence of spheroids composed predominantly of silicon and oxygen with diameters ranging from 20 nm to 70 nm. While silica glaze spheroids co‐exist with manganese‐rich rock varnish in the same sample, the different rock coatings are texturally and physically distinct at the nanoscale. These observations are consistent with a model of silica glaze formation starting with soluble aluminum‐silicon (Al‐Si) complexes [Al(OSi(OH)³)²⁺], mobilized with gentle wetting events such as dew or frost. The transition between complete and partial wetting on silica surfaces rests at about 20–70 nm for liquid droplets. Upon crossing this transition, a metastable wetting film would be ruptured, initiating formation of silica glaze through spheroid deposition. Copyright © 2010 John Wiley & Sons, Ltd.     

成岩过程对五峰-龙马溪组页岩地震岩石物理特征的影响

五峰-龙马溪组页岩是目前国内页岩气勘探的首选层位,而其地震岩石物理特征是利用地震方法进行"甜点"预测的重要基础之一,但对五峰-龙马溪组页岩地震弹性特征变化规律的研究并未考虑沉积、成岩过程的影响,致使相应的规律性认识缺乏地质意义.在对五峰-龙马溪组页岩样品系统声学测量基础上,分析了页岩样品地震弹性性质的变化规律.利用X射线衍射分析、扫描电镜（SEM）、阴极发光（CL）与能谱分析确定了五峰-龙马溪组页岩在不同沉积环境下的成岩过程,并讨论了成岩过程与地震弹性性质变化规律的因果关系.研究结果表明,页岩中有机质（TOC）受高热演化程度的影响,其密度通常高于1.4 g·cm^-3,并接近于有机碳密度上限1.6 g·cm^-3（石墨密度）.五峰-龙马溪组页岩地震弹性性质变化规律整体受沉积环境控制,沉积环境的差异形成不同的成岩过程,致使地震弹性特征也表现出不同的变化规律.表现在五峰-龙马溪页岩样品动态岩石物理特征主要受岩石结构控制（支撑颗粒弹性性质）,而孔隙度、TOC含量以及孔隙形状则为对地震弹性特征影响的次一级因素.五峰-龙马溪组页岩上段为浅水陆棚相,机械压实与化学压实（硅质胶结）为先后两个过程,造成样品表现出高的速度-孔隙度变化率、高速度比（泊松比）、高各向异性以及低TOC含量的特征.五峰-龙马溪组页岩下段为深水陆棚相,机械压实过程中同时伴有生物成因的硅质胶结,造成岩石样品表现出较高TOC含量与孔隙度、各向异性较弱以及较小的速度-孔隙度变化率.研究结果可为五峰-龙马溪页气储层的测井解释和地震"甜点"预测提供依据.     

Impact of subsurface rock fragments on runoff and interrill soil loss from cultivated soils

Field and laboratory studies have indicated that rock fragments in the topsoil may have a large impact on soil properties, soil quality, hydraulic, hydrological and erosion processes. In most studies, the rock fragments investigated still remain visible at the soil surface and only properties of these visible rock fragments are used for predicting runoff and soil loss. However, there are indications that rock fragments completely incorporated in the topsoil could also significantly influence the percolation and water distribution in stony soils and therefore, also infiltration, runoff and soil loss rates. Therefore, in this study interrill laboratory experiments with simulated rainfall for 60 min were conducted to assess the influence of subsurface rock fragments incorporated in a disturbed silt loam soil at different depths below the soil surface (i.e. 0.001, 0.01, 0.05 and 0.10 m), on infiltration, surface runoff and interrill erosion processes for small and large rock fragment sizes (i.e. mean diameter 0.04 and 0.20 m, respectively). Although only small differences in infiltration rate and runoff volume are observed between the soil without rock fragments (control) and the one with subsurface rock fragments, considerable differences in total interrill soil loss are observed between the control treatment and both contrasting rock fragments sizes. This is explained by a rapid increase in soil moisture in the areas above the rock fragments and therefore a decrease in topsoil cohesion compared with the control soil profile. The observed differences in runoff volume and interrill soil loss between the control plots and those with subsurface rock fragments is largest after a cumulative rainfall (P_cum) of 11 mm and progressively decreases with increasing P_cum. The results highlight the impacts and complexity of subsurface rock fragments on the production of runoff volume and soil loss and requires their inclusion in process‐based runoff and erosion models. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydrograph separation in a mountainous catchment — combining hydrochemical and isotopic tracers

Runoff components of the Zastler catchment (18\4 km², southern Black Forest, Germany) were analysed with hydrograph separations using stable oxygen isotopes and dissolved silica. It was shown that event water and components with low silica contributed only small amounts to total runoff. In addition, comparison of the two‐component hydrograph separations showed that the low‐silica components are generated by both event water and pre‐event water fractions, depending on the state of the system. A modified three‐component hydrograph separation method was introduced using dissolved silica and ¹⁸O. During storm events an interaction of three runoff components having distinct silica concentrations could be shown. Based on the geological and geomorphological genesis of the study site, it was appropriate to assign (i) the low silica component to the riparian zones and impermeable areas, (ii) the medium silica component to the periglacial debris cover and (iii) the high silica component to the crystalline detritus and crystalline hard rock. Exact quantification of the runoff components remained difficult. However, runoff components with medium silica concentrations reacted very sensitively and intensely. The contribution of this component to total runoff is comparatively large. This shows the important role of the periglacial debris to runoff generation of the study site and emphasizes the importance of runoff generation processes occurring in this reservoir. Copyright © 2000 John Wiley & Sons, Ltd.     

Tafoni in the El Chorro area,Andalucia, southern Spain

Using a combination of field, laboratory and micromorphological evidence, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the processes responsible for their formation. Twenty-five tafoni were randomly selected for field examination. The morphology of these cavernous rock domes is characterized by a helmet-shaped outer roof and an arched-shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, overhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, obtained using a Schmidt hammer, are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures, alkaline pH values and some base cation enrichment. Micromorphological analysis of thin sections, prepared from undisturbed blocks, reveals large quantities of pore-filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodules with a metallic appearance. In terms of their formation, different processes appear to act on different parts of the landform. On the outer roof surfaces, case hardening, resulting from near-surface cementation and surface varnish development, is dominant. On the inner cavern surfaces, however, core softening, resulting from granular disintegration and flaking, dominates. Exfoliation weathering, running water and wind deflation also appear to play an important role in tafone formation. A phased model of tafone evolution is proposed whereby the features pass through four phases of development–initiation, enlargement, amalgamation and degradation; in the study area there are examples of tafoni in each of these phases. Much of the evidence suggests that the tafoni are actively developing under current environmental conditions. © 1997 by John Wiley & Sons, Ltd.     

基于二维图像的数字岩心电导率计算方法研究

电导率是表征岩石电学性质的重要物理参数,在地质资源勘查和测井解释等领域发挥着巨大作用.快速、准确地确定岩石电导率具有重要的理论和实践意义.作为近年来发展的一种岩石物理数值模拟工具,数字岩心技术在定量计算电导率等物性参数方面应用广泛.三维微观结构的准确获取是数字岩心技术计算岩石电导率的关键,但传统获取岩石三维微观结构的方...     

A comment on ‘a note on the characteristics and possible origins of desert varnishes from Southeast Morocco’ by Drs. Smith and Whalley

This note queries the discussion of the origins of ‘desert varnishes’ (Smith and Whalley, 1988), with particular reference to manganese-rich conditions. The case for a biotic model of varnish development is proposed and illustrated as opposed to the abiotic model put forward by Smith and Whalley.     

Integration of rock physics and seismic inversion for rock typing and flow unit analysis: A case study

Rock typing and flow unit detection are more challenging in clastic reservoirs with a uniform pore system. An integrated workflow based on well logs, inverted seismic data and rock physics models is proposed and developed to address such challenges. The proposed workflow supplies a plausible reservoir model for further investigation and adds extra information. Then, this workflow has been implemented in order to define different rock types and flow units in an oilfield in the Persian Gulf, where some of these difficulties have been observed. Here, rock physics models have the leading role in our proposed workflow by providing a diagnostic framework in which we successfully differentiate three rock types with variant characteristics on the given wells. Furthermore, permeability and porosity are calculated using the available rock physics models to define several flow units. Then, we extend our investigation to the entire reservoir by means of simultaneous inversion and rock physics models. The outcomes of the study suggest that in sediments with homogeneous pore size distribution, other reservoir properties such as shale content and cementation (which have distinct effects on the elastic domain) can be used to identify rock types and flow units. These reservoir properties have more physical insights for modelling purposes and can be distinguished on seismic cube using proper rock physics models. The results illustrate that the studied reservoir mainly consists of rock type B, which is unconsolidated sands and has the characteristics of a reservoir for subsequent fluid flow unit analysis. In this regard, rock type B has been divided into six fluid units in which the first detected flow unit is considered as the cleanest unit and has the highest reservoir process speed about 4800 to 5000 mD. Here, reservoir quality decreases from flow unit 1 to flow unit 6.     

Effects of geological structures on rainfall-runoff responses in headwater catchments in a sedimentary rock mountain

Clarifying rainfall-runoff responses in mountainous areas is essential for disaster prediction as well as water resource management. Although runoff is considered to be significantly affected by topography, some previous studies have reported that geological structures also have significant effects on rainfall-runoff characteristics. Particularly in headwater catchments located in sedimentary rock mountains, dips and strikes may significantly affect rainwater discharge. In this study, the effects of geological structures on rainfall-runoff characteristics were investigated based on observed discharge hydrographs from 12 catchments, which lie radially from the summit of a sedimentary rock mountain. The results obtained were as follows: (1) Even though the topographic wetness index (TWI) distributions of the 12 catchments were similar, there were significant differences in their runoff characteristics; (2) Catchments with average flow direction oriented towards the strike direction (strike-oriented catchments) are characterized by large baseflows; (3) Catchments with average flow direction oriented towards the opposite dip direction (opposite dip-oriented catchments) are steep, and this results in quick storm runoff generation; (4) Catchments with average flow direction oriented toward the dip direction (dip-oriented catchments) are gentle, and this results in delayed storm runoff generation. It was presumed that in strike-oriented catchments, large quantities of groundwater flowing along the bedding planes owing to hydraulic anisotropy, exfiltrate and sustain the large amount of the observed baseflow, that is, in strike-oriented catchments, runoff is directly controlled by geological structures. Conversely, in opposite dip-oriented and dip-oriented catchments, runoff is indirectly controlled by geological structures, that is, geological structures affect slope gradients, which result in differences in storm runoff generation. Thus, this study clearly illustrates that geological structures significantly affect rainfall-runoff responses in headwater catchments located in sedimentary rock mountains.     

Distribution of secondary minerals in crusts developed on sandstone exposures

Previous research on rock weathering crusts has revealed their large variability depending on the type of host rocks and development of weathering processes. The composition of crusts developed on natural sandstone exposures is less documented in the literature in comparison to those developed on architectonic stones. In both cases, previous research has focused mainly on the progress of salt weathering. This study considers the surfaces of sandstone tors in the Polish Outer Carpathians. The exposed parts of the rocks in this area are often covered by crust, which is up to several centimetres thick, and differs from the internal part in colour and composition. The crusts were characterized using light and electron microscopy, X‐ray diffractometry, thermal analyses, Mössbauer spectroscopy, bulk chemical analyses and sequential chemical extractions. Porosity was estimated by digital image processing. The following two hardened zones were observed: (1) thin (up to 30 µm), black, external layer, rich in carbon and composed of opal‐type silica, covered in places by sulphate incrustations and numerous spherical particles of anthropogenic origin; (2) thicker (up to several millimetres), internal part composed of a set of laminae of variable colouration, enriched in iron (oxyhydr)oxides (goethite and hematite) in comparison to the rock interior. Development of the crust results from silicon and iron redistribution during the sandstone alteration. The chief source of silica is hydrolysis of aluminosilicates, whilst that of iron is decomposition of aluminosilicates, carbonates and sulphides. Hematite is probably a result of goethite transformation. However, air pollutants may play an important role in the formation of sulphates. Silica and iron compounds affect the properties of the rock, hardening the surface and lowering porosity by formation of secondary cement. Crystallization of sulphate salts, in turn, may contribute to mechanical disintegration of the rock. Copyright © 2013 John Wiley & Sons, Ltd.     

The hydrological response of soil surfaces to rainfall as affected by cover and position of rock fragments in the top layer

Rainfall experiments have been conducted in the laboratory in order to assess the hydrological response of top soils very susceptible to surface sealing and containing rock fragments in different positions with respect to the soil surface. For a given cover level, rock fragment position in the top soil has an ambivalent effect on water intake and runoff generation. Compared to a bare soil surface rock fragments increase water intake rates as well as time of runoff concentration and decrease runoff volume if they rest on the soil surface. For the same cover level, rock fragments reduce infiltration rate and enhance runoff generation if they are well embedded in the top layer. The effects of rock fragment position on infiltration rate and runoff generation are proportional to cover percentage. Micromorphological analysis and measurements of the saturated hydraulic conductivity of bare top soils and of the top layer underneath rock fragments resting on the soil surface reveal significant differences supporting the mechanism proposed by Poesen (1986): i.e. runoff generated as rock flow or as Horton overland flow can (partly) infiltrate into the unsealed soil surface under the rock fragments, provided that they are not completely embedded in the top layer. Hence, rock fragment position, beside other rock fragment properties, should be taken into account when assessing the hydrological response of soils susceptible to surface sealing and containing rock fragments in their surface layers. A simple model, based on the proportions of bare soil surface, soil surface occupied by embedded rock fragments, and soil surface covered with rock fragments resting on the soil surface, describes the runoff coefficient data relatively well.     

Petrographic and geochemical characterization of weathered materials developed on BIF from the Mamelles iron ore deposit in the Nyong unit,South-West Cameroon

Iron ore deposits hosted by Precambrian banded iron formation(BIF) are the most important source of mineable iron.In Cameroon,they are located in the southern part of the country.This study reports the petrological and geochemical data of iron ores collected from a weathering profile in the Mamelles BIF deposit,SW Cameroon.The profile is composed of three levels which are from the bottom to the top:the saprock,the ferruginous horizon,and the loose horizon.Eight representative iron ore samples(rock fragments and loose clayey material)were collected along the profile and were subjected to petrographic and geochemical analyses.Their mineralogy consists of martite,goethite,quartz,and lesser amounts of hematite,magnetite,kaolinite,and halloysite.The presence of minerals such as kaolinite and goethite in the Mamelles iron ores suggests their supergene origin.Geochemically,the saprock is characterized by high iron content(70.25 wt% Fe_2 O_(3 t)),and low silica(26.38 wt% SiO_2) and alumina(1.14 wt% Al_2 O_3).The rock fragments collected from the ferruginous horizon display higher Fe_2 O_(3 t)(72-76.40 wt%),Al_2 O_3(2.80-5.43 wt%),and lower SiO_2(16.70-18.35 wt%) contents,suggesting the leaching of silica during the enrichment process.The loose clayey samples collected from both the ferruginous horizon and the upper loose horizon show lower iron and higher silica contents.When normalized to the underlying BIF saprock,both rock fragments and loose clayey ores display LREE enrichment,suggesting that they formed through supergene processes.Economically,most of the Mamelles iron ores are classified as medium-grade ores and a few display acceptable contents in contaminants.Overall,this petrological and geochemical study of the Mamelles iron ores revealed encouraging results.Given its strategic location near the deep seaport,the deposit should be investigated in more detail for its mining potential.     

Composition and properties of the iron hydroxides-cemented lenses in Estonian sandstone of Middle Devonian age

The formation of hematite and goethite concretions in different sedimentary rocks including sandstones is an important diagenetic process in the geologic history of the Earth. Its interpretation can also contribute to understanding the diagenetic history of Martian iron hydroxide concretions. A case study of iron-rich concretions from Estonian Middle Devonian sandstones exposed in ancient river valleys in southeastern Estonia was carried out based on the results of mineralogical, petrographical, geochemical, petrophysical and magnetic analyses. It was found that the high Fe₂O₃(total) content (25.0–39.5%), high magnetic susceptibility, bulk and grain density, very low porosity, corrosion and fracturing of the quartz grains of the platy iron concretions are in contrast with properties of the Devonian host sandstones. However the ferrous iron content (measured as FeO) of iron-rich concretions was as low as in the other Devonian rocks, suggesting an oxidizing environment and arid climate during the cementation by iron-hydroxides. The fracturing of quartz grains cemented by iron hydroxides could take place at near-surface conditions including vadose and phreatic zones in arid climate with high evaporation rates. Such climatic conditions have been reported for the Baltic region during Devonian, Upper Permian and Triassic times. We have found that goethite is prevalent in the cement, replacing clay and carbonate minerals. We assume that this iron-rich cement is originated from the mobilization of iron in host sandstones by groundwater, associated with tectonic activity at the end of the Middle Devonian, evidenced by fracturing in Devonian outcrops and caves. Although this mobilization could occur under reducing conditions, precipitation of goethite and hematite for the cementation could take place in oxidizing environment along bedding planes close to the surface during short sedimentation breaks. Another possible time for the formation of iron concretions could be Permian, under the condition of both arid climate and tectonic activity.     

天山北麓岩石漆显微原理层序的初步建立及其年代学意义

对天山地区的调查发现天山北麓有较好的岩石漆发育。利用特殊的超薄片磨制技术,在对１００个岩石漆超薄切片的显微影象分析对比后,初步建立了研究区内晚更新世以来岩石漆显微层理的标准层序,经过年代校正后,给出了该程序的大致年代控制。并对其在估计河流阶地,断层陡坎等地貌面时代方面的应用作了探讨。     

天山北麓岩石漆显微层理层序的初步建立及其年代学意义

对天山地区的调查发现天山北麓有较好的岩石漆发育。利用特殊的超薄片磨制技术,在对100 个岩石漆超薄切片的显微影象分析对比后,初步建立了研究区内晚更新世以来岩石漆显微层理的标准层序,经过年代校正后,给出了该层序的大致年代控制。并对其在估计河流阶地、断层陡坎等地貌面时代方面的应用作了探讨     

Trace elements migrating from tailings to rock varnish laminated sediments in an old mining region from Nelson,Nevada, USA

The dispersion and influence of soluble and particulate trace elements(TEs) present in the materials from abandoned mines is an imminent problem on terrestrial and aquatic environments.The goal of this research is to provide a broader view of such transport,assessing particle size distribution and TE interactions(e.g.,scavenging) with mine tailings and locally derived sediments transported by water in arid region.Sand grains from wash sediment were collected from a dry ephemeral wash in Nelson,N...