Weathering Of Sandstones Research Articles

Sandstone weathering processes in the painted rock shelters of Cerro Colorado (Córdoba, Argentina)

AbstractTafoni, produced by physicochemical weathering of sandstones, are abundant in the Cerro Colorado (Córdoba province, Argentina). Many of the rock shelters are decorated with rock art of high heritage significance. This art belongs to semisedentary communities from the Late Prehispanic Period (ca. 1550–350 AP). They include morphologies inherited from more humid ancient climates combined with current weathering processes. Evolutionary relief study established that tafoni formation is a product of inherited past wetter climates. Mineralogical and petrographic analyses were performed on samples collected from the sandstones and from various morphologies, weathering residues, and wall and ceiling concretions at four rock shelters, Cueva del Indio, Ricci, Quitilipi, and La Trampa. Gypsum and other salts that are not part of the unaltered rock but the result of weathering reveal the magnitude of hydroclastic and haloclastic processes. These activities form active granular disintegration, flaking, and spalling, and impinge on some painted panels. Mapping the distribution of weathering features and processes at each rock shelter enables the planning of interventions to mitigate and alleviate the effects caused by infiltration of water through rock discontinuities, fractures, and overhangs. It also allows decreasing the accumulation of dust and minimizing muddy water flowing over the paintings. These actions are aimed at retarding the progression of rock art degradation and the visible loss of some paintings.

Evaluating chemical weathering of sediments using “R” programming environment – A possible alternate of spreadsheet applications

Evaluation of chemical weathering of sediments is the key factor for provenance study and characterization of various sedimentological processes involved in weathering. The various Weathering Indices (WIs) models are available to estimate major elemental geochemistry of the sedimentary package including CIW, CPA, CIA, PIA, CIX, ICV, WIP, R, V, STI and MWPI. The present investigation is to improve and correlation of the earlier models with our development of a computer program using R programming environment to establish various statistical measures, correlation of chemical indices of weathering and to plot different compositional variation diagrams and their analyses. The present results reveal that the correlation of Different correlation plots suggest a significant level of relationship and positive correlation coefficients in CIW – PIA, CIA – PIA, CPA – CIX, ICV – MWPI pairs while significant level of relationship and negative correlation coefficients in CIW – ICV, CIA – WIP pairs. Besides, boxplot of WIs indicates a skewed left or negatively-skewed distribution of CIW and CIA while a skewed right or positively-skewed distribution of CPA, CIX and WIP. Different compositional space diagrams (A – C – N, A – CN – K, A-K – C – N, M – F – W, A – CNK – FM, A – N – K) constrain sediments and rock/mineral composites major elemental geochemistry and demonstrate weathering profiles and source rock chemical makeup. R environment provides flexibility to alter the code as per the data and user requirement by altering different arguments in the code and to include further relevant charts & plots. The applicability of the programming tool is demonstrated by its performance in evaluating the chemical weathering of Surma and Barail sandstones, Mizoram Foreland Basin, NE India and sediments of Manasbal Lake, Kashmir, India, having contrasting provenance histories.

Chemical weathering rates of clastic sedimentary rocks from the Paraná Basin in the Paulista Peripheral Depression, Brazil

The chemical weathering plays an important role in the processes of surface geochemistry, which is fundamental to understand the landscape evolution. The Paraná Basin is an intracratonic sedimentary basin developed on the South American platform from Paleozoic to Cenozoic, with area of 1.5 × 106 km2 and 5000 km3 of clastic sediments and igneous rocks. The main clastic sedimentary rocks in the Paraná Basin are sandstones and mudstones. Surprisingly, there are no studies assessing the chemical weathering rates of clastic sedimentary rocks from the Paraná Basin under tropical climate conditions in Brazil or in South America. Thus, two small watersheds were chosen to assess the chemical weathering rates of clastic sedimentary rocks from the Paraná Basin in the Paulista Peripheral Depression (São Paulo State, Brazil), i.e. the Monjolo Grande and Jacutinga stream basins. The chemical composition of rain and surface waters, as well as the chemical and mineralogical composition of the fresh rocks (sandstones and mudstones) and soil (horizon C) were characterized. The instantaneous daily flux indicates that ca. 60% of the dissolved solids loads are carried from October to March in both watersheds. The natural fluxes due to chemical weathering (Fw) were ca. 4.5 t km−2 yr−1, with a mixed of local rainwater and chemical weathering of sandstones and mudstones controlling the surface waters composition. The possible chemical weathering processes of sandstones and mudstones occur by quartz dissolution and partial hydrolysis (monossialitization) of albite, microcline and illite, respectively. The chemical weathering rates were of 0.3 and 0.8 m Myr−1 for the Monjolo Grande and Jacutinga Streams basin, respectively, confirming the lower chemical weathering rates for clastic sedimentary rocks in relation to basalts (Paraná CFB) of the Paraná Basin and for the igneous and metamorphic rocks of the Ribeira Belt, other important geological units in the São Paulo State. In addition, these chemical weathering rates are also lower than the suggested for other rock types elsewhere, i.e., carbonates, basalts, granites, schist, gneisses or alkaline rocks. Finally, the effect of climate on [SiO2] was demonstrated, also described in basaltic and granitic watersheds, suggesting that the chemical weathering of sandstones and mudstones are more effective in the wet season.

Petrology and geochemistry of Upper Jurassic Weimei Formation sandstones in southern Tibet: implications for provenance and tectonic setting

Located within the centre of the Tethys Himalayas, the Weimei Formation of the Rongbu area in southern Tibet is a littoral- neritic sedimentary association composed of clastic rocks and a few shales. Petrologic characteristics and detrital component statistics indicate that sandstone types include lithic quartz sandstones, with the average content of quartz, feldspar and debris being 77%, 9% and 14%, respectively. The provenance of the Weimei Formation is a recycled orogeny, and the aforementioned sandstones generally have high SiO2 levels, low CaO, K2O and Na2O levels and wide ranges of CIA and CIW values. The primitive mantle-normalised trace element spider diagrams showed unstable content of some elements and losses of Ba, P, Sr, K, Ti and other elements in partial samples. Chondrite-normalised rare-earth element (REE) patterns indicate that the sandstones are characterised by an enrichment of LREE, significant fractionation of LREE and HREE and negative Eu anomalies. Further, ratios of Sm/Nd and Cr/Zr and discriminant diagrams (i.e., La/Yb versus ΣREE, ΣREE versus SiO2/Al2O3, La/Th versus Hf and function discrimination diagrams) reveal that sandstones from the Weimei Formation were derived from ancient quartzose sedimentary rocks and volcanic rocks of continental island arc. Many important parameters (i.e., La, Ce, Pb, Th, Hf, Zr, ΣREE, La/Yb, (La/Yb)N, ΣLREE/ΣHREE, Eu/Eu*, Rb/Sr and Zr/Hf) of samples and tectonic setting discriminant diagrams (i.e., Th versus Co versus Zr/10, La versus Th versus Sc, La versus Th and function discriminant diagrams) indicate that the tectonic setting of the Weimei Formation sandstones from the Tethys Himalayas has both characteristics of a passive margin and continental island arc. By comparing the detrital zircon U-Pb ages and Hf isotopic characteristics of the Weimei Formation with the strata whose provenances have been identified and analysing the weathering, petrography and geochemistry characteristics of Weimei Formation sandstones and the paleogeography conditions of the northern margin of the India craton within the Late Jurassic epoch, we inferred that Weimei Formation sandstones were mainly sourced from the Precambrian metamorphic rocks of the India craton, simultaneously, the sandstones also received minor sedimentary materials from earlier Tethys Himalaya strata whose provenances contained volcanic rocks of the nearby Gangdese volcanic belt. After experiencing weak to moderate weathering, denudation and long-distance transport, the sedimentary materials eventually migrated to the shelf and the top of the continental slope on the northern margin of the Indian continent, where the materials later participated in diagenesis.

Provenance of the Cretaceous Debre Libanos Sandstone in the Blue Nile Basin, Ethiopia: Evidence from petrography and geochemistry

This work aims to unravel the provenance, depositional tectonic setting and source area weathering of the cretaceous Debre Libanos Sandstone in the Blue Nile Basin, Ethiopia from its petrographic and geochemical signatures. Both the average chemical index of alteration (CIA) and chemical index of weathering (CIW) of sandstones (CIA = 91.08, CIW = 96.75), shales (CIA = 81.89, CIW = 98.17) and siltstones (CIA = 98.10, CIW = 99.35) of the Debre Libanos Sandstone indicate very strong chemical weathering. The low Th/Sc ratio of sandstones (1.46), shales (0.85) and siltstones (0.81), and Th/Sc versus Zr/Sc relationship indicate sediment recycling that leads to a progressive enrichment of heavy minerals such as zircon which is the major host for Zr and Th. The increase of quartz content in sandstones and dense minerals in siltstones and shales can be related to hydraulic sorting, grain-size controlled fractionation and source composition. Trace element concentration and their ratios (La/Th, Cr/Th, Th/Sc, Cr/V, Y/Ni) with rare earth element proxies (Eu/Eu*, LaN/YbN) of the studied samples revealed felsic to mixed source rocks of upper continental crust with some input from recycled old sediments. The source area of the Debre Libanos Sandstone is from the northern and northwestern Ethiopia during the time of marine regression, retreat of the Indian Ocean from NW Ethiopia back to SE Somalia. Tectonic discrimination diagrams revealed continental rift setting for the sandstones and collision setting for shales and most of the siltstones. Furthermore, the studied sandstones were partly deposited in a passive margin setting which had undergone post-depositional K-metasomatic alteration. The shales and siltstones were deposited in active margin setting.

On-site monitoring for better selection of stone repairs: a case study

Weathering of clay-bearing sandstones does not only depend on material properties but also on the environmental conditions they are exposed to. The same is true for repaired stones, in which the compatibility of the repair mortar should be studied not only in terms of material properties, but also in terms of the climatic conditions it will be subjected, in order to maximize this compatibility. This paper proposes a methodology to quantify the thermal and hygric stresses in clay-bearing sandstones and their repair, based on the measurement of temperature and relative humidity at the surface and at several depths in a repaired and a non-repaired stone, as well as wind-driven rain and absorbed water. This is illustrated by a case study in an historical building. The data are used to quantify the stresses in the materials and to propose possible degradation mechanisms.

Effect of parent material on soil acidity and carbon content in soils under silver fir (Abies alba Mill.) stands in Poland

The objective of this research was to investigate the influence of parent materials on soil acidity, particularly pH, fractions of aluminium (AlTA) and hydrogen (HTA), and their effect on soil organic matter accumulation in acid forest soils under silver fir stands. We established 86 investigation plots with soils developed from Triassic sandstones, TS (33 plots), and Quaternary sand derived from weathering of sandstones, QSW (53 plots). Soil samples of the A horizons were analysed for pH measured in water extract (pHw), the content of carbon (Ct), base cations (BCEX) and total acidity (TA). Additionally, TA was fractionated into HTA and AlTA. We also analysed chemical composition of water extracts. Soils developed from TS were significantly richer in clay and Ct than QSW soil. The mean pHw of soils developed from TS and QSW was similar. TA was significantly greater in TS soils; however, this difference was only related to the content of AlTA. Conversely, the content of HTA was similar in TS and QSW soils. The content of Ct and TA soil showed a moderate significant positive correlation with clay content (r=0.50 and 0.60, respectively). The TA correlation was largely attributed to AlTA (r=0.60), while HTA was not correlated with clay content. The positive correlation between clay content and TA suggests that clay fraction increases soil acidity. Higher TA of TS soils was particularly related to the higher AlTA content, while the content of HTA was not related to either parent material or clay content. Small differences in soil pHw between the investigated parent materials may result from the similar relationship between AlTA and Ct. AlTA and HTA appeared to have a positive similar impact on soil Ct.

Rapid and selective clast weathering on an alluvial fan, eastern Karoo, South Africa

A small alluvial fan formed on the surface of a dry infilled dam in January 2010 as a result of a high-magnitude rainfall and flood event. Gravel and boulder-size clasts in the fan comprised dolerite, mudstones and fine-grained sandstones. Within a month of formation some mudstone gravels had disintegrated due to weathering processes. The fan was investigated four years later and the weathering of different lithologies was categorised and quantified. Almost half of the weathered clasts were completely weathered forming small heaps or spreads of particles <4mm in size. The majority of these were mudstones. Dolerites, except for those with signs of weathering prior to transport, were almost all unaffected. Rapid weathering of mudstones which underlie small areas of the local slopes helps explain how fine grained materials are transferred from slopes to the valley bottom gullies. The considerable thicknesses of colluvial and alluvial deposits that have developed in the Holocene and now mantle the footslopes and infill valley bottoms, are composed of dolerite and occasional sandstone clasts and fine-grained material derived from rapid and selective weathering of mudstones and sandstones.

Composition, provenance and source weathering of Mesozoic sandstones from Western-Central Mediterranean Alpine Chains

Forty-two Mesozoic sandstone samples from three different sedimentary successions of the Internal Domains along the Western-Central Mediterranean Alpine Chains (Betic Cordillera, Rif Chain and Calabria-Peloritani Arc) were chemically analyzed to characterize their composition and the degree of weathering in the source area(s). The Rif Chain sandstones have SiO2 contents higher than those of the Calabria-Peloritani Arc and Betic Cordillera sandstones, whereas Al2O3 contents are higher in the Calabria-Peloritani Arc sandstones rather than in the Rif Chain and Betic Cordillera sandstones. The indices of compositional variability (ICV) of the studied samples are generally less than 1, suggesting that the samples are compositionally mature and were likely dominated by recycling. Recycling processes are also shown by the Al–Zr–Ti diagram indicating zircon addition and, thus, recycling processes. The Chemical Index of Alteration (CIA) values are quite homogeneous for the Calabria-Peloritani Arc (mean=76) and Betic Cordillera sandstones (mean=55), whereas the Rif Chain sandstones are characterized by CIA values ranging from 54 to 76. The CIW and PIA values are high for all the studied sandstones indicating intense weathering at the source areas. The different values of weathering rates among the studied sandstones may be related to variations of paleoclimatic conditions during the Mesozoic, that further favored recycling processes. Thus, these differences among the studied samples, may be related to an increase in continental palaeoweathering conditions and sediment recycling effects from the Middle Triassic to the earliest Jurassic due to rising humidity. In addition, regional tectonic movements promoted structural changes that allowed sedimentary recycling and subsidence, which in turn caused diagenetic K-metasomatism. These processes could significantly affect the CIW and PIA weathering indices, which likely monitor a cumulative effect, including several cycles of weathering at the source. The source areas are mainly composed of intermediate-felsic rocks with minor, but not negligible, mafic supply, as suggested by provenance proxies.

Relief-Rock-Soil relationship in the transition of Atlantic Plateau to Peripheral Depression, Sao Paulo, Brazil

This paper presents an atlas of physical geography at a detailed scale for an area located between two large morphostructural units: the Atlantic Plateau, a crystalline shield of rough landforms, and the Peripheral Depression, a transition unit from Plateau to the Parana Sedimentary Basin, whose smooth landforms are generally sculpted over sedimentary terrain. The transition between these two land systems creates contrasting landforms as a function of bedrock structure and soil type. Smooth landforms underlain by sedimentary rocks and reddish clayey soils alternate with small hills with abrupt slopes sustained by igneous or metamorphic rocks. These soils, developed from weathering of siltstones can rapidly change to brownish and pale colors indicating leaching and weathering of stratified sandstones or massive structured quartzites. A geomorphopedological classification at 1:10,000 scale was performed initially by delimiting units according to landforms. Secondly, other themes were added by combining them with different types of rocks and soils. The geomorphological map was produced by using thematic maps including a topographic base map, hypsometric map, slope map and morphological map. Also the geological and pedological maps were created based on field data collection, morphological analysis of soil profiles and physical, chemical and mineralogical laboratory analysis. Finally, the geomorphopedological units were defined using all the mapped data. Each of these units has peculiar features concerning developed or undeveloped soil formation, with mineralogy and structure very often linked to underlying rock. The landform shape is either controlled by geologic structure or originated from weathering processes that make the relief flatter.

Mobilization of aluminum by the acid percolates within unsaturated zone of sandstones

The area of the Black Triangle has been exposed to extreme levels of acid deposition in the twentieth century. The chemical weathering of sandstones found within the Black Triangle became well-known phenomenon. Infiltration of acid rain solutions into the sandstone represents the main input of salt components into the sandstone. The infiltrated solutions--sandstone percolates--react with sandstone matrix and previously deposited materials such as salt efflorescence. Acidic sandstone percolates pH 3.2-4.8 found at ten sites within the National Park Bohemian Switzerland contained high Al-tot (0.8-10 mg L(-1)) concentrations and high concentrations of anions SO4 (5-66 mg L(-1)) and NO3 (2-42 mg L(-1)). A high proportion (50-98 %) of Al-tot concentration in acid percolates was represented by toxic reactive Al(n+). Chemical equilibrium modeling indicated as the most abundant Al species Al(3+), AlSO4 (+), and AlF(2+). The remaining 2-50 % of Al-tot concentration was present in the form of complexes with dissolved organic matter Al-org. Mobilization and transport of Al from the upper zones of sandstone causes chemical weathering and sandstone structure deterioration. The most acidic percolates contained the highest concentrations of dissolved organic material (estimated up to 42 mg L(-1)) suggesting the contribution of vegetation on sandstone weathering processes. Very low concentrations of Al-tot in springs at BSNP suggest that Al mobilized in unsaturated zone is transported deeper into the sandstone. This process of mobilization could represent a threat for the water quality small-perched aquifers.

Characterization, weathering, and protection of sandstones: The case of ‘Agro d'Ardesia’

The conservation of roofing and cladding materials has both environmental and cultural heritage bearing. Our research aimed at comparing the performances of some polymeric products used for the protection of a typical Italian sandstone, the ‘Agro d'Ardesia’, used mostly as cladding, paving, roofing, and building material. These products belong to three different classes: polysiloxanes, acrylics, and fluoropolymers. In addition, an acrylic–silicone mixture was chosen for testing. After the application onto the Agro sandstone by capillary absorption, both efficacy and durability were assessed. Particularly, two types of weathering tests were performed to check the stability over time of the protective products: the exposure to ultraviolet radiation in a climatic chamber and the ageing by exposure to acid rain, expressly designed to represent realistically natural and anthropogenic changes. Detailed information on treatment performances was obtained. The polysiloxane was shown to be the most effective protective material for the Agro sandstone.

Sr–Nd isotopic and REE geochemical constraints on the provenance of fine-grained sands in the Ordos deserts, north-central China

Surface sands and sandstones in the Ordos region, alluvial sands from the surrounding mountains, and fluvial sands in the Yellow River are systemically collected for granulometric, mineralogical, geochemical and isotopic analyses. Fine-grained fractions are small for the Ordos desert sands as well as alluvial sands of the surrounding mountains whereas they are abundant for fluvial sands of the Yellow River. Quartz, feldspar and muscovite are common minerals but their relative proportions are variable for various sediments. REE geochemical compositions are roughly identical but isotopic compositions, particularly ε Nd(0) values of fine-grained sands vary significantly in the Ordos deserts. Based on the integrated Sr–Nd isotopic data, the possible source areas are isotopically divided into four regions for fine-grained sands in the Ordos deserts: region A, the Helan-Yin mountain ranges; region B, the Ordos sandstone areas; region D, the Tarim Basin, the Alxa Plateau, the Hexi Corridor, the northern Qinghai-Xizang Plateau and the Yellow River drainage; and region E, the Jungger Basin and the southern Mongolia. Through geochemical and particularly isotopic comparisons in combination with expert meteorological and geographical knowledge, fine-grained sands in the Ordos deserts originated both from weathering of local sandstones and from blowing of fine sediments in region D. The input from region D is subordinate to the local source for fine-grained sands in the Ordos deserts. Contributions from weathering of local sandstones might be roughly homogeneous from the west to the east of the Ordos region as well as the input of fine-grained sediments from remote areas such as the Tarim Basin, the Qinghai-Xizang Plateau and the Alxa Plateau. Geographical Nd isotopic variations of fine-grained sands in the Ordos deserts might be caused by the input from adjacent places such as the Yellow River drainage due to the near-surface winds.

The effect of weathering in the Buyukmelen River basin on the geochemistry of suspended and bed sediments and the hyrogeochemical characteristics of river water, Duzce, Turkey

The Buyukmelen River is expected to be a water source that can supply the drinking water needs of Istanbul until 2040. The drinking and utility water needs of Istanbul are to transport water from the Buyukmelen River to Istanbul via pipeline and pump it into the Alibeykoy and Omerli reservoirs when their water levels drop. The Buyukmelen River is located in the province of Duzce in 170 km east of Istanbul and its water basin is approximately 2250 km 2. The Buyukmelen River flows muddy in the rainy season and into the Black Sea. The chemical compositions of natural waters alter due to interaction with geological formations, physical and chemical weathering of various rocks and the effects of mining and agricultural production. A research was conducted at the Buyukmelen River basin to determine the effects of hydrological processes. Therefore, the samples of rocks, soil, stream water, suspended and bed sediment were collected from the Buyukmelen River basin. Geochemical and water chemistry analyses of samples were performed at ALS Chemex laboratories, Canada. The bed sediments contain quartz, calcite, plagioclase, amphibole and clay minerals. The clay minerals in the suspended and bed sediment samples are kaolinite, smectite and illite. The water samples collected from the Aksu, Kucukmelen, Asarsu and Ugursuyu streams and the Buyukmelen River in winter (December 2005) and summer (June 2006) periods are rich in Ca and HCO 3 ions. The ions most abundant in rainfall sample are HCO 3 and Na. The ion compositions of surface waters have increased due to the weathering of limestone, agglomerate and volcanic sandstones, light acidic rainfall, semi-arid Black Sea climate in the Buyukmelen basin. The suspended sediment amount of the Buyukmelen River in the rainy season (December 2005) is 174 mg/l. According to the water contamination regulation of Turkey, the Buyukmelen Rivers belongs to quality class 4 based on Al ion and to quality class 3 based on Fe ion in winter period, and to quality class 2 based on Mn concentration in summer period. Chemical index of alteration (CIA) indices observed in the suspended and bed sediments (average of 55) suggest that their source area underwent moderate degrees of chemical weathering processes. According to Upper Continental Crust (UCC) values, the suspended sediment was rich in elements such as Fe 2O 3, CaO, MgO, MnO, TiO 2, P 2O 5, V, Cr, Co, Cu, Zn, As, Cd, Sb, Hg and Pb. The element concentrations of the suspended sediments were related to size fractionation, mainly of clay content. The mentioned enrichment was contributed by agglomerate, basalt, volcanic sandstone and graywacke from rocks in the study area. Source of ions such as Al, Fe, Mn, Ba, Cr, Co, Cu, Ni, Ti and Hg and major in the Buyukmelen River is interaction with rocks such as the agglomerate, basalt, andesite, volcanic sandstone and graywacke. As suggested by Singh et al. (2005), before weathering of some rocks in the Buyukmelen River basin, it was determined that they were graywacke and literanite based on the geochemistry of the suspended and bed sediments.

Weathering of sandstones studied from the composition of stream sediments of the Kupa River (Croatia)

AbstractElemental compositions of the &lt;2mm fraction of three sediments sampled along the Kupa River are presented. On plots of log(SiO2/Al2O3) vs. log(CaO+Na2O)/K2O, the sediments plot closest to average sandstone compositions. On plots of weathering indices A vs. B (Kronberg and Nesbitt, 1981) the sediments plot close to basalt, terrestrial crust and granite. X-ray diffraction of the &lt;63 μm fraction suggests chlorite, vermiculite and mica are present as weathering products in the upper, middle and lower stream sediments. Depletion of some elements in the sediments suggests loss of elements from the sediments due to dissolution of minerals.

Characterisation of weathering of Sydney sandstones in heritage buildings

“Yellow block” sandstone, a colloquial expression used for a locally quarried variety of sandstone, has made an important contribution to the cultural and architectural heritage of Australia’s largest city, Sydney. The golden colour of this dimension stone adds to the attractive appearance of a number of significant landmarks of Sydney. After almost a century of exposure, the progressive decay of the natural consolidant, which is predominantly clay, is causing deterioration of many of these sandstone buildings. While in some cases partial and total replacements have been successful in the restoration work, a proper understanding of the cause of decay of the natural consolidant is necessary if a suitable consolidant is to be developed to preserve the original blocks. This paper reports the findings of a study of the weathering behaviour of the clay matrix in “yellow block” sandstones used in some of Sydney’s historic buildings. Sandstone samples were investigated by the techniques of FTIR spectroscopy, X-ray diffraction, scanning electron microscopy, inductively coupled plasma–atomic emission spectroscopy (ICP–AES) and thermal analysis. The analytical results support the theory that the changes to the original kaolinite clay structure upon prolonged exposure and weathering of the sandstone blocks investigated are at least partly due to the substitution by Fe 3+ for Al 3+ in the octahedral sheet.

La production de limon sur des terrasses de cryoplanation dans les monts Richardson, Canada

The production of silt on cryoplanation terraces in the Richardson Mountains was documented by a sedimentological study. Geochemical, grain-size, and petrographic analyses of the Jurassic Bug Creek sandstone were done. Results show that in the unaltered rock the detrital quartz grains are solidly held by a siliceous cement. In the altered and strongly altered sandstone, there is a marked increase in intergranular porosity and fracturing of the grains. The disaggregation and weathering of the sandstone releases mainly material in the silt-size fraction, with lesser amounts of sand and clay. This material covers the tread of the terraces and constitutes a potential source of loess in glacial time. Runoff water has a PCO2 value two orders of magnitude greater than the atmosphere yet is weakly mineralized, with only amorphous silica approaching saturation. However, the weathering of sandstones is probably accelerated by the chemical action of water which, by attacking the cement that forms the diagenetic overgrowth of the quartz grains, facilitates their release.

Identification of organic compounds extracted from black weathered surfaces of Saxonean sandstones, correlation with atmospheric input and rock inhabiting microflora

The participation of organic compounds in weathering of Saxonean sandstones has been investigated. The organic and elemental carbon content as well as a few microbiological data of black weathered surfaces on historic buildings in Dresden have been obtained during preliminary studies. Organic compounds in the samples were analysed, after extraction, by gas chromatography-mass spectrometry. More than 150 compounds belonging to different chemical classes were detected. Their origin is discussed and correlated with data from the literature about organics in the atmosphere and with the microbiological results. Certain compounds contribute to the weakening of the rock structure.

Mesozoic-Tertiary weathering and soil formation on slates of the Rhenish Massif, Germany

Weathering of Paleozoic slates and sandstones of the Rhenish Massif in western Germany, which occurred during the Upper Mesozoic and Tertiary under warm, humid tropical to subtropical climatic conditions, led to the formation of a regolith up to 150 metres thick. In flat upland areas of the Hunsru¨ck and Eifel, where the tectonic uplift was relatively weak, remnants of this weathering mantle, more or less truncated by Upper Tertiary and Pleistocene erosion, are still preserved. On overlapping sections and drill cores of an autochthonous paleosoil and the saprolite underneath, the morphological, geochemical and mineralogical zonation of a complete profile of the weathering mantle has been reconstructed. A fossil pre-Oligocene Plinthitic Acrisol, covered by Oligocene fluvial sediments, displays characteristics formed under a high groundwater table. On the other hand the underlying saprolite can be subdivided into a lower reduction horizon and an upper oxidation horizon, each of them more than 40 meters thick. This indicates a polygenesis of the weathering mantle during the Tertiary, due to descending groundwater tables caused by the beginning tectonic uplift of the Rhenish Massif and drier climatic periods during the Middle Tertiary. During the Upper Oligocene the deposition of kaolinitic clays derived from removed horizons of the kaolinitic soils started in fault-block depressions. Periglacial processes during the Pleistocene led to a further removal of the weathering mantle and to the deposition of superficial layers on top of the autochthonous remnants of the saprolite. Presently the kaolinitic sediments as well as the saprolite are exploited as kaolin deposits.

The Durability of Sandstone as Building Stone, Especially in Urban Environments

There are several aspects of maintaining the quality of stonework in buildings in urban environments. Two of the most important factors are the durability of stone and the way in which the environment affects the weathering of stone in general, especially the relative affects of natural and pollution weathering. In addition the severity of the conditions to which a stone is exposed depends upon its location within the building itself. Urban atmospheres have generally been polluted with acidic gases and such polluted air is believed to accelerate the process of weathering by many times compared with that in natural rural environments. The effects of this weathering can be both conspicuous and unsightly and include pitting, blistering, spalling and cracking. The general resistance to weathering of sandstones is determined by their chemical compositions. In particular, those sandstones which contain carbonate cements are susceptible to attack by acid rainwater. This occurs through solution of carbonate matrix, allowing many grains of sand to loosen and be removed, even over a relatively short time period. In terms of assessing the durability of sandstones, the tests are more straightforward than those, for example, for limestones. In fact the durability of sandstones can be assessed using two types of acid immersion tests, followed by two types of crystallization tests. The results of these tests allow the sandstones to be classified for use in different environmental conditions and locations in buildings.