The interaction between armouring and particle weathering for eroding landscapes

The interaction between particle weathering and surface armouring and its effect on erosion has been investigated. The effect of soil armouring is to decrease sediment transport with time by preferentially stripping away fine particles. On the other hand the effect of weathering, which breaks down the particles in the armour, is generally believed to increase erosion. By extending an existing armouring model, ARMOUR, and using a variety of published weathering mechanisms this interaction has been explored. The model predicts that while this is generally true, in some cases erosion can be decreased by weathering. When the particles generated by weathering were approximately of equal diameter, erosion increased while armouring decreased. When weathering produced very fine particles by spalling, erosion increased and armouring also increased. When weathering produced a range of particles from fine to coarse, the armour layer broke down and erosion decreased relative to the no‐weathering case. This latter decrease in erosion was due to the high entrainment of coarser transportable materials from the bed decreasing the sediment transport capacity of the flow. In these studies clear regimes could be identified where erosion was limited by either the energy of the flow alone (i.e. ‘transport‐limited’), or the rate of weathering (‘weathering‐limited’); however, for some mechanisms there was an interaction between the two, which we called ‘weathering/transport limited’. Copyright © 2006 John Wiley & Sons, Ltd.     

南京红层桩基持力层承载力初探

南京红层岩性复杂，桩基持力层承载力难确定。依据南京某工程勘察资料，建立南京红层泥岩、泥质粉砂岩不同互层情况的地质模型，运用FLAC3D软件分析，并与载荷板试验结果进行对比表明：桩基持力层应选择在厚度大于两倍桩径的泥质粉砂岩地层上，避免放置于泥岩地层；载荷板试验结果偏于保守，设计时可以把这部分作为安全储备。     

贵州西部中、上二叠统界线附近风化壳类型及成矿作用

贵州西部中、上二叠统界线附近普遍存在不整合面,在不整合面上往往发育厚度不等的风化壳。风化壳主要可归纳为三种类型:茅口组顶部的喀斯特漏斗、洼地中堆积的红粘土风化壳,锰质(铁质)风化壳,峨眉山玄武岩喷发间断面上或玄武岩顶部与龙潭组(宣威组)之间的的高铝高岭石风化壳。喀斯特侵蚀面上的红土风化壳是红土型金矿的赋存层位,峨眉山玄武岩喷发间断面上的高铝高岭石风化壳是铜矿、铝土矿赋存层位,喀斯特侵蚀面上的锰质(铁质)风化壳是锰矿、铁矿赋存层位,而玄武岩顶部与龙潭组(宣威组)之间的高岭石粘土风化壳是稀土、硫铁矿、铝土矿赋存层位。中、上二叠统界线附近风化壳对金、铜、铝土、硫铁、锰和稀土有明显的控制作用。     

初论塔里木盆地砂岩储层中SiO2的溶蚀类型及其机理

根据显微镜观察,塔里木盆地砂岩储层中SiO₂溶蚀分为石英颗粒边缘溶蚀、次生加大边溶蚀和交代溶蚀。通过对石英颗粒和加大边溶蚀特征及其与粘土矿物组合、与碳酸盐胶结、与古油藏破坏以及现今油田水中Si离子含量等关系的综合研究,这些SiO₂溶蚀可能存在两种机理:碱性环境下的SiO₂溶蚀和有机酸(烃类微生物降解产生的有机酸以及有机质成熟产生的有机酸)引起的SiO₂溶蚀。第一种溶蚀类型较普遍,但程度微弱;第二种溶蚀类型主要发生在古油藏破坏和有机质成熟过程中,这种溶蚀较强烈但很局限。SiO₂的溶蚀作用可以为油气提供一定量的储集空间,这对于深埋的时代较老的志留-泥盆系储层具有重要意义。在古油藏中尽管烃类已经充注储层孔隙,但只要孔隙中存在有机酸或碱性孔隙水,SiO₂的溶蚀和石英次生加大的胶结等作用仍可进行。     

Statistical empirical index of chemical weathering in igneous rocks: A new tool for evaluating the degree of weathering

Chemical weathering indices are useful tools in characterizing weathering profiles and determining the extent of weathering. However, the predictive performance of the conventional indices is critically dependent on the composition of the unweathered parent rock. To overcome this limitation, the present paper introduces an alternative statistical empirical index of chemical weathering that is extracted by the principal component analysis (PCA) of a large dataset derived from unweathered igneous rocks and their weathering profiles. The PCA analysis yields two principal components (PC1 and PC2), which capture 39.23% and 35.17% of total variability, respectively. The extent of weathering is reflected by variation along PC1, primarily due to the loss of Na₂O and CaO during weathering. In contrast, PC2 is the direction along which the projections of unweathered felsic, intermediate and mafic igneous rocks appear to be best discriminated; therefore, PC1 and PC2 represent independent latent variables that correspond to the extent of weathering and the chemistry of the unweathered parent rock. Subsequently, PC1 and PC2 were then mapped onto a ternary diagram (MFW diagram). The M and F vertices characterize mafic and felsic rock source, respectively, while the W vertex identifies the degree of weathering of these sources, independent of the chemistry of the unweathered parent rock.

The W index has a number of significant properties that are not found in conventional weathering indices. First, the W index is sensitive to chemical changes that occur during weathering because it is based on eight major oxides, whereas most conventional indices are defined by between two and four oxides. Second, the W index provides robust results even for highly weathered sesquioxide-rich samples. Third, the W index is applicable to a wide range of felsic, intermediate and mafic igneous rock types. Finally, the MFW diagram is expected to facilitate provenance analysis of sedimentary rocks by identifying their weathering trends and thereby enabling a backward estimate of the composition of the unweathered source rock.     

江西黄连坑锰矿成矿特征及成因探讨

江西省武宁县境内分布了众多的小型铁、锰矿床(点),前人称之为“武宁式”铁矿。通过对黄连坑锰矿区地层的含矿性、控矿构造及容矿构造、矿体特征、矿石类型等地质特征的论述,认为震旦系地层是“初始矿源层”,滑脱构造对成矿物质起迁移、富集及容矿作用。并与萍—乐断陷盆地内的沉积加热液叠改型乐华锰矿床进行了对比分析,二者在矿体规模、矿石类型、锰品位等方面存在显著差别,应属不同成因类型的矿床。初步认为黄连坑锰矿床属构造控矿、风化淋滤成因。     

邻二氮菲分光光度法测定碳酸盐相中微量亚铁

建立了在超声波作用下，HAc—NaAc缓冲溶液和邻二氮菲选择性溶解分光光度法直接测定碳酸盐相中微量亚铁的方法。结果表明，在适宜条件下，含2g／L邻二氮菲的HAc—NaAc缓冲溶液（pH3）可选择性溶解并络合碳酸盐相中的Fe（Ⅱ），通过加入EDTA溶液有效地抑制了Fe（Ⅲ）的光还原作用。常见含铁矿物如赤铁矿、磁铁矿及硅酸盐等均对碳酸盐中Fe（Ⅱ）的测定无干扰。样品经30％的H2O2浸泡、过滤、晾干预处理，能有效地消除黄铁矿和有机物的干扰。方法用于石灰岩国家一级标准物质的测定，结果与标准值相符。     

Post-Palaeozoic evolution of weathered landsurfaces in Uganda by tectonically controlled deep weathering and stripping

A model for the evolution of weathered landsurfaces in Uganda is developed using available geotectonic, climatic, sedimentological and chronological data. The model demonstrates the pivotal role of tectonic uplift in inducing cycles of stripping, and tectonic quiescence for cycles of deep weathering. It is able to account for the development of key landforms, such as inselbergs and duricrust-capped plateaux, which previous hypotheses of landscape evolution that are based on climatic or eustatic controls are unable to explain. Development of the Ugandan landscape is traced back to the Permian. Following late Palaeozoic glaciation, a trend towards warmer and more humid climates through the Mesozoic enabled deep weathering of the Jurassic/mid-Cretaceous surface in Uganda during a period of prolonged tectonic quiescence. Uplift associated with the opening South Atlantic Ocean terminated this cycle and instigated a cycle of stripping between the mid-Cretaceous and early Miocene. Deep weathering on the succeeding Miocene to recent (African) surface has occurred from Miocene to present but has been interrupted in the areas adjacent to the western rift where development of a new drainage base level has prompted cycles of stripping in the Miocene and Pleistocene.     

Silicate and carbonate weathering in the drainage basins of the Ganga-Ghaghara-Indus head waters: Contributions to major ion and Sr isotope geochemistry

The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ∼ 77% (Na + K) and ∼ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ∼40% and in most cases it exceeds the carbonate Sr. The observations that (i) the⁸⁷Sr/⁸⁶Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation between⁸⁷Sr/⁸⁶Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The generally low⁸⁷Sr/⁸⁶Sr (< 0.720) and Sr/Ca (∼ 0.2 nM/ μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and⁸⁷Sr/⁸⁶Sr in the headwaters on a basin-wide scale, however, the high⁸⁷Sr/⁸⁶Sr (∼ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of⁸⁷Sr/⁸⁶Sr and Ca/Sr data of the source waters show that they diverge from a low⁸⁷Sr/⁸⁶Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated CO₂ consumption rates and their global significance.     

Bedrock freeze-thaw weathering regime in an alpine environment,colorado front range

Reduced major axis analysis is used to describe monthly temperature averages for daily maxima, minima, means and ranges at a sequence of bedrock microenvironments in the alpine zone of the Colorado Front Range. Seven thermistors buried at 1 cm in bedrock provide comparative data on easterly, southerly and westerly aspects, and also upon the impact of snow accumulation (?0.5m to ≥4.0m deep) against an east-facing rock wall. Intersite temperatures commonly differ by less than 5°C and, rarely, by more than 10°C. The freezing intensity of freeze-thaw cycles occurring within the confines of a seasonal snow patch rarely dropped to ?5°C, while at snowfree, vertical faces freezing dropped to ?5°C quite commonly. Comparison with laboratory established criteria for effective freeze-thaw weathering (abundant moisture and freezing to at least ?5°C) suggests that moisture rich microsites lack adequate freezing intensity, while adequately frozen sites lack moisture. Available data suggest that the overlap between freeze-thaw and hydration weathering requires careful re-evaluation.