四川九寨沟水体景观成因与发展演化趋势分析

探讨了九寨沟典型水体景观的成因机制,并对其演化趋势进行了预测分析,认为九寨沟水体景观相对稳定;但其个别影响因子处于相对活动状态,高山高寒岩溶作用及重力作用将会导致以泉、湖、瀑等为主体的水体景观的消亡与再生。     

舟曲三眼峪沟泥石流松散物质补给特征与稳定性探讨

基于对舟曲三眼峪沟流域固体松散物质的调查勘测,阐述该沟"8.8"特大泥石流形成过程中松散物质的补给特征,对沟内固体松散物质的稳定性及可转化数量进行分析评价。分析认为,三眼峪沟泥石流的形成具有以主沟冲蚀再搬运为主的显著特点,以崩塌及沟床松散物质补给为主。沟内极不稳定易转化为泥石流的固体松散物质540×104m3,不稳定的松散物质1 730×104m3,较不稳定的423×104m3,稳定而难以转化的松散物质1 450×104m3。泥石流的主要补给沟段集中于大眼峪沟的竹塔沟—罐子坪沟段、大峪口段,小眼峪沟的滴水崖—峪支沟沟段、小峪口段,控制了上述沟段固体松散物质,将有效控制该沟泥石流的形成。     

新疆伊犁河谷夏季降水日变化特征

基于2007-2011年伊犁河谷4个气象站逐时降水资料, 分析了伊犁河谷夏季降水日变化特征. 结果表明: 降水量最大值出现在北京时间22:00, 最小值出现在13:00, 其中, 降水量高值区主要集中在21:00至次日08:00. 一天中最易发生降水的时间为23:00至次日10:00, 03:00是降水频数最多的时刻, 16:00则发生频数最少. 降水强度最高值出现在16:00, 最低值出现在13:00. 降水主要以短持续时间的降水为主, 持续1 h的次数最多, 持续2 h的降水量最多, 对总降水量的贡献也最大, 贡献率最小的为持续14 h的降水事件. 伊犁河谷夏季的降水主要发生在夜间, 且以短时间的降水为主.     

青海化隆县独特的峡谷丹山地貌景观及其科学价值

“峡谷丹山”是内外地质营力共同作用而形成的多成因地貌组合,不仅具有非常高的景观价值,而且具有重要的科学价值。青海省化隆县拉木峡为黄河一级支流,发育典型的角度不整合和丹霞地貌,因而成为青藏高原隆升和黄河形成演化研究的良好区域。以该地貌组合类型的形态特征、成因机制及形成年代研究为目标,采用野外地质调查等方法,对组成峡谷的岩石及地层、构造背景开展研究,系统总结与分析拉木峡地貌景观的形成动力和演化过程,进而探讨其对青藏高原隆升及黄河演化的指示意义。拉木峡由约900 Ma前的新元古代化隆岩群片麻岩及角度不整合于其上的约54 Ma前的新生代古近纪西宁群红色砂砾岩层组成。约8 Ma前青藏高原快速隆升,丹霞地貌雏形开始形成。自1.1 Ma黄河贯通化隆盆地后,受拉木峡流水强烈侵蚀下切和重力崩塌的持续作用,发育“峡谷丹山”地貌组合景观。峡谷险峻幽深、丹霞栩栩如生,成为极具典型性、独特性和稀有性的国家级地质遗迹,对青藏高原隆升、黄河形成演化和新生代气候环境的研究具有重要价值。     

Further evidence of marine Cenomanian,lower Turonian and Maastrichtian in the Upper Benue basin of Nigeria (West Africa)

Field investigations in the Upper Benue rift basin of Nigeria highlight the necessity for a review of the Cenomanian-Turonian biostratigraphy of the area. Three ammonite zones of Vascoceras bulbosum (Reyment), Paravascoceras costatum (Reyment) and Pseudotissotia (Bauchioceras) nigeriensis (Woods) have been recognized at sections exposed at the quarry of the cement factory near the village of Ashaka. The sediments within the Vascoceras bulbosum zone also contain specimens of Exogyra olisiponensis (Sharpe), Kanabiceras septemseriatum (Cragin) and Epengonoceras dumbli (Cragin). These are all indicative of an uppermost Cenomanian age and reveal that at least the lower portion of the Gongila Formation as exposed at Ashaka is faunistically equivalent to the Pindiga Formation as exposed in the type section at Pindiga.The shales in the Gombe region in which Libycoceras ismaeli (Zittel) was found are probably part of the Maastrichtian Fika shales outcropping extensively in the northern part of the area. The discovery of Cyclolithes nov. sp. within Gombe sandstones at Dakiti near Kumo confirm that they are at least partially marine in origin and Maastrichtian in age. This could be considered as an additional evidence in favour of the presence of marine waters in the Upper Benue during the last part of the Cretaceous.     

Engineering geologic and geotechnical analysis of paleoseismic shaking using liquefaction effects: field examples

The greatest impediments to the widespread acceptance of back-calculated ground motion characteristics from paleoliquefaction studies typically stem from three uncertainties: (1) the significance of changes in the geotechnical properties of post-liquefied sediments (e.g., “aging” and density changes), (2) the selection of appropriate geotechnical soil indices from individual paleoliquefaction sites, and (3) the methodology for integration of back-calculated results of strength of shaking from individual paleoliquefaction sites into a regional assessment of paleoseismic strength of shaking. Presented herein are two case studies that illustrate the methods outlined by Olson et al. [Engineering Geology, this issue] for addressing these uncertainties.

The first case study is for a site near Memphis, Tennessee, wherein cone penetration test data from side-by-side locations, one of liquefaction and the other of no liquefaction, are used to readily discern that the influence of post-liquefaction “aging” and density changes on the measured in situ soil indices is minimal. In the second case study, 12 sites that are at scattered locations in the Wabash Valley and that exhibit paleoliquefaction features are analyzed. The features are first provisionally attributed to the Vincennes Earthquake, which occurred around 6100 years BP, and are used to illustrate our proposed approach for selecting representative soil indices of the liquefied sediments. These indices are used in back-calculating the strength of shaking at the individual sites, the results from which are then incorporated into a regional assessment of the moment magnitude, M, of the Vincennes Earthquake. The regional assessment validated the provisional assumption that the paleoliquefaction features at the scattered sites were induced by the Vincennes Earthquake, in the main, which was determined to have M7.5. The uncertainties and assumptions used in the assessment are discussed in detail.     

宁芜盆地闪长玢岩的形成时代及对成矿的指示意义

宁芜盆地是长江中下游成矿带的重要组成部分和玢岩型铁矿床的主要产地,盆地内广泛产出闪长玢岩,这类岩体与铁矿床的形成关系密切,但其精确的成岩成矿时代及其形成构造背景的研究仍十分薄弱。本次工作在详细野外地质工作的基础上,系统开展了盆地内与铁矿床有关的7个闪长玢岩体的年代学研究,通过对闪长玢岩锆石LA-ICP-MS同位素定年方法,确定了盆地内主要闪长玢岩体,凹山岩体、陶村岩体、和尚桥岩体、东山岩体、白象山岩体、和睦山岩体和姑山岩体的成岩时代分别为130.2±2.0Ma、130.7±1.8Ma、131.1±1.5Ma、131.1±3.1Ma、130.0±1.4Ma、131.1±1.9Ma和129.2±1.7Ma。定年结果表明盆地内闪长玢岩成岩年龄均为130Ma左右,其成岩年龄可以近似代表铁矿床的成矿年龄。长江中下游地区存在145～136Ma、135～127Ma、126～123Ma三期成岩(成矿)作用,宁芜盆地内闪长玢岩是第二期岩浆活动的产物,其形成时代明显晚于长江中下游成矿带断隆区内与斑岩型-矽卡岩型铜(铁)、金矿床有关的高钾钙碱性岩体。宁芜盆地内闪长玢岩及玢岩型铁矿床形成于区域岩石圈伸展构造环境。     

长江中下游地区深部地球物理调查成果及对地质构造与成矿预测的新认识

在长江中下游地区先后做了八条大地电磁测深剖面,取得了较系统的深部地质结构的实际资料,对一些重要断裂及构造分区提出了新的认识,并用板块观点对板块的拼合、缝合线、洋壳俯冲等问题进行了探讨,提出了区域成矿预测意见,指出了一些新的成矿远景区。     

Morphology and Chemistry of Placer Gold from Attappadi Valley, Southern India

We report the morphological, textural and chemical characteristics of gold grains in stream gravels from the Siruvani River in Attappadi Valley, southern India. The placer gold deposits contain both primary grains with jagged grain contours and secondary grains with smooth grain margins. The primary and secondary gold grains are also distinguished by marked contrast in microtextures with the latter displaying a range of corrosion textures including striations, etch pits and chemical corrosion cavities that coalesce to form honey-comb patterns. Some of these cavities are filled with fine clay derived from lateritic weathering front. While the primary grains are characterized by high silver content (up to 35.77 wt.%) with marginal overgrowths of high purity gold, the secondary grains show exceedingly high fineness (1000 Au/Au+Ag) levels (up to 984) with no marked compositional variation indicating selective extraction of Ag and/or reprecipitation of Au. From morphological and chemical characteristics, we propose that the high purity gold grains were not derived directly from primary sources, but underwent chemical refinement in the weathering front before they were transferred to the fluvial systems. Our findings have important implications for gold exploration in the Attappadi Valley.     

长江中下游成矿带宣城矿集区重磁场特征与找矿启示

宣城矿集区位于长江中下游成矿带东段的安徽省南陵-宣城盆地,是长江中下游成矿带新一轮找矿工作的重点勘查区。为查明成矿深部背景,发掘成矿潜力,亟待了解深部结构特征。本文使用地面测量1：25 000重力和1：10 000磁力数据,通过重磁场分离和密度、磁化强度三维反演,结合岩矿石的密度、磁化率等物性测量结果,以及区域地质和矿床资料,以期揭示深部构造及与成矿关系。研究结果表明,新河庄背斜、陈家湾向斜、马山埠背斜和昆山向斜等褶皱构造横向上剩余密度正负差异显著呈相间分布的深部特征,垂向上褶皱构造的最大深度约为2 km;在褶皱构造下方分布北西浅、往南东逐渐加深的低密度、低磁性层,推测可能为研究区内茅山-九连山推覆构造的滑脱面。马山埠背斜轴部花岗闪长斑岩附近的重高磁高组合异常是铜山-荞麦山矽卡岩型矿床的找矿线索,然而成矿岩体规模较小,深部磁化强度弱,马山埠背斜附近深部找矿潜力有限。茶亭矿区呈重高磁高组合异常特征,同时成矿的石英闪长玢岩岩体与围岩灰岩、上覆火山岩磁性差异明显,这些特征是重要的找矿线索。文章研究结果证实,茶亭下方-400~-1900 m磁化强度正异常区域对应茶亭铜金矿床,这为在研究区其他地点深部进一步寻找斑岩型、隐爆角砾岩型铜金矿床提供了地球物理依据。