Late-glacial and Holocene sedimentation and fluctuations of river water level in the Caquetá River area (Colombian Amazonia)

The sequence of Late-glacial and Holocene alluvial sedimentation in the middle Caquetá River Basin of Colombian Amazonia is described, based on the study of the sediments and palynology of several river bank sections and on 30 radiocarbon dates. An early Late-glacial sedimentation cycle is recognised, followed by a minor late Late-glacial erosion phase. The Holocene valley fill consists of grey clays (often present in the lower part of the sections) deposited in open water and silty clays often with faint yellow mottling, deposited under a regime of seasonal flooding. The base of the Holocene sections is formed by sands, where exposed. In two places the transition of sand to open-water grey clay was dated around 10 000 yr BP and there is a suggestion that open water may have been more common at the beginning of the Holocene than later, when sedimentation by seasonal flooding became important. In many places much of the earlier Holocene sediments may have been removed by erosion and replaced by younger sediments, by a process of lateral aggradation. A considerable part of the present valley fill is younger than ca. 3500 yr. However, in several places older Holocene sediments are found, apparently only little affected by later erosion, lying below younger varzea silty clays. During the Holocene more organic sediments were formed in periods with reduced river discharge, related to drier climates in the Andes and possibly in Amazonia. These dry periods, deduced from data in the Caquetá River area, correspond well with dry phases in other parts of northwestern South America (e.g. between approximately 2700-1900 yr BP and approximately 3200-3800 yr BP). Rates of average net sedimentation, calculated from dated sections that apparently lack major hiatuses caused by erosion, were high in the lower Holocene, low during the middle Holocene and increase again in the upper Holocene. Levee deposits became coarser and the high river level of the Caquetá increased during the late Holocene. These phenomena may be explained by the increasing influence of man on the vegetation cover in the Andean headwater areas and possibly also in the Amazonian catchment area of the Caquetá River.     

Multi-designated geoparks face challenges in China’s heritage conservation

The author selects a thorny issue of doubled designations that bother pol-icy-makers, professional planners and managers in the field of conservation for protected areas. The analyzed case study areas cover 5 National Geoparks of China in Sichuan Prov-ince, 18 UNESCO Global Geoparks in China, and 219 World Heritage sites in China and 8 European nations. Through analysis and synthesis, the author concludes that doubled des-ignation, which also leads to unbalanced park distribution, is particularly prevalent in China other than in Europe. Therefore, on-site management agencies and related governments in China should carefully translate the doubled designations into responsibilities and harmoni-ous partnerships between all stakeholders in order to enhance the management effectiveness and avoid paper park phenomenon.     

Investigating the impact of artificial environmental watering on a semi-arid,highly saline floodplain using electromagnetics and surface nuclear magnetic resonance

Floodplains have ecological and cultural significance and need to be managed properly. However, floodplains along the River Murray in South Australia are showing a substantial vegetation health decline due to increased salinization. To improve floodplain health, water resource managers are experimenting with the delivery of fresh water to the high priority floodplains. However, the salinity impact of watering on the shallow, saline groundwater is not well understood due to the presence of a spatially variable and impermeable surface clay layer. This study uses time-domain electromagnetics (EM) and surface nuclear magnetic resonance (NMR) to assess the impact of watering on groundwater salinity in a South Australian River Murray floodplain. We examined the changes in bulk electrical conductivity (EC) from time-domain EM data collected at five sites before and after a watering event. Only one site showed a bulk EC reduction of up to 5,200 μS/cm, suggesting groundwater was freshened, whilst the remaining sites showed little change in bulk EC. Our results suggest the salinity impact of watering is highly localized and heterogeneous. For ecological management purposes, it is also desirable to estimate groundwater EC after watering. This study presents a method to estimate groundwater EC in a highly conductive environment by coupling EM with surface NMR. We also extended the analysis to an airborne-EM survey to derive spatial distribution of groundwater EC, which provides additional insights into the floodplain processes and shows an overall good agreement with field observations. This study demonstrates the potential benefits of using geophysics to investigate floodplain dynamics. The methodology developed in this study is useful for first-pass assessments of groundwater quality in a non-invasive manner, which is transferrable to many other fresh or saline groundwater systems, especially in ecologically sensitive areas where traditional hydrogeological techniques may be unsuitable due to the potential disturbance of local ecosystems.     

热水沉积岩岩石学特征:以内蒙古二连盆地白音查干凹陷下白垩统腾格尔组为例*

内蒙古二连盆地白音查干凹陷下白垩统腾格尔组—都红木组发育1套热水沉积岩。通过岩心观察、显微镜下鉴定、扫描电镜、全岩X衍射和电子探针分析等手段,对研究区热水沉积岩的矿物组成、结构与构造特征、矿物组合关系进行系统研究,在此基础上进行岩石分类与命名。研究表明: 研究区热水沉积岩在岩心上表现为深灰色、灰色和灰褐色;其矿物组分复杂多样,以白云石和沸石(钠沸石和方沸石)为主要造岩矿物,黄铁矿、菱镁矿、菱铁矿、水镁铁石及重晶石为次要矿物,混有黏土矿物、石英、钾长石和斜长石等泥质陆源碎屑;热水沉积岩结构构造特征复杂多样,根据结构特征可以划分为热水内碎屑结构、泥晶结构和团块结构3种,依据构造特征可以划分为纹层状和条带状构造、网脉状构造、同生变形构造、角砾状构造、蝌蚪状构造、星散状构造、块状构造和韵律性构造8种构造类型;以主要的热水沉积矿物白云石、沸石及陆源的泥质物(黏土矿物、石英及长石)为三端元,以90%、75%、50%、25%、10%为界限,将研究区热水沉积岩划分为泥质白云岩、含沸石白云质泥岩及沸石岩,前两者是研究区的主要岩石类型。本区热水沉积岩岩石学特征的研究可为其他地区的热水沉积岩及其沉积模式研究奠定基础,可以进一步丰富现今的沉积学理论。     

长江漫滩沉积特征与地震效应的工程研究

基于多年大量的长江漫滩勘察实际资料，结合地势、地貌和沉积特征，从空间分布、物理力学性质等方面，系统研究总结了岩土工程地质特性，分析了场地地震效应，并从场地建筑持力层的选择、地摹的处理和基础类型的确定、基坑开挖与支护等方面进行了工程对策分析讨论，以促进城市规划和建设中地质环境的充分合理开发利用。     

Crevasse splays from the rapidly aggrading, sand-bed, braided Niobrara River, Nebraska: effect of base-level rise

Base-level rise of ≈2·35 m on the Niobrara River has resulted in aggradation of the channel belt and a recent avulsion. Overbank areas have become flooded by rising groundwaters, and more than eight crevasse splays have formed between 1993 and 1997. Two crevasse splays, situated on the west and east sides of the Niobrara, have been studied using ground-penetrating radar (GPR), shallow boreholes and topographic surveys. The vibracores and GPR profiles provide a nearly three-dimensional view of the architecture of crevasse splay deposits. The east splay was initiated in the winter of 1993/94 and has expanded to cover an area ≈200 m by 1000 m, with sediment up to 2·5 m thick. The west splay, which was initiated by the opening of a crevasse channel through a levee in the autumn of 1995, covers an area ≈150 m by 250 m, with up to 1·2 m of sand deposited in a single year. The Niobrara splays are sand dominated and characterized by bedload deposition within channels, 5–30 m wide and 0·5–2 m deep, with the development of slipfaces where splays prograde into standing bodies of water. Sedimentary structures in cores include horizontal lamination, ripple lamination and sets of cross-stratification. There is a slight tendency for splays to coarsen up, but individual beds within the splays often fine up. The abundance of crevasse splays on the Niobrara River contrasts with other braided river floodplains. In the Niobrara, crevasse splay formation followed aggradation within the channel belt, which occurred in response to base-level rise. The link between crevasse splays, channel aggradation and base-level rise has important implications for the interpretation of ancient braided river and floodplain sequences. It is suggested that crevasse splay deposits should be an important component of aggrading fluvial sediments and, hence, should be preserved within the rock record. In this case, the aggradation and crevassing have been tied to a rise in base-level elevation, and it is suggested that similar deposits should be preserved where braided rivers are affected by base-level rise, for instance during transgression and filling of palaeovalleys.     

Vegetation disturbances and flood energy during an extreme flood on a sub-tropical river

The advent of 2D hydraulic modelling has improved our understanding of flood hydraulics, thresholds, and dynamic effects on floodplain geomorphology and riparian vegetation at the morphological-unit scale. Hydraulic concepts of bed shear stress, stream power maxima, and energy (cumulative stream power) have been used to characterize floods and define their geomorphic effectiveness. These hydraulic concepts were developed in the context of reach-averaged, 1D hydraulic analyses, but their application to 2D model results is problematic due to differences in the treatment of energy losses in 1D and 2D analyses. Here we present methods for estimating total and boundary resistance from 2D modelling of an extreme flood on a subtropical river. Hydraulic model results are correlated with observations of the flood impacts on floodplain geomorphology and the riparian vegetation to identify thresholds and compute variants of flood energy. Comparison of LiDAR data in 2011 and 2014 shows that the 2011 flood produced 2–4 m of erosion on floodplain bars that were previously forested or grass-covered. Deposition on flood levees, dunes, and chute bars was up to 3.4 m thick. Various hydraulic metrics were trialled as candidates for thresholds of vegetation disturbance. The accuracy of thresholds using metrics extracted at the flood peak (i.e. boundary resistance and stream power maxima) was similar to that using energy as a threshold. Disturbance to forest and grass on vegetated bars was associated with stream powers of >834 W/m² and unit flows of >26 m²/s, respectively. Correlation of the hydraulic metrics with erosion and deposition depths showed no substantial improvement in using flood energy compared to metrics extracted at the flood peak for describing erosion and deposition. The extent of vegetation disturbances and morphological adjustments was limited for this extreme flood, and further 2D studies are needed to compare disturbance thresholds across different environments.     

The impact of single natural vegetation elements on flow characteristics

In this experimental study, measurements were conducted to explore the impacts of different forms of individual natural vegetative elements within the flow domain on velocity and turbulence characteristics. All the experiments were performed in a flume measuring 26 m in length, 0·98 m in width and 0·85 m in depth, and real tree saplings were utilized to represent the vegetation element. In order to analyse this commonly observed nature phenomenon in floodplains, trees with wide trunks were classified into three groups on the basis of their volume versus height relation. Throughout the velocity measurements three acoustic Doppler velocimeters were employed. Time‐averaged velocity, streamwise and vertical turbulence intensities and turbulent kinetic energy parameters were examined. Additionally, a formulation that gives the velocity profile at a certain distance downstream of vegetation was introduced and the validity of the proposed formulation was checked with experimental data. It is seen that despite their porous structures, the presence of vegetation considerably disturbs the flow field and dissipates a remarkable amount of energy by turbulence. Copyright © 2008 John Wiley & Sons, Ltd.     

SOIL GEOMORPHOLOGY AS AN INTEGRATIVE APPROACH IN PHYSICAL GEOGRAPHY: A REVIEW OF FOUR RECENT BOOKS

Relict barrier beaches occur around the margins of many former pluvial lakes in the California desert. In common with modern barrier beaches along ocean coasts, these relict barriers possess geometric and compositional properties indicative of beach-forming processes, notably waves and currents. Because lake waves are a direct response to wind action, past wave climates may in turn be used to infer paleowind regimes. Where beaches of known age occur, wave climates and wind regimes may be invoked for specific time intervals. Using barrier beaches from former Lake Mojave and Lake Manly, this paper explores the extent to which the above theory may be applied to invoke past processes and wind regimes. The data indicate beach emplacement mainly by wave-related accretion and overwash generated by winds similar in direction to those of today, doubtless influenced then as now by topographic corridors. However, textural properties, predicted wave heights and entrainment velocities indicate that sustained southerly winds were probably twice as strong (by a factor ranging from 1.2 to 2.2 or more) and probably more persistent than today, at least towards the close of pluvial conditions. [Key words: barrier beaches, wave climate, paleoclimate, paleowinds, pluvial lakes, Quaternary, California desert.]     

THE MOST PERFECT EXAMPLE OF AN ALIEN ECOLOGY EVER CONSTRUCTED

Much science-fiction literature is based upon the creation and exploration of alternative environments. Stanley G. Weinbaum's short story, “Parasite Planet,”introduces an unusual environment that may be wrong in the light of recent evidence on Venus but still stimulates the geographical imagination.