Microstructures in subglacial and proglacial sediments: understanding faults,folds and fabrics,and the influence of water on the style of deformation

Macroscopic field and micromorphological studies have been carried out on subglacially and proglacially deformed glacigenic sequences at a number of sites throughout Scotland, UK. Examination of microstructures (folds, faults, hydrofractures, plasmic fabrics) aided understanding of the deformation histories preserved in the sediments, but a similar range of structures were developed in both Subglacial and Proglacial settings. Discrimination between Subglacial and Proglacial deformation was only possible when micromorphological data was used in conjunction with larger-scale field observations. Variations in lithology and water content were controlling factors influencing the style and apparent intensity of deformation recorded. Changes in pore-water content and pressure during deformation can lead to liquefaction and hydrofracturing, with early-formed structures locally controlling the pattern of water escape. Liquefaction can also lead to homogenisation of the sediments and the destruction of earlier deformation structures, even at relatively low strains. Beds or zones of liquefied sand and silt may form highly ‘lubricated’ detachments within the sediment pile, resulting in a marked reduction in the amount of shear transmitted to underlying units. A multidisciplinary approach, involving sedimentological, geomorphological, stratigraphical and structural field observations, combined with micromorphological analysis, is recommended to confidently unravel the glacitectonic history and depositional environment of most deformed glacigenic sedimentary sequences.     

Are long subglacial bedforms indicative of fast ice flow?

It has been suggested that extremely long subglacial bedforms (e.g. attenuated drumlins and mega-scale glacial lineations) record former areas of fast-flowing ice and that bedform elongation ratio is a useful proxy for ice velocity. Despite the availability of much data pertaining to the measurement and analysis of subglacial bedforms, these assumptions have rarely been explicitly addressed in detail. In this paper, we demonstrate that long subglacial bedforms (length:width ratios ≥10:1) are indicative of fast ice flow. Using satellite imagery, we mapped over 8000 lineaments associated with a highly convergent flow pattern near Dubawnt Lake, District of Keewatin, Canada. This flow pattern is unusual in that it displays a large zone of convergence feeding into a main 'trunk' and then diverging towards the inferred ice margin. The 'bottleneck' pattern is taken to record an increase and subsequent decrease in ice velocity and we analysed transverse and longitudinal variations in bedform morphometry. The main trunk of the flow pattern (down-ice of the convergent zone) is characterized by mega-scale glacial lineations of great length (up to 13 km) and high elongation ratios (up to 43:1). The down-ice variations in elongation ratio reflect exactly what we would expect from a terrestrial ice stream whose velocity increases in the onset zone passes through a maximum in the main trunk and slows down as the ice diverges at the terminus. It is suggested that any unifying theory of drumlin formation must be able to account for the association between long subglacial bedforms and fast ice flow, although it is not assumed that fast ice flow always produces attenuated bedforms. A further implication of this work is that many more ice streams may be identified on the basis of attenuated subglacial bedforms, radically altering our views on the flow dynamics of former ice sheets.     

Gravelly shoreface deposits: a comparison of modern and ancient facies sequences

The morphology and dynamics of modern gravel shorefaces are poorly documented. This hinders the interpretation of possible ancient counterparts. A comparative study of a modern (Chesil Beach, England) and an ancient (Baytree Member of the Cardium Formation, Alberta) gravel shoreface shows that the two systems are very similar close to and above sea-level, with a high (about 1 m) gravel plunge step lying below plane-bedded sands and gravels of the beachface. The shoreface at Chesil Beach is dominated by asymmetrical gravel wave ripples. These are oriented offshore near the toe of the shoreface, and onshore in shallower depths. This may reflect offshore movement during storms and landward reworking during fair weather. The Baytree Member is over 12 m thick and comprises over 80% conglomerate. Conglomerate is decimetre-bedded, massive or cross-bedded, with sets over 60 cm thick produced by gravel bedforms migrating alongshore. It is interbedded with discontinuous cm- to dm-bedded sandstones which may be cross-bedded. Pebble fabric and cross-bed orientation both indicate strong alongshore sediment transport. Near the base of the section, pebble orientations suggest that gravel wave-ripples developed below the zone of strong longshore flows. Differences between these two examples may be attributed to different directions of wave approach.     

The sediment infill of subglacial meltwater channels on the West Antarctic continental shelf

Subglacial meltwater plays a significant yet poorly understood role in the dynamics of the Antarctic ice sheets. Here we present new swath bathymetry from the western Amundsen Sea Embayment, West Antarctica, showing meltwater channels eroded into acoustic basement. Their morphological characteristics and size are consistent with incision by subglacial meltwater. To understand how and when these channels formed we have investigated the infill of three channels. Diamictons deposited beneath or proximal to an expanded grounded West Antarctic Ice Sheet are present in two of the channels and these are overlain by glaciomarine sediments deposited after deglaciation. The sediment core from the third channel recovered a turbidite sequence also deposited after the last deglaciation. The presence of deformation till at one core site and the absence of typical meltwater deposits (e.g., sorted sands and gravels) in all three cores suggest that channel incision pre-dates overriding by fast flowing grounded ice during the last glacial period. Given the overall scale of the channels and their incision into bedrock, it is likely that the channels formed over multiple glaciations, possibly since the Miocene, and have been reoccupied on several occasions. This also implies that the channels have survived numerous advances and retreats of grounded ice.     

Microstructural interpretations of modern and Pleistocene subglacially deformed sediments: the relative role of parent material and subglacial processes

Analysis of till micromorphology represents a relatively new technique that has been used most frequently to infer the importance of subglacial shear in till genesis. This study aims to calibrate the technique by comparing Pleistocene tills from United Kingdom with a modern till (the UpB till) from beneath Ice Stream B, West Antarctica. Despite the fact that all of the tills examined have been interpreted as deforming‐bed deposits, the modern till has significantly less abundant and diverse microstructures than those found in the Pleistocene tills. Seventeen examined thin‐sections of the UpB till contain recognisable microstructures over only 0–30% of individual thin‐section area. The most common microstructures are: (i) birefringent clay patterns that are interpreted as shear zones and (ii) adherent matrix structures, which we interpret as uncomminuted remnants of the parent glacial/glaciomarine diamictons. Fourteen thin‐sections of the Pleistocene tills were covered by microstructures in 10–95% of their area. The Pleistocene microstructures include birefringent clays and adherent matrix structures, as in the UpB till, but also laminations and deformed pods made of chalk and sorted sediments. We conclude that the same till‐forming process, i.e. subglacial deformation, may result in distinctly different till micromorphology. This is a consequence of the fact that microstructural characteristics are strongly influenced by factors other than shear deformation. We identify three controls that may be important for forming contrasting microstructural assemblages: (i) strain magnitude, (ii) the degree of heterogeneity of parent material, and (iii) relative importance of sediment sorting by flowing water in the subglacial environment. Thus till micromorphology is sensitive to multiple factors, which with proper calibration may enhance the existing capability to interpret past subglacial conditions from microstructures. Copyright © 2001 John Wiley & Sons, Ltd.     

Moraine ridges formed from subglacial frozen-on sediment slabs and their differentiation from push moraines

Throughout the 1980s the annual cycle of ice-front activity along the stationary north margin of the ice-cap Myrdalsjökull, southern Iceland, produced a complex ridge, 4 m high, composed of imbricately stacked slabs of frozen, clast-paved lodgement till dipping up-glacier. Further observations in 1994 revealed that glaciofluvial processes and associated deposits may be involved in the final stage of ridge production depending on local climate and meltwater drainage pattern. It is concluded that at the margin of Myrdalsjökull the progressive stacking of subglacial frozen-on sediment slabs to form a moraine ridge is a fundamentally similar mechanism to that involved in the incremental double-layer model reported from Styggesdalsbreen, southern Norway. This study has also identified internal characteristics which are of potential use for distinguishing between moraine ridges formed by this mechanism and push moraines formed by proglacial thrusting.     

事件沉积:沉积物补给及相对海平面变化的作用

本文为事件沉积综述,要点如下:(1)根据河流悬移载荷和床沙载荷简要总结了不同地形和气候区的剥蚀速率;(2)不同盆地充填物中泥砂的比例;(3)控制沉积物再活化的机理。在大陆环境中,事件沉积(冲积扇上的碎屑流、扇三角洲上的崩塌作用和漫滩上的泛滥作用)的粒度和频率直接反映了物源区的侵蚀作用和剥蚀面积与堆积面积之比率。在海洋环境中,沉积物补给和近滨容纳空间的变化在很大程度上控制着地层层序的性质。在沉积物大量补给和低频率海平面变化的情况下,巨厚的体系域内事件沉积(包括风暴岩)的差异很小。随着沉积物补给量减少,事件沉积在低水位体系内不断聚集。如一些模式(差异性沉降和盆地边缘抬升)所示,相对海平面的迅速下降突出反映了海岸及海底富砂沉积物的再活化(特别是在低水位早期)和延迟河谷下切,所形成的海底扇多半以砂为主。而河流供给的大型扇则以浊积泥岩为主。在海岸抬升的地区,河谷下切的时期比低水位期长,海平面变化可导致"上升脉动"和峭壁削蚀。三级或高频率海平面变化的低水位沉积,在碳酸盐建隆边缘通常为粗粒骨屑和巨角砾岩;在混合体系内通常为硅质碎屑浊积岩。在快速闭合的前陆盆地中,高频率海平面旋回通常只影响盆地边缘两侧;而三级海平面变化直接控制靠近逆冲前缘的沉积层序和事件沉积的可能性也有限。由于沉积物大量补给,各种事件沉积(如碎屑流沉积、砂质和钙质风暴岩和浊积岩)形成的时间间隔一般为几十年、几百年,甚至数千年。在沉积物补给量小或以大规模块体流和巨浊积岩为特征的环境中,其再现的时间间隔还更长。     

Heavy metal concentrations in sediments of streams affected by a sanitary landfill: A comparison of metal enrichment in two size sediment fractions

The purpose of this study was to determine if metal concentrations are enriched in two size sediment fractions of streams that receive landfill effluent and, if so, whether there is a greater extent of metal enrichment in one of the fractions. Sediment samples were collected from three streams adjacent to a sanitary landfill. Sediments representing control for the study were also collected from a stream not influenced by the landfill. All samples were sieved and the <0.0625-mm and <0.25-mm to >0.149-mm size fractions from each sample were used in this study. The concentrations of acid-extractable Cu, Zn, Pb, and Cr for all samples were determined by atomic absorption techniques. Mean concentrations, coefficient of variation values, at test, and the variation of metal concentrations along the stream were used to analyze the data. Results indicated that Cu, Zn, Pb, and Cr concentrations were enriched in both size sediment fractions from the stream whose channel originated at the base of the landfill. Copper, Zn, and Pb concentrations were enriched in the <0.0625-mm size sediments of the stream whose channel did not intersect the landfill. Copper, Zn, Pb, and Cr concentrations appear enriched in both size sediment fractions of the third stream, which formed from the confluence of the other two streams. The extent of metal enrichment was greater in the <0.0625-mm size sediments. A decreasing trend of metal concentrations in a downstream direction was not present in the enriched sediments. This was true for each metal in both size sediment fractions.     

Danian deposits of the Crimea: Facies peculiarities and conditions of sediment accumulation

The Danian deposits of the Crimea are described. Emphasis is placed on the characteristics of the rock of a well-exposed section in the Crimean piedmont. The depositional environment and paleogeography of different sections of the Crimea are reconstructed. The sequences and the system tracts that are distinguished in the Danian deposits fix the alternation of transgressive-regressive episodes. It is revealed that the stratigraphic completeness of Danian sections and variations in their thickness are determined not only by the sea-level fluctuations, but also by the tectonic events that manifested themselves in the growth of the Simferopol’skoe rise and the formation of trough-like structures in eastern Crimea.     

Subaqueous liquefied and fluidized sediment flows and their deposits

A clear distinction must be made between liquefied and fluidized systems. In liquefied beds and flows, the solids settle downward through the fluid, displacing it upward, whereas, in fluidized beds, the fluid moves upward through the solids, which are temporarily suspended without net downward movement. Many recent references to fluidized sediment gravity flows refer, in fact, to flows of liquefied debris. Most uniformly liquefied beds of well-sorted sand- or gravel-sized sediment will resediment as simple two-layer systems. Liquefied flows can originate either by liquefaction followed by failure, as in many retrogressive flow slides, or by failure followed by liquefaction, as in the case of some slumps. Empirical and theoretical estimates of flow velocity, thickness, and travel distance suggest that natural laminar liquefied flows of fine-grained sand will generally resediment after moving a kilometre or less. Laminar flows of coarse-grained sand will resediment after moving only a few metres. Grain dispersive pressure is thought to be of little significance in the development or maintenance of liquefied flows. Many surficial submarine sand beds are apparently susceptible to liquefaction, including submarine canyon and continental rise deposits. Within submarine canyons and narrow fjords, steep slopes and channels promote the evolution of liquefied flows from slumps by liquefaction after failure and of high density turbidity currents from liquefied flows by the development of turbulence. Upon moving into the lower parts of submarine canyons or into proximal fan channels, liquefied flows will resediment and high density turbidity currents will tend to decline to flows transitional between liquefied flows and turbidity currents. The liquefied, coarser detritus within such transitional flows will be deposited while finer-grained debris will remain in suspension and continue downslope as dilute turbidity currents. Resedimentation of the liquefied portions of such flows may be responsible for the deposition of the A-subdivision of many turbidites and many thick, structureless ‘proximal turbidites’ or ‘fluxoturbidites’. Similar units can originate by liquefaction of the traction deposits of normal turbidity currents. Fluidized flows are probably uncommon, thin, and, where formed, originate through fluidization of the fine-grained tops of liquefied graded beds.     

Laurentide and Cordilleran fast ice flow: some sedimentological evidence from Wisconsinan subglacial till and its substrate

With the aid of three case studies we summarize sedimentological and structural evidence in Wisconsinan subglacial till and its substrate that can be used to infer the presence of former fast-moving ice. We compare sedimentological characteristics of till with its stone fabric, stone surface erosional features and orientations of fractures, faults and folds in till and substrate, in order to interpret the processes that formed the till. In the cases we studied, stones in tills that experienced ductile deformation (inferred to be associated with fast-moving ice) show variable alignment of latest surface striae. Moreover, striae are inconsistent with the orientations of lee ends and chattermarks on the same stones, and with the dip directions of shallow shear planes and (or) steeper tension fractures in the till matrix. These observations indicate that stones were rotating in a flowing till matrix prior to their final emplacement. Ductile shear of fine-textured, water-saturated till and Jeffrey-like rotation of stones are invoked where most of the following occur together: (i) delicate striae follow the curvature of stone surfaces; (ii) a-axis fabrics are multimodal to girdle-like; (iii) stone a axis eigenvectors, striae orientation and stone lee end alignments fall between the modes of stone fabrics; (iv) stone pavements are uneven with vertically oriented and inverted bullet-shaped stones; (v) soft sediment clasts and (or) fragile fossils are preserved.     

Quantification of turbate microstructures through a subglacial till: dimensions and characteristics

Despite extensive study and debate regarding the significance of turbate (also known as ‘rotational’) microstructures in glacially deformed sediments, characteristics regarding the dimensions of these features remain unresolved. This study presents the first explicitly quantitative measurement and analysis of turbate microstructure dimensions, and their relation to till texture through thin section analysis. Samples were taken from coarse‐resolution horizontal and vertical transects of a macroscopically homogenous subglacial till, with subset areas of each thin section (30 mm²) analysed. The frequency and apparent a‐axes and b‐axes of both coreless and cored turbate structures (and their corestones) were measured, and simple univariate statistical methods used to establish the (in‐)variability of these dimensions through the till profile. Summarizing findings, (i) the dimensions of both cored and coreless turbate populations display log‐normal distributions when all measurements are analysed together, although not all individual sample populations possess these same distributions; (ii) turbate dimension populations are inconsistent within a sample block, precluding evaluation of turbate variability through a profile from single thin sections; (iii) analysis of turbate morphology and variability provisionally indicate that the three‐dimensional structure of turbates are likely to be cylindrical or flared, while weak relationships are also observed amongst till texture, turbate dimensions and frequency.     

Till mélange at Amsdorf, central Germany: sediment erosion, transport and deposition in a complex, soft-bedded subglacial system

Glacial mélange in the open-cast mine at Amsdorf, central Germany, consists of several square meters of large, sorted sediment blocks embedded in till. The blocks are composed of largely intact to slightly deformed glaciofluvial and glaciolacustrine sand, silt and clay, initially deposited in a proglacial lake (2–3 km up-ice) and subsequently overridden by a glacier. The blocks typically have cuboid to subrounded outlines, are randomly distributed in the till, and the contacts with the surrounding till are distinctly sharp. Underneath the mélange are varved clays which exhibit strong deformations occasionally intervening with entirely undisturbed areas. It is suggested that the blocks were entrained into debris-rich basal-ice by bulk freeze-on when the glacier sole was lowered onto the bottom of an overridden lake. After entrainment the blocks were transported englacially and re-deposited (with far-traveled till matrix) as a melt-out till from stagnant ice. The glacier moved mainly by sliding enhanced by low-permeability varved clays in the substratum. The glacier is believed to have been of a polythermal type. These results show that bulk freeze-on can lead to entrainment of soft sediment blocks at least 20 m² in size, and that these blocks can be englacially transported with little or no deformation for several kilometers and more. The occurrence of deformed and undeformed clays under the till mélange indicates a possible mosaic of coupled and decoupled ice, the latter caused by a thin, transient subglacial water film separating the bed from the glacier.     

Steady-state vertical distribution of cohesive sediments in a flow

Settling velocity of diluted suspended aggregates is examined under steady-state conditions. It is shown that if the local settling velocity of the suspended mass of sediments at the bottom is gamma distributed, then, the vertical variation of the local mean settling velocity $\overline{W}$ is proportional to a power $1/r$ of the local concentration C, where r is the gamma distribution parameter. That is a consequence of the suspended-sediment sorting produced by the vertical dynamics. The parameter r characterizes the range of settling velocity values for all the aggregates simultaneously in suspension. To cite this article: M. Sánchez et al., C. R. Geoscience 337 (2005).     

中巴喀喇昆仑公路(巴基斯坦境内)河床沉积物与泥石流堆积物的关系

统计分析中巴公路沿线红其拉甫河(Khunjerab River)、洪扎河(Huza River)、吉尔吉特河(Gilgit River)、印度河(India River)共计45个河床沉积物样本,对比分析了样本的平均值、中值、分选系数、偏态和峰度等粒度参数,初步探讨了公路沿线河床沉积物粒度分布与区域内泥石流沉积物的关系,结果表明河流沉积物主要来源于泥石流补给,因而具有了类似于泥石流沉积物的分布特征。     

The compressibility of sediments and its importance on Flandrian Fenland deposits

In the Flandrian Fenland sediments of the Nar Valley, Norfolk, an upper clay and thick upper peat overlie lower clay and lower peat units thinning eastwards up the valley. Lithostratigraphic and altitudinal variations in less than thirty metres led to the application of a soil mechanics technique, the oedometer test, to the sediment units in order to estimate amounts of compression and corrections for the altitudinal displacements. The principles and assumptions behind the oedometer test are reviewed and results from compression tests on Flandrian Fenland deposits in the Nar Valley discussed in relation to interpretation of stratigraphic data, especially in terms of reconstruction of former sea levels. Limitations of the technique, particularly with reference to peats, are discussed.     

Properties of debris flow deposits and source materials compared: implications for debris flow characterization

In the Alps, debris flow deposits generally contain <5% clay‐size particles, and the role of the surface‐charged <2 μm particles is often neglected, although these particles may have a significant impact on the rheological properties of the interstitial fluid. The objective of this study was to compare debris flow deposits and parent materials from two neighbouring catchments of the Swiss Alps, with special emphasis on the colloidal constituents. The catchments are small in area (4 km²), 2·5 km long, similar in morphology, but different in geology. The average slopes are 35–40%. The catchments were monitored for debris flow events and mapped for surface aspect and erosion activity. Debris flow deposits and parent materials were sampled, the clay and silt fractions extracted and the bulk density, <2 mm fraction bulk density, particle size distribution, chemical composition, cation exchange capacity (CEC) and mineralogy analysed. The results show that the deposits are similar to the parent screes in terms of chemical composition, but differ in terms of: (i) particle size distribution; and (ii) mineralogy, reactivity and density of the <2 mm fraction. In this fraction, compared with the parent materials the deposits show dense materials enriched in coarse monocrystalline particles, of which the smallest and more reactive particles were leached. The results suggest that deposit samples should not be considered as representative of source or flow materials, particularly with respect to their physical properties.