Sand intraclasts within a diamicton mélange,southern Niagara Peninsula,Ontario, Canada

Sand intraclasts found within diamicton units along the north shore of Lake Erie in the Mohawk Bay area of the Niagara Peninsula would appear to be part of a ‘block-in-matrix’ mélange. The intraclasts are undeformed and many exhibit primary bedding structures. Numerous intraclasts have been rotated and/or tilted and are, in general, subrounded in outline. Examination of the surrounding diamicton reveals that the diamicton clast fabrics exhibit a wide scatter and are not characteristic of any known till clast fabric. Around each intraclast exists an aureole of brecciated diamicton. Other evidence in the form of macro- and microshear structures, and banding within the diamicton indicate that the diamicton has been subject to high strain. Interpretation of the sand intraclasts seems to be intrinsically linked to the origin of the diamicton and together linked to the origin of the mélange. Various hypotheses are suggested separately for the sand intraclasts, diamicton and mélange. A subglacial deformable bed hypothesis is advanced as the most acceptable explanation for the complete sediment sequence in which diamicton and frozen sand intraclasts, the latter mobilised from the substrate, are moved as a mélange below an active fast-moving ice mass. Several implications from this study emerge with regard to glacial sedimentology and stratigraphic interpretations.     

Middle to Late Weichselian Norwegian Channel Ice Stream deposits and morphology on Jæren, southwestern Norway and the eastern North Sea area

Glacial lineations on a bank area and a coastal lowland, both bordering the Norwegian Channel, are studied with regard to morphology and distribution by means of side-scan sonar data, detailed digital maps and fieldwork. Their genesis and age are further elucidated through stratigraphic and sedimentologic information from excavations in one typical coast-parallel drumlin. Four excavated sections revealed four lithologic units: Prodeltaic glaciomarine sand, glaciofluvial gravel, glaciomarine diamicton and deformation till. After Middle Weichselian delta progradation, glaciomarine diamicton was deposited and later subglacially reworked by a northwards flowing glacier. The two upper diamictons form the main volume of the ridge, which is interpreted as a drumlin, and imply a reinterpretation of the Jæren part of the so-called Lista moraine. Preconsolidation of glaciomarine diamicton suggests a maximum ice thickness of 500 m during drumlin formation, indicating an ice surface slope of 1 m/km. The occurrence of sediments that provided low basal shear stresses, and the orientation of drumlins and megaflutes indicating ice confluence both point to high glacier flow velocities and suggest that an ice stream, rather than a slower moving part of the ice sheet, occupied the Norwegian Channel during the Late Weichselian maximum. Deformation till overlying, more or less, undeformed glaciomarine diamicton suggests that high glacier velocities during periods of low driving stresses were possible due to a subglacial deformable layer.     

深部膨胀性黏土层冻结温度场的分布与冻胀力形成规律

防止冻结管断裂是深部膨胀性黏土层在冻结壁形成过程中的一项亟待解决的课题。针对淮南矿区某矿副井深部膨胀性黏土层, 通过热力耦合计算分析, 研究了其冻结温度场分布与冻胀力形成规律。结果表明： 冻结152天、 236天时, 黏土层冻结壁平均温度分别为-14.42 ℃、 -16.58 ℃, 细砂层冻结壁平均温度分别为-15.86 ℃、 -17.32 ℃, 黏土层冻结壁平均温度比同时期细砂层高1.44 ℃、 0.74 ℃。黏土层冻结壁平均厚度分别为8.92 m、 10.25 m, 细砂层冻结壁平均厚度分别为9.54 m、 10.77 m, 黏土层冻结壁平均厚度比同时期细砂层小0.62 m、 0.56 m。细砂较膨胀性黏土易于冻结。冻结90天时, 黏土层外、 中、 内圈三圈冻结管平均冻胀力约为同时期细砂层的1.1倍。冻结151天时, 黏土层三圈冻结管围成的冻结壁内平均冻胀力均达到初始地应力的81.1%, 是同时期细砂层的1.16倍。冻结236天时, 细砂层内圈管的冻胀力为3.91 MPa, 比中圈管3.72 MPa大了5.11%, 而黏土层内圈管的冻胀力为4.81 MPa, 比中圈管4.74 MPa大了1.48%。黏土层三圈冻结管围成的冻结壁内平均冻胀力均达到初始地应力的88.6%, 是同时期细砂层的1.28倍。深部膨胀性黏土层及与细砂层界面处冻胀力均存在显著的不均匀性, 最大冻胀力的主要位置与实际工程中掘进时的断管处基本对应, 不均匀冻胀力是造成冻结管断裂的重要原因。     

Glacial deposits at the Boyne Bay Limestone Quarry,Portsoy, and their place in the late Pleistocene history of northeast Scotland

The glacial deposits at the Boyne Bay Limestone Quarry near Portsoy, a key Quaternary Site of Special Scientific Interest, comprise (i) a sandy, partly weathered diamicton (Craig of Boyne Till Formation, CBTF) resting on decomposed bedrock, (ii) a central, variably glaciotectonised assemblage of dark clay, diamicton and sand, with rafts of sand and weathered diamicton (Whitehills Glacigenic Formation, WGF), and (iii) an upper dark sandy diamicton (Old Hythe Till Formation, OHTF). The CBTF was probably derived from the west or southwest, and the WGF from seawards. Structures within the OHTF conform to deposition by east‐ or southeast‐moving ice from the Moray Firth, but some erratics indicate derivation from the south. The CBTF is believed to pre‐date the last (lpswichian) interglacial, but the WGF and OHTF both post‐date the early Middle Devensian, and are probably of Late Devensian age. It is proposed that the OHTF was deposited by ice from inland which was directed eastwards near the coast by a vigorous glacier in the Moray Firth, and that the complex, Late Devensian glacial history of the south coast of the Moray Firth as a whole is the result of the interplay of these two contemporary ice‐masses. British Geological Survey. © NERC 2000.     

Late Pleistocene glaciolacustrine sedimentation and paleogeography of southeastern Michigan,USA

The geomorphic, stratigraphic and sedimentological characteristics of glaciolacustrine sediments in the metropolitan Detroit, Michigan area were studied to determine environments of deposition and make paleogeographic reconstructions. Nine lithofacies were identified and paleoenvironments interpreted based on their morphostratigraphic relationships with relict landforms. The sediments studied are found southeast of the Defiance and Birmingham moraines lying beneath a lowland characterized by a low morainal swell (Detroit moraine) and a series of lacustrine terraces that descend progressively in elevation southeastward. The glaciolacustrine sediments were deposited approximately 14.3–12.4 kA BP during the Port Bruce and Port Huron glacial phases of late Wisconsinan time, and are related to proglacial paleolakes Maumee, Arkona, Whittlesey, Warren, Wayne, Grassmere, Lundy and Rouge. The glaciolacustrine section is typically 2–4 m thick and consists of a basal unit of wavy-bedded clayey diamicton overlain by a surficial deposit of stratified and cross-stratified sand and gravel. The basal unit is comprised of subaqueous debris flow deposits that accumulated as subaqueous moraine in paleolake Maumee along the retreating front of the Huron lobe. The surficial deposits of sand and gravel were formed by traction, resulting from lacustrine wave activity and fluvial processes, in lakebed plain, beach ridge and deltaic depositional settings. Much of the lake-margin sand and gravel was derived from clayey diamicton by lacustrine wave action and winnowing, and that associated with paleolakes of the Port Huron phase is largely reworked Port Bruce sediment. Paleogeographic reconstructions show that the Defiance, Birmingham and Detroit moraines, Defiance and Rochester channels, and the Rochester delta, were deposited penecontemporaneously as paleolake Maumee expanded northward across the map area. A unique type of wavy bedform is characteristic of clayey diamicton deposited by subaqueous mass flow in the study area that is useful for differentiating sediment: 1) deposited by mass flow in subaqueous vs. subaerial settings, and 2) deposited by subaqueous mass flow vs. basal till. These bedforms are a useful tool for identifying subglacial meltwater deposits, and facilitate the mapping and correlation of glacial sediments based on till sheets. The map area provides a continental record of ice sheet dynamics along the southern margin of the Laurentide ice sheet during Heinrich event H-1. The record reveals rapid glacial retreat (～ 0.8 km/yr) contemporaneous with the discharge of a large volume of meltwater. Evidence in the study area for subglacial meltwater is problematic, but indications that periglacial conditions persisted in the map area until ～ 12.7 kA BP, and extended for 200 km or more south of the ice front suggest that a frozen substrate may have contributed to instability of the LIS.     

Distribution of relict permafrost features in the Pannonian Basin,Hungary

Wedge structures and involutions suggest that Late Pleistocene frozen ground, either permafrost or deep seasonal frost, extended at least as far south as latitude 47°N in central Europe (the Pannonian Basin). Optically stimulated luminescence dating of the sand infill from a number of wedges indicates that emplacement of the sand infill occurred during the Late Pleistocene (22.2–15.7 ka). This suggests that during this time the mean annual air temperature was depressed by at least ～15°C relative to the present. Either continuous or discontinuous permafrost was probably present in the north and NW of the Pannonian Basin. The subsequent thaw of frozen ground is indicated by the widespread occurrence of deformed sediments. The presence of soil (ground) wedges suggests conditions of deep seasonal frost probably existed during the period when climate ameliorated following the Last Permafrost Maximum (LPM).     

青藏粉土单向冻结冻胀率变化特性研究

开展了饱和青藏粉土在开放系统不同温度梯度条件下的单向冻结试验,对冻结过程中冻结深度和冻胀速率的发展变化过程进行了研究.结果表明：不同顶板温度下,土样均在25 h左右达到冻结稳定状态,之后冻结深度基本不再发生变化.土样冻结过程中,冻胀速率表现为快速减小、较小值保持稳定、快速增大、较大值保持稳定四个阶段.基于以上试验结果,对土样冻结过程中冻胀率的变化过程进行了研究,发现冻胀率随冻结深度的增加先减小后增加,冻胀率从减小变为增大的时刻就是冻结深度趋于稳定的时刻,而冻胀率快速增大的时刻为冻胀速率进入较大值保持阶段的时刻.冻胀率变化的内在机理为土样冻结过程中由冰透镜体分凝所导致的未冻区固结和已冻区冻胀共同作用的结果.     

Combined flow origin of edgewise intraclast conglomerates: Sellick Hill Formation (Lower Cambrian), South Australia

Unusually thick, coarse grained edgewise intraclast conglomerates occur at eight or more horizons within subtidal nodular and ribbon bedded wackestones and packstones of the Lower Cambrian Sellick Hill Formation, South Australia. The intraclast beds are flat based and laterally discontinuous, forming bar-like structures that must have exhibited bathymetric relief of as much as 1 m. The internal fabrics of these beds are variable. Thinner beds are dominated by flat-lying intraclasts; thicker beds contain both chaotic, randomly oriented, steeply inclined intraclasts and clusters of fan-shaped, vertically stacked edgewise intraclasts. The Sellick Hill Formation intraclast conglomerates are inferred to have been formed by intense, storm-generated combined flows on a broad, subtidal carbonate ramp. Superimposition of wave-induced oscillatory motions on geostrophic bottom flows during large storms generates short-lived, but exceptionally high instantaneous shear stresses in the bottom boundary layer. Entrainment of the relatively large intraclasts occurs through sliding, rather than pivoting. Edgewise fabrics are a product of asymmetric acceleration and deceleration of intraclasts during passage of waves and the chaotic nature of collisions between intraclasts moving within the boundary layer. Collisions between intraclasts impart a rotating moment, causing intraclasts to tip up during maximum fluid shear stress. Lodgement or packing of clasts in vertical or steeply inclined positions occurs within scours, where intraclasts can wedge between other vertically inclined clasts, or where intraclasts are pinned in steep orientations by collisions with shallowly inclined intraclasts. Differential erosional resistance of the intraclast deposits probably led to the development of sharp lateral changes in thickness. The Sellick Hill Formation intraclast conglomerates record erosion and reworking of subtidal, subfairweather wave base environments by exceptionally intense and presumably rare storm flows. The intraclast horizons represent a substantial loss in stratigraphic resolution due to widespread erosion of the ramp.     

Proglacial glaciotectonic deformation associated with glaciolacustrine sedimentation,Lake Pukaki,New Zealand

The Quaternary glaciogenic sediments exposed on the southwest shore of Lake Pukaki were investigated. The sections consisted of the Pukaki Diamicton, which is composed of four lithofacies: (i) homogeneous facies (PDH)—a grey matrix-supported homogeneous subglacial diamicton; (ii) coarse facies (PDC)—a very coarse matrix-supported diamicton, which was interpreted as a proximal glaciolacustrine sediment; (iii) laminated facies (PDL)—a cream coloured, fine-grained, matrix-supported diamicton, with grade laminations of silt, sand and gravel, interpreted as a more distal glaciolacustrine facies; and (iv) fine facies (PDF)—a cream coloured fine-grained, silt-rich matrix-supported diamicton, with lenses of sand and gravels, which was interpreted as the most distal glaciolacustrine facies. It is suggested that these sediments were produced by two small ice advances during a period of general retreat. Furthermore, the sections showed a combination of three types of glaciotectonic deformation; gravity tectonics, proglacial glaciotectonics and subglacial glaciotectonics. Two of the moraines showed an unusual style of glaciotectonic deformation, i.e. proglacial deformation on the proximal face and gravitational slumping on the distal face. It is suggested that this style of deformation is diagnostic of proglacial deformation into a waterbody associated with a retreating margin.     

Late Quaternary Stratigraphy, Glacial Limits, and Paleoenvironments of the Marresale Area, Western Yamal Peninsula, Russia

Stratigraphic records from coastal cliff sections near the Marresale Station on the Yamal Peninsula, Russia, yield new insight on ice-sheet dynamics and paleoenvironments for northern Eurasia. Field studies identify nine informal stratigraphic units from oldest to youngest (the Marresale formation, Labsuyakha sand, Kara diamicton, Varjakha peat and silt, Oleny sand, Baidarata sand, Betula horizon, Nenets peat, and Chum sand) that show a single glaciation and a varied terrestrial environment during the late Pleistocene. The Kara diamicton reflects regional glaciation and is associated with glaciotectonic deformation from the southwest of the underlying Labsuyakha sand and Marresale formation. Finite radiocarbon and luminescence ages of ca. 35,000 to 45,000 yr from Varjakha peat and silt that immediately overlies Kara diamicton place the glaciation >40,000 yr ago. Eolian and fluvial deposition ensued with concomitant cryogenesis between ca. 35,000 and 12,000 cal yr B.P. associated with the Oleny and the Baidarata sands. There is no geomorphic or stratigraphic evidence of coverage or proximity of the Yamal Peninsula to a Late Weichselian ice sheet. The Nenets peat accumulated over the Baidarata sand during much of the past 10,000 yr, with local additions of the eolian Chum sand starting ca. 1000 yr ago. A prominent Betula horizon at the base of the Nenets peat contains rooted birch trees ca. 10,000 to 9000 cal yr old and indicates a >200-km shift northward of the treeline from the present limits, corresponding to a 2° to 4°C summer warming across northern Eurasia.     

A Pliocene mega-tsunami deposit and associated features in the Ranquil Formation,southern Chile

An exceptionally large tsunami affected the coastline of southern Chile during the Pliocene. Its backflow eroded coarse beach and coastal dune sediments and redistributed them over the continental shelf and slope. Sandstone dykes and sills injected from the base of the resulting hyperconcentrated flow into underlying cohesive muds, assisted in plucking up large blocks of the latter and incorporating them into the flow. Locally, the rip-up intraclasts were fragmented further by smaller-scale injections to form a distinct breccia of angular to rounded mudstone clasts within a medium to coarse sandstone matrix. Sandstone sills in places mimic normal sedimentary beds, complete with structures resembling inverse gradation, planar laminae, as well as ripple and trough cross-lamination. These were probably formed by internal sediment flow and shear stress as the semi-liquefied sand was forcefully injected into cracks. In borehole cores, such sills can easily be misinterpreted as normal sedimentary beds, which can have important implications for hydrocarbon exploration.     

隧道水平冻结施工引起地表冻胀的历时预测模型

隧道水平冻结施工过程中,土体冻结引起体积膨胀,进而会在地表产生冻胀现象。实际工程一般采用多根冻结管形成冻结壁。冻结壁交圈前,地表冻胀由多个冻土柱的叠加膨胀变形引起;冻结壁交圈后,地表冻胀则由整个冻结壁的膨胀变形引起。鉴于此,考虑冻结壁的形成过程,基于随机介质理论,建立了隧道水平冻结施工引起地表冻胀位移的历时预测模型。同时对冻结外锋面半径和冻胀区域外半径这2个关键参数的取值方法进行了相关探讨。最后针对两个工程案例,采用该计算模型对地表冻胀位移进行分析,得到地表冻胀位移随时间的变化规律,并与现场实测结果相比较,验证了模型的可靠性。该模型应用于隧道水平冻结施工前、冻结期内任意时刻的地表冻胀位移预测,可为工程冻结实施方案的合理确定提供有效依据。     

封闭系统单向冻结淤泥质黏土水分迁移特性研究

高含水率饱和淤泥质软黏土在封闭系统中由冻结引起的土体内部水分迁移是影响其冻胀速率的重要因素。为揭示冷端温度对沿海软黏土水分迁移特性的影响,采用上海第四系滨海-浅海相淤泥质黏土,在-5~-20 ℃冷端温度条件下开展了封闭系统单向冻结试验,测定了试样冻胀量及沿着温度梯度方向的试样温度,得到了冻结锋面高度随冻结时间的发展规律、引起水分迁移的临界温度梯度、水分迁移入流通量及入流速率。结果表明:试样冻结锋面高度是关于冻结时间的函数,其拟合公式形如X(t)=t(at+b)-1;冻结区内温度梯度降低至临界温度梯度是水分迁移起始的判据,随着冷端温度的降低,临界温度梯度线性增大;水分入流速率随冻结时间的延长先增大后减小,水分入流通量-冻结时间曲线随冷端温度的降低由“S型”逐渐趋于线性;结合临界温度梯度-冷端温度关系式和冻结锋面高度-冻结时间拟合公式,可预测某一冷端温度条件下封闭系统单向冻结过程中试样内部水分迁移的起始时刻。以上试验结果有助于推进封闭系统单向冻结过程中高含水率软土水分迁移特性的定量研究,为沿海软土地区冻结法施工中冻胀量预警提供重要参考依据。     

竖向直排冻结条件水平冻胀力试验研究

竖向直排冻结方式常用于软弱地层中斜井掘进和深基坑止水帷幕的冻结法施工中,目前国内外对该冻结方式下水平冻胀力的分布研究鲜见报导。以某斜井冻结工程为背景,推导出温度场、应力场和水分场等相似模拟准则,设计了竖向直排冻结模型试验并细化具体试验方案,利用大型三维模拟冻结试验系统,对竖向直排冻结条件下不同深度土体的水平冻胀力分布特性进行了试验研究。结果表明,竖向直排冻结过程中埋深对水平冻胀力的影响明显,在其他条件相近的情况下同一水平面中水平冻胀力的大小与温度密切相关;冻胀力的变化率主要受冻结锋面位置的影响,距离冻结锋面较近时冻胀力的变化率逐步达到峰值;已冻土体的冻胀力随冻结锋面向外发展而趋于平稳。     

饱和软黏土中埋地管道冻结模型试验研究

为研究软土地区埋地管道在土体冻结过程中的管道受力机理, 开展了饱和软黏土中地埋管道冻结模型试验。通过人工冻结技术, 近似还原了管土受冻过程, 研究了人工冻结过程中土体温度场、 水分场、 位移场分布情况, 以及管道的力学特性。结果表明： 在冻结过程中, 土体温度场的变化直接影响着土中水分场的分布; 在冻结锋面前缘存在着剧烈的水分迁移现象, 大量的水分向冻结锋面迁移, 使得土体产生线性冻胀; 冻胀发展速率受外部荷载的直接影响; 当冻结发展到管道处时, 位于冻胀和非冻胀过渡段位置处的管身出现应力最大值。研究结果对于正冻土中管道的安全评估具有重要的意义。     

正冻土冻结缘研究现状及展望

湿土冻结过程中, 生长发育于冰透镜体与冻结锋面之间特殊的区间带称为冻结缘带。冻结缘作为温度场、 水分场和应力场三场耦合作用的结果, 是冰分凝的水源补给站, 冰水相变发生的剧烈区域以及水分迁移的必经之路, 具有重要的研究意义, 也是深入认识冻胀机理的基础。通过系统地阐述冻结缘的形成过程、 相关理论与试验、 微结构特征、 参数特征及冻结缘的模型构建等5个方面的研究进展及成果, 结合各个方向的发展趋势提出了冻结缘研究的重点, 即对冻结缘的研究应回归到试验研究, 利用新型测试技术深入对冻结缘微结构的观测, 结合物理参数及结构性参数变化构建耦合的冻结缘模型, 从而揭示其热力学机理, 为冻胀机制分析、 冻土精确预报提供理论支撑。     

饱和粉土冻结过程中的水分迁移试验研究

改变边界温度和土样高度,对饱和粉土进行了冻结试验,研究其水分迁移、水分重分布、冻胀和冰透镜体的发展规律。试验结果表明：当温度稳定时,水分持续迁移到冻结锋面附近,含水率急剧增大,易形成冰透镜体。饱和粉土冰透镜体几何形态较为规则,无枝状交叉结构,已冻土为整体状且无网状裂隙。冻结过程中存在起始冻胀时间,在起始冻胀时间之前,土中水分被排出,冻胀发生之后水分补给到冻土中,且补给水分产生的冻胀量和总冻胀量数值接近。土样高度影响水分迁移量和冻胀量,土样越高,冻胀量越小,冻土含水率增量越小,但水分增量分布区域越分散,且起始冻胀时间越长。     

Sedimentology and history of a Late Wisconsinan glacial lake, Grande Prairie, Alberta, Canada

The Grande Prairie region of northwestern Alberta was partially covered by glacial Lake Peace, which was dammed against the retreating Laurentide ice sheet. Two levels of glacial Lake Peace are identified in the study are by closely spaced groups of strandlines and minor deltas lying at 805 to 840 m a. s. l., and 655 to 710 m a.s.l. Sedimentation associated with the upper of the two lake levels is marked by rhythmites of silt and clay deposited by turbid underflow, interbedded with diamicton deposited by debris flow. Dropstones and dump structures indicate common ice-rafting. Thick sequences are only found on the axes of major valleys, where sediment gravity flows were concertrated. Thin sequences of ice proximal glaciolacustrine sediments reflect topographic setting and do not indicate a short-lived lake. Retreat of the ice front resulted in a decrease in ice-rafted material and diamicton in sediments. The fall in lake level to the second stage resulted in deposition of sequences of vaguely laminated silt and clay close to the modern Beaverlodge River. These sediments were deposited by suspension settling from interflow or overflow of the Beaverlodge River as it entered the lake. Lake sedimentation was dominated by inflow from unglaciated areas, rather than the ice front.