Seismic interpretation of crustal structure in the flinders‐mt lofty ranges and gulf regions,South Australia

There appears to be little correlation of earthquake epicentres with known surface geological features in South Australia. Seismic wave travel‐time residuals are used to derive corrections for the velocity and depth parameters for the simple uniform crustal model which approximates to that in South Australia. Local studies of Moho depth in the seismic zone and analysis of travel‐time station corrections from both local earthquake and teleseismic data suggest that lateral and vertical variations in the South Australian crust are small. Data presented in this paper appear to be consistent with a plate tectonic model derived from focal mechanism studies (Stewart & Mount, 1972) for the active South Australian seismic zones.     

Assessing hydrogeologic controls on dynamic groundwater storage using long‐term instrumental records of water table levels

This study analyzes a long‐term regional compilation of water table response to climate variability based on 124 long‐term groundwater wells distributed across New England, USA, screened in a variety of geologic materials. The New England region of the USA is located in a humid‐temperature climate underlain by low‐storage‐fractured metamorphic and crystalline bedrock dissected by north–south trending valleys filled with glacial and post‐glacial valley fill sediments. Uplands are covered by thin glacial till that comprises more than 60% of the total area. Annual and multi‐annual responses of the water table to climate variability are assessed to understand how local hydraulic properties and hydrogeologic setting (located in recharge/discharge region) of the aquifer influence the hydrologic sensitivity of the aquifer system to climate variability. This study documents that upland aquifer systems dominated by thin deposits of surface till comprise ~70% of the active and dynamic storage of the region. Total aquifer storage changes of +5 to ?7 km³ occur over the region during the study interval. The storage response is dominated by thin and low permeability surficial till aquifer that fills and drains on a multi‐annual basis and serves as the main mechanism to deliver water to valley fill aquifers and underlying bedrock aquifers. Whereas the till aquifer system is traditionally neglected as an important storage reservoir, this study highlights the importance of a process‐based understanding of how different landscape hydrogeologic units contribute to the overall hydrologic response of a region.     

The use of integrated sedimentological and geophysical methods in drumlin research – a case study of Pigeon Point,Clew Bay,Northwest Ireland

Despite significant advances over the past decades, our understanding of drumlin formation and associated ice‐bed processes is still incomplete. In this paper, we present the integrated use of geomorphological, sedimentological and geophysical techniques as a powerful means to force a breakthrough towards solving the drumlin enigma. We report on investigations of the anatomy of the Pigeon Point drumlin, Clew Bay, Ireland. We found that the bulk of the landform, which displays a classical drumlin shape, consists of silty‐clayey diamicton showing evidence of deformation, hydrofracturing and comminution. The unit is interpreted as a sub‐glacial traction till/comminution till. The thin unit overlying this basal till consists of silty‐sandy diamicton, and is interpreted as a para‐glacially modified melt‐out till. The partly cemented third unit consists of stratified, massive to graded sands and gravels. Its contact with the sub‐glacial traction till consists of a series of concave shapes, which suggests that it was deposited in meltwater channels that flowed in sub‐glacial cavities and that cut laterally into the drumlin. We propose that highs in the undulating rockhead relief, as shown in the seismic profile, have provided nuclei which initiated drumlin formation. This idea is supported by the observation of local detached bedrock slabs that grade upwards into a comminution till. In the long profile, very high normalized induced polarization (IP) values form a wedge‐shape, which is interpreted as a set of conjugate thrusts, or a ‘pop‐up’ structure. The structure is positioned directly above one of the undulations in the bedrock, suggesting a direct relationship. The high values are thought to reflect the presence of pre‐existing clays, which were sheared into the till, thus forming linings in the thrust features. It is concluded that glacitectonic processes, notably differential bedrock weathering and thrusting, have played a key role in the formation of this drumlin. Copyright © 2011 John Wiley & Sons, Ltd.     

高精度磁测在阿勒泰冰碛物覆盖区地质填图中的应用

在阿尔泰高纬度冰碛物等覆盖区,通过分析岩石磁性资料,调查区岩石磁性差异明显,初步建立地质体磁异常识别标志,磁场划分为三大区,Ⅰ-1高平稳场区与黑云花岗岩体对应,Ⅰ-2高跳跃场区反映了志留纪深变质岩区的分布,Ⅱ中等平稳场区对应二云母花岗岩,Ⅲ低磁场区是震旦系—下寒武统弱—无磁的浅变质岩的反应;推断北西、北东向两组隐伏断裂构造8条,为本区构造格架的建立提供了物探依据,地面高精度磁测在覆盖区辅助地质填图效果良好,在实际地质填图中起到了指导作用。     

Distinguishing between tills from Valdaian ice sheets in the Arkhangelsk region, Northwest Russia

Recent studies in the Arkhangelsk region, northwest Russia, have identified at least three consecutive tills all associated with the last Valdaian (Weichselian) glaciation. The Scandinavian ice sheet deposited a Late Valdaian till (ca. 17 ka BP), whereas two tills were deposited in the Early–Middle Valdaian by the Barents/Kara Sea ice sheet (ca. 45–60 ka BP) and an older ice sheet with an eastern centre (ca. 74 ka BP). This article expands on previous stratigraphical work on the discrimination of regional till units by a combination of compositional characteristics and directional properties. Tills associated with the Scandinavian ice sheet were deposited by a glacier advancing from west or northwest, transporting predominantly material from the Fennoscandian shield and the White Sea area. The Barents/Kara Sea ice sheet moved from the north and northeast, whereas the oldest ice advance came from the east–southeast. Although, the two oldest tills both contain material with an eastern provenance, the Viryuga Till is dominated more by local carbonate-rich material. This study demonstrates that detailed investigation of till units facilitate the distinction of glacial events imperative for the reconstructing of the last glaciation in northern Russia.     

The characteristics of glacial tills from Northern England derived from a relational database

Geotechnical investigations have been carried out for many years in order to identify the depth, extent and the properties of the glacial deposits of Northern England. This valuable source of data exists in various forms because of the differences in the design of the investigations, the codes of practice relevant at the time of investigations, and contractors practice. An electronic store of these data would be a valuable asset not only to future development but to help in scientific studies of glacial till. This paper presents an overview of NETDATA (Northern England Till DATA), a relational database which is designed to store that data in a consistent format, allowing flexibility for further use. A preliminary analysis of the data has resulted in a characteristic data set for the tills.     

Field characterization of vertical bromide transport in a fractured glacial till

A study of the fracture distribution, hydraulic properties, groundwater levels and the transport of bromide was conducted to characterize vertical transport in the oxidized and reduced zones of a fractured glacial till. Detailed vertical profiles of groundwater levels and solute concentrations were obtained over a 4.5-year period. Vertical migration occurred at several time scales, as a low concentration front was rapidly transported at rates of 100–500 m/year ahead of a slower moving main plume, which advanced at rates of 0.2–0.8 m/year. Concentrations in the leading edge of the plume displayed a high degree of spatial variability over short vertical distances through day 1,000. Late in the test, the influence of matrix diffusion became apparent as concentration patterns developed from being irregular to more uniform distributions. Calculations show that the mass within the low concentration plume front accounts for less than 1% of the total solute mass. Simulation of the breakthrough curves using a simple one-dimensional advection-dispersion model of transport in porous media indicates that vertical transport is dominated by advection. Furthermore, the results indicate that vertical transport of solutes in oxidized and reduced zones of the till can be adequately simulated using an equivalent porous media.     

Till in the marine environment

Till is of common occurrence in the marine environment and can be both subglacial and proglacial in origin. Former glacial margins are often represented by till tongues, that are interbedded with stratified glaciomarine sediment, and the relationship is readily mappable using seismic reflection techniques. It is inferred that within individual till tongues, sediment-gravity flow deposits form transitional contacts with till of subglacial origin, but these contacts cannot be distinguished on seismic profiles. These unstratified, gravity-induced deposits formed in close proximity to the glacial source at the grounding line are considered to be secondary tills (flow-till complex) in terms of the INQUA classification of tills. Till-tongue successions and till deltas are large, ice-marginal depocentres associated with floating front ice-margins, and appear to be closely related in origin. Tabular and hummocky moraines of the mid-Norwegian Shelf also are considered to be products of floating-front margins and represent a large proportion of the retreat tills formed during ice recession. Seismostratigraphic evidence for channelisation in ice-marginal deposits appears to provide a means of distinguishing between former tidewater and floating-front margins.     

OSL and TL dating of buried podsols and overlying sands in Ostrobothnia,western Finland

In Ostrobothnia, western Finland, buried fossil soils have been recognised in a number of places in the sandy sediments that occur between glaciofluvial deposits and overlying till. Samples from the soil horizons as well as below and above them were taken for optically stimulated luminescence (OSL) dating. The same sites were also sampled for thermoluminescence (TL) dating. Altogether five TL dates and seventeen OSL dates were obtained. The OSL dates can be grouped into two age classes, (i) 120-163 ka and (ii) 76-106 ka, whereas all TL dates are of the order 135-155 ka. A comparison between the results obtained from the two dating methods shows that OSL dates are generally younger than the TL dates for the same sample. The discrepancy may arise partly from problems of setting a correct residual level in TL dating. If this is the case then the TL dates may indicate an upper limit for the true age. The results support the geological interpretation that the fossil soils were formed during the last interglacial, that the soil-forming processes possibly also continued during the first Early Weichselian stadial and interstadial (Brørup sensu lato), and that in general the till-covered glaciofluvial sequences, interpreted as eskers in Ostrobothnia, were deposited during the Saalian or Early Weichselian deglaciation.     

贡嘎山海螺沟冰川冰舌地段的泥石流

海螺沟冰川冰舌地段1976年以来发生过两次大的冰雪雨水泥石流。1989年7月8日和26日观景台西沟的泥石流通过1.7km长的冰面后,在冰川末端前形成一个5.5×10~4m~3的扇形地;1976年8月长草坝3号冰川下部冰碛体边坡的两处滑坡泥石流在开阔地上扩散堆积后,达1317.8m~3/s的泥石流和洪水下到冰川上。今后每年雨季还有可能发生更大规模的泥石流,值得人们警惕。