Stratigraphic Indicators of Subglacial Hydraulic Scouring and Deposition During Surge- Related Outburst Floods, Bering Glacier, Alaska
Interactions Between Groundwater Flow, Ice Sheet Dynamics and Landforming Processes Beneath the Odra Ice Lobe, Central European Lowland
Advance of the Scandinavian Ice Sheet (SIS) into the central European Lowland during the LGM generated a continental-scale unconformity within the Quaternary sequence clearly discernible in the sedimentary and geomorphic record. In peripheral areas, the ice sheet developed non-synchronous, multiple lobes (possibly indicative of ice streams) that project tens of km beyond the main ice margin. Each of these lobes can be considered as largely stand-alone palaeoglaciologic system influenced by local parameters such as bed morphology, hydrogeology and rheology with one central factor being the capacity of the substratum to evacuate meltwater from the ice/bed interface which controls basal coupling and the ice movement mechanism. The Odra lobe occupied an area of over 18,000 km2 in NW Poland and NE Germany characterized by a set of subglacial landforms including open tunnel valleys, a drumlin field, and eskers. We have conducted a series of steady-state and time-dependent 3D numerical experiments on subglacial groundwater flow to constrain the interactions between ice, water and sediment under this ice lobe. The model simulates (1) groundwater flow in 35 time steps corresponding to different positions of the advancing ice margin, and (2) interaction between groundwater and subglacial channels (tunnel valleys). The model shows a complete re-organization of groundwater flow under the ice sheet and some distance in front of it in relation to a non-glacial time with a reversal of the main flow direction, deeper penetration and higher flow velocities. Low conductivity of the bed allows only a fraction of the basal meltwater to drain into the bed and areas exist with groundwater upwelling to the ice sole. We suggest that the surplus of water at the ice/bed interface facilitated fast ice flow due to some combination of basal sliding and sediment deformation leading to the formation of drumlins. Once subglacial channels form, groundwater flow undergoes renewed re-organization. The channels now serve as discharge conduits for the groundwater causing formation of multiple second-order catchments collecting groundwater before it reaches the ice margin. Since tunnel valleys post-date the subglacial landforms indicative of fast ice flow, we suggest that the ice streaming was terminated by rapid drainage of large volumes of meltwater from the ice sole through a channel network.
Shallow High Resolution Seismic Reflection, a Tool to Map Buried Unconformities
Subglacial jokulhlaup erosion, Skeidararjokull, Iceland
Hydrodynamic processes responsible for governing steady-state water flow through glaciers are relatively well understood. However less is known about the erosional impact of glacier outburst floods or jokulhlaups characterised by transient hydraulic processes and routeways. Despite the ubiquity of tunnel channels and tunnel valleys within formerly glaciated areas, their origin remains enigmatic. Few tightly constrained modern analogues exist for event-related subglacial erosion. At Skeidararjokull, Iceland, sudden-onset, high- magnitude jokulhlaups exiting overdeepened subglacial basins have generated a variety of hydro-mechanical erosional forms at and beneath the glacier bed. We present geomorphological and stratigraphic evidence for subglacial jokulhlaup erosion in form of tunnel channels and also sub- glacier-bed erosion associated with confined fracture and channel networks. The spatial distribution of subglacial erosional forms is influenced by sub- and proglacial topography and the distribution of buried glacier ice. Zones of glacier bed erosion are also associated in many cases with hydrofracture-fill and esker ridges demonstrating interconnection of subglacial and sub-glacier bed (subterranean) jokulhlaup flow pathways. Our findings may help explain complexity with the sediment and landform record of formerly glaciated areas where tunnel channels are often found in association with subglacial deposits such as eskers.
Tunnel Valleys in Denmark - Distribution, Architecture, Ages and Possible Origin
At least 50 percent of the water supply in Denmark is based on ground water abstraction from buried tunnel valleys. The tunnel valleys are therefore in focus in the current national mapping campaign that embraces Transient ElectroMagnetic (TEM) and seismic surveys, test drilling and several other methods. These new data have exposed intricate, cross-cutting networks of tunnel valleys in most surveyed areas. The TEM data provide dense data coverage and the valleys appear as elongated bodies of electrical resistivity anomalies. The valleys appear as erosional structures in the seismic data and the internal structures of the valleys also often appear in these data. Data from test drillings and numerous water wells provide valuable control on the geophysical data by providing information about the valley in-fill and the sedimentary surroundings. The valleys are typically between 5 and 30 km long, between ½ and 2 km wide and 50-250 m deep. They are incised into unconsolidated layers of clay, silt, and sand as well as into consolidated chalk and limestone. Their long profiles undulate, often with thresholds of more than 50 m. The valleys occur in numerous cross- cutting or stacked generations, but frequent internal cut-and-fill settings also indicate valley erosion along pre- existing tunnel valley tracks. Each generation is believed to represent one glacial event including subglacial meltwater erosion, withdrawal of the ice sheet and pro-glacial sedimentation. Occurrences of interglacial deposits separating individual generations and the varying preferred orientations of the valley generations indicate that formation of tunnel valleys took place during several glaciations throughout the Pleistocene. Due to frequent and widespread erosion during the glaciations, most of the Early Pleistocene and early Mid Pleistocene record has been removed. But as deeply eroded valleys have a high potential for preservation, remnants of old glacial sediments may have been preserved in the valleys. Thus, a number of soil samples collected from drillings in valleys indicate pre-Elsterian ages, which is older than normally found elsewhere in the Danish Quaternary setting. In some areas, however, a thick succession of Late Pleistocene has kept an untouched picture of the youngest tunnel valley generations. The exact time of valley incision can in general not be dated accurately due to the complicated cut-and-fill settings, but the youngest valleys in particular, enable us to gain valuable insight into the formation of the tunnel valleys. A clear genetic connection between the young tunnel valley generations and end moraines can occasionally be observed, pointing to tunnel-valley formation during or shortly after minor re-advances. The valley generations found in Denmark are composed of more or less discrete valleys that are not interconnected in anastomosing systems. It is therefore not clear if all valleys within each generation were formed simultaneously by one large subglacial meltwater outburst, or if the individual valleys were formed by separate events. However, it is evident from several surveyed sites in Denmark that many tunnel valleys were left behind filled with glacier ice by a retreating ice sheet. The valleys may have become ice filled by creeping ice during the erosion of the substrate. Steady subglacial meltwater flow or recurring minor outbursts of meltwater continuously eroded deeper while the created space gradually were closed by creeping ice during periods of decrease or cessation of meltwater flow.
Bed load and Morpho-Dynamics of Gravel-bed Rivers
The morphology of gravel-bed rivers, and the transfer of bed load, are closely linked. Overall mean gravel flux is determined by total stream energy relative to grain size, and channel gradient and dimensions adjust to the sediment flux and water discharge supplied to the river. Large-scale depositional or erosional reaches reflect overall flux convergence or divergence and the residence times reflect characteristic floodplain morphology. At reach scale local morpho-dynamics is a manifestation (and integral part) of spatial and temporal variation in bed load flux. Movement of individual particles is 'slaved' to the bar- scale morphology and this provides a means of back-calculating bed load flux as well as insight into the transport process at this scale.
Aquifer Characterization Within the Hämeenkangas Interlobate Glaciofluvial Complex in Western Finland by Using Geophysical Data With a Reconnaissance GPR Survey.
The Hämeenkangas glaciofluvial complex (20 km long, 1-3 km wide and mainly 20-50 m thick) is a high yield (30 000 m3/d) groundwater reservoir, including the famous fountain Kuninkaanlähde with a discharge of 10 000 m3/d. The complex has been described either as the western end of the Central Finland Ice-Marginal Formation, or as the eastward turning continuation of the Pohjankangas interlobate esker. The past ice margin terminated (ca. 11 000 BP) in water depths of 60-120 m, and widespread beach deposits imply substantial alternation of the original glaciofluvial deposits by littoral processes. Surprisingly little is known about the internal structure and related groundwater flow conditions of the complex. This is partly due to lack of sediment exposures and borehole data. Increasing demands for groundwater supply and protection have led to a need for comprehensive aquifer characterization. Our aim was to create a geologic conceptual model to understand the spatial heterogeneity of the aquifer. Large-scale depositional architecture and sedimentary structures, and grain-size characteristics of the deposits, as well as groundwater levels and flow directions were studied by a topography-corrected GPR reconnaissance survey (38 km/15 km2/ with 100 MHz antenna). The GPR data was supported by information from gravimetric measurements, seismic refraction soundings, and airborne geophysical survey to define the bedrock topography. The morphology of the complex was examined using a Digital Elevation Model integrated with the map of Quaternary deposits. In addition, 5 boreholes (Southwest Finland Environment Centre) provided preliminary reference data for the interpretation of GPR profiles. The results suggest time-transgressive, interlobate glaciofluvial deposition with a continuous, gravel-rich esker formed in subglacial tunnel to ice-walled, subaqueous environment. The main esker is 200-250 m wide, 20- 40 m thick, and associated with tributary ridges and widening gravel-rich deposits without change to deltaic foresets. The esker path is also indicated by erratic boulders, kettle holes, large-scale deformation structures related to morphologically undetectable kettle holes, and follows the sides of the bedrock depressions. No evidence on ice-marginal till beds or moraines was found. The esker is flanked by low-angle inclined and horizontally stratified sandy to fine-grained deposits that are topped by a silty/clayey bed in the northern side of the esker. This bed supports a widespread perched groundwater table within an extensive paraglacial landsystem that consists of 5-20 m thick shore terraces or spit-platforms with northward dipping foresets. The upper slope on the southern side of the esker shows intense shore erosion. The high discharge of the fountain Kuninkaanlähde is explained by a long, continuous esker aquifer bounded by fine-grained deposits with poor hydraulic conductivity, and in places by bedrock. A possible groundwater divide within the esker is located ca. 6 km to the east of the fountain. Our results will help to guide future groundwater investigations in an efficient and economical way, and forms the necessary framework for 3-D modelling approaches. Furthermore, we were able to throw light on the interlobate depositional style of the complex, which also has wider implications for the ice stream dynamics during the last deglaciation of the Scandinavian ice sheet.
Glaciofluvial Processes and Mineral Exploration in Southern Slave Province, Northwest Territories, Canada
The extent to which glaciated landscapes have been modified by channelized and/or regional subglacial meltwater processes is controversial. The significance of such processes, however, where documented, may have important implications for mineral exploration programs and thus requires further enquiry and evaluation. Esker sampling, for example, played a key role in the discovery of Canada's first diamond mining camp at Lac de Gras. Recent work completed on a number of mineral exploration properties suggests that there has been inadequate testing of hypotheses of glacial landform and sediment origin. Hence, process based models to interpret indicator mineral trends have been poorly developed and applied; particularly in areas affected by cryogenic processes that may confound recognition of glaciofluvial sedimentation. Recent exploration and sampling in Slave Province, has recognized and identified well defined corridors and broader fields or belts affected by subglacial meltwater processes, in addition to classic esker forms. These features appear to be nested or ordered in the landscape, whereby broad fields or belts are older than corridors, with eskers forming the youngest inset features. Eskers are glaciofluvial landforms that form extensive tree-shaped networks of sorted sediment readily observed across the barren lands of the Canadian north. Distinct, narrow (less than 0.5 km) to wide (more than 2 km) corridors, defined by scoured bedrock and till, lags, gravel deposits and landforms, occur adjacent to esker ridge networks. Scoured glaciofluvial corridors also contain a variety of s-forms, potholes, plunge pools, sculpted scarps (e.g. till plateaus), and in places, rock and sediment drumlins. Less obvious, broad fields and belts (greater than 2 km) occur beyond distinct corridors and are marked by glaciofluvial landforms, and gravel lags, blankets, transverse ridges and bars. A viable sequence of events for identified features includes: i) widespread deposition of till; ii) local and regional meltwater erosion/ deposition, and iii) esker deposition in response to intermittent meltwater floods from surface or subsurface reservoirs associated with active and/or stagnating ice. Glaciofluvial processes will erode and concentrate indicator minerals in a variety of terrain settings. Erosion may exhume older or deeper till units, thus confounding simple spatial comparison of near surface sample results. Winnowing of fines may concentrate heavy minerals on erosional surfaces. Glaciofluvial deposits and landforms possess distinct dispersal patterns of indicator minerals and metals. Eskers are recognized and sampled glaciofluvial medium. Little is known, however about indicator mineral sourcing, dispersal and partitioning within esker sediment. Additional exploration targets include gravel bars, transverse ridges and lags. Elongated bars may merge with crag and tail features, and thus form landforms with gravelly tails. Gravel bars may act as traps for heavy minerals. For example, kimberlite indicator minerals have been found concentrated in heads of gravel bars in glaciofluvial settings. Subglacial meltwater processes may also provide a viable mechanism for the formation of dispersal trains that are narrow, continuous, elongate and traceable in a down flow direction from their source, such as identified at exploration properties (e.g. Snap Lake). There is a need to recognize and study glaciofluvial landscapes in more detail and to assess patterns of indicator mineral dispersal with evolving glaciofluvial and glacial process models.