Looking Under the ice: Deciphering Subglacial Processes From the Spatial Arrangement of Erosional Landforms
There is considerable debate about the relative importance of 'glacial' and 'fluvial' processes at the base of ice sheets and their role in creating subglacial erosional landforms such as whalebacks, roches moutonnees and p-forms. However, few studies present detailed maps of former subglacial surfaces that allow analysis of the spatial distribution of erosional landforms, particularly over relatively large geographic areas. The Canadian Shield can be considered as a large unconformity surface ornamented by a variety of glacial erosional landforms. These landforms are particularly well exposed in the area of Whitefish Falls, northern Ontario where ancient quartzites, argillites and diamictites of the Huronian Supergroup have been subject to subglacial erosion beneath the Laurentide Ice Sheet. Detailed mapping of erosional landforms across a 1 km2 study area south of Whitefish Falls, Ontario using a handheld GPS system allowed identification of a range of features including striae, whalebacks, roches moutonnees and p-forms. Documentation of landform characteristics such as scale, long axis orientation, topographic position and bedrock type were recorded in the field and detailed photographs and sketches were taken. Average long axis orientation of all erosional features indicates that ice was moving from the northeast to the southwest across the study area. Quantitative spatial analysis of the distribution of landform types shows that none of the landforms are exclusive to any particular bedrock type or location in the study area. However, all landform types are most abundant, and most well developed, on the relatively soft, well jointed argillite. More resistant substrates such as quartzite and sandstone show little p-form development and are ornamented with fewer glacial striae. The distribution of p-forms across the study area shows them to be concentrated on the up- glacier side of north-facing (argillite) bedrock highs where subglacial meltwater flow was focused. Roche moutonnees are more common than whalebacks on argillite substrates, due to the strongly jointed nature of the argillite which facilitated subglacial plucking processes. Detailed mapping and spatial analysis of glacial erosional landforms in the Whitefish Falls area shows that subglacial abrasion, plucking and meltwater erosion operated across a relatively small geographic area. Landform development was influenced by the nature of the bedrock substrate, jointing patterns, and regional topography which may have controlled the location of subglacial meltwater streams.
Jokulhlaup Hydrofracture Fill - Esker Continuum: Skeidararjokull, Iceland
Quaternary eskers are used to infer large-scale meltwater movements and ice sheet dynamics. Despite the presence of many large esker systems, often extending for 100s of km, they have very few modern analogues. Sedimentary evidence suggests that long eskers are formed synchronously within subglacial, or occasionally englacial, conduits. Although a number of distinctive sedimentary architectures have been attributed to fluvial deposition within dynamic ice conduits, there is uncertainty over depositional timescale and esker evolution style. We discuss the controls on esker development by examining an esker complex where depositional timescale and hydraulic processes are well constrained. Two large englacial eskers at Skeidararjokull, Iceland ascend directly towards a supraglacial ice-walled canyon, excavated within less than 17 hours during the November 1996 jokulhlaup by a peak discharge of 25-30,000 m3s-1. The largest esker ridge has cross-sectional areas ranging from 100 to 1000 m2 and displays an undulating multiple crested surface morphology. Esker ridges also coalesce with a rectilinear pattern of sediment-filled fractures. Main esker ridge sedimentary architecture is dominated by a 20 m thick set of 15-20° back set beds composed of boulder size sediment. These beds support a series of climbing gravel dunes and terminate in large-scale foreset beds. In cross-section, the esker displays an anticlinal structure. Deposits range from bimodal cobble-sand and cobble-silt units to polymodal matrix-supported units displaying signs of boulder imbrication and clustering. Inter-fingering of esker ridge and fracture-fill sediments demonstrates simultaneous deposition. Esker ridge sedimentation evolved rapidly (1-2 hours) from a complex network of hydrofractures, to a pattern of localised conduit expansion and macroform growth. Irregular esker morphology is explained by variability of mechanical tunnel enlargement processes and the presence of primary fluvial bedforms within newly developed cavities. It is clear that substantial eskers can form under non-equilibrium conditions during jokulhlaups as long as there is a suitable supply of readily entrained sediment. Our modern analogue may assist those interpreting the meltwater magnitude and frequency regimes of ancient eskers.
Tunnel Valleys in Vendsyssel, Denmark - Evidence of Rapid Formation Beneath the Receding Late Weichselian Ice Sheet
Data collected in connection with a large groundwater investigation has provided new insight into the occurrence and genesis of the buried tunnel valleys in the central part of Vendsyssel, Denmark. Geological interpretations of numerous Transient ElectroMagnetic (TEM) soundings and wire-line logs have led to the delineation of an intricate pattern of 5-10 kilometres long and 1 km wide buried tunnel valleys. The mapped valleys are locally eroded to depths of more than 180 m b.s.l. The TEM data has also provided evidence of glaciotectonically dislocated layers in recessional moraines associated with the valleys. The valleys are interpreted to have been formed by subglacial meltwater erosion beneath the outermost part of the Late Weichselian ice sheet. The TEM data has been compared with recent results from stratigraphical investigations based on lithological and biostratigraphical analyses along with dating of borehole samples. This has led to an overview of the spatial distribution of the Late Quaternary lithostratigraphical formations in Vendsyssel, and the age of the buried tunnel valleys has been defined. The investigation has revealed that the formation of the tunnel valleys took place after the retreat of the Main advance of the Scandinavian Ice Sheet c. 20 kyr BP and before the Lateglacial marine inundation c. 18 kyr BP. Based on the occurrence of the buried tunnel valleys and the topography, four ice-marginal positions related to the recession of the Main advance from northeast and seven ice-marginal positions related to the recession from following eastern re-advance from the east across Vendsyssel have been delineated. As a consequence of this, all the mapped valleys must have been formed within a time interval of about 2000 years, giving only a few hundred years or less for the formation of the buried tunnel valleys at each ice-marginal position.
Do Eskers Provide Local and Regional Provenance Signals for Mineral Exploration?
Large esker systems have a tree-shaped tributary character similar to fluvial systems and can exceed several hundred kilometres in length. This, among other things, has led to the suggestion that such eskers form "synchronously" within a dendritic system of meltwater stream conduits (R-channels) beneath stagnant or sluggish ice. Perhaps guided by this, some workers in mineral exploration have adopted the viewpoint that eskers act as regional pathfinder vectors that record provenance signals from far up-esker. This may be a questionable assumption. A synthesis of data from available studies (a modest number) reveals that eskers in fact have relatively short clastic dispersal trains--typically less than several tens of kilometres--for gravel and (possibly) coarse sand fractions (i.e., bedload). These short dispersal trains are, apparently, best explained if the long, tree-shaped eskers formed time-transgressively in short segments as the ice back-stepped, in a fashion perhaps somewhat analogous to the transgressive infilling of incised fluvial valleys. If these data are representative, which remains to be tested in most parts of northern Canada, bedload in eskers may typically record a relatively proximal provenance signal, one that sampled adjacent and underlying till and bedrock. Esker bedload samples may therefore in some cases be well suited for property-scale investigations. In contrast to gravel, eskerine dispersal of finer sand and mud is less clearly documented. Because finer particles are more susceptible to traveling in suspension, these fractions should be transported farther down the R-channel before they are deposited. Their provenance may therefore be a more powerful test of the shape and length of esker R-channel networks than that of the bedload population. If investigations reveal that finer grain sizes are in fact regional vectors, eskers could potentially serve the dual function of providing both local and regional provenance information. These and other ideas will be tested over the course of a new multi-year project on glacial sediment dispersal, one whose basic objective is to develop scientifically rigorous, yet easy- to-understand, sampling protocols and conceptual models to help guide mineral exploration in Canada.
Giant loadcast Structures in the Enköping Esker: Evidence of a Second Regional Ice Sheet Readvance During late Younger Dryas in Ryssjön, SE Sweden
A well-exposed section in a gravel pit in Ryssjön, Enköping (Sweden) reveals two rarely documented and associated phenomena. Large loadcast structures (3 x 8 m) occur underneath a strongly compacted diamicton unit. Such a structural configuration probably indicates overriding by an ice sheet of the esker deposits during a regional readvance event in late Younger Dryas. The diamicton deposit shows strong glacitectonic deformations. Such an observation provides, for the first time, evidence for a possible second ice sheet readvance event in the area at that time. Emergence of this evidence, in assosiation with new field data from the study area provide ground for a new model for the retreat of the Scandinavian Ice Sheet in the southeast central Sweden. The model is well in accordance with the accumulating knowledge on the rapid final drainage of the Baltic Ice Lake, an event which initiated the dramatical environmental changes at the beginning of Holocene in central Scandinavia.
Esker hydrostratigraphy and 3D mapping in the St-Lawrence Lowlands
Esker stratigraphy and sedimentology was studied in the area of Châteauguay located south of Montreal, Quebec, Canada. It is part of the St-Lawrence Lowlands, underlied by Paleozoic sedimentary rocks. Stratigraphic sections description, GPR, shallow seismic and drilling permitted the synthesis of the depositional setting and the construction of the 3D hydrostratigraphic model. Esker stratigraphy is characterized by three main sequences. In the central part of eskers, coarse gravel sediments are deposited in subglacial streams. These deposits are overlain by a finer sequence of juxtaglacial to glaciolacustrine sediments, deposited at the mouth of subglacial streams. At some sections, a diamictic sequence is found over the former unit, probably deposited from gravitational flow. A grid-oriented technique has been developed in order to build a 3D hydrostratigraphic model of the Quaternary sediments overlying a regional fractured rock aquifer (relative computation method; Tremblay et al., 2007). It is based on the integration of the surficial sediments map and borehole logs with the use of GIS and grid-calculator software (such as Mapinfo and Vertical Mapper). In this method, the geometric relationships between two layers are used rather than the measured thicknesses for the extrapolation in areas where no information exists. The relative computation method is a powerful tool as, in most cases with rugged terrain and non-uniform rock surface where the variation of the total sediment thickness is important, this method avoids the possible overlapping of the upper and lower contacts. It allows quick and easy 3D mapping of simple Quaternary hydrostratigraphic sequences, such as those commonly found in glaciated Canadian landscape.
Sedimentological Analysis of Quaternary Glaciofluvial and Glaciodeltaic Deposits in the Georgetown Region, Ontario
The Regional Municipality of Halton is undertaking an in-depth study of the aquifers in the Georgetown region of southern Ontario with the aim of identifying new and sustainable sources of potable water for the growing urban population. Aquifers are hosted in thick successions of relatively coarse-grained Quaternary sediment overlying a dissected bedrock surface. Coarse-grained subsurface deposits are thought to be primarily of glaciofluvial origin although little is known of their sedimentological characteristics or three dimensional geometries. Five facies associations (FA1: rippled and cross bedded pebbly sand; FA2: cross bedded gravels; FA3: rippled and laminated sand and silt; FA4: interbedded pebbly sand and gravel; and FA5: interbedded rippled and cross bedded sand, silt and clay) were identified in exposures through the Maple Formation in the Limehouse Pit located west of Georgetown. The lowermost unit exposed in the pit consists of rippled and cross bedded sands (FA1) interpreted as glaciofluvial deposits formed on a delta front. Overlying gravel facies (FA2) are poorly to well sorted and occur as a locally continuous unit of large (>1m), stacked planar tabular cross beds that record deposition in relatively ice proximal glaciofluvial (delta top?) environments. Paleocurrent directions obtained from gravel foresets indicate flow from northeast to southwest and suggest sediment supply from an ice sheet lying to the east of the Niagara Escarpment. Gravel facies are overlain by a coarsening upward succession of laminated and rippled silts and sands (FA3) and interbedded channelized pebbly sand and gravels (FA4). This succession is interpreted to record progradation of deltaic sands and glaciofluvial gravels into a water body ponded between the Niagara Escarpment to the west and an ice margin to the east. The uppermost unit exposed in the pit consists of interbedded rippled and cross bedded sands, silts, and clays (FA5) that form a crude coarsening upward succession. This finer-grained succession may also record sedimentation in delta top/glaciofluvial environments but in positions more distal to the ice margin. Integration of these sedimentological field data with borehole data from the local area allows construction of a three dimensional model of the Limehouse region that may be used to interpret the structure and geometry of more deeply buried aquifers of the Georgetown area.
The Paris Moraine Depositional Elements, Paleoglacial Implications, and Hydrogeology Applications
Moraines have been mapped in Southern Ontario for nearly one hundred years, yet their composition and depositional origin remains poorly understood. Moraines in this area have been assigned to one of two classes, stratified moraines and till or recessional moraines. The Paris-Galt moraines are assigned to the till - recessional moraine class. The Paris and Galt moraines extend northward 130 km, are up to 11 km wide, and have relief of 30 m. They evolve from two distinct ridges in the south to a broad hummocky terrain with multiple ridges and secondary landscape elements (kettle depressions, eskers, subaerial fans, channels. These geomorphic changes are mirrored by changes in geology, thickness, and stratigraphy. Continuous cores reveal that the moraine consists of a succession of intercalated gravel and diamicton units. Depending on the geographic location, a variety of different units can underlie the moraine, including bedrock, older tills, lacustrine sediment, and glacifluvial gravel. Locally, the lower contact is cryptic. Outcrop data suggest northern and southern parts of the moraine are different. Within the southern glacilacustrine basin, large foresets of 10 m height occur at the base of pit exposures. By contrast, horizontally stratified outwash gravel is common in northern pits. The till that covers large parts of the moraine, Wentworth TiIl, is massive to stratified and is locally interbedded with sand and gravel. Where overlain by the surficial gravel, its upper contact can be loaded. Glacifluvial deposits are present in front of and locally underneath the moraine. The moraine strata are interpreted to have been deposited by a fluctuating, retreating ice margin with a highly variable meltwater flux both spatially and temporally. The narrow, southern moraine ridges may represent more rapid retreat within a glacial lake basin, whereas the northern, broader hummocky t errainis interpreted to have been deposited in a terrestrial environment. The moraine is significant hydrogeologically as the hummock terrain may provide enhanced recharge for bedrock and sand and gravel aquifers at depth.
Sedimentary Process Control over Acoustic Responses: Evidence from Quaternary Sediments of Eastern Canada.
It is frequently asserted that the geometry of seismic facies in recent glacial marine sediments is related to the dominant mechanism of deposition. In particular, 'ponded' facies, in which the facies is thick in depressions and is thin over topographic highs, is asserted to be related to turbiditic deposition; whereas 'draped' facies (in which the facies has a relatively constant thickness irrespective of topography) is related to the raining out of material from suspension and the melting of floating ice. To test this assertion, the author has developed techniques to quantify both the acoustic geometry and corresponding sampled sediments. Acoustic geometry can be quantified using digitized single-channel seismic profiles. A seismic facies, bounded by continuous reflections, can be characterized by the slope of a graph of facies thickness against the elevation of the lower reflection relative to a defined datum. Commonly such a graph will appear to consist of numerous segments of common slope but differing intercepts. The common slope becomes the single parameter to quantify geometry. A large magnitude of the slope corresponds to a ponded unit, whereas a small magnitude for slope corresponds to a draped unit. The value of the slope is corrected for thickness so that units of different thickness may be compared. Quaternary sediments of eastern Canada were deposited in a glacial marine setting. Sediments are deposited by ice rafting, suspension rain-out, and as turbidites in dense flows from basal ice. The sediments are extensively reworked by bioturbation and possibly by sediment failure, so that sediments may represent a mixture of sediments deposited by the three mechanisms described above. Grain-size measurements from piston cores can be reduced to simpler quantities by numerically dissecting observed grain size distributions into log-normal components, which approximate the weighting of sediments of different origin. The resulting variance in the weighting of ice rafted and turbiditic components is observed to correlate with acoustic velocity and decompacted acoustic impedance. The weighting in turbiditic components also appears to correlate with the parameter describing acoustic geometry.