Buried Valley Research Articles

BURIED VALLEYS IN MOUNTAIN REGIONS, COASTAL LOWLANDS AND CONTINENTAL SHELF IN THE ASIAN PART OF RUSSIA

In mountain areas, adjacent coastal lowlands and continental shelf in the Asian part of Russia a variety of buried valleys is found very different in their forming conditions and morphology. Classification of buried valleys is suggested that shows a great variety types, their formation conditions, morphometric and morphological characteristics. Buried valleys are divided into two main groups: 1) valleys formed in areas of rapid crustal uplifts and differential tectonic movements; 2) valleys formed in areas of tectonic subsidence such as transient areas between uplifting and sinking structures, inversion or inherited tectonic basins, individual valley sections preserved at the crossings of subsiding tectonic blocks, valleys buried in coastal lowlands, valleys buried in continental shelf due to the global sea level rise. In most areas, buried valleys are represented by a combination of different age generations of the above types. The degree of inheritance of the identified types of valleys varies and depends on the type and amplitude of tectonic movements. The larger the amplitude of tectonic movements is, the poorer is the inheritance of river network. In the areas of differential tectonic movements, inheritance may be detected not only for river network pattern, but also for morphodynamic types of river channels and even individual channel forms.

1D Versus 2D Site Effects from Numerical Analyses on a Cross Section at Barberino Di Mugello (Tuscany, Italy)

The results of 1D and 2D numerical analyses performed to better understand the basin geometry effects on ground seismic response in a site of the Mugello area are presented and discussed. Geological subsoil modelling and site geotechnical characterization were performed by using the results of extensive surveys carried out in the area within in-depth seismic microzonation studies promoted by Regional Government of Tuscany. The selected section has a quite regular and horizontal ground profile so that the topographic effects are negligible with respect to the basin effects due to the buried valley. With the aim of distinguishing morphological (’edge effects’) from stratigraphic effects, the results from 2D analyses were compared to those from 1D numerical modelling on a large number of vertical soil profiles spaced along the examined cross-section. Different amplification factors were considered with reference to both peak ground acceleration and elastic response spectra.

Assemblage of buried and seabed tunnel valleys in the central North Sea: from morphology to ice-sheet dynamics

Tunnel valleys are kilometre-scale, channel-like, incised subglacial landforms usually associated with glaciated lowlands (O Cofaigh 1996). Tunnel valleys related to the Pleistocene glaciations are present onshore and offshore of NW Europe (e.g. Huuse & Lykke-Andersen 2000; Van der Vegt et al. 2012). In the central North Sea, tunnel valleys are found buried in the Quaternary succession and as bathymetric deeps at the seabed. Three-dimensional (3D) seismic data from the central North Sea image extensive networks of buried tunnel valleys that consist of multiple generations of cross-cutting landforms (Stewart et al. 2013). Seabed tunnel valleys are identified in high-resolution bathymetric data, and form another set of younger glacial landforms. Morphological measurements of the buried and seabed tunnel valleys are compared to infer their relationship to past ice-margins and highlight the differences in ice-sheet configuration over time. In the central North Sea, over 180 buried tunnel valleys are clearly imaged within 14 overlapping 3D seismic datasets originally acquired for the hydrocarbons industry (Fig. 1a; Stewart et al. 2013). The survey area lies within 56–59° N and 1° W to 2° E, in water depths of 90–150 m. The buried tunnel valleys form part of the extensive erosive surface at the top of the Aberdeen Ground Formation, usually related to the Pleistocene glaciations of the region from the middle Quaternary onwards (Stoker et al. 1983; Graham et al. 2011). The buried valleys overlie the Bruhnes–Matuyama magnetic reversal (dated at 780±5 ka BP from boreholes in the region, Stoker et al. 1983) and are overlain by sediments dated to 30 14C ka BP (Graham et al. 2010). Fig. 1. ( a ) Location of the study area, three-dimensional (3D) seismic volumes and examples in the central North Sea. Map from GEBCO_08. WB, Witch Ground Basin. ( b ) All the tunnel valley outlines mapped within …

3D geological modelling of a buried-valley network based on AEM and borehole data

In former glaciated areas buried tunnel valleys can often be found. These buried erosional structures can be highly decisive for groundwater recharge and groundwater flow. Delineation of the architecture and infill of the structures are therefore very important in relation to groundwater mapping. The dense data coverage offered by airborne electromagnetic methods makes it possible to map and model the buried valley structures with a high degree of detail. The delineation of the individual valleys and the mapping of the internal cross-cutting relationships are dependent on the geological interpretation, which is founded on the knowledge about geological processes and the regional geology. In this study, we have investigated a relatively small study area in Denmark with SkyTEM and lithological log data as the main data sources. The area is characterised by a network of cross-cutting buried tunnel valleys, which have been incised into impermeable Paleogene clay deposits. The geological interpretation of the SkyTEM data resulted in modelling of 21 buried valleys belonging to at least 7 different generations. Manual voxel modelling of the infill of these valleys, as well as the surroundings, resulted in a geological 3D model consisting of 43 different units. Most of the valleys show heterogeneous infill, characterized by a predominant lithology (for instance meltwater sand) with local occurrences of secondary lithologies (for instance clay till). In the majority of the valleys, meltwater sand is the main lithology, but clay till and meltwater clay deposits are also commonly found. Due to the heterogeneity of the infill, proper modelling of this type of geology requires voxel modelling instead of layer modelling.

3D modeling of buried valley geology using airborne electromagnetic data

Buried valleys are important hydrogeologic features of glaciated terrains. They often contain valuable groundwater resources; however, they can remain undetected by borehole-based hydrogeologic mapping or prospecting campaigns. Airborne electromagnetic (AEM) surveys provide high-density information that can allow detailed features of buried valleys to be efficiently mapped over large geographic areas. Using AEM data for the Spiritwood Valley Aquifer system in Manitoba, Canada, we developed a 3D electric property model and a geologic model of the buried valley network. The 3D models were derived from voxel-based segmentation of electric resistivity obtained via spatially constrained inversion of two separate helicopter time-domain electromagnetic data sets (AeroTEM and versatile time-domain electromagnetic [VTEM]) collected over the survey area. Because the electric resistivity do not provide unequivocal information on subsurface lithology, we have used a cognitive procedure to interpret the electric property models of the aquifer complex, while simultaneously incorporating supporting information for the assignment of lithology in the 3D geologic model. For the Spiritwood model, supporting information included seismic reflection data and borehole records. These data constrained valley geometry and provided lithologic benchmarks at specific borehole sites and along seismic transects. The large-scale AeroTEM survey provided the basis for modeling the regional extent and connectivity of the Spiritwood Valley Aquifer system, whereas the local-scale VTEM survey provided higher near-surface resolution and insight into a detailed shallow architecture of individual buried valleys and their fill.

THE CLIMATE CHANGES IMPACT ON THE RIVER CHANNELS AND VALLEY BOTTOMS IN THE EASTERN SIBERIA

Morphology and morphometry of the buried and contemporary valleys bottoms in two mountain placer districts of the Eastern Siberia were analyzed. The Lena district is located within Vitima–Patom plateau on the right bank of the Vitim river, the Kular one is located in the interfluve of the rivers Yana and Omoloy. Using the topographic maps and paleogeomorphological schemes compiled on the basis of drilling data, we defined the morphodynamic types of channels, morphometric parameters (the width of the bottom step, the bend deflection of the meander) and the morphology of the bottoms of the buried and modern valleys. The patterns of their change were traced during several geological periods with different climatic conditions specified by paleolandscape method. Morphological and morphometric characteristics of the bottoms and beds of mountain valleys depend on the altitudinal zonation. In the mid-mountain relief of the Lensky district, even at the considerable changes of climate, morphodynamic type of riverbed, bottom morphology and morphometric parameters of buried and modern valleys are practically identical. More significant is the impact of climate change on the bottoms of valleys and channels in the Kular district. Here, in the relief of low-mountains and plateaus, with a cooling of the climate and the decrease of rainfall, the width of the bottoms of the modern valleys is significantly reduced compared to buried valleys, formed in warm humid climate of the Paleogene. Using the results of long-term observations on the mountain river Bzyb (Caucasus) the mechanism underlying these differences was revealed. It is associated with the intensity of deformation of bed forms depending on the nature of the relief terrain and water availability.

Influence of Lateral Heterogeneities on Strong‐Motion Shear Strains: Simulations in the Historical Center of Rome (Italy)

Abstract The influence of lateral heterogeneities on alluvial deposits is a topic of particular interest in the field of urban planning and engineering design of structures and infrastructures. This work focuses on the effects of such heterogeneities on the shear strains produced within the recent alluvial deposits of the Tiber River in the historical center of Rome in case of the worst expected earthquake scenario. To this aim, a 3D engineering‐geology model of the subsoil is used to derive four geological sections across the Tiber River valley as well as 48 soil columns to perform numerical simulations. Various models are considered: a viscoelastic equivalent linear rheology in a 1D finite‐difference model for one‐component horizontal input, a nonlinear elastoplastic model in a 1D finite‐element scheme for three‐component input, and a nonlinear viscoelasto‐plastic rheology in a 2D finite‐difference model under one‐component horizontal input. After comparing these different simulations, results have shown that lateral heterogeneities play a key role with respect to the expected shear strains within multilayered soils. To this aim, some specific indexes are introduced to estimate the maximum shear strain (MSS) concentration within the soil layers as well as to highlight their effect due to the stratigraphic position of the layers, within the soil column, independently from its depth. A final differential index leads to the evaluation of the lateral heterogeneity effect on the estimated MSS, demonstrating their prevalent role with respect to the bedrock shape (i.e., the angle of inclination of the buried valley slopes). From these results, an MSS zoning map is obtained for the historical center of Rome, showing that the local seismic response should be modeled by assuming 1D or 2D conditions depending on the location considered.

3D geological modelling of a complex buried-valley network delineated from borehole and AEM data

Buried tunnel valleys are common features in formerly glaciated areas, and where present, they are very important for the groundwater recharge and flow. Delineation of the structures and modelling of the infill is therefore very important in relation to groundwater mapping. Typically, borehole information is too sparse to enable a detailed delineation of the structures, whereas densely covering airborne electromagnetic data have proven to be very useful for this. In the last decades, the mapping approach has been studied carefully, but the 3D modelling of the valley structures has not been described to the same degree yet. In this study, we create a 3D geological model of an area that is characterised by a complex network of buried valleys mapped with a spatially dense airborne electromagnetic survey. Due to the comprehensive dataset, the modelling requires formulation of an advanced strategy. This contains a number of steps, where the AEM-derived resistivity data are initially interpreted based on the geological background knowledge to identify the buried valleys and build a conceptual geological model. Secondly, the age relationships between the valleys are established from the valley orientations and their internal cross-cut relationships. Thirdly, the deep erosional surfaces are modelled. Subsequently, the interpreted age relationships are utilised to trim the valley floor surfaces, such that younger valleys cut older. Finally, a voxel model is built and populated with lithofacies and stratigraphical units. The model is constructed as a combined layer-based and voxel model in order to map both the overall structures as well as the lithological variations within the 3D model domain. The final model contains 20 buried valleys that show a complex cross-cut setting that indicate the presence of at least eight valley generations. Most of the valley infills show lithological variations, and the final voxel model thus contains 42 different geological units.

Integration of numerical models and geoinformatic techniques in the delimitation of a protection zone for the MGB 319 complex multi-aquifer system in southwest Poland

Abstract The study area, situated near the city of Wrocław in southwest Poland, is part of the hydrogeological system of the Quaternary/Neogene MGB 319, inclusive of a buried valley of high water potential, named the Bogdaszowice structure. This structure is an alternative source of water supply for the Wrocław city area. Numerical modelling is the most effective tool in establishing a groundwater protection strategy for Major Groundwater Basins (MGBs) in complex aquifer systems. In the present study, the first step was to assess the hydrodynamic conditions of the Radakowice groundwater intake by analyses of head contours, pathlines, average flow times and capture zones of particular wells. Subsequently, these results were used in combination with other data and compiled as GIS layers. The spatial distribution of hydraulic conductivity was based on the lithology of surface sediments. Other data sets such as the thickness of the unsaturated zone, average soil moisture and infiltration rate were taken either directly from the model or were calculated. Based on the input data obtained, vertical flow time calculations for every model cell were made. The final outcome is a map of the protection zone for the aquifer system of the MGB 319.

A stacked Late Quaternary fluvio-periglacial sequence from the Axe valley, southern England with implications for landscape evolution and Palaeolithic archaeology

The current model of mid-latitude late Quaternary terrace sequences, is that they are uplift-driven but climatically controlled terrace staircases, relating to both regional-scale crustal and tectonic factors, and palaeohydrological variations forced by quasi-cyclic climatic conditions in the 100 K world (post Mid Pleistocene Transition). This model appears to hold for the majority of the river valleys draining into the English Channel which exhibit 8–15 terrace levels over approximately 60–100 m of altitudinal elevation. However, one valley, the Axe, has only one major morphological terrace and has long-been regarded as anomalous. This paper uses both conventional and novel stratigraphical methods (digital granulometry and terrestrial laser scanning) to show that this terrace is a stacked sedimentary sequence of 20–30 m thickness with a quasi-continuous (i.e. with hiatuses) pulsed, record of fluvial and periglacial sedimentation over at least the last 300–400 K yrs as determined principally by OSL dating of the upper two thirds of the sequence. Since uplift has been regional, there is no evidence of anomalous neotectonics, and climatic history must be comparable to the adjacent catchments (both of which have staircase sequences) a catchment-specific mechanism is required. The Axe is the only valley in North West Europe incised entirely into the near-horizontally bedded chert (crypto-crystalline quartz) and sand-rich Lower Cretaceous rocks creating a buried valley. Mapping of the valley slopes has identified many large landslide scars associated with past and present springs. It is proposed that these are thaw-slump scars and represent large hill-slope failures caused by Vauclausian water pressures and hydraulic fracturing of the chert during rapid permafrost melting. A simple 1D model of this thermokarstic process is used to explore this mechanism, and it is proposed that the resultant anomalously high input of chert and sand into the valley during terminations caused pulsed aggradation until the last termination. It is also proposed that interglacial and interstadial incision may have been prevented by the over-sized and interlocking nature of the sub-angular chert clasts until the Lateglacial when confinement of the river overcame this immobility threshold. One result of this hydrogeologically mediated valley evolution was to provide a sequence of proximal Palaeolithic archaeology over two MIS cycles. This study demonstrates that uplift tectonics and climate alone do not fully determine Quaternary valley evolution and that lithological and hydrogeological conditions are a fundamental cause of variation in terrestrial Quaternary records and landform evolution.

A mid-Holocene alluvial deposit beneath blanket peat in the Upper Liffey Valley, county Wicklow, Ireland

A 750m section on a hillside in the upper Liffey Valley, county Wicklow has exposed a small valley buried beneath blanket peat. The section shows two alluvial sequences underlain by a stone line over regional till and weathered granite. 14C dates from wood in the sediments indicate that the older alluvium formed between 4950-4550 cal. BP and the younger between 3470-3000 cal. BP. Wood in the basal layer of the overlying peat yielded a date of 2310-2150 cal. BP. The younger alluvium shows the effects of soil paludification prior to the peat expansion and dated pollen analyses elsewhere in the upper catchment show that blanket peat began spreading over most areas above 350m after 4000-3600 BP. The buried valley contributed sediments to the mid-Holocene floodplains in the upper Liffey valley prior to the extension of blanket peat.

Use of a conceptual hydrogeological model and a time variant water budget analysis to determine controls on salinity in Lake Burrumbeet in southeast Australia

Lake Burrumbeet, which is located in the basalt plains of western Victoria, Australia, has a much lower salinity than other lakes in the region. To determine the factors causing the freshness of this water body, an assessment was carried out using existing geological information to reconstruct the palaeotopography beneath basalt flows, and using hydrogeological data, bore hydrographs and hydrological data together with a time variant lake water balance analysis calibrated from 1998 to 2008. The calibrated water balance of Lake Burrumbeet shows that groundwater outflow has a major influence on lake levels (31 % of water losses). The lake water budget analysis confirms that the interaction of Lake Burrumbeet with the surrounding groundwater system is volumetrically significant. Geological re-construction of the course of the buried river valley sediments (Deep Lead underlying the basalt) shows that they run directly beneath Lake Burrumbeet. Groundwater leakage into these sediments is sufficient to transfer enough salt from the lake to maintain a low to moderate lake salinity. The conceptual hydrogeological model of Lake Burrumbeet, coupled with the time variant water budget analysis, show that the difference in salinity among the lakes in the region can be explained by variations in the configuration and hydrogeological setting of the lakes, and this hypothesis provides new understanding of the relative freshness of the lake.

Investigating property damage along Dublin Port Tunnel alignment

During the winter of 2002/2003, Dublin City Council received various complaints regarding aesthetic and serviceability damage of several residential buildings located within the highly populated residential area of Marino (1ṡ8 km north-east of Dublin centre, in the Republic of Ireland) allegedly caused by rock-tunnelling operations of the southwards tunnel-boring machine in the northbound tunnel carried out in execution of the Dublin Port Tunnel scheme. A holistic engineering–geological ground model for the area north of Dublin centre shows that structural elements crossing the bedrock have markedly affected the geological evolution and post-glacial drainage of the site, including the formation of buried valleys. A review of groundwater monitoring and tunnel construction records has allowed the causes of settlement at ground level to be identified. These include volume loss of the ground due to deformation and local failure of the tunnel excavation, dewatering of the bedrock and of coarse strata embedded in the Boulder Clay, resulting in the removal of fine soil particles, vibro-densification due to tunnel-boring machine ground-borne vibrations, and slow production and possibly consolidation of clayey strata. A comparison between the position of the properties where damages are being claimed and that of post-glacial rivulets and lakes, backfilled in the 1920s following a rapid expansion of Dublin suburbs, clearly indicates a direct linkage between the two.

Erratum: Hydraulic relationships between buried valley sediments of the glacial drift and adjacent bedrock formations in northeastern Ohio, USA

Buried valleys are ancient river or stream valleys that predate the recent glaciation and since have been filled with glacial till and/or outwash. Outwash deposits are known to store and transmit large amounts of groundwater. In addition to their intrinsic hydraulic properties, their productivity depends on their hydraulic relationships with the adjacent bedrock formations. These relationships are examined using a steady-state three-dimensional groundwater flow model through a section of a buried valley in northeastern Ohio, USA. The flow domain was divided into five hydrostratigraphic units: low-conductivity (K) till, high-K outwash, and three bedrock units (Pottsville Formation, Cuyahoga Group and Berea Sandstone). The model input was prepared using the data from well logs and drilling reports of residential water wells. The model was calibrated using observed heads with mean residual head error of 0.3 m. The calibrated model was used to quantify flux between the buried valley and bedrock formations. Mass balance was calculated to within an error of 2–3 %. Mass balance of the buried valley layer indicates that it receives 1.6 Mm3/year (≈40 % of the total inflow) from the adjacent bedrock aquifers: Pottsville Formation contributes 0.96 Mm3/year (60 %) while the Berea Sandstone 0.64 Mm3/year (40 %).

Hydraulic relationships between buried valley sediments of the glacial drift and adjacent bedrock formations in northeastern Ohio, USA

Buried valleys are ancient river or stream valleys that predate the recent glaciation and since have been filled with glacial till and/or outwash. Outwash deposits are known to store and transmit large amounts of groundwater. In addition to their intrinsic hydraulic properties, their productivity depends on their hydraulic relationships with the adjacent bedrock formations. These relationships are examined using a steady-state three-dimensional groundwater flow model through a section of a buried valley in northeastern Ohio, USA. The flow domain was divided into five hydrostratigraphic units: low-conductivity (K) till, high-K outwash, and three bedrock units (Pottsville Formation, Cuyahoga Group and Berea Sandstone). The model input was prepared using the data from well logs and drilling reports of residential water wells. The model was calibrated using observed heads with mean residual head error of 0.3 m. The calibrated model was used to quantify flux between the buried valley and bedrock formations. Mass balance was calculated to within an error of 2–3 %. Mass balance of the buried valley layer indicates that it receives 1.6 Mm3/year (≈40 % of the total inflow) from the adjacent bedrock aquifers: Pottsville Formation contributes 0.96 Mm3/year (60 %) while the Berea Sandstone 0.64 Mm3/year (40 %).

Incorporating ancillary data into the inversion of airborne time-domain electromagnetic data for hydrogeological applications

Abstract Helicopter time-domain electromagnetic (HTEM) surveys often suffer from significant inaccuracies in the early-time or near-surface data—a problem that can lead to errors in the inverse model or limited near-surface resolution in the event that early time gates are removed. We present an example illustrating the use of seismic data to constrain the model recovered from an HTEM survey over the Spiritwood buried valley aquifer in Manitoba, Canada. The incorporation of seismic reflection surfaces results in improved near-surface resistivity in addition to a more continuous bedrock interface with a sharper contact. The seismic constraints reduce uncertainty in the resistivity values of the overlying layers, although no a priori information is added directly to those layers. Subsequently, we use electrical resistivity tomography (ERT) and borehole data to verify the constrained HTEM models. Treating the ERT and borehole logs as reference information, we perform an iterative time-shift calibration of the HTEM soundings to achieve regional-scale consistency between the recovered HTEM models and the reference information. Given the relatively small time-shifts employed, this calibration procedure most significantly affects the early-time data and brings the first useable time gate to a time earlier than the nominal first gate after ramp off. Although time shifts are small, changes in the model are observed from the near-surface to depths of 100 m. Calibration is combined with seismic constraints to achieve a model with the greatest level of consistency between data sets and, thus, the greatest degree of confidence. For the Spiritwood buried valley, calibrated and constrained models reveal more structure in the valley-fill sediments and increased continuity of the bedrock contact.

Architecture of buried valleys in glaciated Canadian Prairie regions based on high resolution geophysical data

Airborne electromagnetic and high resolution seismic geophysical data provide insight into complex buried valley geometry and internal sedimentary architecture of buried valleys in the Canadian Prairies. Buried valleys with an extent in excess of 20,000 km beneath the Canadian prairie landscape have been previously interpolated using borehole data; however, they provide only limited insight into the geometry and spatial relationships of these valleys. The collected high-quality geophysical datasets, a three-dimensional airborne electromagnetic survey and high resolution seismic profile data provide much greater resolution of buried valley geometry and internal sedimentary architecture of buried valleys. Several generations of valleys at different scales are identified. We interpret multiple erosion surfaces bounding thick diamicton sequences that fill the largest valleys; the youngest valleys are filled with variable sediment types. Three valley morphologies are identified and process origins inferred: 1) regional-scale, extensive, subaerial and pre/interglacial named α-type valley, 2) regional-scale, narrow and mainly proglacial, β-type valley and 3) local-scale, apparently discontinuous, subglacial, γ-type valley. Within the glacial sediment stratigraphy filling α-type valleys are thick diamicton sequences bounded by erosional surfaces. The β-type and γ-type valleys are filled with variable sediment types. The proglacial β valleys erode bedrock and also occur within the α-type valley fill. Conversely, the γ subglacial valleys are observed to crosscut other valleys and may be shallow and wide, or deep and narrow. The reported geophysical datasets supported by borehole data are able to map buried valleys in three-dimensions and hence identify the morphologic and stratigraphic relationships that permit improved constraints on process and erosional origin and fill of buried valleys. Results provide insight into the relationship between valleys formed as part of Tertiary fluvial erosion, multiple glaciations, and glaciofluvial events. These data provide significant insights on the distribution and character of potential groundwater reservoirs in Prairie regions.

Morphological analysis and evolution of buried tunnel valleys in northeast Alberta, Canada

Tunnel valleys are large elongated depressions eroded into unconsolidated sediments and bedrock. Tunnel valleys are believed to have been efficient drainage pathways for large volumes of subglacial meltwater, and reflect the interplay between groundwater flow and variations in the hydraulic conductivity of the substrate, and basal meltwater production and associated water pressure variations at the ice-bed interface. Tunnel valleys are therefore an important component of the subglacial hydrological system.Three-dimensional modelling of geophysical and lithological data has revealed numerous buried valleys eroded into the bedrock unconformity in northeast Alberta, many of which are interpreted to be tunnel valleys. Due to the very high data density used in this modelling, the morphology, orientation and internal architecture of several of these tunnel valleys have been determined.The northeast Alberta buried tunnel valleys are similar to the open tunnel valleys described along the former margins of the southern Laurentide Ice Sheet. They have high depth to width ratios, with undulating, low gradient longitudinal profiles. Many valleys start and end abruptly, and occur as solitary, straight to slightly sinuous incisions, or form widespread anastomosing networks. Typically, these valleys are between 0.5 and 3 km wide and 10 and 30 m deep, although the depth of incision along some thalwegs exceeds 100 m. Several valleys extend for up to 60 km, but most are between 10 and 30 km long.Valley fills comprise a range of lithofacies, including stacked sequences of diamict, glaciofluvial sands and gravels and glaciolacustrine silts and clays. Displaced bedrock, presumably of glaciotectonic origin, also occurs within several anastomosing valleys. Several channel bodies are exposed along a number of valley sections suggesting progressive valley development through repeated cycles of sediment discharge. Cut-and-fill structures that are capped by fine-grained sequences of rippled sand and mud-rich drapes within these channel bodies suggest unstable flow regimes within the valley and the discharge of sediment-laden basal meltwater under flood-like conditions followed by wane flow events or periods of lower meltwater discharge, likely concomitant with localized modification by glacial ice. Basal meltwater is inferred to have been released as episodic jökulhlaups beneath the western Laurentide Ice Sheet, which at times re-used existing valley systems, which were spatially and temporally stable features, and at other times incised new valleys.

A slope failure caused by drainage cutoff through the advancement of seasonal frost, Hudson Bay Lowland

A failure occurred between December 14 and 17, 2008 in the upper part of a 45-m high, northwest facing bank of the Nelson River in northern Manitoba (56.687°N, 93.777°W). The slope failure occurred at a spring site in a bay associated with a buried valley. The sediment input to the river from this event is roughly 20,000 to 25,000 m3. The source zone is made up of 25 m of water-bearing sand and gravel confined between ice-rich silty clay at the top of the bank and laminated to rhythmically bedded silt and clay at the base of the section. The collapse was confined to the material above the basal silts and clays and was associated with a perched groundwater flow system. A strong argument for drainage cutoff by the advancement of seasonal frost has been demonstrated through the correlation of the bank collapse with the timing of a significant cold snap recorded at two nearby weather stations. The failure illustrates the importance of stratigraphy in controlling bank erosion in this area. Previously, fluvial erosion was seen as an important control on mass wasting in Horseshoe Bay. However, surface information suggests that no toe erosion except to remove the slide deposit has occurred at this site since 2004.

High-resolution seismic reflection profiling: an aid for resolving the Pleistocene stratigraphy of a buried valley in central Illinois, USA

AbstractHigh-resolution seismic reflection (HRSR) data acquired over the Pesotum Bedrock Valley in central Illinois, USA, helped construct the seismic stratigraphy of a valley fill and the overlying sediments. Integrating these data with drilling and borehole geophysics allowed us to develop a seismo-stratigraphic classification for sediments on undulating and folded bedrock. Seven seismo-stratigraphic units that overlie the bedrock surface were characterized. Seismic units A and B include glacial sediments of multiple Pleistocene glaciations above the Pesotum Bedrock Valley, which completely mask the feature. Seismic units C–F, the valley fill, primarily include tills and glacial lake sediment deposited during the earliest Pleistocene glaciations and preglacial alluvium and colluvium that is draped over in situ weathered bedrock. The preservation of conformable-lying glacial and preglacial deposits and paucity of sand and gravel in the buried valley strongly indicate that little or no incision by glacial meltwaters has occurred. These observations contrast markedly with interpretations from buried valleys elsewhere in North America and northern Europe where valley fills contain significant deposits of sand and gravel in tunnel valleys. The HRSR data assisted the characterization and analysis of heterogeneous sedimentary sequences over a buried valley where existing subsurface information was limited. The extent of Pleistocene-age glacial lakes is inferred from the lateral continuity of silt and clay units.