Glaciolacustrine Research Articles

Dumpstones as records of overturning ice rafts in a Weichselian proglacial lake (Rügen Island, NE Germany)

Dumpstones and dropstones up to 0.8 m in size occur in a silty/sandy Weichselian glaciolacustrine succession near Dwasieden on Rugen Island in the SW Baltic Sea (NE Germany). The deposits are exceptional because two levels of dumpstones and dropstones are present, suggesting two dumping phases interrupting characteristic fine-grained glaciolacustrine sedimentation. Plastic downwarping of sediments below the dumpstones and dropstones result in soft-sediment deformation structures. The distribution and orientation of the long axes of the clasts are useful tools for the reconstruction of the state of the lake bottom, as well as for the water depth. The horizontal position of the gravels and boulders (parallel to the bedding) suggests deposition in relatively shallow-water. The dumping events are linked to iceberg rafting in a glacial lake during the Weichselian Glaciation (MIS 2).

Grain texture as a proxy to understand porosity, permeability and density in Chandra Basin, India

The Himalayan Mountains and valley glaciers within it are coupled in such a way that they maintain the ecosystem along the draining stream from the young to old stage. They produce meltwater and nutrient-rich sediments, which serve as precious resources for low-lying areas. Meltwater is assumed to be an important agent of transportation and deposition of sediments in warm-based valley glaciers. Overall, the glacial environment acts as a good erosional system, and the sediments produced form a good-quality aquifer above the hard bedrock in the outwash plain. The glaciofluvial and glaciolacustrine sediments were collected from the Chandra Basin and analysed for the permeability, porosity, density and statistical parameters. Correlation among the different parameters was explained through the regression analysis. The correlation between permeability and mean grain size showed a good regression coefficient, R2 = 0.86 and R2 = 0.97, for glaciofluvial and glaciolacustrine sediments respectively. Correlation between density and porosity was also established against the mean grain size with a good regression coefficient. The physioempirical parameters, effective diameter (D10) and coefficient of uniformity (Cu) were also examined for their dependability on permeability. Hence, this preliminary study attempts to use sediment grain size and texture as a tool to understand the permeability, porosity, density and movement and mobility of water through the glaciofluvial and glaciolacustrine sediments. Also, the correlation study showed that the mean grain size could be used as a factor for predicting the physioempirical model in that region.

Ice marginal dynamics of the last British-Irish Ice Sheet in the southern North Sea: Ice limits, timing and the influence of the Dogger Bank

The southern North Sea is a particularly important area for understanding the behaviour of the British-Irish Ice Sheet (BIIS) during the last glacial cycle. It preserves a record of the maximum extent of the eastern sector of the BIIS as well as evidence for multiple different ice flow phases and the dynamic re-organisation of the BIIS. However, to date, the known ice sheet history and geochronology of this region is predominantly derived from onshore geological evidence, and the offshore imprint and dynamic history of the last ice sheet remain largely unknown. Using new data collected by the BRITICE-CHRONO project this paper explores the origin and age of the Dogger Bank; re-assesses the extent and age of the glaciogenic deposits across the shallow areas of the North Sea between the Dogger Bank and the north Norfolk coast and; re-examines the dynamic behaviour of the BIIS in the southern North Sea between 31.6 and 21.5 ka.This paper shows the core of the Dogger Bank to be composed glaciolacustrine sediment deposited between 31.6 and 25.8 ka. Following its formation the western end of the Dogger lake was overridden with ice reaching ∼54°N where the ice margin is co-incident with the southerly extent of subglacial tills previously mapped as Bolders Bank Fm. This initial ice override and retreat northwards back across the Dogger lake was complete by 23.1 ka, but resulted in widespread compressive glaciotectonism of the lake sediments and the formation of thrust moraine complexes. Along the northern edge of the bank moraines are on-lapped by later phase glaciolacustrine and marine sediments but do not show evidence of subsequent ice override.The shallow seafloor to the west and southwest of the Dogger Bank records several later phases of ice advance and retreat as the North Sea Lobe flowed between the Dogger Bank and the Yorkshire/Lincolnshire coasts and reached North Norfolk. New optically stimulated luminescence (OSL) ages from Garrett Hill on outwash limit the arrival of the BIIS on the Norfolk coast to 22.8–21.5 ka. Multiple till sheets and chains of moraines on the seafloor north of Norfolk mark dynamic oscillation of the North Sea Lobe margin as it retreated northwards. This pattern of behaviour is broadly synchronous with the terrestrial record of deposition of subglacial, glaciofluvial and glaciolacustrine sediments along the Yorkshire coast which relate to post Dimlington Stadial ice marginal oscillations after 21.5 ka.With respect to forcing mechanisms it is likely that during the early phases of the last glacial maximum (∼30-23ka) the interaction between the southern margin of the BIIS and the Dogger Lake was critical in influencing flow instability and rapid ice advance and retreat. However, during the latter part of the last glacial maximum (22–21 ka) late-phase ice advance in the southern North Sea became restricted to the western side of the Dogger Bank which was a substantial topographic feature by this time. This topographic confinement, in addition to decoupling of the BIIS and the Fennoscandian Ice Sheet (FIS) further north, enabled ice to reach the north Norfolk coast, overprinting the seabed with late-phase tills of the Bolders Bank Fm.

Integrated GPR and ERT data interpretation for bedrock identification at Cléricy, Québec, Canada

Ground-penetrating-radar (GPR) and electrical resistivity tomography (ERT) are two geophysical methods widely used for shallow subsurface structure investigations. Although GPR can provide high precision information, it is very limited by the detection depth, especially when there is a medium with high conductivity. This situation is common in the Abitibi greenstone belt of Canada, where fine-grained glaciolacustrine sediments are widespread. Therefore, GPR and electrical methods were combined to investigate the undulating topography of bedrock beneath the clay, to help an infrastructure installation within an urban area. GPR measurements were carried out with four frequencies (450 MHz, 160 MHz, 100 MHz and 80 MHz) and ERT survey were accomplished using a Syscal Pro Switch from Iris Instruments, with 72 electrodes. Shallow boreholes helped in the recognition of the lithology (up to 4 m deep) and in the calibration of geophysical data interpretation. Known infrastructures (underground pipes) were used to estimate the velocity of the radar wave in the uppermost layers, which is estimated at 0.14 m/ns. The data show an unconsolidated material cover, composed of clay, sand and backfill materials, overlying a rugged bedrock surface. The present study highlighted places where the burial of new infrastructures will require rock excavation, and the observations made during the excavation carried out as part of the underground infrastructure installation work made it possible to test the validity of the geophysical measurements.

Monitoring surface changes in discontinuous permafrost terrain using small baseline SAR interferometry, object-based classification, and geological features: a case study from Mayo, Yukon Territory, Canada

Permafrost-induced deformation of ground features is threating infrastructure in northern communities. An understanding of permafrost distribution is therefore critical for sustainable adaptation planning and infrastructure maintenance. Considering the large area underlain by permafrost in the Yukon Territory, there is a need for baseline information to characterize the permafrost in this region. In this study, the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique was used to identify areas of ground movement likely caused by changes in permafrost. The DInSAR technique was applied to a series of repeat-pass C-band RADARSAT-2 observations collected in 2015 over the Village of Mayo, in central Yukon Territory, Canada. The conventional DInSAR technique demonstrated that ground deformation could be detected in this area, but the resulting deformation maps contained errors due to a loss of coherence from changes in vegetation and atmospheric phase delay. To address these limitations, the Small BAseline Subset (SBAS) InSAR technique was applied to reduce phase error, thus improving the deformation maps. To understand the relationship between the deformation maps and land cover types, an object-based Random Forest classification was developed to classify the study area into different land cover types. Integration of the InSAR results and the classification map revealed that the built-up class (e.g., airport) was affected by subsidence on the order of −2 to −4 cm. The spatial extent of the surface displacement map obtained using the SBAS InSAR technique was then correlated with the surficial geology map. This revealed that much of the main infrastructure in the Village of Mayo is underlain by interbedded glaciofluvial and glaciolacustrine sediments, the latter of which caused the most damage to human made structures. This study provides a method for permafrost monitoring that builds upon the synergistic use of the SBAS InSAR technique, object-based image analysis, and surficial geology data.

First-time landslides in Vashon advance glaciolacustrine deposits, Puget Lowland, U.S.A.

Advance glaciolacustrine (Qglv) deposits from the last continental glaciation are widespread in the densely populated Puget Lowland and are prone to shallow landsliding. Much less frequently in historic time, new landslides propagate deeply within intact Qglv deposits. The exceptional volume and highly mobile nature of the 22 March 2014, State Route 530 (Oso) landslide generated considerable uncertainty and public concern about the likelihood for similar deep-seated failures initiating from most any slope where these deposits crop out. The primary study objectives are to utilize geotechnical data from the small data set of available published and public-domain investigations to characterize, in general terms, hydrogeologic, geotechnical and geomorphic factors and their relative contribution to initiating deep-seated landslides within intact Qglv deposits; and to inform future hazard and risk assessments for first-time landslides within these deposits.Qglv deposits are overconsolidated, very stiff to hard, laminated to massive, silt and clay. Shear strength is anisotropic, with cohesion being a significant component of bedding-parallel strength and friction dominating strength perpendicular to bedding. Hydraulic conductivity is likely also anisotropic. Multi-year records of pore pressures within Qglv deposits show no to minor seasonal flux; only minor pore pressure responses to multi-day to multi-week episodes of heavy precipitation have been detected. Back analyses using limit-equilibrium methods and peak anisotropic strength under drained conditions demonstrate initial stability of the slopes. Instability occurs near the fully softened strength. We conclude that, over the long term, loss of cohesive strength, rather than hydroclimatic pore pressure response, is the more important contributor to diminishing stability and, in some cases, initiation of first-time landslides in Qglv deposits. The shape and location of the slide surface is strongly influenced by strength anisotropy and stress state. Sliding surfaces developed near the basal contact of the Qglv deposits in all of the five studied landslides. Higher stress states associated with increasing sequence thickness of the landslide masses decrease stability. The length of the basal slide surface, and thus the aspect ratio of the landslide mass, appears to increase proportionally with sequence thickness of the landslide mass.

Three-dimensional hydrostratigraphy of the Orangeville Moraine area, southwestern Ontario, Canada

Regional-scale three-dimensional modelling of Quaternary sediments in the Orangeville Moraine area of southwestern Ontario has been completed as part of the Ontario Geological Survey groundwater initiative and provides an improved understanding of the glacial history and conceptual hydrostratigraphic framework for that region. Older (Marine Isotope Stage (MIS) 3–5) diamicton, glaciolacustrine, glaciofluvial, and rare nonglacial deposits forming regional aquitards and local aquifers are found in the northwestern part of the area. Catfish Creek Till, deposited during the Last Glacial Maximum (LGM) (MIS 2), forms a key aquitard and stratigraphic marker at depth. Diamicton, fine-textured glaciolacustrine sediments, and the gravel, sand, and silt conduit and subaqueous fan sediments that constitute the overlying Orangeville Moraine were deposited in an ice-walled lake formed between ice lobes during retreat from the LGM. Diamicton deposited during late-glacial ice margin fluctuations forms the upper aquitard unit and buries the edges of the moraine. The Orangeville Moraine is the largest aquifer in the area, and is partially confined by the upper tills. Thick fine-textured glaciolacustrine deposits, Catfish Creek Till, and older aquitards separate the moraine from bedrock aquifers across most of the area. Depending on hydraulic gradients, buried bedrock valleys with gravel and sand fills have the potential to recharge the bedrock aquifer.

Stratigraphic analysis of Late Wisconsin and Holocene glaciolacustrine deposits exposed along the Nottawasaga River, southern Ontario, Canada

Analysis of 56 outcrop exposures in cut banks along the Nottawasaga River in southern Simcoe County, Ontario, Canada, has led to the identification of eight stratigraphic units (SU1–SU8) that represent a record of changing environmental conditions during deglaciation and exhibit strong controls on shallow groundwater flow in the region. The stratigraphic succession is floored by the Late Wisconsin Newmarket Till (SU1), which is locally overlain by ice-proximal debris flow deposits (SU2). These glacial sediments are overlain by glaciolacustrine silt rhythmites (SU3) that pass upwards into deltaic sand (SU4) and channelized fluviodeltaic sand and gravel (SU5). Lying above the fluvial deposits are widespread interbedded glaciolacustrine sands and silt (SU6), which coarsen up-section toward the ground surface. The succession is locally capped by fluviodeltaic (SU7) and younger fluvial (SU8) deposits. These SUs record sedimentary environments that existed during deglaciation of the region and provide insight into the evolution of glacial lakes Schomberg and Algonquin and the Nipissing phase of the upper Great Lakes. The environmental changes described from sediments along the Nottawasaga River provide insights into basin-scale events that occurred throughout the upper Great Lakes during deglaciation. Qualitative observations of groundwater discharge from sediments at outcrop faces are used to characterize the hydraulic function of the stratigraphic units as well as possible preferential groundwater flow pathways in the shallow subsurface.

Conditions of formation of kame near Zalissia village (Volyn Polissia)

This work is an attempt to explain the morphology and geological structure of glacial forms located within the frontal area of the first post maximal stage of Dnipro Glaciation near Zalissia (Volyn Polissia). We tried to explain their origin and conditions of formation. The research included morphological and morphometric methods and method of lithofacies analysis. Presented kames form isolated and semi-isolated hills. They mostly have an irregular shape, slightly elongated, subparallel of the ice frontal zones. The geological structure of the forms was studied in a large sand quarry and in several unrepresentative small outcrops of anthropogenic origin. They are located near the village of Zalissia. This research revealed that fine-grained sediments dominate in the geological structure of these forms. Texture and lithological features of the deposits of Zalissia section helped to identify a few lithofacies complexes in their composition. Thin horizontally layered glaciolacustrine sediments are lying at the base of the form. They are covered with the unsorted detrital deposits (diamicton). Mainly fine and middle sandy deposits of the fluvioglacial complex occur higher in the section. They are rhythmically constructed. Geological structure of section is finished by a layer of unsorted gravel and pebble ablative deposits with mainly sandy filler. Texture features of the deposits of this section make it possible to conclude that their accumulation took place in zones of increased fracture in the body of inactive, dead ice. Sedimentation was carried out in conditions of low-energy glaciofluvial streams and stagnant basins, occasionally interrupted by the accumulation of ablation masses. Such a combination of different deposits may indicate that the formation of this form could take place in the deepenings on the surface of the ice, possibly in large thawed patches, which reached the glacial bed. The thawed patches lay on the intersections of large cracks. The formation of cracks is a consequence of the uneven block movement of the ice masses in front of the barrier in the form of glacial bed elevation. Cracks had a constant tendency to expand as a result of increased ablation processes. These forms are identified as kames based on the detected sedimentological features. Key words: Volyn Polissia, kame, lithofacies analysis, sedimentation, glaciofluvial and glaciolacustrine deposits.

A re-evaluation of Late Devensian glacial Lake Humber levels in the Vale of York, UK

Proglacial Lake Humber formed in the Vale of York and Ancholme Valley during the Late Devensian (Weichselian) glaciation, but its lake levels and their precise ages are uncertain. Three-dimensional geological modelling, based on 193 borehole sediment logs from the eastern part of the Vale of York, indicates that glaciolacustrine sediments extend no higher than 10 m O.D. By contrast, recent palaeoenvironmental reconstructions for the region that suggest Lake Humber had eight recessional shorelines, extending from 42 m to 5 m O.D. Above 10 m O.D., the sediments become more discontinuous, and comprise clay with occasional chalk and flint gravel, and matrix-rich diamicton interdigitated with sands and gravels. Sedimentary and geochemical analyses of sands and gravels from one of the putative shorelines, at 25 m O.D., indicate an easterly provenance for these sediments. They are interpreted here as colluvial deposits, sourced largely by periglacial weathering of Jurassic and chalk bedrock. Collectively, the geological evidence suggests that the highest level of Lake Humber during the Late Devensian did not exceed 10 m O.D., and therefore reconstructions invoking higher lake levels are thought to be unlikely.

Drainage network reorganization affecting the Nene and Welland catchments of eastern England as a result of a late Middle Pleistocene glacial advance

AbstractBritain's latitude is critical for determining the southerly extent of western European ice sheet expansion. Palaeoclimate and palaeosea‐level reconstruction in Britain, however, is complicated by spatially discontinuous glacigenic deposits and genetic and stratigraphic interpretations have relied on their lithological characteristics. This study adopted a sedimentary facies approach combined with geomorphological analyses for investigating upper Middle Pleistocene deposits in two adjacent catchments: Nene and Welland. Detailed sedimentology determined not only genesis of ice‐contact (Welland) and glaciolacustrine (Nene) deposits but also provided insight on lake surface water levels. The ice‐contact deposits recorded a constant lake level at the same height as alluvial remnants upstream in the Welland valley. These alluvial remnants are located where present tributaries join the Welland, indicating they were formed where influent streams entered a former lake and therefore can be interpreted as fluvial terraces resulting from local baselevel rise. The glaciolacustrine deposits in the Nene valley recorded fluctuating lake levels, indicating that altitudinally separated sand and gravel bodies coincident with these fluctuations and located where influent streams entered the lake also can be interpreted as fluvial terraces, but resulting from fluctuating baselevel. Sand and gravel bodies at the southern end of a dry valley linking the two catchments are interpreted as alluvial fan remnants, and those occurring on interfluves downstream of the alluvial fan remnants as flood deposits emanating from lake overflow. This drainage reorganization probably occurred in marine oxygen isotope stage (MIS) 8, but the ice advance footprint is different to that in MI 6.

Near surface geophysical analysis of the Navamuño depression (Sierra de Béjar, Iberian Central System): Geometry, sedimentary infill and genetic implications of tectonic and glacial footprint

The geometric and genetic characterization of the Navamuño depression peatland system (Iberian Central System) is presented here using results from a geophysical survey. This depression is a ~30 ha pseudo-endorheic flat basin over granitic bedrock. Three geophysical techniques were used to map the subsurface geology, and identify and describe the infill sequence: shallow seismic refraction (SR), Magnetic Resonance Sounding (MRS) and electrical resistivity measurements (VES and ERT). The three main geoelectrical layers (G1, G2, G3) identified in previous research, have also been identified in the present work. Using the data obtained in this new research we have been able to analyse these three geological layers in detail and reinterpret them. They can be grouped genetically into two sedimentary units: an ancient sedimentary body (G3), of unknown age and type, beneath an Upper Pleistocene (G2) and Holocene (G1) sedimentary infill. The facies distribution and geometry of the Upper Pleistocene was examined using the Sequence Stratigraphy method, revealing that the Navamuño depression was an ice-dammed in the last glacial cycle resulting in glaciolacustrine sedimentation. A highly permeable sedimentary layer or regolith exists beneath the glaciolacustrine deposits. Below 40 m depth, water content falls dramatically down to a depth of 80 m where unweathered bedrock may be present. The information obtained from geophysical, geological and geomorphological studies carried out in this research, enabled us to consider various hypotheses as to the origin of this depression. According to these data, the Navamuño depression may be explained as the result of a transtensional process from the Puerto de Navamuño strike-slip fault during the reactivation of the Iberian Central System (Paleogene-Lower Miocene, Alpine orogeny), and can be correlated with the pull-apart type basins described in these areas. The neotectonic activity of this fault and the ice-dammed processes in these areas during the Last Glacial Cycle (MIS2) were the main causes of recent sedimentary infill in this depression.

THE SEDIMENTARY COVER STRUCTURE OF THE KANDALAKSHA GULF OF THE WHITE SEA) ACCORDING TO CONTINUOUS SEISMIC PROFILING DATA

The geological structure of the water area adjacent to the White Sea biological research station of the Moscow State University (the Rugoserskaya bay and the Great Salma strait, the Kandalaksha gulf of the White Sea) was examined for the first time basing on seismoacoustic data. The morphology of top of the Archaean basement, the structure of the Quaternary sedimentary cover and the bottom topography were studied. A sequence of glacial deposits dating back to the last glaciation and a compound sequence of glaciolacustrine, glacial-marine and marine sediments were delineated. Spatial configuration and thickness behavior were examined. It was demonstrated that the recent bottom topography is controlled by the faults, characterized predominantly by north-western and the north-eastern strike, and affecting the entire sedimentary cover including the Holocene sediments.

Timing of lake-level changes for a deep last-glacial Lake Missoula: optical dating of the Garden Gulch area, Montana, USA

Glaciolacustrine sediments in the Clark Fork River valley at Garden Gulch, near Drummond, Montana, USA record highstand positions of the ice-dammed glacial Lake Missoula and repeated subaerial exposure. During these highstands the lake was at greater than 65% of its recognized maximum capacity. The initial lake transgression deposited a basal sand unit. Subsequent cycles of lake-level fluctuations are recorded by sequences of laminated and cross laminated silt, sand, and clay deformed by periglacial processes during intervening periods of lower lake levels.Optically stimulated luminescence (OSL) dating of quartz sand grains, using single-aliquot regenerative-dose procedures, was carried out on 17 samples. Comparison of infrared stimulated luminescence (IRSL) from K-rich feldspar to OSL from quartz for all the samples suggests that they were well bleached prior to deposition and burial. Ages for the basal sand and overlying glaciolacustrine exposure surfaces are indistinguishable within one standard deviation, and give a weighted mean age of 20.9 ± 1.3 ka (n = 11). Based on sedimentological and stratigraphic analysis we infer that the initial transgression, and at least six cycles of lake-level fluctuation, occurred over time scales of decades to ∼2 ka. Bioturbated sandy slopewash dated at 10.6 ± 0.9 ka and 11.9 ± 1.2 ka unconformably overlies the upper glaciolacustrine deposits. The uppermost sediments, above the glaciolacustrine section, are younger than the Glacier Peak tephra (13.7–13.4 cal ka B.P.), which was deposited across parts of the drained lake basin, but has not been found at Garden Gulch.Our study indicates that glacial Lake Missoula reached >65 percent of maximum capacity by about 20.9 ± 1.3 ka and either partially or completely drained twelve times from this position. Rapid lowering from the lake's highstand position due to ice-dam failure likely led to scour in the downstream portions of the glacial Lake Missoula basin and megafloods in the Channeled Scabland.

Glaciolacustrine deposits formed in an ice-dammed tributary valley in the south-central Pyrenees: New evidence for late Pleistocene climate

Combined geomorphic features, stratigraphic characteristics and sedimentologic interpretation, coupled with optically stimulated luminescence (OSL) dates, of a glacio-fluvio-lacustrine sequence (Linás de Broto, northern Spain) provide new information to understand the palaeoenvironmental significance of dynamics of glacier systems in the south-central Pyrenees during the Last Glacial Cycle (≈130 ka to 14 ka). The Linás de Broto depositional system consisted of a proglacial lake fed primarily by meltwater streams emanating from the small Sorrosal glacier and dammed by a lateral moraine of the Ara trunk glacier. The resulting glacio-fluvio-lacustrine sequence, around 55 m thick, is divided into five lithological units consisting of braided fluvial (gravel deposits), lake margin (gravel and sand deltaic deposits) and distal lake (silt and clay laminites) facies associations. Evolution of the depositional environment reflects three phases of progradation of a high-energy braided fluvial system separated by two phases of rapid expansion of the lake. Fluvial progradation occurred during short periods of ice melting. Lake expansion concurred with ice-dam growth of the trunk glacier. The first lake expansion occurred over a time range between 55 ± 9 ka and 49 ± 11 ka, and is consistent with the age of the Viu lateral moraine (49 ± 8 ka), which marks the maximum areal extent of the Ara glacier during the Last Glacial Cycle. These dates confirm that the maximum areal extent of the glacier occurred during Marine Isotope Stages 4 and 3 in the south-central Pyrenees, thus before the Last Glacial Maximum. The evolution of the Linás de Broto depositional system during this maximum glacier extent was modulated by climate oscillations in the northern Iberian Peninsula, probably related to latitudinal shifts of the atmospheric circulation in the southern North-Atlantic Ocean, and variations in summer insolation intensity.

Composition and origin of a lithalsa related to lake‐level recession and Holocene terrestrial emergence, Northwest Territories, Canada

AbstractLithalsas of the Great Slave Lowland, Northwest Territories, occur within fine‐grained glaciolacustrine, lacustrine, and alluvial deposits. Detailed investigations of a lithalsa revealed that it is composed of ice‐rich sediments with ice lenses up to 0.2 m thick below 4 m depth. The observed ice accounted for about 2 m of the 4 m between the top of the lithalsa and adjacent terrain. The ice is isotopically similar to modern surface water, but enriched in δ18O relative to local precipitation. Total soluble cation concentrations are low in the basal, Shield‐derived and unweathered glaciolacustrine sediments of the lithalsa. Higher concentrations in the overlying Holocene‐aged lacustrine and alluvial deposits may be due to greater ion availability in Holocene surface waters. Increasing Cl‐ and Na+ concentrations in clays at depth likely relate to exclusion and migration of these dissolved ions in pore water during ice lens formation though total soluble cations remain comparatively low. The lithalsa developed 700 to 300 cal yr BP. A conceptual model of lithalsa formation and landscape evolution illustrates that this feature and more than 1800 other lithalsas in the region have developed in association with Holocene terrestrial emergence following lake‐level recession. Copyright © 2017 John Wiley & Sons, Ltd.

Influence of tributaries on the longitudinal patterns of benthic invertebrate communities

AbstractThere has been little effort to understand how tributaries influence mainstem rivers at large scales beyond the immediate influence of the tributary and downstream of the mixing zone. Such knowledge is needed to create breaks in stream networks that can aid in the classification of stream valley segments and conservation studies that rely on the delineation of zones. We use benthic invertebrate assemblages to infer longitudinal gradients and discontinuities and relate these patterns to confluence symmetry ratio (CSR; the size ratio of the tributary basin to the mainstem basin upstream of the confluence). In addition, we briefly explore reach and catchment‐scale environmental influences. We found evidence for both gradual and abrupt longitudinal changes in benthic invertebrate communities. There was not a smooth continuous gradient but a sawtooth pattern with an overarching trend. Two major discontinuities were found: one associated with a large CSR = 0.74 and reach scale factors including predominance of sand and an abundance of benthic organic matter that provided a unique habitat; and a second associated with a large CSR = 0.64 and a transition from coarse textured morainal deposits to glaciolacustrine deposits. There were synchronous additions of some benthic invertebrates (e.g., Eukiefferiella brehmi, Antocha, Hydropsyche morosa, and Oligochaeta) showing an affinity for downstream reaches, whereas others showed an affinity for headwater reaches (e.g., Simulium tuberosum, Baetis tricaudatus, and Micropsectra). Benthic invertebrate communities were driven by a combination of confluence symmetry ratio, landscape, and reach scale factors that can confound interpretation.

Longevity of dams from landslides with sub-channel rupture surfaces, Peace River region, Canada

BackgroundWe examined seven landslide dams and their changes over time in the Peace River region of Canada. These landslides had subchannel rupture surfaces in glacial and glaciolacustrine sediments. We assessed the stability of the dams using 6 separate, morphometric-based stability indices (with a total of 10 stability thresholds).ResultsThe landslides caused the streambeds to be elevated from 4 to 30 m forming the dams. The landslide lakes diminished in size over one to several years through stream incision into the dams and sediment infilling. The longest-lived dam persisted for up to 20 years. For two dams, incision into the dams lowered the lake levels by about half of the total depth, while the remainder of the water in the basins was displaced by sediment infilling. After the lakes drain, the sediment accumulations behind the dams can persist for decades. The stability analyses overpredicted unstable conditions which is inconsistent with the observed longterm persistence of the dams. ConclusionsThe landslide dams in our study were relatively stable. Their lakes persisted for up to 2 decades and diminished over time through a combination of slow incision and basin infilling. The stability indices we assessed overpredicted unstable conditions and thus would require modification for these particular types of dams in this regional setting.

High-resolution chronology for deglaciation of the Patagonian Ice Sheet at Lago Buenos Aires (46.5°S) revealed through varve chronology and Bayesian age modelling

Glaciolacustrine varves offer the potential to construct continuous, annually-resolved chronologies for ice-sheet deglaciation, and improved understanding of glacier retreat dynamics. This paper investigates laminated glaciolacustrine sediments deposited around the waning margins of the Patagonian Ice Sheet, following the local Last Glacial Maximum (LGM). Detailed macro- and microfacies analyses confirm an annual (varve) structure within these sediments. The correlation of annual layers (varves) across five sites in eastern Lago Buenos Aires yields a 994 ± 36 varve-year (vyr) chronology and thickness record. The floating chronology has been anchored to the calendar-year timescale through identification of the Ho tephra (17,378 ± 118 cal a BP) in the varve sequences. Using a Bayesian age model to integrate the new varve chronology with published moraine ages, the onset of deglaciation at 46.5°S is dated to 18,086 ± 214 cal a BP. New age estimates for deglacial events are combined with high-resolution analysis of varve thickness trends, and new lithostratigraphic data on ice-margin position(s), to reconstruct ice-margin retreat rates for the earliest ca. 1000 years of ice-sheet demise. Glacier retreat rates were moderate (5.3–10.3 m yr−1) until 17,322 ± 115 cal a BP, but subsequently accelerated (15.4–18.0 m yr−1). Sustained influxes of ice-rafted debris (IRD) after 17,145 ± 122 cal a BP suggest retreat rates were enhanced by calving after ice contracted into deeper lake waters. Ice persisted in eastern Lago Buenos Aires until at least 16,934 ± 116 cal a BP, after which the glacier started to retreat towards the Patagonian mountains.

AN GEOLOGICAL OVERVIEW OF GLACIAL ACCUMULATION AND EROSIONAL OCCURRENCES AT THE VELEBIT AND THE BIOKOVO MTS., CROATIA

Numerous accumulation and erosional forms originating from the activity of small valley glaciers or cirque glaciers occur in the highest mountains in Croatia: Velebit (1757 m) and Biokovo (1762 m). They were produced during the Upper Pleistocene, in the Wurm glacial stage of the Alpine classification.Accumulation forms comprise ground, terminal and recessional moraines, drumlins, eskers, glacial erratics and glaciofluvial and glaciolacustrine sediments. Single ridge eskers are often associated with areas of kame and kettle topography. Among erosional occurrences roche moutonnee (sheepback rocks), U-shaped valleys ranging in size from meso-macro, aretes, hanging valleys, meso-sized cirques, kettles, and striations were noted. In Croatian Dinarides such forms in most cases occur between 900 and 1400 meters altitude. During the early to middle Wurm glacial maximum, the snow line was above 900 m, perhaps even above 1000 m altitude, and sea levels were 120 meters lower than at present day. Considering the features of the present relief, ice cover was probably 200 to 300 m thick. Features of drumlins, eskers and kettles point to warm-based glaciers. The drumlins are small – up to 100 meters long and 50 meters wide, with the most common long axis ranging orientation from 130o – 310o. The near total absence of platy clasts, as well as their stratigraphic affiliation, largely reflects features of source rocks.