Landform Assemblages Research Articles

Fluvial system response to tectonically induced base-level change during the late-Quaternary: The Rio Alias southeast Spain

The Rio Alias of southeast Spain crosses a major tectonically active structure, the Carboneras Fault Zone, within the central portion of the drainage basin. The fault originated in the mid-Miocene and has c.40 km of strike-slip offset. The impact of late Quaternary movement along the fault zone on the evolution of the Rio Alias is addressed. Geomorphological and sedimentological analysis of the landform assemblage preserved suggests the development of highly sinuous meander loops and laterally incising streams during phases of tectonic activity. Continued tectonic activity led to the abandonment of the meander loops and a wave of incision that propagated c.2 km upstream. The propagation of this incision wave was restricted by the development of a nickpoint, itself controlled by an anticlinal structure sub-parallel to the fault zone. Syn-sedimentary and post-depositional faulting is evident within the late Quaternary fluvial sediments and suggests dominantly vertical movement along the fault lines with downthrow to the southeast. Regional climatic patterns of aggradation and incision resulting in a basinwide terrace sequence were complicated (i.e. an additional terrace was created and meander wavelength was changed) through the study reach by tectonic activity. This study highlights the importance of tectonically modified sequences when interpreting climatically generated sequences of aggradation and incision.

Landform assemblages produced by the Laurentide Ice Sheet in northeastern British Columbia and adjacent Northwest Territories — constraints on glacial lakes and patterns of ice retreatThis article is one of a selection of papers published in this Special Issue on the theme Geology of northeastern British Columbia and northwestern Alberta: diamonds, shallow gas, gravel, and glaciers.

The Laurentide Ice Sheet reached the Canadian Cordillera during the last glacial maximum in northeastern British Columbia and adjacent Northwest Territories and all regional drainage to unglaciated areas in the north was dammed by the ice. Converging ice-flow patterns near the mountain front suggest that the Laurentide Ice Sheet likely coalesced with the Cordilleran Ice Sheet during the last glaciation. With deglaciation, the ice masses separated, but earlier ice retreat in the south meant that meltwater pooled between the mountain front and the Laurentide margin. The level of the flooding was controlled by persistent ice cover on the southern Franklin Mountains. Glacial Lake Liard formed when the Laurentide Ice Sheet retreated east of the southern Liard Range and, at its maximum extent, may have impounded water at least as far south as the Fort Nelson River. Deglaciation of the plains was marked by local variations in ice flow caused by a thin ice sheet becoming more affected by the topography and forming lobes in places. These lobes caused diversions in local drainage readily traced by abandoned meltwater channels. Radiocarbon ages from adjacent areas suggest the relative chronology of deglaciation presented here occurred between 13 and 11 ka BP.

Geological constraints on Antarctic palaeo‐ice‐stream retreat

AbstractSubmarine landforms preserved in bathymetric troughs on the Antarctic continental shelf show that the style of ice stream retreat across the shelf following the last glacial maximum varied between different troughs. Three styles of retreat are inferred from the geological evidence: rapid, episodic and slow. Rapid retreat by ice stream floatation and calving is recorded by the preservation of a landform assemblage of unmodified streamlined subglacial bedforms including mega‐scale glacial lineations (MSGLs) that record streaming flow along these troughs. These elongate bedforms are not overprinted by recessional glacial landforms formed transverse to ice flow such as moraines or grounding‐zone wedges, and overlying deglacial sediments are thin. A second type of landform assemblage consists of MSGLs overprinted or interrupted by transverse grounding‐zone wedges. This assemblage implies episodic retreat between successive grounding‐zone positions. The third type of landform assemblage is that of numerous, closely spaced, recessional moraines and intermittent grounding‐zone wedges that overlie and interrupt MSGLs. This assemblage records the slow retreat of grounded ice across the shelf. Variation in the style of ice stream retreat between the different bathymetric troughs indicates that Antarctic palaeo‐ice‐streams did not respond uniformly to external forcing at the end of the last glacial cycle. Rather, their diachronous retreat reflects the dominance of local controls in the form of bathymetry and drainage basin size. More broadly, these data show that retreat of marine‐based ice sheets in areas of reverse bed slope is not necessarily catastrophic, and they provide important constraints for numerical models that attempt to predict the dynamics of large polar ice sheets. Copyright © 2008 John Wiley & Sons, Ltd.

Superimposition of ribbed moraines on a palaeo‐ice‐stream bed: implications for ice stream dynamics and shutdown

AbstractThe sediments and landforms preserved on palaeo‐ice‐stream beds can provide important information about their subglacial conditions and flow mechanisms, and the processes accompanying their shutdown. In this paper, detailed observations of an intriguing subglacial landform assemblage of ribbed moraines superimposed on glacial lineations on the Dubawnt Lake Ice Stream bed (north‐west Canadian Shield) are presented, including their morphometry, internal structure (from ground penetrating radar (GPR) surveys and from glaciogeological analysis) and sedimentological characteristics (from sediment architecture and lithofacies analysis). The observations suggest an abrupt change in ice dynamics that correlates with two phases of glacial landform development. This hypothesis is based on evidence from a deformed lodgement till, which subsequently underwent brittle deformation and developed prominent thrust (shear) structures and tension fractures. Tension fractures are observed in a sediment exposure and thrust structures are observed in GPR surveys, where they occur most prominently in the ribbed moraine ridge crests. The presence of the fractures, and their association with a population of clasts in the till that are orientated with their a‐axes transverse to the inferred ice flow direction, suggests a compressional flow regime. It is therefore inferred that the glacial lineations were formed under an extensional flow regime during ice stream activity, but that at some point patches of till under the ice stream stiffened through dewatering. The subsequent increase in basal shear stress resulted in compressional flow and the development of subglacial thrusting and the building of ribbed moraines. We therefore suggest that ribbed moraines may form in areas of compressional flow under ice streams, i.e. sticky spots, and/or at the transition between slow and fast ice flow along parts of an ice stream. The general absence of ribbed moraines on most other palaeo‐ice‐stream beds suggests that either these ice streams continued operating during deglaciation or processes other than the development of localized compressional flow (sticky spots) led to their shutdown (e.g. ice depletion). Copyright © 2008 John Wiley & Sons, Ltd.

Sediment–landform assemblages and digital elevation data: Testing an improved methodology for the assessment of sand and gravel aggregate resources in north-western Britain

This paper uses the results of a series of sand and gravel resource assessment surveys to refine and improve the methods used for identifying aggregate resources in north-western Britain. Geomorphological surveys of the Cors Geirch and Rhosesmor areas of north Wales and the lower Ribble valley in Lancashire, carried out by 1:10,000 scale field mapping, are compared to resource assessments made using high resolution Digital Elevation Models (DEMs) of the same ground. The DEMs proved useful tools for remote geomorphological survey, but precision was significantly better for the Intermap NEXTMAP and, particularly, the Environment Agency LiDAR dataset. Only the LiDAR data revealed with any clarity small-scale and low relief features, for example scroll-bar palaeochannels on river terraces. The identification of sediment–landform assemblage zones from geomorphological survey permits the identification of the primary sedimentary process responsible for each assemblage and the attachment of an ideal sedimentary facies model to it. Each model can then be used to predict sedimentary composition, the optimal location of boreholes and the location of resource blocks. Combining the use of DEM data and the Spatial Analyst toolset within ArcGIS™ (ESRI) allows rapid accurate delineation of landforms and the use of cut and fill functions allows more accurate estimation of the volume of resources than the traditional calculations based on area and average thickness. These new methods improve the accuracy and cost-effectiveness of geomorphology-based aggregate assessments, notably where the mineral-bodies are concealed beneath minimal overburden and produce characteristic landforms.

Submarine glacial landforms and rates of ice-stream collapse

The rate of deglacial ice-sheet retreat across polar continental shelves, and possible ice-stream collapse and sea-level rise, has been much debated. High-resolution imagery of seafloor morphology is available for many polar shelves and fjords. The rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms, produced at ice-stream sedimentary beds. These landforms, exposed by ice retreat across high-latitude shelves, demonstrate that deglaciation occurs in three main ways: rapidly, by flotation and breakup; episodically, by still-stands and/or grounding events punctuating rapid retreat; or by slower retreat of grounded ice. Submarine landform assemblages imply, through the presence of grounding-zone wedges overprinting mega-scale glacial lineations on many polar shelves, that ice-stream retreat is more often episodic than catastrophic. These observations provide a robust test of the ability of numerical models to predict the varied response of ice-sheet basins to environmental changes.

Impact of multiple glacier surges—a geomorphological map from Brúarjökull, East Iceland

Please click here to download the map associated with this article. Simultaneously with geological field investigations at the surge-type glacier Brúarjökull in August 2003, aerial photographs covering the central part of the glacier including the Kringilsárrani area were recorded with the purpose of mapping glacial landforms. A glacial geomorphological map has been completed at a scale 1:16,000 including more than 20,000 landforms from an area of c. 8 km2. Manual classification of landforms was carried out directly in stereoscopic view using a digital photogrammetric workstation taking full advantage of the texture, topography and spatial context of individual landforms. We mapped subglacial lineations such as flutes and drumlins on till plains, pitted outwash, eskers, minor meandering ridges, crevasse fill ridges, ice-free dead-ice moraine and concertina ridges, outwash fans and lake sediment plains. In addition, erosional drainage channels, ice-marginal ridges, overridden end-moraines, and extra-marginal ice wedge polygons and collapsed palsas were mapped. The mapped glacial landforms originate from at least four surges: pre-1810, 1810, 1890 and 1964. The distribution of landforms on the Brúarjökull fore-eld has close resemblance to landform assemblages of palaeo-ice streams. The present terrain surface at Brúarjökull is the cumulated result of multiple landform generations because each surge has superimposed a new association of landforms on older surfaces.

Structural and depositional evidence for repeated ice-marginal oscillation along the eastern margin of the Late Devensian Irish Sea Ice Stream

Late Devensian glacigenic sediments and landforms along the north-west coast of Wales document the advance and subsequent retreat of the eastern margin of an Irish Sea Ice Stream that met, coalesced and ultimately uncoupled from ice radiating outwards from the adjacent Welsh Ice Cap centred over Snowdonia. Across the boundary between the two former ice masses is a set of sediment–landform assemblages that reflect rapidly changing erosional and depositional conditions during ice interaction. From the inner part of the ice-stream the assemblages range outwards, from a subglacial depositional assemblage, characterised by drumlin swarms; through a subglacial erosional assemblage, marked by prominent bedrock scours and large subglacial rock channels; through an ice-marginal assemblage, identified by closely spaced, glaciotectonised push moraines and intervening marginal sandur troughs; into a freely expanding proglacial sandur and lacustrine delta assemblage. The ice-marginal assemblage provides evidence for numerous oscillatory episodes during retreat and at least 20 ice-marginal limits can be identified. At least 11 of these display multiple criteria for identifying readvance and, in the ideal case, is characterised by a moraine form built by localised tectonic stacking of diamict to the rear, fronted by a clastic wedge of ice-front alluvial fan gravel and intercalated flow till. The distribution of sediment–landform assemblages suggests a highly dynamic, convergent ice-stream flow pattern, with high ice velocity, a sharply delineated lateral shear margin, pervasive ice-marginal glaciotectonic deformation and a tightly focused ice-marginal sediment delivery system; all signature characteristics of contemporary ice streams.

Catastrophic flooding origin of shelf valley systems in the English Channel

Megaflood events involving sudden discharges of exceptionally large volumes of water are rare, but can significantly affect landscape evolution, continental-scale drainage patterns and climate change. It has been proposed that a significant flood event eroded a network of large ancient valleys on the floor of the English Channel-the narrow seaway between England and France. This hypothesis has remained untested through lack of direct evidence, and alternative non-catastrophist ideas have been entertained for valley formation. Here we analyse a new regional bathymetric map of part of the English Channel derived from high-resolution sonar data, which shows the morphology of the valley in unprecedented detail. We observe a large bedrock-floored valley that contains a distinct assemblage of landforms, including streamlined islands and longitudinal erosional grooves, which are indicative of large-scale subaerial erosion by high-magnitude water discharges. Our observations support the megaflood model, in which breaching of a rock dam at the Dover Strait instigated catastrophic drainage of a large pro-glacial lake in the southern North Sea basin. We suggest that megaflooding provides an explanation for the permanent isolation of Britain from mainland Europe during interglacial high-sea-level stands, and consequently for patterns of early human colonisation of Britain together with the large-scale reorganization of palaeodrainage in northwest Europe.

Paraglacial landform assemblages in the Hindukush and Karakoram Mountains

The paper focuses on the evolution of debris landforms in subtropical high-mountain areas. The Karakoram and Hindukush Mountains are characterized by their high volume and variety of debris accumulations. The valley flanks are coated by unconsolidated debris slopes up to 1000 m in height. The valley floors are occupied by expansive debris-flow cones and alluvial fans with escarpment heights of over 100 m. The field investigations in over twenty valleys showed that the major part of those landforms is governed by the late and post-glacial glaciation history. A key role in this glacially-controlled landscape system is played by the slope moraines, which cover the valley flanks up to several hundred meters above the valley floor. The secondary debris supply by resedimentation processes of moraine material and glacially-induced rock failures exceed by far the primary debris production due to weathering processes. Landforms which have been formerly classified as “periglacial talus cones” rather can be considered as glacial or glacially-controlled-landforms. The identified paraglacial landforms can serve as useful palaeoclimatic indicators for the ice-age reconstruction.

Palaeo‐ice streaming in the central sector of the British—Irish Ice Sheet during the Last Glacial Maximum: evidence from the northern Irish Sea Basin

Late Devensian glacial sediments and landforms of the Isle of Man record the advance and deglacial signature of the central sector of the British‐Irish Ice Sheet. Evidence from the area, gathered from striae, erratic trains and drift limits, show ice was routed over and around the island in two flow phases post‐36 kyr BP In the south of the island, streamlined depositional bedforms with low elongation ratios suggest low ice‐flow velocities resulting from one or more of (i) the up‐ice location of the island within a regional onset zone, (ii) flow retardation of ice interacting with the margins of the island and (iii) localized drainage of the deforming bed. The deglacial landform assemblage of lateral marginal sandurs and drainage diversions, coupled with a lack of dead‐ice features, suggests ice did not downwaste in situ but retreated intact along the coastal margins as Manx Upland ice thinned. In the north of the island, however, the Bride Moraine complex indicates a change in deglacial ice‐sheet dynamics, with temporary re‐advance and marginal oscillation causing proglacial tectonism and thrusting of the glacial sediment pile, possibly during the Killard Point Stadial event (18.8–16.4 cal. kyr BP). From a basin‐wide perspective, the Irish Sea Basin sector of the British‐Irish Ice Sheet had many of the characteristics of an ice stream, such as a zone of flow convergence up‐ice, a grounding line in the southern Celtic Sea and recessional limits characterized by proglacially tectonized and thrust dead‐ice landscapes indicative of a rapidly oscillating ice margin.

Loch Lomond Stadial Glaciers in the Uig Hills, Western Lewis, Scotland

Abstract Geomorphological mapping of the Uig Hills provides evidence for four locally‐nourished valley glaciers with a total area of 11.69 km2. A Loch Lomond Stadial (∼12.9 — 11.5 ka BP) age for this glacial episode is indicated by the radial configuration of reconstructed glaciers, the landform assemblage and the absence of frost‐weathered rock inside its limits. Recessional moraines extend upvalley from former glacier termini, implying that glacier retreat was driven by gradual climatic forcing. The mean equilibrium line altitude (ELA) of the glaciers is calculated as 212 m, consistent with a northwards ELA decrease along the western seaboard of Great Britain of 68.5m/100 km, equivalent to a latitudinal summer temperature gradient of ∼0.41°C/100 km. A mean July sea‐level temperature of ∼7.1°C is inferred for the coldest part of the stade. Average accumulation at the ELA is estimated to have been 2585 ± 200 mm, much of which probably comprised snow blown from adjacent ground.

Loch Lomond Stadial Glaciers in the Uig Hills, Western Lewis, Scotland

Geomorphological mapping of the Uig Hills provides evidence for four locally‐nourished valley glaciers with a total area of 11.69 km2. A Loch Lomond Stadial (∼12.9 — 11.5 ka BP) age for this glacial episode is indicated by the radial configuration of reconstructed glaciers, the landform assemblage and the absence of frost‐weathered rock inside its limits. Recessional moraines extend upvalley from former glacier termini, implying that glacier retreat was driven by gradual climatic forcing. The mean equilibrium line altitude (ELA) of the glaciers is calculated as 212 m, consistent with a northwards ELA decrease along the western seaboard of Great Britain of 68.5m/100 km, equivalent to a latitudinal summer temperature gradient of ∼0.41°C/100 km. A mean July sea‐level temperature of ∼7.1°C is inferred for the coldest part of the stade. Average accumulation at the ELA is estimated to have been 2585 ± 200 mm, much of which probably comprised snow blown from adjacent ground.

Snow‐avalanche impact landforms, deposits and effects at urdvatnet, southern norway: implications for avalanche style and process

.An unusual assemblage of landforms and deposits is described from upper Norangsdalen, Sunnmøre region, southern Norway, and interpreted as the product of snow‐avalanche events that vary in magnitude, frequency and debris content. An avalanche impact plunge pool, proximal scar and distal mound are associated with a coarse gravel deposit covering part of the valley floor. Landforms in this debris spread include gravel ridges, boulder lines, beaded ridges, fine sediment banked against and covering large boulders, and gravel clumps. Many of these landforms are aligned, indicating across‐valley transport radiating from the plunge pool. Features were mapped in the field and samples analysed for grain size and heavy‐mineral content. The debris spread is attributed to deposition by high‐energy, debris‐rich snow‐avalanche events that collect debris from large areas of the valley side, lower slopes and plunge pool. Aligned landforms develop through sediment transport in a basal shear zone, and randomly distributed gravel clumps represent melt pits following debris transport in the avalanche body. Air displacement ahead of larger avalanches is thought to have felled and tilted trees on the lower slopes of the distal valley side. Approximate ages of damaged trees allowed estimation of the frequency of snow‐avalanche events: (1) small, frequent events (several per annum) carry debris to the lower valley slopes and the plunge pool; (2) moderate events with an annual to decadal frequency maintain the pool–scar–mound complex; and (3) large, debris‐rich events with a decadal to centennial frequency add material to the debris spread.

Morphostratigraphic principles in glacier reconstruction -a perspective from the British Younger Dryas

Glacier reconstruction enables the calculation of palaeoglaciological and palaeoclimatic variables such as the equilibrium-line altitude and palaeo-precipitation values. Such data are important for our understanding of past atmosphere-cryosphere interactions and as input variables to constrain numerical models effectively. Numerical dating is crucial to constrain the age of glacial events, but, due to absence of dateable material and/or contamination problems, ice masses can frequently be constrained satisfactorily only in a few locations. Thus, extrapolation and interpolation of geomorphological evidence is required to establish the extent of glaciers at a given time for the whole ice mass. Using examples from areas in Britain that were last covered by glaciers during the Younger Dryas, geomorphological approaches are reviewed and potential pitfalls highlighted. A multiproxy morphostratigraphic approach that utilizes clear landsystem contrasts inside and outside dated glacial limits is developed and tested in an area where numerical dates are sparse. Landform assemblages suitable in this respect are the type and frequency of moraines, river terrace sequences, glaciofluvial landforms, raised beaches, the upslope terminations of sediment cover (‘drift limits’), periglacial trimlines and periglacial features such as blockfields, solifluction lobes and thick talus accumulations. It is concluded that, if multiple lines of geomorphological evidence converge, these can be used to confine the extent of past glaciers in a given area and to guide dating programmes.

The impact of a major Quaternary river capture on the alluvial sediments of a beheaded river system, the Rio Alias SE Spain

The Rio Alias is a transverse river system developed within the Sorbas and Almeria Neogene sedimentary basins within the Internal Zone of the Betic Cordillera, southeast Spain. Headwaters of the Rio Alias rise in the southern margins of the Sorbas basin following a superimposed transverse course across the Sierra Alhamilla/Cabrera and its associated boundary faults. A major river capture event within the Sorbas Basin (c.70 ka) created a situation whereby the Rio Alias abruptly lost c. 70% of its drainage area. Such an extensive loss of drainage area led to significant modification of the fluvial system in both upstream and downstream zones on the capturing stream, and downstream on the beheaded system. On the beheaded Rio Alias, a combination of terrace mapping, clast identification/analysis, field descriptions of pedological development and mineral magnetic analysis allows the relative timing of the capture event to be confirmed. Examination of the fluvial architecture of the Quaternary terrace deposits reveals significant changes in bedform geometry, involving a reduction in bedform height, degree of channelisation and maximum clast size in the post-capture sediment assemblage. The post-capture evolution of the Rio Alias has been heavily influenced by the river capture event, highlighting the need for further analysis of beheaded drainage systems and their influence on developing landform assemblages.

Formation and deformation of basal till during a glacier surge; Elisebreen, Svalbard

Elisebreen, a retreating valley glacier in NW Svalbard, experienced a surge in the past, possibly during the Little Ice Age when ice was thicker and the bed warmer. The surge produced an assemblage of subglacial landforms and sediments now appearing in front of the downwasting ice margin. We have analysed an array of sedimentary properties of basal till from glacial flutings, ploughing marks, an interflute ground moraine, and meandering ridges in order to infer processes of till formation and deformation during the unrecorded surge. Till in flutings and ploughing marks reveals higher mechanical abrasion than in other landforms, but grain size distribution and mineralogy are largely the same everywhere. Micromorphological characteristics are also very similar and comprise ductile and brittle deformation structures, indicating fluctuating porewater pressures. Frequent micrograin fracturing suggests stress concentration along grain bridges during the surge. Till was formed by a combination of lodgement and deformation, caused mainly by clast ploughing. Meandering ridges containing till with water escape structures indicate that surge termination was facilitated by rapid water evacuation from the ice-bed interface through R-type channels. Micromorphological similarity of till found in flutings, ploughing marks, and interflute areas suggests that the resolution of the micromorphological signature is insufficient to constrain the different processes that led to the formation of those landforms.

Assemblages of submarine landforms produced by tidewater glaciers in Svalbard

High‐resolution swath bathymetry from the marine margins of several Svalbard tidewater glaciers shows an assemblage of submarine landforms that is probably linked to glacier surging. These landforms are essentially unmodified since their initial deposition over the past hundred years or so because they have not been subjected to subaerial erosion or periglacial activity. Swath images comprise an assemblage of superimposed landforms, allowing reconstruction of relative age of deposition: (1) large transverse ridges, interpreted as recessional moraines overridden by a subsequent ice advance; (2) a series of curvilinear streamlined bedforms orientated parallel to former ice flow, interpreted as lineations formed subglacially during rapid advance; (3) large terminal ridges, marking the farthest extent of ice at the last advance, with flow lobes immediately beyond interpreted as submarine debris flows; (4) a series of interconnected rhombohedral ridges, interpreted as a product of soft sediment squeezing into crevasses formed at the glacier bed, probably formed during immediate post‐surge stagnation; and (5) a series of fairly evenly spaced small transverse ridges, interpreted as push moraines produced annually at tidewater glacier termini during retreat. A simple descriptive landsystem model for tidewater glaciers of probable surge type is derived from these observations. We also show that megascale glacial lineations can form not only beneath large ice streams, but are also produced beneath surging tidewater glaciers lying on deforming sedimentary beds.

Late quaternary paraglacial sedimentation in the Macgillycuddy's reeks, southwest Ireland

Abstract Talus slopes in the Curraghmore cirque basin in the Macgillycuddy's Reeks have developed since the end of the Glenavy Stadial (Last Glacial Maximum; MIS 2). The site lay outside Younger Dryas glacier limits and, as a result, thick rockfall and debris flow accumulations form a significant landform assemblage draping lower hillslopes. Rockwall retreat rates since icesheet deglaciation average 0.08mm per year, and 0.2mm per year up to the beginning of the Holocene.

Lake-ice conveyor deposits: Geomorphology, sedimentology, and importance in reconstructing the glacial history of the Dry Valleys

During the last glacial maximum (LGM), lake-ice conveyors transported glacial debris across the surface of large proglacial lakes in the Dry Valleys region of Antarctica. The resulting sediments and landform assemblages are keys to interpreting the glacial history of the Dry Valleys and, potentially, other areas of Antarctica. Paleoconveyor deposits occur in Taylor, Miers, Wright, and Victoria Valleys, as well as in many other parts of the region. Sediments typically consist of stratified to massive sand and/or silt grading upward into poorly sorted gravel, cobbles, and boulders. These sediments make up a variety of landforms, including mounds and moat line, cross-valley, and longitudinal ridges. The classic landform assemblage also displays an associated moraine bank, as well as deltas and shorelines.