Landform Assemblages Research Articles

Glacial and submarine processes on the shelf margin of the Disko Bay Trough Mouth Fan

Fast-flowing ice streams and outlet glaciers exert a major control on glacial discharge from contemporary and past ice sheets. Improving our understanding of the extent and dynamic behaviour of palaeo-ice streams is crucial for predictions of how the cryosphere will respond to climate warming and the associated implications for global sea level. This paper presents results from two 3D-seismic surveys located on the continental shelf adjoining the Disko Bay Trough Mouth Fan (TMF), one of the largest glacial outlet systems in Greenland. Located at the seaward terminus of the c. 370km long cross-shelf Disko Trough, the Disko Bay TMF was generated by highly efficient subglacial sediment delivery onto the continental slope during repeated ice stream advances. A variety of submarine glacial landform assemblages are recognised on the seabed reflecting past ice stream activity. The 3D-seismic study covers the shallow banks located north and south of the Disko Trough and sheds focus on the seabed and the uppermost stratigraphic interval associated with the late Pleistocene development. The buried section (probably of Saalian age) contains a prominent grounding-zone wedge (GZW) in the northern and low-angle progradational packages in the southern area, indicating a period of major glacial advances to the shelf margin. Subsequently, the outer margin was influenced by glacimarine sedimentation, localized shelf-edge ice advances and sediment transport by contour currents that possibly began in the last interglacial period (Eemian). During the last (de)glaciation, the northern bank appears to have been covered by passive ice leaving a field of dead-ice deposits. In contrast, multiple sets of terminal moraine ridges observed on the southern bank suggest a slow retreat of active, grounded ice from the Last Glacial Maximum (LGM) position on the outer shelf. Isostatic and tectonic influences on relative sea level may have played a role in generating the divergent glacial configurations of the northern and southern bank areas.

Late Quaternary paleoseismicity and seismic potential of the Yilan-Yitong Fault Zone in NE China

The Yilan-Yitong Fault Zone (YYFZ), which is composed of two nearly parallel branches with a spacing of 5–30 km and a length of ∼1100 km, is considered to be the key branch of the Tancheng-Lujiang Fault Zone (TLFZ) in NE China. It was traditionally believed that the YYFZ experienced weak activity or was inactive during the Late Quaternary, without the capability to generate strong earthquakes (M ≥ 7), based on the absence of typical outcrops and large historical or instrumental earthquakes (M > 6). However, our paleoseismic study shows that the YYFZ is the primary seismotectonic structure (M ≥ 7) that poses significant earthquake threats to NE China. The synthesis of data collected from geologic investigations, geomorphic mapping, trench logging and the dating of samples indicates that the YYFZ is an active structure that has undergone segmented strong tectonic deformation since the Late Quaternary with a characteristic assemblage of landforms, including linear scarps and troughs, offset or deflected streams, linear sag ponds, small horsts and grabens. The latest ruptures of the YYFZ migrated from previous boundary faults into the basin interior, forming a left-stepping en echelon pattern in plain view, and the kinematics of these events in the Late Quaternary were dominated by reverse dextral slipping. Multi-segment cluster faulting might have occurred during three cluster periods, i.e., ∼34750–35812 a BP, ∼21700–22640 a BP, and ∼4000 a BP-present, which implies that the recurrence interval of large earthquakes along the YYFZ may be as long as tens of thousands of years.

Submarine landform assemblages and sedimentary processes in front of Spitsbergen tidewater glaciers

New swath-bathymetric data from the inner parts of the three Svalbard fjords Ymerbukta, Trygghamna, and Magdalenefjorden reveal the landform assemblages deposited in front of tidewater glaciers in west and northwest Spitsbergen. Overridden moraines in Ymerbukta, a tributary of Isfjorden in central west Spitsbergen, record several re-advances of the Esmarkbreen glacier at the head of the fjord after deglaciation, and glacial lineations formed in seafloor sediments are indicative of fast ice advance during one of these events. A terminal moraine and associated debris lobe mark the maximum ice extent during the Holocene, which, implied by the presence of crevasse-squeeze ridges, is likely related to a surge of Esmarkbreen. Several De Geer moraines provide evidence for subsequent slow and step-wise retreat. In the adjacent Trygghamna and in Magdalenefjorden in northwest Spitsbergen the landforms are similar but the absence of overridden moraines and glacial lineations shows that the glaciers probably only re-advanced once during the Holocene and that ice flow was relatively slow. Terminal moraines and associated debris lobes mark the maximum extent of these advances and formed during the Little Ice Age (LIA). In Magdalenefjorden the relatively small size of the debris lobe suggests that the ice margin was at its maximum position for only a short period of time, or that sediment availability was restricted during the LIA advance. Similar to Esmarkbreen the retreat phase of the glaciers in Trygghamna and Magdalenefjorden was also characterised by periods of still-stand or small re-advances, although the comparatively small number of De Geer moraines in all three fjords shows that these landforms probably formed much less frequently than previously thought. Sub-bottom profiler data, four sediment cores and six radiocarbon dates from Magdalenefjorden further provide information about the Holocene sedimentary environments in a northwest Spitsbergen fjord. The main source of sediment is glacial meltwater entering the fjord from the surrounding coastline, which has led to the accumulation of thick sequences of fine-grained mud. Stratified and laminated muds record glacier-proximal conditions, probably related to a LIA re-advance of Waggonwaybreen around 300calaBP, where the interplay of a range of glacimarine processes led to the formation of partially rhythmic couplets of one coarser and one finer layer, accumulated at a rate of around 3cma−1. Multiple sandy layers intercalated with the glacimarine mud provide evidence for the occurrence of gravitational mass-flow events like turbidity currents. In ice-distal settings, massive to weakly stratified, occasionally bioturbated mud accumulated at a lower rate of 0.04–0.49cma−1. Occasional clasts and diamictic layers show that the depositional environment in Magdalenefjorden is also influenced by sedimentation from icebergs and sea ice, but the ubiquitous glacimarine mud underscores the predominance of meltwater-related sedimentation in the fjord.

High‐resolution sub‐ice‐shelf seafloor records of twentieth century ungrounding and retreat of Pine Island Glacier, West Antarctica

Pine Island Glacier Ice Shelf (PIGIS) has been thinning rapidly over recent decades, resulting in a progressive drawdown of the inland ice and an upstream migration of the grounding line. The resultant ice loss from Pine Island Glacier (PIG) and its neighboring ice streams presently contributes an estimated ∼10% to global sea level rise, motivating efforts to constrain better the rate of future ice retreat. One route toward gaining a better understanding of the processes required to underpin physically based projections is provided by examining assemblages of landforms and sediment exposed over recent decades by the ongoing ungrounding of PIG. Here we present high‐resolution bathymetry and sub‐bottom‐profiler data acquired by autonomous underwater vehicle (AUV) surveys beneath PIGIS in 2009 and 2014, respectively. We identify landforms and sediments associated with grounded ice flow, proglacial and subglacial sediment transport, overprinting of lightly grounded ice‐shelf keels, and stepwise grounding line retreat. The location of a submarine ridge (Jenkins Ridge) coincides with a transition from exposed crystalline bedrock to abundant sediment cover potentially linked to a thick sedimentary basin extending upstream of the modern grounding line. The capability of acquiring high‐resolution data from AUV platforms enables observations of landforms and understanding of processes on a scale that is not possible in standard offshore geophysical surveys.

Holocene glacial evolution of Mohnbukta in eastern Spitsbergen

Submarine geomorphology is one of the main tools for understanding past fluctuations of tidewater glaciers. In this study we investigate the glacial history of Mohnbukta, on the east coast of Spitsbergen, Svalbard, by combining multibeam‐bathymetric data, marine sediment cores and remote sensing data. Presently, three tidewater glaciers, Heuglinbreen, Königsbergbreen and Hayesbreen calve into Mohnbukta. Hayesbreen surged at the end of the Little Ice Age, between 1901 and 1910. The submarine landform assemblage in Mohnbukta contains two large transverse ridges, interpreted as terminal moraines, with debrisflow lobes on their distal slopes and sets of well‐preserved geometric networks of ridges, interpreted as crevasse‐squeeze ridges inshore of the moraines. The arrangement of crevasse‐squeeze ridges suggests that both landform sets were produced during surge‐type advances. The terminus position of the 1901–1910 Hayesbreen surge correlates with the inner (R.2) terminal moraine ridge suggesting that the R.1 ridge formed prior to 1901. Marine sediment cores display 14C ages between 5700–7700 cal. a BP derived from benthic foraminifera, from a clast‐rich mud unit. This unit represents pre‐surge unconsolidated Holocene sediments pushed in front of the glacier terminus and mixed up during the 1901 surge. An absence of retreat moraines in the deeper part of the inner basin and the observation of tabular icebergs calving off the glacier front during retreat suggest that the front of Hayesbreen was close to flotation, at least in the deeper parts of the basin. As the MOH15‐01 core does not penetrate into a subglacial till and the foraminifera in the samples were well preserved, the R.1 ridge is suggested to have formed prior to the deposition of the foraminifera. Based on these data we propose that a surge‐type advance occurred in Mohnbukta in the early Holocene, prior to 7700 cal. a BP, which in turn indicates that glaciers can switch to and from surge mode.

The geomorphic imprint of glacier surges into open-marine waters: Examples from eastern Svalbard

Seafloor morphology beyond nine tidewater glaciers terminating in open-marine settings in eastern Svalbard has been investigated using multibeam swath-bathymetry. Historical information on tidewater glacier fluctuations over the past century or so shows that the seafloor offshore has been exposed only recently. Most glaciers have been observed to surge or have looped surface moraines indicating past surges. During these ice advances and subsequent retreat, a well-preserved submarine landform assemblage has been produced. (a) Subglacial landforms include: overridden moraine ridges, mega-scale glacial lineations (MSGLs), other streamlined lineations, elongate drumlins and medial moraines, small ridges forming a complex boxwork or rhombohedral pattern, and meltwater-related eskers and channels. (b) Ice-marginal landforms include: large terminal moraines with debris flow lobes on their outer flanks and indentations on their inner sides, small transverse retreat moraines, and crater-like kettle holes. (c) Glacimarine features include: iceberg ploughmarks and an otherwise relatively smooth sedimentary seafloor produced by fine-grained debris rain-out. A schematic plan-view landform assemblage model for tidewater glaciers advancing into open-marine settings is developed. Arcuate terminal moraines marking the ice-advance limit provide a distinctive component. Submarine basin(s) exposed inside these moraine ridges when ice retreats contain suites of individual landforms, produced subglacially and ice-marginally, with a consistent order of timing of deposition and stratigraphic superposition. The new schematic model is compared with earlier models based on submarine landforms associated with surges in the more topographically constrained setting of Spitsbergen fjords. Differences include: (i) the overall arcuate shape of the open-marine assemblage; (ii) the indented ice-proximal sides of terminal moraine ridges; (iii) the fan-shaped pattern of streamlined lineations; (iv) crater-like kettle holes at the inner lateral margins of many submarine basins; (v) prominent systems of eskers and channels. Modern glaciological analogues from ongoing surges of ice caps in eastern Svalbard and Severnaya Zemlya provide contemporary observations of ice advance into open-marine settings, including heavily crevassed ice lobes with finger-like termini.

The glacial geomorphology of the Lago Buenos Aires and Lago Pueyrredón ice lobes of central Patagonia

ABSTRACTThis paper presents a glacial geomorphological map of landforms produced by the Lago General Carrera–Buenos Aires and Lago Cochrane–Pueyrredón ice lobes of the former Patagonian Ice Sheet. Over 35,000 landforms were digitized into a Geographical Information System from high-resolution (<15 m) satellite imagery, supported by field mapping. The map illustrates a rich suite of ice-marginal glacigenic, subglacial, glaciofluvial and glaciolacustrine landforms, many of which have not been mapped previously (e.g. hummocky terrain, till eskers, eskers). The map reveals two principal landform assemblages in the central Patagonian landscape: (i) an assemblage of nested latero-frontal moraine arcs, outwash plains or corridors, and inset hummocky terrain, till eskers and eskers, which formed when major ice lobes occupied positions on the Argentine steppe; and (ii) a lake-terminating system, dominated by the formation of glaciolacustrine landforms (deltas, shorelines) and localized ice-contact glaciofluvial features (e.g. outwash fans), which prevailed during deglaciation.

The periodic topography of ice stream beds: Insights from the Fourier spectra of mega‐scale glacial lineations

AbstractIce stream bed topography contains key evidence for the ways ice streams interact with, and are potentially controlled by, their beds. Here we present the first application of two‐dimensional Fourier analysis to 22 marine and terrestrial topographies from 5 regions in Antarctica and Canada, with and without mega‐scale glacial lineations (MSGLs). We find that the topography of MSGL‐rich ice stream sedimentary beds is characterized by multiple, periodic wavelengths between 300 and 1200 m and amplitudes from decimeters to a few meters. This periodic topography is consistent with the idea that instability is a key element to the formation of MSGL bedforms. Dominant wavelengths vary among locations and, on one paleo ice stream bed, increase along the direction of ice flow by 1.7 ± 0.52% km−1. We suggest that these changes are likely to reflect pattern evolution via downstream wavelength coarsening, even under potentially steady ice stream geometry and flow conditions. The amplitude of MSGLs is smaller than that of other fluvial and glacial topographies but within the same order of magnitude. However, MSGLs are a striking component of ice stream beds because the topographic amplitude of features not aligned with ice flow is reduced by an order of magnitude relative to those oriented with the flow direction. This study represents the first attempt to automatically derive the spectral signatures of MSGLs. It highlights the plausibility of identifying these landform assemblages using automated techniques and provides a benchmark for numerical models of ice stream flow and subglacial landscape evolution.

Submarine landforms and glacimarine sedimentary processes in Lomfjorden, East Spitsbergen

Understanding the role of fjords in modulating the long-term interaction between ice sheets and glaciers with the surrounding ocean requires the investigation of glacigenic landform and sediment archives. In Svalbard, there is a wealth of data from fjords in west Spitsbergen that constrains the glacial history of this sector of the Svalbard-Barents Sea Ice Sheet (SBIS) since the Last Glacial Maximum (LGM), and the nature and timing of subsequent ice retreat. In contrast, however, very little is known about the glacial history of fjords in east Spitsbergen.This paper combines multibeam swath-bathymetry, sub-bottom profiles, lithological data and radiocarbon dates from Lomfjorden, Svalbard, to provide the first insights into the dynamics of tidewater glaciers and associated glacimarine sedimentary processes in a northeast Spitsbergen fjord. At the LGM, a fast-flowing ice stream drained the SBIS through Lomfjorden, serving as a tributary to a south-north flowing ice stream in Hinlopenstretet. Ice advance is recorded by streamlined bedrock, glacial lineations and drumlins. A radiocarbon date of ∼9.7ka BP from the outer fjord provides a minimum date for retreat of this ice stream, and suggests that Lomfjorden was ice-free earlier than fjords in west Spitsbergen. Ice retreat occurred in a slow and step-wise manner, indicated by the presence of recessional moraines and De Geer moraines. By 4.5ka BP the local tidewater glaciers had probably retreated inland of their present positions. The limited extent of glacigenic landform assemblages in front of these glaciers implies that any Holocene re-advances were probably restricted.The principal sedimentary processes during deglaciation were suspension settling from meltwater, causing deposition of weakly stratified, bioturbated mud in ice-distal settings at rates of 0.02–0.08cm a−1, and gravitational mass flows forming sandy turbidites in ice-proximal areas. Iceberg ploughmarks and ice-rafted debris provide evidence for the presence of large icebergs during deglaciation.Our data suggest an early and extensive deglaciation in east Spitsbergen fjords and show that previous reconstructions of the extent of the SBIS need to be revised as new data emerges from east Spitsbergen. The data confirm that tidewater glaciers from different regions of Spitsbergen behaved differently since the LGM, and that variations in landform-sediment assemblages occur even within geographically adjacent fjords.

Geomorphic diversity as a river management tool and its application to the Ganga River, India

AbstractUnderstanding of geomorphic processes and the determination of geomorphic diversity in catchments are prerequisites for the sustainable rehabilitation of river systems and for reach‐scale assessment of river health. The Ganga River system in India is a large, complex system consisting of several long tributaries, some &gt;1,000 km, originating from 2 distinct hinterlands—the Himalaya to the north and the cratons to the south. Traversing through a diverse climatic regime across the Plain and through precipitation zones ranging from 600 mm/year near Delhi to 1,200 mm/year in the eastern plains, the Ganga River system has formed very diverse landform assemblages in 3 major geomorphic domains. We have recognized 10 different river classes for the trunk river from Gangotri (source) to Farakka (upstream of its confluence with the Brahmaputra) based on (a) landscape setting, (b) channel and active floodplain properties, and (c) channel planform parameters. The mountainous stretch is characterized by steep valleys and bedrock channels and is dominated by large‐scale sediment production and transport through hill slope processes. The alluvial part of the river is characterized by 8 different river classes of varying reach lengths (60–300 km) many of which show sharp transitions in landscape setting. We have highlighted the application of this approach for the assessment of habitat suitability, environmental flows, and flood risk all of which have been significantly modified during the last few decades due to large‐scale anthropogenic disturbances. We suggest that the diversity embedded in this geomorphic framework can be useful for developing a sustainable river management programme to “work with” the contemporary character and behaviour of rivers.

Past ice flow in Wahlenbergfjorden and its implications for late Quaternary ice sheet dynamics in northeastern Svalbard

Wahlenbergfjorden is a fjord situated in the western part of Nordaustlandet in northern Svalbard. It leads into the 400 m deep Hinlopen Strait located between Nordaustlandet and Spitsbergen. High-resolution multibeam bathymetric and sub-bottom data, as well as sediment cores are used to study the past extent and dynamics of glaciers in Wahlenbergfjorden and western Nordaustlandet. The submarine landform assemblage in Wahlenbergfjorden consists of landforms characteristic of subglacial, ice marginal and proglacial conditions. Glacial lineations indicate that Wahlenbergfjorden was occupied by streaming ice during the LGM and most likely acted as an ice stream onset zone. Westward ice flow in the fjord merged with the ice stream in Hinlopen Strait. Absence of ice recessional landforms in outer Wahlenbergfjorden suggests relatively fast deglaciation, possibly by flotation of the glacier front in the deeper parts of the fjord. The inner part of Wahlenbergfjorden and Palanderbukta are characterized by De Geer moraines, indicating episodic retreat of a grounded glacier front. In Palanderbukta, longer still stands of the glacier terminus resulted in the formation of larger terminal moraine ridges. The inner part of Wahlenbergfjorden was deglaciated prior to 11.3 ± 55 Cal. ka BP. The submarine landform assemblages in front of Bodleybreen, Etonbreen, Idunbreen, Frazerbreen and Aldousbreen confirm that these glaciers have surged at least once during the Holocene.

Multiple origins of large hummock deposits in Alai Valley, Northern Pamir: Implications for palaeoclimate reconstructions

This paper interprets the origin of several of the largest hummocky landform assemblages in the Alai Valley, Northern Pamir – a formerly glaciated intermontane depression. The vast hummocky deposits in the Koman-Suu and Achik-Tash river catchments are found to be of two contrasting modes of formation: glacial hummocks deposited during Koman and Lenin Glaciers withdraw and avalanche hummocks produced during catastrophic Koman and Lenin rock avalanches. The origins of the deposits we assessed through remote and field-based geomorphological mapping, as well as sedimentological investigations, which included clast analysis and the identification of micro-scale agglomerates indicative of rock avalanche emplacement. Both the Koman and Lenin rock avalanches were large, catastrophic events (with run-outs of 34 and 24km, respectively, and a volume over 1×109m3 each) that occurred subsequent to glacier withdrawal from the area. The complex conditions on the moment of the rock avalanche emplacement promoted unusual deposits geomorphology and extensive run-outs. The landslide landforms formed over the pre-existing glacial hummocks and fluvial deposits, and are geomorphologically and sedimentologically distinct from the larger glacial hummocks. The reconstruction of this sequence of events has implications for how hummock dating should be interpreted. This research illustrates large scale catastrophic landsliding in the glacial environment, and adds to the ongoing debate about the misidentification of rock avalanche deposits as of glacial origin, and their relevance to palaeoclimatological and palaeoseismological reconstructions.

The pattern and style of deglaciation at the Late Wisconsinan Laurentide and Cordilleran ice sheet limits in northeastern British Columbia

This paper reports on the landform assemblages at the northern confluence of the Late Wisconsinan Laurentide and Cordilleran ice sheets with montane and piedmont glaciers in the northern Rockies and southern Mackenzie Mountains. Recent observations in northeastern British Columbia refine our knowledge of the pattern and style of ice sheet retreat, glacial lake formation, and meltwater drainage. At the onset of deglaciation, confluent Laurentide and Cordilleran terminal ice margins lay between 59°N, 124°30′W and 60°N, 125°15′W. From this terminal limit, ice sheets retreated into north-central British Columbia and Yukon Territory, with remnant Cordilleran ice and montane glaciers confined to mountain valleys and the Liard Plateau. Distinctive end moraines are not associated with the retreat of Cordilleran ice in these areas. Laurentide ice retreated northeastward from uplands and the plateaus; then separated into lobes occupying the Fort Nelson and Petitot river valleys. Ice-retreat landforms include recessional end moraines (sometimes overridden and drumlinized), hill–hole pairs, crevasse-fill deposits, De Geer-like ribbed till ridges, hummocky moraines, kames, meltwater features, and glacial lake deposits that fall within the elevation range of glacial Lake Liard and glacial Lake Fort Nelson (ca. 840–380 m). Meltwater and sediment transport into glacial lakes Fort Nelson, Liard, Nahanni, and Mackenzie was sustained by remnant ice in the Liard River and Fort Nelson River drainage basins until the end of glaciation. Optical dating of sand from stabilized parabolic dunes on the Liard Plateau indicates that proglacial conditions, lake formation, and drainage began before 13.0 ± 0.5 ka (calendar years). The Petitot, Fort Nelson, and Liard rivers all occupy spillways incised into glacial deposits and bedrock by meltwater overflow from glacial lakes Peace and Hay.

Chapter 4 Conceptual glacial ground models: British and Irish case studies

Abstract Former glaciation style is dictated by physiography and ice dynamics and is encoded in glacial landsystem imprints. As a holistic evaluation of sediment–landform associations and their genetic relationships to the processes involved in terrain development, glacial landsystems can facilitate a preliminary prediction of expected subsurface conditions using depositional surface morphology and wider physiographic setting. This chapter provides exemplars representative of the widely variable glacial depositional environments of the British Isles. The glacial deposits of the British Isles are viewed in terms of the dominant landsystems in the Quaternary sediment–landform record and can be grouped under four categories: (1) ice-sheet-related deposits and (2) upland (hard bedrock) glacial deposits, organized according to subglacial footprints, ice-marginal complexes and supraglacial assemblages; (3) glaciofluvial sediment–landforms, organized according to whether they are ice-contact or proglacial in nature; and (4) subaqueous depositional sequences, related to ice-proximal and ice-distal environments. These glacial landsystems are related to the concept of Quaternary domains in an attempt to translate sediment–landform assemblages into a format that has practicability in engineering geology. In this respect the regional distribution of landsystems resonates to some degree with the classification schemes of ‘glaciogenic subgroups’ and ‘till formation domains’. Beyond the glaciogenic subgroup and domain classifications, landsystems further identify localized complexities and ensure a higher level of detail for site investigations where intensive Quaternary geological assessments have yielded a range of data including geomorphological mapping and outcrop investigations with three-dimensional analyses of borehole archives.

Internal structure and significance of ice‐marginal moraine in the Kebnekaise Mountains, northern Sweden

Despite a long history of glaciological research, the palaeo‐environmental significance of moraine systems in the Kebnekaise Mountains, Sweden, has remained uncertain. These landforms offer the potential to elucidate glacier response prior to the period of direct monitoring and provide an insight into the ice‐marginal processes operating at polythermal valley glaciers. This study set out to test existing interpretations of Scandinavian ice‐marginal moraines, which invoke ice stagnation, pushing, stacking/dumping and push‐deformation as important moraine forming processes. Moraines at Isfallsglaciären were investigated using ground‐penetrating radar to document the internal structural characteristics of the landform assemblage. Radar surveys revealed a range of substrate composition and reflectors, indicating a debris‐ice interface and bounding surfaces within the moraine. The moraine is demonstrated to contain both ice‐rich and debris‐rich zones, reflecting a complex depositional history and a polygenetic origin. As a consequence of glacier overriding, the morphology of these landforms provides a misleading indicator of glacial history. Traditional geochronological methods are unlikely to be effective on this type of landform as the fresh surface may post‐date the formation of the landform following reoccupation of the moraine rampart by the glacier. This research highlights that the interpretation of geochronological data sets from similar moraine systems should be undertaken with caution.

Cenozoic landforms and post-orogenic landscape evolution of the Balkanide orogen: Evidence for alternatives to the tectonic denudation narrative in southern Bulgaria

Continental denudation is the mass transfer of rock from source areas to sedimentary depocentres, and is typically the result of Earth surface processes. However, a process known as tectonic denudation is also understood to expose deep-seated rocks in short periods of geological time by displacing large masses of continental crust along shallow-angle faults, and without requiring major contributions from surface erosion. Some parts of the world, such as the Basin and Range in the USA or the Aegean province in Europe, have been showcased for their Cenozoic tectonic denudation features, commonly described as metamorphic core-complexes or as supradetachment faults. Based on 22 new apatite fission-track (AFT) and 21 helium (AHe) cooling ages among rock samples collected widely from plateau summits and their adjacent valley floors, and elaborating on inconsistencies between the regional stratigraphic, topographic and denudational records, this study frames a revised perspective on the prevailing tectonic denudation narrative for southern Bulgaria. We conclude that conspicuous landforms in this region, such as erosion surfaces on basement-cored mountain ranges, are not primarily the result of Paleogene to Neogene core-complex formation. They result instead from “ordinary” erosion-driven, subaerial denudation. Rock cooling, each time suggesting at least 2km of crustal denudation, has exposed shallow Paleogene granitic plutons and documents a 3-stage wave of erosional denudation which progressed from north to south during the Middle Eocene, Oligocene, Early to Middle Miocene, and Late Miocene. Denudation initially prevailed during the Paleogene under a syn-orogenic compressional regime involving piggyback extensional basins (Phase 1), but subsequently migrated southward in response to post-orogenic upper-plate extension driven by trench rollback of the Hellenic subduction slab (Phase 2). Rare insight given by the denudation pattern indicates that trench rollback progressed at a mean velocity of 3 to 4km/Ma. The Neogene horst-and-graben mosaic that defines the modern landscape (Phase 3) has completely overprinted the earlier fabrics of Phases 1 and 2, and has been the prime focus of tectonic geomorphologists working in the region. The new narrative proposed here for linking the geodynamic evolution of SE Europe with surface landform assemblages raises issues in favour of better documenting the regional sedimentary record of existing Paleogene basins, which constitute a poorly documented missing link to the thermochronological evidence presented here.

Quantitative paleotopography and paleogeography around the Gibraltar Arc (South Spain) during the Messinian Salinity Crisis

The Malaga basin contains an important geological record documenting the complex paleogeographic evolution of the Gibraltar Arc before, during and after the closure and desiccation of the Mediterranean Sea triggered by the “Messinian Salinity crisis” (MSC). Proxy paleo-elevation data, estimated from the stratigraphic and geomorphological records, allow the building of quantitative paleogeoid, paleotopographic and paleogeographic models for the three main paleogeographic stages: pre-MSC (Tortonian-early Messinian), syn-MSC (late Messinian) and post-MSC (early Pliocene). The methodological workflow combines classical contouring procedures used in geology and isobase map models from geomorphometric analyses and proxy data overprinted on present Digital Terrain Models. The resulting terrain quantitative models have been arranged, managed and computed in a GIS environment.The computed terrain models enable the exploration of past landscapes usually beyond the reach of classical geomorphological analyses and strongly improve the paleogeographic and paleotopographic knowledge of the study area. The resulting models suggest the occurrence of a set of uplifted littoral erosive and paleokarstic landforms that evolved during pre-MSC times. These uplifted landform assemblages can explain the origin of key elements of the present landscape, such as the Torcal de Antequera and the large amount of mogote-like relict hills present in the zone, in terms of ancient uplifted tropical islands. The most prominent landform is the extensive erosional platform dominating the Betic frontal zone that represents the relic Atlantic wave cut platform elaborated during late-Tortonian to early Messinian times.The amount of uplift derived from paleogeoid models suggests that the area rose by about 340m during the MSC. This points to isostatic uplift triggered by differential erosional unloading (towards the Mediterranean) as the main factor controlling landscape evolution in the area during and after the MSC. Former littoral landscapes in the old emergent axis of the Gibraltar Arc were uplifted to form the main water-divide of the present Betic Cordillera in the zone.

Morphological and sedimentary responses to ice mass interaction during the last deglaciation

ABSTRACTDuring decline of the last British–Irish Ice Sheet (BIIS) down‐wasting of ice meant that local sources played a larger role in regulating ice flow dynamics and driving the sediment and landform record. At the Last Glacial Maximum, glaciers in north‐western England interacted with an Irish Sea Ice Stream (ISIS) occupying the eastern Irish Sea basin (ISB) and advanced as a unified ice‐mass. During a retreat constrained to 21–17.3 ka, the sediment landform assemblages lain down reflect the progressive unzipping of the ice masses, oscillations of the ice margin during retreat, and then rapid wastage and disintegration. Evacuation of ice from the Ribble valley and Lancashire occurred first while the ISIS occupied the ISB to the west, creating ice‐dammed lakes. Deglaciation, complete after 18.6–17.3 ka, was rapid (50–25 m a−1), but slower than rates identified for the western ISIS (550–100 m a−1). The slower pace is interpreted as reflecting the lack of a calving margin and the decline of a terrestrial, grounded glacier. Ice marginal oscillations during retreat were probably forced by ice‐sheet dynamics rather than climatic variation. These data demonstrate that large grounded glaciers can display complex uncoupling and realignment during deglaciation, with asynchronous behaviour between adjacent ice lobes generating complex landform records.

Event review: Central Narmada Basin Paleoanthropology Fieldschool

In comparison to other subjects in Indian archaeology and Quaternary studies, paleoanthropology has declined significantly in recent decades as a prominent academic discipline in the Indian Subcontinent. Most archaeological research and teaching in South Asia are largely focused on younger time periods, primarily the protohistoric or Chalcolithic and historical phases. In addition to lack of general interest and lack of popularization, another major reason for this decline in paleoanthropology may be the methodological challenges in interpreting complex paleoanthropological records, e.g., landform assemblage, paleoenvironmental reconstructions, assessing contextual integrity of the archaeological evidence (compared to younger sites) and so forth. Due to inadequate preserved evidence compared to younger archaeological sites, the study of human history during the Quaternary demands a multi-disciplinary approach with highly sophisticated and extensive field-based surveys and integrated scientific analyses from geological perspectives.

Landform assemblages and sedimentary processes along the Norwegian Channel Ice Stream

Several regional and detailed bathymetric datasets together with 2D and 3D seismic data are compiled to investigate the landform assemblages and sedimentary processes along the former path of the Norwegian Channel Ice Stream (NCIS). At the broad scale, the glacial geomorphology and sedimentary architecture reveals three different zones along the ice-stream path, characterized by: (1) glacial erosion in the onset zone and inner shelf area, (2) sediment transport through the main trunk of the ice stream across the mid-shelf, and (3) a zone of deposition towards the outer continental shelf edge. Along the first 400km of the ice stream bed (outer Oslofjord–Skagerrak–Stavanger) a major overdeepening is associated with suites of crag-and-tail features at the transition from the crystalline bedrock to the sedimentary bedrock, together with evidence of glaciotectonic thrusting in the form of hill-hole pairs. Here we interpret extensive erosion of both sedimentary rocks and Quaternary sediments. This zone is succeeded by an approximately 400km long zone, through which most of the sediments eroded from the inner shelf were transported, rather than being deposited. We infer that sediment was transported subglacially and is likely to have been advected downstream by soft sediment deformation. The thickness of till of inferred Weichselian age generally varies from 0 and 50m and this zone is characterized by mega-scale glacial lineations (MSGLs) which we interpret to be formed in a dynamic sedimentary system dominated by high sediment fluxes, but with some localized sediment accretion associated with lineations. Towards the shelf break, the North Sea Fan extends to the deep Norwegian Sea, and reflects massive sedimentation of glacigenic debris onto the continental slope. Numerous glacigenic debris flows accumulated and constructed a unit up to 400m thick during the Last Glacial Maximum. The presence of these three zones (erosion, transport, deposition) is consistent with observations from other palaeo-ice streams and their significance arises from their potential to feedback and impact on ice stream dynamics.