Tunnel Valleys Research Articles

Interpretation of Late Ordovician glaciogenic reservoirs from 3-D seismic data: an example from the Murzuq Basin, Libya

AbstractUnderstanding the recessional behaviour of ancient pre-Cenozoic ice sheets based on seismic reflection studies is generally difficult through scarcity of data. In North Africa, however, hydrocarbon exploration has produced high quality seismic reflection datasets that permit an analysis of the morphology and internal sedimentary architecture of incisions of Late Ordovician age related to the Hirnantian glaciation. Analysis of a high-resolution 3-D seismic dataset covering a small area in western Libya (the N Murzuq Basin) reveals a sharply defined, WNW–ESE-oriented palaeo-escarpment, with a higher (cliff-forming) western margin and a lower (basin-forming) eastern margin. The palaeo-escarpment defines the western flank of a sub-basin extending up to 60 km in width, known as the Awbari Trough. The escarpment and the trough are interpreted as the morphological expression of a major unconformity dividing pre-glacial sediments below from Late Ordovician (?Hirnantian) glacially related sediments above. Two hypotheses are considered for the origins of both the escarpment and the Awbari Trough: (1) as a tectonic feature such as a half graben that was active during sedimentation and (2) a glacially related palaeotopography, with the latter interpretation preferred, owing to the lack of evidence for syn-sedimentary fault activity. The width of the Awbari Trough compares to the large-scale cross-shelf troughs in modern high latitude settings, such as the Barents Shelf, produced by ice streams. The Awbari Trough was progressively filled in by gravity flow deposits throughout the course of the glaciation, until the initial incision became filled in with sediments during an overall glacial retreat phase and ceased to influence sedimentation patterns. Glacial re-advance across the basin produced a second unconformity observed in seismic data. Above this unconformity, meltwater processes incised a shallow (~ 20 m) and wide (~ 5 km) subglacial tunnel valley. Stabilization of the ice front prior to its ultimate retreat resulted in the deposition of a delta complex prior to the Early Silurian transgression.

Litho‐ and chronostratigraphy of the Late Weichselian in Vendsyssel, northern Denmark, with special emphasis on tunnel‐valley infill in relation to a receding ice margin

Lithostratigraphy and chronostratigraphy of samples from 18 deep boreholes in Vendsyssel have resulted in new insight into the Late Weichselian glaciation history of northern Denmark. Prior to the Late Weichselian Main advance c. 23–21 kyr BP, Vendsyssel was part of an ice‐dammed lake where the Ribjerg Formation was deposited c. 27–23 kyr BP. The timing of the Late Weichselian deglaciation is well constrained by the Main advance and the Lateglacial marine inundation c. 18 kyr BP, and thus spans only a few millennia. Rapid deposition of more than 200 m of sediments took place mainly in a highly dynamic proglacial and ice‐marginal environment during the overall ice recession. Mean retreat rates have been estimated as 45–50 m/yr in Vendsyssel with significantly higher retreat rates between periods of standstill and re‐advance. The deglaciation commenced in Vendsyssel c. 20 kyr BP, and the Troldbjerg Formation was deposited c. 20–19 kyr BP in a large ice‐dammed lake in front of the receding ice sheet, partly as glaciolacustrine sediments and partly as rapid and focused sedimentation in prominent ice‐contact fans, which make up the Jyske Ås and Hammer Bakker moraines. In the northern part of central Vendsyssel, at least four generations of north–south orientated tunnel valleys are identified, each generation related to a recessional ice margin. This initial deglaciation was interrupted by a major re‐advance from the east c. 19 kyr BP, which covered most of Vendsyssel. An ice‐dammed lake formed in front of the ice sheet as it retreated towards the east; the Morild Formation was deposited here c. 19–18 kyr BP. Related to this stage of deglaciation, eight ice‐marginal positions have been identified based on the distribution of large tunnel‐valley systems and pronounced recessional moraines. The Morild Formation consists of glaciolacustrine sediments, including the sediment infill of more than 190 m deep tunnel valleys, as well as the sediments in recessional moraines, which were formed as ice‐contact sedimentary ridges, possibly in combination with glaciotectonic deformation. The character of the tunnel‐valley infill sediments was determined by proximity to the ice margin. During episodes of rapid retreat of the ice margin, tunnel valleys were quickly abandoned and filled with fine‐grained sediments in a distal setting. During slow retreat of the ice margin, tunnel valleys were filled in an ice‐proximal environment, and the infill consists of alternating layers of fine‐ to coarse‐grained sediments. At c. 18 kyr BP, Vendsyssel was inundated by the sea, when the Norwegian Channel Ice Stream broke up, and a succession of marine sediments (Vendsyssel Formation) was deposited during a forced regression.

Rapid tunnel‐valley formation beneath the receding Late Weichselian ice sheet in Vendsyssel, Denmark

Interpretation of Transient ElectroMagnetic (TEM) data and wire‐line logs has led to the delineation of an intricate pattern of buried tunnel valleys, along with new evidence of glaciotectonically dislocated layers in recessional moraines in the central part of Vendsyssel, Denmark. The TEM data have been compared with recent results of stratigraphical investigations based on lithological and biostratigraphical analyses of borehole samples and dating with Optically Stimulated Luminescence (OSL) and radiocarbon. This has provided an overview of the spatial distribution of the late Quaternary lithostratigraphical formations, and the age of the tunnel valleys has been estimated. The tunnel valleys are typically 5–10 km long, 1 km wide and are locally eroded to depths of more than 180 m b.s.l. The valleys are interpreted to have been formed by subglacial meltwater erosion beneath the outermost part of the ice sheet during temporary standstills and minor re‐advances during the overall Late Weichselian recession of the Scandinavian Ice Sheet. The formation of the tunnel valleys occurred after the retreat of the Main ice advance c. 20 kyr BP and before the Lateglacial marine inundation c. 18 kyr BP. Based on the occurrence of the tunnel valleys and the topography, four ice‐marginal positions related to the recession of the northeastern Main advance and seven ice‐marginal positions related to the recession from the following eastern re‐advance across Vendsyssel are delineated. All the tunnel valleys were formed within a time interval of a few thousand years, giving only a few hundred years or less for the formation of the tunnel valleys at each ice‐marginal position.

Marine glacial and interglacial stratigraphy in Vendsyssel, northern Denmark: foraminifera and stable isotopes

The marine Quaternary of Vendsyssel has been studied in a series of new boreholes in the area, and the climatic development is discussed on the basis of foraminiferal assemblages and stable isotopes. The foraminiferal zones are correlated with previously published records from northern Denmark, and the spatial local and regional distribution is discussed in details based on the new evidence. The new data show that the marine sedimentation in Vendsyssel was not continuous from the Late Saalian to the Middle Weichselian, as previously thought. For example, there is indication of a hiatus at our key site, Åsted Vest in the central part of Vendsyssel, at the transition between regional foraminiferal zones N4 and N3, i.e. at the Late Saalian (MIS 6) – Eemian (MIS 5e) transition. The hitherto most complete Early Weichselian succession (zone N2) in Vendsyssel is presented from Åsted Vest. Deposits from the Early Weichselian sea‐level lowstands (MIS 5d and 5b) may, however, be missing in parts of the area. Two major breaks in the marine deposition during the Middle Weichselian represent glacial advances into northern Denmark. The first event occurred just after deposition of the regional foraminiferal zone N2 (late MIS 4), and the second event in the middle part of zone N1 (early MIS 3). Zone N1 is succeeded by a series of non‐marine units deposited during the sea‐level lowstand of the Weichselian maximum glaciation (late MIS 3 and MIS 2), including deeply incised tunnel valleys, which have been refilled with non‐marine sediments during the Late Weichselian. Vendsyssel was inundated by the sea again during the Late Weichselian, at c. 18 kyr BP. Subsequently, the marine conditions were gradually changed by forced regression caused by local isostatic uplift, and around the Weichselian–Holocene transition most of Vendsyssel was above sea level. A continuous deposition across the Late Weichselian–Holocene boundary only occurred at relatively deep sites such as Skagen. The environmental and climatic indications for Vendsyssel are in accordance with the global sea‐level curve, and the Quaternary record is correlated with the oxygen isotope record from the NorthGRIP ice core, as well as the marine isotope stages.

Pleistocene tunnel valleys in the German North Sea: spatial distribution and morphology

Pleistocene tunnel valleys in the German North Sea: spatial distribution and morphology

Glaciohydraulic supercooling

In this progress report we review recent work which has investigated the process of glaciohydraulic supercooling and its significance for a range of glaciological and geological phenomena. Since our last report (Knight and Cook, 2008), glaciohydraulic supercooling has been identified in new locations and under hitherto unrecognized conditions, both for modern and formerly glaciated environments. Recent work has examined the record of supercooling preserved in the landform and sediment record, within tunnel valleys and in sequences of melt-out till. We suggest that supercooling may also have important implications for glacier dynamics because of the role it may play in controlling the geometry of subglacial overdeepenings, which in turn acts as a control on the dynamics of tidewater glaciers. Despite these numerous recent scientific advances, there remains a need to critically evaluate the importance of supercooling for the various aspects of sediment entrainment and flux, glacial geomorphology and ice dynamics with which it has become associated. We illustrate how collaboration with researchers in other branches of cryospheric science, such as sea-ice scientists, might shed new light on the significance of supercooling within these various aspects of glacier science.

Regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the Cordilleran Ice Sheet in British Columbia, Canada and northern Washington State, USA

Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated in order to evaluate landscape development, subglacial hydrology and Cordilleran Ice Sheet dynamics along its southern margin. Major landscape elements include drumlin swarms and tunnel valleys. Drumlins are composed of bedrock, diamicton and glaciofluvial sediments; their form truncates the substrate. Tunnel valleys of various scales (km to 100s km length), incised into bedrock and sediment, exhibit convex longitudinal profiles, and truncate drumlin swarms. Okanagan Valley is the largest tunnel valley in the area and is eroded >300m below sea level. Over 600m of Late Wisconsin-age sediments, consisting of a fining-up sequence of cobble gravel, sand and silt fill Okanagan Valley. Landform–substrate relationships, landform associations, and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds. They are best explained by meltwater erosion and deposition during ice sheet underbursts.During the Late-Wisconsin glaciation, Okanagan Valley functioned as part of a subglacial lake spanning multiple connected valleys (few 100s km) of southern British Columbia. Subglacial lake development started either as glaciers advanced over a pre-existing sub-aerial lake (catch lake) or by incremental production and storage of basal meltwater. High geothermal heat flux, geothermal springs and/or subglacial volcanic eruptions contributed to ice melt, and may have triggered, along with priming from supraglacial lakes, subglacial lake drainage. During the underburst(s), sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys. Progressive flow channelization focused flows toward major bedrock valleys. In Okanagan Valley, most of the pre-glacial and early-glacial sediment fill was removed. A fining-up sequence of boulder gravel and sand was deposited during waning stages of the underburst(s) and bedrock drumlins in Okanagan Valley were enhanced or wholly formed by this underburst(s).Subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes. The prevailing conceptual model for growth and decay of the CIS suggests significantly thicker ice in valleys compared to plateaus. Subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner (due to melting and enhanced sliding, with significant transfer of ice toward the ice sheet margin). Subglacial lake drainage may have hastened deglaciation by melting ice, lowering ice-surface elevations, and causing lid fracture. This paper highlights the importance of ice sheet hydrology: its control on ice flow dynamics, distribution and volume in continental ice masses.

Meltwater discharge through the subglacial bed and its land‐forming consequences from numerical experiments in the Polish lowland during the last glaciation

AbstractNumerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in NW Poland: primarily its flow pattern, velocities and fluxes. We have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady‐state and transient conditions. The results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin. All model runs show water discharge at the ice forefield driven by ice‐sheet‐thickness‐modulated, down‐ice‐decreasing hydraulic heads. In relation to non‐glacial times, the transient 3D model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca. 90 m below the ice–bed interface and up to 40 km in front of the glacier. Comparison with empirical data suggests that, depending on the model run, only between 5 and 24% of the meltwater formed at the ice sole drained through the bed as groundwater. This is consistent with field observations documenting abundant occurrence of tunnel valleys, indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system. Groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely. In these areas the relief shows distinct palaeo‐ice lobes, indicating fast ice flow, possibly triggered by the undrained water at the ice–bed interface. Owing to the abundance of low‐permeability strata in the bed, the simulated groundwater flow depth is less than ca. 200 m. Copyright © 2009 John Wiley & Sons, Ltd.

Mapping of buried tunnel valleys in Denmark: new perspectives for the interpretation of the Quaternary succession

Tunnel valleys eroded by subglacial meltwater underneath the Late Weichselian ice sheet are a common feature in the Danish landscape (Ussing 1907; Smed 1998). They occur as undulating elongate depressions with hollows and thresholds and without continuously descending floors. The valleys rise tens of metres before terminating in large outwash fans, primarily along the Main Stationary Line in Jylland, but also along younger ice-margin lines formed shortly after the Last Glacial Maximum. The meltwater was driven by hydrostatic pressure gradients below the glacier towards its margin leading to subglacial erosional features, partly in the form of valleys. The term ‘tunnel valley’ was first used by Madsen (1921), who referred to tunnel-like structures below glaciers that were expected to have carried the meltwater. Worldwide, this term is used for subglacially eroded valleys; however, other terms such as ‘tunnel channel’ and ‘incision’ are also widely used for such valleys. There is general consensus that subglacial meltwater is the primary causative agent that has eroded the tunnel valleys (O’Cofaigh 1996; Huuse & Lykke-Andersen 2000; Jørgensen & Sandersen 2006). The subglacial origin is indicated by: (1) abrupt terminations at former ice margins and the association with the large outwash plains, (2) irregular longitudinal profiles, (3) the occurrence of small channels and eskers in the valleys, and (4) the non-meandering and non-dendritic appearance of the relatively straight-segmented valleys. The exact mode of meltwater erosion remains, however, poorly understood. Valleys are present not only in the landscape; they are also found buried in the subsurface. In Denmark, buried valleys have occasionally been described on the basis of borehole data and early geoelectrical methods (e.g. Sorgenfrei & Berthelsen 1954; Lykke-Andersen 1973; Binzer & Stockmarr 1994). Based on the large amount of newly collected hydrogeophysical data in Denmark, it has recently become possible to define such valleys as tunnel valleys and to acknowledge their wide distribution in the subsurface (Sandersen & Jørgensen 2003; Jørgensen et al. 2005; Jørgensen & Sandersen 2006). Analysis of these data has revealed dense networks of tunnel valleys and has significantly improved our understanding of their distribution, geometry and sedimentary infill. In the following, we review this work and identify new perspectives for the interpretation of the Quaternary succession in Denmark. The work was initiated in 1998 by the former Danish counties (amter), and is currently continued by the Geological Survey of Denmark and Greenland and the ‘miljøcentre’ (environment centres). As part of this project, the buried valleys are continuously being mapped as new data are collected.

Base Quaternary in the Danish parts of the North Sea and Skagerrak

Over the years, several maps of the base Quaternary surface of the Danish area have been published. However, the maps have either been local in character (e.g. Håkansson & Pedersen 1992; Huuse et al. 2001) or have concentrated on special topics such as tunnel valleys (e.g. Huuse & Lykke-Andersen 2000) or glaciotectonic features (e.g. Klint & Pedersen 1995; Andersen et al. 2005). The only published map of a more regional character is that of Binzer & Stockmarr (1994) that covers onshore Denmark and eastern Danish waters. Here we present for the first time a regional map of the base Quaternary surface for the entire Danish sector of the North Sea and Skagerrak based on interpretations of reflection seismic data at the Geological Survey of Denmark and Greenland (GEUS) (Fig. 1). The new map has been depth-converted and merged with the onshore map of Binzer & Stockmarr (1994) and thus the first map covering the entire Danish land and sea areas has been compiled. The definition of the base Quaternary is a current issue of debate. In this article, we follow Gradstein et al. (2004) who place the base Quaternary at base Gelasian, which is dated to 2.59 Ma. In parts of the studied area, glacial tectonic features in the form of thrust complexes can be seen on the seismic data. Here the base Quaternary surface has been placed at the base of the dislocated thrust units, corresponding to the basal décollement horizon. The base Quaternary surface is of both academic and practical interest. The depth to the base Quaternary surface and its morphology are of interest to the understanding of the Quaternary development of the region, but are also important in relation to offshore constructions such as oil and gas platforms, pipelines and wind mills.

Time‐transgressive tunnel valley formation indicated by infill sediment structure, North Sea – the role of glaciohydraulic supercooling

AbstractStructure and lithology of the infill sediments from 16 subglacial buried tunnel valleys of Pleistocene age in the North Sea were analyzed using 3D seismic data and geophysical log data from five hydrocarbon exploration wells. The infill sediments are characterized by three seismic facies: Facies I, the volumetrically most important and structurally most distinct, is composed of clinoform reflections downlapping axially up‐valley (up‐ice), Facies II is composed of near‐horizontal, continuous and well layered reflections onlapping the clinoform reflections and the valley walls and Facies III is composed of clinoform reflections downlapping axially down‐valley (down‐ice). A model of formation of this sediment architecture is proposed, in which valley incision and infill are causally linked. It is proposed that Facies I is related to glaciohydraulic supercooling of subglacial meltwater in the distal parts of tunnel valleys. The valleys formed time‐transgressively during ice retreat, whereby sediment eroded further up‐ice was re‐deposited along adverse subglacial slopes of the valleys close to the ice margin. The formation of Facies II and III is related to the deposition in proglacial basins during final deglaciation. Copyright © 2008 John Wiley & Sons, Ltd.

The northern sector of the last British Ice Sheet: Maximum extent and demise

Strongly divided opinion has led to competing, apparently contradictory, views on the timing, extent, flow configuration and decay mechanism of the last British Ice Sheet. We review the existing literature and reconcile some of these differences using remarkable new sea-bed imagery. This bathymetric data provides unprecedented empirical evidence of confluence and subsequent separation of the last British and Fennoscandian Ice Sheets. Critically, it also allows a viable pattern of ice-sheet disintegration to be proposed for the first time. Covering the continental shelf around the northern United Kingdom, extensive echosounder data reveals striking geomorphic evidence – in the form of tunnel valleys and moraines – relating to the former British and Fennoscandian Ice Sheets. The pattern of tunnel valleys in the northern North Sea Basin and the presence of large moraines on the West Shetland Shelf, coupled with stratigraphic evidence from the Witch Ground Basin, all suggest that at its maximum extent a grounded ice sheet flowed from SE to NW across the northern North Sea Basin, terminating at the continental-shelf edge. The zone of confluence between the British and much larger Fennoscandian Ice Sheets was probably across the northern Orkney Islands, with fast-flowing ice in the Fair Isle Channel focusing sediment delivery to the Rona and Foula Wedges. This period of maximum confluent glaciation (c. 30–25 ka BP) was followed by a remarkable period of large-scale ice-sheet re-organisation. We present evidence suggesting that as sea level rose, a large marine embayment opened in the northern North Sea Basin, as far south as the Witch Ground Basin, forcing the two ice sheets to decouple rapidly along a north–south axis east of Shetland. As a result, both ice sheets rapidly adjusted to new quasi-stable margin positions forming a second distinct set of moraines (c. 24–18 ka BP). The lobate overprinted morphology of these moraines on the mid-shelf west of Orkney and Shetland indicates that the re-organisation of the British Ice Sheet was extremely dynamic — probably dominated by a series of internally forced readvances. Critically, much of the ice in the low-lying North Sea Basin may have disintegrated catastrophically as decoupling progressed in response to rising sea levels. Final-stage deglaciation was marked by near-shore ice streaming and increasing topographic control on ice-flow direction. Punctuated retreat of the British Ice Sheet continued until c. 16 ka BP when, following the North Atlantic iceberg-discharge event (Heinrich-1), ice was situated at the present-day coastline in NW Scotland.

First-order reconstructions of a Late Ordovician Saharan ice sheet

Synthesis of outcrop and subsurface sedimentological and geomorphological datasets across North Africa allows a tentative palaeo-glaciological model of the flow dynamics and recessional character of a 440 Ma old (Hirnantian) ice sheet to be proposed. A system of eight cross-shelf trough depocentres is identified from the Late Ordovician of the Sahara region. These are interpreted to have been carved and occupied by ice streams, providing evidence for widespread heterogeneous flow within the ice sheet. During retreat, two key geological features were produced: (1) laterally extensive, sinuous to linear piles of sediment dumped parallel to the ice margin; (2) large meltwater channels (tunnel valleys) cut near the grounding line.

Glacially influenced petroleum plays in the Kulshill Group (Late Carboniferous–Early Permian) of the Southeastern Bonaparte Basin, Western Australia

Glacial deposits within the Lower Kulshill Group (Late Carboniferous-Early Permian) were initially recognised in cores from onshore wells in the southeastern Bonaparte Basin in the 1960s. Subsequent offshore wells have extended the distribution of the glaciogene units 100 km to the north. Their capacity to entrap oil and gas was proven by the Turtle and Barnett wells, located on the offshore Turtle High. Similar age glaciogene rocks occur within the Cooper Basin of central Australia, where they contain oil and gas reserves, and in the Canning, Carnarvon and Perth basins of Western Australia. Using sparse cores, electric logs, palynology and a sequence stratigraphic interpretation of 2D seismic data, the distribution of potential reservoir sandstones and sealing lithologies of the glaciogenic strata has been mapped for the offshore southeastern Bonaparte Basin. This study highlights the petroleum trapping potential associated with sub-glacial ice tunnel valley features, which are widespread in the offshore part of the basin.

A morphometric analysis of tunnel valleys in the eastern North Sea based on 3D seismic data

AbstractA 1250 km2 3D seismic volume is used to provide a detailed spatial and geometrical analysis of fifteen Pleistocene tunnel valleys in the Danish North Sea. All the valleys are buried; they are up to 39 km long, 3–4 km wide and up to 350 m deep. The valleys are part of a vast tunnel valley province covering an area of some 0.5 million km2 of the formerly glaciated lowland areas of North West Europe. The valleys consist of non‐branching, non‐anastomosing troughs; they exhibit strongly undulating bottom profiles with numerous sub‐basins and thresholds, and are characterised by adverse end slopes. Cross‐cutting relationships and theoretical considerations suggest the occurrence of seven major episodes of valley incision attributed to ice marginal oscillations within a few glacials. Calculations considering the valley end gradients and theoretical ice‐surface profiles suggest that the valleys were formed by pressurised subglacial meltwater erosion. Given a range of theoretical ice‐surface profiles, the adverse end slopes are well beyond the supercooling threshold, which suggests that the water was not in thermal equilibrium with the basal ice and that flow was concentrated in substantial conduits with sufficient mass and flux to maintain water temperature well above the freezing point. Copyright © 2007 John Wiley & Sons, Ltd.

Late Ordovician glacial record of the Anti-Atlas, Morocco

Late Ordovician glaciogenic deposits are exposed intermittently along an 800 km long outcrop belt in the Anti-Atlas mountains of southern Morocco. These deposits are of economic significance as potential oil-bearing sandstones in the Tindouf and Boudenib basins and thus are here re-examined as analogues to subsurface hydrocarbon reservoirs. Glaciogenic deposits of the Upper Second Bani Formation rest unconformably upon underlying shallow marine clastic deposits. The unconformity is characterised by a series of palaeovalleys, some 0.5–1.0 km wide, and up to 100 m deep, which may have been cut under elevated hydrostatic pressures as tunnel valleys beneath a Late Ordovician ice sheet. The valleys and intervalley areas are filled with glaciogene sediments categorized into five facies associations, namely 1) a tabular sandstone association (shallow marine/shoreface deposits), 2) a massive sandstone and conglomerate (ice contact debrites), 3) meandriform sandstone deposits (ice proximal sandur), 4) stratified diamictites (ice-rafted debris) and 5) sigmoidally bedded sandstones (intertidal sandstones). Deformation in these sediments is ubiquitous and includes soft-sediment striated pavements, metre-scale duplex systems, thrust and fold belts of deformation affecting some tens of metres of sediment, and pervasive lineations. These features are interpreted to record the complex nature of deformation processes operating beneath a Late Ordovician ice sheet including sliding at the ice-bed interface, folding and deformation within the sediment column, and a series of complex ramps, detachments and shear zones within an unconsolidated pile of sediment beneath the ice sheet.

Development of gullies and sediment fans in last-glacial areas on the example of the Suwałki Lakeland (NE Poland)

The density of gully network in the Suwałki Lakeland (northeastern Poland) with typical last-glaciation relief is 0.2 km/km 2 on average and locally reaches 1.2 km/km 2. Most gullies are isolated but sporadically they create dendritic patterns. The larger gullies are developed along dellies (bowl-shaped, dry valleys) or melt-out valleys. The smaller and shorter gullies occur on the slopes of melt-out depressions and tunnel valleys. Ages of peat covered by fans at the mouths of larger gullies indicate that gully erosion started between 3520 ± 70 to 2240 ± 100 BP. Two different units build the fans and infilled the gullies. The older unit in the lower part of fans is up to 5 m thick, contains sand and gravel that generally originated from the bottom and bank erosion of the gullies and resembles alluvium. The younger unit, about 2–3 m thick, consists of colluvium. The fans at the mouths of smaller and shorter gullies are mainly built of colluvium. The maximal grain diameter in both units is similar, which testifies to a similar intensity of extreme rainfalls. The analysed sediments have different characteristics, which indicate that the source material and depositional changes are linked to forest clearance and farmland expansion starting in the 7th century AD and continuing in the Middle Ages.

Pleistocene subglacial tunnel valleys in the central North Sea basin: 3‐D morphology and evolution

AbstractFour phases of cross‐cutting tunnel valleys imaged on 3‐D seismic datasets are mapped within the Middle–Late Pleistocene succession of the central North Sea basin (Witch Ground area). In plan the tunnel valleys form complex anastomosing networks, with tributary valleys joining main valleys at high angles. The valleys have widths ranging from 250 to 2300 m, and base to shoulder relief varying between 30 and 155 m, with irregular long‐axis profiles characteristic of erosion by water driven by glaciostatic pressures. The youngest phase of tunnel valleys are smaller and have a thinner infill than the older generations. The fill of the larger valleys comprises three seismic facies, the lowermost of which has high amplitudes and is discontinuous. The middle facies consists of wedge‐shaped packages of low‐angle dipping reflectors and is overlain by a facies characterised by sub‐horizontal reflectors, which onlap the valley margins. The seismic character, and comparison with lithologies identified in other northwest European Pleistocene tunnel valleys both onshore and offshore, suggests that the lower two seismic facies are most likely sand and gravel‐dominated, while the uppermost facies consists of glaciolacustrine and marine muds. The 3‐D morphology of the valley margins combined with the geometry of the infill packages suggest that episodic discharge of subglacial meltwater was responsible for incising the valleys and depositing at least some of the infill. Proglacial glaciofluvial deposits are inferred to account for some of the fill overlying the subglacial deposits. Glaciolacustrine and marine muds filled remaining valley topography as the ice sheet retreated. The preserved valley margins are shown to be time‐transgressive erosion surfaces that record changes in geometry of the tunnel valley system as it evolved through time, implying that valleys associated with each ice‐sheet advance/retreat cycle were dynamic and probably long‐lived. Within the constraints of the existing stratigraphy the oldest tunnel valleys in the Witch Ground area of the central North Sea are most likely to be Marine Isotope Stage (MIS) 12 (Elsterian, ca. 470 ka) in age and the youngest pre‐MIS 5e (last interglacial, ca. 120 ka). If each tunnel valley phase was formed during the retreat of a major ice sheet then four glaciations with ice coverage of the central North Sea are recorded in the pre‐Weichselian, Middle–Late Pleistocene stratigraphy. Copyright © 2006 John Wiley & Sons, Ltd.

Late Ordovician glaciogenic reservoir heterogeneity: An example from the Murzuq Basin, Libya

In North Africa, Late Ordovician glaciogenic reservoirs contain large volume of recoverable hydrocarbon reserves, but their heterogeneity and internal complexity means they are poorly understood. To improve this understanding, this paper presents the case study of the Murzuq Basin, SW Libya, and a synthesis of the stratigraphic architecture of Upper Ordovician glaciogenic reservoirs (the Mamuniyat Formation) within it. Particular attention is paid to regionally extensive stratigraphic boundaries and the geometry of sandstone units of potential reservoir quality. Four disconformity-bound units are recognised, the bounding surfaces of which are flat or of high relief. Unit 1 (the oldest) and Unit 3 are mud-dominated and tend towards non-reservoir. In contrast, Units 2 and 4 (the youngest) are sand-dominated, have poor to excellent reservoir quality but distinctly different sedimentological architectures across the basin. Unit 2 comprises transitions from glaciofluvial/intertidal sandstones to offshore turbidites and formed exclusively during glacial retreat. The occurrence of a significant palaeotopography affected sandbody geometry with glaciofluvial/intertidal deposits concentrating in the lee of these palaeohighs or infilling the bottom of tunnel valleys. Unit 4 is a compound stratigraphic unit recording the effects of glacial retreat and isostatic rebound. In deep marine settings, turbidite fans were deposited during retreat but in shallow water settings, structural reactivation and sediment reworking had a more profound effect on sedimentation. Sedimentological architectures are variable at this level and include prograding coarse clastic wedges (away from a palaeohigh) or the infill of half-graben basins during isostatic rebound.

On the formation of the tunnel valleys of the southern Laurentide ice sheet

Catastrophic releases of meltwater, produced by basal melting and stored for decades in subglacial reservoirs at high pressure, may have been responsible for eroding the broad, deep tunnel valleys that are common along the margins of some lobes of the southern Laurentide ice sheet. We surmise that these releases began when the high water pressure was transmitted to the margin through the substrate. The water pressure in the substrate at the margin would then have been significantly above the overburden pressure, leading to sapping failure. Headward erosion of a conduit in the substrate (piping) could then tap the stored water, resulting in the outburst. In some situations, development of a siphon may have lowered the reservoir below its overflow level, thus tapping additional water. Following the flood, the seal could have reformed and the reservoir refilled, setting up conditions for another outburst. Order of magnitude calculations suggest that once emptied, a subglacial reservoir could refill in a matter of decades. The amount of water released during several outbursts appears to be sufficient to erode a tunnel valley. We think that tunnel valleys are most likely to have formed in this way where and when the glacier margin was frozen to the bed and permafrost extended from the glacier forefield several kilometers back under the glacier, as reservoirs would then have been larger and more common, and the seal more robust and more likely to reform after an outburst.