Tunnel Valleys Research Articles

Seismic investigations of buried tunnel valleys on- and offshore Denmark

Abstract Tunnel valleys on- and offshore Denmark have been investigated based on a database of 1000 km two-dimensional (2D) onshore seismic data, 5600 km 2D offshore seismic data and 1200 km 2 three-dimensional (3D) offshore seismic data. From the 2D data we identified 216 onshore and 674 offshore seismic tunnel valley intersections, and 55 individual valleys were identified from three 3D surveys. The majority of the valleys have depths ranging from 50 to 200 m and widths between 500 and 1500 m. Up to seven generations of tunnel valleys were identified, indicating repeated erosion and deposition within the study area. The valleys were most likely formed by subglacial meltwater erosion during the last three glaciations. Statistical analyses conducted on the data show that there are no significant differences between the onshore and offshore valleys with respect to their depth and shape; they share morphological and structural characteristics. The onshore seismic data have been analysed in conjunction with lithological information from boreholes. The analyses show that tunnel valley bottoms terminate equally commonly in substrates dominated by clay and sand, and that the valley shapes are similar for the two substrates.

3D seismic analysis of buried tunnel valleys in the Central North Sea: tunnel valley fill sedimentary architecture

Abstract Fourteen overlapping 3D seismic reflection data sets, originally acquired by the oil and gas industry, were used to image and investigate complex networks of buried tunnel valleys in the UK sector of the Central North Sea. More than 180 individual tunnel valleys were observed and mapped to horizontal resolutions of 12–50 m and vertical resolutions of 8–10 m. The results presented here identify three recurring seismic facies that locally define stratigraphic organization from base to top of the buried tunnel valley fills: (i) high-amplitude disrupted units, (ii) a unit containing gently dipping disrupted reflections or transparent facies and (iii) well-layered continuous reflections. A comparison to similar works in the region is used to infer tunnel valley infill during changing depositional conditions from high-energy subglacial meltwater to quiet glaciolacustrine or glaciomarine environments.

Substratum control on tunnel-valley formation in Denmark

Abstract Tunnel valleys formed by meltwater erosion underneath the margins of the Pleistocene ice sheets are present in high numbers in the Danish onshore area. The geographical distribution of the buried tunnel valleys is uneven, but when comparing with the substrata lithology we find a large number of valleys in areas dominated by low-permeable sediment and a smaller number in areas with highly permeable substrata. The observations point to the drainage capacity of the ice-sheet substratum as an important factor controlling tunnel-valley formation. Tunnel-valley formation appears to be favoured in areas with low-permeable substrata because meltwater drainage through the sediments is impeded, leading to the formation of a channelized subglacial drainage system. The high transmissivity in areas dominated by permeable substrata facilitates drainage of a part of the meltwater as groundwater. This causes a lowering of the subglacial meltwater pressures, and tunnel-valley formation is less likely. Once formed and filled, the tunnel valleys cause a change of the hydraulic properties of the substratum and if subglacial water pressures underneath a subsequent ice advance are sufficiently high, old tunnel valleys will be prone to reactivation.

Multistage erosion and infill of buried Pleistocene tunnel valleys and associated seismic velocity effects

Abstract This study provides results on buried tunnel valleys from the greatest number of released three-dimensional (3D) seismic surveys in the Danish North Sea sector to date. The valley infill sediments and the fluid content from buried Pleistocene tunnel valleys beneath the North Sea floor cause significant seismic velocity effects. Analysis of conventional 3D seismic data, high-resolution two-dimensional (2D) seismic data and wire-line log data from exploration wells reveal a multistage cut-and-fill history of tunnel valleys in the Danish North Sea. The seismic infill facies of the valleys is analysed and the associated velocity effects quantified. The valley infill generates both velocity pull up and push down of underlying seismic horizons. In contrast to expectations, our analysis shows no distinct correlation between particular seismic facies and a particular travel-time effect. Infill units characterized by the same seismic facies within the same valley can thus provide different travel-time effects. The anomalous velocities can be used as indicators of shallow gas and till content within the valleys leading to low and high velocities, respectively. Detailed investigations suggest that tunnel valley formation and sediment infill occurred over several episodes, with partial reuse of the kilometre-scale valleys and strong indications of ice-free periods between some of the formation episodes.

Late Ordovician tunnel valleys in southern Jordan

Abstract The Upper Ordovician glacial record of southern Jordan (Ammar Fm.) essentially consists of palaeovalley infills and of a subordinate time-transgressive fluvial to shallow-marine succession overstepping both the palaeovalleys and interfluvial areas. Valley size (depth, 60–160 m; width, 1–3 km), steep (20–50°) margins, internal organization and depositional facies point to an origin as tunnel valleys. The tunnel valleys are infilled by either fluvioglacial sandstones or fluviodeltaic coarsening-upward successions including fine-grained clayey sediments. Re-occupation of previous valleys is evident in places. At least three generations of tunnel valleys are inferred from cross-cutting relationships, although they most probably only reflect temporary standstills and minor re-advances related to the overall recession following the main glacial advance recorded in Saudi Arabia. Petrophysical measurements indicate that higher permeabilities are located in the glacially related strata (1.5–3 darcy in fluvioglacial infills), with a somewhat reduced porosity (22–28%) relative to the preglacial sandstones owing to a higher clay content, probably of diagenetic origin. Sandstone amalgamation, however, gives the fluvioglacial sandstones a high reservoir quality.

Depositional architecture and sequence stratigraphic correlation of Upper Ordovician glaciogenic deposits, Illizi Basin, Algeria

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.

The glaciogenic unconformity of the southern North Sea

Abstract The southern North Sea is a shallow epicontinental sea that was glaciated several times during the Quaternary. The area is known for its remarkable record of tunnel valleys, the age and origin of which are debated. The recent availability of continuous three-dimensional seismic data between the coasts of Britain and the Netherlands provides the opportunity to establish a new seismic interpretation workflow adapted to the intracratonic glaciogenic successions. By analysing the geomorphology of the buried basal glaciogenic unconformity, four distinct major ice fronts are identified and correlated onshore. The ice fronts provide robust relative timelines, and the analysis of tunnel-valley orientations and their merging points indicates that the number of glacial phases has been underestimated. By comparing the erosion capacities of sand and chalk substrates, it is suggested that mechanical abrasion processes are also involved during tunnel-valley genesis. The methods and observations used in this study are applicable to the ancient glaciogenic record in general and constitute a basis for the sedimentological analysis of tunnel valleys.

Ordovician proglacial sediments in Algeria: insights into the controls on hydrocarbon reservoirs in the In Amenas field, Illizi Basin

Abstract Ordovician proglacial deposits form gas reservoirs in the In Amenas field, Illizi Basin, Algeria. Depositional models were developed to understand the context and disposition of the main reservoirs through an evaluation of core and analogous outcrops from the Tassili N'Ajjer. Tunnel valleys initially accumulated sandstones with tractional structures. Subsequent failures of subaqueous grounding line sediment deposited proglacial debrites comprising poorly sorted argillaceous sandstone with granules. These were interbedded with high-density turbidity sandstones; their fine grain size indicates they were dynamically disconnected from the lithologically varied debrites. A lobate geometry has been defined for one subsurface composite turbidite. Periodic catastrophic outflows, possibly evacuating subglacial lakes, incised the network of subglacial tunnels and in the process delivered sand to the turbidite outwashes. Bedforms indicate high-energy, transcritical to supercritical outflows that were stable for extended periods. During ice retreat, a period of ice margin stability may have occurred due to grounding over the In Amenas granitic palaeohighs. Outwash fan apices were located along this grounding line with feeder channels developing where the substrate was more easily eroded such as between the palaeohighs. Following further ice retreat, deposition evolved to variably sinuous channels and thence to pelagic fines with dropstones.

Glacial landsystems and dynamics of the Saginaw Lobe of the Laurentide Ice Sheet, Michigan, USA

The surficial terrain of the Saginaw Lobe in southern Michigan is divided into 4 landsystems, numbered sequentially from southwest to northeast, containing related assemblages of sediments and landforms, two of which appear to have a genetic relationship with the bedrock units over which they lie. Landsystem 1 consists of the Sturgis Moraine, a terminal/recessional moraine, and an adjacent drumlin field. The moraine is a high-relief ridge with hummocky topography and ice-walled lake plains at its crest, and thick, coarse outwash fans on its distal margin. The drumlin field extends up-glacier from the Sturgis Moraine across the subcrop of the Mississippian Coldwater Shale to the subcrop of the overlying Marshall Sandstone. The low permeability of the shale may have increased basal pore pressures into a range in which deformation of basal sediment into drumlins occurred. Landsystem 2 overlies the subcrop of the Marshall Sandstone. Stagnation of the lobe behind a discontinuous moraine in this area is indicated by high relief, collapse topography, composed of kames, eskers, and ice-walled lake plains. A system of tunnel valleys terminates and merges in this landsystem with large outwash fans and plains sloping away from the margin. High basal drainage into the sandstone may have facilitated stagnation across a broad area to form this landsystem. Landsystem 3 may have also developed during stagnation of the lobe, but it differs from landsystem 2 by its more subdued relief. It contains a network of evenly spaced, southwest-trending open tunnel valleys that contain discontinuous esker segments. Landsystem 4 consists of till plains and low recessional moraines formed as the Saginaw Lobe retreated downslope into Saginaw Bay of the Lake Huron Basin. Tunnel valleys are absent in this area.

Evaluation of MODFLOW‐LGR in Connection with a Synthetic Regional‐Scale Model

This work studies costs and benefits of utilizing local-grid refinement (LGR) as implemented in MODFLOW-LGR to simulate groundwater flow in a buried tunnel valley interacting with a regional aquifer. Two alternative LGR methods were used: the shared-node (SN) method and the ghost-node (GN) method. To conserve flows the SN method requires correction of sources and sinks in cells at the refined/coarse-grid interface. We found that the optimal correction method is case dependent and difficult to identify in practice. However, the results showed little difference and suggest that identifying the optimal method was of minor importance in our case. The GN method does not require corrections at the models' interface, and it uses a simpler head interpolation scheme than the SN method. The simpler scheme is faster but less accurate so that more iterations may be necessary. However, the GN method solved our flow problem more efficiently than the SN method. The MODFLOW-LGR results were compared with the results obtained using a globally coarse (GC) grid. The LGR simulations required one to two orders of magnitude longer run times than the GC model. However, the improvements of the numerical resolution around the buried valley substantially increased the accuracy of simulated heads and flows compared with the GC simulation. Accuracy further increased locally around the valley flanks when improving the geological resolution using the refined grid. Finally, comparing MODFLOW-LGR simulation with a globally refined (GR) grid showed that the refinement proportion of the model should not exceed 10% to 15% in order to secure method efficiency.

Buried bedrock valleys and glacial and subglacial meltwater erosion in southern Ontario, Canada

Morphometric features from a recently compiled bedrock topography map by the Ontario Geological Survey suggest a glacial erosion origin for the buried large bedrock valleys and troughs in southern Ontario. The bedrock valleys at Milverton, Wingham and Mount Forest are tunnel valleys, resulting from subglacial meltwater erosion beneath the Huron ice lobe, probably during or shortly after the Late-Wisconsinan glacial maximum. Diagnostic features for this interpretation include abrupt valley beginning and termination, uneven longitudinal valley profiles and up-slope gradients. The Dundas bedrock valley is the western extension of the Lake Ontario Basin. No comparable bedrock valleys were found to connect it to the Milverton valley for a joint drainage system as previously suggested. The Laurentian bedrock trough is the southeastward extension of the Georgian Bay Basin, both developed along shale bedrock between the Precambrian shield highlands and the Niagara Escarpment, resulting from long-term mechanical weathering associated with Pleistocene glacial erosion. This bedrock low has a floor that exceeds 50 km in width and is 26 m and more below the current water level of Georgian Bay. It could drain Georgian Bay should the drift cover be removed. There is no evidence to suggest that a preglacial river channel, if it existed, is still preserved in the floor of the Laurentian trough as previously suggested. The framework for an intensely glacially sculpted bedrock surface differs from the previous view for simple modification of a preglacial landscape and is, therefore, important in regional subsurface geological mapping and groundwater studies.

The formation and evolution of an isolated submarine valley in the North Channel, Irish Sea: an investigation of Beaufort's Dyke

AbstractBeaufort's Dyke is a submarine depression located in the North Channel of the Irish Sea. With a maximum depth of 312 m, the dyke is one of the deepest areas within the European continental shelf. Integration and interpretation of 450 km of sparker seismic data and full‐coverage bathymetric data derived from multi‐beam echo sounder surveys allow for the investigation of the formation processes of Beaufort's Dyke and the evolution of geomorphological features within it. The dyke, formed by composite subglacial processes dominated by subglacial meltwater discharge, is interpreted as a tunnel valley. The regional isolation of Beaufort's Dyke may be explained by the bounding of the North Channel by the bedrock masses of Ireland and Scotland, coupled with the exploitation of structural weakness along a fault plane and presence of halite in the eroded substrate enhancing the erosive potential of the overlying glacier. Beaufort's Dyke has probably been maintained as an open feature by strong rectilinear tidal currents. The morphology of lunate sediment waves and a large parabolic bedform towards the south of the dyke contradict the observed dominant S–N mean hydrodynamic flow recorded within the North Channel, suggesting an alternative hydrodynamic regime either within the dyke or during bedform creation. Copyright © 2011 John Wiley & Sons, Ltd.

Stratigraphic evidence for the Hirnantian (latest Ordovician) glaciation in the Zagros Mountains, Iran

High-latitude Hirnantian diamictites (Dargaz Formation) and lower-Silurian kerogenous black shales (Sarchahan Formation) are locally exposed in the Zagros Mountains. The glaciogenic Dargaz deposits consist of three progradational/retrogradational cycles, each potentially controlled by the regional advance and retreat of the Hirnantian ice sheet. Glacial incisions of sandstone packages change laterally from simple planar to high-relief (< 40 m deep) scalloped truncating surfaces that join laterally forming complex polyphase unconformities that scour into the underlying Seyahou Formation. The glaciated source area was to the present-day west, in the region of the Arabian Shield, where numerous tunnel valleys have been reported. Based on a study of palynomorphs and graptolites, the glaciomarine Dargaz diamictites are dated as Hirnantian, whereas the youngest Sarchahan black shales are diachronous throughout the Zagros, ranging from the Hirnantian persculptus to the earliest Aeronian (Llandovery) triangulatus zones. The diachronism is related to onlapping geometries capping an inherited glaciogenic palaeorelief that preserved different depth incisions and source areas. Our data suggest the presence of Hirnantian satellite ice caps adjacent the Zagros margin of Arabia and allow us to fill a gap in the present knowledge of the peripheral extension of the Late Ordovician ice sheet.

Seven glacial cycles in the middle-late Pleistocene of northwest Europe: Geomorphic evidence from buried tunnel valleys

The deep-ocean marine isotope record and the Antarctic and Greenland ice caps record numerous glacial and interglacial cycles since the middle Pleistocene, and yet evidence for similar numbers of ice-sheet advances over the continent and shallow shelves of northwest Europe is absent. Here, we document seven generations of regionally correlatable subglacial tunnel valleys that record the geomorphic imprint of ice sheets traversing the North Sea basin between ca. 500 and 40 ka, consistent with that predicted by proxy records of glacial and interglacial climate change. Over 180 subglacial tunnel valleys that incise into Pleistocene sediments in the North Sea basin were mapped over ∼60,000 km 2 of three-dimensional seismic-reflection data. Using a subset of these data, we identified seven separate episodes of subglacial erosion that can be correlated regionally in the UK sector of the central North Sea. The characteristics of the valley morphologies, orientations, and infill stratigraphy indicate that each set of tunnel valleys formed during a separate ice-sheet advance and retreat cycle. Stratigraphic data suggest that the tunnel valleys formed significantly later than the Brunhes-Matuyama reversal event at 780 ka and before the Last Glacial Maximum (marine isotope stage [MIS] 2; 21 ka). These results imply a more complicated glacial history for northwest Europe and more glaciations than the three-glaciation model traditionally interpreted from the terrestrial record for the past 500 k.y. Our data provide the most complete documentary evidence for repeated advance and retreat of the northwest European ice sheets since the middle Pleistocene, and for the first time indicate that terrestrial ice-sheet advances in the North Sea can be matched in number with the cold events recorded in oceanic and/or ice-core proxies of climate change over the past 500 k.y.

Subglacial tunnel valleys in the Alpine foreland: an example from Bern, Switzerland

The morphology of the Alpine and adjacent landscapes is directly related to glacial erosion and associated sediment transport. Here we report the effects of glacio-hydrologic erosion on bedrock topography in the Swiss Plateau. Specifically, we identify the presence of subsurface valleys beneath the city of Bern and discuss their genesis. Stratigraphic investigations of more than 4,000 borehole data within a 430 km2-large area reveal the presence of a network of >200 m-deep and 1,000 m-wide valleys. They are flat floored with steep sided walls and are filled by Quaternary glacial deposits. The central valley beneath Bern is straight and oriented towards the NNW, with valley flanks more than 20° steep. The valley bottom has an irregular undulating profile along the thalweg, with differences between sills and hollows higher than 50–100 m over a reach of 4 km length. Approximately 500 m high bedrock highlands flank the valley network. The highlands are dissected by up to 80 m-deep and 500 m-broad hanging valleys that currently drain away from the axis of the main valley. We interpret the valleys beneath the city of Bern to be a tunnel valley network which originated from subglacial erosion by melt water. The highland valleys served as proglacial meltwater paths and are hanging with respect to the trunk system, indicating that these incipient highland systems as well as the main gorge beneath Bern formed by glacial melt water under pressure.

Preface: special section of papers dealing with overdeepened basins and valleys in the alpine realm

Overdeepened basins and valleys are common features in most regions that were glaciated during the Quaternary. Such structures are known from areas formerly covered by the Laurentian and Scandinavian Ice Sheet as well as for some parts of the Alps and their foreland. The phenomenon describes subsurface geomorphic features, erosionally cut into bedrock and later filled up by sediments. These structures can reach a depth of several hundred metres below the present valley floor. The sedimentary fillings of overdeepened basins and valleys are of particular interest for applied geosciences, for example in the context of groundwater resource management and subsurface constructions (i.e. tunnels). Recently, the topic of when and how overdeepening occurred in the past has gained additional attention with regard to the long-term stability of nuclear waste disposals, especially in Switzerland. In this context, it is important to note that relatively little research focussed on this issue during the past three decades. The last overview on the topic was a compilation of papers published in a special issue of the journal Eiszeitalter und Gegenwart in 1979. This special section in the present issue of the Swiss Journal of Geosciences comprises six papers related to the phenomenon of overdeepening in the Alps. The article by F. Preusser, J. Reitner and Ch. Schluchter summarises the areal distribution and geometry, reviews the present knowledge of the age of formation, and discusses possible processes responsible for overdeepened features. P. Jordan analyses the overdeepened structures in the foreland of the Swiss Alps using a Digital Elevation Model of bedrock surface based on outcrop and borehole data. It is shown that overdeepening mainly occurs within the limits of the Last Glaciation Maximum (LGM), but with a few overdeepened valleys and basin reaching beyond. M. Durst Stucki, R. Reber and F. Schlunegger interpret the buried valleys beneath the city of Bern as a network of tunnel valleys originating from subglacial high pressure melt water erosion. Valleys found in the surrounding Molasse uplands are regarded to represent proglacial melt water paths. The subsurface characteristics of three deep valleys from the Eastern Alps are investigated by E. Bruckl, J. Bruckl, W. Chwatal and Ch. Ullrich using different geophysical surveys. The authors conclude that deep erosion is not related to basal shear stress and that glacial confluence situations did in these examples not lead to important overdeepening. Rather, it is suggested that deep erosion was related to high pressured melt water at the glacier bed in settings of already tectonically weakened bedrock. The upper part of the sedimentary filling of the overdeepened Wehntal in northern Switzerland is analysed by F. Anselmetti, R. Drescher-Schneider, H. Furrer, H. R. Graf, S. Lowick, F. Preusser and M. Riedi revealing that it reaches back to at least 180,000 a. It is shown that the penultimate glaciation (pre-dating the Last Interglacial) may have reached the area M. Fiebig (&) Institute of Applied Geology, University of Natural Resources and Life sciences, Peter Jordan-Str. 70, 1190 Vienna, Austria e-mail: markus.fiebig@boku.ac.at

Turbidite deposition on the glacially influenced, canyon-dominated Southwest Grand Banks Slope, Canada

The deeply dissected Southwest Grand Banks Slope offshore the Grand Banks of Newfoundland was investigated using multiple data sets in order to determine how canyons and intercanyon ridges developed and what sedimentary processes acted on glacially influenced slopes. The canyons are a product of Quaternary ice-related processes that operated along the margin, such as ice stream outwash and proglacial plume fallout. Three types of canyon are defined based on their dimensions, axial sedimentary processes and the location of the canyon head. There are canyons formed by glacial outwash with aggradational and erosional floors, and canyons formed on the slope by retrogressive failure. The steep, narrow intercanyon ridges that separate the canyons are composite morphological features formed by a complex history of sediment aggradation and degradation. Ridge aggradation occurred as a result of mid to late Quaternary background sedimentation (proglacial plume fallout and hemipelagic settling) and turbidite deposition. Intercanyon ridge degradation was caused mainly by sediment removal due to local slump failures and erosive sediment gravity flows. Levée-like deposits are present as little as 15 km from the shelf break. At 30 km from the shelf, turbidity currents spilled over a 400 m high ridge and reconfined in a canyon formed by retrogressive failure, where a thalweg channel was developed. These observations imply that turbidity currents evolved rapidly in this slope-proximal environment and attained flow depths of hundreds of metres over distances of a few tens of kilometres, suggesting turbulent subglacial outwash from tunnel valleys as the principal turbidity current-generating mechanism.

Did the Late Ordovician African ice sheet reach Europe?

ABSTRACTPaleovalleys and their infi lling successions are described from outcrops and drill cores of the Cantabrian Range (northern Spain). A Hirnantia fauna and associated diamictites with striated lonestones indicate that the paleovalleys are related to the Hirnantian (latest Ordovician) glacial event. Based on overall geometry, depositional facies, and associated deformation structures, the paleovalleys are interpreted as subglacial tunnel valleys. They were most likely related to the North Gondwana ice sheet. The ice sheet therefore reached the Ibero-Armorican domain that was still attached to the Gondwana landmass at least until the latest Ordovician.INTRODUCTION The Hirnantian glacial record documents one or several ice sheets throughout Africa, Arabia, and South America, the extents of which are still controversial (Ghienne et al., 2007; Schonian and Egenhoff, 2007). Although glaciomarine strata are widespread in European peri-Gondwa-nan terranes (Robardet and Dore, 1988; Brenchley et al., 1991), they do not precisely indicate the presence of glaciers at those localities. Because a number of these terranes would have been attached to the Gondwana land-mass during the Late Ordovician (e.g., Robardet, 2003), the Gondwanan ice sheet may have reached some of them. The Hirnantian paleovalleys of the Cantabrian Range (northwest Spain) are the fi rst glacial valleys to be recognized outside the Gondwana landmass. They may be related to a Gondwanan ice sheet lobe or to a peri-Gondwana satellite ice cap.

Late Ordovician climbing-dune cross-stratification: a signature of outburst floods in proglacial outwash environments?

Climbing dune-scale cross-statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium-grained to coarse-grained sandstones that typically involve 0·3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss-depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing-dune cross-stratification related to the migration of lower-flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion-based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/subaqueous transition of related deltaic wedges, while the latter formed in an ice-proximal segment of the outwash plain. In erosion-based sand sheets, climbing-dune cross-stratification results from unconfined mouth-bar deposition related to expanding, sediment-laden flows entering a water body. Within incised channels, climbing-dune cross-stratification formed over eddy-related side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing-dune cross-stratification is a combination between high-magnitude flows and sediment supply limitations, which lead to the generation of sediment-charged stream flows characterized by a significant, relatively coarse-grained, sand-sized suspension-load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse-grained sand fallout in sediment-laden flows following flow expansion throughout mouth bars or in eddy-related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss-sides and generation of widespread climbing-dune cross-stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood-dominated depositional systems of foreland basins.

The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)

Evidences of glaciation at the end of the Ordovician are widespread in western Gondwana. Some authors consider the glaciation was restricted to the Hirnantian time, but occurrences of glacial deposits in the Lower Silurian strata of South America indicate that the Gondwana glaciers did not completely disappear after the Hirnantian glaciation. In addition, numerous studies based on palaeoecology, sedimentology, sequence stratigraphy and stable isotopes have suggested that ice sheets formed as soon as the Early/Middle Katian. The objective of this work is to investigate the high-frequency eustatic signals in a stratigraphic succession corresponding to the Katian and the Hirnantian (∼ 10 My comprising the uppermost part of the Ordovician). The studied section (Bou Ingarf section > 600 m) is located in the Central Anti-Atlas, southern Morocco. It shows an almost continuous succession of siliciclastic platform deposits. The lower package, Katian to early Hirnantian in age, corresponds to a shelf succession, within which storm dynamics prevailed. The upper package, middle to upper Hirnantian in age, comprises a glaciation-related succession made up of a suite of coastal to flood-dominated fluvio-glacial deposits. The latter have essentially filled in large palaeochannels interpreted as subglacial tunnel valleys. From high-resolution facies and sequential analysis, a curve of variation of depositional environments was established at very high, high and low frequencies. Time calibration was performed based on a high-quality biostratigraphic control mainly derived from chitinozoan biozones through the whole succession. Assuming a constant tectonic subsidence and a bathymetric model (shoreface/upper offshore boundary: 30 m; upper offshore/lower offshore boundary: 120 m), changes in facies-based water depths are converted into an eustatic sea-level curve using a one-dimensional backstripping procedure. The eustatic sea-level curve shows that the stratigraphic succession is dominated by the stacking of sequences at very high- (< 400 ka), high- (400 ka) and low- (“3rd order”) frequencies that reflect eustatic variations controlled by ice-cap to ice-sheet growths throughout the Katian and Hirnantian. Three episodes of abrupt sea-level falls (> 40 m) and coeval forced regressions were identified during the Katian. They represent glacial episodes of significant extent prior to the Hirnantian, but essentially lacking a glacial record. Strata reflecting the Hirnantian glaciation include two differentiated events and an intermediate but important transgression. The second Hirnantian glacial event, the only one associated with subglacial erosion in the Bou Ingarf area, corresponds to the Late Ordovician glacial climax characterised by a continental-scale ice sheet. Ice sheets permanently occupying the centre of the Gondwana landmass throughout the Late Ordovician may reconcile moderate Hirnantian eustatic sea-level fall amplitude (40–80 m) and palaeoglacial reconstructions that show a Hirnantian ice sheet covering the main part of western Gondwana.