Abstract Evidence is presented for an Early Pleistocene grounded ice sheet in the Central North Sea (CNS). Dated basal Quaternary deposits from UK well 30/13-2 Josephine coincide with the lowest iceberg ploughmarked horizon in a southern CNS depocentre. This horizon has been traced northwards into the central CNS where it is identified as the Crenulate reflection, which exhibits a series of deep, channel-like features, interpreted as subglacial meltwater channels. Further north, a seismic reflection at the same level is characterized by megascale lineations that are interpreted to be the result of erosion, deposition and deformation of sediment beneath an advancing ice sheet. An overlying seismic reflection is characterized by a well-developed channel system that is thought to have drained the retreating ice sheet prior to a post-glacial marine transgression. A southern CNS Early Pleistocene depocentre is shown to have become inactive sometime in the Early to Middle Pleistocene and was replaced by a developing depocentre in the central and northern CNS that remained active into the Late Pleistocene. Evidence for an Elsterian or older grounded ice sheet in UK Continental Shelf Quad 22 is presented in the form of glaciotectonic thrust features.

Abstract This paper provides documentation of unexpectedly high-reservoir-quality glaciomarine sands found in the Cenozoic succession beneath McMurdo Sound, Antarctica, as an analogue study for evaluations of hydrocarbon prospectivity in basins elsewhere. The Oligocene to Lower Miocene succession of the Victoria Land Basin, an extant portion of the West Antarctic Rift System, comprises diamictites, mudrocks and sandstones with minor conglomerates. These lithologies are arranged in repetitive stacking patterns (cycles), interpreted to record repeated advance and retreat of glaciers into and out of the basin, with attendant eustatic and isostatic effects. Phases of ice retreat within the cycles comprise an array of mudrocks, sandy mudrocks and sandstones, deposited mainly during relative sea-level highstands. Clean, well-sorted, unconsolidated and porous sands <25 m thick from such intervals, which are interpreted to be mainly deltaic in origin, were encountered. Some of these sands, which have visible porosity as high as 41%, flowed into the well bore together with significant volumes of cold formation water. Diagenetic modification of sands in these intervals is minimal, which can be attributed to the low-temperature nature of the subsurface environment. Accordingly, glaciomarine sands in near-field glaciogenic successions should be considered as potential reservoir facies in prospectivity assessments.

Abstract This paper proposes a model for glaciation in Oman during the late Palaeozoic ice age (LPIA) based on sedimentological and provenance analyses of the Late Carboniferous–Early Permian Al Khlata Formation, exposed in the Huqf, Oman. The results demonstrate a complex pattern of glaciation across Oman, not fully recognized in previous models. Striated glacial pavements provide evidence for two phases of ice advance: a phase of ice sheet advance towards the NE, and a second and probably younger phase where an ice centre localized on the Huqf High flowed towards the SW. The stratigraphy is constrained by previous palynological studies and is subdivided into three units, from oldest to youngest: ‘early’ AKP5, ‘late’ AKP5 and AKP5/P1. ‘Early’ AKP5 palaeogeography is characterized by ice-contact glacial lacustrine and deltaic sedimentary environments along the western margin of the Huqf High. Meltwater discharge flowed into the lake from ice margins located to the east, upon the Huqf High, recorded by progradational delta and fan complexes. ‘Late’ AKP5 palaeogeography is characterized by pro-glacial fluvial-deltaic outwash braidplains that record high-magnitude meltwater discharge from an ice margin located to the SW of the study area. The youngest undifferentiated AKP5/P1 palaeogeography is characterized by re-establishment of ice-contact glaciolacustrine conditions.

Abstract The Late Palaeozoic glaciation was the longest of the Phanerozoic era. It is recorded in numerous Gondwanan basins, with some, such as the Chaco Basin, having a high petroleum potential. In this basin, the quality of the available seismic, well and outcrop data permits us to characterize the Late Palaeozoic glacial record. Palaeovalleys that are c . 500 m deep and c . 7 km wide have been analysed here. Focusing on the glaciogenic Carboniferous deposits, seismic data with well ties and outcrop analogues provide new sedimentological insights. The palaeovalley infill is imaged as a chaotic seismic facies overlain by an aggrading–prograding prism, interpreted as tillites covered by a fluvio-deltaic system. Tillites form both under the ice and during rapid ice recession, whereas fluvio-deltaic systems can only originate from a stable ice margin and last until the ice sheets withdraw inland. These two depositional modes are repeated several times, generating a progressive burial of the Carboniferous palaeovalleys. This succession of erosions and fills records major glacial stages, including a series of glacial and interglacial phases from the Late Devonian to the Early Permian. Depicting the Late Palaeozoic glacial history of the Chaco Basin seems crucial for the localization of potential good reservoirs.

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.

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.

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.

Abstract The architecture, distribution and development of channelized sandstone bodies are described from Late Ordovician paraglacial successions of the Tassili N'Ajjer (SE Algeria and SW Libya) based on satellite images and field data (sedimentary logs, photomosaics). Sandstone bodies have a ribbon-like form at outcrop (often referred to as ‘cordons’ in the literature). They typify a fluvioglacial outwash plain deposited between a continental ice front and a marine delta-front zone. Channelized sandstone bodies are straight to sinuous, with widths ( W ) in the 60–600 m range, thicknesses ( T ) in the 5–30 m range and they have a mean W / T ratio of 16.5. They develop within an aggradational–progradational sand-dominated deltaic topset succession including at its distal end a terminal distributary channel and mouth-bar environments. The architecture of channel bodies and the related depositional facies, which includes climbing-dune cross-stratification, indicates that channelized sandstone bodies represent plugs of isolated channels related to high-magnitude flood events (glacier outbursts). These plugs form fossilized networks of both braided channels and interlaced anastomosed channels, offering snapshots of an outburst-related unconfined proglacial outwash braidplain constituted by the amalgamation of adjacent, elongated outwash fans.

Abstract Many Neoproterozoic successions contain viable hydrocarbon source rocks, even though they were deposited before most extant life forms evolved. Eukaryotic microalgae, bacteria, chlorophyte micoalgae, marine pelagophyte algae and dinoflagellates may have contributed organic matter. Major global-scale glaciations, which are commonly attributed to a ‘snowball’ or ‘slushball’ Earth scenario, or deposited under a ‘zipper rift’ scenario, are believed to have played an important role in the deposition of hydrocarbon source rocks during the mid- Neoproterozoic (Cryogenian). Phases of Cryogenian deglaciation may have culminated in the deposition of high total organic carbon shales and ‘cap carbonates’ in restricted anoxic basins, which may have been carved by ice sheets themselves or, alternatively, formed as restricted extensional half graben as Rodinia began to fragment. One example of these organically enriched deglacial sediments comprises shales and dolostones deposited following the Sturtian glaciation in the Centralian Superbasin of Australia, an amalgam of basins that extends almost continent-wide across Australia. Data from the Marmot MMDD-1 drill core on the Stuart Shelf in the southern part of the Centralian Superbasin, together with previously published data on organic enrichment in the Amadeus Basin in the central part of the Centralian Superbasin, suggest that the deposition of organically enriched shales was widespread during the Sturtian deglaciation.

Abstract Glaciogenic reservoirs and hydrocarbon systems occur intermittently throughout the stratigraphic record, with particular prominence in Neoproterozoic, Late Ordovician, Permo-Carboniferous and Late Cenozoic strata. Recent interest in glaciogenic successions has been fuelled by hydrocarbon discoveries in ancient glaciogenic reservoirs in North Africa, the Middle East, Australia and South America. Glaciogenic deposits of Pleistocene age are noteworthy for their content of groundwater onshore and potentially prospective and/or hazardous gas accumulations offshore. The abundant imprints of Pleistocene glaciations in both hemispheres can be used to reconstruct complex histories of repeated ice cover and retreat, and glacier-bed interactions, thus informing our view on the dynamics of older ice caps and predictions of future glaciations. This volume aims to provide a better understanding of glaciogenic processes, their stratigraphic record and reservoir characteristics of glaciogenic deposits. The book comprises 3 overview papers and 16 original case studies of Neoproterozoic to Pleistocene successions on 6 continents and will be of interest to sedimentologists, glaciologists, geophysicists, hydrologists and petroleum geologists alike.

Abstract Glaciogenic reservoirs host important hydrocarbon and groundwater resources across the globe. Their complexity and importance for exploration and palaeoclimate reconstruction have made glaciogenic successions popular subjects for study. In this paper we provide an overview of the palaeoclimatic and tectonic setting for Earth glaciation and a chronological account of glaciogenic deposits since c. 750 Ma, with particular emphasis on their reservoir potential and associated hydrocarbon systems. Hydrocarbon accumulations within glaciogenic reservoirs occur principally in Palaeozoic (Late Ordovician and Permo-Carboniferous) sandstones in South America, Australia, North Africa and the Middle East, with relatively minor occurrences of shallow gas hosted in Pleistocene deposits in the North Sea and Canada. Groundwater reserves occur within glaciogenic sandstones across the northern European lowland and in North America. The main glaciogenic environments range from subglacial to glacier front to proglacial and deglacial. Rapidly changing environments, hydrodynamic regimes and glacier-front and subglacial deformation often result in very complex glaciogenic sequences with significant challenges for reconstruction of their origin and resource importance, which this volume seeks to address.

Abstract The glaciofluvial deposits are by volume and permeability the most important unit in the terrestrial glacial successions, and they are the obvious target for groundwater as well as hydrocarbon reservoir exploration. The dominant glaciofluvial units are related to the proglacial setting in the foreland of an advancing ice margin, which results in a coarsening-upwards sequence with fine-grained beds at the base and glaciofluvial gravel at the top. In a complete sequence a till caps the unit, and at its base a glacitectonite is formed by shearing related to the development of the deformational layer below the ice. The glacial deposits laid down during the same glacial advance represent a glaciodynamic sequence. An important feature added to this is the proglacial glaciotectonic deformation. The glaciotectonic architectural elements comprise thrust faulting, folding of hanging-wall anticlines, thrust-sheet duplexes, hydrodynamic breccias and mud diapirs, the structural style of which define the glaciotectonic complex. The glaciodynamic sequence corresponds to the glaciodynamic event related to one major ice advance. The glaciodynamic processes representing the event comprise deposition as well as deformation, creating a glaciogenic sedimentary succession and a set of glaciotectonic structures. These constitute the elements to be recognized for defining a glaciodynamic sequence.

Abstract About 3 million years ago, major ice sheets developed over Scandinavia and began to deliver large volumes of sediment to the mid-Norwegian shelf. The shelf was built out in a prograding pattern towards the west, and more than 1000 m of sediments was deposited over large areas on the middle/outer shelf. The dominating large-scale depositional pattern is a series of prograding wedges and flat-lying, sheet-like units mainly of glacial origin. On top of these units are flat-lying till units deposited during the last few glaciations, commonly separated from the underlying units by one or several erosional unconformities. The lithology of these layers is generally fine-grained, mainly clay and silt, but with sporadic clasts up to boulder size. Based on regional and detailed bathymetry, the ice-flow pattern from the last glaciation has been reconstructed on the shelf. This involved a very dynamic ice sheet with fast-flowing ice streams in the transverse, cross-shelf troughs, separated by more passive ice domes on the intermediate shallow banks. The ice streams appear to follow the cross-shelf troughs from glaciation to glaciation, but occasionally they switched to new flow paths. The thick Quaternary sediments on the outer part of the mid-Norwegian shelf represent a challenge for hydrocarbon exploration. Several large slides have occurred from the shelf break and down the continental slope. During the last few years there has been an increased focus on investigating the Quaternary succession in order to search for hydrocarbon prospects. The potential for finding reservoirs in these sediments is discussed, and a few examples of gas discoveries are shown. The Peon gas field ( c . 250 km 2 ) is located in the glacially eroded Norwegian Channel above the Upper Regional Unconformity (URU) separating flat-lying glaciogenic sediments from dipping sedimentary units. The reservoir is developed in glaciofluvial sands a few hundred thousand years old, and sealed by flat-lying glaciomarine sediments and till units.

Abstract Tunnel valleys are elongated incisions that are commonly interpreted as being the result of erosional processes by subglacial meltwater occurring under continental ice sheets. The abundance, size and the primarily coarse-grained infill of these features have made tunnel valleys important hydrocarbon and groundwater reservoirs. Although numerous tunnel valleys have been described over the last century, their formation and infill remain poorly understood. This review summarizes and discusses the current knowledge of tunnel valleys, providing an overview of the observations around the world. Morphological aspects that separate tunnel valleys from other landforms are discussed, as well as the wide variety of sedimentary environments found to contribute to the infilling of these features. The depth of the incision and the character of ice retreat significantly determine the final infill architecture. The formational hypotheses proposed in the literature are assessed to test their wider applicability to all other tunnel valleys in order to find a generic model that helps in the prediction of the morphology and infilling of both Pleistocene and pre-Pleistocene age. A quasi-steady-state model, with small meltwater outbursts that erode tunnel valleys near the ice margin, seems compatible with most of the known valleys. Other proposed models require specific geographical or climatic conditions.

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.

Abstract Despite a long history of investigation, several critical issues regarding the glacial history of NW Europe, particularly in currently marine areas, remain unsolved. In this study, we present a comprehensive three-dimensional (3D) seismic interpretation of an area measuring 2000 km 2 in the western part of the Danish North Sea that exhibits several buried Quaternary landforms. Well data are used to assign minimum and maximum ages for the studied sedimentary succession. The most prominent buried landforms are three large-scale tunnel valleys of probable Saalian age that extend over more than 20 km across the western and southern part of the study area. These valleys most probably formed through subglacial meltwater erosion close to the termination of a former ice sheet. In the southern part of the study area, an extensive network of small-scale, dendritic seismic lineations interpreted as a palaeo-drainage system characterizes the landward termination of one major tunnel valley. This drainage system was active either contemporaneously or shortly after the development of the tunnel valley. Interpretation of this system as contemporaneous to tunnel-valley formation suggests that steady-state subglacial meltwater discharge was funnelled through a drainage system into the main tunnel valley. In contrast, interpretation of the drainage network as post-incisional points to the development of a post-glacial river system re-using the pre-existing tunnel valley as a downstream fluvial pathway. This uncertainty in the interpretation has important consequences for prediction of the rock content and reservoir characteristics of the tunnel-valley infill, in that either meltwater deposits or fluvial sediments form a considerable part of the tunnel-valley infill.

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.

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.