Global estimates on the number of submarine mud volcanoes are highly uncertain, as well as their role in the deep-sea biosphere and methane budgets. Here, we report the discovery of ten Arctic mud volcanoes in the Barents Sea (440–480 m depth), where only two had been previously known. The new mud volcanoes form flat-topped mounds on the seafloor and are connected to seismic chimneys rooted within the infilling of a buried Pleistocene mega-slide scar. We suggest informally naming the area the Polaris Mud Volcano Complex. These structures have been active at least since the Late Weichselian deglaciation (< 20 ka), displaying evidence of ongoing methane-rich mud expulsion, i.e. mud pools and flows and chemosynthetic fauna. Finally, we propose a conceptual model for their formation which can be exported to other similar settings. Given the widespread occurrence of mega-slides and associated deposits along (paleo)glaciated continental margins, our findings call for a re-evaluation of mud volcanism potential in such regions.

Here, we present an updated stratigraphic subdivision of the Oligocene to Pleistocene succession (880–610 m) in the newly proposed type well for the Molo Formation, the industrial 6407/9‐5 well, located on the continental shelf in the eastern Norwegian Sea. Furthermore, new data from the Danish North Sea wells Nora‐1, Vagn‐2, and Tove‐1 are presented. The studied succession in the 6407/9‐5 well is composed of five sedimentary units separated by hiati. The dating of these five units is based on correlation to the stratigraphically more complete Neogene succession in the (Danish) central North Sea area. In this study, a robust stratigraphic framework of these five units, based on a combination of dinoflagellate cysts (dinocyst) stratigraphy and seismic data, is established. The Oligocene succession is referred to the NSO zonation of Van Simaeys et al., Review of Palaeobotany and Palynology, 2005, 134, 105–128, while the Miocene–Pliocene succession is referred to the dinocyst zonation of Dybkjær and Piasecki, Review of Palaeobotany and Palynology, 2010, 161, 1–29. In well 6407/9‐5 the two lowermost units, located below the Molo Formation, comprise a Rupelian (Lower Oligocene) succession up to 803 m, and an Aquitanian/Burdigalian (Lower Miocene) succession from 803 to 787 m, respectively. Both of these units are referred to the Brygge Formation. The Molo Formation is separated from the underlying Brygge Formation by an unconformity. Furthermore, and in contrast to previous studies, our study shows that the Molo Formation (787–703 m) has an unconformity within it. The lower part of the formation (787–724 m) is dated to late Tortonian (Late Miocene), and referred to the Hystrichosphaeropsis obscura dinocyst Zone. The upper part (724–703 m) is dated to Zanclean (Early Pliocene), and referred to the Melitasphaeridium choanophorum dinocyst Zone. The uppermost unit studied (703–670 m) in the well is referred to the Gelasian (Lower Pleistocene) and is included in the Naust Formation. The regional correlation of this stratigraphy with the complete succession in the Danish North Sea reveals that the hiati found in the Miocene succession on the Norwegian Sea shelf are controlled by tectonism, while the internal depositional patterns of the Molo Formation were controlled by eustatic sea‐level changes.

Strontium (Sr) isotope data from cores and ditch-cutting samples from hydrocarbon wells from Pleistocene sediments from four areas on the Norwegian continental shelf have been compared with previously published biostratigraphic, lithostratigraphic, seismic and new micropaleontological and palynological data. The aim for this paper is also to provide a review of previous investigations of the use of Sr analysis on this margin, and to give a new assessment of some of the well samples by re-analysing these data and discuss the main limitations of the use of Sr isotopes to date Pleistocene sediments. It is important to obtain accurate dating of the Pleistocene sediments in order to constrain the age of glacial events, indicate the age of neotectonic periods and perform basin modelling. However, obtaining a good chronostratigraphy of the thick Pleistocene successions has proved to be very difficult. Since the sediments have been deposited during a relatively short time, there are few recorded fossil events as first and last appearance datums. Sr isotope stratigraphy based on analyses of calcareous mollusc and microfossil tests (foraminifera and Bolboforma) have proven an effective dating method particularly for Oligocene and Miocene sections on the Norwegian continental shelf. Dating Pliocene and especially Pleistocene sediments by means of Sr isotope stratigraphy has proved more difficult. The Sr isotope curves for the Pliocene to Pleistocene, in general, have lower gradients than the Oligocene and the Miocene parts, and small errors in the Sr-87/Sr-86 isotopic composition have a much larger impact when calculating ages from low-gradient parts of the Sr isotope seawater curves than from high-gradient parts. In addition, possible error sources comprise reworked fossil tests since a large portion of the marine Pleistocene deposits on the Norwegian continental shelf consists of redeposited older sediments originally deposited closer to the coast. These factors are probably the main reasons why the Sr isotope analyses of the investigated Pleistocene sections, with some exceptions, have not yielded very reliable ages.

The almost complete, mainly deltaic, upper Paleogene and Neogene succession in Jylland, Denmark, was previously investigated for 87Sr/86Sr ratios in 143 samples from 18 localities. In the present paper, strontium-isotope data from the Upper Oligocene Lower Miocene parts and foraminiferal and pyritised diatoms data from 94 of these samples were used to correlate with previously published data from Norwegian wells and boreholes and one borehole in the British sector of the North Sea. For the Middle Upper Miocene parts of the succession the correlation is based mainly on Bolboforma data. The ages of the geological formations in the Danish succession correlate readily with lithological units in the Norwegian North Sea, the Norwegian Sea shelf and the East Shetland Platform, which have all been investigated applying similar methods. The Bolboforma assemblages have their origin in the North Atlantic and the Norwegian Sea and confirm the presence of an open strait in the northern North Sea. This strait was the only seaway passage into the North Sea Basin during the Miocene. The glauconitic Utsira Formation sand (approximately 5.7-4.2 Ma), in the threshold area close to the outlet to the Norwegian Sea, overlies erosional unconformities comprising hiati of 21 my in some areas and 13 my in other areas. We believe that the unconformity below the Utsira Formation was mainly related to a fall in sea level in the Late Miocene, contemporaneous with that partly responsible for the Messinian salinity crisis. Bolboforma and dinoflagellate cysts stratigraphy indicate that the base of the Mob Formation in its southern distribution area (Draugen Field, Trandelag Platform) is of Late Miocene age (close to 9 Ma). This part of the Mob Formation was contemporaneous with the middle/upper part of the Kai Formation.

Seven side-wall core samples from the lower part of the Mob Formation in exploration well 6610/3-1 off Vestfjorden/Lofoten have been reanalysed for marine palynomorphs. Description of new species from other studies and access to reference successions from nearby sites with an independent chronostratigraphy have enabled a more reliable depositional age constraint for the Mob Formation in its northern distribution area than previously achieved. The new data (e.g., the lowest occurrence of the dinoflagellate cysts Barssidinium graminosum and Barssidinium pliocenicum and the highest occurrence of the dinoflagellate cysts Minisphaeridium latirictum and Operculodinium piaseckii) provide incontrovertible evidence for a Miocene age for the Mob Formation in well 6610/3-1. Our data suggest that the Mob Formation in well 6610/3-1 started to accumulate after erosional activity which took place between the late middle Miocene and the late Miocene. The data (especially the presence of the dinoflagellate cyst A. andalouisiensis andalousiensis, the co-occurrence of the dinoflagellate cysts Barssidiniuni evangelineae and M. latirictum and the presence of the acritarch L. lucifer) suggest that the Mob Formation in well 6610/3 1 started to accumulate at around 8.8 / 8.7 Ma. The sediments were deposited in a warm temperate, high-energy environment influenced by river discharge. During deposition, older sediments, particularly from the Paleogene, became extensively eroded. The erosion and rapid accumulation were probably associated with the combined effects of eustatic sea-level fall and uplift/updoming of the hinterland/coastal zone.

In recent years there has been an increased interest in Neogene hydrocarbon accumulations in the North Sea. The production of gas from Pliocene–Quaternary deposits in the Dutch sector, the discovery of oil-bearing Miocene sands in the Lille John area and oil accumulation in middle Miocene deposits in the T-1 well in the northern part of the Danish Central Graben area, have documented Neogene hydrocarbon accumulations. Some of these deposits are of economic interest. This study presents an oil-bearing, middle Miocene diatom ooze in the Valhall Field (well 2/8-G-2), within the Norwegian sector (Fig. 1). The Valhall Field is located just north of the Danish–Norwegian sector boundary.

Micropalaeontological analyses Micropalaeontological investigations were based on analyses of planktonic and benthic foraminifera and Bolboforma. The fossil assemblages are correlated with the micropalaeontological zonation for Cenozoic sediments of King (1989). Gradstein and Bäckström’s (1996) faunal zonation from the North Sea and Haltenbanken is also used. The zonations of planktonic foraminifera (Spiegler and Jansen, 1989) and Bolboforma (Spiegler and Müller, 1992; Müller and Spiegler, 1993) from ODP and DSDP drillings in the Norwegian Sea and the North Atlantic are very important for the dating of the sediments. Correlation with these zones yields the most accurate age determinations, because the zones are calibrated with both nannoplankton and palaeomagnetic data.

This study provides the results of the first integrated study of Oligocene–Pliocene basins around Norway. Within the study area, three main depocentres have been identified where sandy sediments accumulated throughout the Oligocene to Early Pliocene period. The depocentre in the Norwegian–Danish Basin received sediments from the southern Scandes Mountains, with a general progradation from north to south during the studied period. The depocentre in the basinal areas of the UK and Norwegian sectors of the North Sea north of 58°N received sediments from the Scotland–Shetland area. Because of the sedimentary infilling there was a gradual shallowing of the northern North Sea basin in the Oligocene and Miocene. A smaller depocentre is identified offshore northern Nordland between Ranafjorden (approximately 66°N) and Vesterålen (approximately 68°N) where the northern Scandes Mountains were the source of the Oligocene to Early Pliocene sediments. In other local depocentres along the west coast of Norway, sandy sedimentation occurred in only parts of the period. Shifts in local depocentres are indicative of changes in the paleogeography in the source areas. In the Barents Sea and south to approximately 68°N, the Oligocene to Early Pliocene section is eroded except for distal fine-grained and biogenic deposits along the western margin and on the oceanic crust. This margin was undergoing deformation in a strike-slip regime until the Eocene–Oligocene transition. The Early Oligocene sediments dated in the Vestbakken Volcanic Province and the Forlandssundet Basin represent the termination of this strike-slip regime. The change in the plate tectonic regime at the Eocene–Oligocene transition affected mainly the northern part of the study area, and was followed by a quiet tectonic period until the Middle Miocene, when large compressional dome and basin structures were formed in the Norwegian Sea. The Middle Miocene event is correlated with a relative fall in sea level in the main depocentres in the North Sea, formation of a large delta in the Viking Graben (Frigg area) and uplift of the North and South Scandes domes. In the Norwegian–Danish Basin, the Sorgenfrei-Tornquist Zone was reactivated in the Early Miocene, possibly causing a shift in the deltaic progradation towards the east. A Late Pliocene relative rise in sea level resulted in low sedimentation rates in the main depositional areas until the onset of glaciations at about 2.7 Ma when the Scandes Mountains were strongly eroded and became a major source of sediments for the Norwegian shelf, whilst the Frigg delta prograded farther to the northeast.

One hundred and fifty-six Sr-87/Sr-86 analyses have been performed on 129 samples from 18 outcrops and boreholes in Oligocene-Miocene deposits from Jylland, Denmark. These analyses were mainly conducted on mollusc shells but foraminiferal tests, Bolboforma and one shark tooth were also analysed. The main purpose of the study is to compare the ages of the Danish succession suggested by the biostratigraphic zonation on dinoflagellate cysts (Dybkjmr and Piasecld, 2010) with the ages based on analyses of the Sr-87/Sr-86 composition of marine calcareous fossils in the same succession. Analyses of samples from the Danish Brejning, Vejle Fjord, Klintinghoved, Arnum, Odderup, Hodde, Ornhej and Gram formations gave ages between 25.7 My (late Oligocene) and 10.3 My (late Miocene). The Sr isotope ages from the lower part of the succession, i.e. Brejning to Odderup formations, agree with the age estimates based on biostratigraphy. However, the Sr-87/Sr-86 ratios of fossil carbonates from the middle-upper Miocene, Hodde to Gram succession consistently indicate ages older than those recorded by biostratigraphy. Post-depositional processes as an explanation for this offset are inconsistent with good preservation of shell material and little reworking. A palaeoenvironmental cause for the observed mismatch is therefore indicated. Search for geological events that could explain the older ages obtained by Sr isotope compositions have not led to any conclusions and we had recognised the same problem in earlier reports and communications. We conclude that this is a general and possibly global, middle-late Miocene problem that has to be reconsidered and explained geologically. (c) 2014 The Authors. Published by Elsevier EN.

AbstractThe Oligocene sedimentary succession in the eastern North Sea is revised and re-interpreted by applying new state-of-the-art reflection seismic data integrated with new bio- and Sr-stratigraphy data from three key wells in the study area. The Oligocene succession in the eastern North Sea is divided into four transgressive–regressive (T-R) sequences, characterized by non-accretional and/or aggradational transgressive systems tracts and prograding regressive systems tracts. Detailed studies of three wells, including biostratigraphy and Sr analysis, constrain the age relationships between the T-R sequences. Internal clinoform geometry indicates that the sediments were sourced from the present southern Norwegian mainland to the north of the depositional area. The direction of progradation shifted from being SE-directed in the earliest Rupelian (early Oligocene) to S- and SW-directed during Chattian time (late Oligocene). Rapid basin subsidence is indicated by the development of non-accretionary transgressive systems tracts, with subsequent progradation into water depths of hundreds of metres. The creation of accommodation space was out of phase relative to eustatic sea-level changes, and mainly controlled by regional-scale differential vertical movements where uplift and exposure of landmasses of the hinterland (southern Norway) occurred concurrently with basin subsidence. Halokinesis had an intra-basinal influence on the main sediment transport direction, but probably did not contribute much in creation of accommodation space.

High-resolution multibeam bathymetry acquired on the steep continental slope off Lofoten-Vesteralen (LV), North Norway, reveals approximately 15 canyons, some of which are incised 500-1100 m into a sedimentary succession of mainly Quaternary-Cenozoic age. The shape and size of canyons vary considerably, particularly in the southwest where the average slope gradient is 3-5 degrees and where two of the canyons have reached a mature stage and are filled by 200-450 m of sediments. Three canyons confined to the lower-mid slope were probably initiated at the lowermost slope. The initiation process is poorly understood. There is a marked change to steeper slopes (5-8 degrees) northwards, where most of the canyons transect the entire continental slope and are V-shaped in cross section. The majority of the canyons are deeply eroded, with branches formed by slides. Erosion by various mass-movement processes along thalwegs have steepened slopes and reduced stability, causing a retrogressive development by sliding both at canyon heads and at sidewalls. The age of the present canyons, and how they started to form, cannot be determined from the present data. We suggest that the initiation of canyons by mass movements from the shelf edge are most likely in the north. New 2D seismic data reveal several palaeocanyons. Some are partly or totally filled by younger sediments, while others have been rejuvenated. The data indicate that a marked uplift of the LV margin occurred in Mid Miocene time, causing reduced slope stability and formation of the first canyons. Successions of submarine fan/slide deposits up to 700 m thick interbedded with hemipelagic/turbiditic sediments occur on the continental rise at the outlets of canyons where the seabed slope gradient is around 1 degrees. The last phase of significant canyon activity was during the Late Pleistocene, while minor mass movements may have occurred into the Holocene. (C) 2013 Elsevier Ltd. All rights reserved.

Pleistocene to Eocene stratigraphy and geological evolution of the thick Cenozoic fan deposits on the western Barents Sea continental margin SW of Bjørnøya are interpreted on the basis of seismic data and the results of biostratigraphic analysis (foraminifera, dinofiagellate cysts and radiolaria) from exploration well 7316/5-1. Strontium isotope ages are also obtained from three levels. The biostratigraphic analysis reveals seven informal zones based on foraminifera, four informal zones based on dinofiagellates, and five zones based on radiolaria fauna. Glacially derived Upper Pliocene and Pleistocene sediments rest unconformably on a Lower Oligocene to Lower Miocene section. An unconformity between the Lower Oligocene and Middle Eocene is also recorded. Prior to this investigation Oligocene sediments had not been encountered in exploration wells in the Barents Sea. The Oligocene benthonic foraminiferal fauna found in well 7316/5-1 is very similar to the fauna recorded in outcrop at Forlandsundet. Strontium-isotope correlation suggests, however, that the Oligocene section found in the Barents Sea is 5-6 m.y. older than that found at F orlandsundet.

The Late Pliocene-Pleistocene Naust Formation and equivalent sediments have been mapped on the Norwegian continental shelf between Sognefjorden and Lofoten (61°N to 68°N). During the last 2.8 million years the shelf has prograded up to 150 km westwards. More than 1000 m of sediments have been deposited in large areas around the outer shelf/upper continental slope. Most of the Naust Formation on the mid-outer shelf comprises prograding wedges, commonly thinning westwards and downlapping onto the top of the underlying Kai Formation. 3D seismic data from the shelf show iceberg ploughmarks on palaeo-surfaces buried below 400 m of sediment at the present-day shelf edge. Tentatively, the oldest observed ice-grooved surface is c. 2 million years old. In several hydrocarbon exploration wells, abundant angular gravel fragments, interpreted as ice-rafted debris (IRD), have been found in the lower parts of the Naust Formation. In addition to these observations, the very high depositional rates compared to the previous Tertiary period, support our view that most of the Naust Formation was deposited under glacial influence. The Naust Formation is subdivided into five sequences. From oldest to youngest these are named N, A, U, S and T. Naust S represents the third last glaciation, whereas Naust T is from the last and second last glaciation. An upper regional angular unconformity (URU), best developed on the inner-mid part of the shelf, marks the boundary upwards to more flat-lying sedimentary units. These units mainly comprise tills, deposited during the last extensive glaciations on the shelf. The upper regional unconformity is over large areas off mid-Norway at the base of sequence Naust T and represents an erosional surface made by the Elsterian Ice Sheet.

Based on a revised chronostratigraphy, and compilation of borehole data from the Barents Sea continental margin, a coherent glaciation model is proposed for the Barents Sea ice sheet over the past 3.5 million years (Ma). Three phases of ice growth are suggested: (1) The initial build-up phase, covering mountainous regions and reaching the coastline/shelf edge in the northern Barents Sea during short-term glacial intensification, is concomitant with the onset of the Northern Hemisphere Glaciation (3.6-2.4 Ma). (2) A transitional growth phase (2.4-1.0 Ma), during which the ice sheet expanded towards the southern Barents Sea and reached the northwestern Kara Sea. This is inferred from step-wise decrease of Siberian river-supplied smectite-rich sediments, likely caused by ice sheet blockade and possibly reduced sea ice formation in the Kara Sea as well as glacigenic wedge growth along the northwestern Barents Sea margin hampering entrainment and transport of sea ice sediments to the Arctic-Atlantic gateway. (3) Finally, large-scale glaciation in the Barents Sea occurred after I Ma with repeated advances to the shelf edge. The timing is inferred from ice grounding on the Yermak Plateau at about 0.95 Ma, and higher frequencies of gravity-driven mass movements along the western Barents Sea margin associated with expansive glacial growth. (C) 2008 Elsevier Ltd. All rights reserved.

This chapter provides a lithostratigraphic correlation and the present knowledge of the depositional history of the Tertiary succession of the Scandinavian countries. The succession records an initial phase of carbonate deposition in the early Paleocene. This was succeeded by deposition of deep marine clays with intercalation of sand-rich mass flow deposits during most of the Paleocene and Eocene. Volcanic activity in the North Atlantic was extensive at the transition from the Paleocene to the Eocene resulting in widespread sedimentation of ash-rich layers in the North Sea area. During the Oligocene, the first prograding deltaic complex developed, sourced from the Fennoscandian Shield. Late Oligocene-Early Miocene inversion and uplift of Norway and the Shetland Platform resulted in major progradation of coastal and delta plain systems. At the end of the Tertiary most of the North Sea basin was filled and the Fennoscandian Shield was flanked to the west by a broad, coalesced coastal plain.

Based on an extensive study of biostratigraphic and strontium isotopic data from wells in the southern Viking Graben and eastern flank of the Utsira High we present an improved chronology of the post-Eocene section of the northern North Sea. Emphasis has been placed on the sandy Utsira and Skade formations. Detailed analyses of foraminiferal and Bolboforma fossil assemblages supported by strontium isotopic data from six exploration and two production wells suggest that the Skade sands were deposited mainly during the Early Miocene whereas the Utsira sands were deposited during the Late Miocene and Early Pliocene. All biostratigraphic data are presented in range charts and have been integrated with wire-line log and seismic data. Strontium isotope stratigraphy has been used as an additional dating tool and has proved powerful in the sandy sections. This work also demonstrates a need for an update or modification of the lithostratigraphic nomenclature of the post-Eocene succession in the Norwegian North Sea, and a proposal for a revision is presented.