Discrepancy between Sr isotope and biostratigraphic datings of the upper middle and upper Miocene successions (Eastern North Sea Basin, Denmark)

AbstractOne hundred and fifty-six 87Sr/86Sr 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 (Dybkjær and Piasecki, 2010) with the ages based on analyses of the 87Sr/86Sr composition of marine calcareous fossils in the same succession.Analyses of samples from the Danish Brejning, Vejle Fjord, Klintinghoved, Arnum, Odderup, Hodde, Ørnhøj and Gram formations gave ages between 25.7My (late Oligocene) and 10.3My (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 87Sr/86Sr 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

Seal bypass at the Giant Gjallar Vent (Norwegian Sea): indications for a new phase of fluid venting at a 56-Ma-old fluid migration system

Highlights: • The Giant Gjallar Vent is still active in terms of fluid migration and faulting. • The Base Pleistocene Unconformity acts as a seal to upward fluid migration. • Seal bypass in at least one location leads to a new phase of fluid venting. The Giant Gjallar Vent (GGV), located in the Vøring Basin off mid-Norway, is one of the largest (~ 5 × 3 km) vent systems in the North Atlantic. The vent represents a reactivated former hydrothermal system that formed at about 56 Ma. It is fed by two pipes of 440 m and 480 m diameter that extend from the Lower Eocene section up to the Base Pleistocene Unconformity (BPU). Previous studies based on 3D seismic data differ in their interpretations of the present activity of the GGV, describing the system as buried and as reactivated in the Upper Pliocene. We present a new interpretation of the GGV’s reactivation, using high-resolution 2D seismic and Parasound data. Despite the absence of geochemical and hydroacoustic indications for fluid escape into the water column, the GGV appears to be active because of various seismic anomalies which we interpret to indicate the presence of free gas in the subsurface. The anomalies are confined to the Kai Formation beneath the BPU and the overlying Naust Formation, which are interpreted to act as a seal to upward fluid migration. The seal is breached by focused fluid migration at one location where an up to 100 m wide chimney-like anomaly extends from the BPU up to the seafloor. We propose that further overpressure build-up in response to sediment loading and continued gas ascent beneath the BPU will eventually lead to large-scale seal bypass, starting a new phase of venting at the GGV

Quaternary evolution of the northern North Sea margin through glacigenic debris-flow and contourite deposition

The Norwegian Channel Ice Stream of the Scandinavian Ice Sheet extended across the northern North Sea margin during the mid- to late Quaternary, eroding older sediment from the continental shelf. Consequently, little is known about early Quaternary sedimentation on this margin. We use two- and three-dimensional seismic-reflection data to investigate changing sediment volumes and sources in the northern North Sea through the Quaternary. The northern North Sea Basin was infilled during the early Quaternary by intercalated glacigenic debris-flows and contourites, which provide a record of the delivery of glacigenic sediment to the slope and the intensity of North Atlantic thermohaline circulation during early Quaternary glacial-interglacial cycles. The infilling of the basin reduced accommodation and led to the deflection of mid- to late Quaternary sediments into the Norwegian Sea, forming the North Sea Fan. Close to the onset of the mid-Quaternary, the south-western Scandinavian Ice Sheet margin was drained by an ice stream located beneath Måløy Plateau, 60 km east of the Last Glacial Maximum Norwegian Channel Ice Stream. The southward-flowing Norwegian Sea Bottom Water current was directed into the partially filled northern North Sea Basin during the early Quaternary, and deflected progressively northwards as the basin became infilled.During this work, C. L. Batchelor was in receipt of a Junior Research Fellowship at Newnham College, Cambridge

Evolution of the central West Greenland margin and the Nuussuaq Basin : Localised basin uplift along a stable continental margin proposed from thermochronological data

ACKNOWLEDGEMENTS Tim Redfield and a second anonymous reviewer are thanked for their constructive comments. The work con-tained in this publication contains work conducted during a PhD study undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training(CDT) in Oil & Gas [grant number NEM00578X/1] and is sponsored by the University of Aberdeen.Peer reviewedPublisher PD

Table 3+4. Strontium isotope ratios of Cainozoic samples from the Utsira region

This study is based on biostratigraphic analysis of Upper Cenozoic strata in eight exploration and production wells from the Tampen area (Snorre and Visund fields), and one well from the Troll field. Dating of the units is based primarily on planktonic and bcnthic foraminifera. Eleven fossil assemblages have been defined in sediments from the Lower Oligocene to the Pleistocene. In addition, strontium isotope, lithologic and petrophysical log analyses have been performed, and the studied wells have been correlated along regional 2-D and 3-D seismic lines. In the Troll area the Pleistocene rests unconformably on the Lower Oligocene. The upper part of the Upper Oligocene is absent in all the Tampen wells. In the Visund area (block 34/8) there is a hiatus of more than 2 m.y. between Oligocene and Lower Miocene strata, and in the Snorre area (blocks 34/4 and 34/7) there is a hiatus of more than 18 m.y. between Oligocene and Upper Miocene deposits. The Neogene section has been subdivided into five major lithologic units. In the Visund area, a Lower Miocene unit (1) of predominantly fine-grained, silty sediments has been identified. A major hiatus separates this unit from the overlying Utsira Formation (2), which in the northern North Sea comprises a thick lower part composed of quartzose sand and a thinner upper part of glauconitic sand. The main sands of the Utsira Formation are not present in any of the studied wells, but preliminary results from well 35/11-1 indicate a Late Miocene to possible latest Middle Miocene age for this unit. The glauconitic part of the Utsira Formation (Late Miocene to earliest Early Pliocene in age) overlies the Oligocene strata in the Snorre area and the Lower Miocene deposits in the Visund area. To the east it may drape over the main Utsira Formation sands or partly interfinger with these. It is overlain by a basal upper Pliocene unit (3) consisting of gravity flow deposits. Cores from this unit exhibit ice- rafted pebbles and have a glacio-marine affinity. A thick complex of Upper Pliocene prograding wedges (4) downlap the basal Pliocene unit in the Tampen area and the Utsira Formation in the eastern part of the basin. It is unconformably overlain by a Pleistocene unit at the top (5). An important feature of the Neogene succession is a large incised valley/canyon system which developed in a north-westerly direction from block 35/8 (off Sognefjorden) to about 62ºN. This erosive system cuts into the basal Upper Pliocene unit in block 34/3 and is thus much younger than has been previously suggested

Table 1-8. Strontium isotopic ratios of samples from 8 wells from the Southern Viking Graben

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 wireline 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