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

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

Table1. Strontium isotope data from investigated wells

The Molo Formation represents a characteristic depositional unit on the inner Mid-Norwegian continental shelf and extends along the coast for about 500 km from Møre to Lofoten. It was deposited by coastal progradation in a wave-dominated environment with extensive long-shore drift. The age and stratigraphic relationships have been heavily debated since it was discovered and first described nearly forty years ago. Based on new age information from exploration wells in the Draugen Field on the Trøndelag Platform, the Molo Formation is now determined to be of Late Miocene to Early Pliocene age. It is interpreted to be the proximal equivalent to the deeper marine Kai Formation in the Norwegian Sea and a lateral equivalent to the Utsira Formation in the North Sea. These formations were all deposited as a result of the compression and uplift of mainland Norway in mid Miocene time. In this paper we describe and document the datings and formally define the Molo Formation as a new stratigraphic unit

Miocene oil-bearing diatom ooze from the North Sea

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