A new stratigraphic framework for the Miocene – Lower Pliocene deposits offshore Scandinavia: A multiscale approach

Abstract

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.

Influence of Post‐danian Sea‐level Changes and Variations in Sedimentation Rate on Overpressure Build Up in the Clay‐rich Overburden in the Danish Sector of the North Sea Central Graben

“…Post-Danian sedimentation in a study area around the North Jens-1 and Fasan-1 wells in the Danish Central Graben was simulated numerically in order to examine the generation and evolution of overpressure in the clay-rich successions overlying the Chalk Group. The simulation incorporated details of post-Danian sealevel changes and variations in the sedimentation rate in this area derived from biostratigraphic and seismic interpretations (Rasmussen, 2004(Rasmussen, , 2017Rasmussen et al, 2005;Ponsaing et al, 2020;Dybkjaer et al, 2021). This model was more realistic than that proposed in a previous study by Orozova-Bekkevold and Petersen (2021).…”

“…The Miocene -Quaternary depositional history of the North Sea can be summarized as follows. Sediments deposited during the Tortonian (11.8 to 7.2 Ma) originated from source areas to the NE, while the main input of sediments was from the east and SE during the Pliocene (5.3 to 2.6 Ma) (Rasmussen, 2004;Rasmussen et al, 2005;Rasmussen et al, 2017;Dybkjaer et al, 2021). After the Pliocene, the North Sea Basin tilted due to uplift of the Fennoscandian Shield which resulted in increased rates of subsidence and sedimentation in the Central Graben area.…”

“…Chronologies and thicknesses of the Miocene (23-5.3 Ma), Pliocene (5.3-2.6 Ma) and Quaternary (2.6 Ma to present-day) successions (Fig. 1b) were from Dybkjaer et al (2021). A simplified mean depositional rate (in metres per million years) for each stage was then obtained by dividing the respective unit thickness by the duration in million years (Table 3).…”

“…This area was chosen because both petrophysical logs and detailed seismic and biostratigraphic interpretations are available (Fig. 2) for the Miocene-Quaternary succession (Dybkjaer et al, 2021). The study therefore investigates: (i) whether variations in sedimentation rate, especially during the Miocene, influenced the onset and magnitude of overpressures in the post-Danian succession in the study area; and (ii) whether water depth also played a role in overpressure development.…”

Sites U1569 and U1570

“…Therefore, the discussion below is presented in two parts. Occurrences of palynofacies categories and palynomorphs in Hole U1569A are listed in Table T9 for the Neogene and uppermost Oligocene portion and Munsterman and Brinkhuis, 2004;Egger et al, 2016;Dybkjaer et al, 2021).…”