Early Oligocene dinocysts as a tool for palaeoenvironment reconstruction and stratigraphical framework – a case study from a North Sea well
Abstract. The lower Oligocene (Rupelian) successions are climate record archives of the early icehouse world in the Cenozoic. Even though the number of studies focussing on the generally cold Oligocene is increasing, little is known about climatic variations in the mid-latitudes to high latitudes of the Northern Hemisphere. One of the major obstacles is the lack of stratigraphically complete uppermost Eocene to Oligocene successions in these regions. This study focusses on dinoflagellate cysts (dinocysts) from a thick nearly complete Rupelian succession in the Syracuse Oils Norge A/S well 11/10-1 drilled in 1969 in the Norwegian part of the North Sea basin. The well provides a record of mid-latitude dinocyst assemblages, which yield key biostratigraphical and palaeoenvironmental information. All the analyses were undertaken on ditch cutting samples. The dinocyst assemblages confirm that the well penetrates about 600 m of Rupelian sediments and (as supported by correlation with the Nini-1 well) that the lowermost Rupelian (below the top or the last occurrence of Areosphaeridium diktyoplokum) is expanded. These assemblages also indicate the presence of two hiatuses: the first extends from the Lutetian to the Priabonian (equivalent to the D9nb–D12nb zones), and the second spans the Rupelian–Chattian boundary (equivalent to the D14nb subzone or the NSO-5 zone). Despite the risk of caving, the dinocyst assemblages support the existing sequence stratigraphic framework. The assemblages reflect a clear transition from distal to proximal deposition in the vicinity of the site (across the regional seismic sequences OSS-1 – OSS meaning Oligocene seismic sequence – to OSS-2). The proximal deltaic deposits of the OSS-2 regressive system tract (RST) are characterised by pulses of high sea-surface productivity and pronounced shifts in the dinocyst assemblages, reflecting a highly dynamic environment in a restricted marine to marginal marine setting. The Rupelian succession penetrated by well 11/10-1 yields one new species, Areoligera? barskii sp. nov., which is described here in detail. The cold-water-tolerant dinocyst Svalbardella cooksoniae is present in two intervals in the studied succession. These intervals are related to the early Oligocene cooling maxima (the Oi-1a and the Oi-2 events). Furthermore, these two intervals correlate with two local sequence boundaries, suggesting that they are most probably of glacioeustatic origin. From these observations, I postulate that the early icehouse climate played an important role in the depositional development of the Oligocene succession in the North Sea basin. Even though the Eocene–Oligocene transition interval is not complete (i.e. Lutetian to Priabonian is either missing or condensed), well 11/10-1 merits high-resolution studies of the early icehouse climate for the North Sea region. Although any detailed studies should ideally be undertaken on conventional cores instead of ditch cuttings, no such samples spanning the Eocene–Oligocene transition exist in this area.