Lower Pliocene dinoflagellate cysts from cored Utsira Formation in the Viking Graben, northern North Sea

The Utsira Formation covers an area of at least 75 × 450 km² with its main, sandy depocentre in the southern Norwegian Viking Graben. Here in the Sleipner area, the formation is c. 300 m thick and pinches out to the west, south and east. It consists of unconsolidated, very fine- to medium-grained sand mainly composed of quartz with small amounts of glaucony, shell fragments and lignite, probably deposited by turbidite flows. Earlier biostratigraphical studies of the Utsira Formation have been based on cuttings and the age has been considered to be mainly Late Miocene potentially including latest Middle Miocene and earliest Pliocene. In the present study, core samples from the well 15/9-A23 situated in the Sleipner area have been studied. Dinoflagellate cysts were prepared from seven samples of very fine- to fine-grained sand taken from a core in the interval of 1080.0-1084.99 m. A mixture of Middle and Upper Miocene stratigraphic marker-species indicate significant reworking of older species into younger strata. The main stratigraphical problem is therefore to distinguish reworked species from the in situ flora. The presumed in situ dinoflagellate flora comprises overall Upper Miocene to Pliocene species with relatively few species of precise and clear stratigraphical significance. A single record of Selenopemphix armageddonensis in the lowest sample suggests top Miocene or younger strata. Several dinoflagellate cysts in the assemblage are well known from the Pliocene or even the Pleistocene of the North Atlantic region. Some acritarchs are particularly characteristic of the Pliocene of the northern North Atlantic region. This age is supported by the presence of species such as Hystrichokolpoma rigaudiae and Reticulatosphaera actinocoronata which are reported to range into the Lower Pliocene in the northern North Atlantic region. But the youngest possible age of these samples based on the content of dinoflagellate cysts is strongly dependent on the interpretation of which species that are considered reworked. However Amiculosphaera umbracula, Invertocysta lacrymosa and Ataxiodinium zevenboomii are generally reported to reach stratigraphically not much higher than mid-Pliocene. The age of the cored interval of the Utsira Formation is therefore considered to be Early Pliocene, and this makes a significant part of the formation younger than indicated by previously published biostratigraphic data.