Palynological and foraminiferal biostratigraphy of (Upper Pliocene) Nordland Group mudstones at Sleipner, northern North Sea

The Nordland Group is an important stratigraphical unit within the upper Cenozoic of the northern North Sea. At its base lies the Utsira Sand, a dominantly sandy regional saline aquifer that is currently being utilized for carbon dioxide sequestration from the Sleipner gas and condensate field. A ‘mudstone drape’ immediately overlies the Utsira Sand, forming the caprock for this aquifer. The upper part of the Utsira Sand was recently dated as Early Pliocene, but the precise age of the overlying Nordland Group mudstones has remained uncertain. Dinoflagellate cyst, pollen and spore, foraminiferal and stable isotopic analyses have been performed on these mudstones from a conventional core within the interval 913.10–906.00 m (drilled depth) in Norwegian sector well 15/9-A-11. The samples lie closely above the Utsira Sand. Results give a Gelasian (late Late Pliocene) age for this interval, with a planktonic foraminiferal assemblage at 913.10 m indicating warm climatic conditions and an age between 2.4 and 1.8 Ma. An abundance of the cool-tolerant dinoflagellate cysts Filisphaera filifera and Habibacysta tectata at 906.00 m, along with evidence from pollen and foraminifera, points to deposition during a cool phase of the Gelasian. Dating the mudstone drape provides useful insights into depositional processes. It seems likely that the Utsira Sand, a basinal lowstand deposit, became progressively starved of clastic input as sea level rose and shorelines retreated. The mudstone drape is interpreted as a highstand deposit, perhaps including a maximum flooding surface. Overlying prograding wedges of the Nordland Group form a regressive succession, characterized by increased sedimentary input and rates of deposition of at least 25 cm per 1000 years, which is more than five times that of the Utsira Sand. This is the first published study of a dinoflagellate cyst assemblage from the Upper Pliocene of the northern North Sea. The new dinoflagellate cyst species Echinidinium nordlandensis Head sp. nov. and Echinidinium sleipnerensis Head & Riding sp. nov. are formally described.