Triassic Stratigraphy of the North Sea: A New Cross-border Chronostratigraphic Framework using Age Equivalent Units from the Tampen Spur Area, Norwegian North Sea

The Triassic interval of the northern North Sea contains important hydrocarbon reservoirs and potential targets for CO2 sequestration. However, understanding of the age and distribution of sedimentary facies within the Triassic succession is hampered by the absence of a robust chronostratigraphic framework. Although recent studies for the UK Central North Sea (UK CNS) have defined a robust Triassic chronostratigraphic framework, significant stratigraphic nomenclature differences coupled with lack of dateable palynomorphs within the Triassic continental Hegre Group of the Norwegian Northern North Sea have led to non-biostratigraphic cross-border correlations. Preliminary results from palynology and heavy mineral studies from the Norwegian Northern North Sea within a newly refined tectonostratigraphic framework have prompted a re-examination of the chronostratigraphic correlations between the Central and Northern North Sea. In the greater Tampen Spur area, a lack of active fault movement during sedimentation suggests that deposition of up to 2 km of Triassic Hegre Group sediments is largely climatically controlled and potentially regionally correlatable. Herein we aim to extend the well-established Triassic chronostratigraphic framework of the UK CNS to the Norwegian Northern North Sea area using distinct and age-equivalent Triassic mudstone units. Well data from the Tampen Spur area reveal a regionally correlatable Norian mudstone which represents the Alke formation of the Hegre Group and an additional unassigned Ladinian mudstone package. These mudstone units were deposited on the distal fringes of a major distributive fluvial system sourced from the Norwegian margin. The Alke Formation seals the progradational motif of the Norian Lunde Formation and the Ladinian mudstone seals the Norian-Ladinian sandstone of the Lomvi Formation. The sandstones record the progradation and mudstones the retrogradation of the distributive fluvial system. Given the age and similar petrophysical properties, we propose that the Alke Formation is equivalent to the Jonathan Mudstone Member and the Ladinian Mudstone is equivalent to the Julius Mudstone Member of the Skagerrak Formation in the UK CNS. Thus, the Lunde formation is Josephine equivalent and the Lomvi formation represents the Joanne Member. This layer-cake reservoir configuration is completed with a sandier and locally truncated Raude Formation which is equivalent to the Rhaetian Joshua Mudstone Member of the UK CNS. This preliminary cross-border chronostratigraphic framework unifies the Triassic stratigraphy of the North Sea within a newly refined tectonostratigraphic framework. Regional chronostratigraphic correlations allow a comprehensive paleogeographic reconstruction between the Central and Northern North Sea enlightening all major climatic alternations from Olenekian to Rhaetian.