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Triassic Lithostratigraphy of the Northern North Sea Basin 93 NORWEGIAN JOURNAL OF GEOLOGY Triassic lithostratigraphy of the Northern North Sea Basin 93 Triassic lithostratigraphy of the Northern North Sea Basin Kjell-Sigve Lervik Lervik, K.-S.: Triassic lithostratigraphy of the Northern North Sea Basin. Norwegian Journal of Geology,Vol. 86, pp. 93-116. Trondheim 2006. ISSN 029-196X. The Triassic strata of the northern North Sea were initially sub-divided into three formations: the Smith Bank, Skagerrak and Cormorant formati- ons (Deegan & Scull 1977), defined within the informal "Triassic group". The succession was little understood at that time as few wells had drilled into the Triassic red beds. In an attempt to unify UK and Norwegian Triassic and Jurassic lithostratigraphical nomenclature Vollset & Doré (1984) revised the nomenclature, defining a group with three formations - the Hegre Group with the Teist, Lomvi and Lunde formations. The objective of this paper is to establish a gross nomenclature, which covers the entire northern North Sea. A revised Triassic lithostratigraphic chart for the Nor- thern North Sea Basin is presented. Kjell-Sigve Lervik, Statoil ASA, NO-4035 Stavanger, e-mail: [email protected] Introduction deals with the entire northern North Sea. Vollset & Doré (1984) treat the northern area of tha Norwegian It should be unnecessary to state that the geology of the North Sea (Triassic part approximately between 60 – North Sea is independent of political and geographical 62O N), while Cameron (1993) concentrated on the Bri- borders. In spite of this, the Triassic lithostratigraphy of tish sector. Numerous recent wells have been drilled the central and northern North Sea has been subject to into the Triassic, and much new information has been such limitations. Previous lithostratigraphical subdivi- obtained since those papers were published. sions and the naming of the succession have been con- sidered without viewing the basin as a whole. Local The Triassic strata of the northern North Sea were initi- subdivisions within the main basin have been erected. ally subdivided into three formations: Smith Bank, Ska- According to international lithostratigraphical rules gerrak and Cormorant formations defined within the and recommendations, lithostratigraphical nomencla- informal "Triassic group" (Deegan & Scull 1977). The ture shall be applied to a basin as a whole, and not in succession was poorly understood at that time as few relation to non-geological borders and boundaries wells had been drilled into the Triassic red beds. In an (Hedberg 1976; Salvador 1994). attempt to unify UK and Norwegian Triassic and Juras- sic lithostratigraphical nomenclature Vollset & Doré Erecting general lithostratigraphical nomenclature (1984) revised the nomenclature, defining a group with based on information from local observations has three formations - the Hegre Group with the Teist, resulted in sub-optimal nomenclature due to the lack Lomvi and Lunde formations. According to the inter- of an overall picture of the geological development of national lithostratigraphical guide (Salvador 1994), the the basin. The intention of this paper is to revise exis- already defined Cormorant Formation has preference ting nomenclature in the light of the overall geological to be upgraded to group level instead of introducing a history of the basin as known at present and to esta- new name. blish a gross nomenclature, which covers the entire northern North Sea. Information from all released Several papers have discussed the Triassic succession of wells on the Norwegian and British sector has been the northern North Sea since Vollset & Doré (1984). utilized. Further, information from selected wells from Lervik et al. (1989) published additional well data and the southern North Sea as well as from the Danish sec- six lithostratigraphic units were suggested for the Trias- tor has been included. sic succession including the Statfjord Formation, which extends into the earliest Jurassic. The Triassic lithostratigraphy of the northern North Sea has been outlined in the following major publicati- Most subsequent papers are reviews of previous publi- ons: Deegan & Scull (1977), Vollset & Doré (1984) and cations and few present new data, except for Steel & Cameron (1993). Neither of the last two publications Ryseth (1990) who brought a new concept to the dis- 94 K. S. Lervik NORWEGIAN JOURNAL OF GEOLOGY ig. 1. of lithostratigraphy PreviouslyTriassic published the Southern Goldsmith et Sea basins (from al. and Northern North 2003). F NORWEGIAN JOURNAL OF GEOLOGY Triassic lithostratigraphy of the Northern North Sea Basin 95 cussion, by proposing a megasequence stratigraphy Microfaunal evidence is often very sparse, but where related to tectonic control rather than using a strictly available it has been used to avoid gross miscorre- lithostratigraphic framework for subdividing the Trias- lation. To "compensate" for the frequent lack of micro- sic. faunal evidence, wells with a relatively continuous Tri- assic section have been chosen as the basis for the litho- The attempt of Vollset & Doré (1984) to unify UK and stratigraphical classification. Areas with wells contai- Norwegian Triassic stratigraphy has not been followed ning only relatively small intervals or discontinuous up. Cameron (1993) claimed that some of the lithostra- Triassic successions have not been included in this dis- tigraphic units could not be recognized in the UK sec- cussion. Wells from southern part of the British conti- tor, e.g. the Lomvi Formation, although Frostick et al. nental shelf have not been included, as information (1992) reported the Formation from the Beryl Basin: from Cameron (1993) has been incorporated. Wells the formation has also been identified in the UK sector from the southern part of the Norwegian continental in this work. Further, the name of the Hegre Group has shelf have not been included, as only few wells have been "translated" into English and named the Heron penetrated limited unassigned Triassic strata. Confi- Group. "The geographic component of a name (in this dence in any lithostratigraphy from this part of the case name of a bird) should not be altered by transla- shelf must be low. Further wells and information are tion into another language. Stratigraphic units are not needed for erection of a reliable lithostratigraphy. limited by international frontiers, and effort should be made to use only a single name for each unit regardless of political boundaries" (Salvador 1994). Basin development Deegan & Scull (1984) defined the Skagerrak and The Triassic basin configuration of the northern North Smith Bank formations in the Central Trough, but did Sea has a tectonic history, which started at least as early not extend them northwards into the Viking Graben. as Devonian time (Valle et al. 2002). The oldest rem- Johnson et al. (1993) stated that the terms have since nant of sediment infill in the Hornelen Basin onshore been used informally and erroneously. In the type area Norway (Maehle et al. 1977) represents the first and in the central North Sea, the Skagerrak Formation response to break-up of the area, which, after several overlies the Smith Bank Formation, but in the Viking extensions phases, resulted in the northern North Sea Graben the Smith Bank Formation is placed stratigrap- failed arm system, a system which developed in con- hically above the Skagerrak Formation. nection with, and was influenced by the overall tectonic break up of the north American Plate from the Euro- Goldsmith et al. (1995) discussed the stratigraphy of pean Plate. the South Central Trough (UK sector) where the Smith Bank Formation is defined as including the Scythian In addition to outcrops onshore Norway (Hornelen (Induan-Olenekian) mudstones, whereas Middle Trias- Basin and others), Devonian sediments have been sic mudstones are included as members of the Skager- penetrated on the Utsira High (Færseth 1996) and the rak Formation. This is not in accordance with the origi- East Shetland Platform (Lervik et al. 1989) as well as in nal definitions of the Skagerrak and Smith Bank forma- the Unst Basin (Johns & Andrews 1985), indicating that tions made by Deegan & Scull (1977) where the Smith the northern North Sea was a basinal area in Devonian Bank Formation was defined as mudstones distal to the time. Few wells have penetrated Devonian strata and associated Middle Triassic proximal sandstones of the very little has been done to understand the depositional Skagerrak Formation. regime of the Devonian sediments. Where cored, the succession often shows conglomerates or pebbly sand- A summary of previously defined lithostratigraphical stones reflecting sedimentation at a relatively early units is shown in Fig.1. This paper discusses the entire stage of basin development. Extensive erosion, >400m northern North Sea. The lithostratigraphic subdivision on the Utsira High (Færseth 1996), characterises the is related to the dynamic evolution of the basin rather Devonian succession. than just to the lithologies. In order to avoid introdu- cing too many names for the same lithostratigraphical Permian sediments are restricted to the northern Per- unit defined in neighbouring basins, only one new lit- mian Basin in the south and Unst Basin in the west. hostratigraphical unit has been defined. Adjustments to Permian sediments have not been penetrated in the the existing units are proposed. northern and eastern parts of the basin, indicating ero- sion or non-deposition during Permian time. The wells included in the study are shown on Fig. 2.
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