Mid-Cretaceous stratigraphy and micropalaeontology of the central North Sea
The middle Cretaceous (sediments of Albian to Turonian) lithostratigraphy and foraminiferal biostratigraphy of the central North Sea has been described using data from fifteen wells. These wells are located in or on the margins of major Cretaceous depocentres, namely the Witch Ground Graben, Dutch Bank, Moray Firth, 'Buchan' and Forth Approaches Basins. The lithology of cuttings, sidewall core, and conventional core samples have been described and with the help of electric logs a lithostratigraphic subdivision of each well has been achieved. In this area of the North Sea the middle Cretaceous sequences are subdivided into five units, these are the Valhall (part), Rodby, Hidra, Plenus Marl and Herring Formations. As an aid to correlation the Hidra and Herring Formations have been further subdivided on lithologic and electric log character. The microfaunal content of cuttings, sidewall core and conventional core samples from eight wells has been analysed and the taxonomy of the stratigraphically valuable foraminifera is briefly discussed. Seven new species (Bigeneria sp.1. and 2. , Textularia sp.1. and 2., Uvigerinamina sp.1., Gyroidinoides sp.1. and Osangularia sp.1.) and two new varieties, (Gavelinella intermedia (Berthelin) var A and var B) of foraminifera are described. The abundance of planktonic taxa allows the erection of a local zonal scheme comprising five bio-stratigraphic zones. The mid-Cretaceous microfaunas have many elements in common with the well documented assemblages recorded in the equivalent onshore sequences. This allows the ages of the middle Cretaceous formations to be interpreted by correlation of their microfaunas (particularly planktonic foraminifera) with those recovered from onshore sequences in which the standard macrofossil zonations have been identified. Lithostratigraphic and biostratigraphic analysis indicates the presence of a significant non-sequence within the middle Cretaceous sections in the area of the Buchan Field. This is correlated with unconformities in the Moray Firth, the Dutch Bank Basin, the Witch Ground Graben, the Fladen Ground Spur and Forties Field area. The mid-Cretaceous geological history of the central North Sea is described and related to that of the surrounding onshore and offshore areas of North West Europe. An overall transgression during the Albian to latest Cenomanian/early Turonian was followed by a brief regression and associated erosion later in the early Turonian. A further episode of transgression which was initiated later in the Turonian probably continued into the Maastrichtian. This transgressive-regressive-transgressive cycle may be related to eustatic fluctuations in sea level in conjunction with the mild tectonic activity which persisted through mid-Cretaceous times.