T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021
Distribution of sediments and Mid Miocene tectonism
The Middle Miocene is represented by a hiatus in many of the studied wells, and the distribution of sediments from this period is strongly related to effects of tectonic movements. The large compressional domes in the Norwegian Sea had their main phase of growth in the Middle Miocene, and there is seismic evidence indicating that other major structures such as the Nordland Ridge (Løseth & Henriksen 2005) and the Lofoten-Vesterålen area (Rise et al. in press) were reactivated at this time. Along the western Barents Sea margin well data are sparse, but there are indications of a hiatus in well 7216/11-1S (Map 2, see discussion section). In the North Sea, seismic data indicate uplift and erosion of the area bounding the west coast of Norway, and reactivation of major faults. In the Central Graben and Norwegian-Danish Basin, the Middle Miocene is characterised by deposition of hemipelagic clays and formation of glauconite. Reactivation of some major structural elements and salt structures occurred and minor unconformities are present.
Norwegian Sea and its continental shelf
After the Middle Miocene tectonism, fine-grained pelagic and hemipelagic sediments of the Kai Formation were deposited on the outer and middle parts of the margin. So far, no wells have sampled Middle Miocene sediments from the Molo Formation, and consequently it is not clear if the Middle Miocene is represented by a hiatus or if the progradation was continuous into the Late Miocene (Map 1, Fig. 5).
The upper part of the Middle Miocene is recorded only in well 6507/12-1 on the Trøndelag Platform, borehole 6403/5-GB1 in the Møre Basin and borehole 6704/12-GB1 on the Gjallar Ridge (Map. 1, Fig. 1 and Fig. 2). Middle Miocene sediments are absent in well 6305/5-1 on the compressional, Ormen Lange, elongated dome structure. Seismic data reveal the presence of an angular unconformity below the Upper Miocene sediments on the top of the dome.
Northern North Sea
Northern North SeaMiddle Miocene sediments in the northern North Sea represent the basal part of the Nordland Group and occur as an infilling unit within the Viking Graben. South of the Viking Graben, these are mainly fine-grained sediments. Middle Miocene units in the wells 15/9-13, 25/1-8 S, 25/2-10 S (southern Viking Graben) and 30/5-2 and 30/6-3 in the northern Viking Graben are sandy and microfossils indicate an inner to middle shelf environment (Fig. 12). The Middle Miocene section in well 25/1-8 S was probably deposited at a very shallow-marine site. In the wells, it may be difficult to distinguish these sands from sands of Utsira above and Skade below, and they are believed to act as a single aquifer system (NPD 2011). Seismic data show that in the Middle Miocene, a well defined delta system was developed in the Frigg area and prograded rapidly to the east. Well 25/1-8 S penetrated the sandy deposits in the delta clinoforms (Fig. 12, Profile P6). A thick depocentre of Middle Miocene sands was developed east and north of the Frigg area in a more distal shelf environment (sands penetrated in wells 30/5-2 and 30/6-3, Fig. 12). The Middle Miocene sandy sections appear to form mappable units which are clearly younger than the Skade Formation and older than the Utsira Formation in the Viking Graben, and we introduce the name Eir Formation, after an Æsir (“god”) in Norse mythology, for these units in the Norwegian sector as a new formation in the Nordland Group (see chapter “Suggestions to an update of the lithostratigraphic nomenclature for the post-Eocene succession”).
In the eastern part of the northern North Sea, the Middle Miocene sediments wedge out into an angular unconformity. In the northernmost North Sea, Middle Miocene sediments are mainly absent, but may be locally present in the depressions between diapiric structures penetrated by injectites. In this northern area, an erosional surface of probable Mid Miocene age was described by Løseth & Henriksen 2005. The surface is overlain by a condensed glauconitic bed deposited close to the Late Miocene/Early Pliocene boundary (Profile P8).
Northern Central Graben
The Middle Miocene in the Central Graben consists of fine-grained deposits. A local hiatus is present in the Middle Miocene in well 2/4-C-11 (Ekofisk Field, Map 1 and Profile 3), and the upper part of the Middle Miocene is missing. This is possibly due to salt tectonics and polygonal faulting.
The Middle Miocene deposits onshore Denmark are characterised by a major transgression (Profile 3). The depositional environment was dominated by clay sedimentation and in the late Serravallian the sedimentation rate was very low, thus permitting widespread formation of glauconite (Rasmussen et al. 2010). Despite the global climatic deterioration, water depth increased in this part of the North Sea Basin.