The Utsira Formation shows a complex depositional architecture (see depositional environment), which varies with latitude. In the southern Viking Graben, around 58° N, Utsira Formation forms a giant mounded sand system pinching out both eastward and westward. Only scattered thin mudstone intervals are present in the blocky sands. Here the base of the sands erodes into the underlying sequence (Rundberg & Eidvin, 2005).
At 59° N, in the vicinity of the type well, the Utsira Formation is characterized by a lower part of dominantly blocky sands forming the main sandbody, and an upper part displaying a clear coarsening upwards trend.
In the northern Viking Graben, around 60°- 61° N, the Utsira Formation forms a large mounded sandbody consisting of predominantly blocky sands with only subordinate thin mudstone intervals. Towards its northernmost extent (Tampen area) the Utsira Formation is present only as a thin unit of glauconitic sand.
Rundberg & Eidvin (in press) have shown that previous maximum thicknesses published were exaggerated due to an error in the original definition of the Utsira and Skade formations.
Logs from well 16/1-1 (pdf)
Logs from well 15/9-13 (pdf)
A detailed re-study of original sample material in the type well was undertaken by E. Anthonissen (Anthonissen 2004, MsC thesis). The observations below are directly taken from this study (see range chart of figure ????.).
Depth range: ?689-698 m
Material: One ditch cutting sample
Age: early Late Pliocene
Lithostratigraphy: Clayey sand-mudstone; micaceous with common fossil fragments, green glauconite present, subrounded quartz (Source: original Esso well-log)
Correlation: Subassemblage NSR12A of Gradstein & Bäckström (1996) and (upper) Neogloboquadrina atlantica (dextral) Assemblage of Spiegler & Jansen (1989).
In situ assemblage: This assemblage contains a fairly rich benthic foraminiferal assemblage, with Planulina ariminensis and Cibicidoides grossus being the most abundant taxa. Other characteristic forms include Cibicides lobatulus. No agglutinated forms occur. Planktic foraminifera are present in much lower abundance with Neogloboquadrina pachyderma (dextral), Neogloboquadrina pachyderma (sinistral), and Neogloboquadrina atlantica (dextral) present. The radiolarian Cenosphaera sp. is also present.
Reworked or caved assemblage: Abundant Bulimina marginata are believed to be caved from overlying Pleistocene material. Late Miocene reworking is evident in the presence of Uvigerina venusta saxonica, Uvigerina venusta deurnensis and Glomospira charoides (all with last occurrences in Late Miocene).
Depth range: 707-762? m
Material: Four ditch cutting samples
Age: Early Pliocene
Lithostratigraphy: (707-725 m) Clayey sand-mudstone; micaceous with common fossil
fragments, green glauconite present, subrounded quartz.
(744-762 m) Sand, greenish-gray, micaceous with some glauconite and common fossil fragments, subrounded-rounded quartz.
Correlation: N.atlantica (sinistral) Assemblage of Spiegler & Jansen (1989) and Assemblage NSR11 of Gradstein & Bäckström (1996).
In situ assemblage: Abundant Melonis affinis, abundant Cibicidoides grossus, abundant Uvigerina venusta deurnensis, rare Pullenia bulloides. Approximately half of the assemblage comprises lagenids, including Nodosaria (Dentalina) koninckii.
The agglutinated forms are represented by rare to common Sigmoilopsis schlumbergeri together with the presence of Textularia decrescens and Siphotextularia sculpturata.
Planktic formaminifera are present in much greater abundance and diversity than in the overlying assemblage (although this may be an expression of small sample size in the overlying assemblage). Neogloboquadrina atlantica (sinistral) is common to abundant, Globigerina bulloides is rare to common, Globorotalia inflata is rare, Neogloboquadrina pachyderma (sinistral) is present. Other characteristic taxa present are Orbulina suturalis, Orbulina universa, Globoquadrina altispira globosa, Neogloboquadrina dutertrei, and Catapsydrax sp.
Reworked or caved assemblage: Abundant and millimeter-large Lenticulina rotulata, equally large and abraded fish otoliths and sponge fragments suggest downslope transport from the inner shelf. Broken individual Spiroplectammina carina var. deperdita, Martinottiella cylindrica and Asterigerina guerichi guerichi specimens suggest Miocene reworking, the presence of Karreriella bradyi suggests Late Oligocene reworking, while even older reworking is evident in the presence of Cretaceous Inoceramus fragments. Caving is minimal.
Using all of above data indicates the Utsira Formation in the type well to be largely Early Pliocene in age, just extending in early Late Pliocene.
The detailed study by Piasecki et al. (2002) using dinoflagellate cysts in seven core samples from the 15/9A-23 well indicate an age of Early Pliocene to early Late Pliocene age for the Utsira sands, with the taxon Invertocysta lacrymosa in the two uppermost Utsira samples and Cyst Type 1 of Vernal & Mudie and Reculatosphaera actinocoronata below.
Head et al. (2004) studied a set of core samples between 906.0 and 913.10m in well 15/9A-11, situated closely above the Utsira sands used by Statoil for carbon dioxide re-injection and storage. Foraminifera indicate the interval to belong in the Cibicidoides grossus Zone, cited above. A brief occurrence of Neogloboquadrina atlantica (dextral) and Cibicidoides pachyderma, observed in older Pliocene strata further north along the Norwegian continental margin, is noted at 913.0m, and interpretated as a relatively warm interval assigned an early Gelasian (late Late Pliocene) age . The cool-tolerant dinoflagellates Filisphaera filifera and Habibacysta tectata indicate a late Late Pliocene, late Gelasian age for the level at 906m. This new information indicates strata immediately overlying the Utsira sands to be Late Pliocene in age, in good agreement with studies cited above.
Based on extensive foraminiferal correlations and direct Sr-isotope dating, Eidvin & Rundberg (2001) concluded that main sand deposition, i.e. the Utsira Formation started after 12 Ma. This is based on the observation that at or just below the very base of the Utsira sands in well 24/12-1 occurs an assemblage with Bolboforma badenensis and B.reticulata of Middle Miocene age, and the presence slightly higher in that well of B.fragori of Late Miocene age, and dated by Sr/Sr to be 10.3-11.7 Ma old (see also Table 2 in Rundberg&Eidvin, 2005). The upper level of the Utsira Formation the authors assign a 5 Ma age, which appears too old in the face of the dinoflagellate and foraminiferal evidence presented above.
Rundberg & Eidvin (2005) explain that an error was made in defining the Skade and Utsira formations, in that the Skade Formation, as defined in its type well, correlates to the lower part of the Utsira Formation as defined in its type well. This error was also in conflict with the common usage of the Utsira Formation, as being part of the Nordland Group.
The Utsira Formation is time-equivalent with the Molo Formation distributed off Mid-Norway (Eidvin et al. in press).
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