In total, 3581 benthic specimens were picked and used in the analyses. Only agglutinated foraminifera are present in the sample set. Fourteen taxa are identified and listed in Table 1. The most common species are Trochammina squamataformis, Ammodiscus pseudoinfimus forma granulata and Kutsevella memorabilis. They are often accompanied by Bulbobaculites vermiculus, B. strigosus, Thuramminoides sp., Lagenammina inanis, Psammosphaera sp. and Reophax metensis.

In the examined part of well 7119/12-1, three biotic events are distinguished which reflect transgressive encroachments of marine shelf conditions in the Stø Formation (Fig. 3). The dark mudstone intervals deposited during these inundations show a range of foraminiferal abundance between 41 and 2 individuals per 10 g dry sediment (Fig. 4). The species diversities are generally low with Fisher α values below 5, indicating restricted environmental conditions (Fig. 5).

The transitional beds between the Nordmela and Stø formations were checked for foraminifera by processing nine samples from the interval 2810 and 2845 m core depth. Only a single sample from the lower Stø Formation contains foraminifera (sample 50, 2811.6 m core depth). Just 15 individuals were found in 10 g of dry sediment (Fig. 4). The assemblage is composed entirely of agglutinated taxa and has an extremely low diversity; only three taxa are present (Fisher α index=0.6). The assemblage is nearly monospecific with Ammodiscus pseudoinfimus forma granulata as the most abundant species (Fig. 6). The sample was taken from a fine-grained silty mudstone interval that appears to represent a minor flooding event.

The middle mudstone interval containing foraminiferal assemblages 2 in the Stø Formation is recognized in samples 19–46, which are located between 2729.3 and 2738 m core depth (Fig. 4). This interval is split by a silty–sandy bed into two foraminifera-bearing subunits (samples 19–29 from 2729.3 to 2732.3 m core depth, and samples 34–37 from 2734.1 to 2737.3 m core depth).

The foraminiferal abundance in this mudstone interval ranges between 2 and 41 individuals per 10 g of dry sediment (Fig. 4), reaching maximum values in the upper subunit (sample 20). The Fisher α diversities are low, with an average of 1.7. Simple species numbers range between 4 and 9. The assemblages contain only agglutinated taxa. The dominant species in both subunits is Trochammina squamataformis, which is accompanied by Ammodiscus pseudoinfimus forma granulata and Kutsevella memorabilis (Fig. 6).

The foraminiferal assemblage 3 is characteristic for the samples 4–17 from 2709.7 to 2714.3 m core depth in the upper mudstone interval of the Stø Formation. Foraminiferal abundances range between 2 and 35 individuals per 10 g dry sediment. The diversities are low; showing species numbers ranging between 3 and 10 and an average Fisher α index of 1.6 (Fig. 4). Dominant species include Trochammina squamataformis, Ammodiscus pseudoinfimus forma granulata, Bulbobaculites vermiculus and Kutsevella memorabilis (Fig. 6).

A sedimentological framework of the Lower to Middle Jurassic deposits of well 7119/12-1 was provided by Olaussen et al. (1984) with assessments of the depositional conditions. This sand-dominated succession was deposited in several environmental settings ranging from delta-influenced shelf mudstones through shoreface sands (Stø Formation) to tidal flat/coastal plain fine-grained sandstones with thin coal layers and rooted horizons (Nordmela Formation).

As mentioned above, 32 samples treated for foraminiferal analysis were barren. Mudstone samples containing no foraminifera may represent freshwater deposits. Marginal marine and offshore sandstones are assumed to have been deposited in hyposaline waters.

The foraminiferal assemblages of the three mudstone intervals are typified by entirely agglutinated species composition, extremely low diversities, and the dominance of Ammodiscus and Trochammina taxa. The low Fisher α indices (all <5) suggest restricted environmental conditions in which low salinity or oxygen depletion were the potentially main controlling factors (Nagy et al. 2011), as indicated by modern analogues (Fig. 5). A compilation of foraminiferal distribution patterns in modern environments (Murray 1973) demonstrates that hyposaline conditions are typified by Fisher α indices lower than 5. Low diversity assemblages dominated by agglutinated taxa such as Ammodiscus and Trochammina are known from some modern brackish environments, although these genera also occur in normal marine habitats (Murray 1991). Of particular interest is the dominance of Ammodiscus and Trochammina taxa in some modern marginal marine environments with bottom water hypoxia developed in a salinity-stratified water column. Foraminiferal taxa adapted to such conditions often show reduced test dimensions (see Nagy et al. 2010 and references therein). This feature was used for the recognition of hypoxic conditions in Upper Triassic to Lower Jurassic deposits of the North Atlantic margin by Nagy et al. (2010).

The investigated well sections of the Nordmela Formation consist of fine-grained sandstones and siltstones with interbedded mudstone intervals. Several parasequences show fining upwards trends and might represent tidal channel fills. In some intervals in well 7119/12-1 (2845 to 2820 m core depth) some rootlet horizons and thin coal seams are present. No foraminifera were found in any sample from the Nordmela Formation. These barren samples and the presence of rootlets suggest extremely brackish tidal flat to coastal plain conditions.

The sedimentary succession composing the Stø Formation reveals shallow marine environments with transgressive trends. The thick and homogeneous sandstones with no or sparse bioturbation are interpreted as shoreface and foreshore sand bars (Fig. 3). Fine-grained sandstones, commonly silty, often bioturbated and locally with storm beds are ascribed to the offshore transition zone. The sandstones are barren of foraminifera.

The three main mudstone intervals containing the described foraminiferal assemblages 1–3 represent transgressive pulses superimposed on the overall transgressive development of the Formation (Fig. 3). The lowermost fine-grained and bioturbated siltstone–mudstone beds of the Stø Formation represent a transgressive event in a still coastal near environment. This is confirmed by the presence of a low diversity foraminiferal assemblage in sample 50 of well 7119/12-1 (foraminiferal assemblage 1). The nearly monospecific composition of this assemblage suggest slightly increased salinity in a possibly lagoonal environment. The upper two mudstone intervals are referred to prodelta shelf environments. The siltstone bed appearing in the middle of the lower mudstone interval is interpreted as a storm deposit, which explains why it is barren of foraminifera (Fig. 4). The water masses of the shelf area with prodelta mud deposition were hyposaline as indicated by the low diversity agglutinated foraminiferal assemblages 2 and 3. In accordance with this, the assemblages include Ammodiscus, Trochammina, Bulbobaculites and Kutsevella among the dominant genera. Development of hyposaline conditions in the prodelta shelf outside the Kapp Toscana deltas is in accordance with the large dimensions of this deltaic system, as it is apparent from the reconstructions presented by Van Veen et al. (1992) and Riis et al. (2008).

As mentioned above, size reduction of foraminifera is regarded as an adaptation to low oxygen conditions. By using taxa of Ammodiscus and Trochammina, this feature is applied as a proxy for oxygen depletion by Nagy et al. (2010) in several Upper Triassic to Middle Jurassic sections including an interval of the Ragnarok Formation of the Mjølnir structure (central Barents Sea). In the foraminiferal assemblages 2 and 3, these genera show no size reduction although a small degree of hypoxia is expected owing to the somewhat increased TOC content of the mudstone samples (Fig. 4). It seems probably the oxygen depletion was too small to cause a test size reduction. The presence of bioturbation in the mudstones also suggests that oxygen depletion was not a significant restricting factor in this environment.

The foraminiferal assemblages of the upper Kapp Toscana Group are of low diversity, and the study of these faunas is still in a pioneering stage. In spite of these factors, the faunal content of the foraminiferal assemblages 1–3 reveals a significant stratigraphic potential as shown by the age distribution pattern of the various species in other regions. Ammodiscus pseudoinfimus forma granulata is recorded from the Pliensbachian of Western Siberia (Azbel & Grigalis 1991) and Ammodiscus glumaceus ranges from Toarcian to Aalenian in western Siberia and occurs in the Toarcian of Middle Siberia (Bulynnikova et al. 1990). Bulbobaculites vermiculus is originally described from Pliensbachian to Toarcian strata from Anholt, Denmark (Nagy & Seidenkrantz 2003). Lagenammina inanis occurs in Pliensbachian to Toarcian beds in Western and Middle Siberia (Bulynnikova et al. 1990) and Reophax metensis ranges from upper Sinemurian to lower Bathonian with an acme in the Pliensbachian and Toarcian (Nagy & Johansen 1989). Trochammina squamataformis is reported from the Pliensbachian and Toarcian of the Caucasus and Ukraine (Azbel & Grigalis 1991). Verneuilinoides subvitreus was first described from the upper Toarcian of the northern North Sea Basin but the closely related Verneuilinoides mauritii (Terquem) occurs in the Pliensbachian (Nagy & Johansen 1991).

The stratigraphical ranges mentioned above reveal the significant Pliensbachian–Toarcian affinities of the foraminiferal assemblages in this study. Therefore, they are referred to these stages. Of particular interest in this connection are the taxa A. pseudoinfimus forma granulata, B. vermiculus, L. inanis and T. squamataformis. For the beds above and below the mudstone levels, we follow the ages proposed by Olaussen et al. (1984).

Striking features of the analysed Stø Formation mudstone intervals are their transgressive nature and increased TOC content, which are coupled with a Pliensbachian–Toarcian age (Fig. 4). These relationships warrant a closer consideration of their possible connection to the global Pliensbachian–Toarcian turnover comprising several biotic extinction events.

The main stages of the Pliensbachian–Toarcian turnover were a regression followed by a transgression leading to oceanic hypoxia (Zakharov et al. 2006; Wignall & Bond 2008). The turnover with its main phase in the Early Toarcian falciferum Zone affected primarily the marine realm expressed by a development and widespread extinction of invertebrates such as ammonites and bivalves. In northern Siberia, the turnover comprises two major intervals with reduction of species diversities, the first in the antiquum Zone and the second in the falciferum Zone (Zakharov et al. 2006).

The Stø Formation mudstones and corresponding stratigraphic levels in the Hammerfest Basin can be regarded as reference sections for the Pliensbachian–Toarcian turnover in the boreal seas. Their foraminiferal assemblages might potentially define key horizons that could be used for basin-wide layer correlations and dating. However, prior to this additional research needs to be conducted to confirm the connection to the global Pliensbachian–Toarcian biotic turnover.