The Kapp Toscana Group - Realgrunnen Subgroup

Early Norian to Bathonian

The Realgrunnen Subgroup

was originally defined in the west central Hammerfest Basin with its type area in block 7121/5. It is subdivided into four formations; Fruholmen, Tubåen, Nordmela and Stø. The thickness in the type well (7121/5-1) is 424m, and 488m in well 7120/12-1. Thicknesses of up to 871m have been drilled in the southern part of the Bjørnøyrenna Fault Complex (well 7219/9-1). The subgroup is thinly developed on the Bjarmeland Platform, and the definition of various formations is therefore unclear. The subgroup is mostly eroded on the Troms-Finnmark Platform. The dominant lithology is pale grey sandstone, especially in the middle and upper parts, while shale and thin coal are more common in the lower parts. The lower boundary is defined by the lower Norian basal shales of the Fruholmen Formation. Following the transgression in the early Norian, deltaic systems developed over the southern parts of the Hammerfest Basin up through the Triassic. In the early Jurassic, coastal marine environments developed, grading into a variety of shoreface, barrier and tidal environments from the Toarcian to the Bajocian. Sediments of the Realgrunnen Subgroup have been deposited in general near-shore deltaic environments, characterized by shallow marine and coastal reworking of deltaic and fluviodeltaic deposits.

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The Fruholmen Formation

(Norian to Rhaetian) consists of grey to dark shale passing upwards into interbedded sandstone, shale and coals. Sandstone dominates in the middle part of the formation, while the upper part is dominated by shales. This lithological development has resulted in a threefold subdivision of the formation with the shale-dominated Akkar Member at the base, overlain by the more sandy Reke Member which in turn is overlain by the more shale-rich Krabbe Member. Depositionally this has been interpreted in terms of open marine shales (Akkar Mb) passing into coastal and fluvial-dominated sandstones of the Reke Formation. These represent northward fluviodeltaic progradation with a depocentre to the south. As the main deltaic input shifted laterally, most of the central and southern parts of the basin became the site of flood-plain deposition, with more marine environments to the north (Krabbe Member). In the type well (7121/5-1) the thickness of the formation is 221m and 262m in the reference well (7120/9-2). The thickest sequence drilled so far (572m, well 7219/9-1) is within the Bjørnøyrenna Fault Complex.

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The Tubåen Formation

(Late Rhaetian to early Hettangian, locally Sinemurian) is dominated by sandstones with subordinate shale and coals. Coals are most abundant near the southeastern basinal margins and fade out towards the northwest. Generally the formation can be divided into three parts with a lower and upper sand-rich unit separated by a more shaly interval. The shale content increases towards the northwest, where the Tubåen Formation may interfinger with a lateral shale equivalent. In the type well (7121/5-1) the thickness of the Tubåen Fm is 65m, and in the reference well (7120/12-1) it is 85m with a maximum thickness of 261m (well 7120/6-1) in the Snøhvit Field. The sandstones of the Tubåen Formation are thought to represent stacked series of fluviodeltaic deposits (tidal inlet and/or estuarine). Marine shales reflect more distal environments to the northwest, while coals in the southeast were deposited in protected backbarrier lagoonal environments.

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The Nordmela Formation

(Sinemurian-Late Pliensbachian) consists of interbedded siltstones, sandstones, shale and mudstones with minor coals. Sandstones become more common towards the top. In the Hammerfest Basin the formation seems to form a west-southwest thickening wedge, similar to the underlying Tubåen Fm. It may be diachronous, becoming younger eastwards. The formation represents deposits in a tidal flat to flood-plain environment. Individual sandstones represent estuarine and tidal channels. In the type well (7121/5-1) the thickness is 62m, and in the reference well (7119/12-2) it is 202m. This thickness variation between the type well and reference well clearly illustrates a southwest oriented thickening wedge. Westward thickening is characteristic for all the three Lower and Middle Jurassic formations and may be the result of early Kimmerian subsidence and tilting towards the Tromsø and Bjørnøya Basins.

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The Stø Formation

(Late Pliensbachian to Bajocian) is defined with the incoming of sandy sequences above the shale-dominated sediments of the Nordmela Formation. The dominant lithology of the Stø Formation is mineralogically mature and well sorted sandstone. Thin units of shale and siltstone represent regional markers. Especially in the upper part of the Stø Fm, phosphatic lag conglomerates can be found. In the type well (7121/5-1) the thickness is 77m, and in the reference well (7119/12-2) it is 145m. In general the Stø Fm thickens westwards in consistence with the underlying Nordmela Formation. The unit may be subdivided into three depositional episodes with bases defined by transgressions. The basal unit is only present in the western parts of the Hammerfest Basin. The middle part (Upper Toarcian–Aalenian) represents the maximum transgression in the area. The uppermost (Bajocian) unit is highly variable owing to syndepositional uplift and winnowing as well as later differential erosion. The sands in the Stø Formation were deposited in prograding coastal regimes, and a variety of linear clastic coast lithofacies are represented. Marked shale and siltstone intervals represent regional transgressive pulses in the late Toarcian and late Aalenian.