Definition
The formation's name was proposed by Skjeseth (1963); its name is derived from Eksberget (PN087355) on the island of Helgøya in the Hamar district. The Ek Formation is approximately 95 m thick in this type area; exposures assigned herein to the Ek Formation are dominated by greenish grey to dark grey (often graptoliferous) shales. We consider the Ek Formation to comprise all units assigned by Kiær (1908) to stage 7c in Hadeland, Toten, Hamar and Ringsaker and we retain Skjeseth's concept of the formation. Exposures in the Hadeland district show a transitional development to the Vik Formation but are more like the development of the Ek Formation in its type area; we therefore incorporate the informal 'Askilsrud shale' of Owen (1978) from Hadeland within our Ek Formation.
Poor exposure and complex tectonics make a satisfactory definition of the formation difficult. The type section proposed by Skjeseth (1963) along the 'new road' Kiær 1908: Fig. 100) at Eksberget is now largely overgrown, but no better exposures are known at present. The base of 7c at Eksberget was described by Kiær at the transition from red nodular limestones and shales (here assigned to the Rytteråker Formation) to greenish grey marly shales with darker grey graptoliferous horizons. Kiær noted approximately 95 m of such shales, the lower 65 m with numerous graptolite horizons and the upper 30 m with a sparse shelly fauna. The nature of the top of the Ek Formation is poorly understood on Helgøya. The boundary with the overlying Bruflat Formation is best seen in this latter unit's basal stratotype in the Toten district.
Other occurrences
Several small exposures are seen in the Ringsaker district around Torsæter Bridge in Brumunddalen (PN077584 to PN077589) approximately 20 km north of Eksberget. Exposures there are highly tectonised and dark grey graptoliferous shales with bentonites appear to have been secondarily reddened, probably during Permian times. Small exposures of dark grey graptoliferous shales are also seen in railway cuttings in the Toten district around Bruflat (NM875238 and NM877257).
The base of the unit was temporarily exposed in 1977 in a trench at Storøya (NN907297), 2.5 km east of Reinsvoll in Toten. The top of the Rytteråker Formation was marked there by a 55 cm‑thick red nodular limestone; this was overlain by a 20 cm thick bentonite, followed by greenish grey shales and bentonites with some graptolite horizons. Exposures in Hadeland (railway cuttings at Netberg, NM842851 and Gunstad, NM787844) indicate a similar thickness to that seen in the type area and a basal hypostratotype is defined at Netberg (Fig. 13). The base of the formation there is marked by red shales overlying grey nodular limestones of the Rytteråker Formation. The lower part of the Ek Formation consists of alternating greenish grey and reddish shales with occasional bentonitic horizons, sparse shelly fossils and occasional thin black graptoliferous horizons. The remainder of the formation consists of greenish grey and red shales with finely nodular limestone horizons.
Depositional environments
The shales of the northern districts were deposited in deep water environments distal to the prograding sandy deposits of the overlying Bruflat Formation. This foredeep, passed laterally southwards into shallower shelf environments with the mixed carbonate and mud facies of the Vik Formation. The Hadeland exposures reflect this transition, but are on balance more akin to the Ek Formation.
Age
Re‑examination of Kiærs collections from his new road on Helgøya and of new collections from Toten suggests that the base of the Ek Formation in its type locality should be correlated with the upper part of the crispus Biozone. Kiær (1908:416) recorded a 'Zone with Monograptus turriculatus and Petalograptus palmeus from a horizon 2.7 m above the base of the Ek Formation. We consider this zone to contain a griestonensis Biozone fauna. Graptolites indicate a probable maximus Biozone age for the base of the Ek Formation in its hypostratotype in Hadeland. A turriculatus Biozone fauna is seen 5 m above the base of the formation. Two poorer faunas 3 m and 3.5 m above the base suggest correlation with the maximus Biozone A low crenulata Biozone age is suggested by graptolites from the top of the unit in Hadeland and Toten; sparse shelly faunas support this conclusion, indicating an equivalence to the C5/C6 transition.