Definition
We here adopt, define in detail and extend the use of the formational unit proposed by Strand & Størmer (1955) and Skjeseth (1963) for Toten, Hamar and Ringsaker, with its name derived from Bruflat (NN877256) in Toten. Kiær (1922) introduced the term 'Mariendal Series' for rocks which we assign to this unit in the Ringsaker district; Kiær's term has never been widely used, whereas the Bruflat Formation is now generally accepted. The informal term 'Mariendal Beds' was used by Ramberg & Spjeldnæs (1978) ‑for units which we assign to the Bruflat Formation. We here geographically extend Skjeseth's (1963) formational concept to include units assigned to 8a and 8b by Kiær (1908) in Ringerike and Hadeland.
Because of incomplete exposure and complex tectonics, a single type section cannot be designated. The type area around Bruflat shows a tightly folded and faulted series of anticlines and synclines exposing various parts of the unit. Best exposures are seen in several road and railway cuttings between Eina (NN875229) and Reinsvoll (NN884283). The base of the formation is defined in the southern end of a railway cutting (NN875238), 800 m north of Eina Railway Station. A composite section has not yet been satisfactorily established in the type area, but a thickness of 400‑550 m is suggested (Fig. 14). The formation is characterised by varying proportions of sandstones, siltstones and silty shales. The lower part of the formation in the type area consists of medium to thickly bedded very fine calcareous sandstones and silty shales, the base of the formation being defined by the first occurrence of sandstones. Many of the sandstones have erosive bases, and show planar lamination passing up into rippled tops. The upper 200 m of the formation consist of irregular and intergrading alternations of fine calcareous sandstones, siltstones and bioclastic limestones, often in thick beds with only thin shaly partings. Relatively diverse shelly faunas occur sporadically. A gradational upper contact with red shales assigned to the Reinsvoll Formation is seen in an exposure north of Bruflat (see p. 30).
Other occurrences
Several exposures in Brumunddalen show only a poorly fossiliferous shaly development of the formation (river bank and road cuttings near Torsæter Bridge (PN0757‑PM0758)). At Bjørgeberget (PN0555) reddened shales uppermost in the Bruflat Formation yield a fauna with Cyrtia exporrecta (Kiær 1922). Approximately 12.5 m of the formation are exposed in Hadeland and neither the top of this unit nor any younger units are preserved in this district. However, comparison with Ringerike indicates that the Hadeland exposures represent most of the formation's original development in this district (railway cuttings at Gulla, NM7882).
Numerous partial sections in Ringerike show an attenuated development of the Bruflat Formation. A composite reference section can be studied on the islands of Purkøya and Storøya. The base is seen on the northwest coast of Purkøya (NM688571), the middle parts in good but discontinuous exposures on Storøya. (NM691571 to NM690574) and the top is exposed at Vintergututangen, the northernmost point of Storøya. (NM710586). A total thickness of approximately 115 m is indicated. The base is taken at the first thick siltstones which sharply overlie the greenish grey shales and nodular marls uppermost in the Vik Formation. The lower part of the Bruflat Formation (8a of Kiær, approximately 70 m thick) consists of a rhythmic sequence of shales (with shelly faunas) and more silty intervals; siltstones become more common upwards and 8b of Kiær (approximately 45 m thick) consists of medium‑bedded siltstones and sandstones with bioclastic limestones and minor thin shales. We have not seen the desiccation cracks reported by Størmer (1942) in the uppermost beds, but the top of the unit is marked by an erosive contact with the limestones of the overlying Braksøya Formation.
Depositional environments
The formation shows a coarsening‑upwards sequence in the type area, reflecting a large‑scale coastal progradation into and over the graptoliferous shale environments of the Ek Formation. Distal storm generated sandstones in the lower parts of the Bruflat Formation are overlain by sediments typical of wave‑dominated clastic shelf environments with faunas assignable to Benthic Assemblage 4 of Boucot (1975).
The formation shows a similar coarsening (and shallowing) upwards trend in Ringerike to that seen in the type area. Storm‑generated siltstones in the lower part of the unit (8a) were deposited below normal wave base in quiet muddy environments with faunas typical of Benthic Assemblage 5. The uppermost part of the formation (8b) shows abundant evidence of wave reworking, with faunas which can be assigned to Benthic Assemblage 4. The erosive contact with the overlying Braksøya Formation is noteworthy: clasts of the Bruflat Formation's sandstones found immediately above this contact may suggest emergence prior to renewed marine deposition in the carbonate dominated environments of the Braksøya Formation.
Age
Skjeseth (1963) tentatively correlated his Bruflat Sandstone of the northernmost districts with stages 8 and 9 of more southerly areas and subsequent workers (e.g. Størmer 1967, Bjørlykke 1974a) generally assumed a Wenlock age for the formation although Bjørlykke noted that 'the age of the Bruflat Sandstone is not very well defined'. Exposures both in the Ringerike and northern districts contain graptolite, conodont, brachiopod and coral taxa which are all characteristic of the uppermost Telychian (C6 or crenulata Biozone). No diagnostic Wenlock faunal elements have yet been found, but we cannot yet rule out the possibility that the formation may extend into the basal centrifugus Biozone of the Wenlock.
Bassett & Rickards (1971) first presented revised datings of the formation in the Ringerike district; however, their results were somewhat confusing as the apparent occurrence of cyrtograptids in beds assigned to 8a near Vik suggested a Wenlock age for this locality in contrast to the Llandovery faunas of nearby exposures of 8b. Although several cyrtograptid taxa have been described from the uppermost Llandovery (Jackson & Etherington 1969) it is now clear that the cyrtograptids in question did not originate from Ringerike. The graptolites found in both 8a and 8b at Ringerike (Monograptus vomerina vomerina, M. parapriodon and Retiolites geinitzianus) clearly indicate correlation with the crenulata Biozone. Aldridge (1974) described an amorphognathoides assemblage of conodonts from 8b in Ringerike; this conodont assemblage ranges into the early Wenlock, but the total fauna, including the brachiopods Costistricklandia lirata lirata, Cyphomenoidea wisgoriensis, Protomegastrophia undata and the rugose coral Palaeocyclus porpita suggests a Telychian age.
Both conodont and brachiopod shelly faunas found in the upper parts of the Bruflat Formation in its type area contain the same key elements as listed above, thus suggesting a close correlation with Ringerike (Fig. 4). The reddened 'Cyrtia shales' found by Kiær (1922) on Bjørgeberget in the Ringsaker district lack distinctive faunas. No typical Llandovery faunal elements occur, however, and these beds (probably the youngest parts of the Bruflat Formation preserved in the Oslo Region) may extend into the early Wenlock.