Definition
The formation is best demonstrated on the west coast of Braksøya (NM727610) in Ringerike, where a complete type section can be designated (Fig. 15); the formation is 2 7 m thick there and consists of a complex carbonate development with minor marls and shales. The Braksøya Formation corresponds to 8c and 8d of Kiær (1908) in Ringerike, Modum and Skien. We provisionally subdivide the formation into two informal members, pending the results of works in progress by S.O. and N.M. Hanken.
The base of the Braksøya Formation in its type section is clearly defined by limestones which overlie the eroded top surface of the Bruflat Formation. Small sandstone and shale clasts from the underlying formation are seen in these basal limestones. A lower biohermal member is 17 m thick and is characterised by thinly bedded marls with massive biomicrite and biolithite structures (up to 7.m high and 15 m wide) which we interpret as small patch reefs (Fig. 16, c.f. Kiær 1908, Fig. 19c). These contain algae, bryozoans, tabulate corals, stromatoporoids and other problematical organisms both as framework and binding element in the reefs' structure (Hanken, Olaussen & Worsley 1979). The patch reefs pass both laterally and vertically into marls of variable thickness which contain a varied brachiopod fauna, and occasional horizons show large, current oriented, rugose corals (Whitaker 1965). Scattered pseudomorphs after celestite or gypsum are also seen in the marly sequence. The upper part of this member consists of a 3.5 m thick, black, bituminous shale with in situ tabulate corals and stromatoporoids; individual colonies over 1 m in size are present.
The upper, well‑bedded member is 10 m thick in the formation's type section and consists of thickly bedded limestones with marly partings and occasional desiccation cracks. Thinly (algal?) laminated limestones (with fenestral fabrics and pseudomorphs after evaporite nodules), coral biostromes and oncolitic biosparites also occur. The top of the Braksøya Formation is taken below the first appearance of thinly interbedded limestones and shales here assigned to the overlying Steinsfjorden Formation.
Other occurrences
The complex and varied development of the Braksøya Formation is well demonstrated in several exposures elsewhere in the Ringerike district (Geitøya NM6956, Storøya NM7057 and Sønsterud NM7251). Wherever it is exposed, the contact with the underlying Bruflat Formation is erosive. As might be expected, the patch reefs and marls in the lower member of the formation show variable local developments, and in some exposures this member consists of only one of these lithofacies.
Exposures in Modum and Skien are provisionally included in the Braksøya Formation. A lower biohermal member is well developed in both districts (especially at Kapitelberget, NL367628 in Skien). However, the upper part of the formation in both districts contains interbedded limestones and shales similar to those assigned to the Malmøya Formation in the Holmestrand and central districts and unlike those of the upper member at Ringerike.
Depositional environments
All exposures assigned to the Braksøya Formation indicate the establishment of marginal marine carbonate depositional environments following a short (and biostratigraphically indefinable) period of erosion (and ?emergence) near the Llandovery/Wenlock boundary. The fauna and flora of the patch reefs indicate shallow, clear water, marine environments; however, both the restricted character of the flora and fauna and the occurrence of evaporites in marl sequences at Ringerike suggest periodic hypersaline conditions. Desiccation cracks and evaporitic pseudomorphs seen in the upper parts of the formation in the type area also indicate intra/supratidal depositional environments. Such features have not been observed in the Modum and Skien districts and this suggests deposition in more distal marine environments. Faunas are comparable to those assigned to Benthic Assemblages 2 and 3 by Boucot (1975).
Age
Brachiopods, corals and ostracodes seen in the type section of the Braksøya Formation contain no diagnostic Llandovery elements, and the base of the overlying Steinsfjorden Formation contains brachiopods (including Eocoelia angelini) with a Sheinwoodian (riccartonensis Biozone age. Faunal elements therefore suggest a low Sheinwoodian (pre‑riccartonensis Biozone) age similar to that of the Upper Visby Beds of Gotland (Bassett & Cocks 1974). The occurrence of Eocoelia angelini in the upper part of the Braksøya Formation in the Skien district suggests a somewhat later transition there to beds assigned to the Steinsfjorden Formation. Thus, the Braksøya Formation in its type area is considered to be a time equivalent of the Skinnerbukta Formation in the Holmestrand and central districts. The development in Skien overlies beds assigned to the Skinnerbukta Formation; the top of the Braksøya Formation in this district is approximately correlative with the junction between the Skinnerbukta and Malmøya Formations of Holmestrand and the central districts (Fig. 5).