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The Store Arøya Formation (from Davies et al. 2006)
The type locality for the new Store Arøya Formation is the extensive exposure across the southern part of the island of Store Arøya, located at the mouth of Langesundsfjorden (NL460397). Although not an ideal stratotype (no known exposures of the Store Arøya Formation exhibit both a lower and an upper contact), this section shows a basal contact with the Steinsfjorden Formation, contains all of the lithologies of the Store Arøya Formation and is also one of the best examples of the typical sheet-like geometry of the units which comprise the formation.

The sheet geometry in the Skien district is so pronounced that the formation can be seen to produce a distinct step-shaped topography to the hills that lie to the east of Skien (Fig. 4). This geometry is a result of the Store Arøya Formation having been deposited by a braided fluvial system that had more lateral mobility than that responsible for the Stubdal Formation (Davies et al. 2005). There is little variation in the thickness of the Store Arøya Formation, although it is thinnest (300 m) at its type area. Elsewhere, it maintains a thickness of 550 m in the Skien area (for a lateral distance of 27 km, from Åfallås [NL308760] in the northwest to Herregårdsstranda [NL405525] in the southeast), and also has major outcrops at Drammen (c. 500 m) and along the revised course of the E18 motorway between Sande and Holmestrand (c. 500 m).

For mapping purposes, the base of the Store Arøya Formation is defined by the first terrigenous sedimentary strata that can be seen above the carbonates of the Steinsfjorden Formation in the areas south of Sylling. In the first of these, the type area on Store Arøya (NL460397), alternating beds of Store Arøya Formation sandstones and siltstones are interbedded with carbonates resembling the Steinsfjorden Formation. In the second locality, on the road which runs north-south through Konnerud (south of Drammen, grid reference: NM655197), the first sandstone beds of the Store Arøya Formation lie conformably on top of the Steinsfjorden Formation. The base is also exposed in Porsgrunn, where fluvial sandstones and distal siltstones of the Store Arøya Formation can be seen to drape carbonate mounds or tidal flats of the Steinsfjorden Formation. These exposures indicate that the southernmost Ringerike Group interfingers with the underlying carbonates.

The top of the Store Arøya Formation is either succeeded by the Holmestrand Formation (as in the exposures of Ringerike Group between Holmestrand and Horten), or, more usually, is the last unit of the Ringerike Group to have escaped erosion by the Carboniferous Asker Group or Permian igneous rocks (as is the case at Store Arøya, Skien, Porsgrunn, and all the outcrops north of Sande).
In effect, the stratigraphic position of the formation means that it is a southern lateral equivalent to the Stub-dal Formation. The northernmost outcrop of the Store Arøya Formation is at Drammen, while the southernmost outcrop of the Stubdal Formation is approximately 15 km further north than this at Sylling. No physical contact is ever seen between the two formations.

In all of the exposures of the Store Arøya Formation the rocks are more accurately described as metasediments due to their proximity to Permian igneous plutons, but where grain-size is visible it can be seen to be dominantly constituted of fine to medium (mostly in the medium grain-size range) mature quartz arenite. The sandstones are usually white or pale grey, though some beds near the boundary with the Permian igneous complex in the Sande-Holmestrand area contain abundant epidote and are particularly green in colour. The formation contains more mud- or silt-grade sediment than the Stubdal Formation and this is usually black in colour. Often the siltstone units are no more than a few centimetres thick but they may be up to a metre in thickness and are laterally pervasive sheets. Another distinction from the Stubdal Formation is that the Store Arøya Formation sediments appear (from palaeocurrent analysis) to have a WNW source area (Davies et al. 2005).