DEPOSITION ALONG A SHALE-CARBONATE FACIES TRANSITION ON THE SCANDINAVIAN ORDOVICIAN SHELF – THE ARNESTAD FORMATION, SANDBIAN, SOUTHERN NORWAY

During the lowermost Upper Ordovician, Norway formed part of a westward-deepening shelf on which the Arnestad Formation was deposited. This 55 m thick unit consists of intercalated shales and thin carbonate beds. The dominating shales are composed of dark grey siliciclastic mudstones and contain lenses of fossil fragments. Enterbedded carbonates are mud- to wackestones, and also contain lenses composed of fossil fragments. All facies show modification by Chondrites burrows and Phycosiphon-like fecal strings. Stratigraphically, the unit is subdivided into a lower portion with shales and continuous carbonate beds, a central part rich in shales with local carbonate and ash beds, and an upper part containing both thick shale beds as well as stacks of intercalated shales and carbonates.

The Arnestad Formation is interpreted to represent sedimentation on a shallow shelf showing proximal carbonate and distal siliciclastic mudstone facies. Fair-weather sediments are represented by mud-rich carbonates and fine-grained siliciclastic mudstones. Storms affected both the carbonate and the siliciclastic mudstones leading to erosion and deposition of shell lenses in both settings. The Arnestad succession records a relatively low sea-level stand during deposition of the lower part of the succession, an overall high sea-level position during its central portion, and another low sea-level position during deposition of its top portion. The abundance of ash beds in the central part of the unit is therefore attributed to better preservation during high versus low sea-level positions. However, storm deposition seems to have increased throughout development of the succession independent of sea-level.

Carbonate-shale couplets are interpreted to reflect small-scale, possibly Milankovich cyclicity that characterizes the Arnestad Formation. Nevertheless, these climate-induced cycles are exclusively visible in parts of the succession showing lithology changes between carbonates and shales. In the thick shale-rich beds bioturbation has likely unified the original diverse siliciclastic mudstone facies so much that none of the original cyclicity, if it was present, is detectable anymore.