CLIMATE INFLUENCE ON THE SEDIMENTARY ARCHITECTURE OF THE ORDOVICIAN ARNESTAD FORMATION, NORWAY: HIGHLIGHTING THE CARBONATE-SHALE TRANSITION ON THE DEEP SHELF

The upper Ordovician Arnestad Formation is an excellent example of a climatically influenced highly cyclic depositional environment on the deep shelf. The formation is about 45m thick and consists of siliciclastic mudstones (shales) with interbedded carbonate beds and isolated carbonate nodules. Notable outcrops are found along the coast of the Oslo-Asker region in Norway that allow for measuring several detailed sections. Thin sections were also used to describe the facies in finer detail. The shales consist of dark grey siliciclastic mudstones with fossil fragments. Chondrites burrows and Phycosiphon-like fecal strings can be found throughout the mudstones. The carbonates range from mudstones to packstones, and form either cm-scale nodules or continuous carbonate beds throughout the Arnestad Formation. These beds contain abundant fossil fragments in random orientations, and show mm-scale Chondrites burrows. In both lithologies, fossil fragments are accumulated in cm-scale lenses. Stratigraphically, the unit can be subdivided into a lower portion with continuous carbonate nodule beds, a central part with rare discontinuous carbonate nodules, and an upper portion with abundant carbonate beds.

The Arnestad Formation is interpreted as representing deep shelf sedimentation of a siliciclastic mudstone in close proximity to a carbonate depositional system. Extensive burrowing by multiple organisms throughout the formation indicates dysoxic to oxic seafloor conditions, allowing for diverse benthic life during deposition. Both lithologies show fair-weather and storm generated deposits. Massive siliciclastic and carbonate mudstones represent fair weather conditions whereas shell accumulations reflect storm deposition. The intercalation of the shales with carbonate beds most likely shows the influence of climate cycles on the deposition of the Arnestad Formation. During sea-level lowstands, carbonate was preferentially introduced into the depositional area, whereas highstands show deposition of shales. This study therefore shows that (1) shale environments are most likely cyclic in nature, and this is reflected best at the interface between shale and carbonate environments and (2) proximal shale environments were not anoxic during deposition but represented hospitable living conditions.