Ordovician - Silurian boundary beds from offshore south Norway

The shallow stratigraphic core 13/2-U-2 drilled northeast of the Farsund Subbasin penetrated 76.5 m of clastics and limestones of Late Ordovician and Early Silurian age. The well represents the first record of Lower Paleozoic rocks offshore south Norway and is the westernmost known extension of the Lower Palaeozoic Scandian foreland deposition area Lithologically the core can be subdivided into four units. The lower Unit A (Late Ashgill, Rawtheyan), comprises nodular limestones and shales. The overlying Upper Ordovician Units B (Rawtheyan) and C (Hirnantian) are dominantly siltstones but also contain shale and limestones. The boundary between Unit B and Unit C seems to correlate with a locally defined reflector P marked on the digital seismic. Chronostratigraphically the Ordovician - Silurian boundary coincides with the boundary between Unit C and the overlying Unit D, although basal Llandovery strata have not been identified. Unit D, of early Silurian (Llandovery, Rhuddanian) age, comprises shales with some siltstone and limestone beds. All the units were deposited in shelf environments. The core displays a regression for the lower three late Ashgill units (A, B and C). This is transgressively overlain by Llandovery sediments deposited in slightly deeper water (Unit D). This development is comparable to what is observed in the contemporaneous successions of the western and northern areas of the Oslo Region.