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Nils-Martin Hanken, University of Tromsø (Norway)
Alfred Uchman, Jagiellonian University (Poland)
Jesper Kresten Nielsen, Sintef Petroleum Research (Norway)
Snorre Olaussen, Eni Norge AS (Norway)
Tor Eggebø, Halliburton AS (Norway)
Reidar Steinsland, StatoilHydro Bergen (Norway)
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The uppermost section of the Rawtheyan and Hirnantian (Upper Ordovician) deposits in the Ringerike area (about 35 km northwest of Oslo) contains abundant trace fossils. They include soft ground Archaeonassa fossulata, ?Arenicolites isp., Chondrites cf. caespitosus, Chondrites ispp., Conichnus ?conicus, Curvolithus simplex, Gyrophyllites isp., Helminthoidichnites isp., Helminthopsis isp., cf. Monocraterion isp., Multina isp., Nereites isp., Palaeophycus cf. tubularis, Palaeophycus isp., Phycodes cf. palmatus, Phycodes isp. (probably a new ichnospecies), Planolites montanus, Polykladichnus isp., Plagiomus arcuatus, Ptychoplasma isp., Skolithos isp., Squamodictyon petaloideum, Thalassinoides isp. and Trichichnus linearis. Trypanites weisei and Vermiforichnus clarkei borings are present in stromatoporoids and brachiopod shells. The trace fossils belong to the proximal, archetypal and distal, Cruziana ichnofacies, thus indicating bathymetrical trends.
The Rawtheyan and Hirnantian section is divided into a lower siliciclastic unit and an upper unit dominated by carbonates. The siliciclastic unit shows a general coarsening upward trend which grades upward from mudstone with strata bound limestone nodules to fine- to medium-grained sandstones. Sedimentological textures and structures combined with trace fossil associations are consistent with a shallowing-upward trend from a transitional/offshore zone to upper shoreface depositional environment. This lower siliciclastic unit is interpreted as a prograding highstand third order system tract followed by a low stand or transgressive system tract. A ravinement surface, probably a third-order sequence boundary defines the base of the upper carbonate dominated unit, which comprises a complex pattern involving patch reefs interfingering with shallow-water sandstones, crinoidal limestones and carbonate mud banks to the north and crinoidal limestones to the south. The entire sequence is truncated by a karstified discontinuity surface spanning the Ordovician-Silurian boundary (second-order sequence boundary) and is overlain by lower Silurian clastic sediments.
Thalassinoides occurs in the lower offshore and disappears in the lower part of the upper offshore. Its lower limit is unknown, as is the case with Chondrites and Trichichnus. Chondrites ranges up to the upper offshore and Trichichnus to the offshore-shoreface transition. Multina ranges from the upper offshore to the offshore-shoreface transition. Phycodes occupies a similar position, but occurs more towards the shoreface. Conichnus, which occurs in the lower shoreface, probably has its lower limit in this zone.
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