The stratigraphy and sedimentology of the Ringerike Group of Norway.

The Ringerike Group is a late Silurian red bed succession confined to the Oslo Graben of Norway. At Ringerike (type area) it contains the Sundvollen Formation (500 m) below and the Stubdal Formation (750 m) above. The Kolsas area shows a southerly decrease in thickness; in the Holmestrand area the base is not exposed and the Group consists of the Holmestrand Formation which is, in part, laterally equivalent to the Stubdal Formation. Evidence from vertebrate faunas indicates that the age ranges from lower Ludlovian (Sundvollen Formation) to Downtonian (top of the Holmestrand Formation). Facies analysis indicates that the Sundvollen Formation consists of fine grained sandstones, siltstones and occasional thin calcarenites whereas the Stubdal Formation consists of fine grained sandstones with intra-formational conglomerates. Sedimentary structures and analysis of cyclothems indicate that sedimentation was mostly in fluviatile environments: meandering in the Sundvollen Formation and braided in the Stubdal Formation. The Sandstones of the Sundvollen and Stubdal Formations are similar. They are feldspathic, lithic and quartzose wackes and arenites; those of the Holmestrand Formation are lithic and quartzose wackes and arenites. The main diagenetic events were the authigenesis of quartz and feldspar and the precipitation of calcite cement. The colouration of the red beds is due to hematite grain coatings and matrix which crystallized post-depositionally. Palaeocurrents, sandstone composition and grain size variation suggest a relatively distant source area to the NW in the Caledonian mountains for the Stubdal Formation and a more proximal source on the NE flank of the Oslo Graben for the Holmestrand Formation. Trend surface analysis shows an increase in % quartz (50-70) and % rock fragments (3-7) to the south and a decrease in % feldspar (13-4). The calcarenites in the Sundvollen Formation are interpreted as shallow marine deposits which formed in and at the mouths of tidal channels. Tidal flat deposits and the fine grain size suggest that the Sundvollen Formation formed a coastal alluvial plain with meandering rivers. The Stubdal Formation was deposited by alluvial braided rivers; so was the Holmestrand Formation but the presence of conglomerates shows that they were more powerful than those of the Stubdal Formation.