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Sammendrag: |
The Steinvika Limestone and related sediments of the Langesund-Skien district were deposited near the western limits of the Baltoscandian epicontinental platform. Fourteen lithofacies are recognised and described. Interpretations of them show deposition was in environments ranging from intertidal thorugh restricted and well-circulated epicontinental shelf to deeper offshore epicontinental shelf. The lower parts of Steinvika Limestone represent a regressive sequence and the development of an extensive inshore shelf area. The onshore to offshore distribution of lithofacies at this time was: intertidal algal laminites and desiccated quartz-silts; low energy restricted inshore shelf muds; high energy skeletal shoal sands and moderate energy subtidal bar sands with small path reefs; and offshore transition zone muddy skeletal sands. The upper parts record an overlapping transgressive sequence on shallow subtidal mixed sand-mud flats abd shoreface sediments. In the lates stage of transgression offshore carbonate and argillaceous muds were deposited uniformly across the district. Development of euxinic conditions resulted in the deposition of dark pyritic shales which make up the Venstøp Shale, before a return to fully marine offshore sedimentation in the Upper Ordovician. The sequential development of these sediments is not matched elsewhere in the Oslo Region. The lower beds are akin to contemporaneous deposits in the Mjøsa district while the upper beds are almost identical to those of the same age in the Ringerike and Hadeland districts. On this basis, a change in the pattern of major sedimentary belts, which can be recognized in the Oslo Region during the Ordovician, is identified and suggested to have resulted from differential epeirogenic movements related to activity in the nearby Caledonian tectonic zone. Glacioeustatic changes in sea level, which influenced se |
