MIOCENE TO QUATERNARY PALEOENVIRONMENTS AND UPLIFT HISTORY ON THE MIDNORWEGIAN SHELF

Based on benthic and planktic foraminifera, Bolboforma, oxygen isotope measurements and seismic data, major changes in Miocene, Pliocene and Pleistocene paleoenvironments on the mid Norwegian shelf are discusse d and a possible scenario of the late Cenozoic uplift history is given . The dating of the Neogene sequence has been done using foraminifera and Bolboforma. Four main assemblage zones have been identified with n ine distinct subzones. Most of the Miocene sequence is preserved. The lower Miocene sediments contain only siliceous microfossils. A period of high fertility and upwelling in the study area prevailed. The early Miocene-early mid Miocene (15 Ma?) change from a siliceous to a calca reous rich microfauna, dominated by Nonion barleeanum, can be related to increased surface-water circulation due to overflow across the Icel and-Faeroe ridge. During the Miocene the temperature decreased in the study area. Evidence of increased amounts of coarser sediments may sug gest that an uplift of the mainland areas occurred during the mid-late Miocene. Lower Pliocene sediments contain a foraminiferal fauna that seems to occur in slightly colder conditions than the late Miocene fau na suggesting a further cooling. Possibly, Arctic waters entered the s tudy area in the early Pliocene. A very marked change in lithology (fr om compacted claystone to unconsolidated diamicton), fauna (from deep dwelling to shallow dwelling species) and seismic signature (from flat lying reflectors to prograding clinoforms) occurs during the mid?-lat e Pliocene. A two step cooling trend is indicated by the microfauna of these prograding wedges. (1) The first wedge buildups might have been associated with an uplift of the mainland during the early late Plioc ene (mid Pliocene, ca. 4 Ma). However, the age determination is somewh at uncertain and may very well be of late Pliocene age. (2) The second step of wedge buildup is associated with a glacial phase where the do minating microfauna exists of arctic species. Large continental ice sh eets might have occurred at this time reaching coastal areas and that possibly many of the geomorphological features such as the strandflat were made during this episode. The Pleistocene epoch is represented by an increased percentage of boreal foraminifera intermingled with high arctic species which indicates that interglacial-glacial cycles preva iled and the dynamics of the glacier system changed.