500 m of rapid base level rise along an inner passive margin – Seismic observations from the Pliocene Molo Formation, mid Norway

Abstract

The positions of late Cenozoic coasts, related base level changes and tectonic events are still an enigma along large parts of the North Atlantic passive margin due to severe glacial erosion and inconclusive biostratigraphic ages. Periglacial paleo-coastlines are best preserved along the mid Norwegian margin from where we show the first 3D seismic images of a transgressive coastal delta (lower Molo Formation) that constrains the late Cenozoic geological history.Well data, 2D seismic sections and a 990 km2 3D survey, located above the Froan Basin and the Draugen oil field, are used to investigate the 60 km-wide and 600 km long coast-parallel Molo Formation. We interpret the Molo Formation to be younger (mainly Pliocene) than the Kai Formation (Mid and late Miocene) and propose a further subdivision of the Molo Formation into a lower, largely transgressive and an upper highstand regressive unit.A relative base level fall of several hundred meters is recorded at the onset of the Miocene compression phase (mid Miocene). Simultaneously, the Mid Miocene Unconformity (MMU) developed and the syntectonic and lowstand Kai Formation was deposited to the west of the study area. Upon termination of the Miocene compression phase a cumulative base level rise in the order of 500 m is interpreted. The lower Molo Formation is interpreted as a coastal delta sequence and characterized by a sand-rich parasequence set that exhibits a largely progradational motif. The deposits record a stepped landward migration of the paleo coastline during the initial transgression. Locally, the relative position of topset-foreset rollovers and trajectory analysis document 90 m of the base level rise. The upper Molo Formation is interpreted as a fluvial-dominated, wave-influenced progradational shelf-edge delta. Fluctuations in base level are interpreted to be largely tectonically driven.