Miocene continental margin growth dominated by deposits from ocean currents – an example from offshore Norway

The earliest Cenozoic evolution of the Mid-Norwegian and Lofoten-Vesterålen continental margin (~65-70^o N) involved rifting, opening and finally seafloor spreading, initiating the Norwegian-Greenland Sea. These events resulted in large morphological and structural variations along the margin, creating accommodation space in a deep- to shallow-marine setting that allowed for accumulation of the Miocene sediments of the Kai- and Molo formations. The Cenozoic seismic stratigraphic correlation between the wide Mid-Norwegian and the narrow Lofoten-Vesterålen margin is poorly established. We therefore here analyze a large database of seismic data and exploration wellbores to give new insights on the sedimentary processes and paleo-environments during the Miocene evolution of this complex continental margin segment.

Steeply dipping clinoforms of the Molo Formation testify to a Miocene coastal outbuilding on the eastern part of the northern Mid-Norwegian margin. West of this, elongated sediment accumulations oriented in an along-slope SSW-NNE direction characterize the palaeo-slope. These are up to ~200 km long, between 40 and 110 km wide and up to ~520 m thick. An internal divergent reflection configuration characterize the elongated accumulations and they typically display a progressive upslope onlapping relationship onto an overall gently westward-dipping underlying morphology that includes domes, highs and ridges. Small incisions are frequently observed in association with the upslope onlap. These characteristics are altogether typical of contourites deposited from ocean currents. In the Vøring Basin, the internal seismic configuration can be described as consisting of low to moderate amplitude parallel-layered reflections, which are interpreted to represent a deep-water hemipelagic setting.

On the much narrower Lofoten-Vesterålen margin, parts of the Kai Formation show a seismic reflection configuration similar to what is observed on the northern Mid-Norwegian margin (e.g. elongated character, divergent internal reflections). These sediments are therefore also interpreted to be contouritic- and hemipelagic deposits. In contrast to the northern Mid-Norwegian contourites, the Lofoten-Vesterålen contourites are generally thinner, and they onlap onto an underlying steeply dipping continental slope, a slope which is also characterized by submarine canyons. Downslope of these, depocenters oriented perpendicular to the margin (i.e. slope-parallel), suggest influence of downslope processes through the canyons.

Our preliminary results show the presence of several contourite build-ups on the investigated margin, indicating the occurrence of a well-established ocean circulation with a persistent current direction along the Norwegian margin during deposition of the Kai Formation. The main source area for these sediments were likely south of the Mid-Norwegian margin. Coastal outbuilding in the Molo Formation and canyon-fed sediment input also testify to a sediment input from the east in the Miocene, and some of these were likely also re-distributed by ocean currents.