THE MJØLNIR EVENT - TRACKING A MARINE IMPACT

The late Jurassic Mjølnir impact happened, when a 1,8 km (diameter) bolide hit the shallow (~400 m deep) paleo-Arctic Sea. The Mjølnir Crater, which is 40 km wide, is today located in the Barents Sea under 350 m of water and more than 50 meter of sediments.

In the early phase of petroleum exploration in the Barents Sea a shallow drilling program (IKU) cored several sub-cropping reflectors. The late Jurassic/early Cretaceous beds were of particular interest, both due to their stratigraphical position and source rock characteristics. These cores were drilled in the late 80-s and early 90-s, before the Mjølnir Crater was discovered. In 1998 we were able to drill a separate, shallow core of the central high of the Mjølnir Crater. Presently this core and several of the original shallow cores are under investigation. Before these core studies were initiated, large amounts of geophysical data (seismic, gravimetric and magnetic)have been analyzed and the results presented in several papers.

The Mjølnir Crater is very well preserved and the close connection between possible ejecta and the crater, makes it a very good example for marine cratering.

The Mjølnir impact generated high waves, tsunamis, slumps and slides, and triggered suspension and mass flows, in addition to the proven wide distribution of ejecta. The impact also ignited the extremely organic rich (>10 % TOC) shales in the target area. Recently resurge gullies have been identified, cutting the crater rim. Numerical models indicate that the water did not return to the impact area before at least 20 minutes after impact.

So far no clear-cut melt or glass material have been found, but enrichments of smectite may represents altered impact glass.

The Mjølnir impact event had great influence on life in the paleo-Arctic seas, but no long lasting biological effects have been found so far. Algal blooms may be an indirect response of the impact. These and similar changes, may be reflected in the geochemical anomalies characterizing the post-impact sediments.

In the search for Mjølnir ejecta, possible other traces of the impact, and the recognition of their biological and geochemical characteristics, analyses of Barents Sea cores, Svalbard and North Greenland sections are presently done.