Mjølnir Impact-related Deposition, Barents Sea

Rikke Bruhn

The Mjølnir crater is a 40-km wide feature thought to have formed by the impact of a large meteor at Volgian-Ryazanian boundary time. Modelling suggests that the tsunami caused by the impact into the shallow intra-cratonic sea was more than 200m high initially and affected most of the Barents Sea. A major ecological crisis followed in the wake of the impact, and the resulting distinct species assemblage can be used as a marker for correlating strata relating to the time of the impact. The impact earthquake probably caused widespread structuration and earthquake and tsunami action destabilised nearshore clastic prisms and caused reworking of deposits far from the impact site. This talk will present a recent study of two successions interpreted to be impact-related and explain their characteristics in terms of their palaeogeographical setting. The first succession in the S. Hammerfest Basin is a seismically distinct unit of overlapping wedges in the basal Knurr Fm. along the Finnmark Fault Zone. In the 7121/9-1 well the unit consists of mudstone deposited by slumping and debris flow processes, with abundant Jurassic and Triassic palynomorphs. The unit is capped by a thin black mudstone containing the distinct species assemblage documented elsewhere as immediately post-impact. The unit is interpreted to result from sudden and significant offset along the Finnmark Fault complex in response to earthquake activity causing Jurassic and Triassic strata exposed in the fault scarps to collapse and slide out and form a fault apron. The second succession is a lobate feature at the SE Loppa High. In the 7122/2-1 well the unit consists of sandstone, most of which is cored. The lower part consists of high-density turbidites with abundant clasts, including pedogenically altered mudstones. Biostratigraphic dating of this part yields a poorly constrained Volgian or Ryazanian age. The unit is interpreted as resulting from collapse of a shelf located on the Loppa High in response to the impact-generated earthquake, and transportation of the sand-rich succession into deep water by high-densty turbidity currents. Presence of pedogenic clasts is interpreted in terms of tsunami inundation of the coastal plain and offshore transportation by the receding flow. The differences in the succesions are attributable to their structural context and proximity to a clastic shoreline, controlling their lithology and architecture.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013