Impact breccia and ejecta from the Mjølnir crater in the Barents Sea - The Ragnarok Formation and Sindre Bed

In this study we present the stratigraphic succession related to the Mjølnir impact in the Barents Sea, based on available cores, detailed sedimentological and palaeontological descriptions, as well as seismic reflection profiles. The Mjolnir impact took place in the palaeoBarents Sea, close to the Volgian - Ryazanian boundary. The epicontinental sea had a water depth of 300 - 500 m, and was characterized by anoxic to hypoxic deposition of organic rich clays, presently with kerogen of types II and I. The bolide, about 1.5 - 2.0 km in size, hit the sea/sea floor and created the 40 km wide Mjølnir crater. The Ragnarok Formation is defined as the locally derived allochthonous (mixture of re-deposited excavated material, fall back ejecta and back wash material) to parautochthonous (structurally uplifted, slumped and inverted target material from deeper levels) breccia deposits that were formed during and immediately after the Mjølnir impact. The uppermost part of the formation has been cored by a stratigraphic drilling (7329/03-U-01). It comprises siliciclastic sediments from claystones to conglomerates and consists of chaotic slump and avalanche deposits, along with different mass flow deposits. The formation is normally overlain by shales and siltstones of the uppermost part of the Hekkingen Formation (Oxfordian -Berriasian), and are succeeded by marls of the Klippfisk Formation (Berriasian-Hauterivian). On seismic reflection profiles the Ragnarok Formation can reach thicknesses of 1.3 km, including uplifted, reworked Lower Triassic fragments which originated about 3.5 km down in the crust and were structurally elevated during the impact cratering stage. The Ragnarok Formation reaches its maximum thickness in a wedge-shaped annular trough beneath the present annular crater basin, pinching out both towards the crater centre and towards the periphery. It can be recognised in seismic reflection profiles at the Mjølnir crater location and up to ∼55 km away from the crater centre, forming a wedge-shaped unit within the Mesozoic siliciclastic deposits of the Bjarmeland Platform. During the oblique Mjølnir impact event, large amounts of material were ejected and widely dispersed. Models of the impact process suggest that the ejecta were mainly spread in a north-easterly direction. The impact-related ejecta bed outside the crater boundaries varies from millimetres to a few metres in thickness and has been named the Sindre Bed. It has been recognized in core 7430/10-U-01, and in other wells of the Barents Sea. An Ir-enrichment in a time-equivalent formation at Nordvik, North Central Siberia, possibly represents a distal variety of the Sindre Bed.