The irregular base Cretaceous reflector offshore Mid Norway : a possible result of the Mjølnir impact in the Barents Sea ?

Introduction The last few decades of oil exploration have dramatically increased the geological database of the North European continental margin, and resulted in a number of oil and gas discoveries on the Norwegian continental shelf. This exploration has answered many questions, but also revealed a number of new geological problems. The development of new investigation methods and interpretation techniques, combined with rapid and intense data collection, have resulted in the accumulation of enormous data sets. Recently, several important review papers have appeared (e.g.,, Fleet & Boldy 1999, Martinsen & Dreyer 2001, Eide 2002). Even so, it is clear that numerous geological correlations and causal connections are still to be discovered. In that context, catastrophic geological events with widespread imprints will be of special interest and importance. The main purpose of this paper is to draw attention to a possible link between the characteristic irregular, base Cretaceous reflector offshore Mid Norway and the Mjølnir impact in the Barents Sea (Fig. 1). The deformed surface will also be compared to the results expected from geotechnical explanation models. If the faulting really has been caused by the impact, the base Cretaceous reflector will represent a very important geohazard marker and could make it possible to further evaluate distant geological effects of extraterrestrial impacts. A deeper evaluation of the regional geological implications, however, falls outside the scope of the present paper. The Mjølnir impact The large circular structure in the Barents Sea at 73o48’N, 29o40 ́E (Fig. 1) was first interpreted as an impact structure by Gudlaugsson (1993) and its impact nature was verified by Dypvik et al. (1996). The 40 km-wide structure has been named Mjølnir after the hammer of the old Norse god Tor, and has later been described in a number of papers (e.g., Dypvik & Ferrell 1998, Dypvik & Attrep 1999, Tsikalas et al.1998a-c, Smelror et al. 2001a, Dypvik et al. 2003, 2004 a, b). The crater was probably created by a 1.5 km to 2 km diameter asteroid, which crushed and melted the target rock and completely vaporized at impact. The results of the impact can be traced both inside and outside the crater as shock metamorphosed grains of quartz in the sediments, possible occurrences of glass fragments and its alteration products, and iridium anomalies. Biostratigraphical evidence from a core drilled in the central part of the Mjølnir crater has provided a stratigraphic age for the impact approximating to the Volgian Ryazanian boundary (Fig. 2). A calibration of the present biostratigraphical datums (zonations) from the Mjølnir impact ejecta to recent chronostratigraphic time-scales suggests a rough age of 142 ± 2.6 million years for the impact (Smelror et al. 2001a). The environment at the impact site was marine with an estimated water depth of about 400 m. The Mjølnir impact caused both major shock waves and huge sea waves (tsunami), resulting in a mixing of water masses and sudden KÅRE ROKOENGEN, ATLE MØRK, MAI BRITT E. MØRK & MORTEN SMELROR