Barents Sea - Far Field Structural Consequences of the Mjølnir Impact at the Jurassic/Cretaceous Bboundary

Richard Pegrum

The Mjølnir impact crater is in located in the Barents Sea at 73 48’N / 29 40’E. With a diameter of 40 km it is amongst the 25 largest impact craters currently recognised on Earth. Published studies, based on seismic data, corehole data and numerical modelling, have confirmed it was formed by the violent impact of a 2 km in diameter bolide with a velocity of 20 km/s. Core samples have dated the impact at 142±2.6 Ma, close to the Boreal Jurassic /Cretaceous boundary. The target site was an open marine sediment starved shelf where organic rich muddy sediments were accumulating in a water depth of around 400m. The impact vaporized or displaced 180-230 cubic km of rocks, the initial shockwave released energy equivalent to an earthquake in the Richter range of 7.7-8.7. The airwave, impact blast and crater rim uplift evacuated the water column and generated a train of giant tsunamis, the leading tsunami modelled as having a height of 200m at a distance of 60km some 300s after impact. Resurge of the water column and overfilling of the crater generated a second chain of giant tsunamis. This presentation concerns far field structural damage considered to have been directly triggered by the impact. Primary focus is on areas south and west of the impact site, including the Bjarmaland Platform, Loppa High and Hammerfest Basin. The postulated relationship between structuration and impact is based on interpretation of an extensive seismic database, including both 2D and 3D datasets. Throug the Triassic and much of the Jurassic the area was a gently subsiding platform with little direct evidence of syntectonic activity. Stratigraphic considerations suggest the Hammerfest Basin began subsiding during the Jurassic, possibly in response to rift propagation from the proto-Atlantic. At end Jurassic there was a short, intensive period of normal faulting, pervading the entire area. Fault throws range from tens to hundreds of metres. A network of deep and narrow grabens were emplaced and along the margins of the Hammerfest basin large fault offsets are locally associated with slumping and footwall collapse. It is argued that widespread faulting was triggered by the impact shock, by shock related hydraulic fracturing of the overburden, and was accentuated by the ‘pumping action’ of cyclic compression/decompression caused by the passage of giant tsunami trains. The tsunami activity also resulted in widespread differential erosion and/ or redeposition of sediments at the Jurassic / Cretaceous boundary. Following the impact generated structuration, Cretaceous deposits progressively infill the fault topography and basinal areas and onlap the surrounding highs, only locally accompanied by rejuvenated activity on major faults.

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