Clay mineral alteration associated with a meteorite impact in the marine environment (Barents Sea)

More than 50 samples from a Barents Sea borehole near the Mjølnir Structure (an extraterrestrial impact feature) were used to investigate changes in the clay assemblage associated with the submarine impact. Seismic evidence, the presence of shocked quartz and a prominent Ir anomaly restricted the potential impact affected zone to a 10 m interval, straddling the Jurassic/ Cretaceous boundary. Increased abundance (up to 30 wt%) of a smectite, a randomly interstratified smectite-illite with 85% smectite layers, forms the basis for a two-layer oceanic impact clay model that differs from published terrestrial cases. The smectite is assumed to represent seawater-altered impact glass from the ejecta blanket material that was mixed with resuspended shelf sediments by the collision generated waves. The smectite-rich interval is almost 5 m thick. It is overlain by a coarser unit (~2 m thick) containing abundant smectite, shocked quartz grains, and anomalous Ir contents at its base. The smectite-rich interval may have originated as a density/turbidity current, generated by the impact and the collapse and erosion of the crater rim. Seawater alteration of volcanic glass and changes in the tectonic regime of the provenance area, or changing oceanic current circulation patterns could produce similar variations in the clay mineral assemblage. The most compelling evidence for the possible impact derivation of this clay assemblage is the direct association with the Mjølnir Impact Structure and associated mineralogical and geochemical anomalies.