GEOPHYSICAL SURVEY OF THE PROPOSED MÅLINGEN MARINE-TARGET CRATER, SWEDEN

Målingen is a 1km wide circular structure situated about 15km to SW of the similar age, 8km wide Lockne impact crater, Sweden. At the time of the Lockne impact the area was covered by a >500m deep sea. Sedimentary breccias occur in the central parts of the Malingen structure in both outcrop and drill core and show similarities to the resurge deposits at Lockne. The ongoing Geophysical survey will provide data for a geophysical modeling that will aid the geological studies to determine the dimensions and shape of the Målingen structure. In turn, the geophysical/geological models will be used as constrains for numerical simulations to evaluate the potential impact formation of this structure and its relation with the Lockne impact crater.

The ongoing geophysical survey comprises gravity and magnetic measurements with portable field equipment (i.e. gravimeter and proton magnetometer). They are complemented with geological mapping, a core drilling to 149m depth at the center of the structure, detailed leveling, and lab/field susceptibility measurements of lithologies in outcrops and drillcore. The gravity data were obtained along two roads crudely oriented N-S and E-W intersecting the apparent center of the structure. The resulting Bouguer anomaly map shows a general gravity low over the interior of the structure as well as a concentric pattern of weak lows outside the apparent topographical rim. The magnetic survey covers the whole structure and extends to a distance of about one diameter outside the apparent rim where the terrain allows it. Similarly to the gravity data there appears a concentric pattern of low magnetic anomalies at some distance outside the topographic rim.

The gravity low over the interior of the structure and low magnetic anomalies are consistent with the magnetic and gravity signature of bowl shaped, simple impact craters described in literature. The distribution of the low anomalies from the gravity and magnetic surveys suggest a circular disturbance zone larger than the apparent structure, possibly due to fracturing. The concentric pattern may be a consequence of the putative impact occurring at relatively deep water, thus obtaining a point of explosion at relatively higher level in the target than at an equal sized land-target crater.