THE RESURGE PROCESS AT MARINE-TARGET IMPACTS AT DIFFERENT TARGET WATER DEPTHS EXEMPLIFIED BY THE LOCKNE AND TVÄREN CRATERS

The Lockne and Tvären craters formed about 455Ma in an epicontinental sea where seawater and mainly limestones covered a crystalline basement. The target water depth for Tvären (apparent basement crater diameter D = 2 km) was probably not over 150 m, and for Lockne (D = 7.5 km) recent best-fit numerical simulations give that it was 500-700 m. Lockne has crystalline ejecta that partly cover an outer crater (14 km diameter) apparent in the target sediments. Tvären is eroded with only the crater infill preserved.

We have line-logged cores through the resurge deposits within the craters in order to analyze the resurge flow. The focus was clast lithology, frequencies and size-sorting. The sedimentological analysis has been complemented with numerical simulation of the resurge dynamics using the iSALE hydrocode.

The combined results show that at both craters there is an initial stage of the resurge (resurge proper) which develops into a central water plume (CWP) that subsequently collapses into an anti-resurge and finally oscillating resurge. The CWP formation and anti-resurge is stronger at Lockne than at Tvären due to the relatively deeper target water. The resurge continues over the rim and the outer parts of the basement crater during the whole process of filling the crater. At Lockne, due to the formation and collapse of the CWP the resurge flow along the crater floor is interrupted and momentarily replaced by a large body of slackwater. This slackwater is in its lower parts disturbed by a strong lower anti-resurge loaded with granitic material that initially had been ripped up into the CWP by the resurge proper. A similar granite-loaded anti-resurge along the crater floor develops at Tvären. Here, however, the resurge proper is stronger than the anti-resurge and manages for a relatively long period to keep the upper anti-resurge separated from the lower flow. In this way the resurge proper can affect the central part of the crater longer than at Lockne. Eventually the energy of the resurge proper in the central parts of the crater is waning allowing a slackwater zone to form.