SEDIMENTARY BRECCIAS AND THE RESURGE STAGE OF THE LOCKNE AND TVÄREN MARINE-TARGET IMPACT CRATERS (SWEDEN)

The Lockne and Tvären craters formed 455Ma in an epicontinental sea. The target is similar: seawater and mainly limestones covering a nearly horizontal crystalline basement, with one possibly important difference: the limestones were thinner and weaker at Tvären. The target water depth for Tvären (apparent basement crater diameter D=2 km) was about 150 m, and for Lockne (D=7.5 km) it was 500-700 m. Lockne has ejecta flaps of crystalline rock that partly cover an outer crater (14 km diameter) developed in the target sediments. The outer crater is characterized by occurrences of locally derived sedimentary breccia (water-blow breccia) that formed during excavation stage, but that got partially incorporated in the resurge deposits. Tvären is eroded and no rim or peripheral features are preserved, but the crater infill show a complete succession up to post-impact secular deposits.

We have line-logged cores through the resurge deposits within the craters in order to analyze the mode of resurge flow and cratering process. The focus was clast lithology, frequencies and size-sorting.

At both craters the resurge deposit begins with a poorly sorted lower unit (interpreted as deposited during main resurge phase) that is followed by a better sorted, normally graded unit (interpreted as deposited after the collapse of the water-plume had reworked the resurge deposit). At Lockne a high content of crystalline is followed upwards by an increase in sedimentary clasts. At Tvären the situation is the contrary. We interpret this as a consequence of the weaker sediments causing an overload of sedimentary ejecta during the excavation of an outer crater giving a relatively high content of sedimentary clasts in the early resurge, whereas at Lockne the outer excavation was relatively less efficient. Instead, the resurge was dominated by rip-up and did first bring in crystalline clasts from the flaps, whereas limestone clasts from the outer crater entered the inner crater last.