Density and magnetic susceptibility of rocks from the Lockne and Tvären marine impact structures

Abstract

The Lockne and Tvären impact craters in Sweden formed in a marine environment during the Ordovician. The contrast in density between the impact breccias and the surrounding target rock of these two craters is significantly lower than what has been found in craters formed in crystalline targets on land. Another marine-target structure, the Estonian Kärdla structure, demonstrates intermediate contrast in impact breccia and target rock, which we attribute to the interpreted shallowness of the sea at the Kärdla impact site. We conclude that the main cause for these low-density contrasts is pore and fracture filling of calcite with subordinate quartz and fluorite. Calcite is the most abundant cement, and its density differs most from that of fractured and brecciated bedrock with a low degree of cementation. Furthermore, from the studied cases, it is concluded that the target rock to impact rock contrast is generally the highest in craters formed on land in crystalline targets and the lowest in craters formed at sea, while craters formed on land in sedimentary targets are intermediate. The low density contrasts should decrease the negative gravity anomalies of marine craters.