The Proximal Ejecta Around the Marine-Target Lockne Impact Structure, Sweden

Abstract

Very few impact craters on Earth have preserved proximal ejecta (ejecta blanket), which when present help us to better understand the cratering processes when asteroid hits Earth. The 458 Ma old Lockne impact structure consists of a 7.5-km wide nested crater in the crystalline basement surrounded by an approximately 3-km wide brim developed in the upper sedimentary target. The asteroid struck a marine environment with 500 m sea water, 50-m lithified limestone, and 30 m of Cambrian clay covering a peneplainized crystalline basement. The transient crater that developed in rock and water obtained a “soup-plate” shape and reached about 7 km from the impact crater center, the farthest on the down-range side. The brim of the soup-plate was partially stripped of Ordovician limestone and water before the emplacement of inner impact crater ejecta. Most of the ejecta rest upon the Cambrian clay (today shale). The asteroid struck obliquely from the east, which is reflected in the ejecta distribution. The proximal ejecta field is divided into two crescent-shaped areas to the northwest and southwest of the nested crater and covers 26 km2. Resistivity profiles, mapping, and core drilling show that the thickness of the ejecta masses range between 30 and 50 m with a total volume of about 1 km3. They were not re-worked by the resurge. They represent roughly 26 vol% of the calculated excavated volume of crystalline rocks. Thus, it can be concluded that the Lockne impact crater has a well-preserved ejecta blanket.