The Lower Palaeozoic of the probable impact crater of Hummeln, Sweden

Abstract

A 1.2 km wide, sub-circular hole in Lake Hummeln (57°22′N, 16°15′E) has been suggested to be an impact crater. The lake has furthermore been suspected to contain an outlier of Cambrian and Ordovician sedimentary rocks. A shallow seismic survey, followed by core drilling to 164.25 m below the lake surface, demonstrated the presence of a strongly slumped succession consisting, from below, of shattered crystalline basement rock, Lower and Middle Cambrian sandstone and claystone, Lower Ordovician limestone, and Middle Cambrian clay-stone. The lithogenesis and original stratigraphy are closely similar to coeval beds on Öland 40 km to the east. Dating was done through acritarchs in the Cambrian, and through conodonts and chitinozoans in the Ordovician. An impact is postulated although no quartz with PDF has been identified in the available lithologies. The drilling had to stop, apparently without reaching the crater floor. If dated by the oldest sediments present, the impact would be Early Cambrian. A late Early or early Middle Ordovician age, however, is more likely because slumping after impact filled the crater with sediments aged Lower Cambrian through Lower Ordovician, and because the slumped sediments bear evidence of not having been originally deposited in a crater.