Mutually constrained geophysical data for the evaluation of a proposed impact structure: Lake Hummeln, Sweden

Lake Hummeln covers a 1.2-km-wide near-circular depression in the Precambrian basement of the Baltic Shield. In the depression, more than 150-m-thick Cambrian and Ordovician marine sediments are underlain by non-volcanic breccia. The only explanation that does not meet contradictions is that the structure was caused by impact, but evidence of shock metamorphism is until now missing. This study focuses on the geophysical characterisation and includes magnetic and gravimetric modelling, constrained by information from resistivity measurements, geological mapping and a drill core. The Hummeln structure shows both a gravimetric and a magnetic low. The magnetic low is not broken by any short wavelength anomalies which would indicate the presence of remanent melt occurrences. The diameter of the magnetic low is about twice that of the proposed crater depression, and probably indicates fracturing. Both the magnetic and the gravimetric models show a structure that is consistent with the impact hypothesis. Resistivity measurements were carried out in the surroundings of the proposed crater. The fractured region indicated by the magnetometry does not produce a detectable resistivity anomaly. However, resistivity measurements proved useful in investigating the tectonic fracture zones crossing the area in the vicinity of the structure. Following the impact model, a reconstruction of the pre-erosional structure resulted in an approximately 50% wider original crater than seen today.