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 Precambri an basement of the Baltic Shield. In the depression, more than 150 m thick Cambrian and Ordovician marine sediments ate underlain by non-volcanic brec cia. The only explanation that does not meet contradictions is that the str ucture was caused by impact, but evidence of shock metamorphism is until no w missing. This study focuses on the geophysical characterisation and inclu des magnetic and gravimetric modelling, constrained by information from res istivity measurements, geological mapping and a drill core. The Hummeln str ucture 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 tw ice that of the proposed crater depression, and probably indicates fracturi ng. Both the magnetic and the gravimetric models show a structure that is c onsistent with the impact hypothesis. Resistivity measurements were carried out in the surroundings of the proposed crater. The fractured region indic ated by the magnetometry does not produce a detectable resistivity anomaly. However, resistivity measurements proved useful in investigating the tecto nic fracture zones crossing the area in the vicinity of the structure. Foll owing the impact model, a reconstruction of the pre-erosional structure res ulted in an approximately 50% wider original crater than seen today. (C) 19 99 Elsevier Science B.V. All rights reserved.