Resurge morphology of the marine Lockne impact crater, Jämtland, central Sweden

Abstract

In the Brunflo area of Jämtland, central Sweden, a Cambrian to Ordovician sedimentary sequence rests on a Proterozoic crystalline basement. The area lies just outside the site of the middle Ordovician Lockne impact, and it later experienced Caledonian overthrusting. The degree of Caledonian tectonization of the Palaeozoic varies, but an autochthonous Cambrian and Ordovician sequence apparently occurs in the greater part of the area, particularly in the north. The pre-impact sedimentary succession is 81 m thick in the autochthon, with the Middle Ordovician Furudal Limestone as uppermost member. Brunflo village is located just outside the crater, 8–9 km north of its centre, but the area was affected by the impact. The impact-generated ejecta and resurge deposits rest on a surface which cuts the sedimentary strata at a low angle. This surface cuts at progressively higher stratigraphic levels at increasing distance from the crater. According to observations in the autochthon the impact generated an erosion surface dipping 1–2° towards the crater centre. The sequence of events that shaped this surface began with bombardment with high-speed ejecta closely followed by resurging water. The resurge mixed ejecta clasts with the products of resurge erosion to form the resurge deposits. No rim wall can be traced at the Lockne impact structure, probably because a rim wall, if it formed, collapsed in the modification stage owing to local lithological conditions and because the remains of it were completely eroded in the resurge phase. It is suggested the rim wall formed in sedimentary strata with unlithified clays at their base, and that this clay was unable to support it.