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.