The Inner Crater

The inner crater forms low ground round the northern part of Locknesjon. It is centred at Tramsta, a couple of hundred meters off the shore of the lake. The diameter is just over 7 km. In the central part of the structure there is a thin (tens of meters) outlier of the lowermost Caledonian overthrust nappe. The near-surface bedrock of the rest of the inner crater is Middle Ordovician Dalby Limestone that formed during a prolonged interval after the impact, with a maximum thickness of at least 88 m. This thickness is reduced to a matter of about 10 m near the rim of the inner crater.

The Dalby Limestone is underlain by resurge sediments that reach at least 203 m thickness in the central part of the inner crater, diminishing strongly towards the rim. These deposits are fining upwards, the lower part consisting of Lockne Breccia in the restricted sense discussed later in this paper, whereas the upper part is loftarstone, beginning with fine gravel and coarse sand, and ending upward with fine silt.

The Lockne Breccia rests on monomictic crystalline (mainly granitic) Tandsbyn Breccia, in which the deeper drill-holes ended, at 335 m in the hole that was deepest. No body of melt rock has been identified. However, this circumstance may be due to the lack of deep drill-holes in the central part of the inner crater, rather than to a genuine lack of melt rock within the crater. The abundant presence of melt fragments in the loftarstone resurge arenite demonstrates that melt was indeed produced by the Lockne impact.

Lindstrom et al. (1996) reported on geophysical evidence for the existence of a central uplift.

Fig. 4. Principal stratigraphy and terminology used for the Lockne crater. The sketch of the crater leaves out any tectonic complications and is intended to show only very generalized features, such as the relation of the principal structures to the sub-Cambrian peneplain. s-C peneplain: sub-Cambrian peneplain

Fig. 4. Principal stratigraphy and terminology used for the Lockne crater. The sketch of the crater leaves out any tectonic complications and is intended to show only very generalized features, such as the relation of the principal structures to the sub-Cambrian peneplain. s-C peneplain: sub-Cambrian peneplain

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