|
Tom Øyvind Jahren, Gardnos Meteorittpark (Norway)
Elin Kalleson, Natural History Museum, University of Oslo (Norway)
Henning Dypvik, Department of Geosciences, University of Oslo (Norway)
|
|
The impact at Gardnos probably happened in Late Precambrian time. Since formation the crater has been first filled with post-impact sediments, and later overridden by kilometre thick nappes during the Caledonian orogeny. During Tertiary uplift and erosion brought the crater structure back to surface, and the major glaciations during Quaternary carved out most of the infill and exposed the impact breccias of the crater floor. Within the crater structure of about 5 km in diameter, presently is exposed an almost complete sequence of impactites; fractured bedrock, lithic breccia, suevite and sedimentary crater infill. The excursion guide to the Gardnos impact structure presents localities of all the major lithologies.
The parautochthonous Gardnos Breccia are more than 200 m thick in central parts of the crater. The Gardnos Breccia makes up large parts of the crater floor, typically consisting of angular white granitic or quartzitic clasts embedded in a dark matrix. The matrix content may vary from mm-thick zones dividing the bedrock into separate fragments and to matrix-dominated breccias where the clasts are few and small, floating in a matrix dominated lithology.
The suevite is an allochthonous, polymict breccia with melt fragments in a fine-grained clastic matrix. In field the suevite appears dark grey on fresh surfaces, but with time the matrix weathers almost yellowish-white, and in many cases rock clasts and melt fragments become easier to distinguish. At one outcrop crystallized melt forms the suevite matrix and the rock should be classified as melt-matrix breccia.
The post-impact sediments consist mainly of impact-generated debris deposited, reworked and redistributed to the crater basin. Some of the post-impact sedimentary successions are presently is preserved in the northern part of the crater-structure, ranging from coarse-grained conglomerates through sandstones into fine-grained sand- and claystones.
|
