NJG92-4-04
2012
Late syn-impact and early post-impact sedimentation in the Ritland impact structure, western Norway
92
4
pp 405-431
978-82-92-39476-2

The 2.7 km in diameter, Ritland impact structure was excavated probably during Early to Mid Cambrian time in the gneissic sub-Cambrian peneplain. Based on field mapping and sedimentological study the crater infills are broadly classified as: A) late syn-impact, B) early post-impact and C) late post-impact sediments. Units A) and B) are subdivided into: i) Lower Breccia Unit (LBU), ii) Sandstone Unit (SU) and iii) Upper Breccia Unit (UBU). The lowermost sediments of the LBU contain dispersed particles of melt origin, suggesting a late syn-impact origin, probably related to rock avalanches.

Debris-flow events took place during the early post-impact phase. Shift from clast-supported to matrix-supported texture in the upper part of the LBU represents flow transformation from cohesive to hyperconcentrated density flows, probably due to a result of resurge of seawater. The SU overlying the LBU represents suspension-dominated sedimentation after the crater was filled with water. The UBU represents sedimentation in the marginal part of the crater basin. Scree, rock-avalanche or debris-avalanche processes dominated in this sequence.

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