Late syn-impact and early post-impact sedimentation in the ritland impact structure, western Norway

摘要

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.