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The Gardnos structure in Hallingdal, Norway, is a 5 km in diameter, eroded impact crater of probably late Precambrian age. Within the structure, both impactites and postimpact crater sediments are preserved. The sediments comprise a wide range of siliciclastics: (1) postimpact breccias, (2) coarse conglomerates, (3) conglomeratic sandstones, (4) sandstones, and (5) interbedded fine sandstones, silt-stones, and shales.

The sedimentary succession reveals a shifting depositional environment. The post-impact breccias covering the crater floor were deposited by rock avalanches. Directly after impact, loose rock debris slid down the crater wall and the central uplift, settling on top of the newly formed impactites (suevite and lithic breccia known as Gardnos Breccia). The overlying conglomeratic and sandy sequence shows significant local thickness variations, consistent with coalescing fan-shaped deposits along the lower crater wall and on the crater floor. Probably the resurging water breached the crater rim at its weakest parts, initiating series of screes and debris flows, which built out into an eventually water-filled crater. Sand-enriched density flows then dominated in the water-filled crater basin. Above fine-grained sandstones, siltstones and shales were deposited, representing the reestablishment of quiet conditions, possibly similar to the preimpact depositional conditions. Carbon-enrichments in the impactites, and partly deformed clasts of sedimentary origin in the strata just above, suggest that the crystalline basement of the target area was covered by a thin layer of organic-rich sediments. This supports a scenario with a target area in shallow, stagnant water.

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