Water-blow and resurge breccias at the Lockne marine-target impact structure
Maurits Lindström, Jens Ormö, Erik Sturkell, 2007. "Water-blow and resurge breccias at the Lockne marine-target impact structure", The Sedimentary Record of Meteorite Impacts, Kevin R. Evans, J. Wright Horton, Jr., David T. King, Jr., Jared R. Morrow
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The Ordovician (early Sandbian) Lockne impact crater in central Sweden formed in a sea at least 500 m deep. The structure and impact stratigraphy are sufficiently well preserved to permit detailed analysis of the cratering process. The target seabed consisted of a partly lithified, 75–80-m-thick sediment cover resting on continental crystalline basement. An over 7-km-wide inner crater formed in the basement. The surrounding sediment cover was almost completely removed within about 2 km from the rim of the inner crater. At 2.5–8.5 km from the rim, the thickness of preserved preimpact sediment increases with the distance to the inner crater. The top of the preserved sediment is mostly limestone breccia. Brecciation was probably driven by extremely forceful flow of water that was charged with sediment to the limit of its carrying capacity, and therefore stirred deeper than it would erode. The resulting lithology, for which we suggest the term “water-blow breccia,” is monomictic, with all clasts deriving from the underlying parent rock. Great volumes of crystalline ejecta were emplaced during and immediately after water-blow brecciation. The resurge, connected with the subsequent collapse of the water crater, deposited a mixed breccia of transported clasts that includes eroded limestone of various local provenances, as well as crystalline ejecta. The resurge breccia occurs extensively in the region, even outside the area characterized by occurrences of water-blow breccia. Its thickness and clast sizes decrease away from the crater.