Facies distribution of post-impact sediments in the Ordovician Lockne and Tvaren impact craters: Indications for unique impact-generated environments

摘要

The Lockne and Tvaren craters formed in the Late Ordovician Baltoscandian epicontinental sea. Both craters demonstrate similarities concerning near-synchronous age, target seabed, and succeeding resurge deposits; however, the water depths at the impact sites and the sizes of the craters were not alike. The post-impact sedimentary succession of carbonates, i.e., the Dalby Limestone, deposited on top of the resurge sediments in the two craters, is nevertheless similar. At least three main facies of the Dalby Limestone were established in the Lockne crater, depending on sea-floor topography, location with respect to the crater, and local water currents. The dominating nodular argillaceous facies, showing low values of inorganic carbon (IC), was distributed foremost in the deeper and quiet areas of the crater floor and depressions. At the crater rim, consisting of crushed crystalline basement ejecta, a rim facies with a reef-like fauna was established, most certainly due to topographical highs and substrate-derived nutrients. Between these facies are occurrences of a relatively thick-bedded calcilutite rich in cephalopods (cephalopod facies). In Tvaren, the lower part of the succession consists of an analogous argillaceous facies, also showing similar low IC values as in Lockne, followed by calcareous mudstones with an increase of IC. Occasionally biocalcarenites with a distinctive fauna occur in the Tvaren succession, probably originating as detritus from a facies developed on the rim. They are evident as peaks in IC and lows in organic carbon (Corg). The fauna in these biocalcarenites corresponds very well with those of erratic boulders derived from Tvaren; moreover, they correspond to the rim facies of Lockne except for the inclusion of photosynthesizing algae, indicating shallower water at Tvaren than Lockne. Consequently, we suggest equivalent distribution patterns for the carbonates of the Dalby Limestone in Lockne and Tvaren.