Post-impact deposits in Tvären, a marine Middle Ordovician crater south of Stockholm, Sweden

Abstract

The well-preserved Tvären crater is noteworthy for being one of a small number of Early and Middle Ordovician impact structures formed in a marine environment. It is demonstrated to be an impact structure by the presence of a breccia lens, consisting of crystalline basement rocks, and shocked quartz. The breccia lens formed under dry-hot conditions after expulsion of sea-water by the impact. Resurging sea-water thereupon deposited a positively graded, 60 m thick turbidite-like unit. This graded resurge deposit is a previously unknown feature, to be expected in open-sea impacts. Breccia in the lower part of this graded deposit contains fragments of a remarkably complete orthoceratite limestone succession that existed at the site of impact, resting on non-lithified sand of probably Early to earliest Middle Cambrian age. A sedimentary succession was deposited inside the crater at depths decreasing from more than 200 m in the initial stages to some 100 m at the time of deposition of the youngest preserved beds. The environment within the crater thus favoured deposition of an essentially complete stratigraphic succession with depth-controlled palaeoecology for a significant time interval after the impact. Whereas planktonic members, like graptolites and chitinozoa, are present throughout the post-impact succession, and asaphids, almost as persistent, became established at an early date, burrowers were somewhat reluctant to enter and remopleuridids and small strophomenids came in at a late stage. We suggest as a result of this study that structures formed by impact may offer unique information about the palaeogeology and palaeoenvironment of the region hit by the impact.

Sedimentological analysis of resurge deposits at the Lockne and Tvären craters: Clues to flow dynamics

“…The resurge deposits at the Lockne and Tvären craters were studied in drill cores recovered in the 1990s (Lindström et al 1994(Lindström et al , 1996. At the Lockne crater, the two deepest (Lockne-1 and Lockne-2) of the six cores that were drilled within the basement crater contain Lockne Breccia (Lindström et al 1996).…”

“…Two cores exist from the Tvären crater (Lindström et al 1994). The Tvären-1 core was drilled at the rim of the basement crater and only contains about a meter of fractured basement below Quaternary sediments.…”

“…The only part where it was impossible to obtain data (fragmented core) occurred in the Lockne-2 core at 207.88-208.30 m depth. We focused on the parts in the Lockne-1 and Lockne-2 cores logged as Lockne Breccia by Lindström et al (1996), and on the part logged as coarse to gravelly limestone breccia in the Tvären-2 core by Lindström et al (1994). The logged sections are overlain in all three cores by carbonate-rich, greywacke-like Loftarstone.…”

“…5), but with the difference that the Cambrian at Tvären was represented by beds of unconsolidated sand (Ormö 1994;Lindström et al 1994) instead of the shales at Lockne. Also the Ordovician limestones at Tvären were thinner and less consolidated representing a facies closer to that of Estonia and northern Öland (Ormö 1994;Männil 1966).…”

“…At Tvären, the resurge breccia forms a several tens of meters thick, normally graded unit that passes upward into upwards-fining arenite, just like the Lockne Breccia and the Loftarstone inside the Lockne basement crater (Lindström et al 1994;Ormö 1994).…”