A marine impact crater as an Ordovician ecosystem; the Tvären crater
Frisk, Å.1, Lindström, M.2 & Holmer, L.E1
1Department of Earth Sciences, Palaeobiology, Uppsala University, Norbyvägen 22, SE-752 36 Uppsala, Sweden, firstname.lastname@example.org, email@example.com
2Department of Geology and Geochemistry, Stockholm University, Sweden, maurits.lindström@geo.su.se
The Tvären crater was formed as a result of an impact in the Ordovician Baltoscandian epicontinental sea, now situated in the Stockholm Archipelago, Sweden. The bolide impact resulted in an approximately 2 km wide crater and the pre-impact sedimentary sequence consists of Ordovician carbonates resting on non-lithified sands of Early to earliest Middle Cambrian age. After the impact event and the settling of the impact ejecta and resurge material, deposition of carbonates continued (Dalby Limestone). The lithology and thickness of the post-impact Dalby Limestone vary depending on the depositional environment relative to the cratered seascape. The crater itself acted like a sheltering rim for the deposition of sediments, also causing a fairly rapid sedimentation rate compared to the normal sedimentation of the Dalby limestone. The area consequently displayed pure new settings for the marine fauna still living in the surrounding sea, though not affected by the impact, and thus creating a new ecosystem. This particular condition makes it possible to get a good precision of the sedimentation and faunal succession occurring in the crater. Drillings in the Tvären crater were conducted in 1991 resulting in an almost complete drill core through the sedimentary succession in the crater. The base of the core consists of crystalline breccia followed by resurge deposits and then the sedimentation of the Dalby Limestone. The post-impact fossil fauna mostly consists of chitinozoans, graptolites, trilobites, bryozoans, ostracodes, echinoderms, cephalopods and brachiopods. Certain groups are restricted to deeper or lower water levels, varying during the sedimentation, while some occur throughout the succession. A detailed biostratigraphy through the post-impact succession of the drill core is being conducted to understand how the abundance of species changed in relation to the varied environments and how the pre-impact faunal groups recovered gradually as life returned onto a sterile seafloor.