Kärdla Impact (Hiiumaa Island, Estonia) — Ejecta Blanket and Environmental Disturbances

The Kardla impact occurred at ca. 455 Ma (Upper-Ordovician, Caradoc) in a shallow (ca. 100 in) epicontinental sea not far (ca. 100 km) from the erosion area on the Baltic Shield (Grahn et al. 1996). The explosion of the meteorite ca. 200 in in diameter generated a complex crater 4 km wide and more than 500 m deep on the sea bed. The crater is surrounded by elliptical ring fault, up to 15 km in diameter, within which the sedimentary target rocks are strongly deformed. The ejected matter was spread almost concentrically around the crater, within a 50-km-radius, on ca 5500 km(2). The ejected matter is found also farther away as an admixture in limestones. Most of the ejecta blanket was covered by limy mud immediately after the impact. The crater was buried somewhat later; therefore the ejecta blanket is well preserved, except the rim wall area. Rate of accumulation of deposits and its facial composition in the crater deep, rim wall area and surroundings was different during some millions of years. The ejecta blanket lies in a succession of Upper-Ordovician carbonate rocks as a 0.01-3.5 m thick southward inclined (from 40 m b.s.l. [below sea level] in the island's northernmost point up to 190 m b.s.l. in the southernmost point) bed of silty and sandy limestones or limy silt- and sand-stones. On the sea bed about 10 km northward of the island the ejecta blanket is cut by the erosion escarpment (Baltic Klint). The distal ejecta layer consists mostly of silt- to gravel-sized debris of the target rocks (mostly Cambrian siliciclastic and Paleoproterozoic metamorphic rocks). In the lower part of the bed and closer to the impact centre coarser clasts occur. Farther from the impact site, the thickness of the ejecta layer, as well as the size of the grains decreases. The size of the ejected matter decreases also from the bottom towards the top of the layer. The ejected matter contains up to 1 vol% shock metamorphosed quartz grains with PDFs. The Kardla impact was too small to cause substantial and long-term global environmental changes and catastrophic shifts in the biosphere. Its long-term effect was restricted mostly to changes in sea bed relief and related facial changes, as well as the changes in the biotic communities of pelagic organisms caused by the latter.