Ordovician impact crater at Kärdla, Hiiumaa Island, Estonia

The buried and well preserved Kärdla crater in Estonia (Southern Fennoscandia) (58°59Ń, 22°40É), with a diameter of approximately 4 km, was discovered in 1967. The subsurface structure of the crater has been studied by gravity anomaly measurements, aeromagnetics and by more than 300 boreholes in the crater, on the rim (“ring wall”) and in the surrounding area. Recent boreholes, reaching 800 m, have revealed that the crater is 540 m deep and has 100 m high central peak.

Barely visible in the present topography, the crater is filled with Palaeozoic and Quaternary deposits and is mostly low marshy land surrounded by a low ridge along the ring wall. Quaternary landforms, such as eskers and post-glacial beach ridges, also complicate the topography. In the buried sub-Quaternary bedrock relief, the crater occurs as a roundish depression bordered with two, 10–30 m high horseshoe-shaped elevations. The bedrock topography was slightly drumlinized by glacial excavation.

The crater formed 455 Ma in composite stratigraphy: middle and lower Ordovician (20 m) and Cambrian (120 m) sedimentary rocks overlying crystalline basement on the bottom of a shallow nearshore sea.

The subsurface structure of the crater is well preserved. In vertical section from the top are: (1) post-impact cover of Ordovician sedimentary rocks, 15–100 m thick; (2) allochthonous breccias (filling the lower part of the crater) and beds of fall-out breccias and conglomerates, sandstones and sandy limestones consisting of debris of reworked fall-out breccia and wall rocks surrounding the crater; (3) a body of autochthonous and subautochthonous breccias forming the bottom and the central peak of the crater and also remnants of its rim. Shocked rocks and minerals from autochthonous and allochthonous breccias have been revealed by optical microscopy.