Kärdla (Hiiumaa Island, Estonia)—the buried and well-preserved Ordovician marine impact structure

The Kärdla marine impact structure (Estonia, 58°58′N, 22°46′E) was formed at 455 Ma (Upper Ordovician), in a shallow epicontinental sea some tens of kilometres from the land and erosion area. The iron-rich projectile about 200 m in diameter approached from the west at an angle of 30–45°. The impactor penetrated about 50-m-thick water layer and the sedimentary cover and exploded in the uppermost part of the crystalline basement. A complex crater, 4 km wide and about 500 m deep, with a central uplift rising 130 m from the crater floor, was formed. The highest point of the rimwall is 110 m above the target level. The rimwall is cut by at least two resurge-excavated gullies. The variable height of the rimwall obviously results from the obliqueness of the impact. Outside the crater an elliptical area was revealed, 12–15 km in diameter, with deformed sedimentary rocks below the target level. The elliptical shape of this area may also be due to the oblique impact. Because the crater and its surroundings were buried directly after the impact, the whole complex of impact-related sediments is preserved there. They are recovered by 160 wells, six of which penetrate the entire complex of impact breccias inside the crater.