Geological, geochemical, geophysical and petrophysical studies for structural modeling of the Kärdla crater, Estonia

Latest events affecting the Precambrian basement, Gulf of Finland and surrounding areas. Geological Survey o f Finland, Special Paper 21, 115-125, 1 figure. Jotnian sedimentary rocks and Postjotnian mafic dykes and sills are regarded as the youngest basement rock types in the map area. Throughout the latest Meso-proterozoic and Neoproterozoic, erosion was the main geological process in the western part of the East European Craton, including the map area. A uniform peneplain surface was developed over all the basement units, with the formation of a kaolinitic regolith prior to the commencement of extensive Vendian and Phanerozoic sedimentation. During Vendian and Cambrian through Silurian time, the lowest part of the sedimentary blanket was deposited; it is likely that these sediments also covered large parts of the present day shield area. After a break in sedimentation that coincided with the youngest phase of deformation in the Scandinavian Caledonian orogenic belt, Devonian sediments were deposited in the southern part of the map area. Tectonic dislocations affecting both the basement and the lower part of the sedimentary cover sequence provide evidence for intracratonic compression during the Caledonian orogeny. Moreover, some Precambrian and Caledonian tectonic zones were again reactivated during and after the Devonian sedimentation. The late Paleozoic subsidence of the Bothnian Sea floor is interpreted as a distal conse­ quence of the extensional processes that formed the early Permian Oslo rift. Continued erosion during the late Phanerozoic produced the present topography, both in sediment-covered and shield areas. The gentle arching of the basement topography between the Gulf of Finland and the Bothnian Sea represents the exhumation of the Pre-Vendian peneplain surface from beneath the lower Paleozoic sedimentary cover as well as limited modification by both pre-Pleistocene conti­nental erosion and Pleistocene glacial processes. The erosional escarpments occurring in the area covered by Vendian and Paleozoic platform sediments were formed by differential erosion due to the variable resistance of alternating soft and hard layers. The gentle dip of the basement surface and depositional layering in the overlying sediments was an important factor in escarpment creation. Pleisto­cene glacial erosion has generally removed from several meters to several tens of meters of material, although local gouging of overdeepened valleys and topograph­ ical depressions has also taken place. The Quaternary glacial and postglacial deposits conceal most of both the basement and sedimentary bedrock.