Architecture of the northeastern rim of the Kardla impact crater, Estonia, based on ground-penetrating radar studies

A ground-penetrating radar (GPR) survey was carried out in order to characterize reflection patterns at Palukula Hill, at the NE portion of the rim of the 4-km-diameter, Upper Ordovician Kardla impact crater. The results allow us to distinguish between the Quaternary overburden and layered postimpact marine sedimentary rocks that cover the crystalline rim. The bedrock is indicated by reflections parallel to bedding that are tilted inward and outward with respect to the crater along the inner and outer rim slope, respectively. It is obvious that the uppermost part of the Palukula crystalline rim was eroded and leveled in the Late Ordovician prior to having been covered by subhorizontally layered sediments. The unexpected position and low height of the crystalline rim at the northernmost edge of Palukula Hill indicate that the rim of the Kardla structure has collapsed to different degrees. This is consistent with the different heights of the crystalline rim. Our results favor the interpretation that the observed geology is not due to erosion by sea resurging into the crater but is rather a result of collapse of the crater rim.