Rim morphology of the Kärdla crater based on reflection seismic and ground penetrating radar investigations

Rim morphology of the Kärdla crater based on reflection seismic and ground penetrating radar investigations A. Jõeleht and J. Plado Department of Geology, University of Tartu, Estonia. E-mail: juri.plado@ut.ee Introduction: An impact into a shallow Late Ordovician sea, covering ~150 m thick layer of siliciclastic rocks and crystalline basement, produced the 4-km complex Kärdla crater [1]. The post-impact marine sedimentation buried the crater causing good preservation. Earlier drilling and gravity data have revealed that the crater s crystalline rim is not uniform. In some locations (NE, SW and NW) crystalline rocks have been found at a depth of few tens of meters, i.e. about 200 m above the pre-impact basement level, but there are also places where the rim height is moderate or low. It has been proposed [2] that the lows of rim in the N and S are gullies that were eroded by the backsurging seawater. However, drilling and gravity data allow to speculate that erosion of rim was widespread and not limited only to the lows. Methods: We performed reflection seismics and ground penetrating radar (GPR) studies on more than 10 rim crossing profiles with the aim to study the crater s morphology in more detail. We used 24-channel Summit CU seismometer with 40 Hz geophone groups at 10 m separation. An 8-kg sledge hammer source was applied inline with near-offsets up to 360 m. Usually, vertical stack of 20 was used to improve the signal to noise ratio. The data were processed with Seismic Unix. The GPR Zond-12e by Radar Systems Inc., Latvia, was used at frequency of 100 MHz. The data were processed with a software package Prism2. Results: The GPR data that revealed boundary between the post-impact Ordovician sedimentary layers and Quaternary soil allowed to identify the exact position and width of the rim. Also, these data were used to process the seismic profiles by revealing the exact thickness of low seismic velocity overburden. Seismic profiles agree with the drilling data showing also that the rim of crystalline rocks is not morphologically uniform. In the NE segment the rim is wide both at its base and top, and higher than elsewhere rising more than 200 m above the basement level in surrounding area. In the eastern segment, the rim becomes less prominent and shows signs of collapse by including the inward dipping reflectors and several peaks. In the SE segment the rim is lower and may have several peaks. In addition to collapse features, the rim seems to be strongly eroded as well. In the western segment the crystalline rim is relatively low, usually with relative altitude of less than 100 m. However, some profiles suggest that the crystalline basement is raised over a considerable distance. Discussion and conclusions: The data indicate that the rim was not formed uniformly leading to differential erosion by resurging sea. In the NE segment the rim has remained high as it is not collapsed as much as elsewhere. However, in other parts the rim is lower and, thus, has being more easily erodable. The variations in the rim morphology can be attributed either to the oblique impact from the NE or differences in the properties of the crystalline target. Lowering of crystalline rim by its collapse created favorable conditions for a widespread erosion, which is not limited to the previously proposed resurge gullies in the north and south. Acknowledgements: We are grateful to all the members of field crew. This study was financed through the Estonian Science Foundation grant no. 5851. References: [1] Puura V. and Suuroja K. (1992) Meteoritics, 216, 143 156. [2] Suuroja K. et al. (2002) Deep Sea Research II, 49, 1121-1144. 230