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Otolkad Kort Profil UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre A shallow reflection seismic investigation and depth determination of Söderfjärden, an impact crater in western Finland Erik Fennvik ISSN 1400-3821 B1001 Master of Science (120 credits) thesis Göteborg 2018 Mailing address Address Telephone Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract The early Cambrian impact structure Söderfjärden is a 6.6-km wide crater which is surrounded by a crater rim with a present height up to 55 meters. Söderfjärden is a well-preserved meteorite crater and it’s filled with lower and middle Cambrian sediments, up to the level of the central peak. The central uplift and the Cambrian sediments are overlayered by 30-80 m of Quaternary deposits. The Cambrian sediments are thought to have deposited all-most directly after the impact. This make the crater well preserved but inaccessible. To explore the crater floor in the moat, drilling and reflection seismic are the two possibilities. The crater floor above the moat is expected to be at 300-400 meters below surface level which makes reflection seismics the to the best option (because refraction seismic line would have to be at least 1,5 km long, which is possible but not practical). Two seismic profiles were created in this survey, one short profile above the central uplift and one longer profile in the eastern part of the crater above the moat. The depth to the deepest part of the crater bottom in the eastern part of the moat was interpreted to be between 330-410m. Terraces and/or megablocks were interpreted to be present on the crater floor at a depth of approx. 280 meters in the northern part of the long-profile. The depth of the Quaternary and Holocene layers was interpreted to be 89-96m in the long profile compared to the maximum depth of 75m from the previous drillings. The Cambrian succession reflectors were close to horizontal and easily distinguished from the chaotic Quaternary reflectors. The depth to the central uplift beneath the short profile was 36-46m and faults could be observed within the central uplift. Two new drilling locations have been suggested. The first location is 150 m in to the long profile where the depth is the greatest. This drilling could be carried out to reach the crater bottom and should be atleast 600 m long. The second borehole location should be located close to the existing borehole (SF6), which was done by H.Hirvas in 1983 and is still unpublished. It was in borehole SF6 that limestone for the first time was discovered in Söderfjärden. This new drilling should drill through the limestone to define its extent and to see if its discordant to the Cambrian sediments like the Quaternary sediments or if it’s the natural transition from the sandstone. Keywords: Reflection seismic, Söderfjärden, Impact crater, Shallow Seismic Survey, Cambrian, Drillings Sammanfattning Den tidigt Kambriska impakt kratern Söderfjärden har en diameter på 6.6 km som är omringad av en krater kant som har en nuvarande höj på upptill 55 meter. Meteoritkratern är väl bevarad och är fylld med tidiga och mellan Kambriska sediment, upp till nivån av den centrala upplyftningen. Den centrala upplyftningen och de Kambriska sedimenten överlagras av 30–80 meter kvartära avsättningar. De Kambriska sedimenten tror man ha snabbt efter impakten. Detta gör att kratern är välbevarad men oåtkomlig. För att kunna utforska kraterbotten i vallgraven krävs då borrning eller reflektions seismiska. Kraterns botten ovanför vallgraven förväntas ligga mellan 300–400 meter under marknivån vilket gör att reflections seismik till det bästa valet (Refraktions seismic är möjligt med en 1,5 km lång kabel men det är inte praktiskt). Två seismiska profiler skapades, en kort profil skapades över den centrala upplyftningen i kratern och en lång seismisk profil skapades i den östra delen över vallgraven. Djupet till den djupaste delen av kraterbotten i den östra delen av vallgraven tolkades vara mellan 330–410 m. Terrasser och/eller mega block har tolkats fram ur data och toppen på dessa ligger på 280 meters djupt i den norra delen av den långa profilen. Djupet ned till de Kambriska avlagringarna är 90–96 meter, det är ca 20 meter djupare än vad de tidigare borrningarna uppnått. De kambriska avlagringarnas reflektorer var nästan horisontala och var enkla att utskilja från de kvartära kaotiska reflektorerna. Djupet till den centrala upplyftningen under den korta seismiska profilen var 38-46m och förkastningar kunde urskiljas i den centrala upplyftningen. Två nya borrhåls platser har rekommenderats. Den första i den djupaste delen av den långa profilen vid in i profilen 150m denna borrning ska utföras för att nå krafter botten och ska vara åtminstone 600m lång. Den andra borrhåls placeratingen ska vara nära där H.Hirvas placerade sitt borrhål SF6, som fortfarande är opublicerat. Det var här man förstagången stötte på kalk i Söderfjärden och denna borrningen ska borra igenom de kalkrika sedimenten för att bestämma tjockleken på dessa sediment och om kalkstenen är naturliga övergången från sandstenen eller om den ligger diskordant precis som de kvartära avlagringarna på de Kambriska sedimenten Nyckelord: Reflektions seismik, Söderfjärden, Impakt krater, Seismisk undersökning, Kambrium. Borrningar Table of content Table of content ...................................................................................................................................... 1 Introduction ............................................................................................................................................. 2 Geological setting .................................................................................................................................... 3 Previous studies....................................................................................................................................... 4 Data acquisition ..................................................................................................................................... 10 Data processing ..................................................................................................................................... 11 Results ................................................................................................................................................... 13 Discussion .............................................................................................................................................. 17 The Short profile ................................................................................................................................ 17 Filter and processing ..................................................................................................................... 17 Velocity layers and depth interpretation ...................................................................................... 17 The long profile ................................................................................................................................. 19 Filtering and processing................................................................................................................. 19 Velocity, layers and depth interpretation ..................................................................................... 19 General discussion ............................................................................................................................. 23 Conclusions ............................................................................................................................................ 24 Acknowledgements ............................................................................................................................... 24 References ............................................................................................................................................. 25 Appendices 1 ......................................................................................................................................... 27 Processing in more detail, The long and short profiles ..................................................................... 27 1 Introduction There are 190 impact structures on Earth and all of them have been exposed to erosion to different degrees. From the new almost untouched to the older and more deeply eroded. Generally, an older crater is more eroded than a young one. Furthermore, the geological environment plays a vital role for a craters preservation; as impacts on land experience erosion in contrast with structures formed in a marine setting where sediments can bury the structure. A structure that is still filled with post- impact sediments gives the best opportunity to reveal the original morphology of the crater. The sediments protect the crater but also make access more difficult. Some can be explored by drilling, but to get the best image of the crater and the nature of the sedimentary fill, reflection seismic methods are to be preferred. In craters with thick layers of fill, seismic reflection surveys will give the best result. Several sediment filled craters have been subjected to reflection seismic surveys. More thorough seismic investigations are made if oil is expected, because the seismic results gives an indication where oil might be trapped. Some examples of craters
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