4.4 Genesis of the Skånings-Åsaka Push Moraine
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UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Skånings-Åsaka push moraine - a formation process, paleo glacial tectonics and sedimentology study Hampus Johansson Brian Karlsson Tufuga ISSN 1400-3821 B1016 Bachelor of Science 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 During the Younger Dryas, the ice sheet in Scandinavia re-advanced and several end-moraines were formed in Middle Swedish end-moraine zone (MSEMZ). One of these end-moraines is the Skånings-Åsaka push moraine. Near Bränningsholm in Skara municipality, an exposed outcrop of the moraine is present where our investigation was conducted. With the help of sketching and structural measurements, the provenance and morphology of Skånings-Åsaka moraine was investigated. The moraine was found to consist of interbedded sand, predominantly fine sand. Multiple clay and silt layers were also found in the exposure. In the sand and clay layers faults and folded structures where found. We suggest that the Skånings- Åsaka moraine was formed in three main steps. During the first step, the ice sheet in MSEMZ advanced and pushed up the southern part of Skånings-Åsaka moraine. In step two, the ice retreated and outwash fans in the east supplied sand which was deposited in a near-shore marine environment. Simultaneously clay was deposited further out in the ocean. The Skånings-Åsaka moraine was now situated above sea level in the area. During the last step, the ice sheet re- advanced and deformed the sediment that builds up the Skånings-Åsaka push moraine. The Skånings-Åsaka push moraine is an excellent example of a subaquatic push moraine, formed of sediment deposited under or near the sea level but now exposed subaerial. Keywords: Push moraine, Younger Dryas, MSEMZ Sammanfattning Under Yngre Dryas så ryckte istäcket i Skandinavien tillfälligt fram igen och flera ändmoräner bildades i södra Mellansverige. En av dessa ändmoräner är Skånings-Åsaka, en push morän belägen i MSEMZ (Middle Swedish end-moraine zone). Nära Bränningsholm i Skara kommun finns en blottad skärning av moränen där denna studie genomfördes. Med hjälp av skissning och strukturmätningar har Skånings-Åsaka moränens ursprung och morfologi undersökts. Moränen fanns till största delen bestå av sand, huvudsakligen finsand. Ett flertal ler- och siltlager fanns också representerade. Sanden visade upp både veckade och förkastade strukturer. Vi föreslår att Skånings-Åsaka moränen har bildats i tre steg. Under det första steget avancerade isen i området fram och tryckte upp södra delen av Skånings-Åsaka moränen. I steg två drog sig isen tillbaka och i öster förde outwash fans med sig sand som avsattes nära strandlinjen i en marin miljö. Samtidigt avsattes lera längre ut i havet. Skånings-Åsaka moränen befanns sig nu över havsnivån i området. Under det sista steget drog isen återigen fram och deformerade sedimenten som utgjorde Skånings-Åsaka moränen. Skånings-Åsaka push morän är ett utmärkt exempel på en subaquatic morän som bildats under havsytan men som nu är exponerad ovan land. Nyckelord: Ändmorän, Yngre Dryas, MSEMZ Table of contents 1. Introduction ........................................................................................................................................ 1 1.1 Purpose and aim ............................................................................................................................ 2 1.2 Location of the study area ............................................................................................................. 3 1.3 Previous studies ............................................................................................................................. 5 2. Methods .............................................................................................................................................. 7 2.1 Field work ...................................................................................................................................... 7 2.2 Laboratory work ............................................................................................................................ 8 2.2.1 Grain size analysis ................................................................................................................... 8 2.2.2 Grain composition .................................................................................................................. 9 3. Results ............................................................................................................................................... 10 3.1 Sketch of the outcrop .................................................................................................................. 10 3.2 Structural geology ....................................................................................................................... 14 3.2.1 Reverse faults ....................................................................................................................... 14 3.2.2 Normal faults ........................................................................................................................ 16 3.2.3 Folds ..................................................................................................................................... 17 3.3 Laboratory results ....................................................................................................................... 18 3.3.1 Grain composition analysis for sand fractions ..................................................................... 18 3.3.2 Dry sieving of sands and pipet analysis results .................................................................... 19 4. Discussion .......................................................................................................................................... 20 4.1 Depositional setting of moraine sediment .................................................................................. 20 4.2 Provenance of moraine sediment ............................................................................................... 21 4.3 Deformation process ................................................................................................................... 22 4.3.1 Folds and faults..................................................................................................................... 22 4.4 Genesis of the Skånings-Åsaka push moraine. ............................................................................ 24 4.5 Error sources ............................................................................................................................... 26 4.5.1 Field work: ............................................................................................................................ 26 4.5.2 Laboratory: ........................................................................................................................... 26 5. Conclusions ....................................................................................................................................... 27 6. Acknowledgments ............................................................................................................................ 28 7. References......................................................................................................................................... 29 8. Appendix ........................................................................................................................................... 31 8.1 Dry sieving ................................................................................................................................... 31 8.2 Pipet analysis ............................................................................................................................... 33 1. Introduction During the Late Weichselian deglaciation, a sudden climate change took place and the rapidly warming global climate got colder again. The abrupt climate change occurred approximately 12900 years ago and is called the Younger Dryas event (Carlson, 2010). This stadial lasted for circa 1300 years and caused a significant cooling. The Scandinavian ice sheet pushed forward again in certain parts, like in south central Sweden, contemporaneously the ice sheet in Finland and Norway created end moraines due to the halting of the ice sheet and the re-advancement of it caused by the Younger Dryas (Johnson, Benediktsson & Björklund, 2013). The climate effect was global, and glaciers grew in many other parts of the world as well (Benn & Evans, 2010). Prior to the Younger Dryas was the Allerød interstadial, which was characterized by warm and humid climate, where the Scandinavian ice sheet was melting and shrinking in size, the climate was almost comparable to today's (Mangerud, 1987). Possible causes for the Younger Dryas event are credited to the reduction of the Atlantic Meridional Overturning Circulation (AMOC) (Leydet et al., 2018). Other causes such as meteorite impacts have been suggested by recent studies, where worldwide meteorite impacts would have aided in the rapid cooling of the climate (Wolbach et al., 2018). During this swift cold chock caused the ice sheet in southern central Sweden to move south across the land and formed the Middle Swedish end-moraine zone