Provenance of Pebble Clasts in Hummocks in the Western South
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UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre PROVENANCE OF PEBBLE CLASTS IN HUMMOCKS IN THE WESTERN SOUTH SWEDISH HIGHLANDS NEAR HÖRDA Karin Grodzinsky Martin Thor ISSN 1400-3821 B955 Bachelor of Science thesis Göteborg 2016 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract The southern part of Sweden contains areas with widespread hummock tracts, and their genesis has been studied for a long time. Using high resolution LiDAR-technique (Light Detection and Ranging), new observations and hypotheses for their genesis are possible. This thesis presents a provenance study of the material in hummocks and eskers in an area on the south Swedish highlands near Hörda. The hummock tract near Hörda is located in a valley, interpreted to be a tunnel valley, where hummocks in places are overlain by eskers. The aim is to investigate if the provenance of the material can help determine the genesis of the hummocks. In total, 1000 clasts were collected, half from five hummock sites and the other half from five esker sites. The lithology was determined, and the results from the sediment of each landform were tested statistically. The statistical test showed that there was a significant difference; however, most of the pebbles from both landforms seem to have been eroded from local sources. The samples also include many clasts with an interpreted provenance directly north of Hörda, with a maximum transport distance of 110km. These have most likely been transported by an earlier ice flow with a north-south direction and subsequently picked up by the latest glacier with the ice-flow direction from northeast and deposited in Hörda. Even though there was a significant difference between the eskers and hummocks, both are most likely derived from basal ice debris. This interpretation is strengthened by the geomorphology in the area, which suggests that the valley as well as the hummocks are formed by subglacial erosion. The eskers on top of the hummocks must have formed later, most likely as the ice retreated, but consist of the same material. Sammanfattning Den södra delen av Sverige består av områden med hummocks, varav deras uppkomst har studerats under en lång tid. Med hjälp av LiDAR-tekniken (Light Detecting and Ranging) och dess höga upplösning kommer nya förslag på deras bildningssätt upp. I denna rapport genomförs en proveniensstudie där materialet i hummocks och rullstensåsar undersöks. Området av intresse är beläget på det sydsvenska höglandet i närheten av Hörda. Området ligger i en dal, där vissa hummocks är överlagrade av rullstensåsar. Syftet är att se om materialets proveniens kan bestämmas och om det kan vara till hjälp att bestämma hummocksens bildningssätt. Totalt samlades 1000 stenar in, hälften från hummocks och den andra hälften från rullstensåsar. Proverna bergartsklassificerades och resultatet för varje landform testades statistiskt mot varandra för att se om det fanns någon skillnad. Det visade sig vara en signifikant skillnad, trots det verkade majoriteten av materialet från båda landformerna lokalt eroderade. De stenar som inte eroderats lokalt har sin proveniens norr om Hörda. Dessa anses ha transporterats från ett tidigare isflöde från norr, som sedan plockats upp av den senaste glaciären med isflödesriktningen från nordöst och slutligen avsatts i hummocks och rullstensåsar nära Hörda. Trots den signifikanta skillnaden mellan rullstensåsarna och hummocksen, är båda landformerna troligen avsatta av subglaciala mekanismer. Denna teori stärks av geomorfologin i området, som tyder på att dalen liksom hummocksen har formats av subglacial erosion. Rullstensåsarna ovanpå hummocksen måste ha avsatts senare, troligen under isens tillbakagång, men består av lika material. Keywords: hummock tract, esker, South Swedish Highlands, provenance, glacial geomorphology, LIDAR 2 Table of contents Introduction ............................................................................................................................................. 4 Background .............................................................................................................................................. 5 Regional bedrock geology ................................................................................................................... 5 Scandinavian geology ...................................................................................................................... 5 Sveconorwegian province ................................................................................................................ 5 Transcandinavian Igneous Belt ........................................................................................................ 6 Deglaciation of Småland ...................................................................................................................... 6 Landform genesis ................................................................................................................................ 6 Genesis of eskers ............................................................................................................................. 6 Genesis of hummocks ...................................................................................................................... 6 The study site: Hörda .............................................................................................................................. 7 Deglaciation of the study site .............................................................................................................. 7 Local bedrock geology ......................................................................................................................... 8 Local topography and geomorphology ............................................................................................... 8 Methodology ......................................................................................................................................... 10 Field ................................................................................................................................................... 10 Laboratory ......................................................................................................................................... 10 Statistics ............................................................................................................................................ 10 Results ................................................................................................................................................... 11 Rock type classification ..................................................................................................................... 11 Rock descriptions .............................................................................................................................. 13 Form analysis ..................................................................................................................................... 18 Discussion .............................................................................................................................................. 21 Provenance study .............................................................................................................................. 21 Form analysis ..................................................................................................................................... 24 Hummock formation ......................................................................................................................... 24 Conclusions ............................................................................................................................................ 25 Acknowledgements ............................................................................................................................... 25 Appendices ............................................................................................................................................ 28 forms that hummocks take. With its high Introduction resolution over the hummocky terrain, the precise shapes of hummocks are more The aim of this study is to provide insight into distinctly defined than before. An on-going how glacial hummocks are formed. The study project (Mark Johnson and Gustaf Peterson, area is in the southern part of Sweden and is University of Gothenburg and Swedish based on lithological provenance studies Geological Survey, SGU) is using LiDAR data to together with form analysis to help determine help re-interpret how the geomorphology and sediment and landform genesis. geology have developed. This Bachelor thesis Is it possible to interpret the provenance of will investigate if provenance studies are a the clasts in the till and, further, to get an useful tool for helping to interpret hummocky insight into how the hummocks were formed? deposit mechanisms, both for the current project but also for the general study of Hummocks are one of the most common hummocky deposits. glacial landforms, but their formation has been a topic of considerable discussion The study will separately examine clasts from (Johnson et al., 1995). The most common idea two different hummocky areas (here at Hörda, is that they