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In Situ-Dating of Saprolites in South East Sweden Using Rb/Sr by LA

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In Situ-Dating of Saprolites in South East Sweden Using Rb/Sr by LA UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre In situ-dating of saprolites in south east Sweden using Rb/Sr by LA-ICP-MS Ellinor Wessel ISSN 1400-3821 B1047 Bachelor of Science thesis Göteborg 2019 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 Precambrian basement of the South Sweden constitute of three main erosion surfaces – The sub-Cambrian peneplain, the sub-Cretaceous peneplain and the South Småland peneplain, formed by episodes of burial and re-exposure of the basement during the Phanerozoicum exposing it to weathering at specific times. Episodes of deep weathering, together with slight Quarternary glacial and glaciofluvial reshaping are the major agents that has formed the relief that is today. Saprolites are remnants of these weathering-episodes and by exploring them we can obtain a better understanding of the time and environment they were formed under. In south of Sweden several sites of deep weathering have been observed and several of these are of gravelly saprolites. These gravelly saprolites have never been dated by radiometric methods but are only stratigraphically interpreted as Plio-Pleistocene of age. From two sites of these gravelly saprolites, Knasekärret and Duvedal, authigenic illite are dated by the Rb/Sr-system in this study in order to see if the stratigraphic constraints of the weathering event can be affirmed. The Rb/Sr-dating is preformed using Laser Ablation (LA)- ICP-MS, a method never before used on weathering material. Illite from the kaolinitic saprolite at Ivö, previously dated by K/Ar, are also dated using the same technique with the aim to confirm LA-ICP-MS as a usable method to date palaeosoils and to strengthen or question the certainty of previously obtained ages. From the gravelly saprolites at Duvedal and Knasekärret large spread of ages with a mean of Neoproterozoic ages were obtained suggesting an illite formation not connected to the episode of deep weathering. The results from Ivö correlate with previous studies and give a strong indication that LA-ICP-MS is a valid method to date palaeosoils and the saprolite being of Mid-Triassic age. Content 1. Introduction – The South Swedish Dome ....................................................................................... 1 1.1 Deep weathering and palaeosoils ............................................................................................ 4 1.2 Illite dating .............................................................................................................................. 6 1.3 Aim of the study ...................................................................................................................... 6 1.4 Sites ......................................................................................................................................... 7 1.4.1 Knasekärret ...................................................................................................................... 7 1.4.2 Duvedal ........................................................................................................................... 8 1.4.3 Ivö .................................................................................................................................... 8 2. Methods ........................................................................................................................................... 9 2.1 Scanning electron microscope ............................................................................................... 10 2.2 LA-ICP-MS ........................................................................................................................... 10 2.2.1 Calibration standards ..................................................................................................... 10 3. Results ........................................................................................................................................... 12 3.1 Knasekärret ............................................................................................................................ 12 3.1.1 Rb/Sr-dating .................................................................................................................. 14 3.2 Duvedal ................................................................................................................................. 15 3.2.1 Rb/Sr-dating .................................................................................................................. 17 3.3 Ivö .......................................................................................................................................... 19 3.3.1 Rb/Sr-dating .................................................................................................................. 21 4. Discussion ..................................................................................................................................... 24 4.1 Gravelly saprolites ................................................................................................................. 24 4.2 Ivö saprolite ........................................................................................................................... 25 4.3 Rb/Sr-dating by LA-ICP-MS ................................................................................................ 25 5. Conclusions ................................................................................................................................... 26 Acknowledgements ............................................................................................................................... 27 References ............................................................................................................................................. 28 1. Introduction – The South Swedish Dome The Precambrian basement in south Sweden constitute of a low dome, the South Swedish Dome which consists mainly of granites and gneisses. The dome is characterised by its erosion surfaces, or, peneplains (Lidmar-Bergström, et al., 1997), formed by episodes of burial and denudation during the Phanerozoic. The three major peneplains are the inclined re- exposed sub-Cambrian peneplain, the re-exposed hilly sub-Cretaceous peneplain (or etch surface) and the epigene South Småland peneplain, a horizontal plain with residual hills (fig. 1; Lidmar-Bergström, 1996; Lidmar-Bergström et al., 2017). fig 1: South Swedish Dome with its peneplains presented. The sub-Cambrian peneplain (blue), the sub-Cretaceaous peneplain (green) and the South Småland stepped peneplain (yellow, orange, brown and red). (Lidmar-Bergström et al., 2017) 1 The formation of the very flat sub-Cambrian peneplain, situated over large parts of Sweden, Norway and Finland, began sometime during the Neoproterozoic. Prior to the Cambrian transgression the peneplain was fully developed (fig. 2a) and was covered by early Palaeozoic sediments (fig. 2b) (Lidmar-Bergström 1996; Japsen et al., 2015). The Sub- Cambrian peneplain is well preserved in the eastern part of the South Swedish Dome (fig. 1b and fig 2h) and is inclined to the east. The eastern border of it is overlapped by Palaeozoic sedimentary rocks with a prominent residual hill of the sub-Cambrian peneplain, the Jungfrun island, protruding through the cover (Lidmar-Bergström, et al., 2017). Some parts of the peneplain has not been re-exposed until recent geological times while other parts of the surface were exposed from cover rocks during Late-Palaeozoic - Mesozoic time and were, as a result of the warm and humid climate during the period, subject to extensive deep- weathering (Lidmar-Bergström, 1982). One episode during Late-Perm – Late-Triassic (fig. 2c), forming the smaller Sub-Triassic peneplain and another episode of even heavier weathering during Late-Jurassic to mid Cretaceous (fig. 2e; Japsen et al., 2015). Deep weathering exploited fractures of the basement and subsequent erosion resulted in the hilly relief forming the re-exposed Sub-Cretaceous peneplain. The hilly relief along the west coast are interpreted as the re-exposed sub-Cretaceous peneplain and constitutes both the undulating hilly relief in Halland and the joint aligned valley landscape in Bohuslän, with peaks reaching close to the Sub-Cambrian peneplain (Lidmar-Bergström et. al. 2017). In connection with the Late-Cretaceous transgression the basement was buried by Late-Cretaceous-Paleogene sediments (Japsen et al., 2015). These sediments were eroded due to Miocene uplift leading to re-exposure and denudation of the basement (fig. 2g) assembling another erosion surface, the South Småland peneplain, a stepped erosion sequence, which is truncating both the hilly Sub-Cretaceous peneplain and the Sub-Cambrian peneplain (fig. 2h) (Japsen et al., 2015 and Lidmar-Bergström, et al. 1999). 2 fig 2: From Japsen et al., 2015 Based on AFTA (Apatite fission track analysis) A) Denudation of the basement during Late Neoproterozoic forming the sub-Cambrian peneplain.B) Palaeozoic sedimentary rocks deposited on the sub-Cambrian peneplain during Middle Triassics. C) Exhumation and destruction of the sub-Cambrian peneplain as a result of Middle Triassic uplift and erosion of Palaeozoic cover rocks. Warm and humid climate in Late Perm - Late Triassics resulted in formation of kaolinitic clays. D) The Triassic
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