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Link to original content: https://pubmed.ncbi.nlm.nih.gov/28452366
The inheritance of a Mesozoic landscape in western Scandinavia - PubMed Skip to main page content
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. 2017 Apr 28:8:14879.
doi: 10.1038/ncomms14879.

The inheritance of a Mesozoic landscape in western Scandinavia

Ola Fredin  1   2 , Giulio Viola  1   3 , Horst Zwingmann  4 , Ronald Sørlie  5 , Marco Brönner  1   6 , Jan-Erik Lie  5 , Else Margrethe Grandal  5 , Axel Müller  7   8 , Annina Margreth  1 , Christoph Vogt  9 , Jochen Knies  1   10
Affiliations

The inheritance of a Mesozoic landscape in western Scandinavia

Ola Fredin et al. Nat Commun. .

Abstract

In-situ weathered bedrock, saprolite, is locally found in Scandinavia, where it is commonly thought to represent pre-Pleistocene weathering possibly associated with landscape formation. The age of weathering, however, remains loosely constrained, which has an impact on existing geological and landscape evolution models and morphotectonic correlations. Here we provide new geochronological evidence that some of the low-altitude basement landforms on- and offshore southwestern Scandinavia are a rejuvenated geomorphological relic from Mesozoic times. K-Ar dating of authigenic, syn-weathering illite from saprolitic remnants constrains original basement exposure in the Late Triassic (221.3±7.0-206.2±4.2 Ma) through deep weathering in a warm climate and subsequent partial mobilization of the saprolitic mantle into the overlying sediment cascade system. The data support the bulk geomorphological development of west Scandinavia coastal basement rocks during the Mesozoic and later, long-lasting relative tectonic stability. Pleistocene glaciations played an additional geomorphological role, selectively stripping the landscape from the Mesozoic overburden and carving glacial landforms down to Plio-Pleistocene times. Saprolite K-Ar dating offers unprecedented possibilities to study past weathering and landscape evolution processes.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Deep weathering and saprolitization in crystalline basement in Scandinavia.
(a) Map of known saprolite locations and sample sites (Utsira High, Bømlo and Ivö). Remnants of clay rich (fuchsia circles) and grussy (green squares) saprolites in Scandinavia are shown together with near coast Jurassic basins (blue hexagons). The black line shows the Weichselian last glacial maximum (LGM) position in formerly glaciated Scandinavia86. The fuchsia line stretching from the Shetland Platform, across Utsira High, Bømlo to south central Norway refers to the location of the profile in Fig. 6. (b) Seismic image showing the fractured and etched top crystalline basement of the Utsira High beneath almost 2 km of Mesozoic and Cenozoic sedimentary cover. (c) LiDAR-derived hillshade map of the Bømlo strandflat in western Norway, exhibiting fractured and weathered crystalline rocks. (d) LiDAR-derived hillshade map of Ivö area showing fractured and weathered crystalline rocks protruding through Cretaceous (orange) and Pleistocene (green) cover.
Figure 2
Figure 2. Characterization of deeply weathered basement in southern Sweden.
(a) Simplifed log and photograph of the sampled saprolite at the Ivö site. (b) SEM image from saprolite thin section showing the main clay mineralogy and weathered biotite (Ill, illite; Sm, smectite; Bt, biotite; Qz, quartz; Kao, kaolin). (c) SEM image from saprolite thin section showing illite-smectite mixed-layer clay forming at the expense of K-feldspar (K-fsp).
Figure 3
Figure 3. Characterization of deeply weathered basement on Utsira High.
(a) Simplified log and sample location within well core 16/3-4. (b) SEM image of illite from sample 16/3-4. (c) Transmission electron microscopy image of illite crystals from sample 16/3-4. (d) SEM-CL image of quartz from sample 16/3-4. (e) SEM-CL image of quartz from overlying Draupne Fm. sandstone. The images in d, e are very similar and suggest that the Draupne sandstone was derived locally from weathered basement.
Figure 4
Figure 4. Characterization of deeply weathered basement on Utsira High.
(a) Simplified log and transition from weathered basement to Åsgard Fm. in well core 16/1-15. (b) Sample 16/1-15 at 1927.5 m b.sl. in well core 16/1-15.
Figure 5
Figure 5. Exposed deeply weathered basement on Bomlø in southwestern Norway.
(a) Sample site on Bømlo. (b) Saprolite and core boulders at sample site, samples Bømlo 3 and 4 are from central portion of the outcrop. (c) Detailed view of leftmost part of the outcrop. Sample 2 was taken close to the fresh granodiorite to the left. Spatula is 200 mm long. (d) Panorama from mount Siggjo (474 m a.sl.) looking north towards the strandflat landscape of Bømlo.
Figure 6
Figure 6. K-Ar geochronology of deeply weathered basement in southwestern Scandinavia.
(a) Illite K-Ar age (Ma) versus grain size (μm) spectra for the analyzed samples from Ivö, Bomlø and Utsira High. Only the finest grain size fractions are interpreted as representing authigenic illite and are used to infer the age of saprolitization. Error bars are ±2σ. (b) Schematic geological profile along an E–W cross-section across southern Norway and part of the Norwegian Sea (see Fig. 1 for profile location) with the new apparent saprolitization ages. We suggest that the deeply weathered landscapes at Utsira High and Bømlo both formed in the Late Triassic.
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