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Widespread Glacial Erosion on the Scandinavian Passive Margin Vivi K

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Widespread Glacial Erosion on the Scandinavian Passive Margin Vivi K https://doi.org/10.1130/G48836.1 Manuscript received 24 September 2020 Revised manuscript received 19 January 2021 Manuscript accepted 9 March 2021 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 12 May 2021 Widespread glacial erosion on the Scandinavian passive margin Vivi K. Pedersen1*, Åsne Rosseland Knutsen2, Gustav Pallisgaard-Olesen1, Jane Lund Andersen1, Robert Moucha3 and Ritske S. Huismans2 1 Department of Geoscience, Aarhus University, 8000 Aarhus, Denmark 2 Department of Earth Science, University of Bergen, N-5007 Bergen, Norway 3 Department of Earth and Environmental Sciences, Syracuse University, Syracuse, New York 13210, USA ABSTRACT erosion (Vshelf) allows us to assess the largely The topography in Scandinavia features enigmatic high-elevation low-relief plateau regions unknown onshore bedrock erosion volume be- dissected by deep valleys and fjords. These plateau regions have long been interpreted as yond glacial troughs, Vonshore: relict landforms of a preglacial origin, whereas recent studies suggest they have been modi- fied significantly by glacial and periglacial denudation. We used late Pliocene–Quaternary VVonshores= ed ()11––ϕϕsedfVVjord ––shelfs()helf , source-to-sink analyses to untangle this scientific conundrum. We compared glacier-derived (1) offshore sediment volumes with estimates of erosion in onshore valleys and fjords and on the inner shelf. Our results suggest that onshore valley and fjord erosion falls 61%–66% short where ϕsed and ϕshelf represent porositiesof gla- of the offshore sink volume. Erosion on the inner shelf cannot accommodate this mismatch, cier-derived offshore sediments and sediments implying that the entire Scandinavian landscape and adjacent shelf have experienced sig- eroded from the inner shelf, respectively. nificant glacial erosion. The offshore sediment volume (Fig. 1B) includes late Pliocene–Quaternary sediments INTRODUCTION cult. Estimates are influenced by extrapolation on the Norwegian margin (Naust Formation; The characteristic high-elevation low-relief of the subcrops onto onshore basement regions Rise et al., 2005), sediment volumes of a simi- plateau regions in Scandinavia have tradition- and any assumptions such extrapolations make lar age from the Norwegian part of the North ally been interpreted as remnants of a preglacial about past topography (Riis, 1996). Particu- Sea (Gołędowski et al., 2012), and the Quater- surface, with glacial erosion limited to over- larly, inner-shelf erosion cannot be assumed nary sediment package from the Danish region deepening of existing valleys (e.g., Lidmar- to be uniform in this narrow zone along the (Binzer et al., 1994; Nielsen et al., 2008). Off- Bergström et al., 2000; Japsen et al., 2018). coast, as it has recently been presumed (Hall shore regions north of Lofoten were excluded On the contrary, others have proposed that both et al., 2013). because the Scandinavian contribution is not valleys and plateau regions have been modified We quantify inner-shelf erosion with an ap- well constrained. On the Norwegian margin, by glacial and periglacial erosion, with pla- proach that is consistent with onshore-offshore the outer parts of the Storegga slides were teau formation in situ at high elevation (Nielsen profiles from the region and extends inner-shelf omitted, resulting in a negligible underestima- et al., 2009; Steer et al., 2012; Pedersen et al., erosion estimates to the whole margin. This al- tion of the sink volume. In the North Sea, we 2016; Egholm et al., 2017; Andersen et al., lows us to include inner-shelf erosion in quanti- included only the Norwegian and Danish sec- 2018a, 2018b). tative source-to-sink analyses. In our approach, tors, corresponding to the northeastern rim of Earlier work has shown that fjord incision we consider flexural isostatic effects related to the basin (Fig. 1B). This conservative estimate alone cannot account for the erosional volumes erosion and deposition as well as dynamic sur- is consistent with recent work on Quaternary deposited offshore during glacial times, indi- face changes from mantle convection (Fig. 1C; sediment infill from the Scandinavian region cating several hundred meters of erosion on Pedersen et al., 2016) that could result in expo- (Ottesen et al., 2018), with other regions being the plateau surfaces (Steer et al., 2012). How- sure and erosion of preexisting shelf sediments. dominated by infill from British river systems ever, others have since argued that this excess Quantifying the contribution from inner-shelf from the west and from the Baltic (Eridanos), material can be accounted for by erosion of erosion to the late Pliocene–Quaternary sedi- the Rhine-Meuse (Europe), and Thames (UK) Mesozoic and Cenozoic sediments on the in- ment budget allows us to infer onshore erosion river systems from the south (e.g., Gibbard and ner shelf (Hall et al., 2013) that are cropped by beyond the glacial troughs. Lewin, 2016; Lamb et al., 2018). the glacial sediments in a prominent angular We use the concept of geophysical relief unconformity of Middle Pleistocene age (e.g., METHODS (e.g., Small and Anderson, 1998) as a proxy for Sejrup et al., 1995). However, constraining the Our comparison of the late Pliocene–Quater- onshore erosion in valleys and fjords (Fig. 1B). volume ascribed to inner-shelf erosion is diffi- nary glacier-derived offshore sediment volume Calculations were based on a 1 × 1 km digi- (Vsed) with estimates of onshore fjord and valley tal elevation model (GEBCO Compilation *E-mail: [email protected] bedrock erosion (Vfjord) and inner-shelf sediment Group, 2019) using a previously calibrated CITATION: Pedersen, V.K., et al., 2021, Widespread glacial erosion on the Scandinavian passive margin: Geology, v. 49, p. 1004–1008, https://doi.org/10.1130/G48836.1 1004 www.gsapubs.org | Volume 49 | Number 8 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/8/1004/5362203/g48836.1.pdf by guest on 02 October 2021 ABC Figure 1. (A) Present-day topography and bathymetry of the Scandinavian region. (B) Offshore sediment thickness compared to estimated fjord and valley erosion from the onshore source region (see details in the text). (C) Dynamic surface changes due to mantle convection (30–0 Ma). Dashed lines in A and B mark base-Pliocene subcrop below Quaternary sediments, used to define the basinward extent of inner- shelf erosion (Figs. 2B and 2C). radius of 2 km (Steer et al., 2012). To match In order to assess erosion volumes on the in- son ratio of 0.25, and densities of 1029 kg/m3, the depositional sink, we excluded catchments ner shelf, we first reconstructed preliminary pre- 2300 kg/m3, 2670 kg/m3, and 3300 kg/m3 for in northern Norway that drain north toward the glacial topography and bathymetry (Fig. 2). We water, sediment, eroded bedrock, and mantle, re- Barents Sea, as well as regions east of the Bay removed the offshore late Pliocene–Quaternary spectively. Guided by studies of effective elastic of Bothnia, where the former Scandinavian Ice deposits and filled in onshore valleys and fjords thickness (Te) in Scandinavia (Pérez-Gussinyé Sheet drained east during glacial times, and using our erosion estimates from geophysical re- and Watts, 2005), we used constant Te values where the Baltic (Eridanos) river system drained lief. We calculated flexural isostatic adjustments between 10 and 25 km. Finally, we considered material to the southern and central North Sea from these load changes using the open-source estimates of dynamic topography from global Basin (e.g., Gibbard and Lewin, 2016; Lamb gFlex version 1.0 model (Wickert, 2016). We mantle convection models that suggest mod- et al., 2018; Ottesen et al., 2018). adopted a Young’s modulus of 70 GPa, Pois- est uplift in southwestern Norway in the late ACD B Figure 2. (A) Schematic section of the current margin architecture on the Scandinavian passive margin, with late Pliocene–Quaternary sedi- ments overlying older Cenozoic and Mesozoic sediments that subcrop an angular unconformity of Middle Pleistocene age. (B) Schematic section of the reconstructed margin, with late Pliocene–Quaternary sediments replaced onshore in valleys and fjords, and reconstructed inner-shelf wedge of eroded sediment (see details in text). Panels A and B are not to scale. (C) Map view of reconstructed shelf wedge thick- ness (effective elastic thickness Te = 15 km, paleo–sea level [PLS] = 150 m, with dynamic surface change). (D) Vertical bedrock motion from dynamic changes (Fig. 1C) and flexural isostatic adjustments owing to offshore deposition (Fig. 1B) and erosion (inner-shelf [C], valleys and fjords [Fig. 1B], and uniformly distributed onshore bedrock erosion volume, Vonshore, resulting in 152 m of uniformly distributed bedrock). Geological Society of America | GEOLOGY | Volume 49 | Number 8 | www.gsapubs.org 1005 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/8/1004/5362203/g48836.1.pdf by guest on 02 October 2021 Cenozoic, thereby exposing preexisting shelf Figure 3. Sediment budget: sediments to erosion (Fig. 1C; Pedersen et al., offshore sediment volume 2016). Because of the uncertainties associated (yellow), offshore matrix with the amplitude of these dynamic changes volume range (brown: 30% porosity, light brown: (Pedersen et al., 2016), we present results with 20% porosity), and fjord and without this component along with an al- and valley erosion (color- ternative scenario proposing additional dynamic coded based on region; changes associated with mantle flow. see inset) for the Scandi- navian passive margin. The Based on the preliminary reconstructed volume range of inner-shelf preglacial topography and bathymetry, we re- erosion is shown in pink constructed a shelf wedge of eroded sediment colors (range is detailed in (Fig. 2C). This wedge is defined by linear inter- the text), with lightest pink polation, with its outer position corresponding to representing inner-shelf erosion with an additional the isostatically corrected base Pliocene subcrop 200 m of dynamic change below the Middle Pleistocene angular unconfor- (Fig.
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