Glacial erosion in the Öregrund archipelago: Potential for headward erosion towards Forsmark in future glaciations?

Adrian Hall, Maarten Krabbendam, Mikis van Boeckel

Research output: Book/ReportCommissioned report

Abstract / Description of output

Glacial erosion in the Öregrund archipelago has led to the formation of rock trenches, valleys and basins, now submerged between the low-lying islands. The depths of these topographic depressions are 10–50 m, greater than in areas to the West and South. This study explores the likely reasons for enhanced glacial erosion in the Öregrund archipelago and the potential for future extension of
topographic depressions ENE towards Forsmark.

The present basement topography of Uppland is inherited from the sub-Cambrian basement unconformity. Previous work has reconstructed its form in summit envelope surfaces as a smooth, block faulted surface. The elevations of the floors of bedrock depressions represent depths of glacial erosion below
the re-exposed unconformity surface found on neighbouring, upstanding basement fault blocks. In the Öregrund archipelago, the presence of Proterozoic sandstones and Ordovician limestones in downfaulted positions indicates that most glacial erosion was a result of erosion in these relatively soft and
closely fractured sedimentary rocks. In the basement gneisses, glacial erosion was generally low across the fault block tops and mainly focussed along the regional deformation zones and faults.

Previous analyses of fracture lengths and spacings at Forsmark and in its surroundings indicate that fractures in the gneissic basement conform to general power-law relationships. Major regional brittle deformation zones have lengths of 10s of km, spacings of 20 to 40 km but widths which do not normally exceed ~100 m but which vary along strike. Together with regional faults, these major fracture zones control the locations of long rock trenches which delineate the edges of exhumed fault blocks. Many individual trenches across scales remain segmented by rock thresholds. Comparisons of fracture and
trench network topologies show relatively poor connectivity of trench networks, with many isolated trenches developed on short fractures on fault block tops.
Previous studies show that in situ joint-bound rock block sizes in fracture zones are < 0.1–0.5 m. As in other crystalline rocks, in situ joint-bound rock-block size can be regarded as the dominant control on patterns and rates of glacial erosion in lowland gneissic terrain beneath the Fennoscandian Ice Sheet.

The faulted surface of the sub-Cambrian unconformity provides a reference surface for estimating depths of denudation, including glacial erosion. The onset of deep glacial erosion around the Baltic Sea basin was at 1.2 Ma. Assuming a similar timing for the Öregrund archipelago, rates of glacial erosion for past 100 ka glacial cycles can be derived. Minimum rates of erosion in sedimentary cover
were 3–4 m/100 ka. Average erosion rates in basement were lower at 1.8–2.0 m/100 ka. Terrestrial cosmogenic nuclide inventories at Forsmark indicate erosion rates for rock surfaces on gneiss bedrock hills of 1.6–3.5 m/100 ka, similar to estimates of average erosion based on geomorphological criteria.
Except for the rim of the Åland Deep, rates of trench deepening were 0.8–4.2 m/100 ka and rates of trench widening were 13–71 m/100 ka. Thalweg steepening remains largely confined to incision of the western rim of the Åland Deep; in the eastern Öregrund archipelago, headward erosion operated
at 0.4–1.2 km/100 ka.

Erosion in past glacial cycles at Forsmark involved downwearing of fault-block tops, backwearing of fault-block edges and incision of trenches and basins along fracture zones (involving both deepening and widening of the trenches); the pattern of erosion in future glacial cycles will likely be similar. Rates of downwearing of rock surfaces during future glaciations are estimated to < 1 m/100 ka based on modelling of cosmogenic nuclides and of future climate/glacial periods. In the past, fault scarps on block edges elsewhere in Uppland have retreated at 42–125 m/100 ka; future backwearing is unlikely
to extend far behind existing fault block edges over the next 100 ka. The Singö, Eckarfjärden and Forsmark deformation zones carry relatively shallow and segmented trenches at Forsmark when compared to their continuations in the Öregrund archipelago. Block removal by future glacial erosion is likely to excavate rock trenches within the main deformation zones at Forsmark. The limited widening of trenches beyond underlying fracture zones during the Pleistocene indicates that future trench erosion will remain largely confined within the deformation zones that delineate the Forsmark tectonic lens.
Future trench development can be expected to extend to depths of 10–50 m and widths of < 1 km over the next 1 Ma under assumptions of a similar future total duration of glacial conditions. Thus, headward erosion of rock trenches from the Öregrund archipelago would require around 1 Ma to reach Forsmark.
Recent modelling suggests, however, that glacial cycles over the coming several 100 ka will be short. Hence, the timescales for headward erosion under this scenario would extend beyond 1 Ma.
Original languageEnglish
PublisherSvensk Kärnbränslehantering AB
Number of pages80
VolumeSKB TR-22-08
Publication statusPublished - 28 Jan 2023


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