Neotectonic faulting and the Late Weichselian shoreline gradients in SW Norway

Abstract

The Younger Dryas (YD) maximum highstand shoreline in SW Norway has traditionally been considered as being slightly concave, gradually steepening in the direction of uplift. This phenomenon is attributed to geoidal and isostatic effects near the former ice-sheet margin. On the basis of isolation basin data from the region, we have reconstructed this shoreline, and a Bølling-Allerød (B-A) lowstand shoreline, along three profiles in SW Norway. Along all profiles there are shore levels which, within the error limits estimated, cannot be captured by a single straight (or curved) shoreline. The anomalous shore levels occur near major fault zones and are interpreted to reflect differential uplift rates on opposite sides of faults, superimposed on the general glacio-isostatic tilting of the region. The inferred faulting is consistent with observations previously reported as neotectonic 'claims' in the region and shed new light on the deformational structures observed in seismic profiles of the fjord sediments. Excluding the anomalous shore levels, a straight shoreline with gradient ca. 1.1 m/km provides the best and most consistent representation of the YD shore levels along the three profiles. The B-A lowstand shoreline is constrained by fewer data points, but is approximately parallel-dipping the highstand shoreline. Our reconstructions imply a less steep YD maximum highstand shoreline compared to previous reconstructions, where gradients up to 1.4 m/km have been inferred. This may imply that the ice load effect on the lithosphere in SW Norway during the YD is less than previously assumed.

Evidence of early deglaciation (18 000 cal a bp) and a postglacial relative sea‐level curve from southern Karmøy, south‐west Norway

“…for location) with isobases for the Younger Dryas (YD) chronozone, adjusted from Helle et al . () (see Supplementary Information for details on the isobase model). Published exposure dates are shown in blue frames as the mean age and standard deviation (in ka) calculated from a given number ( n ) of dates (Svendsen et al , ; Gump et al , ).…”

“…For details, see Paus () (Liastemmen), Paus () (Ø. Kvilhaugmyra) and Helle et al . () (northern Karmøy sites). Altitudes of marine limits dated to the YD are shown in orange, and the reference point used for the Fosen RSL curve in Midtbø () is indicated in black.…”

“…However, the cosmogenic exposure age estimates are significantly older than those obtained from radiocarbon dating of adjacent marine records (~18.5k cal a bp ) (Sejrup et al , , ; Morén et al , ), and the timing of deglaciation is therefore poorly constrained in outer Boknafjorden. Most of the fjord was ice‐free well before the Younger Dryas (YD) period (Gump et al , ), but the marine limits in the inner and middle parts of the fjord are generally younger, coinciding with the culmination of a~10‐m transgression in the YD (Helle et al , ). In the autumn of 2014, we discovered marine, foraminifera‐bearing sediments in the bog Kringlemyr, located on the island of Karmøy (Fig.…”

“…) at an altitude of about 12 m above present sea level. This is well above the height of the YD transgression predicted by the isobase model of Helle et al (), and suggests that the marine limit on southern Karmøy is potentially much older than previously thought. In the present study we therefore address the following main research questions: When did the outer part of Boknafjorden become ice‐free? What is the age and altitude of the marine limit on southern Karmøy? How has RSL varied in this area following deglaciation? Is it possible to disentangle the different components contributing to the reconstructed near‐field RSL signal?…”

“…Here, we use a combination of lithostratigraphic evidence, macrofossils, geochemistry and microscopic phytoplankton (Kaland, ; Vasskog et al , ) to identify isolation contacts in two isolation basins called Grødheimsvatnet (vatnet = lake) and Kringlemyr (myr = bog). This provides two new sea‐level index points (SLIPs) for the Lateglacial period on southern Karmøy, which we combine with previous data from several other sites in the outer Boknafjorden region (Anundsen, ; Helle et al , ; Midtbø, ) to construct a complete postglacial RSL curve. The curve is constructed using a statistical procedure that takes into account uncertainties in both age and altitude for the SLIPs and allows us to extract confidence intervals for the reconstruction.…”