Base Naust S (S1)

The base of Naust S (~0.4 Ma) represent the URU for most of the study area, as it indicate a change in depositional environment and truncates the underlying prograding clinoform. The surfaces within the 3D-surveys represent the paleo-shelf where geomorphological features such as paleo-pockmarks, iceberg plough-marks, flutes and MSGL have been identified. Two large cross-shelf paleo-troughs that merge towards the west are also located within the ST10013 3D-survey. The URU itself is a clear indicator that the ice sheet was present on the paleo-shelf during base Naust S time, eroding large quantities of sediment. It marks a change in depositional geometry from progradational to aggradational within the study area (Fig. 5.1.3). The S sequence has been correlated to the Elsterian glaciation (marine isotope stage 10-8) which Rise et al. (2006) described as a very thick and erosive ice sheet, transporting eroded sediment from the paleo-shelf, depositing it on the paleo-shelf edge and paleo-slope.

Paleo-troughs, flutes and MSGL

The paleo-troughs indicate two drainage pathways for fast-flowing ice streams trough the study area. The ice streams have advanced from two different directions, one from the NE and the other from the SE, merging into one stream after what is now considered to be the Sklinnabanken area. This combined with the flutes and MSGL in the area is a good indication of the paleo-ice flow, which appears to originate from Vestfjorden in the NE before gradually getting a more eastward orientation towards the south. At the same time, ice was also draining to the same area form the SE resulting in a large depocenter where Sklinnadjupet descent down to the Vøring Plateau today (Skjoldryggen area) (Fig.5.1.5b). It was in this area the Sklinnadjupet slide was triggered during Naust S time. This has been interpreted to be a result of the high deposition rate to the area during the Elsterian glaciation, leading to large excess pore pressure in the sediments (Rise et al., 2006). The findings of this study also correlates well with the suggestions of Dowdeswell et al. (2006) which have found MSGL with the same orientation. These lineations were found on a paleo-surface, which originated from the same time and area, also concluding with a drainage pattern from Vestfjorden.

Iceberg plough-marks

The iceberg plough-marks in the area generally have a south to north orientation on the S1 surface. Here as on the previous surfaces the direction is interpreted as an indicator of paleo-ocean currents. The direction is also here correlated to the Norwegian Atlantic Current moving along the Norwegian margin (Bryn et al., 2005; Laberg et al., 2005b). As the plough-marks truncate other geomorphological features such as MSGL they have probably originated from calving icebergs drifting from the retreating glaciers.

5.1.5 Base Naust T (T1)

The base of Naust T (~0.2 Ma) represents the start of Naust T time, consisting of the Saalian and Weichselian glaciations (marine isotope stage 6 and 2), which were the two last glacial cycles on the mid-Norwegian continental shelf (Dahlgren et al., 2002b; Rise et al., 2005; Rise et al., 2010). On the interpreted surfaces, large erosional features have been located along with iceberg plough-marks, pockmarks, small moraines, lateral shear zone moraines, flutes and MSGL. Also on the base of Naust T, two large cross-shelf paleo-trough appears to merge within the ST11013 3D-survey.

Paleo-troughs, flutes and MSGL

The cross-shelf paleo-troughs that indicate the drainage pattern of the fast-flowing ice streams of the time have changed direction from the underlying S1 surface. The two troughs have become more parallel with an E-SE to W-NW orientation perpendicular to the coastline. This change indicates that the ice flow within Sklinnadjupet was mainly dominated by ice from the east during Naust T time, while the influence of the southwards flow from Vestfjorden had decreased (Fig. 5.3.4c). This has also been discussed by Dowdeswell et al. (2006) who suggested a major shift in the large-scale glacial patterns between the Saalian and Weichselian glaciations. This was a result of thick sediments sequences being deposited on the shelf during the Elsterian and Saalian glaciation, building up what is now known as Trænabanken. When the Weichselian glaciation started, the easiest pathway for the Vestfjorden ice flow to take was no longer over the Trænabanken area, leading to the development of Trænadjupet. However, there is also a possibility that the change in ice flow was a result of internal changes in ice sheet dynamics (Dowdeswell et al., 2006). The change in the paleo-troughs as well as the direction of the MSGL and flutes in the study area support this change in ice-flow during Naust T time.

However, there are no direct signs of fast-flowing ice streams going directly over Trænabanken on the T1 surface, only glacitectonic features and moraines (discussed below).

Glacitectonic features

Glacitectonic features such as hill-hole pairs are located on the shallowest part of the T1 surface of the study area. They indicate an ice-flow from the north, which gradually appears to bend towards the west. The area is interpreted to represent the early stages of Trænabanken, which was built up during the Elsterian and Saalian glaciation (Dowdeswell et al., 2006). The buildup resulted in the stagnation of the ice-flow on Trænabanken during Naust T time and Ottesen et al. (2005b) suggested a passive ice dome with an ice divide trending NW–SE across the middle of Trænabanken during the last glaciation. As the ice streams stagnate, the base may freeze to the underlying sediments, resulting in the glacitectonic activity observed on the bank (Nielsen et al., 2005; Ottesen et al., 2005a). Ottesen et al. (2005a) discussed that the slow moving ice may be a result of a deglacial situation where thinning of the ice lead to the decrease in flow velocity and therefore basal freezing. It is also distinguished that the orientation of such features can be a good indication of former ice flows. If this were the case in the study area, the hill-hole pairs would correlate well with the deglaciation of the Saalian, both in time (Naust T time) and direction (ice-flow towards the SW).

Moraines

There are two different type of moraines located on the T1 surfaces, small moraines and lateral shear zone moraines. The small moraines are orientated transverse to the ice-flow direction indicated by the glacial lineations and glacitectonic features discussed above. These features resembles De Geer moraines and the spacing between them likely represent standstills or small charges of the ice front, possibly affected by seasonal changes during deglaciation as discussed based on studies of other parts of the Norwegian continental shelf (Solheim et al., 1990;

Rydningen et al., 2013). The lateral shear zone moraines are located along depth changes on the surfaces indicating where fast-flowing ice streams borders to more stagnant ice. The moraines are therefore a good indication, both of the movement of the former glacial front, as well as the borders of ice streams with higher velocity within the study area.

Iceberg plough-marks

The iceberg plough-marks have a main orientation from SW towards NE. This is a good indication of the paleo-ocean currents moving almost parallel to the Norwegian continental margin. The plough-marks are probably relatively young features, as they truncate older features, created by icebergs calving during the deglaciation.