Fjords are characteristic features of the western and northern coasts of Spitsbergen. They formed during repeated glacial cycles when grounded glaciers drained large ice sheets occupying the Svalbard–Barents Sea region. Since grounded glaciers often erode and remove pre-existing deposits from fjord basins, depositional sequences in fjords are typically limited to the last glacial–interglacial cycle (e.g. Forwick & Vorren 2010). Holocene sediment drapes in Spitsbergen fjords are not usually thick enough to obscure glacial landforms and deposits from the last full-glacial and deglacial period. Glacier advances related to climatic cooling during the late Holocene, together with glacier surges, have led to the formation of complex landform assemblages close to many fjord heads (e.g. Ottesen & Dowdeswell 2006; Ottesen et al. 2008; Streuff et al. 2015).
Description
The floors in the central and outer parts of several Spitsbergen fjords, including Kongsfjorden, Isfjorden, Billefjorden, Sassenfjorden and Tempelfjorden, reveal multiple streamlined sedimentary lineations orientated sub-parallel to fjord axes (Figs 1a, b, c, 2a; e.g. Ottesen et al. 2005; Baeten et al. 2010; Forwick et al. 2010). The ridges are≤5 km long, 15 m high and<500 m wide, with a spacing of up to 800 m; length-to-width ratios exceed 10:1. Elongate landforms with length-to-width ratios smaller than 10:1 are also found in Kongsfjorden and Raudfjorden (e.g. Ottesen & Dowdeswell 2009; MacLachlan et al. 2010).
Examples of glacial landforms and sediments in Spitsbergen fjords. (a) Location of study areas (red boxes; map from IBCAO v. 3.0). Red boxes show locations of (b)–(d) and Figure 2. (b) Sun-illuminated multibeam-bathymetric image from Billefjorden (after Baeten et al. 2010). Acquisition system Kongsberg EM300. Frequency 30–34 kHz. Grid-cell size 5 m. (c) Sun-illuminated multibeam-bathymetric image from Kongsfjorden and southern Krossfjorden (courtesy of Boele Kuipers, Norwegian Hydrographic Service; for details see MacLachlan et al. 2010 and Streuff et al. 2015). Acquisition systems Kongsberg EM 1002 and 3002. Frequencies 95 and 293–307 kHz, respectively. Grid-cell size 5 m. (d) Sparker profile across a submarine ice-contact fan in inner Isfjorden (left) and interpretation (right; after Forwick & Vorren 2010). VE×16. Acquisition system Bennex 700 J multi-electrode sparker.
Examples of glacial landforms and sediments in Spitsbergen fjords. (a) Sun-illuminated multibeam-bathymetric image from Sassenfjorden and Tempelfjorden in Isfjorden (Fig. 1a) (after Forwick et al. 2010). Acquisition system Kongsberg EM300. Frequency 30–34 kHz. Grid-cell size 5 m. (b) Sparker profile across a thrust moraine and surge lobes in inner Tempelfjorden (after Forwick et al. 2010). VE×10. Acquisition system Bennex 700 J multi-electrode sparker. Schematic models for glacial landform assemblages produced during (c) full-glacial conditions, (d) regional deglaciation and (e) the late Holocene. (f) Photograph from inner Tempelfjorden taken on 31 July 2005 showing the terminal moraine marking the surge-maximum position of Von Postbreen and the post-surge 2005 position of Tunabreen.
Straight to sinuous ridges with single or multiple crests up to 30 m high, 7 km long and 200 m wide are observed in van Keulenfjorden, Sassenfjorden–Tempelfjorden and near the mouth of Wijdefjorden (Fig. 2a; Forwick et al. 2010; Kempf et al. 2013). The ridge in Wijdefjorden is part of a system with braided to meandering incisions. In Kongsfjorden, low-sinuosity to straight incisions orientated sub-parallel to the fjord axis occur (Fig. 1c; MacLachlan et al. 2010).
Transverse sedimentary ridges of various sizes are found across several Spitsbergen fjords (Sexton et al. 1992; Ottesen & Dowdeswell 2009; Forwick & Vorren 2010). The largest features in the Isfjorden area are over 1 km long and 50 m high. High-resolution seismic data reveal frequent occurrences of thrust planes within these ridges (Forwick & Vorren 2010). However, a more complex reflection pattern including a chaotic internal signature (Facies 1), continuous to discontinuous undulating and stratified reflections (Facies 2), as well as mounded internal reflections with subvertical displacement planes (Facies 3) characterizes one deposit in inner Isfjorden (Fig. 1d). Most of these ridges are deposited above bedrock or basal till and they often interfinger with proximal glacimarine deposits, indicating that they were formed during the last glacial period. One lobate feature, more than 3 km long, 1.5 km wide and up to 13 m high, occurs close to the head of Tempelfjorden (Fig. 2a, b). This feature was deposited on basal till and contains multiple thrust planes.
Multiple single or stacked sediment wedges/lobes occur in the central and outer areas of several fjords. They are visible as depositional features on the seafloor, for example in Wijdefjorden, and may occur as infill serving to smooth irregularities of the pre-existing seafloor or as intercalations with proximal glacimarine sediments, for example in Isfjorden (Forwick & Vorren 2010). These wedges/lobes are up to 4.5 km long and up to 25 m thick, and may exceed 2 km in width.
Small transverse ridges are much more abundant in central and inner fjords than larger ridges. They are typically between 100 and 150 m wide and≤5 m high (Figs 1b, 2a) (Ottesen & Dowdeswell 2009; Baeten et al. 2010; Kempf et al. 2013). Crest spacing is c. 170 m in parts of Billefjorden (Baeten et al. 2010), whereas spacing increases from c. 80 m in outer Van Keulenfjorden to c. 190 m mid-fjord (Kempf et al. 2013).
Characteristic landform assemblages are found in many fjords with tidewater glaciers at their heads (Plassen et al. 2004; Ottesen & Dowdeswell 2006; Ottesen et al. 2008; Flink et al. 2015; Streuff et al. 2015). These include: fjord-parallel streamlined landforms; sinuous, fjord-parallel ridges; major transverse ridges; sediment lobes on the distal sides of the major transverse ridges; and small, concordant and sub-parallel as well as irregularly orientated (rhombohedral) ridges (Figs 1b, c, 2a).
Interpretation
Fjord-parallel streamlined sedimentary landforms with length-to-width ratios exceeding 10:1, found in the central and outer parts of most fjords, are interpreted as mega-scale glacial lineations (MSGLs), formed through the deformation of soft sediments at the base of fast-flowing glaciers that occupied Spitsbergen fjords during the last full-glacial period (Figs 1b, c, 2a) (Ottesen et al. 2005). Elongate features with length-to-width ratios <10:1, especially those that have bedrock cores at their upstream ends, are suggested to be crag-and-tails and drumlins (Fig. 1c) (Ottesen & Dowdeswell 2009; MacLachlan et al. 2010). These streamlined linear landforms provide evidence of fast-flowing, grounded ice draining the Svalbard–Barents Sea Ice Sheet through many fjords on Spitsbergen during the last glacial cycle (Ottesen et al. 2005). However, the absence of such landforms, for example in Smeerenburgfjorden in NW Spitsbergen, indicates that inter-ice stream areas also existed (Ottesen & Dowdeswell 2009).
Straight to sinuous ridges are interpreted as eskers (Fig. 2a), formed in englacial and subglacial conduits after the termination of fast ice flow (Forwick et al. 2010; Kempf et al. 2013). Incisions related to these eskers (e.g. Wijdefjorden) or separate incisions (e.g. in Kongsfjorden; Fig. 1c) (MacLachlan et al. 2010) are interpreted as former subglacial meltwater channels. This suggests lateral variations in erosion and deposition, reflecting differing hydraulic regimes beneath grounded glacier ice.
Large transverse sedimentary ridges are suggested to be glacier-terminus deposits reflecting significant readvances and/or longer halts in retreat during the last deglaciation (Forwick & Vorren 2010; Forwick et al. 2010). Deposits containing thrust planes are push or thrust moraines formed during glacier advances when significant amounts of sediment were eroded and pushed beyond the ice front (Fig. 2b). However, the landform with a complex internal reflection pattern in inner Isfjorden (Fig. 1d) is interpreted as a submarine ice-contact fan indicating: sediment deformation during a glacier advance (Facies 1); a stillstand and aggradation of foreset beds (Facies 2); and sediment failure after the retreat of the ice (Facies 3; Forwick & Vorren 2010).
Sediment wedges/lobes visible as (positive) depositional features, as infill of the pre-existing seafloor or as intercalations within proximal glacimarine sediments (in Isfjorden) are suggested to be glacigenic debris wedges/lobes that have formed: (1) at the fronts of readvancing and/or stagnating glaciers where reworked proglacial and/or subglacially derived sediments failed; (2) in areas of high sediment accumulation where rapid supply and/or seismic activity related to isostatic uplift led to failure; or (3) as cavity infill beneath grounded ice (e.g. Forwick & Vorren 2010).
The relatively small and abundant transverse ridges in several fjords have been interpreted as recessional moraines; that is, moraines that have formed during minor halts and/or readvances of tidewater glaciers in a period of regional deglaciation (e.g. Ottesen & Dowdeswell 2009; Baeten et al. 2010; Kempf et al. 2013). Establishing reliable chronologies for the formation of such moraines remains challenging. However, Ottesen & Dowdeswell (2006) and Baeten et al. (2010) found strong indications for these deposits being annual recessional moraines, formed when the ice front halted and/or readvanced slightly each winter when sea ice suppressed iceberg calving.
The characteristic landform assemblages in many fjords with tidewater glaciers at their heads are often a result of climatic cooling or glacier surges, as well as subsequent stagnation and retreat, during the late Holocene (Plassen et al. 2004; Ottesen & Dowdeswell 2006; Ottesen et al. 2008; Kempf et al. 2013). They reflect: rapid advances of tidewater glaciers during the active phase of the surge cycle (fjord-parallel streamlined landforms); the maximum extent of a glacier after the termination of an advance (major transverse ridges, i.e. terminal moraines); sediment reworking during and after a glacier advance (sediment lobes on the distal slopes of the terminal moraines; Fig. 2a, b); sedimentary infilling of subglacial conduits after termination of fast ice flow (eskers); and crevasse-fill ridges formed from injection of soft sediments into basal crevasses (irregularly orientated (rhombohedral) ridges). Further details of landforms at the margins of surge-type tidewater glaciers are found in Ottesen & Dowdeswell (2006), Ottesen et al. (2008), Flink et al. (2015) and Streuff et al. (2015).
Discussion: fjord glacigenic landform assemblages
Swath bathymetry and sub-seafloor acoustic data from Spitsbergen fjords reveal glacigenic landforms and deposits providing information on the dynamics of ice streams and fast-flowing outlet glaciers during the last full-glacial and regional deglaciation, as well as the dynamics of tidewater glaciers during the late Holocene. Schematic landform-assemblage models are given for three periods since the Late Weichselian Ice Sheet maximum in Spitsbergen fjords.
Full-glacial conditions (Fig. 2c): MSGLs, crag-and-tails and drumlins provide evidence that fast-flowing grounded ice covered the floors of most fjords on Spitsbergen during the last glacial (e.g. Ottesen et al. 2005). However, the absence of such features in Smeerenburgfjorden, for example, indicates that inter-ice stream areas developed in parts of the last Svalbard–Barents Sea Ice Sheet, even though the ice was drained through a fjord and extended to the shelf break; a relatively small catchment size may explain this (Ottesen & Dowdeswell 2009). Sedimentary eskers and bedrock subglacial channels provide evidence of the development of extensive subglacial drainage systems at the base of grounded ice streams/glaciers during and after fast ice flow (Forwick et al. 2010; MacLachlan et al. 2010; Kempf et al. 2013).
Deglaciation (Fig. 2d): Radiocarbon dates from glacimarine sediments suggest that deglaciation of most fjords on Spitsbergen occurred between about 14.1 cal ka BP and 11.2 cal ka BP (e.g. Forwick & Vorren 2010). Transverse ridges of variable dimensions suggest that repeated halts and/or readvances interrupted deglaciation. Push/thrust moraines, a submarine ice-contact fan and sediment wedges/lobes all provide evidence of marked glacier readvances and/or relatively long-lasting halts. However, smaller recessional moraines may reflect the location of the glacier front during consecutive winters (Baeten et al. 2010; Kempf et al. 2013). Assuming that small transverse ridges in some fjords are formed annually, the approximate deglaciation rate for parts of Billefjorden was 170 m a−1 (Baeten et al. 2010). However, in van Keulenfjorden the deglaciation rate accelerated from 80 m a−1 in the outer parts to 190 m a−1 in the central fjord (Kempf et al. 2013).
Late Holocene (Fig. 2e): Characteristic landform assemblages in many inner fjords with tidewater glaciers at their heads provide evidence that glaciers extended up to several kilometres beyond their present margins and have retreated subsequently. The advances were either related to climatic cooling during the Little Ice Age or to glacier surges during the past 2500 years or so (e.g. Plassen et al. 2004; Ottesen & Dowdeswell 2006; Ottesen et al. 2008; Kempf et al. 2013; Flink et al. 2015; Streuff et al. 2015).
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
References
Glacial landform assemblages in Spitsbergen fjords from the last full-glacial, deglaciation and the late Holocene
