DEGLACIATION OF HIGH-LATITUDE COASTAL BASIN REFLECTED IN THE EVOLUTION OF A FAN-DELTA SYSTEM
LOENNE, IDA . The University Courses on Svalbard (UNIS), Norway.
Nemec, Wojtek . University of Bergen, Norway.
Studies of the Holocene deglaciation of Svalbard, Norwegian Arctic, indicate that the receding ice masses retreat quickly from fjords, but commonly turn into isolated, cold-based stagnant glaciers in high-relief onshore areas, where they then melt very slowly. In the cold climate of polar desert, the glaciers and permafrost active layer release water during brief summer seasons, and this abundant ephemeral runoff leads to high sediment dynamics and erases glacial deposits from the landscape. The resulting colluvial and alluvial fans are dominated by watery debrisflows and streamflow processes and are disproportionally large and voluminous, relative to their small catchments and the region¡¯s meagre annual precipitation.
At the present stage of post-Little Ice Age deglaciation, Svalbard abounds in such stagnant ice bodies, ranging from high plateau and cirque glaciers to valley glaciers, some with marine termini. The rate of their summer melting is greatly reduced by the surficial accumulation of rock debris that has melted out from the ice and locally avalanched from adjacent steep mountain slopes. The glaciers effectively evolve into ice-cored moraines, and continue to melt slowly until the debris cover attains the local thickness of the active layer (averaging ca. 1 m at present). Their complete disintegration occurs only when gravitational foundering or streamflow erosion of the debris cover exposes the ice core. In mountain-slope cirques, the ice-cored moraines become richly covered with debris and often evolve into rockglaciers, whereas on low-gradient topographic surfaces, the flowing meltwater winnows fine-grained debris, leaving a coarse gravelly pavement. The glacigenic debris, whether mixed with scree material or reworked by flowing water, inevitably loses its identity, which leads to a rapid vanishing of the sedimentary record of glaciation.
The region¡¯s morphogenic response to the Holocene climatic changes and deglaciation, in terms of the meltwater runoff, sediment flux and relative sea-level change, appears to be particularly well reflected in the depositional history of coastal sedimentary systems. The importance of this proxy record is demonstrated by the present sedimentological and stratigraphic study of an alluvial-fan delta in Adventfjorden, central Spitsbergen. The fjord became ice-free around 9.9 ka BP, and the fan delta formed during the main phase of post-Weichselian deglaciation and evolved through further major changes later during the Holocene. This Gilbert-type fan delta, up to 25 m thick and heavily modified by sea waves, reached a plan-view radius in excess of 1 km and a surface area of 1.2 km2, although its high-relief coastal catchment, scoured in sandstone and shale bedrock, has a plan-view area of no more than 2.6 km2. The fan apex is at an altitude of ca. 100 m, and the fan¡¯s steep catchment, extending to the mountain top at 923 m, is apparently a deglaciated cirque, hosting a rockglacier today.
The fan delta consists of four distinct chronological parts, or depositional units, representing the main successive phases of its Holocene evolution:
1. The oldest part, composed chiefly of gravel and sand, constitutes the bulk of the fan delta and represents the phase of its maximum progradation. It consists of a foreset of steeply inclined subaqueous deposits (upper ¡Ü6 m thickness exposed), containing Mytilus edulis shells and overlain by a topset of coarse gravelly alluvium, up to 4 m thick. The fluvial topset has an uneven erosional base and pinches out in the medial part of the fan delta, dated to ca. 6.2 ka BP, where the foreset facies change from massflow-dominated into fully wave-worked. The latter facies account for the final 160-m advance of the fan-delta front and correspond to a decline in its mean progradation rate from 0.30 to 0.12 m/yr and eventually to 0.07 m/yr. The delta foreset in the medial to distal part is overlain by a downstepping unit of delta-front beach-ridge gravel, which replaces the foreset facies at the distal end by forming a relatively thick (>4 m), aggradational beach, estimated to be ca. 4.5 ka BP in age. The catchment¡¯s high yield of meltwater and sediment apparently declined around 6.2 ka BP, as the melting cirque glacier probably turned into a cold-based ice body and the Neoglacial stage commenced around 5 ka BP. However, the relative sea level kept falling (forced regression) and the fan-delta front continued to prograde, at a decreasing rate, by accumulating wave-worked sediment supplied by alongshore drift. The relative sea level began to rise around 4.5 ka BP, which marked the onset of a marine transgression.
2. The subsequent depositional part of the fan delta is an overlying unit of the landward-accreted, gravelly foreset deposits of a transgressive spit, up to 2 m thick and climbing onto the fan-delta plain to an altitude of ca. 6 m. The rate of the relative sea-level rise thus finally exceeded the declining rate of the isostatic crustal uplift. A corresponding, wave-cut bedrock escarpment is recognizable at the same altitude along the coast lateral to the fan delta.
3. The overlying surficial deposits indicate that the activity of the fluvial system subsequently increased markedly, leading to a normal (progradational) regression, while the relative sea-level rise declined. The fluvial system expanded and the fan delta prograded again, reaching a radius of ca.1 km, probably in response to the melting of the catchment¡¯s relic glacier, which turned into a creeping rockglacier. An associated, regressive delta-front beach has been dated to ca. 4.3 ka BP, which implies that the preceding transgression was relatively brief.
4. The meltwater discharge then declined and the fluvial system became restricted to the fan¡¯s low flank, where it has incised and remains active today, while the abandoned fan surface was subject to sheetwash processes and covered by tundra vegetation. The stagnation of the fan delta rendered it vulnerable to erosion by wave action, combined with a strong alongshore drift of sediment. As a result, the fan delta had been shallowly eroded, its shoreline retreated by the formation of a wave-cut escarpment (<10 m high), and its subaerial area was reduced to ca. 0.4 km2 (<0.6 km radius). Concurrently, modest progradation occurred in the fan¡¯s incising, modern sector, less than 200 m wide. The modern beach, at the foot of the shoreline escarpment, passes laterally into an associated spit complex sheltering an intertidal lagoon. The spit shoal receives abundant sediment by alongshore drift, which has resulted in shoreline progradation (normal regression) in this part of the fjord¡¯s coast. The runoff today is due to the melting of the catchment¡¯s active layer and snow patches, and the ephemeral streamflow erodes and redistributes slowly the coarse gravel delivered by contemporaneous debrisflows. In the fan-head area, there is also evidence of a modern accumulation of debris derived and spread beyond the active sector by sporadic snow avalanches.
The depositional architecture and dynamic stratigraphy of the fan-delta system thus reveals a valuable, high-resolution proxy record of the coastal region¡¯s deglaciation history and climatic changes.
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