The sedimentation pattern of the last deglaciation and postglacial phase in a high-arctic valley: Endalen, Spitsbergen
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The present study, focused in the valley-side gravel fans in the eastern part of Endalen and the adjoining side-slope of Adventdalen, has contributed to an understanding of the spatial pattern of the late Weichselian deglaciation and associated meltwater drainage in a 3rd-order and a 2nd-order valley and adjacent mountain plateau in central Spitsbergen, Norwegian high-Arctic. The author’s fieldwork in 1998 was focused on the following main topics: (1) the mapping and chronology of postglacial sediment accumulations, with special emphasis on the development history and depositional processes of the valley-side gravel fans; (2) the snow-cover conditions on the valley-side slopes during the winter to autumn season 1998, including comparative photographic documentation and fixed-point snow thickness measurements; and (3) the assessment of the intensity, routes and controlling factors of the downslope transfer of sediment and water, including the recognition and spatial pattern of meltwater palaeoflow scours and their significance for the reconstruction of the ice-sheet retreat. The analysis of infrared aerial photographs has proved to be very useful in deciphering the areal pattern of meltwater palaeodrainage and in the recognition of other glacial/postglacial geomorphic features. Arrays of surficial palaeochannels are recognizable on the eastern plateau of Endalen, trending NW and lacking any obvious water catchment today. These scours are attributed to the meltwater runoff during the early Holocene deglaciation. The morphological mapping revealed also relict late-glacial deposits in the form of a lateral moraine and several raised beach terraces near the mouth of Endalen, but no preserved glacigenic diamicton deposits have been recognized on the adjacent eastern plateau. Erratic cobbles and boulders have been found on western plateau, directly outside the Endalen drainage basin, which indicates that the mountain plateaux were covered by the glacier. Some long-transport erratics, derived probably from the eastern part of Spitsbergen, have also been identified in Endalen and on the valley’s eastern slope. Based on the aerial photographs and direct field observations, a tentative spatial model has been suggested for the deglaciation of the Adventdalen area. The valley-side gravel fans whose formation involved meltwater flow from the plateau are larger, associated with well-incised ravines and show two stories of deposits recognizable in geomorphic terms: a broad mound of older, vegetated base-of-slope deposits (alluvial fan), distinguished as sedimentation stage I, and a smaller, steeper accumulation of relatively fresh younger deposits (colluvial fan) superimposed onto the upper/apical part of the former and distinguished as sedimentation stages II and III. The three stages of deposition are interpreted as: (I) an early Holocene, early postglacial stage dominated by the meltwater runoff from the plateaux and coeval high-viscosity debrisflows on the mountain slopes between the ravines; (II) a middle Holocene stage dominated by rockfall processes and minor debrisflows; and (III) the late Holocene stage dominated by the more mobile, watery or slush-laden debrisflows. The low-viscosity debrisflows of stage III have commonly caused strong incision in the fan apex and upper segment, with channels up to a few metres deep and the associated intersection-point deposits making the fan prograde in a “telescoping” style and aggrade in the lower segment. The tripartite stratigraphy of the valley-side fans is thought to reflect the Holocene regional climatic changes. The snow cover in the study area is generally sparse, due to the low precipitation in the Arctic desert, but the predominance of strong and variable winds renders them a key agent controlling the thickness distribution and local excessive accumulation of snow in Spitsbergen. In Endalen, the winter winds are capable of filling ravines with several metres of snow and forming large snow cornices, although no snow avalanches (nor any other active slope-waste processes) have been observed on the valley slopes during the weekly visits in 1998. Slope-waste processes are infrequent and highly episodic. Most of the snow cover seems to disappear due to the processes of sublimation and gradual melting, with the main phase of melting in the early June. The meltwater runoff from the mountain plateau is very limited and incapable of removing large amounts of debris from the ravines, except for the fine-grained sediment, rarely coarser than sand or perhaps granule to fine-pebble gravel. However, the percolating water almost certainly has the capacity to infiltrate the coarse, ravine-entrapped gravel with fine sediment, and this watery matrix might then lubricate the gravel mass and turn it into a debrisflow. The presence of slush in the ravines may have a similar lubricating effect on the cobbly to bouldery rockfall gravel that tends to accumulate in the steep bedrock ravine.