Paraglacial Sedimentation In Lakes

During deglaciation, many lakes experience an upwards depositional succession from proximal, ice-contact sediment accumulation, to ice-distal sedimentation, to paraglacial sedimentation (e.g. van Husen, 1979; Eberle, 1987; Müller, 1999). Proximal glacilacustrine sedimentation is characterized by the formation of ice-contact deltas, subaqueous fans and moraines, submerged ramps produced by subaqueous mass movement and rhythmic laminated bottom sediments (Ashley, 1995). Distal glacilacustrine sedimentation and paraglacial lacustrine sedimentation are both characterized by delta progradation, often of fine-grained, gently dipping foreset sediments, and the accumulation of bottom deposits (sometimes rhythmic) of sand, silt and clay. It is thus often difficult to distinguish distal glacilacustrine deposition from paraglacial lacustrine deposition on the basis of the sediment record alone. In mountainous areas, slumping of valley-side glacigenic deposits may generate large subaqueous debris flows (Eyles, 1987) that may interrupt the depositional sequence.

The influence of paraglacial sediment input is particularly evident in small lake basins. Studies of Late Holocene varve thickness in a distal glacial lake in Alberta led Leonard (1986, p. 211) to note that the '...highest sedimentation rates occur during the initial stages of glacial recession, rather than at the time of the glacial maximum, indicating the existence of a "paraglacial" peak in the sedimentation rate'. This conclusion is supported by later studies of Holocene sedimentation in distal lakes (e.g. Desloges, 1994; Dirszowsky and Desloges, 1997), and has important implications for the interpretation of periods of enhanced minerogenic sedimentation in Holocene lake sequences. Over a longer timescale, Smith (1975) showed that sediment influx in a shallow lake in the Canadian Rockies has diminished during the Holocene, attributing the trend to a declining input of reworked Pleistocene glacigenic sediment. Conversely, an increase in the sedimentation rate in Nicolay Lake in the Canadian arctic over the past five centuries has been identified as representing the delayed arrival of a wave of paraglacial sediment driven by base-level lowering (Lamoureux, 1999).

The pattern of lacustrine sedimentation during and after the retreat of Late Pleistocene glaciers has been investigated in several of the large, deep inland 'fjord lakes' of British Columbia (e.g. Eyles et al., 1990, 1991b; Mullins et al., 1990; Desloges and Gilbert, 1991; Gilbert and Desloges, 1992). These studies suggest that vast quantities of sediment accumulated on lake floors within decades or a few centuries during and immediately after glacier retreat from lake basins. Seismic studies suggest that the largest of these, Okanagan Lake, is occupied by about 90 km3 of sediment up to 792 m thick. The great majority of this infill was interpreted by Eyles et al. (1990, 1991b) as reflecting rapid proglacial accumulation and associated subaqueous mass movement, but a discontinuous upper stratified unit typically less than 25 m thick appears to represent a deglacial/paraglacial varve sequence. Similar seismostratigraphic sequences are evident in Harrison Lake (Desloges and Gilbert, 1991) and Stave Lake (Gilbert and Desloges, 1992), where accoustically stratified units interpreted as the products of rapid proximal glacilacustrine sedimentation are overlain by an upper sediment unit up to 28 m thick that appears to reflect paraglacial/postglacial sedimentation. A drawback of these studies is the lack of deep borehole records to confirm interpretations of the seismostratigraphic record, or of dated horizons to verify the inferred rapidity of glacilacustrine sedimentation or to establish variations in the rate of postglacial infill. The consensus of interpretation nonetheless suggests very rapid sedimentation during glacier retreat, and that though depositional rates slowed after deglaciation, substantial volumes of sediment continued to accumulate through fluvial influx of reworked glacigenic sediment.

Finally, it is worth noting that glacilacustrine deposits constitute a major source of fluvially reworked sediment. Gordon (1979) attributed the high sediment yield of New England rivers over the past 8 ka to continuing erosion of Late Pleistocene glacilacustrine deposits, and the removal of huge volumes of glacilacustrine sediment from the South Thompson Valley (Clague, 1986; Roberts and Cunningham, 1992) and from the Ain Basin in France (Campy et al., 1998) highlights the importance of glacilacustrine deposits as paraglacial sediment sources. Such deposits occur as valley fills and are thus readily accessible to fluvial reworking, and being predominantly fine-grained are readily erodible. The outcrop of glacilacustrine deposits is extensive (Teller, 1987), and it is likely that in many areas glacilacustrine sediments have provided the most important source of fluvially reworked sediment during the Holocene (Church and Slaymaker, 1989; Ashmore, 1993).

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