Paraglacial Landsystems And Postglacial Landscape Change

Subdivision of paraglacial landscape response into six landsystems (rock-slope, drift-slope, glacier foreland, alluvial, lacustrine and coastal) provides a useful context for analysis of process-landform-sediment relationships. This approach, however, fails to highlight the links between individual landsystems. Ballantyne (2002b) has suggested that this may be achieved by representing paraglacial response as an interrupted sediment cascade with four primary sediment sources (rockwalls, drift-mantled slopes, valley-floor glacigenic deposits and coastal glacigenic deposits), four terminal sediment sinks (valley fills, lake basins, coastal/nearshore settings and shelf/offshore settings) and numerous intervening sediment stores and transport pathways (Fig. 17.15). In the initial stages of the cascade, sediment inputs such as rockfall and reworking of glacigenic sediments by debris flows and rivers produces a range of primary sediment stores, such as talus accumulations, debris cones and alluvial fans, while in littoral settings reworking of glacial drift initiates the formation of coastal barriers such as spits and

Www Paraglacial Processes And Landforms

Figure 17.15 Simplified paraglacial sediment cascade, showing the principal primary and secondary sediment stores and the main sediment transfer routeways. Sediment is likely to enter storage on numerous occasions during source-to-sink transport, for example through multiple episodes of floodplain deposition or storage in barrier structures. All terrestrial sediment stores may survive throughout the Holocene (i.e. over a timescale of I04 years), but most experience a transition from net accumulation to net erosion within this timescale.

Figure 17.15 Simplified paraglacial sediment cascade, showing the principal primary and secondary sediment stores and the main sediment transfer routeways. Sediment is likely to enter storage on numerous occasions during source-to-sink transport, for example through multiple episodes of floodplain deposition or storage in barrier structures. All terrestrial sediment stores may survive throughout the Holocene (i.e. over a timescale of I04 years), but most experience a transition from net accumulation to net erosion within this timescale.

46G GLACIAL LANDSYSTEMS

barrier islands. As sediment input to primary sediment stores slows, it is succeeded by net erosion, primarily by rivers in terrestrial settings and by waves and currents in coastal settings. The subsequent transport history of reworked sediment varies in complexity. Some may be transported directly to its final destination within valley fills, lake basins or nearshore or offshore depo-centres. Other sediments may experience several cycles of deposition and re-entrainment, entering secondary sediment stores (Fig. 17.15) that later experience net erosion. The timing and duration of sediment storage is determined not only by sediment budget and the capacity of transporting agents, but also by a wide range of extrinsic influences such as climate change, extreme climatic events, neotectonic uplift and tilting, sea level change and anthropogenic activity.

The sediment cascade model of paraglacial landscape response provides a useful framework for reconstruction of postglacial landscape evolution. It is possible to distinguish an initial, often rapid, period of paraglacial adjustment that immediately follows deglaciation and is characterized by reorganization of landforms and sediments to a more stable state, sometimes over a few decades (modification of glacier forelands), centuries (adjustment of drift-mantled slopes) millennia (accumulation of talus, large alluvial fans or valley fills) or tens of millennia (reworking of glacigenic sediments by rivers or the sea). Superimposed on this pattern are episodes of rejuvenation triggered by external perturbations (Fig. 17.3). The study of Holocene geomorphic change in glaciated environments can thus be conceptualized entirely within a context of initial, prolonged, delayed and/or renewed paraglacial sediment reworking, in which any part of the sediment cascade may be remobilized long after the initial period of paraglacial adjustment.

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