Paraglacial adjustment of the fluvial system to Late Pleistocene deglaciation: the Milfield Basin, northern England

Abstract

Landform–sediment assemblages in the middle reaches of the River Till in the Milfield Basin, northern England, provide a comparatively rare example of a fluvial system emerging from drainage of a Late Pleistocene ice-dammed lake. This paper reviews the chronology and sequencing of Late Pleistocene lake drainage and early Holocene valley-floor development using new geomorphological, palaeoenvironmental and radiocarbon data, and considers the results in the context of paraglacial models of landscape response. The balance of currently available evidence suggests drainage of the proglacial lake occurred some time between the end of the Dimlington Stadial and the relatively mild climate of the Windermere Interstadial. Fluvial downcutting through glaciodeltaic and glaciolacustrine sediments was associated with recoupling of the fluvial sediment system to lower reaches of the Till and paraglacial development of inset fluvial terraces and valley widening prior to establishment of early Holocene channel systems at least 13–15 m below the equivalent glaciodeltaic surface. This short-lived phase of high paraglacial sediment yield was followed by relatively abrupt relaxation of coarse-sediment reworking as Holocene channels became largely decoupled from Late Pleistocene sand and gravel terraces in the basin. The combination of a bedrock barrier at the basin outlet and relatively gentle valley gradients in the basin has promoted a tendency towards Holocene floodplain alluviation (in central parts of the basin) with little net change in channel elevation until recent historic times. Paraglacial landscape modifications will have continued to exert an influence on the fluvial system during the Holocene, especially with regard to fine sediment yields from localized erosion of glaciolacustrine deposits, but this has most probably diminished considerably with time as sediment supplies to the basin increasingly reflect the impact of anthropogenic catchment disturbance and reworking of Holocene valley-floor deposits.

Vegetation succession in deglaciated landscapes: implications for sediment and landscape stability

“…For example, previous research has often been limited to either short-term (up to 100 years following deglaciation, Gurnell et al, 1999;Orwin and Smart, 2004;Moreau et al, 2005Moreau et al, , 2008, or long-term (e.g. Holocene or Little Ice Age; Passmore and Waddington, 2009;Hobley et al, 2010) timescales and are often limited in spatial area (e.g. 5-1200 km 2 ; Irvine-Fynn et al, 2011;Tunnicliffe et al, 2012).…”

Digital landscapes of deglaciation: identifying Late Quaternary glacial lake outburst floods using LiDAR

“…For example, the incision phase of regional paraglacial response, previously considered to have occurred over c . 3000 years (Passmore and Waddington, ) prior to c . 14 000 cal.…”

“…The morphology of this subsurface topography explains features such as the large Crookham meander, as well as the two palaeochannels indicated in Figure (b) that we interpret as abandoned headcut channels progressing by knick point retreat into the lake basin until the channel occupying the eventual Holocene meander belt of the River Till captures the flow. The terrace surfaces interpreted by Clapperton () and Passmore and Waddington () as re‐worked sands and gravels of the Glen delta, linked to the erosive braid channels identified by Payton (), are likely to be so but it was the flow circulation during lake fall that carried out this geomorphic work. Figure shows how abandoned palaeochannels on the Glen delta can be explained under a flood hypothesis, with rapid channel incision by the proto‐Glen grading the channels to progressively lower lake levels as flow was diverted around emerging topography.…”

“…We argue that in addition to the orientation of the palaeochannels, the morphology of scarp slopes and slumped material (Figure ) can be explained more coherently by a rapid lake level drop rather than longer term meltwater processes, although slumping could also be affected by periglacial activity. We concur with Passmore and Waddington () regarding a paraglacial response with the river adjusting to changes in sediment load and discharge regime, however in line with the GLOF hypothesis the reworking of coarse sediment in the Till basin would have dominated rather than net incision.…”

“…Reviewing their more recent data alongside the previously published evidence, Passmore and Waddington () concluded there is no basin‐wide evidence for the Younger Dryas lake refilling model of Payton, and that the biogenic laminated sediments described must relate to ponding in a local depression. Passmore and Waddington () suggest lake drainage and fluvial incision to 31.5 m asl within a 2–3 ka period between disappearance of ice and the later part of the Windermere interstadial, and argue for episodic lowering due to the abandonment of the deltaic surface and the downvalley terrace levels. Missing from the models proposed to date is detailed consideration of the Till gorge geomorphology.…”