A depositional model for stratigraphic complexes and facies superimposition in lodgement tills

Abstract

1Y82 1201: A depositional model for stratigraphic complexes and facies superimposition in lodgement tills. Boreas. VOI. 11,. OSIO. ISSN 0300-9483.A typical stratigraphy below a streamlined till plain in Northumberland, England, consists of cross-cutting lodgemcnt till units, within and between which occur repeated shoestring interbeds of 'cut and fill' channels. I i I I units have erosional lower contacts; in certain cases marked changes in erratic content and local ice flow direction arc evident from one till unit to another. These lodgement till complexes have hitherto been described by 'tripartite' schemes of lower grey till(s) and upper reddened till(s) identified with respect to 'middle' fluvial horizons; regional correlation proceeding on the basis of matching 'middle' horizons, with the whole sequence commonly interpreted as evidence for multiplc glaciation. Data indicates, by way of contrast, that thLse lodgement till complexes were deposited during a single phase of subglacial deposition. T i 1 1 deposition was not continuous but was interrupted by erosional episodes.Changes in the mix of bedrock lithologies transported by the glacier down a single flow line or by lateral displacement of basal ice flow units within the glacier result in till units of different facies to be emplaced when deposition recommences. a process referred to as 'unconformable facics superimposition'. Subgla-cia1 meltwater flow was also a characteristic of the glacier bed; channeled glaciofluvial sediment bodies are found as ribbon-like inclusions in the till and appear to have been deposited rapidly. These so-called 'middle' fluvial horizons occur repeatedly in section, their lateral extent at any given exposure being dependent upon the orientation of the exposure with respect to former ice flow direction. These lenses act as internal drainage hlankcts and have accelerated postglacial soil formation in the drier climatc of eastern Britain accounting for the reddened colour of upper till(s). It is suggested that this model of subglacial deposition can be employed in other areas of northern England characterized by subglacial (lodgement till plain) terrains.

Interlobate ice‐sheet dynamics during the Last Glacial Maximum at Whitburn Bay, County Durham, England

“…The channels formed at the ice/bed interface and were later buried by till accretion. This interpretation is in agreement with previous process-based research further north in Northumberland (Eyles et al 1982). This type of distributed subglacial drainage system is inefficient, and can result in low energy flows and ponding beneath the ice sheet.…”

“…These were inferred to have originated from the glacier sole, dropping from the roof of the channel. Eyles et al (1982) argued that variable palaeo-discharges were the result of ponding episodes, possibly resulting from the closure of the channel downstream by ice or deformation of the till bed. The tills and glaciofluvial sediments were all deposited during a single, complex, episode of wet-based subglacial sedimentation (Eyles et al 1982).…”

Late Devonian glacigenic and associated facies from the central Appalachian Basin, eastern United States

“…This deformation interval is proposed to represent a glaciotectonized substrate. Such local tectonization has been documented with recent and Pleistocene glacial successions (Eyles et al, 1982;Benn and Evans, 1998) and is uncharacteristic of debrisfl ow deposits.…”

Till—as a glacial “tectomict”, its internal architecture, and the development of a “typing” method for till differentiation

“…Many researchers have investigated till internal structure in terms of clast fabrics (e.g., Holmes, 1941;Harrison, 1957;Krueger, 1970;Evenson, 1971;Mark, 1974;Lawson, 1979;May et al, 1980;Dowdeswell et al, 1985;Dowdeswell and Sharp, 1986;Hart, 1994;Benn, 1995;Benn and Evans, 1996;Hicock et al, 1996;Bennett et al, 1999;Hooyer and Iverson, 2000;Carr and Rose, 2003), bed limits (Virkkala, 1952;Kjaer and Krü ger, 1998;Kjaer et al, 2003;RuszczynskaSzenajch et al, 2003), the presence of stratified subfacies units and intraclasts (Eyles et al, 1982), glaciotectonic structures (Slater, 1926;Berthelsen, 1978;Brodzikowski and van Loon, 1980;Aber, 1988;van der Meer et al, 1999), and structures associated with melt-out tills (Lawson, 1981;Shaw, 1977Shaw, , 1982. Miller (1884) probably was the first to recognise the tendency for clasts to be preferentially oriented in the direction of ice movement.…”

“…Later examination of tills revealed that sorted and stratified units were commonplace in many tills and it was suggested that they are linked to the mechanics of till formation (cf. Menzies, 1981;Eyles et al, 1982;Brodzikowski and Van Loon, 1987).…”