Tuesday, April 11, 2017

Reconstructing the confluence zone between Laurentide and Cordilleran ice sheets along the Rocky Mountain Foothills, south-west Alberta

Daniel J. Utting, Nigel Atkinson, Steven Pawley and Stephen J Livingstone
Journal of Quaternary Science
31(7) 769–787
30 August 2016
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Figure:  Patterns of glacial lineations grouped into flowset components, with flowset Stage 1 being the earliest and flowset Stage 3 corresponding to the final glaciation. It is currently not understood as to which glaciation Stage 1 corresponds with, or the age of Stage 1.  The flowset components were given relative ages in the study area based on cross-cutting relationships, discussed in the text.


We examined the landform record in an area of complex former ice flow within the confluence zone of the Cordilleran and Laurentide ice sheets. The landform record of this area has represented a key line of evidence in recognizing the confluence and mutual deflection of the Late Wisconsinan/Laurentide and Cordilleran ice sheets (Roed, 1975; Bednarski and Smith, 2007), although detailed evidence relating to the build-up, confluence and subsequence desuturing and retreat of these two ice sheets has hitherto remained uncertain.
Using a two-pronged approach of flowset interpretation and reconstruction of former proglacial lake extents, we have recognized three distinctive glaciodynamic stages within the fragmentary and overprinted landform record of this area.
The earliest of these stages (Stage 1) is identified by flowset components consisting of lineations on the foothills and plains, and on high summits. These landforms are overprinted by younger flow-set components and document a stage of unconstrained flow of Cordilleran ice out onto the Alberta Plains which was significantly more extensive than previously recognized in south-west Alberta. Although establishing the numerical age of this event was beyond the scope of the present study, this finding lends support to other reconstructions in the foothills (Moran, 1986; Bednarski and Smith, 2007) which have previously recognized evidence of an extensive CIS developing before the arrival of the LIS in the region.

The second glaciodynamic stage (Stage 2) relates to Cordilleran ice flowing out of the Athabasca Valley and deflecting to the south-east upon encountering the Laurentide ice. The general pattern of flow identified in this study is broadly similar to that recognized by previous workers, but additional complexities have been revealed based on the examination of high resolution imagery and bare-earth LiDAR imagery. We find evidence that the along-the-mountain-front-flow (FSc-9, 15, 21) moved up into some of the smaller valleys, which suggests that some areas west of this flowset were ice free during this time.

The third glaciodynamic stage is represented by the retreat of ice from the mountain front, which is manifested in the development of a sequence of proglacial lakes created by glacial ice blocking the regional drainage. Localized flow-set components demonstrate that overall ice sheet recession was interrupted by re-advances or surges, such as in the Baptiste River (FSc-14, 16), North Saskatchewan River (FSc-18), Edson (FSc-4) and Obed (FSc-1) areas. All these re-advances are closely associated with proglacial lakes, suggesting the lakes induced instability in the ice sheet.

 The three phases of former ice-sheet flow documented by the geomorphological and proglacial lake extent record represent a framework for further ice sheet reconstruction studies. Another line of research expanding from the current study would be to establish the numerical ages of the flowset components and associated proglacial lake stages within he LIS and CIS interaction zone. Such an approach is essential to more broadly investigate how proglacial lake drainages correspond with deglaciation events, including whether the drainage events involved are sufficient to account for rapid deglacial global sea level rises as invoked by recent ice sheet modelling simulations (Gregoire et al., 2012).

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