A Sediment-Mixing Process-Model of Till Genesis, Using Texture and
Clay Mineralogy Data from Saginaw Lobe (Michigan, USA) Tills
Randall Schaetzl, Chris Baish, Patrick Colgan, Jarrod Knauff, Thomas Bilintoh, Daniel Wanyama, Michelle Church, Kevin McKeehan, Albert Fulton, and Alan Arbogast
We present a sediment-mixing process-model of till genesis based on data from surface tills of the Saginaw lobe terrain in Lower Michigan, utilizing a spatial approach to understanding glacial landsystems and till genesis. We sampled calcareous till at 336 upland sites, as well as at 17 sites in lacustrine sediment of the Saginaw Lake Plain. The loamy tills have bimodal grain size curves, with a fine texture mode near the silt-clay boundary, and a sand mode. Spatial grouping analysis suggests that tills can be divided into six groups, each with different textures and clay mineral compositions that vary systematically down-ice. The similarity among groups with respect to the silt-clay mode and clay mineralogy argues for a common origin for the fines - illite-rich lacustrine sediment. Fine-textured sediments were probably entrained, transported, and deposited down-ice as till, which also becomes sandier and enriched in kaolinite, reflecting increasing mixing with shallow sandstone bedrock with distance from the lacustrine clay source. Clayey tills on the flanks of the Saginaw terrain may reflect proglacial ponding against nearby uplands. A process-model of progressive down-ice-mixing of pre-existing fine lake sediments with crushed/abraded sandstone bedrock helps to better explain till textures compared to a purely crushing/abrasion process-model.