Palimpsest tunnel valleys: evidence for relative timing of advances in an interlobate area of the Laurentide ice sheet

Abstract

During retreat from the lateWisconsinan maximum advance in the Great Lakes region of North America, the Laurentide ice sheet margin became distinctly lobate. The Lake Michigan, Saginaw, and Huron—Erie lobes converged in southern Michigan and Indiana, U.S.A. to form a complex interlobate region. Some time after the glacial maximum, the Lake Michigan lobe advanced over landscapes previously formed by the Saginaw lobe. This can be explained by an asynchronous advance of the Lake Michigan lobe during a Saginaw lobe retreat or by an increase in size of the Lake Michigan lobe relative to the Saginaw lobe during a synchronous readvance.Cross-cutting relationships in southwestern Michigan, including palimpsest tunnel valleys, document the overriding of Saginaw lobe terrain. Deep, generally straight trenches parallel to glacial flow lines with hummocky, irregular sides and bottoms are interpreted as tunnel valleys. Saginaw lobe tunnel valleys trend northeast—southwest and Lake Michigan lobe tunnel valleys generally trend east—west.At some time after a Saginaw lobe retreat in southern Michigan, the drumlinized landscape was overridden by an advance of the Lake Michigan lobe to an ice-marginal position at the Tekonsha moraine. Saginaw lobe tunnel valleys in the overridden area were completely filled with ice and debris from the Saginaw lobe retreat at the time of the Lake Michigan lobe advance. Supraglacial and proglacial sediments were deposited over the buried valleys by the Lake Michigan lobe, sometimes by meltwater streams that flowed at high angles to the trends of the valleys. After entrenchment of the Kalamazoo River valley, probably by a subglacial outburst flood, short tributaries were cut nearly at right angles across and through the debris and ice within several buried Saginaw lobe tunnel valleys. After the retreat of the Lake Michigan lobe, subsequent melting of ice in the palimpsest tunnel valleys exhumed the valleys, creating the cross-cutting relationships with the Lake Michigan lobe deposits.

Morphological properties of tunnel valleys of the southern sector of the Laurentide Ice Sheet and implications for their formation

“…Kehew et al, 1999;Sjogren et al, 2002). Indistinct valleys may be due to partial burial during re-advance events or by melt out of debris rich ice obscuring them (Kehew et al, 1999). Sjogren et al (2002) also identified indistinct valleys in Michigan that are eroded into the hummocky terrain.…”

“…drumlins, outwash fans, moraines), record a palimpsest signature of tunnel valley erosion. In the Saginaw Lobe, Kehew et al (1999Kehew et al ( , 2005 and Kehew and Kozlowski (2007) identified a series of palimpsest associations in which partially buried tunnel valleys pass beneath terminal moraines, diamicton and surficial outwash associated with later advances. This palimpsest style is interpreted to result from the collapse of ice and debris into the valley, which becomes (partially) buried by sediment during a re-advance and then re-emerges as the ice melts out (e.g.…”

“…Mooers, 1989), to indistinct valleys often associated with hummocky terrain and characterised by beaded or crenulated planforms, or as a series of aligned depressions (e.g. Kehew et al, 1999;Sjogren et al, 2002). Indistinct valleys may be due to partial burial during re-advance events or by melt out of debris rich ice obscuring them (Kehew et al, 1999).…”

“…There is a rich history of work on tunnel valleys beneath the southern margin of the former Laurentide Ice Sheet (Wright, 1973;Attig et al, 1989;Mooers, 1989;Patterson, 1994Patterson, , 1997Clayton et al, 1999;Johnson, 1999;Kehew et al, 1999Kehew et al, , 2013Cutler et al, 2002;Sjogren et al, 2002;Fisher et al, 2005;Kozlowski et al, 2005;Jennings, 2006;Hooke and Jennings, 2006;Kehew and Kozlowski, 2007). In this section we briefly summarise key observations arising from this work, which need to be incorporated into any model of tunnel valley formation.…”

“…Wright, 1973;Mooers, 1989;Kehew et al, 1999Kehew et al, , 2005Fisher et al, 2005;Kehew and Kozlowski, 2007). Cross-cutting relationships, both between tunnel valleys and with other glacial landforms (e.g.…”