Evidence of Massive Sheet Floods
Researchers have examined the "moraines" of the Saginaw Glacial Lobe and have offered several solutions to their anomalous relationship with the other Wisconsin-Era Ice Sheet lobes. Both the Michigan and the Huron/Erie lobes are seen as having overridden the original terminal margins of the Saginaw lobe, presenting a confusing picture of its advance and retreat. We ask the question: did the "Saginaw Lobe" ever exist?, or could the excision of the central Michigan landscape be the result of a cosmic impact? While there is good reason to expect that the central Michigan Peninsula was covered by a 1 to 2km deep ice sheet - as it traversed down from the north - we question the existence of the "plunge" and focused Saginaw lobe advance as commonly portrayed.
Sedimentary and stratigraphic evidence for subglacial flooding, south-central Michigan, USA
Timothy G. Fishera,*, Lawrence D. Taylorb
a Department of Geosciences, Indiana University Northwest, 3400 Broadway, Gary, IN 46408, USA
bDepartment of Geological Sciences, Albion College, Albion, MI 49224, USA
Extracts from Quaternary International 90 (2002) 87–115
Abstract:
The glacial landscape in south-central Michigan is a dissected surface with streamlined, residual, and upland blocks surrounded by lowland valleys dominated by outwash. The upland surface has ridges, hills, and hummocks composed of boulder gravel, which are transitional to drumlins in a down-ice direction, and is dissected by tunnel channels with eskers within them. Sedimentary descriptions from 14 gravel pits indicate a consistent stratigraphy across a width of15 km. The upland consists ofbedrock covered by a lower till with subglacial melt-out or lodgment characteristics. Above the lower till is a sand-and-gravel facies that passes upward into a boulder-gravel facies. Nestled within depressions on the boulder-gravel surface is a discontinuous facies of laminated sand and silt. Above the gravel is an upper till with supraglacial melt-out and flow till characteristics.
Analyses ofthe till and gravel indicate an upward change in clast lithology from locally derived carbonates to clastics and finally to crystallines from the lower till through the gravel facies into the upper till. The transition mimics the bedrock lithology up-ice of the study area, and is explained by meltwater erosion ofbedrock, previously deposited subglacial sediment, and overlying debris-rich basal ice. The systematic change in clast lithology and the stratigraphic position ofthe gravel facies between a subglacial and supraglacial till strongly suggests that the gravel facies sediment was deposited subglacially and that the entire till–gravel–till sequence resulted from the same glacial advance.
Conclusions :
The landscape in south-central Michigan consists of hilly, hummocky, and streamlined upland surfaces dissected by northeast-oriented tunnel channels and east–west oriented valleys. The east–west oriented valleys contain outwash and break up the uplands into blocks, which in many places have a streamlined appearance, similar to the subglacially formed Spooner Hills (Johnson, 1999). Within some lowland areas, streamlined bars, outwash, residual hills and/or kames document deglacial meltwater activity. The tunnel channels on the northeast facing slopes of the uplands give the uplands their dissected appearance. Although the uplands blocks have previously been interpreted as recessional moraines of the Saginaw Lobe (Leverett and Taylor, 1915; Zumberge, 1960), we question this interpretation because tunnel channels, many with eskers in them, cross-cut the moraines, a relationship that is also evident on the Quaternary Geology of Southern Michigan map (Farrand, 1982). This crosscutting relationship between the uplands and tunnel channels, along with the drumlinization of the uplands, suggests to us, that parts of the Tekonsha and Kalamazoo Moraine do not reflect ice margins during the final retreat of the Saginaw Lobe. Instead, the orientation and topographic prominence of these uplands might simply be palimpsest upon a penultimate ice margin, or even represent features generated by ice advance into the region at the beginning of the late Wisconsin glaciation.
The stratigraphy of these uplands is characterized by a sand-and-gravel facies and boulder-gravel facies encased by till. The gravel facies exposed in pits extend for up to 0.5km and show no cross-cutting bedforms or trough-shaped scours that may be interpreted as channels; this suggests to us that the boulder gravel was deposited by a sheetflood. The similarity of bedforms, grain size, and architecture between the boulder-gravel facies and esker gravel, proximal braided-stream environments including jokulhlaups suggests to us that flow regimes and structures were similar in all three environments. However, the presence of a basal lodgment and subglacial melt-out till, beneath the gravel facies, characterized by a strong fabric and dominance of local clasts, and an upper melt-out or flow till above the boulder-gravel facies, argues for subglacial deposition of the gravel facies.
Supporting a subglacial meltwater origin are: (1) the presence of the boulder gravel only on the upland ridges, (2) the capping of the boulder gravel by supraglacial melt-out till and debris flows, and (3) the systematic change in clast lithology from the lower basal till, up through the gravel facies into the upper till. This upsection change in lithology from locally derived limestone through clastic and finally to crystalline clasts mimics the transition in bedrock lithology up-ice of the study area. In our model (Fig. 27), meltwater first erodes into the carbonate-rich underlying till and basal ice, then deposits a carbonate-rich sand-and-gravel facies. With continued flow and a higher discharge the meltwater erodes into the underlying Marshall Sandstone and upward into the clastic-rich ice, resulting in a clastic-rich boulder-gravel facies. As the subglacial flow peaks, tunnel channels were eroded into the gravel facies and there is a reorganization of the subglacial pluming system as the flow rapidly declines. This result in cavities filled with the laminated silt-and-sand facies and eskers forming in tunnel channels. As the ice melts, an upper till is deposited that is rich in clastic and crystalline clasts. Finally, transverse lineaments may be small moraines on the uplands that represent recession of the ice in the northern part of the study area. The nature of the proposed subglacial meltwater flood is poorly understood. The spatial distribution oft he observed gravel and till indicate that the subglacial meltwater flood may have had a width of15 km and have been 3m or more deep. Silt interbedded within the gravel indicates that the flow pulsed, or that there were fluctuations in the discharge, perhaps similar to what has been observed in modern Alaskan jokulhlaups (Sturm et al., 1987). Subglacial lakes are common in Antarctica (Oswald and Robin, 1973) and have been modeled for the Laurentide Ice Sheet (e.g. Shoemaker, 1991, 1999), and reconstructed based on sedimentary evidence (e.g. Munro-Stasiuk, 2000). Large valleys of outwash at the distal end of a drumlin field, down flow from the study area, and a high density of eskers immediately up-ice of the study area may be related to the subglacial flood. If not, they further indicate the presence and importance of meltwater during the deglaciation of soth-central Michigan.
Outburst flood origin of the Central Kalamazoo River Valley, Michigan, USA
Andrew L. Kozlowski (a), , , Alan E. Kehewb and Brian C. Bird (b)
a Department of Geological and Environmental Science, 514 University Ave, Susquehanna University, Selinsgrove, PA 17870-1001, USA
b Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, USA
doi:10.1016/j.quascirev.2005.03.016 Received 13 January 2004; accepted 2 March 2005. Available online 10 August 2005.
Abstract
Geomorphic evidence and stratigraphic information from boreholes suggest that the oversized Central Kalamazoo River Valley (CKRV) in southwest Michigan resulted from a catastrophic outburst flood emanating from subglacial channels under the Saginaw lobe of the Laurentide Ice Sheet. The CKRV occurs as a deeply incised trench over 2 km wide and in excess of 50 m deep situated in a reentrant formed by the Lake Michigan, Saginaw and Huron-Erie lobes.
The course of the CKRV follows an irregular flow path that bisects the Kalamazoo Moraine of the Lake Michigan lobe. Erosional terraces near the mouth of the channel indicate that Lake Michigan lobe meltwater drained eastward prior to the westward Saginaw outburst.
Prior to valley formation the Lake Michigan lobe had retreated westward to at least the Lake Border Moraine. With the Lake Michigan lobe absent to impede flow, drainage from the CKRV proceeded southwesterly until draining into glacial Lake Chicago near St. Joseph, Michigan.
The outburst originated from a system of Saginaw tunnel channels that display convex-up flow profiles and contain eskers. Meltwater drainage transitioned from subglacial-to-ice marginal and proglacial environments. During the interval represented by the outburst, the Saginaw Lobe appears to have been in a relatively stationary position.
