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Catastrophic glacial-lake outburst spillways: form and process relationships. part of Vignettes:Vignette Collection
Proglacial Lake outbursts along the southern margins of the Laurentide Ice Sheet As the Laurentide Ice Sheet retreated northward through the Great Plains and midwest area of North America, meltwater drainage initially occurred southward toward the Gulf of Mexico through the Mississippi Valley and its tributaries. As the margin receded north of the Mississippi drainage divide toward the isostatically depressed Hudson Bay lowland (which was lowered because of the enormous weight of the ice sheet upon the crust), vast amounts of meltwater ponded in many short- to long-lived lakes. Glacial Lake Agassiz was the largest and best-known of these lakes, but numerous smaller lakes formed in the prairie regions of the U.S. and Canada (Fig. 1). In addition to isostatic depression of the crust by the ice sheet, reasons for these proglacial lakes include regional slope of the land surface towards the north, and the formation of temporary dams of ice and sediment. Most of these lakes were unstable resulting in their sudden and catastrophic drainage. The volumes of water released were huge, on the scale of some of the present-day Great Lakes, and eroded glacial sediments and weakly resistant bedrock predominant in the region. The events themselves are called outbursts and channels eroded by the floods are known as glacial-lake spillways. Studies of the landforms and sediments of many spillways have illustrated the magnitude and processes, both erosional and depositional, which characterized these events (Kehew et al., 2009). Outburst floods in the Great Plains are broadly similar in process and resulting landforms to the megafloods that carved the channelel scabland of eastern Washington and surrounding areas (Baker, 2009 and references therein). Triggers for the outburst floods included failure of ice or debris dams at some point along the rim of the basin, exposure of a lower outlet by ice retreat, and outburst flows from upstream lakes causing the receiving lake to overtop its basin and cut a new outlet or downcut an existing outlet. Once a dam was breached, meltwater released catastrophically, producing large-scale channels and other erosional landforms. The substrate materials along the southern margin of the Laurentide Ice Sheet are dominated by unconsolidated glacial drift and poorly lithified Mesozoic and Tertiary sedimentary rocks, which offer little resistance to erosion by high velocity floodwaters. Incision typically continued until a resistant substrate was reached or the catastrophic flow ceased. Characteristic landforms The spillway is the most distinctive erosional landform formed by glacial-lake outbursts along the southern margin of the Laurentide Ice Sheet. These deep valleys are really trench-shape channels with steep sides and relatively uniform widths (1-4 km) and depths (25-150 m) (Kehew et al., 2009; Kehew and Lord, 1986, 1987). Most spillways begin and end at glacial lake basins. Spillways can sometimes be divided into two major components: a narrow, deep inner channel and a scoured sub-upland surface (Figs. 2,3) The scoured sub-upland surface represents the first stage of erosion, when no channel of sufficient size was available to the flood and water covered a broad area. As flow continued, erosion became concentrated within a smaller cross sectional area and began to erode the inner channel. Additional ersoional landforms of outbursts include anastomosing channels, streamlined erosional forms, longitudinal grooves, transverse ripple forms, potholes and obstacle scour depressions. Streamlined erosional forms (Fig. 4) achieve the teardrop shape of a lemniscate loop (a streamlined shape similar to the cross section of an airplane wing), which is an equilibrium shape that minimizes erosional drag on the object. Longitudinal grooves are linear scours that represent the action of a stable longitudinal vortex in the flow. Vortices are cells of circulating water that develop in the turbulent floodwater. Depositional landforms, although scarce, are equally impressive. Within the inner channels of the spillways, erosion was dominant, but occasional bar-like deposits of coarse gravel occur. The extreme size of boulders in these deposits indicate the tremendous competence of the flows. The presence of a sand-size matrix supporting larger clasts suggests that the flow was at times hyperconcentrated (Lord and Kehew, 1987) (Fig. 5). Hyperconcentrated flow developes when the sediment concentration is greater than that of normal stream flow, but less than that of debris flows. Large clasts tend to be separated from each other and surrounded by a matrix in hyrperconcentrated flow rather than being in contact with each other as they are in stream flow. Where spillways terminate at basins that contained proglacial lakes at the time of formation, outbursts flowing in the spillways inundated the lakes and deposited sediments that can be distinguished from normal meltwater deposits by their thick, coarse-grained, homogenous vertical sections that systematically fine with distance from the inlet (Kehew and Clayton, 1983; Kehew and Lord, 1987; Lord, 1991). Deposition of the flood deposits, mainly well-sorted sands, likely occurred by a mix of low-density and high-density turbidity currents. Maximum discharges and velocities of glacial-lake outburst floods were immense. Typical estimates for outbursts along the southern margin of the Laurentide Ice Sheet range from 0.2-1.0x106 m3/s, although values as high as 5 x 106 have been suggested for the final drainage of Lake Aggasiz intoHudson Bay (Teller et al., 2002; Clarke et al., 2004). Estimated velocities range between 5 and 12 m/s, although larger values have been suggested for some spillways. Glacial Lake Souris outburst spillway One of the best known outbursts in the mid-continent region was the Glacial Lake Regina outburst (Kehew et al., 1986, 1987; Lord and Kehew, 1987) which eroded the Souris Spillway containing a well-defined inner channel, a scoured sub-upland 10-15 km wide, longitudinal grooves, transverse erosional ripple forms, anastomosing channels, and streamlined hills (Figs. 3, 6) (Lord and Schwartz, 2003). The outburst began with the breach of a stagnant or dead ice dam that released about 7.4 x 1010 m3 of water from glacial Lake Regina (Fig. 1) generating a discharge of about 0.5 x 106m/sand velocities from 3 to 12 m/s (Kehew and Clayton, 1983; Lord and Kehew, 1987; Lord and Schwartz, 2003). On the scoured sub-upland, flow obstacles were modified into streamlined hills, and longitudinal vorticies in the flow produced longitudinal grooves. Gradually, flow became concentrated into the inner channel as it was downcut. As erosion of weak substrate progressed, the flow became hyperconcentrated and inner-channel bars are composed of massive, homogenous, matrix-supported boulder gravel. Downstream, the flood inundated Glacial Lake Hind (Fig. 1), depositing thick, well-sorted sand units.
Tunnel Channels of the Saginaw Lobe, Michigan, USA part of Vignettes:Vignette Collection
Introduction Drainage of meltwater from a glacier occurs at the surface of the glacier, as well as internally and between the ice and its bed. Different components of the meltwater drainage system commonly interact with each other, transferring meltwater from the surface to the base of the glacier, for example. Meltwater processes result in a wide variety of landforms and deposits. These erosional and depositional features are concentrated near and beyond the margin of the ice because ablation (melting) is at its maximum in the marginal zone. Among the most interesting types of landforms produced by meltwater erosion are tunnel channels, which are also known as tunnel valleys. These are interpreted to be eroded by high-energy meltwater flowing in a tunnel at the base of the glacier. These tunnels are are similar to ones in which eskers are deposited, although they erode downward into the substrate beneath the glacier rather than up into the ice. Despite this difference, eskers do occur in some tunnel channels. Wright (1973) first interpreted valleys of this type in Minnesota as the result of catastrophic discharges of meltwater that was initially impounded under the ice and then was suddenly released by some mechanism to flow through tunnels toward the margins. Hooke and Jennings (2006) suggested that the subglacial lakes were ponded behind a dam of frozen soil and rock under the margin of the glacier (permafrost) until a channel eroded headward from the margin and drained the lake catastrophically. Brennand and Shaw (1994) explain tunnel channels in Ontario as the result of erosion by huge subglacial sheetfloods. Gradual erosion of the valleys over time has also been suggested as mechanism for formation (Mooers, 1989).The purpose of this vignette is to describe the characteristics of tunnel channels in the Saginaw lobe of the Laurentide Ice Sheet in Michigan and to explore different hypotheses for their origin. Characteristics of tunnel channels The most diagnostic characteristics of tunnel channels include: (1) dimensions reaching >100 km in length and up to 5 km width, (2) generally straight reaches oriented parallel to the subglacial hydraulic gradient of the ice lobe, (3) longitudinal profiles that can rise in elevation in the downstream direction indicating formation by pressurized meltwater, and (4) irregular side slopes and valley bottoms (Kehew et al., 2007). Despite the generally straight reaches of these valleys, braided, dendritic, and anastomosing networks have been described. Tunnel channels have highly variable topographic expression due to burial by glacial or post-glacial deposition after their initial formation. One mechanism of burial is the slumping of debris into the valleys as the ice retreats across the landscape. Deposition of sediment during readvances of the ice also provide a mechanism for filling and burial. The degree of burial by younger sediment can range from minor to nearly complete, which would leave in some cases, no visible signs of the valley at land surface. Tunnel channels of the Saginaw Lobe The Saginaw Lobe is one of three major lobes of the Laurentide Ice Sheet that coalesced in southern Michigan (Fig. 1). Tunnel channels occur non-continuously from Saginaw Bay to the southern border with Indiana. Kehew and Kozlowski (2007) identified five different types based on the degree of burial, the source (glacial lobe) of sediment that buried the channel, and the presence of eskers in the channels. One group of channels consists of straight reaches oriented in a fan shaped pattern around the central part of the Saginaw Lobe (Fig. 2). Cross-cutting relationships indicate the following sequence of events for the formation of a tunnel channel: (1) subglacial erosion of the channel near the margin of the glacier, most likely by a discrete, high energy flow of meltwater, (2) collapse of ice and debris into the valley during stagnation and/or retreat of the glacier, and (3) gradual meltout of the buried ice causing subsidence to form the modern topographic depression (Figure 3). Re-advance of the Saginaw Lobe or advance of the adjacent Lake Michigan Lobe over terrain vacated by retreat of the Saginaw Lobe prior to stage 3 added additional sediment to the channels, completely burying them in some cases. Cross cutting relationships can sometimes be used to work out the history of the channels. As shown in Figure 4, after erosion of a channel and collapse of ice and debris into it from the Saginaw Lobe, the Lake Michigan Lobe advanced into the area. Braided outwash fans from the Lake Michigan Lobe grade smoothly grade across the tunnel channel, indicating that it had to have been filled with a combination of collapsed ice and debris. If there had been an open valley at the time, the fans would not grade continuously across it as they do. Later, meltout of the buried ice exposed the modern valley. Many tunnel channels contain eskers (Fig. 5), suggesting that the tunnels remained active for a relatively long period of time after erosion and were gradually filled with sediment by streams flowing in the tunnel. A bore hole drilled in one of these eskers yielded a fining-upward sequence of sediment suggesting a gradual decline in energy of streams flowing in the channels as the ice stagnated. Much remains to be learned about these channels including (1) what was depth of erosion into the substrate, (2) how long and continuous are individual valleys, (3) what was the relationship between the channels and specific ice-marginal positions, (4) whether erosion was rapid during catastrophic releases of sub-glacial meltwater or more gradual sustained discharge in tunnels that remained in the same location, and (5) what was source of the meltwater?