Surface form of the southern Laurentide Ice Sheet and its implications to ice-sheet dynamics PETER U. CLARK Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506 ABSTRACT ka).1 An accurate reconstruction of the Lauren- surface morphology from the elevation of lateral tide Ice Sheet, therefore, has significant implica- and terminal moraines. Because these moraines Reconstructions of the ice-surface mor- tions for interpreting global sea-level history, the were deposited only along the ice margin, they phology of several lobes of the southern amount of ice volume recorded in the deep-sea do not provide direct information on the form of Laurentide Ice Sheet reinforce previous ar- oxygen-isotope record, the effect of the ice sheet the interior of the ice sheet. Nevertheless, the guments that this sector of the ice sheet was on atmospheric circulation, and mechanisms of reconstructed ice-surface morphology within thin and low sloping. Driving stresses, esti- ice-sheet growth and collapse. The form, extent, 500 km of its margin should distinguish between mated from the geometry of the recon- and dynamics of the ice sheet, however, remain several distinct and contrasting models of the structed ice surfaces, are 0.7-4.3 kPa for the topics of considerable debate (Hughes and oth- Laurentide Ice Sheet (compare Boulton and 14 ka Des Moines Lobe, 0.9-1.2 kPa for the ers, 1977; Shilts, 1980; Denton and Hughes, others, 1985; Fisher and others, 1985; Hughes, 14 ka James Lobe, 0.9-1.7 kPa for the 18-20 1981; Andrews, 1982, 1987; Dyke and others, 1987). Furthermore, such reconstructed mor- ka Lake Michigan Lobe, 1.8-2.9 kPa for the 1982; Fisher and others, 1985; Boulton and oth- phologies offer important glaciological data re- 15-18 ka Chippewa Sublobe, and 17-22 kPa ers, 1985; Hughes, 1987; Dyke and Prest, flecting the behavior and dynamics of the ice for the 15-18 ka Green Bay Lobe. Previous 1987a, 1987b). sheet that provide critical boundary conditions estimates of rates of ice-margin advance One means of constraining the form of the for ice-sheet modeling. (450-2,000 m/yr) indicate moderate-to-fast Laurentide Ice Sheet is by reconstructing ice- Several attempts have been made at recon- ice velocities for the ice lobes. Reconstructed structing the surface morphology of the southern driving stresses and velocity estimates of the 1 AH age estimates are based on the radiocarbon Laurentide Ice Sheet, either from slopes of mo- Des Moines, James, and Lake Michigan time scale. raines (Wright, 1972; Mathews, 1974) or by Lobes are analogous to the distal ends ("ice plains") of low-sloping (0.4 x 10~3) but fast moving (500 m/yr) West Antarctic ice streams, whose dynamics have been attrib- uted to sliding and/or subglacial sediment deformation by pervasive shear. These recon- structions support recent models of the Lau- rentide Ice Sheet which include movement by sliding or by subglacial sediment deformation along its southern, western, and northwest- ern sectors; evidence for either mechanism should be represented in the sedimentologic and geomorphic records. Thin ice in these re- gions indicates that the Laurentide Ice Sheet contained less ice volume and represented less of an orographic obstacle to atmospheric circulation than has been considered in mod- els of the ice sheet on a rigid bed with steep profiles. INTRODUCTION The Laurentide Ice Sheet was the largest of the Northern Hemisphere ice sheets that devel- Figure 1. Major lobes along the southern margin of the Laurentide Ice Sheet between Illinois oped during the last glacial maximum (ca. 18 and Montana. Geological Society of America Bulletin, v. 104, p. 595-605, 10 figs., May 1992. 595 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/5/595/3381535/i0016-7606-104-5-595.pdf by guest on 30 September 2021 596 P. U. CLARK specifying the driving stress of the ice (Fisher ern (1-14 kPa; Beget, 1987) sectors of the Lau- then contoured by projecting elevations from and others, 1985; Hooke and Mooers, 1986; rentide Ice Sheet. Reconstructions from parts of moraines parallel to the form lines (Fig. 2D). Mooers, 1989). These studies have given rise to the northeastern sector of the ice sheet (Buckley, At its maximum extent, the lobate southern two distinct views of this sector of the ice sheet: 1969; Clark, 1988; Klassen and Fisher, 1988) margin of the Laurentide Ice Sheet deposited (1) that it was thin and low-sloping, and thus indicate higher driving stresses (30-125 kPa). moraines that can be traced nearly continuously characterized by low values of driving stress, or Several previous workers have estimated as one system from Illinois to Montana. Esti- (2) that it was characterized by high values of former ice thicknesses and ice-surface slopes of mates of the ages of these moraines suggest that driving stress, and the ice was correspondingly various lobes of the southern Laurentide Ice they range from 14 to 21 ka (see reviews by thicker with steeper surface slopes. Sheet. Using moraine elevations along the mar- Clayton and Moran, 1982; Mickelson and oth- The purpose of this paper is to reconstruct the gin of the James Lobe and the land elevation in ers, 1983). Because ice-surface reconstructions ice-surface morphology of the southern Lauren- the central axis of the lobe, Flint (1955) esti- are made for individual lobes, where moraine tide Ice Sheet from Illinois to Montana (Fig. 1) mated a minimum ice thickness of 500 m. Sim- ages are more nearly the same, I assume that the and to discuss implications to dynamics of the ilarly, Flint (1971) used the elevation of moraines from each lobe were deposited former ice sheet. This reconstruction, based on marginal deposits of the Green Bay Lobe and contemporaneously. moraine elevations and flowlines, supports pre- the depth of Lake Michigan to estimate a min- Because significant differences in depositional vious estimates of low longitudinal ice-surface imum thickness of 580 m for the Lake Michigan environments existed along the southern margin profiles and driving stresses for parts of this sec- Lobe 400 km from its terminus. of the ice sheet, moraines marking the limit of tor of the ice sheet (Wright, 1972; Wright and Wright (1972) estimated the ice thickness of that margin have complex and varied morphol- others, 1973; Mathews, 1974; Beget, 1986). the Superior Lobe during the late-glacial Autom- ogies and sediments (Mickelson and others, ba phase as ranging up to 900 m over the Lake 1983, 1986; Attig and others, 1989). Moraines REGIONAL SETTING Superior basin, 270 km from the terminus. Sur- deposited by each lobe, however, generally have face slopes were -1.2 x 10"3. a similar origin. Moraines from the Lake Michi- As the Laurentide Ice Sheet advanced south- Mathews (1974) estimated low driving gan Lobe have "low local relief.. composed ward, ice flow was increasingly influenced by stresses (0.9-2.3 kPa)2 for ice-surface slopes de- mainly of subglacial till"; moraines of the Green pre-existing topographic lowlands, leading to the rived from the elevation of ice-marginal deposits Bay Lobe are comprised of "thick, sandy, and development of major ice lobes such as those of the James and Des Moines Lobes. loamy supraglacial till forming high-relief hum- examined here (Lake Michigan, Green Bay, Des Hooke and Mooers (1986) reconstructed the mocky topography"; and moraines of the James Moines, Chippewa, and James Lobes) (Fig. 1). Des Moines Lobe based on a specified driving and Des Moines Lobes have low- to high-relief Association of the major lobes with pre-existing stress of 60 kPa, arguing that it "seems to have hummocky topography underlain by clayey su- lowlands has long been known (Chamberlin, been distinctly higher than suggested by Ma- praglacial till (Mickelson and others, 1983, 1883; Leverett and Taylor, 1915; Horberg and thews (1974)" (p. 34). p. 13). Therefore, because ice surfaces are Anderson, 1956). The Lake Michigan Lobe reconstructed for individual lobes, the origin of flowed south out of the Lake Michigan basin in METHODS, ASSUMPTIONS, AND the moraine(s) associated with each lobe is not Illinois, the Green Bay Lobe advanced out of the POTENTIAL ERROR SOURCES considered to be a significant variable in the Green Bay lowland in Wisconsin, and the Chip- reconstruction. pewa Sublobe advanced down the Chippewa Ice-Surface Reconstructions Moraine elevations are commonly used to de- River valley. The Des Moines Lobe moved termine the surface elevation of former glaciers down the Minnesota River valley and then Ice-surface reconstructions presented here are (Mathews, 1967, 1974; Pierce, 1979; Beget, crossed a low divide and continued down the based on moraine elevations and flowlines re- 1987; Klassen and Fisher, 1988; Clark, 1988; shallow Des Moines River valley into Iowa. The constructed from ice-flow indicators, based on Denton and others, 1989; Bockheim and others, James Lobe advanced down the broad James the following assumptions: 1989). In order to measure elevations of mo- River valley in South Dakota. (1) the highest moraine elevation in any one raines of the southern margin of the Laurentide area corresponds to the ice-surface elevation at Ice Sheet that have varied relief and morphol- PREVIOUS WORK that point, ogy, I assumed that, for any given area, the (2) ice-flow indicators used to constrain highest elevation of a moraine within 2 km (ar- Low driving stresses have been inferred from flowlines for any one lobe formed contempo- bitrarily assigned) of the distal edge of the mo- reconstructed ice surfaces of the southwestern raneously, and raine corresponded to the ice-surface elevation (0.4-4.5 kPa; Mathews, 1974)2 and northwest- (3) formlines on the ice surface are perpen- in that area.