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A Glacial-Process Model: the Role of Spatial and Temporal Variations in Glacier Thermal Regime

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A Glacial-Process Model: the Role of Spatial and Temporal Variations in Glacier Thermal Regime A glacial-process model: The role of spatial and temporal variations in glacier thermal regime H. D. MOOERS Department of Geology and Geophysics, 108 Pillsbury Hall and Limnological Research Center, 220 Pillsbury Hall University of Minnesota, Minneapolis, Minnesota 55455 ABSTRACT ucts of glaciation also provide important infor- there are several distinct thermal environments, mation on the physical nature and dynamics of each of which has a dominant set of erosional or Glacial landform assemblages associated the ice. Moran (1971), Clayton and Moran depositional processes. The position and charac- with the retreat of the Rainy and Superior (1974, 1977), and Moran and others (1980) teristics of these zones also vary with time during lobes of the Laurentide Ice Sheet in Minne- used field relationships to reconstruct the glacio- glacial advance and retreat. sota provide a record of spatial and temporal logical conditions necessary for the formation of The glacial sediments and landforms in Min- variations in glaciological processes. The large-scale glaciotectonic features, and they pro- nesota associated with the late Wisconsin St. two lobes advanced contemporaneously to vided a major advance in the understanding of Croix Phase provide a detailed record of the their maximum at the St. Croix moraine. The ice-marginal glacial processes. Mathews (1974) maximum and recessional events of the Superior portion of the moraine formed by the Rainy reconstructed longitudinal profiles of the Lau- and Rainy lobes. During recession, each major lobe is characterized by large frontal outwash rentide Ice Sheet and changed the way subse- lobe broke up into multiple, smaller sublobes, plains, broad ice-stagnation topography, gla- quent researchers have viewed ice lobes along its dynamically independent from one another. The cial thrust systems, and an absence of stream- southern margin (for example, Clayton and oth- well-defined recessional events and their asso- lined landforms and features produced by ers, 1985; Beget, 1986; Hughes, 1987a; An- ciated suites of landforms are well suited to stud- subglacial drainage. Recessional moraines are drews, 1987). Shreve (1985a, 1985b) used ies of the spatial and temporal variations in similar to the terminal moraine, but large- characteristics of eskers along with a simple in- glacial processes. The landform assemblages, scale glacial thrust systems are even more novative numerical technique to determine the along with paleoecological data, were used to common. In contrast, the part of the moraine glacial characteristics responsible for esker for- constrain numerical models of the thermal char- formed by the Superior lobe is composed of mation and to reconstruct longitudinal profiles acter and geometry of the Rainy and Superior closely spaced recessional ridges with little of portions of the Laurentide Ice Sheet in Maine. lobes of the Laurentide Ice Sheet in Minnesota ice-stagnation topography. Recessional ice Most of the aforementioned studies concern (Mooers, 1988, 1989a, 1989b). These recon- margins are marked by the termination of the nature of the ice sheet in one particular area structions are summarized below and are used to eskers and tunnel valleys into subaerially and at one point in time. Mickelson and others develop a conceptual model of the changes in deposited outwash fans. Drumlins are abun- (1983, 1986), however, discussed the impor- glacial processes and landform development dant, and sugblacial drainage features are tance of the spatial distribution of glacial land- during ice recession. well developed, as indicated by the numerous forms and asserted that temporal considerations tunnel-valley and esker systems. The thermal are equally important in interpreting physical THE STUDY AREA regime of the Rainy lobe changed little during characteristics of the ice. The value of such tem- retreat. Landforms indicate that a frozen toe poral reconstructions was convincingly demon- During the St. Croix Phase, the Superior lobe existed at the terminal and recessional posi- strated by Bjorck and Moller (1987) in central advanced to the southwest along the axis of the tions, and that conditions were too cold to Sweden, where glacial sediments and landforms Lake Superior basin, while the penecontempo- allow surface meltwater to penetrate to the were used to reconstruct glaciological character- raneous Rainy lobe traversed relatively higher glacier bed. The Superior lobe, on the other istics of the Scandinavian Ice Sheet during ice terrain in northeastern Minnesota (Fig. 1). The hand, apparently underwent an evolution recession. deposits of these lobes now cover much of cen- from a subpolar glacier at its maximum to a The basal and ice-marginal thermal regimes tral Minnesota, and the terminus of this advance temperate glacier during ice recession. Engla- of ice sheets vary spatially, as do the associated is marked by the prominent St. Croix moraine. cial and subglacial drainage systems became erosional and depositional processes (Weert- This large morainic system extends from Leech better developed during retreat, and drumlin man, 1961; Boulton, 1972; Paterson, 1981, Lake in north-central Minnesota southward to formation was favored during recessional p. 185; Eyles and Menzies, 1983, p. 19-23; an area southwest of St. Cloud (Fig. 1). South of phases. Clarke and others, 1984; Drewry, 1986, this point, the moraine is mantled by younger p. 17-18). Climatic change results in a temporal drift, but it can be traced to the southeast as a INTRODUCTION thermal evolution of ice sheets and further spa- prominent topographic feature cored with the tial variation in glaciological processes (Nye, distinctive St. Croix-Phase drift (Wright, 1972). Glacial sediments and landforms provide a 1960, 1963a, 1963b, 1965; Mickelson and oth- The moraine emerges from beneath the younger valuable record from which to study the glacial ers, 1983; Hughes, 1987b; Mooers, 1988, drift in the vicinity of Minneapolis and abruptly history of the Laurentide Ice Sheet. These prod- 1989a, 1989b); therefore, beneath an ice sheet turns to the northeast into Wisconsin. The part Geological Society of America Bulletin, v. 102, p. 243-251, 6 figs., 1 table, February 1990. 243 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/2/243/3380832/i0016-7606-102-2-243.pdf by guest on 25 September 2021 244 H. D. MOOERS al outwash are present (Figs. 2a, 2b), but evidence of channelized subglacial drainage is rare, as are drumlins (Mooers, 1988). Therefore the meltwater that produced the outwash apron presumably came from surface melt, with little or no contribution from subglacial sources. The possibility that subglacial drainage features ex- isted and are now buried is unlikely. Deposits of the St. Croix Phase upglacier from the moraines are relatively thin (2 to 5 m) and are composed largely of lodgment till with a veneer of glacio- fluvial sediment in low areas (Goldstein, 1985, p. 47, 61; Mooers, 1988). Subglacial drainage features such as eskers and tunnel valleys should still have some surface expression, but none can be identified. Large-scale glacial thrust systems, which can be recognized by the presence of a high hill bounded on the up-glacier side by a deep lake basin of similar volume, are associated with all of the end moraines but are more numerous at recessional ice-margin positions (Fig. 2b). The thrust systems are composed of an interlayered sequence of displaced blocks of drift and bed- rock separated by proglacial outwash, unlike the single thrusted masses of bedrock such as those in North Dakota (Moran and others, 1980). Be- cause of their complex nature, Mooers (1988), following the work of Boyer and Elliot (1982), referred to these features as "thrust systems." One such structure consists of a mass of Cre- taceous lignitic shale overlying glacial outwash (Mooers, 1988). The décollement between the shale and the underlying outwash is marked by a Figure 1. Location and general distribution of landforms in central Minnesota. shear zone ~1 m thick. The Cretaceous sedi- ments, which compose only a part of the total thrust system, were apparently quarried from the of the St. Croix moraine and associated deposits hummocky stagnation topography, glacial thrust lake basin and then transported to their present under investigation here, however, is that south systems, or proglacial fans at the terminations of position by shearing in less-competent underly- of Leech Lake and north of the younger drift particular esker and tunnel-valley segments ing beds. cover, termed the "western sector" of the St. (Mooers, 1988, 1989a, 1989b). These retreatal In marked contrast to the northern part of the Croix moraine. This western sector can be di- features and the contrasts in landscape develop- St. Croix moraine, the southern part has smaller vided into two regions on the basis of sedimen- ment between areas traversed by the Rainy and amounts of frontal outwash, and the moraine is tology, morphology, and drainage relationships. Superior lobes are directly related to the differ- composed of a series of closely spaced transverse These two segments, and the features formed by ent glaciological regimes of the former ice ridges. Hummocky stagnation topography in- the recession of ice from them, are hereafter re- masses. dicative of a superglacial drift blanket is less ferred to as the "northern and southern regions Within the northern region, which is the area common there and is absent at recessional ice of the St. Croix-Phase deposits" (Fig. 1) glaciated by the Rainy lobe, the St. Croix mo- margins, which instead are marked by small (Mooers, 1988). raine is characterized by broad hummocky stag- outwash fans with eskers commonly ending at Previous investigations treated the recession nation topography with a maximum local relief fan apices. Throughout this region, there is of the Superior and Rainy lobes from their max- of 10 to 25 m (Fig. 2a). Recessional ice margins abundant evidence of a well-developed englacial imum at the St. Croix moraine as a single phase, are marked by bands of stagnation topography 2 and subglacial drainage system, for tunnel val- with successive formation of drumlins, tunnel to 3 km wide (Fig.
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