Comparison of esker morphology and sedimentology with former ice-surface topography, Burroughs Glacier, Alaska KENT M. SYVERSON* 1 STEPHEN J. GAFFIELD* > Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 DAVID M. MICKELSON J ABSTRACT An esker observed melting out of the ice was the southeastern margin of Burroughs Gla- initially capped by flat fluvial terraces exhibit- cier and use hydraulic head maps to com- Topographic maps of the Burroughs Glacier ing a braided channel pattern. A sharp crest pare esker paths, morphology, and sedi- ice surface and the surrounding land surface developed as the sediment slumped to either mentology to predictions derived by Shreve from 1948 to 1990 were used to generate sub- side of the ridge. (1985b). glacial hydraulic head maps and compare es- ker paths to hydraulic gradients. Hydraulic INTRODUCTION Esker Genesis gradient at the glacier bed is controlled pre- dominantly by ice-surface slope, but it is also Purpose of Study The term esker in this paper is defined as a influenced by the slope of the glacier bed. Es- long, narrow ice-contact ridge, commonly kers at Burroughs Glacier have been observed Burroughs Glacier is located in the north- sinuous, and composed chiefly of stratified forming in subglacial and englacial tunnels. ern part of Glacier Bay National Park and sediment (Flint, 1971). Eskers have been de- Most of the eskers formed in subglacial stream Preserve, between lat. 58°57' and 59°1'N and scribed and research on them has been sum- tunnels oriented parallel to the calculated hy- long. 136°13' and 136°22'W in southeastern marized by Charlesworth (1957), Flint (1971), draulic gradient. Where the former ice-surface Alaska (Fig. 1). Glacier Bay is a region with Price (1973), Embleton and King (1975), Sug- slope and slope of the bed do not coincide, the a maritime climate characterized by small an- den and John (1976), and numerous other relative influence of these factors on esker nual and daily temperature fluctuations, high journal articles and texts. Eskers generally paths is analyzed. One esker is oriented par- relative humidity and cloudiness, and heavy form parallel to the ice-flow direction in two allel to the land-surface contours and perpen- precipitation (Loewe, 1966). main ways. First, eskers form where engla- dicular to hydraulic head contours, implying A long historical record of rapid déglacia- cial or subglacial streams discharge into an esker path controlled by the ice-surface tion at the southeastern margin of the Bur- standing water at the glacier margin. A series slope. In another area, a set of subglacially en- roughs Glacier makes it possible to compare of small deltas form a segmented ("beaded") gorged eskers trends perpendicular to the land esker paths, morphology, and sedimentology ridge as the ice retreats. Eskers formed in this contours (directly down the slope of the former to known past ice configurations. Shreve way are time transgressive, with the older glacier bed) and parallels the calculated hy- (1985a, 1985b) described the Katahdin esker segments located in the downstream direc- draulic head contours. These eskers formed system in Maine, an area with topography tion (De Geer, 1897; Baneijee and McDon- beneath thin ice in tunnels that were air-filled similar to that surrounding the Burroughs ald, 1975; Hebrand and Amark, 1989). much of the time, and thus the slope of the Glacier. He used land-surface topography The second mode of origin is deposition in glacier bed controlled the esker paths. In a and an estimated ice-surface slope to explain subglacial tunnels (Flint, 1971; Shreve, third area where the hydraulic gradient and esker paths and sedimentology observed. He 1985a, 1985b; Ashley and others, 1991). glacier bed slope in the same direction, a sub- stated that rapid melting was associated with Many of this type of esker, commonly glacially engorged esker trends down the land viscous heating of flowing water in nearly formed in the lee sides of nunataks, have slope and the calculated hydraulic gradient. level or descending tunnels within ice at the been observed melting out of the ice at the Most of the eskers at Burroughs Glacier are pressure melting point. This caused large southeastern Burroughs Glacier during the <6 m high, are sharp crested, commonly cross flows of basal ice and entrained debris into past 25 yr (Mickelson, 1971; Syverson, 1992). small hills, and contain poorly sorted, poorly the tunnel and produced sharp-crested eskers Price (1966) noted that the eskers at Case- stratified gravel and sand. The lack of sedi- that contain poorly to moderately sorted, ment Glacier (—15 km east-southeast of Bur- mentary structures implies high sediment in- poorly stratified sand, gravel, and boulders roughs Glacier) also formed on the lee sides flux rates to the ice tunnel during formation with rock types similar to the adjacent till. He of nunataks. The nature of the conduit and/or rapid deposition late during esker for- also postulated that large amounts of melting through which the stream flows and the site mation. "Anticlinal" bedding is not common. ice within the tunnel could cause changes in of deposition control esker sedimentation the location of the main subglacial stream, (Price, 1966; Koteff, 1974; Baneijee and Mc- eroding sediment and forming discontinuous Donald, 1975; Hebrand and Amark, 1989). *Present address: Syverson: Department of Ge- bedding and angular unconformities. In this Steady-state conditions in the conduit are de- ology, University of Wisconsin, Eau Claire, Wis- study we describe the morphology and sedi- consin 54702-4004; Gaffield: BT2, Inc., 3118 Wat- fined by water pressure controlled by the ford Way, Madison, Wisconsin 53713. mentology of recently exposed eskers near weight of overlying ice, water discharge, ice- Geological Society of America Bulletin, v. 106, p. 1130-1142, 8 figs., 1 table, September 1994. 1130 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/9/1130/3382083/i0016-7606-106-9-1130.pdf by guest on 01 October 2021 ESKER MORPHOLOGY AND ICE-SURFACE TOPOGRAPHY of the study area, in ca. A.D. 1700 (Fig. 1). The ice remained at its maximum until some- time between 1817 and 1842 (McKenzie, 1970). Changing climatic conditions near the end of the Neoglacial ice maximum caused a neg- ative glacier mass balance and ice-margin re- treat. Calving occurred at the narrow mouth of Glacier Bay near Bartlett Cove during the Neoglacial maximum. The calving rate accel- erated when the ice margin retreated north- ward to the wider and deeper parts of Glacier Bay (Fig. 1), and the glaciers rapidly de- creased in areal extent and thickness. The earliest maps of the Burroughs Glacier area, published by Cushing (1891) and Reid (1896), show a thick ice mass called the Cushing Pla- teau covering all but the highest peaks. Mick- elson (1971) suggested that ice was 650-700 m thick in the Burroughs Glacier region dur- ing the Neoglacial maximum. The historical record indicates that an area of —510 km2 has been deglaciated in the Burroughs Glacier re- gion since 1900. During this time, the thinning Cushing Plateau separated into several small glaciers (called the Burroughs, Plateau, Cushing, and Carroll Glaciers) as nunataks and ridges emerged from the ice (Fig. 1). Lar- son (1978) reported ice-surface lowering of 7.1 m/yr at —125 m above sea level in Figure 1. Maps showing locations of (A) Glacier Bay and (B) the study area (arrow). Bu, 1972-1973. Burroughs Glacier; W, Wachusett Inlet; C, Carroll Glacier; Cu, Cushing Glacier; Q, Queen The deglaciation of the Burroughs Glacier Inlet; Be, Bartlett Cove—location of the ice margin during the Neoglacial maximum in the early region has been closely monitored since the 1800s. Modified from Smith (1990). early mapping by Cushing and Reid. Photo- graphs and observations made by the Amer- ican Geographical Society since 1926 have been summarized by Field (1947,1959). Field surface topography, bed topography, ice gested that ice-tunnel sediment deposition studies on the glaciology of the Burroughs temperature, and bed permeability (Shreve, occurs at the ice margin or within 3-4 km of Glacier by Taylor (1962, 1963) and Gaffield 1972; Paterson, 1981). it during déglaciation and is time transgres- (1991), glacial geology by Mickelson (1971) Eskers have been used by numerous work- sive. It is likely that final deposition in eskers and Syverson (1992), and glacial hydrology ers to interpret the déglaciation history of at Burroughs Glacier was late during by Larson (1977,1978) supply additional de- large areas (Shreve, 1985a; Aylsworth and déglaciation. tailed information that forms the basis of this Shilts, 1989; Ashley and others, 1991); how- modern process study. Land- and ice-surface ever, the timing of final sediment deposition DEGLACIATION maps of the southeastern Burroughs Glacier within ice tunnels is problematic. Most re- region in 1948,1960,1970, and 1990 provide searchers agree that final esker sedimentation The Neoglacial (Porter and Denton, 1967) detailed control on the rapid deglaciation in takes place during the waning stages of de- was a time of marked glacier expansion in this area (Fig. 2). The 1948 topographic map glaciation. Shreve (1985b) suggested that the coastal Alaska. A stump pushed over by the (Fig. 2A) shows that nunataks were not ex- 150-km-long Katahdin esker system in Maine ice at the base of Nunatak A (Fig. 2D) indi- posed at the southeastern Burroughs Glacier, formed simultaneously, citing evidence such cates that Neoglacial ice reached this eleva- but they were exerting much control over the as the increase in esker size downstream, es- tion in the Burroughs Glacier area —2500 yr ice-surface topography. Photographs from kers crosscutting later ice-margin positions, B.P. (Mickclson, 1971).