A Holocene history of dune-mediated landscape change along the southeastern shore of Lake Superior

Geomorphology 61 (2004) 303 – 322 www.elsevier.com/locate/geomorph

A Holocene history of dune-mediated landscape change along the southeastern shore of Lake Superior Walter L. Loope a,*, Timothy G. Fisher b, Harry M. Jol c, Ronald J. Goble d, John B. Anderton e, William L. Blewett f a U.S. Geological Survey, N8391 Sand Point Road, P.O. Box 40, Munising, MI 49862, USA Department of Earth, Ecological and Environmental Sciences, University of Toledo, Toledo, OH 43606, USA c Department of Geography and Anthropology, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54702, USA d Department of Geosciences, University of Nebraska, 200 Bessey Hall, Lincoln, NE 68588, USA e Department of Geography, Northern Michigan University, Presque Isle Avenue, Marquette, MI 49855, USA f Geography and Earth Sciences Department, Shippensburg University of Pennsylvania, 1871 Old Main Drive, Shippensburg, PA 17257, USA b

Received 14 July 2003; received in revised form 20 January 2004; accepted 30 January 2004 Available online 23 April 2004

Abstract Causal links that connect Holocene high stands of Lake Superior with dune building, stream damming and diversion and reservoir impoundment and infilling are inferred from a multidisciplinary investigation of a small watershed along the SE shore of Lake Superior. Radiocarbon ages of wood fragments from in-place stumps and soil O horizons, recovered from the bottom of 300-ha Grand Sable Lake, suggest that the near-shore inland lake was formed during multiple episodes of late Holocene dune damming of ancestral Sable Creek. Forest drownings at f 3000, 1530, and 300 cal. years BP are highly correlated with local soil burial events that occurred during high stands of Lake Superior. During these and earlier events, Sable Creek was diverted onto eastward-graded late Pleistocene meltwater terraces. Ground penetrating radar (GPR) reveals the early Holocene valley of Sable Creek (now filled) and its constituent sedimentary structures. Near-planar paleosols, identified with GPR, suggest two repeating modes of landscape evolution mediated by levels of Lake Superior. High lake stands drove stream damming, reservoir impoundment, and eolian infilling of impoundments. Falling Lake Superior levels brought decreased sand supply to dune dams and lowered stream base level. These latter factors promoted stream piracy, breaching of dune dams, and aerial exposure and forestation of infilled lakebeds. The bathymetry of Grand Sable Lake suggests that its shoreline configuration and depth varied in response to events of dune damming and subsequent dam breaching. The interrelated late Holocene events apparent in this study area suggest that variations in lake level have imposed complex hydrologic and geomorphic signatures on upper Great Lakes coasts. D 2004 Elsevier B.V. All rights reserved. Keywords: Holocene; Great Lakes; Dune building; Dune damming; Lake level

* Corresponding author. Tel.: +1-906-227-1955; fax: +1-906-387-4025. E-mail address: [email protected] (W.L. Loope). 0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.01.005

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1. Introduction 1.1. Holocene variation in levels of the upper Great Lakes: changing paradigms The Holocene history of lake levels within the upper Great Lakes basin (Fig. 1A) has been a subject of study for over 100 years. Prominent beach ridges, spits, barriers, and wave-cut terraces now perched high above the lakes are signatures of prior lake level variation. Geophysical and erosional mechanisms tied to this variation were well established by the 1960s (e.g., Hough, 1958; Farrand, 1962). Upon the retreat of Wisconsinan ice from the northern basin, the upper lakes drained easterly through North Bay, Ontario (NB in Fig. 1A) and the Ottawa River to the St. Lawrence Seaway. After deglaciation f 11,600 cal. years BP (Saarnisto, 1975; Fisher and Whitman, 1999; Lowell et al., 1999), rebound of the isostatically depressed North Bay outlet led to the mid-Holocene ‘‘Nipissing transgression’’ of the upper lakes (Farrand and Drexler, 1985; Larsen, 1987; Fig. 1B). The transgression continued with rebound of North Bay until the common water plane of the upper lakes (Superior, Michigan, Huron) encountered outlets at Port Huron and Chicago (PH and C in Fig. 1A) and spilled to the south f 5300 cal. years BP. Erosion of these outlets brought a decline in the water plane and establishment of modern drainage and lake levels (Hough, 1958; Fig. 1A, B). Lake Superior was separated from Lakes Michigan and Huron after f 2000 cal. years BP as rebound of a rock sill at Sault Ste. Marie (SSM in Fig. 1A) rose above the surface of the common water plane (Farrand and Drexler, 1985; Fig. 1B). Additional, finer-scale climatic control of lake levels was discovered (Larsen, 1985; Fraser et al., 1990) and quantified (Thompson and Baedke, 1997; Baedke and Thompson, 2000; Fig. 1C) after 1980. ‘‘Quasi-periodic’’ climate-driven highs and lows in lake level appear to be separated by one to two centuries (Fig. 1C).

1.2. Lake-level change, dune building and coastal hydrology The dynamics of Great Lakes coastal dunes during the Holocene have been linked with variation in geophysical/erosional and climate-driven lake level. While studies of beach ridge foredunes along depositional coasts have correlated ground water dynamics, wetland development, and stream regimen with dune response and lake-level change (e.g., Olson, 1958; Fraser et al., 1990; Keough et al., 1999; Booth et al., 2002), investigations of perched dunes on eroding coasts have, to date, focused primarily on physical mechanisms of change in sand supply relative to lake level (e.g., Arbogast and Loope, 1999; Loope and Arbogast, 2000). In perched dunes, such as the Grand Sable Dunes along the SE shore of Lake Superior (Fig. 1A, D), Holocene high stands of the lake have stimulated multiple episodes of dune building while low lake phases have led to periods of relative quiescence and stabilization of dunes by vegetation (Anderton and Loope, 1995; Fig. 1E). Because 300-ha Grand Sable Lake and its tiny outlet, Sable Creek ( f 5 –25 cfs annual discharge; Handy and Twenter, 1985), lie directly in the lee of the Grand Sable Dunes and strong effective winds (Marsh and Marsh, 1987; Figs. 1F and 2A), hydrologic changes to the stream and lake during episodes of dune building might be expected. Grand Sable Lake, lying 28 m above the Nipissing high stand, has been interpreted as a former bay of Lake Algonquin (Bergquist, 1933) and, more recently, as a kettle lying within a Marquette-age glacial drainage, now partially filled by periodic Holocene incursions of the Grand Sable Dunes (Hughes, 1985). 1.3. A dune-dammed lake? Sediment cores collected from the northern floor of Grand Sable Lake (Loope et al., 1999) suggest that the lake is neither a former bay nor a kettle. Radiocarbon

Fig. 1. Study area and lake-level histories. (A) Location of the study area on the Upper Peninsula of Michigan on the southern shore of Lake Superior. (B) Lake level history of Lake Superior since deglaciation. (C) Lake level history of Lake Michigan, modified from Baedke and Thompson (2000). (D) Oblique aerial view of the Grand Sable Plateau looking to the east. Note the f 80-m-tall bluffs with perched dunes. (E) Perched dune model from Anderton and Loope (1995). Top sketch shows stabilized bluff when lake is low (arrow), and in the bottom sketch the bluff is constantly undermined, exposing sediment that is blown on top of the bluff reactivating the dunes. (F) Location of Grand Sable Lake south of the Grand Sable Banks and Dunes and between meltwater terraces to the east and west. Digital elevation model has a synthetic light source from the north at 35j elevation.

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ages of wood recovered from the cores suggest that the lake may have formed as a result of dune-damming in the Holocene that impounded Sable Creek within a late Pleistocene meltwater channel and drowned extant forests. What is the evidence supporting the dune-damming of Sable Creek? If dammed, how was the local water table affected and the discharge of Sable Creek redirected? What was the fate of subsequent eolian sediment? How did the Nipissing transgression and abrupt post-Nipissing decline in regional base level (Fig. 1B, C) affect Sable Creek and Grand Sable Lake?

2. Purpose In this paper, we test the hypothesis that Holocene dune building, driven by fluctuating levels of Lake Superior, was responsible for the creation of Grand Sable Lake, for accompanying changes in the local water table, and for multiple redirections of surface streams. Testing of this hypothesis will contribute to a general model relating lake-level-driven landscape change with water table and stream dynamics along the upper Great Lakes.

3. Methods 3.1. Study area Grand Sable Lake (Figs. 1 and 2) lies along the SE shore of Lake Superior west of the town of Grand Marais, MI. The lake is fed by De Mull, Towes, and Rhody Creeks and drains to Lake Superior via Sable Creek. Parallel to Sable Creek and about 2 km to the east, First Creek flows north into Lake Superior. A striking series of eastward-graded meltwater terraces, apparent products of final deglaciation (Farrell and Hughes, 1985; Blewett and Rieck, 1987; Blewett, 1994), lie north and east of Grand Sable Lake (Fig. 1F: T1 –T4 in Fig. 2A). These terraces began to form after about 11,600 years BP (Saarnisto,

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1975; Fisher and Whitman, 1999; Lowell et al., 1999) when an eastward-flowing meltwater stream was confined against the north-sloping edge of land by retreating Marquette ice. As ice retreated farther northward into the Superior Basin and briefly stabilized, lower terraces were built graded to the east (e.g., Fig. 2A; T1: f 209 m, T2: f 222 m, T3: f 231 m, T4: f 247 m). The Grand Sable Plateau, a high-standing glaciofluvial platform (Fig. 2), lies west of the terraced topography. Stability of the Grand Sable Plateau for f 3000– 4000 years after deglaciation and prior to emplacement of dunes is indicated by the presence of the Sable Creek Soil, a buried Entic Haplorthod (Anderton and Loope, 1995). The surface of the Sable Creek Soil descends eastward off the Grand Sable Plateau and dips beneath dune-draped sand flats north of Grand Sable Lake (Fig. 2C). 3.2. Strategy for hypothesis testing In testing the hypothesis, we employ a multidisciplinary approach that includes: (i) Stratigraphic analysis of sediments and radiocarbon dating of organic matter in two sediment cores recovered from the floor of Grand Sable Lake using a vibracorer (V1 –V2, Fig. 2A). (For information on vibracoring, consult Smith, 1984, 1992; Thompson et al., 1991; Fisher and Whitman, 1999; Moorman, 2001). One in-place stump was recovered from the southern floor of Grand Sable Lake and radiocarbon dated (V3, Fig. 2A). (ii) Stratigraphic interpretation of reflections from ground penetrating radar (GPR) made for three transects positioned at right angles to the hypothesized paleovalley of Sable Creek (R1 –R3, Fig. 2A). For information on using GPR and its theory see Davis and Annan (1989); Beres and Haeni (1991); Jol and Smith (1991); and Jol and Bristow (2003). (iii) Stratigraphic analysis and radiocarbon and optically stimulated luminescence (OSL) dating

Fig. 2. (A) Location of the coring sites (V1 – 3), ground penetrating radar transects (R1 – 3), soil pits (S1 – 3), elbow of capture (E), terraces (T1: 209 m, T2: 222 m, T3: 231 m, T4: 247 m), springs along the bluff overlooking Lake Superior (SP1 – 4), and locations of cross-sections X – XV and Y – YV. (B) Detailed bathymetry of Grand Sable Lake after Kamke (1985) with locations of V1 – 3. Outflowing Sable Creek is in the NE corner flowing eastward. (C) Topographic profiles for the area shown in A with location of the Sable Creek Soil.

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(see Murray and Wintle, 2000) of surface soils, near-surface buried soils, and other deposits accessed in many soil pits and soil auger transects throughout the study area. Arguments of correlation and cause rely on an integrated interpretation of data.

4. Results and discussion 4.1. Drowned forests and dune building near Grand Sable Lake Sediment cores V1 and V2 (Figs. 2B and 3) record major hydrologic changes within the Sable Creek drainage basin in late Holocene time. Chief among

Table 1 Radiocarbon ages of soil-drowning events (inferred from lake cores) compared with radiocarbon ages of soil-burial events (i.e., dune building) east of the Grand Sable Dunes (highly correlated events are shown in Fig. 9) 14

Lab number

Material

WW-2348

Wood

295 F 65

460 – 290

WW-3086

Wood

300 F 40

430 – 300

GX-18512

Wood

680 F 50

670 – 560

WW-3546

Soil

900 F 40

910 – 740

Beta-162656

Wood

1370 F 60

1350 – 1180

WW-2347

Wood

1645 F 55

1690 – 1420

WW-2691

Wood

1670 F 50

1690 – 1520

WW-1155

Wood

1870 F 50

1879 – 1730

WW2349

Wood

2895 F 55

3160 – 2940

WW-2552

Wood

3660 F 40

4090 – 3900

WW-2550

Wood

3730 F 50

4150 – 3980

Beta-68378

Wood

3910 F 50

4420 – 4250

WW-3544

Charcoal

5160 F 50

5990 – 5760

WW-2692

Charcoal

6390 F 50

7420 – 7260

C age

Calendar age (1r range) (BP)

Eventa

Drowned soil GSL Drowned soil GSL Soil burial (east) Soil burial (east) Soil burial (east) Drowned soil GSL Soil burial (east) Soil burial (east) Drowned soil GSL Soil burial (east) Soil burial (east) Soil burial (east) Soil burial (east) Soil burial (east)

a GSL Grand Sable Lake, GSD Grand Sable Dunes, (east) refers to dunes on east side of lower Sable Creek (dark-gray area in Fig. 2).

Fig. 3. Lithostratigraphic cores of vibracores V1 and V2 from Grand Sable Lake. Core locations are shown in Fig. 2A.

these are drowning of a northern white cedar (Thuja occidentalis) forest at f 2990 cal. years BP (Fig. 3, core V2 collected at 6.4-m depth; Table 1) and later drowning of forests at 1560 cal. years BP (Fig. 3, core V1 collected at 4.5-m depth; Table 1) and at f 300 cal. years BP (Fig. 2A, V3 collected at 1.8-m depth; Table 1). Above the drowned soil horizons in cores V1 and V2 (Fig. 3), accumulation of gyttja and silt and very fine sand suggests that valley drowning was followed by long-term lacustrine sedimentation of debris associated with eolian fines derived from the Grand Sable Dunes (Anderton and Loope, 1995). Migration of a

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dune into the north end of Grand Sable Lake buried near-shore vegetation f 300 cal. years BP (Fig. 3, V2; Table 1). A radiocarbon date from one of a cluster of in-place submerged stumps (Fig. 2A, V3; Table 1) suggests contemporaneous drowning of a white cedar forest f 300 cal. years BP at the SE edge of the lake basin. An apparent Inceptisol, requiring several hundred to a thousand years of subaerial soil development (Birkeland, 1999), is buried in core V1 by medium sand 1.5 m below wood dated at 1560 cal. years BP. The presence of thin peat associated with wood recovered from the bottom of core V2 similarly suggests at least several hundred years of soil development prior to forest drowning. The three radiocarbon ages derived from lake cores correlate closely with radiocarbon ages of dune building events within the Grand Sable Dunes (Anderton and Loope, 1995) and with newly discovered burials of soil adjacent to and east of Sable Creek

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(Table 1). The presence of late Holocene dunes east of Sable Creek is not consistent with Drexler’s, (1979) contention that the creek has limited the eastward migration of the Grand Sable Dunes, but rather suggests that the Grand Sable Dunes blocked Sable Creek on several occasions, backflooding its valley. 4.2. A sediment-filled paleovalley revealed in GPR If dune-building events at f 3000, f 1560, and f 300 cal. years BP left evidence of damming in sediments of Grand Sable Lake, larger-scale events of mid-Holocene (i.e., Nipissing) age (Larsen, 1985; Thompson and Baedke, 1997; Baedke and Thompson, 2000; Fig. 1B, C) must have impacted the Sable Creek watershed to a much greater extent. GPR transects R1– R3 (Figs. 4 and 5) appear to confirm this prediction, revealing a north – south trending paleovalley between Grand Sable Lake and Lake Superior, now

Fig. 4. Oblique aerial view south from Lake Superior (foreground) of the Grand Sable Banks, Dunes, and Grand Sable Lake in the background. Ground penetrating radar transects (R1 – 3), soil pits (S1 – 3), and springs along the banks overlooking Lake Superior (SP2 – 4) are indicated.

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completely filled with sediment. Valley sides are defined by continuous, dipping reflections (Fig. 5A, distance 640 – 870 m, dipping at 3 – 4j; Fig. 5B, distance 0 – 280 m; Fig. 5C, distance 0 – 100 m, dipping at 9– 10j), which are probably soils developed on the late Pleistocene land surface, perhaps contemporaneous with the Sable Creek Soil that underlies the Grand Sable Dunes to the west (Anderton and Loope, 1995; Fig. 2C). The paleovalley appears asymmetric, the western side steeper than the eastern side, suggesting that ancestral (earliest Holocene) Sable Creek flowed along the western edge of the valley (Fig. 6A). Although the GPR transects portray valley sides to a maximum depth of f 25 m, the bottom of the paleovalley probably lies somewhat deeper, perhaps graded to the Houghton low phase of ancestral Lake Superior (Hough, 1958; Fig. 1B). Beneath the sides of the paleovalley, horizontal to sub-horizontal continuous to semi-continuous reflections are evident (Fig. 5B, distance 0– 270 m; Fig. 5C) and resemble those of documented braided channels (Jol et al., 1996; van Overmeeren, 1996). This supports the interpretation that earliest Holocene Sable Creek was incised into glaciofluvial deposits. 4.3. Nipissing damming and infilling of the Sable Creek paleovalley How and when was the Sable Creek paleovalley filled with sediment? As suggested in the prior section, Holocene hydrographs for the upper Lakes (Larsen, 1985; Fraser et al., 1990; Thompson and Baedke, 1997) hint that lake-level/dune-building events associated with forest drownings recorded in the lake-floor sediment cores must have paled in comparison to those caused by the earlier Nipissing high stands. Strong signals of Nipissing I and II phases have been identified along the southern shore of Lake Superior (Larsen, 1994; Anderton and Loope, 1995; Arbogast, 2000; Booth et al., 2002). We suggest that dunes had dammed ancestral Sable Creek by f 7340 cal. years BP, early in the Nipissing Transgression, (Figs. 1B and 6B; Table 1). 4.4. Underfit outlet channels If dune building along terrace T1 (Fig. 2A) dammed Sable Creek as suggested, a narrow reservoir

would have been impounded upstream from the point of aggradation/damming (Fig. 6B). The surface elevation of this reservoir and those of subsequent dunedammed reservoirs (Fig. 6C – J) would have depended upon the elevation of the lowest adjacent reservoir outlet, presumably along the terraced topography to the east. Transects established with a soil auger and soil pits along meltwater terraces T1 –T4 (Fig. 2A) reveal a system of small underfit outlet channels incised into each broad terrace. The channels are 1.5 to 2 m deep and are filled with silt to an extent that the surface expression is muted. Small, riparian trees (tag alder, Alnus rugosa) grow preferentially along these channels, thriving on the enhanced ground water supply they afford. We suggest that these channels sequentially carried the flow of Sable Creek when the prior course of the stream was blocked by sand dunes encroaching from the west. The channels can be traced for several kilometers and some are interconnected with spectacular spillways (A in Fig. 7). Channel sizes are consistent with the channel size of modern Sable Creek. The channel within terrace T1 (Fig. 2A) was likely incised by Sable Creek soon after its initial diversion, early in the Nipissing transgression (Figs. 6B and 7). Most of the northern portion of terrace T1 has been lost to bluff recession. The downstream portion of the channel once carried the discharge of Sable Creek eastward past the present path of First Creek and into the present-day site of the town of Grand Marais (Fig. 1F). Springs flow from the truncated path of the channel along present First Creek. The orientations of small channels cut into terraces T2 and T3 (Fig. 2A) suggest that they carried the flow of early Holocene Sable Creek eastward, perhaps at the peak of the Nipissing Transgression (Figs. 6C, E and 7). In similar fashion, a channel cut into terrace T4 briefly carried the flow of Towes Creek after the creek was diverted northeastward by a sand sheet (SS in Fig. 2A and c in Fig. 6E). Because development of the spodic horizon on the surface of this sand sheet (Carey, 1992) likely required several thousand years of subaerial exposure, it was probably emplaced by a very high sand flux across the frozen lake in winter (McKenna Newman, 1993; Fisher and Whitman, 1999) during Nipissing II time ( f 4500 cal. years BP). The flow of diverted Sable and Towes Creeks and increased ground water flow along terraces T3

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Fig. 5. (A – C) GPR images of transects R1 – R3 (upper) and interpretations of sedimentary structures and buried soils (lower). Eolian valley fills are light gray; lacustrine valley fills, medium gray; and braided outwash deposits, darkest gray.

pp. 311 – 312

pp. 313 – 314

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Fig. 5 (continued).

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Fig. 6. (A – J) Chronology of inferred Holocene events within the drainage of Sable Creek. See the text for more details.

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Fig. 7. Vertical aerial photograph stereopairs showing channels that carried the discharge of Sable Creek across deglacial meltwater channels during dune-damming episodes.

and T4 (Figs. 6E and 7: A and B) likely initiated the gully that now comprises First Creek. 4.5. Sequence of valley infilling, geometry of subaerial and subaqueous depositional terraces We have claimed that small underfit channels cut into deglacial meltwater terraces periodically carried the flow of Sable Creek and that the elevations of these meltwater terraces determined the elevation of water surfaces of a series of reservoirs that occupied the Sable Creek Valley. Reservoirs impounded by dunes would have lain near to and directly in the lee of the high standing Grand Sable Plateau (i.e., Fig. 6B –J). Eolian sediment from the Grand Sable Plateau

would have entered the reservoirs from the NW. GPR transects R1 –R3 contain reflections consistent with such deposition in that they suggest large bed forms with a westerly paleocurrent. These are mapped as eolian valley fill in Fig. 5A and B and are represented as active dunes in Fig. 6C, E, and G. Crossbedding in soil pit exposures (S1 and S3 in Fig. 2A; Fig. 8A, B) and in Fig. 5A (e.g., distance 770– 870 and 1050– 1200 m) indicate that large eolian bedforms entered the Sable Creek valley depositing west to east prograding foresets. OSL ages of samples from such foresets exposed in soil pits S1 and S3 (Table 2, Figs. 2 and 4) are consistent with their deposition during high lake stands at f 3000 and f 600 cal. years BP (Figs. 6G, J and 9). Similar

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317

Fig. 8. (A) Soil pit at S1 showing eolian cross beds dipping 25j to the SE. (B) Mechanisms of eolian transport of sediments from the Grand Sable Dunes to depositional terraces at the northern edge of Grand Sable Lake. (C) Eolian sedimentation in the Persian Gulf based on a series of wells (after Shinn, 1973).

prograding foresets have been observed along the western edge of the Persian Gulf where westerly winds have driven sand dunes into the ocean (Shinn, 1973; Fig. 8C). Because the shorelines of dunedammed reservoirs probably fluctuated considerably,

deposition of eolian sediments in the basins probably included subaqueous and subaerial components. The surfaces into which pits S1 and S3 were dug lie at 222 and 231 m, respectively, and correspond to the elevations of terraces T2 and T3 (Fig. 2A). These

Table 2 Optically stimulated luminescence (OSL) data Sample loc. (Figs. 2A and 4)

Burial depth (m)

H2 O (%)a

K2O (%)

U (ppm)

Th (ppm)

Cosmic (Gy)

Dose rate (Gy/ka)

De (Gy)

Age (ka)

Number of disks

S3 S1

1.0 1.0

6.3 2.3

1.89 1.66

0.6 0.5

2.7 2.1

0.19 0.19

1.89 F 0.05 1.73 F 0.04

1.17 F 0.05 5.12 F 0.16

0.62 F 0.05 2.96 F 0.20

30 33

OSL age is shown in bold italic type. a In-situ moisture content.

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Fig. 9. Holocene history of Grand Sable Lake (A), probability distribution of calibrated age of forest drowning within Grand Sable Lake (B) relative to levels of Lake Superior (C) generalized from Booth et al. (2002), and probability distribution of 40 ages from soil burial events (10 ages shown in Table 1, remainder are in Anderton and Loope, 1995) within and adjacent to the Grand Sable dunes (D). Event label at top relates curves to events portrayed in Fig. 6 and links them in time based on probability distribution the soil burial radiocarbon dates. Probability distribution of calibrated radiocarbon ages was calculated using Oxcal (Bronk Ramsey, 2001).

surfaces are also apparent within R1 at distance 1300 m and elevation 222 m and at the ground surface at elevation 231 m (Figs. 5A and 8B). 4.6. Paleosols near lake superior Two prominent, horizontal reflections in R2 (Fig. 5B: distance 255– 500 m, elevation 200 m and distance 360 –600 m, elevation 214 m) occur near the elevations of several paleosols that crop out in the

adjacent coastal bluff to the north (Fig. 4). On this basis, we interpret these reflections as paleosols. These soils formed as vegetation invaded subaerially exposed, near-planar valley fills. The lowest paleosol lies at 200 m, suggesting that its surface represents an embayment of the Nipissing I level (Larsen, 1985; Fraser et al., 1990) of ancestral Lake Superior f 5300 cal. years BP. The soil formed after these deposits was exposed as lake level receded f 5000 cal. years BP (Fig. 6D). The entrenched valley of ancestral Sable

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Creek during this lower lake phase may be evident in the lowest channel fill reflection pattern between distance 500 and 530 m in R2 (Fig. 5B). Radar reflections immediately above the lower paleosol at 200-m elevation probably represent an eolian fill emplaced as ancestral Lake Superior rose to Nipisssing II levels (Fraser et al., 1990; Fig. 6E). 4.7. Mid- and late-Holocene dune damming, regional base level change, and ground water mediated stream piracy In earlier sections, we suggested that during the Nipissing transgression, perched dunes dammed and diverted Sable Creek onto former meltwater terraces. These events resulted from the rise in regional base level (rising Lake Nipissing), which also reduced the ground water gradient from the upland meltwater terraces to Lake Nipissing. A lower ground water gradient would have lessened the probability of breaking the reservoir dam and terrace sides, permitting Sable Creek to flow along terraces T2 and T3 for f 1000 years during the peak of the Nipissing Transgression (Figs. 2A, 6E and 7). Regional base level fell quickly after f 4500 cal. years BP (Baedke and Thompson, 2000), increased local ground water gradients between Sable Creek and Lake Superior and facilitated piracy of the stream back to the west, toward its original channel. Because the substrate in the area is dominated by eolian sand and a minor coarser fraction, stream piracy is to be expected and was likely a function of headward channel growth though increased ground water sapping (Small, 1978; Pederson, 2001). One precise point of capture is identifiable in a prominent ‘‘elbow’’ (E in Fig. 2A; C in Fig. 7). 4.8. Springs and seeps along Lake Superior Four prominent seepage zones crop out along the base of the bluff facing Lake Superior at the north edge of the study area. The three westernmost seeps lie just north of R2 and R3 (Figs. 2A and 4). GPR reflections from R2 (Fig. 5B) suggest that the position of SP2 corresponds with a stratigraphic sag incised into Pleistocene braided outwash (at distance 50 – 60 m in R2), and SP3 and SP4 are associated with channel fills at 300 – 355 and at

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500– 535 m, respectively. The remaining seep, SP1, lies f 350 m east of SP2 at the base of the bluff. We suggest that the positions of these seeps correspond with earlier courses of Sable Creek. 4.9. Dunes east of Sable Creek and Grand Sable Lake Several suites of large, now forested, dunes and sand sheets occur east of Sable Creek and Sable Lake (dark gray area in Fig. 2A). Dune-building events, revealed in soil auger transects near and east of Sable Creek, are well correlated with similar events in the Grand Sable Dunes (Table 1).

5. Sequence of events Fig. 6A – J presents the hypothesized sequence of Holocene events within the Sable Creek watershed. Fig. 9 shows the interpreted correlation among the Holocene history of Grand Sable Lake, Lake Superior, and local dune building events. This history can be summarized as follows. During the early Holocene, Sable Creek lay to the west of its present location near the eastern base of the Grand Sable Plateau (Fig. 6A). As the Nipissing Transgression of the upper Great Lakes (Farrand and Drexler, 1985) reached the lakeward edge of terrace T1 (Fig. 2A), dune building along the terrace began (Farrell and Hughes, 1985) and Sable Creek was dammed and diverted eastward onto terrace T1 impounding a reservoir at f 209 m (Fig. 6B). After bluff recession during the rise toward Nipissing I had completely destroyed the western portions of terrace T1, the high bluffs of the Grand Sable Plateau were undermined and collapsed, thus providing a much higher-standing source of eolian sand (Figs. 1E and 6C). This sand, driven by northwesterly winds, dammed Sable Creek far upstream from its mouth. The level of this impoundment rose to a low divide at 222 m, the tread of terrace T2 (Fig. 2A), the next-higher meltwater terrace, and was drained by Sable Creek eastward along that terrace (Fig. 6C). The reservoir was partially infilled from the NW by eolian sand (valley fill a, Fig. 6C). Simultaneously, the original mouth of Sable Creek was embayed to 200 m, the peak level of the Nipissing Transgression, and infilled with lacustrine sediment.

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Falling levels of ancestral Lake Superior, after f 5500 cal. years BP, steepened gradients of ground water flow and facilitated piracy of Sable Creek westerly to the embayed (original) mouth of Sable Creek (early Sable Creek channel, distance 500 –550 m, Fig. 5B). The sandy Nipissing I embayment of Sable Creek was exposed by falling lake level and became vegetated (Fig. 6D). Rising lake levels between f 5000 and 4500 cal. years BP led to renewed dune building, re-impoundment of a reservoir along Sable Creek, and building of valley fill b (Fig. 6E). As lake level rose farther, dunes dammed Sable Creek against the side of terrace T3 (Figs. 2A and 6E) and eventually onto its surface and eastward (Fig. 7). Concurrent with this event, a large sand sheet (c in Fig. 6E) was emplaced along the east side of present Grand Sable Lake. This deposition diverted the flow of Towes Creek eastward along terrace T4 (Figs. 2A, 6E and 7). These diversions initiated First Creek, a spectacular gully, trending north to Lake Superior. Falling lake levels after 4500 cal. years BP again led to piracy of Sable Creek to the west (Fig. 6F). Valley fill a at 222 m (Fig. 6F) was exposed at the north end of Grand Sable Lake and became forested (location of V2 in Figs. 2A, B and 6F). Towes Creek was pirated back into Grand Sable Lake. Rising levels of Lake Superior f 3000 cal. years BP caused dune building sufficient to again dam and divert the flow of Sable Creek eastward onto terrace T2 and to raise the level of Grand Sable Lake above valley fill a at 222 m (Figs. 5A and 8B), drowning forests that had invaded it (Fig. 6G). OSL dating of foresets in soil pit S1 (Table 2) suggests that the narrow northern tip of newly constituted Grand Sable Lake was infilled by eolian sand f 3000 cal. years BP (Fig. 8A, Table 1: WW2349). Falling lake levels permitted landscape stabilization after f 3000 cal. years BP. The outlet of Grand Sable Lake was again lowered, and a shelf along its NW corner was exposed and invaded by forest (Fig. 6H). Another rise in Lake Superior f 1560 cal. years BP led to renewed dune building (Fig. 6I). Sable Creek was diverted to the east and occupied a channel that can be seen today east of Sable Falls within terrace T2. The level of Grand Sable Lake rose sufficiently to drown the forest that had previously invaded the exposed shelf (core V1, Figs. 2B and 3). The subsequent fall of Lake Superior

permitted landscape stabilization. A lake-level rise at f 600 cal. years BP led to additional dune building (Figs. 6J and 9; Table 2). Bluff retreat during fluctuating lake levels led to the capture of the Sable Creek drainage to its present position (Fig. 6J). The drowned forest in the south end of the lake and the root dated at f 300 cal. years BP (Figs. 2A and 3) record the most recent impoundment of Grand Sable Lake and reflect a lake level rise of about 2 m at that time (Fig. 9).

6. Conclusions The temporal correlation of known dune building events within and east of the Grand Sable Dunes and the drowning of forests on the floor of Grand Sable Lake (Fig. 9; Table 1) are perhaps the most striking evidence that dune building, driven by variations in levels of Lake Superior, served to impound Grand Sable Lake. Buried underfit channels provide evidence that repeated damming resulted in repeated eastward diversion of Sable Creek. While certain sedimentary structures revealed by GPR (e.g., eolian infills of impounded reservoirs, Fig. 5) can be directly linked with this hypothesis, the implied presence of multiple paleosols requires that the floors of partially filled reservoirs be exposed and forested. We suggest that this occurred several times during the Holocene as dune dams and the valley fills behind them were breached during low stands of Lake Superior. This interpretation suggests that, as levels of Lake Superior fell after 5300 and 4500 cal. years BP, gradients of ground water flow were steepened and Sable Creek was pirated westward toward its original channel. Because Sable Creek comprises the only surface stream in the area, piracy must have occurred through rapid channel extension by ground-water sapping. The circumstances that drove landscape changes in the vicinity of Grand Sable Lake during the Holocene are, perhaps, unusually complex. Evidence presented here, however, suggests that lake levelmediated dune building, a relatively common phenomenon throughout the Holocene (cf. Loope and Arbogast, 2000), may have driven repeated hydrologic adjustment within many small watersheds along erosional segments of the upper Great Lakes coastal zones.

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Acknowledgements Brian Carter of Grand Marais collected samples of wood from in-place stumps from the floor of Grand Sable Lake. Leo Badeaux of Grand Marais provided information on ground water flow on his property on First Creek. Neil Korsmo of the National Park Service in Grand Marais assisted in collection of vibracores from Grand Sable Lake. Henry Loope collected OSL samples. Melinda Stamp assisted in preparation of graphic art. Funding for GPR work came from the USGS and the University of Wisconsin-Eau Claire (Office of Research and Sponsored Programs, Office of the Dean, College of Arts and Sciences). This paper is contribution 1279 of the Great Lakes Science Center, U.S. Geological Survey.

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