Geomorphic expression of Late Holocene lake levels and Paleowinds in the upper Great Lakes.
Late Holocene lake level changes and predominant wind directions can be recognized in the geomorphic distribution of coastal features and deposits throughout the upper Great Lakes, and these geomorphic signatures can be used as valuable reconnaissance tools. Lake level was more than 4 m higher during the Nipissing I and II phases of ancestral Lakes Michigan, Huron, and Superior than today. Deposits associated with the Nipissing phases commonly occur on platforms and consist of large dunes, beach ridges, barriers, and spits. In many areas, Nipissing II phase coastal features separate smaller inland lakes from larger primary lake basins. After the Nipissing II phase, lake level fell 4 m in just over 500 years. Depending on available sediment supply to the shoreline and relative rate of lake level fall, the result of the 4 m fall is often expressed as a variable number of beach ridges to a bluff or scarp. Regardless of the coastal features and deposits present, there is a 4 m or more topographic change between features and deposits on the Nipissing platform and those that formed after the fall. During the past 3,500 years, the upper Great Lakes experienced three relatively long-lived highs: the Algoma phase (2,300 to 3,300 cal. yr. B.P.), and two unnamed high phases (1,100 to 2,000 and 0 to 800 cal. yr. B.P.). Superimposed on these long-term trends are quasi-periodic fluctuations having periodicities of about 160 and 32 years in duration. The ~30-year fluctuation is instrumental in producing individual beach ridges, whereas the 160-year fluctuation produces groups of beach ridges consisting of 4 to 6 ridges in a group. The high from 1,100 to 2,000 cal. yr. B.P. contains very well defined groups that often can be recognized in aerial photographs. In the Lake Superior basin and along the southern shore of the lake, the separation of Lake Superior from Lakes Michigan/Huron about 1,000 to 2,000 years ago can also be recognized in strandplains. The separation point occurs at a topographic change in the ridges from a decline to a rise. Widespread wetland or lagoon development can occur at the topographic low, and drainage development and switches in drainage directions may also be present. Small parabolic dunes commonly occur along beach ridges in sites along the southern and eastern margins of the upper Great Lakes. At the Toleston Beach in southern Lake Michigan, these small dunes show a systematic change in predominant wind direction from westerlies to northerlies throughout the late Holocene.
A hydrograph of late Holocene lake levels by Baedke and Thompson (2000) illustrates the upper physical limit and timing of water level change for the past 4,500 years in the Lake Michigan/Huron basin. This hydrograph can also be applied to the Lake Superior basin before about 2,000 years ago (cf. Farrand 1962) when the upper Great Lakes were confluent. Because lake level is a primary control on shoreline behavior in the Great Lakes, lake level fluctuations observed in the hydrograph should be expressed in the distribution, geomorphology, and sedimentology of coastal features and deposits rimming the basins. This response would be especially true for areas of the upper Great Lakes that received a positive supply of sediment during the late Holocene, producing spits, strandplains of beach ridges, and dunes.
This paper will examine the types of long-term lake-level fluctuations that occur in the upper Great Lakes and their geomorphic expression along the coasts in areas of high sediment supply. We will also describe important "events" that occurred in the late Holocene lake level history and illustrate their geomorphic signatures. This paper is intended as a guide to areas that were not part of the Thompson and Baedke (1997) study and other recent work in the Lake Superior basin (Baedke et al. 2004; Johnston et al. 2004).
RECONSTRUCTING PAST LAKE LEVELS
Strandplains of beach ridges are common depositional features in embayments of the upper Great Lakes. The beach ridges are a product of lake level fluctuations. They form in the final stages of a lake level rise and increase in width and height during the ensuing lake level fall (Thompson and Baedke 1995). Thompson and Baedke (1997) produced five lake level hydrographs for Lake Michigan by studying five strandplains containing about 25 to 100 beach ridges (Figure 1). The beach ridges and the wetlands that separate them were used to determine the elevation of the lake through time. Vibracores were collected at the lakeward margin (footslope) of all accessible beach ridges to recover foreshore, or swash zone, deposits. Specifically, the cores were used to determine the elevation of coarse-grained, plunge-point deposits at the base of the foreshore sequence that accumulated at or near the elevation of the lake when each beach ridge formed. From the elevation of these deposits, the elevation of the lake when each beach ridge formed can be approximated. The age of the beach ridges were determined by radiocarbon dating organic sediments recovered from the base of wetland deposits in the swales between beach ridges. Radiocarbon dates were calibrated to calendar years before 1950 (cal. yr. B.P.), and an age model was created at each strandplain to produce a minimum calendar age for each beach ridge. The age model assumes that a wetland was established and began accumulating organic material soon after the beach ridge lakeward of the swale formed. With both elevation and age known per beach ridge, a hydrograph can be plotted for each site.
Hydrographs for the five study areas in Lake Michigan illustrate long-term patterns of change in water volume in the basin, but they also show long-term differential warping of the ground surface (Larsen 1994; Baedke and Thompson 2000). This warping is attributed to isostatic rebound that is occurring in the Great Lakes area following the removal of glacial ice. Maps of ground movement using lake level gages show that present rates of rebound increase to the northeast across the Great Lakes (Figure 1). Consequently, each hydrograph was modified by local rates of ground warping and shows only lake level changes that occurred in the vicinity of each study area. They are said to be "relative" to the immediate shoreline. Baedke and Thompson (2000) used an iterative approach that minimized differences between residuals to remove the differential vertical movement between sites and collapse all the relative hydrographs into a single hydrograph. This hydrograph represents late Holocene lake level change that occurred at the Port Huron outlet during the past 4,700 cal. yr. B.P. (Figure 2).
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LATE HOLOCENE LAKE LEVEL
The hydrograph of Baedke and Thompson (2000) shows that the upper limit of lake level was a little more than 4 m higher 4,500 years ago (Figure 2). This high phase of the lake is known as the Nipissing II phase of ancestral Lake Michigan. An earlier lake phase about 5,600 to 6,400 years ago (Lewis 1969), the Nipissing I phase of ancestral Lake Michigan, does not occur in the Thompson and Baedke (1997) data set. In a roughly 500-year period after the Nipissing II phase, lake level fell more than 4 m, reaching elevations observed in the historical gage record for the lakes. This rapid fall was followed by three high phases from 2,300 to 3,300 cal. yr. B.P., 1,100 to 2,000 cal. yr. B.P.., and 0 to 800 cal. yr. B.P. The oldest of these high phases is known as the Algoma phase of ancestral Lake Michigan; the other two phases are unnamed. Pervasive in the hydrograph is a quasi-period fluctuation of approximately 160 [+ or -] 40 years in duration that can be extended into the historical record, suggesting that the entire historical record is a 160-year quasi-periodic fluctuation. Observable only in an inset graph to the hydrograph is a shorter-term quasi-period fluctuation of 32 [+ or -] 6 years. This fluctuation is based on the average timing of beach-ridge development in the five study sites of Lake Michigan. The 30-year fluctuation is also reflected in the historical record (Thompson 1992).
Lake level changes observed in the Lake Michigan/Huron basin also occurred in the Lake Superior basin up to about 2,000 years ago. Farrand (1962) calculated the separation of Lake Superior from Lake Michigan/Huron at 2,200 years ago, and Larsen (1994) placed the split at 2,100 cal. yr. B.P. Johnston et al. (2001) placed this separation closer to 1,300 cal. yr. B.P. In Lake Superior, the split from Lake Michigan/Huron had a dramatic influence on the shoreline behavior. Shorelines along the southern shore of Lake Superior that were undergoing a long-term relative lake level fall in response to differential rebound with respect to the Port Huron outlet started to experience a long-term relative lake level rise as the new outlet came into effect (Johnston et al. 2004). An increase in the long-term rate of lake level rise occurs from east to west along the southern shore of Lake Superior in response to decreasing rates of rebound away from the Sault Ste. Marie outlet.
RECOGNIZING LAKE LEVEL "EVENTS"
Several of the lake level changes in the Baedke and Thompson (2000) hydrograph have pronounced geomorphic expressions in strandplains of the ridges throughout the upper Great Lakes. Of particular importance are (1) the lake level fall from the Nipissing II phase, (2) 160-year quasi-periodic fluctuations, (3) the unnamed high phase from 1,100 to 2,000 years ago, and (4) the separation of Lake Superior from Lake Michigan/Huron in the Lake Superior basin. Another important feature observed in the strandplains we have studied are small parabolic dunes that occur along the ridges. The orientation of these stabilized dunes record past wind patterns at the shoreline of the embayment. The following sections describe the geomorphic expression of these lake level changes along the coasts of southern Lake Superior and northern Lake Michigan and parabolic dune distributions along the coast of southern Lake Michigan.
Fall from the Nipissing II High Phase
Nipissing shorelines are prominent features surrounding the upper Great Lakes (Hough 1958). Forming after the transgression to the Nipissing I phase, they occupy re-entrants from the shoreline where sediment was plentiful and could accumulate. Varying in response to sediment supply and the topography of the mainland and predepositional surface, they exist as bluffs, spits, barriers, and strandplains of beach ridges. In the southern areas of the upper Great Lakes, deposits of the Nipissing I and II phases are joined into a single barrier covered by high dunes or have been entirely eroded away by later lake phases (Karrow 1980). Higher rates of rebound in the northern areas spread out depositional features of these two high lake phases and abandons them above the modern lake. In the north, Nipissing depositional shorelines occur on an elevated platform topographically above younger coastal deposits and features. A common coastal feature of the Nipissing I phase is a bluff in the landward part of the platform, whereas coastal features of the Nipissing II phase occur along the lakeward margin of the platform as a barrier, consisting of dune-capped beach ridges and spits. Lakes are common on the Nipissing platform and occur in the landward part of the platform between coastal features and deposits of the two phases.
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The post-Nipissing fall is commonly recognizable where Nipissing II deposits are present. Several sites along Lakes Superior and Michigan show various geomorphic expression of the lowering of lake level. At Batchawana Bay, Ontario (Figure 1), the Nipissing I phase produced a scarp into gravely glacial deposits along the eastern two-thirds of the embayment (Figure 3A). The Nipissing II phase also occupied this bluff, but westward deposition during the Nipissing II phase produced a series of beach ridges that isolated Carp Lake from Lake Superior. Lakeward of Carp Lake, the elevation of the beach ridges decreases 7 m over a distance of 0.6 km, encompassing 16 beach ridges. Of particular importance here is that in very close proximity the Batchawana Bay shoreline was contemporaneously eroding and depositing. Beach ridges that formed while the bluff was eroding contain foreshore deposits that are composed of sandy-gravel and gravel (coarse sand to cobbles). Only after the Nipissing II fall was the bluff completely abandoned. Beach ridges that formed after the fall contain foreshore deposits that are composed of coarse sand and granules. Continued progradation of the shoreline added an additional 49 beach ridges over the next 3,500 years.
The Au Train Bay embayment (Figure 1) at Au Train, Michigan, contains more than 90 beach ridges that arc between friable sandstone uplands (Figure 3B). Au Train Lake occurs in the landward part of the embayment and was separated from Lake Superior during the Nipissing II phase by the development of a 0.25 to 0.5 km wide and 5 to 10 m high barrier of beach ridges and dunes. The fall from the Nipissing II phase is observable along the lakeward margin of the barrier where the elevation of the beach ridges decreases by about 6 m, crossing 10 beach ridges and a distance of only about 0.25 km. To the modern lake and crossing another 80 more beach ridges, beach-ridge elevation only decreases another 4 m in the next 2 km. The Nipissing II barrier has an influence on the Au Train River as it flows westward out of Au Train Lake, whereby the river is prohibited from meandering to the south by the 5 m high barrier. Meandering to the north is unrestricted by lower-relief beach ridges.
The Manistique and Thompson embayments occur southwest of Manistique, Michigan (Figure 1). Here, more than 90 beach ridges arc between glacial and bedrock headlands (Figure 3C). The two embayments were combined until the early part of the Algoma phase when shoreline progradation intercepted a bedrock high in the center of the embayment that extends southeastward to Stoney Point. Prior to this time period, most shoreline development occurred in the northwestern part of the embayment, producing a wedge of beach ridges across the lakeward margin of Indian Lake that join and pinch out to the southwest. Indian Lake was separated from Lake Michigan when a beach ridge formed across the entire embayment in the final stages of lake level rise to the Nipissing II phase. The post-Nipissing fall is represented in this area by 26 beach ridges over a distance of 0.8 km. The post-Nipissing fall at Manistique contains the largest number of beach ridges of any site studied in the upper Great Lakes, suggesting that beach-ridge development at most sites in the upper Great Lakes during this time period was commonly unable to keep up with the rapid lake level fall. Manistique appears to be unique in having sufficient sediment supply to continue to produce beach ridges when lake level was falling more than 0.75 m/century. Such high rates of sediment supply did not continue at Manistique as shown by the two 1000-year-long gaps in the beach-ridge record that occur after the Algoma phase when the Manistique and Thompson embayments were separated (Thompson and Baedke 1995).
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The Toleston Beach is the formal geomorphic name for the largest strandplain of beach ridges in the upper Great Lakes (Thompson 1992). The Toleston Beach occurs at the southern tip of Lake Michigan where beach ridges arc across the Indiana shore into Illinois (Figure 1). Unlike the northern strandplains described above, the Toleston Beach does not have a lake captured landward or within Nipissing II deposits, although a lagoon did exist landward of the Toleston Beach during the Nipissing phases (Chrzastowski and Thompson 1992, 1994; Thompson 1992). The landward 25 ridges formed during the Nipissing phases, and more than 80 ridges formed after the post-Nipissing fall. The post-Nipissing fall is represented by an erosional discontinuity that occurs between Algoma-phase beach ridges and older Nipissing ridges (Figure 4). The discontinuity can be traced throughout the Toleston Beach by a change in topography and by the truncation of older beach ridges by younger ridges. Beach ridges landward of the discontinuity are about 3 m higher than those lakeward. Southwest of Ogden Dunes, Indiana, Long Lake occurs along the erosional discontinuity. During the lake level fall, the southern shore of Lake Michigan apparently had insufficient sediment to build or build and maintain beach ridges. Consequently, the shoreline underwent erosional regression. Under such conditions, the shoreline translates downward with little or no lakeward translation. It is only after the Chicago outlet was closed off by the Graceland and other unnamed spits that sediment supply increased to the southern shore of Lake Michigan in significant quantity to create beach ridges (cf. Chrzastowski and Thompson 1992, 1994).
In summary, the post-Nipissing fall can be represented by a scarp, an erosional discontinuity in the landward part of a strandplain, or the successive lowering of the elevation of beach ridges in the landward part of a strandplain. In some areas it may not be possible to differentiate a scarp of the post-Nipissing fall from a scarp formed during the pre-Nipissing transgression and both Nipissing phases. This is true for the eastern part of the Batchawana Bay embayment where both Nipissing phases and the post-Nipissing fall occupied the same bluff. In strandplains, the post-Nipissing fall may be represented by as many as 26 beach ridges. Sites that contain more ridges had the higher rate of sediment supply during the lake level fall. The topographic change that occurs during beach-ridge development varies with respect to the amount of differential vertical ground movement between the strandplain and the Port Huron outlet, the rate of sediment supply to the shoreline, and the ability of wind at shoreline to create dunes on the ridges. For example the topographic change at Batchawana Bay is approximately 8 m, whereas the topographic change at Manistique is approximately 6 m. It was also possible that no beach ridges formed during the post-Nipissing fall. Careful mapping of beach-ridge orientations may reveal the position of the fall by identifying a low-angle erosional discontinuity and a topographic change. Caution in recognizing this truncation is warranted. Periods of erosion during strandplain development that are not associated with the post-Nipissing fall may also occur (cf. Thompson and Baedke 1995), making it difficult to recognize the position of the fall.
[FIGURE 5 OMITTED]
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160-Year Quasi-Periodic Fluctuations
Many strandplains of beach ridges in the upper Great Lakes contain changes in elevation relief and spacing of ridges that define groups. The groups contain four to six ridges, commonly five, and show sequential rises and falls in the elevation of the ridge crests and relief above the surface of the swales. Swales between successive groups are commonly wider than swales that are contained within the groups (Figure 5). Beach-ridge elevation and relief changes mirror subsurface foreshore-elevation changes within the beach ridges that are associated with the 160-year quasi-periodic lake-level fluctuations.
The groups are recognizable in stereo pairs of aerial photographs, but they can also be recognized in single aerial photographs if the strandplain contains wetlands or standing water in the swales (Figure 5). In many areas, however, aerial photography does not reveal the groups; they are only observable during a field reconnaissance or topographic survey.
Unnamed High Phase
Thompson and Baedke (1997) described an unnamed high phase in Lake Michigan/Huron from 1,100 to 2,000 cal. yr. B.P. (Figure 2). Higher in elevation than the Algoma phase, this high phase centered at 1,600 to 1,700 years ago can be recognized in strandplains by the geomorphic expression of the beach ridges. The 160-year groups are most pronounced and well formed in areas of the strandplain that cross this high (Figure 5). Individual beach ridges and swales commonly extend across the entire embayment, and ridges that are part of a group merge at the edges of the embayment producing a single ridge. Seven 160-year quasi-periodic fluctuations occur during the high, and it is possible that all of these fluctuations will be preserved as an individual group. For many strandplains, this high phase occurs in the middle to lakeward part of the strandplain (Figure 3B).
Separation of Lake Superior from Lake Michigan/Huron
The change in outlet from the Port Huron outlet to the Sault Ste. Marie outlet had a profound impact on shoreline behavior in the Lake Superior basin. All shorelines along the southern shore of Lake Superior experienced a change from a long-term relative lake-level fall to a long-term lake-level rise because the new outlet at Sault Ste. Marie rebounded, and continues to today, more rapidly than the southern coastline. Many shorelines in embayments along the southeastern half of Lake Superior (east of the Keweenaw Peninsula), however, continued to prograde in face of the lake level rise (Johnston et al. 2000, 2004). Because of the change from a lake level fall to a rise, beach-ridge topography shows a similar pattern. That is, beach-ridge topography shows a decline followed by a rise. Many areas show a greater areal extent of wetlands at the location within the strandplain where the topographic lowering is replaced by the rise (Figure 6A). It is also common to have a drainage extend across through the strandplain at the low point (Figure 6B) or change flow direction (Figure 3B). In the western half of the upper peninsula and into Minnesota, the increased rate of lake level rise has forced sediment-rich shorelines into aggradation. The most common coastal geomorphic pattern is a barrier beach with a lagoon landward of the barrier.
Paleo Wind Direction in Southern Lake Michigan
Olyphant and Bennett (1994) showed that the modern, small, parabolic dunes associated with foredune ridges along the southern coast of Lake Michigan are most active during the fall and early winter in response to storms. Although prevailing winds in the area are from the west and southwest, dunes migrate parallel to the strong winds, predominant, from the north. Parabolic dunes associated with beach ridges and dune fields of the Toleston Beach should indicate predominant wind directions during their development. Maps and time lines of Thompson (1992) and Thompson and Baedke (1997) can be used to show changing wind patterns for southern Lake Michigan (Figure 4). Specifically, the southern shore of Lake Michigan shows predominant westerlies during the Nipissing phases, followed by a systematic shift of predominant wind direction from westerlies to north-westerlies and northerlies. Such a shift in predominant wind direction would have had an impact on littoral transport directions. Thompson (1989) noted a west-to-east littoral transport direction in Nipissing upper shoreface deposits exposed west of Michigan City, Indiana. This observation supports the littoral transport direction implied by dune orientations of Nipissing dunes. Today, dominant littoral transport directions at Michigan City are from east to west. Such a change would be expected with the systematic shift of predominant winds from the west to the north.
Lakelevel fluctuations in the hydrograph of Baedke and Thompson (2000) are expressed by the distribution, geomorphology, and sedimentology of coastal features and deposits rimming the upper Great Lakes. Most easily recognized in strandplains of beach ridges are (1) bluffs and beach ridges formed during the lake level fall from the Nipissing II phase at about 4,000 cal. yr. B.P., (2) 160-year quasi-periodic fluctuations that produce groups of ridges, (3) the well-formed groups of the unnamed high phase from 1,100 to 2,000 cal. yr. B.P., and (4) the wetland development, drainage position and deflection, and topographic changes of Lake Superior strandplains associated with the separation of Lake Superior from Lakes Michigan/Huron. Many strandplains of beach ridges also contain parabolic dunes having orientations that indicate long-term patterns of predominant wind direction. Knowledge of parabolic dune orientations in many sites would be useful in understanding long-term paleo wind patterns in the upper Great Lakes.
Work on this study was made possible through several cooperative agreements between the Indiana Geological Survey, the U.S. Geological Survey, and the U.S. Fish and Wildlife Service. Most recent funding was provided by the U.S. Geological Survey-Great Lakes Science Center under USGS Agreement No. 98HQAG2180. The agreement requires the following statement: "The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government." Permission to publish this document was granted by the State Geologist/Director and the Publications Committee of the Indiana Geological Survey.
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TODD A. THOMPSON
Indiana Geological Survey/Indiana University
STEVE J. BAEDKE
James Madison University
JOHN W. JOHNSTON
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|Author:||Thompson, Todd A.; Baedke, Steve J.; Johnston, John W.|
|Date:||Jan 1, 2004|
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