beach-ridge development and lake-level variation in southern lake michigan

Sedimentary Geology, 80 (1992) 305-318 305 Elsevier Science Publishers B.V., Amsterdam

Beach-ridge development and lake-level variation in southern Lake Michigan

Todd A. Thompson

Indiana Geological Survey, 611 North Walnut Groue, Bloomington, Ind. 47405, USA

(Received October 22, 1991; revised version accepted February 1, 1992)

ABSTRACT

Thompson, T.A., 1992. Beach-ridge development and lake-level variation in southern Lake Michigan. In: J.F. Donoghue, R.A. Davis, C.H. Fletcher and J.R. Suter (Editors), Quaternary Coastal Evolution. Sediment. Geol., 80: 305-318.

The most accurate source of information on lake-level fluctuations in the Great Lakes is the historical record from lake-level gauges. Although it can be semiquantitatively extended back into the late 1700's, the historical record is too short to recognize long-term patterns of lake-level behavior. To extend the historical record, information must be obtained from the Great Lakes geologic record. Such information includes the elevation and age of geomorphic features and stratigraphic sequences.

One of the longest geologic records of late Holocene lake-level variation is preserved in a beach-ridge complex along the southern shore of Lake Michigan called the Toleston Beach. This strandplain contains over 150 beach ridges that arc across northwestern Indiana and fan out into northeastern Illinois. Each ridge was formed during the fall from a high lake level, and the elevation of the foreshore deposits in each ridge provides information on the upper physical limit of lake level over the past 4000 years. Three scales of quasi-periodic lake-level variation were determined by radiocarbon-dating basal peats of wetlands between the ridges and by measuring the elevation of foreshore (swash) deposits within ridges. These three scales are: (1) a short-term and small-scale fluctuation of 25 to 35 years with a range of about 0.5 to 0.6 m; (2) an intermediate-term and meso-scale fluctuation of 140 to 160 years and a range of about 0.8 to 0.9 m; and (3) a long-term and large-scale fluctuation of 500 to 600 years and a range of 1.8 to 3.7 m. The short-term and intermediate-term fluctuations are reflected in the historical record.

An increase in the rate of shoreline progradation from east to west across Indiana's shoreline causes differential preservation of the lake-level fluctuations. That is, groups of four to six ridges in the western part of the strandplain that formed in response to the small-scale fluctuations combine eastward into single ridges and groups of ridges representing the meso-scale fluctuations. The large-scale fluctuations produced the most dramatic response in the western part of the Toleston Beach. Here, following each high stand, individual spits prograded southward off of a bedrock headland. The successive spit extensions created several small lakes landward of the spits and started the 20 km eastward stream-mouth deflection of the Grand Calumet River across Indiana's western lakeshore.

Introduction

The historical record from lake-level gauges is the primary source of information on lake levels throughout the Great Lakes. The historical record extends from 1860 to the present, but it can be semiquantitatively extended back into the late 1700's (Bishop, 1990). However, with only about

Correspondence to: T.A. Thompson, Indiana Geological Sur- vey, 611 North Walnut Grove, Bloomington, Ind. 47405, USA.

190 years of data, the historical record is too short to statistically recognize long-term patterns (10's to 100's of years) of lake-level behavior. To extend the historical record, information must be obtained from the Great Lakes geologic record. Such information includes the elevation and age of coastal geomorphic features, and facies and facies contacts within nearshore stratigraphic sequences (Thompson et al., 1988).

The dune and beach-ridge complex along the southern shore of Lake Michigan is one of the

longest late Holocene records of lake-level varia-

tion in the Great Lakes. This strandplain, called the Toleston Beach, contains more than 150 beach

ridges that arc across northwestern Indiana and

fan out into northeastern Illinois (Fig. 1). The

ridges formed during the fall from high stands in

lake level. The elevation of the foreshore deposits

in each ridge, therefore, provides information on

the upper physical limit of lake level over the past

4000 years (Thompson et al., 1991). The purpose

of this paper is to describe relative lake-level

variation in the southern part of the Lake Michi-

gan basin during the late Holocene and the con-

comitant shoreline behavior in northwestern Indi-

ana and northeastern Illinois. Evidence for this

study comes from the internal architecture and

timing of development of the beach ridges in two

parts of the Toleston strandplain.

Study area

The study area is a 68 km long strandplain

along the southern lacustrine plain of Lake

Mich igan that is called the To les ton Bcact~ ~[qg.

l). The Toleston Beach began to form approxi-

mately 6000 years ago as lake level rose I rorn ;tit

extreme low phase of the lake (Hansel et al.,

1985; Thompson, 1989). The Toleston Beach is a

single 45 m high dune ridge from Michigan (21it)',

Indiana westward to Ogden Dunes, Indiana. West

of Ogden Dunes, however, the Toleston Beach

begins to fan out into individual dune-capped

beach ridges. There are about 50 ridges tit Miller,

Indiana that split and decrease in relief into over

150 beach ridges through Gary,, Indiana. In the

western part of the strandplain, the beach ridges

join several southward-projecting spits that ex-

tend from a bedrock high in Illinois called Stoney

Island (Alden, 1902). Meandering through the

Toleston Beach from west to east is the Grand

Calumet River. The Grand Calumet River is an

eastward continuation of the westward flowing

Little Calumet River that crosses the southern

lacustrine plain of Lake Michigan landward of

the Toleston Beach.

The southern shore of Lake Michigan is the

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Fig. 1. Map of northwestern Indiana and northeastern Illinois akmg southern Lake Michigan, showing beach ridges (lines), parabolic dunes (arcs), spits, and major cultural features in the Toleston Beach (modified from Chrzastowski and Thompson, 1992).

Approximate time lines (dashed) indicate the position of several paleoshorelines. Note the locations of Figs. 2A and 2B.

BEACH-RIDGE DEVELOPMENT AND LAKE-LEVEL VARIATION IN SOUTHERN LAKE MICHIGAN 307

sediment sink for longshore and beach drift from the east and west margins of the lake in the southern part of the Lake Michigan basin. Thompson (1987) and Chrzastowski and Thomp.

son (1992) have suggested that throughout the evolution of the coast a feedback exists between a rising lake level and increased sediment yield to the southern shore of Lake Michigan. That is, a

T 37 N

T 36 N

R 8 w R 7 w

\ - "~CHIGAN

30 29

R 9 W R 8 W

Fig. 2. Map of the Miller Woods (A) and Gary Airport (B) areas, illustrating beach ridges (lines), parabolic dunes (arcs), vibracore locations (dots), and cultural features. Arrows in (A) indicate the composite structures of ridges in the Miller Woods area.

rise in lake level would accelerate erosion along the east and west margins of Lake Michigan and

the eroded sediment would be transported to northwestern Indiana and northeastern Illinois.

The increase in sediment supply lags behind the

rise in water level, producing an initial period of landward translation of the shoreline. This situa-

tion, however, reverts to one of transgressive de- position and aggradation as the influx of eroded material nourishes the system. As lake level begins to drop, an episode of rapid progradation

begins. Olson (1958) has shown that this progradation forms a beach ridge when lake level falls from a high stand, and Hunter et al. (1990) have

documented the development of a new beach

ridge in northwestern Indiana following the high lake levels of 1985 to 1987. The ridges in the

Toleston Beach are, therefore, records of past high lake levels throughout the late Holocene.

Two parts of the Toleston Beach were studied: ( l ) the Miller Woods Unit of the Indiana Dunes

National Lakeshore, and (2) several locations near the Gary Airport (Fig. 1). At Miller Woods, the

beach ridges are 3.7 to 6.1 m high and separated

by wetlands (Fig. 2A). The ridges are com~ositc structures that consist of a mare ridge with se~- oral minor ridges that decrease m height on the lakeward side of the main ridge. The composite

structures arc separated by wider than normal

wetlands, and each composite structure indicates a [ong-term lake-level variation with a superim-

posed smaller-scale wtriation (Thompson. 1988). Near the Gary Airport, the beach ridges arc 1.5 to 2.5 m high and can be traced for several kilometers parallel to the modern shoreline (Fig.

2B). The ridges in the Gary Airport area combine eastward or recurve into the Grand Calumet River. The recurves indicate the positions of the

mouth of the river as the lakeshore prograded. Specifically, the recurves show that the mouth of

the river was driven eastward during the late Holocene by longshore drift from the western

margin of the lake (Chrzastowski, 1990). The ridges in the Gary Airport area also have a crude structure that tends to form groups of four to six

ridges. Like the composite structures at Miller Woods, the groups of ridges are separated by

slightly wider wetlands.

MIWP= NGVD Feet NGVD

F 615

310

505

800

595

180- 590

585

580

EXPLANATION Dune la'¢l washover ,16o y om,

Foreshore 200 Ioo o ft

Upper ,~'~r~ace Vertical exaggeration = 1:5

Palus'cine and lacustrine

Fig. 3. N - S cross-section of beach ridges in the eastern part of the Miller Woods area. The diagram illustrates that the beach ridges are cored by foreshore deposits and overlain and underlain by dune and upper shorefaee deposits, respectively. Patustrine and

lacustrine sediments occur in the swales between beach ridges. Vertical lines indicate vibracore locations.

BEACH-RIDGE D E V E L O P M E N T AND LAKE-LEVEL VARIATION IN SOUTHERN LAKE MICHIGAN 309

Vibracores of the beach ridges and intervening wetlands show that the beach ridges are com- posed of a core of foreshore (swash zone) sedi-

ments that are capped by dune deposits and underlain by upper shoreface sediments (Fig. 3). The foreshore deposits consist of 1.4 to 1.8 m of moderately sorted, upper fine- to lower medium- grained sand with laminae and beds of upper medium- to upper very coarse-grained sand. These sediments have a horizontal parallel- lamination and a lakeward-dipping subhorizontal parallel-lamination. Some high-angle landward- dipping parallel-laminae occur in the upper part and occasionally at the base of the foreshore sequence, but high-angle landward-dipping laminations and ripple laminations are rare. The foreshore sequence is ungraded, but the base and top of the foreshore are generally coarser grained and more poorly sorted. The dune and upper shoreface deposits are finer grained than the foreshore sediments, consisting of upper fine- to lower fine-grained sand and lower fine- to upper very fine-grained sand, respectively. The dune sediments are generally unstratified but occasionally contain a high-angle landward- or lakeward- dipping parallel-lamination at their base. The upper shoreface sediments commonly contain low- to high-angle parallel-laminations that dip lakeward and landward, and ripple and micro-trough cross-laminations.

The coarse-grained base of the foreshore sequence represents the gravelly plunge point at the base of the swash zone. Although the elevation of the plunge point is influenced by the wave climate impinging on the shoreline, its elevation is a close approximation of the level of the lake when the beach ridge formed (Thompson et al., 1988). The thickness of the foreshore sequence is also influenced by wave climate (Howard and Reineck, 1981), the greater the wave energy influ- encing the shoreline the thicker the foreshore. Noting the elevation of the top and the bottom of the foreshore sequence in the beach ridges along the southern shore of Lake Michigan, therefore, yields information on the lake level and the range of wave runup at the time of beach-ridge development.

Methods

Sixty-eight vibracores using a 7.6-cm-diameter

core-pipe were collected along the lakeward margin of 59 beach ridges in the springs of 1988, 1989, and 1990. The cores were collected at the breaks in slope at the bases of the beach ridges to ensure that foreshore sediments were recovered in the cores. Twenty-three additional cores were collected from the crests and landward sides of the beach ridges and from within wetlands to define the internal facies relationships within the beach-ridge complex. All vibracore sites were sur- veyed for accurate elevations from U.S. Geologi- cal Survey and National Park Service benchmarks and from U.S. Geological Survey water wells. The vibracores were returned to the laboratory, split, described, photographed, and sampled. More than 1000 samples were wet and dry sieved at 0.5 ~b intervals, and graphs of grain-size distributions for each core were constructed. Latex peels were made of the cores to make permanent records of the cores and to enhance stratification. Genetic facies (dune, foreshore, upper shoreface) were identified using the grain-size distributions and physical sedimentary structures. The elevation of the foreshore deposits was determined by sub- tracting the depth to the top and bottom of the foreshore from the elevation of ground surface at the site. Compaction generated during the vibra- coring was added to the depth measurements before the foreshore elevations were calculated.

To determine the age of the beach ridges, 48 peat and organic-rich sand samples were hand- augered or vibracored from the bases of the wetlands between ridges. These samples were radiocarbon-dated and corrected for variations in t513C (Geochron Laboratories Division of Krueger En- terprises, Inc.). An additional twelve peat and wood samples from within wetland and nearshore sequences were also dated. The radiocarbon dates were calibrated to correct for variations in atmo- spheric 14C (Stuiver and Reimer, 1986) and con- verted to calendar years before 1950 (cal yrs B.P.).

An assumption is made in this study that the wetlands formed in the swales between the beach

ridges soon after the beach ridges formed lakeward of them. This assumption is valid if the wetlands formed and began accumulating organic sediment within 40 to 120 years - - the standard laboratory error on most radiocarbon-date deter- minations (Hedges, 1985) - -o f the development of the swales. Exceptions occur for the wetlands between ridges in the higher elevations of the composite structures in Miller Woods. In these areas, the swates between ridges contain isolated wetlands that may not have been in contact with the water table throughout their history, preclud- ing the development of wetland vegetation and its subsequent accumulation. These wetlands radiocarbon-date significantly younger than wetlands lakeward of them. In the discussion that follows, these samples will be omitted from the construc- tion of the lake-level curve.

Foreshore elevations and shoreline ages

Miller Woods area

Foreshore deposits were collected in 22 cores from the Miller Woods area, and the ages of 23 ridges were determined. The elevation of the foreshore deposits show an overall decrease lakeward, indicating a long-term relative fall in the level of Lake Michigan during the late Holocene (Fig. 4A). Superimposed on this downward trend are several upward and downward deflections of 0.9 to 1.8 m that represent the composite ridge structures. The six beach ridges from 1500 to 1900 m landward of the shoreline were cored twice; foreshore deposits were collected for only two ridges, because I was unable to entirely pene- trate the thick dune sediments capping the ridges.

195

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50o 1600 15'00 Distance landward from shoreline (m)

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170 0 500 1 (~0 15'00 20'00 25'00 30'00 3500 40'00 4500

Distance landward from shoreline (m)

Dune crest , Foreshore top ~ Foreshore base

Fig. 4. Graphs of foreshore elevations for the Miller Woods (A) and the Gary Airport (B) areas. Dune crests of individual beach ridges are also shown in (A).


TABLE 1

Summary of the linear regressions used to calculate the age of the beach ridges and the timing of their development

Slope R 2 Std. error

Miller Woods area

Age vs. distance 1.84 y r /m 0.88 0.025 Age vs. ridge # 151.0 y r / # 0.89 13.9

Gary Airport area

Age vs. distance 0.56 y r /m 0.73 0.012 Age vs. ridge # 30.5 y r / # 0.49 7.5

determine the average timing of beach-ridge development, a weighted least-squares line of best fit of age versus a sequential number assigned to each beach ridge landward from the shoreline was calculated. The slope of the calculated line and standard error indicate that beach ridges formed in the Miller Woods area every 151 + 14 yrs (Table 1).

Gary Airport area

The gaps in data points on Fig. 4A from 1500 to 1900 m are, therefore, an artifact of the sampling.

The calibrated radiocarbon dates for Miller Woods show a lakeward decrease in the age of the ridges (Fig. 5A). This lakeward decrease in age reflects the progradational nature of the shoreline with a calculated rate of progradation at this location of about 0.54 m/yr (Table 1). To

In the Gary Airport area, a continuous transect of cored ridges could not be constructed because of extensive urban development. Thirty- four cores from five sites were combined to produce the Gary Airport transect. All of the cores penetrated foreshore deposits, and most of the cores recovered the entire foreshore sequence. The elevations of the foreshore deposits show little variability, but several broad upward deflec-

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Fig, 5. Graphs of beach-ridge age for the Miller Woods (A) and the Gary Airport (B) areas with weighted least-squares lines of best fit (Table 1).

tions occur (Fig. 4B). The elevations of the fore-

shore deposits are, however, lower than deposits of similar age in Miller Woods.

In the Gary Airport area, the ages of nineteen beach ridges were determined. As do the ridges

in Miller Woods, these ridges too show a lakeward decrease in age but with considerably more

scatter (Fig. 5B). The calculated rate of progradation is 1.7 m/y r . Beach ridges were added to the shoreline in the west-central part of the Toleston

Beach every 31 +_ 8 yrs (Table 1).

L a t e H o l o c e n e l a k e - l e v e l c u r v e s

The linear regressions in Fig. 5 can be used as

a real time frame for plotting the foreshore elevations from Fig. 4. Two curves of late Holocene lake level can be generated. The curves must be

adjusted from the National Geodetic Vertical Datum (NGVD) of 1929 to the International

Great Lakes Datum ( I G L D ) o l 1955 tc+ bc cotn-

pared to the historical record for Lake Michigan. The adjustment to IGLD for northwestern Indi- ana is --0.44 m NGVD.

The Miller Woods curve shows a long-term lake-level fall of 4.6 m over the past 4000 cal yrs B.P. (Fig. 6A), yielding a relative rate of lake-level

fall during the late Holocene of 0.96 mm/y r . This fall reflects the lowering of lake level from the Nipissing 11 Phase of ancestral Lake Michigan

that was as established about 4200 cal yrs B.P. at an elevation of about 180.5 m for southwestern Lake Michigan (Larsen, 1985). The Miller Woods curve suggests an elevation of at least as high as

181 m for the Nipissing II Phase at the southern tip of Lake Michigan. The lowering of lake level from the elevation of the Nipissing II Phase reached an elevation of about 178 m by 2500 to

2700 cal yrs B.P. Although not well documented in the Miller Woods curve, one upward deflection

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1701 + . . . . -5oo o 5oo lO00 15oo 2000 25oo Calendar years (BP)

, , Foreshore top , Foreshore base

Fig. 6. Late Holocene lake-level curves for the Miller Woods (A) and Gary Airport (B) areas with least-squares lines of best fit

through the basal foreshore elevations. The regression lines are anchored on the historical average for Lake Michigan (176.4 m).

Historical record is shown for comparison to the geologic record.

BEACH-RIDGE D E V E L O P M E N T AND LAKE-LEVEL VARIATION IN SOUTHERN LAKE MICHIGAN 313

during the fall from the Nipissing II Phase to the low at 2500 to 2700 cal yrs B.P. occurred about 3100 cal yrs B.P. at an elevation of 179 m. This slight rise in lake level may represent the Algoma Phase of ancestral Lake Michigan. A similar elevation and age for this phase was noted by Larsen

(1985). High stands above the average of the long-term

fall and after the Algoma Phase occurred at about 600, 1700, and 2300 cal yrs B.P. (Fig. 6A). The high stands roughly match the timing of highs in the level of Lake Michigan noted by Larsen (1985) and Fraser et al. (1990). Their studies also recognized a high stand from 1150 to about 1300 cal yrs B.P. This high was not recognized in the Miller Woods curve, however, the curve does flatten from 1100 to 1250 cal yrs B.P. This flattening suggests a slight rise or at least a stillstand in lake level on the long-term fall at about 1175 cal yrs B.P. The high lake levels at 600, 1700, and 2300 and possibly 1175 cal yrs B.P. indicate that episodes of high lake level occur at about 500- to 600-yr intervals in the Lake Michi- gan basin at least since 2300 cal yrs B.P. Such a pattern suggests a long-term quasi-periodic behavior in the level of Lake Michigan of about 500 to 600 yrs throughout the late Holocene.

The lake-level curve for the Gary Airport area (Fig. 6B) differs from the curve at Miller Woods in its long-term trend but does show the same lake-level events. In the Gary Airport area, most of the basal foreshore elevations occur below annual averages of the historical record for Lake Michigan, producing a very slight relative lake- level rise of 0.29 mm/yr. Moreover, the more rapid progradation of the western part of the Toleston Beach and the addition of new beach ridges every 25 to 35 yrs yields a higher resolution curve. High lake levels at 1700 and 2300 cal yrs B.P. in the Miller Woods curve are broad humps at 1500 to 1900 cal yrs B.P., and the high lake levels before 2000 cal yrs B.P. in the Gary Airport curve. Superimposed on these high lake levels are smaller variations comprised of groups of four to six beach ridges. These variations are a reflection of partial preservation the 151 + 14 yr fluctuation instrumental in the development of the beach ridges in the Miller Woods area.

The lake-level curves for Miller Woods and the Gary Airport area show similar patterns of lake-level events (i.e. similar high and low phases), but opposite patterns of long-term lake-level behavior. The divergence of the relative lake-level curves across a distance of only 11.3 km is surpris- ing because the southern shore of Lake Michigan is generally considered to be tectonically and isostatically stable (Goldthwait, 1908). Clark et al. (1990) show that the southern shore of Lake Michigan has undergone isostatic subsidence in historical time of 1.04 mm/yr relative to a lake- level gauge at Goderich, Ontario. Their isobases for Lake Michigan are roughly perpendicular to Indiana's shoreline. If these isobases are correct, the maximum rate of isostatic adjustment in the southern part of the Lake Michigan basin occurs along the northwestern coast of Indiana. The study of Clark et al. (1990) implies that lake level along the southern shore of Lake Michigan should be rising, and that the rate of relative lake-level rise should increase westward along Indiana's coast.

In contrast, the relative lake-level curve for the Miller Woods area shows a long-term lake-level decline. If the southern shore of Lake Michigan was subsiding during the late Holocene, the vol- ume of water in the Lake Michigan basin must have been decreasing at a rate that exceeded the rate of subsidence in the Miller Woods area. Westward, the rate of subsidence was slightly greater than the rate of water loss in the basin and produced the small relative lake-level rise observed in the Gary Airport area. If the regression lines through basal foreshore in each curve are representative of the two relative lake-level curves, the rate of vertical movement between the two areas is 1.25 mm/yr. This rate of adjustment along Indiana's coast is larger than the estimates of Clark et al. (1990) for Goderich to northwestern Indiana.

Assuming the rates of isostatic subsidence of Clark et al. (1990) are accurate for the southern shore of Lake Michigan and representative of vertical movement throughout the late Holocene, some other mechanism or mechanisms were active to produce the differences observed between the relative curves of the two study areas. Two

possible mechanisms are faulting and differential compaction. No data are currently available to evaluate these possible processes. Without better knowledge of isostasy between the southern shore of Lake Michigan and the outlet at Port Huron, Michigan, tectonism in the southern part of Lake Michigan, and differential compaction along Indi- ana's shoreline, neither of these factors can be removed from the relative lake-level curves for southern Lake Michigan. The study of other coastal features in the Lake Michigan basin and the continued study of the southern shore of Lake Michigan is needed to reconstruct water planes across Lake Michigan that can better define long-term rates of vertical movement.

Lake-level behavior

The longest continuous historical record of lake level for Lake Michigan is 130 yrs, starting in 1860 at Harbor Beach, Michigan (Bishop, 1990). Quinn and Sellinger (1990), however, have extended the record an additional 41 yrs by correct- ing intermittent data from a gauge at Milwaukee, Wisconsin to IGLD. The most complete part of the pre-1860 data are the summer months (Fig. 7). The record for Lake Michigan is highly vari- able with a difference of 2.4 m between the maximum monthly means of 1838 and 1964 and a seasonal variation of about 0.5 m. Long-term and short-term smoothings of the average summer elevations suggest two patterns of lake-level vari-

ation. A long-term smoothing (e.g. 50 yrs) shows that lake level was on an average high in the 1850's, rising from lower levels in the early 1800's (Fig. 7). Lake level fell throughout the late 180()'s and early 1900's to a low in the 1930's. The fall may have been enhanced by as much as 0.67 m by dredging of the St. Clair River (Quinn and Sell- inger, 1990). Following the 1930's and except for the early 1960's, lake level has steadily risen with each successive high episode above the previous high. The observed pattern suggests a quasi-periodic behavior of about 160 yrs. This period is within the range of the 151 + 14 yr variation observed in the Miller Woods area, and this trend in the historical record reflects the pattern observed in the geologic record.

A shorter-term smoothing (e.g. 15 yrs) of the historical record suggests that another quasi-periodic behavior of about 28 yrs exists superimposed on the 160-yr fluctuation (Fig. 7). This fluctuation is represented in the historical record by persis- tent groups of high and low lake levels (e.g. the low lake levels in the mid to late 1930's followed by the high lake levels in the early 1950's and the subsequent low levels in the mid 1960's). Lui (1970) using a spectral analysis suggested that a low-frequency cycle appeared to exist in Great Lakes water levels at a period of about 27 yrs, but he dismissed it as noise. Cohen and Robinson (1976) indicated two periodic fluctuations in the historical record at 22 and 36 yrs. Their periods, however, are not statistically valid for the little

178"

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~ 176-

a 1755-

'7~8'oo 10'20 1~.0 18'6o 18'80 1~0 19'20 19'40 19'60 19'80 2000 Year

Summer - - 15 Year 50 Year

Fig. 7. Graph of the historical record of lake level for Lake Michigan from lake-level gauges. The graph shows the average June,

July and August elevation from 1819 to 1990 with 15-yr and 50-yr smoothings. The summer average was used in this graph because

more summer months are represented in the pre-1860 data than winter months.


over 100-yr record used for the study (see Currie

and Fairbridge, 1985). The cycles of Lui (1970) and Cohen and Robinson (1976), however, are

similar to the 31 + 8 yr timing of beach-ridge development observed in the Gary Airport area. Together, they suggest that quasi-periodic high lake-level events occur every 25 to 35 yrs in the Lake Michigan basin.

The geologic record for southern Lake Michi- gan over the past 4000 cal yrs B.P. is a record of recurring high stands in lake level. Consequently, the foreshore elevations within beach ridges and the timing of beach-ridge development provide no information on the elevation of the lake during low lake-level stands. One can only speculate on the range of variation during the late Holocene. If the 160-yr trend in the historical record is a counterpart to the 151 + 14 yr fluctuation observed in the geologic record, the range of fluctuation between beach ridges in the Miller Woods area could have been as high as the 2.4 m observed in the historical record. The more prob- able range of variation for the 151-yr fluctuation would be similar to the 0.8 to 0.9 m range in the 50-yr smoothing of the historical record (Fig. 7). Similarly, the range of fluctuation for the 31-yr variation would be about 0.5 to 0.6 m. Without a modern counterpart with which to compare, the range of lake-level variation for the 500- to 600-yr fluctuation can only be estimated. High levels occur in the Miller Woods curve 0.9 to 1.8 m above the regression line through the basal foreshore deposits (Fig. 6A). It is possible that the variation from one high stand to a low stand was at least double the observed variation (1.8 to 3.7 m).

Shoreline behavior

The Toleston Beach began to form at the end of the Chippewa Phase of ancestral Lake Michi- gan as lake level inundated the southern lacustrine plain of the lake. The later stages of the transgression were depositional (Thompson, 1989), and the landward part of the Toleston Beach in the west-central part of the shoreline may have prograded even as lake level rose (Thompson, 1988). Whatever the case, lake level

attained an elevation of 184 m about 4700 cal yrs

B.P. (Nipissing I Phase of ancestral Lake Michi- gan) and intercepted a smooth lacustrine plain along most of its length. Only in the western part of the lacustrine plain did the shoreline front morainal highlands. At this time, the water over- topped a break in the morainal highlands and flowed out of the Lake Michigan basin to the Des Plains River. This break is called the Sag Chan- nel. Lake level after the Nipissing I Phase began a long-term fall to an elevation of 178 m about 2600 cal yrs B.P. The fall is punctuated by high stands of the Nipissing II Phase at 4200 cal yrs B.P. and the Algoma Phase 3200 cal yrs B.P.

The early Toleston shoreline was fed from sediment derived only from the eastern margin of the lake. An embayment in what is now western Chicago prevented littoral sediment from the western margin of the lake from reaching the early Toleston lakeshore. These western lakeshore sediments accumulated in the Grace- land spit. Only after the distal part of the Grace- land spit extended south across Chicago's lakeshore to the Stoney Island headland at about 3000 cal yrs B.P. was drift from the western margin able to reach northwestern Indiana (Chrzastowski and Thompson, 1992).

Although not easily distinguished, the Nipiss- ing and Algoma Phases of ancestral Lake Michi- gan are preserved as a series of more than 30 beach ridges in the landward part of the Toleston Beach (Fig. 1). The first ridges filled crenulations in the predepositional surface, but later ridges formed across the entire lacustrine plain. In the eastern part of the Toleston Beach where the regional gradient is the steepest, the ridges merged and began to form parabolic dune fields. These dune fields reached their greatest development before 3200 cal yrs B.P. The most striking feature of the parabolic dunes is their consistent orientation with their noses pointing east (Fig. 1). This orientation suggests that westerlies were the predominant storm winds during the initial development of the Toleston Beach.

Lake level from 2600 cal yrs B.P. to present continued to decline with long-term high stands occurring at about 2300, 1700, 1175 and 600 cal yrs B.P. (Fig. 6). Shoreline evolution at this time

echoed three themes: (1) shoreline progradation and beach-ridge development in the central part of the Toleston Beach; (2) active dune growth in the stable eastern part of the shoreline: and (3) southward spit growth from the Stoney Island headland (Fig. 8), After lake level rose to the high at 2300 cal yrs B.P., the central part of the Toleston Beach continued the shoreline progra-

dation established during the Nipissing and AI- goma Phases. 7"hat is, ridges were added to the shoreline following short-term highs it] lakt,' level (3l-yr variation). Groups of four to six ridges in the western part of the Toleston Beach combined eastward to form a single ridge in the east-central part (151-yr variation). Farther to the east, sand continued to bc removed from the beach and

A , ,~[ ......... LAKE LEVEL 181 5M MSL ~ Stuq,~y .......

n Rp;~r'h L ~ - - - - ,.. "" \

I \ i ' \ \ - Dune deve opment

G ~ ~ s~, growth ~ / J - "

I ~ ' . . .

Beach ridge development ~ - - - . ~,i~e~

C "~, LAKE LEVEL 179 M MSL ~1 N ~ 1500 BP

hgbad pndent _ I . ' . : ": , I

D '~ LAKE LEVEL 178 M MSL ~,~ 0 5 Miles ~ 500 BP

, , ~ ' " ~ . . , . : ~ .~.wo, L~,~\~. ~ ' [ ~ t " k - ~ 62 ' ~ ' ' - ' ~ z Beach ridge development ~ j . : ~ I ~'~/~o' : : . . , . '.." "

" Ltttle %~ltlmet

Fig. 8. Schematic diagrams illustrating shoreline behavior along the southern shore of Lake Michigan during the late Holocene (modified from Chrzastowski and Thompson, 1992).

B E A C H - R I D G E D E V E L O P M E N T A N D L A K E - L E V E L V A R I A T I O N IN S O U T H E R N L A K E M I C H I G A N 317

added to dunes. Within the ridged part of the beach, the orientation of small parabolic dunes suggests that throughout the development of the late Toleston Beach the predominant storm winds changed from the westerlies to northwesterlies and ultimately to the northerlies observed today (Fig. 1). This apparent shift in wind direction needs more study to ascertain its significance in the development of the lakeshore.

The western part of the southern lacustrine plain has undergone the most dramatic changes during the past 2300 cal yrs B.P. Individual spits began to build off of the Stoney Island headland, crossing the last embayment along the western margin of the lake. By 1600 cal yrs B.P., the first of these spits had reattached to the southern shore of Lake Michigan (Fig. 8). This reattach- ment captured a part of the lacustrine plain landward of the spit that became Lake Calumet. A similar event about 1100 cal yrs B.P. was instrumental in creating Wolf Lake. The periods of rapid spit growth occurred following an extreme high stand in the level of the lake. The rapid growth suggests that a positive feedback exists between a rise in lake level and an increased sediment supply to the southern lacustrine plain. That is, a rise in lake level inundates all margins of the lake, but along east and west margins of the lake, the water surface intercepts easily eroded glacial bluffs. The eroded sediment is transported to the southern tip of the lake, coun- teracts the landward translation of the shoreline caused by the lake-level rise, and promotes rapid spit growth when lake level falls.

The low lake level following the Algoma Phase had a significant effect on the Little Calumet River. Prior to the low lake level, the Little Calumet River flowed into the remnant basin of a lagoon that existed landward of the Toleston Beach during the Nipissing Phases and drained through the Sag Channel. But after lake level dropped below the elevation of the sill in the Sag Channel (approximately 180 m NGVD), the river recurved back through the opening to the Sag Channel and emptied into Lake Michigan. The short recurved reach of the river became the Grand Calumet River. Although lake level after 2600 cal yrs B.P. attained elevations at or slightly

above the elevation of the Sag Channel sill, the Sag Channel was largely abandoned as an outlet from Lake Michigan after 2600 cal yrs B.P. be-

cause the opening was closed by several spits. The Grand Calumet River initially flowed di-

rectly into Lake Michigan, but after Lake Calumet was formed (about 1600 cal yrs B.P.) the river flowed into Lake Calumet (Fig. 8). The river exited from the lake between the spits prograding from the Stoney Island headland and the beach ridges forming along the southern tip of the lake. At this time, the mouth of the river began to shift from west to east across the southern lacustrine plain. The river-mouth deflection resulted from sediment accumulation along the western part of the mouth of the Grand Calumet River and slight erosion on the eastern part of the mouth. The sediment accumulation on the western part of the mouth formed a series of recurves extending from the beach ridges into the mouth of the river (Fig. 1). The eastward deflection of the river continued until the turn of the 20th century.

Conclusions

Beach ridges along the southern shore of Lake Michigan are records of past lake-level fluctuations. The elevation of foreshore deposits within ridges and timing of their development can be used to construct relative lake-level curves. Two curves constructed for the western and central part of Indiana's shoreline indicate about 4000 cal yrs of lake-level behavior in the southern part of the Lake Michigan basin. The curves show three scales of quasi-periodic lake-level variation: (1) a short-term and small-scale fluctuation of 31 _+ 8 yrs with a range of about 0.5 to 0.6 m; (2) an intermediate-term and meso-scale fluctuation of 151 _+ 14 yrs and a range of about 0.8 to 0.9 m; and (3) a long-term and large-scale fluctuation of 500 to 600 yrs and a range of 1.8 to 3.7 m.

These fluctuations are differentially preserved along Indiana's coast. That is, groups of four to six beach ridges in the western part of the strandplain that formed in response to the smallest-scale fluctuation combine eastward into single ridges and groups of ridges representing the meso-scale fluctuation. The large-scale fluctuation produced

the most change in the western part of the Tole- ston Beach with the development of individual spits off of the Stoney Island headland following each high stand. The spit progradation crealed two lakes and started the 20 km eastward migra- tion of the mouth of the Grand Calumet River across Indiana's lakeshore.

Acknowledgements

Funding for this project was provided by the U.S. Geological Survey as part of the Coastal Geology Program and administered by the Illi- nois-Indiana Sea Grant Program (NOAA) under grants 88-125 and 89-134. Research Assistants who have worked on this study are Paul K. Doss, Steven J. Baedke, Linda D.P. Thompson, Zinta Smidchens and Charlotte Tanner. Figures for this report were produced by Steven J. Baedke and the Drafting Section of the Indiana Geological Survey. Permission to publish this report was granted by the State Geologist and the Publica- tions Committee of the Indiana Geological Sur- vey. The report was reviewed by Curt Larsen, Mike Chrzastowski and two anonymous review- ers. Their comments are appreciated.

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beach-ridge development and lake-level variation in southern lake michigan

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