J. Great Lakes Res. 20(1):73-92Internat. Assoc. Great Lakes Res., 1994

Lake-Level History of Lake Michigan for the Past 12,000 Years:The Record From Deep Lacustrine Sediments

Steven M. Colman,! Richard M. Forester,2 Richard L. Reynolds,2 Donald S. Sweetkind,2John W. King,3 Paul Gangemi,3 Glenn A. Jones,4 Lloyd D. Keigwin,4 and David S. Foster!

JU.S. Geological SurveyWoods Hole, Massachusetts 02543

2U.S. Geological SurveyFederal Center

Denver, Colorado 80225

3Graduate School of OceanographyU. ofRhode Island

Narragansett, Rhode Island 02882

4Woods Hole Oceanographic InstitutionWoods Hole, Massachusetts 02543

ABSTRACT: Collection and analysis of an extensive set of seismic-reflection profiles and cores fromsouthern Lake Michigan have provided new data that document the history of the lake basin for the past12,000 years. Analyses of the seismic data, together with radiocarbon dating, magnetic, sedimentologic,isotopic, and paleontologic studies of core samples, have allowed us to reconstruct lake-level changesduring this recent part of the lake's history.

The post-glacial history of lake-level changes in the Lake Michigan basin begins about 11.2 ka with thefall from the high Calumet level, caused by the retreat of the Two Rivers glacier, which had blocked thenorthern outlet of the lake. This lake-level fall was temporarily reversed by a major influx of water fromglacial Lake Agassiz (about 10.6 ka), during which deposition of the distinctive gray Wilmette Bed of theLake Michigan Formation interrupted deposition of red glaciolacustrine sediment. Lake level then continuedto fall, culminating in the opening of the North Bay outlet at about 10.3 ka. During the resulting Chippewalow phase, lake level was about 80 m lower than it is today in the southern basin ofLake Michigan.

The rise of the early Holocene lake level, controlled primarily by isostatic rebound of the North Bayoutlet, resulted in a prominent, planar, transgressive unconformity that eroded most of the shoreline fea­tures below present lake level. Superimposed on this overall rise in lake level, a second influx of waterfrom Lake Agassiz temporarily raised lake levels an unknown amount about 9.1 ka. At about 7 ka, lakelevel may have fallen below the level of the outlet because of sharply drier climate. Sometime between 6and 5 ka, the character of the lake changed dramatically, probably due mostly to climatic causes, becom­ing highly undersaturated with respect to calcium carbonate and returning primary control of lake levelto the isostatically rising North Bay outlet. Post-Nipissing (about 5 ka) lake level has fallen about 6 mdue to erosion of the Port Huron outlet, a trend around which occurred relatively small (± -2 m), short­term fluctuations controlled mainly by climatic changes. These cyclic fluctuations are reflected in the sed­imentological and sediment-magnetic properties of the sediments.

INDEX WORDS: Lake Michigan, sediments, radiocarbon, ostracodes, sedimentology.

73

74 Colman et al.

INTRODUCTION

In the overall plan to study the problem of coastalerosion in southern Lake Michigan (Folger et al.this volume), fluctuations in lake level were recog­nized as an important control on processes and ratesof coastal change. Although most concern has beenfocused on accelerated erosion related to rising lakelevels, equally serious problems are associated withfalling lake levels, including siltation of harbors,entrenchment of tributary streams, and dispersionof previously deposited pollutants. All issues re­lated to mitigating damage and planning to alleviatefuture impacts depend heavily on the ability to pre­dict future trends in lake level and the response ofthe lake to those trends. Important elements that af­fect the impact of changing lake levels include therate, cyclicity, and duration of lake-level changes.The goals of the lake-level component of the over­all southern Lake Michigan study are to documentthe history of lake level in Lake Michigan over thepast several thousand years, to understand better thecauses of lake-level fluctuations, and to determinehow the lake and its shoreline have responded tothose fluctuations.

This paper describes the record of changes inLake Michigan derived from studying lacustrinesediments deposited in the deep southern part of thelake, focusing on Holocene time, but also includingthe latest Pleistocene. The advantages of studyinglake sediments from the deep basin to document thehistory of lake-level changes include the following:(1) these sediments provide a continuous recordthrough time of conditions in the lake; (2) the tex­tural, magnetic, geochemical, and paleontologicalproperties of the sediments reflect conditions in thelake, including lake level; and (3) when these sedi­ments are dated, they furnish a continuous time se­ries, in contrast to the inherently episodic nature ofshoreline deposits. However, although deep-waterlacustrine sediments can provide detailed informa­tion about the timing of lake-level fluctuations,only rarely can they be used to determine waterdepths precisely. Ideally, water-level estimates de­rived from shoreline deposits can be combined withage and paleoenvironmental information from deeplacustrine sediments to produce a complete recordof changes in lake level.

We will present a reconstruction of lake level inthe southern Lake Michigan basin, based on spe­cific events and the evidence used to reconstructthose events. This evidence includes extensive ef­forts to date precisely the sediments and to deter-

mine the degree to which their various textural,magnetic, geochemical, and paleontological proper­ties appear to reflect changes in lake level. Finally,using the resulting time series of these properties,we will discuss distinct lake-level events and theimplications of the time series for the frequencyand magnitude of lake-level changes.

METHODS AND DATA

During two cruises in 1988 and 1989, we ac­quired 2,150 km of high-resolution seismic-reflec­tion profiles and collected cores at 29 sites (Fig. 1).Both 3.5-kHz and boomer-type seismic-reflectionprofiles were obtained. Navigation was byLORAN-C. We collected multiple cores at eachsite, including (at most sites) a box core, a gravitycore, and either a piston core or a vibracore. Thebox cores contained a complete, undisturbed sam­ple of the uppermost sediments; the gravity cores(10 cm in diameter) were complete, but in somecases, the uppermost sedimentary layers were ap­parently slightly thinner than the corresponding lay­ers in the box cores. Comparisons of thestratigraphy and physical properties of the vibra­cores (10 cm in diameter) and piston cores (6.5 cmin diameter) with the gravity cores and box coressuggest that variable amounts of sediment, rangingfrom a few centimeters to more than a meter, werelost from the top of the piston cores and vibracores.Various cores from each site were correlated on thebasis of color and texture, biostratigraphic andmagnetostratigraphic markers, and radiocarbonages; data are presented here in the form of a singlecomposite core at each site. These correlations sug­gest that some of the piston cores and vibracoresfailed to recover the uppermost part of the sedimentsection. Consequently, 15 cm were added to thedepths in core 9V, 35 cm to those in core 6P, and110 cm to those in core 4P, in comparison with theactual depths given in the original core descriptions(Colman and Foster 1990, Colman et al. 1990).

This study employed a variety of analytical tech­niques, including seismic stratigraphy, core litho­stratigraphy, detailed grain-size analyses, ostracodebiostratigraphy and paleoecology, accelerator-massspectometer (AMS) radiocarbon analyses, stablecarbon- and oxygen-isotope measurements, andother geochemical analyses. We focus on the threecore sites (4, 6, and 9) (Fig. 1) for which we havethe most complete analyses. Details of the seismic,sedimentologic, paleontologic, isotopic, and radio­carbon methods used in this study are given by Col-

Lake Michigan Lake Levels for the Past 12,000 Years 75

88°

ILLINOISINDIANA

50 KILOMETERS, ! ! I ,

CONTOUR INTERVAL' 20 METERS

man and Foster (1990), Colman et ai. (1990), andFoster and Colman (1991). All ages are on the ra­diocarbon time scale, either in years B.P. or ka(thousand years before present).

We have defined five ostracode zones for the pe­riod 5 to 10 ka (Forester et al. this volume), on thebasis of the relative abundances of environmentallydiagnostic species (Colman et ai. 1990). Thesezones contain considerable paleolimnologic and pa­leoclimatic information (Forester et ai. this vol­ume). Of particular interest are two indices(Forester et ai. this volume). One is based on therelative abundance of Limnocythere friabiiis, whichis a species that typically lives in environmentsaround the margins of large lakes, and is thereforean indication of the proximity of the shoreline to agiven site (Forester et al. this volume). We calcu­lated a numerical index of shoreline proximity fromthe following ratio: (L. friabilis)/(Candona subtri­anguiata + Cytherissa iacustris + 1). The otherindex is a measure of the total dissolved solids(TDS) in the lake and is defined as (C rawsoni)/(C subtrianguiata + C iacustris + 1). The defini­tion and interpretation of these ratios are discussedin more detail by Forester et ai. (this volume).

In addition to these methods, we include paleo­magnetic analyses for age control and a variety ofsediment-magnetic properties for paleoenvironmen­tal interpretations. Paleomagnetic directions weredetermined at 3-cm intervals from whole cores byusing an automated, pass-through, superconductingmagnetometer following alternating-field (AF) de­magnetization at 10 or 15 milliteslas (mT). Suchdemagnetization steps were confirmed to be appro­priate on the basis of paleomagnetic results from in­dividually oriented samples that were subjected todetailed AF demagnetization, typically at eightpeak-field steps from 2.5 to 60 mT, and that wereanalyzed by principal component analysis(Kirschvink 1980). Magnetic susceptibility was alsomeasured on most whole cores at 3-cm intervals byusing pass-through coils (Colman et ai. 1990).

FIG. 1. Index map of seismic-reflection tracklinesand core-site locations. Numbered squares are coresthat are discussed in the text. Locations of profiles inFigure 5 are shown as thick lines; profiles in Figures7A and 7B pass through core sites 4 and 9, respectively.Inset is an index map showing locations of outlets forLake Michigan discussed in the text: N, North Bay; M,Straits of Mackinac; I, Indian River lowlands; C,Chicago.

76 Colman et al.

After remanent magnetization was measured, in­dividual samples were used to determine the fol­lowing magnetic parameters, for which themeasurement, principles, and interpretations havebeen summarized by Thompson and Oldfield(1986): (1) magnetic susceptibility (K), a measureof the amount of ferromagnetic minerals, princi­pally magnetite; (2) the ratio of susceptibility of an­hysteretic remanent magnetization to volumesusceptibility (KARM/K), mainly a measure of thegrain size of magnetite, higher values indicatingsmaller grain size; and (3) the ratio of the backwardisothermal remanent magnetization (IRM) acquiredat 0.3 Tesla (T) to the forward IRM acquired at 1.2T (the S parameter), a measure of the content ofmagnetite relative to high-coercivity ferric oxideminerals, such as hematite and goethite. Higher val­ues (maximum of 1.0) indicate higher relative con­tent of magnetite. In this report, we use "hematite"loosely to represent all high-coercivity ferric oxideminerals and "magnetite" to represent the class offerrimagnetic Fe-Ti oxide minerals, such as mag­netite and titanomagnetite, as well as related oxi­dized forms, such as maghemite and titanomag­hemite.

Paleoenvironmental interpretations using suchmagnetic properties are valid only if the sedimentmagnetism is carried overwhelmingly by detritalmagnetic particles from the drainage catchment.Our petrologic and geochemical results support thisrequirement and did not reveal magnetic effectsfrom post-depositional alteration (Reynolds et al.1990).

SETTING AND STRATIGRAPHY

Lacustrine sedimentation in the southern LakeMichigan basin began as the Lake Michigan lobe ofthe Laurentide ice sheet began to retreat frommoraines along the southern margin of the lakeabout 14 ka (Hansel et al. 1985). Relatively coarsesublacustrine outwash (Equality Formation; Fig. 2)was deposited near the glacial terminus while finergrained glacial-lacustrine sediments (lower LakeMichigan Formation) were deposited in more distallocations (Colman and Foster 1990, Foster and Col­man 1991). The period from about 14 to 11 ka wascharacterized by overall retreat of the glacial lobe,interrupted by at least three readvances (Linebacket al. 1974). When ice was extensive in the basin,lake levels were relatively high (Glenwood andCalumet phases) and were controlled by theChicago outlet (Hansel et al. 1985). Conversely,

lake levels were relatively low when the ice marginwas north of the Indian River Lowlands or Mack­inac Straits, and the lake drained through those pas­sages. Although this broad outline is generallyagreed upon, considerable controversy exists con­cerning the details of late glacial lake-level historyin the Lake Michigan basin (compare Hansel et al.1985, Larsen 1987, and Chrzastowski and Thomp­son 1992). We focus here on the period followingthe retreat of the Two Rivers glacier, the last of themajor readvances into the Lake Michigan basin,which culminated about 11.8 ka (Hansel et al.1985).

The lithostratigraphy of the upper Quaternarysediments beneath Lake Michigan was first de­scribed by Hough (1955, 1958) and later formalizedby the Illinois State Geological Survey (summa­rized by Wickham et al. 1978 and Lineback et al.1979) (Fig. 2). These summaries recognized till andoutwash units of several formations and they di­vided the lacustrine Lake Michigan Formation intofive members. In general, we agree with this strati­graphic framework, but we use an informal, modi­fied stratigraphy here for two reasons: (1) Weinterpret the details of the earlier stratigraphy dif­ferently, as described in the following paragraphs,and (2) the earlier formal lithostratigraphy is in theprocess of being revised (Personal communication,A.K. Hansel, Illinois State Geological Survey,1992).

We subdivide the Lake Michigan Formation intoonly two units, a red lower unit and an gray-to­brown upper unit, because we were unable to dis­tinguish consistently between the two redglaciolacustrine members of the Lake MichiganFormation (South Haven and Sheboygan Members)nor among the three postglacial gray-brown mem­bers (Winnetka, Lake Forest, and Waukegan Mem­bers), as these members were defined (Lineback etal. 1970, Wickham et al. 1978).

The separation of the Winnetka, Lake Forest, andWaukegan Members appears to be based mostly onthe abundance of black streaks and mottling in theLake Forest Member (Lineback et al. 1970, Wick­ham et al. 1978), although the Winnetka Membertends to be browner than the other two and theWaukegan tends to be siltier. The black staining ofthe Lake Forest Member is caused by iron monosul­fides, probably hydrotroilite and (or) mackinawite,which are unstable, reduced phases that oxidizerapidly when exposed to air. In our cores, the blackmonosulfides are commonly concentrated in themiddle of the upper unit of the Lake Michigan For-

A

Lake Michigan Lake Levels for the Past 12,000 Years

BAGE FORMATION MEMBER

c77

DARK-GRAY CLAY

LIGHT-GRAY CLAY

BROWN CLAY

SAND AND ~~.~~~.~.~~.~.J.

RED CLAY ~

BLUISH-GRAYCLAY

RED CLAY

SANDY RED CLAY,PEBBLES

TILL

WAUKEGANwzw() LAKE FOREST0-I0I

Zor::( WINNETKAC)

f--- I()

:E SHEBOYGJ\:Nw:x:: WILMETTE Ior::( BED-I

SHEBOYGAN

wz SOUTH HAVENw()0 "0I- Q) CARMIen Ew C'll-I §Xa.. >- TWO RIVERS

!:::-Ior::(:J

MANITOWOC0w

z SHOREWOOD0II:Clw

WADSWORTH3:

UPPER UNIT

(GRAY TO BROWN)

CHIPPEWA

UNCONFORMITYZ

.................... .......0 LOWERI- UNIT«:E WILMETIEa: BED0LLZor::(C)

LOWER UNITI()

:E (RED)w:x::«-I EQUAlITY FORMATION

TWO RIVERS

TILL

MANITOWOCw Z

TILLwOzp:~c(

c(::!: SHOREWO003:0:wo TILL:><::u...

WADSWORTHTILL

FORMATION

FIG. 2. Comparison of stratigraphic units used in this study and in previous studies: A, Hough(1955); B, Lineback et al. (1971; 1974; 1979); C, this study. Stratigraphic units used in this studyare informal, reflecting modifications of the formal stratigraphy in B. Formal revision of thestratigraphy for these units is in the process of being revised (Personal communication, A.K.Hansel, Illinois State Geological Survey, 1992).

mation, although the distribution is commonly irreg­ular and does not correspond to any given age inter­val in the cores. The black material is also abundantin the Wilmette Bed of the Sheyboygan Member,and it occurs sporadically in the lower unit of theLake Michigan Formation where that unit is close tothe sediment-water interface. We interpret the blackmonosulfides to be largely a diagenetic feature asso-

ciated with reducing conditions caused by decayingorganic matter. Because of our inference that theblack monosulfides are a diagenetic feature and thatthey do not correlate with any chronostratigraphicinterval, we conclude that their distribution is not anappropriate stratigraphic criterion.

Consequently, we have informally grouped thered members into the lower unit of the Lake Michi-

78 Colman et al.

gan Formation and the gray-brown members intothe upper unit of the Lake Michigan Formation(Fig. 2). The two are separated by the Chippewaunconformity in relatively shallow water (for exam­ple core 9V) and by a transitional color change indeeper water (for example core 6P). The distinctivegray Wilmette Bed is commonly present within thered lower unit of the Lake Michigan Formation.

Lineback et al. (1979) suggested that the redclays of the lower unit of the Lake Michigan For­mation were not deposited until after the TwoRivers glacier had begun to recede and that the pri­mary source of the red material was the Lake Supe­rior basin. The primary evidence for the depositionof the red clays after retreat of the Two Rivers glac­ier is the lack of north-to-south variation in thick­ness of the red clay (Lineback et al. 1979).However, we believe that the red, lower unit of theLake Michigan Formation was deposited in associa­tion with each of the three reddish tills (the Shore­wood, Manitowoc, and Two Rivers Tills) in theLake Michigan basin, for several reasons. First, theLake Superior basin may have been the ultimatesource of the red materials (Dell 1976, Lineback etal. 1979, Hansel et al. 1985), but the red tills in theLake Michigan basin form a more probable, imme­diate source than the outwash from Lake Superioracross the Upper Peninsula of Michigan (Linebacket al. 1979). Second, deglaciation of the UpperPeninsula during the interval between 11 and 10 ka(Drexler et al. 1983) cut off the source of red sedi­ment and left very little time for deposition of thelower unit of the Lake Michigan Formation after re­treat of the Two Rivers glacier about 11.2 ka(Hansel et al. 1985). Third, the lack of north-to­south variation in thickness may be caused by thewide variations in sediment-accumulation rates inglacial-lacustrine environments and by lake-basinfocusing effects. Finally, we obtained an AMS ra­diocarbon age of 12.4 ka (AA-5900) on ostracodesin the lower unit of the Lake Michigan Formationseveral meters above its base at core site 9, an indi­cation that the red clays were deposited at least inpart before the Two Rivers advance. According toour interpretations, the Equality Formation (proxi­mal, relatively coarse, sublacustrine outwash) andthe lower part of the Lake Michigan Formation aresimply different lateral facies in a proglacial lacus­trine environment.

Drexler et al. (1983) suggested, and Hansel et al.(1985) supported, the idea that the gray WilmetteBed was deposited between 11 and 10 ka, whiledeglaciation of the Superior basin had cut off the

supply of red sediment to the Lake Michigan basin.Teller (1985) suggested that the Wilmette Bedmight reflect the influence of eastward discharge ofLake Agassiz water during this time interval(Moorhead phase of Lake Agassiz). However, thesespeculations lacked supporting dates on the Wil­mette Bed and lacked lithologic evidence of a con­nection between the Wilmette Bed and LakeAgassiz or Lake Superior. The red clay of the upperSheboygan Member (above the Wilmette Bed) mayhave come from meltwater that flowed brieflyacross the Upper Peninsula of Michigan into theLake Michigan basin during the advance and retreatof the Marquette glacier in the Superior basin (10.0to 9.8 ka; Drexler et al. 1983, Teller 1985). Finally,the gradational color change from red to gray (fromthe Sheboygan to the Winnetka Member) in thedeep basins of Lake Michigan may be due to re­working of older red sediments during theChippewa low stage, along with a shift to localrivers and bluffs as sources of sediment (Wickhamet al. 1978, Lineback et al. 1979).

The upper Pleistocene lithostratigraphy in Illinoisis in the process of being revised (A.K. Hansel,written commun., 1992). This revision includesnew Group ranks, and places all the red tills (Shore­wood, Manitowoc, and Two Rivers Tills) into a sin­gle formation, the Kewaunee Formation (Fig. 2).The revision also incorporates many of our inter­pretations of the facies relationships between thetills, the Equality Formation, and the lower unit ofthe Lake Michigan Formation.

AMS RADIOCARBON AGES ANDPALEOMAGNETIC AGE CONTROL

AMS radiocarbon analyses (a total of 51 ages),combined with paleomagnetic measurements thatcan be correlated to dated, regional, magnetic secu­lar-variation curves, provide good age control forthe time represented by the upper unit of the LakeMichigan Formation (10.3 ka to present) (Fig. 3).Radiocarbon ages were obtained on a variety of ma­terials, including biogenic carbonate (ostracode andmollusk shells), total organic carbon (TOC), anddiscrete hand-picked pieces of organic matter. In allcases, carbonate ages are younger than TOC agesfrom the same stratigraphic interval, by amountsranging from about 500 years to more than 1,500years in the Holocene. This contrast demonstratesthat a significant fraction of the organic carbon isdetrital and older than the enclosing sediment. Weused the carbonate ages (22 total), along with the

Lake Michigan Lake Levels for the Past 12,000 Years 79

2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10

RADIOCARBON AGE (ka)

. . . . .COREs/rEg COREs/rEB ++ COREs/rE4

~ + -10ot"-

"-

\ +100 -+t+ 200

• . II -+ + +•

~ . -300 :.w~ + .-•

·200 ++ fo400 •+ +•+ .• • 500•

. . I I I I I I • l'

200

250o

50

o

-E 100o-:J:I­a..w 150C

FIG. 3. Age-depth relations at core sites 4, 6, and 9. AMS radiocarbon ages on biogenic car­bonate (squares), paleomagnetic age correlations (plus signs), and ages based on stratigraphiccorrelations (circles) are shown.

well-dated (l0.3 ka) (Larsen 1987) stratigraphichorizon that marks the Chippewa low phase of thelake, as the basis for our radiocarbon chronology ofthe cores shown here (Fig. 3). For core 6, we alsoused a stratigraphic horizon (discussed later) that isdated in cores 4 and 9 at 9.1 ka. The radiocarbonages on biogenic carbonate are not corrected forpossible reservoir or "hard-water" effects. Our pre­liminary data suggest that this effect may make theradiocarbon ages too old by as much as severalhundred years. However, the close correspondencebetween the radiocarbon ages and the paleomag­netic ages discussed below suggests that any "hard­water" effect on the carbonate radiocarbon ages isno more than about 500 years.

Paleomagnetic directions were measured for eachof the cores, and curves for inclination and declina­tion were plotted with depth. These curves were ap­proximately aligned (within about 2,000 years) withwell-dated regional curves of magnetic secular vari­ation (l.W. King, unpubl. data, 1993), by using thecarbonate radiocarbon ages for each core. Correla-

tion of prominent individual features in the two setsof paleomagnetic curves then provided an indepen­dent estimate of the age of the corresponding depthin each core (Fig. 3).

LATEST PLEISTOCENE (12-10 ka)LAKE LEVELS

Our reconstruction of the history of lake level inthe southern Lake Michigan basin for the periodfrom 12 to 5 ka (Fig. 4) is based on a variety of agecontrol, stratigraphic relations, geochemical data,and ostracode assemblages. In the following sec­tions, we discuss the evidence for each of the majorfeatures in this reconstructed lake-level curve.

Two Rivers Advance-Calumet High Phase

The Two Rivers advance was the last majorglacial readvance into the Lake Michigan basin. Itextended about halfway into the Lake Michiganbasin, overriding a forest bed at Two Creeks, Wis-

80 Colman et aI.

80D

about the timing of discharge through different out­lets and the correlation of those outlets to upliftedterraces in the northern Great Lakes. Correlativeterraces in the southern lake basins are not well pre­served, and the associated shorelines were appar­ently below the Calumet level (Hansel et al. 1985).Larsen (1987) concluded that lake level was wellbelow present lake level in the southern parts of thelake basins, but alternate interpretations based onisostatic rebound of the northern outlets (Chrzas­towski and Thompson 1992) or flooding from LakeAgassiz (Lewis and Anderson 1989) have been pro­posed. Despite the disagreement concerning detailsof the period between 11.2 and 10.6 ka, it is gener­ally agreed that lake levels in the southern LakeMichigan basin eventually fell to well below pre­sent levels (Fig. 4, segment B) as a series of succes­sively more isostatically depressed northern outletswere uncovered. Lake-level lowering culminatedwith the opening of the North Bay outlet (Hansel etai. 1985, Lewis and Anderson 1989), the best datefor which is about 10.3 ka (Larsen 1987, Lewis andAnderson 1989).

Lake Agassiz Influx, Moorhead Phase

From about 10.9 to 9.9 ka (the Moorhead phase),Lake Agassiz drained eastward into the LaurentianGreat Lakes, probably at times catastrophically(Clayton 1983, Drexler et al. 1983, Teller 1985). Bymost recent accounts (Larsen 1987, Lewis and An­derson 1989, Chrzastowski and Thompson 1992),Lakes Superior, Huron, and Michigan were conflu­ent at the time of the Moorhead phase floods, so theadded discharge from Lake Agassiz should havesubstantially raised lake level, perhaps by 20 to 50m (Farrand and Drexler 1985), even if briefly.

Lineback et ai. (1979) correlated the WilmetteBed in Lake Michigan with gray varved clays inLake Superior, which Teller (1985) later ascribed tothe Lake Agassiz influx during the Nipigon phaseabout 9.5 to 8.5 ka. Teller (1985) argued againstthis correlation, but suggested that the WilmetteBed might reflect the influence of eastward dis­charge of Lake Agassiz water during the earlierMoorhead phase of Lake Agassiz.

Our data on the age and character of the WilmetteBed support Teller's (1985) suggestion of its rela­tion to Moorhead-phase flooding from Lake Agas­siz (Fig. 4, point C). The Wilmette Bedstratigraphically overlies the Two Rivers Till and isseparated from that till by several meters of redglaciolacustrine clay. It must therefore be somewhat

125

180

::E 160

z0140i=~ 120wiihoo

Retreat from the Two Rivers Positionand Opening of Northern Outlets

By about 11.2 ka, the retreating ice sheet beganto uncover a series of progressively lower, isostati­cally depressed northern outlets, and lake levelbegan to fall from the relatively high Calumetphase associated with the Two Rivers advance(Hansel et ai. 1985, Larsen 1987, Chrzastowski andThompson 1992) (Fig. 2). Disagreement exists

consin, about 11.8 ka (Hansel et ai. 1985). The TwoRivers advance blocked all northern outlets of thelake, forcing drainage southward through theChicago outlet, and creating the high Calumetphase of Lake Michigan (Fig. 4, point A) untilabout 11.2 ka (Hansel et ai. 1985).

Deposits of the Two Rivers advance and earlierglacial readvances into the Lake Michigan basin arewell shown in our seismic-reflection profiles (Fig.5). They occur as tabular or tongue-shaped, ac­coustically amorphous masses that are intercalatedwith stratified glaciolacustrine sediments (EqualityFormation and lower unit of the Lake MichiganFormation). The preservation of stratified sedimentsbeneath the tills, the fine-grained, stone-poor com­position of the tills themselves, and the configura­tion of the till margins all suggest that thesereadvances were rapid and probably short lived, andthat the ice was thin and had very low basal shearstresses (Colman et ai. 1989, Foster and Colman1991).

6 7 8 9 10 11

RADIOCARBON AGE IN 103 YR

FIG. 4. Reconstructed lake-level curve for the periodfrom 12 to 5 ka. Letters refer to distinct lake-levelevents that are discussed in the text. Arrows indicatethe duration of some of these events. Querries indicatethat the peak lake levels indicated are uncertain.

Lake Michigan Lake Levels for the Past 12,000 Years 81

200011Ows1.111

150

~ NW

A.,DOl ~ ~ ~ ~f ~o_J J_~1~'5o ""~ i + ----;------- 4::=:o:-.::t:::- .... .:;;t'''''~'''~~ ~-Q.. . i 1···· ." 1-(1')w_ 150-i:fiOi!~~1Iiiii ,,~< of - 200 .....J ~C1~ . ! ~8ww ~w1-1- ~~~w I-.....J~~ ~.....J

~ Z 100 I KILOMETER ~ ~8:- VERTICAL EXAGGERATION!:O!I5 R2 f-150 I ZQ.. Olu 0':::::

~ R2 O,lu R3 ~I-

Ow.

300

NW,200

----lI

__._--1---

I I

Rr Qlu R2 011 I KILOMETERVERTICAL EXAGGERATlON.... 18 Olu R2 R3 -

~"..... JlIIrot.Qe Pz h.~Otr

oe"-- "" -~m '"" Qr" - Qwm-

pz

SE150-

200

200-'

B.

FIG. 5. Seismic-reflection profiles (3.5 kHz) of deposits resulting from glacial readvances into LakeMichigan. A, Shorewood advance into the southern basin; B, Two Rivers advance terminating againstbedrock scarp of the mid-lake bathymetric high. Pz, Paleozoic bedrock; Qww, Wadsworth Till(Wadsworth Formation); Qws, Shorewood Till Member of the Kewaunee Formation; Qwm, ManitowocTill Member of the Kewaunee Formation; Qtr, Two Rivers Till Member of the Kewaunee Formation; Qe,Equality Formation; Qll, lower unit of the Lake Michigan Formation; Qlu, upper unit of the Lake Michi­gan Formation; R2, R3, other prominent seismic reflections (Foster and Colman 1991).

younger than the 11.2 ka retreat (Hansel et al.1985) of the Two Rivers lobe. The Wilmette Bed istruncated by the Chippewa unconformity, whichbegan to form at about 10.3 ka during the Chippewalow phase of Lake Michigan. At core site 9 (Fig. 1),the Wilmette Bed has been removed by erosion atthe Chippewa unconformity; an AMS radiocarbon

age on biogenic carbonate just above the unconfor­mity in core 9 is 10.2 ka (AA-4979). We processedmore than 20 samples of the Wilmette Bed for data­ble material, but the bed is nearly barren of bio­genic carbonate and discrete organic matter. Onesample consisting of a fragment of a spruce needleyielded an AMS age of 10.15 ka (AA-5895), but the

82 Colman et aI.

extremely small size of the sample makes the ageunreliable. We conclude that the age of the Wil­mette Bed is between 11.2 and 10.3 ka, most likelybetween 11.0 and 10.5 ka. This age indicates thatthe bed was deposited during the first half of theMoorhead phase of Lake Agassiz.

The gray Wilmette Bed is distinctive because ofits sharp contacts, especially the upper one, with thered clay above and below it. According to Linebacket al. (1979), the Wilmette Bed represents "one ofthe major sedimentological discontinuities in theLake Michigan Formation." In addition, it bears

several striking resemblances to an interval in theupper unit of the Lake Michigan Formation that weascribe to Lake Agassiz influx during the later Nip­igon phase (Colman et al. 1990). Both intervals aremassive (Fig.6A) and gray, and commonly containblack streaks of iron monosulfides. In contrast tothe surrounding sediments, both intervals are nearlydevoid of ostracodes (Fig. 6B). Both intervals alsohave distinctive magnetic properties (Fig. 6C).Taken together, these observations indicate that theWilmette Bed was deposited during a short-livedinflux of water and gray sediment from Lake Agas-

A

610500

300 400 500SUSCEPTIBILITY (JlSI)

c0.2 0.3 0.4

1 2VALVEs/GRAM

650'----

640

620

-E()-

FIG. 6. The Wilmette Bed (WB). Entire section shown is within the lower unit of the Lake Michigan Formation. A,X-ray photograph showing the massive nature of the gray Wilmette Bed in core 4, overlain and underlain by lami­nated red glaciolacustrine clays. B, Ostracode abundance profile for a part of core 4 showing the scarcity of ostra­codes, especially Candona subtriangulata, in the Wilmette Bed. C, Magnetic susceptibility profile for a part ofcore 4showing the decrease in susceptibility of the Wilmette Bed in comparison with that of the red clays that surround it.Dashed lines in Band C enclose zone ofostracode and susceptibility changes related to the Wilmette Bed.

Lake Michigan Lake Levels for the Past 12,000 Years 83

siz, which interrupted the deposition of red glacio­lacustrine clays.

Lake level during deposition of the Wilmette Bedis unknown, but the influx of water from LakeAgassiz must have caused a substantial rise (Fig. 4,point C). Lewis and Anderson (1989) postulatedthat these Lake Agassiz floods raised the level ofLake Huron enough to cause overflow through thePort Huron outlet. If so, Lake Michigan would haverisen to about the level of the Chicago outlet andthe Calumet shoreline. This explanation offers analternate for the ages at the Calumet level of aboutlOA ka, which Chrzastowski and Thompson (1992)ascribe to a second occupation of the Calumet levelcaused by rebound of the Kirkfield outlet during itsuse.

North Bay Outlet and the Chippewa Low Phase

Deglaciation of the North Bay area about 10.3 ka(Larsen 1987) allowed all of the drainage of theupper Great Lakes to flow through this isostaticallydepressed outlet. The result was extremely low lakelevels throughout the upper Great Lakes (Fig. 4,point D), known as the Chippewa low phase inLake Michigan (Hough 1955). Subsequent isostaticuplift of the North Bay outlet caused lake levels torise in Lake Michigan and created a major trans­gressive unconformity (Fig. 2) called the Chippewaunconformity.

The Chippewa unconformity was first recognizedby Hough (1955) as a well-defined sandy zone incores throughout much of Lake Michigan. Thesandy horizon is commonly fossiliferous and ap­pears to truncate underlying units. Hough (1955)traced the sandy zone from cores in shallow waterto those recovered in water as deep as 107 m. Fromthis observation, he suggested that lake level duringthe Chippewa low phase was about 107 m belowpresent lake level. At this level, the northern basinof Lake Michigan would have been separated fromthe southern basin by a river draining across themidlake bathymetric high (Hough 1955, 1958).However, our seismic-reflection profiles across thesaddle in the midlake high (Fig. 1) show no evi­dence of a former river channel, from which weconclude that lake level during the Chippewa lowphase never fell to more than 100 m below presentlake level, and that the water in the northern andsouthern basins of Lake Michigan was always con­fluent.

Although the Chippewa unconformity was notdiscussed in early Illinois State Geological Survey

reports, Wickham et al. (1978) and Lineback et al.(1979) stated that the unconformity truncates boththe South Haven and the Sheboygan Members andthat the associated sandy zone extends to depths of82 m below lake level, as opposed to Hough's(1955) 107-m estimate. The upper limit of the trun­cated red clays is also at this level (Wickham et al.1978, Lineback et ai. 1979). Recognizing that someof the erosion associated with the unconformity oc­curred below lake level, these workers assumed aneffective wave base of 20 m and thus calculatedthat lake level was 62 m below present during theChippewa phase in southern Lake Michigan, a fig­ure that agrees with Buckley's (1975) estimate (61m), which was based on ostracode zonation.

The Chippewa unconformity in our seismic-re­flection profiles is a distinct, high-amplitude reflec­tion that truncates underlying units (Fig. 7). Itsplanar, erosional character indicates that it repre­sents a transgressive unconformity, formed duringlake-level rise from the extreme low of theChippewa phase (Foster and Colman 1991). Fewshoreline features associated with the unconformity,such as wave-cut scarps or relict beach deposits,can be identified in the seismic-reflection profiles,and the few that exist cannot be traced between pro­files (Foster and Colman 1991). Thus, no system­atic reconstruction of discrete lake levels during thetransgression is possible from our data. The reflec­tion that marks the unconformity grades into alower amplitude, conformable reflection in the deeppart of the southern basin. This transition occurs inwater depths of about 80 to 100 m.

A sandy zone a few centimeters thick occurs inmany of our cores at about the depth of the uncon­formable horizon in the seismic-reflection profiles.The sandy zone occurs in cores taken in waterdepths of as much as 100 m, consistent withHough's (1955) observation. However, becausesome erosion and sand deposition must have oc­curred below lake level during the Chippewa phase,the extent of the unconformity and the sandy zoneis insufficient in itself for determining the minimumlevel of the Chippewa phase.

Additional information about Chippewa-phaselake levels comes from our biostratigraphic andlithostratigraphic data (Fig. 8). At core site 12 in 90m of water (Fig. 1), all five of the ostracode zonesthat we have defined (see "Methods") are present,as are the sandy zone, the Wilmette Bed, and redclay between the two. Therefore, the section at coresite 12 appears to be nearly complete; little or noerosion is apparent at the unconformity. In contrast,

84 Colman et aI.

'-150

I KILOMETERVERTICAL EXAGGERATION ~ II

w E

A.50

j i Ii. i i i 50-: . !:~ : . :

~ 125j~" .' '. ,<0" I''''''v .~,:" '~I:·:'.·"":· ':'1 . '~, .' ." '1 ", .1150 ;;

: 150 _' II I I i ~~ : I : L200 ~

~ ww ~c ~r-----------------------------50 -oJ

W

~a::~

>~

I

~~

....L..---------------------------L.200

pzQeq

0.5 KILOMETERVERTICAL EXAGGERATION ~16

-120 ~

>,...140 ~

I

o

~160 ~....L... -1

~

w~

l===~~~~QIIUF~~Q~I~'~C~O~RJE~9~V~~C~u~~r 100 ~«~Qww

FIG. 7. Seismic-reflection profiles (3.5 kHz) of the Chippewa unconformity. A, through core site 4; B,through core site 9. Symbols are the same as in Figure 5; cu, Chippewa unconformity, at base of unit Qlu(upper unit of the Lake Michigan Formation).

Lake Michigan Lake Levels for the Past 12,000 Years 85

EARLY HOLOCENE (10-5 KA) LAKE LEVELS

ential east-west tilting (east side up) of the southernbasin by 11 m or more since the Chippewa phaseminimum lake level.

General Trend

During the period from 10 to 5 ka, isostatic upliftof the North Bay outlet caused an overall increase inlake level (Fig. 4, segment E) from the minimum ofthe Chippewa low phase at about 10.3 ka to the maxi­mum of the Nipissing high phase at about 4.7 ka(Hansel et al. 1985, Larsen 1987, Chrzastowski andThompson 1992). Rising lake level drowned a forestoff Chicago at a present depth of 24 m about 8.3 ka(Fig. 4, point G) (Chrzastowski et ai. 1991, Chrzas­towski and Thompson 1992). The shoreline crossedpresent lake level about 6.3 ka, and reached about 6m above present levels at about 4.7 ka (Larsen 1985).

Comparing time series of ostracode shoreline in­dices (see "Methods" and Forester et al. this vol­ume) with our lake-level curve shows distinctsimilarities between the two (Fig. 9). High valuesof this index suggest that the shoreline was rela­tively close to a given core site and that lake levelwas therefore relatively low; however, the index isclearly a relative measure, not a linear function oflake level. Different sites apparently have differentsensitivities to the index, depending on their posi­tions with respect to the shoreline and their deposi­tional environments. Nevertheless, the timing anddirection of changes in the ostracode shorelineindex are remarkably consistent with the timing anddirection of lake-level fluctuations. The indicesfrom two different cores (core 4, Fig. 9B; core 9,Fig. 9C) are also remarkably consistent with eachother, especially considering the uncertainty of sev­eral hundred years in the time scales (Fig. 3). Theostracode shoreline index for core 4 suggests lowlake levels about 10 ka, consistent with theChippewa low phase of the lake, and the indices forboth core 4 and core 9 show an overall rise to highlake levels about 5 ka, consistent with the Nipissinghigh phase of the lake (Fig. 9).

Detailed grain-size profiles (3-cm intervals) anda variety of sediment-magnetic properties for coresite 4 and 9 also reflect early Holocene changes inlake level (Fig. 9). We have chosen one grain-sizeparameter (silt/clay ratio) and one sediment-mag­netic property (magnetic susceptibility, a measureof the concentration of magnetite) as representa­tives for the comparisons in Figure 9; other grain-

90 M DEPTH

CORE 12G

<- 5KM··>

<- GRAY MUD->

WB->

<- 12.4

<- RED CLAY->

79 M DEPTH

CORE 9V

1(lowerpart)

<- 10.2

'SANO~1->•••

at core site 9, 5 km away in 79 m of water (Fig. 1),the upper red clays and the Wilmette Bed have beenremoved by erosion at the unconformity. In addi­tion, ostracode zone 2 and parts of zones 1 and 3are absent (Fig. 8), presumably because the lowerpart of the missing section was eroded, and theupper part was never deposited. From these rela­tions, we infer that core site 9 was at or above lakelevel for a short time during the Chippewa phase.At the same time, core site 12 appears to have beencontinuously submerged and experienced onlyminor sublacustrine erosion at the Chippewa uncon­formity. We therefore estimate that the minimumlake level during the Chippewa low phase wasabout 80 m below present lake level in the south­western part of the southern basin.

On the eastern side of the southern basin, coresite 4 (Fig. 1), at the same depth as core site 9, con­tains all five ostracode zones, similar to the stratig­raphy of core site 12, which is 11 m deeper. Thisrelation between core sites 4 and 12 suggests differ-

FIG. 8. Diagrammatic representation of the biostrati­graphic and lithostratigraphic relations between coresites 9 and 12. Outside part ofeach column refers to os­tracode zones defined by Forester et al. (this volume).Numbers in the center are radiocarbon ages (ka) fromcore 9V. Query next to core 12 indicates little or no ero­sion at horizon shown. WB, Wilmette Bed of the lowerunit of the Lake Michigan Formation; vertical ruling,gray mud; cross hatch, red clay.

5

4

3(upperpart)

86 Colman et al.

tively high at about 10 ka and decrease over thenext few thousand years. These trends indicate rela­tively coarse grain size and high magnetite contentsduring the Chippewa low phase at core site 4 anddecreases in both grain size and magnetite contentas water depth increased during the ensuing trans­gression. Both parameters are relatively constantbetween about 9 to 8 ka and 5 ka.

o

F : C IEl.G~0.~•..... ? ••..

~: l' · .B

· .. /· . '/;· , . .

80

A 200r---r---r--"'T""""-,--...,.....~-r---:A-,

180

§:160

~ 140i=~ 120wu:l100

FIG. 9. Comparison of our lake-level curve (A) forthe interval from 10 to 5 ka with the ostracode shore­line-proximity index, magnetic susceptibility in 1()4 vol­ume SI units, and silt/clay ratios. All data arethree-point moving averages. B, core 4; C, core 9. Notethat scales in Band C increase downward.

size and magnetic properties generally show time­series fluctuations similar to, or inverse to, the onesshown in Figure 9. Magnetic susceptibility corre­lates well with sediment grain size, particularlywith the sand fraction (Colman et al. 1990). Bothmagnetic susceptibility and silt/clay ratios are rela-

Lake Agassiz Influx, Nipigon Phase

We documented a major influx of isotopicallylight water into the Lake Michigan basin (Fig. 4,point F) on the basis of ()180 values measured onmollusks from core 9V (Colman et ai. 1990). Be­cause of the magnitude of the change in ()180, theonly reasonable source for this isotopically lightwater at this time is glacial Lake Agassiz. Theevent is directly dated in core 9V at 9.14 ka (AA­4615), and dated by interpolation in core 4P atabout 9 ka. These ages correspond to the middle ofthe Nipigon phase (9.5-8.5 B.P.) of Lake Agassiz,during which Lake Agassiz drained eastwardthrough the Laurentian Great Lakes (Teller andThorleifson 1983, Teller 1985).

We have verified the oxygen-isotope signal forthis event for several cores sites (4, 6, and 12) inaddition to site 9 and for several species of ostra­codes in addition to the mixed mollusks in core 9V(Fig. 10). Associated with the isotopic signal ofLake Agassiz water is a drastic decrease in ostra­code abundance (Fig. 10) probably because of dis­solution of the shells (Forester et ai. this volume).Detailed sampling reveals that small ostracodespecies disappear first and large ones last, and thatall shells in this thin interval show signs of dissolu­tion. A possible explanation for the dissolution ofostracodes is that sediment-accumulation rates dur­ing this event may have slowed, because silt andcoarser sediment were trapped in upstream lakes;this slower accumulation rate enhanced dissolutionof shells in the undersaturated water from LakeAgassiz (Forester et ai. this volume).

Some cores (for example core 6, Fig. 10) alsoshow distinct sedimentological and sediment-mag­netic characteristics during the interval of LakeAgassiz influx. Core 6 contains two laminae ofscattered, small clay clasts, which may representice-rafted detritus. The silt/clay ratios in cores 4and 9 also show a decrease at about 9 ka (Fig. 9). Inaddition, many of the sediment-magnetic propertiesshow fluctuations around this interval (Fig. 10). Inmost cores, the concentration of magnetite de-

2

4

3

oo

0.4

0.2

~. . ......

0.6 :..../ •......:•......:- o....-.~ IIIdllil-iI'

0.8 "'1",,".~( ...1It1

SL·_...;·8III=.::i::~' -;;:;':':(:;;:"'J::~----'8~~~9-'--1:1':'~:":": - ....1":.1~-!1~·

RADIOCARBON AGE (ka)

c

Lake Michigan Lake Levels for the Past 12,000 Years 87

140 r---'"T"""-"""'--""""A B c

e-o-J:180~Q.Wo

170

............................................................

~~------------

0.9

S0.84

o

40 80 80

K (J,l.SI)140r--T--r--T'""-"T-r--..,.....,

1801-.............--'--.......---120

D - BIVALVES.-. c.lIdo.. •uIIItII/lfu""

•

I::: ~--~;:_::0­wo

170···········F\

i1BOL..-""'-...a.........._ ......""'-...a.........·5 ·10 0 2 4 8

~ 180 (POB) VALVES/GRAMFIG. 10. Example from core site 6 of isotopic, ostracode, sedimentological, and sediment­magnetic signatures related to the influx of Lake Agassiz water into the Lake Michiganbasin about 9.1 ka. A, magnetic susceptibility; B, KARM/K parameter; C, S parameter; D,oxygen-isotope values for bivalves and Candona subtriangulata; E, total ostracode abun­dance. Heavy dashed lines indicate layers of small, scattered clay clasts.

creases within this interval. Detailed magneticproperties for core 4 suggest an increase in mag­netic grain size and an increase in the ratio of mag­netite to hematite in this interval, trends mirrored inthe clay-clast laminae in core 6 (Fig. 10). Thesechanges in magnetic properties suggest changes in

sediment sources or depositional processes associ­ated with the Lake Agassiz influx.

The ostracode shoreline-proximity indices sug­gest that lake levels were high around 9.1 ka (Fig.9), consistent with the influx of water from glacialLake Agassiz documented earlier. The index for

88 Colman et al.

core 4 suggests the possibility of a second episodeof higher lake level at about 8.5 ka, similar to theone associated with the influx of Lake Agassizwater at about 9.1 ka.

Most recent reconstructions (Hansel et al. 1985,Larsen 1987, Lewis and Anderson 1989) postulatethat the water in Lake Michigan was at a higherlevel than the path of Lake Agassiz dischargethrough Lakes Superior and Huron during the inter­val from 9.5 to 9 ka. Lake Michigan drained towardthe isostatically depressed northern Huron basin viaa river through the Straits of Mackinac (Larsen1987). Given the strong evidence for the influx ofLake Agassiz water into the Lake Michigan basin atabout 9.1 ka, two possibilities exist: (1) isostatic re­covery of the North Bay outlet was more rapid thancommonly inferred, bringing the water level in theLake Huron basin close to that in the Lake Michi­gan basin, or (2) hydraulic damming at constric­tions in the North Bay outlet during Lake Agassizdischarge, as Lewis and Anderson (1989) sug­gested, was sufficient to backflood water from theLake Huron basin into the Lake Michigan basin.The fact that the Lake Agassiz influx into the LakeMichigan basin (about 9.1 ka ) is younger than thebeginning of the eastward drainage of Lake Agassiz(9.5 ka; Teller 1985) suggests that both phenomenamay have occurred together. The level to whichwater in the Lake Michigan basin temporarily roseduring the Lake Agassiz influx is unknown, butcould have been several tens of meters, similar tothe amount suggested for Lake Huron by Lewis andAnderson (1989).

Possible Fall in Lake Level About 7 ka

Both ostracode assemblages and oxygen-isotopevalues of biogenic carbonate (Forester et al. thisvolume) suggest that, at about 7 ka (Fig. 4, point H)Lake Michigan became more chemically concen­trated (higher total dissolved solids [TDS]) than itwas before or has been since. Increased preserva­tion of ostracodes during this interval, despiteslower sediment-accumulation rates in some cores(for example, core 6) suggest that the lake becamesaturated with respect to calcium carbonate. Thesilt/clay ratios in cores 4 and 9 show small in­creases at about 7 ka, suggestive of somewhathigher energy conditions during this time (Fig. 9).

A large increase in the ostracode shoreline-prox­imity indices at this time suggests that lake levelfell (Fig. 9): The level of the lake during this periodshould have been largely controlled by isostatic re-

bound of the North Bay outlet (Hansel et al. 1985,Larsen 1987). If TDS rose and lake level fell de­spite an isostatically rising outlet at 7 ka, then thelake would have become a closed basin-a viablemechanism for increasing TDS. However, some in­crease in TDS could have occurred if lake level hadjust kept up with the rising outlet, because evapora­tion could still have increased relative to outflow(Forester et al. this volume). Regardless of the ac­tual change in lake level, this chemical concentra­tion of the lake water is a major paleoclimaticallycontrolled limnological event that has not been pre­viously described.

Change in Character of LakeSediments About 6-5 ka

Between 6 and 5 ka, some characteristics of theupper unit of the Lake Michigan Formation changedgreatly (Fig. 4, point I). One of the most noticeablechanges is the virtual disappearance of ostracodeshells in the sediments at this time (Fig. 11). As dis­cussed in the previous section, ostracode shells wereabundant before 6 ka, and the assemblages, togetherwith isotopic data, suggest that the lake was satu­rated with respect to calcium carbonate (Forester etal. this volume). The disappearance of ostracodeshells after 6 ka indicates dissolution due to muchmore dilute water and clearly signals a major cli­mate change to higher effective moisture. The NorthBay outlet became the primary control on lakelevel, superimposed fluctuations being controlled byclimate. This condition has prevailed until the pre­sent; ostracodes now live in the lake but are poorlypreserved in even the most recent sediments.

Sediment-magnetic properties, including thoserelated to magnetic grain size, concentration, andmineralogy, all change at about 6 ka (Fig. 11). Insediment representing 6 to 5.5 ka, magnetite con­tent (K) abruptly increases. Magnetite also sharplyincreases in amount relative to hematite (S) whiledecreasing steadily in grain size (increasing valuesofKARM/K).

In addition, sediment-accumulation rates in somecores (for example, core 9V) show an abrupt de­crease at about this time. Apparently, a majorchange in sediment sources and (or) transport pathsoccurred in response to climatic changes indicatedby the ostracode and isotope data. The change insediment source or transport may have been partlycaused by the submergence at this time of the mid­lake bathymetric high to a depth where it no longeraffected waves along the full fetch of the lake.

Lake Michigan Lake Levels for the Past 12,000 Years 89

at. this volume). In addition, lake-level fluctuationsafter 5 ka were much smaller than those in the inter­val from 12 to 5 ka., Consequently, we use the lake­level curve of Larsen (1985; this volume) for thetime younger than 5 ka. Modifications of Larsen'scurve have been made (Fraser et al. 1990), but thesemodifications are tenuous. Thompson (1992) andChrzastowski and Thompson (1992) have producedan alternate lake-level curve for the late Holocene,but we disagree with their time scale, which wasgenerated from a regression line through scatteredminimum radiocarbon ages. For these reasons, weprefer to use Larsen's (1985) original curve.

Nevertheless, we have obtained continuous timeseries for the late Holocene by using other sedimentparameters, particularly grain-size distributions andsediment-magnetic properties. Core sites 4 and 6provide the primary record for the late Holocene;sediment-accumulation rates in core 9 are too slowin the late Holocene to provide useful time resolu­tion. Comparing time series of these parameterswith the late Holocene lake-level record (Larsen1985, Fraser et aZ. 1990) shows at least a generalcorrespondence in all cases (Fig. 12). Grain-size pa­rameters, including silt/clay ratio (Fig. 12B) andpercentage of sand (not shown), show gradual in­creases from 6 to 5 ka, followed by a sharp peakjust younger than 5 ka, roughly corresponding to themaximum of the Nipissing high lake phase. LateHolocene values of these grain-size parameters areclearly lower than they were during the interval of 5to 4 ka.

Sediment-magnetic properties for core 4 and core6 are similar, but core 4 has much more resolutionbecause of smaller sampling intervals and a fastersediment-accumulation rate (Figs. 12C-E). Mag­netic susceptibility, reflecting the concentration ofmagnetite, correlates well with the grain-size para­meters. Susceptibility shows an even closer correla­tion with the lake-level curve than the grain-sizemeasurements do. Given an uncertainty of a fewhundred years in the radiocarbon time scales, it canbe argued that each major peak in the lake-levelrecord matches a peak in the susceptibility record.

The KARM/K parameter (high values reflectingdecreases in the size of magnetic grains) shows asomewhat weaker relation to the lake-level curve.Magnetic grain size decreases in the 6-to-5-ka inter­val, but the curve shows primarily a broad peak onwhich a minimum magnetic grain size occurs about3 ka. If this pattern represents a response to rela­tively high lake levels in the late Holocene, then atime lag occurs between the maximum Nipissing

.,o

2 :E«a:CJ(jjw>....J«>

- Magnetic susceptibility (right)000. K,.",K(loft) ,

- $ parameter (left) /\

.... Ostracode abundance (right) // \\.

/' \ ..~:,:' '.

....../ .......

12r--~-....----....,----r-----,O.3 en....Jo>

>-0.2 !:::

....J

iiii=~w()(JJ

4~:::::::::::::::::::::::::~:::::::::::::::::::::::::~~:::::::::::::::::::::::::~:::::::::::::::::::::::::~0.1~0.96

However, climatic changes, associated vegetationalchanges, and (or) circulation changes in the lakealso may have affected the sediments.

LATE HOLOCENE (5-0 KA) LAKE LEVELS

At about 5 ka, the lake reached the level of theNipissing high phase, about 6 m above present lakelevel (Hansel et aZ. 1985, Larsen 1985). LakeMichigan became confluent with Lake Huron,through the Straits of Mackinac, before the peak ofthe Nipissing phase. For a time during the Nipissingphase, three outlets of the combined lakes-NorthBay, Chicago, and Port Huron (on Lake Huron)­were used simultaneously (Hansel et aZ. 1985,Larsen 1985). Most of the 6-m fall in lake levelsince the Nipissing phase has been caused by ero­sion of the Port Huron outlet, climatically con­trolled fluctuations being superimposed on the slowfalling trend (Hansel et aZ. 1985, Larsen 1985).

Our data provide few independent constraints onthe timing or amount of lake-level fluctuations afterabout 5 ka (Nipissing phase and younger lakephases). This lack is partly because of the fact thatostracodes, one of the most useful paleolimnologi­cal tools, are virtually absent in sediments youngerthan 5 ka, apparently because of dissolution of car­bonate in dilute, undersaturated water (Forester et

0.884 5 6 7 80

RADIOCARBON AGE (ka)

FIG. 11. Changes in ostracode abundance and sedi­ment-magnetic properties (three-point running aver­ages) at about 6 to 5 ka at core site 4.

a:wI-W:E~0.92

«~

(JJ

90 Colman et aI.

DISCUSSION AND CONCLUSIONS

The multidisciplinary data described in this paperhave greatly improved our understanding of lake­level and lake-environment records in Lake Michi­gan. In part, they have confirmed previousreconstructions of lake level in the Michigan basin,and, in part, they require revision or refinement ofprevious reconstructions .

For the first time, we have constructed preciseage models for Holocene sediments in Lake Michi­gan, based on extensive AMS radiocarbon dating ofbiogenic carbonate and on correlation of paleomag­netic secular variation with well-dated regional pa­leomagnetic chronologies. This age control,combined with the continuous deposition of sedi­ments in the deep-lake basin, yields a paleoenviron-

lake level and the minimum in the magnetic grain­size parameter. This lag may be caused by rework­ing of sediment during the slight overall fall fromthe maximum Nipissing high lake level.

The sediment-magnetic S parameter, reflecting therelative proportion of magnetite to hematite in thesediments, appears to correlate well with a smoothcurve drawn through the peaks of the lake-levelcurve. The S parameter also shows a good correla­tion with the overall shape of the magnetic suscepti­bility curve. Consequently, it may be respondingprimarily to periods of relatively high lake level andenhanced erosion of magnetite-rich till bluffs.

All of the sediment-magnetic parameters showindications of higher frequency structure than thelake-level curve does. The lake-level curve, by itsnature, is discontinuous and biased toward lake­level stands higher than present lake level. In con­trast, the grain-size and sediment-magneticparameters form a continuous time series. Althoughthe detailed structure of these time series needs bet­ter definition, they at least suggest cycles of about200 to 500 years (Fig. 12). Thompson (1992) ob­served similar cyclicity in beach ridge formation onthe southern shore of Lake Michigan.

I t Ii I /" '" ,. •• I

i···....····f··/ I ······t..\ I.·····f i ! ! \1 I

i I t··\ I\ I",

I 1:::=I ,! Ij j I j II I ! I I

! iii II +··_······1···········\···. J····l········l I I ··--r·· .

, .

D

10

1 8 Cln4-llo .. i 1lIIII i

4 I \ I-

0 \.: 1 zw

i 2()

V a:4 w

D-

o! 2I

0

30

~ 20

J

A

=2501n--'r"""-"--"--"'T"""~n--..,...--,

"w~200...J

~~150...J

iiit: 100wg::;)(0 50'L--_........._.J---.J---..L-_........._ ......._...J

112

!§1" ,~ 17I.J\ f\ 'iil 171 ~j - 1--\-, -t- ----- MM-- -.

FIG. 12. Comparison of the lake-level curve (A) forthe last 7,000 years (modified from Larsen 1985,Fraser et al. 1990) with grain-size and magnetic-prop­erties. B, Percentage of sand and silt/clay ratio (three­point averages) for core 4; C, magnetic susceptibilityfor cores 4 and 6; D, the KARAIK parameter (see text)for cores 4 and 6; E, the S parameter (see text) forcores 4 and 6.RADIOCARBON AGE (kl)

0.'8 ;, I '

~E r ..\......·....j...······'l'·.. i

ffi 0.'4 ". /,/ l ' , 1-..

Iii '~0.•2: .... =~(0 0.'

Lake Michigan Lake Levels/or the Past 12,000 Years 91

mental record that has significant advantages overthe discontinuous record preserved above presentlake level, particularly with regard to the frequencyof lake-level changes. The remarkable correspon­dence among sedimentologic properties, sediment­magnetic measurements, carbonate isotopic values,and ostracode assemblages clearly demonstratesthat deep-lake sediments record paleoenvironmentalchanges, including lake level.

On the other hand, the deep-lake sediment recordsuffers some deficiencies with regard to lake-levelreconstructions. Many sediment properties are af­fected by variables other than lake level, and manyare not linear functions of lake level. Therefore, al­though the sediments may record the timing andfrequency of lake-level change, they rarely recordactual lake levels.

The post-glacial history of lake-level changes inthe Lake Michigan basin begins at about 11.2 kawith the fall from the high Calumet level, caused bythe retreat of the Two Rivers glacier, whose read­vance had blocked the northern outlet of the lake.During this lake-level fall, a major influx of waterfrom glacial Lake Agassiz caused a temporary risein lake level, at perhaps 10.6 ka, and deposition ofthe distinctive gray Wilmette Bed in the midst ofred glaciolacustrine sediment. Following this event,lake level continued to fall as successively lower,isostatically depressed northern outlets of the lakewere uncovered by the retreating ice sheet, culmi­nating with the opening of the North Bay outlet atabout 10.3 ka. We estimate that lake level wasabout 80 m lower than it is today in the southernbasin of Lake Michigan, during the Chippewa lowphase, on the basis of the distribution of lithologicand ostracode units in closely spaced cores. LakeMichigan did not separate into a northern lake and asouthern lake during this low phase.

As has long been known (see reviews by Hanselet al. 1985, Larsen 1987), early Holocene lake levelwas controlled by isostatic rebound of the NorthBay outlet. Control points on the rising lake-levelcurve are provided by our estimate of -80 m at 10.3ka, -24 m at 8.3 ka for the Olson tree site (Chrzas­towski et al. 1991), and the Nipissing high stand of+6 m at about 4.7 ka (Larsen 1985). This rise inlake level resulted in a prominent, planar, transgres­sive unconformity. Erosion accompanying thetransgression removed most evidence of shorelinefeatures below present lake level. Superimposed onthis overall rise in lake level, our data indicate thata second influx of water from Lake Agassiz tem­porarily raised lake levels at about 9.1 ka, and that

lake level may have fallen to, or slightly below, thelevel of the North Bay outlet at about 7 ka becauseof sharply drier climate (Forester et ai. this vol­ume). Sometime between 6 and 5 ka, the characterof the lake changed considerably, probably formostly climatic reasons, becoming highly undersat­urated with respect to calcium carbonate and return­ing primary control on lake level to the isostaticallyrising North Bay outlet.

The post-Nipissing changes in lake level havebeen relatively small (± -2 m around millennial av­erages) and have been brought about largely by cli­matic fluctuations superimposed on a slight fallingtrend caused by erosion of the Port Huron outlet(Larsen this volume). Time series of sedimentologi­cal and sediment-magnetic properties mirror what isknown of these lake-level changes and suggest cy­cles as short as 200 to 500 years.

This paper, focused on stratigraphy, time control,and lake levels, is complemented by the paper byForester et al. (this volume), which is concernedmainly with paleoclimate and paleolimnology. To­gether the two papers demonstrate the wide varietyof environmental reconstructions that can be madeon the basis of interdisciplinary study of the lake­sediment record of Lake Michigan.

ACKNOWLEDGMENTS

We thank the captain and crew of the R/V RogerR. Simons and of the R/V Laurentian for their en­thusiastic and capable help with the field program.K.E. Parolski was instrumental in collecting theseismic-reflection data. C.E. Franks performed allof the stable isotope analyses. A.R. Gagnon pre­pared and ran the AMS radiocarbon samples.Grain-size analyses were performed by A.M. Mof­fet in L.J. Poppe's sedimentology laboratory,USGS, Woods Hole. Helpful reviews of earlier ver­sions of the paper were provided by D.W. Folger,G.J. Larson, and R.N. Oldale.

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Submitted: 21 April 1993Accepted: 27 August 1993