Strand-plain evidence for late Holocene lake-level variations in Lake Michigan Todd A. Thompson* Indiana Geological Survey, Indiana University, 611 North Walnut Grove, Bloomington, Steve J. Baedke } Indiana 47405-1401 ABSTRACT INTRODUCTION that form at or near lake level; most are caused by fluctuations in lake level (Olson, 1958; Thompson Lake level is a primary control on shoreline Shorelines along Great Lakes coastlines are and Baedke, 1995). That is, beach ridges form in behavior in Lake Michigan. The historical products of the interaction among three factors: many parts of the Great Lakes basin after a fall in record from lake-level gauges is the most accu- wave climate, sediment supply, and lake level. Of lake level from a highstand. Facies and facies con- rate source of information on past lake levels, these three factors, lake level is the primary con- tacts within beach ridges preserve information on but the short duration of the record does not trol on shoreline behavior because of its rapid lake-level maxima, and their sequential develop- permit the recognition of long-term patterns of month to month and year to year variation and ment yields information on their relative age. Ac- lake-level change (longer than a decade or immediate influence on the shoreline system tual ages of the ridges can be estimated with ra- two). To extend the record of lake-level change, (Hands, 1983). Knowledge of lake-level varia- diocarbon dates from basal organic sediments of the internal architecture and timing of devel- tion in the Great Lakes is based on the historical wetlands that commonly occur in the swales be- opment of five strand plains of late Holocene record from lake-level gauges (cf. Bishop, 1990). tween ridges. Because the age (relative or ab- beach ridges along the Lake Michigan coast- The historical record of the lakes extends from solute) and lake-level elevation of formation can line were studied. Relative lake-level curves for A.D. 1860 to the present; the record of some of the be readily obtained from the study of beach each site were constructed by determining the lakes (e.g., Lake Michigan) can be semiquantita- ridges, strand plains of beach ridges may contain elevation of foreshore (swash zone) sediments tively extended to the early nineteenth century one of the best records of late Holocene lake level in the beach ridges and by dating basal wet- (Quinn and Sellinger, 1990). Larsen (1985), how- of all of the coastal geomorphic features pre- land sediments in the swales between ridges. ever, showed that the historical record may under- served around the Great Lakes. These curves detect long-term (30+ yr) lake- estimate lake-level extremes. Furthermore, the Strand plains of beach ridges are common fea- level variations and differential isostatic ad- historical record, 135 yr, may be too short to rec- tures along the margins of Lake Michigan. Most justments over the past 4700 yr at a greater ognize significant long-term patterns of lake- occur in the northern part of the lake, where nu- resolution than achieved by other studies. level behavior (DeCooke and Megerian, 1967; Liu, merous embayments into glacial deposits and The average timing of beach-ridge develop- 1970; Cohn and Robinson, 1975, 1976; Thomp- bedrock are present and where a supply of sand- ment for all sites is between 29 and 38 yr/ridge. son et al., 1991). An alternative data source, the sized sediment is plentiful. The largest strand This correspondence occurs in spite of the em- geologic record, is needed to extend the lake- plain of beach ridges, however, occurs at the bayments containing the strand plains being level record further in time and provide informa- southern tip of Lake Michigan along the entire different in size, orientation, hydrographic tion on long-term lake-level change. coast of Indiana and the southern coast of Illinois. regime, and available sediment type and cal- The onshore geologic record consists of coastal This strand plain contains more than 100 individ- iber. If not coincidental, all sites responded to a geomorphic features and sedimentary deposits ual beach ridges that record the position of the for- lake-level fluctuation of a little more than three that occur at the present lake margin or several mer shoreline of southern Lake Michigan and the decades in duration and a range of 0.5 to 0.6 m. kilometers inland. Such features and deposits in- elevation of the lake at the time of beach-ridge Most pronounced in the relative lake-level clude wave- and stream-cut terraces, delta plat- formation. Other areas of beach-ridge develop- curves is a fluctuation of 120–180 yr in dura- forms, valley and river-mouth fills, spits, dunes, ment occur along the eastern shore of the Door tion. This ≈150 yr variation is defined by and beach ridges. The elevation of these coastal Peninsula of Wisconsin, the southern shore of the groups of four to six ridges that show a rise and features and deposits may be used to estimate the Upper Peninsula of Michigan, and the northwest- fall in foreshore elevations of 0.5 to 1.5 m elevation of lake level; or, where the relationship ern coast of the Lower Peninsula of Michigan. For within the group. The 150 yr variation can be of these features and deposits to actual lake level example, southwest of Manistique, Michigan, correlated between sites in the Lake Michigan cannot be established with certainty, their stratig- more than 90 beach ridges arc between bedrock basin. The ≈30 and 150 yr fluctuations are su- raphy can be used to determine the behavior of and glacial headlands, capturing Indian Lake perimposed on a long-term loss of water to the lake level when the feature or deposit was devel- from the Lake Michigan basin. Smaller embay- Lake Michigan basin and differential rates of oping. The relative ages of the geomorphic fea- ments containing partial chronosequences of isostatic adjustment. tures and sedimentary deposits can be determined beach ridges occur throughout the area. by spatial and stratigraphic relationships, and es- We studied five strand plains along the margin timates of their absolute ages can be approxi- of the Lake Michigan (Fig. 1) to determine, at mated by radiometric dating (commonly 14C). each site, (1) the physical limits of lake-level vari- *E-mail: [email protected] Beach ridges are sandy depositional features ation during the late Holocene, (2) the timing of GSA Bulletin; June 1997; v. 109; no. 6; p. 666–682; 9 figures; 1 table. 666 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/6/666/3382701/i0016-7606-109-6-666.pdf by guest on 27 September 2021 LATE HOLOCENE LAKE LEVEL OF LAKE MICHIGAN high lake levels, and (3) long-term patterns of shoreline behavior and beach-ridge development in response to changes in lake level. The purpose of this paper is to describe the lake-level informa- tion obtained from the stratigraphy and timing of development of the beach ridges within the five strand plains. Data were obtained from a coring program aimed at determining the elevation of foreshore (swash zone) deposits within beach ridges, and the age of the beach ridges from radio- carbon dates on basal wetland sediments in the swales between ridges. Because we rely on radio- carbon dates from basal wetland deposits to deter- mine the age of the ridges, the absolute age control of the beach ridges is not ideal. However, we feel Figure 1. Map of Lake Michi- that the elevation data and our treatment of the age gan showing the five study areas data permits us to detect lake-level variations at a and other sites referenced in the time scale much shorter than those achieved by text. See Figures 3 and 5 for de- other studies of Great Lakes levels (e.g., Dalrym- tailed maps of the study areas. ple and Carey, 1990; Dott and Mickelson, 1995; Fraser et al., 1975, 1990; Flint et al., 1988; Larsen, 1985, 1994). This report provides information on long-term behavior of the shoreline at the five sites under natural conditions. Unfortunately, human modifications to the depositional systems at many of the sites during the nineteenth and twentieth centuries prohibit continuation of the unaltered coastal processes and patterns of sediment accu- mulation that we describe. METHODS Two types of data are needed to construct a lake-level curve from a strand plain of beach ridges. The first is the elevation of the lake when each ridge formed. One of the best methods of measuring lake level is based upon the elevation of foreshore (swash zone) sediments preserved in each ridge (Fig. 2). At the five study sites one or more transects were established perpendicular to the depositional strike of the beach ridges. Vibra- cores were collected on the lakeward margin of accessible ridges along the transects to recover foreshore sediments (Fig. 2). All vibracores were collected at the break in slope between the ridge and the wetland that occurs lakeward of it. At this position, the foreshore sediments are at their highest collectable elevation within each ridge (Thompson, 1992). The elevation of each core site was surveyed with a transit to a precision of 0.003 m and tied into the International Great Lakes Datum (IGLD) of 1955 using benchmarks (3–4.5 m) and actual lake level. The cores (256 total) were returned to the laboratory to be split, described, (1–2.5 m) photographed, and sampled. Latex peels were made of each core to preserve the core and to en- hance the ability to see stratification. Grain-size Figure 2.