Nearshore Sediments of the Illinois Shore of Lake Michigan

J. Great Lakes Res., December 1976. Internat. Assoc. Great Lakes Res., 2(2) :283-293.

NEARSHORE SEDIMENTS OF THE ILLINOIS SHORE OF LAKE MICHIGANl J.B. Graf [niversity of Illinois at Champaign-Urbana~ Urbana~ Illinois

Department of

Geology~

Accepted 23 August 1976

Abstract. Sediment sampling and a bathymetric survey of the nearshore zone of Lake Michigan between Waukegan and Zion, Illinois, provided a detailed description of the area and a basis for comparison to models of textural variation in nearshore zones. The survey, made 15-30 June 1973, yielded evidence that the breaker zone along this shorel ine extends to depths of about 6 m (20 ft), that waves significantly affect the bottom to depths of 6 to 8 m (20 to 25 ft), and that some agent, probably wave action,is able to move silt and clay-sized sediment at the greatest depths of this study (15 m or 50 ft). Although the field area is composed dominantly of fine, well-sorted sands, an area of bedforms composed of coarser, more poorly-sorted sediments was found within the area. Sediments on the crests of these bedforms, now at depths of 12 to 15 m (40 to 50 ft), have characteristics in common with sediments forming the crests of longshore bars, and may represent deposits of a lower-than present lake level.

INTRODUCTION

Thomas et al. 1973). An extremely variable, patchy distribution of these faci es is revealed by deta iled studies of the surficial sediments of small areas (Seibel et al. 1974). \lise use of the shoreline and nearshore areas, \tJhether for recreation, for sand and gravel resources, or for industry should therefore be based upon detailed knowledoe of the character of local sediment distribution and the factors

Previous studies of Great Lakes sediments have shown that the bottom within a few miles of the shoreline is in many cases formed by bedrock, glacial till, and sand and f1ravel ~Jhic;) may ei ther be in equ; 1i bri um \lJi th present-day coastal rrocesses or relict deposits (Coakley 1972; lIough 1935:, Sutton et al. 1974;

lField work carried out in 1971 and supported by the Illinois State Geological Survey provided background information for this study. However, the views expressed here are those of the author and do not necessarily represent those of the Geological Survey. Subsequent fielJ work was supported by a grant from the Penrose Fund of the Geological Society of America, Inc., and funds from Exxon U.S.A. Foundation, the Shell Companies Foundation, and Texaco, Inc., through the Department of Geology, Universityof I II inois.

283

284

J.B. GRAF

controlling that distribution. The area of the present study, which stretches along the western shore of Lake Michigan from 17th Street, Zion, Illinois, to apoint just north of Waukegan, Illinois (Fig. 1), includes both the longest stretch of undeveloped Lake Michigan shoreline in Illinois, Illinois Beach State Park, and the site of the Commonwealth Edison Nuclear Power Plant at Zion. The plant began operation at partial capacity in June 1973, and this study, carried out 15-30 June 1973, is in part a post-construction, pre-operational description of surrounding surficial sediments and bottom topography. The area is one of the few along the western shore where sands of glacia11akep1ains (Willman and Frye 1970) extend continuously into the offshore zone. Till bluffs lie about 1.6 km (1 mi) inland within the study area and outcrop at tile shoreline at Racine, Wisconsin and south of Waukegan. The sand is thickest at the shoreline, where 9 to 11 m (30 to 35 ft) lie over till (Fraser and Hester 1974). The sand body thins to 1.5 m (5 ft) near the outer edge of the study area except in the area of the Zion power plant site, where it remains at least 3 m (10 ft) th-ick to a distance of 2.5 km (1.5 mi) from shore (Fraser and Hester 197L~) . Previous study of the shoreline within the study area (Illinois Division of Waterways 1952, 1958; Hester and Fraser 1973) resulted in the characterization of the northern third as erosional, the central third as relatively stable, and the southern third as accretionary. The Illinois Division of Waterways study (1952, 1958) was conducted from 1946 to 1950 and suggested that little change in bathymetry or sediments takes place in this area at depths

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greater than 6 m (20 ft). Waves in the study area tend to be small (greater than 1.7 m or 5.5 ft only 1% of the time) and of very short period (3 to 5 sec). Tidal range is 4.3 cm (.14 ft) or less. Records of lake levels kept by the U.S. Lake Survey Center show that small variations (.5 m or 1 to 2 ft) in the level of Lake Michigan occur seasonally and larger variations (maximum about 2 m or 6.6 ft) occur over periods of from 5 to 20 years. The present study was carried out during a period of relatively high lake level. At the end of June 1973 the level was 579.2 ft (176.7 m) or about 2.4 ft (.7 m) above low water datum. Large scale investigations of sediments of southwestern Lake Michigan (Hough 1935; Somers and Josephson 1968) provide a regional framework for the present study. At Kenosha, Wisconsin, about 14.5 km (9 mi) north of the area, Hough (1935) found a shelf extending 6.8 km (4.2 mi) offshore and averaging 15 m (50 ft) deep. The inner portion (1.3-4 km or .8-2.5 mi from shore) was composed of gravel and

LAKE MICHIGAN NEARSHORE SEDIMENTS

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ti 11, whereas the outer porti on was covered with sand. The sands were found to fine lakeward away from the till, then to coarsen toward a maxi mum medi an di ameter of .38 mm (1.4 phi) at the shelf edge. No shelf existed at Waukegan, 2.4 km (1 .5 mi) south of the present study area (Hough 1935). There, sands became finer away from shore, and the bottom sloped evenly away from shore. Somers and Josephson (196B) did find an increase in median di·· ameter at distances of between 6 and 16 km (4 and 10 mi) off Waukegan. Median diameters in that zone were in the range .18 to .14 mm (2.5 to 2.8 phi). METHODS A Raytheon model DE-719 recording fathometer operated from a 4-m (13-ft) Boston Whaler provided continuous dept~ data along the lines shown on the bathymetric: map (Fig. 2). Sediment samples

were taken at points along those survey lines and the locations of the samples also serve as control for the bathymetric survey. For each sample, the boat was anchored in position and the location given by intersecting sightings from two shore-based transits located about 1.6 km (1 mi) apart. These locations are shown by small dots on eachsediment map (Fig. 3, 4, 5, and 6). Sediment samples were taken with Petite Ponar and Ekman grab samplers. A visual-accumulation tube, described in detail in Interagency Committee on Water Resources (1957), was used for size ~nalysis of the sand-sized portion of the samples. The sizes corresponding to 5, 16, 25, 50, 75, 84, and 95 percent of total accumulation were read from the resulting curve and used to calculate mean grain size and sorting values according to the formulae of Folk and Ward (1957). Samples with sediment too fine or too coarse for the settling tube were dry-sieved to remove particles

286

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f e an gra d ·1ment onsamples SIZ by of sma se Distributl . 'Ins are th i s an d .In mm, 11 dots . e s, hown 1973. 'on Samp0 Ie mlocation taken fo 110wl~une, ng se diment maps. FIG. 3.

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FIG. 5. Distribution of material coarser than 1.19 mm (-.25 phi) in weight percent.

JUNE, 1973

FIG. 6. Distribution of sediment finer than .063 mm (4.0 phi), in we i ght percent.

288

,1. B. GRAF

of 1.19 mm (-.25 phi) diameter or greater, and wet-sieved to remove particles finer than .063 mm (4.0 phi). The percents by weight of coarse sediment (Fig. 5) and of fine sediment (Fig. 6) were calculated and treated as separate parameters. RESULTS The present study provides a good description of the bathymetry (Fig. 2) and sediment chal"acteristics (Fig. 3, 4, 5, 6) of the area. Within about 1 km (.6 mil of the shoreline (depths less than about 8 m or 25 ft) contours of both sediment si ze and depth arE~ nearly parallel to shore. The contour of 2% fine sediment (Fig. 5) roughly coincides with the 20-ft (6-m) contour and is parallel to shore. Offshore of that zone in the northern third of the area, the depth (Fig. 2), mean grain size (Fig. 3), and sorting (Fig. 4) contours become more complex. An area covered with bedforms begins at depths of about 11 m (36 ft) in the northeast corner of the area. Fathometer records of that area show these features to be smooth-surfaced, sand wave-l i ke bedforms varyi ngi n height from .3 to 3 m (1 to 10 ft) and in spacing from 180 to 450 m (600 to 1500ft). The larger forms, which can be seen on the map, tend to be symmetrical and are aligned subparallel to shore. Smaller, forms, on the order of .5 m (1-2 ft) in height, are also found on the fathometer traces. These are most often asymmetrical, with stE~ep faces toward shore, and may occur in groups of two or three. Anomalously high percentages of fine sediment were found in the troughs of some of these bedforms, and the crests have little fine sediment

nor sediment larger than 1.19 mm (-.25 phi). Sediments of the entire bedform area are coarser than the surrounding sediments. In the south east corner of the study area, the bathymetric survey revealed that at a depth of about 12 m (40 ft) the smooth, evenly sloping bottom increased sharply in slope and gave lakeward onto a plain which had a distinctive fathometer record. The records of that area show a very irregular surface with sharp, abrupt peaks. Samples of the plain were difficult to obtain, but the two that were retrieved contained pebble and cobble gravel, some sand, and chunks of stiff grey clay believed to be till. The plain is interpreted as a till surface with a sparse and irregular cover of gravel and sand. Mean size of sands decreases toward the plain, and sorting improves (Fig. 3, 4). The percent of fine sediment (Fig. 6) increases abruptly toward the plain in the vicinity of the slope increase, and contours of that parameter remain parallel to the depth contours to a depth of about 6 m (20 ft). The correspondence of the amount of fine sediment to depth rather than to di stance from shore suggests that the plain is the source of that material. A single longshore bar was found to be continuous along most of the shoreline. In the northern area the bar is small, about .5 m (1-2 ft) in height, with about 2.5 m (8-9 ft) of water over the crest. Just south of the breakwater at the power plant (Fig. 1), no bar was found. The bar increased in height from the point where it reappeared about .4 km (.25 mi) south of the breakwater, to the south end of the field area. The depth of water over the crest and the di stance from the shore also increased to the south. Inshore of the bar, sediments are coarser and more poorly sorted than sediments of the bar crest

LAKE MICHIGAN NEARSHORE SEDIMENTS

and offshore of the crest. Most samples rich in sediment coarser than 1.19 mm (-.25 phi) were collected close to shore, in the bar trough and inshore from it (Fig. 5). An increase in the amount of coarse sediment to the south in that ZOnE! can be seen (Fig. 5). The dominant longshore drift in the area is to the south, and the increase in bar dimensions and the coarsening of sedi ment may be due in part to thE! increase in longshore current velocity to the south from the breakwater at the Zion power plant, and in part to the increase in wave energy reaching the shoreline because of steepening offshore slopes to the south.

289

consisting of particles coarser than about .25 mm (2.0 phi). Some probable suspension load has been removed from samples of this study and treated separately (Fig. 6). The traction load has been truncated at 1.19 mm (-.25 phi) and that fraction also treated separately (Fig. 5). The range of probabi 1i ty curves found in this study is shown in Fig. 7. Types 2, 3 and 4 are similar to Vi sher s (1969) shoa1i ng zone curves with large, well-sorted saltation loads and more poorly sorted traction loads. Type 2 has no traction load, and the percent of sediment in that fraction increases from Type 3 to Type 4. The average amount of fine sediment increases from 3% for Type 4, to 6% for Type 3, to 14% for Type 2. Visher (1969) found DISCUSSION that the amount of the suspended load varied with proximity to a source of Size distributions of sands that material, and examination of the areal distribution of types withfrom certain depositional environments seem to have characteristic in the study area (Fig. 8) reveals patterns when plotted as cumulative that Type 2 is found along the shorepercent versus grain size (in log ward side of the till surface beunits) on arithmetic-probability lieved to be contributing fines to paper (Visher 1969). Visher found the area. a pattern characteristic of the Types 5 and 6 of this study II wave li or shoaling zone (Fig. 9 match examples of breaker zone in Visher 1969) and another charac- curves given by Visher (1969). teristic of the II surf li or breaker Type 5 contai ns almost enti rely sand zone (Fi g. 10 in Vi sher 1969). sized sediment, is uniformly well From his diagrams, one can also sorted, and compares well with a distinguish between the patterns plot of Visher's from the crest of produced by sands from the plunge a breaker-zone bar. Type 6 is zone and those from the crest of characteristic of samples from the a longshore bar, both within the plunge zone, with a large amount of breaker zone. coarse sediment with poorer sorting Probability plots may be than Type 5. Although not'always divided into straight-line segments, the case, bar crest samples are Type 5, and trough samples Type 6. which Visher (1969) identified with the IIsuspensionli population, Type 5 also includes samples from inshore of the bar trough, and from made up of sediment smaller than .06 to .09 mm (4.0 to 3.5 phi), a offshore of the bar crest to depths II sa ltation li population, or load, of about 3 m (10 ft). Types 1 and 7 can be interpreted made up of ~articles between about as mixtures of other types. Type 7 .25 mm (2.0 phi) and .06-.09 mm (4.0 to 3.5 phi), and a II s1iding (Fig. 7) has a coarse fraction like that of Type 5, a fine fraction like or rolling ll or traction load, I

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LAKE MICHIGAN NEARSHORE SEDIMENTS

that of Type 4, and a short interval of overlap. The fine fraction of Type 1 coincides with the Type 3 curve, whereas the coarsest part of the distribution approaches Type 5. The overlap interval is much wider for this type than for Type 7. All but one of the Type? samples were taken from the crests of bedforms found in the offshore. Other crests in that area have Type 5 distributions, whereas troughs show a variety of types. Type 1 samples were found over a 1arge area at depths at whi ch TYP,e 3 samples occurred in the central and southern parts of the field area. If Type 5 curves are truly characteristic of surf zone processes as Visher (1969) suggested, then sediments of the northeast corner of this study area must have been deposited when lake level was 11 to 12 m (35 to 40 ft) lower than the present level, and left stranded as the level rose. Type 7 distributions could be caused by later modification of those breaker zone sediments by onshore movement of the finer fraction of Type 4 sediments. The later action must have been strong enough to modify crest distributions and to prevent deposition of fine sediments on the crests, but not strong enough to prevent deposition of fines in the troughs or to level the bedforms. Continuing with this hypothesis, Type 1 samples may represent a greater modification of Type 5 distributions caused by increasing ability of waves to transport sediments as the water shallows. TYP,e 4 and 5 sediments either were not available offshore to the south, or their existence has been masked by shoreward movement of fine sediment from the offshore till plain. Whatever the cause, the sequence of curve types from the shoreline to the offshore edge of the map in

291

the southern part of the study area is the expected one for a zone of shoaling waves. Bottom features similar to the offshore bedforms described from the northern part of the present study area have been found by Hough (1935) near Chicago and by Berkson et al. (1975) over much of the southern lake basin, in depths of 8 to 30 m (26 to 98 ft). The bedforms found in this study are of the same size as those described by Berkson et al., but their spacing is somewhat less regular. Hough (1935) found gravel lag deposits or till in the low areas between sandy ridges and took that as evidence that the features were being worked upon by present-day currents or waves. He suggested, however, that the ridges were formed at or near the shoreline at a lower lake level. Berkson et al. (1975) determined from side-scan sonar evidence that the parallel linear features they found had many characteristics of sand waves and glacial fluting as well as of longshore bars. Linear parallel features were found by them where sand or gravel cover over till was thin or absent. Evidence from the present study suggests that there may, in fact, be two different sorts of parallel linear features on the lake bottom. One of these is associated with till surfaces and thin gravel or sand lag deposits and may well be glacial fluting, and the other is found in areas of relatively thick but anomalously coarse sand which has the characteristics of longshore bars. Maximum depths for the present day longshore bars in Lake Michigan are about 6 m (20 ft). Therefore these anomalous areas are now at depths about 9 to 15 m (30 to 50 ft) greater than those at which longshore bars are forming today. The absence of fine sediment in the crests and the presence of large amounts in the troughs of bedfarms found in the Zion area suggest that they are relatively

292

.J.B. GRAF

inactive features and that the erosive agents there are not as strong as those in the vicinity of Chicago where Hough (1935) found bare till or gravel lag deposits between these features. CONCLUSIONS Surf or breaker zone processes within the study area appear to be restricted to depths less than 6 m (20 ft). Little sediment finer than .063 mm (4.0 phi) was found in that zone, i nd'jeating that high energy processes control the sediment distribution. Maximum depth of longshore bars, known to be related to breaking waves, is 6 m (20 ft) within the area but bars more commonly OCCUI" at depths of 3 to 4.5 m (10 to 15 ft). Grain-size distributions of sediments from depths less than 6 m (20 ft) compare well with distributions from known surf zones. Also, evidence from a previous study (Illinois Division of Water"ways 1952, 1958) showed that bathymetric changes at depths greater than 6 m (20 ft) were not significant over a four-year period. Shoaling waves appear to influence the sediment distribution over much of the remainder of the stU(~ area. Evi dence for that i nf1 uenc:e includes the fact that sands coarsen toward shore as predictd for sediment distributed by shoaling waves, that fine sediment appears to be moving shoreward from the offshore till surface, that crests of offshore bedforms have been

winnowed of fine sediment, and that grain-size distributions of sands match those from known shoaling zones. Two kinds of parallel linear bottom features were found in the area, one on a till surface and the other formed in an area of sand. The latter features have characteristics of present-day longshore bars and may have been formed when lake level was at least 9 m (30 ft) lower than present. The area of anomalously coarse sediments in which those bedforms were found dies out to the south within the field area and extends an unknown distance to the north and to the east. It is possible that it represents the southern end of the shelf found by Hough (1935) off Kenosha, Wisconsin. ACKNOWLEDGEMENTS A number of people gave much of their time and energy to help with the field work involved in this study: Susan B. Buckley, Roscoe Jackson, David Macke, Rodney D. Norby, Margaret Leinen Prince, Linda Provo, James Rodgers, Moon Vanko and John Wert. The Illinois Department of Conservation, particularly John Camerio, Robert. Needham and Helen Lyon eased the work by giving access to park property. I am particularly grateful to John B. Southard and Dennis S. Wood for their advice and encouragement. Figures 2 and 3 in this manuscript are reproduced by permission of the Elsevier Scientific Publishing Company.

REFERENCES Berkson, J.M., Lineback, J.A., and Gross, D.L. 1975. A side-scan sonar investigation of small-scale features on the floor of southern Lake Michigan. Illinois State Geoz. Survey~ Environm. Geol. Note, 74, 18 p. Coakley, J.P. 1972. Nearshore sediment studies in western Lake Erie. In Proc. Z5th Conf. Great Lakes Res. ~ I nternat. Assoc. Great Lakes Res., PP. 330-43.

LAKE M1CHIGAN NEARSHORE SEDIMENTS

293

Folk, R.L. and Ward, W.C. 1957. Brazos River bar: a study on the sugnificance of grain size parameters. J. Sed. Petrol., 27:3-26. Fraser, G.S. and Hester, N.C. 1974. Sediment distribution in a beach ridge complex and its application to artificial beach replenishment. Illinois State Geol. Survey, Environm. Geol. Note, 67, 26 p. Hester, N.C. and Fraser, G.S. 1973. Sedimentology of a beach ridge complex and its significance in land-use planning. Illinois State Geol. Survey, Environm. Note, 63, 24 p. Hough, J.L. 1935. The bottom deposits of southern Lake Michigan. J.

Sed. Petrol., 5:57-80. Illinois Division of Waterways. 1952. Interim report for erosion control, Illinois shore of Lake Michigan. Div. Waterways Dept. Pub. Works and Bldgs.• , State of 111 inois, 34 p. 1958. Interim report for erosion control, Illinois shore of Lake Michigan. Div. Waterways Dept. Pub. Works and Bldgs., Stateof Illinois, 108 p. Inter-agency Committee on Water Resources. 1957. Measurement and analysis of sediment loads in streams: the development and calibration of the visual-accumulation tube. Suboomm. Sediment., Inter-agenoy Comm. Water Res., St. Anthony Falls Hydraul.Lab., Minneapolis, Minn., Rept. 11:18-25. Seibel, E., Jensen, R.E., and Carlson, C.T. 1974. Surficial sediment distribution of the nearshore waters in southeastern Lake Michigan. In

The biological, chemical and physical character of Lake Michigan in the vicinity of the Donald C. Cook Nuclear Plant, eds. E. Seibel and J.C. Ayers, pp. 369-432. Univ. Michigan, Great Lakes Res. Div. Spec. Rept. 55. Somers, L.H. and Josephson, P.O. 1968. Bottom sediments of southwestern Lake Michigan. In Proc. llth Conf. Great Lakes Res., Internat. Assoc. Great Lakes Res., pp. 245-52. Sutton, R.G., Lewis, T.L., and Woodrow, D.L. 1974. Sand dispersal in southern Lake Ontario. J. Sed. Petrol., 44:705-15. Thomas, R.L., Kemp, A.L., and Lewis, C.F.M. 1973. Surficial sediments of Lake Huron. Canadian J. Earth Sci., 10:226-75. Visher, G.S. 1969. Grain size distribution and depositional processes. J. Sed. Petrol., 39:1074-106. Willman, H.B. and Frye, J.C. 1970. Pleistocene stratigraphy of Illinois. Illinois State Geol. Survey, Bull. 94, 204 p.