THE GEOLOGIC FRAMEWORK OF SOUTHERN LAKE MICHIGAN Foster, D. S. and Folger. D. W. U. S. Geological Survey Woods Hole, MA 02543 ABSTRACT This paper is based on geologic and geophysical data collected from 1988 to 1990. Most of the Illinois-Indiana lake bottom surveyed, to about 18.5 km from shore, slopes gently (1:770) lakeward. Bathymetry is controlled by the underlying bedrock that dips northeast toward the center of the Michigan Basin and comprises Silurian dolomite or Devonian limestone overlain in the eastern part of the area by Devonian-Mississippian shale. Quaternary sediment, overlying bedrock, thickens from 10 m off Waukegan, Illinois to more than 40 m off Michigan City, Indiana. As much as 4 m of postglacial lacustrine sediment overlies about 10 m of till off Waukegan and 6-10 m of postglacial lacustrine sediment overlies 3- 30 m of till off Michigan City. From Waukegan south to Indiana Harbor, the bottom is floored by till, sand, gravel, and cobbles. Sand, more common within 1-2 km of shore, thins lakeward to a patchy veneer. The lake floor is erosional or nondepositional where till or gravel-cobble pavement is exposed. In contrast, north of Waukegan and east of Indiana Harbor, fine sand covers much of the bottom and grades offshore to muddy sand, part of the modern, lacustrine, Lake Michigan Formation. The complex surficial bottom sediment distribution between Waukegan and Michigan City could be mapped in detail only where we have sidescan sonar mosaics. In those areas, the till, or coarse lag sand-gravel surface, is covered intermittently with a layer of fine sand most often about 0.5-1.0 m thick. The sand appears to be mobile, covering and uncovering the substrate in response to storm-driven waves and currents. INTRODUCTION Purpose This phase of the southern Lake Michigan Coastal Erosion Study was conducted to establish the framework on which to base subsequent studies concerned with lake level and sediment processes, and to provide data for input to sediment budget estimates. The distribution of bottom sediment, and the thickness and physical properties of the strata relate to and provide information for the determination of the sedimentary environment. By studying the geologic framework, we can broadly determine where erosion, non- deposition, and deposition are taking place; then, within this context, detailed studies of the processes that are affecting the most change in the lake floor and shoreline can be carried out. In addition, the background geologic information provides the perspective within which the follow-on studies can be best interpreted. Geologic Setting and Previous Work Southern Lake Michigan lies wholly within the western side of the Michigan Basin. In northern Illinois and Indiana, bedrock formations dip northeast towards the center of the Basin; progressively younger beds are exposed at the bedrock surface to the northeast. The Silurian Niagaran dolomite forms a cuesta around the northern and western boundary of the lake. In northeastern Illinois, Niagaran dolomite dips beneath Lake Michigan (Horberg 1950; Suter et al. 1959; Buschbach and Heim 1972) toward the lower peninsula of Michigan and forms the eastward sloping lake bottom (Hough 1958). In northwestern Indiana, Devonian limestone and Devonian and Mississippian shale overlie the dolomite (Hough 1958; Schnieder and Keller 1970; Gray 1982; Shedlock et al. in press). Contacts between dolomite, limestone and shale on land have been projected beneath Lake Michigan by Hough (1958), Wickham et al. (1978) and Wold et al. (1979). Hough (1935) indicated that bedrock is exposed on the lake bottom close to shore near Chicago. Pleistocene glaciers scoured the eastward dipping Paleozoic bedrock, particularly the comparatively soft Devonian shale, forming the southern basin of Lake Michigan (Twaites 1949; Emery 1951; Hough 1958; and Wickham et al. 1978). At least two till units, the Wadsworth and Shorewood Members (Lineback et al. 1974), were deposited by Late Wisconsin glacial readvances during the overall retreat of the Lake Michigan ice lobe through the southern basin. Wadsworth Till, and possibly older tills overlie bedrock beneath the Illinois and Indiana nearshore, because the younger ice readvance that deposited the Shorewood till terminated to the north in the southern basin (see Colman et al. this volume). The Wadsworth Till, comprising mainly silty clay and lesser sand and gravel, crops out at the lake floor over much of the Illinois and Indiana nearshore or is mantled by glacial outwash sand and gravel (Wilman 1971, Lineback et al. 1974, Wickham et al. 1978). Deep water lacustrine mud accumulating offshore encroaches over the till and feathers out or grades into patchy sandy silt and silty sand, and finally to modern beach sands nearshore (Wickham et al. 1978, Foster and Colman 1992). "The modern investigation of the sediments of Lake Michigan Basin, from the geologist's point of view, was initiated by Pettitjohn [1931] in his study of the beach sands of Southern Lake Michigan" (Hough 1935). Hough (1935) constructed the first map of bottom sediment texture south of a line connecting Racine, Wisconsin and Holland, Michigan. He suggested that the western side of the southern lake, particularly offshore from the Chicago area, is complex because the bottom is characterized by highly variable textures where gravel or gravel-veneered till bottom is widespread and covered with patches of sand. The lag sand and gravel that cover the till were derived from till (Hough 1935), or from glacial outwash (Lineback et al. 1974). In contrast to the erosional or nondepositional character of the nearshore area off Chicago, Hough (1935) pointed out that deposition is taking place in the nearshore area adjacent to Waukegan, Ill., where neither till nor gravel is exposed on the lake floor. Cahill (1981) reported the results of the first systematic bottom sampling that covered all of Lake Michigan, but only 9 samples fall within this study area. Norby (1981) collected 45 cores near the harbors of the Great Lakes Naval Training Station, Waukegan, and Winnetka. Coring and jetting were carried out by Norby and Collinson (1977), and most recently by Shabica and Pranschke (this volume). Several detailed studies of bottom sediment were conducted off Illinois Beach State Park (Graf 1976; Welkie and Meyer 1982). Berkson et al. (1975) partially mapped small scale bottom features beneath southern Lake Michigan with a 48 kHz sidescan sonar system. They distinguished areas of low backscatter as lacustrine clays, areas of medium even backscatter as sheets, patches and bars of sand, and areas of high backscatter as sand ripples, gravel, or till. The existing information broadly defines the pattern of bottom sediment distribution in the southern lake. By using new technology such as the digital sidescan sonar, and by expanding the data coverage we have been able to document the complexity of the boundaries between sediment types, but perhaps more important, we can demonstrate that in much of the area, particularly off Chicago, sediment is highly mobile and hence the observed variabilty over short distances probably changes from season to season and even from storm to storm. METHODS Data were collected in Lake Michigan from a variety of platforms. These included the USGS 12-m Research Vessel (R/V Neecho), Hydrographic Surveys, Inc. 20-m R/V Neptune, Great Lakes Dredge and Dock Company's 14-m R/V Miami River, and several small boats such as Boston Whalers. Data were collected over a period of 3 years from 1988-1990. Vessels were positioned with shore-based systems, Miniranger Falcon IV, and Del Norte, backed up with Loran C. Acoustic surveys were conducted along 24 tracklines that lie approximately perpendicular to the shoreline and extend 18.5 km offshore (fig. 1). A 148-km-long tieline, run roughly parallel to shore, extends from the Wisconsin-Illinois border to the Indiana- Michigan border about 5-10 km offshore. Another tieline extends from Chicago to Waukegan, Ill. within 3 km of shore. Closely-spaced (50-125m) lines were run with sidescan sonar in six areas to mosaic acoustic images of the bottom (fig. 1). Water depths, measured with a 200 kHz Odem echosounder, were recorded digitally at 10-s intervals and on analog paper. A Klein sidescan sonar system (100-500 kHz) was used to image the bottom across a 200 m swath. Surficial sediment thickness and character were assessed with an ORE narrow-band transducer with an output of 10 kW at a frequency of 3.5 kHz. These data were recorded on analog paper at a 0.125-s sweep rate. For deeper strata and bedrock, the profiling system consisted of a Huntec sled-mounted, surface- towed, broad-band boomer source with an output of 135-500 joules. Seismic signals were filtered between 200-5000 Hz and recorded at 0.25-s sweep rate. The interpreted reflections from the analog profiles were digitized and converted to depth assuming sound velocities of 1463 m/s for the water column and 1500 m/s for unconsolidated sediment and shale. Bottom sediments were sampled with a Van Veen grab sampler or described from scuba observations. Sample locations were selected from specific targets observed on the sidescan sonar data. Grain- size analyses were carried out to classify bottom sediment type using Folk's (1974) classification, to provide verification of the sidescan sonar interpretation. Topography from 1:250,000 scale USGS Digital Elevation Model (DEM) data (Elassal and Caruso 1983) and bathymetry (National Geophysical Data Center 1987) were combined to produce a base map for the area. The base map was contoured using Interactive Surface Modeling (ISM) software (Dynamic Graphics Incorporated). Bedrock elevation and sediment thickness maps were gridded and contoured using ISM. The contour maps generated with ISM required editing and smoothing by hand. This was done by exporting contours from ISM to a Geographic Information System (ARC/INFO) where graphic edits were carried out. The methods of computer contouring used are described by Foster and others (in press). RESULTS AND DISCUSSION Bathymetry Isobaths (fig. 1) and bathymetric profiles (fig. 2a-h) show that most of the lake floor in the southwest corner of the southern basin is a broad, relatively flat shelf. South of Waukegan as far as the Indiana border, the lake floor of nearshore southern Lake Michigan forms a gently dipping (a minimum of1:770) slope. A minimum depth of about 24 m at the lakeward end of our profiles, 18.5 km offshore, occurs on a profile off Chicago (fig. 2d). In contrast to the gently dipping shelf, north of Waukegan an abrupt break in slope occurs about 4.5 km offshore, increasing the gradient to about 1:240. Water is as much as 77 m deep 18.5 km offshore (fig. 2a). East of Gary, Indiana, a steep ramp has developed nearshore; water increases in depth from 0 to 15 m within a distance of 1 km from shore producing a slope of 1:67 (fig. 2f,g). The slope is more gentle offshore and water is only 25-27 m deep 18.5 km offshore (slope 1:740); however, to the east, off Michigan City, the water is 47 m deep 18.5 km offshore (slope 1:400) (fig. 2g). Local variation from the smooth, gently dipping profiles is particularly evident in the Indiana Shoals area where bathymetric ridges are as much as 3 m high, spaced 100-300 m apart, and trend north-northeast. The ridges are common between Indiana Shoals out to as far as 15 km east of Chicago (fig. 2d,e). Bedrock Surface We mapped two continuous seismic reflections beneath the Illinois and Indiana nearshore area (fig. 4). The deeper reflection of the two is strong and continuous throughout most of the region. The shallower reflection is not as strong; it is continuous off Indiana but discontinuous off Illinois. We interpret the deeper reflection to be the surface of Silurian dolomite or Devonian limestone (fig. 3). The upper reflection may be: (1) the boundary between two Pleistocene glacial till units; or (2) the contact between Devonian and Mississippian shale and overlying glacial till. We favor the second interpretation. Although no offshore well data are available in this area, a bedrock shoal off Lakeside, Michigan has been sampled and identified as Ellsworth Shale (Devonian- Mississippian)(Meisburger and others, 1979). Seismic-reflection profiles (Foster and Colman 1992) that cross the shoal and extend to the Indiana nearshore show that the upper surface of the shale correlates with the upper reflector on our nearshore seismic profiles off Indiana. The reflector probably is the surface of the Antrim Shale (Devonian) beneath the Indiana nearshore; no seismic reflection marks the contact between the Antrim and Ellsworth Shales. Antrim Shale overlies the carbonate surface beneath most of the Indiana nearshore, except for a few small windows where it is absent (fig. 4). The shale is fairly continuous westward to Indiana Shoals and apparently extends onshore (fig. 4). The bedrock surface beneath the Illinois nearshore is carbonate except where pockets of the shale unit occur particularly within valleys that cut into the carbonate surface (figs. 2h, 4). Foster et al. (in press) have mapped the elevation of the bedrock surface in northeastern Illinois and northwestern Indiana, including the area of Lake Michigan covered by our profiles. On land, bedrock is incised with broad valleys and many closed highs and lows (fig. 4), apparently the remnant of a former drainage pattern that has been modified by glacial erosion. We were able to project these valleys offshore on the seismic profiles (figs. 2h, 4). The bedrock surface beneath the lake broadly controls the Illinois nearshore bathymetry, Except where bedrock valleys occur, in the easternmost region of the Indiana nearshore isobaths parallel the coast and are perpendicular to bedrock surface contours (fig. 4). Quaternary Sediment Quaternary sediment overlying bedrock thickens irregularly from less than 10 m off Waukegan, IL to more than 40 m off Michigan City, Indiana (fig. 2h,5). Till fills bedrock valleys and covers bedrock highs resulting in a smooth lake floor. Only in a few areas between Waukegan, IL, and Chicago does bedrock crop out at the lake floor (fig. 2b). Most of the offshore sediment overlying bedrock is probably Wadsworth Till (Lineback et al. 1974). However, continuous reflectors locally occur within the till unit, suggesting that stratified layers occur within the till, or some till older than Wadsworth may be present. As much as 4 m of postglacial lacustrine sediment overlies about 10 m of till off Waukegan, IL, and 6-10 m of postglacial lacustrine sediment, the upper part of the Lake Michigan Formation, lies above 30 m of till off Michigan City, IN (Foster and Colman 1992). The southern feather-edge of the Lake Michigan Formation can be traced across the survey area; post-glacial sediment thickens to a maximum of 16 m about 40 km northeast of Michigan City (Foster and Colman 1992, Foster and Colman this voume). In contrast, to the west, as far as Indiana Shoals, postglacial deposits are thin and patchy, exposing pebbly, silty clays of Wadsworth till and coarser lag deposits. Nearshore, the sand wedge is usually less than 1 m thick and local exposures of till form a surface of complex ridges and pinnacles. In some areas, sediment thickness is similar onshore and offshore; in others, sediment thickens rapidly onshore (fig. 5). North of Waukegan, IL, for example, sediment thickens only gradually from the offshore to the onshore beach ridge complex. Between Winnetka, IL and Gary, IN, sediment thickness is uniform (10-20 m) from the offshore to the adjacent Chicago Lake Plain. However, along the coastal bluffs from Waukegan, IL to Winnetka, IL, sediment thickens abruptly from about 20 m offshore to about 40 m onshore and in the Indiana Dunes region, sediment thickens from 30-40 m offshore to 50-70 m onshore in the Holocene dunes. Bottom Sediment Texture and Distribution By mapping the sidescan sonar imagery, with supplementary bathymetric, high-resolution seismic, and sample data, we have created a generalized map of bottom sediment texture (fig. 6) from which some observations concerning the distribution of lake floor sediment can be made. Analyses of sidescan sonar data and sediment samples reveal some relationships of backscatter strength to sediment texture. Bottom characterized by strong acoustical returns (dark) is most often covered by boulders, cobbles, gravel, coarse sand or silty clay till. Most commonly the till is patchy and crops out in scarps or ridges through a mantle of sand and gravel (fig. 7a). Areas of cobble pavement over till have a distinct rough, dark, acoustic character (fig. b,c). Large targets with strong reflections and acoustic shadows characterize boulders (fig. 7d). Bottom with little acoustical return most often is covered by finer-grained material such as sand, silt, and/or clay (fig. 7d). Smooth, fine sand is difficult to differentiate from silt or clay without sample data . Megaripples, with wavelengths of about 1 meter, often are composed of medium to coarse gravelly sand and sandy gravel (Schlee et al. 1992) (fig. 7d). The complexity and variability of the surficial bottom sediment distribution can be mapped in detail only in areas where we have overlapping coverage of sidescan data, many bottom samples, direct observations of the bottom, or in a few areas nearshore, more data from a variety of sources. -North of Waukegan North of Waukegan fine sand covers most of the bottom and grades offshore into muddy sand; gravel and cobbles are sparse (fig. 6). The presence of more silt and clay offshore appears to represent the encroachment of modern deep water lacustrine deposits of the Lake Michigan Formation. Seismic profiles show lacustrine sediments that extend from deep water to form a uniform sheet of sand nearer shore that is most often between 1-2 m thick but, close to shore, exceeds 6 m just north of Waukegan Harbor (Shabica and Pranschke this volume). -Waukegan to Lake Forest Between Waukegan and Lake Forest, a wedge of sand covers the bottom within 1-2 km of the shore and thins lakeward to a patchy veneer (fig. 6). The outer margin of the sand wedge is complex. Two northeasterly-trending sand ridges that are about 4 km long and 0.5 to 1 km wide overlie cobble pavement (fig. 8). The ridges are asymmetric in profile and reveal the direction of sand movement at the time our surveys were conducted. Their geometry suggests that these sand bodies are large, flat, southward-moving dunes. They thicken from a feather edge on the northwestern edge (fig. 7b) to a maximum of about 2 m on the southeastern steeply dipping edge (fig. 7c). When our data are combined with data collected less than 1 km from shore by Norby and Collinson (personal communication) in 1974-76, and (Shabica and Pranschke this volume), a more detailed picture of nearshore sand distribution can be constructed (fig. 8). In some areas, close to shore, windows in the sand expose the underlying till. The addition of these older data makes a coherent picture; however, the distribution of sand nearshore has probably changed significantly during the years since some of the data were acquired. The complexity of the sand distribution in this area illustrates the transition from a sand-covered, net depositional area north of Waukegan to a sediment-starved, nearshore area south of Lake Forest. -Lake Forest to Chicago South of Lake Forest to Chicago, the bottom is mainly floored by till, sand, gravel, and cobbles (fig. 6). Silty, clayey sand, muddy gravel, and clay occur only in comparatively small areas; thus, this appears to be an area of erosion or nondeposition. Lag gravel and sand have been eroded from till. Lag boulders (See fig. 7d) are concentrated in a belt that extends to the northwest from 15 km east of Chicago to just east of Fort Sheridan, IL. They may be the remnant of the lake border moraines that are present on land north of Wilmette, IL (fig. 5). Off Fort Sheridan, Wilmette, and Grosse Point, in 0.7 by 5 km areas, sidescan sonar mosaics (Polloni and Brown in press) show that the relatively flat bottom is covered by a cobble till pavement with a few patches of thin sand; the nearshore sand wedge apparently feathers out lakeward within about 550 m of shore. This is consistent with Shabica and Pranschke's fig 5a (this volume) thickness profile at Gilson Park, Wilmette, Illinois. -Chicago to Gary, Indiana Between Chicago and Gary, Indiana the lakebed comprises till, lag sand, and gravel (fig. 6) similar to the area to the north; however, discontinuous sheets of sand are more common and are often associated with north-trending bathymetric ridges (fig. 2d,e). At Indiana Shoals, we have more closely spaced geophysical data including a sidescan sonar mosaic (Polloni and Brown in press; Schlee et al. in press, fig. ) that covers approximately 12 km2 of the lake floor. The bathymetry shows a gently rolling surface with depressions and irregular ridges and shoals (fig. 9). Local relief is as much as 3-5 m in the ridges and shoals. The shallow area extends 10-15 km offshore modifying the gradually deepening shore-normal profile that characterizes most of the southeastern part of the Lake. Adjacent to the Indiana Harbor breakwater, the sidescan sonar mosaic shows several irregular light gray-white (low backscatter) polygons that we have inferred to be dredge scars extending 1.5 km lakeward and narrowing to the southeast (Schlee et al. in press, fig. ). Most of the mosaic to the north and east of the area of polygons is light gray (low backscatter) with irregular N-S trending dark (high backscatter) areas that range in width from 10 to 200 m (Schlee et al., fig. ). On the western side of the mosaic the dark areas are more common (~50% of the bottom) and are even more irregular. Dark areas show no obvious relationship to depth and are distributed randomly across ridges and valleys. The 34 bottom grab samples collected in the area consist of moderately well-sorted, fine to medium-grained sand. A few, however, consist of as much as 98% gravel. The samples suggest that the light gray areas most often are fine sand and the dark areas are most often coarse sand and gravel. To confirm the nature of the boundary between dark and light areas on the mosaic, four areas were investigated by scuba divers. In each area where the lighter (less reflective) returns were observed, the bottom was covered by light gray fine sand; whereas where darker returns were observed the bottom was covered by coarse, brown sand with granules and pebbles as much as 3 cm in diameter. The dark areas also are characterized by east-trending megaripples 60 cm in wavelength, and 20-25 cm in height with coarse sand and occasionally clay in the troughs. In contrast, the less reflective sand is characterized by small ripples with wavelengths of 6 cm and heights of 2.5 cm. We interpret the highly reflective areas as lag deposits over till that have been subjected to large storm waves from the north forming the megaripples. The discontinuous sheet of sand, as much as 3 m thick in only a few areas (fig. 9), apparently has moved across the area partially covering the dredge scars and the coarse lag deposits and till. This pattern of deposition is probably typical of much of the area between Waukegan and Michigan City. -Gary, Indiana to Michigan City, Indiana The feather edge of the the Lake Michigan Formation lies close to Michigan City and trends offshore to the northwest (Colman and Foster, this volume, fig. 2). Thus, the lacustrine silty sands cover most of the bottom to the north and east of Michigan City (fig. 6). These deposits, like those north of Waukegan, are in sharp contrast to the erosional and nondepositional areas that cover most of the study area. Shabica and Pranschke's sand thickness profiles (fig. 6a, this volume) show that the nearshore sand wedge is relatively thick (4 m) and extends more than 530 m offshore. West of Michigan City to Gary, sand cover, while more extensive than off Illinois, is still patchy and thin partially exposing till. The nearshore sand wedge is, most often, less than 1 m thick. Till ridges and pinnacles are commonly exposed on the bottom in waters 5-10 m deep. SUMMARY AND CONCLUSIONS Our interpretation of the sand distribution, though in part qualitative, provides input for sediment budget calculations. North of Waukegan, sand is abundant; nearshore it is as thick as 6 m (see Shabica and Pranschke, this volume) and offshore as thick as 2 m. This net depositional area, thus constitutes a source of sand for the coast to the south. However, much of it appears to be depleted south of Lake Forest. Between Waukegan and Lake Forest, deposits of both offshore and onshore sand are much more patchy. Till and boulder-gravel pavement exposures are more extensive. Beyond a few kilometers from shore, sand appears to be no more than 2 m thick in limited dunes or bars. Nearshore, sand is intermittent ranging from 1-2 m thick on the updrift (north) side of structures to zero on the downdrift side. (See, for example, the Great Lakes Naval Training Station profiles-Shabica and Pranschke this volume). From Lake Forest to Gary, Indiana little sand is present nearshore. Offshore sand is scattered in ridges and patches. The thickest deposits, about 3 m, were measured at Indiana Shoals; because of the sparsity of data between transects other equally thick deposits may be present. From Gary to Michigan City, nearshore sand is less than a meter thick. The feather edge of the Lake Michigan Formation approaches shore east of Michigan City and sand cover of the bottom is more extensive, similar to the area north of Waukegan. South of Waukegan and east of Michigan City, much of the southwestern lake Michigan nearshore area is a dynamic environment; storm-driven currents transport sediment that periodically covers and uncovers the till-gravel pavement. It is, therefore, likely that detailed maps of bottom sediment texture in this area will differ significantly from year to year, possibly from season to season, and even from major storm to major storm. The veneer of sand, as much as 2 to 3-m thick moves along the bottom apparently in response to storm-driven waves and currents. Because of the 500-km fetch to the north, we assume that the net transport direction is southward toward the Indiana Dunes area where it is being lost from the lacustrine system by onshore wind transport from the beaches (see Olyphant this volume) Acknowledgments We acknowledge the able assistance of D. Blackwood, C. Brown, A. Brill, M. Chrzastowski, B. Irwin, L. Maizlisch, D. Mason, D. Nichols, T. O'Brien, K. Parolski, C. Polloni, F. Pranschke, J. Risch, J. Schlee, B. Seekins, R. Tagg, J. Zwinakis, during the many phases of this project. BIBLIOGRAPHY 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 Geological Survey, Environmental Geology Note No. 74. Buschbach, T. C. and Heim, G. E. 1972. Preliminary geologic investigations of rock tunnel sites for flood and pollution control in the greater Chicago area. Illinois State Geological Survey, Champaign, IL, Environmental Geology Notes, no. 52. Cahill, R. A. 1981, Geochemisty of recent Lake Michigan sediments. Illinois State Geological Survey, Champaign, IL, Circular 517. Folk, R. L. 1974. Petrology of sedimentary rocks. Austin, TX, Hemphill Publishing Co. Elassal, A. A. and Caruso, V. M. 1983. Digital elevation models. U. S. Geological Survey Circular 895-B. Foster, D. S., and Colman, S. M. 1992. Thickness and distribution of post glacial deposits beneath Lake Michigan. U. S. Geological Survey Miscellaneous Investigations Map, MI-2202. Foster, D. S., Folger, D. W., Brown, C. L., Seekins, B. L. 1992. Maps and cross-sections showing bedrock surface elevations and Quaternary sediment thickness beneath southwestern Lake Michigan and in the adjacent Illinois and Indiana nearshore region. U. S. Geological Survey Miscellaneous Investigation Map xxxx. Graf, J. B. 1976. Comparison of measured and predicted nearshore sediment grain-size distribution patterns, southwestern Lake Michigan, U. S. A. Marine Geology, 22: 253-270. Gray, H. H. 1982. Map of Indiana showing topographjy of the bedrock surface. Indiana Geological Survey, Bloomington, IN, Miscellaneous Map 36. Horberg, C. L. 1950. Bedrock topography of Illinois.Illinois Geological Survey, Champaign, IL, Bulletin 73. Hough, J. L. 1935. The bottom deposits of southern Lake Michigan. J. Sedimentary Petrology, 5: 57-80. Hough, J. L., 1958. Geology of the Great Lakes. Urbana, IL, University of Illinois Press. Lineback, J. A., Gross, D. L., Meyer, R. P., and Unger, W. L. 1971. High resolution seismic profiles and gravity cores of sediments in southern Lake Michigan. Illinois State Geological Survey, Champaign, IL, Environmental Geology Notes no. 47. Lineback, J. A., Gross, D. L., and Meyer, R. P. 1974. Glacial tills under Lake Michigan. Illinois State Geological Survey, Champaign, IL, Environmental Geology Notes, no. 69. Meisburger, E. P., Williams, S. J., and Prins, D. S. 1979. Sand resources of southeastern Lake Michigan.U. S. Army Corps of Engineers Miscellaneous Report No. 79-3. National Geophysical Data Center 1987. NOAA National Geophysical Data Center Data Announcement 87-MGG-12. Norby, R. D.. 1981. Evaluation of Lake Michigan nearshore sediments for nourishment of Illinois beaches. Illinois State Geological Survey, Champaign, IL, Environmental Geology Notes 97. Pettitjohn, F. J. 1931. Petrography of the beach sands of southern Lake Michigan. J. Geology, 39: 432-455. Polloni, C. F., and Brown C. L. in press. The southern Lake Michigan Coastal Erosion Study CDROM. U. S. Geological Survey Open File Repoprt 92-xxxx. Schlee, J. S., Folger, D. W., Risch, J. S., Seekins, B. A., Foster, D. S., Polloni, C. F., Brown, C. L., Chrzastowski, and Thompson, Todd, in press. Physiography and sediment distribution of Indiana Shoals, southern Lake Michigan. U. S. Geological Survey Miscellaneous Investigation Series Map MI-xxxx. Schneider, A. F., and Keller, S. J., 1970. Geologic map of the 1oX2o Chicago Quadrangle, Indiana, Illinois, and Michigan, showing bedrock and unconsolidated deposits. Indiana Geological Survey, Bloomington, IN, Regional Map No. 4. Shedlock, R. J., Chen, D. A., Imbrigiotta, T. E., Thompson, R. A., and Lindgren, H. A., in press, Hydrogeology and hydrochemistry of dunes and wetlands along the south shore of Lake Michigan, Indiana: U. S. Geological Survey Water Supply Paper. Suter, Max, Bergstrom, R. E., Smith, H. F., Emrich, W. C., Walton, W. C., Larson, T. E. 1959. Preliminary report on ground-water resources of the Chicago region, Illinois. Illinois Water Survey and Illinois Geological Survey Coop., Champaign, IL, Ground-Water Rept. 1. Welkie, C. J., and Meyer, R. P. 1982. Exploration and assessment of offshore sand and gravel, western Lake Michigan. Marine Minerals, 3 (3-4):315-378. Wickham, J. T., Gross, D. L., Lineback, J. A., and Thomas, A. L., 1978. Late Quaternary sediments of Lake Michigan. Illinois State Geological Survey, Champaign, IL, Environmental Geology Note No. 84. Willman, H. B., 1971. Summary of the geology of the Chicago area. Illinois State Geological Survey Circular 460. Wold, R. J., Paull, R. A., Wolosin, C. A., and Friedel, R. J. 1981. Geology of central Lake Michigan. Amer. Assoc. Petroleum Geologists Bulletin, 65, no. 9:1621-1632. FIGURES Figure 1. Map showing southwest Lake Michigan, bounded by Illinois and Indiana. Acoustic survey tracklines are plotted; bold lines labelled with letters show the locations of cross- sections in figure 2a-h. Gray shaded areas depict detailed sidescan sonar mosaics: FS-Fort Sheridan mosaic area; W-Wilmette Harbor and Grossepoint mosaic areas; IS- Indiana Shoals mosaic area; MC-Michigan City mosaic area; O-Olson Tree Site mosaic area. Bathymetric contours (5- m intervals) are referenced to the International Great Lakes Datum (IGLD) of 1955; topographic contours (10-m intervals) represent elevation above mean sea level (modified from Foster et al. in press). Figure 2. Selected cross-sections showing bathymetric profiles and bedrock units. SDc, Silurian and Devonian carbonate; DMs Devonian and Mississippian shale; Q, Quaternary unconsolidated glacial till and postglacial lacustrine sediment. Dashed vertical lines are where cross-sections bend. See figure 1 for cross-section locations. Figure 3. Stratigraphic column for the Illinois and Indiana nearshore framework area. Bedrock, glacial till, and post-glacial lacustrine stratigraphic units and associated lithologies are correlated to nearshore seismic stratigraphic units. Qlu, post-glacial lacustrine deposits of the upper part of the Lake Michigan Formation (after Colman et al. this volume); Qw, Late Wisconsinan glacial till of the Wedron Formation (Willman 1971); DMs, Devonian and Mississippian carbonates and Devonian shale (Ellsworth and Antrim shale) (Schneider and Keller 1970); SDc, Silurian (Niagaran Series dolomite) and Devonian (Traverse and Detroit River Formations) carbonate (Schneider and Keller 1970; Willman 1971). Figure 4. Map showing 10-m contours of bedrock elevation above mean sea level (modified from Foster et al. in press); the distribution of Devonian and Mississippian shales (DMs) (light gray area), Silurian and Devonian carbonates (SDc) (medium gray), and the Des Plaines Complex, a faulted area at the bedrock surface, are mapped (modified from Schneider and Keller 1970; Willman 1972). Figure 5. Map showing 10-m contours of the thickness of the Quaternary sediment overburden (modified from Foster et al. in press). Geomorphic features mapped on land: the Lake Border Moraine (light gray), the Tinely Moraine (medium gray), and the Valparaiso Morainic System (dark gray) (modified from Schneider and Keller 1970; Willman 1971). Figure 6. Map showing sidescan sonar interpretation of bottom sediment types for the Illinois and Indiana nearshore (Modified from Folger et al. in press). The width of the interpretive strips are not to scale, actual swath width is 200 m. Bathymetric contours (5-m intervals) indicate depth below IGLD of 1955; topographic contours (10-m intervals) indicate elevation above mean sea level. Figure 7. Selected sidescan sonar images collected in the southwestern Lake Michigan nearshore area. (A) Image showing till ridges exposed at the lake floor at the Olson Tree Site (see fig. 1). The ridges are surrounded by fine sand (light areas) and small fields of megaripples. (B) Image showing the trailing edge of the same sand ridge referred to in A. (C) Image showing the leading edge of a sand ridge, (fine sand correlates with the light area on the image) that appears to have migrated to the south over a gravel-cobble till pavement lake bed (dark area of the image). (D) Image showing megaripples where a bottom sample indicates a coarse sandy gravel texture; several boulders protrude above the lake floor resulting in acoustic shadows on the image. See figure 8 for location of images B and C and figure 6 for location of image D. Figure 8. Map showing the distribution of sand over till-gravel pavement between Waukegan and Lake Forest, Illinois. (B), Location of figure 7b. (C), Location of figure 7c. Tracklines shown in figure 1. Dashed lines represent tracklines of Shabica and Pranschke (this volume). Figure 9. Interpretive line drawing of a 3.5 kHz seismic profile from the Indiana Shoals mosaic area (fig. 1). A reflection at the sand-till interface defines the thickness of sand that drapes till ridges in the subbottom. The depth scale assumes a sound speed of 1500 m/s for water and sediment.