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. 



 


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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.