MN

MI

GrandTraverseBay

ON

AuTrainBay Tahquamenon

BayWI

Canada

U.S.A.

BatchawanaBay

KEYGTB

ATB

BATB

LAKESUPERIOR

48º

46º

84º86º88º90º92º

84º86º88º90º92º

48º

46º50 100 150 km0

TAHB

Lake Superior Study SitesFour Embayments with Preserved Beach Ridges

Four different areas are being studied along the Lake Superior coastline. Data is collected during a month-long summer fieldseason, and the data is analyzed during the following year until the next fieldseason (summer fieldseasons: GTB-1998, TAHB-1999, ATB-2000-01, BATB-2002-03).

ABSTRACT The internal architecture and formation of lacustrine beach ridges has long been a topic of debate, partly owing to the lack of continuous data through the ridges and their complex stratigraphy. Past interpretations were based on correlating isolated information from strategically placed vibracores across beach ridges. Today, ground penetrating radar (GPR) provides a method to view inside ridges and collect continuous data to define and correlate sedimentary units within beach ridges. Baedke and Thompson (1995) proposed a theoretical model explaining beach ridge development as a product of changing rates of sediment supply and water level change. This model was based on information from numerous vibracores and current shoreline processes. Thompson and Baedke (1995) redrew the Curray (1964) diagram, focusing on the positive rate of sediment supply side of the diagram and placing importance on water level changes crossing the aggradation line for the development of individual beach ridges. Some of these theoretical ideas of beach ridge formation and shoreline development have been verified using GPR. Several continuous GPR reflection surveys were collected across beach ridges in three embayments along the Lake Superior shoreline (Grand Traverse Bay, Tahquamenon Bay, and Au Train Bay). Digital GPR data was collected using a Noggin 250 SmartCart with a fixed 250 MHz antennae and a recording interval of 5 cm between traces. The depth of penetration was from 5 to 8 m. Information from vibracores were used to estimate the velocity of the radar signal and calibrate the GPR data before processing. Beach ridge topography was measured using a transit and used to correct GPR lines. Although each beach ridge has a unique GPR signature, they all contain a series of lakeward-dipping reflectors and a strong concave reflector that extends lakeward from the base of swales. The strong reflector is interpreted as an erosional surface (ravinement) created during lake-level rises while the other reflectors are interpreted as the offlapping part of the progradational development of beach ridges.

Air photographs of the study areas are shown. All study areas consist of an embayment filled in with preserved shorelines called beach ridges. These embayments are ideal areas for beach ridges to form because littoral sediment is trapped between headlands along the margins of the embayment. Each beach ridge records shoreline behavior (water level and sediment supply) and vertical ground movement (isostatic rebound/tectonics) throughout time (5,000 years to present at these sites).

modified from Thompson and Baedke (1995) after Curray (1964)

A

B

-

+

+

Rate ofWater Level

Change

Rate of Sediment Supply

Depositionaltransgression

Depositionalregression

Depositional (Forced)regression

ProgradationRetrogradation

Erosionalregression

Erosionaltransgression

S1 S2 S3

R1

R2

R3Aggradation

12

3

12

3

12

3

123

12

3

12

3 12

3

kilometer(s)meter(s)

LL

Phase Diagram of Shoreline BehaviorRates of water-level change and sediment supply

Shoreline behavior can be represented by a phase diagram that has on its axes rates of sediment supply and rates of water level change. For every rate of sediment supply there is a corresponding water level change that places you into a certain phase or field. The most important series of points in beach ridge creation is the aggradation line where the shoreline vertically aggrades and forms the core of the beach ridge.

Time

Ele

vatio

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rapidrise

stable

rapidfall

stable

increasingfall

decreasingfall

decreasingrise

increasingrise

stable

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e

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- -

Time +

Absolute Elevation versus Rate of Water-Level Change

Rates of water level change are very difficult to understand and are rarely used because most people think of absolute water levels. Tangents along certain portions of the absolute elevation curve (top graph) yield rates, which are plotted in the lower graph. Notice that the curves on the two graphs are out of phase (1/4 wavelength) where the maximum rate of water-level rise is approximately at the average absolute water-level elevation.

-

+

+

Rate ofWater Level

Change

Rate of Sediment Supply

Depositionaltransgression

Depositionalregression

Forcedregression

ProgradationRetrogradation

Erosionalregression

Erosionaltransgression

Aggradation

12

3

12

3

12

3

123

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nochange

somechange more

realistic

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Shoreline DevelopmentRates of water-level change and sediment supply

A water level fluctuation plotted on a phase diagram forms a vertical line when there is no change in the rate of sediment supply. But water level fluctuations are commonly accompanied by changes in sediment supply. The line would then become an oval. A more realistic scenario would be a tilted oval, showing that sediment supply would be lost to the system as water level rises and drowns fluvial systems. At the maximum rate of water level rise sediment supply would increase because waves would be eroding upland areas.

Phase Diagram of Shoreline Behavior

INTRODUCTION The primary focus of our research program is to find certain water-lain sediments, called basal foreshore deposits, inside beach ridges to create hydrographs of paleo lake level relative to each study site area. Previous research only used cores to create a conceptual model explaining how beach ridges formed. Ground penetrating radar (GPR) now provides a window that better defines beach ridge architecture between core sites. GPR not only helps us confirm the conceptual model of beach ridge formation but also allows us to explore model variations at a single site and between sites.

Batchawana Bay

Au Train Bay

Tahquamenon Bay

Grand Traverse Bay

Conceptual Model of Beach Ridge Development

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+

+

Rate ofWater Level

Change

Rate of Sediment Supply

Depositionaltransgression

Depositionalregression

Forcedregression

ProgradationRetrogradation

Erosionalregression

Erosionaltransgression

Aggradation

12

3

12

3

12

3

12

3

1

23

4

12

3

12

3

123

Phase Diagram

A certain combination of the rates of two variables, water level and sediment supply, are important in creating beach ridges. The diagram above shows that a shoreline experiences several different phases of shoreline behavior as a beach ridge is created. The most important phase in creating the core of a beach ridge is the aggradation phase where the shoreline vertically aggrades.

1

2

3

4

1'

Progradation

Depositional regression

Aggradation / Stillstand

Depositional transgression

KEY

Beach ridge forms

large vertical exag

geratio

n

LL

Shoreline Profile History

Equilibrium shoreline profiles created during the formation of a beach ridge are shown. The numbers correspond to the oval on the phase diagram in the previous slide (note the large vertical exageration). A return to the starting point is shown as 1'. A core of the beach ridge may begin to form from 1 to 2 as the rate in water level increases but a later stage bewteen 2 and 3 may erode the sediment deposited between 1 and 2. Therefore, the bulk of the preserved core of the beach ridge forms bewteen 3 and 4 where the rate in water level is decreasing.

The preserved part of the record inside a beach ridge. The ravinement surface is created between 2 and 3 where the rate of water level change is near its maximum and the shoreline is in the depositional transgression field. The core of the beach ridge is created while the shoreline vertically aggrades as the rate of water level decreases. An offlapping sequence forms lakeward of the beach ridge and helps protect the newly formed beach ridge during the next maximum rate in water level rise.

Ravinement

AggradationDep. RegressionProgradation

ForcedRegression

Lakeward

large vertical exag

geratio

n

Preserved Record

Inside a Beach Ridge: Using Ground Penetrating Radar (GPR)

Conceptual Model Confirmed of Beach Ridge Development

CoreATB 1014 Beach Ridge Crest #14Beach Ridge Crest #13

Est

imat

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epth

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eter

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1

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(Velocity = 0.10 m

/ns)

Internal Architecture of Beach RidgesStrong Reflectors are Outlined

Lakeward

swale

Distance landward of Lake Superior (meters)

350 355 360 365 370 375 380 385

ravinementsurface

offlappingprogradationalwedges

water table

aggradationdep. regression

Several strong reflector were traced on the GPR line to define important sequences inside a beach ridge. These sequences fit the simplified conceptual model of beach ridge development. A ravinement surface forms during a rapid rate in water-level rise. The core of the beach ridge forms as the water-level rise decreases and the shoreline vertically aggrades. Offlapping progradational wedges form on the lakeward side of the beach ridge as lake-level falls, protecting it from future water level rises.

CoreATB 1014 Beach Ridge Crest #14Beach Ridge Crest #13

Est

imat

ed d

epth

in m

eter

s

0

1

2

3

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(Velocity = 0.10 m

/ns)

GPR Signal ResponseMean Grain Size Results

Lakeward

swale

Distance landward of Lake Superior (meters)

350 355 360 365 370 375 380 385

The GPR signal response is a function of the material's electrical properties. The electrical properties control how electromagnetic waves travel through a material. Important electrical properties are the dielectric permittivity [primarily controls wave speed] and conductivity [determines the signal attenuation]. Preliminary analysis of the mean grain size results from a core through a beach ridge suggests grain size variations may play a part in GPR signal responses and help define certain sedimentary sequences.

water table

CoreATB 1014 Beach Ridge Crest #14Beach Ridge Crest #13

Distance landward of Lake Superior (meters)

Est

imat

ed d

epth

in m

eter

s

350 355 360 365 370 375 380

0

1

2

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5

(Velocity = 0.10 m

/ns)

GPR Signal Inside a Beach RidgeAu Train Bay, MI

Lakeward

swale

385

A relatively unprocessed GPR line recorded over beach ridges at Au Train, Michigan. The difference between the strong and weak reflectors above and below the water table are a function of signal strength, distance from the source, and receiver and electrical properties of the two materials (i.e. saturated vs. unsaturated). A fairly strong reflector extends underneath the beach ridge at depth to the surface in the next landward-adjacent swale. Several fairly strong horizontal reflectors underneath the beach ridge may represent the core of the beach ridge. Several fairly strong lakeward-dipping reflectors represent offlapping progradational and forced regression sequences.

Steve Baedke running the Noggin 250MHz SmartCart GPR system over a beach ridge at Au Train, Michigan. This unit sends electromagnetic waves into the ground, the waves reflect off of boundaries between materials with different electrical properties. The waves are then collected by the receiver, recorded and displayed live in the digital vodeo logger (yellow box on top).

Conclusions

Ground Penetrating Radar is an excellent tool for studying the internal architecture of beach ridges and verifying conceptual models of shoreline development

Shorelines experience several different types of behavior when constructing beach ridges(progradation to aggradation, depositional transgression and regression, and forced regression)

Beach-ridge development and preservation is dependent upon a positive supply of sediment and a water-level fluctuation where it crosses the aggradational boundary

Acknowledgments

This grant was possible through the cooperative effort between the Indiana Geological Survey, an Institute of Indiana University, and the Biological Research Division/USGS under the Global Climate Change Program of the USGS. The support of Dr. Doug Wilcox for coordinating this grant is appreciated.

Thanks to the Director of the Indiana Geological Survey, Dr. John Steinmetz for the purchase of the GPR system.

Thanks to Sensors and Software Inc. for creating such a user-friendly GPR system, the Noggin 250 MHz Smart Cart.