Preliminary Report of Late Holocene Lake-level Variation in Southeastern Lake Superior Part II: Tahquamenon Bay, Michigan

Indiana Geological Survey Open-File Study 2001-4The internal architecture and age of development of 71 beach ridges in the Tahquamenon Bay embayment, located along the southeastern shore of Lake Superior on the Upper Peninsula of Michigan, were studied to generate a late Holocene relative lake-level curve for Lake Superior. The record from this embayment is important because Tahquamenon Bay is located near the outlet for Lake Superior and may have experienced similar vertical movement (isostatic uplift) rates as the outlet. The lakeward side of beach ridges were cored to obtain the elevation of basal foreshore deposits, which record the elevation of the lake when each beach ridge formed. Basal wetland sediments were collected from swales between ridges and radiocarbon dated to determine the age of the next lakeward adjacent beach ridge. Regression analysis of the calibrated dates was used to approximate the age of the beach ridges. Elevation data and age data were used to construct a relative lake-level curve for Tahquamenon Bay. Beach ridges in the Tahquamenon Bay embayment formed between about 4,300 and 2,000 calendar years before 1950 (cal. yrs. B.P.). The average timing for beach-ridge development of one ridge in the Tahquamenon Bay strandplain is 31 ± 3.7 years. Groupings of four to six beach ridges indicate longer-term fluctuations in lake levels. Basal foreshore elevations indicate relative lake levels dropped rapidly (almost 5 m) from about 4,100 to 3,800 cal. yrs. B.P., lowered gradually (approximately 7 m) from about 3,800 to 2,300 cal. yrs. B.P., and remained fairly constant from about 2,300 to 2,000 cal. yrs. B.P. The rapid drop is associated with a drop in water level from the Nipissing II high water-level phase, and the change from a gradual fall to a fairly constant slope is associated with an outlet change from Port Huron, Michigan, to Sault Ste. Marie, Michigan. Grain-size and foreshore thickness trends may be attributed to variations in sediment source or littoral currents or wave climate or outlet location or outflow characteristics or vertical movement between the study area and the outlet or a combination of these.United States Geological Survey Global Climate Change Program and United States Geological Survey Biological Research Agreement No. 98HQAG218

Preliminary Report of Late Holocene Lake-Level Variation in Southern Lake Superior: Part 1

Indiana Geological Survey Open-File Study 99-18The internal architecture and age of development of 60 beach ridges in the Grand Traverse Bay embayment, located along the southern shore of Lake Superior on the Upper Peninsula of Michigan, were studied to generate a late Holocene relative lake-level curve for Lake Superior. Basal foreshore elevations, collected from the lakeward sides of beach ridges, were used to determine the relative elevation of Lake Superior when each beach ridge formed. The break in slope between each ridge and the lakeward swale was cored to ensure that the foreshore was penetrated and a maximum basal foreshore elevation was obtained. Basal wetland sediments, collected from swales between beach ridges, were dated to determine the age of adjacent lakeward beach ridges. Basal wetland sediments were recovered from the deepest part of selected swales along the coring transect. Basal wetland sediments provided a minimum age for the lakeward adjacent beach ridge and a least squares regression was used to reduce variability in the data and to approximate the age of unsampled wetlands. Beach ridges in the Grand Traverse Bay embayment formed between 900 and 3800 calendar years before 1950 (cal. yrs B.P.). The average timing for beach-ridge development of one ridge in the Grand Traverse Bay strandplain is 36 +/- 7.8 years. Groupings of four to six beach ridges indicate longer-term fluctuations in lake levels. Basal foreshore elevations indicate relative lake levels lowered about 4.5 m from 3800 to 1200 cal. yrs. B.P. and increased about 0.7 m from 1200 to 900 cal. yrs. B.P. A coarsening in foreshore mean grain-size per ridge also occurs at about 1200 cal. yrs. B.P. Foreshore thicknesses increase about 0.4 m from 2000 to 1200 cal. yrs. B.P. and remain fairly constant from about 1200 to 900 cal. yrs. B.P. Increased foreshore thicknesses indicate larger wave setup and may be related to a shift in the predominant wind direction that would produce greater wave setup in the Grand Traverse Bay embayment.United States Geological Survey Global Climate Change Program Agreement No. 98HQAG218

Reconstructing Paleo Lake Levels from Relict Shorelines along the Upper Great Lakes

Shorelines of the upper Great Lakes include many embayments that contain strandplains of beach ridges. These former shoreline positions of the lakes can be used to determine changes in the elevation of the lakes through time, and they also provide information on the warping of the ground surface that is occurring in the Great Lakes after the weight of glacial ice was removed. Relative lake-level hydrographs can be created by coring the beach ridges to determine the elevation of basal foreshore (swash zone) deposits in each ridge and by obtaining radiocarbon dates of basal wetland sediments between ridges to generate an age model for the ridges. Because the relative-level hydrographs are the combination of lake-level change and vertical ground movement (isostatic rebound), the rebound must be removed to produce a graph that shows only the physical limits and timing of past lake-level fluctuations referenced to a common outlet. More than 500 vibracores of beach-ridge sediments were collected at five sites along Lake Michigan and four sites along Lake Superior. The cores showed a sequence of dune deposits overlying foreshore deposits that, in turn, overlie upper shoreface deposits. The base of the foreshore deposits is coarser and more poorly sorted than an overlying and underlying sediment and represents the plunge-point sediments at the base of the swash zone. The plunge-point deposits are a close approximation of the elevation of the lake when the beach ridge formed. More than 150 radiocarbon ages of basal wetland sediments were collected to produce age models for the sites. Currently, age models exist for all Lake Michigan sites and one Lake Superior site. By combining the elevation data with the age models, six relative lake-level hydrographs were created for the upper Great Lakes. An iterative approach was used to remove rebound from the five Lake Michigan relative hydrographs and merge the graphs into a single hydrograph. The resultant hydrograph shows long-term patterns of lake-level change for lakes Michigan and Huron and is referenced to the Port Huron outlet. When the age models are completed for the Lake Superior sites, a hydrograph will be created for the entire lake

Late Holocene Lake-level Variation in Southeastern Lake Superior: Tahquamenon Bay, Michigan

Internal architecture and ages of 71 beach ridges in the Tahquamenon Bay embayment along the southeastern shore of Lake Superior on the Upper Peninsula of Michigan were studied to generate a late Holocene relative lake-level curve. Establishing a long-term framework is important to examine the context of historic events and help predict potential future changes critical for effective water resource management. Ridges in the embayment formed between about 4,200 and 2,100 calendar years before 1950 (cal. yrs. B.P.) and were created and preserved every 28 ± 4.8 years on average. Groups of three to six beach ridges coupled with inflections in the lake-level curve indicate a history of lake levels fluctuations and outlet changes. A rapid lake-level drop (approximately 4 m) from about 4,100 to 3,800 cal. yrs. B.P. was associated with a fall from the Nipissing II high-water-level phase. A change from a gradual fall to a slight rise was associated with an outlet change from Port Huron, Michigan/Sarnia, Ontario to Sault Ste. Marie, Michigan/Ontario. A complete outlet change occurred after the Algoma high-water-level phase (ca. 2,400 cal. yrs. B.P.). Preliminary rates of vertical ground movement calculated from the strandplain are much greater than rates calculated from historical and geologic data. High rates of vertical ground movement could have caused tectonism in the Whitefish Bay area, modifying the strandplain during the past 2,400 years. A tectonic event at or near the Sault outlet also may have been a factor in the outlet change from Port Huron/Sarnia to Sault Ste. Marie

Insight into beach ridge formation using ground penetrating radar.

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

Insight into beach ridge formation using ground penetrating radar.

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