David L. Gross

Origin of regional geomagnetic variations recorded by Wisconsinan and Holocene sediments from Lake Michigan, U.S.A., and Lake Windermere, England

The declination of the Earth9s paleogeomagnetic field, determined from sediments in Lake Michigan deposited during the past 11,500 C14yr, exhibits fluctuations east and west of the mean declination with a period of about 2,090 C 14 yr. Inclination and intensity measurements do not exhibit similar fluctuations. Each extreme in declination occurs in the same stratigraphic position in cores from different parts of the lake. The variations in declination in Lake Michigan are similar to those found in sediments from Lake Windermere, England, deposited during the same time span, but the period of the Windermere cycles is 2,800 C 14 yr. Plots of the paleogeomagnetic poles for inclination-declination pairs representing each east or west extreme for the two lakes are quite different, indicating that the geomagnetic effects were not a result of shifts of the main dipole field. In a model of the standing nondipole field, the foci, represented by radial dipoles located one-fourth of the Earth9s radius from the geocenter, are each allowed to oscillate with a characteristic period. This model gives a satisfactory explanation of the principal features exhibited by the declination and inclination records at the two lakes.

Glacial and postglacial sediments in Lakes Superior and Michigan

The lacustrine sediment sequence above the glacial till under Lake Superior consists of red clays overlain by gray varved clay and nonvarved gray and brown clay. The Lake Michigan sequence also contains red clay at the base and brown and gray clays above. The red clays in Lake Michigan are found in areas overridden by glaciers of Valderan age as well as in the far southern end of the lake. Deposition of red clay took place in the northern part of the lake as the Valderan glaciers were melting and continued during the Algonquin stadial when the ice front reached a stillstand on the northern peninsula of Michigan. Red and gray clay outwash dumped into Lake Superior may also have escaped from the Superior Basin into the Lake Michigan Basin through the Au Train–Whitefish channel and other channels across the upper peninsula of Michigan. The connection of the two lakes began between about 11,000 C14 yr B.P. and ended about 10,000 C14 yr B.P., when the connection was broken by rebound or by lowering of water levels near the end of the glaciation in the Lake Superior Basin.

Rate of Advance of the Woodfordian (Late Wisconsinan) Glacial Margin in Illinois: Stratigraphic and Radiocarbon Evidence

Thirty radiocarbon dates from 18 localities were used to document the rate of advance of the Woodfordian glacial margin in Illinois. These dates were obtained from the Robein Silt (formerly called Farmdale Silt) of the Farmdalian Substage, the top of the overlying Morton Loess (Woodfordian Substage), and the base of the Wedron Formation (Woodfordian Substage). Robein Silt accumulation ended with the initial deposition of the overlying pro-glacial Morton Loess followed by deposition of the Wedron Formation. The base of the Wedron Formation is time-transgressive. In northeastern Illinois it has been dated at 23,000 radiocarbon yrs B.P. However, in south-central Illinois at the southern limit of the Woodfordian glacial advance, it has been dated at 20,000 yrs B.P., and at the western margin it has been dated at 19,000 yrs B.P. For distances up to 250 km in Illinois, therefore, the base of the unit transgresses 3,000 to 4,000 radiocarbon yrs. The net rate of advance of the Woodfordian glacial margin can thus be calculated as being of the order of magnitude of 62 m per radiocarbon yr. Although this rate is approximate, it falls within the range of rates (25 m to 106 m per yr) reported from Ohio.

Palynology and sedimentology of Holocene deposits in southern Lake Michigan

An examination of the fossil pollen in the sediment shows that the upper part of the Waukegan Member (Holocene) of the Lake Michigan Formation can be divided into four pollen assemblage zones on the basis of subtle changes in percentages of the dominant types of pollen and in total numbers of pollen grains per gram of sediment. Three of these zones are distinguished on the basis of the relative abundance of pine and oak pollen, and the fourth is characterized by a high percentage of ragweed pollen. Zone 1, the oldest, has lower percentages of pine and higher percentages of oak than the other zones. The total pollen concentration in the sediment is highest in zone 1. Zone 2 is distinguished from zone 1 by a lower concentration of total pollen. Zone 3 has the highest percentages of pine. Zone 4 is distinguished by high percentages of ragweed, a result of forest clearance for agriculture about 1840 A.D. On the basis of the ragweed increase, calculated sedimentation rates in southern Lake Michigan for the postsettlement period are 0.49 mm/yr for the central area of the southern basin of the lake and 0.9 to 1.02 mm/yr for areas along the east side of the lake. In agreement with results of studies of the other Great Lakes, the pre settlement sedimentation rates in Lake Michigan were generally lower than the postsettlement rates.