Palynology and Sedimentology of Holocene Deposits in Southern Lake Michigan

Abstract

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.