Excerpt from Cretaceous Deposits and the Illinoian Glacial Boundary in Western Illinois

The report is concerned with two major items. First, the stratigraphy of a sequence of sand, clay, and gravel that underlies glacial deposits and overlies Pennsylvanian and Mississippian rocks and is here assigned to the Cretaceous. These deposits extend the area of Cretaceous deposition about 200 miles east of the previous limit in western Iowa. In addition, their presence raises questions about the age of the upland erosion surfaces and of other isolated deposits of similar character. The second item is the need for revision of the Pleistocene stratigraphy and mapping. The Illinoian glacial boundary was found to be more than 15 miles northeast from its formerly mapped position.

The area studied, primarily in Adams and Pike Counties, is shown in fig ure 1. The map shows Cretaceous deposits in an area of 80 to 100 square miles, 59 localities where samples were collected, the locations of 8 measured sections included in this report, and the revised position of the Illinoian glacial boundary. The location and stratigraphic position of all samples used are given in table 1 at the end of the report.

It was impractical to study all size fractions, and mineral analyses were made of one fraction of sand (62 - 250 microns) and of clay (less than 2 microns). This sand fraction was selected because it had been used in previous studies of Pleistocene deposits throughout Illinois (frye, Willman, and Glass, 1960; Frye, Glass, and Willman, 1962; Willman, Glass, and Frye, 1963) and is the coarsest size that is common to most samples throughout the range of materials under con sideration. In this sand fraction, both the heavy and light minerals were indenti fied using the Optical microscope. The samples of sand were acid treated, sieved, and heavy minerals separated from the 62-250 micron fraction using bromoform. The heavy and light fractions were mounted separately in balsam on slides and the percentages determined by counting 200 grains. Light and heavy mineral analyses are given in table 2 and are summarized in table 4.

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