Petrographic Similarity of Wisconsin Tills in Marion County, Indiana

Abstract

Two to four till sheets were deposited in Marion County, Indiana by the East White sublobe of the Ontario-Erie major lobe of the Wisconsin ice sheet. Fraction analyses (mineralogy and lithology versus size) were made for 11 till samples from 4 outcrop localities near the corners of Marion County. The composition of each of 16 Wentworth size fractions (between 0 and 32 mm) was determined for each sample. True weight relationships were obtained from the particle-frequency data following experimental determination of the average grain weight for each mineral and rock type in a given size fraction. The fraction analyses were then expressed in weight percent. Correlation of till samples from the four outcrop localities was attempted according to these assumptions: (1) parts of at least one formerly continuous till sheet are present in Marion County, (2) remaining segments of this till sheet crop out at the four localities chosen for study, (3) individual till sheets are homogeneous and distinguishable one from another, and (4) a given till sheet can be ascertained by finding the group of four samples (one from each outcrop locality) that shows the highest degree of textural and (or) compositional similarity. Plotting of the fraction-analysis data on triangular diagrams indicates that the 11 till samples are too similar in texture and composition to satisfy assumption four, and it is probable that assumption 3 is invalid. The pronounced textural and compositional similarity of the till samples implies uniformity of glacial processes throughout the area of investigation and suggests but slight variation in the materials of the source area between successive glaciations.