VARIATONS OF LAURENTIAN PROVENANCE RECORDED IN THE UPPER CAMBRIAN MOUNT SIMON SANDSTONE AND EAU CLAIRE FORMATION OF THE ILLINOIS BASIN

The Upper Cambrian Mount Simon Sandstone and overlying Eau Claire Formation of the Illinois Basin provide a detrital record of Paleozoic sedimentary transport within an ancient Laurentian marine embayment. Located above 1355 – 2800 Ma Proterozoic basement terranes, and between the Transcontinental arch and 950 – 1300 Ma Grenville province, these strata were deposited in a key location to record detritus shed from central and northern Laurentian sources. Utilizing detrital zircon geochronology, thin section petrography, well log and core analysis, this study investigates fluctuations in the contributions of Laurentian source terranes, characterizes facies architecture, and determines relationships these factors have on detrital composition across the Illinois basin. Data for these analyses originates from 18 well logs, 5 cores, and 120 thin sections taken from subsurface wells in Illinois and Indiana. Core samples yield 955 U-Pb ages (N=10), with populations at 906-1792 Ma, 1819-1949 Ma and 2622-2883 Ma. Core and well log analysis establish 11 lithofacies indicative of a shift in depositional environments from deltaic to marine. Compositional analysis reveals an average modal composition of Q₈₆F₁₃L₁. We test the hypothesis that compositional changes across the Illinois Basin correlate with shifts in lithofacies trends and detrital zircon ages. The primary observations driving this hypothesis are: 1. Across-basin changes in detrital composition, 2. depleted Grenville signals through time, and 3. fluctuations in depositional environment. We argue Late Cambrian sea level encroachment into the ancient Illinois basin obscured source terranes, particularly the Grenville, which contributed to the observed compositional fluctuations.