SURFICIAL GEOLOGIC MAP OF THE MARSEILLES 7.5 MINUTE QUADRANGLE, LASALLE COUNTY, ILLINOIS

The Marseilles 7.5 minute Quadrangle, lies between the towns of Ottawa and Seneca, and occurs in the area between 88°45'00" and 88°37'30" west longitude, and 41°15'00" to 44°22'30" north latitude. This project is part of the long-term research investigation of the geology of LaSalle County, Illinois by Illinois State University faculty and students. This research is supported by grants from the U.S. Geological Survey EDMAP program and the Illinois State Geological Survey workforce development program. A surfical geologic map of the Marseilles Quadrangle and adjacent areas was constructed using soil maps, surficial geologic observations, borehole logs, and geophysical data.

Rock and sediment exposures are common along the Illinois River Valley, which transects the Quadrangle from east to west, and its major tributaries. The uplands adjacent to the river valley consists of comparatively high recessional moraines and comparatively low ground moraines, and consist largely of Wisconsin-age till and loess with well developed soils. The Marseilles Quadrangle occurs on the eastern flank of the LaSalle Anticlinorium. Bedrock units dip 1-2 degrees to the east.

The Pennsylvanian Carbondale Formation is the only bedrock unit exposed in the Quadrangle. It is as much as 100 m (250 ft) thick, and consists of interbedded sandstone, shale, limestone, and coal. The Vermilionville Sandstone member, which is as much as 10 m (30 ft) in thickness, is locally well exposed where it forms conspicuous cliffs and ledges. Boreholes penetrate Cambrian and Ordovician strata, which unconformably underlie the Carbondale Formation.

The Pennsylvanian strata are unconformably overlain by Quaternary sediments. Relief on this surface is complex, and is dominated by the Ticona buried bedrock valley, which occurs along the southern margin of the Quadrangle. As much as 30 m (100 ft) of the Illinoian Pearl Formation occurs within the Ticona Channel. Diamicton of the Lemont Formation (Wedron Group) overlies the Ticona Channel and Pennsylvanian strata. The Lemont Formation is covered by the Peoria Silt over much of the Quadrangle, but is commonly too thin to map. Small areas of sand and gravel of the Henry Formation also occur in the Quadrangle. Extensive sand and silt deposits of the Cahokia Formation occur along the modern Illinois River valley.