Deglacial History and Paleoenvironments of Northeastern Illinois

WISCONSINARCHSandwich Fault Zone.DesPlainesdisturbanceanticlinesynclinemonoclinefault, ticks ondownthrown sidestructure caused bymeteorite impactPeru MonoclineILINAnticlinoriumLa SalleKANKAKEE ARCH0020 40 60 mi40 80 kmNFigure 7 Major structural features associatedwith the bedrock of northeastern Illinois (fromNelson, 1995).and Graese (1983). Although we will not see outcrops of this unit, it was readily comminuted byglaciers to silt and clay-sized particles. These fines were entrained in the glacial bed, later to bedeposited as till, or redeposited as supraglacial or paraglacial debris, such as proglacial lakesediment.Silurian-age rocks at or near the bedrock surface in northeastern Illinois primarily are composedof light gray, fine-grained dolomite and limestone with occasional wavy, greenish gray shale laminae.The oldest Silurian unit is typically the Elwood Formation, which is a slightly argillaceous,light gray, fine-grained dolomite; chert nodules and layers as much as 7 cm thick are presentin most places. This unit is about 7 to 10 m thick where it has not been eroded. The ElwoodFormation grades upward into the non-cherty, relatively pure dolomite of the Kankakee, Joliet,Sugar Run, and Racine formations with a combined thickness of about 115 m (Kolata, 2005).Devonian-age rocks have recently been discovered in Illinois near the Indiana border, but theywill not be seen on the field trip. Soft, easily eroded Devonian black shale subcrop beneath lakesediment and glacial drift in the Lake Michigan basin (Wold et al., 1981), and comprise as muchas 20% of the coarse sand fraction of some grey till units (Willman and Frye, 1970). Microscopicamber discs (spores) of the Devonian green algae Tasmanites are resilient to glacial comminutionand were concentrated in fine-grained diamicton and proglacial lake sediment.Pennsylvanian-age shale, siltstone, sandstone, and coal occur along the Illinois River valley inthe southern part of the study area. Here, the Pennsylvanian Tradewater and Carbondale Formationsoccur above an angular unconformity developed in Ordovician and Cambrian rocks thatform the northern part of the La Salle Anticlinorium (Kolata, 2005; Figure 6).Bedrock TopographyThe regional bedrock surface is characterized by gently sloping highlands cut by steep-walledbedrock valleys (Figure 8). Elevations of the highest bedrock uplands in the study region rangefrom about 210 to 250 m, and the lowest elevations, between about 120 to 135 m. Fluvial andglaciofluvial sand and gravel deposited in the bedrock valleys serve as important aquifers in thewestern suburbs that are reliant on groundwater, such as in McHenry and Kane county. TheTroy, Rock, and St. Charles bedrock valleys are the largest buried bedrock valleys in this region(Figure 8). Maximum relief of the bedrock surface in the area is about 110 m. The overall slopeof the bedrock valley floors is gradual. For example, the slope of the elevation of the St. Charles11