Carlock Drew C.¹, Thomason Jason F.², Malone David H.^1,, Peterson Eric W.¹

Quaternary glacial till, outwash, lake sediments, and loess compose the surficial deposits of McHenry County, Illinois. Much of the landscape of McHenry County were formed by at least three separate advances of the Harvard Sublobe of the Wisconsin Episode Lake Michigan Lobe, which was part of the Laurentide Ice Sheet. This project focuses on the delineation of the stratigraphy and extent of the Pearl-Ashmore Aquifer. The Pearl-Ashmore Aquifer is the combination of the proglacial outwash of the Wisconsin Episode Ashmore Tongue of the Henry Formation and the youngest outwash associated with the Illinois Episode, which is the Pearl Formation. A 3-D geologic model was generated from a number of different subsurface geologic data sets, including geologic borings, and municipal and private water well records. These data were initially visualized and interpreted in the 3-D environment using ESRI’s ArcScene. More than 700 wells were used to construct the model. The stratigraphic picks were imported into Petrel, and horizons were created from the surface data to complete the 3-D geologic model. Isopach maps of each unit were then created. The Pearl-Ashmore Aquifer extends through the eastern two-thirds of McHenry County. The 3-D geologic model predicts that the aquifer has an average thickness ranging from 5 to 15 m and is thickest in the north-central portion of McHenry County where it can reach thicknesses of up to 40 m.

Pearl-Ashmore Aquifer, McHenry County, IL, quaternary, Laurentide Ice Sheet, petrel