The 2020 field team has drilled numerous continuous soil borings throughout Northeast Illinois. We've found a predictable consistency in the geologic framework published by the Illinois State Geological Survey.
However, in 2020, we discovered an entirely new and older sedimentary sequence in Geneva, Illinois.
BACKGROUND: Wisconsinan-Age Sediments
The regional upland glacial sedimentary sequence consists of the Yorkville Member of the Lemont Formation. Four facies of the Yorkville Member are generally consistent: an ice-marginal facies and three subglacial facies (informally mapped as Facies A, Facies B, and Facies C). Below the Yorkville Member is the Batestown Member which can be depositionally variable resulting with a wide range of lithologies and sedimentary properties.
Sorted proglacial sediments of the Henry Formation and/or the Equality Formation are known to occasionally occur between the different facies of Yorkville Member and Batestown Member. One general consistency is when sorted sediments are present, they occur between mapped stratigraphic units which enable us to recognize its stratigraphic position and hydrogeologic context. Without stratigraphic control/context, understanding the relationships between borings (where the sorted sediments are present) is virtually 'mission impossible'. But with stratigraphic context, it gives tremendous certainty, and we can correlate buried sands without much question.
THE DISCOVERY: Early-Wisconsin-Age Sediments
In 2020, a boring was drilled and encountered a buried A-Horizon topsoil beneath the Batestown Member. Below the buried A-Horizon was an oxidized and unleached (OU) weathering zone progressing downward into an unoxidized and unleached zone (UU). The interval included both subglacial and resedimented diamictons along with intervals of sorted sediments. Carbon-14 dating tests of the buried topsoil indicating it was Early-Wisconsin-Age at 29,660 years BP and so it was assigned stratrigraphicall to the Farmdale GeoSol.
This discovery motivated Midwest GeoSciences Group to conduct the 2022 soils workshop in Geneva, Illinois.
The benefit of being familiar with the regional stratigraphy allows field staff to discern subsurface relationships and recognize new intervals. Just classifying soil or relying solely on identifying "preferential pathways" would very likely squander the opportunity for this understanding or discovery.
HYDROGEOLOGIC IMPLICATIONS: Calculating Meaningful Vertical Seepage Rates
The hydrogeologic implications related to calculating seepage rates are significant since most of the sequence classifies as the same in the Unified Soil Classification System. Lumping all of the clayey units as a single "Hydrostratigraphic Unit" would lead to erroneous calculations because each facies within the Yorkville Member, Batestown Member, and underlying Early Wisconsinan deposits yield significantly different vertical hydraulic conductivity (Kv) values.
By differentiating stratigraphy in the field, we can not only discern subsurface relationships between buried sands when they occur, but we can measure and model vertical seepage rates with confident certainty.
