SEDIMENTOLOGY AND 3-D ARCHITECTURE OF SUBSURFACE FACIES OF THE ILLINOIAN DEGLACIATION IN EAST-CENTRAL ILLINOIS, UNITED STATES

Determining the character and distribution of deposits correlated to the deglacial phase of the Illinoian glaciation in central Illinois led to the identification of cut-and-fill deposits that are set into important aquitard materials that provide protection to shallow groundwater resources. However, the nature and mode of this incision, which emplaced coarse and/or fine-grained sediment, informally referred to as the Glasford deglacial unit, into the underlying Illinoian-age till are poorly understood. Consequently, sediment records from continuous cores and seismic facies interpretation of near-surface seismic surveys were used to elucidate these events. The Glasford deglacial unit is found in two distinct subsurface features: 1) a buried valley filled with four deglacial sediment assemblages (V1-V4), which breaches the regional aquitard of the Vandalia Member till; and 2) an overlying tabular body consisting of three facies assemblages (A-C). Specifically, the buried valley is filled by interstratified, massive, and laminated sand (V1 and V3), as well as by laminated to massive silt and clay, and diamicton (V2 and V4). The tabular body that overlies the valley-fill consists of a highly heterogeneous package of interstratified sand (B) and diamicton (A and C), and discontinuous layers of fine-grained material (A and C).

Relatively clean sand and gravel are considered the most productive aquifers in the Glasford deglacial unit. Facies assemblages B, V1, and V3 represent discontinuous aquifer materials that are potential groundwater sources for residents of central Illinois. Overall, groundwater flow in the deglacial unit is defined by the continuity and interconnectivity of permeable units, and to a much lesser extent controlled by deposits having a wide range of hydraulic conductivities that are arranged in a complex configuration within the unit. As a result, this study improves our understanding of these geologic events, and further evaluates sedimentary assemblages found at the margins of fluctuating glacial lobes. We are now able to determine in more detail the subsurface stratigraphic geometry and hydrostratigraphy of the Glasford deglacial unit, which may affect aquifer connectivity, water supply and quality.