An Integrated Architectural Analysis Approach to Three-Dimensional Geologic Mapping of the Huntertown Aquifer System In Northeastern Indiana

A three-dimensional geologic model of the Huntertown glacial aquifer system is being developed. Permeable hydrostratigraphic units within the Huntertown Formation (Quaternary) comprise the principal ground-water resource for an expanding population in northern Allen County, Indiana. The Huntertown aquifer system lies predominantly between the less permeable Lagro and Trafalgar Formations. The Huntertown Formation and corresponding aquifer system are located in an interlobate setting characterized by complex glacial stratigraphy, consisting of coarse-grained proglacial sediments and loamy till intercalated with glaciofluvial and glaciolacustrine facies.

To construct the model, a database of more than 2,500 lithologic logs from public and private water supply wells and more than 200 down-hole natural gamma-ray logs was compiled for the study area (~500 km²). The lithologic logs were plotted to construct 110 hand-drawn cross sections (total length of about 900 km) that were used to map the bounding surfaces of the three formations (high-order architectural elements) as well as to constrain the scale and geometry of intratill sand and gravel aquifer units. The base of the Lagro Formation was determined from well logs by a shift from massive clays to loam-textured sediments or by the presence of laterally and vertically extensive underlying sand and gravel units. A prominent shift toward higher counts in natural gamma-ray profiles was also used to determine the base of the Lagro. The top of the Trafalgar Formation was defined by a ubiquitously present overlying outwash package depicted in gamma-ray logs or by lithologic descriptions reflecting the Trafalgar's overconsolidated nature. Previously mapped areal extents of hydrogeologic facies are currently being used in conjunction with both lithologic and natural gamma-ray cross sections, rendered at various orientations across the study area, to determine the geometry of individual morphosequences or lower-order architectural elements (e.g., ice marginal fans and outwash channels).