A GIS-Based Approach to Modeling Three-Dimensional Geology of Near-Surface Glacial Morphosequences: Huntertown Formation, Northeastern Indiana

Abstract

The Huntertown Formation (Quaternary) in Allen County, Indiana, is located in a continental interlobate landscape position characterized by complex glacial stratigraphy consisting of coarse-grained proglacial sediments and loamy till interbedded with glaciofluvial and glaciolacustrine facies. The goal in this study area is to generate a three-dimensional depiction of the units represented on a traditional geologic map with emphasis on conceptual model(s) of unit relationships, position of bounding surfaces, and morphological characteristics of bounding surfaces. Because we are working in near-surface sediments (i.e., depths less than 200 feet), we are able to constrain the units using multiple data sources, such as borehole lithologic information from water well records and rotosonic cores, natural gamma-ray log data, shallow geophysical surveys, and interpreted cross sections. These data sources also provide information about units that underlie those shown on the geologic map and form the base units of the model. The model of the Huntertown Formation is being built by reconstructing each unit by building from georeferenced GIS layers representing the topography of each major bounding surface, in this case, the surface topography and the top of the overconsolidated glacial till of the Trafalgar Formation. The two-dimensional geologic map guides the horizontal shape of each unit, whereas the morphology on the bottom surface of the model guides the initial vertical placement of the units, and the thickness and position of each unit is determined by the many data sources in our database. Subsurface unit shape and geometry are governed by the conceptual model or interpreted unit relationships (e.g., onlapping, offlapping, interbedded, and so on) in areas with sparse data. A team approach that utilizes geological expertise is useful to provide interpretations where there are gaps in other data sources. The model is being calibrated by supplemental descriptions of characteristics regarding distribution, thickness, position, and geometry of units; well-log and gamma-log interpretations, and georeferenced interpreted cross-sections. Validation of the model will be conducted by statistically analyzing the position and thickness of borehole lithologic units that intersect the reconstructed geologic units in the model.