REGIONAL GEOLOGIC CHARACTERIZATION AND MAPPING USING COMPUTERIZED LOG INFORMATION: AN EXAMPLE FROM SOUTHWESTERN ILLINOIS

Logs of water wells and other subsurface borings are often the most basic data set available for regional mapping purposes. Stratigraphic borings and other high quality data can be rare or scattered over a broad region. The lithologic descriptions found in these logs range from detailed to ambiguous. The range in earth material descriptions can make creation of workable geologic databases difficult. This is particularly true when it is desirable to utilize the information in graphical computer applications. Over the course of several regional geologic mapping investigations, a lithologic coding method was created to categorize earth material descriptions. The units are categorized by material type. Unconsolidated deposits are classified by texture. They are then further subdivided allowing a finer differentiation of the materials. Bedrock units are classified by lithology. Once classified, a cross-sectional correlation of units of interest can be performed. This coding facilitates our computer mapping efforts and gives us greater ability to manipulate large data sets. Graphical visualization of subsurface data assists regional geologic correlation and stratigraphic interpretation.

The aquifers in three southwestern Illinois counties, Madison, Monroe and St. Clair, are being studied as part of a regional evaluation of groundwater resources. Typical of regional evaluations, thousands of records are available for study. Data is irregularly spaced and of varying worth. Analytical techniques are used to evaluate the overall quality of the subsurface well data. Factors such as location verification, depth, well type, and completeness of the drilling record can be used to limit the data set. The aquifers of the region are varied. Quaternary sands and gravels underlie the Mississippi River floodplain. On the uplands to the east, shallow and relatively discontinuous glaciofluvial and other sand and gravel deposits are found within stream valleys. Most upland areas have limited groundwater resource availability. Karstic, Mississippian-aged limestone aquifers provide small to moderate quantities of water and are present primarily in the southern half of the region. Pennsylvanian-aged bedrock units, found along the eastern half of the region, are mostly shale and typically yield very limited quantities of water.