THE ANALYSIS AND TECTONIC IMPLICATIONS OF A PREVIOUSLY UNDESCRIBED NEOGENE UNIT NEAR DIXON SPRINGS, ILLINOIS

In 1997, the Illinois State Geological Survey discovered a small graben in northeast Massac County, Illinois while conducting bedrock mapping. A core taken within the graben, the Weaver core, contained a previously undescribed unit approximately 70 m thick. By examining stratigraphic relationships, clay mineralogy, petrology, seismic profile data, composition and variation within the unit a more accurate estimate of the amount of offset and timing of tectonic movement in the graben was determined. The clay mineralogy results show that the Weaver Core contains four zones of kaolinite/expandable clay (a paleosol indicator), an increase in illite content at a depth of 30.8 m (representing an Ohio Valley sediment source), and an inverse relationship between kaolinite+chlorite (K+C) and expandable clays with depth. The petrology of sand-sized material in the Weaver Core is dominated by monocrystalline quartz with variable amounts of mica. Seismic profile data showed the outline of the graben as well as aiding in estimating the total offset of the graben. Within the core, 35 separate coarsening- and fining-upward successions were identified. These successions are indicative of fluvial and deltaic environments, respectively. In order for these coarsening- and fining-upward successions to be preserved, it was necessary to create accommodation space within the graben. Based on available data, a total offset of about 117 m was determined for the graben.