A PILOT STUDY UTILIZING ONE-DIMENSIONAL CONSOLIDATION TESTS TO ESTIMATE THE ORIGINAL THICKNESS OF THE UPLAND COMPLEX IN DESOTO COUNTY, MISSISSIPPI

The Upland Complex (Lafayette gravel) is interpreted to be a basal sand and gravel facies of the Pliocene ancestral Mississippi/Ohio River system. In this interpretation, an unknown amount of Pliocene flood plain sand and clay used to overlie the basal sand and gravel facies. Previous studies have suggested that the late Pliocene and Pleistocene entrenchment of the Mississippi/Ohio river system would have reduced the vertical stress and may have initiated the New Madrid seismic zone. However, to quantify the amount of denudation and change in the vertical stress due to erosional removal of the Upland Complex, the maximum original thickness of the Upland Complex must be determined.

Five meters of loess overlies the Upland Complex in a sand and gravel quarry in central Desoto County, Mississippi. At this quarry, a clay bed within the Upland Complex was sampled at a depth of 7 m below ground surface and 5.5 m above the base of the Upland Complex using the block sampling method. The purpose of sampling and subsequent consolidation testing of the clay was to estimate the maximum burial depth the clay had experienced since its deposition. Utilizing an oedometer, consolidation tests were performed on two clay specimens to estimate the pre-consolidation stress (past maximum vertical effective stress) that the clay endured since deposition. A value of 470 k Pa for the preconsolidation stress was estimated using the Casagrande method. A preconsolidation stress of 470 k Pa indicates that the clay was under a vertical stress equivalent to approximately 30 m of sediment at some time since its deposition. Therefore, at this locality, the original total thickness Upland Complex is estimated to have been 35 m. Due to the limitations of this particular oedometer test, our pre-consolidation stresses are considered to be a minimum value and thus the thickness of the Upland Complex may have been greater than 35 m when originally deposited. This analysis supports the interpretation that the Upland Complex is the basal facies of a Pliocene flood plain, the Pliocene flood plain was at least as thick as the modern Mississippi River, and thus the Pliocene Mississippi/Ohio River was similar in size to today’s Mississippi River.