STRATIGRAPHY AND BULK PROPERTIES OF A DEPOSIT OF CLAY-RICH MINE TAILINGS FROM A CLOSED PHOSPHATE MINE NEAR BARTOW, FLORIDA

Eight geoprobe continuous push borings were obtained to investigate stratigraphy and variability in bulk and properties of a deposit of clay-rich mine tailings from a closed phosphate mine near Bartow, Florida for the purpose of long term land management. Investigations indicate that sediment is very clay rich and is commonly laminated at the millimeter scale with yellow and gray end members. Generally yellow sediment has more phosphate minerals which gray sediment has more phyllosilicate dominated minerals. Rarely coarse sand material occurs and in some instances such in a single core gravel occurs. Gravel rich units are interpreted to be debris flows form the containment walls of the impoundment. Limited stratigraphic markers of wood fragments and organic matter occur in the central portion. A kaolinite – sand liner was encountered in two borings near the impoundment wall. Moisture content in the gray clay is higher and varies from 27.9 to 57.4 percent. Standard deviation for all moisture content determinations is 9.46 indicating that there is significant variability. The average percent moisture in the grey clay is 42.0%. There is a weak correlation (r^2=0.32) with lower water contents with depth suggesting some dewatering has occurred and this may explain some of the comparatively high (>50%) water contents observed in the middle level of the unit. As water is compressed out of lower layers of the unit it migrates up into more porous clays. Organic matter occurs largely as deciduous leaves and partially decomposed wood fragments. Organic matter content varies from 1.8 to 6.5% with an average of 3.2%. Two populations are observed one which shows higher organic mater content at depth and the other larger trend which shows less organic matter at depth. The larger trend is interpreted as a decomposition trend where organic matter, which appears to dominantly be leaves, gets buried and then decomposes. The higher organic matter contents are related to period large influxes of material, particularly wood. Grain size of the materials is dominantly clay rich with approximately 80 percent of the deposit having 5% or less than >38 micrometer material. This is the first detailed stratigraphic investigation of clay rich phosphate mine tailings in the Polk county, Florida region and provides a baseline for comparison to other tailings.