FLORIDA GEOLOGICAL SURVEY Figure 16. Assemblage of typical Bone Valley Member fossils. Clockwise from upper left: ray spines, shark vertebra, shark teeth, horse teeth, alligator tooth in matrix, (nickel for scale) dugong rib, mammoth tooth and bone fragment. Background is a slab of Avon Park Formation dolostone with Thalassodendron sp. carbonized impressions. (Photo credit: Jon Arthur, FDEP-FGS). Limestone is locally absent (Plate 39). Delineation of the Peace River Arcadia Formation contact is problematic in some localities. In many cores, the two units appear to be conformable, with phosphate-rich siliciclastics grading with depth to more siliciclastic-interbedded carbonates containing generally finer-grained and less abundant phosphorite. Thickness of this transition zone may exceed tens of feet. With increasing depth, Arcadia Formation lithologies become more dominant. In such cases, the lower contact of the Peace River Formation is estimated based on sedimentary structures as well as a best approximation of where the overall lithologic sequence becomes more carbonate dominant. In contrast to the locally gradational contacts, other areas provide strong evidence of an unconformity, where a phosphatic rubble zone occurs at the base of the Peace River Formation (Scott, 1988). Post-Pliocene/Miocene sediments disconformably overlying the Peace River Formation in the study area are comprised of fossiliferous sands, clays and shell beds with variable amounts of limestone and reworked phosphorite (e.g., Plate 11, W-14389 [ROMP 76] and Plate 13, W- 16576 [ROMP DV-1]). The contact of these sediments with the Peace River Formation can be difficult to determine because of lithologic similarities (e.g., clays and phosphorite), especially where the uppermost beds of the Peace River Formation have been leached by groundwater, giving the sediments an appearance similar to that of some post- Hawthorn Group lithologies. In addition, it can be very difficult to distinguish Peace River Formation sediments from those of reworked Peace River sediments (e.g., post-Hawthorn Group undifferentiated sands and clays) when studying cores (the distinction is extremely difficult to impossible when evaluating cuttings).