BULLETIN NO. 68 Dolostone is common as well. The microfossil Helicostegina gyralis (Figure 11) is common but not unique to the unit (Miller, 1986). In the southeastern Florida peninsula, this microfossil is common within a glauconitic bed that serves as an excellent geophysical marker (Duncan et al., 1994). Within the study area, the uppermost Oldsmar Formation contains gypsum as nodules, laminations and as pore-filling material. Stewart (1966) reports "selenite impregnation" in the unit and proposes that the gypsum is altered from anhydrite. In some areas, these gypsiferous, low-permeability carbonates comprise the upper part of the MFCU, whereas deeper in the section, the carbonates comprise the upper part of the LFA. Cander (1994) suggests that the Oldsmar Formation represents a shallow subtidal to supratidal carbonate paleo- environment. Avon Park Formation The Middle Eocene Avon Park Formation (Miller, 1986) occurs in the subsurface throughout the study area and is the oldest lithostratigraphic unit exposed in Florida (Scott et al., 2001). This unit was originally described by Applin and Applin (1944) as two units, the Lake City Limestone and the Avon Park Limestone. Due to the inability (except locally) to distinguish these two formations based on lithology or fauna, Miller (1986) proposed that the term Lake City Limestone be abandoned and formalized the Avon Park Formation to include the two units of Applin and Applin (1944). Lithology of the Avon Park Formation varies between limestone and dolostone. The limestone is generally cream to light brown, poorly to well indurated, variably fossiliferous, skeletal/peloidal wackestone to grainstone with minor mudstone. The limestone can be interbedded with dark brown to tan very-fine to coarse-grained, vuggy, fossiliferous dolostones. Incomplete to complete dolomitization of limestone is also observed. Dolostone textures range from very fine-grained to coarsely recrystalized (sucrosic). Minor clay beds and organic-rich laminations may occur, especially at or near the top of the unit. Although not common, sedimentary structures include cross- beds and burrows. The burrows (Callianassa sp.) generally occur in the uppermost thinly bedded updip part of the formation (e.g., crops out in Levy County). Accessory minerals include chert, pyrite, celestine, gypsum and quartz (some as doubly-terminated euhedral crystals "floating" in vugs). Gypsum tends to be more abundant with depth. Reese and Richardson (2008) report a glauconite marker horizon that occurs intermittently within lower -200 ft (~ 61 m) of the Avon Park Formation. Porosity in the Avon Park Formation is generally intergranular in the limestone section. Fracture porosity occurs in the more densely recrystallized dolostone and intercrystalline porosity is characteristic of sucrosic textures. Pinpoint vugs and fossil molds are present to a lesser extent. Total porosity measured for 16 Avon Park Formation samples averages 31.7 percent (median = 30.0 percent) and ranges from 22.3 percent to 42.0 percent. Diagnostic fossils include the foraminifera Cushmania americana (Dictyoconus americanus) and Fallotella (Coskinolina) floridana and the echinoid Neolaganum (Peronella) dalli (Figure 12). The foraminfer Fallotella (Dictyoconus) cookei occurs in the Avon Park Formation as well as the Suwannee Limestone; however, presence of Cushmania americana is unique to the Avon Park Formation. Other fossils include algae, mollusks and carbonized plant remains (e.g., Thalassodendron sp.) (Scott, 2001). Miller (1986) reports the top of Lower Eocene rocks (the approximate base of Avon Park Formation) at depths ranging from -1,000 ft to -2,400 ft (-304.8 to -731.5 m) MSL within the study area. The Avon Park Formation varies in thickness across the study area, from 1,000 ft to 1,600 ft (304.8 to 487.7 m), with the thickest area occurring along northeastern Polk County (Miller, 1986). Maximum observed elevations exceed 50 ft (15.2 m) MSL in northern Polk County (Plate 20). In the southernmost extent of the study area, the unit is encountered at depths exceeding -1500 ft (-457.2 m) MSL (Plate 38). The area, which is centered along eastern Charlotte Harbor, may be related to structural deformation (Winston, 1996).