SEQUENCE STRATIGRAPHY OF A SOUTH FLORIDA CARBONATE RAMP AND BOUNDING SILICICLASTICS (LATE MIOCENE-PLIOCENE)

SUMMARY OF DEPOSITIONAL TIMING

Peace River Formation

Established chronologic data (Cunningham et al., 1998; Edwards et al., 1998; Guertin et al., 1999; Missimer, 1999; Weedman et al., 1999) and the new biochronology of this study indicate that the Tortonian and Zanclean ages bracket deposition of the Peace River Formation. These chronologic data allow constraints to be placed on the ages of DS1, DS2, and DS3. In southwestern Florida, Missimer (1999) divided the Peace River Formation into one supersequence (lower Peace River Formation) and one depositional sequence (upper Peace River Formation). Deposition of the lower Peace River Formation of Missimer (1999) occurred between the intervals of 11 and 8.5 Ma (Tortonian age) and deposition of the upper Peace River Formation of Missimer (1999) was between 5.2 and 4.3 Ma (Zanclean age).

Biostratigraphic results presented herein indicate that terrigenous mudstones from the base of DS1 of the Peace River Formation in Palm Beach County probably can be assigned to Zone CN8 (Tortonian age). The boundaries of Zone CN8 are 9.4 and 8.6 Ma (Fig. 7). Micropaleontologic results show that deposition of CS2 of the Peace River Formation occurred from late Tortonian and Messinian age. Micropaleontologic results also suggest that CS2 is, at most, 7.2 Ma and likely no younger than 5.6 Ma; however, results from Cunningham et al. (1999) suggest an age ranging between 7.44 and 6.83 Ma.

Missimer (1999) reports a hiatus in deposition of the Peace River Formation between 8.5 and about 5.2 Ma in southwestern Florida--an interval in time that brackets deposition of CS2 in southeastern Florida. Edwards et al. (1998) indicate that the unnamed formation in western Collier County, which is equivalent to the Peace River Formation for the present study, ranges in age from 9.5 to 5.7 Ma based on strontium-isotope chemostratigraphy, but biostratigraphic data suggest it may be as young as Pliocene. Weedman et al. (1999) produced similar results for the Peace River Formation and unnamed formation in eastern Collier and northern Monroe Counties, which are equivalent to the Peace River Formation for the study herein. Weedman et al. (1999) report a late Miocene age for the Peace River Formation based on dinocysts and strontium-isotope chemostratigraphy, and an age for the unnamed formation ranging between 6.9 and 4.6 Ma (late Miocene to Pliocene) based on strontium-isotope chemostratigraphy. Coccolith data from the condensed section of DS3 and the age of overlying mudstones in well W-9104 con-strain the age of DS3 to range from 5.23 to 3.83 Ma.

Depositional sequence 1 (DS1) correlates to the lower Peace River Formation of Missimer (1997; 1999) where the data from the present study are linked to data from Missimer at the W- 17115 corehole (Figs. 5 and 7). Data presented by Edwards et al. (1998) and Weedman et al. (1999) are consistent with deposition of DS1 during the Tortonian and Messinian ages (11.2- 5.32 Ma). Results herein suggest that the age of DS1 is probably at most 11 Ma and no younger than 7.2 Ma, the probable maximum age of CS2. Biostratigraphic data from CS2 and CS3 are consistent with deposition of DS2 during the latest Tortonian and Messinian ages (Fig. 7). Interval I of the Long Key Formation in the Florida Keys (Guertin et al., 1999) is probably equivalent to DS2 (Fig. 7). Guertin et al. (1999) assign Interval I to the Messinian age, suggesting that Interval I may be equivalent to the upper portion of DS2 that occurs beneath the Florida peninsula (Fig. 7). Deposition of the upper Peace River Formation of Missimer (1999) in southwestern Florida may be coincident with DS3 in southeastern Florida as suggested by an early Pliocene age for the upper Peace River Formation (Fig. 7).

Ochopee Limestone Member of the Tamiami Formation

Results presented herein suggest that the Ochopee Limestone or DS3 was deposited during a time spanning the early-late Pliocene boundary and during the eustatic cycle TB3.6 of Haq et al. (1988) as shown in Figure 6. Coccolith data from the base of DS4 in well W-9104 are consistent with assignment to Sub zone CN12aA (3.83-3.62 Ma) as shown in Figures 5 and 6. Cunningham et al. (2000, in press) used silicoflagellate and coccolith data to determine the age of the lower boundary of the Tamiami Formation to be near the early-late Pliocene boundary in the W-18074 and W-18075 coreholes in Glades County (Fig. 1). Cunningham et al. (2000, in press) also show a regional-scale seismic sequence boundary at the contact between the Peace River Formation and the Tamiami Formation. The Tamiami ages at well W-9104 and the two coreholes (W-18074 and W-18075) in Glades County are consistent with determination by Missimer (1999) that deposition of the Tamiami Formation began about 0.2 million years after the Peace River Formation at 4.3 Ma or Tamiami deposition began at about 4.1 Ma. Edwards et al. (1998) and Weedman et al. (1999) determined the Ochopee Limestone was most likely deposited during the early Pliocene, but the margin of error spans the late Miocene to late Pliocene age. A distinctive molluscan assemblage in several coreholes indicates an age for the Ochopee Limestone near the early-late Pliocene boundary (Edwards et al., 1988).

Age determinations of Edwards et al. (1998), Weedman et al. (1999), and Missimer (1999), and correlations for the present study suggest that deposition of the Ochopee Limestone was coincident with deposition of the lower portion of Interval II of the Long Key Formation (Fig. 6). Foraminiferal sandstone beds occurring at the base of Interval II are composed of a lithofacies characteristic of the Stock Island Formation (Cunningham et al., 1998), and may represent a distal portion of the Ochopee Limestone ramp (Fig. 6).

Unnamed Sand

The unnamed sand was probably deposited during the late Pliocene based on age determinations for DS4 (Fig. 6). Correlations shown in Figure 6 suggest that the unnamed sand is coincident with deposition of the middle and upper parts of Interval II and all of Interval III (Guertin et al., 1999) within the Long Key Formation.