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

CARBONATE RAMP AND BOUNDING SILICICLASTICS TEMPORAL AND SPATIAL BOUNDARIES

Ochopee Limestone Member of the Tamiami Formation

Lithostratigraphy and Depositional Environments

The Ochopee Limestone Member of the Tamiami Formation (Hunter, 1968; Meeder, 1987; Missimer, 1992; Edwards et al., 1998; Weedman et al., 1999) includes a regionally extensive limestone facies that can be mapped throughout much of the study area (Fig. 8). The Ochopee Limestone has a sheet-like geometry that drapes over an unconformity at the top of the Peace River Formation (Figs. 5 and 6). The Ochopee Limestone represents a shift in sedimentation on the Florida Platform from the retrogradation of DS3 within the Peace River Formation to aggradation of the Ochopee Limestone. The Ochopee Limestone laps out near the southern margin of the Florida peninsula. The lapout is probably coincident with the edge of the siliciclastic shelf containing DS2 of the Peace River Formation (Fig. 6).

Figure 8. Structure contour map of the base of the Ochopee Limestone Member of the Tamiami Formation. Structure contours show altitude in feet below sea level of base of the Ochopee Limestone. [larger image]

Two lithofacies characterize the Ochopee Limestone in an area shown in Figure 1: (1) pelecypod lime rudstone or floatstone, and (2) pelecypod-rich quartz sand or sandstone (Table 6). The rudstone or floatstone facies is the most common lithofacies, whereas the sand or sandstone facies occurs only locally as thin to thick beds. The quartz sand is typically very fine to fine grained, but locally may range from silt to very coarse sand. Skeletal carbonate grains of the pelecypod lime rudstone or floatstone include fossils listed in Table 6.

The Ochopee Limestone was deposited in a carbonate ramp depositional system (Burchette and Wright, 1992) during a reduction in siliciclastic supply to much of southern Florida. Criteria to support the environmental interpretation include: (1) a low basinward depositional gradient of less than 1 degree without a break in slope, as suggested by the upper and lower lithostratigraphic boundaries (Fig. 6); (2) widespread continuity of facies patterns; and (3) an almost complete absence of internal exposure surfaces. In the study area, most of the Ochopee Limestone was deposited in a mid-ramp depositional environment (Burchette and Wright, 1992). Evidence for this depositional environment is indicated by the common occurrence of coarse-grained lime rudstone that has a well washed, grain-dominated matrix (Lucia, 1995) and limemud-rich floatstone (Table 6). The mixture of these grain-dominated and mud-dominated carbonates and the lack of shallow-water faunal indicators suggest deposition below fair-weather wave base (FWWB) but above storm wave base (SWB). The zone between FWWB and SWB defines the mid-ramp depositional environment of Burchette and Wright (1992). Planktic foraminifera-rich sandstone--similar to lithofacies of the Stock Island Formation of Cunningham et al. (1998)--between depths of 275 and 336 feet below sea level in the W-17157 corehole may represent a distal portion of the Ochopee ramp that accumulated in relatively deep sea water (Fig. 6). Although the Ochopee Limestone contains quartz sand, the overwhelming abundance of carbonate grains represents a period of reduced quartz sand, silt, and mud to the southern Florida Platform.

The benthic carbonate grains of the Ochopee Limestone represent a heterozoan particle association, which James (1997) defined as a group of carbonate particles produced by light-independent, benthic organisms that may or may not contain red calcareous algae. Red algae were not observed in the Ochopee Limestone within the study area. The predominately heterozoan assemblage of carbonate particles and an absence of shallow-marine particles, such as ooids and green algae, is consistent with deposition in a mid-ramp depositional environment with temperate bottom-water conditions. An almost complete absence of exposure surfaces within the Ochopee Limestone is also consistent with mid-ramp deposition at water depths sufficient to minimize changes in water-bottom conditions during low-amplitude changes in relative sea level.