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

CARBONATE RAMP AND BOUNDING SILICICLASTICS TEMPORAL AND SPATIAL BOUNDARIES

Peace River Formation

Sequence Stratigraphy

In developing a regional sequence stratigraphy, it is common practice to initially identify the more easily recognized condensed sections of unconformity-bound depositional sequences (Posamentier and James, 1993). In prior studies of the Peace River Formation and equivalent sediments in southern Florida, only bounding unconformities have been proposed (Missimer, 1997; Guertin, 1998; Guertin et al., 1999; Missimer, 1999).

Figure 5. Geologic cross-section A-A' showing lithostratigraphy, sequence stratigraphy, and biostratigraphy of the upper Tertiary of southern Florida. An additional condensed section and possible depositional sequence may occur between DS3 and DS4 in wells W-9114 and W-9104. The location of the lower boundary of DS4 in wells W-9114 and W-9104 above the lower lithostratigraphic boundary of the Ochopee Limestone Member of the Tamiami Formation may be due to poor samples. Portions of some sequence boundaries are equivalent to the parasequence concepts of shoaling-upward cycles bounded by flooding surfaces (Van Wagoner et al., 1988). All lithologic descriptions were taken from continuous cores, except for wells marked by an asterisk which were taken from well cuttings. Location of cross section shown in Figure 1. [larger image]

In the proposed southern Florida sequence stratigraphy for this study, downdip portions of some sequence boundaries are equivalent to the parasequence concept of shoaling-upward cycles bounded by flooding surfaces (Van Wagoner et al., 1988) instead of unconformities. The framework herein provides guidance for further investigation into recognition of unconformities and a more precise definition of sequence boundaries. Additionally, two newly identified condensed sections of the Peace River Formation are placed into the established framework of unconformities. A condensed section is a relatively thin marine stratigraphic unit composed of pelagic to hemipelagic sediments that accumulated at very low sedimentation rates (Loutit et al., 1988). Condensed sections are important for biostratigraphic dating, defining and correlating depositional sequences, and reconstructing depositional environments (Loutit et al., 1988; Posamentier and James, 1993). In much of the study area, the distinct lithology of the condensed sections facilitates their recognition in the context of the proposed developing sequence stratigraphy.

The diatomaceous mudstone that forms the two condensed sections of the Peace River Formation (Figs. 5 and 6) contains a greater concentration of planktic fossils than overlying terrigenous mudstone, suggesting the upper surface of the diatomaceous mudstone defines the surface of maximum flooding within each couplet. The maximum flooding surface represents a time of maximum flooding within a depositional sequence, and marks the change from a transgressive systems tract to a highstand systems tract (Van Wagoner et al., 1988; Posamentier and James, 1993).

Depositional sequence 1 (DS1) is a wedge-shaped deposit of quartz sand, sandstone, and minor carbonate that thins toward the southern and eastern edges of the Florida peninsula. This depositional sequence laps out north of the W-17273 corehole in Miami-Dade County toward the southern edge of the Florida peninsula (Fig. 6). Downlap of internal strata onto the top of the Arcadia Formation is suggested by correlations shown in Figure 5. The southern lap out thinning toward the east and probable downlap to the east suggest progradation of a siliciclastic shelf toward the east and south (Figs. 5 and 6). A shelf-margin break is postulated to occur between the W-17969 and W-17273 cores (Fig. 6).

Figure 6. Geologic cross-section B-B' showing lithostratigraphy, sequence stratigraphy, and biostratigraphy of the upper Tertiary of southern Florida. Southern lap out of depositional sequence DS1 and condensed section CS2 within the Peace River Formation and southern lap out of the Ochopee Limestone Member of the Tamiami Formation are shown. Depositional Intervals I, II, and III from Guertin et al. (1999). Portions of some sequence boundaries are equivalent to the parasequence concepts of shoaling upward cycles bounded by flooding surfaces (Van Wagoner et al., 1988). All lithologic descriptions were taken from continuous coreholes, for wells marked by an asterisk which were taken from reverse-air core samples. Location of cross section shown in Figure 1. [larger image]

The bottom of DS1 is delimited by a regional unconformity that separates the Peace River Formation from the Arcadia Formation. This unconformity in southern Florida represents a hiatus of about 1.6 to 11.5 million years based mostly on strontium-isotope chemostratigraphy (Guertin et al., 2000). An unconformity, with local evidence of subaerial exposure (discussed later) defines the top of DS1 in the western part of the study area (Fig. 5). In the east and southeast, the base of a condensed section (CS2) delineates the top of DS1 (Figs. 5 and 6). A sequence stratigraphy produced by Missimer (1999) at the W-17115 corehole in Collier County (Fig. 5) was linked to the sequence stratigraphy developed here, suggesting that DS1 is equivalent to a supersequence defined within the lower Peace River Formation (Fig. 7) by Missimer (1999).

Depositional sequences 2 and 3 (DS2 and DS3) contain coarsening upward siliciclastic deposits defined by mudstone (CS2 and CS3) at the base that grades upward into mostly very fine to fine quartz sand and sandstone (Figs. 5 and 6). Depositional sequence 2 (DS2) has a profile in Figure 6 that thins landward, thickens as fill along the marginal slope of DS1 and thins seaward in cross-section B-B’ (Fig. 6). The profile in Figure 5 shows landward thinning of this unit in a sheet like geometry. Most of the base of DS2 is delimited as the base of CS2 (Figs. 5 and 6). Quartz sands in the W-17273 and GB2 coreholes (Fig. 6) form an early transgressive deposit at the base of DS2 that is consistent with palynomorph data presented by Cunningham et al. (1998). These sands and the diatomaceous mudstone of CS2 form the transgressive systems tract of DS2.

Depositional sequence 2 (DS2) is probably equivalent to Interval I of the Long Key Formation (Guertin et al., 1999) in the Florida Keys. This depositional sequence is bounded at the top by an unconformity identified in southernmost Florida by Guertin et al. (1999) at the top of Interval I of the Long Key Formation (Fig. 6). This unconformity is probably regional in extent and may merge to form an amalgamated unconformity with the top of DS1 (Fig. 5). The limits of CS2 in Figure 3 and the cross sections shown in Figures 5 and 6 suggest that southward transport of quartz sand to the Florida Keys during deposition of DS2 was mostly along the southeastern coast of Florida.

Depositional sequence 3 (DS3) is the youngest depositional sequence defined within the Peace River Formation in the study area. The condensed section (CS3) of DS3 seems to be restricted to an area in Martin and Palm Beach Counties and possibly an area mostly contained in Lee County (Fig. 4). Depositional sequence 3 (DS3) contains CS3 as the basal unit in most of the area, and is partly overlain by the Tamiami Formation (Fig. 5). The PB-1703 corehole (Fig. 1) contains an abrupt contact that may be an erosion/truncation surface and may define an unconformity at the base of DS3 (Fig. 5). This potential unconformity may become conformable down dip at the base of CS3 (Fig. 5). An unconformity has not been identified at the top of DS3 in Martin and Palm Beach Counties. An unconformity that bounds the upper Peace River Formation of early Pliocene age in southwestern Florida (Missimer, 1997; 1999) is a depositional sequence possibly equivalent to DS3 based on similar age (Fig. 7).