Abstract Introduction Background Methods >Seismic Sequences > SS1 - SS2 - SS3 Discussion Conclusions & Acknowledgments References Tables PDF Version

Below are descriptions and interpretations of the seismic character, lithology, and chronostratigraphy of three seismic sequences (SS1-SS3; Fig. 1, Fig. 2). Upper boundaries to seismic-sequences SS1 and SS2 correspond to unconformities identified in cores (Fig. 1, Fig. 3, Fig. 4, Fig. 5). Also included are GPR data collected north of the Caloosahatchee River at the Indian Mound site (Fig. 1B) that show reflections within the upper part of SS2 and possibly within SS3.

Seismic Data.-The seismic- reflection characteristics of SS1 include a subhorizontal parallel configuration with an even to broad-wavy pattern (Fig. 3, Fig. 4). In the area of the W-18071 corehole (Fig. 1), reflections have a parallel high-amplitude wavy pattern that may be related to the karst induced collapse of more deeply buried limestone strata. Near the W-18069 corehole (Fig. 3), the uppermost seismic reflections of SS1 are truncated, which is interpreted to represent an erosional event and development of a major seismic-sequence boundary between SS1 and SS2. Reflections of SS2 downlap onto the top of SS1 over much of the SS1-SS2 seismicsequence boundary (Fig. 1, Fig. 3, Fig. 4).

FIG. 2. (right) Relationship between seismic and lithostratigraphic nomenclature, sequence chronostratigraphy, and eustatic curves (Haq et al. 1988). The light and dark gray shaded areas correspond to the Late Miocene(?) to early Early Pliocene and younger Early Pliocene portions, respectively, of the deltaic depositional system shown in Figure 1. [larger image]

Core Data.-The lithology of SS1 includes dolosilt, lime mudstone, pelecypod floatstone and rudstone, bryozoan floatstone, quartz sand with abundant phosphorite grains, and mudstone, and matches that described from the lower Peace River and Arcadia Formations by Missimer (1997), who interpreted these formations to represent a broad, aggradational, mixed carbonate-siliciclastic ramp depositional system. This is consistent with region-wide SS1 subhorizontal parallel reflections that are typically exhibited in the seismic profiles (Fig. 3, Fig. 4). We believe the SS1-SS2 seismicsequence boundary is an erosional unconformity that can be observed in cores W-18069 and W-18070 (Fig. 6A), and corresponds to the sequence boundary that separates the lower and upper Peace River Formations (Missimer 1997; Guertin et al. 2000).

FIG. 3. (above) A) Uninterpreted reflection-seismic profile along coreholes W-18069 and W-18070. B) Interpreted reflection-seismic profile along coreholes W- 18069 and W- 18070. The oblique prograding reflections assigned to the upper Peace River Formation downlap onto a major seismic-sequence boundary that formed by erosional truncation along the break in slope of the distally steepened lower Peace River ramp. The parallel reflections that lie below the unconformity are assigned to the lower Peace River Formation and confirmed by corehole data. Near the W-18070 corehole, clinoformal beds of the upper Peace River Formation probably crop out at the river bottom. To aid visualization, segments of the seismic data showing poor quality are blanked out. [click on images above for larger versions]

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FIG. 4. (above) A) Uninterpreted reflection-seismic profile along corehole W-18075. B) Interpreted reflection-seismic profile along corehole W-18075. A thick section of oblique progradational reflections are assigned to the upper Peace River Formation with overlying subhorizontal reflectors assigned to the Tamiami Formation. The top of the SS2 is well defined by truncated reflections. [click on images above for larger versions]

Strontium-isotope chemostratigraphy delineated for the W-18069 core indicates a range in age between 24.31 and 18.39 Ma for the Arcadia Formation (Fig. 2, Fig. 5; Table 1), which is consistent with age determinations by Missimer (1997) and Guertin et al. (2000). Chronostratigraphic results for the lower Peace River Formation indicate an age range between 12.81 and 8.6 Ma (Fig. 2, Fig. 5), which is somewhat wider than that (~ 11 to ~ 8.5 Ma) reported by Missimer (1999). Coccoliths from the lower Peace River Formation in the W-18070 core are assigned to Zone CN8 (Fig. 5, Fig. 7; Table 2), suggesting an age of 9.4-8.6 Ma or early Late Miocene, whereas strontium-isotope chemostratigraphic data from the same core produced an age range between 12.81 and 10.44 Ma (Fig. 2, Fig. 5; Table 1).

FIG. 5. Geologic cross section A-A' (Fig. 1B) showing new coreholes and one pre-study well drilled with conventional roller- bit (W-17591) used in calibration of seismic data collected in the Caloosahatchee River. Chronologic results of strontium-isotope chemostratigraphy and micropaleontology are shown. FO = first occurrence. Cunningham et al. (2001b) interpreted the top of the upper contact of the lower Peace River Formation somewhat higher in elevation than that shown here in the W-18075 core. [larger image]