Abstract Introduction Background Methods Seismic Sequences >Discussion > Depositional Events - Siliciclastic Conveyor - Siliciclastic Influence Conclusions & Acknowledgments References Tables PDF Version

Three seismic sequences (SS1-SS3) imaged beneath the Caloosahatchee River and data from six new coreholes document a Middle to Late Cenozoic (1) cessation of carbonate-ramp sedimentation on the southeastern part of the Florida Platform, (2) subaerial erosion of the ramp, (3) flooding of the exposure surface by an approximate 100 m rise in relative sea level, (4) filling of resulting accommodation by siliciclastics, and (5) return to carbonate-platform growth. SS1 delineates an aggradational, mixed carbonate-siliciclastic ramp of Late Oligocene to early Late Miocene age comprising the Arcadia Formation and lower Peace River Formation (Fig. 2, Fig. 9A). The Arcadia Formation was deposited during the Late Oligocene to the Middle Miocene in south Florida (Guertin et al. 2000), and its upper surface is bounded by a regional unconformity (Fig. 10A). We speculate that this unconformity is equivalent to the "Middle Miocene unconformity" Mullins et al. (1988) observed on the margin of the west Florida carbonate platform. Development of this unconformity has been attributed to an increase in flow velocity and volume of paleoceanographic currents (Loop Current) flowing across the Florida Platform, which resulted in truncation of seaward-prograding clinoforms at the west Florida margin. Intensification of these currents has been attributed to tectonic restriction of the Isthmus of Panama, climatic change, or both (Mullins et al. 1987). As previously proposed by Guertin et al. (2000), we conclude that this current influenced development of the unconformity bounding the top of the Arcadia Formation in southern peninsular Florida. At the Caloosahatchee River study area, Middle Miocene eustatic highstands (Fig. 2) may have also contributed to shutting down Arcadia Formation carbonate deposition by drowning of the platform top (Schlager 1981).

Deposition of the lower Peace River Formation likely corresponds to the initial aggradation of the "sequence I" carbonate ramp of Mullins et al. (1988) at the west Florida margin. Stratigraphic evidence presented in Guertin et al. (2000) and Cunningham et al. (2001a) indicates that the lower Peace River Formation backsteps northward of the southern limit of the Arcadia Formation at the southern edge of the Florida Platform (Fig. 10B). This backstepping is probably related to recovery of carbonate deposition following Middle Miocene drowning and erosion of the Arcadia Formation ramp. The erosional truncation of reflections at the top of SS1 in the vicinity of the W-18069 and W-18070 coreholes (Fig. 3) is interpreted to be related to subaerial exposure, with collapse of the upper surface of SS1 in the area of the W-18071 corehole (Fig. 1, Fig. 9A) probably related to karst development. Thus the relatively steeply sloping plateau-to-valley erosional transition shown in Figure 9A is suggested to be related to subaerial erosion, but, this erosion may have only enhanced a distally steepened break in slope (cf. Read 1985) of the lower Peace River ramp, which shows a maximum apparent dip of 3° (Fig. 1C, Fig. 3, Fig. 5, Fig. 9A). This major break in slope is shown in Figure 1C, Figure 3, Figure 5, and Figure 9A, where west of the W-18069 corehole there is a high ratio of carbonate rock to mudstone in the lower Peace River Formation of the W-18071, W-17591, and W-18070 coreholes, but at the W-18069 corehole along the ramp break in slope there is a low ratio of carbonate rock to mudstone in the lower Peace River Formation (Fig. 5). The low ratio at the W-18069 core is probably related to the occurrence of the break in slope of the lower Peace River ramp seaward of high-energy, shallow-marine environments. Perhaps the interpreted karst at the top of SS1 is related to the Late Miocene karst surface at the top of "sequence ID" of Mullins et al. (1988) on the west Florida margin. However, our geochronological data suggests roughly a 2.5 Myr difference in surface ages, whereas the upper surface of "sequence ID" of Mullins et al. (1988) was keyed interpretively into the major eustatic lowstand at the end of TB2.6 eustatic cycle of Haq et al. (1988) and not by precise age dating.

FIG. 9. Reconstruction of the depositional history of the Caloosahatchee River study area. A) SS1, consisting of the Arcadia and lower Peace River Formations. B) Late Miocene(?) to Early Pliocene portion of SS2 or upper Peace River Formation. C) Early Pliocene portion of SS2 or upper Peace River Formation. D) Latest Early Pliocene to Late Pliocene portion of SS3. [click on images above for larger versions]

Initial westward and eastward accretion of the siliciclastic clinothems of SS2 over the lower Peace River Formation occurred in the paleo-valley shown in Figure 9A. Deposition is interpreted to have been initiated after a rise in relative sea level of about 100 m (vertical distance from the toes of larger clinoforms to tops, Fig. 1, Fig. 3, Fig. 9B) following exposure of the top of SS1. This interpretation is consistent with the vertical succession of benthic foraminifers in the Caloosahatchee cores that indicate shallowing upward of the SS2 siliciclastic clinoforms from outer-shelf to inner-shelf and possibly lagoon environments. The Late Miocene(?) to early Early Pliocene siliciclastics of SS2 (Fig. 1C, Fig. 9B) are probably correlative, at least in part, to Peace River siliciclastics (DS2 and possibly DS1 of Cunningham et al. 2001a) that extend over a broad area of south Florida (Fig. 10C; Cunningham et al. 2001a). Deposition of the SS2 siliciclastic clinoforms continued into the Pliocene and spilled onto the paleo-plateau shown in Figure 9A after accommodation was filled to the east (Fig. 9C). These Early Pliocene siliciclastics (Fig. 2) are contemporaneous with a siliciclastic depositional sequence (DS3) mapped on the eastern Florida peninsula by Cunningham et al. (2001a) and the upper Peace River Formation of Missimer (1999) in southwestern Florida, which suggests their rather limited extent depicted in Figure 10D. The backstepping of later SS2 siliciclastic deposition as shown in Figure 10D may be related to highstand sea-level events during the Early Pliocene (Fig. 2) or possibly related to diversion of fluvial drainage in the area of the Gulf Trough.

The SS3 sequence or Tamiami Formation indicates that minor carbonate deposition resumed in southern Florida during the latest Early and Late Pliocene (Fig. 2) with a shift from the progradational SS2 siliciclastic clinoforms to aggradation of the vertically mixed carbonates and siliciclastics of SS3 (Fig. 1C, Fig. 9D). The probable CN12aA assignment of the base of SS3 in the W-18075 corehole suggests that the deposition of the mixed carbonates and siliciclastics of SS3 began with the beginning of the TB3.6 eustatic rise (Fig. 2) in sea level of Haq et al. (1988). If this eustatic interpretation is valid, then it is possible that the unconformity bounding the top of SS2 in the area of the W-18075 corehole is related to the eustatic fall in sea level at the end of TB3.5 eustatic cycle (Fig. 2). Deposition of SS3 was widespread throughout southern Florida (Fig. 10E) and included the deposition of a carbonate ramp that is depicted at its base in Figure 10E (see Cunningham et al. 2001a). The siliciclastics of SS3 are present in the northern Florida Keys and may extend out to or near the present Florida reef tract and shelf margin (Fig. 10E).

The siliciclastic "platform" constructed in southern peninsular Florida by the end of the Pliocene provided a relatively shallow, marine substrate for carbonate growth, and the relatively warm climatic conditions of the Early Pleistocene (Willard et al. 1993) coupled with higher frequency sealevel changes (Fig. 2) of the Pleistocene likely contributed to the development of the well-known Quaternary carbonates and rimmed shelf margin (Read 1985) of the Florida Keys (Fig. 10F; Enos and Perkins 1977). Had deposition of SS2 siliciclastics and SS3 mixed siliciclastics and carbonates not filled accommodation on the southeast Florida Platform during the Late Miocene and Pliocene, as shown in Figure 10B, it is likely that the return to shallow- marine carbonate deposition as a reef-rimmed shelf would not have occurred.