Abstract Introduction Material & Methods Lithostratigraphy Petrography, Biostratigraphy & Sr-isotope Analysis Conclusions References Figures Tables Appendix 1 Appendix 2 PDF version

Conclusions

The lithostratigraphy of the southern Florida Miocene to Pleistocene sediments has been investigated for many years. These efforts have been hampered by the paucity of exposures, the predominantly subsurface nature of the units, and the complex nature of the sediment facies. The formations encountered often have been identified on the basis of molluscan fauna rather than lithology. This investigation utilizes lithologic parameters to identify the formations and component members. Biostratigraphic and strontium-isotope analyses aid in the correlation and in determination of ages of the units.

The oldest unit encountered in the study cores is the unnamed formation. The formation consists of variably fossiliferous, clay- to gravel-sized siliciclastics and carbonates. These sediments appear to be equivalent in age to the upper Peace River Formation of the Hawthorn Group and possibly may be an updip portion of the Long Key Formation. The maximum recovered thickness of the unnamed formation was 187 ft, although the entire unit was not penetrated. At least four depositional sequences are present in the carbonate unit below the siliciclastics in the western part of the study area. An orange surface with soil textures, indicating subaerial exposure, was found at the top of the carbonate in several cores. The unnamed formation may or may not be indurated. In all cores, it contains dolomite. It is mostly or entirely late Miocene. Calculated age, based on strontium-isotope analysis, ranges from 9 to 5.7 Ma. Within the unnamed formation there appear to be two genetically and lithologically distinct units: a lower carbonate (dolostone and limestone) that occurs in the western cores of Southern States, Collier- Seminole, Old Pump Road, Picayune Strand, and possibly Fakahatchee Strand-Ranger Station, and an unconsolidated quartz sand that contains varying amounts of shells and calcite mud, and minor amounts of dolomite mud and sandstone. The unconsolidated quartz sand increases in thickness from west to east. Calculated ages based on strontium-isotope analysis suggest that the hiatus between the carbonate unit and the siliciclastic unit is 2 m.y. in the Old Pump Road core. The dinocyst and mollusk ranges in several of the cores allow the possibility of a late middle Miocene age for at least part of the unnamed formation, but where these sediments are dated by strontium isotopes, they are late Miocene. In two cores, Collier-Seminole and Old Pump Road, the uppermost sediments of the unnamed formation are not dated by strontium isotopes and contain fossils that could be as young as Pliocene. In the Fakahatchee Strand-Ranger Station core, the possibility that this unit ranges into the Pliocene must be considered: dinocysts suggest a latest Miocene age, whereas mollusks in the upper part of the unit suggest a mid-early Pliocene age. In the Fakahatchee Strand-Gate 12 core, mollusk assemblages in part of the unit contain mollusks reported from the Miocene, intermixed with species reported from the Pliocene. The fossils do not show signs of reworking, the mixed interval is relatively thick, and the strontium-derived ages suggest that at least some samples containing a mixed Miocene-Pliocene mollusk assemblage are Miocene. Subaerial exposure is indicated for the top of the unnamed formation in the Fakahatchee Strand-Ranger Station core by pisolitic grains.

The Tamiami Formation overlies the unnamed formation throughout the study area and consists of the Ochopee Limestone Member in all the study cores. The lithology varies from an unconsolidated to moderately indurated sand or sandstone to a slightly sandy, moldic, fossiliferous, unconsolidated to well-indurated limestone. A maximum of 101 ft of the Ochopee sediments was encountered in the study area. Deposition appears continuous for the Ochopee Limestone, and all cores show an abrupt decrease in quartz-sand content that indicates a decrease in transport of sand to the site of deposition. The Ochopee Limestone Member of the Tamiami Formation is Pliocene and probably spans an interval of time from the early Pliocene to near the early/late Pliocene boundary. Strontium-isotope analysis indicates an early Pliocene age with a margin of error that includes the latest Miocene and the late Pliocene; calculated ages range from 5.1 to 3.5 Ma. The dinocyst assemblages often are not diagnostic, but where they are, they indicate a late Miocene or Pliocene age. Mollusks indicate a Pliocene age, and a distinctive assemblage that includes the overlapping ranges of Carditimera arata and Chione cortinaria ? may indicate an age near the early/late Pliocene boundary.

Unnamed, undifferentiated sands overlie the Pliocene limestones in two cores in the southern part of the study area. The very fine to medium sands fill a low on top of the Ochopee Limestone Member. This unnamed quartz sand is dated as Pliocene in the Old Pump Road core. This sand overlies material containing late Pliocene and (or) early Pleistocene mollusks.

Artificial fill or natural soil occurs a the top of the cores.

Implications for the structure of the surficial aquifer system

The confining units in the 200-ft interval penetrated by most of these seven cores are different in different parts of the study area. To the west, at the sites of the Southern States, Picayune Strand, Collier-Seminole, and possibly at Old Pump Road cores, a hard tightly cemented dolostone forms the first major confining unit below the water table. In the eastern part of the study area, confinement is more difficult to determine. At Fakahatchee Strand-Ranger Station the confining unit at 61 feet is a tightly cemented sandstone, which is much younger than the dolostones to the west, and which probably is not laterally connected to them. At the other two sites, Gate 12 and Jones Grade, thick zones of poorly sorted, muddy unconsolidated sands form a slight confining unit. These sands probably are not correlative to the sandstone at the Ranger Station site, nor to the dolostones to the west.

These age and sedimentologic observations suggest a complex compartmentalization of the surficial aquifer system in southwestern Florida. The current model for the system is two aquifers separated by a discontinuous confining unit. That model is clearly too simple, and a more realistic model would be several discontinuous confining units of different ages, lithologies, and vertical transmissivities that occur at different depths.

The next phase of this project is to move farther to the east to document changes in lithology and aquifer properties in eastern Collier County.

Biostratigraphic implications

The calibrations of dinocyst and molluscan occurrences with strontium-isotope stratigraphy allows us to expand and document the reported ranges of taxa. Mollusks reported here from the late Miocene (9-6.5 Ma, based on strontium analysis) include: Conus chipolanus ?, Crepidula plana ?, Turritella apicalis, T. pontoni, T. segmenta, T. subvariabilis, Anadara callicestosa, Carditimera apotegea ?, and Costaglycymeris mixoni ?. A distinctive early/late Pliocene assemblage consisting of Turritella apicalis, T. perattenuata, Carditimera arata, Chione cortinaria ?, and C. latilirata is dated as 4.7 to 3.7 Ma by strontium analysis.

Strontium-isotope calibration supports the use of the highest occurrence of Erymnodinium delectabile (Verteuil & Norris) Lentin et al. to mark the top of the Miocene. Dapsilidinium pseudocolligerum extends into the Pliocene.