Abstract Introduction Material & Methods Lithostratigraphy Petrography, Biostratigraphy & Sr-isotope Analysis Collier-Seminole Old Pump Road F.S.-Ranger Station F.S.-Gate 12 F.S.-Jones Grade >Picayune Strand Southern States Conclusions References Figures Tables Appendix 1 Appendix 2 PDF version

Picayune Strand State Forest core

The Picayune Strand State Forest corehole site (fig. 1, table 1) is in the back pasture of a residence on 52nd Street, S.E., in the Southern Golden Gate Estates. The corehole was drilled to 201 ft in August 1996. The core (fig. 8) was sampled for thin sections, X-ray diffraction, strontium analysis of shells, and for mollusk, pollen, dinocyst, foraminifer, and ostracode content.

The unnamed formation occurs from the bottom of the core at 201 ft to 106 ft and is a sandy, dolomitic, moldic packstone that has scattered quartz pebbles in its lower part (from the bottom of the core to 181 ft). From 165 to 106 ft, the formation is an unconsolidated quartz sand, more clay-rich at its base. The Ochopee Limestone Member of the Tamiami Formation was recovered from 106 to 5.2 ft. It is a very sandy moldic limestone and calcareous sandstone that grades upward into a moldic molluscan packstone. The upper 5.2 ft of this core is artificial fill.

Semi-quantitative X-ray diffraction data for this core are shown in table 11. Dolomite occurs as the dominant mineral at 189.6 ft in the unnamed formation. In the Ochopee Limestone Member of the Tamiami Formation, calcite is the dominant mineral in all samples, and quartz is present in minor and trace amounts. Aragonite occurs as a co-dominant mineral at 38.0 ft.

Petrography. Two thin sections were made from the unnamed formation in the core at Picayune Strand State Forest. Both samples (181, 169 ft) are dolostones with dolomicrite matrix and minor quartz sand. Skeletal grains are sparse but include mollusks (now leached), echinoid fragments, barnacles, and coral.

Seven thin sections were made of the Ochopee Limestone Member of the Tamiami from this core. The lower two (85.2 and 75.0 ft) are very sandy with 10 to 40 percent micrite matrix. Skeletal grains include molluscan fragments and molds, benthic foraminifers, bryozoans, red algae, echinoid fragments, and barnacles. Meniscus cement occurs at 85.2 ft, and there is minor dogtooth spar on surfaces in both samples.

The middle three thin sections (53.0 to 31.3 ft) come from a molluscan wackestone to grainstone with only a trace of quartz silt in a micrite matrix of 25 to 40 percent. Skeletal grains are similar to the sandy limestone below, and cements are minor.

Two thin sections were made from 11.8 and 9.0 ft of this core. Both samples are quite similar and are very low porosity molluscan packstones with neomorphosed microspar matrix and about 15 percent micrite matrix; both samples contain blocky calcite-filled skeletal molds, benthic foraminifers, ostracodes, and echinoid fragments, and minor quartz sand (trace to 10 percent).

Lithologic and petrographic summary. As in the other cores from the western part of the study area, there is a lithified carbonate within the unnamed formation in the lower part of the Picayune Strand State Forest core. The dolomite of the unnamed formation here is similar to the dolomite in the cores from the Collier-Seminole and Old Pump Road sites. In the Picayune Strand core, the distinctive yellow to orange color suggests a surface of exposure at the top of the dolostone at 165.5 ft.

The Ochopee Limestone Member of the Tamiami Formation at this site is similar to that in the other sites in that there is a lower sandy limestone and an upper nearly sand-free limestone. Cements are rare here, and there is no relict aragonite observed, no planktonic foraminifer zone, and no evidence for an early fibrous to bladed, now-leached cement as observed in the Collier-Seminole, Old Pump Road, and Fakahatchee Strand-Ranger Station cores. The upper part of the Ochopee looks like caprock, a highly recrystallized limestone of very low porosity commonly observed near the ground surface in the study area. The grains are not extensively leached, but all molds are spar-filled. There is no evidence of relict aragonite nor of an early aragonitic cement.

Biostratigraphy. Seven samples were studied for dinocysts; two of these were barren (appendix 2, table 3). The lowest sample (188 ft) contains Erymnodinium delectabile and is late middle or late Miocene. The sample at 153 ft contains a relatively nondiagnostic assemblage. The sample at 91 ft is late middle or late Miocene or Pliocene, and because this sample is above the highest occurrence of E. delectabile and H. obscura, it is likely to be very early Pliocene. The sample at 34 ft is no younger than Pliocene based on the presence of Invertocysta lacrymosa. The highest sample (13 ft) is not diagnostic.

Twelve samples were examined for molluscan faunal content. Eight samples from 175.5 to 26.5 ft contained mollusks identified for this report (table 4). The lowermost samples contain Turritella perattenuata, a species that has been reported from late Pliocene or early Pleistocene units. The large gap between deposition at 175.5 and 85 ft, and additional data gathered in this study, indicate a longer range for this species. The age of samples containing only T. perattenuata therefore is considered unknown. The sample at 39.2 ft is mid-early to late Pliocene based on the occurrence of Carditimera arata. However, the sample at 31.4 ft is tentatively restricted to the mid-early Pliocene (mollusk zone M6) based on the questionable identification of Chione cortinaria, reported from the Ecphora zone of the Jackson Bluff Formation and the Sunken Meadow Member of the Yorktown Formation (Mansfield, 1932). Chione procancellata (26.5 ft) is from the Cancellaria zone of the Jackson Bluff Formation (Mansfield, 1932). As in other cores, these species may suggest an age near the early/late Pliocene boundary.

Strontium-isotope stratigraphy. Three samples were analyzed for strontium isotopes (table 6). They show a monotonic decrease in calculated ages from 4.9 Ma (84.7 ft) to 4.26 Ma (44 ft) to 3.66 Ma (25 ft), all with a margin of error ± 1.5 m.y. These values are early Pliocene, but the margin of error includes both late Miocene and late Pliocene.

Age summary. The unnamed formation (201-106 ft) in the Picayune Strand State Forest core is late middle or late Miocene based on the presence of the dinocyst species E. delectabile at 188 ft. Above this, the unit could be as young as Pliocene. Mollusks are not diagnostic and no samples were analyzed for strontium in this unit.

The Ochopee Limestone Member of the Tamiami Formation (106-5.2 ft) is Pliocene. Strontium-isotope analysis suggests an early Pliocene age with a margin of error that includes the latest Miocene and the late Pliocene. Dinocysts at 91 ft are consistent with a Pliocene age. Mollusks from 39.2 to 26.5 ft indicate a Pliocene age and may suggest an age near the early/late Pliocene boundary. Material above 13 ft is not diagnostic.