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

Fakahatchee Strand-Gate 12 core

The Fakahatchee Strand-Gate 12 corehole site (fig. 1, table 1) is on Janes Memorial Scenic Drive about 6 miles past the Fakahatchee Strand State Preserve headquarters. The corehole was drilled to 227 ft in June 1996. The core (fig. 6) was sampled for thin sections, X-ray diffraction, strontium analysis of shells, and for mollusk, pollen, foraminifer, ostracode, and dinocyst content.

The unnamed formation is found from the bottom of the core at 227 ft to a depth of 40 ft. Discoidal quartz pebbles were observed in the unconsolidated sand from about 212 to 187 ft. There are aragonitic mollusks preserved from 50 to 46 ft. At the top of the unnamed formation at 40 ft is 5 to 6 inches of sandstone. The Ochopee Limestone Member of the Tamiami consists of a very sandy limestone to calcareous sandstone from 40 to 33 ft and a yellowish-gray, moldic, molluscan packstone from about 33 to 6 ft. The upper 6 ft in this core is artificial fill and is primarily silt and quartz sand.

This corehole site differs from cores drilled to the west in this study in that there is no lithified sediment below the transition from limestone to unconsolidated quartz sand at about 40 ft. However, within the unconsolidated sand are two zones that are dolomitized: one from 133 to 127 ft, and another clayey dolomitic zone from 220 to 217 ft. Hydrologic and geophysical data indicate that the confining zones in this corehole are thicker than in coreholes to the west, although no densely cemented zone occurs in this core.

Semi-quantitative X-ray diffraction data for this core are shown in table 9. Quartz sand is common in most samples analyzed from the unnamed formation. Calcite is observed in minor amounts in samples from 219.0 to 41.2 ft. Dolomite is common only in the samples from 135.6 to 129.9 ft in an unconsolidated clayey quartz sand, and in a trace amount in a sample from 99.0 ft. Aragonite is found in most samples from the unnamed formation. In the Ochopee Limestone Member of the Tamiami, calcite is the predominant mineral in samples from 26.5 to 6.8 ft. Aragonite is found in a sample at 6.8 ft.

Petrography. Five thin sections were made from samples from the Ochopee Limestone Member in this core that range in depth from 36 to 7 ft. The deepest two samples from 36.0 and 32.3 ft are sandy skeletal wackestones to grainstones with minor amounts of micrite. The lower sample has a microspar matrix that apparently has replaced the micrite (neomorphic). Skeletal grains include molluscan molds and fragments, foraminifers (planktonics at 32.3 ft), ostracodes, bryozoan and echinoid fragments, and barnacles. Blocky spar occurs in both samples.

Above the sandy limestone is a molluscan wackestone to grainstone that has a trace of quartz silt and sand, and 5 to 30 percent micrite matrix (at 17.7 ft), and a similar assortment of skeletal grains as observed deeper (36 to 32.3 ft), with the addition of red algae. The shallowest sample at 7.0 ft has preserved original molluscan aragonite remnants.

Lithologic and petrographic summary. As in the Fakahatchee Strand-Ranger Station core, there is no lithified carbonate depositional unit in the lower part of Fakahatchee Strand-Gate 12 core. The Ochopee Limestone Member in the Gate 12 core is thinner than in the other cores but still has the two lithofaces of a lower sandy wackestone to grainstone, and an upper nearly sand-free wackestone to grainstone. Also, planktonic foraminifers occur abundantly at 32.3 ft; similar horizons in the other cores may be correlative. A major difference in this core, as compared to the previous ones discussed, is the absence of dolomitized limestone and the absence of the early leached fibrous to bladed cement in the upper portion of the Ochopee. These differences may be related to location on the Florida platform with respect to paleo-shoreline.

Biostratigraphy. Six samples were examined for dinocysts (appendix 2, table 3). Of the three lowest samples, the middle one was barren but the other two (218, 126 ft) contain dinocyst assemblages of late middle or late Miocene age. The sample at 88 ft is late middle Miocene, late Miocene, or Pliocene, based on the overlap of A. andalousiensis and D. pseudocolligerum, and because it is above the highest occurrence of E. delectabile and H. obscura, it is likely to be Pliocene. The sample at 62 ft contains long ranging forms and the sample at 36 ft is barren.

Thirty-six samples were examined for molluscan faunal content. Seventeen samples from 136.3 to 10.7 ft contained mollusks identified for this report (table 4). Samples from 136.3 to 87.9 ft contain a relatively diverse mixed assemblage of fossils reported from Miocene and Pliocene units. As in the Fakahatchee Strand-Ranger Station core, the most likely explanation for this mixed assemblage is that the full ranges for some of these species have not been discovered yet. Seven samples from 63.2 to 42 ft may be late Miocene and (or) Pliocene. Three species (Crepidula costata, Crucibulum grande, and Arcopsis adamsi) positively identified in this interval range from Miocene through at least Pliocene (Ward, 1992a; Gardner, 1947). Anadara callicestosa (63 ft) is reported from the Ecphora and Cancellaria zones of the Jackson Bluff Formation (Mansfield, 1932), so its reported age is mid-early to late Pliocene, but its presence in the lower portion of this core indicates it may range into the Miocene. Samples from 24.7 to 10.7 ft contain an assemblage no older than mid-early Pliocene based on the occurrence of Carditimera arata.

Strontium-isotope stratigraphy. Seven samples were analyzed for strontium isotopes (table 6). The lowest, at 218.6 ft, yields a calculated age of 8.92 Ma ± 0.3 m.y. (late Miocene). Four higher samples from 126.2 to 48 ft give calculated ages of 6.8 Ma ± 1 m.y. (late Miocene) for the sample at 126.2 ft and 6.1 to 6.0 ± 0.5 m.y. for the higher samples (late Miocene). Samples from 24.5 to 12.5 ft give calculated ages of 4.4 Ma ± 1.5 m.y., early Pliocene, but have a margin of error that includes both latest Miocene and late Pliocene.

Age summary. The unnamed formation (227-40 ft) in the Fakahatchee Strand-Gate 12 core is late Miocene and possibly Pliocene. The strontium-derived ages and the dinocyst assemblage at 126 ft suggest that at least some of the samples that contain a mixed Miocene- Pliocene mollusk assemblage are Miocene. If this age assessment is correct, some taxa whose published ranges do not extend below the Pliocene do indeed have ranges that extend into the late Miocene, as in this unit in the Fakahatchee Strand-Ranger Station core. The unnamed formation above 126 ft is late Miocene and (or) early Pliocene. Strontium analysis yields calculated ages of late Miocene up to 48 ft. Dinocysts indicate an age of Miocene or Pliocene at 88 ft. The mollusks present above 63 ft indicate a Miocene and (or) Pliocene age.

The Ochopee Limestone Member of the Tamiami Formation (40-6 ft) is Pliocene. The unit is dated by using mollusks as mid-early and (or) late Pliocene, and here again, Carditimera arata and Chione cortinaria suggest an age 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. Only nondiagnostic dinocysts were found in the Ochopee.