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-Ranger Station core

The Fakahatchee Strand-Ranger Station corehole site (fig. 1, table 1) was drilled to the east of a group of cypress trees between the ranger's residence and the headquarters office of the Fakahatchee Strand State Preserve. It was drilled to 161 ft in April 1996. The core (fig. 5) was sampled for thin sections, X-ray diffraction, strontium analysis of shells, and for mollusk, foraminifer, pollen, dinocyst, and ostracode content.

The unnamed formation is found from the bottom of the core at 161 ft to a depth of 61.5 ft. It is an unconsolidated, calcareous, medium to coarse, quartz sand that contains a relatively thin dolomite-cemented sandstone from 149 to 139 ft. From 139 to 137 ft, quartz and phosphate discoidal pebbles were observed in a dolomitic quartz sand that continues upward. From 61.5 to 4 ft is the Ochopee Limestone Member of the Tamiami Limestone. The unit is poorly recovered in this core and consists of white to gray to yellowish-gray, moldic, molluscan packstone to wackestone with minor amounts of quartz sand. At 61.5 to 61.0 ft, there is a 6-inch thick, tightly calcite-cemented sandstone, which forms a confining unit of the aquifer at this site (Weedman and others, 1997). At about 48 ft, the limestone grades into a very sandy limestone to calcareous sandstone. The upper 4 feet of sediment is artificial fill.

X-ray diffraction data for the lower part of this core, unnamed formation only, are shown in table 8. Quartz sand is common in most samples analyzed, including the limestones and dolostones. Calcite is rare except at 138.4 ft. Dolomite is present in trace amounts at 158.8, is a major component in samples from 148.8 to 138.4 ft, and is not observed in any shallower samples. Aragonite occurs in small amounts in most samples from 158.8 to 110.3 ft.

Petrography. One thin section was made from the unnamed formation in this core. It is a dolostone from 139.8 ft with minor quartz sand in a dolomicrite matrix. The only skeletal grains that can be recognized are now-leached molluscan molds. There was no additional precipitation of dolomite cement after dissolution of skeletal grains.

Between this dolomite and the Tamiami Formation is a thick interval of unconsolidated quartz sand; some portions are quite shelly with aragonitic mollusks.

One thin section was made of a lithified portion of the Ochopee Limestone Member of the Tamiami Formation at 61.5 ft. It is a calcareous sandstone interbedded with a pisolitic mudstone to wackestone. There are no skeletal grains and the sample has extremely low porosity.

Eight additional thin sections were made from samples of the Ochopee Limestone Member. Samples at 53.8 and 51.5 ft are similar molluscan wackestone to packstones, with 10 to 20 percent quartz silt to medium sand. They contain molluscan fragments and molds, foraminifers, ostracodes, bryozoans, echinoids, and barnacles. The lower sample has some microspar in the matrix, and porosity in both is moldic. Planktonic foraminifers occur at 53.8 and at 34.0 ft. Cement in the lower part of the formation is typically very fine dogtooth calcite.

From 34 to 8.3 ft, the Ochopee Limestone Member is a molluscan packstone to grainstone that has less micrite matrix and considerably less quartz silt and sand than the lower part has. There is evidence for an earlier, nowleached fibrous to bladed cement (aragonite?) in the samples from 22.7 to 8.3 ft. Cements in the upper part of the formation are more variable: there is blocky calcite filling voids at 10.7 and 8.3 ft and lining voids at 17.6 ft, as well as a finely crystalline dogtooth spar at 34.0 ft.

Lithologic and petrographic summary. None of the carbonate depositional units that were observed in cores to the west are observed in the Fakahatchee Strand-Ranger Station core. However, this corehole was drilled to 161 ft and may not have reached those units.

The sample at the base of the Ochopee Limestone Member at 61.5 ft looks very much like the pisoidal wackestone in the unnamed formation in the Old Pump Road core, except that here it lacks skeletal grains.

The depositional pattern within the Ochopee in this core is similar to that in the two previous cores: the lower portion (61.5 to 47 ft) is sandy and the upper portion (47 to 4 ft) has less than 5 percent quartz sand. There are planktonic foraminifers from 50 to 30 ft, overlain by a zone with evidence of an earlier, now-leached fibrous to bladed (aragonitic?) cement.

Biostratigraphy. Nine samples were examined for dinocysts (appendix 2, table 3). The two lowest samples (160, 158 ft) contain long-ranging taxa that are late Oligocene or younger. The next higher samples (137, 99 ft) are late middle or late Miocene based on the overlapping ranges of Erymnodinium delectabile and Achomosphaera andalousiensis. The sample at 79 ft is late in the late Miocene, based on the ranges of E. delectabile and Selenopemphix armageddonensis. Samples above 79 ft had very few, non-diagnostic dinocysts. Reworked Cretaceous material was noted at 56 ft.

Forty-five samples were examined for molluscan faunal content. Twenty-two samples from 133 to 11 ft contain mollusks identified for this report (table 4). Samples from 133 to 123 ft contain a species reported from the Pliocene Pinecrest beds and Buckingham Marl of the Tamiami Formation (Turritella pontoni). The sample at 123 to 126 ft contains both an apparent Miocene (Carditimera apotegea ?) and an apparent Pliocene species (T. pontoni), and the samples from 121 to 96 ft contain an apparent Miocene species (Costaglycymeris mixoni ?). Costaglycymeris mixoni is reported from the upper Miocene Cobham Bay Member of the Eastover Formation, molluscan zone M7 (Ward, 1992b). This entire interval from 133 to 97 ft is therefore a mixed Miocene/Pliocene assemblage based on the published fossil ranges. However, the most likely explanation for this mixed assemblage is that the full ranges for some of these species have not been reported yet. The fossils do not show any signs of reworking and the interval of the mixed assemblage is relatively thick; therefore reworking seems an unlikely explanation for the mixing. The interval from 133 to 97 ft is tentatively assigned a Miocene age. Samples from 87 to 11 ft are no older than mid-early Pliocene, based on the lowest occurrences of Gemma magna and Parvilucina multilineata. Parvilucina multilineata ranges from mid-early Pliocene to the Recent; it has been reported from the Cancellaria and Arca zones of the Choctawhatchee Formation (now Jackson Bluff) in Florida, and from the Yorktown, Waccamaw, Chowan River, and James City Formations in the mid-Atlantic Coastal Plain (Gardner, 1943; Ward and Blackwelder, 1987). Gemma magna ranges from mid-early Pliocene to Pleistocene; it has been reported from the Yorktown, Duplin, Waccamaw, and James City Formations (Ward and Blackwelder, 1987), from the Ecphora zone, Jackson Bluff Formation (Mansfield, 1932), and from the Caloosahatchee faunal unit (Gardner, 1943). Chione cortinaria is questionably identified in eight samples from 60 to 11 ft; this species is restricted to molluscan zone M6 (equals Zone 1 Yorktown) and has been reported from the Sunken Meadow Member of the Yorktown Formation and the Ecphora zone of the Jackson Bluff Formation (Mansfield, 1932; Lyle Campbell, written communication, 1997). Here, as in the Collier Seminole core, the overlap of C. cortinaria and Carditimera arata suggests an age near the early/late Pliocene boundary. Chione latilirata ranges from Miocene? to Recent and has been identified in Bed 11 of the Pinecrest Beds at APAC mine (Ketcher, 1992) and the Caloosahatchee faunal unit (Dubar, 1958).

Twenty-two samples were examined that contained foraminifers. All specimens identified are benthic forms; none are age diagnostic.

Strontium-isotope stratigraphy. Nine samples (including one duplicate pair) were analyzed for strontium isotopes (table 6). The lowest sample at 159-160 ft gives a calculated age of 7.78 Ma ± 1.5 m.y. (late Miocene). Higher samples from 132.5 to 70.0 ft have calculated ages of 6.5 to 5.9 Ma ± 0.5 m.y. (late Miocene). The two highest samples (44, 26.7 ft) have calculated ages of early Pliocene, but have a margin of error that includes both latest Miocene and late Pliocene (4.7 to 4.3 Ma ± 1.5 m.y.)

Age summary. The unnamed formation (161-61.5 ft) in the Fakahatchee Strand-Ranger Station core is late Miocene, based on strontium-isotope stratigraphy. Dinocysts are less diagnostic in the lower part of the unit, but are in general agreement with the strontium-isotope stratigraphy. Above 133 ft, the strontium-derived ages suggest that the mixed Miocene-Pliocene mollusk assemblage is actually Miocene and that Turritella pontoni, whose published range does not extend below the Pliocene, does indeed extend into the late Miocene. At 79 ft, dinocysts indicate a latest Miocene age. Above 70 ft, the possibility that the unnamed formation ranges into the Pliocene must be considered: mollusks suggest a mid-early Pliocene age.

The Ochopee Limestone Member of the Tamiami Formation (61.5 to 4 ft) is Pliocene. Mollusks suggest a Pliocene age, and strontium-isotope analysis indicates an early Pliocene age with a margin of error that includes the latest Miocene and the late Pliocene. As in the Ochopee in the Collier Seminole core, an age near the early/late Pliocene boundary is indicated from 53-11 ft. Only nondiagnostic dinocysts were found in the Ochopee.