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

Petrography, Biostratigraphy, Strontium-Isotope Analysis

Collier-Seminole State Park core

The Collier Seminole State Park corehole was drilled in February 1996 beside a pond near the head of a nature trail in Collier-Seminole State Park (fig. 1, table 1). It was drilled to a depth of 194 ft, and the core (fig. 3) was sampled for thin sections, X-ray diffraction, strontium analysis of shells, and foraminifer, ostracode, mollusk, dinocyst, and pollen content.

The unnamed formation was identified from 194 to 111 ft. From 194 to 128 ft, this unit ranges in lithology from a well lithified, very sandy dolostone and (or) limestone, to an unconsolidated, possibly clayey, dolomitic silt, to a calcareous sandstone with scattered discoidal quartz pebbles; the unit is an unconsolidated calcareous quartz sand from 128 to 111 ft. The Ochopee Limestone Member of the Tamiami Formation was recovered from 111 to 20 ft, and is a white, moldic, molluscan, sandy wackestone to packstone. Undifferentiated unconsolidated quartz sand was recovered from 20 to 5 ft. The upper five feet of this core is man-made fill and unconsolidated quartz sand.

Semi-quantitative X-ray diffraction data for this core are shown in table 2. The unnamed formation contains abundant quartz sand in most samples. Sandy limestones, dolomitic sandstones, and dolostones occur in the lower part of the core. Dolomite is present as a subordinate mineral in most samples below 111 ft and is the dominant mineral in one sample (141.7 ft). Aragonite is rare but was detected in trace amounts at 187.5 and 186.0 ft, and in more abundant quantities at 138.2 and 134 ft. In the unconsolidated sediments of the unnamed formation, from 128 to 111 ft, calcite is subordinate to quartz. Within the Ochopee Limestone Member of the Tamiami Formation, quartz is the dominant mineral in most of the lower samples and calcite is the dominant mineral in the upper samples. A sharp decrease in quartz, from 40 to 25 percent, occurs at about 53 ft and down to 10 percent or less at 46.8 ft and shallower. Samples from the undifferentiated sand above the Ochopee (above 20 ft) are predominantly quartz.

Petrography. Sixteen thin sections (appendix 1) were examined from the unnamed formation (194-111 ft), from samples that range in depth from 188.0 to 128.0 feet. Much of this interval, in contrast to most of the other cores, is lithified, and it provides the highest number of samples appropriate for thin sections from the cores examined. This unit is highly variable, ranging from unconsolidated quartz sand to dolomitic and calcareous sandstone to sandy dolostone and sandy limestone, and includes at least four distinct lithologies. Samples are grouped by their lithologic similarity, although they may be part of a larger, genetically related package.

The deepest sample at 188 ft is a uniform, low-porosity dolostone composed of silt-sized dolomite rhombs, very little quartz sand, and no skeletal grains. It comes from a laminated interval of dolostone interbedded with a coarse dolomitic sandstone. Most of the core below this depth interval is a moderately cohesive, poorly sorted, unconsolidated, fine to very coarse quartz sand.

Overlying the dolostone from 181 to 166 ft is a calcareous sandstone (samples at 177.9, 176.5, and 171.9 ft) of bimodal quartz grains (fine and very coarse sand) with very well rounded, pebble-sized, skeletal (primarily mollusk) grains. This unit exhibits an upward decrease in porosity over this depth range due to an upward increase in calcite cementation in intergranular and moldic pores.

Overlying the calcareous sandstone from 166 to 140 ft, at an apparent sharp contact, is a dolomitic sandstone to sandy dolostone (samples from 163.4 to 140.1 ft); the quartz sand grains are poorly sorted and range from medium to very coarse. The quartz sand content of this unit decreases upward, especially from 146 to 145.4 ft, while the dolomicrite content increases upward. Skeletal-grain content (as indicated by molds, not necessarily calcareous shells) decreases upward, and desiccation cracks occur from 145.4 to 140.1 ft.

Overlying the dolomitic sandstone to sandy dolostone at a sharp contact, is a clayey, unconsolidated fine quartz sand from 140 to 133.0 ft, which is gradational into a moderately well lithified sandy wackestone to packstone (samples examined from 133.0 to 129.8 ft). The wackestone has a highly variable quartz-sand content (from less than 1 to 50 percent) in the three samples examined and a diverse skeletal-grain content that includes molluscan molds and fragments, benthic foraminifers, ostracodes, echinoids, and bryozoans. There is minor blocky calcite cement on void surfaces that is overlain by a thin veneer of dolomite rhombs in the uppermost sample at 131 ft. The packstone at 129.8 ft is similar to the wackestone at 131 ft, but it has less micrite matrix and slightly more dolomite cement in the voids. Dolomite content increases from a small amount of voidfilling cement observed at 129.8 ft to a dolostone with no calcite at 129.5 ft. In the core, the dolostone was observed to extend up to 128.0 ft, where it ends at a blackened and bored surface, and is overlain by an unconsolidated quartz sand (Weedman and others, 1997). Porosity in samples at 129.5 and 129.0 ft is low due to dolomitic cement in both molds and interparticle voids and to a dolomicrite matrix filling interparticle voids; most skeletal grains are leached.

Twenty-five thin sections were made from samples of the Ochopee Limestone Member of the Tamiami Formation, from about 111 to 20 ft. Petrographic examination indicates that the unit ranges from a calcareous sandstone in samples examined from 111 to 81 ft, to a very sandy molluscan packstone in samples examined from 79 to 54 ft, to a molluscan wackestone to packstone in samples examined from 54 to 20 ft. Mollusks and benthic foraminifers are common throughout the formation; planktonic foraminifers are present from 97 to 75 ft.

The calcareous sandstone (samples from 111 to 81 ft) varies from 5 to 20 percent micrite matrix and generally contains molluscan molds and fragments, benthic and planktonic foraminifers, ostracodes, bryozoans, red algae, and echinoids. In general, this interval has a high porosity due to the low micrite content and high interparticle porosity. There is commonly a meniscus micrite cement at quartz grain contacts. In most samples, the mollusk shells are micritized and then leached, but are not infilled with blocky or sparry cement, as is common in other parts of the core. There is commonly a single layer of dogtooth calcite spar on molluscan micrite envelopes and on skeletal grain surfaces.

The very sandy packstone from 79 to 54 ft is similar to the underlying sandstone but with less quartz sand and more skeletal grains. From 79 to about 65 ft, the packstone has only minor blocky and dogtooth calcite cement on surfaces such as skeletal grains and molds. Porosity in that zone is moderate to high, and micrite matrix varies from 10 to 20 percent. However, from 65 to 50 ft, there is a pronounced increase in pore-filling cement, and consequently a decrease in porosity; neomorphism of matrix occurs from 60 to 47 ft.

A molluscan wackestone to packstone occurs from 54 to 24 ft. Sand content decreases abruptly at about 54 ft from 40 to 5 percent. The pore-filling cement is typically blocky. Original molluscan aragonite is preserved from about 54 to 24 ft, and there is evidence for a now-leached, early, probably aragonitic isopachous, bladed to fibrous cement in samples from about 40 to 24 ft.

Lithologic and petrographic summary. There are at least four depositional units in the unnamed formation at this site. From 194.6 to 181 ft is an unfossiliferous, laminated dolostone to unconsolidated dolomitic quartz sand. From 181 to 166 ft is a calcareous bimodal sandstone with rounded molluscan fragments. The top of this sandstone is pale orange suggesting subaerial exposure. This unit is tentatively interpreted as a high energy beach, or perhaps eolian, deposit. It also forms a tight confining unit (Weedman and others, 1997).

Overlying the bimodal sandstone is another dolostone, from 166 to 140 ft, that is quartz-rich at the base (which may be due to remobilized sand from the lower unit) and more dolomicritic near the top. The dolostone has relatively few skeletal grains and some desiccation cracks, and may have formed in a supratidal environment.

The dolostone is overlain by an unconsolidated, clayey siliciclastic unit at 140 ft that grades upward into a calcareous unconsolidated sand, a limestone at 133 ft, and a dolostone at 129.5 ft to the top of the unit at 128 ft. All lithologic transitions are gradational, and the interval from 140 to 128 ft may represent a single upward-shallowing depositional unit. The phosphatic content, the borings, and dense cementation at 128 ft indicate a submarine hardground. At this site, the dense zone from about 133 to 128 ft forms the first major confining unit below the water table aquifer in the surficial aquifer system. Contact with the overlying unconsolidated quartz sand is sharp.

Deposition appears continuous for the Ochopee Limestone Member of the Tamiami, and a sudden decrease in delivery of quartz sand to the depositional environment is evident at about 54 ft. Planktonic foraminifers occur in samples from 97 to 75 ft, and these sediments may represent the most open marine environment in this core, or perhaps a zone where planktonic foraminifers were not dissolved. Samples from all other depths contain only benthic foraminifers.

Most upward porosity and permeability changes in the Ochopee Limestone can be attributed to diagenetic rather than depositional processes. Porosity is controlled by the distribution of micrite matrix and pore-filling blocky cement. The most porous zone is from 110 to 65 ft, and this zone has a low micrite content (5 to 20 percent). The most cemented zone is from about 60 to 24 ft, where blocky calcite fills most pore space. Neomorphic microspar and the abundance of blocky calcite coincide with the preservation of aragonitic skeletal grains, which may have been protected from dissolution by early calcite cement. An early aragonitic cement that preceded precipitation of blocky calcite occurs from 47 to at least 24 ft and is now leached, perhaps by modern meteoric water. The thin, isopachous, aragonitic cement may occur deeper in the core and be overgrown by calcite cement, obscuring its presence.

Biostratigraphy. Eighteen samples were examined for dinocysts; nine yielded dinocysts (appendix 2, table 3). Samples from 188, 179, and 176 ft contain late middle or late Miocene assemblages including Achomosphaera andalousiensis Jan du Chêne, Erymnodinium delectabile (Verteuil & Norris) Lentin et al., and Hystrichosphaeropsis obscura Habib. Samples from 159 and 130 ft contain long-ranging species. Samples from 124, 101, and 88 ft contain Achomosphaera andalousiensis, Dapsilidinium pseudocolligerum (Stover) Bujak et al., and Invertocysta sp. They are of late middle or late Miocene or Pliocene age, and because they are above the highest occurrence of E. delectabile and H. obscura, they may be Pliocene. Higher samples were either nondiagnostic or barren.

Forty-three samples were examined for molluscan faunal content; eleven samples from 86 to 26 ft contained material identified for this report (table 4). Three samples from 85.5 to 83.5 ft contain only questionably identified forms. Eight samples from 76 to 26 ft are Pliocene, based primarily on the occurrence of Anadara lienosa, Carditimera arata, and Chione cortinaria ?. Anadara lienosa occurs in molluscan zone M5 of Blackwelder (1981) as well as younger material. Zone M5 is midearly through late Pliocene and includes the upper portion of the Yorktown Formation. Carditimera arata has been reported from the Pinecrest Beds of the Tamiami Formation and the Caloosahatchee faunal unit in Florida, and from the Yorktown (upper portion), Raysor, Chowan River, and James City Formations in the mid-Atlantic Coastal Plain (Ward, 1992a). These units are late Pliocene (or possibly late early Pliocene) to Pleistocene in age. Chione cortinaria is questionably identified in two samples (38 and 26 ft); this species is restricted to molluscan zone M6 (lower Pliocene = Zone 1 Yorktown) and has been reported from the Sunken Meadow Member, the lowest member, of the Yorktown Formation and the Ecphora zone of the Jackson Bluff Formation (Mansfield, 1932; Lyle Campbell, written communication, 1997). The overlap of species from molluscan zone M6 and M5 indicates that either the identification is incorrect or the published ranges should be extended to include overlap near the lower/upper Pliocene boundary. Turritella apicalis and T. perattenuata have been reported from the Caloosahatchee faunal unit (Dubar, 1958); however, data from the Fakahatchee-Ranger Station and Fakahatchee Gate 12 cores indicate that the full ranges of these species may be much longer.

Eight samples were examined for foraminifers (table 5). Six of these samples yielded planktonic forms, and five of these yielded identifiable, age-diagnostic planktonic species. The sample at 127.5-128 ft contains the early Miocene species Globigerinoides altiapertura Bolli. The sample at 126.5-126.8 ft could be either middle Miocene or late Miocene without definitive late Miocene taxa. Samples from 126 to 119 ft are late, but not latest, Miocene, based on the overlap of ranges of Globigerina praebulloides Blow and Globigerina apertura Cushman.

Strontium-isotope stratigraphy. Eight samples (including one duplicate pair) were analyzed for strontium isotopes (table 6). The lowest sample at 171.5 ft gives a calculated age of 9.17 Ma ± 0.5 m.y. (late Miocene). Higher samples from 105 to 26.6 ft have calculated ages of 5.1 to 3.88 Ma ± 1.5 m.y. (early Pliocene, but with a margin of error that includes both latest late Miocene and late Pliocene).

Age summary. The unnamed formation (194 to 111 ft) is late Miocene in the Collier- Seminole core. Near the bottom, the unit contains late middle or late Miocene dinocysts and has a calculated age of 9.2 Ma. Planktonic foraminifers at 127.5-128 ft indicate reworked early Miocene forms just above a prominent exposure surface. In the upper part of the unit, dinocysts indicate an age of Miocene or Pliocene, and planktonic foraminifers indicate a late, but not latest, Miocene age.

The Ochopee Limestone Member of Tamiami Formation (111 to 20 ft) is Pliocene and spans an interval of time from the early Pliocene to near the early/late Pliocene boundary (the boundary is 3.55 Ma on the timescale of Berggren and others, 1995). The dinocysts indicate that samples from this unit up to 88 ft are no younger than Pliocene. Strontium-isotope stratigraphy indicates an early Pliocene age throughout the unit, and calculated ages range from 5.1 to 3.9 Ma with a margin of error that includes late Miocene and late Pliocene. Pliocene mollusks are found from 76 to 26 ft.

The undifferentiated quartz sand (20 to 5 ft) is undated. It overlies material containing late Pliocene mollusks.