SEQUENCE STRATIGRAPHY OF A SOUTH FLORIDA CARBONATE RAMP AND BOUNDING SILICICLASTICS (LATE MIOCENE-PLIOCENE)

CARBONATE RAMP AND BOUNDING SILICICLASTICS TEMPORAL AND SPATIAL BOUNDARIES

Peace River Formation

Micropaleontology

Taxonomic identification of diatoms, silicoflagellates and coccoliths from the Peace River Formation are limited to the condensed sections contained in DS2 and DS3 (Figs. 5 and 6). Micropaleontologic analyses focused on CS2 and CS3 because these mudstone units contain microfossils that are useful for constructing a chronostratigraphy. Examination of samples for benthic foraminifera was conducted on both mudstones and quartz sands of the Peace River Formation. Both the benthic foraminifera and diatom populations were helpful in defining depositional environments.

Diatoms and silicoflagellates

Biostratigraphic analysis of diatoms was conducted on samples from the W-9110, C-1142, C-1182, and W-17273 cores (Table 2). Diatoms from one sample of CS2 in the C-1142 corehole suggest an age of 6.0 to 5.5 Ma (million years ago). Four diatomaceous mudstones samples of CS2 in the W-17273 corehole and one sample from the C-1182 corehole contain very similar diatom assemblages. A latest Miocene age older than 5.5 Ma is suggested for CS2 in both of these cores based on the absence of Thalassiosira oestrupii, which first occurs at 5.5 Ma (Fig. 7). Other diatoms present in the assemblage (Paralia sulcata, Stephanopyxis sp., Delphineis sp., Actinoptychus sp., Actinocyclus octonarius, Thalassionema nitzschioides, Thalassiosira eccentrica, Thalassiosira leptopus, Koizumia adaroi, and early forms of Koizumia tatsunokuchiensis) are consistent with an age younger than 6.5 Ma (Yanagisawa and Akiba, 1998; J.A. Barron, U.S. Geological Survey, written commun., 2000).

Figure 7. Correlation of the chronostratigraphy of a portion of the late Tertiary geomagnetic polarity time scale (Berggren et al., 1995) and coccolith zonation. From ODP Leg 171B at the Blake Nose east of northern Florida (Shipboard Scientific Party, 1998) with normalized additions of Subzones CN12aA, aB, and aC from Bukry (1991), the eustatic curves of Haq et al. (1988), diatom datums from offshore California, and southern Florida sequence stratigraphy. [larger image]

The diatom analyses herein suggest that the age of CS2 can be constrained to 6.5 to 5.5 Ma. The presence of the silicoflagellate Distephanus frugalis in the W-17273 corehole supports an age younger than 6.5 Ma (Barron, 1976). Alternatively, prior work by Cunningham et al. (1998) reported the age of the diatomaceous mudstones (CS2) of the Peace River Formation in the W-17273 corehole to range from 7.44 to 6.83 Ma (Fig. 6). This time frame brackets the Tortonian-Messinian boundary based on the presence of two cosmopolitan silicoflagellate species Distephanus pseudofibula and Bachmannocena triodon (Cunningham et al., 1998). The 7.44 to 6.83 Ma range in age is consistent with the broader age range for biostratigraphic assignment of coccoliths from CS2 (Zone CN9, perhaps only Zone CN9b) as shown in Figure 7.

The assemblages from the C-1142, C-1182, and W-17273 cores are composed predominantly of shelf-dwelling taxa. The diatomaceous mudstones in the C-1142 and C-1182 cores record a transgressive event, upwelling of nutrients, or possibly both across a siliciclastic shelf as indicated by the dominance of the shelf-dwelling taxa. The diatomaceous mudstones of the W-17273 corehole (Fig. 6) also contain an abundance of shelf-dwelling taxa, but the stratigraphic position of the taxa (Fig. 6) suggests deposition in a shelf-slope or toe-of-slope environment. Environmental conditions such as currents, wave sweeping, or both could explain the transport of shelf-dwelling taxa into this off-shelf environment.

Two samples of well cuttings from upper and lower parts of CS3 were collected for analysis from well W-9110 (Fig. 5 and Table 2). The upper sample (Table 2, 208-218 feet below sea level) is from the terrigenous mudstone facies (Table 1), and the lower sample (Table 2, 238-248 feet below sea level) is from the diatomaceous mudstone facies (Table 1). A maximum flooding surface separates the two samples (Fig. 5).

The sample from the upper part of CS3 in well W-9110 contains Paralia sulcata, Actinocyclus octonarius, Actinoptychus senarius, Stephanopyxis sp., Koizumia tatsunokuchienis, Thalassionema nitzschioides, Thalassiosira eccentrica, Thalassiosira leptopus, Thalassiosira oestrupii, and Rhaphoneis fatula. An occurrence of T. oestrupii suggests an age younger than 5.5 Ma (Fig. 7). A rare presence of R. fatula suggests an early Pliocene age based on comparison with occurrences in California (Dumont and Barron, 1995). An abundant presence of P. sulcata possibly indicates that this is an outer shelf assemblage (Sancetta, 1981).

The lower sample from CS3 in well W-9110 contains an assemblage similar to the sample from the upper portion but yields few Paralia sulcata. In addition to the taxa identified in the upper sample, the lower sample includes the occurrence of Hemidiscus ovalis. The presence of Thalassiosira oestrupii (Fig. 7) and H. ovalis indicates an early Pliocene age (Dumont and Barron, 1995). Planktic diatoms are more common in the sample from the lower part (diatomaceous mudstone facies) of CS3 relative to the upper part of the sequence (terrigenous mudstone facies). Relatively more planktic diatoms in the sample from the lower portion of CS3 is consistent with greater interpreted water depth during deposition of the diatomaceous mudstone relative to the terrigenous mudstone above the maximum flooding surface. This surface is defined by the boundary between the diatomaceous mudstone and terrigenous mudstone (Fig. 5).

Coccoliths

Samples were collected for analysis of coccoliths from wells C-1142, C-1182,W-9104, W-9110, W-9114, and W-17273. These samples were taken from a terrigenous mudstone near the base of the Peace River Formation and the two condensed sections (CS2 and CS3) of the Peace River Formation (Figs. 5 and 6). A single sample from the terrigenous mudstone near the base of the Peace River in well W-9110 (Fig. 5) contains abundant coccoliths that include Discoaster bellus, Discoaster brouweri, Discoaster prepentaradiatus, and a questionable Discoaster bollii. The assemblage of coccoliths probably belongs to Zone CN8 (Fig. 7), suggesting a Tortonian age (Perch-Nielsen, 1985).

Coccolith and diatom occurrences suggest assignment of CS2 to Subzone CN9b, but could be as old as Zone CN9 and as young as Subzone CN10a (Fig. 7). Coccoliths contained in eight samples from CS2 in the C-1182 corehole suggest assignment of CS2 to Subzone CN9b (7.2-5.6 Ma) based on the presence of Discoaster berggrenii, Discoaster quinqueramus, Discoaster surculus, and Amaurolithus primus (Fig. 7). Reworking of coccoliths in samples from the C-1182 corehole was investigated, but is unlikely since no uniquely older or younger taxa were identified. Two samples from CS2 in the C-1142 corehole contain D. surculus and thus are no older than Zone CN9. The upper biostratigraphic range of the C-1142 corehole sample is indefinite and assigned to Zone CN9 or Subzone CN10a, but associated diatoms are late Miocene; therefore, samples of CS2 from both the C-1182 and C-1142 cores suggest a late Miocene age no older than Zone CN9 or probably Subzone CN9b (Figs. 5 and 6). The occurrence of the coccoliths D. quinqueramus and D. berggrenii in one sample of CS2 collected from well W-9104 and another of CS2 from well W-9114 suggests that CS2 in these wells belongs to Zone CN9 (Fig. 5 and Table 3).

Combined diatom and coccolith data suggest assignment of CS3 to Subzone CN10b through Zone CN11 or early Pliocene (Fig. 7). Coccoliths from CS3 in well W-9104 are characterized by the presence of Ceratolithus acutus, Ceratolithus armatus, and Amaurolithus primus (Plate 1 and Table 3). These taxa and especially the presence of C. acutus indicate assignment of CS3 to Subzone CN10b (5.23-5.05 Ma) and an early Pliocene age (Fig. 7; Table 3). Two samples from CS3 in well W-9110 contain a trace to sparse presence of coccoliths including Discoaster surculus, Ceratolithus rugosus, and Reticulofenestra pseudoumbilica. These coccoliths are consistent with assigning CS3 in well W-9110 to Subzones CN10c through Zone CN11 (5.05-3.83) and an early Pliocene age (Fig. 7).

 

Plate 1. Photographs of coccoliths from well W-9104. Photographs 1 to 16 are from a sample interval of 318 to 328 feet below sea level. Photographs 17 to 20 are from a sample interval of 428 to 438 feet below sea level. Click on the thumbnail images below for larger versions. a,b. Coccolithus pelagicus (Wallich) Schiller: (1a) cross-polarized light, and (1b) plane light. a,b. Calcidiscus leptoporus (Murray and Blackman) Loeblich and Tappan: (2a) cross-polarized light, and (2b) plane light. a,b. Calcidiscus macintyrei (Bukry and Bramlette) Loeblich and Tappan: (3a) cross-polarized light, and (3b) plane light. a,b. Reticulofenestra pseudoumbilica (Gartner) Gartner: (4a) cross-polarized light, and (4b) plane light. a,b. Reticulofenestra pseudoumbilica (Gartner) Gartner: (5a) cross-polarized light, and (5b) plane light. a,b. Sphenolithus abies Deflandre: (6a) cross-polarized light, and (6b) plane light. a,b. Ceratolithus armatus Muller: (7a) cross-polarized light, and (7b) plane light. a,b. Ceratolithus armatus Muller: (8a) cross-polarized light, and (8b) plane light. a,b. Ceratolithus armatus Muller: (9a) cross-polarized light, and (9b) plane light. a,b.Amaurolithus primus (Bukry and Percival) Gartner and Bukry: (10a) cross-polarized light, and (10b) plane light. Discoaster brouweri Tan. Plane light. Discoaster brouweri Tan. Plane light. Discoaster pentaradiatus Tan. Plane light. Discoaster pentaradiatus Tan. Plane light. Discoaster surculus Martini and Bramlette. Plane light. Discoaster surculus Martini and Bramlette. Plane light. Discoaster quinqueramus Gartner. Plane light. Discoaster quinqueramus Gartner. Plane light. Discoaster berggrenii Bukry. Plane light. Discoaster berggrenii Bukry. Plane light. 1a 2a 3a 4a 5a 1b 2b 3b 4b 5b 6a 7a 8a 9a 10a 6b 7b 8b 9b 10b 11 12 13 14 15 16 17 18 19 20

Benthic foraminifera

Nine samples from CS2 were examined for benthic foraminifera. The benthic foraminifera of CS2 belong to a marine shelf assemblage. Two samples from CS3 were examined. The assemblage present in CS3 is consistent with deposition on a marine shelf (Tables 4 and 5).