AGE OF MARINE MOLLUSKS FROM THE LOWER MIOCENE POLLACK FARM SITE , DELAWARE , DETERMINED BY 87 SR / 86 SR GEOCHRONOLOGY 1

Analyses of 87Sr/86Sr ratios in the shells of marine bivalve mollusks from the Pollack Farm Site in Kent County, Delaware, indicate that the fossils represent an early Miocene assemblage which lived about 17.9 ± 0.5 Ma. Faunal similarities, as well as Sr-isotopic data, support a correlation between the fossils at the Pollack Site and portions of the Kirkwood Formation to the north (New Jersey) and to the south the Fairhaven Member of the Calvert Formation (Maryland), the Pungo River Formation (North Carolina), and the Chipola Formation (Florida). A strong marine-nonmarine link with terrestrial vertebrates of the Hemingfordian North American Land Mammal Age is also suggested. 1 In Benson. R.N. , ed., 1998, Geology and paleontology of the lower Miocene Pollack Farm Fossil Site, Delaware: Delaware Geological Survey Special Publication No. 21, p. 21–25. 2 Florida Museum of Natural History, University of Florida, Gainesville, FL 32611 3 Virginia Museum of Natural History, 1001 Douglas Avenue, Martinsville, VA 24112 4 Department of Geology, University of Florida, Gainesville, FL 32611 MATERIALS AND METHODS Five valves from articulated specimens of marine bivalve mollusks were selected from four individuals collected by L.W. Ward in November 1991 and by L.W. Ward and D.S. Jones on 15 June 1992. The specimens came from the main shelly unit, the approximately 3.0 m-thick Bed b of Ward (1998) which is the lower shell bed at the site as described by Ramsey (1998). Three individuals (Table 1, AC) of Mercenaria ducatelli (Conrad), with aragonitic shells, and one Crassostrea virginica (Gmelin) (Table 1, D), with a calcitic shell, were used. Each specimen was examined microscopically for evidence of alteration or recrystallization. In addition, X-ray diffraction (XRD) and analysis of Sr/Ca ratios by atomic absorption spectrophotometry were performed on specimens of Mercenaria ducatelli to assess potential diagenetic effects which, in all but one case, were found to be minimal (Stanley, 1992). Scanning electron microscope (SEM) examination of the exterior shell surface of the right valve of specimen A revealed destruction and overgrowth of original shell microstructure. XRD analysis of powdered shell material indicated the presence of both calcite and aragonite, suggesting partial diagenetic alteration of this shell. SEM inspection and XRD analyses of the opposing (left) valve of this specimen indicated pristine microstructure without calcite. Both right and left valves were analyzed for comparative purposes (Table 1 specimens A, A'). Sample powders, drilled from the outer shell layer of each valve, were analyzed for strontium isotopic composition (87Sr/86Sr) in the Department of Geology at the University of Florida using standard techniques of dissolution, centrifugation, evaporation, cation exchange chemistry, and mass spectrometry (Hodell et al., 1991). Five separate analyses, including one “duplicate” (same individual mollusk, opposite valve) were made. 87Sr/86Sr analyses were normalized to 86Sr/88Sr = 0.1194. The NBS standard SrCO3 (SRM-987) was measured at 0.710244 during the course of this study with a long-term analytical precision (2s) of ±2 X 10-5. The ratios reported in Table 1 are corrected to SRM987 = 0.710235 so that they may be directly correlated to the Sr-isotope seawater curve of Hodell et al. (1991) and Hodell and Woodruff (1994). Corrections to SRM-987 = 0.710252 and = 0.710255 permit comparisons with seawater Sr-isotope reference curves developed by Miller et al. (1991) and Oslick et al. (1994), respectively. Measured Sr-isotope ratios were converted to estimates of absolute age using the regression equations of Hodell et al. (1991), Miller et al. (1991), and Oslick et al. (1994). Discrepancies between age estimates arising from these three different equations are relatively small for the early Miocene, within the error of the various age estimates. Detailed discussions of errors associated with Sr-isotopic ages can be found in Hodell et al. (1991) and Miller et al. (1991). For the time interval considered here, errors about single sample Sr-isotopic age estimates typically fall into the range of ±0.5 to no more than ±1.0 Ma at the 95% level of confidence. RESULTS: AGE ESTIMATES The results of the Sr-isotopic analyses are reported in Table 1. Values for 87Sr/86Sr range from 0.70848 (right valve of specimen A with calcitic alteration) to 0.70862. Excluding specimen A (right valve), the ratios of the other samples cluster tightly about the mean ratio value (0.70861), falling well within the range of long-term analytical precision (2s) = ±2 X 10-5. There is very little heterogeneity among the unaltered samples at the site. As indicated in Table 1, the 87Sr/86Sr of the right valve of specimen A (0.70848) was significantly lower than the others. Because the right valve of this shell revealed evidence of secondary calcite overgrowth and replacement near the shell exterior, and because its 87Sr/86Sr was substantially lower than that of the left (unaltered) valve, as well as the other specimens, this ratio was not included in age determinations. The 87Sr/86Sr values for specimens A'-D all gave ages in the range 17.8-18.3 Ma. Errors associated with these ages are on the order of ± 0.5-1.0 m. y. (Hodell, 1991; Miller et al., 1991; Oslick et al., 1994). When all four sample measurements (A'-D) were pooled, and an age calculated for the mean 87Sr/86Sr, the resulting age was 17.9 to 18.2 Ma, depending on which seawater Sr-isotope reference curve was used (Table 1). DISCUSSION: COMPARISON WITH OTHER Sr-ISOTOPIC DATA The early Miocene ocean was characterized by rapidly rising 87Sr/86Sr, making this portion of the Neogene particularly suitable for Sr-isotopic geochronology (Hodell et al., 1991; Hodell and Woodruff, 1994; Oslick et al., 1994). Hence, the ages calculated here from the mollusk 87Sr/86Sr measurements serve to place the fossils recovered from the Pollack Site into a fairly narrow chronostratigraphic context (Fig. 1). The ratios presented in Table 1, with their associated age calculations, suggest that the shells in this deposit range in age from approximately 17.9 to 18.2 ± 0.5 Ma. Without additional samples from multiple specimens collected at successive stratigraphic horizons throughout the sequence, it is impossible to refine the age structure of the deposit. Stratigraphic condensation, which has been observed elsewhere in Coastal Plain shell beds, can result in mixtures of shells of different ages and different Sr-isotopic ratios (e.g., Webb et al., 1989; Jones et al., 1995), but this process does not appear to have played a major role here. However, it was interesting to discover a specimen of Mercenaria ducatelli in which one valve (sample A') appears to have faithfully record22 Table 1. 87Sr/86Sr ratios, within-run error, and age estimates for marine mollusk samples from the Pollack Site, Kent County, Delaware. Columns 1 and 2 are based on the time scale of Berggren et al. (1985) whereas column 3 is based on Cande and Kent (1992) which is essentially the same time scale as Berggren et al. (1995) over this interval. Specimen 87Sr/86Sr Age (Ma) 1 2 3 A. Mercenaria ducatelli 0.708479±6 +++ +++ +++ A’. Mercenaria ducatelli 0.708618±6 17.8 18.0 17.8 B. Mercenaria ducatelli 0.708599±5 18.1 18.3 18.1 C. Mercenaria ducatelli 0.708619±11 17.8 18.0 17.8 D. Crassostrea virginica 0.708604±9 18.1 18.3 18.0 mean 0.708610 18.0 18.2 17.9 1 Hodell et al. (1991); 2 Miller et al. (1991); 3 Oslick et al. (1994). +++ Sample diagenetically altered (see text).