STRONTIUM ISOTOPIC STRATIGRAPHY

Nineteen Sr isotopic age estimates were obtained from mollusk and foraminifer shells (~4-6 mg) at the Ancora borehole (Table T1; Figs. F2, F4, F5, F6). Shells and foraminifer tests were cleaned ultrasonically and dissolved in 1.5 N HCl. Sr was separated using standard ion exchange techniques (Hart and Brooks, 1974), and analyzed on a VG Sector Mass Spectrometer at Rutgers University. Internal precision on the sector for the data set averaged 0.000025, and the external precision was approximately ±0.000020 (Oslick et al., 1994). NBS 987 was measured for these analyses at 0.710255 (2 standard deviation = 0.000008, N = 22), normalized to a ⁸⁷Sr/⁸⁶Sr of 0.1194. Cenozoic ages were assigned using the Berggren et al. (1995) time scale (Table T2), using the early Miocene regression of Oslick et al. (1994) with age errors of ±0.61 at the 95% confidence interval for one analysis. Cretaceous ages (Table T2) were assigned using the regressions of Sugarman et al. (1995) and Howarth and McArthur (1997); Sugarman et al. (1995) conservatively estimated Campanian-Maastrichtian errors of ±1.9 at the 95% confidence interval for one analysis (Table T2). Age errors for the coeval and older sections are purportedly one order of magnitude better according to Howarth and McArthur (1997). Recent evaluation of Campanian-Maastrichtian Sr isotopic correlations suggests that ±1 m.y. is a reasonable estimate of the error (Miller et al., 1999).

Because of the poorly fossiliferous nature of the Miocene and younger sediments of the borehole, the first Sr isotopic ratio was obtained from 243 ft (74.07 m). Marine Cretaceous sediments generally contained sufficient fossils for Sr isotopic analysis.

Three Sr isotopic age estimates from shells at the base of the Kirkwood Formation (243, 255, and 263 ft [74.07, 77.72, and 80.16 m]) ranged from 20.2 to 21.2 Ma and are correlated to the lower Miocene Kw1a sequence (Fig. F2; Sugarman et al., 1993; Miller et al., 1997).

Eleven Cretaceous samples yield ages similar to those based on biostratigraphy. Samples from the Navesink through Wenonah Formations (645.0-713.8 ft [196.60-217.57 m]) gave reliable ages. One reliable age estimate was obtained from each of the Marshalltown (750.8 ft [228.84 m]), Woodbury (863.5 ft [263.19 m]), and Merchantville (932.1 ft [284.10 m]) Formations.

Five samples yielded ages younger than predicted by biostratigraphy; these samples are listed in Table T1, although the ages are not plotted. Three samples have ages that are significantly younger than other Sr isotopic ages bracketing them (740.5, 776.7, and 892.1 ft [225.70, 236.74, and 271.91 m] from the Marshalltown, Englishtown, and Woodbury Formations, respectively). Previous studies of the New Jersey Coastal Plain samples have shown that diagenesis is a potential problem for some Upper Cretaceous samples (Sugarman et al., 1995), particularly for aragonitic shells that have a vesicular wall structure (e.g., Pycnodonte). Two samples from the Cenomanian portion of the Bass River Formation yield one age (82.2 Ma) that is younger than the actual age of >91.3 Ma and one age that could either be younger (85.0 Ma) or older (96.7-101.0 Ma) than its actual age; it is not clear if this is a diagenetic problem or if this results from problems in the Sr isotopic age calibration.