standard deviation 0.000008; n = 22) normalized to 86Sr/88Sr of 0.1194.
Sr isotopic age estimates were obtained from mollusk shells. Approximately 4–6 mg of shells or foraminiferal 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 (New Jersey). Internal precision on the sector for the data set averaged 0.000009 and the external precision is approximately ±0.000020 (Oslick et al., 1994). NBS 987 is measured for these analysis at 0.710255 (2 standard deviation 0.000008; n = 22) normalized to 86Sr/88Sr of 0.1194.
Cretaceous ages were assigned using linear regressions developed for upper Coniacian through Maastrichtian sections by Miller et al. (2004). Using a similar upper Campanian–Maastrichtian regression, Sugarman et al. (1995) conservatively estimated age errors of ±1.9 m.y. at the 95% confidence interval for one Sr isotopic analysis; age errors for the coeval and older sections are purportedly one order of magnitude better according to Howarth and McArthur (1997). We estimate that the maximum Sr isotopic age resolution for this interval is ±1 m.y. (i.e., the external precision of ~0.000020 divided by the slopes of the regressions of ~0.000020/m.y.)
Strontium isotopic ages were obtained from three samples in the Woodbury Formation in the Fort Mott borehole (Fig. F4). The sample at 61 ft (18.59 m) (86Sr/87Sr = 0.707678 ± 12) yielded an age estimate of 73.8 Ma; the sample at 62.6 ft (19.08 m) (86Sr/87Sr = 0.707622 ± 7) yielded an age estimate of 75.6 Ma; and a sample at 84.2 ft (25.66 m) (86Sr/87Sr = 0.707620 ± 6) yielded an age estimate of 75.7 Ma. These ages are substantially younger than the ages that would be expected based on calcareous nannofossil biostratigraphy, and it is concluded that these samples have been altered by diagenesis.
