Sr isotopic age estimates were obtained from mollusk shells. Approximately 4–6 mg of shells was cleaned in an ultrasonic bath and HCl and dissolved in 1.5 N HCl. Sr was separated using standard ion exchange techniques (Hart and Brooks, 1974). The samples were analyzed on an Isoprobe T Multicollector thermal ionization mass spectrometer (TIM). Internal precision on the Isoprobe for the data set averaged 0.000007 and the external precision is approximately cate analyses of standards). National Bureau of Standards (NBS) 987 is measured for these analysis at 0.710241 normalized to 86Sr/88Sr of 0.1194.
Cretaceous ages were assigned (Table T7) using linear regressions developed for upper Coniacian through Maastrichtian sections by Miller et al. (2004). Using a similar late Campanian–Maastrichtian regression, Sugarman et al. (1995) conservatively estimated age errors of 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 of the regressions of ~0.000020/m.y.). For comparison, Table T7 also shows ages derived from the look-up tables of McArthur et al. (2001).
The youngest sediments recovered in the corehole (Eocene to Paleocene) are not suitable for strontium isotopic dating because strontium ratios change too slowly to allow time discrimination. Two samples from the Vincentown Formation were analyzed from these sections and had ages of 57.1 and 57.9 Ma (30 and 47 ft [9.1 and 14.2 m], respectively), confirming the Paleocene age of the sediments, but the ages are not plotted on Figure F2 because of low age resolution.
Strontium isotopic ages were obtained from three samples (at 61, 90, and 93 ft; 18.5, 27.3, and 28.2 m) in the Navesink Formation. Ages range from 66.0 to 67.7 Ma. This is in agreement with calcareous nannofossil biostratigraphy indicating a late Maastrichtian age, and previous age estimates from other localities in New Jersey using Sr isotopes and nannofossils (Sugarman et al., 1995). The sedimentation rate for the sequence is ~8.3 m/m.y.
Six Sr isotopic ages were obtained from the Marshalltown sequence. The youngest age of 72.2 Ma was at the top of the sequence at 101.0 ft (30.6 m) and the oldest age 0f 76.0 Ma was at the bottom of the sequence at 216.0 ft (65.5 m). The two ages yield an overall age for the sequence of 72.2–76.0 Ma and a sedimentation rate of ~9 m/m.y.
Six samples including two duplicates yielded four ages from the upper Englishtown sequence. The four ages range from 75.8 to 78.1 Ma. This is consistent with calcareous nannofossil biostratigraphy, and the ages are similar to those obtained at the Sea Girt site (Miller et al., 2006). Because of the scatter in the data (Table T7), a sedimentation rate cannot be calculated.