standard deviation 0.000008, N = 22) normalized to 86Sr/88Sr of 0.1194 for the mass spectrometer and 0.710241 for the TIM.
Sr isotopic age estimates were obtained from mollusk shells (Table
T13). 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 mass spectrometer for the data set averaged 0.000012, and the external precision is approximately ±0.000020 (Oslick et al., 1994). Internal precision on the Isoprobe for the data set averaged 0.000007, and the external precision is approximately ±0.000010, based on replicate analyses of standards. National Bureau of Standards (NBS) 987 was measured for these analysis at 0.710255 (2 standard deviation 0.000008, N = 22) normalized to 86Sr/88Sr of 0.1194 for the mass spectrometer and 0.710241 for the TIM.
Insufficient carbonate was recovered from the Miocene section to allow Sr isotopic analyses. Correlations in these sections are based on superposition and sparse planktonic foraminifers. The Oligocene was not identified in the Sea Girt corehole.
Cretaceous ages were assigned 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 ±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 from ±0.5 to ±1.0 m.y. (i.e., the external precision of 0.000010 divided by the slopes of the regressions of ~0.000020/m.y.).
Most of the Sr analyses from shells below 691.2 ft (210.68 ft; i.e., below the Marshalltown sequence) yielded ages much younger than those predicted from calcareous nannofossil biostratigraphy. The discrepancy grew more pronounced deeper in the borehole. We conclude that the original Sr ratios in the carbonate have been altered and the Sr ages are not reliable below 691.2 ft (210.68 ft).
Sr isotopic ages were obtained from 10 samples in the Navesink Formation (Table T13). These samples yielded ages in two clusters. Seven samples (520.5–547.2 ft; 155.60–166.79 m) yield ages ranging from 62.8 to 65.0 Ma. These samples are Upper Cretaceous based on nannofossils, foraminifers, and regional correlations, which is within the error of the Sr isotopic ages. Three samples (562.7–563.6 ft; 171.51–171.79 m) yield ages ranging from 68.3 to 69.7 Ma. This is in agreement with calcareous nannofossil biostratigraphy and may indicate that there are two sequences in the Navesink Formation.
Six Sr isotopic ages (some from duplicate depths; 585.8–666 ft; 178.55–203.00 m) were obtained from the Marshalltown sequence (Table T13). The analyses yielded ages ranging from 72.5 to 75.2 Ma. These yield an overall age for the sequence of 72.7–75.8 Ma and a sedimentation rate of ~12 m/m.y.
Eight samples from the upper Englishtown sequence (691.2–837.7 ft; 210.68–255.33 m) yielded seven Sr isotopic dates (including a duplicate sample). Four samples yielded ages much younger (74.6–68.7 Ma) than predicted by calcareous nannofossil biostratigraphy and were affected by diagenesis. They are plotted in gray on Figure F17. The four analyses that agree with calcareous nannofossil biostratigraphy yield ages ranging from 78.2 to 77.7 Ma. These yield an overall age for the sequence of 77.7–72.0 Ma and a remarkably high sedimentation rate of ~63.8 m/m.y. The upper Englishtown sequence might be slightly older at Sea Girt than it is in other coreholes, though age control at other sites is poor to moderate (±2 m.y.). The upper Englishtown sequence was much thinner in other coreholes and the age determined here is believed to be slightly more reliable.
A total of 18 samples from the Sea Girt corehole below 837.7 ft (255.33 m) yielded ages for the sequences much younger (~2–10 m.y.) than predicted by calcareous nannofossil biostratigraphy and were much younger than the ages determined for those sequences in previously drilled coreholes. These analyses are plotted in gray on Figure F17 and ages for those sequences are determined using calcareous nannofossil biostratigraphy.
