Sr isotope age estimates were obtained from mollusk shells (~4–6 mg) at the Cape May Zoo borehole (Table T3; Figs. F2, F4, F6, F8, F10, F13, F15). Shells were cleaned ultrasonically 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 isotope mass spectrometer. Internal precision on the Isoprobe for the data set averaged 0.000007 and the external precision is approximately tional Bureau of Standards (NBS) 987 is measured for these analysis at 0.710241 normalized to a 86Sr/88Sr ratio of 0.1194. Ages were assigned using the Berggren et al. (1995) timescale (Table T3), and the Miocene regressions of Oslick et al. (1994) were used where possible. The Oslick et al. (1994) regressions are only valid to sections older than 9.9 Ma (Sr isotopic values < 0.708930). For the Pleistocene analysis, we derived a linear regression using the data of Farrell et al. (1995), correcting their data to NBS 987 of 0.710255 and fitting linear segments to the data between 0 and 2.5 Ma:
Miller et al. (1991) and Oslick et al. (1994) estimate age errors derived from linear regressions of Sr isotopic records. Age errors are for 15.5–22.8 Ma and –15.5 Ma at the 95% confidence interval for a single analysis. Increasing the number of analyses at a given level improves the age estimate (analyses each in the two intervals) (Oslick et al., 1994). The regression for the late Pliocene–Pleistocene (0–2.5 Ma) has an age error of m.y. (for one analysis at the 95% confidence interval) to three analyses at the 95% confidence interval) (K.G. Miller, unpubl. analysis of data of Farrell et al., 1996).
The upper 280 ft (85.34 m) of the borehole was only locally fossiliferous for carbonate, and age estimates rely primarily on pollen and dinocyst data. Shells were numerous between 50 and 82 ft (15.24 and 24.99 m) in the corehole (Unit 1 of the Cape May Formation) (Fig. F2). Six ratios were obtained from shells in this interval, providing ages ranging from 0 to 1.23 Ma and an average age of 0.4 Ma; however, there is substantial scatter to the data and the average age is in agreement with ages provided by amino acid racemization (~0.3–0.4 Ma).
The Kirkwood Formation was generally fossiliferous for mollusks, and an excellent chronology was obtained using Sr isotopes. Sequence Kw-Ch2 only contained shells in the lower 9.4 ft (2.87 m; between 283.9 and 293.3 ft [86.53 and 89.40 m]). Two isotopic ages were obtained on shells at 285.5 and 292.9 ft (87.02 and 89.28 m) yielding ages of 12.0 and 12.1 Ma, respectively (latest middle Miocene) (Figs. F6, F15) consistent with previously assigned ages of 12.1–11.5 Ma for the Kw-Ch sequence (Miller et al., 1997; 1998a).
Sequence Kw-Ch1 yielded three ages of 13.1 (311.7 ft; 95.01 m), 12.7 (323.7 ft; 98.66 m), and 13.7 Ma (324.4 ft; 98.88 m), with an average age of 13.1 Ma. The oldest age estimate comes from a sample taken very close to the sequence boundary, yielding an age similar to those from samples below the unconformity. The sample may be reworked. The age-depth plot provides a rough estimate of 12.8–12.9 Ma for Sequence Kw-Ch1.
Fourteen age estimates obtained from Sequence Kw3 yielded ages ranging from 11.8 to 14.0 Ma (Figs. F8, F15). There is considerable scatter to the data, and the ages do not monotonically decrease upsection. The average age for these samples is 13.4 Ma, which is in agreement with previous estimates for Sequence Kw3 (Miller et al., 1997, 1998a). The age-depth plot provides a rough estimate of 13.2–13.6 Ma for Sequence Kw3.
Sequence Kw2b (Wildwood Member) is well represented in the Cape May Zoo corehole. Four age estimates were obtained on samples between 415.7 and 479.2 ft (126.71 and 146.06 m), ranging from 16.3 to 15.8 Ma (Figs. F10, F15). These ages are consistent with ages previously assigned to Sequence Kw2b (16.1–15.6 Ma) (Miller et al., 1997, 1998a). No ages consistent with Sequence Kw2c (14.7–14.3 Ma) were encountered in the corehole, suggesting that Sequence Kw2c is not present.
The age of the sediments between 479.2 and 515.7 ft (146.06 and 157.19 m) is difficult to interpret, though we tentatively assign them to Sequence Kw2b. Two Sr ages were obtained (Figs. F10, F15). The sample at 501.9 ft (152.98 m) was run twice, giving ages of 16.7 and 16.8 Ma, and a sample at 512.0 ft (156.06 m) gave an age of 16.6 Ma. These are consistent with previous estimates for Sequence Kw2a (17.8–16.5 Ma; Miller et al., 1997; 1998a), not Sequence Kw2b. Diatoms at 487.2 ft (148.50 m) are assigned to the lower part of ECDZ 3–4, consistent with assignment to Sequence Kw2b. Diatom zones are not well calibrated to the Berggren et al. (1995) timescale, and it may be that the lower part of ECDZ 3–4 is found in the upper part of Sequence Kw2a. There is also the possibility that an irregular contact at 506.6 ft (154.41 m) is a sequence boundary, which would explain the 16.6 Ma age at 512.0 ft (156.06 m). This still fails to explain the 16.75-Ma age at 501.9 ft (152.98 m), though it is possible that this sample could be reworked. We retain the sections from 479.2 to 515.7 ft (146.06 to 157.19 m) in Sequence Kw2b, which has an age of 16.1–15.6 Ma elsewhere (Miller et al., 1997; 1998a). We acknowledge, however, that the sequence boundary could be 506.6 ft (154.41 m) and the one sample is reworked.
Sequence Kw2a (515.7 and 630.7 ft; 157.19 and 192.24 m) is well defined using Sr isotopes. Six isotopic ratios were obtained with ages ranging from 17.8 to 16.6 Ma with an average age of 17.3 Ma (Figs. F10, F15). The ages do not regularly decrease in age upsection, and all of the ages except the oldest age estimate (17.8 Ma at 567.3 ft [172.91 m]) cluster within the previously defined age range for Sequence Kw2a of 17.8–16.5 Ma (Miller et al., 1997; 1998a).
Sediments between 630.7 and 643.5 ft (192.24 and 196.14 m) are assigned to Sequence Kw1c. Seven age estimates were obtained ranging from 18.8 to 19.2 Ma, which is within the ages previously given (18.4–19.4 Ma) (Miller et al., 1997, 1998a) for Sequence Kw1c (Figs. F13, F15). Sediments of this age were previously only known from the Cape May corehole (Miller et al., 1996a).
Sediments between 643.5 and 656.5 ft (196.14 and 200.10 m) are difficult to interpret. Five age estimates obtained range in age from 18.5 to 20 Ma (Figs. F13, F15). Four of the five samples are consistent with an assignment to Sequence Kw1b. A single sample at 651 ft (198.42 m) gave an age of 18.5 Ma, consistent with Sequence Kw1c. The section appears continuous with the section assigned to Sequence Kw1c above. It may be that these are Sequence Kw1b sediments reworked during the deposition of Sequence Kw1c.
The section from 656.5 to 713 ft (200.10 to 217.32 m) is assigned to Sequence Kw1b. Four age estimates were obtained with ages ranging from 20.1 to 19.2 Ma (Figs. F13, F15). All of these ages are consistent with Sequence Kw1b elsewhere (19.5–20.1 Ma) (Miller et al., 1997; 1998a).