Thirty-six Sr-isotopic age estimates were obtained from shells at the Cape May borehole (Table 2; Figs. 2, 3, 4, 5,). Shells were sonified and dissolved in 1.5 N HCl. Sr was separated using standard ion exchange techniques and analyzed on a VG sector mass spectrometer at Rutgers University (see Miller et al., 1991a, for procedures). Ages were assigned using both the Berggren et al. (1985; hereafter cited as BKFV85) and Cande and Kent (1992; hereafter cited as CK92) time scales (Table 2); results are discussed using BKFV85 to compare with previous Sr-isotopic studies of the New Jersey subsurface and Leg 150 studies (Miller et al., 1990, 1994a, 1994b; Sugarman et al., 1993).
The Oligocene regressions are those of Miller et al. (1988) and Oslick et al. (1994), which rely on the BKFV85 and CK92 time scales, respectively. Miocene age estimates were based on Oslick et al. (1994) for both BKFV85 and CK92. The Oligocene regression (applicable to ca. 22.8 Ma; Oslick et al., 1994) has age errors of about 1 m.y. (for one analysis at the 95% confidence interval; Miller et al., 1988). The Miocene regressions for 15.6 to 22.8 Ma have age errors of 0.6 m.y. (for one analysis at the 95% confidence interval) to 0.4 m.y. (for three analyses at the 95% confidence interval), whereas the Miocene regressions from ca. 10 to 15.2 Ma have age errors of 1.2 (CK92) to 1.4 (BKFV85) m.y. (for one analysis at the 95% confidence interval) to 0.8 (CK92) to 0.9 (BKFV85) m.y. (for three analyses at the 95% confidence interval).
Our preliminary studies focused on sequences identified in the lower to middle Miocene Kirkwood Formation by Sugarman et al. (1993) where calcareous plankton biostratigraphy is poor due to rare occurrences of taxa. Seven age estimates from shell beds between 357 and 432 ft (108.8-131.7 m) are 11.1-11.9 Ma, dating this sequence as younger than any definitively dated Miocene section in New Jersey. As discussed above, this sequence could be the downdip marine equivalent of the nearshore Cohansey Formation. It apparently correlates with an undated sequence at Atlantic City (Fig. 5). One age estimate of 12.5 Ma at 358.4 ft (109.3 m) is considerably older than those below (Table 2), and we regard it as anomalous and subject to replication.
No samples were dated for the next sequence (432-503 ft; 131.7-153.3 m). Our correlations (Fig. 5) indicate that the section is equivalent to the Kirkwood 3 (= ECDZ 6) sequence dated as 13.1-13.7 Ma at Atlantic City.
Samples at 529.5 ft and 574 ft date the 503-576 ft (153.3-175.6 m) sequence at ca. 14.3-14.8 Ma. It therefore represents a sequence that is not present at Atlantic City and has not been identified definitely elsewhere in New Jersey.
Age estimates of 16.2, 16.3, and 16.9 Ma were obtained at 582, 600, and 677.3 ft (177.4, 182.9, and 206.4 m), respectively. These ages indicate that the two sequences from 576 to 615 ft (175.6-187.5 m) and from 615 to 680 ft (187.5-207.3 m) correlate with the Kirkwood 2b and 2a sequences, respectively (= ECDZ 2) (Fig. 5). An estimate of 17.8 Ma at 706 ft indicates that the 680-710 ft (207.3-216.5 m) sequence is correlative to the Kw2a sequence recognized at Atlantic City (Fig. 5).
Four isotopic age estimates date the sequence(s) from 710 to 850 ft (216.5-259.1 m) as 18.6-19.3 Ma (Table 2). Once again, this sequence(s) is not represented at Atlantic City.
A group of analyses between 850 and 1062 ft (259.1-323.8 m) are 20.1-20.9 Ma (Fig. 5; Table 2) and are equivalent to the lower Miocene Kirkwood 1 sequence (= ECDZ 1). This agrees remarkably well with ages at Atlantic City (Fig. 5). It may be possible to subdivide Kirkwood 1 into two sequences with ages of 20.1-20.3 Ma and 20.5-20.9 Ma (Fig. 5), although this age discrimination is within the errors of Sr-isotopic resolution.
A thick lowermost Miocene sequence (1062-1180 ft; 323.8-359.8 m) has age estimates of 22.6-23.1 Ma. With the available data, it appears that this sequence correlates with the 21.9-24.1-Ma sequence at Atlantic City (Fig. 5).
Only three Sr-isotopic ages are available for the Oligocene at Cape May. They indicate that the Oligocene sequences between 1180 and 1249 ft (359.8-380.8 m) are upper upper Oligocene (<26 Ma; Fig. 6).