STRONTIUM ISOTOPE CHRONOSTRATIGRAPHY

Sr isotopic age estimates were obtained from mollusk shells. Approximately 4–6 mg of shells were cleaned in HCl, ultrasonically cleaned, and dissolved in 1.5-N HCl. Sr was separated using standard ion exchange techniques (Hart and Brooks, 1974). The samples were analyzed on two different machines at Rutgers University; a VG Sector mass spectrometer (Table T10) and an Isoprobe T Multicollector thermal ionization mass spectrometer (TIMS) (Table T2). Internal precision on the Sector 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 (replicate analyses of standards). National Bureau of Standards (NBS) reference material 987 is measured for these analyses at 0.710255 (2 standard deviation = 0.000008, N = 22) normalized to 86Sr/88Sr of 0.1194 for the Sector and 0.710241 for the Isoprobe.

Miocene Sr isotopic ages were assigned using the Cande and Kent (1995) timescale (Table T2) and the early and middle Miocene regressions of Oslick et al. (1994), with age errors of ±0.61 and ±1.17 m.y. at the 95% confidence interval for one analysis, respectively. The Oligocene–Miocene portion of the Cande and Kent (1995) timescale is identical to the Berggren et al. (1995) timescale that is used to assign calcareous and planktonic foraminiferal ages.

Strontium ages were obtained on 22 samples from the Kirkwood and upper Shark River Formations (Table T10; Figs. F10, F11). No shells were found above 342.5 ft (104.39 m). Samples at 365.9 and 402.3 ft (111.53 and 122.62 m) yielded ages of 8.5 and 0.9 Ma, respectively. These ages are very different from other Miocene samples at this stratigraphic level and are assumed to represent either contamination or diagenesis of these samples. These data points are not plotted on Figure F10. The remaining samples between 342.5 and 407.9 ft (104.39 and 124.35 m) yielded ages ranging in age from 19.0 Ma at 402 ft (122.53 m) to 21.0 Ma at 371.65 ft (113.28 m) (Fig. F10). The heavy line on Figure F10 represents our preferred age interpretations for these sediments. Neglecting 3 samples that fall off the line (stippled circles), the mean age for 11 samples is 20.4 Ma, with a preferred age estimate of 20.2–20.6 Ma. This correlates these sediments to the Kw1a sequence of Sugarman et al. (1993). The sedimentation rate for the Kw1a sequence at Millville is 51.8 m/m.y., which is comparable to the 40 m/m.y. sedimentation rate found by Miller et al. (1997) for this sequence at Cape May and Atlantic City.

Five samples between 425.5 and 457 ft (129.69 and 139.29 m) yielded Eocene ages that are outside of the useful range for Sr isotopic analysis.

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 ±0.5 to ±1.0 m.y. (i.e., the external precision of 0.000010 [Isoprobe] to 0.000020 [Sector] divided by the slopes of the regressions of ~0.000020/m.y.). For Cenomanian through Turonian sections, ages were assigned using the look-up tables of Howarth and McArthur (1997), who measure NBS 987 at 0.710248. Thick mollusk shells were not common in the Millville borehole, and thin shells gave ages that were not in agreement with the calcareous nannofossil biostratigraphy.

Sr isotopic ages were obtained from five samples in the Navesink Formation. These samples yielded ages ranging from 28.4 to 0.4 Ma and therefore they must be altered by diagenesis. These samples are not plotted on Figure F11. Age relations for the Maastrichtian are based upon calcareous nannofossil biochronology.

Thirteen Sr isotopic ages were obtained from Campanian through Santonian sediments. Two samples in the Marshalltown sequence (1033 and 1042 ft; 314.86 and 317.6 m) yielded ages of 72.1 and 74.7 Ma, respectively. (A third age of 0.6 Ma at 1045 ft [318.52 m] reflects diagenesis and is not included on Fig. F10). If both of these ages are correct, the sedimentation rate would be no higher than 3.1 m/m.y. In the Ancora borehole, the Marshalltown sequence had a sedimentation rate between 7.7 and 16.2 m/m.y. We assumed that there was some postdepositional mixing of sediments in the Marshalltown sequence at Millville. We prefer the younger age (72.1 Ma) because it is in agreement with the age assigned using calcareous nannofossils. Age data are too sparse to allow a confident interpretation of this sequence's sedimentation rate.

Two Sr isotopic ages, 76.2 Ma at 1060 ft (323.09 m) and 75.0 Ma at 1073 ft (327.05 m), were obtained from the upper Englishtown sequence in the Millville borehole (an age of 55.9 Ma from a sample at 1084.2 ft [330.46 m] is assumed to have been altered by diagenesis). These ages are compatible with the age of the upper Englishtown obtained in the Ancora and Bass River boreholes. The sedimentation rate of the Englishtown sequence in the Millville borehole is 12.6 m/m.y. based on the age relationships shown in Figure F11. This is comparable to the 15.2 m/m.y. sedimentation rate found at Ancora and the 10.9 m/m.y. sedimentation rate found at Bass River (Miller et al., 2004).

Four strontium ages were obtained on thin-walled bivalve shells from the Woodbury Formation (Merchantville III sequence) at Millville. These provided ages of 75.7–75.0 Ma, which are younger than the Merchantville III sequence at either Ancora or Bass River (Miller et al., 2004). The Millville Sr isotopic ages from the Woodbury Formation do not agree with calcareous nannofossil biostratigraphy. The thin shells found in the Woodbury Formation do not provide reliable ages, most likely because of diagenesis. An age of 82.2 Ma was obtained from a shell at 1237 ft (377.04 m) in the Merchantville I sequence. This is comparable with the age of this sequence found at Ancora and Bass River (Miller et al., 2004).

The Cheesequake sequence at Millville is dated with Sr isotopic ages of 82.1 and 85.2 Ma at 1249 and 1249.5 ft (380.70 and 380.85 m), respectively. The age of 85.2 Ma is in agreement with the assignment to nannofossil Zone CC15.

The Bass River sequences are difficult to date because the Sr isotopic record has minima and maxima in the Coniacian through Cenomanian (Howarth and McArthur, 1997). Two potential ages for most isotopic values are plotted on Figure F11. We prefer the older ages for each sample because they are required by the pollen biostratigraphy and sparse calcareous nannofossil biostratigraphy. A few samples have only one age plotted because values above the maxima in the Sr isotopic curve are undefined. The sample at 1340 ft (408.43 m) yielded an age of 62.6 Ma and is not plotted on Figure F11. The Bass River III sequence had ages of 94.8 and 92.3 Ma at 1314 and 1331 ft (400.51 and 405.69 m), respectively. The Bass River II sequence had an age of 95.8 Ma at 1370 ft (417.58 m). The Bass River I sequence had ages of 98.0 and 99.3 Ma at 1384 and 1413 ft (421.84 and 430.68 m), respectively.

The ages of sequences at Millville are consistent with those at Bass River and Ancora (Fig. F9), although the chronology of the Millville corehole is not as firm. Less of the late Campanian–early Maastrichtian sediments appear to be represented at Millville, although the middle–late Maastrichtian appears to record and date two sequences that were poorly recognized previously (Fig. F9). Similarly, although the Turonian–Coniacian section is less complete at Millville, the Santonian–lower Campanian Merchantville sequences and the Cenomanian–Turonian Bass River sequences are well represented at Millville (Fig. F9).

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