SUMMARY AND CONCLUSIONS

The Cape May site was the third borehole drilled as part of the New Jersey coastal plain drilling project (Leg 150X). It focused on middle Miocene to upper Eocene sequences. Recovery was good (75% mean, 85% median), and gamma and neutron logs were obtained to a total depth (TD) of 1500 ft (457 m). This location is the most downdip onshore location in New Jersey, and the Miocene section is the thickest (823 ft; 251 m) of the three Leg 150X boreholes, with numerous shells suitable for Sr-isotopic age measurements. Unconformities are revealed by erosional surfaces, gamma-ray peaks, lithologic breaks, biofacies shifts, shell beds, indurated zones, and hiatuses determined by Sr-isotopic stratigraphy and biostratigraphy.

Good recovery of sands and clays in the top 357 ft (109 m) provide excellent examples of fluvial-estuarine and marginal marine environments, although age control on this section is poor. The top 90 ft (27 m) of the borehole recovered alternating organic-rich clays and shelly sands deposited in an inlet-marsh environment similar to the modern setting. These Pleistocene to Holocene strata are assigned to the Cape May Formation. Clays from 90 to 140 ft (27-43 m) are tentatively assigned to the Cape May Formation (?Pleistocene). The restricted-marine diatom flora and benthic foraminiferal fauna indicate a lower estuarine environment. This unit overlies a thick (140-357 ft; 43-109 m) unit of poorly fossiliferous estuarine sands and clays. Based on stratigraphic studies of the nearby Cape May airport borehole, we tentatively correlate this thick section with a Pliocene unit mapped in the Cape May peninsula (Owens et al., 1995).

Four middle Miocene sequences representing 258 ft (77 m) were recovered, including the youngest fossiliferous middle Miocene sequence sampled in New Jersey (ca. 11-12 Ma). We are uncertain if this upper unit (the Kirkwood-Cohansey sequence) correlates with the poorly dated Cohansey Formation or older middle Miocene sequences. Three additional middle Miocene sequences were recovered, including the probable equivalent of the Kirkwood 3 sequence of Sugarman et al. (1993; dated as 13.1-13.7 Ma at Atlantic City; Miller et al., 1994a, 1994b), a previously unknown sequence (dated as ~14.3-14.8 Ma), and the Kirkwood 2b sequence of Sugarman et al. (1993; 15.9-16.3 Ma). The significance and age of the 14.3-14.8 Ma sequence requires verification.

The thick lower Miocene section (565 ft; 172 m) represents at least five well-developed sequences. The Kirkwood 2a sequence (650-715 ft; 198-218 m) of Sugarman et al. (1993) is thinner than at Atlantic City where it is well dated (ca. 17.0-18.1 Ma). This sequence is a classic coarsening-upward "New Jersey" cycle: (1) basal shelly, sandy silts and clays (shelf environments); (2) medial micaceous, carbonaceous, laminated clay-silts and fine sands (prodelta environments); and (3) upper interbedded sands and carbonaceous silts (delta front environments). A thick (710-850 ft; 216-259 m), 18.6- to 19.5-Ma sequence was recovered at Cape May, but it is not present at Atlantic City, nor has it been unequivocally identified elsewhere in New Jersey. The thick (850-1062 ft; 259-324 m) Kirkwood 1 sequence (ca. 20-21 Ma) is well represented at Cape May and throughout the surface and subsurface in New Jersey. The Kirkwood 1 sequence may be divided into the Kirkwood 1b (850-942 ft; 259-287 m; ca. 20.1 Ma) and Kirkwood 1a (942-1062 ft; 287-324 m; 20.4-20.9 Ma) sequences. Although no discernible hiatus is present between these sequences, there is clearly a major disconformity indicated by gamma logs, facies shifts, and an irregular surface at the contact.

A lowermost Miocene (ca. 23.1-22.7 Ma) Kirkwood 0 sequence of glauconite sands is the thick equivalent of a sequence bracketed by shell beds at Atlantic City. The transition from the Kirkwood 0 to Kirkwood 1 sequences in the coastal plain boreholes appears to mark a fundamental change in depositional regime from shelfal to deltaic dominated, although offshore seismics suggest deltaic deposition may have locally begun earlier (Greenlee et al., 1988).

At least one upper upper Oligocene sequence (1180-1249 ft; 360-381 m; ca. 25.5 Ma) was recovered; further studies are needed to determine if this is, in fact, two sequences. Another upper Oligocene sequence may be represented by Subzone P21b (ca. 28-29 Ma). A major "middle" Oligocene unconformity separates these sequences from the lower Oligocene, with a hiatus of at least 3 m.y. The presence of "middle" Oligocene strata at Atlantic City and its absence at Cape May is attributed to minor differences in subsidence histories between locations.

Two lower Oligocene sequences represent a thick Zone P19 to P20 sequence and a thin Zone P18 sequence. The lower Oligocene (Zone P18) unconformably overlies the upper Eocene. The upper Eocene comprises uniform, neritic clays (Zone P15-P17) to TD.

Facies changes within Oligocene to middle Miocene sequences (systems tracts) are clear. The bases of the sequences contain shell beds (typically 1-2 ft [0.3-0.6 m] thick) that grade up to clays/silts (typically 15-90 ft [4.5-27 m] thick) and sands (typically 40-120 ft [12-37 m] thick). Capping the Miocene sequences are indurated zones that may reflect subaerial exposure. At the Atlantic City and Cape May boreholes, Oligocene sequences contain glauconite throughout; the glauconite in the medial silts and upper sands is probably recycled from Eocene and older strata and tends to obscure the facies succession. Further biostratigraphic and Sr-isotopic studies are needed to refine the ages of the sequences, whereas further lithostratigraphic and benthic foraminifer biofacies studies should reveal details of the depositional environments and systems tracts of these well-developed sequences. Recovery and dating of upper Eocene to middle Miocene sequences at the three Leg 150X boreholes will allow us to attain our primary goal: to evaluate the roles of glacio-eustasy, tectonics, and changes in sediment supply on the development of sequences.

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