The Ancora borehole was the fifth site drilled as part of the New Jersey Coastal Plain Drilling Project and the second site drilled as part of Ocean Drilling Program (ODP) Leg 174AX, which complements shelf drilling conducted during Leg 174A. Drilling at Ancora, New Jersey (location = 39° 41'31.975"N, 74° 50'56.459"W; elevation = 103.9 ft [31.65 m]; Hammonton 7.5-min quadrangle), targeted middle Miocene and older sequences, with the primary focus on Upper Cretaceous (Cenomanian-Maastrichtian) to Paleocene strata. Between 2 July and 11 August 1998, the United States Geological Survey (USGS) Eastern Earth Surface Processes Team (EESPT) drilled two holes at Ancora.
At Hole A, we recovered 693.82 ft (211.48 m) between the surface and 759 ft (231.34 m; 91.4% recovery), whereas at Hole B we recovered 591.48 ft (180.28 m) between 543 ft (165.55 m) and a total depth of 1170 ft (356.71 m; 94.3% recovery). Total recovery (92.7%) was remarkable and a gamma-ray log was obtained to total depth. The on-site scientific team provided descriptions of sedimentary textures, structures, colors, fossil content, identification of lithostratigraphic units, lithologic contacts, and sequences (unconformity-bounded units) that were integrated with subsequent biostratigraphic, Sr isotopic, and gamma ray-log studies.
Neogene sequences at Ancora are largely unfossiliferous except for the lowermost Miocene. The ?upper-middle Miocene Cohansey Formation consists of two sequences deposited in nearshore (beach, shoreface, and offshore sand ridge) environments. The Kirkwood Formation consists of two shallowing-upward lower Miocene sequences (Kw1a and ?Kw1b) deposited in neritic to prodelta to delta-front environments. Lowermost Miocene (Sequence Kw0) through uppermost Eocene strata are not represented at Ancora.
The upper to middle Eocene Shark River Formation consists of a very sandy upper Toms River Member, a sandy upper Shark River unit, and a clayier lower Shark River unit. The Toms River Member is potentially an important aquifer that comprises the highstand systems tract (HST) of a thick Sequence E9, spanning the middle/late Eocene boundary (Zones NP16 partim to NP18). A sequence that comprises most of the upper Shark River unit is assigned to Zone NP16 partim and correlates with Sequence E8, whereas most of the lower Shark River unit correlates with Sequences E7 and E6. Three thin sequences (E3-E6) span the middle/lower Eocene boundary and the Shark River/Manasquan Formation contact.
The lower Eocene Manasquan Formation at Ancora displays larger lithologic variations than previously drilled locations further downdip. The Manasquan Formation is dominated by calcareous silty clays ("marls") as at other locations, but fine quartz sand and reworked glauconite comprise highstand deposits within Manasquan sequences. Three sequences comprise the Manasquan Formation (E1-E3), with the bulk of the formation represented by Sequence E2 (Zones NP11-NP12 partim and P6b partim to P7).
Silty clays (equivalent in part to the Vincentown Formation) comprise an exceptionally thick (75 ft, 22.87 m) uppermost Paleocene section (Zone NP9). This section encompasses the late Paleocene thermal maximum (LPTM) and is associated with an intriguing interval of cross-bedded clay and silty clay, interrupted by two sections exhibiting convoluted bedding (543.5-545.5 and 559.2-560.0 ft; 165.66-166.27 and 170.44-170.69 m) with mini-flame and mini-ball structures. Both planktonic foraminifer and nannofossil "excursion taxa" occur in this interval and indicate that the carbon isotope excursion (CIE) associated with the LPTM occurs at or just below the base of the lower interval of convoluted bedding. The LPTM section at Ancora potentially represents one of the most expanded records of this global event. The CIE apparently overlies a possible sequence boundary at 562.1 ft (171.33 m). A glauconitic transgressive-regressive sequence occurs below this level in the "Vincentown equivalent."
Sequence boundaries are not readily observable in the lowermost Vincentown equivalent and the Hornerstown Formation because the entire lower part of the Paleocene section is uniform clayey glauconite sands. Three sequence boundaries were inferred in the lowermost Vincentown equivalent and the Hornerstown Formation based on biostratigraphic evidence, gamma ray-log kicks, and regional sequence stratigraphic relationships. The presence of Zones P1b and P2 indicates that the lower Paleocene section at Ancora is expanded relative to other coastal plain sites.
The Cretaceous/Tertiary (K/T) boundary in Hole A contains clay clasts above the uppermost Maastrichtian Navesink Formation; a spherule layer is absent. By contrast, Hole B contains a spherule layer at 617.1 ft (188.09 m). The presence of glauconite grains and foraminifers indicates that the original tektites were redeposited from deeper water. The K/T boundary is tentatively placed at an indistinct surface at 618.3 ft (188.50 m). Below this level, Cretaceous planktonic foraminifers and nannofossils are abundant. Both holes provide an excellent opportunity to assess the recovery of marine microorganisms from the terminal Cretaceous extinction event and the relationship of global sea-level change to the extinctions and recovery.
Cretaceous sequences are thick and well expressed lithologically. The Santonian-Maastrichtian section is dated by calcareous nannofossil biostratigraphy, limited planktonic foraminifer biostratigraphy, and Sr isotopic stratigraphy. The Santonian-Maastrichtian section consists of five sequences deposited in neritic paleoenvironments:
Recovery of the nonmarine ?Turonian-?Santonian Magothy Formation was excellent. The unit appears to consists of two sequences as it does at Bass River. The upper sequence consists of an upper very coarse sand to sandy gravel deposited in a delta plain and a lower slightly sandy silty clay and lignitic sand deposited in a delta-front or estuarine setting. The lower Magothy sequence is a thick unit of predominantly medium to coarse sand; its age and depositional setting are enigmatic, although it may have been deposited in a nearshore marine setting. It is unique in that it lacks the typical stratal succession found in other New Jersey sequences, perhaps reflecting unusually low sea level. The upper and lower sands of the Magothy Formation are both excellent aquifers.
The neritic Bass River Formation consists of fossiliferous, micaceous (chloritic), clayey silts and silty clays with occasional sandy silts. The Bass River Formation consists of three sequences at Ancora. Facies within these units are much more variable and vertical variations are more dramatic than at Bass River, reflecting not only the more updip location but also the preservation of two Cenomanian sequences not found at Bass River. Nannofossils indicate that the Bass River Formation at Ancora is Cenomanian below 1074.5 ft (327.51 m); the section above is barren of nannofossils. Planktonic foraminifers suggest that the upper part may be Turonian. The section from 1064 to 1066 ft (324.31-324.92 m) is darker with a faint petroleum scent; we speculate that this interval correlates with the lowermost Turonian Bonarelli bed equivalent observed at Bass River.
Abrupt facies variations occur within the Potomac Group that is dated as lower to middle Cenomanian by pollen, and as upper Albian to lower Cenomanian Zone CC9 by nannofossils. The section contains marginal neritic clays, marine to estuarine shelly clayey very fine sands and clays, and ?nonmarine kaolinitic clays. Several sequences may occur within the Potomac Group, although age and facies variations are not sufficient for unambiguous recognition of sequences and their global correlations.
The Ancora borehole has provided cores that address various aspects of global sea-level change, sequence stratigraphic development, and local hydrogeology. The Upper Cretaceous (Cenomanian-Maastrichtian) section complements the downdip Bass River borehole in New Jersey; this should allow reconstruction of sea-level variations for this interval and evaluation of sequence stratigraphic succession and facies models. The Ancora borehole also provides material needed to evaluate aquifer-confining unit relationships and local hydrogeology, especially major aquifer units within the Magothy, Englishtown, Mount Laurel, and Kirkwood-Cohansey Formations.