Eocene-Oligocene sea-level changes on the New Jersey coastal plain linked to the deep-sea record KENNETH G. MILLER Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903, and Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 DENNIS V KENT 1 ANDREW N BROWER J Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 LAUREL M. BYBELL U.S. Geological Survey, Reston, Virginia 22092 MARK D. FEIGENSON RICHARD K. OLSSON RICHARD Z. POORE U.S. Geological Survey, Reston, Virginia 22092 | Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903 ABSTRACT We use magnetostratigraphy and Sr-isotope stratigraphy to im- prove stratigraphic control for the Eocene to Oligocene of the New Jersey coastal plain (ACGS4 borehole). Magnetostratigraphy in many cases is complicated in outcrop sections of shallow-water (<200 m paleodepth) sediments by low remanence and weathering; we mini- mize these problems by analyzing large samples obtained from the ACGS4 borehole and construct a firm magnetochronology for the early to middle Eocene. Sr-isotope stratigraphy confirms biostrati- graphic evidence for a previously unknown uppermost Eocene to lowermost Oligocene unit and delineates a "middle" Oligocene hiatus that is unresolvable using biostratigraphy alone. We recognize hia- tuses and associated unconformities on the New Jersey margin near the lower Eocene/middle Eocene boundary, within the middle Eo- cene, and in the "middle" Oligocene and correlate these events with similar hiatuses observed in other continental-shelf, slope, and epicon- tinental settings. In addition, a hiatus probably occurred near the middle Eocene/upper Eocene boundary. We conclude that the in- terregional distribution of these Eocene-Oligocene hiatuses indicates a global cause: eustatic change. INTRODUCTION Relative sea-level changes on continents and their margins reflect variations in global sea level, sediment supply, and basin subsidence and have been inferred from transgressions/regressions of the shoreline, chang- ing water depths, and stratal discontinuities (regional unconformities and hiatuses). Unconformities form on passively subsiding continental margins during eustatic lowerings, and their formation is relatively insensitive to sediment supply (Christie-Blick and others, 1989). Tectonic subsidence along old, passive continental margins generally follows a simple thermal subsidence curve (Steckler and Watts, 1982). Hence, studies of uncon- formities on old, passive margins (such as the Cenozoic margin of New Jersey) potentially provide a way to evaluate the timing, if not the magni- tude, of major global sea-level (eustatic) lowerings. Seismic and sedimentary sequence stratigraphy provides one means for evaluating unconformities on passive margins (for example, Vail and others, 1977; Baum, 1986; Haq and others, 1987). Because of uncertainties in subsidence and paleodepth corrections, the exact relationships among eustasy, seismic stratigraphy, and sedimentary sequence stratigraphy re- main controversial; still, seismic disconformities generally form during lowerings of relative sea level (summary in Christie-Blick and others, 1989). Hiatuses are generally associated with disconformities formed dur- ing these sea-level lowerings (for example, Aubry, 1985; Graciansky, Poag and others, 1985; Miller and others, 1985b, 1987a, 1987c; Poag, Watts, and others, 1987; van Hinte, Wise and others, 1987). In order to differen- tiate regional from global causes, the age equivalency among hiatuses on different continental margins must be firmly established, and this requires detailed detailed biostratigraphy, magnetostratigraphy, and isotope stratig- raphy (discussion in Miller and Kent, 1987). Herein, we focus on Eocene- Oligocene stratigraphic breaks of the New Jersey coastal plain and correlate these with breaks on other margins, the deep-sea record, and other evidence for eustatic lowerings. Deciphering the influence of sea-level fluctuations on the stratigraph- ic record poses a paradox. Shallow-water (neritic) sections are most sensitive to sea-level changes but are generally difficult to correlate to a standard chronostratigraphy because planktonic index fossils are generally rare or controlled by facies changes, and deposition tends to be discontinu- ous. Conversely, deep-sea (bathyal to abyssal) sections in most cases are more complete and have better stratigraphic control, but the link between deep-sea deposition and sea-level fluctuations is not direct (Tucholke, 1981). The solution to this paradox is to develop chronostratigraphic standards correlated to deep-sea sections (for example, Berggren and oth- ers, 1985; Miller and others, 1988); to monitor sea-level changes using shallow, passive continental-margin sequences (for example, Aubry, 1985; Olsson and Wise, 1987); and to use facies-independent means (for exam- ple, magnetostratigraphy, Sr-isotope stratigraphy) to correlate the two regions. The Eocene-Oligocene record on the New Jersey coastal plain con- tains stratigraphic gaps that have been related to relative sea-level lower- ings. Olsson and others (1980) noted a prominent unconformity between the middle Eocene and upper Oligocene that may be related to a "middle" Oligocene glacio-eustatic lowstand (Miller and others, 1985b). Olsson and Wise (1987) recognized six early to middle Eocene hiatuses in the coastal plain associated with paleobathymetric changes. In contrast to this view of a punctuated record, the continuously cored ACGS4 borehole near Mays Landing, New Jersey, indicates that the coastal-plain record may be more complete than previously believed (Owens and others, 1988). At the ACGS4 borehole, Poore and Bybell (1988) identified every nannofossil zone from the upper lower Eocene to lowermost Oligocene (Zones NP12-NP21), including a previously unknown uppermost Eocene- Geological Society of America Bulletin, v. 102, p. 331-339, 3 figs., 1 table, March 1990. 331