The emergence of seismic/sequence stratigraphy since the late 1970s has led to a revolution in stratigraphy and to a renewal of interest in the stratigraphic response to eustasy (global sea-level change; Vail et al., 1977, 1984, 1991; Vail and Hardenbol, 1979; Loutit and Kennett, 1981; Berg and Woolverton, 1985; Haq et al., 1987, 1988; Vail, 1987, 1992; Cross and Lessenger, 1988; Posamentier et al., 1988; Sloss, 1988; Eberli and Ginsburg, 1989; Fulthorpe and Carter, 1989; Christie-Blick et al., 1990; Van Wagoner et al., 1990; Haq, 1991; Loucks and Sarg, 1993; Posamentier and James, 1993; Weimer and Posamentier, 1993; Christie-Blick and Driscoll, 1995; Fulthorpe and Austin, in press). Two arguments were advanced in support of the eustatic interpretation. One involved widespread seismic evidence for the existence of regional unconformities characterized by apparently abrupt basinward shifts in onlap, which were interpreted to imply relatively rapid falls of sea level with amplitudes of up to several hundred meters. The second was based on the purported global synchroneity of resultant sequence boundaries, which if correct would be difficult to explain by other than a eustatic mechanism.

These arguments were not universally accepted for several reasons (Watts, 1982; Thorne and Watts, 1984; Miall, 1986, 1992, 1994; Burton et al., 1987; Hubbard, 1988; Christie-Blick et al., 1990; Reynolds et al., 1991; Christie-Blick, 1991; Christie-Blick and Driscoll, 1995):

1.Basinward shifts in onlap were shown not to require sea-level changes that were either rapid or of large amplitude. Therefore, there was no reason to assume a eustatic causal mechanism or to exclude possible local tectonic mechanisms for sequence-boundary development.

2.No mechanism was discovered for rapid eustatic change during intervals such as the Mesozoic, for which there is little or no evidence for continental glaciation.

3.Limitations in the resolution with which sequence boundaries could be dated and correlated between basins cast doubt on the level to which global synchroneity had been established.

4.At least prior to 1987, the "sea-level curve" first published by Vail et al. (1977) was based primarily on proprietary data (see Haq et al., 1987). Therefore, at the time of the Second Conference on Scientific Ocean Drilling (COSOD II, 1987), there was a great deal of interest in acquiring public data that might be used to establish a sea-level record independent of the Vail et al. (1977) synthesis.

Following COSOD II, the role of scientific ocean drilling in sea-level studies was advanced by means of a Joint Oceanographic Institutions, Inc. (JOI)/U.S. Scientific Advisory Committee (USSAC) Workshop (Watkins and Mountain, 1990) and a JOIDES working group (JOIDES Sea Level Working Group, 1992). Sea-level studies were also prioritized in the JOIDES Sedimentary and Geochemical Processes Panel (SGPP) White Paper (1994) and in the Ocean Drilling Program (ODP) Long Range Plan (1996). These reports differ in detail and emphasis, but they endorse several broad objectives. These include (1) the dating of stratigraphic "events" and associated surfaces that might be related to sea-level change; (2) investigating how sedimentary architecture is related to sea-level variations (local or global); and (3) estimating the magnitudes and rates of eustatic change through time, if a role for eustasy can be demonstrated. All(reports recognized that multiple drilling legs would be required to make comparisons between coeval successions at different locations and that ODP would be able to sample only a small portion of earth's sea-level history. Therefore, three intervals were prioritized within the Mesozoic to Cenozoic span accessible to ocean drilling: (1) the late Oligocene to Holocene "Icehouse" Earth, dominated by the waxing and waning of continental ice sheets; (2) the mid Cretaceous "Greenhouse" or "Hothouse" Earth, when ice sheets were essentially absent; and (3) the intermediate interval from the latest Paleocene to the middle Eocene, for which the degree of glaciation is unknown or uncertain and the term "Doubthouse" Earth was suggested (Miller et al., 1987, 1991a; Watkins and Mountain, 1990; Barron et al., 1991; Frakes et al., 1992; Browning et al., 1997).

The scientific ocean drilling community tacitly assumed that this approach would lead to insights about possible mechanisms of eustatic change, as well as to a broader understanding of the relationships among eustasy and various phenomena, including changes in continental ice volume (and hence global climate), nearshore ecosystems, particle and nutrient transfer to the deep sea, ocean circulation, biological evolution, and patterns of deposition, erosion and hydrocarbon distribution in sedimentary basins. It is now clear that the main control on short term eustasy is the continental ice budget, and that during nonglacial times sea-level change is likely to have been influenced significantly by noneustatic mechanisms, including tectonics (e.g., Christie-Blick and Driscoll, 1995). There is no evidence to support the long-held assumption in sea-level studies that tectonic processes act only at long time scales (e.g., Vail et al., 1991). Thinking has also matured about the stratigraphic response to eustatic change. Modeling studies suggest that this may vary from one basin to another, according to such factors as the local rate of subsidence and sediment supply, the relative abundance of siliciclastic vs. carbonate sediment, compaction history, and the physiography of the depositional surface. The locally determined timing of sea-level events is therefore expected to vary, even when the events are global (Jordan and Flemings, 1991; Reynolds et al., 1991; Christie-Blick, 1991; Steckler et al., 1993); strict stratigraphic synchroneity cannot be assumed as a criterion for judging the role of eustasy in the origin of observed sedimentary cyclicity. Instead, precise dating of stratigraphic successions at a number of well-chosen locations may permit predicted leads and lags to be measured. Ocean drilling has been consistently envisioned as a primary tool for such an approach to studying the history of sea-level change.
The JOIDES Sea Level Working Group (1992) endorsed a three-fold approach to sea-level studies, involving: (1) passive continental margins (primarily siliciclastic); (2) carbonate atolls, guyots and platforms, the so-called "dipstick" approach; and (3) the deep-sea oxygen isotopic record, a proxy for the growth and decay of continental ice sheets. This strategy has since been reaffirmed in the SGPP White Paper (1994) and in the JOIDES Long Range Plan (1996). ODP Legs 133 and 166 addressed "Icehouse" sea-level issues at the seaward margins of carbonate platforms off northeastern Australia and the western Great Bahama Bank, respectively. Legs 143 and 144 studied the "Greenhouse" drowning history of western Pacific guyots. Leg 174A is a continuation of the New Jersey Mid-Atlantic Sea-level Transect (MAT), the first concerted effort to evaluate the effects of "Icehouse" glacial-eustatic change at a passive continental margin characterized by predominantly siliciclastic sedimentation. Leg 174A follows successful sampling of the continental slope and rise during Leg 150 (Mountain, Miller, Blum, et al., 1994; Miller et al., 1996b), and continuing studies of the adjacent New Jersey coastal plain (Legs 150X and 174AX; Miller, et al., 1994, 1996a; Miller and Sugarman, 1995; Pekar and Miller, 1996; Figs. 1 and 2).

To 174A Scientific Objectives

To 174A Table of Contents

ODP Publications

ODP Homepage