The emergence of seismic stratigraphy in the late 1970s (Vail et al., 1977, 1984; Vail and Hardenbol, 1979; Vail, 1987; and Posamentier et al., 1988) led to a revolution in stratigraphy and to a renewal of interest in the role of eustasy in governing sedimentary cyclicity. Two arguments were advanced in support of the eustatic interpretation. One involved 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 argument was based on the purported global synchroneity of sequence boundaries, which if correct, would be difficult to explain by other than a eustatic mechanism.

These arguments were not universally accepted for a number of reasons (Watts, 1982; Thorne and Watts, 1984; Miall, 1986, 1992; Burton et al., 1987; Hubbard, 1988; Christie-Blick et al., 1990; Christie-Blick, 1991; Christie-Blick and Driscoll, 1995): (1) basinward shifts in onlap were shown not to require sea-level changes that are either rapid or of large amplitude. Therefore, there is no reason to assume a eustatic cause or to exclude possible tectonic mechanisms for sequence boundary development; (2) no mechanism exists 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 can be dated and correlated between basins casts doubt on the level to which global synchroneity can be established; (4) 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). So, 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.

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 (Sea-Level Working Group Report, 1992). The working group recommended a threefold approach to sea-level studies involving passive continental margins (primarily siliciclastic); these included carbonate atolls, guyots and platforms, and deep-sea oxygen isotopic records. The working group also recognized the fundamental differences between glacial and nonglacial spans of earth history and suggested focusing on three different intervals: the Oligocene-Holocene (an example of "Icehouse" conditions), the mid-Cretaceous (an example of "Greenhouse" conditions), and the intervening Paleocene-Eocene (fancifully described as the "Doubthouse" Earth, owing to uncertainty about the time of onset of southern hemisphere continental glaciation). This strategy is reaffirmed in the recently published JOIDES Long Range Plan (1996). Ocean Drilling Program (ODP) Legs 133 and 166 addressed "Icehouse" sea-level issues at the seaward margins of carbonate platforms off northeastern Australia and the western Bahama Bank, respectively. ODP Legs 143 and 144 studied the "Greenhouse" drowning history of western Pacific guyots (the so-called "dipstick" approach). The New Jersey Mid-Atlantic Sea Level Transect (MAT) represents the first concerted effort to evaluate the effects of glacial-eustatic changes at a passive continental margin characterized by predominantly siliciclastic sedimentation.

Leg 174A follows successful sampling of the continental slope and rise during Leg 150 (Miller and Mountain, 1994; Miller, Mountain et al., 1996a), and of the adjacent New Jersey coastal plain (Leg 150X; Miller et al., 1994). Leg 174A will sample as many as six locations along the shelf and upper slope. The primary sites are located on the outer shelf in water depths of just under 100 m and are designed to calibrate successions of Oligocene to Holocene age. Four sites on the adjacent continental slope/Hudson Apron will serve as backups in the event that hydrocarbon accumulations are encountered or bad weather makes it impossible to operate safely in shallow water. Additional drilling has been completed through the Cenozoic and uppermost Cretaceous of the coastal plain and will be published as Leg 174AX (K.G. Miller, per. com., 1997). Intermediate sites on the mid- to inner-shelf remain to be tackled.

To 174A Background

To 174A Table of Contents

ODP Publications Homepage

ODP Homepage