SCIENTIFIC OBJECTIVES
Oxygen isotopic variations show that polar ice volume has changed markedly during the last 35
m.y. (the "Icehouse" interval), and perhaps longer (Miller et al., 1991a). As a result, global sea
level has varied by many tens of meters, and such changes have had a profound impact on
shallow-water deposition, nearshore ecosystems, particle and nutrient transfer to the deep sea, and
other components of the ocean-atmosphere system. The primary goals of Leg 174A are to
investigate the stratigraphic responses to sea-level change, and specifically to:
- date sequence boundaries of Oligocene to Holocene age and to compare this stratigraphic
record with the timing of glacial-eustatic changes inferred from deep-sea
d18O variations;
- place constraints on the amplitudes and rates of sea-level change that may have been
responsible for unconformity development;
- assess the relationships between depositional facies and sequence architecture; and
- provide a baseline for future scientific ocean drilling that will address the effects and timing of sea-level changes on this and other passive margins.
Seismic stratigraphic studies reveal a complex array of discontinuities within the mid-Atlantic
continental margin. Leg 174A will provide new information about the arrangement of depositional
facies that is needed to evaluate whether the surfaces provisionally interpreted as sequence
boundaries are in fact related to the lowering of depositional base level. Leg 174A will also place
constraints on the partitioning of unconformity-bounded sequences between so-called lowstand,
transgressive, and highstand units. This is relevant to the controversial issue of how sequence
boundary development is related to the time of most rapid sea-level fall.
To 174A Drilling Strategy and Proposed Sites
To 174A Table of Contents
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