CONCLUSIONS
The holes drilled on the New Jersey continental shelf and slope on Leg 174A form part of a
transect of holes from the slope (ODP Leg 150) to coastal outcrops (150X and 174AX) that
constitute the Mid-Atlantic Sea-level Transect. The primary goals of the transect were as
follows:
1. Date sequence boundaries of Oligocene to Holocene age and compare this stratigraphic record
with the timing of glacial-eustatic changes inferred from deep-sea delta18O
variations.
Four prominent seismically imaged unconformity surfaces of middle Miocene to Pliocene
Pleistocene age are probably related to times of falling sea level. Best estimates of ages for these
surfaces, from shipboard paleontology and paleomagnetics are: < 0.5 Ma [pp3(s)], >1.1-<7.4
Ma [pp4(s)], 7.4-11.2 Ma [m0.5(s)], and >11.4 Ma [m1(s)]. Higher order cyclicity is most
likely present but not resolved with existing seismic reflection data. The precision of ages will
improve with postcruise studies, which will include techniques not available on the ship as well
as the analysis of additional samples. Older Miocene and Oligocene sequence boundaries were
intersected at the slope site, Site 1073, but in an area of marked condensation. Attempts to date
these surfaces at geometrically optimal locations on the shelf (Sites 1071 and 1072) were not
successful because off the difficulty of maintaining hole stability in unconsolidated sandy
sediments in the upper 400 m of the section.
2. Place constraints on the amplitudes and rates of sea-level change that may have been
responsible for unconformity development.
In the case of the late-middle Miocene to Pleistocene surfaces that were intersected at the shelf sites
[pp3(s), pp4(s), m0.5(s) and m1(s)], the water depth fell to close to zero at a point 100 km
seaward of the present shoreline. This is indicated by (1) the prominence of offlap (stratal
truncation) at these surfaces; (2) the presence of several tens of meters of highstand sand in the
vicinity of clinoform breakpoints/rollovers (points at which the gradient of the shallow
paleoshelf steepens seaward from about 0.1° to a slope of about 4-5°), suggesting very
shallow-water conditions [m0.5(s) and m1(s)]; and (3) the recovery of probable
estuarine/lagoonal sediments in the vicinty of m0.5(s), only 3 km landward of its
breakpoint/rollover (Hole 1071F). Sea level probably did not fall significantly below the level of
these breakpoints/rollovers. This is indicated primarily by the lack of significant
incision/downcutting by rivers at unconformity surfaces (less than 5 m). Lowstand sediments
that might have been derived in part by this process are also thin to absent in the vicinity of
clinoform toes, although lowstand deposits may be present in deeper water in the vicinity of the
continental slope/rise. Lowstand units have been imaged in deep shelf seismic reflection data for
several of the older Miocene surfaces. Evidence from benthic foraminifers indicate maximum
water depths on the shallow shelf during times of high sea level were around 50-100 m. This
implies changes in water depth of ~50-100 m, a figure that can be used to estimate amplitudes
of global sea-level change once the local effects of sediment accumulation, compaction, and
loading are taken into account.
3. Assess the relationships between depositional facies and sequence architecture.
One of the surprises of Leg 174A was the discovery of an unusual distribution of sediment types
between unconformity surfaces related to times of sea-level fall. The shallow shelf for each
sedimentary unit between these surfaces is dominated by sediments that accumulated during
overall flooding ("transgressive" deposits). Seaward of breakpoints/rollovers, the deeper shelf is
dominated by sediments that accumulated during spans in which the shelf was building
seaward ("highstand" deposits). One explanation for this arrangement is that the space available
for sediment to accumulate was efficiently filled during times of sea-level rise as a result of an
abundant supply of sediment during the past 12 million years. The observed distribution departs
from the standard conceptual model, widely used in petroleum exploration, in which highstand
sediments extend well landward of associated transgressive ones.
4. 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.
For the first time in almost 30 years, scientific ocean drilling attempted to sample a thickly
sedimented continental margin in water less than 150 m deep. In challenging drilling conditions,
sediments as old as middle Miocene (~12.5 Ma) were sampled on the shelf, and late Eocene
(~35 Ma) on the upper slope. A full suite of geophysical logs (including logging while drilling)
was obtained from one site on the shelf (Site 1072). Logging data were also acquired in
available time at slope Site 1073, including both sonic log and VSP data. The data obtained on
Leg 174A represent an important step toward completion of the Mid-Atlantic Sea-level
Transect, and they also provide valuable information about the technology that will be needed to
drill, core, and log unconsolidated sandy sediments expected beneath the middle and inner shelf.
5. Relationship between sea-level fluctuations and interstitial-water chemistry: an unexpected
result.
Analyses of interstitial water resulted in two discoveries relevant to the overall sea level and
climatic themes of Leg 174A. First, salinity variations with depth observed in pore waters from
the shelf are consistent with alternate exposure of the shelf during the Pliocene-Pleistocene and
then renewed flooding by seawater. Second, changes in alkalinity and phosphate observed in
samples from the thick, late Pleistocene succession of the slope are consistent with variations in
the amount or type of buried organic matter, apparently with a periodicity of close to 100,000
yr, and consistent with the time scale of astronomically forced climate change during the
Pleistocene.
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