1,796 research outputs found
The sediments and stratigraphy of the East Coast continental margin : Georges Bank to Norfolk Canyon
The continental shelf off the northeastern coast of the United States was the first of
our offshore coastal areas to be charted in detail by the Coast and Geodetic Survey,
starting on Georges Bank in 1930. The techniques responsible for this increased accuracy
in offshore waters were first described by Rudé (1938) and have been constantly improved.
From these soundings Veatch and Smith (1939) compiled their set of contour
charts aided by a grant from the Penrose Bequest of the Geological Society of America.
These soundings reopened the submarine canyon problem first commented upon by
Dana (1863), which had gradually lapsed into obscurity from insuffcient data. The
reader is, of course, well aware of the major controversy, with all its far reaching implications,
which has been precipitated since the 1930 surveys of Georges Bank were brought
to the attention of geologists by Shepard (1933).
As more of the new surveys were completed, data from the field sheets were kindly
furnished by the U. S. Coast and Geodetic Survey to the Woods Hole Oceanographic
Institution for use in dredging and coring operations. This field work, first reported in
1936, was continued from time to time until 1941 as new soundings became available.
Rock dredging and coring has been carried out in every major canyon on the slope from
Corsair Canyon at the tip of Georges Bank to Norfolk Canyon off the entrance to the
Chesapeake (Fig. I). Numerous cores have also been taken from the areas in between;
and while the whole slope from Georges to the Chesapeake has not been covered, it is
believed that no significant areas have been missed. In fact, cores from the slope taken
during the summers of 1940 and 1941 have yielded results that are corroborative rather
than new. In 1938 on a cruise from Hudson Gorge to Norfolk Canyon, cores were
taken on the slope in areas which Veatch had considered to be the most important
(personal communication).
In the following report the tows and cores will be described by areas from Georges
Bank southwards, as the same region was revisited in successive years. The various
samples, however, will be referred to by number followed by the year in which they were
taken. The material is in storage in the Woods Hole Oceanographic Institution and in
the Museum of Comparative Zoology at Harvard University.
The late Joseph A. Cushman was kind enough to identify the Foraminifera which
have been obtained in tows from the canyon walls and in cores, except for those described
in Appendix A which is contributed by Fred B Phleger, Jr. Most of the type material
is in storage in the Museum of Comparative Zoology, although at the present writing
some is in the Cushman Laboratory in Sharon, Massachusetts. I am indebted to Lloyd
W. Stephenson for identifying a molluscan fauna from one of the canyons, and to W. C.
Mansfield who has reported on another formation. Numerous discussions with Percy E.
Raymond have, as usual, proved most helpful, and thanks are also due to Eugenia C.
Lambert for performing the mechanical analyses and to Constance French for other
laboratory assistance. Phleger (1939, 1942, 1946) has previously published on the Foraminifera from the slope and deep water cores. This material is, at present, at
Scripps Institution of Oceanography
The sediments of the continental shelf off the eastern coast of the United States
Our knowledge of clastic, shallow-water sediments over any considerable area of
ocean floor is very generalized and leaves much to be desired. The notations concerning
the character of the bottom found on all charts are necessarily limited to a descriptive
word or two, and although suffcient for navigational purposes, are of little use to the
stratigrapher. Of all the marine sediments in the geologic column, those laid down in the
neritic zone bulk the largest. They grade slowly into the sediments of the bathyal zone
with no sharp line of demarcation. The early oceanographers were more interested in the
clays and organic oozes of the deep sea and they added but little information concerning
those materials which to the geologist are the most important. From the charts one is
apt to obtain the impression that bottom deposits, excepting those of the deep sea, are
very patchy in their distribution, and that there is little rhyme or reason in their arrangement.
On the other hand the geological text books are apt to make it appear that there
is an orderly gradation of sediments from coarse to fine in an offshore direction, and that
a sandstone is always an indication of shallow water deposition, with a shale the reverse.
Twenhofel has called attention to the role of environment in sedimentation. Like
organisms, sediments are the resultants of a long sequence of environmental factors to
which they have been exposed: action by currents, wave generated and otherwise,
availability of supply and its type, distance from shore, and depth of water, plus their
combined effect during times of changing sea level in the past. These factors have
operated in the regions of production, during the period of transportation, and at the
place of deposition, and the retention of older characteristics further complicates the
record. The following study was undertaken with the hope that through a detailed and
systematic series of samples not only might something be learned about the characteristics
and distribution of the sediments of a particular area, but something also of the
environmental factors which govern conditions of sedimentation in a major ocean
Surface Brightness and Stellar Populations at the Outer Edge of the Large Magellanic Cloud: No Stellar Halo Yet
We present a high quality CMD for a 36'x 36' field located 8 degrees (7 kpc)
from the LMC center, as well as a precise determination of the LMC surface
brightness derived from the resolved stellar population out to this large
galactocentric radius. This deep CMD shows for the first time the detailed age
distribution at this position, where the surface brightness is V=26.5 mag/sq".
At a radius R=474' the main sequence is well populated from the oldest turnoff
at I=21.5 to the 2.5 Gyr turnoff at I=19.5. Beyond this radius, a relatively
strong gradient in the density of stars with ages in the 2.5-4 Gyr range is
apparent. There are some stars brighter and bluer than the main population,
quite uniformly distributed over the whole area surveyed, which are well
matched by a 1.5 Gyr isochrone and may be indicative of a relatively recent
star formation, or merger, event. The surface brightness profile of the LMC
remains exponential to this large galactocentric radius and shows no evidence
of disk truncation. Combining the information on surface brightness and stellar
population we conclude that the LMC disk extends (and dominates over a possible
stellar halo) out to a distance of at least 7 kpc. These results confirm that
the absence of blue stars in the relatively shallow off-center CMDs of dIrr
galaxies is not necessarily evidence for an exclusively old stellar population
resembling the halo of the Milky Way.Comment: ApJLett, in press 13 pages including 3 color figure
The origin of the LMC stellar bar: clues from the SFH of the bar and inner disk
We discuss the origin of the LMC stellar bar by comparing the star formation
histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar
and in a number of fields in different directions within the inner disk. The
CMDs, reaching the oldest main sequence turnoffs in these very crowded fields,
have been obtained with VIMOS on the VLT in service mode, under very good
seeing conditions. We show that the SFHs of all fields share the same patterns,
with consistent variations of the star formation rate as a function of time in
all of them. We therefore conclude that no specific event of star formation can
be identified with the formation of the LMC bar, which instead likely formed
from a redistribution of disk material that occurred when the LMC disk became
bar unstable, and shared a common SFH with the inner disk thereafter. The
strong similarity between the SFH of the center and edge of the bar rules out
significant spatial variations of the SFH across the bar, which are predicted
by scenarios of classic bar formation through buckling mechanisms.Comment: MNRAS Letters, accepte
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