RESEARCH· ·1970·

GEOLOGICAL SURVEY RESEARCH 1970
THE GLACIATED SHELF OFF NORTHEASTERN UNITED STATES 1
By R. N. OLDALE and ELAZAR UCHUPI, Woods Hole, Mass.
Work done in cooperation with the Woods Hole Oceanographic
Institution, Woods Hole, Mass.
Abstract.-The Continental Shelf off northeastern United
States is made up of basins-the sounds and the Gulf of Maine
-flanl{ed on the landward side by the Fall Zone and on the
seawnrd side by a cuesta. Morphology of the shelf is believed
to be the result of fluvial erosion during pre-Late Cretaceous,
Puleocene, nnd Pliocene-curly Pleistocene time, and glacial
erosion during the Pleistocene. The bathymetry of the shelf and
the topography C?f the basement (which is made up of Triassic
nnd pre-Trinssic rocl{S) show a drainnge system developed
during lute 'l'ertinry-enrly Pleistocene time nnd modified by
glncial erosion during the Pleistocene. Older erosion surfaces
mny huve controlled the late Tertiary-early Pleistocene fluvial
erosion. )j..,olded sedimentary rocks below fluvially carved thalwegs
in the Gulf of Maine indicate structural basins, probably
of Triassic or Carboniferous age. Seismic velocities of the
folded sedimentary rocl{S, however, suggest that the basins may
be as young ns Tertinry.
'"!"'he glaciated shelf off northeastern United States is
made up of lowlands flanked on the landward side by
a gently sloping surface, the Fall Zone of Flint ( 1963),
and on the sea ward side by a cuesta and by end moraines
of Pleistocene age. Long Island, Block Island,
R.hode Island, and Nantucket Sounds are the major
low lands (fig. 1). The islands in this area, together
with Georges and Browns Banks, are the topographic
expressions of the cuesta and end moraines. The lowlands
west of the Gulf of Maine are generally less than
40 m deep. Beneath Long Island Sound a valley slopes
gently eastward. The bottoms of Block Island, R.hode
Island, and Nantucket Sounds have gentle southerly
slopes. The sea floor of Cape Cod Bay slopes north and
that of Massachusetts Bay slopes east toward the Gulf
of Maine (fig. 1). Small shallow, closed depressions
occur in the bottom in some places.
In contrast, the bathymetry of the Gulf of Maine is
1
Contribution No. 2334 of the Woods Hole Oceanographic Institution.
quite complex (fig. 1). The bottom of the gulf forms a
large low area, with an average depth of about 150 m
below sea level (Uchupi, 1965a), bordered on the south
by Georges and Browns Banks. The low area has an
outlet, Northeast Channel, with a threshold depth of
more than 240 m below sea level. In preglacial time
Great South Channel was an outlet, but it is now
choked with drift. Major features of the Gulf of Maine
are the closed depressions such as Wilkinson, Jordan,
and Georges Basins, which have maximum depths of
more than 200 m and enclose areas of 5,000 to 10,000
sq km (Uchupi, 1965a). Separating the basins are
ledges, banks, and swells such as Jeffreys Bank, Cashes
Ledge, Stellwagen Bank, and Truxton Swell. A major
divide, trending south, separates the Gulf of Maine
into two large lowlands.
In the present report, a topographic chart of the
basement has· been compiled from published and unpublished
sources (fig. 3). Data from this chart, and
information from dredge hauls and seismic-pro filer recordings
have been used to reconstruct the geologic history
of the Continental Shelf of the northeastern United
States. As described below, the morphology of the shelf
is believed to be a result of pre-Cretaceous fluvial erosion
deposition during Cretaceous and early Tertiary time,
fluvial erosion during late Tertiary and early Pleistocene
time, and glacial erosion during the Pleistocene. In
the western part of the Gulf of Maine, a fourth period
of erosion, possibly during the Pal~ocene, influenced the
shelf morphology.
METHODS
During 1963, 1964, and 1965, seismic-profiler records
were taken on the shelf east of New York City by the
U.S. Geological Survey-Woods Hole Oceanographic
Institution Atlantic shelf and slope program. These
U.S. GEOL. SURVEY PROF. PAPER 70D-B, PAGES B167-Bl73·
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