Geologic
Origins of Stellwagen Bank
by
Robert Oldale
From
The Cape Naturalist, 1993-94. Reprinted by permission.
Just
six miles north of the Provincetown spit lies a shallow platform
known as Stellwagen Bank. Now famous for whale watching, Stellwagen
Bank was once dry land where mastodon and mammoth roamed
Located
in the western Gulf of Maine, the large bay north of Georges
Bank and between New England and Nova Scotia, the bank is about
26 miles (40 kilometers) long, about 13 miles (21 kilometers)
wide at its south end, and narrows to about 3 miles (5 kilometers)
wide at its northern end (Fig. 1). Water depths to the top of
the bank range from 65 to 100 feet (20 to 30 meters). The bank
rises about 165 feet (50 meters) above the surrounding seafloor.
Stellwagen Basin, with water depths of about 260 feet (80 meters),
is adjacent to the west flank of the bank and is part of the
Massachusetts Bay seafloor. Stellwagen Bank is separated from
the Provincetown spit by Little Stellwagen Basin with water
depths ranging from 130 to 200 feet (40 to 60 meters).
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A
computer-generated northward-looking oblique view of Lower
Cape Cod and Stellwagen Bank. Boston Harbor lies in the
upper left corner of the view. Cape Ann is located north
of the northern tip of Stellwagen Bank. Vertical exaggeration
is about 100 times. Generated by Richard Signell
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Stellwagen
Bank mostly owes its existence to the last great ice sheet (known
as the Laurentide Ice Sheet) and to changes in sea level that
accompanied and followed deglaciation. In a computer-generated
oblique view (see figure), Stellwagen Bank looks like an extension
of Lower Cape Cod. This similarity is not accidental, for Lower
Cape Cod and Stellwagen Bank share, at least in part, the same
geologic history. The submergence of the bank has hidden much
of the evidence of its origin, but Lower Cape Cod provides us
with a model so that we can speculate on how and when Stellwagen
Bank was formed.
Although
the events that shaped the present bank occurred over about
the last 25,000 years and involve the last continental glaciation
and sea-level change that accompanied andfollowed deglaciation,
it is likely that rocks deeply buried by glacial and postglacial
deposits record a geologic history that spans more than500 million
years. A geologic cross section , based on seismic soundings,
shows the inferred structure of the bank. The deepest rock below
the bank (called bedrock) is likely to resemble the bedrock
exposed at the surface in Eastern Massachusetts. If this is
so, the oldest rocks consist of granite (solidified molten rock
formed deep in the earth’s crust) and volcanic rocks that
formed during explosive eruptions (similar to the eruption of
Mount St. Helen's), or as lava flows. The bedrock also would
include sedimentary rocks that formed from deposits laid down
in ancient streams, lakes and oceans, and metamorphic rocks
that were formed from existing rocks by heat and pressure caused
by deep burial and mountain building.
The
bedrock beneath the bank may range from more than 570 million
years, when there was little or no life on Earth, to as little
as about 150 million years, when dinosaurs ranged the landscape.
Together these rocks contain a record of the breakup of super
continents, collisions between continents as they drifted around
the globe, and the birth and death of oceans. The youngest bedrock
beneath Cape Cod contains a record of the earliest opening of
the Atlantic Ocean, which occurred about 200 million years ago.
It is likely that some of the bedrock beneath Stellwagen Bank
contains a similar record.
Above
the bedrock and beneath the glacial deposits of Stellwagen Bank,
there are rock layers that are thought to resemble the strata
under the coastal plain of New Jersey. The coastal plain strata
were deposited in the streams, lakes and swamps or on the shallow
seafloor adjacent to the land. They may range in age from 5
to 140 million years old and were formed during the last part
of the age of dinosaurs and the early part of the age of mammals.
Coastal plain deposits filled the Gulf of Maine. The most seaward
waterfalls of present coastal rivers mark the fall line that
probably delineates the former landward edge of the coastal
plain sedimentary wedge. About 5 million years ago, sea level
fell, and streams and rivers began to erode the coastal plain
deposits. Stellwagen Bank began to take shape as the streams
eroded away the coastal plain deposits adjacent to the bank.
Glacias,
a much more powerful force in erosion than running water, finished
what the streams had started. They removed most of the coastal
plain deposits in the Gulf of Maine, eventually leaving behind
only remnants and carved deep basins in the bedrock. We know
that continental glaciers reached southern New England on two
occasions, but we don’t know how many more times the region
was glaciated. Over the past 1 1/2 million years — roughly
the length of the Pleistocene Epoch or the Ice Age — it
was likely that the Gulf of Maine was glaciated several more
times. However, it was the last continental glacier that deposited
most, if not all, the glacial sediments that underlie Stellwagen
Bank.
The
Laurentide Ice Sheet advanced out of Canada about 25,000 years
ago and reached southern New England about 21,000 years ago.
Shortly after its arrival, the ice sheet began its retreat as
melting along the glacier front exceeded the advance of the
ice. During the retreat, the ice sheet built Cape Cod, and studies
of the geology of the Cape provide the clues needed to infer
how Stellwagen Bank was built.
The
plains of Lower Cape Cod, from Eastham to Truro, were formed
mostly by meltwater streams that drained a lobe of the Laurentide
Ice Sheet that lay to the east of Lower Cape Cod. Known as the
South Channel lobe, it is named after Great South Channel, the
western entrance to the Gulf of Maine that lies between Nantucket
and Georges Bank. To the west of Lower Cape Cod, the Cape Cod
Bay lobe occupied Cape Cod Bay. Evidence from Cape Cod shows
that the retreat of these lobes was not simultaneous and that
the lobe in Cape Cod Bay retreated before the South Channel
lobe.
During
the retreat of the Cape Cod Bay lobe, a lake developed in Cape
Cod Bay. The lake was contained by glacial deposits to the south
and west and by ice on the north and eat. Meltwater streams
of the Lower Cape Cod outwash plains flowed from the South Channel
lobe westward to the lake and deposited sand and gravel on the
outwash plain surface. When the streams reached the lakeshore,
they deposited sand to form deltas in the lake. As the deltas
advanced into the lake, the outwash streams lengthened and the
outwash plains grew westward. Therefore, the outwash plains
of Lower Caper Cod are composed of glacial steam deposits that
cap delta deposits that, in turn, overlie glacial lake deposits
of silt and clay.
When
the worldwide volume of glacial ice was at its maximum, about
20,000 years ago, sea level around the world was about 300 feet
below its present level. This was caused by the continental
glaciers being made mostly of water removed from the world's
oceans. In the Gulf of Maine, the weight of the glacier depressed
the crust of the Earth to below the glacial sea level. Thus,
as the Laurentide Ice Sheet retreated from the Gulf it was immediately
replaced by the sea, which flooded this depressed region.
The
shape, size and location of Stellwagen Bank suggests that it
may have had an origin similar to that of Lower Cape Cod, but
the bank is somewhat younger. We can postulate that the meltwater
streams that built the bank probably drained the South Channel
lobe of glacial ice and that the Cape Cod Bay lobe probably
had retreated away from the bank and into Stellwagen Basin.
However, instead of the meltwater streams entering a glacial
lake, as they did to build Lower Cape Cod, they entered the
sea that had invaded Stellwagen Basin. As on Lower Cape Cod,
meltwater steams deposited sand and gravel on the bank top,
and sand along the seashore, to form deltas.
Thus,
the glacial composition of the bank is likely to be very similar
to Lower Cape Cod, with glacial outwash stream deposits overlying
outwash delta deposits. However, beneath Stellwagen Bank, the
delta deposits mix with and overlie glacial marine mud that
was deposited in Stellwagen Basin. Eventually, as the ice melted,
the South Channel lobe retreated, and the glacial development
of Stellwagen Bank ended.
When
did the Laurentide Ice Sheet build Stellwagen Bank? The answer
to this question comes from Wilkinson Basin, a deep basin in
the Gulf of Maine, located about 40 miles (65 kilometers) northeast
of Stellwagen Basin. Microscopic, hard-shelled single-celled
animals, called foraminifera, which lived in the sea and on
the sea floor, were deposited along with the glacial marine
mud. The remains of these animals can be radiocarbon-dated.
The age of the oldest foraminifera in Wilkinson Basin is about
18,000 years. Because the glacial marine mud was deposited in
the open sea in front of the retreating glacier, the age of
these animals indicates that the glacial part of Stellwagen
Bank can be no younger than the oldest foraminifera, or about
18,000 years.
A
short time later, local sea level began to fall when, along
with the release of the glacial load, the crust rebounded. By
12,000 years ago, the bank stood well above sea level and may
even have been connected to Lower Cape Cod or, at the least,
separated from the Cape by a shallow strait in Little Stellwagen
Basin. Stellwagen Bank, then, closely resembled present-day
Lower Cape Cod. Lakes, swamps and marshes probably dotted the
landscape. Along the shore, there were beaches, sea cliffs,
spits and lagoons. The climate was colder than it now is, and
spruce and poplar forests and park lands of tundra shrubs and
grasses may have covered the bank top. Teeth of woolly mammoths
and mastodons have been dredged up in Stellwagen Basin, evidence
of the animal life of the time. Early humans arrived in New
England about 11,000 years ago, and they may have witnessed
the beginning of the final chapter in the history of Stellwagen
Bank. By then, local sea level was rising as crustal rebound
slowed and as the melting glaciers continued to return water
to the ocean basins. About 10,000 years ago, Stellwagen Bank
slipped beneath the sea.
As
the bank drowned, waves and currents eroded and reworked the
glacial sediments atop the bank. The reworked sediments were
redeposited on the bank or were transported to the edge of the
bank and deposited on the flank. In this way, the bank took
on its final shape.
Even
though Stellwagen Bank is now covered by at least 20 meters
(65 feet) of water, the bank top continues to change. Currents
from large waves, generated by northeast storms and hurricanes,
reach the surface of the bank and move sand and, at times, even
gravel. The sand forms sand waves and sand ridges on the bank
top, and some is likely transported to the bank edge and deposited
on the flanks. However, compared to the dynamic events of glaciation
and submergence, Stellwagen Bank is now a quiet place where
whales, not mammoths and mastodons, find refuge.
Robert
Oldale, recently retired, was a geologist with the US Geological
Survey in Woods Hole for many years.
This
article is reprinted from The Cape Naturalist 1993-94 by permission
of the Cape Cod Museum of Natural History.
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