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).
 |
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
|
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.
