O rio Saskatchewan Norte (em primeiro plano), em sua confluência com o rio Alexandra, próximo às nascentes dos rios nas Montanhas Rochosas canadenses, faz parte do sistema do rio Saskatchewan, que junto com a bacia do Rio Nelson é o último remanescente do outrora poderoso Paleo. -Sistema do rio da Bell. Credit: Yvesgagnon1974/istockphoto.com.
No ensino médio, a maioria dos estudantes aprendeu que o rio Amazonas é o mais poderoso dos rios da Terra. Sua vazão média para o oceano de cerca de 209.000 metros cúbicos por segundo é maior do que as descargas combinadas dos seis maiores rios do mundo. Seus afluentes mais distantes se elevam nos Andes, a algumas centenas de quilômetros do Oceano Pacífico, e drena uma área de cerca de 7 milhões de quilômetros quadrados, ao cruzar a América do Sul para o Atlântico. A Amazônia supera o maior sistema fluvial da América do Norte, o Mississippi, que drena menos da metade da área da Amazônia e libera cerca de um décimo da quantidade de água.
Os cientistas da Terra podem fazer uma pausa para se perguntar por que a América do Norte não tem uma bacia fluvial de escala semelhante à da Amazônia, seguindo a mesma história geológica dos dois continentes: tanto a América do Norte quanto a América do Sul se deslocaram para o oeste com a abertura. do Oceano Atlântico, e teve interações comparáveis com as placas tectônicas subjacentes ao Oceano Pacífico. Essas interações resultaram em uma cordilheira de montanha contínua ao longo das margens ocidentais de ambos os continentes, e em cada caso, a drenagem da cordilheira fluiu para o leste, através de plataformas sedimentares e escudos pré-cambrianos.
De fato, a América do Norte tinha um sistema fluvial tão extenso. De cerca de 50 milhões a 3 milhões de anos atrás, esse sistema drenou uma área maior que a atual Amazônia, antes de ser suplantada pelos modernos sistemas fluviais do continente durante o Pleistoceno. Nos últimos anos, conforme os detalhes desse antigo gigante fluvial surgiram, tornou-se conhecido como o rio Paleo-Bell em homenagem ao geólogo canadense Robert Bell, que apresentou pela primeira vez evidências de sua existência.
Amazônia da América do Norte hipnotizada
Robert Bell nasceu em Toronto em 1841, menos de um ano antes da fundação do Serviço Geológico do Canadá (GSC), uma organização à qual dedicaria 50 anos de serviço. Ele primeiro provou a aventura da exploração geológica como assistente de campo adolescente de Sir William Logan, que fundou o GSC. Após a formação universitária da Bell e vários anos como professor universitário, ele se juntou ao GSC e iniciou pesquisas de reconhecimento da geologia, flora, fauna, povos indígenas e potencial agrícola das vastas regiões que drenaram para a Baía de Hudson.
Todo verão, Bell e outros cientistas do GSC desapareciam na região norte do Canadá, viajando de barco, de canoa ou a cavalo. Como apenas mapas rudimentares das rotas de exploração e comércio associados ao comércio de peles existiam em grande parte dessa região, as partes de campo geralmente faziam mapas topográficos e mapas geológicos à medida que prosseguiam. Doença ou lesão no campo, por vezes, resultou em morte; Um Bell proativo obteve um diploma em medicina em 1878 para lidar com essas eventualidades. De 1884 a 1885, ele foi para o mar como oficial médico e científico em navios de pesquisa explorando a oceanografia da Baía de Hudson e do Estreito de Hudson, que separa Labrador da Ilha de Baffin e é a saída da baía para o Oceano Atlântico.
Robert
Bell (kneeling, center left, with beard) poses with his field party in
1883. Credit: Geological Survey of Canada/Library and Archives Canada.
During their explorations on land and at sea, Bell and his GSC
colleagues keenly observed evidence of past glaciations of the
landscape. They, in association with the U.S. Geological Survey and
university colleagues, eventually outlined the limits of glaciation and
compiled evidence of ice flow directions across North America.
Bell integrated his 25 years of fieldwork and that of fellow GSC
explorer-scientists in reconstructing the patterns of drainage that
existed prior to glaciation. In May 1895, at the annual meeting of the
Royal Society of Canada, he made a bold hypothesis: There had been a
great preglacial river basin that drained into a trunk stream that
traversed what is now Hudson Bay and exited into the Atlantic Ocean
through Hudson Strait. The basin of this river would have extended east
from the Canadian Rocky Mountains and from the north central United
States to what is now the Mackenzie River Basin — North America’s second
largest river basin, which drains into the Arctic Ocean. Bell did not
know how and why the relatively shallow Hudson Bay was once above sea
level, but his paleo-drainage reconstruction indicated that it had been —
and in relatively recent geologic time.
Within a few years, A.W.G. Wilson of McGill University cited
extensive evidence that the Canadian Shield, eroded to a nearly flat
surface, had been broadly uplifted and warped in geologically recent
time. This notion supported Bell’s view that Hudson Bay had been above
sea level prior to glaciation. Why this uplift occurred and why the
contemporary Hudson Bay lay below sea level were still not known to Bell
and Wilson. Despite such knowledge gaps, Bell — like many great
scientists before him — had seen the larger picture. And over the next
century, his “great preglacial river” was slowly corroborated as
geologists investigated North America’s ancient past for a variety of
reasons, from seeking oil and gas to searching for diamonds.
Evidence of Bell’s River in the West
Early in the exploration of the American and Canadian west,
geologists recognized that an interior seaway once stretched from the
Arctic Ocean to the Gulf of Mexico, between the rising cordillera to the
west and the Canadian Shield and Appalachian Mountains to the east. The
seaway existed during the Late Cretaceous from about 90 million to
70 million years ago. Dinosaurs roamed its forested shores, and the
forests shed pollen into the adjacent sea that was buried amid seafloor
sediments. This fossil pollen also played a large role as evidence for
the existence of the Paleo-Bell River.
By the early Cenozoic Era, about 50
million to 60 million years ago, the seaway had become dry land as
erosion of the eastern margin of the young cordillera resulted in the
growth of massive alluvial fans that filled the seaway and spread
eastward toward the Canadian Shield and what is now Hudson Bay. These
fans would have been of comparable scale to the modern delta of the
Ganges-Brahmaputra River in Bangladesh and northeastern India, which
drains much of the Himalayas today. The erosion of the mountains and
growth of the deltas established the west-to-east drainage pattern that
gave rise to the Paleo-Bell River system (drainage had been east to west
prior to the rise of the cordillera).
Lithified
gravel caps the Cypress Hills in southwest Saskatchewan. The gravel was
deposited by a large braided river — likely ancestral to the modern
Yellowstone River — that was one of the main tributaries to the
Paleo-Bell River circa 30 million years ago. The detail view (A) shows
colorful quartzite and argillite clasts derived from the Proterozoic
Belt Supergroup. The nearest likely sources of these clasts are 300
kilometers to the southwest. Credit: David Sauchyn, University of
Regina.
Subsequent regional uplift of the interior plains of North America
resulted in the erosion of much of these fan sediments as well as some
of the underlying seaway sediments. However, scattered across the
interior plains of the northern U.S. and Canada are remnants of some of
the earliest deposits laid down by the Paleo-Bell River system. One of
the best-known examples is from the Cypress Hills in southwestern
Saskatchewan and southeastern Alberta, Canada, about 70 kilometers north
of the Montana border. Today, the hills rise about 300 meters above the
surrounding plains, but in the past, the summits of this upland area
were high enough to rise above the kilometers-thick continental glaciers
of the last several million years.
In the 1880s, pioneering geologist
R.G. McConnell,
a colleague of Bell, noted that the upland was capped by distinctive
reddish, quartz-rich gravels. These gravels are largely derived from the
Belt Supergroup, a massive assemblage of sedimentary and mafic volcanic
rocks deposited about 1.4 billion years ago and which underlie the
spectacular mountains of Montana’s Glacier National Park and much of the
adjacent Rockies. The nearest outcrops of Belt rocks to the Cypress
Hills, however, occur hundreds of kilometers southwest of the hills.
These gravels were recognized as having been transported across the
braided flood plain of a great river — apparently an ancestor of the
Yellowstone River system — that flowed northeastward toward Hudson Bay.
Subsequently, fossil mammal remains from between 30 million and
10 million years ago in the Oligocene and Miocene were identified in
these gravels. And in a 2013
article in GSA Today, geologist
James Sears
of the University of Montana concluded that the drainage basin
associated with these gravels may at times have extended as far south as
what is now the Grand Canyon region. (see sidebar below)
A generalized reconstruction of Paleo-Bell River drainage and evolution
of other major rivers in western and northern North America, after
James Sears and others. By the Miocene, the Paleo-Bell River Basin
reached its greatest extent. Rifts like the Rio Grande and Great Basin
created an ancestral Colorado River that was yet to establish a course
to the Pacific (location 1). Instead, it flowed north from the Grand
Canyon region through structurally controlled valleys and into the
larger Paleo-Bell River Basin via an ancestral Yellowstone River, whose
gravels cap the Cypress Hills. This route was blocked by eruptions of
lava in the Snake River Plain (location 2) associated with the
Yellowstone Hot Spot. Repeated glaciation starting about 2.6 million
years ago diverted north-flowing rivers like the Paleo-Yellowstone along
ice sheet margins (location 3) to form the Missouri River. The ice
sheets also disrupted the Paleo-Bell River Basin, causing river
sedimentation to cease in the Saglek Basin. The Mackenzie River Basin
was created, leaving the Saskatchewan/Nelson River Basin as the last
remnant of North America's Amazon. Credit: K. Cantner, AGI and Lionel
Jackson, based on Sears, GSA Today, 2013.
Uplands similar in age to the
Cypress Hills are found scattered amid the interior plains to north of
the Arctic Circle and document other branches of a former vast river
system that flowed east toward Hudson Bay from the Eocene through the
Pliocene epochs (about 55 million to 3 million years ago). By the start
of regional glaciation about 2.6 million years ago, the areas between
these uplands were eroded down to about the level of the interior plains
and adjacent Canadian Shield today. However, virtual time capsules of
Eocene sediments, complete with vegetation, were preserved by volcanism
that emplaced diamonds in what is today Canada’s Northwest Territories.
Diamond exploration and mining in the Lac de Gras diamond fields, for
example, has inadvertently yielded fascinating information about
climatic conditions in the Paleo-Bell River Basin tens of millions of
years ago (see sidebar below).
Through the Pleistocene, the periodic spread of ice sheets over most
of what is now Canada and the northern U.S. eventually disrupted some
rivers of the Paleo-Bell system, such as the early Yellowstone River,
diverting these flows from their northeastward courses more to the east,
into what would become the Missouri River, the path of which largely
marks the margin of past ice sheets. Rivers farther north in the
Paleo-Bell system, meanwhile, may have re-established their prior routes
during intervals between glacial episodes until they assumed their
current courses.
The Paleo-Bell’s Mouth Is Discovered
During the 20th century, the search for new sources of oil and gas,
and advances in offshore drilling technology, opened up deeper and
stormier seas to geologic exploration. By the 1970s, the continental
shelves of Labrador and Baffin Island had become prospects for
exploration. Drilling and seismic investigations revealed that the area
was underlain by millions of cubic kilometers of sediments deposited
during and after the opening of the North Atlantic, beginning about 140
million years ago. N.J. McMillan, then with Aquitaine Petroleum in
Calgary, compiled existing drilling data and geophysical data for the
region and identified a large package of sandy and muddy sediments in
the Saglek Basin, which lies beneath the Labrador Sea at the mouth of
Hudson Strait. These sediments — totaling an estimated 2.5 million cubic
kilometers — were deposited between about 55 million and 5 million
years ago.
Uninhabited
Salisbury Island, part of the Canadian territory of Nunavut, is in the
Hudson Strait, which flows between Hudson Bay and the Labrador Sea.
Research in the 1970s and ‘80s revealed that the Hudson Strait’s entry
into the sea — an area known as Saglek Basin — was the mouth of the
Paleo-Bell River. Credit: Doc Searls, CC BY 2.0.
Erosion of adjacent uplands in
Labrador and on Baffin Island during this time was inadequate to account
for the vast amount of sedimentary material in the Saglek Basin.
McMillan recognized that one of Earth’s largest rivers must have
discharged into the Atlantic Ocean during that interval. The area
corresponded well with the mouth of Hudson Strait, where Robert Bell had
placed the mouth of his great preglacial river. In his landmark
paper
published by the Canadian Society of Petroleum Geologists in 1973,
McMillan revived and expanded on Bell’s earlier hypothesis. McMillan’s
hypothesis was subsequently corroborated by research carried out in the
1980s by V. Eileen Williams at the University of British Columbia.
Williams separated and studied fossil pollen grains, called
palynomorphs, from drill cores obtained from exploratory oil and gas
drilling in the late 1970s and early ‘80s in the Saglek Basin. Although
the sediments contained pollen as young as about 5 million years old,
there was a significant amount of recycled palynomorphs dating from the
Mesozoic Era throughout younger sediments associated with the Paleo-Bell
River. Studies of modern rivers, such as the Mississippi and Orinoco,
have shown that fossil palynomorphs are commonly eroded from sedimentary
rocks and transported thousands of kilometers and redeposited
(see sidebar below). In the case of the ancient pollen found in the
Saglek Basin, Williams concluded that these palynomorphs had eroded from
sediments originally deposited in the former North American Interior
Seaway. The age range of the younger palynomorphs constrained the
duration over which the preglacial river existed to 40 million to 50
million years. By comparison, the delta of the Amazon River has recently
been
shown to have been in existence for only about 11.5 million years.
The Paleo-Bell Gives Way to the Mackenzie
Alejandra Duk-Rodkin sits in the Painted Mountains of the Northwest
Territories, Canada, a site near the former headwaters of a branch of
the Paleo-Bell River that’s now within the Mackenzie River Basin.
Credit: courtesy of Alejandra Duk-Rodkin.
The Pleistocene ice ages, starting about 2.6 million years ago,
spelled the end for the Bell River. Ten ice sheets formed over North
America during the past million years alone, and many others formed
prior to that. The full extent of glacial erosion over Hudson Bay and
the Canadian Shield over this period is debated, but the Hudson Bay area
likely experienced hundreds of meters of erosion, as well as
substantial isostatic depression from the weight of several kilometers
of overlying ice (Hudson Bay is still rebounding from unloading of the
last ice sheet). As a result, Hudson Bay was inundated by the sea, and
rivers began draining directly into it during interglacial periods,
cutting off sedimentation to the Saglek Basin.
The Saskatchewan and Nelson river
systems, which drain much of Alberta, Saskatchewan and Manitoba, and
part of North Dakota, are the last remnants of the Paleo-Bell River
system. Ice sheet margins allowed the Mississippi River to capture many
formerly northeast-flowing rivers in the American West. However, the
grandest change caused by ice sheets was the creation of the Mackenzie
River.
The sequence of events that led to the Mackenzie’s formation was largely uncovered through the work of
Alejandra Duk-Rodkin,
who joined GSC in the early 1980s. At that time, pipelines and oil and
gas development were slated for the Mackenzie River Valley and the
Mackenzie Delta, and a detailed understanding of the surficial geology
was required. Duk-Rodkin began studying and mapping the surficial
geology of that region, and of the adjacent Mackenzie Mountains (a
northern continuation of the Rockies) and Yukon — an investigation that
would last decades.
She found that the Mackenzie Valley region was a palimpsest of two
drainage patterns: an older one marked by erosional surfaces that could
be traced from the Mackenzie Mountains and Richardson Mountains across
what is now the Mackenzie Valley into the interior plains and the
Canadian Shield; and a younger pattern that paralleled the Mackenzie
Mountains. Additionally, her mapping and stratigraphic work demonstrated
that the last Laurentide Ice Sheet was more extensive in that region
than any of its predecessors had been over the prior two and a half
million years, and that it — spreading westward from northeastern North
America — had diverted drainage from the Mackenzie Mountains, from the
ice sheet itself, and from as far south as Alberta and British Columbia,
to the north. This drainage cut massive canyons leading to the Arctic
Ocean.
As the ice sheet later thinned and retreated eastward, it ponded
immense lakes in what is now the Mackenzie Valley that eventually
spilled northward. The result of all this glacial diversion was the
formation of the Mackenzie River Basin, which today covers an area of
1.8 million square kilometers — about the size of Alaska, or about 60
percent of the size of the Mississippi River Basin.
By the early 1990s, Duk-Rodkin realized that the eastward-dipping
erosional surfaces crossing the Mackenzie River that she’d observed,
indicative of a once mighty pattern of east-directed drainage, were
consistent with Bell’s notion of a “great preglacial river.” She simply
referred to this former river system as the Bell River. North America’s
vanished Amazon had a fitting name.
