Penokean orogeny
Encyclopedia
The Penokean orogeny was a mountain-building episode that occurred in the early Proterozoic
about 1.85 to 1.84 billion years ago, in the area of North America
that would eventually become Minnesota
, Wisconsin
, Michigan
and Ontario
. It was a major event in the formation of the North American continent
, and happened during a worldwide period of mountain building and continent formation.
Before this episode the area was a passive continental margin occupied by a shallow sea, which created large sedimentary deposits including the banded iron formation
s of the Iron Range
s.
The orogeny
happened in two phases. First an island arc
called the Pembine-Wausau terrane collided with the ancient North American craton along with volcanoes formed in its back-arc basin
. The second phase involved a microcontinent called the Marshfield terrane, which today forms parts of Wisconsin and Illinois. The episode lasted about 10 million years.
Hundreds of millions of years later, the Keweenawan Rift
occurred in the same area creating the basin that would eventually become Lake Superior
. The remains of this orogeny
can be seen today as the Iron Ranges of Minnesota and Ontario, the Northern Highland
s of Wisconsin and the Upper Peninsula of Michigan
.
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
about 1.85 to 1.84 billion years ago, in the area of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
that would eventually become Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
, Wisconsin
Wisconsin
Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...
, Michigan
Michigan
Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....
and Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
. It was a major event in the formation of the North American continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
, and happened during a worldwide period of mountain building and continent formation.
Before this episode the area was a passive continental margin occupied by a shallow sea, which created large sedimentary deposits including the banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
s of the Iron Range
Iron Range
The Iron Range is a region that makes up the northeastern section of Minnesota in the United States. "The Range", as it is known by locals, is a region with multiple distinct bands of iron ore...
s.
The orogeny
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
happened in two phases. First an island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
called the Pembine-Wausau terrane collided with the ancient North American craton along with volcanoes formed in its back-arc basin
Back-arc basin
Back-arc basins are geologic features, submarine basins associated with island arcs and subduction zones.They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. Most of them result from tensional forces caused by oceanic trench rollback and the...
. The second phase involved a microcontinent called the Marshfield terrane, which today forms parts of Wisconsin and Illinois. The episode lasted about 10 million years.
Hundreds of millions of years later, the Keweenawan Rift
Midcontinent Rift System
The Midcontinent Rift System or Keweenawan Rift is a long geological rift in the center of the North American continent and south-central part of the North American plate. It formed when the continent's core, the North American craton, began to split apart during the Mesoproterozoic era of the...
occurred in the same area creating the basin that would eventually become Lake Superior
Lake Superior
Lake Superior is the largest of the five traditionally-demarcated Great Lakes of North America. It is bounded to the north by the Canadian province of Ontario and the U.S. state of Minnesota, and to the south by the U.S. states of Wisconsin and Michigan. It is the largest freshwater lake in the...
. The remains of this orogeny
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
can be seen today as the Iron Ranges of Minnesota and Ontario, the Northern Highland
Northern Highland
The Northern Highland is a geographical region covering much of the northern territory of the U.S. state of Wisconsin. The region stretches from the state border with Minnesota in the west to the Michigan border in the east, and from Douglas and Bayfield Counties in the north to Wood and Portage...
s of Wisconsin and the Upper Peninsula of Michigan
Upper Peninsula of Michigan
The Upper Peninsula of Michigan is the northern of the two major land masses that make up the U.S. state of Michigan. It is commonly referred to as the Upper Peninsula, the U.P., or Upper Michigan. It is also known as the land "above the Bridge" linking the two peninsulas. The peninsula is bounded...
.