Penokean orogeny

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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 formations of the Iron Ranges.

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 Highlands of Wisconsin and the Upper Peninsula of Michigan.

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