The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Tuesday, January 21, 2014

January 21. Trans-Hudson and Penokean Orogenies

By Richard I. Gibson

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About 1.85 billion years ago, as the last of the banded iron formations were being deposited, the core of North America, the Superior Craton, was growing. The smaller Wyoming Craton and other nearby blocks collided with the Superior Craton, raising up a mountain range, possibly of Himalayan proportions, in the area we know today as Saskatchewan, Manitoba, North and South Dakota, and western Nebraska.

This collision was much like India’s ongoing collision with Asia, so the idea that the mountains there were like the Himalayas is not out of line. A few hundred million years of erosion, and there’s nothing left of that mountain range, but there is plenty of subsurface evidence for it expressed in measurements of the magnetic and density properties of the rocks that were once the mountain roots. The zone was a persistent weakness in the earth’s crust, and by Ordovician time (which we’ll talk about in March) this area sagged to form a sedimentary depression, the Williston Basin, important today as the host for oil deposits including those of the Bakken Formation, which we'll talk about in May.

Another small rigid block collided with what is today the southern edge of the Superior Craton, in northern Lake Michigan, across Wisconsin and Minnesota and into present-day Iowa. This zone is called the Penokean Fold Belt. It’s probably no accident that those wonderful banded iron formations were along the edge of this zone—they were probably laid down in a deep marine setting, a trench between the two continental blocks that ultimately fused to make the North American craton a bit bigger.

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