January 24. Belt Basin 1.3-1.4 billion years ago

By Richard I. Gibson

Here’s the podcast:



Transcript:

St. Mary Lake, Glacier National Park. Photo by David Restivo, NPS

We’ve talked briefly about continental crustal fragments large and small drifting around the earth’s surface, a process which continues today. Inevitably, at some times a lot of the continents might come together to create what we’ve come to call a supercontinent. While we won’t encounter the best known supercontinent, Pangea, until August, supercontinent assemblies have happened repeatedly in the past.

The Sioux Quartzite was deposited in North America during the time that the supercontinent called Columbia is thought to have existed, around 1.7 to 1.5 billion years ago. We know something more about the break up of that possible supercontinent than its assembly, and there is abundant evidence for a break, separating what is now western North America and eastern Siberia, around 1.4 billion years ago.

That break created a huge basin in what is now the northwestern United States and nearby parts of Canada. It was filled with as much as 50,000 feet of mostly fine-grained sediments, mostly mud and silt that lithified into shale, mudstone, and a few sandstones and limestones. We call this thick package of rocks the Belt Supergroup, named for occurrences in the Big and Little Belt Mountains of Montana.

Armored mud balls in Grinnell Formation,
Glacier Park. Photo by Richard Gibson.

The Belt rocks are beautifully exposed in Glacier and Waterton National Parks, where they display red and green colors sculpted by glaciers that were vastly younger than the rocks themselves. The rocks contain details that give us lots of information about the early earth. For example, thin layers of coarse grains indicate that storms probably stirred up the sediment on the floor of a shallow lake or sea. It may seem obvious that rain fell on the ancient earth, but we KNOW it did because of raindrop impressions found in some of the Belt rocks.

One of the most remarkable things about the Belt rocks is the immense span of time that they represent – maybe as much as 250,000,000 years of relatively undisturbed deposition. Understanding the Belt is challenging, to say the least, and has been the life work of many geoscientists.

From Idaho State University

Lots of geologists try to figure out how the earth’s early continents were combined before they broke up – this has implications for resource exploration as well as pure science – and a lot of effort went into the question of what continent separated from western North America to create the Belt Basin. Antarctica, Australia, and Siberia were popular candidates. But thanks especially to the work of Jim Sears and his students at the University of Montana, and Paul Link at Idaho State, I think it’s pretty well settled now that it was Siberia that was once where western Washington and Oregon are today. UPDATE March 2014: maybe not ... some recent work suggests that it might NOT have been Siberia out there... stay tuned - I'll add information soon.