Jar, from
An honest question for creos regarding dates and dating:
quote:
Actually, you do have the means to check many of these things out and you're in a perfect place to do so. Here are a few things right around you to check out.
Head over to Calumet and the Hill Annex State Park. It's an old open pit mine that goes down to the Coleraine formation from about 60-100 Million years ago. Back then most of the Mesabi Range area was covered by an ancient ocean.
Next head to Taylor Falls where you can see the result of glaciers from 11,000 years ago. The rock that was potholed and scraped away is about 1.1 Billion years old.
In Tower, MN you can visit the Soudan Underground Mine. Some of the rock there is over 2.7 Billion years old.
Head over to the Jay Cooke State Park in Carlton and you can see some of the pre-cambrian shale from well over 2 Billion years ago. It was compressed and hardened into shale and then later bent and twisted.
There are lots of other places right around you where you can check these things out first hand and for yourself.
This is getting into my region, especially the Minnesota stuff. Jar, it sounds like you too lived in this area, somewhere along the line.
The Soudan Iron Formation is a member of the Ely Greenstone, and is Archean (early preCambrian) in age. In general the Archean of northern Minnesota is dated at 2.7 billion years.
Calumet is in the upper peninsula of Michigan. I believe that the referred to iron formation is Middle preCambrian (Proterozoic), having an age of 2.1 billion years. This correlates with the Biwabic Iron Formation of the major Minnesota iron ranges, the largest being the Mesabi Range.
The Thompson Formation (Carlton area) is greywacke sandstone and slates. It correlates with the Virginia Formation to the north, which overlies (is younger than) the Biwabic Iron Formation. The Thompson Formation are turbidite deposits.
In general, the preCambrian geology of northern Minnesota is as follows:
The northeast is Archean, consisting of volcanic/sedimentary belts. These are strongly folded by the Algoman Orogeny (tectonic/metamorphic event), dated at 2.7 billion years. Intrusives of this event include the Giants Range Batholith.
The Archean rocks are unconformably overlain by the Proterozoic Animiki Group, which includes the Biwabic Iron Formation and the Virginia formation. This unconformity marks a major erosional event. The Animiki Groups dips southward at a fairly low angle. As I said above, the Virginia and Thompson Formations correlate. Essentially, the are two areas of the same formation.
But the Thompson Formation was strongly folded by the Penokean Orogeny (1.6 billion years ago), which was centered in central Minnesota over through Wisconsin and into northern Michigan. This orogeny is why the iron formations of northern Michigan are much for folded and metamorphosed than those of Minnesota.
All these earlier rocks are in turn intruded by or overlain by the rocks of the Keweenawan event (dated at 1.1 billion years ago). This includes the intrusive Duluth Gabbro Complex and the North Shore Volcanic Group.
These are in turn overlain by sediments that cover the remainder of the preCambrian era.
Well, it was quick and dirty. The basics are correct, although some of the precise details may be wrong (damn memory).
I recommend the book
Geology of the Lake Superior Region, by Gene L. LaBerge. Penokean Press, 1994. Gene is/was a professor at the University of Wisconsin, Oshkosh. I believe he is now somewhere in the vicinity of retirement age.
Moose
Professor, geology, Whatsamatta U
Evolution - Changes in the environment, caused by the interactions of the components of the environment.
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