The Great Unconformity near Melrose, Montana

This video presents the Great Unconformity exposed in the Camp Creek drainage east of Melrose, Montana. At this location, rocks metamorphosed during the collision of continental plates some 1.8 billion years ago are overlain by ~520 million-year-old beach sandstones of the Flathead Formation. This huge gap in the geologic record is known as the Great Unconformity, and the contact between these different rocks represents over a billion years of erosion and/or non deposition.
The consensus interpretation of the unconformity is that some 1.8 billion years ago the collision of a continental crustal block called the Medicine Hat Block with the Wyoming Province of the North American craton (continent) resulted in metamorphism and uplift of mountains that might have rivaled the Himalayas today. Once the colliding blocks were sutured and the plate collision ended, the mountains began to erode. Over the next 1.0 to 1.3 billion years, they were eroded down to sea level and much of western North America was a flat, low-lying plain. The ocean rose up around 520 to 515 million years ago, drowning the eroded surface with ocean water and depositing ocean sediments.
As the sea transgressed across the eroded surface, it first deposited beach sand of the Flathead Sandstone. This rock contains some fossils and abundant sedimentary structures that indicate a marine beach environment. With continued sea-level rise, the beach moved eastward, and the Camp Creek area was covered by deeper water, where fine-grained mud that couldn't settle on the beach were deposited. These deposits are called the Wolsey Shale, and they are loaded with burrows, trilobite tracks and a few, nearly intact trilobite shell fossils. As the sea continued to rise, the source of sand, silt and clay were moved so far away from the area that organisms that filter the water and secrete calcite flourished, depositing limestone of the Meagher Formation.
Sea-level continued to fluctuate, leaving behind more shale and limestone, marine and non-marine deposits throughout the period from 520 to 245 million years ago, what geologist's call the Paleozoic Era. Many of these rocks are exposed at Camp Creek and throughout the area. There is a map in the video to help you locate the site. You can find specific directions in the 2nd Edition of Roadside Geology of Montana, so get out, explore and enjoy Montana Geology!

[@SeldomSeen-0824]

Thanks! Super interesting to see this. And described so well!

[@wfoybealiii4668]

What a great spot! Structure and sed/strat one can lay one's hands upon! Thanks for sharing!

[@rockdoctorMT]

You bet!

[@georgerisberg8830]

Shawn Wilsey has a nice viideo on the great unconformity west of Cody, Wyoming. Thanks for your video, great!

[@nickraschke4737]

Gday from Australia. Enjoying the channel. Cheers.

[@rockdoctorMT]

Excellent! Thanks…

[@nhragold1922]

Id be sampling it for sure

[@pmm1044]

Thank!! You suggest that some discussion of every static slide would be helpful

[@rockdoctorMT]

I just learned how to do voiceover today, so next time.

[@pmm1044]

⁠Nice!!

[@TheTruthPlease100]

Shows a great calamity that happened. Pole change, great impact?

[@rockdoctorMT]

It’s really shows us how long it takes to erode away mountains formed by continental collisions. Sometimes, no geologic record tells us more than the rock itself! The Earth is old, always changing, and we are newcomers to this ancient story. It’s great!

[@vannemocilac274]

I got to go see that last year!

[@robertodebeers2551]

Melrose, Montana, has always been a great unconformity.

[@richarddavies7419]

Interesting! An even greater gap may be seen just east of Yellowstone Park in the Canyon of the Shoshone River, where a dark red granite, small pegmatite dikes, and metamorphic rocks of 2.8 billion year old Archean age are exposed along the highway. Thinly bedded limestones of the Flathead Formation sit directly on an almost polished surface of the red granite, suggesting glaciation back when the Wyoming Province was part of Rodinia some 700 million years ago.

[@rockdoctorMT]

I know the spot well. I put the Flathead Sandstone contact just before the tunnel in the Roadside Geology of Yellowstone Country book. I didn’t notice a polished basement surface at that location, but the bedding surface is not well exposed in the road cut.

[@richarddavies7419]

@@rockdoctorMT I peeled back a thin-bedded layer specifically to be able to inspect the granite surface, back in the summer of 1952, when that granite was simply "pre-Cambrian" in age.

[@rockdoctorMT]

@richarddavies7419 Super cool!

[@JohnZolla-bp7tl]

I own 20 acres in the Pipestone Pass. Wish I knew more about the geology of the area.

[@rockdoctorMT]

The area is underlain by granitic rock of the Boulder Batholith, an ~80 million year intrusion of magma from the subduction of the Pacific Ocean floor under North America.

[@jeffreypaul734]

A chunk got washed away. Hmmm

[@rockdoctorMT]

?

[@FriendsofTaz]

Cool. Were you a professor at Montana State University back in 1986? If so, I may have taken your class back then

[@rockdoctorMT]

No, I’ve been at Montana Western in Dillon since 1993. I did an MS at UM, and have taught field camp in Dillon almost continuously since ‘86.

[@pmm1044]

The blue color metamorphic rock is what minerals? And how firmed? Is it schist?

[@rockdoctorMT]

The metamorphic rocks are mineralized. I have found copper-bearing minerals like malachite (green) and azurite (blue). Mine adits and tailings are throughout the area, and there are several sills that are likely Cretaceous and possibly the source of mineralization.

[@rockdoctorMT]

The rock varies from biotite schist to gneiss.

[@pmm1044]

⁠thankyou!

[@frankwolf3860]

Did any of your students, or you!, investigate the pegmatite for the presence of "cool" crystals?

[@rockdoctorMT]

It’s rather dull by the standards of other pegmatites in the region. No tourmaline, for example, just quartz and feldspar.