Maroon Bells
Colorado USA
About 310 million years ago during the Pennsylvanian Period, the earth’s crust in what is now Colorado and New Mexico, and parts of adjacent Utah, Texas, and Oklahoma, was breaking, big-time. The huge uplifts that formed were similar to the big ranges in Wyoming and vicinity today – the Wind Rivers, Beartooths, Big Horns, Uintas, and others. The ultimate cause for what is called the Ancestral Rockies was probably the collision between the northern prong of Gondwana (northwest South America, plus various other blocks) with North America. The immediate result of the collision was the Marathon thrust belt in West Texas, but the effects were felt well into the continent.
It’s called the Ancestral Rockies Orogeny because the 310-million-year-old uplifts were in locations similar to, but definitely not quite exactly the same as the uplifts that formed the modern Rocky Mountains about 75 to 50 million years ago. The later breaking sometimes followed old (Ancestral Rockies) weak zones and breaks, and sometimes it didn’t.
As the Ancestral Rockies rose, the rocks that had been deposited previously eroded off the mountains, with the youngest (the ones on top) eroding first. But these uplifts were so high, it wasn’t long (geologically – a few million years) before the ancient Precambrian mountain roots were being eroded off as well. All that eroded sediment was shed onto the flanks of the uplifts, where it eventually solidified into rock.
The mountain-flank environments included rivers and mud flats, channels and lakes, and (early on) some beaches. There are mud-cracks, ripple marks, raindrop impressions, and worm tubes, all indicating shallow water. Because the material was being deposited in shallow, wet settings, the abundant iron in it oxidized to many shades of rust-red: brown, orange, purple, red-brown, gray-red, red – with purplish maroon really common. Those colors gave their name to Maroon Creek and the Maroon Bells peaks near Aspen, Colorado, seen in my photo here.
The obvious relatively thin layers in the Bells contain alternating conglomerates, sandstones, siltstones, mudrocks, and a few limestones, all indicating those shallow-water terrestrial environments I mentioned. The area around the Maroon Bells is on the flank of the Uncompahgre Uplift, one of the Ancestral Rockies uplifts that extended from east-central Utah across southwest Colorado into northern New Mexico. Here, the sediments that were shed off the uplift are called the Maroon Formation. A separate but similar uplift developed in north-central Colorado, and the coeval Pennsylvanian-age sediments that were deposited there are called the Fountain Formation, which makes up the famous flatirons west of Boulder, Colorado, and the Red Rocks amphitheater near Golden.
The tilting you see in the Maroon Formation in my photo is probably the result of the more recent (75 to 50 million years ago) mountain uplift in Colorado.
As a reminder, most of my posts relate to things in my own collections, places I’ve been (and ideally have my own photos), and/or places I’ve worked on. That’s not a hard-and-fast rule, but it does define my focus. Nothing sacred about it, it’s just my general preference. Thanks for your interest!
Excellent synopsis. Thankyou!