Wednesday, June 29, 2016

Living the Dream, Day 6 – Elves Chasm and the Great Unconformity

A rosy–fingered dawn metamorphoses into a vibrantly blue morning sky. In the blink of an eye, shadows and colors shift on the canyon walls. Another plate of scrambled eggs sits balanced on my lap. It is Day Six of our Colorado River rafting adventure in Grand Canyon. A question hangs suspended in midair. Where exactly in this magnificent chasm are we now?
A balanced breakfast (Click on any pic to enlargenate)
We camped last night at river mile 109, just downstream of Bass Rapid and Hotauta Canyon. A block of tilted Grand Canyon Supergroup rocks exposes itself downstream for a few short miles along this stretch of river and then disappears.

Grand Canyon Supergroup tilts at river level
Once upon a time, North America looked much different than it does today. For one thing, the continental landmass was smaller. Studies of the chemical composition of Grand Canyon rocks have led geologists to conclude that in this region the continent grew by collision and accretion of island arcs and other continental fragments during subduction of one ancient tectonic plate beneath another. A modern parallel to this tectonic growth is likely Indonesia, where the Australian plate is sliding northward, subducting beneath the Pacific plate and forming the volcanic island arcs of the Indonesian archipelago.

Upper Granite Gorge
Metamorphic rocks of the Upper Granite Gorge
Early 20th century studies in Grand Canyon identified different “packages” of deformed metasedimentary and intrusive igneous rocks that resulted from this tectonic activity. At 1.84 billion years old, the igneous intrusive Elves Chasm pluton is considered to be “basement,” the oldest rocks upon which subsequent sedimentary and igneous rocks were deposited or intruded.
Faulting? Shearing? Something happened here.

Note convoluted rocks at lower right (click to enlargenate)

Convoluted rocks with a rapid

In the Upper Granite Gorge, between river miles 113 and 120 (from Waltenberg Canyon to Blacktail Canyon, where we will have lunch and a serenade), the contact between the Elves Chasm pluton (1.84 billion years old) and a younger (1.75–1.73 billion years old) Grand Canyon Metamorphic Suite of schists is exposed.

Elves Chasm gneiss? 
A fellow rafter is mesmerized by the enormity of it all
Looks to me like pillow basalt in lower right
Where exactly is this contact? I have no idea. Our guides are not geologists and so there is no one to point these things out to me. My research is done after the fact, long after I’ve returned home from this trip of a lifetime. I study my photos, and read my references, and try to weave a thread of sense from all I have seen. I need another river trip in a bad way!

Igneous dikes cross cut the older gneiss

More cross cutting

In the meantime, I nearly fall out of the boat when we encounter the SE–NW trending Monument Fold. The folded sedimentary layers look to be the Cambrian Tapeats Sandstone, quite a bit younger than the metamorphic Elves Chasm pluton and suite of schists beneath.

I am pretty sure this perky little structure is not the Monument Fold.

On the other hand, I am fairly certain this behemoth is.

Monument Fold near river mile 116
Close up of Monument Fold
Monument Fold 
The fold runs behind Explorer’s Monument and presents itself again downstream, just beyond river mile 118. I consider that it is likely the surface manifestation of a reactivated Precambrian thrust fault buried deep in the ground. There seems to be a lot of that sort of thing going on in these parts. When the folding occurred, I have no idea.

The Great Unconformity at river level, from the mouth of Royal Arch Creek
and Elves Chasm

There is missing time here, between the metamorphic rocks and overlying Tapeats Sandstone. Over a billion years is unaccounted for in the geologic record. This is the Great Unconformity at river level.

A short hike takes us to Elves Chasm
Around river mile 117 we disembark at Royal Arch Creek for a short jaunt. The Elves Chasm waterfall is a major reward for minimal exertion. Somewhere around here is the contact between the Elves Chasm pluton (gneiss) and the Grand Canyon Metamorphic Suite. I wish I could recognize it or have someone to point it out to me. I could be right on top of it and not know it.
Elves Chasm

Explorer's Monument from
Elves Chasm

The two river elves

Soon it is time to jaunt back to the boats. A lunchtime serenade beneath the Great Unconformity awaits us downstream at Blacktail Canyon.

Back to the boats



Karlstrom, K.E., Ilg, B.R., Williams, M.L., Hawkins, D.P., Bowring, S.A., and Seaman, S.J., 2003, Paleoproterozoic Rocks of the Granite Gorges, in Grand Canyon Geology, Beus, S.S. and Morales, M., eds., 2nd edition, Oxford University Press

Stevens, L., 2013, The Colorado River in Grand Canyon – River Map & Guide, Grand Canyon Wildlands Council

No comments:

Post a Comment