Saturday, December 12, 2015

Living the Dream, Day 3 – Whither the Devonian?

It is inevitable, along this downstream Grand Canyon journey. The rock layers we have come to know so intimately must now retreat, above and beyond the water level of the Colorado River. They now graciously extend the limelight to newer (at least to our eyes) yet older layers. We have spent the morning hours enveloped within the sheer walls of the Redwall Limestone, getting to know its Mooney Falls and Thunder Springs members, Vasey’s Paradise, Redwall Cavern, the Bridge of Sighs. Our j-rigs are transporting us down canyon and down section, into deeper geologic time.

The Redwall recedes from river level (click on any pic to enlargenate)
Whenever I stand upon the rim of the Canyon and peer down into its infinity, the Redwall seems so thick, so solid, so eternal. Up close, however, these crumbling walls appear weathered and ephemeral, carbonates dissolving into oblivion by trickles of groundwater. The Redwall Limestone never really surrenders its prominence, though, even as it shifts the limelight to the rocks beneath it. On this adventure I always look for it. Its distinctive walls are a reference point that the eye seeks. Where is it? Where am I? 

Is it lunch time yet?
36 Mile Rapid. I think.
Redwall or not, I think we are heading into the jaws of 36-Mile Rapid. I stop daydreaming about rocks and food for a moment, unravel my dry bag, and whip out my camera. This is a short clip –  my goal here as a promising videographer is to stay inside the boat.


Well, that was a ton of fun! Beyonce thought so, too.

Beyonce having the time of her life
These rocks look like limestones, and indeed they are. According to my handy dandy River Map & Guide, this is the Muav Limestone of Middle Cambrian age, 530 million years old and the upper, youngest formation of the Tonto Group.

Limestone everywhere
Wait a sec…

Have I missed something? Have I not been paying enough attention? There should be at least one other rock formation below the Redwall. Even the standard mnemonic for remembering the stratigraphy of Grand Canyon omits it:

Know
The
Canyon’s
History,
Study
Rocks
(Hey!!! Something is missing here! Whither the Devonian???)
Made
By
Time.

Where is the Devonian? Where are the famous channel deposits of its Temple Butte Formation? Peering precariously over and past my boat-mate’s hatted heads, I kneel and reposition a couple of random appendages as I scan the canyon walls for the elusive structures. Where are those little rascals? Before my inquiries are considered by the guides, however, there is something equally important to address.

Lunch time!
Beverage of choice on the river
38.5 Mile lunch spot
Enjoying the moment
I watch as an assortment of watercraft skips by on a baffle of rapids.


In eastern Grand Canyon, rocks of the Temple Butte Formation outcrop as isolated, lens-shaped structures that have been identified as channel fill deposits. These deposits coalesce and thicken to the west. In late Devonian times, 370 million years ago, shallow marine conditions existed along a passive continental margin in the area that would one day become the Grand Canyon. It was a widespread continental shelf covered by a clear shallow sea, similar to what eastern Florida or the Yucatan is today. Rivers flowing outward from the continent found their way to the sea and formed tidal estuaries along the shallow shelf. What we know today as the Wasatch Hinge Line was the boundary, 370 million years ago, between the shallow continental shelf to the east and a deeper, rapidly subsiding ocean basin to the west. The channel fill deposits I am looking for are evidence of these estuaries.

After lunch, I finally see something suspiciously lens-shaped, and nearly jump out of the boat. Can you see it?

A channel at last!
This red line should help.

Outline of the channel
In the next image, another channel is just above the hats. It is on the other side of the river and might be related to the first one but I cannot be certain. I am clearly too excited to keep track. The other rafters do not know what to make of my frenzy until I point out the lens shapes.

Another Devonian channel fill remnant
An unconformity is a break in the geologic rock record. It signifies a period of time during which erosion occurred, or during which there was no deposition. Or both. These channels are significant in that they mark the top of an unconformity between the older, underlying Middle Cambrian Muav Limestone (530 million years old) and the younger, overlying Late Devonian Temple Butte Formation (370 million years old). The Cambrian-Devonian unconformity is an extensive break in the rock record that represents missing time of the late Cambrian, all of the Ordovician, all of the Silurian, and most of the Early and Middle Devonian geologic periods. It is an enigmatic chunk of geologic time that no longer exists.

We motor on, in shadows and sunshine.

An early dam site
Near river mile 39 the guides cut the engines, tether the boats, and stop for a few moments of togetherness. They speak quietly of the Marble Canyon dam that was proposed for this stretch of river but never built, and of the history of Colorado river allotments among seven particular western states since a 1922 compact was signed.

 Here is another video for your viewing pleasure. No musical accompaniment with this one.


Gravity happens. Rocks pop, detach, plummet and crash with impressive enthusiasm, straight off the sheer walls of the Redwall/Temple Butte/Undifferentiated Limestone/Muav. I wonder how recently this event occurred…


Gravity happens

…or this beauty, not far downstream on the opposite side. Was there an earthquake recently, that caused these events to occur in such proximity to each other? The guides have no idea. The dust is settled. The rock falls look fresh enough to have happened yesterday. Did they make a sound? Did anyone hear?




We sidle up to Saddle Canyon for our second night’s camp. The sun slowly drops behind the towering cliffs but the rocks still glow in its reflection. Exhausted, I am ready to forego a three-mile hike up the canyon when the guides call “Happy Hour!” After giving it some thought, the guides reschedule the hike for the morning. I for one am pleased with the decision.

Sidling up to Saddle Canyon
Kris and I set up and relocate our tents several times, not only to be out of the main traffic pattern but also to avoid a view of “the groover” (more on “the groover” later). I am hot. The cool river beckons so I bring soap, a towel, and a change of clothes down to the beach where the boats are tied. I dunk myself in the 50° shallows and quickly wash my hair with that old campers standby, Dr. Bronner’s peppermint soap.

Beyonce guards the boats
Folks socialize around the tents and at the beach until dinner. Box wine and beer stays chilled in a bucket of that 50° river water. Dusk brings a campfire in the cast-iron fire pit over which the guides had grilled burgers and cooked a dutch-oven carrot cake. I sip one last cup of wine, marvel at white pin points of stars dancing madly against the blue-black sky, say goodnight to the Milky Way, and crawl contentedly inside my tent, certain there are more Devonian channel fill deposits in my future.

View upriver from the beach below Saddle Canyon

****************************************


References:
Beus, S., 2003, Temple Butte Formation in Beus, S. and Morales, M., eds., Grand Canyon Geology, 2nd edition, Oxford University Press
Blakey, R. and Ranney, W., 2008, Ancient Landscapes of the Colorado Plateau, Grand Canyon Association
Collier, M., 1980, An Introduction to Grand Canyon Geology, Grand Canyon Natural History Association

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



2 comments:

  1. The rock falls really catch the eye. I think I photographed nearly all of them as well! Wonderful pictures.

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    Replies
    1. I know! That rockfall was jaw-dropping! It looked really fresh, too.
      I love looking at my river pix, and reliving the trip.

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