East Turks Head Peak and Spencer Mountain

I uploaded this view of East Turks Head to prove it wasn't always storming on my trip through the San Juan Mountains during the 2017 New Mexico Geological Society Fall Field Conference.

Of course, behind me, to the east, it was storming...

 

This is going to be one of those Geology intensive descriptions so be fore-warned.

 

East Turks Head Peak shows several interesting features that help explain the geologic history of Southwest Colorado.

 

Look at the cliff near the top of East Turks Head, See the reddish beds underneath the grey cliffs? One of the beds is relatively flat and the other dips slightly to the left (if you hover your cursor over the photograph, you should be able to see the labels I placed on the photograph).

 

The dipping beds are the Permian Cutler Group or Formation. If you have admired the Fisher Towers north of Moab, you were looking at the Cutler group. These sediments were eroded off of the Ancestral Rockies , in particular, the Uncompahgre Uplift portion of the Ancestral Rockies. This area was uplifted during the Pennsylvanian and Permian boundary about 300 to 275 million years ago.

 

The flat-lying sediments on top of the Cutler Group are the Telluride Conglomerate formation. The Telluride Conglomerate is only about 28 million years old. That means that at the contact between the Cutler and the Telluride (right below the dark red band at the base of the Telluride) there is a gap of about 250 million years of missing sediments. We call this an unconformity and because the beds are slightly dipping below the contact with respect to the overlying beds, we call this an angular unconformity.

 

That 250 years of missing section covers the entire age of the dinosaurs. From th earliest dinosaurs in the Triassic, through the age of Dinosaurs in the Jurassic, through their eventual extinction at the end of the Cretaceous. All gone.

 

So what happened? Were sediments never deposited here? Was this area mountainous throughout the missing interval?

 

Fortunately, by looking at the nearby areas and the nature of the deposits there, we can make a pretty good guess.

 

After the Ancestral Rockies were formed, this area began to be filled with sediments eroded of off them. In addition to the lowering of the land surface from erosion, this are also began to sink regionally possibly from cooling of deep magmas that helped push up the Ancestral Rockies. During the Jurassic, great coastal sand dune desserts formed to the southwest and deposited or layed down the beautiful Navajo Sandstone that is seen in so many of our National Parks and Monuments.

 

Later this area continued to sink and the seaway formed during the Cretaceous. This area was once covered by the famous Cretaceous sediments: The Mancos, Book Cliff, Cliff House, and Dakota Formations were deposited here.

 

So what happened to these sediments? Something big happened and something big usually involves a tectonic plate. It certainly does here.

 

A Pacific plate named the Farallon Plate did something unusual. Ocean plates like the Farallon plate usually get pushed down under a continental plate when they collide. They get pushed down at about a 30 degree angle and start melting as they get near the mantle. If the plate contains water, and oceanic plates usually do, some of the plate melts earlier and molten magma begins to rise through the overlying continental plate and may eventually reach the surface and erupt or cool in place in the upper continental crust rocks. These volcanoes are usually located back from the coast because the oceanic plate needs to travel down at that 30 degree angle for a while until it starts to melt. The Cascades and the Andes are good modern examples of this and are located 60 to 100 miles inland from the subduction zone where the oceanic plate begins to be carried downward.

 

The Farallon plate behaved that way for most of its history and the Sierra Mountains of Eastern California were formed by the Farallon plate subduction.

 

Near the end of the Cretaceous, however, that whole process changed. The Farallon plate instead of being pulled down into the mantle, suddenly broke apart and the new leading edge started moving east just underneath the bottom of the North American plate. It pushed eastward in two main pulses. The oldest created the Sevier Mountains and fault zone of western Utah. When the plate started to move eastward again, it created the Laramide thrust and Rocky mountains.

 

This lifted Southwest Colorado along with a broad swath of Central North America. The inland sea disappeared as this area was raised by the compression caused by the Farallon plate rippling underneath the North American plate.

 

The story doesn't end here however and you can see the results in East Turks Head. After the Laramide compression and thrust marked the eastward progression of the Farallon plate, the compression and eastward movement suddenly stopped. The Farallon plate started to slowly sink down into the mantle. As it sank, some parts started to sink and melt especially those parts that again, contained a lot of water. Eventually magmas formed and began to rise pushing up this area again. The Telluride Conglomerate is thought to be a result of the initial Laramide Rockies being eroded and the eroded parts of the doming San Juan area being pushed up by the now rising magma stocks.

 

The grey rocks at the top of the peak mark another phase

in the story. They are the San Juan Formation volcanics.

 

When I said that magma started to rise from the sinking or foundering Farallon plate I should have said a lot of magma started to rise. This resulted in huge eruptions. Just east of here, there was an explosion to rival the huge caldera at Yellowstone. In fact, the volcano with the highest Volcanic Explosivity Index or VEI is one of the nearby calderas in the San Juan Volcanic Field - La Garita. There were more calderas in this area, each one blasting many hundreds to thousands of square miles of material up into the sky and back down onto the earth creating the San Juan formation.

 

The San Juan formation looks thick on top of East Turks Head but imagine 2.5 kilometers more material piled on top of the peak and imagine how big the explosions must have been to deposit that much material.

 

The magma caused another phenomena in southwest Colorado. The deep waters that came in contact with these huge stocks of magma were heated and infused with heavy metals from the magma including gold and silver. The hot metal laden water rises up through the overlying rocks thermally altering the rock and depositing the metals as it cools near the surface. Red Mountain a little further north of East Turks Head along US 550 is an example of an entire mountain that has been hydrothermally altered by these hot deep waters. If you ever see it, you will immediately understand what the results of thermal alteration are.

 

Finally, this area that includes the San Juan Mountains is the highest average elevation in the continental United States.

 

One of the presentations at the conference presented the results of a seismic study of the southwest corner of Colorado. The results of the study suggest that something special is occurring under this high area. High areas in continents are usually supported by deep roots. In essence, high mountains float on deep roots below them.

 

That doesn't seem to be the case here. The study suggests that the crust is thin here. It is also hotter than normal and this heat is what is expanding the rocks and lifting them up. The lift is a little slower than we can measure just now but the ragged appearance of these mountains indicates that it is a very young landscape still slowly rising and with erosive forces still actively trying to plane these peaks down.

 

I hope this description helps you to think about the interesting story of how this natural beauty evolved. Take another look, I think you will find it worth while.

 

 

D0A0067

Done