Wednesday, February 24, 2021

Geological Destination - Four Corners Monument

Being located at the junction of four states, Four Corners Monument might just as well be considered a political park. However, geology is everywhere and therefore this is a perfect example of a geological park as well. The Four Corners Monument is a Navajo Tribal Park that designates the boundaries between the states of Utah, Arizona, Colorado, and New Mexico. It also designates the boundaries between the Navajo Nation and the Ute Mountain Ute Tribe Reservation. We visited the park on my birthday back in March of 2019. 

Me, laying across all four states at the monument. 

It has been said, by many people, that this point does not actually represent the point of intersection of the four states. These people are wrong. As noted by NOAA:
"... the Four Corners monument was established at the point he [surveyor Chandler Robbins] determined, to the very best of his ability and using the available technology, to be the prescribed location of 109 degrees 03 minutes West longitude and 37 degrees North latitude."
This precise location, however may be off by ~1,800 feet to the west of the monument. However, since this point was surveyed as the junction of the four states, and approved by the governments of the four states as well as the federal government, it IS the location of the junction, regardless of what the original intention of the surveyor may, or may not, have been.

It should also be noted that acclaimed geological landmark, nearby Ship Rock, was used as one of the marker locations for the designation of the Four Corners Monument. My next post will be on Ship Rock.

A closer look at the geodetic survey marker, designating the actual Four Corners point.

The Four Corners Monument sits within the Colorado Plateau. An area that is being forced upwards by the subducted Farallon Plate. 
Location of the Colorado Plateau. Image courtesy of Woodward, 1973.

Starting ~100 million years ago, along the west coast of North America was a subduction zone. This is where one plate goes beneath another plate. In this instance the Farallon Plate subducted, or went beneath, the North American Plate.

Graphic of the Farallon Subduction. Image courtesy of the NPS.

Over time the majority of the Farallon Plate was completely subducted, including the mid-ocean ridge (aka spreading center), leaving behind a new type of plate boundary along the coast of California. Instead of a subduction zone, there now was left a transform plate boundary, where one plate slides passed another one. This plate boundary is better known as the San Andreas Fault. Remnants of the Farallon Subduction Zone still exist along the coasts of northern California, Oregon, and Washington. 

Over the past ~10 million years, the subducted Farallon Plate was still a very hot plate, and therefore wanted to rise up underneath the North American Plate. Because of this, the Farallon Plate started to push upwards on North America, creating a region of the continent that is rising vertically upwards compared to the surrounding regions. This is the Colorado Plateau.

The geological features of the Colorado Plateau surrounding the Four Corners Monument. Image courtesy of Woodward, 1973.

The Colorado Plateau is made up of many of the world's most beautiful landscapes because of these geological forces playing around in the region. Looking specifically at the Four Corners region, anyone who has been to the Four Corners Monument will also notice that this area is a relatively flat, mountainless plain. This region is known as the Four Corners Platform. The platform is a relatively flat region that sits as an intermediary between adjacent basins and uplifts. In an area surrounded by mountains and canyons (and several anticlines (A-shaped bends in the rocks)) this region just happens to be in the middle of it all forming a flat surface. 

Geological Map of the Four Corners Monument. Image courtesy of NGMDB.

Looking at the rock units that cover the Four Corners Monument, they are represented on the map above by the symbols Jmw and Jmb, These are both members of the Upper Jurassic Age (~585 million years old) Morrison Formation. The Brushy Basin Member (Jmb), which covers the actual monument, is an interbedded green, purple, and grey mudstone and siltstone with grey and tan sandstone and conglomeratic sandstones. Known for its dinosaur fossils, the Brushy Basin Member was deposited from prehistoric rivers (fluvial) and lakes (lacustrine). The underlying Westwater Canyon Member (Jmw), is a yellowish-grey sandstone containing conglomeratic lenses and dark-reddish-brown siltstone. The Westwater Canyon Member is also a fluvial (river) deposited unit.


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