This photo/essay is for those with at least a basic understanding of structural geological.
The Sunset Point Rest Area on Interstate Highway I-17 sits on the precipice of a thick lava flow overlooking Black Canyon Basin to the west. Today it's a nice view of the basin and the Bradshaw Mountains but 1.75 billion years ago it was a very different place. You would have been standing on the west coast of the North American Continent looking at the Paleo-Pacific Ocean. Yes! Arizona had beach front property and a seaside view long before the pieces and parts that make up California ever collided with North America.
You are looking at a convergent zone where the Paleo-Pacific Plate was being subducted beneath the North American Plate. Seamounts and islands were sheared off as the oceanic plate ground beneath the continent crust and were added as land to the edge of the continent. Although it happens slowly it is a common occurrence at convergent zones and is how continents add land mass and grow wider.
However, the Black Canyon Basin is somewhat different. What you can't see from the rest area is that the floor of the basin is an ophioltic sequence. You ask, "What is that?" Simply put an ophiolitic sequence or suite is the vertical sequence of rock that makes up oceanic crust, but this raises a another question? What is oceanic crust doing in the center of Arizona?
The floor of the ocean is created as magma extrudes from the earth's mantle along a crack at the bottom of the ocean. The crack is actually a valley (the mid-ocean ridge) at the crest of a very long mountain range. There is such a mountain and valley called the Mid-ocean Ridge that was discovered and mapped by the US Navy during the Cold War. It is 40,000 miles long and encircles the earth through all of the ocean basins.
The sides of the ridge move apart for three possible reasons: First, and least likely, the pressure of volcanic eruption forces the valley to widen. Second, the magma chamber creates a bulge above it and gravity slides the opposing sides of the ridge down the slope from each other in a slow motion avalanche thus widening the valley. Third, and most probable, the convection current in the molten rock under the solidified crust drags both sides away from each other like an endless conveyor.
As oceanic crust is created at a spreading ridge, an equal amount of mass must be subducted back into the mantle somewhere else on earth. This occurs because oceanic crust is made up of denser rock than the continents on which we live, so when the two collide the ocean floor grinds its way underneath the lighter continental land mass. However, when a bump (such as a mountain, island or spreading ridge) encounters the continental land mass it can be sheered off to become part of the continent.
Nearly two billion years ago Arizona was the western edge of the North American Continent. The Falleron Oceanic Plate (now long gone) was subducting under Arizona at the time. When its spreading ridge collided with Arizona it sheered off and became part of the continent. Its metamorphic remains, laden with valuable metals and minerals, is now the floor of the Black Canyon Basin.
There are two exits off of I-17 to get down to Black Canyon Basin, the Bumble Bee exit 248 and Bloody Basin exit 259 to the community of Crown King in the Bradshaw Mountains. It is a hot desert environment in the summer and not the place you want to be, but in July and August the seasonal monsoon rains rumble through Arizona and saturate the land with water. Usually dry washes start to flow and the cactus swell with moisture. By mid-fall it cools off and that is the best time go exploring there. It's generally a good dirt road but if you venture into Black Canyon itself a serious high clearance 4WD vehicle is a must. I use a quad. Be aware of possible flash flooding, impassable washes, wash-outs and rock falls.
The drive from the Bloody Basin exit is the better to view the geology. The ophiolitic sequence is sandwiched between a granite batholith on the east on which I-17 rides and the Bradshaw Mountains on the west, also a granite batholith. Both these granite plutons intruded after the ophiolite was in place. I expected to find rocks in a specific order: horizontal sediments, limestone, pillow lava, vertical sheeted dykes, gabbro, and peridotite. Moving away from the granite I came to vertically layered slate and shale. This baffled me at first then I realized the ophiolitic block had rolled 90 degrees when it was sheered off of the oceanic plate.
The northern end of Black Canyon Basin.
However, the Black Canyon Basin is somewhat different. What you can't see from the rest area is that the floor of the basin is an ophioltic sequence. You ask, "What is that?" Simply put an ophiolitic sequence or suite is the vertical sequence of rock that makes up oceanic crust, but this raises a another question? What is oceanic crust doing in the center of Arizona?
Ophiolitic Suite
To understand how a desert valley in central Arizona had beach front property let's review some basic plate tectonics.
The floor of the ocean is created as magma extrudes from the earth's mantle along a crack at the bottom of the ocean. The crack is actually a valley (the mid-ocean ridge) at the crest of a very long mountain range. There is such a mountain and valley called the Mid-ocean Ridge that was discovered and mapped by the US Navy during the Cold War. It is 40,000 miles long and encircles the earth through all of the ocean basins.
The sides of the ridge move apart for three possible reasons: First, and least likely, the pressure of volcanic eruption forces the valley to widen. Second, the magma chamber creates a bulge above it and gravity slides the opposing sides of the ridge down the slope from each other in a slow motion avalanche thus widening the valley. Third, and most probable, the convection current in the molten rock under the solidified crust drags both sides away from each other like an endless conveyor.
As oceanic crust is created at a spreading ridge, an equal amount of mass must be subducted back into the mantle somewhere else on earth. This occurs because oceanic crust is made up of denser rock than the continents on which we live, so when the two collide the ocean floor grinds its way underneath the lighter continental land mass. However, when a bump (such as a mountain, island or spreading ridge) encounters the continental land mass it can be sheered off to become part of the continent.
Nearly two billion years ago Arizona was the western edge of the North American Continent. The Falleron Oceanic Plate (now long gone) was subducting under Arizona at the time. When its spreading ridge collided with Arizona it sheered off and became part of the continent. Its metamorphic remains, laden with valuable metals and minerals, is now the floor of the Black Canyon Basin.
There are two exits off of I-17 to get down to Black Canyon Basin, the Bumble Bee exit 248 and Bloody Basin exit 259 to the community of Crown King in the Bradshaw Mountains. It is a hot desert environment in the summer and not the place you want to be, but in July and August the seasonal monsoon rains rumble through Arizona and saturate the land with water. Usually dry washes start to flow and the cactus swell with moisture. By mid-fall it cools off and that is the best time go exploring there. It's generally a good dirt road but if you venture into Black Canyon itself a serious high clearance 4WD vehicle is a must. I use a quad. Be aware of possible flash flooding, impassable washes, wash-outs and rock falls.
The drive from the Bloody Basin exit is the better to view the geology. The ophiolitic sequence is sandwiched between a granite batholith on the east on which I-17 rides and the Bradshaw Mountains on the west, also a granite batholith. Both these granite plutons intruded after the ophiolite was in place. I expected to find rocks in a specific order: horizontal sediments, limestone, pillow lava, vertical sheeted dykes, gabbro, and peridotite. Moving away from the granite I came to vertically layered slate and shale. This baffled me at first then I realized the ophiolitic block had rolled 90 degrees when it was sheered off of the oceanic plate.
There are numerous ridges of sedimentary rock down in the basin.
Sedimentary rock next to the granite was metamorphosed into black slate.
A slate seam.
Further away from the granite where it wasn't as hot are beds of shale.
The layers are only a few millimeters thick.
Colorful green and yellow lichen grows liberally on the red shale.
Saguaro Cactus grow out of a bed of limestone chert deposited by deep ocean.
Valuable minerals were deposited by super-heated water and magma. There are many abandoned mine shafts (vertical) and drifts (horizontal) throughout the basin and in the Bradshaw Mountains. Some are still active and being worked. Be careful not to drive into a mine shaft. I found some that were open and unmarked next to roads.
This abandoned shaft is right next to the road.
The Golden Turkey Mine office.
It was a residence up into the 1980's and burned in 2006.
The Golden Turkey tailings pile.
An explosives bunker was located well away from the Golden Turkey Mine.
This one speaks for itself. Everything about it looked dangerous and I didn't go near it.
The tiny town of Cleator on the road up to Crown King.
Just past the town is the road down into Black Canyon.
Just past the town is the road down into Black Canyon.
The St. Johns Mine
Prickly Pear cactus and a lava dike protrude through the shale.
Prickly Pear cactus grow a purple fruit that can be made into jam.
It requires a federal permit to harvest the "pears".
Sheet dikes in Black Canyon.
The entrance of a flooded mine.
Turkey Creek in the bottom of Black Canyon.
An abandoned bicycle at an abandoned cabin. The bicycle is no longer there.
The Spire.
Saguaro cactus next to the road through Black Canyon.
A Crested Saguaro is a genetic mutation that affects only 1 in 200,000 saguaro cactus.
The Howard Copper Mine
A rough section of road cuts through a layer of metamorphic schist.
The greenish tint of the rock is caused by the olivine mineral.
The greenish tint of the rock is caused by the olivine mineral.
Out of the metamorphic depths of Black Canyon and back to the granite of Bumble Bee Creek.
In the Black Canyon Basin shale and slate, limestone and marble can be found in thick layers. Since they sit vertically the ophiolitic sequence had to have been rotated 90 degrees. So, what seem to be vertical lava dikes in the canyon must actually be volcanic sills. The dikes and pillow lava's must still be buried, missing, or I haven't found them yet.
I've been looking for pillow lava on the west side of the basin closer to the Bradshaw Mountains but I'm finding sills there. The sedimentary layers of shale and limestone are further east. Then it occurred to me it makes a difference which direction the ophiolitic suite rotated. If it turned clockwise (to the east) this makes sense; vertical sills to the west and vertical shale to the east. Unfortunately the dikes with the pillows on top may have been consumed by the granite batholith that intruded after the ophiolite was scraped off the oceanic plate. What dikes and pillow lavas remain they would be on a horizontal plane and either eroded. consumed by granite, or still buried possibly under the thick lava flow on the east side of the basin.
In fact there is an exposure of pillow lava just a mile away. A road cut on I-17 north bound at mile marker 249 exposed some pillow lavas. I don't believe they are part of the ophiolitic sequence. A granite batholith separate the two so it isn't likely. The pillows are very eroded and not well defined.
Road cut on north bound I-17 at mile post marker 249.
The exposure is very old and eroded but still shows the bulb like texture typical of pillow lavas.
Best information I found on the area
ReplyDeleteVery cool read. I just moved to Black canyon City. I find this beyond fascinating. Who knew I was buying beachfront property! What a great article thanks so much for doing this!
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