Geology – Joshua R Benton

January 10, 2015August 29, 2019Earth Science, Geology

Geology of Columbia, SC – Peach Tree Rock

Recently while on vacation visiting family for Christmas I had the pleasure of visiting a geologically interesting part of SC called The Peach Tree Reserve. The Reserve is a protected, 466 acre plot of land in the midlands of South Carolina that’s managed jointly by SC’s Department of Natural Resources and a nonprofit organization called The Nature Conservancy. The preserve is part of the Sand Hills physiographic region which marks the upper limit of the Coastal Plain in South Carolina. Within the reserve there are fantastic exposures of the underlying bedrock (Eocene Barnwell Formation) in the form of pyramid-shaped sandstone outcrops, one of which is called Peach Tree Rock.

Peach Tree Rock. Photo Credit: Kristine Hart / The Nature Conservancy

Since the famous rock actually toppled over in December of 2013 I had to use an older photo from the Conservancy’s website for the above picture. Below is a Gigapan I took of Peach Tree Rock in Dec. 2014 to show how it looks now in comparison:

Click on the photo to link to the Gigapan site.

The reserve has other interesting precariously balancing sandstone outcrops such as Little Peach Tree Rock:

What you’re seeing whenever you walk around the reserve are bedrock exposures that have resisted weathering and now poke out above the soil. The bedrock here is a quartz rich sandstone with portions that are more clay rich than others giving the rock a color variation from white to brown. As a result of the bedrock being sandstone this part of the state is rich in silica sands. So much so that some Carolinians have even established profitable businesses (such as Columbia Silica Sand, inc) mining this sand and selling it to be used for various industrial purposes.

These sandstones mark the upper limit of the Coastal Plain before the state transitions into The Piedmont only a few miles north of the reserve in the capitol city Columbia. This transition from Coastal Plain to Piedmont is known as the Fall Line and it is the reason why Columbia is where it is today. If you were to travel upstream along one of South Carolina’s major rivers (such as the Congaree) the Fall Line is typically where you encounter the first turbulent water where the rivers “fall” down from the rocks of the Piedmont into the Coastal Plain. This was crucial to early European settlers who used rivers as their primary conduits to explore and colonize the East Coast. Most of the East Coast’s major cities all sit upon the Fall Line (New York, Baltimore, Washington D.C., Richmond, ect.).

So where did all this sand come from? This is the type of stuff you would expect to accumulate on a beach or in a desert yet there is neither of these in Columbia. As it turns out these sands are all shoreline or near shore deposits and represent where the coastline used to be millions of years ago during a geologic epoch called the Eocene. If you owned property in around Columbia during the Eocene you might have had beach front property.

You can find evidence to support this within Peach Tree Reserve. Upon a more detailed examination of Little Peach Tree Rock you can find trace fossils of burrowing organisms that would have lived within a coastal marine environment. Here is a picture of one such trace fossil called Ophiomorpha that is interpreted to be the burrow of a shrimp:

Ophiomorpha within Little Peach Tree Rock. Note the bumpy exterior.

A sedimentary structure known as cross-bedding can also be observed within Little Peach Tree Rock:

Cross-beds like this are typical among sandstones that formed in beach or shallow-marine environments and from these one can infer the direction of the water current that originally deposited the sand. Using clues such as fossils and sedimentary structures we can learn more about South Carolina’s past, and The Peach Tree Rock Preserve reveals only a small fraction of the Palmetto State’s long and fascinating natural history.

August 23, 2014Earth Science, Geology

Geology of Ribbon Falls

Ribbon Falls is a beautiful waterfall within the The Grand Canyon that’s roughly an 8 mile hike from the North Rim. After walking a short distance off the North Kaibab Trail through a narrow side canyon with towering walls of quartzite on both sides of you, the falls make a dramatic appearance:

Kristi standing in front of the travertine dome at Ribbon Falls.

This splendid pool and shower of cool water was truly an oasis in the dry hot canyon that easily reaches over 100ºF in temperatures; and for or us it was the perfect place to take a siesta during the heat of the day.

What makes Ribbon Falls visually and geologically interested is that huge 30 ft tall moss covered mound of rock that sits directly underneath the falls. In this high energy location where the water hits the ground you might expect erosional forces to dominate but instead deposition reigns here. This is a massive travertine deposit. Travertine is essentially limestone (aka calcium carbonate) that’s deposited by fresh water instead of sea water. As Ribbon Falls Creek passes over one of the many limestone units upstream it dissolves calcium carbonate and then deposits them at Ribbon Falls.

Our rivers and creeks are by no means pure H2O but a mixture containing a multitude of different ions floating around just waiting to bond with one another. In solution calcium, carbon, and hydrogen atoms often bond to form the soluble compound calcium bicarbonate, Ca(HCO3)2 , but given the right pressure and temperature conditions a precipitation reaction occurs and calcium bicarbonate reacts to form the insoluble compound calcium carbonate.

Ca(HCO3)2(aq) → CO2(g) + H2O(l) + CaCO3(s)

In our case the right conditions occur once the water flowing out of Bright Angel Falls comes into contact with the ground thus precipitating it’s calcium and carbon ions. Over time this accumulation has simply built up to for form the mound that we see today

How long did it take for this dome to build up? That’s a great question! As far as I can tell no one has calculated this. I would take an interval of time (such as a year), measure the accumulation rate for that period, determine the size of the dome, and then do the math. Would anyone reading this like to pay for me to go back to The Grand Canyon in the name of science?

August 15, 2014Earth Science, Geology, Uncategorized

Grand Canyon – The Precambrian

The Earth is very very very old. Roughly 4.54 billion years old, or 4,540 million years old, or 4,540,000 thousand years old (however you would like to think of it). Prior to the development of radiometric dating to give us numbers and absolute dates we mostly used fossils and other principles to obtain relative dates of rocks. So all of our subdivisions of time are based upon the fossil record. The Precambrian refers to an immense span of time (4,540 million years ago to 542 million years ago) where the rocks contain very few fossils due to the fact that organisms never developed hard parts that could be fossilized until later in Earth’s history.

Zoroaster Granite and Vishnu Schist

At the bottom of the Grand Canyon we find rocks that formed during the Precambrian, two of which are the Zoroaster Granite and Vishnu Schist. In general a granite is defined as an igneous rock that forms from the crystallization of magma while a schist is a metamorphic rock that forms when shale is subjected to heat and pressure.

These rocks formed roughly 1,700 million years ago when a tectonic collision occurred between an older section of our continent that stretches from Southern California to Wyoming (dubbed Wyomingland) collided with a volcanic island arc. This collision of Earth’s plates provided the heat and pressure to morph the shale that formed the Vishnu Schist while producing the magma that formed the Zoroaster Granite. The Zoroaster literally intruded into the Vishnu as magma before cooling and solidifying to become a rock itself. We came across the first exposures of these rocks within the inner gorge of the canyon along side Bright Angel Creek:

This is a spectacular example where you can see huge sections of the Vishnu schist that broke off into the magma chamber before the magma cooled and solidified locking them into the place they are now.

By observing that there are inclusions of the Vishnu inside of the Zoroaster you can determine which rock is older than the other. The Vishnu must have been there first for the magma to intrude into it and break off pieces.

The Great Unconformity

Further upstream we got some impressive views of an interesting natural phenomena called an unconformity. In fact, this is known as the Great Unconformity which is represented here by an angular unconformity. The Great Unconformity was first identified in the 1800s by John Wesley Powell, a one-armed civil war vet who was the first person to lead an expedition down the Colorado River and later became the second director of the USGS. Here is a picture of the unconformity:

Can you spot it? Does it help that Kristi is pointing directly to it? Clearly there are beds of tilted rock dipping towards the right but are truncated at the top by horizontal bedding of a different rock. What in the world would cause this to happen? These are two different types of sedimentary rocks sitting right on top of each other at different angles. Would natural processes deposit the original sediments like this right on top of each other? The answer is no. The two rocks are not conformable. The “line” separating the two is an erosional surface which represents a gap in time that we call an unconformity. The rock unit on top is known as the Tapeats Sandstone that’s roughly 520 million years old while the unit on the bottom is known as the Dox Formation that’s roughly 1,120 million years old so there’s somewhere around 600 million years of missing time in between the two.

Here’s how this works:

Pretty cool right? Unconformities are found throughout the Grand Canyon and all throughout the world. Sometimes they are hard to identify but this one stands out as a prominent feature along the rim of the inner gorge.

August 9, 2014Geology

The Grand Canyon – Chapter 1

In late June a friend and I had the distinct pleasure of getting to hike across the Grand Canyon from the North Rim (via North Kaibab Trail) to the South Rim (via Bright Angel Trail). Hiking across the canyon is quite an amazing experience that takes you from aspen/ponderosa forests on the North Rim, juniper forests, and then desert scrubland towards the base of the canyon. Here is just one example of all the different flora you get to see on the hike:

Agave ugahenis. A perennial that grows within the desert scrub portion of the canyon.

The plants and animals are not the only thing that’s interesting about this place but there are so many fascinating stories to be told from the rocks themselves. The Grand Canyon contains an extensive record of North America’s natural history going back 1.7 billion years. The canyon itself is an erosional feature that formed from the Colorado River incising down into the landscape.

Rivers carving out huge canyons or gorges like this are typically a result of one of two scenarios: A.) there is a drop in the river’s base level (the lowest point to which a river flows) or B.) the land itself is uplifted causing an increase in the river’s gradient thus an increase in it’s erosional power. In the case of The Grand Canyon it is evident that a large section of the Earth’s crust was uplifted to form what is now the Colorado Plateau. What caused this uplift? The best working hypothesis we currently have is the uplift was caused by a tectonic event we call the Laramide Orogeny where the shallow subduction of oceanic crust off the coast of North America caused mountain building and deformation throughout the Southwest roughly 70 million years ago. I have illustrated a possible sequence of events:

I think it’s interesting how such a huge block of mostly sedimentary rock stayed relatively intact during all of this motion. I would assume it would have broken apart into smaller pieces along different points of weakness or areas of higher stress from below. How The Colorado Plateau staid mostly intact during the uplift still seems to be a mystery in the scientific community.

The uplift explains why the canyons of the colorado plateau are so deep but it doesn’t account for the width of the canyon. The Grand Canyon is deep due to the erosional force of the river but extremely wide due to the erosional force of gravity. After the river carved downward it destabilized the rocks exposed along the canyon walls causing them to mass waste, break apart, fall down into the river, and widen out the canyon over the course of millions of years.

While this may explain how the canyon itself formed it doesn’t answer how the rocks formed that make up the stratification of the canyon. In order to answer that question we need to go much further back in time. In my next blog post we will explore the actual rock units that make up the canyon and find out what stories they tell.

April 20, 2014Earth Science, Geology

Geology of Cerro de Cristo Rey

Just west of El Paso, Texas and the Franklin Mountains there is a mountain called Cerro Cristo Rey (Christ King Hill). During the Border to Beltway Field Exchange I and some fellow students from NVCC and El Paso Community College got to tour this mountain guided by UTEP graduate student, Eric Kappas. This mountain sits right on the border of Mexico and the United States along the banks of The Rio Grande. Here is an aerial photo giving you the general position of the mountain relative to El Paso:

Magma Intrusion

The stratigraphy of the Cristo Rey area consists of Cretaceous aged sedimentary rocks that have been intruded by an igneous mass called the Moleros Andesite which now stands out in high relief, forming the mountain peak. The oldest unit of these sedimentary strata consists of a black shale (Mesilla Valley Shale) overlain by the Anapra Sandstone Formation (Sarten Member of the Mojado Formation), another shale, and then a fossiliferous limestone (Muleros Formation). At one location we visited the Andesite actually has inclusions of shale which supports the assumption that the shale unit is indeed older than the Muleros Andesite. Here is an illustration showing you the supposed sequence of events that would form Cristo Rey as it looks now:

The Muleros Andesite is considered hypabyssal which means it crystallized and intruded under shallow depths below the Earth’s surface. Whenever magma came into contact with The Muleros this weak shale unit acted like pudding under the intense heat and pressure. This led to ductile structures such as ptygmatic folds (chaotic folds) not far from the contact zone. Here is a picture of some of these amazing folds in the Muleros Formation exposed along the edge of an arroyo:

Believe what you’re seeing. This is a syncline that sits on top of an anticline! The only thing I can interpret from the chaotic nature of these folds is how black organic shale must have a tendency to to deform in a ductile manner and act more like a fluid when subjected to pressure and heat even at the “shallow” depths which the Muleros Andesite intruded. This leads me to wonder whether the Mesilla Valley Shale is highly ductile due to its’ rich organic components or fineness of grain size.

Once you walk a few yards upstream from this outcrop the black shale beneath your feet abruptly disappears and turns into this dominantly white felsic rock with dark specks throughout it. This is the contact between the Mesilla Valley unit and the Muleros Andesite with a 50 million year gap in between the two:

The Andesite exposed in Cristo Rey is porphyritic in texture with Diorite xenoliths, specks of Albite, Hornblende, and dark nodules of older Andesite that recrystallized to give it a different texture.

In fact, the andesite intrusion exposed in Cristo Rey is not only found here but there is also another igneous mass with a similiar composition and date exposed roughly 3 miles to the South-East around the campus of UTEP, appropriately named “Campus Andesite”. These two formations are one in the same, both of which are thought to be caused by the Laramide Orogeny, the subduction of the Farallon Plate to the west. The age of the Muleros Andesite (~49 Ma) does fall within the time period of the Laramide Orogeny (70-40 Ma) and the long axis of the Andesite pluton is parallel to the faults of the Laramide.

Crustal Extension

The presence of normal faults around Cristo Rey indicate that other forces have played a role in shaping the region. Normal faults are typically associated with extensional forces and not plate-to-plate collisions or magma intrusions. Here is a picture of one these extensional faults found along the northern side of the laccolith:

Although the direction of movement may not be obvious with this fault I interpreted this to be normal fault where the hanging wall (on the right) has moved down a few meters relative to the footwall (on the left). I have highlighted in blue what I used as a marker bed to get an idea of the amount of displacement and direction of movement:

This outcrop appears to be evidence that Basin and Range Extension (Rio Grande Rifting) has affected this region. Regretfully I did not think to record the orientation of this fault while I was on site, however, judging by the direction of the shadow coming off this escarpment and considering the time of day (somewhere around 2-3 pm) I’m guessing this fault has a roughly east-west orientation. I was under the impression that both Basin and Range Extension and Rio Grande Rifting is typically associated with North-South trending faults so I’m not willing to say for definite that this is a product of either one.