The Earth Story

What have fossils ever done for us?

Palaeontology has provided the inspiration for many great works. But just what is it that palaeontologists actually do with fossils once they’ve got them out of the ground? Of course, you can clean them up and show them off in a museum, and this is the side of palaeontology that the vast majority of the public sees. Nothing is quite as eye-catching as a dinosaur, and the beauty of ammonites is undeniable. This is the realm of curating, though and although it’s the dream job of many palaeontologists, there are only so many museums in the world. Instead, many palaeontologists work on micro-fossils.

As the name suggests, micro-fossils are fossils which are especially small. Whilst they can be spectacular to look at, their main use is for biostratigraphy. Biostratigraphy is the use of fossils to divide rock sequences into time zones. This is done using what are known as index fossils: fossils that are abundant, wide-ranging, and went extinct relatively fast.

Graptolites, an enigmatic group unique to the Palaeozoic, are a fantastic example of this. These organisms can be found in almost all deep-water deposits in the Ordovician and Silurian. Many individual species last for little more than a million years, with some having ranges as short as 300,000 years. Though an incredibly long time from a human perspective, in geological timescales it’s simply the blink of an eye.

But why do we want to know how old rocks are, and how do we know that? Knowing the age of rocks is important for multiple reasons; for example, oil deposits in some areas of the world occur within shales of certain ages. While radiometric dating is an incredibly useful technique for both igneous and metamorphic rocks, it is not a technique that can be reliably used with sedimentary rocks. As such, other methods need to be found. Microfossils can be extracted from boreholes with very little risk of damage and, by determining the biozone of the species, can be used to determine rock age. Microfossils like these can be extracted from rocks sampled through boreholes and matched with microfossils from other areas; matching index fossils would mean the rocks have the same age!

Biozone ranges are determined by radioactively dating overlying and underlying igneous deposits. This gives a maximum and minimum age for the deposit, though the boundaries are difficult to define beyond this. By finding a good biozone defining species entirely within the deposit, it can be correlated across the world with deposits that have not had the good fortune to be bounded by volcanic layers. Once you have a matching age, you can do all sorts of other comparisons, such as figuring out where shorelines were or where units might have generated resources.

Image Credit of various Graptolites - http://geolmag.geoscienceworld.org/cgi/content-nw/full/147/2/253/FIG2 Read More - http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8447431 - http://www.stratigraphy.org/upload/bak/bio.htm - http://the-earth-story.com/post/90435946398/whos-been-writing-on-the-rocks-although-theres

Dip into the past with Foraminifera

These little shell-shaped beauties are Foraminifera under a microscope. They are not only beautiful but fascinating and useful tiny organisms with a huge story to tell.

Both living and fossil varieties of Foraminifera exist, they belong to the Rhizaria; a group of single celled organisms in the domain of eukaryotes (organisms whose cells contain a nucleus, for those of you who are not familiar with biological classification). Out of 10,000 known species a little more than 80% are still found alive in the marine ecosystem. Most living species are found in or on the seafloor sediment living benthically and a smaller part is marine aquatic plankton floating in the water. Even though molecular data suggest that they are probably a lot older Foraminifera are known as fossils as far back as the Cambrian period (560 Ma).

Foraminefra occur in a variety of shapes and sizes, varying from 40 micrometers up to as much as 20 centimeters, although most species are at the small end. Fossil Foraminifera are widespread and their morphology is complex. For that reason these little fossils play an important role in biostratigraphy, a branch of stratigraphy in which relative ages of rocks are determined by means of fossil assemblages contained within them. Because their shells are commonly well-preserved, a huge amount of information can be read out of borehole samples. Therefore Foraminifera are used to define and identify geologic periods and have a huge impact when it comes to determine the age of rocks or telling us more about the environment and its conditions under which they formed.

By examining the abundances of specific trace elements in foram shells, even more information can be revealed, such as the history of the carbon cycle or ancient global temperatures. Living Foraminifera in modern coastal environments or coral reefs say a lot about the condition and health of the ecosystems in those regions and are therefore used as so called bioindicators (species used to monitor the health of an environment or ecosystem).

-Cé

Image Credit: http://commons.wikimedia.org/wiki/File:Foraminifères_de_Ngapali.jpg

Rudolf Röttger, Gunnar Lehmann: Benthic foraminifera In: R. Röttger, R. Knight, W. Foissner (Hrsg.): A course in Protozoology, Protozoological Monographs Bd. 4, 2009, S. 111–123 http://www.marinespecies.org/foraminifera/ http://www.newscientist.com/article/mg19826553.500-sea-creatures-had-a-thing-for-bling.html

Some of the Grand Canyon’s youngest rocks

This photo captured out the window of a plane shows some of the youngest rocks found in the Grand Canyon.

The Kaibab plateau is an area of flat land sitting atop rocks that are Permian in age, over 250 million years old. But within the last few hundred thousand years, the black rocks in this photo have appeared on top of them.

The Uinkaret volcanic field is made of a series of volcanoes mostly on the Canyon’s north rim (although there are a volcanoes in a different volcanic field on the southern side as well). These volcanoes have mostly erupted basaltic lava, which flows easily over the surface. When they’ve reached the canyon, these lava flows have poured over the edge at places like Lava Falls, seen to the right of this image.

When these lavas poured down to the river level, they actually dammed the Canyon, creating lakes behind them not unlike the manmade lakes on the Colorado river today…with the exception that the lava dams didn’t have spillways and thus might have been overtopped or even collapsed catastrophically.

There is actually quite a bit of scientific debate about why these lavas are erupting. The modern Colorado Plateau is a fairly stable place; there aren’t a whole lot of faults actively cutting the Grand Canyon today. There is active movement to the East and the West in the Rio Grande and Basin and Range rift zones, which could bring hot mantle near enough to the surface to melt, but it’s not clear why those would impact the Colorado plateau in this area.

Other alternatives have proposed some sort of plume-like structure, initiating deeper in the mantle beneath the Colorado plateau, carries hot mantle up to the surface and breaks through to the crust to drive the volcanism.

The volcanic fields in the area today are the most active modern phase of rocks being generated in the Grand Canyon area.

-JBB

Image credit: https://www.flickr.com/photos/docsearls/2919774298

Read more: http://geology.gsapubs.org/content/39/1/27.short http://geology.gsapubs.org/content/30/8/739.short Previous articles: https://www.facebook.com/photo.php?fbid=71718732167564 https://www.facebook.com/photo.php?fbid=717596974968016 https://www.facebook.com/photo.php?fbid=718487278212319 https://www.facebook.com/TheEarthStory/posts/718917208169326 https://www.facebook.com/TheEarthStory/posts/719035941490786 https://www.facebook.com/TheEarthStory/posts/719534524774261 https://www.facebook.com/photo.php?fbid=720485404679173 https://www.facebook.com/photo.php?fbid=720916891302691 https://www.facebook.com/TheEarthStory/posts/721282287932818 https://www.facebook.com/TheEarthStory/posts/721455997915447 https://www.facebook.com/TheEarthStory/posts/722212221173158 https://www.facebook.com/TheEarthStory/posts/722332104494503 https://www.facebook.com/TheEarthStory/posts/723288294398884 https://www.facebook.com/TheEarthStory/photos/a.352867368107647.80532.352857924775258/723925267668520/?type=1 https://www.facebook.com/TheEarthStory/photos/a.352867368107647.80532.352857924775258/724756080918772/?type=1 https://www.facebook.com/TheEarthStory/posts/724792024248511 https://www.facebook.com/TheEarthStory/posts/725410850853295 https://www.facebook.com/TheEarthStory/posts/726153457445701 https://www.facebook.com/TheEarthStory/posts/726938514033862 https://www.facebook.com/TheEarthStory/posts/727461423981571 https://www.facebook.com/TheEarthStory/posts/727462763981437 https://www.facebook.com/TheEarthStory/posts/727463347314712 https://www.facebook.com/TheEarthStory/photos/p.727464010647979/727464010647979/?type=1 https://www.facebook.com/TheEarthStory/posts/727464800647900 https://www.facebook.com/TheEarthStory/posts/727465573981156 https://www.facebook.com/TheEarthStory/posts/727465993981114 https://www.facebook.com/TheEarthStory/posts/727466543981059 https://www.facebook.com/TheEarthStory/posts/727466987314348

You’ve probably seen the Redwall Limestone before

The Redwall Limestone might very well be the single most distinctive unit in Grand Canyon. Passengers flying overhead in airplanes can often make out its massive, red-stained faces.

The Redwall is a limestone from the Mississippian period. At this time, a great, warm, tropical seaway covered the Western portion of North America. All sorts of life lived in these seaways, forming all sorts of shells and leaving enormous piles of fossils and carbonates that wound up as hundreds of meters of limestone.

The Redwall contains very little sediment that isn’t carbonate. Most of the rocks in the Canyon have a silicic component even if they are limestones; in the Redwall contains <1%. That implies that the seaway was so extensive the shoreline was just a huge distance away. There just wasn’t any sediment supplied to this area other than the life.

The unit is rich in fossils and sedimentary structures, including crinoids, brachiopods, corals, bryozoans, ooids, cherts, and various types of cross-bedding.

Perhaps the most noteworthy feature of the Redwall limestone is that the rock itself is not red. The color is supplied from above; reddish sediments wash down from the shales above, painting the exposed surfaces of the limestone a brilliant red.

The sharp face of the Redwall occurs as a consequence of the behavior of limestone. Limestone erodes easily with water flow but doesn’t break up easily without water. Thus, in a mostly arid area like the Grand Canyon, the Redwall makes sharp cliffs that even climbers will struggle with unless the rocks are otherwise broken. But, where water does flow, spectacular waterfalls and caves can be found in this unit.

-JBB

Image credits: https://www.flickr.com/photos/grand_canyon_nps/7706754222 https://www.flickr.com/photos/alanenglish/4599793309

Sources: http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/redwall.htm http://www.bobspixels.com/kaibab.org/geology/gc_layer.htm#rl http://www.utahgeology.com/fm_redwall.php

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Temple Butte Formation

Rising or “transgressive” sea levels are often the best time for sedimentary rocks to be deposited. The rising seas erode previous sediments as the shoreline passes and the high sea levels create space for sediments to be deposited.

When sea levels retreat, the opposite is true. The space for sediment deposition is removed and often erosion takes over. That is the case at the top of the Muav limestone. Sea levels across the western U.S. dropped, the sediments just deposited began to erode, and it took over 100 million years before the sea returned to this area. If you’ve watched this series, you might have recognized that this pattern seems to be repeating itself. The seas come in and the seas go out. Based in part on patterns like this in 1963, Lawrence Sloss proposed that these cycles of sediment deposition were related to large-scale sea level changes, which were later recognized as potentially driven by plate tectonic cycles. Today, the cycles of falling and rising sea levels are known as Sloss Cycles.

385 million years ago, the top of the Muav Limestone had been exposed and eroded, forming channels and topography. The seas began to encroach again, and in the low-lying areas where channels had formed, a mixture of limestones and silicic sediments eroded from the surrounding area was deposited. Those rocks are the Temple Butte formation.

The Temple Butte formation gives clear indications of the behavior of ocean levels. In the eastern part of the Canyon, it is only a few meters thick, deposited in channels where the waters were deep enough or sediment to accumulate. In the western parts of the park, the unit is much thicker, up to 150 meters. The seas encroached enough to barely cover parts of the eastern canyon, leaving a huge submerged plateau to the west where thick limestones could be deposited.

-JBB

Image credits and references: http://www2.nature.nps.gov/GEOLOGY/parks/grca/age/index.cfm http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/templebutte.htm

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The Bright Angel Shale and the Tonto Platform

Up section from the Tapeats Shale you will often find this unit, the Bright Angel Shale. I said “often” find it not because it isn’t there, but because this unit is a great example of how easily shale units weather.

Shales are made of fine-grained silt and clay sized particles and are often poorly held together. Their ability to erode so easily creates one of the distinguishing features of the Grand Canyon; the Tonto platform, the flat area seen in the second image.

The inner portion of the Grand Canyon is supported by the strong Tapeats sandstone and the river has only cut into it in small areas. The overlying Bright Angel formation however, erodes easily and has been cut back large distances from the river, creating a flat plateau held up by the top of the Tapeats Sandstone. That platform is known as the Tonto Plateau.

The Bright Angel formation is a Cambrian-aged shale and is up to 150 meters thick. The Tapeats sandstone below recorded beach environments; the overlying shale was likely deposited in much deeper, quiet water, suggesting that sea levels across the western U.S. continued to rise throughout the Cambrian.

The Bright Angel Formation varies in color from greens through red, with patches of gray as well. The colors are produced by iron and minerals like glauconite. Glauconite contains reduced (2+) iron. It is deposited in marine sediments usually as a result of biological processes and gives a greenish color when present. When it gets exposed to air, the iron rusts (turns to Iron 3+), switching the color to a strong red. The units colors therefore are giving hints about how the unit was formed.

A few other interesting details. There are places in Grand Canyon where the Bright Angel sits directly on top of the Great Unconformity and the Tapeats is missing; that is consistent with sea levels continuing to rise and swamping mesas and hills where the Tapeats wasn’t deposited. There are also occasional sandy layers inside the Bright Angel that look like the Tapeats; possibly deposited when debris flows carried more coarse-grained sediment out to sea from the nearshore environment. Finally, as you see here, the Bright Angel also contains a variety of fossils, including Trilobites!

-JBB

Image credits: http://3dparks.wr.usgs.gov/coloradoplateau/photo/tonto.jpg https://www.flickr.com/photos/grand_canyon_nps/8227961772 https://www.flickr.com/photos/grand_canyon_nps/4749628242

Sources: http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/brightangel.htm https://www2.bc.edu/~strother/gc.html http://www.grandcanyonnaturalhistory.com/pages_nature/geology/8-bright-angel.html http://www.bobspixels.com/kaibab.org/pda/rckbs.htm http://www.utahgeology.com/fm_brightangel.php

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Supergroup!

Wikipedia tells me that in music, a supergroup is a group composed of artists who have been successful on their own or in other bands. For the last week+, we have seen a series of geologic formations that make up the Grand Canyon Supergroup. Aside from making beautiful rock music (ba-dum tss), what is a supergroup to a geologist, or for that matter, a formation?

A formation is the building block of the geologic history of a spot. Geologic formations are sections of related rocks, deposited together, where the rocks have similar properties. We started at the bottom of the Grand Canyon in the Vishnu Schist – a formation defined by having been heavily metamorphosed. The Grand Canyon supergroup includes a number of different units – the Dox Formation, the Hakatai Formation, the Cardenas Formation, etc. Every one of these has something in common – the Cardenas formation is a series of igneous rocks, the Hakatai formation is a shale that has mostly turned reddish.

When you look at the Grand Canyon, you see a stairstep sequence on the side. That stairstep pattern is defined by formations. Some formations erode easily and they step backwards, some formations are strong against erosion and they stand up as cliffs.

Formations are named based on some property of where they are first described, often at an example or “type” location. The Shunimo formation, for example, is named for a site in the Grand Canyon known as Shunimo wash.

A “Group” to a geologist is made up of several formations that are somehow related. The Grand Canyon Supergroup contains 2 major “Groups” – the Unkar Group and the Chuar Group. The Unkar Group is made of the Bass Formation, the Hakatai Shale, the Dox Formation, the Shunimo Formation, and the Cardenas Formation. The first 4 formations are sedimentary rocks that formed over a billion years ago – they record sea level rising and flooding the area that is today the Grand Canyon at that time. The Cardenas formation is an igneous rock that sits on top of all those units; these rocks likely represent what was happening in the Canyon Area as a Precambrian Supercontinent formed – the predecessor to Pangaea, a supercontinent that formed about a billion years ago is known as Rodinia.

The Chuar Group contains sedimentary units including the Nankoweap Formation, the Galeros Formation, and the Kwagunt formation. These rocks represent sediments deposited in this area about 800 million to 700 million years ago – they were deposited as Rodinia rifted apart. Sediment stopped being deposited in this area at the start of the Cryogenian, a time when the full Earth’s surface was covered by glaciers.

The Chuar Group, the Unkar Group, and one more formation – the Sixtymile Formation – all of these rocks are considered part of the Grand Canyon Supergroup. The Supergroup is a package of tilted rocks found beneath the flat-lying sediments deposited in the Paleozoic. The rocks in the Grand Canyon Supergroup were once deposited as flat-lying sediments and a basaltic lava flow sequence, but a series of faults allowed those rocks to tilt early in the Cambrian. Today, the Grand Canyon Supergroup tilts off to the East by about 10 to 30°. In this photo, all of the rocks that tilt off to the right side - those are the rocks of the Supergroup.

-JBB

Image credit: https://commons.wikimedia.org/wiki/File:Precambrian_-_Cambrian_Unconformity_in_Grand_Canyon.jpg

The units of the Grand Canyon Supergroup: Bass Formation: https://bit.ly/2sK7QTr Hakatai Shale: https://bit.ly/2W6eMr7 Shunimo Quartzite: https://bit.ly/2TZPTvx Dox Formation: https://bit.ly/2AVKA9d Cardenas Basalt: https://bit.ly/2RS8JY8 Nankoweap Formation: https://bit.ly/2T3YM7p Galeros Formation: https://bit.ly/2MiYnva Kwagunt formation: https://bit.ly/2Hmh3va Sixtymile Formation: https://bit.ly/2T2cyHy

100 years

If you’ve been following this page the last few weeks, we’ve spent a lot of time in the Grand Canyon. We started in the unit at the bottom of this photo – one of the metamorphic rocks that form the core of the continent of North America. We’ve worked our whole way up to that line – one of the most important boundaries in the geologic history of the western US. Tomorrow we’ll cover that great line – the great unconformity.

The Grand Canyon exposes about 2 billion years of geologic history. It tells the story of how this continent was created and how the world evolved afterwards; sea levels rise and fall, supercontinents form and break apart, glaciers advance and retreat. Today, millions of people visit this site to learn that story, but they only are able to do so because of the work of the National Park Service.

The Grand Canyon was first protected as a National Monument by president Theodore Roosevelt using the power of the Antiquities Act – an act that gave the President the right to set aside some areas in the country for protection before Congress could act. Prior to that, mining companies, tourist companies, and many private individuals were trying to claim portions of this spectacular geologic site. Decades later, there was talk of placing a dam on the Colorado River that would flood this magnificent space. But in the end, it has endured because it is Grand Canyon National Park.

Grand Canyon National Park was officially declared on February 26, 1919 – after its approval by Congress. This is one of the jewels of geology on Earth – a single location where you can walk up and down across the history of a continent. As a geology page on social media, what better way to celebrate that anniversary than by marching up the history recorded in these rocks. In the links below you can start at the bottom of the canyon and head all the way up to the Great Unconformity – later this week we’ll head farther upwards still, into the pile of flat-lying sediments that we can hike all the way to the Canyon rim.

-JBB

Image credit: https://flic.kr/p/dERwjd

NPS 100th birthday celebration:https://www.nps.gov/grca/getinvolved/centennial.htm

The first unit in the Canyon, the Elves Chasm Gneiss: https://bit.ly/2RzpJ5Y The Grand Canyon Metamorphic Suite:https://bit.ly/2S2pSyw The Brahma Schist: https://bit.ly/2U0rAxB The deformation and igneous rocks:https://bit.ly/2R4p9by The Rama Schist: https://bit.ly/2Mi7PPg The Precambrian Igneous rocks:https://bit.ly/2FNjKng The first unconformity: https://bit.ly/2ATv5i1 Bass Formation: https://bit.ly/2sK7QTr Hakatai Shale: https://bit.ly/2W6eMr7 Shunimo Quartzite: https://bit.ly/2TZPTvx Dox Formation: https://bit.ly/2AVKA9d Cardenas Basalt: https://bit.ly/2RS8JY8 Nankoweap Formation: https://bit.ly/2T3YM7p Galeros Formation: https://bit.ly/2MiYnva Kwagunt formation: https://bit.ly/2Hmh3va Sixtymile Formation: https://bit.ly/2T2cyHy

The Dox formation: another red shale

The last 2 units in the Grand Canyon were, in order, the Hakatai Shale and the Shunimo Quartzite. These rocks reflected a period of up to 50 million years where the land was extending and faulting. This tectonic motion created low-lying areas that were first filled with quiet water, tidal flats, followed by the overrunning of the area by river sediments.

In other words, the land first subsided slightly below sea level, then moved slightly back above sea level due to high sediment loads delivered by rivers.

The next step? Back down.

Over a period of about 10 meters at the top of the Shunimo Quartzite, the sandstones grade back into reddish shales that are thinly bedded and easily eroded. This sequence is very reminiscent of the Hakatai shale seen previously and represents a similar depositional environment. After the land moved above sea level to deposit the quartzite, the land moved back below sea level and resumed depositing shales in tidal-flat settings.

Just like the Hakatai and the Shunimo, there are tectonic features recorded within the Dox formation, indicating that the land was experiencing earthquakes and faulting which probably helped create the space where the Dox formation was deposited.

An interesting feature of the Dox formation is found in its detrital zircon population. The population of zircons found in this formation comes from a source that didn’t exist when the previous units were deposited. These zircons derive from the Grenville orogeny; an enormous mountain building event clear on the other side of the country; in the area that is today eastern North America and eastern Canada. Sediments from that mountain range, thousands of kilometers away, represent an important new source of material that appears in the Dox formation.

The zircon population also tightly constrains the age of this formation, deposited between 1.14 and 1.10 billion years ago.

Previously we saw that the Hakatai formation erodes easily and the Shunimo Quartzite stood strong and hard to erode. The Dox formation is another fine-grained, reddish shale, and just like the Hakatai, it erodes easily and does not outcrop well.

One other feature is illustrated here; the red color of formations like this is due to the presence of a lot of oxidized, reddish (rusted) iron. In the Dox formation, there are all sorts of white spots like this, created by the presence of some other compound such as organic carbon in the rocks that could instead reduce the iron, changing it from Iron (III) to Iron (II) and removing the red color.

-JBB

Image credits: Wayne Ranney (with permission) www.wayneranney.com http://earthly-musings.blogspot.com/

And http://en.wikipedia.org/wiki/Unkar_Group#mediaviewer/File:Grand_Canyon_Supergroup_showing_Cardenas_Lava.JPG

Details: http://bulletin.geoscienceworld.org/content/117/11-12/1573.full

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The Cardenas Basalt

The last several units in the Grand Canyon have been sedimentary rocks. These sandstones and shales were deposited in basins created as normal faults grew in the area that would one day be the Grand Canyon. On top of the Dox formation, and occasionally even penetrating some of the lower rocks we suddenly find a different rock type: the Cardenas basalt.

The Cardenas is the black unit seen in this image from Cardenas creek. This is an igneous rock, produced by a series of volcanic eruptions around 1.1 billion years ago. Because it is an igneous rock it can be well dated and modern techniques uniformly give that age. The stratigraphy suggests a rapid outpouring of lava started in this area at the end of the Dox formation. There are locations in the upper portion of the Dox suggestive of interactions between basalt and the sediments; sediments that were pushed around or altered by the heat of the lava on top, so the lava must have come in at the end of deposition of the Dox formation.

The Cardenas basalt is thick when it is found in the canyon: up to 300 meters thick. This thickness implies that the location where it erupted is somewhere very near the canyon as deposits of lava usually become thinner the farther from their source they are.

The Cardenas formation has 2 members; an upper and lower portion. The lower portion is represented by olivine rich basalts that have textures like Pahoehoe found within. The upper unit is slightly higher in silica, reaching andesite compositions. The dikes, as seen here, of magma that intrude the lower sediments match the Cardenas lavas in composition, indicating that they have a similar source.

At about this time, 1.1 billion years ago, vast outpourings of lava are found in rocks of the Grand Canyon, Death Valley, and far to the north in Montana’s Belt basin. This continent-wide outpouring of lava probably helps indicate its cause. Some scientists have suggested that the lavas could have been produced by a large mantle plume rising beneath western Laurentia, with its eruption made easier by the rifting processes.

The Grand Canyon Supergroup sediments were deposited in rift basins associated with normal faults and crustal extension. The same processes were occurring far to the north in the Belt Basin and in Death Valley at the same time. Crustal extension can allow the Earth’s mantle to rise towards the surface and begin melting. The presence of basalts in these basins associated with rifting and normal faulting therefore suggests that the lavas erupted as part of the continent was pulled apart or even rifted away, and the same forces which created the basins that the sediments collected in also gave rise to the Cardenas basalt.

-JBB

Image credits: https://www.flickr.com/photos/grand_canyon_nps/5477460388 http://upload.wikimedia.org/wikipedia/commons/f/f9/Grand_Canyon_Supergroup_with_basalt_dike_in_Hakatai_Shale.JPG (Creative commons licensed)

Read more: http://www.sciencedirect.com/science/article/pii/0301926894901082#

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Bass Formation

In the Grand Canyon, the oldest rocks are highly metamorphosed schists and gneisses – considered parts of the continental basement. These rocks are only reached in the inner gorge of the Colorado River, where the river has eroded through the entire sedimentary sequence of the western United States. The top of those metamorphic units is a called a nonconformity – it is a rough, erosional surface, created when the ancient metamorphic rocks were exposed at the surface for millions of years. The metamorphic rocks of the Grand Canyon formed about 2-1.7 billion years ago, then 10% of the age of the planet passed before anything else was recorded at this site. Atop those igneous and metamorphic rocks sits a small set of tilted sedimentary rocks known as the Bass Formation or the Bass Limestone. This is the first fairly pristine sedimentary rock we find while walking up the Grand Canyon. The features in the first photo are mostly stromatolites; layers of limestone and other sediments believed to be created as mats of bacteria grew on the floors of shallow, warm oceans. The second photo was taken at the Phantom Ranch Boat Launch deep in the Canyon’s inner gorge. Look around the people – all the rocks are pretty massive, there’s no obvious layering anywhere near the people. The only place where you see layered rocks is atop the ridge in the distance – those rocks are the Bass formation. All of the lower rocks are the igneous and metamorphic rocks of the inner gorge. The Bass Formation is the lowermost member of what is known as the Grand Canyon Supergroup. The Supergroup is a series of sedimentary rocks formed in the late Precambrian, exposed at the bottom of the canyon as a sequence of rocks that have been tilted and dip off to the northeast.

The Bass is a sequence of sedimentary rocks. It contains many layers of dolomite that probably originally formed in the ocean as limestone and then were altered to dolostone after they were buried, with thin layers of sandstone, siltstone, and the occasional coarse grained conglomerate. The stromatolites are fossil bacteria colonies that formed in the ocean This sedimentary sequence indicates that water levels were changing – from rivers that deposited the conglomerates to shallow ocean waters that formed the dolomite. This unit marks the first step in a transgressive sequence – water levels were rising to cover the exposed Vishnu Schist basement rocks, and those rising waters produced this rock unit.

Geologists look for layers of volcanic ash in sedimentary rocks like this one because we are easily able to produce age dates by measuring isotope ratios in those layers. An isotope measurement on this rock gave an age of 1254 million years old. After the big tectonic events that formed the crust of the western US, that’s when the next stage – the sedimentary stage – began.

-JBB

Image credit: NPS https://www.flickr.com/photos/grand_canyon_nps/6705376193/in/photostream/ https://www.flickr.com/photos/grand_canyon_nps/8229636485/

Reference: https://bit.ly/2REaVCr By the way, we’re going to be in the Grand Canyon Supergroup for a while.

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Cutting through Gneisses of the Grand Canyon

The Rama Schist

This is another part of the complex of twisted, bent, metamorphosed, nearly 2 billion year old rocks that sit at the bottom of the Grand Canyon and make up some of the oldest, deepest rocks exposed anywhere in the western U.S.

The other parts of the schist we’ve seen, the Vishnu Schist and the Brahma Schist, look a lot like metamorphosed parts of the ocean, metamorphosed sedimentary rocks and metamorphosed basaltic rocks. Those sit in contact with granites that look like an island arc that slammed into North America about 1.7 billion years ago.

These rocks fit into that same story. The Rama schist is a third component in the mixture of schists; more felsic, quartz and plagioclase rich metamorphosed rocks.

These rocks have the chemistry of volcanic rocks erupted close to the Earth’s surface, as happens often in island arcs. The rocks may have been faulted or moved around prior to running into the growing continent, but they were pulled into the same mountain building and metamorphic event as the surrounding sediments and basalts.

These rocks were probably volcaniclastic before they were metamorphosed, produced by explosive volcanic eruptions. Some of the layering is even maintained in the second image. Based on those details, the protolith of these rocks (what they were before metamorphism) was probably a tuff.

In other words, it would be fully accurate to call this rock a tuff schist. You’re welcome.

-JBB

Image credit: Tisha Irwin (with permission, taken on sample on GC National Park Rim Trail) https://www.flickr.com/photos/tishairwin/14491015401 Visit her page: http://www.photonsandplutons.com/ Also used: Ilg et al., GSA Bulletin, 1996http://gsabulletin.gsapubs.org/content/108/9/1149.short Previous articles: https://www.facebook.com/photo.php… https://www.facebook.com/photo.php?fbid=718487278212319 https://www.facebook.com/TheEarthStory/posts/718917208169326

The deformation in the Grand Canyon

These are the same two units I showed in the last post moving up the Grand Canyon, but the setting in this photo illustrates an important part of the story; the deformation.

These rocks record the building of what was once a mighty mountain range that has now been worn away. In this photo, the Zoroaster granite and the Vishnu Schist are seen in their typical state. The schist was once sedimentary rock trapped in a continental collision. The granites intruded them, heating and recrystallizing them to metamorphic rocks. Meanwhile, far above these rocks, mountains were being built. The pressures of the mountain building process twisted and folded the hot rocks deep below.

Similar features are observed in the deepest parts of many mountain ranges when they are exhumed; twisted layers of granite and metasedimentary rocks. The directions of the folds vary somewhat but they generally record the direction that the rocks were moving, constraining the impact of an island arc with the continental mass to the North.

The ages of the deformation in these rocks vary from 1.70 to 1.68 billion years ago (a 20 million year pulse of mountain building!).

These rocks started at the Earth’s surface, were buried 20+ kilometers deep, and then within the next 100 million years brought back close enough to the surface to stop the metamorphic processes. A truly twisted path!

-JBB

Image credit Penn State https://www.e-education.psu.edu/geosc10/l10_p4.html Previous posts: https://www.facebook.com/photo.php?fbid=71718732167564 https://www.facebook.com/photo.php?fbid=717596974968016