#clast

Clast-supported conglomerate

This is a type of sedimentary rock where clasts (pebbles, cobbles or boulders) dominate the volume of the rock and are resting on top of each other. The matrix makes up a small percentage of the volume and infills the gaps between the clasts and usually consists of fine to coarse sand.

Conglomerates are awesome rocks. Their clasts can indicate what types of rocks existed in the catchment of the river that deposited them. Very useful, as these rocks may no longer exist, and the clasts in the conglomerate could be all that is left of those rocks!

Just like finding random ripped out pages from the last copy of the book and your job is to tell everyone what the book was about.

Geologist’s job in a nutshell.

The Tapeats Sandstone

Throughout the Grand Canyon area, the Great Unconformity represents about 200 million years of time where no rocks were deposited. The seas retreated, leaving the basement rocks to erode away. But, 525 million years ago, the seas returned and the geologic record resumed. The first unit deposited in the Cambrian was the previous Sixtymile formation, but it was found only in a few select spots. About 500 million years ago, much more extensive deposition of sediments began with this unit.

In many places throughout the Grand Canyon, this unit, the Tapeats Sandstone, is the first unit on top of the Great Unconformity. The unit represents a shallow marine deposit, made up of sediments deposited in settings like modern-day sand bars, beaches, and channels.

The unit’s maximum thickness is about 250 meters, but like the Sixtymile Formation, the Tapeats actually isn’t deposited everywhere. In some places, the next unit up is actually deposited directly on top of the Grand Canyon Supergroup rocks. That means at the time of Tapeats deposition, there was substantial topography in the area. There were mesas and hills made up of the strong quartzites of the Supergroup and of the Vishnu Schist basement standing as islands amongst the channels where the sandstone was deposited.

The Tapeats also, as you see here being pointed at by this helpful person from Flickr, contains a variety of larger pebble and cobble-sized clasts. Many of these are made of familiar rocks; components of the Vishnu Schist and the Shunimo Quartzite, strong units deposited previously, are reworked into the Tapeats. The presence of these large clasts also suggests high-energy flows, like debris flows or turbidites, carried the clasts out to sea. The ripple marks, visible in this shot, also testify to the flow of currents that carried the sediments of this unit.

The Tapeats contains a variety of Cambrian-era fossils and trace fossils, including tracks of creatures like trilobites. It also contains the mineral glauconite, an iron-bearing mineral that forms sand-sized grains and also is likely a remnant of life, either of waste or of bodies after death. Various other sedimentary structures, like cross bedding and ripple marks are found as well.

As a strong, hard to erode sandstone, the Tapeats stands out and forms the strong platform around the central gorge in many areas of the Canyon. The Tapeats also marks the beginning of a long period of marine deposition both here and throughout the Western U.S. Although the rock types change slightly, sandstones correlative to the Tapeats can be found throughout the West, again as far north as Montana, all indicating the seas rising and beginning to claim the continent.

-JBB

Image credits (Creative Commons): https://www.flickr.com/photos/alanenglish/3609981986 https://www.flickr.com/photos/ceedave/4647884862 References: http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/tapeats.htm

References: http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/tapeats.htm http://geology.swau.edu/faculty/tapeats.html http://hikearizona.com/dexcoder.php?PID=1942

Oh you have got to visit the Tapeats on Google Streetview. https://www.google.com/maps/views/view/streetview/colorado-river/tapeats-sandstone/jXK-DOmUBE4PPqCz6A7CBw?gl=u&heading=263&pitch=107&fovy=75

Name: Redheart

Gender: Female

Age: Roughly 3 years old

Regular disclaimer that the information here may change over time.

Metaconglomerate

This texture is a particularly neat one to find in the field. These rocks are clearly metamorphic; there is a strong foliation pattern developed in the rock, but there is still a strong remnant of the protolith in the variation in chemistry from one spot to another.

This rock in northern France started off as a sedimentary conglomerate containing large chunks of rock >10 centimeters in length. That rock was then squished, squeezed and heated to the point that the rocks began to flow. The big clasts in the conglomerate were flattened and new minerals grew, but the metamorphism wasn’t intense enough to fully homogenize the rock. The clast chemistries started off distinct from the surrounding groundmass and they remained distinct throughout the metamorphism.

The shape of the clasts seen now can be used to reconstruct what happened during the metamorphism. On average, the cobbles that went into the conglomerate can be assumed to have started off life either round or randomly oriented. Measuring their length now can allow scientists to estimate how much strain has happened to this rock and how its shape changed during metamorphism.

-JBB

Image credit: Graeme Churchard https://flic.kr/p/tHwTaN