A Brief Geology Lesson


Grand Canyon Geology

The Grand Canyon is one of seven natural wonders, and is one of the most geologically unique places in the world. It has the most complete sequences of rocks layers, with nearly forty major layers exposed. The rocks at the Grand Canyon range from 200 million years old to 2 billion years old.  So how did the canyon form? What can the different layers tell us about the environment when they were deposited?

The rocks at the Grand Canyon are primarily sedimentary rocks, which means they consist of sands, and other fine sediments. These sediments are derived from both marine and terrestrial environments. Long ago shallow seas and oceans used to occupy the southwest sporadically though time. When the seas retreated they often left barren deserts, and fields of sand dunes.

The oldest rock at the bottom of the Grand Canyon is a Precambrian rock that is over two billion years old. This rock is made up of mud, sand, silt, and ash that were deposited in a marine setting. During this time period plate tectonics compressed these sediments and then uplifted them out of the ocean, this is why these units are exposed today. These units, which are called the basement units, consist of the Precambrian sandstone, and metamorphosed rocks deposited by volcanic activity.

 

Grand Canyon Geology

 

The next set of rock units called the Supergroup were deposited between 1.2 billion years and 740 millions years ago. This group of rocks is made up if units deposited in shallow marine and river valley environments. Most of the sediments in this group are gravels and sands. Among this group there is also conglomerate layer, known as the Hotauta Member.. Many of the units in this group and throughout the canyon are red/orange in color. This coloring is due to oxidation or rusting of the minerals within the rock. Just like if leave metal out in the rain and it rusts, it can also happen to rocks over a long period of time.

The Tonto group, was formed during the Paleozoic era, 550 million years ago, is made up of rocks deposited during a time when the southwestern United States was a tropical environment. Shallow seas returned to land, and now invertebrate such as trilobites were in abundance. There are three main rocks formations in the Tonto group, the Tapeats sandstone, Bright Angel Shale, and the Muav Limestone. These three units were laid down over a period of 30 million years. Within the units especially the Muav Limestone, fossilized remains of ancient ocean life can be found.

After the Tonto Group there is what geologist call an unconformity, which essentially just means missing time between rock units. There is a large period of time, 65 million years, which separates the Tonto Group and the Temple Butte formation. It is unknown whether any sediment was actually deposited during this gap of missing time. If there were, then it means that 65 million years worth of sediment was eroded away by water or wind. But it also possible that there was no deposition during this time at all. Deposited on top of the Muav Limestone is the formation called the Temple Butte formation. The interesting thing about this formation is that in the western part of the park you can see fossilized remains of freshwater fish, and in the eastern part you find marine fossils. This indicates that deposition for this unit was formed in two different environments.

 

Grand Canyon Geology

The next major group of rocks is called the Supai Group. This group is made up of both marine and non-marine deposits of mud, silt, sand, and calcareous sediments. These units indicate a warm shallow sea and muddy delta environment. The group consists of shale’s, and limestone’s, and is overlaid by sandstones

The next sets of units are the youngest and can be seen throughout the Northern Arizona region. These are the units that are most visible from the rim of the canyon. The red unit that can be seen is the Hermit formation. It is a soft deep red shale. This unit is significant because plant fossils can be found within this unit. Next is the Coconino Sandstone. This unit is distinguishable from other sandstones because it shows large and defined cross bedding, which indicates this was a time of ancient sand dunes. The Toroweap Formations is red/yellow sandstone, with shaly gray limestones. In this unit you can find marine fossils, which mean the sea returned to land after the Coconino was deposited. Finally at the top Grand Canyon you can see the Kaibab Limestone. The Kaibab is a yellow/tan unit, which is known for bearing lots of marine fossils.

There are other many units that can be seen at the Grand Canyon including the Redwall Limestone and the Moenkopi Limestone. To learn more about the geology of the Grand Canyon you can visit the Geology Museum, and take a walk along the Trail of Time and get an up close look at many of the rock units found in the canyon.

 

 


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