More About Grand Canyon’s Great Unconformity
By Wayne Ranney, Grand Canyon Geologist
In Part I of this posting, The Great Unconformity (TGU) in Grand Canyon was described as a long-lived gap in the rock record with a duration of about 1.2 billion years. It is positioned where the Tapeats Sandstone sits directly above the crystalline rocks of Vishnu Schist and Zoroaster Granite, best seen and touched in beautiful Blacktail Canyon. Part 1 ended with a question – since the rock record has been erased by erosion, can anything be known about what happened during these 1.2 billion years? Surprisingly, the answer is yes!
What Answers Can Be Known About What Happened in that Gap of Time?
Astute readers might think this a trick question and that the answer is simply erosion (duh!). Even though this is right, it’s not a complete answer because additional evidence still exists fortuitously in the Canyon! It is called the Grand Canyon Supergroup, a somewhat enigmatic package of sedimentary and volcanic rocks that were deposited during these 1.2 billion years, but only found in about 10% of the river corridor in the Canyon. At river level, it is exposed between river miles 63 and 78, miles 108 and 110, and intermittently between river miles 131 and 139. The Supergroup is also exposed a bit higher up in the canyon walls above the river, as in the approach to the Phantom Ranch area. The main idea to take away from this posting is that not all of the 1.2 billion years represented by TGU simply involves a long period of erosion. Two major periods of deposition are recorded by the Supergroup rocks, themselves separated by a period of erosion that lasted more 300 million years!
The Vishnu Schist and the Zoroaster Granite
Recall that the crystalline rocks below TGU (Vishnu Schist and Zoroaster Granite) were formed in a mountain building event about 1,700 million years ago (1.7 billion years). These mountains continued to rise for some time afterwards but ultimately the forces that raised them ceased and erosion became dominant. The gradual erosion of the mountains removed a confining weight, such that as more rock was eroded, the mountain root rose up from below. The same thing happens when you get up off of a mattress, the removal of your weight causes the depressed mattress to slowly rise back up. This is how rocks that were formed 12 to 15 miles deep returned back to Earth’s surface.
The Unkar Group
By about 1,250 million years ago (1.25 billion), those 12 to 15 miles of rock were completely eroded, and a low level plain began to receive the very first sediments of the Grand Canyon Supergroup. This initial pile eventually thickened to 6,800 feet over an approximate 150-million-year time period. This initial package of rocks is collectively called the Unkar Group and contains five different named formations (Bass Limestone, Hakatai Shale, Shinumo Sandstone, Dox Formation and Cardenas Basalt). The rocks record deposition in shallow sea environments, river floodplains, sandy deserts, and coastal volcanic eruptions. Not all of TGU is an unknown!
The Chuar Group
After the last Cardenas Basalt erupted, 300 million years of non-deposition passed before the another set of layers came in. Three formations comprise the Chuar Group (the Nankoweap, Kwagunt, and Galeros formations), which are an additional 5,200 feet thick. These cannot be readily seen from the river and are located about 2 miles west only in side canyons on river right between River Mile 52 and 66. Chuar Group deposition began around 780 million years ago and ended “only” about 40 million years later. Rocks of this particular age are relatively rare on planet Earth and fossils have been observed in the Chuar Group that record the evolution of heterotrophic life, defined as life forms that prey on other life for sustenance, instead of passively obtaining nutrition from photosynthesis. Eventually, deposition of the Chuar Group came to an end and another 230 million years passed before the Tapeats Sandstone washed over the area. It was during this 230-million-year erosion period when most of the Supergroup rocks were removed from Grand Canyon when blocks of rock faulted higher were eroded away but the down-dropped blocks escaped erosion to become preserved.
The Snowball Earth Hypothesis
Newer ideas are adding even more details to this final chapter of TGU erosion. A hypothesis called Snowball Earth proposes that two or three worldwide glaciations may have covered most of planet Earth (even in tropical latitudes) beginning 717 million years ago. These glaciations could have scoured the ancient landscape in Grand Canyon, leaving only about 10% of the Supergroup rocks in the Canyon walls. Faulting plus erosion by ice may be the mechanisms that explain why, when the Tapeats Sandstone eventually came in, it did so mostly on top of the Vishnu Schist and Zoroaster Granite but also on eroded remnants of Supergroup rocks. Wonderful examples of Tapeats Sandstone or Bright Angel Shale “lapping onto” the eroded Supergroup remnants can be seen from the river, preserving a moment in time when the Tapeats sea washed sediment onto “islands” of Supergroup rocks (see the photo)!
What is a “Supergroup” anyway?
By the way, two or more Groups of rocks can be further classified into a Supergroup and that is where the name Grand Canyon Supergroup comes from. The aggregate thickness of the Supergroup is over 12,000 feet, nearly two-an-a-half miles of stacked rock! The only way a two-an-a-half mile thickness of rock can fit in a canyon “only” one-mile-deep is if the rocks are tilted at an angle. Everywhere the Supergroup is found it is tilted at about 15 degrees, a result of the faulting that deformed the rocks just before the ice came in.
I hope you have enjoyed this explanation of The Great Unconformity. And how the record of a “nothing” in Grand Canyon actually contains a lot of “something”!