The Great Unconformity
By Larry Larason
John Wesley Powell led expeditions into Grand Canyon in 1869 and 1871. Fascinated by the magnificent display of stacked rocks, he wrote, “. . . the canyons of this region would be a Book of Revelations in the rock-leaved Bible of geology . . . I determined to read the book.” The rocks of the canyon provide a valuable record, but unfortunately several chapters of the book are missing. Within the Inner Gorge sedimentary sandstone lies atop older basement rocks of igneous granite and metamorphic schist. Powell called this the Great Unconformity.
Don’t confuse unconformity with nonconformity. The great nonconformity occurred about 1968 when the flower children were running around barefoot in San Francisco. Unconformity is a rather quaint geological term inherited from the 1700s when earth science was just being formulated. It refers to a break in deposition, a time when no sediment was being deposited. Or, if sedimentation did occur, erosion has removed it from the record. When we try to read the rocks, an unconformity is like a loss of pages in the book of Earth’s history.
Powell recognized the unconformity in Grand Canyon, but he probably did not fully realize just how profound it is. In his time dating the age of rocks by the breakdown of radioactive elements had not yet been invented. But now we know that the igneous/metamorphic complex is roughly 2.0 to 1.5 billion years old. The sediments above them are younger than 543 million years. The gap in the rocks represents more than a billion years. So, when we try to read the rocks as Powell suggested, at the bottom of Grand Canyon nearly a fourth of Earth’s history is missing.
A few decades ago geologists believed that life arose during the Cambrian Period. Most of the Precambrian rocks were igneous and metamorphic, therefore, lacking in fossils. But then some sedimentary deposits were discovered, notably in Australia and Newfoundland, which could be dated to the Precambrian, and fossils were found in them. So a new division of time was designated and named the Proterozoic [early life] Eon [2.5 billion to 570 million years ago]. The gap in strata at the bottom of Grand Canyon encompasses most of the Proterozoic.
The Proterozoic was an interesting time in Earth’s history. It would be nice to have the rocks in place in Grand Canyon, but nowhere in the world is there a complete record of Earth’s entire history in stone. To put the book of Earth’s history together we have to find a page here, a page there, and organize them into chapters. What have geologists been able to piece together from other places about what was happening?
First, the Proterozoic rocks that are present in Grand Canyon tell us something about the environment in which they formed. Metamorphism occurs when rocks are placed under tremendous pressure and heat that alters and deforms them. This usually happens when tectonic plates collide. The metamorphic rocks in Grand Canyon, including the Vishnu Schist, tell us there were one or more collisions more than 1.5 billion years ago. Also, granite magma was intruded into the metamorphic rocks. Granite intrusions tend to form above subduction zones where melted rock ascends toward the surface. All this suggests that North America was very active during the Proterozoic.
Supercontinents formed and broke up every 300-500 million years. Each such event produced mountains, which have mostly eroded away. Erosion would have been faster then because no plant life grew on land to slow it. Also, the continents were growing as plate tectonics moved them around and they intercepted micro-continents. What are now the Four Corners states and Nevada were accreted to North America during the Proterozoic in a series of at least three continental collisions. This increased the size of our continent by 30-40 percent.
Multi-cellular life appeared during the Proterozoic. All life was on the ocean floor at that time. The best known fossils are termed the Ediacaran [ed-i-AC-a-ran] Biota. They were named for the site of the original discovery in the Ediacaran hills of Australia, but have been found in a few other places around the globe. The life forms seem peculiar to us, in part because they had no hard parts — shells or skeletons. Some looked like feathers. One iconic fossil, Dickinsonia costata, has been described as “a quilted bladder.” It seemed to have moved around, so it was an animal, but many of the fossils are so enigmatic that it is not certain whether they were plants, or not.
Multi-cellular life evolved because oxygen was building up in the atmosphere. Cyanobacteria had been spewing it out for some time, but there was a lot of iron dissolved in the ocean. Oxygen combines with iron readily, so as soon as it was released by the bacteria, it was gobbled up by iron. On every continent there are Proterozoic deposits called banded iron formations [BIFs]. They are composed of hematite [Fe2O3] and chert precipitated from seawater. It’s estimated that BIFs contain twenty times the amount of oxygen as what is in the air today. It was only after that oceanic iron was confined in the BIFs that our atmosphere became hospitable to higher life forms. BIFs have supplied, and continue to supply, most of the world’s iron ore. Minnesota is one prominent location of this ore. Such rocks do not form today, because there is so little iron dissolved in the sea. Since the Proterozoic, instead of washing away, iron oxidizes on land. It makes the rusty red color in the sandstones we love in the Four Corners.
There is clear evidence of glaciation about the time that the supercontinent of Rodinia was breaking up 750 million years ago and all the land masses were situated near the equator. Some geologists believe that for glaciers to grow in the tropics, the rest of the world must have been frozen over, as well. The Earth would have been a giant snowball. The arguments for and against the Snowball Earth hypothesis are more complicated than I have space to detail, but even the critics of the idea acknowledge that there was glaciation. [For more information see the Wikipedia entry for Snowball Earth.] There may have been three or more cycles of earth freeze, which probably affected the evolution of multi-cellular life. For example, only a couple of the Ediacaran forms survived into the Cambrian. What caused the icing over is still poorly understood, but these glaciers were not like the ones our ancestors lived through during the Pleistocene. The world was liberated from the ice only by volcanoes spewing carbon dioxide into the atmosphere to create a greenhouse situation.
Unconformities are unfortunate. It would be nice to find all Earth’s history laid out in one place, but that will never happen. We have to piece together the tale by finding clues around the globe. There is another unconformity in the Sandia Mountains where granite about 1.7 billion years old is overlain by Pennsylvanian limestone only 300 million years old. What happened here during that gap in the story?
