Unconformity

From a mile away, or perhaps from a few hundred feet away, disconformities can hide. The layers appear regular, all parallel. However, between the layers, a disconformity can lie. Not until a geologist closely examines the fossils in the layers for the presence or absence of certain organisms can he or she recognize the gap in time—an erosional period when sediment accumulation or deposition halted or perhaps when anything laid down washed away.

As with an angular unconformity, disconformities form in steps. In step one, sediments collect on an ocean floor (or perhaps on the bed of a large lake). They compact and become rock layers. In the second phase, sea level falls or the sea floor rises to expose the layers to weathering and erosion. The main difference in the formation of disconformities and angular unconformities lies in this second step. As the layers of the future disconformity rise above sea level, they remain horizontal—no tilting occurs. If they tilt in this step, they later form an angular unconformity. Then, in step three, the land subsides or sea level rises, and new sediments collect on the older, still horizontal, layers.

Back in the Grand Canyon, where our story paused roughly 550 million years ago, the Tapeats Sandstone draped across the Grand Canyon Series and the Vishnu Schist to form the Great Unconformity. At least two more layers of sediments—the Bright Angel Shale and the Muav Limestone—collected on the Tapeats over the next 50 million years. Other layers may have been formed, too, but disappeared when the land rose and eroded for the next 80 million years. When the area again sank beneath the sea, the Temple Butte Formation, 80 million years younger than the Mauv, accumulated on the disconformity. The cycle of deposition, uplift, erosion, subsidence, and more deposition repeated at least four times from 550 to 250 million years ago.