As the sea continued to rise and flood the land progressively eastward, the locations where sand, mud and carbonates accumulated shifted with it, forming the world renowned, time-transgressive sequences of the Tonto Group. Beginning with the Tonto Group, the remaining rock formations from the Tapeats to the rim-rocks are the products of the assembly of Pangaea, including its rotation, collision and re-assembly with Rodinia’s continental remnants.
Many geologists have employed the use of comparative terms such as "lesser", "great" and "greatest" to describe the relative size of the gaps in time in association with the Precambrian basement rocks. But in the lexicon of geology there exists only one Great Unconformity, a time gap in which the others don't even come close.
According to the geological Principle of Inclusions, clasts are older than the rock in which they are contained. Notice the “loose” fragments of Zoroaster pegmatite from the underlying Grand Canyon Metamorphic Suite incorporated within the contact below the basal-most Tapeats Sandstone. Inclusions can often be utilized to recognize a nonconformity such as this.
PHOTO BELOW: THE GREAT UNCONFORMITY WITHIN THE GRANITE GORGE
This strikingly beautiful beach within the Upper Granite Gorge along the river is littered with erratics of black Vishnu Schist and pink Zoroaster Granite. Notice the pegmatite dikes emplaced into the Vishnu matrix in the massive block along the river's edge. The Great Unconformity is directly below the Tapeats on both sides of the river.
PHOTO BELOW: PLATEAU POINT AND THE CHEOPS PYRAMID
That's me standing on the edge of time on Plateau Point at sunset, located at the end of a promontory on the broad terrace of the Tonto Platform. Five more feet and it's a 1,320 foot free-fall into the Colorado River at the bottom of the gorge. The Point is an easy stroll from Indian Garden, a camping oasis of cottonwood trees 3,120 feet below the canyon's South Rim. The gently undulating bedrock beneath my feet is the Tapeats Sandstone, and under that is the Great Unconformity juxtaposed above the Vishnu Schist.
Across the gorge is some incredibly beautiful and equally complex geology. My gaze is at the Cheops Pyramid. It's another erosion-surviving, down-dropped block of the Unkar Group. One would assume that's the Tapeats Sandstone at the same level across the gorge (barely visible center right), but it's actually the Bass Limestone, the lowermost Unkar unit. Above it is the shaddowed-slope of orange-red Unkar's Hakatai Shale with a prominent cliff of resistant Shinumo Quartzite higher up. This particular unit of Shinumo was an offshore island in the Cambrian sea. The sloping Bright Angel Shale and the cliffed Muav Limestone crown the pyramid, only without the basal Tapeats since it undoubtedly projected too high above the sea to receive Tapeats deposition but high enough to receive the rest of the Tonto Group. The key to interpreting the stratigraphy is to not so much look at one layer but the relationships between the layers. Of course, a bedrock map will serve as a confirmation.
So where is the Great Unconformity? It's a trick question. In the photo, it's only on our side of the river between the Tapeats and the Vishnu Schist where an uplifted block of the Unkar Group was removed by erosion. On the far side of the river we have a "lesser" unconformity (of perhaps 600 million years) between the Shinumo Quartzite and the Bright Angel Shale.
LAST PHOTO: THE ANGULAR UNCONFORMITY AT APOLLO'S TEMPLE
My white line depicting the "great" unconformity at the base of the Tapeats continues off the photo to the right. Beneath the Tapeats here lies the beginning of a lower portion of the Chuar Group deposited in a syncline just to the west of the Butte Fault.
MORE QUESTIONS THAN ANSWERS
Why is the end of the Precambrian often marked by an unconformity? Why was the interval so long? Did global "Snowball Earth" glaciation influence Chuar deposition? What lifeforms, if any, existed during the period? Did the geological events of the period favor or disfavor their evolution and diversification? What caused the Sauk transgression following the rifting of Rodinia? Sounds to me like the introduction to another post.