We left off the last post having seen the complete Supergroup sequence of over two miles of Neoproterozoic and Mesoproterozoic sediments. The river never encounters the Neoproterozoic part, roughly half. These lie above the river on the North Rim side along the northeast shoulder of the Kaibab uplift that defines the first leg of the “W” of the Grand Canyon.
The river essentially makes an end run around the southern end of the Kaibab uplift. Where it turns the corner and heads towards the northwest, the river first encounters the Paleoproterozoic felsic intruded schists of the upper “Granite Gorge”. We don’t know how old these are. They have been baked at high temperature at a very considerable depth, possibly in several episodes. Each melt overprints the isotopic information.
We show above the river leaving the colored and hatched Mesoproterozoic Supergroup and entering the first Granite Gorge. The granite intrusions look like swimming amabae. Red Cardenas Basalt intrusions can be seen scattered about the Supergroup.This sequence from the green Escalante Creek member to Cardenas intruded Bass Formation foreshadows the rest of the Supergroup in the Grand Canyon.
Below we zoom in to the strange relationship between the Bass and the Cardenas intrusion. It is like there is some weak layer in the Bass the Cardenas easily intruded. We will see this same relationship at the very last Supergroup exposure in the Grand Canyon. A date of 1741 million years has been found the most recent melt of the Rama Schist near the left of the image.
The solid hatched Cardenas Basalt (Yc) can be seen sandwiched within the Bass. The Hatakai Shale lies above it; and finally the outlining top of Paleozoic Tapeats.
After turning the corner, the river never encounters the Supergroup again until Shinumo Creek; but up side canyons, particularly on the North Rim side, Supergroup rocks can be found. The shallower canyons have only the lowest Bass and Shinumo members, with maybe some Cardenas Intrusion. The deeper canyons expose the green Escalante Creek, but nothing higher in the Mesoproterozoic sequence is ever exposed in the Grand Canyon again.
Despite the names Bass Rapid and Bass Camp, the Bass Formation only just barely manages to reach the river at the Shinumo Creek Supergroup exposure below. The Bass lies over waning Vishnu Schist near the end of the Upper Granite Gorge, and only reaches the river with the help of a fault.
The Shinumo Creek exposure is tectonically complex. We have shown only the faults that control Supergroup extents. Here the Cardenas has intruded between the Bass and the Hatakai Shale. Over the Hatakai we get the namesake Shinumo and Escalante Creek (greenish blue here). A fault system has left more Shinumo above the Escalante Creek (Dox), including an outlier in the “Mordred Abyss” beyond the usual Tapeats boundary.
Below we zoom into the final Supergroup exposure in the Grand Canyon. Fittingly, it extends down to the river, where river runners may bid it proper farewell. You may recall a couple graphics back how the Cardenas intruded the bass in a ribbon, seemingly in some weak layer, next to the older Paleoproterozoic schists. Here the river decided to follow the ribbon of Cardenas.
The Middle Granite Gorge begins just below Specter Rapid, near the bottom of the graphic. We will explore the relationship between the Supergroup and the older schists and granites in the next post.
When we step back and contemplate the distribution of the Supergroup rocks in the Grand Canyon, we still find ourselves asking the same question that drove us to this exercise: What’s going on here?
Do the Supergroup rocks extend over the top of the Kaibab uplift where they will be exposed when the Paleozoic overburden eventually erodes away? We find no reason to believe they do not. The Mesoproterozoic members, sometimes called the Unkar Group, have no unconformities or intervals of time and erosion. They extend the entire length of the second leg of the Grand Canyon “W” at fairly consistent elevations up the southwest side of the Kaibab Uplift.
Do the Neoproterozoic members overlie the Mesoproterozoic rocks beneath the Paleozoic rocks in the Kaibab uplift? Maybe. Both Karlstrom (2012) and Huntoon (1999) draw sections that have the Neoproterozoic pinch out at the North Rim.
The above is from Karlstrom. It is a transect at about Nankoweap. It shows both the Mesoproterozoic (Y) and Neoproterozoic (Z) both pinching out somewhat up the Kaibab uplift towards the North Rim. It is unlikely that anyone has drilled to verify this and it strikes us as relying heavily on the notion of a flat surface before Paleozoic deposition.
We find the top of the Mesoproterozoic exposures up the creeks on the western side of the uplift routinely at 4200 feet, some 2000′ higher than shown for the Escalante and Shinumo members here.
Do the Supergroup members continue dipping down beneath the Paleozoic members on river left? The Karlstrom section suggests they do.
The next post will tackle another river runner’s bewilderment; the schists.