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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.

A typical visitor to the Grand Canyon looks down from the rim through about a mile of sediments to the river. Most of these are Paleozoic sediments extending back about 515 million years. They appear to be level, although in a prior post we showed this levelness is an illusion of limited perspective.

This post is about over two miles of older sediments the typical visitor rarely sees. The Supergroup sediments are sloped at about 15 degrees due to uplift prior to the Paleozoic sediments deposited on top of them. River runners encounter these rocks in a very bewildering sequence we resolved to clarify on our last trip.

Above we show the eastern distribution of Supergroup rocks on an Arcmap image showing the grand “W” of the Grand Canyon. It can be seen that all of the Supergroup exposures are on the shoulders of the Kaibab Uplift, which reads dark green from the forest. The North and South Rim Visitor Centers are roughly centered on the eastern Supergroup.

Above we zoom into the first iteration on the river, and complete sequence of the Supergroup. This corresponds to the right hand first leg of the “W” before the river turns northwest in the first image. The Supergroup is composed of 17 members. Nine are Neoproterozoic (hatched horizontal) and eight are Mesoproterozoic (hatched diagonal). The colors follow the spectrum from red to violet

Sediments are difficult to date in general and Proterozoic sediments in particular because complex critters used for markers had not yet  evolved. A tiny volcanic ash deposit in the Walcott, the second Neoproterozoic member, dates to 742 million years ago. The Walcott is separated from the Sixty Mile formation above by an unknown amount of missing time and an unknown amount of erosion. The Sixty Mile Formation, currently a maximum of 200 feet thick, is preserved rarely, and is separated from Paleozoic rocks above by  unknown intervals of time and erosion as well.

The oldest and lowest Neoproterozoic member, the Nankoweap Formation, is also bounded by unknown intervals of time and erosion. Interestingly, the lowest Nankoweap contains wisps of sediments derived from the latest Mesoproterozoic Cardenas Basalt, showing that these intrusions got off to an early start.

The river never encounters the Neoproterozoic section (Sixty Mile through Nankoweap). These rocks lie up the Kaibab monocline and are separated at about the top of the Redwall from the river sequence by a large vertical offset along the Butte Fault. Like other places in the Grand Canyon, you can get multiple copies of the sequences where faults have made a mess of things. From the river, the Paleozoic sequence goes from Tapeats to top of Redwall. Above the top of Redwall and the Butte Fault lie members of Neoproterozoic Supergroup. Above the Supergroup it starts over again with Tapeats and climbs up the entire sequence to the North Rim.

The river first encounters the Supergroup in the Mesoproterozoic Escalante Creek Formation, which is not even the youngest. This is because a vertical offset along the Palisades Fault has allowed the younger strata to erode away. Below the fault the river marches through each of the inclined sedimentary strata beginning with the Ochoa Point Formation in turn.

The youngest Mesoproterozoic member, the Cardenas Basalt, is an intrusive rather than sedimentary layer. Below the Palisades Fault it intrudes all the sedimentary layers and only meets the river in dikes intruding the Bass and Shinumo members near the bottom of the second graphic above. The Cardenas is generically dated about 1.1 billion years ago. The oldest Mesoproterozoic member, the Bass Formation, contains some ash dated at 1.254 billion years ago.

In summary, the Supergroup extends from 1.25 billion years ago to some unknown time between 742 and 515 million years ago. Its current aggregate thickness is over two miles. It contains four unconformities, intervals of erosion, where an unknown amount of additional thickness was lost.

The lowest Paleozoic member, The Tapeats Sandstone, overlies the Supergroup and provides an outline. The bottom of the Tapeats is about 515 million years old. We show the top of the Tapeats as a tan line (close to its actual color). We extend it through the entire distribution of Supergroup rocks, even where it overlies even older schists and granites rather than the supergroup, because we feel it ties the narrative of Paleozoic deposition over a highly eroded Proterozoic surface together.

We have found many places where the Tapeats pinches out against the Supergroup, and also against older Paleoproterozoic rocks. We will argue that the Proterozoic surface was not entirely flat, and that these places where the Tapeats pinches out represent islands in the Tapeats Sea.

For context, we show above the distribution of middle and late Proterozoic rocks in the western United States. These formed at the same time as the Supergroup. The warmer colors are igneous granites and volcanics and the cooler colors are sediments. The “W” of the Grand Canyon is shown as a white path in the lower left. The large area of sediments at the top of the image extends well into Canada and is known as the “Belt Supergroup”. Correlation of these units with the Grand Canyon Supergroup is hindered by the same difficulties of dating sediments.

In the next post we will follow the river down through the remaining Supergroup exposures.