A River Runner’s Guide to Grand Canyon Geology V: Transects


River runners follow the water. The water is in our blood, and following it is what we do. We began this series by using the Colorado River as a transect, and following the drainages up to the South Rim as lateral transects showing the elevations of the layers.

We had promised at the end of the last post to next explore the schists, but we had an idea to follow the water from the last Supergroup exposure at Tapeats Creek up and over the Kaibab uplift. The original notion was to illuminate the Supergroup. We followed Tapeats Creek up to the North Rim and then up Quaking Aspen Creek to the divide or watershed high point. From there, the water did not naturally take us back to the southeast across the Supergroup as intended. It took us down North Canyon, continuing to the northeast.

We followed it.

The dark blue transect is quite natural, and the water may eventually cut through the Kaibab Uplift along this line. This would leave the first dogleg of the Grand Canyon “W” as a cutoff meander.

As we followed the water we first encountered a puzzlement in the extra layers of Toroweap and Kaibab Limestone near the top. This appears to be the result of complex faulting and folding that may or not extend to deeper layers. We may eventually dig into this at an appropriate scale. We found the elevations of the strata at significantly higher elevations as we followed the water down North Canyon. Where the grade leveled off we were surprised to find an inverted sequence, where as we went down we encountered the sequence one would expect going up.

The river follows a very different (and much longer) path. It has been known since the Geological Survey in 1923 that the river is pretty much in equilibrium, and can be approximated closely with a straight grade as we have done here. Distances of the members at river level are apportioned according to the actual river length and position.

We are unaware of any published transects approximating ours, and offer here a reasonable hypothesis accounting for the data. The Butte Fault, which separates the Neoproterozoic Supergroup members from the river, transitions to the East Kaibab Monocline, which our transect crosses nearly perpendicular.

The folding of the East Kaibab Monocline must have taken place after Kaibab time, so Supergroup members may also have been folded. We find it interesting that the Escalante Creek member, the highest exposure “around the corner” and where we leave it in Tapeats Creek, is also the first Mesoproterozoic member the Colorado River encounters, even though it is not the youngest Mesoproterozoic member.

This post will be updated as we follow the water in future transects. Hopefully they will shed some light. Likely they will require some modifications to our effort here.

 

 

 

 

 

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