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Standing atop the Kaibab Plateau

Well, we've made it - starting with the oldest rock in the Grand Canyon in December, we have arrived at the Canyon Rim in February. Writing and scheduling all these has almost seemed like climbing the canyon itself. In fact, I think you can climb the Canyon in less time than it’s taken to write all these, or at least make very good progress on it.

We’re officially at the top of the Grand Canyon. The Kaibab Limestone forms a great plateau that surrounds the gorge of the Colorado River, spreading out for many kilometers in either direction. Kaibab is actually a word from the local Native American tribes translating as “Mountain lying down”, a fitting summation of this unit.

The Kaibab limestone was deposited in the middle-Permian. It is a true marine limestone, loaded with fossils, some of which you see here marked by the helpful folks at the Park service.

The Kaibab was deposited after a marine transgression; the seas came in, covering the area that would one day be the Canyon again and creating a warm, tropical seaway where ocean critters could thrive. Those organisms lived their lives, died, and left their shells behind to build the limestone.

The Kaibab predates the greatest extinction known in the geologic record at the end of the Permian. 10 million years or so after this limestone was deposited, the seas had receded again and the great dying took place. There is no record of the end-Permian recorded in the Grand Canyon, but it gives an amazing look through the Paleozoic, as we’ve seen for the last week+.

As we’ve seen with previous limestones, it is difficult to erode them in dry, arid climates. The Kaibab therefore forms fairly steep, rocky cliffs at the edge of the Canyon, and most easily is removed by collapse when the shale units beneath it are eroded away.

The top of the Kaibab is an unconformity, both now and in the past. Right now, the Kaibab forms a fairly flat plateau on which visitors drive to reach the Canyon and occasionally dart out for ice cream after a long day of being rained on while trying to hike around the rim. The top of the Kaibab is eroding right now, even if slowly. In the past, there was erosion on top of the Kaibab as well.

The next unit up does not outcrop in Grand Canyon national park. The Moenkopi formation is a Triassic-aged shale that sits unconformably on the Kaibab, just as shales sat upon the Redwall limestone unconformably well beneath our feet. As we’ve seen with many other shales in the park, they erode easily and so the Moenkopi has been stripped away from this location, leaving the Kaibab standing as a sentinel.

Looking back through time, it was penetrating the Kaibab formation that helped the rivers that we now call the Colorado to form the Grand Canyon. This is a tough unit to erode. Once streams got through it in a few locations, it often became easier to cut downwards than it was to migrate from side to side. To some extent, that is why the Canyon is there; the Kaibab and the other resistant units at the top locked the river in place, and the waters obliged by cutting downward.

The Kaibab is the youngest rock in sequence in the Grand Canyon but it is not the youngest rock in the Canyon. So, even though we've hiked through the entire sequence, from the bottom to the top, we're not quite done yet.

-JBB

Image credits: https://www.flickr.com/photos/jsjgeology/8411890361 https://www.flickr.com/photos/grand_canyon_nps/7706174560 https://www.flickr.com/photos/grand_canyon_nps/7706171986

Sources: http://www.bobspixels.com/kaibab.org/geology/gc_layer.htm http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/kaibab.htm http://sed.utah.edu/Kaibab.htm http://www.grandcanyonnaturalhistory.com/pages_nature/geology/1-kaibab.html

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Beautiful petroglyphs along a drainage on the Kaibab National Forest! #petroglyph #rockart #paleontology #history #rocks #geology #ancientpeople #art #nature #arizona #kaibab #forest (at Williams, Arizona)

#geology #kaibab #paleontology #forest #ancientpeople #arizona #rocks #history #petroglyph #nature #rockart #art