The rim rocks at the Grand Canyon are the Kaibab Formation of Permian age. It’s a package of rocks dominated by limestones, reaching as much as 400 feet thick. Limestones are resistant in arid country, so the upper layers at the Grand Canyon are prominent cliff formers. The rocks that were once above it, younger than the Kaibab, have been eroded away, leaving the broad, relatively flat plateaus that run up to the rims of the canyon.
It’s not all limestone – and the variations, including sand and silt, tell us that the Permian in the Grand Canyon area was a place of fluctuating sea levels, with alternating influx of sandy sediment from lands and chemical precipitates like limestone in shallow water offshore. The age of the Kaibab is late early Permian or early middle Permian, just when glaciers were coming and going repeatedly in the southern hemisphere, so it’s pretty easy to explain these sea level changes in terms of glacial periods.
The Kaibab sedimentation was near the shore at times, but the shallow shelf where the sediments accumulated was as much as 200 miles wide in places and at times, so there was plenty of room for diverse types of sediment to be laid down. Many of the fossils, including brachiopods, corals, crinoids, and mollusks suggest that the Kaibab was in the aerated nutrient-rich intertidal zone around much of what is now the Grand Canyon.
One obvious conclusion you can draw from the Kaibab following the Coconino, which we talked about yesterday, is that when I say things like “The Permian was arid” it’s quite a vast simplification. Even if it was arid at some places and at some times, the situation could change in a few million years.
—Richard I. Gibson
Photo by Richard Gibson
See also The Earth Story - Kaibab