The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Saturday, August 16, 2014

August 16. Kaibab formation

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

No comments:

Post a Comment