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

August 15. Coconino Formation

By early Permian time, about 260 million years ago, what is now southwestern United States, around the Grand Canyon area today, was becoming pretty arid. The Coconino sandstone represents an extensive dune terrane, essentially a Permian desert that formed there. Wind-borne or eolian sand in dunes forms sloping dune faces, and when these are preserved in the sandstone, the sloping forms are called cross-beds, angular curving beds within a single package of sand. We can infer wind direction from the orientation of cross-beds.

The Coconino is typically a white, almost pure sandstone 60 to 100 feet thick. It is so resistant that it forms near vertical cliffs in places, and makes for some of the most difficult passages down into the Grand Canyon today. I climbed through it on a route in the western part of the canyon back in 1987 – a 70-foot section that required the use of ropes to descend.

The Coconino is extensive and forms prominent landscapes across much of southern Utah as well as northern Arizona.

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On August 15, 1950, an 8.6-magnitude earthquake in Assam, eastern India, reportedly killed more than 30,000 people, but other estimates give much smaller death tolls. This quake was definitely related to the ongoing collision between India and Eurasia, pushing up the Himalayas. There are other consequences to that collision, and this location, in northeastern India and adjacent Tibet, is essentially at the corner of the collision, so mountain belts and fault zones change direction here from about east-west to more north-south.

—Richard I. Gibson

See also The Earth Story's report on the Coconino

Photo by Richard Gibson

August 14. Permian ammonites

Cephalopods are the group that includes the octopuses, squids, and chambered nautilus. The extinct ammonites were spiral cephalopods that thrived in the Paleozoic and Mesozoic. Ammonites diversified during the Permian Period, but many varieties including the goniatites were killed off in the end Permian extinction.

Most Permian ammonites were relatively small, a few inches across. Many ammonites are associated with another fossil, called aptychus, which was thought for years to be the shells of a clam-like mollusk. Now it is thought to be part of the ammonite animal, but exactly what isn’t settled yet. It might be a trap-door-like mechanism, an operculum, that sealed the animal inside its shell when it was inactive. Snails have things like that. But it’s also possible that it may be some kind of jaw apparatus that helped the animal munch its prey. Or just maybe, it served both purposes. Paleontologists are still working on that little question.

—Richard I. Gibson

Drawing of Permian ammonite from an old text (public domain)