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.

* * *

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 4. Supai Group and Hermit Shale

First today, I don’t think I’ve given my usual disclaimer for a while, to point out that this calendar is not at a correct scale for the history of the earth. If it were, we’d be in the Precambrian until mid-November and the daily episodes would be pretty boring. So I’ve arbitrarily assigned the Precambrian to January, the periods of the Paleozoic to February through August, the Mesozoic periods to September, October, and November, and the Cenozoic Era gets December.  

Second, as near as I can tell based on downloads, you the listener must be more or less enjoying the posts. But I’d love to hear feedback, whether through a review on iTunes, a comment on the blog, or in an email to rigibson@earthlink.net. And please feel free to ask questions – I have a couple that are pending, about the Late Heavy Bombardment, an update on work on the Belt Basin, and shallow versus deep-water limestones, and I hope to get to those soon.

So back to the Permian.

The last time we talked about the Grand Canyon, it was the Mississippian Redwall Limestone, pretty much the most prominent cliff-former in the canyon. It exemplifies the vast, shallow warm carbonate seas of the Mississippian that spread over much of what is now North America.

Supai Group (National Park Service photo)

The rocks above and younger than the Redwall are the Supai Group, diverse rocks that span time from Late Mississippian into the Permian. They include limestones, as well as non-marine rocks that were probably laid down in a muddy coastal plain setting. Some of the Pennsylvanian sediments were eroded off the Pennsylvanian Ancestral Rockies to the northeast of the Grand Canyon area. Especially in the upper part of the Supai Group, the sandstones and siltstones and shales tend to be reddish – an indication of oxidized iron, which is an indication that the environment was alternately under shallow water and exposed to the atmosphere. The rocks have amphibian footprints in them, further evidence of the coastal nature of these sediments.

Even higher in the stratigraphic section, the Lower Permian Hermit Formation overlies the Supai Group. The Hermit Formation is red and white sandstone and siltstone and especially bright red shale. Because the shale is relatively non-resistant, it erodes easily, and it’s the red material from the Hermit Shale as well as some of the rocks of the Supai Group that have stained the underlying Redwall red. The Redwall limestone is fundamentally a grayish rock, but the bright red color that gives it its name comes from iron oxide eroded out of overlying layers.

The rocks of the Hermit Formation show cross-bedding, angular layers within individual sandstone packages, which are produced by currents moving sand grains in one particular direction. Either wind or flowing water can make cross-beds. Wind-generated cross-beds often represent the faces of sand dunes. Water-borne moving sand can also form dune-like shapes underwater, and most of the Hermit cross-bedding is the result of channelized flow in river systems. The actual channel margins can also be found in the Hermit, where rivers cut into underlying strata as much as 130 feet deep – although channels of that extent are unusual. But the Hermit was laid down on an erosional surface, so there is a distinct unconformity at its base.

The abundance of red colors in the Pennsylvanian and Permian rocks of the Grand Canyon reflect the increasing arid conditions, which were global in extent. Instead of shallow seas, the planet was becoming dominated by land. This is partly a result of the ongoing continental collisions that were creating the supercontinent of Pangaea, and probably also a result of the increasing glaciers across Gondwana in the south polar region. Glaciers take up water, and lower sea level. We’ll be talking more about Pangaea and the glaciation throughout this month.

—Richard I. Gibson

USGS Hermit Formation 
See also The Earth Story
Photo (public domain) from National Park Service. 

February 16. Cambrian rocks of the Grand Canyon

White line marks Great Unconformity,
with Tapeats Sandstone above.

The Grand Canyon of the Colorado River is a geologist’s dream. The rocks scream out their relationships, and as you descend into the canyon, the rocks are older and older. In the inner gorge, the dark-colored rocks are Precambrian in age. They are metamorphic rocks, altered during their long lives by heat and pressure. And the top of the Precambrian rocks is a surface called an unconformity. That means a break in the rock record – a gap in time when sediments were not laid down, or they were eroded away, or sometimes a combination of both. The unconformity in the Grand Canyon is called an angular unconformity, because the layers below it are at an angle to the layers above it – a clear violation of the rule of original horizontality that we talked about a few days ago. Not only were the lower rocks cooked and changed, they were tilted – all before they were eroded off to create that unconformity surface.

The Great Unconformity in the Grand Canyon is part of a nearly continent-wide break. The amount of time it represents varies, even within the Grand Canyon area, from as little as 175 million years to possibly as much as a billion years or more, depending on the age of the rocks beneath the erosion surface.

In the Tapeats Sandstone

But it’s February, and we’re in the Cambrian now. Let’s talk about the rocks above the unconformity – the Cambrian strata. There are three distinct packages of rocks, called formations, in the Cambrian of the Grand Canyon. The lowest, the oldest, is called the Tapeats sandstone. When you look into the Canyon, if you can see the inner gorge, the Tapeats is the relatively thin, resistant lip on the rim of the gorge. It’s probably around 525 million years old, which puts it in the Middle Cambrian, and it averages something like 200 feet thick, pretty thin for the Grand Canyon.

Above, and younger than the Tapeats we find the Bright Angel Shale. Shale is a fine-grained rock that solidified from mud, and it often has really thin beds, sometimes microscopic. All of that adds up to a rock unit that may be a lot less resistant to erosion than something like sandstone, and that’s the case in the Grand Canyon. Consequently, the top of the Tapeats Sandstone is marked by a wide, flattish expanse called the Tonto Platform. It’s the place where the Bright Angel Shale would have been but it’s been eroded away – at least eroded back, pretty far from the rim of the inner gorge. When it’s still present, it tends to form slopes rather than cliffs because it’s more easily eroded. The Bright Angel is reddish and greenish in color because of variable iron content, and it contributes to the beautiful colors deep in the canyon. It’s around 500 feet thick, which gives plenty of room for lots of erosional variety and interesting landforms.

The upper, youngest part of the Cambrian in the Grand Canyon is the Muav Formation. It’s a multi-colored limestone interbedded with mudstone and some other rocks. It’s as much as 600 feet thick, and it’s a resistant cliff-former, making some of the first steep cliffs above the inner gorge and the Tapeats Sandstone.

Traditionally, geologists interpreted a change in rock type from sandstone that might have been deposited on a beach, to shale, which would be the finer sediment carried out into deeper water, to limestone, which could form in very deep water – all that would have been seen as evidence of the Cambrian Transgression that we talked about on February 5, with the seas encroaching and getting deeper and deeper across North America. That’s generally the way it worked, but it’s also possible for things like limestone to form in fairly shallow water – think of the calcareous white sand beaches on the west coast of Florida – so don’t look at it as entirely smooth and continuous. Stuff happened.

Geologists name rock formations, like they name periods of geologic time, to make it easier to refer to them, but it’s not arbitrary – there are distinct characteristics in each formation that make each one relatively easy to identify. Names come from a lot of sources, but all the Cambrian formations, Tapeats, Bright Angel, and Muav, were named for creeks and canyons in the Grand Canyon area.

—Richard I. Gibson