Mid-week Mystery Sample: What is it? Heck if I actually know!

Well, that just beats all. We community college instructors often revel in the fact that we know a little bit about a lot of things instead of a lot about some specific things, but sometimes it helps to actually know a lot. Back when we found the mystery sample of the Chinle Formation, we wondered if it was a coral, and how a coral could have gotten into the Chinle, which is a terrestrial formation.

As I described in the previous post, I thought I had discovered the answer to the mystery when I saw a metoposaurus in the visitor center at Petrified Forest National Park (shown in the photo above), given the unique pits in the skull plates. The metoposaurs were large carniverous amphibians. I couldn't exactly make my sample line up with any particular part of the skull, but it was an adequate answer for me, the non-expert. In the best tradition of bad science, I didn't continue to explore other hypotheses once I had an answer that satisfied me. That's the kind of attitude that drives me nuts when my students are trying to identify rock and mineral specimens ("it's clear so it must be quartz, even if it cleaves").

Of course, running a blog demonstrates your ignorance to anyone who wants to pay attention, so I say thank you to an actual paleontologist, Jerry Harris, who suggested my sample could also be a phytosaur scute. So I am properly chastised for not covering my bases (y'all notice I could have said I knew it was a phytosaur all along....). I am at the same time happy to find that I might have a piece of phytosaur. They were one of the fierce predators of the Triassic: somewhat crocodile shaped, up to 30 feet long, and overall pretty cool.

Picture from Wikipedia

(This post has been revised)

Mid-week Mystery Sample: What is it?

So what is it? In the early 1990's, I was conducting one of our first field trips through the Colorado Plateau region, and at a stop to observe the sediments of the Chinle Formation, one of the students picked up the sample seen in the picture. I am admittedly not a paleontologist, but I looked and looked and couldn't make heads or tails of it. It sat on my desk for seven or eight years, until I walked through the visitor center at Petrified Forest National Park (the park is a showcase for the Chinle Formation), when I got it! Can you?

Time Beyond Imagining - A Brief History of the Colorado Plateau: Into the Triassic

The first period in the Mesozoic Era is the Triassic, which lasted from about 245 to around 200 million years ago. The world's continents had been combined into the supercontinent Pangea, and the biosphere had just been subjected to the worst extinction event ever recorded in earth history, with something like 95% of all species wiped out. It is as if the world were starting over, with a new cast of characters, and new landscapes. In the Colorado Plateau region, two formations were laid down: the Moenkopi Formation, and the Chinle Formation.

The Moenkopi was formed in river floodplains and shallow lakes, the shales and sandstones stained bright red by iron oxides. Distinctive ripplemarks, channel deposits, and mud cracks attest that the environment was in turns wet and dry, under either arid or subtropical conditions. The source of the sediments was from mountains far to the southeast, sloping towards a northwest shoreline near present-day Nevada and western Utah. Numerous trackways attest to the presence of numerous small reptiles and amphibians, but bones are rarely preserved. Great places to see the Moenkope? Try the western and southern part of Zion National Park, the basal cliffs of Capitol Reef National Park, at Cliff Dwellers near the Navajo Bridge downstream of Glen Canyon Dam, the region near Tuba City and Moenkopi, and along the Colorado River near Moab, Utah, which is the subject of today's photograph.

I am kind of cheating with today's picture. It shows the Fisher Towers along the Colorado River near Moab, Utah, along Highway 128. The La Sal Mountains form the backdrop. The Moenkopi Formation is in fact visible in the picture, but only as the darker tops of the towers, and the red slopes to the left. I consider this road to be one of the most spectacular drives in North America, especially in the evening light. I just happen to really love the picture and was looking for an excuse, any excuse, to use it!

The Grand Staircase and the Geologic Time Scale

Sort of a follow-up to the previous post...here is a National Park Service diagram of the Grand Staircase (click on the image to enlarge). The image has been a popular poster in the National Park Service Bookstores for years, and I was pleased to find it on Wikipedia.

Highly Allochthonous made a great point the other day when he commented about how easily we geologists can confuse laypeople when we start describing geological periods and eras without noting the actual years in which the events took place. I am taking advantage of a time scale that Chris provided on his website, and have added to the page elements to the left (scroll down). I hope it clears up any confusion!

Time Beyond Imagining - A Brief History of the Colorado Plateau: On to the Grand Staircase

Picture by Susan Hayes

I have enjoyed exploring the Grand Canyon on this blog over the last few weeks, using the incredible gorge as an introduction to the history of a unique corner of North America, the Colorado Plateau. A layperson could be forgiven for thinking that the history of the world is written in the walls of the Grand Canyon given the rich story told by the rocks there, but there are many parts of the story missing. So Geotripper is going to hit the road and start exploring some other corners of the Plateau country. We will return later to the Grand Canyon because we still have not explained how the canyon itself developed, but that is actually one of the last events in the history of the region.

What is it that is missing from the story? Well, imagine this: standing on the rim of the Grand Canyon, you can look down and see nearly 2 billion years of earth history in the mile of cliffs below you. Now, look up. Today you see only sky and clouds...but not that long ago there was at least another mile of sediments covering this landscape. Another mile! These rocks recorded the Mesozoic and Cenozoic history of the region, the 245 million years leading to the present day. Putting it another way, the entire history of the dinosaurs, the birds, and the mammals.

How do we know they were there? The Mesozoic rocks are found north and east of Grand Canyon, sitting on top of the Paleozoic rocks. The most striking exposure is a series of cliffs visible from the Kaibab Plateau north of Grand Canyon long known as the Grand Staircase. Named for the dominant color of each of the cliffs (the Pink Cliffs, the Gray Cliffs, the Vermilion Cliffs and so on), the region includes several famous national parks (Zion and Bryce Canyon), but also some less familiar national monuments and state parks (Grand Staircase-Escalante NM, Kodachrome State Park, Coral Pink Sand Dunes State Park and others).

Our next post will look at the world of Pangea and the Triassic Period. Some very strange animals are coming up.

Time Beyond Imagining - A Brief History of the Colorado Plateau: A Final Transgression

With today's post we reach the rim of the Grand Canyon, and the end of the Paleozoic rocks of the Colorado Plateau. But these pictures are not of the Grand Canyon, although some similarities exist. Both canyons were carved by the Colorado River, for instance. We are in southeastern Utah at Canyonlands National Park. Our vantage point is the Island in the Sky plateau that overlooks the confluence of the Green and Colorado Rivers. Unlike the Grand Canyon, the oldest rocks visible are Permian in age, and the upper canyon walls are in Mesozoic rocks (the age of the dinosaurs, basically).

Hikers in the Grand Canyon who have a geological background don't measure their progress in miles and feet the way normal people do. We note our location by the strata through which we are strolling. The base of the Supai Group means a long slog through red dirt. The base of the Redwall almost always means a tough climb is ahead. The greatest relief to geological hikers is to arrive at the top of the Coconino Sandstone, where we realize that only two more formations lie between us and a cold beer or iced tea: the Toroweap Formation and the Kaibab Formation. Both are easily visible in yesterday's post, the Toroweap as the forested slope above the sheer Coconino cliff, and the Kaibab as the ledgy cream-colored cliff forming the rim of the canyon. Like the Coconino, both formations are Permian in age, the very latest part of the Paleozoic Era.

The Toroweap Formation is rather variable in composition, ranging from dune sands, evaporites (forming in drying bodies of water), and limestone. It formed in a time of changing sea level, where the shoreline transgressed and regressed repeatedly, forming dune fields, tidal flats and sabkhas (coastal salt flats). Fossils are generally rare, given the harsh conditions in which the rocks formed.

The Kaibab Formation is more marine in origin, being composed of limestone, dolomite, and sand. In many places chert nodules can be found. The chert is a form of altered quartz that can develop long after deposition. The layer is rich with fossils: crinoids, brachiopods, sponges, bryozoans, shark teeth, nautiloids, fish, and some of the last trilobites to inhabit the earth (the Kaibab Formation on the North Rim of the Grand Canyon was the site of my first childhood fossil discoveries). Many of these species were doomed: the end of the Permian was marked by the worst mass extinction event ever recorded. Something like 95% of all the species on the planet disappeared, for reasons that have not yet been determined, although many hypotheses are proposed.

So why the pictures of Canyonlands National Park? Note the white rock layer sandwiched between thick layers of red rock. This layer is appropriately called the White Rim Sandstone (the White Rim is a very prominent terrace in Canyonlands), and being Permian in age, is related to the Coconino, Toroweap and Kaibab formations. The sea that formed the Kaibab and other late Permian marine layers transgressed from the west onto the continent. This is indicated in part by greater thicknesses to the west. On the other hand, the farther east and north that one travels, these layers become thinner. And thinner. By the time we reach Canyonlands National Park, the marine sands are only a few feet thick and then pinch out. We are looking in these photographs at the last beach of the last Paleozoic sea.

We have reached the rim of the Grand Canyon, and the end of the Paleozoic story. The reconstruction of the supercontinent Pangea was complete. World ecosystems had been devastated by extinctions almost beyond imagining. The world had changed completely, and the Mesozoic age had begun. Dramatic changes are coming! In another post, though....

Time Beyond Imagining - A Brief History of the Colorado Plateau: Sand!

Back to our adventure through time on the Colorado Plateau. We have been mostly exploring the Proterozoic and Paleozoic history with a walk up from the bottom of the Grand Canyon where these rocks are so spectacularly exposed. There have been some side excursions to the Paradox Basin and the Ancestral Rockies, but today we reach the rim of the canyon, with a short exploration of the final three formations that make up the canyon walls (in two posts).

One of the most obvious layers in the Grand Canyon is a 300 foot high white cliff just a few hundred feet below the rim. The rock making up the cliff is the Coconino Sandstone, which formed in Permian time as sand dunes in a desert that extended across northern Arizona, New Mexico, and southern Utah (something like 200,000 square miles). The most obvious feature of the formation are the crossbeds, the sloping layers that represent the slip faces of the dunes that have been preserved in the cliffs. It is a beautiful rock formation!

When I was a new student taking my first or second class in geology, I had the opportunity to visit the Raymond Alf Paleontology Museum at Webb School in Claremont, California. I was intrigued by their spectacular collection of fossil trackways, and was curious where such ichnofossils were found. One of the sources was the Coconino Sandstone, which I would see up close only a few weeks later on my first geology field trip. There is nothing quite like finding a fossil trackway like the one illustrated above. Unlike fossil shells and bones, representing death, a trackway preserves a moment in the life of a creature. Was it running or walking, feeding or escaping? What kind of creature was it anyway? The Coconino preserves more than two dozen kinds of tracks, from large amphibians or reptiles to scorpions or spiders.

The Coconino is correlated with the De Chelly Sandstone, which forms the spectacular cliffs of De Chelly National Monument, and the wonderful John Wayne movie towers and spires of Monument Valley. Because the sand is porous, it is an important groundwater reservoir for towns to the south of Grand Canyon like Flagstaff and Williams.