|Publisher:||Chicago Review Press, Incorporated|
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Welcome to the Bighorn Basin
The northwestern corner of Wyoming is home to two of the nation's most famous national parks: Yellowstone and Grand Teton. Each year, these parks see more than 3 million visitors who arrive from all directions, and those who come to Yellowstone from the east by way of Cody must pass through a vast, dry depression known as the Bighorn Basin. The basin's main towns are Thermopolis, Cody, Powell, Lovell, Greybull, and Worland, but we'd be remiss if we didn't mention Shell, Ten Sleep, Meeteetse, Basin, Otto, and Bridger.
From outer space or on the Wyoming Highway Map, the basin appears as a giant oval hole about 150 miles long by 80 miles wide. The high points of the surrounding mountains reach more than 11,000 feet, while the low point of the basin is only 3,500 feet. The Bighorn Basin is a curious bit of topography, and it has one of the best geological stories on the planet.
In fact, the Bighorn Basin may be the best place on Earth to tell the story of our planet. Because of its geology, the Bighorn Basin contains layers of rock older than 2.5 billion years, as well as many, many younger rock layers. What makes this place so amazing is that it has layers of rock from almost every single geologic time period. If you had to pick one place in the world to tell the story of Earth's history, you would pick this place. So we picked this place.
The layered rocks of the Bighorn Basin were once ancient landscapes, and the fossils in the rocks are clues to what these landscapes looked like, what the ancient vegetation was, and what kinds of animals lived here. Because the Bighorn Basin is a dry place, not many plants grow here today, so it is easy to see the rocks. If you can see the rocks, you can find the fossils in the rocks. In this place, the history of the Earth lies on the ground as if it were an open book. And the goal of our little book is to give you the tools to read the big rock book of the Bighorn Basin.
Using layered rocks and fossils, geologists and paleontologists are able to envision what these lost worlds looked like. To share them with you, we studied the rocks; tracked down the fossils; reconstructed the plants, animals, and landscapes; and then employed an artist to paint them, choosing ancient worlds ranging in age from 520 million years to 18,000 years old. There are so many layers of rock in the basin that we could have painted hundreds of them. We chose to paint twelve.
AN EXTREMELY SHORT HISTORY OF ROCKS, THE WORLD, AND THE BIGHORN BASIN
Our universe formed in the big bang around 13.8 billion years ago, and our solar system, with our sun and all of our planets, formed about 4.567 billion years ago. The early Earth was molten, but by 4 billion years ago, it had cooled enough that its surface was rocky, and water had filled large depressions to form the first oceans. Shortly thereafter, the first life in the form of bacteria appeared and began to colonize the planet.
The early continents were small and regularly broke apart and smashed into each other. The chunk that included what is now Wyoming formed about 2.5 billion years ago. By about 1.8 billion years ago, a number of small continents had collided to form the core of what would become North America. One geologist has called this the United Plates of America.
You can think of continents as thin, flat pieces of Styrofoam floating on a swimming pool — except the Styrofoam is hard rock, and the water is soft rock that can flow like super-gooey molasses. If you add a weight to the foam, it will float lower in the water. It is the same with continents: If you add weight to them, they will sink lower; if you remove weight, they will float higher. You can add weight by loading continents with sediments, ice sheets, or water. You can remove weight via erosion, melting ice sheets, or draining water.
When parts of a continent sink below sea level, the sea floods in and covers sections of it, and the weight of the water will make the continent sink more. Sediment carried into the sea from the continent's eroding mountains will add yet more weight and make the continent sink further, creating room for more sediment. As the sediment layers get buried by more layers, they are compacted and become rock. That is how layers of sedimentary rock form and get preserved underground for millions and millions of years.
The Bighorn Basin is one of those singular places on Earth where lots of layers were deposited and stored underground for millions of years. They were later pushed up and exposed at the surface so that today we can see them in all of their glory. These layered rocks that poke out of the ground around the basin tell a pretty amazing story about our planet.
Although the first 3 billion years of life on Earth were ultimately dominated by very simple single-celled organisms like bacteria, by 542 million years ago, a host of more complicated multicelled marine animals had evolved. As the sediment piled up in places like the Bighorn Basin, it also piled up the bodies of the dead sea animals. The planet buries its dead, and the buried dead are called fossils.
Between 542 and 60 million years ago, the northwestern part of Wyoming was on a part of North America that was sinking and accumulating layers of sediment that would become fossil-filled rock layers. About 60 million years ago, the mountains that today ring the Bighorn Basin began to form. As they formed, they lifted up and folded the rock layers that were already there. As those rock layers rose and became exposed at the surface, they also eroded, and the sand and mud from the mountains flowed down into the basin and began to fill it up.
During this time, there were some pretty aggressive volcanoes in westernmost Wyoming. These mountains erupted repeatedly between 50 and 45 million years ago, forming what are now called the Absaroka Mountains.
By 45 million years ago, the Bighorn Basin was fully ringed by mountains. The rivers flowed off of those mountains and into the basin, and continued to fill it with sediment. Eventually the entire basin was full, and the surrounding mountains were buried in their own eroded debris.
Then a very strange thing happened. The whole region began to lift up and rivers began to cut down through both the mountains and the basin. Eventually these rivers carved the Wind River, Bighorn, and Clarks Fork Canyons through the margins of the basin. It was through these outlets that the eroding rivers scoured the basin, exposing the rock layers and creating the topography we see today.
It is in these canyons and in these rock layers that this story is told.
How Rock Layers Relate to Geologic Time
Here we connect the geologic episodes that we discuss in the book to specific rock formations, and the geological age in which they formed. The whole pile of rock layers in the Bighorn Basin (right) represents about 540 million years of time, and each individual formation represents a smaller window within that time.
Bighorn Basin Cross Section
This block diagram shows the Bighorn Basin as if it were sliced down the middle from north (right) to south (left), approximately along the Bighorn River. This perspective shows how the geological formations extend underground from what you can see exposed at the surface.
At the deepest part the basin, the entire thickness of sedimentary rocks is about 17,000 feet (see the diagram on page 10 for details). On the left side of the diagram, you can also see the northern edge of the Wind River Basin, which is just to the south of the Bighorn Basin.
Maps of Ancient Earth
Because the Earth's continents are always moving, different time periods have different maps. These paleogeographic maps show what the land that is now North America looked like and where what is now Wyoming could be found during each of the episodes we describe in this book. In these maps, the dark blue sections are ocean, the light blue is shallow sea, and the solid line is the equator. The red rectangle is Wyoming.
The Bighorn Basin Coring Project
Geologists and paleontologists have been coming to the Bighorn Basin since the 1880s, and they keep coming back because there is so much amazing geology to see and so many incredible fossils to find. Many of them have realized that the geology has direct value in the form of oil, gas, coal, gypsum, bentonite, and groundwater. Others come because they realize that the Bighorn Basin really is one of the best places in the world to study specific events in Earth's long history.
In 2011, a team of forty earth scientists from around the world came to the Bighorn Basin to drill and core three wells in the Willwood Formation. They came here because it is probably the best place on the planet to study what happened on land 55.5 million years ago, when there was a sudden and large burst of global warming known as the Paleocene-Eocene Thermal Maximum. Sometimes things that happened millions of years ago can have great relevance to planet Earth today.CHAPTER 2
AGE: 500 MILLION YEARS, CAMBRIAN
Formation: MAURICE LIMESTONE
Ancient Environment: WARM AND WET
Mounds of slimy algae and photosynthetic bacteria poke up above a muddy bottom in warm, shallow seas. Rocky ridges in the distance are barren of plants and animals – nothing growing, nothing walking, nothing moving, just a thin veneer of soil and lots of rock. In the water, small insect-like trilobites scuttle between the algae towers, and filter-feeding lampshell brachiopods cling to the bottom. The ocean here is that light-blue color that occurs near the equator, and the water leaves a salt stain when it evaporates from a rock. The scene is calm and peaceful, but a biological revolution is under way – a revolution that will forever change evolution of life on Earth and our ability to study it using the fossil record.
What you see today
One of the oldest sedimentary layers on top of the metamorphic and igneous Precambrian "basement" rocks is the Maurice Limestone, which was deposited during the Cambrian Period. Because the oldest layers in a basin are on the edges near the adjacent mountains, that is where you can see the Maurice. The pictured outcrop can be found in the Clarks Fork Canyon west of Clark, Wyoming. Here, the Clarks Fork River tumbles out of the Beartooth Mountains, cutting through the entire sequence of tilted geological layers and exposing them on the sides of the canyon. The Maurice Formation tends to form a cliff because it is hard, well-cemented limestone surrounded by softer, weakly cemented mudstone and shale.
During the Cambrian, Wyoming formed the coastline of North America and was situated near the equator. The present-day land to the west (Idaho, Nevada, Oregon, Washington, and California) had not yet crashed into the North American continent, so there was little geological drama nearby in the form of mountain building or volcanoes. Sea level was rising around the globe, and the Bighorn Basin represented a calm, shallow marine environment. Multicelled organisms with hard parts, such as trilobites and brachiopods, were beginning to rapidly evolve, but simpler single-celled holdovers from the Precambrian, including mats of bacteria and algae called stromatolites, were still common. This was the time of the greatest revolution in the history of life on Earth – the "Cambrian Explosion." During the approximately 3.5 billion years before the Cambrian, the biosphere was dominated by single-celled organisms. The first hints of multicelled life actually appeared just before the beginning of the Cambrian, but these life-forms had no shells or skeletons to speak of and thus were rarely fossilized. Then, multicelled organisms began to secrete hard parts, and within a few million years the fossil revolution had begun. When organisms started growing shells and skeletons, they essentially made their own rock, which means they more easily became fossils for us to find millions of years later (or hundreds of millions of years in this case!). While the shells and skeletons provide the animals with anatomical structure and protection from predators, they provide us with a much clearer window into the biological history of our planet than is available for the soft-bodied world of the Precambrian.
Giants of the Green Deep
Age: 450 MILLION YEARS ORDOVICIAN
Formation: BIGHORN DOLOMITE
Ancient Environment: WARM AND WET
A broad, shallow tropical sea stretches as far as the eye can see. A pod of giant squid hovers near the surface. Some of these squid have huge cylindrical shells that are more than a foot in diameter and up to 30 feet long. They look like floating logs, but they lived long before the first tree. Like today's squid, they are hunters, and they are the largest predators of their time. Gathering together near the surface to school and spawn, they have the sea largely to themselves; there are almost no fish and no other large animals. Stretching across the limy seafloor far below, shelly sea creatures go about their business. Smaller shelled squid, flower-like crinoids, feathery bryozoans, cone-shaped corals, clam-like brachiopods, bug-like trilobites, and coiled snails live in a variety of communities and are the source of food for the giant squid.
What you see today
Today, the Bighorn Dolomite forms one of the more recognizable geologic layers in the mountains that surround the Bighorn Basin. It forms prominent creamy vertical cliffs that rise above the slopes formed by the older underlying shale. When these cliffs fail, they send massive rectangular blocks smashing down to the bottom of the valley. The blocks look like giant dice, and one of them sits right in the middle of the Wind River, six miles north of the Boysen Dam. Up close, the Bighorn Dolomite has a jagged texture, and geologists call it the "tear pants" formation because that's exactly what it does if you sit on it in the wrong place. Fossils are not common in this layer, but there are enough to tell the story of how these towering cliffs were made from what was once the bottom of the sea.
The Bighorn Dolomite formed as limy mud at the bottom of a shallow sea. At the time, North America was far south of its present position, and the state of Wyoming would have been about 10 degrees south of the equator. The animals that lived in this sea were diverse, representing early examples of many groups of organisms that would continue to dominate life in the salty seas for hundreds of millions of years.
Age: 400 MILLION YEARS, DEVONIAN
Formation:BEARTOOTH BUTTE FORMATION
Ancient Environment:WARM AND DRY
Streams are entering a coastal area and have cut into the surrounding bedrock of Bighorn Dolomite. The channels are filling with sediment that has eroded from the surrounding hills. Lurking under the brackish water are armored fish, snails, and brachiopods. A five-foot-long predatory eurypterid is trolling the shallows in search of its next meal. These "scorpions of the water" are some of the largest predators of the Paleozoic and close evolutionary cousins to spiders and horseshoe crabs. They have legs for walking and paddles for swimming, so they can easily move in and out of the water. On land, life is now apparent. Thin, low-stemmed plants are sprouting from the muddy deposits on the edges of the streams. True land scorpions are scurrying among the plants, hunting for other critters that have evolved into this new, wide-open ecosystem outside of the water.
What you see today
The most spectacular outcrop of the Beartooth Butte Formation is perched on the top of the Beartooth Plateau, more than 6,000 feet above the basin floor. This geological remnant is the only bit of post-Precambrian sedimentary rock left on top of the mountains in this area – the rest was eroded away during the rise of the Rockies. The butte preserves horizontal layers of Cambrian, Ordovician, and Devonian shale and limestone, with sediments of the Beartooth Butte Formation filling channels cut down into the Bighorn Dolomite. These channels formed when sea level dropped during the Early Devonian, creating a coastal environment where streams flowed in from the adjacent land. Sediment slowly filled these channels, entombing the pieces and parts of the organisms that were living in this thriving ecosystem.
The beginning of the Devonian was the time when organisms were just emerging onto land. The early land plants were small – no forests existed yet, just low stems and a few small leaves. Plant roots and dead plant debris mixed in with weathered rock to form soils that started to live and breathe like the ones we have today. Arthropods, the evolutionary group that includes crabs, insects, and trilobites, were the first animals to be preserved as fossils from this new land-based ecosystem, but other soft-bodied groups were likely there also, leaving behind evidence in the form of burrows and tracks. The Beartooth Butte Formation contains a mix of marine (brachiopods and snails) and land (scorpions and plants) organisms, providing a perfect window into the very environment where this remarkable water-to-land evolutionary transition was taking place.(Continues…)
Excerpted from "Ancient Wyoming"
Copyright © 2016 Denver Museum of Nature & Science.
Excerpted by permission of Fulcrum Publishing.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
Section 1 Welcome to the Bighorn Basin,
Section 2 Lost Worlds,
Episode 1 Trilobite Towers, 500 million years, Cambrian,
Episode 2 Giants of the Green Deep, 450 million years, Ordovician,
Episode 3 Scorpion Stream, 400 million years, Devonian,
Episode 4 Icehouse Dunes, 300 million years, Pennsylvanian,
Episode 5 Red World, 220 million years, Triassic,
Episode 6 Longneck Lineup, 150 million years, Jurassic,
Episode 7 Ammonite Surprise, 82 million years, Cretaceous,
Episode 8 Herbaceous Cretaceous, 72 million years, Cretaceous,
Episode 9 Mammal Swamp, 57 million years, Paleocene,
Episode 10 Greenhouse Bird, 54 million years, Eocene,
Episode 11 A Bad Day, 640,000 years, Pleistocene,
Episode 12 Dry and Icy, 18,000 years, Pleistocene,
Section 3 Where to Go, What to Do,