Thermopolis Shale facts for kids

The Thermopolis Shale is a rock layer that formed in west-central North America. This happened during the Albian age of the Late Cretaceous period. You can find parts of this rock layer on the surface in central Canada, and in the U.S. states of Montana and Wyoming.

This rock formation was created over about 7 million years. It formed from sediment that flowed into a large ancient sea called the Western Interior Seaway. Geologists sometimes have different ideas about where this formation starts and ends. This is because its boundaries are not always clear. The Thermopolis Shale is special because it contains many fossils of both small and large animals. It also has fossilized animal waste, called coprolites.

Ancient Seas and Rock Layers

The Western Interior Seaway

Imagine a huge inland sea that once covered the middle of North America. This was the Western Interior Seaway. It existed from the Late Jurassic period (about 161 million years ago) until the end of the Paleogene period (about 23 million years ago). This ancient sea stretched from the Arctic Ocean all the way to the Gulf of Mexico. It was about 3,000 miles (4,800 km) long and 1,000 miles (1,600 km) wide. Even though it was so big, the seaway was quite shallow. Its deepest parts were probably only about 660 to 1,640 feet (200 to 500 m) deep.

How the Thermopolis Shale Formed

The Thermopolis Shale formed in a deep area of the Western Interior Seaway. This area was next to a mountain-building zone called the Sevier orogenic belt. Sediment from the eroding mountains to the west piled up in this deeper part of the sea. This created the layers of rock we see today. Changes in sea level, land uplift, and sinking land all affected how the sediment was laid down. This led to different types of rock forming, including those from rivers, marshes, and estuaries, as well as the main shallow and deep sea rocks.

Scientists have studied tiny plant pollen and volcanic ash layers (called bentonite) to figure out when the Thermopolis Shale formed. The lowest part of the formation, called the Lower Thermopolis Member, was deposited between 100.3 and 98.5 million years ago. The upper parts of the Thermopolis Shale formed over about 7 million years.

Discovering the Thermopolis Shale

The Thermopolis Shale was first noticed in 1914 by a geologist named Ferdinand F. Hintze, Jr. He called it the "Lower Benton Shale." He described it as having a "rusty beds" layer at the bottom, a lower shale, a "Muddy Sand" layer, and an upper shale.

The name "Thermopolis Shale" was first used in 1916 by geologist Charles T. Lupton. He described it as a dark-colored rock layer. It lay on top of the Cloverly Formation and below the Mowry Shale. Lupton noted that it was about 710 feet (220 m) thick and often had sandy parts. He named it after the town of Thermopolis, where the rock layers were easy to see. The United States Geological Survey used Lupton's description for the next 50 years.

What the Formation Looks Like

The Thermopolis Shale is part of two larger groups of rock layers: the Colorado Group and the Dakota Formation. Geologists have sometimes disagreed on how to define and name the different parts of these rock groups.

The Thermopolis Shale usually sits on top of the Cloverly Formation or the Kootenai Formation. There's some debate about its lowest layer, the "rusty beds." Some geologists think these beds are part of the Thermopolis Shale, while others place them with the Cloverly Formation or even the Fall River Sandstone.

The top boundary of the Thermopolis Shale is also debated. This makes it tricky to say exactly what rock layer sits on top of it. Depending on how it's defined, the Thermopolis Shale can be very different in thickness. It can be as thick as 800 feet (240 m) in some places, but only 10 feet (3.0 m) thick in others.

Generally, the Thermopolis Shale is a dark gray to black shale. It has thin layers of bentonite (volcanic ash), sandy claystone, and siltstone mixed in. Some definitions also include a gray, thin-layered sandstone between the upper and lower parts.

Layers of the Thermopolis Shale

The Thermopolis Shale is made up of four main layers, called members. These are:

• The "rusty beds" Member

* This is the bottom layer of the Thermopolis Shale. It was first described in 1904. It's called "rusty beds" because of its reddish-brown color, which comes from iron. * These rocks formed as the Western Interior Seaway began to rise. They are made of thin layers of reddish-brown sandstone (3 to 18 inches (7.6 to 45.7 cm) thick) and black shale (1 to 12 inches (2.5 to 30.5 cm) thick). * The very bottom of the "rusty beds" probably formed in estuaries (where rivers meet the sea) or river deltas. This layer can be between 20 feet (6.1 m) and 200 feet (61 m) thick.

• Lower Thermopolis Member

* This layer formed when the northern and southern parts of the Western Interior Seaway connected. This was during a time when sea levels were at their highest. * The change from the "rusty beds" to this layer is gradual. This layer is often described as carbon-rich black shale with some sandy parts. It is similar to the Skull Creek Shale.

Edgarosaurus muddi, a polycotylid plesiosaur found in the Upper Thermopolis Member.

• Muddy Sandstone Member

* This layer was first described in 1904 and became formally known as the "Muddy Sandstone Member" by 1972. It's a widespread, almost white sandstone layer, usually 25 to 40 feet (7.6 to 12.2 m) thick. * The Muddy Sandstone formed when sea levels in the Western Interior Seaway dropped and then rose again. It's made of thin layers of fine-grained, silty sandstone that can be buff, brownish-grey, or grey. * This sandstone often has ripple marks, showing it formed in water. It also contains thin layers of shale, siltstone, and volcanic ash. Its thickness varies a lot, from 10 to 55 feet (3.0 to 16.8 m).

• Upper Thermopolis Member

* This layer formed on top of the Muddy Sandstone as the Western Interior Seaway rose for a second time. It's mostly bluish-black shale. * This layer also has occasional beds of volcanic ash and bentonite. Some parts of this layer are harder and contain sandy lenses. * The very top of this layer gradually changes into the Mowry Shale. It has thin, limy, silty sandstone and silty shale beds.

The Thermopolis Shale can be found in central Canada, Montana, and Wyoming. The rock layers formed in the sea are mostly shale, with some limestone, sandstone, and siltstone. The layers formed on land are mostly sandstone, with some coal, shale, and siltstone.

Fossils in the Thermopolis Shale

The Thermopolis Shale is a treasure trove of marine (sea-dwelling) vertebrate fossils. These fossils include bones, teeth, and coprolites (fossilized poop).

A particularly rich fossil area is found in the lower parts of the Upper Thermopolis Member. Here, you can find many remains of:

• Marine crocodiles
• Plesiosaurs, especially Edgarosaurus muddi
• Rays, like Pseudohypolophus
• Sawfish, like Onchopristis
• Turtles, including Baenidae and Glyptops

You can also find teeth from Hybodont sharks (like Meristodonoides), different types of fish, and invertebrate ammonoids (like Baculites).