Straight Cliffs Formation facts for kids

Quick facts for kids Straight Cliffs Formation Stratigraphic range: Turonian-Campanian ~92–81Ma PreЄ Є O S D C P T J K Pg N
Cedar Canyon Basalt over Straight Cliffs Formation (Iron County, Utah)
Type	Geological formation
Underlies	Wahweap Formation
Overlies	Tropic Shale
Thickness	Max. 750 m (2,460 ft)
Lithology
Primary	Sandstone
Other	Siltstone, mudstone, coal, conglomerate
Location
Region	South central Utah
Country	United States
Extent	3,600 km2 (1,400 sq mi)
Type section
Named for	Straight Cliffs
Named by	Gregory and Moore
Year defined	1931

The Straight Cliffs Formation is a special layer of rock found in south central Utah, in a place called the Kaiparowits Plateau. These rocks formed during the Late Cretaceous period, about 92 to 81 million years ago. They tell us about ancient environments like rivers, swamps, lagoons, and coastlines. You can see these rocks clearly in the Grand Staircase–Escalante National Monument. The formation gets its name from the "Straight Cliffs," which are a long line of cliffs that form a mountain called Fiftymile Mountain.

The Straight Cliffs Formation was laid down along the western edge of an ancient inland sea called the Western Interior Seaway. Below it, you'll find the Tropic Shale, and above it is the Wahweap Formation. Scientists have discovered many different fossils here, including ancient ammonites (like squids in shells), mollusks, tiny foraminifera and ostracods, sharks, fish, amphibians, turtles, lizards, crocodile-like creatures, dinosaurs, and even early mammals.

What is the Geology of Straight Cliffs Formation?

The Straight Cliffs Formation sits on top of the Tropic Shale Formation and is covered by the Wahweap Formation. It holds clues about ancient rivers and coastal areas from the Kaiparowits Basin, which was part of the Cretaceous Western Interior Seaway. The formation is mostly made of sandstone, but it also has smaller amounts of siltstones, mudstones, coals, and conglomerates (rocks made of pebbles). It's about the same age as parts of the Mancos Shale formation found further east. Scientists first studied these rocks for their coal resources. More recently, they've been studied to understand where oil and gas might be found deep underground.

How Were the Straight Cliffs Rocks Formed?

The Straight Cliffs Formation was created in the Kaiparowits Basin, a large bowl-shaped area where sediments collected. This basin received sand, mud, and pebbles from several places. These included the Mogollon highlands (ancient mountains in Arizona), the Sevier fold-thrust belt (a mountain range forming to the west), and the Cordilleran volcanic arc (volcanoes even further west in California).

Even though these rocks formed in an ancient basin, they are now part of a modern flat-topped mountain area called the Kaiparowits Plateau. This plateau covers about 3,600 square kilometers (1,400 square miles). Early studies of the Straight Cliffs Formation focused on its coal. Later, geologists like Peterson and Vaninetti studied it more closely.

The formation is divided into four main parts, from oldest to youngest:

• The Tibbet Canyon Member
• The Smoky Hollow Member
• The John Henry Member
• The Drip Tank Member

Scientists have looked closely at these layers to understand how the ancient sea level changed. They found evidence of times when the sea covered more land (transgression) and times when it pulled back (regression).

Tibbet Canyon Member: Ancient Beaches and Estuaries

The Tibbet Canyon Member contains rocks from shallow seas, beaches, and estuaries (where rivers meet the sea). You can see it best in the southwestern and central parts of the Kaiparowits Plateau. This part of the formation is about 21 to 56 meters (70–185 feet) thick. It's made of yellow and gray sandstone. The bottom of this layer gradually changes from the Tropic Shale, and the top is marked by the mudstones and coal-rich shales of the Smoky Hollow Member. Scientists believe these rocks formed as the ancient Tropic Sea pulled back, leaving behind beach and shallow marine deposits.

Smoky Hollow Member: Coastal Plains and Rivers

The Smoky Hollow Member includes rocks from coastal plains with coal, and even braided rivers. It's fairly easy to see along the southern edge of the plateau. This member is about 7 to 101 meters (24–331 feet) thick and gets thicker towards the north. The top of this layer is marked by a special rock layer called the Calico Bed. This bed is from an ancient braided river and is easy to spot because of its white and orange colors. The Smoky Hollow Member formed in non-marine places like lagoons, coastal plains, and river environments.

John Henry Member: A Mix of Sea and Land

The John Henry Member is the thickest part of the Straight Cliffs Formation. It contains rocks from both rivers and the sea. You can find gray shales, siltstones, sandstones, shales with coal, and even beds filled with ancient shells. This member ranges from 200 to 500 meters (650–1,640 feet) thick. Scientists have studied its sandstone layers in detail, finding connections between river deposits in the southwest and coastal plain coals in the center of the plateau.

Drip Tank Member: Fast-Flowing Rivers

The Drip Tank Member, the youngest part, is made of coarse-grained rocks from a braided river environment. The bottom of this layer often forms a flat area at the top of the plateau. The Drip Tank gradually changes into the Wahweap Formation above it. It's about 43 to 160 meters (141–523 feet) thick and mostly made of yellow to brown sandstone with cross-bedding (layers that show ancient currents).

What Was the Environment Like?

The Straight Cliffs Formation shows us how the environment changed over millions of years as the sea level of the Western Interior Seaway went up and down.

The oldest part, the Tibbet Canyon Member, formed at the edge of the Greenhorn Seaway. It shows us the sandy beaches that formed as the shoreline grew outwards and the sea retreated.

The Smoky Hollow Member has rocks from rivers and lagoons. This was a time when the sea level was lower, and the shoreline was further east of the Kaiparowits Plateau.

The John Henry Member tells a story of changing sea levels. It has a mix of marine (sea) and terrestrial (land) deposits. In the southwest, it shows ancient river systems that carried sediment from the rising Mogollon highlands and Sevier fold-thrust belt. On the eastern side, it has a mix of marine and coastal deposits. By looking closely at how these layers stack up, scientists can tell that the sea was pulling back during the bottom part of the John Henry Member. Then, the sea level rose and flooded the land during the middle part. Finally, the sea level fell again during the last phase of its formation.

A break in the rock layers separates the Drip Tank Member from the John Henry Member below it. This means that the very top layers of the John Henry Member were exposed to the air and eroded away before the Drip Tank rocks were deposited. After this period of erosion, large river systems flowed across the Kaiparowits Plateau, laying down the sandy deposits of the Drip Tank Member.

What Fossils Are Found in the Straight Cliffs Formation?

Invertebrate Fossils

The most common and varied fossils found in the Straight Cliffs Formation are invertebrates (animals without backbones). These include oysters, ammonites, inoceramids, bivalves (like clams), ostracods, and foraminifera. Oysters are very common and are often found in large beds of broken shells in the parts of the rock that formed near the sea.

Only the Tibbet Canyon and John Henry members contain marine (sea-dwelling) invertebrate fossils. This is because the Smoky Hollow and Drip Tank members formed in land environments. For example, the Tibbet Canyon Member was first dated using a fossil called Inoceramus howelli, which helps scientists know its age. The John Henry Member has also yielded various invertebrate fossils, including the ammonite Baculites codyensis and the bivalve Endocostea baltica. Studying tiny fossils like foraminifera and ostracods has helped scientists understand more about ancient lagoons, bays, and estuaries.

Vertebrate Fossils

Fossils of vertebrates (animals with backbones) have been found throughout the Straight Cliffs Formation. These fossils show a wide variety of ancient mammals.

The Tibbet Canyon Member has shark teeth from marine areas and rare mammal fossils from ancient river deltas. Other fossils found here include rays, fish like gar, crocodile-like creatures, and small mammal teeth.

The Smoky Hollow Member also contains many types of sharks, amphibians, reptiles, snakes, crocodile-like creatures, and even dinosaurs. It also has fossils of early mammals called multituberculates and marsupials.

The John Henry Member has more fossils from brackish (slightly salty) and marine environments, as well as some mammals and other land animals, though these are less common.

The Drip Tank Member formed mostly from rivers, so only worn pieces of turtle and crocodile-like fossils have been found there.