The Cedar Mountain Formation is a distinct layer of rock that crops out in western Colorado and eastern Utah. It is made from sediments, such as sand and mud, which were altered into rock over geologic time. The original sediments were deposited mostly in rivers and on flood plains, as well as in small, seasonal ponds. The Formation is made up of five smaller sublayers, called members, which can be illustrated diagrammatically below. From bottom to top these are: Buckhorn Conglomerate, Yellow Cat Member, Poison Strip Sandstone, Ruby Ranch Member, and the Mussentuchit Member.

Representation of the various members of the Cedar Mountain Formation (right columns) compared to their estimated geologic age. Missing strata shown with cross-hatched lines. Column of numbers in the center are dates in millions of years (mya). The various names (left column) are subdivisions given to the the Lower Cretaceous (dark green) and part of the Upper Cretaceous (light green). The Jurassic is also subdivided but only the very late, the Tithonian is shown.

 

Interestingly, not all of the members are evenly distributed across Colorado and Utah. In the Paradox Basin area along the Colorado-Utah border, the Yellow Cat Member is thick, but it thins and disappears to the west. In contrast, the Mussentuchit is thickest southwest of the San Rafael Swell and thins eastward. The most widespread member is the Ruby Ranch. It occurs almost everywhere and is thickest north of the San Rafael Swell.

 

In discussing the ages of the various members, some terms are used that may not be familiar. These terms, such as "Barremian" and "Aptian," are subdivisions of geologic time.

  Buckhorn Conglomerate

 

Buckhorn Conglomerate relative to the other members of the Cedar Mountain Formation, and a field photograph showing the eroded surface (arrow) on the top of the Morrison Formation (brick-red color strata below arrow).

 

This member is composed pebbles and small boulders that form a conglomerate that is up to 25 meters (82 feet) thick. It occurs primarily along the west side of the huge bulge known as the San Rafael Swell (a mountain in development). The cherts contain Paleozoic (570-245 million years ago - mya) marine fossils, such as brachiopods and coral, that were eroded during the Early Cretaceous from mountains farther to the west, nearer central and western Utah. These pebbles and conglomerates were deposited in fast flowing rivers and streams. The finer grained sands and mud were carried farther by the fast water.

 

Many rivers today with similar characteristics are called braided rivers because the river is actually composed of numerous smaller, channels that merge and separate repeatedly. They do so because the large sediment load actually hinders the river by forming gravel bars forcing the water around them. Braided rivers occur where water velocity is relatively high, such as in and near mountains. The ancient mountain chain that the Buckhorn conglomerates eroded off of was called the Seviers, which snaked along from Arizona to Idaho.

 

The age of the Buckhorn has been problematic because until very recently, it produced few age diagnostic or datable fossils. Some geologists have suggested that the Buckhorn belongs to the very top of the sedimentary rocks called the Morrison Formation, hence is very Late Jurassic in age. Other geologists have argued that the Jurassic-Cretaceous boundary lay somewhere within the Buckhorn, implying that deposition of the Buckhorn began in the very Late Jurassic and continued into the Early Cretaceous. Recently, however, a partial skeleton of an armored dinosaur, an ankylosaur, has been found suggesting that at least this part of the Buckhorn is Cretaceous.

Yellow Cat Member

 

Yellow Cat Member relative to the other Members. The field photograph shows it exposed between the brick-red colored mudtsones of the Morrison Formation and the sandstone cliff of the Poison Strip.

 

The Yellow Cat Member is confined to eastern-most Utah, and is thickest near Arches National Monument where it is 24 meters (79 feet) thick. Because it is so far from the San Rafael Swell, the Buckhorn Conglomerate is missing. Instead, the drab, pale-colored mudstones of the Yellow Cat Member Yellow Cat are situated atop the brick-red mudstones of the Morrison Formation. In places the contact between the two is marked by hard, brown and rust-colored rock layer of an ancient caliche (cal-ee-chee) or hardpan. A drop of acid on these lumps usually cause them to fizz indicating the presence of calcium carbonate cement. Similar rocks today form in soils where rain fall is low. As the ground dries under the hot sun, evaporation of mineral-laden ground water causes calcium carbonate to precipitate in a layer below the surface. The presence of caliche in sedimentary rocks usually means the presence of an ancient soil, or paleosol (pale-ee-oh-sol).

 

The caliche at the contact between the Yellow Cat and Morrison represents a 25 million year gap in sediment deposition. It is possible that sediments were deposited during that interval of time and subsequently eroded away. If the erosion was slow enough, then a soil might have developed and continued to form at a rate in pace with the erosion. Once deposition of the Yellow Cat Member began, however, it essentially buried the soil, thus stopping further development. This caliche is not present everywhere and where absent, the Morrison is sometimes marked by another soil feature: a zone of different colored blotches and elongate vertical streaks that may represent where roots once extended downwards. This type of paleosol feature also forms in dry environments, but for a variety of reasons caliches don’t form.

 

The mudstones of the Yellow Cat Member were originally mud deposited on flood plains adjacent to river channels. The presence of caliche layers indicate long intervals of time between the floods that brought mud onto the flood plains. The river channels which brought the floods are today represented by large bodies of sandstones, formed from the sandbars in the rivers.

 

From afar these sandstones form small cliffs because the sandstone tends to break off in large blocks; these then litter the slopes below. Up close, the base of the sandstone is irregular where the river once scoured its channel into the floodplain sediments. The sandstone near the base usually has pale-colored caliche pebbles and cobbles. This caliche was eroded from the river banks by the river and rounded as it was tumbled downstream. 

 

The sandstone in these ancient channels is not uniform throughout, but shows repeated cycles of sediment sizes due to waxing and waning of water level. These cycles suggest that rain fall was seasonal, an idea or hypothesis that is supported by tree rings in fossilized wood. During the rainy season when water volume and flow was at its greatest, the largest sediment sizes (pebbles and cobbles) could be moved along the river bottom. But as the water volume decreased with the passing of the rainy season, water velocity also slowed, and the force of the water also dropped. This steady drop in water force results in a progressive drop in sediment size as well. It is during this waning of the river water that most of the sand is deposited. As a result, there is a gradation of coarser to finer sand grains from bottom to top. Within the sandstones of the Yellow Cat, there are repeated cycles of fining upwards.

 

Some sandstone bodies end abruptly against mudstone and mark where the river was eroding a cut-bank during the Early Cretaceous. Other sandstone bodies feather into the mudstones and this portion probably represent natural levees formed when floods spilled out of the channel. No longer confined, the water velocity rapidly decreased and the sediment load was rapidly deposited adjacent to the channel. Some sandstone bodies look like smaller versions of the river channel sandstone. However, viewed from the air, they sometimes erode into roughly hand-shaped, or lobate structures. Internally, there may be ripples stacked on top one another, called climbing ripples.

 

Today, such lobate structures, called a crevasse splay, are formed when flood waters erode a notch or crevasse through the river bank and the river water spills out. The initial velocity near the river can remain very high and the water can erode a channel into the floodplain. But as water velocity drops progressively finer sediments are deposited. Once a crevasse forms, it may provide an outlet for future floods. Sometimes the river may abandon its original channel through the crevasse and carve a new channel across the flood plain, an event called an avulsion. 

 

Some of the mudstones contain fossils of aquatic plants and creatures, such as fish bones or the tiny reproductive spheres of green algae, called charophytes. These mudstones were deposited in small ponds on the floodplain.

 

The mudstones and sandstones of the Yellow Cat Member indicate that it was deposited by meandering rivers flowing across broad flood plains that were dotted with small ponds and lakes.

 

The age of the Cedar Mountain is difficult to determine because few age datable fossils have been found and no volcanic ashes, which can be dated precisely. Distinctive charophyte species, Atopochara trivolvos and Nodosoclavator bradleyi have been found in pond deposits. Atopochara trivolvos has previously been found in sedimentary rocks as old as upper Barremian (around 122 mya) and as young as lower Aptian (120 mya), whereas Nodosoclavator bradleyi  has been found in Barremian and older rocks (121 mya or older). This Barremian age overlap suggests that the Yellow Cat, then, was deposited roughly 127-121 mya. In addition, some dinosaurs resemble those from the Barremian and older strata of the Isle of Wight in England (more on this in the Dinosaurs of the Cedar Mountain Formation). 

Poison Strip Sandstone

Poison Strip Sandstone in relation to the other members of the Cedar Mountain Formation. The Poison Strip forms a distinctive cap to a line of cliffs that extends from the Colorado-Utah border to the west side of Arches National Monument.

 

Capping the Yellow Cat Member is a cliff-forming interval of sandstones called the Poison Strip Sandstone. It is up to 5.5 meters (16 feet) thick. There is some debate as to whether the Poison Strip Sandstone should be recognized as a separate member of the Cedar Mountain Formation, or whether it is a sandstone interval of the Yellow Cat Member. One objection is that there is not just one thick sandstone, but multiple sandstone bodies separated by Yellow Cat-looking mudstone. In fact, the lowest channels grade into Yellow Cat mudstone.

 

There are two possibilities to explain these sandstones. One is that the slow sinking of the land slowed so that rather than lots of sediments being deposited, the rivers flowed back and forth reworking the older flood plain deposits. The other possibility is that uplift to the west increased so that the rivers were carrying much more sediment than previously. What hasn’t been determined is if the Buckhorn conglomerate becomes the Poison Strip and other sandstones of the Yellow Cat Member to the east. Keep in mind that the finer grained sediments were carried farther from where the Buckhorn Conglomerates were deposited. It may very well be that these finer grained sediments (mud, sand and silt) are preserved as the Yellow Cat Member and Poison Strip Sandstone. As yet, no detailed study has been conducted to invalidate or test this hypothesis, but cherts with Paleozoic fossils do occur in these sandstones, as they do in the Buckhorn Comglomerate.

 

The Poison Strip Sandstone is probably late Barremian because it overlies or is a part of the top of the Yellow Cat. 

Ruby Ranch Member 

 

The Ruby Ranch Member in relation to the other members of the Cedar Mountain Formation. Its most distinctive feature is the carbonate nodules that liter the ground giving it a slight reddish tint from the distance.

 

The most widespread member of the Cedar Mountain Formation is the maroon and chocolate brown weathering mudstones of the Ruby Ranch Member. It is over 33 meters (108 feet) thick. The ground surface is usually littered with irregular caliche nodules that have eroded from the mudstones. Some of the caliche forms thick beds indicating a very long time interval of soil development because of little input of flood sediments. In such soils today, the ground frequently cracks, allowing oxygen to reach deeply. Any organic material, such as buried plant debris, can be oxidized rapidly and break down. Some of the mudstone contains aquatic fossils indicating small, seasonal ponds that formed during the rainy season.

 

Sandstones of meandering rivers also occur throughout the member. Some of these channels near the town of Green River are exposed three-dimensionally because the surrounding softer floodplain mudstones have eroded away. This has left the channels preserved as long, sinuous ridges of sandstone, called exhumed channels. More on this in Pulling the Pieces Together.

 

 An example of an exhumed channel as seen from the air just west of Green River, Utah. The large meander loops are clearly seen.

 

The age of the Ruby Ranch is uncertain, although the lowest portions may be equivalent with the Yellow Cat Member.  

Mussentuchit Member

 

The Mussentuchit Member in relation to other members of the Cedar Mountain Formation. The grey-color and lack of nodules are characterisitic features.

 

The uppermost member of the Cedar Mountain is very different from the other members in that it is predominantly grey mudstone lacking caliche nodules. It is 25 meters (82 feet) in thickness. Near the top of the member occur dark, organic rich, or carbonaceous shales. The organic material is decomposed plant material and in places is concentrated enough to form lignite, a type of low grade coal. Volcanic ash also occurs, indicating volcanism farther to the west. The ash is important because it provides the best age for the Cedar Mountain Formation. The radioactive decay of certain minerals can be precisely measured to tell us the age of a rock.  Volcanic minerals removed from an ash about 18 meters (60 feet) below the top have a date of 98.39 + 0.07 million years old. This date is about at the boundary between the Early Cretaceous and Late Cretaceous. Because this date is near the top of the Cedar Mountain, it means that all of the members below have to be older.

 

Volcanic ash commonly alters into a peculiar type of clay (called smectite) that has a tendency to absorb water readily and thus swell. Where abundant in the Mussentuchit, this swelling causes the surface to become irregular and has a texture that resembles popcorn. Swelling clays also occur in a thin mudstone above the Ruby Ranch near Green River and it can be traced towards Grand Junction in Colorado. Although this mudstone is frequently a pale green, it may be the Mussentuchit also. Alternatively, it may be the top of the Ruby Ranch Member that has had been altered due to downward percolation of seawater when the ocean covered the region.

  

The absence of caliche in the floodplain mudstones of the Mussentuchit is significant and indicates a high water table due to the proximity of the encroaching sea. Damp soil can seal out oxygen, thus plant debris that was destroyed in the Ruby Ranch sediments are preserved in the Mussentuchit. Furthermore, microscopic pyrite can form because of the organic sulfur present in the plant detritus plus iron dissolved in the ground water. Thus, the grey color of the Mussentuchit, from organic material and pyrite, is a clue to the soil environment.  

Naturita (=Dakota) Formation

 

The Naturita (=Dakota) Formation sits ontop of the Cedar Mountain Formation and represents sediments deposited by rivers flowing into the Cretaceous seaway.

 

Although not part of the Cedar Mountain Formation, for practical purposes I have included the Naturita Formation as part of the Cedar Mountain Project. That is because it is a thin unit that is situated between the Cedar Mountain Formation and the Tununk Member of the Mancos Shale and represents the transition zone between land and sea. These strata have been called the Dakota Formation, but that formation was deposited on the east side of the Cretaceous Seaway. Thus, using that term on the west side of the Seaway is incorrect.

 

The Naturita Formation consists of yellowish-orange sandstone and conglomerates. It is considerably variable in thickness and in some places is completely missing so that the Mancos sits directly atop the Cedar Mountain. Where it is missing, there is sometimes a thin gravel bed between Cedar Mountain and Mancos that suggests the sands were winnowed out by wave action. In other places, however, the Mancos sits atop the Cedar Mountain without an abrupt transition (meaning it is gradational). Here, oyster beds are common at the very top of the Cedar Mountain (identified by its green, rather than grey color).

 

The Naturita was deposited by rivers flowing from the mountains located to the west. In places deltas formed where the rivers emptied into the sea. Some deltas remained active until the advancing sea overwhelmed them, but others were abandoned when the rivers avulsed to form a new channel. These deltas were quickly eroded by the back-and-forth sloshing of the waves. This action winnowed out the sands, leaving behind a thin gravel bed. Between the delta were quiet lagoons where oysters thrived. Eventually the sea would cover all of eastern Utah and connect the Arctic Ocean with the Gulf of Mexico.

 

The age of the Naturita Formation is lower Cenomanian, about 97 million years old. Thus, the Naturita was technically deposited at the start of the Late Cretaceous.

Mancos Shale

The Mancos Shale in relation to the Cedar Mountain and Dakota Formations.

 

The Mancos Shale overlies the Naturita Formation, although in some places it lies directly atop the Cedar Mountain Formation. The presence of marine invertebrate fossils indicate that the Mancos was deposited on the seafloor when North America was split in half by the Cretaceous sea.

 

Back to Introduction

Forward to Dinosaurs of the Cedar Mountain Formation