Graneros Shale facts for kids
Quick facts for kids Graneros ShaleStratigraphic range: Cenomanian |
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Graneros Shale at outlet of El Vado Reservoir, New Mexico, USA
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| Type | Formation |
| Sub-units | See text |
| Underlies | Greenhorn Formation |
| Overlies | Dakota Formation |
| Thickness | 114–1,000 feet (35–305 m) |
| Lithology | |
| Primary | Shale |
| Other | Sandstone, limestone |
| Location | |
| Coordinates | 38°16′35″N 104°42′44″W / 38.2763°N 104.7121°W |
| Region | CO,IA,KS,MN,MT,NM,NE,ND,OK,SD,WY |
| Country |  United States |
| Type section | |
| Named for | Graneros Creek, Walsenburg quadrangle, Pueblo Colorado |
| Named by | G.K. Gilbert (from R.C. Hills) |
| Year defined | 1896 |
The Graneros Shale is a rock layer found in the United States. It is mostly made of shale, which is a type of clay-rich rock. This formation dates back to the Cretaceous Period, specifically the Cenomanian Age.
You can find the Graneros Shale in the Great Plains region and in New Mexico. It sits above the older Dakota Formation, which has sand and mud from land. It is found below the younger Greenhorn Limestone, which is a chalky rock formed in an open sea.
Scientists first defined the Graneros Shale in Colorado. This definition is now used in many other states. These include Kansas, Texas, Oklahoma, Nebraska, and New Mexico. It is also found in parts of Minnesota and Iowa. In some northern and western areas, like near the Black Hills, this same rock layer is called the Belle Fourche Shale.
Contents
How the Graneros Shale Got Its Name
Early Discoveries of Rock Layers
In the 1860s, two scientists, F.B. Meek and F.V. Hayden, started naming rock layers in the central Great Plains. They named a large group of shale rocks "Benton." These rocks were found between sandstone cliffs near Dakota City, Nebraska, and chalk cliffs where the Niobrara River meets the Missouri River.
At that time, these rock layers, called Dakota-Benton-Niobrara, were known across the Great Plains. They stretched from Canada all the way to New Mexico, up to the Rocky Mountains.
Naming the Graneros Layer
In southcentral Colorado, near Pueblo, the Benton shales showed clear patterns in the landscape. Rivers and wind easily wear away these shales, creating cliffs. For example, you might see tall cliffs of Niobrara Chalk above long, flat plains of Benton shale.
In 1896, scientist G.K. Gilbert gave a specific name to a lower, clay-rich shale layer. He called it Graneros after Graneros Creek. This creek is a branch of the Arkansas River near Pueblo. This name helped scientists understand how the larger Benton shale group was divided.
Understanding Ancient Sea Levels
The Graneros Shale helps us understand the Western Interior Seaway. This was a huge ancient sea that once covered much of North America. The Graneros layer formed as this sea began to spread out. It is similar to the Blue Hills Shale, which formed as the sea later pulled back.
By 1938, the Graneros Shale had been mapped across many states. These included eastern Wyoming, southeastern Montana, South Dakota, Nebraska, Kansas, and northeastern New Mexico. The same type of landscape and Graneros rock can be found in the Smoky Hills of Northcentral Kansas.
In Iowa and Nebraska, the Graneros changes into a different type of rock. This rock is part of the Dakota Formation. The Graneros Shale shows how rock layers can change over time and space. As the ancient sea spread eastward, the Graneros rock formed in different places at different times. For example, the top of the Graneros in Pueblo is older than the very bottom of the Graneros in Iowa.
Different Names for Similar Rocks
North of Kansas, the name "Graneros" was sometimes used differently. In 1904, N.H. Darton used the name Graneros Group for several layers in the Black Hills of South Dakota. These layers included the Belle Fourche Shale, Mowry Shale, Newcastle Sandstone, and Skull Creek Shale.
Scientists later realized that these layers were often grouped based on the fossils they contained, not always on how the rocks looked. By the 1960s, it was clear that many of these layers were known by their own names. Most of them had little to do with the original Graneros definition. Today, these layers are often considered part of the Dakota Group in Wyoming and Colorado.
What the Graneros Shale Looks Like
The Graneros Shale is mostly dark gray shale. It also has small amounts of sandstone and limestone. You can sometimes see shiny minerals like pyrite, marcasite, selenite, and calcite in it.
This rock layer sits on top of the Dakota Group. The Greenhorn Limestone is found directly above it. The thickness of the Graneros Shale can vary a lot, from about 114 feet (35 meters) to 1,000 feet (305 meters).
Special Features of the Graneros Shale
Even though the Graneros Shale is mostly one type of rock, it has a few special parts:
- Thatcher Limestone Member: The limestones in the Graneros Shale are not formed from minerals that settled out of water. Instead, they are made of broken pieces of shells, mostly from a clam called Inoceramus. These shell pieces show that the seafloor was often disturbed, washing away fine mud and leaving only the shells. In some areas, like southcentral Colorado and northcentral New Mexico, these limestones are thick enough to be called the Thatcher Limestone Member. Fossils of ammonites (ancient squid-like creatures) found here help confirm the rock's Cenomanian age.
- "X bentonite" Bed: The Graneros Shale has several thin layers of bentonite. Bentonite is a type of clay formed from volcanic ash. One specific bentonite layer is very thick and found in many places. Scientists call it the "X bentonite." These bentonite layers are like time markers. Wherever you find the "X bentonite," you know that rock layer formed at the exact same time in Earth's history.
* In the deepest part of the ancient sea, the "X bentonite" is found in the lowest layers of the Greenhorn Formation. * But as you go eastward, the "X bentonite" appears higher up in the Graneros Shale. By the time you reach Iowa, it's at the very bottom of the Graneros. * This pattern shows that the Graneros Shale formed in different places at different times as the sea level rose. The environment where the Graneros formed moved eastward and upward in geologic time.
Fossils Found in the Graneros Shale
The upper parts of the Graneros Shale contain many fossils. You can find lots of oyster fossils here. There are also ammonoids like Tarrantoceras sellardsi, Desmoceras, Anthoceras, and Borissiakoceras.
Other mollusk fossils include Inoceramus rutherfordi, Ostrea beloiti, and Turrilites acutus americanus. As you go into Wyoming, fossils become harder to find in the Graneros Shale.
