Graneros Shale
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The Graneros Shale is a
geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...
in the United States identified in the
Great Plains The Great Plains (french: Grandes Plaines), sometimes simply "the Plains", is a broad expanse of flatland in North America. It is located west of the Mississippi River and east of the Rocky Mountains, much of it covered in prairie, steppe, a ...
as well as
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...
that dates to the
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
Age of the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
Period. It is defined as the finely sandy
argillaceous Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...
or clayey ''near-shore/marginal-marine'' shale that lies above the older, ''non-marine''
Dakota Dakota may refer to: * Dakota people, a sub-tribe of the Sioux ** Dakota language, their language Dakota may also refer to: Places United States * Dakota, Georgia, an unincorporated community * Dakota, Illinois, a town * Dakota, Minnesota, ...
sand and mud, but below the younger, chalky ''open-marine'' shale of the
Greenhorn Greenhorn is a slang for an inexperienced person, or a slur against Portuguese people in New England, United States. It may also refer to: Places * Greenhorn, California, United States * Greenhorn Mountain, a mountain in Colorado * Greenhorn, O ...
. This definition was made in
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of the ...
by G. K. Gilbert and has been adopted in other states that use Gilbert's division of the Benton's shales into Carlile, Greenhorn, and Graneros. These states include
Kansas Kansas () is a U.S. state, state in the Midwestern United States, Midwestern United States. Its Capital city, capital is Topeka, Kansas, Topeka, and its largest city is Wichita, Kansas, Wichita. Kansas is a landlocked state bordered by Nebras ...
,
Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...
,
Oklahoma Oklahoma (; Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a state in the South Central region of the United States, bordered by Texas on the south and west, Kansas on the north, Missouri on the northeast, Arkansas on the east, New ...
,
Nebraska Nebraska () is a state in the Midwestern region of the United States. It is bordered by South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Kansas to the south; Colorado to the sout ...
, and New Mexico as well as corners of
Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over t ...
and
Iowa Iowa () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States, bordered by the Mississippi River to the east and the Missouri River and Big Sioux River to the west. It is bordered by six states: Wiscon ...
.
North Dakota North Dakota () is a U.S. state in the Upper Midwest, named after the indigenous Dakota Sioux. North Dakota is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minnesota to the east, ...
,
South Dakota South Dakota (; Sioux: , ) is a U.S. state in the North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Lakota and Dakota Sioux Native American tribes, who comprise a large po ...
,
Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to t ...
, and
Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...
have somewhat different usages — in particular, north and west of the
Black Hills The Black Hills ( lkt, Ȟe Sápa; chy, Moʼȯhta-voʼhonáaeva; hid, awaxaawi shiibisha) is an isolated mountain range rising from the Great Plains of North America in western South Dakota and extending into Wyoming, United States. Black ...
, the same rock and fossil layer is named Belle Fourche Shale.


Naming and Rank

F.B. Meek and F.V. Hayden originated the scientific names for the series of Cretaceous rocks in the central Great Plains of the
North American Continent North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
. They gave the name "Benton" to the great shale deposits between the sandstone bluffs at
Dakota City, Nebraska Dakota City is a city in Dakota County, Nebraska, United States. The population was 1,919 at the 2010 census. It is the county seat of Dakota County. Tyson Foods' largest beef production plant is located in Dakota City. History Dakota City was ...
, and the chalk bluffs at the junction of the
Niobrara Niobrara may refer to: * Niobrara, Nebraska * Niobrara County, Wyoming * Niobrara River * Niobrara National Scenic River * Niobrara Formation The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that ...
and
Missouri Missouri is a state in the Midwestern region of the United States. Ranking 21st in land area, it is bordered by eight states (tied for the most with Tennessee): Iowa to the north, Illinois, Kentucky and Tennessee to the east, Arkansas t ...
rivers. At that time, the early 1860s, Meek and Hayden's "lower Cretaceous" series of the upper Missouri River, ''Dakota-Benton-Niobrara'', was already widely observed from Canada to New Mexico over the Great Plains up to the foothills of the
Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...
. In southcentral Colorado, southeast of
Pueblo In the Southwestern United States, Pueblo (capitalized) refers to the Native tribes of Puebloans having fixed-location communities with permanent buildings which also are called pueblos (lowercased). The Spanish explorers of northern New Spain ...
, this series expresses topographical patterns that inspired subdivision of the Benton shales. Wind and rivers rapidly erode the shales, producing bluffs: massive blocks of Niobrara Chalk cap high slopes of ''non-chalky'' Benton shale leading down a flatter plain that stretches miles to another bluff of ''chalky'' shale with many thin limestones. Particularly, in places where rivers have cut deeply through this lower chalky shale, and furthermore have cut into the bases of the bluffs, banks of ''non-chalky'' shale can be found. In 1896, G.K. Gilbert named this lower argillaceous shale Graneros (from R.C. Hills) for the exposures in Graneros Creek, a tributary of the
Arkansas River The Arkansas River is a major tributary of the Mississippi River. It generally flows to the east and southeast as it traverses the U.S. states of Colorado, Kansas, Oklahoma, and Arkansas. The river's source basin lies in the western United ...
near Pueblo. Thus, the Graneros records the opening transgression of the Greenhorn Marine Cycle of the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea ...
, and is therefore the complement of the similar Blue Hills Shale of the
Carlile Formation The Carlile Shale is a Turonian age Upper/Late Cretaceous series shale geologic formation in the central-western United States, including in the Great Plains region of Colorado, Kansas, Nebraska, New Mexico, North Dakota, South Dakota, and Wy ...
that records the regression of the same Greenhorn Sea. By 1938, the Graneros had been mapped into eastern Wyoming, southeastern Montana, South Dakota, Nebraska, Kansas, and northeastern New Mexico. The same Benton topography is also found in the
Smoky Hills The Smoky Hills are an upland region of hills in the central Great Plains of North America. They are located in the Midwestern United States, encompassing north-central Kansas and a small portion of south-central Nebraska. The hills are a disse ...
 of Northcentral Kansas, and the same Graneros Formation is found in the river banks there. In Iowa and Nebraska, the Graneros grades into middle-
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
rock of the Woodbury Member the
Dakota Formation The Dakota is a sedimentary geologic unit name of formation and group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and shales deposited in the Mid-Cretaceous opening of the Wester ...
. As discussed in the Description section, the Graneros exceptionally demonstrates Walther's Law of Facies. As the Greenhorn Cycle of the Western Interior Seaway advanced eastward, formation of the Graneros also shifted eastward, but also upward in elevation and time, to such an extent that the top of the Graneros at Pueblo is older than the lowest Graneros in Iowa. North of Kansas, the application of the name has been somewhat different. In 1904, describing the geology of the
Black Hills The Black Hills ( lkt, Ȟe Sápa; chy, Moʼȯhta-voʼhonáaeva; hid, awaxaawi shiibisha) is an isolated mountain range rising from the Great Plains of North America in western South Dakota and extending into Wyoming, United States. Black ...
of South Dakota, N.H. DartonDarton, N.H. 1904. Comparison of the stratigraphy of the Black Hills, Bighorn Mountains, and Rocky Mountain Front Range. Geological Society of America Bulletin, v. 15, p. 379-448. applied the name Graneros Group to descending members; marine Belle Fourche Shale and
Mowry Shale The Mowry Shale is an Early Cretaceous geologic formation. The formation was named for Mowrie Creek, northwest of Buffalo in Johnson County, Wyoming. Description The Mowry Shale or Fish-scale Beds because of the abundance of fish scales, is ...
, terrestrial Newcastle Sandstone (a tongue of Dakota Formation from the southeast of South Dakota), and marine Skull Creek Shale. This group definition was a wide departure from the preceding applications of the early-
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
Graneros name by applying it to late-
Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...
units. Robert M. Grace (1952) explains the basis for this association; previous studies associated these units by their fossil species rather than their lithological sequence. The late-Albian Skull Creek Shale in the Black Hills is seen with "microfauna identical to" that of the Newcastle, which, in sequence, traces to the Dakota type in the east. By the 1960s, Darton's definition was recognized as problematic, but many geologists continued to use this classification. It is understood that the listed formations are widely known individually, most with little relationship to the original Graneros Formation definition; and, the same or equivalent units are classified in Wyoming and Colorado as Dakota Group. As a result, newer reports include the Belle Fourche (Graneros equivalent), Mowry, Newcastle, and Skull Creek within the Dakota Group of this region.


Description

The Graneros Shale is primarily dark gray sandy and silty
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
with minor
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...
and
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
lenses, with visible crystalline minerals (
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...
,
marcasite The mineral marcasite, sometimes called “white iron pyrite”, is iron sulfide (FeS2) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both ...
, selenite,
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
). It also features septarians and other concretions. It rests on the Dakota Group and is in turn overlain by the
Greenhorn Limestone The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle ...
. The thickness varies from . Because the Graneros Shale is nearly monolithologic, only one member and one bed have ever been named. * Thatcher Limestone Member: Rather than calcite precipitates, the limestones of the Graneros (similar to the limestones of the Lincoln "Marble" just above the formation) are sorted skeletal remains of marine animals. These skeletal limestones indicate repeated disturbances of the bottom that washed away the silt, leaving mostly
Inoceramidae The Inoceramidae are an extinct family of bivalves ("clams") in the Class Mollusca. Fossils of inoceramids are found in marine sediments of Permian to latest Cretaceous in age. Inoceramids tended to live in upper bathyal and neritic environm ...
shell fragments. ''
Ostrea ''Ostrea'' is a genus of edible oysters, marine bivalve mollusks in the family Ostreidae, the oysters. Fossil records This genus is very ancient. It is known in the fossil records from the Permian to the Quaternary (age range: from 259 to 0. ...
beloiti'' beds are particular index fossils for the formation. The middle limestones of the formation are sufficiently developed in southcentral Colorado and northcentral New Mexico to be named Thatcher Limestone Member. Ammonites from the Thatcher Limestone Member are indicative of
Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...
age. * X-bentonite: While the formation has several ''thin''
bentonite Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-m ...
beds, one unique bed is sufficiently thick, widespread, and widely used as a marker bed to be informally named "X", the commonly applied quotation marks indicating the lack of formal classification of the bed. A feature of bentonite beds is that they mark specific instants in geological time; that is, all locations where a particular bentonite bed is found are therefore known to record the same date in Earth history. The X-bentonite is radiometrically dated to 96 or 95 million years ago and it identifiable throughout the Western Interior Seaway marine sediments. At the Graneros type location, which was generally the deepest part of the Western Interior Seaway, the X-bentonite is actually in the lowest beds of the overlying Greenhorn Formation. However, progressing eastward from Peublo, the X-bentonite is found within the ''top'' of the Graneros, then lower into the unit, and by Iowa, the X-bentonite is found at the ''bottom'' of the Graneros. This pattern is evidence that the Graneros was deposited in different locations in time and space; as the sea levels of the Greenhorn Cycle rose, the Graneros environment moved eastward in space and upwards in geologic time. The same is observed West of Pueblo; the base of the Graneros equivalent in the
Mancos Shale The Mancos Shale or Mancos Group is a Late Cretaceous (Upper Cretaceous) geologic formation of the Western United States. The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures near the town of Mancos, ...
rise to meet the X-bentinite in Utah and Arizona. In other words, before the X-bentonite ashfall, the Graneros shoreline environment is advancing east and west, leaving the central seaway; and by the time of the X-bentonite ashfall, was in the process of advancing into the regions of modern Iowa and Utah, respectively. As discussed in the previous Naming and Rank section, the Graneros name was applied in South Dakota to a group of other units; however, this has no relation to the description of the Graneros Shale formation.


Hydrocarbon exploration

The Graneros/Belle Fouche Shale is an important hydrocarbon
source rock In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been depo ...
in the High Plains production
basins Basin may refer to: Geography and geology * Depression (geology) ** Back-arc basin, a submarine feature associated with island arcs and subduction zones ** Debris basin, designed to prevent damage from debris flow ** Drainage basin (hydrology), a ...
.


Fossils

The upper part of the formation contains abundant oyster fossils and the
ammonoid Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...
s ''Tarrantoceras sellardsi'' Adkins, ''
Desmoceras ''Desmoceras'' is a genus of ammonites belonging to the family Desmoceratidae. These cephalopods were fast-moving nektonic carnivores. They lived in the Early Cretaceous; Albian epoch. Species * ''Desmoceras alamoense'' * ''Desmoceras argonaut ...
'', '' Anthoceras'', and ''Borissiakoceras''. The mollusks ''
Inoceramus ''Inoceramus'' (Greek: translation "strong pot") is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus '' Pteria''. They lived from the Early Jurassic to l ...
rutherfordi'' Warren, ''
Ostrea ''Ostrea'' is a genus of edible oysters, marine bivalve mollusks in the family Ostreidae, the oysters. Fossil records This genus is very ancient. It is known in the fossil records from the Permian to the Quaternary (age range: from 259 to 0. ...
beloiti'' Logan, and ''
Turrilites ''Turrilites'' is a genus of helically coiled ammonoid cephalopods from the lower part of the Upper Cretaceous (Cenomanian and Turonian The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stag ...
acutus americanus'' are also present. Fossils become more scarce in Wyoming.


See also

* Belle Fourche, the unit's name in its northern range * List of fossiliferous stratigraphic units in Colorado *
Paleontology in Colorado The location of the state of Colorado Paleontology in Colorado refers to paleontological research occurring within or conducted by people from the U.S. state of Colorado. The geologic column of Colorado spans about one third of Earth's history ...


References

{{Clear Shale formations of the United States Cretaceous Colorado Cretaceous Kansas Cretaceous Iowa Cretaceous Montana Cretaceous formations of New Mexico Cretaceous geology of Wyoming