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The Greenhorn Limestone or Greenhorn Formation 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 exp ...
in the Great Plains Region of the United States, dating to the
Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...
and
Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...
ages of the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
period. The formation gives its name to the Greenhorn cycle of the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
.


Description

The formation was named for the Greenhorn Station on Greenhorn Creek in
Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...
in 1896 by
Grove Karl Gilbert Grove Karl Gilbert (May 6, 1843 – May 1, 1918), known by the abbreviated name G. K. Gilbert in academic literature, was an American geologist. Biography Gilbert was born in Rochester, New York, and graduated from the University of Rochester. ...
; and it is the namesake of the Greenhorn Marine Cycle of the Cretaceous
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
. With the underlying Graneros Shale and
Dakota Formation The Dakota is a sedimentary rock, sedimentary geologic unit name of Formation (stratigraphy), formation and Group (stratigraphy), group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and ...
, it records the progressive stage of Greenhorn Marine Cycle while the overlying Carlile Shale records the regressive stage. The Greenhorn unit name is recognized in the Great Plains Region from
Minnesota Minnesota ( ) is a U.S. state, state in the Upper Midwestern region of the United States. It is bordered by the Canadian provinces of Manitoba and Ontario to the north and east and by the U.S. states of Wisconsin to the east, Iowa to the so ...
and
Iowa Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...
to
New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
to
Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
and the Dakotas. In much of Alberta and Saskatchewan, the "Second White-Specked Shale" contains limy equivalents of the Greenhorn. In
Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...
, the Greenhorn Formation is divided into the (lowest) Lincoln Limestone, Hartland Shale, Jetmore Chalk, and (highest) Pfeifer shale members, each noted by changes in chalkiness and limestone rhythmite patterns. In eastern
Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...
and western Kansas
Hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for hydrocarbon deposits, particularly petroleum and natural gas, in the Earth's crust using petroleum geology. Exploration methods ...
, the divisions are Lincoln Limestone, Hartford Shale, and Bridge Creek Limestone. In other states, where the formation is less developed, the unit is not subdivided and is named the Greenhorn Limestone, as a formation or as a member of another formation, e.g., Cody Shale, Colorado Shale, and Mancos Shale. Within Kansas and a small neighboring portion of Nebraska, the Greenhorn Formation is particularly noted for its uppermost bed, the Fencepost limestone, from which the Kansas stone posts were quarried. The combination of the toughness of the Fencepost limestone with the softness of the chalk and shale above and below as resulted in the formation of the main range of 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 diss ...
north and west of Salina. In 2018, Kansas Legislation HB 2650 designated the Greenhorn Limestone formation, specifically "the famous "post rock" limestone" bed of that unit, to be the state rock of Kansas.


Lithologic character

The Greenhorn Formation is characterized as shale to chalky shale, light bluish-gray in color, with rhythmically repeating beds of chalk or limestone that become
marl Marl is an earthy material rich in carbonate minerals, Clay minerals, clays, and silt. When Lithification, hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. M ...
y closer to 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 great-circle distance, straight-line distance from the northernmost part of Western Can ...
. The shale can weather to buff color under hilltops. Exposures show many thin, rust-colored
bentonite Bentonite ( ) is an Absorption (chemistry), 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 swelli ...
beds (named for the Old Benton Limestone classification that the Greenhorn and other names replaced), several of which are consistent and widespread marker beds. These orange seams in the weathered shale and the yellow/orange stainings of some of the weathered limestones in the Greenhorn are associated with volcanic events in the Sevier orogeny. The oceanic iron (Fe) and volcanic sulfur (S) that precipitated with the
volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...
into the calcareous mud (CaCO3) formed
pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...
(FeS2), which later altered to selenite (CaSO4·2H2O),
siderite Siderite is a mineral composed of iron(II) carbonate (FeCO3). Its name comes from the Ancient Greek word (), meaning "iron". A valuable iron ore, it consists of 48% iron and lacks sulfur and phosphorus. Zinc, magnesium, and manganese commonly ...
(FeCO3), and
limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...
(FeO(OH)·nH2O), leading to the yellow to orange staining.


Paleofauna

The formation is recognized for its sequence of
index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...
s, including
oysters Oyster is the common name for a number of different families of Seawater, salt-water bivalve molluscs that live in Marine (ocean), marine or Brackish water, brackish habitats. In some species, the valves are highly Calcification, calcified, a ...
,
Ammonoidea Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
,
Belemnitida Belemnitida (or belemnites) is an extinct order (biology), order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous (And possibly the Eocene). Unlike squid, belemnites had an internal skeleton that made up the cone ...
, and
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 environment ...
. * Ammonoidea and Belemnitida were common, free-swimming
pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...
predators, reflecting abundant fish populations in a broad sea. * The occasionally low oxygen bottom was soft mud to which the broad, flat Inoceramidae were particularly adapted. These stationary
filter feeder Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a s ...
s are evidence of currents sufficient to carry enough food to support such very large invertebrates, but not so strong as to suffocated them with drifting mud. * Oysters are smaller filter feeders. Several species were common. Larval oysters require clear, mud-free hard surfaces to attach and develop into mature oysters. '' Ostrea congesta'' was particular for colonizing the top valve of large Inoceramidae and oyster-encrusted shell fragments litter slopes of eroded Greenhorn. * In 2022, Schumacher & Everhart described '' Plesioelasmosaurus'' based on a partial skeleton previously known as the " Holyrood elasmosaur" from the Greenhorn Limestone (Lincoln Limestone Member, Calcarenite beds). The Greenhorn marine cycle was the deepest and broadest stage of the Western Interior Seaway, and supported large sharks and the largest of marine reptiles, including
Mosasauridae Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains were discovered in ...
and Pliosauroidea. Fossilized
driftwood Driftwood is a wood that has been washed onto a shore or beach of a sea, lake, or river by the action of winds, tides or waves. It is part of beach wrack. In some waterfront areas, driftwood is a major nuisance. However, the driftwood provides ...
and dinosaur remains have been recovered from the formation; such remains are presumed washed into the sea by rivers swollen by the heavy rainfalls of the hothouse Cretaceous.Weishampel, David B; et al. (2004). "Dinosaur distribution (Late Cretaceous, North America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 574-588. . File:Styxosaurus and Xiphactinus.jpg, Paleoart depicting two large marine predators of the late Cretaceous;
Elasmosauridae Elasmosauridae, often called elasmosaurs or elasmosaurids, is an extinct family of plesiosaurs that lived from the Hauterivian stage of the Early Cretaceous to the Maastrichtian stage of the Late Cretaceous period (c. 130 to 66 mya). The taxo ...
and
Xiphactinus ''Xiphactinus'' (from Latin and Greek for "sword-ray") is an extinct genus of large predatory marine ray-finned fish that lived during the late Albian to the late Maastrichtian. The genus grew up to in length, and superficially resembled a garga ...
.


See also

* Volcanic mineralization of the Greenhorn sequence: **
Bentonite Bentonite ( ) is an Absorption (chemistry), 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 swelli ...
, sedimentary volcanic ash, general showing some weathered iron stain in the Colorado Group **
Pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...
, precipitation of volcanic sulfuric acid with oceanic iron as FeS2 **
Limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...
, pyrite in limestone weathered to HFeO2 ( rust stain or
yellow ochre Ochre ( ; , ), iron ochre, or ocher in American English, is a natural clay earth pigment, a mixture of ferric oxide and varying amounts of clay and sand. It ranges in colour from yellow to deep orange or brown. It is also the name of the col ...
) ** Selenite, CaSO4 associated with Bentonite seams and ochre *
List of dinosaur-bearing rock formations This list of dinosaur-bearing rock formations is a list of geologic formations in which dinosaur fossils have been documented. * List of stratigraphic units with dinosaur body fossils * List of stratigraphic units with dinosaur trace fossils ** ...
* List of fossiliferous stratigraphic units in Colorado *
List of fossiliferous stratigraphic units in Kansas This article contains a list of fossil-bearing stratigraphic units in the state of Kansas, Kansas, U.S. Sites See also * Paleontology in Kansas References

* {{DEFAULTSORT:Fossiliferous stratigraphic units in Kansas Fossiliferous st ...
* List of fossiliferous stratigraphic units in Minnesota *
Paleontology in Colorado Paleontology in Colorado refers to paleontological research occurring within or conducted by people from the United States, U.S. state of Colorado. The geologic column of Colorado spans about one third of Earth's history. Fossils can be found almo ...
*
Paleontology in Kansas Paleontology in Kansas refers to paleontological research occurring within or conducted by people from the United States, U.S. state of Kansas. Kansas has been the source of some of the most spectacular fossil discoveries in US history. The fossil ...
* Paleontology in Minnesota


References

{{Chronostratigraphy of Colorado, Mesozoic state=expanded Limestone formations of the United States Cenomanian Stage Turonian Stage Cretaceous geology of South Dakota Cretaceous Kansas Cretaceous Minnesota Cretaceous Colorado Cretaceous geology of Nebraska Cretaceous formations of New Mexico Symbols of Kansas Shale formations of the United States Marl formations