Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...
to early
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58formation
Formation may refer to:
Linguistics
* Back-formation, the process of creating a new lexeme by removing or affixes
* Word formation, the creation of a new word by adding affixes
Mathematics and science
* Cave formation or speleothem, a secondary ...
in the central
High Plains High Plains refers to one of two distinct land regions:
*High Plains (United States), land region of the western Great Plains
* High Plains (Australia), land region adjacent to the Great Dividing Range
See also
* Altiplano (disambiguation)
The ...
of the western United States and the location of the
Ogallala Aquifer
The Ogallala Aquifer () is a shallow water table aquifer surrounded by sand, silt, clay, and gravel located beneath the Great Plains in the United States. One of the world's largest aquifers, it underlies an area of approximately in portio ...
. In
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 so ...
and
South Dakota
South Dakota (; Sioux language, Sioux: , ) is a U.S. state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Lakota people, Lakota and Dakota peo ...
Hemphillian
The Hemphillian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 10,300,000 to 4,900,000 years BP. It is usually considered ...
,
Clarendonian
The Clarendonian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 13,600,000 to 10,300,000 years BP, a period of .
It is ...
, and
Barstovian
The Barstovian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 16,300,000 to 13,600,000 years BP, a period of . It is usu ...
Texas
Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...
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 se ...
was elevated several thousand feet (thousands of meters) above sea level. The previous pattern of erosion of material from the northeast into the midcontinent was largely reversed so that material eroded from the Rocky Mountains was carried eastward on to expansive, geologically weathered plains. This material included quartz and feldspar from
Precambrian
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of th ...
granite, but also residue from the uplifted shale, limestone, and sandstone accumulated from the
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
and
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
periods, as well as igneous rock from
Cenozoic
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...
volcanism.
Many rivers carried this material from the mountains to the east. What each river carried from different parts of the mountains varied, changing also as the geography of the Rocky Mountains evolved.
Owing to the low amount of sloping, vast quantities of sand and gravel were dumped on the plains in broad and overlapping
alluvial fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
s rather than being carried into the Mississippi River.
Over this time, many volcanic ash falls occurred and have been identified to their source, including several from the Yellowstone Supervolcano and two from the Jemez Caldera.
Lithostratigraphy
From bottom to top, the Ogallala was deposited as overlapping
alluvial fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...
s of sand and gravel, displaying bedding resulting from river flows and flooding. Much of this material remained unconsolidated; however, significant modification occurred as the material deepened, particularly with the cementation of the sand and gravel with calcium carbonate and
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...
.
The top of the Ogallala is defined by a thick cap rock of hard limestone or
caliche
Caliche () is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regio ...
, broadly called "algal limestone". Patterns of thin lines in the "limestone" suggested algal origin to frontier geologists (calcite and silica deposited within mats of
algae
Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...
under a shallow lake). However, the material is determined to be mostly caliche that formed after deposition of the sand and gravel from the mountain rivers was sharply reduced and the climate became arid or semiarid. This extensive and resistant material established the broad flat expanses of the High Plains. Most of the material that overlies this bed is wind-blown Pleistocene loess, while the Blanco Formation overlies the Ogallala in Texas and southwestern Kansas. Much of the underlying sand and gravel is unconsolidated or poorly consolidated such that it easily erodes away, leaving in western Kansas, Nebraska, and Texas a range of dramatic high bluffs that define the boundary of a "high plain".
The lowest, oldest deposits filled in the shallow valleys of the weathered rocks that were exposed at that time, which ranged in age from Cretaceous in Nebraska and Kansas to Permian in Texas. As the rivers built up the alluvial fans, subsurface water circulation varied, resulting in localized formation of caliche, and shallow lakes formed between natural levees, which deposited limestone. These consolidated lenses, distributed in within the middle portion, do not represent any particular
horizon
The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether ...
; but do cap intermediate bluffs below valley rims. Because of their bluff-forming nature and resemblance to concrete or mortar, these lenses are described as "caprock" or "mortar beds". Various ash falls were locally deposited, supplying silica for the formation of agate and chert in the upper portion and opal in the lowest portion.
Other than the lower opaline sandstone zone and the "Algal limestone", the Ogallala has no persistent lithological marker beds beyond the stated generalities of local chert beds in the upper zone and local caliche "mortar bed" lenses in the middle zone. It may be the case that consolidation of the caliche of the "Algal limestone" occurred broadly at the same time but in whatever material was at the local surface after deposition of the sand and gravel ceased. And, while the "Algal limestone" is a marker bed, it does not establish correlation of the specific beds in which the caliche formed.
Biostratigraphy
Faced with particularly rich collections of terrestrial Neogene vertebrate fossils, Nebraska geologists recognized the distinct
North American land mammal age
The North American land mammal ages (NALMA) establishes a geologic timescale for North American fauna beginning during the Late Cretaceous and continuing through to the present. These periods are referred to as ages or intervals (or stages when re ...
s (NALMA) within the Ogallala;
Clarendonian
The Clarendonian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 13,600,000 to 10,300,000 years BP, a period of .
It is ...
,
Hemphillian
The Hemphillian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 10,300,000 to 4,900,000 years BP. It is usually considered ...
, and
Blancan
The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of .biostratigraphic basis. The Ogallala's rich grass seed record was also used to develop biostratigraphic successions within the unit. Unfortunately, no correlation with any consistent rock bedding can be established over the Ogallala's range.
These Nebraskan biostratigraphic classifications elevated the Ogallala to group status with three members Kimball (youngest), Ash Hollow, and
Valentine
A valentine is a card or gift given on Valentine's Day, or one's sweetheart.
Valentine or Valentines may also refer to:
People and fictional characters
* Valentine (name), a given name and a surname, including a list of people and fictional char ...
(oldest). Kansas geologists attempted to carry these classifications into the state; but, the state has since abandoned the three divisions. Like the remainder of High Plains states, Kansas considers the Ogallala to be a formation. However, it is convenient to informally divide the Ogallala into the three general zones with these names:
* Kimball - the upper zone with the "Algal limestone", agate, and chert beds
* Ash Hollow - the broad middle zone with the additional caprock "mortar beds"
*
Valentine
A valentine is a card or gift given on Valentine's Day, or one's sweetheart.
Valentine or Valentines may also refer to:
People and fictional characters
* Valentine (name), a given name and a surname, including a list of people and fictional char ...
Paleontology in Nebraska The location of the state of Nebraska
Paleontology in Nebraska refers to paleontological research occurring within or conducted by people from the U.S. state of Nebraska. Nebraska is world-famous as a source of fossils. During the early Paleo ...
*
Blanco Canyon
Blanco Canyon is a canyon located in the U.S. state of Texas. Eroded by the White River into the Caprock Escarpment on the east side of the Llano Estacado, the canyon runs for in a southeasterly direction, gradually widening from its beginning ...