Geological Section of Nebraska
Introduction
This bulletin reviews the geological formations of Nebraska, focusing on their age, lithological characteristics, and uses for oil company geologists studying deep wells in the state. It emphasizes stratigraphic paleontology, economic geology, groundwater, and soils. This detailed examination aids in understanding the subsurface architecture and resource potential of the region.
Composition of the Land
Nebraska's geological formations include:
Unconsolidated sediments (mantlerock) shaped by wind (loess), streams (alluvium), and glaciers (till). These surficial deposits vary in thickness and composition across the state.
Widespread sedimentary bedrock (shale, mudstone, sand, sandstone, limestone). These rocks, formed over millions of years, record different depositional environments and geological events.
Deep-seated granite and granite-like rocks of the Precambrian basement. These ancient rocks form the foundation upon which the sedimentary layers were deposited.
The sequence, nature, and thickness of these rocks are understood through studies of outcropping formations and deep well samples. The Nebraska Geological Survey has published deep well records available for review, providing essential data for subsurface interpretation and resource exploration.
Structure
Bedrock formations generally lie nearly flat with a low westward dip, reflecting the stable tectonic setting of the region. The oldest sedimentary rocks are buried deeply, but exposed in southeastern counties due to regional uplift and erosion. Regional arches (anticlines), faults, and basins modify this attitude, creating structural complexities that influence hydrocarbon accumulation and groundwater flow. Major structures include the Cambridge, Table Rock, Redfield, and Richfield arches, and the Julesburg, Central Nebraska, and Forest City basins. The Humboldt fault in Richardson County has a displacement of about 1000 feet, indicating significant crustal movement. Deep primary granite underlies Nebraska, forming the Nemaha Mountains, a buried granite ridge west of the Humboldt fault. The presence of these buried structures underscores the complex geological history of the region.
Sedimentary Rocks
The rocks were not formed by continuous deposition. Some are of marine origin, deposited in ocean or sea waters. Others were laid down on land, classified as continental origin. Unconformities (breaks in rock deposition) mark system, subsystem, series, and group boundaries, caused by shifts in elevation, erosion, and deposition. These breaks in the geological record provide insights into past sea-level changes, tectonic events, and erosional processes. Sedimentary rock thickness varies from about 500 feet in Pawnee County to 9,000 feet or more in western counties, reflecting differences in basin subsidence and sediment accumulation.
Classification and Correlation
The State Geological Survey has studied and correlated rocks with bordering states. Subsurface formations were classified through deep well cuttings and cores from water supply, and oil and gas exploration. These correlations help to establish a regional stratigraphic framework and understand the geological evolution of the area.
Classification uses both time and rock terms: periods, epochs, ages, stages, and substages. Rock systems, series, groups, formations, and members were deposited during these time units. The periods and systems in Nebraska, from youngest to oldest, are Quaternary, Tertiary, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, Devonian, Silurian, Ordovician, Cambrian, and Pre-Cambrian. This hierarchical classification system provides a framework for organizing and interpreting the geological history of Nebraska.
History of Geologic Investigations
Exploration began with Lewis and Clark (1806) and Major Long (1819). Formal geological exploration started with Meek and Hayden and many persons affiliated with State and Federal geological surveys, colleges, universities and museums or with oil companies have contributed significantly to the present knowledge of the geology of the Northern Mid-Continent Region and the Rocky Mountain Region. Their work has laid the foundation for our current understanding of Nebraska's geological history and resource potential.
Cooperative Survey
Cooperative work has occurred between the Kansas, Missouri, and Nebraska Geological Surveys, assisted by members of state surveys and oil company geologists. Studies of Permian formations involved the Texas, Oklahoma, Kansas, and Nebraska surveys, and the Kansas Geological Society. Cretaceous formations have been studied by multiple state surveys, federal geologists, oil company geologists, and independent contributors. These collaborative efforts have fostered a shared understanding of the region's geology and facilitated the exchange of data and ideas.
Systems and Subsystems
In classifying late Paleozoic rocks, the Carboniferous and Permian are ranked as systems, while the Mississippian and Pennsylvanian are subsystems. This structure maintains existing series groupings and allows flexible classification of the Mississippian and Pennsylvanian as systems or subsystems. This approach reflects the ongoing refinement of stratigraphic classifications as new data become available.
Order of Treatment
The geological section of Nebraska is outlined from youngest to oldest rocks. Columnar sections are used to visualize formation features, thickness, and sequence. Formations are described concisely, including classification, local and regional distribution, and economic relations. This systematic approach provides a comprehensive overview of Nebraska's geological framework.
Pleistocene System
Formations were laid down during glacial or Pleistocene period and include most of Quaternary time. They occur widely in the north-central states. They formed from glacial ice sheet debris, stream deposition of silt, sand and gravel, and wind erosion and deposition, as dune sand and loess. Lugn described them in detail. The Pleistocene deposits are critical for understanding recent geological processes and their impact on the landscape.
Outline of the Pleistocene Deposits of Nebraska
(Grouped by Age, from Youngest to Oldest)
Recent deposits: Alluvium, dune sand and the latest loess
Peorian loess: Yellowish, thickness 5-80', and some dune sand
Loveland loess: Thickness 5-100', and some dune sand, sand and gravel of Loveland age and the Loveland fossil soil
Upland formation: Thickness 5-30' + on the Grand Island formation; age of the fossil soil and re-worked materials on the Kansan drift
Kansan till: Boulders, till and sand of the drift region, 0-75' + ; Grand Island formation
Aftonian formation: Gravel, silt and clay on the Kansan drift, 0-60'; Fullerton formation
Nebraskan till: Glacial area, 50-100', sub-Nebraskan sands and gravels, 9-100' +
Holdrege sand and gravel
Geologic History
Features and occurrences of Pleistocene deposits are reviewed below in order of genesis.
Nebraskan Glacial Stage
Eastern Nebraska was comparatively rough, and the Missouri River Valley and main tributaries occupied current positions. The Nebraskan ice sheet planed high places, filled valleys with sand, gravel, and till, and covered the land with bluish-gray till with pebbles and boulders. It caused streams to fill valleys with sand and gravel wash from wide inwash belt, aggrading Holdrege formation. This glacial episode significantly reshaped the landscape and influenced the deposition of sediments.
Aftonian Interglacial Stage
Nearly flat plains were left as the Kansan ice-sheet retreated with Holdrege formation left to the west. Grasses and woody vegetation grew, and a soil profile was developed over time, forming gumbotil on flatter areas. Surface runoff reopened some pre-glacial valleys, eroding the drift plain, leaving till plains on the divides. The Fullerton formation was formed on the Holdrege sand plain via weathering, reworking, and deposition of loess-like materials. This interglacial period allowed for soil development and the establishment of vegetation.
Kansan Glacial Stage
Changes were similar to Nebraskan invasion i.e., valleys filled with glacial debris, Kansan drift left on Aftonian deposits, and sand plain to west was capped by Grand Island formation. The Kansan glaciation further modified the landscape and contributed to the deposition of glacial sediments.
Yarmouth Interglacial Stage
Drift region and sand plains were left nearly flat, with poor drainage, but surface runoff remained low. Soil, gumbotil, and reworked till deposits formed on flats, with valleys and drift hills in active erosion zones. Grand Island area became mantled with Upland formation, with relatively low surface runoff due to rapid subsurface drainage. Soil formation and erosion continued during this interglacial stage.
Illinoian Glacial Stage
The Illinoian ice-sheet in Illinois and southeastern Iowa caused a climatic shift, leading to land erosion and sand/gravel deposition in central/eastern valleys. Valley phase of Loveland formation clogged valleys and lifted flood plain levels. The Illinoian glaciation had indirect effects on Nebraska through climate change and sediment deposition.
Sangamon Interglacial Stage
This was a dry cycle in the Mississippi Valley region, resulting in the deposition of reddish Loveland Loess. Soil and gumbotil formed on the Illinoian drift in parts of Iowa and Illinois. The Sangamon interglacial period was characterized by dry conditions and loess deposition.
Wisconsin Stage
Leighton (1931 and 1933) classes the Iowan drift with the Wisconsin stage and divides Wisconsin in Illinois as substages, separated by loess deposits:
Mankato drift
Cary drift
Tazewell drift
Iowan drift
The Wisconsin drifts are relatively thin, not much weathered/leached, and represent relatively short substages. The interdrift loesses are thin boundaries of the drifts. Although the Wisconsin ice sheets did not reach Nebraska, substage lobes advanced into northwestern Iowa and southeastern South Dakota, accompanied by wet and dry cycles in Nebraska and adjacent areas. Wisconsin substages had a close relation to post-Sangamon cycles of sedimentation, erosion, loess leaching, soil formation, and terrace development.
Outwash from the Wisconsin ice-sheets was deposited in the valleys leading from the ice, which mainly did not reach Nebraska except locally along the Missouri River. Some effects and relations of glaciation:
Iowan glacial substage
The ice-sheet reached Iowa, Minnesota, and South Dakota. It brought a humid cycle with erosion, valley-excavation, high terrace development in central and eastern Nebraska and a thick soil (one-to-five feet) on the Loveland loess.
Peorian loess
A dry cycle followed Iowan ice retreat, with the Peorian loess proper deposited on the Loveland loess and high alluvial terraces along various valleys. This loess buried the Loveland soil several feet in depth, as may be observed in highway cuts in the Loess Hill Region.
Loess Deposits in General
The Loveland loess lies between the Illinoian and Iowan drift sheets in Iowa, but farther west, where the Illinoian is missing, it lies on the eroded Kansan and Nebraskan drifts and locally on bedrock formations. In Nebraska where the Iowan and other Wisconsin drifts are absent, the term Peorian applies to all post-Loveland loess.
The Peorian loess of Nebraska is being studied to correlate its zones with substages found between Wisconsin drift sheets in South Dakota, Iowa and Illinois. Loess deposits lie nearly flat on the Yarmouth formation west of the drift border, but are hilly where they mantle the drift hills. The topography of the Loess Plain, Loess Hill, and Loess-Drift Hill regions is largely inherited from the pre-loess topography. The loess deposits mantle more than half the area of Nebraska and are best represented in the Loess Plain, Loess Hill and the Loess-Drift Hill regions, but occur (at places) on the table lands and as small areas in the White River and Pierre Hill regions. Loess deposits have significantly influenced the landscape and soil composition of Nebraska.
Valley Fill
Soundings show that valley fills are of composite age, from Nebraskan to Recent. For example, the channel-fill of ancient Missouri River Valley is composed of variable thicknesses of Nebraskan till, Holdrege-Grand Island sand and gravel, Kansan till, alluvial phase of Loveland formation, and shallow recent alluvium. Some rivers are in pre-glacial channels, filled during Nebraskan time, reopened in Aftonian time, refilled during Kansan glacial stage, opened again in Sangamon time, refilled by deposition of the alluvial phase of the Loveland formation and eroded to their present condition during middle and late Wisconsin time, accompanied by the deposition of Peorian loess on the terraces. The amount of recent alluvium in larger valleys of the state is much less than is generally supposed but is underlain by Pleistocene deposits. Valley fills provide important records of past geological events and hydrological conditions.
Buried Valleys
Some pre-glacial valleys were filled during Nebraskan/Kansan ages, as in the Sandhill, Loess Hill, Loess-Drift Hill, and Loess Plain regions, and are now buried beneath later Pleistocene deposits. Fills of Nebraskan/Kansan age are deeply covered by Loveland wash, Peorian loess, or dune sand. Some buried channels extend southeastward from Sandhill/Loess Plain regions, and their locations are being explored by drilling. Located buried channels north of Dorchester and near Aurora contain more than 100 feet of sand and gravel fill. Buried valleys are potential sources of groundwater and may influence subsurface drainage patterns.
Changes in Surface Elevation
Wind, streams, and ice-sheets have lifted the surface of the land from the bedrock floor upward 100 to 200 feet or more. Erosion is lowering the surface, with colluvial materials low on the valley-sides and sediments on flood plains. This changes the topography, but loess and dune sand are still being formed faster than they are removed in some places. The interplay between depositional and erosional processes continues to shape the surface of Nebraska.
Dune Sand
Occupying more than one-fourth of Nebraska, much of dune sand is Pleistocene age, blown from Tertiary formations and Grand Island/Holdrege sand and gravel sheets. Sand-hills during their development contributed to loess deposition, and Loveland loess grades eastward from dune sand. Dune sand is now forming locally, and dust is carried eastward. The Sandhills region of Nebraska is one of the largest stabilized dune fields in the world.
Pleistocene Economic Relations
Sandy Pleistocene deposits are sources of much sand and gravel production and carry vast quantities of groundwater used for domestic purposes and pump irrigation. Thickest water-bearing deposits are the Holdrege, Grand Island sands/gravel, valley-fills, and buried channels. Pleistocene terraces are well-drained, afford good locations for homes, railroads, highways, and cities. Volcanic ash occurs in Love formations, but production is limited. Peorian loess is used in brick and tile manufacture. Loess/till/finer alluvial deposits have fertile soils. Sandy soils support grazing in hilly land and wild hay production in valleys with a shallow water table. The Pleistocene deposits provide valuable resources for Nebraska's economy.
Biological Relations
The advance and retreat of continental glaciers and the change in climate influenced the distribution of the plant and animal population of eastern Nebraska. Many plants perished during the advances of the ice, and others migrated to non-glacial areas. The shift in plant life during the glacial and inter-glacial stages was accompanied each time by a shift in the zonal distribution of the animals such as the fish, birds, and mammals. Skeletons of the musk ox, mastodon, elephant and bison, from the Pleistocene deposits, are now in the University Museum. The fossil record provides evidence of the dramatic changes in flora and fauna that occurred during the Pleistocene.
Tertiary System
The Tertiary rocks of Nebraska are of continental origin and lie unconformably on Cretaceous formations in the central and western areas of the state. Major workers are Hayden, Hatcher, Darton, Peterson, Barbour, Cook, Lugn and Schultz. The most comprehensive study was by Lugn. The Tertiary formations record a period of significant landscape evolution and mammalian diversification.
Classification of Tertiary System in Nebraska
Classification of marine/continental rocks is not on the same basis, so discrepancies arise in ages assigned. Ages given for subdivisions of the Tertiary of Nebraska are approximately the same as those of marine origin. Following is a table of the Tertiary formations of Nebraska and their age correlation condensed after A. L. Lugn, 1939:
I. Pliocene
Ogallala group, Darton 1899, redefined by Lugn 1939, thickness 400' or less:
Kimball limestone formation: Gray, algal limestone, at top, 2'-3'; gray to buff shale in middle, about 20'-30'; caliche sandstone at base, 8'-10'; combined thickness 30'-50'
Sidney gravel, Lugn and others 1939, channel or basin deposit in southwestern Nebraska, northeastern Colorado and at places in northwestern Kansas, yellowish to dark brownish, fine to coarse, with some pebbles, thickness variable, 0-50'
Ash Hollow formation, Engelmann 1858, redefined by Lugn, known as the "mortar beds," consists of light gray to dark gray and buff to yellowish irregular beds or lenses of silt, sand and gravel and of "mortar bed" zones, also some volcanic ash; thickness, 100'-250'
Valentine formation, Barbour and Cook 1917, light gray to slightly buff, friable sand, with some irregular indurated material, early Pliocene age, and probably in part upper Miocene, 50'-200'
II. Miocene
Hemingford group, Schultz 1939, middle Miocene age, about 500' or less:
Sheep Creek formation, Matthew and Cook 1909, predominantly pinkish and greenish sand and sandy clay, partially consolidated into mortar beds; divided by erosional unconformities into three members, but no complete development of all three members is observed in a single area; age, upper Miocene; combined thickness, about 400':
Box Butte member, Cady 1940, upper zone consisting of greenish sandy clay at top and greenish to grayish sand below; lower zone largely pinkish to brownish clay, age, late upper Miocene, combined thickness about 85'
Sand Canyon member, Elias 1942, greenish above, grayish below, largely sand, with irregularly cemented mortar beds throughout the section, and a thin dark-gray volcanic ash bed at base, age, upper Miocene, combined thickness about 145'
Spottedtail member, Elias 1942, largely pinkish to greenish-gray, sand and fine sandy clay; upper part partially cemented sand as friable beds, about 105'; lower part predominantly sandy clay without concretions, about 60'; combined thickness about 165'; age, upper Miocene
Marsland formation, Schultz 1938, the "Upper Harrison,:" buff to reddish brown above and more gray below, consists of soft sandstones; combined thickness, 150' or less; age, early upper Miocene
Arikaree group, Darton 1899, redefined by Lugn 1939, age, lower Miocene, thickness about 400-500':
* Harrison formation, Hatcher 1902, unconsolidated fine gray sand with "pipy" concretions in lower 100', but smaller than those of the Monroe Creek; thickness about 200'. Contains the Agate Springs bone fossil deposits.
* Monroe Creek formation, Hatcher 1902, upper 100'-150' composed of pinkish to buff sandy silt and clay with layers of cemented concretions and scattered large concretions; lower 185'-220' composed of gray medium textured, massively bedded sand with large "pipy" concretions; combined thickness, 285'-370'.
* Gering sand formation, Darton 1889, upper part gray, medium fine, massive sand; lower part gray, bedded to cross-bedded channel deposits, combined thickness, 100'-230'
III. Oligocene
White River group, Meek and Hayden 1858, about 325'-700':
Brule clay formation, Darton 1899, upper member pinkish, massive silty clays with thin layers of volcanic ash and sand, 250'-500'; lower member, pink, sandy clays and channel sandstone, 125'; combined thickness, 325'-600'
Chadron formation, Darton 1899, composed of greenish to buff clay and silt, and a channel sandstone locally at the base, marking an unconformity, 50'-100'
IV. Eocene, absent in Nebraska.
Distribution of Formations
The Chadron formation outcrops north of Pine Ridge, from Wyoming eastward through Sioux and Dawes, and into South Dakota; it also outcrops in the North Platte Valley in western Scotts Bluff County into Wyoming; underlies the western part of the state west of a line between Keith and Cherry counties.
The Brule clay is exposed north of Pine Ridge, and in the North Platte, Pumpkin Creek and Lodge Pole valleys. It underlies western Nebraska farther east than the Chadron and extends into Colorado, Wyoming, and South Dakota. Its thickness decreases southward and eastward from Sioux County.
Formations of the Arikaree group are best exposed in the Pine Ridge Escarpment of Sioux, Dawes and Sheridan counties. They reach southward from Pine Ridge to the Platte Valley beyond which there are outliers of the Monroe Creek in Wild Cat Ridge and in the north border of Cheyenne Table. Arikaree beds thin out to the east in Sheridan County and it is difficult to distinguish and map their formations as such northeastward where it was called Rosebud formation.
The Hemingford formations are exposed in two-thirds of Box Butte and form a belt south of the Pine Ridge Escarpment.
The Ogallala beds rest unconformably on the eroded edge of the Hemingford and Arikaree formations and extend eastward on the Cretaceous formations.
Its Kimball and Ashhollow beds reach southward through eastern Colorado, western Kansas and Oklahoma to Texas and New Mexico, but only a short distance westward in Wyoming.
Evolution of Grasses
Some present-day grasses and larger mammals originated in the Tertiary of Nebraska. M. K. Elias (1942, pp. 1-176), Paleontologist of the Nebraska Geological Survey, prepared the following statement regarding the grasses and their relation to ancient animals. The appearance and rapid spread of prairie grasses soon after the beginning of Miocene caused changes in adaptation of the horses and other herbivorous mammals from browsing to grazing habit, and evolution of teeth changed from cuspidate to hypsodont type. The earlier horse changed rapidly into a horse-like creature of donkey size. Rocks of Nebraska have fossil seeds of grasses, characterized by M. K. Elias, that indicate the evolution of herbs and successive major migrations. Comparative analysis shows, the Late Tertiary rock indicates two major climatic cycles in the late Tertiary time, roughly corresponding to the Miocene and Pliocene. The leveling of the Tertiary prairie and the appearance of extensive swamps and lakes marked the end of Ogallala time and uplift, followed by a cycle of erosion, marked the beginning of the Pleistocene. The evolution of grasses and grazing mammals is a key feature of the Tertiary period in Nebraska.
Extinct Mammals
The skeletons of extinct animals from the Tertiary and Pleistocene of Nebraska in the University of Nebraska State Museum in Lincoln are arranged from more ancient to more recent to show evident evolutionary changes. Many extinct animals lived here during this time. Some originated here but were exterminated chiefly by the severe climatic reverses in the Pleistocene. The fossil record provides valuable insights into the evolutionary history of mammals in North America.
Tertiary Economic Relations
Stone, sand and gravel, some volcanic ash, and large quantities of water occur in the Ogallala and Arikaree formations. Some quarrying, sand, and gravel production occur. Much groundwater is pumped for domestic purposes and irrigation. The deep soils on the Tertiary beds are cultivated, the sandy soils are used for grazing, and the rough stony lands are used for tree land. The Tertiary formations are important sources of water and construction materials in Nebraska.
Cretaceous System
Formations of this system underlie most of Nebraska, except in the southeastern counties. Their combined thickness in the western counties is 4000-5000 feet, but decreases rapidly eastward. The system rests unconformably on pre-Cretaceous rocks and its upper surface was peneplained in early Tertiary time, making the unconformable contact with Tertiary and later rocks. The Cretaceous formations are of marine origin, except the main sandstones, which are fresh water continental deposits. The Cretaceous period was a time of significant marine sedimentation and dinosaur evolution.
Outline of the Cretaceous Groups and Formations in Nebraska
Montana group, Eldridge 1888 and 1889:
Lance formation, Hatcher 1903
Fox Hills sandstone, Meek and Hayden 1862, probably absent in Nebraska
Pierre shale formation, Meek and Hayden 1862
Colorado group, Hayden 1876:
Niobrara chalk formation, Meek and Hayden 1862
Carlile shale formation, Gilbert 1906
Greenhorn limestone formation, Gilbert 1906
Graneros shale formation, Gilbert 1906
Dakota group, Meek and Hayden 1862:
Omadi formation, new name, Condra and Reed
Fuson shale formation, Darton 1901
Lakota sandstone formation, Darton 1899
The Cretaceous group names, except the Dakota, are not used generally. Also the name, Benton, given by Meek and Hayden in 1862 to include the section now classed as Carlile, Greenhorn and Graneros, is no longer in use except at places where the Greenhorn is poorly developed or absent.
Composite Section of the Cretaceous System in Nebraska:
Montana Group - Lance Formation
Exposed in Wyoming, just west of Scotts Bluff County, Nebraska. Composed of greenish gray argillaceous sands, some sandstone, and lignite coal. It underlies western Scotts Bluff County, Banner County, and part of Kimball County, in a thickness of 90-125 feet.
Fox Hills Sandstone
Consists of gray to yellowish sandstones and has been mapped in Colorado near the Nebraska line and at places in Wyoming, South Dakota, and North Dakota. Thus far it has been found neither in deep wells, nor in the outcrops of western Nebraska. A sandy transitional shale exists at the top of the Pierre, which states have correlated as the Fox Hills.
Pierre Shale Formation
From Pierre, South Dakota, it consists of black, dusty gray, and brownish clay shales, thin layers of bentonite and cone-in-cone structure, indurated shaly chalk, thin shaly limes, well defined concretionary zones, and thin sandstones in the upper part. The thickness is less than 100 feet along its easternmost outcrop in the state and increases westward to 2000 or 3000 feet. It is eroded through at places on the Cambridge Arch. The members of the Pierre are recognized by their sequence, lithology and faunal content in which species of Baculites are the best horizon markers.
The following members are recognized from the outcrops of east central South Dakota, east central and southern Nebraska and northwestern Kansas:
Elk Butte member: Located between Wakpala and Elk Butte in Carson County, South Dakota, missing in Kansas and southern Nebraska, but occurs in northeastern Nebraska, Gregory County, SD, and northwestward. It consists of medium dark gray, fine grained argillaceous shales, thin bentonite layers and some calcareous concretions, but it has not been differentiated in western Nebraska nor in southwestern South Dakota.
Mobridge member: Occurs in Gregory County, South Dakota, extending into northeastern Nebraska, boundaries are not well defined in some areas, thickness of about 350 feet, consisting of beds of chalk, chalky shale, sandy shale and layers of sandstone.
Virginia Creek member: Consists of grayish argillaceous shale with calcareous bands and concretions near the top. Its thickness in South Dakota and northeastern Nebraska is approximately 130 to 140 feet. The Salt grass zone has a thickness of about 60 feet in northwestern Kansas.
Sully member: Includes the Verendrye shale, Agency-Oacoma, and the Crow Creek. The basal sands and chalk beds of the Sully member are called the Crow Creek zone by the South Dakota Geological Survey. In northwestern Kansas the Sully member correlates with the Lake Creek shale, and the Weskan.
Gregory member: Correlates with the upper Sharon Springs and includes a shaly zone and a chalky zone.
Sharon Springs member: Is a dark fissile shale carrying bituminous material and the scales and other remains of fish, a good horizon marker.
Colorado Group - Niobrara Chalk Formation
Named from Niobrara, Nebraska this well-known formation. It includes lead-gray shaly Smoky Hill chalk, and gray to yellowish, massive Fort Hayes limestone. This formation occurs from Canada to Mexico, it is exposed as chalk bluffs in the Republican Valley. According to the text. Nebraska has more chalk rock than England. The Niobrara Chalk is a significant geological formation known for its fossil content and economic importance.
Carlile Shale Formation
Includes the Codell sandstone, the Blue Hill shale, and the Fairport shale. The combined thickness in Nebraska is about 150 feet in the east, 250 feet in the southwest, and 400 to 500 feet in the northwest. The fine grained, grayish Codell sandstone underlies southwestern, western, and northern Nebraska. The bluish gray Fairport shale lies between the Greenhorn limestone. The Carlile Shale is a complex formation with varying lithology and thickness across Nebraska.
Greenhorn Limestone Formation
Occurs widely in the Great Plains region. It outcrops at Homer, Milford, east of Hebron and at Hubbell, Nebraska. The Greenhorn formation is composed of thin, medium soft, gray limestones interbedded with gray shales. The upper part includes Inoceramus labiatus. The presence of Inoceramus labiatus is a key marker for identifying the Greenhorn formation.
Graneros Shale Formation
Consists of dark gray plastic shale with thin calcareous layers, some sand and sandy shale, and carbonaceous material in the basal part. This formation changes facially westward. Belle Fourche shale occurs above and gray calcareous Mowry shale below in western Nebraska, eastern Wyoming and southwestern South Dakota. The Graneros Shale exhibits facies changes, reflecting variations in depositional environments.
Dakota Group
It has been separated as three formations. It is proposed the name be changed to Omadi sandstone, for the so-called Dakota formation, to include the section lying between the Fuson and Graneros shales. The type locality is selected and a type reference seciton is set forth.
Cretaceous Economic Relations
Coal occurs in the Lance, Fox Hills, Omadi and Lakota formations, in Colorado, Wyoming, and South Dakota. In Nebraska it has been found, but under conditions not now favorable for economic production.
Bentonite is produced from the middle Cretaceous shales areas adjacent to the Black Hills, but only small quantities of it are produced in Nebraska, from the Pierre shale.
Stone is produced from the Niobrara, Greenhorn, Omadi and Lakota formations. Some of the Greenhorn limestone is sawed as fence posts. Portland cement. Clay from the Fuson shale is used in brick and tile manufacturing. Some sand from the Dakota group is used as moulding sand. The Cretaceous formations provide various resources, including coal, bentonite, and stone.
Jurassic System
Occurrence of Jurassic Rocks in Nebraska
Jurassic rocks do not outcrop in Nebraska But they are present in the subsurface within the vicinity of the western one half of the area of Nebraska. This area occurs just below the oldest Cretaceous and just above the Triassic layers. The thickness of the Jurassic is from 10 ft to 542 feet. It can be satisfactorily correlated with the Morrison and Sundance formations. The Jurassic system represents a period of significant geological and biological changes.
Jurassic Economic Relations-
The sundance formation is an important Oil producer where the upper sundance is the discovery horizon of the ENtrada level. The Basal Sundance sandstone is highly productive where it is the Nugget level. The Jurassic formations are important sources of hydrocarbons in some regions.
Triassic System
The main topic that encompasses the triassic system in Nebraska is over the limit between Triassic and Permian system. It had seemed as thought the permian phosphoria was in the spearfich formation, but it was shown that both where different. Rocks correlated with spearfish appear in North western Nebraska. They were about 125 feet in thickness and disappear due to eastward Truncation. There is not much Economic importance associated except to show good location for either crop or livesstock farming. The Triassic system marks a transition between the Paleozoic and Mesozoic eras.