Tracing the origins and significance of the Miocene Ogallala Formation, Great Plains, USA

Abstract. The Great Plains of North America host a stark climatic gradient, separating the humid and well-watered eastern US from the semi-arid and arid western US, and this gradient shapes the region's water availability, its ecosystems, and its economies. This climatic boundary is largely set by the influence of two competing atmospheric circulation systems that meet over the Great Plains – the wintertime westerlies bring dominantly dry air that gives way to moist, southerly air transported by the Great Plains low-level jet in the warmer months. Climate model simulations suggest that, as CO2 rises, this low-level jet will strengthen, leading to greater precipitation in the spring but less in the summer and, thus, no change in mean annual precipitation. Combined with rising temperatures that will increase potential evapotranspiration, semi-arid conditions will shift eastward, with potentially large consequences for the ecosystems and inhabitants of the Great Plains. We examine how hydroclimate in the Great Plains varied in the past in response to warmer global climate by studying the paleoclimate record within the Ogallala Formation, which underlies nearly the entire Great Plains and provides a spatially resolved record of hydroclimate during the globally warmer late Miocene. We use the stable isotopes of oxygen (δ18O) as preserved in authigenic carbonates hosted within the abundant paleosol and fluvial successions that comprise the Ogallala Formation as a record of past hydroclimate. Today, and coincident with the modern aridity gradient, there is a sharp meteoric water δ18O gradient with high (−6 ‰ to 0 ‰) δ18O in the southern Great Plains and low (−12 ‰ to −18 ‰) δ18O in the northern plains. We find that the spatial pattern of reconstructed late Miocene precipitation δ18O is indistinguishable from the spatial pattern of modern meteoric water δ18O. We use a recently developed vapor transport model to demonstrate that this δ18O spatial pattern requires air mass mixing over the Great Plains between dry westerly and moist southerly air masses in the late Miocene – consistent with today. Our results suggest that the spatial extents of these two atmospheric circulation systems have been largely unchanged since the late Miocene and any strengthening of the Great Plains low-level jet in response to warming has been isotopically masked by proportional increases in westerly moisture delivery. Our results hold implications for the sensitivity of Great Plains climate to changes in global temperature and CO2 and also for our understanding of the processes that drove Ogallala Formation deposition in the late Miocene.

<p>The North American Great Plains are characterized by a sharp aridity gradient at around the 100<sup>th</sup> meridian with a more humid climate to the east and a more arid climate to the west. This aridity gradient shapes the region's agriculture and economy, and recent work suggests that arid conditions on the Great Plains may expand eastward with global warming. The abundant Neogene sediments of the Ogallala Formation in the Great Plains present an opportunity to reconstruct regional hydroclimate conditions at a time when <em>p</em>CO<sub>2</sub> and global temperatures were much higher than today, providing insight into the aridity and ecosystem response to warming. We present new paleosol carbonate δ<sup>13</sup>C and δ<sup>18</sup>O data (n=366) across 37 sites spanning the Great Plains and compile previously published measurements (n=381) to evaluate the long-term hydroclimatic and ecosystem changes in the region during the late Neogene. This study combines a spatial and temporal analysis of carbon and oxygen isotope data with reactive-transport modeling of oxygen isotopes constrained by climate model output, providing critical constraints on the paleoenvironmental and paleoclimatological evolution of the Neogene Great Plains. Carbonate δ<sup>18</sup>O demonstrate remarkable similarity between the spatial pattern of paleo-precipitation δ<sup>18</sup>O and modern precipitation δ<sup>18</sup>O. Today, modern precipitation δ<sup>18</sup>O over the Great Plains is set by the mixing between moist, high-δ<sup>18</sup>O moisture delivered by the Great Plains Low-Level Jet and drier, low-δ<sup>18</sup>O westerly air masses. Thus, in the absence of countervailing processes, we interpret this similarity between paleo and modern δ<sup>18</sup>O to indicate that the proportional mixing between these two air masses has been minimally influenced by changes in global climate and that any changes in the position of the 100<sup>th</sup> meridian aridity gradient has not been forced by dynamical changes in these two synoptic systems. In contrast, prior to the widespread appearance of C<sub>4</sub> plants in the landscape of the Great Plains, paleosol carbonate δ<sup>13</sup>C show a robust east-to-west gradient, with higher values to the west. We interpret this gradient as reflective of lower primary productivity and hence soil respiration to the west. Close comparison with modern primary productivity data indicates that primary productivity has declined and shifted eastward since the late Neogene, likely reflecting declining precipitation and/or a reduction in CO<sub>2</sub> fertilization during the late Neogene. Finally, δ<sup>13</sup>C increases across the Miocene-Pliocene boundary, which, consistent with previous studies, we interpret as a shift from a C<sub>3</sub> to a C<sub>4</sub> dominated landscape. We conclude that, to first order, the modern aridity gradient and the hydrologic processes that drive it are not strongly sensitive to changes in global climate and any shifts in this aridity gradient in response to rising CO<sub>2</sub> will be towards the west, rather than towards the east.</p>

The Cenozoic tectonic history of the Southern Rocky Mountains has been interpreted in some detail, but the corresponding history of the Great Plains has not received similar attention. This evaluation of significant datum levels in the Great Plains at different times during the Cenozoic indicates that during the Laramide, when the mountains possibly were uplifted more than six thousand meters, the Great Plains remained tectonically stable and was only slightly depressed locally by isostatic response. Post-early Oligocene (post-Wall Mountain Tuff) uplift of both the Great Plains and the Southern Rocky Mountains was of the same broad order of magnitude, and most of this uplift occurred after the end of deposition of the Ogallala Formation, 5 to 6 million years ago. Lack of tectonic effects in the Great Plains during Laramide time suggests that Laramide uplift of the mountains was the result of vertical rather than horizontal or compressional forces.

The principal control on landscape evolution in the central Great Plains of the United States over the past 10 m.y. is a contentious subject. New sedimentary data collected from Late Miocene Ogallala Group and Pliocene Broadwater Formation of the Nebraskan Great Plains demonstrates a twofold increase in the median grain size (from 20 mm to >40 mm) exported from the Rocky Mountains across the Miocene-Pliocene boundary. Paleoslope reconstructions derived from these data support the tilting of the Miocene Ogallala Group after 6 Ma, but demonstrate that the transport slope of the lower part of the unconformably overlying Pliocene succession is identical to the present-day slope. These data allow us to constrain the timing of differential uplift in the Great Plains to between 6 and 3.7 m.y.; the wavelength and short duration of this tilting are best explained by the initiation of localized dynamic topography. Our results also suggest a threefold to fourfold increase in specific stream power at this time, meaning that Pliocene rivers draining the central Rockies were considerably more competent than their Miocene predecessors. Incision during this period was not continuous. A significant episode of aggradation from 3.7 to 2.5 Ma is best explained by high rates of sediment supply relating to the warm, wet mid-Pliocene climate optimum. The modern pattern of incision on the Great Plains occurred from 2.5 Ma, and not from the end of the Miocene as is sometimes supposed, reflecting the onset of major Northern Hemisphere glaciation.

Research Article| February 01, 1987 Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension Roy T. Budnik Roy T. Budnik 1Bureau of Economic Geology, University of Texas, University Station Box X, Austin, Texas 78713 Search for other works by this author on: GSW Google Scholar Author and Article Information Roy T. Budnik 1Bureau of Economic Geology, University of Texas, University Station Box X, Austin, Texas 78713 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (2): 163–166. https://doi.org/10.1130/0091-7613(1987)15<163:LMROAR>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Roy T. Budnik; Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension. Geology 1987;; 15 (2): 163–166. doi: https://doi.org/10.1130/0091-7613(1987)15<163:LMROAR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Structural and stratigraphic evidence from the Ogallala Formation (Neogene) documents late Miocene tectonic activity within the Great Plains. Field and subsurface studies in the Texas Panhandle indicate that parts of the Amarillo uplift, a major element of the Pennsylvanian Ancestral Rocky Mountains, were elevated as much as 150 m during initial deposition of the Ogallala Formation. Reactivation of these basement structures occurred in response to Basin and Range extension and opening of the Rio Grande rift in central New Mexico and Colorado. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Origin of the late Quaternary dune fields of northeastern Colorado

Using phytolith assemblages to reconstruct the origin and spread of grass-dominated habitats in the great plains of North America during the late Eocene to early Miocene

The Rocky Mountains and adjacent western Great Plains share a common history of late Cenozoic stream incision. Both epeirogenic uplift and climate change (with no associated uplift) have been proposed as the cause of thissubcontinental-scale erosional episode. However, the lack of a well-defined Cenozoic paleoelevation history for the region has hampered our ability to distinguish between the two. A tilt analysis of the Cheyenne Tablelands in the western Great Plains of Wyoming and Nebraska provides us with a datum from which postdepositional changes in slope can be determined. Miocene to earliest Pliocene gravels of the Ogallala Group (17.5-5 Ma) cap the tablelands, which currently tilt down to the east at slopes as great as 10 - 2 . However, paleohydraulic analysis of the Ogallala gravels indicates depositional slopes of 10 - 3 to 10 - 4 , implying a postdepositional increase in tilt. If a hinge point at the eastern edge of the Cheyenne Tablelands is assumed, this tilt translates into differential uplift of 680 m (815-410 m) at the western edge of the Great Plains. Flexural isostatic rebound of the tablelands due to known young erosion in surrounding basins only produces a few hundred meters of uplift. Therefore, even if all of the recent erosion in the region can be attributed to climate change, the resulting rebound is insufficient to account for the observed uplift of the tablelands. Thus, the tilting of the Cheyenne Tablelands is most consistent with broad-wavelength tectonic uplift centered under the Rocky Mountains initiated during Ogallala deposition and continuing since deposition ceased.

Architecture, heterogeneity, and origin of late Miocene fluvial deposits hosting the most important aquifer in the Great Plains, USA

The Dynamic Holocene Dune Fields of the Great Plains and Rocky Mountain Basins, U.S.A.

Reconstructing late Miocene depositional environments in the central High Plains, USA: Lithofacies and architectural elements of the Ogallala Formation

An often-windy landscape with few major topographic features, poorly consolidated fine-grained geology, and limited and variable precipitation has endowed the Central and Southern Great Plains with the ideal environment for development and repeated reactivation of dune fields and sand sheets. Mapping efforts have documented large numbers, sizes, and a wide distribution of aeolian sand deposits, and the application of soil texture, geochemistry and other data have begun to enhance our perspective on these deposits. In recent decades, numerical dating techniques have spurred inquiry into the development of activation chronologies, which have defined periods of sediment flux and prehistoric droughts, including megadroughts commonly observed in other paleoclimatic records. Nearly thirty dune fields within the region have been investigated and dated with radiocarbon and luminescence techniques. Activation, which has usually been climatically forced, occurs when sediment becomes transportable under the prevailing wind regime. Currently, most dunes throughout the Great Plains are, however, inactive, but those that are active occur primarily along the Texas-New Mexico border region and in areas impacted by human activity such as cattle ranching and off-road vehicle recreation. With the major exception of the Nebraska Sand Hills, most dune fields of the Central and Southern Great Plains are associated geomorphically with and in some cases geochemically-linked to one or more fluvial systems. Dunes composed of fine sediments (silt, clay) also occur in the region—lunettes, or crescentic dunes associated with playa basins (Kansas through to Texas into New Mexico), and the parna dunes of the Oklahoma Panhandle. Adaptation of mineralogical and geochemical finger printing of aeolian sand deposits and potential sand sourcing has made possible the identification of provenances (e.g., the Miocene Ogallala Formation and late Pleistocene-modern river systems) and by implication the directions of formative paleowinds. Given the early indications of global warming and model scenarios for the future, the Central and Southern Great Plains may experience future intense and extended-duration droughts and the attendant reactivation of dune fields and sand sheets.

The present study defines the western limits of the Ogallala Formation (upper Tertiary) and documents the late Cenozoic geology of the region including fragmentary deposits of early Pleistocene age and molluscan faunas of Wisconsinan and Holocene age. The Ogallala was deposited during the latest Tertiary episode of alluvial deposition throughout the Great Plains, where this formation has been documented extensively. However, the Ogallala has not been previously traced to its primary sediment source in the Rocky Mountain belt. Streams feeding the extensive Great Plains alluvial blanket of Ogallala sediments headed in the mountain belt and flowed east and southeast, but identification of these streams and their sediments is obscured in northern New Mexico by extensive volcanic activity. Farther north in Colorado, the extensive erosional belt of the Colorado piedmont has removed evidence of late Tertiary history. Therefore, central and southern New Mexico are the only regions where tracing of headwater sources is possible. This unique situation is largely the result of the late- and post-Ogallala structural history of the region. Extensive warping of the uppermost Tertiary surface occurred here, with less differentiated uplift of the mountain belt to the west than the uplift that occurred farther north.

This report describes the geography, geology, and ground-water resources of Stanton county, in southwestern Kansas. Stanton county has an area of about 685 square miles, and lies in the High Plains section of the Great Plains province. Most of the area is drained by Bear creek and Sand arroyo, which are ephemeral streams. The population of the county was 1,443 in 1940, and is principally rural. Johnson City, the county seat, is the largest community and in 1940 had a population of 524. Wheat farming is the chief industry. During the last several years the county has experienced partial or complete crop failure owing to drought conditions. The climate is the semiarid continental type, the average annual precipitation being about 17 inches. The exposed rocks are sedimentary and range in age from late Cretaceous to Quaternary. Undifferentiated Pliocene (including the Ogallala formation) and Pleistocene sediments lie at or near the surface over nearly all of the county. They are overlain by a thin mantle of loess in the interstream areas, by thin deposits of alluvium in the stream valleys, and by dune sand south of Bear creek. The Cockrum sandstone (Upper Cretaceous) is the oldest formation exposed. Unexposed rocks beneath the Cockrum sandstone comprise the Kiowa shale and Cheyenne sandstone of early Cretaceous age, and rocks of the Triassic (?) and Permian systems. The pre-Tertiary strata in the northern part of the county form a buried trough, the north flank of which has been faulted in post-Tertiary lime. The north flank of the buried trough is the south flank of the Syracuse anticline of southern Hamilton county. The water table beneath Stanton county ranges in depth from less than 25 feet to 250 feet below the surface, and in general has an easterly slope. In the western part of the county, where the Cockrum sandstone is the principal water-bearing formation, the water table slopes as much as 60 feet to the mile, but in the eastern part of the county, where the Ogallala formation is the principal water bearer, the slope is very gentle and locally is as little as 4 feet to the mile. The difference in slope is due principally to the difference in permeability of the water-bearing materials in the two areas. The ground-water reservoir is recharged in three ways; by downward-percolating water that falls within or just west of the county, by influent seepage from Bear creek and possibly a small amount from Sand arroyo, and by water entering the Ogallala formation from the Cockrum sandstone at places where the latter formation thins between the Ogallala formation and the underlying impervious Kiowa shale. Water is discharged from the underground reservoir through wells and by lateral migration into areas to the east. All the public, railroad, and irrigation water supplies and most of the domestic and stock supplies are obtained from drilled wells. At the time the investigation was made there were four active irrigation wells in the county, all of which obtained water from sands and gravels of the Ogallala formation. The most favorable place for future irrigation development is a roughly triangular area in the northeastern part of the county where the water table ranges from less than 50 feet to less than 100 feet below the surface. The ground water is hard, but in general is of satisfactory quality for most purposes. In general the waters from the Ogallala formation and the Cockrum sandstone are slightly better than the water from the Cheyenne sandstone. The Ogallala is the principal water-bearing formation in the county. It consists mostly of calcareous gravel, sand, and silt, which are consolidated locally to form conglomerate, sandstone, or siltstone. In Stanton county the Ogallala (including Pleistocene undifferentiated deposits) ranges in thickness from less than 50 feet to more than 400 feet. Most of the water is obtained from the sands and gravels of the formation. The Cockrum sandstone, which unconformably underlies the Ogallala formation, yields water to many wells in the western and southwestern parts of the county. It consists chiefly of fine- to medium-grained sandstones and light-colored shales or clays. It reaches 100 feet in thickness, but is absent locally. A few wells obtain water from the Cheyenne sandstone, which is made up of loose or cemented, fine to coarse sand and minor amounts of silty shale. It is about 50 feet thick in most places. The basic field data upon which most of this report is based are given in tables, and include records of 147 wells, and chemical analyses of 38 samples of water from representative wells. Logs are given of 11 test holes drilled during the investigation and of several water wells in the county. The monthly water levels since August, 1939, in 17 unused wells are tabulated.

This report describes the geography, geology, and ground-water resources of Graham County, in northwestern Kansas. Records of 344 wells and springs and logs of 31 test holes are given. The outcropping rock formations were studied in the field and a geologic map and geologic cross sections were prepared. Samples of water from 21 wells were analyzed for dissolved mineral content. Graham County is in the High Plains section of the Great Plains physiographic province. The county is moderately to well dissected and South Fork Solomon River, Bow Creek, Saline River, and tributaries to these rivers afford good drainage to the area. The most extensive flat lands in the county are the terrace surfaces in South Fork Solomon Valley. The climate of the area is subhumid, the average annual precipitation being about 21 inches. In addition to ground water, the principal mineral resources are oil and construction materials. Farming and livestock raising are the principal occupations of the county. A very small acreage is irrigated. The outcropping rocks in Graham County are sedimentary and range from late Cretaceous to Recent in age. The oldest outcropping rock is the Smoky Hill chalk member of the Niobrara formation, which underlies the entire county. The Ogallala formation of Tertiary (Pliocene) age overlies the Smoky Hill chalk member, but in several areas erosion has removed the Ogallala and the Cretaceous bedrock is exposed. Along many of the valleys where the Ogallala has been removed, late Wisconsinan terrace deposits mantle the bedrock. Other older Pleistocene alluvial deposits are the Crete sand and gravel member of the Sanborn formation and the Meade formation. The wind-blown silt of the Sanborn constitutes the surficial material over much of the area, particularly in the uplands. The youngest deposits are Recent alluvium along the streams aDd scattered sand dunes. The Ogallala formation is the most wide-spread water-bearing formation in the county and yields water to many wells. In stream valleys the late Wisconsinan terrace deposits supply water to many wells and also the Crete yields water to wells. Small amounts of water can be obtained from the Niobrara formation, and from the upper part of the Carlile shale, which underlies the Niobrara. The Dakota formation, which underlies the surface at depths ranging from about 500 to 1,100 feet, contains considerable amounts of water. However, this water is of questionable quality. The body of ground water contained in the Pleistocene and Pliocene deposits is recharged principally by precipitation that falls in the county or in adjacent areas to the west. Ground-water recharge to the Niobrara formation probably takes place in a similar manner. Some recharge to the Carlile and Dakota formations may result from local precipitation but probably the greater part of recharge to these aquifers takes place in their areas of outcrop. Ground water is discharged from Pleistocene and Pliocene deposits through transpiration and evaporation, by discharge into streams, by subsurface movement into other areas, and by wells and springs. Discharge from Cretaceous aquifers is accomplished principally through subsurface movement. The report contains a map showing the location of wells and springs for which information was obtained and showing the depth to water level at each well. The maximum measured depth to water was 229 feet in a well to the Dakota formation. A contour map showing the shape and slope of the water table indicates that ground water generally moves into the county from the west and out of the county to the east. Geologic cross sections indicate that the Pleistocene and Pliocene water-bearing materials are too thin over most of the county for the development of irrigation wells. The most promising areas for obtaining sufficient water for irrigation wells are in the northwestern part of the county from the Ogallala formation and in places along South Fork Solomon River from the Wisconsinan terrace deposits. Analyses of 21 samples of ground water reveal that water from the Ogallala formation, although moderately hard, is suitable for most purposes. Water from alluvial materials and from the bedrock formations is more highly mineralized than water from the Ogallala formation. Water from the Carlile shale may be unfit for irrigation and water from the Dakota formation is unfit for irrigation and may be unfit for domestic and stock use.