The Pierre-Niobrara Unconformity In Western Nebraska

The Pierre-Niobrara unconformity, one of several significant unconformities in the Cretaceous System of the Western Interior region, has not generally been recognized. Detailed correlation of electric logs of a large number of wells in western Nebraska provides evidence of its existence. Isopach maps of the beds occupying three identifiable stratigraphic intervals - a redefined Niobrara Formation, an unnamed uppermost Niobrara unit, and an unnamed basal Pierre unit, which includes the Ardmore Bentonite - provide recognizable geologic patterns that permit reconstruction of the stages of development of the unconformity. The upper part of the Niobrara Formation has been truncated in several areas of western Nebraska. Truncation is most pronounced in northwestern Nebraska, where the unnamed uppermost Niobrara unit, elsewhere more than 100 feet thick, is entirely absent. Throughout much of western Nebraska, however, such truncation is subtle and not easily recognized. The unnamed basal unit of the Pierre Shale consists of silty shale and bentonite. Although discontinuous, it attains a thickness of more than 60 feet in at least one locality. Early Pierre topography appears to have been structurally controlled, uplands tending to coincide with uplifts. The basal Pierre unit is especially well developed in two south-southwest-trending troughs which appear to coincide with synclines. The silty shale well may be of fluvial origin, having been derived from the Niobrara strata of adjacent uplands. Detailed mapping and analysis of unconformities, such as the one between the Niobrara and Pierre Formations, enables the geologic history of the Cretaceous Western Interior region to be interpreted more fully

Cenozoic Paleogeography of Western Nebraska

The Cenozoic strata of western Nebraska are an extensive sequence of continental deposits that extend eastward from the Hartville, Laramie, and Front Range uplifts and southeast from the Black Hills. The oldest Cenozoic sediments (Chadron Formation, White River Group) are Early Oligocene alluvial valley fills. Subsequent to filling of these drainages and continuing for about the next 7 m.y., landscape development in western Nebraska was dominated by eolian deposition of tremendous volumes of rhyolitic volcanic ash derived from western eruptions. A plain of low relief, with only an occasional narrow drainage heading in the western highlands, was maintained during most of this period. Uplift in the Rocky Mountains and Great Plains (pre-Gering Formation, Arikaree Group) caused erosion and brought epiclastic detritus into the area about 28 m.y. ago. Eolian sediment consisting mostly of pyroclastic detritus continued building the plains during and after Gering alluvial deposition until about 19 m.y. ago when Arikaree deposition ceased. About this time, western volcanic activity declined for several million years and was followed by a marked decrease in the volume of rhyolitic volcanism for the remainder of the Cenozoic. At the end of Arikaree deposition in western Nebraska, a major pulse of erosion (pre-Runningwater Formation, Ogallala Group) was followed by a fundamental change in depositional style and landscape evolution, characterized by a heterogeneous mixture of epiclastic valley fills. Sands and gravels from Rocky Mountain sources were first deposited in a major valley in the northern half of the area and later in valleys to the south. Episodic regional and local structural movements influenced the size and position of many Ogallala valleys. For the past 5 m.y. degradation, in response to major regional uplift, has greatly exceeded aggradation as the dominant f actor affecting landscape evolution in western Nebraska

Cenozoic Paleogeography of Western Nebraska

The Cenozoic strata of western Nebraska are an extensive sequence of continental deposits that extend eastward from the Hartville, Laramie, and Front Range uplifts and southeast from the Black Hills. The oldest Cenozoic sediments (Chadron Formation, White River Group) are Early Oligocene alluvial valley fills. Subsequent to filling of these drainages and continuing for about the next 7 m.y., landscape development in western Nebraska was dominated by eolian deposition of tremendous volumes of rhyolitic volcanic ash derived from western eruptions. A plain of low relief, with only an occasional narrow drainage heading in the western highlands, was maintained during most of this period. Uplift in the Rocky Mountains and Great Plains (pre-Gering Formation, Arikaree Group) caused erosion and brought epiclastic detritus into the area about 28 m.y. ago. Eolian sediment consisting mostly of pyroclastic detritus continued building the plains during and after Gering alluvial deposition until about 19 m.y. ago when Arikaree deposition ceased. About this time, western volcanic activity declined for several million years and was followed by a marked decrease in the volume of rhyolitic volcanism for the remainder of the Cenozoic. At the end of Arikaree deposition in western Nebraska, a major pulse of erosion (pre-Runningwater Formation, Ogallala Group) was followed by a fundamental change in depositional style and landscape evolution, characterized by a heterogeneous mixture of epiclastic valley fills. Sands and gravels from Rocky Mountain sources were first deposited in a major valley in the northern half of the area and later in valleys to the south. Episodic regional and local structural movements influenced the size and position of many Ogallala valleys. For the past 5 m.y. degradation, in response to major regional uplift, has greatly exceeded aggradation as the dominant f actor affecting landscape evolution in western Nebraska

Keith County, Nebraska, Map Series

KEITH COUNTY--LIST OF MAPS AND THEIR AUTHORS Topography--U. S. Geological Survey Index of 7.5\u27 Topographic Quadrangles and Township Boundaries--R. F. Diffendal, Jr. Generalized Soils Map--M. Kuzila and J. Culver Approximate Loess Thickness--R. F. Diffendal, Jr. Bedrock Geologic Map--R. F. Diffendal, Jr. Volcanic Ash Localities--R. F. Diffendal, Jr. Ogallala Vertebrate Faunal Sites--R. F. Diffendal, Jr. Ogallala Group Outcrops--R. F. Diffendal, Jr. White River Group Outcrops--R. F. Diffendal, Jr. Conservation and Survey Division Test Hole Locations--R. F. Diffendal, Jr. Oil and/or Gas Test Hole Locations--R. F. Diffendal, Jr. Mineral Resources Localities--R. F. Diffendal, Jr. Locations of Registered Irrigation Wells--R. F. Diffendal, Jr. Configuration of Top of Bedrock--R. F. Diffendal, Jr. Configuration of Top of White River Group (= Brule Fm.)--R. F. Diffendal, Jr. Configuration of Top of Cretaceous--H. M. DeGraw Configuration of Top of Niobrara Fm.--H. M. DeGraw Configuration of Base of Greenhorn Limestone--H. M. DeGraw Configuration of Top of Permian System--R. R. Burchett Structural Contours on Top of Stone Corral--R. R. Burchett Structural Contours on Top of Pennsylvanian System--R. R. Burchett Depth to Precambrian Surface--M. P. Carlson Configuration of Top of Precambrian--R. R. Burchett and M. P. Carlson Geothermal Projected Temperatures on Top of Dakota Group--D. Eversoll and W. Gosnold Bouguer Gravity Anomaly Map--R. R. Burchett and T. Eversol

Keith County, Nebraska, Map Series

KEITH COUNTY--LIST OF MAPS AND THEIR AUTHORS Topography--U. S. Geological Survey Index of 7.5\u27 Topographic Quadrangles and Township Boundaries--R. F. Diffendal, Jr. Generalized Soils Map--M. Kuzila and J. Culver Approximate Loess Thickness--R. F. Diffendal, Jr. Bedrock Geologic Map--R. F. Diffendal, Jr. Volcanic Ash Localities--R. F. Diffendal, Jr. Ogallala Vertebrate Faunal Sites--R. F. Diffendal, Jr. Ogallala Group Outcrops--R. F. Diffendal, Jr. White River Group Outcrops--R. F. Diffendal, Jr. Conservation and Survey Division Test Hole Locations--R. F. Diffendal, Jr. Oil and/or Gas Test Hole Locations--R. F. Diffendal, Jr. Mineral Resources Localities--R. F. Diffendal, Jr. Locations of Registered Irrigation Wells--R. F. Diffendal, Jr. Configuration of Top of Bedrock--R. F. Diffendal, Jr. Configuration of Top of White River Group (= Brule Fm.)--R. F. Diffendal, Jr. Configuration of Top of Cretaceous--H. M. DeGraw Configuration of Top of Niobrara Fm.--H. M. DeGraw Configuration of Base of Greenhorn Limestone--H. M. DeGraw Configuration of Top of Permian System--R. R. Burchett Structural Contours on Top of Stone Corral--R. R. Burchett Structural Contours on Top of Pennsylvanian System--R. R. Burchett Depth to Precambrian Surface--M. P. Carlson Configuration of Top of Precambrian--R. R. Burchett and M. P. Carlson Geothermal Projected Temperatures on Top of Dakota Group--D. Eversoll and W. Gosnold Bouguer Gravity Anomaly Map--R. R. Burchett and T. Eversol

The Pierre-Niobrara Unconformity In Western Nebraska

The Pierre-Niobrara unconformity, one of several significant unconformities in the Cretaceous System of the Western Interior region, has not generally been recognized. Detailed correlation of electric logs of a large number of wells in western Nebraska provides evidence of its existence. Isopach maps of the beds occupying three identifiable stratigraphic intervals - a redefined Niobrara Formation, an unnamed uppermost Niobrara unit, and an unnamed basal Pierre unit, which includes the Ardmore Bentonite - provide recognizable geologic patterns that permit reconstruction of the stages of development of the unconformity. The upper part of the Niobrara Formation has been truncated in several areas of western Nebraska. Truncation is most pronounced in northwestern Nebraska, where the unnamed uppermost Niobrara unit, elsewhere more than 100 feet thick, is entirely absent. Throughout much of western Nebraska, however, such truncation is subtle and not easily recognized. The unnamed basal unit of the Pierre Shale consists of silty shale and bentonite. Although discontinuous, it attains a thickness of more than 60 feet in at least one locality. Early Pierre topography appears to have been structurally controlled, uplands tending to coincide with uplifts. The basal Pierre unit is especially well developed in two south-southwest-trending troughs which appear to coincide with synclines. The silty shale well may be of fluvial origin, having been derived from the Niobrara strata of adjacent uplands. Detailed mapping and analysis of unconformities, such as the one between the Niobrara and Pierre Formations, enables the geologic history of the Cretaceous Western Interior region to be interpreted more fully