Geologic Map of the valley corridor of the Niobrara National Scenic River, Nebraska

This release of digital geologic map data covers the Niobrara National Scenic River, a 76-mile stretch of river at the northern edge of the Nebraska Sand Hills. The mapped area includes the National Park Service unit of the Niobrara National Scenic River and the adjoining Middle Niobrara Preserve of The Nature Conservancy, and the Fort Niobrara Wildlife Refuge of the U.S. Fish and Wildlife Service. These digital geologic map data focus on the surficial geologic framework, dominated by the late Quaternary geology, of the Niobrara River valley corridor. These data are the result of geologic mapping begun by the U.S. Geological Survey in 2011 and continuing for about a decade. Mapping is based on geomorphic and other surficial characteristics as well as sedimentary and stratigraphic characteristics gleaned from exposures and borehole data. To produce this geologic map, field observations and data collection were combined with interpretation of various available geospatial data sources. The geologic map data released herein is the late Quaternary geology that dominates the surface landforms, modern soils, and substrates of modern ecosystems of the Niobrara National Scenic River. Three groups of riverine terraces are recognized, including a high group near 60 m above modern grade that yielded dates from 16 ka to 10 ka using the optically stimulated luminescence dating method, the youngest and main valley floor terrace group that has formed in the past 6000 year, and a group of intermediate height and age range. Fluvial scarps that separate individual terraces within and between terrace groups were mapped based on morphologic characteristics. Holocene dunes that mantle parts of the high terrace group on the south flank of the river valley are at the northern limit of the Nebraska Sand Hills. Stratigraphic and geomorphic relations in central northern Nebraska demonstrate that 60 m of net incision by the Niobrara River since latest Pleistocene time was interrupted after mid-Holocene by ~10 m of valley aggradation (from a grade of a few meters above the present grade) before resumption of ongoing incision. The Tertiary bedrock stratigraphy is treated to the extent it is essential to the geologic framework and setting. Pre-Quaternary sedimentary formations form scenic outcrops along the river and canyon rims. The map area is underlain by the Cretaceous Pierre Shale, which crops out only in the eastern part of the map area. The Pierre Shale is disconformably overlain by a succession of Tertiary fluvial sedimentary rocks. The dataset includes a geographic information system geodatabase that contains mapped contacts and faults, geologic lines representing the fluvial scarps, and map unit polygons of the mapped geology. Vector data are attributed according to the USGS National Cooperative Geologic Mapping Program’s GeMS digital geologic map schema. The spatial data are accompanied by non-spatial tables that describe the sources of geologic information, a description of geologic map units, a glossary of terms, and a Data Dictionary that duplicates the Entity and Attribute information contained in the metadata file. To maximize usability, spatial data are also distributed as shapefiles and tabular data are distributed as ascii text files in comma separated values (CSV) format.