Geologic map for Pawnee County now available
LAWRENCE — A new, full-color geologic map of Pawnee County showing a range of surface geology — from older rocks formed in marine environments to younger silt deposits and sand dunes shaped by wind and water — is available from the Kansas Geological Survey based at the University of Kansas.
Surficial geologic maps highlight the type and age of rock layers and thick layers of sand, gravel and other unconsolidated sediments found on the surface or immediately below the vegetation and soil.
Mapped by Bill Johnson, KU professor of geography, and Terri Woodburn, adjunct professor in geography at Park University in Parkville, Missouri, the computer-generated Pawnee County map includes shaded relief to provide a three-dimensional quality that accentuates the topography.
Pawnee County is dissected by the Arkansas River running southwest to northeast and the Pawnee River running across the middle from west to east until it joins the Arkansas. Both rivers flow through wide floodplains of unconsolidated silt, sand, gravel and clay created by the meandering streams over millions of years.
A number of intermittent tributaries running into the Pawnee River cut into layers of widespread loess — windblown silt — averaging 6 to 10 feet thick that were deposited over the last 12,000 years. The surface geology in northern and southwestern Pawnee County also includes older sandstone, limestone, chalk and shale deposited in near-shore or shallow seas environments.
“After mapping the geology of about twenty Kansas counties, Pawnee County was one of my favorites because of the tremendous variation in the geology and terrain,” Johnson said. “It has something for everyone, be it the meandering channel of the Pawnee River, the variety of sand forms, the historical post-rock or the variegated colors of the Dakota sandstone.”
Buildings at Fort Larned National Historic Site five miles west of Larned are constructed of Dakota sandstone quarried about 2.5 miles east of the fort. Deposited about 130 million years ago in a near-shore environment, the Dakota is the oldest rock exposed in the county. The fence-post limestone used for the windowsills was deposited in later shallow seas that covered much of the state about 80 million years ago and was used by early settlers to make fence posts.
In contrast, the third of the county southeast of the Arkansas River is dominated by inactive sand dunes, formed from river valley sand carried and reworked into fine grains by the wind, and sand sheets — remnants of older sand-dune topography that have flattened into broad, undulating swales and are covered by thick soil developed in the sand. Most of the dunes and sand sheets have been stabilized by vegetation, although a few localized active dunes can be found.
The water-bearing Ogallala Formation, deposited by streams carrying debris from the Rocky Mountains about 5 million years ago, is far-reaching in western Kansas but confined in Pawnee County to the southwest corner. Although the Ogallala is the main source of groundwater in western Kansas, most water for irrigation, industrial and municipal uses in Pawnee County is drawn from unconsolidated sediments in the Pawnee and Arkansas river valleys. Smaller, localized sources of groundwater for irrigation, domestic and stock use include sandstones in the Dakota Formation.
Playa lakes, transient wetlands fed only by precipitation and runoff, are scattered on the uplands away from the stream valleys north and west of the Arkansas River. When wet, the small, shallow basins — also known as lagoons or buffalo wallows — provide refuge for local and migratory wildlife.
Natural resources in the county extracted for commercial use include sand and gravel, mined from terrace and alluvial deposits in the river valleys and used primarily for road metal, and the Dakota sandstone and fence-post limestone used historically for building materials. In 2014, Pawnee County produced 369,227 barrels of oil from 195 wells and 758,017 million cubic feet (mcf) of gas from 115 wells. The county ranked 39th out of the 93 oil-producing counties and 31st of out of the 56 gas-producing counties in the state.
In addition to geologic and hydrologic characteristics, the map includes towns, roads (from highways to unimproved roadways), elevation contours at 10-meter intervals, and township and range boundaries. The map is at a scale of 1:50,000 so that 1 inch on the map equals about 3/4 mile of actual distance.
Besides the map, the 60-inch by 45-inch sheet contains an illustrated rock column, which shows the order in which the rock units were deposited over time, and a description of each unit.
Copies of the Pawnee County map are available from the Kansas Geological Survey at 1930 Constant Ave., Lawrence, KS 66047-3724 (phone 785-864-3965, email pubsales@kgs.ku.edu) and at 4150 W. Monroe St., Wichita, KS 67209-2640 (phone 316-943-2343, email wwsl@kgs.ku.edu).
The cost is $15 plus shipping and handling. Inquire about shipping and handling charges and, for Kansas residents, sales tax. More information about county geologic maps and other KGS products is available at the Survey's website.
The map, which was funded in part by the U.S. Geological Survey National Cooperative Geologic Mapping Program and the National Park Service, can also be accessed online.