The stratigraphy of the Quaternary alluvium in the Great Bend Prairie, Kansas

Abstract

The Quaternary alluvial aquifer beneath the Great Bend Prairie in south-central Kansas serves as a major source of water for irrigation and domestic use. Low quality saline water, however, intrudes locally from Permian and possibly Cretaceous bedrock and mixes with the less dense freshwater in the aquifer.

Grain-size analysis of samples from three wells of a natural recharge study was correlated to the respective gamma logs. The correlation was then applied to the gamma logs from an observation well network, established by the Kansas Geological Survey and Groundwater Management District No. 5, to improve the stratigraphic interpretation. The stratigraphy of the area is defined from the surface down as (1) dune sands, (2) near surface silt-clay, (3) a section of variable lithology, (4) basal sand, and (5) bedrock.

The alluvium was mainly deposited by the Arkansas River as it aggraded its channel and moved northward laterally by avulsion to its present position. The gamma logs and grain-size analysis show fining upward sequences in the sediments.

The near surface silt-clay and basal sand are the only two lithologies that are continuous and could be mapped across the study area. The near surface silt-clay can influence aquifer recharge. If saline water and then freshwater is allowed to come in contact with the silt-clay, then the permeability of the silt-clay will be significantly reduced. This will decrease recharge to the aquifer. The basal sand tends to be thicker in the paleovalleys and thinner over the paleoridges. This sand serves as an aquifer in the area. The variable lithology section is the result of lateral shifting of the streams in the study area.