PLIO-PLEISTOCENE LANDSCAPE EVOLUTION ON THE HIGH PLAINS OF SOUTHWESTERN KANSAS

Abstract

This dissertation investigates patterns and forcing mechanisms of Plio-Pleistocene landscape evolution in the Cimarron River valley, southwestern Kansas. Particular attention was given to the geomorphic history of the Cimarron River and the response to environmental forcing. During the Pliocene, the ancestral Cimarron River deposited the alluvial sediments of the Ogallala Formation. The Pliocene age is inferred from stable isotope values that indicate 50% C4 biomass - a Pliocene phenomenon in the Great Plains. In order to provide better age constraints, a new technique to date volcanogenic zircons preserved in paleosols was utilized. A concordia age of 64.2±3.9 Ma was obtained but appears to represent detrital populations. Evidence of late-Pleistocene alluviation occurs in the form of distinct valley fills beneath two terrace surfaces (T-1 and T-2). During Marine Isotope Stages (MIS) 5-4, the Cimarron River rapidly aggraded. Paleoenvironmental data from alluvial fills suggest a cool and likely wet environment between ~77-58 ka. Paleoenvironmental data from eolian sediments, however, indicate multiple climatic shifts between ~84-54 ka that resulted in episodic eolian deposition and soil formation on the High Plains surface. During MIS 3, the paleoenvironmental record from both alluvial (~52-28 ka) and eolian (~52-29 ka) deposits indicates a shift to warmer, drier conditions. This shift resulted in slower rates of alluviation and cumulic soil formation in the T-2 fill. Rates of eolian deposition on the uplands also slowed at this time, resulting in the formation of a soil that represents the Gilman Canyon Formation pedocomplex. Chronostratigraphic data indicate that the Cimarron River incised over 25 m before ~28 ka likely due to local base level lowering from the dissolution of Permian evaporite beds during the late Pleistocene. The bulk of the T-1 fill aggraded between ~28-13 ka in response to cooler, wetter climatic conditions. Around 13 ka, sedimentation rates declined due to warmer and probably drier conditions, forming a cumulic soil in the T-1 fill. Overall, this dissertation fills an important gap in our understanding of landscape evolution on the High Plains by shedding light on chronostratigraphic relationships and paleoclimatic change during the period of record.