Advances in Quaternary stratigraphy, unit correlation, and event timing in Iowa

Iowa has been glaciated at least eleven times during the past 2.6 million years. While this provides one of the most complete records of glacial deposits in the world, certain aspects of the distribution and timing of the associated sediments is often poorly understood. The Iowa landscape is mantled with widely varying thicknesses of glacial till, loess, colluvial, and alluvial deposits, and all of these provide us with critical information about the climate and environment of the past. The assembly of historic and new data has made novel insights possible related to the stratigraphy of Iowa. This study focuses on three datasets.

Glacial stratigraphic units can be differentiated based on lithologic and mineralogic characteristics and placed in a regional framework based on stratigraphic position. A till sheet has long been recognized in north-central Iowa but has not been formally defined until now. Previous researchers referred to this till as the Tazewell and more recently the Iowa Geological Survey (IGS) has used the term Sheldon Creek Formation. Radiocarbon ages demonstrate that the glacier depositing the Sheldon Creek Formation advanced twice into Iowa between 42,000 and 30,000 years ago. Sheldon Creek Formation deposits can be differentiated from the much older underlying Pre-Illinois Episode till by sand fraction lithology and matrix grain-size. Stratigraphic position is necessary to differentiate the overlying Dows Formation Alden Member as their lithologic characteristics are similar.

Loess (wind-blown silt) deposits provide a critical repository of environmental information, and their thickness and grain-size characteristics respond to a complex system of sediment supply, wind speed and direction, and distance from the source area. Loess deposits related to the last glacial advance mantle much of Iowa, and a detailed study of four loess cores in east-central Iowa suggests that the timing and rates of deposition were highly variable through time. Grain-size data identify two significant shifts in deposition that were likely responding to changes in source area sediment supply and distance from the source, both of which were responding to environmental conditions and the position of regional glaciers in Iowa and the Midwest overall.

The Ashton Prairie Living Laboratory (APLL) was established on the University of Iowa (UIowa) campus in 2019 to provide an interactive field-based learning environment for students. A stratigraphic study was completed from 15 cores and associated monitoring wells to better understand the hydrogeology of APLL and the potential stratigraphic features that may prove to be hydrologically significant across the site. Two alluvial sequences are bounded by paleosols and a younger terrace, and the entire site is mantled with loess deposits. Preliminary hydrologic data indicates that the site stratigraphy has a significant effect on hydraulic head and groundwater flow directions.