LITHOLOGY AND STRATIGRAPHIC SEQUENCE OF PRE-ILLINOIAN TILLS IN NEBRASKA AND WESTERN IOWA

John Boellstorff revolutionized pre-Illinoian till stratigraphy in the 1970s by documenting 7 individual tills in Nebraska and western Iowa, where earlier workers had forced the entire sequence into a two-fold division of “Kansan” and “Nebraskan.” Boellstorff placed the 7 tills into 3 main groups (A, B and C-types) based on pebble content, magnetic remanence, and fission-track dating of interbedded tephras. These divisions have achieved nearly iconic status as informal units, but surprisingly little is known about their lithologies and how they might correlate to pre-Illinoian tills in other areas.

We relogged and sampled cores from Boellstorff’s original study and collected additional samples from surface exposures with paleomagnetic control. We then characterized the tills with 3 simple but widely used parameters: texture, clay mineralogy, and sand-fraction lithology.

Four mature paleosols with argillic horizons are present within the till sequence, suggesting that the seven tills represent 5 major glaciations. The two oldest tills (C1 & C2- reversed polarity, > 2 Ma) have fine, silt-rich textures, low percentages of kaolinite, and a very low percentage of igneous rock fragments. These two tills are separated by a thin leached zone lacking other evidence of pedogenesis, suggesting that they were both deposited during the same major glaciation. The A4 and B tills (type Nebraskan with reversed polarity, < 1.3 Ma) are lithologically identical and have sandy textures with low percentages of expandable clay. The A4 and B tills are separated by fluvial sediments and/or weakly developed weathering profiles, and seem to represent two stadial advances of the same glaciation. The 3 youngest tills (A1, A2, & A3; normal polarity) are separated by mature paleosols and generally have high expandable clay contents. The A1 has the coarsest texture and highest concentration of igneous rock fragments. The A3 is distinguished from the other A & B tills by a high clay percentage and a lower content of igneous rock fragments. All bonafide examples of the A1 till (type Kansan) occur east of the Missouri River. This suggests that the youngest pre-Illinoian glaciation established the present course of the Missouri River, and ironically that the “Kansan” glaciation did not reach Kansas.