Date of this Version
Transactions of the Nebraska Academy of Sciences, vol. 5 (1978)
The Seward Formation as originally described was known largely from subsurface data; the authors considered it a fine-grained facies of the Ogallala. It was later correlated with the Elk Creek Till of the Nebraskan glaciation and an outcrop selected in northeastern Seward County as a type section of the unit. Recent research has demonstrated that the Elk Creek Till is much older than the classical Nebraskan tills. The upper part of the exposure designated the type section of the Seward Formation is a massive silt unit that has a thick chernozemic paleosol, and the lower part is laminated silt and clay with a thinner CaCO3-rich paleosol. Land snails collected from a sparsely fossiliferous zone near the base of the massive silt unit represent only five species; 88% are about equally divided between Pupilla muscorum and Gastrocopta armifera, both long lived and widely distributed taxa that indicate open grassland.
This faunule and the massive, nearly uniform nature of the silt from which it was collected suggest that part of the sediment may have been deposited as loess. Its thick soil profile, which is buried beneath a laminated silt and fine sand that is overlain by Cedar Bluffs Till a short distance away, is well developed. Judged by post-Wisconsinan soil profile development, it probably would have required several times as long to form. The upper part of the Seward Formation at its type section probably correlates with the Elk Creek Till, which was deposited by the earliest ice to reach the area of eastern Nebraska. The basal part of the formation probably is Pliocene. It is likely that not all the silts identified as Seward Formation from well logs and samples have the same origin as these beds at the outcrop of the type section.