Date of this Version
The Quaternary Provincial Land Mammal Ages (Blancan, Irvingtonian, Rancholabrean, and Recent) are here correlated with the fossliferous valley-fill sequences in the Great Plains of North America, based on faunal, stratigraphic, and geomorphologic evidence.
The fossil mammals from the valley-fills (Broadwater) of the highest terrace (Terrace-5, T-5) are of Blancan age, long known to be equivalent to the Villafranchian of Europe and Asia. The earliest Blancan faunas, chiefly from the middle part of the Broadwater, are equal to the Late Rexroadian of Kansas, whereas the later ones, from the upper or Red Cloud portion of the Broadwater, equate with the Senecan of central Nebraska.
The mammals from the next-younger terrace (T-4), which come from the lower and middle parts (provisionally Sappan) as well as from the upper part (designated Sheridanian, although this term could be applied to the whole), correlate with the Irvingtonian.
The uppermost parts of both the T-5 and T-4, equal to Sangamon ("Last Interglacial") and Wisconsin ("Last Glacial"), together with the T-3 and T-2B sediments, yield fossil mammals that correlate with the Rancholabrean. The Rancholabrean seems to have required not more than 120,000 yr contrasted with perhaps 2 m.y. necessary for the Irvingtonian. The very youngest valley-fills (of T-2A, T-1, and T-0) are Recent or Holocene, representing only the last 10,500 to 12,500 yr or so.
A major unconformity and faunal break separate these Quaternary valley-fills from the Latest Tertiary Kimball Formation of the Ogallala Group, dividing the Blancan from the Kimballian. It seems clear that the "Great Ice Age" may be considered to have begun perhaps 3.2 m.y. ago, and that the Pliocene/Quaternary boundary probably should be drawn between the Kimball and Broadwater. This boundary corresponds very closely to the Pliocene/Quaternary break identified in deep sea cores and elsewhere in the world by many workers.