Late Quaternary loess in northeastern Colorado: Part I—Age and paleoclimatic significance

Abstract

Loess in eastern Colorado covers an estimated 14 000 km 2 , and is the westernmost part of the North American midcontinent loess province. Stratigraphic studies indicate there were two periods of loess deposition in eastern Colorado during late Quaternary time. The first period spanned ca. 20 000 to 12 000 14 C yr B.P. (ca. 20‐14 ka) and correlates reasonably well with the culmination and retreat of Pinedale glaciers in the Colorado Front Range during the last glacial maximum. The second period of loess deposition occurred between ca. 11000 and 9000 14 C yr B.P. This interval may be Holocene or may correlate with a hypothesized Younger Dryas glacial advance in the Colorado Front Range. Sedimentologic, mineralogic, and geochemical data indicate that as many as three sources could have supplied loess in eastern Colorado. These sources include glaciogenic silt (derived from the Colorado Front Range) and two bedrock sources, volcaniclastic silt from the White River Group, and clays from the Pierre Shale. The sediment sources imply a generally westerly paleowind during the last glacial maximum. New carbon isotope data, combined with published faunal data, indicate that the loess was probably deposited on a cool steppe, implying a last glacial maximum July temperature depression, relative to the present, of at least 5‐6 °C. Overall, loess deposition in eastern Colorado occurred mostly toward the end of the last glacial maximum, under cooler and drier conditions, with generally westerly winds from more than one source.