The stratigraphic and climatic significance of Paleogene alluvial paleosols in synorogenic strata of the Denver Basin, Colorado

Abstract

Paleogene strata in the Denver Basin contain an interval about 10 m thick and distinguished by bright red, purple, and yellow-brown colors. The bright-colored interval has been regarded as a paleosol marking an unconformity in the stratigraphic record. The interval consists of mudrocks that show morphologic features indicating ancient soil development. The mudrocks showing paleosol modification are interbedded with coarser-grained deposits that show little, if any, paleosol development. Thus, the interval can be subdivided into a series of vertically stacked alluvial paleosols indicating that pedogenesis was coeval with deposition of the parent material. This interpretation contrasts with that of workers who have considered the paleosol interval to represent a long-lived (millions of years) episode of landscape stability. Three kinds of paleosols are recognized based on the color of the B horizon: red, purple/red, and purple paleosols. Reddening of the B horizon was an important process in developing the paleosols and indicates that the paleosols formed under moderately well-drained and oxidizing conditions in a warm temperate to subtropical climate. The bright red and purple paleosol intervals directly overlie strata dominated by gray colors and organic-rich deposits. The transition in lithology suggests a change in soil drainage conditions possibly caused by a climate change. A similar lithologic change characterizes continental strata in other sedimentary basins in the Rocky Mountain region. The transition is best known in the Bighorn Basin, where it has been attributed to a combination of local tectonic controls and a global climate change that took place from late Paleocene to early Eocene time. We speculate that the lithologic change in the Denver Basin corresponds to that found in the Bighorn Basin and other basins in the Rocky Mountain region. Thus, development of the paleosol intervals in the Denver Basin may have been caused primarily by a global change to a warmer and drier climate and may have taken place at or about the Paleocene–Eocene boundary.