Significance of loessite in the Maroon Formation (Middle Pennsylvanian to Lower Permian), Eagle Basin, northwest Colorado

Abstract

Quaternary loess deposits are widespread on the earth's surface, yet pre-Quaternary loess deposits have rarely been reported. The Maroon Formation (Middle Pennsylvanian to Lower Permian) of the Eagle Basin, northwest Colorado, includes a siltstone-dominated facies interpreted as loessite (lithified loess) along its downwind basin margin. The section of inferred loessite in the Maroon Formation is locally at least 490 m thick and consists in large part of structureless and nearly structureless beds of homogeneous sandy siltstone. Bed contacts are generally planar to undulatory and are either horizontal or are characterized by gentle relief. Loessite beds are separated by common claystone drapes and weakly developed paleosols, and by rare pond deposits, channel deposits, and eolian-ripple-laminated deposits. The loess interpretation is based on 1) the homogeneity and dominance of the sandy silt grain-size; 2) the relative lack of primary sedimentary structures; 3) the gentle character of most bedding contacts and the common mantling of irregular depositional topography; 4) the inferred paleogeographic setting; and 5) the absence of suitable alternative interpretations. The loessite grades laterally into mixed fluvial-eolian deposits of the Maroon Formation in the main part of Eagle Basin, which served as the loessite sediment source. Deposition of the Maroon Formation was probably strongly affected by cyclic climatic changes synchronous with fluctuations in late Paleozoic continental ice sheets. The paleogeography and paleoclimatology of the Maroon Formation depositional system are not unique, suggesting that there are probably many other ancient loessites that have gone unrecognized.