Abstract

Exposures of the upper Eocene to middle Oligocene White River Formation in northeastern Colorado contain ichnofossil-rich paleosols in a meandering alluvial system. The paleoenvironmental, paleoecological, and paleoclimatic significance of these paleosols and ichnofossils record the effects on soil ecosystems of the initial stages of global cooling in the late Eocene and early Oligocene. Previous studies of Eocene-Oligocene paleosols in Wyoming and South Dakota suggest a transition from woodland to grassland ecosystems in response to global cooling and drying. We describe four paleosol types from the study area. Type I paleosols include compound Entisols characterized by shallow networks of fine rhizoliths and ichnofaunal assemblages of Planolites isp., Pallichnus dakotensis, Macanopsis isp., Celliforma ficoides, and vertebrate coprolites. Type II paleosols are compound Inceptisols characterized by elongate rhizoliths and ichnofaunal assemblages of Planolites isp., Pallichnus dakotensis, Macanopsis isp., and vertebrate tracks. Type III paleosols are cumulative Inceptisols characterized by elongate rhizolith and ichnofaunal assemblages of Planolites isp., Pallichnus dakotensis, Macanopsis isp., Parowanichnus isp., Edaphichnium isp., and backfilled burrows. Type IV paleosols include composite Inceptisols characterized by large rhizoliths and ichnofaunal assemblages of Edaphichnium isp., Fictovichnus parvus, and wasp cocoons. The vertical transition from less-well-developed (Type I) to better-developed (Type IV) paleosols records decreasing rates of sedimentation and erosion, increasing rates of pedogenesis, and a transition in landscapes from grasslands to savannahs and woodlands. The effects of global cooling on the paleosols and soil biota, therefore, appear to have been buffered by basin-scale, autogenic, sedimentologic, and hydrologic processes.

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