Late Miocene advent of tall grassland paleosols in Oregon

Abstract

Paleosols in the late Miocene Rattlesnake Formation reveal three successive paleoclimatic regimens that can be dated using cumulative paleosol development prior to the radiometrically dated Rattlesnake Ash-Flow Tuff (7.05 Ma). The basal Rattlesnake Formation (ca. 7.5–7.3 Ma) has a suite of weakly calcareous paleosols indicating subhumid paleoclimate (800–1000 mm mean annual precipitation or MAP) and seasonal waterlogging. A shift at ca. 7.3 Ma to semi-arid paleoclimate (MAP 500–850) is indicated by paleosols with calcic horizons at depths of 105–85 cm. A second shift at ca. 7.2 Ma ushered in an assemblage of soils with shallow (36–53 cm) calcic horizons, indicating semi-arid conditions (MAP 200–600 mm) not much different from today. Along with the climatic shifts documented by the paleosols there also are substantial changes in alluvial architecture and paleotopography. The lower Rattlesnake Formation (ca. 7.5–7.2 Ma) was deposited by a stream flowing northeast in an ancestral John Day Valley. The upper part of the formation (ca. 7.2–7.1 Ma) was deposited within large alluvial fans shed from the growing anticline of Picture Gorge Basalt to the north. Stepwise changes in former vegetation can also be interpreted from paleosols. The basal Rattlesnake Formation (7.5–7.3 Ma) has a suite of paleosols with root traces, clay skins and manganese nodules like those of riparian woodland and seasonally waterlogged riparian meadow. The succeeding paleoclimatic sequence (7.3–7.2 Ma) includes the earliest known mollic paleosols with deep (>50 cm) calcareous nodules, interpreted as former soils of tall grassland. Also found are manganese-stained paleosols of seasonally waterlogged meadows, and weakly developed paleosols of early successional riparian vegetation. A later paleoclimatic sequence (7.2–7.1 Ma) includes desert shrubland paleosols, as well as paleosols of early successional riparian vegetation. Fossil bones and teeth found in many paleosols indicate coeval changes in mammalian faunas. The major shift from woodland to grassland at ca. 7.3 Ma introduced into this area tall grassland ecosystems, including large cursorial horses and pronghorns. Aridification continued with the shift to shrubland ca. 7.2 Ma (early Hemphillian), which corresponds to continent-wide mass extinction of most remaining browsing mammals from the Clarendonian Chronofauna.