Stepwise Climate Change Recorded in Eocene‐Oligocene Paleosol Sequences From Central Oregon

Abstract

Successions of paleosols bounded by erosional surfaces in fluvial sediments of the Eocene‐Oligocene strata of Central Oregon can be interpreted as terrestrial equivalents of the unconformity‐bound units of sequence stratigraphy. In the upper part of the upper Eocene Clarno Formation and in the lower part of the lower Eocene‐lower Miocene John Day Formation, truncation surfaces separate otherwise conformable alluvial deposits and allow for stratigraphic subdivision into informal members (lower and upper “Red Hill” claystones in the Clarno Formation and lower, middle, and upper Big Basin Members and lower Turtle Cove Member in the John Day Formation). Paleosols in each member show a stepwise change in the degree of weathering of the most strongly developed paleosols: kaolinite‐rich, Ultisollike paleosols in lower “Red Hill” claystones (late Eocene, 42‐43 Ma), smectite‐rich Alfisol‐like paleosols in the upper “Red Hill” claystones (late Eocene, 41‐42 Ma), Alfisols and Ultisol‐like paleosols in the lower Big Basin Member (late Eocene, 34–40 Ma), Alfisol and Inceptisol‐like paleosols in the middle and upper Big Basin Members (early Oligocene, 30–34 Ma), and calcic Inceptisol‐like paleosols in the lower Turtle Cove Member (middle Oligocene, 28–30 Ma). These changes across the Eocene‐Oligocene transition are interpreted as representing global cooling and drying of the midlatitudes from Eocene subtropical, humid conditions to Oligocene temperate, subhumid conditions. In central Oregon, these changes appear to be stepwise with climatically stable periods, represented by packages of similar paleosols, of approximately 2‐4 m.y. in duration. Our interpretation of these paleosol packages as non‐marine sequences is not based on correlation with sea‐level changes but on correlation with global climate change events. Geomorphic processes influenced by climate and vegetation, and not base‐level change, basin subsidence, or volcanic supply are thought to have controlled sedimentation rates. Thus, the stepwise increase in sedimentation rates across the Eocene‐Oligocene transition in the central Oregon alluvial strata reflect increased sediment yields due to drying climatic conditions. High‐precision 40Ar/39Ar age determinations of tuffs allow for the correlation of these sequences with the record of global climate change from deep sea cores. Three major paleoclimatic changes stand out. The change from Ultisol‐like paleosols formed in near‐tropical climate to AAlfisol‐like paleosols formed in subtropical climate between 42.8 and 43 Ma) corresponds to a global cooling trend after the mid‐Eocene climatic optimum. The Eocene‐Oligocene boundary (∼34 Ma) is marked by the change from subtropical Ultisol‐like paleosols to Alfisol‐like paleosols formed in temperate humid climate. Global cooling during the mid‐Oligocene (∼30 Ma) is reflected in a change from non‐calcareous, Alfisol‐like paleosols to calcareous Andisol‐like paleosols formed in sub‐humid temperate conditions. These mid‐Tertiary paleosol sequences are evidence of stepwise terrestrial climate change that was strongly coupled with marine events.