Paleoclimatology and paleobotany of the Eocene Chumstick formation, cascade range, Washington (USA): a rapidly subsiding alluvial basin

Abstract

The Middle to Late Eocene Chumstick Formation comprises >12 km of fluvial and lacustrine strata deposited in a strikeslip basin in Washington State. The Chumstick paleoflora is a mixed suite of broad-leaved evergreen and broad-leaved deciduous taxa, with physiognomic characteristics intermediate between paratropical and subtropical rainforest types. The Chumstick paleoflora probably occupied an intermediate paleogeographic position between lowland and montane rainforest, in a setting about 150 km inland from the Pacific coast at elevations up to about 750 m. Most plant macrofossils are found in proximal overbank sequences or on the preserved tops of emergent mid-channel bars. Certain taxa are more typical of floodplain versus channel-margin sites. Key differences were also noted between “upland” (proximal to fault scarps) and “lowland” taxa, which may be related to soil drainage. Chumstick paleosols include entisols (poor horizontal development), inceptisols (incipient Cca or K horizon development) and histosols (histic epipedons containing up to 6% total organic carbon). Paleovegetation had a significant effect on fluvial deposition in the Chumstick Formation. Even in coarse-grained, fault-proximal facies there is evidence for bank cohesion and stability, bank undercutting and failure, soil erosion, and overbank deposits. The dominance of vertical accretion over lateral channel migration in more distal deposits can be attributed to rapid subsidence, coupled with bank stability imparted by vegetation.