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Alluvial Terraces and Paleosols as Indicators of Early Oligocene Climate Change (John Day Formation, Oregon)
Erick A. Bestland
Flights of paleoterraces exposed in the Painted Hills area of central Oregon record periods of alluvial cutting and filling that correspond with early Oligocene climate changes. The terraced interval is situated chronologically between two global climate steps: the Eocene-Oligocene boundary at 34 Ma and the mid-Oligocene climatic transition at 30 Ma. The paleoterraces, and associated paleosols, are in the middle and upper Big Basin Members of the John Day Formation and are bracketed by both 40Ar/39Ar age determinations and Oligocene floral and faunal fossil assemblages. The paleoterraces are defined by truncation surfaces that cut underlying alluvial strata, are nlapped by paleosols, and are mantled locally by colluvium-influenced, iron-rich paleosols. One well-exposed truncation surface has a stepped geometry with nearly horizontal sections (treads) separated by steep sections (risers) with relief of 4-5 m. The truncation surfaces represent episodes of incision of the overbank alluvium, and the overlying disconformable paleosols (mantling the surfaces) represent periods of landscape stabilization following incision.
The strata as a whole can be grouped into three categories according to the degree of development of paleosols in the sequences. These paleosol sequences can be interpreted in terms of climatic-geomorphic-fluvial response. (1) Rapidly deposited alluvium with weakly developed paleosols is interpreted to represent deposition during periods of rapid climate change. During these periods, the vegetation-landscape system was in disequilibrium, thereby causing accelerated soil erosion from the hinterland landscape, which was accompanied by floodplain aggradation. (2) Slowly deposited alluvium with well-developed, clayey paleosols may indicate climatic optimum conditions when vegetation both suppressed soil erosion and enhanced sediment storage in hinterland soils. (3) Most of the alluvial st ata falls into a third category, which consists of alternating moderately and weakly developed paleosols. This alluvium is interpreted as fluvial deposition responding to a geomorphic system in dynamic equilibrium. The largest terrace surface cuts into rapidly deposited alluvium with weak paleosol development and may represent a complex-response incision feature associated with a preceding period of rapid alluviation.
A chronology for the alluvial cut-and-fill episodes is constructed based on stratigraphic relationships, time estimates of paleosol development, and 40Ar/39Ar age determinations. Time estimates from paleosols are made both by comparison of paleosols to modern soils from dated chronosequences and directly from 40Ar/39Ar age determinations of tuffs interbedded with complete sequences. These methods allow estimation of sediment accumulation rates and of the timing and duration of aggradation and degradation episodes. In the Painted Hills area, the Eocene-Oligocene boundary is marked by a truncation surface overlain by rapidly deposited alluvium. This alluviation episode is estimated to have begun at about 33.2 Ma. A 10 m stack of red beds (well developed, clay-rich, and oxidized paleosols) cap the before-mentioned rapidly deposited strata and mark the top of the middle Big Basin Member. These paleosols are estimated to span the interval from 32.0 to 32.6 Ma, which corresponds to an early Oligocene warming trend that immediately followed the Eocene-Oligocene (Oi1) cooling event, both recorded in marine isotopic data. Thus, these red beds may represent an early Oligocene climatic optimum in the Pacific Northwest.
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