THE MINERALOGICAL RECORD OF CLIMATE CHANGE IN THE JOHN DAY FORMATION, CENTRAL OREGON

John Day Formation paleosols formed in volcaniclastic sediment under climatic conditions that changed from humid-subtropical to subhumid-temperate across the Eocene-Oligocene boundary. In this study, we seek to take advantage of the well-constrained paleoclimatic (Retallack et al. 2000) and zeolite (Hay 1963) records of the John Day paleosols to examine (1) the mineralogical record of climate change and (2) the effects of burial diagenesis on pedogenic minerals. XRD analyses of 85 paleosols indicate clear distinction in clay mineral content between late Eocene (39 - 34 Ma) and early Oligocene (34 - 29 Ma) paleosols. Late Eocene paleosols contain abundant kaolinite (K) and lesser kaolinite/smectite (K/S) and halloysite (H). Early Oligocene paleosols are virtually devoid of kaolin minerals and are dominated by smectite. The kaolin in the lower John Day Formation reflects intense subtropical weathering of the late Eocene. Abrupt shift to smectitic soils across the Eocene-Oligocene boundary is attributed to step-like change to cooler, more arid climates of the early Oligocene as described by Bestland et al. (1997). The presence of clinoptilolite in the Turtle Cove Member (30-29 Ma) constrains pre-uplift burial depth to approximately 1000 - 2000 m, an estimate that is consistent with the folded-faulted character of the John Day Formation. In the 39 - 34 Ma paleosols, K is predominant in the 1 - 5 mm size fraction, whereas H and K/S are the dominant clays in the <0.2 mm fraction. We are currently analyzing SEM images to assess pedogenic and diagenetic reaction textures among H-K/S-K.