Eocene and Oligocene Paleosols of Central Oregon (GSA by Gregory J. Retallack, Erick A. Bestland, Theodore J. Fremd

By Gregory J. Retallack, Erick A. Bestland, Theodore J. Fremd

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The type profile is developed on a talus of large boulders of porphyritic dacite and is overlain by boulder breccia of dacite with an additional Pswa paleosol. 3 Ma by 40Ar/39Ar incremental heating (R. A. , 1999). The type Pswa clay is at a stratigraphic horizon 25 m below the columnar basalt that forms the ridge of Black Spur, and thus correlates to a level of about 16 m in the reference section at Hancock Field Station 33 (Fig. 33). , 1996). 5Y5/2): with sparse drabhaloed root traces up to 1 cm diameter of greenish gray (5G6/1): common distinct coarse mottles of dark reddish gray (10R4/1): few irregular ferruginous nodules of brownish yellow (10YR6/8): coarse angular-blocky peds defined by slickensided clay skins (illuviation argillans) of brownish yellow (10YR6/8): noncalcareous: porphyroskelic skelmosepic in thin section, with abundant Figure 38.

A more copious source of local potassium, however, would be interlayered illite in smectite clays of the paleosols. Smectite clays with basal reflections between 14 and 12Å compatible with interlayered illite-smectite have been found in the middle and lower Big Basin Member (Rice, 1994). Where moderately well ordered, these clays may have been pseudomorphs of plagioclase. However, where poorly ordered and mixed with weakly crystalline phases such as allophane and imogolite inferred for the upper Big Basin and lower Turtle Cove Members, these potassic phases would have been consumed by zeolitization during burial.

The dacite dome may have excluded coarse debris and impounded local swamps. Although there are indications of hilly terrane and even large stratovolcanoes nearby, Cmuk paleosols represent locally waterlogged bottomlands. At least locally, these would have been flat and slowly subsiding so that peat could accumulate below the water table faster than it could decay. Parent material. Sandstones and siltstones associated with Cmuk paleosols include abundant grains of little-weathered feldspar and volcanic rock fragments of andesitic composition like those associated with Patat and Sayayk paleosols of the conglomerates of Hancock Canyon.

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