A MIDDLE MIOCENE PALEOSOL FROM CRICKET FLAT, ELGIN, OREGON, USA

The Cricket Flat paleosol (CFP) is named for its occurrence at Cricket Flat, a geomorphic feature east of Elgin in Union County, Oregon, USA. The CFP was formed on a parent material of Middle Miocene rhyolitic lithic tuff. Locally, erosional remnants of an olivine basalt from the Powder River Volcanic Field overlie the weaker tuff, including the CFP. Whole-rock ⁴⁰Ar/³⁹Ar ages on correlative olivine basalt flows to the southeast indicate that the CFP was buried at about 14 Ma. Although workers in the area noticed the CFP, it has never been formally described. In this study we used field description, bulk chemistry, micromorphology, and clay mineralogy to investigate the CFP. In the field the CFP is marked by oxidative reddening and prominent prismatic peds in the B horizon. The A horizon contains mineralized root traces and numerous Fe/Mn oxide nodules, with raphid diatoms and phytoliths visible in thin section. Both geochemically and at the field scale, the most abrupt physical boundary divides the A and B horizons. Because of the high clay content (more than 90 percent in the B horizon) it appears that the B horizon formed a seasonally-impermeable boundary and confined soil water flow to the A horizon. In the B horizon, strongly prismatic peds and a sepic plasmic fabric in thin section demonstrate the importance of shrink-swell processes, while local chemical reduction around aggregates in thin section suggests a seasonally waterlogged site. Using Maynard's CIA-K index with Sheldon et al.'s empirical equation for lowland soils yields a paleoprecipitation estimate in the range of 1100-1200 mm per year, much wetter than today.