BIOSTRATIGRAPHY OF COLUMBIA RIVER BASALT GROUP PETRIFIED FORESTS

Well-known CRBG petrified wood localities in central Washington are placed in a stratigraphic context in an effort to determine their occurrence, lateral extent, and mode of deposition. Three major petrified forests are recognized on the basis of in situ stumps, forest litter, soil horizons and abundance of logs. These forests correspond to and help define temporal lulls in flood basalt activity: Umtanum Petrified Forest at the Umtanum unit/Sentinal Bluffs unit of the Grande Ronde Basalt (~15.6 mya); Ginkgo Petrified Forest at the Vantage interbed/Ginkgo flow of the Wanapum Basalt (~15.5-15.4 mya); and Saddle Mountains Petrified Forest at the Roza flow/Priest Rapids flow of the Wanapum Basalt (~14.5 mya). Widespread localities with abundant petrified wood at the latest Grande Ronde Basalt/Vantage interbed/Ginkgo flow zone suggests that the unique lahar-deposition of logs at Ginkgo Petrified Forest State Park, from a probable northwesterly volcanic event, may have also had a major local derivation. Ginkgo Petrified Forest is tentatively expanded to include several petrified wood assemblages. Three other important sites are catalogued: an unnamed in situ petrified forest at Locke Lake, WA/Mosier, OR, at the Ginkgo flow/Sand Hollow flow level; petrified wood at Roosevelt, WA, possibly in upper Priest Rapids flows; and a probable petrified forest at Sunnyside, at the Umatilla flow/Pomona flow level, making it the youngest occurring site at ~12 mya. Other occurrences of petrified wood are either the result of log and sediment rafting by lava/minor pillow/palagonite complexes (seen at Vantage and Sentinal Gap) or transport and burial within massive pillow/palagonite complexes (Petrified Canyon in Moses Coulee). A number of sites remain to be investigated. Previous attempts to identify petrified forests by wood identification/generic abundance are critiqued. Current research on leaf-impression and permineralized bog floras provide useful constraints on paleoecological interpretations based on petrified wood occurrences/identification.