Palynology and Age of the Alvord Creek Formation, Steens Mountain, Southeastern Oregon
The age of the Alvord Creek Formation of Steens Formation, southeastern Oregon, has been a center of controversy since Axelrod's 1944 paleobotanical assignment of a Lower Pliocene age to the tuffaceous leaf-bearing shales. This date varies from earlier determinations by Chaney and MacGinitie that the flora was Mascall (Upper Miocene) equivalent. However, it is more sharply contradicted by the report of a Middle Miocene (Barstovian) fauna in the apparently overlying Steens Basalt and by a 21.3 m.y. radiometric date obtained on a basalt flow 61 meters above the leaf-bearing beds. Palynomorphs recovered from three exposures of the leaf-bearing beds show only a fair to low correspondence at the generic level with the megaflora previously reported by Axelrod and Wolfe. Several new taxa are encountered in the palynoflora including the gymnosperms Tsuga, ?Taxodium, Podocarpus, and Ephedra and the angiosperms Quercus, Ulmus, Corylus, Juglans, Fraxinus, and Nyssa. Pollen assigned to the undivided Taxaceae-Cupressaceae-Taxodiaceae group, Quercus and Abies dominate the assemblage; monolete and trilete spores are conspicuously rare. The overall composition of the palynoflora lends support to Axelrod's Pliocene assignment. The abundance of Abies and other coniferous pollen combined with the lack of ferns and other typical Miocene taxa indicate a cool, dry climate such as is associated with the latest Tertiary. However, a tuff 15-20 cm above the leaf bed yielded a K/Ar date of 23.8 m.y.b.p., lowermost Miocene or even uppermost Oligocene. Stratigraphic relationships in Steens Mountain are obscure and may be complicated by normal or thrust faults, as has been suggested by some structural geologists. There is also the possibility that the K/Ar date determined for the tuff overlying the leaf bearing shales is in error. Whatever the reason(s) for the discrepant age assignments, there is not now a satisfactory resolution to the problem of discordant dating of the Alvord Creek Formation by paleobotanical and radiometric methods.
Geology, Paleobiology, Geology; Stratigraphic -- Pliocene. Palynology -- Oregon -- Steens Mountain., Loma Linda University. Geological Sciences Program -- Dissertations.
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