MIOCENE FLORA OF THE ALVORD CREEK FORMATION, OREGON; INTERPRETATION OF ANCIENT CLIMATE

The Alvord Creek Formation is located in the southeastern corner of Oregon, in the Steens Mountains. The Alvord Creek beds (light colored tuffs and tuffaceous shales with interbedded andesite flows and basaltic intrusives) are overlain by and interbedded with the Pike Creek Volcanic Series (rhyolite/dacite) flows. The proximity, duration, and intensity of a low shield cone affected the thickness of the ash layers. The Alvord Creek fossil plant localities are in siliceous, tuffaceous shales and are approximately 21.3 Ma. An unconformity exits between these siliceous volcanic formations and the overlying Steens Basalts. Miocene angiosperm plants collected from Alvord Creek are compared with extant plants using leaf morphology. Preliminary field identification indicates cherry (Prunus), hornbeam (Carpinus), juniper (Juniperus), maple (Acer), rose family (Rosaceae), sumac (Rhus) and willow (Salix) are present. Previous work enumerated 26 species in the Alvord Creek paleoflora. Plant physiognomy and habitat requirements of the nearest extant relatives will be used as proxies for temperature and moisture conditions to interpret Miocene climate.