Buechler, Walter K.; Dunn, Michael T.; Rember, William C.
LATE MIOCENE PICKETT CREEK FLORA OF OWYHEE COUNTY, IDAHO
A rich leaf and seed flora, diatoms, and palynomorphs have been recovered from late Miocene lacustrine sediments at Pickett Creek, Idaho. The sediments are part of the lower Chalk Hills Formation of the Idaho Group. Chemical analysis of two ash samples suggests an age range of 8.5-10.5 m.y.. While southwestern Idaho and southeastern Oregon are rich in middle Miocene floras, major late Miocene assemblages are rare and Pickett Creek is the youngest in this area. The examined fossil beds consist of 3 m of lacustrine sediments, overlaid by 3 cm of volcanic ash, 3.7 m of pure diatomite, and 15 m of lacustrine and fluvial sediments. They contain a diverse leaf flora of more than 45 leaf species in addition to fruits, including those of Acer, Ostrya, Salix, Fraxinus, Quercus, Pterocarya and Fabaceae species. In addition to random collections, a stratigraphic megafossil count was done on a surface of 60 x 100 cm, through a total depth of 293 cm below the diatomite. Pollen samples were taken every 30 cm. The most abundant megafossil taxa are Quercus prelobata (50%), Q. columbiana (9%), five Salix species (8%) and five Fabaceae species (5.7%). Based on the predominance of white oaks, the presence of several live oaks and dry-land species of Pinus (pollen), small leafed Fabaceae (leaves and fruits), Amaranthaceae, and Chenopodiaceae (pollen), the ecologically and climatologically most similar modern forest type is the broad-leaved forest of the Californian foothill woodlands. A high proportion of xeric leaf forms (Quercus simulata, Quercus hannibali, Quercus oberhi, Robinia species) indicates drier habitats, possibly on slopes above the lake. Evergreen species (Quercus hannibali, Q. simulata, Mahonia macginitiei, Lyonothamnus cf. parvifolius, possibly Quercus oberhi and some unassignable specimens) amount to about 6.5% of the collection. Two new species (Populus douglasae and Quercus oberhi) are described. Pollen analysis corresponds with most of the leaf families and suggests additional taxa from higher elevation and dry sites (Abies, Pinus, Amaranthaceae; Chenopodiaceae). The sediments contain a rich and variable diatom flora of more than 47 species, including four new Cymbella species. Based on megafossil plant remains and palynomorphs, the present study gives a detailed account of a fossil flora dominated by white oaks and growing under summer-dry conditions. It establishes well documented estimates for early late Miocene climate parameters and provides the earliest evidence for a Neo-gene Snake River Plain lake-system. Its position in the poorly known stratigraphy between the Chalk Hills and Poison Creek Formations makes Pickett Creek an interesting area for future geological and stratigraphic exploration.
Contributions from the Museum of Paleontology University of Michigan, 2007; Vol 31(12) : 305 - 362