NEW AGES FOR BELLEVUE FORMATION - IMPLICATIONS FOR QUATERNARY STRATIGRAPHY OF THE WOOD RIVER VALLEY, IDAHO

The Wood River valley is a basin and range graben on the north margin of the Snake River Plain. At the mouth of the Wood River valley a large alluvial fan formed at the junction with the Snake River Plain. The fan stratigraphy and morphology is complex and represents cycles of aggradation and downcutting in response to Quaternary volcanism in the Snake River Plain and glaciation in the Boulder and Pioneer Mountains. Eruptions of Quaternary basalt flows of the Bellevue Formation of Schmidt (1961) repeatedly dammed and diverted the ancestral Wood River channel both to the east and west. We sampled units of the Bellevue Formation for dating and paleomagnetic analysis. Results of ⁴⁰Ar/³⁹Ar geochronology yield ages of 1.45±0.16 Ma for the Macon basalt, 1.17± 0.21 Ma for the Lower Wind Ridge basalt, and 0.515± 0.063 Ma for the Hay basalt. Paleomagnetic analysis indicates normal polarity for the Hay and Macon basalts, and reversed polarity for the Wind Ridge basalt.

These data indicate the need to reinterpret the Bellevue Formation and the gradational history of the Wood River valley.