LACUSTRINE-FLUVIAL DYNAMICS OF THE WESTERN LAKE IDAHO

Lake Idaho occupied the western Snake River Plain (WSRP) in the Mio-Pliocene as evidence by sediments known as the Chalk Hills and Glenns Ferry Formations. This stratigraphic and mapping study in the Vines Hill region roughly 40 km west of Vale, Oregon on US-20 aims to understand the depositional environments of the transitional strata between the conventionally named formations (Chalk Hills and Glenns Ferry). The primary stratigraphic section represents several fluctuations of lake level, siliciclastic input from the paleo-Malheur river, and an ash fall tuff thought to be the Faust Tuff. Regional mapping illuminates the extent of a previously undescribed or mapped lacustrine oolitic gastropod limestone suggesting significant chemical changes in the western lake. This mapping also shows the limestone unconformably overlying a small rhyolite vent. Geochemical analysis and K-Ar dating to be completed will reveal absolute age of this section and allow us to correlate these strata more closely with better known sections to the east.