The principal objectives of this study are to determine the eruption rates and emplacement mechanisms for the flows of the Sentinel Bluffs Member, Grande Ronde Basalt. The size and extent of the youngest flows of the Columbia River Basalt Group are well established because of their distinct properties, but little is known about the most voluminous formation, the Grande Ronde Basalt, which consists of over 149,000 km3 of basalt that was erupted in a little over 1 million years. This study will use a large existing database of stratigraphic sections and borehole data, lithologies, chemical compositions and magnetic polarities supplemented with new stratigraphic and chemical data to meet these objectives. The data will first be used to refine the stratigraphy of the Sentinel Bluffs Member and determine the extent and volume of the flows. Once a well-defined stratigraphy is established, compositional variations through the flows will be compared to intraflow structures. This technique has been used previously to determine flow emplacement mechanisms, and to constrain eruption rates. The volume of the flows coupled with the compositional variations also will provide new constraints on petrogenetic models for the Grande Ronde Basalt. In addition, the size of the flows and their eruption rates will provide new insights into the impact these huge eruptions had on the Miocene climate.
