ERUPTION AND EMPLACEMENT OF A GRANDE RONDE FLOOD BASALT, COLUMBIA RIVER BASALT GROUP

The eruption of Sentinel Bluffs Member lavas marked the end of the greatest period of Columbia River Basalt Group volcanism, the Grande Ronde Basalt. The Sentinel Bluffs Member lavas erupted from a northerly trending vent system and flowed westward down an ancestral paleoslope covering over 169,700 km² and producing over 10,000 km³ of lava. The member is divided into six eruptions that are distinguished by their compositions. The first eruption was the most voluminous but volumes declined with later eruptions until the final eruption that produced the second largest volume of basalt.

Sentinel Bluffs flows are relatively homogeneous which allows them to be recognized throughout the province. One exception is the ‘Cohassett flow’ where four eruptions combined to form one local inflated lava. Compositional zonation reflects the sequence of eruptions. The original lavas remained intact except for mixing along their contacts, which are separated by thin zones of vesicles. A thick vesicle zone called the ‘interior vesicular zone’ marks the boundary between the last two eruptions and represents volatile-rich lava.

There are two compositional trends in the Sentinel Bluffs Member that are defined best by TiO₂ and P₂O₅. The six eruptions fall along one or the other trend but neither trend is defined by timing of eruptions or location along the vent system. Fractionation or eruption of a zoned magma chamber cannot explain adequately the six compositional types of the Sentinel Bluffs Member. The compositional types and the field relations are best explained by rapid changes in magma composition feeding the flows and rapid eruption and emplacement of the lavas.