EMPLACEMENT RATES OF COLUMBIA RIVER FLOOD BASALT FLOWS-FAST, SLOW OR VARIABLE

Shaw and Swanson (1970) proposed a flow-emplacement model for voluminous sheet flows of the Columbia River flood basalts (CRFB). Based on Swanson’s extensive field experience and Shaw’s modeling skills, they envisioned a scenario of rapidly moving, turbulent flows emplaced in as little as a week or few weeks. In 1996, Self and others proposed an alternative view with emplacement rates of decades to centuries, based largely on the idea of thermal insulation associated with flow inflation, as observed in contemporary lava flows in Hawaii and elsewhere. Ho and Cashman (1997) used heat loss estimates on the Ginkgo flow to constrain such models, concluding that flow emplacement was unlikely to have been turbulent, and more likely associated with fast-laminar flow, with little heat loss (20^oC) from the source to the Oregon Coast, a distance of over 350 km.

The application of emplacement models for the CRFB flows requires detailed field studies and close examination of each stratigraphic unit. In our view, emplacement rates are quite variable throughout the stratigraphic section. The oldest formation of the CRFB, the Steens Basalt, is composed of stacked successions of thin flow lobes combined into thicker compound flows, or flow fields. Flow lobes commonly show evidence of flow inflation but with limited geographic extent, similar to contemporary examples. Evidence for flow inflation in the much larger sheet flows of Grande Ronde Basalt (GRB) and Saddle Mountains Basalt (SMB) is also apparent, but with variable rates of emplacement. For example, the Asotin and Umatilla Members (SMB) and a Sentinel Bluffs Member flow (GRB) erupted distinct compositions along their linear vent systems. In each case, several chemically distinct flows erupted closely in time from their source vents. About 200 km west of their vents, these flows are no longer distinct, but instead they exist as compositional zones of a single, moderately mixed lava flow. Such flows must have been emplaced rapidly in perhaps weeks to months. We conclude that emplacement rates may be quite variable throughout the CRFB province, with thin flow units of Steens Basalt erupting rapidly, and larger inflated sheetflows erupting over variable time spans, some from a few weeks to months, and others over a duration of years.