Origin of vesicle layering and double imbrication by endogenous growth in the Birkett basalt flow (Columbia river plateau)

The 40-m thick Birkett basalt pahoehoe flow at Sentinel Gap in the Columbia River Plateau has an unusually thick (≥15 m) upper vesicular zone. This zone includes a striking layering in which the layers have contrasted vesicle abundances and sizes. Most layers show a reverse grading of vesicle size and abundance. The layering is interpreted to have grown endogenously by the cyclic injection of vesicular lava layers under the growing top crust, accommodated by uplift of that crust. Grading of the layers resulted from vesicle growth and ascent. Each injection occurred at or near the boundary between vesicular and non-vesicular lava of the preceding layer and split that layer into an upper vesicular part and a lower non-vesicular part. Critical to this interpretation are (1) a pervasive foliation and lineation, defined by the parallelism of strongly flattened and elongate vesicles, transects the vesicle layers obliquely; and (2) the magnetic fabric (the anisotropy of magnetic susceptibility) is oriented similarly to the vesicle foliation, and also defines a cryptic foliation in the non-vesicular zone having a dip opposed to that in the layered zone. These foliations are interpreted to be opposed imbrications and indicate the flow azimuth of the lava. They strongly support the concept of lava growth by successive thin sill-like insertions of fresh vesicular lava between hot but static and effectively solid floor and roof.