Subsurface volcanology at Test Area North and controls on groundwaterflow

Abstract

Detailed core logging, paleomagnetic measurements, and an internallyconsistent geochemical database are used to develop a picture of thesubsurface geology above the Q-R interbed at Test Area North in thenorthern part of the Snake River Plain. Our argon dating and minorchanges in paleomagnetic inclination match perfectly those of Lanphereet al., indicating that the 120 m sequence erupted in a short timeinterval (<1 k.y.) over the entire area, ca. 1.13+ or -0.24 Ma. Theflows are inflated pahoehoe lavas, and their brecciated and weatheredflow tops provide preferential flow paths that dominate ground-waterflow. The compositional variation of the basalts is relatively large,the compositional evolution is markedly consistent, and internaldifferentiation of individual cooling units is only locally important.Thus, careful logging coupled with chemical analysis is the idealcorrelation tool. Our cross sections are substantially different fromthose previously constructed because we can demonstrate that most ofthe flows are much smaller than previously supposed and disprovecertain correlations. At least nine flow groups are identifiable andonly two lava flows extend across the area. Several of the flows havewidths <100 m in the line of section. The most important contactfor groundwater flow pinches out to the northwest and thinsdramatically to the east, the direction that the contaminant plume isoriented. Anisotropy of magnetic susceptibility measurements were madeto permit estimates of lava flow directions. These data cluster aboutN70W and N25E, which we believe are the directions to two vents thaterupted these lava flows. Our interpretation is that this terrain wasmuch different than that currently exposed at the surface of the SnakeRiver Plain, and involved many long, narrow flows emplaced on anirregular surface at a very rapid accumulation rate.

48 samples from this reference