Volcanology, geochemistry, and stratigraphy of the F Basalt flow group, eastern Snake River Plain, Idaho.

Abstract

The eastern Snake River Plain (ESRP) volcanic basin in southeast Idaho is underlain by approximately 1 km of dominantly Pliocene-Quaternary olivine tholeiite basalt and interbedded sediment. The F basalt flow group is a stratigraphic marker bed near the top of the regional aquifer and underlies a portion of the Idaho National Engineering and Environmental Laboratory, where radiochemical and chemical wastewater has been discharged to the aquifer. This flow group erupted during an unusual, short-lived period of reversed magnetic polarity approximately 565 ka, probably in < or =200 years. This study uses new petrographic, geochemical, and isotopic analyses of the flow group to refine the subsurface stratigraphy. This sequence of lava flows is uneroded, apparently comagmatic, and is observed in drill core over an area of approximately 75 km (super 2) between approximately 120-220 m depth. Lithologic logs for six sections of F flows in drill-core reveal textural discordance within the sequence and that the thickest ( approximately 55 m) lie in the southwest part of the study region and contain an upper portion ( approximately 15-23 m) that is texturally coarser and significantly enriched in incompatible elements relative to the remainder of the sequence. In addition, lava flows in the lower sequence have lower initial (super 87) Sr/ (super 86) Sr isotopic ratios than the upper flows (0.7068 vs. 0.7071) while all exhibit similar (super 143) Nd/ (super 144) Nd isotope ratios ( approximately 0.5124; epsilon (sub Nd) approximately -4.3). Petrographic, isotopic, and geochemical features support correlations between sampled sections and define two flow groups within the F sequence. Variations in the texture and stratigraphy of the two flow groups indicate that they were derived from multiple coeval eruptive centers aligned along a common rift or fissure system, and not from a central vent complex. The stratigraphy of the entire F sequence is consistent with formation by constructional volcanic processes and is unaffected by post depositional structural offsets.

73 samples from this reference