GEOCHEMISTRY, STRATIGRAPHY, AND PETROGENETIC IMPLICATIONS OF THE F BASALT FLOW GROUP, EASTERN SNAKE RIVER PLAIN, IDAHO
SCARBERRY, Kaleb C., HUGHES, Scott S., and MCCURRY, Michael, Geosciences, Idaho State Univ, Pocatello, ID 83209, scarkale@isu.edu

The eastern Snake River Plain (ESRP) volcanic basin in southeast Idaho is underlain by ~1 km of dominantly olivine tholeiite basalt and interbedded sediment. This study refines subsurface stratigraphy of the F basalt flow group, a stratigraphically important sequence of uneroded, apparently comagmatic lava flows observed in drill core over an area of ~ 75 km2 between ~120-220 m depth. The ~565 ka F flow group erupted during an unusual, short-lived period of reversed magnetic polarity, probably in < 200 years. Detailed lithologic logs for six sections of F flows in drill-core reveal that the thickest (~50-60 m) lie in the southwest part of the study region, which we interpret to be near the vent. In each near-vent section 0.2-0.4 % higher P2O5 relative to values observed in the lower flow units characterize the uppermost lava flows. Chemical breaks are also identified in CaO/Al2O3 and K2O/TiO2 ratios. Preliminary trace element analyses indicate similar breaks at the same stratigraphic interval in incompatible elements. These chemostratigraphic characteristics support correlations between sampled sections. We interpret the F basalt flow group to have erupted from a source which sporadically tapped a periodically replenished magma chamber. Less evolved flows have slightly lower Sr-isotopic ratios than higher flows (0.7068 vs. 0.7071) while all exhibit similar Nd-isotope ratios (~0.5124). We infer that these isotopic signatures reflect magmatic derivation from an isotopically heterogeneous mantle source.

Cordilleran Section - 98th Annual Meeting (May 1315, 2002)
Session No. 30--Booth# 25
Pacific Northwest Geology East of the Cascades: In Honor of George W. Walker (Posters)
LaSells Stewart Center: Agriculture
1:30 PM-5:30 PM, Tuesday, May 14, 2002
 

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