SCIENTIFIC DRILLING IN THE SNAKE RIVER PLAIN, IDAHO: CHEMICAL-STRATIGRAPHIC RELATIONSHIPS IN WENDELL RASA AND SUGAR CITY DRILLCORE

The eastern Snake River Plain (SRP) preserves a 1–2 km record of young and tectonically undisturbed basalt where the time-integrated history of the passage of continental lithosphere over a relatively fixed magma source, the Yellowstone plume, can be traced. Drilling offers the best opportunity to view the entire volcanic record. Work on drill core has been limited exclusively to the Idaho National Laboratory site. We report here geochemical data for two fully cored test wells located at opposite ends of the eastern Snake River Plain.

The Sugar City geothermal test well (~2300 ft total depth), located near Rexburg, ID, cored ~ 500 ft of basalt and 1700 ft of rhyolite lava flows and tuffs on the western margin of the Henry’s Fork caldera complex. We analyzed 16 samples from 6 basalt flows, located within the first 500 ft of core. Basalts can be separated into 3 flow groups stratigraphically, separated by sediment horizons or tuff. Group 1 lavas (120-200 ft) can be divided into 3 distinct lava flows. Group 2 lavas (230-320 ft) can be separated into flows 2a and 2b; they differ in SiO₂ (50 vs. 54), Fe₂O₃ (16 vs. 11), and Al₂O₃ (14 vs. 16). Groups 1 and 2 are located above the Huckleberry Ridge Tuff (320’). Group 3 lavas (360-440 ft) are low in SiO₂ (~48), low in K₂O (<0.5), but high in MgO (~8).

The Wendell RASA hydrologic test well, located near Wendell, ID, is ~1100 ft deep. This well penetrated an upper basalt unit from 1 to 403 ft and a lower basalt unit from 590 to 1072 ft. We analyzed 64 samples from 52 basalt flows. Major element data identified 11 cycles: 5 fractionation cycles, 3 recharge cycles, and 3 cycles that did neither. Major element data are consistent throughout the core; SiO₂ ranges from 46-49, Al₂O₃=12-16, Fe₂O₃=12-16, MgO=6-9, and CaO=9-11. There is an overall increase in K₂O; values of 0.2 wt% are common at the bottom of the hole, while at the top, reaches as high as 0.7 wt%.

These two wells offer different histories into intra-crustal processes of the SRP. Studies from existing core of the eastern SRP show that young basalts form a series of fractionation and reversed cycles. We can only conclude that the Wendell well provides further insights into this type of system. Sugar City, however, provides no such account and can only be concluded that basalts evolved from discrete magma batches or pulses.