TIMING THE POST-CALDERA ONSET OF BASALTIC VOLCANISM ON THE EASTERN SNAKE RIVER PLAIN

After the passage of the North American Plate over the Yellowstone Hot Spot, volcanism on the Eastern Snake River Plain (ESRP) changed from Caldera-erupted rhyolites and tuffs to fissure and shield-volcano erupted basalts. Due to the veneer of Quaternary basalts covering the silicic caldera rocks, the contact between the two rock types is buried. Timing when the silicic volcanism died out and the post-caldera basalts began to erupt is therefore poorly constrained. The Gerrit Basalt (~199 ka) erupted through the Huckleberry Tuff (~2 Ma), which is the oldest member of the Yellowstone caldera group, and also through the Mesa Falls and Lava Creek Tuffs, dated 0.64 and 1.2 Ma respectively. This implies that the lag time between caldera volcanism and ESRP basaltic volcanism at Yellowstone may be between 0.64 and 2 Ma. In an effort to estimate the rhyolitic-to-basaltic volcanic lag time in older parts of the ESRP, previously-published basalt accumulation rates, age dates and depths to silicic caldera rocks for 3 deep wells at the Idaho National Laboratory were used to estimate the age of basalts at the basaltic-to-rhyolitic contact. Ages of near-by exposed rhyolitic tuffs of the Heise volcanic field were used as ages of the caldera rocks at the basaltic-to-rhyolitic contact. For two wells which penetrated silicic volcanic rocks with estimated ages between 6.0 and 6.5 Ma, the calculated ages of the basalts at the contact were 2.69 and 2.70 Ma. For the northernmost well, the estimated age of the silicic volcanic rocks was 4.3 Ma and the calculated age for the basalt at the contact was 3.93 Ma. Since the distribution of the silicic caldera rocks underneath the basalt veneer is poorly defined, it is possible that the uppermost rhyolitic tuff encountered in the most northern well has an age of 6.0-6.5 Ma instead of the original estimate of 4.3 Ma. In that case, if the basalt accumulation rates are correct, then the minimum time lag between silicic and basaltic volcanism is greater than 2 Ma. If the silicic caldera rocks in the northernmost well have an age date of ~4.3 Ma, then the lag time is ~0.4 Ma. This is similar to the minimum possible lag time for the Gerrit Basalt and suggests that the Gerrit Basalt may represent the beginning of the end of caldera volcanism and the onset of ESRP-style volcanism at Yellowstone. Estimating the bounds on the lag time will remain speculative, however, in the absence of more robust data.