ORIGIN OF LAVA BENCHES AND UPPER AND LOWER MESA FALLS IN THE HENRYS FORK AND WARM RIVER CANYONS, ISLAND PARK, IDAHO

The headwaters of the Henrys Fork River—including Warm River—lie in the Island Park area of eastern Idaho. In southern Island Park, these rivers form deep canyons through the flank of the Yellowstone I caldera, then join and flow west onto the Snake River Plain. The Island Park area developed as the Yellowstone I & II calderas formed and then were partially filled—by rhyolite lava domes and flows that developed near the margins of and within the calderas, and by basalt flows, glacial outwash, alluvium, the Lava Creek Tuff and rhyolite lava flows from the Yellowstone III caldera, and loess. The Henrys Fork and Warm River canyons contain lava benches formed when local basalt lava flows entered the canyons, solidified, and were later incised.

We correlate lava benches inside the canyons to local basalt lava fields using field relations, petrography, remanent magnetization, and chemical composition. The five lava-bench remnants in Henrys Fork River canyon are from the Warm River (~835 ka), Elk Wallow Well (~743 ka),Highway 20 (~737 ka), Survey Draw (~450 ka?), Hatchery Butte (~81 ka), and Pinehaven (~29 ka) lava fields. The three lava-bench remnants in Warm River canyon are from the Warm River, Survey Draw, and Hatchery Butte lava fields.

The Henrys Fork River canyon contains Upper and Lower Mesa Falls and other small falls and cascades. We use density variations in the Mesa Falls Tuff in the Henrys Fork River canyon to infer that these falls result from high-density/highly-welded zones in the Mesa Falls Tuff. Changes in gradient of the top of the Pinehaven, Hatchery Butte, and Survey Draw lava benches suggest that Upper and Lower Mesa Falls existed downstream from their current locations in the past. We estimate waterfall retreat rates of 1-20 mm/yr. The migration of the falls was halted at least three times by lava. The density profile of the Mesa Falls Tuff suggests that at least three ash deposits built the Mesa Falls Tuff, as follows: a layer over 150 feet(46 m) thick was emplaced at about 800 °C; after a break of months to years and before the lower layer had finished welding, a layer about 140 feet (43 m) thick was deposited, also at about 800°C; then, following a break of at least days, a layer 135 feet (41 m) thick was emplaced at about 630 °C; finally, after a break of days to months, a 50 feet (15 m) thick layer at about 700 °C capped the unit.