HEISE CLIFFS VOLCANICS: A PRELIMINARY GEOLOGIC MAP OF THE HEISE CLIFFS AREA, ID

The Heise cliffs lie near the southern margin of the Snake River Plain near Ririe, Idaho. Volcanic rocks exposed in this area belong to the Yellowstone Snake River Plain (YSRP) magmatic province. These rocks record the volcanic history of the Heise caldera complex (6.6 – 4.5 Ma). We report detailed mapping of these volcanic units, which lie next to and above Paleozoic and Mesozoic sedimentary rocks and are covered in places by alluvium, colluvium, and loess.

The igneous units exposed in the area record three volcanic episodes: pre-Heise, Heise, and post-Heise. The pre-Heise episode includes the Arbon Valley Tuff (10.2 Ma), a basalt, and the tuff of Newby Ranch (7.5 Ma). The Heise episode contains six units: the Blacktail Tuff (6.6 Ma), the Kelly Canyon Rhyolite (5.7 Ma), the tuff of Wolverine Creek (5.6 Ma), the tuff of Elkhorn Springs (5.6-5.8 Ma), the tuff of Hawley Gulch, and the Kilgore Tuff (4.5 Ma). The post-Heise episode includes the Huckleberry Ridge Tuff (2.1 Ma), the basalt of Antelope Flat and basalt of Lyons Creek (~2 Ma), and the Lava Creek Tuff (~630,000 ya).

Previous workers mapped the southern boundary of the Blacktail caldera in Ririe Reservoir to the west and along Canyon Creek to the northeast. However, our observations do not allow the caldera boundary to extend through the Heise cliffs. Either 1) the Blacktail caldera boundary lies to the north, buried in the Snake River Plain or 2) the caldera experienced nonuniform collapse along its southern boundary, i.e., the Ririe Reservoir area records “trap door” collapse.

Structurally, this area contains a step-over zone, which connects the Grand Valley fault to the south to the Rexburg fault to the north. A series of down-to-the-south normal faults accommodate extension in the step-over zone.