Timing and Episodicity of Middle Eocene Volcanism and Onset of Conglomerate Deposition, Idaho

Abstract

Seventeen $^{40}Ar/^{39}Ar$ incremental release mineral ages from lava flows and tuffs in the Lost River and Lemhi Ranges, Idaho, (1) demonstrate deposition of thick sequences of syntectonic conglomerate in middle Eocene and Oligocene time and (2) reveal two temporally and compositionally distinct phases of the Challis Volcanic Group. Thick sections of coarse conglomerates are preserved in two NNW-trending half graben. These were previously assigned to the Pliocene ( ? ) but are reassigned to the Eocene and Oligocene based on ages from four tuffs interbedded with conglomerates. Andesitic and dacitic lava flows and tuffs of the Challis Volcanic Group, up to 2.5 km thick, were deposited between about 49 and 48 Ma, whereas rhyolite tuffs, only tens of meters thick, accumulated between about 46 and 45.5 Ma. The rhyolite tuffs are intercalated within or underlie the syntectonic conglomerates. Lithologic, stratigraphic, and geochronologic evidence correlate these tuffs with the tuff of Challis Creek, which erup...