Timing and episodicity of Middle Eocenevolcanism and onset of conglomerate deposition
Document Type
Article
Journal/Book Title/Conference
Journal of Geology
Volume
101
Publisher
University of Chicago Press
Publication Date
1993
First Page
603
Last Page
621
Abstract
Seventeen 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 erupted at the end of Challis volcanism from the Twin Peaks caldera in central Idaho. This correlation expands the lateral extent of this tuff southeast of the caldera to at least 100 km. The distribution of the tuff of Challis Creek suggests that it was unimpeded by topographic relief across numerous synvolcanic NE-striking normal faults. Displacements on these faults either were too small to block the tuff, or depressions in the hanging wall had been filled by volcanic rocks before tuff emplacement. In the central part of the Lost River and Lemhi Ranges a magmatic hiatus of about 1-2 m.y. separates deposition of older intermediate-composition lava flows from deposition of younger rhyolite tuffs that may record rhyolite-dominated bimodal magmatism. Previous K-Ar ages of volcanic and intrusive rocks in the northern half of the Challis volcanic field suggest that this hiatus may be widespread. We speculate that the limited intermixing of mafic magmas during younger silicic volcanism, and abrupt decline of Challis volcanism after 48 Ma, reflect to a decrease in rates of magma input from the mantle when convergence between the Farallon plate and North America slowed.
Recommended Citation
Janecke, S. U., a n d S n e e , L. W., 1993, Timing and episodicity of Middle Eocene volcanism and onset of conglomerate deposition, Idaho: Journal of Geology, v. 101, p. 603-621.
