Mid-Miocene record of large-scale Snake River−type explosive volcanism and associated subsidence on the Yellowstone hotspot track: the Cassia Formation of Idaho, USA

Knott, T. R., Branney, M. J., Reichow, M. K., Finn, D. R., Coe, R. S., Storey, M., Barfod, D. and McCurry, M. (2016) Mid-Miocene record of large-scale Snake River−type explosive volcanism and associated subsidence on the Yellowstone hotspot track: the Cassia Formation of Idaho, USA. Geological Society of America Bulletin, 128(7-8), pp. 1121-1146. (doi: 10.1130/B31324.1)

Abstract

The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River−type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (∼1900 km3). It correlates regionally and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Major- and trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.

• Item Type: Articles
• Additional Information: This work was funded by Natural Environment Research Council (UK) grants RP14G0070 and IP-1311-0512
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Barfod, Dr Dan
• Authors: Knott, T. R., Branney, M. J., Reichow, M. K., Finn, D. R., Coe, R. S., Storey, M., Barfod, D., and McCurry, M.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Geological Society of America Bulletin
• Publisher: Geological Society of America
• ISSN: 0016-7606
• ISSN (Online): 1943-2674
• Copyright Holders: Copyright © 2016 The Authors
• First Published: First published in Geological Society of America Bulletin 128(7-8): 1121-1146
• Publisher Policy: Reproduced under a Creative Commons License