GEOLOGIC SIGNIFICANCE OF THE BASE OF THE LATE CRETACEOUS ELKHORN MOUNTAINS VOLCANICS IN THE RADERSBURG AND GIANT HILL 7.5’ QUADRANGLES, SOUTHWESTERN MONTANA

Preliminary results from new geologic mapping (1:24,000-scale) in the Radersburg and Giant Hill 7.5’ quadrangles in southwestern Montana are used to describe the EMV section, which is about 750 m thick in the study area. Furthermore, we have examined the base of the EMV and the underlying volcaniclastic sandstones of the Slim Sam Fm. The Lower Member EMV comprises rhyodacitic to andesitic crystal-rich lavas and ash-flow tuffs, lahars, breccias, and tuff breccias. Lahars of the Lower Member EMV contain rounded pebble- to boulder-sized clasts of older Cretaceous sedimentary rocks and are intercalated with the Slim Sam Fm. Fiamme indicate a few units are pyroclastic and part of a large ignimbrite eruption. While the Middle Member EMV – a sequence of rhyolite ignimbrites up to ~450 m thick – has fairly well-constrained ages between 85–83 Ma, the Lower Member EMV has proven difficult to date but is thought to be older than 85 Ma. Lower Member EMV are intruded and brecciated by hornblende- and/or pyroxene-bearing diorite and granodiorite, which we interpret to be a network of hypabyssal sills, stocks and dikes. The age and geometry of Late Cretaceous mafic intrusions need further study to determine if they are part of the Lower Member EMV or if they belong to a younger (~80 Ma) pulse of mafic volcanism documented to the west in the Elkhorn 7.5’ quadrangle.

The base of the EMV is marked, in places, by a 10–20-m-thick ignimbrite (ash-flow tuff) with a basal vitrophyre lying on the Slim Sam Fm. The Slim Sam Fm is a poorly bedded volcaniclastic sandstone containing abundant broken plagioclase crystals with a very fine-grained groundmass, locally altered to chlorite and sericite. Bivalve fossils and carbonate cement in some exposures of the Slim Sam Fm suggest a partially water-lain and marine depositional environment. We suggest that the basal EMV may represent a gradational transition from a near-shore marine, in which the Cody Shale, Telegraph Creek Fm, and Eagle Sandstone were deposited, to an island arc setting with mafic volcanism.

Below the Slim Sam Fm, we observe mudstone layers of the Telegraph Creek Fm that serve as planes of weakness for E-directed compression and shortening. We propose this zone of slip in the mudstones may be a regionally important, yet unrecognized, thrust fault with undetermined displacement.