THE OLIGOCENE ISOM FORMATION, UTAH-NEVADA: A REGIONAL ASH-FLOW TUFF SHEET CONTAINING FLUIDAL FEATURES OF A LAVA FLOW

The Isom Formation (27-26 Ma), made of at least 8 units of dense, dark, crystal-poor trachydacite totaling >800 m in thickness and distributed over >15,000 km2 in SW Utah and adjacent Nevada, exhibits many of the features usually associated with lava flows. J.H. Mackin and his students interpreted these units as ash-flow tuffs, showing that they are made up largely of welded glass shards and have aspect ratios of <1:10,000. “Tufflavas” or “rheoignimbrites” similar to the Isom have been studied by workers worldwide, notably in Idaho by E.B. Ekren and his colleagues, who in 1984 described “lavalike ash-flow tuffs without calderas.” The Isom, however, is >10X greater in volume than Ekren’s “large” tuffs. Especially noticeable in the Isom are “lenticules,” a non-genetic term Mackin applied to light-colored, pancake-like layers of devitrified tuff as much as 10 m in diameter but usually <2 cm thick. We believe lenticules are devitrified gas-rich layers of tuff that separated and buoyed less gaseous layers, thereby enabling the entire mass of tuff to move by laminar flow. If a homely metaphor may be permitted, we think of the movement of the Isom ash flows as somewhat similar to tossing a hundred opened decks of brand-new playing cards into a basket, then lifting the basketful of cards high and dumping them onto a highly polished floor. This is a mechanism of movement heretofore undescribed, one that we admit to not yet fully understanding. Our tentative conclusions, however, are that the Isom erupted as extremely hot ash flows that initially moved by turbulent flow but, as they quickly deflated while spreading laterally, coalesced into plate-like coherent fluid masses that, buoyed by gas-rich lenticules, then moved by laminar flow for distances up to tens of kilometers. As the thicker masses of tuff deflated, or where tuffs “puddled” in a lowland or against a scarp, friction developed between the still moving viscous laminae, thereby leading to their folding.