EFFECT OF POORLY CONSTRAINED CALDERA LOCATIONS ON IGNIMBRITE VOLUME ESTIMATES, HEISE VOLCANIC FIELD, EASTERN SNAKE RIVER PLAIN, IDAHO

Silicic eruptive centers along the trend of the Yellowstone hot spot are largely concealed by younger basalts. The location of calderas and volume of eruptive products have been inferred for many centers from sparse outcrop and deep well data. Geophysical methods have not proven generally successful in defining Snake River Plain calderas. As a result, uncertainties for eruptive volumes may be much greater than is generally appreciated.

We examine this issue with examples from the 6.6 – 4.5 Ma Heise volcanic field, the second-to-youngest center along the Yellowstone trend. While locally excellent, exposures of Heise rhyolites are commonly obscured by extensive Pleistocene basalt flows and loess deposits. For this reason, relationships between Heise outcrops on the southeast side of the Snake River Plain and those on the northwest, a distance of 70 to 100 km, have been problematic. Only three deep wells (2-2A, INEL-1 and WO-2) in the middle of the Snake River Plain and a fourth (Sugar City) on the southeast margin substantially sample Heise rocks. Over the past decade, advances in ⁴⁰Ar/³⁹Ar dating and micro-sampling of phenocrysts and glass for major, trace element, and isotopic analysis have resolved many correlation problems. Four regional ignimbrites (Blacktail Tuff, Lake Walcott Tuff, Conant Creek Tuff, and Kilgore Tuff) are now securely identified.

Despite this progress, the location of Heise calderas and the nature of deposits within them remain conjectural. Intracaldera facies have been identified at only a handful of localities and the thickness of intracaldera deposits are essentially unknown. This has prompted the use of model or analog data for intracaldera thicknesses. For example, Morgan and McIntosh (2005) used reasonably well-known outflow deposit data with average thicknesses of “50-75 m” and highly approximate intracaldera thicknesses of “>1000 m” for their widely quoted Heise ignimbrite volumes. Improved volume estimates will require continued geologic mapping coupled with innovative geochemical methods, sampling of deep wells, and enhanced geophysical techniques that uniquely identify caldera locations and geometries.

Morgan, L.A. and McIntosh, W.C., 2005, Geol. Soc. Amer. Bull. 117, p. 288-306.