Assembling an Ignimbrite: Mechanical and Thermal Building Blocks in the Bishop Tuff, California

Abstract

Abstract The building blocks that control ignimbrite physical characteristics are here defined as mechanical (flow units and flow packages) and thermal (cooling units). Ignimbrite construction from these mechanical and thermal building blocks is illustrated by a longitudinal profile of the Bishop Tuff in the Owens River Gorge, California. In the Bishop ignimbrite, flow unit boundaries can be defined only locally, but known or inferred multiple flow units combine to form eruptive packages showing imbricate, offlapping relationships such that each package is thickest successively farther from the source. In these packages, the presence or absence of flow unit boundaries does not necessarily reflect directly the presence or absence, respectively, of time breaks; thus massive ignimbrite need not be the product of continuous accumulation (progressive aggradation). The intrinsic thermal properties of each package (or parts thereof) have combined with local thicknesses of accumulated material to control the weld...