Compaction profiles of ash-flow tuffs: Modeling versus reality

Density profiles of 4 ash-flow deposits in Oregon and Idaho are simulated using the model of Riehle et al. (1995) to calculate heat flow, degassing, and compaction. The deposits are all < 45 m thick and most have well defined density reversals and lack substantial alteration or vapor-phase deposition. Model results are made to closely match the measured profiles of 3 simpler deposits by making the assumptions that density reversals represent cooling breaks between ash flows or that subtle density inflections at the base of the profiles are cooler surge deposits. The first assumption is supported by a fossil fumarole pipe truncated at a density inflection, and the second by foreset bedding at the base of the deposit. Deposit temperatures within each unit of thermally stratified deposits are assumed to be homogeneous.The Rattlesnake Tuff—a more complex ignimbrite sheet—was sampled at 3 distal sites near to one another, a medial site, and 4 proximal sites. Model results of distal profiles are similar despite more than twofold variation in thickness and comprise 3 early deposits having emplacement temperatures of 724–732 °C, followed by two hotter deposits (785–790 °C), and a cooler capping deposit of about 735 °C. One distal site has an inferred surge deposit at its base. Density reversals simulated as cooling intervals all range from 3 to 10 days. Correlated cooling intervals agree to within a factor of two, however, the duration of a cooling interval cannot be precisely hindcast owing to potential complications by rainfall or accumulation of chilled airfall ash. Thus, the seemingly wide range of model results for the cooling intervals is perhaps not surprising.Proximal profiles have more density reversals implying more deposits than the distal profiles. They also have greater overall compaction and consequential loss of detail in the density profiles. Some earlier deposits have model temperatures of 736–757 °C and may correlate with the lower distal deposits; at one site these are overlain by two hotter deposits (790–796 °C) and a cooler capping deposit (745 °C) that are almost surely correlatives of the upper distal deposits. Some sites have additional, early hotter deposits and one site has an uppermost deposit that is the coolest of all model deposits. Thus a picture emerges of thermally stratified, near-source flows during early eruptive activity and more far-travelled flows during later eruptions. Shards analyzed by Streck and Grunder (1997) show a compositional range and corresponding magma temperatures of 795–880 °C, which is permissive of thermally layered deposits and implies cooling during fallback and travel of 50–100 °C.Rare textural evidence for internal flow boundaries, as well as our consistent model results among multiple sites despite variation in deposit thickness, support the assumption that density reversals represent partial cooling breaks. If so, then modeling of density profiles can provide important evidence for how large ignimbrites are constructed. Sheridan and Wang (2004) also report stratigraphically consistent results by modeling density reversals in distal Bishop Tuff as cooling breaks. Our results contrast with those of Wilson and Hildreth (2003), who concluded that density reversals in the proximal Bishop Tuff do not represent cooling breaks but instead are evidence for thermally heterogeneous deposits. It may be that the proximal Bishop Tuff is not amenable to modeling due to its great thickness (100–200 m) and deposition on steeply dipping, rugged terrain, factors which would likely cause turbulent flow, obscure subtle stratigraphic details of flow boundaries, and complicate resulting compaction zonation.