Abstract
The Huckleberry Ridge Tuff (HRT) is the first voluminous silicic product erupted from the Yellowstone volcanic center. Differences in the elevation of the basal contact by approximately 500-1000 m between the western and eastern mapped extents have been used to argue for uplift of Yellowstone and subsidence of the Snake River Plain. Because uplift or subsidence of the tuff must have occurred after eruption at approximately 2 Ma, these differential elevations have also been used to estimate geologic uplift rates of the high terrain around Yellowstone that relate to its deeper geodynamics. To date, these estimates are approximate, and here I attempt to reconstruct the basal surface beneath the HRT in more detail using the mapped elevations at the basal contact and spatial interpolation techniques. The elevation of the base contact could represent the shape of the topography at the time of eruption, provided the area has not been substantially deformed by faulting or differential rock uplift. Alternatively, if the basal surface has been substantially deformed since eruption, the shape could also be used to elucidate the deformation patterns, perhaps in addition to paleo-relief. I focus on the northern margin of Yellowstone where remnants of the basal HRT deposits are mapped in the Madison, Gallatin, and Yellowstone River valleys. Their map pattern and thickness imply that there was at least some topographic relief at approximately 2 Ma to guide the flows down valleys. Using available 1:100,000 scale digitized geologic maps and a 90 m Digital Elevation Model (DEM), I first extracted the elevations at the mapped basal contact, and then made a preliminary reconstruction of the surface using universal kriging. The resulting surface has substantial topography. In the Gallatin River Valley, it has on the order of 500-600 m of relief that mimics the ridge and valley structure of the present-day topography, suggesting some topographic relief at the time of eruption. In other places the relief on the surface appears to be more controlled by fault structures that may represent post-eruption deformation. Ongoing work explores the effects of map and DEM resolution on the resulting surface and the variability in uncertainty estimates, but the reconstructed surface contains some interesting clues about the early Quaternary landscape and post HRT deformation around Yellowstone.