Variation of physical properties of alluvium in an arid basin

Abstract

We have used geologic and geophysical data from 139 drill holes in Yucca Flat, an alluvium-filled basin within the Nevada Test Site, to study the ways in which differing sources and transport processes of the alluvium are reflected in variations in physical parameters. Bulk density, grain density, pore water, and carbon dioxide are routinely measured in the subsurface alluvium for each drill hole, and porosity is calculated from the first three of these parameters. The alluvium is derived principally from three source rocks exposed along the perimeter of the basin: Plleozoic elastic and carbonate sedimentary rocks, and Tertiary volcanic tuff. The alluvium of Yucca Flat is subdivided into an older volcanic-rich basal tuffaceous alluvium and an overlying younger mixed alluvium; the latter is composed of both Paleozoic and Tertiary fragments, except near isolated source terranes, where it may be derived from only one of these rock types. The mixing of different sources where individual fans merge and coalesce at the center of the basin is reflected by progressive changes in the physical properties. Alluvium consisting of dominantly Paleozoic debritus has high density and relatively low water content and porosity, whereas alluvium composed of dominantly volcanic debris has low density and high porosity and water content. Mixing of these deposits near the center of the basin is reflected by intermediate densities. Water content and porosity are greatest at the center of the basin, where the alluvial sediments have undergone the greatest amount of distal fining and sorting.