LITHOSTRATIGRAPHIC FEATURES AND POROSITY IN THE LOWER LITHOPHYSAL ZONE OF THE TOPOPAH SPRING TUFF IN THE CROSS DRIFT AT YUCCA MOUNTAIN, NEVADA

Five types of lithostratigraphic features in the lower lithophysal zone of the Topopah Spring Tuff are exposed in the cross drift tunnel at Yucca Mountain, Nevada. These features were mapped using four techniques and porosity of the rocks was calculated from distributions of features. The distribution of features and porosity are used in evaluating the drift design for the proposed high-level radioactive waste repository. The five lithostratigraphic features are groundmass (crystallized rock initially deposited as glass shards and pumice), lithophysal cavities, rims on lithophysal cavities, spots (similar to rims but with no cavity), and lithic clasts. Two-dimensional (2D) map data include 18 panel maps (1x3-m-size maps overlain on photographs) and a large (>0.5 m)-lithophysae inventory. All features in panels are mapped, so data are continuous, but the inventory only documents large lithophysae, so data are discontinuous. Linear traverses across the tunnel are 1D map data and include 22 angular traverses (edges of all features are recorded) and 186 tape traverses (spaced every 5 m, and only lithophysal cavities are measured, with rims and spots estimated). Angular data are continuous and tape data are discontinuous. Discontinuous data are easier to collect, but continuous data provide more spatial details. Tape data are used to develop distributions of features along the tunnel by (1) using moving averages, (2) scaling to angular data, (3) correcting for consistency with panel data, and (4) empirically adjusting for areas of sparse data. Moving 15-m averages for tape data resulted in both a reduced variance and the largest correlation coefficient (R2) compared to angular data, so correlation equations are used for the 1D data. Correlation of 1D and 2D data are poor, so empirical corrections are used. With the amounts of features known (locally, along the tunnel, or as an average of the total zone), the total porosity can be calculated by assigning porosity values to each type of feature. For example, with porosity values (cm3/cm3) for groundmass (0.13), cavities (1.00), rims and spots (0.25), and lithic clasts (0.10), the mean total porosity for the lower lithophysal zone (without large-lithophysae data) is 0.273 with a standard deviation of 0.045.