EVALUATING THE TRANSLATION OF LITHOFACIES TO HYDROFACIES USING OUTCROP DATA

The opportunity to translate lithofacies into hydrofacies (bodies of sediment with distinct hydrologic properties) offers the sedimentologist a role in groundwater studies, particularly in characterizing aquifer heterogeneity. This is not a simple translation, however, but it can be quantitatively evaluated using well-exposed and accessible outcrops such as our field area in the Tesuque Formation (Miocene), near Española, New Mexico. There, poorly lithified sediment crops out in a heterogeneous assemblage of sand and silt with minor gravel lenses, and permits both detailed mapping and in-situ permeability measurements. The depositional setting was a piedmont, comparable to many Basin and Range settings, that accumulated sediment transported by channelized flow, overbank flow, and wind. We mapped depositional units and measured permeability on a stratigraphic panel 370 m long and ca. 30 m thick, defining eleven lithofacies as mapping units and collecting ca. 3200 air-permeability measurements. The range of most of the permeability values is 0.2 to 20 darcys. The quantitative nature of our field data allows us to characterize the lithofacies with both standard statistics and geostatistics, focussing on physical dimensions, geometrical relationships between lithofacies, permeability, and permeability structure. Some important results are that the distributions of permeability values are significantly different for most lithofacies, lending credence to the role of sedimentological information in characterizing aquifer properties. Within some facies, however, the distribution of permeability is too variable to allow straightforward translation of lithofacies into distinct hydrofacies. To complement these results, we will also present spatial statistical analyses of permeability.