This report summarizes the surficial geology and geomorphology of the Tinajas Altas area on the Barry M. Goldwater Air Force Range in southwestern Arizona (Figure 1). The purpose of these investigations is to provide a geologic and geomorphic framework for an archaeological survey of this area. The geologic map included with this report covers parts of several 1:24,000-scale topographic quadrangles that cover the study area (Plate 1). Interpretation of aerial photographs was done primarily by Karen Demsey, with review and field checking by Thomas Biggs, with field assistance from Philip Pearthree of the Arizona Geological Survey. Map compilation and report preparation was done by Biggs; final editing and formatting of this report was done by Pearthree.

Throughout the southwestern United States, vegetation in what historically was grassland has changed to a mixture of trees and shrubs; exotic grass species and undesirable shrubs have also invaded the grasslands at the expense of native grasses. The availability and amount of soil nutrients influence the relative success of plants, but few studies have examined fire effects on soil characteristics in a temporal, spatial, and species group specific fashion. Likewise, few studies have tied fire effects and ecological aspects to the underlying geology. Our research investigates the effects of fire events on selected soil characteristics pH, nitrate (NO3-), plant-available phosphorus (PO4 -3), and total organic carbon (TOC) on native grass-, exotic grass-, and mixed grass-dominated plots distributed on four different geological surfaces. Treated and control plots were sampled prior to burn treatment and at intervals after the burns. In addition to new geologic mapping of the study areas, results indicate the geologic substrate is the most important variable for explaining pH, NO3- and PO4-3 values in the soils. Dominant grass type –native, non-native, or mixed – had little effect on the response of soil geochemistry to fire events: post-burn results indicate vegetation was a significant factor only for TOC. Recovery to pre-burn levels varies with characteristic: there were no significant initial differences between vegetation types, but significant differences in NO3-, PO4-3, and TOC amounts occur as a result of fire events, geological characteristics, and time. The research helps identify the soil response to fire and the recovery times of soil characteristics, further defines which fire frequency is optimal as a management strategy to maximize soil macronutrient contents, and illustrates the important role geology plays in grassland ecosystems.