TOPOGRAPHIC AND HYDROGEOLOGIC INFLUENCES ON DEPOSITION IN THE HUECO BOLSON AQUIFER, RIO GRANDE RIFT

This is an outgrowth of a study designed to help determine the influence of stratigraphy and faulting in an 8 by 20 km area in the Hueco Basin into which wells were drilled to provide brackish water for a desalination plant. During analysis of cuttings and logs from these wells, we defined four units that reflected abrupt changes in the sedimentation patterns in the basin. The abrupt rearrangements of facies that occur in the upper part of the Hueco Basin fill are inferred to result from changes in basin topography and climate, with the study area becoming markedly drier and changing from a topographic low to a local high. The study area evolved from a broad playa that likely received sediment from a large part of the Hueco/Tularosa Basin drainage and discharged groundwater into adjacent basins to a low gradient alluvial fan, with local playas remaining in the southeast corner of the study area. The large playa evident in Unit 1 is segmented and smaller playas associated with fluvial and eolian strata fill form Unit 2, reflecting climate change. The transition to Unit 3 could result from climate fluctuations that resulted in dramatically reduced basin floor deposition, or it could reflect tectonic segmentation of the basin floor. With a broad flat basin floor, even small offsets on fault scarps can redirect basin floor stream systems. The change in relative basin floor elevation from a topographic high, to a low into which the Rio Grande discharged and built a fan occurred during the Middle Pleistocene. This was probably due, not to tectonism, as the Hueco Basin has continued to exhibit greater Late Pleistocene subsidence than the Mesilla Basin to the west, but due to filling of the Mesilla Basin by the Rio Grande, until it overtopped the divide between the basins. Therefore abrupt changes in deposition reflect climatic factors as well as relative changes in elevation of the area related to tectonism within the basin and regionally.