Geologic controls on the occurrence and movement of water in the Lower Cienega Creek Basin

Abstract

The occurrence and movement of water in lower Cienega Creek Basin is controlled mainly by the depth of bedrock and its geometry. Gravity data collected in the basin and its margins was inverted to model geologic cross-sections. Depths from these model cross-sections were then contoured to generate isopach maps and a depth to bedrock map. These maps indicate a depth of over 1,800 feet in the northwest portion of the basin and a thick wedge of denser, less permeable Pantano rocks in the same area. Shallow bedrock and Pantano rocks are thought to block ground water flow on the northwest edge of the basin and force it to the surface to generate perennial flow in Cienega Creek Natural Preserve. In addition, the possible existence of a structural trough that may channel ground water from the upper basin into the San Pedro Basin was detected with gravity data. Temperature and electrical conductivity data support the hypothesis that the source of perennial flow in Cienega Creek Natural Preserve is simple overflow from the lower basin and that the intermittent flow in the Preserve is due to variations in the thickness of the stream channel alluvium.