Preliminary results of modeling the gravity anomaly field in the upper San Pedro Basin, southeastern Arizona

Abstract

A preliminary analysis is presented of the complete (terrain corrected) Bouguer gravity anomaly field in the area of the upper San Pedro Valley from near Fairbank, Arizona southward to the border with Mexico. Fifty-four new gravity stations were acquired in the area to supplement existing coverage. Preliminary models of the gravity anomaly suggest the presence of two northto north-northwest-trending structural subbasins beneath the upper San Pedro Valley. The northern subbasin, named the Huachuca City structural subbasin, contains about 1,100 m of basin-fill sediments, and the southern subbasin, named the Palominas subbasin, contains about 2,300 m of basin-fill sediments. The Palominas subbasin is probably a half graben, deepest on the western side adjacent to the Huachuca Mountains. A north-northwesttrending zone of Pleistocene fault scarps between the San Pedro River and the Huachuca Mountains may mark the location of part of the bounding fault zone. The gravity anomaly map suggests that the Huachuca City structural subbasin is separated and offset westward from the Palominas structural subbasin by a complex northeaseast-trending zone of relatively high bedrock beneath Fort Huachuca and Sierra Vista that contains several local bedrock highs and lows. No evidence was found to support the presence of a proposed buried bedrock high on the west side of the Palominas structural subbasin near Nicksville. Evaluation of well data indicates the bedrock high is an artifact of several mislocated wells on the draft structure contour map prepared by the Arizona Department of Water Resources.