Bedrock Morphology and Structure, Upper Santa Cruz Basin, South-Central Arizona, with Transient Electromagnetic Survey Data By Mark W. Bultman and William R. Page U.S. Geological Survey Open-File Report 2016-1152 ABSTRACT The upper Santa Cruz Basin is an important groundwater basin containing the regional aquifer for the city of Nogales, Arizona. This report provides data and interpretations of data aimed at better understanding the bedrock morphology and structure of the upper Santa Cruz Basin study area which encompasses the Rio Rico and Nogales 1:24,000-scale U.S. Geological Survey quadrangles. Data used in this report includes the Arizona Aeromagnetic and Gravity Maps and Data referred to here as the 1996 Patagonia Aeromagnetic survey, Bouguer gravity anomaly data, and conductivity-depth transforms (CDTs) from the 1998 Santa Cruz transient electromagnetic survey (whose data is included in appendixes 1 and 2 of this report). Analyses based on magnetic gradients worked well to identify the range-front faults along the Mt. Benedict horst block, the location of possibly fault-controlled canyons to the west of Mt. Benedict, the edges of buried lava flows, and numerous other concealed faults and contacts. Applying the 1996 Patagonia aeromagnetic survey data using the horizontal gradient method produced results that were most closely correlated with the observed geology. The 1996 Patagonia aeromagnetic survey was used to estimate depth to bedrock in the upper Santa Cruz Basin study area. Three different depth estimation methods were applied to the data: Euler deconvolution, horizontal gradient magnitude, and analytic signal. The final depth to bedrock map was produced by choosing the maximum depth from each of the three methods at a given location and combining all maximum depths. In locations of rocks with a known reversed natural remanent magnetic field, gravity based depth estimates from Gettings and Houser (1997) were used. The depth to bedrock map was supported by modeling aeromagnetic anomaly data along six profiles. These cross sectional models demonstrated that by using the depth to bedrock map generated in this study, known and concealed faults, measured and estimated magnetic susceptibilities of rocks found in the study area, and estimated natural remanent magnetic intensities and directions, reasonable geologic models can be built. This indicates that the depth to bedrock map is reasonable and geologically possible. Finally, conductivity-depth transforms (CDT) derived from the 1998 Santa Cruz Basin transient electromagnetic survey were used to help identify basin structure and some physical properties of the basin fill in the study area. The CDTs also helped to confirm depth to bedrock estimates in the Santa Cruz Basin, in particular a region of elevated bedrock in the area of Potrero Canyon, and a deep basin in the location of the Arizona State Highway 82 microbasin. The CDTs identified many concealed faults in the study area and possibly indicate deep water-saturated clay-rich sediments in the west-central portion of the study area. These sediments grade to more sand-rich saturated sediments to the south with relatively thick, possibly unsaturated, sediments at the surface. Also, the CDTs may indicate deep saturated clay-rich sediments in the Highway 82 microbasin and in the Mount Benedict horst block from Proto Canyon south to the international border. DISCLAIMERS Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner." Although these data have been processed successfully on a computer system at the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. The USGS or the U.S. Government shall not be held liable for improper or incorrect use of the data described and/or contained herein. FILES INCLUDED IN THIS REPORT ofr20161152_Readme.txt ofr20161152_508.pdf ofr20161152_plate1.pdf ofr20161152_plate2.pdf ofr20161152_Appendix1.zip Area_2 (tif files: 201, 201-NE, 202, 202-NE, 204, 204-NE, 205, 205-NE, 206, 206-NE, 207, 207-NE, 208, 208-NE, 209, 209-NE, 210, 210-NE, 211, 211-NE, 212, 212,NE, 6001, 6001-SE, 6002, 6002-SE, 6003, 6003-SE, 6004, 6004-SE,6005, 6005-SE) Area_4 (tif files: 410, through 436 and 8002) Area_5 (tif files: 501 through 520 and 9001 through 9004) cdt_info_from contractor.pdf CDT_key.pdf ofr20161152_Appendix1_readme.txt ofr20161152_Appendix2.zip CDT_gxf_files Area_1 (gxf files: 101a1 through 158a1 and 5001a1 through 5006a1) Area_2 (gxf files: 201a2,through 212a2 and 6001a2through 6005a2) Area_3 (gxf files: 301a3 through 319a3 and 3051a3, 3052a3 and 7001a3 through 7004a3) Area_4 (gxf files: 401a4,through 436a4 and 8001a4 through 8003a4) Area_5 (gxf files: 501a5 through 519a5, and 9001a5 through 9004a5) cdt_info_from contractor.pdf a1dtm.gxf, a1tc.gxf, a3dtm.gxf, a3tc.gxf, a3tf, gxf, a245dtm.gxf, a245tc, a245tf.gxf, area1.001through area5.001, pta02.out through pta11.out, readme.txt, readme1.txt through readme5.txt, USGS 1998 Santa Cruz GEOTEM.pdf HOW TO OBTAIN THE DIGITAL FILES: The digital files constituting the geologic map database of this report can be obtained via the Internet from the U.S. Geological Survey publications website. Go to the web page at http://dx.doi.org/10.3133/ofr20161152/ or https://pubs.er.usgs.gov/publication/ofr20161152 and follow the directions to download the files. The manuscript product is a Portable Document Format (.pdf) report, which requires Adobe Acrobat for viewing. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com