### Bedrock morphology and structure, upper Santa Cruz Basin, south-central Arizona, with transient electromagnetic survey data

By: Mark W. Bultman**William R. Page**

                            https://doi.org/10.3133/ofr20161152

#### Links

-                                                                                                                             Document:                                                                                                                                                                                                                                                  **Report**                                                                                                     (8.93 MB pdf)                                                                                                                                     
- Plates:                               - **Plate 1 Map showing potential field boundaries plotted over upper Santa Cruz Basin study area geology**                            (135 MB pdf)                        
  - **Plate 2 Map showing conductivity-depth transforms plotted over upper Santa Cruz Basin study area geology**                            (101 MB pdf) 
- Appendixes:                               - **Appendix 1. Santa Cruz Transient Electromagnetic Survey Conductivity-Depth Transforms (CDT) Plots**                            (11.7 MB zip)                        
  - **Appendix 2. Santa Cruz Transient Electromagnetic Survey Data**                            (45.6 MB zip) 
- Read Me:  **Read Me**                            (8.0 kB txt)                        
- Download citation as:  RIS                         |                         Dublin                            Core

#### Abstract

First posted October 31, 2016

**For additional information, contact:**
Center Director, USGS Geosciences and Environmental Change Science CenterBox 25046, Mail Stop 980Denver, CO 80225

http://gec.cr.usgs.gov/

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 include 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 are 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 reason-able and geologically possible.

Finally, CDTs 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.

#### Suggested Citation

Bultman, M.W., and Page, W.R., 2016, Bedrock morphology and structure, upper Santa Cruz Basin, south-central Arizona, with transient electromagnetic survey data: U.S. Geological Survey Open-File Report 2016–1152, 49 p., http://dx.doi.org/10.3133/ofr20161152.

ISSN: 2331-1258 (online)

#### Study Area

#### Table of Contents

- Acknowledgments
- Abstract
- Introduction
- Geologic Setting of the Study Area
- Previous Geophysical Analysis and Depth to Bedrock Estimates
- Potential Field Data and Analysis in the Study Area
- Transient Electromagnetic Data and Analysis
- Conclusions
- Possible Additional Work
- References Cited
- Appendix 1. Santa Cruz Transient Electromagnetic Survey Conductivity-Depth Transforms (CDT) Plots
- Appendix 2. Santa Cruz Transient Electromagnetic Survey Data

Part or all of this report is presented in Portable Document Format                        (PDF). For best results viewing and printing PDF documents, it is recommended that you download                        the documents to your computer and open them with Adobe Reader. PDF documents opened from your                        browser may not display or print as intended. Download the latest version of Adobe Reader, free of charge. More information                        about viewing, downloading, and printing report files can be found at the common download problems FAQ.