OPEN-FILE REPORT OFR 08-06 Arizona Geological Survey www.azgs.az.gov GEOLOGIC MAPPING OF DEBRIS-FLOW DEPOSITS IN THE SANTA CATALINA MOUNTAINS, PIMA COUNTY, ARIZONA Arizona Geological Survey Ann Youberg Michael L. Cline Joseph P. Cook Philip A. Pearthree United States Geological Survey Robert H. Webb Geologic Mapping of Debris Flow Deposits in the Santa Catalina Mountains, Pima County, Arizona Open‐File Report 08‐06 Ann Youberg Michael L. Cline Joseph P. Cook Philip A. Pearthree Robert H. Webb September 2008 Research supported by Pima County Regional Flood Control District, the Arizona Geological Survey, and United States Geological Survey. Cover Photo: Debris‐flow levees (yellow lines) along Finger Rock Wash (blue line) just downstream of the canyon mouth. Photo taken June 4, 2006, by Ann Youberg. Suggested Citation: Youberg, Ann, Cline, M. L., Cook, J. P., Pearthree, P. A., and Webb, R. H., 2008, Geologic Mapping of Debris Flow Deposits in the Santa Catalina Mountains, Pima County, Arizona; Arizona Geological Survey Open‐File Report 08‐06, 41 pp, 11 map sheets on CD, scale 1:6,000. Arizona Geological Survey Table of Contents ABSTRACT ..................................................................................................................................................... iv INTRODUCTION ............................................................................................................................................. 5 Background ............................................................................................................................................... 5 Location ..................................................................................................................................................... 5 Debris flows............................................................................................................................................... 6 Terminology .......................................................................................................................................... 6 Initiation ................................................................................................................................................ 7 Rheology ............................................................................................................................................... 8 GIS‐Derived Basin Characteristics ......................................................................................................... 9 METHODS ...................................................................................................................................................... 9 GIS‐Derived Basin Characteristics ............................................................................................................. 9 Geologic Mapping of Debris‐Flow Deposits ............................................................................................ 10 Generalized Map Unit Descriptions .................................................................................................... 13 Numerical Age Dating of Debris‐Flow Deposits ...................................................................................... 16 Radiocarbon Dating............................................................................................................................. 16 Cosmogenic Dating ............................................................................................................................. 16 Sample Collection ............................................................................................................................... 17 Sample Preparation ............................................................................................................................ 18 Analysis of Targets .............................................................................................................................. 19 RESULTS ...................................................................................................................................................... 20 GIS‐Derived Basin And Stream Characteristics ....................................................................................... 20 Geologic Mapping ................................................................................................................................... 23 Agua Caliente‐La Milagrosa‐Molino Canyons ..................................................................................... 23 i Arizona Geological Survey Soldier Canyon .................................................................................................................................... 24 Gibbon Canyon .................................................................................................................................... 27 Sabino and Bear Canyons .................................................................................................................... 28 Esperero and Bird Canyon................................................................................................................... 29 Ventana Canyon .................................................................................................................................. 30 Finger Rock and Pontatoc Canyons ..................................................................................................... 31 Cobblestone Canyon ........................................................................................................................... 32 Pima Canyon ....................................................................................................................................... 33 Pusch Canyon ...................................................................................................................................... 34 Linda Vista ........................................................................................................................................... 34 Numerical Age Dating ............................................................................................................................. 35 Radiocarbon Dates .............................................................................................................................. 35 Cosmogenic 10Be Dates ....................................................................................................................... 35 DISCUSSION ................................................................................................................................................. 37 ACKNOWLEDGEMENTS ............................................................................................................................... 38 REFERENCES ................................................................................................................................................ 39 GEOLOGIC TIMESCALE ................................................................................................................................ 44 GLOSSARY.................................................................................................................................................... 44 ii Arizona Geological Survey List of Arizona Geological Survey Digital Map Series maps included on accompanying CD: Map Title Map Number Debris‐Flow Deposits at the Mouths of Agua Caliente, La Miligrosa, and Molino Canyons DM‐DF‐1A Debris‐Flow Deposits at the Mouth of Soldier Canyon DM‐DF‐1B Debris‐Flow Deposits at the Mouth of Gibbon Canyon DM‐DF‐1C Debris‐Flow Deposits at the Mouths of Bear and Sabino Canyons DM‐DF‐1D Debris‐Flow Deposits at the Mouths of Bird and Esperero Canyons DM‐DF‐1E Debris‐Flow Deposits at the Mouth of Ventana Canyon DM‐DF‐1F Debris‐Flow Deposits at the Mouths of Pontatoc and Finger Rock Canyons DM‐DF‐1G Debris‐Flow Deposits at the Mouth of Cobblestone Canyon DM‐DF‐1H Debris‐Flow Deposits at the Mouth of Pima Canyon DM‐DF‐1I Debris‐Flow Deposits at the Mouth of Pusch Canyon DM‐DF‐1J Debris‐Flow Deposits at the Mouth of Linda Vista Canyon DM‐DF‐1K iii Arizona Geological Survey ABSTRACT An extremely rare weather pattern occurred over southeast Arizona in late July 2006. This five‐day storm culminated on July 31, 2006, when the last pulse of precipitation generated record floods in several washes throughout the region, and hundreds of hillslope failures and debris flows in at least four southeastern Arizona mountain ranges. In the Santa Catalina Mountains, debris flows occurred in nine canyons, exiting or nearly exiting the mountain front in five of these canyons. Infrastructure in Sabino Canyon, a popular recreational area, was damaged, as were structures and roads in Soldier Canyon. Historically debris flows in the Santa Catalina Mountains were limited in size, and to upper elevation hillslopes. There are no known reports of debris flows from the Santa Catalina Mountains affecting developed areas in historic times. However, evidence of prehistoric debris flows are present in most canyons and associated alluvial fans along the front range of the Santa Catalina Mountains. While debris flows were previously recognized as significant hazards in Arizona’s mountains and canyons , the number and extent of debris flows from the 2006 storm event was surprising. In order to begin to assess debris‐flow hazards along the Santa Catalina Mountains in Pima County, we mapped the extent and character of relatively young prehistoric debris‐flow