GEOLOGY AND SLOPE STABILITY IN SELECTED PARTS OF THE GEYSERS GEOTHERMAL RESOURCES AREA: A Guide to Geologic Features Indicative of Stable and Unstable Terrain in Areas Underlain by Franciscan and Related Rocks 1980 CALIFORNIA DIVISION OF MINES AND GEOLOGY SPECIAL REPORT 142 STATE OF CALIFORNIA EDMUND G. BROWN JR. GOVERNOR THE RESOURCES AGENCY HUEY D. JOHNSON SECRETARY FOR RESOURCES DEPARTMENT OF CONSERVATION PRISCILLA C. GREW DIRECTOR DIVISION OF MINES AND GEOLOGY JAMES F. DAVIS STA TE GEOLOGIST SPECIAL REPORT 142 GEOLOGY ,AND SLOPE STABILITY IN SELEO'ED PARTS OF THE GEYSERS GEOTHERMAL RESOURCES AREA: A Guide to Geologic Features Indicative of Stable and Unstable Terrain in Areas Underlain by Franciscan and Related Rocks By Trinda L. Bedrossian Geologist 1980 CALIFORNIA DIVISION OF MINES AND GEOLOGY 1416 Ninth Street, Room 1341 Sacramento, CA 95814 -~------------------------------------------------------------------------------------------------------------------- CONTENTS Page SUMMARY OF FINDINGS ........................................~ ...................................................................................................................1 RECOMMENDATIONS ..................................................................................................................................................................1 INTRODUCTION· ............................................................................................................................................................................2 Purpose and scope ....................................•....•................•........................•....•..........•................ , ...•...........•................................. 3 Acknowledgments ........................................................................................................................................................................3 GEOLOGY ......................................................................................................................................................................................3 Regional geology ........................................................................................................................................................................3 Franciscan melange ....................................................................................................................................................................4 Geology and geomorphology of Big Sulphur Creek canyon ....................................................................................................5 t;, ... -CHARACTERISTICS OF STABLE TERRAIN ....................................................................................................................................7 Competent formational units ......................................................................................................................................................7 Ridge tops ..................................................................................................................................................................................8 Flat ground .....................................................................•............................................................................................................8 Competent masses of rock embedded in melange matrix ........................................................................................................8 Naturally buttressed landslides ................................................................................................................................................10 CHARACTERISTICS OF UNSTABLE TERRAIN ..••••••••••••••••••.•..•.•..•.•..••••••••••••••••.........•...••••••••.••••••.•..••...•••....••..•...•••••••••••.••••.••.•... 10 Landslides ..................................................................................................................................................................................11 Falls ......................................................................................................................................................................................11 Slides ....................................................................................................................................................................................11 Flows ....................................................................................................................................................................................14 Comple,x landslides ....................................................................~: ........... ~ ............................................................................ 14 Factors and processes affecting landslide movement .......................................................................................................... 16 Erosion ........................................................................................................................; ............................................................. 16 Sh~ared melange matrix ........................................................................................................, ................................................. 18 Hydrothermally altered rock ....................................................................................................................................................19 Unfavorably dipping bedding planes ..............................................................................: .......................................................20 Faults and shear zones ...............................................................................................................: ............................................21 iii Page Sheared serpentinite ........................................................•.....................•...........••.•.••.....................•............•............•.•............. 24 Mines and mine tailings ............................................................................................•.•..........•........•................•...............•.•...... 24 Subsidence ................................................................................................•...............................................•.•...............•............. 26 Expansive soils ...................................................................................•.....•....•.•...........•...................•.........................•.........•..... 26 OTHER FACTORS AND PROCESSES AFFECTING SLOPE STABILITY ............••..•........•....................................................•....... 27 Earthquake shaking ..........................................................................................•.....••...................•..•...........•.............•...••.......... 27 Surface and ground water........................................................................................................................................................ 28 Vegetation ................................................................................................................................................................................ 28· Human activities ........................................................................................................................................................................ 30 REFERENCES CiTED ...................................................................................................................................................................... 32 APPENDICES Geologic time scale ........................................................................................ , ......................................................................... 35 Description of rock units in parts of The Geysers GRA underlain by Franciscan terrain ...................................................... 36 Factors and processes affecting landslide potential ................................................................................................................ 44 Hydrothermal alteration and its effect on soil stability in The Geysers GRA ........................................................................ 46 Searching for records of mines and miners .............................................................................................................................. 48 Earthquake scales .................................................................................................................................................................... 50 Guidelines to geologic/seismic reports .................................................................................................................................... 53 Recommended guidelines for determining the maximum credible and the maximum probable earthquakes ........................ 55 Recommended guidelines for preparing engineering geologic reports ....................................................................................56 Guidelines for geologic/seismic considerations in environmental impact reports ................................................................. ,58 Checklists for the review of geologic/seismic reports .............................................................................................................. 60 Guidelines for evaluating the hazard of surface fault rupture .............................................................................................