University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1984 The geology and tectonic history of the Fourth of July Creek area White Cloud Peaks Custer County Idaho Robert M. Sengebush The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Sengebush, Robert M., "The geology and tectonic history of the Fourth of July Creek area White Cloud Peaks Custer County Idaho" (1984). Graduate Student Theses, Dissertations, & Professional Papers. 7445. https://scholarworks.umt.edu/etd/7445 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. COPYRIGHT ACT OF 1976 Th i s is a n unpublished m a n u s c r i p t in w h i c h c o p y r i g h t s u b ­ s i s t s . An y f u r t h e r r e p r i n t i n g of its c o n t e n t s m u s t b e a p p r o v e d BY t h e a u t h o r . Ma n s f i e l d L i b r a r y Un i v e r s i t y ofo f II^-^ na Da t e :______*. The Geology and Tectonic History of the Fourth of July Creek Area White Cloud Peaks Custer County, Idaho by Robert M. Sengebush B,A. University of California Santa Barbara 1977 Presented in partial fulfillment of the requirements for the degree of Master of Science, University of Montana 1984 ApprovedI by: Q^irman, Board of Examiners an. Graduate School Date UMI Number: EP38246 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMI OiSMrtatkm Publish^ UMI EP38246 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQ^st ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 -1346 Sengebush, Robert M., M.S., June 1984 Geology Geology and Tectonic History of the Fourth of July Creek Area, White Cloud Peaks, Central Idaho (79 pp.) Director: Dr. James W. Sears The Fourth of July Creek area in the White Cloud Peaks, central Idaho, contains two principle lithotectonic units: 1) the black ar­ gillite and minor gray marble of the Upper Mississippian (?) Salmon River sequence (Msr), up to 1400 m thick, and 2) the conglomerate, bioclastic turbidites, calcareous sandstone and siltstone of the Upper Pennsylvanian to Lower Permian Pole Creek Formation Members 1 through 4, (PPpc), 2500 m thick. The Msr was deposited on deen-sea fans in a late-stage Antler foreland basin; the PPpc was deposited on the continental slope by turbulent flow, grain flow (?), trac­ tion and suspension sedimentation processes which create a fining- upward succession and record a diminishing sediment supply due to erosion of the source highland. Geologic mapping and structural interpretation reveals a folded regional detachment fault at the contact between the Msr and the PPpc. This fault is defined by truncation of PPpc strata and by tectonic breccia and myIonite in rocks of the upper and lower plates. This east-directed fault may represent westernmost or hinterland thrusting in the Sevier overthrust belt. Tight folds, penetrative cleavage and intricately folded schis- tosity characterize the deformation of the parautochthonous Msr; deformation of the allochthonous PPpc consists of jointed, open- to isoclinal folds which lack a penetrative fabric. This contrast in deformational styles suggests the Msr was deformed prior to deposition, thrusting and folding of the PPpc. The presence of de­ formed Msr clasts in the PPpc conglomerate further supports this timing. Two north-trending, steeply west-dipping faults cut the Msr, the detachment fault and the PPpc, and are sub-parallel to PPpc bedding. The westernmost of these faults places the Msr against an overturned section of the PPpc Member 4, and the easternmost fault has created asymmetrical megascopic folds in the PPpc. Although these faults are now geometrically normal, facing direction and repetition of strata along the west fault, and fold vergence along the east fault indicates these faults are in fact, west-directed contraction faults which have been overturned eastward. The detachment fault, megascopic folds and contraction faults are cut by the White Cloud stock (83.6 m.y. old). This restricts the age of the regional detachment and associated structures to pre-Late Cretaceous. 11 AKN3WLEDGEMENTS Thanks go to the entire University of Montana geology department faculty and staffs and especially to my thesis advisors Dr. Jim Sears, Dr. Johnnie Moore and Dr. Wayne Van Meter. I also thank Mr. Elmer Enderlin for allowing roe valuable access to remote portions of the field area via his private mining roads, and for unique historical and geologic perspective on the White Cloud Peaks area. Thanks also to Monty Marchus for the open door policy in Boise; to Wendy and the Binghams for a family in Missoula; and to ny parents. This research was financially supported by a University of Montana Graduate Research Grant, by the University of Montana McDonough Stipend for Montana Energy Development Research, and by a grant from Sigma Xi, the Scientific Research Society. Ill TABLE OF CONTENTS LIST OF FIGURES.................................................. vi LIST OF PLATES................................................... vii OÎAPTER ONE INTRODUCTION........................................ 1 PURPOSE OF STUDY............................................ 1 LOCATION AND ACCESS......................................... 2 SUBAREA LOCATICNS........................................... 4 PREVIOUS WORK.......................................... ..... 4 p r e s e n t w o r k ................................................ 7 REGIONAL GEOLOGIC HISTORY.................................... 8 CHAPTER TWO STRATIGRAPHY.........................................11 HISTORICAL REVIEW........................................... 11 S A m O N RIVER SEQUENCE........................................15 POLE CREEK FORMATION.........................................17 MEMBER 1 ................................................. 18 MEMBER 2 ................................................. 22 MEMBER 3 .................................................. MEMBER 4................................................. 25 DEPOSmONAL ..................................... 29 CHAPTER THREE STRUCTURAL GEOLOGY............................ 32 iv INTO)DUCTION................................................. 32 SAUOî RIVER SEQUENCE........................................ 33 MEGASCOPIC FOLDS.......................................... 33 MESOSCOPIC FOLDS.......................................... 35 FOLD STYLE................................................ 39 FOLIATION................................................. 40 MXLONITE.................................................. 41 POLE CREEK FORMATION......................................... 44 MEGASCOPIC FOLDS.......................................... 44 MESOSCOPIC FOLDS.......................................... 46 JOINTS....................................................47 FOLD STYLE.............................. ‘................. 50 BRECCIATED BASAL CONTACT.................................. 52 IHRUST ON THE SALMON RIVER SEQUENCE-POLE CREEK FORMATION CONTACT......................................................55 CONTRACTION FAULTS........................................... 57 CHAPTER FOUR CONCLUSIONS AND SUMMARY........................ .62 REFERENCES CITED............................................ .66 APPENDIX PETROGRAPHIC DESCRIPTION OF THIN SECTIONS.......... 71 LIST OF FIGURES Figure 1. Location of Study Area................................ 3 Figure 2. Subarea Location Map.................................. 5 Figure 3. Age Correlations of Pole Creek and Wood River Formations.................................. 14 Figure 4« Stratigraphie Column................................... 16 Figure 5. Turbidite Divisions.................................... 21 Figure 6 . Mega-ripples in Pole Creek Formationr Member 4.......... 27 Figure 7. Summary of Facies, Flow Processes and Depositional Environments in the Pole Creek Formation................ 31 Figure 8 . Megascopic Folds, Salmon River Sequence.................34 Figure 9. Penetrative Cleavage, Salmon River Sequence........... 36 Figure 10. Tight, Angular Folds, Salmon River Sequence............. 38 Figure 11. f^lonite Texture, Salmon River Sequence.................42 Figure 12. Stereo Net Plot of Poles to Bedding.................... 45 Figure 13. Mesoscopic Fold, Pole Creek Formation................. 48 Figure 14. Stereo Net Plot of Poles to Joint Surfaces............ 49 Figure 15. a-c Joints in Folds................................... 51 Figure 16. Undeformed Conglomerate in Contact with Breccia ted Conglomerate.......................... 53 Figure 17. Contraction Faults.................................... 58 Figure 18. Contraction Faults in Subarea 1 ....................... 60 Figure 19. Contraction