Petrography of the Kootenai Creek Area Bitterroot Range Montana
Total Page:16
File Type:pdf, Size:1020Kb
University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1954 Petrography of the Kootenai Creek area Bitterroot Range Montana Sidney Lavern Groff 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 Groff, Sidney Lavern, "Petrography of the Kootenai Creek area Bitterroot Range Montana" (1954). Graduate Student Theses, Dissertations, & Professional Papers. 7600. https://scholarworks.umt.edu/etd/7600 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]. PET ROGRAPHY OF THE KOOTENAI CREEK AREA, BITTERROOT RANGE, MONTANA by Sidney L. Groff B. A., Montana State University, 1941 Pr« (sented in partial fulfillment of the requirements for the degree of Master of Arts MONTANA STATE UNIVERSITY 1954 Approved by; üK&irmasry board ot Exainers Deani Graduate Schol z ____ uate UMI Number: EP38401 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. UMT UMI EP38401 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Authoi Microform Edition © ProQuest LLC, All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQ^sf ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 -1346 TABLE OF COOTENTS Page ABSTRACT ........................................ .. vi INTRODUCTION .......................................... 1 Purpose of the Investigation ........ # 1 Location ....... ........................... 1 Previous Work in the Area ............. 3 Acknowledgments .......... « ................ 5 GEOGRAPHIC SETTING . ........................... 6 GEmRAL GEOLOGY..................................... 8 THE GEOLOGY OF THE AREA INVESTIGATED.............. 14 Previous Concepts ......... .......... 14 Procedure ......... ....................... 17 Physiography................................ 20 Petrography and Petrology ......................... 21 The gneissic frontal z o n e ...................... 21 The zone of contact m e t a m o r p h i s m « 28 The zone of contorted gneiss ................ 30 The bass Canyon intrusive .......... 34 Structure......................................... 36 CONCLUSIONS......................................... 42 APPENDIX ........................................... 59 BIBLIOGRAPHY .......................................... 78 —li— LIST OF ILLUSTRATIONS Page Plate I. Index, sampling and geologic map of Kootenai and Bass Canyons • . • • in pocket II. Geologic and specimen location map of the eastern portion of Kootenai Canyon ........................... in pocket Photomicrographs Plate III A. Gneiss at location F - 1 ........... 47 B. Gneiss, location F-1, higher magnification . • .......... 47 C. Gneiss, location F-1, detrital zircon , . * • ................ 47 IV A. Gneiss at location F - 2 ............. 4B B. Gneiss at location F - 4 ........... 4â C. Gneiss-schist contact at location F - 3 ............................. 48 D. Apatite and zircon, location F-3 . 48 V A. Gneiss-schist contact at location F—6 49 Dm Gneiss-schist contact, location F-6, polished section 49 C. Gneiss at location F - l O w ........... 49 D. Pegmatite-gneiss contact, location F-9, polished section **.••• 49 VI A. Hornblende and zircon at location F-13 . • ...................... 50 B. Gneiss at location F - 1 3 ........... 50 Cm Sieve texture in diopside at location F - 1 5 .................. 50 VII A. Fine-grained, hornblende gneiss at location F - 1 5 .................. 51 B. Kedium-grained gneiss at location F - 1 5 a ........................... 51 C. kediurn-grained gneiss at location F - 1 6 ............................. 51 -ill- -iv- Page D* Schist or fine-grained gneiss at location F - 1 6 ................. 51 Plate VIII A. Detrital zircon in schist at location F - 1 9 ........................... 52 B. Schist, location F-19, showing structure ........ .. 52 C. Myrraekitic textures in gneiss at location F - 1 7 ................ 52 D. Schist at location F - 2 Û ......... 52 IX A. Augen and augen-like masses in gneiss at location F-2ÛW, polished section *••••••••• •••• 53 B. Injected nature of augen, location F - 2 0 W ......................... 53 G. Quart site at location F - 2 1 f ..... 53 D. Plagioclase in contact rock near location F - 2 1 ................. 53 X A, Porphyritic granite, location M-1, polished section . ............ 54 Quartz monzonite, location M-4 . # # . 54 C« Detrital zircon in gneiss, location iVi—3w •«<!••••••••••••• 54 D* Corroded plagioclase, location M-56 • 54 E« Sillimanite in gneiss, location M-61 • 54 XI A* Contorted gneiss, location M-61 • . • 55 B. Contorted gneiss, location M-61 . 55 C. Contorted gneiss, location M-6 .... 55 D. Pleochroic haloes in biotite, location M-8 ................ 55 XII A. Detrital zircon in gneiss, location K - l .................. 56 B. Quartz monzonite dike rock, location K-l . .................. 56 C. Porphyritic granite, location IB . • 56 XIII A. Cataclastic nature of schist, location B - 1 ................... 57 B. Schist, location B-1, more highly magnified ............ 57 XIV A. Contorted gneiss, location B-3 .... 58 B. Alaskite, location B - 4 .............. 58 -V- Page Figure 1. Index map showing Ravalli County, Montana and adjacent areas of Western Montana and Idaho ........................... 2 2. View of the northern part of the Bitter root Range Front showing topography and the uniform eastern slope ........ 9 3* The northwest termination of the Bitter root R a n g e ............................. 18 4- The mouth of Kootenai Canyon as seen from the southeast .......... 18 5. An outcrop of differentially weathered [ne is 8 ............................... 18 6. Schists and gneisses near location F-3 . 26 7. A small pegmatite mass in gneiss at location F - ? w .......... 26 8. Bedded appearance of schist and gneiss, location F - 3 .............. * .......... 29 9- Joint planes in gneiss at location F-15a . 29 10. Augen gneiss at location F - 2 0 w .... 29 11. Hypothetical structure section through the Kootenai-Bass Creek divide ..... 39 Table I. Sedimentary formations in the Missoula, bitterroot area of western Montana . 13 II. Probable origin of rocks described in appendix............................... 61 PETROGRAPHY OF THE KOOTENAI CREEK AREA, BITTERROOT RANGE, MONTANA Sidney L* Groff ABSTRACT Existing papers concerning the origin of the gneissic east front of the Bitterroot Range, Montana, appear to be in disagreement. Lindgren (1904) considered that the remarkably uniform eastern slope was caused by a great normal fault and that the gneissic zone was the result of fault stresses. Langton (1935) accepted the fault concept and considered the gneiss to be formed from granitic rock older than the Idaho batholith. Pardee (1950) cited geomorphic features indicat ing that such a fault may be present. Ross (1950) believed that the gneiss was a zone of granitized Beltian sediments and that the uniform slope was due to the doming effect of the Idaho batholith. Much of the gneissic rock of the mountain front west of Stevensville, Montana appears to be a raylonitic augen gneiss. Zircon of detrital origin is present, and quartz may be in excess of 90 per cent in the composition of some of the rock. A schist with foliation parallel to the gneiss and having sharp contacts with the gneiss is common in the -vi- -vil- frontal zone. Diopside is locally abundant near the west edge of this zone, and sillimanite is common in a contorted gneiss even farther westward* In this area it is probable that the frontal gneissic zone is composed of both metasediments of Beltian age and later igneous intrusives, and t M t the cataclastic nature of the rock resulted from both the intrusion of the Idaho batholith and subsequent faulting along the range front. INTRODUCTION Purpose of the Investigation This paper is primarily concerned with the establish ment of facts bearing on the "dispute” (Ross 1950, p. 135) as to the origin of the gneiss and the structure of the gneissic east front of the Bitterroot Range in western Montana. It will be shown that none of the previous papers dealing with the geology of this area are in complete agree ment. There is, however, considerable evidence to support some of the differing views of the previous papers- Believ ing that a comprehensive study of a smaller area might uncover pertinent data and either prove or disprove some of the existing views, an extensive reconnaissance and sampling of the Kootenai Creek area was carried out. Location The area (fig. 1 k pi. I) with which this paper is mostly concerned is a small portion of the northern part of the Bitterroot Range. The range begins about eight miles south of Missoula, Montana and extends some sixty miles farther south along the Montana-Idaho border. The area indicated in plate I is about twenty-five miles south of Missoula, generally west of Stevensville,