Stratigraphy and Structure of Part of the Western Sierra Nevada Metamorphic Belt, California GEOLOGICAL SURVEY PROFESSIONAL PAPER 410 Prepared in cooperation with the State of California, Department of Conservation, Division of Mines and Geology Stratigraphy and Structure of Part of the Western Sierra Nevada Metamorphic Belt, California By LORIN D. CLARK GEOLOGICAL SURVEY PROFESSIONAL PAPER 410 Prepared in cooperation with the State of California, Department of Conservation, Division of Mines and Geology UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows: Clark, Lorin Delbert, 1918- Stratigraphy ·and structure of part of the western Sierra Nevada metamorp4ic belt, California. Washington, U.S. Govt. Print. Off., 1964. iv, 70 p. illus., maps (7 col.) diagrs. (1 col.) tables. 30 em. (U.S. Geological Survey. Professional paper 410). Part of illustrative matter fold. in pocket. Prepared in cooperation with the State of California Dept. of Natural Resources, Division of Mines and Geology. Bibliography : p. 58-59. (Continued on next card) Clark, Lorin Delbert, 1918- Stratigraphy and structure of part of the western Sierra Nevada metamor­ phic belt, California. 1964. (Card2) 1. Geology, Stratigraphic. 2. Geology-California-Sierra Nevada Mountains. I. California. Division of Mines and Geology. II. Title. III. Title: Sierra Nevada metamorphic belt, California. (Series) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS Page Page Abstract------------------------------------------- 1 Stratigraphy-Continued Introduction ______________________________________ _ 2 Rocks of Jurassic age-Continued Location, culture, and accessibility _______________ _ 2 Bear Mountains fault zones-Continued Physical features ______________________________ _ 3 Copper Hill volcanics_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 30 Previous work _________________________________ _ 3 Lithology____________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 30 Purpose and scope of investigation _______________ _ 4 Age_______________________________ 31 Field work and acknowledgments ________________ _ 5 Peaslee Creek volcanics _____ - ____ -- ____ -- 31 TerminologY----------------------------------- 5 Lithology _____________ - _________ --- 31 Synopsis of geology ________________________________ _ 6 Age_______________________________ 31 Stages of the Middle and Late Jurassic ___________ _ 7 Stratigraphic relations between the forma- Stratigraphy ______________________________________ _ 8 tions of the western block______________ 31 Calaveras formation ___________________________ _ 8 Stratigraphic relations between the Salt Distribution and character __________________ _ 8 Spring slate and Gopher Ridge vol- Eastern belt _______________________________ _ 8 canics___________________________ 31 Clastic member ________________________ _ 9 Stratigraphic relations between the Salt Volcanic member ______________________ _ 9 Spring slate and Copper Hill vol- Argillaceous member ___________________ _ 9 canics___________________________ 33 Chert member ______________________ .,. __ _ 10 Stratigraphic relations of the Merced Western belt_ _____________________________ _ 12 Falls slate to the Peaslee Creek vol- Cosumnes River exposures ______________ _ 12 canics and Gopher Ridge volcanics__ 33 Mokelumne River and Jackson Creek Rock and mineral fragments in rocks of Jurassic exposures---------------------------- 13 age_____________________________________ 33 Fossils and age ____________________________ _ 13 Composition and shape of fragments_______ 35 Rocks of Jurassic age ____________ :_ _____________ _ 15 Origin of limestone blocks ___ -__ _ _ _ _ _ _ _ _ _ _ 37 Rocks exposed in the central block and Melones Source of fragments ___________________ -- 38 fau~ zone_~------------------------------ 16 Bedding features and conditions of deposition__ 39 Rocks of Middle(?) Jurassic age _________ _ 16 Green schist ___________________________________ - 41 Amador group ____________________ .,. ____ _ 17 Plutonic rocks______________________________________ 41 Cosumnes formation _______________ _ 17 Ultramafic rocks__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 42 Lithology________ - ________ - ___ - 17 Age_______________________________________ 42 Fossils and age ________________ _ 18 Structural relations_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 42 Logtown Ridge formation ___________ _ 18 Gabbroic to granitic rocks ______________________ - 42 Lithology __________________ .___ _ 19 Age_______________________________________ 43 Fossils and age ________________ _ 21 Hypabyssal rocks___________________________________ 43 Pefion Blanco volcanics _________________ _ 22 Structural geology _________________________________ - 44 L~hology _________________________ _ 23 Top determinations___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 45 Age ______________________________ _ 23 Faults----------------------------------------- 46 Mariposa formation ____________________ _ 23 Melones fault zone__________________________ 47 . Lithology _________________________ _ 24 Description of localities________ _ _ _ _ _ _ _ _ _ _ 49 Fossils and age ____________________ _ 25 Bear Mountains fault zone __________________ - 50 Strata of uncertain stratigraphic position _____ _ 26 Direction and amount of movement___________ 51 Volcanic strata ________________________ _ 26 Mother Lode fissure system__________________ 51 Epiclastic strata _______________________ _ 26 Minor faults_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 52 Rocks exposed in the western block and Bear Age of faulting_____________________________ 53 Mountains fault zone _____________________ _ 27 Structure of the eastern block____________________ 53 Gopher Ridge volcanics _________________ _ 27 Structure of the central block____________________ 54 Lithology _________________________ _ 28 Structure of the western block____________________ 54 Age ______________________________ _ 29 Small-scale structures___________________________ 55 Salt Spring slate _______________________ _ 29 Cleavage and schistosity_____________________ 55 Lithology _________________________ _ 29 Minor folds_._______________________________ 56 Fossils and age ____________________ _ 29 Lineations__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 56 Merced Falls slate _____________________ _ 30 References cited_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 58 Lithology ___________ .______________ _ 30 Description of map unitA_____________________________ 59 Age ______________________________ _ 30 Index_____________________________________________ 69 •. Ill IV STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT ILLUSTRATIONS [Plates 1-llin map case] PLATE 1. Geologic map of the western Sierra Nevada between Page the Merced and Cosumnes Rivers, Calif. FIGURE 6. Photograph showing scattered large clasts in 2. Expanded block diagram showing geology of the bedded pyroclastic groundmass_ _ _ _ _ _ _ _ _ _ 21 western Sierra Nevada between the Merced and 7. Photograph of thin-bedded to laminated tuff_ 21 Cosumnes Rivers, Calif. 8. Photograph of thin-bedded tuff showing intra­ 3-8. Geologic maps and composite cross sections along: formational breccia bed below dime and 3. Merced River. small-scale crossbedding under breccia____ 22 4. Tuolumne River. 9. Photograph of clastic sills of medium-grained 5. Stanislaus River. tuff__________________________________ 22 6. Calaveras River. 10. Geologic map and cross section showing struc­ 7. Mokelumne River. ture of slate on Tuolumne River east of La 8. Cosumnes River. Grange, Stanislaus County, California____ 34 9. Stratigraphic sections within the central fault block. 11. Photomicrograph of very coarse graywacke 10. Geologic map showing type section of Amador group. from Cosumnes formation______________ 35 11. Geologic map and cross section of epiclastic rocks 12. Photomicrograph of very coarse graywacke east of Merced Falls, Merced County, Calif. from Mariposa formation_______________ 36 13. Photomicrograph of graded tuff bed________ 45 14. Photomicrograph showing relation of clea v­ Page age to graded bed of fine graywacke in Salt FIGURE 1. Index map showing location of area studied_ 2 Spring slate___________________________ 46 2. Canyon of Stanislaus River_______________ 3 15. Photograph of small asymmetric anticline 3. Block diagram showing intertonguing______ 16 showing axial plane cleavage___________ 47 4. Photomicrograph of mudflow conglomerate_ 16. Geologic map and cross section showing struc- 5. Photograph of volcanic breccia of probable 18 ture of bedded pyroclastic rocks west of mudflow origin________________________ 20 Don Pedro Dam____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 48 TABLES Page Page TABLES 1-6. Description of map units: TABLES 1-6. Description of map units-Continued 1. Merced River ____________________ _ 59 5. Mokelumne River ________________ _ 64 2. Tuolumne River __________________ _ 61 · 6. Jackson Creek ____________________ _ 66 3. Stanislaus River __________________ _ 62 7. Cosumnes River __________________ _ 66 4. Calaveras River __________________ _ 64 STRATIGRAPHY AND STRUCTURE OF PART OF THE WESTERN SIERRA NEVADA METAMORPHIC BELT, CALIFORNIA By LORIN D. CLARK ABSTRACT Formations exposed in the central block