Geology of the Varney and Gameron Quadrangles, O pi O r Madison County > Montana O O < O T! GEOLOGICAL SURVEY BULLETIN 1459 §2! O O 2 H >z I to Geology of the Varney and Cameron Quadrangles, Madison County, Montana By JARVIS B. HADLEY With a chapter on PALEONTOLOGY AND CORRELATION OF THE MADISON GROUP ON BALDY MOUNTAIN By WILLIAM J. SANDO and J. THOMAS DUTRO, Jr. GEOLOGICAL S U R V E Y BULLETIN 1459 Description of the Paleozoic, Mesozoic, and Cenozoic stratigraphy and structure of the northern Gravelly Range, Montana UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Hadley, Jarvis Bardwell. Geology of the Varney and Cameron quadrangles, Madison County, Montana. (Bibliography: 110 p. Geological Survey bulletin ; 1459) 1. Geology Montana Madison Co. 2. Paleontology Montana Madison Co. 3. Stratigraphic correlation Montana Madison Co. I. Sando, William Jasper, joint author. II. Dutro, J. Thomas, joint author. III. Title. IV. Series: United States. Geological Survey. Bulletin ; 1459. QE75.B9 no. 1459 [QE134.M25] 557.3'08s 78-606068 [557.86'663] For sale by Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03229-5 CONTENTS Page Abstract ____________________________________________ 1 Introduction __________________________________________________ 1 Acknowledgments __________________________________________ 2 Geologic setting ______________________________________ 2 Paleozoic rocks ___________ ______________________ _____________ 11 Flathead Sandstone and Wolsey Shale ___________________ 11 Meagher Limestone _______________________ ____ ____ _____ 12 Park Shale _________________________________ 15 Pilgrim Dolomite or Limestone ______ _____-- 16 Red Lion Formation and Bighorn Dolomite _____-________ 20 Jefferson Dolomite or Limestone ______ _____________ ___ 22 Three Forks Formation ___________________________________ 29 Madison Group _______________________________________ 31 Lodgepole Limestone _______________ ____ ______ _____ 31 Mission Canyon Limestone ___ ________ __ __ __ 32 Paleontology and correlation of the Madison Group on Baldy Mountain, by William J. Sando and J. Thomas Dutro, Jr __ 33 Big Snowy Group and Amsden Formation _____________________ 46 Quadrant Quartzite or Formation ___________________ ________ 57 Shedhorn Sandstone _________________________________________ 58 Mesozoic rocks ________________________________________________ 62 Dinwoody Formation __________________________________ 62 Ellis Group ________________________________ 63 Morrison Formation _____________________________ 68 Kootenai Formation _______________________________ 69 Thermopolis Shale ______________________________ 72 Frontier Formation ________________________________________ 75 Upper Cretaceous volcanic and volcaniclastic rocks _____________ 78 Cenozoic rocks ________________________________ 81 Sphinx Conglomerate ______________________________________ 81 Tertiary volcanic rocks and gravel ________________________ 83 Lower Tertiary erosion surface and colluvial deposits __________ 88 Rhyolite ash-flow tuff ____________________________ 89 Freshwater limestone ______________________________________ 91 Quaternary deposits _________________________ _ _ 93 Glacial deposits _________________________________________ 93 Boulder deposits of uncertain origin _____________________ 96 Alluvial fans and gravel of the Madison Valley ____________ 97 Gravel of the Ruby River valley __________________ 98 Landslide deposits _________________________________ 99 m IV CONTENTS Page Structural geology _______________________________ 100 Laramide structural features _______________________ 100 Cenozoic normal faults _____________________________ 103 Selected references _______________________________ 105 ILLUSTRATIONS Page FIGURE 1. Columnar sections of the Bighorn Dolomite, Red Lion For­ mation, and Pilgrim Dolomite or Limestone, Varney quad­ rangle _____________---________________ 18 2. Columnar sections of the Amsden Formation and underly­ ing beds, Varney quadrangle __________ 48 3. Columnar sections of the Shedhorn Sandstone and Phos- phoria Formation, Varney quadrangle _______ __ 60 4. Columnar sections of the Ellis Group and the upper part of the Dinwoody Formation _______________________ 66 5. Columnar section showing position of numbered fossil col­ lections from Cretaceous rocks, Varney quadrangle ____ 74 6. Photograph showing Tertiary volcanic rocks, Varney quad­ rangle ________________________________ 84 7. Photograph showing faulted rhyolite ash-flow tuff, Madison River valley _________________________ 90 8. Photograph showing Tertiary limestone in lower canyon of Wigwam Creek, Varney quadrangle ______________ 92 9. Map showing relation of the Greenhorn and Snowcrest faults in the vicinity of Ruby River Canyon __________ 101 TABLES Page TABLE 1. Stratigraphic units in the Varney quadrangle ___________ 3 2. Stratigraphic units in the Cameron quadrangle ________ 7 3. Invertebrate fossils collected from the Madison Group on Baldy Mountain __________________________ 40 GEOLOGY OF THE VARNEY AND CAMERON QUADRANGLES, MADISON COUNTY, MONTANA By JARVIS B. HADLEY* ABSTRACT In the Gravelly Range, west of the Madison Valley in southwestern Mon­ tana, Paleozoic rocks about 1,500 m thick range in age from Middle Cam­ brian to Permian; Mesozoic rocks about 1,400 m thick range in age from Early Triassic to Late Cretaceous. The lithology, thickness, and age of each of 21 mapped Paleozoic and Mesozoic units and the extent of several locally distributed Tertiary and Quaternary units are summarized. Variations from the usual Paleozoic sequence in southwestern Montana show that 245 m of Middle and Upper Cambrian and Ordovician rocks were eroded from a local uplift in pre-Late Devonian time and that 200 m of Upper Mississippian rocks (Big Snowy Group or its equivalents), present in the western part of the area, are absent in the eastern part, probably because of uplift and erosion before deposition of the Amsden Formation. The structure of the western part of the area is dominated by Laramide folds and thrust faults, including low-angle faults on which Precambrian and Paleozoic rocks have been carried several kilometers eastward over Upper Cretaceous rocks. The eastern part of the area is dominated by high- angle faults associated with downfaulting of the adjacent Madison Valley in late Tertiary and Quaternary time. Tertiary rocks, mainly cobble gravel and various volcanic rocks probably of Eocene to Miocene age, were deposited on a surface now tilted northeastward from the crest of the Gravelly Range to near the present level of the Madison River. Farther south, the Madison Valley seems to have subsided mainly by faulting on both sides. INTRODUCTION The northern part of the Gravelly Range and adjacent areas were studied as a part of the U.S. Geological Survey's investiga­ tions ;of the structure and stratigraphy of the disturbed belt in western Montana. Geologic mapping of the Varney and Cameron quadrangles began in 1956 and was essentially complete by 1960. Final field checking of the geologic maps was completed in 1965. A preliminary report on the geology of the area was published * Deceased. 2 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. as a part of the Billings Geological Society's symposium on the West Yellowstone-earthquake area (Hadley, 1960). The 1:62,500- scale maps of the two quadrangles, containing geologic structure sections, were published in 1969 (Hadley, 1969a, b). This report, left in nearly completed manuscript form at the time of the writer's death in 1974, was revised and completed for publication by J. T. Dutro, Jr. ACKNOWLEDGMENTS The writer was assisted during the field-mapping phase of the work by G. H. Haddock, R. W. Kistler, Harold Masursky, and R. W. Swanson in 1956; J. J. Branco and E. H. McKee in 1957; P. W. Birkeland in 1958; W. D. Long in 1959; and E. J. Young in 1965. This report has benefited from lively discussions in the field, and during the compilation stages, with M. R. Klepper, J. D. Love, W. J. McMannis, John Montagne, G. M. Richmond, G. D. Robin­ son, and I .J. Witkind, all of whom were deeply involved in similar work in nearby areas at the time. The writer was visited in 1956, 1957, and 1958 by J. T. Dutro, Jr., and W. J. Sando, who measured the sections of the Madison Group and the Big Snowy-Amsden interval on Baldy Mountain. The results of their studies have been incorporated as a chapter in this report. W. A. Cobban helped with the field studies of the Cretaceous rocks and collected fossils from them in 1958, and R. W. Imlay gave similar assistance with the Jurassic strata in 1959. Fossils from other stratigraphic units were identified by R. W. Brown, W. A. Cobban, H. M. Duncan, J. T. Dutro, Jr., W. H. Hass, R. W. Imlay, Estella Leopold, A. R. Palmer, John Pojeta, Jr., W. J. Sando, N. J. Silberling, I. G. Sohn, D. W. Taylor, R. H. Tschudy, J. A. Wolfe, and E. L. Yochelson. Thanks are expressed to all these people for their insights into the age significance and environmental implications of the fossils. GEOLOGIC SETTING The stratigraphic sequences in the Varney and Cameron quad­ rangles are similar, but the two areas complement each other so as to provide a complete geologic history of the region (see tables 1 and 2). For example, most of the Tertiary units are best de­ veloped in the Cameron