Geology of the Park County
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Geology of the Gardiner Area Park County Montana Geology of the Gardiner Area Park County Montana By GEORGE D: FRASER, HENRY A. WALDROP, and HAROLD J. HYDEN GEOLOGICAL SURVEY BULLETIN 1277 General geology near the North Entrance of Yellowstone National Park, with special emphasis on recurrent faulting along the boundary of the Beartooth uplift UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Suoerintendent of Documents, U.S. Government Prlntinll Office WashJ.naton, D.C. 10401 CONTENTS Page Abstract---------------------------------------------------------- 1 Introduction------------------------------------------------------ 3 Location----------------------------------------------------- 3 Fieldwork and acknowledgments _____________ ----------_________ 3 Relation to previous geologic work_______________________________ 5 Regional background and. physiography_----------------- ____ ---~ 6 Precambrian geology_______________________________________________ 7 Metasedimentary rocks_________________________________________ 7 Quartz-biotite schist and quartzite ___________ --------------__ 8 Quartz-amphibole schist and related rocks____________________ 9 Phyllite__________________________________________________ 10 Granite______________________________________________________ 10 Metagabbro__________________________________________________ 11 Structure and geologic history ___________________ ----------______ 12 StratigraphY------------------------------------------------------ 15 Paleozoic rocks________________________________________________ 16 MadisonLimestone________________________________________ 16 Amsden Formation _____ ----------------------------------- 16 Quadrant Quartzite ___________________ --------------------- 17 Park City Formation______________________________________ 18 Mesozoic rocks ________________________________________ -------- 19 Dinwoody and Chugwater Formations ___ -------------------- 19 Ellis GrouP----------------------------------------------- 20 Morrison Formation __________________ -----------------·---- 21 Cloverly Formation________________________________________ 22 Thermopolis Shale________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 23 Mowry Shale--------------------------------------------- 24 Frontier Formation ________________ ------------------------ 25 Cody Shale--------------------------------~-------------- 26 Telegraph Creek Formation_______________ ------------------ 27 Eagle Sandstone _________________________________ --------__ 28 Everts Formation ______ ----------------------------------- 32 Landslide Creek Formation_________________________________ 34 Cenozoicrocks------------------------------------------------ 37 Lower Cenozoic volcanic rocks______________________________ 37 Dacitic breccia ________________________________ ------__ 37 Trachyandesite flows _______________________ ----------__ 41 Andesitic breccia __ --------___________ --------___ ------ 44 Intrusive rocks __________________________ ------________ 47 Dacitic intrusives north of the Gardiner fault_________ 47 Intrusives in or near the Gardiner fault_ _ _ _ _ _ _ _ _ _ _ _ _ _ 4 7 Dacite sill south of the Gardiner fault________________ 48 Andesite sill at Mount Everts_______________________ 49 Upper Cenozoic volcanic rocks______________________________ 49 Basalt------------------------------------------------ 49 YellowstoneTuff______________________________________ 53 m IV CONTENTS Page Stratigraphy-Continued Cenozoic rocks-Continued Surficial deposits ____________________________ --__ ---- __ ---- 56 Conglomerate beneath basalt____________________________ 56 Conglomerate younger than basalt_______________________ 57 Travertine____________________________________________ 57 Glacial deposits _____________________________________ -- 57 Undifferentiated deposits ________________________ ;...__ 58 Pinedale deposits ________ -----____________ --_____ -- 59 Landslides-------------------------------------------- 60 Alluvium and colluvium________________________________ 61 Structure--------~------------------------------------------------ 62 Regional relations ______________________________ ----------_____ 62 Gardiner fault _________________________________ --------------- 63 DiP------------------------------------------------------ 63 Laramide structure in the drag zone_________________________ 64 Surficial modification of drag-zone structure___________________ 66 Quaternary reactivation of Gardiner fault_________________________ 67 Minor Quaternary structures______ -------- _________ --------- 69 Age of the young faults ____ -------------------------------- 73 Earthquakes______________________________________________ 74 Transverse faults ________________________________________ ------ 75 Mammoth fault _____________ ------------------------------ 75 Age------------------------------------------------------ 76 Other faults and folds__________________________________________ 77 Economicgeology_________________________________________________ 78 Coal deposits ________ -------__________________________________ 78 Electric coal field ______________________________ ------------ 78 Little Trail Creek area_____________________________________ 79 Gardinerarea_____________________________________________ 80 Economic potentiaL_______________________________________ 82 Other deposits ______________ ---------_________________________ 82 Gold, arsenic, and tungsten _____________________ ------------ 82 Oil and gas prospects ___ ----------------------------------- 83 Nonmetallic deposits ___ ------____________________ ------____ 83 Geologic history ________________________________________ --------___ 84 Measured sections_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 87 References cited___________________________________________________ 110 Index_----------------------------------------------------------- 115 ILLUSTRATIONS Page PLATE 1. Geologic maps and section of the southwest quarter of the Gardiner quadrangle_______________________________ In pocket FIGURE 1. Index maps of Gardiner area______________________________ 4 2. Photograph of irregular channel-filling basalt flow___________ 50 3. Diagram illustrating faults in downthrown drag and shear zone adjacent to a high-angle fault______________________ 66 CONTENTS v Page FIGURE 4. Photograph of Gardiner fault and drag zone showing Qua ternary uplift of Pliocene(?) basalt, Holocene superficial faults, contact between sedimentary rock and Precambrian granite, and Landslide Creek Formation__________________ 68 5. Photograph of Quaternary graben in Pliocene(?) basalt and distant view of Cinnabar Mountain______________________ 70 6. Diagrammatic cross sections showing Quaternary disruption of basalt near Little Trail Creek___________________________ 72 7. Photograph of Holocene tilted fault blocks in Pliocene(?) basalt------------------------------------------------ 73 TABLES Page TABLE 1. Pelecypods from Eagle Sandstone near Gardiner, Mont______ 31 2. Measured sections and total thicknesses____________________ 88 GEOLOGY OF THE GARDINER AREA, PARK COUNTY, MONTANA By GEORGE D. FRAsER, HENRY A. WALDROP, and HARoLD J. HYDEN ABSTRACT The Gardiner area occupies about 52 square miles in the soUJtJhwest quarter of the Gardiner 15-minute quadrangle in .southwestern Park Oounty, Mont., at the North Entrance to Yellowstone National Park. The Gardiner area is divided into two blocks by the high-angle northwest-trending Gardiner reverse fault. North east of the fault the uplifted Beartooth Mountains block is crystalline Precam brian rock stripped of its sedimentary cover and veneered by Eocene and younger volcanic rockis and surficial deposits. Southwest of the faul!t the dropped and deformed Yellowstone valley block has 10,000 feet of Paleozoic and Mesozoic rocks and is also veneered by Oenozoic volcanic rocks and ·surficial deposits. '11l.e sedimentary rocks, mostly Cretaceous at the surface, dip generally toward the fault but are truncated, curled upwal'd by drag, sheared, ·and thinned to form a 1-mile-wide deformed zone rflhat is ·the steep limb Oif a locally overtumed syncline parallel to the Gardiner fault. Oretaceons rocks ·are generally in contact with Precambrian rocks along the faUJlt, although lower Paleozoic rocks were deposited throughout the area and sedimentary remnants as old as Mississippian are exposed locwlly in the di"ag zone. A large gra:be.n, transverse to ·the rtrend Oif the Gardiner fault, is confined to the dropped block and helps to localize spurs of Sepulcher Mountain and Mount mverts that project norflhward into the valley from Yellowstone National Park. Poorly exposed lower Precambrian rocks, which include deformed metasedi mentary rock, ·gneissic grani!te, and weakly metamorphosed gabbro, indicate a complex history as follows: (1) Prolonged deposition, (2) downbuckling, intense folding, regional amphibolite-facies metalm.orphism, ·and Cl"OSIS-!folding, (S) local ized thermal metamorphism, granite intrusion, and mild deformation, (4) ga;bbro intrusion, ( 5) localized quartz-pegm:atite intrusion, hydrothermal metamorphism, gold~amenopyl.'dte-scheelite mineralization, retrograde greenschist-facies meta morphism, and mild deformation accompanied and followed by (6) prolonged uplift and erosion. The uplift, which may have begun during step S, probably con