Big Snowy and Amsden Groups and the Mississippian-Pennsylvanian Boundary in Montana By EDWIN K. MAUGHAN and ALBERT E. ROBERTS SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 554 7 B A study of the Upper Mississippian and Pennsylvanian sedimentary formations with special emphasis on their regional correlation, variation in lit ho logy, and age assignments UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEW ART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Abstract. ______-____--__-_----_______ Bl Mississippian-Pennsylvanian boundary________________ B20 Introduction. ________________________ 1 Tectonic framework.________-_-____-_--______------_ 23 Development of nomenclature._________ 1 Early and Late Mississippian (Madison Group) _ _ _ _ 23 Mississippian System, Big Snowy Group. 5 Late Mississippian (Big Snowy Group)____________ 23 Kibbey Formation._______________ 5 Early and Middle Pennsylvanian (Tyler Formation Otter Formation._________________ 7 and Alaska Bench Limestone)__________________ 24 Heath Formation.________________ 7 Middle and Late Pennsylvanian (Devils Pocket, Regional relations._______________ 10 Quadrant, and Tensleep Formations)____________ 24 Pennsylvanian System, Amsden Group __ 11 Permian to Jurassic events__-___--------_-------- 25 Tyler Formation..______________ 11 References cited.________________-_-_------_-_----_- 25 Alaska Bench Limestone_________ 14 Devils Pocket Formation._________ 15 Regional relations._______________ 19 ILLUSTRATIONS [Plates are in pocket] PLATE 1." Columnar sections showing correlation of Big Snowy and Amsden Groups in central and southern Montana. 2. Columnar sections showing correlation of Big Snowy and Amsden Groups in central and eastern Montana. 3. Paleogeologic map showing distribution of Big Snowy and Madison Groups in Montana. 4. Chart showing stratigraphic distribution and comparative ranges of fossils from the Big Snowy and Amsden Groups in central Montana. Page FIGURE 1. Chart showing development of nomenclature for Mississippian and Pennsylvanian rocks in central Montana____ B2 2. Index map of Montana and adjacent areas____________________________________________________________ 4 3. Chart showing nomenclature and stratigraphic relations of the Upper Mississippian and Pennsylvanian rocks in Montana, Wyoming, South Dakota, and North Dakota.___________________________________________ 6 4. Index map of localities in Big Snowy Mountains and vicinity, Montana.___________________________________ 8 5. Diagrammatic section showing relations of Big Snowy and Amsden Groups in central Montana-------_______ 11 6. Diagrammatic section of erosional unconformity between Heath Formation (restricted) and Stonehouse Canyon Member of the Tyler Formation exposed at west end of Middle Bench, Little Snowy Mountains __________ 12 7. Photograph showing unconformable contact of the Stonehouse Canyon Member of the Tyler Formation upon restricted Heath Formation.______________________________________________________________________ 14 TABLES Page TABLE 1. Comparison of thicknesses of Heath Formation and Stonehouse Canyon Member of the Tyler Formation. B7 2. Pollen and spores recovered from Mississippian and Pennsylvania!! samples from Montana_______________ 22 in SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY BIG SNOWY AND AMSDEN GROUPS AND THE MISSISSIPPIAN-PENNSYLVANIAN BOUNDARY IN MONTANA By EDWIN K. MAUGHAN and ALBERT E. ROBERTS ABSTRACT ble with present understanding of the relations of the The Big Snowy Group is redefined to include the Kibbey, rocks comprising the upper part of the Mississippian Otter, and Heath Formations, as the group was originally estab­ and lower part of the Pennsylvanian of this region. lished. However, the Heath Formation, and consequently the Study of these rocks was begun in 1957 and particular top of the Big Snowy Group, is restricted to strata beneath a attention has been given to the problem of the systemic Late Mississippian to Early Pennsylvanian regional unconform­ ity. This restriction limits the Big Snowy to three closely re­ boundary since 1960. The authors have collected sur­ lated formations that comprise one sedimentary cycle uncom­ face and subsurface stratigraphic information through­ plicated by intraformational structural movements. Also, the out Montana and adjacent States. Surface sections Big Snowy Group, as restricted, closely approximates a time- of the west half of Montana have been visited and stratigraphic unit of Late Misisissippian age. studied in detail; subsurface sections in the east half The Amsden Group unconformaWy overlies the Big Snowy or have been studied from sample and geophysical logs. Madison Groups and consists of three formations in central Montana that are stratigraphically and lithologically nearly In central Montana special emphasis has been given equivalent to.the type Amsden Formation in northern Wyoming. to detailed stratigraphic correlation, description, and The formations, in ascending order, are the Tyler Formation, fossil content of individual units. New information re­ the Alaska Bench Limestone, and Devils Pocket Formation. garding the ages of these rocks is reported, and the The Tyler Formation is locally divided into a Stonehouse Canyon equivocal position of the boundary between the Mis­ Member at the base and a Cameron Greek Member at the top. Spores collected from the upper part of the Stonehouse Canyon sissippian and Pennsylvania!! Systems is resolved. and Cameron Creek Meiribers are of Early Pennsylvanian age. Much of this basic data has been obtained from pub­ Fusulinids from the Alaska Bench Limestone suggest a Morrow lished and unpublished reports and credit is given to and Atoka age, and those from the Devils Pocket Formation are the original sources throughout this report. However, of Atoka or early Des Moines age. interpretations of these data are those of the authors Regional upwarp in south-central Montana and much of adja­ and we assume full responsibility for them. cent Wyoming took place near the end of Mississippian time. The area of uplift was bounded on the north by a system of The boundary between the Mississippian and Penn­ probable faults and monoclinal folds. Rocks of the Big Snowy sylvanian Systems, which is believed to be at the un- Group were stripped south of this structural belt, and were tilted conformable contact of the Big Snowy and Amsden northward and beveled north of this belt. Subsequent to erosion, Groups, is given particular attention. Four groups of the region gradually submerged during Early Pennsylvanian detailed columnar sections (pis. 1, 2) are presented to time. Seas inundated the region and detrital sediments of the basal part of the Amsden Group were deposited uiiconformably illustrate the unconformity between the Big Snowy on Upper Mississippian rocks in central Montana, and on older and Amsden Groups and to show the regional correla­ rocks farther south in southern Montana and northern Wyoming. tion of the stratigraphic units that compose these groups. INTRODUCTION DEVELOPMENT OF NOMENCLATURE Upper Paleozoic rocks of the Big Snowy and overly­ Quadrant Formation was first used in the Three ing Amsden Groups of central and western Montana Forks, Mont., area by Peale (1893) (as shown in fig. 1) show wide variations in lithology and thickness and for strata between underlying Madison Limestone (Mis­ have long presented problems of identification, correla­ sissippian) and overlying Ellis Formation (Jurassic). tion, and dating. This paper summarizes the stratig­ Quadrant Quartzite was applied by Weed (1896) and raphy and presents revisions in nomenclature compati­ by Iddings and Weed (1899) (as shown in fig. 1) at its Bl B2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Weed (1896) Peale Weed Freeman Reeves Scott Sloss Iddings and Weed (1893) (1900) (1922) (1931) (1935) (1952) (1899) Mesozoic Jurassic Jurassic u u Ellis v> ^CarboniferousTriassic Teton formation E lis formation 100 feet of black shale Ellis formation BJ Ellis formation as Ellis group formation 1Permian_.., 3 3 ^~v_^~^ .-^~\_^~^/~^ -^ u C .5 c C <0 yw&^~^ Minnelusa CO (Not described) .5'c 'E C £ <0 a <0 <0 > c ">, > CO U ndifferentiated -?_, 1 Upper Amsden c 1 C 3 CU 0 c M Alaska Bench cu cu Amsden Q. Quadrant Quadrant quartzite <5 c Q. Q. 'E 0 limestone a Q. "5 c formation formation or sandstone o =3 <0 .Q 0 100 feet of c Lower Amsden <0 00 E , jjray shale , O o .5'a. Quadrant Carboniferous c a. QO Meath GO <0 Tyler sandstone Meath formation a c formation >> formation <0 <0 a in =3 Otter shale i Otter c O1 CO Otter shale 'i Otter formation 3 00 formation V) cr Mississippian op CarboniferousLower Kibbey ss Kibbey sandstone Mississippian bo Kibbey fm bo Kibbey formation Charles formation Q. c 3 'a.a 0 Mission Canyon Madison Madison a. Madison Madison M '« C limestone C astle limestone (Not given) 0 limestone limestone limestone limestone ' 5 u> <0 Lodgepole i limestone FIGURE 1. Development of nomenclature for Mississippian and Pennsylvanian rocks in central Montana. Scott (1935) originally placed all the Amsden in the Mississippian but later (1945; 1950, p. 48) placed part in the Pennsylvanian on the basis of f usulinids. type section on Quadrant Mountain in the northwestern is not in the vicinity of the type sections of the Kibbey part of Yellowstone National Park (fig. 2) for strata and Otter