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  CENOZOIC TECTONIC SEQUENCES AND BASIN EVOLUTION IN WESTERN MONTANA by DEBRA L. HANNEMAN, WHITEHALL GEOGROUP, INC., WHITEHALL, MONTANA 59759 and CHARLES J. WIDEMAN, EMERITUS, MONTANA TECH OF THE UNIVERSITY OF MONTANA, BUTTE, MONTANA 59701   ABSTRACT Cenozoic sedimentary and volcanic basin-fill in western Montana can be packaged into sequences related to regional tectonic events. We useinterpreted seismic, borehole, and gravity data, regional mapping, and detailed age constraints on Cenozoic deposits, employing both fossil vertebrates and isotopic ages, to delineate sequences and to generate a more data-integrated paradigm for Cenozoic basin evolution. The “Eocene unconformity” or Rocky Mountain erosion surface underliesthe oldest sequence. This sequence, known as Cenozoic Sequence 1/Challis Sequence, ranges in age from ~50 to 44 Ma. It preserves a record of orogenic collapse and basin development related to metamorphic core complex formation in the Cordilleran hinterland and listric normal faulting in the fold and thrust foreland, as well as abundant arc-magmatism. The overlying sequence, Cenozoic Sequence 2/lower Kittitas Sequence, spanning a time duration of ~38 to 30 Ma, documents a tectonic intervalof extensional collapse that occurred throughout the entire Cordilleran orogen. Significantly, this sequence is bounded by the ~30 Ma unconformity which may be related to eustatic and/or climatic changes rather than to tectonic events. The overlying ~27 to 21 Ma Cenozoic Sequence 3/upper Kittitas Sequence is bounded at its top by the Hemingfordian unconformity, which represents a hiatus from ~20 to 17 Ma. The Heminfordian unconformity is largely isochronous across the western U.S., and marks the change from orogenic collapse to backarc spreading of the Basin and Range event, the next major Cenozoic episode of crustal extension and magmatism. Its overlying sequence, Cenozoic Sequence 4/lower Walpapi Sequence, ranges in age from ~16 to 4 Ma. This sequence records the developmentof the present topographic basins which includes reactivated Paleogene and newly defined basins. Some of these newly delineated basins contain only portions of the older Paleogene basins whereas others extend geographically beyond the srcinal Paleogene basins. The youngest sequence, Cenozoic Sequence 5/upper Walpapi Sequence ranges in age from ~ 2 Ma to the present. This sequence correlates to the tectonic pulse documented in the Yellowstone/Teton area and to the initiation of the Yellowstone Plateau volcanic field at ~ 2 Ma. References Bendick, R., Baldwin, J. , 2009, Dynamic models for metamorphic core complex formation and scaling: The role of unchannelized collapse of thickened continental crust, Tectonophysics, doi:10.1016/j.tecto.2009.03.017. Constenius, K.N., Dawson, M.R., Pierce, H.G., Walter, R.C., andWilson, M.V.H. , 1989, Reconnaissance paleontologic study of theKishenehn Formation, northwestern Montana and southeasternBritish Columbia, in: French, D. E. and Grabb, R. F., GeologicResources of Montana: Montana Geological Society, 1989 Field Conference Guidebook, p. 189-203. Fields, R. W., Rasmussen, D. L., Tabrum, A. R., and Nichols, R. ,1985, Cenozoic rocks of the intermontane basins of western Montanaand eastern Idaho, in Flores, R. M., and Kaplan, S. S., eds., CenozoicPaleogeography of west-central United States: Society of Economic Paleontologists and Mineralogists, p. 9-36. Hanneman, D.L., and Wideman, C.J. , 1991, Sequence stratigraphy of Cenozoic continental rocks. Geological Society of America Bulletin, 103, 1335-1345. Hanneman, D.L., and Wideman, C.J. , 2006, Calcic pedocomplexes-regional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States. in: Alonso-Zarza, A.M.,Tanner, L.H., eds., Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates, Geological Society of AmericaSpecial Paper 46, p. 1-15. Hanneman, D.L., Wideman, C.J., and Halvorson, J. , 1994, Calcic  paleosols: their use in subsurface stratigraphy. American Association of Petroleum Geologists Bulletin 78, p. 1360-1371. Portner, R.A., Hendrix, M.S., Stalker, J.C., Miggins, D.P., and Sheriff, S.D. , 2011, Sedimentary response to orogenic exhumation in the northern Rocky Mountain Basin and Range province, Flint Creek  basin, west-central Montana: Can. J. Earth Sci. 48, p.1131 -1153. Vuke, S.M., Coppinger, W.W., and Cox, B.E. , 2004, Geologic mapof the Cenozoic deposits in the upper Jefferson Valley: MontanaBureau of Mines and Geology Open-file report 505. B   Toston/Three Forks Valleys   Dunbar Creek Formation: Orellan/WhitneyanClimbing Arrow Formation/Milligan Creek Formation: ChadronianSphinx Conglomerate - ? C   Jefferson/Beaverhead/Melrose/Divide Valleys Renova Formation: Dunbar Creek Member: Orellan/Whitneyan Climbing Arrow Member/Bone Basin Member: Chadronian A   Townsend Valley Canyon Ferry Beds: ?Late Chadronian,Orellan, Arikareean D   Upper/Lower Ruby Valleys   Renova Formation: Passamari Member: Whitneyan in lower Ruby; Late Arikareean in upper Ruby Dunbar Creek Member: Orellan/Whitneyan Climbing Arrow Member/Bone Basin Member: Late Unitan/ Duchesnean - Chadronian E   Sage Creek/Blacktail Valleys White Hills Beds: ?WhitneyanCook Ranch Formation: Late Chadronian - Late OrellanDell Beds: Late Unitan F   Grasshopper/Horse Prairie/Muddy Creek Medicine Lodge Valleys   Medicine Lodge Beds: Chadronian - Late Arikareean(Renova Formation for all in Muddy Creek)Sixmile Creek Formation: Early Barstovian - HemphillianSixmile Creek Formation: Arikareean - HemphillianSixmile Creek Formation: Early Barstovian - Early BlancanMadison Valley Formation: Early Barstovian - Clarendonian ISOLATED VALLEY STRATIGRAPHICINTERPRETATIONS Sixmile Creek Formation: Early Barstovian - HemphillianLittle Sage Creek beds: Early BarstovianSixmile Creek Formation (Railroad Canyon beds): Early Barstovian G   Deer Lodge Valley   Cabbage Patch beds: Early Arikareean - early Late ArikareeanRenova Formation: Chadronian - Late OrellanSixmile Creek: Early Barstovian to Hemphillian H   Flint Creek Valley   Cabbage Patch beds: Early Arikareean/Late ArikareeanRenova Formation: Chadronian - Late OrellanGarnet Range/Bearmouth volcanics/Anaconda beds: Early/ Middle EoceneSixmile Creek: Early Barstovian to Hemphillian I   North and Middle Fork of the Flathead Valleys   Kishenehn Formation: Upper Kishenehn: Middle - upper ArikareeanQuaternary sediments/Glacier Peak-Mazama volcanic ashQuaternary sediments/Mazama volcanic ashRed Mountain/Shoddy Springs volcanics: Early EoceneQuaternary sediments/Mazama volcanic ashPioneer/Beaverhead volcanics: Early/Middle EoceneQuaternary sediments/Yellowstone Huckleberry Ridge Tuff Timber Hill Basalt: @ 4 MaVirginia City/Ruby Dam volcanics: Early/Middle EoceneQuaternary sediments/Yellowstone Huckleberry Ridge Tuff Sage Creek Basalt equivalent: @ 5 MaBeaverhead Canyon/Hall Springs volcanics: Early/Middle EoceneQuaternary sedimentsChallis equivalent volcanics: Early/Middle EoceneQuaternary sedimentsLowland Creek volcanics/Anaconda Beds: Early/Middle EoceneQuaternary sedimentsQuaternary sedimentsPaoloa Gravel: ?MioceneMiddle Kishenehn: Chadronian - WhitneyanLower Kishenehn: Bridgerian - middle Uintan   Sequence 1/Challis documents metamorphic core complex extension coupled with basin formation and widespread magmatism. Sequence 1/Challis volcanics, Deer Lodge Valley.Sequence 1/Challis conglomerates and red  beds, Sage Creek Valley.Sequence 1/Challis volcanicash, Three Forks Valley.Calcic paleosol atop Sequence 3/Upper Kittitas mudstone, Jefferson Valley.Sequence 5/Upper Walpapi unconformably overlies Sequence 3/Upper Kittitas, Grasshopper Valley. Newly delineated basins occurred during Sequence 4/Lower Walpapi tectonic pulse as indicated by stranded older Cenozoic deposits.Sequence 4/Lower Walpapi tuffaceous sediments at Anceney in the Madison Valley.Sequence 2/Lower Kittitas mudstone, Pipestone area, Jefferson Valley. CENOZOIC SEQUENCE STRATIGRAPHYOFSELECTED WESTERN MONTANAVALLEYS 5565453525155 Ma JBMDValleys Toston - ThreeForks ValleysSageCreek/Blacktail ValleysRuby Valley(Upper/Lower)Puercan Torrejonian TiffanianClarkforkianWasatcheanBridgerianUintanDuchesnianChadronianOrellanWhitneyanArikareeanHemingfordianBarstovianClarendonianHemphillianBlancanIrvingtonianRancholabrean 6554.833.723.85.31.750.0163.560.556.250.455.547403733323011.515.91994.91.750.165 GH,HP,MC,MLValleys TownsendValley North AmericanLand MammalAgesEpoch PliocenePaleoceneEoceneOligoceneMiocene Holocene Pleistocene Ma Ma Deer LodgeValleyFlint Creek ValleyN/MFork FlatheadValleys S 2/LK S 3/UK S 4/LW S 5/UW (HC) S 1/C  Eocene   Unconformity HemingfordianUnconformity30 Ma Event Plateau VolcanicField  B A  S I   N-R A  N GE  E X T E  N S I   O N CR  U S T A L E X T E  N S I   O N ,ME T A M OR P HI   C C OR E  C OMP L E X F  OR MA T I   O N ,MA  GMA T I   S M Sequence 5/Upper Walpapi (High Cascade)Sequence 4/Lower Walpapi Sequence 3/Upper KittitasSequence 2/Lower Kittitas Sequence 1/Challis  Erosional Boundary & Hiatus Stratigraphic Column Legend  Inferred Erosional Boundary Yellowstone ?? ABCDEFGHI   Sequence 2/Lower Kittitas documents a tectonic interval of extension, as seen by the structural control of the Kishenehn Formation deposition. Sequence 3/Upper Kittitasalluvial fan strata, Toston Valley(Sixmile Creek type section). Huckleberry RidgeTuff  Sequence 5/Upper WalpapiYellowstone Event Tuff unconformably overlies Sequence 4/Lower Walpapi strata, Upper Ruby Valley.Sequence 3/Upper Kittitas lacustrine strata,Townsend Valley.Sequence 4/Lower Walpapi mudstoneat Aelurodon locality, Jefferson Valley. Sequence 2/Lower Kittitas mudstone and cryptic grus channels, Little Pipestone area, Jefferson Valley. Blacktail Deer Creek Formation: Late Arikareean Subsurface distribution of Cenozoic sequences in Jefferson and Beaverhead Valleys as interpreted on unmigrated seismic data. Note that Sequences 2/4 thicken into a structurally controlled eastern basin margin. Highland Mountains Tobacco Root MountainsSilverstar TwinBridges C    h  e  r   r    y    C    r   e  e  k   H    e  l    l    C    a  n   y   o  n   N    o   r   t   h    R    o   c   h   e   s   t   e   r    T   w   i   n   B   r   i   d    g  e  s   S   o  u   t   h  R   o  c   h  e  s   t   e  r    N   e  z   P  e  r  c  e   M    c   C    a   r   t    y    M    o  u   n  t   a   i    n   N02400MetersInterpreted Cenozoic Basin-Fill SequencesPre-Cenozoic RocksSequence2/Lower KittitasSequence3/Upper KittitasSequence4/Lower WalpapiSequence5/Upper Walpapiand Wideman,1991   Normal fault,bar/ballon downthrown side      B   e   a    v   e    r     h   e   a    d      V   a     l     l   e    y GABCCCDDEEFFFIHBozemanButteMissoulaKalispell A Townsend ValleyB Toston- ThreeForks ValleysC Jefferson, Beaverhead, Melrose, DivideValleysDRuby ValleysE SageCreek, BlacktailValleysF Grasshopper, Muddy Creek, MedicineLodgeValleysG Deer LodgeValley HFlint Creek ValleyI North & MiddleFork of  Flathead RiverValleys030ScaleMiles WESTERN MONTANA
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