CENOZOIC TECTONIC SEQUENCES AND BASIN EVOLUTION IN WESTERN MONTANA

by DEBRA L. HANNEMAN, WHITEHALL GEOGROUP, INC., WHITEHALL, MONTANA 59759 whgeol@gmail.com and CHARLES J. WIDEMAN, EMERITUS, MONTANA TECH OF THE UNIVERSITY OF MONTANA, BUTTE, MONTANA 59701 cjwidemans@gmail.com 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 original 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 FieldConference 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 - Hemphillian

Sixmile Creek Formation: Arikareean - Hemphillian

Sixmile Creek Formation: Early Barstovian - Early Blancan

Madison Valley Formation: Early Barstovian - Clarendonian

ISOLATED VALLEY STRATIGRAPHICINTERPRETATIONS

Sixmile Creek Formation: Early Barstovian - Hemphillian

Little Sage Creek beds: Early Barstovian

Sixmile Creek Formation (Railroad Canyon beds): Early Barstovian

G Deer Lodge Valley

Cabbage Patch beds: Early Arikareean - early Late ArikareeanRenova Formation: Chadronian - Late Orellan

Sixmile 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 Eocene

Sixmile Creek: Early Barstovian to Hemphillian

I North and Middle Fork of the Flathead Valleys

Kishenehn Formation: Upper Kishenehn: Middle - upper Arikareean

Quaternary sediments/Glacier Peak-Mazama volcanic ash

Quaternary sediments/Mazama volcanic ash

Red Mountain/Shoddy Springs volcanics: Early Eocene

Quaternary sediments/Mazama volcanic ash

Pioneer/Beaverhead volcanics: Early/Middle Eocene

Quaternary sediments/Yellowstone Huckleberry Ridge TuffTimber Hill Basalt: @ 4 Ma

Virginia City/Ruby Dam volcanics: Early/Middle Eocene

Quaternary sediments/Yellowstone Huckleberry Ridge TuffSage Creek Basalt equivalent: @ 5 Ma

Beaverhead Canyon/Hall Springs volcanics: Early/Middle Eocene

Quaternary sediments

Challis equivalent volcanics: Early/Middle Eocene

Quaternary sediments

Lowland Creek volcanics/Anaconda Beds: Early/Middle Eocene

Quaternary sediments

Quaternary sedimentsPaoloa Gravel: ?Miocene

Middle 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/UpperKittitas 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 STRATIGRAPHY OF SELECTED WESTERN MONTANA VALLEYS

55

65

45

35

25

15

5

MaJBMDValleys

Toston -Three Forks Valleys

Sage Creek/Blacktail Valleys

Ruby Valley(Upper/Lower)

Puercan

Torrejonian

Tiffanian

Clarkforkian

Wasatchean

Bridgerian

Uintan

Duchesnian

Chadronian

OrellanWhitneyan

Arikareean

Hemingfordian

Barstovian

Clarendonian

Hemphillian

Blancan

IrvingtonianRancholabrean

65

54.8

33.7

23.8

5.3

1.750.01

63.5

60.5

56.2

50.4

55.5

47

40

37

333230

11.5

15.9

19

9

4.91.750.1

65

GH, HP,MC, MLValleys

TownsendValley

North AmericanLand MammalAgesEpoch

Pliocene

Paleocene

Eocene

Oligocene

Miocene

HolocenePleistocene

Ma MaDeer LodgeValley

Flint CreekValley

N/M Fork FlatheadValleys

S 2/ LK

S 3/ UK

S 4/ LW

S 5/ UW (HC)

S 1/ C

EoceneUnconformity

HemingfordianUnconformity

30 Ma Event

Plateau Volcanic Field

BASIN - RAN

GE

EXTENSIO

NC

RUSTAL EXTEN

SION

,M

ETAMO

RPHIC

CO

REC

OM

PLEX FORM

ATION

,M

AGM

ATISM

Sequence 5/Upper Walpapi (High Cascade)Sequence 4/Lower Walpapi

Sequence 3/Upper Kittitas

Sequence 2/Lower Kittitas

Sequence 1/Challis

Erosional Boundary & Hiatus

Stratigraphic Column LegendInferred Erosional Boundary

Yellowstone

?

?

A B C D E F G H I

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 Mountains

Silverstar

TwinBridges

Cherry Creek

Hell CanyonNorth Rochester

Twin Bridges

South Rochester

Nez Perce

McCarty M

ountain

N

0 2400

Meters

Interpreted Cenozoic Basin-Fill Sequences

Pre-Cenozoic RocksSequence 2/Lower KittitasSequence 3/Upper Kittitas

Sequence 4/Lower WalpapiSequence 5/Upper Walpapi

and Wideman, 1991

Normal fault, bar/ballon downthrown side

Beav

erhe

ad Va

lley

G A

B

CC

C

DD

E

E

F

F

F

I

H

Bozeman

Butte

Missoula

Kalispell

A Townsend Valley

B Toston- Three Forks Valleys

C Jefferson, Beaverhead, Melrose, Divide Valleys

D Ruby Valleys

E Sage Creek, Blacktail Valleys

F Grasshopper, Muddy Creek, Medicine Lodge Valleys

G Deer Lodge Valley

H Flint Creek ValleyI North & Middle Fork of Flathead River Valleys

0 30Scale

Miles

WESTERN MONTANA