GEOLOGIC MAP OF THE WINNETT 30' X 60' QUADRANGLE

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Montana Bureau of Mines and GeologyGEOLOGIC MAP OF THE WINNETT 30' x 60' QUADRANGLECENTRAL MONTANAbyKaren W. Porter and Edith M. WildeMontana Bureau of Mines and GeologyOpen File Report MBMG 3071993Revisions:Map and text 1999Map 2007This report has had preliminary reviews for conformity with Montana Bureau of Mines andGeology’s technical and editorial standards.Partial support has been provided by the STATEMAP component of the National CooperativeGeologic Mapping Program of the U. S. Geological Survey under Contract Number 98-HQ-AG2080.

GEOLOGIC SUMMARYThe Winnett 30'x60' quadrangle of central Montana is approximately equally dividedbetween the geologic settings of the central Montana uplift and the Blood Creek Basin.Geologic structure, stratigraphy, and geologic history are highly contrasted between these twosettings: the central Montana uplift is a large, west-east structural block that has dominatedcentral Montana since Proterozoic time; the Blood Creek Basin to the north is a broad, generallyfeatureless structure with a poorly defined and unmapped surface axial trace. The boundarybetween these two settings is the steeply dipping north limb of the asymmetric, northwestsoutheast-trending Cat Creek Anticline. Along this anticline 10 domes are recognized, 6 in theWinnett quadrangle, and 3 have produced oil from Lower Cretaceous and Jurassic rocks. TheCat Creek Anticline is terminated on the west by the Judith Mountains whose easternmostexposed pluton, Black Butte, occupies the northwest corner of the map.Bedrock exposures (figure 1) in the area are predominantly marine and nonmarinesedimentary rocks of Lower and Upper Cretaceous age. Paleozoic rocks are poorly exposed ina narrow band on the steep flanks of Black Butte. Paleocene Fort Union Formation beds arepreserved in a few small erosional remnants in the Blood Creek Basin, and Paleocene-Eoceneultramafic intrusions occur as northeast-trending dikes and small plugs, predominantly in thesouthwest area of the map. Quaternary deposits include, in addition to modern stream alluviumand colluvium, continuous deposits of older alluvium, including terrace deposits, occurring alongand slightly higher than present stream courses in the northern part of the map area. Limitedpediment gravels remain on the flanks of the Judith Mountains.Previous mapping was conducted by Reeves (1927), Johnson and Smith (1964), andNelson (1993) in the eastern part of the Winnett quadrangle, along the eastern end of the CatCreek anticline. Mapping of the Judith Mountains (Black Butte) in the northwestern part of thequadrangle was conducted by Goddard (1988). No earlier mapping was available for the BloodCreek basin beyond the work of Ross and others (1955). The effort of Johnson and Smith(1964) to follow Cobban (1951, 1953a) in applying Black Hills stratigraphic terminology to thecentral Montana marine Cretaceous section has been continued in the Winnett quadrangle,permitting more detailed mapping of the formerly mapped Colorado Shale that included the FallRiver Sandstone through the Niobrara Formation. This new mapping has revealed a number ofsmall folds on the broad south limb of the Cat Creek Anticline and a new small dome, SouthFork dome, in the western map area.1

and CambrianO sFigure 1. Age correlation chart of map units for Winnett 30 x 60 quadrangle.2

DESCRIPTION OF MAP UNITSQalFLOOD PLAIN AND CHANNEL ALLUVIUM (HOLOCENE) — Yellow-tan and gray-tandeposits of poorly to well stratified gravel, sand, silt, and clay deposited in floodplains and channels of modern streams. Locally includes some slightly olderHolocene terrace alluvium at slightly higher elevations than modern flood plainalluvium. Thicknesses not measured.QcCOLLUVIUM (HOLOCENE) — Yellow-tan, tan, cream, and gray-tan weathering,nonstratified surface deposits of minor slumps and sheet-wash proximal toalluvium of a few modern drainages, and dried lake deposits; located innorthwest map area. Thickness ranges from 1 to 20 ft.QaoOLDER QUATERNARY ALLUVIUM (HOLOCENE) — Light gray to yellowish whiteweathering deposits of unconsolidated clay, silt, sand, and some small gravel ofpredominantly limestone composition; moderately sorted. Mapped in northernhalf of quadrangle adjacent to modern streams, generally slightly above modernalluvial flood plains; includes unmapped terrace deposits; poorly exposed exceptwhere undercut by streams. Thin soils developed on unit. Thickness ranges from10 to 60 ft.QpgPEDIMENT GRAVEL AND GRAVEL ALLUVIUM OF DISSECTED BRAID PLAINS(QUATERNARY) — Light gray to yellowish-white and gray-brown weatheringdeposits of uncemented to locally cemented, cobbles in pebble, sand, and claymatrix. Cobbles predominantly light gray, rounded clasts of Madison Grouplimestone, commonly with powdery white, calcareous coatings; in western maparea, composition includes green and brown alkalic igneous rocks derived fromJudith Mountains. Cemented intervals 1 to 5 ft thick, with iron-rich calcareousmatrix commonly weathering reddish to yellow-orange or rusty brown. Unitincludes thin veneer of gravel on pediment surfaces and several levels of thickergravels apparently deposited in broad alluvial fans now largely removed. Unitoccurs in two broad areas extending from the Judith Mountains in westernmostmap area, and in several isolated patches along high elevations on both sides ofMcDonald Creek in south part of map; unit equivalent to Qab unit mapped onLewistown quadrangle (Porter and Wilde, 1993; revised 1999) and onMusselshell quadrangle (Porter and Wilde, 1999). Thickness not measured; fromless than 1 ft to 10 ft.TialALKALIC INTRUSIVE ROCKS, UNDIVIDED (TERTIARY) — Medium brown, greenbrown, to yellow-brown, brick red, and orange weathering; ultramafic, porphyritic,and commonly containing small and large (1/2 to 18 inches in diameter) angular,slightly metamorphosed blocks of sedimentary bedrock, generally shale.Intrusions commonly deeply weathered, poorly to moderately resistant, and formcrumbly, coarse rubble; occur as low, slightly resistant dikes forming streaksacross ground surface (secs. 5, 6, T14N, R25E; note this body has been calledthe Winnett sill, but its sill orientation is unclear to the present mappers), as dikeridges with relief of 60 ft or more (sec. 1, T15N, R25E), and as isolated low-relief3

plugs (sec. 7, T15N, R25E); sills have been reported farther south in Elk Creekdrainage of Musselshell 30' x 60' quadrangle (Johnson and Smith, 1964) butwere not observed in this map area. Strike orientation of dikes is north 55 to 70 east. Intrusions within map area are not clearly associated with faulting but thisrelationship is observed by Johnson and Smith (1964) farther to south. Refer to:Johnson and Smith (1964), Kohrt (1991) for Black Butte area); Marvin and others(1980), Ross (1926), and Scambos (1991) for detailed discussion of compositionand emplacement history of these igneous intrusions.TftTKqmpKhcTULLOCK MEMBER, LOWER MEMBER OF FORT UNION FORMATION (LOWERTERTIARY) — Alternating layers of light to medium gray and tan sandstone,siltstone, and claystone; white-weathering surfaces commonly associated withclaystones and siltstones; claystone surfaces occasionally weather to popcorntexture. Beds lens shaped and variable in width and length. Contact withunderlying Hell Creek Formation marked by distinctive color change and placedabove a thin (6 in. to 4 ft), black to dark brownish-gray, carbonaceous claystonecontaining thin coal seams and lenses. Unit occurs in small, isolated erosionalremnants near center of ridge between Cottonwood Creek and Blood Creek andin one small area on ridge between Blood Creek and Dovetail Creek in westcentral part of the map. Dips nearly flat to less than 1 degree. Only basal 40 to 60ft of formation remain.QUARTZ MONZONITE AND SYENITE PORPHYRY (PALEOCENE OR UPPERCRETACEOUS; TKqm of Goddard, 1988) — Phenocrysts of plagioclase(oligoclase and andesine), potassium feldspar, pyroxene, hornblende, and minorquartz in gray groundmass; includes some quartz diorite, monzonite, and rhyoliteporphyry; also forms dikes and sills; large masses of syenite porphyry atMaginnis Mountain and Lewis Peak.HELL CREEK FORMATION (UPPER CRETACEOUS) — Composed of nonmarinesandstones, siltstones, and claystones in three distinct intervals. Lower intervalconsists of very discontinuous, thinly interbedded, olive-gray to brownish-gray, sandysiltstones, and claystones with occasional thin beds of light-gray sandstone; someclaystones bentonitic. Thickness varies from 30 to 60 ft. Middle interval consists oflight-gray to yellowish-gray sandy siltstones and fine- to medium-grainedsandstones; locally calcareous; contains conspicuous, cross-bedded, concretionaryzones that weather light-brownish-gray to yellowish-brown and often form log-likemasses. Contains many shallow channel features with common basal conglomeraticlag; locally, channels have cut through lower interval into top of Fox Hills Formation.Middle interval makes up more than half of entire formation in this area. Unit oftentree-covered. Ranges from 125 to 150 ft thick. Upper interval consists of interbeddedyellowish-gray, medium-grained sandstone and olive-gray to brownish-gray siltstoneand claystone. Upper contact placed below a distinct color change at a very darkbrownish-gray silty claystone. Thickness ranges from 40 to 90 ft. Bones, leaf andstem imprints, and plant fragments occur at many locations and throughout theformation. Accurate dips difficult to obtain, but generally less than 2 degreesthroughout the area, except along Cottonwood Creek and near the Cat CreekAnticline. Formation forms main ridge areas between eastward-flowing streams innorthern part of map area. Total formation thickness is 185 to 225 ft. Johnson and4

Smith (1964, p. 55-56) report thickness of 214 ft just east of Winnett quadrangle.KfhFOX HILLS FORMATION (UPPER CRETACEOUS) — Consists of two distinct intervals.Lower interval consists of interbedded sandstone, siltstone, and claystone beds thatoften contain small ironstone concretions and small, light brownish-gray sandstonenodules. Becomes more clay-rich at base; contact transitional with underlyingBearpaw Shale. Interval generally forms gentle, grass-covered slopes at base ofsteep hills near valley bottoms; ranges from 10 to 30 ft thick. Upper interval, mostconspicuous part of the formation, light gray, fine- to very fine-grained sandstoneweathering to tan or light brownish-gray. Generally unconsolidated to slightlyconsolidated; appears massive. Forms benches above interbedded lower intervaland ledges and small hills where better cemented. Ledge-forming sandstone oftencalcareous, weathers rusty brown, and generally contains either ripple marks or thincross beds; contains fossilized plant fragments in a few locations. Interval 40 to 90 ftthick. Upper contact placed at top of a brownish-black to purplish-black, verycarbonaceous, silty claystone forming conspicuous ledge where visible, but oftencovered; bed contains streaks of coal and often plant fragments; distinct colorchange from generally light-colored Fox Hills to darker olive-brown tones of lowerHell Creek marks contact. Contact throughout most of area appears conformable.Formation forms basal part of steep hills bordering creek valleys. Total formationthickness ranges from 60 to 120 ft. Johnson and Smith (1964, p. 50-53) reportthicknesses of 63 ft to 141 ft just east of Winnett quadrangle.KbBEARPAW SHALE (UPPER CRETACEOUS) — Consists of medium- to dark-gray,fissile shale weathering to lighter gray or brownish-gray surface. Contains a few verythin (1-3 in.), laterally inconsistent, yellowish-gray bentonite beds, and a fewscattered white to light gray weathering limestone concretions that commonly containmarine fossils. Becomes increasingly sandy upward, grading into interbeddedsiltstones and sandstones at base of overlying Fox Hills Formation. Outcrops formflat to very gently rolling topography exposed in bottoms of eastward-flowing creeksin northeastern part of map area. Less than 200 ft of this formation crops out in thisarea, but Cobban (1953, p.101) reported a thickness of 1318 ft east of Winnettquadrangle.KjrJUDITH RIVER FORMATION (UPPER CRETACEOUS) — Composed of three distinctintervals. Lower sandstone unit (approx. 78 ft) weathers yellow-gray, very fine- orfine-grained, massive to poorly bedded, burrowed to bioturbated; more resistant,coarser-grained and containing brown, fist-size nodules in upper part, withuppermost approximately 7 ft composed of hard, light-brown, ferruginous, locallycoarse-grained, concretionary, cross-stratified sandstone forming resistant ledge.Middle unit (approx. 140 ft) is green-gray weathering, fine-grained sandstones,siltstones, mudstones, and brownish carbonaceous shale; common plant imprints,coalified plant debris, and silicified wood; numerous conspicuous rusty-brown topurple-black weathering ironstone concretions. Upper unit (approx. 55 ft) composedof basal 10-ft yellow-gray to yellow-brown weathering fine-grained sandstoneoverlain by sequence of interbedded sandstone, mudstone, and carbonaceous shalewith common small ferruginous concretions. Contact with underlying Claggett istransitional; contact with overlying Bearpaw Shale is relatively sharp aboveuppermost gray, sandy beds. Middle and upper units of formation commonly weather5

to a badlands aspect. Total thickness of 273 ft measured by Cobban (1953, p. 98)east of Winnett quadrangle.KclCLAGGETT SHALE (UPPER CRETACEOUS) — Dark gray fresh surfaces commonlyweathering to soft brown; fissile except on fresh exposures where it is more blockyweathering. Upper middle portion contains characteristic deep orange weathering,smooth, ovoid to discoid, calcareous concretions up to 3 ft in diameter occurrin