Evolution of Resource-Rich Foreland and Intennontane Basins in Eastern Utah and Western Colorado: Salt Lake City, Utah to Grand Junction, Colorado, July 20-24, 1989 PDF
Preview Evolution of Resource-Rich Foreland and Intennontane Basins in Eastern Utah and Western Colorado: Salt Lake City, Utah to Grand Junction, Colorado, July 20-24, 1989
Evolution of Resource-Rich Foreland and Intennontane Basins in Eastern Utah and Western Colorado Salt Lake City, Utah to Grand Junction, Colorado July 20-24, 1989 Field Trip Guidebook T324 Leaders: Karen J. Franczyk Janet K. Pitman Associate Leaders: William B. Cashion Thomas D. Fouch John R. Dyni Ronald C. Johnson Marjorie A. Chan American Geophysical Union, Washington, D.C. COVER Exposures of the sandstone-dominated lower part of the tertiary Green River formation seen in the Gate Canyon area on the afternoon of day 2. Frequently fluctuating lake levels resulted in this sequence of interbedded delta plain in marginal lacustrine deposits. The pronounced lateral acretion bedding common in this part of the Green River formation formed in both fluvial and deltaic environments. Leaders: Karen J. Franczyk Janet K. Pitman U.S. Geological Survey U.S. Geological Survey MS 939, Box 25046, 'DFC National Center, MS 955 Denver, CO 80225 Reston, VA 22092 Copyright 1989 American Geophysical Union 2000 Florida Ave., N.W., Washington, D.C. 20009 ISBN: 0-87590-624-9 Printed in the United States of America EVOLUTION OFRESOURCE-RICH FORELAND AND INTERMONTANEBASINSIN EASTERN UTAHAND WESTERN COLORADO KarenJ.Franczyk,JanetK. Pitman,WilliamB. Cashion,JohnR. Dyni,ThomasD. Fouch, Ronald C. Johnson U.S. GeologicalSurvey,MS 939,Box25046, Denver, CO 80225 Marjorie A. Chan Dept. ofGeologyandGeophysics, UniversityofUtah, SaltLakeCity, UT 84112 JohnR. Donnell 6035 S. MilwaukeeWy.,Littleton, CO 80121 TimothyF. Lawton NewMexicoStateUniversity, Box3AB,LasCruces,NM 88003 RobertR. Remy Dept. ofGeology,LouisianaStateUniversity, BatonRouge,LA70803 UpperCretaceousandlowerTertiaryrocksincentraland easternUtahrecordthetransitionfromdepositioninamarine foreland basin, bounded on the west by a thrust belt, and deposition in a regionally extensive set ofcontinental inter 1 I montane basins. Similar structural domain-linked rock se I ~ quences characterize much ofthe eastern Cordillera of the SD .....--- United States and Canada, but the timing ofevents and the \ local structural and depositionalstylevary from areato area. I Theformationoftheseforeland andintermontanecomplexes I I hasgivenriseto aremarkablydiverseandextensivecomplex Wy : NE ofenergyandmineraldepositswhoseoccurrencescommonly ________J.....---- co aredepositionallycontrolled. InUtah,Albian(?)tolateCampaniantectonismcharacter izedbythrustfaultsandcoevalfoldsandsynorogenicsedimen tation in amarine forelandbasinis apartof, and coincident with,theCretaceousSevierorogenyofAnnstrong(1968)(fig. 1). Rocksreflectingthesesedimentologicandstructuralchar acteristics are commonly referredto as having resulted from "Sevier-style"tectonismandsedimentation. Thedevelopment ofalargeinternaldrainage system oraseriesoflocalbasins bounded by faults ofvaried types is used to indicatenew or renewedupliftonstructuralelementsfartotheeastofthethrust belt, and movement in the thrustterrain, that segmented the 1......,~"--0--2..,.~ 011--__ 0 Miles forelandbasin(fig.2). Thelocalupliftandintermontanesys o 150 300 Kilometers teminthisareaischaracteristicof"Laramide-style"tectonism and sedimentation (Fouch and others, 1983) which, in the Figure1.Generalizedpaleogeographic reconstructionduring Guidearea,extendedfromlateCampanian/earlyMaastrichtian early late Campanian time for part of the Western United tolateEocene/earlyOligocene(?)time. Thetenn"Laramide" States. Thetooth-markeddashedlineshowstheapproximate final position offrontal thrust faults; the position ofactive hasbeenvariablyappliedformanyyearsforUpperCretaceous thrustfaultsinearlylateCampaniantimeisslightlywestofthis toEocenetectonismandsynorogenicrocksofsimilaroriginin line. Dotsindicateareasofsandycoastlines;plantsymbols the Rocky Mountains and is commonly used inthe region's indicate areas ofcoastal-plain swamps. Box encloses field literature. guidearea. T324: 1 thethrust sheets and betweenthe thrustbelt and earlyrising structuralblocksintheforeland. Segmentationoftheforeland ultimately resulted in a complex oflarge, rapidly subsiding depositionalandstructuralbasins. BeginninginMaastrichtian andextendingtolateEocenetime(perhapsearlyOligocenein centralUtah),morethan3,600m (12,000ft)oflacustrineand associated synorogenic sedimentary rocks fonned inthis ev olvingintennontanesystem. Thelake-basinsystemexpanded greatlyinlatestPaleocene/earlyEocenetimefollowingavery pronouncedtectonicepisodeonsomeoftheupliftsbounding theUintaandPiceancebasins(Uinta,WhiteRiver,Uncomph agre,Sawatchuplifts). Mostofthelacustrinerocks(3,050m, or10,000ft,thick)fonnedalongtheUinta/Piceancesubsiding depositional axesfrom thattimeuntillateEocenetime. The synorogenic clastic influxfrom basin-marginuplifts wasnot sufficienttofilltopographicdepressionsandlakestherein,and thus, within a few kilometers ofshorelines, clastic sediment gradeslaterallyto chemicallyprecipitatedcarbonateinopen lacustrine settings. Because thick units of open-lacustrine rocksareexposedalongerodededgesofthesouthflankofboth theUintaandPiceancebasinsandalongtheWasatchPlateau, theoriginalarealextentofthelakesandthemagnitudeoftheir phases arenotknown. Present-daytopographyismostlytheresultofextensional faultingthatbeganintheNeogeneandthatisstillactiveinthe westernpartoftheGuidearea. Thisfaultinghasdisruptedthe CD ~~~~~:~~DT~~~~~~~RT~=~~~NIC ~HIGHRELIEF continuityoforiginalthrustandbasinstructures,thusbothex BELT ~MODERATETOLOWRELIEF posingandconcealingtherecordofLateCretaceousandearly (!) o AXISOFTHEUINTABASIN EJJ~I~~~~~'T~~~~I~~::~TS o o SANRAFAELUPLIFT FLOODPLAINS UNCOMPAHGREUPLIFT E::::;] EXTENSIVESWAMPS 50 100 150 200MILES o 50 100 150 200KM. ~~(ELXoTcaEtNioSnIVoEfriLvAerCsUisScToRnIjeNcEtuEraNl)VIRONMENTS Figure2.Generalizedpaleogeographicreconstructionofupliftsand intennontaneareasduringearlyTertiarytimefortheareashownin figure 1. Boxenclosesfieldguidearea. ModifiedfromMcDonald, 1972. AtthebeginningofLateCretaceoustime,regionalpatterns ofsedimentationinthispartofNorthAmericawerecontrolled primarily by a continuum of thrusting and coeval folding episodeswhosestructuralelementsfonnedabeltthatextended from north-central to southwestUtah. Thisorogenic activity gaverisetoaregionofmoderatetopographicreliefconsisting ofahighlandboundedonthe eastbyasubaerialplainthatin mostplacesslopedgentlyeasttoamarineforelandbasin. The foreland basin extended across west-central North America, andthebasin'swesternmarginwasslightlywestofthewestern edgeoftheCretaceousseaduringitsmaximumphase(fig.3). Variablesubsidenceatthebasin'swesternmargin,upliftinthe orogenichighland,andglobalsealevelchangescontrolledthe ob 500~1I000 k1il0o6m0eters basin'sdepositionalcharacteristics. Theresultofthistecton I miles ism and associated sedimentation in the Guide area was an eastward-thinning wedge of strata that reaches a maximum Figure3.ExtentofepicontinentalseaduringCampaniantime. thickness of4,000m (13,000ft) inthebasinforedeep. Seawas slightlylargerthanshown hereduringitmaximum Internallydraineddepositionalbasins first fonnedinrela expansionintheTuronian. ModifiedfromGillandCobban, tively small depressions partitioned by ramp anticlines upon 1973. T324: 2 eters Miles m 0 o I4 Kil ~ 0 5 I ~ ~ 0 i2 d30fS' 25' ) l& i 0108 Piceance,(T)Creek,Q ~ (T) oLOShire~~.~~h~ oIo eldtriparea. J_ fi Ar- he t n hi wit al) o c g 0109 I.0.0~I<~<:E--..g::JIOIU (excludin s e c ur o s e r n o b ar c o dr y h al p 0110 princi f o e p y t d n a n o 1110 ApproximateoutlineofntaandPiceanceCreekBasins 6Clear~Creek(M)CD(M)EXPLANATION ~GasfieldGilsonitevein~OilfieldTarsanddeposit B~umeninrocksofTertiaryage B~umenofinrocksTertiaryandMesozoicage ProductionfromrocksofTertiaryageProductionfromrocksofMesozoicageProductionfromrocksofMesozoicandTertiaryageProductionfromrocksofPaleozoicageAllunlabledfieldsproducefrom(M) Figure4.Distributi Ui ~ ~ D " T) C TIM)M& P) ((( ( 00 09 4 3 ...-3WI'\)+= w n w ofeek sho neCr ns Fj mateoutliPiceanceBasins crosssectio @ UPLl pproxintaand ettered 6 ~:t=tm :D:2 m:D• AUi dtol 8 n 0 o 1 p s e orr c n o cti e s f o ~«' es 1:Q Lin 1 ~Q~~?~"'~ mber. ~ ~ u 109° II1(')d~~IO>I::r:IO '0I dstopn n a y a d d n a n o ati c o l p o st e 110° S cateth N di n AI si NT ngle OU tria d M t;J~ bere ~ m A ~ Nu 111° UINT ~~.....---#Vjs s triparea. Z' ometerI40Mile hefield z Kil ft o 0 5I p a m ~~$'"§ E!~~~ B 25II20 Locationutthetext. ®ffQ..<'~ oIo Figure5.througho 0° 9° 4 3 t-3wI'\).l=" .+= Tertiaryorogenesisandsedimentation. Thisphaseandstyleof ogy. TheWasatchfaultzonefonnsthewesternboundaryofthe faulting commonly is associated with the Basin and Range rangeandseparatestheBasinandRangephysiographicprov physiographic province of the United States, but it extends ince to the west from the Middle Rocky Mountain and the well to the east into the Colorado Plateau physiographic Colorado Plateau provinces to the east; repeated Holocene province. The eastern boundary ofthe extensional faulting movement has occurred along the fault zone (Machette and terrane is considerably east ofthe Wasatch Plateau that was others, 1987). Mostofthesedimentsexposedinthelowlands elevatedbyit. westofthe WasatchRange were deposited inmarginal- and Energyandmineralresourcesareabundantintheforeland open-lacustrineenvironmentsofPleistoceneLakeBonneville andintermontanesedimentaryrocksoftheGuidearea(fig.4). fromabout32to10ka. Tracesofancientshorelinesarevisible CoaloccursinCretaceouscoastal-plainandTertiaryintermon onthelowerpartofthemountainfront. 1-15isconstructedon tane wetlandrocks; oil andgasis recoveredfrom Cretaceous fine-grained lake bottom deposits from this pointto nearthe coastal-plain,braid-plain,littoral, andopen-marinerocks and northernflankoftheTraverseMountains, about30km (18.6 fromPaleoceneandEocenefluvial-lacustrinerocks(thelatter mi) tothesouth(fig. 5). containtarsands); oil and gas, oil shale, gilsonite, and eva 7.0 7.0 Totheeast,Precambrianquartzitesandschists poritedeposits,mostnotablynacholite,halite,anddawsonite, and Tertiary intrusive rocks are exposed in the Wasatch occurinlacustrine facies ofthe GreenRiverFormation; and Mountains. Tothewest,theOquirrhMountainsarecomposed uraniumoccursincarbonizedwoodinfluvialPaleoceneunits ofthrust sheets ofpredominantly Paleozoic sedimentary se and in sequences ofmarginal-lacustrine and fluvial Eocene quences. Pre-LakeBonnevillealluvialfandepositsoccurnear rocks. the base ofthe mountains and sandy and gravellymarginal Onthefirstday (fig.5),weexaminePleistocenelacustrine lacustrine sediments deposited during the Bonneville and deltaic sedimentsthatare significantlydifferentfrom thePa Provo shoreline phases extend eastward from the Oquirrh leoceneandEocenelacustrinerocksweseeduringmostofthe Mountain front·almost to the Jordan River. Tailings ofthe trip. We then proceed through the western margin of the BinghamPitCopperminearevisibletothewest; mineraliza Western Interior Cretaceous foreland basin and discuss the tionisassociatedwithTertiaryintrusionintoPaleozoicquartz structuralanddepositionalevolutionofthatsystem. Because ite andlimestone. ofgeographicdistribution, wespendmostofthefirstdayex 13.0 6.4 Exit294. Stayon1-15. PointoftheMountain aminingexposuresofuppermostCretaceousandlowerTerti at 1o'clock to the southwest. An extensive Tertiary quartz ary intermontane deposits. Onthe second day, we examine monzoniteintrusion,theLittle Cottonwoodstock,isexposed clasticmarineandnonmarineforelanddepositsinthemorning intheWasatchRangetotheeast. Tothewest,Tertiaryvolcanic and combined clastic and carbonate alluvial and lacustrine units cropoutnearthebaseofthe OquirrhMountains; more intermontanedepositsintheafternoon.Onthethirdandfourth than 4,300 m ofPennsylvanian sedimentary rocks fonn the days, we concentrate onEocenelacustrine oil-shale-rich se mainpartoftherange. quences,theresources associatedwiththesedeposits, andthe 14.5 1.5 UtahStatePrisontotheright. Theeast-west developmenthistoryoftheoil-shaleindustry. Onourlastday, trendingTraverseMountains, directlyahead,arecomposedof we again compare and contrast foreland and intermontane PaleozoicsedimentaryandTertiaryvolcanicrocks. TheBon basindeposits inareasmuchfarthertotheeastthanthosewe nevilleandProvoshorelinesarewellexposedonthenorthern initiallyexamined. flankofthemountains. 16.4 1.9 Junction with Hwy. 140. Stayon1-15. For aboutthenext5km(3mi),theInterstatepassesmainlythrough DAY1 SALTLAKE CITYTOPRICE,UTAH sandyandgravellymarginal-lacustrinedeposits. Alargespit more than 1.6Ian (1 mi) wide fonned along the Bonneville Proceedeaston1-80fromSaltLakeCityairporttojunction shorelineatPointoftheMountaintothenortheast(Huntand with1-15. Gosouthon1-15and1-80forabout7km(4.4mi). others, 1953);depositsofthisspitaremorethan90m(295ft) Roadlogbeginsatjunctionof1-15southboundandexitto1-80 thick. eastbound. 19.1 2.7 Utah County line. Utah Lake (fresh water) ahead. TripMileage 23.1 4.0 Highland Bench, part of the fan-shaped .e.u.m.. In1r. (cumulative) (interval) AmericanForkdeltaiccomplex,istotheeast; thehighwayis builtonlake-bottommud and silt. 0.0 0.0 Totheeast(left),theWasatchRangeiscom 26.1 3.0 Leave1-15atexit281atAmericanFork. Turn posedmainlyofPrecambrianmetamorphic rocks andPaleo right(west)onMain. zoicandMesozoicsedimentaryrocksemplacedanddefonned 27.0 0.9 Tumright(north)neartallexhausttower. bythrustfaultingduringtheCretaceousSevierorogeny. Nor 27.8 0.8 Tumleft(west)onMainatRollerMills. malfaultingassociatedwithNeogeneBasinandRangeexten 28.1 0.3 Tumright(north)ontoPioneerDriveatLehi sionupliftedthemountainstotheirpresent-dayposition; Pleis HighSchool. toceneglaciationinfluencedmuchoftherange'sgeomorphol- 29.3 1.2 STOP1: Turnleft(west)to gateatquarry. T324: 5 LakeBonnevilledepositsatLehi structures. The height, slope, and trend ofthe foreset beds controlthemoderntopographyofthe30to46-m-high(98-130 AnenonnousbodyofliteraturefollowedGilbert's(1890) ft)bluffsattheedgeoftheHighlandBench(Huntandothers, classicpublicationonlatePleistoceneLakeBonneville. Most 1953). recently, Scott and others (1983), Currey and Oviatt (1985), Thebenchrises61m(200ft)tothenortheastin4.8km(3.0 Machette and others (1987), McCoy (1987), and Oviatt and mi),andtopsetbedsfonnitsuppersurface. Thetopsetbedsare others(1987)useddetailedstratigraphicstudiesinconjunction composeddominantlyofpoorlysorted,troughcross-stratified, with refined dating techniques to establish three major epi gravel to gravelly coarse grained sand that contains a rich sodesofLakeBonnevilleexpansioninthepast150kainthis assemblage of pulmonate mollusks (J. H. Hanley, personal partoftheBasinandRangeprovince. Thefirstcycleextended commun., 1983). Braidedstreamsflowed from the adjacent fromabout160to 130ka,thesecondfrom75to60ka,andthe Wasatch Range across the subaerial part of a fan delta and last,theBonnevillelakecycle,from32to10.3ka. Mostofthe depositedthetopsetsequence(FouchandDean,1982, figs.48 lacustrine deposits exposed along the Wasatch Front were and 49). The size and abundance ofgravel decreases from deposited during the Bonneville lake cycle. The maximum topsettoforesettobottomsetbeds. Theentiredeltaiccomplex lake transgression occurred at 15 ka and is marked by the is finer grained to the northwest indicating current transport highestshoreline. Whenthelake'snaturalsillwasbreached, andwinnowinginthatdirection(Huntandothers, 1953). lakeleveldroppedrapidlyby 108m (355 ft). Afterisostatic Coarse-grainedforesetbedsrepresentsubaqueousdeposi rebound resulting from this drop slowed and the lake level tiononamountain-frontfandeltathatextendedintorelatively stabilized, the Provo shoreline was established and it main deeplakewater. Foresetbedsarethintothickbedsofpoorly tainedits positionfrom about 15to 14ka. Duringthistime, towell-sortedgravel,sand, andsilt; they arefinergrainedin sedimentsdepositedduringtheprevioushighstandwereeroded theirdistalparts. Internally,individualforesetbeds arecom andredepositedtofonnextensivedeltas,spits,andbarsgraded monlytroughcross-stratifiedandexhibitabundantevidenceof to the Provo shoreline. After about 14 ka, the lake level gravity-induceddownslopeslumping,scouring,andsediment progressivelydroppedand,by11ka,wasataboutthepresent flow withinabed(FouchandDean, 1982,figs, 48-50). Peri day levelofGreatSaltLake. Inthepast10ka,lakelevelhas odic flood and possibly seismic events induced foreset ava notfluctuatedmorethan10m(33ft). Thelakecyclesmaybe lanchesandsedimentgravityflowsthatledtothedevelopment broadlysynchronouswithmajorglacialexpansions,but local ofmultiple, gradedbottomsetsand andsiltbeds. climatevariations strongly influencedthetiming andmagni tudeoflake-levelfluctuations (Scottandothers, 1983). Returnto1-15southboundviathe sameroute. Thesequenceexposedinthisquarry(fig.6)underliesthe HighlandBenchandfonnedduringtherapidregressionasso 32.5 3.2 JUnctionofl-15 andexit281. Proceedsouth ciated with the Provo shoreline (Machette, 1988). Gilbert on1-15. (1890)firstinterpretedtheHighlandBenchasdeltaicinorigin 38.5 6.0 1-15beginsto crosstheProvoBench,partof (suchdepositsarenowcommonlyreferredtoascomprisinga alargefan-deltacomplexalsodepositedduringtheregression Gilbertdelta)anddescribedlacustrine-deltaicsedimentologic associatedwiththeProvoshoreline. Thehighwaycrossesthe processes and the resulting topset, foreset, and bottomset westernedgeofthemarginal-lacustrinedepositsforaboutthe next 12kIn (7.5 mi). Atthis latitude, the WasatchRange is composedofMississippian,Pennsylvanian,andPermiansedi mentary rocks. To the west, the Lake Mountains fonn the westernborderofUtahLake. 45.8 7.3 1-15crossesProvoRiverfloodplainandlake bottom deposits. The Wasatch Front east ofProvo contains PrecambrianandCambrianmetamorphicrocksandCambrian andMississippiansedimentaryrocksinthelowerslopes, and Pennsylvanian and Pennian sedimentary rocks in the upper slopes. 49.1 3.3 Springvilleexchangeto UtahHwy. 75. The Wasatch fault zone swings to the eastin this area fonning a largereentrant. Facetedspursindicatingrecurrentmovement onthefaultmarkthebaseofthemountainsbetweenMapleton andSpanishForkCanyon. 53.1 4.0 Exitl-15onexit214toUtahHwy.214toward Spanish Fork Canyon. Road crosses open- and marginal Figure6. StratificationwithinLakeBonnevilledeltaicdepositsex lacustrinedepositsthatarepartoftheSpanishForkfan-delta posedonanow-minedquarryfaceatStop1,Day1. Steeplydipping complex. foresetbeds(F)depositedsubaqueouslyshowdifferentialweathering 57.9 4.8 JunctionofUtahHwy. 214andU.S. Hwy. 6. andcolorvariationsresultingfromgrain-sizesegregation. Flat-lying togentlydippingtopsetbeds(1)depositedinbraidedriversoverlie ContinueeastonHwy. 6. theforesetbeds. Manatbottomofphoto(arrow)forscale. 58.7 0.8 Junction ofU.S. Hwy. 6 and U.S. Hwy. 89. T324: 6 Continue east on Hwys. 6 and 89. Cross the Wasatch fault percentofthe monthly normal, for Octoberthrough Aprilof zone,,whichismarkedbya5-10m (16-33ft)highfaultscarp. 1982it was 172 percent.ofnormal, and for Octoberthrough Remnants ofBonnevillelakecycledeposits arepreservedon Aprilof1983itwas152percentofnormal. Themainlandslide thecanyonwallsto anelevationofabout 1,585m (5,200ft). extends 1.8km (1.1 mi) upthe slope; material was removed 59.6 0.9 ExposuresofthemarineandnonmarinePenn- from a bedrock trough in the upper 1.2 km (0.75 mi) and sylvanianandPermianOquirrhFormation. depositedinthelower600m (1968ft). Initialmovementwas 62.5 2.9 Quaternaryterracedepositsareonnorthside rapidandtheratesdeclinedsignificantlyafterthefirst6days. ofroad;thePennianKirkmanLimestoneisexposedincanyons By early May, the landslide toe had moved 150 m (490 ft). tothe south. About22x106m3(29x106 yd3) ofmaterialiscontainedinthe 63.4 0.9 ExposuresofPermianDiamondCreekSand- mainpartofthelandslide,whichhasanaveragethicknessof40 stoneonbothwallsofthecanyon. m (130ft). 64.1 0.7 Permian marine Park City Formation crops, At the slide site, rocks of the Cretaceous and Tertiary outnorthoftheroadandisoverlainbypoorlyexposedTriassic NorthHomFormationandtheTertiaryflagstaffandColton(?) Woodside Shale. Formations,allofwhichdipabout15°to30°NE,unconforma 64.4 0.3 Triassic marine Thaynes Limestone is ex blyoverlieolderrocksthatdipabout40°-60°SEand arepart posed on north side ofthe road. South ofthe canyon, the oftheCharlestonthrustplate. TheNuggetSandstoneformsthe Cretaceous-TertiarynonmarineNorthHomFormationuncon prominentridge to the leftofthe landslide, and the Ankareh fonnably overliestheThaynes. Formationoccupiesabedrocktroughcontainingthelandslide 64.8 0.4 JunctionwithDiamondForkroad. Outcrops debris. TheNorthHornandyoungerrockswerethesourceof aheadtotheleftareoftheTriassicAnkarehFonnation,which thelandslidedebris,and sufficientmovementoccurredinthe was deposited in dominantly fluvial and floodplain environ upperpartoftheslidetoexposetheunderlying,intactAnkareh mentsunderprogressivelymorearidconditions(Brandley,in Formation. Asof1988,movementofthemainlandslidehas press). virtually stopped; however, continued slidingofthe younger 65.7 0.9 TheeolianJurassic(?)andTriassic(?)Nugget rocks,principallytheNorthHomFormation,isaddingdebris Sandstonecropsout, andtheJurassicTwinCreekLimestone tothedepletedupperpartofthemainlandslide. capsthe slopes. 67.1 0.4 Undifferentiated North Hom Fonnation and 66.7 1.0 STOP 2: Thistle landslide overlook; Twin youngerrockunits. CreekLimestoneexposedinroadcut. Thefollowingdescrip 68.2 1.1 Turnright(south) ontoHwy 89. tionismodifiedfrom KaliserandFleming(1986). 69.3 1.1 Thistletownsite; notehighwatermarksfrom Thistlelake. Thistlelandslide 69.7 0.4 BridgeoverSoldierCreek; NuggetSandstone onright. Tumleft(east)ontodirtroadatsouthendofbridge. TheThistlelandslide is directly across the SpanishFork 70.4 0.7 Turnright(south)ondirtroadupLakeFork. CanyonfromtheoverlookonrelocatedU.S. Hwys. 6and 89. 71.8 1.4 STOP 3: Across the creek, outcrops ofthe ThiswasthemostcostlylandslideinU.S.history; direct and steeplydippingCretaceousIndianolaGrouparecappedbythe indirectcosts areestimated atabout200milliondollarseach. gentlydippingNorthHomFonnation. Only arelativelythin PriortotheApril1983slide,boththeDenverandRioGrande (1,750 m, 5,741 ft), partial interval (Cenomanian(?) to Railroad and U.S. Hwys. 6 and 89 ran in the bottom ofthe Santonian(?))ofIndianolaGroupisexposedinthisarea. The canyon. Duringtheinitialdaysofmovement,crewsattempted Indianolarepresents synorogenicclasticdepositsthatformed tokeeptheriverdrainageopen; however,thevolumeofdebris intheforedeep,therapidlysubsidingpartoftheforelandbasin andrateofslidingwastoogreatandthecanyonwasblocked. proximaltotheSevierthrustbelt. TheIndianolasequenceat ImpoundedbehindthelandslidedamwasThistlelake: 4.8km thislocationcontainsdeltaic-plaindepositsoverlainbymarine (3 mi) long and as deep as 55 m (180 ft). The high-water deposits; these intumareoverlainbypebblybraidedfluvial shorelinemarksarestillvisibleonthewallsofthecanyon. An and alluvial-fandeposits (Lawton, 1985). Correlationofthis emergency spillway tunnel initially prevented the lake from sequence with equivalent sequences to the south and west overtoppingthedam,andalowerleveltunnelcompleteddrain showsregionalchangesinmineralogyand depositionalenvi age ofthelake inDecember 1983. The railroad nowpasses ronments that reflect episodic tectonic events and regional throughtwotunnelsintheNuggetSandstone, andthehighway fluctuations inthe Late Cretaceous shoreline. TheIndianola wasrerouted·throughthecutjusteastofthisoverlook. There sequencewasfoldedandpartiallyerodedintheCampanian(?) havebeencontinuinglandslideproblemswiththeslopesofthe duringeastwardpropagationofthrustdeformation. cut. TheThistleslidebeganmovingonApril 13, 1983,andis DevelopmentoftheSevier thrust belt and associated a reactivation of an ancient landslide that probably formed foreland basin about8to 11ka. Althoughslowandgradualmovementofthis ancientslideoccurredpriorto1983,therewasnohistoriclarge East-westtransectsthroughthe southern\VasatchMoun scalemovement. Extremelyhighratesofprecipitationresult tains and the high plateaus ofcentral Utah, which are in the ing in very high piezometriclevels probably reactivated the transitionzonebetweentheBasinandRangeprovinceandthe landslide: in September 1982, precipitation was locally 800 ColoradoPlateau,offersomeofthefinestexposuresoflinked T324: 1 metersA WASATCH MOUNTAINS bendinsection WASATCH PLATEAU A' teet 3000 10000 2000 5000 SL 5000 10000 4000 15000 Jurassic ~ UndifferentiatedQuaternarydeposits G ArapienShale ~ TwinCreekFm 0 CarrnelFm ~ UndifferentiatedTertiaryrocks ~ ~~;~:~cs~~~~:S~~est),NavajoSandstone(east) ~ CretaceousIndianolaGroup B ~~~~e;:~~~I~uPand ~ UndifferentiatedTriassicrocks KKfsvm,,FSunixkmVilaellCeyanFymonFm ~ UndifferentiatedPaleozoicrocks Kav,AllenValleyShale Ksp,SanpeteFm ~ Precambrianrocks a 5 10 ~E=3 E"3 I kilometers ~~ JLuorwaesrs.ICcrreotcakcseousandUpper aE3 e--=-32 E:?'"34 E=36 miles Figure7.StructuralcrosssectionthroughthesouthernWasatchMountainsandnorthernWasatchPlateau. LocationofNebothrustfaultis schematic,showingtransferofshorteningtounderlyingfrontalthrustsystem. NormalfaultsmaysoleintothrustfaultsassuggestedbySmith andBruhn(1984)forothersimilarexamplesinregion. UnitKJuasshownhereincludesTwistGulchandCedarMountainFonnations;Kmm includesDakotaSandstoneatitsbase. Lineofsectionisshowninfigure5. ModifiedfromLawton(1985). allochthonous rocks and coarse-grained synorogenic clastic central Utah was not fully recognized, however, until Arm depositsintheentire Cordilleranfold-and-thrustbelt. Inthis strong(1968)integratedtheinfonnationaboutthruststructures area, aforeland-steppingsequenceofthreethrustallochthons and synorogenicbasinfonnation; henamed the central Utah defonned a pre-existing Paleozoic miogeoclinal section be thrust terrane the Sevier orogenic belt and the genetically tweenlateAlbianandlateCampaniantime. Assuggestedby relatedbasintheRockyMountaingeosyncline. Jordan(1981), theloadofthesethrustsheetscreatedaneigh Structuralfeatures thatformedduringSevierdefonnation boringforedeeporforelandbasinthataccumulatedasedimen inlate Mesozoictime are characterized by ramp-style thrust tarysectionabout4,000m(13,000ft)thickconsistingofcon faultsandassociatedrampanticlinescomplementarytobroad glomeraticmolasse thatthins and fines rapidlyeastwardinto synclinesaboveinter-rampthrustflats. Lesscommonareover intercalatedmarineandnonmarinesiliciclasticrocks. Contin turnedanticlineshavingseveralkilometersofstructuralrelief ueddeformationofyounger,moreeasternthrustscauseduplift anddisplacement-transferstructuresathighanglestothetrend oftheoldestforedeepdeposits,includingthoseintheThistle ofthe thrustfaults (Smith and Bruhn, 1984). AlthoughPre area,thatresultedintheirfolding,partialerosion,andredeposi cambriancrystallinerocksoccurabovethrustrampsalongthe tionintheforelandbasintofonntheyoungerpartsofthesyno west flank ofthe WasatchMountains northeastofthe Great rogenicsection. BeginninginMaastrichtiantime,sedimenta SaltLake(Bruhnandothers, 1983, 1986),theupper-platesin tion occurred in intennontane basins partitioned by ramp the southern Wasatch Mountains consist ofPrecambrian to anticlines uponthe thrustsheets, inbasinslyingbetweenthe UpperCretaceous sedimentaryrocks. thrustbeltandtheforeland, andinbasinsfonnedbysegmen Threemajorallochthonsseparatedbythrustfaultsoccurin tationofthe foreland. RiseoftheUintauplift(fig. 5)during the region; however, exposures are generally poor, and the LaramidedeformationinPaleogenetimeinterruptedtheorigi locationandcorrelationoffault tracesarefrequentlyinferred nalstructuralcontinuityofthethrustedterraneinUtahwiththat from seismicdataandregionalgeologicrelations. Fromwest inWyoming(Gries, 1983;Bruhnandothers, 1986). Neogene to east, these allochthons are the Canyon Range, the Nebo BasinandRangenonna!faultingfurtherdisruptedthecontinu Pavant, and the Charleston-Gunnison. Olderrocks typically ityoftheoriginalthruststructures,bothexposingandconceal occur in upper·plate positions within more interior thrust ingtherecordofLateCretaceousandearlyTertiaryorogene sheets, and the eastern, orexterior, allochthons containpro sis. gressivelyyoungerupperplaterocks. Footwallrockslikewise Baker(1947)andBakerandothers(1949)firstrecognized becomeyoungerinaneastwarddirection. TheCanyonRange theallochthonousnatureofrocksexposedabovethruststruc thrust underlies allochthonous rocks onthe westernexposed tures within the southern Wasatch Mountains (fig. 7) and marginofthethrustbeltand is believedtobethe structurally demonstrated the significant amount oflateral displacement highestthrustoftheforelandsystem(Millerandothers, 1988). involved. Spieker(1946; 1949a, b) first associated the con Precambrianrocksintheupperplateofthisthruststructurally glomerateandcoarse-grainedsandstoneoftheIndianolaGroup overlie lower Paleozoic miogeoclinal rocks and Cretaceous (fig. 8) withnumerousorogeniceventstothe west. Thelink synorogenicconglomerates(Christiansen,1952;Burchfieland between the molasse and the structural features displayed in Hickox, 1972; Swank, 1978). T324: 8
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