Active Geologic Environment of Central Colorado Sept. 12-14, 1997 Friends of the Pleistocene
FIELD GUIDEBOOK 1997 FRIENDS OF THE PLEISTOCENE FIELD TRIP ROCKY MOUNTAIN CELL
THE ACTIVE GEOLOGIC ENVIRONMENT OF CENTRAL COLORADO ASPEN-GLENWOOD SPRINGS-SILT, COLORADO
SEPT. 12-14, 1997 Compiled By: James P. McCalpin GEO-HAZ Consulting, Inc. P.O. Box 1377 Estes Park, CO 80517 ISBN: 978-0-9835382-1-9
Crestone Science Center, Crestone, CO, Guidebook No. 1, www.crestonescience.org 1
Introduction This year's FOP Field Trip visits several sites ofrecent (1990-97) applied geological studies between Aspen, Glenwood Springs, and Silt, Colorado. The impetus for these applied studies includes: 1) the Environmental Impact Statement (EIS) for the 1993-97 expansion ofAspen Highlands Ski Area (GEO-HAZ, 1993, 1995); 2) the EIS for the 1998 Gondola Interconnect project between Highlands, Buttermilk, and Snowmass Ski areas (GEO-HAZ, 1996); and 3) recent 1:24,000 - scale geologic mapping ofthe 1-70 corridor by USGS and the Colorado Geological Survey (CGS). In a slight departure from the "usual" Rocky Mountain FOP trip, this trip will focus more on post-glacial geomorphic processes, and their effects on land use management rather than on Pleistocene stratigraphy or soil development. These dynamic geologic processes, which have operated in the natural montane environment since ca. 15 ka, are now coming into conflict with man's intensifying development ofalpine central Colorado. The expansion of ski area operations into steeper and less stable terrain has led to instances ofartificially induced soil erosion and landsliding. On the gentler slopes oflower elevations, residential developments and roads have been damaged by subsidence, hydrocompaction, sinkhole collapse, piping, as well as rockfalls and debris flows. We will point out how natural geologic processes should be recognized and describe typical engineering schemes for mitigating potential damage. Acknowledgements We especially wish to thank the Aspen Skiing Company for turning on the ski lifts at Highlands and Snowmass for our group, at no charge. Access was also arranged by ASGthrough.privateland tooue stop.near the Buttermilk Ski area. The geological work near Aspen was. assisted ibymanypeople,some ofwhom are listed below: Aspen Ski Company Fred Smith - V.P. Brent Gardner-Smith - Public Affairs Ron Chauner - Mtn. Manager, Aspen Highlands Mack Smith - Maintenance Mgr., Aspen Highlands Peter King - Mtn. Manager, Buttermilk Doug McKenzie - Mtn. Manger, Snowmass USFS Tony Svatos, Andrea Holland-Sears - Glenwood Springs Art Bauer, Marilyn Gredig, Robert Iwamoto, Ray Spencer - Aspen Organization ofGuidebook The trend in FOP Guidebooks over the past two decades has been from informal short compilations offigures toward more formal, citable collections ofpapers. This guidebook represents a half-step backwards, and contains elements ofboth the early, crude volumes and the later more sophisticated ones. The portion ofthe guidebook devoted to Day 1 is merely a collection offigures, meant to supplement discussion at·the field stops. Some ofthese figures have been published, but many are from unpublished consulting reports. There is no text. The log ofDay 2 is taken from a previously-prepared guidebook for the 1996 GSA National Meeting in Denver, The log ofDay 3 has been compiled from published material. The varied format ofthis guidebook results from limited preparation time available to the authors, and the desire for a less formal publication. .
2. DAY 1
1-0 r
Day 1 Summary
During Day 1 we will examine geomrphic features related to slope failures within three ski areas near Aspen.
Mileage (mi) Description
o Depart Ski Sunlight parking area; drive E and than N on Garfield Co. Rd 117
10.9 Junction ofCo. Rd 117 and Colorado Highway 82; tum right (S) towards Carbondale and Aspen; proceed S
39.5 debris flows of3 Sept. 1997
49.5 Junction ofCO 82 and Maroon Creek Road; tum right at stoplight; proceed W for 1.5 miles
51.0 Base parking lot ofAspen Highlands Ski Area; park;
STOP 1 - Slope Failures at Aspen Highlands Ski Area. Assemble in parking lot. McCalpin gives a briefoverview ofthe ski area and recentexpansion plans. Get on chair lifts and ride up to Loge Peak (elev. 11,365 ft).Hike around Loge Peak area for 1 hour and examine landforms caused by deep-seated gravitational failure. Re-board lifts and descend to.the"basecomplex. Leave parking lot and return to CO 82
52.5 Junction ofMaroon Creek Road and CO 82; tum left (N)
53.9 Turn left (W) into base parking area ofthe Buttermilk! Tiehack Ski'Area. Proceed W through the parking lot and onto West Buttermilk Road which diagonally ascends the moraine slope to the north. Follow this road for 2.0 miles
55.9 Gate into private land, and end ofpavement. Continue to the West Buttermilk parking lot
57.0 West Buttermilk parking lot. TAKE YOUR LUNCHES! Hike up private road to Government Trail (about 1 mile).
STOP 2 - Landslides in Cretaceous Rocks and Till The terrain between Buttermilk and Snowmass ski areas is a gigantic dipslope mainly composed ofDakota Sandstone or the overlying Mancos Shale. Below an elevation ofabout 9200 ft. the Mancos Shale is mantled by coarse-grained Quarternary till and outwash; these deposits soak up precipitation like a sponge and keep the underlying shale wet. Landslides cover most ofthe area where till overlies the shale, and are relatively rare at higher elevations. ,- , Quarternary landslides at Stop 2 have evidently crumpled the lower Mancos shale into a series ofanticlines and monoclines. This phenomenon is rare. Return via trail to W. Buttermilk parking lot and drive back to CO 82
60.0 Junction ofButtermilk parking lot and CO 82; turn left (N). Proceed 0.3 miles to Owl Creek Road.
60.3 Junction ofOwl Creek Rd. and CO 82; turn left (W). Proceed 4 miles to East Village area ofSnowmass Ski Area. (Two Creeks ski lift)
64.6 Two Creeks ski lift. Pull into parking lot area. Get on ski lifts to Cafe Suzanne and Elk Camp Summit (elev. 11,325 ft.). Get offlift and hike 0.5 miles down to Stop 3
64.6 STOP 3 - Block Slides inDakota Sandstone We are still on the dipslope ofDakota Sandstone, here in the East Pod of the Burnt Ridge area ofSnowmass. We will examine an incipient dipslope rockslide that has detached in the headscarp area, creating a complex graben. The toe ofthe slide has overridden in-situ Dakota Sandstone farther downslope. A hand-dug soil pit in the graben reveals multiple buriedHsoils and angular unconformities, but the exposure is too small to permit reconstruction ofthe sliding history here. Return to Elk Camp Summit & descend lift. Leave parking lot and 64.9 turn left (NW) onto Highline Road. Continue N to junction wi Brush Creek Rd.
65.8 Brush Creek Road. Turn right (E) and proceed 2.6 miles to CO 82.
68.4 Junction ofBrush Creek Road and CO 82. Turn left (N) and proceed ca. 20 miles to Carbondale.
STOP 4 - (Optional) - Debris Flows and Development ofRiver Valley Ranch River Valley Ranch is a·new residential! golfcourse development ofabout 200 lots on 200 acres. About 50 lots are on three alluvial fans emanating from small ephemeral .streams draining an incised pediment surface to the west. Options for mitigating debris flow hazards included: 1) structural alterations to buildings, 2) debris basins, or 3) debris flow conveyance channels. Test pits show that the larger fans are composed mainly ofdeposits ofhyperconcentrated flow, whereas smaller fans are composed mainly ofdebris flows. Recurrence intervals oftypical 2 ft thick deposition episodes ranges from several hundred to several thousand years, based on C-14 dating ofburied soils exposed in test pits on each fan.
t-~ '.:;'-,..,,';,"
MAROON BELLS '4.••'
.' ;
".615 f••t 3.635 h.t 61' acr., : loNotSf RuN: 3.$ ...... ~:, Z "".I~p•.•d, "uad',.5dou"lech."" 1 ,urfac. lin CAPACItY: ',645 ,~"r' p.r how AYfMCK ANNuAl SNowfAll: JOG mch., SNOWMAtCING CAfO.....tIfS: 110 acr.' (20% of .r••) E...StEIT TEMIAIN: 20% MoRE Dw:ftculT TI"""': 33'" MosT DFACUlT TfJIllAJN: 17'" ..~·T~: 30% SUiONOAJlI! o.~.ftIh.",1"5-Ap'117,'996
HIGHLANDS '.~
TRAil MAP AND SKIER GUIDE C SIIiP.tfol A Tkh'l~ ..... Dllfkutt ...... ' MmIDlfikulI III (lM'fgl'n<:r • ...... l'l @ ,..... DoNo! f.nt., • NASTAI' D WarrnlrtgHut _.- surf-.UfU II ...... ""ok m-'- III "'CMd< D SII1Sc'-l
S.,MIOU AHO COIOII coon '''f)lC'''' 'HI UU.'IVI 'IIING .'''Icun... 'Oil uo.n "HO UIUU ON A~'IN HIGHt"NO' ONLY. fOil ,"OUII 0"'" '1I0llCTlON, 00 "0' ""u OC"''' " n"'l 011 nO'1 UN "I .,OU I[NC'" In Of GilIf Of ."..cun., AND NIVfII Sll:1 A CLOSED nA".
l1.h~/ ~/n",,("'" t 1 ( ,L'r'" 1I,."hltllll1." II ."",.6."rnl'r:,.r;"'Y,r;lt' tIIullrlu FOR Au MOUNTAINS: / ~"a~''':' IUf'lttlltri". Si"a- 195,V. 1t1f'"l,' "J qUi',",' ,'[ikt' balY mj"."." lI~t,h1mlr1.1 /;~,. SICI SCHOOLS 0' ASP'N - lJerotm>ahf.tter:'ikif'randsnowhoonWrwith / /'11"1'01 il•• "ft'('ltl! ('h(Irddfr i,;dmJ,ir.t1'UI trlmm/,,,,;;,{ S~i Schools of ASllen, offering Privates. Sfo,nilll·ivl\If's. C;roIlp. BK'lI~.lmK~h ",\,/,.,."". /f"-,.",I1 mu} "U~II"~'('(W" 1'1;'11',1 /'l/~ II/fin"", &11., m,(I.",r"'Jf",Ji,~" I~.K.I to:'i~' Pmgmms and ChildrensSki St-hooIl....s,";ons. Fnr mol'{" inronnalion and ,,,',,l,,, Ii.'(I ht,i,h·"f((t! "UlUJ" h("h"!Jht ,h( Irhir,6 4th;'./t,I",I,,".• o,l.; lIIytl. A.~ III' rOT das.~ visil Sid Schools ofA,lI;pen lorntt~1 at the ba.... ofearh mountain, Tlwn'an-'Rlin"lttAsp"'nl-lifl;hlatldshk'hKliI~2 ASPEN HICiHt.ANOS L,fTS - GUEST SUVICU • TEU " '0 'HE 'OP - l1"'mnrnw'rsnfAspf'nSkiinR hi~h·sIM ..·tl fl";lll.. lhallalt.· ~'{"'In 11ll' hll' fir til(' momllain in 2U mil1l1ks. !;(Inuhlf' • CompAny W;lIll tn hrliT rmm yOll when )'(KI·n' nn 'hr' mountain, Ski illlol\n.~'llI"' dlairs :111111 sllrl;u'('lill (I(n" inh·l"l1If>flil.llf' tn :lflv:m('f'(1 slck'rs (Hlly). <:OIKUriO"S 1)(>1' ornurGuesl Se~(A:'-nl(''fS on any !nOllnlaili and yCKI11 fil"1 Sf'ninl' mana,(f'ri (·~'r 10 mittiuJ!. alllilh"I.'n I.~innill~at ~H){l am mKt dow helwt'f'n 3:15 and 400 pm, listen IOVOlITrommenl~MWasand sll~sUnns. Y«MI'U AlsofilHllmii mnllS.p;I'fl(Jl1lillJt n'pnr1~ and ('Ourt~ phon('>s. ~ arecommitlM 1o l1Iakinp; 0111' ski an'l'~ tht:· h('si in .f", :.In 1.11'1 Vlltll"11 1l1J.. :n'l IAn V'""I\I nlllt worM. anti we need your hf'lp! l.".~ :lJt IlN rf\t1 1,f,Nl':1lI liN Tl'" 1 I'.'ullInlll''I; X.liOUlt ,.Wllllt !Imill C. OUMI'"" 4)i74fl. 1.2."",,'Ht III min '''tA'''~I'~ ft "'-HI ,t,l;I;AOORS -,,,.,.~pr.n ~~iillACnllll~lIwAml...I....:lf'nr.. m'f' (·Ultllllllllil.'.' \ul· 2 lu:~:l'lu X mill nr"",n I.XlIlI" rll!'NIH-:IlI."'·I. 2.,670n. R.'iOfl. 7mitl. 'lnl('('~ whoworl'on all four m(Mmlain~ aOOIUf' availahle 10 an:'iWf'f (11H':'iIHM1!\; E'''IHlI'''NII ",AA1f'1. !l42f'1. 12n,ilt "7 <:....ltf,:N IIi"'N 2.f1l2rt. 6701'1. 4m'n oITf'r ~Iion~ ~I~k> fR'f" motlntain lours alMI awsl )1MI on II", mounlnin. "4 OUHI!I ·1.2!1Hll H7HIl 7nliIJ R Sll\\U. "RO. 1170. 2nlin. TIIf')' aT{' idrntifiM "'Ihf-ir,Ia.-k hille jac-kf'ls wilh a silvt'r Aml..., ...... 1or ~'OI' Ihf' hark.l1~ hc:Mnc bA.o;e fqr th:e AmhR.wvlors is lite GUMI Servi('e Cenl('ri loralcd on ASPEN HIGHLANDS Rn'... U.... NTS ra(·h mounlain. I IIf:lIl.I\NI IS (:,,\n·; MEHItH.:.: ... nO(lfIIl) _ SICI CHECK - N6w~IC'aIl('hM'k~'OIlr~kisinlhrRnf'rll(l(,"nlourSki(:h('d(." IHMln.Il(~inni'tJ.tjnJllllltnT.\~(:oIonwlos fIEES'YLE fIIO ... Y- b'('n' Fri,la\" at hrsl .. and Ihf'V will hP wAiIi'~ rOf ~'(KI al all~· ofnUT tour s~i afms 11K' tM'xll1""nil'ft. ItHlf" .."illllllll'n'f·sh-I,· S~il'I'S will hump ami jnmp tlwir ",'~' (I"wn St-af.lt'lfllj; nlln in 11K' '3' rfmllspnrt c·hal'RC· $2.) IIlt",1 c'\l'ili,,)!: rm:'shlc'I'utnpf'liticllI in (:nlonH'U. '11K' (Irt:-k it' IIlf'!'1f'rl'\·-t:C,..Huuml 1_ I.·...t:llIfanl i.. II...· ill.,,,. Spll' In "j.,,," Ih,' .. ~if' .... all,1 ~mhl1 hil"lo 11_1. ASPEN I SNOWM"'SS ARE SfIIYIUD IV THE FOLLOWING AIRLINES: I I
"'t/./!::I1""nillc/.,lumrID h%A/Dun/ain. 4 prrmNr I,-,Jnuiu; mountain anil ..nou·· 1,.,.mJ~;:f paraiJi,..c. J/.f pIlL r, ,I/I"~q trrrain. ,,·biJ:iJ fal/.t Ii" ,m _1 ,lummi! ek,'tltiPn 0/ 'j,l)f't'fut, i.f partieuJar/y ",~U "I/(k~ (() kginner... inlermrdi· ,,,,'.', kill.. anil aJult., ..ba ",jay ,b,' l'lrU' tl.1 m.udJ /lc4 tlx ..lope., BUTTERMILK LifTS - 1llere ;11" h (·hairliftsand one Ski School lin ;11 Bunennilkincludingone IHllhlm"o·summit higtrspeed (pl,.d and 5double chairs. ( :I'llditio~spennjtting,all ,-II;liflifi~openat 9:00am and are . "1·lu·dulffitoclose at 3:30pm, ,\",," Uftsl and 6whichcl_ ;,1 ·H 10 pm: (Hoors may vary with I );1\ light Smings lime.)
LIFT VEJmC\l...RUJf: U:NCnI RISf: TIWF. MAlfrl II.mIIMu 1 .... '\1\111 hi'lil-..... RS7urt. 1.8341\. 9.5 min.
.: ",\\10) 2Ji50R 614ft fimin. !,I'\\I'·\ 1'1 H. U80t\. IJ3n TRAil MAP AND SKIER GUIDE :- "'~i SCHOOL ),:;"1 407n. 2')11 .E~~r~~ a SltiPMfOI .. Ticlteb W~5TJ~ MoreDiffICuIt I] SIl:iSc.hool '::t-- 1\1rlHI\llLI\ s.-. MostDiffit..,n .Center \\''1 6.0tKIfI U8!Hl 12min CD -""", . -... \ TI(HACk ~ I ".\\Ht II NAST" rlllt\Cl.: 1.28(lft. 275ft. W.rmingHut - C ',l BlIttt'rmillt 1;1 II \(1\ 5.916f1 1.439lt .",,~ 1.':1 ~.~..: CloMd DoNotEnter ~ """"For'tFrog BUTTERMILk MOUNTAIN ~ter .:C~~ RESTAURANTS HI \11.... CAFEWE.\! ~ ·~bon (llIrIlO\.:S[ Off.. Slow5ltiing MAX THt MooS! Ex'IISS ~.,u, III '\.!I,ler \,otlr kids for Ski School hM.OU ."0 COLOR COOlS 'NOlc...n ,tU UL""IVE UIlNG !_c "'__ d ....·' __- TO ASPEN. HIGHWAY 82. TO SNOWMASS OIH.CUl,--....·-'OIil hO'B .ND n"'IL~ 0'" aUTHhllll:. Hili pllr{'ha~lheirtj('ketsat MOUNT.I'" ONLY. Fo~ YOUII OWN 'ROHCHO"'. 00 NOT UII
1·...(' f11Iht'm to lessonsat Butternrilk "ia Ihe-Max the Moose HELMETS - AspenSliing:Compan~'rerommendsthat COLORADO SKI SAfny ACT OF 1979 - InColonl(i(l~-;Ski NAS TAR - NASTAR ral'esare held at all four mountains. 1- \1 In-,,- hus then ba(.'k lolhe C.ondola Building: at 4:00 pm. parents ronsiderhelmets fortheir skiingchildnon. Helmets are Saft>ty J\rl ofl979. as amt"llded. tht" Colorado C.f'neral A'iSt-'mbh available at most area ski shops. rt"<-oinized thal dangers and tisksare inhen:,nt in thf:' sp(ll1 of ski "l \ 1\ - Silver Dip Swing_ daily 10:00 am • FOR ALL MOUNTAINS: iJ.lp:. rt1!3rd1t"ssof any ,:lnd all rt'a.'i(mahlf> safety mE'3SlIre!' whirh 2:00 pm. 85/2 runs. lOST AND FOUND - Forilemslostonan\'ofourmountains, s - ~ an- not pennitted at any ofour mountains may be employt'fl. r nder tht' Act. f:'ach skier f'xpressh' ;In:ot'pts an2 ii, ," ,,:,'-Exhibilion.dail~·ex('t'pt pl~ase chetK at the base area.... . ,111l11l!! ~ki st'ason. assumes thf' Iisk urand 'llllt'J!'ll rt"sponsibility for am' inillf".\ TllPsda~'s 1100 am· 3:00 pm. $5/2runs. Restf(JomsarelocatedatlhE'ba.~ofe.-ach rt"'iultin~from an.,· oftlw inht"n'nt dangt"rs and lish of .... kiin~. f·. \1' li\: \1'\ - Scnio. Tuesdm·· Saturda\' SHUTTLE BuS SERVICE - FJ"f'eskiershuttlt'bu.'ieS RES1ROOMS - mountain and in all mountain restaurants. Important additional pro\"i!ooions inC'!l,df' tnt- j(lIIO\~'ing: Eal·h IHKlam'300pm,S5/2ru" ' , I ,,:\lwd all four ski areasdail:,-' from 8:00 am .4;2()pm. Buses skier is resp()n~ihlt-' t(1f knm"incanci skiing within thl-' limits of -""1 \1·; -Cabin TraiL dailyexC'ept I: 111 Illlltihilously and depart from desi/malt'd stops located TREE SKIING - Our four mountains offerfabulous tref' skiing. his ability and maintHining a proper luokcllIf so as to bE' ablc" h. Saturd
~#-~.".~'- -- ;Zalit1a:~
I~",,}me to tIN !J<.,t Jkiuj anJJIW.ob.>arik,:. u,,· unU",:m \'fJfIl(".\LRISF. RlOF:TIMf: [.JFT' UFT!J.:'i(:J}l VF.!I'I1C.-\L1lJs[ RlllETIIlolF. SNo.uss 511 AIU RUTAURA.TS 12min. / mountain in North Amerrea. Only /2 mik.· SAM', KIIOI I:! Hll:lI\l.rl'F ·UII:'iIl. t:388R. c.-\FF. Sl7..\;o.<7'fE GWY;o.<·S HIGH ALPINE 18 F\~, ... HILI. ::L)H;31\. 042 ft. 4.5 min. 15 \'l<;t-:rlLIIl\ 7,i""7tt. 1.6.'),'in. 16mi... , from Np.!'n, Snowma.JJ ;", IcIWWn It1#" wng. ,.·rui.,· UU.RHOF Up4Pu.z..\ 2 Bnll.l~;,\ME :jA·'j.11l. UI98I1. II min. be- ing run.' anJ Jk".in!outlotlging. EWf')IfJM -Irom t.,IJk, 19Co"r-:,·CI.\OE .').100 It. 1.2161\. 5mlR. R .\~~W Htl.l 1.77.'1U ;!.,t R. Rmin. Dt·DI.F.Y"S,/S-\M·S K'K>8 KRABUX)NIK to gr,znJparrnt. b - -- ".• T. 9 5. 39° '15' __ 00" I J u , ~ .HayStaCk '\ .Mln IJ 12.~_ ...... i1''3;' -~- '0' • ,~~ 11 _ ~ <- T.l05. " g ChII'M Peak .r-<"A, Pt.,.,.. Lobs ... " B."r 39° 01' I30" T.115.1---"" ~~-- Asher 11' \ ,\~' r ) >. ,~ r2j ~ Vol: 10. No. 177· September 4, 1997· FREE (One copy per customer) rlighway 82 buried by mudslides .""'" t. ,,<.,g"J.l'iiUtif!TZZZZi • Motorists stranded for more than three hours By Melissa Schmitt and Michael Brands Aspen Times Staff Writers ~ Mud and boulders poured like lava from the walls in ... Snowmass Canyon Wednesday night, closing Highway 82 for more than three hours and leaving motorists shak cJ\ en and stranded. • "I just saw this mud shooting out of the mountain boulders, trees - my hands are still shaking," said o Michelle McKeown, who ended up stuck between two slides in Snowmass Canyon on her way home from work. Police were notified at about 7 p,m:'that three mud slides had let loose, closing both the highway and Low'er • River Road. No one was reported to be injured by the slides, but several cars were swallowed up in the mud and water. Hundreds of tourists and commuters, sat stranded in the datk,and rainy night. . McKeown was lucky enough to make it through the first slide in Snowmass Canyon, only to be stopped by a second mudslide a quarter-mile down the road. The sec-' ond slide, much larger than the first; appeared to stretch 50 yards across the highway and was about four 'feet deep. McKeown said she watched a white pickup !rUck car rying a woman and lWO children plow into .the second slide at what she estimated to be about 50 mph. The, ,.., ,woman',pulled 'her kids from the'car and the three fled, McKeown said. Ken Jackson, who lives in a cabin in the area between the two Highway 82 slides said he was at home when the att Belden, left, and his son Grant team up to push'a boulder off Lower River' Road as the owners of a mudslide began, Ikswagen Bug, background; walt for a tow truck. Michael Brands photo. • See Mudsllde on page 1.4 , . Published Geologic Map of the AHSA area IScale: 1"= 2000 ft I ISource: Bryant, 1971, 1972 numbered circles show locations of photographs in the text r I -- -- '-'-'---"--_._-.1- B------te t et of I I ~. ~ ; ,I I ; ~ I ~ I se Maroon BeHs Snowmass Wild~rness, ft, ee Aspen Highlands m Environmental Impacl Slalemenl te , an E-r W ed .. !; ,: ~lOUfC. _.-..c·""'I''iIll __'''"C .:~ l!lflfl DII';_" ~I '''''111 flS ...... MMlfltA.I:n ICIno PIOIII:IJI{"~ ~SIlC 1.110,')'-"1:10 ...... ,~ t ...... ,"'.. (IIOI)~-QO.\,)""'11 1lIllI1i*C; IOtI.lI" M;g,e., INC '-lQ lib 11111111"'''..01. .... 00_,. IlD.')"..Y..ol. Morch 1997 Allern~tive 8 LEGENO: (Propose. Aclion) .,-)! KEY, -EXislin. or /, /.' / . ApproTed (rlf -Skiable Terrain Q"" ...,- Proposed lifts No Aclion Lift - ProposetJ Cotwalks .".-Lifts La be ~~ - Exislinq '" Roods •.' removed under Snowmaking ~ - Proposek. Snowmokinq lhe AILernaliye 8 All 8 ' ( ) _. Propo!lP-d new skiableterroin h~.ZA Figure 2.3. Alternative 'l r- Prop r~q I "~ Aspen Highlands Ski Ana - FmaJ EnvironmentaJlmpocl Statement ~, 2/'" I r .... ,' The Steeplechase, Highland Bowl and Loge Bowl catwalks Lower Temerity maintenance road would cross very traverse slopes averaging 60-70% and all three could be steep colluvial terrain especially where it traverses the affected by dip slope failures in several places if slope cuts Castle Creek Valley wall. This could result in deep -, undermine them. roadcuts that undermine upslope stability. -' --.. -. Rockfalls may also occur in some places. The restaurant would be lOcated in an area subject to gravitational spreading and would require special construction design. .". Rockfalls may also occur in some places. Soil Hazards Soil resources and the management of these Due to the widespread distribution of slopes exceeding 40010, Due to widespread high erosion hazards associated resources within the Aspen Highlands Ski Area severe or high erosion hazards exist for at least part of all with slopes exceeding 40%, the Highland Bowl, would remain unchanged from the existing lift lines and catwalks included in this alternative. Temerity, Lower Temerity, and Loge Bowl catwalks! conditions, except those that' occur as a result of maintenance roads would be affected in a similar natural forces of nature. Mass movement hazard is less problematic but is also high way as the liftlines and catwalks included in for 67% of the Maroon Bowl Lift alignment. Alternative B. Potential for slope failure exists where snowmelt runoff is Since additional snowmaking would be much less concentrated onto slopes with a high saturation potential, under Alternative C, the added risk of landsliding especially those associated with Leadville and possibly Scout would be lower under Alternative C than Alternative type soils. Snowmaking could exacerbate the potential for B. slope failure if drainage systems discharge water onto unstable slopes. .. • Comparison ofAlternatives 2-20 POl~nliallmpaclS Chapter -I - Environmental Consequences Table 4.1. .... P()tential.mitiglltiob measures (()tgeo)ogy!slDd soil resources. bnpact Mitigated Potential Mitlgation Effective Responsible I ness' Partv " Risk ofstructural damage to ski 1. LOcate the top terminals of Maroon Bowl and FSiASC lill and building foundations Steeplechase lifts, as well as the Loge Peak located in the vicinity of Restaurant/ski patrol head"uarters, away from large r1 gravitational spreading (sackung sackung cracks. . ~, features). 2. Require inspection by an; engineering geologist to FSlASC identify smaller sackung cracks during excavation. t 3. Ifsackung cracks cannot bel avoided, lift and building 2 FSiASC foundations should be desIgned to resist or absorb computed cumulative horiz0i'tal and vertical strains over j the lifetime ofthe structure.. 1 1 Risk ofstructural failure oflift 1. Lift tower foundations shoilld be anchored in stable FSiASC towers placed on talus slopes material beneath the activ~ly-moving talus layer, as (e.g., Highland Surface Lift). designed by a qualified engil1fCr. 1 Risk ofslope failure along 1. oh catwalks and access roa"s, cuts should be less than 2 FSlASC catwalks superimposed on dip 8 feet high and the inslope gtades should be no less than II slopes with 60 to 70 percent (30 ISO to 100 percent (i.e., %:11to 1:1 slope ratio). to 35') slope angles. 2. As specified by engineering. design, fill slope instability FSiASC should be reduced through the construction of properly designed tie-back retaining, walls. This would also reduce downslope disturbanc:ie width. Risk ofmass movement or other 1. Construction plans submitted for review for all FSiASC geological hazard resulting from excavation located along ~twalks and lift terminals proposed activities. shall include detailed surveys and designs, ari,d a geotechnical (slope stability) assessment performed'py an approved firm. 2. End-haul soil iffull-bench construction is used. . ASC 3. Construct retaining structures to reduce cut and fill FSlASC heights for roads built actoss steep slopes (typjc8Jly required for slopes exceedi" 550/, or as specified in the geotechnical assessment). i Risk ofdebris flows resulting 1. Design and construct a drail'fage system to handle spring FSiASC from saturation ofrelatively thin runoff incorporating BMPs. soils on steep slopes (e.g., 2. Design and construct debris flow crossings to FSiASC Leadville soils on slopes of>40 accommodate potential flow) volumes. percent) due to rapid melting of artificial snowpack or from I. II roadways cutting across alluvial fan'- Risk oferosion resulting from 1. Implement BMP for erosioh control on disturbed areas 2 FSiASC disturbance ofsoils with high immediately. Revegetate by the end ofthe construction erosion hazard ratings (e.g., soil season. I units 34C and 35C along parts 2. Avoid concentrating runolf along mapped potentially FSiASC ofthe Highland Bowl Lift). linstable areas to prevent larldsliding. Hazard from rockfalls on natural 1. ~Iace signs warning sumfler visitors of the rockfall 3 FSiASC cliffs (particularly along the ~d. ! Loge Bowl Catwalk). 2. Construct a continuous ch~n-link fence on the upslope ASC side to catch falling rocks. ~ 3. Close affected areas to summer visitors. FSlASC I I""highly effective; 2""ffioderately effective; 3""somewhat eff¢tive; 4=uncertain I I below. Due to the slow nature ofgravitational spreadingl (fractions of a milljmeter per year), this process does not pose a threat to human life, but it could 'rWse foundation distress unless structures were Geology andSoils Potential Mitigation Mcasuns 4-11 11+....lllI01l . I ~ ~ !", Maroon Bells - I Snowmass Wilderness ," Aspen Highlands Environmental Impact Statement .--- Source: - 1iIU9"II'C,Il{JO·'i.IJltlll~ 1!lID CItt.SI[. 'II 1{\ll!i(Df:lr>lIMfINfQ ...... 00 «III" ...... ,.(NIIIltONM(lIl'... n\IC(sl«. lJOJ)_l-tll'" ""_I800~" l...... U'....U'l!l OIllITlhtC: m..1It ••S1IOC. Me. rllOl}J»--~1 ,",. "'OCflll) lllll.flO't, ooan:m (_11"'_&&" March 1997 Authorized Elements LEGEND: KEY: <>'" - Approved lifts -E_nc or / Appraftd 'Skiable Terrain" No Action Wl •••••••~"- Approved Cotwotks & Roods .'"•-Lifts to be I!J#'- Existing ,,- Approved Snowmoking ii" removed under , Snowmoking the ApproYal ) - Approved New . Skiable Terroin Figure 1, Authorized Developments for Aspen Highlands Ski Area, 5 I-\lb GEO·HAZ Consulting, Inc. --_._.._- ,~5teep -forested s£ope. abandoned 5 road Go Ide n Barre~ IN5ET ski '-" YU SCALE. n ( Qpproll..) I 2.0 ft SC.ALE (Q 1't't'Oll.) ~'~i\~~st\ 100 ft "- ---.------'" " '. Fig. 4. Sketch map of the Golden Barrel slides. Inset shows detail of the roadway slump. E:\GEOHAZ\PIONEER\ASPEN\GEOL9S.RPT Febroary 15, 1996 8 f- (3 GEO·HAZ Consulting, Inc. ---~------ / / UIlIe.r no ...... 111•• ..... • rosion ...... Inllllr.'lon'ur.'.d ...... p.r....' • .... in _.-d zone Fig. 5. Diagrammatic cross-section ofa hillslope, showing the buildup ofa zone ofsaturation and a perched water table (temporary piezometric surface) in a colluvial soil during rapid infiltration (from rainfall in this case). From Campbell, 1975. E:\GEOHAZ\PIONEER\ASPEN\GEOL9S.RYr Fcbruaty 15, 1996 10 ( /S~eeplecha.se /t-I,ft line ( proposed) Loge Bowl & Maroon Bowl Ridge Area Maroo,,! ( dashed where Bow.9. a r rro~i IY'IQtel", lift· I'lY\e locQted) ( (pro posed) ... ~ It '!' figurefeatur~s6 itap of sackung ( on the south of Loge gravitational s 1 10 ft Lridge crest is /efak. Contour i preading) • ocatio t. el nterva to coincide Winat~of sackung crasewhereCkS it is depressions , n saddlesthe axesonof throUghS.t is inferredclosed e ridge. ... N .. 12 GEO-HAZ Consulting, Inc. E..t Top of . Stur~dte.s. lift A A' ",5S0 ft \ \ \ ",soo H ~ \ \ \\ \ \ \ ",'\'50 H: B' SCALE. ( vntita 2 ",~o.33eration :: 2.5')( } 250ft \~ 100ft ",SS() f \ \ Top of Maroon Bow~ Jif~ C ~ (' 11,1000 ft ",5'SOft FiJ. 7. TGpOII'apbie eross-seetioDs aerou tile Loae Peat- HipIaNl PeatJ'idIe, south ofLoge Peak, in the vicinity of proposed lift terminals. A-A', cross-section near the top terminal of the Steeplechase lift; B-B', section showing symmetrical double-crested ridge; C-C', cross section near the top terminal of the Maroon Bowl lift. Subsurface geometry of sackung faults is inferred. E:\OEOHAZ\PIONEER\ASPEN\OEOL9S.RPT February 15, 1996 13 l- lu r , '"gg !i' ~, ~ ~ ~ ?i. Ro £':;' ~ ~ ~. 00 ~ i\l Q-- D ~ I 00 ~' :;' .~~ " ~ c." :< .- -:;' Z ? " .'" ~ .'"'" =?...., ..., I" • g Figure 4. Stereo aerial photographs of sackungen at the AHSA. Arrows show scarps, troughs, and notches. The areas between arrows labelled "A" through "0" show locations of ground photographs in Figure 5A through D, respectively. "E"s indicate closed depressions. The short heavy line marked by "r' indicates the location of the backhoe trench. LP ; Loge Peak, HP ; Highland Peak, • rL~,7 .. A B Figure 6. Sketch of typical profiles across spreading ridges in massive crystalline rocks. A) Summit graben and uphill-facing scarps; B) Double-crested ridge and uphill-facing scarps (from Varnes et al., 19B9,Figure 3); , ,. 282 Environmental & Engineering Geoscience, .... _. .. _~ ... "., ...... -- ..•.,_.. .' ~ . 11-.------'------::----:1..------. 10 ... SW 9 ...... E 8 Q) (,,) 7-+············· c: ItS iii 6 is 5 ItS (,,) 4 t Q) 3 > 2 1 O+-.a.L.----,-----=='--,------~----,....,...----r------f '., o 10 20 30 40 50 60 Horizontal Distance (m) Figure 7. Generalized geologic cross-section across the uphill-facing scarp at the trench site. Small numbers above the ground surface show inclination of surface slope segments in degrees. The backhoe trench is shown by crosshatched area. The thickness of colluvium (unshaded) is inferred except at the crest of scarps and in the 'trench. Environmental &: Engineering Geoscience, Vol. I, No.3, Falll99S, pp. 277-290 283 >\ '> . II ...... _~ ...... - ... . 1- 1ct McCalpin and Irvine A m 3 ~S75·W EXPLANATION -.mm. organic horizon ~:.=---:: SOCl-pond silt , 2 p L:': '·:1 sandy colluvium ~ pebbly colluvium , sandstone clast 1I void space b bedding plane joint 0 2 3 4 5 6m ~\.~. \0 '.~ ...... ,.•.__ ...... -..... ,,,~,, . ...... ~- .. -~- .... -' A 6 E 5 -~ e 4 f= '0 Q) 3 1J ...::::J 1ii c: 2 0 0 a:Q) 1 1 2 3 4 5 6 7 8 9 10 11 12 Deposit Age (ka) 1 2 3 4 5 6 7 8 9 10 11 12 Deposit Age (ka) Figure 9. A) Reconstructed throw for three soils and total surface offset, as a function ofdeposit age. Horizontal dimension ofboxes II is defined by I-sigma limits ofcalendar-corrected ages (Table 2). The age for soil6Ais derived from extrapolation in Figure 9B. Ver tical dimensions of boxes represent the range of throws resulting from uncertainty in maximum dip angle for each soil used in the trigonometric calculations (see text). The dotted line shows appar ent vertical slip rates on the sackung fault. B) Deposit age (Table 2) as a function of depth below the ground surface in the trough fill. The dotted line shows apparent deposition rates: Rf =fast deposition rate, Rs =slow deposition rate. Depending on which deposition rate is extrapolated to the bottom ofthe trough fill, the inception of sackung movement ranges from 11-11.5 lea. 'Ice, Vol. I, No.3, Fall 1995, pp. 277-290 ...... -" w~ .JIIi"'" .. M •• '·· .....• ,. Chapter J• Affected Environment I I Table 3.•. Aspen Highlands Ski Area Geologic Hazards. Maroop Creek Castle Creek Hazard Description Distribution acres % acres % Unstable rock slides Area rock slides are mostly dip-slope .00 wedge 138.5 20.4 18.9 /7.4 Maroon Bowl, slopes failures in the Maroon .00 State Bridge slopes ofCastle Formations; most slide blocks have moved less Creek "~ than 300 feet downslope; also includeS mixed till and rock slides ofvarious sizes on the. steep valley sidewalls oflower Maroon Creek; these slides detached and reached the valley floor. debris Thin slab failures ofglacial till or CO~Vium 0.0 0.0 0.4 0.2 Heads of 1st- slides overlying bedrock; usually triggered rapid order drainages, I precipitation or snowmelt; found at thie heads of Castle Creek I first-order drainages and on steep unvegetated I slopes, such as at lower Maroon Creek. erosion! Surface soil erosion and rilling on steep slopes 15.6 2.3 0.0 0.0 Steep ski runs rilling where the natural forest vegetation ~ been removed. Potentially rock slides Includes dip slopes adjacent to or sim;lar to 97.1 14.3 40.9 16.1 Maroon Bowl, unstable locations ofpast rock slides and wedge failures. below Moment slopes ofTruth catwalk debris Includes steep slopes with a thin mantle of (included with rock slides) Heads offirst- slides! colluvium or till, especially those areas adjacent to order drainages accelerated past debris slides, and areas above debris fan and creep alluvial fan deposits; the mapped extent ofpotential debris slide areas probably underestimates its true extent. Debris-flow and debris- Slopes where debris flows and debris avalanches 301.7 44.5 171.0 67.5 Basins fronted avalanche-prone slopes deposit their sediment; typically the siJrfaces of by alluvial fans or debris fans, found at ihc base of alluvial/debris steep glacial valley sidewalls. fans Rockfall Includes rockfall source areas (cliffs) and 124.6 18.4 12.7 5.0 Maroon Creek active/inactive talus deposits below ~ cliffs. side ofridge, high elevation Gravitational spreading Slow, deep-seated horizontal spreadihg ofsteep (dispersed linear features) Ridgecrest (Sackung) mountain ridges due to gravity; results in betwcenCloud topographic features such as upslo~ and Nine and downslope-facing scarps, double-crested ridges, Highland Peak and closed depressions; found along ~ ridge crest between Cloud Nine and Highland P~. Soil and talus creep The slow; natural downslope! moveni:nt ofregolith 0.0 0.0 9.2 3.6 Isolated talus .. due to ~thaw and shrink-swell ~ses; and rock occasionally results in pistol-butted. and tilted glaciers, Maroon fence posts. I Creek Totals i 677.6 100 253.2 100.0 as potentially unstable is prqbably low, but is dif$cult to quantify since the geometry of the deep failure surfaces in bedrock is unknoWn. During the spring of 1995, heavy snows in April aJld May, combined with cold temperatures, resulted in up to 300% normal snowpack in many areas ofColorado. Sudden wanning in early June led to rapid snowmelt, ground saturation, and several slope failures at Aspen Highlands. Slides occurred near the Moment ofTruth Catwalk and Golden Barrel Trail. I ·~q.IIB 3-5 l- '2,.,1; \J . ... :,:. 1:, : ~ I I I / / / I, I ,,./ // I I / . +~.. / i .' ..,§ " lAoroon Bells / Snowmass Wilderness I / Aspen Highlands Environmental Impact Statement l [ -N-W I IIM'I'WG ~'~,~.~:~ ~frlMr. Pf·"'fn"~II"_G II"M.., '"(I MHO (!tlH) ..... ",'it) Unstable slopes: "~'.~ :':".r:..':":~ ;:.,~::a$ INC 1..,.,.,.111.... 0\.111'1 ',.'~~,'~ ..:;;~;~~~ (!lOl)I'1l-0'H' IlIf,l1l,..._ • lItr. e -- Bedrock slides and U"II"", VJ """'" wedge 'ojlures lll"f ,... ~'14' • - ShoUi.w debris slide Moreh 1997 : o -Surface erosion and ·rilli"9 €) -Potentially unslable slopes LEGEND: o -Debris fjow & debris ovalancl1e - prone slopes ~"-) of~ Action Elements -Areas soil creep or - ExislIIlfl or • talus cljeep KEY: / ApproYed Q -Rockfall 'oreos No Action Lilt hochu"e~ show edges of ..... - Propo""d lift. source zone; dashes show ... ~ • grodatiol1al contact "ith -' -WIts to be stable 11IIus ..••••,. - Proposed Catwalks e" removed under lhe & Roods Action Allernalives _-.:::; ~ -lro~hs (\ -Snow Avalanche Areas o - Restaurant Upgrades .- teeth point to \J I toPographic depreSSIon <: -Artificial. droinage I '- (Ditth) :----'=-'-'------' ------~------' Figure 4.1. Location of action elel\nenls relative lo geologic hazards. 4-3 Episodic Continuous Creep Displacement Continuous Deposition. o. b. Episodic Deposition c. d. Fig. 3. Hypothetical cross-sections through a sediment-filled trough (at left) adjacent to a sackung scarp (at upper right). In the trough, thin lines indicate bedding, short vertical : lines indicate soils. (a) Continuous creep and continuous deposition yield increasing folding (drag) with depth; no soils are present. (b) Episodic displacement and continuous deposition. yield three packages of strata bounded by angular unconformities; no soils are present. The angular unconformities are "event horizons" in the terminology used by paleoseismologists. (c) Continuous creep and episodic deposition yield three unconformity-bounded packages of strata. Each package is topped by a soil, the upper parts of which have been eroded nearest the sackung fault plane. The upper contact of each soil is an event horizon. (d) Episodic displacement and episodic deposition yield discrete unconformity-bounded packages of strata, but soils may be found at any position within the stratigraphic sequence. In this scenario, the angular unconformities are event horizons, but do not coincide with buried soils. _E:\GEOHAZ\GEOMATRI\SACKUNO\SACKUNG1.RPT April 13, 1996 21 1.A LANDFORM INTERNAL STRUCTURE GENTLE SLCFE BOJNDED BY A ~, \((((~~I( RIDGE-CROSSING DEPRESSION~ f7~ .__ ,. Fig. 1. Typical landforms and their subsurface structures formed by mass rock creep. Roman numerals show the structure types defined by Chigira (1992). Interpretive cross sections at lower right (Arabic numerals) are derived from: (1) Mahr and Nemcok, 1977; (2) Ando et aI., 1970; (3) Tabor, 1971; (4) Radbruch-Hall, 1978 and Shimuzu et al., 1980; (5) Jahn, 1964. _E:\GBOHAZ\GBOMATRI\SACKUNG\SACKUNG1.RPT April 13, 1996 19 ...... BRUS~ v· \SA r--~ '" (\1- SNOWMASS SKI AREA FEIS V White River National Forest I ~ WILLOW CAEEK /--- ~/ r- r-- ~~~ I ~'>, ,"-.- -~ " -fYIJ I' ..... _...... ,WI£DE"RII~S~.,...I'!!fI".-~~_80UHOAfI'i~-..: '";e .-...... ~O )~ \~ 1.'._"9 SkI Ufta Wlkh,n••• ~ry 'en,'" a04.lnclwy ....on-I 'or..' OwM.llhitt ".1. H6gh••y lld.l", 'av.d ...... 1",.1lng Din R_ I ...... Il. LItIo, P_ 110 __ ," P,~.d.I-'Ilil ...... P I.d L'" P'_ H.-....T'.... P,..,.." ...... Tra•• "0"-" UIWIH P_ - f2C , '_...., ...... 04C 110 _ • __,- 13 T,"_lIe' I PVT P...... 0-...., NfSL -'_"_.~ l....,// 8 ':. ·:;1 - e-, .A. -' NI ~ J-'GTITIl=:::-=:sA4~ ". a.-. SOUTH ...... ,.,, I 0 1000 2000 I. _.: '-III - !!T[[[[JJl F.., Map prepar14 lly: Fabr'*Y 10. 1194 Figure 11-3: \lES~ A._rces, Inc., Redding, CA l ~-----~--~------::~'T"~;~"'''''tf..S;;;:;;S;:;P,:",:;. __::_.'_§1.S":'~,.. ,,,;-!'t¢!,, "t"..,,,' -);#1.¥...._:e::.1t,:;..;..."'i; •.~W-'~l~i.H,."?·!~~_4# . "'%",5'~:~~" . '~;q(, ~~>1";:', '/::;:::" ~~~~~~fOREST i-· I i-i?'. tf\~·IS.13 C-;UtiWMA~$ r--- ';KI ARf.A \ I I l I'II'MII i I H{lllt.lIJAR'Y \ D Em1iD&RUD I) I .I \ rI -- ---K. [- J. GMorrison Formation 1 ,'" --- -j / j-- _/~. I L, r ,..- J "- 8 Dakota SandstonelBurro CanyOll L I fv"~ r- \ Fonnalion _'I t _.. /.",c,~ i i l/~~"r:§Jr-J 8 Mancos Shale L_l' I / r-- J ~ I I Kmu I \. /r---...J, ... -----_.-."-/ .- ~A1luviwn I _.---;::.---;Y'· ( .1/-,,// o Alluvial-Fan Deposits - I""~' l ' / J ,,' ]0.'ILandslide Deposits ~/I:,'''y/ 1 -¥ ' ".-, 1 I Qmo IGlacial Deposits ~'- \1 , t I Clmb I / 4--- I CImc I l ,: I Omd I f·,...... : ~Older Alluvial-Fan Deposits " ' "');:c"'.'-....., :-: "Y", '""-~'. I~" ~Talus ":.fl 6',,\,)" / '~llTl[RMIIK -r' '\(.- JKI ARfA PEHMII , BOllNDAH, IT]Hornblende Grandiorire " \ / " \ ~) 1'1·:( '11 i I j ", 1!,;. \ \ ',IJUWklL.' <; .'/ ( \ C' \ ,\'.:,1 ,kll IJI ARl/\ Vill /\(,,[ \ . \ (~ \ / , 'f \ / r'i.o,F\1' \' ~ ,I-'_\:~l( I \ ~'" './ ~ ~;AI- SOUTH.. o , 2 1,:I,[I!!i .... LEGEND ---...... , ~ ...... , .....::c~=e==-_...... 0wIwIIlIp ....._.--_...... ,~ ,.--;::J.- j ...... _1IIOdI .. - ...... 1lIIt .... _lond-", I _~PIlIl ...". ~m ...... \. ij ....,-.-.y ~ PVT ,.. o.n-tdp NFSl -...... ,...... ~ j /' / Figure m-2: 1 / Landforms and Geologic Hazard Classifications ~15.1984 .. GEO-HAZ Consulting, Inc. (b) Sharply defined components. (A) H\STORK. Slopewash and shallow mass movements modify sharp edges, but drainage lines are not established. (B) . YOUN~ Drainage follows rifts and sags on slide mass, internal blocks are slightly (c) dissected, material is eroded from slide MATURE mass. Slide mass is almost completely removed, drainage networK shows _ weak structural control, valley drainage re-establishes its pre-slide profile. (0) OLD Fig. 13. The age classification of landslides used in this study. E:\GEOHAZ\PIONEER\GONDOIARPf November 29, 1996 41 1- oct. To Glenwood Springs R.85W. " 39.0 Miles, : I ·'OJl· •• : /, ''\ I t ./'"'\", <, Aspen '.to Miles T.105. EXPLANATION Old Lar1dslide Topography ,. -.-...... ,....-..- Liltline and Tower Locations SLOPE STABILITY INVESTIGATION N TIEHACK SKI LIFT NO.5 I Vicini~Y Map Scale 1",2000' c en and associates, inc. CONSULTING ENGINEERS .a s. ZUNI, DENVER, eo. tOIU Job Numb., Dot.: FiG. 18 B (UVA1KU4 rHl 1- I rI ;; ·o ·~ ...... ,..... ; .. ··.. ··~ ~ .. ~ /1 ·".. " · o ; o' ·.~ 0 " • N ."N" : . I"o.: f .:; .. ·.: ~ · .. : 0 ~; ,;; o • DE 0" : ..:.. : ; ..o. o! S 0" . ·: :.. : ·• ..·0 ..o'- ·:: :. iio" • · ~ I r--~·-f ----..,....-.. ---~ ~ .UJt.OtIYA1H .. Q • " A Axis B Axis IN 45 II) ~ (N 45 E) HORlzo~rrAL ulSPLACEMENT (INCHES) (5 45 '.I) (N 45 II) HORIZONTAL DISPLACEMENT (INCHES) (5 45 E) 3 2 o 2 10 9 8 7 6 5 4 2 o ..,:;:.. " \ /" ..•./ II \ " ':!/ \\ ' 1/ ! \ 1/1 :' \ ! ,I . \ I II ! \ ~ If ./ \ Lo //..1 .: \\ ~ :' \ ~ I .: " t -/----- t E \~~-\ ':::..-..- - "'~';;'" ~ --r--lv,' '1 . ----.... " Shear -_._=--~"'~\' {Shear I ~ "l'"- Zone ...... ::: Zone~ i 20 ~ t:: f \oJ Q -. -'~\\ --'-t ...J ~-~------~---- \ \') \\ ~ I'CLINa-lETER 3i.:RVEYS . '. \ . \: \ t READINGS I \ I \\ \ Survey l~n. Symbol D~te \\ x, 30 1 1-16-80 t \~, '\~.. 2 5-7-80 I 3 5-21-80 ~~ 4 6-4-80 ~~O 40 5 , ';',; 7-14-80 {- 6 ;. 8-20-80 .~ 4.29-81 ':i SLOPE STABILITY INVESTIGATION Ii No Dat., Casing Sheared at 20' TIEHACK SKI LIFT NO.5 ···~.\~l[ INCLINOMETER NO. I \ f't~. 2Jll C of \CU1 sliJLe b~ Whrte\.s 1- ?;") GEO-HAZ Consulting, Inc. Bedding fault Transgressive fou!t Fig. 18. Schematic diagram of a translational landslide. From Braddock, 1978. E:\GEOHAZ\PIONEBR\GONDOIA.RPT November 29, 1996 46 1- -:/f GEO-HAZ Consulting, Inc. ------PULL-AWAY ZONE OF TRANSLATIONAL SUDE, BURNT MTN. N (vertical exaggeration=2.2 x) S 9.,.------. f i~~~~~~~~;~ _1-+------r'---":-----,------.---.L.-.----'---,------l o 10 20 30 40 50 60 Hori~ontal Distance (m) Fig. 20. Topographic profile and inferred slide planes in the pull-away zone of the larger dipslope landslide, upper Burnt Mountain. E:\GBOHAZ\PIONEER\GONDOLARPT November 29, 1996 49 1- L(O GEO·HAZ Consulting, Inc. 5ED'ME"TA~V (2) P~C.KA~fS ~L,--t--r--L..---r""-"--t- ...... -J..,"""""'r--=,,..., I rec~Y\t eo\ia.V\~ d~v-'l'led CO II u. VivWl 20 collu.vivwz deillfd frcWl B- Sovt.\-t $(.(.\V'f 00 sa~ pond silt (u\d eo C- 50ils (Vlot tIlted) 100 so.~ pOVld silt llnd 120 soib (tilted) <0 II ",Vll/WI a.l'\d (~sidlAuW\ deV'lved fl'"oYl1 Ll.l\d12dyiY\4 bdt'"ock (pY'e- 51 ide) .., Fi&- 21. Log of the soil pit excavated in the younger pull-away trough, larger dipslope slide, upper Burnt Mountain. E:\GEOHAZ\PIONEER\GONDOlARPT November 29, 1996 50 t- L{( . GEO.HAZ Consulting, Inc. . ~----~~-- - -_. _._~-- ---~~~- -~~~~ S Topographic profile in area of suspected creep o (vertical exaggeration =1.4 x) N 1 Top terminal, Burnt Mtn. West lift I ~2100 ::::::::::::::::::?§:::::::::::::::::::::::: ,. 5 . -30 ········20········· . -40 7 . J-50 20 . -60 ~ . I -70 23 . -80;-----r------r------r------.----~l....--.:::::::tJ o 50 100 150 200 250 300 Horizontal Distance (m) Fig. 28. Topographic profile across an area of suspected soil creep, immediately below the summit of Burnt Mountain. The ground surface is composed of a series of slopes (20°-25°) with intervening benches sloping from 5°_10° (slope angles are shown beneath each slope segment). This regular undulation of topography cannot be ascribed to fluvial -processes, so creep of the colluvial/regolith layer is suspected. E:\GEOHAZ\PIONEER\GONDOIARPf November 29, 1996 58 , " ~", ... ~ \ ~~ i~~ , "- --.. .._ ~" ~: ) ....' I , , I ,,--=-~':, ~" ;; _ - ~ 1/ _~, ' ...... ~ *:~~>~ 1 -l ...._ '.._==~"'. :7;~"" ll-> '- lu o· 1 -.:Jc= :,J;.",,- 1 o ( ,.' .. .",~ :-::'~.' ~ 35 11 6·· • I ~ q: "", 11,. Il.. " " M... I I . :: -<>,,? 'I • ) ;- " I : ",l 11~ r , n I I IBN • ~ , 0:' l C\J c', C' C\J I (\.j c' I f..£.1 cr.1 -, cr:~ ~VJ-n '&! ~ ~ I1 ! I. \ 39"22'3(1' 1...-" ~·r~~ 11~j:' ' ...... 1 If=-~' --":::~BlejIfIV 11 I ,.- ;-~!i', y; '1'1 lorl5' ., ------.. 71...... ·roe"r"....-=: -,,: .~n.'AI Mapped. edited. and published I (;(lotlol b, USGS and USC&GS F F?+3 E:=C=: TOllO&1aphy by phGtogramrnelric methods from aerial pfMJtographs I3km 19tiO, f"lefd c1tlded 1961 PolYCDllic prDjection, 1921 Norlh American d.tUfll 10,OOO-foollrid based on CoIoriido coordina~ system, central zone l000-rneler ,!"iveryl Transverse Nerc.alor arid tDs, I lone 13, shf'MI in blue II. fine red dashed lines indicate selected fena and lielcllirles where UTtl Gl!IO AND 1961 MMl'IEIIC IIOATH DECUNIIl"OII A' C€..rEIl Of' SHUT - - --_ ••-..... -;...:LI,...... ,..,,:,1 ,.,... ~,....~.",< Thi-; i"{",m..tin,, i~ 1Jna":hoP.C~ptl ",ne; ....P rOMPI IF"'~ WtTH NAT' kr ~( lAT J 17' 7 T 11~ ~ T 'J 'I P.1Al t,_ 4'. ~ ~1\/\l 1 'J'i'i\/A \l'JA1\l .... ~ ~ ~ N '-' ~ OJ (tI1- J. - .:;j::;z. -slJ 1: {i ...... )J . .'" ~ E ~~ },.Ji ·f -, 1:' ~ :~ 0.- .~ .J .~.~ .-I - 1- L(9- \. I l"'~ ,'0 _.f' c • -t- -.... 1/1 ...... I\) ""'-.J . ~ ~ ~. ...... t Q o o o N 1 o \I) o \.0 - N N <) - r- N DAY2 .. GUIDEBOOK FOR DAY 2 OF THE 1997 FIELD TRIP ROCKY MOUNTAIN CELL OF THE FRIENDS OF THE PLEISTOCENE LATE TERTIARY AND QUATERNARY COLLAPSE RELATED TO DISSOLUTION AND FLOWAGE OF PENNSYLVANIAN EVAPORITIC ROCKS IN THE GLENWOOD SPRINGS AREA, COLORADO FIELD TRIP LEADER: BOB KIRKHAM COLORADO GEOLOGICAL SURVEY P.O. Box 112 Monte Vista, CO 81144 (719) 588-1280 This field trip emphasizes the Neogene structural and depositional history of the Lower Roaring Fork Valley (LRFV), which is interpreted as being controlled by regional and local collapse resultant from dissolution and flowage of underlying Pennsylvanian evaporitic rocks. The collapse is an on going process that is active today, as demonstrated by the historic development of sinkholes and very high salinity levels in hot springs and rivers in the region. The field trip consists of eight stops which will emphasize various aspects of the collapse model. For those folks with a fascination for geologic hazards and an endurance for long field trips, there will be an optional stop (time permitting) to view the 1994 and 1995 debris flows on Storm King Mountain that developed subsequent to the forest fire in which 14 firefighters tragically lost their lives. The guidebook includes a location map which shows the field trip stops (figure 1), a brief description of the geologic setting, and the field trip schedule. Those of you interested in a good road log for the area should refer to the field trip guidebook by Kirkham and others (1996a), which was prepared for a field trip that was run in conjunction with the 1996 Geological Society of America meeting held in Denver and is available for purchase through the Colorado neological Survey. The Geological Society of America guidebook was produced prior to development of our salt-dissolution collapse model, and therefore does not describe that theory. The Colorado Geological Survey (CGS) has recently undertaken several projects that involve geologic mapping of selected 7.5-minute quadrangles with high societal or economic importance. Funding sources for the 1:24,OOo-scale mapping projects have included state general funds and mineral severance taxes, the U.S. Geological Survey STATEMAP program through several cooperative agreements, the U.S. Bureau of Land Management, and the u.s. Forest Service. Figure 2 shows the 7.S-minute quadrangles where geologic mapping has recently been completed or is scheduled for 1997. During the past four years the CGS has mapped the geology of seven 7.5 minute quadrangles in the Glenwood Spr1ngs-Carbondale-Dotsero area of west central Colorado (figure 3). We plan to map at least four additional quadrangles in this region during the next two years. A number of geologists and field assistants with CGS have been involved in the development of the 1- 1 o I i 5 km. Figure 1. Map showing locations ofstops for Day 2 ofthe 19f}7 FOP field trip. 2- 2 109" 1OS0 107" 106° 105° 104° 103° • Craig [7.J u 40°1 I I I 1- • Denver Limon Grand • • Junction tv, ~ w 39°, I I I :::~. 38 °1 r I I ~ • Completed STATEMAP Quadrangles ~ STATEMAP Quadrangles to be mapped in 1997 [] Completed Mineral Severance Tax Quadrangles Cortez • nm Mineral Severance Tax Quadrangles ~ to be mapped in 1997 Trinidad ~ Completed mapping funded in part by • BLMor USFS Figure 2. The Colorado Geological Survey's geological mapping program (scale 1:24,000). 107° 30' 107° 00' ICGSOFR 97 2 :"':,:0', ... ;.,'; , °45' "..•: :< ::. ~ 1"'" ]39 ~:'::~:i~·~i:;:&q).·.'.:": : ~. ~ '.::.. .< ~ ",:" g~~ ~,,'~. 953 ?i:-:..1. 4' ..,;.,.,""_..... : ...... • OFR. 5-4 OFR 97 ...... ';":}::_'7: i,?:I =ri~~~~Od" ·4 ~ stIoehone CcIIIonweod PaD 39° 30' c CGS ICGS~. CGS CGS ~ OFR 96-2 OFR 96-1 OFR 97-3 OFR in 1998 .I ··~rbondaie,. -- Hunter M••• cent... Mounlllin I C8tIIe Creek I cerbon.. .CGS :OFRln 1999 N I. • • Mt.SoprIa Basalt t 39° 15' o 2 4Mi I iii .' o 2 4 6Km :.: ;; ~ () r Figure 3. Status ofgeologic mapping of 7.S-minute quadrangles in the vicinity of Glenwood Springs. collapse theory. They include Randy Streufert, Jim Cappa, Beth Widmann, Pete Stelling, and Tim Schroeder. The USGS is mapping several quadrangles in adjacent areas. Discussions, seminars, and field trips with several of their geologists, including Bruce Bryant, Bob Scott, Dick Moore, Mick Kunk, Bill Perry, Dave Lidke, Paul Carrara, Ralph Shroba, Mark Hudson, Karl Kellog, and Larry Snee, have contributed greatly to our understanding of this fascinating theory. GEOLOGIC SETTING During the Pennsylvanian Period the LRFV lay within the Eagle Basin portion of the Central Colorado Trough (figure 4). Sediments within the basin I W~NE ------.------_ 1 - UT I , I I Glenwood \ Springs- ~ 'NE I Carbondale ~~ Denver I Area '( Basin F- ' I I ~e (l , O Eagle Basin \, I \ , I , I I \ , I I I I Paradox \, Basin , I I ..!!!.L..' _ ----~ ...>Z.~. NM \ OK AZr NM o hnson and others, 1990 Figure 4. Pennsylvanian paleogeography. The Glenwood Springs-Carbondale area lies within the Eagle Basin portion of the Central Colorado Trough. 2-5 were deposited upon a sequence of Early and Middle Paleozoic sedimentary formations that overlie Proterozoic basement rock. Pennsylvanian age sedimentary rocks deposited within the Glenwood Springs-Carbondale part of the Eagle Basin include: (1) the Belden Shale, a basal sequence of black marine shale and thin limestones about 600 to 800 feet thick, (2) the Eagle Valley Evaporite, an overlying evaporitic unit composed of thick beds of gypsum, halite, anhydrite, and associated clastic and carbonate rocks, (3) the Eagle Valley Formation, a stratigraphic interval in which the older evaporitic rocks intertongue and grade into younger, overlying red beds, and (4) the Maroon Formation, a 2,500- to 4,00o-feet-thick sequence of predominantly red beds. Thickness of both the Eagle Valley Evaporite and Eagle Valley Formation vary widely. The Shannon Oil Rose no. 1 well, which was drilled near the mouth of Cattle Creek, penetrated over 3,000 feet of evaporitic rocks, of which the bottom 900 feet was almost entirely halite, before being abandoned while still in halite. This well led Mallory (1971) to suggest the evaporitic rocks might exceed 10,000 feet in thickness near Carbondale. We suspect that the evaporitic rocks may have been tectonically thickened beneath the Grand Hogback Monocline during Laramide crustal shortening, but. will await speculation upon the thickness of the unit until several recently acquired seismic lines that cross the monocline are interpreted. The evaporitic rocks playa major role in the Neogene deformation which will be seen during this field trip. The Pennsylvanian rocks are overlain by a sequence of sedimentary rocks ranging in age from Permian to Eocene. This package of rocks contains evidence of one or more periods of tectonism during the Permian and Triassic, the Cretaceous interior seaway, and also the Laramide orogeny. Subsequent to creation of a widespread erosion surface during the late Eocene or early Oligocene which beveled off the tops of the Laramide structures, a series of mafic lava flows of Miocene age were emplaced across the erosion surface. Basaltic lava flows range in age from 22.4 Ma to 4,160 years B.P. in this part of Colorado and serve as excellent marker horizons which define the widespread Neogene deformation found in this region. A regional tectonic map for the LRFV and surrounding area is shown in figure 5. The Grand Hogback Monocline separates the Piceance Basin from the White River Uplift. There is over 20,000 feet of structural relief across the monocline, most if not all of which occurred very late during the Laramide orogeny, probably in middle to late Eocene time. A prominent belt of faults and folds trend easterly from Glenwood Springs, cutting across the south flank of the White River Uplift par.a11e1 to Glenwood Canyon. The LRFV coincides with the axis of the Cattle Creek Anticline from Glenwood Springs to about Carbondale. The Cattle Creek Anticline appears to be a Laramide structure that has been modified by salt diapirism during the Neogene. N~ENE DEF~TI~ Abundant evidence of Neogene structural deformation occurs within the LRFV. Types of structures include (1) linear to horseshoe-shaPed arcuate synclinal sags, some with faulted limbs, (2) intrusive or piercement contacts between evaporitic rocks and overlying clastic formations, (3) sets of 1080 1070 RIVER UPLIFT 3go 45' • Rifle PICEANCE BASIN Colbran • 390 15' ~ N Anticline + Syncline t ~ Monocline 0 10mi y High-angie Treasure fault I I Mountain 0 10 20km Dome X ~Thrust fault Figure 5. Regional tectonic map. (Modified from Tweto, 1979 and Tweto and others, 1978) Stipple pattern indicates area where evaporite rocks lie at or near the ground surface. 2.-7 orthogonal faults and a series of parallel faults in late Tertiary basalts, (4) circular, elliptical, rectangular, and irregularly shaped bowl-like structural troughs of varying sizes, (5) a large, arcuate, half graben whose floor was occupied by a lake until drained by homesteaders, (6) valley anticlines, (7) complexly deformed, highly broken and brecciated blocks of randomly oriented bedrock which we classify as Pleistocene and/or late Tertiary collapse debris, (8) sharp, monoclinal1y folded basalts, and (9) broad tilting of basalt-capped plateaus. Various types of late Tertiary and early Quaternary sedimentary deposits are preserved within the LRFV. They include fluvial, lacustrine, and deltaic deposits that may attain thicknesses as great as 800 feet locally. These distinctive structures and deposits are pervasive wherever evaporitic rocks lie at or near the land surface (figure 6). Many of the structural sags, troughs, and bowls contain locally derived sediments eroded from adjacent uplands; the Lava Creek B volcanic ash is preserved in at least one trough. These sediments also are deformed, but much less so than underlying bedrock formations. We contend that this unusual deformation is restricted to the Eagle Valley Evaporite and overlying deposits. Bedrock older than the Eagle Valley Evaporite has not been affected by salt tectonism and dissolution. Geomorphica11y these unusual structures have combined to create a landscape that has the classic characteristics of karst topography. Open voids and caverns can be seen in many outcrops of the Eagle Valley Evaporite, and sinkholes, some quite large, have developed in bedrock formations and surficial deposits overlying the evaporitic rocks. As first reported by Mallory (1966) and later documented by Piety (1981), Soule and Stover (1985), and Unruh and others (1993) Quaternary deposits may be affected by this unusual type of deformation. River terraces tilt away from modern river channels. Large, broad, closed or nearly closed depressions and swales that we interpret as subsidence troughs have developed in many outwash terraces. Sinkholes are locally abundant in surficial deposits. Drainage patterns and the extent of many of the basin-filling surficial deposits are influenced by the sinkholes and subsidence troughs, folded river terraces, folded and faulted bedrock, and collapse debris. Neogene deformation along the Grand Hogback Monocline was first described by Murray (1966; 1969) and later by Soule and Stover (1985), Stover (1986), Unruh and others (1993), Kirkham and others, 1995a, 1996a, b; and Carroll and others, 1996). It involves (1) regional down-to-the-east tilting of Miocene basalt flows that unconformably overlie moderately west-dipping sedimentary formations within the monocline and (2) a series of subparallel bedding plane faults that closely follow the bedding of the moderately west dipping sedimentary beds and offset overlying Neogene deposits. As shown in figure 7, the bedding plane faults are downthrown to the west, but blocks of Miocene basalt between the faults dip as much as about 30° eastward, opposite that of the underlying pre-Laramide sedimentary formations. This type of deformation can be explained by dissolution of evaporitic rocks that underlie the monocline or by flowage of them towards the Roaring Fork Valley. As the evaporites are removed, the monocline relaxes or "unfolds", causing regional tilting to the east. Dips of sedimentary rocks within the monocline decrease 1.-- 8 L--.--- _ -----~~-----j 107·15' 107·07' 30' ", --. Sunlight Mesa (tilted east and faulted), .., 39·15' Basalt Mountain Volcano I 39· 22'30' N f 0 2 4MI I I i I I 0 2 4 6Km Quaternary landslide caused by collapse of bedrock Limit of shallow evaporite 1';-.9T.~·tl Major collapse depression; Early Quaternaryllate Tertiary Sediments includes synclinal sags, structural troughs, subsidence troughs, and Early Quaternaryllate Tertiary collapse debris a half graben. Late Tertiary volcanic rocks Late Tertiary sedimentary deposits Figure 6. Distribution ofshallow evaporites, Neogene volcanic rocks, collapse debris, and major collapse depressions (modified from Tweto and others, 1978; Kirkham and others, 1995a, 1996b; Streufert and others, 1997a, 1997b). Some map units are not labeled. 2....,9 FEET sw NE Grand Hogback Monocline Sunlight Peak 10,000 8,000 ~ ....I o 6,000 o 1 2 Mi Vertical exaggeration 4x I I I I I I o 1 2 3Km Figure 7. Schematic cross section from Sunlight Peak to the Roaring Fork Valley. MEVF-Pennsylvanian Maroon Eagle Valley Formations; DSB-Cretaceous Dakota Sandstone through Triassic-Permian State Bridge Formation; Tw-Paleocene andEocene Wasatch Formation; Tb-Miocene basalt. I __------ll as relaxation progresses, and the overlying Miocene basalt cap is tilted eastward. While the monocline relaxe~ strain also occurs as differential slippage on bedding planes, creating a series of subparallel faults along which the unconfonmably overlying Miocene basalts are downdropped to the west. These unusual evaporite-related structures occur within a prominent regional topographic depression that lies within an area where evaporitic rocks are at or near the ground surface. The topographic depression is as much as 4,000 ft lower in elevation than.surrounding terrane, and late Tertiary basalts are downdropped 3,000 to 4,000 feet within the collapse block. We interpret the topographic depression as a large collapse block that has resulted from dissolution and flowage of evaporitic rocks from beneath the area and have named this regional collapse feature the Carbondale collapse center (figure 8). Based on preliminary geologic mapping in the Gypsum-Eagle Limit ofevaporite Eagle Collapse Center Dillon. Carbondaiij:iliJil:!' Collapse """~{",,, N Center 0 1 50Km, I , 0 30MI Figure 8. Distribution ofshallow evaporitic rocks andgeographic relationships with the Carbondale and Eagle collapse centers (after R. Scott, 1997, written commun.). 1. ... 11 area, the USGS suspects another regional collapse feature, the Eagle collapse center, may exist in that area (R. Scott, 1997, personal commun.). Neogene igneous rocks are well preserved within the down-dropped Carbondale collapse center (figure 9). The synclinal sags, intrusive contacts between sedimentary formations, orthogonal fault sets, bowl-like structural troughs, arcuate half graben, valley anticlines, collapse debris, folded Pleistocene outwash terraces, and thick deposits of Neogene sediments all occur within and are limited to the coll~pse block. SALT-DISSOLUTION COLLAPSE MODEL We have developed a model called the salt-dissolution collapse theory to explain the regional collapse found in the Glenwood Springs-Carbondale area. Evaporite flowage plays a major role in the model, but the ultimate way in which the evaporite is removed from the collapse area is by dissolution. The dissolved salts are then carried down the Colorado River. The process initiated as the Roaring Fork River downcut into the evaporitic rocks, creating differential overburden loadings on the evaporite and causing it to flow from beneath the adjacent uplands towards the river valley. As the evaporite thickness diminished when it flowed out from beneath the uplands, deposits overlying the evaporite slowly dropped downward. When the low-density evaporitic rocks reached the valleys they rose upward, piercing into and/or doming overlying rocks and surficial deposits. Wherever the evaporite encountered fresh ground water it was dissolved, creating caverns into which younger, overlying materials subsided. Synclinal sags, the bowl-like structural troughs in bedrock, the half graben of Spring Valley, the presence of collapse debris, and locally very thick sequences of Neogene sediments all can be explained by locally intense subsidence within the regional collapse block. Features such as valley anticlines and intrusive sedimentary contacts are probably a result of diapirism. Folded outwash terraces are a product of either dissolution induced, localized subsidence and/or diapiric processes focused along river channels where overburden pressures are at a minimum. If our salt-dissolution collapse model is correct, then up to 1800 square miles in west-central Colorado may be affected by collapse. R. Scott (1997, personal commun.) has estimated the total volume of collapse at 550 cubic miles, based on structural contouring of deformed late Tertiary lava flows. Direct evidence for dissolution includes the widespread occurrence of voids and caverns within the Eagle Valley Evaporite and sinkholes, subsidence troughs, and synclinal sags in deposits overlying it. Hot springs in the region, such as those at Glenwood Springs and Dotsero, have sodium, chloride, calcium, and sulfate concentrations ranging from about 9 to 20 grams per liter and'combined discharges around 3,000 gallons per minute (Barrett and Pearl, 1976), suggesting the dissolution is an active, on-going process. On the basis of analyses and flow rates in Barrett and Pearl (1976), Yampa Spring, which supplies water for Glenwood hot springs pool, discharges about 260 short tons or about 120 cubic yards of salt each day to the Colorado River system. Yampa 1..-12 L------~~--~~~~~~~~~~~~- 1070 30' 1070 00' r-----..I------~~~--.....,~~~~~~~--'-----_r_400 00' o ~ Holocene II Quaternary • Pliocene lliill1] Miocene 390 30' N 10.4 Ma f 0 5 10 Mi I I I 0 10Km Figure 9. Generalized distribution of Neogene igneous rocks (after Tweto, 1978; Tweto and others, 1978; Larson & others, 1975; Kirkham & others, 1995a, b; Carroll and others, 1996; Streufert and others, 1997a, 1997b). Dashed line with barbs indicates approximate extent of collapse block. Spring alone would be responsible for the dissolution of one cubic mile of salt in about 110,000 years. Water quality monitoring by the National Water Quality Assessment Program of the USGS indicates the total dissolved solid load in the Colorado River at Cameo downstream of the collapse areas has averaged 1.6 million tons per year during the past ten years, most of which is sodium, chloride, calcium, and sulfate (N. Bauch, 1997, personal commun.). Above the collapse centers the streams have very low dissolved- solid loads. Assuming the modern dissolved solid load in the river has remained constant during the recent geologic past, it would take 3.7 million years to dissolve the 550 cubic miles of evaporite that has been removed from beneath the collapse area since the eruption of 8 to 10 Ma basalts. This rate is roughly twice as high as the long-term geological rate. When combined with the observation that most recent sinkholes have occurred in irrigated fields and in or near irrigation ditches and reservoirs, one could tentatively conclude that the current dissolution rates and hence the salinity in the Colorado River might be affected by human activities. If correct, this interpretation may potentially spark mitigation techniques that could reduce salinity loadings to the river that heretofore were considered entirely "natural". As demonstrated by Larson and others (1975) and Tweto and others (1978), Neogene igneous rocks are prevalent across much of this part of west-central Colorado (figure 8), which leads one to assume that the crust in this region must have been fractured and probably was faulted during the Neogene. Igneous activity has occurred as recently as 4,150 years B.P. at Dotsero volcano (Giegengack, 1962; Streufert and others, 1997a). Volcanic activity has also occurred during the Quaternary at several other locations in this area, including Triangle Peak (Larson and others, 1975) and Willow Peak (Streufert and others, 19978). However, much and perhaps all the demonstrable Neogene deformation within the area where evaporitic rocks are at shallow depths and in the adjoining Grand Hogback Monocline appears to be directly due to salt tectonism and salt dissolution. Neogene crustal tectonism related to regional extension, if present within this area, is masked by the salt-related deformation. This complicates seismotectonic evaluations in the LRFV. SOUTH CANYON FIRE AND DEBRIS FLOWS ON STORM KING MOUNTAIN South Canyon fire burned slowly for several days before exploding into a fire storm on July 7, 1994 as a cold front passed through. Fourteen firefighters were killed in the blazing inferno, which burned about 8 k.2 of pinyon, juniper, and gambe1 oak on the south side of Storm King Mountain. Temperatures were locally high enough to melt and fuse quartz grains in soil. large clouds of blowing ash were reported on the mountain in the days following the fire. Ash drifted into and accumulated on the bottoms of the drainages within the burn area (Cannon and others, 1995). Torrential rains the night of September 1, 1994 created floods which washed debris onto I-70 at several locations as a slurry of mud, rocks, and burned trees. Deposition occurred as hyperconcentrated floods and debris flows. Detailed (1:5,000 scale) mapping of the burn area indicates most debris mobilized during the 1994 event was derived by 1) flushing out dry~ravel deposits that had accumulated on the channel floors, 2) erosion of older surficial deposits 1-14 including previous debris-flow deposits, landslides, and co11uvium/s10pewash immediately adjacent to the channel floors, and 3) rilling, minor gullying, and debris avalanching of loose surficial deposits on steep slopes cut into old landslide deposits, old valley-filling co11uvium/s10pewash, and Maroon bedrock (Kirkham and others, 1996). Thirty vehicles were trapped by the debris flows, and a few people were swept into the river by the floods. Fortunately, there were no deaths and only a few serious injuries resulting from the debris flows. Although the burn area was aerially seeded in November, 1994, additional debris flows poured out of Basin F during 1995. The revegetation effort was very successful on gentle slopes where the original soil profiles were preserved, but steep slopes are revegetating poorly due to absence of productive soil horizons and removal of seed by sheetwash during rainstorms prior to sprouting. On a positive note, burned oakbrush is rapidly developing new shoots which "sucker up" from roots. Abundant unconsolidated materials remain within these basins and will be susceptible to mobilization during future storm events. A few small landslides have moved since the last debris flows in 1995 and partially block drainage channels. No evidence was observed to indicate large-scale reactivation of the huge, old landslide complex. An interesting way in which to mitigate the flood hazards was clearly demonstrated during the flooding. Sediment-laden waters ran out of Basins C and D (Cannon and others, 1995), spilling out onto the entrance and exit ramps at the South Canyon exit on 1-70. Debris then flowed down the ramps towards the underpass beneath 1-70. The underpass provided a pathway for the debris to flow on to the Colorado River without interfering with traffic on the highway, effectively mitigating the hazard to highway. SELECTED REFERENCES Barrett, J.K., and Pearl, R.H., 1976, Hydrogeological data of thermal springs and wells in Colorado: Colorado Geological Survey Information Series 6, 124 ~ass, N.W., and Northrop, S.A., 1963, Geology of Glenwood Springs quadrangle and vicinity, northwestern Colorado: u.S. Geological Survey Bulletin 1142-J, 74 p. Bryant, B., 1979, Geology of the Aspen 15-minute quadrangle, Pitkin and Gunnison Counties, Colorado: u.S. Geological Survey Professional Paper 1073, 146 p. Bryant, B., and Shroba, R.R., 1997, Revised preliminary geologic map of the Storm King Mountain quadrangle, Garfield County, Colorado: u.S. Geological Survey Open-File Report (in preparation). Cannon, S.H., Powers, P.S., Pih1, R.A., and Rogers, W.P., 1995, A preliminary evaluation of the fire-related debris flows on Storm King Mountain, Glenwood Springs, Colorado: u.S. Geological Survey Open-file Report 95-508, 38 p. Carroll, C.J., Kirkham, R.M., and Stelling, P.L., 1996, Geologic map of the Center Mountain quadrangle, Garfield County, Colorado: Colorado Geological Survey Open-File Report 96-2. Dames &Moore, 1996, Draft report--Debris flow assessment and preliminary recommendations for mitigative measures for the River Valley Ranch development, Carbondale, Colorado: unpUblished consulting report prepared for Hines Interests Limited Partnership, 39 p. 2-15 DeVoto, R.H., Bartleson, B.L., Schenk, C.J., and Waechter, N.B., 1986, Late Paleozoic stratigraphy and syndepositional tectonism, northwestern Colorado, in Stone, D.S., ed., New interpretations of northwest Colorado geology: Rocky Mountain Association of Geologists, 1986 symposium, p. 37-49. Ellis, M.S., and Freeman, V.L., 1984, Geologic map and cross sections of the Carbondale 30' by 60' quadrangle, west-central Colorado: u.S. Geological Survey Coal Investigations Map C-97A. F.M. Fox &Associates, 1974, Roaring Fork and Crystal Valleys-An environmental and engineering geology study, Eagle, Garfield, Gunnison, and Pitkin Counties, Colorado: Colorado Geological Survey Environmental Geology 8, 64 p. Giegengack, R.F., Jr., 1962, Recent volcanism near Dotsero, Colorado: Boulder, Colo., University of Colorado, M.S. thesis, 43 p. Gile, L.H., Peterson, F.F., and Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulation in desert soils: Soil Science, v. 101, no. 5, p. 347-360. Hunt, C.B., 1969, Cenozoic history of the Colorado River: U.S. Geological Survey Professional Paper 669-C, 130 p. Izett, G.A., and Wilcox, R.E., 1982, Map showing localities and inferred distributions of the Huckleberry Ridge, Mesa Falls, and Lava Creek ash beds (Pear1ette family ash bed) of Pliocene and Pleistocene age in the western United States and southern Canada: U.S. Geological Survey Miscellaneous Investigations Map 1-1325. Johnson, S.Y., 1987, Sedimentology and paleogeographic significance of six fluvial sandstone bodies in the Maroon Formation, Eagle Basin, northwest Colorado: U.S. Geological Survey Bulletin 1787-A, p. A1-A18. Johnson, S.Y., Schenk, C.J., Anders, D.L., and Tuttle, M.L., 1990, Sedimentology and petroleum occurrence, Schoolhouse member, Maroon Formation (Lower Permian), northwest Colorado: American Association of Petroleum Geologists Bulletin, v. 74, p. 135-150. Johnson, S.Y., Schenk, C.J., and Karachewski, J.A., 1988, Pennsylvanian and Permian depositional cycles in the Eagle Basin, northwest Colorado, in Holden, G.S., ed., Geological Society of America field trip gUidebook: Colorado School of Mines Professional Contributions 12, p. 156-175. Kirkham, R.M., Bryant, Bruce, Streufert, R.K., and Shroba, R.R., 1996a, Fie1dtrip guidebook on the geology and geologic hazards of the Glenwood Springs area, Colorado, in Thompson, R.A., Hudson, M.R., and Pi11more, C.L., eds., Geologic excursions to the Rocky Mountains and beyond; Fie1dtrip guidebook for the 1996 annual meeting of the Geological Society of America: Colorado Geological Survey Special Publication 44. Kirkham, R.M., Cannon, S.H., and Powers, P.S., 1997, Geology of the South Canyon fire area, south flank of" Storm King Mountain, Garfield County, Colorado: Colorado Geological Survey Information series (in preparation) Kirkham, R.M., Streufert, R.K., and Cappa, J.A., 1995a, Geologic map of the Glenwood Springs quadrangle, Garfield County, Colorado: Colorado Geological Survey Open-file Report 95-3. _____ 1995b, Geologic map of the Shoshone quadrangle, Garfield County, Colorado: Colorado Geological Survey Open-file Report 95-4. Kirkham, R.M., Streufert, R.K., Hemborg, T.H., and Stelling, P.L., 1996b, Geologic map of the Catt1'e Creek quadrangle, Garfield County, Colorado: Colorado Geological Survey Open-File Report 96-1. Kirkham, R.M., and Widmann, B.L., 1997, Geologic map of the Carbondale quadrangle, Garfield County, Colorado: Colorado Geological Survey Open-File Report 97-3. 2-16 Langenheim, R.L., Jr., 1954, Correlation of Maroon Fonmation in Crystal River Valley, Gunnison, Pitkin, and Garfield Counties, Colorado: American Association of Petroleum Geologists Bulletin, v. 38, no. 8, p. 1748-1779. larson, E.E., Ozima, M., and Bradley, W.C., 1975, Late Cenozoic basic volcanism in northwest Colorado and its implications concerning tectonism and origin of the Colorado River system, in Curtis, Bruce, ed., Cenozoic history of the Southern Rocky Mountains: Geological Society of America Memoir 144, p. 155-178. Lincoln-Devore Testing laboratory, 1976, Garfield County land use studies: unpublished series of maps prepared for Garfield County Land Use Planning Department. Lovering, T.S., and Mallory, W.W., 1962, The Eagle Valley Evaporite and its relation to the Minturn and Maroon Formation, northwest Colorado: U.S. Geological Survey Professional Paper 450-0, p. 045-048. Mallory, W.W., 1966, Cattle Creek Anticline, a salt diapir near Glenwood Springs, Colorado: U.S. Geological Survey Professional Paper 550-B, p. B12-B15. _____ 1971, The Eagle Valley Evaporite, northwest Colorado-a regional synthesis: U.S. Geological Survey Bulletin 1311-E, 37 p. Murray, F.N., 1966, Stratigraphy and structural geology of the Grand Hogback Monocline, Colorado: Boulder, Colo., University of Colorado, Ph.D. dissertation. _____ 1969, Flexural slip as indicated by faulted lava flows along the Grand Hogback Monocline, Colorado: Journal of Geology, v. 77, p. 333-339. Perry, W.J. Jr., Grout, M.A., Hainsworth, T.J., and Tang, R.L., 1988, Wedge model for late Laramide basement-involved thrusting, Grand Hogback Monocline and White River Uplift, western Colorado [abstr.]: Geological Society of America Abstracts with Program, v. 20, no. 7, p. 384-385. Pierce, K.l., 1979, History and dynamics of glaciation in the northern Yellowstone National Park area: U.S. Geological Survey Professional Paper 729-F, 90 p. Pierce, K.L., Obradovich, J.D., and Friedman, I., 1976, Obsidian hydration dating and correlation of Bull Lake and Pinedale glaciations near west Yellowstone, Montana: Geologic Society of America Bulletin, v. 87, no. 5, p. 703-710. Piety, L.A., 1981, Relative dating of terrace deposits and tills in the Roaring Fork Valley, Colorado: Boulder, Colo., University of Colorado, M.S. thesis, 209 p. Poole, F.G., 1954, Geology of the southern Grand Hogback area, Garfield and Pitkin Counties, Colorado: Boulder, Colo., University of Colorado, M.S. thesis, 128 p. Richmond, G.M, 1986, Stratigraphy and correlation of glacial deposits of the Rocky Mountains, the Colorado Plateau and the ranges of- the Great Basin, in Sibrava, V., Bowen, D.Q., and Richmond, G.S., eds., Quaternary glaciations in the northern hemisphere: Quaternary Science Reviews, v. 5, p. 99-127. Richmond, G.M., and Fullerton, O.S., 1986, Introduction to Quaternary glaciations in the United States of America, in Sibrava, V., Bowen, D.Q., and Richmond, G.S., eds., Quaternary glaciations in the northern hemisphere: Quaternary Science Reviews, v. 5, p. 3-10. Schenk, C.J., 1987, Sedimentology of an eolian sandstone from the Middle Pennsylvanian Eagle Valley Evaporite, Eagle Ba$in, northwest Colorado: U.S. Geological Survey Bulletin 1787-B, p. 19-28. _____ 1989, Sedimentol,ogy and stratigraphy of the ,Eagle Valley Evaporite (Middle Pennsylvanian), Eagle Basin, Colorado: Boulder, Colo., University of Colorado, Ph.D. dissertation, 172 p. Soule, J.M., and Stover, B.K., 1985, Surficial geology, geomorphology, and general engineering geology of parts of the Colorado River Valley, Roaring Fork River 2,-17 Valley, and adjacent areas, Garfield County Colorado: Colorado Geological Survey Open-File Report 85-1. Stover, B.K., 1986, Geologic evidence of Quaternary faulting near Carbondale, Colorado, with possible associations to the 1984 Carbondale earthquake swanm, in Rogers, W.P., and Kirkham, R.M., eds., Contributions to Colorado Seismicity and tectonics-A 1986 update: Colorado Geological Survey Special Publication 28, p. 295-301. Streufert, R.K., Kirkham, R.M., Schroeder, T.S., and Widmann, B.L., 1997a, Geologic map of the Dotsero quadrangle, Eagle and Garfield Counties, Colorado: Colorado Geological Survey Open-File Report 97-2. Streufert, R.K., Kirkham, R.M., Widmann, B.L., and Schroeder, T.S., 1997b, Geologic map of the Cottonwood Pass quadrangle, Eagle and Garfield Counties, Colorado: Colorado Geological Survey Open-File Report 97-4. Szabelak, S.A., 1984, Colluvial slope estimate and driven anchor field experimenting portion of Glenwood Canyon, Colorado: Golden, Colo., Colorado School of Mines M.S. thesis, 180 p. Tweto, 0., 1977, Tectonic history of west-central Colorado, in Veal, H.K., ed., Exploration frontiers of the central and southern Rockies: Rocky Mountain Association of Geologists, 1977 symposium, p. 11-22. Tweto, 0., Moench, R.H., and Reed, J.C., 1978, Geologic map of the Leadville 1° x 2° quadrangle, northwest Colorado: u.S. Geological Survey Miscellaneous Investigations Map 1-999. Unruh, J.R., Wong, I.G., Batt, J.D., Silva, W.J., and Lettis, W.R., 1993, Seismotectonic evaluation, Rifle Gap Dam, Silt Project, Ruedi Dam, Fryingpan Arkansas Project, northwestern Colorado: unpublished report prepared by William R. Lettis & Associates and Woodward-Clyde Consultants for u.S. Bureau of Reclamation, 154 p. Welder, G.E., 1954, Geology of the Basalt area, Eagle and Pitkin Counties, Colorado: Boulder, Colo., University of Colorado, M.S. thesis, 72 p. Widmann, B.L., 1997, Evaporite defonmation in the Dotsero, Gypsum, and Cottonwood Pass 7.5 minute quadrangles, Eagle County, Colorado: Boulder, Colo., University of Colorado M.S. Thesis, 93 p. '2.-18 FIELD TRIP SCHEDULE DAY 2 OF THE 1997 FIELD TRIP ROCKY MOUNTAIN CELL OF THE FRIENDS OF THE PLEISTOCENE SEPTEMBER 13, 1997 FIELD TRIP LEADER: BOB KIRKHAM COLORADO GEOLOGICAL SURVEY P.O. Box 172 Monte Vista, CO 81144 (719) 588-1280 CAR POOL----CAR POOL---CAR POOL STOP 1 (7:30 to 8:00 AM)-- Sunlight Ski Area. Overview of today's field trip activities and the Colorado Geological Survey (CGS) mapping program. Discuss the regional tectonic setting, with emphasis on the abundant evidence of Neogene deformation, and an introduction to the salt-dissolution collapse theory, which appears to account for much and perhaps all of the known Neogene structure in the region. Directions to stop 2: Depart Sunlight Ski Area and head east on County Road (CR) 117 (Fourmile Creek Road) as if returning to Glenwood Springs. About 3 1/4 miles past the Sunlight Bavarian Inn, which is at the entrance into the ski area, turn very sharply to the left (almost a U-turn) into the west entrance of Argonaut Fanms. There is a small set of iron gates at the entrance. If you pass by the sign for Oak Meadows subdivision, you've gone too far. Drive past the barns, cross the creek, and continue uphill on the main gravel road. Park near the upper barns. STOP 2 (8:15 to 9:30 AM)-- Argonaut Farms. Overview of Neogene defonmation associated with the Grand Hogback Monocline, including subtle fault scarps in late Pleistocene debris-flow deposits. Discuss the unfolding or relaxation of the Grand Hogback Monocline. Discuss and view tilting and faulting of 10 Ma basalts. View faulted early Quaternary/late Tertiary basaltic gravels and late Pleistocene debris-flow deposits. Directions to Stop 3: Return to CR 117 (Founmile Creek Road). Turn left and head towards Glenwood Springs. In about 4 1/2 miles the road passes between an old ranch house and barn. This is John Bershenyi's ranch. Pull off the road and park, being careful not to block the county road or John's driveway. If it 1s dry, we can park in his corral. If it's been raining, this will be a tough place to find parking spots for everyone. You might be able to back track along Founmi1e Creek road to find other parking spots. STOP 3 (9:45 to 10:45 AM)-- Bershenyi terrace, a diapirica11y upwarped pre Bull Lake terrace with overlying debris fan deposits. Overview of Cattle Creek Anticline and other folded terraces in the LRFV. 2-19 Directions to Stop 4: Continue north on CR 117 to junction with Colo. 82 in Glenwood Springs. Turn right (south) on Hwy 82. Go about 2 1/4 miles to CR 115 (Red Canyon Road) and turn left (east). This road junction is opposite the south end of the Glenwood airport. The turn is a bit tricky, but there is a center turn lane. The road runs between a couple of closely spaced buildings. BE VERY CAREFUL ON RED CANYON ROAD. It is a very narrow, yet busy shelf road that is prone to closure due to rockfall and lands1iding. The road currently is posted with a "road closed" sign, but local traffic continues to use it. Drive about 2 1/2 miles up CR 115 to a pull-out with mail boxes and dumpsters. Park in the pull-out or on the shoulder of the county road. If you get separated from the group for any reason, you can meet us for lunch in Sopris Park in Carbondale. Since we will return to Hwy 82 using this same road, you could also wait for us below the shelf road. STOP 4 (11:15 to Noon)-- North end of Spring Valley, a large, arcuate-shaped half graben into which a large part of the hills to the east are collapsing. The 22 Ma basalt exposed in the roadcut has been downdropped around 4,000 feet into the Carbondale collapse center. Nearby 7.7 and 10 Ma basalt flows have been downdropped 3,000 to 4,000 feet. The floor of Spring Valley was occupied by a lake until being drained by homesteaders prior to 1900. Directions to lunch: Return to Colo. 82 via Red Canyon Road. Turn left (south) onto Hwy 82. Continue south on Hwy 82 for about 8 miles to the junction with Colo. Hwy 133. Watch for folded Pinedale, Bull Lake, and pre-Bull Lake terraces along the Roaring Fork River. An excellent exposure of the intrusive or piercement contact between the evaporitic rocks and overlying, younger red beds can be seen in the east valley wall just south of Cattle Creek. Turn right (south) onto Hwy 133. Go about 1/2 mile to the stoplight and turn left (east) onto Main Street. Follow Main Street to 7th Street and turn right. Park next to Sopris Park. There are bathrooms available here. LUNCH (12:30 to 1:30 PM)-- Sopris Park in Carbondale. Directions to Stop 5: Return to the stoplight at Main Street and Hwy 133. Continue straight through the intersection. Stop 5 is about 1 1/4 miles from this stoplight. Main Street turns into CR 106. Continue past the Colorado Rocky Mountain School. CR 106 turns into CR 108 at about this point. Cross over the Crystal River. Continue west on CR 108 at the junction of CR 108 and 109, which is just past the Crystal River. Park on the south side of the road opposite the prominent roadcut that is about 1/4 mile past the intersection of CR 108 & 109. Alternative parking is available back near the river. STOP 5 (1:45 to 2:15 PM)-- Edgerton Creek roadcut. View highly contorted Eagle Valley Evaporite in outcrops on the south side of the valley. Examine roadcut into a subsidence trough developed in middle Pleistocene Bull Lake outwash gravel and overbank deposits. Numerous sinkholes have developed on the terrace surface above this exposure, causing the rancher to halt irrigation of the meadow. A large, over 2 miles long, regional subsidence trough occurs immediately to the south of this location. The 620 ka Lava Creek B ash and overlying reddish sediments derived from bedrock hills to the west are preserved within the subsidence trough. 1-20 Directions to Stop 6: Retrace route to junction of Hwys 133 and 82. Turn right (east) onto Hwy 82. Drive 2.8 miles to a private road on the left (north) side of the highway. The private road is 1 mile past the turnoff to Crystal Spring Creek Road (CR 103). Park at base of hill at the entrance to the gravel pit or on the shoulder of the highway. Be very careful when parking along or walking across the highway. STOP 6 (2:45 to 3:15 PM)-- Blue terrace. The prominent topographic depression in this Pinedale outwash terrace is a subsidence trough that is 27 feet deep. Directions to Stop 7: Continue east on Hwy 82 for about 1 mile to the Catherine Store Road (CR 100). Turn left (north) on Catherine Store Road and drive generally northward for about 4 2/3 miles to reach Stop 7. You will pass several road junctions, but stay on CR 100, following the signs for Cottonwood Pass. Park on the shoulder about 1/4 mile past the turnoff to Panorama Drive (CR 170). This rolling countryside that you have been driving across is underlain collapse debris. STOP 7 (3:30 to 4:00 PM)-- View tilted lacustrine sediments deposited within a localized lake environment in the Carbondale collapse center. We have informally named the various types of surficial sediments deposited in the Carbondale collapse center the sediments of Missouri Heights and suspect they are of Pliocene or early Quaternary age. Fossiliferous material from this deposit is currently being studied in an attempt to date the unit. The sediments of Missouri Heights unconfonmab1y overlie moderately to severely deformed basalt flows. Slight tilting of the sediments indicates post deposition defonmation. Directions to Stop 8: Return to Hwy 82. Turn left (east) onto Hwy 82. Go 3.5 miles to the stoplight at El Jebel. Turn left (north) on Upper Cattle Creek Road at the stoplight. Drive 0.6 miles to where the Upper Cattle Creek Road makes its first bend and heads up hill. Park on the shoulder of the road. Be very careful parking and walking at the stop, as the shoulder is quite narrow and there is considerable traffic on the road. STOP 8 (4:30 to 5:00 PM)-- Roadcut on Upper Cattle Creek Road between El Jebel and Missouri Heights. Examine exposure of collapse debris and a recent sinkhole and spring in evaporite deposits. A good exposure of a tilted sequence of 7.8 Ma basalt flows can be seen in the roadcut further up the hill. Directions to Optional Stop 9: Return to the 1-70 exit in Glenwood Springs. Take 1-70 westbound for about 6.1 miles to South Canyon exit, which is the first exit past the West Glenwood Springs exit. Park at the bottom of the exit ramp. OPTIONAL STOP 9 (5:30 to 6:00)-- Underpass beneath 1-70 for South Canyon Road. Discuss Stonm King Mountain forest fire and debris flows. End of Day 2. Return to Sunlight ski area. 2.- 21 T DAY 3 L------ QUATERNARY LOESS STRATIGRAPHY ALONG THE COLORADO RIVER BETWEEN GLENWOOD SPRINGS AND RIFLE. COLORADO: PREUMINARY FINDINGS . SHROBA, Ralph R., U.S. Geological Survey. MS 913. Box 25046. Denver Federal Center. Denver, CO 80225 Geologic mapping and preliminary stratigraphic studies indicate that increasingly order terrace and pediment deposits along the Colorado River, between Glenwood Springs and Rifle. Colo.• are mantled by a progressively greater number ofQuaternary loess sheets. Terrace deposits of Holocene age lack loess mantles; those of Pinedale age (about 35-12 ka) are mantled by one loess sheet; terrace and pediment deposits of Bull Lake age (about 150-140 ka) are mantled by two loess sheets; and those of pre-Bull Lake age (> 150 ka) are mantled by three to five loess sheets. The loess mantres are as much as 6 m thick and consist of one or more loess sheets that commonly are 1-3.5 m thick. Loess thickness decreases with increasing slope gradient. Some of the variation in loess thickness is probably also due in part to local differences in the amount of primary eolian deposition as well as the amount of subsequent erosion. The greater thickness of loess mantles on terraces of Pinedale and Bull Lake ages on the south side of the Colorado River suggests that the predominant wind direction was from the northwest and that the flood plain of the Colorado River was the main source of the loess. The unweathered loess is commonly pink (7.5YR 7/4 and 8/4. dry), calcareous (6..9 percent calcium carbonate), slightly clayey, sandy silt Ooam and silt loam). The grain-size distribution of the carbonate-free fraction of the unweathered loess commonly consists of 22-39 percent sand (2-0.05 mm), 43-62 percent silt (0.05-0.002 mm), and 15-18 percent clay «0.002 mm). About 55-65 percent otthe unweathered loess is composed of very fine sand (0.01-0.05 mm) plus coarse silt (0.05-0.02 mm). Median grain size ranges from 0.03 to 0.05 mm. The loess sheets have surface or buried soils formed in them. The buried soil formed in the penultimate loess sheet (penultimate soil) is strongly developed, has a Bw/Btk/K/Bk profile, and typically contains more pedoQenic clay and calcium carbonate (CaC93) than the soils that are formed In the other loess sheets. Commonly, the soils in the other loess sheets are less developed and have Bt/Bk or Bt/Btk/Bk profiles. Possible sources of the loess include flood-plain sediments of the Colorado River and its major tributaries and sparsely vegetated . outcrops of fine-grained, clastic, sedimentary rocks of Tertiary age in the Piceance Basin, JUst west of the study area. The relatively high content of very fine sand plus coarse silt and the relatively high coarse silt/total silt ratios (about 0.7) of the unweathered loess suggest: (1) a relatively short distance of eolian transport and (2) that flood plains of the Colorado River and its major tributaries, which aggraded primarily during glacial times in response to glacial and penglacial activity farther upstream, are likely sources of much of the loess. Shroba, R.R., 1994, Quaternary loess stratigraphy along the Colorado River between Glenwood Springs and Rifle, Colorado-preliminary findings (abs.): American Quaternary Association, Biennial Meeting, 13th, Minneapolis, MiM., Program and Abstracts. p. 246. ~-\ ~~--~------~------~------_.---_._------~-~------ PLEISTOCENE SURFICIAL DEPOSITS ON PEDIMENTS SURFACES ALONG THE COLORADO RIVER BETWEEN NEW CASTLE AND RIFLE, COLORADO: PRELIMINARY .FINDINGS Shroba, Ralph R., U.S. Geological Survey, MS 913, Box 25046, Federal Center, Denver, CO 80225 Gently sloping pediment surfaces are present at four or more levels along the Colorado River between New Castle and Rifle, Colo. These surfaces are cut on Wasatch Formation and Mancos Shale, slope toward the Colorado River, and commonly have as much as 6 m of local relief. Interbedded gravelly alluvium and debris-flow deposits, referred to herein as pediment deposits, overlie these surfaces. The total thickness of the pediment deposits is commonly 2-12 m and locally as much as 15 m. Alluvium is commonly poorly sorted, clast~supported, bouldery, cobbly, pebble gravel with a silty sand matrix. Debris-flow deposits are very poorly sorted and commonly consist of boulders to granules in a matrix of silty sand to sandy clayey silt. Debris-flow deposits appear to be more common in the upper part of the pediment deposits. Hyperconcentrated flow deposits may be locally present. Clasts are chiefly angular to subrounded sandstone. Some of the boulders in the debris-flow deposits are as much as 4 m in length. Stage III soil K horizons have formed in the top of the pediment deposits. Some of the sandstone pebbles and cobbles in the upper 2 m are disintegrated and weathered to sand-size particles. Pediment deposits lack buried soils and are commonly mantled by 2-3 m of loess that locally consists of two or more sheets. The lower limits of the pediment deposits are about 35-50, 60-85, 110-120, and 190 m above the Colorado River. The pediment deposits on the two lowest levels appear to be graded to terrace deposits. Soil development in the pediment and terrace deposits that are 35-50 m above the Colorado River suggests that they may be of Bull Lake age (about 140-150 ka). Shroba. R.R.• 1996, Pleistocene surficial deposits on pediment surfaces along the Colorado River between New CasUe and Rifle, Colorado: preliminarY findings [abs.]: American Quaternary Association, Biennial Meeting, 14th, Flagstaff, ArIz., Program and Abstracts, p. 187. a: 0 w w w ro.~ Z z ... w S~NbA~1 CJ Tertiary Rocks 14-5tSP 0 I- I------_.... Fie.~~ T~'P i ~To? NEM RttLE " Bulletin SO EXPLANATION Quaternary History of Lower fpa Flood plain alluvium ytg Younger terrace gravels Debris Flows Near Rulison Itg Lower main-stream terrace gravel A most significant stratigraphic succession 4 kIn tgb Basalt-boulder facies of lower terrace gravel east of Rulison suggests that a large debris flow ydf Younger debris flow . crossed the Colorado River near a southward Idf Lower debris-flow deposits, Idf1 is oldest projecting bedrock high known as Webster Hill mdf Middle debris-flow deposits, mdf1 is oldest (Figure 19). Debris-flow deposits rest on a high yaf Younger alluvial fan deposits point north of the present river. Lack of rounding of Tw Wasatch Formation basalt clasts which are supported in a fine-grained X Debris may have obstructed river for a short matrix, coupled with a lack of Colorado River grav period at its narrowest point els, suggests that the river did not occupy the high point after deposition of the debris flow. Part of a debris fan that may have been contiguous with the • Location of gravel pit above debris-flow deposit is present directly south Line of section in Fig. 20 across the river channel. 1I1t1l .....tt.1I II II 1111111111 Trench A gravel pit and prospect trench expose a strati Inter-debris flow contact graphie section (Figure 20) that suggests that the Levee crest debris flow crossed the river and came to rest on a terrace on the north bank. The gravels in the terrace Figure 19. Locality 4 km east ofRulison (sec. 28, are Colorado River gravels; they are the oldest sur T. 6 Sy R. 94l'v.) near Webster Hill (triangle) where fidal deposits at this locality, and they are overlain debris flowed across the river onto a bedrock bench. by fine-grained alluvium andloess. In the upper Debris may have obstructed river for a short period part of the loess is a calcic soil with a well developed at its narrowest point (x). Map units are described Bt horizon, and a carbonate horiZOn. with stage III in text and on Plate I. (DIM 24CN C077F, photo morphology (Cile et aI., 1966). More loess overlies graph 1-8-184) the soil, and the upper unit is the debris-flow de posit. The hiatus represented by the soil suggests A A· • • F • weak soil '. i;' l::.t:::L:to, •• , ii' "!-' .. ..••.•.. e·•.••.•..,.~." .,.• es . ...,..•.•:.. #...... Idf· .• • Cca soil --.- --=-~ ...... -:---:- --:- -- -:- -: ~- -: --:- -.--:- -..:--.---::----=:---.-~:- -:- -- ...... :. :·t.···.····.·.·.········...... • 0 • '. 0 • • ·0·0·°.0•••0. 0 . 0 .. 0 • • 0 •• .0 ••• 0·· o· ..0· EXPLANATION o '. o· . 0 •••• 0 •• .' ° 0 b· o' 0 ., Itg Lower Colorado River terrace gravel .0. '.. Itg •• :. • 0.0 "00 '00' es Fine-grained terrace alluvium 0.' . 0 • es Eolian sand and loess Idf Debris-flow deposit ,Cca Carbonate soil Figure 20. Sketch ofstratigraphic section exposed in gravel pit and trench near Webster Hill, north ofCol orado River (NWJ/4. sec. 28, 1: 6 5., R. 94 l'v.). Section orientation shown on Figure 19. Not Drawn to scale. _Colorado Ceological Survey 27 - Ch3pter 3 Interpretations of Surfid.3I.Cco!ogic History t Tw N yal Ipa ydl 19b Ipa / yal Idl I \ \ mdl '\ : '\ : : Ilg o 2000 It '\ II----l-"..JI \ mdl o 500 m 26 Colorado Geological Survey - EXPLANATION fpa Flood-plain alluvium . Itg Lower main-stream terrace gravel tgb Basalt-boulder facies of lower terrace gravel Idf Lower debris-flow deposits, Idf1 is oldest Tw Wasatch Formation Figure 21. Sketch ofinterpretation ofevents from stratigraphy exposed in gravel pitand trench, and field mapping. A) pre-debris flow landscape, river is entrenched several metns below terrace deposits (ltg). B) debris flow (ld!> flows into river, spilling onto part ofterrace deposit north ofriver, then flows downstream in valley axis 0.5 km. Dashed lines outline possible(?) spillway location where part ofriver may have flowed. C) Riverhas flushed outfine debris leaving boulders in tgb unit, and downcut to present level ofstream. that the river was entrenched below the terracelevel when the debris flow;was;deposited (Figure 21-A). The debris-flow materials Contain large angular basaltboulders mixed with rounded river cobbles, all in a poorly sorted matrix; perhaps the latter were derived from the flood plain as the debris flow crossed the river. A lower main-stream terrace forms the edge of the pointalong the edge of the point along the river, and contains many large (2.5-3-m diameter) basalt boulders ofdebris flow origin (unit tgb). The boulders have been smoothed and rounded bythe river, buthave not had all their pedogenic(?) Cac~ coatings worn off (Figure 4), indicating thatboulders deposited in the valley axis by the debris flow have not moved far from their original site of deposition. This natural rip-rap ex plains why the Colorado River is so narrow (40 m) at this locality. The debris appears to have flowed onto the ter race north of the river during a major debris-flow event which poured millions ofcubic meters of mud and rock into the river valley. As the debris be came diluted with river water and lost much of its original viscosity and strength, it probably began to flow downstream in the valley below the terrace (Figure 21-B). The debris on the terrace was thus left stranded above the river. An east-west trending val ley lies northeast of the gravel pit 54 m above the river, and is floored with thin patch deposits of river gravels veneered by loess. The valley could have been a spillway through which part of the 28 Colondo Geological Survey 'b-fo Bulletin SO river flowed during the time the main channel was means that the debris fan on the south bank of the obstructed by the debris-flow materials. river is not part of the one on the high point north Eventually, the finer grained matrix of the ob of file river, suggesting that the latter debris is the structing debris in the river was flushed away, leav remains of a much older event no longer preserved ing behind large basalt boulders in the flood plain. on the south side of the valley axis. A better under These boulder-laden flood plain deposits are now standing of the history of this locality mustawait preserved above the present river as the tgb terrace more detailed soils, stratigraphic, and relative dat gravel facies (Figure 21-0. ing studies. Other interpretations of the Webster Hill local Regardless of the details, it appears that large ity are possible. The strong development of the debris flows thatstarted on the middle and upper buried soil beneath the debris flow suggests that the flanks of Battlement Mesa often reached the river un-reworked debris flow resting on the terrace and sometimes crossed it. The huge masses of de gravels may have been perched on the peninsula bris, once setin motion, rarely stopped before above. the flood plain prior to a younger debris flow reaching the lowest points in the landscape (Rodine that entered the river, and now its only remains are and Johnson, 1976). the reworked basalt boulders of unit tgb. This Colorado Ceological Survey 29 Bulletin 50 Summary and Conclusions The results of this surficial-mapping project suggest the Roan Plateau is about 750 m lowerin altitude that debris flows are important geologic agents re than Battlement Mesa, and may not have experi sponsible for shaping the "mesa" landscape of the enced the same precipitation and climatic conditions north slopes of Battlement Mesa. The gently sloping which prevailed there in the past. "mesa" surfaces, previously interpreted as pedi The constructional nature of the huge debris ments overlain by outwash gravels are, in fact, the flow deposits that created the sloping surfaces peri surfaces of old debris-flow fans, which have been pheral to Battlement Mesa urges cautionin the use muted and smoothed through time (Figure 22). of these surfaces as time orriver position indicators. Studyand comparison with well-preserved The pre-debris flow landscape"present wheri"the younger debris flows in the present valley axis is surfaces were formed lives buried under 20 to 70 m the strongest evidence for the above interpretation of debris. Gradients of these constructional surfaces of the older higher surfaces. The deposits and strati should not be used to project former river levels or graphic relations of younger debris-flow deposits rates of downcutting, because the gradients are mirrors thatfor the older "mesa"-forming flows. morea function of the physics of high-density de- . A crude cyclicity ofmajor debris-flow periods is brisflowage, and oftenare not related to the under suggestedby the positions of debris-flow remnants lying surfaces. on the presentflanks of the valley. The surfaces of The fonner valley elevationat the time when an these individualflows cannot be correlated as the individual surface was formed more closely corre remains ofa once continuous surface as was previ sponds to the present position of thelower edge of ously attempted, but do suggest cycles of debris that surface on the slopes ofBattlement Mesa. If flow events with respect to former river positions. dates can be obtained for some of these debris-fan Thecycles are probably related to a combination of surfaces, accurate rates of Colorado River downcut geologic andclim8ticfactors that periodically re ting could becalculated using the elevations of sulted in massive debris flows off the sides of Bat buried main-stream terrace gravels within the debris- tlement Mesa. ,- flow sequence. Many levels ofdebris-flow deposits other than Further detailed relative weathering studies are those present may have existed in the past. H there needed to more confidently determine ages of de were others, itwould suggest that the debris flows positional units, and for correlation with other Qua occurred more continuously in closer cycles, or that ternary deposits in the region. Future studies should no cyclicity was involved. Itis difficult to determine include soil carbonate-horizon development, car if there were any additional debris-flow levels be bonate coating developmenton cobbles and boul cause of the extensive landsliding and erosion that ders, surface-boulder weathering studies, and per has occurred on the valley slopes. haps weathering rind studies on some types of Debris flows of the type above did not occur basalt clasts found within the debris flows (Birke from the upper Roan Oiffs. There is no evidence land, 1974). An appendix describing field locations that the incompetent, unnamed claystone unit was of soil profiles and key stratigraphic exposures ever present here, and thus geologic conditions useful in further studies is included, and these loca were not as conducive to massive debris flows. Also, tions are plotted on the map. Colondo Ceological Survey 31 ---~-",,...-,---,il,, ---~r ~ ~ , 1... Ipa Flood-plain alluvium Itg Lower main-stream terrace gravel Dot 0 mtg Middle main-stream terrace gravel C') J Upper main-stream terrace gravel .. . utg ::I ! Do ~. Ntg Neogene-age main-stream terrace gravel g ydf Younger debris flow Idf Lower debris-flow deposits,Idf1 is oldest i Middle debris-flow deposits, mdf is oldest go -~ Ndf mdf 1 .. udf Upper debris-flow deposits i' : ~ ~ .Ntn - ...... ___I __ .. ___!_ • ..J ____!&_ .. (,)..) ..b Figure 22.ldea1iud block diagram showing relationships among constructional and erosional landforms and the succession ofdebris-f'ow and alluvial deposits in tire Colorado River valley area. !, I Ir.... Bulletin SO -'. EXPLANATION Quaternary History of Lower fpa Flood plain alluvium ytg Younger terrace gravels . Debris Flows Near Rulison Itg Lower main-stream terrace gravel A most significant stratigraphic succession 4 kIn tgb Basalt·boulder facies of lower terrace gravel east of Rulison suggests that a large debris flow ydf Younger debris flow crossed the Colorado River near a southward Idf Lower debris-flow deposits. Idf1 is oldest projecting bedrock high known as Webster Hill mdf Middle debris-flow deposits, mdf1 is oldest (Figure 19). Debr~-flow deposits rest on a high yaf Younger alluvial fan deposits point north of the present river. Lack of rounding of Tw Wasatch Formation basalt clasts which are supported in a fine-grained X Debris may have obstructed river for a short matrix, coupled with a lack of Colorado River grav period at its narrowest point els, suggests that the river did not occupy the high point after deposition of the debris flow. Part of a debris fan that may have been contiguous with the • Location of gravel pit above debris-flow deposit is present directly south line of section in Fig. 20 across the river channel. IIIIII'Utllllllllllllllll"" Trench A gravel pit and prospect trench expose a strati Inter-debris flow contact graphic section (Figure 20) that suggests that the Levee crest debris flow crossed the river and came to rest on a terrace on the north bank. The gravels in the terrace Figure 19. Locality 4 km eastofRulison (sec. 28, are Colorado River gravels; they are the oldest sur T. 6 S., R. 94 lV,) near Webster Hill (triangle) where ficial deposits at this locality, and they are overlain debris flowed across the river onto a bedrock bench. by fine-grained alluvium andloess. In the upper Debris may haw obstructed river for a short period part of the loess is a calcic soil with a well developed atits narrowest point (x). Map units are described Bt horizon, and a carbonate horizon with stage III in text and on Plate I. (BIM 24CN C077F, photo- morphology (Cile et al.,l966). More loess overlies graph 1-8-184) . the soU, and the upper unit is the debris-flow de posit. The hiatus represented by the soil suggests A , weak soil '. ,-, , '::::i:::c::::r 'in, i , , t , , • ! . . ~ ~...... " -.,...... ;" ..•.••., .e' Idf .• es • • Cca soil ---....---: -- ....----:--:- ---:- -: ~- --: --:- -.- --:- --:"---. -=-~..-:- ...... -~:- ":'" -- .-: :·18···.····.·.·.·.······ • 0. 0 •.• 0 .• o· ·0·0· .0 ••·0· ,. 0 .' 0 • - 0 .' .0 0.' 0" o' ·.0 • EXPLANATION '0' 0.0.' •..' 0 b:.···0 0 •• Itg lower Colorado River terrace gravel .0. '.' 0ltg_':. • 0. 0 es Fine-grained terrace alluvium '. '00. 0 . 0 0 . ° . es Eolian sand and loess Idf Debris-flow deposit Cca Carbonate soil _ Figure 20. Sketch ofstratigraphic section exposed in gravel pit and trench near Webster Hill, north ofCol orado River (NWI/4. sec. 28, T. 6 5., R. 94 l'v.). Section orientation shown on Figure 19. Not Drawn to scale. Colorado Geological Survey 27 ?1-\o GUIDEBOOK FOR DAY 3, PART II OF THE 1997 FIELD TRIP ROCKY MOUNTAIN CELL OF THE FRIENDS OF THE PLEISTOCENE LATE TERTIARY, QUATERNARY, AND HOLOCENE GEOLOGIC mSTORY OF GLENWOOD CANYON, COLORADO· A STORY OF GEOMORPHIC DEVELOPMENT AND RESULTING OBSTACLES IN GEOTECHNICAL ENGINEERING OF INTERSTATE 70. FIELD TRIP LEADERS: JON WHITE AND BOB KIRKHAM COLORADO GEOLOGICAL SURVEY 1313 SHERMAN STREET, ROOM 715 DENVER, CO 80203 (303) 866-2611 GEOMORPHIC DEVELOPMENT OF GLENWOOD CANYON Glenwood ~anyon is a 3,000 foot deep gorge cut into Lower Paleozoic and Precambrian rocks across the south limb ofthe White River Uplift. The canyon extends from Glenwood Springs at its downstream end nearly to Dotsero at the confluence with the Eagle River. See Figure #1. Several researchers have speculated on the geomorphic development of Glenwood Canyon. Hunt (1969) thought the canyon was cut during the Pliocene and Quaternary as an antecedent stream that was cut into softer Pennsylvanian and Permian rocks between the core ofthe White River Uplift and Miocene basalt rocks on the south rim of the canyon. Szabelak (1984) described three distinct canyon rims and inferred that each represented primary episodes of river downcutting. A thick sequence of7.7 Mabasaltic lava flows cap Dock Flats on the south rim ofGlenwood Canyon (Kirkham and others, 1995; Streufert and others, 1997). A thin remnant of what appears to be this same 7.7 Ma flow, along with a thin deposit ofcobbly, mainstem Colorado River gravel, lie on Spruce Ridge about 200 feet below the south rim ofthe canyon. (Streufert and others, 1997) This suggests that only a very shallow ancestral Glenwood Canyon had been incised by 7.7 million years ago. Gobble Knob, the next ridge east from Spruce Ridge, is capped by another, somewhat thicker, basaltic flow which lies about 500 feet below the south rim ofthe canyon. This flow has been dated at 3.0 Ma and has been correlated to a small, eroded cinder cone built upon the 7.7 Ma rocks that cap Dock Flats. If the Gobbler Knob flow was erupted onto a flat surface at or near the level of the Colorado River three million years ago, then only minor downcutting of the river occurred between 7.7 and 3.3 million years ago, whereas more than 2,000 feet of canyon has been cut during the past 3.3 million years. At the close of the Pleistocene the canyon bottom was cut to its lowest extent. Flo~ rates during the glacial age eroded narrow gorges and scoured deep pools into the basement rock floor. Throughout the Holocene the canyon bottom has been aggrading and the sco1,1red bedrock surface has been buried by more than 200 feet of sediment. This sediment aggradation is the result ofrapid rock mass wasting (rock slides, rockfall), debris fans, and limited-transport alluvial deposits from the tributary creeks and intermittent drainages. Along the canyon side today are many FOP, Day 3, Part II, Page 1 b-\1-- ~---~~~~------ examples of thick talus aprons, shear canyon cliffs descending into the river, and large rockslides that cover the canyon floor. The more recent slides are barren rock that still have a fresh broken appearance. The older slides show rocks that are more worn and lichen covered, and have developed a soil mantle. GLENWOOD CANYON 1·70 PROJECT SUBSURFACE INVESTIGATIONS Construction offuterstate Highway 70 through Glenwood Canyon was a major engineering feat that took 13 years to complete at a total cost of $480,000,000. The Colorado Geological Survey was fortunate to be a part of this project and, by contractual arrangement with the Colorado Department of Transportation, staffed the Canyon Geotechnical Office during the duration of the project. Prior and during the construction ofthe Glenwood Canyon futerstate, preliminary engineering geologic investigations were conducted along the proposed highway alignment. The bulk of the subsurface investigations were test borings, with more than a thousand drilled. The borings were generally taken down to a depth where packed river gravels (that offer excellent bearing capacity for highway foundations) lie, or bedrock was entered and verified, whichever was encountered first. This wealth of information has been useful in examining the Late Pleistocene and Holocene depositional record within the canyon and to determine the geologic history that resulted in the varying sediments found. While investigatory work progressed, it grew apparent that surficial deposits overlying bedrock were very complex within the canyon's interior. fuvestigative borings showed that in many . areas ofthe canyon the overlying sediments were exceptionally thick, not what would be expected in a mountain canyon environment. Often drill borings were advanced more than 200 feet into colluvial, alluvial, and probable lacustrine deposits. Most test holes were stopped without encountering basement rock. The hundreds of drill logs by CDOT that did reach bedrock showed highly variable depths in short lateral distances. At some bridge locations buried bedrock elevation changed 100 feet or more from one hole to the next, just a few feet away at the same pier location. As investigations continued it became apparent that thick deposits of interfingering and mixed alluvial and colluvial sediments had aggraded in the canyon. Most startling was that the entire eastern half ofthe canyon contains a deposit ofclay, silt and fine sand that ranged from 30 to 60 feet thick. This deposit, known during project construction as the notorious 'Gray Layer' is underlain by river alluvium with mixed colluvial rocks, and overlain by roughly 30 feet of colluvial rocks in alluvial fines. See Figure #2. These deposits were increasingly complex as interfingering and lateral mixing occurred at slope ~dges where talus and debris flows were incorporated, and at the mouths of tributary streams (i.e., French Creek, Tie Gulch, Dead Horse Creek, Cinnamon Creek). It was unclear early on what caused the deposition of the clay and silt sediments and speculation arose as to whether rockfall, debris flooding, peri-glacial activity, or perhaps even fault movement had occurred. (Hynes, 1983) FOP, Day 3, Part II, Page 3 ~-\3 As the extent of such deposits became known, geotechnical and foundation engineers were forced to change initial assumptions ofdesigning shallow foundations on packed river gravels with excellent bearing capacities thatoverlie shallow bedrock. The actual profile consisted of thick soft compressive sediments that require consolidation times and expensive deep foundations. These compressible deposits were not evident at the surface and required increasingly detailed subsurface investigations for highway structures. It was the detailed investigations at the Shoshone Interchange, where the highway crosses the river into the Hanging Lake Tunnels, that verified the prevalent theory that a rockslidecompletely dammed the Colorado River and was responsible for a Holocene lake and resultant lacustrine deposits. See 19cation in Figure #1. LATE PLEISTOCENElHOLOCENE DEPOSITIONAL HISTORY At the close of the Pleistocene Epoch post-glacial aged river flow rates likely decreased. Holocene history within the canyon is marked by deposition rates that exceeded the rate ofriver bed load transport. A major contribution to the aggradation of the canyon floor was the rock slide that occurred near the Hanging Lake Rest Area in a narrow portion of the canyon. See figure #3. The Holocene lake that the rockslide created silted in and filled quickly from the sediment load of the Colorado River. Concurrently, colluvial material from the canyon side continually fell onto the canyon floor. That lacustrine deposit, the Gray Layer, is predominantly dark gray to gray black in color. Commonly within the deposit are reddish brown bandings or laminae. Presumably, these red bandings were the result offines settling from single storm events. The storms occurred up-river in the McCoy, Bond, State Bridge area where red mudflows from steep, rugged tributaries within the Maroon Formation redbeds enter the Colorado River. On several occasions, during the history of the Glenwood Canyon Project, we have observed the river turning a chocolate-red color as it FOP, Day 3, Part II, Page 4 Water level Cottonwood Rapids r A'--__-..\ . •.q P.:.~ (j ...... '=?: :i? '0·'0 I "._"'-'... _ .., . "'-"'--1... _." ",-,,---1... _ ...... - ...--: Figure #3. Schematic Longitudinal section ofrockslide that created Gray Layer. Based on COOT boring logs. Not to scale. becomes hyper-concentrated with clay and silt. The river sediment load at these times gives it the appearance and flow characteristics ofchocolate mille Partially carbonized mats ofvegetal material are also common. They are frequent enough to suppose that vegetation floating in on the Holocene lake from tributary debris flows and sinking en-masse were typical Holocene events. Conventional 14C dating was done on this organic material recovered from near the top and bottom of the Gray Layer deposit. Age dating indicated that the lake existed from 9,820%130 14C yearsB.P. to 3,890%120 14C years B.P. (J.B. Gilmore, 1984, person. comm.) From4,000± y.a. to the present the gray layer has been buried by a mix of alluvial sand and pebbles and colluvium from the canyon sides. Drill hole data seems to suggest that the natural dam formed by the rockslide was never breached. The lake behind completely silted in and was covered over by the mixed alluvium/colluvium such that the river bottom rose to the top of the rock slide, creating the Cottonwood Falls that are seen today. ~ee figure #3. GEOLOGIC HAZARDS OF GLENWOOD CANYON Glenwood Canyon is affected by several geologic hazards. They include rockslides(falls), debris flows, ice falls, and avalanches. Rockfall is the most critical geologic hazard and $10,000,000 was spent on rockfall control during the Glenwood Canyon Project. As recently as 1995, fatalities have occurred within the canyon from falling rocks striking vehicles. Rockfall mitigation techniques developed and used in the canyon include protection devices such as roadway grade or alignment change, catchment ditches, elephant-trap ditches, slope reconstruction, reinforced earthen impact walls, a variety offences, ice attenuation posts, hanging chains, and hanging tire attenuators across active rock chutes. Rock stabilization methods such as rock scaling, trim blasting, rock bolting, anchored cable/cable netting/wire mesh, ~hotcrete, and anchored concrete buttresses were also used for rockfall mitigation. Rock scaling and trim blasting involves removal of loose and unstable rock from a cliff or steep rocky slope by technical rock climbers using steel bars as levers, jacks, or blasting. Earthen walls reinforced by geotextiles are FOP, Day 3, Part II, Page 5 capable of stopping large rocks at high velocity and impact energies. Large basins, referred to as elephant traps, are excavated into rock slopes above the roadway, and catch falling rocks from above. Hanging tire attenuators consist of closely spaced columns of stacked tires on steel posts, hung vertically from a horizontal cable that spans an active rock chute. Rocks falling down the chute impact these columns that either stop or considerably slow down the rock on the slope above the roadway. A single innovative rubber tire wall was installed near the west portal of Hanging Lake Tunnel. About 400,000 tires went into the fill and fabricated rubber blocks that face the wall. Unfortunately, the wall spontaneously caught fire in 1995. The upper portion ofthis tiered wall has been removed and the exposed eroded cut slope has been draped with wire mesh. Two methods we used to control falling ice were hanging heavy chains and ice attenuation posts. The chains prevent large ice chunks from falling en masse. The array of grouted posts in the ice fall runout path are positioned to break up the ice into manageable sizes before being stopped by a fence. STOP #1 Hanging Lake Rest Area, Interstate 70 Park at Hanging Lake Rest Area and walk westward onto the bike path. From Public Service Bridge several geologic hazard mitigation methods and structures (rockfall) can be seen protecting the highway and on-ramps. The tiered tire wall is on the south side of the slope above the west bound on-ramp. Continue down the bike path to bridge piers. The rockslide location that dammed the river 10,000 y.a. can be seen across river. A broad, joint defined, wedge failure on the steep canyon side resulted in a slide onto the canyon floor of approximately 450,000 cubic yards. The railroad cuts into the large fan ofrockslide material. Adry stack rock retaining wall supports these cuts. Below, next to the Hanging Lake off-ramp bridge piers, is Cottonwood Falls, a class VI rapid, even after alteration by construction.and grouted rip-rap; it is unnavigable. The detailed subsurface investigations for the overpass mainline bridges and the eastbound off-ramp showed conclusively that the gray layer lacustrine sediments began where this large fan ofmaterial and Cottonwood Falls exist. Downstream, further borings show up to 200 feet of random deposits of very large angular rocks in an unsorted matrix, all oflocal Precambrian origin. Upstream the Gray Layer is relatively constant in thickness and depth to the opening ofthe canyon. Currently the extent ofthese Holocene lake deposits up-valley, outside Glenwood Canyon is unknown. STOP #2 optional Hike to Hanging Lake Massive steep ledges ofHolocene and Pleistocene(?) Tufa, precipitated from mineral-charged spring water, has dammed Dead Horse Creek to form a small lake with exceptionally clarity. (Kirkham and others, 1995). The hike on maintained USFS trail from Hanging Lake Rest Area is 1.8 miles round-trip and covers 1,000 feet in elevation. FOP, Day 3, Part II, Page 6 b-l4> SELECTED REFERENCES Colorado Department of Transportation, 1983-1989, Glenwood Canyon Interstate 70 Project Plans, Ralph Trapani - Project Manager. Colorado Department of Transportation, Materials Laboratory, Geology Unit, Glenwood Canyon Project archived drill logs, John B. Gilmore, CDOT Chief Geologist. Hynes, J.L., 1983, Geology of the Glenwood Canyon along 1-70, in Hynes, J.L., ed., Proceedings of the 33rd Annual Highway Geology Symposium: Colorado Geological Survey Special Publication 22, p. 136-145. Hunt C.B., 1969, Cenozoic history of the Colorado River: U.S. Geological Survey Professional Paper 669-C, 130 p. Kirkham RM., Streufert, RK., and Cappa, lA., 1995, Geologic map of the Shoshone quadrangle, Garfield County, Colorado: Colorado Geological Survey Open-file Report 95-4. Streufert, RK., Kirkham, RM., Widmann, B.L., and Schroeder, T.S., 1997, Geologic map of the Cottonwood Pass quadrangle, Eagle and Garfield Counties, Colorado: Colorado Geological Survey Open-file Report 97-4. Szabelak., S.A., 1984, Colluvial slope estimate and driven anchor field experimenting portion of Glenwood Canyon, Colorado: Golden, Colo., Colorado School ofMines M.S. thesis; 180 p. FOP, Day 3, Part II, Page 7 ?J-\1