Title:

. Authors:

ABSTRACT

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Recognition of active but "non-seismogenic" salt-related. deformation within an extensional environment, west-central Colorado

Robert H. Kirkham and Randall K. Streufert, Colorado Geological Survey

During the past five years the Colorado Geological Survey has mapped the geology of seven 7.5-minute quadrangles in the Glenwood Springs-Carbondale­Dotsero area of west-central Colorado, which lies about 45 km west of the northern end of the Rio Grande Rift. This region contains widespread late Cenozoic volcanic rocks, including the 4,15O-year-old Dotsero volcano, it has experienced historic seismicity, and several previous investigators have reported evidence of late Tertiary and Quaternary structural deformation partly attributed to salt tectonism.

During the initial phases of our project we discovered several geologically young structures which could be interpreted as evidence of significant paleoseismicity, suggesting this area had potential for large future earthquakes. As geologic mapping progressed, we encountered many more young structures but began to realize that these were unusual types of structures unlike those typically found in extensional environments. They included diapiric valley anticlines which fold late Pleistocene terraces, synclinal sags, ductily folded, yet brittle volcanic flows, intrusive contacts between evaporitic and clastic formations, large topographic troughs in outwash terraces, and extensive deposits of bro~en and brecciated rock. These unusual structural features lie within a 650 knf area where Pennsylvanian evaporitic rocks occur at or near the ground surface. Late Tertiary volcanic rocks are downdropped up to 1,300 m within this collapsed block. Deformation along the boundaries of the collapse block include monoclinal folding of late Tertiary volcanic flows and relaxation or unfolding of the Laramide Grand Hogback Monocline. Based on preliminary mapping, the U.S. Geological Survey suspects that another large collapse center may exist in the nearby Eagle-Gypsum area.

We interpret this widespread, geologically young structural deformation as resulting from regional and localized collapse due to flowage, dissolution, and diapirism of underlying halite and gypsum in the Pennsylvanian Eagle Valley Evaporite. Flowage and diapirism play important roles in the process, but long-term dissolution is the principal mechanism that is responsible for the huge crustal volume loss that leads to the regional collapse. This tectonic interpretation greatly affects the seismogenic potential of these structures. If these features were related to crustal extension, they could be capable of generating potentially damaging earthquakes of magnitude 6.5 to 7.0. If, as we suspect, they are a result of salt tectonism and collapse into dissolution cavities in the evaporite, then their seismic potential is much lower. Evidence of Neogene extensional tectonism, if present, is masked by the salt-related deformation.

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".,' .: Figure 1. Location map of the Glenwood Springs-Carbondale-Dotsero area~.west-, central Colorado. Project area lies about 45,km west'of the northern end of the Rio Grande Rift. . 1.,

This figure has been prepared as a slide for the GSA talk, but I cannot find a paper copy of it in my files. As I recall, it should be suitable as a page size or preferable a slightly smaller figure.'

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Figure 2. Status of geologic mapping of 7.5-minute'q~adrangles. . ~ - ' . - , .. ~. . ' . . ;" ;,.", ;',

The Colorado Geological Survey (CGS) has mapped, the . geology of seven ' 7 .5-minutequadrangles in the Glenwood Springs"7CarborlCfala:-oOtsero'irea' during the past five years. Project work has been fund8d:1n',pa'ri;'bythe U~S~, ~' ". Geological Survey STATEMAP program of the National:,Geologic Mapping Act of . 1992, Agreement No. 1434-HQ-96-AG-01477. The CGsplan:s':;'t6'map;fouradditional

'quadrangles ,in this region'during 1997 and 1998~:The':USGshasan active, ' mapping program 'underway in adjacent areas and'ha!{been,:;workingclosely with the CGS during the past two years. ',. ' .

. We have 2~e~~ions of this figure avai lable:1) done~,~s'a:Sl'ide for the GSA: talk and, 2) one being prepared as Figure 1 ofthe:Carbondale'sxplanation. I prefer to use the latter for this poster session,· but 'the: stipple . pattern ' highlighting Carbondale quadrangle should be delete~.:", . ' .' '

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'." . Figure 3. Historical earthquakes in the Glenwood Spring-Carbondale-Dotsero [,:.:" area (from Kirkham and Rogers, 1985). ! I

t' DATE LOCATION INTENSITY HAGNITUDE \ I ,

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10/19/1990 12/12/1990

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The historical earthquake record indicates that small earthquakes·are relatively common in the region. A swarm of 34 earthquakes in 1984 were in part recorded by a portable seismic network operated by the USGS (Gote~ and others, 1988). Another swarm of earthquakes occurred in 1986 in the vicinity of Treasure Hountain Dome (see Figure 5), a short distance south of the project ,area (Bott and Wong, 1995).

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Figure 4. Occurrence and ages of Neogene igneous rocks (larson and others, 1975; Tweto and others, 1978; and Kirkham and Widmann, 1997). Dashed line with barbs indicates approximate extent of Carbondale collapse center.

Neogene igneous rocks are widespread and locally voluminous 1n this area. They include the 4,15O-year-old crater and flow at Dotsero and the Holocene? cinder cone at Willow Peak and its associated flow. The Neogene igneous history suggests significant extensional crustal fracturing has occurred in this region during the late Cenozoic.

Use Figure 4 of Carbondale quadrangle explanation, but delete the box that outlines Carbondale quadrangle.

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Figure 5. Regional tectonic map (modified from Tweta, 1977; Tweto and others, 1978, and Kirkham and Widmann, 1997). Stippled pattern indicates area where evaporitic rocks lie at or near the ground surface.

A number of major structural features are found in this region. Prior work by Hurray (1966, 1969), Mallory (1966), Tweto (1977), Soule and Stover (1985), Stover (1986), and Unruh and others (1993) described evidence of late Tertiary and/or Quaternary defonmation on several structures in the area. Host studies suggested diapirism and flowage of evaporite was responsible for the defonmation. Stover (1986) and Unruh and others (1993) proposed that dissolution could also cause some of the individual structures.

Use Figure 2 of Carbondale Explanation for this figure, but delete the box outlining Carbondale quadrangle.

Figure 6. Geologic map of the Carbondale quadrangle..,

Based upon the earthquake record, Neogene igneous history, and previous reports of Neogene defonmation,' we began our mapping project expecting to find young faults and folds related to both salt tectonism and crustal extension. Earlier reports of Neogene structures were confinmed, 'and abundant new evidence of geologically young defonmation was discover9d during the mapping program. , .

Much of the defonmation consists of structures such· as (1) tilted and faulted basalt, (2) linear or arcuate synclinal sags, sOme with faulted limbs, (3) intrusive contacts between 'evaporitic rocks and clastic rocks, (4)'sets of orthogonal' faults, (5) circular, elliptical, rectangular, and irregularly shaped bowl-like structural troughs of varying sizes,~. many of which contain locally derived sedimentary deposits, (6) a large, arcuate half graben whose floor was occupied by a lake until drained by homesteaders, (7) valley anticlines, some of which affect late Pleistocene outwash terraces, (8) complexly defonmed, highly broken and brecciated blocks of randomly oriented bedrock that we infer is collapse debris, and (9) abundant sinkholes, including both older ones and active ones.

Use full size copy of CGS OFR 97-3 (Carbondale Quad)

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Figure 7. Distribution of shallow evaporitic rocks and geographic relationships with the Carbondale and Eagle collapse centers (after R. Scott, 1997, written commun.)

We believe that much and perhaps all of the Neogene structures documented in the Glenwood Springs-C8rbondale-Dotsero area are due to dissolution, flowage, and diapirism occurring within thepennsrlvanian Eagle Valley Evaporite. The deformation is localized within a'650 km topographic depression which we call the Carbondale collapse center. The collapse center coincides with an area where evaporitic rocks are at or near the ground surface. Late Tertiary volcanic rocks are downdropped 1,000 to 1,300 minto the collapsed area. Based on preliminary mapping, the USGS suspects there is a second collapsed block, the Eagle collapse center, near the towns of Eagle and Gypsum.

Use slide 2 (collapse center and evaporite distribution) that is being prepared for the NAWQA talk.

Figure 8. Schematic cross section across the Grand Hogback Monocline.

Use Figure 5 of Carbondale quad text for Figure 8.

Figure 9. Photograph of uphill-facing fault scarps in early Quaternary/late Tertiary basaltic gravel.

Scan attached slide or ask me for a laminated photograph for Figure 9.

The following caption should be centered beneath both Figure 8 and Figure 9.

The Grand Hogback Monocline is a late Laramide feature with over 6,000 m of structural relief. It separates the Piceance Basin from the White River Uplift. It also forms the western margin of the Carbondale collapse center. A sequence of 10 Ma basalts that unconformably overlie an erosion surface cut across the monocline have been tilted eastward into the Roaring Fork Valley and faulted down-to-the-west by a series of bedding plane faults. Early Quaternary/late Tertiary basaltic gravel derived from the basalt cap is also offset by the faults, and late Quaternary deposits are locally broken by some of the faults (Stover, 1986). Maximum throw on individual faults in the Miocene basalt 1s about 100 m, whereas 1n the basaltic gravel it is about 30 m. Late Quaternary deposits are offset up to 4 m.

Murray (1969) suggested that flexural slip could explain the faulted basalt that overlies the monocline. Unruh and others (1993) proposed that this deformation was result of flowage of evaporitic rock out from beneath the monocline. We concur with the flowage model, but add that dissolution may also playa role in the relaxation or unfolding of the Grand Hogback Monocline.

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Figure 10. Monoclinally folded basalt flows 'along the margin of Cottonwood Bowl.

The flat surface in the left-center of the photograph is Dock Flats, and the hurrmocky ground in the center is eottonwoOdBow1. Dock Flats is capped by a flat-lying sequence of 7.7 Ma basalt flOws, that' are monoclinally draped over the rim of Cottonwood Bowl at dips of up to"44°.'Tt.e flows return to a subhorizontal attitude beneath the floor of the boWl. Locally derived sediments were deposited in the bowl over the 'basalts post-collapse and have since been sculpted by erosion, creating thehurlmocky ground on the floor of the bowl. '

Scan attached slide. . ' ---------------------------------------------------------------------------

Figure 11. Photograph of Spring Valley, a large half graben formed by dissolution or flowage of underlying evaporitic ,rocks. The basalt cap on the right is broken by several faults and loeallyfolded by synclinal sags. Hurrmocky terrain in the foreground is collapse, debris which is sliding into the subsiding half graben. The floor of Spring:;Valley was occupied by a lake when homesteaders settled the region, suggesting the collapse is active. The homesteaders hand dug a ditch to drain the lake and, fanm the fertile lake sediments.

Scan attached slide.

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Figure 12. Photograph looking north across~USChkel, Park, a faulted synclinal sag. <,~"

Scan attached slide.

Figure 13. Schematic north-south cross section shawi'rig, the Heuschke1 Park synclinal sag.' , ",

Use the GSA slide, but add meters to the vertical scale. . . . -.

The following paragraph should be centered beneath both Figures 12 and 13.

Heuschkel Park is a faulted synclinal sag with about 300 m of structural relief. The north limb of the sag is fonmed by a fault near the top of the prominent escarpment. North of the fault the basalt cap dips moderately northward and is broken by an orthogonal set of faults. On the south side of the fault the basalt flows dip steeply southward paralleling the north wall of the park. The flows are essentially flat-lying beneath the axis of the sag and then rise sharply upwards on the south flank of the sag.

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Figure 14. Folded Pleistocene terraces along'the,Roaring Fork River • .... ,. ,.' ;, ..

The surface in the sunlight in the left2center of the photograph is a pre-Bull lake? outwash terrace that overl1esthe,Eagle Valley Evaporite' and is capped by basaltic gravel deposited in a fan ,at the mouth of Fourmile' Creek. The distal and downstream end of this surface ,has been uparched by diapirism at least 30 m. The late Pleistocene terrace'across the river also has been folded but to a much lesser degree, demOOstrat~ng that the diapiric upwelling is recurrent or episodic.' ,"" ',' ..

Scan slide. ' ..

Figure 15. Photograph of Shippes Bowl. . ' ,"

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This praninent, oval, bowl-shaped topOgraphic depression has the appearance of an impact crater, but we believe ,it is a result of~collapse a dissolution cavern or from flowage ofevaPo~itic,rock out from beneath.

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Figure 16. Photograph of a recent sinkhole, fo~:,in Pennsylvanian' Eagle Valley Evaporite. "','0',',' ' ..... '. Scan slide, or use the print that I have. ;~,; .' , ....

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Figure 17. Photograph of a recent sinkhole de~el~ped in fan d~its'overlYing evaporitic rocks.';,::" "",'"

Sca'n sl ide.

The following paragraph should be centered beneath the two photographs.

Recent sinkholes demonstrate that dissolution is an on-going.process. They pose hazards to buildings, highways, and other structures.

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Figure 18. Salt loadings' in' Yampa hotspr:ings and the Colorado; Riv~r:'.'~ (,:~J;;;:';./ , ". \,' . .' '. . ' ~'.: ... :' ,~',:,', ... .' ..

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, To evaluate the. feasibility of sait dissolution as being the primary, mechanism for regional ,collapse, the modem salt loadings of area hot springs and the Colorado River>were reviewed. About' 240 tons of sodium, chiloride, calcium, and sUlfat'e are : discharged everyday from a single spring, Yampa hot spring. This is equiyalen(,to one sinkhole measuring 1 m wide by1 m long by about 80 m deep forming every. day! , ' ' '

, The Colorado River at:' CameO~ 'downstream of the collapse area, carries a total 'dissolved solid:Jood,:of'1.5 million tons per year. At this'rate one ' cubic mile (4.2',cubiC km)'of,haliteandgypsum is dissolved in only 6,700 years. ThesewatBrqualitydata indicate dissolution is a reasonable mechanism to account forthe1arg~scale, regional" collapse theory. . . . ,", .

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CONCLUSIONS. '

, " ··,We '~in~~pret, the~~i~~spre~d, Qeological1y young striJctu';~f:'~eformation observed An ,theG1eowooct Springs-carbondal'e-Dotsero'area as resulting' from , regional "and local ized,:collapse 'due' to flowage, disso1ution~::and diapir,isrD of under.1y·ing,·hante 'and,-gypsuin';;in the;Pennsy1vanian Eagle V&.l1ey, EV,8Pof:ite. ,This interpreta:tion:greatlY,'affects'the'seismogenic potential Of,ttJese'st'ructures. ' If: thes9'featlires:werere.late(J, to cru'sta1 extension, they,:c0u1d~:bS"C8pab1e of· generatingpotEmtiallYY'damagingearthquakes of magnitude6.5'tO 7~oi :If~~'aswe

. suspect,':they" area result; of : salt- tectonism and collapse' intodi'ssoliJti'on',-:' cavitieswfthin': the,ev8Porite;then, their seismic potential 'is much'~lower.' Evidence of Noogene extensi'onal 'tectOnism, ',if present, is, maskedbY.the' 'salt;:", re1ated·:dt~forination~ , ,: ' ,",' ,,' " ",' .' .

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REFERENCES . ,.'

Barre~t, J.K;~ .·~nt~atJ, R.H. ,.197~~~Hydro'~010g1Cal data of the mal' springs and . :;,wells"1n.,CO.lorado:,'.Colorado. Geri.1ogjcafsurvey I nfo rmat 1 on Series 6,· 124 p.

Bass; N.W.~)md NorthrOp~.,·S·.iA.~.:1963{.Geologyof Glenwood Spr1ngs:quadrangle and .,' " "'. 'l~";" : .~,~,l·~· (. ,\.' ...... , • " . "I', ," .' ,,~, ""'., ", " ..... , ' ,

'. ' :v1cinity, ':northWestern. Colorado: ~U~.S.·~Geolog1cal SUrvey. Bullet1n··1142-J, 74 p. Bott;J. D~J.;,: :an(FWong,':I~'G~·,,·.1995";Ttie::!1986i,Crested Butte earthquake .swarm and its

. . :1mplications;;for3seiSniogeriesis';{iri:Coiorado: Seismological' Soc1etyof' America , ," T;'B,~11~t,in};y~,.:.:~5{Y~~.<5~/p~':,:.14.~~vj~90Y::,:~<':" ;. ',' .. ;' ,',';':"''', " ....... . Bryant,"B.·"'and;'Shfoba·,-t'R~R.;::'1997 ~;':Revise(Lprel1m1nary. geologic map of the Stom'

,.:."·-' ... K1rl'~;.HOu~~1'n.~~.~~~~r:~~~.1~l:,:Gar.:(!~i~;h~~nty, .. ~10rado:.'·U~S~>Geo10g·ical Survey '. Ope~Fil.e; .. ~RePO""';:'(:1n':·pre·pa· r'ation) 'i:: \'.'7': ,; '::.'.,:, c·.'. ..... .. ,,' ,,-,. ':". . ' .

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Carroll ,·;'C.J~':~.::K1r:kfi8m'~~{R~'tt·~./amrstell1rig·;;:pa,~·,1996, Geolog1c'!,map'of:the' center . • •.. ' ',: ,;. '... .• ~,~ " ', ... y, l t' ,. , '. . ' ,. -,~ ;" .' .' .,. :""" ~ .',,' :'i- •• . • (- ..

. . .: .·Mounta1n.~.quadrangle~; .. Gar.f,1el(t,;COunty;i,~,Colorado: Colorado;Geologica1.Survey . . :.:.:':.ope~,f.}j,~~"~P9~{.P~2,~;·j.;:;:L:~;··:';;),H~-;~>::\:;~'~;~~:':~:;~{':"", . ,.,;,;', , ,.:. '.: ~'j'::>" . .',;: .. , .. ' .. ' Goter.~S. K.;,.f>resgrave,~:B.W.';Henr::isey,;:iR.F.,,,:.·and· Langer,. C.l,·.;1988,The Carbondale,

. , . ". ' .. ,".:Co:l·orado~:earitIQ'uaj(e:swarm\of~Aph1.'~'~"aY~'984:" U ~ S~"!'GeO 1 og1 c8 t'Survey . Operi-F1l e .... ... '. 'RepOrt;,;88+41,7,~'\1'~,:~'p/;~: ::.: .. ;,j ;:!,;:>\?t;~;.;~)J{(~!;:;fJ~~:;i:»t~:; i:;'':,: :.', :i >::)',\:>,,:)c:' .'. . Kir.kham~, R~M.'·~:,·:~Bryant,·:Bruce,~;St.reufert;iR;K~:t-:arid/ 'Shroba"'· R~R~ ,; ;j 996a~;'fie ldtri p .

I .... ..''': " ";gu1d~POOk' 9n(the:;geology.,lan:~i9Ej,Qlogj:Cttiazar,~'s.: of~,;the'GleriWoOd;spr.ings area,

!L,.~;-.' .. : :" .C~l~t~d#~Lin:cJ~~~'.l~U~~:~~:i,:~I:I~~~§~'~~~",~.~.:i.~:aij~.'~~'.llm.o~~:;~P~.~~:,:')!d~~" .Ge~l,?gic . " c, "::';if~,xc:~r.s~i~~~,;\y»~~,~~kY<!4~,~~~~IJ~:f!r-"d,~~~.eI1~;,,::Rie,1~~r1~,:gu,~.e.~~,for:the, ·1996 i... • '.>." annual·'meet1ngof.:too'~eologicaUSociE¢y;;'Of.Amer1ca:,ColoradoGeological' . i~ " .:: .':';s~urveY;·~~,~1a~~b11cat,o.~~;.~;.;+·{:.;':;:>-?:;;z~1t:~~:W'.,'I(;>".: ';;:,: -:'::;':,:~":':~'-J'i~~;" .,.'. . r" .. K~~k~~,.,.~;~~~;::\an,9·;R.~gersi;·W~~~~cJ~~.~~0.£.g.J'1qr~~~'te~ljttJquak~ ,d~~~l~a,nd·7;·i;\:,"~i .. :: ... :'., r, .... < ':·;.;·::interp~tat~ons71867 :to··.198~::.;·Col.onldo~GeOlog1cal:Survey.:.'Bulletin'.46~."105 p.' f:'. '. : Kirkham'~; R~,M~'~;I~streufert,' R. K~:',! and :iC8P·pa;.;;1I;~7A';i;iU~1995a·;;~GeolOgi:(t'niap;:'cJ.f·::.the Glenwood' ,. .'. ':.' " ~ ,:;Spr.ings:quadrang1e": Garfield'COlJrity~::.8co'r6rido:;:'COlonidO:G901ogical-survey: ' V '. f" . .O~~ffi e':R~~o.·ft .. 95~3 .;:,' ': .. ~', '; '{ '}~::::~i~'; ::~r'l~~:"~j1:1r~::~'f';':\:":}"::~' : :'(, :\;":'~Ut;0.;:~r>;; '::: :. ;'~:, :...... . r .. ..1995b~;'Geo10gic·map of:the .. ShQshOne~'g~adrangl~~';,Garfield5Co.Ul1ty'''·:;Qo]orad0: ' . ~""". . , . 'Colorado "Geologica1 Survey :0p8n;f~~1·e~ReP9·rt'~9!r.4·~:,' ~.~.:~/;'t,::"'~::'<'<"'?':':> ,,'. 'f' " •.• '.' ••

r;::· .. I<i rkham,R~M. ~ 'Streufert, 'R. K,~.);'Hem¥:tgX;~,t~!H~rz~~.n(fst~JlJ~g~o:I~~'~:~,;',~:1~~~~'~:,~Geo,10g1c t ,map of :the Cattle Creek quadrangJe; ··~G~r:f,1,eJd·. County,; I. :Cf;lJor~do:,'~~,Cf;ll~rado,: ',' I';'> Geological SurveY,Open.:-File Re'PO~·;9fki~:~1\·Y:·'· .. ' ',. . .>:':', ' ':':'.' .

Kirkham, R.M., and Widmann, B.L., 1997, Geolog1c'map of the Carbondale,quadrangle, , .. i Garfield County, Colorado:.COlorado ,Geol()g1'eal:Survey' Ope~f.i1'e\Rep(;rt',97~3.·

Larson, E.E., Ozima, M., and Bradley,'W~.C~~>,t975,'~,Latecenozoic,'b8sic;Vo1canism,in northwest Colorado and its implicationst!c~oncerning 'tecton1sm·:'anci'.or1gin of the Colorado River system, in Curtis, Bruce;: .... ec.C, 'cenozoic.'.~1storyof··the Southern Rocky Mountains: Geological Society'of'Aiilerica Memoir 144;. p-':·'15~178.

Mallory, W.W., 1966, Cattle Creek Anticl ine;: a salt diapir near' Glenwood Springs, Colorado: u.S. Geological Survey Profess'i'cnal Paper 5S€rB,p.B12-B15.

_____ 1971, The Eagle Valley Evaporite,northwest Colorado-a regional synthesis: u.s. Geological Survey Bulletin 131 kE; 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.

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.

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

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,,' ':," ,~. Valley; ~lJd .adjacent.,areas·, . ~ilin+v Colorado:Colorad(fGeological:'J:~:'; .. ' .: ... . . . . '; , : "',surVeY·Open~fi1e/Report 85-71..... . .. ' . . .'. ./.' '.".:-".:.: '~."" ... . . : ~·:Stoveh:.B~K: ~r1986~~~eo.l0giC· evidence 'of: . .rnary faulting near 'CarbOndale, .

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Streufert, R.K., Kirkhani~·R.M., SchrOeder,::r'.S., and Widmann, B.L~, 1997a, Geologic :. map of the: Dotsero quadrangle, Ea'gleand Garfield Counties,' Colorado: Colorado

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quadrangle, "northWest :colorado::::~U;.;S~!2.~Ge·olog1calSurveYJHiscel1a:neous'; ":.' " ; Investigations ·Hap';I7999'~·::./:;;'<:;';;'~':;;,:':)·:~f~): ,'. " '::'''~'' .' 'Y··'... '

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Arkansas .·Project',··northwes'tern;C019radQ:;,:unpublished report prepared, by' William R.· Lettis'& .. AssC>Ciates:,and<wOodVard-Clyde' Consultant.Sdfor U~S. Bureau·· of Recl8mat1on,'.)~~~·i~~,:;;.,~ '. ";",,< ;·';St:y.~> ',:' '" .:;:' , .. ..

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