U.S. DEPARTMENT OF THE INTERIOR GEOLOGIC INVESTIGATIONS SERIES U.S. GEOLOGICAL SURVEY I-2656 Kp Kp in northerly dipping shale of the Niobrara. Principal bedding-plane zone of failure com- the map area, contact was mapped at base of 0.6–1-m-thick, light-gray dolomite that DISCUSSION OF STRUCTURAL FEATURES depositional voids or soluble minerals such as evaporitic minerals (J.M., Soule, Colorado Geological Qcg Kp 14 QTg QTdi CORRELATION OF MAP UNITS Kp Qcg Qcg monly occurred in shale just above basal limestone of the Niobrara Formation. Proba- pinches out abruptly northward (South Canyon Creek Dolomite?). Farther north, con- Survey, oral commun., 1998) (table 1). The Eagle Valley Evaporite (hee) and the Eagle Valley For- Kn Qcg Kp Alluvial Alluvial and Structural features in the map area include faults and folds of differing ages and origin (fig. 1). 20 Qcg QTg 13 bly formed principally by slow, deep creep that probably was most active during wet pe- tact was placed where finer grained, orangish-red beds of State Bridge Formation mation (he) are respectively the principal evaporite-rich and evaporite-bearing rock units in the Kp Artificial fills deposits Colluvial deposits Colluvial deposits Most faults and folds appear to be related to deep-seated, regional tectonism. Some folds and a Qac Qtl 15 riods of the late and middle Pleistocene. Slide masses may be susceptible to creep; overlie interbedded, reddish-brown and light-grayish-green, coarser grained sandstone map area. These two units are widely exposed in the southern part of the map area. QTg large sag in the southwestern part of the map area, however, probably are local features related to Qcg Qcg Kp Kp however, active creep unknown. Maximum thickness estimated to be about 20–25 m assigned to Maroon Formation (Phm). The State Bridge Formation thins abruptly in In Eagle and Garfield Counties, evaporite-related collapse features range in scale from local, rel- gravity block-glide landsliding, and to collapse or upheaval of rock units due to dissolution and dia- Qac Qac Kp Qtl Kps af lf Qlsr Qlsb Landslide of Benton Shale (Holocene to middle Pleistocene?)—Relatively intact, but eastern part of the map area, north of the Eagle River. If the thin, light-gray dolomite atively small and widely scattered sinkholes, to relatively large regional collapse centers (Kirkham Kp piric movement of evaporitic rocks of the Eagle Valley Evaporite (hee). The north and central Qac deformed Benton Shale (Kb), in extensive slide masses that formed on northerly-fac- present there is equivalent to South Canyon Creek Dolomite Member, then the lower and others, 1997; Scott and others, 1998). The sinkholes commonly are a few meters to tens of Kn Qlsd Qac parts of the quadrangle contain gently north-dipping Pennsylvanian to Late Cretaceous strata that 18 Kn 16 12 Qac Qtl Qlsg Qlsg ing, bedding-plane dip slopes. Similar in all fundamental characteristics, origin, and part of the State Bridge Formation is absent as well as the Schoolhouse Member of meters in length and width, and show surface collapse of about one to several meters; many of 13 Qalc Holocene are deformed by three north- to northeast-trending folds. From west to east they are the Horn age to landslides of Niobrara Formation (Qlsn). Principle bedding-plane zone of fail- the Maroon Formation (Phms) and perhaps part of the main body of the Maroon For- these have formed relatively recently, a few within the last several years. In contrast, the regional Qcg Qal Qfy Ranch Anticline and the Wolcott and Ute Creek Synclines. Rock units of the Middle Pennsylvani- Qtl Qcg ure commonly occurred in shale just above basal, sandy transition zone with underly- mation (Phm)—suggesting the presence of an unconformity in the middle to lower collapse centers are a few to many kilometers in length and width and appear to have collapsed as Qc Qcd Qcg an Eagle Valley Formation and the Eagle Valley Evaporite are exposed in the southern part of the Kb QTg Qac ing Dakota Formation (Kd). Slide masses may be susceptible to creep; however, active part of the State Bridge Formation or unidentified faults. Total thickness ranges from much as a thousand meters. The regional collapse centers apparently have collapsed slowly, and Kp Kps 13 quadrangle; these strata are complexly deformed by tight, closely-spaced folds that are cut by nor- creep unknown. Soil and other surficial materials derived from Benton slide masses about 145 m in western part of map area to about 100 m in eastern part perhaps continually, during the last few to several million years. Much of the evidence for the pres- 10 Qcg mal and reverse faults. In the southwestern part of the quadrangle, early Miocene basaltic flows un- Qlsg Qac 18 Kps Qac may contain expansive clay. Maximum thickness estimated to be about 20–25 m Maroon Formation (Lower Permian and Pennsylvanian)—Predominantly red beds of ence of these collapse centers comes from deformation and sagging of 24 to 8 Ma basaltic flows Kn Kp conformably overlie previously folded rocks of the lower part of the Eagle Valley Formation and Kb Kn Qac Qlsd Landslide debris of Dakota Sandstone (Holocene to middle Pleistocene?)—Uncon- sandstone and sandy siltstone with minor shale and sparse lenticular beds of pebble where they overlie evaporitic rocks. The basaltic flows (Tb) in the southwestern part of the map Qtl Kp the Eagle Valley Evaporite. These basaltic flows apparently have collapsed as much as 430 m due Qal Qty solidated debris primarily derived from the Dakota Sandstone (Kd) mixed with other conglomerate. Light-colored sandstone of Schoolhouse Member (Phms) at top of for- area overlie the Eagle Valley Evaporite (hee) and Eagle Valley Formation (he), and these basaltic 13 Qcg to dissolution and removal of underlying evaporitic rocks. In the central part of the quadrangle, a rocks appear to have collapsed or sagged (Hubert, 1954) as much as 430 m during some period of 15 Qcg surficial material. Includes both clast- and matrix-supported material with sandstone mation mapped separately where present. Usage of the nomenclature “Schoolhouse 15 Qlsg Qlsv Qlsn Qlsb Qlsd crescent-shaped anticline and syncline pair and smaller cross folds formed in a large, block-glide 23 clasts as much as 5 m in length. Large slide masses present mostly along north flank time after about 22 m.y. ago when they were initially deposited. This area of regional collapse in 25 Qac Qtl Member of the Maroon Formation,” after Johnson and others (1990) Qac Qfo h landslide that consists mainly of the Dakota Sandstone. These structural features are labeled on the map area is considered part of the recently identified, but incompletely defined, Eagle collapse 9 Kp Qfo Qal of Bellyache Ridge and on slope east of Ute Creek Many deposits show some scarps P ms Schoolhouse Member of Maroon Formation—White- to buff-weathering, grayish- figure 1 and described, from oldest to youngest, in greater detail below. center (Kirkham and others, 1997; Scott and others, 1998). 23 14 Qlsd near their bases and internally that suggest reactivation of parts of the slide masses af- white to pale-grayish-green, feldspathic and micaceous, medium- to coarse-grained, QTg Qto Qfo Qdu QUATERNARY ter major slide activity ceased. Probably formed mainly during moist periods of the and locally pebbly sandstone with some interbeds of grayish-red and pale-reddish- Areas underlain by evaporitic rocks have some potential for diapiric upwelling (rising ground) re- Qlsb QTdi SOUTHERN FOLD AND FAULT ZONE 16 Qcg Qac late and middle Pleistocene. Deposits prone to slumping where destabilized by road brown sandstone and siltstone. Well exposed in cliffs north and south of Eagle River lated to differential loading, or perhaps, hydration of evaporitic material. Diapiric upwelling, howev- er, also appears to be principally a slowly evolving, regional-scale process, most notable in some river Qtl 6 cuts. The westernmost slide mass of Bellyache Ridge shows several small, unmapped in western part of map area. Thins to east and northeast. Can be traced discontinu- Complexly deformed strata of the Eagle Valley Formation and Eagle Valley Evaporite in the Qlsb valleys where rapid downcutting by rivers in the last several million years has decreased the overlying Qc Qcg ? ? ???? Pleistocene slumps where cut by Interstate 70, and the recent landslide (Qlsr) formed where the ously from just east of basalt field (Tb) eastward around nose of Bellyache Ridge to southern part of the map area comprise a structural domain referred to here as the “southern zone 12 Qcg load above evaporitic rocks at depth and thereby triggered the anticlinal upwelling, or rise, of the Qlsn Kn QTg Qlsd western edge of this slide mass is cut by Highway 6. Thickness variable; westernmost where Schoolhouse Member pinches out abruptly northward, about 0.8 km south of of folds and faults” (fig. 1). Folds in this zone are tight and have variable to sinuous trends, and in landslide of Bellyache Ridge may exceed 80 m Eagle River. Member is entirely absent north of Eagle River in eastern part of map places are cut by steep to moderately dipping normal and reverse faults. Fold axes trend east- evaporitic rocks beneath the river valley (Kirkham and others, 1997; Kirkham and Widmann, 1997). Qtl Qdu Debris-flow deposits, undifferentiated (late and middle(?) Pleistocene)—Consists most- area. Where exposed along east flank of Bellyache Ridge, consists of white, medium- southeast to east-west in the western and central part of the zone, but they are bent and refolded The Horn Ranch anticline, in the west-central part of the map area, may reflect diapiric upwelling of 10 Qac 8 Qac evaporitic rocks at depth beneath the steep canyon of the Eagle River (Scott and others, 1998). Kb 14 Qac QTdi ly of debris-flow deposits and minor amounts of stream alluvium; may include earth- grained, noncalcareous sandstone that grades downward through limey, white sand- into a northerly configuration in the eastern part of the zone where they wrap around the east limb flow deposits. Includes both matrix- and clast-supported, subangular to rounded, peb- Areas underlain by and near evaporitic rocks also have potential for high-salinity surface and 14 16 Kn Qcg Kp stone into white, crystalline limestone. Directly north of Eagle River a thin bed of of the Wolcott syncline. Folds in the eastern part of the zone are cut by north- to northeast-strik- Qcg ble- to boulder-size clasts of locally derived bedrock in a poorly sorted silty and sandy d ground water from dissolution of the evaporitic rocks and surficial deposits derived from them. Qcg Qcd light-gray dolomite, assigned to base of the State Bridge Formation ( Psb), occupies ing reverse faults. Early Miocene basaltic rocks are unaffected by the folds and many of the faults Qlsb 32 Kn 45 The Eagle and Colorado Rivers are known to have high salinities downstream from the map area, Kn 43 matrix. Composition, form, and character of deposit variable and dependent primarily about same stratigraphic position as the absent Schoolhouse Member. Maximum that deform the underlying Pennsylvanian rocks. This indicates that the folding and faulting in the Kn 9 QTdi 16 18 11 10 QTg QTdi on source material and age of deposit. The two deposits in the Squaw Creek drain- thickness about 14 m southern zone is Pennsylvanian or younger but older than the early Miocene basaltic rocks that and a large percentage of that salinity is probably derived from dissolution of evaporitic rocks and 45 Qlsb 30 50 Qc Qac age, southeastern part of map area, are the most eroded and dissected, contain the Phm Main body of Maroon Formation—Mostly interbedded, brick-red, reddish-brown, and truncate these structures. evaporitic surficial deposits (Kirkham and others, 1997; Kirkham, 1997; Scott and others, 1998). 9 Kp Qcd Kn lf most alluvium, and are the only debris-flow deposits that contain granitic and gneissic some orangish-red, micaceous, fine to medium-grained, arkosic sandstone, siltstone, Folds and many of the faults in the southern zone are the oldest structures identified in the map Qac Qcg Qac Qc clasts; these deposits probably are late to middle Pleistocene in age. The debris-flow and shale. Shale is sparse and present mainly as thin beds and partings at silty tops of area, because they are deformed by younger structures like the Wolcott syncline. Folds in the EXPANSIVE AND CORROSIVE SOILS 10 Qlsb Qlsg Kp deposit directly south of the Eagle River consists primarily of Dakota Sandstone (Kd) blocky sandstone beds. Locally contains coarse-grained to pebbly sandstone lenses, southern zone may have initially developed during (1) Pennsylvanian to early Permian deformation clasts, shows little dissection, and on aerial photographs shows some preservation thin, mottled gray and red, argillaceous limestone interbeds, and light-greenish-gray to related to formation of the ancestral Rocky Mountains, (2) Laramide deformation (Late Cretaceous Soils that tend to swell or shrink due to changes in moisture content are commonly known as 15 expansive soils. Expansive soils are common in those areas underlain by sedimentary rock contain- Kb Pliocene where the flow formed natural levees; it appears to be the youngest such deposit in white sandstone interbeds. Green reduction spots and mottles locally common in red to Eocene) that initiated the formation of the present Rocky Mountains, or (3) flowage and diapiric 10 Kn Qlsd ing expansive clay minerals, or in areas underlain by colluvial and alluvial deposits derived from Qlsn the quadrangle and probably is latest Pleistocene in age. The form and dissection of beds. Cliff-forming and blocky weathering. Sedimentary structures include ripple activity of evaporitic rocks in the Eagle Valley Evaporite at sometime during Pennsylvanian to pre Qcg Qcd those rocks. Expansion and contraction of these soils can cause structural damage to rigid materi- Qcg QTg ? ? the debris-flow deposit north of the Eagle River suggests it is intermediate in age with cross-laminations, low-angle parallel and tangential crossbedding, and, locally, synere- late Laramide time. The folds in the southern zone are principally confined to the lower part of 20 Kb Qac als, such as foundations, pipes, and septic systems sitting on or buried in these expansive soils. In Qlsb respect to the deposit directly south of the Eagle River and those in Squaw Creek sis cracks. Contact with underlying Eagle Valley Formation (he) is gradational and the Eagle Valley Formation and the underlying Eagle Valley Evaporite, and they appear to have 12 Qlsb Unconformity the map area the Benton Shale (Kb) and Pierre Shale (Kp) are known to contain bentonite, which 13 Qac drainage. Thickness variable; locally may exceed 15 m placed at prominent color change between bright-red rocks of the Maroon Formation had consistent west-northwest trends prior to refolding by the northerly trending folds. The pat- Qcg Qac is an expansive clay mineral (table 1). Soils developed directly on these units or on alluvium, collu- 9 6 16 Kn QTdi Diamicton (middle Pleistocene to Pliocene?)—Isolated and highly eroded, colluvial de- and pale-reddish-brown rocks of upper Eagle Valley Formation. Color change also co- tern of consistent initial trends of these folds probably argues against an origin from diapiric move- Kb TERTIARY vium, and landslide deposits derived from these units should be tested for expansive properties pri- 15 10 posits that locally cap ridge crests in the northeast part of the map area. Consists of incides closely with uppermost medium- to dark-gray, micritic limestone in underlying ment of the evaporite, which might be expected to produce folds with more chaotic trends and less Qac Tb early Miocene Kb lf Qfo Kn angular to subangular pebble- to large boulder-size clasts mostly of Dakota Sandstone Eagle Valley Formation. Hubert (1954) used same criteria to define base of Maroon, continuity. Both Pennsylvanian to early Permian and Laramide deformation are known to have or to new construction. 9 Potentially corrosive soils, surficial deposits, and bedrock in the map area are associated with 10 Qcg (Kd) and some Morrison Formation (Jm) clasts in matrix- to clast-supported, poorly- but mapped underlying rocks as Minturn Formation. Hubert measured 323 m of Ma- had components of north-northeast/south-southwest-oriented compression in the region (Hon- h 11 B' sorted, sandy and silty matrix; some clasts as much as 5 m in length. Present only in roon Formation below Schoolhouse Member in the western part of map area, in cliffs gzhuan and others, 1996; Warner, 1956; Burbank and Lovering, 1933), either of which could ac- those areas underlain by evaporite-bearing rocks—the Eagle Valley Evaporite ( ee) and, to a lesser Kd Qac 5 Qcd h Qc Qfo northeastern part of map area where occurs on erosionally beveled ridge crests. Com- directly south of the Eagle River. Main body of the Maroon formation thins rapidly to count for the origin of these features. Near the contact of the Eagle Valley Formation and the Ea- degree, the Eagle Valley Formation ( e). In combination with moisture, the readily dissolved evap- Qlsd Qac Qc early Miocene orite minerals (principally gypsum) can be highly corrosive to conventional concrete and uncoated Tg positionally similar to younger landslides of Dakota Sandstone (Qlsd), but lacks hu- east. Total thickness ranges from about 335 m in western part of map area to 160 m gle Valley Evaporite, smaller scale folds were locally observed that are cut or detached along low- Kd Qc Kn or Oligocene Kb Kn mocky surface morphology. Origin is uncertain. Adjacent Qlsd deposits in northeast- in eastern part where Schoolhouse Member is absent angle faults. These relations may suggest that the relatively incompetent rocks of the Eagle Valley metal pipes. Testing of soils, rock, and surficial deposits for the presence of corrosive evaporite 15 10 Qcg minerals and proper selection of construction materials can mitigate evaporite-related corrosion. Qac ern corner of map area are, in part, derived from diamicton deposits. Holocene and he Eagle Valley Formation (Middle Pennsylvanian)—Reddish-brown, reddish-gray, gray, Formation and Eagle Valley Evaporite served in part as de’collement horizons during compression- 5 Je Jm ? Pleistocene erosion and downcutting of 250 m or more by local streams isolated ero- light-green, and tan shale, siltstone, and fine-grained to very fine grained sandstone in- al deformation related to Pennsylvanian to Permian and (or) Laramide tectonic episodes. During wet conditions, dirt roads become extremely slippery to impassable in rock units that Qc form clay-rich or evaporite-rich soils. The most notable units in the map area that form these road Jm 9 SYNCLINE Unconformity sional remnants of diamicton deposits from Dakota Sandstone (Kd) source areas to terbedded with distinctive, dark- to light-gray micritic to finely crystalline limestone. 16 Qcd conditions are Tertiary basalt (Tb), Pierre Shale (Kp), Niobrara Formation (Kn), Benton Shale (Kb), 15 east. Deposits locally appear to overlap and cover remnants of high-level Quaternary- Limestone beds range from about 5 cm to 2 m thick and are present throughout for- NORTHERLY TRENDING FOLDS dc Kps h h Kp Tertiary, gravelly alluvium (QTg). Diamicton may represent old debris-flow or other mation, but particularly common in middle and lower part of unit; seventeen individual Eagle Valley Formation ( e), Eagle Valley Evaporite ( ee), and alluvial and colluvial deposits de- 14 Kb Mesozoic and Paleozoic strata in the north and central parts of the quadrangle are deformed by 10 landslide deposits derived from highlands adjacent to alluvial lowlands. Maximum pre- limestone beds were identified in a partial section measured in the south-central part rived from these rock units (Qac, Qc, Qlsv, Qlsn, and Qlsb). Qalc Kd three north- to northeast-trending folds; the Ute Creek syncline, Wolcott syncline, and Horn Ranch A dPsb Kn served thickness estimated to be about 10 m of the map area, on south flank of Bellyache Ridge near axis of Wolcott syncline 7 58 Upper anticline. In the eastern part of the map area, the Ute Creek syncline has a very gentle dipping 9 77 (C.D. Blome, U.S. Geological Survey, oral commun., 1997). Locally contains evapor- ENVIRONMENTAL HAZARDS Kn Qfo Cretaceous west limb, but a very steep to overturned east limb that contains at least two northeast-trending re- Je Qalc CREEK CRETACEOUS ite interbeds; most are near base and are part of a 50–90-m-thick basal transition into af Qlsr Kb BEDROCK UNITS verse faults. The northern part of the Ute Creek syncline trends northeast, but its southern part The Eagle County landfill is in the map area, about 2.5 km north of Wolcott, near the head of Qac thick evaporite of the underlying Eagle Valley Evaporite (hee). Tweto and others 8 Qty Qcd Kn Jm ? bends to a north-northwest trend. The Wolcott syncline is a broad, northeast-trending fold in the the west fork of Ute Creek, which is an intermittent tributary of the Eagle River. Of the two land- 8 Qc 15 Tb Basaltic flows (early Miocene)—Dark- to light-gray and orangish-red, vesicular to non-ve- (1978) first mapped the Eagle Valley Formation in this region and defined it as fine- 12 central part of the quadrangle. The Horn Ranch anticline is a broad, northeast-trending anticline in fills (lf) shown on the map in that area, the southern one is an older reclaimed landfill, whereas the 7 62 sicular basaltic flows. Contains phenocrysts of olivine and pyroxene as much as 5 mm grained clastic rocks transitional between coarse-grained clastic rocks of the Minturn 12 Kb Kp Kd the western part of the quadrangle that shares its western limb with the Wolcott syncline. The Ute northern one is an active landfill. These sites are in shaly rocks of the Pierre Shale (Kp) and Nio- Lower in length in a devitrified and altered, microgranular to pilotaxitic groundmass of plagio- and Maroon Formations and evaporitic rocks of the Eagle Valley Evaporite. Precise Qlsd Cretaceous clase, pyroxene, olivine, and iron oxides. Olivine commonly altered to iddingsite. Lo- Creek syncline affects rocks at least as young as the Late Cretaceous Pierre Shale, and the Wolcott brara Formation (Kn). These shaly rocks, unless fractured, should be relatively impervious to any Qc Qlsd 20 13 Minor unconformity? correlation among these Pennsylvanian and Permian Formations remains unresolved. 10 Qalc Qc Qcd cally, vesicles are filled by calcite amygdules. X-ray fluorescence analyses of two sam- syncline and Horn Ranch anticline deform rocks as young as the early to late(?) Cretaceous Dakota seepage towards underlying ground water. Unusual precipitation events related to major storms Phms 25 Hubert (1954) mapped rocks assigned to the Eagle Valley Formation and Eagle Valley Jm ples gave chemical compositions of basaltic trachyandesite (J.R. Budahn, U.S. Geolog- Sandstone. Therefore, these folds are all Cretaceous or younger in age and probably mostly related might potentially cause some erosion of these landfills and, in a worst-case scenario, transport ma- d Qalc Upper Evaporite in the Wolcott quadrangle as Minturn Formation and speculated that the up- c 13 Kn JURASSIC ical Survey; written commun., 1997). Forms large field of basaltic flows occupying to Laramide (Late Cretaceous to Eocene) episodes of deformation; Locally, however, these folds terial down Ute Creek and into the Eagle River. 19 Qac 12 Jurassic per limestone of his Minturn correlated with the Jacque Mountain Limestone, which is 70 may be related to, or modified by, Cretaceous or younger salt tectonism. Qto af 66 Je much of southwest part of map area and small, isolated remnant along eastern part of defined as the top of the Minturn Formation farther east (Tweto and Lovering, 1977). ECONOMIC GEOLOGY Qalc Qcd southern map border. Consists of two or more flow units that were not mapped sepa- The northerly trending folds and reverse faults and an inferred northwest-trending tear fault Qcd Schenk (1992), however, presented evidence that the Jacque Mountain Limestone is dPsb 7 15 Qc Qlsd h rately. Weathers to rounded knobs and rubble. Best exposed on crest of Bellyache (fig.1) all appear to be related in age and collectively suggest west-northwest/east-southeast-orient- The map area has a low potential for extractable mineral resources. There have been no ex- 6 13 Kd P m Unconformity present near the bottom of the sequence mapped as Eagle Valley Formation in this re- ANTICLINE Ridge. Erratic flow-foliation orientations combined with an elevation difference of ed compression during their formation. I have inferred the presence of a buried, northwest-trend- ploratory oil and gas wells drilled in the map area and no producing wells are located nearby. The Qalc UTE 12 Je port. Rocks assigned to the Eagle Valley Formation in this report and in Tweto and Kp dc Upper nearly 600 m between the topographically highest flow exposures along crest of Belly- ing tear fault directly south and southeast of the Ute Creek syncline in the Eagle River Valley. Tear few exploratory wells that have been drilled nearby to the west have all been reported as dry (Tom Qac Kb Qty dPsb dc others (1978) differ distinctly from the Maroon and Minturn Formations and from the Qlsd Kb Triassic ache ridge and the lowest flow exposures farther south, suggest collapse caused by faults commonly form nearly perpendicular to related folds and reverse faults and they serve as Hemborg, Colorado Geological Survey; oral commun., 1997). No sign of mining prospects was 16 Eagle Valley Evaporite. In the map area, the Eagle Valley Formation everywhere un- TRIASSIC withdrawal or dissolution of the underlying Eagle Valley Evaporite as suggested by Hu- partitions between domains of rock in which a common magnitude of shortening has been ach- found, no active mining claims are present, and no evidence could be found for intrusive rocks, hm 70 60 h derlies the Maroon Formation (Phm) and overlies the Eagle Valley Evaporite (hee). P e ieved in different ways (Davis, 1984). In my interpretation, the inferred tear fault provided a parti- veins, and associated mineralization and alteration. Qalc af Qlsd Unconformity bert (1954). Cinder-bearing basaltic rocks are present in Warren Gulch, but no obvi- In the south-central part of the area, along the south flank of Bellyache Ridge, a typi- 40 39 tion (zone of lateral slip) that allowed different sets of northerly trending folds and reverse faults, The map area has some potential for small deposits of sand and gravel resources along the Ea- Qac ous cinder cone or other vent sources were identified. Ar/ Ar age spectrum analy- cal section of Eagle Valley Formation was found to be present between the Maroon d Lower Triassic north and south of the tear, to accommodate shortening related to west-northwest/east-southeast- gle River and for gypsum in the southern part of the map area. The largest and potentially most CH h 20 Psb ses were obtained for two samples; sample L6681b, collected in the map area directly Formation (Phm) and Eagle Valley Evaporite (hee), where reconnaissance mapping P ms 20 14 Unconformity? and Permian ¼ oriented compression. In the Red Canyon area, about 22 km to the north, Beckett (1955) found exploitable sand and gravel deposits are in the younger terrace alluvium of the Eagle River (Qty). 13 north of Warren Gulch (NW , sec. 31, T. 4 S., R. 83 W.), and sample L6844, locat- by Tweto and others (1978) had shown the Eagle Valley Formation as absent. Unit is RAN d Qc 20 c 16 80 ed about 1.5 km south of the map area in Trail Gulch (SE¼, SW¼, sec 18, T. 5 S., similar relations among northeast-trending folds that deform northwest-trending folds, which are all Mining of this gravel has occurred in the past. A golf course along the north side of the Eagle Riv- Kb 60 well exposed along south flank of Bellyache Ridge and in cliffs south of Eagle River di- Qac 25 Qlsb Unconformity PERMIAN R. 83 W.). Sample L6844 yielded a plateau age of 22.3 ± 0.1 Ma with 67% of the cut and truncated by a northwest-trending tear, or strike-slip fault. Folds and faults in the Red Can- er, in section 23, now occupies one of the larger terrace remnants. The Eagle Valley Evaporite dPsb Qlsb Kn rectly west of map area. Basal contact with underlying Eagle Valley Evaporite (hee) is Qac 15 39 yon area are Laramide in age (Beckett, 1955). Similar to the Red Canyon area, north- to north- (hee) is exposed or shallowly buried in several localities in the southern part of the map area. This 85 AND Ar released on the plateau. Inverse isotope correlation of the age spectrum data conformable, gradational over 50–90 m, and commonly deformed by salt diapirism of 12 Qlsd Qcd 40 36 east-trending folds and reverse faults and the tear fault in the map area may be related to the latest Qfy Phm 37 Phms PENNSYLVANIAN yielded an age of 22.2 ± 0.2 Ma, with an initial Ar/ Ar ratio of 341 ± 15, and h unit is actively mined for gypsum to the west near the town of Gypsum. Although the Eagle Valley 70 50 evaporite interbeds and evaporite in the underlying Eagle Valley Evaporite ( ee). 10 65 episodes of Laramide deformation, and the northwesterly trending folds in the southern zone Evaporite is a potential gypsum resource in this part of the map area, most of the localities under- HORN 40 mean standard weighted deviation (MSWD) = 0.967. Although the inverse isotope Contact difficult to locate precisely or consistently; placed approximately at base of Phm might similarly be related to the earliest phase of Laramide deformation. lain by the evaporite are undergoing rapid residential development. Larger undeveloped areas of 20 Je correlation analysis suggests the presence of a small amount of extraneous argon in dominantly clastic sequence and above thick, massive evaporite. Evaporite interbeds 10 75 The Horn Ranch anticline may have also formed during the west-northwest/east-southeast, evaporite are exposed in several areas west of the map area. 15 Qac 15 45 Qac the sample, the agreement of the correlation age with the plateau age, within the lim- may be susceptible to ground subsidence from solution voids and to diapiric swelling. Qalc Kn he its of analytical precision, suggests that this basaltic flow was emplaced about 22.3 ± Laramide compression discussed above, however, the Horn Ranch anticline appears to connect 15 25 87 Soils and other surficial materials derived from evaporite interbeds may also be sus- Qc 20 45 Middle 0.1 m.y. ago. Sample L6681b yielded a disturbed age spectrum that did not have a with an unnamed, easterly trending anticline that deforms early Miocene basaltic rocks near Gyp- 28 Qty 80 85 PENNSYLVANIAN ceptible to subsidence from hydrocompaction and piping of evaporitic material. Evap- Qlsd 5 Pennsylvanian plateau age. Inverse isotope correlation of the age spectrum data yielded an age of sum and Eagle directly west of the map area. This unnamed anticline appears to reflect diapiric 5 Qac hee orite-bearing solutions derived from bedrock, soils, or surficial deposits may be corro- 20 40 36 upwelling of the Eagle Valley Evaporite (hee) along the Eagle River Valley (M.R. Hudson, U.S. ACKNOWLEDGMENTS Qc 20 30 22.8 ± 0.3 Ma, with an initial Ar/ Ar ratio of 329 ± 28, and MSWD = 1.303. sive to materials such as conventional concrete and metal pipes. Total thickness Qalc 18 Kb Geological Survey, unpubl. mapping, 1997; Scott and others, 1998). The Horn Ranch Anticline The age of this sample is not as certain as that of sample L6844, because it did not about 700 m also exists in the Eagle River Valley and may also reflect salt tectonism from diapiric rise of the Ea- This mapping project was funded by the National Cooperative Geologic Mapping Program. 17 have an age plateau. Nonetheless, its correlation age is similar to the apparent ages hee Eagle Valley Evaporite (Middle Pennsylvanian)—Light- to dark-gray and white evaporitic 10 Qdu 50 h gle Valley Evaporite (hee) at depth, which would account for this anticline as a product of salt tec- Kevin Tyson assisted with field studies during the summer of 1996. Jim Budahn and Dan Unruh Qc 30 Qdu e of L6844 and suggests that it was emplaced 22–23 m.y. ago (M.J. Kunk, U.S. Geo- sequence consisting mostly of gypsum with interbeds of tan-weathering, light- to dark- Jm tonism and not Laramide compressional deformation. Laramide compressional deformation and provided geochemical analyses, Mick Kunk provided isotopic age analyses, and Frank Byers provid- Kd 8 55 Qalc logical Survey; written commun., 1997) gray shale and argillaceous limestone, tan, very fine grained sandstone, and sparse, red 16 43 Neogene salt tectonism appear to be equally valid interpretations for the origin of this broad anti- ed petrographic analyses of basaltic rock samples. Ralph Shroba and Paul Carrara provided advice 25 15 Jm dPsb 40 Kp Pierre Shale (Upper Cretaceous)—Medium- to dark-gray, olive-gray, and brownish-gray silty sandstone. Exposures commonly poor and consist mostly of dirty-gray gypsum 50 and information concerning classification and delineation of surficial deposits. Scott Minor, Ric 25 10 margins. Unit is probably equivalent in part to outwash of the Pinedale glaciation, slightly to non-calcareous shale and silty shale. Contains numerous calcareous concre- cline Qc Kd DESCRIPTION OF MAP UNITS commonly weathered to rounded slopes and covered by thin residuum of gypsum-rich Page, and Bob Kirkham contributed insightful and helpful technical reviews, and Craig Brunstein Je which is about 12–35 k.y. old (Richmond, 1986, chart 1A). Unit is a potential gravel tions present as irregular masses and lenses. Locally contains thin, impure, and dis- 15 silt that shows a popcorn-like texture from wetting and drying. Fresh exposures are COLLAPSED BASALTIC FLOWS did the technical edit. All graphics were digitized in GSMCAD (Williams and others, 1996), and 10 27 75 resource. Thickness commonly 5–10 m continuous fine-grained sandstone and siltstone layers.. Top of formation not exposed he 20 55 35 80 SURFICIAL DEPOSITS light to dark gray. Evaporitic rocks and interbedded limestone are commonly brecciat- h 35 af in map area, only about 50 m of poorly exposed shale is preserved above the upper In the southwest part of the quadrangle, early Miocene basaltic flows overlie highly folded Mid- the polygons were exported through ARC INFO, and then into Illustrator using MAPublisher. P m 16 22 Qty Qto Older terrace alluvium of Eagle River (middle Pleistocene)—Alluvium deposited by Ea- ed and breccia may be collapse-related, resulting from dissolution of evaporitic rocks. 20 80 sandstone marker bed in northeast corner of map area. Directly north of the map dle Pennsylvanian rocks (fig. 1). Mapped relations suggest the basaltic rocks collapsed into a sag GSMCAD was also used to export DXF files, which were imported into Illustrator using MAPublish- Qc 45 Surficial deposits shown on this map are generally at least 1 m thick, thinner deposits are not shown. gle River that underlies small terrace remnants about 25–30 m above Eagle River. Unit Unit is also commonly contorted and faulted by salt diapirism, flowage, and volume 25 15 er. Denny Welp, Nancy Shock, Craig Brunstein, Jim Hoffman, Alex Donatich, Deke Young, Van 25 Qlsd Qlsb Jm Thin, discontinuous colluvial deposits, residual material on bedrock, and some artificial fills were not consists of well-sorted, rounded and well-rounded, pebble-cobble gravel. Clasts consist area, about 245 m of silty shale overlies the upper marker bed and is unconformably changes related to hydration and dehydration reactions of gypsum and anhydrite, re- feature that formed in the underlying Middle Pennsylvanian strata. Collapse of the basaltic flows 22 9 45 Williams, and Karen Morgan all assisted, advised, or were principally responsible for different as- 9 16 20 Jm Qfy 30 mapped. Many contacts of surficial deposits are approximate because the contacts are poorly exposed or mostly of white and pink, coarse-grained granite, gray, fine-grained granite and grano- overlain by Tertiary basalt (Wanek, 1953). Mollusk fossils are locally present through- appears to have resulted from dissolution and removal of evaporitic rocks from the underlying Ea- 10 57 spectively. Much of the deformation was likely episodic and related to burial depths, pects of the manipulation of the digital files. Informal seminars, field trips, and discussions with 25 80 gradational (for instance; the contacts between landslides and colluvium, or gravelly colluvium and other diorite, dark gneiss and schist, white, pink, and tan quartzite, fine-and medium-grained out formation but most abundant in middle to upper part of unit. Marine fossils, char- gle Valley Evaporite (hee) (Hubert, 1954). On Bellyache Ridge, these basaltic rocks are at an alti- Tg 35 Qlsd 33 52 uplift and erosion, and water saturation. Some deformation in the unit probably is re- many members of the Colorado and U.S. Geological Surveys (Bob Kirkham, Randy Streufert, Qac Qcg units). red sandstone, and light- to dark-gray basalt. Matrix consists of pale-brown, silty sand. acteristic of the Pierre Shale in the map area and areas nearby, have been described lated to Laramide and younger tectonic events, but distinction of deformation related tude of about 2,750 m and dip about 20–25 degrees southwest toward the central part of the col- 10 15 20 Chris Carroll, Ralph Shroba, Paul Carrara, Bob Scott, Karl Kellogg, Bruce Bryant, Bill Perry, Jim 17 dc Divisions of Pleistocene time correspond to those of Richmond and Fullerton (1986): Holocene, 0 to Gravel is commonly overlain by about 1 m of overbank materials consisting of mas- by Cobban and Reeside (1952), Stauffer (1953), and Wanek (1953). Basal contact of lapse or sag feature. In this area, Middle Pennsylvanian to Jurassic strata that underlie the basalt 45 17 to regional tectonics from local, salt-related deformation, or from tectonic-induced salt Budahn, Dan Unruh, Mark Hudson, Tom Steven, and Frank Byers) influenced or contributed to as- 22 Qc 11,560 yr ago; late Pleistocene, 11,390 to 127,000 yr ago; middle Pleistocene, 127,000 to 778,000 yr sive, light-yellowish-brown, silty fine sand. Unit found at two localities in the map area: Pierre Shale with underlying Niobrara Formation is poorly exposed and gradational. dip in the opposite direction, to the northeast, at about 20–30 degrees. In the central part of the Qc 30 10 62 deformation is difficult and speculative in map area. Locally contains dissolution voids 15 65 collapse feature, the base of the basaltic flows is at an altitude as low as about 2,320 m, suggesting pects of the information presented. Will Henderson and Ted Archibeque of the Eagle County 9 20 15 25 ago; and early Pleistocene 778,000 to 1,806,000 yr ago (D.S. Fullerton, U.S. Geological Survey, (1) underlying a terrace remnant just west of junction of Squaw Creek and Eagle River, Contact approximately located where medium- to dark-gray shale assigned to upper as much as a few meters in diameter and several meters deep. Unit is susceptible to that the basalt collapsed about 430 m relative to its exposure along the crest of Bellyache Ridge as Community Development Department and Assessor's Office, respectively, helped with maps and Qc 12 14 Qalc written commun. 1997). southeast part of map area, and (2) a few patches of terrace remnants are present on Niobrara Formation is overlain by dark-gray, silty, slightly calcareous to non-calcare- ground subsidence from solution voids and may be susceptible to diapiric swelling. Qlsv 55 20 other information concerning land status and ownership. Numerous landowners graciously grant- 23 30 Age assignments for surficial deposits are based chiefly on stratigraphic relationships, the degree of spur ridges north of Eagle River in west-central part of map area. A wedge of undivid- ous, concretion-bearing shale assigned to Lower Pierre Shale. About 825 m thick in Soils and surficial deposits overlying or derived from unit may also be susceptible to was suggested by Hubert (1954). The orientations of flow foliation of flows in the central part of 40 Jm 6 ed permission to enter their land during the field studies. Qc 20 erosional modification of original surface morphology, height above stream level, and degree of soil northeast part of map area but top not exposed. Directly north of map area, Wanek the collapse feature are erratic and the rocks are commonly rubbly and poorly exposed, but it ap- he 10 ed alluvium and colluvium (Qac) overlaps and covers much of the terrace near Squaw settlement from hydrocompaction and piping of evaporitic materials. Evaporite-bear- 25 h h 20 55 development. Age assignments for some units are inferred chiefly on the basis of regional rates of Creek. Unit probably equivalent in part to outwash of the Bull Lake glaciation, which (1953) measured a total thickness of about 1,070 m beneath the Tertiary unconformi- ing solutions derived from bedrock, soils, or surficial units may be corrosive to materi- pears that the folds in the Eagle Valley Formation ( e) and Eagle Valley Evaporite ( ee) were ero- Qac af 15 25 stream incision of about 13 m/100 k.y. (k.y. = thousand years) and 190 m/m.y. (m.y. = million years). is about 140–150 k.y. old (Pierce and others, 1976). Terrace remnants too small to be ty present there als such as conventional concrete and metal pipes. Base of unit not exposed in map sionally beveled and partly covered with Early Miocene or Oligocene gravel prior to emplacement Qac Phm The first incision rate is based on a value for stream incision since the deposition of the 660 k.y. Lava potential gravel resource. Preserved thickness commonly 1–3 m Kps Sandstone marker beds in Pierre Shale—Two prominent ledge-forming sandstone and later collapse of the basaltic flows. Jm area, and poor exposures and contortion of unit precludes accurate thickness esti- REFERENCES CITED he Creek B volcanic ash (Izett and others, 1992), where it is present about 80 m above the White River near marker beds (Kps) are present about 700 m above base of Pierre Shale (Kp). Marker Structural relations between the northeast-dipping Pennsylvanian-Jurassic rocks and southwest- Qc 9 Qty QTg Alluvium (early Pleistocene to Pliocene?)—Gravelly alluvium containing clasts likely de- mates, but exposures directly east of map area suggest unit is at least 500 m thick 25 dipping basaltic rocks along the northeastern margin of the collapse feature suggest that the Pennsyl- Qac Meeker, Colorado (Whitney and others, 1983). The second incision rate is based on about 1,830 m of posited by an ancestral Eagle River or its tributaries. Present as several gravelly patch- beds are wavy-bedded, shaly, silty, micaceous, fine-grained to very fine grained, salt- American Geological Institute, 1982, Grain-size scales used by American geologists, modified Kd vanian-Jurassic rocks dipped more steeply (by about 20–25 degrees) to the northeast when the basal- Kd 27 downcutting by the Colorado River since the deposition of 9.7±0.05 m.y. basalt on Grand Mesa (Marvin es in northeast part of map area where it is characterized by poorly exposed gravelly and-pepper sandstone that contains rusty-weathering nodules, fucoid casts and impres- Wentworth scale, in Data sheets (2nd ed.): Falls Church, Virginia, American Geological Insti- tic flows were originally deposited. Restoration of the southwest-dipping basaltic flows to their original 10 and others, 1966). rubble and yellowish-brown, silty, fine and medium sand matrix material. Consists of sions, and some thin, discontinuous, silty shale interbeds. Upper sandstone marker tute, sheet 17.1. 24 Grain-size terminology for the surficial deposits is based on visual estimates and follow the modified bed is about 45 m thick and lower sandstone marker is about 10 m thick. Marker sub-horizontal orientation would require that the underlying Pennsylvanian-Jurassic rocks also be re- 20 rounded and well-rounded, pebble-boulder gravel capped by and mixed with subangu- Bass, N.W., and Northrup, S.A., 1963, Geology of the Glenwood Springs (30-minute) quadrangle 22 stored to a pre-collapse orientation where existing northeast dips would increase from about 25 to 45 Tb Qcg Qac Wentworth grade scale (American Geological Institute, 1982). In descriptions of surficial deposits, the lar to subrounded pebble-boulder clasts of basalt. Rounded and well-rounded gravel beds separated by about 70 m of dark-gray, silty, wavy-bedded shale that contains and vicinity, northwestern Colorado: U.S. Geological Survey Bulletin 1142–7J, 74 p. 28 18 22 80 term “clasts” refers to the fraction greater than 2 mm in diameter, whereas the term “matrix” refers to clasts consist primarily of white and tan quartzite, red sandstone, and sparse clasts of thin, lenticular sandstone interbeds Contact—Dashed where approximately located; dotted where concealed degrees or greater. The most likely explanation for this collapse feature is Neogene removal of a Qac 22 Beckett, R.L., 1955, Structural Geology of the Red Canyon area, Eagle County, Colorado: Bould- 27 70 particles less than 2 mm in size. Kn Niobrara Formation (Upper Cretaceous)—Light- to dark-gray and brownish-gray, calca- large volume of evaporitic rocks of the underlying Eagle Valley Evaporite by groundwater dissolution 20 Qc granite and granodiorite and dark gneiss and schist; clast composition similar to ter- T er, University of Colorado, M.S. thesis, 36 p. Tb 21 Qcg 10 reous shale and very light gray, dense, conchoidal-fracturing, sparsely fossiliferous High-angle fault—Dashed where approximately located; dotted where concealed; ball and and transport. Neogene removal of evaporite apparently was most active beneath the central part of Qc 17 race deposits along Alkali and Muddy Creeks (Qtl) and to Tertiary alluvium (Tg) in Braddock, W.A., 1978, Dakota Group rockslides, northern Front Range, Colorado, U.S.A., in L6681b 15 limestone. Upper 70 m is mostly medium-gray and brownish-gray, silty, calcareous bar on apparent downthrown side. Labeled T, where concealed and inferred to be a the collapse feature, but evaporite removal was probably also active beneath the northeastern margin hee SYNCLINE Phm 68 ARTIFICIAL FILLS southwestern part of map area. Locally may include gravelly colluvium (Qcg). Rem- Voight, Barry, ed., Rockslides and avalanches, 1; natural phenomena: Developments in Geo- 30 Je 45 shale. Middle 185 m is dark-gray shale that commonly weathers to paper shale with tear fault, based on apparent, inconsistent lateral offsets of the collapse, which would account for subsequent down-dropping and related rotation of both the T nants of unit about 150–300 m above modern Eagle River. Maximum preserved technical Engineering 14A, Elsevier Scientific Publishing Company, p. 440–479. Je af Artificial fill (latest Holocene)—Compacted and uncompacted fill material composed sedimentary and basaltic rocks. Similar large areas of collapsed early Miocene basaltic flows have Qac Qc 50 thickness about 5 m white to light-gray surface coatings. Lower 75 m is interbedded light-gray, fissile, cal- Reverse fault—Dashed where approximately located; dotted where concealed; hachures Burbank, W.S., and Lovering, T.S., 1933, relation of stratigraphy, structure, and igneous activity 30 17 65 mostly of varying amounts of silt, sand, and rock fragments. Unit includes fills beneath h 35 15 Alluvium (early Miocene or Oligocene)—Gravelly alluvium containing clasts likely deposit- careous shale, shaly limestone, and dense, blocky- to massive-weathering limestone. on upthrown side of fault been recognized in the vicinity of Carbondale, Eagle, Gypsum, and State Bridge; existing information ee he 33 Qc Tg to ore deposition of Colorado and southern Wyoming, in Ore deposits of the Western States 25 Qc h Interstate 70 and the tracks of the Denver and Rio Grande Western Railroad. Thick- ed by an ancestral Eagle River or it's tributaries. Unit present as several remnant, grav- Limestone forms ledges, and base of unit marked by prominent ledge-forming, light- suggests that these collapse features resulted from evaporite removal after about 8 Ma over a period 45 he Tg 32 Qc Qac 80 e Block-glide fault—Approximately located; dotted where concealed; double hachures on (Lindgren volume): New York, American Institute of Mining and Metallurgical Engineers, p. d 40 ness ranges from about 1 to 15 m elly patches in southwest part of map area where unit locally appears to underlie basalt gray limestone with some interbedded, light-gray, limey shale. Contains marine mol- of a few to several million years, and suggests that evaporite removal is still active in some areas (Kirk- 42 Psb 42 76 lf block-glide slab above fault 272–316. Qlsv 45 dc Landfill (latest Holocene)—Compacted and uncompacted fill material composed mostly of ham and others, 1997; Kunk and others, 1997; Scott and others, 1998). Collapse features and 46 Qc 55 (Tb). Poorly exposed, mainly as numerous patches of gravelly rubble in southwestern lusks and, in upper part, cycloid fish plates and bone fragments (Stauffer, 1953; Wa- Cobban, W.A., and Reeside, J.B., Jr., 1952, Correlation of Cretaceous Formations of the Western Jm 28 varying amounts of silt, sand, and rock fragments and human refuse. Unit includes Folds—Showing approximate trace of axial surface and direction of plunge; dotted where processes characterisic of the Carbondale region, are described and interpreted in Kirkham and Wid- af Qac part of map. Consists of rounded to well-rounded pebble-cobble gravel (sparse bould- nek, 1953). Conformably overlies black shale assigned to the top of the Benton For- Interior of the United States: Geological Society of America Bulletin, v. 63, p. 1011–1044. Qto two deposits north of Wolcott and the Eagle River; southern deposit is reclaimed and mation. Unit may be susceptible to translational sliding and creep on dip slopes. To- concealed mann (1997) and Kirkham (1997). Some of the same features and processes appear to characterize Qcg WOLCOTTPhm ers) of weather-resistant rock types. Clasts are primarily white, tan, and pink quartzite; Colton, R.B., Holligan, J.A., Anderson, L.W., and Patterson, P.E., 1975, Preliminary map of land- 25 38 compacted landfill, northern deposit is compacted and uncompacted, active landfill, tal thickness about 335–365 m Anticline collapse in the map area and elsewhere in the Eagle-Gypsum region (Scott and others, 1998). Tb Phms dc Qty 70 af red, fine- to medium-grained sandstone; and very sparse clasts of granite and gneiss. slide deposits, Leadville 1˚ × 2 ˚ quadrangle, Colorado: U.S. Geological Survey Miscellaneous and its outline is approximate. Thickness ranges from about 3 to 10 m Benton Shale (Upper Cretaceous)—Dominantly black and dark-gray, slightly calcareous, Qcg 24 30 Qc Commonly covered by or littered with debris from overlying basalt. Locally may in- Kb Field Studies Map MF–697, scale 1:250,000. 38 Syncline BLOCK-GLIDE FOLDS Tg 32 24 dPsb clude gravelly colluvium (Qcg). Remnants of unit about 300–670 m above modern platy shale with thin bentonite stringers and thin, buff, carbonaceous, fine-grained Qac 28 Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes, in Turner, A.K., and he ALLUVIAL DEPOSITS Eagle River; large elevation range of unit may reflect post-depositional collapse of this sandstone lenses. Upper part consists of 6–10 m of black shale that underlies Nio- 20 Qalc Minor folds—Showing trend of axial surface and, locally, direction and amount of plunge In the central part of the quadrangle, a crescent-shaped anticline-syncline pair and associated Schuster, R.L., eds., Landslides-investigation and mitigation: National Research Council, Qcg 30 68 Qalc region related to dissolution or movement of the underlying Eagle Valley Evaporite brara Formation. Beneath upper shale is about 3 m of thinly and unevenly laminated, smaller-scale, west-trending folds are in a large landslide block composed mostly of the Dakota For- Transportation Research Board Special Report 247, p. 36–75. Stream-channel and flood-plain deposits along Eagle River (Holocene)—The upper 1- h sandy, gray, brown-weathering, fetid, crystalline limestone that contains abundant ma- Anticline h 30 ( ee). Similar in appearance and clast composition to gravelly alluvium north of Eagle 5 mation and parts of the underlying Morrison Formation and overlying Benton Shale. I interpret Davis, G.H., 1984, Structural geology of rocks and regions: New York, N.Y., John Wiley & Sons, P ms 68 60 to 2 m of the unit is commonly a light-yellowish-brown, massive, silty fine sand to me- Tg Qcg 35 T River (QTg and Qtl). Maximum preserved thickness about 5 m rine mollusk shells and some small fish teeth (Stauffer, 1953; Wanek, 1953; Hubert, Qcg 38 these folds to represent internal deformation related to block-glide landsliding. The block slid essen- 492 p. Qcg Tb dium sand that locally contains minor amounts of pebbles and cobbles in lenses gener- 1954). Remainder of unit is mostly black shale. Lower 10–15 m contains numerous 15 Syncline Phm 60 Qac ally less than 20 cm thick. The lower part of the unit, poorly exposed and of unknown tially intact down-slope to the northeast on a glide plane consisting of lithologically unstable shale, Dubiel, R.F., 1992, Sedimentology and depositional history of the Upper Triassic Chinle Forma- 25 63 75 63 ALLUVIAL AND COLLUVIAL DEPOSITS 1–15-cm-thick beds and lenses of buff sandstone and siliceous, dull-greenish-gray to Qcg thickness, consists of well-sorted, rounded and well-rounded, pebble-cobble gravel de- Strike and dip of beds mudstone, and siltstone in the upper part of the underlying Morrison Formation. The slip surface at tion in the Uinta, Piceance, and Eagle Basins, northwestern Colorado and northeastern Utah: 40 30 black siliceous shale; fucoid casts locally present in lower shale. Basal contact appears Qcg Tg rived from a variety of sedimentary, igneous, and metamorphic rocks. Clasts consist 35 the base of the slide is primarily a bedding-plane fault that is poorly exposed or covered throughout U.S. Geological Survey Bulletin 1787–W, 25 p. 45 35 60 Qfy Younger fan alluvium and colluvium (Holocene and latest Pleistocene)—Unit forms al- gradational and conformable with underlying Dakota sandstone. Weathers to rounded Inclined 80 he the map area. The southeast margin of the block appears to have slipped along a steeper edge of Freeman, V.L., 1971, Stratigraphy of the State Bridge Formation in the Woody Creek quadrangle, 45 he 43 52 predominately of fine-grained black basalt, white quartzite, red sandstone, light-colored luvial and colluvial fans along Eagle River. One small, low-gradient fan is present along slopes and valleys and generally poorly exposed. Best exposed, particularly upper hee Qcg 30 the plane, which cut the Dakota Sandstone and separated Dakota Sandstone involved in the block- Pitkin and Eagle Counties, Colorado: U.S. Geological Survey Bulletin 1324–F, p. F1–F17. 55 40 granite and granodiorite, and dark-colored schist and gneiss. Some clasts have a thin, Vertical Qc east side of Eagle River in east-central part of map area. Several steeper-gradient, co- part, in cliffs directly north and east of Wolcott along Eagle River. Unit may be sus- glide from Dakota Sandstone that was not involved. Folds are present in similar block-glide land- Hongzhuan, Ye, Royden, L.H., Burchfiel, B.C., Schuepbach, M.A., 1996, Late Paleozoic defor- 50 Qc white carbonate coat. Matrix in lower, gravelly part consists of very pale brown, sandy 43 40 alesced, alluvial and colluvial fans present along south side of Eagle river in west-cen- ceptible to translational sliding and creep on dip slopes. Total thickness 115–125 m 20 Qac Qcg he 35 silt and silty sand. Unit may include low terraces along the flood plain. Unit subject to Overturned slides composed of the Dakota in several localities along the Colorado Front Range (Braddock, mation of interior North America; the greater ancestral Rocky Mountains: American Associ- Qac Qcg M tral part of map area were collectively mapped as a single fan complex. Consists of Kd Dakota Sandstone (Upper and Lower? Cretaceous)—Cliff-forming, crossbedded, tan, 36 1978). Braddock has shown that those folds are essentially fault-bend folds that reflect the ramp-flat ation of Petroleum Geologists Bulletin, v. 80, no. 9, p. 1397–1432. he periodic flooding. Thickness estimated to be 5–10 m white, and light-greenish-gray, medium- to coarse-grained, silica-cemented, well-sorted 15 30 Qal pebble- to boulder-size, angular to subrounded, locally derived clasts that are mostly geometry of the underlying slip surface. The crescent-shaped axes of the anticline-syncline pair in Hubert, J.F., 1954, Structure and stratigraphy of an area east of Brush Creek, Eagle County, Colo- Alluvium along tributaries of Eagle River (Holocene and latest Pleistocene)—Includes sandstone and orthoquartzite interbedded with black, yellowish-gray, and sparse, pale- Strike and dip of inclined flow foliation in volcanic rocks 45 Qcg 30 42 55 matrix supported in massive fine sand, silt, and clay. Unit may be subject to channel the map area suggest that these folds may reflect an underlying crescent-shaped ramp along the bas- rado: Boulder, University of Colorado, M.S. thesis, 104 p. he stream-channel, flood-plain, and some sheetwash deposits. Very pale brown to very purplish-red shale. Contains scattered lenses of chert and quartz-pebble conglomer- Qcg 30 modification and debris flows during intense or prolonged rainstorms. Maximum h al, block-glide fault, a segment of which is depicted in cross section B-B’. The west-trending cross Izett, G.A., Pierce, K.L., Naeser, N.D., Jaworowski, Cheryl, 1992, Isotopic dating of Lava Creek B pale gray, massive, sand, silt, and clay with matrix-supported pebble- to cobble-size ate; commonly a thin conglomerate bed is present at or near base of unit. Where well M Red marker bed in Eagle Valley Formation ( e)—Shown locally in southeastern part of Qac thickness estimated to be about 7–10 m folds, which are present between the crescent-shaped folds, may be mainly buckle folds that indicate Tephra in terrace deposits along the Wind River, Wyoming; implications for post 0.6 Ma up- clasts of locally derived Cretaceous shale and sandstone. Locally bedded and laminat- exposed, divisible into three parts (not distinguished on the map) that vary in thick- map area to help depict structure B 41 Qac Undivided alluvium and colluvium (Holocene to middle Pleistocene)—Sediments de- most of the movement and buckling of the block was down-dip to the north-northeast and at an ob- lift of theYellowstone hotspot [abs]: Geological Society of America Abstracts with Programs, hee 45 42 38 ed and contains clast-supported, pebble to small cobble gravel lenses. Mapped only ness. Upper part consists of tan, well-sorted, well-cemented, medium- to coarse- Gravel pit—Showing approximate outline of mined area h 44 posited in tributary valleys of Eagle River and as broad alluvial and colluvial aprons that lique to nearly perpendicular angle to western segments of the crescent-shaped, ramp-related folds. v. 24, no. 7, p. 102. Tb e Tb 60 along some tributaries of Eagle River, north of Eagle River. Thickness estimated to be grained, blocky- to massive-bedded sandstone and orthoquartzite. Middle part is inter- he Qcg 30 40 overlap and cover terrace deposits along the Eagle River. Includes stream-channel, Johnson, S.Y., Schenk, C.J., Anders, D.L., and Tuttle, M.L., 1990, Sedimentology and petroleum 42 M 2–7 m bedded, lenticular, dark-gray to black and yellowish-gray shale and lenticular, tan, pale- Mass movement structures and features GEOLOGIC HAZARDS 40 10 flood-plain, fan, sheetwash and colluvial deposits that interfinger. Character and com- occurrence, Schoolhouse Member, Maroon Formation (Lower Permian), northwestern Colo- 60 Qtl Terrace alluvium along tributaries of Eagle River (late and middle Pleistocene)—Allu- 40 35 37 position variable and dependent on local source rocks, topography, and relative posi- greenish- to yellowish-gray, and white sandstone. Lower part is crossbedded, thin- to Landslide scarp—Hachures on downslope side LANDSLIDES rado: American Association of Petroleum Geologists Bulletin, v. 74. no. 2, p. 135–150. Qac vium deposited by Alkali and Muddy Creeks that underlies at least two different terrace Qcg Qcg Qac tion within drainage. In northern part of quadrangle, where Cretaceous shales under- medium-bedded, light-gray to tan sandstone interbedded with thin lenticular, gray shale Kirkham, R.M., 1997, Late Tertiary and Quaternary collapse related to dissolution and flowage of 30 levels that are undifferentiated on the map. Exposed mainly as gravelly rubble with Qcg 70 43 41 lie most valley areas, deposits are chiefly light-gray to grayish-brown silty clay and and sparse lenses of pebble conglomerate; conglomerate consists of black chert and Pressure (buckle) ridge in landslide As used in this report, “landslide” is a general term that includes a wide variety of mass-move- Qac hee 60 Qcg 50 yellowish-brown, silty, fine and medium sand. Consists of rounded and well-rounded Pennsylvanian evaporitic rocks in the Glenwood Springs area, Colorado, in McCalpin, J.P., 12 30 44 clayey silt with some matrix-supported, angular to sub-rounded shale and sandstone quartz clasts in iron-stained matrix of medium-grained to very coarse grained, silica- 40 ment deposits and processes involving the slow to rapid downslope transport of surficial materials 36 pebble-boulder gravel that is capped by and mixed with subangular to subrounded peb- Strike and dip of contorted beds in landslide compiler, The active geologic environment of central Colorado-Aspen-Glenwood Springs-Silt, Qcg 70 clasts; deposited principally as interfingered colluvium and sheetwash. These northern cemented sand. In the vicinity of Wolcott, the stratigraphic position of the boundary and bedrock blocks by gravity. This definition includes various types of flows, slumps, slides, and he 85 45 Colorado: Field Guidebook, 1997 Friends of the Pleistocene field trip, Rocky Mountain cell, Qc 35 30 50 35 ble-boulder clasts of basalt. Rounded and well-rounded gravel clasts consist primarily between Upper and Lower Cretaceous rocks is not known. Existing data suggests that combinations thereof. In the map area, landslides and landslide deposits are very common along 60 Qac 20 deposits are commonly overlain by gravel clasts that appear to be lag gravels of older, Day 2, p. 1–18. 20 Qcg 85 50 hee 80 of white and tan quartzite and red sandstone, with sparse clasts of white granite and the boundary probably is present within the Dakota Sandstone (Wanek, 1953; W.A. 5 Minor fold in shale beds in landslide the north flank of Bellyache Ridge and in the northern part of the quadrangle; they commonly oc- 40 36 high-level, Quaternary-Tertiary alluvium (QTg) present nearby. In southern part of Kirkham, R.M., Streufert, R.K., Scott, R.B., Lidke, D.J., Bryant, B., Perry, W.J., Jr., Kunk, M.J., 75 he Qcg 75 55 granodiorite and dark gneiss and schist. Clast composition is similar to Quaternary to Qc hee Qac 20 quadrangle unit is more variable: along Squaw Creek, gravelly alluvium interfingers Cobban, U.S. Geological Survey, oral commun., 1998). Dakota Sandstone is best ex- T cur on north-facing slopes in Cretaceous-age shale and sandstone whose bedding layers also dip 30 Qcg Tertiary alluvium (QTg) mapped nearby and to Tertiary alluvium (Tg) mapped to south- Fault—In cross section; arrows show directions of relative movement in plane of section; Driver, N.E., and Bauch, N.J., 1997, Active salt dissolution and resulting geologic collapse in 50 h posed in cliffs along north side of Eagle River north and west of Wolcott. Basal con- A north at moderate inclinations. Landslide deposits cover about 10% of the map area. Landslide Tg Qcg 30 ee 35 with small fan and colluvial deposits; most other southern deposits are fine sand, silt, T, movement towards observer; A, movement away from observer the Glenwood Springs region of west-central Colorado [abs.]: Geological Society of America 50 west. Mapped only near junction of Alkali and Muddy Creeks in north-central part of tact with underlying Morrison Formation (Jm) is sharp but rarely exposed, and is asso- types that are present in the map area include; earth flows, earth slumps, debris flows, debris 40 and clay derived principally from the Eagle Valley Formation and Eagle Valley Evapor- Abstracts with Programs, v. 29, no. 6, p. A-416. Qac 42 60 A' area. Terraces present at about 6 m and about 27–30 m above present Alkali Creek. Qc Tg 55 ciated with pebble conglomerate; no angular unconformity is apparent. Forms north- Bedding formline—Shown in subsurface of cross sections slumps, rock-block slides, translational slides, rock falls, and complex landslides (Varnes, 1978; 22 h 55 ite, and many of these deposits are evaporite-rich. Gullies within unit may be subject Kirkham R.M., and Widmann, B.L., 1997, Geologic map of the Carbondale quadrangle, Garfield Qcg e 50 28 Maximum preserved thickness about 3 m erly dipping slope along northern flank of central and eastern parts of Bellyache Cruden and Varnes, 1996). Many of the extensive landslide deposits in the map area were first h 85 75 h Qty to flash floods and, near canyon mouths, may be subject to debris flows related to un- County, Colorado: Colorado Geological Survey Open-File Report 97-3, scale 1:24,000. ee 47 38 Qcg 52 ee Younger terrace alluvium of Eagle River (late Pleistocene)—Alluvium deposited by Eagle Projected contact and fault formlines—Shown above topographic surface in cross 18 65 usually intense rainstorms. Fine-grained deposits of unit may be susceptible to hydro- Ridge. Highly resistant to weathering and forms abundant blocky landslide deposits mapped by Colton and others (1975) in their overview of Colorado landslides. 45 Qcg 30 h Kunk, M.J., Kirkham, R.M., Streufert, R.K., Scott, R.B., Lidke, D.J., Bryant, B., and Perry, W.J., he e 48 75 River that underlies terraces about 6–8 m above river. Unit consists of well-sorted, im- and other colluvial deposits in many localities in map area. On the map, Colluvial de- sections Landslide deposits were recognized by a combination of field observations and air photo analy- 40 Qcg 82 Tg compaction- and piping-related subsidence (particularly evaporite-bearing deposits), Jr., 1997, Preliminary constraints on the timing of salt tectonism and geologic collapse in the 45 bricated, rounded and well-rounded pebble-cobble gravel. Clasts consist predominately sis. Basically, landslide deposits were identified by (1) hummocky topography, including closed de- 30 Qcg 45 45 63 and expansive clays may be present in deposits derived from Benton and Pierre Shales posits composed primarily of Dakota Sandstone clasts are labeled Qcd and landslide Carbondale and Eagle collapse centers, west-central Colorado [abs.]: Geological Society of he 68 of white and pink coarse-grained granite, white and gray fine-grained granite and gran- deposits labeled Qlsd; diamictite deposits (QTdi) are also composed mostly of Dakota pressions, (2) the deflection of stream channels at the toes of deposits, (3) headwall scarps and de- 25 he (Kb and Kp). Maximum thickness estimated to be about 10 m America Abstracts with Programs, v. 29, no. 6, p. A-416. hee 15 60 odiorite, dark gneiss and schist, white, pink, and tan quartzite, fine- to medium-grained Sandstone clasts. Commonly jointed, and Dakota Sandstone cliffs are very susceptible pressions, (4) vegetation differences between landslide and adjacent stable areas, (5) anomalous 35 20 he Qcg Qfo Older fan alluvium and colluvium (middle Pleistocene)—Light-brown, poorly sorted silt, Lovering, T.S., and Johnson, J.H., 1933, The meaning of unconformities in the stratigraphy of 20 red sandstone, and basalt. Some clasts have a thin, white carbonate coat. Matrix con- to rock falls. Total thickness about 60–85 m strikes and dips of bedrock blocks, and (6) intact masses of material downslope of their sources. 55 sand, and clay with matrix-supported, subangular to rounded, pebble-cobble gravel of central Colorado: American Association of Petroleum Geologists Bulletin, v. 17, p. Qdu 75 85 sists of pale-brown, silty sand and sand. Gravel is commonly overlain by about 1 m of Qcg Qcg 50 h 60 locally derived sandstone and other rock types. Forms low-gradient fan deposits at Jm Morrison Formation (Upper Jurassic)—Interbedded, calcareous sandstone, siltsone, mud- Low sun angles in the early morning or late afternoon highlighted the subdued topography of Tb P m 5 45 Qdu 58 353–374. overbank materials consisting of massive, light-yellowish-brown, silty fine sand and fine stone, shale, and limestone. Upper 90–120 m is dominantly interbedded and lenticu- 106˚45' many old landslide deposits and facilitated their identification in the field. Tg h Tb base of some landslide deposits in northeastern part of map area. The older fan de- 106˚37'30" Marvin, R.F., Mehnert, H.H., and Mountjoy, W.M., 1966, Age of basalt cap on Grand Mesa: U.S. h ee sand. Wedges of undivided alluvium and colluvium (Qac) overlie and cover terrace 39˚ For the most part, landslides in the map area were subdivided based on source material, which P m Qcg posits cut and are younger than these landslide deposits. Present adjacent to intermit- lar red, green, and purplish-gray siltstone, mudstone, and shale interbedded with Geological Survey Professional Paper 550–A, p. A81. 45' appears to exert a fundamental control on the types of landslide deposits developed (table 1). The tent tributaries of Eagle River at elevations 25–45 m above active channels of Ute and greenish-gray calcareous sandstone and gray limestone. Lower 30 m consists of four Murray, H.F., 1958, Pennsylvanian stratigraphy of the Maroon trough, in Curtis, B.F., ed., Sym- most common and largest landslide deposits are derived from the Cretaceous Formations. Specifi- Cache Creeks. Some undifferentiated alluvium and colluvium (Qac) in northwestern or more 1–3-m-thick, white, lenticular, calcareous, fine-grained, crossbedded sand- posium on Pennsylvanian rocks of Colorado and adjacent areas: Rocky Mountain Association 1 o 12 / 2 cally the Dakota Sandstone (Kd), Benton shale (Kb), and Niobrara Formation (Kn). Base from U.S. Geological Survey, 1962 SCALE 1:24 000 Geology mapped by Dave Lidke in 1996; assisted by Kevin M. Theissen part of map area may be remnants of similar older fan deposits. Maximum thickness stone beds interbedded with pale-greenish-gray, sandy siltstone and shale; oolitic, light- of Geologists, p. 47–58. Shaly rocks of the Benton and Niobrara fail similarily and commonly form large masses of rela- Photorevised 1987 1 1/ 2 0 1 MILE estimated to be about 5 m gray limestone beds are locally present in lower part. White cross-bedded sandstone Pierce, K.L., Obradovich, J.D., and Friedman, I., 1976, Obsidian hydration dating and correlation

H Map, sections, correlation, and index map digitized by Dave Lidke T beds in lower part are similar to white sandstone at top of Entrada Sandstone (Je). tively intact, but folded or rumpled shaly beds that have detached and slipped downhill above mod- Lambert conformal projection. 10,000-foot grid ticks based on R CONVERSION FACTORS of Bull Lake and Pinedale glaciations near West Yellowstone, Montana: Geological Society of O COLORADO Layout design, and digital layout by Denny Welp and F. Craig Brunstein Basal contact with underlying Entrada Sandstone (Je) mapped beneath lowermost erately dipping bedding surfaces of the underlying stable bedrock. The humocky surfaces that de- Colorado coordinate system, central zone. 1,000-meter N 1 .5 0 1 KILOMETER COLLUVIAL DEPOSITS America Bulletin, v. 87, p. 703–710. IC T Multiply By To obtain greenish-gray siltstone or shale of sandy lower part. Poorly exposed throughout most velop on these deposits are most spectacular on aerial photographs, where the entire extent of the E Manuscript approved for publication August 14, 1998

Universal Transverse Mercator grid ticks, zone 13. N Richmond, G.M., 1986, Stratigraphy and correlation of glacial deposits of the Rocky Mountains,

G TRUE NORTH rumpled surfaces of individual landslide masses can be viewed. Particularly striking in this regard A centimeters (cm) 0.3937 inches (in.) Colluvium (Holocene and late Pleistocene)—Clast and matrix supported, mostly angular of map area; commonly weathers to rounded slopes covered by colluvium and rock-fall the Colorado Plateau, and the ranges of the Great Basin, in Sibrava, V., Bowen, D.Q., and 1927 North American Datum M CONTOUR INTERVAL 40 FEET MAP LOCATION meters (m) 3.281 feet (ft) to sub-angular, pebble- to boulder-size clasts in sand, silt, and clay matrix derived from deposits derived from Dakota Sandstone. Best exposed along ridge west of Red Can- are the Benton Shale landslide masses (Qlsb) in the Milk Creek drainage in the northwest part of DASHED LINES REPRESENT 20-FOOT CONTOURS Richmond, G.M., eds., Quaternary glaciations in the United States: Quaternary Science Re- APPROXIMATE MEAN yon near eastern border of map area. Total thickness 120–155 m quadrangle. The type of slope failure and movement suggested by the surface character of these DECLINATION, 1998 NATIONAL GEODETIC VERTICAL DATUM OF 1929 kilometers (km) 0.6214 miles (mi) weathered bedrock. Clast lithology, size of clasts, and character of clast support varia- views, v. 5, p. 99–127. deposits is deep creep (Varnes, 1978). Deep creep slide masses characteristically move imper- ble and dependent mostly on source rocks, slope gradient, and relative position on Je Entrada Sandstone (Upper Jurassic)—Primarily salmon-pink, fine- to medium-grained, Richmond, G.M. and Fullerton, D.S., 1986, Introduction to Quaternary glaciations in the United ceptibly slow, but over long periods of time they may move tens to hundreds of meters, causing de- slope. Easily eroded source rocks tend to form finer grained, matrix-supported depos- well-sorted, well-rounded, crossbedded, calcareous, cliff-forming sandstone. Upper States of America, in Sibrava, V., Bowen, D.Q., and Richmond, G.M., eds., Quaternary glaci- formation of shaly bedding layers within the slide masses. In the map area, much of the move- its with fewer and smaller clasts; resistant source rocks, such as those of the Dakota 1.5–3 m consists of white, crossbedded, calcareous sandstone similar to sandstone in ations in the United States: Quaternary Science Reviews, v. 5, p. 3–10. ment is probably Late Pleistocene or older in age; however, imperceptible, slow creep, might local- Formation (Kd), tend to form blocky, matrix-supported, coarser-grained deposits with lower part of Morrison Formation. Large-scale, low- to moderate-angle, tangential Schenk, C.J., 1992, Stratigraphic Correlations Between the Eagle Valley Evaporite and Minturn ly still be active in some deposits. Human activities or natural events, such as road cuts, stream in- more and larger clasts. Colluvium derived dominatly from Dakota Formation (Kd) and crossbedding is common and, combined with frosted sand grains, suggests eolian dep- Formation, Eagle Basin, northwest Colorado: U.S. Geological Survey Bulletin 1787–GG, p. from gravelly alluvium (Qtl, QTg, and Tg) are distinctive and are differentiated as such osition in dune fields. Weathers to form distinctive, salmon-colored, blocky to round- cision along the toes of landslides, and increased precipitation, may aid in reactivating creep in SYNCLINE GG1–GG8. parts of these deposits. In the Milk Creek drainage these deposits locally appear to be as much as A A' in most places on the map (Qcd and Qcg). Locally may include talus, landslide depos- ed, cliff faces in which crossbedding is often obscured. Entrada sandstone is best ex- Scott, R.B., Lidke, D.J., Shroba, R.R., Hudson, M.R., Kunk, M.J., Perry, W.J., Jr., and Bryant, 30 m thick, whereas in the eastern part of the north Flank of Bellyache Ridge the deposits proba- its, sheetwash deposits, and debris-flow deposits that are too small or indistinct to map posed in cliffs directly north of Eagle River in western part of map area. Contact with Bruce, 1998, Large-scale active collapse in western Colorado: interaction of salt tectonism METERS FEET separately. Areas of colluvium may be susceptible to debris flows, rock falls, and small underlying red shale and siltstone of Chinle Formation (dc) is sharp and unconforma- bly do not exceed about 15 m. ANTICLINE and dissolution, in Brahana, J.V. and others, eds. Gambling with groundwater—physical, Bellyache Ridge 10,000 The Dakota Sandstone commonly forms landslide deposits (Qlsd) composed of broken and 3,000 slumps and landslides; fine-grained deposits may be prone to settlement from piping ble; locally shows slight angular discordance. Total thickness 17–25 m chemical, and biological aspects of aquifer-stream relations: American Institute of Hydrology, d chaotic debris mixed with surficial materials and locally these deposits contain sandstone blocks as and hydrocompaction, particularly where derived from evaporitic rocks. Maximum c Chinle Formation (Upper Triassic)—Brownish-, purplish-, and orangish-red, calcareous Proceedings of the joint meeting of the XXVIII Congress of the International Association of RANCH

WOLCOTT SYNCLINE much as a few meters in length. These deposits formed by slope movement processes ranging thickness as much as 15 m, but generally much thinner shale, siltstone, mudstone, and very fine grained sandstone. Contains distinctive lime- Hydrogeologists and the annual meeting of the American Institute of Hydrologists, Las Vegas, Qc Colluvium undifferentiated—Dominantly fine-grained deposits of sand, silt, and clay that stone-pebble conglomerate in beds ranging from 15 cm to 2 m thick and some thin, from debris and rock slides to debris flows as defined by Varnes (1978). Individually mapped slide Kd Nevada, USA, p. 195–203.

Jm 9,000 CREEK Kd include matrix-supported, angular, pebble- to cobble-size clasts of underlying bedrock mottled red and gray limestone beds. Limestone-pebble conglomerate consists of lime- HORN masses commonly are complex in that they include components of slides, flows, and even rock Jm Qc B-B' Sheridan, D.S., 1950, Permian (?), Triassic, and Jurassic stratigraphy of the McCoy area, west-

SECTION Qcd Colluvium of Dakota Sandstone—Angular, pebble- to large boulder-size clasts of white stone and red sandstone pebbles in light-gray or reddish-brown, sandy limestone ma- falls and topples near cliffs and cliffy headwall scarps. Most of the large slide masses derived from Je Qac Qlsd Qac central Colorado: Boulder, University of Colorado, M.S. thesis, 88 p. d Kd to tan sandstone and orthoquartzite that are mostly clast supported in matrix of silty trix. Paleosols identifiable in good exposures have been described by Dubiel (1992). the Dakota Sandstone appear to be relatively old and relatively stable, probably principally late 2,500 c Qac A Je he h h he Stauffer, J.E., 1953, Geology of and area west of Wolcott, Eagle County, Colorado: Boulder, Uni- HORN RANCH ANTICLINE A e ee T sand. Locally contains blocks as much as 3 m in length Gartra Sandstone member (not mapped separately) forms base of unit and consists of Pleistocene and older, but unconsolidated material in these slides has the potential for becoming dPsb Jm T d Qac 8,000 Block-glide versity of Colorado, M.S. thesis, 56 p. c destabilized by human activity such as road cuts and by natural or human-induced increases in wa- h Je Qcg Gravelly colluvium—Derived from Quaternary-Tertiary, gravelly alluvium (Qtl, QTg, and 3–8 m of purplish-red and reddish-brown, fine-grained to very coarse grained and peb- folds Thomas, C.R., McCann, F.T., and Raman, N.D., 1945, Mesozoic and Paleozoic stratigraphy in P ms Squaw Creek d

Eagle River ter saturation. For example, a road cut (Highway 6, now the frontage road of I-70) destabilized the I-70 Psb dc Tg). Forms distinctive gravelly slopes near depositional sites of gravelly, source alluvi- bly, silica-cemented sandstone that locally is conglomeratic; commonly weathers to Phm Qac northwestern Colorado and northeastern Utah: U.S. Geological Survey, Oil and Gas Investi-

blocky or massive sandstone ledges. In and adjacent to the map area, Stauffer (1953), UTE edge of an old landslide (Qlsd) and that edge reactivated to form the recent landslide deposit Qfy dPsb um. Distinguished from alluvial deposits principally by position on slope; locally may gations, Preliminary Chart 16. Qac Qalc (Qlsr), which episodically reactivates during moist periods. Several smaller, recent slumps and Phms 7,000 include some stable alluvium Wanek (1953), and Hubert (1954) all mapped the Shinarump Formation at the base of Tweto, Ogden, and Lovering, T.S., 1977, Geology of the Minturn 15-minute quadrangle, Eagle slides (not mapped) along Interstate 70 east of the Qlsr deposit are also examples of recent slope Phms Landslide deposits (Holocene to middle Pleistocene)—Includes a variety of materials: the Chinle Formation, apparently based on previous nomenclature (Lovering and and Summit Counties, Colorado: U.S. Geological Survey Professional Paper 956, 96 p. 2,000 failures that have resulted from destabilization of these relatively old, unconsolidated, landslide de- hee from relatively intact masses of bedrock to unconsolidated surficial material and rock Johnson, 1933; Thomas and others, 1945) and on the presence of petrified wood Tweto, Ogden, Moench, R.H., and Reed, J.C., Jr., 1978, Geologic map of the Leadville 1˚ × 2˚ , posits. Other unconsolidated deposits on slopes, such as colluvium (Qc, Qcd, Qcg) and diamicton fragments that have moved down slopes. Deposits mostly formed as rock falls and fragments. Their descriptions, thickness estimates, and stratigraphic placements of the quadrangle, northwest Colorado: U.S. Geological Survey Miscellaneous Investigations Series Phm (QTdi), also have some potential to become destabilized and form small slump or slide masses. 6,000 topples, translational and rotational rock and debris slides, and rock and debris flows. Shinarump at the base of the Chinle suggest that the Gartra Sandstone, identified at Map I–999, scale 1:250,000. Near the crest of Bellyache Ridge, the Dakota Sandstone (Kd) and the uppermost part of the he These differing types of mass movement deposits are broadly lumped under the term base of Chinle in this report and by Dubiel (1992), comprises most of their Shinarump T Varnes, D.J., 1978, Slope movement types and processes, in Schuster, R.L., and Krizek, R.J., h Morrison Formation (Jm) have also failed as a large, essentially intact, block-glide mass that slid e "landslide" after Varnes (1978). Many of the landslide deposits mapped are complex in Formation. Petrified wood fragments are present in float at a few localities, but are eds., Landslides, analysis, and control: National Academy of Sciences, Transportation Re- d northward down slope. The toe of this large block-glide appears to have been folded above its un- that they include two or more of the general types mentioned. Scarps and remnants of sparse in outcrop. Basal contact with underlying State Bridge Formation ( Psb) is WOLCOTT search Board Special Report 176, p. 11–33. 1,500 5,000 SYNCLINE derlying block-glide fault as it slid (see map, cross section B-B’, and discussion of structural fea- scarps within many of these deposits suggest some parts of the deposits moved recur- sharp and easily located where the distinctive Gartra Sandstone is well exposed; the Wanek, L.J., 1953, Geology of an area east of Wolcott, Eagle County, Colorado: Boulder, Univer- tures). Braddock (1978) has discussed and described similar large block-glide masses that involve rently. Scarps and pressure (buckle) ridges shown on the map were mostly identified Gartra rests unconformably on reddish-brown and orangish-red, very fine grained sand- sity of Colorado, M.S. thesis, 62 p. d the Dakota Sandstone and Morrison Formation in several locations along the Colorado Front on aerial photographs. In the map area, landslide deposits were subdivided broadly stone and siltstone of the underlying State Bridge Formation ( Psb). Where the Gar- Warner, L.A., 1956, Tectonics of the Colorado Front Range, in American Association of Petrole- based on age of movement, material, and dominant type of slope movement tra Sandstone is poorly exposed or absent, contact appears gradational and is difficult Collapsed Range. The Bellyache Ridge block glide formed before formation of the landslide deposits (Qlsd) 4,000 um Geologists Rocky Mountain Section, Geologic Record, 1956, p. 129–144 Recent landslide deposit (latest Holocene)—Heterogeneous mixture of unconsolidated to locate precisely. In map area, unit thickness ranges from about 80 to 110 m; unit that cover its northern edge; therefore, movement of the block-glide mass probably occurred in the h hee Qlsr basaltic flows Whitney, J.W., Piety, L.A., and Cressman, S.L., 1983, Alluvial history in the White River basin, north- ee early Pleistocene or earlier. surficial material and rock fragments showing a fresh (unvegetated) scarp headwall. thins to east and northeast. Hubert (1954) reported a total thickness of as much as west Colorado [abs.]: Geological Society of America Abstracts with Programs, v. 15., p. 328. 1,000 The potential for rock-fall hazards exist along nearly all of the steep cliffy slopes in the map Only one deposit mapped, which is prominent along Highway 6 on south side of Ea- 253 m directly south of map area and noted rapid thinning of Chinle to northeast Williams, V.S., Selner, G.I., and Taylor, R.B., 1996, GSMCAD, a new program that combines the area, particularly those containing sandstone ledges. The Dakota Sandstone cliffs appear to be par- gle River in west-central part of map area. Present along edge of older and larger dPsb State Bridge Formation (Lower Triassic and Permian)—Orangish-red to reddish-brown, functions of GSMAP and GSMEDIT programs and is compatible with Microsoft Windows and ticularly susceptible to rock falls as is evident from the abundant large sandstone boulders on slopes landslide (Qlsd) where cut by Highway 6. Other recent small slumps and slides are even-bedded, calcareous siltstone and very fine grained sandstone. Near top of forma- ARC/INFO: U.S. Geological Survey Open-File Report 96-007, one 3.5-inch IBM-compatible and at the base of slopes beneath cliffs of the Dakota Sandstone. present, but they are too small to map near the base of the north flank of Bellyache tion is a 3–6-m-thick, fine-grained, crossbedded, orangish-red sandstone that com- diskette. Ridge where old landslide deposits have been destabilized by road cuts of Highway 6 monly forms ledges. Lower part locally contains a 0.5–1-m-thick bed of medium-gray, B B' and Interstate 70. Maximum thickness about 10 m finely crystalline limestone or light-gray dolomite that may be correlative with the FLOODING FEET Qlsg Landslide debris of alluvial gravels (Holocene to middle late Pleistocene?)—Uncon- South Canyon Creek Dolomite Member (Hubert, 1954), which is mapped farther west 39˚ The flood plain of the Eagle River is very narrow throughout most of the map area, as de- METERS solidated debris consisting primarily of rounded and well-rounded gravel clasts of Qua- (Murray, 1958; Bass and Northrop, 1963; Freeman, 1971). North of map area, 37' 10,000 fined by the stream channel and flood-plain deposits (Qalc) along the Eagle River. The Eagle 3,000 30" Table 1.—Map units associated with potential geologic hazards in the Wolcott quadrangle ternary-Tertiary gravels (QTg) mixed with locally derived bedrock and surficial material. Sheridan (1950) subdivided the State Bridge Formation into three members, a subdivi- 0 0.5 1 MILE River floodplain has potential for flood hazard during prolonged or high precipitation and Bellyache Ridge Present as two deposits in north-central part of map area. Base of these gravelly land- sion that defines the thin South Canyon Creek Dolomite Member as the middle mem- runoff events (table 1). Other areas of possible flood hazard during such events include low- slides may extend beneath gravel into underlying shale of the Niobrara Formation (Kn). ber between much thicker upper and lower members. In western part of the map Expansive and Qc 1 KILOMETER lying areas along Squaw Creek and it’s tributaries and low-lying areas along Alkali, Muddy, Erosion A-A' 0 0.5 Ground 9,000 Maximum thickness about 5–10 m area Hubert (1954) measured a 50-m-thick lower unit, overlain by a 0.6-m-thick mid- corrosive Flooding Jm SECTION and Red Canyon Creeks. In addition to these locations, low-lying areas near the mouths of Warren Gulch Warren Qc Landslide debris of volcanic rocks (Holocene to middle Pleistocene?)—Unconsolidat- dle unit of gray limestone (South Canyon Creek Member?), which is, in turn, overlain subsidence Qcd Qlsv Rube, Milk, and Alkali Creeks may also be prone to rare floods and debris-flow events caused Mass wasting1 Gullying soils dPsb Qac Qcd EXPLANATION Tg Kd ed debris consisting primarily of angular to subangular, pebble- to large boulder-size by an 88-m-thick upper unit. The State Bridge Formation was not subdivided in map by intense thunderstorms in their headward regions. Numerous relatively low-lying locations Tb Tb Je UTE CREEK SYNCLINE Reverse fault 2,500 Qac clasts of basalt, minor gravel, and unconsolidated surficial material. Only two deposits area because the thin, middle limestone-dolomite does not appear to be present con- in the map area underlain by undivided alluvium and colluvium (Qac), particularly fine-grained QTg Qlsg QTdi Qac Qac hee Qac Kb Qalc Qc Qlsd mapped in basalt (Tb) field in southwestern part of map area. Other small basalt slides sistently and the upper and lower parts are not sufficiently distinct from one another. Southern zone of tight folds dc Qcd 8,000 deposits in the northern part of the quadrangle, may be susceptible to local flooding accom- Tg Qlsv Tb Qfy Qc he Qc Kp Qal Eagle River Block-glide fault hee Qcg Tb Phm Jm I-70 Qlsd and rubble are present along the south flank of Bellyache Ridge and suggest some po- The basal contact of the State Bridge Formation is sharp and easily located in most of Qlsd Qcd panied by rapid erosion and gullying during cloudburst or prolonged thunderstorm conditions. Qc Qlsn Kn hee Qac Qlsb Qac Kp tential for future slides and rockfalls along this flank. Maximum thickness about 10 m map area where the underlying Schoolhouse Member of the Maroon Formation High-angle fault h h af Qty Qalc Qlsn h Anticline or syncline h e P ms Kn Landslide of Niobrara Formation (Holocene to middle Pleistocene?)—Niobrara For- (P ms) is present. Where the Schoolhouse Member is present, the contact was map- GROUND SUBSIDENCE Qcd Qlsb Kb e Qfy 7,000 Tear fault mation (Kn) shale and limestone beds in extensive slide masses that formed on northerly ped at the base of reddish-brown to orangish-red State Bridge sandstone and siltstone General bedding or flow foliation orientation Qcg Qlsd Kd Qty? facing, bedding-plane dip slopes. Niobrara rocks appear relatively intact, but they are and directly above distinctive, light-colored, fine to coarse-grained sandstone of the T Nearby the map area and in localities elsewhere in Eagle and Garfield counties, significant prop- 2,000 Qlsr Qdu folded or warped above one or more northerly dipping zones of landslide failure planes Schoolhouse Member. Where Schoolhouse Member is absent, in easternmost part of Figure 1. Sketch map of structural features in the Wolcott quadrangle, Colorado erty losses have resulted from ground subsidence related to solution voids in evaporitic rocks, and ? from subsidence related to hydrocompaction of soils and low-density, surficial materials containing 1Includes landslides, debris flows, rockfalls and rock topples. ? ? 6,000 Kd Kb Jm Je Any use of trade names in this publication is for REGION OF EVAPORITE REMOVAL AND COLLAPSE dc 1,500 5,000 descriptive purposes only and does not imply he Phms GEOLOGIC MAP OF THE WOLCOTT QUADRANGLE, EAGLE COUNTY, COLORADO endorsement by the U.S. Geological Survey dPsb This map was produced on request, directly Phm 4,000 By from digital files, on an electronic plotter. he 1,000 David J. Lidke For sale by U.S. Geological Survey Information Services hee Box 25286, Federal Center, Denver, CO 80225 3,000 1998 This map is also available as a PDF file at http://greenwood.cr.usgs.gov