Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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The foliation in the granodiorite is generally conformable with the Proterozoic gneiss along the southern contact and becomes more variable away from the contacts and in the central part of the body. The predominant orientations are northwest with moderate to steep northeast dips and east-west with moderate to steep north dips. Farther to the northeast near the northern border of the quadrangle, the Early Proterozoic granodiorite (Xgdf) contact with the Early Proterozoic gneiss is truncated by the range-front Shavano fault zone. Two northeast-trending, elongated pendants of amphibolite gneiss (Xag) are present in the granodiorite in this area. The pendants are about 1,200 to 2,400 ft long in the Maysville quadrangle and extend into the southern part of the Mount Antero quadrangle. Gneissic layering in the pendants and foliations in the granodiorite are oriented about N75° to 85°W with moderate to steep dips to the northeast and southwest. Folding of foliations in the granodiorite was not observed, but variations in strike and especially local dip reversals suggest that foliations in the granodiorite and the contact with the gneisses are probably warped and folded. A large body of foliated granodiorite is present in the Mount Aetna cauldron collapse structure in the Garfield quadrangle. Crawford (1913) and Dings and Robinson (1957) interpreted it as a foliated Tertiary intrusion. However, strong similarities to Proterozoic foliated granodiorite (Xgdf) in the Maysville quadrangle led Shannon (1988) to suggest affinities with Routt-type Proterozoic intrusions. The foliated granodiorite at these localities is similar to Proterozoic foliated granodiorite in the Buena Vista West quadrangle (McCalpin and Shannon, 2005), Buena Vista East quadrangle (Keller and others, 2004), Castle Rock Gulch quadrangle (Wallace and Keller, 2003), and Mount Harvard quadrangle (Brock and Barker, 1972). A hybrid border facies (Xgdh) with finer grain size and higher mafic mineral content is locally developed in the foliated granodiorite along the Proterozoic gneiss contact but the zones are too small and discontinuous to map. The hybrid border rocks are variable, ranging from chilled, fine to medium-grained, mafic-rich, non-foliated to weakly foliated granodiorite with or without small microcline phenocrysts to strongly foliated gneissic granodiorite to quartz monzonite. 122
Xgd Granodiorite (Early Proterozoic) – Five intrusions of biotite granodiorite are present in the southwestern part of the Maysville quadrangle. The two larger bodies, on the west edge of the quadrangle, are apophyses connected to two of three irregular intrusions that were mapped as Pikes Peak Granite in the southeastern part of the Garfield 15’ quadrangle by Dings and Robinson (1957). They were later interpreted to be Early Proterozoic Denny Creek Granodiorite (Tweto, 1987, p. A26 and A27). The granodiorite is light to medium gray to pinkish gray and medium to very coarse grained (fig. 28). It is characterized by large 0.5 to 1.0 inch, tabular, subhedral to euhedral microcline phenocrysts in a medium-grained matrix of plagioclase, quartz, and biotite, with accessory magnetite and sphene. Microcline phenocrysts are randomly oriented to rarely, very weakly flow aligned. The mineralogy of the granodiorite is variable and the composition changes with proximity to contacts and to a lesser extent with variations in the amount of microcline phenocrysts. Estimates of mineral modes indicate 15 to 30 percent quartz, 25 to 40 percent microcline, 25 to 35 percent plagioclase, and 5 to 10 percent biotite and magnetite. Most of the samples are granite b but are close to the granite b-granodiorite join (IUGS classification). Rocks from the borders of intrusions are typically more mafic and finer grained and are locally transitional with hybrid granodiorite (Xgdh) locally developed in the contact zone. The largest body is a 2,500-ft by 3,000-ft by 4,000-ft triangular area across the mouth of Fooses Creek in the west edge of the Maysville quadrangle. The second apophysis is a 2,000 ft long, east-west oriented body that is about 6,000 ft north of the southwest corner of the quadrangle. Two smaller, isolated bodies of granodiorite, about 1,500 ft long are present in the area between and east of the two apophyses. The fifth intrusion of granodiorite (Xgd) is present in a 4,000-ft long east-west zone on the north side of the South Arkansas River between the North Fork and Lost Creek. Numerous small, dike-like bodies of granodiorite, too small to map, are present throughout the Proterozoic terrane in the southwest part of the quadrangle. The contacts of the granodiorite intrusions with the older Proterozoic gneisses are rarely exposed. However, float trends and most of the dikes are parallel, or subparallel and concordant to the gneissic layering. 123
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OPEN-FILE REPORT 06-10 Geologic Map
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TABLE OF CONTENTS Introduction…
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mylonitic, augen textures………
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INTRODUCTION LOCATION AND ACCESS Th
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SAWATCH RANGE RIFT-SHOULDER UPLIFT
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of the South Arkansas River (subseq
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EXPLANATION Rio Grande Rift Neogene
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in descriptions of the geologic uni
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During the Early and Middle Paleozo
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Mountains, and White River uplifts
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field are the largest remnants of t
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Figure 5. Detailed geologic-structu
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with the Colorado mineral belt (Boo
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Shannon, 2005). Widmann and others
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the Upper Arkansas graben (fig. 5)
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15-minute quadrangle, which is adja
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etween the northern and southern se
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on U.S. Forest Service color aerial
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We use fractional map units on the
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Qpt Pinedale till, undivided (late
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weathered, but Mount Princeton quar
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stratified, matrix-supported, bould
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as rockfalls, rock avalanches, rock
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stream-channel deposits of all pere
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Figure 10. View west up South Arkan
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Figure 11. View west up South Arkan
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valley near the eastern map boundar
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lithologies are variable and depend
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Arkansas River in parts of Redman C
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angular to subangular. Deposits gen
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elations, age constraints from foss
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elated to the inferred Squaw Creek
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Figure 14. Crude bedding in coarse
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microcline, and muscovite. The latt
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Volcanic Ash Figure 17. View of eas
Page 71 and 72: Figure 18. Close-up view of intense
Page 73 and 74: The western zone of brecciated Pale
Page 75 and 76: Tr Rhyolite dikes (early to late Ol
Page 77 and 78: as the overall N47°E trend of the
Page 79 and 80: One age determination on the rhyoli
Page 81 and 82: northwest-trending fault. Thus, fie
Page 83 and 84: on the North Fork leucogranite intr
Page 85 and 86: A small 400 by 400 foot body of Nor
Page 87 and 88: N12°E, 62°NW (about 6,000 ft sout
Page 89 and 90: The age of the quartz latite porphy
Page 91 and 92: Tma Mount Aetna quartz monzonite po
Page 93 and 94: of the Maysville quadrangle and the
Page 95 and 96: Mount Aetna ring dikes (Tma), one i
Page 97 and 98: the margins of the pluton, includin
Page 99 and 100: Shannon (1988). The average of seve
Page 101 and 102: indicating a granite b composition
Page 103 and 104: analysis by Crawford (1913), two an
Page 105 and 106: locally tanish to pinkish gray. It
Page 107 and 108: quadrangle (tables 1 and 3) were fr
Page 109 and 110: slices of Paleozoic sedimentary roc
Page 111 and 112: south-southwest of Maysville is a 9
Page 113 and 114: There is one additional Paleozoic s
Page 115 and 116: near the southwest margin of the qu
Page 117 and 118: eastern contact of the diorite intr
Page 119 and 120: endoskarn zones. The two widely sep
Page 121: Xhig) along the southeastern contac
Page 125 and 126: Hybrid granodiorite is locally deve
Page 127 and 128: Xcs Calc-silicate gneiss (Early Pro
Page 129 and 130: Many of the calc-silicate gneiss zo
Page 131 and 132: typically expressed by intermittent
Page 133 and 134: The field relations and characteris
Page 135 and 136: the north side of all three segment
Page 137 and 138: eplaced by muscovite. The western m
Page 139 and 140: sillimanite-quartz-muscovite that a
Page 141 and 142: sillimanite gneiss (Xmsg) and the m
Page 143 and 144: Figure 35. Outcrop of Proterozoic h
Page 145 and 146: samples) indicates they are compose
Page 147 and 148: Xal Lineated amphibolite (Early Pro
Page 149 and 150: of the Proterozoic metamorphic rock
Page 151 and 152: that the present structural configu
Page 153 and 154: elated to rotation of domain blocks
Page 155 and 156: sometimes terminate at a cross stru
Page 157 and 158: limestone that is exposed in a wind
Page 159 and 160: quadrangle are intimately associate
Page 161 and 162: of the quadrangle. The north-northw
Page 163 and 164: quartz monzonite ring dikes (Tma).
Page 165 and 166: that truncates the Upper Arkansas g
Page 167 and 168: as at the mouth of Squaw Creek. How
Page 169 and 170: adjacent to the southwestern edge o
Page 171 and 172: graben and the Upper Arkansas Valle
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GEOLOGIC HAZARDS Potential geologic
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In addition, older landslides may h
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sparse vegetation. One such area is
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Table 7. Historical (pre-instrument
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4262272N. ECONOMIC GEOLOGY Currentl
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A number of the larger pegmatite bo
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River placers near Buena Vista (Van
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The small mines and prospects along
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elatively recent (post-1970) explor
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contact of two Quaternary gravels (
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quadrangle. The first area is a clu
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y Sharp (1976) to be a fault slice
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quadrant of the Maysville quadrangl
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the large Paleozoic rock outlier) i
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WATER RESOURCES Water resources on
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water near Maysville is being produ
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to Rudy C. Epis: Colorado School of
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University, M.Sc. thesis, 50 p. Din
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Hutchinson, R.M., and Hedge, C.E.,
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Litsey, L.R., 1958, Stratigraphy an
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County, Colorado, in Berg, R.R., an
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Stark, J.T., and Barnes, F.F., 1935
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Tweto, O., 1987, Rock units of the
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quadrangle, Colorado: U.S. Geologic
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94A 391996 4263422 Prospect YXp;Xbf
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336A 396189 4274961 Prospect Xgdf B
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854 391849 4268852 42 Adit; Geochem
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Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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