Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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The southeastern contact of the Mount Pomeroy subunit is in intrusive contact with Early Proterozoic granodiorite (Xgdf) for about 10,000 ft across the Maysville quadrangle (plate 1). The contact trends about N60°E and extends from about 1,500 ft southeast of the summit of Mount Shavano to Lake Shavano along Squaw Creek. The overall relation between the contact and topography suggest that the Mount Pomeroy subunit contact is vertical to steeply inward (northwestward) dipping across the Maysville quadrangle. The contact is mostly discordant and oblique to foliations in the Early Proterozoic granodiorite (Xgd). No finer-grained, chilled textures are evident in the Mount Pomeroy subunit along the contact. Evidence for this being an intrusive contact is supported by dikes (2 to 100 ft thick) of Mount Pomeroy subunit that cut Proterozoic rocks along the contact zone. Whole-rock chemical analyses were compiled on the various subunits of the Mount Princeton pluton (Shannon, 1988). Three analyses of the Mount Pomeroy subunit (from St. Elmo and Whitepine quadrangles) show the most within-group variation. A new whole-rock chemical analysis of the Mount Pomeroy subunit from the Maysville quadrangle is given in table 1. This sample has the lowest SiO 2 and K 2 O contents and highest Fe-total and MgO contents compared to all Mount Princeton pluton samples. The sample is chemically classified (De la Roche and others, 1980) as tonalite. The tonalitic composition of the Mount Pomeroy subunit contrasts with the composition of the interior of the Mount Princeton pluton which is predominantly chemically classified as granodiorite. Tmpf Mount Princeton finer-grained quartz monzonite subunit (late Eocene) – The Mount Princeton finer-grained quartz monzonite subunit is a light-gray to slightly pinkish-gray, medium- to coarse-grained equigranular rock (fig. 23). Plagioclase is white in contrast to the purplish-gray plagioclase in the Mount Pomeroy subunit. Biotite and hornblende, and relatively coarse and abundant accessory sphene, are conspicuous in hand sample. Thin section studies of this subunit showed minor clinopyroxene in one sample and accessory allanite, apatite, and zircon (Shannon, 1988). The average of three modal analyses showed 23.6 percent quartz, 29.3 percent alkali feldspar, 36.5 percent plagioclase, 9.8 percent hornblende-biotite, and 0.8 percent magnetite (Shannon, 1988), 100
indicating a granite b composition (IUGS classification). Specific sample location information is lacking for published and unpublished whole-rock chemical analyses thus it is difficult to determine if they include analyses on this subunit. The Mount Princeton finer-grained quartz monzonite subunit is present in a 900 ft by 1,400 ft by 1,700 ft triangular area in the northwestern corner of the Maysville quadrangle. The relationships between this subunit and the Mount Pomeroy subunit are complicated by structures related to the Mount Aetna cauldron ring zone and younger Rio Grande rift related faulting. The general field relationships suggest that the Mount Princeton finer-grained quartz monzonite is structurally and stratigraphically below and interior to the Mount Pomeroy subunit. Thus, the finer-grained quartz monzonite subunit is slightly younger than the Mount Pomeroy subunit. Miscellaneous magmatism (Late Cretaceous to late Eocene) – In the Maysville quadrangle this group includes two lithologic units that have uncertain genetic relations to the other Tertiary igneous rocks described above. The first is a quartz monzodiorite intrusion (Tqm) that has a possible genetic relationship to the Mount Princeton pluton. This tentative correlation is supported by a 40 Ar/ 39 Ar age determination conducted for this study. The second lithologic unit includes a low-density andesitic dike (Ta) swarm in the Proterozoic terrane in the southwest part of the Maysville quadrangle. The overall characteristics of the andesite dikes suggest they may be Cretaceous, late Eocene, or early Oligocene, but no cross-cutting relations even support a post-Proterozoic age. The andesitic dikes are described here but an age assignment is uncertain. Tqm Quartz monzodiorite (middle to late Eocene) – A quartz monzodiorite dikelike body is present about 13,000 feet south of Mount Shavano, on the south side of the North Fork, in the western part of the Maysville quadrangle. The intrusion was first described by Crawford (1913) and a similarity to a quartz diorite intrusion in the Whitepine quadrangle was noted. Cross-cutting relations, showing the quartz diorite is younger than Paleozoic sedimentary rocks and is older than the Mount Princeton pluton, led Crawford to suggest the same pre-Mount Princeton age for the intrusion near the 101
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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
Page 49 and 50: Figure 10. View west up South Arkan
Page 51 and 52: Figure 11. View west up South Arkan
Page 53 and 54: valley near the eastern map boundar
Page 55 and 56: lithologies are variable and depend
Page 57 and 58: Arkansas River in parts of Redman C
Page 59 and 60: angular to subangular. Deposits gen
Page 61 and 62: elations, age constraints from foss
Page 63 and 64: elated to the inferred Squaw Creek
Page 65 and 66: Figure 14. Crude bedding in coarse
Page 67 and 68: microcline, and muscovite. The latt
Page 69 and 70: 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: Shannon (1988). The average of seve
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 and 122: Xhig) along the southeastern contac
Page 123 and 124: Xgd Granodiorite (Early Proterozoic
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
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that the present structural configu
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elated to rotation of domain blocks
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sometimes terminate at a cross stru
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limestone that is exposed in a wind
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quadrangle are intimately associate
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of the quadrangle. The north-northw
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quartz monzonite ring dikes (Tma).
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that truncates the Upper Arkansas g
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as at the mouth of Squaw Creek. How
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adjacent to the southwestern edge o
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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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