Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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poorly exposed. Individual sills are up to hundreds of feet wide and form continuous outcrop-float zones that are thousands of feet long. On the north side of the North Fork the granite and pegmatite sills and hornblende intermediate gneiss are oriented about N30°W with moderate (30° to 55°) dips to the northeast and on the south side of the North Fork they are oriented about N35°E with moderate-steep (61 to 81°) dips to the southeast and northeast. Thus, the Xgp/Xhig zone appears to bend or warp as it crosses the North Fork. The strongly concordant nature of granite and pegmatite (Xgp) sills and the finegrain size and presence of weak penetrative metamorphic fabrics in the granite contrasts with the characteristics of the Berthoud-type granite and pegmatites (YXgp). The granite and pegmatite described here are interpreted to be older than the Berthoud Plutonic Suite and possibly related to, or older than, the Routt Plutonic Suite. The Xgp/Xhig unit is distinctive because of its bimodal mafic-felsic character. No relict textures that might provide indications of protoliths are preserved in these rocks. Xmd Microdiorite dikes (Early to Middle Proterozoic) – Two augite microdiorite dikes cut the Proterozoic rocks in the western half of the Maysville quadrangle. They are dark gray to dark greenish gray, very fine to fine grained, lack phenocrysts, and contain abundant, fine-grained, randomly-oriented, subhedral, elongated plagioclase laths. Abundant clinopyroxene with subophitic texture and minor brown hornblende are the main mafic constituents. Estimated modes are about 60 percent plagioclase, 30 percent augite, 5 percent skeletal magnetite, and less than 5 percent hornblende, quartz and biotite. Plagioclase is andesine (An 44), determined by the Michel-Levy method (15 grains). The rock is classified as a diorite (IUGS classification). Minor to trace blue-green actinolitic amphibole, quartz, and biotite are irregularly developed, appear secondary, and may be related to a very weak, metamorphic overprint. However, there are no preferred metamorphic fabrics. The dikes are weakly to moderately altered, including weak to moderate saussuritic (epidote-sericite) alteration of plagioclase, weak uralitic alteration of augite, and weak to moderate chloritization of hornblende. The southern dike contains trace disseminated pyrite and hematite and is cut by epidote and calcite veinlets. The longer western dike segment locally contains small, irregular pervasive epidote-garnet 118
endoskarn zones. The two widely separated augite microdiorite dikes are very similar and are probably of similar age. The northern dike is present in the range front area on the north side of Squaw Creek near the northern boundary of the quadrangle. It occurs as a 1,500 ft long concentrated float zone. Float patterns and trends suggest the dike is less than 3 ft thick, is oriented roughly north-south, and has a steep to vertical dip. It cuts a large linear pendant of Proterozoic amphibolite gneiss (Xag) and cuts the foliated biotite granodiorite (Xgdf), indicating that the microdiorite dikes are younger than, or are a late part of, the Routt-type intrusions. The second dike is present as two northwest-trending segments on the north side of Willow Creek in the Proterozoic terrane in the southwest part of the quadrangle. The two segments are in different lithologic-structural domains. The western dike segment is 2,600 ft long, from 15 to 25 ft thick, and cuts muscovite-sillimanite gneiss (Xmsg) and amphibolite gneiss (Xag). The contacts are not exposed, but the dike trends about N40°W, paralleling the strike of the gneisses. The east segment of microdiorite dike is about 400 ft long, 25 ft thick, and cuts interlayered amphibolite gneiss (Xag), calc-silicate gneiss (Xcs), and biotite felsic gneiss (Xbfg). The dike strikes N63°W and has a vertical dip, which is slightly oblique to the gneisses. This dike has narrow chilled margins and crops out near a northeast-trending Tertiary () andesite dike (Ta). Cross cutting relations could not establish relative ages due to poor exposures. ROUTT PLUTONIC SUITE- DENNY CREEK EQUIVALENT (~1.66 Ga) – The Routt-type coarsely porphyritic granodiorite (Xgd and Xgdf) is one of the most distinctive Proterozoic units in the region. The Routt-type intrusions in the Maysville quadrangle are interpreted to be satellite bodies of the largest Routt-type intrusion (about 25 miles long and 3 to 5 miles wide) that is present in the southern Mosquito Range. These Routt-type intrusions were previously referred to as the Trout Creek Augen Gneiss (Hutchinson and Hedge, 1967). Similar rocks in the central Sawatch Range were referred to as the Denny Creek Granodiorite Gneiss (Brock and Barker, 1972). Tweto (1987) recommended abandoning the term gneiss for these units and referred to them as Denny Creek granodiorite. Hutchinson and Hedge (1967) reported a 1,665 Ma Rb-Sr age for the 119
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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
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 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: eastern contact of the diorite intr
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
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
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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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