Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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higher proportion of hornblende than amphibolite gneiss and are locally riddled with fine net veinlets of white plagioclase. Minor small zones of moderate to strong mylonitic amphibolite gneiss are present in the southwest part of the quadrangle but no continuity was established. Amphibolite gneiss is commonly intimately associated with areas of calc-silicate gneiss (Xcs) or has a weak to moderate calc-silicate overprint. The calc-silicate assemblage is dominated by epidote and, locally, actinolite. In areas of well-developed calc-silicate gneiss the adjacent amphibolite gneiss typically has a weak to moderate calcsilicate overprint characterized by garnet-magnetite-epidote-actinolite +/- clinopyroxene and biotite-chlorite assemblages. Four of the larger, continuous areas of calc-silicate gneiss (Xcs) are shown on the map. The amphibolite gneisses are locally cut by abundant sills and dikes of Berthoudtype granite and pegmatite (YXgp) and possibly older Routt-type granite and pegmatite (Xgp). They are also cut by Middle Proterozoic () microdiorite dikes and Tertiary () andesite dikes. The rocks were completely recrystallized during amphibolite-grade metamorphism. Rare relict textures include small plagioclase eyes that may represent remnant plagioclase phenocrysts or crystal fragments. The regular gneissic layering is generally interpreted to represent remnant, primary layering (stratification) in the protolith. This is supported by the presence of interlayered amphibolite agglomerate (Xaa) with well-preserved remnant breccia structures, the presence of randomly oriented amphibolite gneiss clasts in the amphibolite agglomerate, and the presence of massive amphibolite layers. The composition of the amphibolite gneiss suggests a mafic, basaltic protolith and remnant textures support an interlayered sequence of basaltic volcaniclastics, flows, and minor breccias. STRUCTURAL GEOLOGY The Maysville quadrangle is located in a tectonically complex region that is dominated by the Late Paleogene-Neogene Rio Grande rift (figs. 4 and 5). The structural interpretations presented here strongly corroborate the views of many earlier workers, 150
that the present structural configuration of the main rift components (axial basins, rift shoulder uplifts and accommodation zones) are largely influenced and controlled by preexisting structure (Taylor, 1975; Tweto, 1977, 1979a, and 1980a; Chapin, 1979 and 1988; Lindsey and others, 1983; Chapin and Cather, 1994). The strongest support for this interpretation comes from the similarity in the orientation of the main rift components and their bounding structures and regional structural trends produced by Proterozoic, Late Paleozoic, and Laramide tectonic events. Evidence of control of specific rift-related faults by reactivation of older structures is rare and difficult to prove. PROTEROZOIC DEFORMATION The Proterozoic terrane exposed in the western half of the Maysville quadrangle is lithologically and structurally complex. Most of this terrane, especially in the southwest quadrant, can be viewed as a complex arrangement of lithologic-structural domains that are bounded by faults. Most of the domain boundaries in the southwest part of the quadrangle have northwest to north-northwest orientations and a few have northeast orientations. Less evidence of multiple lithologic-structural domains is found in the Proterozoic terrane in the northwest part of the quadrangle, but the faults show the same two predominant north-northwest and northeast orientations. In general, the faults are poorly exposed and it is difficult to prove that any of them have Precambrian ancestry. No evidence for through-going, major Proterozoic shear zones analogous to the mylonitic-ductile shear zones common in the Front Range (Idaho Springs-Ralston Creek and Loveland Pass-Berthoud Pass shear zones) and in the northern Sawatch Range (Homestake shear zone) was found during this study. Minor zones of strong mylonitic fabrics were found in the southwest part of the Maysville quadrangle, but no continuity could be demonstrated for these zones and no indication that they are associated with the domain boundaries was found. The mylonite zones are interpreted to be the remnants of narrow Proterozoic shear zones that have been highly disrupted by subsequent deformation events. 151
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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
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Figure 18. Close-up view of intense
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The western zone of brecciated Pale
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Tr Rhyolite dikes (early to late Ol
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as the overall N47°E trend of the
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One age determination on the rhyoli
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northwest-trending fault. Thus, fie
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on the North Fork leucogranite intr
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A small 400 by 400 foot body of Nor
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N12°E, 62°NW (about 6,000 ft sout
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The age of the quartz latite porphy
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Tma Mount Aetna quartz monzonite po
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of the Maysville quadrangle and the
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Mount Aetna ring dikes (Tma), one i
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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 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: of the Proterozoic metamorphic rock
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
Page 173 and 174: GEOLOGIC HAZARDS Potential geologic
Page 175 and 176: In addition, older landslides may h
Page 177 and 178: sparse vegetation. One such area is
Page 179 and 180: Table 7. Historical (pre-instrument
Page 181 and 182: 4262272N. ECONOMIC GEOLOGY Currentl
Page 183 and 184: A number of the larger pegmatite bo
Page 185 and 186: River placers near Buena Vista (Van
Page 187 and 188: The small mines and prospects along
Page 189 and 190: elatively recent (post-1970) explor
Page 191 and 192: contact of two Quaternary gravels (
Page 193 and 194: quadrangle. The first area is a clu
Page 195 and 196: y Sharp (1976) to be a fault slice
Page 197 and 198: quadrant of the Maysville quadrangl
Page 199 and 200: 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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