Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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that Tertiary () andesite dikes (Ta) mapped in the Proterozoic terrane in the southwest quadrant of the quadrangle could be that old. No volcanic rocks related to the Middle Tertiary or Late Tertiary magmatic pulses are exposed in the quadrangle, although it is possible that some Middle Tertiary volcanic rocks could be preserved on the floor of the Upper Arkansas and South Arkansas grabens below the base of the Tertiary Dry Union Formation. Some of the faults mapped in the Proterozoic terrane in the western half of the Maysville quadrangle are interpreted to be Laramide faults or at least have Laramide heritage. Laramide folds and faults with clear evidence of pre-Mount Princeton pluton ages (about 36.6 Ma) are closely associated with the N40°W-trending belt of Paleozoic sedimentary rock outliers that extends from Aspen to the Kerber Creek area (fig. 5). The north-northwest-trending folds and north-northwest- and north-trending reverse and thrust faults responsible for preservation of the Paleozoic rock outliers described by Dings and Robinson (1957) in the nearby Monarch area were interpreted to be related to the Laramide orogeny. A number of these faults and associated Paleozoic sedimentary rocks are present about 6,000 feet west of the Maysville quadrangle boundary. The small Paleozoic sedimentary rock outliers in the southwest part of the Maysville quadrangle are interpreted to be the erosional remnants of a larger outlier that was disrupted by younger faulting related to the Rio Grande rift. It is suggested that this large Paleozoic rock outlier was originally down faulted into the Proterozoic basement during the Laramide. Thus, some of the faults in the area of the Paleozoic rock outliers probably have Laramide offsets. The lithologic-structural domains in the southwest quadrant of the quadrangle are bounded by northwest-, north-northwest-, and northeast-trending faults and broken rock (BR) zones. Some of the bounding faults are expressed by a combination of linear broken rock zones and linear lithologic-structural discontinuities that correlate with topographic features including saddles and swales on ridges and small gulches on the sides of the ridges. Other bounding faults are inferred, with no surface expression, and are placed at locations of lithologic and/or structural discontinuity. A number of these bounding faults are shown with limited strike extent because the evidence for a lithologic-structural discontinuity could not be demonstrated on the strike projection. These inferred faults 154
sometimes terminate at a cross structure, but some are shown to terminate with unclear structural relationship. Some of the north-northwest- and northwest-trending faults that have no topographic expression and no supporting evidence other than a lithologicstructural discontinuity may be healed faults as old as Proterozoic. BROKEN ROCK (BR) ZONES – Discrete zones of finely broken rock (BR) occur as linear, mappable zones with northwest, northeast and north-south trends in the Maysville quadrangle (plate 1). These only occur in the Proterozoic terrane and only affect various Proterozoic lithologies. The zones are characterized by the almost complete lack of outcrop and the rock occurs as a float concentration of finely comminuted chips generally smaller than ½ inch and commonly less than ¼ inch. The other important characteristics of BR zones are the presence of a weak, orangish to reddish-brown limonite staining on the rock chips and the presence of slickenside surfaces on some of the chips. The slickensided material coating the surfaces of the rock chips is variable and ranges from limonite and/or hematite to less common phyllosilicates including chlorite and sericite. The linear BR zones are best observed where they cross ridges and are topographically expressed as flats, swales, or saddles. The relatively straight trend pattern of the northwest-trending BR zones relative to topography suggests they have steep to vertical orientations. The overall character and float pattern of BR zones suggest they are brittle fault zones that were broken or shattered and represent fluid circulation zones that deposited light Fe-oxide and locally phyllosilicate coatings on fracture surfaces. The slickensided surfaces on the chips suggest recurrent movement on the fault zones. The BR zones are finely brecciated rocks; however, they are not true breccias because they never have evidence of a matrix or cement that binds the fragments. The BR zones are similar to the reactivated broken zones that are associated with Precambrian shear zones in the Front Range (Shannon, 1983, unpub. mapping). However, the BR zones in the Maysville quadrangle are not cored with mylonitic, ductile shears and the broken rock is much more finely comminuted. The size of the broken rock fragments may be related to the rock type. Berthoud-type granite and pegmatite dikes 155
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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
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Shannon (1988). The average of seve
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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 and 150: of the Proterozoic metamorphic rock
Page 151 and 152: that the present structural configu
Page 153: elated to rotation of domain blocks
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
Page 201 and 202: WATER RESOURCES Water resources on
Page 203 and 204: 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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