Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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major fault structures that bound the western side of the Rio Grande rift grabens. The timing of formation is also critical to the interpretation of BR zones. Do they cut any of the Tertiary intrusions including the Mount Princeton pluton, Mount Aetna cauldron-related rocks or the younger Mount Antero leucogranites It may be possible with detailed studies to date the age of slickensided phyllosilicate coatings in the broken rocks. The reason that they do not cut and affect the Paleozoic rock outliers (for a Laramide origin or post-Laramide origin) and the Oligocene rhyolite dikes (for a Neogene origin) is still a problem. The overall relationships support a two-stage structural process for formation of BR zones involving early Laramide faulting and reactivated Neogene faulting. POST-MIDDLE TERTIARY AND RIO GRANDE RIFT DEFORMATION The most obvious major faults on the Maysville quadrangle are the bounding faults that frame the western side of the Upper Arkansas graben (Shavano fault zone) and South Arkansas graben (Willow Creek fault) and the concealed fault (Salida-Maysville fault) that separates the Upper Arkansas graben from the South Arkansas graben. These faults clearly have major Neogene offsets related to formation of the rift grabens and juxtapose rift fill against Proterozoic and Tertiary rocks. In the northwest quadrant of the quadrangle the presence of the late Eocene-early Oligocene intrusions related to the Mount Princeton pluton and the Mount Aetna cauldron, and late Oligocene intrusions related to the Mount Antero leucogranites, provides a datum to evaluate the ages of faulting (fig. 38). The faults in this area include three subparallel north-northwest-trending faults, one north-south-trending fault, one west-northwest-trending fault, and two parallel, major northeast-trending faults. The north-northwest- and north-south-trending faults cut Proterozoic rocks and at least two of them appear to be truncated where they obliquely intersect the northeast-trending-range front Shavano fault zone. The north-northwest-trending faults continue northward where they cut the late Eocene-early Oligocene Mount Princeton pluton in the northwest corner 160
of the quadrangle. The north-northwest- and northeast-trending faults cutting the Mount Princeton pluton continue northward into the Mount Antero quadrangle where they cut the early Oligocene Mount Aetna ring dikes and the Oligocene Mount Antero leucogranite intrusions (Shannon, 1988). Thus, some of the faults have significant offsets that are younger than about 29.8 Ma (average age of leucogranites). The north-northwest-trending faults are the southern end of a zone of similar north-northwest-trending faults that define the western margin of a long, rift-shoulder horst block that is informally referred to as the Collegiate Peaks horst (figs. 4 and 5; Shannon and others, 1987c; Shannon, 1988). The down-to-the-west sense of offset across the western margin of the horst block was estimated to be about 2,600 ft on the basis of offset of internal textural igneous-stratigraphic horizons in the Mount Princeton pluton and other lithologic discontinuities. Additional support for differential uplift of the horst block is indicated by the distribution of peaks higher than 14,000 feet and the resetting of apatite fission-track ages. The down-to-the-west sense of apparent offset on the northnorthwest-trending fault (which offsets the Proterozoic Xag/Xhig-Xgdf contact in the northwest quadrant of the Maysville quadrangle) is compatible with it being related to the western margin of the Collegiate Peaks horst. The north-northwest-trending faults in the northwest quadrant of the quadrangle are of similar orientation to faults in the southwest quadrant, some of which are interpreted to be Laramide faults. It is suggested that the north-northwest trend of the Collegiate Peaks horst may be influenced by Laramide basement structures (and possibly even earlier Late Paleozoic and Proterozoic structures) that have been superimposed on the Oligocene intrusions. The northeast-trending faults in the northwest part of the Maysville quadrangle also fit into a larger regional picture of complex faulting in the Sawatch Range riftshoulder uplift. The northeast-trending fault crossing the very northwest corner of the Maysville quadrangle, herein informally referred to as the McCoy Creek fault, is situated between Mount Shavano and Tabeguache Peak and extends into the southwest corner of the Mount Antero quadrangle. This fault is oblique to the southeast margin of the Mount Aetna cauldron ring zone defined by various ring zone features including Mount Aetna 161
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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
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analysis by Crawford (1913), two an
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locally tanish to pinkish gray. It
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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 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: quadrangle are intimately associate
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
Page 205 and 206: to Rudy C. Epis: Colorado School of
Page 207 and 208: University, M.Sc. thesis, 50 p. Din
Page 209 and 210: 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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