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Cenozoic Geology of the Arkansas Hills Region of the Southern Mosquito Range, Central Colorado Gary R

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Cenozoic Geology of the Arkansas Hills Region of the Southern Mosquito Range, Central Colorado Gary R New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/22 Cenozoic geology of the Arkansas Hills region of the southern Mosquito Range, central Colorado Gary R. Lowell, 1971, pp. 209-217 in: San Luis Basin (Colorado), James, H. L.; [ed.], New Mexico Geological Society 22nd Annual Fall Field Conference Guidebook, 340 p. This is one of many related papers that were included in the 1971 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. 209 CENOZOIC GEOLOGY OF THE ARKANSAS HILLS REGION OF THE SOUTHERN MOSQUITO RANGE, CENTRAL COLORADO by GARY R. LOWELL Southeast Missouri State College Cape Girardeau, Missouri INTRODUCTION IONAL VOMmit cr" LOCATION Mt Hvver3 1.20. tem Jet The Arkansas Hills area is located northeast of Salida, ev Colorado, in the southernmost portion of the Mosquito Range. The study covers about 100 square miles within F E the southern half of the Cameron Mountain 15-minute 0 quadrangle. Elevations range from about 6,800 feet at Sa- lida to 10,993 feet atop Cameron Mountain. The major geologic features bounding the area include the upper Arkansas segment of the Rio Grande depression on the west, the Thirtynine Mile volcanic field on the north and east, and the canyon of the Arkansas River on the south. Adjacent communities include Salida, Wellsville, Howard, Cotopaxi, and Guffey. Accessibility is provided mainly by the Ute Trail, which connects Salida with Colorado State 11530 Highway 9, and by numerous mining and logging roads which branch from it (see figure 1 ) . ot;;;t8s 31 We Ichf PRE-CENOZOIC ROCKS 440110 A The pre-Cenozoic rocks in the Arkansas Hills area range • AI E Crestane in age from Precambrian to Permian 'and include rocks %fr.= C rr from all periods of the Paleozoic Era except Cambrian and FOREST La Silurian. The Precambrian rocks consist, for the most part, arRa IMA,M s3 3=, of metamorphics which have been intruded by granitic MONUMENR and gabbroic bodies and invaded by their associated dikes Hoye, and sills. Unconformably overlying the Precambrian ero- Del NOrt sion surface is a Paleozoic section which includes, from MCCCa bottom to top; Manitou Dolomite (Early Ordovician ), A t M O S A I Monte Vista Harding Sandstone (Middle Ordovician), Fremont Dolo- R r l c mite (Late Ordovician ), Chaffee Formation (Devonian) ), R A Leadville Limestone (Mississippian), and a very thick mmnygle sequence of Permo-Pennsylvanian strata. Preservation of blocks of Paleozoic strata is largely due to down faulting during Late Paleozoic and Late Cretaceous-Early Ter- H 500551 tiary orogcnies and subsequent burial by Cenozoic sedi- 10 ments and volcanic rocks. Pre-Cenozoic stratigraphic and structural features are shown on the enclosed geologic map, Ra (see back pocket). Las C. Lobfos. PANSY Monde PRE-VOLCANIC EROSION SURFACE Ar;Yero The Arkansas Hills are located on the western edge of an SCALE Of MILES extensive pre-volcanic erosion surface described by Epis and 0 5 10 30 40 Chapin (1968, p. 56-59) . This surface covers an area of ONE INCH EQUALS APPROXIMATELY 18 MILES about 5,000 square miles and extends from the foot of NA Kenosha Pass in northeastern South Park to the northern Wet Mountains and the Wet Mountain Valley. In an FIGURE 1. cast-west direction it stretches from the upper Arkansas Location map. 210 NEW MEXICO GEOLOGICAL SOCIETY TWENTY-SECOND FIELD CONFERENCE Valley to the Cripple Creek area, a distance of about 50 Pleasant Valley syncline to form the Whitehorn stock. miles. According to Epis and Chapin (1968, p. 57) the Sills, dikes, and apophyses pierce the inclosing rocks, par- pre-volcanic erosion surface was a relatively smooth plain ticularly the Permo-Pennsylvanian strata within which of low relief upon which small hills, as much as 800 feet laterally-continuous sills are common. The nature and high, were present. The surface slopes southward from an significance of the dikes and abundant xenoliths have been elevation of about 9,500 feet in South Park to about 8,500 discussed by Osborn and Rainwater (1934, p. 33-35) and feet in the northern Wet Mountains. The erosion surface Bhutta (1954, p. 85). Pyrometasomatic iron-ore deposits appears to have formed by Middle to Late Eocene time which formed along the contacts of the stock were de- and was subsequently broken by Late Cenozoic faulting scribed by Behre, Osborn, and Rainwater (1936). (Chapin, Epis, and Lowell, 1970). The central portion of the Whitehorn stock is character- In the area of investigation, volcanic rocks preserve an ized by knobby, bouldery outcrops, whereas towards the anomalous drainage pattern which trends east-west and periphery massive, sheeted zones are common. The rock crosses the Laramide structural grain and the present drain- typically has a bluish "salt and pepper" appearance on age at nearly a right angle. The Salida-Waugh Mountain fresh surfaces and a dull greenish-gray cast on weathered or and Gribbles Run paleovalleys arc remnants of east-flow- altered surfaces. The rock is holocrystalline and the grain ing palcodrainages incised in the pre-volcanic erosion sur- size varies from fine to coarse. Ouartz and orthoclase sur- face and later filled by Early Oligocene volcanic rocks (see round and embay the earlier-formed plagioclase and ferro- geologic map, back pocket ). The Salida-Waugh Moun- magnesian minerals. Accessory apatite, zircon, and sphene tain paleovalley has been traced for 11 miles from near are prominent in hand specimens and thin sections. A de- Salida eastward to Waugh Mountain. It averages 2 miles tailed petrographic analysis is given by Bhutta (1954, p. in width and has a maximum depth of about 1,000 feet 89). (Lowell, 1969, p. 62-63; Chapin, Epis, and Lowell, 1970). The Whitehorn stock is younger than all of the Paleo- The Gribbles Run paleovalley is about 4 miles long, 1/4 to zoic rocks and much of their deformation is older than the 1/2 mile wide, and about 500 feet deep. Both paleovalleys last phase of Laramide tectonism. By Late Eocene time the trend east-west, have westward or northwestward-forking stock had been unroofed and beveled by an erosion surface tributaries, and are cut by steep north-trending post- of moderate relief. volcanic faults. ASH FLOW 1 CENOZOIC ROCKS The basal volcanic unit in the Arkansas Hills is an ex- A composite table of stratigraphic units with correspond- tensive (chemically rhyolitic), mineralogically latitic to ing ages and thicknesses is shown in Table 1. A brief de- trachytic, ash-flow sheet which is the earliest major extru- scription of the various lithologies is given together with sive rock in the adjacent Thirtynine Mile volcanic field It is other pertinent data. a multiple-flow, simple cooling unit informally designated as Ash Flow 1 by Epis and Chapin (1968). Member flows WHITEHORN STOCK of the cooling unit are indistinguishable in the field and Prior to the onset of volcanism, granodioritic magma generally form prominent, ledgelike exposures with a strik- was intruded along the axial plane of the asymmetric ing eutaxitic fabric. The fabric is made conspicuous by the TABLE 1. Cenozoic stratigraphic units in the Arkansas Hills region of the southern Mosquito Range, central Colorado. AGE LITHOLOGIC UNIT THICKNESS Quaternary Alluvium Landslides, slump blocks, and colluvial deposits Late Miocene- Boulder gravels Pliocene Dry Union Formation Tenderfoot I Till Facies of Dry Union Formation ±450 Oligocene- Silicic Felsite of Section 36 0-200 Miocene Andesite of Big Baldy 0-200 (?) Upper Andesite (18.9 ± 1.2 m.y.) 300-1200 Latite of Waugh Mountain 200-700 Ash Flow 7 (34.8 ± 1.4 m.y.) +600 Latitc of East Badger Creek 250-300 Tuff of Badger Creek + 313 Antero Formation + 368 Ash Flow 1 (40.0 ± 1.4 m.y.) Pre-volcanic Erosion Surface (37.3 ± 1.9 m.y. and 35.4 ± 1.1 m.y.)-t Late Cretaceous- Whitehorn stock Late Eocene Epis and Chapin (1968) Van Alstine (1969) NEW MEXICO GEOLOGICAL SOCIETY-TWENTY-SECOND FIELD CONFERENCE 211 presence of subparallel pumice lapilli.
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