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Geology of the Eldorado Springs Quadrangle Boulder and Jefferson Counties, Colorado

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Geology of the Eldorado Springs Quadrangle Boulder and Jefferson Counties, Colorado Geology of the Eldorado C_-J / Springs Quadrangle Boulder and Jefferson , Counties, Colorado By JOHN D. WELLS CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY BULLETIN 1221-D Petrography, stratigraphy, and structural geology of the Precambrian metamorphic and igneous rocks, Paleozoic and Mesozoic sedimentary rocks, and Cenozoic surficial deposits UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.G. 20402 CONTENTS Page Abstract_____ _________ __ __ __ __ _________ Dl Introduction___________ ___ __ _ ___ 2 Location and geologic setting______--_______--__--___-_-_-_--___ 2 Previous and present work and acknowledgments._________________ 4 Description of rocks.____-----___----_____-_-------_-------_-______ 5 Precambrian rocks__________________-_____________ ______ 5 Metasedimentary rocks_____-__-_______ __________________ 5 Hornblende gneiss__-___________________ ____________ 7 'Biotite gneiss____-_______---------_-_____-____________ 7 Quartzite.. __ ____________________ ______ 8 Contact effects with igneous rocks.__________________ 11 Schist layers in quartzite___--___--_---____-____--______ 14 Calc-silicate rocks in schist_______--_--______-_-_-__-___ 17 Older igneous rocks_-__--_---_ __________________________ 18 Boulder Creek Granodiorite__________________________ 19 Inclusions in the Boulder Creek Granodiorite_________ 24 Quartz monzonite____________________________________ 24 Cataclastic rocks_________________ _____________________ 27 Augen gneiss _______________________________________ 27 Cataclastic gneiss __________________________________ 27 Younger igneous rocks-_______-_-___________-_-___-___-_-_- 28 Silver Plume Granite_________________-___-____________ 29 Aplite and pegmatite________________________________ 30 Chemical composition of Precambrian igneous rocks___________ 31 Paleozoic and Mesozoic sedimentary rocks._______________________ 31 Paleozoic rocks__________________________________________ 33 Fountain Formation___________________________________ 33 Lyons Sandstone_____--__--__-_-___---__- __________ 35 Lykins Formation.__________________________________ 36 Mesozoic rocks__________________________________________ 37 Ralston Creek Formation.. __________________________ 37 Morrison Formation____________________________ ____ 38 Dakota Group________________________________________ 39 Benton Shale___________________________ 40 Niobrara Formation____________________-_-___-________ 41 Pierre Shale_____________ _ ____ _ 42 Cenozoic rocks________________________________________________ 43 Tertiary intrusive rocks____ ___________ _________ 43 Diabase. _____________________ ________ ____ 43 Latite porphyry________________________-____----_-____ 44 Tertiary or Quaternary deposits-____-__-___---_____---___-_- 44 High-level alluvium.____________________________ __ 44 Pre-Rocky Flats alluvium______________________________ 45 m IV CONTENTS Description of rocks Continued Cenozoic rocks Continued Quaternary deposits.______--___-__---_-_--__--_____-______ D45 Pleistocene deposits... _____________________________ 45 Rocky Flats Alluvium ... 45 Verdos Alluvium_______-_-___--___________________ 47 Slocum Alluvium._________________________________ 48 Terrace alluvium__________________________________ 49 Recent deposits...____________________________________ 49 Colluvial deposits.--- _________________________ 49 Valley alluvium._____-_-_---_---_-_-_-____________ 50 Structural geology._______________-----------------_---------_---__ 50 Precambrian structural geology.._--___----___-_____________-__ 51 Lineation and foliation _____________________________________ 51 Folds.....____________.__...__.._._._._____.__ 52 Shear zones and faults.______---_--_-_-_-------_--___-_____ 52 Orientation of lineations.. _______________________________ 54 Sequence of structural events.___.____________________ ______ 55 Laramide structural geology.____________----_-__-_______-______ 57 Folds... _______________-_-.______________.__ 57 Faults.....___.-.._.._-_-._.---_______________ 57 Summary of geologic history...______________________________________ 60 Economic geology.________________----_---_-_-___-__________-_____ 61 Metallic resources.__________----_-_--__--______-____________'__ 61 Nonmetallic resources._________________________________________ 63 Ground-water resources._______________________________________ 67 Stratigraphic sections..__________________________________________ 68 References cited_______________________________________________ 82 ILLUSTRATIONS Page PLATE 1. Geologic map of the Eldorado Springs quadrangle..___.__In pocket FIGURE 1. Index map showing location of the Eldorado Springs quadrangle. _ D3 2. Triangular diagrams showing mineralogical composition of Precambrian igneous rocks______________________________ 22 3. Triangular diagram showing chemical composition of Precam­ brian igneous rocks._____________________________________ 33 4. Contour diagram of foliations in Precambrian igneous rocks____ 53 5. Diagram summarizing attitudes of lineations in rocks of the Coal Creek syncline_______.__________________________________ 55 6. Contour diagrams of lineations of Precambrian rocks on Scar Top anticline__-____-_-----_____-----_--_--___---_--.__ 56 CONTENTS TABLES Page TABLE!. Modes of biotite gneiss______._----__.-------.-_____. D8 2. Modes of quartzite___-----_--_---------_----------___ 10 3. Modes of specimens taken across the selvage zone at the contact of quartzite and quartz monzonite____________ 13 4. Calculated chemical composition of specimens from modal analyses shown in table 3__________________________ 14 5-11. Modes of 5. Schist layers in quartzite-_____________________ 16 6. Boulder Creek Granodiorite_-_------___----___- 20 7. Inclusions in the Boulder Creek Granodiorite____ 24 8. Quartz monzonite____________-__-_--__---___ 26 9. Augen gneiss----_---___-__-__--__-_-__----__- 28 10. Silver Plume Granite_________________ 30 11. Aplite.___________-____-____-_--.________ 31 12. Chemical analyses of Precambrian igneous rocks.________ 32 13. Correlation of terms used for Quaternary alluvial deposits. _ 46 14. Mineralogy of clay deposits determined from X-ray diffrac- tometer patterns___________________________________ 64 CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGY OF THE ELDORADO SPRINGS QUADRANGLE BOULDER AND JEFFERSON COUNTIES, COLORADO By JOHN D. WELLS ABSTRACT The Eldorado Springs quadrangle lies across the eastern margin of the Front Range immediately south of Boulder and about 25 miles northwest of Denver. The western two-thirds of the quadrangle is mountainous and is underlain by Precambrian metamorphic and igneous rocks. The upturned Paleozoic and Mesozoic rocks form The Flatirons and the hogback ridges to the east. The eastern part of the quadrangle is a plain covered by Pleistocene gravels. The valleys of Coal Creek, South Boulder Creek, and their tributaries, and the tribu­ taries of Boulder Creek dissect these rocks and form steep-walled canyons as much as 2,000 feet deep. The Precambrian rocks are a folded and sheared layered sequence of high- grade metasedimentary quartzite, schist, hornblende gneiss, and biotite gneiss. These rocks were intruded by Precambrian igneous rocks Boulder Creek Qrano- diorite, quartz monzonite, Silver Plume Granite, aplite, and pegmatite. Cat- aclastic gneisses were formed in the Idaho Springs-Ralston shear zone. The hornblende gneiss consists mostly of hornblende and plagioclase. The biotite gneiss is well foliated and is largely biotite, quartz, and plagioclase. Locally the gneiss contains sillimanite in contact with microcline; this feature indicates high-grade metamorphism. The quartzite and schist commonly con­ tain andalusite and locally contain cordierite, garnet, and sillimanite. The Paleozoic rocks are Pennsylvanian and Permian in age and consist of sandstone, shale, and conglomerate, generally red, in the Fountain, the Lyons,, and the lower part of the Lykins Formations. The Mesozoic rocks consist of gray and varicolored shale, sandstone, and limestone in the upper part of the Lykins Formation, Ralston Creek and Morrison Formations, Dakota Group, Benton Shale, Niobrara Formation, and Pierre Shale. A small remnant of a Tertiary gravel deposit occupies a high level in the mountains. Four pre-Wisconsin Pleistocene gravels the pre-Rocky Flats, Rocky Flats, Verdos, and Slocum alluviums form veneers on successive pedi­ ment surfaces east of the mountains. Wisconsin alluvial deposits occur im­ mediately above modern stream terraces. Recent alluvial deposits occur in the streams, and colluvial deposits occur on the slopes of the mountains and hills. Three periods of Precambrian metamorphism and structural deformation have been recognized in the area. Evidence of the oldest period, which was one of plastic deformation, is found as relict staurolite grains and warped lineation of the rocks. The second deformation was also plastic; it formed the Coal Dl D2 CONTRIBUTIONS TO GENERAL GEOLOGY Creek syncline and the Scar Top anticline and was accompanied by the intrusion of the Boulder Creek Granodiorite and the quartz monzonite. The third de­ formation, characterized by shearing, formed the relatively restricted Idaho Springs-Ralson shear zone and certain minor structures. The Silver Plume Granite and associated aplite and pegmatite were intruded relatively late in the
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