Geology of the Eldorado 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 STATE.S GOVER_N:MENT 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 Printin~ Office Washington, D.C. 20402 CONTENTS Page Abstract---------------------------------------------------------­ D1 Introduction------------------------------------------------------ 2 Location and geologic setting ___ - __ ---------------------------__ 2 Previous and present work and acknowledgments _________________ _ 4 Description of rocks ____________ - ___ -_---_---------------------_-__ 5 Precambrianrocks--------------------------------------------- 5 ~etasedimentary 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 Olderigneousrocks---------------------------------------- 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 ~esozoic sedimentary rocks _______________________ _ 31 Paleozoic rocks ___________________________________________ _ 33 Fountain Formation __ -------- __ ----------------------­ 33 LyonsSandstone-------------------------------------- 35 Lykins Formation _______________ ----_---_____________ _ 36 ~esozoic rocks ___________________________________________ _ 37 Ralston Creek Formation______ ---------------_--------- 37 ~orrison 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 ·I IV CONTENTS Description of rocks-Continued Page 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----------------------------------------------------Faults __________________________________________________ _ 57 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 __________ Jn 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 Creeksyncline __________________________________________ 55 ~ 6. Contour diagrams of lineations of Precambrian rocks on Scar Top anticline _____________________________ -.____________ 56 '• CON'l'ENTS v TABLES Pag~ TABLE 1. 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 immedintely south of Boulder and about 25 miles nortllwest 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 ea;stern 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 Grano­ diorite, quartz monzonite, Silver Plume Granite, aplite, and pegmatite. Cat­ aclastic gneisses were formed in the Idaho Spring&-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. -i 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