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Joints in Precambrian Rocks Central City-Idaho Springs Area, Colorado

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Joints in Precambrian Rocks Central City-Idaho Springs Area, Colorado A 75" Joints in Precambrian Rocks Central City-Idaho Springs Area, Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 374-B Prepared on behalf of the U.S. Atomic Energy Commission and published with the permission of the Commission Joints in Precambrian Rocks Central City-Idaho Springs Area, Colorado By J. E. HARRISON and R. H. MOENCH SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 374-B Prepared on behalf of the U.S. Atomic Energy Commission and published with the permission of the Commission UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961 UNITED STATES DEPARTMENT OF THE INTERIOR Stewart L. Udall, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 20 cents (paper cover) CONTENTS Page Page Abstract-__________________________________________ B-1 Accumulation and interpretation of the joint data.__ _ ___. B-4 Introduction _______________________________________ Methods and problems of interpretation__________. 5 General geology.___________________________________ Precambrian joints in igneous rocks_______________ 5 Precambrian rocks._____________________________ Precambrian joints in metamorphic rocks._________ 7 Tertiary rocks._________________________________ Post-Precambrian joints _________________________ 12 Folds_______________________________________ References cited____________________________________ 14 Faults_____________________________ ILLUSTRATIONS Page FIGURE 1. Index map of Colorado-____-_-__--____--____________________-_-_-__----___-_---_--_--_---------_-__-_ B-1 2. Generalized geologic map______-_______________________. 3. Contour diagram of joints in granodiorite_______________ 4. Contour diagram of dikes in granodiorite _________________ 5. Contour diagram of joints in biotite-muscovite granite. ___. 6. Contour diagrams of joints in metamorphic rocks__________ 7. Diagrams showing highs interpreted from contour diagrams. 8. Theoretical joints__________________________ __________ 11 9. Contour diagram of shear joints.________________________ 12 10. Stereographic diagrams_______________________________ 13 TABLE Page TABLE 1. Summary of attitudes of joint sets in metamorphic rocks. B-10 ill 673504 61 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY JOINTS IN PRECAMBRIAN ROCKS, CENTRAL CITY-IDAHO SPRINGS AREA, COLORADO By J. E. HARRISON and R. H. MOENCH ABSTRACT Attitudes of about 9,000 joints in Precambrian intrusive and metasedimentary rocks in the 50-square-mile area studied re­ veal that some joints sets are systematically distributed. Each of the principal Precambrian intrusive masses contains pri­ mary joints. Joints related to folds have formed during two periods of Precambrian deformation and probably during Laramide (Late Cretaceous and early Tertiary) arching of the Front Range. A time sequence of geologic events and jointing can be worked out as follows: First, granodiorite was intruded during the older Precambrian deformation; primary joints formed in the granodiorite, and joints related to sinuous and doubly plunging folds formed in the metasedimentary rocks. Second, biotite- muscovite granite was intruded late in an older period of Precambrian deformation; primary joints formed in the granite. Third, Precarnbrian metamorphic and igneous rocks were locally deformed during a younger period of Precambrian folding; joints related to this folding formed in all Precambrian rocks, and a unique set of slickensided joints formed predominantly FIGURE 1. Index map of Colorado showing relation of the Central City- in the more massive granitic rocks. Fourth, a system of four Idaho Springs area to the Front Range highland. After Levering and Goddard (1950, fig. 14). joint sets was formed that locally cuts Precambrian rocks but not the Tertiary rocks that intrude them. Joints of this system In the course of the general study it became apparent were locally followed by Tertiary dikes, and therefore can be to us that several joint sets are related to Precambrian dated in outcrop only as post-Precambrian but pre-Tertiary. folds and intrusive bodies and that four joint sets are The conformity of the system with that expected from arching of the Front Range highland leads us to conclude that the persistent but apparently independent of these Pre­ post-Precambrian system probably is Laramide in age. cambrian structures. We studied the joints for the following reasons: (a) Joints obviously represent apart INTRODUCTION of the geologic history of any region; (b) multiple joint sets in rocks of complex structural history have, The Central City-Idaho Springs area is about 30 in our opinion, been ignored too often or dismissed as miles west of Denver and is a small part of the Front too complicated to interpret; (c) this discussion may Range of Colorado (fig. 1). The area was mapped in encourage other geologists working in the Front Range detail during the field seasons of 1952-54 as part of the to test an hypothesis set forth here concerning a regional U.S. Geological Survey's studies on behalf of the Divi­ joint system; and (d) a comprehensive knowledge of sion of Raw Materials of the U.S. Atomic Energy Com­ this proposed regional joint system may lead to a better mission. The principal purpose of the study was an understanding of Front Range tectonics. exhaustive investigation of the geology of the uranium- The data presented in this report have been drawn bearing veins, and this report presents only a small part from our work and from that of seven of our colleagues. of the data gathered during the investigation. We sincerely thank A. E. Dearth, A. A. Drake, Jr., C. C. B-l B-2 SHORTER CONTRIBUTIONS TO GENEGBAL GEOLOGY Hawley, F. B. Moore, P. K. Sims, E. W. Tooker, and ing much of the Late Cretaceous epoch, all the J. D. Wells for their generous contributions of data and Front Eange was submerged. stimulating discussions of the problem. Many details 2. The present Front Eange was uplifted and arched, of the geology of the area have been summarized from generally along the old Precambrian anticline; the data compiled by us and by our colleagues. For the arching began in Late Cretaceous time and culmi­ interpretations of the data we, however, accept full nated during the early Tertiary. The bedrock was responsibility. folded along the margins, faulted, and intruded by The general geology of the area has been described porphyry stocks and dikes during early Tertiary previously in several reports, the most comprehensive time. Mineralization of the Colorado mineral belt of which are three U.S. Geological Survey Professional accompanied and followed emplacement of most Papers (Spurr and others, 1908; Bastin and Hill, of the intrusive rocks. 1917; and Lovering and Goddard, 1950). 3. Minor renewed movement along some of the early Tertiary faults and continued erosion bring the GENERAL GEOLOGY geologic history up to date. The Precambrian bedrock in the Central City-Idaho Springs area consists of a generally conformable series PRECAMBRIAN HOCKS of folded metasedimentary gneisses, metaigneous rocks, The Precambrian rocks in the Central City-Idaho and igneous rocks. The Precambrian rocks have been Springs area are an interlayered and generally con­ faulted and intruded during Tertiary time by a series formable sequence of metasedimentary gneisses, gneissic of calc-alkalic to alkalic porphyry dikes and small plu- metaigneous rocks, granite, and pegmatite (fig. 2). tons. As this report concerns only the Precambrian The oldest rocks are metasedimentary gneisses, which rocks, no extensive discussion of the Tertiary intrusive are principally biotite-quartz-plagioclase gneiss, rocks will be given. sillimanitic biotite-quartz gneiss, and microcline-quartz- A generalized summary of the Precambrian geologic plagioclase-biotite gneiss. Minor amounts of lime- history in the area shows the following events: silicate gneiss, cordierite-bearing gneiss, quartz-mag­ 1. Precambrian sediments were deeply buried and re­ netite gneiss, and amphibolite are also found in the area, constituted into high-grade gneisses. and these form lenses or pods in the more abundant 2. The foliated metasedimentary rocks were plastically biotite-rich gneisses and microcline-bearing gneiss. The deformed into major folds with north-northeast- mineral assemblages in the metasedimentary gneisses trending axes. The deformation was accompanied are those described by Turner (1948, p. 76-88) for sev­ by the intrusion of granodiorite, and then minor eral subf acies of the amphibolite f acies. amounts of quartz diorite and associated horn- The granite gneiss and pegmatite unit consists prin­ blendite. cipally of granite and granite pegmatite that grade into 3. Biotite-muscovite granite was intruded near the end each other and into the metasedimentary gneisses. The of the period of plastic folding. unit is virtually conformable and forms large mappable 4. Uplift and erosion of several thousand feet of cover. units as well as thin layers in the biotite-rich metasedi­ 5. The Precambrian rocks were deformed locally. mentary gneisses. This unit is probably of metasomatic Where deformed, the more massive rocks were origin. crushed and granulated; the more foliated gneissic The metasedimentary gneisses and the granite gneiss rocks were formed into small terrace, monoclinal, and pegmatite are cut by a series of younger intrusive or chevron folds; also some foliated metasedi­ plutons, sills and dikes. The oldest of this series, a mentary rocks were
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