DEPARTMENT OF THE INTERIOR Hubert Work, Secretary U. S. GEOLOGICAL SURVEY George Otis Smith, Director Bulletin 777 PRE-CAMBBIAN BOCKS OF GUML30N BIVEB COLOBADO BY J. FRED HUNTER WASHINGTON GOVERNMENT PRINTING OFFICE 1926 CONTENTS Tage --------------. .__. ________-___________-___-_---__.. 1 Purpose and extent of the work------__-----_---_-------------- 1 / Field work and acknowledgments.-.------.--------------------- 2 Geography __________________________________-_-----__-__---- 3 Literature------------------------_-----.-------_-------<------ 6 { Adjacent pre-Cambrian areas,._____.__--___.--__.___-_------__ 6 Relations to the overlying formations.__________________________ 7 Outline of the pre-Cambrian geology-___---_-__-____-------_-_-_ 8 Metamorphic complex. --_-__-_-___-_-_-_____-_-______------_---_-- 8 Age and correlation.__-__--_-_-----___---_________------_---__ 8 Divisions of the complex._.--_.____._____._--__---------------- 9 Black Canyon schist...-_.---__..--.-----.-.------------------- 10 1 Occurrence__-_._-__---______-_-_____-_-- ________________ 10 \ General character.________________________________________ 11 1 Biotite schist-..---.--. ____------__-_--_----__------------- 12 Quartz-muscovite schists._________________________________ 14 Granite gneiss._______-___--______--_--___-___-__------_-_ 17 Amphibole schist.-__--___._-_-__--___-----__------___-_-- 18 Ultrabasic rocks._____-___-___-___----.___.___-__--__--___ 20 River Portal mica schist-__--__-____-__-_-____--__--_-.-__--___ 22 Occurrence ______--_----_-_---_-_---_-------------------- 22 General character and relations.___-__---__-_______-___-___ 23 Mica schist._-___-__---__-__-___--_-_-__----_---_---_-__- 24 i Quartzose phase of mica schist.____________________________ 25 f Quartz-mica schist of Mesa Creek__---_-__---__---_---___-- 26 Derivation __-_______-___-___,___---_--_-_____-______-___ 26 j Dubois greenstone-----___-_-_-_-_-__---__--__-____--_-----___ 28 Occurrence __-_-_____--__-_._--___-_-_. ________ .___ 28 I General character..--.. _--___-.--_.__-.__-_______.___--_- 29 Relations-----------.--.----------------------------- __ 30 Hornblende gneiss.__.-____-_________--___._____--____.__- 31 Amphibole schist and amphibolite..-__---__-___------__-___ 32 Chlorite schist -----__---__-___-___-__--_-_--__-_-_--__-__. 34 Basic associates---___--_--_-_-_____-_-__--___.-----__-___ 35 Igneous intrusions --__-_____-_-_________.____._______.__-_-____-- 36 General features __----_-_-__--_______-_____________-_-_______ 36 Granites of the area _--_----__-__-____-__________.____-_______ 38 Powderhorn granite group ----_-----_-_--__..____-_-----_-_--___ 38 Occurrence ______________________________________________ 38 General character ____-___-_______..--___.__--___-.___-__- 39 Relations __.--_-____--______._ __-_-.______-_______.-__- 40 Granite porphyry ________________________________________ 41 Porphyritic biotite granite -_---_-_-__---_____--_-_-__--___ 44 Gray biotite granite _.____-._-_-___-________-__.___-_______ 46 Vernal Mesa granite. ______-___-_____________________ ______.___ 47 Occurrence __-___-___-___-__-.___-___-___________-_-_--__ 47 General character ____-___-__-____-_-.____ _._.___._____... 48 Petrography _-___-_______-___-___-___.._.____._._--_.-__- 48 in IV CONTENTS Igneous intrusions Continued. Page Curecanti granite _____________________________________________ 49 Occurrence _---__-----_-_-___________-___________________ 49 General character __________________________________________ 49 Relations _______________________________________________ 50 Inclusions ----_-_-----__--___--__________-___--__________ 51 Jointing ________________________________________________ 51 Petrography _--__------______-_______-_________________... 52 Smaller granite bodies ________________________________________ 53 Occurrence _______________________________________________ 53 Age and relations ________________________________________ 54 Normal microcline granite ________________________________ 55 Soda granite _____________________________'_______..________ 55 Fine porphyritic granite _______--__--_-____---_---.____-___ 56 Metagabbro _________________________________________________ 56 Occurrence ______________________________________________ 56 General character ________________________________________ 57 Petrography _____________________________'________________ 58 Metagabbro proper ___________________________________ 58 Feldspathic phase of metagabbro _______________________ 60 Quartzdiopite -___________________--__--___-____-_-___ 60 Quartz diorite and diorite_-___________________---__--__-_____ 61 Occurrence and relations ______-____---_-_----_--______-___ 61 Petrography _____-__..-_______-_-_--__-_---___-_-_.___-___ 62 Olivine gabbro__-___--___--______-___-__--__--_______________ 63 Occurrence and relations __________-_-_-___-_-__-_-____-___ 63 Petrography _________-_______-___---_-___--_-_--_________ 63 Augite syenite _______-_-___^_____-___--__-___--___--_____-___ 64 Occurrence and relations _____________--_-_-_--_----__--___ 64 Petrography _____________-___-___-_---________-_--___-___ 65 Augite syenite of Wildcat Gulch and Goose Creek ________ 65 Augite syenite in Willow Creek ________________________ 69 Syenite near the mouth of Wolf Creek __________________ 70 Mica syenite near Lot mine____-___-_-----_--------__ 70 Shonkinite _-_-__--_-_-___-_-_-_____-_------------------_----_ 71 Occurrence __-___-___-___-___-___-_-__-_--------_________ 71 Petrography _____--__________-____-_-____----------_---_- 71 Chemical composition_-_______-______________^_----__--___ 72 Other syenites ___________________-___-____-_--_--------_----- 75 Occurrence and relations __________-__--__----__-_-..___-___ 75 Petrography _____________________________________________ 76 Syenite porphyry_-______________----__--------__---_- 76 Soda syenite_____-___-____-_--_---_-----------__----_ 76 Pegmatite bodies_____-___-__---___----_-----------_------_--_ , 77 Occurrence ______-__--__..-___---_-_--------------___----_ 77 Relations ________________-__--_-_-_---------- -----_------ 77 Petrography _____________________________________________ 78 Aplite__--_-____----_----------_---------_------------------_ 79 Occurrence ______--__.-_---__--_--------------_-_-------_ 79 Petrography _____-_______-___-___-_---------------------_ 80 Diabase _________-__------_---------------__----------------- 80 Occurrence ______-_-_-_----_------_-----_-----------l--_- 80 Petrography _____-___-___-_-_--__-_---------------------_ 81 Lamprophyre ___________--_---___-_.---------------_--------_ 82 CONTENTS V / Page Structure....-.---.__----._________________.______..__...._.__. 82 Schistosity _____--_--__-_____...__--__..__---______-___-_______ 83 General features.-____-_______-___-___-_______-___-___-__. 83 Local details.--__-___-_---___..__ _-___-_______..___--__-___ 85 Faulting.__________________________-_____________________ 86 }. Age..__..__._-__. ___________________.__..______> 86 Nelson Gulch fault___-_______.-____._____-_-___---.____ 86 Mesa Creek fault.________________________________________ 87 I Cimarron fault.-_-___---_____-____-__-___-___..___-_______ 88 Red Canyon fault-------_--___-_--_ -_--_---__--_------_-_ 91 Other faults______________________________ 91 Index-_--___--_--_-__---____-.___________-__-__..-__.___ 93 ILLUSTRATIONS Page PLATE I. Geologic map showing pre-Cambrian rocks of the Gunnison River region, Colorado___________._______________________ In pocket II. A, Black Canyon and Curecanti Needle; B, Black Canyon east of the mouth of Cimarron Creek..______________________ 4 III. Black Canyon below the mouth of Cimarron Creek -___..____ 4 IV. A, View across the Gunnison Canyon from the northwest end of Bostwick Park; B, View down the lower portion of the Gunnison Canyon from the northwest end of Vernal Mesa__ 4 V. A, Banded biotite schist of the Black Canyon schist; B, Photo­ micrograph of biotite schist. ____________________________ 5 VI. A, Biotite schist of the Black Canyon schist; B, Photomicro­ graph of biotite schist._-______.____-_____-___-_-___--__ 12 VII. A, Amphibole schist of the Black Canyon schist; B, Photo­ micrograph of amphibole schist._________--__-----_._-__ 12 VIII. A, Amphibole schist of the Black Canyon schist; B, Photo­ micrograph of amphibole schist-_ _-______-___-___-_--_._ l2 I.. A, River Portal mica schist from Vernal Mesa; B, Photomicro­ graph of mica schist_____---___ ________________________ 13 X. A, Granite porphyry of Powderhorn granite group; B, Photo­ micrograph of granite porphyry.________________________ 64 XI. A, North wall of Black Canyon 2 miles east of Curecanti Needle; B, Black Canyon from north rim half a mile west of Curecanti Needle; C, Black Canyon from point half a mile west of Cottonwood Gulch.._....__-.__._-_-_..-_-_ 64 XII. A, Photomicrograph of olivine gabbro; B, Mica schist showing two distinct injections of granitic material-___.___-____--_ 64 XIII. A, Augite syenite; B, Photomicrograph of augite syenite ..__. 64 XIV. Photomicrograph of shonkinite___.___-___-___-__________- 64 XV. A, View from east rim of Cebolla Canyon opposite Tolvar Peak; B, View from Cimarron Creek road showing Cimarron fault escarpment ___________________________________________ 65 FIGURE 1. Index map of Colorado, showing location of area described.... 2 2. Two injections of granite___.-___.___.___-___--__.___ ..-. 54 3. Side view of a thin wall of schist intersected and buttressed by granite intrusions.________._______.__.-__.-___-___-___ 54 4. Typical shapes of