Clay Minerals in Triassic Rocks of the Colorado Plateau GEOLOGICAL SURVEY BULLETIN 1147-C Prepared on behalf of the U.S. Atomic Energy Commission Clay Minerals in Triassic Rocks of the Colorado Plateau By LEONARD G. SCHULTZ CONTRIBUTIONS TO THE GEOLOGY OF URANIUM GEOLOGICAL SURVEY BULLETIN 1147-C Prepared on behalf of the U.S. Atomic Energy Commission JNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 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, D.C., 20402 CONTENTS Pare Abstract________________________________________________________ Cl Introduction._____________________________________________________ 2 Laboratory analysis______________________________________________ 3 X-ray diffractometer examination._____________________________ 3 Microscopic examination.______________________________________ 5 Chemical and base-exchange analysis.___________________________ 5 Clay minerals distinguished in the Triassic rocks._____________________ 7 Illite-montmorillonite group.___________________________________ 7 Illite_________ ____________________________________________ 8 Mixed-layer illite__________________________________________ 8 Mixed-layer illite-montmorillonite___________________________ 8 Mixed-layer montmorillonite_._-_-_--________-__-______--___ 9 Montmorillonite__ _ ________________________________________ 9 Geologic interpretation_____________________________________ 9 Kaolinite_ _ ___________________________________________________ 10 Well-crystallized kaolinite__________________________________ 10 Poorly crystallized kaolinite_______________________________ 10 Geologic interpretation.____________________________________ 10 Chlorite______________________________________________________ 11 Well-crystallized chlorite___ ________________________________ 12 Mixed-layer chlorite_______________________________________ 12 Geologic interpretation..___________________________________ 13 Palygorskite-_________________________________________________ 13 Presentation of data_ _____________________________________________ 13 Clay minerals.________________________________________________ 13 Feldspars__ _--_-_______-__-_____--______--____-__---_------- 14 Carbonates- __________________________________________________ 14 Color________________________________________________________ 17 Moenkopi formation-______________________________________________ 18 Description ___________________________________________________ 18 Clay-mineral distribution.______________________________________ 19 Interpretation of geographic distribution of clay minerals,__________ 22 Interpretation of abundance of kaolinite and chlorite in coarse-grained rocks_ ______________________________________________________ 24 Chinle formation__________________________________________________ 27 Temple Mountain member______________________-__----------_ 27 Shinarump member.___________________________________________ 27 Monitor Butte member______________-_-__-_-_-_-----_---------- 30 Moss Back member______________________________________---___ 35 Petrified Forest member______________________-__--_--__-------- 36 Owl Rock and Church Rock members_____________-__--_------_-- 40 IV CONTENTS Page Mottled and bleached rocks_____________________________________ C43 Mottled rocks_____________________________________________ 43 Description._ ___________________________________________ 43 Moenkopi formation_____________________________________ 45 Chinle formation________________________________________ 48 Altered Precambrian crystalline rocks._____________________ 50 Interpretation. ___ _______________________________________ 50 Bleached rocks in the Moenkopi____ ___________________________ 54 Relations between clay minerals and uranium-vanadium mineralization. 56 General distribution of clays_ _________________________________ 56 Clay pebbles, bleached zones, and feldspar ______________________ 57 Secondary clay minerals._____________________________________ 58 Vanadiferous chlorite________________________________________ 58 Relation of altered volcanic debris to mineralization.________________ 61 Conclusions_ _ __________________________________________________ 64 Sample localities_______________________________________________ 65 References cited_________________________________________________ 69 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Sample localities and Triassic outcrops on the Colorado Plateau. 2. X-ray diffractometer traces of Triassic clays. 3. Stratigraphic sections showing common clays in Triassic rocks. 4. Fence diagram showing common clays in Triassic rocks. Page FIGURE 1. Sedimentary facies and structures in the Moenkopi formation. C20 2. Distribution of kaolinitic and chloritic clays in the Moenkopi formation. ____________________________-----__-----__ 21 3. Probable area where Moenkopi sediments were altered from chloritic to kaolinitic varieties____________---_--______ 23 4. Pseudomorphic and radial textures of mixed-layer chlorite___ 26 5. Distribution of members and orientation of sedimentary structures in the Chinle formation._____________________ 28 6. Rounded tuff particles in the Chinle formation_____________ 32 7. Particle with seemingly perlitic cracks in a palygorskite- bearing siltstone____________________________________ 39 8. Distribution of clays in the Owl Rock and Church Rock members of the Chinle formation_______________________ 41 9. Generalized sketch showing the occurrence of mottled rocks. 44 10. Clays in mottled and bleached rocks._________---___-_---_ 46 11. Kaolinite books in the Moss Back member of the Chinle for­ mation on Temple Mountain.________________________ 59 12. Structural differences between mixed-layer chlorite and vanadiferous chlorite_______-__-_-_________-----_--__-- 60 CONTENTS TABLES Pare TABLE 1. Triassic stratigraphy in southeastern Utah_________________ C3 2. Chemical analyses of Triassic clays^________________________ 6 3. Base-exchange analyses of Triassic clays____ _________ ________ 6 4. Feldspars in samples of Triassic rocks_______________________ 15 5. Carbonate content and clay minerals_______________---______ 16 6. Color and clay-mineral composition of samples__________-__-_ 17 7. Clay minerals in mottled Moenkopi rocks__________________ 47 8. Clay minerals in mottled Chinle rocks_____________________ 49 9. Clay minerals in altered Precambrian crystalline rocks on the Uncompahgre Plateau____ _______________________________ 51 10. Mineralogy of bleached rocks______________________________ 55 11. Clays in mineralized and nearby unmineralized rocks_________ 57 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM CLAY MINERALS IN TRIASSIC ROCKS OF THE COLORADO PLATEAU By LEONARD G. SCHULTZ ABSTRACT Kaolinite, illite, montmorillonite, chlorite, palygorskite, and mixed-layer clay composed of different proportions of illite, montmorillonite, and chlorite were identified in samples from the Moenkopi formation and the overlying Chinle formation using X-ray-diffractometer methods. Predominantly illitic sediments, inferred to have come from an eastern source, characterize the Moenkopi formation, and the upper part of the Chinle formation in the northern part of the Colorado Plateau. While the illitic sediments were being transported westward, some of the less resistant accessory components, such as chlorite, were destroyed and kaolinite was formed. Montmorillonitic clays, inferred to have come from a southern source, charac­ terize most of the rocks in the lower part of the Chinle formation and also are abundant components of the upper part of the Chinle in the southern part of the Colorado Plateau. They were derived mostly by alteration of volcanic debris. Part of the volcanic debris probably was altered before deposition, but rounded tuff particles, now composed of aggregates of swelling clay, must have altered after deposition. Latite tuff altered to highly montmorillonitic clays; rhyolite tuff altered to mixed-layer illite-montmorillonite clays. The altered rhyolite tuff is the less abundant of the two types and generally occurs stratigraphically below the altered latite tuff. Clay assemblages in some rocks near the Chinle-Moenkopi contact are mostly the result of weathering. The effects of weathering are most pronounced in mottled soil zones formed on rocks at the surface during the Chinle-Moenkopi hiatus and in rocks laid down during the initial intermittent stage of Chinle deposition. Characteristic clays of the weathered rocks are kaolinite, par­ ticularly a poorly crystallized variety, and a type of mixed-layer illite-mont­ morillonite. In contrast to the mottled soil zones, clays in bleached zones just below the Chinle-Moenkopi contact are unaltered; the bleached zones appar­ ently were caused by ground water circulating in the overlying Chinle sandstones. Palygorskite is locally abundant near the middle of the Chinle formation. Its origin is undetermined. No one clay or assemblage of clays is characteristically associated with ura­ nium deposits in the Triassie rocks. Clays from some mines are predominantly kaolinitic; some are chloritic; some are illitic; and others are montmorillonitic, but in each deposit they are similar to the clays found in nearby barren rocks. Secondary kaolinite and chlorite occur both in mineralized and unmineralized Cl C2 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM rocks and seem to represent recrystallization or slight modification