Areal Geology of the Little Cone Quadrangle Colorado By A. L. BUSH, O. T. MARSH, and R. B. TAYLOR CONTRIBUTIONS TO ECONOMIC GEOLOGY GEOLOGICAL SURVEY BULLETIN 1082-G Description of the stratigraphy and struc­ ture of a quadrangle that contains some of the signifie ant vanadium deposits of the Placerville district. Prepared on behalf of the U.S. Atomic Energy Commission and published with the permission of the Commission UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. CONTENTS Page '* Abstract. --.-.-.----------------------------------------------- 423 Introduction. ____._._____-__--___-_-__------_------__---__----____ 425 Location, accessibility, and culture__-_-_________-_--_--_._._._. 425 Topography,.climate, and vegetation_--____________-_-._-_._-_. 426 Scope and purpose of the work._________________________________ 427 Previous work.._______________________________________________ 427 Fieldwork and acknowledgments____-_-_________-__---_________ 428 Regional geology_---__-__------------_-------_--_-_-_-____--___._. 429 Sedimentary and igneous rocks ________---_-__---__-__------_.___- 429 Relations of the sedimentary rocks____-_-_--_---_..______._____ 430 Permian system_--__----------------------------_-----_____--- 430 Cutler formation..________.____________.____.___.___.____> 433 Triassic system_..___-_..---_-_-__------___-____--_-__________ 433 Upper Triassic series.__--_-_-___-----___-____.---_______-.. 433 Dolores formation.____________________________________ 437 Jurassic system___.___-_-___-__-----_-___-_-___---__.-._._._ 437 Upper Jurassic series ____-____---_____-_____-_---___---__ 437 Entrada sandstone___________________________________ 438 Wanakah formation__________________________________ 439 Pony Express limestone member.. ___________________ 439 Bilk Creek sandstone member___-____-______________ 441 Marl member.____________________________________ 441 Morrison formation.___________________________________ 443 Salt Wash sandstone member._._______.__._.__.____ 443 Brushy Basin shale member._-__--_-_-_---___.____ 444 Cretaceous system.________-_-__-___--_---__-_-_-_-_-------___- 446 Lower Cretaceous series.___________________________________ 446 j Burro Canyon formation._______________________________ 446 Upper Cretaceous series.________-_____-_-___-_-__-______-__ 447 Dakota sandstone.___-_--__---_-------_---_-------_-__ 447 Mancos shale.___---___-___---_-__----_-_-----_-__._._ 449 Tertiary system._______________-____-______---____----_____-__ 450 Granogabbro and microgranogabbro.____--_____-__.__-__.___ 451 Mineraology and petrolgraphy__________________________ 451 Distribution and occurrence______-__-__-__---_________ 453 Little Cone..._._..._.....__.____.._...._.._.._ 453 Flat Top Peak....._.......____.....______. 455 Dikes and sills._-____-_----_-_-----_--_---_-_-_.__ 457 Microgranodiorite and rhyodacite__________________________ 458 Mineralogy and petrography.___________________________ 458 Distribution and occurrence.__-____----_-_--_____-_____ 459 j Porphyritic microgabbro__________----___--____.-_--___--__-- 460 Rhyolite(?) and porphyritic rhyolite(?)__________.______._._._ 461 Age of the Tertiary igneous rocks_-___-__--_-----------_--__- 462 Tertiary(?) system...._._....____.._._.._.____._.........___.. 462 » Clastic dikes-__-_.------------------_--------------------- 462 in IV CONTENTS Sedimentary and igneous rocks Continued Page Quaternary system.___________________________________________ 463 Pleistocene series-_________________________________________ 463 Porphyritic basalt-____________________________________ 463 Glacial drift._______________________________ 464 Terrace gravel and valley fill____J_______________________ 466 Pleistocene and Recent series_______________________________ 466 Colluvium________________________________ 466 Recent series._______ ________________________________-__J_ 467 Talus.....______________________________ 467 Alluvium.._--___--___-__--_-_____________ 467 Spring deposits______________________-_---__--__--_-_ 468 Structure_____________________________ - _______ 468 Regional setting.______________________________________________ 468 General features.._-_-__-____-________-____-_---_-----_-__--___ 469 Folds.__-__-_________--____________--__--__--_----_---_-_-_-_ 471 Little Cone syncline.______________________________________ 471 Placerville anticline._______________________________________ 472 Little Cone anticline_______________________________________ 473 Woods Lake syncline_____________________________________ 473 Other folds..________________________________ 474 Faults_____________________________________ 475 Specie Creek graben_______________________________________ 475 Ross graben______________________________________________ 476 Little Cone fault-____________.______.__'_____ 477 Lou Hallfault.______________________________ 478 Other faults_________________________________ 478 Age relations of the faults_________________________________ 479 Age of deformation._________________________________________ 479 Geomorphology.__________________________________________________ 480 Upland surfaces_____________________________________1__L______ 480 Drainage systems______________________________j_____________ 481 Landslides__________________________ _____________ 482 Mineral deposits______________________________________! _______ ___ 483 Vanadium-uranium deposits.________________________________;___ 483 Placer gold deposits.._________________.______________________ 486 Literature cited-__-_-_- ___-______________________.________________ 487 Index___---. ----- ________________________.. 491 ILLUSTRATIONS [Plates in pocket] PLATE 18. Generalized columnar section of the sedimentary rocks of the Little Cone quadrangle. 19. Geologic map and sections of the Little Cone quadrangle.'' Page FIGURE 42. Index map of part of southwestern Colorado.._____________ 425 43. Index map of the Placerville district, San Miguel County, Colo._._______._.___.__.___________ 440 44. Index map of part of the western San Juan Mountains area.. 452 45. Index map showing the major structural features of the region surrounding the Little Cone quadrangle _________________ 469 t6. Index map showing the major folds and faults in the Little Cone quadrangle_--__-___--____________--___-____-__- 470 CONTRIBUTIONS TO ECONOMIC GEOLOGY AREAL GEOLOGY OF THE LITTLE CONE QUADRANGLE, COLORADO By A. L. BUSH, 0. T. MARSH, and R. B. TAYLOR ABSTRACT The Little Cone quadrangle includes an area of about 59 square miles in eastern San Miguel County in southwestern Colorado. The quadrangle con­ tains features characteristic of both the Colorado Plateaus physiographic province and the San Juan Mountains, and it has been affected by geologic events and processes of two different geologic environments. ' The continental sedimentary rocks of the Cutler formation of Permian age are the oldest rocks exposed in the quadrangle. Deposition of the Cutler was followed by a long period of erosion and peneplanation. There is no marked angular discordance between the Cutler and the overlying Dolores formation in the Little Cone quadrangle, but there is in areas some tens of miles east and west of the quadrangle where some crustal warping took place. The continental sedimentary rocks of the Dolores formation of Late Triassic age are red beds that are similar in gross lithology to those of the Cutler. The Dolores formation is subdivided into five general units that persist throughout the quadrangle and for some tens of miles to the north, south, and east. A second long period of erosion followed deposition of the Dolores. ' The Entrada sandstone of Late Jurassic age overlies the Dolores forma­ tion, and is in turn overlain by the Wanakah formation, also of Late Jurassic age. The Wanakah consists of the Pony Express limestone member at the base, the Bilk Creek sandstone'member near the center, and a "marl" member at the top. The Morrison formation, which overlies the Wanakah, consists of the Salt Wash sandstone member in the lower part and the Brushy Basin shale member in the upper part. A period of erosion, probably of relatively short duration, followed deposition of the Brushy Basin member. The Burro Canyon formation of Early Cretaceous age occurs as discontinu­ ous bodies that fill channels cut in the top of the Morrison formation. Deposi­ tion of the Burro Canyon formation was followed by another period of erosion, which in turn ended with deposition of the Dakota sandstone of Late Cretaceous age. The Dakota sandstone grades upward into the Mancos shale, also of Late Cretaceous age. The Paleozoic and MesozOic formations were broadly folded during Lara- mide time as part of an orogeny of regional extent, and the San Juan Moun­ tains area was uplifted as a broad dome. Extensive erosion followed deforma­ tion, and the Cretaceous rocks in the area of the Little Cone quadrangle and the Mesozoic and Paleozoic rocks eastward from the quadrangle were suc­ cessively bevelled. The Telluride conglomerate of Oligocene(?) age was laid down on this surface. In the Little Cone quadrangle several hundred feet 423 424 CONTRIBUTIONS TO ECONOMIC GEOLOGY of the Telluride was deposited upon a considerable thickness (probably 3,000 feet or more) of the Mancos shale. At Telluride, about 12 miles east of the quadrangle, the Telluride conglomerate