Stratigraphy of the Star Peak Group (Triassic) and Overlying Lower Mesozoic Rocks Humboldt Range, Nevada GEOLOGICAL SURVEY PROFESSIONAL PAPER 592 Stratigraphy of the Star Peak Group (Triassic) and Overlying Lower Mesozoic Rocks Humboldt Range, Nevada By N. J. SILBERLING and ROBERT E. WALLACE GEOLOGICAL SURVEY PROFESSIONAL PAPER 592 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress cata~og-card No. GS 68-262 For sale by the Superintendent of Documents, U.S. Government Printing Offic~ Washington, D.C. 20402 - Price $1.25 (paper cover) CONTENTS Page Page Abstract___________________________________________ 1 Star Peak Group-Continued Introduction ______________. _______________ -.-________ 2 Paleogeographic interpretation-Continued Fieldwork and geologic mapping__________________ 6 Facies relation between the Prida and Natchez Pass Formations_________________________ 28 Acknowledgments_______________________________ 6 Terminology___________________________________ 6 Relationship between volcanic and carbonate rocks in the Nat chez Pass Formation_______ 29 Star Peak Group___________________________________ 6 Regional relations of the upper member of the Prida Formation________________________________ 8 Natchez Pass Formation__________________ 30 Lithologic description_______________________ 10 Grass Valley Formation_____________________________ 31 Lower member_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 Lithologic description___________________________ 31 Middle member_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 13 Age___________________________________________ 34 Upper member_________________________ 13 Paleogeographic interpretation_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 36 Fossils and age_____________________________ 15 Dun Glen Formation________________________________ 40 Natchez Pass Formation_________________________ 20 Lithologic description___________________________ 41 Fossils and age_________________________________ 42 Lithologic description ___________________ '____ 21 Post-Dun Glen lower Mesozoic strata in the Pershing Fossils and age_____________________________ 24 district_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 44 Paleogeographic interpretation of the Star Peak Lower unit_____________________________________ 44 Group_______________________________________ 25 Upper unit__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 46 Lateral variation at the base of the Star Peak References_________________________________________ 47 Grouv----------------------------------- 25 Index_____________________________________________ 49 ILLUSTRATIONS Page PLATE 1. Fence diagram showing the reconstructed section of the lower part of the Prida Formation along the east side of the Humboldt Range between Congress and Star Canyons, N ev ____________________ - ___ - _ In pocket 2. Fence diagram showing the reconstructed section of the Star Peak Group along the length of tlie Humboldt Range, Nev ____________________________________________________________________________ In pocket FIGURE 1. Index map showing location of the Humboldt Range_________________________________________________ 3 2. Diagram showing succession of lower Mesozoic rock units in the Humboldt Range _________ --_----------- 4 3. Diagram showing time-stratigraphic correlation and lateral variations in the age of the lower Mesozoic rock units in the Humboldt Range _____________________________________________________ - ____ - __ --- 5 4-8. Photographs showing: 4. East flank of Star Peak ____________________________________________________ - ___ ---_------- 8 5. Talus characteristic of the brown calcareous sandstone unit; lower member of the Prida Formation_- 11 6. Weathered block composed partly of metavolcanic rock and partly of impure dolomite; lower member of the Prida Formation_______________________________________________________________ 12 7. Outcrop of dark laminated dolomite; upper member of the Prida Formation __________________ --- 14 8. Carbonate rocks from the upper member of the Prida Formation on Fossil HilL ____________ -_-__ 16 9. Isopach map of pre-upper Anisian strata at the base of the Star Peak Group _______ ~ __ - _________ --_--- 27 10. Very thin and thin-bedded impure sandstone of the Grass Valley Formation _____________________ - __ --- 32 11. Linguoid ripple marks on the top surface of an impure sandstone bed in the Grass Valley Formation______ 33 12. Sedimentary structures on the bottom surfaces of sandstone slabs from the upper part of the Grass Valley Formation_________________________________________________________________________________ 35 13. Sedimentary structures of a single slab of thin-bedded impure sandstone from the Grass Valley Formation_ 37 14. Diagrammatic comparison of the thickness, composition, and paleocurrent directions of the Grass Valley Formation in the Humboldt Range and nearby ranges to the east_________________________________ 38 15. Photograph showing carbonate breccia and conglomerate from Upper Triassic rocks of the Pershing district_ 43 TABLE TABLE 1. Composition and biostratigraphic assignment of the ammonite faunas from the lower part of the Prida Forma- tion in the northeastern part of the Humboldt Range ____________________ --- _------- ---------- -- 17 III STRATIGRAPHY OF THE STAR PEAK GROUP (TRIASSIC) AND OVERLYING LOWER MESOZOIC ROCKS, HUMBOLDT RANGE, NEVADA By N. J. SrLBERLING and RoBERT E. WALLACE ABSTRACT Prida are compensated for by -rhanges in the thickness and age The Star Peak Group and other, still younger, lower Mesozoic of the massive organic-detrital carbonate rocks that form the rocks of the ""Winnemucca sequence" of Silberling and Roberts lower part of the overlying Natchez Pass Formation. The upper form roughly half the bedrock exposures in the Humboldt Range, Prida and part of the lower Natchez Pass are therefore inter­ which occupies parts of the Imlay, Unionville, and Buffalo preted as interfingering facies of one another, the upper Prida Mountain quadrangles in northwestern Nevada. These strata, being the deposit that accumulated farther from shore and in aggregating more than 10,000 feet in thickness, are largely ma­ deeper water. rine sedimentary rocks that overlie with subtle angular uncon­ Within the Natchez Pass Formation, which forms the upper formity the predominantly volcanic Koipato Group of Permian formation of the Star Peak Group, two members are recognized to earliest Triassic age. Exposures of the Star Peak Group throughout most of the Humboldt Range. The lower member roughly encircle the central part of the range and are most ex­ consists mainly of massive thick and very thick bedded car­ tensive at its north and south ends. The Star Peak Group, bonate rocks except in the southern part of the range, where comprising the Prida and Natchez Pass Formations, is char­ roughly half of its 1,500-foot thickness is made up of mafic acterized by calcareous rocks whose composition contrasts volcanic rocks. Farther north in the range, where the lower sharply wth that of the overlying formations, which are com­ Natchez Pass is largely supplanted by the upper Prida, the posed mainly of fine-grained terrigenous clastic rocks. thickness of the lower Natchez Pass is as little as 400 feet. The Prida Formation, the lower formation of the Star Peak The upper member of the Natchez Pass is about 1,000 feet Group, is divided into three informal members. Its lower mem­ thick whereV1er recognized in the Humboldt Range. In most of ber is lithologically heterogenous and includes a variety of cal­ the range, brown-weathering impure silty limestone forms its careous terrigenous clastic rocks and impure carbonate rock~ lower part and abruptly, and perhaps disconformably, overlies above a basal clastic unit of noncalcareous sandstone and con­ the lower member. These impure limestones grade upward into glomerate. Lateral lithologic changes within the lower member an approximately equal thickness of massive thick-bedded rela­ and variation in its thickness from a few tens of feet to several tively pure carbonate rock resembling that of the lower member. hundred feet reflect the original slope and topographic irregu­ Although these two members cannot be recognized among the larity of the eroded surface of the underlying rocks of the structurally deformed carbonate rocks of the Natchez Pass at Koipato Group. In the northern part of the Humboldt Range, the northeastern tip of the range, about 400 feet of interbedded much of the lower Prida is locally cut out against two promin­ chert conglomerate, sandstone, and calcareous siltstone in the ent basement highs that coincide in location with intrusive parts middle part of the section there may be a coarse clastic equiva­ of the Koipato volcanic pile and with the principal silver depos­ lent of the impure limestone that forms the lower part of the its in the Star Peak rocks. Elsewhere in the northern part of upper member farther south. the range, where the lower member of the Prida is thickest and In the southern part of the range, the Natchez Pass contains was first deposited, the lower member includes at least two fossils of probable early Karnian (earliest Late Triassic) age successive ammonite faunas of Spathian (latest Early Triassic) in its middle part and probably includes beds ranging from age. late Ladinian to late Karnian in age. Farther north,