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Igneous and Tectonic Structures of the Stillwater Complex Montana

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Igneous and Tectonic Structures of the Stillwater Complex Montana By W. R. JONES, J. W. PEOPLES, and A. L. ROWLAND CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY BULLETIN 1071-H A study of the structures in a belt of upturned layered noritic and ultramafic Precambrian rocks on the northeast margin of the Beartooth Mountains -UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON t 1960 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director U For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. CONTENTS * Abstract____..._..__---___.._-__--..___-----_-__---_--____---__ 281 Introduction. __________-_____________--__-_-_--_-_--_--___---__-__ 282 Igneous structures.__._-_._..._._.-.--__-___-___-______--___-____._ 283 i Characteristics of layered complexes.____________________________ 283 i Geologic setting and age of the complex__-_____-_-________-_.____ 284 Stratigraphy of the complex.___________________________________ 286 Major subdivisions_____.___-_.___-_-_______--_-_-.______-- 286 » Distinctive boundaries and layers__--______---_-____-_.____ 289 Serpentinized rock_____._____-____----_______-__.__________ 291 I Characteristics of layers.-____--------_-----____-_______________ 291 Igneous lamination.___-______-_-_-_--___._-___-____-___-__ 291 Rhythmic layering..___-_-__-____._-----___--_-__-______.__ 293 Variations in mode------------------------_-------.-__ 293 Variations in texture.--------------------------.------- 294 ^ Thickness and form. _______________________________________ 297 Discordant masses.-____-____-_---_-_------__-_--_______-_-__._ 299 Primary basins and folds in Ultramafic zone._-___--_-___-________ 301 Tectonic structures and igneous activity____________________________ 303 T Regional structural setting____________________________________ 303 Previous work______.________ __ ____________________________ 303 Outline of igneous and structural history -_________._-__.__.-____ 303 Pre-Middle Cambrian igneous activity and deformation_____-_-__._ 304 Igneous activity.._--__-_--_---_--_--_---__--_---_-----_-__ 304 ,v Deformation- ______________________________________________ 305 Paleozoic and Mesozoic interval.________________________________ 306 Laramide igneous activity and deformation_______________________ 307 Igneous activity..___--___-_---------_-_-_---------_---.--- 308 A Deformation--_----_---_------_------_-_---------_-------- 308 Major structural features.-____--________-_--__-_-______ 309 Trend-.-.---.------------------------------------ 309 Tilt.._------_------------------.-------------..-- 309 ** Folds..._--_.----_-----------------------------. 310 Faults__-.__._.._-_-___--__-____----___-._----__ 312 Longitudinal ramp and thrust faults of northeast ^ dip-...-.---------------------------------. 313 Iron Creek-Brownlee Creek fault._._________ 313 South Prairie and North Prairie faults. _._-_ 314 Castle Creek fault..-----...------.________ 315 "1 Dry Fork thrust..-------..--------_..----. 315 Suce Creek fault...________________________ 316 " Longitudinal thrust and steep reverse .faults of j south dip_-----------------_---------------- 316 Horseman thrust..-.-......-.-.___.__.____ 316 Lake-Nye Creek fault......__.___ ____.___ 317 Bluebird thrust________________________ 318 «' Thrust east of Fishtail Creek___..____..___ 319 ' Graham Creek fault............-.-.----_--m 319 IV CONTENTS Tectonic structures and igneous activity Continued Laramide igneous activity and deformation Continued Deformation Continued Major structural features Continued Faults Continued Page East-trending vertical faults.____-_-_-__-__--__- 319 Mill Creek-Stillwater fault zone_..__..____ 319 Lost Creek fault_____-______-_-____--____ 320 Transverse or cross faults_____________________ 320 Development of Laramide structural features _____________ 321 Regional development____________________________ 321 Development of deformation in vicinity of Stillwater complex.__.-_.._____-__-____--___^___-__--____- 323 Early phase_____-_-___---_-_--__-__-_---_-. 324 Late phase______-_____--________--_-----__. 326 Stillwater River area_____________________ 326 Boulder River area._______________________ 329 Summary_ _ _________________________________ 331 Post-Laramide igneous and deformational history__________-_-_-_ 331 Regionalevents-____-____-_-__-------__----------_-------- 332 Events in the Stillwater area________-______________---_--- 332 References cited.___________________________________________________ 333 Index_________._________________-_-______-___---_-._ 337 ILLUSTRATIONS [Plates 23-26 are In pocket PLATE 23. Geologic map showing regional setting of Stillwater complex. 24. Generalized geologic map of Stillwater complex. 25. Cross sections of Stillwater complex. 26. A, Laminated hypersthene gabbro from upper part of Banded Page zone; B, Anorthosite layer banded with norite________.__ 296 27. Alternation of pyroxene-rich and plagioclase-rich layers____ 296 28. Gravity stratification of norite layer above an anorthosite layer. _ _ 296 29. A, Contact between granular harzburgite and poikilitic harzburgite; B, "Grating structure" thin layers of augite in anorthosite.________________________________________ _ 296 30. Branching chromitite layers on small scale___-__-___--__- - 296 31. Dunite "intrusive" into layered harzburgite______________ 297 FIGURE 38. Generalized columnar section of the Ultramafic zone--------- 288 39. Columnar sections of Stillwater complex_______________ 290 40. Columnar sections of B chromitite zone.______________ 295 41. Section showing chromitite layers in poikilitic harzburgite . --- 298 42. Isopach map and vertical section showing interval between G and H chromitite zones..__________-_--_---____-__~_ 43. Irregularities in norite layers, plan view._.____---____- _ 44. Sections showing possible development of Lake-Nye Creek fault and dike___________-__-_--__-----__---_-_.----- 327 45. Sections showing possible development of Graham Creek fault, Gish mine..--..___.._______._._._______ __... 330 IGNEOUS AND TECTONIC STRUCTURES OF THE STILL- WATER COMPLEX, MONTANA By W. E. JONES, J. W. PEOPLES, and A. L. HOWLAND ABSTRACT Igneous structures. An upturned and previously beveled edge of the Still- water complex, a Precambrian stratiform sheet about 18,000 feet thick and comprising a sequence of noritic and ultramafic rocks and chromite layers, crops out in a belt 30 miles long on the northeast margin of the Beartooth Mountains in Montana. According to the scheme of subdivision used in this paper, the complex consists of the following four zones from bottom to top: (a) the Basal zone, 200 feet thick, medium-grained noritic rocks; (b) the Ul­ tramafic zone, 4,000 to 6,000 feet thick, a succession of layers of bronzitite, granular harzburgite, poikilitic harzburgite, and chromitite; (c) and (d) the Banded and Upper zones, in aggregate 14,000 feet thick, layers of norite, an- orthosite, troctolite, and gabbro. The position of the contact between the Banded and Upper zones is in doubt because of uncertainty in lateral corre­ lation of several troctolite layers. The rocks of the Ultramafic zone are easily distinguished from those of the Banded and Upper zones; however, the nature of the upper contact of the Ultramafic zone is uncertainly known in some places. Local discordances at that contact are probably the result of deforma­ tion after consolidation of the complex, but possibly they resulted from defor­ mation during crystallization. Layers within the major zones exhibit two types of primary structures: igneous lamination and rhythmic layering. Lamination, a function of the platy habit of the clinopyroxene, orthopyroxene, and plagioclase, is most frequently observed in the upper part of the Banded zone and in the Upper zone. Rhyth­ mic layering, involving variations in mineral composition or mode and in tex­ ture, is more conspicuous in the Banded and Upper zones but is also common in the Ultramafic zone. Rhythmic layers range greatly in thickness from a fraction of an inch to hundreds of feet. Many thin layers appear to branch and, farther on, to coalesce, forming primary lenses concave upward in the columnar section. All the layers are truly lenses, ranging from minute to large lenses thousands of feet long. Where the curvature of the lens is great, a primary basin or trough is formed. Such structures involving chromitite layers may be of eco­ nomic significance, for detailed mapping will reveal trends in thickening and thinning. In primary basins, individual layers thicken toward the axis of the trough. 281 282 CONTRIBUTIONS TO GENERAL GEOLOGY Irregularities in the trend of certain layers and groups of layers are believed to be due to movements before crystallization had been completed. Irregulari­ ties were observed in norite layers, and in the upper boundaries of some troctolite layers. Tectonic structures and igneous activity. Before Middle Cambrian time the metasedimentary rocks beneath the complex had been locally displaced by granite, and the complex may have been arched within an ancestral range. Diabase intruded the granite, the metasedimentary rocks, and the Stillwater complex, forming sills, dikes, and small irregular masses. Erosion, accelerated by the uplift, beveled the arched crustal block, exposing its core of granite, metasedimentary rocks, and an edge of the Stillwater complex. During Paleo­ zoic and Mesozoic times, the Precambrian erosional surface was buried beneath 8,000 to 10,000 feet of sedimentary
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