Cenozoic Geology of the Yerington District, Nevada, and Implications for the Nature and Origin of Basin and Range Faulting
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Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting JOHN M. PROFFETT, JR. Anaconda Company, 1849 West North Temple, Salt Lake City, Utah 84116 ABSTRACT steeply east-dipping normal faults and Mackin, 1960, p. 119-122; Kurie, 1966, moderate to steep westward tilting of the p. 869). The steeply dipping normal faults In the Yerington district, western rocks. The gently and moderately dipping of the Yerington district control the present Nevada, pre-Tertiary rocks are overlain by faults were steep when they were active Basin and Range topography of the region, an Oligocene ignimbrite sequence and (Proffett, 1971a) and are not to be confused and the older moderately and gently dip- Miocene andesites. Basin and Range nor- with large landslide or gravity-slide type ping normal faults were Basin and Range mal faulting began in Miocene time, as an- faults related to high topographic relief, faults in the past. Description and interpre- desitic volcanism died out (17 to 18 m.y. that have been described in some parts of tation of the complex history and geometry ago), and has continued to the present. The the Basin and Range province (see, for of Basin and Range structure at Yerington faults dip east and are curved, concave up- example, Longwell, 1951; Drewes, 1959, has been made possible through detailed ward, with net displacements in a nearly 1963, p. 48—52, and references cited there; surface mapping and abundant subsurface east-west direction. Movement on the information. It is necessary to understand curved faults has resulted in steep westward the complex Basin and Range structure in tilting of the Miocene andesites and of all the Yerington region in order to understand older rocks. Alluvium and 8- to 1 l-m.y.-old the pre-Tertiary geology and ore deposits, basalt flows deposited during the period of which will be discussed in other papers. faulting are tilted gently west. The oldest Most of the time I worked in the faults, which dipped steeply east when they Yerington district (June to September 1966, were active, are now inactive and dip gently January 1967 to February 1971) was spent eastward as a result of westward tilting on mapping the geology of approximately 200 other faults. Younger faults dip more km2, (75 mi2) at a scale of 1 in. = 1,000 ft, steeply east, and the youngest faults, those logging cores and cuttings from more than responsible for present Basin and Range to- 30,000 m (100,000 ft) of drilling, and com- pography, are the steepest. More than 100 piling and interpreting these data. percent of east-west extension has taken place across the district because of normal GEOLOGIC SETTING faulting. The rate of extension was most rapid between 17 and 11 m.y. ago and was The oldest rocks of the Yerington district slower after 11 m.y. ago. The extension is are metamorphosed volcanic and sedimen- deep seated rather than thin skinned and tary rocks of Triassic and Early Jurassic apparently involves thinning of the crust. age. The other pre-Tertiary rocks of the dis- Several theories of origin for Basin and trict include intrusive and volcanic rocks of Range structure can be rejected because of Jurassic and possibly Cretaceous age. A the field data at Yerington, and the theory prominent widespread unconformity was that Basin and Range structure was caused developed in the pre-Tertiary rocks by Cre- by a continental spreading axis best fits the taceous(?) and early Tertiary erosion. The data. Basin and Range spreading seems to oldest rock units that overlie the uncon- have been most active between the pro- formity consist of conglomerate, sedimen- jections of the Mendocino and Murray tary breccia, and minor basalt, all of prob- fractures. It may have first started south of able early Tertiary age (Fig. 3). These untis the Great Basin, when these fractures were are overlain by a sequence of widespread farther south relative to the continent and silicic ignimbrite sheets as much as 1,400 m when the oceanic spreading axis that had (4,500 ft) thick of Oligocene1 age (Fig. 3; been between these fractures was interact- ing with the continent. 'In this paper a date of 22.J m.y. B.P. is used for the INTRODUCTION Oligocene-Miocene boundary (Turner, 1970; Bandy and Ingle, 1970). Because dates of 5.0 to 13.0 m.y. B.P. Geologic mapping in the Yerington dis- have been determined by various workers for the con- Figure 1. Map of part of western North troversial Miocene-Pliocene boundary (Harland and trict, Nevada (Figs. 1, 2), has revealed America, showing Basin and Range province, others, 1964; Evemden and others, 1964; Bandy and strongly developed late Cenozoic structure, Great Basin part of province, and Yerington dis- Ingle, 1970; Kulp, 1961), rocks that fall into this age which includes gently, moderately, and trict. range are referred to here by their radiometric age. Geological Society of America Bulletin, v. 88, p. 247-266, 18 figs., February 1977, Doc. no. 70212. 247 &&&&&& Quaternary deposits I H ! t <p Late Oligocene and Miocene dacite, 1H111 To basalt and andesite extrusive rocks £ Alluvium and basalt dated at 8 toll m.y.old <f Oligocene silicic ignimbrlte sequence O O O O O O O Xb TI and basal Tertiary deposits Miocene andesite and dacite intrusive + + Pre-Tertiary rocks 1± pT t.-T- Strike and dip of bedding Hanging wall-east normal fault showing dip, dashed where inferred,dotted where buried Strike and dip of overturned bedding West side down fault, dotted where buried Contact, dashed where approximate, dotted Possible buried strike slip fault,arrows show where buried direction of displacement Figure 2. Generalized geologic map of Yerington district, Nevada, showing major Cenozoic features. THICKNESS COLUMN DESCRIPTION NAME Alluvium, fanglomerate, sandstone, lake and flood- variable plain deposits. MINOR ANGULAR UNCONFORMITY — ±270' Flows of pyroxene olivine basalt. Coarse basal conglomerate, overlain by fanglomerate, in ±400' turn overlain by pebble-cobble conglomerate, sandstone, and siltstone. v. MAJORMA ANGULAR UNCONFORMITY ÎVWWWWW WWWWWWWSAAAAAAAAAAAA/VVVVWWWV Flows, breccia, tuff-breccia, sediments, and intrusions of grey andesite with abundant hornblende or hornblende HORNBLENDE and plagioclase phenocrysts. Hornblende pyroxene andé- ANDESITE sites common in northern Singatse Range. Associated OF intrusions of hornblende biotite dacite quartz porphyry LINCOLN present. FLAT — EROSIONAL OR SLIGHT ANGULAR UNCONFORMITY• Flows and breccia of olivine pyroxene basalt. EROSIONAL UNCONFORMITY TUFF AND Crystal-rich dacite tuff-breccia, breccia, and ash-flow 0-520' 3 BRECCIA OF v * tuff with plagioclase, biotite, and pyroxene. GALLAGHER <1 qV PASS EROSIONAL UNCONFORMITY MAP UNIT 10 - White to pale-colored crystal-poor, BLUESTONE 200- unwelded tuff, tuff-breccia, and sediments interbedded MINE with two crystal-poor, pale brown to reddish,poorly 980' TUFF welded tuffs. EROSIONAL UNCONFORMITY MAP UNIT 9 — Brown to red-brown, strongly to 800- / moderately welded, crystal-rich ash-flow tuff with 1350' plagioclase, quartz, sanidine, biotite,and hornblende phenocrysts and sparse pumice fragments. Relatively biotite rich. SINGATSE TUFF MAP UNIT 8 - Similar to unit 9 but with buff 100-300' color and abundant foreign rock fragments. 0-150' MAP UNIT 8a - Similar to unit 9. MAP UNIT 7 — White, pale green, or pale pink rhyo- 100-370' litic unwelded tuffs and sediments. <s> 6MA P UNIT 6 — Buff to lavender to reddish brown o> 200- moderately welded, moderately crystal-rich ash-flow 2tc£ luOÇO 470' tuff with plagioclase, sanidine, quartz, biotite, and 5 abundant white pumice fragments. SS X u. 0-70' MAP UNIT 5 - Rhyolitic sediments. D 0-90' MAP UNIT 4 - White, poorly welded tuff. </<) 0-160' MAP UNIT 3 - Unwelded rose pink tuff. O. > MAP UNIT 2 — Brownish-pink to lavender-gray U* J crystal-rich ash-flow tuff with sanidine, quartz, 0-800' o plagioclase, minor biotite,and large white pumice Zee \ fragments. Moderately welded. 2 00 OS -I UJ MAP UNIT 1 — Crystal-rich, strongly welded, brown s ash-flow tuff. Plagioclase, biotite, and pyroxene 5 0- phenocrysts at the base become less abundant up- o 1700' wards. Sanidine, quartz, some plagioclase, and biotite phenocrysts are present near the top. EROSIONAL UNCONFORMITY Erosional remnants of moderately welded ash-flow tuff 0-400' with quartz, plagioclase, and sanidine. BASAL 0-300' Flows, breccia, ana tuff of pyroxene basalt. TERTIARY DEPOSITS 0-200' Coarse, unsorted, unstratified landslide breccia. (Unnamed) Conglomerate. Well rounded cobbles, pebbles, and 0-400' boulders in sandy matrix. (W^WVWWWW VW VWWWWWVA >/ MAJOR t/IVCO/VFOflM/ry'Wwwwwvw\+ MESOZOIC Granitic ana metamorphic rocks. Figure 3. Stratigraphie section of Cenozoic rocks of Yerington district, Nevada, after Proffett and Proffett (1976) (courtesy of Nevada Bureau of Mines and Geology). 250 J. M. PROFFETT, JR. Table 1). This sequence contains three major ignimbrite cooling units, several thin WEST ignimbrites, and thin interbeds of rhyolitic sedimentary rocks and tuff. For mapping purposes, the sequences has been divided into 10 units, which are fairly uniform and continuous throughout and beyond the dis- trict. Individual mapping units are distinc- tive enough to be identifiable, even in drill holes and isolated outcrops. This has helped immeasurably in working out the SEA LEVEL geometry of late Cenozoic structure. The 10 map units have been grouped into three formations (Fig. 3; Proffett and Proffett, QUATERNARY DEPOSITS 1976) that can be identified well beyond the Œ district, but to facilitate detailed description I ^To,"-! TERTIARY DACITE AND HORNBLENDE ANDESITE of the structure, the numbered units will be [xYV:;: TERTIARY IGNIMBRITE UNITS 8,9 8 10 used in most of the following discussion and illustrations. The ignimbrite sequence i&Cfs&i TERTIARY IGNIMBRITE UNITS 4,5,6 8 7 is disconformably overlain by the late ia°J?*>:'1 TERTIARY CONGLOMERATE 8 IGNIMBRITE UNITS 1,2 8 3 Oligocene tuff and breccia of Gallagher // pT>/ PRE-TERTIARY ROCKS Pass, a dacite unit about 150 m (500 ft) Figure 4.