Character and Chronology of Basin Development, Western Margin of the Basin and Range Province
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CHARLES M. GILBERT 1 Department of Geology and Geophysics, University of California, MITCHELL W. REYNOLDS* J Berkeley, California 94720 Character and Chronology of Basin Development, Western Margin of the Basin and Range Province ABSTRACT parallel to the Mono Basin-Excelsior zone to the south and the Carson lineament to the Near the western margin of the Basin and north. Range Province in an area encompassing some 1,500 km2 between Mono Lake, California, and INTRODUCTION Yerington, Nevada, six structural basins con- The region studied for this report includes a tain thick accumulations of Miocene-Pliocene thick sequence of stratified rocks, both sedi- sedimentary and volcanic rocks. From ap- mentary and volcanic, that is exposed over an proximately 22 to 18 m.y. ago, the area was a area large enough to encompass six existing highland from which ignimbrite flows of structural basins in the western part of the Oligocene age were generally eroded. Sub- Basin and Range Province. The sedimentary sequent eruptions of andesitic rocks blanketed rocks in the sequence vary sufficiently in the area with flows and breccia. Between about lithology and facies characteristics to provide a 12.5 and 9 to 8 m.y. ago, the area became an stratigraphic basis for interbasin correlation and integrated basin of sedimentation in which for interpretation of depositional environ- some 2,500 m of strata accumulated. During ments. Furthermore, many of the rocks are this period, faulting, along west and north- datable by radiometric means, and the sequence west trends, and volcanism occurred. Within is nearly continuous from about 12.5 to 6 m.y. the basin, surface environments varied from ago. Coupled with the younger sequence in- fluvial to lacustrine, and basin margins fluctu- volved in studies made in the adjacent Mono ated, the maximum extent of the basin having Basin area (Gilbert and others, 1968; Christen- been reached about 10.5 m.y. ago, but a single sen and others, 1969), a nearly continuous integrated basin persisted. By approximately record of the last 13 m.y. is provided for this 7.5 m.y. ago, the region had been disintegrated general region. The report that follows by normal faulting into existing structural describes first the character, age, and relations blocks. Faults of this episode generally trend of the various rock units, and then considers northeast, east, and northwest. Relative the chronology and character of the late tectonic quiescence ensued for about 4 m.y. Cenozoic deformation. During this time a well-graded erosional sur- The area that has been mapped extends more face evolved and was locally covered by basic or less continuously from Fletcher and Lucky volcanic flows and silicic protrusions, com- Boy Pass on the south to Smith and Mason monly emplaced along faults of the earlier Valleys on the north, and from the western episode. Broad upwarping and block faulting flank of the Wassuk Range to the western part during the Quaternary Period produced the of the Pine Grove Hills (Fig. 1). Mapping on a present topography. In contrast to trends of scale of 1:62,500 or larger was completed in faulting prior to 7.5 m.y. ago, Quaternary most areas where late Tertiary sedimentary and normal faults have a north orientation. These volcanic rocks are exposed; less attention was faults terminate en echelon in structural warps given to older volcanic rocks, and basement or by abrupt decrease in displacement to define rocks were not differentiated. a northeast-trending lineament across the area, An extensive gravity survey was made to supplement surface mapping, the results of 'Present address: U.S. Geological Survey, Denver, which will be reported in a later publication. It Colorado 80225. is expected that the gravity data will provide Geological Society of America Bulletin, v. 84, p. 2489-2510, 7 figs., August 1973 2489 Figure 1. Index map showing part of the western heavy lines are Quaternary normal faults referred to in margin of the Basin and Range Province. Dots outline the text. the approximate area mapped during this study, and BASIN DEVELOPMENT, WESTERN BASIN AND RANGE PROVINCE 2491 additional information on the depth and con- Grove Hills were determined by Eastwood figuration of existing structural basins in which (1969) as 22.8 and 28.8 m.y. Farther south, the younger sedimentary rocks are preserved. similar rocks having similar ages were mapped east of Mono Lake (Gilbert and others, 1968). GENERAL CHARACTER AND During early Miocene time, rhyolitic ignim- AGE OF ROCK UNITS brites probably covered most of this part of western Nevada and adjacent parts of Cali- Regional Basement fornia. Within approximately 5 to 7 m.y., how- Granitic, metavolcanic, and metasedimen- ever, they had been deformed and extensively tary rocks of Mesozoic age are the basement eroded, for Miocene andesite overlaps them rocks of the region. Over large areas they are and rests directly on eroded basement over buried unconformably by thick accumulations most of the area mapped for this study. of Cenozoic sedimentary and volcanic rocks. Andesitic flows and clastic rocks of Miocene Most of the exposed basement within the age are widespread and locally thick in western region studied for this report consists of quartz Nevada and in the Sierra Nevada northward monzonite or granodiorite together with a from Mono Lake (Ross, 1961; Slemmons, 1966; variety of more mafic intrusive rocks. Meta- Gilbert and others, 1968; Bonham, 1969; morphic rocks occur as small remnants or Moore, 1969). Flows and coarse breccia pre- pendants in the granitic intrusive rocks. They dominate, but tuff and andesitic sandstone and are undoubtedly of Triassic and Jurassic age conglomerate are interbedded in many places. (Moore, 1960, 1969), but they were not dif- Locally, at the base of the sequence in the ferentiated. southern Singatze Range and in the western Quartz monzonite masses in the western Pine part of the Wassuk Range, bouldery sedi- Grove Hills have been dated radiometrically as mentary lenses containing a variety of base- 87 and 90 m.y. old (Krueger and Schilling, ment rocks represent drainage channels across 1971, p. 11). These dates correlate the Pine the underlying surface. These channels appear Grove Hills pluton with the Cathedral Range to trend roughly east. intrusive epoch of Late Cretaceous age in the The K-Ar age of a hornblende andesite flow Sierra Nevada (Evernden and Kistler, 1970). A about 15 m above granitic basement in the Late Cretaceous age has also been reported for central Cambridge Hills is approximately 15 plutons in the Wassuk Range at Lucky Boy m.y. (KA2493, Table 1). Proffett (1972) re- Pass and near Powell Mountain, but a pluton at ported K-Ar ages of andesite flows in the the northern end of the Wassuk Range has a northern Singatze Range from 17.0 to 18.9 Late Jurassic age of 140 m.y. (Evernden and m.y. In the Mono Basin area, the oldest Kistler, 1970). Clearly, more than one intrusive andesite flows are interbedded with rhyolitic epoch is represented by the granitic rocks of the ignimbrite about 22 m.y. old (Gilbert and area. others, 1968; samples KA1974, KA2000, KA2074). Older Volcanic Rocks Andesitic rocks of equivalent age in the The oldest Cenozoic rocks are rhyolitic southern part of the area mapped have been ignimbrite flows that are probably correlative silicified, argillized, and propylitized almost with the Hartford Hill rhyolite tuff in the beyond recognition. Extensive exposures of Virginia Range (Moore, 1969). Within the these altered rocks occur in the canyon of the area mapped, these ignimbrites occur in thin East Walker River southeast of the Pine Grove sequences in the western Pine Grove Hills, near Hills, and along the western flank of the Wassuk Morgan Ranch, and at two localities along the Range for several kilometers north and south of eastern margin of Coal Valley (Fig. 2). They Lucky Boy Pass. Near the East Walker River are more widespread and thicker to the north (N. 38°26.45', W. 118°58.5'), hydrothermally along the northeast margin of East Walker altered andesite is overlain unconformably by Valley and on the northern flank of the Gray unaltered remnants of a hornblende-biotite Hills, southeast of Yerington. West of Yering- andesite flow having an average K-Ar age of ton, in the northern Singatze Range, a thick 12.2 m.y. (KA2362, KA2364, KA2368, Table sequence of rhyolitic ignimbrites was mapped in 1). A dikelike mass of similar andesite in the detail by Proffett (1972), who reported K-Ar same area, also unaltered, was intruded along a ages for them ranging from 25 to 28 m.y. The fault between altered andesite and basement; it ages of two ignimbrites in the western Pine has an average age of 12.65 m.y. (KA2372, 2498 GILBERT ANI) REYNOLDS TABLE 1. ANALYTICAL DATA FOR POTASSIUM-ARGON AGE DETERMINATIONS y ROCK TYPE MINERALS K SAMPLE 40 . 40. CALCULATED Ar Ar SAMPLE LOCATION ANALYZED WEIGHT WEIGHT rad ra,| * ¿¿e (latitude - longitude) PERCENT IN GRAMS moles/g x 10-11 40 106 YEARS Older andesite Andésite N38°43.1' - W119°01.8' Hornblende 0.6550 2.76115 1.7956 41.0 14.88 * 0.47 Hornblende-biotite andesite dike and flow y. ' KA2362 Andesite (flox) I Plagioclase 0.5377 5.193B9 1.1611 40.95 ! 12.11 - 0.39 KA2364 N38°26.45'- W118°58.35' Hornblende 0.6159 4.63701 1.3254 28.53 12.07 * 0.50 KAZ368 IBiotite 6.123 1.05060 13.4685 31.36 12.33 » 0.31 KA2372 Andesite (dike) I Biotite 5.489 0.99832 12.1607 41.66 12.42 * 0.17 KA2373 N38°27.3' - W118°58.55' I Hornblende 0.631 5.14858 1.4556 36.03 12.93 * 0.45 Aldrich Station Formation KA2379R