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Generalized Geologic Map of the Basement Rocks of the Southern Sierra Nevada, California

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Generalized Geologic Map of the Basement Rocks of the Southern Sierra Nevada, California DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Generalized geologic map of the basement rocks of the southern Sierra Nevada, California by Donald C. Ross U.S. Geological Survey Open-File Report 8?-2?6 This report is preliminary and ha$ not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Menlo Park, California 198? CONTENTS INTRODUCTION, DESCRIPTION OF MAP UNITS........................................2 SUMMARY OF BASEMENT GEOLOGIC FEATURES..........................21 REFERENCES CITED...............................................25 ILLUSTRATIONS Plates 1. Geologic map 2. Coverage of the area Figure 1. Intrusive relations for some granitic rocks 2o Generalized geologic map of the basement rocks of the southern Sierra Nevada, California ' INTRODUCTION The basement geology of the southern Sierra Nevada north to lat. 36°00f N. (plate 1) has generally been shown on previous compilations as basement rock undivided or as granitic rock containing some metamorphic bodies. Other maps have shown much more detail, but only for limited areas of the southern Sierra r\ Nevada. This map, which encompasses some A900 knr of basement exposures, is an attempt to present a somewhat standardized coverage of the basement rocks of this large area and, at the same time, to point out some of its petrograph- ic complexity, and to a lesser extent, to show some of the variety in the metamorphic pendant rocks. Certainly more complexity in the geology is pre­ sent than is grossly summarized here at a scale of 1:250,000. Coverage of the area has been somewhat uneven as shown on Plate 2 that indicates all localities where samples or other data have been collected. For example, coverage is notably sparse in the northeast part of the map area and, in the northwest, is mostly limited to scattered road traverses. The geologic study has concentrated on granitic rocks and has focused more on their petro­ graphy than on their structure. Some of the granitic units shown on Plate 1 have been distinguished on other than geologic mapping evidence. For example, the Bear Valley Springs (Kbv), Poso Flat (Kpf), Alder Creek (Kal), and Dunlap Meadows (Kdm) granitic bodies are much alike in the field. All four bodies are texturally similar and have abundant hornblende and biotite. However, in the study of the thin sections and stained slabs there were distinguishable differences, principally in the amount of K-feldspar. Also, distinctions can be made on the basis of differences in °'Sr/°°Sr ratios. Based on these factors, these units have been delineated on Plate 1, but it should be empha­ sized that no mappable contacts have been seen between them. Hence, the dis­ tinctions are strictly based on petrography and chemistry. The Poso Flat unit (Kpf) may be a relatively K-feldspar-rich fades of the Bear Valley Springs (Kbv) and the Alder Creek unit (Kal) may be similarly related to the Dunlap Meadows (Kdm) unit. Early in the geologic mapping the Dunlap Meadows body -1- (Kdm) was considered a correlative of the Bear Valley Springs (Kbv). Subse­ quent Rb/Sr studies showed that though both bodies have a considerable range of ^ST/^ST ratios, the Dunlap Meadows mass (Kdm) is somewhat higher (average of 0.07069) than the Bear Valley Springs (Kbv) (average of 0.7054) and the two units probably are not correlative. Nevertheless, neither field nor micro­ scopic studies revealed any obvious textural or mineralogic differences be­ tween them. The showing of almost all geologic contacts as solid lines is more a convention for simplicity than it is a recognition of easily observable contacts. Seldom were geologic contacts obvious, except in very local areas. The intrusive relations shown on figure 1 for a limited number of granitic units are based mostly on diking relations and map pattern. Correlations (?) are based on mineralogic and petrographic similarities as well as textural resemblance in the field. DESCRIPTION OF MAP UNITS SURFICIAL DEPOSITS Alluvial, sedimentary and volcanic rocks, undivided (Quaternary and Tertiary). PLUTONIC ROCKS GRANITE (radiometric ages shown) Knc GRANITE OF NONAME CANYON (LATE CRETACEOUS)--leucocratic alaskite with minor amounts of muscovite and garnet. K-Ar age on muscovite, 80.1 +_ 5.3 Ma (Diggles, 1984). Rb/Sr on whole rock isochron on 4 samples is 80.5 +^ 1.7 Ma (R.W. Kistler, written commun., 1986). Kcc GRANITE OF CANNELL CREEK (LATE CRETACEOUS) coarse-grained, strongly sheared rock. U-Pb age on zircon of 80 ^ 2 Ma (Saleeby and Busby- Spera, 1986). Rb/Sr ages on whole rocks for 5 samples average 107.5 _+ 5 Ma. K-Ar ages on biotite of 50.5 Ma (Evernden and Kistler, 1970) and 55 Ma (Jenkins, 1961 location uncertain). -2- Kkr GRANITE OF THE KERN RIVER (LATE CRETACEOUS) relatively dark-colored hornblende and biotite-bearing rock with distinctive centimeter-sized dark clots. U-Pb age on zircon of 80 +_ 3 Ma (Saleeby and Busby- Spera, 1986, p. 92). K-Ar ages on biotite of 86.8, 87.2, and 88.8 Ma (Evernden and Kistler, 1970). Rb/Sr whole rock isochron on 4 samples gives an age of 81.1 _+ 11.1 Ma (R.W. Kistler, commun., 1986). Kgo GRANITE OF ONYX (LATE CRETACEOUS) fine grained sugary textured rock with about 4 percent biotite. K-Ar age on biotite, 80.7 Ma (Evernden and Kistler, 1970). Ksp ALASKITE* OF SHERMAN PASS (CRETACEOUS) coarse-grained with minor biotite and some hornblende. Rb-Sr whole rock age of 91 Ma from samples just to north of the map area in the Hockett Peak quadrangle (R.W. Kistler, written commun., 1987.) May be correlative with alaskite of Coyote Pass and/or alaskite of Hells Hole in the Golden Trout Wilderness Area to the north of the map area (du Bray and Dellinger. 1981). Kbp GRANITE OF BAKER POINT (LATE CRETACEOUS) fine-grained biotite bear­ ing rock. Essentially a large dike into the granite of the Kern River. Questionable K-Ar age on biotite of~92 Ma (Jenkins, 1961). Ktj GRANITE OF TEJON LOOKOUT (LATE CRETACEOUS) coarse-grained, forms craggy orange-to-yellow outcrops that weather to distinctive yellow grus. May be correlative to the similar granite of Brush Mountain. Rb/Sr determinations on 3 whole rock s^Vnples gives an age of 96.0 + 1.9 Ma (R.W. Kistler, written commun., 1986). *In the IUGS classification the term "alaskite" is restricted to rocks with a plagioclase fraction of An Q _ 5 . This is unusually restrictive and virtually eliminates the term from the Cordilleran batholithic belt. I prefer the time-worn usage of "alaskite": felsic granitic rock with less than 5 percent mafic constituents, and normally subequal amounts of plagioclase, K-feldspar, and quartz. -3- Kbm GRANITE OF BRUSH MOUNTAIN (LATE CRETACEOUS) relatively coarse­ grained rock. Many outcrops extensively altered. U/pb age on zircon of 98 Ma (James, 1986). Rb/Sr date on whole rock gives age of 91.5 +_ 1.1 Ma (R.W. Kistler, written commun., 1986). Kpp GRANITE OF PORTUGUESE PASS (EARLY CRETACEOUS) coarse-grained bio- tite-bearing rock with some hornblende. Rb/Sr determinations on whole rock samples gives an age of 105.6 +_ 9.2 Ma (R.W. Kistler, written commun., 1986). GRANITE (no radiometric ages) Kac GRANITE OF ARRASTRE CREEK (CRETACEOUS) coarse-grained felsic rock. Kba GRANITE OF BASKET PEAK (CRETACEOUS) coarse-grained felsic rock that forms craggy orange-weathering outcrops on peak and sides of Basket Peak. Mapped outline very tentative. Kbc GRANITE OF BEAN CANYON (CRETACEOUS) five irregular shaped bodies south of the Garlock fault that may well be felsic differentiates of the granite of Tejon Lookout. Kbr GRANITE OF BISHOP RANCH (CRETACEOUS) contains only sparse biotite (alaskitic) and forms craggy orange to salmon-pink outcrops. Kblm GRANITE OF BLACK MOUNTAIN (CRETACEOUS) a small, nearly round plug of biotite granite intruded into the granite of the Kern River and the tonalite of Woffird Heights. Kbra GRANITE OF BOB RABBIT CANYON (CRETACEOUS) felsic, coarse-grained biotite bearing rock with minor hornblende. Intrudes granodiorite of Rabbit Island and probably granodiorite of Castle Rock. -4- Kbo GRANITE OF BODFISH CANYON (CRETACEOUS) typically coarse-grained but some variation, contains a few percent of biotite and only local hornblende. Kb GRANITE OF BROWN (CRETACEOUS) small plug on east fron of the Sierra Nevada. Distinctly different from nearby felsic dikes and plugs. Has some textural resemblance to quartz diorite of Walker Pass. Kgg QUARTZ SYENITE OF GRAPEVINE GRADE (CRETACEOUS) small plug about 6 km east of White River that is surprisingly quartz-poor (13 percent). Klm GRANITE OF LONG MEADOW (CRETACEOUS) an ovoid pluton of fine- to medium-grained felsic rock containing a few percent of biotite. Intrudes the granodiorite of Castle Rock. Kmf MISCELLANEOUS FELSIC BODIES IN GRANODIORITE OF SACATAR (CRETACEOUS) vari-textured dikes and small stocks on east face of the Sierra Nevada. Granite of Brown and stock correlated with Granite of Onyx about 6 km to the northwest of Granite of Brown may be related to the small stocks and dikes. Some of these dikes may be part of the Jurassic Independence dike swarm of Moore and Hopson (1961). Khs GRANITE OF THE OLD HOT SPRINGS ROAD (CRETACEOUS) small medium- grained heart-shaped alaskitic stock with traces of garnet. Kpd POLKA-DOT DIKES (CRETACEOUS) conspicuous felsic dikes intruding tonalite of Bear Valley Springs. Dikes contain abundant spherical dark clots to several centimeters in diameter composed of biotite, muscovite, sodic plagioclase, quartz, and scattered pink garnet. Common white rims (haloes) around cores. Krr ALASKITE OF ROBBERS ROOST (CRETACEOUS) coarse-grained orange weathering craggy rock with small amounts of biotite, opaque minerals, and garnet. -5- Kss GRANITE OF SADDLE SPRINGS ROAD (CRETACEOUS) fine-grained, vari- shades of gray, and characterized by small mafic clots and inclusions to several millimeters.
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