| Abstract: | The Fontana 7.5‘ quadrangle is located in the northern part of the Perris Block (English, 1926, Woodford, and others, 1971), a rectangular shaped and relatively tectonically stable structural block located between the San Jacinto and Elsinore Fault zones at the north end of the Peninsular Ranges Provinces (Jahns, 1954). Included in the southern part of the quadrangle are the Jurupa Mountains, that along with rocks exposed in the San Bernardino South 7.5‘ quadrangle to the east, are underlain by exposures of the northernmost Peninsular Ranges province basement rocks. The oldest rocks are a sequence of amphibolite grade metamorphic rocks consisting of predominantly biotite-quartz schist and lesser amounts of quartzite, marble, and amphibolite. The marble forms relatively thick layers, most of which have been quarried, mainly for manufacturing of cement. Pyroxene hornfels facies skarn occurs along contact zones between marble and intrusive granitic rocks. Major marble quarries include the Crestmore quarries at the eastern part of the quadrangle and the Jensen quarry at the eastern end of the Jurupa Mountains; smaller quarries are the Henshaw quarry located between the Crestmore quarries and the Jensen Quarry, and the Glen Avon Quarry located in the western part of the Jurupa Mountains. Much of the marble at the Crestmore quarries has a characteristic blue color. The Crestmore quarries are known worldwide for the large number of rare minerals found in the contact metamorphic rocks (about 150 mineral species, e.g., Murdoch and Webb, 1966). Most of the unusual minerals occur in a metasomatic zone between marble and a monzogranite intrusive in the Commercial Quarry (e.g., Burnham, 1959, Woodford and others, 1941). Based upon the abundance of marble in the section, these metasedimentary rocks are considered to be of Paleozoic age.
Intruding the metamorphic rocks are Cretaceous-age granitic rocks, the northern part of the Peninsular Ranges batholith. These batholithic rocks range in composition from gabbro to granitic pegmatite dikes. Biotite-hornblende tonalite is the most abundant granitic rock, but there is a large biotite granodiorite body located near the central part of the Jurupa Mountains. Emplacement age of the tonalite, based on U/Pb analyses of zircon, is 104 +/- 0.4 Ma (W.R. Premo, Written Com., 2003). Sr/Rb isochron whole rock age is 117.6 +/- 6 Ma (R.W. Kistler, Written Com., 2000). Granodiorite (Kgd) gave a zircon U/Pb age of 108 +/- 6 Ma (W.R. Premo, Written Com., 2003).
Tonalite and granodiorite commonly have two planar fabrics, a planar fabric oriented northwest that is typical Peninsular Range Batholith and one that is atypical striking east and northeast; both planar fabrics are produced by oriented hornblende and biotite and ellipsoidal to pancake-shaped mafic inclusions. East and northeast oriented inclusions are generally noticeably more attenuated than the northwest oriented inclusions. Commonly a single outcrop includes both fabric planes; where discernible the northwest striking fabric is older than the east and northeast oriented fabric.
The granitic rocks are some of the closest granitic rocks to the Los Angeles Metropolitan area that can be used for construction purposes. As such, the tonalite and granodiorite have been extensively quarried for the construction of breakwaters and a variety of other engineering projects. A major quarrying operation was located at the north central part of the Jurupa Mountains at Declezville where a rail line spur was constructed for transportation of tonalite. Another large quarry is located in the hills immediately east of the Jurupa Mountains. Currently the only active quarry is the Stringfellow quarries developed in granodiorite and located in the south central part of the Jurupa Mountains.
The northern half of the quadrangle is covered by unconsolidated boulder and cobble-bearing alluvium of the distal part of the Lytle |
| Genre: | USGS Numbered Series |
| ProdID: | 53254 |
| Citation Author: | Morton, Douglas M. |
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| Citation Edition: | Version 1.0 |
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| Citation Language: | ENGLISH |
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| Citation Phsyical Description: | digital geologic data set |
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| Citation Series: | Open-File Report |
| Citation Series Code: | OFR |
| Citation Series Number: | 2003-418 |
| Citation Search Results Text: | Preliminary Geologic Map of the Fontana 7.5? Quadrangle, Riverside and San Bernardino Counties, California; 2003; OFR; 2003-418; Morton, Douglas M. |
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| Citation Year: | 2003 |
| Type: | citation/reference |
| Text: | Preliminary Geologic Map of the Fontana 7.5? Quadrangle, Riverside and San Bernardino Counties, California; 2003; OFR; 2003-418; Morton, Douglas M. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/usgs_thumb.jpg |
| URL (INDEX PAGE): | http://geopubs.wr.usgs.gov/open-file/of03-418/ |
| URL (INDEX PAGE): | http://ngmdb.usgs.gov/Prodesc/proddesc_61860.htm |
| Date Other: | Thu, 1 Jan 2004 00:00 -0600 |
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