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Chemical analyses of waters of streams that drain the semiarid eastern slope of the southern Coast Ranges in California demonstrate that differences in the anion composition, especially in the ratio of bicarbonate to sulfate, are related chiefly to the lithologie character of the rocks exposed in the tributary drainage area. Where more than half the drainage area of a typical eastern-slope stream is underlain by elastic marine sedimentary rocks of Jurassic and Cretaceous age, bicarbonate generally predominates over sulfate; the ratio of bicarbonate to sulfate, both expressed in equivalents per million, in samples of the streams at low-flow stage ranges from 0.8 to 6. Conversely, where more than half the drainage area is underlain by marine and continental deposits of Tertiary age and continental deposits of Quaternary age, sulfate predominates over bicarbonate, and the ratio of bicarbonate to sulfate in samples taken during the low-flow stage ranges from 0.02 to 0.7. Organic siliceous marine shale of Tertiary age deposited in a reducing environment is probably the primary source of sulfate in the region. Secondary deposits of sulfate minerals, chiefly gypsum, which are abundant in the continental deposits of late Tertiary and Quaternary age, also contribute sulfate to the stream waters. INTRODUCTION Chemical analyses of waters of the streams that drain the semiarid eastern slope of the southern Coast Ranges of California indicate that, despite similarities of climate and distribution of precipitation in the several drainage basins, the chemical character of the stream waters differs greatly. Bicarbonate predominates over the other anions in many of the waters and sulfate predominates in other waters; chloride predominates only in the waters of two streams. This paper shows that, contrary to an assumption commonly made for stream waters, the chemical character of the waters of typical streams in the southern Coast Ranges is determined less by climatic influences than by the lithologie character of the rocks exposed in the drainage basins. Chemical analyses of samples from most of the principal streams and from many of the minor streams of the eastern slope of the southern Coast Ranges are presented in table 2. Determinations are given B-l B-2 GEOCHEMISTRY OF WATE.R of the principal cations calcium, magnesium, and sodium and the principal anions bicarbonate, carbonate, sulfate, and chloride and for most samples of boron as well. Silica, potassium, fluoride, nitrate, iron, and aluminum were determined also in many of the analyses. However, these constituents, which were generally present in relatively small concentration, and the pH were omitted from table 2 because they showed little relation to the geology of the drainage basin. In figure 4, the analyses are compared to the percentages of different geologic units exposed in the respective drainage areas (table 1), which were determined b}^ planimeter from the "Geologic Map of California" (Jenkins, 1938; Kundert, 1955). ACKNOWLEDGMENTS The writer wishes to express his gratitude to his colleagues of the U.S. Geological Survey who have offered many helpful suggestions and criticisms. Special thanks are due Messrs. William Back and D. E. White for their constructive advice on the geochemical features. None of these men, however, should be held responsible for the author's interpretations and conclusions. PHYSIOGRAPHY AND CLIMATE The southern Coast Ranges comprise the coastal mountains of California extending from San Francisco Bay southward 250 miles where they are terminated by the Transverse Ranges. The southern Coast Ranges are characterized by longitudinal ranges rising 2,000 to 6,000 feet above sea level, and by intervening valleys trending generally N. 30°-40° W. Folding and faulting control the trend of the ranges. The easternmost ranges, which include the area discussed in this report, are the Diablo Range, which extends from San Francisco Bay to Polonio Creek (fig. 1), and the Temblor Range, in echelon to the Diablo Range, which extends southward from Polonio Creek to the Transverse Ranges. The topography of the Diablo and Temblor Ranges is controlled largely by faulting and folding in rocks of Cenozoic, Cretaceous, and Jurassic age. The drainage basins of the eastern slope of the ranges are characterized by rugged hills and steep stream gradients. Along the east border of the Coast Ranges the sediments dip steeply beneath the San Joaquin Valley, thus forming an abrupt boundary that virtually lacks transitional foothills. Precipitation in the Coast Ranges and San Joaquin Valley generally decreases southward and with declining altitude in the eastern part of the mountains and adjoining valley areas. The average annual precipitation on the west side of the San Joaquin Valley ranges from MINERAL COMPOSITION, STREAM WATERS, COAST RANGES B~3 about 5 inches at Buttonwillow near the south end of the valley to about 9% inches at Tracy at the north end. Precipitation in the mountains differs greatly with altitude. At Priest Valley (altitude 2,400 feet) about 22 miles west of Coalinga, the average seasonal (October through September) precipitation is 19.56 inches, but at Buttonwillow (altitude 295 feet) on the floor of the San Joaquin Valley, the seasonal total is only 4.88 inches. ROCKS OF THE EASTERN SLOPE OF THE SOUTHERN COAST RANGES The drainage basins of the eastern slope of the southern Coast Ranges are underlain chiefly by sedimentary rocks ranging in age from Late Jurassic to Quaternary. Volcanic rocks are present, chiefly in the Franciscan formation of Jurassic and Cretaceous age; locally, ultramafic intrusive rocks, largely serpentinized, are closely associated with the Franciscan formation. Near the contacts of the intrusive rocks with the Franciscan formation, the Franciscan has been altered into many varieties of metamorphic rocks. The rocks of the area may be conveniently subdivided into five generalized units as shown in the following table: the Franciscan formation of Jurassic and Cretaceous age, ultramafic rocks intrusive Generalized geologic unit Continental deposits of Tertiary and Quaternary age Marine sedimentary rocks of Tertiary age Marine sedimentary rocks of Cretaceous age Ultramafic intrusive rocks Franciscan formation Formations and age Older alluvium (Recent? and Pleistocene), terrace deposits (Pleistocene), Tulare formation (Pleistocene? and Pliocene), continental deposits of the McKittrick group. San Joaquin, Etchegoin, and Jacalitos formations, (Pliocene); San Pablo formation, Neroly formation, Cierbo sandstone, Santa Margarita sandstone, Temblor formation, Reef Ridge shale, Monterey formation, and Vaqueros sandstone (Miocene); Kreyenhagen shale (Oligocene and Eocene); Domengine sandstone, A venal sandstone, and Tejon formation (Eocene); Lodo formation (Eocene and Paleocene; and Martinez formation (Paleocene). Moreno shale (Paleocene? and Upper Cretaceous) and Panoche formation (Upper Cretaceous). Serpentine (post-Eranciscan) Franciscan formation (Cretaceous and Jurassic), locally includes small bodies of post-Franciscan ultramafic intrusive rocks B-4 GEOCHEMISTRY OF WATE,B into the Franciscan formation, marine sedimentary rocks of Cretaceous age, marine sedimentary rocks of Tertiary age, and continental deposits of late Tertiary and Quaternary age. The areal distribution of these simplified geologic units in the principal drainage basins is shown on figure 1 and the areas, determined by planimeter, are shown in table 1. The Franciscan formation and sedimentary rocks of Cretaceous age predominate in the northern part of the area, but younger deposits underlie a greater part of the area from Panoche Creek southward. This distribution influences the geochemistry of the stream waters, determining not only the percentage composition but also the total mineral concentration of the waters. The Franciscan formation consists of a heterogeneous but characteristic assemblage of shallow marine clastic, chemical, and organic sediments and mafic volcanic rocks. Although composed chiefly of sandstone, shale, and mafic volcanic rocks, it contains also minor amounts of chert, limestone, glaucophane schist, and actinolite schist. According to Taliaferro (1943, p. 123), the lower part of the Franciscan consists largely of arkosic sandstone, thin subordinate shale partings, a few basalt flows, and lenses of radiolarian chert; the upper part consists of the same type of sandstone, but it contains more shale partings, basalt flows, and lenses of chert. The bulk of the formation is sandstone that contains 30 to 60 percent orthoclase and plagioclase feldspar. The ultramafic rocks intrusive into the Franciscan formation are commonly called serpentine, because they have been almost completely altered to minerals of the serpentine group. In outcrops where the primary textures and minerals are evident, Walker and Griggs (1953, p. 45) have identified peridotite, saxonite, dunite, Iherzolite, and pyroxenite among the parent rocks. As these rocks are composed chiefly of magnesium silicates, they constitute a major source of magnesium in the stream waters. The Cretaceous rocks underlying the eastern slope of the southern Coast Ranges consist chiefly of shale and sandy shale, sandstone containing conglomerate members, and, locally, thin limestone beds most of which were referred to the Panoche formation by Anderson and Pack (1915, p. 39-46). According to Reed (1933, p. 98), the rocks of Cretaceous age are remarkably similar throughout the area. As in the sandstone of the Franciscan formation, the Cretaceous sandstone has a high proportion of feldspar. A notable exception to the general lithology of the rocks of Cretaceous age is the Moreno shale (Paleocene? in part), which is predominantly siliceous organic shale and partly diatomaceous. North of Coalinga it ranges in thickness from 1,000 to 2,000 feet and includes the uppermost GretaMINERAL COMPOSITION, STREAM WATE

Geologic control of mineral composition of stream waters of the eastern slope of the Southern Coast Ranges, California