'Sci.LlB.* 
 
 PHYSICAL SCIENCES 
 UBRARY 
 
 UC DAVIS 
 
 STATE OF CALIFORNIA 
 
 DEPARTMENT OF NATURAL RESOURCES 
 
 GEOLOGY OF THE 
 SEBASTOPOL QUADRANGLE 
 
 CALIFORNIA 
 
 BULLETIN 162 
 1952 
 
 DIVISION OF MINES 
 
 FERRY BUILDING, SAN FRANCISCO 
 
STATE OF CALIFORNIA 
 EARL WARREN. Governor 
 
 DEPARTMENT OF NATURAL RESOURCES 
 
 WARREN T. HANNUM, Director 
 
 DIVISION OF MINES 
 
 FERRY BUILDING, SAN FRANCISCO 11 
 OLAF P. JENKINS. Chief 
 
 San Francisco 
 
 BULLETIN 162 
 
 October 1952 
 
 GEOLOGY OF THE 
 SEBASTOPOL QUADRANGLE 
 
 CALIFORNIA 
 
 By RUSSELL B. TRAVIS 
 
^ 
 
LETTER OF TRANSMITTAL 
 
 To His Excellency 
 
 The Honorable Earl Warren 
 
 Governor of the State of California 
 
 Sir: I have the honor to transmit herewith Bulletin 162, Geology of 
 the Sevastopol Quadrangle, prepared under the direction of Olaf P. 
 Jenkins, Chief of the Division of Mines, Department of Natural Re- 
 sources. The report is accompanied by various maps, charts, and illus- 
 trations, particularly a colored lithograph geologic map of the Sebastopol 
 quadrangle, wliich lies in southwestern Sonoma County and north- 
 western Marin County. Attention is called to the relationship of the 
 geology to the occurrence of economic mineral materials, such as sand 
 and gravel, crushed rock, ground water, chromite, and other substances 
 which have previously been exploited or may have potential possibilities. 
 
 The author, Russell B. Travis, mapped the geology of the area and 
 prepared the report in partial fulfillment of the doctorate requirements 
 at the University of California. 
 
 Respectfully submitted, 
 
 Warren T. Hannum, Director 
 Department of Natural Resources 
 August 20, 1952 
 
 (3) 
 
CONTENTS 
 
 Page 
 Abstract '7 
 
 Introduction 7 
 
 Geology 10 
 
 Stratigraphy 10 
 
 Jurassic system 10 
 
 Tertiary system 16 
 
 Quaternary system 22 
 
 Structure 23 
 
 Faults 23 
 
 Folds 25 
 
 Geologic history 26 
 
 Economic geology 28 
 
 Chromite . 28 
 
 Petroleum 29 
 
 Sand and gravel 29 
 
 Crushed stone 29 
 
 Water 30 
 
 Bibliography 31 
 
 ILLUSTRATIONS 
 
 Page 
 Plate 1. Geologic map of the Sebastopol quadrangle In pocket 
 
 2. Economic map of the Sebastopol quadrangle In pocket 
 
 3. Geologic sections through the Sebastopol quadrangle In pocket 
 
 4. A. Photo of Franciscan-Knoxville fossiliferous conglomerate. 
 B. Photo of interbedded green and white Franciscan-Knoxville 
 
 chert bet. pp. 16-17 
 
 5. A. Photo of quarry in Franciscan-Knoxville silty chert. 
 
 B. Photo of pillow structure in Franciscan-Knoxville variolite bet. pp. 16-17 
 
 6. A. Photo of serpentine boulders in sheared serpentine. 
 
 B. Photo of outcrop of Franciscan-Knoxville metadiabase bet. pp. 16-17 
 
 7. A. Photo of triangular facets on upthrown block north of 
 Bloomfield fault. B. Photo of lava flow of Sonoma volcanic rocks bet. pp. 16-17 
 
 Figure 1. Index map showing location of Sebastopol quadrangle 8 
 
 2. Stratigraphic column, Sebastopol quadrangle 12 
 
 (5) 
 
GEOLOGY OF THE SEBASTOPOL QUADRANGLE 
 
 CALIFORNIA 
 
 By Russell B. Travis * 
 
 ABSTRACT 
 
 The Sebastopol quadrangle, comprising an area of about 245 square miles, is in the 
 western Coast Ranges in Sonoma and Marin Counties about 45 miles north of San 
 Francisco. It occupies for the most part a broad, uplifted, differentially dissected plain. 
 
 The roclis in the area, predominately sedimentary, range in age from Upper Jurassic 
 to Recent. The oldest rocks, the Franciscan-Knoxville group, form the basement in 
 the region and crop out primarily in the northwestern quarter of the quadrangle. They 
 consist chiefly of sandstones of the graywacke type and interbedded shale, chert, and 
 altered volcanic rocks ; they have been intruded by diabase and serpentine. Meta- 
 morphic rocks of the glaucophane schist type are locally developed in the vicinity of 
 serpentine and in a few places where no serpentine is apparent. In the southeastern 
 part of the quadrangle, greenish clay of the Petaluma formation (middle or upper 
 Pliocene) is in fault contact with Franciscan-Knoxville rocks. Both of the formations 
 are unconformably overlain by the Merced formation (upper Pliocene) or by thin 
 basalt flows which locally are interbedded with the Merced formation. The basalt flows 
 are part of a thick series, the Sonoma volcanic rocks (upper Pliocene), which crop 
 out primarily east of the Sebastopol quadrangle. The Merced formation, consisting 
 chiefly of very fine-grained, massive, marine sandstone and some fresh water sand 
 and gravel, forms a thin blanket over most of the central and southern part of the 
 quadrangle. In many places it includes a thin bed of tuff-breccia. Ovei-lying these 
 formations are marine and river terrace deposits and valley alluvium. 
 
 The Franciscan-Knoxville rocks are complexly folded and faulted, but some broad 
 northwest-ti-ending folds can be recognized. The Petaluma formation is sharply folded, 
 probably by the late Pliocene orogeny. The Merced formation has been broken by a 
 series of high-angle faults as a result of Pleistocene uplift, northeast tilting, and 
 settling movements of the San Francisco-Marin block which the quadrangle occupies. 
 The large streams draining the area flow westward across the regional structure. 
 Recent subsidence (or rise in sea level) is evidenced by the drowned mouths of these 
 streams. 
 
 Most important mineral resources are sand and gravel and crushed rock. Chromite, 
 copper, coal, building stone, gold, magnesite and garnets have been exploited at times. 
 
 INTRODUCTION 
 
 The Sebastopol quadrangle, comprising an area of approximately 245 
 square miles, is located in the Coast Ranges about 45 miles north of San 
 Francisco. It occupies a local area of subdued topography and low alti- 
 tude bordered on the north, east, and south by higher more rugged hills 
 typical of the Coast Ranges. For about 3 miles in the southwestern 
 corner, it borders the ocean. The quadrangle is readily accessible by 
 several paved highways, most of which lead to Sebastopol, near the center 
 of the area. Within the quadrangle good roads are numerous, and very 
 few places are further than a mile from an all-weather road. 
 
 Field mapping of the Sebastopol quadrangle was done during the 
 summers of 1948, 1949, and during short periods until June, 1950. The 
 geology was plotted on a 15 minute topographic sheet (scale: 1:62500) 
 of the United State Geological Survey. 
 
 Acknowledgments. This investigation was done under the direction 
 of N. L. Taliaferro of the Department of Geological Sciences, University 
 
 * Condensation of a thesis submitted in partial fulfillment of the requirements for the 
 degree Doctor of Philosophy, University of California, Berkeley, California. Manu- 
 script submitted for publication February 1952. 
 
 (7) 
 
8 
 
 SEBASTOPOL QUADRANGLE 
 
 [Bull. 162 
 
 10 
 
 SCALE 
 
 10 
 
 20 Miles 
 
 Figure 1. Index map showing location of Sebastopol quadrangle. 
 
1952] INTRODUCTION 9 
 
 of California, Berkeley. Other members of this department who gave 
 helpful criticism and suggestions are C. M. Gilbert, Adolf Pabst, F. J. 
 Turner and Jean Verhoogen. Mahlon Kirk of the Department of Paleon- 
 tology, University of California was very helpful in preparing and 
 identifying fossils. J. W. Durham and D. E. Savage of the same depart- 
 ment also identified many fossils, and suggested probable environments. 
 Well drillers who gave freely of subsurface information are W. A. Duer, 
 N. K. Keyt, and C. R. Woodbury. Land owners in the quadrangle were 
 cooperative in allowing the writer access to private roads and property, 
 and some gave helpful information about outcrops and fossil localitites. 
 
 Geography. The climate in this area is temperate, with well-pro- 
 nounced wet and dry seasons. The rainy season is from November to 
 March, and the average annual precipitation is about 30 inches. Temper- 
 atures range from about 90 degrees in the summer to about 40 degrees 
 in the winter, although greater variations may occur in Santa Rosa 
 Valley. Although most of the quadrangle is well populated, the bulk of 
 the population is in Santa Rosa Valley and its bordering hills. Agricul- 
 tural and resort operations are the chief sources of income. 
 
 The most conspicuous geomorphic feature in the Sebastopol quad- 
 rangle is the broad dissected plateau surface recognizable throughout 
 the area between Santa Rosa Valley and the Pacific Ocean. The surface 
 is best preserved in the southern half of the quadrangle, where it is ex- 
 pressed by flat topped hills and ridges. It rises northward and westward 
 to a maximum elevation of about 1200 feet near Camp Meeker. In the 
 northwestern corner of the quadrangle, where deeply entrenched mean- 
 ders of the Russian River cut across the regional structure, the plateau 
 surface has almost been obliterated by stream dissection. 
 
 The northwestern part of Santa Rosa Valley, a broad alluvial plain 
 about 15 miles long and 7 miles wide, is in the Sebasopol quadrangle. 
 Except for its northernmost part, which has been slightly dissected to 
 form a series of low rolling hills, it is a nearly featureless plain. 
 
 The largest stream in the region is Russian River, which flows along 
 the northern border of the quadrangle, entering it in only a few places. 
 Tributaries of this river drain most of the northwestern part of the quad- 
 rangle as well as Santa Rosa Valley and its bordering hills. The rest of 
 the quadrangle is drained by westward flowing Salmon, Americano, 
 and Stemple Creeks and their tributaries. 
 
 A series of steps in the evolution of the present drainage system may 
 be outlined. First the pre-Merced drainage system was obliterated by 
 subsidence of the area and blanketed by deposition of the Merced for- 
 mation. Following uplift, westward flowing consequent streams were 
 established, and some subsequent tributaries developed as the Francis- 
 can-KnoxviUe rocks were exposed. The next step was the eastward tilting 
 of the region, which tended to reverse the flow of the consequent streams 
 and increase the power of the tributaries to flow along structural fea- 
 tures. Only the most powerful streams could maintain their westerly 
 courses by cutting canyons through the growing barrier of Franciscan- 
 Knoxville rocks near the present coast, thus becoming antecedent streams. 
 Later tilting and uplift further modified the drainage to some extent, 
 and as the most recent step, subsidence of the land or rise in sea level 
 has drowned the mouths of major streams along the coast. 
 
10 SEBASTOPOL QUADRANGLE [Bull. 162 
 
 Previous Geologic Investigation. Early references to the geology of 
 the Sebastopol quadrangle were made by J. D. Whitney ^ and A. C. Law- 
 son.^ The first detailed study was done by Osmont ^ in the preparation of 
 his geologic sections across the Coast Ranges north of San Francisco 
 Bay. Physiographic features are discussed in detail by Holway in his 
 studies of the evolution of the Russian River ^ and of geomorphic fea- 
 tures related to San Francisco Bay.'^ Johnson ^ made the first areal geo- 
 logic map of the quadrangle in connection with his study of the Merced 
 formation. He did not attempt to differentiate units in the Franciscan- 
 Knoxville group or work out the structure of these formations. Weaver "^ 
 mapped the geology of the Santa Rosa, Petaluma, and Point Reyes quad- 
 rangles, which adjoin the Sebastopol quadrangle on the east, southeast, 
 and south. The geology of the Healdsburg quadrangle, immediately to 
 the north, has been mapped by Gealey.^ Higgins ^ has made a recent 
 study of the geomorphology of the lower Russian River area, including 
 the Sebastopol quadrangle. 
 
 GEOLOGY 
 
 Stratigraphy 
 
 Only a few formations, ranging in age from Upper Jurassic to Recent, 
 are present in the Sebastopol quadrangle. The accompanying strati- 
 graphic column shows the principal features of these formations. The 
 most extensive exposures are of the Franciscan-Knoxville group, the 
 Merced formation, and alluvium, which together cover almost the entire 
 quadrangle. Other formations are limited to small scattered exposures. 
 The Franciscan-Knoxville group forms most of the western part of the 
 Coast Ranges north of San Francisco and is the basement rock through- 
 out most or all of Sebastopol quadrangle. The Merced formation forms 
 a thin blanket over much of the area and has a gentle north to northeast 
 dip. Sandstone is by far the most abundant rock in the quadrangle, but 
 other sedimentary, igneous and metamorphic rocks are abundant also. 
 
 Jurassic System 
 
 Franciscan-Knoxville Group. The basement rock in the Sebastopol 
 quadrangle consists of a heterogeneous series of sandstone, siltstone, silty 
 shale, conglomerate, radiolarian chert, and basic lavas, the lavas now 
 largely altered to greenstone. This series is intruded by serpentine and 
 by small bodies of diabase and gabbro. Metamorphic rocks of the glauco- 
 
 1 Whitney, J. D., Geology of California : California Geol. Survey, vol. 1, 1865. 
 
 2 Lawson, A. C, Geomorphogeny of the coast of northern California : Univ. California, 
 
 Dept. Geol. Sci. Bull., vol. 1, pp. 241-271, 1894. 
 
 » Osmont, V. C, A geological section of the Coast Ranges north of the Bay of San Fran- 
 cisco : Univ. California, Dept. Geol. Sci. Bull., vol. 4, 1904. 
 
 * Holway, R. S., The Russian River, a characteristic stream of the California coast : Univ. 
 California Publ., Dept. Geog., vol. 1, 1913. 
 
 6 Holway, R. S., Physiographically unfinished entrances to San Francisco Bay : Univ. 
 California Publ., Dept. Geog., vol. 1, 1914. 
 
 'Johnson, F. A., The Merced Pliocene formation: unpubl., thesis, Univ. California, 1934. 
 Johnson, F. A., Petaluma region : California Div. Mines Bull. 118, pp. 622-627, 1943. 
 
 ^ Weaver, C. E., Geology of the Coast Ranges immediately north of the San Francisco 
 Bay region, California: Geol. Soc. America Mem. 85, 1949. 
 Weaver, C. E., Geology and mineral deposits of an area north of San Francisco Bay, 
 California : California Div. Mines Bull. 149, 1949. 
 
 8 Gealey, W. K., Geology of the Healdsburg quadrangle, California : California Div. Mines 
 Bull. 161. 1951. , . ^ . ^ ,., 
 
 e Higgins, C. G., Jr., The lower Russian River, California : Unpubl. thesis, Univ. Califor- 
 nia, 1950. 
 
1952] GEOLOGY 11 
 
 pliane schist group are extensively developed in the vicinity of these 
 intrusions and in a few places where no intrusions are apparent. On the 
 basis of lithology, this series is referred to the Franciscan-Knoxville 
 group of Upper Jurassic age.^° 
 
 Detailed mapping in the Franciscan-Knoxville group is hampered by 
 thick covers of vegetation or soil, and also by the structure of the rocks, 
 which are severely crushed and sheared throughout much of the area 
 mapped. In many areas, chert, sandstone, metamorphic rocks, and green- 
 stone crop out together in small apparently unrelated masses, separated 
 by pulverized material that is readily eroded and exposed only in cuts 
 or cliffs. Large road cuts along the Kussian Eiver afford some exposures 
 of this material and included blocks of other rocks. 
 
 Franciscan-Knoxville rocks are exposed in about one-third of the quad- 
 rangle, principally in the northwestern part. Throughout most of the 
 remainder of the area, they are covered by Merced sandstone. Sandstone 
 and siltstone predominate in the extreme northern part of the area, 
 whereas intrusive and metamorphic rocks have their greatest develop- 
 ment in the Camp Meeker-Occidental area. Exposures of these rocks in 
 the southern half of the quadrangle are confined to valley bottoms, to 
 blocks uplifted by faulting, or to high points on the pre-Merced erosion 
 surface. 
 
 It is not possible to determine the total thickness of the Franciscan- 
 Knoxville group because the base is not exposed, and overlying forma- 
 tions are separated from these rocks by an angular unconformity'. How- 
 ever, between Camp Meeker and the Mountainview school to the north, 
 about 7000 feet of probably unrepeated strata are involved in folds. 
 
 Megafossils were found in one locality. Aucella piocliii (Gabb), a char- 
 acteristic Franciscan-Knoxville fossil, has been identified from fragmen- 
 tarv material that was found in the sandv matrix of a conglomerate 
 southwest of Occidental. Radiolaria are abundant in the chert, and in 
 thin section several genera may be distinguished, although preservation 
 is generally poor. However, stratigraphic significance of the radiolaria 
 is not known at present. 
 
 Exposures in the Sebastopol quadrangle are too poor to determine 
 defijiitely whether Franciscan-Knoxville rocks exist as separable litho- 
 logic units. Only one relatively small outcrop of black shale is designated 
 in the report as Knoxville ( ?) shale. 
 
 Sandstone and siltstone of the "gra^-wacke suite" ^^ is by far the most 
 abundant constituent of the Franciscan-Knoxville rocks in the Sebas- 
 topol quadrangle, comprising probably three-fourths of the total. The 
 sandstone is characterized by firm induration, grain angularity, abun- 
 dant rock fragments and fresh feldspar, small amount of matrix, and 
 gray or green color. For the most part, it is medium-grained, but in the 
 northwestern corner, fine-grained sandstone and siltstone predominate. 
 Coarse-grained sandstone occurs in several places but is of limited extent. 
 Both the coarse- and fine-grained sandstones are strongly resistant to 
 erosion, but the fine-grained sediments weather more readily and are 
 exposed only in road cuts or cliff's. Much of the coarse- and medium- 
 grained sandstone, however, is so resistant that weathering produces 
 topographic knobs nearly free of soil. 
 
 10 Taliaferro, N. L., Franciscan-Knoxville problem : Am. Assoc. Petroleiim Geologists — 
 
 Bull. vol. 27, pp 109-219, 1943. 
 iiPettijohn, F. J., Sedimentary rocks; Harper and Brothers Pubis., New York, 1949. 
 
12 
 
 SEBASTOPOL QUADRANGLE 
 
 [Bull. 162 
 
 AGE 
 
 FORMATION 
 
 COLUMN 
 
 FEET 
 
 DESCRIPTION 
 
 QUATERNARY 
 
 UPPER 
 
 PLIOCENE 
 
 ALLUVIUM 
 
 a 
 
 TERRACES 
 -Unconformity 
 
 MERCED 
 
 FORMATION 
 
 Up to 
 300 
 
 Up fo 
 1000 
 
 MIDDLE 
 
 OR 
 UPPER 
 
 PLIOCENE 
 
 ^-Unconformity 
 
 PETALUMA 
 
 FORMATION 
 -Fault contact- 
 
 Unknown 
 
 UPPER 
 
 JURASSIC 
 
 FRANCISCAN- 
 
 KNOXVILLE 
 
 GROUP 
 
 Gravel, sand, clay 
 
 Chiefly fine-grained, buff and gray, 
 marine, feldspathic sandstone. Massive, 
 friable, fossiliferous. Conglomeratic 
 marine sandstone in western part of 
 quadrangle. Some freshwater sands and 
 gravels on west side Santa Rosa Valley. 
 Thin bed of tuff breccia intercalated in 
 marine sandstone in central part of 
 quadrangle. Thin basalt flow of Sonoma 
 volcanics at base in southeastern part of 
 quadrangle, locally intercalated in sands 
 and gravels. 
 
 Thin-bedded, green and buff, discoidal 
 clay in southeastern part of quadrangle. 
 Base not exposed. 
 
 7000+ (?) 
 
 •ciSii. 
 
 Sandstones of the graywacke suite with 
 minor amounts of dark shale and con- 
 glomerate. Dark gray, thin-bedded, 
 silty shale in Purrington Creek canyon 
 assigned to Knoxville(?) formation, 
 Jk. Chert, volcanic rocks, diabase. 
 Serpentine intrusions with associated 
 metamorphic rocks of the glaucophane 
 schist series. 
 
 base 
 nnaxposed 
 
 Figure 2. Stratigraphic column, Sebastopol quadrangle. 
 
1952] GEOLOGY 13 
 
 Bedding is restricted to the fine-grained sediments, and is especially- 
 well developed where fine-grained sandstone and siltstone alternate. The 
 siltstone has a well-developed parting parallel to the bedding and a 
 micro- joint system which, together, cause it to break into small rectan- 
 gular blocks. Medium- and coarse-grained sandstone is generally massive, 
 and gives no suggestion of attitude, except where it includes thin layers 
 of shale. Jointing in the coarser-grained sediments is common, and is 
 suggestive of bedding in some places, but, wherever the bedding is deter- 
 minable, the two structures are independent. Sorting is poor in all the 
 sandstone and graded bedding is rare. 
 
 Weathering transforms the gra}^ or greenish-gray color of the fresh 
 rock to brown or dark greenish-brown and, in many places, penetrates 
 several feet into the rock. Thick mantles of decomposed sandstone cover 
 many of the hills, especially those composed of fine-grained sediments. 
 
 The Franc iscan-KnoxviUe sandstone has a relatively uniform compo- 
 sition in the Sebastopol quadrangle. It is composed essentially of quartz 
 and feldspar with minor but significant amounts of rock fragments, 
 biotite, muscovite, opaque minerals and carbonaceous material in a 
 fine-grained matrix. In the hand specimen, quartz, plagioclase, rock frag- 
 ments, and muscovite are readily recognized. Estimates of the propor- 
 tions of minerals in thirteen thin sections average quartz 35 percent, 
 feldspar 45 percent, and rock fragments 7 percent. The proportions of 
 quartz range from 32 percent to 40 percent ; of feldspar, from 30 percent 
 to 54 percent. Alkali and plagioclase feldspar occur in the ratio of 1 :2 
 or 1 :3. The proportion of rock fragments ranges from 3 percent to 10 
 percent, about half of which is chert. Fragments derived from dense 
 glassy lava and from chlorite-clinozoisitic rocks (?) are common, and 
 shale fragments up to 2 inches long are locally abundant. 
 
 Biotite, more or less chloritized, is common throughout the sandstone. 
 The amount ranges from 1 percent to 10 percent, increasing with decreas- 
 ing grain size. Muscovite (less than 1 percent) as large pearly flakes is 
 conspicuous in the hand specimen. Black opaque constituents include 
 magnetite, ilmenite, hematite, pyrite, and carbonaceous matter. Carbon- 
 ate minerals, predominantly calcite, ranging in amount up to 10 percent, 
 occur in veins with quartz or in minor amounts as cement in some sand- 
 stones. Other minerals present in most of the thin-sections are : apatite, 
 common ; chlorite, alteration of biotite and in matrix, common ; epidote- 
 clinozoisite, common ; garnet, colorless or pale pink, rare ; rutile, rare ; 
 sphene, dark bro-^ra, slightly pleochroic, abundant ; zircon, abundant. 
 
 Thin-bedded, silt- and clay-shale is sporadically interbedded with 
 sandstone throughout the quadrangle. It is largely restricted to layers 
 only a few inches or feet thick. Most of the shale is dark gray or black, 
 but dark olive-green shales are common. 
 
 The section of dark-gray, very thin-bedded, brittle shale in the canyon 
 of Purrington Creek about 1^ miles northeast of Occidental is designated 
 as Knoxville ( ?) shale. It is at least 500 feet thick, and probably thicker. 
 Although its exposure is restricted, it far exceeds the extent of any 
 other shale in the area mapped. It is unfossiliferous, and no other rock- 
 types were found interbedded with it. 
 
 The shale occupies the central part of a portion of the Camp Meeker 
 s^mcline, and overlies the Camp ]\Ieeker serpentine sill and Franciscan- 
 KnoxviUe sandstone of unknown thiclmess. It is separated from this sill 
 by a few ( ?) hundi'ed feet of sandstone and metamorphic rocks, but the 
 
14 SEBASTOPOL QUADRANGLE [BuU. 162 
 
 contact with these rocks is not exposed. As may be seen in the accompany- 
 ing geologic section, the sediments overlj-ing the sill occupy a high strati- 
 graphic position in the Franciscan-Knoxville rocks in this part of the 
 quadrangle, although the stratigraphy of these rocks is imperfectly 
 kno-wai. Therefore, although no fossils were found, it is designated Knox- 
 ville ( ?) shale on the basis of its stratigraphic position and lithology. 
 
 Franciscan-Knoxville conglomerate in the Sebastopol quadrangle is 
 restricted to a few thin lenses of pebble and cobble material which are 
 vaguely expressed in the topography. Pebbles are black and well- 
 rounded ; they consist, for the most part, of several kinds of porphyries, 
 quartzite or recrystallized chert, and black chert. Pebbles of Franciscan- 
 Knoxville sandstone and volcanic rock are present but rare. The porphy- 
 ries include quartz porphyry, quartz-feldspar porphyry, and feldspar 
 porphyry. A count of pebbles in some of these conglomerates reveals the 
 following composition : 
 
 porphyries 80% 
 
 quartzite or recrystallized chert 10% 
 
 black chert 7% 
 
 other 3% 
 
 Chert is widely distributed throughout the exposed Franciscan-Knox- 
 ville group in the Sebastopol quadrangle, except in the extreme north- 
 western corner where it is conspicuously rare. Numerous large outcrops 
 occur in the southern part of the quadrangle. The largest mass is about 
 a mile east of Aurora school and is nearly a mile long and from 50 to 100 
 feet thick. No other large continuous masses were found. Few are over 
 100 feet long, and most of them are too small or too poorly exposed 
 to map. 
 
 The character of the chert and its occurrence in this quadrangle are 
 the same as elsewhere in the Coast Eanges. It is rhythmically-bedded 
 radiolarian chert and crops out conspicuouslj'- in irregularly shaped 
 masses of bare rock jutting abruptly above the ground. Individual beds of 
 chert are from 1 inch to 4 inches thick and are generally separated by 
 thin layers of shale. These beds are not of uniform thickness, but pinch 
 and swell and lens out in short distances. Most commonly the chert is red 
 in color, but in a few places it is green, black, brown, or white. Many of 
 the chert lenses are severely contorted, brecciated, and veined with 
 quartz, but others are quite undeformed. 
 
 Association of chert with greenstone is common, but most of the chert 
 lenses are interbedded with sandstone and siltstone, and appear to be in- 
 dependent of greenstone. 
 
 Igneous Rocks. Franciscan-Knoxville volcanic rocks are widely dis- 
 tributed in the quadrangle, but do not comprise a great total thickness. 
 They have been hydrothermally altered to fine-grained rocks of dull 
 dark-green or greenish-brown color. As it is not possible to identify the 
 original rock type in hand specimens, the term ' ' greenstone ' ' is used as a 
 convenient field designation. The large bodies of greenstone have been 
 mapped (pi. 1), but numerous other masses are too small to be included 
 on a map of this scale. Much of the greenstone is intensely sheared, and 
 commonly contains veins of quartz or calcite, or both. 
 
 Diabase occurs sparingly in the Franciscan-Knoxville group in the 
 northwestern quarter of the quadrangle. It is altered to greenstone, and 
 in the hand specimen most of it is indistinguishable from the volcanic 
 rocks. 
 
1952] GEOLOGY 15 
 
 Serpentine, derived largely from alteration of the intrusive ultrabasic 
 rocks peridotite and pyroxenite, is common in the Franciscan-Knoxville 
 group in the quadrangle. The largest body is a sill exposed in both limbs 
 of the Camp Meeker syncline. In the syncline two northwest-trending 
 sub-parallel bands of serpentine crop out, exposed almost continuously 
 over a length of about 6 miles. They coalesce about 1:^ miles north of 
 Camp Meeker. The sill has an average thickness of about 1000 feet, with 
 a maximum of about 2500 feet. None of the other serpentine bodies have 
 outcrop lengths of over 1 mile. 
 
 Unweathered serpentine is dark olive-green when massive to light 
 bluish-green when sheared, and has greasy to dull earthy lustre. Field 
 appearance is largely determined by the amount of shearing. Outcrops 
 of massive serpentine weather to rusty-brown or buff, rough blocky 
 masses. When sheared it erodes easily, and exposures of such material are 
 largely limited to road cuts. 
 
 Small disseminated grains of magnetite and chromite are sparingly 
 present in all of the serpentine, although locally chromite is concentrated 
 to form small deposits which were worked sporadically during World 
 Wars I and II. Veinlets of chrysotile up to 6 or 7 millimeters wide trellis 
 most of these rocks, and small veins or pods of magnesite are common. 
 
 Conspicuous and widespread, but not abundant, is the so-called silica- 
 carbonate rock, a product of the hydrothermal alteration of serpentine. ^^ 
 As the name indicates, it is composed essentially of silica and a carbon- 
 ate of calcium, iron, and magnesium. Because of its silica minerals it is 
 much more resistant to erosion than the enclosing serpentine, and stands 
 in conspicuous knobs and dikelike ridges, few of which are over 20 feet 
 long in this quadrangle. It forms highly irregular buff or dark-brown 
 outcrops. Leaching of the carbonate leaves a cellular meshwork of ocher- 
 coated silica, producing a cavernous, jagged structure, which combined 
 with the color, readily distinguishes it from other rocks. 
 
 Metamorphic Rocks. Metamorphic rocks are widely distributed and 
 locally abundant in the Franciscan-Knoxville group in the Sebastopol 
 quadrangle. They crop out in the vicinity of most of the serpentine bodies 
 and in some places where no intrusive rock is apparent. Although not all 
 these rocks are schistose, and glaucophane is absent from some, it is a 
 major constituent in most of them and they are commonly referred to as 
 glaucophane schists. The principal types are eclogite ;^^ a series of rocks 
 composed of some combination of quartz, glaucophane, actinolite, albite, 
 epidote-clinozoisite, hornblende, garnet, lawsonite, chlorite, muscovite, 
 stilpnomelane, zoisite, and pumpellyite ; and metaspilitic rocks and 
 metadiabases composed almost entirely of chlorite-clinozoisite or chlorite- 
 lawsonite (or pumpellyite) with but minor amounts of glaucophane. 
 
 Well-crystallized, lustrous schists consisting largely of glaucophane, 
 actinolite, or hornblende crop out conspicuously in knobs or short ridges. 
 They are extremely hard and resistant to erosion, and in areas where they 
 are abundant, form numerous rounded masses 20 or 30 feet in diameter. 
 The most striking feature of the metamorphic rocks is their erratic dis- 
 tribution and apparent discontinuity over short distances. Masses of 
 
 12 Knopf, Adolph, An alteration of Coast Range serpentine : Univ. California, Dept. Geol. 
 
 Sci. Bull., vol. 4, pp. 425-530, 1906. ^ ^ . 
 
 ^ Switzer, George, Eclogite from the California glaucophane schists : Am. Jour. Sci., 
 
 vol. 243, pp. 1-8, 1945. 
 
16 SEBASTOPOL QUADRANGLE [BuU. 162 
 
 these well-crystallized schists in some places are abudantly distributed, 
 yet no trace of metamorphism can be found in the intervening rocks which 
 may include sandstone, chert, and greenstone. Although they are most 
 abundant in the vicinitj'^ of serpentine bodies, no direct connections to 
 the serpentine can be traced. Even where the rocks are in contact, or 
 where metamorphic rocks are included in the serpentine, the relation is 
 not gradational but abrupt, and the contact surfaces slickensided. These 
 relations probably are the results of strong orogenic movements on rocks 
 of diverse resistance. 
 
 Glaucophane schist is developed in the vicinity of all the serpentine 
 bodies, but the most extensive development is near the Camp Meeker 
 serpentine sill where all the varieties mentioned in this report may be 
 found. Schist is locally abundant both above and below the sill, although 
 along Dutch Bill Creek, below the sill it crops out almost continuously. 
 
 Eclogite occurs in abundant small masses and irregular veinlike bodies 
 in the glaucophane schist. In general, eclogite is presumed to have been 
 formed at extremely high pressures and temperatures. However, its close 
 association with glaucophane schists indicates that here it probably 
 represents relatively low grade metamorphism. 
 
 Most of the metamorphic rocks in the Sebastopol quadrangle have 
 probably been formed by metasomatism related to the intrusion of basic 
 and ultrabasic rocks, that involved the addition of lime and alumina, and 
 the extraction of soda and silica. 
 
 Tertiary System 
 
 Petaluma Formation (Middle or Upper Pliocene). The Petaluma 
 formation crops out only in a small area on the edge of the quadrangle 
 7 miles southeast of Sebastopol. Here it is exposed for a short distance 
 in a narrow gully which cuts across the axis of an anticline. However, 
 the presence of this formation is indicated in some low places along Gos- 
 sage Creek by thick, black, sticky soil. In its only exposure it is composed 
 of greenish and buff, well-bedded clay and is unconformably overlain by 
 gravelly sand of the Merced formation and a thin flow of basalt of the 
 Sonoma volcanic rocks. Absence of this formation over the greater part 
 of the area is probabl}^ due to a period of uplift and erosion antedating 
 deposition of the upper Pliocene Merced formation. 
 
 The base of the Petaluma formation is not exposed, making thickness 
 determination impossible. Morse and Bailey ^"^ suggest a thickness of 4000 
 feet based in part on well data from the southeastern part of the Santa 
 Rosa quadrangle. Northwest of Roblar, the formation is buried beneath 
 the Merced formation, and southwest on the south side of the Tolay fault 
 the Merced formation and Sonoma volcanics rest directly on Francisean- 
 Knoxville rocks. 
 
 The age of the Petaluma formation is in doubt because of lack of good 
 fossil evidence. However, the best evidence seems to point to a middle or 
 upper Pliocene age.^^ 
 
 " Morse, R. E., and Bailey, T. L., Geological observations in the Petaluma district, Cali- 
 fornia : Geol. See. America Bull., vol. 46, pp. 1437-1455, 1935. 
 15 Johnson, F. A., The Merced, Pliocene formation: Univ. California, unpub. thesis, 1934. 
 Stirton, R. A., Cenozoic mammal remains from the San Francisco Bay region : Univ. 
 
 California, Dept. Geol. Sci. Bull., vol. 24, pp. 339-409, 1939. 
 Grant, U. S. IV, and Hertlein, L. G., Pliocene correlation chart: California Div. Mines 
 Bull. 118, pp. 201-202, 1943. 
 
DIVISION' OF MINES 
 
 BULLETIN 162. PLATE 4 
 
 -■1. FRANCISCAX-KNOXTVaLLE FOSSILIFEROUS 
 CONGLOMERATE 
 
 B. INTERBEDDED GREEN AND WHITE FRANCISCAN- 
 KNOXVILLE CHERT 
 
 
DIVISION OF MINES 
 
 BULLETIN 162, PLATE 5 
 
 A. QUARRY IN FRANCISCAN-KNOXVILLE SILTY CHERT 
 
 B. PILLOW STRUCTURE IN FRANCISCAN-KNOXVILLE 
 
 VARIOLITE 
 
DIVISION OF MIXES 
 
 BULLETIN* 162. PLATE 6 
 
 
 A. SERPENTINE BOULDERS IN SHEARED SERPENTINE 
 
 B. OUTCROP OF FRANCISCAX-KNOXVILLE 
 METADIABASE 
 
DIVISION OF MINES 
 
 BULLETIN 162, PLATE 7 
 
 A. TRIANGULAR FACETS ON UPTHROWN BLOCK 
 
 North of Bloomfield fault. 
 
 
 
 
 B. LAVA FLOW OF SONOMA VOLCANIC ROCKS 
 On flank of Washoe Creek anticline. 
 
1952] GEOLOGY 17 
 
 Sonoma Volcanic Rocks. In the southeastern part of the Sebastopol 
 quadrangle are several small exposures of thin lava flows. Some of them 
 may be traced eastward into the Santa Rosa quadrangle where they 
 become more continuous and thicker, and are associated with pyroclastic 
 material. The volcanic materials represent the western fringe of the great 
 mass of lavas, tuifs, and interbedded sands and gravels of the Sonoma 
 volcanic rocks. 
 
 The lava flows in most places in this quadrangle rest unconformably 
 on Franciscan-Knoxville rocks and are overlain by the Merced forma- 
 tion. However, north of Dunham school a flow of black olivine basalt is 
 interbedded with gravelly sandstone of possible Merced age, which in 
 turn overlies the Petaluma formation with strong angular unconformity. 
 Thus at least part of the Sonoma volcanics may be contemporary with 
 the Merced formation. The largest exposure of olivine basalt is southwest 
 of Dunham school on the north flank of hill 713. Here it rests unconform- 
 ably on Franciscan-Knoxville rocks and is overlain by the Merced for- 
 mation. Elsewhere in this corner of the quadrangle small patches of 
 basalt and olivine basalt crop out sporadically, many of them bordering 
 faults. 
 
 All these lavas are light- to dark-gray to black. Porphyritic olivine 
 basalts predominate but basalts occur also. Phenocrj^sts of olivine, augite, 
 and plagioclase, up to 2 or 3 millimeters in maximum dimension, are 
 megascopically recognizable in many of the lavas. In some, large pheno- 
 crj^sts of olivine and augite have been replaced by iddingsite, serpentine, 
 chlorite, and iron oxide, forming conspicuous brown or green spots in 
 the rock. Microscopically, most of the rocks are seen to be medium- 
 grained, and to have a strong fluidal arrangement of crowded plagioclase 
 laths, with abundant granules of augite and opaque minerals, and small 
 amounts of interstitial glass. A typical mineral composition is : labrado- 
 rite microlites 50 percent, groundmass augite 15 percent, labradorite 
 phenocrysts 3 percent, augite phenocrysts 15 percent, olivine pheno- 
 crysts 10 percent, opaque minerals 5 percent. 
 
 A small plug (?) of hexagonally jointed tuff breccia of rhyolitic com- 
 position forms a small hill 2000 feet north of Dunham school. It intrudes 
 the Petaluma formation and possibly the Merced formation, but its rela- 
 tion to the Sonoma volcanic rocks is not clear because of erosion. The 
 rock consists of abundant white pumice fragments up to 3 inches in diam- 
 eter, in a greenish-yellow groundmass of glass, quartz, sanidine, clastic 
 grains, and opaque minerals. Probably the vent-breccia intrusion is about 
 the age of the Sonoma volcanic rocks. 
 
 Merced Formation (Vpper Pliocene). The Merced formation, con- 
 sisting of both fresh-water and marine buff and gray sandstones and 
 lenses of conglomerate and sandy shale, is the most widely distributed 
 formation in the Sebastopol quadrangle. The marine sandstone, which 
 interfingers with the fresh-water sediments and Sonoma volcanic rocks, 
 includes a thin bed of tuft' breccia. It is predominanth' very fine-grained, 
 argillaceous, feldspathic, and fossiliferous. The fresh-water part of the 
 Merced formation consists chiefly of gravel and medium-grained sand- 
 stone intergrading within short distances, but includes a few lenses of 
 sandy clay. Calcium carbonate and iron oxide concretions, commonly 
 containing fossils, are numerous and widely distributed in the formation. 
 
 3—60369 
 
18 SEBASTOPOL QUADRANGLE [Bull. 162 
 
 Tlie formation covers about throe-fii'tlis of tlio Sebasto]iol (luadraiitile, 
 inc'Uiding' the sonth-eentral part of the northern haff and ahnost all the 
 southern half. Thin patches of Merced cap most of tlie flat-topped hills in 
 the western part of the quadranpjle, except in the northwestern corner. 
 The exposures of the fresh water part of the Merced formation are con- 
 fined to the western ed.u'e of Santa Kosa Valley. The most extensive ex- 
 posures are alonp' the Gnerneville llifihway northeast of Sonsa Corners, 
 in and around Sebastopol, and the area east of Cadwell and Hessel near 
 the eastern marp'in of the quadran<ile. The Merced formation is found 
 also along the northern border of the Ft. Reyes quadrangle and in the 
 Santa Rosa, Petaluma, and Mare Island quadrangles, northwest and 
 southeast of Petaluma.'" The exposed thickness of the Merced formation 
 ranges from a few feet to about 500 feet. The persistent gentle northeast 
 dip indicates that the thickness is greatest near the western edge of Santa 
 Rosa Valley. Fresh-water deposits interfinger with the marine part of 
 the Merced formation along the western edge of Santa Rosa Valley and 
 probably thicken eastward but are not over 200 or 800 feet thick where 
 they are exposed. 
 
 Total cumulative thickness of the Merced formation cannot be reliably 
 estimated because of the general absence of horizon markers and the 
 irregularity of the surface upon which it rests. An interbedded tuft' 
 breccia, which is a good time marker over part of the area, is underlain 
 by a maximum of 250 feet and overlain by a maximum of 200 feet of 
 marine fine-grained sandstone. In the southwestern part of the quad- 
 rangle, about 300 feet of conglomeratic marine sandstone rests on the 
 Franciscan-Knoxville group and grades upward into the typical fine- 
 grained marine sandstone. 
 
 Tahle 1. Composition of Coarse-Grained Merced Sediments. 
 
 Rock type 
 
 Marine 
 
 conglomeratic 
 
 sandstone (percent) 
 
 Fresh water 
 
 conglomerate 
 
 (percent) 
 
 Chert .. 
 
 Quartz 
 
 Sandstone and schist (Franciscan-Knoxville) 
 
 Quartzite 
 
 Othcr_ _. 
 
 50 
 
 30 
 
 10 
 
 8 
 
 2 
 
 6.5 
 
 25 
 
 5 
 
 4 
 
 1 
 
 
 
 Throughout most of its extent in the area, the Merced formation rests 
 on an irregular erosion surface of considerable relief cut in rocks of the 
 Franciscan-Knoxville group. Because the slopes of this erosion surface 
 are in many places steep, there is a great irregularity in thickness of the 
 Merced, and the Franciscan-Merced contact is erratic in nature. The 
 maximum relief on this surface exposed at the present time is about 500 
 feet. In a small area north of Dunham school, and for an unknown dis- 
 tance to the northwest, the Merced formation unconformably overlies 
 the Petaluma formation. In some places the Merced formation is uncon- 
 formably overlain by Quaternary alluvial gravels and terrace deposits. 
 
 Almost all the Merced sandstone is massive, loosely-packed, and poorly 
 cemented by argillaceous material and iron oxide. Locally'', zones con- 
 taining fossils are firmly cemented with calcium carbonate. Crossbedding 
 is not uncommon in the fresh water part of the Merced formation. 
 
 18 Weaver, C. E., op. cit., California Div. Mines, Bull. 149, 1949. 
 
 \ 
 
1952] 
 
 GEOLOGY 
 
 19 
 
 In the southern half of the qnadranofle, the sandstone includes more 
 resistant beds ^vhich are conspicuous in gullies but are rarely traceable 
 between them. 
 
 Cou<rlomeratic, gritty sandstone, common in the southwestern corner 
 of the quadrangle, caps small buttes near Valley Ford, and has been 
 carved by erosion into large, bold structures which stand out conspicu- 
 ously on the landscape. However, these more resistant phases are of local 
 extent and grade laterally into non-resistant gravels and sands which 
 readily disintegrate. 
 
 The bulk of the marine Pierced sandstone is subangular. well sorted, 
 and very fine-grained. Mechanical analyses of five samples showed that 
 75 to 94 percent of the sand is composed of grains between 0.2 and 0.05 
 millimeter in diameter. ^lost of the remaining material is finer grained. 
 Both the marine conglomerate sand and the fresh-water sand and gravel 
 range considerably in sorting. 
 
 F. A. Johnson ^' made an extensive petrographic study of the Merced 
 formation. Some of his results have been confirmed by the writer and are 
 included in table 2. This table shows the average mineral composition of 
 the principal types of Merced sandstone, which does not differ signifi- 
 cantly from place to place within the area studied. Pebble counts from 
 the coarser deposits indicate that well-rounded and highly polished chert 
 and quartz predominate. The results of several counts are summarized 
 in table 1. Most of the chert pebbles are radiolarian and similar to that 
 found in the Franciscan-Knoxville rocks. 
 
 A highly tuffaceous sandy conglomerate occurs in the hills just east of 
 Sousa Corners. Over half of this conglomerate is composed of fine tuff- 
 aceous material and rounded pebbles of decomposed pumice as much as 
 
 Table 2. Mineral Composition of Merced Sandstone. 
 
 Mineral 
 
 Fiae-grained 
 
 marine 
 
 sandstone 
 
 Plagioclase 
 
 Quartz 
 
 Orthoclase 
 
 Rock fragments 
 
 Chert 
 
 Volcanic glass 
 
 Hea^'J" minerals 
 
 Magnetite 
 
 Non-magnetic iron ore 
 
 Lan"sonite 
 
 Glaucophane 
 
 Zircon 
 
 Green hornblende 
 
 Actinolite 
 
 Sphene 
 
 Garnet 
 
 Epidote 
 
 Rutile 
 
 Zoisite 
 
 Basaltic hornblende 
 
 TourmaUne 
 
 Musco^^te 
 
 Biotite 
 
 Augite 
 
 Hypersthene 
 
 45 
 
 20 
 
 17 
 
 10 
 8 
 
 pr 
 0.5 
 5.0 
 
 15.0 
 6.5 
 3.0 
 3.5 
 
 3.C 
 
 l.c 
 l.c 
 O.c 
 
 l.c 
 
 pr 
 
 pr 
 
 pr 
 
 pr 
 
 pr 
 
 pr 
 
 rare 
 
 rare 
 
 Conglomeratic 
 
 marine 
 
 sandstone 
 
 38 
 22 
 15 
 15 
 10 
 
 1.0 
 1.0 
 
 11.0 
 o.o 
 3.0 
 2.5 
 
 pr 
 2.0 
 1.0 
 1.5 
 1.0 
 
 pr 
 
 pr 
 
 rare 
 
 pr 
 
 rare 
 
 rare 
 
 rare 
 
 rare 
 
 Fresh-water 
 sandstone 
 
 30 
 
 35 
 
 15 
 
 10 
 
 10 
 
 pr 
 0.5 
 3.5 
 
 15.0 
 5.0 
 3.5 
 1.5 
 5.0 
 2.0 
 1.5 
 1.0 
 
 pr 
 
 pr 
 
 pr 
 1.0 
 
 pr 
 
 rare 
 
 pr 
 
 pr 
 
 pr 
 
 " Johnson, F. A., The Merced Pliocene formation : Univ. California, Unpub. thesis, 1934. 
 
20 SEBASTOPOL QUADRANGLE [Bull. 162 
 
 1 inch in diameter. The remainder corresponds approximately to the 
 composition of fresh water conglomerate given in table 1. 
 
 A few small exposures of lignitic coal occur in Merced fresh-water 
 sand and gravel from a half to three-fourths of a mile west of Trenton 
 on the Uiver road. Water wells in this area penetrate lignite beds 5 to 10 
 feet thick at variously reported depths. 
 
 Tuf Breccia. A thin but persistent bed of water-laid tuff breccia, 
 interbedded witli Merced sandstone, crops out over wide areas in the 
 central antl northern part of the area. The white spots it foi^ms on hill- 
 sides stand out conspicuously. In many areas, however, it is obscured by 
 vegetation or soil. It can be traced almost continuously from a point \% 
 miles west of Dunham school, where it is but a few inches thick, to Jonive 
 school, east of Occidental. Near Jonive school it averages about 10 feet in 
 thickness, and is interbedded with marine, buff, fine-grained sandstone 
 with little tuffaceous material. Here it ranges from 50 to 250 feet strati- 
 graphicall.y above the base of the Merced formation. In the hills on either 
 side of Mark West Creek northwest of Trenton, sandj^ tuff breccia crops 
 out abundantly. On the south side of the creek outcrops are small and 
 erratically distributed, but on the north side the tuff breccia crops out 
 continuously for about a mile and has an estimated thickness of 50 feet. 
 Careful searching by the writer failed to reveal any trace of tuff breccia 
 between the two areas, and it is doubtful that the two occurrences repre- 
 sent a single bed. 
 
 Glass and pumice fragments as much as 4 inches long are predominant 
 constituents in the tuff breccia from both areas. Near Trenton, the tuff 
 includes about 25 percent of sand and pebbles, but, elsewhere it con- 
 tains only minor amounts of clastic material. Some tuff, composed 
 exclusively of fine ash, occurs locally. Fossils occur in the tuff breccia 
 near Freestone and, although "marine in character," are indeter- 
 minate.^^ 
 
 The source of the pyroclastic material in the tuff breccia is not known. 
 The thickest accumulation of the Sonoma voleanies is east of the Sebas- 
 topol quadrangle, and it is possible that the pyroclastics came from that 
 direction. Johnson ^'■* found a volcanic neck in the northeastern part of 
 the Bodega Head quadrangle, and suggested that as a possible source. 
 However, there is no evidence to support this view. The vent breccia 
 near Roblar may represent the source, but the tuff breccia is thicker to 
 the north and west. 
 
 The Merced formation is probably upper Pliocene in age. Although 
 fossils are abundant in the formation, they are not diagnostic. Specimens 
 w^ere collected from several localities and submitted to J. W. Durham and 
 Mahlon Kirk, in whose opinion they indicate Pliocene age, more likely 
 upper than lower. -"^ Species determinations, as well as localities from 
 which the specimens were collected, are listed in table 3. 
 
 The most prolific source of Merced fossils is in the Healdsburg quad- 
 rangle at Wilson Grove, 2 miles north of Trenton. The fauna of this 
 
 18 Osmont, V. C, A geological section of the Coast Ranges north of the Bay of San Fran- 
 
 cisco : Univ. California, Dept. Geol. Sci. Bull., vol. 4, 1904. 
 
 19 Johnson, F. A., op. cit. 
 
 20 Personal communication. 
 
1952] 
 
 GEOLOGY 
 Table 3. Fossils From Merced Formation. 
 
 21 
 
 Pelecypoda: 
 
 Anadara trilineata (Conrad) 
 
 Cryptomya californica Conrad 
 
 Glycymeris sp 
 
 Macoma nasuta (Conrad) 
 
 cf M. secta (Conrad) 
 
 sp 
 
 Marcia angustifrons (Conrad) 
 
 Mytilus sp 
 
 Prototheca staleyi (Gabb) 
 
 Pecten (patinopecten) healeyi Arnold. 
 
 Solen sp 
 
 Spisula cf S. albaria (Conrad) 
 
 sp 
 
 Tellina sp 
 
 Location* 
 
 C-13 
 
 Gastropoda: 
 
 Crepidula princeps Conrad 
 
 Miopleionia sp 
 
 Nassarius californicus (Conrad). 
 
 Olivella pedroana 
 
 Polinices sp 
 
 Coral (?) 
 
 Coronula (?) sp.. 
 
 X 
 X 
 
 X 
 X 
 
 M-2 
 
 L-16 
 
 C-17 
 
 X 
 X 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 X 
 
 X 
 X 
 
 X 
 X 
 
 X 
 
 X 
 
 X 
 
 • Locations: 
 
 C-13 Creek bed, three-quarters of a mile south of Vine Hill School, near Trenton. 
 
 M-2 Road cut, 2J miles northeast of Dillon Beach. 
 
 L-16 Road cut, 2i mUes west of Dunham school. 
 
 C-17 Creek bed, 1,800 feet N. 58 E. of hill 426, 1 mile west of Trenton. 
 
 X Fossils found commonly in concretions in southwestern part of quadrangle. 
 
 Y Fossils found commonly as sand casts and rarely composed as original material throughout the 
 
 fine sand. 
 
 locality has been collected by Staley,^^ Osmont ^- and Dickerson,-^ all of 
 whom regarded them as Pliocene, and at lea.st in part ecinivalent to the 
 type Merced of the San Francisco peninsnla. Charles Merriam suggested 
 that the Wilson Grove fauna indicates middle Pliocene age. Weaver con- 
 cluded that the fauna from Wilson Grove and other jMerced localities 
 "is probably of middle Pliocene age."-^ However, in the Sebastopol 
 quadrangle the Merced formation rests unconf ormably on the middle to 
 upper Pliocene Petaluma formation, and it is considered here to be upper 
 Pliocene in age. 
 
 The ]\Ierced formation was derived primarily from rocks of the Fran- 
 ciscan-Knoxville group, and was deposited in a relatively shallow marine 
 embayment which opened to the ocean to the west. The eastern border of 
 this embayment was near the present western edge of Santa Rosa Valley, 
 but was not constant in its position. No marine ]Merced is known on the 
 east side of this valley, and hence the maximum eastern transgression 
 of the Merced sea was located somewhere in the site of the present Santa 
 Rosa Vallev. 
 
 21 Gabb, W. jM., Paleontology of California : California Geol. Surv., vol. 2, 1869. 
 
 22 Osmont, V. C, A geological section of the Coast Ranges nortli of the Bay of San Fran- 
 
 cisco : Univ. California, Dept. Geol. Sci. Bull., vol. 4, 1904. 
 *s Dickerson, R. E., Tertiary and Quaternary history of the Petaluma, Point Reyes and 
 
 Santa Rosa quadrangles: California Acad. Sci. Proc, 4th ser., vol. XI, pp. 527-601, 
 
 1922. 
 2* Weaver, C. E., op. cit., California Div. Mines Bull. 149, 1950. 
 
 4—60369 
 
22 SEBASTOPOL QUADRANGLE [Bull. 162 
 
 Taile 4- Typical Composition of Pleistocene Gravel in Santa Rosa Valley. 
 
 
 Percent 
 
 Acidic lavas 
 
 45 
 
 (including vesicular, pumiceous, and banded varieties) 
 
 
 Intermediate lavas 
 
 20 
 
 (gray-colored, dense, non-vesicular types) 
 
 
 Basic lavas _ 
 
 12 
 
 (black, dense, fine-grained rocks possibly including some andesites) 
 
 
 Franciscan-Knoxville chert 
 
 10 
 
 (red, green, and brown) 
 
 
 Other Franciscan-Knoxville rocks 
 
 5 
 
 Obsidian 
 
 3 
 
 Tuff or tuff-breccia - - 
 
 3 
 
 Petrified wood 
 
 2 
 
 
 
 The presence of Crepidula princeps Conrad in conglomeratic sandstone 
 from a mile north of Bodega to Dillon Beach indicates that this stretch 
 was on the open ocean, at least during early Merced time. All the other 
 fossils are of forms which prefer quiet, relatively shallow water, pro- 
 tected from heavy waves. The fineness of the sand also attests quiet water. 
 According to Clark ^^ the climate during Merced time was ' ' cold-temper- 
 ate to boreal" and on the basis of the fossils collected by the writer, 
 Mahlon Kirk ^^ suggests the climate was probably somewhat cooler than 
 at present. Carbonaceous material and fragments of wood are common 
 in the Merecd and well-preserved logs have been penetrated by drilling. 
 
 Quaternary System 
 
 Pleistocene deposits are neither abundant nor thick except in Santa 
 Rosa Valley. Here they compose a 100 foot terrace which is being 
 dissected by streams. The terrace is most conspicuous where Mark West 
 Creek has cut a shallow valley in it about half a mile wide. Northwest of 
 Trenton the 100 foot terrace is clearly expressed on most of the bordering 
 slopes, where the terrace gravels rest on Merced sandstone or tuff breccia. 
 Southward and eastward, this terrace merges imperceptibly with the 
 alluvial deposits of the greater part of Santa Rosa Valley. Therefore, 
 it is included on the map with older alluvium. 
 
 These terrace deposits are composed of gravel and gravelly sand, and 
 were largely derived from volcanic rocks. 
 
 Buff or reddish-brown gravelly sands are common on marine terraces 
 north of Dillon Beach. These deposits are 10 or 15 feet thick and consist 
 exclusively of Franciscan-Knoxville and Merced debris. They are well- 
 stratified, and some have primary dips up to 20 degrees. 
 
 Small patches of stream deposits were found at several levels along the 
 Russian River, but were not mapped. Notable occurrences are in the 
 hills west of Northwood in the northwestern corner of the quadrangle, 
 and at about the 200 foot level in the hills southeast of Mirabel Park. 
 
 26 Clark, B. L.., Stratigraphy and faunal horizons of the Coast Ranges of California : Pri- 
 vate publication, 1929. 
 ^ Personal communication. 
 
1952] GEOLOGY 23 
 
 They consist of buff to dark-broAvn gravel and coarse sands composed of 
 Franciseau-Knoxville and volcanic rocks. A large terrace deposit at an 
 altitude of about 200 feet covers the hill a mile northeast of the Mark 
 "West Creek-Russian Eiver confluence. Many vertebrate fossils of "later 
 Pleistocene" age have been found in the gravels along Estero San 
 Antonio, about a mile from its mouth.-" Among them a bison, Mammu- 
 thus, cf., columhi, and Mammut cf., amcricanum have been recognized. 
 
 Recent alluvium fills many of the stream valleys in the quadrangle 
 and consists of gravels, sands, and clays. In the lower part of Laguna 
 de Santa Rosa, in Green Valley Creek, and in the esteros to the south, 
 alluviation has developed marshes. Landslides are abundant in the Sebas- 
 topol quadrangle. Although most are small, some of moderate size occur 
 in the Franciscan-Knoxville rocks southwest of the Camp Meeker 
 syncline. 
 
 Structure 
 
 The geologic structure of the Merced formation in the Sebastopol quad- 
 rangle is relatively simple, but the structure of the underlying Fran- 
 ciscan-Knoxville group is complex. Although the details of structure of 
 these basement rocks are difficult to obtain, it is clear that they are com- 
 plexely folded and faulted, and, like the Franciscan rocks in other parts 
 of the Coast Ranges, have a general NW-SE structure. The ]\Ierced for- 
 mation forms a thin blanket over this basement and, in most of the 
 quadrangle, is nearly horizontal and undulating, although northeast 
 dips greatly predominate. This may be accounted for by a series of north- 
 west-trending faults of small to moderate displacement that break the 
 region into several blocks tilted to the northeast. Although the structure 
 of the Merced formation is simple, details are obscured by poor expo- 
 sures, homogeneity of much of the formation, and lack of bedding. An 
 intercalated tuff breccia in the central part of the quadrangle and thin 
 lava-flows of the Sonoma volcanics in the southeastern part outline the 
 structure in those regions. 
 
 Although the San Andreas fault passes through the water-covered 
 southwestern corner of the quadrangle, no other large Tertiary fault- 
 systems are present and the entire region is confined to Lawson's San 
 Francisco-Marin Block. 
 
 Faults 
 
 Faults in the Sebastopol quadrangle may be separated into two groups ; 
 those of pre-]\Ierced, and those of post-Merced age. The pre-Merced 
 faults, with the exception of the Tolay fault involve only rocks of the 
 Franciscan-Knoxville group. Although these rocks are intricately 
 faulted, only a few pre-Merced faults have been shown on the map. In 
 a strict sense, many of the contacts between members of the Franciscan- 
 Knoxville group are probably fault contacts, as most of them show signs 
 of movement. This applies to the Camp Meeker serpentine sill as well as 
 to smaller bodies. However, as these movements are probably of small 
 displacement, it is felt that a more accurate impression is given by plot- 
 ting the contacts as unfaulted. In the Franciseau-Knoxville rocks west of 
 Forestville, two faults are shown. The location of the larger one is deter- 
 mined by gouge and crush zones which appear to belong to a single fault 
 or narrow fault zone. This fault extends northeastward into the Healds- 
 
 ^ Personal communication. 
 
24 SEBASTOPOL QUADRANGLE [Bull. 162 
 
 burg quadrangle and is part of the Mt. Jackson fault zone.^® The shorter 
 one has brought chert and metamorphic rocks which are, in part, brec- 
 ciated into contact with sandstone. This fault can be recognized for only 
 a short distance beyond the exposure of the metamorphic rocks, which 
 are rare in this part of the quadrangle. Another of these faults is located 
 a mile north of Dillon Beach. Here a shear zone, 50 to 100 feet wide in- 
 cluding blocks of brecciated, recemented graywacke, is clearly exposed 
 along a cliff. It can be recognized for about half a mile to the south- 
 east, where it passes beneath the Merced formation. None of these faults 
 has affected the Merced formation. 
 
 The Tolay fault, best expressed in the Santa Rosa and Petaluma quad- 
 rangles extends from San Pablo Bay for over 20 miles into the Sebas- 
 topol quadrangle north of Dunham school. Here it brings Franciscan- 
 Knoxville rocks on the southwestern side of the fault into contact with 
 the Petaluma formation on the northeastern side. The post-Merced move- 
 ment of this fault consisted of small, mainly vertical, displacement ^^ 
 and has been insufficient to expose the rocks underlying the Merced for- 
 mation beyond a mile northwest of Dunham school. The main activity 
 along this fault took place after Petaluma time and before Merced time. 
 
 The post-Merced faults have many characteristics in common. With few 
 exceptions they are nearly vertical, trend northwestward, and have 
 small displacements uplifting the northeast blocks. As a result the blocks 
 have been slightly tilted to the northeast. These faults bring Franciscan- 
 Knoxville rocks into contact with the Merced formation. In the Trenton- 
 Forestville area, movements along two nearly vertical faults have pro- 
 duced a series of steep hills. One of these faults borders the range of hills 
 about a mile east of Forestville, and extends northwestward into the 
 Healdsburg quadrangle. To the southeast, it is lost in the Merced forma- 
 tion. It probably has controlled the drainage of the small creek south of 
 the Vine Hill school, and intersects, or is continuous with, the nearly 
 north-south fault east of Sousa Corners. The movement on the fault east 
 of Forestville has been vertical, as far as can be ascertained, and has up- 
 lifted the northeastern block and tilted it northeastward. The maximum 
 displacement is over 300 feet but probably less than 400 feet. The fault 
 contact is exposed in a small road cut just south of hill 516 and at this 
 place it dips steeply to the east. However, the soft incoherent Merced 
 sand makes such evidence inconclusive and it would not be safe to say 
 that the fault is a high-angle thrust. Another fault, just east of Trenton, 
 passes beneath terrace deposits to the north and beneath valley alluvium 
 to the south. It brings up Franciscan-Knoxville metamorphic rocks, 
 chert, sandstone, and a small patch of serpentine on the northeastern 
 side of the fault. The Merced formation borders the fault on the south- 
 west. The maximum displacement is at least 250 feet and is probably over 
 300 feet. 
 
 Most of the post-Merced faults are in the southeastern quarter of the 
 quadrangle. Two of these, the Bloomfield fault and the Americano Creek 
 fault, have developed a graben structure in the Bloomfield area. The 
 Bloomfield fault extends from a mile north of Walker school northwest- 
 ward about eight miles, where it apparently dies out. It has a strong ex- 
 pression in the topography. Over most of its extent this fault has brought 
 
 28Gealey, W. K., Geology of the Healdsburg- quadrangle, California: California Div. 
 
 Mines Bull. 161, 1951. 
 =» Weaver, C. E., op. cit., California Div. Mines Bull. 149, 1950. 
 
 X^ 
 
1952] GEOLOGY 25 
 
 up Franciscan-Knoxville rocks on the northeastern side into contact with 
 ]\Iereed sandstone. The maximum vertical displacement is probably over 
 600 feet. The Americano Creek fault extends along the range of hills 
 south of Americano Creek. It is the only fault known in the quadrangle 
 in which the south block has been upthrown. It extends from a mile east 
 of Two Rock to a mile southwest of Bloomfield. a distance of about 6 miles. 
 This fault is nearly vertical, and has a maximum vertical displacement of 
 at least 300 feet south of Bloomfield. 
 
 The Dunham fault, just south of Dunham school, has upthrown Fran- 
 ciscan-Knoxville rocks on the northeast into contact with the Merced 
 formation and Sonoma volcanic rocks. The maximum displacement is 
 over 300 feet but is probably less than 400 feet. There are three other 
 high-angle faults in this area, but they are much smaller than those 
 discussed above. The upthrown north-side blocks bring Franciscan- 
 Knoxville rocks into contact with Merced sandstone. The displacements 
 are at least 100 feet, and probably near 200 feet. They are about 2 or 3 
 miles long. A small fault of 100 or 200 feet maximum displacement 
 occurs north of "Walker school. 
 
 Another series of faults occurs north of Bodega in the west-central 
 part of the quadrangle. The tracing of these faults is greatly handi- 
 capped by thick soil and timber, and by the absence of the usually com- 
 mon rocky outcrops of the Franciscan-Knoxville group. In this area, 
 three faults have been mapped. They are high-angle, of moderate dis- 
 placement, with the north blocks upthrown. The largest of these, called 
 herein the Joy Woods fault, is about 3i miles long. Metamorphic rocks 
 are abundant on its north side. The maximum vertical displacement is 
 about 300 feet. The other two faults in this group are similar to but 
 smaller than the Joy Woods fault. The larger of the two is well expressed 
 in the topography and has a maximum displacement of about 300 feet. 
 
 The similarities of the post-Merced faults in the Sebastopol quadrangle 
 clearly indicate a common origin. This region occupies a block which is 
 an extension of the San Francisco-Marin block bounded on the west by 
 the San Andreas fault system and on the east by the possible extension 
 of the Hay ward fault. During Pleistocene time, probably during the 
 mid-Pleistocene orogeny, this block was uplifted and tilted to the east. 
 It is probable that concurrent and subsequent settling movements devel- 
 oped the series of faults described above. Some of these are perhaps 
 reactivations of older faults, as the general strike of the post-Merced 
 faults is about the same as the regional strike in the Franciscan-Knox- 
 viUe group. 
 
 Folds 
 
 Some broad structural features can be deciphered in the Franciscan- 
 Knoxville rocks. The most clearly defined structure is the Camp Meeker 
 syncline. delineated by the Camp Meeker serpentine sill. The exposure 
 of metamorphic rocks and a few small serpentine masses along the general 
 strike of this sill strongly suggest its extension to the area around Two 
 Rock in the southeastern part of the quadrangle. It plunges gently to the 
 south, disappearing beneath the Merced formation southeast of Occiden- 
 tal. Its strike ranges from about N. 50° "W. in the southeastern part of its 
 outcrop to about X. 30° TV. in the northwestern part. It is somewhat 
 asymmetric, the steeper northeastern limb dipping to the southwest 
 
26 SEBASTOPOL QUADRANGLE [BuU. 162 
 
 nearly 90 degrees, but the dip of this limb is difficult to determine. Two 
 other broad folds are shown in the Francisean-Knoxville rocks, but their 
 locations are vaguely defined and can only be approximated. Both are 
 located in heavily timbered regions and no details of their structure can 
 be given. 
 
 In spite of the prevalence of northeastward dips in the Merced forma- 
 tion, there is evidence of undulation, as indicated by the fact that over 
 much of the area the thickness of the formation is relatively uniform. A 
 small, sharply asymmetric syncline involving a lava flow of the Sonoma 
 volcanic rocks and the Merced formation has been caused by fault drag 
 on the south side of the Dunham fault. The lava adjacent to the fault 
 dips 60 to 70 degrees to the west whereas the lava half a mile southwest 
 dips about 6 degrees to the east. 
 
 The only clearly defined folding in the Merced formation occurs north 
 of Dunham school. Here, the Washoe Creek anticline of the Santa Rosa 
 quadrangle broadens out and splits into two low anticlines and included 
 syncline. A basalt flow of the Sonoma volcanics, intercalated with Merced 
 gravels, delineates the structures in the area. 
 
 Intensely folded Petaluma clays form the core of the folds, and farther 
 east are overlain directly by basalt, but they have too limited an exposure 
 here to determine much of their structure. Dips of the basalt on the flanks 
 of these folds do not exceed 15 degrees. The axis of the north anticline, 
 which is a direct extension of the Washoe Creek anticline, strikes about 
 N. 65° W., whereas that of the south anticline passes through Roblar and 
 strikes about N. 75° W. These folds are not traceable northwest of this 
 area. 
 
 Santa Rosa Valley seems to be a structural trough brought about by the 
 uplift and eastward tilting of the San Francisco-Marin block, and the 
 uplift of the Berkeley Hills block, bordering Santa Rosa Valley on the 
 east. It may be in part synclinal, because there are steep west dips on the 
 eastern margin of the valley and gentle east dips on the western margin. 
 Minor folding or faulting in the valley is revealed by drilling. 
 
 Geologic History 
 
 The earliest geologic event recorded in the Sebastopol quadrangle is 
 the deposition of the Francisean-Knoxville group of Upper Jurassic age. 
 However, just west and south of the quadrangle, at Bodega Head and 
 Point Reyes peninsula, there are exposures of granitic and metamorphic 
 rocks of the Sur series, which are possibly Paleozoic or older. Although 
 these older rocks only crop out west of the San Andreas fault, they 
 probably comprise, at least in part, the basement upon which the Fran- 
 cisean-Knoxville group rests. 
 
 In Upper Jurassic time this area was apparently part of a broad slowly 
 sinking geosyncline in which was deposited the great thickness of sand- 
 stone and shale of the Francisean-Knoxville group. Submarine volcan- 
 ism accompanied deposition of these sediments, as basic lava flows were 
 poured out on the sea floor and in places intruded into the sands at 
 shallow depths. Silica, concentrated in the sea water as a result of vol- 
 canism, was precipitated to form numerous lensoid bodies of chert. At a 
 later stage in their depositional history the deeper portions of the sedi- 
 ments were intruded by basic and ultrabasic igneous rocks, which in some 
 places metamorphosed the intruded rock. 
 
1952] 
 
 GEOLOGY 
 
 27 
 
 Taile 5. Pariial Log of Ratnondo No. 1 (Elev. 85 feet). 
 (Courtesy of E. IF. Lenhart) 
 
 Rock 
 
 Buff sand 
 
 Blue gumbo clay with sand streaks 
 
 Thin layers of asphaltic mat 
 
 Blue clay 
 
 Grit in blue clay 
 
 Gas show 
 
 Bright greenish-blue clay 
 
 Logs; pulpy, soft mat; redwood-Uke bark. 
 Pea gravel, (3-s" average diameter) 
 
 Shell layer 
 
 Shells in blue sandy clay 
 
 SheUs in reef 
 
 3ot area 
 il show, pebbles up to 2 inches in diameter. 
 
 Blue fine sand and shale 
 
 Hot area 
 
 Log, redwood? 
 
 Blue sand, clay, shale 
 
 Turquoise clay with black matter 
 
 Blue clay sand shale 
 
 Spots of bentonite 
 
 Depth 
 (feet) 
 
 35 
 65 
 66 
 
 290 
 
 300 
 
 700 
 
 750? 
 
 785 
 
 825 
 
 900 
 1,190 
 1,260 
 1.450 
 1,460 
 
 1,475 
 1,530 
 1,630 
 
 2,100 ± total 
 
 The absence in this quadrangle of deposits representing the long 
 interval between Upper Jurassic and middle Pliocene time makes it im- 
 possible to outline in detail the events which took place during this 
 interval. In middle Pliocene time a basin was developed along the present 
 site of Petaluma Valley and in it clay, with interbedded fine-grained 
 conglomerate and sand, was deposited. The presence of brackish-water 
 mollusks in these sediments indicates that the basin was depressed enough 
 at times to allow sea water to enter, probably from the southeast. At 
 other times a fresh-water lake probably occupied this basin. 
 
 Following deposition of the Petaluma formation, tectonic activity 
 increased and culminated in a severe diastrophism in late Pliocene time, 
 when the block south of the Tolay fault was uplifted several thousand 
 feet and the Petaluma formation removed from it. On the north side of 
 the fault, the Petaluma formation was strongly folded. Erosion of the 
 San Francisco-Marin block during and after this orogeny developed a 
 topography of late maturity or early old age, marked b}'^ relatively low 
 rounded hills and broad valleys. 
 
 In upper Pliocene time the area was depressed, possibly by broad 
 warping, and covered by the sea in which the Merced formation was 
 deposited. At this time, farther eastward, volcanic activity began, and 
 lava flows and pyroclastics of the Sonoma volcanics were emitted in great 
 volume. These flows spread out over the Petaluma formation and the 
 Franciscan-Knoxville rocks bordering the Tolay fault, which was appar- 
 ently still active. Fluvial sands and gravels were deposited between 
 flows. A burst of explosive volcanic activity spread ashes and pumice over 
 the ]\Ierced sea. These ashes, combined with debris brought in by streams, 
 formed a tuff breccia as much as several feet thick in marine Merced 
 sediments. A volcanic vent developed at Roblar, but apparently did not 
 erupt any large amount of material. The land bordering the Merced sea 
 
28 SEBASTOPOL QUADRANGLE [Bull. 162 
 
 was relatively low and, in part, wooded. Flnvial and strand-line con- 
 glomeratic and medinm-grained sand and some silt accumulated along 
 the eastern margin of the sea. Carbonaceous material including some 
 logs, were buried in these deposits and locally formed coal beds. 
 
 The deposition of the Merced formation may have extended into Pleis- 
 tocene time, for there is no adequate means to date the upper limit. The 
 history of the region in the Pleistocene epoch has been mainly one of 
 uplift and minor settling. Patches of Merced sandstone west of Occi- 
 dental at elevations over 1100 feet testify to the great amount of uplift 
 that has occurred. 
 
 Several stages in the Pleistocene uplift of the San Francisco-Marin 
 block are represented by numerous marine terraces along the coast and 
 by a 100 foot terrace in Santa Rosa Valley. Recent minor sinking (or 
 rise in sea level) is evidenced by the dro^^^led features of Estero San 
 Antonio, Estero Americano, and the mouths of Salmon Creek and the 
 Russian River. These streams are affected by the tide far inland from 
 their mouths, and have alluviated valleys, even where they pass through 
 narrow canyons near the ocean. 
 
 ECONOMIC GEOLOGY 
 
 The chief mineral products mined in the Sebastopol quadrangle are 
 sand and gravel, produced chiefly along the Russian River, and crushed 
 stone, produced from numerous small cpiarries. Chromite, copper, coal, 
 building stone, tuff, gold, and garnet have been mined at one time or 
 another. Quicksilver, oil and gas have been prospected for, but no eco- 
 nomic deposits of these commodities have been reported. All known mines, 
 quarries, and prospects are located on the accompanying economic map 
 (pL2). 
 
 Chromite 
 
 Several chrome workings are located in outcrops of the Camp Meeker 
 serpentine sill. All but one are prospects, having no recorded shipments 
 of ore. 
 
 Sonoma Chrome, Inc., mine is in sees. 15, 16, and 17, T. 7 N., R. 10 W., 
 M.D., 3^ miles north of Occidental. It is owned by Florence M. Button, 
 of Camp Meeker, California. 
 
 The ore occurs as small kidneys of chromite which strike about 
 N. 40° W. and are parallel to the schistosity of the serpentine country 
 rock. A shaft 55 feet deep, drifts, pits, and trenches were used to develop 
 this mine. Production came mostly from a large open cut. 
 
 In 1918 the property, then called the Meeker mine, was leased to 
 S. H. Dolbear, who mined and shipped 644 long tons of ore containing 
 39 to 42 percent Cr20.s. In 1942 a 2000-acre tract of this land was leased 
 to Bentley Newman of Santa Clara, who, after obtaining a loan from the 
 Reconstruction Finance Corporation, cleaned out the shaft and old 
 chromite workings and repaired the road. Evidently not enough ore was 
 developed to warrant further work, because on expiration of the lease in 
 1952, no ore had been mined. 
 
 The high-grade ore is said to contain from 42 to 51 percent Cr203 with 
 about 3 : 1 ratio of Cr : Fe. Dow and Thayer estimated the reserves to be 
 less than 5000 long tons. The property is idle. 
 
1952] ECONOMIC GEOLOGY 29 
 
 Petroleum 
 
 Gas seepages and small oil slicks occur in Santa Rosa Valley northeast 
 of Sebastopol. and gas is reported in -wells in the lower part of Green 
 Valley Creek. Residents near Bloomfield claim that gas is a serious prob- 
 lem in pumping their deep water-wells. In 1937 a well was drilled a mile 
 or so south of Two Rock by the Two Rock Oil Corporation. After drilling 
 475 feet into Franciscan-Knoxville rocks, it was abandoned. In 1948 a 
 weU was drilled 300 feet south of Hall school, northeast of Sebastopol. 
 This well has been temporarily abandoned after reaching about 2100 
 feet, the limit of the portable rotary-rig being used. A partial log of this 
 well is given in table 5. 
 
 Sand and Gravel 
 
 Mirabel Sand & Gravel Company is located in the XAV^ sec. 31, T. 8 X., 
 R. 9 "W.. M.D.. half a mile northwest of Mirabel Park. Operator is Simon 
 Camgros. San Rafael. California. 
 
 Sand and gravel has been taken from this area since 1927 by Simon 
 Camgros. who is the present owner and operator of the plant. Most of 
 the product of the washing plant is used as concrete aggregate by a 
 ready-mix concrete plant in San Rafael which is also owned by 
 Mr. Camgros. 
 
 Sand and gravel is taken from the river and beach of the Russian River 
 by two slackline cable excavators and one Northwest dragline. One 
 scraper delivers directly to the washing plant, and the other scraper and 
 the dragline load into trucks which dump into an elevator at the base of 
 the washing plant. The river-run material is washed and sized, then 
 stored in concrete bunkers from which it is loaded by gravity into trucks 
 for delivery. The washing plant has a capacity of about 400 cubic yards 
 per shift, and the products are as follows : 
 
 Size • Cubic yards 
 
 Sand 250 
 
 ■*46-inch pea gravel 25 
 
 i-inoh pea gravel 15 
 
 f-inch gravel "5 
 
 lY-inch rock 30 
 
 Each year the sand and gravel previously removed is replaced during 
 the period of high water. The material consists of sandstone, chert, 
 quartzite. and serpentine. The particles are well-rounded and have a 
 high strength. 
 
 TVheu working behind the dams the slackline cable excavators have 
 limited use and most of the loading into trucks is done by the dragline. 
 When the dragline is inoperative the washing plant is supplied with 
 river-run gravel by a bulldozer which feeds from a stock pile into the 
 elevator boot. The capacity of the present equipment of this company is 
 about 1000 cubic yards of river-run gravel per shift. Eight men are 
 employed in this area by the ^Mirabel Company. 
 
 Crushed Stone 
 
 Crushed stone is produced intermittently from many small quarries, 
 largely located in outcrops of Franciscan-Knoxville rocks. Rock types 
 used include chert, silty cheit. greenstone, sandstone, serpentine, and 
 tuff breccia. The product is used locally in road construction. At one time 
 a tunnel was driven into a quartz vein in Franciscan-Knoxville rocks 
 
30 SEBASTOPOL QUADRANGLE [Bull. 162 
 
 just north of Dunham school, the quartz being crushed and sold for 
 chicken grit. 
 
 Water 
 
 Problems of water supply vary widely in the Sebastopol quadrangle. 
 The rapid growth in population in recent years has developed water 
 supply problems in areas where water at shallow depth has previously 
 been plentiful. 
 
 The porous sands and gravels of the valley alluvium in Santa Rosa 
 Valley and of the Merced formation in the eastern valley border section 
 are excellent aquifers and, in general, adequate water supph^ may be 
 obtained at reasonable depths in these areas. In most places a supply of 
 water adequate for domestic use may be secured from wells between 25 
 and 50 feet deep. Larger supplies, as for small ranch use, may be obtained 
 between 100 and 200 feet. For irrigation purposes, for 5 or more acres, 
 wells from 400 to 600 feet deep are necessary in most places. Near the 
 mouth of Mark West Creek, a hop grower uses a 1300 foot well (in the 
 Merced formation) to supply irrigation water. In the valley border sec- 
 tion much of the water from the deeper wells is unfit for drinking and, 
 in many places, two wells are maintained. Areas where Franciscan- 
 Knoxville rocks are exposed present the most difficult problems of water 
 supply. Natural springs and shallow wells in valley alluvium are de- 
 pended on for water in these areas, but many are barely adequate for 
 domestic use. A thick mantle of soil and rotten rock serves as an aquifer 
 to some extent, but it also supports a lush vegetation which is a powerful 
 drain on the water supply. Fault and shear zones, fractures, and joint 
 systems control ground water to some extent, but are so erratic that it is 
 not safe to project them very far with the hope of tapping a reservoir 
 at depth. The best sources of water in this area are developed springs and 
 wells drilled after a detailed examination of the locality for spring 
 patterns or shear zones. 
 
 The plateau section is also one in which water supply is a problem for 
 most residents. The plateau is covered by the Merced formation, but the 
 formation does not contain the abundant supplies of water found else- 
 where. Although water is more easily obtained here than in areas of 
 Franciscan-Knoxville rocks, the need for larger amounts for dairies, 
 ranches, and cultivation emphasizes the limited quantities. Most wells for 
 domestic use in this area are sunk in valley alluvium and are about 25 
 feet deep. Springs are not uncommon due to the alternation of pervious 
 with impervious, nearly horizontal beds, and many residents develop 
 these, especially for stock use. Other wells for domestic use are commonly 
 200 or 300 feet deep. East of Valley Ford, a copious supply of water may 
 be obtained between 300 and 400 feet (Merced formation), according to 
 Mr. Clarence Woodbury, a driller who has drilled many wells in this area. 
 At shallower depth a supply barely sufficient for domestic use is all that 
 is obtained. Northeast of Two Rock, the Merced formation includes small 
 lenses of gravel and coarse sand which are excellent aquifers, especially 
 where they rest on basalt flows or Franciscan-Knoxville rocks. 
 
 Some residents west of Dunham school have tried horizontal drilling 
 into hillsides near springs. Only two small borings had been made up to 
 the time of this writing, but satisfactory results were reported and plans 
 were being made for more extensive drilling. 
 
1952] BIBLIOGRAPHY 31 
 
 BIBLIOGRAPHY 
 
 Bailey, E. H., Quicksilver deposits of the western Mayacmas district, Sonoma County, 
 California: California Jour. Mines and Geology, vol. 42, pp. 199-230, 1946. 
 
 Briggs, L. I., Geology of the Ortigalita Peak quadrangle, California : Univ. California, 
 
 unpublished thesis, 1950. 
 Clark, B. L., Stratigraphy and faunal horizons of the Coast Ranges of California : 
 
 Private publication, 1929. 
 Clark, B. L., Notes on California Tertiary correlation : California Div. Mines Bull. 
 
 lis, pp. 187-191, 1943. 
 Crittenden, M. D., Jr., Geology of the San Jo.se-Mount Hamilton area, California :- 
 
 California Div. Mines Bull. 157, 1951. 
 
 Davis, E. F., The radiolariau cherts of the Franciscan group : Univ. California, Dept. 
 Geol. Sei. Bull., vol. 11, pp. 235-432, 1918. 
 
 Dewey, H., and Flett, J. S., Some British pillow-lavas and the rocks associated with 
 them : Geol. Mag., vol. 8, pp. 202-209, 241-248, 1911. 
 
 Dickerson, R. E., Tertiary and Quaternary history of the Petaluma, Point Reyes and 
 Santa Rosa quadrangles : California Acad. Sci. Proc, 4th ser., vol. XI, pp. 527-601, 
 1922. 
 
 Dow, D. H., and Thayer, T. P.. Chromite deposits of the northern Coast Ranges of 
 California : California Div. Mines Bull. 134, pt. 2, chap. 1, pp. 1-38, 1946. 
 
 Gabb, W. M., Paleontology of California : California Geol. Survey, vol. 2, 1869. 
 
 Gealey, AY. K., Geologv of the Healdsburg quadrangle, California : California Div. 
 Mines Bull. 161, 1951. 
 
 George, W. O., The relationship of the physical properties of natural glasses to their 
 chemical composition : Jour. Geol., vol. 32, p. 353. 
 
 Gilluly, James, Keratophyres of eastern Oregon and the spilite problem : Am. Jour. 
 
 Sci., 5th ser., vol. 29, pp. 225-252, 336-352, 1935. 
 Grant, U. S. TV, and Gale, H. R., Catalogue of the marine Pliocene and Pleistocene 
 
 mollusca of California : San Diego Soc. Nat. History, Mem. 1, p. 1036, 1931. 
 
 Grant, U. S. IV, and Hertlein, L. G., Pliocene correlation chart : California Div. Mines 
 Bull. 118, pp. 201-202, 1943. 
 
 Greenly, Edward, Geology of Anglesey : Geol. Surv. Gr. Britain, Mem., 1919. 
 
 Higgins, C. G., Jr., The lower Russian River, California : Univ. California, unpublished 
 thesis, 19.50. 
 
 Hinde, G. J., Note on the radiolarian chert from Angel Island : Univ. California, Dept. 
 Geol. Sci. Bull., vol. 1, pp. 236-240, 1894. 
 
 Holway, R. S., Eclogites in California : Jour. Geol., vol. 12, pp. 344-358, 1904. 
 
 Holway, R. S., The Russian River, a characteristic stream of the California coast : 
 Univ. California Publ., Dept. Geog., vol. 1, 1913. 
 
 Holway, R. S., Physiographically unfinished entrances to San Francisco Bay : Univ. 
 
 California Publ.," Dept. Geog., vol. 1, 1914. 
 Honke, M. T., Jr., aad Ver Planck, W. E., Jr., Mines and mineral resources of Sonoma 
 
 County, California : California Jour. Mines and Geol., vol. 46, no. 1, pp. 83-141, 1950. 
 Hutton, C. O., and Turner, F. J., Metamorphic zones in north-west Otago : Roy. Soc. 
 
 New Zealand Trans., vol. 65, pt. 4, pp. 504-406, 1936. 
 Johnson, F. A., The Merced, Pliocene formation : Univ. California, unpub. thesis, 1934. 
 Johnson, F. A., Petaluma region : California Div. Mines Bull. 118, pp. 622-627, 1943. 
 Joplin, G. A., An interesting occurrence of lawsonite in glaucophane-bearing rocks 
 
 from New Caledonia : Mineral. Mag., vol. 24, pp. 534-537, 1937. 
 
 Knopf, Adolph, An alteration of Coast Range serpentine : Univ. California, Dept, 
 
 Geol. Sci. Bull., vol. 4, pp. 425-430, 1906. 
 Kruckeberg, Arthur, The flora of serpentine : Univ. California, unpublished thesis, 
 
 1950. 
 Kuenen, P. H., and Migliorini, C. I., Turbidity currents as a cause of graded bedding: 
 
 Jour. Geol., vol. 58, pp. 91-127, 1950. 
 Lawson, A. C, Geomorphogenv of the coast of northern California : Univ. California, 
 
 Dept. Geol. Sci. Bull., vol. 1, pp. 241-271, 1894. 
 Lawson, A. C, Sketch of the geologv of the San Francisco Peninsula : U. S. Geol. Surv. 
 
 15th Ann. Rept., 1893-1894, pp. 399-476, 1895. 
 
A ^ 
 
diituC iiiiajouy-iiiMiijuaj^ 
 
 »i\uK>]oa Duiows dnojB uomijuimj 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 5 S 
 
 p- 
 
 1 
 
 u 
 
 5 
 
 i 
 
 1 
 
 o 
 
 5 
 
 b 
 1 
 
 Ij 
 
 1 
 
 ■s'"' 
 
 1- 
 
 5 
 
 'i 
 
 i i 
 
 i e 
 
 1! 
 
 1 
 
 
 
 |i 
 
 s 1 
 
 ^ 
 
 il 
 |t 
 
 a = 
 
 -S 
 
 it 
 
 '-^1 
 
 ¥ Il 
 
 if! 
 
 i I5 
 
 p^ 
 
 i 
 
 Hi 
 
 y 
 
 Is 
 
 "1- 
 
 1 
 
 1 
 
 1! 
 
 il 
 
 
 
 II 
 1 
 
 
 Is 
 
 = 
 
 1 : 
 
 1" 
 
 a — 1 ^ 
 
 1 
 
 1 
 
 I Id \ 
 
 1 
 
 i "5 
 
 €i 1 
 
 i| 
 
 5|« 
 
 
 
 si 
 
 .'if . 
 
 S - 
 
 -11 - 
 
 
 
 
 
 
 tt 
 
 ? 
 
 t 
 
 
 
 
 
 
 ?-- 
 
 
 
 ? I ^ 
 
 S.. <'52 
 
 >< Jc X HJ>- 11" 
 
 «) "*<;) Jtttati 
 
 il 
 
 ? I 
 
DIVISION OF MINES 
 OLAF P. JENKINS, CHIEF 
 
 STATE OF CALIFORNIA 
 DEPARTMENT OF NATURAL RESOURCES 
 
 BULLETIN 162 
 PLATE 3 
 
 ./ 
 
 ■J" SANTA ROSA 
 Qai ^^'-'-EY 
 
 Trtl?: 
 
 A 
 
 SEA LEVEL - 
 
 SANTA ROSA VALLEY 
 
 Ool 
 
 SEA LEVEL- 
 
 ■ SEA LEVEL 
 
 B' 
 
 
 C 
 
 SCALE 
 
 2 3 
 
 :^ 
 
 Tsu 
 
 Peloluma formarion 
 Franciscon-Knoxville group 
 
 I breccio 
 Sonoma volconics 
 
 t ■• Jsp+ * Serpentine 
 
 UliKARY 
 
 UNIVERSITY OF CALIFORNIA 
 
 DAVIS 
 
 GEOLOGIC SECTIONS OF THE SEBASTOPOL QUADRANGLE 
 
 BY RUSSELL B. TRAVIS 
 
i 90! ill 
 
 dlUUH i((liJOU_y-UIU«IJUlUJ 
 
 I 
 
 =1 . Il 
 
 i i 
 
 = 3 it 
 
 iiti 
 
 it 8 
 
 ^1=^^. 
 
 53 5^ ^ i z 
 
 si I 
 
 iuiv)M nuiticH,' dnaiC ua/rtjuiuj 
 
 I 
 
 * 
 
 
 
 
 
 
 
 
 
 , 
 
 
 ? 
 
 
 
 
 3. 
 
 
 
 1 
 
 
 m 
 
 
 
 S 
 
 
 «i 
 
 
 
 fe 
 
 
 § 
 
 If 
 
 si 
 
 r 
 
 If |i-Ul . 
 
 I 4-=l ^ii 1 : 
 
 
 
 
 
 i 
 
 jKiaa 
 
 IKUHJ 
 
 "^fPPfff 
 
 
 
 *ii»o;w 
 
 
 ionmr-itdds} 
 
 
 Aidviliiai 
 
 
 aisswunr 
 
 UiJxt^ «*W 
 
 11 
 
 ii 
 
> 'X 
 
Ill llmiTl^rnf^^.?.:,M^ORNIA 
 
 3 1175 niRooWil"-' 
 
 DAVIS 
 
 U" 
 
 , K' 'Z'^ 
 
 QEOLOGt 
 
 r^ 
 
 .^ 
 
 
 tCf 
 
 r 
 
 COLLATE : 
 
 PIECES