Stratigraphy and Structure of Part of the Western Sierra Nevada Metamorphic Belt, California
GEOLOGICAL SURVEY PROFESSIONAL PAPER 410
Prepared in cooperation with the State of California, Department of Conservation, Division of Mines and Geology Stratigraphy and Structure of Part of the Western Sierra Nevada Metamorphic Belt, California
By LORIN D. CLARK
GEOLOGICAL SURVEY PROFESSIONAL PAPER 410
Prepared in cooperation with the State of California, Department of Conservation, Division of Mines and Geology
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 UNITED STATES DEPARTMENT OF THE INTERIOR
STEWART L. UDALL, Secretary
GEOLOGICAL SURVEY Thomas B. Nolan, Director
The U.S. Geological Survey Library has cataloged this publication as follows: Clark, Lorin Delbert, 1918- Stratigraphy ·and structure of part of the western Sierra Nevada metamorp4ic belt, California. Washington, U.S. Govt. Print. Off., 1964. iv, 70 p. illus., maps (7 col.) diagrs. (1 col.) tables. 30 em. (U.S. Geological Survey. Professional paper 410). Part of illustrative matter fold. in pocket. Prepared in cooperation with the State of California Dept. of Natural Resources, Division of Mines and Geology. Bibliography : p. 58-59. (Continued on next card)
Clark, Lorin Delbert, 1918- Stratigraphy and structure of part of the western Sierra Nevada metamor phic belt, California. 1964. (Card2) 1. Geology, Stratigraphic. 2. Geology-California-Sierra Nevada Mountains. I. California. Division of Mines and Geology. II. Title. III. Title: Sierra Nevada metamorphic belt, California. (Series)
For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS
Page Page Abstract------1 Stratigraphy-Continued Introduction ______2 Rocks of Jurassic age-Continued Location, culture, and accessibility ______2 Bear Mountains fault zones-Continued Physical features ______3 Copper Hill volcanics______30 Previous work ______3 Lithology______30 Purpose and scope of investigation ______4 Age______31 Field work and acknowledgments ______5 Peaslee Creek volcanics _____ - ____ -- ____ -- 31 TerminologY------5 Lithology ______- ______--- 31 Synopsis of geology ______6 Age______31 Stages of the Middle and Late Jurassic ______7 Stratigraphic relations between the forma- Stratigraphy ______8 tions of the western block______31 Calaveras formation ______8 Stratigraphic relations between the Salt Distribution and character ______8 Spring slate and Gopher Ridge vol- Eastern belt ______8 canics______31 Clastic member ______9 Stratigraphic relations between the Salt Volcanic member ______9 Spring slate and Copper Hill vol- Argillaceous member ______9 canics______33 Chert member ______.,. __ _ 10 Stratigraphic relations of the Merced Western belt______12 Falls slate to the Peaslee Creek vol- Cosumnes River exposures ______12 canics and Gopher Ridge volcanics__ 33 Mokelumne River and Jackson Creek Rock and mineral fragments in rocks of Jurassic exposures------13 age______33 Fossils and age ______13 Composition and shape of fragments______35 Rocks of Jurassic age ______:______15 Origin of limestone blocks ___ -______37 Rocks exposed in the central block and Melones Source of fragments ______-- 38 fau~ zone_~------16 Bedding features and conditions of deposition__ 39 Rocks of Middle(?) Jurassic age ______16 Green schist ______- 41 Amador group ______.,. ____ _ 17 Plutonic rocks______41 Cosumnes formation ______17 Ultramafic rocks______42 Lithology______- ______- ___ - 17 Age______42 Fossils and age ______18 Structural relations______42 Logtown Ridge formation ______18 Gabbroic to granitic rocks ______- 42 Lithology ______.___ _ 19 Age______43 Fossils and age ______21 Hypabyssal rocks______43 Pefion Blanco volcanics ______22 Structural geology ______- 44 L~hology ______23 Top determinations______45 Age ______23 Faults------46 Mariposa formation ______23 Melones fault zone______47 . Lithology ______24 Description of localities______49 Fossils and age ______25 Bear Mountains fault zone ______- 50 Strata of uncertain stratigraphic position ______26 Direction and amount of movement______51 Volcanic strata ______26 Mother Lode fissure system______51 Epiclastic strata ______26 Minor faults______52 Rocks exposed in the western block and Bear Age of faulting______53 Mountains fault zone ______27 Structure of the eastern block______53 Gopher Ridge volcanics ______27 Structure of the central block______54 Lithology ______28 Structure of the western block______54 Age ______29 Small-scale structures______55 Salt Spring slate______29 Cleavage and schistosity______55 Lithology ______29 Minor folds_.______56 Fossils and age ______29 Lineations______56 Merced Falls slate ______30 References cited______58 Lithology ______.______30 Description of map unitA______59 Age ______30 Index______69
•. Ill IV STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT ILLUSTRATIONS [Plates 1-llin map case]
PLATE 1. Geologic map of the western Sierra Nevada between Page the Merced and Cosumnes Rivers, Calif. FIGURE 6. Photograph showing scattered large clasts in 2. Expanded block diagram showing geology of the bedded pyroclastic groundmass______21 western Sierra Nevada between the Merced and 7. Photograph of thin-bedded to laminated tuff_ 21 Cosumnes Rivers, Calif. 8. Photograph of thin-bedded tuff showing intra 3-8. Geologic maps and composite cross sections along: formational breccia bed below dime and 3. Merced River. small-scale crossbedding under breccia____ 22 4. Tuolumne River. 9. Photograph of clastic sills of medium-grained 5. Stanislaus River. tuff______22 6. Calaveras River. 10. Geologic map and cross section showing struc 7. Mokelumne River. ture of slate on Tuolumne River east of La 8. Cosumnes River. Grange, Stanislaus County, California____ 34 9. Stratigraphic sections within the central fault block. 11. Photomicrograph of very coarse graywacke 10. Geologic map showing type section of Amador group. from Cosumnes formation______35 11. Geologic map and cross section of epiclastic rocks 12. Photomicrograph of very coarse graywacke east of Merced Falls, Merced County, Calif. from Mariposa formation______36 13. Photomicrograph of graded tuff bed______45 14. Photomicrograph showing relation of clea v Page age to graded bed of fine graywacke in Salt FIGURE 1. Index map showing location of area studied_ 2 Spring slate______46 2. Canyon of Stanislaus River______3 15. Photograph of small asymmetric anticline 3. Block diagram showing intertonguing______16 showing axial plane cleavage______47 4. Photomicrograph of mudflow conglomerate_ 16. Geologic map and cross section showing struc- 5. Photograph of volcanic breccia of probable 18 ture of bedded pyroclastic rocks west of mudflow origin______20 Don Pedro Dam______48
TABLES
Page Page TABLES 1-6. Description of map units: TABLES 1-6. Description of map units-Continued 1. Merced River ______59 5. Mokelumne River ______64 2. Tuolumne River ______61 · 6. Jackson Creek ______66 3. Stanislaus River ______62 7. Cosumnes River ______66 4. Calaveras River ______64 STRATIGRAPHY AND STRUCTURE OF PART OF THE WESTERN SIERRA NEVADA METAMORPHIC BELT, CALIFORNIA
By LORIN D. CLARK
ABSTRACT Formations exposed in the central block are conformable and include the Cosumnes formation, of probable Late Jurassic The area described in this report lies on the lower western age, ·consisting largely of epiclastic rocks ; the Logtown Ridge slopes of the Sierra Nevada due east of San Francisco Bay and formation, ranging from Callovian to late Oxfordian or early northwest of Yosemite National Parle It is about 90 miles long Kimmeridgian in age, and consisting of volcanic, chiefly pyro and 30 miles wide and is tmderlain by steeply dipping meta clastic rocks ; the Penon Blanco volcanics, of Late Jurassic age, morphic rocks of Paleozoic and Mesozoic age lying between the consisting of lava and pyroclastic rocks; and the Mariposa for Sierra Nevada batholith on the east and overlapping Tertiary mation, of late Oxfordian or early Kimmeridgian age or both, deposits of the Central Valley of California on the west. The consisting chiefly of epiclastic rocks. Conglomerate is Yery historically famous Mother Lode gold belt extends northwest abundant in the Cosumnes formation and part of the Mariposa ward through the center of the area. formation and contains many pebbles and boulders of limestone Distribution of the different units of metamorphic rocks is derived from the Calaveras formation. Coarse, poorly sorted profoundly influenced by two nearly vertical major fault zones conglomerates, previously thought to be tillite, are probably of that divide the region into three structural blocks. The orienta mudflow origin. The section is everywhere truncated by faults tion and amount of net slip along the fault zones have not been or erosion, but the total thickness of the Jurassic strata is in definitely established but the horizontal component is probably excess of 15,000 feet. much greater than the vertical component; the apparent vertical In the western f&ult block, a lower formation of volcanic component of movement amounts to many thousands of feet. rocks, a middle formation of epielastic rocks, an,d an upper Rocl{S of Paleozoic age constitute the eastern fault block, formation of volcanic rocks can be distinguished. Names here which lies east of the Mother. Lode. All the Paleozoic rocks proposed for these formations are, respectively, the Gopher have previously been assigned to the Calaveras formation, and Ridge volcanics, the Salt Spring slate, and the COpper Hill this usage is retained. The upper part of the Calaveras forma volcanics. These units form a conformable and, in part, inter tion is of Permian age, but the formation is perhaps tens of tonguing, sequence. 1Fossils have been described from only one thousands of feet thick, and the age of lower parts is unknown. locality; these are from the Salt Spring slate and indicate late Five lithologic members have been distinguished within the Oxfordian or early Kimmeridgian age. The Salt Spring slate Calaveras formation, but no revision of the nomenclature is is therefore about equivalent to the Mariposa formation, and proposed. The Calaveras formation consists largely of orig the Gopher Ridge volcanics are presumably about equivalent to inally argillaceous rocks and chert and is notable for a paucity the Logtown Ridge formation. ,The youngest unit, or Oopper of detrital material as coarse as sand, except in the lowermost Hill volcanics, may be equivalent to the upper exposed part part. Volcanic rocks form lenses in most of the formation and of the Mariposa formation but may be younger. Two other thiclt, extensive sequences in some places. Limestone also forms formations, here named the Peaslee Creek volcanics and the widespread lenses, but is prominent only in the east-central Merced Falls slate, of probable Late Jurassic age, have been part of the area. distinguished in part of the block. . Mesozoic rocks constitute the western fault block and most 'The metamorphic rocks are on the west limb of a faulted of the central block. Paleontologically dated Mesozoic rocks synclinorium, the axial part of which is occupied by the Sierra are of Late Jurassic age. They are divisible in a general way Nevada batholith. Large folds having essentially horizontal lnto sequences of epiclastic rocks, largely slate, graywacke and axes were formed during development of the synclinorium but conglomerate, and volcanic rocks, although epiclastic and vol wide homoclinal belts also occur. Axial planes of the folds dip canic rocks are commonly iliterbedded and in some places inter steeply eastward or are vertical, andl most beds dip east tongue. The graywacke and conglomerate of all the formations ward more steeply than 60°. !The folds and homoclinal sec are similar in composition. The most abundant clastic frag tions are truncated by the two major fault zones, which strike ments are of volcanic rocks, slate, and chert, but fragments of nearly parallel to bedding and are parts of a fault system that metamorphic rocks, plutonic rocks, and quartzite are wide extends throughout and beyond the western Sierra Nevada spread. The various volcanic formations also have many fea metamorphic belt. ~e fault zones are characterized by map tures in common; most are composed largely of tuff and volcanic pable belts of cataclastically deformed and in part recrystallized breccia, but lavas, in part having pillow structure, form thick rocks and by thin elongate pods of ·serpentine. ·The general sequences locnlly. 1!-.elsic volcanic rocl.:s nre common in one of sequence of strata within each fault block is from older on the the volcanic formations, but lacldng in the others. Type local west to younger on the east. This sequence was reversed by ities of previously named formations are within the central movements on the major fault zones, so that the youngest ex fault block, and becanse of the difficulty of correlating between posed strata are in the western fault 'block and the oldest in the two fnult blocks new nnmes are proposed for formations of the eastern fault block. Pervasive shearing that accompanied the western fault block. the faulting resulted in the development of steeply plunging 1 2 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT minor folds and b-lineations, particularly in the fault zones and 1). It extends from a short distance south of the the eastern fault block. Merced River to a short distance north of the Cosumnes Whether extensive deformation accompanied emplacement of the Sierra Nevada batholith during middle Cretaceous time has River, or from north latitude 37°30' to about north been neither established nor disproved. Most of the deformation latitude 38°35'. The area is bisected by the historically apparently took place during part of Late Jurassic time as famous Mother Lode gold mining belt and the towns major folds involving upper Jurassic strata are truncated by and villages of the region were established as centers the major faults, some of which are in turn cut by a Late of mining activity during the 19th Century. The Jurassic pluton. largest town is Sonora, with 2,500 people. The popu INTRODUCTION lation of the region has decreased markedly since the
LOCATION; CULTURE, AND ACCESSIBILITY days of active mining. The road and highway net work is still much the same as shown on topographic The area covered by this report lies on the lower maps published in 1900 and before, but grades and sur western slope of the Sierra Nevada in California (fig. faces of the highways have been modernized and most
N
INlJEX MAP OF CAI.IFORNIA
Sacramento @
EXPLANATION D Volcanic and unconsolidated rocks ~ Metamorphic rocks Includes some rocks of very low metamorphic grade in coast ranges
Granitic rocks 50 0 50 100 MILES .
FIGURE 1.--Inde::x: map showing location ot area studied. INTRODUCTION 3
FIGURE 2.-Canyon of the Stanislaus River about two miles northeast of Parrotta Ferry bridge, looking northeast. The canyon is cut into a rolling upland surface underlain by the Calaveras formation of Paleozoic age. The cliffs are formed in limestone. Flat-topped ridge in left background is part of Table Mountain, which is composed of river-channel de posits of Tertiary age. These deposits consist of alluvial gravels and rhyolite tuffs capped by latite lava.
of the secondary roads also have hard surfaces. Houses ered. It is buried in some places by younger deposits, are sparse between towns. which, in the western part of the area, project as buttes At the lower altitudes near the western edge of the and cuestas. The upland surface is dissected by the area are extensive treeless semiarid grasslands. With rivers and their larger tributaries, which have cut deep, increasing altitude, evergreen and deciduous oaks oc youthful canyons. In many of these the average slope cur, and near the Mother Lode belt open oak forests of the walls from bottom to top is more than 40°, and are interspersed with grasslands dotted by scattered the bottoms are little wider than the stream itself at trees and by dense thickets of manzanita and chamise intermediate water levels. The Calaveras River is an covering areas of poor, thin soil. At altitudes of about exception, as it flows in a broader valley inherited from 2,000 feet, oaks give way to a conifer forest in which an earlier erosion cycle, except for a few miles -above pines predominate. Little of the land is tilled, and the point where it leaves the bedrock. Part of the up most of the nontimbered land is used for grazing. land surface and canyon of the Stanislaus River are Precipitation is directly proportional to elevation, so shown in figure 2. The western edge of the area stands that the annual rainfall increases from about 20 inches about 300 feet above sea level, but near the eastern edge at the west edge of the area, to 30 inches or more on the upland surface stands 5,000 to 6,000 feet above sea the Mother Lode, and to 40 inches farther east. It oc level. curs mostly during the fall, winter, and spring months In the western part of the area long ridges carved rain is rare betwe~n about the first of ,June and from resistant parts of the bedrock extend parallel to ' the middle of September. Snow seldom whitens the the range front and project as much as 1,000 feet above ground west of the Mother Lode. the surrounding country. These are probably monad nocks on the upland surface (Eric, Stromquist, and PHYSICAL FEATURES Swinney, 1955, p. 5). The western slope of the Sierra Nevada in this region PREVIOUS WORK is for the most part a gently rolling upland with occa sional scattered higher hills. This surface largely was Numerous publications on the geology of the western developed prior to Eocene time, and is d~ply weath- part of the Sierra Nevada have appeared, beginning 4 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT
shortly 'after the 'Gold Rush of 1849-50 ; the earlier of through standard mapping procedures would be very these were listed and summarized by J.P. Smith (1894). costly in time and manpower. At the same time the In the present account only those which bear on the feasibility of a different approach to the regional subject of this investigation will be mentioned, but some geology was demonstrated. others will be referred to later in the report. The first comprehensive investigations were those of PURPOSE AND SCOPE OF INVESTIGATION H. W. Turner, Waldemar Lindgren, and F. L. Ransome. The investigation described in this report is designed The results were published between 1893 and 1900, in to supplement previous geologic mapping with new a series of geologic folios covering most of the western data for interpreting major features of the regional Sierra Nevada, and in separate papers dealing with structure and stratigraphy of the ·metamorphic rocks specific areas or problems. The geologic maps of the ( pls. 1 and 2). Plutonic rocks were not studied nor folio series are shown by later work to have distin were the nearly flat lying Tertiary strata that locally guished the various lithologic units with much accuracy. cover the ancient surface. Field work consisted of de Besides the positions of intrusive rocks, these maps show tailed study of the geology along the major streams, that the western part of the area is underlain by north which in general cross the regional structure at about west-striking alternating belts of volcanic rocks ( dia right angles. The folio maps indicate the continuity base and porphyrite of the folio maps) and sedimentary of formations between the streams. This approach is rocks, and that the eastern part of the area is underlain reasonable not only because of the existing map cover largely by sedimentary rocks, among which only lime age, but because of the physiographic and geologic stone is differentiated. Structural interpretations in features of the region. the folios are diagrammatic, and bedded volcanic rocks Exposures, particularly of sedimentary rocks, are are shown as massive igneous bodies, without strati generally poor on the upland surface, and contacts~ graphic significance. tween sedimentary rocks in the valleys and volcanic Adolph Knopf (1929), in studying many of the gold rocks in the ridges are commonly concealed beneath mines of the Mother Lode region, found that the quartz extensive surface debris. In contrast, the major stream veins with which the gold ore bodies are associated are canyons provide exposures that are continuous for con in reverse faults. He also recognized that many of the· siderable distances. volcanic rocks were bedded and that tuff was intimately Fossils are so sparsely and unevenly distributed in interbedded with some of the sedimentary sequences. the region that stratigraphic sequences must be wor~ed The results of N. L. Taliaferro's extensive investiga out by systematic observation of structures showing tions in the region are presented in a series of abstracts, the direction of tops of beds; such structures are a bun papers, and a quadrangle geologic rna p published be dant in the region. They are readily observed on tween 1942 and 1949. Taliaferro (1942, p. 96) treated stream-polished surfaces but in many places where ex the ·volcanic units as formations and has presented a posures are weathered and cove~ed with lichen~ t~ey concise su~ary of the sequence of major geologic can be discovered only after tedwus search. Similar events in the western Sierra Nevada. He has also sum field methods have been used for many years in areas marized the regional aspects of the structure and stra of Precambrian rocks, particularly in the Lake Superior tigraphy (Taliaferro, 1942, p. 77-81, 89-105; 1943b, p. region, but have been little used in California. 280-286, 302-323), but his interpretations are sup In the field, the metamorphic rocks were mapped ported by little specific locality data. During World and described in terms of the numbered map units War II, a number of copper mines in the region were shown on plates 3 through 8, and the basic rock descrip mapped by G. R. Heyl, M. W. Cox, and J. H. Eric tions in tables 1 through 7 (p. 59 to 66) are independent (in Jenkins, ed., 1948). of structural and stratigraphic interpretations. Dur About 1946, the U.S. Geological Survey began a pro ing compilation each unit was assigned a number in gram of geologic mapping for the purpose of gathering sequence from west to east and the units w.ere grouped the field data necessary to develop an understanding into formations. The rock descriptions do not repre of the regional structure and stratigraphy. Results of sent columnar sections, for each outcrop belt resulting this program appeared in the geologic rna ps and de.:. from repetition of a unit by a fold or fault is described scriptions of the Sonora and Angels Camp (Eric, separately. Stromquist, and Swinney, 1955) and Calaveritas This investigation is limited to the metamorphic
(Clark, 1954) 7lj2 minute quadrangles. Although rocks of the region-rocks of Tertiary age and the plu these maps contributed to the desired end, it became tonic rocks are not considered. This is a preliminary apparent that to approach the larger scale problems report on a continuing study, and some of the interpre- INTRODUCTION 5 tat ions may be changed as a result of further work. At them is the origin and manner of comminution of the present, only some of the sedimentary rocks have been fragments. In recognition of this, shale, sandstone, studied petrographically, and these not fully. Petrog conglomerate, and the like will be designated "epi raphy of the volcanic rocks probably has less bearing clastic" rocks to indicate that the component debris was
on the stratigraphy and has not yet been studied.. formed by weathering of pre-existing rocks. I) The tenn "graywacke" designates poorly sorted sand FIELD WORK AND ACKNOWLEDGMENTS stone with a matrix of silt-size and originally clay-size Field work covered a total period of 51h· months dur material (Pettijolm, 1954; 1957, p. 290-292). Sand ing 1953 and 1954. Field mapping was done on aerial grains are mostly chert or other lithic fragments, but photographs and topographic maps at a scale of various amounts are mineral grains, most commonly quartz. Rocks in which the sand-size fraction consists 1!24~000. The data were compiled at 1/48,000 for pub hcatiOn at 1/62,500 scale. In addition, short sections entirely of volcanic rock and mineral detritus are called have been mapped by plane table at a scale of one inch tuff, but those containing even widely· scattered grains to 500 feet. The investigation was performed in co of non volcanic material are _called graywackes. No opei·ation with the California Division of Mines and fundamentally important aspect of the interpretation Geology. In compiling the map and cross section along of geologic history of the region is masked by this usage, the ·Calaveras River free use was made of an unpub as sequences of definite volcanic material alternate with lished map and report on the San Andreas northwest sequences of definite epiclastic material on both the quadrangle prepared by A. A. Stromquist and C. M. regional scale and the scale of a single fonnation. The Swinney. A detailed map of the type section of the term "tuffaceous sandstone" is not appropriate unless Amador group in the Cosumnes River area prepared the rock contains both grains fonned by epiclastic proc by G. R. Heyl, J. H. Eric and M. W. Cox is incorporated esses and first-c.ycle grains fonned by pyroclastic in this report. Richard Pack assisted in the field work processes. during the early part of the 1953 field season, and E. H. In the absence of a purely descriptive tenn, the name "chert" is applied to microcrystalline, granoblastic rocks ~a?1:peya1: later i~ that season. C. D. Rinehart par tiCipated In mappmg the shores of the reser:voirs in the composed of quartz. In this area, these rocks have been referred to by most previous geologists as either phtan area .. ~he East Bay Municipal Water District granted permiSSIOn to enter the closed area around the Pardee ite or quartzite. Phtanite has fallen into disuse in this Reservoir. country, ~nd although quartzite is often used as a de scriptive term, it is more properly applicable to a rock TERMINOLOGY fonned from quartz sand. Most of the chert is seri citic, and rock containing so much sericite so that it can . Discussion of some of the tenninology used in this be scratched with a hammer point is here referred to as report is necessary for clarity. All the sedimentary and quartzose slate. vo~c~nic rocks in the region are metamorphosed, but "L'1mes t one " IS . use"d 1oose 1y to Inclu . d e carbonate rock original structures and textures are commonly well pre ranging in composition from limestone to dolomitic served, -and the emphasis of this study is on their struc limestone and possibly to dolomite, and in texture from ture and stratigraphy. For this reason the rocks are aphanitic to coarsely crystalline. generally designated by such names as chert and tuff Three tenns-cleavage, schistosity, and slip cleav rather than metachert and metatuff. Lack of consist age-are used to distinguish different kinds of parting ency will be apparent, however, for originally argilla surfaces of metamorphic origin. "Cleavage," without ceous rocks are referred to as slate, phyllite, and schist, a modifier, designates the sort of planar structure char and some volcanic rocks in which original textures are acteristic of slate, the slaty cleavage of ·many· authors. destroyed are distinguished by the tenn "metavolcanic." In this region development of cleavage is widespread. This nomenclature has proved convenient and under It has formed in fine-grained pyroclastic rocks and standable to geologists of my acquaintance. sandstones as well as slate. It commonly crosses bed In this region, original textures and structures of pyroclastic rocks forming much of the volcanic forma ding, fonns a simple regional pattern, and in most t-ions are indistinguishable from those of rocks of similar places is about parallel to the axial planes of folds. grain size composing the "sedimentary" formations. "'S ch' IS t os1ty . " d esignates . a planar ~tructure forined by This similarity indicates that both sandstone and tuff the parallel arrangement of tabular mineral grains and for example, were introduced to the area of deposition' clastic fragments in rocks sufficiently coarse grained to u.n~ were deposited in the same manner-they are both be termed schist. In strongly sheared schist, slip sur- sedimentary rocks. The significant difference between . faces parallel to grains and fragments accentuate the 724-275 0-65-2 6 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT cleavage. "Slip cleavage" consists of closely spaced area have been designated from west to east, the clastic, crinkles and micro-faults (White, 1949) cutting earlier volcanic, argillaceous, and chert members. These terms schistosity and cleavage. Previously formed mica do not adequately describe the various sequences of beds, flakes are. commonly bent to a direction parallel to the but emphasize the distinguishing features of each. micro-faults. The clastic member, exposed between Bagby and the North Fork of the Merced River, consists in its lower SYNOPSIS OF GEOLOGY part of thinly interbedded slate and graywacke. In Rocks of the western Sierra Nevada are divisible into its upper part it consists largely of slate, with inter two groups which have long been known as the bedrock bedded pyroclastic rocks, and a thin-bedded chert unit complex and the superjacent series; the latter, consist several hundred feet thick. The volcanic member con ing of nearly flat-lying alluvial and volcanic deposits, sists mostly of volcanic breccia and tuff, but contain is not considered here. The surficial rocks have been some pillow lava. The argillaceous member is com described by Storms (1894); Piper, Gale, Thomas, and posed largely of black carbonaceous slate and siltstone, Robinson (1939); Clark (1954, p. 9-11) ; and Eric, but contains appreciable amounts of bedded chert. Stromquist and Swinney (1955, p. 16-19). Except in deep road cuts along the Merced River, the The bedrock complex consists of metamorphosed sedi chert is preferentially exposed and for this reason the mentary and volcanic rocks of Paleozoic and Mesozoic argillaceous member is not readily distinguishable from ages, and plutonic rocks of Mesozoic age. The dis the chert member in most places. The chert member tribution of the metamorphic rocks and the interpreta consists dominantly of interbedded black carbonaceous tion of their stratigraphy is profoundly influenced by quartzose slate, phyllite, schist, and chert, but limestone two northwesterly-trending fault zones that divide the lenses are larger in the. lower part of the chert mem metamorphic rocks into three structural blocks (pl. 2). her than elsewhere in the form.ation and north of For convenience these are referred to as the eastern, Sonora limestone forms a prominent part of the sec central, and western blocks. The fault zones are not tion. The chert member underlies a much greater area obscure features drawn to explain anomalous distribu than the other three members combined. tion of formations, but are mappable belts of cata A fifth member of the Calaveras formation is exposed clastically deformed and recrystallized rock ranging in a small part of the central fault block, where it un from a few hundred feet ·to a few miles wide. Most of derlies the Cosumnes formation, of probable Late the metamorphic rocks of Paleozoic age are in the Jurassic age. This member has long been referred to eastern block and all the metamorphic rocks of Meso as the western belt of the Calaveras formation. It in zoic age are in the western and central blocks and cludes lenticular clastic limestone which at several local within the fault zones. ities has yielded fossils of Permian age, and probably The oldest rocks in the region are those of the Cala includes quartzose slate and minor pyroclastic rocks. ve:vas formation, which has been defined to include all Other rocks may be part of the Calaveras formation of the metamorphic rocks of Paleozoic age in the area here, but the central block is structurally complex, and (Turner, 1893a, p. 309; 1893h, p. 425). This usage is it is uncertain whether some of the rocks in this vicinity followed in this report, although five lithologically dis belong to the Calaveras formation or to younger forma tinct members within the formation have been dis tions. The member in the central block is unlike any tinguished. Four of these mem1bers are in the eastern of those exposed in the eastern block. block and one is exposed in a narrow belt in the central The western and central fault blocks are characterized block. All the metamorphic rocks of the eastern struc-· by northwesterly trending alternating belts of epiclastic tural block .are included in the Calaveras formation. and volcanic rocks (pl. 1). Many of these belts are sev The most complete section of the formation is exposed eral miles wide and continuous for great distances, but along the Merced River, where four members have been the stratigraphy is complex. Rocks were correlated by recognized. Two members are separable from one earlier geologists on the basis of lithologic similarity, another near the Merced River, but not elsewhere. although fossils are sparse and have not been found in Strata of the Calaveras formation are apparently some belts. To avoid unjustified correlation and pos progressively younger eastward along the Merced sible oversimplification of the geologic history of the River. Diagnostic fossils have been found at only one region, previously proposed formation names are used locality in the eastern block, near the top of the Merced only in belts which contain their type localities. New River section of the Calaveras formation, and suggest names are proposed for the formations of other belts. a Permian age. For convenience, the four members of Mesozoic stratified rocks in the central block are con the Calaveras formation in the southern part of the formable and consist of the Amador group, comprising SYNOPSIS OF GEOLOGY 7 the Cosumnes and Logtown Ridge formations, the tinuous with the Mariposa formation, at least not in all Pefion Blanco volcanics, the Mariposa formation, and places. epiclastic and volcanic rocks of uncertain stratigraphic Graded bedding, widely and abundantly developed position. No fossils have been found in the Pefion in the tuff and graywacke of Mesozoic age, indicates Blanco volcanics. But fossils indicate that the age of that these beds accumulated in quiet and probably deep the upper part of the Cosumnes formation is within water. Textures and structures indicative of deposition the interval extending from middle Bajocian to late in agitated water are rare or absent. Callovian, that the Logtown Ridge is of Callovian to Plutonic rocks in the region include ultramafic rocks late Oxfordian or early Kimmeridgian age, and the which are mostly altered to serpentine, hornblende gab Mariposa is of late Oxfordian and early Kimmeridgian bro, diorite, and granodiorite. All the plutonic rocks age. are younger than the metamorphic rocks. IDtramafic STAGES OF THE MIDDLE AND LATE JURASSIC rocks form narrow, elongate bodies in major fault zones, and less commonly form somewhat thicker bodies After Arkell ( 1956, p. 7-8) except that th~ Callovian that extend beyond the limits· of the fault zones. The is here included in the Late Jurassic. proximity of ultramafic rocks to fault zones, including Late Jurassic possible sill-like bodies, and the general absence of such Tithonian rocks elsewhere suggests that emplacement of the ultra ICimmeridgian mafic rocks was controlled largely by the fault zones. Oxfordian As cataclastic deformation associated with these fault Callovian zones is much more extensive, both along and across the Middle Jurassic strike, than are the ultramafic rocks, it is not likely that .Bathonian the serpentine controlled the locus of faulting. Bajocian Only a few individual plutons in the region have The Cosumnes formation consists of dark-gray slate, been dated, but most of the granitic rocks appear to abundant conglomerate, graywacke, and tuff. Much of fall into two separate series, those of the western and the conglomerate contains limestone fragments derived central blocks apparently being appreciably older than from the Calaveras formation. The Mariposa forma those of the eastern. tion is similar to the Cosumnes formation except that The easternmost of the two major fault zones is the conglomerate is much less abundant in most places and Melones fault zone and the western one, the Bear the Mariposa contains a thick volcanic member. The Mountains fault zone. The Melones fault zone coin Logtown Ridge formation consists largely of coarse cides throughout much of its length with the Mother porphyritic volcanic breccia, but contains some tuff and .. Lode gold belt. Schistosity within the fault zones sug minor pillow lava. The Pefion Blanco volcanics con gests that they dip eastward, at angles between 65 ° and sist largely of volcanic breccia and tuff, but a thick mass 90°. The two fault .zones form part of the Foothills of lava, some showing pillow structure, forms the lower fault system (Clark, 1960a), which extends from south part of the section. of the area ·studied to the northern limit of exposures Stratified rocks of Mesozoic age in the western block of metamorphic ·rock in. the western Sierra Nevada, a are also conformable. They consist in· most of the area distance of about 200 miles, ·and perhaps farther; of three formations, here named the Gopher Ridge vol Steeply plunging lineation and minor fold axes suggest canics, Salt Spring slate, and Copper Hill volcanics. that the dominant component of movement along the Two other formations, the Peasle~ Creek volcanics and Melones and Bear Mountains fault zones was horizon Merced Falls slate, ar.e distinguished in the southern part of the area. The Salt Spring slate is about equiva tal. Some faults of the system are cut by granitic lent to the Mariposa formation and of late Oxfordi~an plutons of the older series. Previously mapped reverse or early ICimmeridgian age, and the Gopher Ridge vol faults that control the quartz veins of the Mother Lode canics are presumably about equivalent to the Logtown are within the Melones fault· zone in most, but not all, Ridge formation. The Copper Hill volcanics may be of the area. These faults also dip eastward but less of ICimmeridgian age, or may be younger. The only steeply than the Melones fault zone, and in the north area where continuity of these formations across the ern part of the area are exposed as ·much as one mile strike can be observed is in the north where the Salt west of the· Melones fault zone. They are probably Spring slate thins and intertongues eastward with the younger than the Melones fault zone, and may have Gopher Ridge and Copper Hill volcanics. This sug formed concurrently with emplacement of the Sierra gests that the Salt Spring slate was not laterally con- Nevada batholith. 8 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT Although large folds repeat the outQrop of forma Upper Carboniferous rooks of the Taylorsville region tions in places, older beds are generally exposed in the (Turner, 1893a, p. 309; 1893b, p. 425). This broad_ western part and younger beds in the eastern part of usage is followed here. The formation was named for each of the three fault blocks. This arrangement sug beds in the central block about 2 miles east of the crest gests that the region lies on the west side of a large of the Bear Mountains in Calaveras County that in this synclinorium, but is reversed by the Melones and Bear. report are included in the Mariposa formation.
Mountains fault zones, so that the youngest exposed DISTRIBUTION AND CHARACTER metamorphic rocks of the region are in the western Within the report area, most of the Calaveras forma block, the oldest in the eastern block, and rocks of inter tion lies between the Melones fault zone on the west and mediate age in the central block. tne=Sierra Nevada batholith on the east. A much Only general features of the structure of the eastern smaller belt of Calaveras formation in tlie central fault block have been worked out because of the monotonous ~=@Ocl{ extends from south oithe-MOKelumne Rive~~to lithology of a great part of the Calaveras formation north of the Cosumnes River. Other small, iSOlated and because bedding has been destroyed by shearing blocks derived from the Calaveras formation, sur in much of the region. Much of the Calaveras for rounded by younger rocks, occur in major· fault zones mation is schistose. Near the Merced River, where it at widely separated· localities, and in conglomerate of is iess recrystallized, the structure of the eastern block the overlying Cosumnes and Mariposa formations. is apparently homoclinal with tops east. "Where bed The Calaveras formation within the area can be ding is preserved, it nearly everywhere dips steeply. divided into five members. One of these is in the cen Large folds may occur near the Merced River, but none tral block and forms the western belt of the formation ; are large enough to repeat any of the four ·members the other four are in the eastern block: The Calaveras distinguished in the Calaveras formation. In the formation of the western belt consists largely of cherty southern part of the area, the northwest regioJ?.~l trend phyllite and thin-bedded chert, with minor amounts of of belts of rock in the Calaveras formation is truncated calcarenite. The eastern block comprises, from older to at an acute angle by the Melones fault zone. Near San younger, (1) a member__composed of sl~te, graywacke, Andreas, a large easterly plunging complex anticline is chert, and tuff, here designated the clastic member; (2) truncated at a large angle by it. an upper volcanic memoer;T3) ·an argillaceous mem The amount of deformation in the central and west ber, recognized only on t~ River; and (41 a ern blocks is about equal. I'Soclinal folds occur in the ffieffiber composed oftliill-booaedCh==ert,Eiack carbo central block near the middle of the area, but the beds ~aceous slate, and limestone,:Ji:ere desiggat_eut~ are homoclinal with tops east farther north and south. member.. The fourr members of the eastern block are In the north half of the western block are large open exposed along the Merced River. Proposal of formal folds overturned to the west. In the southern part of stratigraphic names for these units is deferred until the western block, folds also occur, but their form and named subdivisions of the Calaveras formation exposed position are obscure because they occur in thick se quences of sheared volcanic rocks with little bedding north of the area covered by this report have been and no graded bedding. Intersections of bedding and studied. cleavage, the plunges of minor fold axes, and the areal EASTERN BELT patterns of outcrop show that the major folds plunge The most complete section of the eastern belt of the at low angles, generally less than 25°. Cleavage occurs Calaveras formation is in the southern part of the area, widely and schistosity locally in rocks of the central where it is divisible into four members. The section and western blocks. Most rocks of the eastern block is apparently conformable, and contacts between mem ate schistose. bers are drawn to group beds· of similar lithology. Most of the deformation within all three fault blocks Graded beddin and_a.t · e~o.f... he..d uggest that suc- preceded the major faulting and probably took pia essively_YQ!!!! er mem _ ~sed=;{~om w.est}o during an orogeny that appreciably antedates intrusioll:l · east along the Merced River.~ Northwest of the Merced of the Sierra Nevada batholith. River the lower members of the Calaveras formation STRATIGRAPHY are successively truncated by the Melones fault zone; the argillaceous and chert members that form the upper CALAVERAS . FORMATION part of the formation are more extensively exposed. The term Calaveras formation was used for all the The northeastern part of. the eastern belt includes un metasedimentary and metavolcanic rocks of Paleozoic differentiated rocks of the Shoo Fly formation of prob age in the Sierra Nevada, except the Silurian and able Silurian age (Clark and others, 1962, p. B15-B18). STRATIGRAPHY 9
CLASTIC MEMBER north. It is about 5,000 feet thick east of the mouth of The clastic member is exposed in the ~ierced River the North Fork, assuming a uniform dip of 80°. between Bagby -and--the mouth of th~-~2~th_-F9rk of In the Merced River, the volcanic member consists tl~d!tivei~:- ··n iSbounded on the west~ almost entirely of coarse volcanic breccia, similar to peiltfnearuf oi1-tlu; east by the volcanic member. It some breccias of Jurassic age in color, grain size, and is trunditea"a.loflg tife Melones fault zoiie a fe,~ miles the presence of pyroxene phenocrysts as much as 1 em north of the Merced River (pl. 1). It is inferred to in diameter. Two layers of black slate, ~'lch about 50 extend south of the Merced River to a point near the feet thick, with interbedded volcanic conglomerate and boundary of the map area where it is truncated by a tuff lie in the breccia near its western side. Fragments granitic pluton. of black, carbonaceous slate are common in the eastern ~grained detrital rocks, chiefly black slate part of the volcanic breccia. n.nd siltstone, make up most of this member, but eyro The volcanic rocks exposed on the North Fork of the cJaSfic rocks and-ooclcted chert constitute important Merced River, less than a mile from the section exposed parts of it. Abunaant thin beds of graywacke are in the main stream, are more varied. Besides volcanic interbedded with the black slate in the western part breccia they include green phyllite, volcanic conglomer of the section. Small lenses of black · ~O- cur ate, and tuff. Differences in the two sections suggest at several hori~~ns an crinoic"!_ p!_~tes .al'e _apullii.&nt in very rapid interlensing of different rocks, or small folds O!le. Poorly sorted conglomerate or breccia, consisting that cannot be demonstrated by the observed bedding of pebbles and boulders of feldspathic graywacke in a attitudes. matrix of sandy argillite, forms units 31 and 33 (pl. 3). Gradually waning volcanisn1 and conformable rela These are probably mudflows, as suggested by the poor tions between the volcanic mmnber and the overlying sorting, argillaceous matrix, and absence of internal argillaceous member are suggested by the presence of bedding. The total thickness of the clastic. member in interbedded epiclastic and pyroclastic rocks in the south the Merced River is more than 10,000 feet. western part of the section included with the argilla The black slate (unit 29, pl. 4) west of the bridge ceous member. In sees. 25 and 26, T. 3 S., R. 17 E., on on State IIighway 49 across the Tuolumne River is the North Fork of the Merced River, pillow lava and arbitrarily included with the clastic member, following layers of volcanic breccia and tuff too thin to map indi Turner (1897), but may be of Mesozoic age. The slate vidually are interbedded with argillaceous slate and silt contains two small limestone masses. stone. On the west side of the Merced River near the
VOLCANIC MEMBER south end of McCabe Flat, the thick porphyritic breccia of the volcanic member is succeeded on the north by The pyroclastic rocks exposed in the Merced River conglomerate less than 20 feet thick consisting of peb east of the mouth of the North Fork of the Merced are bles of volcanic rock in a black slate matrix. This is continuous to the northwest with a belt of metavolcanic succeeded by another volcanic breccia layer less than 100 rocks mapped previously (Turner, 1897) as amphibolite, feet thick and this by black slate containing some chert. green schist, and, locally, as gabbro. The belt extends Another volcanic breccia iayer is exposed at the north northwestward as far as State Highway 108 (pl. 1); end of McCabe Flat. Farther east along the Merced between State Highway 108 and San Andreas meta River is fine tuff and a green phyllite that was probably volcanic rocks that form separated masses are included. a very fine tuff. Although the stratigraphic position of most of those ARGILLACEOUS M:EMBER rocks has not been established, their distribution sug gests that they are near the same stratigraphic level. A member composed dominantly of bedded~ black The 1netavolcanic rocks may be infolded with or inter argillaceous rock which locally contains be.Clde.d_c.he.rt tongue with the surrounding rocks. The mass east of occurs on the Merced River east of the volcanic member. San Andreas forms the core of an anticline and is over It differs from the clastic member to the west, as it con lain by interbedded black carbonaceous schist and chert tains almost no graywacke and less variety of lithologic sin1ilar to that forming the argillaceous and chert mem· types, and it differs from the chert memh&i.n...c.onta.in hers in the Merced River (Clark, 1954, p. 8-9). It also ing much less chert. Nevertheless, tJ:le argillac~()US includes thin, probably interbedded layers of chert and !?~ember can_!lQkhe distil}guis~QJJ:Lth,~g}w~tJ!llmlber black schist. Small masses of volcanic rock north of along _the North_ Fork q;LtJl~ 1\ierced R~'rer, .P~~haps be San Andreas ( pls. 1 and 2) are included, but may be cause_o~ i_f.1Crease in al:>undance o_f chert northw~Jit)YaJ:d. interbedded with the argillaceous or chert member of It may, hmYever, be due to the nature of the exposures, this report. The volcanic member extends southeast as the extensive fresh highway cuts aJong the Merced ward from the Merced River beyond latitude 37°30' River are not comparable to the stream-polished expo- 10 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT
sures examined elsewhere. The resistant chert and stone, ~uartzose slate, and carbonaceous slate quartzose slate may be preferentially exposed in some ~chcould haye been deriyed from underlllllg narts of streams, giving an erroneous impression of the amount the Calaveras formation; no crystalline. meta.m~, of chert. or other rocks foreign to the Calaveras fonnatiou occur. Existence of a distinct member of argillaceous rocks A local source is suggested by contrasting composition is suggested also by the normally weathered rock in of adjacent conglomerate beds; in some the pebbles are road cuts immediately north and south of Briceburg, almost all volcanic rocks, and in others these are rare and about 3 miles northeast of Coulterville, which con or absent. The argillaceous matrix of the conglom sist largely of gray- and brown-weathering slate, at erates, the extremely poor sorting and lack of internal least superficially similar to weathered exposures of the bedding in some layers, and the grading of, associated Mariposa formation. Rocks of this sort do not occur in sandstone beds suggest deposition by mudflows and tur other parts of the Calaveras formation. bidity currents. Very coarse slump breccia composed The total thickness of the argillaceous member is un of sandstone blocks more than 15 feet long in a slate certain, because at several places in the Merced River matrix is exposed along the South Fork of the Merced bedding has been destroyed by shearing for several hun River about 1 mile east of the mouth of Devil Gulch dred feet across the regional strike. The widest un (pl. 1). broken block with apparently simple structure is be CHERT MEMBER tween Briceburg and McCabe Flat. In this interval the The easternmost member of the Calaveras formation beds are more than 5,000 feet thick. This represents in this area is characterized by abundant chert inter only about one-fourth of the width of outcrop of the bedded with black carbonaceous slate and siltstQne. A argillaceous member, and may represent a comparable belt of lenticular limestone masses lies near the western fraction of its total thickness in the Merced River. Vol boundary of the member, and it also includes the large canic rock to the west and the belt of limestone to the mass of limestone northeast of Sonora. Some quartz east converge northward, suggesting that the argilla rich siltstone resembling the chert also occurs.· Sand ceous member thins to the north. stone beds exposed near El Portal on the Merced River Rocks of the argillaceous member are chieflJr slate and along the South Fork of the Merced are here· in and siltstone. the latter partly massiye~ ~thin cluded with the chert member ·although their strati .bedded. Seams of thin-bedded chert are present lo graphic relations are uncertain. Lenses of volcanic ably derived from rhyolite; lies within a thick unit of abundant and are largest in an area extending about limestone in the NEl)..,NW% sec. 34, T. 3 N., R. 14 E. on 2 miles north from the Mokelumne River. On the banks the Stanislaus River. The schist contains rudely ellip of Pardee Reservoir (Mokelumne River, pl. 7), the tical pods (amygdules?) of coarse-grained carbonate. rocks associated with the limestone lenses are quartzose Massiye dark-g,my_guartzite is exposed on the south slate and chert with minor volcanic rock and, together west side of the Merced River about half a mile south with the limestone, are included with the Calaveras y~st of Clearing House, and thick- to thin-bedded quartz formation in this report. Also included with the Cala sandstone is exposed on the south side of the river in veras formation here is a belt extending northward in . the south part of sec. 13 ana'the SW% sec. 14, T. 3 S., strike with these exposures to a point north of the R. 19 E. Similar sandstones were found on the South Cosumnes River (pl. 1). Fork of the Merced in the northwest part ofT. 4 S., COSUMNES RIVER EXPOSURES R. 20 E. The stratigraphic ,and structural relations of Rocks exposed in and near the Cosumnes River in the sandstone to the chert-bearing strata are not clear. sees. 20, 21, and 28, T. 8 N., R. 10 E. (units 33-41, pl. 8) In thin section, the quartzite from the first locality is are, tentatively included with the Calaveras fonnation, seen to consist largely"'Of wen-rounded quartz grains in following Taliaferro (1943b, p. 283, fig. 2). Several a matrix of fine-crystalline quartz. biotite, and m:usco- . shear zones, and the absence of means of determining vite. About 10 pp,rntmt of the rock consists of irregu tops of beds, within the area leave open the possibility larl;x ·shaped graJns Of cm:dierite gr feldspar containin f alternative interpretations of the structure and strat abundant inclusions of quartz and abundant minute igraphic relations. Taliaferro (1943b, p. 283) states rains of an o ue material. The sand-size fraction that on the Cos~mnes River, a thick basal conglomerate comprises a out40 percent of the sandstone in theSE% of the Cosumnes formation overlies the Calaveras for sec. 14. The sand grains are mostly quartz but un mation on both sides of an anticline. Conglomerates twinned feldspar is common; twinned plagioclase and containing limestone debris occur through severa.I hun garnet are rare. The matrix is sericite, untwinned feld dred feet of section in the Cosumnes formation (see spar, and quartz. WESTERN BELT pl. 10) and there is little reason to designate any par ticular conglomerate bed, even though it be thicker and Turner ( 1894a) included with the Calaveras forma lower in the section than the others, as the base of the tion of the western belt all of the sedimentary and inter formation. Similar conglomerates west of the beds bedded pyroclastic rocks between the Copper Hill vol here placed in the Calaveras formation are interbedded canics and the belt of volcanic rocks formed by the Log with rocks in which bedding attitudes and sedimentary town Ridge formation and the Brower Creek volcanic features indicate homoclinal structure with tops east. member of the Mariposa formation. Most of the major These beds are probably equivalent to the Cosumnes or categories of epiclastic rocks are represented and the Mariposa formations, and if intervening beds are Cala belt is cut by several faults of the Bear Mountains f.ault veras formation they" are repeated by faulting rather zone. From the assemblage of rocks originally assigned than folding, as postulated by Taliaferro. to this part of the Calaveras formation (and from other The rocks here included with the Calaveras forma unit$) ·Taliaferro separated some which he included tion include slate and graywacke, volcanic rocks, with the Consumnes formation, of Jurassic age, but his quartz-rich sandstone and granule conglomerate, thin published descriptions do not provide an adequate basis bedded chert·and slate, and minor calcarenite. Macro for correlation of individual mappable units. scopic descriptions of these rocks are given on plate 8. LimeStone, forming scattered discontinuous bodies, is Petrographic descriptions of thin sections of quartz the only rock that has yielded Paleozoic fossils. All sandstone and, granule conglomerate are given below of the other rock types are known to occur in the Juras for comparison with similar rocks included with other sic section, but are not known to be restricted to the units. Jurassic. Some of the fossiliferous limestone forms A thin section of the granule conglomerate associated reworked blocks in the Jurassic strata or slivers in fault with the calcarenite on the southwest bank of the river zones and it remains to be proven whether any of the near the center of sec. 21, T. 8 N., R. 10 E. (unit 40, Paleozoic rock is in place in the belt mapped by Turner pl. 8) shown well-rounded clastic grains in a matrix as Calaveras formation. Nevertheless, .the writer be of coarse-grained clastic carbonate containing oolites lieves that the largest, most concentrated limestone and comminuted fossil debris. The clastic grains are bodies are among the rocks with which they were orig composed of feldspathic quartzite, strongly sheared inally deposited. According to Turner's ( 1894a) map ·aggregates of quartz grains, chert with scattered car of the Jackson quadrangle, limestone lenses ·are most . bonate rhombs, perthite, and a fine-grained granitic STRATIGRAPHY 13 rock consisting of granoblastic quartz surrounding sub The chert has a dull luster and conchoidal or splintery hedraJ, much-altered feldspar crystals. The feldspar fracture. Fine-grained volcanic breccia (Jackson :in some of the feldspathic quartzite clasts is mostly Creek section unit 7, pl. 7) is interbedded with quartzose potassium feldspar, and in others is mostly sodic plagi phyllite in sec. 1, T. 5 N., R. 10 E. A thin conglomerate oclase. The feldspathic quartzites strongly resemble is interbedded with quartzose phyllite in the same sec rocks of the Shoo Fly formation (Blue Canyon forma tion. The conglomerate contains clasts of chert, slate, tion of Lindgren, 1900) exposed in the eastern part of and volcanic rocks, and may be of intraformational the ColftlX 30-minute quadrangle. · origin. The quartz-rich sandstone exposed near the NW cor. The differences between the sections exposed in Jack sec. 28, T. 8 N., R. 10 E. (unit 37, pl. 8) consists largely son Creek and on Pardee Reservoir about 3 miles away of grains of quartz, and lesser amounts of chert, sericite reflect the complexity of the pattern of distribution of slate, quartzose slate, siltstone, orthoquartzite, volcanic rock types in the belt underlain by the Calaveras and rocks, and quartz aggregates showing elongate quartz Cosumnes formations. grains with strongly sutured contacts. The matrix is In thin section, the quartz-rich phyllite is seen to con sericite, but the amount of matrix is difficult to deter tain 60 to 98 percent quartz, mostly in the form of a mine because not all of ·it is distinguishable from the granoblastic groundmass. Angular, silt-size quartz sln.te fragments. Most of the quartz grains wre angular grains are common. These grains are clear and show but some are well rounded and a few show overgrowths, no undulatory extinction. Sericite and minute tremolite as do s01ne grains in the orthoquartzite clasts. Most of crystals form 40 percent to less than 15 percent of the the quartz grains contain vacuoles arranged in planes rock. Sparse grains of a carbonate mineral, pyrite, and and show tmdulatory extinction, but at least part of epidote, generally larger than the grains of the ground the undulatory extinction has been developed in the mass, are found in some specimens. Brown tourmaline present rock, as shoWn by the relation of the extinction in nearly equidimensional grains is rare. Carbonate to point contacts between grains. Rare quartz grains and epidote as well as quartz form veinlets. Nearly all have needle-like inclusions of rutile and one grain has thin section:s show round to elliptical areas as much as inclusions of biotite. Some of the chert is microcrystal 0.2 mm long that may be replaced Radiolaria. They line and some cryptocrystalline. The siltstone is com consist of granular quartz, generally coarser grained. posed of quartz and sericite. Included under the term and containing much less of the associated minerals "quartzose slate" are grains showing a considerable than the quartz of the groundmass. range in the proportions of the two minerals. Volcanic Thin sections show the black chert to consist of about rock :Vragments are dark colored and cryptocrystalline 98 percent microcrystalline granoblastic quartz with to microcrystalline and are prdbably devitrified glass. minor sericite. The round structures attributed to MOKEL U1MNE RIVER AND JACKSON CREEK EXPOSURES Radiolaria are common also in the chert, but no recog nizable organic structures were found. Rocks included with the Calaveras formation are ex posed on the banks of Pardee Reservoir on the Moke FOSSILS AND AGE lumne River and on Jackson Creek. Near Pardee Res he Calaveras formation was thought by Turner ervoir, they are separated from rocks correlated with ( 1893a, p. 309 ; 1893b) to be of Carboniferous age an other formations by exposed shear zones, but the con is assigned to the Mississippian in Wilmarth (1938, tacts on Jackson Creek are not exposed. Almost all p. 315). Taliaferro (1943b, p. 280) surmised tha.t. the of the rock included with the Calaveras formation near Carboniferous is represented in the Calaveras formation Pardee Reservoir is quartzose phyllite that weathers but that part is of Permian age and P-art is pre-Carbonif gray to light brown. The phyllite is aphanitic and eron.s._ During the present investigation well preserved bedding is not ·apparent in most exposures. Other Foraminifera were found at three localities and indicate rocks included with the Calaveras formation ne.:'lr Par .!J!aLpart of the formation is of Permian ag~ large dee Reservoir are small lenses of limestone and a single parts of the formation have not yielded diagnostic fos exposure of n.mygdaloidal lava. The lava is exposed sils and may be oidw. erlloict plates are abundant in near the limestone lens in the western part of sec. 17, limestone of the Calayeras formation at manx..places T. 5 N., R. 11 E. (unit 28, pl. 7). Rocks exposed near but the localities are not listed here because the pla.tes Jackson Creek that ttre included in the Calaveras are of littJe yalue in dating. formation differ from those near Pardee Reservoir in The first three fossil localities represent the western that quartzose phyllite is rare; black chert forming belt of the Calaveras formation, as follows: layers about 1 to 2 inches thick with interbedded black One is in sec. 10, T. 4 N., R. 11 E., at the west bound phy Bite is the most common rock in Jackson Creek. ary of the San Andreas quadrangle where Foraminif- 724-276 0-65-3 14 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT era compose most of a block of limestone about five Foraminifera discovered by Turner ( 1893a, p. 309) near feet in diameter exposed in the north wall of a railroad Rite Cove, in the El Portal quadrangle, which were cut. Boulders of similar limestone, probably removed identified as Fusulina cylindri FIGURE 4.-Photomicrograph of mudfiow conglomerate from Mariposa formation, NElA,NE~ sec. 2, T. 3 N., R. 11 E. Only a small fraction of the total size range of clastic fragments is shown. Felsic volcanic rock with quartz (white) phenocrysts, fv; mafic volcanic rocks, mv, in part with plagioclase and pyroxene phenocrysts; quartz, q; granophyre composed of intergrown K-feldspar and quartz with minor biotite, a; volcanic rock partly re placed by pyrite, v; mica-quartz phyllite, p; chert-carbonate rock, c; clastic limestone, 1. which are several times as large as the other boulders zoic locality 22175, U.S. Geol. Survey collections). in the layer. These conglomerates are probably of mud This specimen was compared by Imlay ( 1952, p. 975) flow origin. Both kinds of conglomerate occur in the with the Callovian genus Grossouvria, but he now con Cosumnes formation on the Cosumnes River and are siders (written communication, January 1960) that: well exposed in a small canyon cut into rocks of the its coiling is too involute for that genus and tllat it agrees Mariposa formation in the SW1,4 sec. 1, T. 3 N., R. better in coiling and rib pattern with Pseudocadoceras which 11 E., about 4 miles southwest of San Andreas. occurs in considerable numbers in the lower part of the over lying Logtown Ridge formation.' The speciman suggests, there Fossils and age fore, that tlle upper part of the Cosumnes formation is not older Taliaferro ( 1943b, p. 284) ascribed to the Amador than Callovian. Furthermore, considering that the Logtown group a late Middle to early Late Jurassic age, but does Ridge formation grades downward into the Cosumnes formation, the presence of Pseudocadoceras in the basal beds of the Log not give a specific age for the Cosumnes formation. He town Ridge formation implies that tlle Cosumnes formation is mentions several fossil localities but does not describe at least in part of Callovian age * * *. the localities or faunas in detail. Later collections indi LOGTOWN RIDGE FORMATION cate that the age of the upper part of the Cosumnes The name Logtown Ridge formation is applied in formation is within the interval extending from middle this report to the sequence of volcanic rocks that ever Bajocian to late Callovian. Eric, Stromquist, and lies the Cosumnes formation in the Cosumnes River Swinney ( 1955, p. 10, 11) report finding an ammonite in the Cosumnes River section about 500 feet below the 1 While this report was being prepared for publication, a report de scribing late Jurassic fossils from the western Sierra Nevada in more top as the formation is described in this report (Meso- detail was published by Imlay ( 1961). STRATIGRAPHY 19 southwest 'of Huse Bridge. It extends from north of diameter ranges widely. Clasts are angular to sub the area studied to a point between Drytown and the rounded or well-rounded (fig. 5). The mudflow de ~{okelumne River where it is overlapped by the Brower posits range widely in composition-some contain only Creek volcanic 1nember of the l\tlariposa formation. one or two textural vrurieties of lava and others contain The southern boundary of the Logtown Ridge forma many textural and perhaps even compositional varie tion is drawn arbitrarily on the regional geologic map ties. In some layers part of the fragments are ·amyg and block diagran1 ( pls. 1 and 2), for detailed mapping daloidal but scoriaceous clasts are infrequent. Rare that might show the position of the contact between layers contain fragments of pyroxenite and plagio the Logtown Ridge formation and the younger volcanic clase-pyroxene rocks. Although clasts larger than two sequence is lacking. The Logtown Ridge formation inches in diameter are abundant in most of the prob has been examined only in the type section, where it is able volcanic mudflow breccias, a layer of finer volcanic about 4,300 feet thick and consists largely of volcanic breccia about 0.8 ~iles southwest of Huse Bridge con breccia and tuff. Two thin layers of pillow lava are tains widely scattered chunks as much as several feet present in the type section. long of somewhat contorted, thin-bedded, coarse to very The name Logtown Ridge formation was used by fine tuff. These chunks are consistent with the notion Eric and others (1955, p. 11-12, pls. 1, 2) to include that this layer is also a mudflow. In stream polished volcanic rocks extending as far south as Chinese Camp, exposures the matrix of most of the volcanic breccias and by Taliaferro and Solari (1949) to include vol wastes away at about the same rate as the fragments canic rocks in the vicinity of Copperopolis. These se (fig. 5). In one breccia layer about 100 feet thick, quences will be given new names because some occupy however, the matrix weathers much more rapidly than a different stratigraphic position "from the Logtown the fragments, leaving them in bold relief; it is exposed Ridge formation and the position of others relative to about 300 feet west of Huse Bridge on the Co~umnes the Logtown Ridge formation is unknown. River. Clasts are angular to subrounded and range Lithology from sand size to boulders more than 4 feet long and 2 feet thick. The large boulders are elongated parallel In the Cosumnes River, tuff and lapilli tuff dominate to the bedding in this vicinity, but no layering sugges the lower part of the Logtown Ridge formation and tive of bedding was observed. The clasts have little volcanic breccia dominates the upper part. Nearly all lithologic variety as nearly all contain pyroxene pheno the volcanic fragments are dm green, witlLan.-a.pha crysts more than 2 mm in diameter-about a third of nific texture. Pyroxene phenocrysts in both clasts and them are amygdaloidal (quartz and calcite amygdules). matrix of some beds range from less than one milli The matrix is composed of sand size grains of lava and meter to 1nore than one centimeter in diameter. A few breccia beds contain scattered clasts of medium- to pyroxene cemented by calcite. Although this layer is coarse-grained pyroxenite and coarse-grained rocks apparently the product of a mass movement process, composed of plagioclase and pyroxene in various pro "mudflow" seems hardly an appropriate term for a portions. No megascopic features tha't indicate dif mass lacking in clay-size material. This layer is sep ferences in chemical composition between the various arated from an underlying pillow lava by about 10 feet breccia beds were observed, but the amount of por of rubble atop the lava, and about 4 feet of bedded tuff phyritic or amygdaloidal fragments differs from bed immediately under the breccia. It is overlain by a thin to bed. Beds may also differ in the ~ize of the pheno amygdaloidallava. crysts and in the proportion of clasts that contain The mudflow deposits represent material that has phenocrysts or amygdules. moved to a permanent site after temporary accumula Original textures and structures of the pyroclastic tion in another place. A different kind of volcanic rocks are well preserved along the Cosumnes River and breccia, probably the result of direct deposition from are probably representative of features in other volcanic formations of the region that were studied in less detail. explosive vents, is represented by bedded tuff and lapilli The Cosumnes River exposures show that the detritus tuff layers containing scattered boulders of volcanic composing various beds of pyroclastic rock was em rock (fig. 6). Flattish blocks generally are parallel to placed in several different ways. the bedding. The shape of these fragments suggests M~ch of the volcanic breccia of the Logtown Ridge that they are not volcanic bombs, but rather are blocks formation was apprurently emplaced by subaqueous of lava. Their well-defined regular bedding indicates 1nudflows. Mudflow layers have poor sorting, lack in that the poor sorting is not the result of mass flow; and ternal bedding, and some contain boulders several feet the absence of graded beds, as well as extremely wide long. Packing of fr3.ooments larger than one inch in range in the size of the clasts indicates that they were 20 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT FIGURE 5.-Photograph of volcanic breccia of probable mudflow origin. Closely packed fragments. Dark spots in clasts are pyroxene phenocrysts. Fragment of plagioclase-pyroxene rock (light colored with scattered black crystals) in lower left corner between two pyroxenite (black) fragments. Part of block more than two feet long shown in upper left corner. Logtown Ridge formation, Cosumnes River, about 2,000 feet southwest of Huse Bridge. not formed by turbidity currents. Such deposits prob ded, medium- to very fine-grained tuff and filled with ably record the sporadic activity of the source vent. the same kind of material. Load casts (Kuenen, 1957, p. 246-248, fig. 13), shown Many beds in the lower part of the Logtown Ridge in figure 7, and more wildly contorted structures result formation contain angular fragments of very fine black ing from contemporaneous deformation are common in tuff; like that in place in associated beds (fig. 8). In the lower part of the Logto,,n Ridge formation and some beds the fragments are widely scattered, suggest have been noted else"·here. Cut-and-fill structures ing that they ''ere ''ell stirred into the enclosing coarser measurable in inches are widespread in volcanic rocks tuff and may have been carried a considerable distance of the western Sierra Nevada, but one erosion channel by mudfimYs. In other beds the fragments are closely noted in the lower part of the Logtown Ridge formation packed and have moved only a short distance from their in the Cosumnes River is about two feet deep and six source, apparently as result of restricted subaqueous feet wide at the top. The channel is cut into thin-bed- slumping that involved a single layer or possibly two STRATIGRAPHY 21 lows by R. W. Imlay (written communication, 1959) : The ammonites belong to species of Pseudocadoceras that have been found in Alaska, British Columbia, and the Taylors ville area * * *. The genus Pseudocadocm·as in Europe ranges from * * * [lower to upper Callovian]. Imlay (1952, p. 975-976) reported the occurrence of an ammonite in volcanic breccia of the Logtown Ridge formation near Ruse Bridge, apparently in the upper part of the formation. He then described this am monite as being "similar to Divisosphinctes from the upper Oxfordian and to some of the finer ribbed species of Paohysphinctes from the Kimmeridgian" but later (Imlay, written communication, January 1960) stated that "This ammonite is now identified with the genus FIGURE 6.-Photograph of volcanic breccia showing scattered I doceras of late Oxfordian to early Kimmeridgian age." large clasts in bedded pyroclastic groundmass. Tops of beds Because the Logtown Ridge formation was previously are toward the left. Logtown Ridge formation, Cosumnes assumed to be overlain by the Mariposa formation, of River, about 1,000 feet west of Huse Bridge. Kimmeridgian age, Imlay concluded (1952, p. 976) that this specimen is of upper Oxfordian age. The slates that or three adjacent layers of the very fine tuff that con stitutes the fragments. Small clastic sills of medium-grained tuff that in truded very fine tuff (fig. 9) can be observed in the lower part of the Logtown Ridge formation along the Co sumnes River. Beds in the photograph are of subaque ous origin and graded beds are abundant stratigraphi cally above and below. That the medium-grained tuff forms sills, rather than lenticular beds, is shown by the associated cross-cutting dikes and by the separation and breaking of septa of very fine dark-colored tuff visible below the coin in the photograph (fig. 9). Intrusion of the sills apparently took place after deposition of the beds but before they were indurated, for although the very fine tuff was sufficiently coherent to break into an gular fragments the coarser tuff was able to flow. Clastic dikes more than a few inches long are rare and both these and the sill-like bodies are the results of processes that apparently operate only locally. Fossils and age The Lo~Ytown Rid e formation apparently ranges in a e from Callovian to upper x or Ian or ower Kimmeridgian. Ammom es co ec e y ar and R.. W. Imlay (Mesozoic localities 24710 and 27397, re spectively, in the U.S. Geological Survey collections) from above an abandoned ditch on the hill slope north of the Cosumnes River at the abrupt southward bend FmunE 7.-Photograpb of thin-bedded to laminated tuff showing of the river about 4,550 feet southwest of Ruse Bridge graded bed and load casts. Dime shows scale. Elongate hard and about 1,000 feet above the base of the Logtown areas are concretions. Logtown Ridge formation, Cosumnes Ridge formation (figs. 4 and 28) are described as fol- River, 134 miles southwest of Huse Bridge. 724-276 0-65-4 22 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT tuff with some interbedded chert or silicified ash and the unnamed epiclastic rocks is exposed on the margins of Lake McClure in the eastern part of sec. 7, T. 4 S., R. 16 E. The Pefion Blanco volcanics were mapped as porphyrite and amphibolite by Turner (1897) except that he differentiated a unit of thin-bedded tuff and chert which is here included in the Penon Blanco vol canics. The Pefion Blanco volcanics of this report were placed by Taliaferro ( 1943b, p. 283) in the Amador group of his southern type section although he was un able at that time to establish complete correlation with the Cosumnes River section. Taliaferro (1933, p. 149- 150; 1943b, p. 283) divided the Amador group in the FIGURE B.-Photograph of thin-bedded tuff showing intraforma· tional breccia bed below dim and small-scale crossbedding under breccia. overlie the Logtown Ridge formation, however, have not been demonstrated to be of Kimmeridgian age, so it is possible that the Logtown Ridge formation extends into the Kimmeridgian. PENON BLANCO VOLCANICS The name Penon Blanco volcanics is here applied to the sequence of volcanic rocks that underlies the Mari p.osa formation from the south end of the area shown on plate 1 to a point west of the town of Melones. The formation underlies and is named for Pefiop Bla~o Ridge between the Merced and Tuolumne Rivers. ~ .cype section is along the Merced River. Of the three sections studied, this is the only complete one.; in the Stanislaus and Tuolumne Rivers the Pefion Blanco volcanics are in fault or intrusive contact with serpen tine and part of the formation is missing. The Penon Blanco volcanics consisL&J.._most entjx;ely of aark gteen m'lffic~k-s. In thetype section, where the for m"ati''n is nearly 15,000 feet thick, tuff, lapilli tuff, and volcanic breccia constitute the upper part of the forma tion, and lava, mostly massive but some with pillow ,, structur.e, constitutes the lower part. The areal pattern ·- I south of the Merced River suggests that the Penon Blanco volcanics thin rapidly southward, beyond the edge of the map area. The top of the Pefion Blanco volcanics is well ex posed on the banks of the Merced River near the north east corner of sec. 27, T. 3 S., R. 16 E. (fig. 23). Bed ding in the Mariposa formation is parallel to that of the Pe:fion Blanco volcanics near the contact. The con tact of the Pefion Blanco volcanics with an underlying, unnamed sequence of epiclastic rocks is in sec. 15, T. 3 FIGURE 9.-Photograph of clastic sills of medium-grained tuff S., R. 15 E., but is not exposed at the abandoned railroad {light gray) in very fine tuff (dark gray). Dime shows scale. Lowest light-gray band may be a bed. Note wedge shape of grade on the south side of the Merced River where the various light-gray layers and brecciation of dark-gray traverse was made. A conformable contact between material. STRATIGRAPHY 23 southern area into .five formations but he did not define tion is of late Oxfordian or early Kimmeridgian age. his formations and they are not used here. The Pefion The age of the lower part of the Penon Blanco volcanics- Blanco volcanics of this report were included in the is unknown. Part of the formation is probably of the Logtown Ridge formation by Heyl, Stromquist, Swin same age· as the Logtown Ridge formation. ney, and Wiese (pl. 2, in Eric, Stromquist, and Swinney, MARIPOSA FORMATION 1955). The Penon Blanco formation is here distin The Mariposa formation wa.s pamed for exposures on guished from the Logtown Ridge formation because the the former Mariposa esta.te, southeast of Bagby two are not continuous on the surface nor are the epi (Becker, 1885, p. 18-19). As used by Turner (1894a, clastic rocks that overlie the Logtown Ridge formation 1897), the Mariposa slate included all the dominantly continuous with the Mariposa formation. epiclastic rocks in the yarjous sequences west of the LITHOLOGY Melones fault zone. except those which he assigned to Most of the Penon Blanco volcanics are medium- to the western belt of the Calaveras formation. Talia dark-green mafic nyroclastic rocks~pt along the ferro (for example, in Taliaferro and Solari, 1949) Tu_olumne River where massiye layas form most of the gave the formation a similar scope, except that he as incomplete section. Some of the pyroclastic rocks, no signed some of the epiclastic rocks to the Cosumnes tably those near the Stanislaus River, contain pyroxene formation. Eric, Stromquist, and Swinney ( 1955, p. phenocrysts. Felsic crystal tuff with interbedded chert 12) substituted the term Mariposa formation for Mari or silicified ash, apparently in the basal part of the posa slate because other rock types form important Penon Blanco volcanics, is exposed in the western part parts of the unit. Here, the name Mariposa formation of sec. 8, T. 4 S., R. 16 E. Original textures are well is restricted to rocks in the central fault block and the preserved, except on the Stanislaus River where orig Melones fault zone that are continuous on the surface inal textures and structures have been almost oblit with the type area, or are repeated across the strike by erated by recrystallization, and in the central part of adequately documented structures. All previously de the section along the Merced River, which has been scribed strata attributed to the Mariposa formation that strongly epidotized. Massive a hanitic lavas in part have yielded diagnostic fossils and are within the area ~lgdaloidal o_c_c_ · - ctwns studied ut iorm of figure 21 are retained in the Mariposa formation, .2!!]y a smafl part oJ the incomplete sec i'Oiial'Oiiifthe but some rocks in the central block that were previously Stanislaus Riv~r: Pillow lava forms-three units near correlated with the Mariposa formation are excluded the top of the lava sequence on the Merced R1ver; pil and will be discussed later. New names are proposed low structure is ossible at places in lower arts of the for rocks in the western block that were previously cor lava §equence, but very poor y developed. related with the Mariposa formation. Two thin epiclastic units (units 30 and 33, pl. 5) are Exclusive of the Brower Creek volcanic member, the exposed near the Stanislaus River and are interpreted Mari osa formation is chiefly black slate and silty sla~ to be interbedded with the Penon Blanco volcanics. but tuff and I surrounded on three sides by epidastic rocks continuous considerably from place to place. In most of the-sec on the surface with the type area of the Mariposa for tions studied, a unit of tuff about 300 feet thick is inter mation. Exposures of the member near Fowler Look bedded in the lower part and a unit of graywacke about out and to the northwest along the strike are on the 50-n feet thick farther ull:- Eric, Stromquist, and western flank of a syncline, and the beds dip steeply. Swinney (1955) have differentiated the slate, tuff,. and Accordingly, the surface pattern portrays· the true graywacke of the Mariposa formation on their maps of stratigraphic relations of the Brower Creek member to the Angels Oamp and Sonora quadrangles. other parts of the Mariposa formation. The conglomerates of the Ma.ci.pGsa-J€H!matioD...lll!>stly The top of the Mariposa formation is not preserved, resemble those of the Cosumnes formation, but one type as it is everywhere truncated by faults or erosion. is peculiar to the MariJ.:!oSa forrna.tio.n._ This type is North of the Stanislaus River, the Mariposa formation exposed at the south end of the area underlain by the lies in a syncline whose eastern and western limbs are Brower Creek volcanic member, and west of the north truncated by the Melones and Bear Mountains fault end of the north-trending arm of Melones Reservoir. zones. South of about the latitude of Chinese Camp, ~es and cobbles of the gopgJomBf(!tth as much as 5 the eastern limb of the syncline is absent and the Mar~ inches in diameter in some beds, are well rounded and iposa formation forms part of an eastward-dipping consist-largely of resistant rocks, such as w~ and homocline. The partial section of the Mariposa forma~ dark-gray vein quartz, f51ttJM!'1Tm'e;-"f1ff}TOlflT~te, and tion exposed near the Merced River is about 2,700 feet quartz conglomerate. Less resistant antt-'~ndant thick. Near the Stanislaus River the formation seems pebbles consist of gray chert and slate, medium-grained to be much thicker, but the structure is not well enough granitic rocks, and fine-grained gneissic granitic rocks. known in the eastern part of its outcrop to justify more Pebbles of volcanic rocks are rare or absent. The con than a crude estimate here. Near the Mokelumne River, glomerate and associated sandstones contain little Silt the Mariposa formation is about 4,000 feet .thick, of or clay-size material. _ which nearly all is the Brower Creek volcanic member. The Mariposa formation includes poorly exposed The Mariposa formation is identified on all streams sandstone and fine conglomerate rich in quartz and studied except the Cosumnes River, but the most readily chert and with little detrital matrix near the Calaveras accessible good exposures .are on the Merced River, River (unit 26, pl. 6). The quartz grains are angular northwest of Bagby and in roadcuts along State High to well rounded and the chert grains are angular to way 49 south of Bagby. The lower part of the forma subangular; the. sandstone also contains fragments of tion is well exposed in the Tuolumne River west of slate and probable quartzite made up of quartz grains Jacksonville, and higher parts of the formation are with sutured contacts. The associated conglomerate exposed in State Highway 49 cuts northwest of Jackson has a larger proportion of slate fragments than the ville. The railroad grade on the north side of the sandstone. Calaveras River about 21h miles west of San Andreas The quartz and chert sandstone surrounds an ex provides large exposures of both the epiclastic rocks and posure of limestone in the NW1;4NE% sec. 22, T. 4 N., the Brower Creek volcanic member. R. 11 E., about 700 feet southeast of an abandoned A departure from the usual practice of this investiga farmhouse. The limestone is black, thin-bedded cal tion has been made in tentatively correlating with the carenite, some beds of which contain finely commi Mariposa formation a belt of epiclastic ·and volcanic nuted organic detritus and scattered quartz grains. rocks extending from west of Altaville to south of the The shape and attitude of the limestone and its con Stanislaus River. These rocks lie east of the main belt tained quartz grains suggest that the limestone may be interbedded with the sandstone. underlain by the Mariposa formation and ·are separated These rocks differ from others of the Madposa for from it by a fault of the Melones fault zone; neverthe~ mation in being better sorted and containing a more less, correlation of rocks in this area seems more certain restricted range of pebble types. However, they do not than that of rocks in other outlying areas. contain fragments of rocks foreign to the graywacke LITHOLOGY and other congtomerates of the Mariposa formation. The Mariposa formation. exclusive of the Brower Thin beds of megascopically similar sandstone are Qreek member, consists la.rg~ of slate: tuff: graywacke interbedded with limestone pebble-bearing conglomer and conglomerate in the same manner as the Cosumnes ate in rocks of uncertain stratigraphic position near formation. In most places, thin beds of tuff and gray the Cosumnes River (unit 31, pl. 8). Although the wacke are interbedded with the slate throughout-the sandstone is not known to be interbedded with lime formation. but proportions of these rocks to slate varies stone pebble-bearing conglomerate near the Calaveras STRATIGRAPHY 25 River, such conglomerates are exposed in strike with As a result of subsequent investigations, Imlay (written the sandstone about 3 miles to the southeast (A. A. communication, January 1960) believes that faunas at Stromquist, written comn1unication 1952). localities 902-903 below are probably also early_ No chert has been observed in the M:ari posa except Kimmeridgian. in the lower part west and southwest of San Andreas, Faunal localities in the Mariposa formation from a where it forms pa.rts of units 23, 25, and 28 of the compilation by Imlay, are listed below. Numbers des Calaveras River section. I-Im·e, it is thin bedded and ignate Mesozoic locality numbers in collections of the light to dnrk gray nnd is interbedded with slate and quartzose slate of the same colors. Some is sericitic, U.S. Geological Survey. References to current (1959) with varied composition, and probably there is a com maps have been added in parentheses to old locality plete gradation between chert m1d sericite slate. In descriptions. thin section, much of the sericitic chert can be 243 Near Pine Tree Mines, Mariposa Estate, Mariposa seen to contain small elliptical bodies composed of County. (See topographic map of the Coulterville 15- quartz more coarsely crystalline than the surrounding minute quadrangle.) Buohia oonoentrioa (Sowerby) material; perhaps they forn1ed from Radiolaria, as they Perisphinotes? sp. differ frmn detrital quartz grains in their shape and 253 Left bank of Merced River :14 mile below Bentons Mills, polycrystalline character. Mariposa County. (Bentons Mills known as Bagby in The Brower Creek volcanic member of the Mariposa 1959 ; see pl. 1.) formation consists largely of coarse dark-green volcanic B uchia. oonoentrica (Fischer) 719 Texas Ranch, Calaveras County. (Near Texas Charlie breccia, in places containing blocks 2 or more feet in Gulch, pl. 5.) diameter. Rarely, clasts of pyroxenite and limestone Amoebooera.s (Amoebites) dubium (Hyatt) occur in the breccia of the upper tongue. Pyroxene Buchia conoentrica ( Sowerby) phenoorysts as large as 1 centimeter in diameter are 901 Trail on south side of Stanislaus River, opposite Bosticks Bar, near Reynolds Ferry, NW%, sec. 3, T. 2 N., R. 13 E., common in fragments and matrix of many breccia Tuolumne County, flooded by Mel<>nes Reservoir (see pl. layers. The upper tongue of the Brower Creek mem 5 or topographic map of Copperopolis 15-minute quad ber contains some tuff and lapilli tuff and one small rangle, edition of 1916). exposure of pillow lava. The only epiclastic detritus Perisphinotes (Discosphinotes) virgulatiformis Hy att in the Brower Creek member, other than the rare lime P. (Diohotomosphinotes) cf. P. (D.) milhlbaohi stone fragments, occurs on the mrurgins of Pardee Reser Hyatt voir on the Mokelumne River. Here, thin beds of slate TarameUioeras? dentioulatum (Hyatt) "Belemnites" paoificus Gabb are interlayered with the volcanic breccia of unit 31; Turbo? sp. (pl. 7) on the west limb of a syncline, and sm.all con.: Oerithium? sp. glomerate lenses with a few pebbles of epiclastic origin A vicula? sp. (Hyatt, 1894, p. 429) form part of unit 35 on the east limb of the syncline. 902 South bank <>f Tuolumne River at Moffit Bridge site, Tuolumne County (near Jacksonville, pl. 4) and east of J. H. Eric (written communication, 1950) reports sim contact between Peiion Blanco volcanics and Mariposa ilar conglomerates north of the Teservoir about on the formation. strike of unit 35. Buchia conoentrica ( Sowerby) Subdichotomocera.s? aff. S. filiplem ( Quenstedt) FOSSILS AND AGE 903 Stanislaus River near canyon opposite m<>uth of Bear According to R. W. Imlay (written communication, Creek, near center of sec. 11, T. 1 N., R. 13 E., Calaveras 1959) : County (pl. 5). The Mariposa formation is definitely of late Oxfordian to early Buchia oonoentrica ( Sowerby) Kimmeridgian age throughout most of its extent. The most "Belemnites" pacifious Gabb common fossil is Buchia conce1tt?·ioa ( Sowerby) which in north Su.bdiohotomoceras '? aff. S. filiplem ( Quenstedt) ern lDurasia and in Alaslm ranges from upper Oxfordian into 904 Six miles from Copperopolis on road to Sonora and on middle Kimmeridgian (Imlay, 1959, "' "' * p. 157, 158) "' "' "'. grade t<> Angels Creek, Calaveras County. (Probably in The Oxfordian age assignment is based on the occurrence of the sec. 33, T. 2 N., R.13.E., pl. 5.) lllmnonites Diohotonwsphinctes and Discosphinotes. Of these Snbdichotomooeras aff. S. fili1Jlem (Quenstedt) D,iohotomos7Jhinctes ranges through the Oxfordian, but is ~'lrtie B1tehia oonoentrioa ( Sowerby) ulnrly chnrncteristic of the upper Oxfordian. Disoosphinotes "Belem?'llites" lJacificus Gabb occurs in the upper Oxfordian and questionably in the lower A.m1tsinm aurari1tm Meek Kimmeridgian "' "' "'. 'l'he early Kimmeridgian age assignment 1982 Hell Hollow, north of Bear Valley, Mariposa County of part of the Mariposa formation is based on the ammonite (pl. 3). A.mocbUes, a subgenus of A.moebooeras [see locality 719 Perisphinotes '! sp. below]. "' "' "' "Belemnites" paoifious Gabb 26 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT 1983 5 miles southeast of Princeton, Mariposa County. also unknown, owing to a lack of detailed mapping in (Princeton is shown as Mount Bullion on the topo this part of the region. The age of the unit is estab- graphic map of the Coulterville 15-minute quadrangle.) _lished by an ammonite recovered from a ground-sluice Buchia sp. .Amusium aura.rium Meek mine on top of a narrow ridge 600 feet southeast of the 20554 Woods Creek Canyon 3 miles northwest of Jacksonville, northwest corner of sec. 25, T. 4 S., R. 17 E., Bear Val Tuolumne County. (Pl. 1 and topographic map of So ley 7¥2-minute quadrangle, California. The mine was nora 15-minute quadrangle.) cut into bedrock and the dumps contained no material Buchia coocentrica (Sowerby) foreign to the immediate area. The ammonite, identi 22176 Left bank of Mokelumne River, 400 feet west of the east edge of sec. 17, T. 5 N., R. 11 E., Sutter Creek 15-minute .tied as Perisphinotes (Diohotom.osphinotes) cf. P. (D.) quadrangle, Calaveras County (pl. 7). miihlbaehi Hyatt, is of late Oxfordian age (Imlay, writ Ammonite fragment ten communication, 1959). 24317 Loose boulder in bed of Cherokee Creek, sec. 22, T. 3 N., Another large body extends from north of the R. 12 E., San Andreas 15-minute quadrangle, Calaveras Cosumnes River to southwest of Mokelume Hill im County. Matrix is identifiable with beds in the Mariposa formation west of Cherokee Creek. mediately west of the Melones fault zone. Near the Perisphinctes ( Dichotomosphinctes?) sp. Cosumnes River, this body is in fault contact with the 27311 Moffit bridge site. Railroad cut on south side of Tuol epiclastic rocks to the west and with sheared grano umne River 200 to 225 feet east of contact between diorite. on the east. In this section ·the western part Penon Blanco volcanics and Mariposa formation. Near of the volcanic body is composed of dark-green mafic Jacksonville, Tuolumne County, plate 4. Buchia coocentrica (Sowerby) volcanic breccia with pyroxene phenocrysts. A thin 27312 North bank of Merced River, NW corner, sec. 26, T. 3 S., unit of interbedded tuff and black slate occurs in the R. 16 E., Mariposa County, plate 3 eastern part. Volcanic rocks of this narrow belt might Buchia coocentrica ( Sowerby) belong to the Logtown Ridge formation, the Brower 27398 Crest of Woods Creek Ridge 1 mile northwest of Jack Creek volcanic member of the Mariposa formation, or sonville, Tuolumne County. Con. by George R. Heyl, 1946. (See topographic map of Sonora 15-minute to neither. quadrangle.) An elongate area in the eastern part of the Bear B uohia concentrica ( Sowerby) Mountains fault zone near J ack!?on Creek is shown on Lima? sp. plate 1 as volcanic rocks of uncertain stratigraphic posi 27459 Woods Creek, Tuolumne County. Received from Becker. tion but includes some bands of epiclastic rocks to small (See topographic map of Sonora 15-minute quadrangle.) Buchia sp. to differentiate. Near Jackson Creek, the rocks of the Lima? sp. area are tuff, with some interbedded slate and porphy 27460 From Mount Bullion, Mariposa County, collection by W. ritic volcanic breccia. These rocks are faulted against D. McLearn, Coulterville, California. Received May serpentine on the west and to the east lie with undeter 3, 1922. mined relations against the quartzose phyllite assigned Perisp·hinctes (Dwhotomosphiootes) cf. P. (D.) to the Calaveras formation. milhZbachi Hyatt The elongate fault-bounded block west of Fowler STRATA OF UNCERTAIN STRATIGRAPHIC POSITION Lookout, consists mostly of mafic pyroclastic rocks but VOLCANIC STRATA includes an amygdaloidal mafic lava flow, a thin-bedded The Jurassic of the central part of the region includes chert unit, a thin unit of slate with some interbedded various bodies of volcanic rock that have not been corre conglomerate, and small lenticular bodies of very fine lated with the standard formations because they are sep grained massive limestone. The geographic position of arated from them by faults, or have been insufficiently this block suggests that it is part of the Copper Hil1 appraised during this investigation. A body that volcanics or the Penon Blanco volcanics. underlies Mount Bullion constitutes the only known EPICLASTIC STRATA occurrence of Jurassic rocks east of the Melones fault The epiclastic unit exposed along the Cosumnes zone within the area covered by this report. Its true River east of Ruse Bridge consists largely of black extent is unknown, but volcanic rocks extending from near the Merced River to the south edge of plate 1 have slate and siltstone but contains some tuff, graywacke, been included arbitrarily. Poor exposures on Mount and fine conglomerate. The epiclastic belt conform Bullion .suggest that the unit consists largely of coarse ably overlies the Logtown Ridge formation for several mafic volcanic breccia, in part containing pyroxene miles north and south of the Cosumnes River. Absence phenocrysts, but it also includes some mafic tuff and . of a break in deposition at the top of the Logtown Ridge pillow lava. The internal structure of. the unit and formation is shown by top determinations along the the kind of contact with the Calaveras formation are Cosumnes River, and parallelism of beds in both units. STRATIGRAPHY 27 The unit was previously mapped as Mariposa forma Mariposa formation, but its stratigraphic position is tion (Lindgren and Turner, 1894; Taliaferro, 1943b, incompatible with this assignment. No new name is fig. 2), but detailed mapping in the area between the proposed for this sequence because it extends south of l\{okelumne and Cosumnes Rivers will be necessary to the area shown on plate 1 and the exposures studied determine whether this correlation is valid; for the constitute but a small part of the formation. present, therefore, the unit is not designated by a for ROCKS EXPOSED IN THE WESTERN BLOCK AND mation name. Existing maps (Lindgren and Turner, BEAR MOUNTAINS FAULT ZONE 1894; Lindgren, 1900) show the epiclastic rocks east of Huse Bridge to be part of a belt that extends con The western block includes the area lying between the tinuously more than 40 miles north of the Cosumnes Bear Mountains fault zone and the western limit of River. There may, however, be structural complica bedrock exposures. In the northern part of this area, tions in part of the belt, and Imlay ( 1952, p. 975) be the rocks consist of a lower volcanic formation, a middle lieves that strata near its north end are of Callovian age, epiclastic formation, and an upper volcanic formation. or more nearly equivalent in age to the Amador group These formations extend to the southern part of the than to the Mariposa formation. B'U(Jhia concentrica, area studied, where additional igneous and epiclastic Entolium ~ sp., and Nucula ~ sp., were identified by Im rock formations may occur. Relations between the ln,y (written communication, 1959) in a collection from three most extensively exposed formations of the west a narrow projection of this belt exposed in a sm.all gully ern block are best displayed in the Cosumnes River %mile north-northwest of Plymouth. (pl. 8). In the northern part of the region another belt of Volcanic rocks of the western block resemble those of epiclastic rocks lies along the western side of the cen the central block in that they are dominantly of pyro tral fault block from the north end of the area shown clastic origin and are mostly of probable andesitic or on plate 1 to south of the Mokelumne River. These basaltic composition. Rhyolite or dacite, however, is rocks are black slate, graywacke, conglomerate, and much more common in the westernmost belt of volcanic mafic volcanic rocks. Pebbles in the conglomerates are rocks than in others. The epiclastic rocks of the western composed chiefly of volcanic rocks, slate, chert, and vein block apparently contain less graywacke and conglom quartz ; some beds contain limestone pebbles. Sand erate than the Cosumnes and Mariposa formations of stone consisting largely of well-rounded quartz grains the central block, but are otherwise simil·ar. is interbedded with the conglomerate of unit 31 (pl. 8) The stratigraphic sequence of the western block has along the Cosumnes River. not previously been distinguished from that in the These rocks were mapped as Calaveras formation by central block. Structural data collected during the Turner (1894a, and Lindgren and Turner, 1894), and current investigation, however, indicate that some of Taliaferro (1943b, fig. 2) included them in the Am.ador the rocks of the western blocks may be younger than any group. They resemble both the Cosumnes and Mari exposed in the central block and that correlation of posa formations and it cannot be determined whether other units is complicated by the difficulty of establish they belong to one or the other. ing lateral continuity between the two blocks. New A narrow belt of conglomerate (unit 11, Jackson names are proposed for subdivisions of the western Creek section, pl. 7) exposed along Jackson Creek prob block. ably belongs to the Mariposa formation but may be part GOPHER RIDGE VOLCANICS of the Cosumnes formation. It was included by Turner The name Gopher Ridge volcanics is here used for the (1894a) in the Calaveras formation. The conglom westernmost exposed volcanic formation in the part of erate, like those of the Cosumnes and Mariposa forma the area shown on plate 1 that lies north of the Stanis tions, contains varied p~bbles, including limestone. It laus River. The formation_extends nor_th_w_estwa:r:d"'and apparently lies conformably below the Brower Creek p_r_Qbahlvoutheastward be ond the area studied, but volcanic member of the Mariposa formation. The thin the stratignmhic relations in____the_s_outhern pa_rt o the ar__e_a studie_d___ar_e_]es~a r The formation underlies bedded black chert and slate of unit 10 on Jackson and is named fQr___QoJ?JLer Ridg_~, a prominent topo Creek, exposed immediately west of the conglomerate, gr.aphic feature near th_(UY_es_Lside_oLthe e;w-~_ AGE ~ tiin limestone fra~ts.= No fossils have been found in the Gopher Ridge vol Dark-~ydimes~ns,e.s=-Qn~tod:.wo-=feekthiclr canics. They are probably of Late Jurassic age, ~d_a fe.w___fee.t.Jo.n~~=B1s.Gk=q,uar.tzose_s!_ate-.ar~- cterhedde SALT SPRING SLATE FOSSILS AND AGE The name Salt Spring slate is here used for the dom The Salt Spring slate is of Late J u!assic ag~. Fos ~aiitli epiclastic rocks that overlie and intertongue sils are rare in the Salt Spring slate and the few found with the Gopher Ridge volcanics. The for_mJ~tion1S previously are not diagnostic. During the present in named for Salt Spring Valley, whlch lieaimm..e.d.iately vestigation, fossils were found at four localities, but eas_LoLQouher Ridge (pl. 1). Exposures of the for fossils from only one of these have been dated. In col mation are poor in Salt Spring Valley and the section lections by Clark and R. W. Imlay (Mesozoic localities exposed in the Cosumnes River near the Michigan Bar 25638 and 27318, respectively in U.S. Geological Sur Bridge (pl. 8) is designated as the type section. Black vey collections) from the south side of the Cosumnes sericite slate dominates in the formation, but gray River, 0.1 mile east of the Michigan Bar Bridge, in the wacke and tuff are widespread and thin conglomerate SE:t;4 sec. 36, T. 8 N., R. 8 E., Imlay (written communi layers occur in some places. The SalLS:wing'=Sla.tS=is cation, 1959) identified ?Buchia concentrica (Sow of abo_!!t t,he same ag~ as the Mariposa formation, but erby), "Belewnites" sp., and Phylloceras sp. !!_e the_t\~9.- fq~·~~a~i()nS may have oeenueposited in sepa;rate C()nsidere(Lthem diagnostic of_late Oxfor.dian_t_o-=earJy ~asins, Jtssug_g_estedl5y Turner ( 1894b, p. 457). J(immeri dgiaJJ ag~ The Salt Spring slate iseij)Osed in a continuous belt, Belemnite molds were found in the Salt Spring slate throughout the length of the area mapped, according near Don Pedro reservoir on the Tuolumne River, and 724-276 0-65-5 30 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPIDC BELT part of a plesiosaur ··skeleton was found on Lake Mc 'Y.ith the Salt_Sp.ri.ng slate. It is named for the inactive Clure, on the Merced River. The belemnite molds were Copper Hill mine, in the NE14 sec. 34, T. 8 N., R. 9 E., found in slate in the south part of sec 35, T. 2 S., R. in Amador CountJ:,_The type section of the formation 14 E., below the high-water line on Don Pedro Reser is on the Cosumnes River (pl. 8), in Amador and El voir. The plesiosaur skeleton was found at the high Dorado Counties. The Copper Hill volcanics are water mark of Lake McClure on the south line of sec. 1, mainly pyroclastic rocks, probably mostly andesitic. T. 4 S., R. 15 E., enclosed in coarse-grained black clastic The formation extends from north of the Cosumnes limestone that forms a lens about 1 foot thick in the River to south of the Stanislaus River within and west black slate. The skeleton has not yet been studied~ of the Bear Mountains fault zone; volcanic rocks south 'MERCED FALLS SLATE ward as far as the Merced River are included, but this correlation is less adequately supported. The Copper The name Merced Falls slate is here used for the epi Hill volcanics are truncated on the east by faults and clastic rocks shown by Turner {1897) in the belt ex the top of the formation is probably not preserved in tending southeastward from La Grange to beyond the the area. The formation is exposed along the Stanis southern boundary of the area (pl. 1). The type sec laus River and all rivers to the north. The best ex tion is along the Merced River east of the town of posed and most complete section is along the Cosumnes Merced Falls (see pl. 11). The formation is well ex River, but exposures are also excellent along the Moke posed along the Tuolumne River and weathered ex lumne River. posures are plentiful on the north side of the Merced The Copper Hill volcanics were mapped as diabase River. The top of the formation is not preserved but and amphibolite by Turner ( 1894a) and by Lindgren about 5,000 feet of strata form the type section. The and Turner ( 1894) . They were placed in the Logtown Merced Falls slate overlies and intertongues with the Ridge formation by Taliaferro (1943b, fig. 2), and by Gopher Ridge volcanics. It apparently underlies and Taliaferro and Solari ( 1949) . The part near. Copper intertongues with the Peaslee Creek volcanics. opolis was tentatively assigned to the Amador group LITHOLOGY by Heyl {1948b, p. 99). Parts of the Copper Hill vol Near the Tuolumne River the formation is almost en canics near the Newton mine, about 6 miles west of tirely thin-bedded dark gray slate and siltstone, and J a·ckson, were named the Mountain Spring volcanics only one thin bed of graywacke was observed. Along and Newton Mine volcanics by Heyl and Eric {1948, the Merced River, however, it includes considerable p. 52-53), and were tentatively correlated with the graywacke and some felsic tuff. Graywacke is inter Amador group {1948, p. 51). bedded throughout the Merced River section, but is Partial sections show that the volcanic rocks of this coarser grained and forms thicker beds in the eastern unit are more than 7,000 feet thick on the Cosumnes upper part of the formation, where tuff is also common, River, and perhaps equally thick on the Stanislaus than in the western lower part. Rare thin beds of River, although here the rocks are more strongly foli conglomerate occur south of the Merced River. a ted so that the thickness is less certain. The formation is more than 3,000 feet thick on the Mokelumne River. AGE In the Calaveras River, the ·formation is strongly schist L. E. Mannion of Stauffer Chemical Co. (oral com ose and the thickness cannot be measured. munication, 1954) found Buehia in the Merced Falls slate about 1.5 miles southeast of La Grange in the LITHOLOGY south part of sec. 21, T. 3 S., R. 14 E. The present The Copper Hill volcanics are moderately deformed writer found none, but this genus indicates that the in the northern part of the area, but mucfi'O"'fliie forma beds containing it are not older than late Oxfordian, tion is strongly sheared near the Calaveras RIVer and as Buchia is unknown in older beds (Ralph W. Imlay, to the south. In some of the strongly sheared rocks written communication, 1957). the primacy textures and structures are obscure or com Belemnite molds, not adequate for dating, were pletely destroyed. In the northern part of the area, found in black slate about 20 feet west of the contact pyroclastic rocks form most of the formation, and tuff between the slate and the Gopher Ridge volcanics, near is more abundant than the coarser pyroclastics. Pre served textures suggest that similar proportions exist the top of the penstocks of the La Grange power farther south. Minor pillow lava is interbedded near plant (fig. 10). the Cosumnes and Stanislaus Rivers. COPPER HILL VOLCANICS Most of the Copper Hill volcanics are dark to medium The name Copper Hill volcanics is here..u~:fi(tr-ehe green In color, without..il.u.ax±z-p1l.ewlCJ.:Y.at5., and are sequence of volcanic rocks that overlies and intertongues probably andesitic o;fi basaltic. Porphyritic rhyolite STRATIGRAPHY 31 or dacite with quartz phenocrysts occurs on the Cosum~ other volcanic sequences. The southwestern higher nes and Mokelumne Rivers; some of it is massive and part of the unit consists of massive rocks without either of hypabyssal or volcanic origin. Other parts on amygdules but with small feldspar phenocrysts and the Cosumnes River (unit 21, pl. 8), are bedded and hardly with quartz phenocrysts. Most of the massive show pyroclastic textures and are certainly extrusive. rocks are fresh-appearing and may be of hypabyssal A distinctive or hyritic lava containing green saus origin, perhaps appreciably younger than the bedded suritize p agwclase phenocrysts as muc 1 as 8 mm ong pyroclastic rocks in the northeastern part of this belt. in an aphanitic groundmass is interlayered with the AGE more common yolcanic rock t.y.pes in the Cosumnes Rix,er (unit 25, pl. 8). Some of this forms pillow lava, No fossils have been found in the Peaslee Creek vol with pillows as much as 5 feet in diameter. The di canics, but its stratigraphic relations to the Merced ~on..oLto.ps indicatedoy the iiillQiY.s..is-consis.t&nt Falls slate indicate that it is of Late Jurassic age, and probably Oxfordian or younger. with that indicated b~jacent graded beds. This p.QI.p.h.y.r.y_is..extrusiye,Jmt..similaL-.I:ocks-foml.. · ·n STRATIGRAPHIC RELATIONS BETWEEN THE FORMATIONS OF the Cosumnes and Logtown Ridge forroations. and in THE WESTERN BLOCK t~ype section of the A rna do~g:z:o.up_alo.ug_the STRATIGRAPHIC RELATIONS BETWEEN THE SALT SPRING SLATE AND Cosunmes River (pl. 10). West of Amador CitY-_!t GOPHER RIDGE VOLCANICS sheet of simila-r rock, apparently a sill (Knopf, 1929, Stratigraphic relations between the Salt Spring slate p. 17), is interiayered in greenstone of either the Log and the Gopher Ridge volcanics are shown best near town Ridge formation or the Brower Creek member the Cosumnes River. Moreover, this section is the only of the Mariposa formation. one where facies changes normal to the strike can be AGE examined. Less complete data in the rivers as far No fossils-ha.v.e..b..e.en..fo.und..in_th.e_.QQ.JWer Hill vol outh ·as the Stanislaus supplement those in the Cosum canics. The lower part of the formation intertongues es River, and show that the Gopher Ridge volcanics with the Salt 'Spring slate and may thex:~e..b..e. as 111}derlie and intertongue with the Salt Spring slate. old as late Oxfordian. Higb&LJ>arts of the formation ~ar the Tuolumne River, however, some of the epi- ar._e probablY. of IGmmeridgian age or youn~t i csJ.astic rocks previously interpreted as continuous with a arently the youngest formation of the bedrock com those in Salt Spring Valley apparently underlie· the plex in the area st' g c.anic rocks Gopher Ridge volcanics. In the Merced River, the con sou.th..oi.La..Gxange..o.r..the.sla.tQ.ex.p.osed.east. of Merc.e tact between the main belts of these formations is Falls may conceh~ahly..he.3.oung&- faulted. The more significant sections are described below. PEASLEE CREEK VOLCANICS Oosumnes River ewposures.-The Gopher Ridge vol The name Peaslee Crook volcanics is given here to canics and Salt Spring slate are exposed near the west an isolated mass of volcanic and possible hypabyssal ern end of the Cosumnes River section and are repeated rocks, at the western edge of the area of exposed bed by folding east of the Michigan Bar Bridge (pl. 8). rock south of La Grange. The type area is about 2 Bedrock is concealed near the contact between the two miles south of La Grange. These volcanics were formations east and west of the Michigan Bar Bridge. mapped as porphyrite by Turner (1897). The base of Enough observations can be made of tops of beds, bed the formation, lying on the :Merced Falls slate, is ex ding attitudes, a~d kinds of rock to show that the epi posed immediately south of the Tuolumne River (fig. clastic rocks are conformable with the volcanic rocks 10) but its top is not preserved. The thickness of the and that there is little if any interbedding. No slate formation as exposed is uncertain, but is probably more body of comparable thickness occurs on the e'ast limb of than 3,000 feet. Its structure is shown diagrammati the anticline east of Michigan Bar Bridge, and the cally on plate 4. approximate stratigraphic equivalent of the slate at LITHOLOGY Michigan Bar Bridge is interbedded slate .and pyro The Peaslee Creek volcanics are not exposed in the clastic rocks. Gross aspects of the interbedding are Tuolumne River and rela,tively poor exposures about shown on plate 8, but the two rocks are also much more 1 1nile south of La Grange were examined to complete thinly interbedded, and exposed contacts are common. the Tuolunu1e River section. Here, bedded pyroclastic Here bedding dips consistently eastward and graded rocks form the n@rtheastern and lower part of the unit, beds .are abundant. Although precise correlations can and consist of tuff and volcanic breccia of probable maf not be made between the two limbs of the anticline, ic or intermediate composition, similar to those of the the stratigraphic interval corresponding to the epi- 32 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT clastic sequence at Michigan Bar Bridge almost cer than infolded, with epiclastic rocks. Small folds, such tainly lies within the more than 12,000 feet of strata as are common where rocks of differing competency are on the east limb of the anticline. tightly folded are rare or absent. The tuff and epi The contrast between the lithology in the syncline at clastic rocks alternate with each other in layers ranging Michigan Bar Bridge and that on the east limb of the down to less than one inch in thickness so that the units anticline indicates that the Salt Spring slate inter shown on plate 5 are generalized. tongues eastward with volcanic rocks. In the Co Tuol!utmtne River ewposures.-Exposures near the sumnes River area, the volcanics interbedded with slate Tuolumne and Merced Rivers contribute little to an un on the east limb of the anticline have been divided derstanding of the relations between the Gopher Ridge arbitrarily between the Gopher Ridge volcanics and volcanics and the Salt Spring slate to the east. Near the Copper Hill volcanics. The outcrops suggest that the margins of Don Pedro Reservoir on the Tuolumne the Salt Spring slate may be absent and the two River (pl. 4), epiclastic rocks on the trend of the Salt volcanic formations in contact with one another at Spring slate are cut out by a plutonic body. Those be depth at the latitude of the Cosumnes River. tween the pluton and Don Pedro Dam, tentatively in Mokelumne River ewposures .-Near the Mokelumne cluded in the Salt Spring slate, project southwestward River, the Salt Spring slate forms three belts in a part into the belt of the Gopher Ridge volcanics and appar of the section that is interpreted to be homoclinal with ently underlie at least part of them. An apparently younger beds on the east. Each belt of slate must depositional contact between the two is exposed in the therefore be a tongue in the volcanic rocks. The vol NE% sec. 2, T. 3 S., R. 14 E., where the rocks are folded canic rocks at Pardee Dam have been included arbitrar on a small scale but are not much sheared. The part of ily with the Gopher Ridge volcanics. The volcanics the volcanic section that appears to be structurally con at the dam are not continuous either northward or formable with the epiclastic rocks immediately north south ward according to Turner ( 1894a), and might be east of the dam extends southwestward from the dam a volcanic member within the Salt Spring slate, or a for nearly a mile and is at least 700 feet thick. Volcanic tongue of the Copper Hill volcanics. Thin layers of rocks farther west are separated from this part of the tuff interbedded in dominant slate units and thin layers section by shear zones and possible faults and their of slate interbedded in dominant pyroclastic units are relations to the rocks nearer Don Pedro Dam are less well exposed in the spillway and river canyon immedi certain. ately west of Pardee Dam. Again, only gross relations As suggested on plate 4, the epiclastic rocks northeast could be mapped. The eastern belt of Salt Spring slate of Don Pedro .Dam may form the core of an anticline is poorly exposed around the margins of Pardee Res and be a tongue of the Salt Spring slate overlain by a ervoir and yields little stratigraphic information. tongue of the Gopher Ridge volcanics that includes Calaveras River ewposu11es.-Exposures of the Salt units 13, 16, 17, and 19. The tongue of epiclastic rocks Spring slate in the Calaveras River are generally poor would correspond approximately to the lower part of but top determinations within both the slate and the the slate in Salt Spring Valley, and the tongue of pyro Gopher Ridge volcanics near the western contact of the clastic rocks to the upper part of the Gopher Ridge slate indicate tops towards the east so that the slate volcanics southeast of the Stanislaus River. However, overlies the Gopher Ridge volcanics. Near Hogan if the fault extending toward the northwest corner of Dam the structure has arbitrarily been interpreted as T. 3 S., R. 15 E. (pl. 1) continues into the approximate homoclirial with· the volcanic rocks forming a tongue axis of the inferred anticline, epiclastic rocks southwest of the Gopher Ridge volcanics, but definitive structural of the fault might be appreciably older than the Salt data a~e lacking. Spring slate. Stamislaus River ewposures.-The belt underlain by Merced River ewposures.-Near the Merced River the Gopher Ridge volcanics is wide, and the belt under epiclastic rocks tentatively placed in the Salt Spring lain by the Salt Spring slate is narrow, south of the slate are exposed on the shores of the northwest-trend Stanislaus River. Near the river the map pattern (pl. ing arm of Lake McClure and west of Exchequer Dam. 5; Taliaferro and Solari, 1949) indicates that the two Of these, only those west of the dam are known to be formations intertongue or are infolded. Graded beds in depositional contact with the Gopher Ridge volcanics. are scarce and exposures do not certainly show the rela The epiclastic rocks on the shores of Lake McClure tions between the · two formations. Taliaferro and and in railroad cuts on the east side of the lake are on Solari (1949) interpreted the pattern as being the result the extension of the belt of epiclastic rocks that under of tight folding, but details within single exposures in lies Salt Spring Valley. No long uninterrupted sections dicate that pyroclastic rocks are interbedded, rather across the strike are available in this vicinity and STRATIGRAPHY 33 scattered bedding attitudes and top determinations in of an anticline (fig. 10}. The Merced Falls slate thus dicate that the rocks in this area are folded. At least probably overlies the Gopher Ridge volcanics and part of the contact between these epiclastic rocks and underlies the Peaslee Creek volcanics. The western the Gopher Ridge volcanics to the west is faulted as contact of the slate is not exposed near the river, but well. the eastern contact is well exposed at the La Grange The epiclastic rocks west of Exchequer Dam (units powerhouse (fig. 10). Neither this exposure nor a 6-8, pl. 3) form a syncline in· the Gopher Ridge vol smaller one in the canal on the north side of the river canics and are probably separated by a fault from the indicate the stratigraphic relations although they sug· epiclastic rocks that underlie the western part of Lake gest that the two formations are conformable. Below McClure. The western contact of the epiclastic rocks the headgates of the penstocks, the massive volcanic is not exposed, but about one foot of the eastern con rocks east of the slate form a vertical face, and the slate tact is exposed immediately below the road. Here, un is exposed in a gully at the base of the face. The vol- disturbed slate is in contact with a layer of massive, canics are not bedded, but bedding in the slate parallels amygdaloidal, nonporphyritic lava about 25 feet thick. the surface of the volcanic rocks. Within a few feet Graded beds in the tuff immediately east of (below) the of the contact, fragments of volcanic rock are present lava indicate that tops are to the west and that the slate in the slate-they might be interpreted as either the overlies the lava. Graded graywacke beds in the west first or the last small episode of volcanism related to ern part of the slate belt indicate that the graywacke and the accumulation of volcanic rocks east of the slate. associated slate overlie the volcanics to the west. In the bank of the canal north of the river a similar STRATIGRAPHIC RELATIONS BETWEEN THE SALT SPRING relation is shown. Here, a layer of amygdaloidal vol SLATE AND COPPER HILL VOLCANICS canic rock about two feet thick is interlayered with Stratigraphic relations between the Salt Spring slate pumice-bearing slate near the contact of the slate with and Copper Hill volcanics are shown best on the the volcanic sequence. Cosumnes River. The contact between the two forma Abundant graded bedding between tightly folded tions on the Mokelumne and Calaveras Rivers is poorly areas in the slate east of Merced Falls shows that tops exposed and top determinations have not been made are consistently westward in noncrumpled parts of the belt (plate 11} • This is consistent with the gently nearby. The Salt Spring slate and Gopher Ridge vol 1 canics are exposed near, but not at, the contact on the plunging minor folds in the westernmost slate belt ex Stanislaus River and top determinations have not been posed in the Tuolumne River. At the eastern side of made near the contact. On the Tuolumne River, the the belt underlain by the Merced Falls slate, the rocks formations are separated. by intrusive rocks and near are strongly crumpled, and the volcanic rocks with the Merced River they are separated by a fault. which they are in contact are schistose, suggesting that On the Cosumnes River, the volcanics interbedded the two formations may be in fault contact. The slate with epiclastic rocks on the east limb of the anticline belt becomes narrower," and the Gopher Ridge volcanic east of Michigan Bar Bridge have been divided arbi belt wider, northeast of Merced Falls. This probably trarily between the Gopher Ridge volcanics and the results from intertonguing of the slate with the vol Copper I-Iill volcanics. The stratigraphically lowest canics, for the most southerly projection of volcanic unit included with. the Copper Hill volcanics is unit 18, rock extends nearly to the Merced River, and tops are plate 8. vVithin this unit and the epiclastic units on the to the west in the epiclastic rocks on both sides of this two sides graded beds are common and exposures are projection. In addition, thin tuff beds are commonly excellent, showing an interbedded, conformable rela interbedded with the slate and graywacke near the tion. Graded beds were not found for several hundred tongue. feet stratigraphically above the uppermost epiclastic ROCK AND MINERAL FRAGMENTS IN ROCKS OF unit (unit 19, plate 8). JURASSIC AGE STRATIGRAPHIC RELATIONS OF THE MERCED FALLS SLATE TO Field examination of conglomerate pebbles was ·made THE PEASLEE CREEK VOLCANICS AND GOPHER RIDGE at about 50 localities and laboratory study was made VOLCANICS of 34 thin sections of graywacke and fine conglomerate. No graded beds were found in the Merced Falls slate More information is available from the Mariposa and along the Tuolumne River, but 1ninor folds near La Cosumnes formations than from other units which con Grange suggest that the slate is on the western limb tain fewer graywacke and conglomerate beds. c.,.., ~ EXPLANATION .; J"iC]pc Peaslee Creek volcanics ·~ \ ~ ~::JmJ~ ·., .. ~ :;;;; .... a .... ~ Merced Falls slate ~ 500' ~ ~ > Approximate altitude f~ :j .· of river surface - ~ 0 Gopher Ridge ~ volcanics SEA LEVEL r; / Contact ~ Dashed where approximately located 500' ~ Fault t:=' I r.ll Anticline, showing crest line 1-3 ~ ~ t Syncline, showing position of trough ~ \ -3--+ 38°40' (...... j ~ Bearing and plunge of t;l minor anticline ~.5 [/l Bearing and plunge of H minor syncline t;l ~ ~35 Bearing and plunge of minor folds, ~ showing trace of bedding > 70_.Y Strike and dip of beds ~ Overturned beds on section a.re not ~ distinguished as such on map because t:=' evidence(minor folds) is indirect > •y Strike of vertical beds a:: t;l ~ -17~- Strike and dip of cleavage 1-3 •y 120°27'30" Geology and base mapped by L. D. ~ Clark and Richard Pack, 1953 Strike of vertical cleavage 0 ~ ~ '"d Bearing and plunge of inter section of bedding and cleavage ~ 500 0 500 1000 1500 FEET 0 DATUM IS MEAN SEA LEVEL t::d APPROXIMATE MEAN t;l t"4 DECLINATION. 1962 1-3 FIGURE 10.--Geologic map and cross section showing structure of slate along Tuolumne River east of La Grange, Stanislaus County, California. STRA'IIIGRAPHY 35 COMPOSITION AND SHAPE OF FRAGMENTS nearly all well rounded, but they range widely in de The pebbles are simila,r in all formations studied and gree of roundness. Degree of rounding is not every consist largely of volcanic rocks, shte, and chert, but where directly related to hardness; some beds contain those of quartzite and high-grade metamorphic rock are both well-rounded pebbles of vein quartz, and angular ubiquitous also (figs. 11, 12). Pebbles and boulders of pebbles of chert, quartzose phyllite, and volcanic rocks. felsic plutpnic rocks have been found in the Cosumnes Carbonate rock fragments occur only in the Cosumnes and Mariposa formations, and some sand grains in the and Mariposa formations. They are ·chiefly calcarenite Salt Spring slate may have been derived from the same containing much rounded organic detritus, including rocks. Although the various formations are alike in abundant crinoid fragments. Two fragments of oolitic composition 9f fragments, proportions between various limestone were found in coarse breccia consisting mostly rock types in adjacent beds of single formations vary of volcanic rocks in the western part of unit 28 on the strikingly, even in beds of similar grain size. More Calaveras River (pl. 6). A large boulder of fusulinid over, slate is less abundant and chert and quartz are limestone of Permian age was mentioned previously more abundant in graywacke than in conglomerate. (p. 14). The metamorphic rocks include quartz-mica Most of the Jurassic rocks are in the greenschist or schist, nonmicaceous aggregates of strongly strained higher metamorphic facies, and most fragments derived quartz, and crinkled sericite slate. Granitic detritus from terranes of lower metamorphic grade have now is rare in all the conglomerates. In addition to frag been altered to the same metamorphic grade as the ments of coarse-grained rocks composed of quartz and enclosing rock. potassium feldspar, separate grains of untwinned feld Pebbles in the conglomerate are well rounded to angu spar and one grain showing a micrographic intergrowth lar. In some beds nearly all are angular, in others of quartz in feldspar were observed. FIGURE H.-Photomicrograph of very coarse graywacke from Cosumnes formation, Cosumnes River, NW1,4 sec. 22, T . 8 N. , R. 10 E., showing mixture of detritus of volcanic, metamorphic, and sedimentary rocks. Chert with carbonate spicule ( ?) , C,; schistose quartz sandstone partly replaced by carbonate, Q; chert with scattered carbonate, C,; granular limestone with scattered em bayed quartz grains, L; scoriaceous mafic volcanic rock, sv; pyroxene, p ; chlorite, ch; mafic volcanic rock, mv. Plain light. 36 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT FIGURE 12.-Photomicrograph of very coarse graywacke from Mariposa formation, SE1,4 sec. 16, T. 3 N., R. 12 E., showing association of detritus from diverse terranes. Chert with carbonate rhombohedrons, C; quartzite ( ?) , Q; pyroxene, P; porphyritic mafic volcanic rock, mv ; slate with quartz veinlet, S; devitrified volcanic glass ( ?) , G; chlorite pseudomorph, ch; microcrystalline epidote rock, E. Plain light. Pebbles and boulders of granitic rocks are rare hut slate also contain more undefcmned quartz than those occur as fragments in the conglomerates of the Cosum of other formations. The Salt Spring slate overlies nes and Mariposa formations; they have not boon found and intertongues with the Gopher Ridge volcanics, in the Salt Spring slate or Merced Falls slate, where which contain more felsic volcanic and hypabyssal rock conglomerates are less abundant. Most of these than the other volcanic formations. are fine-grained aplite or slightly coarser. In the Mari Pebbles of coarse quartz-mica schi~t occur in the posa formation pebbles of medium-grained granitic Cosumnes and Mariposa formations, and books of mus gneiss occur in conglomerate on the south flank of the covite, probably derived from aTeas of high-grade meta ridge topped by Fowler Lookout, and Knopf (1929, morphic rocks, are widespread in all the epiclastic for p. 13) reports a pebble of coarse diorite near the con mations. Fragments of fine-grained quartz schist tact of the Mariposa formation and the Penon Blanco containing very little mica is sparsely present in the volcanics "one-half mile west of Kittridge" in the Mariposa· formation and Salt Spring slate. canyon of the Merced River. Fragments of chert and quartzose slate are ahundant Untwinned feldspar, in part perthitic, is common in in the graywackes and conglomerates. The cherts graywJLckes of the three epiclastic formations of Meso range in color from nearly white to nearly black as well zoic age, and granophyre clasts are widespread. They as several shades of green, and very rarely red. The are probably derived from felsic volcanic or hypabyssal chert varies in purity and textUTe. The silica in nearly rocks as they are most abundant in exposures of the all the chert is microcrystalline quartz, but chert con Merced Falls slate east of Merced Falls. Here, felsic sisting of opal or chalcedony occurs in graywacke of tuff of the Peaslee Creek volcanics intertongues with the Mariposa formation west of Angels Camp and near the Merced Falls slate. Graywackes of the Salt Spring the Tuolumne River. Chert, mostly phyllitic, with STRAT-IGRAPHY 37 abundant finely disseminated carbonaceous material Sandstones with a carbonate martix show similar varia was found in all formations, but in a larger proportion tions between individual grains in proportion of quartz of thln sections of the Salt Spring slate. Chert with to feldspar. The carbonate of the matrix is coarse scattered carbonate rhombs was observed in several thln grained and generally twinned. Recrystallization has sections of the Mariposa formation and Salt Spring destroyed many of its original features. slate. Chert, in part phyllitic, with ovoid structures ORIGIN OF LIMESTONE BLOCKS (Radiolaria~) occurs in all formations. Isolated small masses of limestone, surrounded by Nearly all of the clast-forming slate and siltstone other rocks, are widely distributed in the region studied. resembles at least superficially the rocks with which Some are lenses formed in place in the enclosing rocks the graywackes and conglomerates are interbedded. and others are probably horses in faults. However, Some slate and siltstone clasts, however, contain abun other blocks were probably emplaced into younger beds dant finely disseminated black material, probably car by sliding or slumping. Many of these lie in con bonaceous, which is absent from most of the Jurassic glomerates of the Mariposa formation ; some lie in slate rocks of the region. of the Mariposa formation. Contacts of the blocks are Fragments of volcanic rocks and minerals are abun rarely exposed but their shape and character is such dant in most of the graywackes and conglomerates (fig. as to indicate that they neither developed simultane 11) and in some form more than 90 percent of those ously with surrounding rocks nor were emplaced by megascopically visible. Most of these fragments are plastic flow; available evidence suggests that they were microcrystalline or porphyritic with a microcrystalline emplaced by slumping during deposition of the sur groundmass. Their present texture, except for the rounding beds. Modern subaerial mudflows contain phenocrysts, results largely from the regional metamor ing boulders 40 to 50 feet long have been described phism which has affected the Jurassic rocks, and they by Blackwelder (1928, p. 471) and much larger blocks may have been derived either from volcanic glass, lava, emplaced by slumping in other sediments have been or both. The phenocrysts are pyroxene and sericitized described by King ( 1937, p. 66-68, 89-92), by Renz, plagioclase, and these minerals also form discrete grains Lakeman, and VanDerMeulen (1955), and others. in some of the graywackes. Limestone masses are commonly grouped. Two Quartz is abundant in most of the graywackes and groups of limestone blocks have been mapped by A. A. conglomerates, but adds little data about source ter Stromquist (written communication, 1952) on the San ranes. Both white and gray vein quartz have been Andreas northwest quadrangle, in the western belt of distinguished in the conglomerates of the Cosumnes and the Mariposa formation near the Calaveras River. Mariposa formations, but vein quartz has not been dis One group (in strike with unit 23, plate 6) is about tinguished from quartz of other origin in thin sections :14 mile northwest of the river in sec. 15, T. 4 N., R. 11 of graywacke. Many of the quartz grains seen in thin E. and as much as 1;2 mile southeast of the river in section are strongly deformed aggregates that may have sec. 22, T. 4 N., R. 11 E. Another group (in strike been derived from quartzite, veins, or granitic rocks. with unit 28, figure 26) occurs in the SW:l4 sec. 14, T. Nearly all the quartz shows strain shadows and 4 N., R. 11 E. within 14 mile both northwest and south vacuoles that are commonly arranged in planes. east of the river. Sandstone fragments (figs. 11 and 12):· occur in all three formations but especially in the Cosumnes forma In unit 28, plate 6, contact relations of smaller lime tion. They are of three sorts: orthoquartzite, or quartz stone blocks, about 5 feet and less in diameter, are ex sandstone with quartz cement (Pettijohn, 1957, p. 295- posed in two places. One block is in the previously 296), gi·aywacke, and sandstone having a carbonate mentioned 14 railroad cut in sec. 10, T. 4 N., R. 11 matrix. The orthoquartzite ranges from fine sandstone E., where fusulinid limestone of Permian age is to fine conglomerate, and consists of well-rounded surrounded by slate and conglomerate of the Mariposa quartz grains, many of which show overgrowths, but formation. A narrow zone of gouge surrounds the are not markedly sutured. Most of the component limestone and can be traced from the bottom to the top quartz grains show undulatory extinction and lines of of the cut. The rocks on the two sides o£ the zone vac"\t:>les. The graywacke consists of angular grains of are similar, however, and the displacement along the quartz or quartz and feldspar in a sericite matrix fault marked by the gouge is probably minor. On the which commonly contains a minor amount of carbonate. north bank of the Calaveras River (plate 6), near the The proportion of quartz to feldspar ranges widely, west side of unit 28, is a very coarse sheared breccia half of the grains in some clasts being feldspar. Po consisting largely of porphyritic andesite like that of tassium feldspar is more abundant than plagioclase .. the Brower Creek volcanic member to the east but con- 38 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT taining scattered blocks 6 feet and less in length of localities described above. Several of the blocks con limestone and bedded chert. tain fossils but the visible original textures indicate. that Another group of limestone blocks, within the. Me the limestones are bioclastic deposits rather than reefs. lones fault zone in the. NW%, sec. 22, T. 3 N., R. 12 E., SOURCE OF FRAGMENTS about 7 miles slightly east of south of San Andreas, has been mapped by D. B. Tatlock (written communica Some of the fragments in the Cosumnes and Mari tion, 1952). These are surrounded by strongly sheared posa formations and the. Salt Spring slate were appar conglomerate in which the less resistant rock frag ently derived from older formations exposed in the ments are much elongated. The fragmental rock is Sierra Nevada, and others from nearly contemporane probably not a tectonic breccia, as the. fragments in ous beds in or near the Upper Jurassic basin of deposi clude a considerable variety of rocks including widely tion of the western Sierra Nevada. Derivation from scattered granitic rocks. sources west and northwest of the Sierra Nevada has A group of limestone blocks exposed near the Stan been neither established nor disproved. No evidence islaus River in sec. 26, T. 2 N., R. 13 E. (pl. 5) was has been found by the writer to support Taliaferro's mapped by Eric and Stromquist (in Eric, Stromquist, ( 1942, p. 89) contention that sediments of Jurassic age and Swinney, 1955, pl. 1). A block composed of lime become coarser westward. Furthermore, when consid stone on the west and carbonaceous quartz-mica schist ering deposits transported by turbidity currents and on the east is distinguished as unit 46 on plate 5. The mudflows, the site of deposition of coarse material pro contact of the schist with conglomerate on the east is vides no reliable evidence of its source, as coarse detritus poorly exposed. On the west, the limestone is in fault carried down submarine canyons may bypass finer de contact with pyroclastic rocks of the Brower Creek posits laid down on the continental shelf on either side volcanic member. The limestone and schist may be. a of the canyon. tiny remnant of the basement on which the Mariposa Other than the region of the Sierra Nevada, con formation was deposited, fortuitously exposed on the ceivable sources include the sites of the present Klam present land surface. Relations of other limestone ath Mountains, California Coast Ranges, and Central masses in the area are not clear. Valley. The eastern part of the Klamath Mountains, The limestone blocks at the localities mentioned are about 200 miles northwest of the Cosumnes River, may associated with conglomerate. Of the group west of have been exposed during Late Jurassic time. It is San Andreas a limestone block has been found to be in about on the projection of the strike of the Upper contact with conglomerate only at the railroad cut ex Jurassic rocks under discussion. The strike of the Up posure, but lenticular conglomerates are in strike with per Jurassic rocks may be parallel to the axis of the the beds surrounding the limestone blocks in the two basin in which they were deposited but some debris groups west of San Andreas. Conglomerate surrounds laden currents probably move parallel to the axis of a the limestone southwest of San Andreas and overlies basin of deposition as well as across it. Common rocks the exposure in the Stanislaus River. exposed in the eastern Klamath Mountains include The shape and size of the limestone blocks, together mafic, intermediate and felsic volcanic rocks, chert, with their internal texture and structure and their rela shale, graywacke, and intrusive rocks ranging in com tions to surrounding rocks, are not consistent with the position from ultramafic to granitic. Chert and quartz hypotheses that they were. formed in place or were em mica schist are not abundant. placed by faulting. The limestone blocks range in Taliaferro ( 1942, p. 103) believes that the site of the shape from very elongate to nearly equidimensional. California Coast Ranges was a land mass during depo They apparently range in size from the smallest pebbles sition of the Amador group and the Mariposa forma~ of the conglomerate to blocks more than 100 feet long. tion. In that region, sedimentary rocks shown to be Bedding is not apparent in most of the blocks. Breccia older than the Mariposa formation are rarely exposed structure is common, but whether it is of epiclastic or except in the Santa Lucia Range, about 100 miles south cataclastic origin was not determined. No conglomer west of the Merced River. These rocks, the Sur series, ate or other rock is interbedded with the limestone of consist of "quartzose schists and gneisses, quartz-biotite the individual blocks, nor are there tongues of lime schists, marbles, and plagioclase amphibolites" (Reiche, stone extending into surrounding rocks. Although 1937, p. 118) and quartzite (Trask, 1926, p. 127). They some limestone blocks that are apparently horses in may have been more widely exposed in the Coast faults are coarsely crystalline and have planar struc Ranges during Late Jurassic time. On the other hand, tures suggestive of great deformation and plastic flow, Hill and Dibblee ( 1953, p. 449), suggest that displace these features have not been found in the blocks at the ment of pre-Cretaceous rocks by the San Andreas fault STRATIGRAPHY 39 may total 350 miles since Jurassic time, so that these carenite with abundant organic debris. In addition, rocks may not then have been in their present position. Permian fossils have been identified in one limestone Well-borings in the Central Valley provide some in boulder in the Mariposa formation (p. 14). Clasts formation on the basement rocks but are too widely of black carbonaceous slate, phyllite, and chert strongly spaced in most plac"es to provide complete information. resemble rock types that are abundant in the chert unit The well borings have penetrated slate, amphibolite, of the eastern belt of the Calaveras formation and may schist, granitic rocks, serpentine, and quartzite, sug well have been derived from this unit. gesting that the basement rocks are similar to those ex The granules and pebbles of orthoquartzite, gray posed in the western Sierra Nevada (Taliaferro, 1943b, wacke, and quartz-.feldspar-carbonate sandstones may p. 129). have been derived from the Blue Canyon formation, ex Evaluation of the possible sources is hindered by posed in the Sierra Nevada in a wide belt extending factors other than the extensive cover of post-Mariposa beyond the north and south limits of the Colfax 30- strata. There is insufficient knowledge of the petrog minute quadrangle (Lindgr.en, 1900). All these rocks raphy and composition of both the transported frag are in the Blue Canyon formation, including the unusual ments and possible source rocks in place and the pos sandstone consisting of angular quartz grains in a seri sibility that some rocks exposed during Late Jurassic cite matrix. Of the rocks already discussed, exposures time may have since been either removed by erosion or of all but the calcarenite of the Calaveras formation are emplacement of plutonic rocks, or metamorphosed be east of the exposures of the Upper Jurassic formations. yond recognition. Large parts of the Klamath Moun However, these rocks may also have been exposed west tains and Sierra Nevada have not been mapped in of this belt during Late Jurassic time. The abundance detail and possibly not all the distinctive lithologic of calcarenite boulders and blocks, however, indicates varieties in these areas are known. that the source of the calcarenite was close to the site of Some geologists, for example (Taliaferro, 1943b, p. deposition. 187) have suggested that areas now buried beneath the Fragments of slate, noncarbonaceous chert and vol sea, or younger deposits are more attractive sources of canic rocks may have been derived from penecontempo epiclastic rocks in northern California than exposed raneous or older Mesozoic beds in the western Sierra areas of older rocks, when these areas apparently do Nevada. These rocks might also have been derived in not include the lithologic types found in the clasts. part from the Calaveras and other formations of Paleo Nevertheless, erosion has removed much material from zoic age in the western Sierra Nevada, the Central Val the exposed areas and all the kinds of rocks exposed ley area, or the l{lamath Mountains. Many of the there during Late Jurassic time are not necessarily still fragments of volcanic rocks might be the result of represented. Moreover, the Paleozoic rocks now ex contemporaneous volcanism. posed in the Sierra Nevada, are almost certainly of The source of the granitic rocks, schist, and gneiss is higher metamorphic grade than the Paleozoic rocks ex unclear. Little can be added to Knopf's ( 1929, p. 19) posed during Late Jurassic time, as the batholith was thesis that the granitic pebbles were derived from since then emplaced and eroded to considerable depth. masses exposed within the area of this investigation. The metamorphism would probably affect the texture Other possible sources for these materials are the previ of much of the chert. The volcanic rocks are charac ously mentioned regions northwest and west of the teristically lenticular, and distinctive varieties orig Merced -Cosumnes River area. inally present in modest volume might have since BEDDING FEATURES AND CONDITIONS OF disappeared, owing to erosion and emplacement of DEPOSITION plutonic rocks, or their appearance may have changed owing to extensive recrystallization. Other rocks less The Cosumnes and Mariposa formations and the Salt notably lenticular might also have been removed or Spring slate have similar ·bedding features which indi altered beyond easy recognition. cate that all three were deposited in like environm·ents. Fragments of sandstones, and carbonaceous slate, The three volcanic formations are likewise similar to siltstone, and chert were probably derived from forma one another in their primary structural and textural tions exposed in the Sierra Nevada, and limestone clasts features. Besides, structures of some of the pyroclastic in the Cosumnes and Mariposa formations were de rocks are almost identical with those of the epiclastic rived from the part of the Calaveras formation that is graywackes and conglomerates and indicate that both represented by the western belt of exposures. Nearly kinds of rock were carried to the site of deposition in a all of the limestone in both the clasts and the masses be similar manner. Fossils in the Logtown Ridge and lieved to be in place in the western belt . is coarse cal- Mariposa formations and Salt Spring slate indicate 40 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT that the deposits ar~ marine; the Cosumnes formation, bed. The upper boundary of some coarse layers is well Copper Hill volcanics, and Gopher Ridge volcanics dif marked by the base of the overlying coarse layer or by fer from them in no way that would suggest a different an abrupt change to finer grain size. In other beds origin. Structures in the sands and finer sediments in the upper boundary of the coarse layer is ill-defined be dicate deposition in quiet water, but structures in coarser cause changes in grain size from sand to clay take place sediments are not diagnostic. Wide distribution of over appreciable parts of the beds. The sediments con graded beds in both epiclastic and pyroclastic rocks is stituting graded beds range from fine conglomerate to indicated by the maps of individual traverses. Current shale, but the complete range between these size grades ripple bedding (small-scale cross-beds), in places as seldom occurs in a single layer or unit. Pebble con sociated with minor cut-and-fill structures, is wide glomerate. forms the lower parts of rare graded layers. spread but not ·common. Load casts and crumples Most of the well-graded beds are 1,4 inch to 6 inches generally associated with contemporaneous deformation thick, but some well-graded beds are as thin as 2 mm or are also widespread but are only locally common. They as thick as 30 inches. The bottoms of most graded are strikingly exposed in the lower part of the Log beds are smooth, but some are irregular from minor town Ridge formation and the upper part of the Co channeling of the underlying finer material, form.ation sumnes fonnation in the Cosumnes River (pl. 8). of load casts, and from post-depositional crumpling. Flutes and grooves have not been observed. Compositional sorting is lacking in graded gray Most of the graywacke forms beds less than 3 feet wacke layers but is pronounced in some graded tuff thick but tuff is generally thicker bedded and some tuff layers. Thin sections indicate that such beds range and lapilli tuff layers several tens of feet thick show from crystal tuff to lithic or vitric tuff. Rock frag no internal discontinuities in grain size. At least some ments are microcrystalline, in part at least, because of volcanic breccias have no textural discontinuities metamorphism, and it is difficult at best to distinguish through stratigraphic intervals of several hundred feet. originally glassy from originally lithic material. Some beds of slate and very fine tuff are only a few Crystal fragments are much more abundant in the inches or millimeters thick. coarser lower parts of these beds and lithic fragments Graded beds have been described elsewhere (Petti are equally or more abundant in the finer upper parts. john, 1943, p. 949; Kuenen and Migliorini, 1950; and The original composition of the very fine tuff is uncer Shrock, 1948, p. 78-84) and have been noted previously tain, as it consists of microcrystalline mixtures of quartz, in the western Sierra Nevada by Hey I, Cox, and Eric zoisite( n' white mica, and chlorite, with scattered ( 1948, p. 68), but they are fundamental to the structural plagioclase and pyroxene crystal fragments. and stratigraphic interpretations of the region and are In thin sections studied, proportion between rock and therefore described here in some detail. crystal fragments does not always vary directly with Graded units are marked by progressive changes in the grain size. In some, contiguous layers of similar color as well as grain size. In graywacke-slate se grain size have different proportions of rock and crystal quences, the coarser material is of lighter color than the fragments, and in others an upward increase in propor fine, but among the pyroclastic rocks either the coarser tion of fairly coarse rock fragments reverses the size or the finer material can be lighter, depending on the grading in small thicknesses of the bed. mineral content. Some beds appear megascopically to Current-ripple bedding is generally associated with grade uniformly from the coarsest to the finest mate small cut-and-fill structures and is common in sequences rial, but in others the grading is apparent in only having many graded beds. Nearly all cross-laminated part of a bed and is obscure or absent in other parts. In units are less than 2 inches thick and the thickness these, size-grading may be in the lower, middle, or upper of cross laminae is measurable in millimeters. Grain part of the bed. There are no single isolated graded sizes in most cross-laminated layers range from very fine beds; instead, they form sequences commonly many tens sand to clay. of feet thick in· which more than half of the beds are Features suggestive of deposition in agitated water graded. are rare or ambiguous. Distorted ripple marks are Graded graywacke 2 layers are common both in dom exposed on many surfaces of thin-bedded tuff near the inant graywacke and dominant slate. In both, bottoms gaging station about 1 mile west of Don Pedro Dam of the coarse layers precisely mark one boundary of a on the Tuolumne River (fig. 16) but have not been found elsewhere. Graded beds are absent or rare near 2 The terminology used in this paragraph to describe phenomen'a re lated to size sorting is that of the epiclastic rocks. AlthoUigh these the ripple marks. No diagnostic bedding features were features are equally common in the pyroclastic rocks the nQmenclature noted in the quartz-rich sandstone included with the for comparable size grades is different and verbiage can be reduced by d'escrlbing a single genetic type. Mariposa formation near the Calaveras River (unit 26, PLUTONIC ROCKS 41 pl. 6), but the scarcity of clay and silt in this rock sug downwarping might have preceded significant accumu gests deposition in agitated waters. In contrast with lation of detrital material, and at the other subsidence other sandstones in the Jurassic formations, there is might have coincided exactly with filling. If the first, little fine detrital material in the matrix of this rock, each succeeding bed would be laid down in shallower and labile constituents are scarce. If these isolated beds water whereas in the second all would be laid down at do result from a shallow water enviromnent, they were equal depth. The history of most filled basins is prob probably forn1ed in places where the depositional basins ably between these two extremes. l\{oreover, lava and te1nporarily filled to wave base. coarse pyroclastic rocks characteristically are built up Structures and textures of the graded sandstones and as mounds or ridges on the sea floor so that volcanic tuffs and the clean, bedded conglomerates suggest that material in a given basin might be deposited at shal they 'are turbidity current deposits. The mud-matrix lower depths than contemporaneous epiclastic sedi conglomerates were probably emplaced by submarine ments. slides. Similar deposits of Pliocene age in the Ventura GREEN SCHIST Basin have been ·ascribed a sinular origin (Natland Green schist, derived from volcanic rocks, has been nnd ICuenen, 1951, p. 102-105). Lawson (1933, p. 10) mapped within the Melones fault zone near ~an held that unsorted conglomerate in rocks near Colfax Andreas and in the Bear Mountains fault zone at the equivalent to the Mariposa formation is tillite, and north end of the area studied. Because of their occur reported a glaciated surface beneath them. The alleged rence in fault slices these rocks are of unknown age. glaciated surface may, however, be a fault surface, as The mass near San Andreas probably includes rocks of faults are comn1on in the region. both Paleozoic and Jurassic ages because in this vicin The thickness of the pris1n of Jurassic deposits sug ity volcanic rocks of both ages lie adjacent to the gests that the basement surface upon which they were Melones fault zone. laid formed a basin, as the thickness of the stratigraphic Most of the rock included with this unit near San section is as great or greater than the depth of the Andreas is markedly schistose. In most places, the mode1n ocean except in the deeps. The aggregate thick~ primary textures and structures are obliterated, but ness of the Gopher Ridge volcanics exposed in the volcanic breccia texture is :preserved locally. The mass Mokelumne River and the Copper Hill volcanics ex at the north end of the map area as exposed in the posed in the ·cosumnes River is about 19,000 feet. The Cosumnes River is locally schistose, but consists largely total apparent thickness of the Penon Blanco volcan of massive fine- to medium-grained amphibolite. ics and Mariposa formation in the Merced River is nearly 20,000 feet. The bedding of some of the depos PLUTONIC ROCKS its, especially of volcanic rocks, may have been in clined, so these values may be greater than the depth Plutonic bodies exposed in the region consist of of the floor below sea level. Nevertheless, the figures ultramafic rocks and rocks ranging in composition from are 1ninimun1 values because the complete stratigraphic gabbro to granite. The ultramafic rocks form linear section is nowhere exposed and some allowance must be belts and are most abundant west of the central part made for the water column above the uppermost de of the area. The less mafic plutonic rocks form the posits, which must have been laid down at consider Sierra Nevada batholith in the eastern part of the area able depth. The deposits were probably bathyal or and small isolated plutons elsewhere. l\{ost of the abyssal, but the absolute depth probably cannot be plutonic rocks· east of the Melones fault zone are ascertained in the absence of diagnostic fossils granodioritic to granitic whereas most of those west of (ICuenen, ·1950, p. 203). The wide distribution of the fault zone, except for the ultramafic rocks, are gab graded beds, and the general absence of sorted clastics broic to quartz dioritic, although all types occur in of other than clay size indicate not only transporta varying amounts on both sides of the fault zone. The tion by turbidity currents and mudflow, but also that composition, origin, and mode of emplacement of the the bottmn water was not sufficiently agitated by cur plutonic rocks were not studied in detail for this proj ect. General descriptions of the plutonic rocks are rents or wnve action to destroy the characteristic given by Turner (1894a, 1897, 1899) Turner and Ran structures. some ( 1897) and ICnopf ( 1929, p. 18-21). Calkins The depth of the basement beneath the mass of de (1930) discusses the petrology and sequence of emplace posits is only indirectly related to the depth of water ment of some plutons of the western·.part of the Sierra over the accmnulating deposits, for the two depend on Nevada batholith near the Merced River. Several of the rate and time of downwarp and the rate and time the ultramafic bodies have been mapped and described of accumulation of the deposits. At one extreme, by Cater ( 1948a, 1948b) . 42 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT ULTR~FIC ROCKS Ultramafic bodies that extend beyond the margins of known fault zones are the two large masses east of Ultramafic rocks include serpentine, talc-antigorite Latrobe, the mass exposed in the eastern part of Pardee ankerite schist, and less commonly, peridotite, dunite, Reservoir, the mass about 3 miles west of Fowler Look and saxonite. Serpentine is much more abundant than out, the large mass crossed by the Stanislaus and the otlier rocks in most places, but talc-antigorite Tuolumne Rivers, and the mass near Bagby. These a~kerite schist is more abundant east of the Melones have irregular_ contacts which locally cross the struc fault zone near the latitude of San Andreas. Peri ture of layered rocks. Although these masses are dotite, dunite, and saxonite occur in some of the serpen largely serpentine like the rest, they differ from most tine masses. The ultramafic rocks in the region form of the smaller masses in containing some preserved elongate masses from less than a foot wide and several peridotite, dunite, and saxonite, (Cater, Rynearson, and feet long to nearly four miles wide and more than 15 Dow, 1951, p. 120-125; and Cater, 1948a, 1948b). miles long. Most of the serpentine is blocky, with each In contrast to these large masses the smaller ones surface strongly slickensided, but part is strongly foli are characterized by straight contacts and are confined a ted. AGE to zones of sheared rocks. On the regional map (pl. 1) several of these are indicated west of the Bear Moun The ultramafic rocks of the area are probably of Late tains, but many more are· shown on individual river Jurassic age. The faults which a;pparently controlled sections. Small sheets of serpentine, some less than a the emplacement of the serpentine cut nearly all the foot thick, are widely distributed in the shear zones. Jurassic and Paleozoic stratified rocks of the region. The serpentine bodies with straight contacts are prob The youngest of these, the Copper Hill volcanics, over ably horses or masses that were injected plastically into lie beds of Kimmeridgian age. Serpentine is cut in turn their present positions. by felsic plutonic rocks north of the area of this report. The close areal relation of ultramafic rocks and the A felsic pluton northeast of Folsom (see Lindgren, shear zones shown on the regional maps (pis. 1 and 2) 1894) which cuts serpentine has been dated by Curtis, suggests that the serpentine and fault zones are some Evernden, and Lipson (1958, p. 6) by the :potassium how related. By far the greater part of the ultramafic argon method as 131 million years and quartz diorite rocks of the region are within the area encompassed by from the same pluton as 142.9 million years old. Ser- · the Melones and Bear Mountains fault zones and many pentine is also cut by another, undated, pluton near of even the sill-like bodies are bounded on at least one Placerville (see Lindgren and Turner, 1894). side by faults. Ultramafic rocks are scarce east of the STRUCTURAL RELATIONS Melones fault zone and are absent west of the Bear Mountains fault zone. It is more likely that the major In plan, ultramafic bodies are elongate parallel to faults controlled the position of the ultramafic rocks the regional strike of bedding and fault zones. Where than that the ultramafic rocks controlled the position adjacent to schistose rocks, the contact of the ultramafic of the faulting (Clark, 1960a, p. 488) . rocks is parallel to the schistosity. Surface exposures It is true that the ultramafic rocks are strongly suggest that the bodies dip steeply, but whether more sheared, probably owing to later movements on the fault or less steeply than adjacent bedded rocks is unknown. zones, but they have not localized the shearing, as the The extent and magnitude of the fault zones has not belts of sheared rocks marking major fault zones are been reeognized previously and the orientation of the more persistent than the ultramafic rocks. The large ultramafic bodies has been cited as evidence that they mass of serpentine exposed on the Calavar.as River is were intruded as sills (Taliaferro, 1943b, fig. 2). Fer thus flanked on the w~st by a wide belt of schistose guson and Gannett (1932, p. 21-22), on the other hand, rocks, and the serpentine southeast of Bagby is flanked recognized as a major fault a large part of the structure on the west by a helt of sheared slate about 200 feet here referred to as the Melones fault zone and sug wide exposed in cuts of State Highway 49 about 11h gested that it determined the position of serpentine miles southeast of Bagby. The Melones fault zone is emplacement. Distribution of the ultramafic rocks believed to extend the length of the metamorphic belt, supports this, as many of the smaller masses are confined but in much of the segment covered here it contains lit to belts of cataclastically deformed rocks and even the tle serpentine. larger masses in the area that intrude layered rocks GABBROIC TO GRANITIC ROCKS are adjacent to fault zones. The strongly sheared con Hornblende gabbro and diorite are abundant in the dition of the serpentine, however, indicates that it was isolated plutons of the western Sierra Nevada. They emplaced before fault movement ceased. form nearly all the mass exposed in the northeast part HYPABYSSAL ROCKS 43 of Don Pedro Reservoir, the part of the mass south Evernden, and Lipson, 1958, fig. 1). Quartz monzonite east of Jackson that is cut by the Mokelumne River, from the Guadaloupe intrusive about 5 miles south of the mass 5 miles southwest of Sonora that is associated the southern boundary of plate 1 and immediately east with serpentine, and many smaller masses. These Tocks of the projection of the Bear Mountains fault zone was also form the marginal parts of some less malic plutons originally dated as 142.9 million years old but~ later such as the one east of San Andreas (Clark, 1954, recalculated as 136 million years (Best, 1963, p. 113). p.11), and the whole of many smaller bodies (for exam A granitic body west of Columbia has been correlated ple, Eric, Stromquist, and Swinney, 1955, pl 1). The by Baird ( 1962, p. 18) with the older series on the basis hornblende gabbro and diorite range in texture from of structural evidence. The younger series composes medium to fine grained. Masses of gabbro and diorite the Sierra Nevada batholith in the Yosemite region east are generally adjacent to or are surrounded by meta of El Portal and ranges in age from about 77 to about volcanic rocks, and in places grade into them texturally, 95 million years. Evidence of Carboniferous age of suggesting that they may have been formed in part by some granitic intrusions is indirect and inconclusive, metamorphism of the volcanics. Eric, Stromquist, and but has not been disproved. Swinney ( 1955, p. 21) suggest that they are at least Curtis, Evernden, and Lipson (1958, p. 10-11) partly differentiates from the same magma as that of pointed out that according to the Holmes A time scale the serpentines, hecause of gradational contacts. Dur (in Zeuner, 1952) the older series is of Early Cretaceoll:S rell ( 1940, p. 74) found evidence of similar origin of age and according to the Holmes B time scale it is of the gabbro in the western Sierra Nevada about 100 miles Early Jurassic age. They cite stratigraphic evidence to south of Sonora. Compton ( 1955, p. 18-20) on the show that the older series is of Late Jurassic age and other hand found that some of the gabbroic rocks in that the Holmes scales need revision. the western Sierra Nevada, north of the area covered, The intrusions assigned by Knopf (1929, p. 18-19) are products of replacement or recrystallization in place to the Carboniferous are those east of Plymouth, at and that others are intrusive. Mokelumne Hill, and southeast of San Andreas. They Granitic rocks form most of the larger isolated plu intrude the Calaveras formation and the plutons east tons of the area, the Sierra Nevada batholith, and the of Plymouth and at Mokelumne Hill are in fault con cores of some smaller masses that are mostly mafic. tact with rocks of Late Jurassic age. Knopf's age The granitic plutons, except near their borders, are assignment was based on (a) the occurrence of granitic much more uniform in texture .and composition than pebbles in the Mariposa formation, (b) cataclastic de the mafic bodies. Contacts of the isolated plutons with formation of rocks in the plutons, (c) higher meta metasedimentary rocks are sharp, and in many places morphic grade of the Calaveras formation as ~ompared the metasediments at the contact are but slightly coarser with the Mariposa formation, and (d) exceptionally grained than elsewhere. Contact metamorphism is strong pleochroic haloes in the micas of these plutons. more pronounced at the border of the batholith. Con This evidence is suggestive rather than conclusive, and tacts of isolated plutons with metavolcanic rocks are less the present author believes that all intrusives in the area sharp and the grain size of the metavolcanics increases are of Late Jurassic to middle Cretaceous age, but the markedly toward the contact. Granitic dikes com occurrence of Carboniferous intrusives has not been monly cut the wall rocks near the plutons. disproved. AGE The plutons east of Plymouth and at Mokelumne Hill are truncated by the Melones fault zone and are there Although most geologists previously considered all fore apparently older than the dated pluton that trun the granitic rocks of the area covered here to be of about cates faults of the Foothills system north of the mapped the same age, Knopf (1929, p. 18-19) suggested two area. On the other hand, emplacement of granitic rocks sets of jntrusions, one of late Carboniferous age and certainly; and fault movements probably, occupied finite another of post-Mariposa age. Curtis, Evernden, and Lipson (1958, p. 6-9) more recently showed that two parts of geologic time, and it is unlikely that granites distinct sequences of emplacement of granite rocks can of a single series of intn1sions would everywhere· show be distinguished by potassium-argon dating. the same relations to the faults. Two bodies of the older series near the map area have been dated. Granodiorite from Rocklin, dated as 131 HYPABYSSAL ROCKS million years old, and quartz diorite from Horseshoe Massive rocks of possible hypabyssal origin, because Ba.r, dated n,s 142.9 million years old, form parts of a of their uncertain relations, are grouped with the for pluton north of the area that extends to within 4 miles mations with which they are associated and are not of the northern map boundary (for location, see Curtis, discussed here. Most of the rocks known to be of 44 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT hypabyssal origin fonn dikes. Swarms of fine-grained lones fault zones, which are elements of the Foothills leucocratic dikes cut the metamorphic rocks west of fault system (Clark, 1960a). In most homoclinal se the gaging station downstream from Don Pedro Dam quences, tops of beds are eastward, and in large parts of on the Tuolumne River (fig. 16) and along the Merced each of the three blocks separated by the BearMoun- River between Bagby and the mouth of the North Fork ta1ns and Melones fault zones older beds are on the west of the Merced River (pl. 3). A porphyritic leucocratic and the younger beds on the east. J:hi_s distri.hutio.njs rock with fine-grained groundmass, probably forming ]feversed by the maJor fault zones, as the youngest bed a dike, is poorly exposed in the south bank of the Ca-la rook_unit of the region, the Qmper Hill volca,nics,is.,in veras River at the western side of bedrock exposures the western block and the oldest, the CalaYeras_fonna (pl. 6). Knopf ( 1929, p. 21-22) has described sev tion, is in the easten1 block. Metamorphic rocks exposed eral albitite dikes from other localities in the region. in the central block are of intermediate age. Some mafic sills consisting of saussuritized plagio Axes of major and minor folds related to develop clase phenocrysts in a fine-grained groundmass intrude ment of the snyclinorium are essentially horizontal; the lower part of the Amador group near the Cosumnes they plunge northwest and southeast at angles of 35° River (see pl. 10). A similar rock is intruded at the and less. The folding was followed by a second stage base of the upper tongue of the Brower Creek volcanic of deformation that involved the major faulting and member of the Mariposa formation south of the Cala development of slip cleavage and steeply plunging veras River. The dike rock is not exposed along the minor folds and b-lineations, particularly east of the river, and is represented only by boulders. Melones fault zone (Clark, 1960b). Most of the minor A vertical dike of porphyritic rhyolite or dacite in structures in the region are identifiable with these two trudes unit 2 of the Calaveras River section (pl. 6). stages, but minor structures in some small parts of the The unit shows horizontal columnar jointing. · ma;p area have different attitudes than those so iden- Dikes and sills intruding the metamorphic rocks can tified. Baird ( 1962, p. 35-44), for example, has de not be precisely dated. The upper age limit is defined scribed a prominent slip cleavage and lineation direc by the land surface which truncates the dikes. The tion in limestone northwest of Columbia. He found surface apparently developed largely during the Late these structures to be younger than the first regional Cretaceous. The oldest Mesozoic rocks cut by dikes are deformation but older than the second. probably not older than Middle Jurassic and the . The structure of the western and central blocks is youngest are of Late .Jurassic~ age. The sills in the similar, but the eastern block is deformed more severely lower part of the Amador group post-date the con and differently than the others. 'These differences have solidation of enclosing beds, for dike walls are straight led some geologists to infer that the Calaveras forma for considerable distances, bedding is not more dis tion was strongly folded before deposition of the Co torted at dike contacts than elsewhere, and some dike sumnes formation, but this is not assured. The greater walls show sharply angular offsets. Some dike rocks age of the metamorphic rocks in the eastern block rela in the eastern block may be of Paleozoic age. The tive to those farther west indicates that the eastern vertical rhyolite or dacite dike exposed near the Cala block has been relatively uplifted by faulting, hence that veras River is tentatively interpreted as of Tertiary age the exposed metamorphic rocks of the eastern block on the basis of its fresh appearance, jointing and were deformed under greater pressure at lower levels attitude. of the earth's crust than metamorphic rocks exposed STRUCTURAL GEOLOGY farther west. of the deformation of the region is older than Metamorphic rocks of the western Sierra Nevada are ~ost the Sierra Nevada batholith, and is part of a zone of de on the west limb of a faulted synclinorium, the axial formation that coincides with a belt of eugeosynclinal part of which is occupied by the Sierra Nevada batho deposits and granitic rocks of Jurassic age that extends lith (Bateman and others, 1963) . The strike of bed with some interruptions from the western Sierra N e ding and major fault zones, about parallel to the trend vada through the western part of the l{]amath Moun of the axis of the synclinorium, is generally northwest except in the northern part of the map area where bed tains into southwestern Oregon. The Jurassic rocks of ding and the fault zones turn northward. Beds in most the western Sierra Nevada were folded and the folds places dip eastward at angles of more than 60° or are were truncated by major steep northwest-striking faults vertical. Isoclinal folds occur in some laces and more before emplacement of the Upper Jurassic plutons. Qpen f6lds_e1s~, but there are wide areas w1 h Much of the deformation of the eastern block was also homoclinal dip. The... :maior folds and homoclinal se earlier, for regional structures in the block are truncated guenc_es_ar:_e t.I:lillC'a.ted__buhe Bear Mountains and Me- by the Melones fault zone, and slip cleavage related STRUCTURAL GEOLOGY 45 to the :faulting locally offsets bedding and previously :formed schistosity in the eastern block. Little evidence :for large-scale deformation accom panying emplacement o:£ the Sierra Nevada batholith has been :found. Faults, probably thrusts, in and near the M~lones :fault zone are later than the strike-slip deformation and may be related to emplacement o:£ the batholith. Perhaps spacial adjustments o:£ considerable magnitude were accommodated by previously :formed structure~') without leaving a record that is readily dis- . tinguishable. Structures available :for such adjust ments include :folds, wide shear zones in the Melones and Bear Mountains. :fault zones, and an infinitude o:£ slip surfaces in schists o:£ the Calaveras :formation. TOP DETERMINATIONS Structural interpretations presented here, as well as those o:£ stratigraphic sequence, depend greatly on de terminations o:£ tops o:£ beds, based on graded bedding, crossbedding, pillow lavas, minor :folds, and relations between cleavage and bedding (Shrock, 1948). More determinations have been based upon graded bedding than on all other :features. As tuff is more abundant than graywacke, most o:£ the determinations are on tuff beds (fig. 13) . No plotted determination has been based on less than three graded beds, and some rep resent scores o:£ them. Exposures showing obscure, or spurious, grading and apparently conflicting top direc tions within stratigraphic intervals o:£ a :few :feet have been ignored. Curved cleavage planes, convex strati graphically upward, distinguish some steeply dipping graded beds showing no megascopically apparent size gradation, as well as many beds with such gradation (fig. 14). In these beds, the angle between cleavage FIGURE 13.-Photomicrograph of graded tuff bed in Copper Hill and bedding is a :ftmction o:£ grain size so that cleavage volcanics, NEJ,4 NEJ,4 sec. 28, T. 2 N., R. 12 E. Light-colored planes make a large acute angle with bedding in the grains are pyroxene and sodic plagioclase crystal fragments. Dark grains are chlorite after lithic or glassy fragments. coarsest part o:£ the bed, and approach parallelism to Dark layer in upper part of photograph-originally dust-tuff the bedding as the material becomes finer. This phe is also chlorite. Most of the coarse layer is about of the same nomenon is more common in graywacke and slate than grain size, but this layer grades into the fine material through in tuff. an interval that is appreciable relative to the thickness of the bed. Note sharp contact of succeeding coarse layer at Throughout the region relation o:£ cleavage to bedding extreme top of photograph. Gray band in uppermost layer indicates. that cleavage is parallel or nearly parallel to is an epidote veinlet. Plain light. the axial planes o:£ the :folds (fig. 15). In much o:£ the area o:£ Jurassic rocks, this relation should provide Minor :folds were used to interpret structures and reliable top determinations. However, locally devel stratigraphic sequence in areas on the Tuolomne River oped cleavage which is at least superficially similar to near Don Pedro Dam and La Grange. Those in the the pervasive cleavage is probably related to a stage, or Gopher Ridge volcanics near Don Pedro Dam (fig. 16) several stages, o:£ deformation later than that which are consistent :for a mile west o:£ the slate contact near :formed the regional :folds and may have a random rela the dam and suggest that this area is on the west limb tion to axes o:£ these :folds. Because o:£ these uncertain o:£ an anticline. The :folds near La Grange (fig. 10) are ties only two top determinations based on cleavage were much less extensive. At both places the minor :folds recorded. plunge as gently as the major :folds o:£ the region and 46 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT FIGURE H.-Photomicrograph showing relation of cleavage to graded bed in fine graywacke of Salt Spring slate, NE%NE14 sec. 14, T. 4 S., R. 15 E. Cleavage direction, influenced by grain size, is about 60° from bedding in coarse part but changes progressively to about 25° from bedding in finer part. Such curved cleavage, convex stratigraphically upward, commonly indicates the existence of graded bedding where the gradation is not other wise recognizable in the field. Most of the clastic grains are quartz and the matrix is sericite. Original texture is well preserved although cleavage is well developed. Plain light. are probably related to the deformation that produced younger deposits of the Central Valley, and others may them. have once been present in the area now occupied by FAULTS the Sierra Nevada batholith. The Melones and Bear Most major faults are components of the Melones and Mountains fault zones have components of reverse Bear Mountains fault zones which in turn are part of movement, but the orientation of their net slip and that the Foothills fault system (Clark, 1960a). The Melones of the Foothills fault system as a whole is not and Bear Mountains fault zones are the dominant struc established. tural features of the region. Faults that control the Both faults and shear zones shown on the accompany quartz veins and gold ore deposits of the Mother Lode ing maps express loci of deformation that disrupted belt are younger than the Foothills faults and are much structural or stratigraphic continuity, or produced less important as structural features. Faults with only cataclastic deformation, crumpling, or recrystallization. a few feet or a few tens of feet of displacement do not Most zones of dislocation of less than mappable width notably affect rock distribution and few have been are indicated as faults whereas dislocation belts that mapped. are of mappable width are indicated as shear zones; The Foothills fault system extends more than 100 some structures shown as shear zones on the detailed miles northwest and an unknown distance southeast of maps are indicated as faults on the regional map and the area shown on plate 1. The width of the fault diagram. Other faults are drawn arbitrarily at con system within the latitudes considered here is unknown. tacts of sheared ultramafic rocks "here they occupy Additional fault zones parallel to the exposed Melones the position of major faults and at stratigraphic dis and Bear Mountains fault zones may underlie the continuities where the shearing extends for many hun- STRUCTURAL GEOLOGY 47 nonschistose serpentine, platy structure is caused by the slip surfaces alone. Original textures and structures are generally obscure or absent in the schists in the fault zones. The schists and phyllites are commonly lineated. Amplitudes of crumples and minor folds in the shear zones are generally measurable in millimeters or inches, and limbs of many are truncated by slip surfaces. In some places rock fragments are rotated and mineral grains broken. MELONES FAULT ZONE The Melones fault zone trends about N. 30° W. through most of the area studied, but near Plymouth the strike changes to northerly. From south of the area studied to about the Mokelumne River, it coincides with the area known as the Mother Lode belt. Within the area covered by the regional map (pl. 1) the Me lones fault zone has been mapped in detail for a total distance of about 30 miles, through the Sonora, Angels Camp, and San Andreas NW 7ljz-minute quadrangles (Eric, Stromquist, and Swinney, 1955, pls. 1, 2; A. A. Stromquist, written communication, 1952). The Me lones fault zone is cut about 3lj2 miles south of the map area by a granitic pluton that crystaJlized after the last movement on the fault zone ( Cloos, 1932a, p. 303), but may cut metamorphic rocks exposed south of this pluton. FIGURE 15.-Photograph of small assymetric anticline showing Existence of the Melones fault zone is indicated by axial plane cleavage. Fold is in bedded tuff of the Gopher both a break in stratigraphic and structural continuity, Ridge volcanics on the north side of the Cosumnes River in and by mechanical deformation and recrystallization. the west part of sec. 31, T. 8 N., R. 9 E. Cleavage is most pronounced in axial region and gently dipping east limb of Through an 80-mile segment of the report area, the the fold. Fold axis plunges north about 15° (away from fault zone separates plutonic rocks and rocks of Paleo observer). zoic age on the east from rocks of Jurassic age on the west. Southeast of San Andreas, the regional trend of dreds of feet in both directions, as east of Angels Camp the volcanic rocks and limestone lenses in the Calaveras (pl. 1; Eric, Stromquist, and Swinney, 1955, pl. 1). formation is truncated at an acute angle by the fault Shear zones were mapped on the basis of thin platy zone, and successively younger strata of the Calaveras structure; of schistose belts, some markedly more formation are cut by the fault zone to the northwest. crystalline than rocks on the two sides; of zones of The plutonic bodies north of Mokelumne Hill are ter crumpling; and of cataclastic deformation; or of a minated on the west by the fault zone. The Mariposa combination of these. Rocks in the shear zones-ex formation bounds the western side of the fault zone cept the metavolcanic and granitic rocks-are generally except in the north part of the report area. too much fractured for collecting hand specimens; some The most striking truncation of structure of the phyllitic rocks in shear zones separate into paper-thin eastern block is southeast of San Andreas, where an layers. Schistosity in the shear zones results from easterly trending anticline having a core formed by parallel arrangement of tabular minerals and rock frag ments, fiat mineral pods, and closely-spaced slip sur volcanic rocks of the Calaveras formation is terminated faces. Plates have feather edges due to acute inter abruptly on the west by the fault zone. The Melones section of slip surfaces, and are distinct from cleavage fault zone is more nearly parallel to structures of the plates of slate. Although similar schistosity is widely rocks to the west, but it cuts at a small angle a syncline developed in rocks of the Calaveras formation east of formed in the Mariposa formation. The axis of this the Melones fault zone, it is not there restricted to such syncline is about two miles west of the fault zone near well-defined belts. In some phyllite and fine-grained the Mokelumne River, and intersects the fault east of 1-J::. 00 EXPLANATION \ -~ v·. r ~ ~ Gopher Ridge volcanics ~ ...., Mostly tuff and fine volcanic breccia. <( "l~ Coarse volcanic breccia distinguished In: DAM fr by symbol brand~ by pattern in vertical :::> '(Jl ::::> projection 1 8 '··\% ~ <77 > \~ 8 Dike rock of Cretaceous or ~ ~ \~ Jurassic age 0 ~ '..\s --~ w\\ \. Quartz vein of Cretaceous or ~ '1: Jurassic age /f"' 3 ~ --~~ 7"42'30" ~l:\,. ------~ I::! l7•'< Contact Dashed where projected - -- '(Jl ~- .---6~~eo'l ~r(l~ ,__:I: ------8 FIGURE 16.-Geologic map and vertical projection showing structure of bedded pyroclastic rocks of the Gopher Ridge volcanics of Late Jurassic age west of Don Pedro Dam, Tuolumne River, Mariposa County, California. STRUCTURAL GEOLOGY 49 Chinese Camp. Farther southeast, the trough and east DESCRIPTION OF LOCALITIES limb of the syncline are missing. The Melones fault zone is crossed by the Cosumnes The west side of the Melones fault zone is marked by River east of Ruse Bridge (pl. 8), where there are three an abrupt transition fron1 moderately to strongly zones of shearing, separated by non-sheared rocks. The cleaved rocks that coincides with a break in the strati west sides of these zones are about 1,400 feet, 2,700 feet, graphic sequence, whereas on the east side of the zone, and 3,600 feet east of the mouth of the North Fork of slip cleavage or schistosity parallel to the fault zone the Cosumnes River. The westernmost shear zone, commonly persists thousands of feet northeastward be about 200 feet wide, contains carbonaceous sericite yond the stratigraphic discontinuity. The greatest schist quite unlike the rocks on either side, which retain width of the fault zone in the map area, about 4 miles, their original textures. The central shear zone is was measured between the major breaks in strati marked by green schist that is probably formed by de graphic and structural continuity northwest of Alta formation and recrystallization of part of the volcanic ville, that is, between the northeast side of the Mariposa breccia immediately west of the shear zone. A narrow formation and the contact between the green schist of block of slate with interbedded tuff having well-pre the fault zone and Calaveras formation. Across most served original textures and structures (too small to of this interval, the rocks are greatly sheared. In addi map separately) separates the chlorite schist from meta tion, slip cleavage parallel to the schistosity in the fault morphosed granitic rock of the eastern shear zone. zone can be recognized as much as 6 miles east of the The granitic rock, exposed for about 2,500 feet on the fault zone in the southern part of the Calaveritas quad river, is schistose throughout and contains a pro rangle (Clark, 1954, pl. 1). The slip cleavage is most nounced lineation resulting from parallel arrangement pronounced near the fault zone, and less prominent east of amphibole crystals. In thin section, the feldspars ward. In the northeast part of the Calaveritas quad are seen to be completely saussuritized and although rangle and other places where the schistosity of the textures suggest that some of the altered feldspar crys Calaveras formation is parallel to that of the Melones tals have been broken, this has not been established. fault zone, movements related to the faulting cannot Quartz forms microcrystalline lenticles with grano be distinguished. blastic texture; undulatory extinction is weak or absent. The fault zone is much narrower near the Merced Epidote also forms lenticles. Cataclastic deformation River. At Bagby, near the river, exposures are inade is shown by broken amphibole crystals. The schistose quate to determine the width, but the shear zone is granitic rock was included by Lindgren and Turner certainly less than 200 feet wide. At the Pine Tree ( 1894) and by Turner ( 1894a) with a much larger mass mine, about 1112 miles south of the river, the shear zone lying east of the Melones fault zone. Exposures ex west of the serpentine is about 100 feet wide. The ser amined during reconnaissance on roads east of the fault pentine, n1ore than 600 feet wide near the Pine Tree zone suggests that the granitic rocks become progres 1nine, n1ay have displaced sheared rocks and has no sively less sheared to the east-the granitic rocks north doubt itself absorbed some of the movement. of Fiddletown are apparently undeformed. Dip of the Melones fault zone is indicated by internal The granitic rock abutting the Melones fault zone planar structures which in most places are vertical or on the Mokelumne River also is schistose on the western dip to the east at an angle of 70° or more. Lineation side, but the feldspars are megascopically little altered, within the fault zone is in the planes of schistosity and and no lineation is apparent. Epidote is abundant and n1arked by hornblende crystals in sheared granitic suggests metamorphism of the granitic rocks. The rocks, widespread elongate fragments iri pyroclastic fault zone extends more than 3,000 feet westward from rock and conglomerate, and axes of small folds. With the granitic rock. Immediately west of the granitic rare exceptions it bears eastward and is nearly normal to the strike of the schistosity; the plunge is 60° to rock is a narrow, poorly exposed belt of chlorite schi"st, vertical, and in most places about 80°. hut west of this is a belt several hundred feet wide The Melones fault zone is not uniformly sheared. It underlain by black Jurassic quartzose slate with inter includes large blocks in which original structures are bedded tuff. Another chlorite schist belt extends west well preserved and the cleavage is no more strongly ward from the interbedded slate and tuff nearly to Mid developed than in the Jurassic rocks west of the fault die Bar Bridge, but two narrow belts of black carbona zone. In some places most of the movement was near ceous quartzose mica schist, very similar to parts of the western side of the zone, in some places near the the Calaveras formation, occur within the chlorite eastern side, but in others the locus of greatest displace schist 1,200 feet and 2,000 feet, respectively, northeast ment cannot be established. of the bridge. 50 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT On the Calaveras River, the fault zone is marked by Calaveras Rivers where it is marked by a belt of greatly a belt of green schist and serpentine with a total width sheared volcanic rocks as well as by the individually of about 4,500 feet. Schistosity in the green schist dips mapped faults. It is narrowest in the southern part of eastward about 70°. Within the Calaveras formation the area. Rocks in the Bear Mountains fault zone are east of the fault zone as delineated on the accompany not as well exposed as those in the Melones fault zone, ing maps, strong schistosity parallel to the Melones because four reservoirs extend across it. When the res fault zone extends an unknown distance eastward. ervoirs are filled the original exposures on the former This schistosity may be at least in part related to the banks of the river are submerged and at low water they fault. are covered with slime. The Bear Mountains fault zone On the Stanislaus River (pl. 5) the fault zone is include~ the Bostick Mountain fault zone, the Copper divisible into two parts separated by a body of Jurassic opolis fault, and the Hodson fault of Taliaferro and rocks that are no more strongly defoTined than those Solari ( 1949), the first-named being near the east side outside the fault zone. The westernmost fault of the and the last-named near the west side. Melones zone truncates the easternmost belt of the Schistosity has not been measured in the individual Brower Creek volcanic member of the Mariposa forma shear zones in the Cosumnes and Mokelumne Rivers, but tion. The fault is marked in the river !by a narrow farther south it dips eastward about 70° to 80°' al shear zone immediately east of the Brower Creek mem though both steeper and more gentle dips occur. On ber, and a narrow block of Calaveras formation lies the banks of Lake McClure the schistosity in shear zones against the west side of the fault. The top of th~ is vertical. In general, the schistosity of the shear zones Brower Creek member west of the fault is westward, is as steep as, or steeper than, the dip of the rocks, on but tops in the Jurassic strata to the east are eastward, either side. indicating that displacement along this fault is at least Near the Cosumnes and Mokelumne Rivers, the fault great enough to eliminate the crest of an anticline. zone consists of two main parts-a belt of serpentine Rocks of the Mariposa formation are not notably de and metavolcanic rocks on the west, and a belt of nar formed and graded beds are well preserved. The main row shear zones causing repetition of strata on the east. part of the Melones fault zone is between the highway The meta volcanic rocks associated with the serpentine bridge at Melones and the mouth of Coyote Creek be on the Cosumnes River are fine grained. cause west of the bridge are characteristic Mesozoic On the Cosumnes River (pl. 8) the position of the rocks and east of the creek mouth are characteristic main part of the Bear Mountains fault zone is marked by Calaveras rocks. Exposures in the banks of the river two bodies of serpentine, separated by schistose and are poor in this interval. massive metavolcanic rock of unknown age. The shear Exposures of the Melones fault zone in the Tuolumne zone cutting the Mesozoic rocks east of the serpentine River add little to the information ava.ilable elsewhere and the shear zone separating this block of Mesozoic but excellent exposures of the strongly sheared rock strata from the rocks tentatively included with the characteristic of the fault zone are readily accessible Calaveras formation are interpreted to be less impor near the State Highway 49 bridge. •Only the part of tant· parts of the Bear Mountains fault zone. the fault zone composed of serpentine is exposed on Near the M:okelumne River (pl. 7) the locus of great the Merced River. est displacement is again marked by the serpentine. Whether it is at the largest body or the ·smaller mass BEAR MOUNTAINS FAULT ZONE farther east depends on correlation of the volcanic The Bear Mountains fault zone is parallel to the rocks (unit 22, pl. 7) east of the largest serpentine here Melones fault zone, and like the Melones fault zone, arbitrarily included with the Copper Hill volcanics. extends beyond the limits of the area shown on plate Displacement on lesser faults of the eastern part of the 1. Schistose rocks along the projection of the Bear zone has produced horst and graben structure. Mountains fault zone have been described by R. J. P. The fault zone as exposed near the Calaveras River Lyon 3 from near Miles Creek, about 8 miles south of differs from that farther north in that the volcanic the southern boundary of plate 1, suggesting that the rocks west of the serpentine are strongly sheared for fault zone extends at least this far south. Near the 11h miles west of the serpentine. The largest strati Cosumnes River, the fault zone divides into two parts. graphic break is apparently about 1 mile east of the The fault zone is widest between the Stanislaus and serpentine, at the east side of exposures of the Copper 3 Lyon, R. J. P., 195"-, Studies in the geology of the weste.rn Sierra Hill volcanics. Near the Stanislaus River, the volcanic Nevada: (I) Tecton.ic analysis of the Miles Creek area, Mariposa rocks west of the serpentine are also sheared, but much County, California: (II) Mineralogy of the ores of the .southern foot hill copper belt in California : California Univ., Berkeley, Ph. D. thesis. less so, nearly as far westward as O'Byrnes Ferry. STRUCTURAL GEOLOGY 51 Taliaferro and Solari (1949) have mapped blocks of the fault zones, and parallel to the b tectonic axis. The Cala vents formation within the serpentine immediately steeply plunging fold axes and lineations suggest an south of the Stanislaus River. important strike-slip component in fault movement., but Near Don Pedro Reservoir on the Tuolumne River, many more fold axes must be measured to provide a sheared rocks are limited to a belt about 500 feet wide reliable basis for interpretation. Limbs of most of the near the northeast corner of sec. 24, T. 2 S., R.14 E., and minor folds so far observed are sheared off, so these within a volcanic sequence arbitra.rily included with folds do not provide evidence of sense of movement. the Peiion Blanco volcanics. West of the shear zone is Evidence for strike-slip deformation elsewhere in the a granitic pluton which may have displaced other exten western Sierra Nevada has been reviewed previously sively sheared rocks. A shear zone along the strike (Clark, 1960a, p. 492). of the one exposed in the Tuolumne River is exposed in The apparent vertical component of movement on the a quarry where the road between La Grange and Coul Melones and Bear Mountains fault zones is large. Be terville crosses Piney Creek. The shear zone exposed cause rocks of Jurassic age west of the Melones fault in Piney Creek, presumably controlled the straight zone are juxtaposed against rocks of Paleozoic age east course of Piney Creek, and short segments of faults of the fault zone for a distance of more than 90 miles, along this trend are found on the banks of the north the apparent vertical component of movement is westerly trending arm of Lake McClure (pl. 3). An equivalent to, or greater than, the thickness of the other shear zone, about 1,000 feet wide, is exposed in Jurassic section :west of the fault zone. Partial sections cuts alongside the La Grange-Coulterville road about of these rocks range from about 3,000 to almost 15,000 2 miles south of the quarry at the position indicated feet thick. Strata along both sides of the Bear Moun by a f.ault on plate 1. No corresponding shear zone was tains fault zone are of Jurassic age, but those adjacent found in Don Pedro Reservoir at a period of low water. to the east side of the zone are at least 10,000 feet strati DIRECTION AND AMOUNT OF MOVEMENT graphically lower than those adjacent to the west side. Age relations of the rocks exposed in the three main MOTHER LODE FISSURE SYSTEM fault blocks indicate that the vertical component of The term "Mother Lode fissure system" is used here movement in the Bear Mountains and Melones fault for the system of eastward dipping faults with which zones was in the reverse direction, but direct determi the quartz veins and gold ore bodies of the Mother Lode nation of the orientation and amount of net slip along belt are associated. The controlling faults have usually the fault zones is impossible, as corresponding points been described as reverse (Knopf, 1929, p. 45) or thrust or combinations of planes on opposite sides have not faults, although evidence for direction of movement is been identified. The structure here referred to as the scarce (Eric, Stromquist, and Swinney, 1955, p. 27). Melones fault zone has been described as a reverse fault During the present investigation little study was made by Ferguson and Gannett ( 1932, p. 21) and Knopf of the Mother Lode fault system and most of the fol ( 1929, p. 45, 46). Ferguson and Gannett used the term lowing discussion is based largely on previous descri p "reverse fault" strictly in a descriptive sense, but Knopf believed the dominant component of net slip to be ver tions. The resulting picture may be biased, for pub~ tical. Taliaferro (1942, p. 90), states that "The Mother lished descriptions are, appropriately, of the large Lode represents a great U ppet Jurassic thrust along mines where veins and ore zones have been explored for which the Paleozoic was thrust westward over the distances of several thousand feet along the dip or Mesozoic." Cloos (1932b, p. 392-394; 1935, p. 234) dis strike, or both. Veins and ore zones within the Melones cussed chiefly faults that controlled the quartz veins fault zone that meet this qualification post-date signif of the western SierraNevada and compares these faults icant movement along the major fault zone; otherwise to marg.inal thrusts related to batholiths. Cloos ( 1935, the veins or their controlling structures would have p. 238, 247) also, however, identified a long continuous been broken by fault zone movements into trains of fault (apparently the Melones fault zone of this report) horses or boudins extremely difficult to trace success as the "Mother Lode proper" and said that "It seems fully in mining. If small remnants of ore bodies dat~g possible that movement along this fault began long be fore intrusion of the larger batholithic masses.'' Clark from an earlier stage do exist, they would receive slight (1960a, p. 491-492) suggested the possibility that the consideration in any but the most exhaustive investiga dominant component of movement along major fault tion of gold ore deposits of the Mother Lode belt. zones is horizontal (strike-slip). J{nopf (1929) gave excellent descriptions of the faults Lineations within the fault zone plunge steeply, are exposed in many of the mines, and the areal pattern of parallel to axes of most minor folds measured within the quartz veins and gold mines is shown by Ransome 52 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT (1900) and Eric, Stromquist, and Swinney (1955, pis. displacement of 375 feet in the Gover shaft of the Fre 1 and 2). mont mine (Knopf, 1929, p. 25, 54). Cloos (1932b, p. Most of the Mother Lode fissures are in or near the 393-394) believed the fissures to be related to emplace Melones fault zone, but eastward-dipping faults that ment of the Sierra Nevada batholith and comparable may be part of the same system have been recognized to marginal thrusts. in the Penn mine (Heyl, Cox, and Eric, 1948, p. 75) and The mine maps and sections prepared by Knopf pro in the adjacent Grayhouse area (Heyl, 1948a, p. 89-90), vide evidence that some of the Mother Lode veins and near Campo Seco. The distribution of mapped east gold ore bodies post-date most of the movement along ward dipping faults closely parallels that of the gold the Melones fault zone. In several of the mines within and copper mines, suggesting that their recognition may the Melones fault zone the principle ore bodies replace depend on detailed study of well-exposed areas, and schist. Schistosity is much stronger in the Melones that they may be more extensive than known. fault zone than in most of the bordering rocks and is From a point about half way between the Merced apparently related to fault movements. Replacement and Tuolumne Rivers northward to the Mokelumne of schistose rock by gold ore accordingly suggests that River, most of the Mother Lode fissures are within the the ore is later than fault movement. Ore bodies of this Melones fault zone, but near the southern end of the kind are found in the Eagle Shawmut and Clio mines belt and from the Mokelumne River to the north end which are in the Melones fault zone near the Tuolumne of the Mother Lode belt, northwest of Placerville, River. In these mines Knopf interprets the sequence most of the veins crop out one mile or more west of of events as follows: (a) reverse faulting, (b) emplace the Melones fault zone. Although the strike of some ment of peridotite or similar rock, (c) serpentinization veins is moderately sinuous, the strike of most veins of the peridote and renewed fault movement, (d) depo throughout the Mother Lode belt is about parallel to sition of ore. The ·first faulting was no doubt part that of the Melones fault zone. Veins generally dip less of the Melones fault zone movement-the later move steeply than cleavage and bedding west of the fault ment noted by Knopf may be related to the same tec zone and schistosity within the fault zone. The veins tonic episode or to a later episode. dip eastward, mostly at angles between 50° and 70°, al In contrast with ore bodies at the Eagle Shawmut and though one dips as gently as 20° for a considerable Clio mines, some of those at Carson Hill, in the Melones distance (Knopf, 1929, p. 24). Among the veins whose fault zone about one mile north· of the Stanislaus River, attitude is given by Knopf (1929) or Ransome (1900), are associated with smaller faults that cross and offset no significant difference was noted between the dips of structures characteristic of the chief movement along veins within the Melones fault zone and those west of the major fault zone. Here, gold-quartz ore bodies were the fault zone. Veins west of the fault zone cut and found at the intersection of gently dipping veins with offset bedding in the country rock at acute angles. Some a steeply dipping vein. Along at least one of the gently veins and zones of replacement ore within the Melones dipping veins, contacts of the wall rocks, including ser fault zone bear similar relations to schistosity and tab pentine and schist, have been offset in a reverse direction ular bodies of rock drawn out parallel to the schistosity (Knopf, 1929, p. 76-77, fig. 23). Mapping by Eric and of the fault zone but others are parallel to schistosity Stromquist (pl. 1 in Eric, Stromquist, and Swinney, and contacts within the fault zone. Of the veins that 1955) shows that in the vicinity of Carson Hill serpen crop out west of the fault zone, none have been described tine and schist fonn narrow bodies elongate parallel to as having been traced down dip into the fault zone. schistosity in the Melones fault zone as they do else The movement picture of the Mother Lode fissure where in the zone. system evidently is not simple. Knopf (1929, p. 45) believed as a result of finding older rocks on the hang MINOR FAULTS ing wall side of many veins that most of the veins The minor faults discussed here are those found dur occupied dip-slip reverse faults. He points out that ing this investigation which are apparently not related flutings on post-mineral gouge indicate movement par to the fault systems discussed previously. Their dis allel to the dip in the Plymouth mine (Knopf, 1929, p. placements probably do not exceed a few hundreds of 52), but faulting that has produced the main fissure of feet at the most. the central Eureka mine had horizontal displacement of The westward-dipping faults exposed in the north 120 feet (p. 62), and a strong lateral component of wall of the Tuolumne River gorge below Don Pedro movement is suggested by structural relations in the Dam (see fig. 16) are apparently thrust faults related Argonaut mine (p. 67). The greatest measurable dis to the folding. The apparent offset of the westernmost placement found by Knopf was an apparent dip-slip .fault is shown by the coarse volcanic breccia bed. If STRUCTURAL GEOLOGY 53 the volcanic breccia exposed near Don Pedro Dam is Eocene age (see discussion by Eric, Stromquist, and the same layer, the .displacement of the easternmost Swinney, 1955, p. 16). fault is much greater. The faults were observed only from the south side of the river, and the true dip .and STRUCTURE OF THE EASTERN BLOCK direction of movement were not measured. Only some of the larger aspects ·of the geologic struc In the cut immediately north of Ruse Bridge steeply ture of the eastern block can be described, as most of the dipping slickensided surfaces separating several blocks metamorphic rocks of this area lack distinctive marker of volcanic breccia suggest a ·minor fault. Most of the units and in large areas bedding has been destroyed by slickensides plunge less than 30°, both north and south, shearing. Bedding is preserved in some isolated small but on one surface the plunge is steep. No comparable blocks within sheared areas, but in such places it is slickensides were found immediately south of the bridge almost invariably parallel to the regional schistosity, but here about an inch of gouge separates the Logtown leading to the suspicion that it is preserved only be Ridge formation from the overlying epiclastic rocks. cause of this orientation. Attitudes of bedding in these No evidence was found to suggest that this fault has isolated blocks have not been recorded. The structure more than a few feet of movement. of the eastern block is particularly obscure in the part The steeply dipping fault exposed in the Cosumnes that lies north of the latitude of San Andreas, except River near the SW corner of sec. 31, T. 8 N., R. 9 E., near the Mokelumne River where a northerly trend on the west flank of the large anticline, is near where is suggested by the distribution of limestone. Regional Taliaferro (1943b, fig. 2) showed a major reverse fault. trends can be distinguished south of this latitude. The fault is prominent in stream-bed exposures as it In the southeast half of the eastern block, the vol separates moderately folded but generally gently dip canic member of the Calaveras formation and the belt ping beds on the east from nearly vertical beds on the of limestone lenses both trend northwest at an acute west. The fault is marked by a fracture less than an angle to the Melones fault zone. This trend is further inch wide, and the beds on either side are apparently substantiated by the fact that the lowermost unit of not crumpled. This fault is in such striking contrast the Calaveras formation in this region, exposed east of to those lo1own to be 1najor faults in the region that Bagby, is missing north of the Tuolumne River, where it is here interpreted as a minor structure. it is truncated by the Melones fault zone. West and AGE OF FA.ULTING northwest of Sonora, distribution of volcanic rocks in the Calaveras suggests that they are repeated in folds, Movement on faults of the Foothills system began or that they form large lenses at various stratigraphic during Late Jurassic time and may have continued into positions instead of a single continuous sequence. The Early Cretaceous time. The youngest rocks cut by the eastward-trending mass of volcanic rock southeast of lVfelones fault zone include early IGmmeridgian strata San Andreas forms the core of a large anticline (Clark, of the Mariposa formation, and the youngest rocks cut 1954). The volcanic rocks and limestone lenses con by the Bear Mountains fault zone conformably overlie verge north of the Merced River suggesting that one strata of late Oxfordian or early IGm1neridgian age unit or the other, or both, thickens at the expense of but are probably pre-Tithonian. Branches of the Bear intervening rocks. This is further suggested by the Mountains fault zone are truncated north of the report fact that limestone is in contact with volcanic :rocks area by the Rocklin granodiorite and Horseshoe Bar north of the Stanislaus River. quartz· diorite which have yielded potassium-argon More data on the· structure of the eastern block were dates of 131 and 142.9 million years, respectively, and obtained near the Merced River than elsewhere. Here, are believed by Curtis, Evernden, and Lipson (195-8, p. top determinations, absence of areas of general low dip, 6, 10-12) to be of Late Jurassic age. The latest sig and lack of repetition of lithologic members of the nificant m6vement along the Melones fault zone is less Calaveras formation, suggest that the structure of the closely dated: the zone is truncated south of the map block is .essentially homoclinal with younger heds on area by a pluton that is presumably a lobe of the Sierra the east. Nevertheless, folds with amplitudes of a few Nevada batholith (Cloos, 1932a) and of middle Cre hundred or even a few thousand feet may be present taceous age. in parts of the area lacking distinctive marker units. Faults of the Mother Lode fissure system may he of The data are most complete between Bagby and Brice about the same age as the Sierra Nevada batholith. burg, .and the possibility of undetected large folds is They are younger than the Melones fault zone, but greatest between Briceburg and the abandoned cement older than the unconformably overlying auriferous plant where the argillaceous unit yielded few top deter gravels of the region, generally considered to be of minations or lithologically distinctive horizons. Also, 54 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT exposures on canyon walls are poor in this interval, so Ridge volcanics, which there constitute much of the that the trace of bedding is not readily observed. East block, show little bedding. of the abandoned cement plant, long limestone lenses In the western block, structure is best documented and abundant chert horizons can be traced readily on near the Cosumnes River (pl. 8), where the syncline the canyon walls, and these show regular, near vertical at Michigan Bar Bridge, the anticline to the east, and dips. An area of tight, complex minor folding well the wide homoclinal section on the east flank of the shown in riverbank exposures in the northeast corner anticline are adequately controlled by top determina of sec. 19, T. 3 S., R. 19 E., may indicate the axis of a tions and attitudes of beds. On plate 8, the structure larger fold, but the limestone exposed west of this area of parts of the anticline is generalized, as the transi is not repeated to the east. Structure of the meta~ tion from gently dipping to steeply dipping beds on morphic rocks east of Indian Flat Guard Station is both sides of the crestal region is marked by small tight o?scure due to poor exposures and extensive crumpling. folds. Axes of subsidiary folds near the crest of the anticline indicate that it plunges northwest at an angle STRUCTURE OF THE CENTRAL BLOCK of about 25 °. The nearly horizontal attitude of beds at the west end of the Cosumnes River section is inferred In the northern and southern parts of the area the from exposures in the banks of the river near Bridge structure of the central block is homoclinal with tops House. Although no bedding was found here, lava east. Near the Cosumnes River, beds are vertical and is exposed at the low-water level for a distance of rubout near the Merced River they mostly dip eastward 50° 200 feet, whereas coarse volcanic breccia is exposed to 75 o. In the central part of the block ·are isoclinal higher on the banks in the same area, suggesting a hori folds, the largest of which is the syncline whose core zontal contact. ~s formed by the Mariposa formation. It has been On plate 7 the epiclastic and volcanic rocks west of traced down the Mokelumne River to the Stanislaus Campo Seco are interpreted to be a homoclinal section River. The trough of the fold is at an acute angle with tops east. This implies that the volcanic rocks in to the Melones fault zone, lying 2 miles west of the fault the core of the anticline at the Cosumnes River plunge zone on the Mokelumne River, and intersecting the beneath the bedrock surface near lone, and that the fault zone hetween the Stanislaus ·and Tuolumne Rivers. volcanic rocks near Campo Seco are on strike with the The areal pattern and generally very low plunge of westernmost belt of volcanic rocks exposed on the the lineation formed by intersection of bedding and Cosumnes River. However, if it be assumed that the cleavage indicates that the fold axis is nearly hori volcanic rocks near Campo Seco (pl. 7) are a contin~ zontal. The fold has been verified by repetition of beds uation of the belt that is overlapped by Tertiary de on the Mokelumne, Calaver.as, and Stanislaus Rivers, •' posits near lone, the distribution of epiclastic and vol~ and also by top determinations in the first two areas. canic rock units suggests, instead, that the large folds Most of the bedding on the limbs of the folds is nearly found on the Cosumnes River extend southward beyond vertical. the Mokelumne River. Heyl, Cox and Eric (1948, p. An isoclinal anticline is exposed east of this syncline ~7) state that tops are westward at the Penn mine (pl. near the Calaveras River, where it has been established 7) which may support this interpretation, although by top determinations and attitudes of bedding. The they do not indicate the size of the area covered by their anticline is shown as far north as the Mokelume RiveT observation. If this indicates a major fold, the epi on the accompanying maps, but here the structural clastic rocks west of the mine should be in a syncline. interpretation, is not well established. This anticline Nevertheless, these epiclastic rocks dip consistently east is truncated by the Melones fault zone south of San ward at an angle of about 35 °, and no trace of a trough Andreas. is apparent. Search for graded beds on the surface in The structure of the Peiion Blanco volcanics west of the Penn mine area during this investigation revealed Melones Dam is uncertain, as only one top determina only one small group consisting of about 5 dubiously tion was made and bedding is obscure in much of the graded beds. These are on the north bank of the area between the dam and the serpentine to the west. Mokelumne River near the west side of unit 7. The west parts of the beds were consistently lighter colored STRUCTURE OF THE WESTERN BLOCK and apparently finer grained than the east parts but The northern part of the western block is charac the light-colored parts apparently are fine-grained ag terized hy open folds overturned to the west. Large gregates resulting from alteration of feldspar crystals folds are also formed in the south part of the block, that originally constituted the coarser parts of the beds. but the shapes of folds are unknown because the Gopher If so, tops of the beds would be toward the east. STRUCTURAL GEOLOGY 55 Structural data are meager along the Calaveras River in metamorphic rocks of the shear zones and the eastern within the interval between A and B on the section block. Slip cleavage (White, 1949) pervades large (pl. G) and near liogan Dam. Between unit 8 and the parts of the eastern block, and occurs locally in rocks western part of unit 15 the structure is indicated by top of the western and central blocks. Nearly all the slate, determinations and attitudes. In the SWl~ sec. 14, T. and much of the graywacke, conglomerate, and tuff of 3 N., R. 10 E., beds dip westward at angles between the western block are cleaved. Much of the lava and 65° and 90° or are vertical, and the relation of cleavage coarse volcanic breccia seems massive in fresh exposures to bedding suggests tops west. In the eastern parts of but cleavage becomes apparent in weathered exposures. sees. 22 and 27, gross layered structure, suggesting su Regionally, and in single exposures (fig. 15), cleavage is perimposed lava flows, strikes northeast, parallel to the about parallel to the axial plane of folds. In the east river, and dips northwestward about 50°. The structure ern block, argillaceous rocks east of Bagby on the near !-logan Dam is arbitrarily interpreted to be homo Merced River show only cleavage, but most of the rocks clinal. east of the volcanic member of the Calaveras forma Near the Tuolunme and Merced Rivers, tops are west tion are phyllitic or schistose. erly near the western side of the area of bedrock expo In all rocks here described as schistose, a large sures and beds are folded near the eastern side of the proportion of the tabular minerals are parallel to block. Structure of much of the intervening area is the parting surfaces. In some, however, the parting is obscure because the Gopher Ridge volcanics consist attributable solely to parallel mineral grains and in .largely of massive volcanic and pyroclastic rocks in others it results in part from parallel rock fragments which bedding can be distinguished only locally. In of epiclastic or cataclastic origin. In most places, the the slate east of La Grange, on the Tuolumne River, no schistosity results from slip parallel to the parting graded beds occur, but minor folds in the western part planes as well as recrystallization. Within major fault of the slate belt (fig. 10) suggest that these beds are on zones slip is indicated in conglomerate and sandstone the west limb of an anticline. The epiclastic rocks east by microfaults and mortar structure with trains of of l\{erced Falls, on the Merced River, are tightly folded fragments and in phyllite by small drag folds with in places, but m.any graded beds show tops west wher sheared-off limbs. In much of the eastern block slip ever the structure straightens out (pl. 11). parallel to the schistosity is indicated by fragmentation Drag folds exposed for nearly a n1ile west of Don of beds, the development of spindle-shaped fragments, Pedro Dam on the Tuolumne River (fig. 16) indicate and minor folds with sheared-off limbs. In large areas that the eastern part of the Gopher Ridge volcanics lie bedding in the chert and argillaceous sequences of the on the west limb of an anticline. The axis of the anti Calaveras formation has been completely destroyed in cline is apparently a.Iong the northwest-trending part of the development of strongly lineated fragmental schist. Don Pedro Reservoir, for tops are toward the east im In other parts of the eastern block, especially in some lnediately east of this arm. However, the cr~st of the places where schistosity is parallel to bedding, the sig anticline may be cut out by a continuation of a fault nificance of slip in the development of schistosity is extending fr01n Lake McClure toward the northwest not apparent. trending arm of Lake McClure. Slip cleavage that is younger than schistosity can Because tops are westward on the west side of the be distinguished in some places east of the Melones main belt of Gopher Ridge volcanics on the Merced fault zone. It is most apparent southeast of San An River, and eastward on the east side, the Gopher Ridge dreas where it is parallel to the schistosity in the Me volcanics probably form an anticline, but part of the lones fault zone, but cuts eastward trending schistosity 11nticline may be faulted out as suggested by strong and related structures in the Calaveras formation schistosity in sec. 2, T. 5 S., R. 15 E. A syncline with (Clark, 1954, p. 12). There schistosity is commonly a core of slate is suggested by graded bedding west of parallel to bedding and both are deformed along the Exchequer Dam on the Merced River (pl. 3), and folds slip cleavage planes. The slip cleavage consists of of moderate size near the western shores of Lake Mc microfaults and crinkles. In thin section mica is seen Clure are indicated by the differing directions of tops to be sharply bent into the slip cleavage direction. The of beds at various localities. slip cleavage becomes less prominent eastward, where the trend of the earlier schistosity is more nearly paral SMALL-SCALE STRUCTURES lel to the Melones fault zone. Where slip cleavage CLEAVAGE AND SCHISTOSITY surfaces are closely spaced, they can be distinguished Cleavage is widely developed in rocks of the western only with difficulty from schistosity. Slip cleavage is and central blocks, t1nd schistosity is well developed shown by the same symbol as the schistosity on the map 56 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT of the Angels Camp quadrangle (Eric, Stromquist, and are chiefly in the eastern block and the two fault zones. Swinney, 1955, pl. 1, p. 29) although it is described Most lineation in the less deformed Mesozoic rocks separately in the text as shear cleavage. In some places that constitute the central and western blocks results the only remaining foliation in the rocks of the eastern from intersections of bedding and cleavage; a pencil block may be related to the slip cleavage. structure occurs where bedding and cleavage are nearly at right angles near the crests or troughs of moderately MINOR FOLDS to steeply plunging minor folds in epiclastic rocks of Minor folds have amplitudes of a few inches to a few Mesozoic age. Pencil structure in the slate east of Mer tens of feet. Minor folds resulting from both stages of ced Falls provided a useful guide in locating axes of regional deformation are found throughout the meta minor folds. morphic rocks of the map area, but most of the minor Lineations in the greatly sheared rocks are of several folds west of.the Melones fault zone plunge gently and kinds: in schists some lineations are marked by flat are related to the first stage, whereas most of those east triaxial ellipsoids derived from fragments that are of of the Melones fault zone plunge steeply and are re pyroclastic, cataclastic, and epiclastic origin, others by lated to the second stage. Axes of most minor folds elongate flat pods of chlorite or mica or by parallel here interpreted to be related to the first stage of de amphibole crystals. Lineations marked by minerals are formation plunge at angles of less than 20°, but axes commonly associated with those marked by rock frag of some plunge as much as 35°. Gently plunging minor ments, but are not restricted to fragmental schists. folds can be readily observed on the western limb of the Each kind has been found to be parallel to minor fold anticline east of Michigan Bar Bridge on the Cosumnes axes and hence parallel to b. River, and near both Don Pedro Dam and La Grange The most common lineations east of the Melones fault on the Tuolumne River. Gently plunging minor folds zone consist of elongate fragments of chert in a matrix are rare east of the Melones fault zone, but are asso of carbonaceous and commonly quartz-rich mica schist. ciated with gently plunging lineations on the north side Associated with them in many places are thin elongate of the Mokelumne River in sec. 32, T. 6 N., R. 12 E., mica pods. The lineations are very similar to some of and along the Tuolumne River in sec.1, T. 1 S., R.15 E. the mullion structures in the Moine series of Scotland Axes of minor folds interpreted to be related to the described by Gilbert Wilson (1953.) The chert frag second stage of deformation plunge more steeply than ments (granoblastic microcrystalline quartz) take many 60°. The steeply plunging minor folds occur in all forms depending upon the relations of cleavage and three fault blocks and the fault zones but are scarce bedding and the amount of deformation. Axial ratios in the central and western blocks. Too few steeply range from the order of 1 :1 :200 or 1 :5 :200 iri the more plunging folds were ·mapped to provide independently elongate fragments to about 1 :1 :5 in spindle-shaped a regional movement picture, but the folds served to fragments. In cross section, the fragments are circu indicate the tectonic orientation of lineations which are lar in some places and angular in others, but more com much more abundant. Axial planes of steeply plung monly show streamlined shapes. Lineations lie in the ing minor folds are parallel to slip cleavage and plane of schistosity where schistosity can be identi schistosity. fied, but in rare places where the rock consists of closely Further work will be necessary to establish the sig packed cylinders of chert no prevailing direction of nificance of minor folds having different orientations schistosity was found. Although chert fragment linea than folds characteristic of the two stages of regional tion is of the same .kind throughout areas underlain by deformation. An example of such folds is in the Mer the chert-bearing rocks east of the Melones fault zone. ced Falls slate along the Merced River. Here, minor they are not necessarily everywhere related to the sam~ fold axes are of diverse orientation except near the stage of deformation. east side of the slate belt where the plunge is consist The chert fragments were formed by cataclastic ently steep and to the southeast (pl. 11). The diversity processes; they are not elongated epiclastic pebbles of orientation possibly results from a local modification (Clark, 1954, p. 6-7). The abundant and widespread of regional deformation patterns by a buttress of vol lineation in the Calaveras formation probably results canic rocks that ends north of the river (pl. 1), or from the great extent of rocks consisting of alternating from superimposed folding. thin beds of chert and relatively incompetent carbona ceous quartz-mica schist. In exposures and speci LINEATIONS mens showing the relation of cleavage to bedding, linea Lineations discussed in the following paragraphs are tion is parallel to axes of minor folds and is formed in in the more strongly deformed and sheared rocks that two ways; both result in lineation parallel to the b STRUCTURAL GEOLOGY 57 tectonic axis. Where bedding crosses the schistosity, that followed rotation of the beds into a near vertical the chert beds are broken into long prisms by offset position. along schistosity surfaces, but where bedding is parallel Intersection of slip cleavage that is parallel to schis to the schistosity elongate fragments are formed by tosity of the Melones fault zone with earlier schistosity boudinage. 'i\There offset is slight, the fragments of and bedding forms a prominent set of lineations con chert beds are angular. With further movement they sisting of axes of minor shear and flexure folds, crinkles, n.ssume streamlined or roundish forms in cross section. and crenulations on schistosity surfaces. These struc A third process depends upon the tendency, common tures are developed southeast of San Andreas, where in rocks consisting of alternate layers of competent rock the eastward-trending anticline in the Calaveras forma and slate, for cleavage which is nearly parallel to the tion is at a large angle to the Melones fault zone. bedding in the slate to be refracted at a large angle to Although the slip cleavage and associated lineations the bedding in the competent layer. Such refracted are prominent, earlier schistosity and bedding can be cleavage breaks the competent layer into prisms elon readily traced in most exposures. The strike of the gate parallel to b. slip cleavage ranges as much as ~59 and attitudes of the In much of the area deformation is so severe that bed bedding and schistosity range much more widely. ding is completely destroyed, and the origin and tec Consequently, the plunge of lineations related to slip tonic orientation of the chert fragment lineation is ob cleavage is apparently at random, even in areas of less scure; no pattern that is consistent throughout the east than a square mile. Lineations of this sort can be ern block is apparent. Nevertheless, the attitude of expected in the Calaveras formation within a few miles lineation is reasonably consistent over large areas within of the Melones fault zone wherever slip cleavage is de which it is possible to show that the lineation is parallel veloped. Linear structures related to slip cleavage can to axes of small folds. Not all small folds, however, be seen conveniently in the floor of the Calaveritas Hill were necessarily formed during the same stage of de Consolidated hydraulic pit, about 4 miles southeast of formation. Lineations with gentle to moderate dip are, San Andreas (see Clark, 1954, pl. 1) . at least h1 places, related to large folds apparently Lineations within the Melones fault zone lie within formed during the first stage of severe deformation of the planes of schistosity and shearing, and plunge east the Calaveras formation. Steeply plunging lineations, ward and southeastward at angles of 60° to 80°. The also characteristic of large areas, seem to be best ex plained as the result of a second stage of deformation bearing of the lineation is nearly normal to the strike acting on previously folded beds. Relation of moder of the schistosity. Abundant parallel amphibole crys ntely plunging lineations to major folds is best shown tals form a well-marked lineation in the sheared west east of San Andreas where the lineations generally ern part of the granitic pluton north of Plymouth. plunge ESE. 30° to 50°. The pattern of volcanic rocks The lineation is developed throughout the width of the southeast of San Andreas show·s that the anticline in western tongue of the pluton near the Cosumnes River, which they are exposed plunges eastward, but the angle a distance of more than half of a mile, but no lineation of plunge is not known directly. Lineations in the is visible in unsheared granodiorite of the same pluton Mokelumne River also plunge at moderate angles in a several miles east of the Melon~s fault zone. Between direction slightly more southerly than mo~t of the linea San Andreas and Angels Camp, lineation is marked tions east of San Andreas and are probably related by elongated fragments of volcanic breccia and elongate to the same fold system. blebs of chlorite in the metavolcanic rocks and by elon The few measurements on the Tuolumne River show gated pebbles and boulders in the mixed-pebble con both gentle and steeply plunging lineations and form a glomerate of the Mariposa formation. Within the pattern that is not readily interpreted. Those on the Bear Mountains fault zone lin~3-tion is formed by elon Merced River and the North Fork of the Merced River gated fragments in volcanic breccia, elongate amyg fall into two groups, both lying in a vertical plane that dules, and thin elongated blebs of chlorite. strikes N. 70° W. and is close to the average for schis In summary, lineations that show a consistent pat tosity in this region. Lineations in the argillaceous tern seem to be parallel to a b tectonic axis, but pr.esent sequence nearest the volcanic sequence plunge north information indicates that it will be necessary to dis west at nngles of 50° to 80° whereas those farther east tinguish a b1 related to the first important folding and plunge southeast .at similar angles. The steepness of a b2 related to a subsequent stage of deformation. It plunge of these lineations and the associated minor folds cannot be assumed that all b2 lineations mapped in dif suggest that they are related to a stage of deformation ferent places result from the same tectonic episode. 58 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT REFERENCES CITED Eric, J. H., Stromquist, A. A., and Swinney, C. M., 1955, Geology and mineral deposits of the Angels Camp and Sonora quad Arkell, W. J., 1956, Jurassic geology of the world: New York, rangles, Calaveras and Tuolumne Counties, California: Hafner Pub. Co. California Div. Mines Spec. Rept. 41, 55 p. Baird, A. A., 1962, Superposed deformation in the Central Sierra Ferguson, H. G., and Gannett, R. W., 1932, Gold quartz veins of Nevada foothills east of the Mother Lode: California Univ. the Alleghany district, California: U.S. Geol. Survey Prof. Dept. Geol. Sci. Bull., v. 42, p.l-70. Paper 172. Bateman, P. C., Clark, L. D., Huber, N. K., Moore, J. G., and Heyl, G. R., 1948a, The Grayhouse area, Amador County, Cali Rinehart, C. D., 1963, The Sierra Nevada batholith-a fornia: California Div. Mines Bull. 144, p. 85-91. synthesis of recent work across the central part: U.S. Geol. --- 1948b, Ore deposits of Copperopolis, Calaveras County, Survey Prof. Paper 414-D, p. Dl-D46. California: California Div. Mines Bull. 144, p. 93-110. Becker, .G. F., 1885, Notes on the stratigraphy of California: --- 1948c, The zinc-copper mines of the Quail Hill ·area, U.S. Geol. Survey Bull, 19, 28 p. Calaveras County, California: California Div. Mines Bull. Best, M. G., 1963 Petrology and structural analysis of meta 144, p. 111-126. morphic rocks in the southwestern Sierra Nevada foothills, Heyl, G. R., Cox, M. W., and Eric, J. H., 1948, Penn zinc-copper California: California Univ. Dept. Geol. Sci. Bull., v. 42, mine, Calaveras County, California: California Div. Mines no. 3, p. 111-158. Bull. 144, p. 61-84. Heyl, G. R., and Eric, J. H., 1948, Newton copper mine, Amador Blackwelder, Eliot, 1928, Mudflow as a geologic agent in semi County, California: California Div. Mines Bull.144, p. 49-60. arid mountains: Geol. Soc. America Bull., v. 39, p. ~65-483. Hill, M. L., and Dibblee, T. W., Jr., 1953, San Andreas, Garlock, Calkins, F. C., 1930, The granitic rocks of the Yosemite region: and Big Pine faults, California-a study of the character, U.S. Geol. Survey Prof. Paper 160, p. 120-129. history and tectonic significance of their displacements : Cater, F. W., Jr., 1948a, Chromite deposits of Tuolumne and Geol. Soc. America Bull., v. 64, p. 443-458. Mariposa Counties, California : California Div. Mines Bull. Hyatt, Alpheus, 1894, Trias and Jura in the Western States: 134, pt. 3, chap. 1, p. 1-32. Geol. Soc. America Bull., v. 5, p. 395-434. ---1948b, Chromite deposits of Calaveras and Amador Coun-, Imlay, R. W., 1952, Correlation of the Jurassic formations of ties, California: California Div. Mines Bull. 134, pt. 3, North America, exclusive of Canada: Geol. Soc. America chap. 2, p. 33-60. Bull., v. 63, p. 953-992. Cater, F. W., Jr., Rynearson, G. A., and Dow, D. H., 1951, --- 1959, Succession and speciation of the pelecypod Auoellao: Chromite deposits of El Dorado County, California: Cali U.S. Geol. Survey Prof. Paper 314-G, p. 155-169. fornia Div: Mines Bull. 134, pt. 3, chap. 4, p. 107-167. ---1961, Late Jurassic ammonites from the western Sierra Cayeux, Lucien, 1929, Les Roches sedimentaires de France: Nevada, California: U.S. Geol. Survey Prof. Paper 374-D, Roches Siliceuses: Paris, Imprimerie Nationale. p. Dl-D30. Clark, L. D., 1954, Geology and mineral deposits of Calaveritas Jenkins, 0. P., ed., and others, 1948, Copper in California: quadrangle, Calaveras County, California: California Div. California Div. Mines Bull.144, 429 p. Mines Spec. Rept. 40, 23 p. King, P. B., 1937, Geology of the Marathon region, Texas: U.S. ---1960a, The Foothills fault system, western Sierra Nevada, Geol. Survey Prof. Paper 187, 148 p. California : Geol. Soc. America Bull., v. 71, p. 483-496. Knopf, Adolph, 1929, The Mother Lode system of California : U.S. Geol. Survey Prof. Paper 15.7, 88 p. ---1960b, Evidence for two stages of deformation in the Kuenen, P. H., 1950, Marine geology: New York, John Wiley western Sierra Nevada metamorphic belt, California: U.S. and Sons, 568 p. · Geol. Survey Prof. Paper 400-B, Art. 148, p. B 316-B 318. --- 1957, Sole markings of graded graywacke beds: Jour. Clark, L. D., Imlay, R. W., McMath, V. E., and Silberling, N. J., Geology, v. 65, p. 231-258. 1962, Angular unconformity between Mesozoic and Paleozoic Kuenen, P. H., and Migliorini, C. I., 1950, Turbidity currents as rocks in the northern Sierra Nevada, California: U.S. Geol. a cause of graded bedding: Jour. Geology. v. 58, p. 91-127. Survey Prof. Paper 450-B, Art. 6, p. B15-B19. Lawson, A. C., 1933, The geology of middle California: Internat. Cloos, Ernst, 1932a, Structural survey of the granodiorite Geol. Cong., 16th, Washington 1933, Guidebook 16, p. 1-12. south of Mariposa, California: Am. Jour. Sci., 5th ser., v. Lindgren, Waldemar, 1894, Sacramento, Calif.: U.S. Geol. Sur 23, p. 289-304. vey Geol. Atlas, Folio 5, 3 p. ---1900, Colfax, Calif.: U.S. Geol. Survey Geol. Atlas, Folio ---1932b, "Feather joints" as indicators of the direction of 66,10 p. movements on faults, thrusts, joints, and magmatic con Lindgren, Waldemar, and Turner, H. W., 1894, Placerville, tacts: Natl. Acad. Sci. Proc., v. 18, no. 5, p. 387-395. Calif.: U.S. Geol. Survey Geol. Atlas, Folio 3, 3 p. Cloos, Ernst, 1935, Mother Lode and Sierra Nevada batholiths: Natland, M. L., and Kuenen, P. H., 1951, Sedimentary history Jour. Geology, v. 43, p. 225-249. of the Ventura Basin, California, and the action of turbidity Compton, R. R., 1955, Trondhjemite batholith near Bidwell Bar, currents, in Soc. Econ. Paleontologists and Mineralogists, California : Geol. Soc. America Bull., v. 66, no. 1, p. 9-44. Turbidity Currents and the transportation of coarse sedi Curtis, G. H., Evernden, J. F., and Lipson, Joseph, 1958, Age ments to deep water-a symposium : Soc. Econ. Paleontolo gists and Mineralogists, Spec. Pub. no. 2, p. 76-107. determination of some granitic rocks in California by the Pettijohn, F. J., 1943, Archean sedimentation: Geol. Soc. Amer potassium-argon method: California Div. Mines Spec. Rept. ica Bull., v. 54, p. 925-972. 54,16 p. ---1954, Classification of sandstones: Jour. Geology, v. 62, Durrell, Cordell, 1940, Metamorphism in the southern Sierra p. 360-365. Nevada northeast of Visalia, California: California Univ. ---1957, Sedimentary rocks: New York, Harper and Dept. Geol. Sci. Bull., v. 25, p. 1-117. Brothers, 718 p. DESCRIPTION OF MAP UNITS 59 Piper, A. M., Gale, H. S., Thomas, H. E., and Robinson, T. W., Wilson, Gilbert, 1953, Mullion and rodding structures in the 1939, Geolot:,"Y and ground-water hydrology of the. Mokelumne Moine series of Scotland: Geologists' Assoc., Londo.n, Pr. v. Hill m·ea, California: U.S. Geol. Survey Water-Supply 64, pt. 2, p. 118-151. Paper 780, Zeuner, F. E., 1952, Dating the past; an introduction to Ransome, 1!"". L., 1900, Mother Lode district: U.S. Geol. Survey geochronology: London, Methuen and Co., Ltd., 495 p. Geol. Atlas, Folio 63, 11 p. DESCRIPTION OF MAP UNITS Reiche, Pan-y, 1937, Geology of the Lucia quadrangle, Cali· fornia: Cnlifornia Univ. Dept. Geol. Sci. Bull., v. 24, no. 7, TABLE !.-Description of map units, Merced River (pl. 3) p. 115-168. Map Af,n~:i;:8ate Rem;, 0., Lakeman, R., and VanDerMeulen, E., 1955, Submarine unit (feet) Lithology sliding in western Venezuela: Am. Assoc. Petroleum Geolo· l_ __ _ 2000 ____ _ Top not exposed. Black slate, locally gists Bull., v. 39, no. 10, p. 2053-~067. pyrite bearing, with interbedded gray Shrock, R. R., 1948, Sequence in layered rocks: New York, wacke and siltstone. Graywacke beds McGraw-Hill, 507 p. commonly less than one-half . in. thick below Merced Falls dam, several inches Smith, J. P., 1894, Age of the a.uriferous slates of the Sierra thick elsewhere, locally thick bedded. Nevada: Geol. Soc. America Bull., v. 5, p. 243-258. -Interbedded rhyolite(?) tuff eastern part. Storms, W. H., 1894, Ancient channel system of Calaveras Graywacke beds commonly graded County: California State Mining Bureau, Rept. 12, p. 482- throughout the section. 2 ___ _ 492. Coarse rhyolite tuff. 'l'aliaferro, N. L., 1933, Bedrock complex of the Sierra Nevada, 3 ___ _ 1000+ --- Like unit 1. west of the southern end of the Mother Lode [nbs.] : Geol. 4 ___ _ 1000+ --- Schistose volcanic rocks with some rhyo- Soc. America Bull., v. 44, no. 1, p.149-150. lite(?) tuff on west side; mafic and inter ---1942, Geologic history and correlation of the Jurass-ic of mediate flows, volcanic breccia and minor southwestern Oregon ru1d California: Geol, Soc. American tuff elsewhere. No graded tuff beds Bull., v. 53, no.l, p. 71-112. found. 5____ 3000+--- Mafic or intermediate thick-bedded pyro ---1943a, Franciscan-Knoxville problem : Am. Assoc. Petro clastic rocks. leum Geologists Bull., v. 27, no. 2, p.109-219. 6_ _ _ _ 500:.. _ _ _ _ Laminated black slate with interbedded thin ---1943b, Manganese deposits of the Sierra Nevada, their graywacke beds. Includes thick-bedded genesis and metamorphism : California Div. Mines Bull. 125, graywacke about 100 ft thick on west p. 277-332. side. Rare graded beds in thick-bedded Taliaferro, N. L., and Solari, A. J., 1949, Geology of the Copper graywacke. Interpreted to be equiva opolis quadra.ngle, California: California Div. Mines Bull. lent to unit 8. 145 (map only). 7____ 200_____ Bedded fine conglomerate with interbedded Trusl{, P. D., 1926, Geology of the Point Sur quadrangle, Cali slate. Pebbles angular to well rounded, fornia: California Univ. Dept. Geol. Sci. Bull., v. 16, p. as much as 1 in. long in some beds, but 119-186. smaller in most beds. Slate matrix. Turner, H. W., 1893a, Some recent contributions to the geology Rock fragments include slate, dark of California: Am. Geologist, v. 11, p. 307-324. and light-gray chert, white· and gray vein quartz. ---1893b, Mesozoic granite in Plumas Colinty, California, g____ 500 __ --- Laminated black slate with minor inter ru1d the Calaveras formation: Am. Geologist, v. 11, p. 425- bedded graywacke on west side. 426. 9 __ -- 400_ _ _ _ _ Thin- to thick-bedded rhyolite tuff, lapilli ---1894a, Jacl{son, Calif.: U.S. Geol. Survey Geol. Atlas, tuff and silicified ash. Lapilli tuff con Folio 11, 6 p. tains fragments of chert or silicified vol ---1894b, The rocks of the Sierra Nevada: U.S. Geol. Survey, canic glass or ash. Large proportion of 14th Ann. Rept., pt. 2, p. 435-495. beds are graded. ---1894c, Geological notes on the Sierra Nevada: Am. Geolo 10___ 1500+--- Locally faulted on east side. Silicified ash gist, v. 13, p. 228-249, 297-316. and volcanic breccia with quartz pheno ---1895, The age and succession of the igneous rocks of crysts; in part, at least, rhyolitic(?); Slerrn Nevada: Jour. Geology, v. 3, p. 385-414, map. dark-gray massive rhyolite(?) porphyry; and minor black chert. Turner, H. W., and Ransome, F. I.J., 1897, Sonora, Calif.: U.S. ll___ 500+ _ _ _ Structure uncertain; faulted on east side. Geol. Survey Geol. Atlas, Folio 41. Laminated black slate with interbedded ---1897, Description of the Big Trees quadrangle: U.S. thin beds of graywacke or tuff in some Geol. Survey Geol. Atlas, Folio 51. places. Contains interbedded conglom --- 1899, 'l'he granitic rocks of the Sierra· Nevada: Jour. erate near south quarter-corner of sec. Geology, v. 7, p. 141-162. 12, T. 4 S., R. 15 E. Slate is locally White, '\V. S., 1949, Cleavage in east-central Vermont: Am. quartzose, with subvitreous luster. Near Geophys. Union 'l'rans., v. 30, p. 587-594. south quarter corner of sec. 1, T. 4 S., R. 15 E., contains pods about 1 ft thick Wilmarth, M. G., 1938, Lexicon of geologic names of the United of dark-gray calcarenite. One such pod States: U.S. Geol. Survey Bull. 896, pt. 1. contains Plesiosaur bones. 60 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT TABLE 1.-Description of map units, Merced River {pl. 3)-Con. TABLE 1.-Description of map units, Merced River (pl. 3)-Con. Approximate Map thickneaa Map A~rrc~~:;~e unit (feet) Lithology unit (feet) Lithology 12 __ _ 1000+ --- Faulted on west side. Mafic or intermediate 3L __ 0-300 __ _ Poorly sorted schistose conglomerate con volcanic breccia and tuff; some tuff beds taining angular to well-rounded frag graded. Includes chlorite schist of the ments of graywacke as much as 18 in. in fault zone. diameter in a mottled yellow-green and 13 __ _ 1000+--- Thin-bedded black slate and tuff. black sandy argillite matrix. Not bedded. 14 __ _ 300+--- Thin-bedded rhyolite(?), crystal tuff, and 32 __ _ 1100_--- Interbedded dark-gray chert and black silicified ash or chert. Top not mapped. slate. Most chert and slate beds are 15 __ _ 1000 ____ - Coarse dark-green volcanic breccia. about 1 in. thick. 16 __ _ 5000 ____ _ Massive aphanitic dark-green mafic lavas. 33 __ _ 400 ____ _ Poorly sorted conglomerate, like unit 31. Three bands of pillow lava present in 34 __ _ 600 ____ _ Black slate with minor interbedded tuff upper 2000 ft; rare suggestions of pillow and volcanic breccia near center of unit. structure in lower part. 35 __ _ 400 ____ _ Massive dark-green recrystallized tuff. 17 __ _ 1500 ____ - Thin-bedded tuff and silicified ash; graded 36 __ _ 4000 ___ _ Interbedded laminated black slate, massive bedding common. Hunter Valley cherts black siltstone, and dark-green slate. of Taliaferro ( 1933). Individual rock types form subunits 18 __ _ 3000 ___ _ Recrystallized and epidotized mafic or 50 to 100 ft thick. intermediate pyroclastic rocks; top poorly 37 __ _ 5000 ____ - Coarse dark-green volcanic breccia, volcanic exposed. conglomerate, and very fine grained 19 __ _ 2000+-- Coarse massive dark-green volcanic breccia metatuff with well-developed slaty cleav with large augite phenocrysts. age Volcanic breccia contains fragments 20 __ _ 2500 ___ _ Thick-bedded coarse tuff and volcanic with pyroxene crystals as much as one breccia. A few thin-bedded zones with half inch in diameter, and locally contains graded bedding are present. abundant black slate fragments Two 2L __ 600 ____ _ Black laminated slate with rare Aucella. interbedded units about 50 ft thick near 22 __ _ 300 ____ _ Coarse dark-green volcanic breccia. base consist chiefly of black slate with 23 __ _ 500 ____ _ Interbedded thin-bedded graywacke and interbedded tuff and conglomerate con black slate. taining volcanic rock fragments in black 24 __ _ 20Q-500_ Thin-bedded tuff and silicified volcanic ash. silty slate matrix. 900 ____ _ Graded bedding common. Unit thickens 38 __ _ Fine-grained green phyllite and phyllitic to the southeast. tuff with some interbedded black phyllite. 25 __ _ 500+ --- Interbedded black slate and graywacke with 39 __ _ 4000 ____ _ Interbedded black slate, massive black thick-bedded graywacke horizon at base. siltstone, green slate, and fine-grained Graywacke contains fragmental plant tuff. remains in Highway 49 roadcut on south 40 __ _ 5000+ --- Thin-bedded black slate and phyllite, gra side of Merced River at Bagby. Fault phitic in part. Contains thinly inter contact with serpentine on east. 26 __ _ 2000 ___ _ bedded metachert locally. Faulted at base-in contact with serpentine. 4L __ 1000 ____ - Conglomerate, argillaceous sandstone, and Laminated black silty slate in lower part, massive black sandy siltstone. Con containing lens of black massive aphanitic limestone. One exposure of volcanic glomerate contains fragments of mafic breccia with pyroxene phenocrysts as volcanic rocks, some of which are amyg much as one-quarter inch in diameter at daloidal,. limestone, chert, and calcareous Bagby. Upper part is interbedded black orthoquartzite. Fragments in some layers silty slate and graded graywacke. Beds are nearly all volcanic rocks, in others, are about one-half inch thick. Parts of nearly all chert. some graywacke beds are cross-laminated. 42 __ _ 1500 __ --- Thinly interbedded chert and black car 27 __ _ 5Q-400 __ Interbedded thin-bedded green tuff, silicified bonaceous slate. Contains massive black volcanic ash, and black slate. Volcanic quartz-mica phyllite. Contains knots of breccia occurring as float on northeast sericite, probably after andalusite, near side contains pyroxene phenocrysts as small plutonic body southwest of much as one-quarter inch in diameter. abandoned cement plant. 28 __ _ 500 ____ _ Dark-green quartzose slate. Bedding not 43 __ _ 9Q-200 __ _ visible. Cleavage fairly well developed. Black aphanitic thin-bedded limestone. 29 __ _ 300 ____ _ Black slate, with lenticular dark-gray lime Locally crossbedded. 44 __ _ •.J stone containing abundant crinoid debris. 1000 __ --- Thinly interbedded chert and black car 30 __ _ 1000_--- Interbedded tuff and black slate. Includes bonaceous phyllite. 45 __ _ thin unit of interbedded limestone (in 1500 ____ - Fine-grained hornblende-feldspar schist. part calcarenite), black slate, and silty Probably a metavolcanic rock. sandstone exposed at sand bar in NEH 46 __ _ Like unit 44. 35 ______SWH sec. 10, T. 4 S., R. 17 E. 47 __ _ Dark-gray aphanitic limestone. DESCRIPTION OF MAP UNITS 61 TABLE !.-Description of map units, Merced River (pl. 3)-Con. TABLE 2.-Description of map units, Tuolumne River (pl. 4)-Con. Approzimate Approximate thickness Map thickness :r;::c (feet) Lithology unit (feet) Lithology 10 __ _ 48 __ _ 4000+--- Thin-bedded chert interbedded with black Phyllitic light-colored volcanic rock; includes carbonaceous phyllite and schist and mas some tuff and slate or very fine tuff. sive black quartzose argillite. Chert ll __ _ 500+ --- Massive light-colored volcanic rock. In layers range in thickness from about cludes so.me tuff and volcanic breccia. H to 10 in. May include some quartz Schistose on east. siltstone. In easternmost part, massive 12 __ _ Thick-bedded pyroclastic rocks, massive dark-gray sandstone with widely dispersed light-gray silicified volcanic rocks and roundish quartz grains in very fine minor amygdaloidal lava. grained quartzose matrix. 13 __ _ 1500+--- Thin- to thick-bedded light-green tuff and 49 __ _ Massive microcrystalline black quartz horn lapilli tuff with minor volcanic breccia. fels in western part; thick-to thin-bedded Thickness uncertain because of folding quarz-mica hornfels with scattered round and faults. quartz grains, probably sandstone, in 14 __ _ 1000+--- Thin-bedded black slate with interbedded eastern part. tuff near west side. Interbedded gray 50 __ _ Massive argillite, thinly interbedded chert, wacke with abundant volcanic rock frag and black phyllite or schist. Some ments; g.raded beds .O:P. ~a~t s~de. feldspathic quartzite near west side. 15 __ _ 300-----, Interbedded tuff and black slate. 5L __ Lile unit 50 but contains limestone, in part 16 __ _ 500± --- Massive dark-green amygdaloidallava. 17 __ _ silicated or altered to tactite at eastern 300± --- Interbedded tuff and volcanic.breccia. margin. 18 __ _ 300±--- Black slate. 19 __ _ Dark-green volcanic breccia, cut by grano TABLE 2.-Description of map units, T1wlumne River (pl. 4) diorite sills and porphyritic dikes with feldspar phenocrysts. Map APfhfcO:~:~te unit (feet) Lithology 20 __ _ Schistose greenstone. l_ __ _ 21_ __ 500+ --- Massive black aphanitic porphyritic rhyo 2000+ --- Porphyritic rhyolite, locally sheared. Prob lite(?) with feldspar phenocrysts about 1 able fault on west side. 22 __ _ mm long. Possibly intrusive. West bound Massive dark-green metavolcanic rock. ary not exposed. 23 __ _ Massive dark-green metavolcanic rock con 2 ___ _ 4000 ____ - Volcanic and metavolcanic rocks. From taining small masses of hornblende gabbro. west to east these are: fine-grained, mas 24 __ _ 3000+--- Dark-green massive porphyritic volcanic sive greenstone; massive aphanitic fresh rock with feldspar phenocrysts 1 to 2 mm looking rhyolite or dacite porphyry with long. In part silicified. Eastern part quartz phenocrysts 0.5 mm in diameter amygdaloidal. 25 __ _ and feldspar phenocrysts 1 to 3 mm long; 700----- Medium-green volcanic breccia. Amygda medium-bedded tuff and silicified ash; loidal fragments abundant in places. 26 __ _ interlayered light- and dark-green vol 300----- Black slate with interbedded graywacke beds canics including some pyroclastics and 1 in. to 1 ft thick. Graywacke beds are massive rhyolite porphyry. commonly graded. Conglomerate at base 3 ___ _ 1000+--- Thin- to medium-bedded black slate and of formation consists of pebbles of volcanic siltstones, rare graywacke. No graded rocks in black slate matrix. Conglom beds found. Contains belemnites east of erate lenses about 45 ft above the base of LaGrange powerhouse. the unit contain pebbles of vein quartz. 4 ___ _ 500+ --- Medium-green massive silicified aphanitic Invertebrate fossils abundant 200 to 225 ft volcanic rock. Shows vertical columnar above the base of the unit. jointing locally. 27--- 600----- Thick-bedded coarse graywacke. Graded 5 ___ _ Schistose, light-green metatuff and volcanic bedding not common. Only the basal breccia; contains fragments of silicified contact exposed on line of traverse-upper ash and volcanic rocks. Fault on east contact projected from mapping by G. R. side. Heyl (written communication). 6 ___ _ Medium-bedded silicified volcanic ash and 28 __ _ 1000+--- Black slate; very poorly exposed along line 7 ___ _ tuff, lapilli tuff, and volcanic breccia. of traverse. Contains medium-bedded in Schistose, silicified epidotized volcanic rocks, . terlayered graywacke and slate in sec. 20, with some volcanic breccia, lapilli tuff, T. 1 S., R. 15 E. Many graded gray chert or silicified ash, and massive rhyo wacke beds. li_te{ ?) porphyry with feldspar phenocrysts. 29 __ _ 500± __ _ Thin- to medium-bedded black slate. Con 8 ___ _ Sheared agglomerate or pillow lava consisting tains two small limestone lenses. Faulted of ellipsoidal bodies of amygdaloidal on east and west sides. aphanitic lava as much as 6 in. thick and 30 __ _ Schistose metavolcanic rock. 3 ft long. Contains rare fragments of 31_ __ Black carbonaceous quartzose mica schist. medium-bedded tuff. 32 __ _ Schistose metavolcanic rock. Fault on east g ___ _ 100 ____ _ Dark-green sheared pillow lava. side. 62 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT TABLE 2.-Description of map units, Tuolumne River (pl. 4) TABLE 3.-Description of map units, Stanislaus River (pl. 5) Continued Continued Approximate Map Aff;~~;1:;:ee Map thickness unit (feet) Lithology unit (feet) Lithology 2 ___ _ 33 __ _ 2000±.-- Black thin- to medium-bedded interlayered Light-green rhyolite or dacite porphyry with metachert and carbonaceous mica schist. aphanitic groundmass. Quartz pheno 34 __ _ 2QQ ____ _ Volcanic conglomerate and schistose meta crysts rare. Locally sheared. volcanic rock. 3. ___ _ 200_-.-- Dark-green massive metavolcanic rock, 35 __ _ 15QQ_--- Black quartzose mica schist; bedding obscure probably lava. in most places. 4---- 400_-- ·- Light-green volcanic breccia with abundant 36 __ _ 2QQ ____ _ Interbedded limestone and black quartzose fragments of white chert or silicified ash. mica schist. Feldspar crystals 1 to 2 mm long are 37--- 15QQ ___ _ Medium to very thin bedded interlayered abundant in groundmass. chert and black carbonaceous schist with 5 ___ _ Light-green thick-bedded lithic tuff and minor interbedded volcanic . breccia and volcanic breccia. massive metavolcanic rock. Coarse volcanic breccia with interbedded tuff. 76---- ___ _ 38 __ _ 1500+-- Black thin- to medium-bedded interlayered 2000 ____ _ Medium to very thick bedded medium- to chert and carbonaceous schist. Includes dark-green coarse tuff. some medium-bedded quartz ·siltstone. g ___ _ 200 ____ _ Dark-green volcanic breccia. Locally includes pods of limestone and 9 ___ _ 200 ____ _ Massive amygdaloidal volcanic rock. ca_lcareous qua.rtz-mic~. schist .. 10 __ _ 3000 ___ _ Dark-green tuff .and volcanic breccia. 39 __ _ Limestone with interbe.dded black, carbo Medium to very thick bedded. Some naceous quartzose mica schist. Includes tuff beds graded. Schistose except in limestone breccia at the mouth of the eastern part. North Fork of the Tuolumne River. 1L __ 300 ____ _ Medium-bedded silicified ash and tuff. 4Q __ _ 0-5QQ __ _ Dark-green tuff and volcanic breccia. Thins Some graded beds. 12 __ _ rapidly west of the mouth of the North 500+--- Black slate. Fork of the Tuolumne River. Includes 13 __ _ Volcanic breccia and coarse tuff. thin zone of black quartzose carbonaceous 14 __ _ 500 ____ _ Quartz-bearing rhyolite(?) tuff. schist at contact. 15 __ _ 200 ____ _ Black slate. Includes a medium-bedded 41_ __ 200 ____ _ Thin-bedded limestone. tuff and silicified ash sequence about 10 42 __ _ 1100_ --- Thin- to medium-bedded interlayered black ft thick. metachert and carbonaceous schist. Con 16 __ _ 300 ____ _ Very fine grained tuff. tains zone of limestone breccia in middle 17 __ _ 1000 ___ _ Black slate with interbedded tuff layers 1 part of unit. mm to about 1 m thick. 18 __ _ 43 __ _ 1800_--- Thick-bedded limestone-probably includes 400+--- Quartzose slate. some dolomite. 19 __ _ 1000_--- Medium to very thick bedded volcanic 44 __ _ 3QQ ____ _ Massive very fine grained black meta breccia, tuff, and silicified ash with minor volcanic rock. amygdaloidal lava. Contains abundant 45 __ _ 2QQ ____ _ Medium-bedded limestone. feldspar phenocrysts as much as 5 mm 46 __ _ 3QQ ____ _ Thin- to medium-bedded vitreous metachert long. and black, carbonaceous schist. 2Q __ _ 4QQ ____ _ Interbedded black slate, volcanic breccia, 47 __ _ 1000_--- Limestone and dolomite with interbedded and tuff. metachert and black, carbonaceous schist. 21_ __ 1400 ___ _ Black slate with interbedded graywacke. 48 __ _ 1500_--- Thin- to medium-bedded black, carbo 22 __ _ 1000_--- Coarse volcanic breccia; contains fragments naceous quartzose mica schist. of white chert or silicified ash on east side. 49 __ _ 300~)_--- Coarse crystalline limestone and medium Minor tuff. grained dolomite with interbedded quartz 23 __ _ 300_-- -·- Medium-bedded tuff and silicified ash. ose mica schist. Graded beds common. 50 __ _ Black, carbonaceous quartzose mica schist 24 __ _ 1100 ___ _ Black slate with interbedded tuffaceous gray and gneiss with rare pods of coarse wacke, graywacke, and minor conglomer crystalline limestone. Cut by granitic ate. Conglomerate has angular to well rock and pegmatite dikes. rounded fragments of vein quartz, chert, and silicified ash(?), and angular fragments TABLE 3.-Description of map units, Stanislaus River (pl. 5) of black slate. Approximate 25 __ _ Map thickness Coarse dark-green tuff on west; dark-green unit (feet) Lithology volcanic breccia in eastern part. l_ __ _ Includes volcanic rocks of mafic or inter 26 __ _ 300 ____ _ Medium- to thick-bedded tuff and very mediate composition and rhyolite (?) fine tuff. porphyry. Most mafic and intermediate 27 __ _ 5000+-- Coarse yellow-green tuff with scattered thin rocks are schistose, whereas most of the layers of volcanic breccia. Bedding ob rhyolite is fresh looking and massive. scure. Contains zone about 25 ft thick ·Contacts of rhyolite cross schistosity of of pillow lava in the NE~ sec. 32, T. 1 N., more mafic rocks at least locally. R. 13 E. DESCRIPTION OF MAP UNITS 63 TABI..E a.-Description of map units, Stanislaus River (pl. 5) TABLE 3.-Description of map units, Stanislaus River (pl. 5) Continued Continued Approximate Approximate thickness Map thickness ~ft (feet) Litholoull unit (feet) Litholouu 28_._ Includes massive fine- to medium-grained 48 __ _ Black slate with minor interbedded tu.ff and metavolcanic rock; interbedded chlorite chert. schist and black phyllite with rare lime 49 __ _ Conglomerate, like unit 4 7. stone beds about 4 in. thick; foliated 50 __ _ Medium· to thick-bedded tuff with inter.. medium-bedded interlayered black chert, bedded black slate. Slate most abundant dark-gray phyllite, and metavolcanic rock; in eastern part. Tuff beds commonly and dark-gray quartzose phyllite. Lith graded. ology and structure complex. Correla 5L __ Volcanic conglomerate. Contains scattered tion uncertain; structure shown diagram pebbles of slate. matically. 52 __ _ 29 __ _ Melones fault zone-mostly green schist Volcanic breccia and tuff, faulted against containing 'blocks of black carbonaceous serpentine on west side. quartzose schist, and volcanic conglom 30 __ _ 200 ____ _ Medium-bedded black slate with interbedded erate. tuff. Minor interbedded graywacke and 53 __ _ Thin- to medium-bedded chert and black conglomerate. Most pebbles are black carbonaceous very fine grained schist. slate and volcanic rocks; contains some 54 __ _ Mostly derived from medium to very thi~k chert and rare limestone pebbles. bedded mafic volcanic rocks with some vol 3L __ Volcanic rocks, chiefly pyroclastics. Bed canic breccia, but western part is interbed ding obscure. Zone of massive amyg· ded black carbonaceous schist and green daloidal volcanic rock on west side. 32 __ _ schist; and central part is quartzose Pyroclastic rocks. Bedding and texture sericite schist. In part fine-grained and obscure. schistose, and in part massive with 33 __ _ 100 ____ _ Black medium·bedded slate. Limestone medium-grained gabbroic texture. East float on surface. ern part is massive amphibolite, becoming 34 __ _ 1000+-- Coarse dark·green volcanic breccia. Lo coarser grained to the east. Thin zone of cally contains augite phenocrysts as much as one-quarter inch in diameter. black carbonaceous schist in SWXNEX -35_-- Coarse dark-green volcanic breccia. sec. 17, T. 2 N., R. 14 E. Brecciated and 36 __ _ 200 ____ _ Black slate. cut by granitic dikes near contact with 37 __ _ Coarse dark-green tuff. Eastern contact granodiorite. 200+ --- 55 __ _ concealed. Limestone. 56 __ _ 38 __ _ Black slate with interlayered thick to very Limestone. 1000+-- 57 __ _ thick bedded graywacke. Some gray Thin- to medium-bedded coarse- to medium wacke beds graded. Slate contains abun grained limestone. 58 __ _ dant plant remains in the SW% sec. 33, Aphanitic dark-green metavolcanic rock. 59 __ _ T. 2 N., R. 13 E, and invertebrate fossils Massive limestone, in part coarse-crystalline, in addition to plant remains in the NWX and massive dolomite or dolomitic lime sec. 3, T. 1 N., R. 13 E. stone. 60 __ _ 39 __ _ 400 ____ _ Medium- to thick-bedded graywacke; graded Medium- to thick-bedded dolomite ·and bedding common. limestone. Contains interbedded black 40 __ _ 500+-- .. Black slate. Folded in NE!~ sec. 3, T. 1 N., carbonaceous quartzose schist on west R. 13 E. side. Contains thin zone of quartz 41_ __ 500 ____ _ Massive dark-green tuff. mica-magnetite schist, probably meta 42 __ _ 300 ____ _ Black slate with interbedded tuff. rhyolite, in the NEXNWX sec. 34, T. 3 43 __ _ 500 ____ _ Tuff. N., R. 14 E. 44 __ _ Interbedded tuff and black slate, poorly 61_ __ Massive limestone. Includes thin zone of exposed at high-water mark. Contains coarse calcareous quartz-biotite schist on tuff units as much as 300 ft thick, but in east side. general slate and tuff zones are much 62 __ _ Alphanitic dark-green metavolcanic rock. thinner. 63 __ _ Coarse crystalline limestone. 45 __ _ Medium- to thick-bedded tuff with augite 64 __ _ Thin- to medium-bedded chert and black, crystals. carbonaceous quartz-mica schist. Chert 46 __ _ Coarse crystalline limestone and black is metamorphosed and locally vitreous. carbonaceous quartzose schist. Includes some limestone. Bedding de 47 __ _ Conglomerate, containing pebbles of vol stroyed in most places: Lineation ex canic rocks, slate, chert, vein quartz, and tensively developed. limestone. 65 __ _ Like unit 64. 64 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT TABLE 4.-Description of map units, Calaveras River (pl. 6) TABLE 4.-Description of map units, Calaveras River (pl. 6)-Con. Approximate Map thickness -Map A~rrco:~r;:~te unit (feet) Lithology unit (feet) Lithologu l_ __ _ Massive quartz-feldspar porphyry, contain 19 __ _ 2000+-- Black slate with some interbedded gray ing subhedral quartz phenocrysts as much wacke-poorly exposed. as 2 mm in diameter, euhedral plagioclase 20 __ _ Schistose meta volcanic rocks. Includes some phenocrysts as much as 1.5 mm in diam metatuff and feldspar porphyry on east eter, and ferromagnesian minerals in a side. Poorly exposed. white. very fine grained groundmass. 21_ __ Metamorphosed volcanic breccia containing 2 ___ _ 3000+-- Very fine grained dark-green massive por phenocrysts probably composed of am phyritic lava with abundant feldspar phibole after augite. phenocrysts averaging 1 mm in diameter. 22 __ _ 3000____ _ Tuff, with some interbedded volcanic breccia. Feldspar partly replaced by epidote. 23 __ _ 500+--- Slate, in part quartzose. Weathers gray. Includes vertical rhyolite(?) dike about Minor interbedded very coarse gray 15 ft thick with horizontal columnar joints. wacke. Dike rock contains quartz phenocrysts 3 24 __ _ Tuff. mm in diameter and plagioclase pheno 25 __ _ Not exposed in river. Probably dark-gray crysts 2 mm long in aphanitic dark-gray quartzose slate and sericitic chert as groundmass. exposed south of river. 3 ___ _ 200 ____ _ Aphanitic light-green volcanic rock. 26 __ _ Well-indurated dark-gray coarse sandstone 4 ___ _ 500±--- 1000+-- Massive fine-grained drark-green volcanic and fine conglomerate composed chiefly rock. of fairly well-rounded fragments of vein 5 ___ _ 1000+-- Dark-green coarse metamorphosed volcanic quartz and chert. Slate fragments com breccia. mon. 6 ___ _ 800 ____ _ Massive dark-green porphyritic metavol- 27 __ _ 400±--- Coarse dark-green volcanic breccia contain canic rock, in part brecciated. Pheno ing augite phenocrysts more than 2 mm crysts are of augite or amphibole after in diameter. augite and are 1 to 2 mm in diameter. In 28 __ _ 1000____ _ Dark-gray quartzose slate and sericitic chert. cludes zones of volcanic breccia and pillow Includes scattered zones of fine con lava. glomerate containing fragments of chert, 7 ___ _ 1000 ___ _ Massive dark-green metavolcanic rock, slate, and volcanic rocks. Conglomerate mostly brecciated. Some breccia contains is possibly infolded. amygdaloidal fragments widely scattered 29 __ _ Dark-green massive porphyritic dike rock among nonamygdaloidal fragments. with tabular saussuritized plagioclase Breccia fragments are separated by epi phenocrysts. Not exposed on river banks. dote-rich seams. Some nonbrecciated 30 __ _ 2000+--- Coarse dark-green volcanic breccia contain parts that are not amygdaloidal contain ing augite phenocrysts more than 2 mm in amygdaloidal bodies 1 to 2 ft long. Con diameter. tacts with unit 8 are poorly defined. 31_ __ Thin- to medium-bedded black slate with 8 ___ _ 1500 ___ _ Massive dark-green metavolcanic rock like 1000+--- interbedded tuff. unit 6, [but] contains zones of volcanic 32 __ _ breccia and pillow lava. 2000+--- Thin- to medium-bedded black slate with 9 ___ _ 400 ____ _ Volcanic breccia containing some coarse tuff interbedded tuff. Includes lenticular con zones with some graded beds. glomerates and possibly some chert. 33 __ _ Massive dark-gray rhyolite(?) porphyry Green schist. 10-~- 34 __ _ containing quartz and feldspar pheno Green schist. crysts about 1 mm in diameter. 35 __ _ Black medium-bedded interlayered chert 11_ __ 150 ____ _ Dark-green coarse volcanic breccia. In and carbonaceous schist. Some inter cludes some massive lava. bedded limestone. 12 __ _ 300 ____ _ Coarse porphyritic rhyolitic(?) volcanic breccia with abundant feldspar pheno TABLE 5.-Description of map units, Mokelumne River (pl. 7) crysts and widely scattered quartz pheno crysts. Approximate 13 __ _ Map thickness Massive dark-gray porphyritic rhyolite(?). unit (feet) Lithologu Contains common plagioclase phenocrysts l_ __ _ 1500+--- Massive dark-green metavolcanic rocks; in 1 to 2 mm in diameter and widely scat cludes lavas and volcanic breccia. Pillow tered quartz phenocrysts. lava present in SW1/4sec. 5, T. 4 N, R. 10 14 __ _ 1800±-- Light-green volcanic breccia, tuff, lapilli tuff, E. Some lavas and volcanic breccia in and silicified ash. eastern part contain feldspar phenocrysts. 15 __ _ 1500 ___ _ Black slate with some interbedded gray- Poorly exposed. wacke and minor interbedded tuff. 2 ___ _ Massive feldspar (rhyolite?) porphyry with 16 __ _ 800 ____ _ 700_~_- -- Pyroclastic rocks. very dark gray aphanitic groundmass. 200 ____ _ 17 __ _ Black slate. Quartz phenocrysts 2 mm and less in 18 __ _ 500 ____ _ Pyroclastic rocks. diameter. DESCRIPTION OF MAP UNITS 65 TABLE 5.-Description of map units, Mokelumne River (pl. 7) TABLE 5.-Description of map units, Mokelumne River (pl. 7) Continued Continued Approximate thickness Map A~ffcf~";r;te ~::c (feet) Lithology unit (feet) LUholoov 3 ___ _ 1500____ - Thin- to very thick bedded medium-green 25 __ _ 500+--- Interbedded black slate, conglomerate, gray rhyolite(?) tuff, lapilli tuff, and light-green wacke, and tuff. silicified ash. Tuff contains abundant 26 __ _ 100±--- Limestone. feldspar phenocrysts and scattered quartz 27 __ _ Interbedded black slate and tuff on west. grains. Graded beds common. Coarse dark-green volcanic breccia with 200 ____ _ 4---- Medium-green volcanic breccia. Feldspar augite phenocrysts as much as 5 mm in crystals and crystal fragments common diameter on east. Not exposed on north and quartz grains scattered in groundmass. west side of reservoir. 5 ___ _ 600 ____ _ Tuff and lapilli tuff, like unit 3. 28 __ _ Quartzose phyllite, weathers light gray to 6 ___ _ 600+--- Thin- to medium-bedded black silty slate light brown. Contains light-green foliated with some· interbedded graywacke. Con amygdaloidal lava about· 6 ft ·thick· and tains a tuff horizon about 15 ft thick near limestone about 20 ft thick in the SE}~ the east side. NEH sec. 18, T. 5 N., R. 11 E. Lime 7 ___ _ 800 ____ _ Foliated medium- to thick-bedded meta stone is calcarenite and breccia with lime andesite and metadacite pyroclastics. stone and dolomite fragments. No non 8---- 500±--- Foliated metamorphosed volcanic breccia. calcare-ous detrital grains noted. Includes minor intrusive rhyolite por 29 __ _ 1500+-- Coarse dark-green volcanic breccia contain phyry. ing augite crystals more than 5 mm in 9 ___ _ 200 ____ _ Medium- to dark-green basaltic pillow lavas. diameter. 10--- 500±--- Dark-green foliated pyroclastic rocks. 30 __ _ 500 ____ _ Tuff, poorly exposed. n __ _ 1000±--- Interbedded rhyolite and mafic or inter- 31_ __ 500 ____ _ Medium- to thick-bedded tuff with inter mediate pyroclastic rocks. Contains layered black slate. many vertical quartz veins. 32--- 1500_--- Medium- to very thick-bedded tuff and 12 __ _ 500 ____ _ Porphyritic rhyolite or dacite. lapilli tuff. 13 __ _ 600 ____ _ Mafic or intermediate volcanics. 33--- 100+--- Black slate with interbedded tuff. 14__ _ 1500+--- Porphyritic rhyolite, foliated in eastern part. 34 __ _ 1300_--- Medium- to very thick-bedded tuff and Quartz phenocrysts are as much as 2 mm lapilli tuff. in diameter. Vein quartz, sericite schist, 500+--- Tuff with interbedded conglomerate or and much-altered strongly foliated vol breccia. Conglomerate consists chiefly of canic rocks in eastern part. fragments of volcanic rock in a tuffaceous 15 __ _ 1000+--- Light-green foliated fine- to medium-grained matrix, but also contains scattered frag tuff. ments of black slate and vein quartz. 16 __ _ 2000 ____ - Interbedded medium- to thick-bedded tuff Most volcanic rock fragments are angular. and black slate. Predominantly slate. 36 __ _ Black slate with interbedded tuff, very Tuff beds commonly graded. coarse graywacke, and fine conglomerate. 17 __ :.. 1500 ____ _ Medium- to very thick-bedded fine tuff, 37 __ _ Medium-bedded coarse to fine tuff. lapilli tuff, and volcanic breccia. 38 __ _ Black slate with interbedded tuff. 18 __ _ 700+--- Black slate. Poorly exposed around margins 39 __ _ Tuff with interbedded slate. Poorly ex of reservoir. posed. 19 __ _ 1500+--- Tuff with minor interbedded slate. In 40 __ _ Chlorite schist. Fault zones contain thin cludes chlorite schist and sericite schist in units of black carbonaceous quartzose the NE~SW~ sec. 23, T. 5 N., R. 10 E. schist, probably derived from the Cala Thickness measured in sec. 15, T. 5 N., veras formation. R. 10 E. 41_ __ Black carbonaceous quartzose phyllite with 20 __ _ 200 ____ _ Porphyritic foliated metarhyolite or metada lamellae of metatuff. cite. 42 __ _ Interbedded tuff and black phyllite. In 2L __ 2500+-- Dark-green tuff and volcanic breccia in cludes chlorite schist on east side. western part; massive metavolcanic rocks 43 __ _ Chlorite schist, coarse crystalline limestone, with some interbedded metachert and and black quartzose schist. Intruded by metatuff in eastern part. gabbro dikes. 22 __ _ 500+--- On northwest side of reservoir; from west to 44 __ _ Medium-bedded metachert and black car east: coarse massive dark-green metavol bonaceous quartz-mica schist. Possibly canic rock, massive amygdaloidal lava. includes quartzose siltstone. Includes Exposed on southeast side of reservoir, chlorite schist and limestone pods on east light-gray thin- to medium-bedded very side. fine grained limestone about 30 ft thick, 45 __ _ Interbedded black carbonaceous schist, meta and bedded tuff. On southeast side of chert, and possibly quartzose siltstone. reservoir, undifferentiated volcanic rocks. 46 __ _ South part like unit 45, north part coarse 23 __ _ 200+ --- Tuff with interbedded conglomerate. crystalline limestone. 24 __ _ Quartzose phyllite, ~eathers light gray. 47 __ _ Like unit 45. 66 STRATIGRAPHY AND STRUCTURE, SIERRA NEVADA METAMORPHIC BELT TABLE 5.-Description of map units, Mokelumne River (pl. 7) TABLE 7.-Description of map units, Cosumnes River (pl. B) Continued Continued Approximate Map thickneBs Aff;~f~'::'e unit (feet) Litholo(111 ~:: (feet) Lithologf/ 5 ___ _ 48 __ _ Coarse crystalline limestone with interbedded 500+--- Medium-green rhyolite volcanic breccia and quartz-mica schist. coarse tuff. Contains abundant roundish 49 __ _ Like unit 45. quartz grains mostly less than 1 mm in so __ _ diameter. Coarse crystalline limestone. 6 ___ _ 51_ __ Like unit 45 .. 500+--- Fine to coarse medium- to very thick-bedded 7 ___ _ tuff and lapilli tuff. 1000 ___ _ Black slate with interbedded graywacke. Includes fine-grained metatuff on east side. TABLE 6.'----Description of map units, Jackson Creek g ___ _ 200+--- Graywacke. 9 ___ _ Unit No. Thickness Litholo(111 200+--- Black slate with interbedded graywacke. 1______Dark-green metavolcanic rock. Includes thin crystal tuff unit on east side. 1000 ___ _ 2 ______Coarse- to fine-bedded tuff. 10 __ _ Thick- to thin-bedded graywacke, minor 3 ______Interbedded black slate and graywacke. slate. Graded beds common. Coarse to 4______Interbedded black slate and tuff. fine tuff about 100ft thick at western side. 1L __ 2000 ___ _ Black slate with minor interbedded gray 5 ______Tuff and porphyritic volcanic breccia with wacke. Eastern part fossiliferous. pyroxene phenocrysts. 12 __ _ 1000+-- Medium- to thick-bedded tuff and lapilli tuff 6 ______Light-green buff-weathering quartzose phyl- with minor silicified ash. Graded beds lite. common. 7 ______Gray -green fine-grained volcanic breccia 13 __ _ 200 ____ _ Black slate with interbedded tuff. with feldspar phenocrysts less than 2 mm 14 __ _ 500 ____ _ Volcanic breccia and tuff. 15 __ _ 300 ____ _ long. Black slate with interbedded tuff. 16 __ _ 8 ______.:. _ _ Light-green quartzose phyllite with thin in- 1000_--- Medium-bedded tuff. 17 __ _ 700 ____ _ terbedded conglomerate. Conglomerate Medium-bedded interlayered graywacke and contains pebbles of chert, slate, gray black slate. 18 __ _ 600 ____ _ graywacke, and volcanic rocks. Tuff and volcanic breccia. 19 __ _ 300 ____ _ 9_-- ______Interbedded thin-bedded gray quartzose Black slate with interbedded tuff. 20 __ _ 500 ____ _ phyllite and black chert. Includes lime Mafic or intermediate pyroclastic rocks. 21_ __ stone lens, probably bioclastic, with 400_-- _._ Porphyritic rhyolite(?), including massive Foraminifera locally abundant, in eastern lava and pyroclastic rocks. 22 __ _ 1200 ___ _ part. Mafic or intermediate pyroclastic rocks with 10_- ______Thin-bedded black chert, with phyllite minor pillow lava. 23 __ _ Massive porphyritic rhyolite(?) with some partings. Limestone lens on east side. 700+--- interlayered mafic or intermediate volcanic 11_-______Interbedded coarse and fine conglomerate, rocks. containing pebbles of volcanic rocks, chert, 24 __ _ 3000+-- Massive fine- to medium-grained tuff with vein quartz, quartz schist, limetone, and some volcanic breccia, lapilli tuff, and rare granitic (?) rocks. pillow lava. Zone of lapilli tuff and vol 12 ______Volcanic breccia. canic breccia 80 ft thick about 1600 ft north of the south quarter-corner of sect 27, T. 8 N., R. 9 E., contains feldspar . crystals as much as 1 in. in diameter and TABLE 7.-Description of maps units, Cosumnes Rit1er (pl. 8) abundant scoria fragments. Pillow lava Approximate Map thtckneBB is porphyritic, with saussuritized euhedral unit (feet) Lithology feldspar phenocrysts about 5 mm long. l_ __ _ Coarse dark-green volcanic breccia contain Zone of black carbonaceous slate about 10 ing augite phenocrysts as much as 5 mm ft thick occurs about 1500 ft west of the i.a diameter. Probably equivalent to unit east quarter-corner of sec. 27, T. 8 N., R. 3. Exposed in upper part of river banks; 9 E. 25 __ _ not shown on map. 1100_--- Porphyritic lava containing sa~ssuritized euhedral feldspar phenocrysts and clusters 2 ___ _ Massive dark-green very fine grained lava. of phenocrysts as much as 8 mm long in Base not exposed. 3 ___ _ aphanitic matrix. Much of this lava has 1100_--- Coarse dark-green volcanic breccia. Frag pillow structure. ments have quartz amygdules as much as 26 __ _ 1000+-- Mafic or intermediate tuff and volcanic .1 in. in diameter, and chalcedony amyg breccia. Some graded beds and cross dules as much as 1% in. in diameter. laminated beds. 4 ___ _ 400 ____ _ 27 __ _ Massive dark-green very fine grained lava. 1000+-- Tuff in western part, green schist in eastern Contains some epidotized breccia. part. DESCRIPTION OF MAP UNITS 67 TARLE 7.-Description of map units, Cosumnes River (pl. 8) TABLE 7.-Description of map units, Cosumnes River (pl. B) Con~inued Continued Approximate Map Map thickneaa unit Lithologu unit (feet) Lfthologu 28 __ _ Metavolcanic rocks, including massive green 42 ___ 300 ____ _ Interbedded black slate, vol stone, green schist, and medium-bedded canic conglomerate, and metatuff. tuff. 29 __ _ Green schist. 43 __ _ 200 ____ _ Fine conglomerate. 30 __ _ Medium-bedded fine- to coarse-grained tuff. 44 __ _ 500 ____ _ Coarse conglomerate. 31_ __ 700+--- Thick- to very thick-bedded conglomerate 45 __ _ 600 ____ _ Interbedded black slate, silt- with interbedded sandstone on east side. stone, with subordinate Conglomerate consists mostly of fragments graywacke and conglom of volcanic rocks but also contains vein erate. Includes massive, quartz, black and gray chert, and black porphyritic sill rock with slate, in a tuffaceous matrix. Fragments plagioclase phenocrysts. of volcanic rocks and slate are angular to 46_,._ 300 ____ _ Coarse conglomerate. well rounded; chert and quartz fragments 47 __ _ 800 ____ _ Black slate, medium- to thick are well rounded. Some beds contain bedded graywacke, and len pebbles of fine-grained gray limestone. ticular conglomerate. The sandstone is well indurated and con 48 __ _ 1000 ___ _ Tuff, black slate, coarse con sists mostly of well-rounded quartz grains, glomerate, graywacke. In but contains some less resistant grains. cludes porphyritic dike rock 32 __ _ 1000+-- Thin- to medium-bedded interlayered fine to with plagioclase pheno coarse graywacke and black slate. Gray crysts. wacke beds commonly graded. Contains 49 __ _ 1500 ___ _ Thin- to thick-qedded tuff and lapilli tuff rare limestone beds less than 1 ft thick. and some volcanic breccia. Pyroxene Near west side, includes massive porphy crystals abundant in lower part. Many ritic lava or hypabyssal intrusive rock with graded beds, some slump structures. feldspar phenocrysts. Eastern part in Upper part fossiliferous. Includes several cludes volcanic rocks about 200 ft thick. 33 __ _ porphyritic sills with plagioclase pheno 1000+-- Interbedded graywacke, massive gray slate, crysts. and fine-grained tuff or tuffaceous sand so __ _ 900 ____ _ Very coarse thick-bedded dark-green vol stone. canic breccia with abundant augite pheno 34 __ _ 600 ____ _ Tuff and volcanic breccia. 35 __ _ 300 ____ _ crysts. Volcanic rocks. 51_ __ 60 ______36 __ _ Pillow lava. Black slate with interbedded pyroclastic 52 __ _ 1900 ___ _ Very coarse thick-bedded to massive vol rocks. 37 __ _ canic breccia with abundant augite pheno Well-indurated dark-gray sandstone con crysts. Minor tuff. sisting chiefly of quartz grains. Chert 53 __ _ 500 ____ _ Medium- to thin-bedded black slate with and black slate grains are common. interlayered graywacke. Interbedded Grains are angular to well rounded. Con slate and tuff near H use Bridge. tains some fine conglomerate. 54 __ _ 300 ____ _ 38 __ _ Medium-bedded fine- to medium-grained Black medium-bedded interlayered chert tuff. and carbonaceous phyllite. Some quartz 55 __ _ 400 ____ _ Thin-bedded black slate with interlayered schist. graywacke. 39 __ _ Quartz sandstone, like unit 37. 56 __ _ 300 ____ _ Coarse thick-bedded graywacke with inter 40 __ _ 1000+-- Black slate. Contains beds of black argil laceous very .fine grained limestone 1 in. layered black slate. 57 __ _ to 1 ft. thick on southeast side of river Black sericitic carbonaceous(?) schist. near the north quarter-corner of sec. 28, 58 __ _ Dark-green volcanic breccia with augite T. 8 N., R. 10 E. Contains poorly ex phenocrysts commonly 4 mm in diameter. posed fine conglomerate with interbedded 59 __ _ Metavolcanic rock. Green schist with crys arenaceous limestone (calcarenite) on the talloblasts of augite or amphibole re southwest side of the river near the center placing augite. of sec. 21, T. 8 N., R. 10 E. Conglom 60 __ _ Schist, consisting chiefly of sausseritized erate contains well-rounded granules of feldspar, quartz, and hornblende. De volcanic rocks, vein quartz, chert, and rived from granitic rock. quartzite. 61_ __ Medium-bedded chert and black carbona 400 ____ _ 4L-- Very fine-grained dark-green massive meta ceous quartzose schist with interlayered volcanic rock. chlorite schist. INDEX [MaJor references are In italic] A Pa~ Page Page Acocsslblllty of tho nroa ..• -····------s Copper Hill mine.•... ------30 Graywacke, defined.------5 Acknowledgments ••••••••••••••••• --••••• _----. 6 Copper Hill volranics .. _. __ 7, 12, 26, SO, 32, 40, 41, 44, 50 Green schist...•.. _••..•..•. ____ ._ •• _._. ______._ 41 Ago, Calaveras formation...... 1S Copperopolis fanlL .. ------· 50 Grossouvria .• _.•.•• _.• _. _•••• ______• ______• __ 18 Copper Hill volcanics ..• ------Sl Cosumnes formation •.. ------6, 7, 8, Guadaloupe intrusive ••••.•• ------•. 43 faulting .•••• ------•••••••••••• ------._____ 6S 12, 17, 23, 27, 31, 33, 35, 36, 37, 38, 39, 40, 44 Gopher H.ldgo volcanics .. ------B9 Cosumnes River .••..• ------2, 8, 53 H Logtown H.ldgo formation .• ------B1 Cosumnes River rock exposures._.------S1 Henbest, L. G., quoted ••• ------.14 Morlposa formation ... ----·------S6 Coulterville .. ------10 and Douglass, R. G., quoted------14 Merced Falls slnto...... SO Crinoid plates------13 Hodson fault._------.------50 Pensleo Crook volcanics •. ------31 Hogan Dam•• ------32,55 Petlon Blanco volcanics...... BS D Horseshoe Bar ____ ------43 Salt Spring slate...... eo Horseshoe Bar quartz diorite------53 ultmmafto rooks...... 4B Deposition, rocks or Jurassic age .. ------39 Hunter Valley cherts--.------······· 23 Amador group ••••••••••••.•••• 6, 17, 22, 27, 28, 30, 38, 44 Devil Gulch .. ------10 Ruse Bridge.••...... ••.....•.•...•.•• 17, 49,53 Amoeboceras••••••• ••••••••••• ------__ _ _ 25 Dichotomosphinctu. __ ------25 Hypabyssal rocks...... 4S Amoebites) dubittm •••••• ------25 Direction and amount of movement, fault zones. 61 Amoebltes ••••• ------25 Discosphinctes __ . ------25 Amusittm aumrfum .••••••••••• ______25, 26 Distribution, Calaveras formation______8 Idoceras...... 21 Argillaceous member, Culavorns formation...... 6, 9 Di!lisosphictes. ___ ------____ ------. 21 Imlay, R. W., quoted •... ------·------18,21,25 Argonaut mine.------52 Don Pedro Dam. __ ------32, 45, 53, 55, 56 Indian Flat Guard Station._------54 Ar,lcula BP------25 Don Pedro reservoir .. ------29,32 Introduction. __ .••. ------S B E K Bagby ••.• ------.------6, 9 Eagle Shawmut mine------52 Klamath Mountains .. ------38,39, 44 Boar Mountains .• ------8, 42 Ens tern belt, Calaveras formation._------8 Boar Mountnlns fault zone •.. ------7, 8, 12, 15, 17, 24, Eastern block, Calaveras formation •.. ------8 L 26, !7, 29, 30, 41, 42, 43, 44, 45, 46, 60, 53, 57 Eastern fault block.------8, 44, 6S · La Grange power plant.------30 Bedding features, rooks of Jurassic age______S9 El PortaL------10 Lake McClure •••••• ------22, 30, 32,55 Bedrock complex •• ------6 Entolium sp. ------27 Lima sp ______---- ____ ------26 Belemnite mold!; .• ------.------30 "Epiclnstlc" rocks, defined______5 Limestone, defined...••• ------5 Belemnites pacijicus .• ·---· ------25 Epiclnstic strata.. ------S6 Limestone blocks, origin.••• ------S7 Belemnites SP------29 Eureka mine .. ------52 Lineations •••..• ------66 Blue Canyon formation of Lindgren •••••••••••• 13,39 Exchequer Dam·------32, 55 Lithology, CopperHill volcanics______30 Bostick Mountain fault zone .. ------50 Cosumnes formation •.... ------17 Bower cave------11 F Gopher Ridge volcanics______!8 Bridge House .••.. ------54 Fault blocks .•.. ------8 Logtown Ridge formation •.•• ------19 Brower Crock volcanic member, Mariposa for Set also Central, Eastern, and Western fault Mariposa formation ...•------B4 mation •...... •• 12, 19, es, 26, 27, 31, 37, 38, 50 blocks. Merced Fans slate••.. ------SO Buchta •••••••••• --.--.---__ •• ------.______30 Fault zones, generaL_------6, 7 Peaslee Creek volcanics•••.• ------S1 concentrica ______25, 26, 27, 29 Fault zones, direction and amount of movement. 61 Penon Blanco volcanics______SS sp ••• ----.------______• ____ _ 26 Faulting, age. __ ------6S Salt Spring slate..•. ------$9 Faults·------46 Logtown Ridge formation .. ------7, c Field work._------5, 12, 16, 17, 18, 23, 26, 28, 29, 30, 31, 39,53 Calaveras formation______6, 8, Foothills fault system._.------7, 44, 46 23, 26, 27, 39, 43, 44, 47, 49, 50, 53, 55, 56. 57 Foraminifera.------11, 13,14 M Calaveras H.lvor ------3, 50 Fossils, Calaveras formation______1S McCabe FlaL.------9 Calavorns ll.iver rock oxposmes .••• ------SB Cosumnes forml\tlon______18 Mariposa formation .... ------7, California Const Ranges •• ------38 generaL_------4, 7, 11, 12 8, 10, H, 16, 17, 18, 21, 22, SS, 26, 27, 29, Campo Seco •• ------54 Logtown Ridge formation______S1 31, 33, 35, 36, 37, 38, 39, 41, 43, 47, 49, Carson Hill______52 Mariposa formation._------S6 50, 53, 54, 57. Central fault black.------8, 23, 27, 44, b4 Slllt Spring slate.------· S9 Mariposa slate .. __ ---"------23 Central Valley ..•. ------38, 39, 46 Fowler Lookout.._------23, 36,42 Melones Dam._------54 Cerlthinm sp. ------25 Fremont mine .. ------52 . Melones fault zone ... _.------7, 8, 14, 23, 24, 26, Character, Calaveras formation .•• ------8 Fttsttlina culindrica. _------14 38, 41, 42, 44, 45, 46, 47, 52, 53, 54, 55, 56, 57, Chert, dotlned______5 Merced Fans slate ______7, SO, 31, 36, 56 Chert member, Calaveras forml\tlon______6, 10 0 Merced River. __ ·------2, 6, 8 Merced River rock exposures ______--- St Clastic member, Calaveras formation ..• ------6, 9 Geology, synopsis..•• ------6 Cleavogo •• ------5, 66 Gold Rush .. ------4 Metamorphic rocks, structural blocks ..... ------6 Clio mine •.. ·------52 Gopher Ridge.------·------_ 27,29 Michigan Bar Bridge ... ------29, 31, 54,56 Oloos, Ernst, quoted .. ------51 Gopher Ridge volcanics_ 7, S7, 30, S1, 36, 40, 41, 45, 54, 55 Middle Bar Bridge.------49 Composition of fragments In rocks or .Turnsslc Gopher Ridge volcanism •••.... ------28 Minor faults ____ ------6$ age.------··------S6 Granitic rocks------4B, 4S Minor folds _____ ------56 69 70 INDEX Page Page Page Mi&ellina sp. ------14 Pseudofusulina. __ ------~------14 Stanislaus River ____ ------3, 50 Moine series of Scotland______56 Purpose of investigation------4 Stanislaus River rock exposures ______~ St Mokelumne River------· 8, 49 Strata of uncertain stratigraphic position.______t6 Mokelumne River rock exposures. ___ ------St R Stratigraphic nomenclature .•• ------15 Mormon sandstone ______------16 RadioliU'ia .•------11, 37 Stratigraphic relation.'!, Copper Hill volcanics. ss Mother Lode fissure system.------61, 53 Rocklin granodiorite._------53 Gopher Ridge volcanics. __ ------SJ Mother Lode gold mining belt ______2, 7, 46, 51 Rock and mineral fragments in rocks of Jurassic Gopher Ridge volcanics. ___ ------SS Mount Bullion..------26 age_------SS Merced Falls Slate. __ ------SS Mountain Spring volcanics______30 Rocks exposed in central fault block and Melones Peaslee Creek volcanics______SS fault zone______16 Salt Spring Slate. __ ------·------SJ, SS N Rocks exposed in western fault block and Bear western fault block.------SI Mountains Fault zone______t7 NagatoeUa orentis_ ------14 Stratigraphy ___ ------8 Rocks exposed on Cosumnes River.------Structural blocks of metamorphic rocks______6 Neofusulinella mantis.------: 14 te Rocks exposed on Mokelumne River and Jack- Newton mine. __ ------30 Structural geology .... ------44 Newton Mine volcanics._------30 son Creek._------IS Structural relations, ultramafic rocks.------4S North Fork, Merced River______6, 9 Rocks of Jurassic age_------16 Structure, central fault block.------54 Rocks of Mesozoic age______7 eastern fault block______6S Nucula sp. _------·--- 27 Rocks of Middle Jurassic age_------16 western fault block.------54 Rocks of Paleozoic age______6, 8 0 Subdichotomoceras filiplex. _------25 Origin, limestone blocks______S7 Sur series _____ ------38 s Sutter Creek._------17 Salt Spring slate______7, Sweetwater Creek------.------10 p Zl, 28, !9, 30, Sl, 35, 36, 37, 38, 39 Pachysphinctes ••. ------21 Salt Spring Valley------29, 32 T Parafusulina. __ ------14 San Andreas ... ------8 turgida. ____ ------14 San Andreas fault_------38 Taliaferro, N. L., quoted.------51 sp ______------14 San Andreas metavolcanic rocks______9 Taramelliceras denticulatum ___ ------25 Pardee Dam. ____ ------32 Santa Lucia Range_------38 Taylorsville region. ___ ------8 Pardee Reservoir.------13, 42 Schistosity, defined.------5 Terminology used in report______6 Peaslee Creek volcanics ______7, 30, SJ, 36 Schistosity, shear zones and eastern fault block___ 66 Tertiary strata._------4 Pebbles in rocks of J'urassic age______S6 Schubertella kingi. _------"------14 Textrataxas SP------14 Penn mine. __ ------52, 54 Scope of investigation______4 Tops of beds, determinntion______46 Pefion Blanco Ridge______22 Schwagerina sp ___ ------14 Tuolumne River.------9, 10,50 Pefion Blanco volcanics. ______7, tt, 26, Zl, 36, 41, 51, 54 Shape of fragments in rocksofJurassicage. ______So Tuolumne River rock exposures.------~-- ss Perisphfnctes (Dichotomosphinctes) mtlhlbachL __ 25,26 Shoo Fly formation______8, 13 Turbo sp _____ ------21) (Dichotomosphinctes) SP------26 Sierra Nevada batholith______7, (Discosphinctes) virgulatiformis______25 8, 41, 43, 44, 45, 52, 53 u sp. _------______------25 Slip cleavage, defined ____ ------6 Ultramafic rocks ____ ------41 Phulloceras sp ___ ------29 Slip cleavage melones fault zone.------55,57 Physical features of the area.------_____ s Small-scale structures _____ ------66 v Pine Tree mine______49 Smedley, Jack E., quoted._------14 Ventura basin------41 Plesiosaur skeleton, Snit Spring slate._------30 Snow ___ ------3 Volcanic member, Calaveras formation.------6, 9 Sonora. ___ ------2 Volcanic strata.------t6 Plutonic rocks------4, 7, 41 Plymounth mine______52 Sonora limestone.------~------6 Source offragments in rocks ofJurassaic age______S8 w Precipitation.------· ------3 South Fork, Merced River ___ ------10 Previous work done in area._------S Spiriferina sp. _ ·------·------14 Western belt, Calaveras formation ______6, 8, tt Pseudocadoceras ______------18, 21 Spirorbis sp ... ------_------14 Western fault block. ___ ------23, !1, SJ, 44 U.S. GOVERNMENT PRINTING OFFICE: 1965 0-'124-276