Geology of the Sierra Foothills Melange 

and Adjacent Areas, Amador County, 

California 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 827 

IUREAU OFUBRARY 

SPokANE, WASff. 

NOV191975 

t PltAsf RETURN 

\ lO liBRARY 
Geology of the Sierra Foothills Melange 

and Adjacent Areas, Amador County, 

California 

By WENDELL A. DUFFIELD and ROBERT V. SHARP 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 827 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STANLEY K. HATHAWAY, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 74-600083 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-001-02616-3 

-. 
CONTENTS 

Page Abstract __ __ _ ___ __ _ _ _ _ __ _ _ ____ ____ __ __ _______ _ ___ _ _ _ _ _ ____ 1 

Introduction ---------------------------------------------- 1 

General geology -------------------------------------- 4 

Nomenclature ---------------------------------------- 5 

~elange belt---------------------------------------------- 5 

General interrelations of rocks in the melange belt ______ 8 

Problems of regional correlation------------------------ 8 

Eastern belt ---------------------------------------------- 9 

Logtown Ridge-~other Lode belt-------------------------- 10 

Logtown Ridge Formation______________________________ 10 

~ariposa Formation ---------------------------------- 13 

Rocks of the ~elones fault zone ------------------------ 15 

VVestern belt-----------------------------------~---------- 16 

Intrusive rocks-------------------------------------------- 17 

~elange belt------------------------------------------ 17 

Logtown Ridge-~other Lode belt---------------------- 18 

Eastern belt ------------------------------------------ 18 

Ages of intrusion------------------------------------------ 18 

' 

Page 

Serpentinite and related rocks______________________________ 19 

Superjacent rocks ____________________ --------------________ 20 

Valley Springs Formation______________________________ 20 

~ehrten Formation------------------------------------ 20 

Structure ------------------------------------------------ 21 

~elange belt------------------------------------------ 22 

Bear ~ountains fault zone ---------------------------- 23 

Evidence for faulting of Logtown Ridge strata in 

pre-~ariposa time ------------------------~--------- 24 

Tilting and folding ------------------------------------ 24 

Deformation after regional tilting ---------------------- 25 

~elones fault zone -------------------------------- 25 · 

~other Lode fault system-------------------------- 26 

Regional crustal flexure____________________________ 26 

Structural cross sections ---------------------------------- 27 

Regional implications-------------------------------------- 27 

References ____ -------------------------------------------- 30 

ILLUSTRATIONS 

Page 

PLATE 1. Geologic map of the western Sierra foothills between the Cosumnes River and the ~okelumne River, Amador County, 

California ------------------------------------------------------------------------------------------- In pocket 

FIGURE 1. Index maps of the west-central Sierra foothills-------------------------------------------------------------- 2 

2. Sample of unedited field data from which the geologic map was compiled ____________ ...:_______________ ________ 3 

3. ~ap showing the belts of rocks mapped on plate 1 ---------------------------------------------------------- 4 

4. Diagrams of bedding, regional cleavage, and axes of minor folds for slate and graywacke of the ~ariposa Formation 

between Plymouth and Amador City --------------------------------------------------------·---------- 25 

5. Diagrams of bedding, regional cleavage, and axes of minor folds for slate and graywacke of the melange belt south of 

flighway 16 ------------------------------------------------------------------------------------------ 25 

6. Diagrams of bedding, regional cleavage, and axis of a minor fold for slate and graywacke of the melange belt north of 

flighway 16 --------------------------------------------------------------------~--------------------- 25 

7. Cross sections showing the ey_olution of interpretations of geologic structures in the pre-Cenozoic rocks along the 

Cosumnes River-------------------------------------------------------------------------------------- 28 

TABLE 

Page 

TABLE 1. Chemical analyses of greenstone from the Logtown Ridge-~other Lode belt -------------------------- _______ _ 11 

Ill 
GEOLOGY OF THE SIERRA FOOTHILLS MELANGE AND ADJACENT 

AREAS, AMADOR COUNTY, CALIFORNIA 

By WENDELL A. DuFFIELD and RoBERT V. SHARP 

ABSTRACT 

Detailed outcrop mapping in the western Sierra foothills of Amador 

County, Calif., has resulted in some major changes in the interpretation of stratigraphy and structure. The Amador Group was originally 

defined at its type locality on the south bank of the Cosumnes River in 

Amador County to include the Cosumnes Formation and the conformably overlying Logtown Ridge Formation, but the new data indicate 

that the lower boundary of the type Logtown Ridge should be located 

600 m farther west (downsection) than originally designated and that 

this boundary is a fault. The strata that were originally called the 

Cosumnes Formation are part of a lithologically diverse assemblage 

of tectonically intermixed rocks that constitute a newly recognized 

melange and thus are not a formational rock-stratigraphic unit as the 

earlier workers believed. Thus, the names Cosumnes Formation and 

Amador Group are both inappropriate in their type area and are 

abandoned. 

The Logtown Ridge Formation is here divided into four members, 

some of which cross what earlier was considered to be a formational 

boundary of the Logtown Ridge with overlapping pyroclastic strata. 

The outcrop mapping requires additional changes, although oflesser 

importance, in the identification and correlation of other Mesozoic 

rocks in Amador County. 

The newly recognized melange forms a 4-km-wide belt underlying 

the Logtown Ridge Formation. In addition to the type section of the 

abandoned Cosumnes Formation and scattered fault-bounded blocks 

of strata ofCosumnes lithology, the melange comprises rocks heretofore mapped as "western belt of Calaveras Formation," considered to 

be of Paleozoic age. Single clasts of this huge tectonic breccia range 

from a few centimeters to a few kilometers in maximum dimension. 

Distinctive strata are generally disrupted, and pervasive shearing is 

common. In the absence offossils, no age of original deposition can be 

assigned to any clast or matrix of the melange, but on the basis of 

indirect structural evidence, the intermixing that formed the 

melange probably took place during the Late Jurassic or before, and 

therefore the now sheared and faulted strata must originally have 

been at least this old. 

Available data are ambiguous but suggest that rocks were intermixed to form the melange when the strata were horizontal or nearly 

so. Similarly, the overlying Logtown Ridge and Mariposa Formations 

were faulted when these rocks were essentially horizontal. The entire 

section was subsequently tilted to its present, nearly vertical position. 

Traditional syntheses of the tectonic history of the Sierra foothills 

argue that the faults there have always been steeply dipping. Although this may be true for some faults, the new interpretation 

suggests that most faulting occurred before the section was steeply 

tilted. Neither suggestion can yet be proved, but we maintain that the 

highly deformed rocks mapped in Amador County represent primarily the effects of subduction at a continental margin, possibly augmented by gravity tectonics in a trough of sediment accumulation 

there. On the basis of the ages of B.ffected strata, this period of subduction was Late Jurassic but possibly began at an earlier time. If this 

interpretation of the melange in Amador County is correct, a belt of 

similarly deformed rocks should extend far beyond the limits of the 

study area. 

INTRODUCTION 

Since the gold deposits of the well-known Mother 

Lode country of California were first discovered and 

mined over a century ago, the geology of that area has 

been of considerable interest. Prompted by the gold 

rush, initial geologic. fieldwork emphasized the relation 

of the general geology to gold-bearing quartz veins of 

the Mother Lode system. More recent field investiga~ 

tions have concentr~ted on the structural and stratigraphic complexities of the pre-Tertiary metamorphic 

rocks to explain the origin and history of the entire 

Sierra Nevada province. 

Relatively recent work has resulted in detailed interpretations ofthe geologic history of some small areas 

(for example, Baird, 1962; Best, 1963) and also more 

comprehensive treatments of a much larger part of prebatholithic rocks of the Sierra Nevada province (Clark, 

1960, 1964; Kistler and others=' 1_9_7__1). Nonetheless, 

much fundamental information, such as the stratigraphic thickness of rock systems, is still unknown, and 

the number and nature of tectonic episodes that have 

affected these rocks are not generally agreed upon. 

Attempts to answer these and other fundamental 

questions in the western Sierra foothills traditionally 

have been frustrated by such conditions as (1) the low 

ratio of outcrop area to total field area, (2) marked 

lithologic changes over short horizontal distances, with 

the result that important geological contacts commonly 

are more closely spaced than outcrops, and (3) a conspicuous absence of fossils to aid in dating and correlating strata. These conditions are not likely to change, but 

methods of fieldwork are. Large-scale mapping of all 

available outcrops in western Amador County (fig. 1) 

has enabled us to make fundamental changes in some 

earlier defined stratigraphic units and to demonstrate 

the presence of a previously unrecognized melange 

there. 

We studied only a small part of the total foothills 

area; very detailed mapping elsewhere in the foothills is 

required for a general understanding of the geology. We 

recognize, however, that sufficient natural exposures of 

1 
0 

SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

1206 57'30" 120° 47'30" 

38°35' .------------------------------------------------, 

A 

6KM 

I 

FIGURE 1.-Index maps ofthe west-central Sierra Foothills, showing approximate area of plate 1 (shaded area in B). 

B 
,' 

INTRODUCTION 3 

bedrock generally are not available, and we urge all 

interested workers to make detailed observations at 

manmade exposures as they occur. 

Nearly all the evidence from which the conclusions in 

this paper are drawn is summarized on the geologic 

map, plate 1. Since contacts are rarely exposed in this 

area, the lines on the geologic map represent conclusions about the nature of the contacts based almost 

entirely on indirect evidence. Thus, a special explanation of how the map was constructed is helpful to its 

interpretation. The working scale was 1:12,000, and for 

most map units, except those in the eastern belt, individual outcrops were plotted. Contacts were draw~ to 

interpret the distribution of lithologies at the working 

scale, then the entire map was reduced to a 1:24,000 

scale without showing the outcrops. Figure 2 is an example of the unedited large-scale field data that provided the control for compiling plate 1. 

We have shown the distribution of lithologies by the 

simplest possible arrangement of contacts on the map. 

The trend and linearity of some contacts are closely 

bracketed by field data. Comparison of these contacts 

FIGURE 2.-Sample of unedited field data from which the geologic map, plate 1, was compiled. This area is about 1.2 miles square; 

Drytown is located at the east-central border. A few initial interpretations of the data are shown by straight dashed lines. Most 

outcrops are so small that they can be shown only as a dot o.n the map. Large outcrops are outlined with dashed lines. Letter 

symbols are field shorthand and do not correspond directly to the symbols of rock units used on plate 1. 
4 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

with others whose form is not so well controlled suggests that the less well known ones should also be drawn 

as approximately linear where outcrop d~ta permit, 

rather than as curved, sinuous, or even more complex. 

boundaries. Nevertheless, it should be remembered 

that many of the contacts could be redrawn with different shapes and consequently would have different implications. However, the basic conclusions about structure, especially in the melange, would remain unchanged. 

The subjacent series is divided into four belts of rocks 

(fig. 3, pl. 1); in the text, each belt is described in a 

separate section to permit the reader to move from the 

text to the geologic map with ease. 

The authors shared equally in ·the fieldwork and 

manuscript preparation for this study. Donald Swanson 

0 

.-

0 

C) 

"!I 

0 

~ 

z 

5KM 

MELONES FAULT ZONE 

Jackson 

FIGURE 3.-Belts of rocks mapped on plate 1. 

and Othmar Tobisch reviewed the manuscript and offered many helpful suggestions. 

GENERAL GEOLOGY 

Western Amador County, as all the western Sierra 

foothills area, is underlain by folded and faulted 

metasedimentary and metavolcanic rocks of Paleozoic 

and Mesozoic ages (the subjacent series) upon which 

isolated erosional remnants of a formerly extensive 

capping ofTertiary volcanic conglomerate and tuff(the 

superjacent series) lie with angular unconformity. Bedding and metamorphic planar structures in the subjacent rocks dip steeply and generally trend parallel to 

one another and to the crest of the adjacent Sierra 

Nevada. Bedding in the Tertiary rocks dips gently to the 

-west. 

Paleozoic and Mesozoic rock units form northnorthwest-trending bands that have suggested a 

homoclinal structure to some geologists (for example, 

Clark, 1964, pl. 1), but our work shows that this interpretation is valid only locally where the internal 

stratigraphy and structure of a few formations are 

reasonably well documented. A more nearly correct interpretation of the structural geology includes abundant faulting and shearing, both within and between 

lithologic units. Accordingly, although the earlier recognized arrangement of mappable rock units into 

north-northwest-trending bands is preserved, the ages 

and the nature of juxtaposition of certain map units 

must now be reevaluated. 

To date, the most thorough treatment of the structure 

of the area has been that of Clark (1960, 1964), who 

mapped a homoclinal sequence, with tops facing east, 

interrupted by two steeply dipping north-northwesttrending pre-Cenozoic fault zones, the Bear Mountains 

and Melones. The Melones fault zone juxtaposes 

Paleozoic rocks upon Mesozoic, whereas the Bear Mountains fault zone is characterized by discontinuous masses of serpentinite. In the terminology ofthis paper, the 

Melones fault zone separates the Logtown RidgeMother Lode belt from the eastern belt, and the Bear 

Mountains fault zone separates the western belt from 

the melange belt. (See fig. 3.) On the basis of fossil 

occurrences, Clark believed that the Bear Mountains 

fault zone also marked a reversal of stratigraphic succession, but new data indicate that this need not be so. 

Both fault zones undoubtedly mark zones of major tectonic dislocation and are discussed in more detail in 

later sections. 

All the subjacent rocks have been metamorphosed to 

some degree, generally to the greenschist facies. The 

most abundant minerals of the various lithologies include white mica, epidote, chlorite, actinolite, quartz, 

and sodic plagioclase. Small areas of higher grade rocks 

occur locally. In the bedded rocks and in many lava 
MELANGE BELT 5 

flows, primary structures and textures, and even some 

original minerals, are excellently preserved in spite of 

the metamorphism. Because these primary structures 

often are the most useful identifying features of the 

rocks, we often use such terms as graywacke and volcanic rocks, even though metagraywacke and metavolcanic rocks would be more precise. 

NOMENCLATURE 

Before the work of Clark (1964), almost all the subjacent rocks of the Sierra foothills were assigned to one of 

three formally named rock-stratigraphic units: the 

Calaveras Formation, the Mariposa Formation, and the 

Amador Group. The Calaveras Formation was originally defined to include all the metasedimentary and 

metavolcanic rocks of Paleozoic age (Turner, 1893a, p. 

309; 1893b, p. 425). The Upper Jurassic Mariposa Formation was named by Becker (1885, p. 1S-19; 1900, p. 3) 

for fossiliferous and highly metamorphosed auriferous 

slates in Hell Hollow just southeast of Bagby (fig. 1), 

about 12 miles northwest of the town of Mariposa. The 

Upper Jurassic Amador Group was named by Taliaferro 

(1942, p. 89--90; 1943; p. 282-284) to include several 

formations; those of interest to this report are the 

Cosumnes Formation and the overlying Logtown Ridge 

Formation, whose type locality is along the Cosumnes 

River (fig. 1) immediately west of Highway 49. 

Most workers in the foothills have correlated their 

map units with one of these three formally named rockstratigraphic units wholly on the basis of lithologic 

similarity. Such correlations were made despite the 

patchy, discontinuous nature of outcrops, the structural 

complications, and the general absence of a fossil record; they are often unsatisfactory because none of the 

formations has a unique lithologic makeup. Clark 

(1964) recognized this problem and generally restricted 

his use of formation names to rocks at their type 

localities and adjacent areas where stratigraphic continuity can be documented. 

In general, we follow Clark's nomenclature (1964). 

However, our mapping indicates that the name Amador 

Group must be abandoned (Sharp and Duffield, 1973). 

Briefly, although the Amador Group was originally 

defined as two conformable formations, the Cosumnes 

and Logtown Ridge, new mapping indicates that (1) 

these two lithologic units are in fault contact, (2) the 

single fossil locality originally used to date the 

Cosumnes Formation is within the Logtown Ridge 

Formation, and (3) most rocks of the Cosumnes Formation form scattered, fault-bounded blocks in a melange. 

Accordingly, we herein abandon the name Cosumnes 

Formation; only the Logtown Ridge Formation remains 

a demonstrable rock-stratigraphic unit in Amador 

County, and the use of the term Amador Group there is 

inappropriate. 

Our use of the name Calaveras Formation follows 

Turner's (1893, a, b) original definition-all rocks of 

Paleozoic age of the Sierra Nevada-and rocks that are 

assigned to the Calaveras Formation in this report are 

so named for one or more of the following features: (1) A 

characteristic assemblage of lithologies, (2) a relatively 

complex set of structures, and (3) a degree of 

metamorphism higher than Jurassic rocb of the western Sierra foothills. We found no fossils during our 

work, and no previously described fossil localities are in 

the study area. Thus, our assignment of rocks to the 

Calaveras Formation is not beyond question. In general, we simply follow the designations used by Clark 

(1964), although the formational assignment of rocks in 

the western belt of Calaveras of all earlier workers is 

now questioned in view of the newly recognized 

melange there. · 

The melange forms a 4-km-wide belt of chaotically 

intermixed rocks that previously have been mapped as 

a homoclinal sequence of the Calaveras Formation 

(Lindgren and Turner, 1894), Calaveras plus complexly 

infolded Cosumnes Formation (Taliaferro, 1943, and 

unpub. map), and a homoclinal sequence of Calaveras 

Formation conformably overlain by Cosumnes Formation (Clark, 1964, pl. 8). One clast in the melange, a 

fragment of limestone a short distance south of the map 

area (pl. 1), contains Permian fossils, suggesting 

Calaveras Formation. But because no other fossils have 

been found and because the melange is a tectonostratigraphic unit of the type described by Hsu (1968), 

essentially all the rocks that compose the melange are 

of unknown age. In the absence of new age data, however, we retain the original Calaveras and Cosumnes 

designations for some rocks of the melange. The reader 

should realize that these designations are mainly for 

convenience in describing generally distinctive 

lithologies, and the implied correlation to the type 

Calaveras and Cosumnes Formations may, in fact, be 

incorrect. We further emphasize this ambiguity and 

also stress the fundamental chaotic character of the 

melange by referring to the rocks there as showing 

(!Calaveras affinities" or <~Cosumnes affinities," rather 

than calling them formations. We recognize that, in 

general, the law of superposition and the concept of 

rock- and time-stratigraphic units should not be;applied 

to the chaotic mass of rocks that forms the melange, and 

we use the terms Calaveras and Cosumnes there only 

for convenience of discussion. 

MELANGE BELT 

The melange crops out in a north-northwest-trending 

belt about 4 km wide, and we have mapped it for about 

15 km across Amador County. The melange includes 

chaotically intermixed clasts of both Calaveras and 

Cosumnes affinities. Common lithologies of Calaveia's 
6 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

affinities include quartzose slate-phyllite, chert,quartz- · 

ose sandstone, limestone, quartz-muscovite-albite. 

phyllonite, and massive to blocky mudstone. Those of 

Cosumnes affinities are black clay slate, graywacke, 

and slate-clast conglomerate. Intergradation between 

rock types precludes unambiguous assignment of all 

rocks of the melange to one of these two groups, but 

where possible, a distinction seems valuable and may in 

fact reflect two or more sources of fundamentally different character. 

~rtually none of the map units in the melange consists of a single hthologv. Each unit contains assemolages of lithologies that generally are not unique, but 

ffie assemblages can be distinguished bx the proportions of different lithologies and. locallyo4..b)l..the exclusion of orie or more litholo~ The most widespread 

assemblage is quartzose slate-phyllite and intercalated 

thin-bedded chert, with or without more massive siliceous rocks (thick-bedded chert?), quartzose sandstone, 

and limestone. The most common association in a single 

outcrop is quartzose slate with interbedded chert and 

lesser amounts of quartzite. The slate generally is tan 

and gives a characteristic ring when hammered; cleavage surfaces commonly exhibit a phyllitic sheen. Intercalated gray chert beds range in thickness from about 2 

to 20 em, but most do not exceed 6 em. Cleavage surfaces 

in chert are less closely spaced than in the associated 

slate, and they are refracted at the boundary between 

the two lithologies. Thin beds of poorly sorted impure 

quartzose sandstone are locally intercalated with the 

slate to the exclusion of chert. Also, local rhythmically 

bedded chert with thin slaty partings crops out to the 

exclusion of both slate and sandstone. A metamorphic 

rock cleavage is apparent at nearly every outcrop in the 

melange, but bedding often is not evident. Rocks at 

many outcrops are pervasively sheared; the original 

beds of chert and quartzite are so thoroughly disrupted 

that they form isolated pods in a matrix of sheared 

pelitic material. 

Quartzite and poorly sorted impure quartzose sandstone form many outcrops both within and adjacent to 

the slate-chert terr~nes .. These quartzites are bedded, 

invariably with slaty partings, and are at least partly 

the lateral equivalent of associated slate. Typically, the 

brown to tan quartzose sandstones are characterized by 

sparse plates of mica and by scattered well-rounded 

medium-grained clasts of quartz in a brown matrix. 

Locally, this matrix is displaced completely to form beds 

of quartzite. 

Scattered highly siliceous outcrops are common locally. Some of this material is nearly pure massive microcrystalline quartz of light- to dark-gray color. However, much of it (for example, in sec. 21, T. 7 N., R. 10 E) 

displays a distinct color lamination that trends generally parallel to elongate outcrops or a group of closely 

spaced outcrops, suggesting that the color bands reflect 

original stratification. 

Limestone occurs as scattered outcrops within terrain 

underlain by quartzose slate, chert, quartzose sandstone, quartzite, and clay slate. Exposures range from 1 

to about 40 min maximum dimension and often are 

clustered in relatively ~mall north-northwest-trending 

elongated areas. Typically, the limestone is light gray 

and nearly pure carbonate; grain size is relatively uniform for outcrops at one locality, but it varies from very 

fine (lithographic limestone) to coarse throughout the 

area. At one outcrop(SW14 sec. 9, T. 6 N., R. 10 E.), thin 

beds of chert are intercalated with limestone, and a 

distinctive calcarenite in sec. 22, T. 7 N., R. 10 E., 

contains about 10 percent sand-size quartz, plagioclase, 

and lithic fragments. 

Some limestone outcrops are massive, whereas others 

show a color banding, which, like that in the laminated 

highly siliceous rocks, is believed to parallel original 

stratification. By projecting color bands, some outcrops 

oflimestone (for example, in sees. 4, 5, 8, 9, T. 7 N., R. 10 

E., and sec. 16, T. 7 N., R. 10 E.) can· be fi~ted into 

complex folds; scattered outcrops oflaminated chert can 

be correlated in the same manner, and at one area (SE14 

sec. 21, T. 7. N., R. 10 E.) similar folds are so outlined in 

both chert and limestone. These folds, together with the 

other structural characteristics of the melange belt, / 

suggest that linear bands of limestone outcrops represent limbs of sheared, disrupted folds. 

Irregularly shaped masses of gneissose to schistose 

phyllonite crop out about 0.8 km southwest of Drytown 

and locally elsewhere. These rocks are composed 

primarily of quartz, muscovite, and albite, commonly in 

that order of abundance, although muscovite is locally 

predominant. The plane of schistosity is locally folded 

and commonly diverges from the north-northwest trend 

of the regional metamorphic rock cleavage that pervades nearly the entire map area. Also, the plane of 

schistosity is locally crosscut by a fracture cleavage 

with a north-northwest trend that may have formed at 

the same time as the regional foliation. Schistosity apparently developed before regional cleavage and occurs 

only in a few clasts of the melange. 

Medium-grained phyllonite, which closely resembles 

the schistose variety and probably differs from it only in 

degree of recrystallization, is associated with both the 

schistose phyllonite and phyllitic slate. At outcrops it is 

characterized by about 10 percent subrounded quartz 

grains 1 mm in diameter, which are embedded in a tan 

to gray matrix. Parallel surfaces of rock cleavage are 

evenly spaced at about 3-mm intervals and are lined 

with plates of muscovite that impart a phyllitic sheen to 

the rock. These surfaces appear to have resulted from 
MELANGE BELT 7 

concentrated shearing on evenly spaced planes. Locally, 

they are folded, and some fracture cleavage is developed 

parallel to the axial planes of such folds. 

In thin section, these rocks are obviously sheared and 

crushed. About 80 volume percent of the rock is composed of quartz and feldspar, both as porphyroclasts and 

'finely crushed matrix. The rest of the matrix is primarily muscovite, chlorite, and accessory sphene, apatite, 

and an opaque mineral. 

Tan to dark-olive mudstone crops out along the west 

margin of the melange belt and generally underlies 

relatively small, northwest-trending elongate areas 

within the belt of serpentinite exposures. Chert, limestone, immature conglomerate, and graywacke crop out 

within areas that are underlain principally by the mudstone, but they are scattered and volumetrically very 

small. 

The mudstone is unique among the epiclastic rocks of 

clay- and silt-size material, for it lacks the penetrative 

cleavage that is so widespread elsewhere. A weakly 

developed rock cleavage occurs locally, but rocks in· 

most outcrops are either massive or fractured into 

blocks. Bedding generally is not evident, but where 

graywacke and immature conglomerate are 

interstratified, bedding strikes north-northwest and 

dips steeply eastward. 

Moderately well sorted mature quartzose conglomerate and sandstone crop out along the east and northeast 

margins and locally elsewhere in the melange belt. 

These rocks appear to grade both laterally and vertically from one to another. Conglomerate is most abundant south of Highway 16 near the latitude of Drytown 

(pl. 1). The clasts are well-rounded cobbles and pebbles 

of quartz and chert, which are cemented by silica to form 

very resistant outcrops. Rocks in most exposures are 

light colored, commonly a pale-pinkish tint. Where outcrops can be reasonably related to one another, they 

form north-northwest-trending bands. 

The quartz sandstone occurs primarily north of 

Highway 16 and extends in discontinuous northnorthwest-trending fingers for about 8 km toward the 

north boundary of the map (pl. 1). The clasts are wellsorted medium-grained rounded to subrounded particles of quartz, minor chert, and rare feldspar (albite?) 

and slate, all of which show some granulation at their 

edges and, commonly, undulatory extinction in thin 

section. Outcrops are light gray and, where weathered, 

exhibit a few volume percent of evenly distributed pore 

spaces of the same general diameter as the clasts. Generally massive beds range from a few centimeters to a 

few meters thick. 

Tan-weathering slate is associated and interbedded 

with both the quartzose conglomerate and sandstone. It 

forms outcrops in roadcuts at the intersection of Highways 49 and 16 about 2 km north of Drytown; the ridge 

to the south is underlain by the more resistant rocks. 

About 6 km to the north-northwest, the slate and interbedded quartz sandstone are well exposed in cuts along 

a dirt road (not shown on pl. 1) that follows the west 

flank of the conspicuous north-trending ridge in the 

west half of sections 28 and 33, T. 8 N., R. 10 E. Typically, bedding and cleavage are nearly parallel and 

trend north-northwest, but they are greatly divergent 

in the narrow axial regions of rare minor isoclinal folds. 

Poorly sorted unbedded conglomerate, the largest 

clasts of which are subangular to subrounded pebbles 

and cobbles, is abundant within the melange belt, especially along the west margin. This conglomerate and 

associated clay slate and graywacke closely resemble 

slate and graywacke in the type Cosumnes Formation 

on the Cosumnes River. The conglomerate in some outcrops contains numerous clasts of black slate and an 

argillaceous matrix; in others, it is characterized by 

abundant volcanic clasts with no fragments of slate. 

These types form two end members of which most of the 

conglomerates are mixtures. The argillaceous conglomerate is relatively concentrated in the ·eastern half of the 

melange belt, and the volcanic conglomerate in the 

western half, although there are many exceptions. In 

decreasing abundance, the lithologies represented by 

the clasts of a utypical" conglomerate are (1) 

fine-grained and porphyritic intermediate to basic volcanic rocks, (2) black clay slate, chert, quartzite, limestone, and milky quartz, and (3) granitic rocks, mica 

schist, and serpentine. The only known serpentine clast 

occurs in the conglomerate that underlies the conspicuous hill 0.6 km southwest of Drytown. 

In the volcanic conglomerate, nearly all the clasts are 

fine-grained or porphyritic volcanic rocks of intermediate to basic composition. These clasts are firmly 

cemented by tuffaceous material to form very resistant 

massive dark-colored outcrops. Locally, outcrops of 

structureless augite-feldspar porphyry several meters 

in maximum dimension are included with these volcanic conglomerates. Such bodies probably represent 

exceptionally large clasts in a fragmental rock, although they may be parts of intrusive bodies or flows. 

Some of the conglomerate appears structureless; 

however, deep weathering commonly etches the resistant varieties in a preferred north-northwest-trending 

direction, and those rocks with abundant clasts of slate 

and an argillaceous matrix often show a well-developed 

planar structure unaided by weathering. This structure 

is reflected both in the fine-grained matrix (as slaty 

cleavage) and in the general parallelism of the larger 

clasts, many of which are discoidal. Locally, deformed 

clasts also define a lineation that plunges steeply in the 

plane of foliation. The total strain that affected the 
8 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

original rock, as seen in this flattening and elongation, 

is difficult to assess because the slaty matrix likely has 

undergone· considerably more deformation than the 

coarser material. Bedding is generally absent, but the 

original surface of deposition is probably parallel to the 

metamorphic foliation because the regional trends of 

the conglomerate, the metamorphic foliation, and the 

bedding in associated graywacke are all parallel, or 

nearly so. 

Available outcrops indicate that the entire spectrum 

of epiclastic rocks from cobble conglomerate to clay 

slate is represented in the conglomerate map units of 

plate 1. Slate and intercalated graywacke possibly underlie a major part of the areas mapped as conglomerate, but since the conglomerates are relatively resistant, they are preferentially exposed. Slate and 

gray~acke form many outcrops in the easternmost zone 

of conglomerate perhaps because there they are 

shielded by the resistant volcanic rocks of the adjacent 

Logtown Ridge Formation. The slate is black and contains interbeds of graywacke ranging in thickness from· 

about 3 to 20 em. The clasts of the graywacke are subangular to subrounded grains of plagioclase and quartz, 

together with fragments of aphanitic and porphyritic 

intermediate to basic volcanic rocks, slate, and chert. 

Bedding and slaty cleavage are apparent at most outcrops, and with few exceptions, they diverge by 15° or 

less in either strike or dip. Graywacke beds commonly 

are graded with nearly all stratigraphic tops facing 

eastward. Known west-facing beds are everywhere associated with small tight to isoclinal folds. 

A relatively thin (230-500 m in outcrop width) yet 

quite persistent band· of black slate with thinly interbedded graywacke and minor immature slate-clast conglomerate roughly bisects the melange belt in a northnorthwest direction. These rocks are indistinguishable 

from the slate and graywacke of much less volume, 

which are associated with similar conglomerate as described in the preceding paragraph. 

Beds of graywacke in this narrow .. band range in 

thickness from 3 to 20 em and provide abundant opportunities for direct measurements of bedding, cleavage, 

and stratigraphic top. Nearly all tops face eastward; 

they are most commonly indicated by noticeable grading, but a few examples of refracted cleavage were 

noted. Bedding planes are even and regular and seldom 

diverge from the attitude of associated cleavage by more 

than 20° in either strike or dip. Known folds in bedding 

are few; they are very tight to isoclinal with axial plane 

cleavage and measure only 1-2m in wavelength. 

The continuity of this map unit is interrupted for a 

short interval near its intersection with Highway 16 (pl. 

1). The exact nature of this break is not well 

documented, although it almost certainly is structural 

rather than stratigraphic. Regardless of the nature of 

the break, this sequence of rocks provides the most 

nearly throughgoing marker unit of the entire melange 

belt and thereby represents the largest known clast in 

the melange. 

GENERAL INTERRELATIONS OF ROCKS 

IN THE MELANGE BELT 

The melange belt of this report was originally 

mapped as the uwestern belt of Calaveras Formation" 

on the U.S. Geological Survey folios. The Cosumnes 

Formation was later separated from these rocks by 

Taliaferro (1943, p. 306. fig. 2), and still later Clark 

(1964, p. 17) refined Taliaferro's original designation of 

the Cosumnes Formation. Although these earlier workers differed on the number of formations and the structural relations of one to the other, they all believed that 

the formations formed normal stratigraphic successions 

which had been tilted or folded or both. 

Our work shows that such simplicity does not exist. 

As plate 1 indicates, individual marker beds or sequences of beds do not extend far, normally no more 

than a few kilometers. Lithologies of distinctive appearance, such as limestone and laminated chert, generally persist for far shorter distances and locally are 

greatly contorted. Many outcrops are pervasively 

sheared. Although the exact nature of contacts is unknown because they are not exposed, all this evidence 

suggests the presence of a chaotic tectonic intermixture 

of rocks. 

Thus, a normal stratigraphic succession_.cannot be 

deciphered from rocks within the m~lange~- In fact, the 

age of rocks that compose the melange, except for clasts 

which actually contain diagnostic fossils, must now be 

considered unknown. Indirect structural evidence discussed in a later section suggests that the melange 

formed in Late Jurassic or older time; thus the rocks of 

the J?elange presumably also are Late Jurassic or older. 

PROBLEMS OF REGIONAL CORRELATION 

.The·· retention of the Calaveras designation for some 

·rocks of the melange is supported by lithologic similarity of these rocks to generally accepted Calaveras rocks 

described by others elsewhere and by the discovery of 

Permian fossils from the Allen marble quarry (Clark, 

1964, p. 14) within an area directly on strike with what 

. is now recognized as the melange. Clark (1964, pl. 1) 

indicated that this band of Paleozoic rocks (the uwestern 

belt of Calaveras Formation") pinches out southward 

from Amador County between fault slices of Jurassic 

rocks. However, inspection of other maps south of 

Amador County and comparison of detailed rock descriptions for river profiles by Clark (1964, pls. 6, 7) 

suggest a southeastward continuation of the melange 

because of similarity in rock types, such as shown in the 

Jackson folio (Turner, 1894a). A brief excursion we 
EASTERN BELT 9 

made· along highways near the Calaveras River substantiates the similarity of the rocks there to the 

ucalaveras" rocks exposed in the melange of Amador 

County. Although south of northwestern Calaveras 

County, Clark (1964) and Clark, Stromquist, and Tatlock (1963) uniformly assigned these rocks to the Jurassic Mariposa Formation, isolated blocks of limestone 

there locally have yielded Permian fusulinids (Douglass, 1967). Clark (1964, p. 14) interpreted these as 

blocks of Permian limestone that probably slumped into 

massive beds of the Mariposa Formation during Juras-· 

sic time, but in view of the new data, the possible southward extension of Paleozoic rocks within the melange in 

Amador County must be considered. It now seems probable that sufficiently detailed mapping will show intimate tectonic intermixture within all the rocks labeled 

uwestern belt of Calaveras Formation" by Turner 

(1894a) and considerable extension of the melange belt 

southward from Amador County. 

A comparable problem exists for recognition of the 

melange northward from Amador County. Lindgren 

and Turner (1894) showed northward continuity of the 

Calaveras Formation into El Dor·ado County to the 

latitude of Placerville in their Placerville folio, but subsequent regional compilation by Clark (1964, pl. 1) replaced much of this map unit in ElDorado County with 

the designation uepiclastic rocks of uncertain stratigraphic position." Unfortunately, newer and more detailed maps than the original folio sheet have not been 

made in the critical area to solve this problem of continuity, and structural intermixing of the type found in 

the melange belt in Amador County cannot yet be demo~strated there. 

Possible lateral extension of the melange beyond the 

western boundary as shown on plate 1 is suggested by 

aeromagnetic surveys. Within the map area, magnetic 

anomalies form north-northwest-trending patterns 

with especially strong positive anomalies directly over 

serpentinite bodies (Henderson and others, 1966; U.S. 

Geological Survey, 1969). The most pronounced bands 

of magnetic highs are concentrated along the Melones 

fault zone and the west boundary of the melange belt, 

where serpentinite is most abundant. Similar bands of 

magnetic anomalies are located along the east edge of 

the Great Valley of California, over the relatively flat 

lying Cenozoic sedimentary rocks that floor the valley. 

These data suggest that north-northwest-trending l;>elts. 

of serpentinite lie beneath the Cenozoic rocks of the 

valley and imply the existence ofm~;tjor zones of faulting 

there. Thus the zone of chaotically intermixed rocks in 

the foothills region may have a far greater width than 

we suggest. 

EASTERN BELT 

Most of the metamorphic rocks in the eastern belt are ...... ~ 

phyllite, phyllonite, and chert. which occur separately 

or in various proportions in single outcrops. Lithologic 

similarity precludes delineation of separate map units 

among these rocks by the methods of this study, although known bedding attitudes generally trend 

north-northwest and dip steeply to the east, in accord 

with the regional structural grain of the foothills. Assignment of these rocks to the Calaveras Formation 

follows the usage of earlier workers. 

The phyllitic and phyllonitic rocks appear very similar in the field. Both exhibit a dark-gray to silver-gi&Y 

color. a sheen from oriented plates of muscoyite a.L9ck 

cleavage parallel to this mica, and a steenly-Rlunging .. 

color streaking in the plane of the cleavag!h.Jn thin 

section, however, the rocks show clear evidence of different histories. Phyllite is composed principally of 

granoblastic quartz, muscovite, albite, chlorite, and actinolite and pr.obably represents recrystallized 

sedimentary rocks of appropriate composition. Phyllonite shows grain granulation textures. One phyllonite 

sample consists of quartz, microcline, perthite, and 

minor muscovite as strained and fractured grains that 

are in turn surrounded or partly bounded by more finely 

crushed material. 

Phyllite is more common than phyllonite or chert, 

and locally it exhibits two foliations. The older foliation 

trends north-northwest, dips steeply to the east, and is 

paralleled by plates of mica and the dominant direction 

of rock cleavage. The younger trends about north-south, 

dips steeply to the west, and appears either as crenulations on the older foliation or as distinct planar fractures. Steeply plunging color-streaking lies in the plane 

of the older foliation and generally parallels the axes of 

associated minor folds. 

The chert is indistinguishable from that in the 

melange belt. It is locally massive but more commonly 

occurs in beds from 3 to 20 em thick, separated by thin 

"'pfiylhbc partings. Minor isoclinal folds as much as 1 or 

2 m in wavelength occur in the chert; their axes plunge 

from 20° to aoo to the southeast. averaging about 608 • 

Two relatively large masses of coarsely porphyroblastic white-mica hornfels lie within and adjacent to a 

granodioritic pluton near the Cosumnes River. The 

textural and mineralogical character of the hornfels 

indicate a higher grade of metamorphism than that 

found in any rocks around the eastern periphery of the 

pluton. In addition to white mica, the hornfels contains 

medium-grained granoblastic biotite, quartz, and calcic 

oligoclase, with weak development of planar fabric 

defined by biotite grains. White mica forms relatively 

large (6 mm) poikiloblastic grains without preferred 

orientation, through which the earlier biotite foliation 

extends without interruption. Tourmaline and opaque 

minerals form small accessory grains. 
10 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA· 

Although no similar rock is known elsewhere in the 

Calaveras terrane of the eastern belt, the hornfels is 

included with. the Calaveras Formation because one 

mass is continuous with typical Calaveras rocks in El 

Dorado County to the north, according to relations 

shown on the Placerville folio (Lindgren and Turner, 

1894). Probably because of their geometry, the bodies of 

hornfels are more extensively recrystallized than other 

weakly hornfelsed rocks found along the eastern contact of the pluton. One mass is surrounded by granodiorite, and the other forms a deeply penetrating appendage of wallrock. 

Minor bodies of serpentinite, gabbro, diorite, and 

limestone (for example, in sec. 6, T. 7 N., R. 11 E.) also 

crop out in the eastern belt, but most are too small to be 

shown on plate 1. 

LOGTOWN RIDGE-MOTHER LODE BELT 

Most of the Logtown Ridge-Mother Lode belt is underlain by marine stratified rocks of Jurassic age. These 

rocks are divided into two formations, the Logtown 

Ridge and overlying Mariposa, which trend northnorthwest and dip steeply to the east. Detailed 

lithologic descriptions of outcrops along the Cosumnes 

River haye been reported by Clark (1964); the present 

discussion emphasizes significant new stratigraphi 

subdivisions, refinements, and changes relating 

Clark's work. 

LOGTOWN RIDGE FORMATION 

The Logtown Ridge Formation, a se uence of Late 

Jurassic rna c vo can1c se imentary rocks and interlayered flows and sills, has been substantially revised 

by our detailed mapping in Amador County. The formation has been removed from the Amador Group, a term 

no longer retained, and its base at the type locality has 

been redefined (Sharp and Duffield, 1973); the-formation is herein divided into four distinctive members, 

most of which are traceable on a regional scale. The 

distribution of the various members sheds· much light 

on original depositional features within the formation, 

as well as the character of subsequent deformation. 

Strata of the Lo own Rid e Formation form a near y 

vertical homoc 1ne with its top..to.ward the easUpl. 1). 

The thickness of the formation yaries greatly within 

Amador County, ranging from a maximum of about 

~.OOQ m near Sutter Hill to a minimum of about 600 rn 

at Dr)d,own At the type locality, along the Cosumnes 

River, the thickness is about 2,000 m. As will be discussed later, much of the variation in thickness may have 

resulted from complex tectonic truncation along the 

base of the formation, rather than original stratigraphic irregularity. 

The basal unit of the Logtown Ridge Formation in 

Amador County is here designated the Rabbit Flat 

Member for good exposures on the north bank of the 

Mokelumne River, sec. 17, T. 5 N., R. 11 E., about 3 km 

south of Rabbit Flat. It consists of coarsely ·porphyritic 

augite basalt breccia and massive flows or sills, as well 

as minor bedded pyroclastic rocks. The base of the 

member is in fault contact with the melange (pl. 1). The 

Rabbit Flat Member reaches a maximum thickness of 

about 2·,000 m west-southwest-of Martell and thins less 

abruptly to the south than to the north. As shown on 

plate 1, the thickness at the Mokelumne River is about 

640 m. At that ·location, the Rabbit Flat Member was 

called unit 29 of the Brower Creek Member of the 

Mariposa Formation by Clark (1964, pl. 7). 

Clasts in the breccia range from subangular to subrounded pebbles and boulders of coarsely porphyritic 

augite basalt. The clasts and matrix are generally similar in lithology, having pheno~rysts as much as 1 em in 

diameter, but are distinguished chiefly by abrupt 

though slight contrasts in abundance or size of the 

phenocrysts. The breccia appears unstratified in single 

outcrops but probably is coarsely bedded at a scale 

larger than most outcrops. A · few well-bedded 

fine-grained pyroclastic rocks similar to those of the 

overlying member crop out near the base of the member 

at Sutter Creek. 

he Rabbit Flat Memb~ali@ compo~i!ioll .1lJl 

shown by chemical analy~wo representative samples of massive augite porphyry (table 1). The composition of these samples is nearly the same as those for all 

of the overlying members of the formation; it is especially similar to that of the Pokerville Member, which 

consists chiefly of the same rock types. 

The age of the Rabbit Flat Member is unknown, insofar as reported discoveries of fossils have not been 

definitely located within it. The only possible fossil occurrence from this unit is a specimen of the Callovian 

ammonitePseudocadoceras cited by Imlay (1961, tables 

2, 3, loc. 15). However, the exact site of the fossil collection is unknown. Nonetheless, since the Rabbit Flat 

Member is conformably overlain by the Goat Hill 

Member of documented Callovian age, both are considered to be of Late Jurassic age. 

The Goat Hill Member is here named for exposures on 

the north bank of the Mokelumne River, sees. 16 and 17, 

T. 5 N., R. 11 E., about 1 km south of Goat Hill. This 

member is composed chiefly of well-bedded marin~ 

pyroclastic rocks, locally forming as much as half the 

thickness of the Logtown Ridge Formation. In Amador 

County the member is exposed along a continuous belt 

ranging in width from 335 to 1,465 m. Because the 

underlying Rabbit Flat Member is structurally truncated at Drytown, the Goat Hill Member from that point 

northward to beyond the Cosumnes River is the basal· 

unit of the formation. Southward stratigraphic thin-
LOGTOWN RIDGE-MOTHER LODE BELT 11 

TABLE !.-Chemical analyses of greenstone from the Logtown Ridge-Mother Lode belt 

[Analyses by rapid-rock method. Analysts: Lowell Artis, G. W. Chloe, P. L. D. Elmore, J. L. Glenn, James Kelsey, Hezekiah Smith. Analyses recalculated with H20-omitted] 

Logtown Ridge Formation 

Unknown 

Rabbit 

Flat 

Member 

Goat Hill Pokerville strati!P"aphic position Member Member 

Specimen 

Si02 ---------------------- Al20a----------------------

F e20:J --------------------

FeO ----------------------

~~~ ====================== 

~;gC:==:::::::::::::::::::: Ti02 ----------------------

P205 ---------------------- MnO ----------------------

C~ -----------------------

2 

49.2 48.7 

15.2 15.5 

.9 =- 1.7._ 

7.9 ===-6.4 

7.1 6.3 

11.0 ==H.4 

2.1 3.2 

1.1 1.1 

3.2 2.7 

1.0 .6 

.2 .2 

.1 .2 

1.0 1.9 

Total ---------------- 100.0 99.9 

3 

51.2 

16.6 

-=-.5 

9:o9 

4.8 

-=-==6':"7-

2.8 

==t.3 

4.5 

1.7 

.5 

.2 

.4 

100.1 

4 5 

51.1 50.0 

17.0 17.9 

1.9 ~-3.2 

~.9 s~ 

2.9 3.3 

1.7 ~872 

3.3 3.8 

=2.8 o=fl.. 7 

3.4 2.6 

1.2 1.3 

.2 .5 

.2 .2 

.3 1.0 

99.9 99.9 

6 7 8 9 

49.1 49.6 49.2 55.5 

14.6 10.6 14.8 16.3 

-===--;1.0 -1.7 ---=4..5 ~.0 

80 68 8.0 2J. 

8.5 11.8 6.9 3.4 - -8~9 <=1.~.6 =- 10.7 - 9.5 

3.6 2.4 2.8 1.7 

--.9 -1.2 =.9 _1.1 

3.8 2.3 3.0 2.1 

1.1 .7 1.0 .8 

.3 .4 .2 .3 

.2 .2 .2 .2 

.2 .8 .9 .1 

100.2 100.2 100.1 100.1 

1. 900 fl. cast of west boundary and 300 fl. south of north boundary of sec. 2, T. 6 N., R. 10 E. Massive augite J?Orphyry. 

2. 1,600 fl. cast of west boundary and 1,000 fl. south of north boundary of sec. 2, T. 6 N., R. 10 E. Massive augite porphyry. 

3. South bank Cosumncs River, 50 fl. west of east boundary of sec. 21, T. 8 N., R. 10 E. Very fine grained tuff. 

4. Roadcut, Old Sacramento Road, 750 fl. east of west boundary of sec. 10, T. 7 N., R. 10 E. Non porphyritic volcanic breccia. 

5. North side ofTonzi Road, 90 fl. west of east boundary of sec. 2, T. 6 N., R. 10 E. Coarse plagioclase porphyry pillow lava. 6. Roadcut, west side of State Highway 49, 100-150 fl. south of south end ofHuse Bridge across Cosumnes River. Sample location and analysis from Clark (19.70, table 2). Coarse 

augite porphyry. 7. Roadcut, west side State Highway 49, 1,390 fl. north of south boundary sec. 15, T. 7 N., R. 10 E. Coarse augite porphyry. ' 8. Augite porphyry dike(?) west of Jackson (precise location unknown). Sample location and analysis from Turner (1894b, p. 473). Coarse augite porphyry. 

9. East side Plymouth Ditch, 340 fl. north of south boundry of sec. 26, T. 8 N., R. 10 E. Phyllonitic greenstone. 

ning of the overlying Pokerville Member leaves the 

Goat Hill Member as the uppermost unit of the formation for the 3-km interval immediately north of the 

Mokelumne River (pl. 1) and perhaps for a considerable 

distance farther south in Calaveras County. 

Some of the rocks of the Goat Hill Member are here 

incorporated into the Logtown Ridge Formation for the 

first time. At the Mokelumne River, the member corresponds to Clark's (1964, pl. 7) units 30, 31, and 32, all 

designated by him as the Brower Creek Volcanic 

Member of the Mariposa Formation. Furthermore, the 

Goat Hill Member at the Cosumnes River is equivalent 

to most of Clark's (1964, pl. 8) unit 47 of the type 

Cosumnes Formation, combined with units 48 and 49 

assigned by him to the Logtown Ridge Formation. The 

structural and lithologic evidence supporting the latter 

reassi'gnment has been discussed in detail by Sharp and 

Duffield (1973). 

Three principal rock types constitute the Goat Hill 

~ Member: (1) ~thin- to thick-bedded very fine to mediumgrained tuff, (2) ~c.e..]apilli tJJff in commonly 

t_hickJ.o_callx,.gx:a.ded bed_s that ax:e interlaY.ered.with.tbe 

finer tuff, and (3) thick-bedded fine to coarse_yolcanic 

breccia that r~~es \l W~l d_ · edJJJ.JJl- d 

~gr · dditional but relatively minor types 

of rock found within the Goat Hill Member and mapped 

separately on plate 1 include massive and pillowstructured very coarse plagioclase porphyry and 

augite-plagioclase porphyry· (Duffield, 1969). These 

rocks occur in thin sheetlike flows and sills north of 

Sutter Creek but have not been found farther south in 

Amador County (pl. 1). Known outcrops are either 

within or at the base of the Goat Hill Member. Similar 

rocks that form sill-like and other irregular masses 

intrude the melange. 

The interbedding of the various lithologies of the 

Goat Hill Member, as well as other sedimentary features including cut-and-fill structures and softsediment deformation, is dramatically ~xposed along 

the banks of the Cosumnes River. Most of these features 

have been described and depicted in detail by Clark 

(1964, p. 19-22). The common occurrences of uncleformed highly vesicular fragments of pumice, graded 

beds, abundant examples of soft-sediment deformation, 

and known associated marine fossils indicate that the 

bedded sequence of this member is of subaqueous pyroclastic origin of the type described by Fiske (1963) and 

Fiske and Matsuda (1964). 

Chemical analysis of typical specime_ns_of bedded 

Goat Hill rocks sug ests that the redominant c m osition ofthe membe · " table 1). Samples of very 

fine grained tuff and volcan1cbreccia with granule- to 

pebble-size clasts in very fine grained matrix that is 

similar in lithology to the clasts strongly resemble one 

another in composition. However, these rocks, as well as 

the coarse plagioclase porphyry separately mapped 

within the member, contain slightly more SiOl and 

AbOJ and less MgO and CaO than sample's from other 

members of the formation. 

The age of the Goat Hill Member is well established 

by fossils along-the Cosumnes River. Specimens of the 

ammonite Pseudocadoceras, of Callovian age, have 

~ 
12 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

been collected from strata about 395 m above the base 

and within 275m of the top of the member (compare 

Eric and others, 1955, p. 10; Imlay, 1961, p. 3, 6; Clark, 

1964, p. 18, 21; pl. 1, this report). Although a slightly 

older age than Callovian is possible for the basal part of 

the member, the uppermost part probably is also Callovian in age, on the basis of fossil occurrences in the 

overlying Pokerville Member. 

The Pokerville Member is here named for exposures 

on the south bank of the Cosumnes River in sees. 14 and 

15, T. 8 N., R. 10 E. Pokerville is an early name of the 

nearby town of Plymouth (Gudde, 1969, p. 252). Nearly 

all the Pokerville Member is coarsely porphyritic augite 

basalt; it forms massive flows, pillow lavas, and coarsebedded pyroclastic deposits, but volcanic breccia with 

virtually monolithologic clasts and matrix is dominant. 

Even the few clasts of different lithology observed in the 

breccia were mostly mafic volcanic rocks. The clasts and 

matrix in the breccia are essentially identical to those 

found in the Rabbit Flat Member. 

Chemical analyses of sam les of the Pokerville 

Member ar_e~n~early: idJmfu_aLwith thos_e_ma e_on__; e 

lilli2!ggic~allx similar Rabbit Flat M_embeUS_ee table 

1.) Analyzed samples from widely scattered localities 

within the Pokerville from the Cosumnes River on the 

n - . the latitude of J ackso..D_Qn_t_h_e_s_o_uth,. are all 

as~ iid vary little in bulk chemistry .. 

Throughout most of Amador County, the Pokerville 

Member is the uppermost map unit of the Logtown 

Ridge Formation, but east and southeast of Drytown it 

is overlain by the New Chicago Member. West of 

Plymouth, the Pokerville forms nearly three-fourths of 

the formation, but its proportion and thickness diminish stepwise southward through a series of three 

- tectonic blocks. In the southernmost block, south of 

Drytown, the member commonly forms less than 10 

percent of the formation, and it gradually thins southward and pinches out about 3 km north of the 

Mokelumne River. Its maximum thickness in the 

county is approximately 1,370 m 0.8 km north of 

Plymouth. 

The Pokerville Member overlies the Goat Hill 

Member with apparent conformity south of Drytown. 

North of Drytown, however, the irregularity of the contact between the members suggests disconformity. At 

two localities northwest and southwest of Plymouth, 

the members seem to intertongue, suggesting that the 

relief on the contact may be a depositional rather than 

an erosional feature and that the contact is everywhere 

conformable. Strong evidence, to be discussed more 

fully elsewhere in this report, suggests that faulting 

occurred in the Rabbit Flat and Goat Hill Members of 

the Logtown Ridge Formation when sediments of the 

Pokerville Member were accumulating. 

The age of the Pokerville Me.mb.etis=PLOhabJY-_Callovian, but it may extend upward into latest Oxfordian or 

early Kimmeridgian, depending on the actual locations 

of two previously recovered and identified ammonites 

described by Imlay (1961, table 3). Although the locational data for one latest Oxfordian or early Kimmeridgian ammonite,Idoceras, led Imlay to believe that it 

came from the upper part of the Logtown Ridge Formation near the Cosumnes River (l~lay, 1961, table 3, loc. 

10), the description of the e~closing rock as 

((metamorphosed andesitic tuff'' makes it difficult to 

assign it to any part of the Logtown Ridge except the 

Goat Hill Member in the lower half of the formation. It 

is more likely that the andesitic tuff is one of the lenticular tuffaceous greenstone bodies that we assign to the 

Mariposa Formation, immediately overlying the 

Pokerville Member at that location. (See pl. 1.) This 

interpretation resolves an additional dilemma regarding the other ammonite, Peltoceras, recovered from the 

nearby Mariposa Formation, according to original records (Imlay, 1961, p. 6-; compare to his table 3, loc. 13). 

Despite the original notes referring this specimen of 

Peltoceras to a location near Big Indian Creek, which 

parallels Highway 49 north of Plymouth, Imlay argued 

that it probably came from a position near the middle of 

the Logtown Ridge Formation, in order to avoid placing 

the Callovian specimen above the Oxfordian or early 

Kimmeridgian one. No inversion of the time sequence 

results if this specimen of Peltoceras is assigned to the 

Mariposa Formation but at a stratigraphically lower 

position than the Idoceras-bearing tuff unit within the 

Mariposa. Such a reconstruction is consistent with all 

the published data and suggests that the entire type 

Logtown Ridge Formation on the Cosumnes River is 

Callovian in age. 

The uppermost unit recognized in the Logtown Ridge 

Formation is here named the New Chicago Member, for 

exposures in and near the roadcut of Highway 49, sec. 

36, T. 7 N., R. 10 E. This member is named after the 

settlement of New Chicago, about 1.6 km north of the 

type locality. The member is composed chiefly of fine 

volcanic breccia with vesicular fine-grained to aphanitic clasts in a similar matrix. Vesicular fine-grained 

massive greenstone, sparsely and finely porphyritic augite porphyry, and coarse volcanic breccia containing 

clasts of both fine-grained to aphanitic greenstone and 

minor amounts of vesicular coarse augite porphyry are 

also intermixed in this member. 

Volcanic breccia of the Pokerville Member that immediately underlies the New Chicago Member also possesses a distinctive vesicular character. Although scattered clasts of such vesicular augite porphyry can be 

found through much of th~ Pokerville breccia, such 

clasts_ arP. especially abundant throughout a thickness 
LOGTOWN RIDGE-MOTHER LODE BELT 13 

of several meters where overlain by the New Chicago. 

This close spatial association of unusually vesicular 

rocks assigned to both members suggests that a single 

source or vent ·which produced abundant vesiculated 

debris possibly existed locally from Pokerville into New 

Chicago time. 

The New Chicago Member is restricted to the south 

half of the Logtown Ridge Formation (pl. 1). It forms 

thin, discontinuous, elongate bodies; the thickest is 150 

m, and the longest is about 3 km. 

Fossils have not been found in the New Chicago 

Member, but its conformable relationship with the underlying Pokerville Member indicates that it is only 

slightly younger. If the basal beds of'the directly overlying Mariposa Formation are Callovian, as those near 

the Cosumnes River may be, then the New Chicago 

Member would be additionally restricted to a Callovian 

age along with the remainder of the formation; however, the actual age may be somewhat younger. 

Published maps indicate the continuity of the Logtown Ridge Formation northward from Amador 

County; southward continuity into Calaveras County is 

not agreed upon. Clark (1964, pl. 1) believed that the 

Logtown Ridge Formation thinned southward stratigraphically and disappeared in Amador County under 

an overlapping tongue of lithologically similar rocks, 

the Brower Greek· Volcanic Member of the Mariposa 

Formation. Our mapping, however, shows that the Logtown Ridge Formation extends southward across this 

boundary to the Mokelumne River and probably 

beyond. Examination of relatively detailed maps of 

Calaveras County by Eric, Stromquist, and Swinney 

(1955), Clark, Stromquist, and Tatlock (1963), and 

Clark (1964),.as well as the early folio maps of the U.S. 

Geological Survey, strongly suggests that the Logtown 

Ridge rocks in Amador County are equivalent to at least 

some of the metavolcanic strata to the south. Indeed, all 

existing data suggest that the Logtown Ridge designation by Eric, Stromquist, and Swinney (1955, pl. 1) for 

rocks exposed northwest from Brower Creek along 

Hogback Mountain in Calaveras County and depicted 

on many maps (for example, Clark and others, 1963; 

Clark, 1964, pl. 1) along a belt connecting with what is 

now definitely Logtown Ridge Formation at the 

Mokelumne River is valid by reason of stratal continuity. Proof of this correlation, as well as the possible 

inclusion of other rocks deemed Logtown Ridge by Eric, 

Stromquist, and Swinney (1955) in Calaveras County, 

must necessarily await more detailed subdivision and 

mapping. However, regional stratigraphic correlation 

of these rocks is now open to multiple interpretation, 

and future detailed work may invalidate some of the currently favored stratigraphic nomenclature of the Upper 

Jurassic metavolcanic rocks south of Amador County. 

MARIPOSA FORMATION 

Rocks immediately overlying the Logtown Ridge 

Formation west of the Melones fault zone are here designated the Mariposa Formation. These rocks include 

black cla~ slate i;ntgx:bedded ~itb ~~acK:e, congiomerate, and fine-grained mafic tuffaceous rocks and are 

intruded b~ QOrQh~ritic volcanic rocks OI h~aby.ssal 

~· Although these rocks were origina-lly termed the 

MariQOSa Slate b~ IuiDer.il BS~a) on the Jackson folio, 

Clark (19641 Q. 27llater assigned them an unknown but 

Eost-Log!own Ridge stratigraQhic :eosition ,becausg the;x 

..could not be clearl~ co;nnecte.d-w.itLthe »::ell-

_documented Mariposa Formation i;n Calaveras C.o.u.nty 

_to the south Ho,~ver,_la.teraJ--eontinilit)! with rocks 

..] Marinosa Formation at the Mokelumne River 

bx Clark (19641 Qls. 12 7~ is now established b~ detailed 

maooing (Seg pl. 1.) 

Following the usage established by Clark (1964, p. 

23), volcanic rocks within t.hP M Formation are 

here designated the Brower Creek Volcanic Member. 

·These volcanic rocks aQQarentl~ ggminate the Mari:eosa 

section in the southern half of Amador Count~ 1 although the eastern extent of the MariQOSa Formation 

has not been defined in glate ] Northwaz:d_from 

Amador City, bodies of Brower Creek rocks diminish in 

abundance and are virtually absent midway between 

Drytown and Plymouth, but north of Plymouth small 

bodies of Brower Creek crop out as numerous thin bands 

in the Mariposa Formation. 

Although the g+satest midth of the belt of Mariposa 

F~mation where both o£its edges ha:y:e been man~d is 

about 2,100 m measured east-northeastward from nea r 

Drytown, it is not possible to establish the true stratigraphic thickness there owing to isoclinal folding and 

possible repetition of section by faulting. Northward 

from that location, the formation thins by tectonic truncation to ~bout 800 m at the Cosumnes River. Uni-

. formly eastward stratigraphic top determinations o n 

nearly vertical bedding near the Cosumnes River, com 

bined with a distinctive stratigraphic succession, suggest that the 800-m width ·of the Mariposa belt there 

approximates a true stratigraphic thickness. The 

Mariposa Formation along the Mokelumne River possibly forms a nearly 915-m-wide belt (Clark, 1964, pl. 8), 

but abundant small-scale folds there suggest that the 

section may be considerably repeated. (See pl. 1.) 

Field evidence indicates that there was little, it any, 

hiatus in deJ2osition across the Log:town RidgeMariposa boundarx. This is supported, first, by appar 

ent stratigraphic conformity at the only clear exposure 

of the contact in the roadcut of Highway 49 immediately 

south of the Cosumnes River (see also Clark, 1964, p. 

26), second, by the general conformity of mapped 

marker beds within the Mariposa Formation to the con-
14 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

tact, and third, by the continuity of a very thin strip of 

Mariposa slate between the Logtown Ridge and Brower 

Creek rocks from Martell southward to the Mokelumne 

River (pl. 1). Such a conformable relationship is consistent with the ages of fossils discussed in the section 

uLogtown Ridge Formation." 

The dominant nonvolcanic rock type in the Mariposa 

Formation is black clay slate, enerall with s arse 

In In er e s o fine- to medium-grained graywacke. 

At most locations along the contact, these rocks rest 

directly on the Logtown Ridge Formation. Between 

Plymouth and Drytown, these interbedded rocks constitute nearly the entire thickness of the Mariposa Formation, but to the north and south other lithologies, principally greenstones, dominate the section. 

Detailed descriptions of the lithologic character and 

sedimentary features found in Mariposa slate and its 

interbedded graywacke were given by Clark (1964, p. 

24, 39-41). This slate and graywacke are vir.tu.all~un.­

distinguishable in hand specimen and outcrop from 

some rocks of Cosumnes affinitY. in the melange belt. 

However, Mariposa slate contrasts strongly with both 

the siliceous and nonsiliceous slates of Calaveras 

affinity found in the melange. The chief differences are 

the total absence of chert beds and silicified lentils or 

other irregular masses that are nearly ubiquitous in 

Calaveras siliceous slate and the presence of graywacke 

interbeds that have not been observed in the Calaveras 

nonsiliceous slate. Although we think that these contrasts are noteworthy and consistent with other major 

differences between the Mariposa and Calaveras Formations, Clark, Stromquist, and Tatlock (1963) and 

Clark (1964) have not made such distinctions south of 

Amador County; there, much of the slate shown on their 

maps as Mariposa Formation is lithologically equivalent to slate of Calaveras affinity in the melange. 

Two types of conglomerate have been mapped in the 

Mari osa F'ormation in Amador County Conglomerate 

with. abundant well-rounded clasts of quartzite_an.d 

similarly resistant rock types fm::nLlocall¥-¥61!-Y--tl:Mek. 

beds near tha.base..of..the.Maciposa-~rmatien between 

Amador City and Enterprise (near the Cosumnes 

River). Thick beds of immature conglomerate, made 

chiefly of granule-sized clasts of shale chips and subordinate amounts of cherty and quartzose clasts, are interbedded with coarse. graywacke and slate for 3.2 km 

south arid an unknown distance north from the 

Cosumnes River. 

The quartzite-cobble conglomerate intertongues and 

locally grades laterally into finer conglomerate, coarse 

gt:ay.wacke and:Siafe An unUSii8Jly tfiick bea:Qf such 

conglomerate with intercalated coarse graywaAe partings forms a section about 27 5 m thick along Dry Creek 

northeast of Drytown, but this unit abruptly thins to the 

north and south and intertongues with slate and 

graywacke. The lateral extent of individual thin beds of 

quartzite conglomerate is remarkable, more than 3 km 

for some beds. Well-rounded quartzitic clasts everywhere predominate, but there is an appreciable fraction 

of more angular clasts of volcanic rocks as well. The 

matrix is poorly sorted coarse graywacke containing 

substantial amounts of relatively well rounded quartz 

grains. The quartzite clasts are fine to very coarse 

grained and contain some relatively coarse white mica 

that defines foliation surfaces randomly oriented from 

clast to clast. These clasts were derived from a 

metamorphic terrane possible of relatively high grade, 

judging by the coarseness of the white mica and apparently annealed textures. The matrix and interbedded 

slaty rocks show only slight evidence of metamorphism, 

and so the foliation in the clasts is definitely pre- . 

Mariposa in age. 

The distribution of the quartzitic conglomerate suggests that the main deposits are fillings of channels th~t 

followed depresssions on the upper surface of the Logtown Ridge Formation. The principal channel was east 

and north of Drytown, where the conglomerate is thickest. The fact that this channel lies directly above a zone 

of structural dislocations within the Logtown Ridge 

Formation (pl. 1) suggests that faulting affected the 

area at or just before the time of Mariposa deposition. 

This conclusion is critical to our interpretation of structure in Amador County, and its implications are more 

fully discussed under the section ttStructure." 

The second kind of conglomerate in the Mariposa 

Formation, the immature shale-clast granule to pebble 

conglomerate, is mostly restricted to a small area 

within 3 km of the Cosumnes River in Amador County. 

Elsewhere, similar rocks have been recognized only in a 

small lens near Sutter Creek. These rocks are interlayered with thick-bedded fine- to coarse-grained 

graywacke and black clay slate, both of which resemble 

similar rocks from the main slate zone in the Mariposa 

belt but lack the common thinly interbedded relationship. Although most of the clasts in the conglomerate 

appear to be angular very fine grained argillaceous 

fragments which are similar to the matrix of the rock, 

except for slight variations in color, an appreciable fraction of the clasts consists of chert, quartzite, and 

fine-grained volcanic rocks. Metamorphic rock cleavage 

generally is almost as well developed within the clasts 

as in the matrix, rendering them markedly fissile. The 

shale-clast conglomerate is virtually identical lithologically to some rocks of Cosumnes affinity in the 

melange. 

Volcanic greenstones of chiefly pyroclastic origin interbedded with epiclastic rocks consbtute-tb-e::BJ:ow.eL 

Creek Volcanic Membez:, which forms a majQL!llirt of 
LOGTOWN RIDGE-MOTHER LODE BELT 15 

the Mariposa Formation in Amador_C.o.unt¥--P.ru:.t.ic..ularly south of Sutter Creek . ...The member also includes 

bodies of intrusive and extrusive greenstone. Although 

much of the Brower Creek appears to be small lenticular masses entirely enclosed in sedimentary contact 

w.ithin Mariposa slate, the largest bodies as depicted on 

plate 1 probably are separated from the remainder· of 

the formation by faults, and thus their assignment to 

the Brower Creek Volcanic Member is tentative. Locally, however, epiclastic rocks similar to those of the 

Mariposa Formation are widely intercalated within the 

larger bodies mapped as the Brower Creek. The 

justification for including many of these rocks in the 

formation therefore is chiefly based on these interbedding relations and cannot yet be substantiated by age 

dates from fossil occurrences. 

The most prevalent rock in the Brower Creek Volcanic Member in Amador County is fine- to mediumgrained thin- to thick-bedded tuffaceous greenstone 

that is intercalated with black clay slate and graywacke 

similar to that found in other parts of the Mariposa 

Formation. The largest body of this rock extends from 

about midway between Plymouth and Drytown southward to the Mokelumne River, where it corresponds to 

Clark's (1964, pl. 7) units 34 through 37. Many smaller 

lenses of tuffaceous greenstone are scattered throughout the Mariposa section north of Plymouth. The tuffaceous rocks form beds from less than 1 em to more 

than 1 m thick. Grading within these beds has only 

rarely been detected. No chemical analyses are available, but on the basis of lithologic resemblance to the 

Copper Hill Volcanics of the western belt, these rocks 

probably are andesitic to basaltic in composition. The 

proportion of graywacke and slate interbedded with the 

tuffaceous greenstone varies greatly, and no attempt 

has been made to separate these rocks on plate 1 because of the complexity and scale of interbedding. 

Volcanic rocks of basaltic composition with coarse 

phenocrystic augite form relatively thick massive flows 

and flow breccia within the large body of bedded pyroclastic rocks of the Brower Creek Volcanic Member at 

least as far south as Sutter Creek. Other lenticular 

bodies of augite porphyritic basalt probably representing flows or shallow sills occur locally in the Mariposa 

Formation north of Plymouth. These rocks are lithologically identical to the augite porphyry greenstone of the 

Rabbit Flat and Pokerville Members of the Logtown 

Ridge Formation. An analysis of one specimen of augite 

porphyry from the Brower Creek Volcanic Member 

shows that the composition of these rocks is basaltic and 

nearly equivalent to the lithologically similar Logtown 

Ridge Formation (table 1). 

Volcanic breccia containing blocks of fine-grained 

volcanic rocks is a minor rock type in the Brower Creek 

Volcanic Member. A single lenticular mass of such rock 

occurs along the base of a bedded tuffaceous greenstone 

section about halfway between Plymouth and the 

Cosumnes River·. It is superfically indistinguishable 

from similar breccia in the Goat Hill Member of the 

Logtown Ridge Formation. 

Molluscan and ammonite faunas from scattered locations throughout the Mariposa Formation indicate the 

late Oxfordian and early Kimmeridgian Stages of Late 

Jurassic age (Imlay, 1961, p. 7-8; Clark, 1964, p. 25-26). 

Fossils previously discovered in Amador County generally corroborate this age assignment, but as discussed 

earlier, the lower part of the Mariposa might possibly be 

as old as Callovian. Recovery of the pelecypod B uchia 

concentrica at a position probably within 152 m strati- · 

graphically above the base of the Mariposa ~ormation . 

. about 1.2 km north-northwest of Plymouth (Imlay, 

1961, table 2, loc. 14) demonstrates a late Oxfordian and 

early Kimmeridgian age there. The question remains, 

however, as to whether basal Mariposa strata laterally 

transgress time and whether beds somewhat older than 

late Oxfordian might also exist, or, alternatively, 

whether the lower range ofBuchia concentrica is somewhat older than presently recognized. 

ROCKS OF THE MELONES FAULT ZONE 

A narrow band (about 25~800 m) of distinctive 

sheared rocks separates the Mariposa Formation on the 

west from the large granitic pluton and the Calaveras 

Formation of the eastern belt to the east. Widespread 

evidence of pervasive shearing in this band of rocks, as 

well as regional stratigraphic relations, suggests that 

large fault offset occurs across the zone. This zone includes the Melones fault as defined by Clark (1960, 

1964). However, Clark believed that the width of the 

fault zone in Amador County was considerably less than 

we show on plate 1, and he (1964, p. 27) referred many of 

the rocks that compose the fault zone of plate 1 to a 

volcanic unit of unknown stratigraphic position. (See 

Clark, 1964, pl. 1.) 

In general, we assign to the fault zone rocks that show 

noticeably greater shearing and recrystallization than 

adjacent rocks, on the assumption that this band of 

relatively highly metamorphosed rocks is coextensive 

with a zone of faulting. Within Amador County most of 

the rock types fo:und in this band are unique to it; they 

include phyllitic and phyllonitic greenstone as the dominant types, hornblende-bearing rocks of metaigneous 

aspect ranging from leucogabbro to hornblendite, 

mylonitized rocks probably derived from all the previous types of rock, and medium-grained alaskitic rocks 

that commonly show mylonitic effects. Bodies of rock 

similar to material of Calaveras affinities in the 

melange are also incorporated within this band; they 

are described with the rocks of the melange, although 
16 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

the setting of their occurrence here is markedly differ- pyroxene in the cores of large hornblende crystals. 

ent. White mica and epidote grains, as in the phyllite and 

The phyllitic and phyllonitic greenstone generally phyllonite, form felted masses generally without preappear nearly massive on the scale of outcrops. In hand ferred orientation, even though a pronounced planar 

specimen, they are distinctly more coarse grained than fabric is defined by the other minerals. This relation 

greenstone found in all other formations in the area suggests that at least one stage of recrystallization 

except some rocks assigned to the Copper Hill Volcanics postdated much of the shearing evident in these rocks. 

of the western belt. Typical specimens contain abun- Some bodies of strongly mylonitized rocks of the fault 

dant plates of relatively coarse white mica (easily visi- zone with abundant large hornblende augen have been 

ble to the unaided eye) that give a pronounced sheen to mapped separately on plate 1 because of their great size 

cleavage surfaces. Some of the rocks have relict and apparent internal homogeneity. These rocks rehornblende phenocrysts, but most phenocrysts have semble the phyllitic and phyllonitic rocks in mineralogy 

been completely or partially replaced by very fine and probable bulk composition and also possess the 

grained aggregates of white mica, epidote-chlorite, and cataclastic texture that is commonly but less strikingly 

quartz. Bedding has not been observed in any outcrops developed in the other rocks. Moreover, the mylonitic 

of this rock, but in thin section, compositional layering rocks appear to be gradational with many of the porin some specimens is evident. Most of this layering phyritic rocks included in the phyllite-phyllonite unit 

results, however from late vein formation or possibly in terms of the abundance of the hornblende phenofrom metamorphic segregation developed during shear- crysts. 

ing of the rock. The field distinction between phyllite A few dikelike or sill-like bodies of light-colored 

and phyllonite is made on the basis of whether obvious foliated alaskite lie within the phyllite-phyllonite belt. 

fluxion shown by milled-down hornblende augen is evi- These rocks are granitic in composition and are reladent in the rock. However, on the microscopic scale, all tively coarse grained. Nearly all outcrops exhibit foliasamples studied show generally pervasive shearing and tion to some degree, and thin sections show that it is 

thus possess a phyllonitic aspect. related to cataclastic effects of widely ranging intensity. 

The parent from which the phyllite and phyllonite In some mylonitized phases, compositional layering is 

formed probably consisted primarily of intermediate to formed by thin epidote-rich and white mica-rich bands 

basic porphyritic volcanic rocks. Analysis of a typical separating layers composed almost entirely of potasspecimen (see table 1) shows an andesitic composition, sium feldspar and quartz. Contacts of alaskite with the 

but the high proportion of dark minerals in some of surrounding materials are not exposed, and so the inthese rocks, generally exceeding 50 percent and reach- trusive nature shown on plate 1 is interpretive. Aling locally as much as 9<;> percent, suggests that basaltic though the generally concordant .geometry suggests 

compositions also occur. Inasmuch as hornblende por- that these bodies were sills prior to the cataclastic 

phyritic rocks are unknown as large-scale masses else- shearing, they conceivably could represent fault slivers 

where in the immediate region, this belt of rocks ·ap- of much larger intrusive masses not presently exposed. 

pears to be lithologically unique. WESTERN BELT 

Other rocks associated with the phyllite and phyllon- All 'rocks of the western helLare assigned to the Co.pite distinguish the fault zone, too. Irregularly shaped per Hill volcanics. asdefi11ed bx ClarltJ1964...P.~_3J):-3.1), 

masses of fine- to coarse-grained metaplutonic rocks of and consist chiefl of fine- ained yroclastic greengenerally gabbroic affinity are widely scattered in the stones. lthou h t e strati a hie base of these rocks 

zone. In detail, these rocks apparently grade from rela- has not been mapped in this study, C ar 1964) state 

tively leucocratic rocks (leucogabbro) to very dark rocks that the Copper Hill Volcanics 1ntertongue to t~~st 

approaching hornblendite. Many of these rocks display with the Sal.t-£pring Slate. which has been dated as late 

abundant irregular greenish veins consisting almost Oxfordian and early Kimmeridgian age on- the basi~Lof 

entirely of very fine grained aggregates of epidote; the fossils fopnd along the Cosumn~s Riyer in Sacramento 

veins are generally small and follow irregular fractures County StJ:~hic.....top dJ~.terminations. noted by 

in the host rock. Greenschist-facies metamorphic over- Clark along the Cosumnes River thus showJh_e up~r 

print, as well as some cataclastic shearing and post~ er Hill Volcanics to be youn er than this slate l?_ut 

shearing recrystallization, gives these rocks an excep- 11 ~~~~~~a,mg~e~s...,reM~p~r~e-Jt~x tliese .. volcanic rocks.is 

tionally complex aspect both in hand specimen and i unknown at present. It should be added that tectonic 

thin section. All the rocks consist of the same minerals intermixing of strat~ in a manner equivalent to that 

as the associated phyllite and phyllonite: hornblende, found within the melange shown on plate 1 may conoligoclase, epidote, white mica, chlorite, quartz, and ceivably have occurred much farther west than has 

some pyrite. A few specimens contain small remnants of been recognized in our study; if so, then the Copper Hill 
INTRUSIVE ROCKS 17 

Volcanics may not rest in undisturbed stratigraphic 

position on the Salt Spring Slate. Detailed mapping in 

the Copper Hill Volcanics and far.ther west should answer this question. 

roclastic rocks of the Copper Hill Volcanics consist 

typically of relatively fine grained and near y massive 

~yllitic greenstone. Although some of the rocks along 

the east margin Qrtfie formation contain relatively 

large quartz grains, most are nearly eguigranular. tocally, on a microscopic scale, faint compositional banding is defined by white mica-chlorite-quartz-albiteepidote laminae interlayered with bands lacking white 

mica. Larger scale bedding, as well as other sedimentary structures, are apparently absent in the Copper 

Hill Volcanics. Presumably, these features, if once 

present, have been masked during the low-grade 

metamorphism that affected these rocks. Slaty argillaceous rocks interbedded with the Copper Hill Volcanics have been recognized at several locations lying 

west of the map area. 

Parallel bands of greenschist and amphibolite, possi~ 

bly part of the Copper Hill Volcanics, crop out in the 

northwest section of plate 1. Field evidence suggests 

that the transition from greenschist to amphibolite is 

gradational, but tectonic juxtaposition cannot be ruled 

out. Although t}:lese rocks form north-northwesttrending bands, schistosity diverges greatly from this 

direction. The plane of schistosity has been deformed 

into chevron folds at some outcrops, broader open folds 

at others, and locally (NEIA sec. 18, T. 7 N., R. 10 E.) into 

isoclinal folds with superposed broad open folds. A few 

outcrops show interbeds of chert folded into a similar 

shape as the enclosing greenschist, suggesting that 

schistosity parallels original stratification of the parent 

volcanic rocks. 

u Contacts within and surrounding the greenschistamphibolite terrane have not been completely delineated by our mapping, but available evidence suggests that these rocks are fault bounded rather than in 

:p.ormal stratigraphic sequence with underlying Copper 

Hill strata. The structural relations shown by Clark 

(1964, pl. 8) suggest that the greenschist-amphibolite 

rocks form a fault-bounded block of the melange that 

has been transported from deeper levels of the crust 

than adjacent rocks of the Copper Hill Volcanics. 

INTRUSIVE ROCKS 

Plutonic masses of both relatively deep seated and 

hypabyssal origin intrude rocks of all but the western 

belt. Part of a large coarse-grained granitic intrusion 

crops out in the eastern belt, but nearly all the other 

plutons are relatively small, irregular in outline, markedly discordant to host rocks, and fine grained or porphyritic. Compositions of the plutons range from 

ultramafic to granitic, although ultramafic rocks occur 

only in the melange. Original mineralogy has been altered in many plutons, but despite local overprinting of 

metamorphic texture or cataclastic structural fabric, 

the-initial textures commonly are intact. Two masses of 

coarsely recrystallized hornfels lie within the large 

eastern pluton, but marked contact metamorphic effects usually are lacking around intrusive boundaries; 

where present, such effects are restricted to apparent 

annealing, as shown by obliteration of rock schistosity 

without obvious recrystallization. 

MELANGE BELT 

Many types of intrusive rocks, ranging in composition 

from pyroxenite and serpentinite to granodiorite, have 

been recognized in the melange. Relative ages of the 

various intrusive rocks are generally impossible to determine because of isolation or lack of exposure along 

contacts. 

Outcrops of coarse-grained pyroxenite and 

hornblende pyroxenite are localized in a small area 

near the southwestcornerofplate 1 (W¥2 sec.15, T. 6 N., 

R. 10 E.); these ultramafic rocks are completely bounded 

by serpentinite or other intrusive rocks, including 

augite-hornblende gabbro and aphanitic fine-grained 

greenstone. Contact relations are not exposed, and the 

shape and distribution of these ultramafic bodies permit 

them to be interpreted either as tectonic blocks or inclusions in surrounding rocks, or as intrusions lying essentially in their original positions. 

Other equigranular rocks suggesting relatively deep 

seated crystallization are widely scattered in the 

melange. These rocks occur chiefly as small masses 

commonly associated with serpentinite and include 

pyroxene gabbro nearly completely altered to the 

greenschist facies, fine- to coarse-grained augitehornblende gabbro (in a 1.6-km-long intrusive mass 

near the southwest corner of sec. 9, T. 7 N., R. 10 E.), 

brecciated medium-grained hornblende quartz diorite 

(known only from a single outcrop in southern sec. 9, T. 

6 N., R. 10 E.), and relatively fine grained brecciated 

biotite granodiorite that forms very thin sill-like bodies 

(western sec. 27, T. 7 N., R. 10 E.). This last rock is 

extensively altered locally to dolomite, magnesite, and 

quartz. 

The bodies of ultramafic to mafic composition may be 

genetically related to the Pine Hill layered gabbro complex, located on strike with the melange belt about 13 

km north of the area in El Dorado County. Springer 

(1969) and Emerson (1969) showed that the Pine Hill 

igneous complex is partly composed of sill-like intrusive 

masses that became gravitationally layered during 

crystallization. The conformable relation of this layering to the steeply eastward dipping country rock there 

indicates that intrusion and crystallization took place 
18 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

while the enclosing rocks were still subhorizontal, that 

is prior to development of the steep regional dip. 

Diverse types of aphanitic to porphyritic intrusive 

rocks are widely distributed in the melange. Lithologically, some of these rocks are virtually indistinguishable from flows and sills in the Logtown Ridge Formation. Such rocks include coarse plagioclase porphyry 

greenstone and plagioclase-augite porphyry greenstone 

that are mineralogically and texturally equivalent to 

sills and pillow lavas. in the· Goat Hill Member of the 

Logtown Ridge Formation, as well as coarse augite porphyry greenstone that is identical to flows and breccia 

clasts in the Rabbit Flat and Pokerville Members. 

Plagioclase porphyry greenstone similar to that reported here was also recognized by Clark (1964, p. 31) in 

the Copper Hill Volcanics along the Cosumnes River a 

few miles west of the map area. The porphyritic rocks at 

that location are at least partly extrusive pillow lavas 

(Clark, 1964). 

Other porphyritic intrusive rocks have no counterparts in the Logtown Ridge Formation. Plagioclase porphyry, hornblende-plagioclase porphyry, and biotitehornblende-plagioclase porphyry form relatively large 

but extremely irregular intrusive masses that are particularly abundant in the eastern half of the melange 

belt. Quartz is sparse but ubiquitous in these rocks. The 

first two types of rock apparently grade into one another 

and into fine-grained aphanitic greenstone. All these 

rocks locally exhibit pervasive brecciation, metamorphism to greenschist facies, and rock cleavage concordant with the adjacent metasedimentary rocks. 

The geometry of some plutons in the melange belt 

strongly suggests that their emplacement followed the 

main structural disturbances there. For example, the 

finely porphyritic and aphanitic rocks associated with 

the gabbroic body at the corner of sees. 8, 9, 16, and 17, 

T. 7 N., R. 10 E., appear to cut across boundaries of 

structural block~ within the melange. The brecciation 

of these intrusive rocks, moreover, does not appear to be 

related to brecciation in the wallrocks that the intrusive 

EASTERN BELT 

The large coarse-grained granitic pluton underlying 

most of the eastern belt is part of a much larger mass 

that extends nearly 32 km to the northeast. It is separated from the main body of the Sierra Nevada 

batholith by intervening metamorphic rocks, but its 

composition, texture, and structural setting suggest 

·that it is simply an outlying part of the batholithic 

complex. 

The rock generally is fine- ·to medium-grained 

hornblende-biotite granodiorite, but at one locality 

near the Cosumnes River, it has a distinctly more mafic 

character, with a color index over 50. This variant, 

probably a quartz gabbro ·in composition, appears to 

grade laterally into the more typical and much lighter 

colored (color index 10--20) granodiorite. A north- to 

northwest-trending foliation of mafic minerals is common through much of the pluton but is more strongly 

developed near the west margin. Prismatic crystals of 

hornblende form a steeply plunging lineation in the 

west marginal zone. 

Although there is abundant evidence of cataclasis 

along the west contact, other contacts appear to have 

formed by intrusion of a liquid or liquid-crystal mixture, with no subsequent crushing effects. Strongly discordant contact relations in sees. 6, 7,. and 18, T. 7 N., R. 

11 E., and the weak to absent aureole of contact 

metamorphism in some parts of the adjacent Calaveras 

Formation suggest only a moderately deep level of emplacement. 

AGES OF INTRUSION 

Intrusive rocks in the west-central Sierra foothills 

generall~aheen assigned a Late~urassic age. by 

previous workers (Clark, 1964; Bateman and Wahrhaftig, 1966, p. 117; Evernden and Kistler, 1970). A Jur_a.s-" 

sic age has been determined (R. W. Kistler, written 

commun., 19~0) .Qy the potassium-argon method for two 

rock types in western Amador County. A specimen of 

the granodioritic pluton in tha_eastern belt. collested 

.near__ihe center_ofsec~2 T ~~_LLE., about 4.8 km 

masses locally transect; brecciated intrusive rocks gen- ~ast of the_areainclude.cLoa,plat~ c_ontains biotite 

erally are magmatically healed and tightly coherent, in d_at_ed at 157.4±4. 7 million years. A small quartz diorite 

strong contrast to the marked fissility of the apparently mass in southern sec. 9, T. 6 N., R:" 10 E., withiiitlie 

earlier sheared metasedimentary rocks. ..melange, gives_lL:h~de age ~4; miHion 

LOGTOWN RIDGE-MOTHER LODE BELT ¥-ear.,s.~aiteLdatesugg-est&all---E$l-:Y,rat er ffian 

A single small intrusion is emplaced in this belt about L.ate Jurassic age, and owing to possible disturbance of 

1.2 km east of Sutter Creek. This rock consists of ~otassium--a~~..adiometci.~clo_c_Kl>y faulting, this 

medium-grained hornblende quartz monzonite; rocks of ~~inimum. 

this composition have not been found elsewhere in the Springer (1969) and Emerson (1969) described a commap area. plex of gabbroic sills near Pine Hill, about 13 km north 

Other intrusive rocks of generally mafic composition of the map area and on strike with the melange belt, 

lie at the east edge of this belt along the Melones fault that has compositional layering of gravitational origin. 

zone. They have been thoroughly metamorphosed and The sills originally were nearly horizontal as indicated 

are described in the section uMelones Fault Zone." by the layering, but they now dip steeply eastward in 
SERPENTINITE AND RELATED ROCKS 19 

conformable relation with the preintrusion 

metasedimentary rocks. Thus, at least some plutonism 

in the Sierra foothill belt had begun prior to regional 

tilting of strata to nearly vertical positions. Moreover, 

serpentinite at the same locality predates the gabbroic 

sills, and thus its empl~cement also predates regional 

tilting. 

Structural and mineralogical features of hypabyssal 

intrusive rocks in the melange belt suggest that these 

bodies may have been emplaced earlier than most of the 

coarse-grained rocks in this and other belts. All the 

porphyritic rock types at least locally are altered to 

greenschist facies, suggesting that the regional 

metamorphic overprint has affected them as well as the 

stratified rocks. Rock cleavage locally present in the 

hypabyssal intrusive bodies further substantiates this 

interpretation. However, no radiometric ages have been 

obtained for these rocks. 

Most coarse-grained intrusive rocks apparently postdate regional metamorphism, as shown by the general 

absence of greenschist facies metamorphism. Moreover, 

some slight hornfelsing has occurred along the east 

margin of the large granodioritic body in the eastern 

belt; the zone of hornfelsic rocks is subparallel to the 

margin of the pluton, but it is discordant to the 

metamorphic foliation of the wallrocks and hence is 

younger. Foliation is evident in some coarse-grained 

rocks, but it probably is a primary igneous feature unrelated to cleavage in the wallrocks, despite subparallelism of both with the regional structural grain. 

SERPENTINITE AND RELATED ROCKS 

Serpentinite forms a 14.5-km-long sheared and 

faulted sill-like body that trends north-northwest along 

the west margin of the melange. It ranges in outcrop 

width from about 1.6 km to several tens of meters. 

Smaller satellitic bodies of serpentinite crop out both to 

the east and west of the main mass and are thought to 

define crudely the lateral limits of a zone of significant 

tectonic dislocation. Two approximately equant bodies 

of serpentinite are associated with the large plutons in 

sees. 8, 9, 16, and 17, T. 7 N., R. 10 E.; these are thought 

to be genetically related to the plutons or their emplacement. 

.M2s,.Lo.utCI:ops-oLs.eJ:p.en.tinite in the sill-like mass 

have many features in common. In plan, nearly all are 

elongate in a north-northwest direction, parallel to the 

~ional structural grain. Some outcrops are massive, 

Jm.d_Qthel:s_sho_w_a layer~d structure with scattered 

_grains of bastit.e.;..h.o..~av.er.,..most consist of slickensided 

lozenge-shaped fragments about 2 em to 1 or 2 m in 

averag~ameter and generally flattened in one plane. 

The resulting-foliation-is-steeply dipping but shows 

considerable variation in strike among outcrops, generqliy--llUlging between.N. 30° E. and N. 60° W. and ap-• 

pearin~aFallel-t;he..trend of the nearest wallrock 

contact. Locally, contacts are irregular in plan, but 

more commonly they are linear, or nearly so, and 

change trend abruptly, suggesting that they are steeply 

dipping, planar, and likely tectonic. However, they 

rarely are exposed for direct examination. 

The contact of one small serpentinite mass is exposed 

in a roadcut along Highway 124 (not shown on pl. 1) 

about 520 m north of the highway crossing over Dry 

Creek ,(E¥2 sec. 28, T. 7 N., R. 10 E.). Exposures there 

suggest that the contact between serpentinite and wallrock conglomerate has an antiform shape that is discordant to wallrock bedding and foliation in the axial 

region of this arched surface. Contact metamorphic effects are clearly evident in a zone a few meters wide. The 

unmetamorphosed conglomerate has a black slate matrix that surrounds relatively unrecrystallized clasts of 

slate and porphyritic to fine-grained volcanic rocks of 

intermediate to mafic composition. The metamorphosed 

conglomerate has retained all original structures and 

textures, but it is pale green, more difficult to fracture, 

and almost completely replaced by chlorite and actinolitic amphibole. Similar metamorphic effects are not evident at the contacts of other bodies of serpentinite. 

Rare inclusions of conglomerate and slate as much as 

10m in maximum dimension occur within some bodies 

of serpentinite. Original structures and textures are 

preserved in these inclusions, but the rocks are hardened and show a gray to gray-green color suggesting 

some recrystallization. Gneissic amphibolite locally occurs as both inclusions and narrow marginal bodies to 

serpentinite. 

Two occurrences of rodingite in serpentinite form 

steeply dipping dikes from 1 m to several centimeters 

thick; they have sharp contacts and generally are conformable with foliation in the host serpentinite. At one 

locality, slickensided shear planes pass from serpentinite through the rodingite dike, but elsewhere undeformed dikes are exposed for as much as 60 m along 

strike. Well-preserved relict textures in thin section 

suggest that the original rock was diabasic. 

Rare minor bodies of altered gabbro and related mafic 

rocks occur both marginally and intern.ally to 

serpentinite, and large masses of brecciated red and 

gray carbonate with or without incrustations of 

chalcedonic silica show a similar association. This 

carbonate also has small scattered clots of chlorite. 

The presence ofbastite and associated gabbroic rocks 

suggests that at least some of the serpentinite formed 

from preexisting ultramafic plutons, but evidence for 

the general origin of many bodies of serpentinite is 

lacking. If ultramafic rocks were the common precursors of serpentinite, the serpentinite cannot now be in 

the original intrusive position of the plutons because 
20 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

contact metamorphic effects that would be expected in 

sedimentary rocks intruded by ultramafic melt are not 

evident. 

The spatial association of nonfissile mudstone of 

hornfelsic aspect near some serpentinite bodies sug:-

gests the possibility that the serpentinite or its precursor rocks were emplaced as at least moderately warm 

bodies later than the regional metamorphism. Alternatively, baking of the wallrocks bordering some of the 

serpentinite. bodies could have preceded regional 

metamorphism if the resultant mechanical properties 

effectively prevented the development of slaty cleavage 

during later folding and metamorphism. Although the 

evidence in our study area is inconclusive and no 

definite assessment ofthe age of the now serpentinized 

bodies relative to metamorphism is p9ssible, Springer 

(1969) and Emerson (1969) showed that ultramafic and 

mafic plutons, including serpentinite, are at least in 

part older than regional metamorphism in western El 

Dorado County, on strike with and within 13 km of the 

melange belt in Amador County. 

SUPERJACENT ROCKS 

Shallow-dipping sediments and sedimentary rocks of 

Tertiary age rest in angular unconformity on rocks of 

the subjacent series. Although such deposits once 

formed extensive aprons that may have completely 

covered the older rocks at the lower elevations of the 

Sierra foothills, only scattered and relatively small erosional remnants of this blanket are preserved in western Amador County. Stratification in these deposits 

dips gently to the west, essentially parallel to the regional orientation of the surface of unconformity at 

their base. Detailed examination of the unconformity 

shows abundant evidence of marked local topographic 

irregularity and incision on the regional surface. 

Gale, ·Thomas, and Robinson (1939) to include welded 

rhyolite tuffs and associated sedimentary rocks exposed 

in northern Calaveras County near the Mokelumne 

River. Geochronologic age determination of correlative 

rocks higher in the Sierra Nevada by Dalrymple (1964, 

fig. 3) showed that the Valley Springs Formation probably ranges in age from 30 to 20 million years (late 

Oligocene to middle Miocene). 

A unit of Tertiary quartzitic gravel and conglomerate 

mapped on plate 1 is considered part of the Valley 

Springs Formation because it overlies at several places 

rhyolitic tuff that is unknown in the pre volcanic series 

(Slemmons, 1966, p. 201). Although we cannot demonstrate that all the quartzitic gravel is younger than the 

tuff, the close spatial distribution and lithologic uniformity justify its inclusion with the Valley Springs. 

Quartzitic conglomerate and equivalent unconsolidated gravel form most of the Valley Springs Formation. These strata are poorly bedded and contain wellrounded pebbles and cobbles of quartzite and bull 

quartz, with only very minor amounts of other types of 

clasts. Although the conglomerate is known to be as 

much as 12 m thick in the map area, only a thin veneer 

generally less than 2m thick is preserved at most locations. Many of the unconsolidated deposits have been 

extensively worked for gold by placer operations. 

At a few locations, Valley Springs conglomerate is 

underlain by massive white fine-grained tuff. Elsewhere, deeply weathered clay deposits have been found 

under the Valley Springs conglomerate, suggesting 

that tuff was once considerably more common than now 

and that it has weathered to clay. Tuff exposed at the top 

of the hill 3 km east-northeast of Plymouth is strongly 

indurated, suggesting that at least some of the tuff may 

be welded. 

MEHRTEN FORMATION 

Two units of Tertiary age have been differentiated on The name Meh.rten Formation was applied by Piper, plate 1. Both are included with rocks called the intervolcanic gravels by Lindgren (191 1) and Bateman and Gale, Thomas, and Robinson (1939) and Curtis (1951; 

Wahrhaftig (1966, p. 133). Other gravels called prevol- 1965) to andesite-clast mudflows and conglomerate at 

canic gravels (Bateman and w ahrhaftig, 1966, p. 133) widely separated points on the west slope of the Sierra 

are exposed in Amador County, but they lie entirely Nevada. Dalrymple (1964) radiometrically dated these 

west and downslope of the area covered by plate 1. The rocks at 19-5 million years. Conglomerate .and poorly 

lowermost. intervolcanic deposit consists of two consolidated gravel containing nearly monolithologic 

andesitic clasts within the map area are referred to this lithologies: quartzitic gravel, and conglomerate underlain by rhyolitic tuff; it is here assigned to the Valley formation, although they could be equivalent to the 

Springs Formation of Piper, Gale, Thomas, and Robin- Relief Peak Formation ofSlemmons (1966, p. 203). The 

Relief Peak Formation includes rocks previously asson (1939). Unconformably overlying this formation at . d h 1 f h M h F t' · · · d' I h b' s1gne to t e ower part o t e e rten orma Ion In 

many locations, but resting 1rect y on t e su ~acent h co h'll h fA d C (D 1 I 196 · 4 1 d 1 t e tOOt 1 s sout o rna or ounty a rymp e, ; rocks at most places, are volcanic grave s an cong om- Sl 1966) Th · th' k f th d · h h M h F · f emmons, . e maximum 1c ness o ese eerate correlated wit t e type e rten ormatiOn o .t . b t 60 b t bl h th · k 

Piper Gale Thomas and Robinson (1939). post s Is a. ou m, u presuma Y muc IC er ' ' ' I accumulatiOns, perhaps as much as 300m, once covered 

VALLEY SPRINGS FORMATION the area. (See Slemmons, 1966, fig. 4.) 

The Valley Springs Formation was defined by Piper, Andesitic gravels and conglomerate in the map area 
STRUCTURE 21 

are generally poorly bedded and sorted where e~posed. 

Clasts ·range in size from pebbles to very large boulders 

and are subangular to subrounded. The matrix is graycolored finer volcanic debris, presumably also of andesitic composition. The andesitic rock that forms nearly all 

the clasts contains phenocrysts of plagioclase, 

clinopyroxene, hornblende, and olivine. Although the 

clasts are very nearly monolithologic, scattered fragments of granitic and metamorphic rocks are present 

locally. STRUCTURE 

Traditional reconstruction of geologic history in the 

western Sierra foothills involves the following succession of principal events: (1) Dru>osition of strata from 

Paleozoic through at least Late Jurassic time (including 

poss16le periods of nondeposition or erosion, local 

metamorphism, and earlyfOlaing episoaes)-;-t2rlowgraaeregi0i1al me amorp ism with steep ti ting associatedWi.Tii complex maj.w.:..is.Qclinal folding and;;ccompanied by intrusion of plutonic and rela ed 

hypabyssal rocks of the Sierra Nevada batholith, and 

(3) ma]or fiijilting_e.pisodes..a!Q.ug well-defined northnorthwest-trending belts that have substantially alt.ru:.e.d-the-spatiaLclistcib.utio.n...o.Lthe..stJ:ataLunits. 

Field relations and structural data obtained in our 

detailed study of the Sierra foothills in Amador County 

suggest an appreciably different scheme of events that 

may be applicable to a much larger area, even perhaps 

to most of the foothills west of the Melones fault zone. 

These events include (1) accumulation of volcanic and 

epiclastic material on the sea floor, accompanied by 

large- and small-scale differential movement on 

numerous gently_dip.ping_faulta, (2) regional steep tiltill;g associated with small-scale very tight to isoclinal 

folding, greenschist-facies metamorphism, and intrusion by plutonic_ancLh.)tpab¥ossaLr.o,cks, and (3) largeand small-scale local faulting and minor refolding_of 

strata. 

These two solutions to the complex structural relations differ mainly in the timing and nature of the 

·large-scale movements that caused the gross redistribution of the principal rock types into their present 

patterns. This paper demonstrates that many of the 

large displacements probably occurred while the sediments were essentially flat lying and that sediments 

continued to accumulate while this disruption of strata 

was in progress. Thus the mechanism of the major dislocations is thought to have operated in a relatively shallow marine environment essentially within the basin of 

accumulation (near the continental margin?), perhaps 

resulting strictly from gravitational and subductional 

stresses rather than from compressional stresses and 

attendant shearing associated solely with the emplacement of the Sierra Nevada batholith. 

Abundant literature has been devoted to melanges in 

the Franciscan terrane of coastal California (for example, Hsu, 1968; Page, 1970) and the probable relations to 

lithospheric plates subducting at the continental margin there. In this paper, we suggest, specifically on the 

basis of newly obtained field data, the possibility that 

such events took place east of the Great Valley of 

California. in Mesozoic time. Others (-for example, 

Hamilton, 1969; Davis, 1969) attempted to explain the 

.tectonics of this. area using these processes, but such 

attempts have simply interpreted grand-scale rock distribution on the basis ofthe geological literature of this 

region. While we agree with this general type of interpretation, we believe that data published from previous field studies in the Sierra foothills are not adequate to support such interpretations. 

For many geologists melange means literally a tectonic mixture of rocks, without connotations of genesis. 

For others, however, it carries a definite implication of 

how the mixture originated. We propose a specifically 

subduction related genesis for the melange of this report, but regardless of whether our proposal is correct or 

not, the existence of the melange is demonstrated by the 

outcrop mapping and must be accounted for in any comprehensive explanation of the tectonic history of the 

foothills. 

In general, the Sierra foothills melange is indeed a 

chaotic tectonic mixture of rocks, but a limited degree of 

internal order is apparent from the geologic map (pl. 1) 

and gives some clues to the extent of mixing that the 

originally undisturbed rocks have undergone. To some 

geologists, the degree of order shown by the map may 

not justify using the term melange; we believe that this 

viewpoint is incorrect, for if we have erred in accurately 

portraying structure within the melange, the map suggests a lesser amount of tectonic chaos than is actually 

present. 

Despite the apparent simplicity of some of the map 

units in plate 1, as well as the relatively undisturbed 

bedding visible in many outcrops within such map 

units, exposures in roadcuts and along stream courses 

reveal a marked degree ·of internal complexity in most 

units. In fact, these exposures suggest that many, and 

perhaps most, of the natural outcrops between roadcuts 

and streambeds are individual uclasts" in a great 

megabreccia. Their unfragmented state makes them 

relatively resistant to erosion, whereas the sheared 

matrix surrounding them is easily eroded. Thus, it is 

important to recognize that simplicity and continuity as 

recognized on older geologic maps may in many places 

mask faulting and shearing that are nearly pervasive 

between outcrops but cannot be proven generally because of incomplete exposure. To interpret the geology 

of the melange strictly on the basis of natural outcrops 
22 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

could lead to important misconceptions because the key 

evidence is largely concealed between the exposures. 

MELANGE BELT 

Inspection of plate 1 reveals that individual map 

units possess both extreme complexity and a limited 

degree of order and continuity on a local basis within 

the melange. By mapping essentially all outcrops, we 

have determined that most of the map units probably 

have abrupt, rather than gradational, contacts with 

one another and that in many places the possible locations of the contacts essentially rule out sedimentary 

intertonguing as a common relationship. It is important 

to reemphasize here that most contacts have not been 

observed directly but rather are inferred from the distribution of outcrops. Almost all those few contacts that 

have been observed are exposed in stream gullies and in 

relatively new roadcuts along Highways·'!~ and 124 

(the latter is not shown on pl. 1). 

Certain associations of rock types have been critical 

in establishing some of the map units within the 

melange. Some lithologic assemblages contain rock 

types that are not found in other assemblages. Still 

other assemblages that consist chiefly of widely distributed lithologies are themselves distinctive because of 

the absence of certain rock types that are relatively 

resistant to erosion and hence should be exposed if present. Although the definitions of several of the units in 

the map explanation might imply virtually complete 

overlap, we think that each of the divisions is real and 

justifiable. Such divisions demonstrate the existence 

and the general geometry of each assemblage in the 

melange. To explain the distribution of lithologies and 

the sheared nature of many outcrops by lateral facies 

changes or folding does not seem tenable to us. 

The exact geographic extent of pervasive shearing in 

rocks of the melange is difficult to establish, although 

all indications suggest that it is widespread and general. Relatively few sheared outcrops are known in the 

western part of the melange, but they are quite common 

to the east, where good examples can be found on the 

banks of Amador Creek (sec. 34, T. 7 N., R. 10 E.), in the 

banks of the Cosumnes River (sees. 20, 21, 28, T. 8 N., R. 

10 E.), and in roadcuts along Highways 16 and 124 (sec. 

21, T. 7 N., R. 10 E.). Elsewhere, we believe, exposures of 

sheared rock are less common simply because such 

rocks are not resistant enough to form outcrops. 

The pervasive shearing is characterized in pelitic 

material by countless steeply dipping subparallel shear 

surfaces along which considerable displacement appears to have occurred. The trend of shear surfaces 

generally is constant at an outcrop but ranges from 

about N. 30° E. toN. 60° W. over the entire melange. 

This style of deformation is best developed in quartzose 

slate with associated chert or quartzose sandstone. 

Chert and sandstone typically form subrounded fragments or pods-remnants of once continuous beds, now 

floating in a mass of thoroughly sheared pelitic 

matrix-and these fragmented lithologies apparently 

remained relatively competent as the pelitic material 

flowed plastically to fill spaces left by disruptions in 

bedding. 

Several broad generalizations may be made concerning the distribution of rock types and distinctive associations of rocks within the melange. South of the latitude 

of Drytown and along the west margin of the melange 

belt, irregularly sized and shaped blocks of Calaveras 

affinity, Cosumnes affinity, serpentinite, and intrusive 

rocks of various kinds are intermixed in a zone 1 km 

wide. (See pl. 1.) North of the latitude of Drytown, however, this zone appears to be missing; its absence is 

perhaps partly reflected by the shorter distance between the western serpentinite-belt and the base of the 

Logtown Ridge Formation. East of this zone and south 

of the latitude of Drytown, another distinctive band 

consisting chiefly of conglomerate of Cosumnes affinity 

is intermixed with a small 'proportion of rocks of 

Calaveras affinity and a few small serpentinite bodies. 

The continuity of this band is broken and displaced at 

the latitude of Highway 16, but a similar band of rocks 

continues northward, in contact with the massive block 

of serpentinite, to the Cosumnes River. Both north and 

south of Drytown, this zone gives way eastward to a 

regionally distinctive map unit consisting of thinly and 

rhythmically interbedded graywacke and clay slate of 

Cosumnes affinity. These strata apparently rest in depositional contact on the underlying conglomerate, although the geometry of the contact is quite probably 

complicated by cross-faulting at low angles to the strike 

of the unit. Very locally, thin slices of silicic rocks of 

Calaveras affinity lie. between the two bands, such as in 

sec. 3, T. 6 N., R. 10 E., and in sec. 5, T. 7 N., R. 10 E., 

suggesting that further tectonic complication is possible along the contact. Graded bedding in the graywacke 

layers is prevalent throughout the unit and almost uniformly shows the top of the section to be eastward; 

where the top directions are not eastward, small-scale 

folding is evident. 

Another distinctive zone of rocks south of the latitude 

of Drytown lies in essentially continuous fault contact 

with the graywacke-slate band on the west and consists 

principally of rocks of Calaveras affinity that give way 

to rocks of Cosumnes affinity near the base of the Logtown Ridge Formation. Rocks with similar characteristics also extend northward from the latitude of 

Drytown, although the zone there has a considerably 

greater east-west dimension. This zone is further 

characterized, both north and south of Drytown, by 

many irregularly shaped small to relatively large 
STRUCTURE 23 

bodies of hypabyssal igneous rocks that are lacking in 

the more western zones. 

Contacts between and often within these zones of the 

melange are tectonic. None of t_hese faults, however, 

transect the contact between the melange and the overlying Logtown Ridge Formation, suggesting that intermixing within the melange predates the Logtown 

Ridge Formation. It might be argued that intermixing 

occurred after eruption and deposition of the Logtown 

Ridge Formation but did not affect them because of. 

their greater strength or because they were effectively 

separated from underlying rocks by the fault at the base 

of the Logtown Ridge. In view of the nature of faulting 

within the Logtown Ridge, however, it seems more 

likely to us that the melange formed either before the 

Logtown Ridge was deposited (before Callovian time) or 

at a somewhat later time but before the melange and 

the Logtown Ridge were juxtaposed. 

If this latter explanation is correct, then the two units 

must have been subsequently juxtaposed from very different geologic environments. The two are indeed in 

fault contact, but since both show greenschist-facies 

metamorphism, only huge lateral tectonic dislocation 

would permit this explanation; if one unit had been 

buried much deeper than the other, grade of 

metamorphism should reflect the difference. Large lateral motion would fit our scheme if faulting were along 

subhorizontal surfaces, but we find no evidence for huge 

lateral displacement on steeply dipping faults, as the 

traditional interpretation would require. Large displacement along a subhorizontal thrust fault (subduction surface?), however, is a distinct possibility. 

BEAR MOUNTAINS FAULT ZONE 

The west boundary of the melangdelt_c.oo:r:esponds to 

part of the Bear Mountains fault zone of Clark (1960, 

Hf64). Clark inferred the presence of this fault zone 

throughourtfie Sierra foothills from the gross strati-

~phic relations and distribution pattems..oLM.e.s.oWc 

and presumably Paleozoic rocks, the localizati o er-

_Eentinite in a relatively narrow linear band, and the 

~Currence of sheared rocks at apn.ropriate locations.ln 

particular, the homoclinal sandwiching of what Clark 

and others before him believed to be a belt of Paleozoic 

rocks between Mesozoic rocks required major faulting 

to account for the repetition of Mesozoic strata; thus 

thousands .of meters of reverse-slip displacement on a 

group of steeply dipping faults, the Bear Mountains 

fault zone, was inferred. 

The new evidence obtained in the present study, however, allows a very different interpretation of the field 

relations. In fact, with recognition of the melange, no 

evidence now prohibits the accumulation and intermixing of all rocks of the melange C'western belt of 

Calaveras" of earlier workers) after the accumulation of 

the underlying Mesozoic Copper Hill Volcanics, and 

thus no compelling evidence remains for a reversal of 

stratigraphic succession across the Bear Mountains 

fault zone in Amador County. 

Regardless of the presence or absence of this stratigraphic reversal, however, shearing and fault displacement occur at the west edge of the melange belt. The 

serpentinite bodies that are localized there provide 

much evidence of faulting. But there is no evidence of 

net fault displacement across a serpentinite mass. If the 

serpentinite bodies and their associated deep-seated 

igneous rocks were solidified before they intruded overlying rocks, the abundance of shearing within and along 

their margins is explained. The driving force for such a 

process need have been no more than gravitational instability of low-density serpentinite, but other forces, 

perhaps reflecting horizontal compressive stress, could 

have been involved as well. The boundaries of the serpentinite masses may have been controlled by preexisting faults, as Clark (1964, p. 42) argued, but these faults 

may have formed during melange development, shortly 

after sedimentary accumulation rather than later, as 

initially steeply dipping urooted" faults. 

In the. sense that a fault should have a demonstrable 

net displacement across it, we conclude that the existence of the Bear Mountains fault zone cannot be ascertained at present. The dissimilarity of stratigraphic 

units on opposing sides of the zone makes it difficult to 

prove net displacement. Nevertheless, the possibility 

that such displacement has occurred and that serpentinite masses subsequently have invaded these preexisting shear zones, as suggested by Clark (1964), cannot be discounted. Ultimately, the distinction between 

''conventional" faulting as suggested by Clark and 

melange movements proposed by us probably must be 

based on whether the fault surfaces were rooted within 

the crust or were essentially parallel to the earth's surface at the time of their activity. We have·already offered some evidence favoring the second interpretation; 

additional support is provided in following sections. 

Even though the existence of large net displacement 

across the Bear Mountains fault zone is now questionable, an important reversal of stratigraphic succession 

apparently does exist in the foothills of Amador County 

between the Salt Spring Slate of Clark (1964), west of 

our map area, and the Mariposa Formation. Both of 

these rock units are late Oxfordian and early Kimmeridgian in age (Clark, 1964, p. 25, 29), and the somewhat older Logtown Ridge Formation lies between 

them. All the rocks dip steeply to the east in an upright 

position. This stratigraphic reversal and repetition of 

time units may have resulted from displacement on the 

fault at the base of the Logtown Ridge Formation, on the 
24 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

Bear Mountains fault zone, or on presently unknown 

faults west of the map area. 

EVIDENCE FOR FAULTING OF LOGTOWN RIDGE 

STRATA IN PRE-MARIPOSA TIME 

Near Drytown, all the distinctive members in the 

Logtown Ridge Formation are displaced abruptly along 

a linear east-southeast trend (pl. 1). Because four members of the formation are involved, it is unlikely that 

this offset resulted from primary differences in stratigraphic thickness. A fold in the strata seems unlikely 

because contacts between members are nearly linear up 

to the discontinuity. A fault offset seems to be the only 

reasonable interpretation. Unfortunately, no direct 

evidence of faulting has been observed because of poor 

exposures, but the offsets of the easily mapped units are 

clear and, in our opinion, constitute sufficient evidence 

for the existence of the fault. -·~- . The top of the Logtown Ridge Formation north of this 

transecting fault is downdropped as much as 250 m. 

Distinctive quartzitic conglomerate at the base of the 

Mariposa Formation abuts this positive topographic irregularity on the top of the Logtown Ridge, but higher 

Mariposa beds are essentially flat and continuous 

across the irregularity. The continuity of these higher 

beds indicates that the fault does not extend from the 

Logtown Ridge into the Mariposa Formation. The top of 

the Logtown Ridge Formation most probably is not a 

fault surface, in view of its irregularity and direct evidence from a single exposure where Highway 49 crosses 

the.Cosumnes River; thus the fault transecting the Logtown Ridge terminates at the top of the section and must 

be pre-Mariposa in age. A similar though less well 

documented pre-Mariposa fault in the Logtown Ridge is 

located about 3 km north of Drytown (pl. 1). 

The age of faulting along the base of the Logtown 

Ridge Formation is difficult to determine. All field data 

indicate that the fault crossing westward through 

Drytown terminates at the base of the formation, suggesting that the basal fault is relatively younger. However, the cross-fault and the basal fault may have been 

simultaneously active, with the cross-fault splaying 

from the principal plane of dislocation as a minor finger. 

Some support for coeval activity of this sort is seen in 

field relations at and near the base of the Logtown Ridge 

Formation west and southwest of Jackson. (See pl. 1.) 

For example, about 3 km west of Jackson, the base of the 

Logtown Ridge Formation is displaced about 550 m by a 

northeast-trending cross-fault. The distribution of 

nearby outcrops in the melange belt suggests that the 

cross-fault does not extend west of the basal fault, and 

the undisturbed contact between the Rabbit Flat and 

Goat Hill Members of the Logtown Ridge Formation 

indicates that the fault does not extend far on trend to 

the northeast. Several reconstructions of the time relations involved with this faulting are possible, but considering the characteristics of faulting in the Logtown 

Ridge Formation as a whole, we believe that this tectonic dislocation occurred simultaneously on the basal 

fault and cross-fault in pre-Goat Hill time. 

Other fault relations show that soine movement at 

the base of the Logtown Ridge Formation probably 

postdated the Mariposa Formation. At the latitude of 

Plymouth, a northeast-trending fault transects the 

Logtown Ridge Formation and continues through the 

Mariposa Formation before converging with the 

Melones fault zone. On the southwest this structure 

apparently terminates at the base of the Logtown 

Ridge, as suggested by the distribution of rock types 

nearby in the melange. (See pl. 1.) This implies that at 

least part of the fault at the base ofthe Logtown Ridge 

was active either coevally with deposition of the 

Mariposa Formation or at a later time. 

Episodes of faulting involving the Logtown Ridge 

Formation apparently began while Logtown Ridge deposition was underway and continued locally during 

and after accumulation of the Mariposa sediments. Because the initial slopes on which deposition occurred 

probably were relatively flat, the faults that are now 

essentially vertical must have been much less steeply 

dipping when they were active. 

TILTING AND FOLDING 

After the formation of the melange and the Logtown 

Ridge and Mariposa faulting, all the rocks were tilted to 

their present steeply dipping positions, a process that 

was probably contemporaneous with late Mesozoic 

magmatism ofthe Sierra Nevada province. As the rocks 

we:r:e tilted, some were very tightly to isoclinally folded, 

and a slaty cleavage developed, which is locally axial 

planar and pervades rocks of the entire foothills area. 

Detailed statistical studies of these and associated 

structures elsewhere in the foothills (for example, see 

Best, 1963; Baird, 1962) indicate multiple episodes of 

folding, and a more recent analysis of regional deformation extends the multiple episodes to the entire Sierra 

Nevada province (Kistler and others, 1971). Structural 

data from our map area generally suggest only two 

widespread periods of deformation-subhorizontal dislocations and later tilting and folding-although the 

subhorizontal displacements continued for a long period 

of time, judging from the complex relations within the 

melange belt and Logtown Ridge and Mariposa Formations. 

Metamorphic rock cleavage that parallels the dominant structural grain of the foothills is evident at nearly 

all outcrops, and its general orientation is north trending (pl. 1). Paired measurements on bedding and cleavage and on axes of folds are restricted to a few sequences 

of interbedded slate and graywacke in the melange belt 
STRUCTURE 25 

and to Mariposa slate and graywacke; elsewhere, either 

most rocks are not visibly bedded, or original 

stratification has been thoroughly disrupted. The 

paired measurements summarized in figures 4, 5, and 6 

show the essential parallelism of bedding and cleavage . 

. Contoured stereographic projections of poles to bedding 

reflect the very tight to isoclinal nature of known folds, 

whereas measurements of fold axes suggest one episode 

ofnoncylindroidal folding. Clearly, the data are too few 

to develop strong statistical arguments against multiple episodes of folding, but the simplest explanation of 

available data is that axial plane cleavage and isoclinal 

folds formed simultaneously; the plunge of resulting 

fold axes varies greatly along a uniform strike. Such 

variation might have resulted from locally anomalous 

N N 

A B 

FIGURE 4.-Summary of bedding, regional cleavage, and axes of 

minor folds for slate and graywacke of the Mariposa Formation 

between Plymouth and Amador City. Projection on the lower 

hemisphere of an equal-area net. A, One hundred sixty-six poles to 

bedding contoured at 0.6, 3.0, 5.4, and 7.8 percent per 1 percent total 

area. Fold axes are marked by x. B, Fifty-seven poles to cleavage 

contoured at 1.8, 5.3, and 19.3 percent per 1 percent total area. 

N N 

A B 

FIGURE 5.-Surnmary of bedding, regional cleavage, and axes of 

minor folds for slate and graywacke of the melange belt south of 

Highway 16. A, Fifty-five poles to bedding contoured at 1.8 and 12.7 

percent per 1 percent total area. Fold axes are marked by x. B,, 

Fifty-seven poles to cleavage contoured at 1.8, 12.3, and 29.8 percent per 1 percent total area. 

N N 

A !J 

FIGURE 6.-Surnmary of bedding, regional cleavage, and axis of a 

minor fold for slate and graywacke of the melange belt north of 

Highway 16. A, Sixty-two poles to bedding contoured at 1.6, 4.8, 

14.5, and 21.0 percent per 1 percent total area. Fold axis is marked 

by x. B, Sixty-five poles to cleavage contoured at 1.5, 4.6, 13.8, and 

35.4 percent per 1 percent total area. 

fluid pore pressure_ and the uneven vertical or horizontal strain of material during dewatering of what probably was predominantly a sequence of water-saturated 

sedimentary rocks. 

A word of caution is offered here for structural studies 

of cleaved slate and graywacke sequences elsewhere in 

the Sierra foothills. Even though cleavage generally I 

parallels axial planes of known folds, and, statistically, 

zones of cleavage-bedding intersections are parallel to 

zones of fold axes, the relation of bedding to cleavage at 

an outcrop is not a dependable indication of stratigraphic top. This is evident from our study at numerous 

outcrops where stratigraphic top was determined independently from multiple graded beds. A similar situation probably exists throughout the foothills. 

DEFORMATION AFTER REGIONAL TILTING 

The map area contains evidence for both local and 

more widespread deformation subsequent to the episode 

of tilting and folding. Fracture cleavage in phyllite was 

described i:q. the section uEastern Belt." The areal extent of this cleavage is unknown, but it appears to be 

restricted to rocks east of the Melones fault zone in the 

map area. Other posttilting structures are better 

documented. 

MELONES FAULT ZONE 

Clark (1960, p. 493-494; 1964, pl. 1) described the 

Melones fault zone as a major Late Jurassic zone of 

dislocation that juxtaposes Paleozoic rocks upon 

Mesozoic ones and truncates major folds in rocks of both 

ages. Rocks within the zone of faulting are also generally characterized by relatively great shearing. 

Lacking any substantial new evidence to the contrary, we follow Clark's suggestion that the Melones 

fault formed in Late Jurassic time after the rocks of the 

foothills had been tilted to their presently steep at-
26 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 

titudes. Future studies, however, may indicate ~ subhorizontal origin, similar to that proposed by us for the 

faulting in the melange and in the Logtown Ridge and 

Mariposa Formations. Davis (1969) has already proposed such an origin with broad correlations between 

major structures and map units in the Klamath Mountains and Sierra Nevada. 

In Amador County, the Melones fault zone ranges 

from about 250 to 800 m in outcrop width and generally 

separates the Mariposa Formation on the west from a 

granodiorite pluton and Calaveras Formation on the 

east. Most rocks within the fault zone are pervasively 

sheared and recrystallized phyllitic greenstone of unknown age. The phyllitic fabric and the fault zone itself 

are parallel. Minor amounts of other lithologies are also 

contained within the fault zone, especially in the southeastern part of plate 1; these are described elsewhere in 

the text. 

Contacts of the fault zone with adjacent rocks generally are gradational over a width on the order of tens of 

meters. However, the eastern contact, along the 

granodioritic pluton of the eastern belt, is difficult to 

determine because cataclastic texture of the pluton is 

structurally conformable to phyllitic greenstone of the 

fault zone there. Moreover, these cataclastic structures 

closely resemble shear structures in rocks of the fault 

~one, suggesting that the age of the pluton is prefaultIng and that marginal cataclasis is due to Melones faulting. This interpretation is consistent with radiometr{c 

and fossil-derived ages. Near Indian Gulch east of 

Amador City, however, the granodiorite transects the 

boundary between Calaveras rocks of the eastern belt 

and the phyllonitic greenstone belt, indicating that the 

intrusion is younger than the juxtaposition of 

Calaveras and the phyllonitic greenstone. If that juxtaposition resulted from movement on the Melones 

fault zone, then the marginal cataclasis in the pluton 

must postdate at least some of this movement. However 

it is conceivable that this Calaveras-phyllonitic green~ stone contact is essentially an original irregular strati-' 

graphic boundary and that subsequent shearing has occurred but has not substantially altered the relative 

positi?ns of the two rock types. If this were true, then 

sheanng of all the rocks, including the granodiorite, 

could be coeval with or younger than the emplacement 

of the pluton. We arbitrarily chose to exclude the margin of the pluton from the fault zone. 

The direction and amount of slip on the Melones fault 

zone are not well known. Clark (1964, p. 51) concluded 

that a vertical component of900-4,500 m is indicated by 

the juxtaposition of Paleozoic strata upon Mesozoic 

ro.ck~; he also believed that steeply plunging lineations 

within the fault zone owe their origin to a major component of strike-slip displacement. Similarly, our work 

suggests vertical displacement on the order of hundreds 

and possibly thousands of meters, but no substantive 

support for strike-slip movement was discovered. 

MOTHER LODE FAULT SYSTEM 

The term Mother Lode fault system is used here for 

the eastward-dipping faults that are locally occupied by 

gold-bearing quartz veins of the Sierra foothills. This 

system of faults is minor in terms of offset but is of 

considerable importance for the great wealth that came 

from its ore bodies. Little new information on the nature 

of this system of faults was found during this study. 

Therefore, most of the pertinent information comes 

from previously published reports, primarily from the 

study within mine shafts by Knopf (1929). 

The Mother Lode fault system traverses the Sierra 

foothills in a north-northwest direction for at least 190 

km, from Mariposa to Georgetown (Knopf, 1929, p. 2, 

fig. 1). For most of its length, the Mother Lode fault 

system is in or very near the Melones fault zone (Clark, 

1964, p. 52). However, locally the two structures diverge, and there is evidence that formation .of the 

Mother Lode veins postdates movements on the 

Melones fault zone. (See Knopf, 1929, p. 24-25.) Movement on faults of the Mother Lode system apparently 

wa~ neither simple nor great, but Knopf (1929, p. 5) 

beheved that offset was primarily in a reverse sense 

and he cited a maximum displacement of 115m alon~ dip. 

In the southern half of the area of plate 1, most of the 

Mother Lode faults lie about 1.6 km west of the Melones 

fault zone, but from about 1.6 km south of Plymouth to 

the north boundary of the map, the two structures overlap extensively. Surface evidence of the Mother Lode 

fa~lts includes scattered massive veins-ofmilky quartz, 

th~n. bands of altered wallrocks, and folded cleavage 

originally formed during regional tilting. The veins 

trend roughly north-northwest and dip steeply to the 

east, and the chief evidence that deformation accompanied their formation is seen locally where nearby 

slaty cleavage has been folded and relatively thorough 

shearing is apparent in massive greenstones. Such effects rarely extend more than several meters away from 

known veins. 

In the southern part of the area, most of the major 

veins are located at or very near the contact between 

slate and greenstone of the Mariposa Formation. However, Knopf (1929, p. 54, 55, 57, 58) showed that these 

veins dip less steeply to the east than the lithologic 

contact, and so at depth they are enclosed entirely 

within greenstone. 

REGIONAL CRUSTAL FLEXURE 

The map area includes the axial zone of a large-scale 

flexure of possibly fundamental significance to under-
REGIONAL IMPLICATIONS 27 

standing late Jurassic or post-Jurassic crustal movements for the Western United States. This flexure is 

evident on geologic maps ofthe Sierra Nevada province 

as a change in trend of pre batholithic rock systems and 

the batholithic axis itself. South of the general latitude 

of Highway 16, the trend is approximately northnorthwest, whereas to the north the trend is more 

nearly due north (pl. 1; also, compare figs. 5, 6). Since 

the mapped belts of rocks, bedding and regional cleavage, and the Melones fault zone are all affected, the 

flexure can be no older than Late Jurassic. Its origin is 

unknown. 

STRUCTURAL CROSS SECTIONS 

Throughout this paper, many references have been 

made to the work of previous investigators, especially 

the mapping published in the late 19th century as U.S. 

Geological Survey folios, the subsequent changes in 

stratigraphy and structure by N. L. Taliaferro, and 

finally the revisions offered by L. C. Clark. These principal investigators shaped the present understanding of 

pre-Cenozoic foothills geology, and fortunately, each of 

these workers summarized his ideas of the fundamental 

nature of the geology in Amador County with a cross 

section drawn along the Cosumnes River, the north 

boundary of plate 1. 

For comparison, each of the earlier cross sections together with our interpretation are reproduced in figure 

7. Although greatly generalized, these diagrams summarize the evolution of thought pertaining to the geology of the central foothills region. They do not indicate 

the importance of subhorizontal tectonic dislocations 

inferred from the present study; this point has been 

emphasized adequately in earlier parts of the text. The 

succession of such divergent interpretations is graphic 

evidence of the extreme complexity of Sierra Foothills 

geology. 

REGIONAL IMPLICATIONS 

Our explanation of the tectonic history of the western 

foothills of Amador County closely resembles that of 

Davis (1969), but our conclusions are drawn from a very 

detailed study of a small area, while his interpret the 

large-scale distribution of rocks in both the Klamath 

Mountains and western Sierra Nevada, before the 

Sierra foothills melange was known. 

In developing the iqea that the melange formed and 

subsequent Logtown Ridge-Mariposa faulting occurred 

during active subduction at a continental margin before 

the strata were tilted to their present steeply dipping 

attitudes, we have tried to avoid overinterpreting the 

new data. However, some degree of overinterpretation 

is necessary, for neither our reconstruction of structural 

history nor the more traditional synthesis can be proved 

from the presently available data. We believe, 

nevertheless, that the usubhorizontal" explanation is at 

least as reasonable as the traditional one, and implications of both must be entertained equally. Ultimately, 

the choice between this and the traditional type of 

synthesis can probably be made only after many more 

data are available. Recent advances in understanding 

motions of the earth's lithospheric plates and processes 

at plate boundaries as embodied in the rapidly expanding literature of global plate tectonics provide some 

indirect support for the foothills structural history we 

propose. The general foothills setting-belts of greatly 

deformed rocks that face parallel belts of time-related 

igneous rocks-is appropriate for the generally agreed 

upon features and related processes of subduction zones, 

and the melange is just the sort of structural chaos that 

might be expected to form at or near the surface of such 

a zone. This line of reasoning is, of course, largely circular but is at least partly satisfying in that it relates 

observations to known processes of tectonism and 

mountain building. 

The first modern comprehensive synthesis of the 

geologic history and origin of rocks of the Sierra Nevada 

province was attempted by Bateman and his coworkers 

(Bateman and others, 1963; Bateman and Eaton, 1967). 

These investigators believed that Paleozoic and 

Mesozoic prebatholithic rocks collected in a subsiding 

trench, a ·complex· synclinorium, whose root eventually 

melted in part to form magma; the relatively less dense 

magma subsequently pierced the overlying strata, 

forming a batholithic spine along the major axis of the 

trench. This is essentially a specific application of the 

traditional tectogene model of Griggs (1939). 

The recent popularity of global plate tectonics, however, has given rise to several new and entirely different 

interpretations for the origin of structures and plutons 

in the Sierran province (for example, Davis, 1969; 

Hamilton, 1969; Dickinson, 1970; Kistler and others, 

1971; Shaw and oth~rs, 1971). In general, these new 

syntheses relate known data to the presumed dynamics 

at plate boundaries with a tantalizing degree of success. 

In our opinion, neither type of genetic model can be 

proved or disproved with the present state ofknowledge 

of rocks of the province. We hope that this paper demonstrates the need for many more detailed field observations before any well-founded choice between the two 

·models (or a compromise or a completely new model) can 

reasonably be made. We have now shown, for example, 

that fundamental stratigraphy and structure of the 

least deformed subjacent rocks of the foothills are not 

yet accurately established, which encourages the suspicion that even larger errors have been made in the 

presently depicted geology of the more deformed units. 

If additional detailed mapping demonstrates the persistence of the melange belt throughout the foothills, 
28 

~ .ii 

Ill 

.Q 

~ ...J 

I 

WSW 

Caiaveras Formation 

(Carboniferous) 

D Calaveras Formation, 

undifferentiated 

(Paleozoic) 

SIERRA FOOTHILLS MELANGE,-AMADOR COUNTY, CALIFORNIA 

0 

l~{i:1ii1:i:ll Mariposa Slate 

(JurassicCretaceous) 

0 

Basal conglomerates 

and sandstones 

A 

APPROXIMATE SCALE 

Amphibolite 

Derived from diabase, 

gabbro, and otller mafic rocks 

Middle and Upper Jurassic 

Amador Group 

Cosumnes Formation 

Well-bedded tuffs, cllerts, 

slates, sandstone, and 

black limestone 

Logtown Ridge 

Formation 

CIJiefly augite andesite, 

agglomerate, [lnws. 

and tuffs 

5 KM 

Age of Mariposa Slate and older 

Serpentine 

5KM 

Mariposa Formation 

Slates, sandstone, and. 

conglomerates 

[illillJ 

Diabase 

Serpentine and gabbro 

(Upper Jurassic) 

FIGURE 7.-See facing page caption for explanation. 

Granodiorite 

Granodiorite 

(Upper Jurassic) 
REGIONAL IMPLICATIONS . 29 

WESTERN 

FAUL.T 

Bl.OCK CENTRAL. FAUL. T Bl.OCK AND FAUL. T ZONES 

Eplclaulc 

rock a 

Calaveras 

Formation 

Cosumnes 

Formation 

l.ogtown Ridge Eplclastlc Volcanic 

Formation rocks rocks 

0 3 5KM 

Amador Gro~p APPROXIMATE SCAL.E' 

Calaveras 

formation 

(Permian) 

Cosumnes Formation 

(Upper Jurassic) 

Logtown Ridge 

Formation 

(Upper Jurassic) 

Epiclastic 

~ 

~ 

rocks 

of uncertain 

stratigraphic 

position (Upper 

Jurassic) 

Volcanic rocks 

of uncertain 

stratigraphic position 

(Upper Jurassic) 

Greenschist Ultramafic rocks 

(Upper Jurassic) 

Plutonic rocks 

(Jurassic and 

Cretaceous) 

WESTERN l.OGTOWN RIDGE- EASTERN 

~ 01 BE;l.T 

-;:g~ 

MI;LANG.E BELT MOTHER l.ODE BELT BELT 

..Jill 

r-----------------~~--~------------------------~----------~-- ~ 

.I 

w~w 

BEAR MOUNTAINS 

FAUL.T ZONE 

Not moppod 

D 

MELONES 

FAULT 

ZONE 

,..,.._, ENE 

Sedimentary and volcanic rocks Logtown Ridge Formation 

0 4 5KM 

APPROXIMATE SCALE 

Amphibolite ·serpentinite 

(Jurassic) 

Sedimentary and 

volcanic rocks 

Logtown Ridge 

Formation 

(Upper Jurassic) 

Mariposa Formation 

(Upper Jurassic) 

Greenschist Granodiorite 

(Jurassic) 

FIGURE 7.-Four cross sections illustrating the evolution of interpretations of geologic structures in pre-Cenozoic rocks along the 

Cosumnes River. Sources of information: A, Lindgren and Turner 

(1894); B, Taliaferro (1943, fig. 2); C, Clark (1964, pl. 8~D, Present 

study. The lithologic symbols do not correspond from one section 

to another, but each author generally uses them to depict the 

orientation of bedding. Light lines represent normal stratithis will constitute important new data for selecting the 

most probable among various models of rock deformation, mountain. building, and plutonism for the Sierra 

Nevada province. The melange may well mark the locus 

of convergence between an oceanic plate that abuts and 

plunges beneath a continental margin, but it only suggests this conclusion and must be weighed together 

with all other data to select the most probable solution . 

graphic contacts, and heavy lines represent faults. Light broken sinuous lines in sections C and D depict shearing of certain zones, and 

unbroken sinuous lines in A, C, and D depict folding in amphibolite 

and shearing in the melange (D only). With the exception of the newly 

discovered melange, the section from the present study is most nearly 

like section A, which was the first to be published. 

Considered alone, however, the melange does require 

that those who adhere to the ideas of a subsiding trough 

and anatexis explain the apparent absence of similarly 

deformed rocks on the east limb of the trough. The 

presence of only a western melange gives a pronounced 

asymmetry to the trough that is not easily explained by 

a simple subsiding basin in which there is sediment 

. accumulation. 
30 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA. 

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