Correlation of Late Cenozoic Tuffs in the 

Central Coast Ranges of California 

by Means of Trace- and 

Minor-Element Chemistry 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 972 

Correlation of Late Cenozoic Tuffs in the 

Central Coast Ranges of California 

by Means of Trace- and 

Minor-Element Chemistry 

By Andrei M. Sarna-Wojcicki 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 972 

A geochemical approach to the correlation of 

tuffs in Pliocene and Pleistocene marine and 

nonmarine strata of central California 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1976 
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Sf'af)lat}' 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Sarna-Wojcicki, Andrei M. 

Correlation of late Cenozoic tuffs in the central coast ranges of California by means of trace- and minor-element chemistry. 

(Geological Survey Professional Paper 972) 

Bibliography: p. 29-30. 

Supt. of Docs. no. I 19.16:972 

1. Volcanic ash, tuff, etc. California-Coast Range. 2. Stratigraphic correlation-California-Coast Range. 3. GeochemistryCalifornia-Coast Range. 4. Geology, Stratigraphic-Cenozoic. I. Title: Correlation of late Cenozoic tuffs in the central 

coast ranges of California ... II. Series: United States Geological Survey Professional Paper 972. 

QE461.S324 552'.2 76-608254 

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

Washington, D.C. 20402 

Stock Number 024-oot-o2880-8 
CONTENTS 

Page 

1\bstract---------------------------------------------------- 1 

Introduction and scope of study------------------------------ 1 

i\cknowledgments _ _ ___ _ _ _ _ ___ _ _ _ _ _ _ __ _ _ _ _ _ _ _ ___ _ _ ____ _ _ 1 

Geolo.gic setting _ _ ______ ____ ____ ______ _________ _ _ _ _____ _ __ __ 3 

Sonoma volcanic field ---------------------------------- 3 

Clear Lake volcanic field -------------------------------- 4 

Southern Cascade Range volcanic field __________________ 4 

i\nalytical methods __ ____ _ _ ______ __ _______ _ _______ _ _____ _ __ 4 

Refractive indices of glass ------------------------------ 4 

Mafic-mineral analysis---------------------------------- 4 

X-ray fluorescence spectrometric analysis ________________ 4 

Methods of evaluating chemical data ________________________ 7 

Graphic methods________________________________________ 7 

Similarity coefficient------------------------------------ 8 

Cluster analysis ---------------------------------------- 8 

Results of analyses__________________________________________ 9 

Tuff correlations-------------------------------------------- 14 

Tuff in the Merced(?) Formation of Sonoma County ________ 15 

PinoleTuff -------------------------------------------- 16 

Lawlor Tuff-------------------------------------------- 16 

Putah and Nomlaki Tuff Members of the Tehama Formation 21 

Tuff in the type section of the Merced Formation __________ 23 

Page 

Discussion __________________________________________________ 24 

Description of units ---------------------------------------- 26 

Thick sections of volcanic deposits ______________________ 26 

Sonoma Volcanic, Monticello Road section, southeastern 

part of Sonoma volcanic field, east of Napa ________ 26 

Sonoma Volcanics, southernmost Sonoma volcanic field 

north of Suisun Bay (Goodyear Station section) ____ 26 

Pinole Tuff ---------------------------------------- 26 

Widespread tuffs interbedded with detrital sedimentary deposits __________________________________________ 27 

Western part of the main study area ---------------- 27 

Tuff in the type section of the Merced Formation __ 27 

Tuff in the Merced(?) and Petaluma Formations of 

SonomaCounty ------------------------------ 27 

Southeastern part of the main study area ------------ 27 

Lawlor Tuff ------------------------------------ 28 

South of Mount Diablo -------------------------- 28 

Northeastern part of the study area------------------ 29 

Putah Tuff Member of the Tehama Formation ____ 29 

Nomlaki Tuff Member of the Tehama Formation-- 29 

References cited _______________________________________ ----- 29 

ILLUSTRATIONS 

Page 

FIGURE 1. Generalized geologic map showing location of samples, central Coast Ranges ---------------------------------------- 2 

2. Stratigraphic section exposed along Monticello Road, southeastern part of Sonoma volcanic field ---------------------- 4 

3. Cross section showing structure of Sonoma Volcanics in vicinity of Monticello Road ---------------------------------- 5 

4. Similarity coefficient matrix comparing trace- and minor-element analyses of all glass samples analyzed ______________ 10 

5. Dendrogram from cluster analysis of all analyzed samples----------------------------------------------------------- 12 

6. Ternary diagram showing provincial differences between tuffs in central Coast Ranges and southern Cascade Range ____ 15 

7. Binary diagram showing provincial differences between tuffs in central Coast Range~ and southern Cascade Range ____ 15 

8. Maps showing maximum known areal distribution of five major eruptive tuff units ---------------------------------- 16 

9. Correlation chart of late Cenozoic tuffs----------------------------------------------------------------------------- 18 

10. Histogram of principal mafic mineral frequencies of three size fractions of tuff in the Merced(?) Formation of Sonoma 

County _______________________________________________________________________________________________________ 20 

11. Ternary diagram showing correlation of the Pinole Tuff with volcanic deposits of the Sonoma Volcanics exposed along 

Monticello Road east of Napa _______________ _________ __ ___ ______ _ ____ __ ____ ___ _____ _ _ ________ _ ___ _ __ __ ____ ____ 20 

12. Ternary diagram showing correlation of the Pinole Tuff with tuffs in Tessajara area south of Mount Diablo ____________ 20 

13. Diagram showing correlation of the Pinole Tuff at Rodeo with tuffs south of Mount Diablo, in the Highland syncline section, Tassajara area___________________________________________________________________________________________ 20 

14. Histograms of concentrations of minor and trace elements in volcanic glass of the Lawlor Tuff and its correlatives------- 20 

15. Histograms of concentrations of minor and trace elements in volcanic glass of the Putah and Nomlaki Tuff Members 

theTehamaFormation --------------------------------------------------------------------------------------- 21 

16. Similarity coefficient matrix comparing trace- and minor-element analyses of glass samples of the Putah Tuff Member 

and tuff in the Merced(?) Formation of Sonoma County---------------------------------------------------------- 22 

17. Dendrogram from cluster analysis of trace- and minor-element data, using distance function---------------------------- 23 

III 
IV CONTENTS 

TABLES 

Page 

TABLE 1. List of units studied ___________________________________________________________________________________________ ---- 6 

2. Analytical data on potassium-argon dates ------------------------------------------------------------------------- 7 

3. Summary of X-ray fluorescence spectrometer analytical conditions -------------------------------------------------- 7 

4. Comparison of glass compositions of a silicic and an intermediate tuff------------------------------------------------ 9 

5. Chemical analyses and petrographic data _____________________________________________________________ -------------- 13 

6. Average minor- and trace-element composition of volcanic glass of the Lawlor Tuff and correlative localities------------ 21 

7. Trace- and minor-element analyses of volcanic glass of two thin water-laid tuffs in the Livermore Gravels of Clark 

(1930) ------------------------------------------------------------------------------------------------------- 21 

8. Average similarity coefficients comparing trace- and minor-element analyses of volcanic glass of two thin water-laid tuffs 

in the Livermore Gravels of Clark (1930) ----------------------------------------------------------------------- 22 
CORRELATION OF LATE CENOZOIC TUFFS IN THE 

CENTRAL COAST RANGES OF CALIFORNIA BY MEANS OF 

TRACE- AND MINOR-ELEMENT CHEMISTRY 

By ANDREI M. SARNA-WOJCICKI 

ABSTRACT 

Deformed late Cenozoic tuffs in the central Coast Ranges of 

California have been correlated by means of trace- and minorelement chemistry of volcanic glass, supported by potassium-argon 

dates, petrographic data, and stratigraphy. Cluster analysis of the 

chemical data indicates that four orders of chemical variability exist 

in the trace- and minor-element composition of volcanic glass. The 

greatest differences are between tephra of silicic and intermediate 

composition. Considering silicic tephra alone, the greatest differences are observed between tephra erupted in different volcanic 

provinces. Differences between samples of silicic tephra erupted 

within the same volcanic field are smaller, while the smallest differences are observed between samples of tephra from individual eruptions. 

Five widespread tuffs and composite tephra units erupted during a 

period from approximately 1 to 6 million years ago have been recognized in the study area. These include the tuff in the type section of 

the Merced Formation, the Putah Tuff Member of the Tehama Formation, the Lawlor Tuff, the Pinole Tuff and the tuff in the Merced(?) 

Formation of Sonoma County. All except the first were erupted from 

local central Coast Range sources, probably in the Sonoma volcanic 

field; the tuff in the type section of the Merced Formation was derived from the southern Cascade Range, about 320 km north of the 

main study area. 

Tuff correlations indicate that Suisun Bay and Mount Diablo, in 

the eastern part of the main study area, were formed less than 4 

million years ago and that drainage from the Great Valley of 

California to the ocean in the vicinity of the San Francisco Bay was 

established some time between 0.6 and 3.3 million years ago. 

INTRODUCTION AND SCOPE OF STUDY 

Correlations between the discontinuous exposures of 

deformed Pliocene and lower Pleistocene strata in the 

central Coast Ranges of California are important to an 

understanding of late Cenozoic development of the region because the distribution and structure of these 

strata record the tempo and style of ongoing deformation in the Coast Ranges. However, late Cenozoic 

sedimentation has been too rapid and has occurred 

under environmental conditions too diverse to permit 

refined correlation on the basis of fossil chronology. 

Radiometric dating of these geologically young deposits lacks sufficient precision as a correlation tool 

owing to the large errors involved in correcting for atmospheric argon-40 in the potassium-argon method, 

the effect of detrital contamination, and the general 

scarcity of material suitable for dating within 

sedimentary sections. 

Because of the problems in fossil and radiometric age 

correlation, many of the late Cenozoic deposits in the 

central Coast Ranges are lumped into a broad, illdefined "Plio-Pleistocene" category. A more detailed 

regional correlation of these deposits on the basis of 

tephrochronology, supported by radiometric, stratigraphic, and paleontological data, can serve as one of 

the basic tools in the solution of several longstanding 

geologic problems, such as the late Cenozoic 

paleogeography of the Coast Ranges and the nature 

and rates of late Cenozoic deformation. 

The principal approach to correlation discussed in 

this paper is that of geochemical tephrochronologycomparison of tuff beds in these deposits by means of 

minor- and trace-element compositions of the volcanic 

glass (chemical fingerprinting method of Jack and 

Carmichael, 1968). Other comparative methods, such 

as mafic-mineral frequency analysis, refractive indices 

of glass, and potassium-argon dating of some of the 

main tuff units, serve to support the correlations based 

on this chemical fingerprinting of the tuff units. 

This paper is mainly concerned with the results of 

tuff ·correlations and their geologic implications. A 

more detailed discussion of methods and the relative 

merits of various correlation criteria can be found in 

the author's dissertation (Sarna-Wojcicki, 1971). 

This study is limited to middle and late Pliocene and 

early Pleistocene deposits in the central Coast Ranges 

of California, between lat. 37°30' N. and 38°45' N. 

(fig. 1). Some tuff units and volcanic sources outside of 

this region, in the Clear Lake area and northern Great 

Valley, were also studied (inset map, fig. 1). 

ACKNOWLEDGMENTS 

This report is based partly on a doctoral dissertation 

submitted in 1971 to the Department of Geology and 

Geophysics, University of California, Berkeley, and 

1 
2 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

..,., 

I 

Late Cenozoic 

-

sedimentary deposits 

EXPLANATION 

~ 

~ "'{ Sonoma ~ volcanic 

"" field 

Major -widespread tuff 

unit 

Major fault 

Dashed where approximately located; 

dotted where concealed 

43 

• Sample 

locality 

FIGURE !.-Generalized geologic map showing location of samples, central Coast Ranges, Calif. Geology from Strand 

and Koenig (1965), Koenig (1966), Rogers (1966), Jennings and Burnett (1961), and Ross Wagner (written commun., 1974). 
GEOLOGIC SE'ITING 3 

partly on research performed during 1971-72 at the 

U.S. Geological Survey. 

I thank the following people at the Department of 

Geology and Geophysics at Berkeley: Robert Jack, who 

instructed me in XRF analytical procedures and 

helped in the analyses; Frank H. Brown, who did electron microprobe analyses of volcanic glass and phenocrysts; Richard L. Hay, Clyde Wahrhaftig, and Donald 

Savage, who advised me in this research; Garniss H. 

Curtis, who ran potassium-argon dates on several of 

the tuff samples in this study; Leonard Vigus, who 

helped with construction of research equipment; 

Leonard Leudke, who prepared and polished microprobe samples; and Katherine Condon, who helped in 

laboratory preparation of tuff samples. 

Thanks are given to the following people at the U.S. 

Geological Survey in Menlo Park: Alan Bartow, who 

gave me information on the tuff in the Merced(?) Formation of Sonoma County; Brent Fabbi, who ran XRF 

analyses on several tuff samples; Kenneth F. Fox, Jr., 

who advised me on the use of the cluster analysis program; and Paul C. Russell, who did glass separation on 

tuff samples and helped with the drafting. 

I also thank Donald 0. Emerson, of the University of 

California, Davis, for information on the Putah Tuff 

Member. 

I am grateful for a Penrose Grant from the Geological Society of America and for a Graduate Research 

Grant from the University of California. 

GEOLOGIC SETTING 

Late Cenozoic deposits in the central Coast Ranges 

of California are discontinuous sedimentary and volcanic prisms occupying axes of northwest-trending 

post-Miocene 'basins, dissected remnants mantling uplifted northwest-trending ranges, elongate truncated 

edges of sedimentary prisms upwarped along zones of 

flexure or faulting between areas of uplift and subsidence, and elongate northwest-trending fault-bounded 

slivers situated along major fault zones. Throughout 

much of the central Coast Ranges, late Cenozoic deposits lap onto deformed rocks of Jurassic to Miocene 

age along major unconformities and are themselves 

strongly deformed. 

Late Cenozoic deposits include both sedimentary and 

volcanic rocks. In the western part of the area, marine 

Pliocene and Pleistocene sediments are associated with 

brackish-water deposits and freshwater alluvial and 

lacustrine deposits of approximately the same age, 

though the contact relations between them in most 

places are concealed by younger alluvium or severed 

by faults. For instance, the marine Merced(?) Formation of Sonoma County probably interfingers to the 

east and northeast with the brackish-water and freshwater Petaluma Formation and with the freshwater 

alluvial Glen Ellen Formation (Bartow and others, 

1973), but the transition from one formation to the 

other is nowhere continuously exposed. In the eastern 

basins, the late Cenozoic sediments are mainly alluvial, fan, and lacustrine deposits. Throughout most of 

the study area, the western basins are separated from 

the eastern by northwest-trending ranges underlain by 

older rocks. The volcanic rocks, restricted primarily to 

the northern part of the study area, form thick and 

extensive fields and interfinger locally with sedimentary rocks. Pyroclastic rocks in the study area are 

found as thick deposits interbedded with flow rocks 

within the volcanic fields and thinner tuff units interbedded with sedimentary deposits. Most of these thinner units wete probably derived from volcanic fields 

within the study area. Such tuffs are fairly widespread 

in late Cenozoic sections throughout the central Coast 

Ranges, though they constitute but a small percentage 

of the total sediment volume. These tuffs are a useful 

tool in correlating stratigraphic sections between 

widely separated areas, especially between marine and 

continental sections and between sedimentary and volcanic sections for which fossil and radiometric data are 

often inadequate. 

Tuffs interbedded in thick volcanic piles were closer 

to eruptive centers; consequently they contain coarser 

material than tuffs in the outlying areas. Two such 

thick volcanic piles within the study area are the 

Sonoma volcanic field and the Clear Lake volcanic 

field, but a major potential source of tuffs from outside 

the Coast Ranges is the area southwest of Mount Lassen, in the southern Cascade Range of northeastern 

California (inset map, fig. 1). 

SONOMA VOLCANIC FIELD 

The term Sonoma volcanic field, as used herein, 

refers to an area of middle and late Pliocene volcanism 

in the northern part of the study area. The Sonoma 

Volcanics, deformed and partially eroded, is the rock 

formation defined by Weaver (1949) which represents 

the present distribution of the Sonoma volcanic field. 

This formation is composed of pyroclastic deposits and 

lava flows with associated intrusive dikes. The rocks 

are predominantly silicic but range in composition 

from basalt to rhyolite. Two sections in the southeastern part of the field were studied in detail: the Monticello Road section east of Napa (fig. 1, locs. 17, 45-51) 

and the Goodyear Station section a few miles northwest of Suisun Bay (fig. 1, locs. 19-21). The Monticello 

Road section consists of more than 600 m of tuff, breccia, and flows of predominantly dacitic composition 

(figs. 2 and 3). 
4 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

CLEAR LAKE VOLCANIC FIELD 

The Clear Lake volcanic field (Becker, 1888; Anderson, 1936; Brice, 1952), northeast of the Sonoma volcanic field, was active during late Pliocene and 

0 

z 

• Q. 

E 

~ 

47 

48 

> 

e 

·= 

4.7±0.21 

50 -5.4 ±0.22 

600 v 

" 

v 

v 

500 " " V' 

51 ---,)i3~~;1 

Lithology 

Pebble, cobble,and boulder conglomerate 

in channels in top of tuff 

Reddish paleosol developed in top of unit 

Tuff with rounded pumice lapilli as 

much as 2.5 em long in fine-grained 

matrix 

Tuff with angular to subrounded lithic ? 

fragments as much as 0.6 em in dia- a 

meter, and pumice lapilli as much as ·.: 

2 em in diameter; lithic fragments are 'ai andesite and rhyolite or dacite(?) c 

Pumice-lapilli tuff- breccia and pumice- ~ 

block breccia. Massive beds, 0.6-6 m -5 

thick, of unsorted, angular, tubular, t 

light-bluish-gray to dark-gray pumice • 

fragments 1.3-5 em in average dia- ~ meter. Pumice blocks as much as40cm & 

long. Pumice has pearly sheen, is rather g. dense and hard, with small vesicles. &. 

Matrix consists of pink to brown, .~ 

soft opaline(?) material and mont- ~ 

morillionite clay, together with vitric ~ 

ash. Locally, this unit contains welded ·~ 

tuff near top ~ 

Blocks, as much as 1 m in diameter, of 0 banded dacite and perlite in matrix u 

of pumice breccia and vitric ash ~ 

Perlite, approximately 3 m thick ~ Light-purplish-gray to gray flow-banded ~ 

vescicular dacite. Flow banding is very 0 irregular, forming dome-and-basin 

shapes several metres across. Flow € bands are spaced 1-8 em apart. Ve- ., 

sicles are localized along flow bands a. Massive bluish-gray flow-banded dacite ,! Dacite > 

Black, porphyritic pitchstone, 10-60 em E 

thick, containing angular to round ... 

lithic fragments (=welded zone?). Un- ., 

derlain by a zone of welded dacite(?) a. 

breccia approximately 1.5 to 2m thick, Gi 

containing lenses of gray porphyritic ~ 

glass 

Lithic breccia and tuff. breccia, 6-9 m 0 

thick; decrease in amount of lithic ... 

fragments downward, with increase ~ 

in amount of vitric ash and pumice Gi 

lapilli a. Cream y,l ight-gray to white chalky pumice- a. l·apilli and vitric-ash tuff 2 

Andesite(?) dike c 

0 

Narrow zone of welded dacitic(?) pumice- ·~ 

lapilli tuff containing lenses and stream- l!i ers of glass 0 Dark grayish-brown andesitic ash tuff, ..,. 

12 m thick, containing black to very • 

dark brown scoria lapilli and bread- a. 

crust scoria bombs as much as 30 em j in diameter o 

Light-gray to tannish-gray pumice-lapilli .J 

and vitric-ash tuff, 1.5 m thick, in 

thin beds ranging from a few centimetres 

to 0.6 m thick, interbedded with 

tuffaceous sandstone containing rounded pumice lapilli as much as 1.2 em 

long 

Quaternary time (Brice, 1953; G. H. Curtis, oral commun., 1971). The volcanic rocks erupted in the Clear 

Lake area range in composition from basalt to rhyolite. 

Pyroclastic rocks are rare, suggesting that explosive 

volcanic activity was unimportant in the development 

0 

z 

Ql 

Q. 

E 

~ 

> 

e 

·= 

u Ill 

= c ~ Ql 

·- E c 

0 c 

·~ ·~ 

w ii 0 ... Lithology 

45-3.8 ±0.1 2 

Light-gray to white porphyritic rhyolite 

flow or welded tuff(?) containing an46------i 

17-(4.0 ±0.2)3 " v v v v 0 

1 Mankinen, 1972 

gular lithic volcanic fragments c 

Black porphyritic pitchstone or obsidian ·2 with angular lithic fragments and pia- a 

gioclase feldspar phenocrysts (welded 0 top of tuff sequence?) ... 

Light-tan to gray indurated welded tuff, :U 

with flattened pumice-lapilli as much a. 

as 1.3 em long ~ 

Gray crudely stratified poorly sorted a. 

pumice -lapilli tuff :::> 

Gray, coarse pumice-lapilli tuff. Lapilli 

as much as 1.3 em long 

Et 

;,/. ~ 

It) 

.. .. 

White massive unstratified coarse pumice-}~ ~ lapilli tuff, with pumice lapilli as much ~ ~ 5 as 2 em in diameter. Ash flow tu.ff(?) 8. o ·;::; 

~.s~ 

::::~~ :I Ill 0 

1-

2 G. H. Curtis, oral commun., 1971 

3 Age inferred on basis of correlation with 

chemically similar Lawlor Tuff, dated at 

4.0 ± 0.2 m.y. 

FIGURE 2.-Stratigraphic section exposed along Monticello Road, southeastern part of Sonoma volcanic field (locs. 17, 45-51). 
ANALYTICAL METHODS 5 

sw 

Older 

St. Helena Rhyolite 

Tuff with Oldest Membe_r of Sonoma 

scoria bombs tuff Unconformity(?) Volcanics 

NE 

K-Ar ages, in million years 4.2 ± 0.1 1 4.7 ± 0.2 1 5.4 ± 0.22 4.0 ± 0.23 3.8 ± 0.1 2 

METRES FEET 

1 MILE 1000·11500 

500 1000 

500 

0 0 

0 .5 1 KILOMETRE 

0 

FIGURE 3.--Structure along a section roughly parallel to Monticello Road (sample locs. 17, 4~51), southeastern part of Sonoma volcanic field. Stratigraphic relations between monoclinal section in southwest and St. Helena Rhyolite Member of Sonoma 

Volcanics in northeast are not clear, but the St. Helena appears to be youngest unit in section. 

of the field. However, a small area of tuff is exposed at 

Cobb Mountain and near Siegler Canyon in the Lower 

Lake quadrangle (Brice, 1953). At the Siegler Canyon 

locality, the tuff is a poorly vesiculated pumice breccia 

forming part of the late Pliocene Cache Formation of 

Anderson (1936). 

SOUTHERN CASCADE RANGE VOLCANIC FIELD 

The large volcanic field in the vicinity of Mount Lassen, northeast of Sacramento Valley (inset map, fig. 1), 

has been active throughout late Cenozoic time, from at 

least the late Pliocene to Holocene time (Macdonald, 

1966). Tephra from some of the more explosive eruptions, such as the Nomlaki Tuff Member -of the Tehama· 

Formation, was deposited in northeastern and northwestern Great Valley (Russell, 1931; Anderson and 

Russell, 1939). Although the southern Cascade Range 

in the vicinity of Mount Lassen is approximately 320 

km to the northeast of the main study area, it is a 

potential source area for some of the water-transported 

tuffs in the central Coast Ranges because this area has 

drained through the San Francisco Bay area since at 

least Pleistocene time (Hall, 1966; this study). 

All tuff units examined in this study are listed in 

table 1. A more detailed discussion of the texture, stratigraphy, and structure of the tuffs and the stratigraphic sections is given in the description of units. 

Basic data on radiometric age determinations for this 

study are given in table 2. 

ANALYTICAL METHOI)S 

Samples of tuff were collected both laterally and vertically in a stratigraphic section wherever possible to 

test for random and systematic variations within each 

unit. Between 200 and 500 grams of sample were taken 

for physical and chemical analysis. In addition, larger 

samples, up to several kilograms, were taken at several localities for radiometric dating. Laboratory work 

consisted of five main operations: (1) physical description of the tuffs on the basis of microscopic examination, (2) measurement of the refractive indices of glass, 

(3) mafic-mineral analysis, (4) X-ray fluorescence spectrometric analysis for minor and trace elements, and 

(5) radiometric dating of the tuffs. 

REFRACTIVE INDICES OF GLASS 

The range of refractive indices for several samples of 

each tuff were measured under the petrographic microscope after the samples were treated with 8 percent 

hydrofluoric acid to remove altered and hydrated surfaces. The Becke line method, aD-line filter, and Cargille immersion liquids calibrated to 0.002 R.I. intervals were used in determinations, and the ranges of 

refractive indices were corrected for temperature variations. Precision attained for the average refractive 

index of glass in the sample was ±0.001. 

MAFIC-MINERAL ANALYSIS 

Samples were crushed and sieved, and the 120- to 

60-mesh fraction (0.125 to 0.250 mm) was separated in 

a Frantz magnetic separator. The magnetic fraction 

was separated in bromoform, the heavy separate 

placed in optic oils, and mafic mineral frequencies were 

determined by line count under a petrographic microscope fitted with a mechanical stage. 

X-RAY FLUORESCENCE SPECTROMETRIC ANALYSIS 

Relative and absolute concentrations of trace- and 

minor-element concentrations in tuff samples were determined by means of a Norelco Universal Vacuum 

Spectrograph using the analytical procedures described by Jack and Carmichael (1968; see table 3). 

Samples were analyzed for iron, titanium, barium, 

manganese, zirconium, rubidium, strontium, zinc, yttrium, gallium, niobium, copper, and nickel. Two types 

of analyses were run: (1) rapid-scan analyses of acidtreated whole-rock samples, which indicate relative 

proportions of the rubidium-niobium group (rubidium, 

strontium, yttrium, niobium) on the basis of relative 
6 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

peak intensities (Jack and Carmichael, 1968) and (2) 

ttabsolute" analyses of glass separated from the wholerock sample, which indicate concentrations of all elements listed above by comparison with analyzed 

standards. Powdered whole-rock samples for rapid 

scan were treated with 10 percent hydrochloric acid in 

order to remove strontium in carbonate form, a common groundwater contaminant (Sarna-Wojcicki, 

1971). 

scatter of the data, again resulting in apparently overlapping sample groups. In either case, the results make 

correlation on a fine scale difficult or impossible. In 

such situations a procedure that has better resolution 

than rapid scanning of whole-rock samples is necessary. For this reason, volcanic glass of selected samples 

was separated and scanned to see if the spread of data 

for individual units could be reduced. 

Selected samples were crushed and sieved, and glass 

from the 120- to 60-mesh fraction (0.125 to 0.250 mm) 

was separated using the Frantz magnetic separator. 

The glass was treated with 10 pe;rcent hydrochloric 

acid, etched with 5 percent hydrofluoric acid, and 

cleaned in an ultrasonic vjbrator to remove adhering 

fine particles. 

Rapid-scan analyses can be useful in indicating the 

correlation between specific units. However, when considering a large number of units within a single petrographic province, such as those erupted from the 

Sonoma volcanic field, chemical similarities between 

units often result in overlap of data for sample groups 

from different tuff units. Variations in ground-water 

and detrital contamination and in concentrations of 

crystals and lithic fragments may cause a spread or 

The magnetic properties of some samples prevented 

complete separation of the salic minerals and alteredglass fragments from the clean glass. Some of these 

Sample 

locality 

1 

2 

3,4 

5 

6 

7 

8--11 

12-15 

16 

17 

18 

19-21 

22-28 

29,30 

31,32 

33, 34 

35-37 

38--40 

41,42 

43 

44 45-49 

50,51 

52, 52A-C 

53, 54 

55-62, 79 

63, 63A-C 

64 65,66 

67 

68 

69 

70 

71 

77 

78 

Material Unit 

TABLE 1.-Units studied 

[See figure 1 for locations of samples] 

Geologic age 

Radiometric age 

<K-Ar), in m.y. References 

Water-laid tuff ___________ _ Merced Formation (type section! __ Late Pliocene and early Pleistocene 1.49±0.75 ____ Hall (1966). do _____________ _ Alameda Formation ________________________ do ------------------------------------------------do. do _____________ _ Sal}ta ClariJ. Formation ____________ Pliocene and Pleistocene -------------------------- Dibblee (19661. Ash-fall(?) tuff ___________ _ Tassajara Formation______________ Pliocene or Pleistocene---------------------------- Clark (1943). Water-laid tuff ___________ _ Ohlson Ranch Formation---------- Pliocene(?)________________________________________ Bartow, (oral commun., 1972). 

Ash-flow tuff __ ---------- Unnamed formation ______________ Pleistocene ---------------------- 0.25±1.1±0.5 Wilson (1961); Gilbert (1969).* Curtis (oral commun., 1971).* 

do -------------- Nomlaki Tuff Member of Tehama Formation. 

Late Pliocene -------------------- 3.3±0.4 ______ Anderson and Russel (1939); 

Evernden, Savage, Curtis, and James (1964).* Water-laid tuff ____________ Putah Tuff Member of do ---------------------- 3.28±0.10 ____ Miller (1966)*; Sims and Tehama Formation. Ash-tuff__________________ Sonoma Volcanics __________________________ do ____________________ ------------------

Ash-flow tuff ____________ Sonoma Volcanics --------------------------do -------------------------------------- do ________________________ do ________________________________ do _____________________________________ _ 

Ash-flow tuffs ______________________ do --------------------------------do -------------------------------------- do -------------- Lawlor Tuff------------------------ ________ do ---------------------- 3.96±0.16 ___ _ do -------------- Tassajara Formation______________ Pliocene or Pleistocene____________ 4.00±1.00 ___ _ 

Water-laid tuff____________ Livermore Gravels Late Pliocene -------------------- 4.46±0.45 ___ _ of Clark (1930). do ________________________ do ________________________________ do _____________________________________ _ 

do -------------- Merced(?) Formation______________ Early and late Pliocene____________ 5.68±0.68 ___ _ 

Various types of do --------------------------------do ______________________ 6.0±0.1 _____ _ tuff. 

Water-laid tuff____________ Petaluma Formation______________ Early or middle and late Pliocene------------------

do _____________ _ Tassajara Formation or_ _____________ Pliocene or Pleistocene---------------------------- Green Valley Formation. do _____________ _ Unnamed tuff-------------------- Miocene or Pliocene ______________ 8.18±2.0 _____ _ Tuff ________________ ------ Sonoma Volcanics ________________ Middle or Upper Pliocene __________ 13.79±0.08 ___ _ 

Tuff with scoria bombs ______________ do ------------------------ ________ do ______________________ 25.36±0.16 ___ _ Tuff______________________ Pinole Tuff ---------------------- Middle Pliocene __________________ 35.2±0.1 _____ _ 

Tuff with scoria bombs ______________ do --------------------------------do _____________________ _ Tuff ________________________________ do ________________________________ do ------------------------

do ______________ Tassajara or ________ do ------------------------

Green Valley Formation of Clark (1943). 

do ------------------------do ------------------------ ________ do ---------------------- do ______________ Green Valley Formation Pliocene--------------------------

do _____________ _ 

Vent breccia _____________ _ 

Pumice at Napa Glass Mountain. Obsidian at Napa Glass Mountain. 

Pumice breccia ----------

Ash-fall(?) tuff ___________ _ 

Ash-flow(?) tuff _________ _ 

of Clark (1943). 

Tassajara Formation______________ Pliocene or Pleistocene------------ Unnamed breccia ____ ______ ______ Pliocene(?) _______________________ _ Sonoma Volcanics ________________ Pliocene _________________________ _ 

do -------------------------0-~---- do ----------------------

Cache Formation of Anderson (1936) Unnamed tuff ___________________ _ 

Sonoma Volcanics _______________ _ 

Upper Pliocene ------------------

Pliocene(?) _______________ - __ ------ Pliocene _________________________ _ 

Sarna-Wojcicki (1974). 

Present study. Weaver (1949). 

Do. 

Do. Patten (1947); present study.* 

Clark, (1943); present study.* 

Huey, (1948); present study.* 

Do. 

Travis (1952); present study.* Travis (1952); Bartow, Sarna-Wojcicki, 

Addicott, and Lajoie (1973).* 

Bartow, Sarna-Wojcicki, Addicott 

and Lajoie (19731. Clark (1943). 

Curtis (oral commun., 1971).* Weaver (1947); Curtis (oral commun., 1971)* 

Do. Weaver (1947); Evernden, Savage, 

Curtis, and James (19641.* 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. Travis (1952). Weaver (1949); Jack and Carmichael (1968). 

Do. 

Brice (1953). 

Patten (1947). Weaver (1947). 

*Reference to radiometric age date. 'At top of unit. 2At base of unit. 3Near base of unit. 
METHODS OF EVALUATING CHEMICAL DATA 7 

samples were separated in bromoform-alcohol mixtures, but the use of heavy liquids was generally 

avoided in order to avoid contamination with bromine, 

which affects the rubidium analyses in the X-ray 

fluorescence procedure. 

The separated glass was then mixed with 20 percent 

by weight fibrous cellulose binder and pressed into 

3.2-cm-diameter discs in a hydraulic press at pressures 

of about 2,500 kg/cm2 (35,000 lb/in2). The standards 

were similarly prepared in order to provide uniform 

surfaces for both sample and standard. Glass separates 

were then analyzed for ~~absolute" concentrations of 

other elements that cannot be easily analyzed by the 

rapid-scan technique: iron, titanium, barium, manganese, zinc, copper, nickel, and gallium, as well as the 

rubidium-niobium group. The position for each of these 

elements was calibrated with pure element standards 

(for example, RbCl for rubidium), and element concentrations were determined by fixed-time counts at 

fixed 28 positions. Additional counts were made at adjoining 28 positions to determine the shape and intensity of the background curve. U.S. Geological Survey 

standards used were G-1 and G-2 for all elements except for gallium, zinc, copper, and nickel, for which 

W-1 was used (Fleisher, 1969; Flanagan, 1969). 

METHODS OF EVALUATING 

CHEMICAL DATA 

Trace- and minor-element analyses were compared 

(1) graphically, utilizing binary and ternary diagrams 

and histograms, (2) by calculation of similarity coefficients, using a computer program to perform analyses 

of similarity, and (3) by cluster analysis, using a computer program to calculate Q-mode cluster analysis on 

distance function, and shown on a de.ndrogram. 

GRAPHIC METHODS 

Rapid-scan analyses of whole-rock samples for the 

three most abundant elements in the rubidiumniobium group (rubidium, strontium, and zirconium) 

TABLE 2.-Analytical data on potassium-argon dates 

[Age determinations were made at the potassium-argon laboratory of the Department of Geology and Geophysics, University of Califomia, Berkeley. 8pectrometric analyses by 

N. Gilbert. Potassium analyses by J. Hempel] 

Sample Sample loc. no. 

22 KA2310 

30 KA2319 

31 KA2323 

37 KA2321 

Element Analytical line 

Fe ---------- K 

Ti ---------- K Mn __________ K 

Ba ---------- L 

Ni ---------- K 

Cu __________ K 

Zn __________ K 

Ga __________ K Rb __________ K 

Sr ---------- K 

y ---------- K 

Zr ---------- K Nb __________ K 

1Jack and Carmichael (1968). 

K 40 Ar atm. Age 

Unit Material <percent) (percent) (million years) 

Lawlor Tuff, Contra Costa Co. Coarse plagioclase crystals 0.7225 75.0 3.96±0.16 

(24-48 mesh) 

Tuff in Tassajara Formation, Plagioclase crystals 0.6060 96.0 4.00±1.00 

south of Mount Diablo, Con- (60-120 mesh) 

tra Costa Co. 

Lower tuff bed in Livermore Plagioclase crystals 0.6030 89.9 4.46±0.45 

Gravels of Clark (1930) south (60-120 mesh) 

of Livermore, Alameda Co. 

Tuff in the Merced(?) Formation Coarse plagioclase crystals 0.7642 92.1 5.68±0.68 

near Roblar, Sonoma Co. (24-48 mesh) 

TABLE 3.~ummary of X-ray fluorescence spectrometer analytical conditions 

[Standard value of iron in percent; all other values in ppm. Modified in part from Jack and Carmichael (1968)] 

Exciting Analyzing radiation 

crystal (50 kilovolts) 

LiF 200 w 

LiF 200 w 

LiF 200 w 

LiF 200 w 

LiF 200 Mo 

LiF 200 Mo 

LiF 200 Mo 

LiF 200 Mo 

LiF 220 w 

LiF 220 w 

LiF 220 w 

LiF 220 w 

LiF 220 w 

Primary 

beam 

filter 

--------

--------

--------

0.001"Ti 

0.001"Ti 

0.001"Ti 

0.001"Ti 

--------

--------

--------

--------

--------

Detector 

(with pulse-height discrimination) Path 

Standard (assumed value) weight percent or parts per million 

Scintillation ____________ Air ________ G-2 (1.844) 

Flow-proportional________ Vacuum ____ G-1 (1560); G-2 (2930) 

Scintillation ____________ Air ________ G-1 (200); G-2 (280) 

Flow-proportional________ Vacuum ____ G-1 (1040); G-2 (2030) 

Scintillation ____________ Air ________ W-1 (78) ____________ do __________________ do____ W-1 (110) 

____________ do __________________ do____ W-1 (82) 

____________ do __________________ do____ W-1 (16) 

____________ do __________________ do_____ G-1 (220); G-2 (175) 

____________ do ___________________ do____ G-1 (250); G-2 (465) 

____________ do __________________ do____ G-1 (13); G-2 (10) 

____________ do __________________ do____ G-1 (210); G-2 (320) 

_ ___________ do __________________ do____ G-1 (20); G-2 (16) 
8 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

were recalculated to mutual percentages and plotted 

on a ternary diagram. Analyses of absolute concentrations of trace and minor elements in the purified glass 

were plotted on histograms and on binary diagrams of 

one element plotted against another. 

SIMILARITY COEFFICIENT 

A coefficient that allows all analyzed variables for a 

pair of samples to be compared has been derived by 

Borchardt, Aruscavage, and Millard (1972). This similarity coefficient, which is 1 for identical analyses, is 

given by: n 

LRi 

d i=l (A,B)=-n-, (1) 

where 

d(A,B) = d(B,A) = similarity coefficient for comparison between sample A and sample B, 

element number, 

n 

R. ~ 

X.A ~ 

number of elements 

XiA!XiB if XiB~XiA; otherwise XiB/XiA, 

concentration of element i in sample A, 

and 

XJ~ concentration of element i in sample B. 

The similarity coefficient is a simple and effective 

way of comparing any quantitative parameters for any 

group of samples, and the method is readily adapted to 

a simple computer program. The only disadvantage to 

this method involves comparison of a large number of 

samples. For 50 samples, 1,225 comparisons must be 

made; for 100, 4,950, and for 200, 19,900 since the 

number of comparisons increases exponentially with 

increase of the sample population. For large sample 

populations, provisions must be made to extract coefficients within a selected range. 

Borchardt, Aruscavage, and Millard (1972) have also 

introduced weighting coefficients in order to minimize 

the effect of the least accurately determined elements 

on the similarity coefficients. In this study, weighting 

coefficients were not used. Instead, only those elements 

were used that were considered reliable, both with respect to the precision of the analysis and the natural 

variability of the elements within the volcanic glass. 

The reliability of any particular element was 

evaluated by multiple analyses of samples from a 

single, extensive tuff bed (locs. 22-28). Analyses were 

made on samples collected both laterally and vertically 

in the section in order to test the internal consistency 

of chemical and physical properties within a single depositional unit. Analyses for only those elements that 

showed a high degree of consistency (iron, titanium, 

barium, manganese, zirconium, rubidium, strontium, 

and zinc) were used in calculating similarity coefficients and the cluster analysis described in the next 

section. Comparisons of multiple analyses of a single 

unit indicated that copper, nickel, gallium, and, to ~ 

lesser extent, yttrium were not reliable for correlation 

purposes, mainly because these elements occur in low 

concentrations not much above the detection limit of 

the X-ray fluorescence method. Consequently, the precision for analyses of these elements was low and these 

elements were not included in the comparison proce~ 

dure. In addition, strontium, though abundant, varies 

greatly owing to ground-water contamination, concentration in feldspar and mafic microlites and 

phenocrysts, and concentration in mafic and intermediate lithic volcanic fragments. However, strontium 

is important in distinguishing between tuffs derived 

from different volcanic fields, so analyses of similarity 

were run both with and without strontium. 

CLUSTER ANALYSIS 

In addition to calculation of similarity coefficients, 

the chemical data were also compared by means of 

Q-mode cluster analysis using a computer program by 

Parks (1970). According to Parks, the program ~~computes an R-mode principal components analysis (factor 

analysis with unities in the principal diagonal)* * *" 

using the simple distance function. Factor scores are 

then calculated, forming a set of ~~new orthogonal (uncorrelated) variables."1 The formula used for the simple distance function for R-mode analysis is 

d12= 1.0- [ I (Xl . -X2 .)2/N] lh. ' l l i = 1 

Using the new orthogonal variables, the program 

~~computes a Q-mode similarity matrix, comparing 

each sample with all other samples across all variables," using the distance function as similarity coefficient. The formula for distance function for Q-mode 

analysis is 

d,:>= [~ (Xi,-X,2)2/M ]~>. 

The Q-mode similarity matrix now contains the euclidian distance between all possible pairs of samples 

measured in a space with dimensionality equivalent to 

the number of factors found in the R-mode principal 

components analysis. This matrix is then searched for 

the two samples with the least distance between them, 

which are then combined to form a cluster and the 

measurements of the pair averaged. All distances be1Some of the elements analyzed in this study, for instance, iron and manganese, are not 

independent variables but, on the contrary, show a high degree of correlation for certain 

sample groups (see fig. 7). 
RESULTS OF ANALYSES 9 

tween either member of the pair and other samples are 

recalculated using the newly averaged measurements 

of the cluster. The process is repeated until all the 

samples are grouped into a number of clusters of progressively greater distance (higher values of the distance function). The program plots all the groups on a 

dendrogram that shows the relations of all individual 

samples and all groups of samples to each other, with 

respect to the distance function. 

The advantage of this method is that several orders 

of chemical variability and relations between samples 

are readily apparent in a two-dimensional format. 

Since the outcome of the clustering procedure is influenced by the composition of every sample present in 

the sample group, the actual values of the distance 

function and the resulting clusters formed are affected 

by the range of compositional types included in the 

comparison. Inclusion of particularly unusual composional varieties in the cluster analysis results in 

tighter clustering (smaller values of the distance function) for samples of similar composition, while exclusion of such unusual varieties results in a greater 

spread (higher values of the distance function) between 

the remaining samples. Inclusion or exclusion of the 

unusual compositional varieties serves as a device 

with which to focus on compositional variations between sample groups of particular interest. 

RESULTS OF ANALYSES 

Before individual tuff units can be correlated by 

means of trace- and minor-element chemistry, the variability within and between individual eruptive units2 

and within and between individual volcanic fields 

must be established. Greatest differences in trace- and 

minor-element chemistry were observed between 

silicic tuffs on the one hand and intermediate pumicelapilli tuffs on the other. These differences are illustrated by the two analyses shown in table 4. The group 

average similarity coefficients between silicic and intermediate tuff groups range between 0.42 and 0.61, 

with an overall average of 0.51, the lowest average 

coefficient for all sample groups considered (fig. 4). 

Large differences between the silicic and intermediate 

tephra are also indicated by the dendrogram derived 

from cluster analysis (fig. 5). Silicic and intermediate 

samples are grouped at highest values of the distance 

function (0.25-0.48). 

Among silicic tuffs, greatest differences are observed 

between tuffs derived from different volcanic fields. 

Tuffs that are definitely known to have been erupted in 

2An eruptive unit is defined here as a collection of all those outcrops that, on the basis of 

several criteria, primarily trace- and minor-element chemistry of the volcanic glass but also 

petrographic characteristics, radiometric age, stratigraphic position, fossil data, and other 

pertinent information, are considered to be products of a single eruption or of multiple 

eruptions closely spaced in time. 

TABLE 4.-Comparison of glass compositions of a silicic and an 

intermediate tuff 

[Concentrations of iron and silica, in percent; all other concentrations in parts million] 

Si02 FeO+Fe20.. Ti Ba Mn Zr Rb 

Sample 501 __________ 62.70 6.86 7362 423 1596 207 54 

Sample 372 __________ 71.43 1.62 803 698 237 247 181 

1lntermediate tuff with scoria bombs. 

2Silicic tuff. 

the southern Cascade Range, such as the N omlaki Tuff 

Member of the Tehama Formation (Anderson and Russell, 1939; Lydon, 1967) and a pumiceous tuff near 

Mineral (Wilson, 1961; Gilbert, 1969), have higher 

concentrations of strontium and lower concentrations 

of iron, zirconium, zinc, and yttrium than tuffs erupted 

in the Sonoma volcanic field (table 5). The samples of 

pumice tuff near Mineral (fig. 1, loc. 7) and the Nomlaki Tuff Member (fig. 1, locs. 8--11) have low similarity 

coefficients when compared with silicic tuffs erupted 

from the Sonoma volcanic field (fig. 4). Group average 

similarity coefficients comparing tuffs from the southern Cascade Range and the Sonoma volcanic field 

range from 0.48 to 0.64, with an overall average of 

0.60. Tuffs from different volcanic provinces have distance function values of between 0.135 and 0.210 (fig. 

5). Differences between tuffs derived from the Sonoma 

volcanic field, those from the southern Cascade Range, 

and those of inferred southern Cascade Range provenance are graphically illustrated in figures 6 and 7. 

Silicic tuffs of different ages that are known to have 

been erupted from the same volcanic field show smaller 

compositional differences than those erupted from different volcanic fields. Age and source criteria independent of chemical analyses are available to test the 

validity of this assertion. Relative and absolute ages of 

many of the units studied are known from stratigraphic position and radiometric dates. With respect to 

source, it is possible to identify those tuffs that were 

erupted, for example, from the Sonoma volcanic field 

by several criteria. First, coarse tuffs, tuff-breccias, 

and agglomerates are interbedded with flow rocks in 

the Sonoma volcanic field itself, indicating proximity 

of the tuffs to vents. For instance, north of Suisun Bay 

(fig. 1, locs. 19--21) pumice bombs as much as 30 em in 

diameter were found in coarse ash flow tuffs. Second, in 

outlying areas beyond the Sonoma volcanic field, the 

source of the more widespread silicic tuffs is indicated 

by coarseness of the tuffs and by size gradients that 

increase towards the Sonoma volcanic field. Independent confirmation of the source of these tuffs is obtained 

from the trace- and minor-element analyses of the volcanic glass. Most analyses of tuffs in the central Coast 

Ranges show strong chemical similarities between 

tuffs of the Sonoma volcanic field and those of the out-
10 

Widespread silicic tuffs erupted in southern Cascade Ran&e 

Widespread silicic tuffs erupted in central Coast Ran&es 

Silicic tuffs and flows of Sonoma volcanic field 

Heterogeneous tephra of intermediate com· position of Pinole Tuff and Sonoma volcanic field (Monti- cello Road section) 

CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

Sample Loc. I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 26 26 27 28 29 30 31 32 33 34 

1 

Tuffs in central 2 

{

1 

::!11 861G01 Coast Ran&es de- 3 

rived from south- 4 

em Cascade Ran&e 5 :~~::~!51 75 66 57 

6 

7 76 73 84 83 801 ~-========================================~ 

Nomlaki Tuff Member{ 8 

of Tehama Forma- 9 

tion 10 

11 

80 

67 64 

78 76 

7574 77 6681 

76 72 65 1 1 82 []I :! ~: ! ~ ! ! ~: ~: :: : ~ ! 3 1 63 

82 8374 75 73 71 659194 90 I ~-========================================~ 

63 

Putah Tuff Member 13 

~~~~lali~~da- 14 

{

12 

:: :: :: : ~ : ~ :: :: : ~ :: :: :: : ~ ~ 5 I 90 

5461636366615462676665 1 I 

78 correlative tuff (16 ) 16 65 68 69 71 72 66 54 66 71 71 70 83 86 89 1 

Lawlor Tuff correla· tives in Sonoma volcanic field 

Lawlor Tuff 

Lawlor Tuff correla· tives south of Mount Diablo 

Tuff in Merced (?) Formation of Sonoma County 

Southern correlatives of tuff in the Mer- ced(?) Formation 

16 6 0 59 6 4 6 6 6 8 6 4 54 59 6 4 6 4 6 3 8 8 9 3 9 6 8 8 1 ~======================================~ 17 6163616162615460656664 82828282821 

18 63 63 61 61 61 58 53 63 67 67 66 78 78 77 82 77 91 1 

19 62 62 58 68 62 58 6163 66 66 65 80 78 77 81 77 93 911 

20 63 63 56 58 62 58 50 64 68 65 67 81 78 78 81 78 93 90 96 1 

21 66 66 58 57 60 55 50 64 68 65 69 77 74 73 79 73 88 92 93 93 1 

22 636359596258 5263676766 8180788178 91939694921 93 23 63 63 58 58 61 57 52 63 68 66 65 82 80 71 80 79 93 91 95 94 92 96 1 

24 60 6159 59 59 58 53 61 65 60 64 80 8180 79 81 94 93 93 9190 95 961 

25 62 62 58 58 61 57 51 63 66 65 65 79 77 76 80 76 91 92 96 92 92 95 96 92 1 

26 63 64 59 59 63 58 51 64 67 67 66 80 77 77 82 77 91 91 97 96 94 91i 95 91 96 1 

27 63 63 59 59 61 57 52 63 67 67 66 80 78 78 79 78 94 91 97 95 94 95 96 94 94 96 1 

28 62 62 59 69 60 57 52 63 66 66 65 79 79 77 80 77 92 93 97 96 92 97 95 96 95 96 96 1 

29 65 65 60 60 63 58 50 66 69 67 68 77 74 73 81 74 89 91 93 92 93 91 91 89 93 93 92 92 1 

30 62 62 58 58 61 57 51 63 66 66 65 80 78 78 80 78 93 91 96 94 91 95 94 95 96 95 97 95 92 1 

31 64 64 61 61 63 59 53 63 68 68 67 80 78 77 83 77 92 94 95 92 94 95 94 94 95 94 94 95 94 94 I 

32 63 63 59 58 61 56 52 63 68 66 66 80 78 76 80 76 90 94 95 93 93 97 96 94 95 95 95 96 92 94 95 I 

33 62 62 69 59 60 55 51 64 65 65 64 74 73 72 78 72 87 94 91 87 90 91 89 89 93 90 90 92 91 91 93 92 1 

34 64 64 61 60 62 56 52 66 67 67 65 76 75 73 79 73 85 92 89 86 88 91 89 88 91 90 89 91 88 89 92 92 96 1 

35 li5 57 62 64 66 64 57 59 63 63 63 82 90 91 82 90 77 75 72 72 68 75 74 77 71 71 72 74 68 72 72 74 71 70 

36 53 li6 61 61 64 63 5li 58 62 62 62 83 90 90 80 90 76 74 72 72 68 74 74 76 71 71 72 73 68 72 72 73 70 70 

37 66 58 63 64 66 64 67 60 64 64 64 81 90 89 80 88 75 72 71 72 68 74 73 76 70 70 71 73 67 71 71 72 67 68 

38 54 58 62 64 67 64 57 59 64 64 63 87 89 86 77 86 74 69 70 70 66 71 72 72 68 69 70 70 64 70 70 69 6li 66 

39 55 57 63 62 67 62 li5 63 68 66 69 87 87 84 7li 83 76 73 71 7168 74 73 75 70 71 7173 67 71 7172 68 69 

40 52 55 61 62 65 63 56 58 62 62 62 86 87 84 75 84 74 71 70 70 67 73 72 74 69 69 70 71 66 70 70 71 67 68 

41 56 59 64 64 69 66 58 6166 66 65 80 86 85 78 85 76 73 7172 68 74 74 76 70 7172 73 67 72 717368 69 

·12 52 li4 60 61 63 62 52 57 62 62 61 84 87 85 76 84 74 7170 70 67 72 72 74 69 69 70 71 66 70 70 70 66 68 

43 65 58 63 64 67 63 55 59 63 63 63 78 84 86 85 85 76 76 75 74 71 77 73 75 74 74 73 75 70 73 74 76 72 72 

44 53 55 61 62 62 62 55 56 61 61 60 87 90 90 82 90 80 78 76 76 72 78 78 81 75 75 76 77 71 76 76 77 74 74 

Uncorrelated tuffs of { 6 4 central Coast Range 6 5 provenance south 6 6 

of Mount Diablo 6 7 

56 58 65 64 66 63 55 58 63 63 62 84 90 92 84 91 83 78 78 78 73 80 79 83 77 77 78 79 73 78 78 78 76 75 

70 65 7170 75 66 57 69 7172 70 80 80 79 84 76 74 77 74 75 75 76 74 73 74 75 73 74 7li 73 77 7676 76 

69 67 70 70 74 65 li6 70 72 72 71 818180 85 77 75 77 74 75 76 76 74 73 74 7li 73 74 7li 73 77 77 76 78 

58 58 54 53 56 4 9 4 8 6 4 6 4 6 2 6 4 6 3 6 2 61 6 8 6 2 70 74 74 74 7 6 7 3 74 7 0 7 7 7 5 7 3 7 3 8 0 7 6 7 6 74 7 7 7 8 

Silicic Pinole tuffs { : ~ (52 , li 7 , 58 ) and 57 correlative tuff (6 3 ) 58 

{ 

Silicic tuffs exposed { 4 5 along Monticello 4 6 Road 48 

Napa Glass Mountain { ~ ~ 

Intermediate tuffs { ! ~ exposed along Mon- 5o ticello Road 4 7 

56 

53 Intermediate Pinole 54 

{

55 

tuffs 59 

60 

61 

62 

Volcanic vent, Sonoma County - 68 

Tuff-breccia, Cache - 7 1 Formation of Ander- son (19 3 6 ), Clear Lake area 

60 63 66 67 71 65 56 64 70 70 69 81 87 86 82 83 76 73 73 73 68 75 75 76 72 72 73 74 68 73 72 74 71 70 

59 61 66 68 68 64 55 62 67 67 66 80 85 87 87 84 78 77 7li 76 71 78 7li 77 74 74 75 77 71 75 75 76 73 73 66 67 62 64 66 59 60 63 67 66 68 77 78 80 87 78 74 77 73 74 77 76 73 73 74 74 72 74 74 72 76 76 74 74 

63 66 63 62 65 59 64 62 67 67 66 82 80 78 84 79 85 86 85 85 85 87 84 83 84 87 86 86 83 83 86 87 85 83 

46 46 65 66 59 51i 46 50 62 64 49 59 62 62 66 64 68 66 69 68 66 66 66 61i 68 69 67 67 69 67 68 66 60 63 

56 57 58li9 65 58 5159 63 6162 9189 86 79 87 82 75 80 83 79 79 80 79 77 80 80 79 76 78 78 78 73 73 

47 49 55 56 li5 55 50 54 58 57 57 72 70 71 64 69 64 63 62 62 60 63 62 65 61 62 62 63 58 61 61 62 61 60 

40 42 48 49 54 liO 48 46 50 50 50 68 69 68 63 59 64 64 61 61 59 62 63 64 60 61 61 62 58 61 62 62 61 61 

39 41 47 48 li3 49 43 46 49 49 49 66 66 66 61 67 63 63 61 60 68 61 61 63 60 li9 60 60 58 60 62 62 60 60 

64 67 46 47 42 43 43 68 67 67 59 62 50 49 51 49 59 63 62 62 65 61 61 61 63 62 62 62 64 62 62 63 67 66 51 50 4 7 47 4 7 4 7 44 54 54 54 53 6 8 6 4 6 2 6 0 61 69 7 2 7 3 71 71 7 2 7 2 7 2 74 7 2 7 3 7 3 71 7 3 71 7 3 7 6 74 

38 40 32 33 31 36 35 44 44 43 44 36 37 34 35 33 37 41 39 38 40 39 39 39 40 38 38 39 40 39 40 40 43 43 

50 50 46 45 44 40 43 65 54 52 55 59 61 51 67 57 68 67 70 70 70 69 71 70 70 69 71 70 68 71 67 70 71 68 

li4 li7 47 46 45 39 39 53 53 51 54 52 liO 49 54 49 60 64 64 63 68 63 62 61 66 64 63 63 66 63 64 64 69 69 

55 55 47 47 46 40 39 li6 54 52 55 53 51 50 li6 50 61 65 65 64 69 64 63 62 66 65 64 64 68 64 66 66 70 70 

45 48 38 39 37 40 45 51 61 49 51 46 46 44 46 44 51 li5 63 61 54 63 53 53 64 52 li2 li3 52 53 64 55 67 67 

liO li3 42 43 40 44 42 57 66 54 57 5148 47 49 48 67 61 60 59 6160 60 60 6160 60 60 60 60 6162 65 64 

62 65 55 54 53 47 45 61 61 59 62 60 58 56 60 56 68 73 73 72 76 72 71 71 74 72 72 73 7 5 72 73 73 77 77 

58 61 48 49 46 45 42 60 60 57 60 48 46 46 51 47 57 57 59 69 61 57 ii7 56 60 60 li9 58 63 69 59 58 61 61 

49 52 42 42 39 43 48 56 li5 53 56 53 51 49 51 50 55 60 58 57 59 60 59 58 59 59 liS 69 68 58 59 60 63 64 

50 52 44 44 41 37 38 49 49 48 50 46 44 44 47 44 63 li7 66 66 59 55 54 54 57 56 66 56 58 56 66 56 60 60 

56 52 66 66 70 66 58 60 60 64 57 61 67 68 69 68 62 66 62 61 60 64 61 62 63 62 60 64 59 60 63 64 62 62 

71 69 74 74 73 78 65 72 74 75 72 70 72 73 75 71 66 66 62 62 62 64 64 64 63 63 63 63 64 63 66 64 63 65 

FIGURE 4.-Similarity coefficient matrix. Similarity coefficients are calculated for each sample pair in study group. Individual values of 

similarity coefficients are given in lower left half of matrix, while average values for main sample groups are given in symmetric 

positions across the identity diagonal in upper right half of matrix. Calculations were made by comparison offollowing eight elements: 

Fe, Ti, Ba, Mn, Zr, Sr, Rb and Zn. No iron analyses were available for samples 1 and 2; consequently these samples were compared 

with other samples using remaining seven elements. Values of similarity coefficient are given in hundredths, with decimal point 

omitted. 

lying areas, including specific correlatives (fig. 1, locs. 

17-34, 4 7, and 55). Similarity coefficients for silicic 

tuffs erupted in the Sonoma volcanic field range from 

0.66 to 0.85, with an average of 0. 75, and values of the 

distance function are between 0.05 and 0.135. 

Smaller differences in chemical composition of volcanic glass are observed between samples of the same 

tuff unit. The similarity coefficient for seven samples of 
RESULTS OF ANALYSES 

r------'-----r-......... - ...... -...L....---. .------1..--, ,--J--, ,.-L-, , ..... --'-----. .-------'-----. 

311 II 37 88 II 40 U U U U 14 U U 17 13 U 1i7 118 41i U 48 U 70 U 49 liO 47 1i1i U 1i3 1i4 51 &0 U 12 U 71 

==== 3 ====!IG0GGGI:=::=::: ===== 44 ~188 

lc:Jc:JGGGI 52 18~ 61 

:=:::::=====::::! IGc:JGGGI so 18~ ~~ -- ~~ 

85 

73 76 77 69 61 61 61 ~ ~ 

================:::: :=======! :=:=====: :=======: :=:============: 1 

nn :! ~4 1 90 88 90 91 1 

88 90 88 94 1 

88 90 90 96 95 1 74 80 71 72 49 47 0 70 

88!1291949195881 

12!12949091911 u 92 91 90 86 86 86 90 87 1 

93 93 90 86 84 87 87 87 88 1 

93 91 90 83 83 83 89 83 89 93 l IG 0

- 0- G 

1

:=:::::=::==::=:::::::::::::

1 

n n 79 77 76 73 74 73 77 72 81 79 80 1 80 78 77 73 74 73 77 72 82 81 81 97 1 79 66 59 55 55 [J68 LJ71 58 li8 57 57 57 57 58 fl6 58 58 58 6 7 6 7 1 . . 

r-::1 Q 1:1 :=:::::::::=====:::::::::::::1 n n ~~L~_j S3lJLl 88 86 87 83 81 83 89 82 86 85 

90 88 87 81 79 81 86 82 90 87 

79 78 76 7170 7174 72 8178 

77 76 74 72 75 71 75 71 80 81 

60 59 57 59 58 ri8 59 58 62 59 

80 80 79 86 83 85 78 84 76 83 74 75 71 74 76 74 72 74 73 75 

73 74 71 74 74 76 72 76 74 74 

70 71 69 71 71 73 70 73 72 71 

49 49 47 45 48 47 47 46 61 63 

69 59 57 59 n u 57 so 66 62 

35 36 36 34 34 36 36 35 35 35 

56 lili 63 liO 62 61 lili 61 66 59 

48 48 46 44 46 4li 47 45 50 61 

50 49 4 7 4 5 47 4 6 48 46 52 63 46 46 44 44 45 45 45 44 46 46 

47 47 45 u 47 4& 4& 46 48 51 

lili 5 5 54 5 2 6 5 5 4 5 4 5 3 56 "5 7 

43 43 42 43 44 43 43 42 44 45 

49 49 47 47 49 48 48 48 50 53 

44 43 45 40 42 41 42 41 46 47 

898685591 

898681i60 911 

79 88 89 64 83 86 1 

82838366 8083831 

63 57 57 57 59 60 56 63 EJ~I Inn 84 75 77 62 78 75 74 80 62 50 48 [J67 .58. 74686944 73736767 . . u 70 62 63 u &s 69 64 63 53 70 67 --.---,1 44 II 47 159 56 69 &1 u 45 &6 6s 63 &2 53 67 &4 92 1 IL..Jo 

63 63 56 64 48 60 64 60 48 64 46 44 44 

63 69 li9 66 69 68 67 68 56 67 64 63 60 

34 39 39 62 38 38 41 39 31 33 29 36 36 

6160 60 68 61 61 60 69 57 61 54 46 45 

52585966 

53606168 

44 48 49 66 

51515264 

58606269 

46464763 

62525255 

47505159 

51536163 

53 54 64 6li 

46485251 

47496458 

53556167 

41434652 

49515559 

45475366 

53 54 48 54 55 50 

36 45 37 

41 51 42 60 62 62 

60 60 41 

37 52 43 

474343 

4 7 4 7 

4 7 4 7 

48 48 52 52 

48 4 7 

39 39 

61 5 I 

4444 

1 

63 1 

60 43 1 

7777521 

78686684 

78706587 

73477259 

84 63 66 67 80714874 

84 li9 58 69 

74636462 

86676672 

I 68 188 1 

96 1 

66 65 1 

7372861 

807962711 

75 

70 7 0 71 7 8 7 7 1 

66 6li 86 86 68 7 6 1 

78 78 76 82 76 79 72 1 

nn 00 7 5 74 7 2 7 1 69 7 0 76 7 1 7 1 7 1 7 1 74 7 3 li 2 74 7 6 7 0 6 7 6 7 6 1 74 6 0 6 8 43 48 3 4 4 9 44 4 6 3 9 4 0 47 3 6 4 0. 4 0 1 IUl 72 717170 68 6970 68 70 71 71 78 7766 72 73 70 67 49 66 60 56 56 49 5142 46 46 46 48 60 62 62 61 42 64 1 

11 

the Lawlor Tuff, for example, range from 0.91 to 0.97 

with an average of 0.95. Values of the distance function for samples within the Lawlor Tuff range between 

0.015 and 0.050. Values of the distance function for 

duplicate analyses of single samples and on replicate 

analyses of samples from single exposures are in the 

lower part of this range (for example, lacs. 3 and 4, 33 

and 34, 65 and 66, fig. 1). 

Graphic analysis of the chemical data was usually 

sufficient to distinguish between most eruptive units 

because variations in chemical composition within 

units were considerably smaller than variations between units. However, in a few instances, for example, 

the Putah Tuff Member of the Tehama Formation and 

the tuff in the Merced(?) Formation of Sonoma County, 

differences between tuffs erupted from a single volcanic province were rather subtle and could be resolved 

only by statistical comparison of analyses. 
z 

0 

SAMPLE LOCALITY 

* * * ~ ;cl ~ ::1 ~ ~~ ~ g: ~ ~ ~ ~ ~ Ol ~ <j ij ~ ~ ~ 01 en ..J ~woo o 10::: ~~~aU~:~ iO ~ ~ ~ ~ ~ ~ tin;~~~ a;~ r~n~ ~ ~ ~ ~ ~ t ~ ~ c;; ~;;:; ~ c:; a; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ m 0 . 

+-

.11-+-

F,;ant and 

omlaki 

~1 Tuff,Member I of Tehama F'm. 

'-v-'~1. I, 

---..,_..., I I ~~ correlative I ~ I tuffs 

flllJLW,UI~tt JJ Jl t14 ~pq:u Tuff and I I J ~ )"-'-"' , correlative tuffs p h ~T ff · I Pinole Tuff, , uta u m : 

tuff near Tuff Merced(?) Fm. 

Lafayette, and Member 

tuff in Tassajara of Tehama 

Fm. Fm. 

I ~I 

Heterogeneous 

J ~ 

tuffs 

1~----------~----------~ and breccias, Sonoma 

Volcanics and Pinole 

Tuff '-v-' 

All silicic tuffs of 

Cascade Range 

source 

All silicic tuffs and flows of Sonoma Volcanics or central 

Coast Range source 

-

j: .2 1- - u 

z 

::> 

IL. 

w 

u 

z 

<( 

1-

~ 

0 

.3 

1-

.4 I+ 

+-

Andesitic tuffs and 

altered and weathered tuffs having 

unusual compositions 

____.J~ 

Sierra Nevada 

tuffs 

All silicic tuffs and flows 

All samples 

-

-

-

-

-

FIGURE 5.-Dendrogram from printout of cluster analysis program. Samples are grouped against distance function for 79 samples analyzed for Fe, Ti, Ba, Mn, Zr, Rb, 

Sr and Zn. Five samples from the Sierra Nevada (72-76) have been included for comparison but are not part of present study. Starred (*) samples are probably 

misgrouped because of high variability of some of eight elements analyzed (fig.17). Sample 15 is a weathered sample from Putah Tuff Member of Tehama Formation; sample 37 is from a tuff in Merced(?) Formation of Sonoma Co.; sample 76 is Holocene surface pumice from Red's Meadow in south-central Sierra Nevada. 

......... 

t-,j 

0 

0 

~ t::l:j 

t"" 

> t-3 

1-1 

0 

z 

0 

l-%j 

t"" 

> t-3 

t::l:j 

0 

t::l:j 

z 

0 

N 

0 

1-1 

0 

t-3 

e 

~ 

~ 

sn 

0 

0 

> U1 

t-3 

~ z 

ga 

sn 

0 

~ 1-1 

l-%j 

0 

~ 

z 

> 
RESULTS OF ANALYSES 

TABLE 5.-Chemical analyses and petrographic data 

[Sample numbers are same as locality numbers shown in figure 1. ND, not determined. Concentration of iron in precent; all other concentrations in parts per 

million. P, mineral present but not abundant] 

Sampie _______ c_o_nc_e_n_tr_a_ti_on_s_o_f_m_i_no_r_a_n_d_t_ra_c_e_e_le_m_e_nt_s_i_n_v_ol_ca_n_i_c_gl_a_ss ____ ___ 

No. Fe Ni 

Main refractive 

Index of glass 

Principal mafic mineral frequencies (percent) 

Gr. hblde Br. hblde Hypersth. Augite 

Fine-grained tuffs in Coast Ranges derived from the southern Cascade Range 

1 ---- ND 1,628 1,166 395 107 125 130 26 ND 13 4 35 7 2 ---- ND 1,867 1,130 377 111 120 131 34 ND 12 9 21 40 3 ---- .71 1,010 1,061 281 102 115 113 25 9 13 13 39 13 4 ---- .71 1,014 1,045 260 105 123 131 27 9 12 12 15 7 5 ---- .82 1,205 1,132 287 125 199 106 25 12 13 7 22 11 6 ---- .62 918 948 305 109 166 201 23 15 8 36 15 15 

1.499±0.001 

1.499±0.001 ND 

ND 

1.499±0.001 

ND 

1 72 

80 

77 

ND 

90 

96 

'0 

0 

0 

ND 

2 

0 

1 28 

20 

21 

ND 

0 

3 

'0 

0 

2 ND 

8 

1 

Rock 

type 

Tuff 

Do. 

Do. 

Do. 

Do. 

Do. 

13 

Ash-flow tuffs near Mount Lassen and northern Great Valley, erupted from the southern Cascade Range (Nomlaki Tuff, ~11) 

7 ---- .61 885 790 327 8 ---- .90 1,372 1,052 518 

9 ---- .94 1,332 1,055 419 

10 ---- .87 1,168 965 387 11 ____ 1.08 1,453 1,003 420 

104 79 414 169 102 177 

182 107 169 181 103 162 

161 99 168 

18 

28 

33 

28 

32 

9 

9 

12 

14 

9 

14 

12 

14 

13 

13 

33 

30 

35 

4 

20 

6 

9 

43 

10 

11 

1 

10 

12 

7 

12 

1.496-1.500±0.001 

1.500--1.501±0.001 

1.501±0.001 

1.501±0.001 

1.501-1.502±0.001 

148 

24 

15 

26 

30 

'0 

0 

13 

8 

6 

152 

68 

60 

55 

51 

'0 

8 

13 

10 

13 

Tuffs erupted in the Sonoma volcanic field, or of inferred Sonoma volcanic prolvenance, central Coast Ranges 

(Putah Tuff Member, 12-15, and correlative, l6) 

12 ____ 1.36 1,251 13 ____ 1.34 1,088 

14 ____ 1.39 1,018 15 ____ 1.40 1,131 

16 ____ 1.44 1,006 

814 278 276 794 262 256 

838 244 261 981 244 263 

875 247 274 

153 

174 

170 

158 

186 

27 

35 

37 

68 

38 

45 

43 

39 

37 

41 

28 

27 

21 

24 

17 

13 

13 

14 

13 

16 

3 

14 

0 

0 

7 

14 

20 

15 

12 

17 

15 

14 

9 

8 

10 

1.503±0.001 

1.502±0.001 

1.501±0.001 

1.503±0.001 

1.502--1.503±0.001 

0 

0 

0 

0 

1 

3 

3 

1 

1 

4 

91 

93 

93 

98 

85 

6 

4 

6 

1 

10 

Tuff erupted in the Sonoma volcanic field, or of inferred Sonoma volcanic provenance, central Coast Ranges 

(Lawlor Tuff, 22-28, and correlative tuffs, 17-21, 29-34) 

17 ____ 1.62 1,074 

18 ____ 1.60 1,064 19 ____ 1.73 1,174 20 ____ 1.62 1,283 

21 ____ 1.75 1,473 

22 ____ 1.72 1,174 23 ____ 1. 72 1,211 24 ____ 1.68 1,048 

25 ____ 1.81 1,180 26 ____ 1. 78 1,203 27 ____ 1.73 1,173 28 ____ 1.72 1,129 29 ____ 1.68 1,258 

30 ____ 1.83 1,183 

31 ____ 1.70 1,134 32 ____ 1.75 1,213 

33 ____ 1.82 1,136 34 ____ 1.82 1,137 

836 366 

711 441 817 459 

840 448 

801 436 

758 433 812 440 

755 428 796 470 845 447 830 437 768 454 913 456 

849 449 

809 422 733 431 

684 502 

718 517 

312 

301 

326 

336 

339 

301 

297 

303 

304 

326 

329 

319 

313 

304 

305 

297 

306 

290 

154 

140 

154 

157 

143 

148 

145 

147 

149 

152 

143 

146 

144 

148 

148 

143 

135 

137 

51 

64 

59 

56 

68 

58 

50 

48 

62 

62 

54 

57 

71 

53 

71 

60 

74 

82 

60 

60 

58 

57 

59 

54 

57 

59 

63 

56 

59 

56 

67 

61 

59 

58 

60 

56 

47 

34 

26 

24 

27 

26 

24 

25 

25 

25 

18 

18 

32 

24 

29 

25 

23 

24 

17 

15 

19 

18 

18 

16 

17 

17 

20 

17 

18 

17 

22 

17 

17 

17 

14 

13 

24 

13 

18 

20 

15 

25 

13 

15 

16 

23 

18 

15 

3 

14 

14 

16 

21 

9 

27 

0 

12 

5 

9 

8 

9 

14 

16 

2 

7 

4 

18 

4 

9 

11 

13 

8 

15 

10 

11 

10 

11 

9 

10 

11 

12 

9 

15 

8 

11 

10 

12 

13 

1~ } 

1.506±0.001 

ND 

1.504±0.001 

1.505±0.001 

1.508; 1.512±0.001 ND 

1.505±0.001 

1.505±0.001 

1.505±0.001 

1.505±0.001 ND 

ND 41.504; 1.506±0.001 41.500; 1.506±0.001 

41.503; 1.505±0.001 

41.499; 1.505±0.001 

1.505±.001 

ND 

0 ND 

20 

30 

ND 

ND 

3 

0 ND 

7 ND 

0 

28 

•o 

34 

0 

0 

0 

ND 

98 

ND 

211 

3 50 

ND 

ND 

64 

52 

ND 

49 

ND 

73 

22 

535 

3 59 

23 

39 

23 

ND 

2 ND 

2 78 

347 

ND 

ND 

4 

8 ND 

6 ND 

6 

0 5 27 

3 21 

56 

29 

52 

ND 

0 ND 

211 

33 

ND 

ND 

29 

40 

ND 

39 

ND 

21 

50 

5 38 

3 16 

21 

32 

24 

Tuffs erupted in the Sonoma volcanic field, or of inferred Sonoma volcanic provenance, central Coast Ranges 

(tuff in the Merced(?) Formation of Sonoma Coun.ty, 35-40, and correlatives, 41-44) 

35 ____ 1.18 36 ____ 1.18 

37 ____ 1.21 

38 ____ 1.13 

39 ____ l.G9 

40 ____ 1.11 41 ____ 1.07 

42 ____ 1.16 43 ____ 1.16 

44 ____ 1.27 

876 686 237 247 181 803 698 233 242 176 

710 759 250 216 174 

766 817 259 225 190 

750 751 310 249 179 755 755 285 228 183 729 724 278 221 181 

775 767 215 219 177 738 667 240 261 179 857 654 250 279 163 

45 ____ 0,58 1,105 897 26 402 214 

46 ____ 1.36 1,348 817 266 333 190 

47 ____ 2. 70 1,369 585 751 447 94 

48 ____ 1.05 818 625 97 418 152 

49 ____ 2.08 1,178 694 576 530 132 

50 ____ 5.12 7,362 423 1,596 207 54 51 ____ 2,78 3,299 657 784 342 93 

41 40 39 43 

40 40 

28 44 27 44 

27 48 44 41 29 48 

59 39 39 46 

24 

21 

19 

28 

24 

28 

23 

29 

20 

41 

17 

19 

15 

18 

19 

18 

15 

14 

14 

18 

17 

7 

9 

4 

9 

17 

9 

10 

8 

11 

11 

13 

43 

11 

6 

34 

23 

1 

15 

12 

14 

12 

20 

8 

6 

13 

10 

11 

14 

1.498; 1.501±0.001 

1.498; 1.501±0.001 

1.502±0.001 

ND 

ND 

ND 

ND 

ND 

1.503±0.001 

1.501; 1.503±0.001 

2 ND 

{ 

60 

71 

so 

ND 

ND 

ND 

51 

so 

10 

7 

96 

ND 

61 

7 12 

5 29 

ND 

ND 

ND 

53 

81 

11 

17 

Tuffs and flows of the Sonoma Volcanics, Monticello Road section 

64 75 

29 51 

50 107 

22 33 

20 113 208 114 

151 88 

46 

35 

24 

24 

46 

22 

31 

17 

14 

25 

22 

23 

22 

24 

51 

46 

79 

73 

49 

33 

42 

30 

15 

16 

18 

23 

34 

26 

12 

17 

16 

6 

10 

14 

18 

ND 

ND 

1.515±0.001 

1.507±0.001 

1.513±0.001 

1.565±0.005 

1.511±0.001 

•p 

"P 

0 

0 

p 

0 

0 

0 

3 

1 

ND 

6 70 

757 

530 

ND 

ND 

ND 

536 

895 

36 

57 

0 

0 

p 

0 

0 

3 

86 

0 ND 

629} 

766 

5 13 

ND 

ND 

ND 

560 

"4 

43 

18 

0 

0 

0 

0 

p 

96 

7 

Tuff. 

Do. 

Do. 

Do. 

Do. 

Tuff. 

Do. 

Do. 

Do. 

Do. 

Tuff. 

Do. 

Do. 

Do 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Tuff. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Welded 

tuff(?). 

Tuff. 

Pumice 

breccia. 

Dacite 

flow. 

Tuff. 

Do. 

Do. 

In summary, graphic and statistical evaluation of 

the chemical data indicates that four orders ofvariability exist in trace- and minor-element compositions of volcanic glass. The greatest variability exists between 
14 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

TABLE 5.-Chemical analyses and petrographic data-Continued 

Sample Concentrations of minor and trace elements in volcanic glass Main refractive Principal mafic mineral frequencies (percent) 

No. Fe Ti Ba Mn Zr Rb Sr Zn y Ga Nb Cu Ni Index of glass Gr. hblde Br. hblde Hypersth. Augite 

Tuffs erupted in the Sonoma volcanic field or of inferred Sonoma volcanic provenance, central Coast Ranges 

(tuffs of the Pinole Tuff and correlative sample, 63) 

52 ____ 1.20 1,014 584 236 230 160 55 37 26 16 17 9 11 1.501±0.001 "7 77 12 4 53 ____ 3.48 4,596 444 855 238 69 262 60 30 16 14 5 4 1.533±0.001 "0 4 34 62 

54 ____ 3.15 3,935 435 781 307 90 191 58 26 16 21 15 31.509; 1.513; "0 0 31 70 

1.528±0.001 55 ____ 2.70 1,798 413 814 444 107 87 95 61 22 20 17 17 1.509±0.001 0 77 0 23 

1 48 6 46 56 ____ :l.74 1,621 470 826 456 104 84 97 65 20 21 3 8 1.501±0.001 0 25 15 60 57 ____ 1.47 1,615 529 232 244 151 69 33 16 17 12 5 5 1.501±0.001 0 16 73 11 58 ____ 1.50 1,206 587 330 328 136 64 44 19 18 7 11 11 1.502±0.001 ND ND ND ND 59 ____ 2.30 2,308 743 616 405 108 107 70 54 18 17 8 3 1.506±0.001 10,120 p 0 p 

60 ____ 2.99 4,663 1,135 643 381 87 156 83 25 17 23 10 8 1.509±0.001 lOQ 0 p p 61 ____ 2.82 4,890 439 598 272 82 266 49 16 15 15 10 8 ND 13Q 0 3 97 

0 5 30 64 62 ____ 3.63 3,624 498 983 371 88 118 89 20 20 14 9 15 31.509; 1.512;} 0 0 46 55 1.517±0.001 63 ____ 1.12 1,146 582 267 218 171 44 34 28 13 14 22 12 ND 4 7 6 83 140 145 148 1487 157 159 155 1579 

Uncorrelated tuffs of the Tassajara area, south of Mount Diablo 

64 ____ 1.25 989 686 250 306 172 44 40 32 16 23 6 10 31.501; 1.502±0.001 1 21 22 57 6516 __ 1.28 1,316 601 276 247 152 78 26 15 14 8 4 

1n {Ng 0 2 98 66 17 __ 1.30 1,355 622 270 251 154 82 29 14 15 4 6 1.501±0.001 ND ND ND 67 ____ 1.95 1,360 908 498 235 40 79 90 26 17 17 7 8 1.501±0.001 1 32 36 32 

Vent tuff-breccia, 68; associated pumice and obsidian of Napa Glass Mountain, Sonoma County, 69, 70 

68 ____ 0.75 851 651 79 221 192 64 26 22 15 10 18 9 ND ND ND ND ND 

69 ____ 1.08 555 447 202 283 199 11 57 30 16 26 8 12 ND 18Q 0 70 ____ 1.08 555 443 211 291 207 6 59 34 16 24 9 14 ND 18Q 0 

Tuff in the Cache Formation of the Clear Lake area, north-central Coast Ranges 

71 ____ 1.28 919 1,368 269 235 138 209 50 23 15 4 11 8 1.503±0.001 0 0 

Altered and chemically contaminated tuff south of Suisun Bay 

77 ____ 1.07 831 808 844 189 148 109 37 14 16 10 18 17 

Hydrothermally altered tuff in the Sonoma Volcanics north of St. Helena 

78 ____ 0.21 5,400 682 20 526 142 57 35 8 44 12 

Tuffaceous lake clays in the Pinole Tuff 

79 ____ 1.97 1,167 1,375 1,037 166 83 54 124 14 15 

1Data from Hall (1965). 2 48-120 mesh. 

11 8 

10Very few transparent mafic minerals present. 

"Separate contains mostly biotite and ilmenite. 

p 

p 

p 

3120 mesh. 4More than one glass type in sample. 560-120 mesh. 

12Mafic mineral separate contains mostly magnetite or ilmenite. 13Mafic mineral separate from scoria lapillus in tuff. 

628-48 mesh. 7 35-100 mesh. 828-32 mesh. 9Separate consists mostly of magnetite and ilmenite plus a few grains of pink zircon. 

14Angular mafic mineral grains. 15Rounded mafic mineral grains. 16Coarse glass separate, 60-120 mesh. 17Fine glass separate, 120-200 mesh. 18Dark-green to bluish-green amphibole (arfedsonite?). 

0 

0 

0 

Rock 

type 

Tuff. 

Agglomerate 

tuff. 

Tuff. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Vent 

breccia. 

Pumice. 

Obsid- ian. 

Tuff- breccia. 

tephra of silicic and intermediate compositions. Considering silicic tephra alone, the greatest chemical variability exists between units erupted from different 

volcanic fields. Smaller chemical differences are observed between silicic tuffs erupted at different times 

from a single volcanic field, while the smallest variability is observed within a tuff layer representing a 

single eruption. Of the silicic tuffs analyzed, variations 

in the trace- and minor-element compositions of the 

glass are discontinuous from one order to the next, 

making it possible in most instances to identify tephra 

of individual eruptions, as well as the volcanic field 

from which the tephra was erupted. 

TUFF CORRELATIONS 

Correlations were made on the basis of chemical fin-
TUFF CORRELATIONS 15 

Sr 

I 

I \ 

\ 

& // • •• 

& / •. • •• 

A& .. ~/ ••• :/.·:~ • • t..c:: .. ? ; .. :.:· .. \ I ,..-. · ... t ... .,.,.,., ..... ·. . ---·--- .. . 

EXPLANATION 

• 

Zr 

Pyroclastic deposits of 

central Coast Range 

source, erupted mainly 

from Sonoma volcanic 

fielq 

Tuffs of northern Great 

• 

Valley, erupted from 

southern Cascade Range 

Tuffs in central Coast Range of 

inferred Cascade Range provenance 

FIGURE 6.-Provincial differences between tuffs in central 

Coast Ranges and southern Cascade Range. Mutual percentages of net intensity peaks for Rb, Sr, and Zr. Intensity peaks were obtained by using rapid X-ray fluorescence scans on powdered whole-rock samples treated with 

10 percent HCI. Dashed line separates tuffs of Cascade 

Range provenance from those of central Coast Ranges 

provenance. 

gerprinting of the volcanic glass, supported by additional evidence such as tuff petrography, stratigraphic 

position and sequence, potassium-argon ages, and 

available fossil data. Five widespread tuff units have 

been recognized within the central Coast Ranges. Figure 8 shows the known maximum areal distribution of 

these units. In order of decreasing age, the tuffs and 

their chemical correlatives are: (1) the tuff in the 

Merced(?) Formation of Sonoma County (fig. 9, locs. 

35-44), (2) the Pinole Tuff and tuffs in the lower part of 

the Monticello Road section in the Sonoma volcanic field 

(fig. 9, locs. 47,50--56, 59-63), (3) the Lawlor Tuff(fig. 9, 

locs. 17-34), (4) the Putah Tuff Member of the Tehama 

Formation (fig. 9, locs. 12-16), and (5) the tuff in the 

type section of the Merced Formation of western San 

Francisco peninsula (fig. 9, locs. 1-4). In addition to the 

above tuff units, there are approximately twelve more 

known tuff units within the central Coast Ranges that 

at this time have not been correlated with any widespread eruptive unit. 

TUFF IN THE MERCED(?) FORMATION OF SONOMA COUNTY 

Exposures of correlative tuff have been found in the 

marine Merced(?) Formation of Sonoma County (fig. 1, 

locs. 35-40), in the fresh- or brackish-water Petaluma 

Formation north of San Pablo Bay (fig. 1, locs. 41 and 

42), and near the base of the continental Tassajara 

Formation or the upper part of Clark's (1943) Green 

Valley Formation south of Mount Diablo (fig. 1, loc. 43). 

A tuff that overlies the Neroly Formation and underlies 

z 1800 

0 

:::i 

...J 1600-

~ 

a: 

w 1400- a.. 

en 

1- 1200- a: 

<( 

a.. 

~ 1000 

..: 800 z 

w 

1-

z 600- 0 

u 

w 400- en 

w 

z 

<( 200- (!) 

z 

<( 

~ 00 

. . • I 

1.0 

I 

• 

J 

2.0 

I 

~ • • • 

I 

3.0 

I 

I 

4.0 

I RON CONTENT, IN PERCENT 

EXPLANATION 

• Central Coast Ranges, Sonoma volcanic 

field (Monticello Road section) 

• Central Coast Ranges, Pinole tuffs 

Central Coast Ranges: Putah Tuff, Lawlor 

Tuff, and tuff in Merced (?) Formation 

of Sonoma County 

0 Southern Cascade Range provenance 

• Central Coast Ranges of probable Cascade 

Range provenance 

I 

• 

-

-

-

I 

5.0 

FIGURE 7 .-Provincial differences between tuffs from the central 

Coast Ranges and from the southern Cascade Range, as illustrated by variations in concentrations of iron and manganese in 

volcanic glass. 

Ham's (1952) Mulholland Formation in the eastern 

Berkeley Hills (fig. 1, loc. 44) is a tentative correlative. 

The tuff in the Merced(?) Formation of Sonoma County 

has been dated at 6.1±0.1 m.y. (fig. 1, loc. 40, Bartow 

and others, 1973) and at 5. 7±0.5 m.y. (fig. 1, loc. 37; 

table 2), while the tuff in the eastern Berkeley Hills (fig. 

1, loc. 44) has been dated at 8.2±2.0 m.y. (G. H. Curtis, 

oral commun., 1971); consequently, its correlation with 

the tuff in the Merced(?) Formation is uncertain. 

Although the refractive indices of the glass are very 

similar for those samples examined, the mafic-mineral 

frequencies between fine and coarse facies are particularly obvious. For instance, sample 35 (table 5), a fine 

water-laid ash tuff, is considerably enriched in 

hornblende with respect to sample 37, from a coarser 

facies of the same tuff containing pumice lapilli and 

cobbles. Presumably, the differences in mafic-mineral 

frequencies vary considerably (table 5). Differences in 

mafic-mineral frequencies between these samples are 

due to hydraulic sorting. Frequency counts of mafic 

minerals on three size fractions of the coarser facies of 

this tuff indicate that the homblende is finer grained 
16 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

than augite and therefore would tend to be concentrated in the finer grained facies (fig. 10). 

PINOLE TUFF 

The Pinole Tuff, a 275-m-thick sequence oftuffs, contains both silicic tuffs (fig. 1, locs. 52, 57 and 58) as well 

as pumice-lapilli tuffs of intermediate composition (table 1, locs. 53-56, 59--62). Glass composition of the intermediate tuffs is heterogeneous. Pumice lapilli of 

several different compositions may be present within a 

single sample, as indicated by presence of several glass 

B : 

types with different refractive indices (table 5, locs. 54, 

62). This heterogeneity introduces considerable scatter 

into the analytical data owing to sampling errors and 

makes it difficult to make specific correlations on the 

basis of glass chemistry. 

The Pinole Tuff can be correlated in a general way 

with the lower part of the Sonoma volcanic field in the 

Monticello Road section east of Napa. At both localities, 

andesitic tuffs containing scoria lapilli and bombs (fig. 

9, locs. 50, 51, 53, 54, 60, 61, and 62) are exposed near 

the base of the sections. At both sections, volcanic dec : 

0 10 20 MILES I I I I I I 

0 10 20 KILOMETRES 

FIGURE 8.-Maximum known areal distribution of five major eruptive tuff units (horizontal lined areas) identified in present study. 

A, Tuff at type locality of Merced Formation. B, Putah Tuff Member of Tehema Formation. C, Lawlor Tuff. D, Pinole Tuff. 

E, Tuff in the Merced(?) Formation of Sonoma County. 
TUFF CORRELATIONS 17 

posits grade upwards into more silicic types. Figure 11 

shows rapid-scan analyses on whole-rock samples from 

both sections. Similarity coefficients for heterogeneous 

intermediate tephra from the two localities are low, 

probably for the reasons mentioned above. The highest 

similarity coefficients for sample pairs from the two 

sections range from 80 to 87 (locs. 4 7, 53, 56, 59--62), 

about the same as the highest internal coefficients for 

the Pinole Tuff, except for samples from localities 55 

and 56, which have a coefficient of 0.96. Intermediate 

tuffs from the Pinole and Monticello Road sections of the 

Sonoma volcanic field are more similar to each other 

than they are to any other analyzed tuff (fig. 4). In 

addition, radiometric ages support the correlation of 

these tuffs. A potassium-argon date of5.2±0.1 m.y. was 

obtained on a sample from the Pinole Tuff near the base 

of the section (loc. 61) (Evernden and others, 1964), 

whereas a 5.4±0.2-m.y. date was obtained on a sample 

of tuff with scoria bombs near the base of the Monticello 

Road section (loc. 50) in the Sonoma volcanic field (G. H. 

Curtis, oral commun., 1971). 

Since the pyroclastic rocks in the Monticello Road 

section are thicker and generally coarser and contain 

interbedded flow rocks and intrusive dikes, we might 

suspect that they are closer to the eruptive source than 

are the Pinole Tuff (fig. 1, locs. 52--62). The tuff containing the scoria bombs in the Pinole Tuff, however, is 

coarser at locality 53 than at the Monticello Road locality 50, suggesting that for this unit at least the eruptive vent was closer to the Pinole localities. 

Similarities in stratigraphic sequence and glass 

chemistry at the Pinole and Monticello Road localities, 

as well as radiometric dates, suggest that the sections 

are contemporaneous and had a common source. However, because niobium content in volcanic glass of the 

Monticello Road volcanic rocks is systematically 

higher (table 5, samples 45--62), it is still possible that 

tephra was erupted from a series of separate but related vents and need not have had a common source. 

The Pinole Tuff is correlated in a general way with a 

60-m-thick sequence of tuffs and tuffaceous sediments 

south of Mount Diablo. Rapid-scan analyses again 

show vertically systematic changes in chemical composition in both sections (figs. 12 and 13). The similarity coefficient between the only complete ((absolute" 

analysis made on a sample from the locality south of 

Mount Diablo and a tuff in the Pinole Tuff section is 

0.91 (locs. 52 and 63). The sample at locality 52, however, was obtained from near the top of the Pinole Tuff 

at Rodeo, while the sample at locality 63 was obtained 

from near the base of the section south of Mount Diablo; consequently these tuffs may not be correlative. 

Nevertheless, since the tuff at locality 63 is massive, 

while that at locality 52 is composed of rounded pumice 

lapilli in a tuffaceous matrix, it is possible that the tuff 

at locality 52 was reworked from a more massive unit 

lower in the Rodeo section. 

LAWLOR TUFF 

The known extent of the Lawlor Tuff (fig. 1, locs. 

22--30) and its correlatives (fig. 1, locs. 17-21, 29--34) is 

shown in figure 8. The Lawlor is an ash flow tuff, the 

distant end of which has been found as far south as the 

southwestern flank of Mount Diablo (fig. 1, locs. 29, 

30), interbedded with the uppermost(?) part of theTassajara Formation; a reworked, water-laid correlative 

has been found even further south on the north flank of 

the Diablo Range, south of the town of Livermore, 

where it is interbedded with the Livermore Gravels of 

Clark (1930) (fig. 1, locs. 31-34). Three outcrops of ash 

flow tuff correlative with the Lawlor have also been 

found within the upper part of the Sonoma Volcanics 

(fig. 1, loc. 17-21). 

The average similarity coefficient for analyses of the 

Lawlor Tuff and its correlatives is 0.93, while values of 

the distance function in cluster analysis range from 

0.015 to 0.035 for all but one sample (29) and from 

0.015 to 0.060 for all samples (fig. 5). The similarity of 

the chemical analyses can be easily visualized when 

histograms of the analyses are compared (fig. 14). The 

similarity of the analyses becomes even more remarkable when average values of samples from the physically continuous Lawlor Tuff are compared with average values of samples from all other known correlative 

but physically separate localities (table 6). Comparisons of average values indicate greater consistency in 

concentrations even for some of those elements (yttrium, gallium, niobium, copper, and nickel) that show 

considerable scatter in individual sample comparisons. 

Radiometric ages support correlations made on the 

basis of chemical fingerprints (fig. 9). A potassiumargon date of 3.96±0.16 m.y. has been obtained for the 

Lawlor Tuff at its type locality in Lawlor Ravine in the 

hills south of Suisun Bay, north of Mount Diablo (fig. 9, 

locs. 22, 27, 28), a date of 4.0± 1.0 m.y. on an exposure 

in the Tassajara area, south of Mount Diablo (fig. 9, 

table 2, loc. 30), and a date of about 4.46±0.45 m.y. 

from its southernmost correlative in the Livermore 

Gravels of Clark (1930) (fig. 9, table 2, locs. 31, 32). The 

tuff exposure correlative with the Lawlor Tuff near the 

top of the Monticello Road section (fig. 9, loc. 17) is 

overlain by the St. Helena Rhyolite Member of the 

Sonoma Volcanics, dated at 3.8±0.1 m.y. (G. H. Curtis, 

commun., 1971), and, if my interpretation ofthe structure in this area (fig. 3) is correct, is underlain by dacite dated at 4.2±0.1 m.y. (Mankinen, 1972). 

At the southernmost locality correlative with the 

Lawlor Tuff, two thin tuffs in the Livermore Gravels of 
18 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

Clark (1930) are exposed; the lower bed is approximately 3m thick (fig. 9, locs. 31 and 32), and the upper 

bed is approximately 1.8 m thick (fig. 9, locs. 33 and 

34). Both tuffs are water laid, well bedded, laminated, 

and, locally, crossbedded. The two tuffs are separated 

by 7.6 m of tuffaceous sediments and were probably 

erupted within a short period of time of each other from 

a common vent in the Sonoma volcanic field. Samples 

of the glass separated from the two tuffs have very 

similar minor- and trace-element concentrations and 

1----------------------------------------- Central Coast Ranges------

0 

2 

> 

~ 

z 

w· 

C) 

<( Uppermost 3 w occurrence of 

1- Scutellaster 

<( interlineata 1 

~ 

X 

0 

II: 

D.. 

D.. 

<( 

4 

5 

6 

0 

~g-j 

1--'C-1 Rancholabrean-1. ·.<C. :1 - fauna3 L c: -J ~ 

I-..:...-! I L.il_, ::: r-: :-J Uncon-L--1 J ~--I formitV\- • 

- -~ 1-1 ~ r--:~ 

'0-

rh 

r~l r-_-:J 

t-_--i 

~-~ 

i 

" c:: 

·;: 

e 

> 

i 

·e 

0 

0 

1 Hall, 1966 

'Hall, 1966 3 Louderback, I 9 5 I 

0 

" 

~ 

c 

.. 

en 

• .- > '0~ 

·.;; c '0 ~ 

0 0 ou 

s: 

4 Dennis Sorg and R. J. McLaughlin, 

oral commun., 19 7 4 5 G. H. Curtis, oral commun., I 9 7 I 6 Miller, 1966 

'This study 

•savage, 1951 

c 

.. 

en 

9 Edward Mankinen, oral commun., 19 7 4 1 0 Edward Mankinen, I 9 7 2 1 1 Evemden and others, I 9 6 4 1 ' Bartow and others, 1 9 7 3 1 'Janda, 1966 1 4 Jean Firby-Durham, oral commun., 19 7 4 

0 

0 

>-

~-~ 

~; 

> 0 

u 

.. > c .. 0 

mu 

c . ~ 0 

·3 ~ eno 

0 c 

.<: ~ > ~o~ 

~ :2 5 ~ ~ 0 

ou 

~ .<: .. ..... ~ t: 0 0 

·- c u J: 

Tuff in type section of Merced Formation 

Pinole Tuff and tuffs in lower 

part of Monticello Road 

section 

e 

g ~ enz 

~~4 

~~ St. Helena 1- o-1 Rhyolite t... lL _j 

Member t.. ~j 

e;~;~~~:~ L ~ J 

3 

J: 

u 

-~ 

0 

> 

FIGURE 9.-Summary of correlations of late Cenozoic tuffs based on chemical fingerprinting and petrographic characteristics of tuffs, 

stratigraphic position and sequence, potassium-argon ages, and fossil evidence. Solid horizontal lines indicate correlation certain; 

dashed horizontal lines indicate correlation probable; and queries indicate correlation uncertain. Location of sections shown in 

figure 1. 
TUFF CORRELATIONS 19 

are hard to distinguish by these analyses (table 7). 

Neutron activation analyses, however, indicate that 

the glass composition of the two tuffs is distinct (H. R. 

Bowman and Sarna-Wojcicki, unpub. data). Similarity 

coefficients comparing these two tuffs with tuffs at 

other localities correlative with the Lawlor show greater similarity in every instance between the lower bed 

and the Lawlor eruptive unit (table 8). 

-----------------------------------+----Sacra~~~~~er~alley---ti---Sanv~~=~uin----1 

c 

.. 

"' 0 

~ 

"' 

,. >~ 

~ c 

:£ g "U 

> 

I 

"' I 

"' 

.. 

c 

0 

u 

6 

u 

>.c 

.! 0 

; ~ c 0 

·- .r:. ::> .. 

io 

0 c ...,_ 

...... , :_ __ I 

Erosional surface -- ~ 7 -·-

----------------------------------------,., ±0.53 

11

Pum~~~a~~h at 

v Ash flow tuff ~-: ~ 

Rhyolite r. ~; ~ 

Nomlaki Tuff 

Member 

(type locality) 

v r-:::i!:-J...J 

~-~, 

1.2 ±0.7 

Irvingtonian'~~ fauna t ~3 

EXPLANATION 

Volcanic flow 

Clastic sedimentary deposits 

Tuff 

Approximate position of fossil 

assemblage 

Location of chemically analyzed sample. See fig. I 

52C 

Location of samples analyzed by 

rapid scan method 

R 

Reversed magnetic polarity 

3.4 ± 0.4 

K-Ar age, in m.y. 

0 

u 
20 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

1-

2 

~ 80 a: 

~ 60 

2 

-_ 40 

>-

u 

~ 20 

::::> 

~ 0 a: 

u. Q) 

"'0 

c: 

Q) 

:0 

c: 

0 

.c: 

c: 

Q) 

~ 

(!) 

A 

Q) 

"'0 

c: 

Q) 

:0 Q) 

c: c: 

0 Q) 

.c: -E c: ~ 

~ Q) 

a. 0 > m I 

Q) Q) 

"'0 "'0 c: c: Q) Q) 

:0 :0 

c: c: 

0 0 

.c: .c: Q) 

.... 

·c, 

:l 

<( 

c: c: 

Q) ~ 

~ 0 

(!) m 

B c 

Q) Q) 

"'0 "'0 

c: c: Q) Q) 

:0 :0 Q) Q) 

c: c: c: c: Q) 0 0 Q) 

.c: .c: .c: .... .c: ~ E Q) 

c: c: Q) 

Q) .... ~ Q) .... a. ·c, Q) 

a. ·c, Q) 0 > :l :l (5 > I <( m I <( 

FIGURE 10.-Principal mafic mineral frequencies of three size fractions 

oftuffin Merced(?) Formation of Sonoma County. Frequency counts 

were made on coarse pumice-lapilli tuff (loc. 37, table 5). A, 28-48 

mesh. B, 35-100 mesh. C, 6~120 mesh. 

A 

B 

I 

Rbt I 

• 

I /-I ., I . I'-.~ •• ,, e ~-~ . FIGURE H.-Correlation of the Pinole Tuffwith the Sonoma 

Volcanics. Mutual percentages of net intensity peaks for 

Rb, Sr and Zr. Intensity rapid peaks obtained by using 

rapid X-ray fluorescence scans on powdered whole-rock 

samples treated with 10 percent HCLA, Tuffs in the Pinole 

Tuff at Rodeo, Wilson Point and south of town of Pinole. B, 

Tuffs and flows of Sonoma Volcanics, Monticello Road, east 

of Napa. Tuffs of intermediate composition containing 

bread crust scoria bombs are included within dashed lines. 

Although the refractive indices of glass for the Lawlor eruptive unit are virtually the same for all samples, 

the mafic-mineral abundances vary considerably (table 

5). These differences cannot be explained by eolian or 

hydraulic sorting since the tuff at all but one of the 

localities correlative with the Lawlor appears to be an 

ash flow. Variations in mafic-mineral frequencies may 

be due to inhomogeneous distribution of crystals in the 

magma prior to eruption. Alternatively, the Lawlor 

• \ b ....... 

EXPLANATION 

Pinole Tuff 

\ ~uffs in Ta~sajara area, 

\ \outh of Mount Diablo 

c •. 

Rb 

FIGURE 12.-Correlation of the Pinole Tuff (loc. 52A, B, and C) with 

tuffs south of Mount Diablo (loc. 63A, B, and C). Mutual percentages of net intensity peaks for Rb, Sr and Zr. Intensity peaks 

obtained by using rapid X-ray fluorescence scans on powdered 

whole-rock samples treated with 10 percent HCL Samples from a, 

gray pumice-lapilli tuff near the base of the Pinole Tuff (loc. 52A), 

and basal tuff south of Mount Diablo (loc. 63A); b, intermediate 

tuff with scoria bombs in the Pinole Tuff (loc. 52B), and similar 

finer grained tuff south of Mount Diablo (loc. 63B); c, silky silvery 

pumice lapilli from tuffaceous matrix near top of Pinole Tuff (loc. 

52A), and from near top of tuff sequence south of Mount Diablo 

(loc. 63C). 

Tuff with rounded, 

silky, silvery pumice 

lapilli (loc. 52C) 

Gray pumice-lapilli 

tuff (loc. 52A) 

Contra Costa Group 

-::-._ ~Tassajara Formation 

'-::-:::-:::-,__ ;ruff with rounded, 

~~~ ~ silky, silvery pumice 

lapilli (loc. 63C) 

0 _, Tuff with scoria lapilli 

_;;;;;;~-:::: \ (loc. 638) 

--II!=>=~;;:~:~:>_... Gray pumice-lapilli 

_... tuff ( loc. 63A) 

Green Valley FormaNeroly Formation tion of Clark (1943) 

Rodeo section Highland syncline section 

FIGURE 13.-Correlation of Pinole Tuff at Rodeo (loc. 52A-C) with 

tuffs in Highland syncline section, south of Mount Diablo (loc. 

63A-C). 

z z Q 

~2.0~2000 

.::; 1.6 ~ 1600 1- a: ~ 1.2 ~ 1200 

~0.8~ 800 

~0.4 ~ 400 

~ 0 .o ~ 0 

i 1.6 i 1600 8 1.2 Q 1200 

~ 0.8 ~ 800 

~ 0.4 ~ 400 w z ~ 0 .o ~ 0 0 z u 8 

FIGURE 14.-Concentrations of minor and trace elements in volcanic 

glass of Lawlor Tuff and its correlatives. A-C, Lawlor Tuff (fig. 5, 

locs. 22, 23, 25). D-H, Lawlor correlatives (fig. 5, locs. 17-19, 30, 

31). Iron in percent; remaining elements in parts per million. 
TUFF CORRELATIONS 21 

Tuff and its correlatives may actually represent several separate eruptions closely spaced in time. 

The Lawlor Tuff and its correlatives form an eruptive unit extending over 112 km, interbedded with the 

upper part of the Sonoma Volcanics, the base of the 

Tehama Formation, the uppermost(?) part of the Tassajara Formation, and the Livermore Gravels of Clark 

(1930). 

PUTAH AND NOMLAKI TUFF MEMBERS 

OF THE TEHAMA FORMATION 

A thin tuff bed (fig. 9, loc. 16) correlative with the 

Putah Tuff Member of the Tehama Formation of the 

western Great Valley (fig. 9, locs. 12-15) has been 

found south of Suisun Bay, stratigraphically above the 

Lawlor Tuff. This thin tuff bed is interbedded with sediments which have been referred to as the Los Medanos 

Formation (Clark, 1943) or the Wolfskill Formation 

(Weaver, 1949) but have been recently designated as 

the Tehama Formation (Sims and Sarna-Wojcicki, 

1975) on the basis of lithologic correlation with the 

type Putah Tuff Member. This correlation extends the 

maximum distance between correlative localities (locs. 

15, 16, fig. 1; fig. 9) of the Putah Tuff Member to approximately 97 km. 

The Putah Tuff Member is a composite unit of 

water-laid tuffs and probably represents several eruptions closely associated in time. Similarity coefficients 

for unweathered samples within the Putah (fig. 4, locs. 

12, 13, and 14) are 0.91, 0.89, and 0.95. Coefficients 

between these samples and a weathered sample (fig. 4, 

loc. 15) are lower (0.83, 0.86, and 0.89) possibly owing 

to higher concentrations of barium and strontium in 

the form of insoluble authigenic sulfate in the weathered sample. The southernmost correlative sample (fig. 

4, loc. 16) is most similar (similarity coefficient of 0.96) 

to the sample (fig. 4, loc. 14) obtained from the lowerz - 0 

~ ~ ::-1.6 ::!: 1600 

~ 1.2 ffi 1200 

~ 0.8 ~ 800 

~0.4 ~ 400 

z 0 .o ~ 0 

i 1.6 ~ 1600 

~ 1.2 ~ 1200 

~0.8;::: 800 

~ 0.4 ~ 4 00 Ill .... 

~ 0.0 ~ 0 

8 ~ (J 

LLL__L_ 

L_L_L_L:_ 

~~~~~~~~>~ ~~~~~~~~>~ :~;~~~~~>~ :~~~~~~~>~ 

FIGURE 15.-Concentrations of minor and trace elements in volcanic 

glass of the Putah Tuff Member, a Putah correlative south of 

Suisun Bay, and the Nomlaki Tuff Member (locs. 8-11) from its 

type locality. A-C, Putah Tuff member of Tehama Formation (locs. 

12-14). D, Putah correlative, south of Suisun Bay (loc. 16). E-H, 

Nomlaki Tuff Member of Tehama Formation (locs. 8-11). 

most emplacement unit at its type locality. 

Miller (1966) has shown that the N omlaki Tuff 

Member (fig. 1, locs. 8--11), exposed within and near 

the base of the Tehama Formation in northern Sacramento Valley, is not correlative with the Putah (locs. 

12-16) although both tuffs are about the same age3 and 

in similar stratigraphic position. Miller's conclusions 

were confirmed during this study by chemical fingerprinting of glasses from the two tuffs (fig. 15). 

Average within-unit similarity coefficients for the 

Putah and Nomlaki Tuff Members of the Tehama 

Formation are 0.90 and 0.92, respectively, while the 

average similarity coefficients between the units is 

0.63 (fig. 4). Samples of the Putah (locs. 12-15) and its 

correlative (loc. 16) are clustered at distance function 

values of about 0.050; the samples from both the Putah 

and Nomlaki Tuff Members are grouped at values of 

0.260 (fig. 5). 

3The Nomlaki has been dated by potassium-argon methods at 3.3±0.4 m.y. (Evernden and 

others, 1964); the Putah at 3.3±0.1 m.y. (Miller, 1966; G. H. Curtis, oral commun., 1971). 

TABLE 6.-Average minor- and trace-element composition of volcanic glass of the physically continuous Lawlor Tuff unit compared with 

average compositions of correlative but physically separate outcrop localities 

[Sample localities shown in figure 1. Concentrations of iron in percent; all other concentrations in parts per million] 

Lawlor Tuff, average of seven analyses (locs. 22-29) _____ _ 

Lawlor correlatives, average of 

eleven analyses (locs. 17-21 

29-34) ------------- ' 

Fe Ti 

1.73 1160 

1.72 1193 

Ba. Mn Zr 

795 444 311 

791 448 311 

Rb Sr Zn y Nb 

147 56 58 23 19 

146 64 60 29 15 

Ga Cu 

17 9 

17 11 

TABLE 7 .-Trace- and minor-element analyses of volcanic glass of two thin water-laid tuffs in the Livermore Gravels of Clark (1930) 

[Concentrations of iron in percent; all other concentrations in parts per million] 

Sample Joe. Fe Ti Ba Mn Zr Rb Sr Zn y Ga Nb Cu 

Upper bed ____________ 34 1.82 1137 718 517 290 137 82 56 24 13 9 8 do ________________ 33 1.82 1136 684 502 306 135 74 60 23 14 21 13 

Lower bed ____________ 32 1.75 1213 733 431 297 143 60 58 25 17 16 11 do ________________ 31 1.70 1134 809 422 305 148 71 59 29 17 14 9 

Ni 

11 

11 

Ni 

8 

11 

13 

12 
22 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

TABLE B.-Average similarity coefficients comparing trace- and 

minor-element analyses of volcanic glass of two thin water-laid tuffs 

in the Livermore Gravels of Clark (1930) with all other outcrop 

localities of the Lawlor Tuff 

Tuffs in Livermore Gravels 

Upper bed _________________________ _ Lower bed _________________________ _ 

South of Mount Diablo 

(sanaple 30) _____________________ _ 

South of Mount Diablo (sanaple 29) _____________________ _ 

Lawlor Tuff (sanaples 22-28) _______ _ 

Southeast of Sononaa Volcanics • (sanaples 19-21) _________________ _ 

Sononaa Volcanics Monticello Road (sanaple 17) _____________________ _ 

Sononaa Volcanics near Schellville (sanaple 18) _____________________ _ 

Upper bed 

Oocs. 33, 34) 

10.95 

.92 

.90 

.90 

.90 

.89 

.86 

.93 

1Sample from locality 33 compared with sample from locality 34. 2 Sample from locality 31 compared with sample from locality 32. 

Lower bed 

Oocs. 31, 32) 

0.92 

2 .96 

.94 

.93 

.95 

.94 

.91 

.94 

Mafic-mineral frequencies of samples of the Putah 

Tuff Member (table 3) are similar, as are the refractive 

indices of glass (table 5); the same is true for the Nomlaki Tuff Member. There are significant differences in 

mafic-mineral frequencies between the two tuffs; however, differences in refractive indices of glass between 

the Putah and Nomlaki are very slight (table 5). 

Trace- and minor-element composition of the volcanic glass in the Putah Tuff Member is very similar to 

that of the tuff in the Merced(?) Formation of Sonoma 

County. Although the average similarity coefficient 

between the Putah and the tuff in the Merced(?) Formation of Sonoma County is 0.85, individual sample 

Sample 

loc. 14 13 12 15 

Putah Tuff Member of {" l Tehama Formation 13 .9 6 1 (loc. 12-15) and a 12 .9 3 .9 4 1 correlative tuff (loc. 1 5 .9 2 .91 .9 0 1 16) 16 .9 7 .9 2 .9 2 .9 3 

pairs from the two tuffs may have similarity coefficients as high as 0.91 (fig. 4) for the eight-element 

comparison used. It was not possible to distinguish 

clearly between the two tuffs using cluster analysis on 

the main group of eight elements or on a second run 

using seven elements, omitting strontium. This is due 

both to the chemical similarity of the glass of the two 

tuffs and to a rather high variability for manganese, 

rubidium, and strontium. It appears that chemical variability of the glass for some elements can differ for 

different eruptive units. Elements with consistent concentrations in one unit may be more variable in 

another. For instance, amounts of rubidium in the 

Lawlor Tuff are very consistent, while those in the 

Putah and the tuff in the Merced(?) Formation of 

Sonoma County are more variable. The two last tuffs 

are known from independent evidence to be of different 

age: the Putah was dated by potassium-argon analysis 

at 3.3±0.1 m.y. (fig. 9, locs. 12-15; G. H. Curtis, oral 

commun., 1971), while the tuff in the Merced(?) Formation of Sonoma County was dated at 5.7±0.6 (fig. 9, 

locs. 35-37) and 6.1±0.1 m.y. (fig. 9, locs. 38--40). Calculations of similarity coefficients and cluster analysis 

were performed on samples of these tuffs using only 

the four most consistent variables: iron, titanium, 

barium, and zirconium (fig. 16). By this procedure it 

was possible to distinguish differences in composition 

between the two tuffs. On the basis of these calculations, the average similarity coefficient between samples of the Putah and the tuff in the Merced(?) Formation is 0.82, while the average internal within-unit 

similarity coefficient for both units was 0.93. The highest similarity coefficient for a pair of samples from the 

two tuffs was 0.88. By means of cluster analysis with 

16 35 36 37 38 39 4-0 4 1 42 43 44 

/" 

.8 7 .8 8 .8 3 .8 0 .8 4 1 36 .8 5 .8 6 .81 .7 6 .8 3 .9 7 1 Tuff in the Merced(?) 37 .8 3 .8 4 . 7 9 • 7 6 .8 0 .8 9 .9 2 1 

Formation of Sonoma 

County 38 .8 5 .8 5 .8 1 . 7 8 .8 3 .9 0 .9 2 .9 4 1 

39 .8 4 .8 6 .8 1 . 7 8 .8 2 .9 3 .9 4 .9 3 .9 4 1 

40 .8 3 .8 4 . 7 9 . 7 6 .8 0 .9 5 .9 4 .9 5 .9 7 .9 7 

Tuff in the Petaluma { 41 .8 0 .8 1 • 7 7 . 74 . 7 8 .9 0 .9 2 .9 5 .91t .9 5 .9 6 1 Formation 4 2 .8 4 .8 5 .8 0 . 7 7 .8 1 .9 1 .9 4 .9 6 .9 7 .9 4 .9 7 .9 5 

Tuff south of Mount -43 .8 4 .8 4 .8 0 . 7 8 .8 1 .9 4 .9 5 .91 .9 0 .9 4 .9 2 .9 2 .9 2 Diablo 

Tuff near Lafayette - 44 .8 7 .8 7 .85 .81 .8 7 .9 4 .9 2 .85 .85 .8 7 .8 6 .8 5 .8 6 .92 1 

FIGURE 16.-Sinailarity coefficient naatrix conaparing trace- and nainor-elenaent analyses 

of glass sanaples of the Putah Tuff Menaber of Tehanaa Fornaation (locs. 12-16) and the 

tuff in the Merced(?) Fornaation of Sononaa County. Calculations of coefficients for the 

four naost consistent elenaents, Fe, Ti, Ba, and Zr. 
TUFF CORRELATIONS 23 

SAMPLE NUMBER 

m ... 00 1\) 

0 r---~-.----r--r----r-,-,----,---,-,-.---.-----.--------.-r----o-~1 ----, ...........,._. 

z 

0 

i= 

0.1 f-

< 

CD 

;::. 

Fine 

ash 

-----

1 

~ 

I 

Coarse 

pumice 

:I 

-t 

~ 

-

u 

z 

::J 

u.. 0.2 ,- w 

u 

z 

Putah Tuffl 

Member of 

Tehama 

Formation 

Tuff in Merced(?) 

Formation of 

Sonoma County 

Ql 

Qj' 

iii 

, -

<( 

1-

(/) 

Q 

0.3-

0.4-

...____,___... 

Pinole 

Tuff 

0 

3 

Dl 

.... 

a· 

:I 

Tuff in 

Tassajara 

Formation 

-

-

FIGURE 17 .-Dendrogram from cluster analysis of trace- and minor-element data, using distance function, for Fe, Ti, Ba, and Zr. 

See text for explanation. Sample number is same as locality number shown in figure 1. 

only these four elements, it was also possible to distinguish between the two eruptive units. To emphasize 

differences between samples, the cluster analysis in 

this instance was run only on samples of the Putah 

Tuff Member, the tuff in the Merced(?) Formation of I 

Sonoma County, plus several close compositional relatives (fig. 17). 

TUFF IN THE TYPE SECTION OF THE MERCED FORMATION 

A correlative of the tuff in the type section of the 

Merced Formation of western San Francisco peninsula 

(fig. 1, loc. 1) has been obtained from a drill hole 10 

miles to the east, in sediments of the Alameda Formation beneath San Francisco Bay (fig. 1, loc. 2). The 

similarity coefficient for analyses of seven elements 

(titanium, barium, manganese, zirconium, rubidium, 

strontium, and zinc) in the volcanic glass of these two 

tuffs is 0.93. No iron analyses have as yet been made 

on these two samples, so they are not included in the 

cluster analysis (fig. 5). A tuff similar to that in the 

Merced and Alameda Formations was exposed in a 

trench in the Santa Clara Formation, in the town of 

Woodside (fig. 1, locs. 3 and 4) approximately 20 miles 

south of the former localities. Though the tuff at the 

Woodside locality has the same mineralogy as the tuffs 

in the Merced and Alameda Formations (table 5), earlier analyses had indicated that there are considerable 

differences in titanium and manganese content of the 
24 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

glass. Similarity coefficients between samples 1 and 2 

on the one hand, and 3 and 4 on the other, range between 0.81 and 0.87, with an average of 0.84. On the 

basis of the earlier analyses alone, the correlation of 

the tuff in the Santa Clara Formation with the tuff in 

the Merced and Alameda Formations must be considered uncertain. However, new analyses by Brent 

Fabbi, U.S. Geological Survey, Menlo Park, of tuff in 

the Santa Clara Formation and in the type Merced 

Formation show a high similarity coefficient of 0.97, 

indicating that the tuffs are correlative (Fabbi and 

Sarna-Wojcicki, unpub. data). The earlier analyses are 

probably inaccurate owing to incomplete separation of 

crystals and lithic fragments from the fine-grained volcanic ash. 

The tuff in the Merced Formation is fine grained and 

probably was deposited by water. It has no coarsegrained facies within the central Coast Ranges that 

might indicate local eruptive sources, and it differes in 

glass chemistry from tuffs of local provenance. For instance, the similarity coefficients between the tuff in 

the Merced Formation (sample 1) and tuffs of local 

Coast Range provenance, such as the Putah Tuff 

Member (samples 12-15), the Lawlor Tuff (samples 

22-28), and the tuff in the Merced(?) Formation of 

Sonoma County (34-40), are low (0.59, 0.62, and 0.54, 

respectively). The similarity coefficient between the 

tuff in the Merced Formation and a tuff-breccia in Anderson's (1936) Cache Formation within the Clear 

Lake volcanic field (sample 71) is also low (0.66). This 

indicates that the Merced tuff probably did not have its 

source in the Clear Lake area either, even though the 

period of volcanism in the Clear Lake area encompasses the age of the tuff in the type ·section of the Merced 

Formation (G. H. Curtis, oral commun., 1971). A 

1.5±0.8 m.y.-potassium-argon date on tuff in the 

Merced Formation (fig. 9, loc. 1; Hall, 1966) is younger 

than the youngest known potassium-argon age obtained on rocks of the Sonoma Volcanics (2.9 m.y., 

Mankinen, 1972). These ages, if correct, further 

exclude the Sonoma volcanic field as a likely source of 

this tuff. However, the similarity coefficient between 

the tuff in the type section of the Merced Formation 

and the average for the Nomlaki Tuff Member, a unit 

of known Cascade Range provenance (Anderson and 

Russell, 1939; Lydon, 1967), is 0.81, a value typical of 

provincial ((relatives," suggesting that the tuff in the 

type section of the Merced Formation was erupted in 

this volcanic province. 

On the basis of glass chemistry and petrography, as 

well as radiometric ages, it seems likely that the 

source of the tuff in the type Merced, Alameda, and 

Santa Clara Formations was in the southern Cascade · 

Range, near the town of Mineral, where two Pleistocene pumice ash-flow units crop out (Wilson, 1961). 

These two pumice ash-flow units yielded scattered 

potassium-argon ages ranging from 0.26 to 1.1 m.y. 

(Gilbert, 1969).4 Though earlier analyses have indicated that the tuff in the type Merced Formation (fig. 9, 

loc. 1) is not correlative with one of the ash-flow units, 

exposed near Lassen Lodge (fig. 1, loc. 7), new analyses 

(Fabbi and Sarna-Wojcicki, unpub. data) indicate that 

the tuff in the type section of the Merced Formation is 

probably correlative with another ash-flow unit, exposed near Manton Lodge; the similarity coefficient for 

these two tuffs is 0.90. 

The mafic mineralogy of the tuff in the type section 

of the Merced Formation (fig. 1, loc. 1) and its local 

correlatives (fig. 1, locs. 2-4) is the same as that of the 

pumice ash flows near Mineral (fig. 1, loc. 7). Both 

groups of tuffs have dark-green hornblende and pale 

pleochroic hypersthene as the principal mafic phenocrysts (table 5). The mafic mineral frequencies are different; the pumice tuff at Mineral has more 

hypersthene and less hornblende than the tuff in the 

Merced Formation. These differences can be explained 

by either hydraulic or eolian sorting of these minerals 

away from their source. Fragments of the thinner, 

tabular, and more cleavable hornblende would probably be carried further by wind or water than the stubbier, equant hypersthene crystals. The refractive indices of glass of the tuff in the type section of the Merced Formation and its correlatives are nearly the same 

as those at Mineral (table 5). 

DISCUSSION 

Five widespread late Cenozoic tuffs, ranging in age 

from approximately 1 to 6 m.y., have been identified in 

the central Coast Ranges of California by means of 

chemical analyses of volcanic glass, potassium-argon 

dating, and petrographic and stratigraphic evidence 

(fig. 9). These units provide five temporal horizons that 

make it possible to correlate late Cenozoic volcanic, 

alluvial, lacustrine, and marine deposits. 

Four of the five widespread units were erupted from 

source areas within the central Coast Ranges, probably 

within the area of the Sonoma volcanic field. These 

tuffs include the tuff in the Merced(?) Formation of 

Sonoma County, the Pinole Tuff, the Lawlor Tuff, and 

the Putah Tuff Member of the Tehama Formation. 

Several tuffs in the central Coast Ranges (fig. 1, locs. 

1-6), including one of the five widespread units, the 

tuff in the type section of the Merced Formation (fig. 1, 

4The 1.5±0.75-m.y.-date on the tuff in the type section of the Merced Formation is very 

imprecise owing to high content of atmospheric argon and possible detrital contamination. 

Ages on its probable chemical correlative in the southern Cascade Range are scattered, 

ranging from 0.26 to 1.1 m.y. (Gilbert, 1969). Consequently, the age of the tuff in the Merced 

Formation is not well determined but is probably about 1 m.y. or younger. 
DISCUSSION 25 

locs. 1-4), were probably erupted in the southern Cascade Range. All are fine-grained water-laid or ash-fall 

tuffs that, unlike tuffs of local derivation, have no 

coarse-grained correlatives in the central Coast 

Ranges. These tuffs are similar in composition to tuffs 

of the southern Cascade Range and differ from tuffs of 

the Sonoma volcanic field, the Clear Lake area, and the 

Sierra Nevada. For example, similarity coefficients for 

Coast Range tuffs of inferred Cascade Range provenance compared with tuffs of the Sonoma volcanic field 

range from 0.50 to 0.66, with an average of0.61 (fig. 4). 

Within-province similarity coefficients for tuffs of the 

Sonoma volcanic field average 0. 75, while coefficients 

for southern Cascade tuffs compared with Coast Range 

tuffs of suspected Cascade Range origin are also 0.75, 

strongly suggesting that the latter two sets of tuffs are 

provincial relatives. 

The tuff in the type section of the Merced Formation 

is 15--60 em thick wherever it is found in the bay area 

and contains glass-coated green hornblende laths and 

glass shards up to 0.5 mm long. It is unlikely that the 

ash was carried by air 320 km to the Merced embayment, in view of its considerable thickness and relatively coarse maximum crystal and shard size. Furthermore, prevailing wind directions must necessarily 

have been northerly, rather than westerly as they are 

at present, in order to carry the tuff southward by 

wind. The purity of the tuff at some of its exposures 

may be due to the low density of the glass shardsa drainage basin blanketed with ash of low density 

will probably first move that material before moving the denser normal bedload. Furthermore, if the 

tuff had been carried by wind to the Merced depositional basin, the tephra lens of such an eruption would 

be at least 320 km long and several tens of kilometres wide. The record of such an eruption would likely 

be preserved throughout the Pleistocene deposits of 

the northern Great Valley, yet no such tuff has been 

reported. 

If this line of reasoning is correct and the tuff in the 

Merced Formation was erupted in the southern Cascade Range, then it seems most likely that the ash was 

brought to the marine Merced embayment by the ancestral Sacramento River, rather than transported by 

air. The presence of a water-transported tuff of Cascade Range provenance in the marine Merced Formation therefore indicates that Great Valley drainage to 

the Pacific Ocean in the vicinity of the bay area had 

been established by late Merced time, or about 1 m.y. 

ago. 

Whether the tuff in the type section of the Merced 

Formation was actually transported to the Merced embayment by air or water, another line of evidence indicates that Great Valley drainage to the ocean in the 

vicinity of the San Francisco Bay area had been established by the time the tuff was deposited. Hall (1966) 

studied the mineralogy of sediments in the type section 

of the Merced Formation and found an abrupt 

mineralogical change in the upper part of the formation. Heavy-mineral grains below this change are of 

local provenance, while those above this change indicate a sudden influx of Great Valley sediments into the 

Merced embayment. The tuff in the type section of the 

Merced Formation lies 46 m stratigraphically above 

this mineral change. 

The Putah and N omlaki Tuff Members of the 

Tehama Formation and the Lawlor and Pinole Tuffs 

(3.3, 3.4, 4.0 and 5.2 m.y. old, respectively) are restricted to the eastern part of the Coast Ranges and 

have not been found in coeval marine formations in the 

western part of the central Coast Ranges, suggesting 

that a north-south drainage divide existed in the central part of the Coast Ranges prior to about 1 m.y. ago 

and that Great Valley drainage flowed to a southerly 

outlet during this period. A southerly connection between the ocean and the Great Valley was in existence 

until late Pliocene time, as indicated by the presence of 

the upper Pliocene continental and marine San Joaquin Formation in the southern San Joaquin Valley 

(Woodring and others, 1940). Subsequently, this 

southerly connection was closed off, as evidenced by 

the presence of the extensive lacustrine Corcoran Clay 

Member of the Tulare Formation beneath Pleistocene 

alluvium in the San Joaquin Valley (Janda, 1965). The 

presence of this unit in the southern Great Valley indicates that Great Valley drainage was temporarily 

ponded. The Corcoran interfingers to the west withalluvial deposits of the Tulare Formation (Wahrhaftig 

and Birman, 1965), which in turn overlies the marine 

San Joaquin Formation. The exact age range of the 

Corcoran is not known, but the member is overlain at 

Friant by ash and pumice dated at 0.6±0.02 m.y. 

(Janda, 1965). The presence of Pleistocene alluvial deposits above the Corcoran suggests that drainage from 

the Great Valley to the ocean had been reestablished, 

probably via a more northerly outlet. The drainage 

change from a southerly outlet to one near the present 

site of San Francisco Bay probably took place sometime after 3.3 m.y. ago, after the eruption of the Nomlaki and Putah Tuff Members, but before about 0.6 

m.y. ago. 

The texture of the Lawlor Tuff at all its outcrop 

localities, except for the southernmost locality south of 

Livermore (fig. 1, locs. 31-34), is typical of an ash flow. 

No stratification, lamination, or vertical size gradients 

have been observed. The presence of the ash-flow phase 

of the Lawlor Tuff south of Mount Diablo suggests that 

neither Suisun Bay nor Mount Diablo existed at the 
26 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

time of the eruption, about 4 m.y. ago, and that these 

features were formed later. The tuff was erupted in the 

southeastern Sonoma volcanic field, where its coarsest 

facies are found, and it would have been difficult or 

impossible for the ash flow to cross these topographic 

features had they existed at the time of the eruption. A 

gentle topography or a slope component probably 

existed along the line between the southeastern margin of the Sonoma volcanic field and the southernmost 

exposure of the ash flow, south of Mount Diablo (fig. 1, 

loc. 30). 

Correlative localities of the tuff in the Merced(?) 

Formation of Sonoma County trend diagonally across 

the central Coast Ranges (fig. 9) and correlative tuffs 

are found both in the southeast, interbedded with continental sediments, as well as in the northwest part of 

the study area, interbedded with marine deposits. 

Heavy minerals in sediments of the Merced(?) Formation of Sonoma County are of local Coast Range provenance; there is no evidence that Great Valley material 

was brought into the embayment during Merced(?) 

time (Johnson, 1934). 

Correlatives of some of the tuffs examined in this 

study, such as the Putah Tuff Member of the Tehama 

Formation and the Lawlor Tuff, are undoubtedly preserved to the east under the Quaternary sediments of 

the Great Valley, beneath which both tuffs dip. 

Likewise, new correlative localities of the Putah Tuff 

Member of the Tehama Formation and the Lawlor and 

Pinole Tuffs will quite likely be found in the future in 

the Sonoma volcanic field, which has been only 

peripherally examined in the present study. The tuff in 

the Merced(?) Formation of Sonoma County, the oldest 

of the five major eruptive units, may antedate the main 

period of volcanism in the Sonoma volcanic field or 

may represent the inception of that volcanism and lie 

buried under the younger volcanic deposits. 

Radiometric ages on the youngest tuffs and flows 

erupted in the Sonoma volcanic field cluster in the 

range 3.0 to 4.0 m.y. (2.9, 3.3, 3.3, 3.8, 4.0, 4.0, 4.2 

m.y.). This cluster of dates defines a maximum age for 

a late Pliocene or early Pleistocene orogeny that deformed and uplifted the formations containing the volcanic units. The eastward shift of progressively 

younger eruptive units from the Sonoma volcanic field 

at the western margin of the Great Valley (fig. 8) 

suggests that uplift and volcanism were proceeding 

simultaneously. This orogeny, or perhaps displacement on the San Andreas fault, or both, closed off the 

sea connection between the southern Great Valley and 

the ocean. Sometime after this pulse of deformation 

and perhaps as a consequence of it, drainage in the 

Great Valley found an outlet to the ocean in the vicinity of the San Francisco Bay area. 

DESCRIPTION OF UNITS 

THICK SECTIONS OF VOLCANIC DEPOSITS 

SONOMA VOLCANICS, MONTICELLO ROAD SECTION, 

SOUTHEASTERN PART OF SONOMA VOLCANIC FIELD, 

EAST OF NAPA (FIGS. 1-3, 9, LOCS. 17, 4S-51) 

Approximately 1 m of gray pumice lapilli tuff at the 

base is overlain by at least 12 m of a dark andesitic tuff 

containing round scoriaceous lapilli and bombs as 

much as 20 em in diameter. This is overlain in turn by 

approximately 240m of light-gray to cream dacitic vitric pumice-lapilli and lithic tuff, which is locally 

welded and intruded in at least one place by andesitic 

dikes. 

This group of beds is overlain by approximately 215 

m of massive flow-banded dacite. The uppermost 3m of 

the unit consists of gray porphyritic perlite, overlain 

by a jumble of angular dacite and perlite boulders in a 

tuffaceotJ.s breccia matrix derived by infilling from the 

overlying unit. 

The dacite flows are overlain by 150 m of coarse dacitic pumice lapilli and pumice blocks as much as 40 em 

long. The pumice is gray, rather hard, and dense. Interstices between the pumice clasts are often filled 

with a creamy soft opaline(?) material and darkorange-brown clay (nontronitic montmorillonite). The 

top of this unit is locally channeled, and the channels 

contain boulders of andesite, dacite, and rhyolite as 

much as 1m in diameter. 

The pumice-block breccia is overlain by approximately 18m of a lithic pumice-lapilli tuff, with coarse 

boulders of andesite, dacite, and rhyolite at its base. 

This unit is overlain in turn by about 24 m of tuffaceous sediments containing rounded pumice lapilli. A 

reddish paleosoil(?) is developed at the top. The unit is 

channeled at the top; the channels contain pebbles of 

basalt, andesite, andesite scoria, dacite, and rhyolite. 

SONOMA VOLCANICS, SOUTHERNMOST SONOMA 

VOLCANIC FIELD, NORTH OF SUISUN BAY (GOODYEAR 

STATION SECTION; FIG. 1, LOCS. 19-21) 

A sequence of pumice-lapilli and pumice bomb tuffs, 

approximately 150m thick, is exposed in a deep long 

roadcut along the frontage road of California Highway 

21, just north of Suisun Bay. The entire sequence is 

massive and shows no stratification except for local 

changes in particle size and a few zones, presumably at 

the base of massive pumice-ash flows, where some of 

the larger lithic fragments have accumulated. The 

tuffs are overlain by a jointed andesite flow 15m thick. 

PINOLE TUFF (LOCS. 52-62) 

The Pinole Tuff is a sequence of deformed layered 
DESCRIPTION OF UNITS 27 

pyroclastic deposits and tuffaceous sediments about 

270 m thick, exposed northwest and south of the town 

of Pinole and at the town of Rodeo south of San Pablo 

Bay (Lawson, 1914; Vitt, 1936; Weaver, 1949). The 

Pinole Tuff overlies andesitic sandstone of the N eroly 

Formation of late Miocene age. There is no angular 

discordance between the two formations at Rodeo, but 

south and west of Pinole the tuffs rest on tuffaceous 

shales of the lower part of the Monterey Formation 

(middle Miocene). The section at Rodeo is thickest and 

at the time of Vitt's work (1936) was well exposed. At 

present much of the section has been concealed by construction. 

A similar though somewhat thinner section is exposed northwest of Pinole near Wilson Point, along the 

south shore of San Pablo Bay and south of Pinole 

where, according to Vitt, the upper part of the tuff is 

cut off by the Pinole fault. The Pinole Tuff is actually 

composed of several tuffs, breccias, and tuffaceous deposits. These various units differ considerably in bedding structures, textures, mineralogy, and chemistry. 

The Pinole Tuff does not contain any flow rocks or 

intrusive rocks. It does, however, contain scoriaceous 

andesite bombs as much as 1 m in diameter in the 

utuffaceous breccia" unit, which suggests that at least 

this particular unit was deposited fairly close to source. 

There are strong similarities between the Pinole section and the Monticello Road section to the north. Both 

sections contain fine- to medium-grained gray pumicelapilli tuffs near the base, are overlain by darkercolored andesitic tuffs containing scoria bombs, and 

are overlain in turn by lighter-colored more silicic 

pyroclastic deposits. 

WIDESPREAD TUFFS INTERBEDDED WITH 

DETRITAL SEDIMENTARY DEPOSITS 

WESTERN PART OF THE MAIN STUDY AREA 

TUFF IN THE TYPE SECTION OF 

THE MERCED FORMATION (LOC. 1) 

On the west side of the San Francisco peninsula, a 

30- to 60-cm-thick fine-grained hornblende-bearing 

vitric tuff is exposed in the cliffs along the beach just 

south of Fleishhacker Zoo, in the type section of the 

Merced Formation (loc. 1) (Lawson, 1914; Hall, 1966). 

The tuff was deposited in marine water, is crossbedded 

and laminated, and in places contains considerable 

amounts of detrital material. Another outcrop of apparently the same tuff is exposed in a steep hillside 

north ofWestmoor School southeast of locality 1. A tuff 

similar to the one in the Merced Formation has been 

found in a drill hole at 83 m below sea level in the 

Alameda Formation beneath west central San Francisco Bay (Trask and Rolston, 1951; fig. 1, loc. 2). Still 

another tuff (locs. 3 and 4) similar to the one in the 

type section of the Merced Formation has been uncovered 35 km to the southeast of locality 1, in the 

Santa Clara Formation, in a trench cut near the San 

Andreas fault zone at the town of Woodside. 

Louderback (1951) and Hall (1966) suggested that 

the tuff in the type section of the Merced Formation 

and the tuff in the Alameda Formation are the same on 

the basis of refractive indices of glass, grain size, and 

mineralogy. Hall further suggested that the tuff may 

have had its source in the southern Cascade Range, 

near the town of Mineral, where two extensive pumice 

tuff flows crop out (Wilson, 1961; Gilbert, 1969). 

TUFF IN THE MERCED(?) AND PETALUMA FORMATIONS 

OF SONOMA COUNTY (LOCS. 35-42) 

Northwest of San Francisco in Sonoma County, a 

tuff of variable texture and grain size is interbedded 

with the lower and upper Pliocene Merced(?) Formation (Johnson, 1934; Weaver, 1949; Travis, 1952; Bartow and Addicott, 1971) .. This tuff is exposed almost 

continuously for a distance of 14.5 km. The tuff, deposited by water and in places containing detritus and 

invertebrate marine fossils, is coarsest in the east near 

Trenton, where pumice bombs as much as 20 em in 

diameter are found, and near Roblar, where pumice 

cobbles several centimetres in diameter are found. To 

the west it becomes progressively finer and contains 

more detrital contamination. Johnson (1934), Louderboack (1951), and Hall (1966) have pointed out that the 

tuff in the Merced(?) Formation of Sonoma County and 

the tuff in the type section of the Merced Formation 

differ in mineralogy and refractive indices of glass and 

are consequently not correlative. The source of the tuff 

in the Merced(?) Formation of Sonoma County is not 

known, although judging from the coarse pyroclasts it 

contains, the source must have been nearby. 

Still another exposure of a similar tuff has been 

found at Sears Point, in the Petaluma Formation, just 

north of San Pablo Bay, approximately 43 km southeast of the Trenton locality (Bartow and others, 1973). 

This tuff, also deposited by water, ranges in grain size 

from vitric-ash tuff to pumice-lapilli tuff. 

SOUTHEASTERN PART OF THE MAIN STUDY AREA 

In the east and southeast part of the study area (fig. 

1), there are several tuffs exposed in late Cenozoic deposits most of which have been correlated with the 

Pinole Tuff by Vitt (1936) on the basis of refractive 

indices of glass and heavy mineralogy. However, many 

of these correlations are not justified since the refractive indices of glass and heavy-mineral species and 

frequencies differ widely between many of these tuffs. 
28 CORRELATION OF LATE CENOZOIC TUFFS, COAST RANGES, CALIFORNIA 

LAWLOR TUFF (LOCS. 22-28) 

The Lawlor Tuff, named for its type locality in Lawlor Ravine, N% sec. 23, T. 2 N., R. 1 W., Contra Costa 

County, by Weaver (1949), is well exposed in the hills 

northeast of Mount Diablo. It trends east-west from 

near Markley Canyon in the east to near Port Chicago 

in the west, a distance of 19 km. The tuff is exposed for 

much of this distance, except for a short interval near 

Arnold Industrial Highway where, according to Patten 

(1947), it is cut off from its eastern and western exposures by normal faults. 

The tuff is approximately 18 m thick south of Port 

Chicago and thins to the east, toward Markley Canyon, 

where it is about 4.5 m thick. The unit is massive and 

contains coarse light-bluish-gray to white pumice 

lapilli and some angular lithic volcanic fragments that 

are primarily bluish gray and brown felsite. The 

pumice lapilli are angular and tightly interlocked. The 

size of the lapilli decreases from west to east, from 

about 5-8 em in maximum diameter near Port Chicago 

to about 0.6-1.2 em at Markley Canyon. There is no 

obvious sorting or stratification in the unit. Textural 

features indicate that the Lawlor was most probably a 

pumice ash flow. 

The Lawlor Tuff disconformably overlies the upper 

Miocene Neroly .Formation along much of its length 

except in the vicinity of Arnold Industrial Highway, 

where it rests unconformably on the upper Eocene 

Markley Sandstone Member of the Kreyenhagen Formation. The uppermost part of the N eroly Formation 

co'ntains white reworked pumice-lapilli tuff. 

The Lawlor Tuff is overlain by the upper Pliocene 

Tehama Formation (Sims and Sarna-Wojcicki, 1975), 

formerly the Los Medanos Formation of Clark (1943, p. 

189) or Wolfskill Formation of Weaver (1949), consisting primarily of sand and gravel. A thin discontinuous 

tuff, approximately 0.6 to 1.2 m thick, is about 8-9 m 

stratigraphically above the Lawlor Tuff west of Arnold 

Industrial Highway (loc. 16). The tuff contains rounded 

pumice lapilli in a matrix of fine glass shards. On the 

basis of its trace- and minor-element chemistry, mafic 

phenocryst abundances, and index of refraction, it is 

correlated with the Putah Tuff. 

SOUTH OF MOUNT DIABLO 

In the foothills immediately south of Mount Diablo 

several tuffs are interbedded with tightly folded late 

Cenozoic continental deposits. These strata are shown 

on the Geologic Map of California (Rogers, 1966) as 

~~middle and/or lower Pliocene nonmarine sedimentary 

rocks," but the middle and upper part of the section 

may be as young as late Pliocene or Pleistocene, at 

least partly contemporaneous with the Pliocene and 

Pleistocene Livermore Gravels of Clark (1930) to the 

south, on the basis of correlation of the Lawlor eruptive 

unit. 

The relative stratigraphic position of at least four 

tuff units is known. Perhaps three or four additional 

tuffs have been distinguished during the present study 

on the basis of their chemistry and mineralogy. Most of 

the tuffs are discontinuous. Oestreich (1958) has attempted to use the tuffs as marker beds for the contact 

between the Tassajara Formation and the Green Valley Formation of Clark (1943). Vitt (1936), again without sufficient justification, has correlated all but one of 

these tuffs with the Pinole Tuff on the basis of refractive indices of glass and heavy-mineral evidence. 

Three of the tuffs (fig. 13) are exposed along Collier 

Canyon Road, along the flank of Highland syncline 

(Oestreich, 1958) (loc. 63, 63A-C). -

The tuff at the base of the Highland syncline section 

(loc. 63A) is unsorted and contains pumice lapilli with 

some angular lithic fragments. This unit, about 2 m 

thick at most, is probably the peripheral part of an 

extensive ash flow. The other two tuffs in this section 

(loc. 63B, C) are reworked pumice-lapilli tuffs and tuffaceous sediments. The exposed thicknesses of these 

tuffs and tuffaceous sediments range from about 1 to 2 

m, but the units are probably thicker. These three tuffs 

(loc. 63A-C) are correlated with the Pinole Tuff (loc. 

52A-C) on the basis of rapid-scan data. A fourth and 

youngest tuff is exposed farther west, about 1 mile east 

of Danville, stratigraphically far above the other tuffs 

in the area (loc. 5). It is a massive very fine grained 

biotite-hornblende vitric tuff, about 1 m thick, and is 

probably a product of direct ash fall. Th~ minor- an_d 

trace-element composition of this tuff indicates that It 

was probably erupted in the southern Cascade Range. 

On the south side of Livermore Valley, in the Tesla 

quadrangle, two tuffs are interbedded with the Livermore Gravels5 of Clark (1930) (fig. 9, locs. 31-34). 

These tuffs, first mentioned by Huey (1948), are well 

exposed at only one locality, a roadcut in a ridge between Arroyo Mocho and Arroyo del Vaile. The lower 

tuff is approximately 3 m thick, the upper about 2 m 

thick. The tuffs are separated by a zone of tuffaceous 

sediments approximately 8 m thick. The tuffs were deposited by water: They are well stratified and show 

graded bedding, laminations, and soft-sediment deformation structures. A lower, massive part of both 

tuffs, up to about 30 em thick, may be directly watersThe dominant clastic sediments exposed here are clay and mud, with some lenses of 

gravel. 
REFERENCES CITED 29 

laid ash-fall material. The lower bed is correlated with here confirmed by trace- and minor-element chemistry 

the Lawlor Tuff on the basis of trace-element chemis- of the glasses in the tuffs. 

try and petrographic criteria. 

NORTHEASTERN PART OF THE STUDY AREA 

(FIG. 9, LOCS. 8-11, 12-15) 

Two extensive units, the Nomlaki and Putah Tuff 

Members of the Tehama Formation, are interbedded 

with continental deposits of the late Pliocene Tehama 

Formation along the foothills bordering the west side 

of Sacramento Valley. 

PUTAH TUFF MEMBER OF THE TEHAMA FORMATION 

(FIGS. 1, 9, LOCS. 12-15) 

The Putah Tuff Member (Sims and Sarna-Wojcicki, 

1975) has been well described by Miller (1966). According to Miller, the Putah crops out almost continuously 

from near Vacaville in the south to a few kilometres 

south of the Yolo-Colusa County boundary, a distance 

of about 64 km. The tuff is thickest (about 15 m) south 

of its type locality, Putah Creek, thinning to the north 

and south. The tuff is well stratified and in places contains rounded hard pumice lapilli, together with detrital sedimentary material, indicating that it is water 

deposited or reworked. 

NOMLAKI TUFF MEMBER OF THE TEHAMA FORMATION 

(FIGS. 1, 9, LOCS. 8-11) 

The Nomlaki Tuff Member is exposed discontinuously along the western side of northern Sacramento 

Valley for a distance of approximately 93 km, from 

north ofNye Creek to Cottonwood Creek. This tuff has 

been described by Russell (1931), Anderson and Russell (1939), Lydon (1967), and Miller (1966). 

At its type locality at the former headquarters of the 

old N omlaki Indian Reservation, about 6 miles northeast of Paskenta, the tuff is approximately 4 m thick 

but elsewhere ranges in thickness from about 1 m to 

about 30m. 

On the basis of its texture, sorting, lack of bedding, 

and absence of any lateral gradation in particle size, 

Russell (1931) concluded that the Nomlaki was produced by an ash flow that had its source to the east in 

the Mount Lassen area, where several exposures of its 

presumed correlative were found interbedded with the 

late Pliocene Tuscan Formation. 

On the Geologic Map of California (Ukiah S:h,eet, 

Jennings and Strand, 1960; Santa Rosa Sheet, Koenig, 

1963, scale 1:250,000) the Putah is shown as the Nomlaki. However, Miller (1966) has concluded that the 

two tuffs are different on the basis of refractive index of 

1 glass and feldspar composition, conclusions that are 

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