The Geology, Carbonaceous 

Materials, and Origin of the 

Epigenetic Uranium Deposits in the 

Tertiary Sespe Formation in 

Ventura County, California 

U.S. GEOLOGICAL SURVEY BULLETIN 1771 

·rhe Geology, Carbonaceous 

JV1aterials, and Origin of the 

!Epigenetic Uranium Deposits in the 

Tertiary Sespe formation in 

Ventura County, California 

By KENDELL A. DICKINSON and JOEL S. LEVENTHAL 

A description of and model for a group of Tertiary epigenetic 

uranium deposits the formation of which was controlled by 

humate deposition at the fresh water-marine water interface in the 

coastal subsurface of California 

U.S. GEOLOGICAL SURVEY BULLETIN 1771 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1988 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Dickinson, Kendell A., 1931-

The geology, carbonaceous materials, and origin of the epigenetic uranium 

deposits in the Tertiary Sespe Formation in Ventura County, California. 

(U.S. Geological Survey bulletin ; 1771) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1771 

1. Uranium ores-California-Ventura County. 2. Sespe Formation 

(Calif.) 3. Sedimentation and deposition-California-Ventura County. 

4. Carbon. 

I. Leventhal, Joel S. II. Title. Ill. Series. 

QE75.B9 no. 1771 [QE390.2.U7] 557.3 s 87-600092 

553.4' 932 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 3 

Geology 3 

Sespe Formation 3 

Structure 5 

Uranium deposits 5 

Superior Ridge 5 

Laguna Ridge 6 

Upper Sespe Creek 7 

Elemental associations 7 

Carbonaceous materials 8 

Analytical techniques 8 

Results and discussion of organic matter analyses 10 

Model for uranium mineralization in the Sespe Formation 12 

Source of uranium 12 

Leaching 15 

Paleohydrology 15 

Host rock 16 

Reductant 16 

Conclusions 17 

References cited 17 

FIGURES 

1. Map showing area of report and sample localities, Ventura County, 

California 2 

2. Stratigraphic columns showing uranium occurrences 4 

3. Stratigraphic section of lower transitional Sespe Formation, Superior Ridge 

area 6 

4. Geologic map of Superior Ridge and Laguna Ridge uranium areas 9 

5. Cross section, Superior Ridge 10 

6. Pyrolysis-gas chromatograms of mineralized organic carbon veins (lower 

trace) and Summit School oil seep (upper trace) 13 

7. Photographs showing carbonaceous veins in the lower transitional Sespe Formation at the uranium occurrence at Beer Can #12 14 

8. Schematic diagram showing hypothetical conditions resulting in uranium 

mineralization in lower Sespe Formation 16 

TABLES 

1. Summary of uranium claims 8 

2. Sample data from the Sespe Formation 11 

3. Organic carbon and related analyses 12 

4. Comparison between ICP analyses of organic carbon-rich veins and enclosing 

sandstone 13 

Contents Ill 

The Geology, Carbonaceous Materials, and Origin of the 

Epigenetic Uranium Deposits in the 

Tertiary Sespe Formation in Ventura County, California 

By Kendell A. Dickinson and Joel S. Leventhal 

Abstract 

Uranium deposits of potentially commercial tonnage and 

grade have been known in western Ventura County, California, since about 1959. These epigenetic uranium deposits are 

mainly in the lower part of the upper Eocene to lower Miocene, 

predominantly nonmarine Sespe Formation. Uranium-bearing 

rocks are exposed in a faulted anticline on Superior Ridge. 

The uranium is associated with carbonaceous material, 

vanadium, and probably molybdenum, in mineralized rock, 

which is defined here as rock containing more than 10 ppm 

uranium. 

At least part of the carbonaceous material and uranium 

was deposited in veins from aqueous solution. Later alteration 

added quartz, which is disseminated in the veins, and jarosite 

and clay minerals that were deposited in central voids in the 

carbonaceous veins. The carbonaceous material is partly 

soluble in sodium bicarbonate, and has a pyrolysis-gas 

chromatography signature similar to that of terrestrial carbonaceous material rather than to those of marine organic 

material or oil. Carbon isotope values from the Sespe carbon 

are generally lighter than those of oil from the same general 

area. The carbonaceous material was most likely derived from 

terrestrial humate. 

The source of the uranium for these deposits was probably the Sespe Formation itself. The Sespe consists of arkosic 

sedimentary rock that contains much felsic-volcanic and granitic 

clastic material. High thorium-to-uranium ratios suggest that 

uranium leaching occurred in an alkaline, oxidizing environment during semi-arid climatic conditions, during or after 

deposition of the middle and upper parts of the Sespe. The 

uranium-host sandstone was formed from sediment deposited 

in streams that flowed directly into the ocean or into a 

westward-flowing trunk stream. The sandstone-fi lied paleochannels served as ground-water conduits that carried uraniumbearing solutions to the depositional site. The uranium 

mineralization took place when the humate was deposited or 

perhaps shortly thereafter. The humate may have been deposited in a zone of mixing between terrestrial and marine 

water. 

INTRODUCTION 

Epigenetic uranium deposits of potentially commercial value have been found in the lower part of the upper 

Eocene to lower Miocene Sespe Formation near Ojai, in 

Ventura County, Calif. This report discusses the geological and geochemical setting of these deposits and 

postulates a model for their origin. 

Several uranium deposits are located on Superior 

Ridge, a topographic high about 3 mi long that is located 

just south of White Ledge Peak and from 6 to 9 mi west 

of Ojai, Calif. A single deposit on Laguna Ridge is 

located about 3 mi south of Superior Ridge, and was included with the Superior Ridge deposits in what Bowes 

and Myerson (1957) called the White Ledge Peak district. 

A few small deposits are located near the Hartman Ranch 

on upper Sespe Creek (fig. 1). 

Most of the uranium exploration and development 

activity in Ventura County took place between 1954 and 

1960. Airborne radiometric surveys and hand-held geiger 

counters were the primary exploration tools. Trenches 

dug by hand or by bulldozer were used to expose the 

deposits. Exploration reached the drilling stage only on 

Superior Ridge where about 30 holes were drilled. The 

deepest hole was drilled to 380 ft. 

A preliminary model for uranium mineralization 

in the Sespe was presented by Dickinson (1982). He postulated, as we do here, that the organic material necessary 

for concentrating the uranium by chemical reduction or 

precipitation originated as terrestrial humic acid or 

humate. Because of the importance of the chemical nature 

of the organic material to the genesis of the deposit, a 

study of the material was carried out and is presented here 

as a separate section. 

Mineralized samples of host rocks, nonmineralized samples of potential host rocks, and samples of 

some possible source rocks were collected (fig. 1). All 

samples were analyzed for uranium and thorium by the 

Introduction 
-~-~ 

i L---... -l_l 

!~, r · L ____ FRA"i/£;---- ___ _r--, BORAX \ 

MINE 'X' \ 

\ 

White 

Ledge 

1 Superior Peak 

6781·11 0 Ridge 6381-3 Jj 6381-1,2 

61381-41 0 ,6581-1, 2. 

(

61381-6 3, 4, 5 

61281-1 

0 

} 

61281-4, 5 

Figure 1. Area of the report and sample localities, Ventura County, Calif. 

2 Epigenetic Uranium Deposits, Sespe Formation, California 

/ 

/ / 

/ /o 

\ 

\ 

\ 

I 

Simi Valley I 

,---1 

I 

I 

I 

,------_j 

/ 

10 MilES 

1 5 KilOMETERS 
delayed-neutron method (Millard, 1976). Mineralogy of 

the samples was studied mainly by X-ray diffraction. 

Chemical analyses were made by six-step semiquantitative 

emission spectroscopy or by inductively coupled plasma 

spectroscopy (ICP) (Crock and others, 1983) of both 

mineralized and nonmineralized host rocks (Dickinson 

and others, 1984). 

ACKNOWLEDGMENTS 

The help received from the following people is 

gratefully acknowledged: Neely Bostick helped with confirmation of the nonlignite or nonvitrinite nature of the 

organic material. Mark Stanton, April Vuletich, and Ted 

Daws helped with organic matter analysis. James Nishi 

aided in the operation of the SEM-XES apparatus. Virgil 

Frizzell and Gil Bowles consulted with the authors on the 

geology of the area, and Albert Hess of the U.S. Forest 

Service (U.S. Department of Agriculture) provided access to the national forest lands. 

GEOLOGY 

The uranium occurrences described in this report 

are located in a folded and faulted area north of the Ventura basin along the southern flanks of the Santa Ynez 

Mountains, which are a part of the east-west-trending 

Transverse Ranges of southern California. A sequence 

of largely marine sedimentary rocks underlies the area. 

The oldest exposed rocks in the area are Upper 

Cretaceous. These rocks are overlain by an Eocene sequence that comprises, in ascending order, the Juncal 

Formation, the Matilija Sandstone, the Cozy Dell Shale 

and the Coldwater Sandstone (Page and others, 1951). 

The Eocene sequence began with deep-ocean sedimentation during deposition of the Juncal and Matilija Formations and ended with shallow-water sedimentation 

during deposition of the Coldwater Sandstone. The mostly terrestrial Oligocene Sespe Formation, which is an exception to the otherwise marine character of the sequence, 

overlies the Coldwater Sandstone. The Sespe is the host 

rock for the uranium deposits. Marine sedimentation 

followed deposition of the Sespe and continued until the 

Pleistocene in the Ventura River Valley. Formations involved are the Oligocene and Miocene Vaqueros, the 

Miocene Rincon, Monterey, and Santa Margarita, and 

the Pliocene Pico Formations (fig. 2). According to 

Dickinson and Lowe (1966), the Rincon Shale is missing 

in an otherwise similar sequence in the upper Sespe Creek 

area to the northwest, but mapping by Moser and Frizzell 

(1982) indicates its presence there. 

The geology of the Superior Ridge and Ojai area 

has been compiled by Moser and Frizzell (1982). The 

general stratigraphic sequence in two areas of Ventura 

County is shown in figure 2. 

Sespe Formation 

The Sespe Formation was named in the Sespe Creek 

area 6 mi north of Fillmore by Watts (1897), who called 

it the "Sespe Brownstone." The formation was later 

revised by Kew (1924), who extended the term throughout 

the depositional basin. The Sespe rests on the Eocene 

Coldwater Sandstone with apparent conformity in most 

areas, but in a few areas the contact is a disconformity 

or a slight angular unconformity. The upper contact of 

the Sespe with the Oligocene and Miocene Vaqueros Formation is conformable in most areas, but a slight disconformity or angular unconformity is found in others. 

Bailey (1947) reported a maximum thickness of the 

Sespe Formation of 7,500 ft, but in most of the Ventura 

basin, it ranges from 3,000 to 5,000 ft in thickness. The 

Sespe generally thins westward. The late Eocene to early 

Miocene age of the Sespe Formation is based mainly on 

its stratigraphic position, although it contains some 

vertebrate fossils (Reinhart, 1928; Page and others, 1951). 

The lower part of the Sespe may be predominantly late 

Eocene. 

The Sespe Formation is a thick red-bed sequence 

consisting mostly of arkosic shale, sandstone, and conglomerate. The ratio of plagioclase to orthoclase in the 

Sespe generally exceeds 1.0, based on whole-rock X-ray 

analyses. The ratio increases to the east, where a more 

mafic igneous provenance was demonstrated by Bohannon (1976). Total feldspar exceeds quartz in most of the 

samples. The lower part of the Sespe is more conglomeratic than the upper part, which contains more red 

beds. In the Superior Ridge area, the lower part consists 

mainly of yellowish-brown or pinkish-gray, medium- to 

coarse-grained, poorly sorted to well-sorted arkosic sandstone that exhibits plane bedding as well as crossbedding. 

The predominant clay mineral in the uranium host rocks 

at Superior Ridge is montmorillonite (Bowes and Myerson, 1957), but the clay suite also includes chlorite, and 

possibly mixed-layer chlorite-montmorillonite and illite. 

A basal conglomeratic member of the Sespe Formation was recognized by Dickinson and Lowe (1966) in 

the area of upper Sespe Creek. This basal member is apparently limited in areal extent to the western margin of 

the Sespe depositional basin. Bowes and Myerson(1957) 

recognized a lower member of the Sespe in the Superior 

Ridge area that was interpreted to be transitional with 

the underlying Coldwater Sandstone and that is apparently below the main conglomeratic facies included in the 

basal member of the Sespe by Dickinson and Lowe 

(1966). In the remainder of this report, the lower member 

of Bowes and Myerson (1957) will be referred to as the 

Geology 3 
VENTURA RIVER AREA 

Modified from Bailey (1952) in 

Vedder and others ( 1969) z z 0 

0 ~ ~ en en a: a: w :2 w u a: a: t- en w 0 w w en u. :2 Cl 

I Santa I Margarita Massive diatomaceous mudstone, 

I thin limy beds, and concretions 

I Fm 

I 

UPPER SESPE CREEK 

I Siliceous shale, laminated, I organic Modified from Dickinson I 0 I Monterey 

and Lowe (1966) z I ~c:' I Fm Za.. >- 0 I ~~I Alternating laminated shales 

Q:::> (!) i= I ~I and hard limestone 

~~ 0 en a.. en ....J a: I ~t I w a: :2<!:J 0 w u I~~ I Massive mudstone, dolomite a: C::c:: :::c t- en t- w I I c w eo w c concretions en :J :2 Cl I In 

I .~0 I N 

Upper: sandstone, mudstone, • J 

Santa c 1 ~,f I .~0 c 

J:! i ~ i 1!1l; t·· ~ c conglomerate, gypsiferous c 

Margarita c I I ~ N 

o•o•o J Middle: siltstone, phosphatic I I ~'li Fm . ~~-·~:2.<. and phosphorite. lower. sand- ~:·~·.:.::.;.~i~· c w -- ~ ............ ~ 

C) stone white siliceous z l! 1./' / w -~~. rf/ u I 0 o o I o c Mudstone, sandstone and I -7 0 In siltstone, dark gray and ~ Monterey ·~L.;,_:~:·~: '<t I // / J brown, siliceous; contains 

Fm diatoms, foraminifers, I / \~e / In I/~~<.. / =...:..-==-~~ C) pelecypods, and gastropods j/ // ~ ~ / w 

~ Vaqueros ').::..:-::=-_.::-_:; Jc !~~~~~~e, siltstone, and / z Variegated mudstone and sandi:"" ~············ c~ // w 

u Sespe stone, grit and conglomerate, 

i~::~:~{~ 0 

z Sandstone, tan, laminated (!) Fm massive, very thick bedded 

w c and crossbedded; contains :J u c Continental 0 

0 Sespe QO red and green sandstone, (!) Fm ) .. ..... J siltstone, and claystone, and ::J 

** ~ brown conglomerate; mostly _Q_ 

["' ~'?;;-:;:~ :::- conglomerate at base 

~ Coldwater ... . .. c!,g Sandstone, gray and tan, S~: ~In massive and crossbedded Shallow marine 

~~1t~~: Lithologically similar to " Hard, fine- to coarse-grained ' Coldwater c 

Topatopa c Cozy Dell and Matilija " c sandstone and silty claystone, w c ' Ss In 

Gp 

~·"·"'"' c Formations in Ventura " N interbedded z In 

w River area u " 0 ---- " " w Fault 

·.:;_~:_~::-~~.:..~:. \ ~,. Juncal 0 0 I I ~ 0 0 Sandstone and siltstone, \ 'I)& 

Fm gray, calcareous \ Cosy --- -- \ Massive silty shale and micaceous 

......... \ Dell mudstone 

\ \ w Sh z \ \ w 

\ \ u 

0 

\ \ w 

\ ~~ \ 

\ 

\ ~ \ c 

\ c Thin- bedded hard sandstone; \ ~· c 

\ ~ \ M siltstone beds in upper and J lower parts; middle part massive, 

\ \ c 

In hard sandstone 

\ \ N 

\ \ 

\ 

\ 

\ Juncal 

\ Fm 

\ 

Figure 2. Stratigraphic columns showing uranium occurrences (** 1 >0.1 percent U30 8; *I >0.001 percent <0.01 percent 

U3

0 8). Thicknesses and correlations approximate. 

4 Epigenetic Uranium Deposits, Sespe Formation, California 
lower transitional Sespe. The lower transitional Sespe is 

nonmarine, but it intertongues with the underlying marine 

Coldwater Sandstone (Moser and Frizzell, 1982), and is 

the main uranium host rock at Superior Ridge. The lower 

transitional Sespe ranges from 225 to 450 ft in thickness. 

Additional stratigraphic details of the lower transitional 

Sespe are shown in figure 3, which was originally published by Bowes and Myerson (1957). 

On the basis of paleocurrent work by Bohannon 

(1976), the sediment source of the Sespe in the upper 

Sespe Creek area was from the west, northwest, and 

north; farther east in the Canton Canyon area (fig. 1), 

the source was from the north, northeast, and east. These 

data are not corrected for tectonic rotation as proposed 

by Luyendyk and others (1980). The conglomerate clasts 

in the Sespe Creek area are mainly volcanic rock of 

rhyodacitic composition, granitic rock, and sandstone. 

In the Canton Canyon area to the east, the main clast 

types are granite, syenite, anorthosite, and gabbro 

(Bohannon, 1976). 

The Sespe Formation was deposited in a dominantly fluvial environment (Reed, 1929; Reinhart, 1928; 

Bailey, 1947; Dickinson and Lowe, 1966). Bohannon 

(1976) described fining-upward sequences in the upper 

Sespe Creek area that he attributed to meandering and 

braided streams on a flood plain. In the Canton Canyon 

area, Bohannon (1976) suggested deposition in a broad 

alluvial fan. Some possible lacustrine beds have been 

reported (Reed, 1929, p. 503). At Dennison Park east of 

Ojai, laminated beds containing gypsum indicate that 

minor amounts of the Sespe were deposited in evaporitic 

lakes. 

Bailey (1947) suggested that a major stream flowed 

westward through the Sespe depositional basin with 

tributaries entering from both the north and the south. 

The paleocurrent data of Bohannon (1976) and 

McCracken (1969a) generally support this hypothesis. 

McCracken (1969a) hypothesized that a trunk stream existed in the approximate position of the ntodern Santa 

Clara River and that it reached the present coast near 

Point Rincon. 

Structure 

The structural evolution important to the history 

of uranium mineralization began with the end of the 

Eocene uplift that created the nonmarine basin in which 

the Sespe Formation was deposited. Minor folding may 

have occurred in the Miocene as evidenced locally by a 

slight disconformity between the Sespe Formation and 

overlying units and perhaps also by conglomeratic lenses 

within the Sespe. These movements were relatively small 

and are difficult to measure or map because of the largescale folding and faulting that occurred later. However, 

they may have affected uranium mineralization because 

of their effect on paleohydrology. The greatest orogeny 

occurred during the Pleistocene prior to deposition of the 

Santa Barbara Formation (Weber and others, 1973; 

Jackson and Yeats, 1982). This orogeny produced the 

Superior anticline and related faults, as well as the overturned anticlines both east and west of the Superior anticline. It probably modified existing uranium deposits 

and uplifted and subsequently exposed the uranium 

deposits along Superior and Laguna Ridges. 

URANIUM DEPOSITS 

Superior Ridge 

The Superior Ridge deposits, the largest and most 

extensive known uranium occurrences in Ventura County, were first discovered during 1954 by using aerial 

radiometries (Bowes and Myerson, 1957). During 1954 

and 1955 many claims were filed in an area about 5 mi 

long by Y2 mi wide (fig. 4). These claims included Gamma 

Queen, Coyote, Chismahoo, Payoff (later consolidated 

into the Payore), Lamar, and Beer Can (fig. 1, table 1). 

All the Superior Ridge deposits (White Ledge Peak 

deposits of Bowes and Myerson, 1957) are in the lower 

transitional Sespe Formation (figs. 4, 5). 

The Superior Ridge uranium deposits are located 

along the crest and on both flanks of a faulted anticline 

(figs. 4, 5). The fault parallels the Arroyo-Parida fault 

to the south and the Santa Ynez fault about 3 mi to the 

north. Whereas it is tempting to suggest that mineralization is related to these Pleistocene structural features, the 

uranium occurrences probably were more widespread in 

the lower transitional Sespe at an earlier age, and the anticline only served to expose a mineralized area to erosion and other modification. 

The most common uranium mineral in the Superior 

Ridge deposits is carnotite [K(U02) 2(VOJ2 • 3H20)]. Becquerelite [CaU6

0 19 ·11H20], autunite [Ca(U02)(POJ2 

·10-12H20], and a black uranium oxide (possibly pitchblende) are also present. These uranium-bearing minerals 

are found in lenses and concretions, most commonly 

associated with carbonaceous material. Bowes and Myerson (1957) described an occurrence at the Payore claim 

as follows: "The concretions, along with irregular carbonaceous streaks and disseminations, form local lenticular clusters of ore-grade material along bedding of a 

size approximating 10 by 10 by 2 feet." At the Beer Can 

#12 deposit, we found uranium associated with an irregular 7 x 4 ft pod of altered sandstone containing 

siliceous carbonaceous veins. The carbonaceous vein 

material, which resembles lignite in hand specimens, is 

anomalously high in both uranium and vanadium. The 

veins are present along the bedding plane for about 50 ft. 

Uranium Deposits 5 
LITHOLOGY 

·o~ v"-~ t;0 

~~ 0~ 

• Anomalous radioactivity 

LITHOLOGY 

Marine 

Light yellow-brown to olivegray massive sands and silty 

sand 

Nonmarine 

Predominantly reddish shale, 

sands, and conglomerate 

Nonmarine 

Lower Sespe sandstone. Pinkishand yellowish-gray ledge-forming 

sandstone. Red shale at base 

Shallow water marine 

Mostly light gray and variegated, 

fine- to coarse-grained arkosic 

sandstone with interbeds of 

silty shale and gravel 

Marine 

Massive to lamellar, black 

mudstone and shale with few 

interbeds of gray sandstone 

Marine 

Vaqueros not complete 

Gray, mostly well sorted and 

graded, fine- to coarse- grain~d 

arkosic sandstone with lt~::;~zl[:!~~: interbedded shale 

Matilija not complete 

Q) 

~ 

0 

0 

1.0 

N 

E 

u.. 

E 

u.. 

Q) 

c. 

Cll 

~ 

Cll 

(/) 

.... 

Q) 

'!ii 

s: 

"0 

8 

~ 

(/) 

Qi 

a 

> N 

8 

w 

z 

w 

g 

(!) 

:::; 

0 

w 

z 

w 

u 

0 

w 

Figure 3 (above and facing page). Stratigraphic section of the lower transitional Sespe Formation in the Superior 

Ridge area (modified from Bowes and Myerson, 1957). 

The carbonaceous material in the veins probably was 

precipitated from humate-bearing ground water as discussed later. 

Nine holes drilled on the Payore claims near the east 

end of Superior Ridge encountered discontinuous ore 

horizons at a depth of between 320 and 380 ft. Homestake 

mining Company estimated undiluted reserves of 

57,167 lb U30 8 based on data obtained from the claim 

6 Epigenetic Uranium Deposits, Sespe Formation, California 

owners. These data consisted of eU30 8 measurements 

from drill holes. 

Laguna Ridge 

One claim, the Hoot Mon (fig. 4, table 1), was filed 

on Laguna Ridge, about 3 mi south of Superior Ridge. 

The Hoot Mon deposit is also in the lower transitional 
EXPLANATION 

Sandstone, arkosic, light-pinkish-gray, coarse- to very 

coarse grained, crossbedded; containing radioactive 

orange-pink iron-rich concretions and carbon trash 

2 Sandstone, arkosic, gray-yellow, medium-grained, 

micaceous, irregularly iron oxide stained 

3 Sandstone, yellow-gray, medium-grained 

4 Sandstone, arkosic, yellow-gray, coarse-grained 

5 Sandstone, gray to yellow, thick-bedded 

6 Sandstone, brown, very coarse grained to conglomeratic; 

contains chert pebbles 

7 Sandstone, pale-yellow-orange, medium-grained 

8 Sandstone, arkosic, friable, medium- to coarse-grained; 

locally contains thin to medium beds of brown shale 

9 Sandstone, quartzose, light-yellow-gray, coarse-grained, 

thick-bedded, locally crossbedded, iron-oxide-stained 

10 Shale, brown-gray 

11 Sandstone, arkosic, gray-orange, medium-grained, locally shaly 

12 Sandstone, coarse-grained, thick-bedded, iron-oxidestained 

13 Shale, brown-gray; interbedded with yellow-gray mediumgrained arkosic sandstone 

14 Sandstone, light-yellow-gray, coarse-grained 

15 Sandstone, arkosic, yellow-gray, medium-grained, friable 

16 Sandstone, arkosic, yellow-orange, coarse-grained 

17 Shale, brown-gray, sandy; interbedded with fine-grained 

shaly sandstone 

18 Sandstone, arkosic, micaceous, pale-yellow-orange, 

coarse-grained; lower part is radioactive and locally contains carbon trash 

19 Conglomerate, light-pinkish-gray, crossbedded, thickbedded; coarse- to very coarse grained arkosic sandstone; 

and iron-rich concretions. Locally unit is stained with 

orange-pink iron oxide, is radioactive, and contains carbon trash 

20 Mudstone, red and brown-gray 

21 Sandstone, brown, conglomeratic 

22 Shale, brown-gray, silty 

23 Sandstone, yellow-gray, iron-oxide-stained 

24 Shale, red, silty 

25 Sandstone, yellow-gray 

26 Mudstone, red and brown-gray 

27 Siltstone, brown-gray 

28 Sandstone, gray 

Sespe, which is exposed where it is thrust to the surface 

along the Arroyo-Parida fault. According to Bowes and 

Myerson (written commun., 1956), this deposit, which 

was not sampled for this study, contains carnotite 

associated with carbon trash and is similar in occurrence 

to the Superior Ridge deposits. 

Upper Sespe Creek 

Two uranium claims, Bertram Lode and Lucky 

Saddle #1, were staked on upper Sespe Creek near Hartman Ranch near State Highway 33 (table 1). These 

deposits were not located during the present study, but 

samples with weak uranium mineralization were collected 

in the area (table 2). The host rock is micaceous arkosic 

sandstone of the lower part of the Sespe Formation. The 

lower transitional Sespe has not been recognized in this 

area (Dickinson and Lowe, 1969). Sandstone samples 

from the Lucky Saddle #1 contained as much as 0.1 percent U3

0 8

, and carbonaceous samples from this deposit 

contained as much as 0.8 percent U30 8 (uranium contents were chemically determined). Radioactivity of the 

Bertram Lode was associated with iron-stained concretions (Dickinson, 1982). Such isolated occurrences are 

probably common in Eocene through Miocene sedimentary rock in Ventura County. They may have resulted 

from redeposition of minor amounts of uranium during 

modification of other deposits and they are believed to 

be commercially insignificant. 

ELEMENTAL ASSOCIATIONS 

For weakly mineralized rocks, uranium content 

alone is not sufficient criterion for determining which 

rocks have been mineralized and which have not. 

However, a significant correlation exists between uranium 

and vanadium and probably between uranium and 

molybdenum in rocks that have been mineralized (table 

2). For this reason, higher vanadium and molybdenum 

values in a particular sample are used as evidence that 

it has been mineralized. In addition, the Th:U ratio is 

low for mineralized rocks, and generally only an upper 

limit for thorium can be determined (Millard, 1976). Examination of the data in table 2 suggests that samples containing more than 10 ppm uranium have been 

mineralized, although some samples containing 5-10 ppm 

uranium may have also been slightly mineralized. 

The linear correlation coefficient between uranium 

and vanadium contents in samples containing more than 

10 ppm uranium is + 0.86, placing the correlation above 

the 99-percent confidence level for 10 samples (table 2). 

The correlation between uranium and vanadium in 

samples containing less than 10 ppm uranium is -0.03, 

which is not significant for 25 samples. An apparent correlation also exists between uranium and molybdenum. 

Molybdenum was undetectable in only one sample with 

more than 10 ppm uranium and was detectable in only 

one sample with less than 10 ppm uranium. The correlation between both uranium and vanadium and uranium 

and molybdenum is common in some epigenetic uranium 

deposits (Shoemaker and others, 1959). 

Elemental Associations 7 
Table 1. Summary of uranium claims 

[NA, not available] 

Maximum 

Area and uranium 

claim values Uranium Percent Petrology of host rock Stratigraphy Strike/dip Sec.!T. N./R. W. (pet. minerals V20s 

eU30 8) 

Superior Ridge: 

Beer Can #12 0.19 Carnotite 0.67 Sandstone, gray, light-brown, arkosic, Lower transitional Dip 40° S. 

2 4 24 crossbedded, fluvial; contains black Sespe Formation. 

carbonaceous veins. 

Payoff1 2.18 Carnotite NA Sandstone, gray, light-brown, ---do ------------ Strike 

10 4 24 micaceous, poorly sorted, fluvial; northeast. 

contains carbonaceous trash. 

Chismahoo East #1 .23 Carnotite NA Sandstone, gray to light-brown, poorly ---do ------------ N. 70° E.; 

10 4 24 sorted; contains conglomerate lenses 40°N. 

and carbonized wood. 

Coyote #1 .16 Carnotite NA Sandstone, gray, coarse-grained, ---do ------------ N. 45°-70° E.; 

4 4 24 micaceous, fluvial. 30°-40° s. 

Gamma Queen .07 Autunite .09 Sandstone, buff-colored, fine-grained, ---do ------------ N. 85° W.; 

4 4 24 micaceous; contains pockets of 45°-50° SW. 

carbonaceous material and 

interbedded conglomerate, siltstone, 

and red and green shale. 

Lamar .03 NA NA Sandstone; contains carbon trash ---do ------------ NA 

7 4 24 

Laguna Ranch: 

Hoot Mon #4 NA Carnotite NA Sandstone, buff-colored to light-gray, ---do ------------ NA 

2 4 24 fluvial; contains carbon trash. 

Sespe Creek area: 

Bertram lode .64 NA NA Sandstone; contains uraniferous Upper Eocene---- N. 60°-70° E.; 

2 5 23 concretion.s and interbedded shale. 30° s. 

Lucky Saddle .51 NA NA Sandstone, gray, micaceous, fluvial; Lower Sespe 

31 6 22 contains carbonaceous material along Formation. 

bedding planes. 

1 Later included in the Payore claims. 

CARBONACEOUS MATERIALS 

Analytical Techniques 

The pyrolysis-gas chromatography method was 

described by Leventhal (1981) and is summarized here. 

A powdered sample (~2 mg) is placed in a 2 mm X 2.3 

em quartz tube which is put in the spiral of the Chemical 

Data Systems pyroprobe1• The sample is then heated for 

10 s at 750 or 900 °C. The pyrolysis products are 

separated on a 2 m X 1 mm stainless steel gaschromatography column packed with Supelcoport coated 

with OV-101. The separated compounds are detected by 

a flame ionization detector. 

Rock-Eva/ pyrolysis is described by Tissot and 

Welte (1978). It consists of continuous heating of a 

powdered sample from 225 oc to 550 °C at 25 °C/min 

and measurement of evolved C02 and hydrocarbons 

(HC). Volatiles released at temperatures below ~300°C 

are called S1 (mg HC/g sample). Pyrolysis products 

released above ~300 OC are called S2 (mg HC/g sample). 

The quantity S/percent org C is the hydrogen index 

(HI) which is similar to H:C ratio. Carbon dioxide released up to 390 OC is S3; S/percent org C is the oxygen 

1 Any use of trade names and trademarks in this publication is 

for descriptive purposes only and does not constitute endorsement by 

the U.S. Geological Survey. 

8 Epigenetic Uranium Deposits, Sespe Formation, California 
Ted 

Tcw 

Tcw ~ GAMMA QUEEN 

2 MILES 

1 2 KILOMETERS 

EXPLANATION 

Surficial deposits (Quaternary) 

Sespe Formation (Tertiary)-Undivided 

Lower transitional Sespe Formation 

Coldwater Sandstone (Tertiary) 

Cozy Dell Shale (Tertiary) 

Formation boundary-Dashed where 

approximate 

Fault-Dashed where approximate --t-- Foldaxes 

T Strike and dip of beds 

A--A' Location of cross section shown on 

figure 5 

~ Uranium claim 

Figure 4. Geologic map of Superior Ridge and Laguna Ridge areas. Geology modified from Bowes and Myerson (1957) and 

Moser and Frizzell (1982). 

index (OI) and is similar to the O:C ratio. The temperature at which S2 is a maximum is called Tmax and relates 

to thermal maturity. Temperatures below 420 °C indicate 

that the sample is thermally immature with respect to 

petroleum generation, whereas Tmax above 460 °C indicates rocks that are thermally overmature and have 

already yielded petroleum. The results are displayed on 

a plot of HI vs. OI which is similar to a van Krevelen 

plot of H:C vs. O:C. 

In addition to these techniques, the carbon-rich materials were studied by scanning electron microscopy-Xray energy spectrometry (SEM-XES), X-ray diffraction, 

Carbonaceous Materials 9 
FEET A 

3000 

2500 

2000 

1500 

1000 

.5 1 KILOMETER 

1 MILE 

A' METERS 

900 

800 

700 

600 

500 

400 

300 

200 

Figure 5. Cross section along Superior Ridge; viewer is looking west. Tcw, Coldwater Sandstone; Tsp, Sespe Formation; 

Tsl, lower transitional Sespe Formation (fig. 4). Locations of uranium occurrences (crossed pick and hammer) are projected at right angles to plane of cross section (fig. 4). Modified from Bowes and Myerson, 1957. 

chemical analysis by ICP, and microscopy of thin and 

polished sections. 

Results and Discussion of Organic Matter Analyses 

In order to test for soluble organic matter, a small 

portion of sample was crushed and added to two test 

tubes, one with sodium bicarbonate and another with a 

mixture of toluene and methanol. Organic material that 

is soluble in sodium carbonate solution (alkaline pH) is 

humic (Schnitzer and Kahn, 1972), whereas material soluble in the organic solvent mixture is bitumen. The 

uranium-related samples show mainly humic material 

and only slight bitumen. A portion of the Beer Can #12 

alkaline extract was transferred to another tube and 

acidified to pH 2; it gave a flocculant precipitate (humic 

acid), leaving a nearly clear solution (fulvic acid). Thus 

the Beer Can #12 extract is mainly humic-acid-type 

material and contains only a small amount of fulvic 

acid, which is characteristic of very recent organic 

matter. 

Pyrolysis-gas chromatography was used to fingerprint the largely insoluble organic matter from the suite 

of samples (table 3). The Beer Can #12 (fresh and 

weathered), east outlier, west outlier, and Payore samples 

all give very similar results; the pyrograms for these 

samples give the same retention time and relative intensities for the 25 pyrolysis products that are observed. The 

irregular spacing of the peaks (lower trace, fig. 6) is not 

typical for bitumen or marine organic matter. These 

results can be contrasted with the pyrolysis-gas 

chromatography results of the Summit School oil seep 

(upper trace, fig. 6) with a predominance of evenly spaced 

n-alkanes (nC15_

25) that are typical of petroleum. 

10 Epigenetic Uranium Deposits, Sespe Formation, California 

The extremely high organic carbon content (table 

3) of these uranium-enriched samples is quite anomalous 

compared to many uranium deposits where the organic 

carbon is in the range of 0.5-3 percent. The Rock-Eval 

pyrolysis shows this organic matter to be type III (terrestrial) that has been partly oxidized (as shown by the 

high oxygen index values); such organic matter is typical 

of uranium deposits (Landais and others, 1984; Leventhal and others, 1986). The Rock-Eval results reported 

here are similar to those of Frizzell and Claypool (1983) 

from nearby unmineralized organic material. 

Although the Tmax and low HI (table 3) indicate 

that this uranium-rich organic material is in the oilgenerating window, it is still capable of yielding additional 

pyrolysis products. The o13C value (table 3) for the 

organic carbon in these uranium-rich samples is typical 

of Tertiary to Holocene terrestrial organic matter 

(Degens, 1969). 

The oil seep sample gives a carbon isotope value 

slightly heavier than the uranium-related organic matter 

and typical of oil from the Monterey Formation and 

related oils (Sofer, 1984). The oil seep shows a low Tmax 

and high HI as expected. 

Another unusual aspect of these samples is their 

high sulfur content. The iron content of the Beer Can 

#12 sample is relatively low, based on the ICP analysis 

(table 4). No pyrite was identified in the SEM-XES or 

optical petrography observations. Thus approximately 

half of the sulfur may be present as organic sulfur. The 

non-pyrite sulfur content is not far from the 0. 7 percent 

reported in Holocene Florida humate (Swanson and 

Palacas, 1965, p. B21) and is within the range for modern 

humic substances (Schnitzer and Kahn, 1972, p. 31). 

The analyses of the uranium-enriched samples show 

high organic carbon (table 1), not typical of normal soil 
Table 2. Sample data from the Sespe Formation 

[N, not detected; L, detected but below limit of determination; detection limit for molybdenum 3 ppm, for vanadium 7 ppm (Millard, 1976)] 

Proximity Van a- Molyb- Mineralogy as 

Sample 1 Uranium Thorium to dium denum determined by 

Locality No. (ppm) (ppm) Th:U mineral- (ppm) (ppm) Field description whole-rock 

ization3 X-ray 

diffraction4 

Superior Ridge ---- 6381-1A 162.0 <27 .0 e> M 1500 30 Sandstone, light-yellow-brown, medium-grained; Q, P, 0, S, 

contains carbonaceous fragments. I. 

6381-1C 84.5 <16.0 M 1000 30 Sandstone, light-yellow-brown, medium-grained ---------- No data 

6381-1E 93.8 <17 .0 M 500 50 -----do--------------------------------------------- No data 

6381-2 8.8 13.2 1.5 p 50 N Sandstone, pink, fine- to medium-grained, friable -------- No data 

6381-3 2.8 6.2 2.2 p 30 N Sandstone, light-yellow-brown, fine-grained, hard, Q, P, 0, S, 

slightly calcareous. I, ch . 

6581-1 10.7 <3.9 M 50 N Sandstone, gray, fine- to coarse-grained, conglomeratic, Q, P, 0, S, 

cross bedded. I, ch . 

6581-2 6.4 <3 .2 M? 50 N Siltstone, light-yellow-brown, hard --------------------- Q, P, 0, 

S(?), I, ch. 

6581-3 2870.0 <640.0 M 5000 50 Sandstone, light-yellow-brown, medium- to coarse- Q, 0, P, 

grained; contains carbonaceous fragments. S(?), I, ch. 

6581-4 6.2 8.2 1.3 p 30 N Sandstone, light-yellow-brown, medium- to coarse- Q, 0, P, 

grained, thin-bedded. S(?), I, ch. 

6581-5 5.2 <2.7 M? 70 N Sandstone, tan, medium- to coarse-grained, poorly Q, C, P, S. 

sorted, calcareous. 

61381-3 1.8 7.3 4.0 R 30 N Conglomerate, reddish-brown, argillaceous; contains C, Q, P, ch, 

seashell fragments. s. 61381-4 1.0 3.9 3.9 R 30 N Sandstone, red, coarse-grained, conglomeratic, crumbly Q, P, 0, ch, 

s. 61381-6 19.2 <5.3 M 70 L Sandstone, buff-colored, medium-grained, hard; Q, 0, P, I, S. 

contains contorted bedding . 

Wilsie Canyon 6881-1 7.9 10.9 1.4 P? 20 N Sandstone, reddish-brown, conglomeratic, Q, 0, P, S. 

weathered. 

Hartman Ranch --- 6982-2A 15 .7 <6.1 M 50 N Sandstone, reddish-gray, medium-grained; contains 0, P, Q, I, 

carbonaceous fragments. ch. 

6982- 1B 3.9 13.0 3.4 R 30 N -----do -------------------------------------------- Q, P, 0, S, I. 

Dennison Park ---- 61181-1 2.3 9.2 3.9 R 70 N Siltstone and very fine grained sandstone, red, Q, P, 0, G, 

laminated, calcareous. s, I. 

61181-2 1.5 5.3 3.6 R 30 N Sandstone, pink, evenly bedded, calcareous, Q, P, 0, G, 

micaceous. S, ch. 

61181 - 3 2.0 8.4 4.2 R 30 L Sandstone, red, medium-grained, medium- to thick- Q, P, 0, C. 

bedded. 

61181-4 2.5 9.5 3.7 R 70 N Sandstone, reddish-brown to buff, fine- to medium- Q, P, 0, I, 

grained, crossbedded. ch. 

Sespe Oil Field 61181-5 1.8 10.8 6.1 R 30 N Sandstone, red, fine- to medium-grained, medium- Q, P, 0, I, 

area . bedded, conglomeratic. ch. 

61181-6 2.1 9.0 4.3 R 50 N Sandstone, buff-colored, fine-grained, silty, Q, P, 0, I, S. 

non-calcareous. 

61181-7 3.3 14.0 4.3 R 70 N Sandstone, gray to brown, weathered, very fine to Q, P, 0, ch, 

fine-grained, banded. s, I. 

61181-8 1.8 8.7 4.8 R 30 N Sandstone, red, hard, medium-grained, micaceous -------- Q,P,O,S,I. 

Laguna Ridge -- - - - 61281-1 1.7 9.1 5.4 R 50 N Sandstone, light-tan, hard, evenly thin- to thick- Q, P, 0, S. 

bedded. 

West Casitas Pass. 61281-4 2.6 11.0 4.2 R 30 N Sandstone, buff-colored, medium-grained, evenly Q, P, 0, S, 

layered, thin- to thick-bedded. I. 

61281-5 2.1 8.1 3.9 R 50 N Sandstone, light-reddish-brown, evenly layered, thin- Q, P, 0, 

to thick-bedded. S, ch, I. 

1See figure 1. 2

If blank: ratios not computed if thorium is less-than value . 3

R, remote; P, proximal; M, mineralized. Queried where uncertain. 4

1n general order of decreasing proportion. Q, quartz; P, plagioclase; 0, orthoclase; S, smectite; I, illite or mica; ch, chlorite; G, gypsum; C, calcite. Queried where uncertain. 

organic matter. The zones of enriched organic carbon suggest that the carbon was mobilized and redeposited. Based 

on the solubility, pyrolysis-gas chromatography, and 

Rock-Eval results, this organic matter apparently is of 

humic origin and was reprecipitated as a humate. 

The organic material at Beer Can #12 was originally 

described by Bowes and Myerson (1957) as follows: 

"Radioactivity is associated with an irregular 7 -by-4-foot 

pod of black carbonaceous lignite-like material that occurs in veinlets near its margin. Similar material is found 

in irregular narrow streaks along the bedding plane for 50 

feee' The pod of black carbonaceous lignite-like material 

was not found, but a pod of altered sandstone about 

7X 4ft (fig. 7A) was found on the outcrop. This pod contained veins of shiny black material that range from 112 

to 1 inch in width and from a few inches to about 3 ft 

in length (fig. 7 B). The veins lack plant cell structure but 

have a banded structure similar to inorganic veins, with 

corresponding bands on opposite sides. A central void is 

found in places along these veins (fig. 7C). Reflected light 

microscopy confirmed that the material was not lignite, 

vitrinite, or coal (Neely Bostick, oral commun., 1985). As 

shown by our autoradiograph studies, bands of radioactive material (uranium) in the veins are nearly coincident 

with the mineral bands (fig. 7 D). In addition, transversely oriented veinlets are present in some of the bands. 

Carbonaceous Materials 11 
Table 3. Organic carbon and related analyses 

[Parentheses indicate anomalous data resulting from instrumental limitations. Rock-Eva! analysis was done by Ted Daws, sulfur analyses by 

Mark Stanton; and carbon isotopes by April Vuletich; leaders ( ---), no data] 

Gamma Organic 

Sample counts/second carbon 

at site percent 

Beer Can #12 

carbon veins >10,000 35.5 

hand picked sample 

(primary occurrence). 

Beer Can #12 

carbon veins >10,000 68.4 

(weathered) 

(primary occurrence). 

Beer Can #12 

carbon veins (fresh) >10,000 70.4 

(primary occurrence). 

Beer Can #12 

carbon veins (30 ft east 2,500 63.6 

of primary occurrence). 

Beer Can #12 

carbon clasts (30 ft west 1,500 11.3 

of primary occurrence). 

Beer Can #12 clay 

clasts (30 ft west of 230 0.24 

primary occurrence). 

Beer Can #12, shale 

layer approximately 30 230 0.01 

ft below uranium deposit. 

Payore claims, 

carbonaceous fragments. 7,500 58.3 

Summit School oil seep 

black heavy oil. 120 75.4 

The black material making up the veins contains 

as much as 70 percent organic carbon (table 3). Most of 

the remaining material is quartz. Only quartz was identifiable in X-ray diffractograms of the black vein 

material. An ashed sample of the black material also 

showed quartz and a trace of a clay mineral, probably 

smectite. Material from the central void of the vein was 

identified, using X-ray diffraction, as jarosite and clay 

minerals (fig. 7E, F). Jarosite was previously identified 

in some of the other deposits (Bowes and Myerson, 1957). 

The mineralogy of the veinlets was not determined, but, 

based on XES, they contain silicon, vanadium, potassium, aluminum, iron, and uranium, generally in that 

order. In addition, tiny inclusions of silica-rich material 

are diffused through the organic matter (fig. 7£). 

In general the paragenetic sequence of the veins is 

as follows: (1) Veins of organic matter and uranium are 

deposited from humic-acid-bearing ground water in fractures of already lithified or partly lithified Sespe Formation. Some if not all of the uranium entered in aqueous 

solution with the organic matter as suggested by the banding as mentioned previously. (2) Various elements (Si, V, 

K, AI, and Fe) and minerals, most notably quartz, were 

12 Epigenetic Uranium Deposits, Sespe Formation, California 

Rock-Eva I Sulfur 13Corg 

T max HI 01 percent permil 

446 95 72 0.72 

458 50 26 1.11 -23.5 

445 82 20 0.96 -24.5 

(501) 14 40 0.50 -24.3 

4 76 0.05 -23.5 

(<4) (650) <.02 -24.0 

(<4) (700) <.02 

473 16 64 0.45 -22.9 

(421) 634 8 1.51 -22.9 

deposited penecontemporaneously in the veins. (3) Clay 

minerals, jarosite, and probably iron oxide were deposited 

in the voids. 

MODEL FOR URANIUM MINERALIZATION 

IN THE SESPE FORMATION 

The model for uranium mineralization in the Sespe 

Formation consists of several elements. These elements 

are source of uranium, leaching, paleohydrology, host 

rock, and reductant, and are discussed herein. Hypothesized physical aspects of the model are shown in figure 8. 

Source of Uranium 

Much of the clastic material in the Sespe was derived from volcanic and granitic rock from uplands that 

surrounded the depositional basin (Bohannon, 1976; 

McCracken, 1969a; Reed, 1929). This clastic material apparently was leached by surface and ground water to provide the uranium for deposits in the Sespe (fig. 8). 
t 

UJ 

(/) 

z 

0 

a.. 

(/) 

UJ 

a: 

a: 

0 

b 

UJ 

~ 

UJ 

Cl 

50 

17 

16 

15 

14 

13 

100 150 200 250 

COLUMN TEMPERATURE, IN DEGREES CELSIUS 

Figure 6. Examples of pyrolysis-gas chromatograms of mineralized organic carbon veins from the Beer Can #12 uranium 

occurrence (lower trace) and the Summit School oil seep (upper trace). Numbers are n-alkanes in oil seep (upper trace). 

Table4. Comparison between ICP analyses of organic carbon- Table 4. Comparison of analyses-Continued 

rich samples and surrounding altered sandstone 

Carbonaceous veins Altered sandstone 

[All values, average of two samples. Elements not determined or reported (hand-picked samples) from near 

only as less-than values were omitted. These include Ag, Au, B, Be, carbonaceous veins 

Bi, Cd, Eu, Ge, Ho, Sn, Ta, W, and Zr] In parts per million-continued 

Carbonaceous veins Altered sandstone Co 5 6 (hand-picked samples) from near Cr 22 83 carbonaceous veins Cu 7 5 

In percent Ga <4 15 

AI 0.21 6.0 La 8 30 

Li 60 18 Ca .08 0.9 Mo 100 25 Fe .46 1.6 Nb 4 25 K .08 2.5 

Mg .05 .47 Nd 27 23 

Na .03 1.8 Ni 11 18 

Pb 28 19 p .008 .25 Sc <2 6 Ti .008 .20 Sr 15 210 

In parts per million Th <4 6 

u 1850 <100 

Mn 24 220 v 2700 154 

As 140 <10 y 44 17 

Ba 18 790 Yb 2 2 

Ce 32 55 Zn 5 24 

Uranium Mineralization Model 13 
A B 

c 

E F 

14 Epigenetic Uranium Deposits, Sespe Formation, California 

E 

0 
Samples of the Sespe are divided into two groups. The 

first is taken from localities or stratigraphic positions that 

are remote from mineralization. The second group is in 

the lower transitional Sespe proximal to, but not within, 

areas of mineralization. The Th:U ratio for the proximal 

group averages about 1.6 and for the remote group it 

averages about 4.1. This relationship suggests that the 

part of the Sespe represented by the remote-group 

samples has lost uranium through leaching and could be 

the source of uranium for the deposits. 

Most of the samples designated in the remote group 

on table 2 (14 samples) are reddish brown, and, based 

on color, are more highly oxidized than the proximal 

group. These samples average 9.1 ppm thorium and 

2.2 ppm uranium. Two of these samples, 61381-3 and 

61381-4, were collected near the uranium deposits on 

Superior Ridge, but their color suggests that they were 

oxidized and probably leached. 

The hypothesis suggested here is that the proximal 

group of samples represents rock that was neither leached 

nor enriched in uranium; thus these samples provide a 

baseline for comparisons. Only four samples were included in this group (table 2), which averages 6.4 ppm 

uranium and 9.6 ppm thorium. Three of the four samples 

in this group are light yellowish brown. Chemically reducing conditions apparently prevailed in parts of the Sespe, 

not only where uranium mineralization occurred, but also 

in the surrounding areas represented by the proximal 

samples. 

Figure 7 (facing page). Carbonaceous veins in lower transitional Sespe Formation at Beer Can #12 uranium claim. A, 

Photograph of vein of carbonaceous material in the altered 

sandstone shown in B. Light streaks above knife are pick marks. 

Knife 7.5 in. long. B, Photograph of pod of altered fluvial 

arkosic sandstone containing veins of carbonaceous material. 

Hammer (lower center) 13 in. long. C, Photomicrograph 

(reflected light) of central part of polished section from vein 

(0) showing jarosite and clay minerals. Lighter areas showing 

fractured fragments are carbonaceous material. 0, Photograph (upper) of vein fragment consisting primarily of 

carbonaceous material. Banding represents matching depositional layers on opposing sides of vein. Light-colored layers are 

siliceous. Void in central part was filled with jarosite, 

KFe(S04)

3(0H)6; quartz, Si02; and clay minerals. Autoradiograph (lower) of sample shown in upper part. Notice similarity between banding shown in photograph (upper) and lighter 

colored bands showing high radioactivity in autoradiograph 

(lower). E, Electron micrograph of polished carbonaceous vein 

material showing transverse veinlets. Veinlets contain silica, 

vanadium, potassium, aluminum, iron, and uranium, in 

decreasing order. Note also numerous tiny inclusions(?) of silicarich material. F, Electron micrograph of central part of 

carbonaceous vein (f) showing porous texture of jarosite and 

clay minerals. Smooth grain in central part of picture is a 

molybdenum mineral. 

Neither felsic tuff nor alkaline granite, generally the 

most favorable uranium source rocks, is present in great 

abundance near any of the uranium deposits in Ventura 

County. Late Tertiary volcanic rock in the Ventura basin 

and Western Transverse Range area is mostly basaltic 

(Moser and Frizzell, 1982), and such rocks are not a rich 

source of uranium. A Miocene felsic tuff is found near 

the Frazier Borax Mine in Cuddy Valley, and Pliocene 

felsic rocks in the Wagon Road Canyon area have been 

evaluated as potential source rocks by Dickinson (1982). 

Although these rocks contain significant amounts of 

uranium, their location is probably outside the Sespe 

depositional basin and they are therefore ruled out as 

potential sources. Volcanic ash is a strong component of 

phosphatic sediments of the Santa Margarita Formation 

in the upper Sespe Creek area (Lowe, 1969), and samples 

of these sediments from near Pine Mountain collected by 

T. Vercoutere (written commun., 1983) averaged about 

30 ppm uranium. The possibility that this uranium played 

a role in the Sespe mineralization cannot be eliminated, 

but the distant location is not favorable for the Superior 

Ridge and Laguna Ridge deposits. 

Leaching 

Uranium is mobilized from source rock in its oxidized valence state ( + 6) and commonly travels in solution as a di- or tricarbonate anion complex. For this 

reason, uranium leaching commonly occurs under oxidizing and alkaline environments. Such environments are 

favored by arid or semi-arid climatic conditions (which 

preserve the abundance of relatively soluble alkalies) and 

by the presence of volcanic glass that tends to release 

alkaline metals upon hydration. Reed (1929) suggested 

that the Sespe was deposited under humid or subhumid 

conditions because of the commonness of red beds in the 

sequence. However, the red color apparently resulted 

from diagenetic alteration of ferromagnesium minerals 

to hematite (McCracken, 1969b). Therefore, it is likely 

that the Sespe was deposited under semi-arid conditions 

as suggested by Reinhart (1928), who based his opinion 

on freshness of the feldspar. In addition, the upper part 

of the Sespe contains gypsum and lacustrine limestone, 

indicators of an arid climate. 

Paleohydrology 

A certain amount of structural warping may have 

occurred during the deposition of the Sespe Formation. 

As a result, accurate knowledge about the paleohydrology 

during the main period of mineralization, which is believed to be the Oligocene, is not available. The best 

assumption, however, is that the hydrologic head was 

Uranium Mineralization Model 15 
MOUNTAINS 

(temperate 

climate?) 

··~·.··.· 

SESPE FORMATION DEPOSITIONAL BASIN 

(semi-arid climate?) 

~~ Itt~,. . Alkaline env· :--... tic,., SOiuticm Of 0 ,.,"0 n!l"'ent and 

',...._ ~.· Zone of leach 9lUliC. m. aterial ...... , eel 8rkose ( ............ , (remotez Tb:(J ::::r4} ..... .._ Fresh al"-t· one) .,. ..__ ~ ~rtheg 

~Ohe Of o;:;;;U -:::":''":'"~ ~ ro ground Water 

. m-unleache"d-----~- · . .........-'r. 1 

(pro · . arkose(.,.... ----- _....... 7. ante 'omaJzonet .. q:tJ::::::l.6) ---....:.--:----. -. · matenaJ,... ' ------ ,..reservat' . ------ 100 · Nonmarine 

Zone~ 

and uraniu:~~ deposition 

mera/iz:ation 

Nearshore marine 

saline ground water 

Figure 8. Hypothesized conditions for solution of uranium and humic material, transportation in ground water, and resulting 

uranium mineralization in lower transitional Sespe Formation (Dickinson, 1982). 

generally parallel to streamflow (Butler, 1969). Bailey 

(1947) suggested that a major westward-flowing river was 

draining the Sespe depositional basin and that tributaries 

entered this stream from both the north and south. 

Paleocurrent directions determined in the upper Sespe 

Creek area by McCracken (1969a) and by Bohannon 

(1976) confirm this pattern. The Superior Ridge deposits 

probably result from uranium deposition in sandstonefilled paleochannels formed by streams flowing south to 

the main river or southwest to the seacoast. South-flowing 

paleostreams in the upper Sespe Creek area (Bohannon, 

1976) deposited sandstone bodies that may have also 

served as conduits for uranium-bearing ground water and 

that now contain uranium. 

In the Superior Ridge area, ancestral ground-water 

flow in the lower transitional Sespe was probably 

southward or southwestward inasmuch as this area lies 

north of Bailey's (1947) major stream and the seacoast 

was to the west. The uranium host rock in this area was 

deposited close to the beach as indicated by marine shells 

found in lower Sespe gravels. Ground-water movement 

in these sediments would have been sluggish shortly after 

deposition because of the low hydrologic head near sea 

level. This sluggishness may have contributed to establishment or preservation of the reducing environment in these 

rocks. 

16 Epigenetic Uranium Deposits, Sespe Formation, California 

Host Rock 

The main host rocks for uranium deposition are 

beds of yellowish- or pinkish-gray, poorly sorted to wellsorted medium- to coarse-grained arkosic sandstone of 

the lower transitional Sespe Formation. These lower beds, 

deposited in south- to southwest-trending fluvial channels, laterally intertongue with nearshore marine sandstone beds of the underlying Coldwater Sandstone. No 

porosity or permeability measurements were made on the 

uranium host rocks, but they are locally well sorted fluvial 

sandstones and, as such, probably had high initial 

permeability. 

Reductant 

Bowes and Myerson (1957) pointed out that 

uranium in the Sespe Formation is associated with 

carbonaceous material. This led to the conclusion that 

carbonaceous material provided the reductant that concentrated the uranium. At most of the uranium occurrences, the carbonaceous material was reported as 

carbonized wood or carbon trash. An exception is the 

Beer Can #12, where the organic matter occurred in veins. 

An additional exception is one sample of organic matter 
from the Payore claims (table 3) that does not appear to 

have originated as carbonized wood. In addition, carbonaceous material from some deposits not examined 

may have been in a form other than carbonized wood. 

In a preliminary hypothesis, Dickinson (1982) suggested 

that the veins were derived from humate deposited by 

ground water in a manner similar to that of the Holocene 

Florida humates, which formed when humic-acid-bearing 

stream or ground water came into contact with marine 

water (Swanson and Palacas, 1965). The analyses 

presented herein are in general agreement with this 

hypothesis. 

CONCLUSIONS 

1. The uranium host rock is arkosic sandstone deposited 

in channels occupied by south- and southwestflowing streams emptying directly into the sea or 

into a west-flowing trunk stream near the marine 

coast. 

2. The source of the uranium was predominantly 

granitic and volcanic clasts that make up much of 

the Sespe Formation. 

3. The sandstone bodies that provided conduits for 

uranium and humic-acid-bearing ground water 

maintained approximately the same structural attitude that existed at the time of sand deposition. 

Considering the stratigraphic distribution of the 

uranium occurrences (at the base of the Sespe), 

these sandstone bodies could not have functioned 

as conduits for ore-bearing solutions after 

Pleistocene tectonism. 

4. Carbonaceous material, vanadium, and molybdenum 

are associated with uranium in the uraniummineralized rocks in the Sespe and were probably 

transported penecontemporaneously in the 

uranium-mineralizing or related fluids. 

5. Much, if not all, of the carbonaceous material that 

is associated with the uranium was formed by the 

precipitation of humate from humic-acid-bearing 

ground water in a chemically reducing zone where 

subsurface waters of terrestrial and marine origin 

intermingled. 

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, U.S. GOVERNMENT PRINTING OFFICE: 1988-573-047/66,061 REGION NO.8