Cooccurrence of Fe-, Fe-Ca-, and Ca-Phosphate 

Minerals in Concretions Within the 

Monterey Formation: A Record of Uplift of the 

Santa Maria Basin, California

Geophysical section offshore Santa Maria basin

£;>' *< iw/i<r./ *< cv.y^'i -"' *-.-."

'^i'J^7-^J/^'f:^Jfjft:

v ->'.'..- ? .^o -^ ' .< . - > e, **', .}*!* ,

Geologic section onshore Santa Maria basin

U.S. GEOLOGICAL SURVEY BULLETIN 1995-C
COVER

TOP Seismic-reflection profile at approximately 350-m water depth through the Queenie 

structure in the offshore Santa Maria Basin. Section shown here, from line 812, was processed 

by Patrick H. McClellan. This profile is part of a larger set of seismic data collected in the 

offshore Santa Maria Basin during 1978 by the U.S. Geological Survey research vessel Samuel 

P. Lee.

BOTTOM Northeast-southwest cross section through the Casmalia Hills and Santa Maria Valley areas of the onshore Santa Maria Basin, modified slightly from Woodring and Bramlette 

(1950). The Cretaceous, Tertiary, and Quaternary sedimentary sequence overlies igneous and 

metamorphic rocks of Mesozoic age.
Chapter C

Cooccurrence of Fe-, Fe-Ca-, and Ca-Phosphate 

Minerals in Concretions Within the 

Monterey Formation: A Record of Uplift of the 

Santa Maria Basin, California

By D.Z. PIPER, C.M. ISAACS, and M.D. MEDRANO

U.S. GEOLOGICAL SURVEY BULLETIN 1995

EVOLUTION OF SEDIMENTARY BASINS/ONSHORE OIL AND GAS INVESTIGATIONSSANTA MARIA PROVINCE

Edited by Margaret A. Keller
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

and does not imply endorsement by the U.S. Government

Text and illustrations edited by George A. Havach

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993

For sale by

Book and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25286 

Denver, CO 80225

Library of Congress Cataloging in Publication Data

Piper, David Z.

Cooccurrence of Fe-, Fe-Ca-, and Ca-phosphate minerals in concretions within the Monterey Formation : a record of uplift of the Santa Maria Basin, California 

/ by D.Z. Piper, C.M. Isaacs, and M.D. Medrano.

p. cm. (Evolution of sedimentary basins Santa Maria Province ; 

ch. C) (U.S. Geological Survey bulletin ; 1995)

Includes bibliographical references.

Supt. of Docs. no. : I 19.3:1995C

1. Phosphate minerals California Lompoc Region. 2. Geology, Stratigraphic Miocene. 3. Geology California Lompoc Region. 4. Monterey 

Formation (Calif.) I. Isaacs, Caroline M. II. Medrano, M.D. III. Title. IV. Title: Co-occurrence of Fe-, Fe-Ca-, and Ca-phosphate minerals in concretions 

within the Monterey Formation. V. Series. VI. Series: U.S. Geological Survey 

bulletin; 1995-C. 

QE75.B9 no. 1995-C 

[QE389.64] 

557.3 s dc20

[553.6'4'0979491] 92-28029

CIP
CONTENTS

Abstract Cl 

Introduction Cl 

Analytical techniques C2 

Results C3

Major-element oxides C3

Minor elements C5

Mineralogy C5 

Mineral stoichiometry C6

Francolite C7

Mitridatite C9

AFP and vivianite C9

Mn-Ni-Zn oxide C9

Mineral distributions C9 

Pelletal phosphate grains CIO 

Discussion CIO

Mineral paragenesis CIO

Elemental sources Cll

Mass transport C12 

Conclusions C13 

References cited C13

FIGURES

1. Index map of California, showing location of the Celite mine C2

2. Photograph showing outcrop of Monterey Formation at the Celite mine C3

3. Photographs of thin-section billets of concretions examined by microprobe and 

SEM-XRF C4

4. Plot of REE patterns in concretion 5-1 C6

5. Plot of XRD patterns in concretion 5-1 C7

6. Backscatter SEM photographs of concretions within the Monterey Formation C8

7. Plot illustrating stability relations between Fe-Mn oxides and phosphate 

minerals Cll

TABLES

1. Bulk composition of individual layers in concretion 5-1 C5

2. Partial listing of microprobe analyses of concretions 5-1, 6-A, and 6-B and of 

phases within the enclosing sedimentary rocks C5

3. Trace-element composition of concretion 5-1 C6

4. Stoichiometry of major phosphate phases in concretion 5-1, of a pelletal phosphate 

grain within diatomite, and of a metal oxide in concretion 6-B C6

5. Partial listing of semiquantitative SEM-XRF analyses of concretions 5-1, 6-A, and 

6-B C7

6. Major mineral fractions of individual layers in concretion 5-1 C9

Contents III

Cooccurrence of Fe-, Fe-Ca-, and Ca-Phosphate 

Minerals in Concretions Within the Monterey Formation: 

A Record of Uplift of the Santa Maria Basin, California

By D.Z. Piper, C.M. Isaacs, anc/M.D. Medrano

Abstract

Phosphatic concretions occur within diatomite in the 

upper part of the Miocene Monterey Formation near Lompoc, 

Calif. Absence of disruption of fine laminar bedding in the 

associated sediment by the concretions shows that they 

formed after complete compaction of the enclosing sediment.

The concretions exhibit a strongly concentric color, 

chemical, and mineralogic zonation. Many of them are composed of a nucleus in which vivianite is the dominant mineral. Amorphous ferric phosphate, mitridatite, and francolite 

are the dominant phosphatic phases in successive layers toward the surface of the concretions. Cd and As contents increase tenfold from the nucleus outward, reaching a 

maximum of 2,000 ppm, whereas Ni content, with a maximum of 720 ppm, and Co content show the opposite trend.

This mineralogy and elemental composition favor accretion under conditions of continuously increasing Eh and pH, 

during uplift into the fresh-ground-water zone of the 

terrestrial environment. Shale-normalized rare-earth-element 

patterns, however, suggest a marine source for the elements biogenic debris consisting of opal-A, organic matter, 

and carbonates of the enclosing sediment.

INTRODUCTION

Concretions recovered from marine sedimentary rocks 

can range in size from a few tenths of a millimeter to several tens of centimeters across. The structural relation of 

the enclosing sediment to the concretions, as viewed in 

outcrop, can reveal the timing of their precipitation: (1) 

under marine conditions at the sediment surface during 

deposition, at a shallow depth of burial during early diagenesis, or at a late stage of burial and diagenesis after 

compaction of the enclosing sediment or (2) under freshground-water conditions during uplift into the terrestrial 

environment.

Manuscript approved for publication, May 26, 1992.

Their composition and mineralogy reflect the chemistry of the water within which they formed. For example, 

ferromanganese nodules occur within pelagic marine sediment (Elderfield and others, 1981) under relatively high 

Eh and pH conditions, whereas concretions composed of 

vivianite require reducing freshwater conditions of approximately neutral pH (Hearn and others, 1983). Other mineralogies for concretions in sedimentary rocks include silica 

(opal-CT and quartz), carbonates (calcite, manganocalcite, 

dolomite, and siderite), hydroxide (bauxite), oxides (todorokite, 5-MnO2, and goethite) and sulfides (pyrite), to name 

but a few, which reflect still other water chemistries.

In this study, we examine the mineralogy and chemical 

composition of phosphate concretions collected from the 

Monterey Formation at the Celite (formerly Johns Manville) mine, a diatomite quarry located 5 km south of the 

city of Lompoc, Calif, (fig. 1). The diatomite is thinly laminated (fig. 2) and consists of as much as 95 weight percent 

biogenic silica. Whereas concretions at other localities of 

this formation are composed predominantly of silica, calcite, dolomite, and (or) francolite (Bramlette, 1946; Murata 

and others, 1969, 1972; Pisciotto, 1981), those examined 

here have a unique mineralogy. In one concretion, the dominant phases in successive envelopes from the nucleus to 

the outer surface are vivianite, amorphous ferric phosphate, 

mitridatite, and francolite; in another concretion, precipitation of an Mn-Ni-Zn oxide followed that of francolite.

We interpret this complex mineralogy of phosphate 

and oxide phases and their chemical and structural relations to the host sediment in terms of the sequence of 

chemical changes that controlled the composition of the 

concretions and the approximate timing of their formation.

Acknowledgments. Reviews of the manuscript were 

generously provided by R. Koski, W. Carothers, and G. 

McClellan. The X-ray-fluorescence and inductively coupled plasma emission-spectroscopic analyses were carried 

out at the U.S. Geological Survey's laboratory in Denver, 

Colo. R. Erd provided X-ray-diffraction analyses. The personnel at the Celite mine provided us access to the site 

and several of the concretions examined.

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C1
ANALYTICAL TECHNIQUES

The concretions were analyzed for mineralogy by Xray diffraction (XRD); for major-element oxides by X-ray 

fluorescence (XRF), inductively coupled plasma opticalemission spectroscopy (ICP-OES), XRF scanning-electron microscopy (XRF-SEM), and microprobe; and for 

minor elements by ICP-OES. The precision of the XRF

and ICP analyses, which is better than 10 percent for all 

analyses, has been established by repeated analyses of 

U.S. Geological Survey rock and ore standards over several decades (Crock and Lichte, 1982; Lichte and others, 

1987; Taggart and others, 1987), as well as by concurrent 

analyses. The precision of the microprobe analyses is better than 7 percent, as determined by our analyses of 10 

rock and ore standards. The precision of microprobe fluo120°30' 120°15f 120°00'

35°00' -

34°45' -

34°30' -

Figure 1. Index map of California, showing location of the Celite mine. 

C2 Evolution of Sedimentary Basins Santa Maria Province
rine analyses, however, is poorer than 10 percent at F 

contents less than about 1.5 weight percent, and poorer 

than 50 percent at F contents less than about 0.3 weight 

percent. ICP-OES analyses of sample aliquants for minor 

elements showed less reproducibility than the approximately 15-percent precision of earlier studies, except for 

rare-earth elements (REE's). This apparent loss in precision likely reflects sample heterogeneities, rather than 

larger analytical errors.

Total carbon and organic carbon were measured by an 

induction-furnace carbon analyzer; precision is better than 

10 percent. Analyses of calcite-spiked samples have a precision of 5 percent.

RESULTS

The concretions occur within an approximately 50- 

m-thick section of the Monterey Formation (upper Miocene). They are spherical to lenticular and 1 to 6 cm in 

diameter. Polished surfaces exhibit a concentric color zonation. In concretion 5-1 (fig. 3), layer 1, the outermost 

envelope, is white to olive yellow (5Y6/6); layer 2 is 

grayish olive (10Y4/2); layer 3 is light olive brown (5Y5/ 

6); layer 4 is moderate reddish brown (10R4/6) but with a 

well-defined, 1-mm-thick, dusky-red (10R3/6) zone at its 

outer edge; and layer 5, the nucleus, is dusky blue (5PB3/ 

2). Visually, part of the nucleus changes color on exposure to resemble layer 3. Under transmitted light, the

phosphatic matrix in layer 1 is colorless; in layers 2, 3, 

and 4 it is deep red; and in layer 5 it is blue and, where 

microcrystalline, strongly pleochroic; it remains so even 

after exposure to air.

Most of the concretions examined in the field consist 

of two to four layers; a dusky-blue nucleus is absent. 

Many concretions enclose interiors filled with friable diatomaceous silica (concretions 6-A, 6-B, fig. 3), but none 

contain an identifiable nucleating solid phase.

Major-Element Oxides

The major-element-oxide composition of the five layers in concretion 5-1 (table 1) strongly reflects the color 

differences of individual layers. CaO decreases in content 

from 23.2 weight percent in layer 1 to 2.4 weight percent 

in layer 5, whereas Fe2O3 (considered to be present as 

Fe2+ in layer 5, owing to its mineralogy; see below) shows 

the opposite trend. The F content exceeds 0.2 weight percent only in layers 1 and 2, and inorganic CO2 is restricted 

to these two layers (that is, its concentration in layers 3, 4, 

and 5 is below the limit of detection of 0.05 weight percent). Other major oxides show no consistent trends, with 

the possible exception of SO3. Microprobe analyses suggest that the bulk SO3 content, which could include both 

S2~ and SO42~, is slightly higher in the outermost layer, 

although more analyses are required to fully document its 

distribution (tables 1, 2). However, S content as SO3 is

Figure 2. Outcrop of Monterey Formation at the Celite mine, showing relation of concretion occurrence to sediment laminations. Hammer at lower right shows scale.

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C3
409

101

211 102- 

213 104 S 107 

219 202 

204

143

305

306

127

131

- 709

* 710

.^^^^ ; <

Figure 3. Thin-section billets of concretions 5-1 (A), 6-A (B), 

and 6-C (Q examined by microprobe and SEM-XRF. Threedigit numbers refer to samples listed in table 2. Numbers 1 

through 5 in figure 3A denote layers referred to in text and 

in tables 1, 3, 5, and 6; light-colored rings denote areas of 

microprobe analyses. Alternating light- and dark-colored layers in figures 36 and 3C reflect alternating dominance of 

mitridatite and francolite, respectively, and interpretation 

based on microprobe analyses for Fe and Ca (see table 2) 

but not on actual XRF analyses. Laminae of dark-colored 

grains in concretion 6-B represent a high concentration of 

detrital plagioclase and quartz.

C4 Evolution of Sedimentary Basins Santa Maria Province
Table 1. Bulk composition of individual layers in concretion 

5-1

[All values in weight percent; dashes, not analyzed. Major-element oxides determined by XRF analysis, and organic and inorganic CO2 by induction-furnace carbon analyzer. Fe reported as Fe2O3 except for layer 5. 

F and SO3 determined by microprobe; values represent averages of all 

analyses of matrix material, that is, excluding analyses of discrete grains 

of opal, quartz, and Al silicates. LOI, loss on ignition. Layers refer to 

concretion shown in figure 3. Clay-shale represents the terrigenous fraction (Isaacs, 1980)]

A1 2°3 

Fe 2 03 --

FeO 

F 2°5

C02 (org) 

CO2 (inorg)

°3

2.8

8

0

2.9

1.5

14 . 9

2.2

1 . 1

.4

14 .1

4 .0 

0

25.

1.

1.

1. 

0

4

5

3.6 

26.9

1.8 

0

 !

less than 0.55 weight percent in all microprobe analyses, 

and no metal sulfides were detected with the SEM, microprobe, or petrographic microscope.

Minor Elements

The REE contents are similar in all layers except layer 4 (table 3). Shale-normalized REE patterns of layers 1, 

2, 3, and 5 show a negative Ce anomaly (a depletion in Ce 

relative to La and Nd) and an enrichment in heavy REE's 

(fig. 4) relative to light REE's; this pattern is characteristic 

of seawater (DeBaar and others, 1985). Normalized REE 

contents in layer 4 decrease from light to heavy REE's.

Other trace-element contents vary sharply among the 

layers of concretion 5-1 (table 3). As, Cd, Cu, Zn, and V 

decrease in content toward the center, whereas Ni and Co 

show the opposite trend, and Cr content reaches a maximum in layer 3. In all the layers, trace-element contents 

exceed a possible clay-shale contribution, weighted according to A12O3 content (table 1).

Mineralogy

The mineralogy of each layer in concretion 5-1 (fig. 

5) is consistent with variations in the contents of CaO, Fe 

(both Fe2O3 and FeO), F, and inorganic CO2, although absolute mineral abundances cannot be inferred from the Xray diffractograms. Carbonate fluorapatite (McClellan,

Table 2. Partial listing of microprobe analyses of concretions 

5-1, 6-A, and 6-B and of phases within the enclosing sedimentary rocks

[All values in weight percent. Locations of samples are shown in figures 

3 and 6 except for the 900 series of analyses, which represent the enclosing sedimentary rocks: analyses 900, pelletal phosphate; analyses 910 

and 911, dark- and light-colored layers in diatomite, respectively. Asterisks denote analyses used to make calculations listed in table 4]

Analysis 1

Si02 30

A1 2 03 -

FeO 1

MgO 

Na 2 0 

K2 0 

CaO 6

P 2 05 5

S03 

01

.2

45

20

01

08

04

36

81

04

1

20

2

3

31

23

32

1

83

33

46

28

34

8

6

24

103

64.5

21.9

.11

.01

9.32

.12

4.71

.39

.04

104

32.1

.98

1.28

.14

.08

.09

11.2

8.36

.12

1

3

1

1

38

30

37

56

97

81

22

35

05

7

8

42

12

11

5

2

40

29

7*

7

38

01

75

33

90

3

6

14

1

27

2

4

23

19

31

2

09

20

34

17

19

9

5

14

1

17

3

2

37

27

33

8

53

26

54

32

36

5

4

17

2

26

1

15

12

17

32

2

54

4

28

19

20

6

4

09

2

18

1

21

13

22

34

5

24

7

18

14

21

4

2

11

Analysis

Si02 

A1 2 03 

FeO 

MgO 

Na 2 0 

K2 0 

CaO 

P 2°5 

Fso3 '

20.1

1.21

23.7

.19

.17

.29

12.5

24 .0

.31

.05

23.0

2. 67

19.5

.40

.46

.25

15.3

23.0

. 68 

.09

15.7 30

2.36

13.5 9

.41

.29

.25

26.3 3

26.2 8

1.56 

.17

1

83

67

14

16

10

42

87

12 

19

33.8 34.5 26.6 51.5 9.36 21.4

2.31 2.72 1.12 1.64 1.08 1.37

9.37 19.8 16.9 12.5 43.3 34.1

.38 .56 .15 17 .31 .32

.21 .43 .08 54 .33 .41

.25 .65 .12 41 .15 .18

3.26 7.20 3.99 4 45 4.55 3.62

8.00 18.0 9.55 11 2 22.4 22.8

.14 .24 .09 08 .03 .05

Analysis

Si02 

A1 2 03 -

FeO 

MgO 

Na2 0 

K2 O 

CaO 

P 2 05 

S03 -

Analysis

Si02 

A12 03 -

FeO 

MgO- -

Na2 0 

K2 0 

CaO 

P 2°5 

S03 -

409*

4.20

.50

43.1

.22

.40

.19

4 . 61

25.9

.01

706

17.9

.11

26.5

.10

.42

.41

13.5

25.0

.13

4:

1

1

32

12

30

7(

20

10

7.

12

30

.75

.85

.8

.39

.41

.38

. 1

.2

.13

37

.6

.29

.5

,11

.09

08

41

.0

11

504

.98

.27

46.8

.24

.07

.02

.28

32.4

.16

708

18.0

.52

1.33

.44

.36

.09

34.8

26.6

.55

5

10

1

36

26

7:

16

38

26

11

.7

.26

.9

.41

.13

.19

.95

.2

.09

LO

.9

.30

.30

.40

.40

10

.6

.8

40

51

0

42

28

7:

6.

1.

5.

32.

25,

8*

12

.2

12

Ob

01

37

.6

07

LI

01

00

50

.40

40

40

.0

.5

30

5:

37

9.

30

7:

19

1,

30

21

19

.74

, 11

. 1

.42

.32

.15

.30

.8

04

12

.7

.60

.90

.30

.30

20

. 1

. 1

51

521

0

.25

42.0

.19

.09

.01

.26

29.5

.13

801

7.72

.50

27.8

.30

.40

.50

13.9

24.6

.11

71

17

2

38

29

8(

2.

20

11

19

33

.6

.40

.11

.37

.39

.11

. 1

.0

.51

32

.51

.30

.3

. 10

.10

10

.1

.6

.01

7(

20

23

13

23

8(

5.

29

15

28

34

.4

.40

.2

.22

.28

.38

.0

.2

19

33

.70

.30

.5

.40

.50

80

.5

.1

02

705

18.5

1.30

1.57

.63

.44

.31

37.2

27.2

.64

805

26.0

.40

22.3

.30

.30

.20

12.3

23.4.

.09

Si02 

A1 2 03 

FeO 

MgO 

Na 2 0 

K2 0 

CaO 

P2 05 

F 

so3 

24

1

14

8

14 

0

.5 27

10

.7 2

20

20

20

30 1

2 2

0

02

7

60

60

10

10

10

60

70

01

18

5

35

27 

1

0

80

60

40

40

20

3

9

70

52

21

1

1

37

27 

1

2

30

80

60

50

30

1

1 

90

38

51

36

3

1

31

21

02

10

65

07

8

1 

73

91

52.9 45

6.30 2

1.39

.95 0

.10

.56

.50

.15 

.07

.13

.0

76

19

03

04

19

10 

02

06

1980), commonly referred to as francolite, is the dominant 

mineral in layer 1. Mitridatite, a calcium-ferric phosphate 

(Moore, 1980), is the dominant mineral in layer 2, but 

francolite is also present. Both of these minerals are 

present in layers 3 and 4, as well as in layer 5. Individual 

deep-red grains, which are concentrated in an outer 1- to 2- 

mm-thick band of layer 4, consist of a pure Fe phosphate 

(their deep-red color indicates that iron is in the 3+ valence

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C5
Table 3. Trace-element composition of concretion 5-1

[All values in parts per million. Dashes, not analyzed; n.d., no data available. Trace-element analyses (duplicates in parentheses) by inductively 

coupled plasma optical-emission spectroscopy (ICP-OES); limits of detection: As, 1; Cd, 2; Ce, 4; Co, 1; Cu, 1; La, 20; Nd, 20; Ni, 2; V, 2; Yb, 

1; Zn, 4 Lichte and others, 1987; Taggert and others, 1987). Rare-earthelement analyses (in brackets) by ICP-OES after group separation; limits 

of detection: Ce, 0.15, Er, 0.04; Eu, 0.004; Gd, 0.09; Ho, 0.02; La, 0.02; 

Lu, 0.009; Nd, 0.01; Pr, 0.3; Sin, 0.15; Yb, 0.01 (Crock and Lichte, 

1982). Layers refer to concretion shown in figure 3. Analyses of clayshale from Haskin and Haskin (1966) and Wedepohl (1969); analyses of 

organic matter from Elderfield and others (1981) and Brumsack (1986)]

Layer 

La 

Ce 

Pr 

Nd 

Sm 

Eu 

Gd 

Ho 

Er 

Yb 

Lu 

AS 

Cd 

CO 

Cr 

Cu 

Ni 

V 

Zn 

1

(27)

(35)

(30)

--

--

--

--

--

(7)

(80)

(650)

7

(6)

55

(56)

36

(51)

76

(150)

220 

(320)

1,250

(1,200)

2

117]

(23)

[24]

(27)

[3.6]

[18]

(22)

[3.5]

[0.9]

[5.8]

[1.2]

[4.8]

[5.2]

(7)

[1.0]

2, 000

(450)

1,500

(950)

17

(14)

150

(89)

46

(50)

180

(170)

290 

(340)

2,000

(1, 400)

3

[15]

[20]

[2.1]

[11]

[2.1]

[0.6]

[5.4]

[0.58]

[2.7]

[3.0]

--

[0.6]

1,500

700

11

--

520

--

37

--

210

220

470

4

[28]

[38]

[3.3]

[8.1]

[0.8]

[0.2]

[0.5]

--

[0.2]

[0.24]

--

--

640

300

--

21

--

96

--

20

--

330

150

700

--

5

[7.7]

[11]

[1.4]

[4.8]

--

[0.7]

[0.2]

[1.4]

[0.37]

[1.5]

[1.2]

--

[0.2]

580

70

--

51

--

17

--

12

720

ft a o y

500

Clayshale

41

83

10.2

38

7 .5

1.61

6.5

1.34

3.75

3.53

.61

10

.3

19

83

35

42

110

100

Organic 

matter

0.014

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

6

12

1

2

11

7

3

110

state) that proved to be amorphous when X-rayed separately (R. Erd, oral commun., 1991); this phase is referred to 

below as amorphous ferric phosphate (AFP). Vivianite, a 

ferrous phosphate, is the dominant mineral in layer 5.

MINERAL STOICHIOMETRY

The stoichiometry of each phosphate phase (table 4) 

was calculated from microprobe analyses of concretion 5- 

1 (table 2) by using a normative scheme. Individual analyses selected for making the calculations represent the most 

phosphate enriched analyses with the largest sum of total 

oxides. In layers 1 and 2, we could not locate individual 

grains of pure francolite and mitridatite, possibly owing to 

their smallness and the pervasiveness of fine-grained, detrital Al-silicate minerals and diatomaceous material. In 

layers 4 and 5, the analyses used were of what appeared 

under the microprobe and SEM (fig. 6) to be individual 

mineral grains.

Our calculations are based on several assumptions. (1) 

All A12O3 is present in a terrigenous Al-silicate fraction,

Table 4. Stoichiometry of major phosphate phases in concretion 5-1, of a pelleta! phosphate grain with diatomite, and of 

a metal oxide in concretion 6-B

[See table 2 (asterisks) for microprobe analyses. CO2 contents in concretionary francolite from table 1. CO2 was added to achieve charge balance 

for pelletal phosphate, H2O was determined by forcing sums to 100 percent, and OH~ was added to achieve charge balance]

Concretionary francolite:

This study Ca 10.1 (P0 4>5.8< C03 ) 0.4 F2.0 

Mitridatite:

This study 1 Ca 4 (H 2 0) 6 Fe 10 _ 6 0 6 (P04 ) 9>3 -

Moore (1980) Cae (H20) sFegOg (PC>4) 9-3H2 0 

Amorphous ferric phosphate (oxykertschenite):

This study Fe3 (OH) 4 ^ 2 (P0 4 ) ^ 6 -5.5H2 0 

Moore (1980) Fe3 (OH) 3 (P0 4 ) 2 -5H20

Vivianite:

This study Fe3 (P0 4 ) 2 7 . 8H2 0 

Dana (1935) Fe 3 (P0 4 ) 2 -8H2 0

Metal oxide:

This study ------ 

Pelletal francolite:

This study 

McClellan (1980) -

MnNi n 10 c ZnO26-.S

(P0 4 ) 5 . 5 <S04 ) 0 _

'Additional analyses suggest that mitridatite contains as much as 

30 mol H2 0.

2 The value of x corrects the stoichiometry of this phase for the 

valence of Mn, which, on the basis of laboratory studies, is 

likely between 3+ and 4+ (Hera and others, 1987, 1989) .

with the same major-element-oxide proportions as those of 

clay-shale (table 1; Isaacs, 1980). Discrepancies in the oxide:Al2O3 ratios with clay-shale reflect variations in the 

composition of the clay-shale fraction and analytical errors 

at low K2O, Na2O, and MgO contents. Short-range variation in composition is clearly represented by discrete laminae composed largely of fine-grained quartz and plagioclase 

(fig. 3). The bulk of the terrigenous fraction, however, is 

considered to have a composition close to that of ideal clayLa Ce Pr Nd Sm Eu Gd Ho Er Yb Lu

Figure 4. REE patterns in concretion 5-1. Numbers refer to 

layers 1 through 5 in figure 3A Analyses normalized to shale 

values on an element-by-element basis (see table 3).

C6 Evolution of Sedimentary Basins Santa Maria Province
Table 5. Partial listing of semiquantitative SEM-XRF analyses 

of concretions 5-1, 6-A, and 6-B

[All values except for SI (SiO2 :Al2O3 ratio) in atomic percent; dashes, 

not detected. Na2O, K2O, and MgO were not detected in any analyses; 

the limit of detection for these major-element oxides was approximately 

0.7 weight percent, as determined by analysis of pure Al-silicate phases. 

See figure 6 for locations of samples]

Analysis- A-l

Ca 55.3

B20. 1

.9

B-2

20.3

B39.

3 C-l

.9 21.4

D-l D-2 E-l E-2 E-3

4.60 12.0

F2-1

.43

shale. In support of this interpretation, the range in SiO2: 

A12C>3 weight ratios of material filling diatom frustules (fig. 

6; table 5) is within the range of the estimated ratios for 

terrigenous clay-shale of 3.5:1 and its Al-silicate fraction of

2.1:1; the difference between these two values represents 

the amount of detrital quartz present in clay-shale. (2) The 

SiO2 in excess of the 3.5:1 ratio corresponds to the amount 

of biogenic opal-A (that is, diatomaceous material) present 

(Isaacs, 1980). (3) All of the F is in francolite. (4) The Fe in 

excess of an Fe2O3:Al2O3 ratio of 0.202, the clay-shale 

contribution (table 1), is in a phosphate phase. Finally, (5) 

water was added to bring all totals to 100 weight percent; 

the amount present as OH~ was assessed to balance charge 

in the phosphate phases, except for francolite.

Francolite

The stoichiometry of francolite was determined from 

analysis 127 of layer 1 (table 2). Iron in this analysis can be 

partitioned into the Al silicates with a residual Fe2O3 

content of only 0.8 weight percent. Charge balance in 

francolite was achieved by including inorganic CO2, as

I I I I I I I II

s s

Layer 4

Layer 1

I i i i i I i i i i I i i i i I i i i 

60° 50° 40° 30°

I I I i I I i i I 

20° 10°

Figure 5. XRD patterns in concretion 5-1. Numbered layers are shown in figure 3A F, francolite; M, mitridatite; S, feldspar; V, 

vivianite.

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C7
Figure 6. Backscatter SEM photographs of concretions within the Monterey Formation, showing locations of samples analyzed in tables 2 and 5. A, 

Francolite in layer 1 of concretion 5-1. B, Francolite 

(light-colored areas) and mitridatite (dark-colored areas) in a radiolarian (black areas) filling in concretion 6-B. An intimate association of these two 

minerals is visible along lower outer edge of radiolarian. C, Mitridatite lath in a diatom test, with porous background. D, AFP (light-colored area in a 

diatom filling) associated with CaO-depleted mitridatite^) in layer 4 of concretion 5-1. £, Vivianite 

(light-colored area) associated with mitridatite (darkcolored area) in a radiolarian filling in layer 5 of 

concretion 5-1. f, Mn 10Ni5ZnO2 6_x lining a cavity 

in concretion 6-A.

C8 Evolution of Sedimentary Basins Santa Maria Province
*y

CC>3 , in the amount measured in layer 1 (table 1). Inorganic CO2 was partitioned into this mineral (table 4), owing to its absence in the bulk analyses of layers 3, 4, and 5 

(table 1) and our failure to detect any carbonate minerals 

within the concretion. A difference in spacing of the 004 

and 410 XRD reflections of 1.5° 26 further supports the 

inclusion in francolite of approximately 1.5 weight percent 

CO2 as CO32~ (Gulbrandsen, 1970; Schuffert and others, 

1990).

SEM-XRF analyses of a relatively pure mineral grain 

(fig. 6A; table 5) confirmed the Ca:P atomic ratio of this 

calculation. Also, no Mg, K, or Na was detected by SEMXRF in excess of the Al-silicate contribution. This stoichiometry agrees closely with the published values for pelletal marine francolite (table 4), except for the absence of 

Na+, Mg2+, and SO42~ substitutions (McClellan, 1980; 

Nathan and Neilsen, 1980).

Mitridatite

The stoichiometry of mitridatite was determined from 

analysis 211 (table 2) by assuming that it and francolite 

are the only mineral phosphates in layer 2 (fig. 5). SEMXRF analysis of a single mineral grain (fig. 6) gave an 

Fe:Ca:P ratio of 10:6.7:12 (table 5), which approximates 

the literature value (table 4). Microprobe and SEM-XRF 

analyses of other grains indicate, however, that this Fe:Ca 

ratio represents a minimum.

The backscatter SEM image shows an intimate association of francolite and mitridatite in concretion 5-1 and 

the other concretions as well. This association is best 

shown in figure 6B, along the outer part of the diatom 

test, where francolite is the brighter phase. Its higher reflectivity requires that it has a higher mean atomic number than mitridatite, but the microprobe analysis (tables 

2, 4) requires the opposite. One possible way to lower 

the mitridatite mean atomic number is to add water. Other microprobe analyses of concretions 5-1, 6-A, and 6- 

B suggest that mitridatite's formula contains as many as 

30 mol H2O, or Ca4(H2O)6Fe 10 3O6(PO4)9-30H2O. Although its mean atomic number is less than that of francolite, its H2O content, which is very weakly constrained 

by the method we have used to estimate it in this and 

other phases, greatly exceeds that reported by Moore 

(1980).

AFP and Vivianite

The stoichiometry of AFP (analysis 409, table 2) approaches that of oxykertschenite, an amorphous phase 

formed through the oxidation of vivianite (Moore, 1980). 

Vivianite, despite its blue color, has the classical composition of 8H2O, with no OH~ ions needed to balance charge, 

in contrast to the stoichiometry favored by Tien and 

Waugh (1969) for a blue vivianite.

Table 6. Major mineral fractions of individual layers in concretion 5-1

[All values in weight percent, rounded to the nearest 5 weight percent. 

See table 1 for major-element-oxide analyses]

Francolite 

Vivianite 

Biogenic silica35

5

0 

0 

35

15

25

35

0 

0 

25

0

3

5 

20

10 

0 

40 

25

0

5

50 

0 

15 

25

0

5

5 

0

0

60 

15 

20

0

Semiquantitative SEM-XRF analyses of vivianite (table 5) suggest that some Fe may be present, which 

would be balanced by the addition of OH~. The Fe:P 

atomic ratio is 9:8, somewhat larger than that of 9:6 from 

microprobe analyses. In both data sets, the ratio is the 

same in the two phases.

Mn-Ni-Zn Oxide

The only metal oxide or sulfide detected (fig. 6F; 

analysis F-l, table 5) is an Mn-Ni-Zn oxide, which occurs as the lining of a cavity within concretion 6-A and 

has an Mn:Ni:Zn atomic ratio of approximately 10:5:1. Its 

high electron backscatter precludes its presence as a carbonate. Charge balance was achieved with 26 oxygen atoms (table 4), by assuming that Mn is in the 4+ valence 

state, although it is much more likely to have an average 

valence between 3+ and 4+ (Hem and others, 1987, 

1989).

Mineral Distributions

The average abundances of minerals and clay-shale in 

the layers of concretion 5-1 were determined from bulk 

analyses (table 1) by assuming a constant stoichiometry 

for each phase (table 4). These calculated mineral distributions (table 6) are supported by X-ray diffractograms (fig. 

5). We note a change of the dominant phosphate phase, 

from vivianite in layer 5 to francolite in layer 1, as well as 

minor amounts of francolite and mitridatite in layers 3, 4, 

and 5. Also, the clay-shale:biogenic-silica ratio, based on 

major-element-oxide analyses, is significantly smaller in 

the outer than in the inner two layers.

Microprobe analyses of concretions 6-A and 6-B 

(fig. 3; table 2), in which only francolite and mitridatite 

were detected by XRD, show a different distribution. The 

two minerals alternate in dominance between successive 

layers, but the abundance of francolite still increases in the 

outermost layers, on the basis of an outward overall increase in Ca:Fe ratio and F content (700 and 800 series of 

analyses, table 2).

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C9
PELLETAL PHOSPHATE GRAINS

Grains of pelletal phosphate also were observed in 

thin sections of the diatomite. The pellets are 1 mm in 

diameter and constitute less than 1 volume percent of the 

total sedimentary rock. The absence of Fe, a high F content, and an F:P atomic ratio of 0.26 (analysis 900, table 

2) all require that francolite is the only phosphate mineral 

present. Its composition, however, corrected for a minor 

contribution of oxides from Al silicates, differs from that 

of francolite within the concretions (table 4): The pelletal 

francolite has Na+ and SO42~ contents typical of marine 

francolite (McClellan, 1980; Nathan and Neilsen, 1980).

DISCUSSION

The host sediment of the concretions consists mainly 

of diatomite, diatomaceous shale, and diatomaceous mudstone (Isaacs, 1989) that contain traces of pelletal phosphate. The abundance of terrigenous debris ranges from as 

little as 10 weight percent in some mined crudes to 50 

weight percent or more in other beds. Strata are thinly 

laminated (fig. 2) in the area from which the concretions 

were recovered. Darker colored laminae contain slightly 

more terrigenous material and organic matter than lighter 

colored laminae (Donegan and Schrader, 1981). Donegan 

and Schrader favored accumulation on an open shelf, impinged by an intense O2-minimum layer, similar to conditions in the Gulf of California (Calvert, 1964). Govean 

and Garrison (1981) and Pisciotto and Garrison (1981) favored a poorly aerated, deep-water marine basin, as proposed initially by Bramlette (1946).

The laminar structure of the diatomite is unaffected 

by the presence of the concretions we have examined (fig. 

2). Laminae exhibit no compaction features about the concretions and pass .through several concretions with no 

change in thickness. Because the porosities of diatomaceous sediment are unusually high (Hamilton, 1976; 

Isaacs, 1981), laminae in the diatomite would show distinct draping around concretions that formed in uncompacted sediment, with thicknesses as little as a fourth to a 

third those of laminae possibly preserved within the concretions. Thus, these concretions must have formed after 

the sediment was fully compacted.

The concretions could have formed near maximum 

depth of burial within the marine environment, where an 

absence of bedded opal-CT strata restricts depth of burial 

to less than approximately 800 m for a geothermal gradient of 50 to 67°C/km (Keller and Isaacs, 1985). Conversely, they could have formed during uplift into their present 

terrestrial setting, beginning 4-2 Ma (Namson and Davis, 

1990). Thus, their mineralogy and minor-element composition might represent a part-freshwater, part-seawater origin. Alternatively, the concretions could have formed

totally within a freshwater environment, despite the strong 

marine signal. In either case, their mineralogy and elemental composition are unrelated, at least directly, to the environment of sediment deposition.

Mineral Paragenesis

On the basis of mineral distributions within the concretions (table 6), paragenesis of the phosphatic and metaloxide phases is represented by the following scheme:

vivianite - AFP - mitridatite - francolite - 

Mn 10Ni5ZnO26_x,

where x corrects for an average valence of less than 4+ for 

Mn. The natural occurrences and thermodynamic properties of these minerals require that this sequence represents 

a change in pore-water chemistry from reducing and virtually neutral pH to oxidizing and alkaline. Vivianite, which 

constitutes the nucleus of concretion 5-1, was the initial 

precipitate; it has been reported in lake and estuarine sediment and in soils (Rosenqvist, 1970; Nriagu, 1972; Hearn 

and others, 1983; Henderson and others, 1984). On the basis of these occurrences, vivianite precipitates under the 

most reducing conditions and at the lowest pH of all the 

minerals observed. The absence of Al-phosphate phases 

probably limits the pH (Altschuler, 1973) to no less than 

about 6.5.

AFP might represent a separate phase or, alternatively, an oxidation product of vivianite. The similar Fe:P ratio 

for these two phases (table 4) and the similarity of their 

blocky morphology (compare the habit of AFP in fig. 6D 

along the top and right edges of the diatom filling with 

that of vivianite in fig. 6£) argues strongly for the interpretation of AFP as an oxidation product, whereas the 

strong difference in REE patterns of the two phases (fig. 

4) that is, REE fractionation between different solid 

phases favors its interpretation as a separate phase. If 

AFP represents a separate phase, then its precipitation 

might have required a change in pore-water pH but, more 

clearly, would have required an increase in Eh.

Both increasing Eh and increasing pH, as well as 

freshwater conditions, are required for the precipitation of 

mitridatite (Nriagu and Dell, 1974; Moore, 1980). The 

freshwater conditions for all three Fe-phosphate phases 

could have been achieved by uplift into the terrestrial 

ground-water environment beginning 4-2 Ma (Namson 

and Davis, 1990).

The dominance of francolite in the outer layers of all 

concretions requires that it was the final phosphate to precipitate. Its dominance would have required a continuation 

in the trend of increasing pH but not necessarily Eh. Within modern sediment on the Peru shelf, francolite precipitates within the uppermost few centimeters of the sea floor 

(Froelich and others, 1988) within the suboxic zone of

CIO Evolution of Sedimentary Basins Santa Maria Province
denitrification (McArthur and others, 1986) and, possibly, under oxic conditions on the sea floor. In major marine phosphorites, in which francolite is the dominant 

phosphate phase (McClellan, 1980), the 534S-SO42~ value 

of francolite requires formation throughout the Eh range 

from oxygen respiration to sulfate reduction (Piper and 

Kolodny, 1988). If these depositional environments define 

the chemistry of francolite precipitation in the concretions 

within the Monterey Formation, then the pore water must 

have had a pH higher than approximately 7.6 that is, 

strongly alkaline but an Eh that could have varied over 

the entire range of Fe-phosphate stabilities. They also 

would seem to require marine conditions.

Seawater intrusion into fresh-ground-water systems of 

southern California is a well-documented phenomenon (Los 

Angeles County Flood Control District, 1957). Such intrusion into the Monterey Formation during uplift might account for the distribution of francolite in these concretions.

The difference between the compositions of concretionary francolite and of pelletal francolite dispersed in the 

diatomite suggests that an origin requiring introduction of 

seawater may not be the simplest explanation. The pelletal 

francolite has high Na+ and SO42~ contents (tables 2, 4), 

similar to those in francolite of major marine deposits. 

Both ions are highly concentrated in seawater. Thus, the 

absence of these ions in the concretionary francolite indicates that it precipitated within the same freshwater environment in which vivianite and mitridatite precipitated.

The trace amount of Mn 1 oNi5ZnO26_x in concretion 

6-A (fig. 6F) required an increase in both Eh and pH for 

its precipitation. Hem and others (1987, 1989) precipitated 

several Mn oxides in the laboratory. Under fully oxidizing, 

freshwater conditions and at a pH of approximately 7.8 

and higher, the average Mn valence is between 3+ and 4+. 

Although these experiments support an interpretation of 

continuously increasing Eh and pH, they probably are only 

weakly analogous to natural systems. For example, the 

high Ni content in this precipitate greatly exceeds that in 

laboratory studies, which tend to show very limited substitution in the |3-MnOOH lattice from a solution with an Ni 

concentration sufficient to precipitate NiO2 (Hem and others, 1989).

Paragenesis of the concretions can be depicted on a 

stability diagram (fig. 7), adapted from Nriagu and Dell 

(1974), for which Eh and pH are variables. Other topologic projections plot different variables (Viellard and others, 

1979). All of these projections, however, are limited by 

the requirement in their construction to hold constant activities of various aqueous components.

The broad stability field of francolite, albeit based on 

its occurrence in the marine environment, and the absence 

of Fe oxides in association with Mn oxide suggest that 

Ca2+, Fe2+, F~, and Mn2+ contents, as well as Eh and pH, 

changed continuously. For example, francolite occurs with 

pyrite in Peru shelf sediment (Piper and others, 1988) at

the one extreme of Eh, and with ferromanganese deposits 

(Manheim and others, 1980) at the other extreme. Using 

Nriagu and Dell's (1974) values for Ca2+ and PO43~ activities and an F:Ca ratio of 0.01 in solution (as estimated 

from the mass ratio of biogenic calcite), the ion-activity 

product should have exceeded the solubility product of 

francolite (Jahnke, 1984). Thus, francolite should have 

precipitated throughout the Eh-pH field of stability of Fe 

phosphates, and it should be abundant in all layers, not 

merely in the outermost layer. Its minor occurrence in the 

interior of concretion 5-1 (table 6) probably represents 

precipitation at the time of accretion of layer 1, by merely 

an infilling of residual porosity, rather than coprecipitation 

with vivianite, although we cannot rule out this possibility. 

Its precipitation in the absence of Fe oxides, unequivocally 

after mitridatite in all concretions, is most easily explainable by a decrease in Fe2+ activity and an increase in Ca2+ 

and F~ activities.

Despite the stability diagram's (fig. 7) limitations to a 

natural system, it nonetheless identifies the changes in Eh 

and pH required to precipitate vivianite initially and an 

Mn oxide ultimately. Failure to detect the other phosphate 

minerals reported by Nriagu and Dell (1974), such as lipscombite and foucherite, may be due to the poor detection 

limit of XRD, or the substitution of lipscombite by AFP.

Elemental Sources

REE patterns further define the conditions under 

which the concretions formed. The REE patterns of vivianite,

Fe-Mn oxides

Laueite/Strunzite

0.5 

0.4 

0.3 

0.2 

0.1 

0.0

-0.1

-0.2

10 11 12

pH

Figure 7. Stability relations between Fe-Mn oxides and phosphate minerals at PO43~, Fe2+, Ca2+, and Mn2+ activities of 

10~6 , 10~4, 10~3, and 10~4, respectively (adapted from 

Nriagu and Dell, 1974). Shaded area, approximate field of 

occurrence of francolite in modern marine environment; 

dashed line, possible trend of pore-water chemistry during 

formation of concretion 5-1 (fig. 3).

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C11
mitridatite, and francolite (fig. 4) strongly resemble that of 

seawater (De Baar and others, 1985) and marine biogenic 

phases CaCO3 (Palmer, 1985), opal, and, possibly, organic matter (Elderfield and others, 1981) showing a 

strong negative Ce anomaly and an enrichment of heavy 

over light REE's. Because Fe phosphates have been reported only in freshwater deposits, to our knowledge, the 

original source of the REE's could not have been seawater. 

Concretion formation late in the diagenetic history of the 

diatomite would have allowed for the immediate REE 

sources to have included pyrite, various alteration products 

of organic matter (Odermatt, 1986), and adsorbed phases. 

The most likely original source for REE's, however, was 

remobilized biogenic phases. For example, marine siliceous ooze, consisting of opal-A, can have an La content 

as great as 5.8 ppm (Elderfield and others, 1981). Opal-A 

is the dominant phase in these rocks, which show abundant evidence of its dissolution (fig. 6). Organic matter, 

with an La content of only 0.14 ppm (Elderfield and 

others, 1981), and CaCO3, with an La content of 0.13 ppm 

(Palmer, 1985), would have supplied only a minor 

fraction.

The source of other minor elements also was likely 

biogenic phases. Within the open ocean today, the Ni and 

Zn contents of seawater increase with depth. Their distributions closely parallel the seawater profile for silica 

(Sclater and others, 1976), suggesting that their pathway 

in seawater is coupled to that of biogenic silica. The distributions of Cd, Cr, Cu, and V in seawater resemble those 

of NO3~ and PO4 , the two essential nutrients for biologic productivity.

The relative concentrations of trace elements are unusual. Within marine organic matter, the content of Cu is 

approximately the same as that of Cd and significantly 

greater than those of other minor elements (table 3) except 

Zn (Martin and Knauer, 1973; Collier and Edmond, 1984; 

Brumsack, 1986). The average content of Cu in concretion 

5-1, however, is much smaller than those of other minor 

elements. The phases in which minor elements occur, as 

well as their sources, clearly must be ascertained to fully 

understand their incorporation into the concretions. We 

propose that both REE's and Ni had a largely biogenic 

silica source, and so they might have been expected to 

show similar trends in concretion 5-1; yet they show opposite trends (table 3) if the REE contents of layer 4 are 

discounted. REE and Ni contents must be influenced by 

their source, although the distribution of REE's may be 

dominated by their substitution for Ca2+ in francolite 

(Altschuler and others, 1967).

The source of major constituents also was probably 

the biogenic phases. The average P content of marine organic matter is 0.76 weight percent (Brumsack, 1986). Although the organic-matter content of the Monterey 

Formation is as great as 25 weight percent, it averages 

only about 3 weight percent in unweathered strata at the

diatomite quarry (C.M. Isaacs, unpub. data, 1991). Even at 

this lower concentration, marine organic matter within the 

sedimentary rocks would have been an adequate source for 

P in the dispersed concretions of this formation. These 

concretions have been reported only in this section of the 

Monterey Formation, and they are widely spaced (fig. 2). 

A possible alternative immediate source for P was an adsorbed phase on clay minerals (Schultz and others, 1980), 

but the original source still was likely organic matter.

The source of Fe could have been either organic matter or pyrite, which has been widely reported in the Monterey Formation and can decompose before the complete 

oxidation of organic matter (Littke and others, 1991). The 

Fe content of marine organic matter ranges from approximately 50 to 3,000 ppm (Martin and Knauer, 1973) but 

commonly is less than 500 ppm. Thus, the Fe:P weight 

ratio in organic matter (approx 0.1) is less than in vivianite (approx 5). If the major source of both Fe and P was 

organic matter, rather than pyrite and an adsorbed component on clays, then this difference in the weight ratios 

suggests that initial formation of the concretions that is, 

the precipitation of vivianite and AFP was Fe limited 

rather than P limited. Although the probable availability 

of Fe and P from other phases weakens this argument, the 

absence of Fe-oxide and Fe-sulfide phases seems to support it.

Dissolution of CaCO3 during continued uplift into the 

freshwater environment could have been the source of increasing pH, as well as of Ca2+. In the diatomite, the 

CaCO3 content is less than about 0.1 weight percent, 

whereas CaCO3 is a major component in subjacent strata 

of the Monterey Formation (Isaacs, 1980, 1989; Govean 

and Garrison, 1981; Pisciotto and Garrison, 1981).

Dissolution of CaCO3 also could have provided F~. 

The average F content in carbonate tests of marine plankton is approximately 0.1 weight percent (Carpenter, 1969). 

Piper and others (1990) showed that CaCO3 dissolution 

easily provided the F~ for precipitation of francolite in the 

insular phosphate deposit on Nauru, also under freshwater 

conditions. The F~:Ca2+ ratio in carbonates, however, is 

much lower than in concretionary francolite (0.002 versus 

0.2, respectively). Thus, precipitation of francolite may 

have been limited by the availability of F~.

Mass Transport

We have addressed above the chemical properties of 

the environment and the source of ions. We might ask why 

the concretions within the Monterey Formation formed at 

all and why they formed where they did, instead of the 

ions merely diffusing through the system and then being 

lost at the surface to streams, as must be the rule rather 

than the exception. Clearly, diffusion through the diatomite was interrupted and precipitation enhanced by some

C12 Evolution of Sedimentary Basins Santa Maria Province
chemical boundary, or chemical gradient. This condition 

must prevail for all concretionary deposits, whether they 

formed on the sea floor, within the sediment after burial, 

within the sedimentary rock after compaction, or at the 

Earth's surface after uplift and erosion. Such a boundary is 

more easily visualized in the first and last cases, but they 

are present throughout sediment and sedimentary-rock sections as well.

The chemical gradient that is, mass transport is 

the essential ingredient for concretionary growth; it could 

have been one of Eh, pH, alkalinity, or concentrations of 

other ions. Both ground-water recharge and tectonic uplift 

could have acted to shift up or down such gradients. The 

size of the concretions and the thickness of individual layers, however, must somehow reflect the stability of the 

system. Without any knowledge of the rate of concretionary growth, of tectonic uplift, or of ground-water recharge 

at the time of accretion, we can only speculate in general 

terms about conditions at the sites of accretion. The distribution of minerals alone, however, suggests that the site 

of formation of such concretions as 5-1 (fig. 3) differed 

from that of concretions 6-A and 6-B. The environment 

of accretion for concretion 5-1 moved in a single direction, in steps, with intervening periods of stability. The 

flux of ions might have been dominated by diffusion, 

which was interrupted by tectonics that is, uplift. In contrast, the environment of accretion for such concretions as 

6-A and 6-B (fig. 3) fluctuated often between two chemical environments. One possible explanation is that episodic variations in freshwater recharge (advection) were 

superimposed on diffusive flux changes brought about by 

uplift.

CONCLUSIONS

Phosphatic concretions, recovered from the Monterey 

Formation at the Celite mine near Lompoc, Calif., contain 

vivianite, mitridatite, francolite, and traces of an Mn-NiZn oxide. They occur in finely laminated diatomite, which 

shows no deformation about the concretions. This structure and the paragenesis of these minerals require that the 

concretions precipitated under conditions of continuously 

increasing Eh and pH, after complete compaction of the 

sediment. On the basis of modern occurrences of vivianite 

and mitridatite and the difference in composition of the 

concretionary francolite and associated marine pelletal 

francolite, precipitation evidently occurred within a freshground-water environment. The dominant source of majorelement-oxides and REE's was marine biogenic debris. 

Our interpretation of this complex chemistry is that the 

concretions precipitated as a result of the decomposition 

of marine phases organic matter, biogenic silica, and CaCO3 during uplift into the phreatic and (or) vadose zones 

of the terrestrial environment.

REFERENCES CITED

Altschuler, Z.S., Berman, Sol, and Cuttitta, Frank, 1967, Rare 

earths in phosphorites geochemistry and potential recovery, 

in Geological Survey research, 1967: U.S. Geological Survey Professional Paper 575-B, p. B1-B9.

Altschuler, Z.S., 1973, The weathering of phosphate deposits 

geochemical and environmental aspects, in Griffith, E.J., and 

others, eds., Environmental phosphorus handbook: New 

York, Wiley, p. 33-96.

Bramlette, M.N., 1946, The Monterey Formation of California 

and the origin of its siliceous rocks: U.S. Geological Survey 

Professional Paper 212, 55 p.

Brumsack, H.J., 1986, The inorganic geochemistry of Cretaceous 

black shales (DSDP leg 41) in comparison to modern upwelling sediments from the Gulf of California, in Summerhayes, C. P., and Shackleton, N.J., eds., North Atlantic 

paleoceanography: Geological Society of America Special 

Publication 21, p. 447^62.

Calvert, S.E., 1964, Factors affecting distribution of laminated 

sediments in the Gulf of California, in van Andel, T.H., and 

Shor, G.G., eds., The marine geology of the Gulf of California: American Association of Petroleum Geologists Memoir 

3, p. 311-330.

Carpenter, Roy, 1969, Factors controlling the marine geochemistry of fluorine: Geochimica et Cosmochimica Acta, v. 33, no. 

10, p. 1153-1167.

Collier, R.W., and Edmond, John, 1984, The trace element chemistry of marine biogenic paniculate matter: Progress in 

Oceanography, v. 13, no. 2, p. 113-199.

Crock, J.G., and Lichte, F.E., 1982, Determination of rare earth 

elements in geological materials by inductively coupled argon plasma/atomic emission spectrometry: Analytical Chemistry, v. 54, no. 8, p. 1329-1332.

Dana, E.S., 1935, A textbook of mineralogy (4th ed., revised by 

W.E. Ford): New York, Wiley, 851 p.

De Baar, H.J.W., Bacon, M.P., Brewer, P.G., and Bruland, K.W., 

1985, Rare earth elements in the Pacific and Atlantic 

Oceans: Geochimica et Cosmochimica Acta, v. 49, no. 9, p. 

1943-1959.

Donegan, Dave, and Schrader, Hans, 1981, Modern analogues of 

the Miocene diatomaceous Monterey Shale of California 

evidence from sedimentologic and micropaleontogic study, 

in Garrison, R.E., and Douglas, R.G., eds., The Monterey 

Formation and related siliceous rocks of California: Society 

of Economic Paleontologists and Mineralogists, Pacific Section Book 15, p. 149-157.

Elderfield, H., Hawkesworth, C.J., Greaves, M.J., and Calvert, 

S.E., 1981, Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments: Geochimica 

et Cosmochimica Acta, v. 45, no. 4, p. 513-528.

Froelich, P.N., Arthur, M.A., Burnett, W.C., Deakin, M., Hensley, 

V., Jahnke, R., Kaul, L., Kim, K.H., Roe, K., Soutar, A., and 

Vathakanon, C., 1988, Early diagenesis of organic matter in 

Peru continental margin sediments; phosphorite precipitation: Marine Geology, v. 80, no. 3^4-, p. 309-343.

Govean, KM. and Garrison, R.E., 1981, Significance of laminated and massive diatomites in the upper part of the Monterey 

Formation, California, in Garrison, R.E., and Douglas, R.G., 

eds., The Monterey Formation and related siliceous rocks of

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C13
California: Society of Economic Paleontologists and Mineralogists, Pacific Section Book 15, p. 181-198.

Gulbrandsen, R. A., 1970, Relation of carbon dioxide content of 

apatite of the Phosphoria Formation to regional facies, in 

Geological Survey research, 1970: U.S. Geological Survey 

Professional Paper 700-B, p. B9-B13.

Hamilton, E.L., 1976, Variations of density and porosity with 

depth in deep-sea sediments: Journal of Sedimentary Petrology, v. 46, no. 2, p. 280-300.

Haskin, M.A., and Haskin, L.A., 1966, Rare earths in European 

shale; a redetermination: Science, v. 154, no. 3748, p. 507- 

509.

Hearn, P.P., Parkhurst, D.L., and Callender, E., 1983, Authigenic 

vivianite in Potomac River sediments; control by ferric oxyhydroxides: Journal of Sedimentary Petrology, v. 53, no. 1, 

p. 165-177.

Hem, J.D., Lind, C.J., and Roberson, C.E., 1989, Coprecipitation 

and redox reactions of manganese oxides with copper and 

nickel: Geochimica et Cosmochimica Acta, v. 53, no. 11, p. 

2811-2822.

Hem, J.D., Roberson, C.E., and Lind, C.J., 1987, Synthesis and 

stability of hetaerolite, ZnMn2O4, at 25 °C: Geochimica et 

Cosmochimica Acta, v. 51, no. 6, p. 1539-1547.

Henderson, G.S., Black, P.M., Rodgers, K.A., and Rankin, P.C., 

1984, New data on New Zealand vivianite and metavivianite: New Zealand Journal of Geology and Geophysics, v. 27, 

no. 3, p. 367-378.

Isaacs, C.M., 1980, Diagenesis in the Monterey Formation examined laterally along the coast near Santa Barbara, California: 

Stanford, Calif., Stanford University, Ph.D. thesis, 329 p.

 1981, Porosity reduction during diagenesis of the Monterey Formation, Santa Barbara coastal area, California, in 

Garrison, R.E. and Douglas R.G., eds., The Monterey Formation and related siliceous rocks of California: Society of 

Economic Paleontologists and Mineralogists, Pacific Section 

Book 15, p. 257-271. 

-1989, Field trip guide to the Miocene Monterey Formation, Salinas and Santa Barbara areas, California: International Geological Congress, 28th, Washington, D.C., 1989, 

Field Trip Guidebook, p. 21-50.

Jahnke, R.A., 1984, The synthesis and solubility of carbonate fluorapatite: American Journal of Science, v. 284, no. 1, p. 58-78.

Keller, M.A., and Isaacs, C.M., 1985, An evaluation of temperature scales for silica diagenesis in diatomaceous sequences 

including a new approach based on the Miocene Monterey 

Formation: GeoMarine Letters, v. 5, p. 31-35.

Lichte, F.E., Golightly, D.W., and Lamothe, P.J., 1987, Inductively coupled plasma-atomic emission spectoscopy, chap. B of 

Baedecker, P.A., ed., Methods for geochemical analysis: U.S. 

Geological Survey Bulletin 1770, p. B1-B10.

Littke, R., Klussmann, U., Krooss, B., and Leythaeuser, D., 

1991, Quantification of loss of calcite, pyrite, and organic 

matter due to weathering of Toarcian black shales and effects on kerogen and bitumen characteristics: Geochimica et 

Cosmochimica Acta, v. 55, no. 11, p. 3369-3378.

Los Angeles County Flood Control District, 1957, Sea-water intrusion in California, app. B of California Department of 

Water Resources, Sea-water intrusion in California: Bulletin 

63, 141 p.

Manheim, F.T., Pratt, R.M., and McFarlin, P.F., 1980, Composition and origin of phosphorite deposits on the Blake Plateau, 

in Bentor, Y.K., ed., Marine phosphorites: Society of Economic Paleontologists and Mineralogists Special Publication 

29, p. 117-137.

Martin, J.H., and Knauer, G.A., 1973, The elemental composition 

of plankton: Geochimica et Cosmochimica Acta, v. 37, no. 7, 

p. 1639-1653.

McArthur, J.M., Benmore, R.A., Coleman, M.L., Soldi, C., Yeh, 

H.-W, and O'Brien, G.W., 1986, Stable isotopic characteristics of francolite formation: Earth and Planetary Science Letters, v. 77, no. 1, p. 20-34.

McClellan, G.H., 1980, Mineralogy of carbonate fluorapatites: 

Geological Society of London Journal, v. 137, pt. 6, p. 675- 

681.

Moore, P.B., 1980, The natural phosphate minerals crystal 

chemistry: International Congress on Phosphorus Compounds, 2d, Boston, 1980, Proceedings, p. 105-130.

Murata, K.J., Friedman, Irving, and Madsen, B.M., 1969, Isotopic composition of diagenetic carbonates in marine Miocene 

formations of California and Oregon: U.S. Geological Survey Professional Paper 614-B, p. B1-B24.

Murata, K.J., Friedman, Irving, and Cremer, Marcelyn, 1972, Geochemistry of diagenetic dolomites in Miocene marine formatians of California and Oregon: U.S. Geological Survey 

Professional Paper 724-C, p. C1-C12.

Namson, Jay, and Davis, T.L., 1990, Late Cenozoic fold and 

thrust belt of the southern Coast Ranges and Santa Maria 

Basin, California: American Association of Petroleum Geologists Bulletin, v. 74, no. 4, p. 467-492.

Nathan, Y., and Neilsen, H., 1980, Sulfur isotopes in phosphorites, in Bentor, Y.K., ed., Marine phosphorites: Society of 

Economic Paleontologists and Mineralogists Special Publication 29, p. 73-78.

Nriagu, J.O., 1972, Stability of vivianite and ion-pair formation 

in the system Fe3(PO4)2-H3PO4-H2O: Geochimica et Cosmochimica Acta, v. 36, no. 4, p. 459 470.

Nriagu, J.O., and Dell, C.I., 1974, Diagenetic formation of iron 

phosphates in recent lake sediments: American Mineralogist, 

v. 59, no. 9-10, p. 934-946.

Odermatt, J.R., 1986, Transition element chemistry of organic 

matter in the Monterey Formation, Union Leroy 51-18 well, 

Santa Maria Valley field, Santa Barbara Co., California: Los 

Angeles, University of California, M.S. thesis, 213 p.

Palmer, M.R., 1985, Rare earth elements in foraminifera tests: 

Earth and Planetary Science Letters, v. 73, no. 2-4, p. 285- 

298.

Piper, D.Z., Baedecker, P.A., Crock, J.G., Burnett, W.C., and 

Loebner, B.J., 1988, Rare earth elements in the phosphaticenriched sediment of the Peru shelf: Marine Geology, v. 80, 

no. 3^1, p. 269-285.

Piper, D.Z., and Kolodny, Yehoshua, 1988, The stable isotopic 

composition of a phosphorite deposit, 5 13C, 634S, and 618O: 

Deep Sea Research, v. 34, no. 5-6, p. 897-911.

Piper, D.Z., Loebner, B., and Aharon, Paul, 1990, Physical and 

chemical properties of the phosphate deposit on Nauru, 

western equatorial Pacific, in Burnett, W.C., and Riggs, S.R., 

eds., Phosphate deposits of the world: Cambridge, U.K., 

Cambridge University Press, v. 3, p. 177-194.

C14 Evolution of Sedimentary Basins Santa Maria Province
Pisciotto, K.A., 1981, Review of secondary carbonates in the 

Monterey Formation, California, in Garrison, R.E., and 

Douglas, R.G., eds., The Monterey Formation and related siliceous rocks of California: Society of Economic Paleontologists and Mineralogists, Pacific Section Book 15, p. 273-284.

Pisciotto, K.A., and Garrison, R.E., 1981, Lithofacies and depositional environments of the Monterey Formation, California, 

in Garrison, R.E., and Douglas, R.G., eds., The Monterey 

Formation and related siliceous rocks of California: Society 

of Economic Paleontologists and Mineralogists, Pacific Section Book 15, p. 97-122.

Rosenqvist, I.T., 1970, Formation of vivianite in Holocene clay 

sediments: Lithos, v. 3, no. 4, p. 327-334.

Schuffert, J.D., Kastner, M., Emanuele, G., and Jahnke, R.A., 

1990, Carbonate-ion substitution in francolite a new equation: Geochimica et Cosmochimica Acta, v. 54, no. 8, p. 

2323-2328.

Schultz, L.G., Tourtelot, H.A., Gill, J.R., and Boerngen, J.G., 

1980, Composition and properties of the Pierre Shale and 

equivalent rocks, northern Great Plains region: U.S. Geological Survey Professional Paper 1064-B, p. B1-B114.

Sclater, F.R., Boyle, E., and Edmond, J.M., 1976, On the marine 

geochemistry of nickel: Earth and Planetary Science Letters, 

v. 31, no. l,p. 119-128.

Taggart, J.E., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, A.J., 

and Stewart, K.C., 1987, Analysis of geologic materials by 

wavelength-dispersive X-ray fluorescence spectrometry, 

chap. E of Baedecker, P.A., ed., Methods for geochemical 

analysis: U.S. Geological Survey Bulletin 1770, p. E1-E19.

Tien, P.-L., and Waugh, T.C., 1969, Thermal and x-ray studies of 

earthy vivianite in Graneros shale (upper Cretaceous), Kansas: American Mineralogist, v. 54, no. 9-10, p. 1355-1362.

Vieillard, P., Tardy, Y., and Nation, D., 1979, Stability fields of 

clays and aluminum phosphates; parageneses in lateritic 

weathering of argillaceous phosphatic sediments: American 

Mineralogist, v. 64, no. 5-6, p. 626-634.

Wedepohl, K.H., ed., 1969, Handbook of geochemistry: Berlin, 

Springer-Verlag, 2 v.

Woodring, W.P., and Bramlette, M.N., 1950, Geology and paleontology of the Santa Maria district, California: U.S. Geological Survey Professional Paper 222, 185 p.

Phosphate Minerals in Concretions Within the Monterey Formation, Santa Maria Basin, California C15