STEPHEN J. BURNS AND PAUL A. BAKER. Duke University. Department of Geology. Durham, North Carolina 27708. ABSTRACt: Isotopic and trace-element ...
A GEOCHEMICAL
STUDY OF DOLOMITE
IN THE MONTEREY
FORMATION,
CALIFORNIA
l
STEPHEN J. BURNS AND PAUL A. BAKER Duke University Department of Geology Durham, North Carolina 27708
ABSTRACt: Isotopic and trace-element analyses of dolomites from the Miocene Monterey Formation of California show that there is a strong correlation between the ~3C values and the iron, manganese, and strontium contents of dolomites from different localities. Dolomites with negative 5t~Cvalues have low trace-element contents, averaging less than 1 mole % iron, 300 to 400 ppm manganese, and 200 to 250 ppm strontium. Those with positive 6'3C values have much higher trace-element contents, averaging greater than 2 mole % iron, with values as high as 10 mole %, 1,200-1,400 ppm manganese, and 600 to 800 pm strontium. Monterey sections with dolomites with low trace-element contents contain higher percentages of dolomite and have lower sedimentation rates and lower detfital mineral contents than sections with dolomites with high trace-element contents. Differences in iron and manganese contents of dolomites from different sections are probably attributable to variation in the amount of readily available iron and possibly manganese oxide coatings on detrital minerals. Whether a dolomite forms in or below the zone of organicmatter oxidation by microbial sulfate reduction also may affect the availability of iron and manganese. In the zone of sulfate reduction, reduced iron, and possibly manganese, may be precipitated as sulfide minerals rather than be incorporated into dolomite. Differences in the strontium contents of Monterey dolomites are probably the result of different reaction stoichiometries of the dolomitization process. Calculations of the maximum depth of dolomite formation based on a difffusion-limitedseawater source of MS2÷ for dolomitization show that for sections with high percentages of dolomite (10-20%), dolomitization must take place within the uppermost few meters of the sediment column. INTRODUCTION
The Miocene M o n t e r e y F o r m a t i o n o f California is a sequence o f organic-rich, pelagic a n d hemipelagic sedim e n t s ranging from a few h u n d r e d to over a t h o u s a n d meters thick. It contains considerable a m o u n t s o f dolomite, which can occur as beds, nodules, o r i n d i v i d u a l grains disseminated throughout the sediments. There have been a n u m b e r o f recent studies which have described the petrology and geochemistry o f the M o n t e r e y dolom i t e s a n d d o l o m i t e s from similar organic-rich sediments (e.g., Bramlette 1946; F r i e d m a n a n d M u r a t a 1979; G a r rison et al. 1981, 1984; a n d Hennessey a n d K n a u t h 1985). These studies have led to a fairly g o o d understanding o f s o m e o f the i m p o r t a n t controls on d o l o m i t e f o r m a t i o n in this setting; however, there are still i m p o r t a n t p r o b l e m s which r e m a i n unsolved. These include 1) the timing a n d d e p t h o f d o l o m i t e formation; 2) the sources o f the reactants for d o l o m i t e formation; a n d 3) the controls on the trace-element contents o f these dolomites. In an a t t e m p t to answer some o f these questions we have e x a m i n e d the isotopic a n d trace-element contents o f d o l o m i t e s from six widely d i s t r i b u t e d sections o f the M o n t e r e y F o r m a t i o n . Variations in the geochemistry o f d o l o m i t e s from different sections a n d from different facies can be u n d e r s t o o d in t e r m s o f sedimentological differences between sections. W e present here our hypotheses o f s o m e o f the m a j o r controls on the process o f d o l o m i t i z a t i o n in an organic-rich c o n t i n e n t a l - m a r g i n setting a n d o f the m a j o r controls o f the trace-element contents o f d o l o m i t e forming in such a setting. PREVIOUS WORK
T h e M o n t e r e y F o r m a t i o n can generally be s u b d i v i d e d into three distinct facies: a lower calcareous facies, a m i d Manuscript received 14 October 1985; revised 18 March 1986.
dle phosphatic a n d organic-rich facies, a n d an u p p e r siliceous facies (Bramlette 1946; Pisciotto 1978; Isaacs 1980; Pisciotto a n d G a r r i s o n 1981). T h e calcareous facies is c o m p o s e d p r i m a r i l y o f n a n n o p l a n k t o n i c a n d foraminiferal shales a n d mudstones. T h e organic-rich facies is s i m ilar in c o m p o s i t i o n to the calcareous facies but is m a r k e d b y the c o m m o n occurrence o f small, irregular nodules a n d stringers o f p h o s p h a t e (Pisciotto 1978; Isaacs 1980; Pisciotto a n d G a r r i s o n 198 l). T h e siliceous facies consists m a i n l y o f d i a t o m a c e o u s m u d s t o n e s a n d d i a t o m i t e s which are frequently altered to porcellanites a n d cherts, depending on the diagenetic history o f a particular section. T h e transitions from one facies to a n o t h e r are gradational, a n d distinction between facies is s o m e w h a t subjective. There is also considerable facies v a r i a t i o n from section to section d e p e n d i n g on the paleogeographic location o f a particular section (Pisciotto a n d G a r r i s o n 1981). D o l o m i t e is a c o m m o n mineral in a l m o s t all sections (Bramlette 1946). T h e lower calcareous facies generally contains the m o s t d o l o m i t e , usually in discrete beds. Lesser a m o u n t s o f d o l o m i t e , usually occurring as nodules, are found in the organic-rich a n d siliceous facies (Pisciotto a n d G a r r i s o n 1981). The precise location o f i n d i v i d u a l beds or nodules within a section m a y be controlled b y the a m o u n t o f precursor c a r b o n a t e f o u n d within a particular layer (Bramlette 1946). Bramlette (1946) was the first to recognize that the b e d s a n d nodules o f d o l o m i t e found in the M o n t e r e y are diagenetic in origin. T h e d o l o m i t e s c o n t a i n the s a m e d e t d t a l constituents as the surrounding shales, a n d l a m i n a e in s e d i m e n t s adjacent to nodules continue through, a n d thicken in the nodules, indicating that the d o l o m i t e f o r m e d p r i m a r i l y as a p r e c o m p a c t i o n pore-filling c e m e n t (Bramlette 1946; Pisciotto 1978). Because they are diagenetic, the isotopic a n d trace-element chemistry o f the d o l o m i t e s can p r o v i d e valuable i n f o r m a t i o n a b o u t the chemical env i r o n m e n t in which they formed. N u m e r o u s recent studies o f d o l o m i t e s from the M o n t e r e y a n d from s i m i l a r
Jou~AL or SEvnu~rrARY I~rROLOGV,VOL. 57, NO. 1, JANUARY,1987, P. 128-139 Copyright© 1987,The Societyof EcenomicPaleontologistsand Mineralogists 0022-4472/87/0057-128/$03.00
DOLOMITE IN THE MONTEREY FORMATION
m o d e m continental-margin settings have shed considerable light on where and why they form (see, e.g., Murata et al. 1969; Friedman and Mumta 1979; Pisciotto and Mahoney 1981; Kelts and McKenzie 1982). One of the best understood aspects of these "organic" dolomites is their carbon isotopic composition and the relationship between the carbon isotopes and the diagenesis of organic matter buried with the sediments. The carbon isotopic values of dolomites from the Monterey and similar organic-rich sediments show a very large variation, normally from about - 15%0to +20%0 PDB (Spotts and Silverman 1966; Murata et at. 1969; Deuser 1970; Pisciotto and Mahoney 1981; Kelts and McKenzie 1982; Kulm et al. 1984; Kelts and McKenzie 1984; and others). The present consensus is that dolomites that have isotopically light carbon are formed where there is input to the porewaters of carbon derived from the oxidation of organic matter by microbially mediated sulfate reduction. The heavy carbon dolomites probably form below the zone of sulfate reduction in the zone of methanogenesis, where methanogenic bacteria preferentially reduce isotopically light-dissolved bicarbonate to form methane, leaving the residual porewater bicarbonate isotopically heavy (Claypool and Kaplan 1974; Friedman and Murata 1979; Pisciotto and Mahoney 1981). One control on whether a dolomite forms in the zone of sulfate reduction or in the zone of methanogenesis is generally thought to be the sedimentation rate, with light carbon dolomites forming in areas with lower sedimentation rates compared to the areas containing heavy carbon dolomites (Pisciotto and Mahoney 1981; Kelts and McKenzie 1982, 1984). The oxygen isotopic composition of most organic-rich dolomites is also quite variable, usually ranging from + 5%0, to about -5%0 PDB. 6~sO values are often used to deduce depths and temperatures of formation for Monterey-type dolomites (e.g., Pisciotto 1978, 1981; Pisciotto and Mahoney 1981; Kelts and McKenzie 1982, 1984; Mertz 1984; Garrison and Graham 1984; Kablanow et al. 1984; Henderson et at. 1984; Isaacs 1984; Kushnir and Kastner 1984). There are, however, several uncertainties involved in making such a calculation (see Land 1980). Most of the studies cited above use the protodolomite-water curve of Fritz and Smith (1970) and also generally assume a porewater ~180 value to calculate temperatures of formation. These are coupled with an assumed geothermal gradient to calculate depths of dolomite formation. Typical calculated depths of formation range from 100 to 1,000 m (Pisciotto 1978; Pisciotto and Mahoney 1981; Kelts and McKenzie 1982; Kulm et al. 1984; Kushnir and Kastner 1984; Hennessey and Knauth 1985). Studies of the variation in ~180 across single nodules show that a nodule may begin to grow at relatively shallow depths and continue to grow as it is buried to calculated depths of many hundreds of meters (Irwin 1980; Pisciotto and Mahoney 1981; Kelts and McKenzie 1982; Kushnir and Kastner 1984). Relatively little work has been done on the trace-element contents of Montereytype dolomites. Only iron has been studied in any detail (Murata et al. 1972; Kushnir and Kastner 1984). Mort-
•
129
rroyo Seco
\
Chico Martinez eCreek
~ Shell Beachj~ Mussel RockT 5U ^ gm . '
LSan -~ . Augu =~ stine Cyn
...... ' El Capitan B e a c h ~
FIG. l.--Locations of the six Monterey Formation sections studied.
terey dolomites have been subdivided into three groups based on their 613C and iron compositions: 1) light carbon (< -5%0 PDB)-Iow iron (< 2 mole % Fe) dolomites; 2) heavy carbon-low iron dolomites; 3) heavy carbon-high iron dolomites (Murata et al. 1972; Pisciotto 1981). Pisciotto (1981) proposed that Group 1 dolomites formed in the zone of sulfate reduction where available iron was precipitated as pyrite, Group 2 dolomites formed in the zone of methanogenesis after iron had become depleted in the zone of sulfate reduction, and Group 3 dolomites formed deeper in the zone of methanogenesis, where the Mg 2+ supply in porewaters was limited and Fe 2+ replaced Mg 2+ in precipitating dolomites (Irwin 1980). Kushnir and Kastner (1984) attributed differences in iron contents of dolomites to differences in the detntal mineral contents of the surrounding sediments. METHODS
Complete or nearly complete sections o f the Monterey Formation were measured and described at the following locations: Shell Beach, Mussel Rock, E1 Capitfin Beach, San Augustine Canyon, and Chico Martinez Creek (see Fig. 1). At one additional location, Arroyo Seco, only the lowermost portion of the Monterey was studied. At each locale, samples of dolomite beds and nodules were collected; for some beds and nodules, multiple samples were taken perpendicular to bedding. Descriptions of the measured sections and locations of the samples used in this study are shown in Figures 2-6. About 130 samples from the various localities were selected for study and subjected to the following analyses. First, the obviously weathered portion of each sample was broken away, and a portion of the remaining sample was crushed and powdered. Second, a wet powder mount of each sample was analyzed for mineralogy by X-ray
S T E P H E N J. B U R N S A N D P A U L A. B A K E R
130
El Capitan Beach
Shell Beach ppm Sr
Foeies B F S _ Late Mohnian
Me ,?o ,
ppm Mn
ppm Fe
t 'T ? , ~,
~'~
?
'~
Facies O F S
'
0--"
-~
ppmMe
Mol %. Fe
_ Early
--
~_-
Minion
o
"-t
.-.42
~ ~. ~
- L.ui~a~ -- Relzian
ppm Sr Ma
-I0
_8
- ~isi=.
T
~
~15.5
,~'
',,h,o' . . .iooo .
zooo ~,
~
~o~,,~'
FIG. 2.--Stratigraphy and trace-element contents of dolomites from the Shell Beach section. For Figures 2..-6, the stratigraphic column shows the general locations of dolomite beds and nodules. Alongside the column are the thicknesses of individual facies: SC = siliciclastic; S = siliceous; O = organic rich; C = calcareous; CO = calcareous-organic, as explained in the text; BFS = California Benthic Foraminiferal Stages, as identified by Surdam and Stanley (1981)--Shell Beach; Woodfing and Bramlette (1950)--Mussel Rock; Isaacs (1980)--E1 Capitfin Beach; Bramlette (1946)--Chico Martinez Creek, and Garrison and Graham (1984)--Arroyo Seco. Ages for the Benthic Foram Stages use the time scale of Obradovich and Naeser (1981). The strontium, manganese, and iron trace-element contents of dolomites from throughout each section are also shown.
o, ~ I;
_gaay ROIIzlon
~15
/c
/
l
FIG. 4 . - Stratigraphy and trace-element contents of dolomites from the El Capithn Beach section. (Figure explanation as in Fig. 2.)
lomitic limestone) and lc (limestone), and reagent grade calcite (Baker Analyzed lot no. 946180) were used to check the accuracy and precision of the AAS elemental analyses. Analyses of the standards are well within the estimated uncertainty of the NBS analyses, and replicate analyses yield values within 2-3 percent of each other. About 50 thin sections from various localities were analyzed petrographically. One-half o f each thin section was stained with both Alizarin-Red S and potassium-ferricyanide (Lindholm and Finkelman 1972). About 50 samdiffraction using a Phillips X-ray Diffraction Unit with ples from throughout four of the sections, E1 Capit~n Cu-Kc~ radiation. Samples containing only calcite or doBeach, Shell Beach, Mussel Rock, and San Augustine lomite as the carbonate mineral were then leached in 1 Canyon were analyzed for carbon and oxygen stable isoM HCI for 15 minutes and filtered through 0.45-micromtopes. The samples were roasted at 320"C for 1 hour and eter membrane filters. Samples with both calcite and dothen dissolved overnight at 50°C in 100 percent H3PO4 lomite were first leached in acetic acid buffered with am(McCrea 1950). Isotopic analyses were done on a Finmonium acetate (pH = 5) for 30 minutes. This treatment negan MAT-251 stable isotope ratio mass spectrometer removed all the calcite and only a small fraction of the at North Carolina State University (in the laboratory of dolomite. These samples were then filtered and rinsed William Showers), and all results are reported relative to and the remaining solid was leached in 1 M HCI and the PDB standard. The standard deviation of the isotopic filtered as decribed above. The leaches for all samples value of a standard run with each set of samples is 0.07c7~ were then analyzed for the elements Ca, Mg, Fe, Mn, and for 313C and 0.12%o for ~lso. No phosphoric acid fracSr by flame atomic absorption spectrometry (AAS) on a tionation factor was applied to the isotope values. Perkin-Elmer Model 5000 spectrophotometer. Replicate analyses of NBS reference standards 88a (doRESULTS AND DISCUSSION
Mineralogy and Petrography Mussel BFS
Focies T PIIocene
s
'G 4oo
Sl
Rock
ppm Sr Ma
ppm Mn
=P
4p
ep?
I =
I ~
~
~opo , ~ ?
tool % Fe ~
4
i aeo
race
Late Molmlan
o $
J =~ t CO l
Ectdy Mohrdan
--.10
Lulslan to .~,,IZ Lall --15 I~tl,den
i
i o
i i I moo 2oooo
I
ppm Fa
FIG. 3.--Stratigraphy and trace-element contents of dolomites from the Mussel Rock section. (Figure explanation as in Fig. 2.)
General descriptions ofthe measured sections and their sedimentologic characteristics are shown in Figures 2--6 and Table 1, respectively. In describing the measured sections we use the stratigraphic subdivisions of Pisciotto (1978) and split the Monterey into three facies: a lower calcareous facies, a middle organic-rich facies, and an upper siliceous facies. However, because the Monterey is quite variable in composition from place to place, not every section is easily subdivided. The section at Shell Beach is very rich in carbonate both above and below a middle organic-rich section and is classified entirely as being the calcareous facies. The lower part of the section at Mussel Rock is calcareous and organic-rich with scattered phosphate throughout and is herein called the cal-
careous organic facies.
DOLOMITE IN THE MONTERE Y FORMATION
Chico Mortinez Creek Facies
B F S
ppm Sr
Ma
A r r a n Seco ppm Mn
ppmSt
MOl %Fe
Facies
f
Pliocene - 5
,1 ~ooa
131
pprn Mn
Mol% Fe
B F S
Late • Mdmian
If
i=oo
(
, Mohntan
~',h'g'
~14.5
o' ~
,~'o i i ~ i ,o
FIG. 6.--Sttratigraphy and trace-element contents of dolomites from the Arroyo Seco section. (Figure explanation as in Fig. 2.)
