carbon ratios and extractable organic matter of the middle proterozoic nonesuch formation, north american midcontinent rift

Precambrian Research, 54 (1991) 65-79

65

Elsevier Science Publishers B.V., Amsterdam

Sulfur/carbon ratios and extractable organic matter of the Middle Proterozoic Nonesuch Formation, North American Midcontinent rift G.B. Hieshima and L.M. Pratt Department of Geology, Indiana University, Bloomington, IN 47405, USA (Received May l, 1990; revised and accepted August 27, 1990)

ABSTRACT Hieshima, G.B. and Pratt, L.M., 1991. Sulfur/carbon ratios and extractable organic matter of the Middle Proterozoic Nonesuch Formation, North American Midcontinent rift. In: M.R. Walter Editor), Proterozoic Petroleum, Precambrian Res., 54: 65-79. The Nonesuch Formation (ca. 1.1 Ga), host to the White Pine copper deposit, is a generally organic-rich unit of relatively wide extent. Selected strata, away from mineralization, were analyzed for their organic carbon and total sulfur contents and for the amount and composition of extractable organic matter. Sulfur to carbon ratios are covariant, but the ratios of total sulfur to organic carbon are generally higher than those found for marine Phanerozoic black shales deposited under oxic bottom water conditions. The sulfur to carbon ratios and the intimate association fo framboidal pyrite and organic matter are suggested to he the result of bacterial sulfate reduction in the Nonesuch sediments. Activity of sulfate reducing bacteria requires a supply of sulfate in the water column and implies that the depositional environment was not a low-sulfate aqueous environment. A marine embayment dominated more by sea water than riverine input is suggested as the environment of deposition based on the correlation of total sulfur and organic carbon, the relatively high total sulfur content, and on the absence of sedimentologic and geochemical criteria for saline lacustrine environments. Deposition of highly reactive organic matter in low concentrations coupled with high concentrations of reactive iron and the absence of bioturbation resulted in unusually efficient pyrite formation. Extractable organic matter in unmineralized Nonesuch strata is interpreted to be autochthonous in origin. Homologous series of n-alkanes and cyclohexyl alkanes, regular isoprenoids and monomethyl-branched alkanes were identified by gas chromatography. Ratios of pristane/phytane, pristane/n-C 17 and phytane/n-C18 suggest relatively low thermal maturity for the overall unit with a slight increase in the level of thermal maturity from Michigan westward to Wisconsin. The aliphatic character and high extractability of organic matter indicate that the petroleum potential of the Nonesuch Formation will be good to excellent if the organic-rich facies thicken down-dip under Lake Superior or into the southern extension of the rift.

Introduction

The Nonesuch Formation, deposited approximately 1.1 billion years ago (Chaudhuri and Faure, 1967; Ruiz et al., 1984; Davis and Paces, 1990), has been the focus of intensive study for many years because of economic interest in copper mineralization and associated oil seeps and tar in mines near White Pine, Michigan. Previous studies of the organic geochemistry of the Nonesuch Formation have 0301-9268/91/$03.50

concentrated on rocks and seep samples from White Pine Mine. In the last five years, several studies have included samples from outside of the principal zone of mineralization (e.g., Imbus et al., 1989; Pratt et al., 1989). Our purpose in studying non-mineralized Nonesuch strata is to document: ( 1 ) the relationship between organic carbon and total sulfur content and (2) the abundance and composition of extractable organic matter. Based on these data, a reinterpretation of the depositional environ-

© 1991 Elsevier Science Publishers B.V. All rights reserved.

66

G.B. HIESHIMA AND L.M. PRATT

ment of the Nonesuch is suggested and a realistic assessment of the petroleum potential of the Nonesuch Formation outside of the zone of mineralization is proposed. The Nonesuch Formation, in the middle portion of the Keweenawan Oronto Group, is part of the sedimentary fill in the North American Midcontinent rift and is exposed in outcrops in northern Wisconsin and Michigan (Fig. 1). The Nonesuch is underlain by brownish to reddish conglomerates and sandstones in the Copper Harbor Conglomerate which is generally interpreted as the product of alluvial fan sedimentation (Elmore, 1984). Overlying the Nonesuch Formation in gradational contact are red sandstones of the Freda Sandstone, generally interpreted as fluvial deposits (e.g., Daniels, 1982). The Nonesuch varies between 75 and 215 m in thickness and

comprises greenish gray to dark gray siltstones and shaley siltstones with minor sandstones (Daniels, 1982). Given the apparently conformable contact of the Nonesuch with the over- and underlying continental deposits, a lacustrine depositional environment is most compatible with the stratigraphic sequence (Daniels, 1982; Elmore, 1984; Elmore et al., 1989). Based on sedimentology alone, however, the absolute distinction between lacustrine and marginal marine settings is difficult (Picard and High, 1972). This is especially true with respect to Proterozoic strata which lack diagnostic metazoan fossils. Four depositional environments have been suggested for the Nonesuch Formation including estuarine (White and Wright, 1954; Moore et al., 1969 ), lagoonal or interdistributary bay (Burnie et al., 1972), marine (Hieshima et al., 1989) and la-

MINN

--

FAULT

Fig. 1. Generalizedgeologicmap (Pratt et al., 1991 ) of northern Wisconsinand Michigannear Lake Superiortaken from Morey (1978) and White and Wright (1954).

S/C RATIOS AND EXTRACTABLE ORGANIC MATTER OF NONESUCH FORMATION

custrine (Daniels, 1982; Elmore, 1984; Elmore et al., 1989). In addition, Daniels (1982) cited unpublished interpretations of deltaic (Erlich and Vogel, 1971, unpubl. Tech. Rep., White Pine Copper Company, White Pine, Michigan) and marine (Jost, 1968, unpublished Ph.D. thesis, Johannes Gutenberg University, Mainz, Germany) depositional environments. Methods

Our sample set was collected from outcrop, core and mine excavations in northern Michigan and Wisconsin. Core samples were taken from mineral exploration cores drilled by the Bear Creek Mining Company in the 1950's. Six cores from Michigan were described and sampled at the core repository of the U.S. Bureau of Mines in Minneapolis, Minnesota. Seven cores from Wisconsin were sampled at the core repository of the Wisconsin Geological and Natural History Survey in Milwaukee, Wisconsin. Samples of rocks and oils collected at the White Pine Mine in Michigan were taken from both the shallow part of the mine active in the 1960's and from new deeper excavations. Non-mineralized samples were identified petrographically as those containing only framboidal pyrite intimately associated with organic matter. Mineralized samples contain coarser-grained aggregates of chalcocite and native copper often concentrated in silty or fine-sandy layers that may have acted as conduits for mineralizing fluids. No obviously mineralized samples were encountered outside of the recognized zone of mineralization near White Pine, Michigan. Contents of total carbon and total sulfur were measured by combusting whole-rock powders in a Leco C / S 244 modified for high sulfur contents. Samples with greater than 0.15 wt.% total carbon were decalcified by reactions with 1N HC1 overnight. The acidic solution was decanted and hot 1N HC1 was added to remove refractory carbonate phases, the sample was

67

concentrated on a glass fiber filter, neutralized with distilled water, dried overnight at 50°C and combusted to determine organic carbon and acid-resistant sulfur contents. The degree of pyritization ( D O P ) was calculated following the methods outlined by Raiswell et al. (1988 ). Pyrite iron was determined by calculating the stoichiometric value assuming that total sulfur approximates pyritic sulfur. Whole rock samples were pyrolized in a Rock Eval instrument at the U.S.G.S. in Denver, Colorado. Samples of powdered rock were extracted in a Soxhlet apparatus (48-60 h) using dichloromethane as solvent. The soluble organic matter (bitumen) was concentrated using a Bucchi rotovapor and then brought to a volume of 10 ml. An aliquot of bitumen was transferred to a glass vial, dried at room temperature under a stream of N2 gas and weighed to determine yield. A second aliquot was utilized for gas chromatographic analysis of total bitumen. A third aliquot of bitumen was separated into saturated hydrocarbons, aromatic hydrocarbons and polar compounds (nitrogen, sulfur and oxygen bearing compounds) on a column (13 m m diameterX50 m m length) of activated silica gel (150°C) by sequential elution with hexane, benzene and benzene: methanol ( 1 : 1 ). Asphaltene compounds were not removed separately because they were present in very low concentrations. Gas chromatography of the saturated hydrocarbon fraction was carried out on a Hewlett Packard 5890 A using a fused silica capillary column (HP Ultra 1 : 2 5 m X 0 . 3 2 m m i.d. × 0.52/lm film thickness) and a splitless injector (325°C). The oven of the gas chromatograph was programmed from 50°C to 130°C at 10 ° per minute, from 130 ° to 300 ° at 4 ° per minute and held at 300 ° for 15 minutes. The gas chromatograph was fitted with a flame ionization detector (FID) for hydrocarbons and a flame photometric detector ( F P D ) for sulfur-bearing organic compounds. Column flow was split in a 1 : 1 ratio just prior to entering the detectors.

G.B.HIESHIMA ANDL.M.PRATT

68

50 40[ ~ 30 ~ 2o E

~ 10 0-0.15

0.15-05 0.5-10 10-20 Organic Carbon (wt %)

203o

Fig. 2. Histogram showing organic carbon contents for unmineralized samples of the Nonesuch Formation. Although contents of organic carbon are generally low in the Nonesuch, values up to 2.8 wt.% are recorded.

Results

Organic carbon and lithology Organic-carbon contents in non-mineralized Nonesuch strata range from less than 0.01 to nearly 3 wt.% ( 132 samples; Fig. 2 ). Clayey siltstones and graded siltstones are the two rock types that constitute the majority of samples having contents of organic carbon greater than 0.15%. Clayey siltstones (Fig. 3A) are composed of alternating laminae of silt (dominantly quartz with lesser feldspar and lithic fragments) and equal to subordinate amounts of fine-grained material (clay+organic matter). Laminae range from 0.05 m m to 0.25 m m and average around 0.1 m m in thickness. Clayey siltstones are a widespread component of the Nonesuch Formation and have the highest concentrations of organic carbon (up to 2.8 wt.% ). The other principle organic-rich lithology consists of graded microlayers generally

overlying a scoured base. Graded laminae are composed of normally graded silt in most cases overlain by clay and, in a few examples, by matlike organic matter. Only strata with mat-like caps on the graded packages contain significant quantities of organic carbon (up to 0.3% ). Thicknesses of these packages vary from about a m m to several cm (Fig. 3B). Both types of organic-rich rock are recognized in core material and mine excavations but not in outcrop, probably because of incomplete exposure. Flat ovoidal, clayey clasts with a mat-like fabric are present in a medium-grained sandstone from one outcrop in Michigan (Fig. 3C). The texture of these clasts is similar to that observed capping some graded laminae and suggests that the sediment was cohesive at the time of erosion. Although the organic carbon content of the whole rock was low, sampling of individual clasts yielded organic carbon contents up to 1.8%. The incorporation of organic-rich clasts into m e d i u m to coarse-grained sandstone can be interpreted as formation by ripping up organic-rich mats in shallow water and depositing them in a high energy deposit. Alternatively, the clasts may represent rounded pieces of desiccated organic material later incorporated into the sandstone.

Total suljur/organic carbon ratios The content of total sulfur in non-mineralized Nonesuch strata is correlated with the content of organic carbon, but the sulfur to carbon ratio is not constant and decreases sig-

Fig. 3. (a) Photomicrograph of laminated shaly siltstone from the Nonesuch Formation. Light colored laminae are composed primarily of quartz silt. Darker colored laminae are composed of clay, mica, pyrite and organic matter. The opaque appearance of the darker layers is due in part to the presence of small spherical framboids of pyrite that are 5-20/zm across and are concentrated in the organic-rich laminae. (B) Photomicrograph of graded siltstones from the Nonesuch Formation showing a scoured surface overlain by graded fine sand and silt fining upward into clay or mat-like organic matter. (C) Photomicrograph of a medium-grained sandstone in the Nonesuch Formation showing a cross section of a rip-up clast composed of mat-like organic material. This cross-sectional view illustrates the laminar internal structure of these clasts. The texture is similar to the mat-like organic matter that caps some graded silt laminae (3B).

69

NOI.LVI~I~IO_qHDFIS3NON -IO ~I3.L.LVI6IDINVD~O ::Iq~IV.LDV~IJ_X'::IO N ¥ SOIiV~I D/S

70

G.B. HIESHIMAAND L.M. PRATT TABLE 2

3 17

Michigan

•

Wisconsin

[]

--00K 3 0

0

0

Ratios of selected geochemical components Sample#

Locality

Pr/Ph

Pr/n-Cl7

Ph/n-C18

877-OIL 3

Copper Range Mine

1.66

0.25

0.12

887-31 688-29 688-36 688-59 688-71 688-72

Michigan Michigan Michigan Michigan Michigan Michigan Michigan

3.30 3.29 4.21 2.55 1.97 3.67 3.43

1.08 0.83 0.70 0.63 0.54 1.08 1.19

0.35 0.33 0.21 0.39 0.47 0.37 0.43

589-25 589-27 589-29 589-41 589-42 589-43 589-53

Wisconsin Wisconsin Wisconsin Wisconsin Wisconsin Wisconsin Wisconsin

1.86 1.88 2.16 2.28 1.61 1.84 2.25

0.68 0.40 0.40 0.27 0.20 0.24 0.69

0.36 0.27 0.22 0.14 0.14 0.16 0.41

y=1.4857+ L2273*log(x)

1

2

Organic Carbon (wt %) Fig. 4. Crossplot o f total sulfur content versus organic carbon content. Open squares are unmineralized core and outcrop samples from Michigan, dots are unmineralized core and outcrop samples from Wisconsin. Total sulfur contents of unmineralized samples are covariant with organic carbon contents. The curved line is a logarithmic fit whose equation is shown in the plot. The straight line is the trend o f normal marine sediments based on Goldhaber and Kaplan (1974).

Average Pr/Ph: Average Pr/n-Cl 7: Average Ph/n-C18:

nificantly in samples with greater than 0.5% organic carbon (Fig. 4). Total sulfur is interpreted to approximate pyrite sulfur because there is little or no acid-volatile sulfur (monoTABLE 1 Rock Eval analyses of selected whole rock samples Sample

Tmax*

SI (mgHC/ g rock )

$2 (mgHC/ g rock )

$3 (mgHC/ g rock )

TOC*

887-31 887-32 887-41 887-46 887-48 688-26 688-34 688-36 688-38 688-41 688-45 688-51

431 430 437 430 220 264 291 421 439 421 307 427

0.08 0.01 0.35 0.02 0.01 0.00 0.00 0.01 0.10 0.05 0.01 0.05

2.40 0.02 9.67 0.05 0.02 0.00 0.00 0.26 9.49 0.08 0.02 2.07

0.41 0.33 0.77 0.50 1.62 0.16 0.22 0.20 0.79 0.36 0.20 0.30

0.81 0.15 2.66 0.05 0.32 01.08 0.08 0.31 2.20 0.09 0.08 0.86

*Tmax values may be depressed because of low $2 yields or because of assymetrical peak shapes caused as a result of high molecular weight components of bitumen contributing to $2. *Organic carbon determined as outlined in methods.

Michigan = 3.20 Wisconsin= 1.98 Michigan = 0.86 Wisconsin = 0.41 Michigan = 0.36 Wisconsin = 0.24

sulfides), sulfur soluble in organic solvents (elemental sulfur), and organo-sulfur compounds in the bitumen (Fig. 5). Pyrite is the only sulfur-bearing mineral recognized by reflected light petrography and X-ray diffraction. Degree of pyritization ( 18 samples) varies from 0.15 to 0.51 and averages 0.30 (Fig. 6). Extractable organic matter

Yields of extractable organic matter are low and range from essentially zero to greater than 300 ppm rock, averaging approximately 160 ppm total rock for samples with greater than 0.15% organic carbon. Although extraction yields are poorly correlated with organic carbon (Fig. 7), rocks with below about 0.15% organic carbon contain extremely small amounts (less than 20 ppm) of potentially extractable organic matter as indicated by RockEval pyrolysis (Table 1 ).

S/C RATIOS AND EXTRACTABLEORGANIC MATTER OF NONESUCH FORMATION C

15

A

71

15 I I

......... B

,I

JILl, ....

15 D ~. . . . . . . . .

III III I I

.

.

.

.

.

.

.

.

,

I

II

. . . . .

Fig. 5. Gas chromatograms of bitumens extracted from the Nonesuch Formation. (A) shows F1D response for the saturated hydrocarbon fractions of bitumens from Michigan; (B) shows FID response for the saturated hydrocarbon fractions of bitumens from Wisconsin; (C) shows FID response for the saturated hydrocarbon fraction ofoil seep from the Copper Range Mine; (D) shows the flame photometric detector (specific for sulfur-bearing compounds) response for saturated hydrocarbon fraction of sample C. Pr= pristane, Ph= phytane; 15, 20, 25 denote normal alkanes. 12-

400 300-

aE

r

o

Michigan ]

•

Wi~etmain

200100-

0

0.2

0.4

0.6

0'.8

1.0

Degree of Pyritization Fig. 6. Histogram of degree of pyritization of iron. Although DOP values have been calibrated only for Phanerozoic strata (e.g., Raiswell et al., 1988; Raiswell and AIBiatty, 1989), low DOP's for the Nonesuch Formation suggest that the water column was not euxinic.

Gas chromatography of total bitumen and saturated hydrocarbons N o r m a l - a l k a n e s ( n - a l k a n e s ) , acyclic isop r e n o i d s , alkyl c y c l o h e x a n e s a n d m o n o m e t h y l b r a n c h e d a l k a n e s w e r e i d e n t i f i e d b y gas c h r o -

0

1

2

Organic carlton (wt %)

Fig. 7. Crossplot of bitumen content versus organic carbon content. The lack of correlation between bitumen and organic carbon content is due in part to analytical problems in handling samples with yields less than l0 mg. m a t o g r a p h y o f the s a t u r a t e d h y d r o c a r b o n fraction of bitumen from the Nonesuch Form a t i o n (Fig. 5 ). T h e i d e n t i f i c a t i o n o f a l k y l cyclohexanes and monomethyl branched alkanes has been confirmed by metastable reaction m o n i t o r i n g u s i n g gas c h r o m a t o g r a p h y - m a s s

72

spectrometry (R.E. Summons, unpublished data). A homologous series of n-alkanes is found in Nonesuch extracts with chain lengths of up to 34 carbon atoms. Within the series of n-alkanes, odd carbon preference is noted in some samples between carbon numbers 15 to 25. Oils also contain a homologous series of n-alkanes but with maximal chain lengths of 30 carbon atoms and weaker odd-carbon preference than the extracts. Pristane, phytane and regular isoprenoids of 18, 16 and 15 carbon atoms have been identified in extracts. The same isoprenoids are present in Nonesuch oils but in significantly lower concentrations relative to the n-alkanes. Gas chromatograms of total bitumen are similar to those of the saturated hydrocarbon fraction. Distributions of n-alkanes, odd-carbon preference, and selected acyclic isoprenoid/n-alkane ratios vary with respect to geography. Samples from Wisconsin have lower ratios of pristane/phytane and pristane/n-C17 than samples from Michigan (Table 2 ). Ratios ofphytane to n-C 18 are only slightly higher in Michigan than Wisconsin. Discussion

Early diagenesis of carbon and sulfur in sediments Early diagenesis of organic matter in sediments can be summarized as bacterially induced oxidation of labile organic compounds in concert with reduction of coexisting inorganic or organic compounds. Bacterial assemblages that mediate such reactions utilize electron acceptors in a sequential fashion that is related to the free energy of the redox reaction (e.g., Claypool and Kaplan, 1974; Stumm and Morgan, 1981 ). Electron acceptors yielding the highest free energy are utilized first resulting in the successive depletion of oxygen, iron, manganese and sulfate. The redox reaction coupling oxidation of organic matter to reduction of sulfate is quantitatively significant on local

G.B. HIESHIMA AND L.M. PRATT

and global scales because of the amount of sulfate in seawater. Oxidation of organic matter by bacteria using sulfate as an electron acceptor can be described by the simplified expression: SO 2- + 2 C H 2 0 - . H S - + 2 HCO~- + H +

(1) Equation 1 illustrates the importance of the variable valence states of sulfur. As written, there is a net transfer of eight electrons during the reduction of sulfate ( + V I ) to sulfide ( - II). Rates of sulfate reduction are influenced by a number of factors in m o d e m sediments including sulfate concentration, temperature, organic matter reactivity, sedimentation rate and bioturbation (e.g., Goldhaber et al., 1977; J~rgensen, 1977; Berner, 1978; Nedwell and Abram, 1979; Capone and Kiene, 1988). Sulfide produced as a byproduct of bacterial sulfate reduction can have several fates: (1) inorganic or bacterially mediated oxidation, (2) reaction with ferrous iron, or ( 3 ) reaction with organic matter resulting in organosulfur molecules (e.g., Jorgensen, 1977; Bauld, 1986; Francois, 1987; Morse et al., 1987). The formation of pyrite generally is interpreted as the reaction of sulfide with reactive ferrous iron through a monosulfide intermediate and later reactions with elemental S (Berner, 1980; Morse et al., 1987): Fe 2+ + HS-__+FeS+H + FeS + S°--+FeS2

(2) (3)

These reactions are interpreted to be controlled by the availability of sulfide produced during sulfate reduction, the rate of sulfide loss by diffusion and oxidation, and the availability of reactive iron and elemental sulfur (e.g., J~rgensen, 1977; Raiswell and Berner, 1985; Swider and Mackin, 1989). The relationships among organic matter, reactive iron and bacterial sulfate reduction in Recent marine sediments have been studied in many environ-

73

S/C RATIOS AND EXTRACTABLE ORGANIC MATTER OF NONESUCH FORMATION

ments, but with emphasis on vertical variation of solute concentration and reaction rate (e.g., Froelich et al., 1979; Goldhaber and Kaplan, 1980; Jorgensen, 1982 ).

Bacterial sulfate reduction in the Nonesuch Formation Evidence for bacterial sulfate reduction in depositional environments of the Nonesuch is provided by the positive covariance of total sulfur with organic carbon and by the intimate association of framboidal pyrite with organicrich laminae. If sulfate-reducing bacteria were active in Nonesuch sediments, then there was a source of sulfate in the water column and the paleoenvironment was not a low-sulfate body of water. Possible explanations are: ( 1 ) Nonesuch sediments were deposited in marine environments, (2) there was intermittent connection between Nonesuch waters and seawater, (3) Nonesuch sediments were deposited in sulfate-rich lacustrine depositional environments, or (4) there was a significant flux of sulfur (either sulfate or sulfide) into a lake from hydrothermal fluids. The well-constrained relationship between total sulfur and organic carbon suggests that sulfate concentrations in the water column did not fluctuate widely or at least never dropped to concentrations below the minimum requirement for sulfate reduction. We hypothesize that the depositional environment was a marine embayment dominated by seawater rather than riverine discharge. Above the basal Nonesuch, the absence of sedimentary structures normally associated with saline lakes (e.g., evaporite nodules or laminae, mudcracks) supports this interpretation. Additionally, sulfur to carbon ratios in modern and ancient sulfate-rich lacustrine deposits are scattered widely reflecting the geochemical complexity expected in lacustrine systems (Fig. 8; Great Salt Lake, Walker Lake, Green River Formation; Turtle et al., 1990; Turtle and Goldhaber, in press). Nonesuch S/C ratios are significantly higher

2 a [ o [ •

•

Great Salt Lake WalkerLake GreenRiv~Fm.

•

I.

o

• --

"i

0

•

o • m "~0,.,~~, ,

0

•

0 ~ 0 ~&O • ~ A0 • 0 •

•

•

..00 •.

1

--

•

•

~ 25e T

0

&I . " t

~o "

-

*

|

*

, *i

2

!

o"

3

Organic carbon (wt % )

Fig. 8. Crossplot of disulfidesulfur versusorganiccarbon content for sedimentsdeposited in two modern and one ancient sulfate-rich lacustrine systems. Modified from Tuttle and Goldhaber (in prep.) and Tuttleet al. (1990). Walker Lake, Nevada contains sulfate concentrationsapproximately equal to seawater (28 mMol). Great Salt Lake, Utah presentlycontainshigh concentrationsof sulfate relativeto seawater (200 mMol), but the salinity has varied significantly.The EoceneGreen River Formation (central U.S. ) was depositedin a lacustrineenvironment of variable salinity. Nonesuchsulfur to carbon ratios are more consistentwith deposition in a marine settingthan a sulfate-richlake.

than Recent and ancient normal marine sediments. Possible reasons for sulfur enrichment are deposition of highly metabolizable organic matter, the absence of bioturbation and more efficient pyrite formation. Anoxic bottom waters containing H2S could yield similar sulfur to carbon ratios. In sediments deposited in euxinic bottom waters, however, migration and reaction of dissolved sulfide with iron in organic-poor strata would be reflected by a positive intercept on the total sulfur/organic carbon crossplot (e.g., Raiswell and Berner, 1985), which is not observed for Nonesuch samples. Additionally, DOP's for Nonesuch samples are not high, indicating that iron was not limiting. For post-Silurian strata, Raiswell et al. (1988) have suggested that DOP's less than 0.42 indicate oxygenated bottom waters. Although DOP's have not been calibrated for paleoenvironmental conditions in the Proterozoic, low DOP's in the Nonesuch suggest that bottom waters were not euxinic.

74

Organic matter reactivity Dominant sources of organic matter to Nonesuch sediments were bacteria and algae, components of which are very reactive with regard to sulfate reduction. Water-column oxidation of organic matter was probably minimal because facies relationships suggest relatively shallow water depths (less than a few hundred meters ). If a greater proportion of total organic matter being deposited in Nonesuch sediments was reactive (labile) compared to Recent normal marine sediments, then proportionately more sulfate could be reduced and more pyrite precipitated relative to organic carbon input. Raiswell and Berner (1986) suggested that high sulfur to carbon ratios in normal marine strata of Cambrian age reflect the lack of terrigenous organic matter input prior to the Silurian. However, Donnelly et al. (1988) reported sulfur to carbon ratios in Middle Cambrian strata that are indistinguishable from younger Phanerozoic deposits.

Bioturbation Qualitatively, the bioturbation in Nonesuch sediments should affect sulfate reduction and pyrite formation in offsetting ways. First, organic matter or sulfate would not be advectively transported to deeper parts of the sediment column (Goldhaber et al., 1977; Jorgensen, 1977; Berner and Westrich, 1985) and second, oxidative loss of sulfide would be decreased because oxygen would not be mixed into sediments (Berner and Westrich, 1985). The relative importance of these factors would determine whether pyrite formation was enhanced or diminished. If sulfate reduction is limited by amount or reactivity of organic matter or amount of sulfate, then the lack of advective transport of these components into the sediment column would have resulted in low S/C ratios. Alternatively, if the concentrations of reactive organic matter and sulfate were relatively high, then high S / C ratios

G.B. HIESHIMA AND L.M. PRATT

would result from decreased oxidation of sulfide.

Enhanced pyrite formation The formation of pyrite from sulfide produced by sulfate reduction may have been more efficient in Nonesuch sediments than recent marine sediments because of high concentrations of reactive iron and the establishment of environmental conditions favorable for pyrite formation. Studies of Recent sediments demonstrate that pyrite formed in many marine environments represents 10% or less of sulfide produced by sulfate reduction (J~rgensen, 1977; Swider and Mackin, 1989) and most sulfide is reoxidized to sulfate. For sediments deposited under oxygenated bottom waters, an important factor determining the amount of pyrite sulfur produced per unit of organic carbon is the abundance of reactive iron available for reaction with sulfide (e.g., Raiswell and A1Biatty, 1989). Siltstones and sandstones of the Nonesuch Formation contain abundant volcanic rock fragments and opaque minerals (e.g., Daniels, 1982) from which the derivation of significant quantities of reactive iron can be postulated. The availability of SO may also limit or enhance pyrite formation (Swider and Mackin, 1989 ). In environments of highly negative Eh, elemental sulfur may not be available for pyrite formation because of the lack of electron acceptors available for conversion of sulfide to elemental sulfur. Consequently, iron monosulfides accumulate in the sediment. The reactive composition but low concentration of organic matter in the Nonesuch may have established anoxic but not highly reducing conditions. Coupled with abundant reactive iron, these factors resulted in a setting of unusually efficient pyrite formation.

Extractable organic matter in the Nonesuch Formation A persistent problem associated with studying bitumen in Proterozoic sedimentary rocks

S/C RATIOS AND EXTRACTABLE ORGANIC MATTER OF NONESUCH FORMATION

is determining whether the samples have been contaminated anthropogenically or by migration of oil from younger strata. In the Nonesuch Formation, geological and geochemical evidence suggests that the bitumen is not a contaminant or migrated product. The geological evidence against migration of hydrocarbons from younger strata at the White Pine Mine was first outlined by Barghoorn et al. (1965) who noted the lack of organic matter in the overlying Freda Sandstone. Other evidence for the autochthonous origin of bitumen and lack of contamination: ( 1 ) the similarity between gas chromatograms and biomarkers in bitumen and hydrous pyrolysates of kerogen (Hieshima and Summons, unpublished data), (2) the lack of bitumen in coarse-grained lithologies with low organic carbon contents, (3) few anthropogenic products contain the spectrum of compounds observed in Nonesuch bitumen and (4) variation exists in the distribution of compounds from sample to sample. Alkanes in the Nonesuch Formation

Meinschein et al. (1964), Eglinton et al. (1964) and Barghoorn et al. (1965) reported n-alkanes, pristane and phytane from petroleum seeps in the Nonesuch Formation. Johns et al. (1966) identified iso- and anteiso-alkanes and an homologous series of alkyl cyclohexanes. Hoering ( 1976, 1981 ) confirmed the presence of n-alkanes, isoprenoids, monomethyl branched alkanes and cyclohexyl alkanes in seep petroleum. Recently, Imbus et al. ( 1988 ), using organic petrography and pyrolysis-gas chromatography-mass spectrometry on kerogens isolated from non-mineralized Nonesuch strata, identified two organic matter types, a filamentous, aliphatic type and an aromatic-phenolic type. Imbus et al. (1988 ) attributed differences in the organic matter types to "differential preservation superimposed on postulated maturity differences". In this study of non-mineralized strata, the smoothly decreasing series of n-alkanes with

75

slight odd carbon preference from n-C17 to n-C25 in most of the Nonesuch bitumens suggests that thermal maturity of the unit is relatively low, probably within the upper to middle part of the oil window. The relatively high ratio of pristane to n-C17 corroborates this interpretation. However, significant variation in the odd carbon preference and pristane/ n-C 17 ratios may reflect different thermal maturities or kerogen compositions. The distribution of n-alkanes and pristane/n-Cl 7 ratios in seep petroleum are substantially different than those of the bitumen. The narrower range and predominance of n-alkanes in seep petroleum indicate a higher level of thermal maturity than bitumens from non-mineralized samples (Hunt, 1979; Tissot and Welte, 1984). Using characteristic distributions of n-alkane and isoprenoids in bitumens, three geographic zones can be established: ( 1 ) west of the mineralized zone in Michigan, (2) further west in Wisconsin, and (3) the zone of mineralization around White Pine, Michigan. The former two are regions defined by Imbus et al. ( 1988 ) as those separating the different types of organic matter. In Michigan west of the mineralized zone, the wide range of n-alkanes, relatively high pristane/n-C17 and phytane/ n-Cl 8 ratios, and odd carbon preference are interpreted to be evidence of the lowest thermal maturity in the Nonesuch. Further west in Wisconsin, lower pristane/n-C17 ratios and less pronounced odd-carbon preference are interpreted to indicate higher levels of thermal maturity than in Michigan. Within the mineralized zone, the narrower range of n-alkanes, lower pristane/n-C 17 and phytane/n-C 18 ratios and lower odd-carbon preference are interpreted to indicate higher thermal maturity than interpreted for bitumen samples from outside the mineralized zone. Although n-alkanes of 25 to 30 carbon atoms often are interpreted to reflect input of terrestrial organic matter (e.g., Tissot and Welte, 1984), n-alkanes in Nonesuch bitumens probably were derived from lipids of

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bacteria and algae because of the absence of higher plants in the Proterozoic. Precursors to n-alkanes in living bacteria include homologues of up to least 30 carbon atoms (Han and Calvin, 1969). Algae contain ketones with chain lengths of up to at least 39 carbon atoms (e.g., prymnesiophyte algae; Marlowe et al., 1984). Monomethyl branched alkanes in Nonesuch bitumens are interpreted to represent bacterial contributions to the sediment (e.g., Summons et al., 1988). Pristane, phytane and the other isoprenoids may have arisen from the diagenesis of the phytol or farnesol side chains of chlorophyll (e.g., Volkman and Maxwell, 1986). However, low phytane to n-C18 and high pristane/phytane ratios may reflect greater input of pristane precursors such as tocopherols into Nonesuch sediments (Goosens et al., 1984), derivation from methanogenic bacteria (Rowland, 1990), or a paleoenvironment favoring the conversion of phytol to pristane (Didyk et al., 1978). It is very difficult to evaluate these alternatives. The use of isotope ratio monitoring GCMS may help solve this problem (Freeman et al., 1990).

Petroleum potential of the Nonesuch Formation The high extractability relative to organic carbon content, aliphatic character and relatively low thermal maturity contribute favorably to the petroleum potential of the Nonesuch Formation. If deeper water facies deposited at lower sediment accumulation rates are present along the basinal axis, then black shales with higher organic carbon contents could be present under Lake Superior and could be productive petroleum source rocks. If thicker sections of the organic-rich facies are present under Lake Superior or in southeastern or southwestern extensions of the North American Midcontinent rift, then the petroleum potential is excellent. The penetration of greater than 400 m thickness of Nonesuch-like lithologies several hundred kilometers to the

G.B. H I E S H I M A A N D L.M. P R A T T

southwest in the M.G. Eischeid # 1 well in Iowa (Anderson, 1990) makes exploration in other parts of the rift favorable in spite of the section being overmature. Conclusions

Unmineralized strata of the approximately 1.1 Ga Nonesuch Formation have S/C ratios that are higher than those from Phanerozoic normal marine and lacustrine rocks. The reactive composition but low concentration of organic matter in Nonesuch sediments coupled with high concentrations of reactive iron resulted in unusually efficient pyrite formation. The depositional environment is interpreted to be a marine embayment dominated by seawater rather than river discharge. Extractable organic matter in unmineralized strata of the Nonesuch Formation is autochthonous in origin and probably derived from mixed bacterial and algal inputs. Saturated hydrocarbon fractions of Nonesuch extractable organic matter contain homologous series of n-alkanes and cyclohexyl alkanes, isoprenoids and monomethyl-branched alkanes. Compared to oils seeping from mineralized strata, extracts from unmineralized samples have higher ratios of pristane/phytane, pristane/ n-C 17 and phytane/n-C 18, more pronounced odd carbon preference and n-alkanes of higher carbon numbers. Overall, the thermal maturity of the Nonesuch Formation in northern Michigan and Wisconsin is interpreted to be relatively low, probably within the upper part of the zone of petroleum formation and preservation. If thicker sections of organic-rich facies exist under Lake Superior or in the southern extension of the North American Midcontinent rift, then the petroleum potential of the Nonesuch Formation is good to excellent given the high extractability and aliphatic character of the organic matter. The penetration in Iowa of rock types similar the Oronto Group and including greater than 400 m of Nonesuch-like strata makes future exploration favorable.

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Acknowledgements We thank D.A. Zaback, J.M. Hayes and M.L. Tuttle for helpful discussions of carbon and sulfur systematics in marine and lacustrine sediments. R.E. Summons is acknowledged for his analytical and intellectual contributions. J.L. Mauk provided some seep oil samples. J. Phillips, W. Boberg and D. Petersen contributed analytical assistance. T.H. Donnelly and an anonymous reviewer made useful suggestions for improvement of the manuscript. The U.S. Bureau of Mines, Minneapolis, the Michigan Department of natural Resources and the Wisconsin Geological and Natural History Survey provided core samples. This work is supported by the U.S. Department of Energy through grant DE-FG02-88ER 13978.A000. References Anderson, R.R., 1990. Review of current studies of Proterozoic rocks in the Amoco Eischeid # l petroleum test well, Carroll County, Iowa. In: R.R. Anderson (Editor), The Amoco Eischied # 1 Deep Petroleum Test Carroll County, Iowa, Iowa Dept. Nat. Res., pp. 175-184. Barghoorn, E.S., Meinschein, W.G. and Schopf, W.J., 1965. Paleobiology of a Precambrian shale. Science, 148: 461-472. Bauld, J., 1986. Transformation of sulfur species by phototrophic and chemotrophic microbes. In: M. Bernhard, F.E. Brinckman and P.J. Sadler (Editors), The Importance of Chemical "Speciation" in Environmental Processes, Berlin, pp. 255-273. Berner, R.A., 1978. Sulfate reduction and the rate of deposition of marine sediments. Earth Planet. Sci. Lett., 37" 492-298. Berner, R.A., 1980. Early Diagenesis: A Theoretical Approach. Princeton University Press, Princeton, 241 pp. Berner, R.A. and Westrich, J.T., 1985. Bioturbation and the early diagenesis of carbon and sulfur. Am. J. Sci., 287: 177-196. Burnie, S.W., Schwarcz, H.P. and Crocket, J.H., 1972. A sulfur isotopic study of the White Pine Mine, Michigan. Econ. Geol., 67: 895-914. Capone, D.G. and Kiene, R.P., 1988. Comparison of microbial dynamics in marine and freshwater sediments: contrasts in anaerobic carbon catabolism. Limnol. Oceanogr., 33: 725-749. Chaudhuri, S. and Faure, G., 1967. Geochronology of the

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