An Organic Origin for the Carbonate 

Concretions of the Ohio Shale 

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An Organic Origin for the Carbonate 

Concretions of the Ohio Shale 

By R.E. CRISS, G.A. COOKE, and S.D. DAY 

U.S. GEOLOGICAL SURVEY BULLETIN 1836 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988 

For sale by the Books and Open-File Reports Section, U.S. Geological Survey 

Federal Center, Box 25425, Denver, CO 80225 

Library of Congress Cataloging In Publication Data 

Criss, R.E. 

An organic origin for the carbonate concretions of the Ohio Shale 

(U.S. Geological Survey bulletin ; 1836) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1836 

1. Concretions-Ohio River Valley. 2. Rocks, Carbonate-Ohio River Valley. 

3. Organic geochemistry-Ohio River Valley. 4. Ohio Shale. 5. Geology, 

Stratigraphic-Devonian. I. Cooke, G.A. II. Day, S.D. Ill. Title. 

IV. Series. 

QE75.89 no. 1836 557.3 s[552'.5] 87--600470 

[QE471.15.C58] 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Methods 3 

Field and petrographic relations 3 

Lithology and petrology 3 

Time of growth of the concretions 6 

Stratigraphic zonation of the carbonate concretions 9 

Isotopic and chemical relations 9 

Chemical composition 9 

Carbon and oxygen isotope data 12 

Interpretation of the o

13C values 13 

Interpretation of the o18 0 values 14 

Discussion and conclusions 17 

References 18 

FIGURES 

1. Map showing outcrop belt of Devonian Shales in Ohio 2 

2. Photographs of typical concretions from the Huron Member of the Ohio Shale 

near Worthington, Ohio 4 

3. Schematic diagram showing mineralogical zonation in the concretions from the 

Huron Member of the Ohio Shale 6 

4. Gray shale dike crosscutting the Huron Member of the Ohio Shale along the 

Huron River near Milan, Ohio 7 

5. Concretions incorporated in clastic dike and sill complex, along the Huron 

River near Milan, Ohio 8 

6. Graphs showing radial distribution of chemical contents and o

18 0 and o

13C 

values in concretions from Worthington, Ohio 10 

7. Graph of o18 0 vs. o13C values of carbonate concretions from the Huron 

Member of the Ohio Shale 13 

8. Photographs of concretions with fossil fish from several localities 15 

TABLES 

1. X-ray spectrochemical data for concretions from the Huron Member of the 

Ohio Shale 11 

2. Isotopic composition of concretions from the Huron Member of the Ohio 

Shale, Worthington, Ohio 12 

3. o18 0 determinations of coexisting vug minerals in concretions of the Huron 

Member 13 

4. Concretions from other localities-This study and others 14 

Contents Ill 

An Organic Origin for the Carbonate Concretions of 

the Ohio Shale 

By R.E. Criss, 1 G.A. Cooke,2 and S.D. Day3 

ABSTRACT 

Pyrite-rimmed carbonate concretions of the lowcarbonate Huron Member of the Ohio Shale are distinguished by their large size (to 3-m diam.), their high degree 

of sphericity, and their common association with the remains 

of arthrodire fish. Field relationships and the high ratio of 

authigenic to detrital components in the concretions indicate 

that they underwent rapid, early diagenetic growth as soft, 

low-density bodies that cemented the void space of extremely porous (81 < t < 94 percent) sediment near the 

sediment-water interface. This early growth phase (stage 1) 

was probably associated with precipitation of small amounts 

of barite and whewellite and was followed by a protracted 

interval that featured (stage 2) formation of the pyrite rims 

and of small pyrite concretions, concurrent with compaction 

of the shale and with replacement of the concretions by 

calcite, followed by replacement of the calcite by Ca- and 

Fe-rich dolomite (except within the central zones of the 

largest concretions); (stage 3) formation of joints in the shale, 

followed by intrusion of clastic dikes, severe deformation of 

concretions incorporated in the dikes, and continued compaction of the shale; and (stage 4) complex recrystallization 

and replacement of the concretions, represented by partial 

dissolution and recrystallization of the calcite centers, growth 

of quartz, dolomite, and calcite in vugs and veins, and 

approximately concurrent growth of cone-in-cone layers in 

the shale. Oxygen isotope data (-5.5<o180< -2.5) of the 

dolomitic matrix of the concretions are consistent with the 

early rapid growth and replacement stages (1 and 2) occurring at low but progressively increasing temperatures in 

marine pore fluid. o180 values of about -10 for recrystallized 

matrix calcite, most vug calcites, and a cone-in-cone layer 

suggest that stage 4 growth occurred at moderate temperatures (perhaps 75 °C) in the presence of marine pore fluid or, 

perhaps more likely, formation water that penetrated the 

shale at a later time. A noteworthy paradox is that, in the early 

portions of the proposed sequence, the highly porous, 

low-density (p < 1.5) sediment could seemingly not have 

1

U.S. Geological Suxvey, Reston, VA 22092. 2

Dept. of Geological and Planetary Science, University of Pittsburgh, PA 15260. 8

National Refractories and Minerals Corporation, Pleasanton, 

CA 94566. 

hydrostatically supported the large, rapidly grown concretions, which now have pee3 g/cc. Furthermore, as is typical of 

carbonate concretions worldwide, the o13C values of the 

carbonates are strikingly different from those of normal 

marine limestones but are similar to those of organic carbon 

and diagenetic carbonates. We demonstrate that the 

observed o13C values are similar to those of fish-bearing 

concretions from several other marine sequences, and we 

propose, along with Berner (1968), that such concretions 

may have originated as bodies of adipocere (low-density 

organic soap) that formed rapidly during decay of proteinaceous matter in the sediment. 

INTRODUCTION 

The Upper Devonian Ohio Shale crops out along a 

north-south band through central Ohio (fig. 1 ). In the 

lower part (Huron Member) and upper part (Cleveland 

Member) of this black shale, there are several horizons 

that contain carbonate and pyrite concretions, as well as 

a few cone-in-cone layers. These large, unusual, and 

commonly fossiliferous concretions (fig. 2) were 

described by the earliest geologic investigators in Ohio, 

and several proposals were made for the conditions ana 

causes of their formation. Much disagreement arose over 

the temporal relations of the concretions relative to the 

degree of compaction of the surrounding shale (Newberry, 1873; Daly, 1900; Westgate, 1926), although subsequently Clifton (1957) presented convincing arguments 

for their penecontemporaneous growth (see below). 

Other investigators focused on the chemistry and mineralogy of the black shale and made conflicting suggestions for the source of the organic matter, the postdepositional conditions that led to its preservation, and 

the environment of deposition (see summary by Hoover, 

1960). 

Studies of concretions from other localities and 

application of modern analytical techniques to concretions have greatly enhanced our understanding of these 

enigmatic bodies. Weeks (1957) demonstrated that fishbearing carbonate concretions from Colombia formed 

shortly after deposition of the sediment and proposed 

Introduction 1 
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INDEX MAl' OF OHIO 

I I II II I II I ' I ~00 MILES 

0 !10 IDD KILOMET[RS 

Figure 1. Map showing the outcrop belt of Devonian shales 

in Ohio, modified after Bownocker (1965). Localities discussed in text are as follows: W, Interstate 270 freeway cut in 

the Huron Member of the Ohio Shale at Worthington; M, cliff 

in Huron Member along Huron River near Milan; B, mine 

shaft at Barberton; P, Pugh Quarry in Devonian dolostone. 

Another sample was collected along the Interstate 71 freeway 

cut in the Cleveland Member of the Ohio Shale near Cleveland, Ohio. 

that decay of proteinaceous material led to rapid precipitation of carbonate and hence to the remarkable preservation of the fossils. Lippmann (1955) proposed that 

the ratio of acid-soluble to acid-insoluble components in 

concretions may be used to estimate the sediment porosity at the time of concretion growth and argued that his 

results demonstrated extremely high porosities (t=:;;75 

percent) in some instances. Oertel and Curtis (1972) 

provided novel structural data that generally substantiate 

Lippmann's (1955) deductions, but they also presented 

evidence that a stage of slow, protracted growth followed 

the initial stage of rapid growth of their concretions. 

Girin (1967) and Galimov and others (1968) suggested 

that concretions preserve a sequential record of the 

diagenetic processes to which the host rock was subjected. Raiswell (1971) outlined criteria that distinguish the 

relative ages of diagenetic concretions. 

Application of stable isotopic techniques to concretions has shed much new light on their origin and 

conditions of formation. Weber and others (1964) proposed that the o13C values of siderite nodules are dependent on the environment of deposition of the enclosing 

sediment. The carbon isotopic data of Hodgson (1966) 

demonstrated that organic matter was typically involved 

in the formation of carbonate concretions, and his findings have had significant influence on recent models 

explaining concretion origin (Berner, 1968; Raiswell, 

1976; Irwin and others, 1977). Although some concretions are known to form in freshwater environments 

(Degens and others, 1962), oxygen isotope data from 

carbonate concretions from numerous localities are similar to those of normal marine limestones and indicate 

that growth commonly occurred at relatively low but 

progressively increasing temperatures in marine pore 

fluid (Irwin and others, 1977; Coleman and Raiswell, 

1981; Hennessy and Knauth, 1985). However, the common existence in such concretions of late-stage calcites 

that are rather strongly depleted in 180 relative to normal 

marine limestones has led to varied interpretations, 

including substantial changes in pore fluid o180 values 

during diagenesis (Coleman and Raiswell, 1981; also see 

Lawrence and Geiskes, 1981), or precipitation during 

subsequent incursion of meteoric water (Hudson and 

Friedman, 1976) or basin-derived connate water (Dix 

and Mullins, 1987), or a combination of all the above 

(Siegel and others, 1987). 

This study attempts to determine the conditions of 

formation of the concretions of the Huron Member of 

the Ohio Shale and to determine how their character 

relates to the environment of deposition and to the 

subsequent diagenetic modification of the host shale. We 

direct particular attention to correlating concretion 

occurrence and growth to stratigraphic relations and to 

diverse stages that occurred during lithification of the 

enclosing shale. We also attempt to resolve the conditions of formation of late-stage vug and vein minerals in 

the concretions with o18 0 measurements of coexisting 

phases (quartz and dolomite or calcite), and we compare 

these data with that of vug calcite from a well-known 

mineral-collecting locality in Devonian dolostone at Pugh 

Quarry, Wood County, Ohio. Our results also disclosed a 

need to test Berner's (1968) adipocere model for concretion growth, and we accordingly made a comparative 

stable isotope study of selected fish-bearing concretions 

from several other localities in the world. 

ACKNOWLEDGMENTS 

This study originated in 1972 as an undergraduate 

research project at Case Western Reserve University. 

Professors Samuel M. Savin and the late John Hower 

contributed abundant and much-needed advice and 

instruction. Elizabeth Kurowski, Mark Hower, Hsueh 

Wen Yeh, Thomas Durst, Matt Stairs, and Karl Criss 

provided instruction and assistance in the laboratory. We 

2 An Organic Origin for the Carbonate Concretions of the Ohio Shale 
also thank M.E. Williams, the late D.H. Dunkle, and 

Virginia Heisey of the Cleveland Museum of Natural 

History for their valuable discussions and samples of 

fish-bearing concretions. S.M. Savin, J.R. O'Neil, Elliott 

Spiker, and W.L. Peterson made helpful critical comments. The Marathon Oil Company and the U.S. Geological Survey supplied financial support. 

METHODS 

Samples for x-ray spectrochemical studies were 

pre ignited at 1100 °C and then fused with Li2B4 0 7 

(sample: flux ratio 2:1 by weight; Hower and others, 

1964). Standards approximating the sample compositions 

were made from Fe2 0 3 , silicic acid, Al20 3 , CaC03 , 

MgO, KN03 , and Ti02 • Analyses were performed on a 

wavelength dispersive x-ray fluorescence unit using 

standard methods. The analyzed elements are reported 

as oxides and are K, Ca, Mg, AI, Ti, Si, and Fe (reported 

as Fe2 0 3 ). Corrections were made for matrix absorption 

effects, but enhancement effects were assumed to be 

negligible (Hower and others, 1964). 

Mineralogy of selected samples was determined 

using standard x-ray diffraction (Grim, 1968; Carroll, 

1970) and petrographic techniques. The mineralogy of 

clays was determined on samples that had been acid 

washed and H20 2-treated to remove calcite and organic 

matter; oriented slides of the < 21!-m material were then 

prepared by centrifugation. X-ray diffractograms were 

made of these slides before and after they were treated by 

KCl saturation, glycolation, and heating to 550 °C and 

650 °C. 

Carbon dioxide for mass spectrometic analysis was 

extracted from the calcite and dolomite of concretions by 

reaction with 100 percent phosphoric acid at 25 °C 

(McCrea, 1950). In most cases where both carbonates 

were present in the same sample, collection of the C02 

from each mineral was attempted by the method of 

Epstein and others (1964). Using the latter technique, 

C02 from the predominant phase of the mixture is not 

significantly contaminated by that from the minor phase, 

but if the carbonates are not present in approximately 

equal proportions, the opposite is not true (Epstein and 

others, 1964). The matrix of the concretions in all cases 

comprises either calcite with minor dolomite or dolomite 

with minor calcite. The most accurate analyses will 

therefore be the "calcite cut" of a dominantly calcareous 

rock and the "dolomite cut" of a dolomitic rock; the 

complementary analyses of the minor phases are of lower 

reliability and are accordingly reported in brackets. The 

isotopic measurements have all been corrected using the 

method of Craig (1957), and are reported relative to the 

PDB standard (unless otherwise stated) in the usual 

delta notation. Following a recommendation by Land 

(1980), no corrections have been made for the different 

acid-extraction fractionations of calcite and dolomite 

(Sharma and Clayton, 1965). Four a180 analyses of 

quartz samples were made using the technique of Taylor 

and Epstein (1962) and are reported in table 3 relative 

to the SMOW standard. Precision is ±0.1 and ±0.2 per 

mil for the carbonate and the silicate analyses, respectively. 

Most samples were collected from a roadcut along 

Interstate 270 about 2 km north of Worthington, Ohio. 

The locations of samples collected elsewhere are given in 

the text and tables (also see fig. 1). 

FIELD AND PETROGRAPHIC RELATIONS 

Lithology and Petrology 

The Huron Member of the Ohio Shale is a grayishblack, fissile shale that generally contains pyrite and more 

than 10 percent organic matter (Hoover, 1960). Carbonized fossils of marine algae and terrestrial plants are 

common, but other fossils are notably scarce with the 

exception of conodonts and occasional brachiopods 

(Westgate, 1926; Wells, 1947). The rank of the shale in 

the area of study is rather low. Observed HJC ratios in 

phytoclasts and Protosalvinia (Foerstia) from the Worthington locality are similar to those of high-volatile 

bituminous coals (Romankiw and others, in press). Conodont coloration ( < 1.5 CAl) also indicates a history of 

rather low temperature conditions in the upper Devonian 

rocks of central Ohio (Epstein and others, 1977). 

Nelson (1955) determined that the dominant mineralogy of the Huron Member is quartz, illite, and chlorite, and that kaolinite is normally absent. Hosterman 

and Whitlow (1983) report that, in drill holes representing a large area of the Appalachian basin, the lower unit 

of the Huron Member has an average composition of 25 

percent quartz silt, 15 percent chlorite, 60 percent illite, 

20 percent mixed-layer illite-smectite, 5 percent kaolinite, and a trace of mixed-layer illite-chlorite. However, 

kaolinite was found in only about half the samples, being 

most abundant within and east of eastern Ohio and, 

accordingly, indicating an eastern and northeastern 

source for this detrital clay (Hosterman and Whitlow, 

1983). Our x-ray diffractograms of the shale at Worthington generally confirm these results, but our observations 

of a continuous spectrum between 10 A and 14.4 A 

suggest that mixed-layer illite-chlorite is more abundant. 

We did not observe significant changes of this material on 

glycolation, on heating to 550 °C, or on saturation with 

KCI. Carbonate minerals were not detected in our x-ray 

study of untreated shale samples except in regions adjacent to concretions. We also found that the clay fraction from the center of the large concretions is 

Field and Petrographic Relations 3 
Figure 2. Photographs of typical 

concretions from the Huron Member 

of the Ohio Shale near Worthington, 

Ohio. A. Concretion from the Huron 

Member, Interstate 270 cut, Worthington, Ohio, broken along the stratification plane through centerline. Dark, 

angular material in center is an arthrodire plate (apatite). Enclosing material 

is predominantly calcite spar (lightcolored inner zone), in sharp contact 

with surrounding, weathered, ochrecolored dolomite zone. Most concretions also have a thin ( < 1-cm thick) 

pyrite rim. B. Huron Member concretion, approximately 1 m in diameter, 

broken in vertical section. Note the 

irregular but sharply defined boundary between the light-colored calcite 

zone and the surrounding dolomite 

zone, which formed by replacement. 

Also note that the calcite zone, 

although bounded laterally, extends 

from the top to the bottom of the 

concretion. C. Huron Member concretion, about 1.5 m in diameter, at the 

Narrows near Worthington, Ohio. 

Note the stratification planes passing 

through the concretion. Shale layers 

are wrapped around the concretion 

due to differential compaction, as discussed in the text. D. Typical concretion in the Huron Member, Interstate 

270 freeway cut, Worthington, Ohio. 

Painted, 5-foot (1.5-m) rod shows 

scale. Note the ellipsoidal form, the 

stratification planes passing through 

concretion, and the •tunnel-shaped• 

depression in top. E. Small dolomitic 

concretions from the Huron Member 

at Worthington, Ohio. One sample 

has been broken to reveal the rim of 

radially oriented pyrite crystals. F. 

Unusually large pyrite concretion from 

the Huron Member at Worthington. 

Note that the bedding in the shale, 

highlighted by pyrite, can be readily 

traced into the granular pyrite zone 

that constitutes the interior of the concretion. Also note the decreased width 

of the layers peripheral to the concretion (as compared to the inside), and 

the well-developed, symmetrical compaction features in the shale peripheral to the concretion. All these features indicate preservation of original 

pore space by concretion formation 

during progressive compaction of the 

sediment (see text). However, the 

perimeter of the concretion comprises 

larger, euhedral, radially oriented 

pyrite crystals that incorporated little 

or no sediment, but rather displaced it 

mechanically. Sample is 12 em long. 

4 An Organic Origin for the Carbonate Concretions of the Ohio Shale 
l

lllljllllllllljllllllllljllllllllljllllllllljllllllllljllllllllljlllll""l""l""l""l''lljllllllllljllllllllljllllllllljlllll'"'l''' em 1 2 3 4 5 6 7 8 9 10 ll 12 13 11 

F 

Field and Petrographic Relations 5 
CENTRAL ZONE 

coarse ~rained coldhl 

qiKirtZ 

clay minerals 

barltt 

!NTERMEQ!AIE ZONE 

fine ~rained dolomite 

(with some groins 

having colette centers) 

clay minerals 

quartz 

pyrite 

OUTER LAYER 

pyrite 

clay minerals 

carbonate minerals 

quartz < IOcm 

Figure 3. Schematic diagram showing mineralogical zon~­

tion in the concretions from the Huron Member of the Ohio 

Shale. Compare figures 2A, 2E, and 2F. 

mineralogically similar to that of the shale, except that 

the 7.2 A110 A peak-height ratio is somewhat larger in the 

concretion clays. 

The large (0.25- to 3-m diam) concretions are 

nearly spherical, with minor elongation along the horizontal axis, and are composed primarily of calcite, dolomite, quartz, pyrite, barite, clay minerals, and organic 

matter (Westgate, 1926; Hoover, 1960; see fig. 2). 

"Funnel-shaped" depressions occur at the tops and bottoms of some concretions (Newberry, 1873; Clifton, 

1957; fig. 2D). Veins and vugs in the concretions are most 

commonly lined with calcite and less commonly contain 

dolomite or quartz; pyrite and barite are not present in 

these late-stage features. Selenite (gypsum; Stauffer and 

others, 1911 ), fluorite and celestite (Clifton, 1957), and 

fine-grained magnetite (Cooke, 1976) have also been 

reported in the concretions. Hyde and Landy (1966) 

reported whewellite (CaC20 4 .H20) in the concretions at 

Milan Ohio and we found this oxalate mineral in one ' ' concretion at Worthington. Fossils of arthrodire plates 

(fig. 24) and terrestrial plants commonly occur in the 

centers of the concretions (Newberry, 1873). Foreman 

(1959) reported that some concretions contain abundant, 

well-preserved radiolaria. 

We observe three distinct mineralogical zones with 

abrupt boundaries in the large concretions (figs. 

24,2B,3). The central zone is predominantly ( > 60 

weight percent) coarse-grained (==I em) calcite, with 

quartz, clay minerals, and small amounts of barite and 

pyrite. This barite occurs either as clear euhedral crystals 

or as dendritic forms that are completely enclosed within 

and are probably replacing the matrix. These barite 

crystals probably originated in the rapid growth stage or 

in the earliest part of the subsequent replacement stage 

(see below). This center is surrounded by an intermediate zone that is more than 80 weight percent finegrained (==0.1 mm) dolomite with appreciable pyrite, 

quartz, clay minerals, and minor calcite (Westgate, 

1926). This zone commonly has a weak radial structure, 

which, together with relict stratification planes in the 

concretion (see below), defines planes of weakness along 

which cracks and joints preferentially develop. Most 

concretions also have a thin ( < 2-cm thick) outer layer 

comprising radially oriented pyrite crystals ~ithin a cla~­

carbonate matrix. Exterior to the concretions there IS 

generally an indistinct zone of dolomite-cemented shale 

that grades continuously into the carbonate-poor shale. 

Large (up to 10 em long) euhedral crystals of bar!te, ~s 

well as our single whewellite sample, were found m this 

indistinct zone, which is principally developed as an 

annular "girdle" (pressure shadow?) about the concretion centerline. These barite crystals are again completely 

enclosed by the shaley matrix material and are commonly 

outlined by pyrite. We found delicate calcite boxworks of 

the cleavage cracks of former, large barite crystals in 

some concretions. Clifton (1957) reports that cone-incone structure occurs in the peripheral zone of some 

concretions. 

Small (5 to 25 em diam) concretions in the Huron 

Member consist only of the dolomitic "intermediate" 

zone with an exterior shell of pyrite; also present are 

small ( < 10 em diam), slightly ellipsoidal to disc-shaped 

concretions composed almost entirely of pyrite and shale 

components (figs. 2E,2F,3). In a few cases we found both 

types of these small concretions enclosed wit~in the outer 

parts of the dolomite zone of large concretions, a relationship indicating partial temporal overlap of the growth 

of large and small concretions. . 

Several lines of evidence indicate that the dolomite 

zones were formed from preexisting calcite by replacement. The boundary between the central and intermediate zones of the large concretions is extremely sharp 

and therefore consistent with replacement. This boundary is nearly circular along horizontal sections through 

the concretions (fig. 24) but is rather irregular and 

elongate on vertical sections (fig. 2B), commonly extending to the "funnel-shaped" depressions that commonly 

occur at the top and bottom of the concretions. This 

shape clearly indicates that the calcite core zones are 

remnant, having been replaced by dolomite at a sta~e 

when sufficient compaction had ensued to produce significant anisotropy in the permeability of the shale. 

Stained thin sections provide equally clear-cut evidence 

for replacement in that the dolomite grains of the intermediate zone often have centers of calcite. The replacement of calcite by dolomite might be associated with 

superimposed, more reducing conditions ~hat are ~videnced by the pyrite rims, insofar as dolomite formation 

is strongly inhibited by aqueous sulfate (Baker and 

Kastner, 1981). 

Time of Growth of the Concretions 

The temporal relations of the concretions in the 

Huron Member of the Ohio Shale were first investigated 

6 An Organic Origin for the Carbonate Concretions of the Ohio Shale 
by Newberry (1873), who noted the pronounced arching 

of the shale above and below the concretions (fig. 2C). 

Newberry (1873) reasoned that the concretions formed 

in incompletely consolidated sediment, and that the 

arching resulted from subsequent compaction that 

deformed the sediments but not the rigid concretion. 

Daly (1900) noted similar arching of shale around the 

carbonate concretions at Kettle Point, Ontario, but proposed that the fully compacted shale had been deformed 

during epigenetic growth of the concretions. Stauffer and 

others (1911) noted that the bedding planes of the shale 

could commonly be traced into the concretions, and 

Westgate (1926) observed that the width between any 

two such bedding planes increases from the edge to the 

center of the concretions (fig. 2F). Westgate (1926) 

proposed that these layers had been physically pushed 

apart during the deposition of the concretionary material; thus his view is similar to Daly's (1900). Clifton 

(1957) concluded that concretion growth was penecontemporaneous, reasoning that compaction of the sediments progressed as the concretions grew outward and 

filled the pore space. According to Clifton's (1957) 

hypothesis, the width between enclosed bedding planes 

would progressively decrease from the center to the 

periphery of the concretion; eventually, when growth 

stopped, the shale would warp around the concretion as 

suggested by Newberry (1873). 

Several independent lines of evidence support the 

interpretations that Newberry (1873) and Clifton (1957) 

advanced for the Huron Member concretions. First, 

Clifton (1957) noted that uncrushed Tasmanites occur in 

the concretions but not in the shale, and Foreman (1959) 

made similar observations about radiolaria. Second, our 

chemical data demonstrate that a significant shale component is present within the concretions and that the 

fractional amount of this component increases radially 

outward in the concretions, as described below. Furthermore, the high porosities indicated by our data (81 to 94 

percent) are in accord with observed values for recent 

uncompacted argillaceous muds (generally 70 to 90 

percent; MUller, 1967). 

In addition, we found several small, rotated and 

sometimes severely crushed, pyrite-rimmed, Huron 

Member-type concretions in a clastic dike near Milan, 

Ohio (figs. 4,5). This unique locality permits many 

important temporal relations to be discerned. Part of the 

dike occupies joints with the same NE orientation as the 

principal joint set in the area (fig. 4). However, a major 

offshoot of the dike is nearly concordant with the enclosing black shale; note that the joints in the black shale do 

not cut the clastic intrusion (fig. 5A). The intrusive 

material is lithologically similar to the light grey shale 

(Olentangy Shale?) that underlies the Huron concretionary zone. In one case the dike material is visibly warped 

around a concretion (fig. 5C), indicating that joint forFigure 4. Gray shale dike crosscutting the Huron 

Member of the Ohio Shale along the Huron River near 

Milan, Ohio. This part of the dike occupies one of the 

prominent, NE-trending joints in the host black shale; 

however, a major sill-like offshoot of the dike (not shown) 

is only locally, but clearly, discordant. The dike and sill 

complex contains fragments of the black shale as well as 

small, rotated and deformed, pyrite-rimmed concretions 

(see fig. 5). 

mation and intrusion occurred before compaction was 

complete. The concretions were obviously formed before 

their incorporation in the dike, and thus the warping 

could not be the result of concretionary growth. Severe 

deformation of most of the concretions in the dike (fig. 

5A,5B) indicates that they still retained some degree of 

their inferred initial soft character; moreover, their 

deformable nature was apparently retained (1) after the 

pyrite rims were formed, and (2) after much of the matrix 

material had become, or was destined to become, dolomite. Even so, the density of the concretions at that time 

probably exceeded that of the intrusive material, insofar 

as at least one specimen (fig. 5C) had sunk to the bottom 

of the subhorizontal part of the dike. Calcite overgrowths 

occur on several of these incorporated concretions (fig. 

5B). 

On the other hand, there is also some petrographic 

evidence for small-scale, mechanically displacive growth 

Field and Petrographic Relations 7 
8 An Organic Origin for the Carbonate Concretions of the Ohio Shale 

Figure 5. Concretions incorporated in 

clastic dike and sill complex, along the 

Huron River near Milan, Ohio. A. Highly 

deformed, 50-em-long concretion within 

sill-like part of clastic dike. Note that the 

prominent, nearly vertical joints in the 

black shale do not cut the intrusive material. B. Highly sheared (U-shaped), 

pyrite-rimmed dolomitic concretion at 

lower interface of clastic sill and host 

black shale. A prominent calcite overgrowth occurs along the concave part of 

the concretion perimeter. Quarter dollar 

shows scale. C. Rotated, slightly 

deformed, pyrite-rimmed dolomite concretion in clastic sill. Weak, differential 

compaction features are developed in 

the dike peripheral to the concretion. This 

concretion appears to have sunk to the 

bottom of the sill. Quarter dollar shows 

scale. 
of authigenic minerals. Examples are the large, perfectly 

clear, barite crystals within and immediately outside the 

concretions, and the euhedral, 1-cm pyrite crystals on the 

outside of some small concretions (fig. 2F). Cone-in-cone 

layers in the formation provide another example of 

possible displacive growth (Brown, 1954). It is therefore 

reasonable to infer that at least some displacive growth 

occurred in the Huron Member concretions, and this 

might partly explain why the "porosities" calculated from 

our chemical data are so high (also see discussions in 

Raiswell, 1971, and Hennessy and Knauth, 1985). 

Stratigraphic Zonation of the Carbonate 

Concretions 

Several workers have noted that the large carbonate concretions tend to occupy definite stratigraphic 

horizons that generally occur in the lower part, perhaps 

the lowermost 10 to 30 m, of the Huron Member of the 

Ohio Shale (Newberry, 1873). This feature led Westgate 

(1926) to suggest that channelized fluid circulation above 

the underlying "impervious" Olentangy Shale played a 

role in concretion development. However, all the original 

observations were made along the extensive Devonian 

outcrop belt in Ohio and do not necessarily have any 

bearing on the distribution of concretions in the subsurface to the east, insofar as the outcrop belt is approximately parallel to the original shoreline (Barrell, 1913). If 

the concretions always occur near the base of the Huron 

Member, then the concretions occupy a diachronous 

stratigraphic zone, because the Ohio Shale is widely 

regarded to be part of a transgressive sequence related to 

the Catskill delta (Barrell, 1913; Allen and Friend, 1968; 

Schwietering, 1977; Dennison and Textoris, 1977). However, a comparison of the concretionary horizons with a 

biostratigraphic zone suggests that the concretions 

occupy an interval that is at least approximately synchronous. 

Studies by Hass (1956), Winslow (1962), Winder 

(1966), Schopf and Schwietering (1970), and Murphy 

(1973) have established that a relatively thin ( < 45-m 

thickness) Foerstia zone occurs at widely distributed 

localities in the Huron Member (Ohio and Kentucky), 

Chattanooga Shale (Tennessee), Chagrin Shale (Ohio), 

Kettle Point Shale (Ontario), and Ellicot Shale (Pennsylvania and New York). Where comparisons are possible, 

the published descriptions indicate that the carbonate 

concretions occur near and not more than 30 m below 

the base of the Foerstia zone (see Hass, 1956; Winder, 

1966; Schopf and Schwietering, 1970; also see figure 6 of 

Broadhead and others, 1980). Unfortunately, the presence or absence of the concretions east of the Devonian 

outcrop belt cannot be inferred from drill-hole data. 

However, Stauffer (1944) noted large (to 0.6-m diam) 

carbonate concretions in a 670-m-deep mine shaft at 

Barberton, Ohio, approximately 90 km downdip from the 

outcrop belt (see fig. 1). The latter concretions occur 

within the middle part of the Huron Member (==145m 

above the base), and none were described in lower levels 

of the formation (Stauffer, 1944). Furthermore, the 

Barberton concretions occur only ==30 m below the base 

of the Foerstia zone identified by Hass (1956) and 

Winslow (1962). Thus, the available evidence suggests 

that the large carbonate concretions have a much closer 

spatial relationship to the presumably synchronous Foerstia zone than to the diachronous base of the Huron 

Member. This evidence implies that a temporal factor, 

perhaps a decrease in the rate of sedimentation, or 

large-scale stagnation of bottom waters, is responsible 

for the stratigraphic distribution of the carbonate concretions. 

ISOTOPIC AND CHEMICAL RELATIONS 

Chemical Composition 

X-ray spectrochemical analyses made of samples 

from traverses across two large concretions (fig. 6) are 

given in table 1. Also included are wet chemical analyses 

of a concretion and a shale sample from the Huron 

Member of the Ohio Shale (Westgate, 1926) that for 

comparison purposes have been recalculated to 100 

percent for the indicated constituents. The analyses generally confirm that the concretions consist predominantly 

of carbonate minerals and shale constituents, and they 

clearly demonstrate the chemical zonation of the bodies. 

Because volatile constituents were not determined 

(except by Westgate, 1926), mineralogic interpretation of 

the oxide concentrations can be made only in conjunction 

with petrographic data. 

Analyses of three central zone samples (T3A-1, -2 

and T3C-1) indicate a predominance of calcite, but the 

high Si02 :Al2 0 3 ratios suggest that authigenic quartz is 

present. Most of the iron reflects the presence of pyrite, 

and the clay-related oxides (Al2 0 3 , K2 0, Ti02 ) represent 

shale components incorporated in the concretion. 

Our analyses of six samples from the "intermediate 

zone" (T3A-3, -4, -5 and T3C-2, -3, -4) agree closely 

with the concretion analysis by Westgate (1926), and all 

are significantly different from those of the central zone. 

The markedly greater MgO and Fe20 3 contents primarily reflect the presence of ferroan dolomite and a radial 

increase in pyrite. Westgate's (1926) complete chemical 

analysis in fact suggests that the composition of the 

dolomite is (Ca.56Mg_39Fe.05 )C03 , although the calculated Ca content may be slightly high because small 

amounts of calcite are also commonly present in the 

intermediate zone. However, a Ca- and Pe-rich compoIsotopic and Chemical Relations 9 
100 40 

.... aa !z 30 

·-..... 1-

aJ I ' z LIJ I '. 0 0 ~ a: ffi I ffi 10 UJ 

D.. 60 D.. I \ ll.. 

~ ~ I I !i: 

(!1 ~ 20 I \ (!) 

~ ~ jjj 

I \ 3: ~ 40 \ i; ' ~ 

~ \ I Js \ I tG 

\ I LL 

A I X A 

0 

0 20 40 80 00 80 20 40 80 

10 

~ 8 !Z w 3 

0 LIJ a: 0 UJ a: 

:: 6 w 

ll.. 

:I: 

(!) ~ 2 [ij (!) 

~ iii 

~ 4 :t 

a ~ 

0 < 

-5" /. ~ 

2 --·-·/ ~ 

20 00 20 40 60 80 00 40 80 

RADIAL DISTANCE IN CENTIMETERS 

-10 

-2 

• ,, !Z 90 I ' UJ _, ..... -4 I :2 ~ ~ I ~ ffi-14 a: !z 80 0.. UJ I UJ 

~ 11... 0 ~-6 I a: 

LIJ 

rs I ll.. 

fa ~ 70 

:::l 

-8 ....1 

§2 

60 

-10 . X 

-20 0 500 20 80 0 20 40 60 80 80 

RADIAL DISTANCE IN CENTIMETERS 

10 An Organic Origin for the Carbonate Concretions of the Ohio Shale 
Table 1. X-Ray Spectrochemical Data For Concretions From the Huron Member of the Ohio Shale (in weight percent) 

Radial Distance Wt.,., Vol.% 

Saq>le Zone on Si~ AltJJ Fet>3 WI' Cal K,fJ Ti~ Total Ce1Ent Ce1ent 

T3A-l Central u 43.42 3.57 1.96 2.33 46.93 .81 .19 99.21 87.9 85.3 

2 Central 25 36.14 8.81 1.55 1.86 45.32 .69 1.19 9.>.56 69.2 66.0 

3 lntem81i ate 3) 9.41 3.29 8.69 28.00 48.79 .55 .15 99.68 91.1 89.1 

4 lntenredi ate 45 11.?6 4.Z3 8.23 29.<Xl 47.~ .77 .17 1£Yl.04 88.6 87.1 

5 lntennedi ate 60 7.7.92 5.06 9.61 17.56 32.18 1.30 .33 93.96 83.2 81.1 

6 .. Shale•• 65 63.57 10.50 5.8\ 6.57 9.:1) 2.74 .46 ~.~ 57.3 53.8 

TJC-1 Central 0 11.52 2.35 1.35 3.04 76.72 .34 .24 95.56 93.0 92.0 

2 Intenredi ate aJ 2.71 1.97 7.70 32.49 57.73 .35 .rJl 99.rrl 94.8 94.1 

3 lntennedi ate 3J 6.04 1.98 6.88 29.47 50.02 .36 .09 94.84 94.5 93.7 

4 Intemaii ate 40 B.ffi 2.79 9.8J 26.52 48.56 .65 .14 (/1.31 92.2 91.1 

5 ~rite 45 2.24 1.89 fi8.73 5.58 15.03 .26 .11 gJ.84 93.8 ~.8 

Concretion* Int? 15.ffi 3.7.7 8.RJ 24.13 47.7l. 0.21 (100.) 9).9 89.6 

{Westgate, 1926) 

lllio Shale* Shale 63.64 ?2..77 6.30 1.71 0.48 5.10 (100.) 0 0 

(Westgate, 1926) 

*Wet chelrical analvses recalaJlate:t to 100 percent for the indicated constituents. 

sition (apparent CaC03 content= 56 to 57 mole percent) 

for the dolomite would be consistent with the 2.906 A 

spacing of the 104 peak that we have observed in several 

samples (see Murata and others, 1972). Of all the 

intermediate zone samples, only T3A-5 and Westgate's 

(1926) concretion have Si02 :Al2 0 3 ratios sufficiently 

large, relative to Westgate's shale, to suggest that significant authigenic quartz is present. Furthermore, the 

clay-related oxides, in this case including most of the 

Si02, all show a clear-cut radial increase in the intermediate zone. Note that Ti02 :Al2 0 3 is essentially constant 

(:;:0.05) in all but one of these samples (T3A-2). However, there is some evidence for a radial increase in 

K20:Al20 3, indicating either that a record of progressive 

diagenetic fixation of potassium by clays is preserved 

within the concretions or that an (assumed) initially 

~ Figure 6. Graphs showing radial distribution of chemical contents and o180 and o13C values in concretions 

T3A (solid line) and T3C (dashed line, in part inferred) 

from Worthington, Ohio. Open circles represent centralzone samples, solid circles represent samples of the 

dolomitic intermediate zone, solid triangle represents 

pyrite band, and x represents carbonate-cemented 

shale immediately peripheral to concretion. The data 

indicate the marked differences in the chemical and 

isotopic compositions of different zones in the concretions. Except for the calcite centers and the thin pyrite 

shell, the amount of shale incorporated in the concretions increases radially outward, while the proportion of 

authigenic constituents decreases. See text and tables 1 

and 2. 

uniform clay composition was subsequently modified by 

diffusional influx of potassium from a chemically evolving 

pore fluid. 

An analysis of "shale" immediately outside one of 

the concretions (T3A-6) shows the expected higher 

concentrations of the clay-related oxides, but comparison 

with Westgate's (1926) shale analysis strongly suggests 

that authigenic dolomite, quartz, and pyrite also occur in 

this sample. Last, an analysis of the thin pyrite shell on 

one of the concretions (T3C-5) indicates that dolomite 

and shale constituents are present, in addition to pyrite. 

An important feature of our analyses is that they 

permit approximate computation of the relative proportions of authigenic constituents and incorporated shale 

material in radial traverses through the concretions. 

Although there is no completely satisfactory way to make 

such computations, particular~y from our partial chemical analyses, we propose that it is reasonable to (1) 

assume that the oxide ratios of shale constituents in the 

concretions are exactly the same as those in Westgate's 

(1926) complete analysis of the shale; (2) determine the 

quantity of incorporated shale in the concretion samples 

from the Al2 0 3 contents and the above assumption; and 

(3) recalculate any unaccounted CaO, MgO, Fe2 0 3 and 

Si02 as authigenic carbonates and quartz. Note that 

computation of all authigenic iron as carbonate is acceptable for our purpose because siderite has nearly the same 

formula weight as pyrite. 

The results of the calculations are given in table 1. 

The proportion of the authigenic constituents in most 

concretion samples ranges from 83 to 95 weight percent, 

Isotopic and Chemical Relations 11 
and would translate to approximately 81 to 94 volume 

percent, given reasonable assumptions of the mineral 

densities. Furthermore, there appears to be a progressive 

radial decrease in authigenic constituents, particularly if 

the central zone samples are excluded. This result 

accords with the observations of Lippmann (1955), Raiswell (1971 ), and Oertel and Curtis (1972), who interpreted similar data in terms of concretion growth by cementation of pore space during progressive compaction of the 

host shale. The relatively low cement contents of the 

calcite centers might indicate late-stage dissolution of 

calcite. Such dissolution might ultimately have been 

responsible for the septarian cracks and the peculiar 

"funnel-shaped" depressions at the top and bottom of 

some concretions. Last, in partial support of our calculations we point out that Westgate (1926) recalculated 

his complete chemical analysis in terms of 85.2 weight 

percent carbonate, and this, along with the small amount 

of authigenic quartz and pyrite in the sample, would 

approximately equal the value of 91 weight percent that 

we calculated by our method for all authigenic components (table 1 ). 

Carbon and Oxygen Isotope Data 

o

13C and o

180 analyses were made of several large 

and small concretions from the Huron Member of the 

Ohio Shale at Worthington, Ohio (table 2). The o13C 

(-19.9±0.5) and o

180 (-10.2±0.2) values of calcite 

from the centers of the large concretions are strikingly 

uniform. The "late calcites" that fill cracks and vugs in 

the concretiol'is typically have similar values, as does one 

sample of a 3-cm-thick, cone-in-cone layer in the shale 

(table 2). 

Isotopic analyses of dolomite from the intermediate zone of the large concretions (-11.0so13C 

s -13.3; -4.6 s o180 s -2.5) are significantly different 

from those of the calcites. A distinct radial decrease of 

o

18 0 in the dolomitic matrix is evident in the large 

concretions (T3A and T3C) and in some of the small 

concretions. The o-values of the dolomite in the small 

concretions are similar to those of the outermost parts of 

the large concretions (T3C-4 ), although it is notable that 

the largest of these small concretions (T2S; 0.25-m diam) 

is isotopically similar to dolomite well inside the large 

concretions (T3A-3,4). Dolomite from a small, pyriterimmed concretion from Milan, Ohio (o13C= -10.1, 

o

18 0=-5.4) is isotopically very similar to the small 

concretions from Worthington. Taken together, the o13C 

and o

18 0 values of all the concretionary dolomites form 

a common trend line (fig. 7). 

In cases where analyses were made of different 

C02 fractions from a single sample, both the o13C and 

o

180 values of the minor fractions (in brackets, table 2) 

Table 2. Isotopic composition of concretions from the 

Huron Member of the Ohio Shale, Worthington, Ohio 

ltxlule ~le 

~ lb. 1Jescr1 pti Cl'l 

T2A 

t:t 

1 centra 1 zone 

2 Central zone 3 Inteme:l1ate zone 

4 Jnt:enmd1ate zone 5 Intenrediate zone 

2 Jntenred1ate zone 3 Interm:liate zone 4 Intenra11ate zone 

1 central zone, with 

fa;sil wood nucleus 

i! lntenrediate zone 

1 Central zone, With arthrodfre plate rucleus 

2 Intenrediate zone 

1 Imer inteliiildlate ZCTe 

2 Mer 11"ternm 1 ate zme 

1 l"ner i ntenredi ate zone ?. ()Jter 1nternl'd1ate ZCTe 

1 lmer 1ntemed1ate zone 2 lllter 1 nteme:li ate zone 

1 Imer 1 ntet1red1 ate zone 2 ~r 1ntemlldiate zone 

T3SV 1 Vug 1n large naille . j! • 

:i 

4 

!l 

m 1 crack. ff111rg 1n 

large norule 

clc 1 cone:1n-cme l11115tone 

~lcite" 

Mlneral!J1Y(1) &lJc &1!\J 

I. !..Mil: KlULS 

u:(O'i'Z,rnn) 

CC(OTZ,JU.O) 

rnJ1((1TZ,CC) 

IXl..fi(OTZ,CC) 

rn.Jl(QTZ,CC) 

tiio 

IXlO 

rno 

it(QfZ) 

IXl.O 

it(Q'ft) 

rno 

I I. 9-WJ. fUU.fS 

-19.5 -1D.4 

-19.4 -10.:i 

(-11.2 - 3,6) 

(-12.2 - 4,1) 

( -1?~6 - 3.~) 

(-13.4 - 3,3) 

( -11.4 - 4.8) 

-Mo -W:t 

::ro:4 -to.2 

( -18.0 -9.3; 

-12,5 -2.6 

-13,2 -2.5 

-13.5 -3,9 

-13,3 -2.5 -13.4 -3.7 

-11.0 -4.6 

-12.1 -4.1 

-12.8 -3,3 

00.0 (- 7.7 - 5.4) - 9.3 =5.3 

Dl.O - 9.2 -5.3: 

IJl.O (- 9.0 - 5.7) - 9,3 -5.1 

00..0 (- 8.4 - 5.6) - 9.2 -6.5 

rn.J) (- 8.8 - 5.5) - 9.4 -5.3 

lll.O (- 1.(.1 - 5.5) - 11.3 -5.1 

iiiO(IlTZ,oc> t-1204 - 4,9) -12." -?-R lll..O(OTZ) (-12.9 - 3.9) -1?~6 -3.0 

Ill. 'tAft." QI.CntS 

tt(QTZ) 

a: 

cc 

a: 

u: 

cc 

tt 

tt(Qtt) 

lt(QfZ) 

:Z0:2 -10.3 

-15.2 -10.5 

-23.0 -10.7 

-17.3 -10.4 

-&3 -10.7 

-16.7 - 9.8 

-18.6 - g,q 

=zu -m.9 

-ilt9 - 9.6 

-21.5 - 9.7 

(1) Detennined by X-t"il)' diffraction; CC = calcite; tQ.O • <blOlllte; OfZ • q.&rtz. 

{*) Samle tt:-1 was la;t. 

tend to mirror those of the principal phase and may 

reflect significant contamination. Thus, no systematic 

differences between the o13C values of coexisting concretionary calcite and dolomite are observed. However, in 

all but one instance the dolomite has a slightly higher 

o

18 0 value than the coexisting calcite. Because this minor 180 enrichment in the dolomite C02 could primarily 

reflect the different acid extraction fractionations of the 

two carbonate phases (Sharma and Clayton, 1965), primary 18 0 fractionation between the phases is not necessarily indicated by the data. 

Four analyses (o180= +23.4 to +25.3 rei. SMOW) 

were made of quartz crystals that line vugs in large 

concretions from Worthington, Ohio (table 3). Analyses 

of calcite in two of these vugs are highly distinct ( approx. 

o13C= +16; o

180= -5) from those of the other vug and 

vein calcites we have measured (table 2). Analyses were 

also made of calcite from the center (o13C= -11.4; 

12 An Organic Origin for the Carbonate Concretions of the Ohio Shale 
-8 

-10 

-12 

-14 ...J 

i: 

a::: 

w 

a.. -16 :z 

..... 

~ 

~ -18 

-20 

-22 

-24 

-12 

~ 

CALCITE 

A 

6. [jjjil 

WVA 

A 

A 

6> 

A€ 

-10 -8 -6 

DOLOMITE LARGE CONCRETIONS e SMALL CONCRETIONS • SAMPLE FROM MILAN + DOLOMITE IN VUGS [jjjil 

CALCITE CONCRETION CENTERS 0 VUG AND VEIN CALCITE A CONE IN CONE CALCITE 0 

-4 -2 

& uo. IN PER MIL 

0 

Figure 7. Graph of o180 vs. o13C values of carbonate 

concretions from the Huron Member of the Ohio Shale. Note 

that the o13C values are generally low and indicate an organic 

source for the carbon. The o180 values of the dolomitic matrix 

of the large concretions are a few per mil lower than values 

typical of unaltered marine limestones, and indicate concretion formation at low to moderate temperatures. Dolomite 

samples from small concretions, including the sample from 

Milan, as well as dolomite crystals from vug WVB, lie on a 

trend line that includes samples of the dolomitic matrix of the 

large concretions. o180 values of calcite from concretion 

centers, vugs, veins, and a cone-in-cone layer in the shale 

are isotopically uniform, indicating that late-stage recrystallization and isotopic reequilibration occurred at elevated temperatures in the presence of infiltrating formation water. This 

event may have been similar to that which produced calcite 

in vugs in dolostone at Pugh Quarry (sample P); however, the 

o

13C values of the concretionary carbonates are significantly 

lower than sample P, probably reflecting exchange of the 

late-stage fluid with the abundant, 13C depleted organic 

matter in the shale. Vug calcites WVC (o180= -4.8; 

o

13C= + 15.9) and WVD (o180=-5.2; o13C= + 16.4) are offscale, and probably formed at a very late stage. Data from 

tables 2 and 3. 

o180= -5.6) and outer part (o13C= -1.3; o180= -6.0) of 

1-m-diameter calcite concretion from the Kettle Point 

Shale, Kettle Point, Ontario. Last, calcite from a vug in 

Devonian dolostone at Pugh Quarry, Ohio, had 

o

13C= -11.2 and o18 0= -8.2. 

Published o13C values of organic matter in the 

Huron Member of central Ohio are quite low 

(==-30±0.2), and indicate the presence of both marinederived and terrestrially derived organic matter 

(Maynard, 1981). The o13C value of Foerstia collected a 

few meters above the uppermost carbonate concretion 

horizons at Worthington was -25.4, typical of values of 

Devonian terrestrial organic matter (Romankiw and 

others, in press). 

The o13C and o18 0 values of the Huron Member 

concretions are similar to those of diagenetic carbonate 

concretions from many other marine sequences 

(Hodgson, 1966; Tourtelot and Rye, 1969; Hudson and 

Friedman, 1976). Such concretions almost invariably 

have low o13C values that are much closer to those of 

marine organic matter than to those of normal marine 

limestones, yet they typically have o180 values that are 

either similar to, or a few per mil lower than, those of 

normal marine limestones, as demonstrated by Hodgson 

(1966). Interpretations of the o13C and o180 data from 

the Huron Member concretions are discussed below in 

turn. 

Interpretation of the o13C Values 

A variety of mechanisms has been proposed whereby the carbon in organic materials may become incorporated in diagenetic carbonates (Cheney and Jensen, 

1965; Hodgson, 1966; Hathaway and Degens, 1969; 

Berner, 1968; Irwin and others, 1977). None of these 

mechanisms can be completely ruled out as contributory 

to growth of the concretions in the Huron Member of the 

Ohio Shale, because most are consistent with the low 

o13C values. Simultaneous operation of more than one 

process could have occurred. However, when taken together, several characteristics of the Huron Member concretions point to Berner's (1968) suggestion that rapidly 

grown, low o

13C calcium carbonate concretions may have 

originated as bodies of adipocere (organic soap; 

Ca(RCOOh). 

Several investigators have described adipocere 

concretions that have formed about decaying proteinaceous organisms on short time scales (Bergmann, 1963; 

Sondheimer and others, 1966). Berner (1968) produced 

this material in laboratory experiments of anaerobic 

decomposition of fish in seawater, although Jackson 

(1973, p. 31) pointed out that organic gels can form when 

samples that have not been treated to remove calcium 

Table 3. o180 Determinations of coexisting vug minerals in 

concretions of the Huron Member 

SN-R.£ &lllo ()Jartzt !Mll &IBn 961 (PM) Carbonate &13c Carimate Relation 

'INA +:!.3.4 +?!2.0 ( -8.6) ruo -17.5 lli.U IU.O on Q1Z 

IoNS +;!5.3 +28.0 ( -Z.R) IXl.O -13.Z rno QTloniXl.O 

WIK: +25.1 +:i5.9 ( -4.8) cc +15.9 cc t:C on QTZ 

ww +24.4 +25.5 (-5.2) a: +16.4 oc 

Isotopic and Chemical Relations 13 
Table 4. Concretions from other localities-This study and 

others 

~1ello. 

MS-1 

Im34 

66-229 

(Keith and 

Weber. 1964) 

~14 

82/2 

!escription 

E11ipsoida1, fish-hearing ca1cite 

concretion. LQoft!r Creta- (quartz?) 

cerus mr1 ne set1i111'11ts, 

1\!hia Series, llah1a, 

~razil. 

Ellipsoidal concretion calcite 

containing the fish 

lharmis 3!:.. Crel:aCA!oJs 

IIBMne (?) sedifl1!1lts. 

Santana Fm. , Ceara, Brazil. 

"lill'eStone", fish-bearing 

concretim. Milrine Cretacerus. Grizzly Bear 

l>t.ns. , Grt. Bear lake, N. W. T. , canada. 

:911111, e111pso1dal, fish 

t Man otus? > beaM ng con- cretion. Pleistocene 

glacial clay [lmMne), 

leda Clay Fl'lo, carlton, 

eo., ~rio, canada. 

Ellipsoidal concretion 

containing hone of 

i)rtleosteus. ~Wer 

Oi!YOI'Iian Cli!Veland l'tllrber, 

Clevelan:!, Cltlo. 

calctteZ with 

ca. 9 wt.% 

~in solid 

sotn. (quartz, 

feldspar) 

dolerite, 

<partz 

Fl..f5 "'.iiii!Stone" concretion 

(Sass and with fish skeleton. 

Kolomy, 1972) 1-I:Jper Creta::erus 

Mishash l1n. (llllrl ne), 

ISI'Ilel, 

lblonite coocretion dolOlrite, 

containing Cladosellld-e sp. quartz 

with well-preservEd trunk 

m1srulature, lle'«lnilll'l 

Cleveland Shale, Cleveland, 

fllfo. 

-13,8 ..J.R 

-15,R -4.5 

-1!1.1 -6.2 

-22.5 -4.5 

- 9.5 to -1.5 to -13.8 -3,0 

~ned by x-ray diffraction. 

2ci(l00)-• 3.010A. r.a..,osttion detennfned by the lll!thod of Cllave (1~). 

Descriptions ~ these Slllples and formM:ions III1I.Y be foum in Weeks (1957), Gulnaraes (1964), RorEr (1971}, l:eith an:l Weber (1964), J'lilwson (1894), lblver (1960), Sass al1d Kolcmy (19n), and ~leon (1CJl2). 

~les IS-1, 10!134, 8Jl4, an:l 1!272 an:! 8325 ~ pn:wld!Jj ~ th@ Clevelam ltlsam of Nrtural History. 

To avoid f1aiiBge to fossils within specimens MS-1, 10034, an:lllll.4, 5111ples ~ :taken _near th@ exterior 

of the cmci'P.f:lons. 

are treated with H2 0 2 • Berner (1968) pointed out that 

the decomposition of adipocere to calcium carbonate 

would necessitate considerable shrinkage of the concretions because of the large density difference between the 

two materials. 

The observational and inferential features of the 

Huron Member concretions that are consistent with 

Berner's (1968) hypothesis include (1) concretionary 

nuclei of arthrodire fish; (2) evidence for rapid growth of 

the concretions near the sediment-water interface; (3) 

evidence for shrinkage and recrystallization of the concretions; ( 4) evidence for low-oxygen fugacity in the black 

shale; and (5) low o13C values of the carbonate. Furthermore, a paradox that is associated with the second of the 

above features may be uniquely indicative of a lowdensity precursor for the concretions; notably, the density 

of the extremely porous (~ > 80 percent) sediment near 

the sediment-water interface would have been much too 

low (p < 1.5) to hydrostatically support the large, rapidly 

grown carbonate concretions that now have p=3g!cc. 

As a partial test of Berner's (1968) hypothesis, we 

analyzed fish-bearing concretions from several other 

localities and present the isotopic data in table 4 along 

with pertinent determinations made by Keith and Weber 

(1964) and Sass and Kolodny (1972). Rapid concretionary growth is suggested by the excellent preservation of 

fish within samples MS-1, 10934, 8014, 8272, and 8325 

(fig. 8). Of particular note are sample 10934, which 

resembles the extraordinary concretions described by 

Weeks (1957) in that the original, three-dimensional 

external shape of the fish is preserved; and sample 8325, 

in which the external form and trunk musculature of a 

cladodont shark are preserved (fig. 8A,B; also see Dean, 

1902). The o13C values of these samples are generally 

similar to one another and to those of the Huron 

Member concretions. Because an adipocere origin is 

probably quite likely for at least some of these concretions, and because all our samples have numerous petrographic and isotopic similarities, the available evidence is 

compatible with an adipocere origin for all fish-bearing 

concretions examined thus far. 

Although we think that the circumstantial evidence 

pointing to an adipocere origin for the Huron Member 

concretions is very strong, some of our observations seem 

to be incompatible with this model. First, most of the 

shrinkage and cracking of the Huron Member concretions is confined to the calcite inner zone, and therefore 

probably occurred after the dolomite replacement stage, 

which should significantly postdate the conversion of 

adipocere to calcite. Furthermore, no indications of 

significant shrinkage were observed in any of the fishbearing concretions described in table 4. Second, several 

investigators report that bone material associated with 

adipocere tends to dissolve or decalcify (Weigelt, 1935; 

Sondheimer and others, 1966), but no definite evidence 

for this has been found in the Huron Member concretions. We cannot fully account for these discrepant 

observations, although it is noteworthy that reports of 

bone decalcification are confined to freshwater environments and might not apply to marine environments 

because abundant calcium is available in the pore fluids. 

Last, our analyses clearly show that the isotopic compositions of the Huron Member concretions do not seem to 

depend on the type of fossil nucleus present ( cf. T3R and 

T3F, table 2). However, Sondheimer and others (1966) 

proved that most of the adipocere in the concretions he 

described was derived from sources external to the 

nuclei, and this might account for the latter observations. 

Interpretation of the o180 Values 

In general, the o18 0 values of carbonate minerals 

reflect the temperature and isotopic composition of the 

fluid from which they precipitated (Epstein and others, 

1953). The sequentially grown carbonate minerals in 

concretions may therefore preserve a temporal record of 

diagenetic changes in pore fluid characteristics, as suggested by Hudson and Friedman (1976). The isotopic 

record of the concretions in the Huron Member of the 

14 An Organic Origin for the Carbonate Concretions of the Ohio Shale 
0 

~ "'0 

a· 

• :::J 

a. 

0 

:::J" 

CD 

3 

a· 

!. 

::0 

CD 

a 

c;· 

:::J 

fl) 

.... 

(1'1 

CIRvelatui M uMurn of Naturol iilalory {)e~:t oi ?t4-or.toloqy 

Nn. h''!J.~ 

S)>t(" .. L' .;_jrj ,., •, Jl :f./ 

imm-:mr,.n 

~Lly C.t.c>•"' ..... f~"J.l 

..:....ll"'•-~...>1 ~.t- • ~ /.,..: ~ 

lr.tt'l>. v •.. 1 ·..:..~ 

CLEVELAND MUSEPM cw NATI'BAL lllSTOHY 

PalP<mtology 

Cat No. 0014 Ace. No. 1' ·f)h: 2 

Vt>llo+us v:·l 11!"1S "••l 1 ···r 

Horimn J 1 8 s tc. c e> n e 

Figure 8. Photographs of carbonate concretions with fossil fish from several localities. All samples are curated by the Cleveland Museum of Natural History. 

Scale bar is 10 em long on A, B, and D and 15 em long on C. See table 4 for isotopic analyses and additional information. A. Ellipsoidal dolomitic concretion from 

the Cleveland Member, Cleveland, Ohio, containing shark (Ciadoselache sp.) with well-preserved trunk musculature (sample 8325). B. Ellipsoidal calcite 

concretion containing fish (Tharrhis sp.) with external, three-dimensional shape preserved, Ceara, Brazil (sample 10934). C. Ellipsoidal, fish-bearing calcite 

concretion, Bahia, Brazil (sample MS-1). D. Ellipsoidal to irregular, fish-bearing calcite concretion, Ontario, Canada (sample 8014). 
Ohio Shale has been greatly modified by recrystallization 

and replacement and cannot be interpreted independently of field and petrographic data, but most of our 

samples record an overall pattern of progressive 18 0 

depletion in the concretionary carbonates. Such a ternporal pattern has been found in concretions from several 

other localities (Sass and Kolodny, 1972; Coleman and 

Raiswell, 1981) and probably represents the major and 

most common trend, although complex patterns (Hennessy and Knauth, 1985; Siegel and others, 1987) and 

even reversed trends (Degens and others, 1962) can 

occur. 

The isotopic similarity of almost all of the "late 

calcites" and of the calcite centers of the concretions, 

together with the coarse grain size of the latter, strongly 

suggest that recrystallization and complete isotopic reequilibration of the calcite centers occurred at a late stage. 

The original isotopic character of the concretions is 

therefore probably best preserved in the dolomite of the 

intermediate zone, but this dolomite grew by replace• 

ment of preexisting calcite, and this process will closely 

(Degens and Epstein, 1964), but in general not perfectly 

(Land, 1980), replicate the o180 value of the precursor, 

depending on the water/rock ratio during replacement. 

Furthermore, isotopic fractionations attendant on the 

conversion of adipocere to calcite are not known, and 

such a process may have occurred in the Huron Member 

concretions, as previously discussed. In spite of these 

complexities, it is probably not accidental that the o18 0 

values of the matrix dolomites are either close to, or only 

a few per mil lower than, the values observed in Quaternary marine limestones (Keith and Weber, 1964), and 

that these dolomites appear to preserve a progressive 

(radial) record of o

180 depletion (see table 2). The o18 0 

values of the dolomite zone are accordingly compatible 

with crystallization at low but progressively higher ternperatures in the presence of a pore fluid with a o18 0 

value similar to that of seawater. 

Similarly, the o

180 values of the carbonates ( -0.9 

to -6.3) of fish-bearing concretions from other localities 

also suggest precipitation at low temperatures from 

fluids with o18 0 values similar to, or somewhat lower 

than, seawater (table 4). It is significant that the lowest 

values in table 4 represent the carbonate from the 

outermost part of the concretions and probably reflect 

growth at a late stage when temperatures were elevated 

and pore fluid o

180 values may have been somewhat 

lower than the seawater value. 

The geologic history of the "late calcites" and other 

vug minerals in the Huron Shale concretions would 

seemingly be straightforward compared to that of the 

matrix carbonates. The relatively low o18 0 values 

c~-10) of most of these "late calcites" are compatible 

with either (1) formation in seawater-like fluid at elevated temperatures (~75 °C); (2) formation at low temperatures in fluid with a o18 0 value approximately 10 per 

mil lower than that of seawater, for example, from 

meteoric waters (although -7 would probably be a 

better average value for modern meteoric waters in 

Ohio); or (3) formation under many other combinations 

of temperature and fluid o180 values, with the restriction 

that only one variable is independent. 

In an attempt to resolve this history, we made o

18 0 

analyses of quartz crystals coexisting with either calcite or 

dolomite from four vugs in the large concretions (table 

3). Although the o180 values of the quartz samples are 

quite uniform ( +23.4 to +25.3 rei. SMOW), in three of 

the four cases the carbonates have higher o180 values 

than the quartz; the marked isotopic disequilibrium in 

these three vugs indicates that the minerals did not 

coprecipitate. Only quartz and dolomite from vug WV A 

can possibly be in oxygen isotope equilibrium; unfortunately, the temperature dependence of the isotopic fractionation between quartz and dolomite has not yet been 

well determined. If it is assumed that the 18 0 fractionation between dolomite and calcite at low to moderate 

temperatures is 3 to 4.5 per mil (see Land, 1980; also see 

fractionations compiled in Friedman and O'Neil, 1977), 

then the data from WV A translate to temperatures of 45 

oc to 95 oc and a fluid o180 value of -7.2 to +1.2, 

utilizing the quartz-water fractionation curve of Clayton 

and others (1972) with the calcite-water fractionation 

curve of O'Neil and others (1969). Note that the dolomite in WV A has an isotopic composition that is generally compatible with that of most of the observed vug and 

vein calcites (table 2; fig. 7). In contrast, the dolomite in 

vug WVB is isotopically similar to that of the dolomitic 

matrix of the concretions (see fig. 7), but is quite different from that of the other vug and vein carbonates, and 

probably precipitated much earlier than the quartz and 

the other vug and vein minerals. 

An additional surprise is that the o18 0 and o

13C 

values of the calcites in vugs WVC and WVD are both 

dramatically different from the values observed in all the 

other vug and vein calcites (table 2). It is unfortunate that 

temporal relations between these unusual calcites and 

the other vug minerals could not be completely determined, although calcite appears to postdate quartz in vug 

WVC. Also, the observations of Siegel and others (1987) 

strongly suggest that their calcite IV, which is isotopically 

similar to calcites WVC and WVD at Worthington, 

formed during a late stage of diagenesis subsequent to 

the precipitation of the main masses of vug carbonates. 

Thus, our paragenetic and isotopic evidence, and especially that of Siegel and others (1987), indicate that the 

o180 record of sequentially precipitated vug minerals in 

concretions is complicated in detail, and is not entirely 

consistent with a simple pattern of progressive depletion 

of 18 0. Note that relatively high o18 0 values could occur 

in very late stage vug minerals that precipitated under 

16 An Organic Origin for the Carbonate Concretions of the Ohio Shale 
conditions of elevated temperatures and low water/rock 

ratios, provided that the a 1

s0 value of the fluid was 

substantially increased by interaction with the rock, for 

example with the fine grained, high 180 matrix carbonates. Irwin and others (1977) suggest that bacterial 

fermentation reactions can form carbonates with unusually high a

13C values. 

Although our attempt to definitively resolve the 

origin of the late-stage minerals met with some difficulty, 

we believe that the most likely possibility is a case only 

marginally different from (1 ), above. For example, if it is 

assumed that the vug quartz and the ordinary (ca. -10; 

table 2) late calcites formed simultaneously, which is 

probably not rigorously true because these minerals tend 

to precipitate under different acid-base conditions, both 

the temperature and the also value of the fluid can be 

calculated from the average a

1

s0 values of these minerals. Thus, using available fractionation factors (O'Neil 

and others, 1969; Clayton and others, 1972) and the 

average a1

s0 values, we calculate that crystallization of 

these minerals occurred at T=-105 °C in a pore fluid with 

a180= +3.6. Alternatively, the largest observed 180 difference between any of these quartz and calcite samples 

would suggest a temperature of 55 oc and a fluid a18 0 

value of -3.5, whereas the smallest quartz-calcite 18 0 

difference would indicate an unreasonably high temperature of 225 °C and a fluid a1

s0 value of + 12.8. 

Because the quartz, dolomite, and calcite samples 

utilized in the above calculations cannot be proved to 

have coprecipitated, we cannot claim to have definitively 

calculated the conditions under which crystallization of 

the vug minerals occurred. However, we believe that our 

calculated conditions for the precipitation of these "ordinary" late-stage minerals are mostly quite reasonable, In 

part because generally similar conditions have been 

commonly inferred for the development of late-stage 

carbonate cements in limestones (Choquette and James, 

1987). Also note that fluid inclusion measurements indicate elevated temperatures (50 °C) for late-stage calcite 

crystallization in septarian concretions from the Fayetteville Formation, Arkansas; later stage fluids were as hot 

as 100 °C (Burruss and Goldstein, 1980; see Zangerl and 

others, 1969, for description of concretions from this 

formation). The calculated a1

s0 value of the fluid in the 

latter case strongly suggests that it was either slightly 

modified marine pore fluid or formation waters. Marine 

pore fluids within 1 km of the sediment-water interface 

almost invariably have a1

s0 values only slightly less (by a 

few per mil) than seawater (Lawrence and Gieskes, 

1981). On the other hand, it is probably highly significant 

that present -day formation waters from the Gulf Coast 

and from the Illinois and Michigan basins generally have 

a

18 0 values of about -3 to +6 if temperatures are 

moderate, as our calculations suggest (see Clayton and 

others, 1966). 

In summary, we believe that the rather low a

1

s0 

values of the vug minerals in the concretions of the 

Huron Member of the Ohio Shale reflect their precipitation at moderate temperatures from marine pore fluids 

or formation waters. A formation water origin is supported by the rather low a1

s0 and a13C values that we 

observed in vug calcite at Pugh Quarry, Ohio. Also, the 

a18 0 and a13C values observed by Hall and Friedman 

(1969) in the late-stage calcites from Mississippi Valley 

type ore deposits are quite similar to those of Pugh 

quarry calcite; Hall and Friedman suggested that these 

calcites precipitated from hot oil-field brine mixed with 

meteoric water (their calculated fluid had o180= -3.0 to 

+0.4). Under the low water/rock conditions at the latest 

stages of diagenesis, vug calcites with high a

13C values 

and with a18 0 values approaching those of the finegrained matrix carbonates could be formed. We do not 

concur with suggestions (Hudson and Friedman, 1976; 

Hudson, 1978) that similarly low a1

s0 values of "late 

calcites" in concretions from many other localities principally reflect growth in low 18 0 pore fluids derived from 

meteoric waters. 

DISCUSSION AND CONCLUSIONS 

The carbonate concretions of the Huron Member 

of the Ohio Shale are structurally and petrographically 

complex bodies that commonly contain well-preserved 

fish remains. We concur with the opinions of Newberry 

(1873) and Clifton (1957) that the concretions originated 

by rapid, early diagenetic growth near the sedimentwater interface. Presuming that the concretionary material filled and preserved void space in the sediment, 

chemical analyses can be used to calculate that the 

sediment originally had a porosity of approximately 81 to 

94 volume percent. Similarly large porosities are typical 

of modern marine sediments in the uppermost few 

meters of the sediment column. Such values have also 

been calculated for early diagenetic concretions from 

numerous other localities (Lippmann, 1955), but the 

indicated sediment densities are extremely low and pose 

a problem for hydrostatic support of the large, dense 

concretions. It must be concluded that either (1) such 

porosity calculations are unrealistic, perhaps because 

much of the original sediment contained carbonate or 

silica that dissolved, or perhaps because the crystals of 

the growing concretion have mechanically displaced 

much of the sediment; or (2) the concretions originated 

as some type of low-density material. However, even if 

the porosity calculations are in error by a factor of two, 

the paradox of hydrostatic support of the concretions 

would remain. Also, only the second possibility can easily 

account for the severe deformation observed in the 

concretions that were incorporated in the clastic dikes at 

Discussion and Conclusions 17 
Milan, Ohio. Observations at Milan also prove that joints 

and clastic dikes formed in the shale at a surprisingly 

early stage, before compaction and carbonate precipitation were complete. 

The concretions have a closer spatial correspondence with the Foerstia biostratigraphic zone than they 

have with the diachronous base of the Huron Member. 

The concretionary horizons may therefore represent 

timelines in the formation. 

Complex recrystallization and replacement phenomena have affected the carbonate concretions of the 

Huron Member. The concretions appear to have been, at 

an early stage, composed principally of calcite, which, 

except for remnant cores in the largest bodies, was 

subsequently replaced by Ca- and Pe-rich dolomite. 

Euhedral crystals and dendrites of barite probably 

formed during or before this stage. The replacement 

stage was accompanied by the growth of small pyrite 

concretions in the shale and the development of pyrite 

rims on some of the large concretions. Subsequently the 

cone-in-cone layers in the shale formed. Concurrently or 

subsequently the remnant calcite cores recrystallized and 

formed the "funnel-shaped" depressions in the top and 

bottom of the concretions. During the late stages of 

development, crystals of calcite and, less commonly, 

quartz and dolomite, filled vugs and veins in the concretions. 

o13C values indicate that the source of the carbon 

in carbonate in the Huron Member concretions, like that 

of early diagenetic carbonate concretions worldwide 

(Hodgson, 1966), was dominantly derived from organic 

matter. The values are similar to those of fish-bearing 

concretions from many other marine sequences, and 

these concretions might in many cases have originated as 

bodies of adipocere. An adipocere origin is compatible 

with the rapid early diagenetic growth of the concretions 

and would explain the unique paradoxes related to the 

low density and deformable nature inferred for the 

concretions during the early growth stages, but such 

origin is seemingly incompatible with the excellent preservation of bone within the concretions. It is noteworthy, 

however, that descriptions of bone destruction by adipocere are confined to freshwater occurrences and might 

not apply in marine environments where abundant calcium is available. 

o

180 values of the dolomitic matrix of the Huron 

Member concretions are only a few per mil lower than 

values for unaltered marine carbonates and are compatible with formation in marine pore fluid at low but 

progressively increasing temperatures. o180 values of the 

recrystallized remnant calcite cores and most calcite 

veins and vugs in the concretions are low and uniform 

and most likely reflect late-stage recrystallization and 

growth at moderate temperatures in the presence of 

formation fluid with a o18 0 value reasonably close to that 

of seawater. Because concretion growth and replacement 

processes are complex and appear to occur at different 

stages under varying conditions, we agree with Hodgson 

(1966) that paleoenvironmental conclusions derived 

from stable isotopic data of diagenetic carbonates must 

be viewed with skepticism. 

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References 21 



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