Geochemical and textural evidence for early (shallow) diagenetic growth of stratigraphically confined carbonate concretions, Upper Devonian Rhinestreet black shale, western New York

Chemical Geology 206 (2004) 407 – 424 www.elsevier.com/locate/chemgeo

Geochemical and textural evidence for early (shallow) diagenetic growth of stratigraphically confined carbonate concretions, Upper Devonian Rhinestreet black shale, western New York Gary G. Lash *, David Blood Department of Geosciences, State University of New York-College at Fredonia, Fredonia, NY 14063, USA Received 17 March 2003; accepted 23 December 2003

Abstract Laterally persistent carbonate concretionary horizons are conspicuous to the Upper Devonian (Frasnian) Rhinestreet black shale of the western New York State Appalachian Plateau. Field observations, including randomly tilted concretions and differential compaction of host sediment laminae around concretions, are consistent with early diagenetic growth in unconsolidated sediment. Further, estimates of pre-cementation host sediment porosity based on the volume percentage of CaCO3 cement (74 – 93%) and, perhaps most importantly, the preservation of a cardhouse clay fabric in mudstones collected from concretion pressure shadow regions suggest that concretionary growth occurred rapidly within perhaps a meter of the seafloor. Petrographic examination (SEM) of matrix carbonate samples collected from six concretions reveals a relatively porous (f4 – 7%) framework of calcite micrite and microsparite, euhedral pyrite (most abundant near concretion rims), disseminated pyrite framboids and randomly oriented detrital clay platelets and domains. C isotopic compositions of Rhinestreet concretion matrix carbonate range from
13.9xto +1.7xPDB suggesting derivation of C from a mixed source that included depleted and isotopically heavy C. Slightly to moderately depleted O isotopic compositions (
3.8xto
6.8xPDB) do not corroborate the shallow depth of concretion growth suggested by field and textural observations. The vertically confined nature of most Rhinestreet concretions, their large size, oblate ellipsoidal shape, variable C isotope compositions and absence of any evidence that concretions were sited at local concentrations of organic matter suggest an origin by anaerobic CH4 oxidation. Early pre-concretion sulfate reduction and associated pyrite framboid formation that occurred throughout the host sediment was renewed in thin, shallow zones of uncompacted sediment—the concretionary horizons—by CH4 and isotopically heavy dissolved carbonate diffusing upward from the zone of biogenic methanogenesis to the base of the sulfate reduction zone, probably little more than a meter below the seabed. Local variations in the mix of isotopically light CH4 and heavier dissolved carbonate may have been responsible for the wide range of d13C isotopic compositions of Rhinestreet concretions. Crucial to this mechanism is a pause in deposition that would have held the zone of carbonate precipitation at a fixed distance below the ocean floor long enough for the large concretions to grow. Evidence that sedimentation slowed or even stopped during concretion growth includes (1) laminae that can be traced through concretions with no observed systematic changes in thickness and (2) minimal concretion center-to-edge variations in estimated pre-cementation porosity. A resumption of sedimentation and related burial brought concretion growth in each horizon to an end. The moderately depleted d18O values

* Corresponding author. Tel.: +1-716-673-3842; fax: +1-716-673-3347. E-mail address: [email protected] (G.G. Lash). 0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.12.017

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of Rhinestreet concretions probably reflect the ‘‘resetting’’ of O isotopic compositions by warm fluids migrating through the permeable concretions during burial. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbonate concretions; Sulfate reduction; Anaerobic CH4 oxidation; C and O isotopes; Upper Devonian

1. Introduction Carbonate concretionary horizons are conspicuous in Middle and Upper Devonian black shale units of central and western New York State (Dix and Mullins, 1987), southwestern Ontario (Daly, 1900; Coniglio and Cameron, 1990) and Ohio (Clifton, 1957; Criss et al., 1988) suggesting that diagenetic conditions favorable to the formation of these features existed regionally. Concretions are especially useful in the analysis of the early post-depositional history of encapsulating shale, for not only can they preserve the original or depositional texture of the host sediment (e.g., Woodland, 1984; Duck, 1990), but they commonly provide a record of the evolution of pore fluids carried by the sediment (e.g., Hudson, 1978; Coniglio and Cameron, 1990). This paper presents results of a study of two stratigraphically confined carbonate concretionary horizons in the Upper Devonian Rhinestreet black shale of western New York State. We describe field, textural, petrographic (scanning electron microscopic) and geochemical evidence for concretionary growth at shallow depth in uncompacted sediment; isotopic data is used to model pore fluid evolution. Finally, we offer a burial history of the Rhinestreet black shale that involves episodic sedimentation and carbonate precipitation by anaerobic CH4 oxidation followed by late diagenetic recrystallization of the carbonate concretions by warm fluids that migrated through what must have been an open system. 1.1. Rhinestreet shale The Upper Devonian (Frasnian) Rhinestreet shale of western New York State comprises 60– 80 m of dark-gray to black fissile and thinly laminated pyritic shale, sparse thin siltstone beds and carbonate concretions (Fig. 1). The entire unit can be studied in excellent creek exposures and along the Lake Erie shoreline (Fig. 1). Analysis of X-ray diffraction traces of more than 180 Rhinestreet shale samples from

throughout the Appalachian Basin yields an average mineralogy of 25% detrital quartz, 75% clay and trace amounts of pyrite and calcite. Clay minerals include illite (60%) and lesser but equal amounts (20%) of chlorite and illite-smectite mixed-layer clay (Hosterman, 1993). The Rhinestreet shale is underlain by the Cashaqua gray shale, the contact being sharp and easily recognized in the field, and passes upward through a zone of interbedded black and gray shale into the Angola shale (Buehler and Tesmer, 1963; Fig. 1). Total organic carbon (TOC) of the Rhinestreet shale ranges from 8.0% at its base to 1.8% within its transitional contact with the Angola shale (Fig. 1). Comparison of the S2 Rock-Eval parameter (a measure of the hydrocarbon generative potential of a source rock) with TOC indicates that organic material in the Rhinestreet is dominantly Type II (liquid-and gas-prone) kerogen of marine origin (Langford and Blanc-Valleron, 1990), consistent with the plot of hydrogen index (HI) versus Rock-Eval Tmax (Fig. 2). The same plot indicates a thermal maturity of the Rhinestreet shale roughly equivalent to its measured vitrinite reflectance of 0.72%, which places this organic-rich unit within the oil-generating window (Tissot and Welte, 1984; Espitalie, 1986). The laminated nature, abundance of pyrite and high Corg content of the Rhinestreet shale suggest that deposition occurred under anoxic conditions, at least below the sediment – water interface. Wilkin et al. (1996) were the first to propose that the size distribution of pyrite framboids provides a reliable indicator of bottom water redox levels, a premise subsequently confirmed by Wignall and Newton (1998) through their work on the Kimmeridge Clay. Our electron microscopic analysis of more than 200 framboids in Rhinestreet shale samples reveals a range in framboid size of 2.7– 23.2 Am, a mean size of 6.4 Am and a mode of 5.9 Am. Though the size distribution of observed Rhinestreet framboids is grossly similar to framboid size distributions documented from modern and ancient environments (Wilkin et al., 1996; Wilkin

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Fig. 1. Generalized stratigraphy of the Rhinestreet shale showing locations of the lower and upper concretion horizons and TOC content of the shales.

et al., 1997; Wignall and Newton, 1998), the mean size of Rhinestreet framboids falls between that of syngenetic framboids that formed within euxinic bot-

Fig. 2. Van Krevelen plot of HI versus Tmax for the Rhinestreet shale data.

tom water and framboids that formed diagenetically within sulfidic sediment under dysoxic or oxic bottom water (Wilkin et al., 1996). Still, we believe that the bulk of the Rhinestreet framboids (f6 Am) grew in a euxinic water column before settling to the seafloor. The relatively large size of these framboids (compare with framboids that formed in anaerobic and lower dysaerobic environments described by Wignall and Newton, 1998) suggests the occasional existence of a density contrast in the Late Devonian Catskill Sea water column capable of holding the syngenetic framboids close to the redox boundary longer than would seem normal based on published size distributions. The reconstructed position of the Appalachian basin west of the Acadian orogen in the southern subtropics and trade belt (Scotese and McKerrow, 1990) would favor an arid to semi-arid climate and at least occasional salinity stratification of the basin (Woodrow, 1985) that may have enabled framboids to grow to perhaps 6.0 Am or more before settling to the seabed with detrital particles. However, the occasional

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Fig. 3. Field photographs of (A) ellipsoidal concretions in the upper horizon (rule=1.5 m), (B) tilted concretion in the lower horizon (bedding in shale is parallel to the long dimension of photograph). Note laminated nature of the concretion (rule=1 m) and (C) flat-topped concretion in the upper concretion horizon (hammer in dashed circle=0.3 m long).

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over-size framboids (>12 Am) observed in Rhinestreet samples are too large to have grown syngenetically in the water column. Instead, these framboids probably grew formed diagenetically at very shallow depth (<0.5 m) within sulfidic sediment during brief periods of bottom water dysoxia when the redox boundary dropped close to the sediment – water interface (e.g., Wilkin et al., 1996; Wignall and Newton, 1998). 1.2. Rhinestreet carbonate concretions 1.2.1. General characteristics and depth of formation The majority carbonate concretions of the Rhinestreet shale are found in two stratigraphically confined but laterally persistent horizons—one a few m above the base of the unit and the other slightly more than halfway up the Rhinestreet (Fig. 1). Most concretions are oblate ellipsoids with maximum diameters and thicknesses ranging up to 2.7 and 1.1 m, respectively (Fig. 3A). The largest concretions are found in the

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roughly 1 – 1.5-m-thick upper concretionary horizon (Fig. 4). Comparison of concretions of both horizons indicates that the maximum vertical dimension of the bulk of the concretions studied by us is in the range of 80 cm (Fig. 4). However, concretions of the upper horizon are as much as a meter larger in the horizontal dimension (i.e., more oblate) than their counterparts in the lower horizon (Fig. 4). Petrographic and geochemical analysis and energy dispersive X-ray (EDX) spectroscopy of Rhinestreet concretion matrix samples reveal a high percentage (>74%) of calcium carbonate (Table 1) and minor detrital (quartz, clay) and authigenic (pyrite) components. Septarian fractures, equally common to concretions of both horizons, extend outward from concretion centers, narrowing to termination near the edges. Brown Fe-poor calcite lines fractures walls and is post-dated by yellow siderite. Pyrite, marked by low but uniform abundance throughout much of the interior of concretions, is concentrated along concretion edges as well as halos along septarian

Fig. 4. Plots of horizontal versus vertical dimensions of carbonate concretions of the lower (n=50) and upper (n=52) horizons of the Rhinestreet shale.

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Table 1 Carbonate, TOC and carbon and oxygen isotope data (PDB), Rhinestreet concretions Concretion

Location

CaCO3 (vol.%)

TOC (wt.%)

d13C

d18O

RSCa

center middle edge center interior interior edge center middle edge center middle edge center middle edge center interior middle interior edge

87.7 87.3 84.4 80.4 84.3 79.9 77.4 82.6 79.1 74.4 80.5 76.1 77.7 83.7 79.0 76.7 83.4 92.7 92.9 92.2 84.5

0.40 0.53 0.42 0.23 0.39 0.42 0.33 0.71 0.54 0.67 0.91 0.76 0.76 0.92 0.84 0.78 0.90 0.89 0.80 1.03 1.00


11.3
11.0 1.7
13.9
13.7
12.3
12.6
12.6
6.0
0.8
11.1
10.2
0.4
11.5
11.2
0.3
2.2
5.7
8.9
2.4
2.1


4.5
4.4
5.0
4.3
4.3
4.6
4.3
3.8
5.0
6.8
5.0
5.3
6.5
4.5
6.8
6.4
6.6
4.7
4.7
5.0
6.5

17Ca

EMC29a

UC

LC1

UC5

a

Lower concretion horizon.

fractures. Broken concretions observed in outcrops reveal depositional laminae inherited from the host Rhinestreet black shale (see Fig. 3B,C). Moreover, laminae show no obvious systematic changes in thickness across concretions, though locally they are wavy and occasionally faulted. Scanning electron microscope observations show the Rhinestreet concretions to be composed of a relatively open framework of irregularly distributed domains of micrite (<5 Am; Folk, 1965) and microspar (>5 Am) calcite (Fig. 5A). Indeed, studied concretion samples routinely retain f4 – 7% residual porosity (Fig. 5B). Pyrite framboids (Fig. 5C) are evenly distributed throughout concretions and are roughly the same size as those observed in the host shale. Euhedral pyrite (Fig. 5B), locally as overgrowths on framboids, is noticeably more abundant in concretions than in encapsulating black shale, especially along concretion edges and septarian fractures. Detrital quartz and clay grains and organic particles occur throughout concretion samples. Locally, the open or random cardhouse arrangement typical of newly deposited flocculated

clay has been preserved by diagenetic carbonate precipitation (Fig. 5C). Field observations suggest that the Rhinestreet carbonate concretions formed in uncompacted sediment. The wrapping of shale laminae around concretions (Figs. 3A and 6) demonstrates that concretions resisted compaction relative to surrounding unconsolidated clay. Indeed, comparison of the thickness of layers within more than 100 concretions (i.e., original layer thickness) with the same stratigraphic interval traced into encapsulating shale (i.e., compacted layer thickness) suggests that the Rhinestreet shale experienced roughly 60% burial-related compaction strain relative to concretions. The long axes of most Rhinestreet concretions lay parallel to bedding, yet some concretions are tilted as much as 15j to the plane of bedding (Fig. 3B). The lack of consistency in tilting direction and angle over horizontal distances of less than 10 m of flat-lying strata rules out systematic rotation of concretions, suggesting, instead, that the tilted concretions subsided rather haphazardly into weak, unconsolidated sediment. Canfield and Raiswell (1991) maintain that the timing of concretion growth relative to host sediment compaction can be best deduced by assessing the degree of compaction of the encapsulating shale at the time of concretion growth. The shielding effect of the apparently incompressible Rhinestreet concretions is obvious at the macroscopic scale (Fig. 6). Argillaceous rock within concretion pressure shadows is not fissile (Fig. 6, see ‘‘ps’’) and is properly classified as mudstone (Pettijohn, 1975). However, these same mudstones become fissile several decimeters distant from the pressure shadows, necessitating their reclassification as shale (Fig. 6, see ‘‘sh’’; Pettijohn, 1975). Scanning electron microscopic analysis of mudstone samples collected from concretion pressure shadows reveals a porous fabric of randomly oriented platy particles, which higher magnification proves to be face-to-face clay flake stacks or domains (Fig. 7A). Domains typically are arranged in a low-density network or cardhouse fabric of edge-to-edge and edge-to-face contacts marked by large voids relative to the thickness of clay flakes and domains (Fig. 7B). Secondary electron images of fissile shale samples collected only 20 – 30 cm from pressure shadows; however, reveal a generally low-porosity microfabric marked by a moderately to strongly preferred orien-

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tation of clay flake domains (Fig. 7C). The almost negligible degree of compaction sustained by pressure shadow mudstone confirms that gravitational compaction of the Rhinestreet shale was minimal until after carbonate concretions had become incompressible, pointing to a shallow diagenetic origin of the concretions (Lash and Blood, 2004).

2. Geochemistry Six concretions ranging from 0.8 to 1.5 m wide, three each from the lower and upper concretionary horizons, were analyzed for total CaCO3 and C and O isotopes. Three to five matrix samples were collected at roughly equal intervals from the center to the edge of each concretion for analysis. 2.1. Total carbonate The most commonly cited explanation for the formation of carbonate concretions involves the passive (i.e., non-displacive) precipitation of diagenetic carbonate in void spaces of the host sediment (Lippmann, 1955; Raiswell, 1971). Thus, a minimum estimate of host sediment porosity at the time of concretion growth is obtained by conversion of the weight percent carbonate cement to volume percent (Raiswell, 1976; Coleman and Raiswell, 1981; Gautier, 1982). Total carbonate of the Rhinestreet concretion matrix samples from both horizons varies from 74% to 93% (mean=83%; Table 1), a range that encompasses the high end of porosity of modern marine clay deposits (e.g., Muller, 1967; Velde, 1996). Recently, though, Raiswell and Fisher (2000) cautioned that widespread application of the passive infill model might not be warranted. Indeed, electron microscopy has revealed textural evidence of some degree of displacive carbonate growth during Rhinestreet concretion formation (see Fig. 5D). Still, the presence of f4– 7% residual porosity and common septarian fractures in Rhinestreet concretions indicate that void space was not completely filled during concretion growth; thus, displacive growth appears to have been minimal during precipitation of the carbonate cement (Raiswell, 1971; Gautier, 1982). The conventional growth model for carbonate concretions assumes that growth occurred concentrically

413

by addition of successive layers of cement during burial (Oertel and Curtis, 1972; Mozley, 1996). Such a sequential growth history has been recognized by decreasing diagenetic carbonate percentage from the centers to edges of concretions reflecting progressively decreasing pore-space volume of the host sediment during concretion growth (e.g., Raiswell, 1971; Oertel and Curtis, 1972). The six studied Rhinestreet concretions show diminishing carbonate in core-to-edge traverses, the maximum reduction being 8% to a value of 74% at the edge of concretion EMC29 (Fig. 8), suggesting only a minor reduction in porosity of the host sediment during concretionary growth. Moreover, concretion UC5 is characterized by more than 90% carbonate in samples collected from the interior of the concretion while center and edge samples contain f84% carbonate (Fig. 8; Table 1). The minimal radial carbonate gradients exhibited by Rhinestreet carbonate concretions indicate that the concretions grew largely pervasively by rapid near-uniform simultaneous precipitation of carbonate cement throughout the concretion body (Raiswell and Fisher, 2000). Further supportive evidence for this interpretation includes the lack of center-to-edge deviation in laminae thickness (e.g., Raiswell, 1971). 2.2. C and O stable isotopes Field observations, textural evidence and carbonate volume percentage suggest that Rhinestreet carbonate concretions grew primarily by pervasive precipitation of a compaction resistant yet porous framework of calcite micrite in organic-rich clay that had a porosity of at least 83% at perhaps a meter or so below the seafloor (e.g., Seibold, 1962; Rieke and Chilingarian, 1974; Velde, 1996). The open framework of the concretions provided strength yet reduced the mean density of most of the concretions precluding their sinking into the weak, low-density clay (e.g., Wetzel, 1992; Raiswell and Fisher, 2000). However, stable element isotopic data suggests a more complex diagenetic history of the Rhinestreet shale and its concretions. The range of d13C values is rather wide (
13.9xto +1.7xPDB) while that of d18O is limited (
6.8xto
3.8xPDB) (Table 1; Fig. 9). Isotopic values of concretions of the upper and lower horizons are roughly similar though less depleted 13C values appear to be

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somewhat more common to concretions of the upper horizon (Fig. 9). Four of the six studied concretions show radial enrichment in 13C (Fig. 10A). Nevertheless, concretion 17C displays minimal radial variation in d13C, and center and edge d13C values of concretion UC5 are more enriched in d13C than samples collected

from interior regions of the concretion (Fig. 10A). O isotope compositions of the studied Rhinestreet concretions show minor radial variations (Fig. 10B). Four concretions display slight center-to-edge depletion in 18 O, and one concretion (17C) shows no change in d18O. Center and edge samples collected from concre-

Fig. 5. Secondary electron images of Rhinestreet carbonate concretions: (A) micrite (m), microsparite (ms) and clay domains (c). Note the open framework; (B) cluster of euhedral pyrite crystals and void space (p) (polished sample); (C) view of open framework of Rhinestreet carbonate concretions showing a framboid (F) randomly oriented clay grain domains (c) and microsparite (ms); (D) framboid wedged apart during displacive concretion growth (polished sample). Note: samples shown in A and C are lightly etched with dilute hydrochloric acid; those shown in B and D are polished.

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415

Fig. 5 (continued).

tion UC5 are characterized by equally depleted d18O compositions whereas samples recovered from the interior of the concretion are less depleted (Fig. 10B). Concretions UC and EMC29 display center-to-edge enrichment in 13C that is complemented by depletion in 18 O. However, concretions LC1 and RSC show almost identical radial increases in heavy C but dissimilar center-to-edge variations in d18O.

3. Discussion Integration of field, petrographic, geochemical and isotopic data provides clues to the diagenetic history of the Rhinestreet concretions and their encapsulating organic-rich shale for that time period well before maximum burial and attendant thermal maturation was attained. Specific aspects of the concretions

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Fig. 6. Field photograph of carbonate concretion showing differential compaction of encapsulating black shale. Note lack of fissility in pressure shadow mudstone (ps), which can be traced laterally into fissile shale (sh).

accounted for in the proposed growth history include their very early formation along stratal horizons, minimal radial pre-cementation porosity gradients, wide range in d13C values and narrow, slightly to moderately depleted O isotopic compositions. 3.1. Pre-concretionary growth The organic-rich clay that would become the Rhinestreet shale accumulated as flocculated sediment, probably as a result of electrostatic and organic cohesive forces (Syvitski and Murray, 1981; Bennett et al., 1991). Initial porosity appears to have been quite high, probably in the range of 80– 85%, resulting in a high effective permeability. Small syngenetic framboids that had formed in a euxinic water column accumulated with the clay floccules. Periodic lowering of the redox boundary close to the sediment – water interface during periods of bottom-water dysoxia led to shallow (<0.5 m) diagenetic growth of larger framboids within the sulfidic sediment (e.g., Wilkin et al., 1996; Wignall and Newton, 1998). 3.2. Concretion growth Precipitation of carbonate cement within the porous and permeable clay occurred rapidly at very shallow depth, perhaps within a meter of the seafloor based on estimated pre-cementation host sediment porosity.

Minimal center-to-edge carbonate volume gradients in all studied Rhinestreet concretions suggest that the concretions grew primarily by pervasive growth of a compaction resistant porous framework of calcite micritic. The concretions were strong enough to preserve the depositional fabric of the flocculated organic clay in concretion pressure shadows during burial. The growth of Rhinestreet concretions along specific horizons, an occurrence common to other black shale units (Raiswell, 1971; Hudson, 1978; Raiswell and White, 1978; El Albani et al., 2001), requires explanation. The siting of carbonate concretions has typically been related to the microbial reduction of locally concentrated organic matter, including skeletal remains and tissue (e.g., Weeks, 1957; Zangerl et al., 1969; Berner, 1980). However, we observed no textural evidence to suggest that such a mechanism accounted for the siting of Rhinestreet carbonate concretions, nor do the concretions show a preference for the organic-rich base of the unit (see Fig. 1). Moreover, organic matter within concretions is strikingly less than that in encapsulating shales (Table 1), though compaction-related volume loss of the shale no doubt elevated host rock TOC. Nevertheless, depleted C isotopic compositions (Figs. 9 and 10A) indicate that concretionary carbonate originated in part from the anaerobic bacterial reduction of organic matter. The large size of the Rhinestreet concretions suggests that limited in situ Corg was augmented by C

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417

Fig. 7. Secondary electron images of black mudstones collected from adjacent to carbonate concretions: (A) pressure shadow sample revealing a porous microfabric composed of randomly arranged clay flake domains; (B) detail of clay domains and general mudstone microfabric in a pressure shadow sample. Note face-to-face clay domains (black arrow); (C) strongly planar fabric in a shale sample collected approximately 15 cm laterally from a concretion pressure shadow (bar=8 Am).

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Fig. 8. Plot of diagenetic volume percent carbonate versus sample position (C=center, M=middle, E=edge). Refer to Table 1 for the stratigraphic locations of the concretions.

Fig. 9. d18O and d13C scatter diagram for Rhinestreet carbonate concretions (filled squares=lower concretion horizon; open squares=upper concretion horizon). The plot includes the data of Dix and Mullins (1987) represented by triangles for comparison.

derived from other sources during diagenetic precipitation of carbonate cement. A likely source of the additional C was the oxidation of biogenic CH4 at the base of the sulfate reduction zone by anaerobic CH4 oxidation (Fig. 11; Raiswell, 1987, 1988). According to this mechanism, biogenic CH4 produced by microbial decomposition of organic matter in the zone of methanogenesis diffuses upward to the base of the sulfate reduction zone where carbonate precipitation occurs, usually within less than a meter of the seabed. The isotopically light CH4 leaves behind a pool of 13 C-enriched CO2 (Irwin et al., 1977). The siting of concretions within a concretionary horizon reflects the locations of permeable vertical pathways capable of conducting CH4 to the base of the sulfate reduction zone (Raiswell, 1987). One may reasonably expect that carbonate cement precipitated by anaerobic CH4 oxidation should reflect the highly fractionated nature of the biogenic CH4 source (Rosenfeld and Silverman, 1959). Yet Whiticar et al. (1986) and Raiswell and Fisher (2000) point out that d13C of C in the zone of biogenic methanogenesis ranges from
30xto +15x, with a mean of
7.9x, a span that encompasses the C isotope values of the Rhinestreet concretions. The

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419

Fig. 10. Plots of d13C (A) and d18O (B) versus sample position (C=center, M=middle, E=edge). Refer to Table 1 for the stratigraphic locations of the concretions.

wide range of d13C values of Rhinestreet concretionary cement reveals the range in the mix of isotopically light biogenic CH4 and heavier dissolved carbonate supplied to the zone of anaerobic CH4 oxidation (Fig. 11). Of the six Rhinestreet concretions studied, one (concretion 17C) shows minimal radial change in d13C, while concretion UC5 comprises relatively heavy C in its center and edge and more depleted 13 C values in its interior (see Fig. 10A). The remain-

ing concretions display d13C gradients of increasing 13 C enrichment toward edges (see Fig. 10A), which could be interpreted to reflect passage of the concretion into the zone of biogenic methanogenesis. Such an isotopic gradient would be expected of concretions that had formed concentrically (e.g., Mozley, 1996), a model not compatible with field observations and textural evidence. Moreover, concretions UC5 and 17C show no isotopic evidence of such a diagenetic

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Fig. 11. A schematic representation of the inferred role of sedimentation (subsidence) rate in the growth of carbonate concretions by anaerobic methane oxidation (modified from Raiswell, 1987, 1988). The column on the left shows the diagenetic zones in anaerobic sediment; AMO=zone of anaerobic methane oxidation. Depth and thickness of the zone of AMO is based on Raiswell (1987) and references therein. Column A: sedimentation and subsidence rates are high enough to carry sediment through the zone of AMO too fast to allow for little more than the diffuse precipitation of diagenetic CaCO3. Column B: a reduced sedimentation rate enables concretions to grow in a relatively wide zone as sediment slowly passes through the zone of AMO (e.g., the lower concretionary horizon in the Rhinestreet shale and level 3 in column C). Column C: a near or complete cessation of sedimentation (very slow passage of sediment through the zone of AMO) results in the formation of a narrow concretionary horizon (e.g., the upper concretionary horizon in the Rhinestreet shale and level 1 in column C). Level 2 in column C reflects an increase in sedimentation (and subsidence) rate following formation of the concretionary horizon represented by level 3.

history. We suggest that differences among d13C gradients of the studied Rhinestreet concretions reflect local variations in the flux of CH4 and dissolved carbonate diffusing upward from the zone of methanogenesis (e.g., Raiswell, 1988). Carbonate precipitation in the zone of anaerobic CH4 oxidation was accompanied by growth of euhedral pyrite as both isolated grains and overgrowths on framboids. Abundant euhedral pyrite along concretion edges and as halos along septarian fractures indicates that concretion growth continued to its end within the sulfate reduction zone, at depths shallow enough to be affected by the downward diffusion of seawater sulfate (Raiswell, 1971; Hudson, 1978; Coleman and Raiswell, 1981). Moreover, the concretions were permeable enough that sulfate-bearing seawater could migrate through them.

Crucial to the precipitation of diagenetic carbonate from anaerobic CH4 oxidation is a marked reduction in, or complete stoppage of, sedimentation, which would stabilize the anaerobic CH4 oxidation zone thereby enabling protracted carbonate precipitation (Fig. 11; Raiswell, 1987, 1988). Two observations provide indirect evidence of a reduction in sedimentation rate during concretion growth: (1) minimal change in estimated porosity (based on carbonate volume percentage) outward from the centers of studied concretions and (2) negligible variation in laminae thickness within concretions (see Fig. 3B,C). Both points suggest that the concretions grew in the near-absence of sedimentation and associated compaction (e.g., Raiswell, 1971). The upper and lower concretion horizons of the Rhinestreet shale differ in two ways that yield indirect information regarding such parameters as the magni-

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tude and duration of the change in sedimentation rate. The upper horizon, 1 –1.5-m-thick, carries concretions that are distinctly larger than the largest concretions of the roughly 2.5-m-thick lower horizon (see Fig. 4). The thickness of a concretionary horizon is interpreted to be proportional to the change in sedimentation rate; i.e., a hiatus in sedimentation yields the minimum vertical range of carbonate precipitation because subsidence is halted and the zone of anaerobic CH4 oxidation is stabilized (Raiswell, 1987, 1988). The upper Rhinestreet concretion horizon, then, records a reduction in sedimentation rate of greater magnitude than that which led to formation of the thicker lower horizon (compare columns B and C, Fig. 11). Similarly, the sizes of the concretions that grew along a specific stratal horizon reflect the duration of the reduction or break in sedimentation, a measure of the amount of time that isotopically light CH4 and 13Cenriched dissolved carbonate were delivered to the stationary anaerobic CH4 oxidation zone (Raiswell, 1987, 1988). The occurrence of more oblate, occasionally flat-topped, concretions in the upper horizon (see Fig. 3C) and the fact that the concretions appear to have grown preferentially in the horizontal dimension (see Fig. 4) may reflect the restriction of carbonate precipitation to a narrow, fixed zone defined by the upper limit of CH4 diffusion and the maximum depth to which seawater sulfate can diffuse (Raiswell, 1988). Thus, the larger horizontal dimensions of concretions of the upper horizon indicate that the break in sedimentation persisted longer at this time than during formation of the lower concretionary horizon. Specifically, the upper concretion horizon of the Rhinestreet shale marked something approaching a stoppage in sedimentation (column B, Fig. 11), while the thicker lower horizon (column C, Fig. 11), and its less oblate concretions, records a smaller reduction in sedimentation rate that persisted over a shorter period of time. Concretionary growth in both horizons was terminated by a resumption of ‘‘normal’’ (i.e., pre-carbonate precipitation) sedimentation rates (level 2 of column C, Fig. 11). 3.3. Late diagenetic alteration of concretions O isotopic compositions of calcite are known to be a function of the temperature of precipitation and the d18O of the water from which the calcite precipitated

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(Hudson, 1977). If one variable is reasonably well known, the other can be estimated (Hudson, 1978). We used the equation of Anderson and Arthur (1983) to calculate paleotemperatures of concretionary calcite precipitation: T o ¼ 16:0
4:14ðdC
dWÞ þ 0:13ðdC
dWÞ2 ð1Þ in which dC=d18O PDB of diagenetic calcite and dW=d18O of seawater on the SMOW scale. Estimates of seawater d18O for the Late Devonian include
1x SMOW of van Geldern et al. (2001) and
2xSMOW of Hudson and Anderson (1989). A Devonian seawater isotopic composition of
2xSMOW yields a temperature range of carbonate precipitation of 24 –39 jC, slightly to moderately in excess of the inferred 20 jC bottom water temperature of the subtropical Catskill Delta basin (e.g., Gerlach and Cercone, 1993) and, therefore, incompatible with a shallow depth of concretion growth argued for by textural and field observations. Indeed, the range of O isotopic values of Rhinestreet concretionary carbonate suggests that diagenetic precipitation occurred 130– 630 m below the seafloor, assuming a geothermal gradient of 30 jC/km. An influx of 18O-depleted meteoric water through the forming Rhinestreet concretions could account for their isotopic compositions, yet the location of the Rhinestreet depocenter far distant from the Devonian shore would seem to preclude such a scenario. Direct precipitation of the Rhinestreet concretions from seawater at the temperatures suggested by the depleted O isotopic values would require abnormally warm bottom waters during the Late Devonian, a hypothesis not supported by any geological data. We suggest, instead, that the O isotope compositions of the Rhinestreet concretions resulted from the incomplete recrystallization of primary calcite micrite to microspar (e.g., Folk, 1965) and re-equilibration of isotopic values by warm fluids migrating through the section. Although four of the Rhinestreet concretions studied by us show minor center-to-edge depletion in 18O (Fig. 10B), estimated temperatures of precipitation of the centers of these concretions are too high to suggest precipitation from normal seawater. Further, concretion 17C shows no radial O isotope gradient, and the center of concretion UC5 is marked by an especially depleted (
6.6x PDB) value (Table 1, Fig. 10B). It appears, then, that the warm fluids had some degree of access to concre-

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tion interior. Our results confirm that C isotopes are much less sensitive to temperature-induced fractionation than are O isotopes (e.g., Veizer, 1983; Gautier and Claypool, 1984). Dix and Mullins (1987) described a similar occurrence of recrystallized carbonate concretions in the more deeply buried Middle Devonian sequence of central New York State that they attributed to the migration of warm connate fluids, as much as 30 jC warmer than those fluids inferred to have affected the Upper Devonian Rhinestreet concretions of western New York state (see Fig. 9).

or pause in sedimentation that would have held the zone of anaerobic CH4 oxidation in place long enough for concretions to attain their great dimensions. Variations in the thickness of the two studied Rhinestreet concretionary horizons and the size of the concretions themselves reflect differences in (i) the duration of the breaks in sedimentation recorded by the concretionary horizons and (ii) the magnitude of the reductions in sedimentation rate.

Acknowledgements 4. Conclusions (1) The high carbonate percentage (74 – 93%), differential compaction of host shale laminae, local occurrence of tilted concretions and the preservation of an open, flocculated clay texture in concretion pressure shadows indicate that Rhinestreet carbonate concretions formed rapidly along stratally confined horizons within perhaps a meter of the seafloor. (2) Minimal center-to-edge variations in laminae thickness and inferred pre-cementation host sediment porosity gradients suggest that the concretions grew pervasively as compaction resistant porous frameworks of calcite micrite. (3) Rhinestreet carbonate concretions did not form at obvious concentrations of organic matter. Rather, the additional C required to form the large concretions may have originated within the zone of biogenic methanogenesis, diffusing upward to the zone of anaerobic CH4 oxidation at the base of the sulfate reduction zone where diagenetic carbonate precipitated. The wide range in C isotopic compositions of Rhinestreet concretions was caused by local variations in the mix of isotopically light CH4 and heavier dissolved carbonate migrating upward from the zone of methanogenesis. Slightly to moderately depleted d18O values are contrary to the shallow depth of concretion growth suggested by field and textural observations and estimated porosity and appear, instead, to be the result of incomplete recrystallization of the porous concretions by warm connate fluids during burial. The confinement of most Rhinestreet concretions to stratal horizons requires that sedimentation was episodic, each horizon recording a marked reduction

Peter Bush and his staff at the University of Buffalo, South Campus Instrumentation Center, School of Dental Medicine are acknowledged for trusting us with their scanning electron microscope. We thank Neal O’Brien for his discussions of shale fabric, and Peter Wilkin is acknowledged for fruitful discussions regarding framboid growth. DRB was supported by a SUNY Fredonia Summer Undergraduate Research Fellowship. The manuscript benefited from the reviews by Stanley Paxton, John Doveton and Paul D. Howell. [LW]

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