E A R L Y B U R I A L Q U A R T Z A U T H I G E N E S I S I N S I L U R I A N P L A T F O R M C A R B O N A T E S , N E W B R U N S W I C K , C A N A D A 1

J. P. A. NOBLE Department o f Geology

University of New Brunswick Fredericton, N.B. E3B 5A3, Canada

AND D. R. V A N S T E M P V O O R T

Department of Earth Sciences University of Waterloo

Waterloo, Ontario N2L 3G1, Canada

ABSTRACT: Several types of quartz cement and replacive quartz fabrics occur in the Silurian platform carbonates of nor them New Brunswick. Petrographic relationships with calcite, pyrite, dolomite, and intraformational erosional clast surfaces indicate that this quartz formed during early burial at depths ranging from the upper few meters to perhaps tens of meters. The quartz cement microtextures suggest that it is a primary precipitate.

A close spatial association of fossils and authigenic quartz suggests some kind of organic control on silica precipitation and that the quartz authigenesis was controlled more by molecular diffusion than fluid flow. Several lines of evidence point to bacterial sulfate reduction coupled with oxidation of organic matter as a key process leading to quartz authigenesis, including: 1) documented dominance of this kind o f reaction in shallow buffed marine sediments, 2) close spatial association of authigenic quartz and pyrite, 3) quartz replacement of gypsum, and 4) experimental evidence that sulfate-reducing bacteria mediate silica precipitation (Birnbaum 1984).

INTRODUCTION

Previous studies (Greiner 1970; Noble 1976; Lee and Noble 1977; Noble and Howells 1979) have established the general depositional history of the mixed carbonate- clastic Silurian platform and adjacent basins in northern New Brunswick (Fig. 1). The general stratigraphy of the area is shown in the cross-section of the Silurian (Fig. 2) from Pointe Laroche (locality 1) in the northwest to Petit Rocher (locality 8) in the southeast. This cross-section includes a portion of the carbonate-rich Chaleur platform, and passes southeasterly into a dominantly siliciclastic basin near Bathurst (Noble 1976).

Current studies are attempting to unravel the diagenetic histories of these sediments and to relate them to their burial histories, tectonic events, and sea-level changes. In that context, this paper describes the extensive dissemi- nated authigenic quartz and minor nodular chert in the platform carbonates of the Limestone Point and La Vieille Formations. The petrography described below is based on a study of many (100+) thin sections and polished slabs sampled from these formations at the stratigraphic sections shown in Figure 2. Based on relationships to other diagenetic minerals and fabrics, a model of quartz authigenesis is proposed which may have general validity for many marine platform sequences.

GEOLOGIC SETTING

The Limestone Point Formation was deposited as an array of distinctive biofacies and lithofacies during a shal- lowing up cycle (Fig. 3) (Noble and Howells 1979). Mixed siliciclastic and carbonate lithofacies include near-shore sandstones and shales, bioclastic grainstones, packstones

' Manuscript received 27 September 1984; revised 8 June 1988.

and lime mudstones. More off-shore lithofacies are silt- stones and carbonate wackestones.

The overlying La Vieille Formation was deposited first at the beginning of the Wenlock during a deepening and widening of the shelf, and later during gradual shallowing which culminated in widespread algal tidal fiats (Fig. 3). Lithologies in the lower part of the formation are silty limestones and calcareous siltstones dominated by di- verse brachiopod biofacies. These pass upwards into wackestones and packstones with minor grainstones and diverse coralline-brachiopod-stromatoporoid biofacies. The uppermost lithofacies include stromatolitic algal beds and sandstones with low faunal diversities.

The Petit Rocher Formation (Fig. 2) represents the slope and basin facies adjacent to the Limestone Point and La Vieille Shelf(Noble 1976). Black shales and nodular lime- stones, variegated shales and silts, and graded sandstones are the dominant Petit Rocher lithologies.

Acadian deformation has affected these Silurian sedi- ments, producing upright folds, thrust faults, and pene- trative cleavage. The maximum grade of metamorphism is lower greenschist facies, but is generally less.

AUTHIGENIC QUARTZ PETROGRAPHY

In carbonate fossils and lithologies of the Limestone Point and La Vieille Formations, authigenic quartz is ubiquitous as millimeter to centimeter scale disseminated aggregates of cement and replacive quartz. Its abundance is strongly correlated with fossil diversity (Fig. 3). Locally, it occurs as large chert nodules up to several cm in di- ameter; very rarely, it occurs as larger lensoid chert beds. The authigenic quartz in carbonates is not strongly con- centrated at a particular stratigraphic horizon or uncon- formity (Fig. 2) and is approximately 5% by volume over- all.

JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 59, NO. 1, JANUARY, 1989, P. 65--76 Copyright © 1989, The Society of Economic Paleontologists and Mineralogists 0022-4472/89/0059-65/$03.00

66 J. P. A. N O B L E A N D D. R. V A N S T E M P V O O R T

New / " i Btun~.wlc k

," Maine "',

Dalhousie

2

CHALEUR BAY

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FIO. l.--Location map of the study area showing location of pr incipal measu red strat i- graphic sections and distribu- tion of Silurian outcrops/sub- crops on New Brunswick side of Chaleur Bay (striped pat- tern).

QUARTZ CEMENT FABRICS

The quartz cement in carbonates includes chalcedony and megaquartz.

Chalcedony Cement

Radially-fibrous length-fast chalcedony is the most common type of quartz cement (Fig. 4). Crystals occur in limpid to cloudy botryoidal masses with concentric growth bands. These growth bands become isopachous as pores become filled (Fig. 5). Fiber size, abundance of inclusions and extinction pattern is very consistent within bands but extremely variable between bands. The ex- tinction depends on the period of twisting of crystals about the [ 1120] direction of elongation (Frondel 1978), and in some examples extinction banding occurs in a striped, zig-zag pattern characteristic of zebraic chalcedony (Folk and Pittman 1971).

Radially-fibrous quartzine consists of intersecting bot- ryoidal masses of length-slow fibers, which often have a brown tint in plain light. This type of chalcedony is much less common than length-fast chalcedony. The length- slow fibers are elongated along the [0001 ] direction (Fron- del 1978).

A third type of chalcedony cement consists of thin, uniformly isopachous bands of pale yellow length-fast crystals, oriented subparallel to each other and perpen- dicular to the host walls, rather than radiating (Fig. 5). Bands of this parallel-fibrous chalcedony often alternate sharply with broader zones of botryoidal chalcedony, and tend to become more common towards the centers o f

pores. However, some larger shelter cavities and some intergranular pores are lined only or dominantly by a thin band of this parallel-fibrous chalcedony (not illustrated).

Since all these types of chalcedony fill pores, never replace skeletal or other grains, and often coat intact rims of earlier calcite cements, they are interpreted as cements and not replacive fabrics.

These chalcedony botryoids are identical in growth habit to those of organic compounds precipitated experimen- tally from solution (Frondel 1978). Furthermore, they are similar to botryoids of other minerals such as aragonite, hematite and malachite, which are known to precipitate directly from solution. This is good evidence that the chalcedony is a primary precipitate and not a product of inversion from another silica precursor.

Megaquartz Cement

This cement consists of pore-filling mosaics of equant anhedral or polygonal grains and subgrains (Fig. 6). Where pores have been incompletely filled with quartz, crystal terminations may be prismatic. Mutual grain boundaries are subplanar. Crystal diameters are generally 0.2 m m to 0.5 m m but may be smaller in small pores. Crystals have slightly to strongly undulose extinction, and are generally clear with minor fine-grained inclusions, including fluids.

The following characteristics suggest that this quartz is usually a pore-filling cement and not a replacement fabric:

1) Megaquartz never replaces skeletal material. 2) It occurs preferentially in intraskeletal cavities within

tabulate corallites, and in shelter cavities, fractures, and intragranular pores.

E A R L Y B U R I A L Q U A R T Z A U T H I G E N E S I S I N C A R B O N A T E S 67

1 2 3 4 5 6 7 8

CHERT

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m ~Lmandover~on ~m L--~..°o~m T.

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KEY TO LITHOLOGY SYMBOLS

Iy:T:FICDNOLOMERAT~S'ITH ~C~S.~OOEO F~_I.O~ARL.M~STONES ~ISHALES. MINOR SANDSTONES SANDSTONES flk LIMESTONES Om SILTSTONES FINE SILTSTONES

MINOR SILTSTONES (BEDDED) SILTSTONES VOLCANIC ROCKS

FIG. 2.--Stratigraphic cross-section of principal measured sections showing approximate distr ibut ion o f autlaigenic quartz in carbonate fossils and lithologies in relation to stratigraphic position, ] i tbology and facies.

8_

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t ~, 8rachiopod Sst.

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: Dolerorthis

FIG. 3.-- Distribution of au- thigenic quartz in carbonates in relation to sedimentary and faunal parameters and inter- preted depositional environ- ments at locality 3 (Fig. I). Bio- facies zones include one or more biofacies (Noble and Howells 1979).

68 J. P. A. NOBLE AND D. R. VAN STEMPVOORT

FiG. 4.--Radially fibrous chalcedony filling a bivalve shell. Note that chalcedony post-dates early mechanical compaction. La Vieille Fm., section 3. Crossed nichols. Scale bar equals 1 mm.

FiG. 6.--Megaquartz cement (Q) infilling corallites and post-dating isopachous dogtooth calcite cement (C). La Vieille Fm., section 3. Crossed nichols. Scale bar equals 1 ram.

3) A crystal size gradient is frequently observed, in- creasing from the wall o f the pore towards the center o f the pore (Fig. 5).

4) The megaquar tz rarely replaces calcite cement lining pore walls, but frequently fills a round euhedral calcite cements (Fig. 6).

5) Quartz crystal t e rmina t ions are euhedral in incom- pletely filled pores.

6) Quartz is generally clear, lacking inclusions, or con- tains few very fine inclusions.

7) No relict patches o f unsilicified sediment have been observed within the megaquartz mosaics.

Relationships Between Megaquartz and Chalcedony Cements

T i m i n g . - - W h e r e both cements occur in the same pore, the chalcedony has always precipi ta ted first against the

pore wall. The megaquar tz then fills or par t ia l ly fills the remaining space (Fig. 5). N o reversals o f this sequence have been observed, and the boundary between the two types o f cement is usually sharp.

This chalcedony to quartz sequence occurs in chert nodules and geodes o f var ious rocks (Meyers 1977; Fron- del 1978; Mill iken 1979). The reason for the abrupt change in crystal habi t f rom finely fibrous to equant megaquartz is not known, but p robab ly indicates a drop in silica con- centra t ion or another change in pore solution chemist ry (Keith and Padden 1963; Frondel 1978). Fabric . - -Both chalcedony and megaquar tz cement crys- tals occur in the following ways:

a) mos t c o m m o n l y as intraskeletal cement , especially within the corall i tes o f tabulate and rugose corals, but also in s t romatoporo id galleries, ar t iculated b rach iopod shells, gastropods, bryozoa, and borings

Fro. 5.--Chalcedony cement as hemispherulitic masses and isopach- ous bands (Chi) lining tabulate coral cavity (highlighted). Final infilling of cavity by megaquartz cement. La Vieille Fm., section 3. Crossed nichols. Scale bar equals 1 ram.

Fio. 7. - Microquartz replacing pclmatozoans (p), and cementing ma- trix (m) of calcarenite in extensively silicified zone, followed by equant calcite cement (c). Some of the disseminated dark inclusions in the quartz are pyrite grains. Note lack of compaction. La Vieille Fm., section 3. Scale bar equals 1 ram.

E A R L Y B U R I A L Q U A R T Z A U T H I G E N E S I S 1N C A R B O N A T E S 69

FiG. 8.--Microquartz replacing pelmatozoan fragment (P); the adja- cent synlaxial calcite cement (cs) is also partly replaced by microquartz (q). Note microdolomite rhombs (white arrows) scattered in the micro- quartz. La Vieille Fm., section 3. Crossed nichols. Scale bar equals 1 1Tim,

b) as intergranular cement in chert nodules in grain- stones of certain stratigraphic horizons (Fig. 7)

c) cement filling molds of pelecypods (Fig. 4) or gas- tropods at certain stratigraphic horizons within the Lime- stone Point and La Vieille Formations

d) occasionally as shelter cavity cements, especially be- low stromatoporoids and brachiopods.

Other occurrences include rare infill of gypsum molds by chalcedony, and fine-grained megaquartz in fractures within extensively silicified zones.

REPLACIVE QUARTZ FABRICS

Fossil fragments are preferentially replaced by micro- quartz, spherulitic chalcedony, or pseudomorphic chal- cedony, whereas nonskeletal grains, matrix, and calcite cements generally remain unreplaced. The susceptibility of different fossil types to quartz replacement shows a consistent order, with rugose and tabulate corals and bra- chiopods showing most frequent quartz replacement, stromatoporoids and pelmatozoans somewhat less, and bryozoa and trilobites least. Pelecypods and gastropods are generally leached and thus not replaced by quartz.

Microquartz

Microquartz consists of aggregates of minute, equant to slightly elongate subgrains of quartz. These aggregates have irregular crenulate boundaries (Fig. 8) and undulose extinction. Inclusions are abundant and include scattered rhombic dolomite crystals.

This microquartz usually replaces fossils and fossil frag- ments. Sometimes it also replaces calcite cements (Fig. 8) and micritic matrix, especially in extensively silicified beds. And microquartz also replaces gypsum (Fig. 9). Small patches ofsphernlitic replacement chalcedony (described below) often occur scattered within the microquartz, and the two types are gradational.

FiG. 9.--Microquartz replacing gypsum (G). Top of La Vieille Fm., section 3. Crossed nichols. Scale bar equals 1 ram.

Spherulitic Replacement Chalcedony

Small spherulites of radiating fibrous chalcedony (Fig. 10) occur as isolated or coalescing masses, often with extremely irregular geometry. The most common replace- ment is of fossils and fossil fragments. Replacement spherulites are often near perfectly spherical to elliptical in cross-section, and individual fibers are usually length- slow and occur as irregular interfering bundles with wavy undulose extinction. Inclusions, such as large rhombs of dolomite, are often abundant.

Pseudomorphic Replacement Chalcedony

This calcite-replacing silica is confined to irregular areas of brachiopod shells and tends to preserve the original microstructures (Fig. 11). The microstructures include laminar and prismatic crystallites and punctae and pseu- dopunctae which were replaced by similarly shaped and oriented chalcedony fibers or microquartz. The organic sheaths that surround the crystallites are usually pre- served in their original alignment as inclusions in the replacement silica. The chalcedony replacement fibers are both length-fast and length-slow, often in alternating bun- dies. This fabric is frequently associated with spherulitic chalcedony in the same shells. It appears similar to the pattern V quartz described by Schmitt and Boyd (1981) and to the fine-scale replacement noted by Holdaway and Clayton (1982) in brachiopods of other formations.

Relationships Between Replacive Quartz Types and Quartz Cements

Timing. -The textural relationships between the different types of replacive quartz do not indicate a precise relative age. In some samples, a central zone of pseudomorphic chalcedony is surrounded by an outer rim of spherulitic replacement chalcedony (Fig. 11). If the silica precipita- tion proceeded outward, the pseudomorphic quartz is

70 J. P, A. NOBLE AND D. R. VAN S T E M P V O O R T

FIo. 10.--Spherulitic replacement chalcedony (Chs) postdated by sty- lolites (st) which also cut syntaxial calcite cement (Cs). Limestone Point Fro., section 3. Scale bar equals 1 ram.

FIO. 11.--Pseudomorphic replacement chalcedony (Chp) as zone be- tween unreplaced brachiopod shell calcite (C) and areas of spherulitic chalcedony (Chs). La Vieille Fm., section 3. Scale bar equals 1 ram.

slightly older (cf. rep lacement fronts o f Holdaway and Clayton 1982).

Some chalcedony cement has apparent ly nucleated on replacement microquartz , but generally evidence o f rel- at ive ages of replacive quartz and quartz cement are ab- sent. However , the dis t r ibut ions o f quartz cements and replacive quartz are strongly correlated on the micro- scopic scale, suggesting that they are approx imate ly syn- chronous, or formed in rapid succession to each other.

RELATIONSHIPS BETWEEN AUTHIGENIC QUARTZ AND OTHER DIAGENETIC MINERALS AND FABRICS

The authigenic quartz in carbonates of the Limestone Point and La Vieille Fo rma t ions occurs in associat ion with a number o f carbonate cements and other minerals such as gypsum and pyrite. The diagenetic in terpre ta t ion o f the authigenic quartz depends largely oil its textural re la t ionship to these other minera ls and o ther diagenetic fabrics.

Calcite C e m e n t s

There are seven recognized calcite cements in these sediments (Table 1). Detai ls of these cements will be pub- l ished separately.

Isopachous dogtooth calcite (type 1) (Fig. 6) and Fe- poor syntaxial calcite (type 2) (Fig. 8), which are widely dis t r ibuted, and radiaxial-f ibrous calcite (type 3) (Fig. 12) are interpreted as early mar ine phreat ic cements on the basis o f the following:

1) They are the init ial coatings on host grains, and the high minus -cement porosi ty indicates that they formed before compact ion.

2) Thei r crystal textures and cloudy internal fabrics are s imi lar to some m o d e m phreat ic cements (e.g., James et al. 1979).

3) Some syntaxial calcite crystals have eroded surfaces and contain microbor ings (Fig. 13), which presumably formed syndeposi t ional ly .

TABLE 1 .--Approximate sequence of principal diagenetic events: Silu- rian carbonates of northern New Brunswick

S u c c e s s i v e S t a g e s o f B u r i a l

V e r y E a d y - - - - ~ L a t e

Isopachous Dogtooth Calcite Fe-poor Syntaxial Calcite Radiaxial-fibrous Calcite Fe-rich Syntaxial Calcite Nodular Micrite Authigenic Quartz Pyrite Microdolomite Dull-luminescing Fe-nch Calcite Brightly luminescing Calcite Fe-rich Saddle Dolomite Stylolites

- - B

m -

B - -

? . . . .

? - - - - - -

FIG. 12.--Interior brachiopod shell (B) with radiaxial-fibrous calcite cement (Cr) partiany replaced by microquartz (arrows). Dolomite (D) replaced both radiaxial-fibrous calcite and the quartz. La Vieille Fm., section 3. crossed nichols. Scale bar equals 2 ram.

E A R L Y B U R I A L Q U A R T Z A U T H 1 G E N E S I S I N C A R B O N A T E S 71

Fic. 13.--Pelmatozoan fragment (P) with partially eroded syntaxial calcite cement (Cs) which has been bored and borings filled with he- matite (arrow). Limestone Point Fro., section 3. Scale bar equals 5 mm.

FIG. 15.--Microquartz (q) replacing isopachous dogtooth calcite ce- ment (C) lining corallite wall which is also largely silicified. La VieiUe Fm., section 3. Scale bar equals l mm

4) Radiaxial-fibrous calcite cement is truncated on the surfaces o f intraformational clasts (Fig. 14) in the lower portion o f the La Vieille Formation (17 m from base o f the formation, section 3, Fig. 2). These reworked bioclasts occur in deeper shelf sediments (Costistricklandia bio- facies) and are interpreted as submarine storm rip-ups. There is no evidence o f subaerial exposure, such as so- lution collapse features, or vadose diagenetic signatures in the clasts or their host beds. The depositional history inferred from stratigraphy (lithofacies, biofacies) suggests no subaerial exposure occurred until much later during deposition o f the uppermost La Vieille.

Nodular micrite cement (type 4) is present in calcareous siltstones o f the La Vieille Format ion (Costistricklandia biofacies). These micrite nodules are interpreted as being early marine burial in origin, because they contain un- compacted burrows, while burrows outside the nodules are flattened (Noble and Howells 1974).

Where timing relationships can be observed on the basis o f textures, forms of quartz cement and replacive

FIG. 14. -- Intraformational clast of silicified brachiopod shell (Ch) and attached radiaxial-fibrous calcite cement (Cr). Note eroded crystals of this cement (arrows). La Vieille Fm. Scale bar equals 1 ram.

quartz always postdate the above marine calcite cements (Figs. 6, 8, 12, 15).

Some Fe-rich syntaxial calcite (type 5) postdates the Fe-poor syntaxial calcite, but it is partly replaced by quartz or coated with quartz cement. Although this type o f calcite is difficult to distinguish petrographically from type 6 (below), it apparently is approximately time-equivalent with Fe-rich crystal terminations o f isopachous dogtooth calcite (type l) and Fe-rich radiaxial-fibrous calcite (type 3).

Dull-luminescing Fe-rich calcite cement (type 6) and brightly-luminescing calcite cement (type 7) postdate the above calcite cements and also the authigenic quartz (Fig. 16). These cements probably formed during intermediate to deep burial, and they occlude remaining porosity.

D o l o m i t e s

Two types o f dolomite occur in these Silurian platform sediments: Fe-poor microdolomite (Fig. 8), and Fe-rich saddle dolomite or ankerite (Figs. 12, 17). Both these types are widely distributed.

The Fe-poor microdolomite consists o f subhedral rhombs scattered in replacement quartz. The microdo- lomite is always confined to the silicified portion of skel- etons and is absent f rom adjacent non-silicified areas. This intimate association o f microdolomite and authi- genic quartz, which is present in other rocks as well (Die- trich el al. 1963; Jacka 1974; Decelles and Gutschick 1983), suggests that the dolomite and authigenic quartz are approximately synchronous.

The microdolomi te is mos t c o m m o n in s t romato- poroids and echinoderm ossicles. In echinoderms the mi- crodolomite crystals have a preferred orientation and ex- tinguish together under crossed nichols, suggesting that they "exsolved" from a single high-Mg calcite crystal pre- cursor (Lohmann and Meyers 1977). This calcite stabi- lization likely occurred in early to moderately early dia- genesis (Gavish and Friedman 1969; Schroeder 1969).

72 J. P. A, N O B L E AND D. R. FAN S T E 3 4 P V O O R T

Fi~. 16.--Botryoidal chalcedony (Ch) lining cavity within stroma- toporoid (S). Cavity filled with later Fe-rich calcite (CO, and chalcedony partially replaced by brightly lumninescent calcite (Cb). La Vieille Fm., section 3. Crossed nichols. Scale bar equals 0.5 ram.

The above combined evidence suggests that microdolo- mite and replacive quartz are of early burial origin.

Fe-rich saddle dolomite or ankerite postdates all au- thigenic quartz types including microquartz, botryoidal chalcedony (Fig. 17) and megaquartz. This is most evi- dent where fractures cut the authigenic quartz and are filled with saddle dolomite. Elsewhere saddle dolomite replaces partially silicified radiaxial-fibrous calcite ce- ment (Fig. 12). No examples ofsilicified saddle dolomite have been found. Saddle dolomite is generally interpreted as a relatively late burial precipitate which forms at tem- peratures exceeding 60°C (Gregg 1983).

Mechanical Compaction

The shapes of some intraskeletal pores were modified by mechanical compaction. In one of these modified pores, intact botryoidal chalcedony nucleated along the walls (Fig. 4), indicating that some authigenic quartz postdates some mechanical compaction.

Stylolites

Some spherulitic replacement chalcedony predates sty- lolites, which truncate the spherule fabric (Fig. 10). These stylolites formed during intermediate to deep burial.

Intraformational Clasts

The La Vieille Formation contains large intraforma- tional clasts that are interpreted as submarine storm rip- ups, with no evidence of prior subaerial exposure. As noted above, the margins of these clasts truncate marine calcite cements (Fig. 14). They also erosively truncate authigenic quartz, including individual chalcedony ce- ment botryoids (Fig. 18). This is a very strong piece of evidence, and indicates that at least some, if not all, of the authigenic quartz formed during submarine burial, within the upper few meters.

FEe. 17.--Zoned Fe-rich saddle dolomite (D) replacing botryoidal chalcedony (Ch) lining cavities in tabulate corals. La Vieille Fm. Scale bar equals 0.1 mm.

Pyrite

Pyrite and authigenic quartz often occur together. In some samples, euhedral pyrite truncates and postdates chalcedony botryoids (Fig. 19); elsewhere, the pyrite is disseminated in the authigenic quartz, suggesting that the pyrite formed earlier or at the same time (Fig. 7).

Gypsum

As noted earlier, quartz sometimes replaces gypsum crystals (Fig. 9) or fills euhedral gypsum molds.

A MODEL OF QUARTZ AUTHIGENESIS IN MARINE PLATFORM CARBONATES

Source of Silica

Studies of modern marine platform sediment profiles (Calvert 1974; Berner 1981) indicate that silica concen-

FiG. 18.-- [ntraformational elast of La Vieille Formation tabulate cor- al fragment showing eroded chalcedony spherulites (arrows) at margin. Scale bar equals 1 mm.

E A R L Y BURIAL QUARTZ AUTHIGENESIS IN CARBONATES 73

Ft6. 19. --Euhedral pyrite replacing botryoidal chalcedony (Ch) filling cavities in tabulate coral. Note hemispherical masses (arrows) of quartz. La Vieille Fm. Scale bar equals 0.2 mm.

trations of the interstitial waters increase rapidly with depth for some tens of centimeters, then decrease slowly. The increased dissolved silica in the uppermost sediments may result from dissolution ofbiogenic opal and volcanic glass, and perhaps also by the alteration of clay minerals. Any evidence for biogenic or volcanic glass precursors of authigenic quartz in ancient platforms would be obscured by dissolution. Rare sponge spicules occur through most sections of the Limestone Point and La Vieille Forma- tions in northern New Brunswick. Volcanic glass is even rarer, especially in the more silicified sediments, but could have been dispersed as a part of the fine-grained matrix. Clay minerals are common throughout most sections. Since the amount of silica needed for the Silurian platform authigenic quartz is not great (approximately 5% of car- bonate lithologies), any or all of these three sources of authigenic quartz silica are possible.

Alternatively, before the Cretaceous bloom of marine diatoms, the concentration of ocean silica was probably much higher, and its diffusion into sediments was an important source of authigenic silica (Siever 1988).

Key Reactions for Silica Precipitation

There is much evidence to suggest that organic remains are important in silica precipitation during the formation ofauthigenic quartz in carbonates. In this and many other studies, authigenic quartz is noted to occur mainly in fossils, often as replacement. Even the quartz cements occur preferentially in closed interskeletal pores (e.g., cor- allites) and are least common in open, intergranular pores. SEM studies of other ancient platform sediments suggest that quartz crystals in brachiopods nucleated on organic sheaths surrounding calcite crystallites (Holdaway and Clayton 1982) and that microquartz in pelecypods nu- cleated on organic shell layers (Schmitt and Boyd 1981). Experimental evidence also indicates that silica precipi- tation is sometimes favored by complexing of silica with organic matter (Francis et al. 1978; Iler 1979), although organic matter may also reduce rates of silica nucleation and precipitation (Hinman 1987).

FIG. 20.-- Partially silicified rugose coral (Paleocyclus) showing typical pattern of silicification (Ch) with peripheral zone of calcite (C) free of chert. Limestone Point Fro. Scale bar equals 1 mm.

During burial, bacterial oxidation of organic matter in fossil microenvironments produces CO2, which can lead to localized dissolution of aragonite and calcite. The silica solubility in these microenvironments might decrease be- cause of their increased salinity (Marshall and Wara- komski 1980) as organic compounds break down and carbonates dissolve, or because of decreased pH (Birn- baum and Wireman 1984). This may lead to localized silica precipitation. These concurrent reactions (oxidation of organics, dissolution of carbonates, and precipitation of silica) may explain the fossil carbonate replacement texture of authigenic quartz.

Oxidation of organic matter in buried sediments pro- ceeds by different sets o f reactions as burial increases (Froelich et al. 1979; Emerson et al. 1980). First, bacteria use free oxygen, then successively manganese oxides and nitrate, iron oxides, and sulfate with increasing burial, as each oxidant is depleted or reduced. Finally, fermentation bacteria take over, producing methane. Holdaway and Clayton (1982) argue that the most important reaction in the precipitation of platform silica is the oxidation of organic matter by free oxygen. The other reactions are considered unimportant, either because they result in pre- cipitation of calcite (Mn 4+ and Fe 3÷ reduction) or because they are quantitatively unimportant (nitrate reduction). Sulfate reduction is dismissed on the basis of some evi- dence for concurrent calcite precipitation by sulfate re- duction (Coleman and Raiswell 1981). Holdaway and Clayton (1982) also discount sulfate reduction and later bacterial fermentation reactions, because in the sediments studied by them some authigenic quartz apparently pre- dates pyrite formation.

The model we propose differs from the Holdaway and Clayton (1982) model in suggesting that the post-oxic reactions of shallow to moderate burial depths, specifi- cally those involving sulfate reduction by bacteria (Birn- baum 1984; Birnbaum and Wireman 1985), play a key role in platform authigenic quartz formation. The prin- ciple evidence is as follows:

1) In many recent sediments over a large variety of

74 J. P. A. N O B L E AND D. R. VAN S T E M P V O O R T

environments, free oxygen is consumed in the upper few centimeters (e.g., Vanderborght et al. 1977; Emerson et al. 1980; Claypool and Threlkeld 1983). Yet the authi- genic quartz of this study postdates the formation of a number of synsedimentary marine calcite cements. There is no reversal of this order as one might expect if the calcite cements and authigenic quartz were approximately synchronous during the first few centimeters of burial.

2) A frequent authigenic quartz fabric in the Limestone Point and La Vieille Formations is replacement of interior skeletal material, while peripheral areas (Figs. 11, 20) remain unsilicified. This pattern is ditftcult to explain as silicification in the near-surface oxic environment, be- cause peripheral areas of skeletons should have greater access to free oxygen. It is possible that the organic matrix in peripheral areas was oxidized in a O2-rich environment prior to silicification.

3) Holdaway and Clayton's contention (1982) that Fe 3+, Mn 4+, and sulfate reduction lead to calcite precipitation is too simplistic. Calcite could be dissolving and precip- itating in distinct mieroenvironments in these early burial reduction zones. The geochemical microgradients would be complex and at present are not clearly understood.

4) The iron content of some Fe-rich syntaxial spar that precipitated prior to silica (see above) indicates that the authigenic quartz is post-oxic and either synchronous with or later than Fe 3÷ reduction.

5) The above cited textural evidence for approximate synchronous precipitation of silica and pyrite fits the sul- fate reduction model very well. Indeed, Holdaway and Clayton (1982) also found textural evidence for contem- poraneous origin of some of the pyrite and silica in their samples. Furthermore, similar textural association of chert and pyrite in silicified lacustrine carbonates (Wells 1983) suggests that sulfate reduction may be an important pro- cess for silica precipitation in a wide array of environ- ments.

6) As noted above, quartz replaces gypsum, and quartz cement fills gypsum molds, indicating that silica precip- itation postdated or accompanied sulfate reduction and/ or dissolution. Silica replacement of sulfate minerals is very common in numerous other sedimentary units (Birn- baum and Wireman 1985).

7) There is some experimental evidence that sulfate may form a complex with silicic acid (Marshall and Chen 1982), thus sulfate reduction could lead to a decrease in silica solubility.

8) Finally, there is some direct experimental evidence (Birnbaum 1984; Birnbaum and Wireman 1985) that sil- ica precipitation is mediated by sulfate-reducing bacteria in the presence of decomposing organic matter.

DISCUSSION

In this bacterial sulfate reduction model of platform quartz authigenesis, the only general process necessm'y is marine burial of fossiliferous calcareous sediments con- taining siliceous skeletons, volcanic glass, clays, or a rel- atively high concentration of marine-dissolved silica (as in pre-Cretaceous oceans) leading to diffusion of silica

into the sediment. As the heterogeneously distributed skeletal organic matter decays in the shallow to moderate burial sulfate reduction zone, disseminated authigenic quartz or nodular chert forms.

Knauth (1979) has proposed an alternative, marine- meteoric mixing zone model for quartz authigenesis (chert) in platform sediments. In this model, silica saturation builds up in the groundwater of the supratidal zone by dissolution of biogenic opal and precipitates as quartz in the mixing zone, either as cement where porewaters are saturated with respect to calcite, or as replacement where the waters are undersaturated with respect to calcite, as in a CO2 closed system. We do not think this model applicable to the Silurian of New Brunswick, or perhaps to many other platform sediments, for the following rea- sons:

1) This mixing model does not explain the close as- sociation of most authigenic quartz, including cement, with fossils and pyrite.

2) The fact that quartz cement and replacive quartz are usually found together in the same microenvironments does not agree well with Knauth 's model of distinct zones of formation of quartz cement and replacive quartz.

3) The mixing model relies on significant lateral flow of silica-rich pore fluids, and one would expect a strati- graphic concentration ofauthigenic quartz in key mixing zones. However, the disseminated distribution of authi- genic quartz in dist inct fossil mic roenv i ronmen t s throughout the Silurian Limestone Point and La Vieille Formations suggests that silica sources were disseminated internally and silica precipitation was controlled by dif- fusion processes associated with skeletal microenviron- ments.

4) The peritidally exposed uppermost portion of the La Vieille Formation has relict gypsum crystals, indicative of evaporative brines rather than a brackish mixing zone.

Following Knauth (1979), Geeslin and Chafetz (1982) have also suggested a peritidal mixing zone for the origin of some Ordovician ribbon cherts. But they do not present evidence of a meteoric influence on chertification. The fact, which they noted, that the algal mats were prefer- entially silicified at an early burial stage can perhaps be explained by the sulfate reduction model.

lnitial Platform Silica Precipitate

The initial authigenic silica precipitate in deep sea ooz- es is generally opal-CT (Wise and Weaver 1974; yon Rad and Rosch 1974) in the form of porcellanite and lepi- spheres. This silica is later transformed via dissolution- reprecipitation during burial into chalcedony and micro- quartz. Whether quartz or opal-CT precipitates depends on the ratio of dissolved cations to silica (Lancelot 1973), and the total concentration of dissolved silica (MacKenzie and Gees 1971), among other factors.

The textures of the quartz cements in the Limestone Point and La Vieille Formation suggest primary quartz precipitation (as noted above). However, opal-CT to quartz transformations may preserve primary opal-CT fabrics (Williams et at. 1985). Unfortunately, there is no

E A R L Y BURIAL Q U A R T Z A U T H 1 G E N E S I S IN C A R B O N A T E S 75

documentation of recent marine platform authigenic quartz for comparison.

CONCLUSIONS

From petrographic relationships with other diagenetic fabrics and stratigraphic information, we conclude that the authigenic quartz in carbonates of the Limestone Point and La Vieille Formations formed at burial depths of a few meters to perhaps tens of meters. This is also the range of depths below the sea-floor over which microbial sulfate reduction is the dominant reaction in the oxidation of organic matter (Claypool and Threlkeld 1983). Sulfate reduction seems to be the key reaction type for precipi- tation of silica in platform sediments, provided a certain minimum level of silica saturation is obtained by dis- solution of biogenic opal, volcanic glass, or the release of silica from clays. Diffusion of silica from overlying sea- water may have been important in pre-Cretaceous time (Siever 1988).

We conclude that burial of heterogeneously distributed skeletal organic matter to post-oxic depths in platform sediments leads to the development of microenviron- ments which are the sites of significant microbial decom- position and sulfate reduction, and to local gradients in alkalinity, pH and salinity. These gradients lead to lo- calized concurrent calcite dissolution and quartz cemen- tation, and preferential replacement of fossils by quartz (a characteristic of many ancient platform carbonate sed- iments).

ACKNOWLEDGMENTS

This paper benefitted from the critisms and comments of the following reviewers: Bill Meyers, John Kaufman, and 2 anonymous reviewers. Financial support was given by NSERC in the form of a Summer Research Award to the second author and an operating grant to the senior author.

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