Diagenesis and porosity evolution of the Upper Silurian ...uregina.ca/~chiguox/s/2001 Bourque et al CPG.pdf · Diagenesis and porosity evolution of the Upper Silurian-lowermost Devonian

BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 49, NO. 2 (JUNE, 2001), P, 299-326

Diagenesis and porosity evolution of the Upper Silurian-lowermost Devonian West Point reef limestone, eastern Gasp6 Belt, Qu6bec Appalachians

PIERRE-ANDRI~ BOURQUE 1 D~partement de G~ologie et de GEnie gdologique,

Universitd Laval Qudbec, QC

G1K 7P4

MARTINE M. SAVARD 1 AND GUOXIANG CHI 1 Geological Survey of Canada (Qudbee)

880 Chemin Sainte-Foy, C.P. 7500 Sainte-Foy, QC

GIV 4C7

PAULINE DANSEREAU 1 D~partement de Gdologie et de GEnie gdologique,

Universit~ Laval QuEbec, QC

G1K 7P4

ABSTRACT

Diagenetic analysis based on cathodoluminescence petrography, cement stratigraphy, carbon and oxygen stable isotope geochemistry, and fluid inclusion rnicrothermometry was used to reconstruct the porosity history and evaluate the reservoir potential of the Upper Silurian-Lower Devonian West Point limestone in the eastern part of the Gasp6 Belt. The West Point limestone was investigated in two areas:

1) In the Chaleurs Bay Synclinorium, the limestone diagenesis of the lower and middle complexes of the Silurian West Point Formation was affected by repeated subaerial exposure related to late Ludlovian third-order eustatic low- stands, which coincided with the Salinic block tilting that produced the Salinic unconformity. The Anse Mclnnis Member (middle bank complex) underwent freshwater dissolution, and mixed marine and freshwater cementation during deposition. Concurrently, the underlying Anse ~ la Barbe and Gros Morbe members (lower mound and reef complex) experienced dissolution by fresh water percolating throughout the limestone succession. Despite this early development of karst porosity, subsequent meteoric-influenced cementation rapidly occluded all remaining pore space in the Gros Morbe, Anse h la Barbe, and Anse Mclnnis limestones. In contrast, the overlying Colline Daniel Member limestone (upper reef complex) does not show the influence of any freshwater diagenesis. Occlusion of its primary porosity occurred during progressive burial and was completed under a maximum burial depth of 1.2 kin.

2) In the Northern Outcrop Belt, the diagenesis of the Devonian pinnacle reefs of the West Point Formation followed a progressive burial trend. The primary pores of the reef limestone were not completely occluded before the reefs were buried at a significant depth (in some cases, to 6 kin). Therefore, hydrocarbon migration in subsurface buildups before primary porosity occlusion might have created reservoirs. Moreover, the presence of gaseous hydrocarbons in Acadian- related veins attests to a hydrocarbon source in the area.

RESUME

L'analyse diag6n6tique bas6e sur la p6trographie en cathodoluminescence, la stratigraphie des ciments, la g6ochimie des isotopes stables du carbone et de l'oxyg6ne, ainsi que la microthermom6trie par inclusions fluides est utilis6e pour d6crypter l'histoire de la porosit6 et 6valuer le potentiel r6servoir des calcaires de la Formation silurienne sup6rieure-d6vonienne inf6rieure de West Point dans la pattie orientale de la Ceinture de Gasp6sie. Les calcaires de West Point sont 6tudi6s dans deux r6gions.

(1) Au synclinorium de la Bale des Chaleurs, la diagen6se des calcaires des complexes inf6rieur et m6dian du West Point silurien a 6t6 profond6ment marqu6e par des expositions a6riennes r6p6t6es durant le bas niveau matin de troisi6me ordre au Ludlovien tardif, lequel a coincid6 avec des basculements de blocs tectoniques, le tout ayant produit la discordance salinique. Le Membre de l'Anse Mclnnis (complexe m6dian) a subi une dissolution par les eaux douces, ainsi qu'une cimentation mixte, marine et m6t6orique, durant son d6p6t. En m~me temps, les membres sous-jacents d'Anse h la Barbe et de Gros Morbe (complexe inf6rieur) subissaient une dissolution par ces m&nes eaux douces qui

I GIRGAB, Groupe interuniversitairre de Recherches en G6odynarnique et Analyse de Bassins.

299

300 P.-A. BOURQUE, M.M. SAVARD, G. CHI and P. DANSEREAU

percolaient ~ travers la pile de calcaires. MalgrE le dEveloppement prEcoce de cette porositE karstique, une cimentation subsEquente d'influence mEtEorique a andanti rapidement toute la porositE qui restait dans les calcaires de Gros Morbe, d'Anse ~ la Barbe et d'Anse McInnis. Par contre, les calcaires sus-jacents du Membre de Colline Daniel (complexe rEcifal supErieur) ne montrent aucun signe de diagenbse mEtEorique. La perte de sa porositE primaire s'est faite pro- gressivement durant l'enfouissement et fut totale 5 une profondeur maximale de 1.2 km.

(2) Dans la Bande du Nord, la diagen~se des rEcifs pinacles du West Point dEvonien refl~te un enfouissement pro- gressif. La porositE primaire des calcaires rEcifaux ne fut pas dEtruite compl~tement tant que les rEcifs ne furent pas enfouis profondEment, dans certains cas, jusqu'h 6 km. Par consequent, toute migration d'hydrocarbures avant cette perte de porositE peut avoir crEE des reservoirs. De plus, la presence d'hydrocarbures gazeux dans des veines d'affinitE acadienne atteste d'une source d'hydrocarbures darts la region.

Traduit par les auteurs.

INTRODUCTION

The West Point reef limestone forms a nearly 1000-kin long reef tract that developed during the Late Silurian-earliest Devonian along the margin of Laurentia. These are among the most promising potential reservoir rocks of the Silurian-Devonian GaspE Belt. However, the limestone shows little porosity in outcrop. Because the hydrocarbon charge of a reservoir rock is related to the presence of sufficient porosity at the time of hydrocarbon migration, it is necessary to understand the rock diagenesis and porosity history before making conclusions about any reservoir potential. This paper attempts to reconstruct the porosity evolution and evaluates the reservoir potential of the Upper Silurian-Lower Devonian West Point limestone in the eastern part of the GaspE Belt (Fig. 1).

In order to achieve this goal, we proceeded with a diagenetic analysis of the limestone by first establishing a cement stratigraphy under transmitted light and cathodoluminescence (CL) microscopy, and subsequently sampling each distinct cement phase for carbon (C) and oxygen (O) stable isotope content, in order to identify the diagenetic conditions under which each phase was precipitated. Fluid inclusion analyses were car- tied out in an effort to constrain the interpretation of the isotope geochemistry results.

METHODS

Conventional petrographic thin sections were stained with Alizarin Red-S and potassium ferricyanide (Dickson, 1965) to determine carbonate mineralogy. Cement stratigraphy was established on thin sections polished with 1 mm-diamond paste for CL examination (Nuclide Corporation model EEM2E luminoscope). Operating conditions for CL were 10 kV, 0.5 mA, and a beam focused at 5 to 10 mm.

Over 200 microsamples (< 10 mg) were taken for 5180 and 813C analysis from pore- and fracture-filling cements, calcitic brachiopod skeletons and crinoids, and carbonate muds. The material was micro-drilled (using 0.34 to 1.02 mm diameter bits) under the binocular microscope with a Jansen microsam- pler from polished cut-offs corresponding to thin sections studied under CL. The purity of individual samples was controlled through CL observation of the drilled zones. Contaminated samples were rejected. Stable isotope analyses were performed

at the Derry-Rust Laboratory, University of Ottawa (portion of the Gros Morbe and part of the Colline Daniel Member samples), and the Delta-Lab of the Geological Survey of Canada in QuEbec (remaining samples). At the Derry-Rust Laboratory, CO 2 liberated from the powder samples was analyzed with a VG-SIRA 12 mass spectrometer, whereas at the Delta-Lab, it was analyzed with a Prism-III mass spectrometer. The O and C isotopic ratios, corrected for 170, are expressed in the conventional notation and given in per mil (%0) relative to the Vienna NBS-19 standard (VPDB). Precision obtained for the C and O isotopic data was always better than + 0.05 and 0.06 %o, respectively, for the Delta-Lab analyses, and 0.1%o for both isotopes at the Derry-Rust Laboratory.

I 65"W

-Z

rce

' N -

C H A L E U R S BAY 0 30 km

I I I I 6 5 " W I

F i g . 1. Location map showing the two outcrop areas of the Silurian-lowermost Devonian Chaleurs Group (shaded) in which the diagenesis of the West Point Formation is studied: the Northern Outcrop Belt and the Chaleurs Bay Synclinorium, in eastern Gasp~. See Bourque et al. 2001, Fig. 2 (this issue) for a detailed map.

DIAGENES1S AND POROSITY EVOLUTION OF THE WEST POINT REEF LIMESTONE 301

Fluid inclusions showing clear relationships with crystal growth zones are rare. Nevertheless, we considered the following occurrences of fluid inclusions to be of primary or pseudo- secondary origin: isolated, clustered, and randomly distributed in three dimensions. Secondary fluid inclusions in microfrac- tures crosscutting crystal boundaries were not analyzed. Microthermometric measurements on fluid inclusions were car- fled out with a USGS heating/freezing stage made by Fluid Inc. The homogenization temperature (Th) and final ice-melting temperatures (Tm_ice) were measured with a precision of _+ 1 °C and _+ 0.2°C, respectively. The T h and Trn.ice data are reported for fluid-inclusion assemblages (Goldstein and Reynolds, 1994; Chi et al., 2000). Oil inclusions were checked with a fluores- cence lmicrosope under blue light excitation.

WEST POINT INTERNAL STRATIGRAPHY

The West Point Formation of the Gasp6 Belt is composed of two reefal limestone packages (Bourque et al., 2001, Figs. 3, 4, this issue): the Upper Silurian reef and bank complexes, which are well developed in the southern part of the Gasp6 Belt (Chaleurs Bay Synclinorium), but also locally present in the northeastern part of the Gasp6 Belt; and the Lower Devonian

pinnacle reefs mostly known from the Northern Outcrop Belt. In order to account for both the temporal and spatial relationships of the various facies of the West Point Formation, our discussion on the diagenetic history and porosity evolution of the West Point Formation is presented in this paper under two headings: 1) the Silurian West Point Limestone of the Chaleurs Bay Synclinorium; and 2) the Devonian West Point Limestone in the Northern Outcrop Belt.

SILURIAN WEST POINT LIMESTONE OF THE CHALEURS BAY SYNCLINORIUM

The Silurian West Point is particularly well exposed in the Chaleurs Bay Synclinorium, where it is composed of three superposed complexes (Fig. 2): a lower complex of deep water mounds and microbial-algal-bryozoan shallow water reefs; a middle complex of crinoidal sand and gravel banks; and an upper complex of stromatoporoid reef-rimmed platforms.

LOWER MOUND AND REEF COMPLEX

The lower mound and reef complex is composed of four members (Fig. 2): the Bouleaux Member, an early cemented, thin- to medium-bedded, mixed, fine-grained siliciclastic and

UPPER REEF COMPLEX

MIDDLE BANK - - COMPLEX

LOWER MOUND AND REEF COMPLEX

calarituba- osmoraphe Facies

oophycos Facies

\ 4 shallowing sequences

6 shallowing sequences

\ 2 shallowing- upward sequences

Fig. 2. Stratigraphy and sequence analysis of the West Point and other formations in the Chaleurs Bay Synclinorium. Stratigraphy from Bourque et al. (1986); sequence analysis from Bourque 2001, Fig. 4 (this issue).


limestone facies; the Gros Morbe Member, a crudely bedded, red stromatactis limestone facies; the Anse h la Barbe Member, a grey massive limestone facies; and the Anse h la Loutre Member, a debris facies laterally equivalent to both the Gros Morbe and the Anse h la Barbe members, and composed of limestone clasts enclosed in a fine-grained siliciclastic matrix. Of these four members, the diagenesis of only the two pure limestone members, the Gros Morbe and the Anse ~ la Barbe, was examined.

Gros Morbe M e m b e r

Depositional Facies

The lowermost part of the Gros Morbe Member (about 5 m) is composed of a rugose coral framestone, whereas the bulk of the member (up to 110 m) consists of a very slightly fossilifer- ous stromatactis limestone (Bourque and Gignac, 1983; Bourque et al., 1986; Bourque and Raymond, 1989). It is a homogeneous red lime mudstone, with ubiquitous stromatactis constituting from 10 to 30% by volume of the facies. The Gros Morbe Member has been interpreted as sponge-constructed, mud-rich mounds that formed below the photic zone and storm wave base, in a siliciclastic-dominated environment (Bourque and Gignac, 1983; Bourque et al., 1986; Bourque and Boulvain, 1993).

Diagenesis

The microfacies of this red mudstone typically consists of agglomerates of pellet-like bodies (pelletoids), which form an irregular pelletoidal network pattern surrounded by homogeneous lime mudstone (Bourque and Gignac, 1983, Figs. 6 to 9). Pelletoids of the network are either closely packed or cement-

supported. Sponge spicules are ubiquitous, occurring largely in the pelletoidal network, but no sponges identifiable by gross morphology were observed in the mound facies.

Bourque and Gignac (1983, 1986) have proposed that a significant portion of the red stromatactis limestone of the Gros Morbe Member originated from the massive calcification during microbial decay of a sponge network, and that stromatactis (interconnected spar bodies with flat floors and digitate tops) represent the cementation of a cavity network. The network permitted sea water to circulate efficiently in the sediment, cen- timetres to a few metres below the sea floor during the earliest diagenetic stage, and resulted in a high 'primary' porosity system (over 30 to 40%).

The full spectrum of the Gros Morbe stromatactis limestone diagenesis was studied by Bourque and Raymond (1994) and is not re-examined here. Based on a cement stratigraphy established under CL and by C and O stable isotope signatures, they concluded that most of the stromatactis cavity system had been filled by numerous isopachous layers of early marine cement that were subsequently altered, possibly by meteoric fluids in the shallow burial environment at depths of a few hundreds of metres. The Gros Morbe Member stable isotope results confirm a meteoric alteration hypothesis.

Anse ~ la Barbe M e m b e r

Depositional Facies

The Gros Morbe Member mounds are capped by several tens of metres of grey lime mudstone to wackestone of the massive Anse h la Barbe Member limestone. The microfacies is an open pelletoidal spar and microspar network, generally structureless but locally vaguely laminoid, where pelletoids are closely

LIGHT-MICROSCOPE PETROGRAPHY

Cement Calcite type characteristics

Non- Inclusion- Microspar ferroan rich

Radiaxial and/or Non- Inclusion- fascicular optical ferroan rich

Non- Inclusion- ferroan bearing

Blocky neomorphic

Saddle dolomite

Inclusion- Ferroan poor

Non- Inclusion- ferroan free

Non- Inclusion- ferroan free

Cement number

1

2a

2b

3

4a

4b

5

6

7

CATHODOLUMINESCENCE PETROGRAPHY DIAGENETIC

ENVIRONMENT Cement Luminescence shape

Non-lum,/dul l l Microspar blotchy

Fibrous to bladed Non-lum. Marine isopachous crusts

Splayed Non- lum.

Stubby to bladed Non-lum. Shallow burial

Yellow bright Thin band on cement 3 crystals

Equigranular Orange bright

Brownish dull Equigranular xenomorphic

Meteoric

Non-lum. to dark red lum,

Yellow to Xenomorphic orange bright ?

Automorphic I ?

Fig. 3. Summary of the cement phases and petrography for the Anse & la Barbe Member, West Point Formation.

DIAGENESIS AND POROSITY EVOLUTION OF THE WEST POINT REEF LIMESTONE 303

Fig. 4. Thin-section photomicrographs illustrating cement phases. (A) Plane-polarized light. (B) Under CL, showing typical isopachous crust of cement 2a, stubby to bladed, non-luminescent cement 3, bright-luminescent equigranular cement 4b, and dull-luminescent equigranular cement 5. Scale bar = 500 pm. Anse& la Barbe Member, Anse a la Barbe quarry, Gascons Village, Chaleurs Bay. Sample AB2-58.

Fig. 5. Thin-section photomicrograph of the cement succession inside a brachiopod shell. (A) Plane-polarized light. (B) Under CL. Same notation as for Figure 3. br = brachiopod shell, 7 = saddle dolomite. Note extensive alteration of cement 4b (alt) by cement 5. Scale bar = 500 pm. Arise

la Barbe Member, Anse& la Barbe quarry, Gascons Village, Chaleurs Bay. AB2-122A.


packed or 'cement-supported' (Bourque et al., 1986, Fig. 20). Cavities of the network are infiltrated with lime mud and/or isopachous fibrous calcite crusts, and blocky calcite cement. Larger cavities (centimetre- to metre-sized) are filled with skeletal grainstone-packstone, and their walls are locally lined by botryoidal calcite cement crusts. The Anse h la Barbe has been interpreted as microbially dominated reefs that developed on top of the Gros Morbe mounds as the mounds grew into the shallow, well agitated photic zone during the late Ludlovian shallowing-upward phase (Bourque, 2001, this issue) (Fig. 2).

Cement Stratigraphy and Chronology o f Diagenetic Events

The dominant component of the Anse ~ la Barbe Member is a pelletoidal spar and microspar network. There is no unequivocal interpretation about the origin of this pelletoidal network, but most hypotheses imply early cementation processes (Monty, 1976, 1995; Bourque and Raymond, 1989; Reitner, 1993; several papers in Reitner and Neuweiler, 1995; Neuweiler et al., 2000).

Petrographic study of the diagenesis of these rocks was based on 170 standard thin sections and 30 polished thin sections taken from the cores of 10 drillholes, drilled throughout the Anse ~t la Barbe and Gros Morbe members at the Anse ~ la Barbe quarry (Lachambre, 1987). Six cement phases were recognized, based on light and CL microscopy, in the Anse ~ la Barbe Member (numbered 1 to 6 in Fig. 3). Phase 1 is a non- ferroan, inclusion-rich calcite spar coating the pelletoids and forming crusts up to several millimetres thick, locally developed as large botryoids (Bourque et al., 1986, Fig. 21) on the pelletoidal network. Under CL, these crusts are seen as a microspar that has a variable luminescence, from non-, to dull- to blotchy luminescence. The crusts have the same attributes

under CL as the spar and microspar surrounding the pelletoids. Cement phase 2 is a non-ferroan, inclusion-rich, radiaxial or fascicular optic calcite. Under CL, most of it is isopachous, non-luminescent, fibrous to bladed (2a in Figs. 3 to 5), whereas in places it is represented by coeval non-luminescent splayed cement (2b, Fig. 3).

Cement phases 3 to 6 are neomorphic blocky calcite. Cement 3 is non-ferrroan with fewer inclusions than in cements 1 and 2 (Fig. 4A). Under CL, it is a distinctive, relatively thin crust of non-luminescent stubby to bladed crystals (Figs. 4, 5). Cement 4 is an inclusion-poor ferroan calcite (Figs. 4, 5) that can be divided into two zones under CL (4a and 4b, Figs. 3 to 5). Cement 4a is a thin, yellow, bright-luminescent band that lines crystal faces of cement 3, whereas cement 4b is made up of orange, bright-luminescent equigranular crystals (Figs. 3, 4). Cements 5 and 6 are non-ferroan, inclusion-free calcite, equigranular xenomorphic, with brownish dull and yellow to orange, bright luminescence, respectively (Fig. 5).

Other diagenetic events occurred during the cementation history of the Anse 5 la Barbe limestone (Figs. 6, 7). Apart from aragonite leaching that followed cement phase 2, two distinct phases of dissolution were identified. The first (Soll, Fig. 6), which is not extensively developed, occurred between cementation phases 2 and 3 and is expressed by local smoothing of cement 2 crusts and growth of cement 3 non-luminescent crystals on smoothed surfaces. The second, better developed, dissolution event (So12, Fig. 6) followed cementation phase 3 and was concurrent with cementation phases 4 and 5. It is seen under CL either as veinlets composed of cements 4 and 5 filling conduits derived from dissolution-enlarged fractures that cut across the pelletoidal network and cements 1 to 3 (Fig. 8), or as extensive dissolution-replacement of non-luminescent cement crusts 2a by orange, bright-luminescent cement 4b (Fig.

Diagenetic Environments and Time

M A R I N E

C1 C 2 a - C 2 b

IS1

I

I I I

F r e s h w a t e r I flushing ~t

r - I I I I

Aragonite leaching S o l l

C3

S H A L L O W B U R I A L

Fresh w a t e r f l u s h i n g I F -

D E E P E R B U R I A L (? )

IS2 S o l 2

C 4 a C 4 b

C5 m

C6 " ~ " S t y l .

~ m m m

S a d . d o l .

Fig. 6. Diagenetic history of the Anse & ia Barbe Member, West Point Formation, in the Chaleurs Bay Synclinorium. Cementation history was affected by two main phases of dissolution. C = cementation, IS = internal sedimentation, Sol = dissolution, Styl = stylolitization, Sad,dol. = saddle dolomitization.


. . . . .

6

4 a

Fig. 7. Sketch illustrating temporal and spatial relationships of the various cementation and dissolution phases in the Anse & la Barbe Member, West Point Formation. Cement notation as for Figure 3. ssl = solution surface related to dissolution phase 1 (Sol1 in Fig. 6), sc2 and sc3 = solution conduits related to dissolution phase 2 (Sol2 in Fig. 5).

Fig. 8. Sketch illustrating temporal and spatial relationships of the various cementation and dissolution phases in the Arise & la Barbe Member, West Point Formation. Cement notation as for Figure 3. ssl = solution surface related to dissolution phase 1 (Sol1 in Fig. 6), sc2 and sc3 = solution conduits related to dissolution phase 2 (Sol2 in Fig. 5).

Fig. 9. Thin-section photomicrograph illustrating extensive replacement of non-luminescent cement 2a by cement 4b during dissolution event Sol2 (Fig. 6), post-dating stubby to bladed non-luminescent cement 3. Outer fringe of cement 2a and crystals of cement 3 resisted dissolution and replacement more than cement 2a. Note dissolution of pelletoidal network and cementation by cement 4b (upper left of photomicrograph), pn = pelletoidal network. Scale bar = 500 pm. Anse& la Barbe Member, Anse & la Barbe quarry, Gascons Village, Chaleurs Bay. Sample AB2-22.


9). Dull-luminescent spots and patches of orange, bright-luminescent cement 4b (Figs. 4, 5), likely resulting from partial replacement by cement 5 during dissolution, also testify to dissolution before and during cementation phase 5.

Stylolitization post-dated all cementation phases (Fig. 6). The last diagenetic event was the development of rare replacement saddle dolomite crystals in cements 4 to 6.

Isotope Geochemistry and Diagenetic Environments

Cements 1 to 5, and internal sediment found as geopetal filling in brachiopod shells or intercalated between cements 1 and 2, and 2 and 3, were sampled and analyzed for C and O stable isotope content (Fig. 10, Table 1). Taken together, cements 1, 2 and 3, and internal sediment isotope results are grouped in a field (5180 = -5.83 to -3.02 %°; 813C = +5.16 to +9.38 %e) com- patible with the field of Ludfordian (upper Ludlovian) marine calcites (Wenzel and Joachimski, 1996; Samtleben et al., 1996; Bickert et al., 1997; Azmy et al., 1998; Veizer et al., 1999; see discussion below) and, therefore, interpreted as early marine cement and sediment. Grouped together, blocky cements 4 and 5 occupy a field with a wider and lower range of 8180 values (= -11.00 %o to -6.45 %0). At one end of the trend, the 813C values form a very narrow range (+7.93 to +8.73 %0) coinciding with the highest range of 8180 values. At the other end, the widest change in 813C values occurs for samples that have 8180 values of about -10 %o (between -10.26 and -8.71%0). This trend sits on the meteoric calcite line for eastern Gaspr, as established using the meteoric water cement from the karstified White Head Formation (Kirkwood et al., 2001, this issue). The isotopic trend for the Anse-h-la-Barbe Member corresponds to an increasing water-to-rock ratio (arrow in Fig. 10B) and is typical of freshwater-marine limestone interaction (Land, 1986; Lohmann, 1988). The shape of the trend is inherited from the relative proportions of C and O in the freshwater system (low and high, respectively) to the reacting carbonates (high and low, respectively). This interpretation is supported by the observed dissolution features associated with cementation phases 4 and 5.

MIDDLE BANK COMPLEX

The middle bank complex of the West Point Formation is relatively thin (about 30 m) compared with the underlying and overlying complexes (Fig. 2). It is composed of two members: the Anse Mclnnis Member, a platformal crinoidal bank facies, and the Cap de l'Enfer Member, a forebank debris facies (Bourque et al., 1986).

Depositional Facies The Anse Mclnnis Member consists of a series of six super-

imposed biostromal units of crinoidal limestone, each a few metres thick (Gosselin, 1981; Bourque et al., 1986, Fig. 27), alternating with thinner siliciclastic units. The entire member was built in response to six transgressive-regressive cycles interpreted as the response to local tectonism (Bourque et al., 1986; Bourque, 2001, this issue). It is likely that the limestone

and the siliciclastic units were at times subaerially exposed, allowing freshwater recharge not only in the Anse Mclnnis limestone, but also in the underlying limestone strata (Anse h la Barbe and Gros Morbe members) (see below).

The Cap de l 'Enfer Member represents resedimented deposits at the margin of the Anse McInnis platformal bank complex. The presence of large cemented fragments of crinoidal beds identical to those of the Anse Maclnnis Member biostromes in the debris flows (see Bourque et al., 1986, Figs. 29, 30) testifies to very early cementation of the bank margin.

Cement Stratigraphy and Chronology of Diagenetic Events

The diagenetic study was conducted on limestone of only the Anse Mclnnis Member. Light and CL microscopy were used to establish a pore cement succession and other diagenetic features on 75 large (5 x 7.5 mm) stained thin sections and 30 polished standard (2.5 x 4.5 ram) thin sections. Five cement phases were recognized (numbered 1 to 5 in Fig. 11). Under plane- polarized light, all cements appear inclusion-poor to inclusion- free. The limestone is commonly crosscut by solution conduits filled by clear blocky neomorphic calcite (Fig. 12). Figure 13 depicts the temporal and spatial relationships of the various diagenetic events. Cement phases 1 and 2 correspond to a non-ferroan, inclusion-poor, syntaxial spar cement developed on the crinoid bioclasts. Under CL, cement 1 is non-luminescent, and cement 2 is a thin, yellow, bright-luminescent band lining crrys- tal faces of cement 1 (Fig. 14). Cements 1 and 2 are volumetrically unimportant. Cement 3 is inclusion-free calcite. It appears under CL either as large, stubby, orange, dull-luminescent crystals when developed in dissolution conduits (Fig. 15), or as xenomorphic crystals when developed in, and filling the interskeletal pore space (Fig. 14). Cement 4 is an inclusion-free calcite, seen under CL as large, stubby, non-luminescent crystals with distinctive thin, yellow, bright-luminescent bands (Fig. 15). They grew exclusively in dissolution conduits, on cement 3 crystals. Cement 5 is similar to cement 4 with distinctive, thin, bright-luminescent bands, except it has an orange to brownish, dull luminescence.

Dissolution features are obvious in the Anse Mclnnis Member limestone. They are represented by dissolution conduits and cavities, and by patchy dissolution of cements 1 and 2 (Figs. 14, 15). Dissolution occurred after cementation phase 2 and before cementation phase 3, when the lime sand was poorly cemented and primary porosity was high.

Isotope Geochemistry and Diagenetic Environments

Our microsampling for C and O stable isotope analyses is mostly from cements 4 and 5 (Fig. 10). Only one sample of a mixture of cements 1 and 2 could be obtained, and no sample from cement 3. The results for the blocky cements show a large variation in 8180 values (from -11.33 to -5.95 %o). The 813C values are mostly invariable for the highest 5180, but show the most important changes when the 5180 values are about -10 %o. As for the Anse h la Barbe Member limestone, results obtained


I C. A n s e M c l n n i s M e m b e r I

(~180%0pDB • • ~ ~ - - - - 9 ) - - ~ , . , . , . . , , / . , . J , .............. ,:, , ,

-16 -14 -12 ,-Io -8 ...6 P -4 -2 I I zi I A

6130%OpDB

- 2

- -2

L.4

O Crinoids

13 Syntaxial non-luminescent cement (AM-C1 & C2)

• Blocky banded dull- luminescent cement (AM-C4)

A Coarser blocky banded lighter dull-luminescent cement (AM-C5)

- - water/rock ratio

I B. A n s e - 6 - 1 a - B a r b e M e m b e r I

, o -D .-o- I

&& &l I I &

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8 1 8 0 % 0 p D B

I , I , I , I , i , i , i ~ i , i

-16 -14 -12 -10 -8

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• .6 P -4 -2

8130%OpDB

- 1 0

- 8

- 6

- 4

- 2

O Brachiopod

V Microspar crust (AB-CI)

+ Fibrous/lamellar isopachous/splayed non- luminescent cement (AB-C2)

<> Granular non-luminescent cement (AB-C3)

a Internal sediment

z~ Blocky bright-luminescent cement (AB-C4)

a Xenomorphic dull- luminescent cement (AB-C5)

• ~ - - water/rock ratio

I A . G r o s M o r b e M e m b e r I

A o D &

0

81800/00PDB

I ' I ' I ' I " I = I ' I

-16 -14 -12 -10

0 O ÷

° 1 ' 1

-8

L .i!"

÷ ÷ll,.xj t.,mll,~

÷

G ' I ' ' ~ ' I ' I ' I " I '

-6 -4 -2

~ 1 3 0 % 0 p D B

-4

-2

0

- - 2

'--4

! Infiltrated mud

x Pelletoidal matrix

O Crinoids

• Bladed non-luminescent cement (unaltered) (GM-C1)

+ Fibrous-like dull-luminescent cement (altered) (GM-C2)

o Blocky dull- luminescent cement (GM-C3)

& Vein blocky dull-luminescent cement (GM-C4)

Fig. 10. Crossplot of 1~18OvPDB and 513C values for cements of the lower mound and reef, and the middle bank complex limestones of the West Point Formation in Chaleurs Bay Synclinorium. (A) Gros Morbe Member, including results from Bourque and Raymond (1994) and new results. (B) Anse ~. la Barbe Member. (C) Anse Mclnnis Member. Shaded boxes indicate marine calcite fields: G = late Gorstian (mid-Ludlovian), L = Ludfordian (late Ludlovian), P = Pridolian; all based on brachiopod shell analyses (Wenzel and Joachimski, 1996; Samtleben et al., 1996; Bickert et al., 1997; Azrny et aL, 1998; Veizer et al., 1999). Note marked increase of ~13C values during Ludfordian time• See Table 1 for values.

308 P.-A. BOURQUE, M.M. SA VARD, G. CHI and P. DANSEREAU

Table 1. Oxygen and carbon stable isotope data for the Silurian West Point Formation, Chaleurs Bay Syncl inorium.

Cementtype I Sample I 613C I 61sO ] I Cementtype Sample I ~13C I 6180 I

GROS MORBE MEMBER

Early marine cements-unaltered (GM-Cl) Bladed, non-lum. AB4-296 2.4 -4.9 Bladed, non-lure. AB3-291 2.5 -5.1 Bladed, non-lurn. AB9-220 2.39 -5.18 Bladed, non-lum. 86GM-11 2.34 -5.66 Bladed, non-lure. 76HG-140 2.69 -5.30 Bladed, non-lure. AB4-296 2.30 -8.83

Mean 2.44 -5.33 Standard deviation 0.14 0,35

Early marine cements-altered (GM-C2) Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, lum. Fibrous dull/compos, tum. Fibrous dull/compos, lum.

AB7-26B 2.5 -7.5 83GM-2 2.4 -6.4 74L-23-3A 1.3 -7.0 74L-23-3A 1.8 -6.9 74L-23-3A 2.0 -6.8 74L-23-3A 2.3 -6.2 74L-23-3A 2.2 -6.4 AB3-291 2.3 -6.2 AB3-291 2.6 -8.2 AB3-291 3.6 -6.3 AB9-220 3.40 -6.38 86GM-11 3.12 -6.86 76HG-140 2.68 -6.34 AB4-246 2.35 -6.13

Mean 2.47 -6.69 Standard deviation 0,61 0.59

Pelletoidal matrix Microspar, dull-lum. AB4-296 2.4 -5.7 Microspar, non-lure 86GM-11 2.45 -5.39 Microspar, non-lum. 76HG-140 2.50 -5.64 Microspar, dull-lum. AB9-220 2.86 -6.11 Microspar, non-lum. 89GM-1 2.93 -5.67


Infiltrated mud Microspar, dull-lum. AB7-26B 2.5 -5.4 Microspar, dull-lum. 83GM-2 2.8 -6.1 Microspar, dull-lum 74L-23-3A 2.6 -5.8 Microspar, non-lum. 74L-23-3A 2.3 -6.1 Microspar, non-lum. 74L-23-3A 2.4 -6.0

Mean 2.5 -5.9 Standard deviation 0.2 0.3

Meteoric cements (GM-C3) Blocky, dull-lum. 86GM-11 2.58 -13.28 Blocky, dull-lure. JM 2.66 -12.75 Blocky, dull-lure. AB9-227 2.81 -8.64 Blocky, dull-lure. 74L-23-3A 1.88 -10.26 Blocky, dull-lure. AB5-223 2.78 -9.39


Vein cements (GM-C4) Blocky, dull-lum. 86GM-3 2.06 -10.83 Blocky, dull-lure. 76HG-83 2.50 -15.48 Crinoid grain, non-lure. 2.09 -7.14 Crinoid grain, bright-lure. 2.49 -7.38

ANSE A LA BARBE MEMBER

Marine cements Cement AB-C1 Micritic crust, non-lum. AB1-53 9.23 -3.02 Micritic crust, comp.-lum. AB6-82 8.28 -4.74 Micritic crust, comp.-tum. AB9-125 7.08 -4.33 Micdtic crust, comp.-lum. AB6-82 7.89 -4.70 Cement AB-C2

Fibrous/lamellar, non-lum. AB1-22 7.92 -4.85 Fibrous/lamellar, non-lum. AB4-198 9.38 -3.54 Fibrousllamellar, non-lum. AB9-125A 6.68 -5.17 Fibrous/lamellar, non-lum. 6.70 -5.10


Marine to shallow burial cement Cement AB-C3 Granular, non-lum. AB4-19B 6.09 -5.37 Granular, non-lum. AB4-32A 7.47 -5.35 Granular, non-lum. AB4-32 9.08 -3.33


Infiltrated mud Red mud, composite-lum. AB4-19 5.46 -5.83 Red mud, composite-lure. AB4-19B 5.16 -4.80 Red mud, composite-lum. AB4-19B 5.47 -5.07

Meteoric cements Cement AB-C4 Blocky, banded bdght-lum. AB1-22 7.80 .-8.36 Blocky, banded bright-lum. AB1-53A 8.16 -11.00 Blocky, banded bright-lum. AB1-53B 8.46 -7.00 Blocky, banded bright-lure. AB1-118 7.24 -10.44 Blocky, banded bdght-lum. AB2-22 7.82 -8.63 Blocky, banded bright-lum. AB2-122 7.73 -9.23 Blocky, banded bright-lum. AB5-138 6.92 -9.95 Blocky, banded bdght-lum. AB6-82 6.99 -10.74 Blocky, banded bright-lum. AB9-125A 5.81 -8.71 Blocky, banded bdght-lum. AB9-125A 5.55 -10.26


Cement AB-C5 Xenomorphic, dull-lure. AB1-53A 8.31 -10.81 Xenomorphic, dull-lure. AB3-53B 8.73 -6.45 Xenomorphic, dull-lure. AB1-82B 8.25 -10.07 Xenomorphic, dull-lum. AB2-22 7.93 -7.18 Xenomorphic, dull-lure. AB6-82 8.35 -10.02 Xenomorphic, dull-lum. AB1-53 8.17 -8.94


Brachiopod shell, non-lum. AB2-122 7.51 -3.18

ANSE MCINNIS MEMBER

Crinoid grain, comp.-lum. 93JA-1.23 Crinoid grain, comp.-lum. 93JA-2.2 Crinoid grain, comp.-lum. 93JA-2.2

Marine (?) cement Syntaxial, non-lum,

Meteoric cements Cement AM-C4 Blocky, banded non-lum. 93JA-1.26 Blocky, banded non-lum. 93JA-1.23 Blocky, banded non-lum. 93JA-1.23 Blocky, banded non-lum. 93JA-1.23 Blocky, banded non-lum. 93JA-1.20 Blocky, banded non-lum. 93JA-1.23 Blocky, banded non-lum.

Cement AM-C5 Coarser blocky, banded dull-lum.

Mean Standard deviation

93JA-1.16

0.78 -6,95 1.07 -7,15 1.07 -5.84

1.07 -5,84

0.56 -11.33 0.71 -11.08 1.12 -5.95

-0.67 -10.75 0.03 -10.57 0.66 -10.46 0.84 -7.14 0.46 -9.61 0.60 2.14

0.78 -10.26


Table 1. continued

[ Cemen, pe I samp,e I "cl 'oI COLLINE DANIEL MEMBER

Early marine cements (cements CD-C1 to C5) Fibrous, non-lum. CD2-351 1A2 -4.64 Palisade-bladed, non-lum. CD3-55.5 3.83 -3.08 Fibrous, non-lurn. CD3-216.3 1.33 -4.94 Fibrous, non-lum. CD3-323.5 1.37 -4.92 Palisade-bladed, non-lum. CD14-286.2 2.15 -5.19 Fibrous, non-lure. CD14-397.6 1.42 -5.37 Fibrous, non-lum. CD15-449 1.60 -5.22 Fibrous, non-lum. CD15-476.9 1.61 -5.18 Fibrous, non-lure. CD17-348 1.24 -6.33 Fibrous, non-lum. CD17-366.4 1.98 -5.13 Fibrous, non-lum. CD4-98.2 1.90 -4.95 Microcrystalline, dull-lum. 85MS-56.1-1a 1.5 -6.2 Microcrystalline, dull-lum. 85MS-56.1-1b 1.9 -5.7 Splayed, non-lum. 85MS-56.1-1b 1.9 -5.6 Microspar 85MS°21-X-3 2.3 -5.9 Microcrystalline, dull-lum. 87-OT-1-1 2.1 -5.2 Splayed, non-lure. 87°OT-1-2 1.9 -5.5 Splayed, non-lure. 85MS-56.1-2a 1.9 -6.0 Bladed, non-lure. 87-OT-1-3 -0.1 -7.0 Fibrous, non-lum, 85MS-27-10-4 1.8 -6.8 Granular, non-lure. CD2-174 1.6 -5.8 Granular, non-lure. CD15-290.1 1.5 -6.1


Infiltrated mud Geopetal mud CD2-345.6 1.54 -4.68 Geopetal red mud CD3-216.3 1.44 -5.08 Geopetal mud CD4-98.2 1.91 -4.90 Geopetal red mud CD15-162.4 0.62 -7.21

Burial cements Cement CD.C6 Equigran., bdght/dull-lum. CD3-9 1.27 -5.64 Equigran., bdght/dull-lum. CD14-3853 1.33 -7.45 Equigran., bdght/dull-lum. CD14-391.4 1.94 -9.54 Equigran., bdght/dull-lum. CD15-209.4 1.56 -8.37 Equigran., bright/dull-lure. CD15-290.1 1.36 -9.25 Equigran., bdght/dull-lum. CD15-449 1.38 -9.08 Equigran., bdght/dull-lum. CD15-476.9 1.57 -8.18 Equigran., bdght/dull-lum. CD15-526.1 1.57 -7.80 Equigran., bdght/dull-lum. CD17-314 1.36 -7.98


Cement CD-C7 Xenomorphie, dull-lure. CD15-282.8 1.47 -7.70 Xenomorphie, dull-lum. CD2-17.2 0.77 -6.91 Xenomorphic, dull-lum. CD3-106.9 1.31 -10.26 Xenomorphic, dull-lum. CD15o209.4 1.19 -9.90 Xenomorphic, dull-lum CD15-290.1 1.21 -10.25 Xenomorphic, dutl-lum. CD17-132 1.25 -10.18 Xenomorphic, dull-lum. 85MS-21-X-7 2.1 -9.3 Xenomorphic, dull-lum. 85MS-21-X-7 2.0 -10.1 Xenomorphic, dull-lum. 85MS-27-10 1.9 -10.3

Mean ' 1.46 .9.43 Standard deviation 0.43 1.26

for cements 4 and 5 that fill dissolution conduits and cavities follow the inverted 'J'-trend, typical of freshwater-marine limestone interaction (Land, 1986; Lohmann, 1988).

UPPER REEF COMPLEX

Depositional Facies The upper reef complex of the Silurian West Point

Formation is the thickest of the three complexes, attaining thicknesses of nearly 600 m. Its internal stratigraphy, facies

mosaic, and depositional environments and dynamics are the subject of previous papers (Bourque and Lachambre, 1980; Bourque et al., 1986; Bourque and Amyot, 1989). The complex is divided into four laterally equivalent members (Fig. 2). Our diagenetic analysis has been restricted to the Colline Daniel Member, which is composed of an assemblage of boundstone, grainstone, rudstone, and mudstone, forming the reef margin facies. The member developed in four distinct shallowing- upward phases (Fig. 2).

Cement Stratigraphy, Isotope Geochemistry and Diagenetic Environments

Diagenetic history and porosity evolution of the Colline Daniel Member were studied by Savard and Bourque (1989). Additional work was carried out to further examine the marine- to-burial cementation history of the rocks. Eight new cores (Lachambre, 1987) representing the four phases of the Colline Daniel Member were systematically sampled for study under light and CL microscopy. Cement stratigraphy was not significantly different from that of Savard and Bourque (1989), and is not repeated here. Cement notation is as in Savard and Bourque (1989, Table 2) (Table 1). j

Thirty-two microsamples of cements were analyzed for C and O stable isotope content, and results were combined with the 12 results of Savard and Bourque (1989) (Fig. 16; Table 1). All the values of 513C, except for two samples, are about +1.5%o (between +0.62 and +2.15%o). The 8180 values obtained for the early cements, averaging -5.49%o, are typical of marine conditions. In contrast 8180 values for the younger blocky and xenomorphic cements vary between -10.26 and -5.64%0. The fractionation factor of the oxygen isotopes is much more sensi- tive to temperature change than that for carbon isotopes; hence, a significant decrease of 8180 can occur in calcite as the temperature increases, without changing the composition of the parent water. Therefore, the Colline Daniel isotopic trend showing a decrease in 8180 for invariable values of 813C is typical of marine-like water undergoing a progressive temperature increase as the sedimentary sequence is buried. The new set of points supports the previous interpretation of Savard and Bourque (1989). Temperature calculation (Savard and Bourque, 1989) based on the lowest 8180 value indicates that the latest cement 7 precipitated at a maximum burial depth of 1200 m.

Other diagenetic events occurred during the cementation history of the Colline Daniel Member. Our new results better constrain a fracturing event that Savard and Bourque (1989, Fig. 17) placed following cementation phase 6 and before cementation phase 7. Locally, however, cement 6 fills fractures, imply- ing that fracturing started earlier, and more likely occurred throughout burial cementation phase 6. Stylolitization and sul- fatization occurred during late burial cementation phase 7, and volumetrically insignificant post-tectonic saddle dolomite locally replaced cement 7.

310 P.-A. BOURQUE, M.M. SAVARD, G. CH1 and P. DANSEREAU

FLUID INCLUSION MICROTHERMOMETRY

In order to better constrain interpretations of diagenetic environments, based on stable isotope geochemistry, six samples from the West Point limestone of the Chaleurs Bay Synclinofium were studied for fluid inclusions (Fig. 17, Table 2). The fluid inclusions studied were mainly in the latest pore- filling cements in individual samples (GM-C3, AB-C5, AM- C5, and CD-C6, Fig. 17), although a few inclusions from earlier phases were studied (AB-C4 and AM-C3). They have overlapping homogenization temperature domains, mainly from 50 to 100°C (Fig. 17). Some inclusions from AB-C5 contain only liquid at room temperature, suggesting homogenization temperatures lower than 50°C.

Fluid salinity values vary considerably between samples and between cements, as reflected in the range of ice-melting temperatures (Table 2, Fig. 18). Fluid inclusions from Gros Morbe Member sample GM-C3 are characterized by fairly low salinity values (0.2 to 1.9 wt % NaCl-equivalent). Fluid inclusions from Anse ~ la Barbe Member sample AB-C4 show salinity values from 1.0 to 7.4 wt %, whereas those from sample AB-C5 have salinity values from 0.2 to 13.5 wt % NaCl-equivalent. The Anse Mclnnis Member sample AM-C3 is characterized by consistently high salinity values (21.6 to 23.0 wt % NaC1 equivalent), whereas sample AM-C5 has low salinity (0.0 wt %, with one outlier of 5.5 wt % NaCl-equivalent). Colline Daniel Member cement 6 is characterized by relatively high salinity in

Table 2. Fluid-inclusion microthermometric data of samples from West Point Formation, Chaleurs Bay Synclinorium.

Sample Host Occurrence Size T~ ~(°C) Salinity (wt % NaCl-equiv.) Th (°C) mineral (mm) Range Mean (n) Range Mean (n) Range Mean (n)

GROS MORBE MEMBER GM-JM-65B GM-C3 Isolated 16 -0.3 -0.3 (1) 0.5 0.5 (1) 73.5 73.5 (1)

Isolated 10 -0.9 -0.9 (1) 1.6 1.6 (1) 78.5 78,5 (1) Isolated 6 -1.1 -1.1 (1) 1.9 1.9 (1) 76.3 76,3 (1) Isolated 6 -0.1 -0.1 (1) 0.2 0.2 (1) 70.9 70.9 (1) Isolated 7 -0.2 -0.2 (1) 0,4 0.4 (1) 92.8 92.8 (1)

ANSE ~. LA BARBE MEMBER AB4-119 AB-C4

AB-C5

AB3-23 AB-C5

ANSE MCINNiS MEMBER SB-10 AM-C3

Randomly distributed 6 ~ 12 -3.8 ~ -5.5 -4.7 (2) 6.1 ~ 8.6 7.4 (2) 59.0 ~ 69.0 62.9 (3) Randomly distributed 8 - 8 -0.6 -0.6 (1) 1,0 1.0 (1) 61.3 ~ 73.4 67.4 (2) Randomly distnbuted 7 55.9 55.9 (1)

Isolated 8 -5.9 -5.9 (1) 9.1 9.1 (1) 65.4 65.4 (1) Isolated 9 -0.2 -0.2 (1) 0.4 0.4 (1) 69.0 69.0 (1) Isolated 11 -0.1 -0.1 (1) 0,2 0.2 (1) 74.4 74.4 (1) Isolated 12 -0.1 -0.1 (1) 0.2 0.2 (1) 85.3 85,3 (1) Isolated 12 -6.9 -6.9 (1) 10.4 10.4 (1) 45.5 45.5 (1) Isolated 12 -6.9 -6.9 (1) 10.4 10.4 (1) all-liquid all-liquid Isolated 7 -9.5 -9.5 (1) 13.5 13.5 (1) 64.4 64.4 (1) Cluster 5 ~ 6 46.5 ~ 46.9 46.7 (2) Cluster 5 ~ 14 -3.4 -3.4 (1) 5.5 5.5 (1) 58.0 ~ 63.3 60.7 (2) Cluster 9 ~ 10 -7.4 -7.4 (1) 11.0 11.0 (1) 62,8 ~ 63.5 63.2 (2)

Isolated 9 -20.9 -20.9 (1) 23.0 23.0 (1) 65.1 65.1 (1) Isolated 7 -18.9 -18.9 (1) 21.6 21.6 (1) 72.5 72.5 (1) Isolated 9 74.1 74.1 (1) Isolated 10 -19.2 -19.2 (1) 21.8 21.8 (1) 80.3 80.3 (1)

AM-C5 Cluster 3 ~ 7 0.0 0.0 (1) 0,0 0.0 (1) 98.5 ~ 98.8 98.7 (2) Isolated 10 0.0 0.0 (1) 0.0 0.0 (1) 100.8 100.8 (1) Isolated 6 0.0 0.0 (1) 0.0 0.0 (1) 97.3 97.3 (1) Isolated 9 101.9 101.9 (1) Isolated 14 99.7 99.7 (1) Isolated 6 -3.4 -3.4 (1) 5.5 5.5 (1) 90.7 90.7 (1)

COLLINE DANIEL MEMBER CD1-534.4 CD-C6 Isolated 6 -7.1 -7.1 (1) 10.6 10.6 (1) 85.6 85.6 (1)

(Mb de Isolated 7 -7.7 -7.7 (1) 11.4 11.4 (1) 77.2 77.2 (1) Colline Daniel) Cluster 6 ~ 7 -8.4 ~ -9.1 -8,8 (2) 12.2 - 13.0 12.6 (2) 87.7 ~ 91.6 89.7 (2)

CD3-205.8 CD-C6 Isolated 3 76.6 76.6 (1) (Mb de Cluster 3 ~ 7 -0.2 -0,2 (1) 0.4 0.4 (1) 79.7 ~ 89.8 84.0 (3)

Colline Daniel) Isolated 5 80.5 80.5 (1) Isolated 5 -0.4 -0.4 (1) 0.7 0.7 I1) 91.2 91.2 (1)


one sample (CD1-534.4:10.6 to 12.6 wt % NaC1 equivalent), and by low salinity in the other sample (CD3-205.8:0.4 to 0.7 wt % NaC1 equivalent). The presence of fluid inclusions with salinity lower than sea water suggests the incorporation of meteoric water, and the range of salinity values probably reflects varying degrees of mixing with saline diagenetic fluids.

These results support our interpretation, based on 813C and 6180 data, of a mixed influence of freshwater and sea water for Anse McInnis, Anse ~ la Barbe and Gros Morbe Member cements. Salinity values for the Gros Morbe Member late cements are lower than those for sea water (-3.5 wt %), whereas those for late cements of the Anse ~ la Barbe Member are mixed: lower than for sea water in one sample, and higher in the other. Early cement 3 of the Anse Mclnnis Member features salinity values that are much higher than those for sea water (evaporation effect?), whereas later cement 5 has salinity values lower than those for sea water. The two values obtained for late pore-filling cement 6 of the Colline Daniel Member, which we interpret as burial cement, based on isotope data, are contradic- tory: one sample has higher salinity than that for sea water, in agreement with evolved marine water, the other has values lower than that for sea water--a situation not readily explained.

SUMMARY OF SILURIAN WEST POINT DIAGENESIS AND

POROSITY HISTORY IN THE CHALEURS BAY SYNCLINORIUM

The Gros Morbe, Anse ~ la Barbe and Anse McInnis members show evidence of early, freshwater diagenesis. The Gros Morbe to the Anse ~ la Barbe members represent progressive shallowing related to the late Ludlovian eustatic fall. The sequence begins with relatively deep-water facies, below the photic zone, and ends with shallow algal-rich facies (Bourque, 2001, this issue). The overlying Anse McInnis Member facies

developed in shallow water environments strongly influenced by local, high-order, sea-level changes, and was periodically subaerially exposed and flushed by fresh water.

The inverted 'J'-trend, similar for both the Anse h la Barbe and the Anse McInnis members, suggests they were both recharged by fresh water. The Anse h la Barbe cements have 813C values that are significantly higher than those of the underlying Gros Morbe and the overlying Anse McInnis and Colline Daniel members: a mean of +2.44%0 for the Gros Morbe Member; +7.90%o for the Anse ~ la Barbe Member; +l.07%o for the Anse McInnis Member; and + 1.74%o for the Colline Daniel Member. This change in carbon isotope ratios likely reflects the Ludfordian (late Ludlovian) shift in 613C recognized in several localities of the southern paleotropical seas (Wenzel and Joachimski, 1996; Samtleben et al., 1996; Bickert et al., 1997; Azmy et al., 1998; Veizer et al., 1999) (Fig. 19A). The marine 613C values of the Gros Morbe to the Colline Daniel members follow the 613C values predicted for their stratigraphic position, from the late Gorstian (late early Ludlovian) to the Pridolian, based on literature data (Fig. 19).

As for the Gros Morbe Member, Bourque and Raymond (1994) concluded that precipitation of the blocky, dull-luminescent cement was likely influenced by freshwater-derived fluids at a burial depth of about 350 m, based on a calculation using the equation of Friedman and O'Neil (1977). This agrees with a freshwater recharge related to subaerial exposure during the sedimentation of the Anse McInnis Member; a recharge that affected both the underlying Anse ?a la Barbe limestone in a very shallow burial environment, and the Gros Morbe limestone, in a deeper burial environment. This situation is also in accordance with the interpretation (Bourque, 2001, this issue) that Salinic subaerial erosion occurred sometime between depo-


Cement type

Syntaxial spar

Blocky neomorphic

Calc i te charac te r is t i cs

Inclusion- Non- LP oor f e r r oan

Inclusion- poor

Inclusion- free

Inclusion- Ferroan free

Inclusion- free

Cement number

2

CATHODOLUMINESCENCE PETROGRAPHY

Cement shape

Syntaxial (?) spar on crinoids

Thin band on cement 1 crystals

Large stubby crystals

Large stubby crystals

Equigranular xenomorphic spar

L u m i n e s c e n c e

Non-lum.

Yellow bright.

Orange dull

Non-lum., with thin bright bands

Orange to brownish dull, with thin bright bands

DIAGENETIC ENVIRONMENT

Marine (?)

Marine (?)

Meteoric

Fig. 11. Summary of the cement phases and petrography for the Anse Mclnnis Member limestone, West Point Formation.

312 P.-A, BOURQUE, M.M. SAVARD, G. CHI and P. DANSEREAU

sition of the lower and the upper complexes of the West Point Formation in the eastern Chaleurs Bay Synclinorium. This ero- sional event corresponds to the latest Ludlovian lowstand in the Gasp6 Belt; the lowstand is recognized globally (Bourque, 2001, this issue).

Primary porosity and secondary early dissolution porosity of the lower mound and reef, and the middle bank complexes of the Silurian West Point in the Chaleurs Bay Synclinorium were lost in the shallow burial environment (less than a few hundreds of metres). Moreover, as no significant subsequent

A

B Fig. 12. Scanned thin section photograph presenting an overview of the Anse Mclnnis microfacies, dissolution conduits (sc), and indicating mag-

nified areas shown in Figures 14 and 15. (A) Sample JA-1.57. (B) Sample JA-1.23. Both thin sections are from Road 132 section, Port-Daniel East, Chaleurs Bay.


dolomitization is present, it is doubtful that sufficient porosity was available for hydrocarbon-bearing fluid migration, unless local fracture porosity developed during the Salinic disturbance or the Acadian Orogeny. The fracture reservoir potential remains to be studied in the Silurian West Point Formation of the Chaleurs Bay Synclinorium (see Kirkwood et al., 2001, this issue).

The Colline Daniel Member of the upper reef complex developed at the beginning of the latest Silurian-earliest Devonian sea-level rise (D 2 transgression of Bourque, 2001, this issue). No traces of early freshwater diagenesis were observed. The diagenetic study by Savard and Bourque (1989) in the Chaleurs Bay Synclinorium dealt particularly with porosity history. They concluded that the porosity decreased from a

Fig. 13. Sketch illustrating temporal and spatial relationships of the various cementation and dissolution phases of the Anse Mclnnis Member, West Point Formation. Cement notation as for Figure 11. Cr = crinoid ossicle, sc = dissolution conduit.

Fig. 15. Thin-section photomicrograph under CL from sample JA- 1.23 (see Fig. 12B) showing large stubby crystals of cement 3, coated by banded luminescent crystals of cement 4 in a dissolution conduit. Centre part of the conduits (oc) is an open crack related to thin section fabrication. Scale bar = 500 pm.

Fig. 14. Thin-section photomicrograph from sample JA-1.57 (see Fig. 12A). (A) Plane-polarized light. (B) Under CL, illustrating cement phases 1 to 3. Cement 3 fills all interskeletal pore space. Note local dissolution and replacement of cement 1 by cement 3 (arrow). Scale bar = 500 pm.


IColline Daniel Member]

Burial Marine

18 0 _ ~ _ . . . . + ~ > ~ 6 -- , . . . . . . . . . . . . . . . . . . . . . .

~ 1 3 0 % O p D B

• •

- 4

- -I-

.-2 - - +

- 0 ZX

[] =

- - - 2

Palisade-bladed / splayed / fibrous isopachous cement (CD-C1 to C4) Granular to bladed non-luminescent cement (CO-C5) Infiltrated mud

Blocky, bright to dull- luminescent cement (CD-C6)

Xenomorphic, dull- luminescent cement (CD-C7

Fig. 16. Crossplot of 618OvPDB and 1~130 values for cements of the Colline Daniel Member of the upper reef complex of the West Point Formation in the Chaleurs Bay Synclinorium. Results from Savard and Bourque (1989) and the current study are combined (see text for explanation). Shaded box, P, is the Pridolian marine calcite field based on brachiopod shell analyses (Azmy et al., 1998; Veizer et al., 1999). See Table 1 for values.

primary porosity of 30% in reef margin facies, to 18% during shallow burial (pre-chemical compaction), and to 8% in a deeper burial environment (post-chemical compaction). All porosity was occluded at 1.2 km for the base of the complex and a few hundred metres from the top, before the maximum burial depth of the reef complex for this region was attained (based on chi- tinizoan colour alteration and organic matter maturation). No karst or secondary dolomitization porosity is present, except for the extensive, post-Acadian Carboniferous karst.

8

g

G r o s M o r b e Member [3 G M - C 3

n . . . . I . . . . I I t I

0 50 100 150 200 250

I 0 50 1 O0

Anse ~ l a Barbe Member [ ] A B - C 4 [ ] A B - C 5

i i 1

150 200 250

!t nnnnn ,-- on°.--eto[]AMo AMc3 . . . . I . . . . I I

0 50 100 150 200' 250

i t H ~ C o l l i n e D a n i e l Member o C D - C 6

, . . . . . I 7 . i , 51 i i i

0 0 100 150 200 250 Homogenization temperature (°C)

Fig. 17. Histograms of homogenization temperatures of fluid inclusions from Silurian West Point Formation limestones, Chaleurs Bay Synclinorium.

DEVONIAN WEST POINT LIMESTONE OF THE NORTHERN OUTCROP BELT

This section is concerned with pinnacle reefs of the West Point Formation that outcrop along, and south of, the Northern Outcrop Belt in the northeastern part of the Gasp6 Peninsula (Fig. 20). All dated West Point reefs are Devonian in age. However, locally, some undated facies in the lower part of the succession strongly resemble the Silurian West Point Formation of the Chaleurs Bay Synclinorium, suggesting the reefs may be coeval in part (see discussion below).

The pinnacle reefs are composed mostly of well-bedded crinoidal limestones and massive stromatoporoid limestones that form buildups up to 300 m thick and about 23 km wide (Lesp6rance and Bourque, 1970; Bourque, 1977; Bourque et al., 1986). Facies and facies architecture of the Madeleine River and the Lac Brfil6 buildups (Fig. 20) were the object of detailed sedimentological studies (Bourque, 1972, 1977; Bourque et al.,

2

E~ E:

Q E .9

0 0

-5

-10

-15

-20

-25

-30

Homogenization temperature (°C)

50 100 150 200 - - J ~ l l ~ l i t l i ~ I I

D

DO D D 0•

D •

250

AA A

o GM-C3 ZXAM-C3 o AB-C4 zx AM-C5 oAB-C5 •CD-C6

Fig. 18. Crossplot of homogenization versus ice-melting temperatures for fluid inclusions from Silurian West Point Formation limestones, Chaleurs Bay Synclinorium.


Sea-level curve

G L o w H igh

Prido. lian ~i ~ !

°;

6 1 3 C % O p D B ~180°/00pDB- , -2 ~ _ _ , 2, , I , T 6 , 8t

-\ A. Upper Silurian paleotropical seas

~13C%opDB Anse & la Barbe ~ 8

- 4

s Morbe Colline Daniel ~ 2

C~" Anse Mclnnis

I ' I ' I ' I ' I ' I ' I ' 0 -6 -4 -2

( 3 1 8 0 % O p D B

B. Chaleurs Bay Synclinorium

Fig. 19. Upper Silurian carbon (C) and oxygen (O) isotope ratios. (A) Curves showing the variations in 5180 and (313C values for brachiopods from paleotropical seas, after Azmy et al. (1998). For each data set, circles refer to mean values, and horizontal bars to maximum and minimum values. Sea-level curve after Johnson et al. (1998). Numbers 6 to 8 associated with the curve indicate sea-level highstands. G = graptolite zones. (B) Mean 8180 and 613C values for the West Point members in the Chaleurs Bay Synclinorium, discussed in this paper. The Gros Morbe to Colline Daniel member succession is inferred to span the late Gorstian to Pridolian interval [Bourque, 2001 (this issue)].

1986; Lachambre, 1987). Although not mapped in detail like the two other buildups, a third buildup, the Madeleine Lake buildup, which is the westernmost known occurrence of these pinnacle reefs, was sampled in the summer of 1997. Our diagenetic analysis concentrated on the limestones of only these three buildups.

The basal few metres of the Madeleine River buildup are a sponge-spicule-rich lime mudstone facies that resembles the Gros Morbe Member of the Silurian West Point in the Chaleurs Bay Synclinorium. This is a new observation as, until now, all buildups of the Northern Outcrop Belt west of the northern fork of the Bras Nord-Ouest Fault (Fig. 20) were thought to be entirely Devonian in age. Field observations of the Madeleine Lake buildup suggest that the lower half of this buildup is Silurian, based on the similarity of facies with the Silurian West Point Formation in the Chaleurs Bay Synclinorium. However, we do not have unequivocal documentation of a Silurian age, and therefore can only suggest that reefs, possibly similar to

those of the Chaleurs Bay Synclinorium, developed at the edge of a block fault north of the Bras Nord-Ouest Fault during the Silurian, a possibility postulated by Bourque (2001, this issue). For simplicity, we refer to the pinnacle reefs of the Northern Outcrop Belt as the Devonian West Point, keeping in mind that their base is possibly uppermost Silurian.

CEMENT STRATIGRAPHY AND CHRONOLOGY OF DIAGENETIC EVENTS

Over 350 thin sections (stained and unstained) from field and drill core samples of pinnacle reef limestones were examined. Out of these, 140 thin sections were polished and studied under CL to establish cement stratigraphy. Thereafter, 73 microsamples of cements and matrix were drilled from a collection of 104 polished blocks for 813C and 5180 analyses.

Cement phases are more numerous and interpreted as being more representative in the Lac Br~16 buildup than in the Madeleine River and Madeleine Lake buildups. Seven phases of

75 o o222--.. Ma.deleine Lake ~ B ~ ~

Fig. 20. Known surface occurrences of Devonian West Point pinnacle reefs in the Northern Outcrop Belt, eastern Gasp6 Peninsula. See Bourque et al. (2000, Fig. 6) for a detailed distribution of the reefs in a cross-section along the belt.


calcite cements (numbered 1 to 7, Figs. 21, 22) were recognized. Figure 23 summarizes cement succession and other diagenetic events, such as fracturing and stylolitization.

Two episodes of fracturing, commonly accompanied by dissolution, are represented by calcite-filled veins. A first episode of fracture-dissolution (FS5 veins; Fig. 24) postdates cement 4, crosscutting cements 1 to 4. Pores were cemented by cement 5 (shown zoned under CL). A second episode postdates the orange, dull cement 6 and is cemented by the last, brownish, dull cement 7 (FS7 veins; Fig. 22). Dissolution related to this last episode is obvious in the form of swarms of narrow seams (Fig. 25). Corrosion of cements 2 and 3 that took place before deposition of cement 4 is observed locally.

Internal sediment was deposited during three discrete diagenetic episodes: during early marine cementation 1 and 2 (SI1, Fig. 26); during cementation 3 (SI3, Fig. 26); and during and after cementation 6, before cementation 7 (SI6, Fig. 26). Brecciation occurred during and after cementation 6. Cement 6 fills fractures associated with this brecciation. Locally, it is not possible to discriminate between fracture-filling cements 6 and 7. All cement phases and veins pre-date stylolites, except in one sample where we observed a cement 7 vein (FS7) crosscutting stylolites. Figure 26 summarizes the chronology of the main diagenetic events for the Lac Brfil6 buildup.

The Madeleine River and Madeleine Lake buildups have similar cement stratigraphy. In the Madeleine River buildup, veins of bright-luminescent cement (FS5) are uncommon, and those with cement 7 (FS7), although present, are much less common than in the Lac Brfil6 buildup. Only cements 5 and 6 were sampled in the Madeleine Lake buildup.

ISOTOPE GEOCHEMISTRY AND DIAGENETIC ENVIRONMENTS

All 513C and 6180 results (Table 3) for the three buildups are shown in Figure 27. Marine calcite values, based on brachiopod shell analyses, are known for the Pridolian (Azmy et al., 1998; Veizer et al., 1999) and the Pragian-Emsian (Lavoie, 1993), but unknown for the Lochkovian (earliest Devonian). The microsamples of early cements 1 to 3 of the Lochkovian buildups show narrow ranges of values (8180 = -5.93 to --4.85%0; 813C = +1.38 to +2.26%0). They plot near the Pridolian and Pragian-Emsian marine calcite fields, and therefore are interpreted as marine (Fig. 27), as are the associated infiltrated mud and muddy matrix samples. These results clearly indicate that marine diagenesis affected the two buildups during and after deposition. Cement 5 has 8180 values ranging between -10.08 and -6.08%0, and 813C values between +0.54 and +2.59%0. Cements 6 and 7 have verry broad ranges of 8~80 and 813C values: -11.62 to -3.50%0, and -2.75 to +3.02%0, respectively.


Cement Calcite type characteristics

Radiaxial and/or fascicular optical

Large syntaxial crystals

Blocky neomorphic

Inclusion- rich

Non- ferroan ' Inclusion-

poor

Non- Inclusion- ferroan poor

Ferroan Inclusion- free

Ferroan Inclusion- to non-

ferroan free

Cement! number

la

lb

2

3

4

5

6

7

CATHODOLUMINESCENCE PETROGRAPHY

Cement shape

Fibrous-like, isopachous crusts

Luminescence

cement 2 crystals

Non-lum./ composite

Syntaxial on Non-lum., local crinoids lum. banding

Stubby to bladed Non-lum.

Thin crust on Yellow bright

Syntaxial on C3 Orange dull

Zoned,yellow Large pinacoid to bright/orange stubby crystals dull

Orange dull, Xenomorphic local banding

Xenomorphic Brownish dull

DIAGENETIC ENVIRONMENT

Marine

Burial

Fig. 21. Summary of the cement phases and petrography for the Lac Brfile buildup limestones, Devonian West Point Formation, Northern Outcrop Belt.


Fig. 22. Thin-section photomicrograph under CL showing cementation history, including fracture-dissolution phase FS7 infilling with cement 7 in the Lac Br01~ buildup. Vein of cement 7 clearly cuts cements 1 to 6. Numbers 1 to 6 refer to cements. Facies E, Lac Br016 buildup, Devonian West Point Formation, Northern Outcrop Belt. Sample DY1- 98.7. Scale bar = 400 pm.

Fig. 23. Sketch illustrating temporal and spatial relationships of the various cementation and fracture-dissolution and stylolitization phases of the Lac BrOle buildup limestones, Devonian West Point Formation, Northern Outcrop Belt. Cement notation as for Figure 21. FS5 and FS7 as in Figure 26.

Fig. 24. Thin-section photomicrograph under CL showing the first (FS5) and second (FS7) phases of fracture-dissolution. Note the vein of cement 5 (FS5) cutting across the coral wall (Co) and cements 2 and 4 (black arrow), and the vein of cement 7 (FS7) cutting across cement 2 to 6 (white arrows). Numbers 2, 5 and 6 refer to cements. Facies D, Lac Br001~ buildup, Devonian West Point Formation, Northern Outcrop Belt. Sample DY4-45.5. Scale bar = 400 iJm.

Fig. 25. Thin-section photomicrograph under CL showing a solution conduit (FS7) cutting across cements 1 to 5. Note swarm of narrow solution seams clearly visible in cement 5. Numbers refer to cements. Facies D, Lac Br01e buildup, Devonian West Point Formation, Northern Outcrop Belt. Sample DY4-45.5. Scale bar = 400 #m.


Diagenetic Environments and Time

C1

MARINE

SI1

C2 H I

C3

SI3 i , i

Corrosion

C4

FS5

C5

C6

BURIAL

SI6

Brecciation

C7

Stylolites

FS7

- - ? -

Fig. 26. Diagenetic history of the Lac BrOl~ buildup limestones, Devonian West Point Formation, Northern Outcrop Belt. C = cementation phase, SI = internal sedimentation, FS = fracture-dissolution phase.

The general isotopic trend in time (from cements 1 to 3, to cement 5, and to cements 6 to 7) is marked by a progressive decrease in 8180 values, and by minor variability in 813C values. Only two samples show 13C-depletion relative to marine values (813C = -2.75 and -2.10%o for cement 6).

Isotopic values taken from hand samples are more reliable for isotopic time-trend analysis of these limestones (i.e. water- rock variations). For example, microsamples of pore cements from hand samples (e.g. samples DY6-86.5, LF-200-2b, and LM-8, Fig. 28) show isotopic trends in time typified by 180- depletion in the latest cements, as in the general trend (Fig. 27). These trends likely reflect progressive temperature increase with burial, under marine conditions to deep burial under evolved sea water, i.e. higher salinity formation waters.

Exceptions to the general trend are present in the Madeleine River and Madeleine Lake buildups, where some late cements show a significant increase in 180 relative to the cements they postdate (Fig. 28). At the Madeleine River buildup, sample LF200-2a (a sample close to LF200-2b) shows a shift of 8180 in time from marine-like in early pore cement 2 and mud matrix to heavier 8x80 values in late cement 7 in an FS7 vein (Fig. 28). Sample CP1-92.7 from the same location exhibits a significant shift from a light 8180 in cement 6 to a heavy one at the latest stage of cementation 7 (Fig. 28). There are also 8180 values of late cements 6 and 7 from some samples (LF200-11, LF320-5, 96LM-6 and 96LM-7) at Madeleine River and Madeleine Lake buildups that are near marine values (Fig. 27). The heavy 8180 end-member for late cements 6 and 7 can be

explained either by a return to shallower burial conditions, or by invasion of fractures by exotic, ~80-enriched waters, without a significant change in burial depth (Sheppard, 1986).

At the Madeleine River buildup, eight analyses of late cements 6 and 7 from an individual sample (CP1-39.5; Fig. 29) show a trend from the normal burial heavy 8~3C end-member (1.53%o) to lower 813C values (0.20%o). Such a decrease may be related to calcite precipitation from formation water containing light carbon from decarboxylation of hydrocarbons (see also Table 4).

FLUID INCLUSION MICROTHERMOMETRY

Fluid inclusions in cements filling primary pores and fractures were studied in samples from the Lac Brfilr, Madeleine River and Madeleine Lake buildups. The identification of different cement phases follows that of the cement stratigraphy established under CL (Fig. 21). In addition, Cp and Cf notation is used to clearly distinguish between pore-filling and fracture- filling cements, respectively (e.g. Cp6 versus Cf6, to identify cement phase 6 in pores and fractures, respectively).

L a c B r ~ l ~ B u i l d u p

Four samples from the Lac Brfil6 buildup were studied for fluid inclusions, in pore-filling cements (Cp4, Cp5, and Cp6) and fracture-filling cement (Cf6) (Table 5; Figs. 29, 30). From Cp4 through Cp5, Cp6 to Cf6, there is a trend of increasing homogenization temperatures (from 87.6 to 209.3°C) and slightly decreasing salinity, although some overlaps exist. Most of the salinity values are higher than that of sea water (-3.5 wt %).


(3180%0 PDB • I ~ ~ I

-12 •

I S i l u r i a n ( ? ) - D e v o n i a n p i n n a c l e r e e f s - N o r t h e r n Outcrop Belt

Marine cements ~ •

,~ + ~ . . .

4 • ~i •~ • ~-~ ,~i II•A

,Ill ,v , I ' ' ' .... i ' ...... i' ' I '

-10 -8 -6 P -4

~130%0 PDB 4

3

2 PE

I

, 0

-I

-2

-3

@ Isopachous fibrous non- luminescent/blotchy cement (C1)

O Stubby to bladed non- luminescent cement (C2)

0 Bright luminescent cement (C3)

X Microspar matrix

+ Internal sediment

Blocky zoned bright/light dull- luminescent cement (C5)

• Blocky light dull-luminescent cement (C6)

• Mixed cement C6-C7

• BIockydark dull-luminescent cement (C7)

Fig. 27. Crossplot of ~18OvPDB and ~130 values for cements of the Devonian West Point pinnacle reefs of the Northern Outcrop Belt. Shaded box, P, is the Pridolian marine calcite field (Azmy et al., 1998). Shaded box, PE, is the Pragian-Emsian (Early Devonian) marine calcite field, based on Gasp@ Belt brachiopod shells (Lavoie, 1993). Cement notation as for Figure 21. See Table 3 for values.

0 Cement 1

0 Cement 2

X Mud matrix A . 1

S i l u r i a n ( ? ) - D e v o n i a n p i n n a c l e r e e f s - N o r t h e r n O u t c r o p B e l t

LF200-2b Z~ Cement5 ~ ' " ' ~

/ , , , - - - - ~ ' IB f Cement 6 O~/_ _ CP1-92;7 _ _ _ / _

[ ] Cement 7 / "~P1 39 5 ~ LF200-2a PE , - - . . . . . . . . . . . . . . . . . . . . . . . . . .

"518OpDB f

I I / i I I i = . r I T f f I I I f I I I I I I I -1 2 / -1 0 -8 - 6 -4

P

~130pD B

- - 3

- - 2

- - 1

I 0

-I

Fig. 28. Results from individual samples showing isotopic trends through time (arrows). "f" preceeding a point indicates fracture-filling cement. Shaded box, P, is the Pridolian marine calcite field (Azmy et al., 1998). Shaded box, PE, is the Pragian-Emsian (Early Devonian) marine calcite field based on Gasp@ Belt brachiopod shells (Lavoie, 1993). Cement notation as for Figures 21 and 27.

320 P.-A. BOURQUE, M.M. SAVARD, (.7. CH1 and P. DANSEREAU

Table 3. Oxygen and carbon stable isotope data for the Devonian West Point buildups, western sector of the Northern Outcrop Belt.

I Cement type I Sample [ ~13C I ~lsO I I Cement type I Sample I ~13C ] 5180 [

MADELEINE RIVER BUILDUP LAC BROLE BUILDUP

Early marine cements Cement 1 Fibrous, non-compos, lum. LF-200-2B 2.26 -5.35 Fibrous, non-compos, lum. LF-200-11 1.56 -4.85 Fibrous, non-compos, lum. CP1-21 1.77 -5.93 Cement 2 Bladed, non-lum. LF-200-2A 1.79 -5.26 Cement 3 Syntaxial, bright-lum. 70L-200-12A 1.38 -5.88


Muddy matrix Microspar, dull-lum. LF-200-2A 1.50 -5.50 Microspar, dull-lum. LF-200-2A 1.50 -6.70

Infiltrated mud Microspar, dull-lum. 70L-200-12A 1.43 -6.01 Microspar, dull-lum. 96RM-17 1.71 -5.30 Microspar, dull-lum. 96RM-18 1.96 -4.64 Microspar, dull-lum. 96RM-18 1.80 -4.30


Burial cements Cement 5 Blocky, zoned bright-lum. Blocky, zoned bright-lum. Cement 6 Blocky, dull-lum. Blocky, dull-lure. Blocky, dull-lure. Blocky, dull-lurn. Blocky, dull-lum. Blocky, dull-lure. Blocky, dull-lum. Blocky, dull-lum. Blocky, dull-lum. Blocky, dull-lum. Blocky, dull-lum. Blocky, dull-lum. Blocky, dull-lure. Blocky, dull-lum. Blocky, dull-lum. Blocky, dull-lum. Blocky, dull-lum. Cement 6-7 Blocky to xeno. dull-lum. Blocky to xeno. dull-lum. Blocky to xeno. dull-lum. Blocky to xeno. dull-lum. Cement 7 Xenomor. darker dull-lum. Xenomor. darker dull-lum. Xenomor. darker dull-lum. Xenomor. darker dull-lum. Xenomor. darker dull-lum. Xenomor. darker dull-lum. Xenomor. darker dull-lum.

96RM-7 0.84 -10.08 96RM-7 1.16 -6.08

LF-320-5 2.00 -3.98 CP1-21 1.59 -8.82 CP1-21 1.77 -7.06 CP1-39.5 1.68 -8.96 CP1-39.5 1.56 -9.38 CP1-39.5 1.53 -9.94 CP1-92.7 1.87 -9.73 CP1-94.5 1.30 -9.43 CP1-94.5 1.83 -8.76 CP1-98.5 1.69 -7.63 CP1-39.5 1.40 -9.20 CP1-5 1.20 -9.90 CP1-70 1.40 -8.80 CP4-118.8 1.50 -8.10 96RM-4 1.18 -11.19 96RM-8 1.23 -8.90 96RM-13 1.04 -8.14

LF-200-11 1.60 -5.72 CP1-92.7 1.70 -9.60 CP1-39.5 -0.10 -8.90 CP1-86.5 1.70 -9.40

LF-200-2A 2.14 -3.85 LF-200-2B 2.31 -9.42 CP1-39.5 1.04 -9.07 CP1-39.5 0.03 -9.36 CP1-92.7 1.90 -4.48 CP1-39 0.10 -9.40 70L-200-2A 2.30 -3.50


Early marine cements Cement 1 Fibrous, non-compos, lum. DY4-152 Fibrous, non-compos, lum, DY6-86.5

1.87 -5.38 1.74 -5.33

Burial cements Cement 5 Blocky, zoned bright-lum. DY3-8.5 1.78 -7.01 Blocky, zoned bright-lum. 96LB-5a 0.86 -9.26 Blocky, zoned bright-lum. 96LB-5 0.54 -9.14 Cement 6 Blocky, dull-lum. DY2-75 1.27 -7.83 Blocky, dull-lure. DY3-73 0.35 -9.52 Blocky, dull-lure. DY6-62.5 -0.16 -10.05 Blocky, dull-lum. DY6-66.5 0.83 -9.19 Blocky, dull-lum. DY1-68.6 1.71 -6.97 Blocky, dull-lurn. DY1-31.5 1.70 -9.00 Blocky, dull-lum. DY3-73 -2.10 -8.60 Blocky, dull-lum. 96LB-4 -2.75 -11.39 Blocky, dull-lure. 96LB-4 0.41 -10.77 Blocky, dull-lure. 96LB-6 -0.24 -11.46 Blocky, dull-lum. 96LB-6 -0.03 -11.62 Blocky, dull-lum. 96LB-6 1.40 -9.52 Blocky, dull-lum. 96LB-6Ca 1.26 -9.25 Cement 6- 7 Blocky to xeno. dull-lum. DY4-63 1.58 -7.96 Blocky to xeno. dull-lum. DY4-45.5 -0.10 -9.40 Cement 7 Xenomor., darker dull-lure. DY6-87 0.19 -10.06 Xenomor., darker dull-lure. DY4-20.5 0.40 -9.90


MADELEINE LAKE BUILDUP

Burial cements Cement 5 Blocky, zoned bright-lum. 96LM-1 1.52 -7.50 Blocky, zoned bright-lum. 96LM-8 2.59 -9.07 Cement 6 Blocky, dull-lure. 96LM-6 1.62 -5,53 Blocky, dull-lum. 96LM-7 1.38 -5.61 Blocky, dull-lum. 96LM-8 -0.59 -11.56 Blocky, dull-lum. 96LME-4A 1.91 -4..82 Blocky, dull-lum. 96LME-4A 3.02 -4.62 Blocky, dull-lum. 96LME-5 1.56 -9,42


Infiltrated mud Microspar, dull-lum. DY4-152 2.40 -7.61 Microspar, dull-lure. 96LB-6Ca 1.14 -9.33


Madeleine River Buildup

Six samples from the Madeleine River buildup were studied for fluid inclusions, in late pore-filling cements (Cp6 and Cp7), and fracture-filling cement (Cf6 and Cf7). Fluid inclusions from different cements have largely overlapping ranges of homogenization temperatures and salinity (Figs. 29, 30) (from 81.6 to 171.2°C and from 1.9 to 16.9 wt % NaCl-equivalent, respectively). Except for fluid inclusions in Cf6, which show salinity values slightly lower than that of sea water, most of the salinity values are higher than that of sea water (-3.5 wt %) (Fig. 30). Compared to Cp6 and Cf6 from the Lac Brfil6 buildup (homogenization temperatures from 121.4 to 209.3°C), fluid inclusions of late cements from the Madeleine River buildup show relatively low homogenization temperatures (from 81.6 to 146.8°C).

Hydrocarbon inclusions were found in the Cf6 cement from one sample (96RM-8). Both light hydrocarbon inclusions and oil inclusions were observed. The light hydrocarbon inclusions show homogenization temperatures (homogenized to liquid phase) from -84.8 to -72.3°C (Table 5, Fig. 30). The homogenization temperatures of oil inclusions range from 77.4 to 148.3°C, which is in the same range as the aqueous fluid inclusions (81.6 to 146.8°C) (Fig. 30). This implies that the oil inclusions were entrapped near the bubble point curve. Assuming that the oil inclusions have API values similar to those found elsewhere in the northeastern part of the Gasp6 Peninsula (Lavoie et al., 2001, this issue; Kirkwood et al., 2001, this issue), and elsewhere along the margin of Laurentia (Chi et al., 2000), i.e. around 40 API, the fluid pressure can be estimated at approximately 300 bars.

Madeleine Lake Buildup

Only one sample from the Madeleine Lake buildup was studied for fluid inclusions, in the late fracture-filling cement Cf7. A number of light hydrocarbon inclusions were observed, with homogenization temperatures ranging from -60.2 to -49.7°C (Table 5). Only a few aqueous fluid inclusions were found, showing homogenization temperatures from 174.6°C to higher than 250°C, suggesting heterogeneous trapping. Because of the limited number of aqueous inclusions studied, the trapping temperature is poorly constrained and could have been lower than 174.6°C. The trapping pressure is potentially relatively high.

Implications for Diagenetic Environments

The fact that nearly all fluid inclusions from the various cements show salinity values higher than sea water suggests that most cementation took place in burial environments. The gradual increase in homogenization temperatures from cement 4 through cement 5 to cement 6 is consistent with increasing burial depths. However, the relatively low homogenization temperatures recorded in the latest cement 7 in the Madeleine River buildup may suggest a return to shallower environments.

The occurrence of hydrocarbon fluid inclusions in the fracture- filling cements (Cf6 and Cf7) indicates relatively late hydrocarbon migration.

SUMMARY OF DEVONIAN WEST POINT DIAGENESIS IN THE NORTHERN OUTCROP BELT

Cements 1 to 3 clearly are diagenetic marine products, whereas cements 5 to 7 generally have attributes of burial products. At the Madeleine River buildup, the decrease of 813C values in fracture-filling late cements is likely related to the influence of hydrocarbon decarboxylation in the burial environment. At the Madeleine Lake and Madeleine River buildups, some high 6180 values of late cements 6 and 7, mostly in fractures, reflect high salinity (low Tice) and high temperature of precipitation (Th). We therefore suggest that exotic water with high 818OsMow values locally saturated the late fractures. The departure of late cements from the normal burial trend, i.e. increase in 5180 values at the Madeleine Lake and Madeleine River buildups, and decrease in 513C values at the Madeleine River, are likely related to late fracturing that post- dated significant burial of the Devonian reefs. Therefore, the fractures are likely of Acadian affinity.

Estimated Burial Depth and Pore Occlusion

Temperatures of homogenization give an approximation of the precipitation conditions for the cements filling primary and secondary pores. A burial depth for pore occlusion can be estimated using a surficial temperature of 20°C, and a geothermal gradient of 25°C/kin, estimated for the Sunny Bank core (Bertrand, 1987). It is relevant to point out here the limitations of this kind of depth estimation, because of the uncertainty of the geothermal gradient, and the assumption that temperatures of homogenization reflect burial temperatures during cementation.

Cements 6 and 7 are the latest cements within pore spaces. Their precipitation generally occurred at burial depths greater than 2 kin, but in some cases, as deep as 6 km (Table 4).

Table 4. Burial depths for late cements 5 to 7 based on homogenization temperatures.

S a m p l e C e m e n t Th D e p t h 1 °C Km

LB-4 fC7 157 5.5 LB-5 C5 122 4.1 LB-6 C6 171 6.0 LB-6B C6 105 3.4 LB-6C C6 148 5.1 70L-200-2A C7 125 4.2 RM-8 fC6-7 115 3.8 RM-13 fC6-7 82 2.5 CP1-39.5 fC7 Min 36 0.6

Max 53 1.3

f = cements in fractures.


Table 5. Fluid-inclusion microthermometr ic data of samples from Devonian West Point buildups, western sector of Northern Outcrop Belt.

Sample Host Occurrence Size Tm. ic,(°C) Salinity (wt % NaCl-equiv.) Th (°C) mineral (ram) Rankle Mean (n) Rankle Mean (n) Rankle Mean In)

LAC BROLE 96LB-4 Quartz (Cf6) Isolated 10 -1,9 -1,9 (1) 3.2 3.2 (1) 170,3 170.3 (1)

Isolated 4 166.8 166.8 (1) Isolated 5 186.7 186,7 (1) Isolated 7 -1,9 -1,9 (1) 3,2 3.2 (1) 164,6 164,6 (1)

Cluster 4 - 6 190.3 - 197.1 194.2 (3) Cluster 4 163.6 - 168.8 166.2 (2) Isolated 7 -2.0 -2.0 (1) 3.4 3.4 (1) 209.3 209.3 (1) Isolated 8 -2.1 -2.1 (1) 3.5 3.5 (1) 171.6 171,6 (1) Cluster 8 ~ 9 -2,0 -2.0 (2) 3,4 3.4 (2) 179.9 ~ 181.7 180.8 (2) Isolated 6 -2.0 -2.0 (1) 3.4 3,4 (1) 171.3 171.3 (1) Cluster 4 -2.1 -2.1 (1) 3.5 3.5 (1) 155.8 - 158,4 157.1 (2)

Growth zone 8 -8.8 -8,6 (1) 12,7 12.7 (1) 87,6 87,6 (1) Randomly distributed 6 ~ 8 -7.8 ~ -11.8 -9,8 (2) 11.5 - 15.8 13.7 (2) 93.2 ~ 96.2 94.7 (2)

Cluster 5 ~ 10 -12.8 -12.8(1) 16.7 16.7(1) 116.0- 123.3 119.7(2) Cluster 5 ~ 13 -2.1 -2,1 (1) 3.5 3.5 (1) 151.5 - 162.7 157.1 (2) Cluster 9 ~ 25 -2.0 ~ -2.2 -2.1 (2) 3.4 - 3.7 3,5 (3) 166.5 ~ 177.6 171.4 (5)

Scattered 6 ~ 12 -2.3 - -2.7 -2.5 (2) 3.9 - 4.5 4.2 (2) 134.8 ~ 145.2 140.3 (6) Cluster 6 ~ 8 -3.9 -3.9 (1) 6.3 6.3 (1) 121.2 ~ 123.5 122.6 (2) Isolated 4 -2.7 -2.7 (1) 4.5 4.5 (1) 169.6 169.6 (1) Isolated 5 -14.8 -14.8 (1) 18.5 18.5 (1) 145.5 145.5 (1) Cluster 6 - 7 -2.7 - -2.9 -2.8 (1) 4.5 - 4.9 4,7 (2) 150.7 ~ 174.2 162.5 (2) Isolated 8 -2.8 -2.8 (1) 4.9 4.9 (1) 161.2 161.2 (1)

Randomly distributed 9 -2.2 -2.2 (1) 3.7 3.7 (1) 135.9 135.9 (1) Randomly distributed 7 113.6 113.6 (1)

Isolated 9 -1.9 -1.9 (1) 3.2 3.2 (1) 166.6 166.6 (1) Isolated 8 -6.4 -6.4 (1) 9.7 9.7 (1) 145.8 145.8 (1) Cluster 4 - 6 -13.9 -13.9 (1) 17.7 17.7 (1) 104.9 ~ 107.4 106.2 (2) Isolated 12 -12.9 -12.9 (1) 16.8 16.8 (1) 147.3 147.3 (1)

Cluster 4 ~ 8 -1.4 -1.4 (1) 2.4 2.4 (1) 156.4 - 158.0 157.2 (2)

Isolated 6 166.0 166.0 (1) Isolated 14 -2.5 -2.5 (1) 4.2 4.2 (1) 170.2 170.2 (1) Isolated 12 -1.4 -1.4 (1) 2.4 2.4 (1) 153.7 153.7 (1) Isolated 12 -5.7 -5.7 (1) 8.8 8.8 (1) 156.1 156.1 (1) Cluster 7 - 15 -5.4 -5.4 (2) 8.4 8.4 (2) 141.7 ~ 152.1 148.9 (4)

Isolated 7 -13.6 -13.6 (1) 17.5 17.5 (1) 136.6 136.6 (1) Isolated 12 -2.2 -2.2 (1) 3.7 3.7 (1) 170.4 170.4 (1)

Isolated 7 -2.1 -2.1 (1) 3.5 3.5 (1) 151.4 151.4 (1) Isolated 12 121.4 121.4 ( 1 )

Calcite (Cf6) 96LB-5B Calcite (Cp4)

Calcite (CpS) Calcite (Cp6)

Calcite (Cf6)

96LB-6B Calcite (Cp4)

Calcite (Cp6)

96LB-6CA Calcite (Cp6)

RIVIERE MADELEINE 96RM-8 Quartz (Cf6)

Calcite (Cf6)

Cluster 4 - 12 Oil: Isolated 8 Oil: Cluster 4 - 10 Light HC: Cluster 2 - 8 Light HC: Isolated 10 Light HC: Isolated 8 Light HC: Isolated 9 Light HC: Cluster 6 - 10 Light HC: Isolated 7 -1.7 Isolated 5 -3.1 Cluster 12 - 20 -1.6 - -1.7 Cluster 6 ~ 12 -1.8 - -2.2 Isolated 6 Oil:

Isolated 15 Oil: Cluster 6 ~ 9 Oil: Cluster 9 ~ 10 Oil:

Thl=-80.6V --82.5L

-1.7 (1) 2.9 2.9 (1) -3.1 (1) 5.1 5.1 (1) -1.7 (2) 2.7 - 2.9 2.8 (2) -2.0 (2) 3.0 ~ 3.7 3.4 (2)

116.5- 162.0 116.5"(11) 148.3 148.3 (1)

-75.3 ~ -79.3 (V)-77.3 (V) (2) -76.0 - -78.9 (V) -77.4 (V) (4)

-77.0 (V) -77.0 (V) (1) -72.3 (V) -72.3 (V) (1) -73.3 (V) -73,3 (V) (1)

-73.9 - -75.9 (V) -74.8 (V) (3) 129.0 129.0(1) 119.4 119.4(1)

97.5 - 109.3 103.4 (2) 121.6 - 121.9 121.8 (2)

122.0 122.0 (1) 95.7 95,7 (1)

95,3 ~ 95.8 95.6 (2) 77.4 - 117.8 77.4* (2)


Table 5. cont inued

Sample Host Occurrence Size T~ i=,(°C) mineral (mm) Range Mean (n)

96RM-8 Calcite (Cf6) Cluster 6 - 8 Oil: Isolated t 0 Oil: Isolated 5 Light HC: Isolated 8 Light HC: Isolated 7 Light HC: Isolated 9 Light HC: Isolated 10 -1.7 -1.7 (1) Cluster 4 - 5 -1.1 -1.1 (2) Isolated 9 Isolated 7

96RM-13 Calcite (Cf7) Scattered 13 - 15 -4.1 -4.1 (1) Isolated 15 -3.3 -3.3 (1) Isolated 12 -3.2 -3.2 (1) Cluster 10 - 18 -2.9 -2.9 (2) Cluster 8 Isolated 12 -2.5 -2.5 (1)

96RM-14 Calcite (Cf6) Randomly distributed 4 - 30 -6.5 - -6.7 -6.6 (4) Randomly distributed 8 - 14 -4.6 - -7.0 -5.8 (2)

Isolated 15 -2.0 -2.0 (1) Isolated 10 -9.0 -9.0 (1) Isolated 8 -9.2 -9.2 (1)

CP1-39.5 Calcite (Cp6) Isolated 5 -5.9 -5.9 (1) Isolated 12 Cluster 13 -8.7 -8.7 (1)

Randomly distributed 7 - 8 -6.4 -6.4 (1) Randomly distributed 6 - 9 -6.6 -6.6 (1) Randomlydistnbuted 8 - 13 -7.6 - -7.8 -7.7 (1)

Calcite (Cf6) Isolated 16 -7.7 -7.7 (1) Isolated 8 ~.7.8 -7.8 (1) Cluster 7 - 13 -7.8 -7.8 (1)

70L-200-2A Calcite(Cp7) Cluster 8 - 13 -10.3--11.2 -10.8(2) Isolated 12 Isolated 9 -12.9 -12.9 (1) Cluster 8 - 12 -12.8 - -13.1 -13.0 (2) Cluster 6 - 11

Calcite (CfT) Randomly distributed 6 - 12 -9.6 ~ -10.8 -10.2 (2) Randomly distdbuted 4 - 9 -6.7 ~ -8.1 -7.4 (2)

CP1-92.7 Calcite (Cp7) Cluster 11 ~ 18 -8.5 -8.5 (1) Cluster 4 - 12 -5.6 -5.6 (1)

LAC MADELEINE 96LM-6 Calcite (Cf7)

Salinity (wt % NaCI-equiv.) Th (°C) Range Mean (n) Rankle Mean (n)

95.7 - 118.7 110.2 (4) 106.7 106.7 (1)

-75.7 (V) -75.7 (V) (1) -84.1 (V) -84.1 (V)(1) -84.8 (V) -84.8 (V) (1) -81.7 (L) -81.7 (L) (1)

2.9 2.9 (1) 171.2 171.2 (1) 1.9 1.9 (2) 142.7 - 150.9 146.8 (2)

115.8 115.8(1) 95.7 95.7 (1)

6.6 8.6 (1) 70.3 - 92.8 81.6 (2) 5.4 5.4 (1) 136.3 136.3 (1) 5.2 5.2 (1) 117.7 117.7 (1) 4.8 4.8 (2) 140.3 - 141.4 140.9 (2)

103.9 103.9 (1) 4.2 4.2(1) 116.1 116.1 (1)

9.9 - 10.1 10,0 (4) 99.5 - 144.3 126.7 (7) 7.3 - 10.5 8.9 (2) 121.4 - 132.0 128.1 (4)

3.4 3.4(1) 111.3 111.3(1) 12.9 12,9 (1) 129.2 129.2 (1) 13.1 13,1 (1) 125.4 125.4 (1) 9.1 9.1 (1) 106.2 106.2 (1)

116.3 116.3 (1) 12.6 12.6 (1) 110,8 110.8 (1) 9.7 9.7 (1) 96.6 - 100,0 98.3 (2) 10.0 10.0(1) 117.6- 127.9 122.8(2)

11 .3- 11.5 11.4(2) 91 .6 - 96.6 94.1 (3) 11.4 11.4(1) 118.7 118.7(1) 11.5 11.5(1) 111.6 111.6(1) 11.5 11.5 (1) 87 .6 - 105.2 94.1 (3)

14.3 - 15.2 14.8 (2) 120.3 - 126.0 123.2 (3) 125.6 125.6 (1)

16.8 16.8 (1) 124.3 124.3 (1) 16.7 - 17.0 16.9 (2) 117.0 - 125.0 121.3 (3)

130.6 ~ 131.5 131.1 (2) 13.6 - 14.8 14.2 (2) 108.0 ~ 128.4 118.9 (6) 10.1 - 11.9 11.0 (2) 116.9 - 127.0 122.2 (3)

12.3 12.3(1) 102.3- 121.9 111.0(3) 8.7 8.7 (1) 110.0 - 118.4 114.9 (4)

Cluster 4 - 10 Light HC: Light HC: .50.7- -60.1 (L) -56.8 (3) Cluster 9 - 10 Light HC: Light HC: .52.5 - -71.4 (L) -62.0 (2) Cluster 6 Light HC: Light HC: -52.8 (L) -52.8 (1) Cluster 9 Light HC: Light HC: -60.2 (L) -60.2 (1) Isolated 11 Light HC: Light HC: -49.7 (L) -49.7 (1) Isolated 10 Light HC: Light HC: -54.2 (L) -54.2 (1) Cluster 6 Light HC: Light HC: -58.4 (L) -58.4 (1) Cluster 3 - 9 234.6 - 251.6 245.3* (5) Isolated 5 -14.9 -14.9 (1) 18.6 18.6 (1) 174.6 174.6" (1) Isolated 4 191.3 191.3" (1) Isolated 6 >250.0 >250.0* (1)

*The wide range in T h within a FI assemblage is interpreted to reflect ge terogenous trapping; minimum T h is use to represent trapping temperature, f = cements in fractures.

324 P.-A. BOURQUE, M.M. SAVARD, G. CH1 and P. DANSEREAU

Exceptions occur for fracture-filling cement 7 that was precipitated at temperatures as low as 36°C, and at burial depths of about 0.6 km. The calculated isotopic composition of the 180- depleted parent water (-5.1%o SMOW) can be used further to estimate the temperature of precipitation of the isotopically heaviest cement 7, for which there were no measurements for temperatures of homogenization (Table 5). This estimation indicates near-surface conditions.

In summary, porosity of the West Point Formation remained open until burial depths exceeded 3 km, and in some cases, were as deep as 6 km. Locally in succession, the West Point Formation underwent deep (4 to 6 km) burial diagenesis, then experienced shallower, marine-like conditions. Late fractures were locally filled by high-temperature, exotic water.

SUMMARY AND CONCLUSIONS

SILURIAN WEST POINT

The Silurian West Point Formation is particularly well exposed in the Chaleurs Bay Synclinorium in the southern part of the Gasp6 Belt, where it comprises three superimposed complexes. The main conclusions from our diagenetic analysis of these limestones are based on the following: 1) Recognition of an extensive freshwater diagenetic event in

the Anse ~t la Barbe and Gros Morbe members (lower mound and reef complex of West Point Formation) and the Anse McInnis Member (middle bank complex). However, this event is absent in the Colline Daniel Member (upper reef complex). These data suggest that the Salinic unconformity

L a c B r O l 6 a r e a

O , . . . . , , , ,I~,I~

I > o 5o

E R i v i 6 r e M a d e l e i n e a r e a U3

:g

idL z -85 -75 -65 100 150

L a c M a d e l e i n e a r e a

2 0 ' ' ' t . . . .

-65 -55 -45 100

[] cp4 HH t [] cp6 o cf6

Finn . . . . I . . . . I ' ' I

100 150 200 250

Q Cp6 [ ] C f 6

C p 7

[] cf7 • cf6 (light HC) [] Cf6 (oil)

. r T . . i . . . . i

200 25O

[] cf7 • Cff (light HC)

, . .F1..F1, . . . .IG,

150 200 250

Fig. 29. Histograms of homogenization temperatures of fluid inclusions from Devonian West Point buildups, Northern Outcrop Belt.

developed between deposition of the lower and upper complexes of the West Point Formation, and during the late Ludlovian third-order eustatic sea-level lowstand (Bourque, 2001, this issue).

2) Repeated subaerial exposure during deposition of the Anse McInnis Member (middle bank complex) related to the late Ludlovian eustatic lowstand has profoundly influenced limestone diagenesis of the middle and lower complexes of the West Point Formation. The Anse McInnis Member underwent freshwater dissolution, and mixed marine and freshwater cementation throughout its deposition. Concurrently, the underlying marine-cemented Anse ?a la Barbe and Gros Morbe members experienced dissolution and cementation by fresh water percolating throughout the limestone strata. This water altered the early marine stromatactis cements of the Gros Morbe Member in less than a few hundred metres of burial (Bourque and Raymond, 1994).

3) Despite this karst porosity development, early meteoric- influenced cementation rapidly occluded all remaining pore space in the Gros Morbe, Anse ~ la Barbe, and Anse McInnis members in a shallow burial environment.

4) The important shift of 513C toward higher values in the Anse h la Barbe Member limestone, and a difference of 5.46%o in the Gros Morbe and the Anse h la Barbe members, may be the same as that described from the Ludfordian limestone of the southern paleotropical belt (Wenzel and Joachimski, 1996; Samtleben et al., 1996; Bickert et al., 1997; Azmy et al., 1998; Veizer et al., 1999).

~d

Q .

E

O 3

$

0

-5

-10

-15

-20

0 0 -

- 5 -

-10

-15

-20

Homogenizat ion temperature (°C)

0 50 100 150 200 250 _ _ _ 1 J L _ _ . L _ _

[ ]

• [3 • [] [ ]

L a c B r O l 6 a r e a ..F• Cp4 [] Cp6 Cp5 a Cf6

50 100 150 200 250 ] L 1 1

A

L a c M a d e l e i n e a r e a a Cp6 z~ Cp7 [] Cf6 A Cf7

Fig. 30. Crossplot of homogenization versus ice-melting temperatures of fluid inclusions from Devonian West Point buildups, Northern Outcrop Belt.


5) In contrast with the underlying limestone, the diagenesis of the Colline Daniel Member (upper reef complex), whose deposition commenced at the beginning of a transgressive phase, does not show any freshwater diagenetic influence. Rather, occlusion of its primary porosity occurred during progressive burial and was completed at a maximum burial depth of 1.2 kin. In the Chaleurs Bay Synclinorium, the Silurian West Point

limestone does not occur in subsurface (Bourque and Lachambre, 1980), and it may therefore appear irrelevant to have studied porosity evolution in those limestones. However, based on the model proposed by Bourque (2001, Fig. 10, this issue) for the southern Gasp6, the Silurian West Point mounds, reefs and banks may occur in the subsurface toward the south at the margin of synsedimentary block faults. Moreover, the Silurian West Point Formation is likely present in the subsurface at the periphery of the tilted block bounded by the Bras Nord-Ouest Fault south of the Northern Outcrop Belt (Fig. 20). This interpretation is based on three main lines of evidence: 1) the occurrence of the Silurian West Point Formation in the eastern part of the Northern Outcrop Belt and likely at the base of the Madeleine River and Madeleine Lake buildups in the western part of the belt; 2) the recognition of the Gros Morbe, Anse

la Barbe, and Colline Daniel members farther west in the Neigette breccia (Dansereau and Bourque, 2001, this issue); 3) the model developed for the northeastern part of the Gasp6 Belt (Bourque, 2001, Fig. 11, this issue).

Whether or not these limestone units underwent the same diagenetic evolution as the one we established for them in the Chaleurs Bay Synclinorium is a matter for debate. It is likely that the limestone below the Salinic unconformity (Gros Morbe and Anse ~ la Barbe members, if present) was affected by meteoric diagenesis as in the Chaleurs Bay Synclinorium. If the Colline Daniel Member limestone followed the same burial trend as in the Chaleurs Bay Synclinorium, some primary porosity would have been maintained until the limestone attained a burial depth of about 1.2 kin. Hydrocarbon emplace- ment would thus have had to occur before burial of the limestone to a depth of 1.2 kin. Finally, the Silurian West Point limestone may have undergone the same Acadian-related diagenesis as the overlying Devonian West Point (see below).

DEVONIAN WEST POINT

The Devonian West Point Formation is represented by pinnacle reefs mainly in the Northern Outcrop Belt. Our diagenetic analysis of three of these pinnacles has shown the following: 1) The relatively high homogenization temperature values for

fluid inclusions in the late pore-filling cements (mainly 140 to 180°C in the Lac Brfil6 buildup and 100 to 140°C in the Madeleine River buildup) indicate that the primary pores were not completely occluded before significant burial of the West Point Formation (in some cases, to 6 kin). Hydrocarbon migration into subsurface buildups before total primary porosity occlusion might have produced reservoir rock.

2) In the case of the Madeleine River and Madeleine Lake buildups, three fluid systems were responsible for precipitation of fracture-filling cements: a) formation waters at normal burial temperature; b) marine-like waters at low temperature; and c) tSO-rich waters at high temperatures. In the three cases, gaseous hydrocarbons were involved in Acadian-related veins. The presence of these hydrocarbons indicates a hydrocarbon source in the area. Further diagenetic investigation, as well as structural and organic matter studies, are required to understand carbonate diagenesis and to locate potential source rocks in these areas.

ACKNOWLEDGMENTS

We are much indebted to Denis Lavoie and to Bulletin reviewers Mario Coniglio and Terry Sami, whose in depth reviews and constructive comments greatly improved the qual- ity of this paper. We thank M.R. Luzincourt for technical support with isotope analysis. This research was supported in part by an individual research grant to Bourque from the Natural Sciences and Engineering Research Council of Canada (NSERC), and in part by Shell Canada, who also contributed to publication costs. Savard acknowledges the Geological Survey of Canada (Natural Resources Canada) for supporting part of the analyses through its A-base funding. This is Geological Survey of Canada Contribution No. 2000279.

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Diagenesis and porosity evolution of the Upper Silurian ...uregina.ca/~chiguox/s/2001 Bourque et al CPG.pdf · Diagenesis and porosity evolution of the Upper Silurian-lowermost Devonian