Isotopic evidence for carbonate cementation and recrystallization, and for tectonic expulsion of fluids into the Western Canada Sedimentary Basin

H. G. Machel P. A. Cavell Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada K. S. Patey

ABSTRACT recrystallization (e.g., Veizer, 1983; Schneidermann and Harris, 1985; Choquette and James, 1990; Machel, 1990). (Throughout this paper, we Carbon, oxygen, and strontium isotope data for calcites and dolo- use the term “recrystallization” rather than the term “neomorphism” mites from the Devonian Obed platform in Alberta, Canada, demon- although these terms are not strictly synonymous [Folk, 1965], because strate that (1) both limestones and dolostones of the Obed platform differentiation into the four recognized subgroups of neomorphism is irrel- underwent significant deep-burial cementation and recrystallization, evant for our paper and the term “neomorphism” is not widely used.) (2) calcites experienced more extensive geochemical alteration than Burial diagenesis of dolostones is also common, but the extent and did dolomites under deep-burial conditions, and (3) the fluids that importance of dolomite formation, i.e., dolomite cementation and dolomi- facilitated deep-burial carbonate diagenesis probably were partially tization (replacement of calcite), and of dolomite recrystallization during derived from the Rocky Mountain fold-and-thrust belt. The more burial, are controversial (Shukla and Baker, 1988; Mazzullo, 1992; Purser extensive degree of recrystallization of calcite is shown especially by its et al., 1994). Most penecontemporaneous to early-diagenetic, marine to higher 87Sr/86Sr ratios. A lesser degree of 13C depletion in dolomites hypersaline dolomites tend to recrystallize within hundreds to thousands of indicates that dolomite recrystallization partially coincided with years (e.g., Gregg and Shelton, 1990; Montañez and Reid, 1992), and bur- hydrocarbon oxidation. Evidence supporting interpretation 3 (above) ial recrystallization of such dolomites, i.e., progressive recrystallization includes fractures and vugs bearing late-diagenetic calcite cements with increasing burial, has been conclusively documented (e.g., Malone et that have extremely high 87Sr/86Sr ratios, including the highest ratios al., 1994). However, there are notable cases where such dolomites retained reported thus far for any diagenetic carbonates from western Canada their hypersaline textures and geochemical character for hundreds of mil- (0.7252). In carbonates, values this high are found only in tectonic lions of years even through burial to several kilometres (e.g., Packard, veins in Proterozoic clastic rocks in the Rocky Mountains and in the 1992) and/or heating to low-metamorphic temperatures (e.g., Tan and Obed platform about 100 km into the foreland basin. The late- Hudson, 1971). Hence, it is not justified to assume a priori that all buried diagenetic calcite cements also have highly depleted δ13C values (min- dolomites are recrystallized. The available textural and geochemical evi- imum –27.1‰ relative to PDB [Peedee belemnite]), indicating incor- dence suggests that, in general, burial dolomites are much more resistant poration of oxidized carbon from thermochemical sulfate reduction. to recrystallization than penecontemporaneous to early-diagenetic, marine The process of carbonate cementation and recrystallization in and hypersaline dolomites and limestones (e.g., Machel et al., 1993, and strata of the Obed platform probably occurred during deep burial references therein). The identification of recrystallization in dolomites is (maximum 5Ð7 km) and was effected by a hot (>100 °C) mixture of particularly important where their genetic interpretation hinges on recog- connate brines and hydrothermal or metamorphic fluids that were nition of the extent of recrystallization. At present, most textural evidence expelled from the Rocky Mountain fold-and-thrust belt during the does not permit an unequivocal identification of dolomite recrystallization, Laramide orogeny. The data also suggest that (1) the common practice and interpretation of dolomite recrystallization is usually based on geo- of using limestones to establish marine or original geochemical base- chemical data or on circumstantial evidence (e.g., Mazzullo, 1992; Machel lines for stable and radiogenic isotope interpretations must be con- et al., 1993). ducted with caution, and (2) replacement burial dolostones are quite A case in point is the widespread dolostones in the Middle and Upper resistant to burial recrystallization. Finally, the geochemical trace of Devonian of the Western Canada Sedimentary Basin. In the Alberta part of tectonically expelled fluids may be limited to about 100 km into the the basin (shown in Fig. 1; the open ocean was to the north, the evaporitic foreland basin, implying that the volumes and/or fluxes of fluids pro- termination of the basin was to the east), most of these dolostones are duced by tectonic expulsion are rather low. replacements of carbonate platforms and reefs and probably formed dur- ing intermediate burial (≈300Ð1500 m) in the Late Devonian to Mississip- INTRODUCTION pian from chemically modified seawater (Amthor et al., 1993; Machel et al., 1993; Mountjoy and Amthor, 1994). These dolomites or dolostones are Burial diagenesis of limestones, including cementation and recrystal- commonly referred to as “burial dolomites” or “burial dolostones.” The lization of marine-equilibrated calcite, is a common and widely recognized genetic interpretation for these dolostones is partially based on the infer- phenomenon. Awareness of this fact has led to the identification of a large ence that they are “insignificantly recrystallized,” i.e., if these dolomites number of petrographic and geochemical criteria that can be and have been recrystallized, the extent of isotopic and trace element alteration was so used to recognize the occurrence and, within limits, the extent of limestone small that the geochemical signature of these rocks is still representative of

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Figure 1. Simplified distribution map of Devonian platform and reef carbonates of the Upper Devonian Woodbend Group in the Western Canada Sedimentary Basin. Map on right is modified from Mossop and Shetsen (1994). Coarse dot pattern—undifferentiated “D-3” Cooking Lake and/or Leduc Formations (subsurface, east of the limit of the disturbed belt) and their outcrop equivalents (west of the limit of the dis- turbed belt, palinspastically restored). Light gray—West Shale basin and off-reef equivalents. Dark gray—Cooking Lake platform in the East Shale basin. The Grosmont platform and shelf rocks in the northeast part of the basin consist of D-3 and D-2. The location of the edge of the underlying D-4 Swan Hills platform is shown around the northeastern limit of the Southesk-Cairn Complex. East of the limit of the disturbed belt, the regional structural dip is to the southwest, with present subsurface depths ranging from 0 to 300 m for the Grosmont platform, increas- ing to more than 4 km close to the limit of the disturbed belt. AÐB marks the location of the cross section in Figure 8. The Obed platform is part of the Southesk-Cairn Complex. Study areas referred to in the text are AMA—Anderson (1985) and Machel and Anderson, Ka—Kaufman et al. (1990), Ko-1 and Ko-2—Koffyberg (1993), Dix—Dix (1993) and Pa—Packard et al. (1990). R-M reef trend—Rimbey-Meadowbrook reef trend. Boxes for Ko-1 and Ko-2 are placed at their present locations with respect to the disturbed belt. Approximate palinspastically restored locations of these areas are indicated with arrows and would be located probably about 100Ð200 km southwestward.

the dolomitizing fluids. Furthermore, many of these dolostones appear to isotopic compositions that are typical of penecontemporaneous hyper- have formed during regional fluid flow through Devonian paleoaquifers, saline dolomites (e.g., Machel and Hawlader, 1990; Luo et al., 1994). but the extent and hydrologic intercommunication of these aquifers are not In this study, we report new C, O, and Sr isotope data of Devonian well understood (Amthor et al., 1993; Machel et al., 1993; Mountjoy and limestones and dolostones from the Obed sour gas field in the Western Amthor, 1994). Exceptions to these generalized patterns are the relatively Canada Sedimentary Basin and compare them with the isotope data of few cases of early-diagenetic, peritidal to supratidal dolostones in the Dev- Devonian carbonates and clastic rocks elsewhere in the basin. The major onian of western Canada, such as the Grosmont shelf and platform in the objectives of this paper are (1) to demonstrate that both limestones and northeastern part of Alberta (Fig. 1); these dolostones have textures and dolostones may experience cementation and recrystallization during deep

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burial (conditions under which dolostones are much more resistant to recrystallization than limestones), and (2) to determine the source and extent of fluid flow responsible for deep-burial carbonate diagenesis. The second objective broadens the scope of this study to an evaluation of deep-burial brine migration in the Canadian Rocky Mountain foreland basin, i.e., the Western Canada Sedimentary Basin, and, by inference, in tectonically similar foreland basins elsewhere. Several studies have pro- vided circumstantial or mathematical modeling evidence for the presence and/or efficacy of tectonic expulsion of fluids into foreland basins and for topography-driven fluid flow subsequent to orogenic deformation (e.g., Oliver, 1986; Bradbury and Woodwell, 1987; Garven, 1989; Ge and Gar- ven, 1989, 1992, 1994; Bethke and Marshak, 1990; Deming et al., 1990; Deming and Nunn, 1991; Nesbitt and Muehlenbachs, 1994, 1995; Bachu, 1995). In conjunction with previous data, our new data are suitable to evaluate, at least partially, the viability of tectonically induced fluid flow and the associated fluxes from the Canadian Rocky Mountains into the Western Canada Sedimentary Basin.

METHODS Figure 2. Schematic northeast-southwest cross section through the Core material was sampled and investigated in the Energy Resources Devonian section in the study area (see Fig. 1), indicating stratigraphic Conservation Board (ERCB) Core Research Center, Calgary. A total of ages and relative hydraulic conductivities. Reefs occur in three posi- about 200 samples was taken for petrographic and geochemical investi- tions: (1) directly on the platform (Leduc); (2) above Leduc reefs (Blue gations. Sample locations and depths are documented in Patey (1995). Ridge), commonly separated from them by a thin layer of tight Nisku Hand specimens and thin sections were examined by standard optical carbonates; and (3) off the platform (left). Where thin, the Nisku car- methods, including staining with Alizarin-red and cathodoluminescence bonates directly above the Leduc reef may be relatively permeable. imagery. Powder samples were obtained with a dental drill and used for XRD (X-ray diffraction) and stable and radiogenic isotope analyses. XRD analyses of the solids were performed with a Rigaku Geigerflex (Co Kα the two being separated by tight, argillaceous carbonates of the Nisku = 1.790 26 Å) using internal standards. Formation (Patey, 1995) (Fig. 2). In general, the platform and reef car- The δ13C and δ18O analyses of carbonates were conducted by standard bonates have high porosities and permeabilities and form aquifers,

methods (McCrea, 1950; Epstein et al., 1964); CO2 was extracted by whereas all other facies, argillaceous carbonates and carbonate-rich clay- phosphoric acid at 25 °C. The samples were not roasted prior to analysis. stones (marlstones), have low porosities and permeabilities and form Small amounts of calcite contained in the dolomitized samples were aquitards (Fig. 2; the recognition of these aquifers and aquitards is based removed with 0.5M HCl prior to phosphoric acid digestion of dolomite. on a number of hydrogeologic studies: Bachu, 1995, and references Analytical precision based on replicate analyses of NBS-19 was ±0.1‰ therein). On top of some Leduc reefs the Nisku is thin and is an ineffec- for both δ13C and δ18O. tive barrier to fluid flow because it tends to be fractured and/or has a 87Sr/86Sr analyses of carbonates were performed according to the higher carbonate content (Fig. 2). In such cases, the Leduc and Blue method of Baadsgaard et al. (1986). Repeated measurements of NBS- Ridge reefs form a vertically contiguous reservoir. In the northeast part of SRM-987 gave values of 87Sr/86Sr of 0.710 15 ± 0.000 02 to 0.710 26 the study area, isolated reefs also occur off the main platform, essentially ± 0.000 03 (n = 10, 1σ), with a mean value of 0.710 21. Precision of indi- encased in off-reef marlstones (Figs. 1, 2). vidual runs was better than ±0.000 07 (2σ). Ratios are normalized to The general geometry and distribution of facies and rock types of the 87Sr/86Sr = 0.1194. Obed platform are similar to those of most other Devonian carbonates in The complete data set can be obtained from us upon request (more than the southern Western Canada Sedimentary Basin (Fig. 1). The only impor- 90% of the isotope data are tabulated in Patey, 1995). In the plots shown tant difference, in the present context, is that the Obed strata are located in the figures in this paper, the size of the symbols used is greater than the very close to the Rocky Mountain fold-and-thrust belt. Consequently, the accuracy of the data plotted. Obed rocks were buried much deeper, probably to a maximum depth of about 5500Ð6500 m, as estimated from thermal maturation and isotopic GEOLOGIC SETTING AND STUDY AREA studies (Deroo, 1977; Connolly, 1989; Dawson and Kalkreuth, 1994). Our study is limited to three stratigraphic levels within the Devonian The Obed field consists of a Late Devonian carbonate platform and section and to the southern part of the Western Canada Sedimentary associated reefs that are located in the Western Canada Sedimentary Basin Basin. Stratigraphically, we restrict our arguments to fluid flow in the about 80Ð100 km beyond the outer limits of the disturbed belt of the Southesk-Cairn Complex and its hydrologic communication with the Rocky Mountains (Fig. 1). According to the most recent palinspastic immediately underlying and overlying carbonate platforms. The reconstruction available (Mossop and Shetsen, 1994), the Obed platform Southesk-Cairn Complex is part of the stratigraphic Leduc level “D-3,” as constitutes a promontory of the Southesk-Cairn Complex (consisting of a are all the other Upper Devonian carbonate platforms and reefs shown in carbonate platform and associated reefs), most of which was located in Figure 1. The D-3 is sandwiched between lithologically and hydrostrati- what now are the Rocky Mountains (Fig. 1). In the study area, the Obed graphically similar carbonate platforms and reefs, the underlying Swan platform is overlain by reef carbonates of the Leduc Formation that, in Hills platform (informally called “D-4”) and the overlying NiskuÐBlue several locations, are overlain by smaller reefs of the Blue Ridge Member, Ridge (“D-2”) and Wabamun (“D-1”), which, however, differ in size and

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geometry (Mossop and Shetsen, 1994). In Figure 1, the approximate out- line of the underlying Swan Hills (D-4) platform is shown around the northeastern limit of the Southesk-Cairn Complex. Geographically, we restrict our discussion to the southern part of the Western Canada Sedimentary Basin, i.e., south of the Peace River Arch (Fig. 1), because the arch separates the basin into two parts with different tectonic, geothermal, and hydrogeologic histories. The Peace River Arch is a southwest-northeastÐoriented flexure in the Precambrian basement that formed an emergent landmass throughout much of the Paleozoic until it col- lapsed in the MississippianÐPermian (e.g., Ross, 1990). Detached from the orogenies that affected the entire region, e.g., Antler (DevonianÐCarbonif- erous), Sonoma (Late Permian?), Columbian (JurassicÐEarly Cretaceous), and Laramide (mid-Late CretaceousÐTertiary), the Peace River Arch area proper had its own tectonic and geothermal dynamics (Stephenson et al., 1989; Ross, 1990; Mossop and Shetsen, 1994). These led to the deposition Figure 3. Hand specimen of typical reef limestone with fragments of of a fringing reef complex (Fig. 1) and a number of other distinct sedimen- corals and stromatoporoids in sand- to silt-sized carbonate matrix. tologic and diagenetic-hydrothermal phenomena (e.g., Packard et al., 1990; Large primary void near center is lined with banded, isopachous Mountjoy and Halim-Dihardja, 1991; Dix, 1993). North of the Peace River fringes of marine-equilibrated calcite cements. Matrix, fossils, and Arch, regional easterly flow of formation fluids deposited Pb-Zn mineral- such banded cements are herein collectively called “early calcites.” ization at Pine Point (located far to the north of the area shown in Fig. 1; The center of the large primary void is filled with coarse-crystalline, Qing and Mountjoy, 1992, 1994). Whatever the timing of regional fluid milky-white calcite spar, herein called “late calcite.” Scale bar is in flow leading to the mineralization at Pine Point (recent interpretations range centimetres and is placed at stratigraphic top. from Late JurassicÐPaleocene [Qing and Mountjoy, 1992, 1994] to Late DevonianÐLate Carboniferous [Cumming et al., 1990; Nakai et al., 1993; Nesbitt and Muehlenbachs, 1994; Morrow and Aulstead, 1995]), there is no evidence for significant Pb-Zn mineralization and a corresponding regional Most Obed carbonates are dolomitized. All transitions from 100% lime- flow system south of the Peace River Arch. Also, the general burial and stone to 100% dolostone are present; hence, Obed dolostones resulted geothermal histories appear to have been significantly different north and from dolomitization of the above limestones. Petrographically these dolo- south of the arch. To the north, a regional event of high heat flow with max- stones are composed of five main dolomite types: (1) medium-crystalline imum temperatures between the end of the Paleozoic and the Jurassic planar-s mosaic, (2) coarse-crystalline planar-s(e) mosaic, (3) coarse- caused oil maturation and migration in the Late DevonianÐCarboniferous crystalline planar-s(e), (4) coarse-crystalline nonplanar-a, and (5) coarse- (at least far to the north in the Liard subbasin area, off our Fig. 1 to the crystalline to very coarse crystalline, nonplanar-c dolomite, similar to the northwest, and most probably farther south). Also, there is circumstantial types identified by, and illustrated in, Amthor et al. (1993) in the adjacent evidence for late Paleozoic large-scale hydrothermal fluid convection via Rimbey-Meadowbrook reef trend (Fig. 1). The textural variations found in faults up and through the Devonian carbonates when they were at very shal- these dolomites appear to be largely inherited from the limestone precur- low burial depths (Morrow et al., 1993; Morrow and Aulstead, 1995). In sors. Types 1, 2, and 4 are due to replacement and approximately correlate contrast, in the Western Canada Sedimentary Basin south of the Peace River with precursor micrite, microspar, and pseudospar found in the limestones, Arch, geothermal gradients appear to have been near normal from the late respectively. In hand specimens, these three types often appear as “matrix Paleozoic to the Laramide orogeny, and then again after the orogeny, which dolomite,” whereby larger biochems and allochems are either preserved led to oil and gas maturation and migration in the Late CretaceousÐearly as calcite or dissolved to leave molds and/or vugs (Fig. 4). Cathodolumi- Tertiary (Hacquebard, 1977; Stoakes and Creaney, 1984; Longstaffe, 1986; nescence reveals mostly homogeneous red images, but some images have Connolly, 1989; Dawson and Kalkreuth, 1994). Also, there is no evidence orange-red blotchy patterns. Late-diagenetic, milky-white, concentrically for large-scale late Paleozoic or Mesozoic convection of hydrothermal flu- zoned saddle dolomite cements occur in a few late-diagenetic vugs (Fig. 4) ids in the southern part of the Western Canada Sedimentary Basin. For these and/or fractures. reasons, we make only passing reference to data from Devonian carbonates Distinctive, albeit volumetrically minor, components of both lime- around the Peace River Arch and from the basin north of the arch. stones and dolostones are late-diagenetic, milky-white sparry calcites that constitute as much as ≈2% of the rock over any 1 m core interval. These PETROGRAPHY calcites occur in three modes: as a cement in large voids (Fig. 3); as a cement in late-diagenetic fractures that crosscut petroliferous reef rocks Obed limestones are typical Upper Devonian marine mudstones to (Fig. 5); and as a replacement of late-diagenetic anhydrite nodules. In iso- grainstones that contain variable amounts of macrofossils, such as stro- lated cases, this late-diagenetic calcite occurs as overgrowths of the white matoporoids, corals, and brachiopods (e.g., Machel and Hunter, 1994; saddle dolomite; hence, the calcite is the paragenetically later phase. Wendte, 1994) (Fig. 3). The matrix is a mixture of micrite, microspar, and Some of these calcite crystals are up to 2 cm in diameter and are concen- pseudospar. Many of the larger crystals display crystal shapes and mosaics trically zoned, with no textural evidence of recrystallization. that are commonly recognized to form as the result of recrystallization (“aggrading neomorphism” sensu Folk, 1965). It is not clear, however, how ISOTOPE DATA much of the observed grain-size variation is inherited from variations in grain size of the original sedimentary particles. Cathodoluminescence Obed calcites have δ18O values that range from –4.7‰ to –11.9‰ (all images reveal black to orange-red blotchy patterns that are common in isotopic values are reported relative to PDB [Peedee belemnite] unless recrystallized marine limestones (e.g., Machel and Burton, 1991). otherwise stated) and δ13C values that range from +2.8‰ to –27.1‰. In

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Figure 4. Hand specimen of typical dolostone, consisting of medium- crystalline, replacement matrix dolomite (lower right) and white sparry dolomite cement in large vug. In the vug, the matrix dolomite is coated with solid bitumen (black); the white dolomite is not. The replacement matrix dolomite is herein called “early dolomite”; the white dolomite cement is called “late dolomite.” Average width of white dolomite crystals is 2 mm. Figure 6. δ18O vs. δ13C plot of Obed calcites and dolomites (symbols explained in insert) and compositional ranges of calcites equilibrated with Late Devonian seawater (SWC: calcite of brachiopods and cements formed in isotopic equilibrium with normal seawater at near- surface temperatures of 25 °C), replacement matrix dolomites (MD) from Devonian carbonates in the Western Canada Sedimentary Basin, and calculated values for dolomites equilibrated with Late Dev- onian seawater (SWD: dolomites formed in isotopic equilibrium with normal seawater at near-surface temperatures of 25 °C) (from Hur- ley and Lohmann, 1989; Carpenter and Lohmann, 1989; Carpenter et al., 1991; Mountjoy et al., 1992; Amthor et al., 1993; Machel et al., 1993; and sources cited therein). The ranges for SWC and MD include ≈98% of all data (a few samples with extraordinary values have been omitted for clarity).

Figure 5. Replacement dolostone with fracture that is filled with Early dolomites range from 0.7089 to 0.7113, and saddle dolomites range white “late calcite” cement. Scale bar in centimetres is placed at strati- from 0.7118 to 0.7152. In δ18O-87Sr/86Sr space, the dolomites form two graphic top. discrete groups (Fig. 7). For comparison, Figure 7 also shows the δ18O and δ13C values of calcite in equilibrium with Late Devonian seawater, δ18O-δ13C space (Fig. 6), the data form two distinct groups that are dis- common values of replacement matrix dolomites in Devonian carbonates tinguished mainly by their δ13C values, whereby one group represents from elsewhere in the Western Canada Sedimentary Basin, calculated val- matrix and fossils (“early calcites”) and the other represents the white, ues of dolomite in equilibrium with Late Devonian seawater, the Late late-diagenetic cements (“late calcites”). The data of the early calcites dis- Devonian seawater 87Sr/86Sr ratio, and the measured 87Sr/86Sr ratios and play a weak positive correlation; those of the late calcites display an δ18O ranges of selected, petrographically similar carbonates. ≈30‰ δ13C variation over a fairly small δ18O interval. Obed dolomites have δ18O values that range from –2.4 to –6.7‰ and DISCUSSION AND INTERPRETATION δ13C values that range from +4.4 to –1.1‰. In δ18O-δ13C space, the data of replacement matrix dolomites (“early dolomites”) and saddle dolo- Data from pristine, unrecrystallized or insignificantly recrystallized car- mites (“late dolomites”) form a tight cluster with a weak positive correla- bonates can be used to characterize the fluids from which these carbonates tion (Fig. 6). A number of early dolomites have significantly lower δ13C originally formed. “Insignificant recrystallization” is here defined as a values than the early calcites. modification via recrystallization (including neomorphism) of the original The 87Sr/86Sr ratios of Obed calcites range from 0.7082 to 0.7252. isotope ratios that is so small that the new composition is within the origi- Early calcites range from 0.7082 to 0.7088, whereas the late calcites range nal range. For example, isotopic changes via recrystallization in dolomite from 0.7086 to 0.7252. The 87Sr/86Sr data display a negative correlation from an original δ18O = –1‰ and δ13C = +2.0‰ to δ18O = –2‰ and δ13C when plotted against δ18O (Fig. 7), but no correlation when plotted = +2.5‰ would be insignificant, as both the pristine and the recrystallized against δ13C (not shown). dolomites plot in the field of dolomite in equilibrium with seawater The 87Sr/86Sr ratios of Obed dolomites range from 0.7089 to 0.7152. (Fig. 6). Conversely, “significant recrystallization” is here defined as a

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with Late Devonian seawater, and the Obed dolomites form separate groups (Fig. 6). (5) Obed dolomites have stable isotope values similar to those of petrographically similar dolomites elsewhere in the basin, albeit Obed dolomites tend to cluster toward the high end of the δ18O range, and several Obed samples have slightly lower δ13C values (Fig. 6). (6) Obed dolomites have 87Sr/86Sr ratios greater than those of dolomites elsewhere in the basin (Fig. 7).

Obed Calcites

The fact that pristine, marine-equilibrated calcites form an end member of the trend through the stable isotope data from Obed early calcites (fea- ture 1, above) implies that nearly all early (i.e., marine-equilibrated) Obed calcites are recrystallized. The weak positive correlation of the early calcites from Obed stata probably is the result of equilibrium fractionation with increasing temperature toward lower δ18O and δ13C values, in which case these calcites, with a measured δ18O range of ≈3‰, recrystallized over a temperature range of about 13Ð15 °C (based on O’Neil et al., 1969). Alter- Figure 7. δ18O vs. 87Sr/86Sr plot of Obed calcites and dolomites (sym- natively, recrystallization may have taken place over a very narrow temper- bols explained in insert), and compositional ranges of seawater- ature range that is represented by the lowest δ18O and δ13C values. In this equilibrated calcite (SWC), seawater-equilibrated dolomite (SWD), and case, the observed trend is due to various degrees of buffering by the host matrix dolomite (MD) (as in Fig. 6). Also plotted are the measured ranges rock, and the increasing deviation from marine δ18O and δ13C values toward of selected, petrographically similar carbonates: Late-diagenetic calcite lower values represents an increasing degree of isotopic reequilibration dur- cements (SHLC) and dolomite cements (SHDL) from the underlying ing recrystallization. Either interpretation can account for the 87Sr/86Sr (D-4) Swan Hills Formation (Kaufman et al., 1990). Late-diagenetic cal- ratios, which indicate a significant uptake of 87Sr during recrystallization for cite cements from the overlying (D-2) Nisku Formation (NLC) (Ander- two of the “early calcite” samples (feature 3). The observed range of son, 1985). Late-diagenetic calcite (PPC) and dolomites from Pine Point 87Sr/86Sr ratios of the early calcites may reflect that calcite recrystallization (PPD) that lie on an exponential trend having early-diagenetic fine- occurred over a depth and time interval within which the fluids acquired crystalline dolomites (bottom right) and late-diagenetic saddle dolomites 87Sr. Assuming a near-marine δ18O composition of the pore fluids and a (top left) as its end members (Mountjoy et al., 1992; Qing and Moun- near-normal geothermal gradient, the δ18O values of the calcites suggest tjoy, 1992, 1994). Three early dolomites from the Leduc reef that fringe that they recrystallized at a depth of less than 1000 m. This burial depth was the Peace River Arch (hollow circles; three samples plot within the scat- achieved in the Late DevonianÐMississippian (Patey, 1995). ter for Obed early dolomites, whereas one sample has a much higher The late calcites, which are the paragenetically youngest phase, formed 87Sr/86Sr ratio and a much lower δ18O value). One sample of fracture- from a significantly altered formation fluid. They are characterized by sig- filling saddle dolomite cement (PRALD) from the (D-3) Leduc reef fring- nificantly lower δ18O and δ13C values and by much higher 87Sr/86Sr ratios. ing the Peace River Arch (Dix, 1993). The areas from which these sam- The very high 87Sr/86Sr ratios of the late calcites (feature 3) further sug- ples were retrieved are shown in Figure 1, except for Pine Point, which is gest that a significant amount of Sr in Obed late calcites was probably off the map far to the north. The Late Devonian seawater 87Sr/86Sr ratio derived from the Rocky Mountains. On the basis of the Sr isotope values of 0.7080Ð0.7083 is taken from Burke et al. (1982) and Smalley et al. published to date, such high values correlate best with values from meta- (1994). A maximum 87Sr/86Sr ratio of 0.7120 (dashed line) is tentatively morphosed Proterozoic clastic rocks (slates) and limestones of the Miette considered to be representative of the basinal shale background. Group near Jasper and late calcite veins therein. These clastic rocks have 87Sr/86Sr ratios of 0.7560Ð0.7783 (n = 2), associated limestones have val- ues of 0.7335Ð0.7355 (n = 2), and late calcite veins in the clastic units modification via recrystallization (including neomorphism) of the original have values of 0.7387Ð0.7741 (n = 10) (Koffyberg, 1993; outlined area isotope composition that is so large that the new composition is outside the labeled Ko-1 in Fig. 1). These data indicate that the pore fluids of the original range. Data of significantly recrystallized carbonates can be used Miette Group were much enriched in 87Sr when the vein calcites formed, to determine the degree of recrystallization by comparison with data of and that this 87Sr probably was released from the host clastic rocks and/or pristine samples. Both pristine and significantly recrystallized samples can from the crystalline basement (as further discussed below). be used to infer the extent of flow in paleoaquifers. This finding further suggests a temporal and geochemical link between The available data reveal six relevant features. (1) Compared to pristine the formation of the calcite veins in the Miette Group and the late calcites calcites formed in equilibrium with Late Devonian marine conditions, from Obed platform rocks. We interpret the available data to indicate that most early calcites from Obed platform strata have similar δ13C values but fluids with 87Sr/86Sr ratios similar to those that formed the calcite veins in lower δ18O values (Fig. 6). (2) Late calcites from Obed rocks have lower area Ko-1 entered the Southesk-Cairn Complex and then migrated through δ18O and much lower δ13C values than those of Obed early calcites and/or this carbonate paleoaquifer into the Obed area (Figs. 1, 8). If so, these flu- pristine Late Devonian marine-equilibrated calcites (Fig. 6). (3) All but ids must have been injected into the connate brines of the aquifer, and the two early calcites from Obed strata have 87Sr/86Sr ratios higher than that mixed brines migrated toward the Obed strata. The injected 87Sr-rich flu- of Late Devonian seawater (Fig. 7). Late calcites from Obed rocks have ids could have been derived (1) from the palinspastically restored location the highest 87Sr/86Sr ratios reported thus far in carbonates of the Western of area Ko-1 (Fig. 1, arrow), (2) from surrounding areas that contained the Canada Sedimentary Basin. (4) The Obed dolomites have δ18O values up same strong 87Sr enrichment, or (3) from a hitherto unknown source. We to about 5‰ lower than calculated δ18O values of dolomite equilibrated suspect that the particular source of these fluids is somewhere south to

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Figure 8. Schematic fluid flow after Laramide orogeny in the southern Western Canada Sedimentary Basin (see Fig. 1 for location of cross section). The general flow patterns probably were similar during the Laramide orogeny, except that flow system 2 was absent. Flow systems 1, 2, and 4 are taken from Bachu (1995). His flow system 3 is not present and/or is not identifiable in this part of the basin. The zone separating the upper from the lower flow systems is interpreted to be a zone of mixing and interference between flow systems in the pre-Cretaceous and post-Jurassic megahydrostratigraphic successions. We divided the Devonian part of Bachu’s flow system 4 into four separate stratigraphic units (D-1, D-2, D-3, D-4) that approximate hydrostratigraphic units (each of these stratigraphic units contains at least one major carbonate aquifer and one major aquitard). The D-3 is interpreted to carry hydrothermal or metamorphic fluids expelled from the fold-and-thrust belt during the Laramide orogeny.

southeast of the location of Jasper (Fig. 1), because this is where the Miette suggests that Obed late calcites formed during the same orogenic event. Group underlies much of the Southesk-Cairn Complex. Combining the available evidence indicates, therefore, that late calcite Although the 87Sr/86Sr ratios of Obed late calcites suggest a possible fluid cementation in the Obed strata probably occurred during tectonic expulsion source, the timing of fluid flow during late calcite cementation is con- of 87Sr-rich fluids from the Miette Group into the foreland basin during the strained by the δ13C values and 87Sr/86Sr ratios in the Obed samples. The Laramide orogeny, when the Obed section was deeply buried to about highly depleted δ13C values (mininum, ≈–27‰ ) of these calcites (feature 2) 5500Ð6500 m (see above). indicate the incorporation of C derived from the oxidation of hydrocarbons. It could be argued that late-diagenetic calcite cementation in the Obed Considering further that these calcites are located within a sour gas field rocks was related to an earlier tectonic event that displaced fluids into the

(Obed gas contains about 30 mol% H2S), and that they typically occur as ancestral foreland basin, i.e., the Columbian orogeny in the JurassicÐEarly cements in late-diagenetic vugs and fractures and/or as replacements of Cretaceous (E. W. Mountjoy, 1995, personal commun.). However, this late-diagenetic anhydrite, they must have formed during and/or after ther- interpretation would require that thermochemical sulfate reduction in the mochemical sulfate reduction. Similar textures and δ13C depletions are Obed strata took place at burial depths of only a few hundred metres (deter- known from other sour gas fields that formed via thermochemical sulfate mined from the thin CarboniferousÐJurassic section overlying the Obed reduction (Krouse et al., 1988; Heydari and Moore, 1989; Machel, 1989; rocks: Mossop and Shetsen, 1994) via hydrothermal fluids (thermochem- Kaufman et al., 1990). Thermochemical sulfate reduction, which typically ical sulfate reduction requires temperatures in excess of about 100Ð135°C: results in the formation of 13C-depleted carbonate (e.g., Machel, 1987; Machel et al., 1995; Worden et al., 1995). We consider this to be a highly Machel et al., 1995), occurred in the Obed strata near their maximum bur- unlikely possibility, as there are no known independent indications of an ial in the Late CretaceousÐearly Tertiary, i.e., during the later phases of the increased geothermal gradient and/or a hydrothermal flow system(s) dur- Laramide orogeny (Patey, 1995). Also, the significantly lower δ18O values ing the Columbian orogeny in this part of the basin, i.e., in the undeformed of Obed late calcites, when compared to the early calcites, are consistent part of the Southesk-Cairn Complex. with elevated temperatures. Furthermore, Koffyberg (1993) interpreted the Last, the late calcites probably did not recrystallize, considering that 87Sr-rich calcite veins in the Miette Group to be of Laramide age, which they are concentrically zoned and that they had little thermodynamic drive

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to recrystallize after their formation. The area has only been slightly Regional Influence of Tectonically Induced Fluid Flow— uplifted since maximum burial, and the formation waters did not change Previous Studies significantly since the early Tertiary (Connolly et al., 1990a, 1990b). Previous studies have provided circumstantial or mathematical model- Obed Dolomites ing evidence for the tectonic expulsion of fluids into foreland basins and/or The observation that calculated δ18O values of seawater-equilibrated for topography-driven fluid flow associated with or subsequent to orogenic dolomite are up to about 5‰ higher than those of Obed early dolomites deformation. A critical evaluation of these studies is necessary before the (feature 4) can be interpreted in two ways: (1) Obed early dolomites are not geochemical data can be integrated to assess the possibility of tectonically marine-equilibrated dolomites; (2) Obed early dolomites are recrystallized induced fluid flow into the foreland basin of the Western Canada Sedi- marine-equilibrated dolomites. The overlap of most Obed early dolomite mentary Basin and, by inference, into other foreland basins. data with those of petrographically similar replacement dolomites in the Oliver (1986), based on circumstantial evidence, speculated that fluids basin (feature 5) suggests a similar origin. Most of these replacement dolo- expelled from continental-margin sediments travel into the foreland basins mites probably formed during a basinwide dolomitization event from chem- and the continental interiors when continental margins in zones of conver- ically modified seawater at burial depths of about 300Ð1500 m and at com- gence are buried beneath thrust sheets (this is commonly referred to as the mensurate temperatures of about 50Ð80 °C (Amthor et al., 1993; Machel et “squeegee model”). He further inferred that these fluids propel hydrocar- al., 1993). This interpretation is based on an integration of 11 separate lines bon migration and are involved in transport of minerals, faulting, magma of evidence, including the fact that the dolomitizing fluids had a δ18O value generation, metamorphism, and paleomagnetism. similar to that of Late Devonian seawater in the Western Canada Sedimen- Ge and Garven (1989, 1992, 1994) and Deming et al. (1990) evaluated tary Basin (0‰ to about –2.5‰ relative to SMOW: Popp et al., 1986; Greg- the squeegee model quantitatively through two-dimensional modeling. ory, 1991; Holmden and Muehlenbachs, 1993) and the observation that Their results suggest that, on a basinwide scale, (1) the flow rates of fluids nearly none of these dolomites displays evidence of significant recrystal- expelled tectonically into a foreland basin probably are on the order of cen- lization (as defined earlier). Hence, the available petrographic and geo- timetres per year (or metres per year in extreme cases of high-permeability chemical data suggest that the replacement early dolomites in the Obed aquifers); (2) the tectonically induced pressure change causing such flow platform rocks are burial dolomites and probably formed during the same, dissipates after about 103 to 104 yr; (3) the temperature increase in the areas nearly basinwide dolomitization event. intruded by tectonically expelled fluids is less than 5°C, unless the fluids The δ13C and 87Sr/86Sr isotope data of Obed early dolomites (features 5 are funneled into relatively small areas via faults (in such cases, the tem- and 6) indicate, however, that several samples have subsequently undergone perature in the relatively small intruded rock volume may rise by up to significant recrystallization. The significantly depleted δ13C values of several about 50 °C); and (4) the tectonically expelled fluid volume probably is early dolomite samples imply recrystallization during thermochemical sul- only a small fraction of the total brine volume in the foreland basin. Bachu fate reduction when 13C-depleted carbonate was available, and the signifi- (1995, p. 1174) further contended that, although the excess pressure gen- cantly higher 87Sr/86Sr ratios of Obed dolomites are best explained by recrys- erated by tectonic compression dissipates so fast, “the residence time for tallization when much 87Sr was available. By analogy with the interpretation the tectonically expelled waters is much larger (i.e., it would take advanced for Obed late calcites, it is most likely that Obed early dolomites 5Ð50 m.y. for a fluid particle to travel 500 km, with the calculated and recrystallized near maximum burial during the Late CretaceousÐearly inferred velocities of 1Ð10 cm/yr).” From a regional synthesis of several Tertiary. hydrogeologic studies, Bachu (1995) suggested that the deep Devonian The above data also are the first conclusive evidence for significant strata in the Western Canada Sedimentary Basin act as an aquifer system recrystallization of replacement burial dolomite(s) in the Western Canada for basin-scale, updip, northeasterly brine migration of “probable tectonic Sedimentary Basin (except for a single sample from the Leduc reef fring- origin” (flow #4 in his Fig. 9). One of his key arguments for inferring a tec- ing the Peace River Arch, analyzed by Dix, 1993, as further discussed tonic origin for this flow system is that it has no apparent recharge area(s), below). To date, there is no conclusive evidence for significant recrystal- as the shallow meteoric recharge area(s) in the Rocky Mountains are lization of burial dolomites anywhere else in the basin (Machel et al., hydrologically decoupled from the deep Devonian flow system. 1993). Our data indicate, therefore, that burial dolomites, once formed at On a much smaller scale, Ge and Garven (1994) modeled fluid expul- shallow to intermediate depths, appear to be much more resistant to burial sion of a single thrust fault with about 30 km displacement, the McConnell recrystallization than are early-diagenetic, shallow-burial calcites. Appar- thrust in southern Alberta, which is approximated by the southern portion ently, the burial dolomites significantly recrystallized only in the deepest of the line labeled “limit of disturbed belt” in Figure 1. For their study part of the basin near maximum burial depths of about 5500Ð6500 m. domain (volume of rock affected by porosity loss through thrusting), they A similar conclusion regarding the relative resistance toward recrystal- calculated that the total volume of fluid expelled during thrusting is on the lization during burial was advanced by Epstein et al. (1964) and by Tan and order of 1% of the total volume of pore fluid that moved through the sec- Hudson (1971). Tan and Hudson (1971) detected this phenomenon via C tion after thrusting (i.e., by topography-driven flow after cessation of oro- and O isotope data in a study of Jurassic carbonates of Scotland, and no genic activity: Garven, 1989) and that the tectonically expelled fluids did distinctive petrographic data were found to discriminate recrystallized not travel more than a few kilometres into the foreland basin. Isotopic stud- from unrecrystallized samples. The dolomites in their study appear to have ies on vein carbonates in the McConnell thrust zone by Bradbury and retained their early-diagenetic, shallow-burial isotope compositions even Woodwell (1987) support the theory that the fault zone acted as an aquifer through a hydrothermal or low-temperature metamorphic event, whereas during thrusting. the calcites display evidence of isotopic reequilibration. Although these results appear to contradict those of Bachu (1995), both Obed late (saddle) dolomites appear to have formed concomitantly with studies are consistent with one another. Garven (1989) and Ge and Garven recrystallization of Obed early dolomites, considering their similar stable (1994) merely calculated what the relative fluxes of tectonically expelled isotopic compositions. The significantly higher 87Sr/86Sr ratios of the sad- fluids and topography-driven flow would be, if the deep Devonian strata in dle dolomites is best explained by a much higher water/rock ratio, i.e., there the basin were affected and flushed by topography-driven recharge after was much less buffering of Sr in the open vugs than in the tight rock matrix. cessation of thrusting. These authors did not comment on the relative fluxes

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of tectonically expelled fluids and connate brines, as inferred by Bachu 0.7116 in saddle dolomites, and 0.7173 (only one sample) in late calcites (1995), who discounted the possibility that these strata were affected by (Fig. 7) (Kaufman et al., 1990). Apparently, the prominent, strong 87Sr sig- topography-driven flow. Furthermore, Bradbury and Woodwell (1987) nal in the Obed samples is not present in saddle dolomites, and present in inferred that ascending hydrothermal or metamorphic fluids may have been only attenuated form in late calcites, of the underlying Swan Hills platform. injected into the McConnell thrust sheet, that “ponding” of hydrothermal or This finding implies the presence of a fairly effective aquitard between the metamorphic and connate fluids may have taken place, and that each thrust Middle and Late Devonian aquifers in the Southesk-Cairn Complex region, sheet acted as a “separate hydrodynamic unit” for expulsion of fluids into if our interpretation is correct that the 87Sr-rich fluids that affected the Obed the foreland basin. rocks were derived from the disturbed belt to the southwest. Finally, Nesbitt and Muehlenbachs (1994, 1995), who conducted stable Similarly, late calcites in the overlying D-2 platform near the center of the isotope and fluid-inclusion studies of quartz and carbonate vein fillings in West Shale Basin (Fig. 1, area AMA) are not nearly so enriched in 87Sr as the southern Rocky Mountains (approximately the area southwest of the Obed late calcites (Nisku late calcites, Fig. 7). Nisku saddle dolomites in “limit of the disturbed belt” in Fig. 1), asserted that there was a pre- this part of the basin have values similar to those of Nisku late calcites, and Laramide southwest-to-northeast flow of brines through a Middle Cam- Nisku replacement matrix dolomites have values that plot in the field of brian aquifer. This brine movement led to downflow magnesitization, talc matrix dolomites in Figure 7 (Anderson, 1985; Machel and Anderson, mineralization, and Pb-Zn mineralization over a flow distance of some 1989). Nisku carbonates in this part of the basin are essentially encased in 40Ð80 km (and saddle dolomite formation along the entire flow path), but Ireton marlstone aquitards; hence, it is to be expected that they were not this flow system did not affect areas outside of the disturbed belt. Syn- to influenced by tectonically expelled fluids. For this reason, the Nisku sam- post-Laramide fluid flow involved meteoric water convection within the ples serve to define the nontectonic 87Sr/86Sr background delivered by the disturbed belt and formed subvertical calcite and/or quartz veins. Regarding clastic sedimentary units in the basin. Therefore, the highest 87Sr/86Sr ratios post-Laramide fluid flow in the Canadian Rocky Mountains, Hitchon from this area, about 0.7120 (Fig. 7), approximate the “basinal clastic sig- (1969) and Hitchon and Friedman (1969) had previously suggested that nal” in the carbonates (this value is tentative—we currently are analyzing recent groundwater flow has a cellular convection pattern and that these shales and marlstones from the basin to better characterize their 87Sr/86Sr flow systems discharge locally, including the well-known hot springs (e.g., ratios). Higher values are herewith interpreted as a Sr isotope signal that is Miette, Banff, Radium). either “tectonic,” i.e., from tectonically expelled fluids from the disturbed In summary, these studies suggest the following scenario for fluid flow belt, or “from the basement,” i.e., from fluids that ascended via faults from in the southern Rocky Mountains and its foreland basin to the south of the the crystalline basement. Peace River Arch: (1) pre-Laramide west-to-east brine migration with min- The few 87Sr/86Sr data reported by Dix (1993) from the Leduc reef fring- eralization, as discussed by Nesbitt and Muehlenbachs (1994, 1995); ing the Peace River Arch range from 0.7092 to 0.7145, which is within the (2) Laramide tectogenesis, including thrusting, with tectonic expulsion of range of data from the other Devonian study areas farther south in the basin. fluids into the foreland basin (squeegee model) within thrust sheets (which Four of these values are from replacement matrix dolomites; one is from may have been hydrodynamically relatively isolated from one another) saddle dolomite in a fracture. Dix (1993) argued that most of the geochem- and/or along some relatively permeable thrust faults (as suggested by the ical similarities between samples from his study area and those farther south various modeling studies discussed above); (3a) post-Laramide meteori- in the basin are due to a similar shallow- to intermediate-burial (<1 km) dia- cally recharged deep convection in the disturbed belt that did/does not feed genetic evolution, particularly with respect to replacement dolomitization. into the deep Devonian aquifers (as suggested by Hitchon, 1969, and Nes- However, Dix (1993) and Packard et al. (1990) also demonstrated respec- bitt and Muehlenbachs, 1994, 1995); (3b) basinwide, updip, tectonically tively that the Leduc and the directly overlying Wabamun carbonates induced northeasterly flow of connate brines in the deep Devonian aquifer around the Peace River Arch underwent a later-diagenetic evolution unlike system (these brines probably contained a hydrothermal or metamorphic that of any strata farther south in the basin, i.e., extensive fracture-controlled fluid component that was injected along the thrust sheets) (synthesized from hydrothermal karst development and hydrothermal dolomitization during Bradbury and Woodwell, 1987, and Bachu, 1995). relatively shallow burial (1Ð1.7 km). (Note that Packard’s study area [Pa in Fig. 1] is contained within Dix’s; see also Mountjoy and Halim-Dihardja Integration of Hydrogeologic Studies with Isotope Data [1991].) This finding further supports the notion that the Peace River Arch area proper had a tectonic and late-diagenetic and hydrothermal evolution In light of the above studies, the influence of tectonic fluid expulsion into different from the basin farther south. the southern Rocky Mountain foreland basin can be estimated from the Dix (1993) and Packard et al. (1990) also argued convincingly that the 87Sr/86Sr data presented in this study. Considering the geometry of the Dev- hydrothermal features observed in their cores were caused by hot fluids onian carbonate aquifers and the fact that the Miette Group extends in the that ascended via deep-reaching, subvertical faults. Hence, the fact that the Rocky Mountains along the entire map view in Figure 1 and beyond, with 87Sr/86Sr ratios in their cores are relatively low (<0.7145) compared to thicknesses between about 1500 and more than 10 000 m (Ross et al., those of the Obed samples could be used as an argument in favor of our 1989), one might expect a significant 87Sr/86Sr increase not only in the Obed interpretation that the extremely high Obed 87Sr/86Sr ratios are derived platform rocks but also (1) in other parts of the Southesk-Cairn Complex; from the disturbed belt via migration through the Southesk-Cairn aquifer, (2) in other D-3 carbonate aquifers, such as the Fairholme Complex or the rather than from fluids that ascended via subvertical faults from the base- Leduc reef fringing the Peace River Arch; (3) in the D-4 and D-2 carbonate ment. If the strong 87Sr signal in the Obed rocks was derived from the base- platforms beneath and above the Southesk-Cairn platform, respectively; and ment, then there should be an equally strong 87Sr signal at the Peace River (4) in the Cooking Lake platform rocks with the juxtaposed Rimbey- Arch, where the basement is much closer to the carbonates and where fluid Meadowbrook reef trend (Fig. 1). flow via subvertical faults has been demonstrated. In contrast, at present There are relatively few 87Sr/86Sr data available from these areas. The there is no direct evidence for deep-reaching faults acting as fluid conduits data closest to the Obed results are from the underlying Swan Hills (D-4) to the Obed platform, as no hydrothermal features were found in Obed platform (Fig. 1, area labeled Ka), where 87Sr/86Sr ratios reach 0.7090 in cores (Patey, 1995). This observation also implies that the basinwide recti- replacement dolomites (plotting in the matrix dolomite field of Fig. 7), linear fracture pattern discussed by Edwards et al. (1995), and inferred by

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them to have controlled sedimentary patterns and to have acted as fluid fluid-inclusion homogenization temperatures from late dolomites and cal- conduits elsewhere in the basin, had little or no influence on the diage- cite veins in Proterozoic clastic rocks near Jasper, which range from about netic and/or hydrothermal evolution of the Obed strata. 120 to 200 °C (Nesbitt and Muehlenbachs, 1994; B. E. Nesbitt, 1996, per- The present data base is insufficient, however, to prove or discount an sonal commun.), and the minimum temperature is suggested by homoge- influence of tectonically derived fluids in the Leduc reef fringing the Peace nization temperatures of 100 to 130 °C in late calcites of the D-4 Swan River Arch. On the one hand, most of the samples analyzed by Dix (1993) Hills platform (Ka in Fig. 1). Regional vitrinite reflectance data (Dawson and all of Packard et al.’s (1990) study area are located more than 200 km and Kalkreuth, 1994), as well as apatite fission-track and 40Ar-39Ar data east of the limit of the disturbed belt, and the Leduc reef fringing the Peace (McDonough et al., 1995), from this part of the basin are consistent with River Arch is not connected to the Southesk-Cairn Complex (Fig. 1). maximum regional burial temperatures in this temperature range for the Hence, if the strong 87Sr signal in the Obed platform rocks is indeed Devonian section. These considerations further imply that the tectonically derived from the area between the boxes labeled Ko-1 and Ko-2 (or their expelled fluids cool rapidly along their flow path, consistent with the mod- palinspastically restored equivalents; see Fig. 1), one would not expect a eling results of Deming et al. (1990). similar strong 87Sr signal in the Leduc carbonates fringing the arch. How- The only region where fluids from the Rockies appear to have moved ever, the most 87Sr-enriched sample from Dix’s (1993) study is a recrys- several hundred kilometres across the basin and left a significant tallized matrix dolomite from within 100 km of the disturbed belt (G. Dix, hydrothermal and geochemical trail (Presqu’ile saddle dolomites and Pb- 1995, personal commun.). Although its 87Sr/86Sr ratio is not nearly as high Zn mineralization) is in the Pine Point region and its westerly extension as that of the Obed calcites, its 87Sr/86Sr ratio is much higher than the ratios into the disturbed belt far north of the Peace River Arch (Qing and Moun- of the recrystallized matrix dolomites from Obed strata. This finding tjoy, 1992, 1994). However, these fluids did not carry a 87Sr signal nearly implies that the tectonically expelled fluids may well have taken the Leduc as strong as that found in the Obed samples (see PPD and PPC in Fig. 7). reef fringing the Peace River Arch as another northeasterly escape route. Most recent studies argue for west-to-east regional brine migration in the [This aspect is being further investigated.] “Presqu’ile aquifer” during the Late DevonianÐLate Carboniferous (Cum- Farther south, 87Sr/86Sr ratios from replacement dolomites and cements ming et al., 1990; Nakai et al., 1993; Nesbitt and Muehlenbachs, 1994, in the Strachan and Ricinus sour gas fields, which are in reefs located on the 1995; Morrow and Aulstead, 1995) (note: Morrow and Aulstead, 1995, Cooking Lake platform of the Fairholme Complex (Fig. 1), reach 0.7104 worked on the Manetoe dolomites, which are similar to the Presqu’ile (Marquez, 1994). In the Rimbey-Meadowbrook reef trend, which is con- dolomites, and they interpreted the Manetoe to be hydrologically and nected with the Fairholme Complex via the Cooking Lake platform (Fig. 1), genetically correlative to the Presqu’ile; this interpretation may or may not 87Sr/86Sr ratios of replacement dolomites are nearly at marine values and be correct: Mountjoy et al., 1996). Even if the much younger estimate of scatter mostly between 0.7080 and 0.7090 (Mountjoy et al., 1992; Machel Late JurassicÐPaleocene (Qing and Mountjoy, 1992, 1994) is correct, et al., 1993; Q. Ning, unpublished data). Furthermore, a calcite vein in Prot- regional Pb-Zn brine migration in the northern part of the basin obviously erozoic clastic rocks below the Fairholme Complex (area Ko-2 near bot- preceded the tectonic expulsion of fluids into the southern part of the basin tom of Fig. 1) has 87Sr/86Sr = 0.7402, i.e., much lower than in equivalent by a long time. These regional fluid migration events appear to be geneti- veins near Jasper (area Ko-1 in Fig. 1; Koffyberg, 1993). Calcite veins in cally and temporally unrelated. lower Paleozoic strata in that area have 87Sr/86Sr ratios ranging from 0.7091 Last, it is interesting to note that whenever reefs are located off the main to 0.7202. There are two possible explanations for the much lower, nearly platform aquifers and/or the hydrologic connection to the underlying plat- marine 87Sr/86Sr ratios of replacement dolomites in the Fairholme Complex form is poor, the reefs appear to be sheltered from regional diagenesis and its northern extension: (1) these dolomites did not recrystallize; driven by regional fluid flow. Cases in point are the Miette Complex, Stra- (2) these dolomites did recrystallize, but the diagenetic fluids were not chan, Golden Spike, Redwater, and a small reef just northeast of the Obed charged with Sr that was radiogenic enough to change the 87Sr/86Sr ratios of platform (Figs. 1, 2). Notwithstanding marginal dolomitization at Miette the dolomites significantly. (Mattes and Mountjoy, 1980) and along the western edge of Redwater, These considerations imply that, in general, the geochemical influence these reefs essentially escaped early replacement dolomitization. Similarly, of tectonically expelled fluids on carbonates in the foreland basin was these reefs escaped the later influence of 87Sr-enriched fluids where such fairly minor. In other words, the volumes and, by inference, fluxes of the fluids migrated through the regional aquifers. tectonically expelled fluids must have been fairly low compared to those of the “connate” brines in these strata, at least in the southern part of the West- CONCLUSIONS AND IMPLICATIONS ern Canada Sedimentary Basin (as previously suggested by the various modeling studies cited above and by the study of Nesbitt and Muehlen- (1) Marine-equilibrated calcites (matrix, fossils) and replacement matrix bachs, 1994, 1995). Significant recrystallization with isotopic resetting of dolomites in the Obed strata recrystallized during burial. The calcites calcites and dolomites is developed only in the Obed part of the section, recrystallized at fairly shallow burial depths of probably less than 1000 m, which is quite close to the limit of the disturbed belt and was probably very whereas the matrix dolomites were much more resistant and did not recrys- well connected hydrologically to the sources of hot, tectonically expelled tallize until they reached burial depths of 5500Ð6500 m. fluids. A similar argument can be made, albeit with much caution (only (2) To date, the Obed platform rocks and the deeper parts of the Leduc reef one data point), for the southwesternmost part of the Leduc reef fringing that fringe the Peace River Arch are the only locations in the Western Canada the Peace River Arch. The fluids responsible for recrystallization with iso- Sedimentary Basin where significant recrystallization (as defined above) of topic resetting are perhaps best characterized as having mixed sources: tec- burial dolomites has been conclusively demonstrated. Similar dolomites at tonically expelled hydrothermal or metamorphic fluids must have been shallower depths may be recrystallized, but that cannot be demonstrated, and injected into the connate brines; then the “mixed brines” migrated north- significant geochemical alteration is not likely (Machel et al., 1993). Hence, eastward as a result of the tectonically induced pressure increase (Fig. 8). replacement burial dolostones buried to lesser depths and subjected to cooler These mixed brines probably were hotter than 200 °C near the edge of the diagenetic fluids, like those in the Rimbey-Meadowbrook reef trend, are disturbed belt and cooled to about 100 °C along their flow path from the likely to have stable and radiogenic isotope signatures that closely resemble Rockies to the Obed region. The maximum temperature is suggested by those at the time of dolomitization.

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(3) The common (and necessary) practice of using presumably marine- Connolly, C. A., 1989, Thermal history and diagenesis of the Wilrich Member shale, Spirit River Formation, northwest Alberta: Canadian Petroleum Geology Bulletin, v. 37, p. 182Ð197. equilibrated calcites to establish “marine” geochemical baselines for sta- Connolly, C. A., Walter, L. M., Baadsgaard, H., and Longstaffe, F. J., 1990a, Origin and evolution of forma- ble and radiogenic isotope interpretations must be conducted with caution tion waters, Alberta basin, Western Canada Sedimentary Basin. 1. Chemistry: Applied Geochemistry, v. 5, p. 375Ð395. because marine limestones may be highly recrystallized. Connolly, C. A., Walter, L. M., Baadsgaard, H., and Longstaffe, F. J., 1990b, Origin and evolution of forma- 13 tion waters, Alberta basin, Western Canada Sedimentary Basin. 2. Isotope systematics and water mix- (4) Late calcites and dolomites in the Obed strata carry a C signal from ing: Applied Geochemistry, v. 5, p. 397Ð413. hydrocarbon oxidation via thermochemical sulfate reduction and a 87Sr Cumming, G. L., Kyle, J. R., and Sangster, D. F., 1990, Pine Point: A case history of lead isotope homogene- ity in a Mississippi ValleyÐtype district: Economic Geology, v. 85, p. 133Ð144. signal from fluid input from metamorphosed Proterozoic clastic rocks in Dawson, F. M., and Kalkreuth, W., 1994, Coal rank and coalbed methane potential of Cretaceous/Tertiary coals the Rocky Mountains. These signals fix the timing of late calcite and dolo- in the Canadian Rocky Mountain foothills and adjacent foreland: 1. Hinton and Grande Cache areas, Alberta: Canadian Petroleum Geology Bulletin, v. 42, p. 544Ð561. mite formation as Late CretaceousÐearly Tertiary. Deming, D., and Nunn, J. A., 1991, Numerical simulations of brine migration by topography-driven recharge: (5) Circumstantial evidence suggests that the 87Sr-rich brines that Journal of Geophysical Research, v. 96, p. 2485Ð2499. Deming, D., Nunn, J. A., and Evans, D. G., 1990, Thermal effects of compaction-driven groundwater flow formed the Obed late calcites were mixed from two sources: tectonically from overthrust belts: Journal of Geophysical Research, v. 95, p. 6669Ð6683. Deroo, G., Powell, T. G., Tissot, B. M., McCrossan, R. G., and Hacquebard, P.A., eds., The origin and migra- expelled hydrothermal or metamorphic brines and connate brines. At pres- tion of petroleum in the Western Canada Sedimentary Basin: Geological Survey of Canada Bulletin 262, ent, with a fairly limited data base, these 87Sr-rich brines are not detectable p. 11Ð22. Dix, G. R., 1993, Patterns of burial and tectonically controlled dolomitization in an Upper Devonian fringing- in other carbonate platforms below, above, and to the south at the same reef complex: Leduc Formation, Peace River Arch area, Alberta, Canada: Journal of Sedimentary Petrol- stratigraphic level. Considering further that the strong 87Sr signal has been ogy, v. 63, p. 628Ð640. Dorobek, S., 1989, Migration of orogenic fluids through the SiluroÐDevonian Helderberg Group during late detected only within about 100 km of the disturbed belt, we infer that the Paleozoic deformation: Constraints on fluid sources and implications for thermal histories of sedimen- component of tectonically expelled brine is fairly minor relative to the total tary basins: Tectonophysics, v. 159, p. 25Ð45. Edwards, D. J., Lyatsky, H. V., and Brown, R. J., 1995, Basement fault control on Phanerozoic stratigraphy in volume and/or flux of brine in these aquifers. the Western Canada sedimentary province: Integration of potential-field and lithostratigraphic data,in Ross, G. M., ed., 1995 Alberta Basement Transects Workshop: Calgary, Alberta, Lithoprobe Report #47, (6) If these interpretations are correct, the Obed data provide the first p. 181Ð224. geochemical evidence of tectonic fluid expulsion during the Laramide Epstein, S., Graf, D. L., and Degens, E. T., 1964, Oxygen isotope studies on the origin of dolomites,in Craigh, H., Miller, S. L., and Wasserburg, G. J., eds., Isotopic and cosmic chemistry: Amsterdam, North-Holland orogeny into the Western Canada Sedimentary Basin. Publishing Co., p. 169Ð180. (7) Tectonic fluid expulsion similar to that inferred here for the Western Folk, R. L., 1965, Some aspects of recrystallization in ancient limestones,in Pray, L. C., and Murray, R. C., eds., Dolomitization and limestones diagenesis: Society of Economic Paleontologists and Mineralogists Canada Sedimentary Basin may be a common phenomenon. For example, Special Publication 13, p. 14Ð48. Dorobek (1989), using mainly fluid inclusion data of quartz and calcite Garven, G., 1989, A hydrogeological model for the formation of the giant oil sands deposits of the Western Canada Sedimentary Basin: American Journal of Science, v. 289, p. 105Ð166. cements in veins, advocated migration of orogenic fluids through the Ge, S., and Garven, G., 1989, Tectonically induced transient groundwater flow in foreland basin, in Price, R. A., ed., Origin and evolution of sedimentary basins and their energy and mineral resources: Ameri- SilurianÐDevonian Helderberg Group in the central Appalachians during can Geophysical Union Geophysical Monograph 48, p. 145Ð157. late Paleozoic deformation. The likely sources for the hot brines in this Ge, S., and Garven, G., 1992, Hydromechanical modeling of tectonically driven groundwater flow with appli- cation to the Arkoma foreland basin: Journal of Geophysical Research, v. 97, p. 9119Ð9144. study area are inferred to be overthrusted terranes of the Appalachian oro- Ge, S., and Garven, G., 1994, A theoretical model for thrust-induced deep groundwater expulsion with appli- gen. Also, tectonically induced fluid flow through thrust sheets and along cation to the Canadian Rocky Mountains: Journal Geophysical Research, v. 99, p. 13851Ð13868. Gregg, J. M., and Shelton, K. L., 1990, Dolomitization and dolomite neomorphism in the back reef facies of thrust planes was invoked by Montañez (1994) to account for dolomitiza- the Bonneterre and Davis Formations (Cambrian), southeastern Missouri: Journal of Sedimentary tion and lead-zinc mineralization in the Ordovician Knox carbonates of the Petrology, v. 60, p. 549Ð562. Gregory, R. T., 1991, Oxygen isotope history of seawater revisited: Timescales for boundary event changes in southern Appalachians. the oxygen isotope composition of seawater, in Taylor, H. P., Jr., O’Neill, J. R., and Kaplan, I. R., eds., Stable isotope geochemistry: A tribute to Samuel Epstein: Geochemical Society Special Publication 3, p. 65Ð76. ACKNOWLEDGMENTS Heydari, E., and Moore, C. H., 1989, Burial diagenesis and thermochemical sulfate reduction, Smackover For- mation, southeastern Mississippi salt basin: Geology, v. 12, p. 1080Ð1084. Hitcheon, B., 1969, Fluid flow in the Western Canada Sedimentary Basin, 2. Effect of geology: Water This study was funded by AMOCO Canada Petroleum Company Ltd. Resources Research, v. 5, p. 460Ð469. Hitchon, B., and Friedman, I., 1969, Geochemistry and origin of formation waters in the Western Canada Sed- and by the Natural Sciences and Engineering Research Council of Canada imentary Basin. I. Stable isotopes of hydrogen and oxygen: Geochimica et Cosmochimica Acta, v. 33, (NSERC) through a strategic grant jointly held with E. W. Mountjoy and p. 1321Ð1349. Holmden, C., and Muehlenbachs, K., 1993, The origin of oxygen isotope variations in Paleozoic carbonates: Geo- a LITHOPROBE grant. We are grateful for the analytical and technical logical Association of CanadaÐMineralogical Association of Canada Program with Abstracts, v. 18, p. A45. assistance provided by H. Huebscher, H. R. Krouse, B. K. “Sunshine” Hurley, N. F., and Lohmann, K. C., 1989, Diagenesis of Devonian reefal carbonates in the Oscar Range, Can- ning basin, Western Australia: Journal of Sedimentary Petrology, v. 59, p. 127Ð145. Manzano, and Q. Ning. We thank P. Erdmer, E. W. Mountjoy, and B. E. Kaufman, J., Meyers, W. J., and Hanson, G. N., 1990, Burial cementation in the Swan Hills Formation (Dev- Nesbitt for helpful discussions. The constructive reviews by J. K. 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