Isotopic Evidence for Carbonate Cementation and Recrystallization, and for Tectonic Expulsion of Fluids Into the Western Canada Sedimentary Basin

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 5–7 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 during intermediate burial (≈300–1500 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 GSA Bulletin; September 1996; v. 108; no. 9; p. 1108–1119; 8 figures. 1108 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/9/1108/3382514/i0016-7606-108-9-1108.pdf by guest on 25 September 2021 CARBONATE CEMENTATION AND FLUID EXPULSION, WESTERN CANADA 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 disturbed 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, increasing to more than 4 km close to the limit of the disturbed belt. A–B 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 100–200 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 Geological Society of America Bulletin, September 1996 1109 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/9/1108/3382514/i0016-7606-108-9-1108.pdf by guest on 25 September 2021 MACHEL ET AL. 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

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