Evolution of Fluid Flow in the Canadian Rocky Mountains Foreland Fold and Thrust Belt : A Study Integrating Structural Geology and Diagenesis

Veerle Vandeginste¹, Rudy Swennen¹, Jean-Luc Faure², Philippe Robion³, Frédéric Schneider², Francois Roure², and Kirk Osadetz^4
1Fysico-chemische geologie, Katholieke Universiteit Leuven, Celestijnenlaan 200C, 3001 Heverlee, Belgium
²Institut Français du Pétrole, 1&4 avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France
³Sciences de la Terre, Université de Cergy-Pontoise, (CNRS UMR 7072), 95031 Cergy Cedex, France
⁴Geological Survey of Canada, 3303 – 33rd Street N.W., Calgary, Alberta

The study of foreland fold and thrust belts (FFTB) is not only of interest with respect to hydrocarbon and ore exploration, but also for example to refine kinematic models and to onravel for example groundwater circulation mechanisms, … . In order to make predictive models, a good understanding of the geological processes in the PVTXt-domain of these tectonised systems is necessary. Here, fluids may play a major role in many of the geological processes whereby significant fluid migration induced during orogenesis may occur as reported by Oliver (1986), however this is questioned by others (Swennen et al., 2003). Besides burial compression, also increasing tectonic compression may cause circulation of fluids. Related increasing temperatures (and possibly also pressure) control water/rock interactions which may induce specific diagenetic processes. While the physico-chemical interactions can be unravelled by studying the diagenetic products, the physico-mechanical rock deformation will be expressed in the structural characteristics of the rocks. Both geological aspects should be linked while working out a good hydrogeological model of fluid migration.

In the Canadian Cordillera, development of a foreland fold and thrust belt links to the paleotectonic setting of convergence between the North America craton and the subduction zones along its western margin. NE-dipping subduction of oceanic lithosphere along the western side of North America started in the Late Triassic and a strip of oceanic lithosphere (±13 000 km wide) has been consumed over the last 150 Ma (Engebretson et al., 1992). The Cordilleran foreland fold and thrust belt began to form in the late Early Jurassic, at ±187-185 Ma (Murphy et al., 1985) in the region behind this NE-dipping, SW-verging oceanic subduction system, as a SW-dipping, NE-verging, accretionary wedge.

To be able to reconstruct the evolution of fluid migration in the Canadian Cordilleran fold and thrust belt, we are investigating different carbonate dominated thrust units in the Rocky Mountains (mainly along the Trans-Canadian Highway N°1). Research is mainly focussed on constraining the physico-chemical conditions which prevailed during the different steps reflected in the diagenetic history. Therefore, field, petrographical and geochemical analyses (i.e. trace element, stable carbon and oxygen isotope, Sr radiogenic isotope geochemistry and fluid inclusion analysis) were carried out. These studies lead to retracing the path of migrating fluids, based on data gathered from regional hydrothermal dolomite bodies, zebra dolomites, dolomite layers characterized by a significant secondary porosity, Mississippi Valley-type Pb-Zn deposists, etc … . A major attempt is also made in investigating calcite cemented veins (in relation to thrust faults). At the same time, structural data have been collected, as well as paleomagnetic data, to provide meaningful input data to work out a PVTXt-model. This modeling has been done by the use of CERES software which was recently developed at IFP (France) (Schneider, 2003).

In general it is assumed that fluid circulation in the studied area is limited (Swennen et al., 2003). However, the zebra dolomites of Middle Cambrian age (Cathedral and Eldon Formation), which have been studied in the area around Field and between Lake Louise and Jasper, reflect focalised fluid flow. The sharp bordered discordant zebra dolomite bodies are characterized by a pattern of alternating mm-thick dark (a) and light (b) coloured sheets (bedding and cleavage oriented), constructing an “abba” cyclicity. Grey dolomite layers consist of fine (<250µm) non-planar crystalline dolomite while the white dolomite layers are constructed of coarse (up to several mms) saddle dolomites. The outer pore lining rim of the latter exhibit a zoned luminescence pattern. Contrary to the replacement origin of the dolomite, the zoned rims are interpreted as cement. All studied zebra dolomites are (nearly) stoichiometric and are characterized by enriched Na and depleted Sr concentrations. Fe and Mn concentrations in these dolomites differ quite a lot depending on the sampling area. Depleted ¹⁸O values (-20 to -14‰VPDB) support a late diagenetic high temperature origin. The range of ¹³C values partially overlaps with the ¹³C signature of Middle Cambrian seawater, suggesting dominant rock buffering during dolomitization. Fluid inclusion data represent hot (T_H = 120 to 210°C) saline (20-23 eq. wt% CaCl₂) fluids.

The zebra dolomitization is explained by episodic expulsion of channeled hot basinal brines along faults. In this process, fluids follow preferential permeability pathways (bedding and cleavage) and reach suprahydrostatical pressures generated in a tectonic compressional regime. As for the zebra dolomites occurring at the Kicking Horse Rim (Field), paleo-facies transitions were preferential locations of fluid interaction, fluid expulsion or structural discontinuities.

The Kicking Horse and Monarch ore deposits in the area of Field are currently being studied. Their relative timing is not fully unravelled, but they most likely developed after the zebra dolomites. Formation of these ore minerals is related to brecciation of host rock dolomites, of which fragments are often bordered by fine crystalline green to greenbrown sphalerite. Between those fragments, ankerite and dolomite is cemented with medium to coarse crystalline green and red sphalerite and galenite. The aim of this study is to deduce the (driving) origin and timing of possibly basin-wide or focalised fluid circulation generating the deposits and regional hydrothermal dolomitization. One of the proposed models is the Cretaceous-Tertiary Laramide model, which was developed by Garven (1985). The latter author discussed a topographically driven regional fluid flow causing dolomitization and mineralization of the Presqu’ile barrier as far as Pine Point. This model combined with tectonically driven fluid flow related to Laramide thrusting has been refined and discussed by Qing & Mountjoy (1992) and others. Nesbitt & Muehlenbachs (1994) advocate the Devonian-Mississippian Antler orogenic model of hydrothermal fluid migration from western shale basins towards eastern and northern permeable carbonate units producing epigenetic dolomite and Pb-Zn mineralizations. Another model is proposed by Nelson et al. (2002), who suggested that sulphide mineralization and saddle dolomite development is caused by mid-Paleozoic tectonic and metallogenetic events of the continent margin during which fluids were driven by thermal convection along both reactivated back-arc structures and permeable stratified units. The data gathered in this study are used to carry out a basin-wide PVTXt-CERES modelling, which will allow to evaluate the validity of the above mentioned models.

Another important example of significant fluid/rock interaction is the development of secondary porosity, created by dissolution of stromatoporoids, in massive dolomite beds of the Cairn Formation. Light grey dolomite rims (mesodolomite with a mosaic extinction) bordering stromatoporoid related cavities (cemented by saddle dolomite, white dolomite and white, greenish and brown calcite) are recognized within the dark grey dolomite host rock. Contrary to the dolomites, the calcite phases display relatively low Na and high Sr values, both decreasing with time in function of the paragenetic evolution. Fe concentration (contrary to the Mn concentration) is high in the dolomite host rock, while it is below the detection limit (<8 ppm) in the dolomite and white and greenish calcite infill generations. Host rock dolomite ¹⁸O values (–8 to –4 ‰VPDB) overlap with the ¹⁸O values of the light grey dolomite rims bordering the biomolds (–8 to –6 ‰VPDB), which indicates that host rock dolomite is affected by mesodolomite recrystallization. ¹³C values around +1,5 ‰VPDB fall within the range of time equivalent marine values. The white and greenish calcite and the dolomite cavity infill phases show more depleted ¹⁸O values (around –11 ‰VPDB) suggesting a high temperature origin, which is supported by fluid inclusion data in the calcites (T_H = 110 to 160°C and salinity = 20 to 23 eq. wt% NaCl). ¹³C values of the white and green calcites cover a large range from -2 ‰ till –26 ‰VPDB, which indicates the involvement of CO₂ derived from thermogenic decarboxylation of organic matter in the presence of sulphate during thermogenic sulphate reduction processes.

The remarkable pervasive development of macroporosity, which is interpreted to be due to the preferential dissolution of non-dolomitized stromatoporoids, could either be explained by ‘host rock cooling’ generated by uplift during thrust emplacement or by acidic fluids generated during the maturation of hydrocarbons. The latter explanation is related to the involvement of depleted CO₂ in the cementation of white and green calcites.

The study of veins in diagenetic research is mainly based on cross-cutting relationships to work out a paragenetic history. Thereby, pre-, syn- and post-tectonic fluid migration can be revealed by relationships between fractures, veins, joints, hydraulic breccies and two types of stylolites, namely bed parallel (due to burial compaction) and tectonic (due to tectonic compression) stylolites. Bed parallel stylolites are assumed to have developed at burial depths of more than 600-800m in function of the nature of the carbonates (Swennen et al., 2003). Tectonic stylolites develop immediately prior to thrust emplacement (Averbuch et al, 1992), which in our case is in relation with the Laramide orogeny. Based on the evaluation of stable oxygen and carbon isotope characteristics of different vein families (based on their structural characteristics) taken from different thrust sheets, some general conclusions can be deduced, but need to be further constrained especially by fluid inclusion analysis. The majority of calcite veins in each analysed thrust zone determines a rather narrow ¹⁸O - ¹³C range. Deviated values are more depleted in ¹⁸O (with ¹³C-signatures pointing rather towards rock buffering), suggesting late-diagenetic high-temperature interactions in semi-closed settings. Hydraulic breccia veins, reflecting expulsion of overpressured fluids along thrust faults, represent the least depleted ¹⁸O signatures of the vein families, unless they are open-cemented. The latter and also cemented joints have very depleted ¹⁸O values, reflecting involvement of meteoric fluids. Although there are exceptions, most slickolites also display similar depleted signatures. Dolomite veins, displaying a ¹⁸O signature which is often more depleted than the calcite veins and less depleted than the open cemented joints, have only been sampled in the Kicking Horse area. Their development is thought to be related to the occurrence of siliciclastics (Swennen et al., 2003) since the Kicking Horse Rim defines the abrupt facies transition of deep-water siliciclastic to shallow-water carbonate layers. Fluorite occurs along tectonic stylolites and suggests existence of evaporite strata in the subsurface, as was recognized in many other FFTB studies by our research team (Swennen et al., 2003). 

Integration of paleomagnetic, structural and diagenetic results, help to constrain the PVTXt-variations for the evolution of fluid migration in the Canadian Rocky Mountain foreland fold and thrust belt. The latter will be used as input data to develop a CERES-modelling. 

References

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