Evaluating new fault‐controlled hydrothermal dolomitization models: Insights from the Cambrian Dolomite, Western Canadian Sedimentary Basin

Ardiansyah Koeshidayatullah,Hamish Robertson,Peter K Swart,Adrian Boyce,Jack Stacey,Cathy Hollis,Fiona Whitaker,Hilary Corlett

doi:10.1111/sed.12729

Ardiansyah Koeshidayatullah, Hamish Robertson + Show 6 more

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AbstractFault‐controlled hydrothermal dolomitization in tectonically complex basins can occur at any depth and from different fluid compositions, including ‘deep‐seated’, ‘crustal’ or ‘basinal’ brines. Nevertheless, many studies have failed to identify the actual source of these fluids, resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitization. With development of new concepts in hydrothermal dolomitization, the study aims in particular to test the hypothesis that dolomitizing fluids were sourced from either seawater, ultramafic carbonation or a mixture between the two by utilizing the Cambrian Mount Whyte Formation as an example. Here, the large‐scale dolostone bodies are fabric‐destructive with a range of crystal fabrics, including euhedral replacement (RD1) and anhedral replacement (RD2). Since dolomite is cross‐cut by low amplitude stylolites, dolomitization is interpreted to have occurred shortly after deposition, at a very shallow depth (<1 km). At this time, there would have been sufficient porosity in the mudstones for extensive dolomitization to occur, and the necessary high heat flows and faulting associated with Cambrian rifting to transfer hot brines into the near surface. While the δ18Owater and 87Sr/86Sr ratios values of RD1 are comparable with Cambrian seawater, RD2 shows higher values in both parameters. Therefore, although aspects of the fluid geochemistry are consistent with dolomitization from seawater, very high fluid temperature and salinity could be suggestive of mixing with another, hydrothermal fluid. The very hot temperature, positive Eu anomaly, enriched metal concentrations, and cogenetic relation with quartz could indicate that hot brines were at least partially sourced from ultramafic rocks, potentially as a result of interaction between the underlying Proterozoic serpentinites and CO2‐rich fluids. This study highlights that large‐scale hydrothermal dolostone bodies can form at shallow burial depths via mixing during fluid pulses, providing a potential explanation for the mass balance problem often associated with their genesis.

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Dolomitized Palaeozoic carbonate strata in the Western Canada Sedimentary Basin (WCSB) have attracted many studies over the last few decades, within Devonian strata, due to their significance as hydrocarbon reservoirs (Amthor et al, 1994; Mountjoy et al, 1999; Duggan et al, 2001; Packard et al, 2001)
The sandstones are typically cemented by dolomite with the occasional presence of dolomite cemented fractures (Stacey et al, 2017)
The Mount Whyte Formation directly overlies the Gog Group and is principally composed of five different lithologies that make up a total succession of ca 78 m thickness with both lateral and vertical diagenetic contacts between dolostone and limestone (Fig. 3A and B)

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Introduction

Dolomitized Palaeozoic carbonate strata in the Western Canada Sedimentary Basin (WCSB) have attracted many studies over the last few decades, within Devonian strata, due to their significance as hydrocarbon reservoirs (Amthor et al, 1994; Mountjoy et al, 1999; Duggan et al, 2001; Packard et al, 2001). Strontium and barium The mean of strontium (Sr) values measured from two different dolomite fabrics (RD1 and RD2) in the Mount Whyte Formation are considerably lower than the host limestone samples, between 28 ppm and 31 ppm, respectively, while the limestone mean value is 808 ppm (Table 1; Fig. 7B).

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