Has Burial Dolomitization Come of Age? Some Answers from the Western Canada Sedimentary Basin
- Bruce Purser,
- Maurice Tucker and
- Donald Zenger
Published Online: 14 APR 2009
Copyright © 1994 The International Association of Sedimentologists
Dolomites: A Volume in Honour of Dolomieu
How to Cite
Mountjoy, E. W. and Amthor, J. E. (1994) Has Burial Dolomitization Come of Age? Some Answers from the Western Canada Sedimentary Basin, in Dolomites: A Volume in Honour of Dolomieu (eds B. Purser, M. Tucker and D. Zenger), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304077.ch13
- Published Online: 14 APR 2009
- Published Print: 25 MAY 1994
Print ISBN: 9780632037872
Online ISBN: 9781444304077
- burial dolomitization - Western Canada sedimentary basin;
- early dolomites and possible neomorphism;
- Upper Devonian Leduc and Swan Hills reefs and carbonate platforms;
- Winnipegosis Formation buildups - crop out at Dawson Bay, Manitoba;
- sea-floor or very shallow-burial dolomitization;
- deeper-burial (late) dolomites;
- petrographic and geochemical characteristics of replacement dolomites in Devonian buildups
Many, if not most, of the dolomites in the Western Canada Sedimentary Basin appear to have formed in the subsurface during two or more stages: intermediate (between 500 and 1500 m) and deep (1500–3000 m or deeper). These subsurface dolomites exhibit similar textural characteristics, isotopic and geochemical trends, and paragenetic relationships in different parts of the basin. Dolomitization varies from partial to complete, and is texturally preserving or destructive, although most is destructive. Significant calcite dissolution is often coincident with, or postdates, dolomitization. Seafloor dolomitization, occurring centimetres below the sediment–water interface, or very shallow-burial dolomitization (less than 500 m) is suggested in some cases by microcrystallinity (5–20 µm), mimetic textures, marine carbon, oxygen and strontium isotopic compositions.
Massive replacement dolomites form between 50 and 90% of all dolomites, yet their origin is still problematic. Oxygen isotopic compositions of these dolomites suggest that they formed at between 50° and 70°C in the shallow to intermediate subsurface. Strontium isotopic compositions are slightly more radiogenic than Devonian seawater. A key problem is explaining how the vast quantities of formation waters (probably modified seawater) required for dolomitization were pumped through intermediately buried sediments.
Late-stage deeper-burial coarse-crystalline dolomite cements, including saddle dolomites, occur as vug and fracture fillings, and generally constitute between 1% and 5% of the rock. Temperatures derived from fluid-inclusion analysis range from 80°C to 200°C, and salinity estimates range from 18 to 31 wt/% NaCl equivalent, respectively. Given the temperature, the depth can be determined assuming normal geothermal gradients and, in turn, the approximate geological time of dolomitization. Generally, fluid inclusions indicate higher temperatures than expected from maximum burial, suggesting that hydrothermal formation waters moved to higher levels along fractures and conduit systems. Indeed, saddle dolomites are often, but not always, associated with Mississippi Valley-type ore deposits, which probably resulted from the circulation of hydrothermal fluids. Locally, dissolution preceded or is associated with this late phase of dolomitization. In some petroleum reservoirs and many Pb-Zn deposits (e.g. Pine Point), solution by hydrothermal fluids was extensive and overprinted many of the earlier dolomites. These examples of extensive solution and diagenetic overprinting also appear to be related to the upward movement of deep-basin brines through conduit systems.
The latest carbonates to precipitate in the Western Canada Sedimentary Basin tend to have the most radiogenic Sr compositions. High 87Sr/86Sr ratios indicate that some radiogenic Sr was introduced to these carbonates from adjacent and underlying clastic strata and/or the Precambrian basement. The saline formation waters that precipitated these late dolomites and calcites were radiogenic and similar in terms of their Sr isotope ratios to the present-day deep-basin formation waters in the Western Canada Sedimentary Basin, supporting the idea of their late timing. There is little or no direct evidence for suitable heat sources in the Western Canada Sedimentary Basin to drive thermal convection. Therefore, most late-stage dolomites appear to be related to basinwide fluid flow, probably caused by a combination of sedimentary and tectonic loading and topographically driven fluid flow during the Cretaceous and Early Tertiary. Some other basins also exhibit two or more phases of dolomitization, except that the early replacement dolomites are interpreted to have formed much earlier during the seaward progradation of tidal flats, or in a marine–meteoric mixing zone.