Abstract: Limited Reflux Dolomitization in Restricted Upper Devonian (Leduc and Nisku) Carbonate-Evaporite Shelves, Southeastern Alberta
TAN, WENBIN and ERIC W. MOUNTJOY, Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC, Canada, H3A 2A7
The Upper Devonian Leduc and Nisku formations of southeastern Alberta form two unconformity bounded megasequences which consist of stacked, meter-scale and tans of meter-scale shallowing upward cycles. Unconformities and cycles have been mapped over a distance of 350 km from the international border to Township 40. An idealized cycle consists of calcareous shales at bottom, overlain by limestones, dolomites, and bedded anhydrites at the top. The Leduc Formation contains eight (I to VIII) tens of meter-scale cycles varying from 20 m to 70 m thick. In the Lower Leduc, cycles I and II are partially dolomitized, and thin to the northwest. Cycles III to V in the middle Leduc have the least lateral thickness variation, and are completely dolomitized. The upper Leduc thickens from about 50 m in the southeast to about 100 m in the northwest. Cycles VI, VII and VIII contain about 50% bedded anhydrites in the southeast and about 20% in the northwest. In the Nisku Formation, four 10-25 m cycles (NI to NIV) change from restricted evaporites in the southeast to porous dolostones and tight dolostones in the northwest. The upper two Nisku cycles (NIII and NIV) thicken and prograde to the north.
Three types of pervasive dolomites occur in the Leduc and Nisku formations: microcrystalline, sucrosic and mosaic dolomites, all of which are largely related to primary facies. Microcrystalline dolomites are commonly mudcracked and brecciated, associated with fenestral voids, quartz silt, underlie bedded anhydrite, and thus represent sabkha type dolomites. Sucrosic dolomites appear to have replaced porous peloidal, sorted packstones and grainstones with low argillaceous content, and low angle cross laminations. Mosaic dolomites most commonly replace fossiliferous wackestones and argillaceous mudstones. The sucrosic dolomites form excellent reservoir rocks with porosities of 25% and permeabilities of 100 md, except where Cemented by anhydrites.
Pervasive replacement dolomites formed in the shelf interior during shallow burial presumably from evaporated sea water that refluxed downwards and laterally. This is evidenced by: 1) dolomites overlain by anhydrites in meter-scale shallowing upward cycles; 2) the decrease downwards of the amount of dolomites away from bedded anhydrites; 3) reworked dolomites present in transgressive shaly limestones of the overlying depositional cycles; 4) shelf interior dolomites having 87Sr/86Sr ratios between 0.7082 and 0.7087, and 5) dolomites relatively enriched in 18O but depleted in 13C compared with reefal Leduc and Nisku replacement dolomites in the central part of the Alberta Basin, suggesting an evaporated sea water source.
Evaporated sea water, generated during gypsum/anhydrite deposition, may have moved downdip and laterally to areas where porous carbonate sediments had not been overlain and sealed by evaporites. The presence of bedded gypsum would cause overlying brines to move across sloping shelf interiors, or be ponded on those parts of the shelf with little slope. These fluids would have the potential to dolomitize near surface porous and permeable carbonates. Reflux of evaporated sea water appears to have been mainly limited to meter-scale and tens of meter-scale successions because of the presence of underlying impermeable anhydrite and shale layers, except where porous facies of different cycles were stacked one above the other along the western and northwestern shelf margins and/or shoals.
Mass balance calculations of evaporated brines indicate that there would have been more than enough Mg available to dolomitize all of the limestones in the Leduc and Nisku platforms of the southeastern Alberta shelf, provided these brines could gain access and reflux through them. Shields and Brady (1995) suggested that if evaporated sea water could be continuously refluxed from southeastern Alberta shelf downdip into the isolated reefs and reef trends in the deeper basin it could be responsible for most of the replacement dolomites there. However, the geology shows this is not realistic and that there are many anhydrite and shale permeability barriers in the Leduc and Nisku formations, which probably prevented the downward reflux of brines. The brines would not be able to reflux deeper in the Alberta basin and any signficant reflux could only take place where porous facies were stacked above one another. Reflux of sea water appears to have been mainly limited to meter-scale and tens of meter-scale cycles. Therefore, because of these permeability barriers insufficient brines were able to reflux downdip from the southeastern Alberta Leduc and Nisku shelves to form the extensive replacement dolomites in the deeper central and western part of the Alberta basin, leaving the origin of these dolomites problematical.
Shield, M.J. and Brady, P.V., 1995, Bulletin of Canadian Petroleum Geology, v. 43, p. 371-392
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah