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The Utility of Palynofloral Assemblages for the Interpretation of Depositional Paleoenvironments and Sequence Stratigraphic Systems Tracts in the McMurray Formation at Surmont, Alberta*
Thomas D. Demchuk1, Graham Dolby2, David J. McIntyre3, and John R. Suter1
Search and Discovery Article #50061 (2008)
Posted February 22, 2008
*Adapted from extended abstract prepared for AAPG Hedberg Conference, “Heavy Oil and Bitumen in Foreland Basins – From Processes to Products,” September 30 - October 3, 2007 – Banff, Alberta, Canada.
1 ConocoPhillips Ltd., Subsurface Technology-Reservoir Description-Sedimentary Systems, Houston, TX ([email protected])
2 G. Dolby and Associates, Calgary, Alberta
3 Calgary, Alberta
Palynofloral assemblages from the McMurray Formation are generally rich and diverse. They reveal insights into the depositional paleoenvironments and can be used to discern elements of sequence stratigraphic systems tracts, specifically candidate maximum-flooding surfaces. The character of the palynofloral assemblages and the identification of these candidate flooding surfaces allow subdivision of the McMurray Formation into palyno-chronostratigraphic units which can be identified locally in the Surmont area (Figure 1), and subsequently used for mapping purposes. Additional investigations suggest these palyno-chronostratigraphic units may have regional utility.
The palynofloral assemblages from the McMurray Formation are dominated by terrestrially-derived forms: gymnosperm pollen and pteridophyte spores are abundant. This domination and accompanying species richness suggest that the depositional paleoenvironments were fluvial-dominated draining a vegetationally diverse hinterland (Figure 2). Freshwater algae (e.g., Schizosporis, Pediastrum) are occasionally abundant, indicating short-lived freshwater lacustrine habitats were present. Importantly, dinoflagellates of brackish-water origin are prevalent throughout much of the strata of the McMurray Formation ,indicating varying and persistent marine influence on deposition. This is corroborated by sedimentary structures of tidal origin and varied ichnologic assemblages of brackish trace-makers. These brackish-water dinoflagellates include species of Nyktericysta, Vesperopsis, Balmula, and Pseudoceratium. Their abundance and diversity can be related to the extent of marine influence on deposition; the greater the abundance and diversity, the greater the marine influence (Figure 2). Peaks in abundance can be interpreted as candidate maximum-flooding surfaces.
In the Surmont region, at least five distinct palyno-chronostratigraphic units can be identified based on unique palynofloral assemblages (Figure 3). In the local Surmont area these units undoubtedly reflect unique depositional paleoenvironments, the evolution of diverse paleohabitats, and the influence of marine waters on deposition on these strata. At least three candidate maximum-flooding surfaces can be interpreted, based on the abundances of dinoflagellates, calibrated to sedimentologic and seismic data.
Although the majority of palynoflora from the McMurray Formation are non-age-diagnostic, some tentative correlations can be made to worldwide chronostratigraphies. The McMurray Formation is interpreted as being primarily late Aptian in age, although the lowermost stratigraphy present at Surmont may be as old as early Aptian (Figure 4). No diagnostic Barremian or older palynofloras have been identified. The top of the McMurray Formation can be no younger than earliest Albian based on the presence of earliest Albian dinoflagellates in the overlying Wabiskaw Formation. Simple correlation of candidate maximum-flooding surfaces to worldwide sequence chronostratigraphies for the Aptian indicate that the McMurray Formation at Surmont may have been deposited in approximately 6.0 Ma. The unconformity with the overlying Wabiskaw may represent as much as a 2.5-3.0 my. hiatus/time gap.
Barson, D., Bachu, and S., Esslinger, P., 2001, Flow systems in the Mannville Group in the east-central Athabasca area and implications for steam-assisted gravity drainage (SAGD) operations for in situ bitumen production: Bulletin of Canadian Petroleum Geology, v. 49, p. 376-392.
Michoux, D., 2002, Palynology of the Lower Cretaceous McMurray Formation, Alberta, Canada: Thirty-fifth annual meeting, American Association of Stratigraphic Palynologists, London, UK, September 11-13; also Palynology, v. 27, p. 237, 2003 (abstract).