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Dolby, Graham, Thomas D. Demchuk, and John R. Suter,
The Significance of Palynofloral Assemblages from the Lower Cretaceous McMurray Formation and Associated Strata, Surmont and Surrounding Areas in North-central Alberta
Graham Dolby,1 Thomas D. Demchuk,2 John R. Suter3
1Graham Dolby and Associates, 6719 Leaside Dr., SW, Calgary, Alberta, T3E 6H6, Canada (e-mail: [email protected])
2ConocoPhillips, Subsurface Technology, PO Box 2197, Houston, Texas, 77252, U.S.A. (e-mail: [email protected])
3Oil Sands, ConocoPhillips Canada, PO Box 130 Station M, Calgary, Alberta, T2P 2H7, Canada (e-mail: [email protected])
We thank ConocoPhillips Oil Sands Business Units (Calgary) and Subsurface Technology (Houston) for their permission to publish these data and interpretations. We also thank Total Canada and Cenovus for their agreement to publish information from the Surmont and Foster Creek–Christina Lake properties. The studies benefited greatly from the excellent palynological slides prepared by Russ Harms of Global Geolab Ltd. The manuscript has benefited greatly from suggestions from Henrik Nohr-Hansen and Stan Stancliffe.
Palynofloral assemblages associated with strata of the McMurray Formation, Wabiskaw Member, and Clearwater Formation can be placed into a classification scheme based primarily on dinocyst content. Although most of the palynofloral assemblages are dominated by terrestrially derived pollen and spores, the dinocysts can be used to characterize fresh water through a marine continuum in which to place these diverse paleoenvironments. Freshwater and slightly brackish paleoenvironments are most characteristic of the McMurray Formation, whereas stressed, shoreface, and nearshore paleoenvironments are most characteristic of the Wabiskaw Member and Clearwater Formation strata. Dinocyst assemblages from the McMurray Formation are characterized by the freshwater algae Hurlandsia rugara and rare Holmewoodinium sp., with varying abundances of Nyktericysta spp. group dinocysts. The relative abundance and diversity of these Nyktericysta spp. dinocysts can be correlated with increased brackish influence. Locally within the McMurray Formation, the presence of Vesperopsis spp. may indicate significant brackish influence. Within the overlying Wabiskaw Member and Clearwater Formation, dinocyst assemblages are indicative of the southward-transgressing Clearwater Sea. Assemblages may be dominated by species of Circulodinium (C. deflandrei and C. brevispinosum), Odontochitina operculata, Oligosphaeridium spp., Palaeoperidinium cretaceum plus a host of accessory taxa indicative of stressed paleoenvironments, including several new undescribed species. Significantly, the distribution and nature of the palynofloral assemblages do not validate the historic threefold division of the McMurray Formation into lower, middle, and upper members, nor do the palynofloral assemblages reflect a gradual upward increase in marine influence. Instead, the palynofloral assemblages indicate much more regionally diverse paleoenvironments, with brackish influence recognized throughout.
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