This chapter deals with the youngest Frasnian and oldest Famennian rocks in the Western Canada Sedimentary Basin, comprising, in succession, the Woodbend and Winterburn groups (Fig. 12.1). These two groups occupy the interval between the Beaverhill Lake and Wabamun groups and contain several distinct chronostratigraphic sequences composed largely of cyclic carbonate-clastic and, to a lesser extent, cyclic carbonate-evaporite deposits.
During early phases of Woodbend Group deposition the Western Canada Sedimentary Basin underwent gradual deepening. This resulted in the development of a thick aggradational succession of carbonates with substantial topographic relief between shelf and basinal areas. By the end of Woodbend deposition, during a period of reduced rate of subsidence and/or sea-level fall, most of the basin was filled by shales. Winterburn Group deposition was characterized by an overall shallowing and filling of the basin.
A number of notable changes occurred during deposition of the Woodbend-Winterburn strata, which resulted in distinct features that set these groups apart from earlier Paleozoic strata. These changes and features include: 1) an apparent increase in the rate of accumulation and preservation of sediment, 2) a dramatic increase in the occurrence of basin-filling shales, 3) the development of thick and extensive reef complexes, 4) the deposition of widespread and prolific source rocks, and finally 5) the significant accumulation of economic reserves of hydrocarbons hosted largely by numerous reefal carbonate reservoirs.
According to Harland et al. (1982) the Frasnian Stage represents approximately seven million years. Considering that the Woodbend and the majority of Winterburn strata were deposited during only part of this time (Fig. 12.1), their maximum thicknesses are extraordinary, in excess of 850 m (Fig. 12.2). The relative contribution of each group is indicated in Figures 12.3 and 12.4. During no other Paleozoic period in the Western Canada Sedimentary Basin was such a thick depositional record accumulated and preserved within such a short time frame. This thick accumulation reflects the interplay between eustatic sea-level changes, rapid sedimentation, and most importantly, a high rate of basin subsidence.
In conjunction with this dramatic increase in sedimentation, Woodbend and Winterburn strata display the lithological complexities of earlier Devonian strata. However, in addition to limestones, dolomites and evaporites, these groups show an increase in the thickness and areal extent of basin-filling shales (Ireton, Fort Simpson, Duvernay and Majeau Lake formations).
The two most distinctive features of these groups are the stacked reef complexes (Leduc Formation) that exceed 275 m in thickness, and the highly bituminous source rocks (Duvernay and Muskwa formations), considered by some (Allan and Creaney, 1991) to have generated the majority of hydrocarbons found within the Upper Devonian reservoirs of the Alberta Basin.
From an economic standpoint, these groups have received a great deal of attention for their significant reserves of conventional hydrocarbons. The Woodbend and Winterburn groups contain approximately 32 and 11 percent, respectively, of the initial established conventional oil plus oil-equivalent gas reserves found within the Paleozoic strata of the Alberta Basin (Energy Resources Conservation Board, 1990). The majority of these oil and gas pools are associated with ancient reef complexes of widely varying size and shape, depositional setting and facies composition (Alberta Society of Petroleum Geologists, 1960, 1966 and 1969). In addition, significant accumulations of heavy oil and bitumen are present at the Grosmont and Winterburn subcrop edges. As a consequence, over the past 45 years a great deal of emphasis has been placed on their study, including reviews of the architectural evolution of reef stages, depositional facies, paleontology, post-depositional alteration, relations between reef development and basin filling and controls on reef localization (see references in Belyea, 1964; Watts, 1987; Moore, 1989; Dix, 1990). Despite this emphasis, there still remains considerable uncertainty in establishing the time equivalence between various reef complexes from different areas of the basin and their relation to basin-filling. This aspect is discussed in some detail in a subsequent section of this chapter.
Considering the economic significance of these strata, it is remarkable that aside from Belyea (1964), little additional regional work has been published on the geology of the Woodbend and Winterburn groups on the scale of the Western Canada Sedimentary Basin. Only Basset and Stout (1967), Ziegler (1967), Nelson (1970), Porter et al. (1982), Burrowes and Krause (1987), Stoakes and Wendte (1987), and Moore (1989) have attempted to enhance the understanding of paleogeography beyond that of Belyea's original work. In an attempt to build upon these previously published works, the aim of this chapter is to illustrate, at a greater level of detail, the sequential basin-wide development and depositional history of the Woodbend and Winterburn groups.
Last modified: August 12, 2008
