Stratigraphy and Sequence Stratigraphic Surfaces of the McMurray Formation

Michael J. Ranger¹ and Murray K. Gingras²
¹808 West Chestermere Drive, Chestermere, AB, Canada T1X 1B6, [email protected]
²University of Alberta, Edmonton, AB, Canada T6G 2E3, [email protected]

The McMurray Formation is the host reservoir for a large portion of the Lower Cretaceous Athabasca Oil Sands. The McMurray Formation has long been casually subdivided into lower, middle and upper members. These remain informal, and our perception is that their identity still seems confusing and uncertain to many geoscientists who work with and study this succession. We suggest here that these units should be raised to formal status as members, specifically allomembers. We observe that the lower, middle and upper McMurray members can be reasonably distinguished in core or outcrop, they have sequence stratigraphic significance, and they can be related to events in the development of the basin. We also propose formal nomenclature for these members based on geographic type localities of surface outcrop or subsurface core.

For the lower McMurray, we propose the name “Daphne Member” after the island in the Athabasca river. Daphne Island is directly adjacent to partial exposures of the lower McMurray member that crop out along the mainland river bank. We also propose the name “Steepbank Member” for the middle McMurray. Outcrops along the Steepbank River were the study site of the modern interpretation of the middle McMurray member as brackish-water point bars. (Flach and Mossop, 1985; Pemberton, et al., 1982). A complete section of middle McMurray is exposed here. The Steepbank River also has well-exposed and complete sections of the upper McMurray member. We propose, however, that the upper McMurray member be referred to as the “Chard Member”. In the Chard area of southwest Athabasca the McMurray is present only in the subsurface. It is in the Chard area that parasequences in the upper McMurray were first recognized as correlateable subsurface units (Ranger, 1994; Ranger and Pemberton, 1997), and it is this area that underwent intensive stratigraphic scrutiny related to the gas-over-bitumen legal issues (AEUB, 2003).

The Daphne (lower) Member was deposited only in the deeper parts of the valley system that makes up the Athabasca subbasin. It is distinguished with high reliability where it is capped by an alluvial unit. The alluvial unit consists of paleosols, coals, underclay and rooted muds, and has the only terrestrial trace fossil assemblage (insect burrows) recognized to date in the McMurray Fm. The alluvial unit is also associated with abundant coarse-grained, apparently freshwater, fluvial channels. In some outcrops, the fluvial channels are seen to interfinger with the alluvial unit, while in other outcrops they incise the alluvial unit. At the Daphne outcrop the alluvial unit lies directly and unequivocally on brackish-water, Epsilon, point-bar deposits (the so-called IHS or Inclined Heterolithic Stratification). The Daphne (lower) Member is commonly described as being dominantly fluvial (i.e. freshwater fluvial). However, we observe dominantly brackish-water elements in the Daphne Member. The only fluvial element observed appears to be that associated with the alluvial unit that caps the Daphne Member. The bias towards a dominantly fluvial interpretation in many studies may be a consequence of the widespread preservation of the upper alluvial unit where it onlaps the sub-Cretaceous unconformity, or, in our experience, many researchers mistake the lower part of the Steepbank (middle) Member for fluvial channels, and frequently assign that to the lower McMurray. (We interpret the lower part of the Steepbank (middle) Member as tidal in origin, not fluvial.)

The Steepbank (middle) Member lies directly on the alluvial unit of the Daphne Member or, higher in the valley system where the Daphne Member is not present, it lies directly on Paleozoic carbonates at the sub-Cretaceous unconformity. Most (but not all) of the significant reservoirs lie in the Steepbank Member. In outcrop, the contact of the Steepbank Member with the overlying Chard (upper) Member is commonly easily distinguished from a panoramic perspective. Whereas the upper part of the Steepbank Member almost invariably preserves brackish point-bar deposits with a distinctive lateral accretion geometry, the Chard (upper) Member appears to preserve dominantly vertically-accreting, coarsening-upward parasequences. Therefore, the contrast in geometry between the Steepbank and Chard Members is typically quite distinct. On close examination of outcrop, however, the contact can be difficult to pinpoint because the base of the Chard Member is almost invariably muddy, weathered, and recessive. In cores the precise contact between the Steepbank (middle) and Chard (upper) is, more often than not, somewhat cryptic. The Chard Member is generally distinguished by a domination by oscillation structures (wave, hummocky, swaley). Here the tidal signature typical of the Steepbank Member is missing, reflecting an apparent evolution from a tide-dominated system (Daphne and Steepbank Members) to a wave-dominated system (Chard Member). The nature of the erosional contact between the Steepbank and the Chard members is uncertain but, where observed, the erosional contact indicates that at least locally they are separated by a transgressive surface of erosion.

The upper contact of the Chard (upper) Member with the overlying Wabiskaw Member of the Clearwater Formation is a formally defined lithostratigraphic contact. That contact is defined as a change from the quartz arenite of the McMurray Formation to a glauconitic litharenite containing abundant dark chert of the Clearwater Formation, which reflects a change in provenance. The contact is typically easy to pick in outcrop and core and is probably an unconformity. In some cores however, where an interpretation of deep Wabiskaw incision is contentious, the contact is less certain.

Each of the Daphne (lower), Steepbank (middle), and Chard (upper) members contains internal stratigraphic surfaces that are correlateable over a wide area, and which we interpret to have sequence stratigraphic significance.

The Daphne Member is composed of at least one parasequence. This parasequence comprises a cross-bedded sandstone that grades upwards into sparsely bioturbated, sand-dominated IHS. Ichnofossils include Planolites and adhesive-backfilled meniscate traces (insect traces) upwards (CNRL Horizon geological team, pers. comm.). A regionally mappable pedogenic unit abruptly caps the Daphne Member IHS, demarcating the top of the Daphne Member. The pedogenic unit interfingers with bedded granular, channelized sands, interpreted as freshwater fluvial. The granular sediments both intercalate with sands of the IHS unit and occur as lenses in pedogenically altered silts and clays.

The Steepbank Member erosionally overlies the Daphne Member. Two major facies associations dominate the Steepbank Member. These are thick, crossbedded, megarippled sands (the main reservoir unit), and sand- to mud-dominated IHS, which always overlie the megarippled sands. It is common thought that this succession represents a single genetic unit of thick channel fill, wherein the lower megarippled sands comprise the traction load in the thalweg, merging upwards into the IHS point-bar deposits. We do not agree with this interpretation. Close observation shows that there is everywhere a sharp discontinuity between the IHS and the megarippled sandstones (Fig. 1). We interpret the succession to represent a trans-regressive couplet. The lower megarippled sands represent tidal-bar and -channel deposits whereas the erosionally overlying IHS represents brackish-water (deltaic), distributary channel deposits. The sharp discontinuity between the units is a regressive surface of erosion suffered during the deltaic phase. Locally a second couplet appears to be present.

The Chard (upper) Member abruptly overlies the Steepbank (middle) Member. Within the Chard Member are at least three coarsening-upwards parasequences, bounded by flooding surfaces that exhibit transgressive erosion. These surfaces are typically marked by a granular lag several centimeters in thickness. In its most common form the lag has been incorporated into the basal part of a transgressive marginal-marine mud apparently due to bioturbation. Where the coarsening-upwards gamma ray signature of the parasequences is missing in the subsurface, some researchers prescribe an incised valley fill.

A summary of the proposed stratigraphic relationships is presented in Figure 2. The depositional units shown therein are mappable and possess stratigraphic continuity. Moreover, the surfaces separating the depositional units lend themselves to an interpretation of stacked parasequence sets throughout the deposition of the McMurray Formation.

REFERENCES

Alberta Energy and Utilities Board (AEUB) 2003. Report 2003-A: Athabasca Wabiskaw-McMurray Regional Geological Study, 187 p.

Flach, PD. and Mossop, G.D. 1985. Depositional environments of the Lower Cretaceous McMurray formation, Athabasca Oil Sands, Alberta. American Association of Petroleum Geologists Bulletin, v. 69, pp 1195-1207.

Pemberton, S.G., Flach, P.D. and Mossop, G.D. 1982. Trace fossils from the Athabasca Oil Sands, Alberta, Canada. Science, v. 217, pp. 825-827.

Ranger, M.J. 1994. A basin study of the southern Athabasca Oil Sands Deposit. Unpublished PhD. thesis, University of Alberta, Edmonton, Alberta, 290 p.

Ranger, M.J. and Pemberton, S.G. 1997. Elements of a stratigraphic framework for the McMurray Formation in South Athabasca. In: Pemberton, S.G. and James, D.P. (eds.), Petroleum geology of the Cretaceous Mannville Group, western Canada. CSPG Memoir 18, Canadian Society of Petroleum Geologists, Calgary, pp 263-291.

Figure 1. Erosional contact between IHS point bar deposits and megarippled tidal sands of the Steepbank (middle) Member.