The Role of Ichnology in the Stratigraphic Interpretation of the Athabasca Oil Sands
Michael J. Ranger1, Murray K. Gingras2, and S. George Pemberton2
1Geoscience Consultant, Chestermere, Canada
2University of Alberta, Edmonton, Canada
Ichnology (i.e. trace fossils: the study of animal-sediment relationships) has played a pivotal role in the facies and stratigraphic interpretation of the McMurray Formation, the main reservoir unit of the Athabasca Oil Sands Deposit. Trace fossils are abundant in many facies of the McMurray Fm., yet for many years sedimentological studies relied almost entirely on physical structures and wireline logs. Subsequent field studies (Pemberton et al., 1982) showed that an impoverished trace fossil suite associated with the ubiquitous IHS beds (epsilon cross-stratified point bar deposits) indicated a brackish and, by implication, estuarine environment for the McMurray Formation.
We discuss here how trace fossils have guided our interpretation of the facies and the stratigraphy of the McMurray Formation. Furthermore, facies analysis, reinforced and integrated by the trace fossil assemblages, has allowed us to observe flaws in some of the commonly held interpretations of the McMurray Formation. Recognition of some widespread stratigraphic surfaces provides insight to the stratigraphy, but also opens up more questions about the sequence stratigraphic interpretation of the McMurray Formation.
Core and field evidence now demonstrate that the brackish elements of the McMurray Fm. are largely confined to the IHS point bar beds, mostly of the middle McMurray (Steepbank Member), but present also in IHS beds in the lower McMurray (Daphne Member). The impoverished trace fossil assemblages that are characteristic of the IHS beds have been well documented, and consist of abundant but generally diminutive and monospecific assemblages of Cylindrichnus or spiral forms (Figure 1) such as Gyrolithes and Spirophyton, as well as micro-helical forms (see description and nomenclature recently proposed by Lettley, 2004).
Thick megarippled sand beds (commonly known as the “trough-cross-bedded sands”), which are always observed to underlie the IHS beds, were once thought to be the channel bottom, traction load that merges upwards into point bars (IHS) of estuarine channel fills. However, the megarippled sands are everywhere separated from the IHS beds by an erosional discontinuity. They are not a continuum through a single genetic unit (i.e. the “mega-channels” of Flach and Mossop, 1985), a commonly accepted model which should now be refined.
The interpretation of the megarippled sands is of high priority, because these constitute a large proportion of the prime bitumen reservoirs. The megarippled sands are observed to contain a sparse but unequivocal marine trace fossil signature, as well as physical structures whose weight of evidence strongly indicates a high-energy, tidal environment. Furthermore, a meso- to macro-tidal regime is intuitive for the early McMurray depositional setting, given the entrenched, elongate, basin trends that can be mapped as deep valleys within the carbonates at the sub-Cretaceous unconformity. Considering the extreme energy stress apparent from the physical structures, ichnofossils within the megarippled sands are extremely rare, as would be expected. However in several locations, at widely-separated outcrops of the megarippled sands, one can observe conical plugs assigned to Conichnus (Figure 2), understood to be the domicile/resting-trace of filter-feeding anemone-like organisms in a normal marine environment (n.b. not brackish). Thin, mud-draped, hiatus surfaces that occasionally cap megarippled bedsets contain a very low diversity trace fossil assemblage, mostly Cylindrichnus but also Skolithos.
Cylindrichnus is also the most common ichnogenera observed in the impoverished assemblage within the brackish IHS beds that overlie the megarippled sands. We suggest that Cylindrichnus results from the filter-feeding strategy of an early coloniser, multiplying and growing rapidly to occupy an newly opened niche, only to be exterminated when environmental conditions underwent sudden, periodic, and severe changes. In the case of the IHS beds, that change was probably a rapid accumulation of mud, associated with salinity fluctuation; and in the case of the megarippled sand, a resumption of extreme tidal energy levels. (“doomed Pioneers”, Föllmi and Grimm, 1990)
The lower McMurray (Daphne Member) is capped by a typically thick (5-15m) alluvial unit, which is one of the most easily correlateable units within the McMurray formation. It is limited in its areal distribution, however, because the lower McMurray accumulated only as an early valley fill on the Sub-Cretaceous unconformity, which it onlaps. We use the term “alluvial” to include coals, dark organic (and rooted) muds and very light grey, leached, pedogenic mud, as well as sandy fluvial channel fills. Rooting is by far the most abundant trace fossil in the lower McMurray alluvial unit. However, the pedogenic muds are also known to contain insect burrows (Figure 3), which have been observed in both core and outcrop. The contact between the lower McMurray alluvial unit and the base of the middle McMurray is believed to be a transgressive surface of tidal ravinement.
The upper McMurray (Chard Member) appears to record a major change in basin configuration. Once the deep valleys within the sub-Cretaceous unconformity had become filled, basin topography became attenuated, releasing the delta from tidal control, and converting to a wave- (and possibly fluvial-) dominated system within a large open bay, much more quiescent than the lower and middle McMurray. In many areas the upper McMurray is preserved as stacked parasequences, each of which is floored by a transgressive surface of erosion, commonly demarcated by a granular grit. This is interpreted as a flooding surface upon which a transgressive mud has accumulated. The mud exhibits a strong marine trace fossil signature, dominated by Teichichnus and Helminthopsis (Figure 4). The distinctive fabric and trace fossil assemblage of this mud are easily recognisable and widespread, and can be used as a reliable stratigraphic marker.
Within a parasequence, bioturbation gradually becomes less abundant and less diverse upwards within a facies of normally-graded event beds overprinted with synaeresis cracks. This evidence suggests an stressed environment for marine organisms caused by extreme salinity fluctuation combined with pulses of turbid flow resulting in rapid deposition of mud. A thin coal or organic mud containing abundant rooting is commonly preserved at the top of the parasequences.
The upper McMurray parasequences have been interpreted as a series of stacked, prograding, shoreface units (Ranger and Pemberton, 1997) within a shallow basin fed by deltaic lobes of fresh water fluvial systems (Caplan and Ranger, 2001). It is reasonable to conjecture that the thickness of each parasequence (6-10m discounting compaction) represents the magnitude of periodic sea-level rise within the upper McMurray (and by inference, perhaps the lower and middle McMurray as well), reflecting the incremental accommodation space produced at each such event.
Trace fossil evidence, as well as the observation of an erosional discontinuity between the IHS point bars and the underlying megarippled sands, lead us to discard the “mega-channel” interpretation for the middle McMurray. Brackish channels are indicated only by the IHS beds, and do not include the underlying megarippled sands. The megarippled reservoir sands represent a separate genetic unit, more marine and distal than the overlying, proximal, brackish, point bars of the IHS unit. Nonetheless these two genetic units are almost always observed to occur together as a macro-couplet, proximal overlying distal, suggesting a regressive succession. Current debate revolves around whether the erosion surface between the units represents an unconformity, indicating a forced regression, or simply a regressive ravinement of a prograding tidal system. An alternative interpretation is that the megarippled sands represent the transgressive systems tract on sea level rise in a strongly tide-dominated basin, whereas the IHS brackish channels record the prograding systems tract at highstand.
We also note a long- and commonly-held belief that the lower McMurray (Daphne Member) represents a fluvial environment, the middle McMurray (Steepbank Member) an estuarine environment, and the upper McMurray (Chard Member) a marine environment.
We feel that trace fossil evidence does not support this generalisation. Indeed we note brackish elements in the lower McMurray, whereas much of the unequivocal fluvial systems are a constituent only of the alluvial unit that caps the lower McMurray. In the middle McMurray the megarippled reservoir sands have a strong marine/tidal fingerprint, whereas the only strong brackish element is the thick, overlying IHS beds. In the upper McMurray, we note parasequences that are floored by transgressive marine muds, grading upwards through a zone of fluctuating salinity, and capped by proximal coals and rooted muds. Overall the succession is much more complex than a simple aggradation through fluvial, estuarine and marine environments.
Caplan, M. and Ranger, M.J., 2001. Description and interpretation of coarsening-upward cycles in the McMurray Formation, northeastern Alberta: Preliminary results. Canadian Society of Petroleum Geologists (core conference), Expanded Abstracts, Calgary, pp. 010-1 - 010-9
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.
Lettley, C.D. 2004. Elements of a genetic framework for Inclined Heterolithic Strata of the McMurray Formation. Unpublished MSc. thesis, University of Alberta, Edmonton, Alberta, 150 p.
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. 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.
AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada