Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 479

that some aspects of the McMurray fill are deltaic and others are estuarine. We
focus here on the bioturbated units within the McMurray Formation that have
been interpreted to have an estuarine affinity. The flooding of theMcMurray sub-
basin led to the establishment of a large complex of estuarine and deltaic deposits
that are observable andmapped over an area exceeding 16,000 km 2 . Owing to the
presence of notable bitumen resources, the deposit is well known fromoutcrop as
well as from a very large core dataset. Although the stratigraphy is exceedingly
complex, it is clear that estuary-associated valley fills represent a large propor-
tion of the strata assigned to the McMurray Formation.
Trace fossils are common in many facies of the McMurray Formation.
Pemberton et al. (1982) showed that impoverished trace-fossil suites associated
with the ubiquitous IHS, defining tidally influenced point-bar deposits, indi-
cated a brackish-water, estuary environment. Later work (e.g., Ranger and
Gingras, 2008 ) differentiated outer and inner estuary deposits, each character-
ized by discrete ichnological assemblages.

4.3.1 Valley Margins and Substrate-Controlled Suites
McMurray Formation strata sit variably on argillaceous limestones and calcareous
shales of the Devonian. Regionally, this erosional discordance represents an angu-
lar unconformity, and the valleyswere carved intomore recessive levels. Evidence
for trace-fossil omission suites are rare in the lower levels of the McMurray For-
mation. However, parasequences at the top of the McMurray Formation can be
identified by the presence of rare, low-diversity firmground suites of Thalassi-
noides and Gastrochaenolites , attributable to the Glossifungites Ichnofacies.
The Clearwater Formation lies on top of the McMurray Formation, and the con-
tact between the two formations is easily identified by the presence of firmground
suites of Diplocraterion , Thalassinoides , Rhizocorallium , and Skolithos . Within
the Clearwater Formation, there is a switch to marine sedimentation and biotur-
bation associated with shoreface and deltaic depositional environments, indicat-
ing that the ongoing relative sea-level rise led to the filling of the subbasin.

4.3.2 Trace-Fossil Distributions by Subenvironment
No bay-head delta deposits are observed in the estuary portions of the McMurray
Formation. Rather, there appears to be a gradational change from fluvially influ-
enced sedimentation to tidally influenced sedimentation in a northward direction.
In landward deposits, the strata are current-ripple laminated to cross-
stratified. Mudstone beds and mudstone drapes are absent to moderately
abundant, and intraformational mudclasts and carbonaceous detritus are locally
common. Importantly, bioturbation is rare (BI 2), and trace fos-

sils ( Planolites and Skolithos ) are sporadically distributed. Local vertical and lat-
eral trends in trace-fossil distributions have not been discerned. These deposits
are likely associated with the tidally influenced part of the fluvial system.

¼ 1; rarely BI ¼