Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 480

The inner estuary deposits in the McMurray Formation are dominated by
brackish water (i.e., oligohaline to perhaps mesohaline). Strata of the inner estu-
aries characteristically comprise IHS. The impoverished trace-fossil assem-
blages that are characteristic of the IHS have been well documented
( Pemberton et al., 1982; Ranger and Pemberton, 1992 ) and consist of abundant
but generally diminutive and monospecific assemblages of Planolites , Sko-
lithos , and Cylindrichnus , or spiral forms such as Gyrolithes and undiagnosed
microhelical forms ( Figs. 3 D and 4 K). Trace fossils are typically restricted to
either the sandstone or the mudstone member of the IHS, which is potentially
indicative of seasonal larval recruitment and colonization ( Gingras et al., 2011 ).
Within the IHS units, the bioturbation intensities characteristically increase
upward: BI

2-3 throughout much of
the medial part of the IHS ( Fig. 3 D and K), and locally grading to BI

¼ 0 at the base of IHS successions, BI ¼

5at
the top of the succession. This vertical distribution is interpreted to represent
lowered sedimentation rates upward and the transition to bioturbated
intertidal-flat-associated strata.
The middle- to outer-estuary deposits of the McMurray Formation are
characterized by amalgamated decimeter- to meter-bedded, high-angle cross-
stratified sandstone, which is interpreted to represent laterally accreted
mid-channel tidal bars. The cross-bedded sandstones contain a sparse,
low-diversity marine-influenced trace-fossil assemblage. Considering the
high-energy stress indicated by the physical structures, trace fossils
within the cross-stratified sands are, not surprisingly, uncommon. Nonetheless,
conical plugs assigned to Conichnus and Siphonichnus are locally present,
but rare. Thin, mud-draped hiatal surfaces, that locally cap cross-stratified
bedsets, contain a very low-diversity trace-fossil assemblage consisting of
Cylindrichnus and/or Skolithos . The trace-fossil evidence (the largest
trace fossils observed and the local presence of large Siphonichnus and
Conichnus ) as well as the presence of higher-energy physical sedimentary
structures are most consistent with meso- to macrotidal regimes. The
differentiation between middle and outer estuary remains unclear in the
McMurray subbasin.

¼

5. DISCUSSION
Although the ichnological content varies from estuary to estuary, trends in
trace-fossil distributions are discernible. Important commonalities that relate
to our ability to identify estuary deposits in the rock record can, therefore, be
established. First, in all of our above-reported examples, the trace-fossil assem-
blage is influenced in landward locales by the presence of brackish water. As
such, a salinity gradient is always evident at the estuary scale, and this can
be established through simple observations of trace-fossil sizes, diversities,
and types, particularly when compared to their fully marine counterparts