Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 477

an erosional unconformity in all but the westernmost locales. The basal beds of
the Montney Formation reflect a regional marine transgression and a shift of the
shoreline hundreds of kilometers to the east. The Montney Formation records
several regional fluctuations in relative sea level, permitting subdivision of this
unit into several unconformity-bound sequences.
4.2.1 Valley Margins and Substrate-Controlled Suites
The sequence boundary in the Montney Formation occurs approximately
between the Induan and Olenekian stages ( Davies et al., 1997; Kendall,
1999; Markhasin, 1998; Moslow and Davies, 1997; Panek, 2000 ). This surface,
referred to as the mid-Montney sequence boundary, resulted in significant ero-
sional incision into an older clastic ramp succession. In the Simonette-Kaybob
area of west-central Alberta, this erosional incision includes several incised
paleovalley complexes ( Buatois et al., 2005; Markhasin, 1998 ). A low-density,
low-diversity firmground omission suite consisting of Skolithos , Thalassi-
noides , and rare Rhizocorallium of the Glossifungites Ichnofacies characterizes
the basal contact of the estuarine IVF succession.
4.2.2 Trace-Fossil Distributions by Subenvironment
Planar- to wavy-parallel-laminated siltstones with thin, sharp-based sandstone
beds are interpreted as subtidal bayfill and storm-washover complexes
( Fig. 11 B-E). These beds contain a low-diversity, locally high-density assem-
blage consisting of Phycosiphon , Gyrochorte , Lingulichnus , Planolite s, and
Trichichnus ( Fig. 11 ). Heterolithic interlaminated siltstone and very fine-
grained sandstone beds are characterized by low-angle inclined surfaces and
display low-diversity but locally high-density assemblages consisting of Phy-
cosiphon , Conichnus , Lingulichnus , Palaeophycus , Planolites , Psilonichnus ,
Skolithos , and Thalassinoides. These heterolithic facies are interpreted as
intertidal-flat/estuarine-bar deposits ( Fig. 11 F and G).
Lingulide brachiopod dwelling traces ( Lingulichnus ) are notably abundant
in the Montney estuarine deposits ( Fig. 11 B and C) and occur in adjacent shore-
face successions as well. Lingulide brachiopods, both modern and ancient,
exhibit wide environmental tolerances and, thus, occur in settings that range
from distal offshore through shoreface and intertidal flat ( Emig, 1997;
Zonneveld and Pemberton, 2003; Zonneveld et al., 2007 ). Consequently, the
presence of Lingulichnus in Montney estuarine successions is not diagnostic.
Notably however, the Montney Lingulichnus are smaller within estuary-fill
successions and diminish in size to the east/southeast (paleo-upstream).
4.3 Tide-Dominated(?) Estuary, McMurray Formation
(Aptian to Albian), Alberta, Canada
TheMcMurray Formationwas deposited in a series of North-South trending val-
leys during the Aptian and Albian transgression of the Boreal Sea, which inun-
dated the setting fromNorth to South. The drainage was complex, and it is likely