Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 159

hand, are marked by regionally mappable extents and separate genetically
unrelated successions possessing discrete stratal geometries. It remains the
responsibility of the geoscientist to determine the significance of any
substrate-controlled omission suites encountered in the rock record, and not
to blindly assign sequence-stratigraphic importance to them.
1.4 Facies and Ichnofacies Juxtaposition
Sedimentary facies are well positioned to characterize depositional trends and
to recognize changes in those trends. As such, changes in facies lend insight into
the presence and possible origin of the stratigraphic surfaces that bound the
facies. Ichnological assessments can be critical in evaluating and interpreting
such sedimentary facies. As well, changes in trace-fossil suites can be useful
in discerning shifts in RSL, because they record organism responses to changes
in a number of physico-chemical conditions, such as sedimentation rate, depo-
sitional energy, salinity, and subaerial exposure. Such ichnological assess-
ments, however, can be challenging as they rely heavily on the interpretation
of tracemaker behaviors, determination of the ichnological diversity (particu-
larly with respect to the proportions of facies-crossing elements), and paleoen-
vironmental implications of the trace-fossil suites in the context of Walther's
Law. Juxtaposed softground suites or ichnofabrics that do not reflect originally
adjacent depositional environments (i.e., successions of suites that contravene
Walther's Law) can be critical in the sequence-stratigraphic assessment of the
ancient record (e.g., MacEachern and Pemberton, 1994; Pemberton et al.,
1992a; Taylor and Gawthorpe, 1993; Taylor et al., 2003 ). Used in conjunction
with physical sedimentology, they constitute a powerful tool in the character-
ization and interpretation of facies and facies associations.
RSL fall commonly leads to the juxtaposition of more proximal conditions
over more distal, because it records the onset of negative accommodation. Shore-
line trajectories change from an aggradational or progradational stacking pattern
to a degradational one ( Bhattacharya, 2011 ). Ichnologically, this negative accom-
modation leads to continental trace-fossil suites or marginal-marine suites to
overlie fully marine associations (e.g., Bann et al., 2004 ), but is also commonly
accompanied by localized higher sedimentation rates (leading to lower bioturba-
tion index values), increased sediment calibers (leading to changes in animal/
sediment interaction as reflected by ethology), and/or widespread subaerial expo-
sure (favoring development of continental ichnocoenoses).
RSL rise, by contrast, is accompanied by positive accommodation and com-
monly leads to distal settings overlying more proximal environments, provided
that the rate of positive accommodation exceeds sediment supply. This favors
aggradational and backstepping shoreline trajectories. Erosion associated with
TSE also commonly leads to top-truncated shoreline successions (e.g., Bhatta-
charya, 2010, 2011 ), which are commonly demarcated by ichnological