Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 156

Omission suites are generally most apparent when developed in hard-
grounds ( Trypanites Ichnofacies) and woodgrounds ( Teredolites Ichnofacies).
Such suites become progressively more challenging to discern where they form
in firmgrounds and stiffgrounds ( Glossifungites Ichnofacies). Palimpsest soft-
ground suites are commonly the most difficult to identify. Considerable care
must be taken in differentiating palimpsest softground suites from cross-cutting
tiers in an association of contemporaneous traces; detailed appraisal of the
ichnofabric may facilitate this distinction (cf. Buatois and M´ngano, 2011;
McIlroy, 2004; Taylor and Goldring, 1993; Taylor et al., 2003 ).

1.3 Autogenic Allogenic Discontinuities

In sequence-stratigraphic analysis, differentiation of breaks of autogenic deri-
vation from those of allogenic origin is paramount. Autogenically driven
changes in deposition are part of the dynamic nature of the sedimentary envi-
ronment and generally affect only that setting and those to which it supplies
sediment. The scale can be quite variable, such as distributary avulsion and
autogenic lobe switching in large deltas; nevertheless, such surfaces are not
regional and do not correlate along strike into adjacent settings. Hiatal surfaces
or localized discontinuities may be generated and colonized by omission suites,
but are of limited temporal significance and more spatially limited (e.g.,
Gingras et al., 2000, 2001 ).
Changes induced allogenically (e.g., via tectonics, eustasy, or climate), by
contrast, affect RSL, and therefore are imposed upon multiple depositional
settings both along strike and down depositional dip by external forces. The
resulting discontinuities tend to record fundamental changes in depositional
conditions that affect a number of contemporaneous environments. While such
surfaces are produced by autogenic processes characteristic of various deposi-
tional settings (e.g., wave erosion in a shoreface setting, fluvial incision in a
river), the changes that occur are a consequence of allogenic mechanisms.
The resulting discontinuities tend to be of greater temporal significance and
are more regionally extensive, allowing correlation through stratal units of a
number of depositional environments. Surfaces corresponding to allogenic
changes are the most relevant to sequence stratigraphy.
Differentiation of autogenically and allogenically induced surfaces is, how-
ever, challenging at the scale of outcrop and core studies, and omission suites
may be occur at both. Surfaces observed at these local scales must be demon-
strated to possess the geometries and regional extents consistent with allogenic
discontinuities in order to have relevance to sequence-stratigraphic architecture.
One exception to this is the tendency to employ autogenically induced marine
flooding surfaces associated with delta-lobe switching to bound parasequences
(e.g., Bhattacharya, 2010; Catuneanu et al., 2011 ). This arises from the recog-
nition that deltas fail to achieve equilibrium profiles during stable sea level
and normal regression, and that autogenic controls continue to dominate the

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