Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 155

normal regression during the HST. The TST directly overlies the MRS formed
at the end of regression and is bounded at the top by the “MFS” formed when
marine sediments reach their most landward position. Stacking patterns exhibit
backstepping and onlapping, as well as retrogradational clinoforms that thicken
landward in siliciclastic systems.
The HST encompasses progradational deposits formed when sediment accu-
mulation rates exceed the rate of accommodation increase during the later
stages of RSL rise. The HST directly overlies the MFS and is capped by the
SU and its CC sensu Posamentier and Allen (1999) . Stacking patterns display
prograding and aggrading clinoforms that typically show downdip thinning,
capped by a topset of fluvial, coastal plain and/or delta plain deposits.
1.2 Recognition of Trace-Fossil Omission Suites
Ichnological omission highlights a discontinuous record of sediment accumula-
tion and/or of its preservation. Omission generally corresponds to sediment
bypass, erosional exhumation, sediment starvation, and/or subaerial exposure,
typically leading to changes in substrate consistency. Substrate consistency con-
stitutes one of the most important factors controlling the distribution of organisms
in sedimentary environments (see Dashtgard and Gingras, 2012 ), and therefore,
changes in the substrate owing to omission lead to fundamentally different ich-
nocoenoses. Consequently, omission suites can help to recognize some of the key
bounding surfaces in sequence-stratigraphic frameworks, because such surfaces
are commonly associated with erosion, prolonged non-deposition, or subaerial
exposure (e.g., RSME, TRS, WRS, and MFS). Much of the existing literature
has focused on the identification of RSME, TRS, and WRS, as these show
erosional exhumation or significant sediment bypass within a marine or
marginal-marine environment (cf. MacEachern et al., 2007a,b ). The potential
exists, however, for the SU to be identified through the presence of continental
omission suites (e.g., suites of the Coprinisphaera or Scoyenia ichnofacies)
overprinting marine or marginal-marine facies of the earlier HST (e.g., Buatois
and M´ngano, 2011; Buatois et al., 1998; Pemberton et al., 1992c ).
Omission suites are recognized on the basis of some combination of the fol-
lowing features: (1) trace fossils cross-cut both the primary sedimentary fabric
and the palimpsest trace-fossil suites; (2) trace fossils are passively filled with
sediment derived from above the discontinuity; (3) trace fossils are commonly
unlined; (4) trace fossils are commonly sharp walled and may contain bioglyphs
(e.g., scratches); (5) trace fossils may occur in lithologies that would not or
could not host such ethological groupings in their original softground consisten-
cies (e.g., dwelling/suspension-feeding structures in shale); (6) trace fossils are
borings; (7) trace fossils show little or no post-depositional compaction; and (8)
trace fossils are associated with mineralized crusts and/or encrusting biota
(e.g., Bromley, 1975; MacEachern et al., 1992a; McIlroy, 2004; Pemberton
and MacEachern, 1995, 2005; Pemberton et al., 1980; Taylor et al., 2003 ).