Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 173

complexes, reflecting fluvial channels, distributary channels of the bay-head
delta, tidal channels, and/or tidal inlets. This distribution of facies has led to
the designation of the classical tripartite zonation of facies ( Dalrymple et al.,
1992 ). Each valley complex reflects a phase of incision during RSL fall, accom-
panied by sediment bypass to the lowstand shoreline and delta complexes lying
further seaward ( Pattison andWalker, 1994 ). Valley fill may have been initiated
during the LST with the aggradation of a fluvial topset ( Fig. 7 ), but occurred
mainly during early transgression. Successions are characterized by predomi-
nantly estuarine conditions (see Gingras et al., 2012 ) and show onlap of
brackish-water bay deposits along valley margins as well as aggradation of
bay-head delta lobes. Pattison (1991) and Pattison and Walker (1994) describe
bay-head delta successions that locally reach 27.4 m in thickness.
3.2.1 Stratigraphic Discontinuities of the Viking Incised Valleys
The bases and margins of the incised valleys are commonly demarcated by firm-
ground suites attributable to the Glossifungites Ichnofacies ( MacEachern and
Pemberton, 1994; Pemberton and MacEachern, 1995; Pemberton et al.,
1992b ). The valley mouth and margins correspond to the SU, which may be
eroded or draped by different TSs ( Figs. 8-11 ). Tidal scour generates TRSs near
the mouth of the estuarine valley ( Fig. 9A and B ) and are associated with tidal
inlets and tidal channels ( Fig. 9 C and D). TRS tend to show erosional relief and
may erode through the SU to form a composite surface (TRS/SU). TRSs are
generally not widespread and may form spatially discontinuous surfaces, par-
ticularly when associated with tidal inlets that backstep across older central-
basin mudstones during erosional retreat of the barrier complex at the mouth
of the system ( Fig. 8 ).
WRSs are more widespread and are formed along the seaward face of the
backstepping barrier complex fronting the valley ( Fig. 8 ). As RSL continues
to rise, wave erosion bevels a relatively flat, seaward-dipping WRS that initially
truncates the earlier lowstand fluvial and transgressive estuarine infill of the
valley ( Figs. 8 and 11A, C ), but eventually cuts across the SU along the valley
interfluves to generate the WRS/SU once the valley is filled ( Fig. 11D and E ).
Within the valley, the TS may flood across older lowstand deposits, or may
onlap the SU along the valley margins to produce TS/SU, particularly below the
bay-head delta and the central-basin mudstones ( Fig. 8 ). In such positions, the
TS shows minimal scour, with bay-head delta deposits showing downlap and
aggradational trajectories and the central-basin mudstones showing onlap of
the valley margins ( Figs. 10 and 11 A, C ).
TRS/SU, TS/SU, andWRS/SU are readily colonized by substrate-controlled
tracemakers and are widespread in many stratigraphic successions ( Figs. 9-11 ).
The SU cut during RSL fall typically produces widespread firmground, hard-
ground, and woodground surfaces, which can then be colonized by marine
and marginal-marine tracemakers during RSL rise, provided the SU is not