Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 172

basal discontinuity, coupled with a study of the overlying facies and the overall
stacking patterns, is therefore key to determining whether the shoreface or del-
taic succession occupies the FSST, LST, or TST.
3.2 Estuarine Incised-Valley Fill, Viking Formation,
Alberta, Canada
The Early Cretaceous Viking Formation of Alberta, Canada, has served as a hydro-
carbon exploration target since the 1930s. The 1978 discovery of the Crystal Field,
however, represents the first Viking interval to be recognized as an incised-valley
complex ( Reinson et al., 1988 ). Pattison (1991) and Pattison and Walker (1994)
placed theVikingdeposits at Crystal into an allostratigraphic/sequence-stratigraphic
framework, employing the wave-dominated estuary model of Dalrymple et al.
(1992) . Pattison (1992) , Pemberton et al. (1992b) ,and MacEachern and
Pemberton (1994) addressed the ichnological signature of the various depositional
subenvironments of the incised valley, integrating the trace-fossil data with the sed-
imentological and sequence-stratigraphic models. The Viking Formation of the
Willesden Green Field was also assigned an estuarine valley interpretation by
Boreen and Walker (1991) .
Although the resulting model is relatively robust, detailed resolution of the
internal facies architecture has remained elusive, despite the presence of 395
wells in the Crystal Field (149 of them cored). This architectural complexity
is a consequence of the valley having served as the locus of more than one phase
of valley reincision, leading to the juxtaposition of non-contemporaneous fluvial
and estuarine erosional remnants. The wave-dominated estuary model shows
spatially discrete facies complexes associated with the bay-head delta, central
basin, and estuarymouth ( Fig. 8 ). These deposits are locally cross-cut by channel
FIGURE 8 Schematic longitudinal section through an idealized wave-dominated estuarine incised
valley, showing the distribution of facies associations and internal discontinuities. SU, subaerial
unconformity; WRS, wave ravinement surface; TRS, tidal ravinement surface; MFS, maximum
flooding surface. See text for explanation. Model modified from Zaitlin et al. (1994) .