Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 170

in the Joffre Field. These WRS are cut through an underlying SU to form the
composite surface WRS/SU ( Figs. 3 and 7A, B ). These shoreface successions
record short-lived periods of shoreline progradation across the WRS/SU during
an overall transgression. This surface is particularly susceptible to the genera-
tion of ichnological omission suites, because units subaerially exposed during
formation of the SU are dewatered, erosionally exhumed, and/or cemented.
During transgression and wave ravinement, these units are exposed to
marginal-marine or marine conditions, favoring colonization of the erosional
discontinuity. Such omission suites are most commonly attributable to the Glos-
sifungites Ichnofacies, but hardground suites of the Trypanites Ichnofacies can
FIGURE 7 Incised shoreface deposits of the transgressive systems tract. Core box photo (A) and
close-up (B) from a transgressively incised shoreface. Lower offshore mudstones of the underlying
regional parasequences of the highstand systems tract are cross-cut by bioturbated lower shoreface
muddy sandstones containing a gravel lag. The discontinuity is a composite surface representing a
wave ravinement surface cutting across an earlier subaerial unconformity (WRS/SU). The discon-
tinuity is demarcated by firmground Thalassinoides ( Th ) of the Glossifungites Ichnofacies. Core box
photo (C) and close-up (D) of the same transgressively incised shoreface, but lying in a more distal
position. The underlying lower offshore bioturbated mudstones of the highstand systems tract are
overlain by pervasively bioturbated, grit-, and gravel-bearing upper offshore mudstones associated
with the incised shoreface. Unlike the BSFR of the forced regressive shoreface shown in Fig. 5 D,
however, these mudstones are separated by a firmground suite of Thalassinoides ( Th ) attributable to
the Glossifungites Ichnofacies. As such, the discontinuity represents erosional conditions prior to
deposition, corresponding to the WRS/SU, despite the fact that the overlying facies reflect deposi-
tion below fair-weather wave base. This shows that the surface was cut while RSL was lower, and
was subsequently reworked by waves during transgression, indicating that the incised shoreface
comprises part of the transgressive systems tract. Early Cretaceous Viking Formation, Joffre Field.
See text for explanation. Scale bar ¼ 15 cm (A and C) and 3 cm (B and D).