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who point out the importance of local tectonics to local relative sea-level change.
However this may be, the general concept of the sequence stratigraphic model has
proved useful in predicting the lithological succession at a continental shelf margin
during a single cycle of relative sea-level change. Depositional systems may be de-
scribed in terms of systems tracts, containing contemporaneous depositional systems
that pass laterally from fluvial to deltaic to deep-water systems.
These systems tracts are often interpreted in terms of their position in the sea-level
cycle, consisting of a major sea-level fall, a lowstand, a sea-level rise and a high-
stand. Sea-level falls result in the formation of unconformities that form the sequence
boundaries, and exhibit sub-aerial exposure and a downward shift in coastal onlap. In
the initial sea-level fall, there is erosion of the coastal system and deposition is con-
fined to basin floor fans. During formation of the lowstand systems tract, a lowstand
wedge of sediment is deposited that consists of leveed channel complexes of slope
fans and shelf-edge deltaic complexes. As sea-level rises, a transgressive systems tract
is formed; deposition is reduced in the basin and transgressive systems form on the
shelf and the coastal plain. At the top of the system there is a maximum flooding
surface, and the highstand systems tract is marked by systems that aggrade and eventu-
ally prograde seaward as accommodation space created by the sea-level rise decreases.
Sequence boundaries can be recognised on seismic from the onlap patterns, but can
be difficult to pick. Study of isolated 2-D seismic sections may miss significant fea-
tures because they ignore the map view and do not see lateral changes parallel to the
coast.
Each systems tract presents its own reservoir associations. Thus, in the lowstand
tract, most sediment bypasses the fluvial and delta plain environments; shelf-margin
delta lobes will offer reservoir targets in the delta front, and sand bodies associated
with distributary channels. In the transgressive tract, stratigraphic traps may be formed
in strike-parallel sand bodies such as wave/tide reworked shelf sand bars and barrier
islands. The highstand systems tract often contains fluvial channel-fill sands encased
in overbank mudstones.
One of the key elements of sequence stratigraphy as formulated by Vail
et al.
in
AAPG Memoir 26 was the assertion that seismic reflections generally follow chrono-
stratigraphic surfaces. Although this seems to be correct in many cases, it is not im-
mediately obvious how to relate this to the way that seismic reflections are caused
by impedance contrasts across layer boundaries, which is fundamental to the type of
resonance between the seismic pulse and cyclic sea-level change has been emphasised
by Anstey & O'Doherty
(2002)
. For typical sedimentation rates, a cyclic sea-level
variation with a period of 1-5 million years would give rise to cyclic sedimentation
patterns with thicknesses in the range 15-300 m; for a typical seismic velocity of
3000 m
s, they would have a TWT thickness of 10-200 ms, which is about the same as
the period range for typical seismic waves (frequencies 5-100 Hz). The effect of the
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