Page 104 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 6: FUNDAMENTALS OF SEQUENCE STRATIGRAPHY 95

The term A’ is almost entirely a function of relative sea-
level change. One caveat must be made, though: A’ as well
SYSTEM TRACTS, ACCOMMODATION, SUPPLY
as S’ represent changes of volume in time, the derivative
dV/dt. Sequence stratigraphic models consider either the
change in the vertical dimension only, the partial deriva-
tive ∂z/∂t, or the change in two dimensions, (∂z/∂t).(∂x/∂t). S' > A'
One tacitly assumes these partial derivatives to be good ap- highstand ST
proximations of the change in accommodation volume - a
premise that does not always hold. In fact, one limitation is
immediately obvious. If slope height increases as the system
progrades, the sediment volume required to maintain a con- mfs: S' = A'
stant slope also increases. This reduces the rate of progra-
dation and eventually leads to retrogradation in siliciclas-
tics (Jervey, 1988, p. 54). Rimmed carbonate platforms ex-
perience the same effect but respond somewhat differently transgressive ST S' < A'
by forming an elevated rim, the empty bucket, and finally
drowning completely (Schlager, 1981).
The term S’, rate of sediment supply, is only remotely re- ts: S' << A'
lated to sea level and is a major control on sequences that
may be completely independent of sea level. In siliciclas-
lowstand ST
tic systems, sediment supply is governed largely by con- S' > A'
ditions in the hinterland. The growth term, G’, of carbon-
ates (p. 107f) is tied to the ocean environment and to or-
ganic evolution. The Holocene is particularly important in
this respect because sea-level history is well constrained.
Holocene coasts abound with examples of highstand and
A' < 0
transgressive tracts developing side by side in response to lowstand exposure, (fans)
differences in sediment supply. For instance, many ma-
jor river deltas currently build prograding highstand tracts
while the adjacent shorelines, undersupplied with sediment, A' = dA/dt = rate of change in accommodation ST = systems tract
retrograde and develop transgressive systems tracts. S' = dS/dT = rate of sediment supply mfs = maximum flooding surface
ts = transgressive surface
The standard model of sequence stratigraphy treats sed-
iment supply in a somewhat inconsistent way. On the one
side, supply is being acknowledged as an important control
Fig. 6.12.— Sedimentologic interpretation of systems tracts in
on sequences (Vail, 1987; Jervey, 1988; Van Wagoner et al.,
siliciclastics. They are generated by the interplay of the rate of
1988; Posamentier et al., 1988; Haq, 1991; Emery et al., 1996).
change in accommodation, A’, and the rate of change in sediment
On the other side stands the categoric statement that rela-
supply, S’. Exposure surfaces that extend into formally marine ar-
tive sea-level change dominates sequence stratigraphy (e.g.
eas are diagnostic of relative sea-level falls and cannot be gener-
Vail et al., 1977; Vail, 1987, p. 3). The standard model of sys- ated by variations of supply. Colors: green – alluvial to littoral, light
tems tract assumes that “ ... sediment supply is constant ... blue – neritic to bathyal, dark blue – turbidite fans. After Schlager
” (Posamentier et al., 1988, p. 110) or that sediment supply, (1992), modified.
changing more slowly than sea level, has only a modifying
effect on sequences (Vail, 1987). This assumption is also a
prerequisite for correlating the transgressive and highstand
tracts as well as the maximum flooding surface to specific the figures certainly are relevant for sequence stratigraphy
2
1
parts of the relative sea-level curve (e.g. Posamentier et al., in the sense that the spatial scale, 10 -10 km, is compara-
1988; Vail et al., 1991; Handford and Loucks, 1993; Emery et ble to that of many sequence-stratigraphic studies. Limiting
al., 1996). this comparison to regions within one climate belt increases
First principles of sedimentation provide little support for their relevance for sequence stratigraphy: individual third-
the notion that accommodation effects generally dominate order sequences are likely to form within one climate belt
5
6
over supply effects (with the exception of the exposure un- as the characteristic time scale of sequences, 10 -10 years,
conformities and downsteps, of course). Much of the spread generally is too short for plate tectonics to move the area
in sedimentation rates is probably generated by supply vari- into another climate belt.
ations in space. Schlager (1993) shows that the sediment The geologic record also shows that sediment supply to
yield of rivers in Italy varies by a factor of 5 and the yield the ocean has varied through geologic history. For instance,
of rivers of Taiwan varies by a factor of 40. Both regions sedimentation rates of the latest Cenozoic are sigificantly
are small and reside entirely within one climate belt. Thus, higher than in the earlier Cenozoic - perhaps as a conse-