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In-Depth Information
the two options; they ultimately chose the second
option because the unconformity on top of the
FST can be expected to be more pronounced and
easier to trace than the surface at the base of the
FST. In this study, the fi rst option is favoured for
reasons given below.
The differentiation into transgressive and
highstand systems tracts is a function of the bal-
ance between the rate of accommodation creation
and the rate of sediment supply (Jervey, 1988;
Schlager, 1993). Constructing a sea-level curve
from these patterns requires independent informa-
tion on the variations of sediment supply or data on
the variations in the rate of aggradation. Lowstand
and falling-stage systems tracts, on the other hand,
offer unambiguous evidence of a fall of relative
sea level independent of the rate of sediment
supply. The surface at the base of the FST provides
the timing of the onset of this fall; timing of the
termination of fall as well as the range of fall can
be gleaned from the lowstand systems tract. Thus,
FST and the LST of the standard model have very
similar functions in sea-level reconstructions and
this similarity is a strong argument in favour of
drawing the sequence boundary between HST and
FST (Posamentier & Allen, 1999; Posamentier &
Morris, 2000) and considering the FST as a subdi-
vision or a close relative of the lowstand tract.
i.e. the part supported by geological
observation. Falling-stage systems tracts and
the continuous unconformity of the standard
model should, therefore, both be common in
the sequence record of tropical carbonates.
This study found no support for two commonly
5
expressed generalizations: (1) The FST should
always be present, albeit diffi cult to recognize.
(2) The anatomy of the standard model can
serve as a prototype for all sequences.
ACKNOWLEDGEMENTS
We thank David A. Waltham and Bart Wassing for
advice on modelling matters and Henk Droste for
data on the Cretaceous of Oman. C.G. St. C. Kendall
and J.A. Simo provided important reviews.
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CONCLUSIONS
1
Numerical modelling linked to fi eld observa-
tions indicates that the falling limb of a sea-
level cycle commonly generates a sediment
accumulation in tropical carbonates - the
falling-stage systems tract (FST). This sequence
anatomy differs sharply from the standard
model (STM) of sequence stratigraphy where
highstand and lowstand systems tract are sepa-
rated by an erosional unconformity.
Whether tropical carbonates develop FST or
2
STM anatomy depends on the balance of the
rates of sea-level fall, erosion and carbonate
production. Slope angle is a fourth, modulating
control parameter.
The FST is favoured by slow sea-level fall, slow
3
erosion and high carbonate production as well
as by small slope angles.
Numerical modelling allows one to defi ne the
4
stability fi elds of FST and STM in a parameter
space of sea-level, production and erosion.
The FST domain fi lls about half the geologic-
ally probable part of this parameter space,
