Falling-stage systems tract in tropical carbonate rocks - Perspectives in Carbonate Geology - page 299

input settings for specifi c runs are given in the
captions or the accompanying text.
The C3D program simulates the major processes
operating in carbonate systems after input of
time and spatial scales, initial three-dimensional
surface, and sea-level history. In the program,
accommodation can be controlled by sea-level
changes, differential subsidence and compac-
tion during the model run. Carbonate sediments
are simulated to be produced in platform mar-
gin, platform interior and pelagic environments.
Each of these carbonate factories has production
rates that vary with water depth and with respect
to a restriction effect related to distance from
the platform edge. Platform-margin production
increases to a maximum toward the margin,
platform-interior production increases with dis-
tance inshore from the margin and pelagic sedi-
mentation increases basinward away from the
margin (for details see Warrlich et al ., 2002).
Simulated strata may be eroded at a user-defi ned
rate to give coarse and fi ne reworked sediment.
If the shear stress is above a critical value speci-
fi ed for a particular grain size, sediment becomes
entrained and transported. Transport is governed
by a shear stress vector fi eld that is a combination
of the shear stress due to currents and waves as
well as slope. As only the shear stress due to slope
is grain-size dependent, coarse-grained sediments
tend to follow slope and fi ne sediments follow
currents and waves (Warrlich et al ., 2002). Thus
the program simulates bed-load and suspended-
load transport respectively. Deposition occurs in
areas where shear stresses are below the critical
value. The C3D program calculates sedimentary
geometries as time surfaces and simulates facies.
The facies assigned to a cell is either that of the
process that forms the largest fraction of the accu-
mulated sediment within that cell or the facies
defi ned by the user as a fi xed combination of grain
types from a particular production belt or from
erosion (see 'digital facies' of Boylan et al . (2002)
for details).
The set-up used to investigate the FST was
a topographic surface 0.6-5 km long and
0.1-0.5 km wide, consisting of fl at top and adja-
cent slope, with initial sea level covering the
shallow part; one carbonate production func-
tion with production rates of 100-6000

HST

FST

LST

Basin-floor fan

Sequence boundary sensu Hunt & Tucker (1992)
Sequence boundary sensu Posamentier & Allen (1999)

Fig. 2. Model of forced-regressive wedge systems tract
(here called falling-stage systems tract, FST) according
to Hunt & Tucker (1992). The FST consists of (1) down-
stepping pairs of topsets and foresets and (2) slope and
basin-fl oor fans already included in the standard model;
both units represent sediment bodies formed during sea-
level fall. In colour: two common ways of positioning the
sequence boundary if FST exists.

that some form of 'falling-stage systems tract'
(FST) is rather common in the sequence record
even though the systems-tract terminology and
the position of the sequence boundary remain
controversial (Fig. 2).
This study examines the situation with respect
to carbonate rocks, in particular the depos-
its of the tropical, or T factory sensu Schlager
(2003, 2005). The key parameters controlling the
presence or absence of the FST were identi-
fi ed via basic principles of sedimentation and
numerical modelling. Well-documented case
studies were used as 'ground truth' for the
validity of the numerical models. Data from
numerical models and case studies were com-
bined to establish the parameter space for the
development of FST and the STM respectively.
Finally, it was estimated what portion of the
stability domains of FST and STM is geologic-
ally probable, i.e. supported by geological
observation.

METHODS

CARBONATE-3D (C3D)

All modelling was done with CARBONATE-3D
(C3D), a fi nite difference, forward-modelling
program that simulates the evolution of carbon-
ate platforms and mixed carbonate siliciclastic
systems up to millions of years with time steps of
tens to thousands of years. The program is writ-
ten in IDL (Information Data Language). The basic
mathematical structure and its justifi cation in
view of sedimentological principles is given in
Warrlich et al . (2002). Below follows a brief sum-
mary of the program structure. The most relevant

m yr 1 ,
decreasing with depth as described in Warrlich
et al . (2002). Information on sea-level curves,
production rates and erosion rates are indicated
with the specifi c runs.