Geology Reference
In-Depth Information
1985), Tucker (1985), Tucker and Wright (1990), James
and Kendall (1992), Handford and Loucks (1993),
Sanantonio (1993), Read et al. (1995) and Lukasik et
al. (2000) provided
enlarged and
more differentiated
models of carbonate depositional environments.
These
papers discuss the subdivision and internal subdivision
of ramps and emphasize the importance of non-rimmed
shelves, drowned and pelagic platforms, as well as
epeiric platforms and ramps. Some of these models are
discussed in Sections 14.1.4 to 14.1.8.
sections. The value of the models is usually tested by
comparison with well-studied outcrops (e.g. Harris
1991).
Most computer models are based on algorithms that
simulate processes acting during the development of
carbonate buildups and determining their shape and
internal structure. Simulations are performed on a spa-
tial grid in a sequence of time steps from a prescribed
set of initial conditions defined by various input pa-
rameters. During program application, these parameters
are varied within the limits of known geological and
sedimentological constraints until the model is consis-
tent with all available data and observations. The pro-
grams developed for modeling carbonate and/or car-
bonate-siliciclastic depositional systems differ in their
main goals, input parameters used, and the way mod-
eling results are shown.
14.1.1.2 Dynamic Models
Depositional systems are inherently complex, yet an-
cient successions are often reduced to very simple
models considering just one major control factor, most
commonly sea level. These
dynamic models
stress the
changes in depositional systems and environmental con-
trols through time and combine different scales of ob-
servations from microfacies to sedimentary sequences
(Aigner 1984). Dynamic models have their merits be-
cause they try to avoid strict categorizations such as
those involved in the Wilson model and because
dynamic models describe the development of facies
zones during time. Systems tracts deciphered by
sequence stratigraphy are the result of gradual changes
in marine environments caused by variations in water
depths related to sea-level fluctuations, accommoda-
tion space and biogenic production. These changes are
characterized by shifts in the relative importance of
limestone grain types. The shifts reflect temporal con-
tinua or relays that can be derived from optimized simi-
larity matrices (see Sect. 14.4).
Carbonate computer models are two-dimensional or
three-dimensional. Principal key
input parameters of
2-D modeling
(see Enos 1991) are sea-level fluctua-
tions, subsidence, clastic sediment input and depth-re-
lated carbonate growth potential. Other parameters are
sedimentation and erosion rates (Bosence et al. 1994),
platform shape and slope angle, and early diagenetic
processes and compaction (e.g. Strobel et al. 1990).
Model outputs
are 2-D cross sections explaining
•
internal and external platform geometries (Bosence
and Waltham 1990; Read et al. 1991),
•
the growth and geometry of reef structures (Gerhard
1991; Bosscher and Schlager 1992; Hüssner 1997),
•
the evolution and geometry of shelf-basin transitions
(Aigner et al. 1990; Kendall et al. 1991; Bosence et
al. 1994),
•
the development of carbonate slopes (Bosscher and
Southam 1992),
14.1.1.3 Numerical Models
•
the estimation of paleobathymetric conditions (Aurell
et al. 1995),
A third category of facies models are
computer models
conditioned with appropriate geometric and strati-
graphic data from outcrops or wells and process data
from modern carbonate depositional environments.
These models simulate temporal development, stratal
geometries and the architecture of carbonate buildups,
and evaluate the rates of processes responsible for the
formation of carbonate platforms and slopes. Computer
models provide a visual image of the internal architec-
ture of carbonate platforms (e.g. Bosence and Waltham
1990) and platform-slope geometries and the shape of
slopes (Aigner et al. 1989; Bosscher and Southam
1992). Modeling allows quantification and evaluation
of the major controls on the evolution of platforms and
ramps studied in outcrops and derived from seismic
•
the sea-level control on cyclic sedimentation (Gold-
hammer et al. 1991),
•
the evaluation of depositional models based on se-
quence stratigraphic concepts (Burgess 2001).
Problems involved in computer models are the dif-
ficulty in separating global sea-level signals from local
or regional processes, obtaining a high time resolution,
and quantifying the controls on the biogenic carbonate
production potential. In addition, many models provide
only rough lithologies or strongly generalized lithofa-
cies types. Thin-section data on microfacies scale are
important parameters in modeling component cycles
(see papers in Franseen et al. 1991).
