Geology Reference
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depositional systems; Schlager 1991). The interpreta-
tion of sequences in carbonate platforms requires more
basic controls (facies and core analysis) than sequence
stratigraphy analysis of siliciclastic rocks (Schlager
1992). Siliciclastic and carbonate sequences differ with
regard to the additional control of sediment supply by
changes in carbonate productivity (Schlager 1991;
Driscoll et al. 1991). Siliciclastics shed most of their
sediment during sea-level lowstands, shallow-water car-
bonates during sea-level highstands. Input of siliciclas-
tics is strongly controlled by tectonics (influencing the
relief and erosion of the hinterland). Carbonate sys-
tems are strongly controlled by environmental factors
governing biogenic carbonate production. Carbonate
and siliciclastic systems generate their own specific
relief on the sea floor.
Tropical carbonate platforms and reefs are often ter-
minated by drowning unconformities. Drowning may
be caused by environmental stress (caused, for example,
by shifts in currents or hinterland drainage) reducing
the growth potential of carbonate systems rather than
by sea level. In contrast, sequence-stratigraphic pat-
terns of temperate carbonates exhibit similarities with
those of siliciclastic sediments (Read 1995).
cal climates favor shallow-marine carbonate deposi-
tion. Arid climate favors high rates of evaporation and
the deposition of evaporites. Humid climate promotes
karstification of exposed carbonate platforms during
sea-level lowstands by rainfall.
16.1.2.1 Sequence Analysis: Some Basics
Sequence stratigraphic analysis is based on the subdi-
vision of sedimentary sequences into units (sequences)
that are bounded by surfaces of stratal discontinuities
or their correlative conformities. These units are verti-
cally arranged into
depositional sequences
(genetically
related packages).
The development of a sequence-stratigraphic frame-
work for carbonate rocks requires integrating observa-
tions from core, wireline logs and outcrop analogs. The
sequences (systems tracts; parasequences) are bounded
updip by unconformities and downdip by correlative
conformities. Systems tracts consist of all facies de-
posited during either lowstand, transgression or high-
stand sea levels. Lowstands and highstands are sepa-
rated by the transgressive surface (flooding surface).
The transgressive stand is separated from the highstand
by the maximum flooding surface.
Controls
The basic assumption for sequence stratigraphy is
that sequences and their systems are essentially con-
trolled by sea-level change and tectonics (Vail et al.
1977; Van Wagoner et al. 1988; Posamentier et al. 1988;
Emery et al. 1996). However, the original and classi-
cal interpretation in terms of sea-level cycle is not the
unique solution. The volume and composition of the
sediment will have effects similar to sea-level fluctua-
tions, especially in carbonate depositional systems
(Schlager 1992). Environmental factors and not only
sea level may represent a primary control of the inter-
nal architecture of sequences. Therefore, dominant con-
trolling factors may change over time (Posamentier and
James 1993).
Systems tracts
An ideal sequence can be divided into three major
components that deposit at different phases of the rela-
tive sea level. These systems tracts are defined mainly
on the basis of their stratal geometries, using termina-
tions of seismic reflectors of onlap, downlap, and offlap,
but must be also defined in terms of facies and micro-
facies (see Sect. 16.1.2.2).
•
Lowstand systems tract (LST).
Includes all deposits
accumulated after the onset of relative sea-level fall.
The shelf is exposed to erosion and/or karstification
producing secondary porosity. Clastics will be brought
closer to the basin of accumulation. The carbonate fac-
tory is moved basinward and is productive over a nar-
rower area, while deeper basins may be starved of car-
bonates and exhibit condensed horizons. Characteris-
tic features of LST are common to abundant siliciclas-
tic input, exposure surfaces on marine carbonates, karst,
caliche horizons and increased influx of siliciclastics.
•
Transgressive systems tract (TST).
Comprises the
deposits accumulated from the onset of coastal trans-
gression until the time of the maximum transgression
of the coast (maximum flooding surface), just prior to
Carbonate stratigraphic sequences may be regarded
as a composite record of
• sea-level fluctuations (causing changes in accom-
modation space),
• changes in sediment supply (product of carbonate
production and erosion) controlled by
• environmental factors (climate; growth potential of
organisms) independent of sea level. Changes in the
water temperature and nutrients, e.g. control the for-
mation of reef carbonates. Warm tropical and subtropi-
