Page 114 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7

Sequence stratigraphy of the T factory

CARBONATE FACTORIES AND THE PRINCIPLE OF dipping shelf - called ramp by carbonate sedimentolo-
DEPOSITIONAL BIAS gists - evolves to a rimmed platform where an offshore
belt of reefs or sand shoals protects a lagoon. Large
Depositional systems resemble newspapers that all report
on the events of the day but each with a different editorial parts of this lagoon may fill with sediment right up to
sea level because large oceanic waves are filtered out by
bias. It behooves the reader to learn about the editorial bias the rim and cannot affect the lagoon floor.
of his paper. Similarly, the geologist ought to know about ➤ Rimmed platforms are flatter than siliciclastic shelves or
the bias of depositional systems in recording sea level and may show a reversed dip from the rim into the lagoon.
other environmental factors. If the rim is drowned during sea-level rise, facies belts
The three carbonate factories each have their own bias usually jump landward by a considerable distance.
in recording sequence-stratigraphic events and all three dif-
➤ T carbonates are rich in metastable minerals. Lithifica-
fer to varying degrees from the siliciclastic standard model.
tion by dissolution of these metastable components and
This is not to say, however, that sequence stratigraphy does
precipitation of stable calcite is rapid. Once lithified, the
not apply to these systems. On the contrary, comparative
limestones develop karst surfaces where mechanical
sedimentology of depositional systems clearly shows that denudation is minimal and chemical denudation is rela-
the basic features of the standard model are shared by all tively slow because much rainwater percolates through
depositional systems, showing once again the power of se- the young, porous rock rather than flowing over its sur-
quence stratigraphy as a unifying concept. face. As a consequence, the highstand systems tracts
In this chapter, we base our discussion of carbonate se-
of T carbonates are often better preserved than in silici-
quence stratigraphy primarily on the deposits of the T fac-
clastics or cool-water carbonates where they get washed
tory. They are volumetrically dominant in the geologic
down as sea level falls.
record and their sequence stratigraphy is best known. The
➤ Large amounts of particulate material from clay- to
sequence stratigraphy of the C and M factories is developed
granule size are shed down the slope and into the basin
in chapter 8 by comparison with the T factory and the stan-
because accommodation on the very shallow T plat-
dard model.
forms is limited and the factory normally produces far
more than can be stored on the platform top.
T FACTORY – KEY ATTRIBUTES FOR SEQUENCE
STRATIGRAPHY
T FACTORY - SEQUENCE ANATOMY
The T factory was characterized and compared with the
other factories in chapter 2. Below is a summary of attributes Importance of platform rims
that are crucial for sequence stratigraphy.
➤ The T factory is highly productive but the depth win- The ability of tropical carbonate platforms to develop
dow of production is very narrow, usually the upper- rigid, wave-resistant structures at the shelf break normally
most 100 m (or less) of the water column. leads to elevated margins at sea level that protect a slightly
➤ The factory can build wave-resistant structures in the deeper lagoon that gently rises to the littoral zone farther
depositional environment either by organic framebuild- landward (see chapter 3). These “defended” platform mar-
ing or by cementation of sands on the sea floor or dur- gins are probably the most important feature in the sequence
ing brief exposures. These structures are independent anatomy of tropical carbonate accumulations. When these
of shoreline position. They may form close to shore or rims are overwhelmed by sea level, they cause facies belts
far offshore. Once formed, they tend to stay in place to jump and interrupt the gradual shift in onlap. This jump
and build upward rather than migrating laterally. must not be confused with a sea-level fall. Furthermore,
➤ Because of the ability to form rigid structures, the classi- elevated rims have a strong tendency to stack vertically,
cal seaward-sloping shelf in equilibrium with sediment putting reef on reef, sand shoal on sand shoal, because the
supply, sediment caliber and wave energy is a transient environmental conditions favor the establishment of a new
condition rather than a stable equilibrium as in silici- reef on top of an old reef, an oolite shoal on top of an older
clastics or cool-water carbonates. In the T factory, the shoal. Fig. 7.1 shows the combined effects of rim-building

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