Page 48 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 3

Geometry of carbonate accumulations

Depositional geometry of recent accumulations is an im- the wave-swept platform top is the preferred location
portant tool for predicting the anatomy of sedimentary of framebuilders and thus of barrier reefs that form a
rocks in the subsurface. One advantage of sediment anal- rim (Fig. 3.2, 3.3). The organic reef structures are fur-
ysis by depositional geometry is that it can be performed on ther strengthened by abiotic cementation that is partic-
remote images, such as seismic or radar profiles and photos ularly extensive there because of high primary porosity
of distant or inaccessible outcrops. Geometry contains sig- and the pumping effect of heavy seas. Furthermore, the
nificant information on the internal structure and the depo- upper slope environment is a preferred location of mi-
sitional history of a formation. This chapter deals first with crobial crusts and cements that stabilize the underpin-
basic controls on the geometry of carbonate accumulations nings of the shoal-water barriers. The production of the
and the suitable terminology for their description. Subse-
quently, we turn to characteristic patterns associated with
A) SILICICLASTIC SHELF
the three carbonate factories or specific depositional envi- bottom profile in
equilibrium with
ronments. continental slope
wave action
BASIC TRENDS IN GEOMETRY OF CARBONATE
ACCUMULATIONS

The geometry of carbonate deposits results from the spa-
tial patterns of production and the superposed effects of
sediment redistribution by waves and currents. Four com-
B) RIMMED PLATFORM
monly occurring patterns in carbonate geometry are directly sand shoals and reefs elevated by
protected lagoon organic framebuilding and
related to principles of carbonate production and the hydro-
syndepositional llithification
dynamics of the water column.
➤ “The rich get richer”. Carbonate factories tend to
build elevated, localized accumulation because biotic
and abiotic precipitation operates best where little other
sediment disturbs the local environment. Once a pro- C) CARBONATE RAMP
duction site has risen above the adjacent sea floor, pre- high energy deposits bottom profile in equilibrium
close to shore with wave action
cipitation is likely to accelerate and build up the ac-
cumulation even faster. This effect is felt in a wide
range of scales, from decimeter-size stromatolite heads
to Bahama-size platforms.
➤ “The sea is the limit”. Carbonate production is high-
est in the uppermost part of the water column but the
terrestrial environment immediately above is detrimen- Fig. 3.1.— Shore-to-slope profiles of siliciclastics, cool-water car-
tal to carbonates (Fig. 2.3). Consequently, carbonate ac- bonates and rimmed carbonate platforms. A) Siliclastics with abun-
dant sediment supply. Result is seaward dipping surface in equi-
cumulations tend to build flat-topped platforms close
librium with deepening wave base. B) Rimmed carbonate plat-
to sea level. Small differences in production are lev-
forms. On tropical platforms, the wave-equilibrium profile is grossly
eled out through sediment redistribution by waves and
distorted by the construction of wave-resistant reefs and quickly
tides.
lithifying sand shoals, both occurring mainly at the platform mar-
➤ “The bucket principle”. The boundary of the platform gin. Platforms are basically dish-shaped and equilibrium profiles
top shaped by waves and the slope shaped by gravity develop only locally in parts of the lagoon. C) Carbonate ramps
transport is a significant juncture in all depositional sys- are accumulations without rims that resemble the siliciclastic equi-
tems (Fig. 3.1). Tropical platforms tend to form a dis- librium profile. Cool-water carbonate follow this pattern. Tropical
crete rim at the platform-slope boundary. Several ef- platforms commonly show ramps as a transient stage during rapid
fects contribute to rim construction. The outer edge of transgressions before a rim can develop.

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