Page 52 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 3: GEOMETRY OF CARBONATE ACCUMULATIONS 43

Ramp basin fills up and slope height and slope angle decrease (Fig.
3.8B; e.g. Biddle et al., 1992).
Ramps are shoal-water carbonate systems that lack the
steep slope seaward of the platform margin on rimmed plat- Slope, rise, basin floor
forms. They show a seaward-sloping surface with dips of
◦
0.1 - 1.5 instead (Fig. 3.1). According to many authors, Slopes and debris aprons around platforms are important
ramps may possess an offshore rim and a lagoon. This com- elements of the edifice and largely determine the extent and
plicates the definition of ramp. Originally, ramps were de- shape of the top. These areas also act as sinks for much of the
picted as systems devoid of an offshore rim (Ahr, 1973; Wil- excess sediment produced by the platform top. In sequence
son, 1975. Under these conditions, the distinction of ramps stratigraphy, platform margins and slopes play a crucial role
and rimmed platforms is easy: rimmed platforms have a as they hold much of the information on lowstands of sea
high-energy facies belt offshore, ramps lack this belt; their level.
only belt of high-energy sediments is in the littoral zone Geometry and facies of slopes and rises rich in mud are
close to shore. governed by several rules:
Subsequent studies produced a substantial number of ➤ The volume of sediment required to maintain a constant
ramp surfaces that are not hung on the shoreline but on slope increases as a function of platform height. The
an offshore rim with a protected lagoon between the high- increase is proportional to the square of the height for
energy belt of the offshore rim and the coastal one. For conical slopes of isolated platforms, such as atolls, and
those ramps, the high-energy-only-nearshore criterion does it is proportional to the first power of the height of lin-
not work. These “detached ramps” are rimmed systems that ear platform slopes, such as on passive margins (Fig.
do not have a slope but a ramp surface seaward of the rim. 3.9). These geometric laws limit platform growth, par-
(Fig. 3.7) ticularly on high atolls (Fig. 3.10).
Read (1985) recognized homoclinal and distally steepened ➤ The upper parts of platform slopes steepen with the
ramps. This distinction is useful. Distally steepened ramps height of the slope, a trend that siliciclastics abandon
sit on top of a slope, commonly a continental slope. The at early stages of growth because they reach the angle
slope may be the inactive slope of a rimmed platform that of repose of mud. As a consequence, the slopes of most
has backstepped. The homoclinal ramp with a uniformly platforms, notably the high-rising ones, are steeper than
dipping surface is typically found in shallow intracratonic siliciclastic slopes (Fig. 3.11). Changes in slope angle
basins or foreland basins. Even in these settings, the ramp during platform growth change the sediment regime on
morphology is normally a transient feature. Carbonates the slope: the balance between erosion and deposition
with rim-building capability have a strong tendency to pro- of turbidity currents shifts such that the slopes evolve
grade, steepen their slope and thus differentiate into a plat- from the accretionary to bypass and finally to erosional
form top, a rim and a steep slope (Fig. 3.8A). The reverse conditions. This change in depositional regime in turn
transition, from rimmed platform to ramp, occurs when a changes sediment geometry on the slope and the rise
(Fig. 3.12).
➤ The angle of repose of loose sediment is a function of
ATTACHED RAMP: from shoreline to below
storm wave base or beyond grain size. This relationship has been quantified in the
high-energy facies
ramp to rim (slope height generally increasing)
sea level

DETACHED RAMP: from high-energy zone at
platform rim to storm wave base or beyond rim to ramp (slope height generally decreasing)

high-energy facies
sea level

Fig. 3.7.— Number and location of high-energy sand belts allow
one to distinguish between the classical attached ramp with high- Fig. 3.8.— A) Transition from ramp to rimmed platform with sharp
energy deposits only close to shore (top panel) and the detached shelf break is commonly observed where a basin deepens and
ramp that starts from a high-energy shoal that lies offshore (lower slopes become higher and steeper as they prograde. B) Transi-
panel). The latter system has two high-energy belts separated by tion from rimmed platform to ramp occurs where a basin fills up
a lagoon. and the relief flattens.