Page 54 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 3: GEOMETRY OF CARBONATE ACCUMULATIONS 45

EROSION
-6 Bahama Escarpment
>25°
carbonates
-5

-4
tangent top-third of slope -2 siliciclatics BYPASS (neutral)
-3

Exuma transect ODP
-1
10-12°
1 2 3 4 5
height of slope (km)

Fig. 3.11.— Modern submarine slope angles of carbonate plat-
forms and siliciclastic systems in Atlantic and Pacific. Contours of
1, 2 and 4% of total sample (N) in unit area. N (carbonates) =
413, N (clastics) = 72. Carbonates steepen with height by building ACCRETION
slump-resistant slopes. After Schlager (1989), modified. Blake transect ODP
2-3°

duces erosional slopes where the sediment budget of the
slope is entirely negative: more material is exported by ero-
sional turbidity currents and by slumping than the slope re-
ceives from the platform. The change from accretionary to Fig. 3.12.— Slope angle and the balance of erosion and deposi-
bypass to erosional slopes with increasing declivity is well tion on slopes. In the accretionary stage, slope angle is a function
supported by observations on mud-rich slopes. It is un- of rate of sediment supply and rate of relative sea-level rise. As
clear if slopes dominated by rubble and sand or slopes con- slope angle increases, the vigor of turbidity currents also increases
trolled by automicrite accretion change in a systematic way and changes the depositional regime from accretion to erosion. By-
as slope angle increases. It is well established, however, that pass slopes represent an intermediate stage. They receive mud
such slopes can maintain much steeper slope angles than the from the perennial rain of sediment, but large turbidity currents by-
mud-rich ones (Figs 3.13, 3.14, 3.15). pass them. After Schlager and Camber (1986), modified.

Slope curvature

The curvature of many slope profiles has been shown vex part that reflects the smoothing of the shelf break
to obey rather simple mathematical functions that provide by storms and sea-level fluctuations (Fig. 3.17).
clues to the transport regime on the slope (Kenyon and Tur- In carbonates, concave profiles that culminate in a very steep
cotte, 1985; Adams and Schlager, 2000; Schlager and Adams, upper slope and sharp shelf break correspond to rimmed
2001). On reasonably smooth slopes, three types of curva- platforms with reefs or lithified sand shoals at the margin.
ture have been observed: Sigmoidal profiles, on the other hand, indicate predomi-
➤ Concave slopes normally fit a negative exponential nantly detrital accumulations where reefs or early-lithified
function; this probably reflects the exponential decay sands occur only as lenses. Straight-line profiles have been
with distance from source in the sediment volume found to be controlled by lime sand or rubble at the angle
moved by creep, slumping and sediment gravity flows. of repose (Kenter, 1990; Adams et al., 2002). Straight slopes
➤ Straight-line profiles are normally steep and were of the M factory may also contain significant portions of au-
found to correspond to non-cohesive sediment piled up tomicrite layers besides sand and rubble. These rigid lay-
to the angle of repose. ers of automicrite episodically slide and break up to form
➤ Sigmoidal profiles are interpreted as a combination of megabreccias at the toe-of-slope.
an exponentially curved lower part and an upper, con-