Page 53 - Carbonate Sedimentology and Sequence Stratigraphy

44 WOLFGANG SCHLAGER

Recent and the same numerical relationships have re- ment gravity flows also increases and significant volumes of
cently been shown to apply to large-scale, geological sediment start to bypass the slope and come to rest on the
features of the past (Figs 3.13, 3.14, 3.15). The most im- adjacent basin floor. The geometric expression of a bypass
portant control on the angle of repose is the degree of slope are downslope gullies with erosional flanks, scours
cohesion of the sediment. and gravel lags; intergully sediment is mainly mud with
The change in depositional regime with changing slope only few turbidites. Basinward of the gullied slope a sed-
angle is largely caused by the degree of sediment bypassing iment apron develops as a series of laterally coalescing tur-
on the slope. The sediment source of most carbonate slopes bidite fans (Schlager and Chermak, 1979; Mullins et al., 1984;
is the platform. Thus, sediment enters the slope at the up- Harwood and Towers, 1988). These aprons are compara-
per end and is distributed downslope by sediment gravity ble to the continental rises of deep ocean basins. They are
transport. At gentle slope angles, the competence and ca- dominated by turbidite sheets; erosional channels (fan val-
pacity of the transporting agents decreases steadily away leys) are shallow and scarce. In a global survey, Heezen et
from the sediment source at the platform margin. Conse- al. (1959) suggested that the boundary between slopes and
◦
quently, sedimentation rates decrease and time lines con- rises in the ocean typically lies at about 1.4 (tanS = 0.025).
verge basinward. At this stage, slope angle is a function of Carbonate data indicate higher variability but overall simi-
therateofsediment supplyand thegrain sizeofthe sedi- lar averages for the slope-rise boundary. (Fig. 3.16)
ment (Fig. 3.13). As declivity increases, the vigor of sedi- Steepening of slopes beyond the realm of bypassing pro-
Atoll "cone" Platform "prism"
Fig. 3.9.— Upward growth of carbonate plat-
forms with constant slope requires deposition of
∆h ∆h
ever larger volumes of sediment on the flanks. In
the case of a cone-shaped atoll, the growth of vol-
∆h ∆h
ume (V) is proportional to the square of the height
∆V ∆V
2 2 of the cone. In case of a linear platform margin, the
∆V 1 ∆V 1 growth in volume of the slope prism, V, is propor-
h h tional to the height of the platform. Schlager (1981),
α α modified.
h h
volume V volume V
tan α tan α
∆V , ∆V = increments in volume caused
1
2
by vertical growth ∆h of platform or atoll
πh³ volume of slope prism of h²
volume of atoll cone V = V = l
3tan²α length "l" 2tanα
∂V π ∂V l
growth of volume = h² growth of volume = h
∂h tan²α ∂h tanα

0

1000 m

5 km 10 km

Run time = 400 ky, line spacing = 40 ky Maximum production, C = 7.835 m/ky at 0-5 m depth
1
2
Diffusion: K nonmarine = 100 m /y, K marine = 1 Subsidence = 2 m/ky
Fig. 3.10.— Effect of increasing slope height on two atolls of different size, modeled by program STRATA ( appendix B). Carbonate
production of both atolls starts in area marked by black bars. Both atolls first aggrade, then retrograde as slopes get higher. Retrogra-
dation reduces the production area at the top and leads to gradual drowning. Time of drowning depends on the size of the production
area. After 400 ky, the large atoll still is healthy and its flat top built to sea level. The small atoll is virtually dead. Its top lies 100 m below
sea level where production is so low that it no longer compensates for sediment loss by downslope transport – aggradation ceases and
the flat top changes into a mound.