Page 51 - Carbonate Sedimentology and Sequence Stratigraphy

42 WOLFGANG SCHLAGER

A C
G b =C-(D-F) F= sum of frictional losses G a =C-D C = construction growth potential
D = destruction
b a
x G = reef growth
rate of rise increased subsidence

distance from hinge line
B lagoon reefs
position of hinge line
sealevel rise

beach ridges & dunes lagoon differential subsidence (tilt)
reef

Fig. 3.5.— Carbonate platforms backstep when faced with relative sea-level rise that slightly exceeds their growth potential. This figure
shows reasons why backstepping may be advantageous for a platform under stress. A) Destruction of the margin by waves is less in the
backstepped position because waves have lost energy by bottom friction. B) Backstepping to higher ground. C) Backstepping to area of
lower subsidence (on passive margin).

Platform rim. The geometric effect of rim-building is best il- the zone of wave action is the presence of a flat top. De-
lustrated by direct comparison of rimmed platforms and gree of continuity of the rim may be expressed as the rim
siliciclastic shelves that lack constructional rims (Fig. 3.1). index, defined as the fraction of the platform perimeter that
The rim is more productive than the adjacent lagoon or up- is occupied by reef or sand shoal (Fig. 3.6). The rim index
per slope. Excess sediment is shed into the lagoon and down is highly variable and should be quantitatively estimated
the slope. The geometric expression of the high production wherever possible, for instance via seismic data, maps or
and the export of vast quantities of sediment is the common large outcrops. Quantification of rim continuity provides an
pattern of bi-directional progradation away from the rim: estimate of the fraction of oceanic wave energy that enters
progradation of the backreef apron into the lagoon and si- the lagoon. In first approximation, the fraction of wave en-
multaneous seaward progradation of the rim and the slope. ergy that passes through a leaky rim is the inverse of the rim
The degree of wave resistance of the rim varied in time index.
and space. During most of the Phanerozoic, shoal-water
carbonate systems were able to build rims in the zone of
perennial wave action. Currently, some reef communities
Rim Index = (R1+R2+R3)/L
can build into the intertidal zone even in settings that face
the full power of oceanic waves in the trade-wind belt. The
system can build into the supratidal zone by forming “is-
lands” of storm ridges that may contain freshwater lenses Platform margin
Land
and be capped by terrestrial (carbonate) eolianites.
The platform rim need not be a reef. Carbonate sand R3
shoals can also form wave-resistant barriers at the platform Platform
margin. They may consist of oolites, precipitated locally in
the mixing zone of normal marine and platform waters, or R2
of skeletal debris from the outer, winnowed parts of the plat-
form. Either type can build into the supratidal zone and is R1 L
prone to early lithification in the submarine or the suprati-
dal environment. This early lithification greatly enhances Open Sea
the wave resistance of the shoal and reduces the rate of lat-
eral migration to almost zero. Consequently, the hydrody-
namic effect of these shoals is similar to that of reefs and reef
aprons, both are wave-resistant, stationary structures near Fig. 3.6.— Rim index is the fraction of the platform margin occu-
the platform margin. pied by reefs or shallow sand bars. Sketch depicts land-attached
The efficiency of reefs and shoals as barriers against wave platform with leaky rim of reefs (R1 etc.) shown in black. Fraction
energy depends on the elevation of the crest and the con- of wave energy passing the leaky rim is the inverse of the rim index.
tinuity of the rim. A geometric criterion for elevation into