Page 117 - Carbonate Sedimentology and Sequence Stratigraphy

108 WOLFGANG SCHLAGER

HST progradation + aggradation A) Bahamas
sea-level rise
G ', G ' > A'
r
p
reef(?)
downlap
reef(?)
surface
HST empty bucket SB
G ' > A' SB
r
G ' < A'
p
B) Cap Blanc, Mallorca

TST backstepping
G ', G ' < A'
r
p
100 m
0
TST drowning 5 km
Heterostegina unit
G ', G ' < A'
p
r
Fig. 7.5.— Downstepping platform margins are one of the best
geometric indicators of sea-level fluctuations in tropical carbonates.
In both examples, red shading indicates the minimum extent of the
lowstand systems tract that can be defined with respect to the red
LST lowstand wedge, prograding reference profiles. A) Bahamas, Cenozoic, based on seismic data
and some bore holes. After Eberli and Ginsburg, (1988), modified.
G ' > A' B) Miocene, Mallorca, based on continuous outcrops and borings.
r
Upstepping and downstepping of the reef belt (black) reveals a hi-
erarchy of rhythms. After Pomar (1993), modified.

Sea-level movements deduced from seismic images of carbonate
SB exposure
platforms
A' < 0
With their flat tops built to sea level and their biota very
sensitive to water depth, carbonate platforms are one of the
most reliable dip sticks in the ocean. This quality is en-
A' = rate of change in accommodation
G ' = growth rate of platform interior hanced by the resistance to erosion of exposed platforms.
p
G ' = growth rate of platform rim Reefs are “born” as rock-hard structures, other platform de-
r
HST, TST, LST = system tracts posits frequently lithify within a few thousand years when
SB = sequence boundary exposed. Subsequent erosion is largely chemical and oper-
ates within the rock rather than at its surface (see above).
Fig. 7.4.— Basic geometries of tropical platforms and their inter- Surface denudation is generally less than in siliciclastics and
pretation in terms of rate of change in accommodation, A’, and rate a reasonable sea-level record can be gleaned from platforms
of sediment supply, G’. The letter G stands for carbonate growth, by determining overall subsidence and measuring the thick-
indicating that most material is produced within the depositional en- ness of marine intervals plus position and timing of expo-
vironment even though lateral transport may be significant. Panels sure horizons (see Fig. 7.4 and Ludwig et al., 1988; McNeill
1,3,5 and 6 are strictly analogous to the systems tracts of the stan-
dard model (Fig. 6.12). Panels 2 and 4 labelled in red, are specific et al., 1988).
to tropical carbonates. Empty bucket and complete drowning illus- The combination of defended margins and enhanced re-
trate the importance of the growth potential of the various elements sistance to erosion creates some special opportunities for
of the system. Submergence below the euphotic zone means that sequence stratigraphy. Rapidly prograding platform mar-
parts or all of the production system are shut down. gins tend to preserve the original elevations of the shelf sur-
faces particularly well, including the very important low-
block and therefore experienced the same relative sea-level stand systems tracts. Eberli and Ginsburg (1988), Eberli et al.
changes. The pronounced difference between windward (2001), Sarg (1988; 1989) and Pomar (1993) have contributed
and leeward platform margin must be entirely due to dif- excellent examples of sea-level curves gleaned from carbon-
ferences in carbonate production and supply. ates using the technique of the fluctuating shelf surface (Fig.