Page 69 - Carbonate Sedimentology and Sequence Stratigraphy

60 WOLFGANG SCHLAGER

tally steepened ramps show slope facies seaward of facies ramp. If the buildups coalesce to a continuous belt, the sys-
2. Using Wilson’s (1975) standard facies for description of tem becomes a rimmed platform. Whether a belt of reefs
ramps is advantageous because ramps often grade upward or mounds acts as an effective rim can be determined from
into rimmed platforms. Rimmed profiles and ramp profiles the facies pattern in plan view. If the facies patterns within
may also alternate in space where the rim index is low. each buildup are nearly concentric and the surrounding fa-
In most instances, the analogy with siliciclastic shelf mod- cies shows no systematic difference between the landward
els is not perfect because carbonate systems maintain some and the seaward sides of the buildup belt, the barrier func-
rim building capability. Offshore highs may be capped by tion problably is negligible. In effective barriers, the facies
reefs, such as in the Persian Gulf, and mud mounds or pin- patterns of buildups show landward-seaward asymmetry
nacle reefs may develop on the homocline of facies belt 2. and the surrounding sediments are different on the land-
If these buildups are isolated features in facies belt 2, their ward and seaward side.
presence is compatible with classifying the system as a

Fig. 4.6.— Recognition of ecologic reefs in the geologic record Fig. 4.7.— Rimmed margins are common on carbonate plat-
depends on a variety of features; most of them require outcrops forms. Reefs (facies 5) are important rim builders, but early-
or cores as an observational basis. After James and Macintyre lithified, stacked sand shoals (facies 6) can be equally effective
(1985). A) Characteristic aspects of reefs. Note particularly the in defending platforms. A rim’s effectiveness in protecting the in-
multiple successions of different encrusters and frame builders as terior depends largely on its continuity, measured by the rim index
well as cavities with internal sediment and cement. B) Character- (see p. 42). Reef and sand shoals may also occur together on a
istic sediment textures in reef deposits. platform margin. After James and Macintyre (1985).