Page 387 - Carbonate Facies in Geologic History

374 Summary

Solution rims developed at the edges of karst platforms form the substrate for
barrier reefs, chains of islands develop over old karst drainage divides, and the
atoll and faro forms may result from submergence and carbonate growth around
isolated blocks or towers whose steep sides develop their own solution rims.
Spurs and grooves and major passes between the reefs may result from a rejuve-
nated carbonate growth on the edges offormer lagoon channels and solution rills.

Hydrology and Climate

The importance of hydrologic factors in causing and modifying buildups has been
stressed by Stanton (1967) and Wilson (1974). Except for tides, climate and hy-
drologic factors are closely interrelated. A particularly good analysis is given by
Stanton. Examples of how some of these controls interrelate follows:
1. Winds and waves produce several effects to encourage carbonate buildups as well as to
destroy them. The bringing of fresh nutrient-laden water causes organisms to flourish. The
loss of CO 2 in agitated water encourages CaC0 3 production with resultant cementation of
debris. The piling effect of waves furnishes higher areas on which organisms may thrive. Even
onshore winds along the coasts may encourage carbonate production by inducing upwelling
water to bring nutrients. Of course, severe storms destroy carbonate accumulations and make
difficulties for the growth of certain more delicate calcareous organisms, but the positive
effects of water energy generally outweigh the destructive tendencies.
2. Combinations of climate and hydrology control salinity. Arid climate and continental
winds result in an increased rate of evaporation; this, in turn with low tide range and sluggish
circulation caused by complicated coastal configuration, results in evaporative conditions
limiting the abundance and types of organisms capable of forming carbonate. The same
combination enhances precipitation of carbonate cement in the splash zone and in shallow
marine bays.
3. Proper combination of tectonic stabilization, eustatic sea-level fluctuations, and cli-
mate offer important controls over carbonate diagenesis. Lithification of carbonate may be
brought about by subaerial exposure caused by long-term sea-level lowering. Seasonal rain-
fall and periods of aridity cause intense cementation ofthe carbonate mass. Conversely, under
conditions of high rainfall and tropical plant cover, considerable solution of carbonate forma-
tions takes place.

Organic Composition

Controls partly dependent and partly independent of physical processes are af-
forded by variations in growth potential and framework construction of various
organisms during geologic time.
Heckel (1974) presents a diagram showing the importance of various groups of
organisms as producers of carbonate mass and framework throughout geologic
time (Fig. XII-6). Table XII-1 shows six major phases of this evolution as well as
the grouping of organisms into those with sediment-trapping, sediment-binding,
and framework-building potential. Calcareous algae have continuity through the
Phanerozoic, holding about the same ecologic positions and affecting carbonate
sedimentation the same way, whereas waxing and waning of coelenterate groups
such as corals and stromatoporoids have occurred, with special culminations of
framebuilders in the Middle Paleozoic and from the Jurassic to the Holocene.