Page 382 - Carbonate Facies in Geologic History

Interrelations of Parameters Controlling and Modifying Carbonate Accumulations 369

merely carry sediment across the flat top to be trapped and stabilized by blue-
green algal growth. A third type of veneer is known in some mounds which
originated in deeper water, and accumulated just to wave base. Here gradual
winnowing may have concentrated worn and resistent bioclasts at the top of the
bamestone unit. In some mounds extensive vertical fissures are filled with the
products of this winnowing, accumulated as blackened, worn, and coated parti-
cles washed in from above.
5. Flanking beds: If the mound top remained for a long time, just at wave
base, and became colonized by more or less fragile, stalked organisms, moderate
water movement and normal organic decay may have resulted in extensive flank
beds. These include echinoderms, fenestrate bryozoans, small rudists, dendroid
corals or stromatoporoids, branching red algae, and tubular foraminifera encrust-
ing noncalcareous organisms. The flank beds lap up on the sides of the mound
and are composed exclusively of bioclastic debris. Under conditions of only slight
subsidence, the original mound core may be almost buried in flank beds which
accreted out from it on all sides. Volumentrically such beds may be greater than
the core itself. This is to be observed in some Silurian reefs in the Midcontinent as
well as in Pennsylvanian algal plate mounds in New Mexico.
6. Talus: A rarer, but widespread flank facies, is that of marine talus com-
posed of lithoclastic and bioclastic debris, the former being the key component.
The lithoclasts represent chunks of partly or wholely lithified micrite torn from
the surface of the mound by collapse or wave action and slumped or carried down
by currents along the sloping sides of the mound into normal flank beds. Since
carbonate mud mounds are generally found in areas of low wave energy, this
deposit is often missing. Exactly what processes formed these local lithoclastic
conglomerates still constitutes a mystery. The smooth, even shapes of most
mounds do not indicate much erosion off the tops.
7. Capping grainstones: When sea level remained stable and the intermound
areas filled in with sediment, often a shelf deposit formed across the top of the
mounds. Commonly this is a single stratum of cross-bedded grainstone over the
whole area. In many places this high energy, post-mound sediment usually con-
tains a specialized fauna consisting of robust gastropods and dasycIadacean algae.

Interrelations of Parameters Controlling and Modifying
Carbonate Accumulations

Clear, well-lit, warm, marine tropical water producing carbonate sediments sus-
tains a very ancient system of chemical and biochemical processes. It is as old as
2.5 billion years. This system, discussed in Chapter II and diagrammed in Fig. 11-
12, is responsible for thousands of meters of mostly shallow-water limestones and
dolomite typical of the geological record. Carbonate production operates most
efficiently in marine areas adjacent to tectonically stable land and in provinces
where there is little fresh-water runoff. In such areas limpid shallow marine water
borders and covers extensive shelves around land areas.